Operation and Optimisation of Activated Sludge Wastewater Treatment Plants

Practical Guide to the Operation and Optimisation of Activated Sludge Wastewater Treatment Plants

R i c h a r d B r i c e a n d Pe t e r M o s s e

Acknowledgements Many people contributed to the preparation of this Guide. The principal authors of each chapter are shown below.

Chapter 1 Waste Treatment and the Development of Activated Sludge Systems Peter Mosse Chapter 2 Hazards Risks and Control Points Peter Mosse Chapter 3 The Bugs. The Good, Bad and Ugly Peter Mosse Chapter 4 Biological Nutrient Removal Peter Mosse Chapter 5 Chemical Phosphorus Removal Peter Mosse Chapter 6 Activated Sludge Plant Configurations Peter Mosse Chapter 7 Operation and Optimisation of Activated Sludge Plants Richard Brice & Peter Mosse Chapter 8 Aeration Ken Hartley & Richard Brice Chapter 9 Prefermentation Ken Hartley Chapter 10 Intermittent Systems Kay White & Peter Mosse Chapter 11 Monitoring the Activated Sludge Process Peter Mosse Chapter 12 Secondary Clarifiers Richard Brice Chapter 13 Trouble Shooting Richard Brice Chapter 14 Online Monitoring and Instrumental Control Rob Dexter, Richard Brice & Peter Mosse Chapter 15 Modelling the Activated Sludge Process Richard Brice Sincere thanks to Dr John Messenger the principal technical reviewer.

Thanks also to the following people for their contributions and/or for reviewing all or part of the Guide: • Adrian Harper • Paul Keating • Kim Mosse • Charlie Suggate • Peter Tolsher • Chris West Thank you also to George Wall for assisting with the review of the content and overseeing the publication of the final book.

WIOA would like to thank the sponsoring partner organisations: • Australian Water Association (AWA) • Government Skills Australia (GSA) • International Centre for Excellence in Water Resources Management (ICE WaRM) • National Water Commission (NWC) • New South Wales Water Directorate • Pipes Wagga Wagga • Queensland Water Directorate (qldwater) • Water Industry Training Association Queensland (WITA) • Water Services Association of Australia (WSAA)

Publications Details First Edition, August 2014.

Ordering More Copies An order form is available from the WIOA website (www.wioa.org.au) for people wishing to order additional copies, any of the other Guides in the wastewater series or any of the Practical Guides to Water Treatment.

General Introduction The Water Industry Operators Association of Australia (WIOA) with the assistance of a number of supporting organisations and industry experts, produced a series of Practical Guides to the Operation of Water Treatment Plants.

As a result of strong demand, WIOA has now extended the project to work with industry experts to create a new series of Practical Guides encompassing wastewater treatment. The topics covered in this series include: 1. Odour control in sewage transport systems.

2. Operation and optimisation of activated sludge wastewater treatment plants.

3. Operation and optimisation of lagoon treatment systems.

4. Operation and optimisation of reclaimed water systems.

An underlying objective of each of the Guides is that on each and every page, operational staff should be able to make mental notes to themselves: •• That’s a good idea.

•• We should be doing that.

•• We can start on that immediately.

The Guides are not about identifying costly solutions but simply identifying ways to get the best out of what is available at any particular time. The aim is to provide suggestions and hints that can be implemented easily, cheaply and immediately in the field. They are about getting the absolute best quality effluent out of the system at all times.

Feedback Requested These Guides have been written to help operators perform their jobs effectively and to help operators understand the vitally important roles they fulfil within the water industry. The authors would be pleased to receive any feedback on this Guide. Suggestions for further information or areas where additional explanation would have aided understanding, can all help to make any second edition stronger and more useful for operators in our industry. Any suggestions can be directed by e-mail to the Water Industry Operators Association of Australia (WIOA) at info@wioa.org.au or by post to PO Box 6012, Shepparton, Victoria, 3632.

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Contents Contents General Information Abbreviations and Acronyms........................................................................................................................ 6 Chapter 1 Wastewater Treatment and the Development of Activated Sludge................... 8 •• The Importance of Wastewater Treatment........................................................................................ 8 •• The Scope of the Problem................................................................................................................. 8 ◦◦ Sewage Composition.................................................................................................................... 8 ◦◦ Concentrations of Ammonia, Nitrate and Phosphate in Wastewater............................................ 9 ◦◦ Flows.......................................................................................................................................... 10 ◦◦ Inflow and Infiltration................................................................................................................... 10 ◦◦ Population Equivalent (PE)......................................................................................................... 11 •• Some of the Solutions to the Wastewater Disposal Problem.......................................................... 11 ◦◦ Lagoons...................................................................................................................................... 12 ◦◦ Fixed Film Treatment (Trickling Filters)....................................................................................... 12 ◦◦ Activated Sludge......................................................................................................................... 13

Chapter 2 Hazards Risks and Control Points................................................................................... 14 •• Hazards........................................................................................................................................... 14 •• Hazardous Events........................................................................................................................... 16 •• Control Points.................................................................................................................................. 16 ◦◦ Critical Limits and Target Criteria................................................................................................ 17 ◦◦ Corrective Actions, Incident and Emergency Management ....................................................... 18 ◦◦ Monitoring................................................................................................................................... 18 ◦◦ Sampling..................................................................................................................................... 19

Chapter 3 The Bugs. The Good, Bad and Ugly................................................................................ 20 •• The Activated Sludge Ecosystem.................................................................................................... 20 ◦◦ Mixed Liquor............................................................................................................................... 20 ◦◦ Microscopic Examination of the Mixed Liquor............................................................................. 21 ◦◦ Bulking and Foaming.................................................................................................................. 25

Chapter 4 Biological Nutrient Removal................................................................................................ 29 •• Basic Activated Sludge Carbon Chemistry...................................................................................... 29 ◦◦ Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)........................... 29 •• The Role of Bacteria in Wastewater Treatment............................................................................... 30 ◦◦ Carbon (COD) Removal............................................................................................................. 31 ◦◦ Nitrogen Removal....................................................................................................................... 32 ◦◦ Nitrification.................................................................................................................................. 33 ◦◦ Denitrification.............................................................................................................................. 34 ◦◦ Simultaneous Nitrification Denitrification (SND).......................................................................... 35 ◦◦ Phosphorus Removal................................................................................................................. 36 ◦◦ Biological Nutrient Removal Summary ...................................................................................... 42

Chapter 5 Chemical Phosphorus Removal........................................................................................ 43 •• Dosing Points.................................................................................................................................. 44 •• Mixing.............................................................................................................................................. 45 •• Alkalinity and pH.............................................................................................................................. 46 •• Theoretical Dosing Required........................................................................................................... 46 •• Jar Testing....................................................................................................................................... 48 •• Total Dissolved Solids and MLVSS.................................................................................................. 48 •• Residual Iron and Aluminium Levels............................................................................................... 48 •• Monitoring....................................................................................................................................... 49 •• Anaerobic Digester Performance.................................................................................................... 49 •• Sludge Volumes.............................................................................................................................. 49

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Chapter 6 Activated Sludge Plant Configurations......................................................................... 50 •• Continuous Plants........................................................................................................................... 50 •• Oxidation Ditch Systems................................................................................................................. 54 •• Intermittent Systems ....................................................................................................................... 55

Chapter 7 Operation and Optimisation of Activated Sludge Plants...................................... 57 •• Principles of Operation.................................................................................................................... 57 ◦◦ Sludge Age................................................................................................................................. 57 ◦◦ Wasting....................................................................................................................................... 57 ◦◦ Separation.................................................................................................................................. 57 ◦◦ Nutrient Balance......................................................................................................................... 58 ◦◦ Protection from Toxic Material.................................................................................................... 58 •• Optimisation of the Plant................................................................................................................. 58 1. Collect Relevant Drawings and Manuals.................................................................................. 58 2. Label all Pits, Valves, Sampling Points and Directions of Flow................................................. 58 3. Control Wasting to Achieve the Required Sludge Age ............................................................. 59 4. Maintain a Good Settling Biomass............................................................................................ 63 5. Control DO and Monitor the Oxygen Uptake Rate (OUR) ....................................................... 69 6. Ensure Sufficient Alkalinity and Suitable pH............................................................................. 71 7. Ensure Necessary Nutrient and Nutrient Ratios are Available.................................................. 73 8. Manage Internal Recycles and RAS in Continuous Plants....................................................... 75 9. Observe Reactors and Clarifiers Regularly............................................................................... 76 10. Manage any Return Streams ................................................................................................... 76 11. Manage Peak and Holiday Population Flows........................................................................... 77 12. Know the Hydraulic Retention Time (HRT)............................................................................... 78 •• Summary Requirements for N removal........................................................................................... 79 •• Summary Requirements for EBPR.................................................................................................. 79

Chapter 8 Aeration.......................................................................................................................................... 81 •• Mechanical Aerators........................................................................................................................ 82 •• Diffused Air...................................................................................................................................... 84 •• Oxygen Transfer.............................................................................................................................. 88 •• Oxygen Transfer Efficiency............................................................................................................. 88 •• Mixing.............................................................................................................................................. 89 •• Carbon Dioxide Stripping and pH.................................................................................................... 89

Chapter 9 Prefermentation.......................................................................................................................... 90 •• Activated Primary Tanks and Sidestream Fermenters.................................................................... 90 •• Process Principles........................................................................................................................... 92 ◦◦ Activated Primary Tank (APT)..................................................................................................... 92 ◦◦ Side-stream Fermenters............................................................................................................. 94 •• Additional Operational Checks........................................................................................................ 94 ◦◦ APT and Side-stream Fermenters.............................................................................................. 94 ◦◦ Side-stream Fermenters............................................................................................................. 95 ◦◦ Monitoring................................................................................................................................... 95 •• RAS Fermentation........................................................................................................................... 95

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Chapter 10 Intermittent Systems .............................................................................................................. 96

•• Basic Features of SBR and IDEA Plants......................................................................................... 96 •• The Intermittent Cycle..................................................................................................................... 98 ◦◦ Differences between intermittent and continuous plants.......................................................... 100 •• Types of Decant Systems.............................................................................................................. 100 •• Modifications for Nutrient Removal............................................................................................... 104 ◦◦ Denitrification............................................................................................................................ 105 ◦◦ Simultaneous Nitrification Denitrification (SND)........................................................................ 107 ◦◦ Phosphorus Removal............................................................................................................... 107 ◦◦ Consequences of Operating Intermittent Plants for Nutrient Removal .................................... 108 •• Operation of Intermittent Plants..................................................................................................... 109 ◦◦ Depth Variance with Time ........................................................................................................ 109 ◦◦ Increased Outflow from each Tank .......................................................................................... 110 ◦◦ Clearance Depth during Decant................................................................................................111 ◦◦ Staggering of Cycles ................................................................................................................ 113 ◦◦ Conditions in the Biomass Change during Settling and Decant .............................................. 113 ◦◦ Changes to Operational Parameters ....................................................................................... 115 •• Modifications for Peak Flow (Storm) Events.................................................................................. 118 •• The Bio-Denitro and Bio-Denipho Hybrid Systems....................................................................... 122

Chapter 11 Monitoring the Activated Sludge Process.................................................................. 126 •• Sampling....................................................................................................................................... 126 •• Types of Samples.......................................................................................................................... 126 ◦◦ Grab Sample............................................................................................................................. 126 ◦◦ Composite Sampling................................................................................................................. 127 •• Sample Storage, Preservation and Transport............................................................................... 128 •• Laboratory Analysis....................................................................................................................... 128 •• Online Continuous Monitoring: Avoiding Sampling, Storage and Transport Errors ...................... 128 •• Process Monitoring........................................................................................................................ 129 ◦◦ Influent Control Point................................................................................................................ 129 ◦◦ Anaerobic Zone Control Point................................................................................................... 130 ◦◦ Anoxic Zone Control Point........................................................................................................ 130 ◦◦ Aerobic Zone Control Point....................................................................................................... 131 ◦◦ Clarifier Control Point................................................................................................................ 131 ◦◦ Return Stream from Solids Processing Control Point............................................................... 131 •• Monitoring Raw Sewage............................................................................................................... 132

Chapter 12 Secondary Clarifiers.............................................................................................................. 133 •• Types of Clarifiers.......................................................................................................................... 133 •• Rectangular Clarifiers ................................................................................................................... 133 ◦◦ Removal of Sludge................................................................................................................... 134 •• Circular Clarifier............................................................................................................................. 134 •• Clarifier Operation......................................................................................................................... 136 ◦◦ Operational Parameters............................................................................................................ 136 •• Monitoring Clarifier Performance................................................................................................... 139 ◦◦ Final Effluent Turbidity and Suspended Solids......................................................................... 139 ◦◦ Return Activated Sludge (RAS) Solids...................................................................................... 139 ◦◦ Depth of the Sludge Blanket..................................................................................................... 139 ◦◦ Scraper Checks........................................................................................................................ 140 •• Optimisation.................................................................................................................................. 140

Chapter 13 Trouble Shooting..................................................................................................................... 141 •• 1. General Activated Sludge Plant Trouble Shooting.................................................................... 141 •• 2. Control of Bulking and Foaming................................................................................................ 145 •• 3. Clarifier Trouble Shooting.......................................................................................................... 146

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Chapter 14 Online Monitoring and Instrumental Control .......................................................... 150 •• Influent Load Measurement (BOD, COD, CODf, RBCOD)............................................................ 151 ◦◦ How is it measured?................................................................................................................. 152 •• Dissolved Oxygen......................................................................................................................... 154 ◦◦ Measurement Scenarios........................................................................................................... 156 ◦◦ DO Control................................................................................................................................ 157 •• pH and EC..................................................................................................................................... 160 •• MLSS............................................................................................................................................. 160 •• Ammonia....................................................................................................................................... 161 ◦◦ Measurement of Ammonia in Wastewater................................................................................ 161 ◦◦ Measurement Scenarios........................................................................................................... 161 ◦◦ How Does Ammonia Based Control Work?.............................................................................. 162 ◦◦ Location of Ammonia Probes.................................................................................................... 162 •• Nitrate............................................................................................................................................ 163 ◦◦ Measurement of Nitrate in Wastewater..................................................................................... 163 ◦◦ Measurement Scenarios........................................................................................................... 163 •• Phosphorus................................................................................................................................... 165 ◦◦ Measurement of Phosphorus in Wastewater............................................................................ 165 ◦◦ Use of Online Phosphorus Analysers....................................................................................... 165 •• Installation of Online Analysers..................................................................................................... 166 ◦◦ Sample Location....................................................................................................................... 166 ◦◦ Instrument Turn Down.............................................................................................................. 166 ◦◦ Sample Pre-preparation............................................................................................................ 167 •• Calibration of Online Analysers..................................................................................................... 168

Chapter 15 Modelling the Activated Sludge Process.................................................................... 171 •• What is a Process Model? ............................................................................................................ 171 •• Why Use a Model?........................................................................................................................ 172 •• Building a Model............................................................................................................................ 172 Step 1. Define the Project ........................................................................................................ 172 Step 2. Configure the Model..................................................................................................... 173 Step 3. Data Collection and Checking...................................................................................... 174 Step 4. Verify the Model............................................................................................................ 176 Step 5. Apply the Model to Investigate Optimisation or Proposals for Upgrades...................... 178 •• Some Examples of Modelling Outcomes...................................................................................... 180 Case Study 1: Solids Capture .................................................................................................. 180 Case Study 2: Nutrient Removal Optimisation ......................................................................... 181

Chapter 16 Closing Words.......................................................................................................................... 183 Appendix

•• A. Method to Determine Flocculated Filtered COD and Readily Biodegradable COD.................. 184

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Abbreviations and Acronyms

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An

Anaerobic

A/O

Anaerobic/Oxic

A2/O

Anaerobic/Anoxic/Oxic

Ax

Anoxic

ADF

Average Daily Flow

ADWF

Average Dry Weather Flow

AHF

Average Hourly Flow

APT

Activated Primary Tank

BNR

Biological Nutrient Removal

BOD

Biochemical Oxygen Demand

BWL

Bottom Water Level

COD

Chemical Oxygen Demand

DO

Dissolved Oxygen

DS

Dry Solids

DSVI

Diluted Sludge Volume Index

EBPR

Enhanced Biological Phosphorus Removal

EC

Electrical Conductivity

EPA

Environmental Protection Authority

F:M

Food to Microorganism ratio

g/p/d

Grams/person/day

HRT

Hydraulic Retention Time

IDEA

Intermittently Decanted Extended Aeration

L/p/d

Litres/person/day

MCRT

Mean Cell Residence Time (= sludge age)

MLE

Modified Ludzack Ettinger

MLSS

Mixed Liquor Suspended Solids

MLVSS

Mixed Liquor Volatile Suspended Solids

OFR

Overflow Rate

OTE

Oxygen Transfer Energy

OUR

Oxygen Uptake Rate

PAO

Phosphate Accumulating Organism

PDF

Peak Daily Flow

PDWF

Peak Dry Weather Flow

PE

Population Equivalent

PFS

Polymerised Ferric Sulfate

PHB

Polyhydroxybutyrate

PHF

Peak Hourly Flow

PIF

Peak Instantaneous Flow

PLC

Programmable Logic Controller

Abbreviations and Acronyms

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PST

Primary Sedimentatrion Tank

PWWF

Peak Wet Weather Flow

RAS

Return Activated Sludge

RASSS

Return Activated Sludge Suspended Solids

RBC

Rotating Biological Contactor

RBCOD

Readily Biodegradable Chemical Oxygen Demand

RR

Respiration Rate

SAE

Standard Aeration Efficiency

SBCOD

Slowly Biodegradable Chemical Oxygen Demand

SBR

Sequencing Batch Reactor

SCADA

Supervisory Control And Data Acquisition

SND

Simultaneous Nitrification Denitrification

SOP

Standard Operating Procedures

SOTE

Standard Oxygen Transfer Efficiency

SOUR

Specific Oxygen Uptake Rate

SRT

Solids Retention Time (= sludge age)

SS

Suspended Solids

sSVI

Stirred Sludge Volume Index (Standard Methods test)

SSVI

Stirred Sludge Volume Index (Adjusted to 3,500 mg/L MLSS concentration)

STP

Sewage Treatment Plant

SV30

30 minute Settled Volume

SVI

Sludge Volume Index

TKN

Total Kjeldahl Nitrogen (Sum of organic nitrogen + ammonia)

TN

Total Nitrogen (Sum of TKN + nitrate + nitrite)

TON

Total Oxidised Nitrogen (Sum of nitrate + nitrite)

TP

Total Phosphorus

TS

Table Solids

TSS

Total Suspended Solids

TWL

Top Water Level

UCT

University of Cape Town

UV

Ultraviolet

upCOD

Unbiodegradable Particulate Chemical Oxygen Demand

VFA

Volatile Fatty Acids

VSD

Variable Speed Drive

VSS

Volatile Suspended Solids

WAS

Waste Activated Sludge

WASSS

Waste Activated Sludge Suspended Solids

WLR

Weir Loading Rate

Chapter 1 Wastewater Treatment and the Development of Activated Sludge The Importance of Wastewater Treatment The provision of sanitation and the separation of water and sewage functions in developed nations has been the single most important step to improving public health at the population level.

The cholera epidemics in London in 1839 and 1849 together killed over 100,000 people. The cause of the outbreaks was shown to be contamination of the River Thames with human sewage. The microorganism involved was identified in the early 1880’s as being Vibrio cholerae, a bacterium that is excreted in the faeces of humans. Infection resulted from the unintentional ingestion of faeces in the water, the so-called faecal oral route.

More recently, the health of the environment has become increasingly important. Wastewater contains carbon (C), nitrogen (N) and phosphorus (P) nutrients, as well as pathogenic bacteria. Carbon in the wastewater provides an energy source for the growth of microorganisms. Where the carbon material is readily available to bacteria and there is sufficient oxygen, the bacteria will consume the organic material and rapidly deplete the oxygen. At this point, other bacteria that do not require oxygen take over and continue to use the carbon.

These anaerobic bacteria produce a range of gases, including methane and carbon dioxide, but most noticeably hydrogen sulfide and other odorous sulfur compounds. In this oxygen depleted environment, fish, amphibians, invertebrates and plants die, a situation that does not please human beings.

In addition to the action of these bacteria changing the subtle ecological balance in the receiving environment, P (phosphate) and N (ammonia and nitrate) provide a stimulus for the growth of algae, in particular green algae and blue-green algae (cyanobacteria). This process is called eutrophication.

Eventually the aquatic life dies off and as it decomposes, oxygen is consumed which lowers the dissolved oxygen level of the water body. This prevents any further growth and creates a stagnant, lifeless environment.

In addition to its role as a nutrient, ammonia is quite toxic to many aquatic organisms. It can also be toxic to land plants where water with high ammonia levels is used for irrigation.

The Scope of the Problem Sewage Composition Each of us contributes a fairly consistent amount of waste products to a sewerage system Table 1.1.

Table 1.1.

Typical individual daily contributions to sewage.

Component Flow BOD COD SS TKN Ammonia Organic Nitrogen Phosphate * grams per person per day

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Amount (g/p/d)*

220 (L/p/d) 60 - 70 125 65 12 8 4 2-5

Chapter 1 - Wastewater Treatment and the Development of Activated Sludge The combination of people in a town or city results in sewage with a typical composition shown in Table 1.2.

The figures are indicative and there are significant differences between towns and cities, as well as between countries.

Table 1.2.

Typical composition of domestic sewage in Australia and similar developed countries.

Component

Concentration (mg/L)

BOD

280

COD

600

SS

320

VSS

75% to 95% SS

Ammonia

30 - 40

Total Kjeldahl Nitrogen

40 - 60

Total Phosphate

8 - 12

Phosphorus levels vary. Historically, in more affluent countries, levels of phosphate were higher than in less affluent countries. However with the realisation that phosphorus has an adverse effect on the environment, affluent countries have generally worked to reduce phosphorus loads to sewers and promote the use of nonphosphate detergents.

During drought with water restrictions, the “strength” of sewage is likely to increase, with the concentrations of Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) increasing. Similarly, in less developed countries with limited water, the sewage is stronger.

Trade wastes also have a significant impact on sewage composition. Cattle saleyards, wineries and breweries, and dairy related food factories are all likely to add significant loads of BOD. Any food industries with rigorous cleaning systems are likely to add salts and have a direct impact on the pH of the raw sewage based on the use of acid and alkaline cleaning agents. In countries with significant industrial output, such as China, the impact of trade waste can be profound.

The composition of sewage varies significantly throughout the day especially in large towns and small cities.

These variations generally parallel the changes in flow.

Concentrations of Ammonia, Nitrate and Phosphate in Wastewater In the wastewater industry, measurement of concentrations of ammonia, nitrate and phosphate is a little different. Licences, and results of testing of wastewater samples issued by laboratories, mostly use the abbreviations NH3-N (ammonia nitrogen), NO3-N (nitrate nitrogen) and PO4-P (phosphate phosphorus). This recognises the fact that it is the amount of N and P that impacts the environment. The other elements such as oxygen (O) and hydrogen (H) are less important.

So if a test result states NO3-N as 5 mgN/L, that means there are 5 mgN/L of nitrogen in the sample in the form of nitrate.

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Chapter 1 - Wastewater Treatment and the Development of Activated Sludge Flows Flow into a Sewage Treatment Plant (STP) typically shows a diurnal (daily) pattern with a peak flow somewhere around 0900 hours but this can be as late as 1200 hours and a second peak around 1600 to 1900 hours. Where there are multiple pump stations, there may be regular short term fluctuations imposed on the basic pattern (Figure 1.1).

Figure 1.1.

An inflow trend for 3 days showing the daily variation and pumped flows to an STP.

These morning and evening peaks simply represent people’s toilet and living habits; showers and toilet visits in the morning, and cooking, bathing and toilet visits in the early evening. In larger cities because of the huge sewer networks, the flow variations are somewhat flattened out.

As well as daily variations, there may also be increased flow during wet weather events when leaking pipes allow infiltration into the sewerage system. There are a large number of terms used to describe the flows.

These are summarised in Table 1.3.

Table 1.3.

Common flow measurements and their abbreviations.

Flow Type

Abbreviation

Average daily flow

ADF

Average hourly flow

AHF

Average dry weather flow

ADWF

Peak daily flow

PDF

Peak wet weather flow

PWWF

Peak hourly flow

PHF

Peak instantaneous flow

PIF

Inflow and Infiltration These terms refer to water entering a sewerage system but by different routes. Inflow is the flow of stormwater into a sewerage system by a direct connection. This results in a rapid change in the flow and is normally the result of storms or high rainfall events. Infiltration refers to ground water that enters the sewerage system via cracks and leaks in the pipe system itself. Compared to inflow, infiltration is slow and does not occur immediately as a result of storms and heavy rains, but rather shows a much slower increase in response to these events.

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Chapter 1 - Wastewater Treatment and the Development of Activated Sludge The peak factor, the difference between the ADWF and PWWF is typically 2 to 6 times but may be higher than 11 times in a “leaky” sewerage system. A number of factors such as the quality and materials of construction and the local geophysical conditions influence the leakiness of a sewer system. Tree roots often enter old clay pipes, cracking them and creating pathways for water infiltration. At one extreme, the sandstone sewer trenches in Sydney fill with water which surrounds the pipes during heavy rainfall events leading to infiltration as much as 10 times the average. By contrast, in the sandy soils of Perth, water quickly drains past the sewers and down into the groundwater resulting in wet weather peaks seldom exceeding 2 times the average.

Population Equivalent (PE) The population equivalent or PE, is a concept used to rate the size of a treatment plant based on either the organic load or hydraulic load. Typically 140 - 240 L/p/d is used as the flow basis for PE or 60 - 70 g BOD/p/d for the organic load basis. This means that a plant receiving trade waste can be rated as having a PE that is significantly greater than the actual population of the town. For example, the City of Shepparton in central Victoria has a population of around 35,000 people but a PE of approximately 1,000,000 due to the food industries located in the city.

Some of the Solutions to the Wastewater Disposal Problem In the earliest times, waste was deposited on the ground or covered with soil, and natural systems of bacteria, fungi, beetles and worms and even scavenging birds and vertebrate animals such as pigs and dogs, reduced and removed the product.

As the human population increased and the nomadic life was replaced by fixed habitation, localised natural ecosystems became stressed. Commonly, wastewater was conveyed to rivers in one way or another, the logic being that the natural flow would carry the wastewater away.

Allowing the sewage to directly or indirectly reach a river or ocean is unsustainable, at least in a way that is acceptable to human communities. Eventually the aesthetics of the river or coastal area will be compromised and destroyed. Wastes discharged at a low enough rate could sustain satisfactory environmental quality.

However problems arise when the amount of waste exceeds the natural capacity of the ecosystem to provide oxygen.

One solution to the problem was to deliberately set up an isolated body of water, a lagoon, control the waste load and let the natural systems “do their work”. The necessary treatment would then occur in the lagoon rather than in the river, bay or ocean.

Environmental Regulators in all developed countries have tried to define acceptable and sustainable limits for discharges to water for the main pollutants originating from sewage. Table 1.4 provides some typical discharge limits to inland waters in Australia.

Table 1.4.

Typical limits for discharge to inland waters in Australia.

Indicator

Median

90th percentile

BOD (mg/L)

5

10

Suspended solids (mg/L)

10

15

Ammonia-N (mgN/L)

2

5

Total N (mgN/L)

10

15

Total P (mg/L)

0.5

1.0

E coli (orgs/100 mL)

200

1,000 11

Chapter 1 - Wastewater Treatment and the Development of Activated Sludge Climate may have a large bearing on the standards that a treatment plant is able to achieve. Thus in tropical North Queensland, a plant may readily achieve a Total N of less than 5 mg N/L whereas in temperate Tasmania, exactly the same plant design might only achieve less than 10 mg N/L.

Lagoons Once plumbing was developed, and the “flushing toilet” widely installed, the waste could be conveyed out of town to a lagoon site where natural processes could take place and improve the quality of the effluent discharged to waterways. These early lagoons were often under sized and were associated with considerable odours, however the odour had been shifted from the town to an area out of town with less people.

Modern lagoon systems may be aerobic, facultative or anaerobic. There are also a range of specialist proprietary lagoon systems such as Pond Enhanced Treatment Operation (PETRO), Advanced Integrated Waste Pond System (AIWPS) and Biolak that have been developed to meet modern requirements.

Lagoons are generally poor at removing nutrients and as a result encourage growth of algae. The algae in turn contribute to increases in Suspended Solids (SS) and BOD. Blue green algae (cyanobacteria) also produce a range of toxins that can contaminate the final effluent. The large surface area of lagoons encourages evaporation which results in increased concentrations of nutrients and increased salinity.

Fixed Film Treatment (Trickling Filters) Wastewater flowing over surfaces allows growth of biofilms and these can provide a higher rate of treatment in a smaller land area than lagoons. The biofilms represent concentrated layers of bacteria and algae that can carry out treatment. The earliest fixed film reactors were the so called trickling filters (Figure 1.2). These were usually circular, rock filled structures, many with exquisite rock walling around the outside. Trickling filters require settled sewage as the solids in raw sewage would overload and block the filter leading to profound operating problems and odour. Thus any trickling filter plant usually has an anaerobic digester, generally unheated, to stabilise the primary sludge and the humus sludge. There were generally two or four distributor arms that rotated continuously, applying the settled sewage from primary sedimentation tanks to the rocks and driven solely by the hydraulic power of this sewage. The biofilm on the rocks grows as it purifies the sewage, and this growth of biofilm continually sloughs off as humus sludge, settling out in the downstream humus tanks.

Figure 1.2.

Examples of a circular and rectangular rock trickling filter.

There are also so called High Rate Trickling Filters (HRTF) that consist of plastic packing in towers to allow more rapid flow and provide increased surface area. Trickling filters are designed to remove carbon from wastewater and are poor at nutrient removal.

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Chapter 1 - Wastewater Treatment and the Development of Activated Sludge Another biofilm system is the Rotating Biological Contactor (RBC) (Figure 1.3) which has large discs placed along a central shaft that rotates. Half the disc is submerged in the influent sewage and the other half is in the air. Oxygen passes into the biofilms while they are exposed to the air. RBCs can achieve some nutrient removal.

Figure 1.3.

A Rotating Biological Contactor (RBC).

Many biofilm systems are proprietary and are sold by international companies - these systems include Integrated Fixed Film Activated Sludge (IFAT), Kaldnes, Linpor and Moving Bed Biofilm Reactor (MBBR).

Activated Sludge The development of activated sludge is generally attributed to Arden and Lockett in the UK. They published a paper in 1914 entitled “Experiments on the oxidation of sewage without the aid of filters” describing their results working with bench scale “fill and draw” systems very similar to modern Sequencing Batch Reactors (SBRs). They found that by aerating sewage and then allowing the solids to settle and drawing the effluent off the top, a high quality effluent could be produced. New sewage was then introduced and the aeration step repeated. They felt that in this process the sludge that had settled and was then remixed in with the raw sewage had been activated in some way. Hence the name Activated Sludge. Subsequently it was found that the mechanism of retaining the sludge by settling effectively concentrated the microbes responsible for the treatment.

The most important aspect of this work was that it separated Hydraulic Retention Time (HRT) from Solids Retention Time (SRT). Liquid would pass through the system in a matter of hours while solids (bacteria) were retained in the system for days. From this arose the concept of Sludge Age. These concepts are covered in detail in Chapter 7.

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Chapter 2 Hazards Risks and Control Points Both the Australian Guidelines for Water Recycling (2006) and the Australian Drinking Water Guidelines (2011) include a section entitled “Framework for the Management of Water Quality”. In the area of wastewater management, there are no similar guidelines; however the principles developed in the Frameworks provide a very sound and effective way to manage wastewater treatment. There are separate guidelines for biosolids management.

Traditionally, the operation and performance of the wastewater treatment process has been governed by a range of licences or permits, generally managed and overseen by a State based regulatory agency; in most cases the Environment Protection Authority (EPA). For some Sewage Treatment Plants (STPs), the licences are prescriptive and define the regulatory requirements for the effluent produced by that plant, while at others a risk management plan provides the basis for operation.

For many years wastewater was seen as a waste product that was treated and dumped into the local river or ocean, or was “irrigated” but often without any real recognition of sustainable agricultural practices. “Waste” water is now seen more as an asset than a problem and there have been major advances in treatment which allow this water to be used for a wide variety of recycling purposes. In some cases, recycled water is being used to replace the use of drinking water for industrial purposes and for the irrigation of urban recreational areas.

The traditional approach to wastewater treatment involved establishing treatment process steps to ensure the treated effluent complied with the licence. Associated with this approach was a reliance on the testing of the final product that was discharged from the STP. The problem with this approach is that should analysis show the final effluent to be non-compliant, by the time the laboratory results were returned it would usually be too late to do anything about it. The non-compliant effluent would have already been discharged to the environment or to the recycled water system. An alternative approach is needed to ensure that we know the effluent complies with specifications before it is discharged or reused. The traditional approach is now being progressively replaced with a system based on the recognition of hazards and the management of risks by the application of preventive measures. This is the same system that has been used extensively in Australia for the management of drinking water for the past 10 years.

Hazards The aim of wastewater treatment is to reduce the risks posed by domestic or industrial wastewater to the environment and to public health. The key element of this approach is firstly the recognition and identification of the hazards, and then the risks associated with those hazards. Preventive measures are then identified to control the hazards and thereby eliminate or reduce the risks.

A hazard may be defined as any potential danger to the environment or public health. The main hazards associated with wastewater and common preventive measures are summarised in Table 2.1.

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Chapter 2 - Hazards Risks and Control Points Table 2.1.

Summary of the main hazards and preventive measures in wastewater treatment.

Hazard

Risk

Preventive Measure

Pathogenic microorganisms

Infection and spread of disease.

Clarification and disinfection.

Media or membrane filtration in some cases.

Aerobic or anaerobic microorganisms

Aerobic microorganisms exert an oxygen demand on the environment contributing to oxygen depletion in receiving waters.

Clarification and disinfection.

Media or membrane filtration in some cases.

Anaerobic microorganisms contribute gases that may smell e.g. sulfides and methane.

Carbon (as BOD or COD)

Provides an energy source for growth of bacteria that exert an oxygen demand. Results in oxygen depletion in the receiving water.

Carbon (C) removal through a biological process such as activated sludge.

Nitrogen (as NH3 or NO3)

Acts as a “fertiliser” and promotes algal growth (eutrophication) in receiving waters.

Removal of nitrogen (N) as nitrogen gas through nitrification and denitrification in the activated sludge process.

Ammonia is toxic to some aquatic plants and animals.

Phosphorus (as PO4)

Acts as a “fertiliser” and promotes algal growth (eutrophication) in receiving waters, particularly fresh water.

Removal of phosphorus (P) biologically in biosolids or precipitated as iron or aluminium phosphate.

pH

Many organisms, for example several species of fish, are very sensitive to small changes in pH.

Control the pH of the final effluent to minimise the impact on a receiving environment.

Temperature

Many organisms, for example several species of fish, are very sensitive to small changes in temperature.

Control the temperature of the final effluent to minimise the impact on a receiving environment.

Suspended Solids (SS) and turbidity

Suspended solids and colloidal particles cause turbidity which limits light penetration and inhibits natural plant growth.

Remove solids by clarification or coagulation and filtration to a level that prevents smothering of organisms in a receiving environment and doesn’t limit light penetration.

Buries and suffocates plants and bottom dwelling animals leading to death.

15

Chapter 2 - Hazards Risks and Control Points Based on the identified hazards, the aim of wastewater treatment is to manage those risks in the following ways: •• Removing the C, N and P, to concentrations that do not lead to oxygen depletion and eutrophication of a receiving environment.

•• Disinfecting the discharged effluent to kill the majority of pathogenic organisms and reduce the seeding of microorganisms to a receiving environment.

•• Removing the Suspended Solids (SS) to limit any change in turbidity and stop carryover of C, N and P. Typically 10 mg/L SS contains 0.7 mgN/L TN (total nitrogen) and 0.25 mg/L TP (total phosphorus). These amounts, while small, can have a significant impact on license compliance.

•• Controlling the pH and temperature so that there is essentially no change in the pH or temperature of a receiving environment. Temperature problems are uncommon in domestic wastewater, occurring more frequently in industrial wastewater.

Hazardous Events In addition to the hazards described above, it is useful to identify an additional problem, that of a hazardous event. A hazardous event is an incident or situation that can contribute to the presence of a hazard.

A hazardous event may include: 1. A wet weather event leading to overloading of a plant and bypassing of untreated wastewater direct to the environment.

2. A large trade waste discharge leading to inhibition of nitrifying bacteria.

3. Loss of power leading to failure of the aeration system leading to an increase in BOD, NH3 and TP in the treated effluent.

The best way to manage an identified hazard or hazardous event would be to remove the hazard or prevent the hazardous event. If this cannot be done then the best option is to control the hazard or hazardous event by reducing the likelihood of the event happening, or by reducing the consequences if the event does occur.

If a hazard or hazardous event cannot be controlled, then there is a risk that the quality of the treated effluent discharged to the environment will be adversely affected, which in turn may have an adverse impact on the health of the receiving waterway and possibly public health.

Control Points A control point can be considered as a step in a multi-step process where control can be applied or a hazard prevented, removed or reduced to acceptable levels. An alternative way to consider a control point is that it is a step in a process where if the control of the process is poor, the quality of the final product is likely to deteriorate.

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Chapter 2 - Hazards Risks and Control Points Activated sludge systems include a wide range of treatment types from the basic extended aeration process for the reduction of suspended solids and COD, to the various nutrient removal processes incorporating anoxic and anaerobic zones. Typical control points for a wastewater system include: •• Primary treatment. o Screening and grit removal. o Gross solids removal.

•• Secondary treatment. o Anaerobic zones. o Anoxic zones. o Aeration (Oxic) zones.

•• Plant recycles.

•• Chemical dosing.

•• Solids separation (clarification).

•• Further solids separation (filtration).

•• Disinfection.

•• Biosolids management.

If the process step is absent, it cannot be a control point for that system.

Critical Limits and Target Criteria To be a control point, there must be something that can be measured or observed to allow the operator to know if the process step is “in control”, “heading out of control” or is “out of control”. By setting numerical limits, the operator can make this assessment and take whatever action is necessary to maintain control at the control point.

The Australian Drinking Water Guidelines put it this way, “A control point has several operational requirements including: •• Operational parameters that can be measured and for which critical limits can be set to define the operational effectiveness of the activity (e.g. chlorine residuals for disinfection).

•• Operational parameters that can be monitored frequently enough to reveal any failures in a timely manner (online and continuous monitoring is preferable).

•• Procedures for corrective action that can be implemented in response to deviation from critical limits.”

Two different target levels are often set.

Target criteria: The target criteria represent the day-to-day operational limits. Any deviation of performance from the target should be regarded as a trend towards loss of control of the process and should result in appropriate actions being taken to resolve the problem and restore control. How to retain the process within the target limits would normally be described in well-written Standard Operating Procedures (SOP). Note that written procedures are not necessary for all preventive measures. Where an action falls into a normal operator competency, a formal procedure may not be necessary.

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Chapter 2 - Hazards Risks and Control Points Critical limit: This is the level that must not be exceeded. Deviation from a critical limit indicates loss of control of the process and should be regarded as representing a potentially unacceptable environmental or health risk. Critical limits represent the level that must not be exceeded. These limits may be defined by the licence limits or in Risk Management Plans prepared for each site.

Many Risk Management Plans also include an alarm limit as an intermediate warning between the target and critical limit. These are often summarised in the form of traffic lights with green being the target limit, amber being the alarm limit and red being the critical limit. The relationship between these limits is shown in Figure 2.1.

Figure 2.1.

Summary of the relationship between target, alarm and critical limits for ammonia, and the actions taken when these are exceeded.

Corrective Actions, Incident and Emergency Management Target criteria are always more stringent than the critical limits. By setting target criteria that are more stringent than critical limits, corrective actions can be taken well before the treated effluent quality exceeds the critical limit and an unacceptable health or environmental risk occurs (i.e. exceeding the critical limit).

Once the critical limit is exceeded (Figure 2.1), incident and emergency plans need to be activated. The level of response required depends on the magnitude of the variation, the duration of the variation and the potential consequences.

Monitoring The testing and observations that are carried out to check whether the control point targets are achieved is called process monitoring. If all the control points in the process from the inlet to the discharge of the final effluent are within the recommended range, and the targets have been set carefully, then the final product must be within the specified limits.

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Chapter 2 - Hazards Risks and Control Points In contrast to this, testing of the final product quality is called verification monitoring and simply confirms that the total system is working. Quality control should not rely on verification monitoring alone, since results of verification monitoring are obtained after the effluent has already been discharged. Process monitoring of control points allows the operator to detect problems before they can have an impact on final effluent quality.

The frequency of monitoring needs to be set based on the rate of change for each control measure. There is no point in having a control point and testing once per month if the parameter being measured can vary significantly from day to day!

Further improvement of overall plant performance can often be achieved by carefully optimising each individual control point and tightening the limits.

Sampling The reliability of data obtained from a sample varies. Just because the sample is analysed and tested in an accredited laboratory doesn’t mean the reported value is reliable. There are many possible sources of error along the way to getting the final result.

Sampling methods and monitoring are covered in more detail in Chapter 11.

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Chapter 3 The Bugs. The Good, Bad and Ugly All life is a cycle of building up and breaking down. Plants take energy from the sun, and carbon dioxide (CO2) from the atmosphere, and form complex organic molecules such as carbohydrates. The energy stored in this way can then be used to form proteins and lipids and nucleic acids (DNA and RNA). The carbohydrates, proteins and lipids are the main organic molecules necessary to form cells. Cells in turn are built up to form tissues and organs, and organ systems combine to make up the body of an organism.

Plants are eaten by animals and during the processes of digestion and metabolism, the complex organic molecules from the plant are broken down and then rebuilt as new carbohydrate, protein, lipids and nucleic acids in the animals. Animals eat other animals and so the process goes.

In death, the structures that made up the cells, and therefore the bodies, are available for breakdown, usually by microbes such as bacteria and fungi. Similarly, all living things produce waste products as part of their living. These are released to the environment as gases or as liquids (urine and faeces) or solids (faeces).

While it may be an unpleasant thought to most people, these wastes are the food sources for other life. Once again the complex organic materials in the wastes need to be broken down before they can be built up again.

The main ‘building blocks’ in the process are CO2 and water as well as some simple compounds such as ammonia, nitrate and phosphate.

The Activated Sludge Ecosystem Mixed Liquor The liquid that is found in activated sludge plants is a mixture of water, bacteria and other microbes. There is also inorganic material such as sand and grit, and breakdown products of the microbes themselves. There are paper and cotton fibres as well as hair. For this reason it is called mixed liquor.

Mixed liquor consists of a large number of different types of microbes living together in a complex microbial ecosystem with the cycles of build-up and breakdown going on continuously, albeit at a level that needs a microscope to be observed. Bacteria, viruses, protozoa, fungi, nematode worms, rotifers and possibly algae are all involved in breaking down the waste; however it is the bacteria that are the dominant microbe responsible for treatment. Put simply, they are better at it than the other organisms. The protozoa, nematodes and rotifers are higher level predators that graze on the bacteria. In turn it is the free swimming, stalked and crawling ciliates that are the most important predators.

Bacteria breakdown most of the waste. To understand how they do that and how we harness them in the activated sludge process, and in particular the nutrient removal process, more information is required about bacteria.

There are broadly two different types of bacteria based on their different primary carbon sources.

Autotrophic Bacteria Autotrophic bacteria use CO2 as the main carbon source to produce their necessary macromolecules.

Photosynthetic autotrophs use sunlight as an energy source (in the same way plants do), whilst chemosynthetic autotrophs rely on energy rich compounds such as nitrate, hydrogen sulfide, sulfate or hydrogen instead of sunlight to provide the necessary energy for the synthesis of macromolecules.

Heterotrophic Bacteria Heterotrophic bacteria rely on breaking down large organic molecules to provide the building blocks for production of their own macromolecules. Heterotrophic bacteria also rely on these macromolecules as their energy source.

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Chapter 3 - The Bugs. The Good, Bad and Ugly In breaking down the carbon based macromolecules, there are essentially two steps.

Macromolecule  Volatile Fatty Acid (VFA)  methane (CH4) and CO2.

The first step is carried out by heterotrophic bacteria known as acidogens (they form the volatile fatty acids) and the second step is carried out by methanogens (they form the methane and CO2).

As well as being classified according to their metabolism, bacteria can also be classified as to whether or not they require oxygen.

1. Obligate aerobes must have oxygen or they will die.

2. Facultative aerobes prefer to have oxygen present but can exist for extended periods without oxygen and can therefore adapt to anoxic and even anaerobic conditions.

3. Obligate anaerobes must live in an oxygen free environment or they will die.

Microscopic Examination of the Mixed Liquor Regular microscopic examination of the mixed liquor is an important process monitoring step. There is no doubt that by observing the biomass regularly, operators at a plant will get to know what the healthy biomass is like when the plant is running well. Daily observation is preferable because conditions can change very rapidly and can be missed by less frequent observation. Changes to the microbial composition can provide an early warning sign of changes within the plant. In conjunction with other plant performance indicators, the observations allow proactive management of problems as they arise.

The following sections describe many of the features of mixed liquor visible under a microscope.

Floc Formation The microbial population in activated sludge becomes organised as floc. Why floc forms and the factors controlling it are poorly understood.

Floc consists of living and dead bacteria and debris all stuck together with large polymeric molecules produced by the bacteria that act somewhat like glue. The polymeric material almost certainly contributes to difficulty in dewatering sludge.

Floc size can vary from small to large and may be quite brittle and weak, or be strong enough to resist the disruptive forces of aeration and pumps. Floc may be compact or quite loose or dispersed (Figure 3.1).

Figure 3.1.

Examples of compact floc (left) and dispersed floc (right).

21

Chapter 3 - The Bugs. The Good, Bad and Ugly The interaction of the microorganisms and the extent to which they form floc can have a big impact on the operation of an activated sludge plant.

•• Where dispersed growth occurs, and floc is formed poorly or not at all, settling in the clarifier will be poor and the final effluent quality will also be poor.

•• Low pH, low DO and some toxic shocks can result in “deflocculation” that in turn results in poor settling and poor quality effluent.

•• Some microorganisms under certain circumstances (for example, poor C:N:P ratio, particularly with low P and N, see Chapter 4) produce large amounts of thick polymer which binds water and results in a horrible sticky mess. The resultant biomass is almost impossible to settle.

•• Plants operating with a low F:M ratio (see Chapter 4) tend to produce some smaller pin floc which does not settle well and again results in turbid effluent.

•• Denitrification releases nitrogen into the floc. When this occurs in a clarifier, the result is rising sludge that contributes to a poor quality effluent.

•• Increasing numbers of filamentous bacteria in floc can result in both foaming and bulking. These conditions are considered in more detail below.

The challenge for the operation of activated sludge plants is to maintain the floc in a healthy state of sufficient size and composition to allow rapid settling in a clarifier (or the settling stage in intermittent plants) leaving behind sparkling clear effluent.

Filamentous Bacteria Most bacteria are roughly spherical or rod shaped. There are however some bacteria that appear as filaments (Figure 3.2). Filaments occur normally in activated sludge and are essential to the formation of a strong, good settling floc by providing a “backbone” for the floc.

Figure 3.2.

22

Floc with a small amount of filaments (blue arrows) associated with it.

Chapter 3 - The Bugs. The Good, Bad and Ugly Under certain conditions however, filamentous bacteria can start to out compete the other forms of bacteria and grow and increase in numbers very rapidly (Figure 3.3). The number of filaments in the individual floc increases and they start to bridge between the floc. Once bridging occurs, the floc cannot settle close to each other and poor settling results. This is known as bulking.

Figure 3.3.

Floc with a large amount of filaments. The right panel is a higher magnification of the material in the left panel. The slides have been stained to show the filaments more clearly.

Alternatively, certain types of filaments produce copious amounts of foam on the surface of bioreactors and clarifiers. Not surprisingly this is called foaming. These filaments appear to have a hydrophobic surface which doesn’t like water. Therefore the filaments try to get out of the water and accumulate as foam on the surface of the water.

The presence of filamentous bacteria does not usually affect removal of COD, nitrogen or phosphorus.

Indeed in some cases it might improve. The problems that do arise relate to carry over of solids from the clarifier (bulking) and physical problems with foam that can literally cover the reactors and clarifier and get blown over walkways and tanks and leave sticky unpleasant residues that are difficult to remove.

Protozoa The main types of protozoans (Figures 3.4) seen in activated sludge systems are: •• Stalked ciliates.

•• Free swimming ciliates.

•• Flagellates.

•• Amoeba.

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Chapter 3 - The Bugs. The Good, Bad and Ugly

Figure 3.4.

Some examples of higher organisms in activated sludge. The photograph at left shows a small green algal cell and a free swimming ciliate. The photograph on the right shows a colony of stalked ciliates.

The differences in these, apart from their shapes, relate largely to how they move and feed. Ciliates have many small hair like projections (cilia) that move in a controlled way to either move the cell or create currents of water for feeding purposes. In flagellates there is one or more long whip like flagellae that undulate back and forth to create movement.

In amoeba, the cell has no cilia or flagellae but rather they simply move by changing the shape of the cell and sending out arms that the rest of the cell “flows” towards. They glide across the surface changing their shape as they go.

The protozoa are bacterial grazers and seem to feed on free or loosely attached bacteria. As a result these are removed from the mixed liquor leaving only well-defined floc. Protozoa contribute to the formation of a clear final effluent with low turbidity and are considered to be an important part of the microbial ecosystem.

The presence of protozoa is often used as an indicator of the health of the mixed liquor.

An efficiently operating plant would tend to have the following protozoan community: •• High numbers of protozoa readily visible in each 100 x microscope view.

•• A mixture of many different crawling and attached ciliates.

•• Few or no flagellates.

Other Organisms As well as the organisms considered above there are other types of higher organisms present (Figures 3.5 and 3.6). The most common are: •• Rotifers.

•• Nematodes.

•• Oligochaete worms.

•• Tardigrades (water bears).

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Chapter 3 - The Bugs. The Good, Bad and Ugly

Figure 3.5.

A tardigrade.

Figure 3.6.

Compact floc, stalked ciliates and part of a bristle worm.

These higher organisms reflect the diversity of the activated sludge ecosystem and are not key indicators of any particular aspects of sludge health. They are thought to prey on bacteria and ciliates and their presence tends to indicate longer sludge ages.

Bulking and Foaming As filament numbers increase, the sludge doesn’t settle as well. The filaments simply get in the way. This is called bulking, and generally results in a poor quality final effluent. In most situations, the actual treatment of the waste is not compromised. That is, the carbon removal and nutrient removal continues.

Foam may be caused by detergents in the influent as well as by over growth of filamentous bacteria. In general, detergent foams are lighter and white in appearance (Figure 3.7). Filamentous foam appears more like “chocolate mousse”, is not generally bright white and tends to have a slightly greasy appearance (Figure 3.8).

25

Chapter 3 - The Bugs. The Good, Bad and Ugly

Figure 3.7.

An example of non-filamentous foaming. This was due to a large amount of detergent dumped into a sewerage system.

Figure 3.8.

An example of filamentous foaming due to Microthrix parvicella. This is an example of moderate foaming. In severe foaming, the foam can “creep” off the top of the aeration basin and onto surrounding paths.

Considerable effort has gone into identifying the different filaments, trying to find out what causes them to grow and perhaps more importantly, how to prevent them. Despite this work over decades, there is still no clear outcome, although much has been learned.

26

Chapter 3 - The Bugs. The Good, Bad and Ugly Many conditions have been considered to encourage growth of filamentous bacteria.

•• Low F:M ratio (see Chapter 4) result in microbial starvation and can favour organisms with a large surface area, i.e. filaments.

•• Alternating anoxic and aerobic conditions with long sludge ages in biological nitrogen removal plants.

•• Low DO (one of the most common).

•• High sulfide (mainly H2S) in the influent. This is usually from a trade waste source or septic sewage from long delivery pipelines.

•• High fats in the influent from grease trap or pump station washout, dairy wastes, abattoir waste, fish processing wastes and even the use of proprietary fat removal products.

•• Variable influent loads.

•• Unfavourable BOD:N:P ratios generally seen as N or P deficiency in the mixed liquor (see Chapter 4).

•• High concentrations of carbohydrates in the influent, usually from a trade waste source.

•• pH < 6.

•• Trapping of foam or return of scum in the system.

•• High ammonia returns to the anoxic zone.

The first important step is to identify the types of filamentous bacteria. Samples should be collected and sent off to a laboratory specialising in identification of activated sludge organisms. Alternatively, with specialist training, plant operators can make their own identifications. The type of organism sometimes suggests a possible treatment.

There is no universally agreed way to control bulking or foaming. Control methods can be specific or nonspecific. The nonspecific controls may achieve some short term relief but are unlikely to provide a long term solution.

The most commonly used control methods in a study of 46 Australia plants were found to be: •• Reduction of sludge age (46%).

•• Increased or decreased aeration (33%).

•• Water sprays (28%).

•• Chlorine sprays (26%).

•• Chlorination of RAS (20%).

Chlorination of the Mixed Liquor Volatile Suspended Solids (MLSS) / Return Activated Sludge (RAS), reduction of sludge age and water sprays are used widely in the US, while in France, reduction of sludge age is most commonly practiced, but only with limited success.

Another method involves foam harvesting. Figure 3.9 shows a foam harvester at a large activated sludge plant. A chain mechanism drives scrapers that move across the reactor surface and scrapes the foam onto a beach and then into a waste channel.

27

Chapter 3 - The Bugs. The Good, Bad and Ugly

Figure 3.9.

A foam harvester at a large activated sludge plant.

More details on the control of bulking and foaming are provided in Chapter 13 (Trouble Shooting).

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Chapter 4 Biological Nutrient Removal Basic Activated Sludge Carbon Chemistry Understanding wastewater treatment and in particular the activated sludge process requires some understanding of chemistry.

Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) In our diet we eat carbohydrates, fats and proteins that are all large molecules containing carbon (C). While we take in a lot of this material during digestion and metabolism we also excrete amounts of it in our faeces.

Therefore wastewater contains these same organic (C containing) molecules. Note that carbon in carbon dioxide is referred to as inorganic carbon, while the carbon in plant and animal matter such as sugar or protein, is referred to as organic carbon.

In breaking down wastes containing these carbon molecules, oxygen is used.

The organic carbon that we eat serves two purposes. Some is used as building blocks to construct and repair our body tissue. The remainder is “burnt up” (oxidised) to provide us with energy. While this oxidation occurs in our body tissues, it is no different to combustion; it uses oxygen and generates carbon dioxide and water, and releases energy just as a fire does. Some is released as heat that helps us maintain our body temperature of 37oC. Our waste (faeces, kitchen waste, and bathwater) is used by bacteria in activated sludge as a food source in exactly the same way as we use food. Thus a portion is used to grow more bacteria and a portion is oxidised to generate energy for the bacteria.

The Biochemical Oxygen Demand (BOD) test was developed to measure how much oxygen biological organisms would need to break down the waste. In the BOD test, a sample of sewage is added to a bacterial culture in a standard sized bottle and held at a standard temperature of 20oC for a standard time of 5 days (some tests are done over 7 days and even 21 days as well). The amount of oxygen present in the sample is measured at the start of the test and again at the end of the test period. The difference between the two is how much oxygen has been used by the bacteria in treating the waste, the so called biochemical oxygen demand.

BOD is not a precise quantitative test, although it is widely used as an indication of the organic strength of a wastewater. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20oC. Herein lies another problem, some of the oxygen is actually being used to oxidise or breakdown ammonia so there are modifications to the BOD test to get just the amount of oxygen used to oxidise the carbon containing molecules. An inhibitor (allylthiourea) is used in the BOD test to stop any oxygen being used for oxidation and breakdown of ammonia. The result of this test is called carbonaceous BOD and if the test was done over 5 days it is given the abbreviation cBOD5.

Partly because of the variability associated with the BOD test and also because it takes 5 days to get a result, an additional test has been developed. By using strong oxidising chemicals (usually potassium dichromate) and acid, then heating the sample, absolutely all the carbon in the sample is converted to CO2. The test is also fairly quick to do (several hours as opposed to 5 days). This is called Chemical Oxygen Demand (COD).

There is no standard relationship between BOD and COD because of the different methods used. For average quality domestic sewage, COD is very roughly 2 to 3 times BOD but such figures should be used only with great caution. If a more accurate relationship is required, specific samples can be taken for COD and BOD testing and a ratio determined.

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Chapter 4 - Biological Nutrient Removal In the design of sewage treatment plants, the COD is characterised as either soluble or particulate, and biodegradable or unbiodegradable. Thus the total COD measured for a sewage sample is actually made up of the following components: •• Soluble, readily biodegradable COD. •• Soluble, unbiodegradable COD. •• Slowly biodegradable. o Colloidal. o Particulate.

•• Particulate, unbiodegradable COD. •• Bacterial biomass.

Quantifying these components is important in the design and operation of sewage treatment plants, particularly the soluble biodegradable COD because this is generally very easy for the bacteria to use.

Hence it is known as the Readily Biodegradable COD or RBCOD. As it is easy to use, it gets “eaten” very quickly. This characteristic is important in the biological removal of nitrogen and phosphorus.

The RBCOD is largely made up of what are called Volatile Fatty Acids (VFA) that include acetic acid (2 carbon atoms), propionic acid (3 carbon atoms) and butyric acid (4 carbon atoms). They all smell. Acetic acid is vinegar; butyric acid is present in vomit, parmesan cheese and rancid butter; propionic acid is significant constituent in body odour. SCVFAs also give silage its characteristic smell.

The Role of Bacteria in Wastewater Treatment Wastewater treatment has three main objectives for the production of a high quality final effluent: •• Removal of dissolved (soluble) and particulate (insoluble) organic carbon matter. If this is not removed it will consume oxygen in the receiving environment (ocean, lake, river or estuary), killing aquatic life and generating odour.

•• Removal of the nutrients nitrogen and phosphorus so that they do not enter the receiving environment and promote the growth of algae (eutrophication).

•• Removal of pathogens which might spread disease amongst communities if released into the receiving environment.

Bacteria operate under different conditions in carrying out these processes. The terms aerobic, anoxic and anaerobic are used widely in operating the activated sludge process. It is convenient to think of them in terms of dissolved oxygen, nitrate and redox potential. The conditions are summarised in Table 4.1.

Table 4.1.

Summary of the main operating conditions found in activated sludge plants.

Condition

Dissolved Oxygen

Nitrate

Redox (indicative)

Aerobic

yes

yes

positive

Anoxic

zero

yes

negative (-200 mV)

Anaerobic

zero

zero

very negative (< -400 mV)

Note that the redox values are only rough indications. Different sources will quote different values. It is also important to note that redox can be quite difficult to measure consistently. Different probes and different calibration systems can give different values. For these reasons, redox is not often used in monitoring programs, but it can still be useful in trouble shooting.

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Chapter 4 - Biological Nutrient Removal To understand the redox values for your plant, take a redox probe and suspend it in an aerated tank for 10 minutes and note the value. Do the same in an anoxic zone in your plant if there is one. Finally, for anaerobic conditions, find a stored sludge tank or sewage pit where sewage or sludge has been stored without aeration for several hours. Place the probe in that and leave for 10 minutes. Note the value. This will provide good working values for the different conditions using your instrument.

Carbon (COD) Removal A major part of wastewater treatment is the removal of the dissolved and particulate organic matter.

Biodegradable matter is used as building material to grow bacterial biomass (activated sludge) and is oxidised to carbon dioxide (CO2) and water (H2O) to provide those bacteria with energy. Unbiodegradable particulate organic matter is enmeshed in the activated sludge floc and is removed with the Waste Activated Sludge (WAS). Soluble unbiodegradable organic matter passes through the process unchanged and into the receiving environment.

In aerobic conditions it is the heterotrophs that remove the COD, producing mainly CO2 and H2O (through energy generation) and new biomass (through growth). In doing so they use oxygen.

COD + O2  CO2 + H2O The rate at which oxygen is used is called the Oxygen Uptake Rate (OUR). It is interesting to note that for a batch of sewage, the rate of COD removal changes over time. If this is plotted as a graph (Figure 4.1), the amount of COD falls rapidly at the start and then slows down. At a later stage it slows down even further. If the OUR is also plotted on the same graph, three distinct phases can be seen.

Figure 4.1.

Changes in the Oxygen Uptake Rate (OUR) and COD in a batch reactor where a fixed amount of sewage is acted on by a fixed amount of biomass.

Initially the OUR is high and steady and the COD decreases fairly rapidly. There is then a marked decrease in the OUR and the rate of COD removal decreases. Finally there is another drop in OUR when the rate of COD removal becomes slower again.

Based on experiments that produce graphs like that shown in Figure 4.1, COD is considered to consist of Readily Biodegradable COD (RBCOD) and Slowly Biodegradable COD (SBCOD).

31

Chapter 4 - Biological Nutrient Removal Domestic sewage typically has about 8% to 25% RBCOD. RBCOD is soluble and is the favourite food of bacteria. It is rapidly used up by all sorts of bacteria in what appears to be fierce competition to get as much food as possible. In doing so, the bacteria use a lot of oxygen quickly resulting in a high OUR (see the first part of Figure 4.1). In the second phase, SBCOD, which is almost only particulate matter, is slowly broken down consuming oxygen at a lower rate (see the middle part of Figure 4.1). The OUR at this stage also includes oxygen used for nitrification. The final phase shown in Figure 4.1 is when all of the food in the wastewater is used up and the bacteria are feeding off the breakdown products of the other organisms. This is called endogenous respiration and represents a slow rate of oxygen use.

Nitrogen Removal Raw sewage contains nitrogen in the form of ammonia and organic nitrogen, the sum of which is measured by the Total Kjeldhal Nitrogen (TKN) test.

Organic nitrogen molecules (mainly proteins, urea and the nucleic acids DNA and RNA) also make up a considerable part of sewage. The nitrogen needs to be removed to a form that will not promote growth of algae and other plants in the receiving environment. Figure 4.2 shows the Nitrogen Cycle whereby nitrogen gas in the atmosphere is fixed as ammonia and then becomes available to plants and animals and ultimately via nitrite and nitrate can be returned back to nitrogen gas in the atmosphere.

Figure 4.2.

The Nitrogen Cycle.

The important parts of the Nitrogen Cycle as far as waste treatment is concerned are the steps of ammonification where the organic molecules containing nitrogen are broken down, releasing their nitrogen content as ammonia. Ammonia is then converted to nitrate in the process of nitrification and in denitrification the nitrate is converted to nitrogen gas. These processes will be considered in more detail below.

The steps from ammonia to nitrate and nitrate back to nitrogen gas occur through intermediates, nitrite (NO2) and nitrous oxide (N2O) (Figure 4.2). These intermediates are consumed very rapidly and very little nitrite or nitrous oxide is actually formed, however nitrous oxide is a potent greenhouse gas so many water utilities and governments are studying its formation and release. Nitrite is sometimes formed when the aerobic zone in an activated sludge plant is under-aerated.

32

Chapter 4 - Biological Nutrient Removal Nitrogen removal in an activated sludge plant involves manipulating conditions that favour firstly the nitrifying bacteria, followed by the denitrifying bacteria.

Nitrification Nitrifying bacteria are autotrophic and aerobic. They use inorganic carbon from carbon dioxide as the building blocks of their cell matter and use oxygen to convert ammonia to nitrate (i.e. nitrification) in order to generate their energy. Nitrifying bacteria are slow growing, they need oxygen at all times to function and they do not like high or low pH. In an activated sludge plant, if the sludge age is too short then there is not enough time for the nitrifiers to establish a healthy population and nitrification will fail.

The microbiology of nitrification is not well understood, however there is no single bacteria capable of converting ammonia all the way to nitrate. There are two key populations required to complete the nitrification process, and individual plants vary greatly with some dominated by just one species and others containing several different species.

There are two steps: 1. Ammonia oxidising bacteria (e.g. Nitrosomonas) oxidise the ammonia to nitrite.

2. Nitrite oxidising bacteria (e.g. Nitrobacter and Nitrospira) oxidise the nitrite to nitrate.

The overall reaction can be written as: NH3 + O2 + OH  NO3 + H2O (Overall reaction) Don’t be put off by the chemical symbols. Have a look and see that the important things to note are: 1. Ammonia (NH3) is converted to nitrate (NO3).

2. Oxygen (O2) is required (aerobic conditions).

3. Alkalinity (OH) is consumed.

Nitrification Implications for Sewage Treatment •• Alkalinity is consumed and decreases the pH unless sufficient alkalinity is present. This can be a problem in areas where the water is soft with a low alkalinity. As a rough rule of thumb, the alkalinity of the raw waste should be > 150 mg/L CaCO3. If the alkalinity of the final effluent is < 75 mg/L CaCO3, alkalinity must be added. This can be done using lime, caustic soda or soda ash.

These add different amounts of alkalinity (Table 4.1). (Note that denitrification returns half of the alkalinity that is consumed by nitrification. This is considered in more detail below.)

Table 4.1.

Alkalinity contributed per mg of pure product.

Alkali

Alkalinity Added (mg CaCO3/ mg pure chemical)

Soda Ash (Na2CO3)

0.94

Hydrated Lime (Ca(OH)2)

1.35

Caustic soda (NaOH)

1.25

Magnesium hydroxide (Mg(OH)2)

1.72

Sodium bicarbonate (NaHCO3)

1.19

33

Chapter 4 - Biological Nutrient Removal •• Nitrifiers prefer an optimum pH of 7 - 8. o If a plant nitrifies without any or sufficient denitrification then there is a risk that the pH will decrease. This low pH will stop or inhibit nitrification and the pH then increases until nitrification starts up again and the cycle repeats itself. A record of pH over a few weeks in a plant with some nitrogen removal problems is likely to show the pH profile rising and falling over time in a saw tooth fashion. To avoid this, alkalinity should be added or proper denitrification should be introduced. If the raw sewage contains a low alkalinity (say 120 mg/L as CaCO3) and a high TKN (say 80 mgN/L) then even full denitrification will not return sufficient alkalinity and dosing of alkalinity will be required.

Adjustment of pH closer to the optimum also serves to improve nitrification.

•• Nitrifiers prefer an optimum temperature of 15oC - 30oC. o There is generally no control possible over temperature. If the sewage temperature in winter is very low, bacterial growth is slowed. It may be necessary to increase the sludge age as winter approaches to ensure that the population of nitrifiers is maintained.

•• Nitrification requires a DO between 1 - 2 mg/L. Higher DO simply uses more air with little increase in the rate of nitrification. Lower DO can be tolerated, but time is required to acclimatise the nitrifier population to this.

•• Oxidation of 1 kg NH3 requires 4.6 kg O2.

•• Nitrification is inhibited by. o Low (and high) pH. o Low alkalinity. o Low DO. o Ammonia. o Industrial pollutants (e.g. heavy metals in landfill leachate). o N2O nitrous oxide. o Short sludge age.

There are laboratory methods available for assessing toxicity of waste streams and also the actual nitrification rate. These are important parameters in the design and operation of activated sludge plants.

Denitrification Denitrifying bacteria are heterotrophic facultative aerobes (e.g. Pseudomonas), the same bacteria that convert COD to CO2. They prefer to use oxygen for respiration when they are oxidising organic matter (COD), but are able to use nitrate if there is no oxygen available. In order for them to remove nitrate from sewage, they require a carbon source (e.g. organic matter in raw sewage) and anoxic conditions.

What happens can be summarised in the following general reaction.

NO3 + Carbon + H2O  N2 + CO2 + OH Again, do not get put off by the chemical symbols. The important things to note are the following: 1. Nitrate (NO3) is converted to Nitrogen (N2).

2. Carbon is needed.

3. Alkalinity is produced and therefore returned to the system.

34

Chapter 4 - Biological Nutrient Removal The earlier description of the different types of biodegradable COD in sewage is important in understanding denitrification. Since RBCOD is favoured by bacteria that oxidise it very quickly, it is useful to ensure that the RBCOD in raw sewage is fed into the anoxic zone of an activated sludge plant so that it can be used for denitrification. That is, in a nitrogen removal plant the first zone should be an anoxic zone (primary anoxic zone). Denitrification will proceed quickly and the nitrate will be denitrified to nitrogen gas through the consumption (oxidation) of the RBCOD. Once the RBCOD is used up, denitrification continues, but much more slowly as the SBCOD is oxidised. Ideally an anoxic zone should be operated so that the nitrate concentration is only just zero leaving the anoxic zone.

If the raw sewage is fed into an aerobic zone then the RBCOD and the SBCOD will be consumed with dissolved oxygen being used for respiration by the bacteria. Should the following zone be anoxic (secondary anoxic zone) it will be very ineffective at removing nitrate because all of the food will have been depleted and the denitrifying bacteria will have to rely on the breakdown products of other organisms as a carbon source (i.e. endogenous respiration). This occurs very slowly and so very little nitrate is removed. The secondary anoxic zone would have to be huge to remove any significant amount of nitrate, or another source of RBCOD such as methanol, ethanol, sucrose, molasses or glucose would have to be added. Such dosing of additional COD can become quite expensive.

Thus, nitrogen removal plants are almost always designed with a primary anoxic zone followed by an aerated (aerobic) zone, which is sometimes followed by a secondary anoxic zone to try and get a bit more denitrification. One immediate problem with this configuration is that the nitrate is produced through nitrification in the aerated zone which comes after the primary anoxic zone. The question then is how does one get that nitrate back to the primary anoxic zone? The answer is to recycle the aerated zone mixed liquor containing the nitrates back to the primary anoxic zone. But because this recycle flow is drawn from the aerated zone it also contains dissolved oxygen which gets used to oxidise the RBCOD in the primary anoxic zone in preference to the nitrate. To minimise this impact, the mixed liquor recycle flow rate should not be too large and the DO in the aerated zone should be kept low at the point where the recycle is drawn. Activated sludge plant configurations for nitrogen removal are discussed in Chapter 6.

Denitrification Implications for Sewage Treatment •• For each NO3 used, alkalinity (OH) is produced. In fact, half the alkalinity used in nitrification is returned to the reactor during denitrification.

•• Denitrifiers prefer an optimum pH of 7 - 7.5, but can get approximately 80% efficiency outside of this range between pH 6 to 8.

•• Denitrification requires zero or very low DO. •• Denitrification requires a carbon source. This is usually provided by using raw influent or dosing with methanol, ethanol, acetic acid, molasses or sucrose.

•• Denitrification is less susceptible to inhibition than nitrification.

Simultaneous Nitrification Denitrification (SND) Experimental work and full scale operation of plants has shown that nitrification and denitrification can occur at the same time in carefully controlled conditions. In compact floc under the right DO conditions, there can be an aerobic zone around the outside of the floc and deeper inside the floc where the oxygen doesn’t reach there is an anoxic zone (Figure 4.3), sort of like a mini treatment plant. Some of the nitrate produced in the outer zone moves into the inner zone and is denitrified.

35

Chapter 4 - Biological Nutrient Removal

Figure 4.3.

Diagram of a floc showing an aerobic area around the outside of the floc and an anoxic zone in the middle of the floc.

It is not possible to give firm advice on DO levels to achieve Simultaneous Nitrification Denitrification (SND), however a suitable starting point might be a DO of 0.5 - 1 mg/L. By carefully monitoring DO, ammonia and nitrate levels, the best DO set point can be determined. However it is important to note that with subtle changes in the biomass and floc structure the amount of SND will vary. For example if small dispersed floc replaces compact floc, SND is unlikely to work as well.

However it is achieved, with dedicated aerobic and anoxic zones or with SND, the removal of nitrogen to low levels reduces the environmental impact in two ways.

1. Removal of NH3. Ammonia is toxic to many aquatic organisms.

2. Removal of NH3 and NO3 reduces the total load of N to the environment thereby reducing the potential for eutrophication.

In a well-run activated sludge plant which has been designed for nitrogen removal, it should be possible to achieve a Total N in the final effluent of < 5 mgN/L in warmer states like Queensland and < 10 mgN/L in the more temperate states of Victoria and Tasmania. This assumes there is sufficient carbon available.

Phosphorus Removal Phosphorus (P) can exist in 3 main forms.

1. Simple soluble phosphate (PO4) (Note that this is sometimes called orthophosphate in other books but will only be called phosphate in this Guide).

1. Chains of phosphate joined together as polyphosphate (Poly P).

2. Organic P where the phosphate is part of organic compounds.

All living organisms require P as it is part of the structure of their cells. As such, in an activated sludge system, the normal growth of bacteria will remove about 20% to 30% of the P in the raw sewage. Conversely when the bacteria die, some of the P is returned to the system. In aerobic or anaerobic digestion, a larger amount of P may be returned to the system.

Phosphorus is also important in the provision of energy for the cells. Energy is stored in a molecule called ATP. The ATP provides the energy for growth, movement, reproduction and cell repair. In providing the energy, the ATP loses a phosphate molecule and becomes ADP, releasing energy in the process.

ATP  ADP + PO4 + energy 36

Chapter 4 - Biological Nutrient Removal The energy in food is used to convert ADP back to ATP to replenish the cells energy store.

ADP + energy + PO4  ATP

Only a small amount of energy can be stored as ATP. Energy derived from organic matter can also be stored in other less readily available ways. Bacteria store glycogen and “fat” in a similar way to us. Glycogen is the same, but in bacteria the main form of stored fat is called polyhydroxybutyrate (PHB). At any time these stores can be used to convert the ADP back to ATP for immediate use.

Observations in the 1960’s, primarily in South Africa, showed that under anaerobic conditions (no oxygen or nitrate) some bacteria released P in quite large amounts. Similarly it was observed that if aerobic conditions followed the anaerobic conditions, large amounts of P could be taken into the bacteria again. The principal requirement was alternating anaerobic and aerobic conditions with the anaerobic period prior to the aerobic period. Much work went into understanding the conditions that favoured P release and P uptake, particularly those that resulted in uptake of more P than was released. Such conditions would result in Enhanced Biological Phosphorus Removal (EBPR) from waste.

The bacteria that are involved are heterotrophs and essentially require aerobic conditions but are able to tolerate anaerobic conditions for several hours. Under aerobic conditions they are able to grow and reproduce. The bacteria involved in the process of RBCOD uptake and accumulation of polyphosphate and PHB have been called Polyphosphate Accumulating Organisms (PAO). There are likely to be multiple species (Acinetobacter is one species that has been known for some time). Alternating anaerobic and aerobic conditions and a substantial source of RBCOD favour the PAOs in some way, either increasing their numbers relative to other heterotrophs, or enhancing their P release and P uptake capability. EBPR occurs best in a pH range between 6.6 and 7.4 and a DO in the aerobic zone of around 1 - 2 mg/L. The principal reactions are summarised in Figure 4.4.

Figure 4.4.

Basic anaerobic and aerobic reactions associated with EBPR. Note the different sizes of the PHB and Poly P stores in the cells in the different environments.

37

Chapter 4 - Biological Nutrient Removal If RBCOD is available under anaerobic conditions, PAOs have a feature which gives them a competitive advantage over other organisms. These other organisms cannot use any of the available food, including the RBCOD, because they require oxygen and under the anaerobic conditions there is none. So these organisms basically have no energy to do anything, they are living in a sort of suspended state until they get into anoxic or aerobic conditions. But the PAOs do not need oxygen immediately; they can obtain energy from internal polyphosphate stores under the anaerobic conditions. This enables them to utilise the RBCOD. The PAO’s break off a phosphate molecule from the stored polyphosphates to release some energy. That energy is then used to convert some of the RBCOD to PHB and then store it within their cell. This is exactly the same as our system where excess calories in our diet are stored as glycogen and fat, indeed PHB is just a bacterial form of stored fat.

The phosphate molecule is released from the cell into the mixed liquor thereby explaining the observation in the 1960’s of increased phosphate concentration in anaerobic zones.

When aerobic or anoxic conditions recur, the PAOs are able to utilise oxygen or nitrate in conjunction with some of their stored PHB to grow and reproduce. A portion of the energy made available from the PHB is also used to take up the phosphorus that they released earlier and resynthesise polyphosphate in preparation for when they encounter anaerobic conditions again.

As a result of this competitive advantage, the PAOs are now able to grow and reproduce and it is this growth that allows more phosphate to be taken up than was released, the process called enhanced phosphorus removal. Wasting the sludge from the aerobic zone then allows the phosphorus to be removed from the process, caught within the reconstructed polyphosphate chains in the PAO cells.

The amount of phosphate released in the anaerobic stage depends to a large extent on the amount of RBCOD available, but soluble phosphate levels may reach 20 mgP/L or even significantly higher at some times. They may also be consistently lower. It is therefore important to monitor these levels on a regular basis.

If DO or nitrate enter the anaerobic zone, the other non-PAO heterotrophs are able to quickly take up some of the RBCOD denying the PAOs the opportunity. The PAOs lose some of their competitive advantage and enhanced phosphorus removal decreases. The impact of nitrate on P removal is also clearly seen when it is considered that denitrification of 1 mg of nitrate requires 8.6 mg of RBCOD. So every mg/L of nitrate that might leak into an anaerobic zone uses up some of the available RBCOD and therefore has a noticeable impact on the ability to remove P, especially if the influent RBCOD concentration is not that large. Therefore it is important that oxygen and nitrate are excluded from the anaerobic zone otherwise the normal heterotrophs will out-compete the PAOs. So how can DO or nitrate get into the anaerobic zone?

•• The main sources of DO into the anaerobic zone are from the raw influent where the sewage passes over weirs and over screens, and from the RAS which may be aerobic.

•• The main source of nitrate is from the RAS recycle.

While nitrate can be reliably measured in the RAS using standard test kits, measurement of nitrate in the anaerobic zone using standard kits is not good enough to detect nitrate leakage reliably. Redox potential can be used and can be more reliable. Using redox was described earlier in this Chapter. Nitrate can also be measured directly using a nitrate probe.

38

Chapter 4 - Biological Nutrient Removal So how much P can be taken up?

Looking at it another way, a simple extended aeration activated sludge plant will remove approximately 20% - 30% P, whereas a BNR plant optimised for EBPR will remove up to 97%. As a result, sludge wasted from a simple extended aeration plant typically contains 1% - 2% P, whereas sludge from EBPR plants can contain up to 5% - 12% P. It is the removal of the microorganisms from the system that brings about the removal of the P. Table 4.2 summarises this for a plant operating under extended aeration and nutrient removal conditions.

Table 4.2.

Simple phosphorus balance sheet for operation of a plant under two different conditions.

Biological Phosphorus Removal : 1 ML/d Condition

Treatment Flowsheet

Extended Aeration

EBPR

#

Phosphorus, kg/d Influent

WAS*

Effluent

Screenings/Grit Removal  Activated Sludge  Disinfection

12.0

3.8#

8.2

Screenings/Grit Removal  Anaerobic Pre-conditioning  Activated Sludge  Disinfection

12.0

11.4*

0.6

Based on WAS with approximately 2.5% w/w P

* Based on WAS with approximately 7.5% w/w P

The Unwanted Release of Phosphate Given that the P is captured in the sludge, it is important that it is retained and not released again in an unplanned manner. Release outside of the anaerobic zone is mainly due to one of three conditions.

•• If WAS, taken from the aerobic zone, is subject to a period of anaerobic conditions, P release will occur. Correct design of the WAS processing should avoid this situation.

•• Aerobic or anaerobic digestion of WAS will cause P to be released, more so in the case of anaerobic digestion. In aerobic digestion, the release is due to the gradual die-off of the PAO’s whose cells rupture, releasing the contents. Experience has shown that the actual release during aerobic digestion is less than might have been expected, but the plant design must allow for the P released in both forms of digestion to be captured or it will be returned to the process via recycles from thickening and dewatering. Usually chemical dosing with alum or ferric coagulants or lime is used for this purpose.

•• At times, reportedly when RBCOD in the anaerobic zone is exhausted and the retention time under anaerobic conditions is too long, the PAOs release phosphorus through a process known as Secondary Release. This is undesirable as when it occurs there is no subsequent EBPR in the aerobic zone.

39

Chapter 4 - Biological Nutrient Removal Provision of RBCOD Anaerobic release of phosphate requires a source of RBCOD. It is well known from the long history of anaerobic digester operation that breakdown of COD occurs in 2 steps. The first step carried out by acidogenic bacteria produces Volatile Fatty Acids (VFA) and is a fairly quick step. The second step carried out by methanogenic bacteria takes the VFA produced in the first step and converts it to methane, and this is a fairly slow step.

The amount of RBCOD coming into a plant can be increased by a pre-fermentation step. The fermentation needs to be controlled so that only the acidogenic bacteria are active and the methanogenic bacteria are prevented from forming methane by not giving them sufficient time to carry out the methane forming reactions.

In some EBPR plants this is achieved by the use of a prefermenter or an Activated Primary Tank (APT).

Provision of this process step most commonly involves conversion of a primary clarifier into a prefermenter or construction of a purpose built APT. Volatile fatty acids formed in the prefermenter or APT are “washed” into the anaerobic zone by the flow of sewage through the tank thereby providing RBCOD to ensure maximum efficiency of P release and therefore P removal. Care should be taken to ensure that solids from the primary treatment are not carried into the main reactors as well as the RBCOD. Operation of prefermenters and APTs are covered in Chapter 9.

Another possibility to enrich the influent to the anaerobic zone with RBCOD that is used in some Scandinavian plants is to ferment the RAS. Fermentation of raw sewage produces significantly more RBCOD than fermenting RAS, however depending on the plant, it may be an option. There are several plants in Australia with RAS fermentation included.

The effect of RBCOD on the final effluent concentration of P is shown in Table 4.3.

Table 4.3.

Effect of RBCOD on final effluent P concentration.

Situation 1

Situation 2

Influent P

12 mg/L

12 mg/L

Normal bacterial requirements (~ 25% removal)

3 mg/L

3 mg/L

Influent RBCOD

45 mg/L

60 mg/L

Additional P able to be removed assuming 10 mg/L RBCOD facilitates uptake of 1 mg/L P

4.5 mgP/L

6 mgP/L

Effluent P concentration

4.5 mgP/L

3 mgP/L

Clearly, providing additional RBCOD has the potential to lower effluent P concentrations significantly.

Additional RBCOD can be provided by dosing chemicals such as acetic acid (vinegar) but this is expensive.

Some trade wastes also provide RBCOD and where possible this should be taken advantage of. Unfortunately many trade waste pre-treatment steps remove some RBCOD and leave only the more difficult SBCOD for the municipal STP. Negotiations with the company discharging the trade waste can broker a more useful relationship where the trade waste helps the treatment of all the wastes at the municipal plant.

40

Chapter 4 - Biological Nutrient Removal How much Phosphorus can be removed biologically?

As with nitrogen removal, EBPR works better in warmer climates. Hence EBPR plants in Queensland can consistently achieve levels of phosphorus less than 1 mgP/L with biological removal only. While not common, temperatures in excess of 30oC can be problematic; however there is evidence of North Queensland plants operating successfully above this temperature.

In colder climates, EBPR can achieve levels of phosphorus below 2 mgP/L in the final effluent. There are however problems with achieving this level consistently and very often chemical polishing of effluent by adding either alum or ferric salts to the activated sludge process is required.

EBPR plants do have a reputation for inconsistent performance, with some notable exceptions. EBPR plants should always have a chemical dosing system to remove P in the event that an upset occurs to the biological process. A risk is that once operators start to dose chemical it can become difficult to stop for fear of breaching licence conditions.

Where do the removed N and P go?

In the processes of nitrification and denitrification, the end product nitrogen gas is released to the atmosphere.

The atmosphere consists of 70% nitrogen and any added nitrogen makes essentially no difference and has no known adverse impact.

In the case of phosphorus removal, the P remains. It might have been removed from the wastewater but it is bound up in the cells that make up the WAS and biosolids. Wasting of sludge must occur from the aerobic zone to ensure the extra P that was taken up is in fact removed, and care must be taken to ensure it is not returned to the plant.

The challenge is to use the removed P sustainably. It must not be viewed as a waste product. There is only a finite amount of P on earth. We cannot synthesise it and we cannot get it from the atmosphere. Agriculture uses a lot, therefore we must not waste what is available. One possible source of P for the future is in the waste from STPs, in particular EBPR plants. However an efficient means to retrieve P from the sludge is still being researched.

Implications for EBPR Design and Operation •• P removal requires anaerobic and aerobic conditions. In the design of EBPR plants the anaerobic zone is always before the aerobic zone and should receive most of the sewage feed.

•• The anaerobic zone mass fraction is typically 10% - 12% of the total mass fraction.

•• The HRT in the anaerobic zone has an impact on P release, too short and too long cause the zone to perform poorly. The Hydraulic Retention Time (HRT) is typically 1 - 2 hours. The HRT can be controlled to some extent by varying the RAS and other recycle rates.

•• Temperature is not usually limiting.

•• Nitrate and DO need to be excluded from the anaerobic zone.

•• P removal requires high levels of RBCOD. To achieve P < 1 mgP/L may require a ratio of total COD to P of > 40:1.

•• Storm flows should bypass the anaerobic zone if the design allows for this.

41

Chapter 4 - Biological Nutrient Removal Biological Nutrient Removal Summary Biological nitrogen removal requires alternating aerobic and anoxic conditions.

Biological phosphorus removal requires alternating aerobic and anaerobic conditions.

Figure 4.5 summarises the basic reactions in the three different stages.

Figure 4.5.

Summary of the basic nutrient removal steps in a BNR plant. The RAS return is not shown.

BNR plants are simply modifications of this basic layout to try to optimise and maximise N and P removal taking into consideration what is known about the bacteria carrying out the treatment. For example a primary anoxic zone might be inserted on the raw sewage before it enters the anaerobic zone. This is to ensure that no nitrate or DO reaches the anaerobic zone as it is known that nitrate and DO inhibits P release. Chapter 6 describes a number of plant designs used for both nitrogen removal, and nitrogen and phosphorus removal.

42

Chapter 5 Chemical Phosphorus Removal Total phosphorus entering in the influent is made up of phosphate, polyphosphates (chains of phosphate joined together) and organic phosphate (phosphate attached to large organic molecules). Through the biological treatment process, the polyphosphates and most of the organic phosphates are converted to phosphate (Figure 5.1).

Phosphates

Polyphosphates

Phosphates Figure 5.1.

Organic phosphates Organic phosphates

The different fractions that make up total phosphorus in the influent sewage (top) compared to those in the effluent from an STP (bottom).

Phosphorus (P) can be removed during sewage treatment biologically or chemically. Even in chemical P removal plants, up to 30% of the P is still removed biologically, simply by the action of the bacteria growing and dividing and being wasted with the biosolids. The bars in Figure 5.1 reflect the 30% reduction in total P in the effluent compared to the influent.

Chemical dosing followed by settling can produce total P concentrations below 0.5 mg/L. To consistently achieve levels around 0.25 mg/L, filtration of the final effluent is necessary. To achieve levels of less than 0.1 mg/L, a combination of chemical dosing to the activated sludge process and to a tertiary clarification step followed by dual media deep bed filtration is required.

The most common chemicals used for P removal in Australia are metal salts such as: •• Aluminium sulfate (alum).

•• Ferric sulfate. o Polymerised Ferric Sulfate (PFS®) is also commonly used due to it having less effect on the pH of the sewage than other ferric salts.

•• Ferric chloride.

•• Occasionally ferrous chloride in “pickle liquor”. Pickle liquor is produced as a byproduct in steel making. For this chemical to be used efficiently, it requires some dissolved oxygen and reaction time to first convert the ferrous to ferric iron.

Note that iron salts (ferrous or ferric) may cause corrosion of concrete structures and will stain all concrete structures a dark rust red colour.

Lime has also been used overseas and at the Lower Molongolo plant in Canberra. Using lime for P removal requires a high pH and results in the production of a large amount of sludge.

P is removed chemically in two ways. Firstly the iron (Fe) or Aluminium (Al) can react with any soluble phosphate in solution to rapidly produce a fine ferric phosphate (FePO4) or aluminium phosphate (AlPO4) precipitate that settles slowly. Secondly the Fe or Al reacts with available alkalinity in the water to produce iron or aluminium hydroxides, Fe(OH)3 and Al(OH)3. These hydroxides are insoluble and produce floc that interact with organic phosphates and the AlPO4 or FePO4, and incorporate them into the floc. The flocculated metal salt species will also interact with the biological floc present in mixed liquor to form a mixed floc that is then removed during settling, and filtration if present. Organic polymers are sometimes used to assist a poor settling mixed floc.

43

Chapter 5 - Chemical Phosphorus Removal Chemical dosing increases the amount of sludge produced in a plant because it generates a solid inorganic precipitate which gets incorporated in the sludge mass. If the sludge age is kept constant following the commencement of chemical dosing, then the Mixed Liquor Suspended Solids (MLSS) will increase, while the Mixed Liquor Volatile Suspended Solids (MLVSS) concentration effectively remains constant. If wasting is increased to keep the same MLSS that was present before the commencement of dosing, then the MLVSS concentration will be lower. Lower MLVSS effectively means less biomass to carry out the treatment. This can be done safely as long as the new, shorter, sludge age is above the minimum sludge age which has been determined for the plant.

Dosing Points The iron or aluminium salts can be dosed at different points in a treatment plant: 1. Raw or screened influent.

2. Activated sludge reactor.

3. Upstream of a clarifier (secondary or tertiary).

4. Upstream of a tertiary filter.

The amount of metal salt required to remove the P varies according to where it is dosed and the number of dosing points used (i.e. multipoint dosing). The metal salts are more efficient at removing simple phosphate than either polyphosphate or organic phosphate. Therefore the ideal arrangement is to let as much bio-P removal occur as possible in the activated sludge process, and then dose metal salt into a clarifier or filter at the end of the plant where most of the P is in the phosphate form and available to react with the metal salt.

When the iron or aluminium salt is dosed into the raw influent, a certain percentage of it will be “wasted” by reacting with chemicals other than phosphates in the raw sewage. Typically the iron reacts with sulfides present in the raw sewage. Indeed iron salts are often dosed into the sewer system to protect the sewers and to reduce odour through the precipitation of sulfides. To avoid “losing” the iron to these other reactions, it is best to dose into or after the aerobic treatment process.

As dosing lines are generally small and relatively inexpensive, plants are often designed with multiple dosing points so that operators can choose the location where dosing is most efficient. Typically a plant might be designed to allow dosing to the aeration zone of the reactor or to the outlet of the reactor just upstream of the clarifiers. The advantages of dosing prior to a secondary clarifier are that this generally provides a point (such as a weir) where good mixing can be achieved, and that the metal salt acts as a coagulant in the same way that occurs in a water treatment plant. This will help settle the solids, producing a higher quality final effluent.

Multiple dosing points in the one plant can be used, however this is generally more complicated to control than dosing at a single point and it is likely to result in use of more chemical than necessary. Where tertiary filters are used, and low effluent phosphorus concentrations are required (< 0.1 mgP/L), it is not uncommon to use two dosing chemicals, iron salt in the main process and alum to the tertiary filters.

A comparison of the advantages and disadvantages of dosing at different points is provided in Table 5.1.

44

Chapter 5 - Chemical Phosphorus Removal Table 5.1.

A comparison of metal salt dosing points.

Dosing Point

Advantages

Disadvantages

Raw sewage

Also removes BOD (RBCOD) and SS if primary settling tanks are installed, which can help an overloaded plant.

However removal of BOD (RBCOD) is a disadvantage for a nutrient removal plant where the RBCOD is required for N and P removal.

Least efficient dosing point resulting in higher use of metal salt.

More chance for residual Fe or Al to be included in the WAS rather than in the final effluent.

Aeration “reactor”

Prior to a clarifier or tertiary filter

Sedimentation of the metal phosphate precipitate can be enhanced by the settling activated sludge.

Overdose of metal salt can cause toxic shock to the biomass due to low pH.

Recycling of some of the Fe or Al can occur via the RAS.

A pH control system is usually required.

Best dosing point to achieve the lowest possible P with the least chemical.

Increased risk of pH in the final effluent dropping below the licence limit.

More efficient removal since lower pH’s that would inhibit nitrification in the aeration reactor can be used.

pH control may be required depending on the dose and final discharge pH limits.

Aids settling of SS resulting in improved effluent quality with lower SS and turbidity.

Can result in increased Fe and Al levels in the final effluent which can reduce performance of UV disinfection systems.

If metal salt is dosed prior to a tertiary filter, the filtration rate needs to be reduced to allow for the increased amount of solids onto the filter.

Mixing Effective dosing of any chemical requires vigorous mixing. Dosing can be carried out directly into the side of a pipe as long as the dosing pump has sufficient head to overcome the pressure in the pipe and there are a number of bends or valves or flow meters in the flow path to create turbulence. Dosing to a mixed liquor flow over a weir at the outlet of the activated sludge reactor is generally good due to the turbulence created by the rapid flow and the drop over the weir. Where adequate mixing cannot be achieved, an inline mixer should be used. The mixer design should not allow hair and rags to foul the mixer.

An example of very poor mixing is shown in Figure 5.2.

45

Chapter 5 - Chemical Phosphorus Removal

Figure 5.2.

An example of poor mixing, with undiluted ferric salt dripping into a slow moving waste stream.

Metal salts can be dosed undiluted if the mixing is good. If not, carrier water can be used to aid mixing.

The ratio of dilution water to coagulant must be low enough to prevent hydrolysis of the metal salt in the water. Simple mixing tests can be carried out and the mixture observed closely for the formation of floc. If “milkiness” or floc forms, the dilution ratio is too high. For PFS®, dilution is recommended at no more than 20:1. Another way to check the dilution is to ensure that for PFS® the diluted solution pH is < 2 and for alum the diluted solution pH is < 3.3.

Alkalinity and pH The precipitated phosphate molecules are quite insoluble, with AlPO4 having minimum solubility at pH 6.3 and FePO4 at pH 5.3. While these would be the theoretical best pH’s to work at, effective removal can be achieved in the pH range 5.5 - 7.0. The effect of pH needs to be considered particularly if the metal salts are being dosed into the aeration tank where nitrification is required. Nitrification requires a pH of around 7 - 8 for best results. Thus there is some conflict between the pH necessary for nitrification and the best pH for chemical P removal. This is another reason why dosing prior to a secondary clarifier might be preferable.

Chemical P removal consumes alkalinity. Where chemical P removal is used, it is important to ensure that sufficient alkalinity is available for the remaining biological processes. As a guide, the alkalinity should be between 100 and 150 mg/L CaCO3 after the dosing point. The pH of the aeration basin or tank should also be maintained above 7.0. Where additional alkalinity is required, it should be dosed approximately 30 seconds after the dosing of the metal salt.

Theoretical Dosing Required Based on the chemistry of the reactions, it is possible to calculate how much metal salt is needed to remove the P from the solution; however in most situations more metal salt is required than theory predicts. The amount of metal salt required in excess of that predicted by theory increases significantly when trying to achieve levels of phosphorus lower than about 0.5 mgP/L. To achieve effluent P levels of < 0.5 mgP/L, a typical starting point would be 3.6 grams of Fe or 1.7 grams of Al for each gram of P to be removed (i.e. a mole ratio of 2:1). Table 5.2 provides some examples of necessary alum or PFS® doses for a number of different influent P concentrations with 0.5 mgP/L being the target effluent P concentration.

46

Chapter 5 - Chemical Phosphorus Removal Table 5.2.

Example dose rates for PFS® and alum.

Influent P (mg/L)

Fe required (mg/L) based on 3.6 mg/L Fe per mg P

Dose PFS® (mL/kL)

Al required (mg/L) based on 1.7 mg/L Al per mg P

Dose alum (mL/kL)

6

21.6

117

10.5

190

8

28.8

156

13.9

253

10

36.1

195

17.4

317

The doses in mL/kL are based on PFS® having an SG of 1.5 and 12.2% Fe and alum having an SG of 1.3 and 8% Al2O3 (= 4.23% Al).

Addition of metal salts results in consumption of alkalinity and increased sludge production.

Figure 5.3 shows the reduction in P as the dose of PFS® was increased at an activated sludge plant with two separate treatment trains consisting of a reactor and clarifier with separate PFS® dosing into the influent to each clarifier. The dose was increased slowly to ensure equilibrium was reached before increasing the dose.

The graph shows that the changes were made over a period of several months. This was important to allow the biomass time to adjust to the PFS® and also to ensure that the end point wasn’t missed.

Figure 5.3.

The effect of increasing PFS® dose on the concentration of Total P in the final effluent from the North and South treatment systems.

47

Chapter 5 - Chemical Phosphorus Removal

Jar Testing Jar testing can be used to provide some information on the necessary dose of metal salt.

Either raw influent or mixed liquor can be tested depending on the chemical dosing point. In each case, the results of the jar test are assessed by allowing the sludge to settle and measuring the phosphate in the clarified supernatant. Observation of the clarity of the supernatant and settling rates is also important.

This method of testing assesses the removal of P using chemical precipitation only, and does not allow for biological P removal. As such it probably represents the maximum dose required for the Fe or Al. Results for one such jar test is provided in Table 5.3. No attempt was made to optimise the pH. Slightly better results may have been obtained at the lower doses if the pH had been lower. The table suggests a maximum dose of around 400 mL/kL would be necessary to consistently achieve a P of < 0.5 mgP/L in the final effluent.

Table 5.3.

Results of a PFS® dosing jar test using raw influent.

Jar

1

2

3

4

5

200

250

300

350

400

mL PFS /L

0.20

0.25

0.30

0.35

0.40

pH

6.3

6.0

5.9

5.7

5.5

Floc at 20 minutes

good

good

good

very good

very good

Floc settling

poor

poor

poor

good

very good

Clarity

poor

poor

poor

good

very good

Phosphate (mgP/L)

-

4.0

3.0

0.64

0.22

®

PFS (ppmV) ®

Details on jar testing method can be found in the Practical Guide to the Optimisation of Chemical Dosing, Coagulation, Flocculation and Clarification also published by WIOA.

Total Dissolved Solids and MLVSS Adding metal salts to the activated sludge process will increase the Total Dissolved Solids (TDS) in the final effluent. This may have some implications for beneficial reuse of the final effluent or indeed discharge of the effluent to the receiving environment.

The addition of the inert precipitate to the mixed liquor as a result of the interaction of the metal salt with phosphate will decrease the percentage of Mixed Liquor Volatile Suspended Solids (MLVSS) in the Mixed Liquor Suspended Solids (MLSS).

Residual Iron and Aluminium Levels Care must be taken when using metal salts for P removal to ensure that Fe and Al levels in the treated effluent do not exceed discharge limits set by the regulator. Excessive Fe or Al concentrations discharged to inland waters may be detrimental if downstream coagulation and precipitation can occur.

The metal salt precipitates produced in an activated sludge plant dosing metal salts are all trapped within the sludge matrix and leave the plant in the wasted sludge. Some regulators limit the use of sludges produced if Al concentrations are too high. Higher Fe concentrations are less likely to limit beneficial reuse; however both Al and Fe based solids can be used beneficially on land.

48

Chapter 5 - Chemical Phosphorus Removal

Monitoring To adequately control the chemical P removal process, comprehensive monitoring is required. This should include: •• Total P in the final effluent.

•• Phosphate in the final effluent.

•• Turbidity or SS in the final effluent. Turbidity contributes to a higher total P in the effluent.

•• Al or Fe residual in the final effluent.

•• UV transmissivity in the final effluent. For those plants with UV systems, overdosing of metal salt can have an adverse effect on UV disinfection.

•• Al or Fe concentration in the WAS or biosolids to check for potential impact on beneficial reuse.

•• Alkalinity after secondary treatment to ensure there is sufficient alkalinity for nitrification (see Chapter 4).

Anaerobic Digester Performance If the sludges are fed to an anaerobic digester, care should be taken during start-up of chemical P dosing systems to ensure that sludge pH remains within acceptable ranges for the digester.

Sludge Volumes Chemical P removal will add as much as 30% to the quantities of sludge produced at an STP. The increased volume may impact adversely on the sludge handling systems as well as the ability to manage the final volume either beneficially or to landfill.

The increased sludge production will also be noticed as an increase in MLSS concentration in the activated sludge process. If the MLSS concentration increases to a point where it compromises the performance of the clarifiers under peak flow conditions, then more sludge will need to be wasted (i.e. the sludge age reduced) in order to reduce the MLSS concentration. Care needs to be exercised to ensure that the sludge age does not fall below the minimum target value necessary to achieve the required level of treatment in the plant, in particular nitrification.

One advantage of chemical P removal is that there will be no P release during sludge handling and therefore any return streams from the sludge processing section of the plant should not increase the load of P into the plant.

49

Chapter 6 Activated Sludge Plant Configurations Continuous Plants The earliest activated sludge plants were designed to remove organic matter and solids. Some of the organic matter was first removed by settling out gross solids as primary sludge in primary sedimentation tanks. The settled sewage was fed to an aerated (aerobic) reactor and mixed with activated sludge that resulted in the removal of more organic material by the mixed liquor biomass. The activated sludge was then settled in a clarifier and returned back to the aerated reactor as Return Activated Sludge (RAS). Activated sludge was wasted as Waste Activated Sludge (WAS) from the clarifier (Figure 6.1). The WAS was blended with the primary sludge and digested in an anaerobic digester.

Figure 6.1.

Basic activated sludge plant configuration.

The first nitrogen removal configurations were proposed in the 1960’s. Ludzack and Ettinger proposed a configuration where an anoxic reactor was situated immediately upfront of an aerobic reactor. The anoxic zone was not completely separated from the aerobic zone and depended on mixing between the two zones to provide the nitrate rich product from the aeration zone back into the anoxic zone (Figure 6.2).

Figure 6.2.

The Ludzack Ettinger nitrogen removal system.

While this configuration achieved some nitrogen removal, lack of control of the nitrate rich recycle meant performance was variable. In the early 1970’s, the system was improved when James Barnard modified the configuration by: •• Completely separating the anoxic zone.

•• Returning the RAS to the anoxic zone.

•• Adding an Internal Nitrate Recycle (called “a” recycle in some engineering texts) from the aerobic zone to the anoxic zone. This typically operates at 200% - 600% of the inflow.

This configuration became known as the Modified Ludzack Ettinger (MLE) configuration (Figure 6.3).

50

Chapter 6 - Activated Sludge Plant Configurations

Figure 6.3.

The Modified Ludzack Ettinger (MLE) configuration for carbon and nitrogen removal.

(Ax Anoxic.)

The MLE was able to achieve improved nitrogen removal but was not able to achieve a zero nitrate concentration because the final effluent was drawn from the aerobic zone which always contained some nitrate. Effluent nitrate typically varied from 5 mgN/L to 15 mgN/L in Australian MLE plants. Attempts to reduce nitrate further by increasing the Internal Nitrate Recycle had little success because the recycle not only returns nitrate, but also dissolved oxygen from the aerobic zone which oxidises carbon in the anoxic zone that would otherwise have been used for denitrification.

To achieve improved nitrogen removal, Barnard developed the Bardenpho process by adding a second anoxic zone and second aerobic zone after the main aerobic zone (Figure 6.4). The secondary anoxic zone provided for denitrification of any nitrate from the aerobic zone that was not returned to the primary anoxic zone. The second aerobic zone was provided to nitrify any ammonia that was formed in the secondary anoxic zone through the breakdown of organic matter. Compared to the MLE process, the Bardenpho process can achieve better than 90% nitrogen removal but still does not easily achieve zero nitrate concentration. This is because the organic carbon which serves to denitrify nitrate is mostly used up in the preceding zones and what remains degrades very slowly, resulting in a very low rate of denitrification. To overcome this problem when very low nitrate is required, a supplementary source of organic carbon (methanol, ethanol, sugar or molasses) is sometimes added to the secondary anoxic zone.

The final aeration zone could also strip any nitrogen bubbles attached to the floc after the anoxic zone and therefore limit formation of floating sludge in the clarifier.

Figure 6.4.

The Bardenpho configuration for improved N removal. (Ax Anoxic, Ae Aerobic).

In South Africa in the 1970’s, phosphorus removal became increasingly important due to the eutrophication of inland drinking water supplies. Observations of various full scale plants around the world showed that anaerobic stages appeared to be necessary to stimulate P removal. Working in a laboratory, James Barnard made observations which convinced him that adding an anaerobic zone to the front of a nitrogen removal plant, upstream of the primary anoxic zone, caused phosphorus to be removed. Without ever performing a pilot trial, a large new wastewater plant which was under construction was modified by including an anaerobic zone and the process was proved at full scale.

51

Chapter 6 - Activated Sludge Plant Configurations In another variant, the Phoredox process, an anaerobic zone was placed upstream of an aerobic zone (Figure 6.5) which was operated at a very short sludge age so that nitrification would not occur. This same configuration also became known as the Anaerobic/Oxic or A/O configuration.

Figure 6.5.

The Phoredox or A/O configuration for P removal. (An Anaerobic).

To combine P removal with N removal, the Phoredox concept was coupled with the MLE process to produce the 3 stage Anaerobic/Anoxic/Oxic or A2/O configuration. The Phoredox system was then combined with the Bardenpho configuration as the 5 stage modified Bardenpho configuration (Figure 6.6). The first such plant was commissioned in 1978 in Johannesburg, South Africa.

Figure 6.6.

The 5 stage modified Bardenpho configuration for N and P removal. The Johannesburg configuration includes a dedicated anoxic zone on the RAS flow. (An Anaerobic, Ax Anoxic, Ae Aerobic).

The 1980’s was a period of experimentation and development of a range of nitrogen and phosphorus removal systems. The most important of these are the University of Cape Town (UCT) and Modified UCT (M-UCT) configurations.

In response to the recognition that nitrate had an inhibitory effect on the anaerobic zone, the UCT configuration aimed to eliminate the recycle of nitrates to the anaerobic zone (Figure 6.7). The main difference between the 5 stage Bardenpho and UCT is that the RAS return no longer goes to the anaerobic zone but rather directly to the primary anoxic zone together with the Internal Nitrate Recycle from the aerobic zone and there is no secondary anoxic or aerobic zone.

The raw sewage is introduced into the anaerobic zone, which is the first reactor in the process. Given that the RAS no longer returns the mixed liquor to the anaerobic zone, a new recycle is introduced to provide biomass to the anaerobic zone. The Internal MLSS Recycle (called “r” recycle in some engineering texts) returns mixed liquor from the primary anoxic zone to the anaerobic zone. The recycle needs to be carefully operated to ensure the anaerobic zone has zero nitrate concentration to improve the reliability of phosphorus removal.

52

Chapter 6 - Activated Sludge Plant Configurations

Figure 6.7.

The UCT configuration for both N and P removal. (An Anaerobic, Ax Anoxic).

The disadvantage of this configuration is that the MLSS concentration in the anaerobic zone is about half of that in the rest of the process and thus the zone needs to be twice as big as in the 5 stage modified Bardenpho process to achieve the same effect. Also it was found to be very difficult to control the Internal Nitrate Recycle to ensure that the nitrate concentration in the primary anoxic zone was zero and it was found that there was still some nitrate bleed in the Internal MLSS Recycle to the anaerobic zone. To counter this problem, the primary anoxic zone was split so that the RAS was returned to the first, smaller portion and the Internal Nitrate Recycle to the second, larger portion. The Internal MLSS Recycle was drawn from the first, smaller portion, returning MLSS to the anaerobic zone. This change made it far easier to ensure that the first portion of the anoxic zone had zero nitrate that could be returned to the anaerobic zone via the Internal MLSS Recycle. This configuration is known as the Modified UCT (M-UCT) configuration (Figure 6.8).

Figure 6.8.

The Modified UCT configuration for both N and P removal. (An Anaerobic, Ax Anoxic).

After the M-UCT process, an alternative known as the Johannesburg configuration was developed. In this, the 5 stage modified Bardenpho configuration was altered by including an anoxic zone (pre-anoxic) for the RAS stream only. This anoxic zone removed nitrate from the RAS stream and stopped it from entering the anaerobic zone where it would reduce the amount of phosphorus removal.

Further refinements have been developed and trialled. One in particular is worth mentioning. The Kelowna plant in Canada splits the influent to provide COD to both the pre-anoxic and the anaerobic zones (Figure 6.9) although it may be argued that the COD which is then consumed in the pre-anoxic zone is lost to phosphorus removal.

53

Chapter 6 - Activated Sludge Plant Configurations

Figure 6.9.

The Kelowna configuration.

In many modern plants, valves and pipework are provided to allow the operator to modify the recycles and the split of the influent.

Oxidation Ditch Systems In parallel with the development of the systems described above, principally in South Africa, similar developments were occurring in Denmark which also had stringent requirements for discharge of treated waste to the coastal water of Denmark. These developments involved modifications of the oxidation ditch (Pasveer Channel) style of plant. By controlling the amount of air introduced, the contents of the “ditch” will pass from an aerobic area to an anoxic area and finally an anaerobic area (Figure 6.10). These can be retrofitted with a dedicated anaerobic zone that receives the influent and the MLSS Recycle thereby resembling to a certain extent the UCT configuration.

Figure 6.10. The basic “Nutrient Removal” ditch system.

The Danes have further developed this concept into two distinct systems, one for N removal (Bio-Denitro) and one for both N and P removal (Bio-Denipho). Both systems involve two carousel style ditches (reactors) next to each other Figures 6.11 and 6.12.

54

Chapter 6 - Activated Sludge Plant Configurations

Figure 6.11. The Bio-Denitro configuration. (Ax Anoxic, Ae Aerobic).

Figure 6.12. The Bio-Denipho configuration. (Ax Anoxic, Ae Aerobic).

Both reactors have an aerator that can be operated continuously to provide DO. There are also mixers to keep the biomass suspended when the aerator is not running. The reactors switch between aerobic and anoxic conditions. Whenever a reactor is in an anoxic phase the influent flow is directed to that reactor to provide COD for denitrification. When both reactors are in the aerobic phase the influent is fed to one of the reactors. This means that each reactor has an anoxic period and an aerobic period so that nitrate produced during aeration is denitrified during the anoxic period. The total anoxic period equals roughly 1/3 of the total aerobic period. The control system allows the phases to be changed to optimise N removal. This configuration is essentially the same as that shown in Figure 6.3, the MLE process.

In Bio-Denipho systems, the reactors are the same as that described above with additional anaerobic zones at the front of the system. Control weirs can direct the influent to the anaerobic zones or allow it to pass to a reactor in the anoxic stage. Many Bio-Denipho systems have been modified to return the RAS to the first anaerobic zone that acts as a RAS Fermenter to produce volatile fatty acids to facilitate P release in the following anaerobic reactors. RAS fermentation is provided where the influent COD and in particular influent RBCOD is low.

Intermittent Systems Intermittent systems include Sequencing Batch Reactor (SBR), Intermittently Decanted Extended Aeration (IDEA) and Intermittently Decanted Aerated Lagoon (IDAL) systems. The operation of these plants is essentially the same.

The feature that distinguishes intermittent plants from continuous plants is the processes of aeration and sludge settlement occur in a single tank. This can represent a considerable saving in civil works.

55

Chapter 6 - Activated Sludge Plant Configurations A Sequencing Batch Reactor (SBR) consists of at least two identical reactor tanks. Raw sewage can be fed to either one or the other. The SBR operates in fill and draw mode. There are five stages to the treatment in the basic SBR. 1. Fill: Sewage is added to the tank to fill it from the level it was drained to in the previous decant.

2. React: The contents are aerated. (In more sophisticated modifications for nutrient removal there may be additional steps.)

3. Settle: The aerators and mixers are turned off and the contents allowed to settle.

4. Decant: The clarified effluent is decanted off the top of the reactor tank.

5. Idle: This is a brief period that allows the system to return to the starting point.

The basic 5 stage sequence can be modified for nutrient removal (Figure 6.13).

Figure 6.13. A modified SBR sequence to include both React 1 aeration (aerobic stage) and React 2 mixing (anoxic stage) for nitrogen removal.

The Intermittently Decanted Extended Aeration (IDEA) process is an Australian adaptation of the SBR system. It was developed by the Public Works Department in NSW in the mid-1960’s. There are now well over 100 of these plants operating in Australia. The stages are essentially similar to those for the SBR but there is no isolation of the individual tank. Raw sewage continues to flow to the tanks during all stages of Fill, React, Settle and Decant. The basic IDEA sequence is 2 hours aeration, 1 hour settle and 1 hour decant.

In the simplest operation sequence, the plants resemble a basic activated sludge plant (see Figure 6.1).

However they can and have been modified for nutrient removal. For example introduction of an additional phase after aeration where the contents are kept mixed and the oxygen is all used up introduces an anoxic stage and the process resembles that of an MLE configuration. This works best in the IDEA because raw sewage continues to be fed to the reactor. In the case of a conventional SBR there is no additional feed however methanol or some other source of RBCOD can be fed to the reactor after the completion of the aeration stage to drive the denitrification phase. EBPR can also be achieved to some extent by further manipulation of the basic cycle to include an anaerobic stage. This can be done by extending the anoxic stage until all the nitrate has been converted to nitrogen gas. Alternatively some IDEA plants have been retrofitted with a dedicated anaerobic zone at the front. In some plants these have been called selectors and possibly serve the dual purpose of P release and inhibition of filamentous bacteria. Larger anaerobic reactors have also been built.

A wide range of activated sludge plant configurations exist in Australia.

56

Chapter 7 Operation and Optimisation of Activated Sludge Plants This chapter provides information on the basic operation and optimisation of activated sludge plants. To get consistent effluent quality it is necessary to actively manage and monitor the process. Wastewater treatment is a biological process and therefore is subject to the variability found in all biological systems. It cannot be operated in a “set and forget” manner, but instead requires continuous active involvement on a daily basis.

So how do you operate an activated sludge plant?

Get to know the plant and be alert to changes. The following sections should assist you in getting to know your plant and how to get the best out of it.

To start with, there are several basic principles that apply to the operation of an activated sludge plant. These are described briefly below and then in more detail later in the Chapter.

Principles of Operation Sludge Age The defining principle behind activated sludge is the separation of the Solids Retention Time (SRT) or sludge age from the Hydraulic Retention Time (HRT). The right bacteria need to be kept in the system for long enough to do the necessary work of carbon (COD), nitrogen (N) and phosphorus (P) removal. Some are slower growing than others and therefore we must be sure to keep them in the system. The end result is that liquids pass through the system in a matter of hours, while solids (bacteria) are retained in the system for periods of days to weeks.

In practice it is wasting that determines the sludge age so this will be considered in more detail below.

Wasting Wastewater provides food for bacteria. Bacteria grow, reproduce and die thereby contributing to the bulk of solids in a reactor. To allow the system to be sustainable, some of the biomass needs to be wasted; otherwise the sludge would get thicker and thicker until it wouldn’t settle in the clarifiers.

Biomass can be wasted from settled sludge in a clarifier or from the “settle” stage in an intermittent plant (see Chapter 10). Alternatively, it can be wasted from the aeration zone in continuous plants, or during the aeration stage in intermittent plants. Wasting from aeration zones allows for much easier calculation of sludge age, however greater volumes need to be wasted compared to wasting settled sludge.

Separation To ensure the clearest possible effluent (lowest possible solids) and to capture the solids to allow return of the active “bugs” to the head of the plant, it is necessary to separate the solids from the liquids. The most common way to do this is to stop the mixed liquor moving, allowing the solids to settle and then removing the clarified effluent from on top.

Every effort needs to be made to encourage production of a good settling sludge. This includes carefully setting a suitable sludge age for the type of treatment required, ensuring sufficient aeration is available (see Chapter 8) and avoiding situations that can lead to bulking or foaming (see Chapter 3).

Some work has been done on producing a granular floc that settles very rapidly and leaves behind a very clear effluent. The conditions that consistently generate this floc are difficult to maintain in a typical Sewage Treatment Plant (STP) so this is still really an area of scientific research. It has been shown however that the granular sludge is more likely to form in plants with a high COD and high RBCOD such as in abattoir waste.

If you have a plant with high COD in the raw sewage, it may be worth investigating this possibility.

57

Chapter 7 - Operation and Optimisation of Activated Sludge Plants Nutrient Balance Just as human beings need the correct balance of nutrients (protein, carbohydrate, lipid and salts), the right bacteria need the right balance of C:N:P to allow them to grow and prosper and to “do their work”. Trace elements such as magnesium (Mg), manganese (Mn), calcium (Ca) and potassium (K) are also required.

Typical domestic sewage normally contains this correct balance, however many industrial or trade wastes do not.

Protection from Toxic Material Again, just as human beings can be poisoned or even killed when exposed to certain chemicals, so too can the microbes in activated sludge. Toxic shock delivered in a sewerage system, often from illegal dumping or loss of control at an industrial plant can “knock over” the biomass. Sometimes it will recover slowly (e.g. a pH shock) but at other times the reactors need to be reseeded with biomass from another basin or perhaps an aerobic digester.

The sections below are numbered to provide a logical sequence to the process of optimising the plant.

Optimisation of the Plant 1. Collect Relevant Drawings and Manuals Activated sludge plants can be quite complex and at times a little confusing. It is important to collect all the design drawings and specifications and where possible photographs of the various structures without water in them.

Current operations manuals should be available on site. Any process changes should be documented and included in the manual, explaining the basis for the changes and why the settings might differ from the original settings.

If new capital works are carried out at a plant it is essential that the plant drawings and manuals are fully updated.

2. Label all Pits, Valves, Sampling Points and Directions of Flow In many plants there can be a bewildering array of pipes underground with valves “all over the place”. Figure 7.1 shows an example of a valve area at a plant that is called the “valve graveyard” by the operators. The yellow lines have been painted on the ground to help new operators understand the complex pipework at this plant. This is a good idea but needs to be made more permanent.

58

Chapter 7 - Operation and Optimisation of Activated Sludge Plants

Figure 7.1.

An example of a confusing array of unlabelled valve pits.

A lack of labelling can create confusion and inappropriate actions during normal operation. Such confusion is made worse during developing incidents or emergency, or when maintenance is being carried out. Complete labelling is essential when new operators are learning the operation of the plant, but can also make it easier for more experienced operators.

3. Control Wasting to Achieve the Required Sludge Age The sludge age, also known as the Solids Retention Time (SRT) or Mean Cell Residence Time (MCRT) is perhaps the most important characteristic of an activated sludge plant. In this Guide, the term sludge age will be used. The sludge age is the average time a bacterium stays in the system before it is wasted and can be calculated in the following way.

Sludge Age (d)

=

Solids Inventory (kg) WAS flow (ML/d) x WAS TSS (mg/L)

The Solids Inventory is the total mass of solids in the reactor

Solids Inventory (kg) = MLSS (mg/L) x Reactor Volume (ML).

So Sludge Age (d)

=

MLSS (mg/L) x Reactor Volume (ML)

WAS flow (ML/d) x WAS TSS (mg/L)

If the WAS flow is drawn from the fully mixed reactor (rather than from the clarifier underflow) then the TSS in the WAS and the MLSS in the reactor are the same and the sludge age equation simplifies to: Sludge Age (d)

=

Reactor Volume (ML) WAS flow (ML/d)

This is one reason why wasting from the reactor is preferable to wasting from the clarifier underflow.

59

Chapter 7 - Operation and Optimisation of Activated Sludge Plants Technically the solids inventory should include the mass of solids in the clarifier but in most instances this is very small and is omitted from the calculation for the sake of simplicity. Similarly, the mass of sludge leaving the process may include the TSS in the final effluent but this should be small and is usually omitted. Solids can also be returned to the reactor in return streams from solids processing however this input is also usually omitted from the calculation.

What Sludge Age Should Be Used?

The sludge age determines the biological health and capability of the biomass to remove COD, N and P. Therefore the necessary minimum sludge age required in an activated sludge plant depends on what level of treatment is required and the temperature (Table 7.1). There are many books written on activated sludge and there are many different sludge ages quoted. The figures in Table 7.1 represent good basic information for Australian conditions. Note that with the temperatures typically experienced in most parts of Australia, it is difficult to run a plant without nitrification occurring. Denitrification will occur readily at any sludge age that supports nitrification provided there is sufficient COD available.

Table 7.1.

Minimum recommended sludge ages for different levels of treatment.

Treatment Required

Reactor Temperature < 14oC

> 14oC

COD removal only

5 days

3 days

N Removal

15 days

10 days

COD and P removal

5 days

3 days

N and P removal

15 days

10 days

Extended aeration to produce a stabilised biosolids product

>20

> 20

An aerobic mass fraction > 50% is recommended for each of the situations in Table 7.1. The aerobic mass fraction refers to the proportion of the total biomass that is exposed to aerobic conditions. So for a continuous plant, the aerobic mass fraction is the percentage of the overall reactor volume that is aerobic.

For an intermittent plant, it is the percentage of the time that the biomass is aerated.

Unless there are specific requirements around sludge stabilisation which requires longer sludge ages (see Table 7.1), plants should be run at the lowest sludge age that will achieve the necessary treatment objectives.

When reducing sludge age in a nitrogen removal plant, progress toward that minimum sludge age should be monitored by regularly measuring the ammonia (NH3) at the end of the final aerobic zone over at least one sludge age. Any increase in NH3 may suggest the sludge age may have been shortened too much, assuming all other parameters such as DO, pH, COD and temperatures are unchanged and there is sufficient alkalinity.

This can be corrected by simply decreasing the wasting (increasing the sludge age) and continuing to monitor the NH3.

One consequence of running at shorter sludge ages is that a greater volume of unstabilised sludge will result.

Such sludges are more difficult to handle and smell badly. This is why some plants operate in the extended aeration range of sludge ages (generally > 20 days) rather than maximising nutrient removal with a shorter sludge age. However, nutrient removal efficiency, in particular phosphorus removal, will be less efficient at the longer sludge ages.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Calculation of Sludge Age Wasting controls the sludge age. If one tenth of the biomass (solids inventory) is wasted each day then the sludge age is 10 days. So an extension of this is that operation at a fixed sludge age simply requires that the daily wasting is equal to (1/sludge age) of the mass of solids in the process. Most domestic treatment plants receive, on average, a relatively constant load of COD each day. If such a plant is operated at a constant sludge age then the MLSS measured in the plant reactor will be constant from week to week and month to month, varying slightly with winter and summer temperatures. Decreasing the sludge age will decrease the MLSS concentration and vice versa.

Example A reactor has linear dimensions 20 m x 5 m x 4 m and an MLSS of 3,500 mg/L. Volume of reactor 20 x 5 x 4 = 400 m3 (= 400,000 L).

Total Solids = 3,500 x 400,000 / 1,000 x 1,000 (kg) = 1,400 kg.

So to set a sludge age of 20 days, the plant needs to waste 1,400 / 20 = 70 kg/d.

If wasting is from the reactor, then calculating the volume of sludge to be wasted to achieve the sludge age of 20 days is simple (see earlier equation) and is calculated as: Volume to be wasted = Reactor volume / 20 days = 400 m3/20 days = 20 m3/day (note that 20 m3 x 3,500 mg/L = 70 kg) Day to day fluctuations in COD loading automatically result in minor variations in the MLSS concentration, but this is seldom noticed if a sludge age of more than 10 days is employed.

A change in the operating sludge age can be made by simply adjusting the wasting rate. The MLSS concentration then gradually moves to its new equilibrium value over a period of 2 to 3 sludge ages.

Sludge Production Cross Check of Sludge Age A good cross check of the wasting set up and calculation of sludge age is to calculate the daily sludge production. To do this check, there must be some way of measuring the flow and concentration of solids wasted or alternatively, the mass or volume of cake and the solids content of the cake.

1. Calculate the mass of solids wasted per day (kg). This can be calculated from the WAS flow rate and the WSS SS or from the mass or volume of cake produced per day and the % dry solids.

2. Calculate the COD load (kg) coming into the plant (Flow ML/d x COD mg/L).

3. Divide the solids mass by the COD load. Compare the result with the appropriate line for raw sewage or settled sewage in Figure 7.2.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants

Example A small activated sludge plant treats approximately 0.19 ML/d. The flow is pumped (via a grinder pump) to the treatment plant and passes through an inlet screen before entering the activated sludge plant. In summary: 1. Average Daily Flow = 0.19 ML/d.

2. Average BOD5 concentration = 320 mg/L (no COD data is collected).

3. The sludge is dewatered using a belt press (2 hours operation per day, 3 days per week).

4. Sludge cake concentration = 15.5% dry solids (or 155 kg/m3).

5. Observed approximately 2 m3 of sludge cake per week in the skip bin.

Step 1 - Calculate the solids mass.

Solids mass = 155 kg/m3 x 2 m3/wk = 310 kg/wk = 44 kg/d.

Step 2 - Calculate the inlet COD load.

Assume a COD:BOD ratio of 2.5:1 (Typical domestic wastewater is around 2.1:1 - 2.3:1 however with the grinder pump we expect more inert solids material/particulate COD through the plant as well as higher inorganic mass from shredded screening).

Inlet BOD load = 0.19 ML/d x 320 mg/L = 60.8 kg BOD5/d.

Inlet COD load = 60.8 x 2.5 = 152 kg COD/d.

Therefore the daily sludge production rate = 44 kg/d / 152 kg COD/d = 0.29 kg TSS / kg COD/d.

Next, refer to figure 7.2. The estimated sludge age given the raw sewage feed to the treatment plant is approximately 30 days.

Figure 7.2.

Relationship between sludge age and sludge production with the example calculation point.

Note that this example applies only to wastewater that has the same characteristics as those used to derive the relationship in Figure 7.2 and hence this method is only a rough approximation. A good and accurate understanding of the sludge age is best obtained by maintaining a constant measured wasting rate, and regularly measuring the reactor MLSS and the TSS in the WAS flow.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Wasting Calculation of the sludge age is easier if mixed liquor is wasted directly from the reactor of a continuous plant, or from the react - aerate or mix phase in an intermittent plant. While it can be done, calculation of the amount wasted from clarifier Return Activated Sludge (RAS) or from the settle phase in an intermittent process is challenging as the concentration of solids varies greatly throughout the day or the settling phase respectively.

The advantage of wasting from the clarifier underflow or settle phase in the intermittent process is the solids are thicker and hence require less “work” to produce a final solids cake. However it is recommended to waste direct from the aeration zone or the react phase if possible (i.e. there are no limitations on solids processing) as this allows better process control of the plant.

Control of Sludge Age While many plants are controlled based on the MLSS, this is not the best way to control the plant. Using the sludge age is a better way.

The key for control of an activated sludge plant is that any changes to the sludge age should be limited to around 5%. The effect of the change should then be evaluated over at least one sludge age. It is important to note that for a given sludge age, the MLSS will increase and decrease based on the inlet parameters, in particular the COD and the temperature. Therefore, any attempt to track the MLSS concentration on a daily basis will only lead to problems and poor overall plant control.

Rather than try to keep a constant MLSS level by varying sludge wasting frequently, it is best to keep sludge wasting (and therefore sludge age) constant and allow the MLSS level to naturally vary due to changes in influent load. However if the MLSS levels start to increase or decrease outside of the range of the plant design, then the sludge age should be changed accordingly.

If the MLSS does continue to rise unexpectedly over a period of several days, the solids capture in the dewatering or the WAS pumping may be an issue or the influent characteristics may have changed significantly. The cause should be identified and corrected.

4. Maintain a Good Settling Biomass A fundamental requirement for high quality effluent is to maintain excellent separation of the biomass from the liquid.

At typical mixed liquor concentrations, settling starts out as simple settling of individual floc under the force of gravity. As this simple settling continues, the floc interact with each other and can combine to form larger floc that settle more rapidly.

As the thickening continues, the larger floc start to interact and they no longer settle independently but rather settle together as a mat.

At still higher concentrations of mixed liquor the particles start becoming compressed by the force of other floc on top, squashing those below.

A number of tests have been developed to allow monitoring of the settling properties of the mixed liquor.

These are described below. They all have some limitations and as such are best used as an indicator of developing problems such as toxic shock, impact of temperature changes or dumping of oil and grease rather than as an absolute control measure.

All the tests rely on taking a representative well-mixed sample of the mixed liquor.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Settled Volume (SV30) The simplest test is to take a representative 1 L sample of the mixed liquor and let it settle for 30 minutes in a 1 L measuring cylinder. Plastic cylinders are preferred to avoid breakage. The volume of sludge at the end of the settling period is measured and expressed as mL/L. While in general practice the most common 1 L measuring cylinder used is like the ones shown in Figure 7.3, there are 1 L measuring cylinders with different diameters. It is better to use the ones like those below.

Figure 7.3.

Two sludge settling tests in standard 1 L plastic measuring cylinders. Filtering apparatus for determining MLSS is also shown.

A high SV30 usually indicates a poor settling sludge while a low SV30 indicates a better settling sludge. While it is a simple test and therefore attractive to use regularly, the SV30 is a fairly useless control parameter. The reason is that there is no consistent relationship between the amount of sludge and its settled volume.

Conceptually it may seem that twice as much solids should result in double the SV30 value. Unfortunately this is not the case. The final settled volume will be much more than twice the volume of half the solids.

The actual SV30 value depends on the: •• Initial sludge concentration.

•• Viscosity of the sludge.

•• Cylinder height and diameter.

The effect of the initial sludge concentration can be reduced by determining the volume occupied by 1 gram of MLSS. This allows more meaningful comparison in a given plant from day to day, and to some extent comparisons between different plants. This standardised measure is the Sludge Volume Index (SVI). To calculate the SVI and other settling measures it is necessary to know the concentration of solids in the mixed liquor. This is called the Mixed Liquor Suspended Solids (MLSS).

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Mixed Liquor Suspended Solids (MLSS) The MLSS is measured by taking a sample of fully mixed, mixed liquor, filtering through a filter paper and weighing the dried solids. The method is provided in more detail below.

The test result can be quite variable, so for careful measurements, duplicate tests should be carried out and an average result calculated.

A suitable sample volume is 10 mL however the method below can be modified for any volume by simply altering the calculations: 1. Dry 2 GF/C filter papers in a 105oC oven for 1 hour.

2. Weigh each of the dried filter papers and record the weight in grams.

3. Ensure the sample to be tested is well mixed.

4. Take a 10 mL subsample using a 10 mL pipette and bulb pipettor (samples must not be pipetted by mouth). A pipette with the opening slightly ground off is best as it increases the size of the opening thereby allowing larger solids to be sampled.

5. Filter the sample through one of the weighed glass fibre filter papers using the vacuum filtering apparatus (see Figure 7.3).

6. Remove the filter paper from the vacuum apparatus, taking care not to lose any of the filtered solids and place the filter paper and its contents on a foil tray.

7. Repeat steps 2 to 6 for the second sample.

8. Place the foil trays in a 105oC oven for a minimum of 1 hour.

9. Remove the trays from the oven, allow them to cool in a desiccator and reweigh the filter papers.

Make sure the dried material is not lost. Record the weights in grams.

10. Determine the weight of dried solids by subtracting the initial weight of the filter paper from the final weight.

11. Repeat the 1 hour drying and reweigh. If the weight has not changed, the sample can be considered to be dried. If it has decreased by more than 4% repeat the drying until the weight change is < 4%.

12. Multiply each value by 1,000 to give mg.

13. Multiply each value by 100 to give mg/L for each sample.

14. Take the average of the two figures.

Alternatively, drying balances (Figure 7.4) are available that make measuring MLSS very simple. The filtered wet sample is placed on the zeroed balance and the lid closed. When constant weight is reached, the balance beeps and the weight can be read off the digital screen.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants

Figure 7.4.

A drying balance. The heating coils are shown in the raised lid.

MLSS vs MLVSS The MLSS test simply measures the total weight of solids in the mixed liquor. This is made up mainly of microbes but also includes sand, grit and inert breakdown products of the biomass. The important thing to realise is that it is the living microbes that carry out the treatment. So to work out how much active biomass is present and how much inorganic non-active material is present, the Mixed Liquor Volatile Suspended Solids (MLVSS) test is used. The test is essentially the same as the MLSS test however the sample is placed in a muffle furnace at 550oC. This effectively burns off all living material and just leaves the inert material. This can be cooled, weighed and subtracted from the MLSS to give the MLVSS. In most plants the MLVSS makes up about 70% - 80% of the total mass. However in plants with very old sludge, those receiving alum sludge from a WTP, or plants dosing aluminium or ferric salts for P removal, the volatiles may be much lower around 50% - 60%. In these situations just measuring MLSS gives a false indication of the biomass that is available to carry out the waste treatment. In some cases, particularly if the sludge age is very low (< 6 days) the MLSS may in fact have to be increased to provide the necessary active biomass.

Sludge Volume Index (SVI) The SVI is the volume occupied by 1 gram of MLSS. It is simply the 30 minute Settled Volume (SV30) divided by the MLSS. SVI (mL/g) = SV30 (mL/L) x 1,000

MLSS (mg/L)

The 1,000 factor is included to convert MLSS in mg/L to g/L so that the final calculated result is mL/g.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants There are a number of limitations with the SVI test. These are the same as described above for the SV30 test. In addition, the SVI comparison between plants is weak. Two sludges with the same SVI may have different settling and dewatering properties. In addition, the SVI is influenced by the concentration of the sludge sample being tested. Thus a sample with a high concentration can yield a higher SVI than one with a lower MLSS concentration when in reality both sludges have the same settling characteristics. It should be noted that the SVI is also affected by the cylinder diameter and as such, the standard method includes slow stirring.

Over years of operation of activated sludge plants some general relationships between SVI and sludge settling properties have been developed. Table 7.2 shows the relationship between SVI and settling.

Table 7.2.

Relationship between SVI and settling behaviour.

SVI (mL/g)

Settling Behaviour

< 120

Good settling

> 180

Bulking

> 200

Major settling problems

To attempt to overcome some of the deficiencies in the SVI method, two other “settling” measurements have been developed.

Stirred Sludge Volume Index (SSVI) This method was developed in an attempt to overcome one of the main deficiencies of the SVI method, in particular the wall effect where settling is slower next to the walls of the cylinder.

The method uses a 10 cm diameter 3.5 L cylinder (Figure 7.5) with a low speed (1 rpm) stirrer to overcome the wall effects. In addition, settling is based on a standard concentration of 3,500 mg/L (3.5 g/L) to allow more meaningful comparisons.

Figure 7.5.

Equipment for measuring SSVI. 67

Chapter 7 - Operation and Optimisation of Activated Sludge Plants To achieve the 3,500 mg/L MLSS can involve some fiddling. If the MLSS is above 3,500 mg/L the sample to be tested needs to be diluted with clarified effluent from a parallel sample. If on the other hand the MLSS is < 3,500 mg/L, a large sample can be taken in a bucket and allowed to settle for a short time and some of the clarified effluent removed. The amount to be removed is pretty much guesswork. As long as the result is within say +/- 300 mg/L it should be OK. The cylinder is filled with the adjusted mixed liquor sample and the stirrer and timer started. At the end of the 30 minute period, the level of the sludge is recorded in the same way as for the SV30 test (mL/L) and the SSVI calculated and reported as SSVI3.5 .

SSVI values are 40 for a sludge with good settling properties, 80 for an average plant that is operating well and 120 for sludge with poor settling characteristics. sSVI sSVI is the Standard Methods test where a stirrer is used in the cylinder. This can be differentiated from SSVI where the test MLSS is adjusted to 3,500 mg/L. Diluted Sludge Volume Index (DSVI) The DSVI test was developed to remove the sensitivity of the sludge settling to solids concentration.

The DSVI is defined as the volume (mL) occupied by 1 gm of sludge after 30 minutes of settling in a 1 L unstirred measuring cylinder with the proviso that the final settled volume is between 150 mL and 250 mL. If not, a different dilution needs to be tried until the value is between those values.

Finding the correct dilution can be tedious, but in general, once it is established for a particular plant, the same dilution can be used over and over again. Alternatively 3 or 4 cylinders may be used simultaneously each with a different dilution.

The calculation then becomes: DSVI = SV30 of the diluted sample

SS of the diluted sample

This might suggest that in routine operation of a plant two measurements of “MLSS” need to be carried out each time. The MLSS of the mixed liquor and the SS of the diluted mixed liquor. In practice this is not necessary. As long as the MLSS is determined on a well-mixed representative sample of mixed liquor and the dilution used is done carefully, the MLSS can simply be divided by the dilution factor to give the SS of the diluted sample.

Which one is best?

Each method has limitations. For practical reasons, despite its known problems, for routine ease of measurement in an STP, the SVI has probably been the most common measurement however, the DSVI is increasingly being used at more plants.

An operator is not really interested in making comparisons between plants. All that is really needed is a measure that reflects what they know of their plant and that might help predict an increased chance of solids carry over from the clarifier, or perhaps problems with a thickener or belt press or centrifuge. In many ways it is a local measure for just the treatment plant in question. The key thing is that any sustained change in the SVI or DSVI indicates something has changed with the mixed liquor biomass and the reason for the change needs to be investigated.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants 5. Control DO and Monitor the Oxygen Uptake Rate (OUR) Oxygen is the single most important requirement for treatment. While COD can be removed effectively with anaerobic treatment, both nitrogen and phosphorus removal require aerobic conditions during treatment.

The oxygen requirements for various stages of biological nutrient removal are summarised in Table 7.3.

Table 7.3.

Oxygen requirements for nutrient removal.

Substance and amount removed

Oxygen Required

1 kg BOD

1 kg

1 kg TKN (NH3 makes up approximately 66% of TKN in the influent)

4.6 kg

1 kg NO3

2.9 kg

1 kg Biomass (Endogenous Demand)*

0.03 - 0.06 kg

* The endogenous demand for the biomass is the demand for the biomass to survive. For long sludge age systems this demand is lower (i.e. > 20 d use 0.03 kg/kg and lower sludge ages, < 20 d, use 0.06 kg/kg).

In general, aeration should be minimised as much as possible so that nitrification occurs reliably and ammonia is removed to the required level. The key is to only use as much oxygen as necessary and save as much degradable carbon for denitrification and for phosphorus release.

Monitoring and comparing blower speed information for each day of the week is useful to develop an understanding of what is normal for specific times on specific days. Check the ammonia levels at the end of the aerobic zone an hour or so after peak blower speed occurs. High ammonia indicates either excessive inlet ammonia or inadequate conversion rates to nitrate.

Active biomass consuming COD or nitrifying ammonia consumes oxygen. The rate will be higher if there is a large amount of COD and/or ammonia available. However, if the Oxygen Uptake Rate (OUR) is determined during a period of low load in a continuous plant, or at the end of the aeration stage in an intermittent plant, the OUR will be lower. The OUR is relatively constant for a healthy biomass in a given plant under the same conditions of load and MLSS. Therefore the OUR can be used as an indicator of the health of the biomass.

For example, if a toxic shock has been experienced by the plant, or if nitrification is inhibited because the sludge age is too short, or the temperature is too low, then the OUR will be lower than usual.

Once again, OUR is not an absolute measure but rather one that can be used to indicate a change in plant operating conditions and warn that something may be going wrong.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants The simplest method to determine OUR is described below. It does require a small aquarium aerator pump and aeration stone (Figure 7.6). These can be easily purchased from an aquarium supplies shop. Alternatively, if there is a compressed air line to the plant laboratory, that can be used.

To determine OUR: •• Take a 1 L sample of biomass during a low load period for example the middle of the afternoon. o At the end of the aeration basin in a continuous plant. o At the end of aeration stage in an intermittent plant.

•• Place the sample in a clean 1 L beaker.

•• Place the beaker on a magnetic stirrer and add a magnetic flea. Start the stirrer going at the lowest possible rate necessary to keep the mixed liquor suspended. If the stirrer moves too rapidly it will add oxygen to the sample.

•• Place a DO and pH probe into the mixed sample. This is best done by securing the probes to a stand.

•• Place the aeration stone in the sample and turn on the aeration pump.

•• Allow aeration to continue until the DO reaches approximately 3.5 mg/L. •• Switch off the aeration.

•• Once the DO reaches about 3.0 mg/L start recording the DO every 30 seconds for 5 minutes or so.

•• Work out the slope of the line in mg/L/min and convert to mg/L/h by multiplying by 60.

If you suspect problems, repeat the test a second time and take the average of the results.

Figure 7.6.

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Laboratory set-up for determining the oxygen uptake rate.

Chapter 7 - Operation and Optimisation of Activated Sludge Plants By measuring the OUR regularly it will be possible to get a feel for the average OUR for a given plant and also the range of normal values for that plant. Any consistent change away from these values suggests problems.

What you are looking for is a significant change, in particular a significantly slower uptake rate i.e. a flatter less steep slope. This suggests loss of nitrification or a toxic load to the plant. Higher rates probably reflect a high input of food, perhaps from a new industrial site.

An alternative in an intermittent plant is to measure the slope of the DO trend after the aerators switch off.

This could be easily programmed into a SCADA system and plotted as a trend. The system could be set to do this once a day during a peak load period and a trend produced.

Any decrease (or increase) in the OUR indicates a problem. If detected, the following should be checked if it isn’t already monitored closely: •• The alkalinity needs to be > 60 mg/L although 75 mg/L is a good safe set point for safe operation.

•• Final effluent ammonia. If a toxic shock has happened, the nitrifiers will almost certainly be first hit and the effluent ammonia will rise.

•• Sludge age.

•• Temperature.

•• Microscopic examination of biomass.

Respiration Rate Some plants monitor the Respiration Rate (RR) or Specific Oxygen Uptake Rate (SOUR).

RR or SOUR =

OUR MLVSS

While it is not possible to give typical figures, determining the RR or SOUR and plotting this on a trend will quickly alert the operator to any changes and give time to investigate the cause and take corrective action.

The respiration rate or specific oxygen uptake rate can be used to verify whether an industrial waste input (or other input such as return streams from solids processing) is affecting the process.

6. Ensure Sufficient Alkalinity and Suitable pH Activated sludge processes require alkalinity. Nitrification consumes alkalinity and denitrification returns some alkalinity to the mixed liquor.

•• Nitrification requires 7.14 mg alkalinity/ mg N nitrified.

•• Denitrification “recovers” 3.57 mg alkalinity/ mg N denitrified.

Therefore effluent alkalinity is a key measurement parameter. This should be measured weekly.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants The amount of N denitrified can be roughly calculated by determining the difference between the average TKN in the raw sewage and the average concentration of NO3 + NO2 (NOx) in the effluent.

Example A 2 ML/d plant has an inlet TKN = 60 mgN/L, inlet alkalinity of 265 mg/L and effluent NOx (nitrate + nitrite) = 10 mgN/L. Calculation Alkalinity demand (assuming complete nitrification) = 2 (ML/d) x 60 (mgN/L) x 7.14 (mg alkalinity/ mg N nitrified) = 857 kg/d Alkalinity recovered (assuming complete denitrification)

= 2 (ML/d) x (60 -10) mgN/L x 3.57 (mg alkalinity/ mg N denitrified)

= 357 kg/d

Influent alkalinity

= 2 (ML/d) x 265 mg/L

= 530 kg/d

Alkalinity balance

Alkalinity available - alkalinity required

= (530 + 357) - 857

= 887 - 857

= 30 mg/L

Conclusion Alkalinity dosing is required. As a rough guide, effective treatment requires an alkalinity of between 150 - 175 mg/L CaCO3. Anything below 60 mg/L is likely to compromise operation. It is recommended to retain effluent alkalinity at or about 75 mg/L. Low pH can also indicate low alkalinity, however the alkalinity drops before the pH drops. If this occurs, ammonia breakthrough is likely due to loss of nitrification. To prevent a drop in pH, it is important to maintain sufficient alkalinity to act as a “safety” buffer. If the pH drops below 6.5, alkalinity should be added.

A pH trend from an activated sludge plant that was experiencing periodic high levels of NH3 in the effluent is shown in Figure 7.7. When the pH trend was displayed using a sensible scale for pH, in this case 6.1 - 7.0 rather than the default 1 - 14, the periodic dips in pH became apparent. These coincided with the periods of loss of nitrification. This example emphasises the importance of ensuring the scales on SCADA trends are appropriate.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants

Figure 7.7.

A 3 month pH (yellow line) trend from an activated sludge plant showing periodic dips in pH (arrows). The vertical scale is 6.1 to 7.0.

Nitrification can also be inhibited at pH values above 8.5. High pH can result from trade wastes, in particular alkaline Clean In Place (CIP) waste. If this is a regular event, the pH should be reduced by acid dosing.

7. Ensure Necessary Nutrient and Nutrient Ratios are Available As with any human diet it is necessary to know just how much food is needed and how much is provided.

The total amount isn’t very useful. It is the amount per microbial mass that is important. It is not surprising that individual bacteria and mixed liquor ecosystems behave differently according to how much food they get.

The food: microorganism (F:M) ratio defines the amount of food available for the biomass.

F:M (Food to Microorganism) Ratio The food: microorganism (F:M) ratio equals the daily mass of BOD5 entering the plant divided by the total mixed liquor solids.

F:M =

kg BOD5 per day

kg Biomass

=

BOD5 (mg/L) x Inflow (ML/d)

MLSS (mg/L) x Reactor Volume (ML)

The F:M ratio is related to MLSS. If wasting is increased, the MLSS will decrease and the F:M ratio will increase meaning there is more food available for the bacteria. On the other hand if wasting is decreased the MLSS will build up and the F:M ratio will decrease meaning there is less food available for the bacteria.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Similarly, the F:M ratio is related to the sludge age. As the sludge age gets longer the F:M ratio is smaller.

For this reason the F:M ratio can theoretically be used to control an activated sludge plant however there are significant limitations in using this approach and it is not recommended. The main limitations are the variability in the BOD test and the fact that the MLSS includes inert material and possibly also old “tired” bacteria.

As a result of these limitations, sludge age is the preferred control parameter.

As a guide, for COD removal plants, the F:M is typically 0.2 - 0.5 while for nutrient removal plants the F:M ratio is closer to 0.02 - 0.12 based on BOD5 and MLSS. C:N:P Ratios The nutrient requirements for operation of an activated sludge plant are often summarised as a C:N:P ratio.

Different text books quote different values. The values quoted here should provide sufficient general information to ensure sound operation of a plant. The ratios don’t necessarily need to be measured regularly.

However, it is very useful to measure them occasionally, particularly where a plant has a significant trade waste input or if the activated sludge plant is treating only industrial waste, such as winery wastewater. For plants receiving industrial waste, it is not uncommon to have an imbalance in the C:N:P ratio which is typically corrected by the addition of either superphosphate fertiliser or urea nitrogen.

Plants running on purely domestic sewerage usually don’t have a problem.

General operation of a “standard” activated sludge plant requires a C:N:P ratio of approximately 100:25:5.

Nutrient requirements for N and P removal can be further defined. Table 7.3 summarises the general COD requirements for biological nutrient removal.

Table 7.3.

COD requirements for biological nutrient removal.

Substance and amount removed

COD Required

1 mg/L NO3

8.6 mg/L

1 mg/L PO4

40 - 60 mg/L

Impact of Inlet Works on Nutrients The operation of the inlet works is beyond the scope of this Guide; however there are certain aspects of the inlet works that directly impact on the operation of an activated sludge plant.

Inlet works generally include screens. Screen sizes vary from the older manually raked 20 mm screens to the newer 3 mm or 10 mm stepping screens. Screens take out the large items in sewage including a certain amount of faecal material. The finer the screen, the more of this material is removed. Faecal material is COD. In fact much of it is degradable COD. If this is not returned to the system then there is potentially less degradable COD available to drive the nutrient removal processes. For many Australian plants that are COD limited, it is essential that all screenings and grit are thoroughly washed and pressed and the washings returned to the process to help nutrient removal processes.

Screenings and grit present a significant risk of damage to aeration systems and also instruments. Excess grit often settles on or around diffusers and can result in reduced aeration efficiency. Taking diffusers out of service is a labour intensive and costly exercise. It is therefore better to remove grit up front if possible.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Micronutrients Bacteria also require phosphorus (P), potassium (K), sulphur (S) and magnesium (Mg). There are also a large number of trace elements that are essential for microbial growth. The most important ones are calcium (Ca), manganese (Mn), iron (Fe), cobalt (Co), copper (Cu) and zinc (Zn). These are generally available in sufficient quantities in domestic sewage, however one or more may be limiting in industrial wastes. Therefore for a plant treating purely industrial waste, full characterisation of the micronutrients may be necessary, particularly if there are problems with the operation of the plant.

8. Manage Internal Recycles and RAS in Continuous Plants Depending on the return site, internal recycles and RAS can deliver oxygen and/or nitrate to zones where the presence of DO or nitrate can inhibit the function of that zone. For example: •• Oxygen returned to the anoxic zone inhibits denitrification.

•• Oxygen and nitrate returned to the anaerobic zone inhibit phosphorus release and therefore Enhanced Biological Phosphorus Removal (EBPR).

There are a number of actions that can be taken to minimise these potential problems: •• In continuous plants, the DO concentrations can be slightly tapered in the aerobic zone to help reduce oxygen recycled to the anoxic zone via the Internal Nitrate Recycle, or the anaerobic zone via the Internal MLSS Recycle. This may be possible where there are multiple aerators in a reactor or in the case of membrane diffusers, separate banks of diffusers with separate controls. Less aeration is provided towards the end of the aerobic zone.

•• Check the levels of DO about one third of the way down the anoxic zone at peak morning inflow.

Any remaining oxygen tells you the anoxic zone is not anoxic at all and that the Internal Nitrate Recycle may be too high or the DO set point in the aerobic zone is too high.

•• Internal recycles should be maximised until DO is detected in the anoxic zone or anaerobic zone.

The recycles can then be reduced slightly to prevent return of any DO. •• In some continuous configurations, RAS can return nitrate to the anaerobic zone. Check nitrate at the clarifier outlet around 10 am, particularly if pin floc or rising sludge is present at this time (due to denitrification in the clarifier). The nitrate has probably not been denitrified during periods of low load to the plant. It may be necessary to reduce the RAS at this time to prevent the return.

•• In EBPR systems, nitrate concentrations should be reduced to < 3 mgN/L prior to the anaerobic zone to reduce inhibition of the EBPR process.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants 9. Observe Reactors and Clarifiers Regularly In many ways an activated sludge plant is a living system. Changes in colour and odour and overall appearance signify a change that may be detrimental to the ongoing operation of the plant. The following list provides some of the things that should be checked daily using our own senses of sight, smell and hearing: •• Smell of the inlet works.

•• Colour of the aeration basin.

•• Smell of the aeration basin.

•• Pattern of aerator throw.

•• Diffused air pattern in the aeration basin.

•• Noises from the aerators or blowers.

•• Appearance of foam on the reactor.

•• Clarity of water flowing out of the clarifier or over the weir.

•• Turbidity of water flowing out of the clarifier or over the weir.

•• Appearance of foam on the top of the clarifier.

•• Smell of the clarifier.

Any change should be investigated and any problems corrected promptly.

10. Manage any Return Streams The impact of return streams on the operation of a STP is often not adequately considered. Return streams include: •• Returns from solids processing.

•• Returns from digesters.

•• Returns from lagoons.

Water removed during the dewatering process is generally returned to the plant. Depending on the efficiency of the dewatering process, solids can also be returned to the plant. A return with high suspended solids can have a significant impact on the operation of the plant, in particular increased aeration costs and in some cases, overloading of the clarifier (see Chapter 12). Dewatering systems can be quite variable in their performance and therefore the solids returned can also vary considerably. Since most plants do not directly measure the efficiency of sludge thickening and dewatering systems, operators are generally unaware of these changes that may be responsible for other problems in the plant.

While cake Dry Solids (DS) are often measured, the impact on the plant of producing 14% DS cake compared with producing 16% DS is not normally considered. A better understanding of the performance of the solids processing elements can be obtained by accurately measuring the feed flow and SS concentration, and the filtrate (return to plant) flow and SS concentration. The comparison is a useful measure of the solids processing efficiency but also the potential for excess return of solids to the plant. Online measurement is preferable however multiple measurements during operation of the dewatering unit will provide valuable information. Without appropriate monitoring, one of the reasons for deteriorating plant performance may be missed.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Digesters can be aerobic or anaerobic. At the completion of digestion, the solids are dewatered and the supernatant usually returned to the head of the plant. The returns can contain nitrogen and phosphorus. In the case of aerobic digesters, aeration is usually intermittent to allow some denitrification and therefore minimise return of nitrogen to the bioreactor. Anaerobic digesters result in release of phosphorus. Similarly, once the solids are removed from an aerobic digester, phosphorus release can also occur. Return of phosphorus in the supernatant return from digester operation will result in an increased load to the bioreactor. The impact of the released phosphorus on the bioreactors is usually minimised by chemical dosing of the digester returns.

Many plants have sludge lagoons that return supernatant to the head of the plant. These returns can contain ammonia and phosphorus and also algae that commonly thrive in the lagoons. The impact of phosphorus can again be minimised by dosing with alum or ferric salts. The algae are not easily removed but seem to form part of the biomass and do not appear to have any major adverse effect on the operation of the activated sludge process.

Return from digesters and lagoons should not occur at the same time as peak flows as the treatment capability of the plant is likely to be exceeded with the result that ammonia breakthrough may occur as well as increased levels of phosphorus in the final effluent. Similarly, return flows from digesters and lagoons should not coincide with minimum flows to the plant. In the absence of inflow, there will be insufficient COD to allow removal of nitrate and phosphorus and therefore ammonia and phosphate levels are likely to rise in the final effluent.

11. Manage Peak and Holiday Population Flows Wet Weather Flows Sewage treatment plants are typically designed to fully treat up to around three times the average dry weather flow through the plant. Flows above this figure can cause wash out of solids from the reactors and, in continuous plants cause the clarifier sludge blanket to lift and overflow the weir resulting in solids wash-out of the clarifiers. This can be catastrophic in terms of solids discharge in the effluent and can also potentially reduce the sludge age to a point where full nitrification is no longer achievable.

There are a number of options to manage wet weather flows above the design capacity of the plant.

•• If a bypass is available, the plant inflow should be bypassed at the point where the solids blanket in the clarifiers is at risk of carrying over. Alternatively, the flow that needs to be bypassed can be based on the calculation of clarifier solids flux and overflow rate (see examples in Chapter 12). Once the maximum flow possible through the clarifier is established, all flows above that level need to be bypassed.

Flows can be set to bypass the entire system or can be returned to any tertiary filters depending on the capacity of those filters. With tertiary filters, there is a risk of blocking the filter media with excess solids loading onto the filters (filter suppliers will generally guarantee their filters up to a feed SS concentration of 30 mg/L).

In some plants, the bypassed flow is stored in a lagoon and can be slowly fed back to the plant once the wet weather event is over. Any return should not coincide with the normal diurnal peak inflows.

•• If a bypass is not available, aerators and mixers should be switched off and the solids allowed to settle in the reactor to minimise wash-out of the solids.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Holiday Populations A common mistake made by operations staff in holiday areas is to stop wasting sludge prior to holiday periods to build up the MLSS concentration. This does not build up active biomass. The biomass will only grow in response to an increase in the influent load. What does build up is inert inorganic particulate material.

There are two main methods for building up active biomass: 1. Addition of WAS from another plant the day before the holiday starts. The necessary amount can be calculated. a. Determine a suitable F:M ratio for the plant during holiday periods based on past experience. b. As the population numbers increase, compare the required value with the actual value and calculate the difference in biomass required to achieve the required value. c. Calculate how many truck loads are required to provide the amount of biomass and commence transport. d. Check the solids flux on the clarifiers for this solids mass (see Chapter 12) to ensure the clarifier capacity is not exceeded.

2. Addition of a carbon source increasing slowly for a period of say 1 month prior to the peak load period. a. Ideally a trade waste customer should be investigated first to provide additional RBCOD either by truck or via sewer. b. If a trade waste source is not available, methanol, sucrose, molasses or acetic acid can be used with a temporary dosing system.

12. Know the Hydraulic Retention Time (HRT) While it is not an important control parameter, the Hydraulic Retention Time (HRT) should be understood simply to show the difference between the time it takes liquid to pass through the plant compared to how long the solids are held in the plant.

The HRT may be as short as 5 hours in a high rate plant but is more typically 20 to 30 hours. The HRT can be calculated by dividing the volume of the reactor by the flow. The easiest way to do this is to calculate the volume of the reactor in ML and use a flow in ML/d.

HRT (d)

=

Reactor Volume (ML) Inflow (ML/d)

Example If the tank volume is 6 ML and the flow rate is 5 ML/d.

HRT = 6 ML/5 ML/d.

HRT = 1.2 days (1.2 x 24 = 28.8 hours).

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Summary Requirements for N removal Most activated sludge plants in Australia are required to achieve nitrogen removal to an effluent TN around 10 mgN/L. The key to achieving nitrogen removal is to ensure nitrification occurs reliably and consistently. This requires: •• An appropriate sludge age.

•• Sufficient aeration.

•• Sufficient alkalinity.

•• An appropriate aerobic mass fraction.

Subsequent denitrification then requires: •• An appropriate plant configuration depending on the TN target in the final effluent. Given sufficient carbon availability, a two stage system is capable of achieving effluent TN around 10 mgN/L (median). To achieve lower effluent TN, a four stage system is required. A two stage system has a single anoxic zone and single aerobic zone whereas a four stage system has two anoxic and two aerobic zones (see Chapter 6).

•• Sufficient carbon (C:N ratio > 8:1).

•• Operation of both RAS and internal recycles. Recycle rates can be increased to achieve lower effluent TN concentrations. This should be undertaken via either RAS flows or internal recycles (or both). It is recommended that recycle rates above 6 times influent flow are not used because of the excess energy costs. Care should be taken when increasing the RAS flows that the clarifier solids loading is not exceeded (see Chapter 12) and that oxygen is not returned to the anoxic zone(s).

Summary Requirements for EBPR Conventional plants will achieve 15% - 25% phosphorus removal through simple biological growth alone.

EBPR plants require a dedicated anaerobic zone (or anaerobic stage in intermittent plants). EBPR is inhibited by oxygen and nitrate and as such many plants contain a configuration for de-aeration and denitrification prior to recycling sludge to anaerobic zones (see Chapter 6).

The amount of EBPR can be assessed to some extent by measuring the increase in soluble phosphate in anaerobic zones and the increase in phosphorus concentration of the biomass (usually from 1% - 3% in conventional plants and 5% - 12% in EBPR systems).

The “sink” for phosphorus removal is the wasted biomass (biosolids). As the removed phosphorus is in the wasted biomass, a common issue with EBPR is the re-release of phosphorus in solids processing streams.

Sludge must be kept aerobic at all times until it is fully removed from the system. Monitoring of phosphate in the return streams will provide an indication of any re-release. All return stream phosphate concentrations should be kept low (at or around the target effluent quality).

There are many plants that also achieve biological phosphorus removal due to complete denitrification in anoxic zones thereby converting the anoxic zone to an anaerobic zone.

Table 7.5 summarises the process requirements to achieve different effluent total P concentrations.

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Chapter 7 - Operation and Optimisation of Activated Sludge Plants Table 7.5.

Summary of unit processes necessary to achieve different effluent total P limits.

Process Required

Total P limit required (mg/L) <1 A

Chem P EBPR with good settling

B



< 0.5 C

D

 





< 0.1 < 0.05 < 0.01 E

F









Post coagulation Sand Filtration

80







G

H











Adsorption



Membranes



Chapter 8 Aeration The aerators are the lungs of the treatment process. They transfer oxygen to the water and strip the carbon dioxide produced by the bacteria. The aerators also provide mixing that ensures the full volume of the reactor is able to be used. They are one of the plant’s major energy consumers and therefore represent one of the biggest operational costs of a Sewage Treatment Plant (STP).

There are two main types of aerator: •• Mechanical surface aerators (Figures 8.1 and 8.2).

•• Fine bubble membrane diffusers (Figure 8.3).

Figure 8.1.

Examples of vertical shaft mechanical aeration systems.

Figure 8.2.

A horizontal shaft “brush” mechanical aerator.

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Chapter 8 - Aeration

Figure 8.3.

A fine bubble membrane diffuser system in place in a reactor (left) and a close up of a membrane diffuser (right).

For an aeration system using surface aerators, the amount of aeration can be varied by adjusting the number of aerators operating or the speed or immersion depth of the aerators. For a fine bubble diffuser system, the amount of aeration can be varied by altering the blower speed or number of blowers online. Where aeration is stopped as part of the normal operating cycle, as in intermittent plants, diffused air systems need to be of a membrane type to prevent backflow of mixed liquor into the air manifold.

Mechanical Aerators The most common types of mechanical aerators include: •• Vertical, slow-speed surface aerators.

•• Vertical, high speed surface aerators.

•• Horizontal brush aerators (often used in oxidation ditches).

•• Aspirators.

Surface aerators break the liquid up to form a mixture of droplets of water and bubbles of air. Some of the small droplets also get thrown up into the air. Oxygen diffuses into the droplets from the air and the air bubbles thereby oxygenating the liquid.

Aspirating systems use a pumped flow that sucks air into a tube and injects the air and water mix into the water usually at an angle to the water.

The vertical aerators generally have a rotor attached to them. Figure 8.4 shows an example of two types of rotor. The efficiency of this style of aerator can be adjusted by altering the depth of the rotor. Figure 8.5 shows a rotor adjusted for maximum efficiency with the top of the rotor level with the surface of the liquid. In a situation where too much air is delivered, the rotor can be raised slightly or the water level lowered; however the best way to adjust the aeration is by using Variable Speed Drives (VSD) to alter the rate of rotation of the aerator.

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Figure 8.4.

Two examples of aerator rotors. The one on the right is a non-clogging design.

Figure 8.5.

A rotor style aerator with the rotor adjusted for maximum rotor efficiency.

Mechanical surface aeration is generally less efficient than fine bubble diffused air aeration. Aerators have a typical life of around 15 years if they are well maintained.

Unlike diffusers, surface aerators have the advantage of being fairly readily accessible. There are a number of things to look out for with surface aerators: •• Leaking floats that could sink or capsize the aerator.

•• Rotors operating backwards. There have been examples where aerators had operated (poorly) for some time and were eventually found to be rotating backwards.

•• Secure mooring cables. This is particularly important in wild weather.

•• The accumulation of rags and hair on the rotor (ragging). Depending on the efficiency of the inlet screens, ragging can be a major or minor problem. In either case, ragging will occur with most rotating aerators. Ragging decreases the efficiency of the aerator and if bad enough can impact on the balance of the unit.

•• Stainless steel construction is more likely to harden and fail.

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Chapter 8 - Aeration •• Be alert to noises. o Growling noises low in the unit (gear box). o Rumbling noise high up in the unit (electric motor). o Rattling noises on top (Shroud fouling the cooling fan).

•• Always follow manufacturer’s recommendations for servicing to ensure longevity and to maintain the warranty.

•• Consider using fully synthetic oils for the gearboxes. Mineral oils need to be changed every 6 months whereas synthetic oil only needs changing roughly every 2 years.

•• Sludge (and weeds) can accumulate on the surface of the aerator. This should be removed as the weight builds up and makes the rotor sit lower in the water resulting in lower efficiencies.

•• Beware of stepping out onto older style pontoon aerators because of their inherent instability.

•• Ensure a full risk assessment is undertaken prior to access and maintenance.

Diffused Air Diffused air systems generally use fine pore “diffusers” that can be either made of a hard ceramic or a flexible membrane fed by air blowers.

Flexible membrane diffusers are usually circular and mounted as part of a PVC header (Figure 8.3). Tiny holes (the pores) punched into the membrane create small slits which open up when pressure is applied through the PVC headers. This stretches the flat discs into a shallow dome shape and allows small bubbles through the pores. When pressure is removed (i.e. the blowers are turned off), the membrane resumes its flat disc shape, which closes the holes to minimise backflow of mixed liquor into the header.

The tiny holes generate clouds of fine bubbles, and oxygen transfers from the bubbles to the liquid between the bubbles.

Diffused air systems are not easy to access. In most cases the only way is to empty the tanks which is quite expensive, introduces OHS management issues and requires the tank to be taken off line for quite some time. The manifolded systems can be modularised such that sections of the manifolds with the diffusers can be lifted from the tank while it remains full and operating. These are more expensive however the operational costs over the life of the plant may still make this attractive. Whenever a new plant is designed with diffusers, accessibility for cleaning and maintenance needs to be carefully considered.

The life of membranes has improved as understanding of suitable materials has increased, but most guarantees on life are still around five years even though the actual life is normally longer than this. The life of the diffusers is largely determined by hardening of the membranes, but some fouling can also occur and ultimately cause failure. The rate of loss of elasticity and degree of fouling depends to some extent on the nature of the water. The hardness, alkalinity, use of iron salts and lime and possibly the presence of manganese in the water all contribute to hardening of the membrane. Figure 8.6 shows the upper and lower surface of membrane diffusers removed from an intermittent plant. The plant doses iron salts for the removal of phosphorus and the iron accounts for the red brown colour of the upper surface. The white colour of the lower surface suggests a calcium product, probably calcium carbonate. The plant doses lime for provision of alkalinity.

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Figure 8.6.

The upper surface (left) and under surface (right) of a fouled membrane.

Fouling increases with time and can vary widely depending on the chemical composition of the mixed liquor.

Fouling rates tend to be higher when the sewage contains a significant organic industrial component such as abattoir, dairy or poultry wastewater.

Fine pore diffusers generally suffer from some degree of external fouling by biological slimes and chemical precipitates. Internal fouling can also occur from excessive dust in the airflow or from sludge which enters the piping through leaks during air outages.

Some information on fouling may be obtained by monitoring flow and pressure over the years of installation.

Table 8.1 shows the flow and pressure in a plant when all 5 blowers were operating. The data suggests there has been a decrease in the peak flow associated with an increased back pressure. The results are consistent with fouling of the diffusers. It is difficult to use this information to predict when the membranes need replacing, however, over time, some very useful predictive information could be obtained, so monitoring of this type is recommended.

Table 8.1.

Flow and pressure data for a two year period from a plant with membrane diffusers.

Flow (L/sec)

Pressure (kPa)

New Diffusers

1,741

46.7

2 years old

1,733

50.8

Removing the membranes and pressure washing them at intervals of around five years may be beneficial, however this necessitates taking a reactor off line. Membranes can also be acid cleaned in situ using formic acid vapour or phosphoric acid. Typically, in situ cleaning is carried out annually. In the case of formic acid vapour cleaning, liquid formic acid is introduced into the “down pipe” of an aeration grid during aeration through an injection point attached to the pipe. The formic acid vaporises quickly and the gas is forced through the pipework and diffusers. Cleaning with phosphoric acid is more difficult since the phosphoric acid doesn’t vaporise under those conditions. There are companies in Australia that specialise in such cleaning.

Some manufacturers recommend aggressive agitation of the membranes by deliberately cycling full flow on and off several times to try to physically disrupt hardened materials on the surface by flexing the membrane repeatedly.

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Chapter 8 - Aeration Fine pore diffusers need periodic cleaning to restore aeration capacity and reduce energy consumption.

A study of three plants using ceramic diffusers concluded that refiring was the most effective cleaning technique, and provided sufficient blower capacity to meet the increase in diffuser back-pressure was available, the most economic cleaning interval was 5-10 years. Another study of plants using membrane or sintered plastic diffusers which could be cleaned satisfactorily by hosing, concluded that annual cleaning was the most economic.

Leaks can develop. Large leaks can be seen as violent bubbling in a particular location. Smaller leaks are more difficult to detect, however at one intermittent plant with a denitrification cycle where the blowers are run for short periods, continuous bleeding at a particular location in the tank can be seen (Figure 8.7).

Figure 8.7.

Small air leaks from a few diffusers. Note how the biomass is stirred up around the escaping bubbles.

Condensation in the headers is common, and provisions for blowing this out are often provided (Figure 8.8).

However it is clear from the operation of membrane diffusers that condensation is not the only source of water inside the manifolds. In an intermittent plant with 3 cells, each fitted with bleed valves, the amount of water that blows out at the start of each aeration cycle is quite different. The plant was originally constructed without bleed valves and the problem was only detected when the impact on aeration efficiency was such that nitrification was compromised. Once the bleed valves were fitted and with no changes in set points, the nitrification was recovered.

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Chapter 8 - Aeration

Figure 8.8.

Bleed valve systems on a diffused air manifold system.

While diffused air systems should only allow passage of air out of the system, items of debris that could only have come from raw sewage have been found inside the manifold (Figure 8.9).

Figure 8.9.

Debris found inside the bleed valves at one plant.

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Chapter 8 - Aeration

Oxygen Transfer In clean water where there is no oxygen demand, the oxygen transfer rate can be measured by the rate of change of Dissolved Oxygen (DO).

In a bioreactor where there is an oxygen demand, the DO stabilises at the value where the oxygen transfer rate equals the oxygen demand. Then, if the aeration rate remains constant, the DO will go down if the oxygen demand increases and will go up if the oxygen demand goes down. Alternatively, the aeration rate can be varied to maintain a constant DO. Although there is a slight increase in the amount of oxygen transferred with increasing temperature, water at that higher temperature saturates at a slightly lower level, therefore there is little net effect of temperature on the amount of oxygen transferred.

Aerator efficiency can be compared with a number known as the alpha factor. This compares the oxygen transfer in wastewater to that in clean water. Alpha values range from 0.4 - 0.8 for diffused air systems. Lower alpha factors are indicative of fouled diffusers.

Oxygen saturation (the maximum amount of oxygen that can be dissolved in a volume of water) varies with the chemical composition of the wastewater. Generally the oxygen saturation possible in wastewater is about 95% of that possible in clean water. Oxygen saturation also decreases with decreasing atmospheric pressure. This is of some significance for plants located at high altitude.

Oxygen Transfer Efficiency Two parameters are used to describe the efficiency of oxygen transfer, the Standard Oxygen Transfer Efficiency (SOTE) and the Standard Aeration Efficiency (SAE).

•• SOTE is used for diffused air systems to describe the percentage of oxygen which dissolves in clean water under standard conditions. SOTE depends on water depth, air flow rate per diffuser pore (which governs bubble size) and diffuser layout (which governs bubble travel time to the surface).

Typical values are 5% - 8% per metre depth for fine bubble systems and 2% - 3% per metre depth for coarse bubble systems. Under field conditions this is simply called Oxygen Transfer Efficiency (OTE). OTE is greater at higher sludge ages as oxygen transfer inhibitors such as surfactants are more completely degraded.

•• SAE is used for both mechanical surface aeration and diffused air to describe the energy efficiency (kg O2/kWh) at standard conditions. That is how much energy is necessary to dissolve a certain amount of oxygen. Standard conditions refer to clean water at a temperature of 20oC and atmospheric pressure of 101.3 kPa with 0 mg/L DO. A comparison of aeration efficiencies is provided in Table 8.2.

Table 8.2.

Comparison of standard aeration efficiencies for common aeration systems (Water Environment Federation).

Aerator Type

88

Standard Aeration Efficiency (kg O2/kWh)

High speed surface aerator

0.9 - 1.3

Low speed surface aerator

1.5 - 2.1

Coarse bubble

0.6 - 1.5

Turbine or jets (fine bubble)

1.2 - 1.8

Fine bubble or fine pore

3.6 - 4.8

Chapter 8 - Aeration The degree of diffuser fouling and the effect on aeration cost can be tracked by monitoring blower specific energy consumption (kWh/m3 of air), the overall air requirement (m3 air / kgCOD treated) or the standard aeration efficiency (kg O2/kWh). The diffuser back pressure can also be useful as described above.

Mixing Bioreactors generally consist of a series of so-called complete mix reactors, each being fully mixed so that the reactor contents are of uniform quality throughout, and the quality of the contents and the quality of the effluent leaving the reactor are identical.

Mixing can be provided by aerators or alternatively, purpose designed mixers that are designed to mix efficiently while not aerating the liquid. The mixing intensity maintains the solids in suspension thereby ensuring the mixed volume is fully active and the biomass is uniformly distributed. However mixing also has a shearing effect on the flocculated biomass which can be detrimental to flocculation and subsequent settling.

Carbon Dioxide Stripping and pH One other function of aerators that is often not considered is their role in carbon dioxide stripping. Carbon dioxide is produced as a result of the oxidation of COD. The amount of CO2 can have an impact on the mixed liquor pH. The higher the CO2, the lower the pH. In industrial waste plants with high COD, the potential impact of the produced CO2 on pH can be considerable.

Carbon dioxide is stripped by the air bubbles. However, the stripping rate and therefore the mixed liquor pH varies with the air flow rate which in turn is governed by the oxygen transfer efficiency. As the oxygen transfer efficiency increases, the air flow rate is reduced and as a result, less stripping occurs and the pH falls due to the higher CO2 levels.

Mixed liquor leaving the reactor is still supersaturated with carbon dioxide and the pH rises through the secondary clarifier stage due to further CO2 stripping at the surface and as the effluent passes over weirs.

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Chapter 9 Prefermentation Enhanced Biological Phosphorus Removal (EBPR) depends on the availability of Volatile Fatty Acids (VFA) and other Readily Biodegradable COD (RBCOD) which can be used by the Phosphorus Accumulating Organisms (PAOs) in the anaerobic zone. However, the amount of RBCOD in the sewage can be too low to support EBPR when there is little prefermentation activity in the sewers due to short travel times or low temperatures. Where there is insufficient RBCOD for EBPR removal, carbon can be added using chemicals such as acetic acid, methanol, sucrose or molasses. Alternatively, precipitants such as alum or iron salts can be dosed for chemical P removal.

Another approach used in some plants is to preferment the influent to increase the RBCOD level ahead of the anaerobic reactor.

The basic principle of fermentation is to take a volume of raw sewage and hold it under anaerobic conditions that favour fermentation. The breakdown of COD occurs in 2 steps. The first, carried out by acidogenic bacteria, breaks down the large organic molecules in raw sewage to produce VFA such as acetic, propionic and butyric acid. The second step, carried out by methanogenic bacteria, takes the VFA produced in the first step and converts it to methane. Prefermenters have to be operated at a short Solids Retention Time (SRT) to avoid conversion of VFA to methane.

Activated Primary Tanks and Sidestream Fermenters There are two common process formats used for prefermentation (Figure 9.1): 1. A Primary Sedimentation Tank (PST) with the primary sludge directed to a sidestream fermenter with solids/liquids separation.

2. An Activated Primary Tank (APT) which ferments the primary sludge internally.

The side-stream fermenter is basically a complete mix reactor in which the SRT can be controlled by the sludge feed rate. The APT is a PST in which the primary sludge is recirculated to the inlet and fermented internally as shown in Figure 9.2. The SRT is controlled by periodic discharge of sludge from the recirculation line to the APT effluent. The operation is essentially similar to the operation of an activated sludge plant with RAS and WAS. (a)

(b)

Figure 9.1. Alternative prefermentation formats, (a) Primary Sedimentation Tank (PST) and sidestream Fermenter (F), and (b) inline Activated Primary Tank (APT).

In both systems, odour control is necessary because of sulfide and VFA generation.

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Chapter 8 - Aeration

Figure 9.2.

Diagram of an APT.

Figure 9.3 shows the APT at the Thorneside STP in Queensland. The tank has a diameter of 23 m, a side water depth of 4 m and is operated with a recycle ratio of 0.5.

Figure 9.3.

30,000 PE APT at the Thorneside STP in Queensland.

In addition to providing RBCOD for P removal, prefermentation can provide an additional benefit of improving sludge settleability in the BNR process (Figure 9.4).

The figure shows that the BNR Stirred Sludge Volume Index (sSVI) increased when the APT was taken off-line for maintenance (3-Jul-01 to 3-Mar-03) and decreased again when it was returned to service.

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Chapter 9 - Prefermentation

Figure 9.4.

Effect of prefermentation in an in-line APT on sSVI in the Coolum biological nutrient removal plant.

Process Principles Activated Primary Tank (APT) The APT consists of a covered primary clarifier fitted with a high-capacity scraper and operated with a high sludge blanket (Figure 9.2). There may also be a scum removal system and/or scum sprays.

Sludge is recycled continuously to the tank inlet. A typical recycle ratio is 0.5:1. Solids in the incoming raw sewage mix with recycled sludge, settle and travel through the sludge blanket to the underflow. The blanket solids inventory is maintained constant by wasting of excess solids. Wasting occurs naturally in the APT effluent and by periodic deliberate wasting from the underflow (Figure 9.2). Wasting occurs most commonly to the APT effluent. The wasted solids then pass to the bioreactor along with the VFA that have been produced.

The sludge blanket behaves as an acid-phase anaerobic digester. Acidogenic bacteria (in both the sludge blanket and the settling zone above) degrade Volatile Suspended Solids (VSS), producing approximately equal quantities of volatile acids and other forms of soluble COD. The solids retention time in the APT sludge blanket must be held low enough to prevent the growth of methane-forming bacteria which would convert the generated RBCOD to methane.

Sulfate reducing bacteria also generate sulfide in the sludge blanket. This activity reduces VFA formation, generates odour and increases corrosion. Therefore, the solids retention time must also be held low enough to limit the formation of sulfide.

The continuous recycle of sludge circulates sewage through the sludge blanket, maximising RBCOD generation potential and washing the RBCOD into the APT effluent.

The goal in operating an APT is to maximise the concentration of VFA and total RBCOD fed to the anaerobic reactor while limiting sulfide and methane generation.

92

Chapter 9 - Prefermentation Optimisation The aim of the initial optimisation and any subsequent checks of APT operation is to establish the optimum combination of recycle rate, solids inventory and Solids Retention Time (SRT) that maximises VFA in the APT effluent while limiting sulfide to an acceptable level (based on odour and corrosion considerations) and minimising production of methane.

•• As the SRT increases: o VFA production increases. However, beyond a critical SRT, VFAs are converted to methane.

For prevention of methane formation, the critical SRT is about 10 days at a sewage temperature of 25oC, increasing to about 40 days at 15oC. o Sulfide production increases, inhibiting bio-P removal (Figure 9.5). The SRT needs to be controlled to limit effluent sulfide to an acceptable value of about 5 mg/L. An SRT of 3 - 4 days (sludge wasting rate about 1% of sewage flow) generally results in acceptable sulfide levels. Corresponding VFA generation is around 25% of sewage total COD.

Figure 9.5.

Effect of sulfide concentration in the anaerobic reactor on the amount of P removed in the biological process in two STPs.

•• High recycle rates minimise VFA concentration in the sludge blanket, reducing methane-former growth rates and potentially increasing the optimum SRT. High recycle rates also maximise the opportunity for fermentation of recycled soluble COD (in addition to the settled particulate COD).

•• Excessive solids inventory will reduce the settling efficiency of the tank and thereby reduce the quantity of settled solids available for fermentation.

The basic procedure for optimising an APT is: 1. Operate the scraper continuously.

2. Run the ventilation fan and odour scrubber continuously.

3. Keep all access hatches closed. H2S and possibly CH4 in the APT headspace and ventilation air are safety hazards.

4. Select the highest recycle rate. A rate of 0.5:1 is a suitable starting point.

5. Set the daily wasting rate to maintain a chosen SRT. As described above at a temperature of 25oC this is likely to be < 10 days, while at 15oC it can be as high as 40 days.

93

6. In practice, the true SRT is difficult to estimate because the percentage of recycled solids in the APT effluent is unknown. The solids inventory can be monitored by periodically stopping the recycle and allowing the solids to settle. The depth of the settled solids is then measured, and the settled solids concentration measured by sampling the sludge recycle when it is restarted. Calculate the SRT. Monitor the VFA and sulfide concentrations in the APT effluent.

7. Repeat for other recycle rates and wasting combinations.

8. The optimum operating condition is the combination giving the maximum VFA generation and acceptable sulfide level.

Side-stream Fermenters Side-stream fermenters differ from APTs in that there is no recycle and only a portion of the inflow to the plant is directed to the fermenter. Typically 2% - 5% of the total flow is fermented. Continuous pumping from the primary clarifiers limits development of anaerobic conditions in the supernatant zone of the fermenter.

Optimisation Optimisation of a side-stream fermenter is essentially the same as for an APT without the variable of the recycle rate. Feed rate and wasting rate can be varied and the optimum settings for VFA production and minimal sulfide and methane production determined.

Additional Operational Checks Depending on the set-up and the instrumentation and trending available, some common checks for the operation of a prefermentation system are listed below.

APT and Side-stream Fermenters •• Confirm the fermenter level is at the designed set point.

•• Confirm fermenter supernatant flows to the bioreactor(s) are within range. If they are too low, the line will need flushing and fermenter scum may need clearing.

•• Confirm duration/interval times are set correctly for the fermenter sludge pump.

•• Confirm that the sludge pump has been operating regularly as per the duration/interval timers and that the flow rate is within range.

•• Check the “smoothness” of the fermenter level trend. If it starts becoming more “jagged” than usual, it may be a sign that too much scum/rising sludge is forming in the fermenter.

•• Confirm that the fermenter scraper is running and is operating correctly.

•• Look at the surface of the fermenter. If bubbles or excess scum are apparent, the desludging should be increased.

•• Flush the supernatant lines and launder with water when flows fall below normal levels.

•• Check the sludge blanket level (make sure the scraper is turned off while this is done).

Note that septic sewage entering the facility will accelerate gas formation in the fermenter and necessitate a very short sludge age. Conversely, following wet weather events when the sewage tends to be less septic, fermenter sludge age may need to be increased to encourage more VFA production in the fermenter.

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Chapter 9 - Prefermentation One of the most common problems with operation of the fermenter system is the fouling of the system with a build-up of gross solids. Therefore the feed sludge to the fermenter should be screened or macerated to minimise particle size to eliminate fouling.

Side-stream Fermenters •• Confirm that the primary feed pump is operating and fermenter feed rate is at the designed set point.

•• By selecting a primary sludge flow rate slightly greater than the fermenter supernatant and desludging flow rate combined, the fermenter will continuously overflow a small portion into its scum trough, which gravitates back to the primary sludge pump station. This recycle helps keep the fermenter surface free of excessive scum which may otherwise block the supernatant pipe launder system.

•• Solids should be screened prior to feeding the anaerobic zone.

Monitoring A process monitoring schedule for the tuning phase of an APT is set out in Table 9.1. A reduced frequency can be implemented for normal operation.

Table 9.1.

APT monitoring schedule during the tuning phase.

Parameter

Location

Sample Frequency Type

Daily Flow

Sewage

Continuous

Meter



Weekly

CompA

Daily

CompM

Weekly

CompA

Continuous

Meter

Weekly

Grab

Waste sludge

Daily

Meter

Settled blanket

3/week

---

Blanket Depth

1/week

Grab

TS

APT overflow Sludge recycle

Depth/ TS2

COD Total 

COD VFA1 Soluble  

SS

VSS

pH

























 



CompA: automatic 24h composite.

CompM: composite of several manual grabs.

1 A simple titration method can be used to measure VFA concentration. The method and calculation spreadsheet (Mod 3 - VFA Analysis) is available on the WIOA website.

2 Blanket Total Solids (TS) can be measured by sampling the sludge recycle upon startup after settling.

RAS Fermentation Another possibility to enrich the influent to the anaerobic zone with RBCOD that is used in some Scandinavian plants is to ferment the RAS. Fermentation of raw sewage produces significantly more RBCOD than fermenting RAS however, depending on the plant, it may be an option. There are several plants in Australia with RAS fermentation included.

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Chapter 10 Intermittent Systems The term intermittent relates to the fact that the mixed liquor in the reactors is first aerated and then allowed to settle prior to the treated effluent being decanted off the top. Therefore aeration, settling and decanting occur intermittently. This is in contrast to the continuous process where mixed liquor moves continually from one tank into the next and finally to a clarifier, where settling and decanting is continuous. Hence we get the common names for the two basic types of activated sludge plants, “Continuous” and “Intermittent”.

One of the advantages of the intermittent system is that the processes of aeration and solids separation occur in one tank. This represents a significant saving in construction costs and a smaller footprint (Figure 10.1).

Many intermittent plants have no Return Activated Sludge (RAS), so there is a further saving on pumps, and pipework. In later designs, RAS is often included where an anaerobic zone is provided upstream of the main tank for Enhanced Biological Phosphorus Removal (EBPR) or where anoxic or aerobic “selectors” are provided for the control of filamentous bacteria.

Figure 10.1. Aerial photographs of a continuous (left) and intermittent plant (right). The greater complexity and footprint of the continuous plant is apparent.

There are two main types of intermittent plants, Sequencing Batch Reactor (SBR) and Intermittently Decanted Extended Aeration (IDEA) (including Intermittently Decanted Aerated Lagoon (IDAL)) plants. The main difference is that in an SBR, flow to the tank ceases during treatment, whereas in the IDEA (and IDAL) processes, influent flow to the tank continues while treatment occurs.

Basic Features of SBR and IDEA Plants Treatment in an SBR is a batch process. A batch of wastewater is taken in, treated and then the treated effluent discharged and the cycle starts again. Once the SBR is full at Top Water Level (TWL), inflow is directed to another reactor and there is no additional input of raw sewage during the treatment stages (Figure 10.2). As such an SBR plant cannot have only one tank. At the very least there needs to be a holding tank to store the inflow while treatment of the batch is completed. Depending on the inflow to the plant, the fill cycle will vary with the flow. A higher flow results in a shorter fill cycle.

In contrast, in the IDEA process, inflow continues to the reactors while treatment is occurring (Figure 10.2).

Therefore theoretically an IDEA plant could be based on a single tank.

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Figure 10.2. Comparison of the essential differences between SBR and IDEA systems.

The IDEA tank is typically designed with a length to width ratio of at least 4:1 to better establish plug flow. This is in part to try to control filamentous bacteria by maintaining a high F:M ratio at the start of the tank where the influent enters. The high F:M ratio favours floc formers over filamentous forms that survive and proliferate in low F:M conditions.

As the oxygen demand will be higher at the start of the tank, aeration needs to be concentrated at the head of the tank. As a rough guide, 50% of the air is required in the first 25% of the tank. There are modifications to the IDEA system where the inflow enters across the whole length of the tank, thereby reducing the need for increased aeration at the start of the tank. This modification is described in more detail below.

The IDEA process was developed in Australia by the NSW Public Works Department. The first plant was built in 1965 as a Pasveer Channel at Adelong NSW. The first of the more common rectangular concrete tanks was built at Bathurst in 1976 (Figure 10.3).

There are now well over 100 of these plants operating in Australia.

Figure 10.3. The original “Bathurst Box”.

Intermittent plants are typically sized to manage wet weather flows. In Victoria, the design peaking factor is typically around 4 times the Average Dry Weather Flow (ADWF), however in Queensland, with heavier rainfall, the peaking factor can be around 6 to 8 times ADWF. In older sewerage systems that are more prone to infiltration, plants are often designed with higher peaking factors.

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Chapter 10 - Intermittent Systems The control system of intermittent plants can be modified to include a trigger time and trigger level for the initiation of a storm cycle that allows high volumes of partly treated diluted wastewater to pass through the plant in a short time. When inflow is such that the TWL will be exceeded at some point prior to the end of the normal cycle, a storm cycle is triggered. This results in a reduced treatment regime that allows more water through the plant. The trigger point needs to be before TWL is reached (Figure 10.4), to allow reduced treatment and at least 30 minutes settling time. Such control systems can be retrofitted fairly easily to a plant.

Figure 10.4. Diagram showing how a storm cycle would be triggered during high flows that would result in the TWL being reached before the programmed decant time.

The left hand panel of Figure 10.4 shows that at the trigger time with “normal” flows, the level is less than the trigger level so there is no need to trigger a storm cycle. The right hand panel shows that at trigger time with higher flows, the water level is greater than the trigger level and a storm cycle needs to be triggered.

The dashed line in the right hand panel shows where the TWL would be reached. Clearly the treatment cycle needs to be changed to allow for this. More details on the actual storm cycle changes are explained later in the Chapter.

Continuous plants don’t have a similar option of using a storm cycle. By turning off aerators and/or mixers, and settling the biomass, excess flow can be allowed to pass through the plant and not wash out too much biomass. The effluent though untreated, is mostly very dilute sewage.

IDALs are a modification to lagoon systems that mimic to a certain extent an IDEA plant. A lagoon is fitted with a decant mechanism, and aerators and a control system included to control the cycle and DO set point.

To some extent, the IDAL terminology just highlights a different method of construction. Sometimes the plants are constructed with vertical sided concrete boxes while in others, concrete lined earthen embankments are constructed. These latter structures are often called IDAL.

The Intermittent Cycle The operation of SBRs and IDEAs is similar enough for them to be considered together.

The principles of operation share some common points with continuous processes, for example DO set points, sludge age and Sludge Volume Index (SVI). However the main variation is that the stages of treatment occur at different times in the same reactor in an intermittent plant, rather than in separate compartments of a multi-chambered reactor in a continuous plant.

The required treatment is achieved by setting the times for the different stages in the intermittent cycle.

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Chapter 10 - Intermittent Systems There are five stages to the treatment in the basic SBR. Note that filling only occurs in the first stage.

1. Fill: Raw sewage is added to the reactor tank. This raises the level from the Bottom Water Level (BWL) after the previous decant to the TWL. Aeration may occur for all or part of this stage.

2. React: The contents are aerated.

3. Settle: The aerators and mixers are turned off and the contents allowed to settle.

4. Decant: The clarified effluent is decanted off the top of the reactor tank.

5. Idle: This is a brief period that allows the system to return to the starting point.

In more sophisticated modifications for nutrient removal there may be additional steps in the React stage.

This is covered in more detail below.

The stages in the IDEA process are essentially similar to those for the SBR, however raw sewage flows to the tanks during all stages.

1. Aerate.

2. Settle.

3. Decant.

4. Idle.

Typical cycle time is 4 hours (sometimes 6 hours) for non-storm flows. In storm flows, the cycle time might be reduced to 3 hours.

For a 3 cell IDEA plant with a typical 4 hour cycle consisting of 2 hours aeration, 1 hour settling and 1 hour decanting, the stages in the different tanks are shown in Figure 10.5. Note that decanting occurs from only one cell for 3 out of the 4 hours, that is, the flow of treated effluent is also intermittent. This can have implications for downstream processes such as UV disinfection where the lamps could be shut off 6 times a day. In reality this switching may impact on the life of the lamps such that it is better just to leave them on and protect them from overheating.

Figure 10.5. Diagram showing how the stages would fit together in a 3 cell intermittent plant. The red arrows show where the decant occurs for each cell and that decanting occurs for 3 out of every four hours.

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Chapter 10 - Intermittent Systems Addition of a fourth cell would allow flow of treated effluent to be more or less continuous, except for a few minutes at the start and end of decant periods.

Differences Between Intermittent and Continuous Plants Table 10.1.

Comparison of the major features of continuous and intermittent plants.

Aspect

Continuous

Intermittent

Number of reactor tanks Minimum two (aeration + settling)

One

Need for upstream balancing tank Need for downstream balancing tank

None

Pipework

Required to allow transfers between tanks.

Larger footprint.

Necessary if only single cell SBR. None if IDEA. Yes if downstream facilities such as disinfection are not sized for peak outflow.

Simpler. Less pipework required.

Area requirements (see Figure 10.1) Hydraulic Retention Time (HRT) Change of operational mode

Management of peak flows

None

Varies as flow varies. Difficult to adjust.

Smaller footprint.

HRT is calculated at BWL. Therefore long term and seasonal variations in flow can be catered for by adjusting the BWL. Can vary HRT, DO and sludge age.

Can vary DO and sludge age.

Difficult to vary anoxic and aerobic Anoxic and aerobic fractions can be fractions. easily varied by changing treatment cycle times.

Increased flow reduces HRT Can adjust the length of the sequence to unless upstream peak flow provide some treatment without bypass. balancing is provided.

Types of Decant Systems The function of the decant system is to remove the treated effluent from the top of the tank, or at least from above the sludge. There are a number of different systems in use: •• Vertically travelling overflow weir.

•• Rotating overflow weir.

•• Floating decant.

•• Gas locked siphon decant.

The vertically travelling overflow weir is typically made up of a horizontal trough, with one or both sides used as the overflow weir. The trough is attached to a vertical pipe, which slides up and down inside a larger pipe, and uses a flexible (“rubber”) boot to prevent the settled sludge or mixed liquor entering through the gap between the two pipes (Figure 10.6).

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Decant pipe Flexible boot

Scum baffle

Figure 10.6. A vertically travelling overflow weir decant system showing the decant pipes, flexible boot and scum baffle. The actual overflow weir is hidden behind the scum baffle.

The most common drive arrangement is shown in Figure 10.7 with the drive motor, the vertical shafts and the horizontal connecting shaft.

Drive motor Vertical Shafts

Figure 10.7. A vertically travelling overflow weir decant system with drive motor (actuator) and vertical shafts visible.

In addition, it is common for such weirs to be fitted with a baffle to prevent scum on the surface entering the trough and contaminating the effluent. These are typically floating baffles, attached to a hinge in such a way that, as the weir lowers into the liquor it pushes the scum away (Figure 10.8.).

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Figure 10.8. A vertically travelling overflow weir decant system showing the scum baffle (white arrow), clear effluent (star) next to the overflow weir and ballast blocks (red arrow).

The overflow weir trough shown in Figure 10.8 has the overflow weir on one side only. While this is the more common design, it is possible to get overflow troughs which have weirs on both sides. Such arrangements may be used where the weir is located in the centre of the IDEA cell, or where higher than usual decant rates are required.

Also visible in Figure 10.8 are ballast blocks which are intended to reduce the buoyancy of the trough, and thus reduce the load on the driving motors.

With the vertical travelling overflow weir, the trough is lowered to the water level and once the top of the weir reaches water level overflow commences (Figure 10.8). The trough is then lowered at a suitable rate to allow the trough to reach BWL at the end of the decant stage.

Rotating overflow weirs typically have a pipe manifold system with a cross pipe with openings or a horizontal trough (Figure 10.9) that allows clarified effluent to flow. As the drive motor moves, a shaft connected to the trough or pipe, lowers the pipe or trough into the liquor.

Figure 10.9. Diagram of a rotating overflow weir.

(From www.iwawaterwiki.org/xwiki/bin/view/Articles/Sequencing Batch Reactor) With vertically travelling overflow weir systems and rotating overflow weirs, it is common for the weir to be moved to the TWL position and “parked” there after the decant. In the parked position, weirs act as an emergency overflow. Just prior to the start of the decant period, the weir is usually lowered rapidly to the current water level so that decanting can start as soon as the PLC triggers the start of the decant period.

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Chapter 10 - Intermittent Systems Floating decant weir systems (Figure 10.10) typically consist of a perforated pipe floating at a slight distance below the surface with an actuated valve used to control when decant occurs.

Figure 10.10. An example of a floating decant system.

The initial decant from such systems is likely to be contaminated with mixed liquor that can enter the decant pipework during the aeration cycle and then settles out. Therefore the initial decanted volume may not comply with licence requirements for the final effluent from the site.

The extent of solids carryover can be minimised by: •• Selecting an aeration type and layout in relation to the decanter position to minimise solids uplift directly into the perforated decant pipework.

•• Returning the initial decanted volume back to the head of plant. The effectiveness of this first flush depends on whether scouring velocities are reached at the start of each decant.

•• Including a mechanism to ensure the decanter is removed from the reactor during the aeration cycle.

Floating decant systems are more common in small industrial systems that provide pre-treatment of industrial waste, however they are occasionally found on larger plants treating domestic wastewater.

An alternative decant system is the gas locked siphon decanter (Figure 10.11). This typically comprises a manifolded set of horizontal pipes with offtakes running vertically to a position between BWL and the top of the settled sludge.

Figure 10.11. Diagram of a gas locked siphon at the point where the valve is just opened allowing fluid to displace the trapped air and start discharging treated effluent.

(Originally sourced from www.cdstech.com) 103

Chapter 10 - Intermittent Systems The manifolded system is connected to a siphon with a solenoid valve. To commence decant, the solenoid valve is opened and clarified effluent flows due to the hydrostatic head and creates a siphon. Once the siphon is made, the solenoid valve is closed and decanting continues until BWL is reached at which point the siphon is broken by the opening of the solenoid valve.

Figure 10.12 shows the pipes of a gas locked siphon decant located above the water surface. Many gas locked siphon decant systems are located below the water surface.

Figure 10.12. A gas locked siphon decant system.

As these systems normally draw from below the water surface, scum baffles are generally not provided.

Problems may be encountered with entry of some mixed liquor into the decanter system during aeration.

Care needs to be taken with selection and replacement of decanters, particularly the length to width ratio.

Short or small decant weirs can result in high approach velocities and solids carry over in the decant.

Modifications for Nutrient Removal The normal stages of aeration and non-aeration (settle and decant) provide aerobic and anoxic conditions that are necessary for nitrogen removal. Once the aeration is stopped, the DO will be used up quickly and anoxic conditions established. Therefore in its simplest form, the intermittent process resembles the Modified Ludzak Ettinger (MLE) process of continuous plants (see Chapter 6). However, denitrification is limited because the settling sludge removes the bacteria away from the nitrified liquor. This is undesirable as the solids need to be in contact with the nitrified liquor to bring about the removal of nitrate. Figure 10.13 shows how nitrate is only really removed in the deeper layers within the sludge blanket. Therefore the amount of denitrification in a conventional IDEA or SBR will not be as good as in a continuous plant. However by introducing a mixing step, denitrification equivalent to that seen in a continuous plant can be achieved.

Even better nitrogen removal can sometimes be achieved by introducing two, or even three, aeration and denitrification periods at the start of the IDEA or SBR cycle.

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Figure 10.13. Nitrate levels at various depths at the midpoint of an IDEA cell.

Denitrification By adding a mixing stage at the completion of the aeration stage, the biomass can be maintained in contact with the nitrified liquor. This results in a marked improvement in the amount of denitrification. Figures 10.14 and 10.15 show nitrate levels in the decant of an IDEA plant before and after the introduction of a mixing stage.

Figure 10.14. Effluent nitrate levels in the decant prior to introduction of a denitrification cycle.

Figure 10.15. Effluent nitrate levels in the decant after the introduction of a denitrification cycle. The values shown in this figure should be compared with those in Figure 10.14.

Mixing for the denitrification cycle can be achieved using dedicated mixers, however by simply running the aerators intermittently or by introducing short pulses of air in diffused air systems, the mixed liquor is kept mixed and nitrate is removed efficiently.

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Chapter 10 - Intermittent Systems The duration and frequency of mixing required to keep the solids suspended without too much addition of oxygen depends to a large extent on the settleability of the sludge and must be established by direct observation. The aim is to keep the sludge within a few centimetres of the surface during the denitrification cycle. In diffused air systems, pulses of approximately 10 seconds every 10 minutes are usually sufficient.

Similarly, surface aerator operation of approximately 1 minute every 10 to 15 minutes is usually sufficient.

The time can simply be adjusted by observing whether the mixed liquor stays adequately mixed. It may also be useful to probe the bottom of the reactor with a long PVC conduit to see whether any sludge has accumulated during the anoxic period. If sludge is felt with the conduit, the mixing can be increased slightly and the test repeated until little or no settling occurs.

In summary, to achieve improved denitrification, the basic SBR or IDEA sequence is modified to include 2 React stages (Figure 10.16).

1. React 1, Aerate (COD removal and Nitrification).

2. React 2, Mix (Denitrification).

Figure 10.16. Modified SBR sequence to include both React 1 Aerate (aerobic phase) and React 2 Mix (anoxic phase). The same modifications can be applied to an IDEA plant by providing pulses of air equivalent to the React 2 Mix phase.

The addition of a mixed anoxic phase actually works best in the IDEA process because raw sewage continues to be fed to the reactor thereby providing the COD necessary to support denitrification. In the case of a conventional SBR, there is no additional feed of raw sewage to provide the carbon source, however methanol or some other readily biodegradable COD source (ethanol, acetic acid, sucrose, molasses) can be fed to the reactor after the completion of the aeration stage to allow denitrification. Even without supplementary carbon dosing, nitrogen removal can be maximised by carefully monitoring the aeration stage and stopping aeration as soon as the ammonia level is low enough. This would typically be 1 or 2 mgN/L depending on the discharge licence. By stopping aeration as soon as the ammonia reaches a target level, some of the COD may be saved to assist denitrification. Another alternative is to include a React Mix stage at the start of the cycle for denitrification of nitrate left over from the previous cycle using the fresh COD delivered to the system in the influent.

It is important to note that in IDEA (but not SBR) plants, inflow during settling and decanting allows some ammonia to “bleed” through to the decant even though the reactors are designed with a high aspect (length: width) ratio. The aim of this design is to try to limit complete mixing along the length of the reactor and to try to set up plug flow where the ammonia coming in with the influent won’t reach the decant during the decant period. However some will, and because of this, an IDEA plant is unlikely to achieve the same low levels of ammonia seen in SBR or continuous plants. This ammonia bleed does not represent poor nitrification but rather, just one of the features of the design of the IDEA process.

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Chapter 10 - Intermittent Systems Simultaneous Nitrification Denitrification (SND) Simultaneous Nitrification Denitrification (SND) can be achieved by carefully setting the DO set point. If the DO level is just right, the outside of a floc is aerobic while deeper inside where the oxygen doesn’t reach is anoxic (Figure 10.17). One possibility is that the nitrate produced in the outer layers can then be denitrified in the inner deeper layer. Research has shown that in fact nitrate may not be formed at all and only nitrite or even nitrous oxide (N2O) formed as an intermediate between ammonia and nitrogen gas (see Figure 4.2 in Chapter 4). So in this case, full nitrogen removal occurs without the formation of nitrate. Nitrous oxide is of particular interest as it is a potent greenhouse gas and has been measured at significant levels in some plants.

Figure 10.17. When the DO level is set so that the DO moving into the floc is not enough to reach the middle, aerobic and anoxic zones can occur together in the floc.

There is no magic DO setting for SND. By carefully monitoring ammonia and nitrate and possibly nitrite levels, the DO set point can be adjusted to achieve the necessary nitrogen removal. As a starting point, DOs between 1.0 mg/L and 1.5 mg/L should be tested. It would be best to do the testing when the peak loads are experienced by the plant, in particular the peak ammonia loads.

Phosphorus Removal Enhanced Biological Phosphorus Removal (EBPR) does not usually occur in conventional intermittent plants because the basic treatment sequence does not include a true anaerobic stage necessary for growth of Phosphate Accumulating Organisms (PAOs).

EBPR can be achieved to some extent by further manipulation of the basic cycle to include a truly anaerobic stage. This can be done by extending the anoxic stage until all the nitrate has been converted to nitrogen gas. Alternatively, some IDEA plants have been retrofitted with a dedicated anaerobic zone at the front.

In some plants these have been called selectors and possibly serve the dual purpose of P release, and inhibition of filamentous bacteria. Larger anaerobic (and anoxic) reactors have also been built. The Idle stage after the decant stage that is often included (but sometimes not used in a cycle) can also serve to create an anaerobic stage. As the biomass settles it uses any oxygen and then converts nitrate to nitrogen and finally when the nitrate is used up, the biomass becomes truly anaerobic and P release may occur if sufficient COD (in particular RBCOD) is available. P release requires RBCOD and in the IDEA process this is provided by the incoming sewage that continues to flow to the reactor at all stages of the cycle. This is not the case in a normal unmodified SBR.

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Chapter 10 - Intermittent Systems In SBRs, a Non Mixed Fill stage (Figure 10.16) can be introduced where raw sewage is fed to the settled biomass. The settled biomass quickly becomes anoxic and then anaerobic. Thus conditions in the sludge layer are essentially those in an anaerobic zone in a continuous plant. The high RBCOD combined with anaerobic conditions favours P release.

The way the raw sewage enters the reactor is important if nutrient removal is required. If the influent is fed to the top of the reactor particularly during the aeration stage, any RBCOD in the influent will be diluted and quickly removed by bacteria aerobically. As a result it will not be available for EBPR or even optimum denitrification. For best nutrient removal, the influent should be fed to the bottom of the reactor and if possible at multiple points below the sludge blanket.

Many changes to the design of IDEA and SBR plants have been made and are likely to continue to be made.

The University of Queensland developed the UNIFED system where a manifold of influent pipe work was fitted to the bottom of the reactor (Figure 10.18).

Raw influent pipe

Feed pipes

Figure 10.18. An empty IDEA cell showing the UNIFED influent feed manifold system.

One of the advantages of the UNIFED system installed at the Moe STP in Victoria is that the manifolds distributing the screened sewage into the base of the treatment cell and into the sludge blanket have eliminated the ammonia short circuiting, even during high and storm flow conditions.

There are also hybrid designs for example where a Johannesburg process is overlaid on an IDEA process.

Consequences of Operating Intermittent Plants for Nutrient Removal Chapter 3 described the problems of foaming and bulking that can occur in low F:M nutrient removal plants.

Foaming and bulking can occur in both continuous and intermittent plants and the same general management practices apply.

In an SBR, if raw sewage is fed into a non mixed Fill stage, the fresh raw sewage will come in contact with the settled biomass. The high F:M ratio established in the sludge blanket may tend to favour floc formers over filamentous forms. Another alternative that applies to both intermittent and continuous plants is to include a selector (Figure 10.19) where RAS can be mixed with incoming raw sewage to set up the same high F:M conditions. Such selectors have been used with mixed success in controlling bulking and foaming.

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3 2 1

Figure 10.19. Covered selectors at the inlet to a 3 cell IDEA plant. There is one selector for each cell.

In an intermittent plant, if the standard “long sludge age” extended aeration cycle is modified to a short sludge age nutrient removal regime, the main change that is likely is that the SVI would increase. This shouldn’t be a problem in a well-designed IDEA system with an appropriately high (4:1) aspect ratio. However if it does become an issue in either an SBR or IDEA plant, the operating cycles may need to be adapted perhaps by providing a longer settling cycle or an increased DO set point during aeration.

Operation of Intermittent Plants There are certain operational features unique to intermittent plants. The main ones are listed below: •• The depth in the reactor will vary regularly with time.

•• The outflow from an IDEA reactor during decant will be higher than the inflow to the reactor over the same period of time. For example, for the typical 4 hour cycle, the inflow for 4 hours needs to be removed in one hour.

•• The biological processes occurring in a reactor will vary with time.

•• The settled sludge will undergo biological processes during settling and decant.

Each of these will be considered in more detail below.

Depth Variance with Time The variation of depth means that the mixed liquor concentration and results of settleability tests will vary during the cycle. As the depth increases, the mixed liquor solids are effectively diluted by the influent entering the reactor. The amount of dilution depends on both the change in depth and the influent SS. Therefore the only way to be able to measure the MLSS and settleability with acceptable repeatability to allow control of the process is to sample the mixed liquor at a fixed point in the cycle. In the case of IDEA plants, that is the BWL, the one depth that the process should return to during every cycle. In contrast, the TWL will vary for each cycle depending on how much inflow occurred during the cycle. In contrast, in SBR plants that always fill to the TWL, either the TWL or BWL can be used.

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Chapter 10 - Intermittent Systems Ideally, the operator of an IDEA plant will need to know the times when BWL is reached each day, and arrange to be at the plant at that time in order to take samples and perform any necessary testing. Another alternative is to note the level in the reactor cell at the time of sampling, measure the MLSS and then adjust it back to BWL using the following calculation.

MLSS (standardised) = MLSS (measured) x Level at time of sampling

BWL

Note that the SVI result still uses the measured MLSS not the standardised MLSS as shown below.

SVI = Settled volume x Dilution factor x 1,000 MLSS (measured) The variation in depth with the cycle does not affect the measurement of DO or pH. The rate of change of depth can also be used to check that the flow to the IDEA cells is equal. If the distribution is relatively equal, the rate of increase of level shown on the SCADA screens should be the same, i.e. the slopes should be the same. If not, there is likely to be something wrong with the flow split mechanism.

Increased Outflow from each Tank There are two other important features of IDEA plants that relate to the outflow from each tank.

1. Because inflow to a plant varies during the day, the amount that enters and therefore needs to be removed during a decant varies.

2. The rate of outflow is significantly greater than the rate of inflow. For an IDEA plant with a typical cycle of 2 hours aeration, 1 settling and 1 hour decant, outflow during the 1 hour must equal the total inflow to the cell during the 4 hour cycle. The following example shows this.

Example If the plant has 4 cells and the flow to the plant averages 12 L/sec this means that: •• Each cell receives 12/4 = 3 L/s = 10,800 L/h.

•• Over 4 hours of the cycle, each cell will receive 4 x 10,800 = 43,200 L. •• Therefore to reach the BWL at the end of the decant, the decant flow from each cell must be equal 43,200 L /h = 12 L/s.

For each cell the outflow is 4 times the average inflow.

The increased outflow requires downstream facilities such as polishing filters or UV disinfection to be sized for the higher flow unless a balancing tank is provided downstream of the intermittent process.

It is also very important to understand that changes in cycle times can have a big impact on the decant flow and therefore the necessary size of downstream processing units.

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Example Consider the example above and a situation where additional aeration or an extended denitrification cycle is required and the decant is reduced to 0.75 hours.

•• Each cell still receives 3 L/sec = 10,800 L/h.

•• Total flow to the cell is still 4 h x 10,800 L/h x 2.5 L/s = 43,200 L. •• However the total volume must be discharged in 0.75 hours = 43,200 / 0.75 L/h = 16 L/sec.

This higher outflow would not occur continuously (it would generally be in 45 minute blocks, with 15 minute pauses in between), but when it did occur, the flow rate would be higher.

This must be considered when making changes to the operating cycles.

All of this is based on the assumption that the actual time that the decant occurs is close to the total decant period that is set in the control system. In practice, it will be slightly shorter, as it may take some time for the decant weir to travel from the parked position (typically TWL, which is set by peak flow requirements and thus will not be reached in most cycles) to the actual water level. It is common for this initial period of travel to be at the highest speed available, and for the speed of lowering to then reduce when the weir reaches a point just above the actual water level. This ensures that the decant occurs over as long a portion of the decant period as possible.

In cases where relatively little inflow has occurred to the plant, it may not be possible to reduce the rate of lowering of the decant weir to have the actual decant occur throughout the entire decant period. This generally has little impact on the outflow rate because the inflow rate was low and well below the design values.

This issue does not apply to submerged, gas locked siphon decant weirs, since they commence decanting immediately.

Clearance Depth during Decant The other issues which must be considered during decant are the clearance between the water surface and the top of the settled sludge, and the “approach velocity”.

Based on tests during the development of the standard 4 hour IDEA plants in NSW, a distance of 0.7 m between the BWL (which is the fixed point in the IDEA cycle) and the top of the settled sludge was found to be sufficient to ensure that settled solids were not scoured into suspension and discharged in the treated effluent.

Such disturbance would result in very poor quality effluent and failure to meet effluent quality requirements. A factor of safety was applied to these results, leading to the standard recommendation that the top of settled sludge be at least 1 m below BWL. Also, by ensuring the recommended 1 m of clear water between the sludge blanket and the decant weir, the decant weir can “chase down” the sludge as it settles further.

The actual distance should be checked periodically using a sludge blanket detector. Figure 10.20 shows a portable sludge blanket detector being used during the settling phase at an intermittent plant. This detector emits a “beep” when sludge enters the U shaped sensor indicating that the sludge level has been reached.

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Figure 10.20. A sludge blanket detector being used to measure the depth of the sludge blanket.

The other parameter which needs to be considered is the “approach velocity”. This is defined as the velocity of outflow above the sludge at BWL. Again, tests during development of the IDEA concept found that the flow should be no more than 72 m/h to ensure that the settled sludge is not disturbed.

Example A single IDEA cell is 20 m long by 5 m wide, with 3.5 m depth at BWL and operating with a standard 4 hour IDEA cycle.

If the design inflow is 840 kL/d (35 kL/h) then outflow during the decant = 4 x 35 = 140 kL/h (the total volume that flowed in during the 4 hours must be discharged in the 1 hour decant period).

So in the 1 hour of the decant: •• Volume flowing =140 kL (= 140 m3).

•• Area the water is flowing through assuming the 1 m clearance = 5 m x 1 m = 5 m2.

So the approach velocity is 140 m3/5 m2 = 28 m3/m2 = 28 m in one hour = 28 m/h.

This is much lower than 72 m/h so it should be OK and scouring of the settled sludge should not occur.

The commonly used method of lowering the weir rapidly to the water level and then slowing it as much as possible when overflow commences is important to reduce the approach velocity and maintain maximum clearance between the water level and the top of the settled sludge during the decant. Both of these reduce the risk of solids carry over into the decanted effluent.

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Chapter 10 - Intermittent Systems Staggering of Cycles Where there are multiple reactors, it is possible to stagger the stages in the different reactors so there is nearly continuous decant (when considering the whole plant) and to spread aeration out as much as possible so that it is not necessary to provide larger blowers to provide all the aeration capacity in one period of a few hours. Figure 10.5 shows the different stages for a 3 reactor IDEA plant and that decanting occurs in 3 hours out of the 4 hours and the maximum aeration capacity required is for 2 reactors at a time.

For example, at the Moe STP in Victoria, the offset between the start of the cycles is 80 minutes.

Altered Cycle Times The early IDEA and SBR plants were controlled by simple timers. With the appearance of modern Programmable Logic Controller (PLC) systems it is now easy to alter the duration of the different stages and the overall cycle times. It is also possible to have different treatment sequences and stage times in different tanks.

Some examples are provided below.

•• Longer aeration stages during times of increased influent organic load and shorter stages during low loads.

•• Longer aeration stages when adjusting from seasonal off-peak flows to peak flows.

•• Longer anoxic periods after morning peak flows or trade waste flows.

•• Two, or even three, aeration and denitrification periods to achieve lower nitrogen levels in the effluent. For example: o Aerate 1, Denitrify 1, Aerate 2, Denitrify 2, Settle, Decant.

But remember, prior to implementing control program changes, consider the impact they may have on the outflow rates from the tanks as described above.

Another option for improved treatment is instrumental control which is considered in Chapter 14.

Conditions in the Biomass Change during Settling and Decant When aeration ceases, DO in the settling sludge will be consumed and eventually reach zero. The biological activity will therefore change progressively from aerobic to anoxic and eventually anaerobic. While this feature aids biological nutrient removal, it means that at the commencement of the next aeration cycle, aeration needs to be of maximum intensity for a short period (up to a few minutes) to resuspend settled material and to cater for the initial high oxygen demand.

In many plants this results in a what is called a DO sag at the start of aeration particularly during peak loads.

Figure 10.21 shows trends for 3 complete IDEA cycles. The plant operates with 3 aerate/denitrification cycles and this is why there are 3 DO peaks for each cycle. During the period of higher load, shown by the steeper red line (arrow), the DO peaks start off low (in fact the first one doesn’t reach the DO set point of 2 mg/L) and then get higher. In contrast during the lower load period (less steep red line) to the left, the DO peaks overshoot the set point of 2 mg/L.

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Figure 10.21. Trend showing DO, cell level and weir position during 3 cycles. The arrow shows increased flow during the second cycle.

One of the problems with diffused air systems is that the sludge settles below the level of the diffusers (Figure 10.22) and is difficult to resuspend from that position. While much will eventually resuspend, over time there will be an accumulation of compacted sludge that necessitates periodic draining so it can be removed. Some idea of the amount of settled sludge can be gauged by carefully probing the bottom of the reactor with a long PVC conduit and feeling for the compacted sludge.

Figure 10.22. Accumulated compacted sludge between the fine bubble diffusers.

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Chapter 10 - Intermittent Systems Changes to Operational Parameters Any changes to operating parameters should be made gradually, with a general rule of thumb being no more than 10% - 15% change per sludge age and only one parameter at a time.

Sludge Age Intermittent plants are no different from continuous plants with respect to sludge age.

If no nutrient removal is required, a basic 4 hour cycle with a > 20 day sludge age will result in an effluent low in COD and suspended solids and a stabilised sludge. A minimum of a 10 day sludge age will typically be adequate for achieving a secondary quality effluent, but will produce a greater volume of less well stabilised sludge.

For nitrification, a sludge age of at least 17 days (including a reasonable safety margin) is recommended for cooler conditions. The sludge age will be less for summer conditions. Exactly how much shorter is dependent on summer wastewater temperatures, but a 3 - 4 day reduction may be a useful guide. The actual amount that it can be reduced can be determined by simply monitoring NH3 and NO3 levels over a full sludge age after a change has been made. Any increase in NH3 and a decrease in NO3 suggest the sludge age may have been shortened too much. Denitrification will occur readily at these sludge ages.

In general, plants with sludge ages < 6 days will probably not nitrify reliably, and treatment of COD may also be deficient. EBPR may require sludge ages of around 10 - 12 days, however this may limit the extent of nitrification possible particularly at lower temperatures.

If the operating conditions are stable, there is some advantage to operating at the minimum sludge age necessary to achieve all the treatment objectives. The primary advantage is some reduction in the aeration required, however this has to be balanced against producing a larger quantity of less well stabilised biosolids.

A compromise will need to be found where the best outcome for the plant will be achieved. This requires quite a lot of monitoring and quite a long time to test out options, remembering that any change should be no more than about 10% - 15% and then evaluated over a sludge age (approximately 2 - 3 weeks). However, the time invested is worth it. The operational team will learn much about the plant and its capabilities.

The impact of proposed changes can also be assessed using an activated sludge process model (see Chapter 15).

Sludge Wasting Biomass can be wasted during the decant stage when the biomass has fully settled, or during the aeration stage. If wasting occurs during aeration, the WAS SS will be equal to the MLSS (typically 3,000 mg/L to 4,000 mg/L). However if it occurs during the settle stage the WAS SS may vary, typically between 8,000 mg/L and 11,000 mg/L (i.e. approximately 2 - 3 times the MLSS).

In the case of an EBPR plant, excess sludge must be wasted during the aeration period as once settling occurs, P release will commence. If however EBPR is not being used, wasting can occur during settling or if the draw off point is well away from the decant weir, possibly during decant. In both cases, WAS will be drawn off from settled sludge, which will give a reasonably consistent mass, and also enable reduced WAS pumping periods as the sludge is settled, and thus thicker than the MLSS. Sludge age can generally be more accurately controlled in intermittent plants by wasting during aeration. As the mixed liquor is fully mixed and a larger total volume needs to be wasted, it is easier to more accurately control the total biomass wasted. The calculation of the amount to be wasted is then simply based on the MLSS value.

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Chapter 10 - Intermittent Systems In plants that have difficulty managing the larger volume of WAS that would result from wasting during aeration, it may be necessary to waste from the settled sludge. In this case, a sample of the WAS should be taken approximately 5 minutes after WAS flow starts and the WAS SS quickly measured. Depending on the result, the total wasting time can be set to achieve the required target sludge age. Since settling may continue during wasting, the actual biomass wasted may vary somewhat. This combined with the smaller total volume wasted generally results in less accurate control of sludge age.

Another risk associated with wasting during settling or decant is that of “rat holing” where the WAS offtake creates a channel in the settled sludge and starts to draw clarified liquor through the “rat hole”, rather than drawing out sludge. Some plants use WAS pipework with multiple offtakes to manage this risk.

The combination of aeration and settling into one tank enables a simple approximate calculation of sludge age to be made based on the volume of WAS as a fraction of the volume at BWL. Example Consider an IDEA plant with a volume of 1,000 kL at BWL. If 100 kL of WAS is withdrawn during a react phase when the WAS SS = MLSS, the simplified estimation is that the sludge age will be 1,000 / 100 = 10 days (ignoring the effects of a potentially different depth at the time of WAS withdrawal).

If wasting occurs from the settled sludge, the relative volume is effectively doubled since the WAS SS is approximately twice the MLSS concentration. Thus the calculation becomes 1,000 / (2 x 100) = 5 days.

Remember that these calculations are approximations but have been shown to provide satisfactory process control. The preferred option however is to measure the suspended solids in the waste stream and base the sludge age calculation on that.

Dissolved Oxygen During settling and decant, biological activity continues in the settled sludge. In addition, untreated COD and ammonia from the raw sewage will accumulate in the surface liquor near the inlet of the reactor. As a result, when aeration commences the DO may take some time to reach the set point (see Figure 10.21). There is no inherent problem with this, although a brief period of more intense aeration may help to resuspend settled solids and achieve the DO set point more quickly.

During low load periods, the DO may be higher than the set point since the plant aeration system and control are such that too much air is provided for the amount of oxidisable material in the plant at the time (see Figure 10.21). During this period the aerators could be operated intermittently, provided the “off” time does not allow the solids to settle away from the DO meter, which would result in a situation where the DO instrument indicates DO levels are satisfactory, whereas the mixed liquor is suffering an oxygen deficit and thus will not provide the required treatment.

Where such changes in aeration strategy are used, it is beneficial to install DO meters below the surface, rather than use floating installations.

Adjustment of RAS Where RAS has been provided in IDEA plants, it is common practice for it to be operated continuously.

Despite the fact that the RAS SS will vary between aeration (when RAS SS = MLSS) and settling and decant (when RAS SS will increase to a higher figure, probably around 2 - 3 x MLSS), in most plants no adjustment of the RAS flow is made to compensate for the amount of biomass returned.

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Chapter 10 - Intermittent Systems Determining Anoxic Fraction Where a denitrification stage has been included in the treatment cycle, one of the first decisions to be made is how long to set the denitrification stage. As a guide, the denitrification time can be set at 20% of the overall react time. This, combined with the anoxic conditions that the MLSS will undergo during settling and decant, is likely to provide a total anoxic fraction of around 25% - 30%, which is similar to that provided in many nitrogen removal continuous plants.

Having set the denitrification time, and the aeration-on (pulse) and aeration-off periods, it is necessary to monitor the effluent nitrate and ammonia and compare these with the target effluent requirements and make appropriate changes as necessary.

•• If nitrate is too high, but ammonia is acceptable, the denitrification period can be increased.

•• If the ammonia increases despite aeration, while the pH, DO and alkalinity are satisfactory, or if settleability becomes poor owing to loss of floc integrity, the denitrification period can be reduced.

To compensate for the reduced anoxic fraction, the MLSS could be increased slightly to provide more efficient denitrification during the denitrification cycle.

By maximising the denitrification cycle, anoxic removal of COD (during denitrification) is maximised and aeration costs are minimised.

Decanting Just as with continuous plants and clarifiers, the quality of the final effluent can vary for a number of reasons: 1. The minimum clearance of at least 0.7 m (preferably 1.0 m) between settled sludge and BWL is not achieved resulting in carryover of solids into the decanted liquor.

2. Scum or foam on the surface of the reactor may be drawn over the weir during decant. Most intermittent plants have some sort of scum baffle (see Figure 10.8) but if not, it is relatively easy to retrofit a baffle.

3. The peak daily flow is greater than design peak but not enough to trigger the storm cycle.

4. Wind can hinder settling and result in a higher solids load in the effluent. This is one of the reasons IDEA cells are often limited to around 50 m in length. In some overseas plants, transverse reinforced plastic plates have been installed to reduce the length of fetch and thus reduce the size of waves formed by the wind activity.

5. If an anoxic stage is included and this occurs just prior to the settling stage, rising sludge can occur during settling and decant resulting in a turbid effluent. The simplest way to stop this is to ensure that where anoxic stages are used, there is a brief aeration stage prior to the settling stage.

Recovering from Loss of Biomass Activity All activated sludge plants are susceptible to “poisoning” with the result that the activity of the biomass can be badly reduced or in fact the biomass may be killed outright. The usual cause is an illegal discharge to the sewer but there have been examples where sewer cleaning chemicals have also poisoned the biomass.

In this event, the biomass needs to be re-established as quickly as possible. There are 2 main possibilities: 1. Transfer of sludge from an aerobic sludge digester.

2. Transfer of mixed liquor from another activated sludge plant.

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Modifications for Peak Flow (Storm) Events Peak flow events require modification of operational cycles to allow the high volume being received to pass through the plant. After the first flush, when settled sludge materials are dislodged and flushed through the sewer, the influent wastewater is diluted by infiltration and inflow and requires much less treatment.

The simplest way to manage storm events is by reducing the cycle time. To understand how this works, the concept of hydraulic throughput needs to be demonstrated by some examples.

Example Intermittent reactors (SBR or IDEA) operate between Bottom Water Level (BWL) at the end of the decant, and Top Water Level (TWL) after filling.

A single IDEA cell is 45 m long and 11.1 m wide with a BWL of 3.5 m and TWL of 4.7 m. The volume between TWL and BWL is therefore approximately: L x W x H = 45 x 11.1 x (4.7 - 3.5) m3 = 600 m3 Based on the standard 4 hour cycle, 6 decants will occur each day.

So the maximum total volume passed through the single IDEA cell = 600 x 6 = 3600 m3 per day.

If, however, the operating cycles are changed to a storm cycle of 1.5 h aeration, 30 minutes settling and 1 h decant (total cycle time of 3 hours), there will now be 8 decants per day.

So the total volume passed through the single IDEA cell = 600 x 8 = 4800 m3 per day.

This represents an increased capacity of 1,200 m3.

So for an IDEA plant operating with a 4 hour cycle and two IDEA tanks in service, that means 600 m3 can be passed through every 4 hours for each tank meaning a total of 600 x 6 x 2 = 7,200 m3 per day.

Under a storm flow cycle of 3 hours that means 600 m3 can be passed through every 3 hours for each tank, meaning a total of 9,600 m3 - an increased capacity of 2,400 m3.

Therefore hydraulic throughput can be increased for peak flow conditions (i.e., storm event or wet weather flow) by temporarily shortening the overall length of the cycle.

Plants are usually designed to operate with a storm cycle up to an inflow of between 4 and 6 times ADWF. Beyond that, if no bypass or upstream storage is available, tanks can be simply used for settling with no aeration. In this situation, aeration is turned off and the decanter parked at TWL. Raw sewage then simply passes to the tanks and overflows the decant weir. The effluent will be of a lower quality but licence maximum limits are unlikely to be exceeded. By leaving the biomass settled and not aerated, the mixed liquor suspended solids are prevented from being washed out of the cell and are available for full treatment as soon as the wet weather event is over.

Where final discharge from the plant goes to an aquatic environment, it may be necessary during storm flow conditions to sacrifice NO3 levels to ensure NH3 levels are acceptable and as close to licence requirements as possible. This is because ammonia is more toxic in the aquatic environment than nitrate.

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Chapter 10 - Intermittent Systems Managing Ammonia Bleed in IDEA Plants Case Study (Aurora STP Victoria) The Aurora STP was constructed as part of a dual pipe recycled water scheme in Victoria in 2005. However, plant commissioning did not occur until 2008 due to delayed development and limited sewage flows from the new housing estate. The facility includes an activated sludge plant designed for an ADWF flow of 3.5 ML/d of domestic sewage. This plant is a modified IDEA plant, combining the IDEA process with an MLE sequence. The original design includes two IDEA reactors each with dedicated anoxic tanks (3.5 h HRT) and a high ratio mixed liquor recycle (up to 9 x flow) to achieve enhanced nitrogen removal. Other elements of note include an influent distribution header and effluent decanters that span across the entire width of each cell.

During process commissioning and the plant acceptance/performance testing, the plant was initially operated on a 4 hour cycle. This resulted in significant breakthrough of ammonia into the effluent, with concentrations of 15 mgN/L or higher recorded. The high ammonia combined with a high nitrate resulted in failure of the plant to meet the performance criteria for the contract and could have hindered downstream treatment processes.

Typically, a conventional IDEA plant is capable of producing an effluent with a TN of around 5 - 7 mgN/L so what was the problem?

The first step was to check whether in fact nitrification was occurring. The findings are summarised below.

•• DO was adequate (this was checked by modifying the set points to successfully provide up to 3 - 4 mg/L during aeration periods).

•• Oxygen Uptake Rates (OURs) of 7.5 mg/L/min implied no inhibition was occurring.

•• The calculation of sludge age as an indication of what might be happening was impracticable as wasting initially occurred directly to sludge tankers at various sequence times and therefore variable MLSS concentrations.

Despite the problems with the calculation of sludge age, the conclusion was that nitrification was occurring.

Analysis of the SCADA trends showed that much of the inflow in the morning peak occurred during the settle and decant stages of the 6.30am to 12.30pm cycle (Figure 10.23). This was likely to result in short circuiting with the unaerated influent passing directly to the decant resulting in high ammonia concentrations in the effluent.

The peak loads also coincided with the emptying of sewage tankers into a nearby pump station.

The resulting high pulsed flows from the pump station created surges through the IDEA reactor during the settle and decant stages, especially during the morning extended peak.

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Case Study (Aurora STP Victoria continued)

Figure 10.23.

A 24 hour trend showing the significant inflow that was occurring (white arrow) during the settle and decant of the 6.30 to 12.30 am cycle.

A number of strategies were implemented: •• The pumping rate from the pump station was reduced so that the load was spread out more evenly across the day.

•• A 6 hour and then an 8 hour cycle was adopted, timed to commence aeration at the start of the morning peak. The 8 hour cycle consisted of 315 minutes Aerate, 75 minutes Settle and 90 minutes Decant. This change reduced the effluent ammonia to 5 - 10 mgN/L. Subsequently, further development in the housing estate has resulted in increased flows that have allowed a standard 4 hour cycle to operate effectively. The different cycles are summarized in Figure 10.24.

Figure 10.24.

A summary of the stages in the different cycles used.

•• A denitrification cycle was implemented, which resulted in only minor improvement.

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Case Study (Aurora STP Victoria continued) The exceptionally high recycle rate was found to be completely turning over the volume of liquor above the sludge blanket (pump rate ~2.2 ML in 1.5 hours) and mixing it with influent during the settling and decant period. This resulted in a mixture of treated (nitrified) effluent and raw wastewater being decanted. As it was not possible to reduce the pump rate low enough to avoid this problem, the pump was operated intermittently during aeration (10 minutes on, 10 minutes off), and turned off during settling and decant. This resulted in an improvement in the effluent quality with Total Nitrogen levels of 4.8 mgN/L. Figures 10.25 to 10.27 summarise the progress in overcoming the problems experienced at the plant.

Figure 10.25.

Effluent ammonia levels in the decant before and after optimisation in the commissioning period.

Figure 10.26.

Effluent nitrate levels in the decant before and after optimisation in the commissioning period.

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Case Study (Aurora STP Victoria continued)

Figure 10.27.

Effluent Total N levels in the decant before and after optimisation in the commissioning period.

Throughout this period the plant was under low loading, and no significant wet weather peak flows occurred.

The Bio-Denitro and Bio-Denipho Hybrid Systems There is another type of plant operating at a number of locations in Australia that is a hybrid between a continuous plant and an intermittent plant. The plants in Australia are mostly the Kruger Bio-Denitro (biological nitrogen removal) and Bio-Denipho (biological nitrogen and phosphorus removal) plants. Flow enters the plant continuously but is switched periodically to one of two bioreactors which are configured as oxidation ditches (Figure 10.28). As well as the flow being directed intermittently to alternate bioreactors, the conditions in the bioreactors are varied between aerobic conditions and anoxic conditions. During the aerobic phase the aerators operate, while during the anoxic phase the aerators are switched off but mixers keep the biomass suspended and moving around the bioreactor. Unlike continuous plants, and like the IDEA and SBR plants, there is no internal recycle.

Figure 10.28. A small Bio-Denitro plant.

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Chapter 10 - Intermittent Systems Figure 10.29 shows diagrammatically how the flow is switched between the two bioreactors in a Bio-Denitro configuration. It also shows how the conditions in the two bioreactors are varied. The process sequence is once again an MLE activated sludge process.

Figure 10.29. The basic double bioreactor Bio-Denitro configuration.

The basic Bio-Denitro configuration can be modified for P removal by the addition of an anaerobic zone at the front of the double bioreactor system (Figure 10.30). Raw sewage and RAS enters the anaerobic zone and then flow from the anaerobic zone is directed to one of the two bioreactors in the same way as shown for the Bio-Denitro plant. The flow from the anaerobic zone alternates between the two bioreactors in exactly the same way as shown for influent sewage in the Bio-Denitro process shown in Figure 10.29.

Figure 10.30. The Bio-Denipho configuration with an anaerobic zone at the front of the double bioreactor system (Ax Anoxic, Ae Aerobic, An Anaerobic).

The double bioreactor system can be modified in larger plants as triple bioreactors or more.

Operation of Bio-Denitro and Bio-Denipho Plants A typical control philosophy for Bio-Denitro and Bio-Denipho plants is to divide each 24 hour period into 8 x 180 minute (3 hour) cycles. Therefore there are 8 cycles each day. During each 180 minute cycle the two bioreactors will rotate in order through each of the phases shown in Figure 10.29. The time in each phase for each cycle is an operator adjustable set point. An example of a table of set points that has proven to be successful over many years at the Warragul STP in Victoria is provided in Table 10.3. The table has required very little modification during wet weather events and varying loads. In this example, Cycle 1 starts at 11 pm.

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Chapter 10 - Intermittent Systems Table 10.3.

Summary of cycles in a Bio-Denipho plant at Warragul in Victoria.

Cycle

Phase 1 time

1 23000200 70

2 02000500 75

3 05000800 75

4 08001100 15

5 11001400 40

6 14001700 40

7 17002000 40

8 20002300 70

Phase 2 time

20

15

15

75

50

50

50

20

Phase 3 time

70

75

75

15

40

40

40

70

Phase 4 time

20

15

15

75

50

50

50

20

Total cycle time

180

180

180

180

180

180

180

180

Anoxic fraction

0.39

0.42

0.42

0.08

0.22

0.22

0.22

0.39

Period of day

Cycles 1, 2, 3 and 8 typically occur during the lowest flow and load periods of the day so the majority of the time in each cycle is spent in phase 1 or 3 where anoxic conditions are present as well as aerobic conditions.

Cycle 4 is typically when the highest flows and loads occur and it is necessary to have a lot of time in phase 2 and 4, where there are aerobic conditions in both bioreactors, to maintain compliance with the ammonia limit in the effluent. Cycles 5, 6 and 7 occur during a moderate flow and load period and hence the times spent in phase 2 and 4 are midway between the two extremes of cycle 4 and cycles 1, 2, 3 and 8.

The time that the total bioreactor volume is anoxic during each cycle can be calculated by adding the phase 1 and 3 time, dividing by 2 (only 1 of the bioreactors is anoxic) and then dividing by 180. This value is included in Table 10.3 as the anoxic fraction. The overall anoxic fraction for the plant for the 24 hours is 0.3 which is typical of other biological nitrogen removal processes.

In addition to the phase times above, the other primary control variables are DO, RAS flow and sludge age.

Each cycle can have individual DO set points which are operator adjustable. The RAS flow rate is typically controlled as a percentage of the inflow and is recommended by the designer to be 40% - 70%. After many years of operation, the Warragul STP has experienced its most consistent operation at a sludge age of 15 20 days.

Apart from some brief periods of a few months, the biological phosphorus removal at the Warragul STP has proven to be inadequate to comply with a total phosphorus limit of 0.5 mg/L (Figure 10.31). Supplemental iron salt dosing is added in between the bioreactors and the clarifier at a typical dose rate of 30 - 60 mL/kL. The results are shown in Figure 10.32.

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Figure 10.31. Long term trend showing P levels during periods of EBPR and during periods of chemical dosing at a Bio-Denipho plant.

Figure 10.32. 7 day trend showing phosphorus levels at an EBPR plant with chemical polishing.

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Chapter 11 Monitoring the Activated Sludge Process Sampling Monitoring is an essential part of operating the activated sludge process. This is generally understood reasonably well, however the importance of correct sampling is often not so well understood and often poorly practiced.

Accurate data on which process changes can be reliably based require representative samples taken from appropriate sampling points, at appropriate times, and at appropriate sampling intervals.

Before any sample is taken, it is essential to ensure that the sample point is appropriate. A sample must be taken from a site where the liquids are well mixed. Therefore a location where the flow is turbulent is most suitable.

In general, sampling should be from below the surface to avoid surface scums but not too deep to ensure that old settled sludge is not sampled. Somewhere in the middle is best (Figure 11.1).

Figure 11.1. Sampling from different depths in a slow flowing stream can give quite different sample characteristics.

However, where sampling is undertaken for Chemical Oxygen Demand (COD) monitoring and if there is oil in the influent, not sampling at the top will result in a low estimate of the COD entering the plant.

At times of low flow, it may be difficult to ensure adequate mixing, but as long as this is recognised, the data can be interpreted carefully at these times. For example, results for Total Suspended Solids (TSS) at low flow times might be pretty meaningless because the solids may settle to the bottom of the pipe or well.

Types of Samples Grab Sample A grab sample involves dipping a beaker or similar container into the liquid to be sampled, typically at one point of time during the day. Because of the variation in raw sewage quality over the day, this method is of limited value in determining the average quality of raw sewage.

Grab samples are however often used for verification monitoring of the final effluent. Environmental regulators seem to accept this, but it is important to realise that grab samples taken at different times may vary considerably, sometimes complying with targets and other times not.

If grab samples are used to monitor plant performance, it is essential that the samples are taken at different times of the day and on different days.

Grab samples can be more useful if they are taken regularly throughout the day. This is best done by using a refrigerated autosampler and taking a sample each hour. Each of the hourly samples can then be analysed.

This method can become very expensive so where there is limited budget, composite samples can be used.

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Chapter 11 - Monitoring the Activated Sludge Process Composite Sampling Composite sampling is most commonly used for the influent to a plant. In this method, individual grab samples are typically taken each hour for 24 hours and mixed together to give an overall representation of the influent for that day. While this is an improvement on grab samples, it does not give good information as to what is coming into the plant, when it is coming in and how much of it there is.

A better system is to take a flow weighted or flow proportional sample. There are two ways to do this.

Calculation Method 1. An autosampler with 24 sample bottles is set up to take a sample every hour.

2. The total flow for the day and the flow each hour are obtained from the flow meter or the SCADA system.

3. The flow for each hour is then calculated as a percentage of the total flow for the day.

4. A volume, proportional to the percentage of the flow for each hour is taken from each of the hour samples and mixed together. An example is provided in Table 11.1.

Table 11.1.

An example calculation for flow proportional sampling to provide a 5 L composite sample.

The calculation is based on 50 mL of sample for each 1% of flow.

Start Time 9:00 am 10:00 am 11:00 am 12:00 pm 1:00 pm 2:00 pm 3:00 pm 4:00 pm 5:00 pm 6:00 pm 7:00 pm 8:00 pm 9:00 pm 10:00 pm 11:00 pm 12:00 am 1:00 am 2:00 am 3:00 am 4:00 am 5:00 am 6:00 am 7:00 am 8:00 am Total

Flow (ML) 2 3 3.5 3 1.5 1.2 1.2 1.5 1.8 2 2 1.5 1.2 1 1 0.8 0.7 0.6 0.5 0.5 0.7 1.3 1.5 2 36

% of Total 5.6 8.3 9.7 8.3 4.2 3.3 3.3 4.2 5.0 5.6 5.6 4.2 3.3 2.8 2.8 2.2 1.9 1.7 1.4 1.4 1.9 3.6 4.2 5.6

Sample Volume (mL) 278 417 486 417 208 167 167 208 250 278 278 208 167 139 139 111 97 83 69 69 97 181 208 278 5,000 127

Chapter 11 - Monitoring the Activated Sludge Process Flow Meter Method Most flow meters send out an electrical pulse at regular intervals related to the flow rate at a particular time.

Therefore during high flow periods there are more pulses than during low flow periods. By connecting the autosampler to the flow meter output, the autosampler simply takes a fixed sample for each pulse (more during high flow and less during low flow periods) to generate a flow proportional composite sample.

While both of these methods give a representative composite sample, there is a risk that short bursts of discharge of a high strength trade waste may be missed. If this is the case, it is probably best to organise a separate sample, even better, a separate composite sample, directly from the trade waste site.

Sample Storage, Preservation and Transport Assuming a well-mixed representative sample has been obtained, the way the sample is stored and transported to an accredited laboratory can have a significant impact on the final result. During this time the sample can change. The aim is to minimise the amount of change that occurs.

In general, samples should be collected and immediately refrigerated. Most autosamplers come with a refrigeration option and these should be used. Samples should be dispatched in good quality eskies (chilly bins) with as many ice packs as necessary to ensure the sample arrives at the laboratory cold.

Laboratory Analysis While accredited laboratories use approved standard methods, samples are usually held in the laboratory for a variable period and then diluted to allow batches of different samples to be analysed together. All these steps can introduce errors into the final result.

Even accredited laboratories can make mistakes. If a result appears to be wrong or puzzling, ask the laboratory to check the report and even repeat the testing of a sample.

Another check that is worth using sometimes is to take a single sample and carefully split it into two or more samples and send them without any realistic sample identification to the laboratory or perhaps even better, send the samples to two different accredited laboratories and compare the results. The results can vary significantly. If they do, it is worth trying to establish which result is correct by further sampling and testing of multiple samples.

Online Continuous Monitoring: Avoiding Sampling, Storage and Transport Errors One way to avoid the errors introduced by sampling, storage, transport and laboratory handling is to use online analytical equipment. This equipment has developed rapidly in the last 5 to 10 years and a large array of sensors and probes are now available. One of the biggest advantages of online equipment is that they effectively take many samples at short intervals therefore providing better data. However, the data is only superior if the instrument is calibrated correctly and maintains that calibration for a reasonable time.

Calibration usually involves taking a grab sample at the same time the instrument samples the flow and then sending the sample to the laboratory as quickly and carefully as possible. Once the result comes back, the value is entered into the instrument to serve as a baseline calibration. It is almost impossible to do this accurately especially with raw sewage as it changes so rapidly. Most of the online meters can be taken out of the sampling location and used in “sample” mode. In this mode, a grab sample is taken and keeping it well mixed, the sample is fed into the online instrument at the same time a sample is bottled for dispatch to an accredited analytical laboratory.

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Chapter 11 - Monitoring the Activated Sludge Process It is probably better to send 2 or even 3 identical samples to the laboratory so an average value can be used.

If this step is repeated for say high and low load periods, the online instrument can provide excellent data.

Laboratory controlled standards can also be used to calibrate online analysers very effectively and can act as secondary checks on the accuracy of the values determined.

One absolute requirement of online instruments is that they are cleaned regularly. Figure 11.2 shows the effect of cleaning an online analyser for BOD and COD. Immediately after cleaning there is an increase in the concentration of BOD and COD. The message is simple, “keep them clean”!

Figure 11.2. An example of online COD and BOD data. The arrows show where the probe was cleaned.

Process Monitoring Process monitoring requires the identification of control points and appropriate targets and control limits for those control points (see Chapter 2). Some targets and critical limits for activated sludge control points are provided in Table 11.2.

Table 11.2.

Some target and critical limits for activated sludge operation.

Influent Control Point Parameter Odour Flow

Alkalinity (mg/L)

Target Noticeable in inlet works area only.

PDWF

> 200

Critical Limit

Comment

Noticeable at plant STPs do not need to smell. boundary.

PWWF High flows compromise the process and can lead to wash out of biomass and possible pollution of the receiving environment.

Low alkalinity can inhibit nitrification.

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Chapter 11 - Monitoring the Activated Sludge Process Anaerobic Zone Control Point Parameter

Target

Critical Limit

pH

7 - 9.5

<6 >10 >0

DO (mg/L) Redox potential

0 As negative as possible.

Nitrate N (mgN/L)

0.05

>1

Soluble phosphate P (mgP/L)

>30

< 20

Comment

DO inhibits anaerobic bacteria.

Redox potential can vary significantly however regular measurement will allow an operator to know the range of values that work best for that plant. The measurement of redox is affected by oxygen, nitrate and phosphate.

Nitrate inhibits PAOs. Sampling the Internal MLSS Recycle to the anaerobic zone is preferable in which case a target < 1 mgN/L and a critical limit of 3 mgN/L applies.

P release occurs in the anaerobic zone.

Anoxic Zone Control Point

130

Parameter

Target

Critical Limit

DO (mg/L) Redox potential

0 As negative as possible.

> 0.5

Nitrate at the end of zone pH

As close to zero as possible.

7 - 7.5

<6 >8

Comment DO inhibits anoxic bacteria.

Redox potential can vary significantly however regular measurement will allow an operator to know the range of values that work best for that plant. The measurement of redox is affected by oxygen and nitrate.

Depends on the plant configuration and may be as high as several mgN/L or as low as zero.

Low pH inhibits denitrification.

High pH inhibits denitrification.

Chapter 11 - Monitoring the Activated Sludge Process Aerobic Zone Control Point Parameter

Target

Critical Limit

DO (mg/L)

1-2

< 0.7 > 2.5

7.0 - 8.0

< 6.5 > 8.5

MLSS (mg/L)

2,300 - 4,500

< 1,200

DSVI (mg/L)

100

150

SVI

120

180

MLVSS %

70 - 80

Ammonia N (mgN/L) Nitrate N (mgN/L) Soluble phosphate P (mgP/L) Total phosphate P (mgP/L) Iron (soluble) (mg/L) Turbidity (NTU) Alkalinity (mg/L)

<2

5

Sufficient oxygen is necessary to ensure adequate nitrification.

DO above 2 mg/L wastes power.

Low pH inhibits nitrification.

High pH inhibits nitrification.

A sufficient and stable biomass is necessary to ensure effective treatment.

Too low inhibits settling.

Bulking sludge makes operation difficult.

Too low leads to pin floc carryover.

Bulking sludge makes operation difficult.

Too low leads to pin floc carryover.

Depends on SRT to some extent.

Chemical dosing decreases % volatiles.

Depends mainly on licence limits.

< 10 < 0.25 - 0.5

> 15 1

Depends mainly on licence limits.

Depends mainly on licence limits.

< 0.5

3

Depends mainly on licence limits.

< 0.5

3

< 10 > 75

20 < 60

Relevant for plants using iron salts for chemical P removal.

Reflects the clarity of the final effluent.

pH

Comment

Clarifier Control Point Parameter

Target

Critical Limit

Comment

Sludge blanket depth

< 30% of clarifier sidewall depth.

50% of clarifier sidewall depth.

Depends to some extent on the depth of the clarifier.

Return Stream from Solids Processing Control Point Parameter Ammonia N Soluble phosphate Nitrate N TSS

Target

Critical Limit

Comment These should be as low as possible. Be alert for changes.

Where the return is supernatant from an anaerobic digester then the ammonia may be expected to be high.

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Chapter 11 - Monitoring the Activated Sludge Process

Monitoring Raw Sewage Raw sewage is not a control point in the sense that it was defined in Chapter 2, however it is important to monitor the influent sewage carefully. Data from raw sewage can be important for a number of reasons: 1. Determining the potential for biological N and/or P removal.

2. To detect changes in the influent that may have a negative (or positive) impact on the operation of the plant.

3. Designing a new plant.

4. Upgrading an old plant.

5. Allowing a process model or mass balance to be developed for the plant to assist in predicting performance and the impact of process changes.

For each of these purposes a different monitoring program is required. For designing a new plant or an upgrade or even determining the potential for biological nutrient removal, a comprehensive short term (at least 2 weeks) monitoring program is required. At a minimum, this should include flow proportional composites tested for the indicated parameter. The best option is to use online instruments wherever possible. For a relatively short monitoring program of several weeks, one option is to initiate a data supply contract with a company that supplies the instrument and simply let them collect the necessary data for the allocated time period. A summary of a comprehensive raw sewage monitoring program is provided in Table 11.3.

Table 11.3.

An example of a raw sewage monitoring program.

Parameter

Frequency*

Total COD Soluble COD

Flocculated Filtered COD#

TSS & VSS Ammonia N Soluble phosphate P Total P TKN pH Alkalinity Temperature Electrical Conductivity

Comment

W M or W depending Note that in the case of raw sewage, soluble COD should be on variability. measured using a GF/C (1.2 µm) filter and not a 0.45 µm filter.

In typical domestic sewage, soluble COD should be approximately 50% of total COD. Soluble COD < 50% will generally lead to high MLSS for the same sludge age due to the higher level of particulate COD present.

Q In typical domestic sewage, flocculated filtered COD makes up 10% - 25% of total COD. For Bio P and N removal, ffCOD should be > 15% of total.

For Bio N removal only, ffCOD should be > 10% of total.

W VSS is measured to allow higher level calculations relating to the biomass.

D Important for determining C:N:P and treatability potential.

D Important for determining C:N:P and treatability potential.

W Important for determining C:N:P and treatability potential.

W Important for determining C:N:P and treatability potential. continuous Can detect dumping or other recurring trade waste problems.

W Important for determining treatability. continuous Temperature effects treatability.

W Can detect dumping or other recurring trade waste problems.

* Where a D (daily), W (weekly) or Q (quarterly) sample is suggested, it is important that the day and time of day is varied. # A method for flocculated filtered COD is provided in Appendix A.

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Chapter 12 Secondary Clarifiers The term “secondary” may seem strange at first but it reflects the history of sewage treatment. Most early Sewage Treatment Plants (STPs) had primary clarifiers at the front of the plant to settle the raw sewage prior to digestion of the settled solids in an anaerobic digester. The clarified liquid from the top of the clarifier went to a trickling filter or other treatment processes. So there were two clarifiers, the primary one at the front of the plant and the secondary one at the end of the plant just prior to discharge (Figure 12.1). Primary clarifiers are now rare, however the terminology remains.

Figure 12.1. Secondary clarifiers at a large STP. There are three main functions of a secondary clarifier: 1. Produce a clarified final effluent suitable for discharge to the environment.

2. Concentrate the sludge to allow easy return to the plant to achieve the required sludge age (or SRT).

3. Concentrate the sludge to allow easy wasting and processing.

Types of Clarifiers Clarifiers are commonly circular, but there are also instances of rectangular clarifiers.

Rectangular Clarifiers Rectangular clarifiers have a high aspect ratio being much longer than they are wide (Figure 12.2 and 12.3).

They usually have an inlet baffle that slows the flow into the clarifier and allows the floc to start settling out.

A sludge hopper is usually situated at the influent end of the clarifier (Figure 12.2).

Figure 12.2. A rectangular clarifier.

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Chapter 12 - Secondary Clarifiers Removal of Sludge Sludge is removed by a chain and flight system. Horizontal boards (flights) are attached to chains that revolve in the clarifier and move the flights along the bottom of the tank (Figure 12.3). The chain system is a little like the chain on a bicycle. The flights scrape the sludge to the sludge hopper where return activated sludge and waste activated sludge (RAS/WAS) pumps, pump it out.

Figure 12.3. A primary sedimentation tank with a chain and flight system.

Circular Clarifier Circular clarifiers vary in diameter from 10 - 40 m and can be centre feed or peripheral feed.

The depth of clarifiers varies considerably. Shallow clarifiers with a sidewall depth around 2.5 - 3 m sometimes struggle to maintain adequate settling and are more susceptible to solids overloading and therefore, poor effluent quality due to solids carryover. In these cases it is important to maintain a low sludge blanket, at least 1 m below the weir overflow level. Site specific experience may allow less depth to the sludge blanket however, the depth should be monitored closely. Sludge blanket level sensors and meters (Figure 12.4) may be useful to control RAS rates and achieve a constant sludge blanket depth.

Figure 12.4. A diagram of an online sludge blanket meter.

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Chapter 12 - Secondary Clarifiers In centre feed clarifiers, mixed liquor is fed to a centre well (Figure 12.5). The central well is a cylindrical structure that only extends part way to the bottom. The mixed liquor flows out around the edges of the well and generally below or into the sludge blanket.

Inlet diffuser Scraper arm

Inflow pipe

Scraper arms

Figure 12.5. Two examples of clarifiers showing a horizontal inlet pipe in the upper clarifier and a centre feed pipe in the lower photograph. Note also the different scraper arm assemblies.

Sludge removal in both centre feed and peripheral feed clarifiers is similar, with rotating scraper arms that transport and remove the sludge from the bottom of the clarifier. However there are two different mechanisms for removal. In one, scrapers deliver the thickened sludge to a central pit (Figure 12.5) from where the RAS is pumped. In the other, pipes with openings at regular intervals along the scraper arms suck the sludge from the bottom using a siphon system.

Most clarifiers, whether circular or rectangular, have a scum trough to remove scum that accumulates on the surface of the clarifier.

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Chapter 12 - Secondary Clarifiers Regardless of design, all clarifiers have a launder or overflow weir. Clarified effluent overflows the launder usually through V notches in the weir. The launder design often incorporates a baffle (Figure 12.6) that is positioned inside the main launder and extends down into the clarified effluent a few centimetres. Any scum, foam or floating sludge gets held back by the baffle while the clarified effluent passes under the baffle and then over the weir. This helps to ensure the clearest possible effluent.

Scum baffle

V notch weir

Figure 12.6. A circular clarifier showing the V notch weir and scum baffle.

Clarifier Operation Clarifier operation should never be considered in isolation as it is intrinsically linked to the biological treatment process and vice versa. Problems occurring in the clarifier such as poor settleability are typically a result of poor process control of the biological treatment.

Clarifier function is sensitive to inflow as in continuous plants the flow into the clarifier is proportional to the inflow to the plant with a slight time delay. Peaks in plant inflow result in peak flow into and out of the clarifier some hours later.

There are four main operational tasks associated with the operation of a clarifier: 1. Ensure a good settling sludge.

2. Maintain the sludge blanket at a sufficient depth to minimise solids carryover.

3. Ensure sufficient RAS to return activated sludge to the plant.

4. Ensure correct wasting (WAS) to achieve the target sludge age.

Operational Parameters There is a direct relationship between the design of the activated sludge system and the design of the clarifier(s). The necessary clarifier surface area depends on the solids loading to the clarifier. This in turn is determined by the size of the reactors, the flow, and the operating MLSS.

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Chapter 12 - Secondary Clarifiers A number of parameters are used to define clarifier performance. These include: 1. Overflow rate.

2. Weir loading rate.

3. Solids flux.

When problems arise with a clarifier, one of the first decisions often made is that the clarifier is not adequate and that a new clarifier is required. This may not be correct. For this reason it is worthwhile being able to check some of the design features to actually evaluate the performance of the clarifier. An example is provided below.

Overflow Rate The Overflow Rate is defined as the volume of water passing through each m2 of area per hour (m3/m2/h).

Overflow Rate = Flow Rate (m3/h)

Surface Area (m2)

Weir Loading Rate The Weir Loading Rate is the volume of water passing over each linear metre of the overflow weir per hour (m3/m/h) Weir Loading Rate = Flow Rate (m3/h)

Weir Length (m)

Solids Flux Solids Flux is the mass of solids passing through each m2 of surface area per hour (kg/m2/h) Solids Flux = (MLSS (mg/L)/1,000) kg/m3 x Flow rate (m3/h)

Surface Area (m2)

Some actual calculations are included in the example on the next page.

Targets for Clarifier Operation Target values for the main operating parameters for a typical circular clarifier are provided in Table 12.1.

Table 12.1.

Typical clarifier operation targets.

Parameter

Conditions

Target Range

Overflow Rate (m3/m2/h)

Average daily flow (or peak dry weather flow during holiday periods).

0.34 - 1.4

Overflow Rate (m3/m2/h)

Peak hourly flow.

1.4 - 2.5

Weir Loading Rate (m3/m/h)

Average daily flow (or peak dry weather flow during holiday periods).

7.8 - 8.3

Weir Loading Rate (m3/m/h)

Peak hourly flow.

7.8 - 8.3

Solids Flux (kg/m2/h)

Average daily flow (or peak dry weather flow during holiday periods).

4.0 - 7.1

Solids Flux (kg/m2/h)

Peak hourly flow.

10 - 15

Note that the peak hourly flow can only usually be sustained for a few hours. Any longer than this and the target for the average daily flow applies.

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Chapter 12 - Secondary Clarifiers

Example A conventional 11.2 ML/d activated sludge plant operates in an extended aeration mode and has some problems with nitrification. There was concern over the clarifier capacity and a new RAS pumping system was being designed. The question was asked, is the current clarifier arrangement over loaded?

The following information (Table 12.2) was gathered at the plant.

Table 12.2. Relevant plant operational data for assessment of clarifier performance.

Parameter

Value

Units

Influent Average Daily Flow*

11.4

ML/d

475

m3/h

Peak Hourly Flow**

1,200

m3/h

RAS Rate (Max)

160

m3/h

MLSS

6,000

mg/L

6

kg/m3

Clarifier Surface Area (total)

1,850

m2

Clarifier Depth

4.9

m

Clarifier Weir Length (total)

370

m

* In this case the average daily flow is equivalent to the peak dry weather flow. ** Peak hourly flow is the flow to full treatment prior to bypass during wet weather events and is close to the PWWF for the site. This flow is only 2.75 times the average flow.

Based on the data in Table 12.2, the clarifier overflow rate, weir loading rate and solids flux were calculated (Table 12.3) and compared to the target values in Table 12.1.

Table 12.3. Calculated Overflow Rate (OFR), Weir Loading Rate (WLR) and solids flux.

Parameter

Calculation

Target Range

Conclusion

Overflow rate (average/ peak dry weather flow) m3/m2/h

475/1,850 = 0.26

0.34 - 1.4

Acceptable

Overflow rate (peak hourly flow) m3/m2/h

1,200/1,850 = 0.64

1.4 - 2.5

Acceptable

Weir loading rate (average/peak dry weather flow) m3/m/h

475/370 = 1.3

7.8 - 8.3

Acceptable

Weir loading rate (peak hourly flow) m3/m/h

1,200/370 = 3.2

7.8 - 8.3

Acceptable

Solids Flux (average/peak dry weather flow) kg/m2/h

(6,000/1,000 x 475)/1,850 = 1.5

4.0 - 7.1

Acceptable

Solids Flux (peak hourly flow) kg/m2/h

(6,000/1,000 x 1,200)/1,850 = 3.9

10 - 15

Acceptable

Based on the calculations shown in Table 12.3, there is significant spare capacity in the clarifier system to fully process peak wet weather events. In addition there is sufficient capacity to handle increased reactor MLSS concentrations that might be required for treatment of increased loads, or to provide longer sludge ages necessary for improved nitrification.

The observed problems at the plant are more likely to be caused by long detention times during average flows resulting in rising sludge.

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Chapter 12 - Secondary Clarifiers

Monitoring Clarifier Performance Final Effluent Turbidity and Suspended Solids The amount of solids overflowing the clarifier is measured in the majority of plants. In fact this is usually a licence requirement. In the final effluent from a well performing clarifier, the Total Suspended Solids (TSS) should be less than 20 mg/L.

Return Activated Sludge (RAS) Solids RAS solids provide an indication of the performance of the scraper and the RAS pumping system of a clarifier. Low RAS solids concentration can be an indication of rat holing. Rat holing occurs when RAS pumping creates a channel in the sludge and the liquid stream is pumped in preference to the solids. For example with an MLSS concentration of 2,500 mg/L and a RAS rate equal to the influent flow (1:1 recycle rate), the MLSS should effectively be doubled in concentration and should be in the order of 5,000 mg/L. Concentrations significantly lower than this may indicate the occurrence of rat holing.

Depth of the Sludge Blanket The sludge blanket is made up of two zones. In the upper settling zone, the floc particles are settling slowly due to the force of gravity. In the lower compaction zone, settling has essentially finished, however the weight of the sludge from above and the upward flow generated by the water flow from below compacts the sludge. The relative thickness of the two zones varies depending on the MLSS concentration feeding the clarifier, the settleability of the sludge and the flow into the clarifier.

At the same time as the flow through the plant is tending to lift the sludge, the RAS withdrawal from the bottom of the clarifier tends to draw it down. Therefore the sludge blanket moves up and down within the clarifier depending on the flow to the clarifier and the RAS flow.

Other factors affecting the depth of the sludge blanket include: •• Varying flow patterns from day to day.

•• Longer term seasonal changes in flows, for example higher flows during winter due to infiltration.

•• Differences in nutrient levels producing different biomass growth rates.

•• The settleability of the biomass, in particular whether there are filamentous organisms present.

Portable sludge blanket detectors (Figure 12.7) provide an easy way to measure the depth of the sludge blanket. The probe is simply lowered into the clarifier until the sound or light signal indicates that the sludge level is reached. The depth is then simply read off the electrical cable.

Figure 12.7. A portable sludge blanket detector.

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Chapter 12 - Secondary Clarifiers Scraper Checks The scraper systems should be checked regularly or whenever observation of clarifier function suggests something may be wrong. The only real way to check the scrapers is to empty the clarifier. Rotation of the scraper arms can be checked by lowering a plastic conduit to the floor of the clarifier and waiting to “feel” the arms hit the conduit. The clarifier design drawings should be consulted before doing this test.

Optimisation Optimisation of clarifier performance is often linked to the overall performance of the plant.

Poor clarifier performance may be due to one or more of the following: •• Excess flow into the plant.

•• A very long sludge age resulting in poor floc settleability.

•• A long sludge age resulting in excess solids flux to the clarifier.

•• Unstable sludge age and MLSS. •• Poor scum removal or failure of scum baffles.

•• Rising sludge.

•• Wind and heat effects on the clarifier.

Inflow to a plant may be able to be controlled by investigation within the network to identify whether there are cost effective options to reduce inflow and infiltration. If the flows cannot be controlled adequately, it may be possible to bypass high flow around the secondary treatment process and into the clarifier, but only if the solids flux and overflow rate are not exceeded.

Unfortunately sludge age is not well controlled in many STPs. It is an important parameter as it allows operation to be based on the minimum biomass necessary to achieve the required level of treatment, thereby maximising the capacity of the clarifier.

Unstable operation can also come from poor control of internal recycles, RAS, DO and sludge age. These often result in poor settling, foaming and/or bulking sludge. It is important to evaluate the potential reasons for these problems which can often be diagnosed through microscopic analysis of the mixed liquor (see Chapter 3).

RAS flows can be increased or decreased in order to optimise clarifier performance. RAS control is generally by one of three methods: fixed RAS flow, RAS flow proportional to the inlet flow and variable RAS flow controlled by sludge blanket level. The recommended RAS rate (as well as the control method) varies for different process designs. A fixed RAS flow is the simplest control method and it produces the least variable flow rate to the clarifier. RAS flow proportional to the inlet flow is generally recommended for secondary treatment process control but will produce more variable flow rates to the clarifier, in particular very high flows during daily peaks.

Regardless of which RAS control method is used, typical average daily RAS rates are 50% - 100% of the inflow. RAS flows can also be increased or decreased based on settleability. Sludge blanket level control attempts to control the sludge blanket level via the RAS flow rates.

Weather can impact on clarifier performance. Wind can create surface turbulence that can reach the level of the sludge blanket. Similarly, direct heating on the side of a clarifier caused by the sun can create thermal currents that can increase the solids carryover. This is more pronounced in an above ground steel clarifier found typically in package plants, than in a large concrete clarifier.

Clarifier trouble shooting is covered in Chapter 13.

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Chapter 13 Trouble Shooting An activated sludge plant is essentially a complex biological system. Many things can affect its operation and many things can go wrong. Chapter 11 emphasised the need for extensive regular monitoring. Without monitoring it is often difficult to detect early warning signs that things may be going wrong.

This chapter provides an extensive list of trouble shooting checks that should be made either if the plant appears to be moving out of control or once the plant is out of control.

This chapter should not be read in isolation from the rest of the book. Many of the actions will only make sense with the background information provided in the previous chapters.

Trouble shooting is considered in three sections below: 1. General plant trouble shooting.

2. Control of bulking and foaming.

3. Clarifier trouble shooting.

1. General Activated Sludge Plant Trouble Shooting A general guide for all activated sludge plants and Table 13.2 some additional suggestions for intermittent plants is provided below.

Table 13.1.

General activated sludge trouble shooting guide.

Symptom

Cause

Action

Inlet Gravity Main and Inlet Works Inlet storm flow > design peak wet weather flow.

Storm event in the catchment.

Ensure storm bypass is activated if present.

Ensure storm flow controls are activated if present.

Turn off the aerators to allow settling in the reactor.

No or Low Flow.

Normal diurnal variation.

Upstream blockages.

Find and clear any blockages.

Low pH (< 6.7).

Low alkalinity influent.

Acidic influent.

Low flows resulting in fermentation in the sewer upstream of the plant.

Check for trade waste discharge (e.g. acid clean or spill or possible dumping).

Correct with alkali dosing as required (e.g. caustic, lime or magnesium hydroxide).

141

Chapter 13 - Trouble Shooting Table 13.1 (continued). General activated sludge trouble shooting guide.

Symptom

Cause

Action

Uneven air pattern, quiet areas or air rising in clumps.

Blocked diffusers.

Inspect the diffusers.

In situ formic acid vapour clean (see Chapter 8).

Cycle full air flow on and off several times to try to clear the blockage (see Chapter 8).

Excessive DO (> 2.5 mg/L) in the aeration tank.

Over-aeration. This may not affect the process but will waste power.

Toxic event resulting in a low OUR by the biomass.

Check the blowers and aeration equipment.

Clean and calibrate the DO probes.

Check the response of the DO probe to saturated mixed liquor (high DO) and a sample from a sludge lagoon or anaerobic zone (very low DO).

Review the DO control program.

Measure the OUR and compare with historical data (see Chapter 7).

Check the effluent quality. A toxic event will often show as an increase in ammonia concentration at the end of the aeration zone due to inhibition of nitrification.

Check any trade waste discharges.

Low DO (< 0.5 mg/L) in aeration tank.

Insufficient aeration.

High organic loading.

Check the blowers and aeration equipment.

Clean and calibrate the DO probes.

Increase the frequency on the VSDs if present.

Increase the number of blowers or aerators online.

Review the DO control program.

Check for anything unusual in the sewage catchment.

Test the raw sewage and check against historical data.

Check the effluent quality, in particular ammonia levels.

White foam covering more than 25% of the surface of the reactors.

High organic loading.

Toxic or inhibitory materials entering the plant.

Detergent or surfactants entering the plant.

Low MLSS.

Check the DO is within normal range of 1 - 2 mg/L. Check any trade wastes.

Test the raw sewage and check against historical data.

Measure the OUR and compare with historical data (see Chapter 7).

Check the MLSS and wasting.

Brown foam covering more than 25% of the surface of the reactors.

Low organic loading (Low F:M ratio).

Long sludge age.

Lower than “normal” wastewater inflow.

Check the wastewater inflow.

Review target sludge age.

Examine the biomass under a microscope and if possible identify the filamentous microorganisms if present.

Increase the DO set point.

Increase wasting.

If foaming persists, consult a wastewater treatment specialist.

Very dark or black foam.

Insufficient aeration.

Excessive organic load.

Long sludge age.

Insufficient DO - see above.

Test the raw sewage and check against historical data.

Check and reduce the sludge age.

Reactor Tanks

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Chapter 13 - Trouble Shooting Table 13.1 (continued). General activated sludge trouble shooting guide.

Symptom

Cause

Action

High effluent ammonia concentration.

Increased influent ammonia loading.

Loss of nitrification.

Insufficient or inefficient aeration.

Low alkalinity.

Low pH. Low reactor temperature.

Short sludge age (insufficient MLSS).

Toxic or inhibitory materials entering the plant.

Fats, oil and grease entering the plant.

Insufficient aerobic mass fraction.

Check the DO. If lower than usual, ensure the aeration equipment is operating correctly.

Increase the DO set point.

Check the oxygen mass flow meter (if fitted) is calibrated correctly.

Check the effluent alkalinity. If the alkalinity is < 60 mg/L, dose with alkali (caustic, lime, magnesium hydroxide). Check the solubility prior to dosing.

Check the influent ammonia concentration (the event may have already passed unless online monitoring is in place). If higher than normal levels, increase the DO set point on the blowers. Remember to reset the DO set point after the event has passed.

Directly control the plant based on online ammonia measurement.

Check the reactor temperature. If low increase the sludge age.

Check and increase the sludge age.

Check any trade wastes.

Check the tank surface for fats, oils and grease or excessive foaming.

View the mixed liquor under a microscope and compare with previous observations.

High effluent nitrate.

Excess aeration.

Insufficient anoxic mass fraction.

Insufficient carbon. C:N ratio < 8:1.

Two stage process incapable of meeting license requirements. TN < 10 mgN/L is difficult to achieve with a two stage process even with sufficient carbon.

Reduce the DO set point.

Increase the anoxic mass fraction to a maximum of 40% of the total reactor volume by switching aeration zones air off and using pulsed air, or install mixers, to keep the biomass suspended.

Add a carbon source such as methanol, ethanol, molasses or sucrose or a suitable industrial waste to the anoxic zone.

Review plant configuration and alter to a four stage system as a last resort.

High effluent TKN.

Industrial waste containing high soluble undegradable TKN. Solids carry over.

Effluent solids contain approximately 7% TKN.

Measure effluent soluble TKN in the filtrate from the TSS test.

Check and test trade wastes.

Review solids settlement performance (see clarifier section below).

Effluent

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Chapter 13 - Trouble Shooting Table 13.1 (continued). General activated sludge trouble shooting guide.

Symptom

Cause

Action

High effluent P (EBPR plant).

Low C:N:P ratio.

Insufficient anaerobic volume (should be around 10% of the total volume).

DO carry over to anaerobic zone.

Nitrate carry over to anaerobic zone. > 3 mgN/L in the Internal Nitrate Recycle.

Solids carry over in the effluent. Effluent solids contain between 2.5% 15% P. Re-release of P in the return stream from solids processing.

Add carbon (methanol, ethanol, molasses, or sucrose), nitrogen (urea) or phosphorus (superphosphate) to establish the correct ratios (see Chapter 7).

Review zone setup and plant configuration.

Review aeration control. Ensure tapered aeration if possible to limit DO carryover.

Review clarifier performance (see clarifier section below).

Ensure the waste sludge is always aerobic until removed from the site. Do not digest or thicken WAS anaerobically if a liquid return stream is present.

Dose the return stream with alum or ferric to remove the P.

High effluent TSS.

Short sludge age.

Long sludge age.

Toxicity.

High organic loading.

Solids flux overload in the clarifier.

Check and adjust the sludge age.

Check trade wastes.

Test raw sewage and check against historical data.

Increase the aerobic cycle.

Check the solids flux in the clarifier. Adjust RAS control, or decrease the sludge age.

Particulate TSS in effluent due to rapid settling of sludge blanket.

Long sludge age.

Low F:M ratio.

Decrease the sludge age.

Cloudy effluent.

Low MLSS < 1,200 mg/L. DO < 0.5 mg/L. Temperature < 100C. High organic loading.

High TN load.

Nutrient limitation.

Increase the sludge age.

Increase the DO set point.

Check trade wastes.

Test raw sewage and check against historical data.

Add carbon (methanol, ethanol, molasses, or sucrose), nitrogen (urea) or phosphorus (superphosphate) to establish the correct ratios (see Chapter 7).

Test raw sewage and check against historical data.

Excessive BOD or COD in effluent.

High organic load.

Insufficient aeration.

Toxic inhibition of the biomass .

Test raw sewage and check against historical data.

Check the DO probe and operation of the aeration system.

Increase the DO set point.

Review the aeration control program.

Effluent ammonia is likely to increase so monitor effluent ammonia regularly, preferably continuously.

Effluent (continued)

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Chapter 13 - Trouble Shooting Table 13.2.

Additional trouble shooting guide for intermittent plants.

Symptom

Cause

Action

MLSS different in the different tanks.

Unequal flow distribution.

Unequal load distribution.

Measure individual flows and correct if possible.

It is very likely that one tank will always receive flow during the peak load period. Try offsetting the times somewhat overnight to share the load. Alternatively set up different cycle times in the different tanks.

High TSS content in the effluent.

Solids carryover from the reactors.

Hydraulic overload.

Poorly designed decant system.

Check the sludge age and adjust if necessary.

Increase settling time to allow more solids to sink to the bottom.

Increase decant time to allow slower decant of the necessary volume.

Check influent flow rates to the cells.

Review the decant system approach velocities.

SBR or IDEA Tanks

If significant problems occur that are not covered in the table, advice should be sought from a wastewater engineer, or other experienced plant operators.

2. Control of Bulking and Foaming A summary of the main methods that have been used to control bulking and foaming is provided in Table 13.3. There is no single method that works. There are also considerable site specific differences in responses to the control measures.

Table 13.3.

Summary of the main control methods for foaming and bulking.

Control Method

Comments

Non Specific Do nothing.

In the case of bulking, some plants can operate at a high SVI and still produce compliant effluent as long as foaming doesn’t occur at the same time or there is significant solids carryover in the clarifier or IDEA/SBR.

Chlorinating the RAS up to 8-10 mg/L max continuous or up to 60 mg/L slug dose.

Care must be taken to ensure this doesn’t adversely affect the biomass or result in loss of nitrification or denitrification. Regular measurement of OUR can be useful in detecting adverse impacts on the biomass.

Use of chlorine is also likely to result in production of disinfection by products in the final effluent.

There may be OHS considerations in using chlorine.

If used, start the dosing with a low concentration of chlorine and observe the impact on the foam. Increase the dose as required.

Continuous dosing of low concentrations appears to be more successful than a single high concentration slug dose and less risky.

Spray chlorine onto the foam directly.

Use a 1% solution of sodium hypochlorite. (Concentrated sodium hypochlorite is approximately 12.5%). See also comments above regarding OHS and disinfection by products.

Spray with water.

This will not control the filamentous organisms but may reduce overflow into the final effluent or even out of a reactor onto walkways.

Spray with anti-foam chemicals.

Not very successful and relatively expensive.

Addition of specific bioadditives.

Generally unsuccessful although some poorly substantiated claims are regularly made.

Expensive.

Foam and scum harvesting.

Expensive but has the advantage that as well as removing foam it also wastes biomass. (see Chapter 3).

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Chapter 13 - Trouble Shooting Table 13.3 (continued). Summary of the main control methods for foaming and bulking.

Control Method

Comments

Specific Increase the DO to at least 2 mg/L.

Note that in the case of foaming, increased aeration can in fact worsen the problem because the gas bubbles get trapped in the foam and make it “grow”, however increasing the DO is commonly used with some success.

Decrease the sludge age.

This is commonly used with some success.

Preferentially waste foam.

Good concept but not that easy to do. Foam harvesters can be installed on aeration tanks (see Chapter 3). Makes determination of sludge age difficult.

Check trade wastes and limit sources of oil, grease and sulfide.

Oil, grease and sulfide encourage particular types of filamentous bacteria. Accurate identification of organisms in the foam can suggest this as an appropriate action.

Check nutrient ratios. Add carbon (methanol, ethanol, molasses, or sucrose), nitrogen (urea) or phosphorus (superphosphate) to establish the correct ratios (see Chapter 7).

Not usually a problem with domestic sewage but more likely to be a problem where large amounts of trade wastes are introduced to the system.

Can have a remarkable effect particularly with thick sticky foams.

Supplement trace elements such as iron, Mn, Mg.

Can have a remarkable effect particularly with thick sticky foams.

Install aerobic, anoxic or anaerobic selector zones.

Expensive option.

Theory is that mixing RAS with incoming wastewater in a selector creates a high F:M ratio which favours floc forming bacteria.

Aerobic, anoxic and anaerobic selectors have been tried with generally inconsistent results.

3. Clarifier Trouble Shooting Table 13.4.

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Clarifier trouble shooting guide.

Problem and Cause

Possible Solutions

Uneven flow distribution in plants with more than one clarifier results in hydraulic overload to one of the clarifiers. This in turn results in poor effluent quality and consistently higher sludge blanket level in one clarifier even though the sludge management settings are identical.

Check the flows. Visual assessment of the flow through the weirs may provide evidence of differences.

Flows to the clarifiers can sometimes be measured by lowering the level in the clarifier(s) using the RAS/WAS pumps and then measuring the rise rate and calculating the volume added in the time of the measurement.

If flow distribution is via a flow splitter or flow distribution box, it may be possible to adjust weir levels or feed valves.

Check overflow rates and weir loading rates to ensure the clarifier isn’t permanently overloaded (see example calculations in Chapter 12).

Uneven weir loading due to uneven clarifier weir levels causing more flow through one area of the clarifier resulting in poor effluent quality.

If possible, level the clarifier weirs.

Erratic scraper arm movement possibly due to damaged scrapers, build-up of compacted sludge on the bottom or foreign objects such as bricks.

Drain and inspect the clarifier and scraper arm.

Chapter 13 - Trouble Shooting Table 13.4 (continued). Clarifier trouble shooting guide.

Problem and Cause

Possible Solutions

Erratic or no bridge rotation can be caused by foreign objects blocking the bridge wheel, unevenness in the clarifier ring beam or wheel slippage due to rain.

Bridge rotation is often monitored by a proximity sensor arrangement and a timer so that an alarm is generated if the bridge doesn’t do a full revolution in a certain time period.

It is important to respond to these alarms quickly as it can adversely affect the treatment process and final effluent quality.

Foreign objects can be removed by a brush or scraper that is fixed ahead of the bridge wheel.

Unevenness in the clarifier ring beam can be fixed by surveying and then grinding or building up the problem areas.

Wheel slippage can be fixed by providing more friction in the problem areas.

Recycle Problems “Rat holing” can occur when the RAS flow creates a channel through the sludge blanket and returns the clarified effluent above the sludge blanket.

Rat holing can be detected by measuring the RAS SS concentration. As a general rule, the RAS SS will be approximately twice the MLSS assuming a RAS flow equal to the average inflow i.e. RAS ratio 1:1. If the measured RAS SS is significantly less than this, the most likely explanation is rat holing.

If rat holing is suspected, reduce the RAS by 5% - 10% and assess the effect by monitoring the RAS SS concentration at intervals. It may be necessary to turn the RAS off for a period to allow the “rat hole” to repair.

If rat holing is a persistent problem, installation of a larger diameter RAS pipe might be necessary. A larger diameter pipe means slower flows and less disruption of the sludge blanket.

It is also worth checking the scraper (if there is one) to ensure it is working properly.

Recycle Problems In clarifiers with a siphon suction system, the suction lines are prone to loss of suction if the RAS gets too thick which ceases the operation of the RAS.

Determine the minimum rate where suction is lost and operate at a flow rate 5% greater than this as the minimum RAS flow rate. This flow should be checked against the clarifier solids flux calculations to ensure the clarifiers are operating in the appropriate range so as not to overload the clarifiers (see Chapter 12).

Rising Sludge In nitrogen removal plants, bubbles of nitrogen caused by denitrification in the clarifier can carry floc to the surface of the clarifier.

Sometimes the bubbles are due to air attached to the floc from the aerobic bioreactor. In this situation it may be possible to reduce the aeration somewhat; however the primary treatment objectives must not be compromised.

Optimise the process to maximise denitrification rather than allow this to occur in the clarifiers. Use of a process simulation model may help (see Chapter 15).

Increase the RAS so that the solids are not held in the clarifier as long.

Pin Floc Pin floc may result from an over long sludge age and old sludge or possibly from a toxic shock event.

Check the required sludge age and sludge age calculations.

Reduce the sludge age.

Dose polymer into the pipe between the reactor and clarifier.

Examine the microscopic appearance of the biomass for healthy protozoa. If absent, the sludge age may need adjusting or there may have been a toxic shock event.

Billowing Sludge Sometimes turbid areas are apparent. These may be always located at the same position or the position may vary.

Possible causes include: • Where the delivery pipe passes through the clarifier (see Figure 12.5), holes in the pipe may cause jetting into the settled sludge.

• Poor operation of sludge scrapers, a faulty scraper or too high scraper speed.

• Localised thermal currents generated by the effect of direct sunlight on the clarifier.

• Hydraulic surges that result from widely variable inflow.

• Extreme wave action on the clarifier due to wind.

Regularly inspect and maintain the scraper systems.

Decrease the rate of rotation of the scraper arm if this is possible.

Inspect the delivery pipe.

Keep the sludge depth as low as possible.

Paint the clarifier with reflective paint. This is particularly relevant for metal clarifiers.

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Chapter 13 - Trouble Shooting Table 13.4 (continued). Clarifier trouble shooting guide.

Problem and Cause

Possible Solutions

Anaerobic Clumps Dark clumps of old sludge may rise to the surface. This may indicate a fault in the sludge scraper system. Sludge left in the bottom of the clarifier becomes anaerobic. Anaerobic bacteria generate methane that can cause the sludge to rise in clumps.

Regularly inspect and maintain the scraper systems.

Increase the rate of rotation of the scraper arm if this is possible.

Even though a plant may be operating well with suitable RAS and WAS rates, over time some compacted sludge may build up on the bottom of the clarifier. This can be assessed by probing the bottom of the clarifier with a length of conduit.

Compacted sludge can usually be felt on the bottom. If this is the case, it may be necessary to take the clarifier offline and thoroughly clean it.

View a sample of foam under a microscope and identify any Surface Foam filamentous bacteria. Develop a strategy for control. (see Foaming can occur for two main reasons, filamentous bacteria or detergents. Detergent foam is crisp white in colour Table 13.3). while filamentous foam tends to be greasy and grey looking.

High Turbidity Due To Bulking Sludge

View a sample of mixed liquor under a microscope and identify any filamentous bacteria. Develop a strategy for control. (see Table 13.3).

Effect of Wind Strong winds can affect the flow pattern in larger diameter clarifiers. This can create problems for removal of scum and may cause billowing (see above).

Fencing or hedging can reduce the effect of wind.

Covering the clarifier may reduce this problem.

Effect of Temperature Paint the clarifier with reflective paint. This is particularly A significant temperature difference between the clarified relevant for metal clarifiers. effluent (top layer) and the sludge on the bottom of the clarifier can lead to lifting of the sludge layer.

Local warming on one side of a clarifier can cause generation of thermal currents in the clarifier which results in billowing and solids carry over.

148

Blocking or ragging of RAS pumps Depending on the level of screenings removal at the STP, there may be screenings present in the RAS that can foul RAS pumps.

The obvious solution is to improve the screening but this may be expensive and may take a long time.

Program a flushing/reversing cycle into the RAS pump controls several times each day so that the RAS pump(s) increase up to 100% speed and/or run in reverse to clear any small blockages before they become big enough to foul the pump(s).

Algal Growth Algae often grow quickly on clarifier weirs and can block V notches (Figure 13.1). Blocked V notches alter weir overflow patterns causing more flow through one area of the clarifier than other areas resulting in poor effluent quality.

Attached algae itself is not really a problem, however algae sloughs off and adds solids, BOD and nutrients to the final effluent which may lead to a non-compliance.

Clarifier brush systems work to some degree however additional manual cleaning is usually required.

V notches need to be kept clear with high pressure water sprays.

Surface spray with chlorine. Full strength hypo (~12.5%) can be sprayed using a spray pack and left for 15 minutes. The dead algae can then be pressure washed off.

Covering the launders to stop the growth of algae. Figure 13.2 shows a permanent cover over the launder and weirs and Figure 13.3 show some results of a simple experiment to cover clarifier weirs.

Chapter 13 - Trouble Shooting

Figure 13.1. An example of V notches badly fouled with algae.

Figure 13.2. An example of a clarifier with permanently covered launders and weirs.

Figure 13.3. The effect of covering the overflow weirs of a clarifier. The difference in algal growth between the uncovered section (left) and covered section (right) is dramatic.

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Chapter 14 Online Monitoring and Instrumental Control The influent to a Sewage Treatment Plant (STP) varies on a daily basis and is related to water usage patterns in the community supplying the plant. Likewise, heavy rains can lead to massive increases in flow.

What is less well known is how much the composition of sewage varies. Chemical Oxygen Demand (COD) concentrations vary, again often reflecting water use patterns in communities. Poorer countries with lower water availability have strong sewage (high COD concentrations), whereas in communities with no water shortage, the sewage tends to be weaker. Even in Australia during the drought, water saving restrictions resulted in decreased flows to STPs and increased concentrations of many of the components of the sewage.

The nature of the COD varies. The proportion of Readily Biodegradable COD (RBCOD) to total COD varies. A community with food industries can have higher percentages of RBCOD. Other industries produce COD that is poorly biodegradable. The more industries present, the greater the variability of the waste, and because industry tends to discharge intermittently, slugs of sewage with quite different composition can enter a plant.

Nitrogen comes from both domestic and industrial sources. Nitrogen can be in a complex organic form as large molecules, or smaller molecules such as urea and ammonia. Take a bucket of fresh urine. It may have a slight smell of ammonia but leave it overnight in warm conditions and the stench of ammonia is memorable.

Fresh sewage will have higher organic nitrogen content than aged sewage where there are much higher levels of ammonia. Some of the organic nitrogen has been converted to ammonia and released as gas.

Nitrate is usually very low; however where sodium nitrate is used for odour control in a sewer, the nitrate levels can vary and at times be quite high. There may even be fertiliser plants or food industries using nitric acid discharging to the sewer.

Phosphorus levels also vary. In the past, one of the most significant factors in determining the level of phosphorus in the influent sewage was the use of phosphorus based detergents. These have generally been phased out of use in developed countries.

There are many other chemicals in sewage that vary and can have an impact on treatment. For instance, in a plant with UV disinfection as a final step, intermittent discharges from industries with difficult to digest waste with high UV absorption can reduce the effectiveness of the UV lamps. Examples are cattle yards, photographic processing companies and food companies producing caffeine or molasses.

There are also a whole range of chemicals that can poison or inhibit the biomass.

So how do you operate a plant to cater for all the variability?

If operating conditions are set up for the “average” influent, then during high loads, treatment will be inadequate and the final effluent is unlikely to meet discharge requirements. Alternatively, if a plant is operated for the influent peaks then massive amounts of energy and therefore money will be wasted except during the short peak periods.

Clearly the plant controls need to be varied to cope with the changes and only incur the necessary costs to achieve the treatment objectives at all times.

Traditionally, control of activated sludge plants has been based on Dissolved Oxygen (DO) and in the case of intermittent plants, time. Both of these are indirect measures of what is actually happening in the plant.

Data derived from laboratory analysis of grab samples for Mixed Liquor Suspended Solids (MLSS), Sludge Volume Index (SVI), pH, alkalinity, ammonia, nitrate and phosphate can be used to make changes to plant controls that will result in long term changes in the operation of the plant, but will not help with short term load changes.

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Chapter 14 - Online Monitoring and Instrumental Control Adjustment of hydraulic retention times by raising a weir in a continuous plant or increasing a cycle time in an intermittent plant can be used to allow time for the biochemical reactions to occur. However since the actual reactions are not really monitored, the effectiveness of the change is not really known. It would be much better to actually track the chemicals of interest. This is likely to yield better process control and improved effluent, sometimes at a lower cost. For example, aerator power input, which constitutes the major cost in the operation of an STP, is often unnecessarily high. The exception being during the peak load period when it may be insufficient to allow the DO to reach the required set point at all points in the bioreactor. This limits biomass activity. The result is that aeration costs are higher than they need to be and the nutrient removal capability of the plant is compromised.

Such simple control also fails to adjust to variations in load on a daily, weekly or seasonal basis. This in turn may cause the biology to change, adding another variable.

A much better result can be achieved if an instrument is used to measure the relevant chemicals parameters directly, and the information used to control the part of the process responsible for their treatment.

Consider the aeration stage. The role of the aeration stage is three fold: •• Removal of COD. •• Oxidation of ammonia (nitrification).

•• Uptake of P (in EBPR plants).

Removal of RBCOD generally occurs quite rapidly, while the oxidation of ammonia and non RBCOD takes longer. Why not monitor the removal of ammonia and once it is clear the required level will be reached, then reduce aeration regardless of the DO? To move away from basic DO control requires real time measurement of the main chemical parameters so that reaction rates in addition to absolute levels can be used to enhance plant operation.

Online instruments for a large range of chemical parameters have been available for decades. Many of the early models failed or fell into disuse for many different reasons. The newer online instruments have improved significantly and are now more reliable to operate and provide higher quality data.

Experience in the Australian wastewater industry is sadly that online analysers are rarely in operation four years after purchase. This is quite different to overseas experience where the same online analysers are widely used. To maximise the life of the instrument and ensure the reliability of the data, a knowledgeable and interested operator (and manager) and support from a knowledgeable service or laboratory technician is required.

The goal of online monitoring is to assist operators to understand their plant and operate it in a way that stabilises the microbiology.

Some of the main online instruments available for monitoring wastewater are considered below.

Influent Load Measurement (BOD, COD, CODf, RBCOD) Load is a combination of the inflow and the concentration of the various chemical parameters in wastewater.

Measurement of influent load is fundamental to managing the entire plant. Nutrient removal processes are driven largely by availability of carbon for phosphorus removal, denitrification and biomass growth. No process can be totally effective without knowing the inlet loading in real time. The great thing about measuring inlet loads in real time is it allows an operator to reduce operational cost while improving effluent quality. In some ways it is like how an efficient fuel management system in a car delivers both power and fuel economy.

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Chapter 14 - Online Monitoring and Instrumental Control For example, a feedback loop controlling carbon dosing for denitrification uses nitrate levels in the main internal recycle. In order to work effectively, the control system must know both the amount of carbon entering the plant and the C:N ratio. If the carbon load is already high at the inlet, it is likely that no carbon needs to be added. If inlet carbon is very low, the amount of carbon needed to drive denitrification is much higher and is often added at high cost.

Inlet loads may peak at many times the average concentration for periods of an hour or more during industrial discharges. When this occurs, the same amount of biomass is expected to consume many times as much carbon in the same time period. This is unlikely to happen unless the amount of biomass present is increased during the high load conditions, for example by increasing the RAS. To achieve this requires sufficient pre-warning of high load conditions to allow the operator to take action or an automated control system to respond. Prior warning of load changes is essential for good control. Online measurement of the inlet load measurement provides this.

How is it Measured?

COD or COD equivalent Inlet load can be measured using biological, chemical or UV/Vis absorbance methods to measure COD or a parameter equivalent to COD. UV/Vis absorbance methods have become the most common as they require the least technical knowledge, lowest maintenance and no chemicals. As with any system it is important to understand what it will and will not measure. Not all chemicals absorb in the UV/Vis range and some chemicals such as methanol, that contribute to load, will not be measured.

The systems that use UV/Vis absorbance usually come loaded with factory software that calculates COD equivalent. They all work reasonably well for influents with little or no industrial components however, tend to under-read or over-read during periods of industrial discharge. The instruments can sometimes be customised based on specific sampling regimes targeting these industrial discharges. They clearly reveal periodic discharges from industry making trade waste management easier. In one example, blood components from a meat processing plant were easily detected. The important thing in this example was not whether the discharges were legal or not, but rather that the discharges were characterised by a low C:N ratio but high N load which exerted an increased oxygen demand. If sufficient oxygen wasn’t provided, the final effluent was out of specification for N. This discharge also has a higher carbon requirement for denitrification possibly requiring carbon addition.

Blower Speed or Air Flow Much can be learned from blower speed and air flow trends. When the blowers are on automated DO control, an STP is effectively a real time BOD analyser! The blower speed, power or air flow trend will be representative of the loading. When the food load is higher, the oxygen demand increases. The blower speed will therefore increase to maintain a given DO value. Watching the DO trend alone when it is on set point control gives the false impression of a stable load. By watching the blower power or speed trend or air flow, you will see how much the load is varying and how close you are coming to maximum blower capacity during morning high load periods.

Figure 14.1 shows a trend for COD mass load and the air flow from the blowers. There is a clear link between the inlet load and the air flow. The DO trend would have been much less informative and not as well correlated.

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Chapter 14 - Online Monitoring and Instrumental Control

Figure 14.1. A one month trend from a large continuous plant (> 1M PE) showing inlet mass load (kg COD) and air flow (m3/h) from the air supply system.

By knowing the typical daily trend, an operator can rapidly detect when industrial shock loads are present (Figure 14.2).

Figure 14.2. A three day trend showing blower header pressure in black and reactor DO in green.

The two shock loads are easily recognised as rapid increases in blower header pressure. The DO value only drops during the first event when the blowers topped out at 100%. The DO profile along the aeration tank was very different during these events resulting in ammonia being present in the effluent.

By monitoring blower speed, air flow or header pressure, it is also possible to see how much of the time the blower is at minimum speed. An elevated DO value is sometimes seen at low load because the blower speed cannot be turned down enough. This is effectively a waste of power and should stimulate investigations into the blower setup and controls.

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Chapter 14 - Online Monitoring and Instrumental Control Trends from intermittent plants (SBR or IDEA) are a little different as the blowers often run at 100% of capacity at the beginning of each cycle. This is caused by the DO being zero after the settle and decant periods. While this is expected in an intermittent plant, what is important is how long after the aeration cycle starts before the blowers ramp back to slower speeds and what the shape of this trend is. The blower power trend on these plants is very instructive. A series of steps will be seen. These steps represent the points where the biomass completes consumption of the carbon, then the ammonia, and then finally drops down to the endogenous respiration rate. The blower speed trend for each batch will vary as a different load is contained in each batch. As a rule of thumb, blower speeds should not be at a maximum value for over 30 minutes during “normal” cycles. If this is the case, the blower setup should be reviewed. Recording and reviewing blower trends as weekly overlays is very instructive when optimising DO control.

Dissolved Oxygen Oxygen is a fundamental requirement for aerobic life. The higher the DO level, the less it limits aerobic biological activity. Low DO limits the rate at which nitrifiers can convert ammonia to nitrate. Low DO also favours microorganisms that can obtain oxygen easily such as filamentous bacteria and this can result in bulking or foaming.

Under high load conditions, the biomass is very active and consumes more oxygen than when it has less food. This means that for a constant DO in the water, a variable DO is seen in the different layers of the floc.

This is a very important point as the biomass sees only its local conditions. In addition to the DO in the water, the operator needs to understand the loading on the plant as this defines the rate of oxygen consumption and provides an understanding of the conditions the biomass in the floc is seeing. A change in the DO within the floc will cause a change in the microbiology of the floc.

If DO is too low, the ammonia is generally not fully removed, while if DO is too high, this generally results in excessive nitrate levels. The relative loads and DO levels at different times of the day explain the quite commonly seen periods of excess ammonia followed by periods of excess nitrate in the plant effluent.

In general terms, DO levels above 2 mg/L in any area of the bioreactor effectively wastes power because the rate of improvement in the rate of nitrification is quite limited above 2 mg/L. Measurement of Dissolved Oxygen in Wastewater A large number of DO sensors are sold in the wastewater market. Most operators swear by particular brands.

This is often based on the level of maintenance required and the consistency of their readings compared to the set point. The problem here is that neither of these criteria are good indicators of the quality of the data.

A summary of the major types of DO sensors and their features is provided in Table 14.1.

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Chapter 14 - Online Monitoring and Instrumental Control Table 14.1

Types of DO sensors and their features.

Fluorescence

Polarographic - often called Membrane

Open Electrode- often called “Zullig Type”

Measurement mechanism

Oxygen permeates through a gas permeable membrane into a ruthenium layer and alters its reaction to an internal light source.

Oxygen permeates through a gas permeable membrane and is reduced on the gold cathode. Current flow relates to the rate of oxygen reduction and therefore the amount of oxygen.

Oxygen directly oxidizes (rusts) the iron or zinc anode generating a current.

Mechanical grinding maintains fresh electrode surfaces in direct contact with the wastewater.

What is being measured

Either time between activation and emission, or intensity of fluorescence emission.

Current flow.

Current flow.

Best installation

Surface aeration systems.

In fine bubble membrane systems, air bubbles contacting the membrane give false high reading. An offset may need to be applied.

Surface aeration systems.

In fine bubble membrane systems, air bubbles contacting the membrane give false high reading. An offset may need to be applied.

At optimal measurement point and depth.

Worst installation

Close to diffusers.

High fat/oil applications e.g. industrial influents.

Abrasive applications.

Close to diffusers.

High fat/oil applications e.g. industrial influents.

Abrasive applications.

Influent containing lots of hair (livestock saleyards) or other screenings.

Sites with electrical interference e.g. cathodic protection present or poor site earthing.

Advantages

Robust and low cost.

Low cost.

Robust.

Disadvantages

Interference from direct transfer of oxygen from air bubbles through the membrane.

Biofouling of the membrane unless fitted with a wiper.

Interference from direct transfer of oxygen from air bubbles through the membrane.

Biofouling of the membrane.

Grindstone must not be turned off if the sensor is removed from wastewater.

Grindstone must be run when submerged.

Hair can accumulate on the rotor or tangle in the grindstone if not well maintained.

Calibration method

Air or saturated water.

Air or saturated water

Saturated wastewater

Recommended calibration frequency

Weekly membrane wipe.

Weekly membrane wipe.

Two weekly check for fibrous material wrapped around the sensor head.

General reliability of the result

Medium.

Medium.

High.

General reliability of the instrument

High.

High.

High.

The first point to note is that DO sensors do not actually measure DO in the liquid. With fluorescence and polarographic sensors they actually measure oxygen gas that has permeated through the sensor membrane.

Why does this matter? Primarily because there is no way the sensor can tell whether the oxygen came from fine bubbles hitting the membrane face or from the liquid itself! During aeration, particularly with diffused air systems, the sensor is immersed in a seething cauldron of fine air bubbles so the only time you can be sure they are seeing only the DO is when the blowers are off.

This can be seen when different types of sensors, all checked as being accurate by their suppliers, are located at the one point in a STP aeration zone (Figure 14.3).

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Figure 14.3. Trend showing output from multiple, correctly calibrated (as per manufacturer’s instructions) DO probes strapped together in an aeration tank.

Despite all yielding correct values in saturated water and in zero DO solution, the readings in the process vary dramatically. The differences here would result in very different operational costs and nutrient removal performance. It is also important to note that the size of the difference between the different meters is not constant but rather varies with the amount of aeration applied.

Measurement Scenarios The scenarios discussed below relate to a system with a DO set point of 2 mg/L. The two most common scenarios are: •• The meter reads low because the sensor has a biofilm on it.

•• The meter reads high due to direct transfer of oxygen from fine bubbles contacting the sensor face.

If the meter reads low… what are the implications?

If the meter is reading 1 mg/L DO when there is actually 2 mg/L DO in the aerobic zone, the aeration system will increase, or attempt to increase the airflow to achieve the 2 mg/L set point. This effectively wastes power as the blowers run harder when it is not required.

The cause is usually a poorly calibrated DO probe or a probe that isn’t clean. In the case of a dirty probe, oxygen transfer into the sensor is limited.

As a check on aeration, an ammonia profile can be made by measuring ammonia in a grab sample every 4 hours through the day from the outlet of the aeration zone to ensure the ammonia is between 0.5 mgN/L and 1 mgN/L at all times. Lower ammonia levels generally indicate over aeration. Higher levels generally indicate under aeration. Daily composites hide these issues. Ammonia profiles should be prepared every few months or when treatment problems are suspected.

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Chapter 14 - Online Monitoring and Instrumental Control If the meter reads high… what are the implications?

If the meter is reading 2 mg/L and the DO is actually only 1 mg/L, the rate of nitrification will be slower than optimal, resulting in high ammonia levels at the end of aeration.

The problem is often not noticed in a daily composite ammonia sample as most plants over aerate at night, reducing the ammonia level in the daily composite to within compliance levels. This is most likely to happen through poor calibration or excessive oxygen transfer from fine bubbles into the sensor.

Elevated DO values for several hours when the blower input power is low but incoming load is normal may be an indication of toxicity affecting the biomass. This is common at sites with industrial plants in the catchment.

Knowing the expected relationship between blower power and inlet load, or overlaying this week’s trend in real time over last week’s trend, provides a good indication of this type of event.

DO Control Generally one or more DO probes are installed within the aerobic zone. For small plants < 1.5 ML/d, a single DO probe is typical.

The operator defines a set point in the SCADA system and the PLC works to maintain the set point by operation of the aerators, blowers or air control valves to the diffusers. As aeration power is usually the largest operational cost for most STPs, the location of the DO probe/probes, the type of control and the sensitivity of the control system, along with the sensor choice should reflect the significance of the job they do. Correspondingly, the aeration system must have a wide enough operational band and sufficient controllability to ensure that DO values over 2.5 mg/L do not occur, particularly in the clarifier end of the aeration zone.

Location of DO Probes The location of the DO probes in a plant is very important. A well-mixed aeration tank does not mean that the DO is the same everywhere.

A DO trend from an IDEA tank with 2 separate DO probes is shown in Figure 14.4. One DO probe is nearer the inlet than the second probe. The trend shown in blue is from the probe closest to the inlet of the tank. The aerators in the tanks have fixed aeration on times.

Figure 14.4. DO trends from an IDEA plant with 2 DO probes. DO probe 1 (blue) is the sensor nearest the inlet.

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Chapter 14 - Online Monitoring and Instrumental Control The trend shows that: 1. During the high load period (cycles 4, 5 and 6), the DO is lower near the inlet to the tank.

2. During the low load period in the early morning (cycles 1, 2 and 3) the DO rises rapidly and stays very high during the majority of the cycle, thereby wasting power.

3. During the high load period around midday and the early afternoon (cycles 4, 5 and 6) the DO struggles to rise above 1 mg/L for most of the cycle.

Based on the recorded DO levels, higher nitrate levels are likely for cycles 1, 2 and 3 where the DO is high during most of the cycle, while high ammonia is likely where the DO remained low at the end of the aeration cycle (cycles 4, 5 and 6). Shortening aeration cycles 1, 2 and 3 and lengthening cycles 4, 5 and 6 would improve the quality of the discharge and reduce the overall cost of aeration.

A plot of DO along the aeration tank of a continuous plant (Figure 14.5) shows how the DO varies with the Oxygen Uptake Rate (OUR) at each point. The DO set point is 1.6 mg/L. Note that only in sector 5 is this achieved. Sectors 1 to 4 are well below the set point while sectors 6, 7 and 8 are well above the set point.

This means that while the biomass concentration is relatively even all over the tank, the DO levels are not.

Figure 14.5. DO profile along an aeration tank in a continuous plant.

So the lesson is, DO NOT ASSUME FULL MIXING FOR ALL PARAMETERS even though the biomass seems evenly mixed!

Sensor Fouling Most sensors are immersed for long periods. One of the effects of this is that they can become scratched by abrasive particles in wastewater, particularly after high flow wet weather events. Minute surface damage on sensors encourages growth of biofilms. Fouling is generally claimed to not happen and if we believe the outcomes of automated calibration routines, it doesn’t. But as with non-stick fry pans we know they become damaged over time. To expect them to be just as non-stick after 6 months continuous use is not realistic! The longer the membrane “lasts” in the process, the greater the surface damage and the greater the potential for fouling to occur.

The effect of fouling can be seen when trends of blower power necessary to maintain a DO set point from consecutive days are displayed together on the same trend (Figure 14.6). The impact of cleaning of the sensor on the blower power can also be easily seen.

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Figure 14.6. Effects of sensor fouling on blower power on 5 consecutive days with each day overlaid on the previous days.

The sudden decreases in blower loading seen on two of the days around 2pm (14:24:00) corresponded with the sensors being physically wiped to evaluate the effects of fat or bio-fouling. Note that the blower power decreased dramatically each time, despite being only 3 days apart. This indicates significant potential for aeration power savings simply by cleaning the sensors. The data suggests that the DO probes should be physically cleaned at least every two days to minimise power wastage. Unfortunately, fouled sensors are all too common in treatment plants.

So why is this issue not recognised?

The main reason is that the DO system is on set point control. The set point is nearly always met with the blower power changing as fouling increases. So everything seems OK to the operator. Watching the blower air flow, or blower power, or blower current trend and comparing it to a historic average for the same day of the week is much more educational than looking at the DO value when it is on set point control!! When you get sudden changes from “normal” ask why? And keep the sensors clean.

So how do we know we have a good DO reading?

The reality is you don’t. Ongoing experience with diurnal blower trends combined with results of tests for nutrients coming into and leaving the STP are critical to managing DO set points.

So does this mean DO probes have no value in the process?

No, it just means you have to understand their limitations and use them as comparative devices.

The value of DO data will always be highest where the sensor is installed in the optimum position. To determine the optimum position in a particular wastewater process requires a clear understanding of the oxygen transfer process.

Positioning the DO sensor at a point in the wastewater where the DO reading will be most affected by changes in incoming load will provide optimum sensitivity to plant oxygen uptake rate changes. This will result in maximum blower response to changing load and will therefore provide the most information on the wastewater present at the time. The optimum installation point is rarely on the surface as the maximum oxygen transfer rate in a fine bubble aeration system is at depth. For surface aerators, which are effectively point aerators, the optimum measurement point tends to be about 3 m out from the point of impact of the water spray and approximately 1 m down below the surface.

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Chapter 14 - Online Monitoring and Instrumental Control Continuous plants with a significant recycle volume often struggle to maintain anoxic conditions in much of the anoxic tank due to DO being too high in the tail end of the aeration zone. This is particularly true during low load conditions where DO is being measured in the front end of the process. Oxygen demand is always high near the inlet and without comparative values from the tail end where DO can rise to very high levels (Figure 14.5) over aeration results. Two sensors well-spaced along the aeration zone give a lot more detail on the oxygen uptake rates being seen in the different zones. The probe at the front end can be used for control to a varying set point. Recycle rates can be modified to prevent excessive oxygen being carried back to the anoxic zone by monitoring the second DO probe.

An additional use of the DO probe in intermittent plants is to use it to indicate the sludge blanket position. By positioning the DO probe approximately 0.5 m below normal Bottom Water Level (BWL) after decant, the DO trend can provide additional information about the decant. When the biomass settles to below the DO probe, the previously very low DO level rises as there is no longer any biomass to consume the available oxygen.

If the sludge blanket starts to lift during decant, the DO will drop indicating a potential sludge carryover problem. Some experimentation may be required to get the most appropriate depth to mount the DO probe for a given plant.

Such simple changes to aeration control can result in major operational cost savings while actually improving effluent quality.

pH and EC pH and EC probes can be used to indicate possible problems with the pH and alkalinity of the influent or possibly even contaminants in the influent that may have an adverse effect on the biomass. In this case it makes sense to place the probes at the inlet works and use the information to bypass high or low pH or EC streams.

Low pH in the aeration basin may provide warning that nitrification has or will be inhibited (for example see Figure 7.7). However, by the time the pH drops it is generally too late and high effluent ammonia concentrations will result. Changes in alkalinity, particularly decreasing alkalinity, can provide early warning of a likely decrease in pH. The alkalinity in the aeration basin or effluent should be monitored regularly as well as the pH in the aeration basin. The alkalinity at the end of the aeration basin or in the final effluent should be at least 60 mg/L and preferably > 75 mg/L at all times (see Table 11.2).

As with DO probes, different brands of correctly calibrated pH and EC probes will give different results.

Also, some probes respond more quickly than others. However, the main problem with obtaining reliable pH readings is fat or biofouling of the probes. To minimise this, the probes must be cleaned regularly, and at approximately the same interval as for DO probes.

MLSS Mixed Liquor Suspended Solids (MLSS) can be readily measured using optical sensors. These sensors are generally low maintenance and robust. The key to remember here is that they are most accurate when the biomass structure is consistent. The better the plant control and more consistent the floc, the more accurate the sensors will be. The MLSS measurement in conjunction with inlet load information can be used to determine F:M ratios. It may also be useful in detecting problems with the RAS system such as clarifier rat holing.

Solids probes can also be used in the feed to dewatering units, filtrate from dewatering units and from the inlet and outlet of digesters. The information can be used to optimise these processes.

In intermittent plants, careful positioning of the MLSS sensor can show lifting of the biomass during the decant.

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Chapter 14 - Online Monitoring and Instrumental Control The MLSS values provided by an online solids instrument will not always agree with the results from a laboratory mainly because of the different methods used to measure the parameter. This does not mean the instruments are wrong. The information from the instruments can be very useful to improve the control of plants.

Ammonia Measurement of Ammonia in Wastewater Ammonia can be measured using colorimetric, gas selective or specific ion electrode methods.

All work well but require different levels of technical knowledge and support.

A summary of the major types of ammonia sensors and their features is shown in Table 14.2.

Table 14.2.

Types of ammonia sensors and their main features.

Colorimetric

Gas selective

Specific Ion

Chemicals are added to create a coloured liquid. The intensity of the colour is then measured.

Chemicals are added to cause ammonia gas to enter the ammonia sensor.

Ammonia ions attach to a polymer matrix and produce a small voltage.

Colour

mV

mV

Advantages

Accurate and stable.

Accurate and stable.

No chemicals or pre-preparation. Low cost.

Disadvantages

Requires chemicals. Sample pre-preparation is usually required.

Requires significant technical support.

Requires chemicals. Some sample pre-preparation may be required.

Requires significant technical support.

Do not use on saline wastewater.

Badly affected by detergents and other wastewater chemicals e.g. discharges from car washes.

Potassium ions interfere.

Calibration method

Automatic against a calibration standard solution.

Automatic against a calibration standard solution.

Manual grab sample comparison or standards addition in most cases.

Recommended calibration frequency

Fortnightly

Fortnightly

Weekly 2 point check.

General reliability of result

High

High

Medium

General reliability of instrument

Medium

Medium

High

Cost

High

Medium

Low

Measurement mechanism What is being measured

Measurement Scenarios There are two possible scenarios where the ammonia measurement is in error. The online ammonia analyser may read less than the true value or greater than the true value. The scenarios below are based on a licence requirement for 1 mgN/L of ammonia in the final effluent and the ammonia being measured at the end of the aerobic zone.

If the meter reads low… what are the implications?

If the ammonia analyser is reading 1 mgN/L when there is actually 2 mgN/L, the aeration system will not ramp up to increase the DO to remove the excess ammonia. The result is the plant may fail to comply with the licence.

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Chapter 14 - Online Monitoring and Instrumental Control If the meter reads high… what are the implications?

If the meter is reading 2 mgN/L and the ammonia concentration is actually only 1 mgN/L, it is likely that the controller will set the aeration input at an unnecessarily high level. This will result in some wasting of power and probably an increase in effluent nitrate.

The risk of both these errors can be reduced by regular laboratory comparisons of the ammonia concentration and ensuring the instrument is maintained and calibrated. The samples for comparison should be taken during peak and low loads as the wider the spread, the more likely the comparison is valid. If a comparison shows a significant difference, repeat the sampling and testing before disbelieving the online equipment.

Sampling and laboratory errors do occur.

How Does Ammonia Based Control Work?

Ammonia control is often based on feedback from the outlet end of the aeration zone. The measured ammonia levels are used to control the aeration set point which in turn controls the amount of nitrification.

As the ammonia increases, the ammonia probe sends a signal to the PLC to increase the blower speed or alter the valve position. This makes more DO available for the nitrifiers to increase the rate of conversion of ammonia to nitrate.

Location of Ammonia Probes For a continuous process, the best place to monitor ammonia with an online instrument is at least three quarters of the way along the aeration zone. Measurement at this point confirms the outcome of any control action taken at earlier stages of the process by the DO systems. Over aeration will result in the ammonia level being zero. Alternatively when under aeration has occurred, the ammonia levels will remain elevated.

Knowing the ammonia is high or low this late in the process does limit the ability to fine control the process but overall, this is the best location for simple ammonia based DO set point control.

For intermittent plants the location of the probes is less critical. As long as it is in the SBR or IDEA tank, useful results can be obtained.

Online ammonia analysers can also be installed in the final effluent for compliance monitoring but not process control due to the lag times.

Figure 14.7 shows discharge ammonia data from a continuous plant which has quite variable nitrification performance and major difficulty complying with licence limits.

Figure 14.7.

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10 days of online ammonia data from the outlet of a continuous plant aeration zone.

Chapter 14 - Online Monitoring and Instrumental Control The variability seen in the figure was not due to differences in the influent but rather to problems with DO measurement as described above, and lack of effective real time control. It is interesting to note that many plants that “comply” with licence limits based on monthly or even weekly grab samples show major diurnal and daily variation similar to that shown in Figure 14.7. Regular sampling at 10.30 am for example, would fail to identify the variability.

Nitrate Measurement of Nitrate in Wastewater Nitrate can be measured using colorimetric sensors, UV absorption or specific ion electrodes.

A summary of the major types of nitrate sensors and their features is provided in Table 14.3.

Table 14.3.

Types of nitrate sensors and their features.

Colorimetric

UV

Specific Ion

Measurement mechanism

Chemicals added to create a coloured liquid. The darker the colour, the higher the nitrate level.

Calculation of nitrate based on absorption of UV light.

Nitrate ions attach to a polymer matrix and produce a small voltage.

What is being measured

Colour

UV absorption

mV

Accurate and stable.

No chemicals or pre-preparation.

Accurate and stable.

No chemicals or pre-preparation.

Advantages

Disadvantages

Requires chemicals.

None in aeration applications.

Usually sample pre-preparation is required.

Requires significant technical support.

Affected by wastewater components causing drift.

Calibration method

Automatic against a calibration standard solution.

Grab sample comparison or standard addition to wastewater.

Grab sample comparison or standard addition to wastewater.

Fortnightly

Monthly

Weekly 2 point check

General reliability of result

High

High

Medium to low

General reliability of instrument

Medium

High

High

Cost

High

Medium

Low

Recommended calibration check frequency

Measurement Scenarios Nitrate based control systems are less common than ammonia based systems. Where nitrate analysers are used, they are usually used to monitor the performance of anoxic zones and in some cases, to control carbon dosing systems.

There are two possible scenarios where the nitrate measurement is in error. The online nitrate analyser may read less than the true value, or greater than the true value.

In the scenarios below based on a generalised UCT plant (see Chapter 6), the objective is to achieve a nitrate level of < 3 mgN/L at the end of the anoxic zone to ensure that the Internal MLSS Recycle does not inhibit P release in the anaerobic zone. Previous work at the plant has shown if the Internal MLSS Recycle is < 3 mg/L, the level of nitrate in the anaerobic zone is < 1 mgN/L which is consistent with the critical limit in Table 11.2. Note that suitable levels would need to be determined at individual plants.

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Chapter 14 - Online Monitoring and Instrumental Control If the meter reads low… what are the implications?

If the nitrate analyser is reading 3 mgN/L when there is actually 5 mgN/L at the measurement point, the plant may have difficulty meeting the TN limits in the licence. In an EBPR plant, high nitrate in the Internal MLSS Recycle will inhibit P removal resulting in higher doses of alum or ferric being necessary to meet compliance.

Where the nitrate analyser is linked to a carbon dosing control system, the system will not increase the carbon dosing sufficiently to reduce the nitrate to the required level.

If the meter reads high… what are the implications?

If the analyser is reading 5 mgN/L and the nitrate is actually only 3 mgN/L there are fewer nutrient problems.

However in a system where the nitrate analyser is linked to a carbon dosing system, it is likely that the controller will set the carbon dose at an unnecessarily high level. This will result in an excess of carbon and an increase in the aeration power to remove the carbon, thereby increasing both chemical and aeration costs.

The risk of both these errors can be reduced by regular laboratory monitoring of the nitrate concentration and ensuring the instrument is maintained and calibrated. The samples for comparison should be taken during peak and low loads and in particular, when industrial influent such as food or meat processing waste or other high nitrogen waste such as fertiliser is present.

How does nitrate based control work?

The simplest nitrate control is focused on ensuring denitrification is complete. In such a system, nitrate is most commonly measured at the end of the anoxic zone. Depending on the levels of nitrate recorded, the operator can make changes to the DO set points and RAS rates to try to maintain a low nitrate level at all times.

In plants where the amount of available carbon may be limiting denitrification, the nitrate analyser can be coupled to a carbon dosing system to ensure there is sufficient carbon available to drive the necessary denitrification. In practice, such control is difficult. Depending on the source of the problem, adding carbon may not have the desired effect.

Finding high nitrate levels at the end of the anoxic zone does not tell us why it is there. There may be a number of reasons for this. Was it a surge of influent industrial nitrate overloading the anoxic zone? Is the RAS recycle low in biosolids due to rat holing in the clarifier? Are we recycling too much nitrate to the inlet of the anoxic zone for the inlet conditions? The more volume recycled the shorter the time available to denitrify it. Is the retention time too low in the anoxic zone due to abnormally high flows? Is there a lack of degradable carbon to drive denitrification? Is excessive oxygen being returned to the anoxic zone in the recycle from the aeration zone? Without a DO probe at the end of the aeration zone there is no way of knowing how much oxygen is being returned. As there are lots of possibilities for why nitrate may be high at the outlet of the anoxic zone, taking control action based on the nitrate level alone is not usually practical. As with all aspects of STP operation, the analyser cannot replace good fundamental operation and monitoring.

Nitrate can also be measured at the end of aeration zone to fine tune the DO set point and to determine the amount of nitrate being sent back by the internal recycle to the front of the anoxic zone.

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Phosphorus Measurement of Phosphorus in Wastewater Phosphate can be measured using colorimetric sensors. There are two methods, one that results in formation of a yellow colour which tends to be used for high range (0.1 mgP/L to 16 mgP/L) determination and one that results in formation of a blue colour for low range (0.05 mgP/L to 5 mgP/L) determination.

There are relatively few analysers of this type being used successfully in wastewater processes. This is mainly due to the relative complexity of the analysers and the lack of knowledge and technical capability of support staff. These instruments are best supported by a service contract with the supplier.

A summary of the two types of colorimetric analysers and their features are shown in Table 14.4.

Table 14.4.

Summary of the two types of colorimetric phosphorus analysers.

High Range Colorimetric

Low Range Colorimetric

Measurement mechanism

Chemicals added to create a yellow coloured liquid. The darker the colour, the higher the phosphate level.

Chemicals added to create a blue coloured liquid. The darker the colour, the higher the phosphate level.

What is being measured

Colour

Colour

Accurate and stable.

Accurate and stable.

Advantages Disadvantages

Calibration method

Requires chemicals. Sample Requires chemicals. Sample pre-preparation is usually required. pre-preparation is usually required.

Require significant technical support. Require significant technical support.

Possible interference from yellow humic material.

Automatic against a calibration solution.

Automatic against a calibration solution.

Recommended calibration check frequency

Fortnightly

Fortnightly

General reliability of result

High

High

Medium to low

Medium to low

General reliability of instrument

Use of Online Phosphorus Analysers There is limited potential for control of activated sludge plants based on online phosphorus measurement.

While in theory it could be used to control alum or ferric dosing, this is seldom very successful. The aluminium or iron coagulant is also consumed in coagulating any solids in the effluent as well as precipitating the phosphorus and some of the remaining organic material.

Online measurement of phosphorus is generally used for compliance reasons with sensors located in the final effluent. Even in this location it can be useful to assist in process fine tuning. Figure 14.8 shows continuous time series data for phosphate in the final discharge from an intermittent plant. The significant reduction in the phosphate level in February occurred as a result of some simple reductions in over aeration. The result was lower nitrate and phosphate in the discharge, savings in aeration power and reductions in the amount of alum dosed for chemical P removal.

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Figure 14.8. A one week trend showing phosphate levels in the discharge of an IDEA plant.

Note with one exception, the drop in P levels at the start of February.

Installation of Online Analysers Correct installation of online analysers is critical to obtaining useful data from the analyser. There are many different brands of analysers and each tends to have their own set of requirements for installation. Some general information relating to installation is provided below.

Sample Location While the instruments can be placed directly in a raw sewer main, the risk of ragging and possible breakage due to high flows is very high. In most applications, a sample of the raw sewage is provided to the instrument in a bypass configuration.

The instruments can be placed directly into mixed liquor or final effluent. However, OHS requirements associated with lifting and removing the instrument for cleaning is requiring the instruments to be installed in a bypass configuration where a sample of mixed liquor or final effluent is delivered to the instrument.

In general, to get the best out of the analyser, the supplier should be required to install the unit according to their requirements for sample delivery, sample pre-preparation and discharge of the sample after measurement.

It is also advisable to engage the supplier in a service contract for the first 12 months of operation of the instrument. If the operators are actively involved in maintenance with the contracted service person during the initial service contract, it will very quickly be evident the operators can take over the requirements at the end of the 12 months.

Instrument Turn Down No instrument can reliably measure a given chemical parameter over a large range from very low to very high concentrations. For example, to expect an instrument to accurately measure 20 mg/L and 0.1 mg/L is unrealistic. In most cases they are only really accurate over a 10:1 range. This is sometimes called the instrument “turn down”. Wastewater variability results in large variations in the parameters being measured so it is important to define the 10:1 range that is most important to control or know accurately. Then define the nice to know value. One tip is NEVER to ask for 0 to something, but rather define the lowest value required as a positive number! So for a target of 0.5 mgP/L for phosphate, a range of 0.1 to 1 mgP/L would be ideal with the accuracy dropping off slightly above 1 mgP/L but still good enough to know whether it is 2.5 or 3 mgP/L.

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Chapter 14 - Online Monitoring and Instrumental Control It is therefore essential that at the time of initiating a project involving an online analyser that a realistic assessment of the required range is made. In most cases this should be based on careful laboratory measurements of the required parameter at the proposed sampling location over a significant period of time covering high load and low load periods. In addition, a decision may need to be made as to which is more important, high level readings or low level readings. The instrument can then be selected to best cover the actual range and also ensure that the instrument range definitely covers the end of the range required for the application.

Sample Pre-preparation The most common form of sample pre-preparation is filtering. Filtering presents problems when filters become clogged and are not cleaned or changed. It is better to spend the money on dual automatically back flushed filters than lose confidence in the instrument’s performance. Blockage of sample lines is also a common problem.

Many instrument manufacturers have tried to develop instrument methods that don’t require pre-preparation and “compensate” for the solids that may be in the sample e.g. mixed liquor or raw sewage. These are only partially successful.

The following general comments provide some information about the different types of measurement methods: •• Colorimetric methods usually require pre-preparation. As these methods rely on measuring the colour of the reacted sample, any solids in the way of the light beam used to measure the intensity of the colour will tend to suggest a darker colour. The best way to obtain reliable data is to take the solids out first.

•• Ion specific, UV and gas selective methods are less susceptible to interference from solids, but in some applications, and with some manufacturers, samples may still require filtering.

Ion specific methods are very susceptible to permanent damage if detergent or some industrial chemicals bind to the measurement sites on the probe. Instrument manufacturers will normally provide specific information on problem chemicals. It is important to note that while general monitoring of raw sewage might suggest that detergent or specific chemicals may be in relatively low concentration, periodic discharge of a high strength waste may be enough to damage the electrode. For example, most car washes now recycle water and progressively concentrate up detergent residues and grit. These are then discharged as a concentrated slug. This may be enough to permanently damage an ion specific probe if it is exposed to the full concentration.

•• UV systems tend to use mathematical methods to compensate for the solids based on the absorbance information from the instrument.

•• In the case of ammonia analysers, gas selective membranes can be irreversibly damaged by precipitation of chemicals onto them. Precipitation occurs when NaOH is added to raise the pH of the sample for measurement. Ammonia forms a gas above a pH of 10.5 and can then cross the membrane into the sensor itself. Many “salts” precipitate at these high pH values and the precipitate will fill the sample chamber with salt crystals. There is no pre-preparation appropriate to prevent this. Gas selective membranes work very well on general wastewater but are just not suitable for applications with significant salt water ingress, some industrial wastes or leachate.

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Calibration of Online Analysers One of the keys to obtaining good results from nutrient analysers is good calibration. Calibration grab samples need to be taken at times when the nutrient being analysed is at a low concentration and also at a high concentration. This ensures the calibration is being done over the necessary range of concentrations in the actual wastewater rather than as a standard made up in deionised water. Where high concentrations of the nutrient being measured are rare, it is possible to spike a wastewater sample that only has low to medium levels with a pure concentrated standard.

Additional samples should also be taken for calibration if the background nature of the wastewater is known to have changed. For example, after a new trade waste discharge has been allowed into the system.

The samples need to be handled quickly and appropriately to prevent or minimise changes in the sample that can occur quite rapidly. For example, a sample taken at the end of the anoxic zone for nitrate will continue to denitrify and result in a lower nitrate concentration than the value the online analyser recorded. For this reason, samples for laboratory testing are best filtered immediately, the sample bottle completely filled to exclude air and the sample immediately placed on ice or refrigerated. Samples for certain tests require preservatives or stabilisers to be in the sample bottle at the time of sampling. It is important to follow the laboratory’s instructions about the use of stabilisers in the sample bottle.

Chapter 11 provides further information on monitoring and checking of data obtained from laboratories.

One important point is that the calibration of an online instrument should not be changed based on a single test sample. This is likely to result in a “tail chasing” exercise and loss of confidence in the analyser.

It is better to carefully consider 8 to 10 samples taken over a two week period capturing both high and low concentrations before altering the calibration.

Case Study: Ammonia Control of an Intermittent Plant The simplest control which is most easily implemented with an intermittent plant is online ammonia control.

The Moe STP in Victoria is a 20 ML/d plant consisting of 3 IDEA cells, 50 m x 20 m x 5 m deep.

Average water level is 4 m. The plant was commissioned using the standard IDEA cycle of 2 hours aeration, 1 hour settle and 1 hour decant. During commissioning, ammonia bleed through and high nitrate in the discharge presented as problems.

A post aeration anoxic phase using pulses of air from the fine bubble diffusers to keep the biomass mixed was implemented with considerable success in reducing NO3.

Figure 14.9 shows a two day trend of IDEA cell level, DO, ammonia and pH. The aeration stages can be identified by the positive DO. The decant stages can be identified by the periodic decrease in cell level. The trend clearly shows high ammonia levels during some decants.

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Case Study: Ammonia Control of an Intermittent Plant (continued)

Figure 14.9. Normal IDEA cycle showing some examples of high ammonia levels during some decants (red arrows). The vertical scale for ammonia is 0 - 14 mgN/L. The horizontal scale is 48 hours. Cell level (green), DO (red/brown), ammonia (dark blue) and pH (yellow).

Figure 14.10 shows in more detail what happens to the ammonia levels during the aeration stage. In most of the cycles, the ammonia (now in green!) is in fact completely oxidised before the end of the aeration cycle (red) indicating the cycle is unnecessarily long, wasting power, and probably consuming RBCOD that could be used for denitrification and P removal.

Figure 14.10.

Normal IDEA cycle showing ammonia removal is complete before the end of the aeration period in all cycles other than during the peak daily flows (compare the cycles shown by the black arrow (high decant ammonia) and red arrow (low decant ammonia)). Flow (blue), DO (red/brown), ammonia (green) and phosphate (orange). The vertical scale for ammonia is 0 - 14 mgN/L. The horizontal scale is 48 hours.

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Case Study: Ammonia Control of an Intermittent Plant (continued) The PLC at the plant was modified so that the aeration cycles were controlled by an online ammonia analyser. Aeration was switched off when the ammonia level reduced to 1 mgN/L with aeration remaining on until this value was achieved. The settle and decant stages were fixed at 1 hour each. There was also a minimum aeration period of 40 minutes regardless of the ammonia level at the time.

The results in Figure 14.11 show that the ammonia (now blue again!) was < = 1 mgN/L for each decant. The decant periods can be identified by the periodic decrease in cell level (green trend).

The vertical scale for ammonia is 0 - 14 mgN/L. The horizontal scale is 48 hours.

Figure 14.11.

Modified IDEA cycle under ammonia control. Note the reduced aeration and the low ammonia at the end of each aeration stage. The vertical scale for ammonia is 0 - 14 mgN/L. The horizontal scale is 48 hours. Cell level (green), DO (red/brown), ammonia (dark blue) and pH (yellow).

The overall result here was a significant net reduction of discharged ammonia and an approximately 25% reduction in power for aeration. This was achieved using one online analyser and a simple modification to the PLC. Significant and sustained financial gains are clearly achievable. The low ammonia during peak flow discharges provides a better environmental outcome at the same time.

Unfortunately this control system and many others like it are no longer in operation despite the clear benefits demonstrated.

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Chapter 15 Modelling the Activated Sludge Process What is a Process Model?

An activated sludge process model is a computer program that aims to model the operation of an activated sludge plant. For a given influent quality and set of operating conditions, the model should accurately predict the aeration demand, the effluent quality and the solids and sludge production.

A model can be a simple excel spreadsheet or an advanced modelling package.

Microsoft Excel based models generally use only a few inputs such as the average daily flow and load, and sometimes the minimum and maximum flows and loads. As such these models provide a good indication of performance under steady state conditions, but provide no indication of the effect of daily variations in loads or trade waste inputs. They do not take into account the differences in the detailed composition of the raw sewage or operating conditions in the plant. These models therefore only provide an indication of general plant performance and average effluent quality and sludge production.

More advanced models use a large number of inputs including daily flow variation, detailed composition of the raw sewage and the sizes of reactor tanks and clarifiers. The output is also quite detailed including air flow, sludge production, and time based effluent quality changes. These are called dynamic models as they can model for the effects of changes, not just a single combination of flow and concentration as described above for steady state models.

In general, dynamic models are far more useful but are more expensive and require a lot more monitoring data. Models are only really as good as the data put into it.

Dynamic models can also benefit from being linked to process models of the sewer network. For example, detailed modelling of trade waste flows can extend the capability of the dynamic model for the treatment plant.

A good example is that trade waste agreements often encourage or force industries to remove degradable carbon while continuing to discharge high nutrient concentrations to the sewer. Modelling can test the best trade waste characteristics for the operation of the whole plant and also what is the best way to discharge the trade waste to the sewer. For example, rather than getting the industry to remove Chemical Oxygen Demand (COD), and usually all they remove is the readily biodegradable fraction, it may be better to get them to install a buffer tank and only discharge to the plant during periods of low C:N ratio when the carbon is useful, and/ or during low load periods overnight when the load has less effect on the plant performance.

There are several different models available. The most commonly used are BioWin and GPSX. They all use similar basic assumptions and produce similar results for the same set of inputs.

One important feature of the models is their use of COD. While licence compliance in Australia is generally based on Biochemical Oxygen Demand (BOD), most models are based on COD. This is because laboratory measurement of BOD is quite variable, whereas COD is a more accurate measure of the amount of carbon coming into a plant. Use of COD also allows a mass balance to be completed for sludge production and nutrient removal which cannot be done using BOD.

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Why Use a Model?

Models are used widely in the design of new plants or the upgrade of older plants. They can also be used to optimise operation of existing plants and help operators get the best out of the plant they operate. Some specific examples include: •• Reviewing the overall performance of aeration tanks, anoxic zones or clarifiers.

•• Evaluating proposed changes to the operation of the plant. For example what might happen to nitrogen removal if the Dissolved Oxygen (DO) set point was lowered in the aerobic zone?

Alternatively, what might be the impact of lowering or raising the Return Activated Sludge (RAS) return rate or Waste Activated Sludge (WAS) rate.

•• Evaluating the effect of alternate plant configurations.

•• Helping to design advanced control systems.

•• Reviewing the process design particularly relating to peak loads and wet weather flows.

•• Providing operator training.

Changes can of course be tested in the whole plant but generally only one change can be made at a time and the result needs to be observed over a period of at least one sludge age, typically 14 to 20 days. Models allow testing of many changes, generally still only one at a time, but the responses can be assessed in minutes rather than days.

From both an operator’s and a manager’s perspective, there is often concern over altering plant operational parameters, particularly if the plant is already consistently meeting license limits. A process is often set up based on ensuring license compliance and continues to be operated based on these set points year after year without really considering whether the plant could be operated better. The concern is that a change may result in non-compliance. Modelling allows confidence to be built up to actually make a change in the plant.

Building a Model The key to modelling is that good data provides good information and leads to good decisions.

The models have been developed over long periods of time and have been extensively tested and verified.

While the constants and assumptions in the models can be changed to force fit any data set, this should not be done without further specific investigation and testing. If the model isn’t predicting the actual conditions well, it generally indicates that more and better data is required, not that the constants in the model need to be changed.

The following steps describe how a model is built.

Step 1. Define the Project What is the reason for building the model? Is it just to provide a general check of performance or is it to be used to test changes in influent, changes in operational settings or options for upgrading the plant or even whether the plant might be able to meet new licence requirements.

At the very least, a clear definition of the project will allow you to decide whether a steady state or dynamic model is required.

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Chapter 15 - Modelling the Activated Sludge Process Step 2. Configure the Model The model set up is relatively simple. It relies on knowing the flow path through the plant, the dimensions of the unit processes, and the control set up including DO set points, internal recycle rates, RAS and WAS flow rates and return flow rates from any dewatering facilities. The following steps are then taken: •• Produce an accurate detailed process flow diagram. A simple process flow diagram is shown in Figure 15.1 and a much more complex one in Figure 15.2.

•• Enter the sizes of the unit processes e.g. length, width and height of reactors and the diameter and height of clarifiers.

•• Enter the process control parameters e.g. DO set points, RAS and WAS rates.

•• Enter the aeration input capacity e.g. blower air flow or surface aerator power.

•• Enter the influent quality data.

•• Enter quality data for any other inputs e.g. the Total Suspended Solids (TSS), ammonia and phosphate levels in the return flows from dewatering.

Figure 15.1. A simple process flow diagram for an MLE process.

Figure 15.2. A more complex process flow diagram for a Johannesburg Process.

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Chapter 15 - Modelling the Activated Sludge Process Step 3. Data Collection and Checking At most plants, licence compliance data is monitored. For example an EPA licence may require monthly or weekly monitoring of the final effluent for a range of parameters possibly including those listed in Table 15.1.

Table 15.1.

Example of typical EPA Licence discharge and notification limits.

Indicator

Median

Notification

Biochemical oxygen demand (5 day) (mg/L)

10

20

Total suspended solids (mg/L)

10

20

Ammonia (mgN/L)

2

5

Total nitrogen (mgN/L)

10

20

Total phosphorus (mgP/L)

0.7

2

E. coli

Dependent on reuse

pH

6-9

At many plants, apart from some in-plant measurement of Mixed Liquor Suspended Solids (MLSS) and pH, the licence monitoring data is all that is available. Surprisingly, testing of the influent is often not carried out at all. Sometimes weekly or monthly grab samples may be analysed for some parameters such as BOD and ammonia and possibly phosphorus. However this may be the limit of monitoring. SCADA trends will provide additional information on flows, and hopefully DO and pH levels but generally not much more.

This is not enough data to build a good model. In fact the only model that could be built with this data would be a static model. More data is required.

So what is required to allow a good dynamic model to be built?

1. Detailed Influent Quality At an absolute minimum, two weeks of comprehensive data is required. Where possible, online instruments should be used (see Chapter 14), for example, online instruments are available that can measure pH, COD, filtered COD, flocculated and filtered COD, ammonia (NH3), nitrate (NO3) and nitrite (NO2). A very important part of influent monitoring is to fully characterise the COD. A good model requires reliable measurements of soluble COD, particulate COD and total COD. This is called COD fractionation, working out what fraction of the total COD is in the different forms (see Chapter 4).

New instruments such as the S::CAN UV Vis spectrophotometer, ZAPSLiquID, LAR QuickCOD, and a range of online ammonia analysers can also be used to gather large volumes of data including importantly, daily variations in composition. This data can then be matched to online flow data. The large amount of data makes the models all the more powerful.

If online monitoring is not possible then a refrigerated auto sampler (Figure 15.3) should be used to collect hourly samples for analysis.

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Figure 15.3. An automated sampler in trade waste discharge outlet.

Figure 15.4 shows an example of the variation in COD, TN and TP in the influent to a 22 ML/d plant with a combined domestic and minor trade waste input over a one month period.

Figure 15.4. An example of detailed monitoring data for a one month period. Note the regular peaks and troughs but also the significant increase around the 4/9/2010.

2. Detailed Biomass Characteristics Mixed Liquor Suspended Solids (MLSS) data alone is insufficient for a detailed model. To assist with the mass balance necessary for a good model, Mixed Liquor Volatile Suspended Solids (MLVSS) and also the COD of the biomass is required. As these parameters are often not measured routinely, it is necessary to obtain the results from at least three separate samples. These can be taken during the two week intensive monitoring program described above.

Determining the MLVSS and COD of the biomass should then remain part of normal routine monthly monitoring at the plant (see Chapter 11).

3. Solids and Nutrient Inputs from Sludge Processing Often liquid removed in the sludge thickening and dewatering steps is returned to the plant. Depending on the solids capture rate, quite a large amount of solids can be returned and repeatedly recycled into the plant, artificially increasing the MLSS and also potentially overloading the clarifier. Solids capture in the sludge processing stage is often an area of considerable inefficiency in a plant. In fact it is one of the most important areas for modelling.

Similarly sludge supernatant returns can have quite high ammonia and phosphate levels that should also be measured. These values are actually calculated within the models and therefore the actual measurements allow a cross check to verify the model.

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Chapter 15 - Modelling the Activated Sludge Process The solids capture through each of the solids processing steps, and the overall mass of solids removed from site should be evaluated. The target should be to model the MLSS/MLVSS ratio and also the final volume and mass of biosolids removed from the site. This is one step that is often not reviewed accurately when setting up a model. In most cases this is due to the lack of accurate data relating to the solids concentration and volume of the biosolids. In this situation, a useful “rule of thumb” is that the solids production should be in the order of 0.25 - 0.4 kgTSS / kgCOD removed. The lower value of the sludge production ratio is usually for higher sludge ages (typically > 20 days) and for WAS from a plant with a primary tank while the upper value in the range is for plants with lower sludge ages and no primary tank.

Step 4. Verify the Model Once the process model is “built” it is important to check that it actually predicts what currently happens in the plant, in particular the following parameters: •• Influent BOD/TSS. •• Solids mass balance.

•• Air flow rate.

•• Effluent quality (particularly TSS, TKN, ammonia, nitrite/nitrate and phosphorus).

•• Effluent alkalinity.

Each of these is considered in more detail below.

Influent Given the models are based on COD and the COD fractionation, the models actually calculate as part of their output, the influent BOD and TSS. It is a useful check of the model to see if the model results agree with the actual measured values for these parameters. Some flexibility with the accuracy of the predicted BOD values may be necessary since the actual measurement of BOD is a fairly non-specific and somewhat variable method. As long as the model gets it somewhere near right that will be OK. Solids (MLSS/MLVSS/Final Solids Mass) If the COD fractionation, the wasting volume and timing, and the solids capture in the solids processing step is set up correctly, the model should match the actual MLSS concentration, the final effluent solids, the sludge cake % solids and the sludge volume produced very well.

Most plants measure MLSS, however the final sludge cake mass is often not determined as it is difficult to measure what is taken off site accurately. The simplest method to determine sludge cake mass, and therefore allow comparison with the predictions of the model, is summarised below.

•• Measure the solids concentration of the final dewatered product.

•• Estimate as accurately as possible, the volume of solids produced per day. Taking the average over a week helps to smooth out daily variations. Where skip bins are used, the volume of the bin itself is a useful starting point for the estimation.

•• Calculate the mass of cake (kg) = concentration of solids (%) x 10 x volume (m3) x 7 d/wk.

•• Calculate the mass of COD (kg) removed for the week. = (Ave COD (mg/L) influent - Ave COD (mg/L) effluent) x average daily flow (ML/day) x 7d/wk.

•• Calculate the ratio of kg solids produced per kg COD removed and compare it to the range stated above of 0.25 - 0.4.

This calculation should be completed using the actual plant data to check on the data being used for the model in the first place.

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Chapter 15 - Modelling the Activated Sludge Process Air Flow Rate Dynamic models require air flow data, however many plants do not measure air flow. If it is not available the model can estimate it based on the DO set points, the number and density of diffusers and the aerobic fraction of the total tank. While the DO is indicative of air flow it is better not to rely on DO probes because of their variable ability to accurately measure DO. Air flow is much more reliable. For plants with surface aerators, the kW rating is used instead of air flow.

The model predicts the peak aeration demand. To check the validity of the prediction, it is necessary to go to the blower curve for the blower to determine what the maximum air flow from the blower is.

•• If the maximum air flow the blower can produce is greater than the peak demand (from the model), and the DO set point is reached during peak demand periods, then this gives some confidence in the model.

•• If on the other hand, the blower maximum flow is less than the peak demand and the set point is being reached then there must be something wrong with the model.

•• If there is a mismatch, it is important to go back and check the actual loads, air flow and DO at the time. It is also important to check that the DO meter is calibrated and the probe(s) are clean. In the situation where DO is used rather than air flow it is likely that close agreement will not be possible.

•• If there is still a problem, it is well worth installing an air flow meter, and possibly an online instrument on the inlet stream, even if it is only for a period of 2 or 3 weeks just to allow better calibration of the model. Aeration capacity is key to the findings of the model and it is important for the quality and usefulness of the model that this is modelled correctly.

Effluent Quality The next exercise is the match of the effluent quality. This is dependent on the level and quality of information gathered at the start of the exercise.

An example of model outputs against an online probe for clarified effluent is presented in Figure 15.5. Whilst this match is not exact, the model generally predicts the variations. Given the data available and the model purpose which was to evaluate opportunities for improved nutrient removal, the match was deemed to be adequate. It is very difficult, time consuming and expensive to get absolute agreement and in most cases it is not necessary.

Figure 15.5. A comparison of modelled values for ammonia (red) and measurements from an online ammonia probe (blue).

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Chapter 15 - Modelling the Activated Sludge Process Some improvement may be possible with some further simple checks. If for example the effluent ammonia is elevated in the model and this is not the case on site, then the same checks that might be conducted in the plant can also be checked in the model. For example: •• Check the aeration meets the DO set points.

•• Check the predicted MLSS concentration matches the plant.

•• Check the predicted effluent alkalinity and pH against those in the plant.

•• Check the predicted influent characteristics.

If these don’t match then the model needs further work. All the inputs need to be carefully checked and possibly more data collected.

Once the model fairly closely matches the actual plant performance it can be thought of as a baseline model.

But, remember that often the model is based on a single two week intense monitoring program. This won’t really represent what happens at different times of the year. For the model to be really useful you should be able to use it at any time of the year. This can be verified through auditing of the model during the different periods of the year and if problems are encountered with model predictions, the model can be further refined by specific monitoring programs.

Step 5. Apply the Model to Investigate Optimisation or Proposals for Upgrades The process model once fully verified can be used to test theories developed for optimisation. The advantage of the model is the ideas from operations staff can be “run off line” so that if the plant fails in the model the alteration doesn’t need to be implemented in the plant and vice versa.

One important area of modelling is to investigate whether changes in operational strategies will achieve improvements in effluent quality, solids production and operating costs.

Examples of different operational strategies that can easily be tested with the model include: •• Changes in the RAS rate with the objective of improving denitrification.

•• Changes in sludge age.

•• Modifications of diffuser layout and density. Increasing the diffuser density improves oxygen transfer efficiency which in turn leads to lower airflow requirements.

•• Changing surface aerators and reviewing the impact of different surface aerator efficiency values.

•• Management of peak flows. Flows can be bypassed and recombined prior to clarifiers or around the plant completely and the effect on the final effluent quality assessed. Flows can also be bypassed, stored and returned to the plant when peak inflows subside.

•• Peak load bypass and storage. This is particularly useful if aeration capacity is limited.

•• Online ammonia feedback aeration control. The online ammonia probe can be situated in the reactor or even after the clarifier, however control will generally be better if it is in the reactor.

•• Forward feed PLC control using online COD and ammonia analysers. The analysers measure the load into the plant which can then be used to evaluate the aeration demand and the recycle requirements.

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Chapter 15 - Modelling the Activated Sludge Process Increasing Model Accuracy The basic models described in this chapter involve making a number of assumptions, in particular around the solids mass entering the plant and stored within the plant. Depending on the purpose of the model, it may be necessary to undertake further specialised testing to increase the accuracy of the model. Some examples include: •• Measurement of nitrification rates. This may be required to more accurately model ammonia and nitrate levels and ammonia bleed through into the final effluent.

•• Careful measurement of the undegradable particulate fraction of COD. This fraction can build up in reactors and just “take up space”. Determination of this fraction is important for upgrades and capacity assessments.

These measurements are often completed using a bench top reactor (Figure 15.6). Once again this would only be completed if the modelling purpose demanded the extra accuracy associated with these methods.

Figure 15.6. A bench top reactor test.

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Some Examples of Modelling Outcomes Case Study 1: Solids Capture A 20 ML/d Johannesburg nutrient removal plant in Victoria has been upgraded several times and at the time of the investigation, the plant was limited hydraulically to 30 ML/d to protect the clarifiers from solids carryover.

The requirement was to increase the flow to full treatment up to 60 ML/d at the minimum cost.

New clarifiers had been proposed as being necessary for the future flows, however after site inspections it was felt that solids processing and in particular poor solids capture may have had a significant impact on the plant.

Points of note about the plant: •• The system includes an overflow to a flow balance tank.

•• Alum is dosed for final phosphorus removal.

•• Sludge is taken from under the primary tank and through a prefermenter.

•• Solids overflow the prefermenter and the drum thickener directly into the reactor.

•• WAS is transferred and blended with digested sludge in a sludge holding tank which is unaerated.

•• Solids processing at the plant consisted of: o Primary tanks and digestion of primary sludges. o Prefermentation.

o Solids/liquids separation with DAF.

o Belt press/centrifuge for the combined thickened sludge stream with the filtrate returning to the process.

The model appeared to be well calibrated given the match of effluent quality, solids produced and aeration input. The initial model identified poor solids capture around the primary solids system of around 50%. This meant that 50% of the solids removed in the primary tank were actually being returned to the process. The output from the model for the reactors is shown in Figure 15.7. Of note for this exercise is the MLSS concentration of 4,722 mg/L as it is currently operating.

Figure 15.7.

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Biowin output showing the value of 4,722 mg/L for the MLSS (arrow).

(Note that in Biowin, MLSS is reported as TSS.)

Chapter 15 - Modelling the Activated Sludge Process

Case Study 1: Solids Capture (continued) A proposal was developed to improve the solids capture by stopping the primary solids bypassing around the prefermenter. This required very minor pumping and pipework alterations.

When the model was run with these alterations and a 90% solids capture, a number of parameters altered, however the main point of focus for this exercise is the MLSS reducing to 3604 mg/L (Figure 15.8). The lower MLSS allowed the plant to be operated up to 60 ML/d with one less clarifier.

Figure 15.8. The outcome of the revised model based on improved solids capture.

The modelling showed that by running the primary solids separation system efficiently (90% solids capture compared to 50%) the plant was able to treat to the same level but using 25% less biomass. This reduces the pressure on the clarifiers and allows for increased flows to be transferred through the process (thus reducing the need for peak bypasses). It also results in significant capital savings.

Case Study 2: Nutrient Removal Optimisation A small 1 ML/d oxidation ditch treatment plant in Victoria had consistently struggled to meet licence limits for either ammonia or total nitrogen. The plant operation was unstable. The final effluent quality would vary such that low ammonia could be achieved at the expense of high total nitrogen and vice versa. If the plant was operating to meet 0 mgN/L ammonia, the effluent total nitrogen would generally be greater than the licence median limit of 10 mgN/L. Reducing the aeration resulted in the ammonia being above the 2 mgN/L median (and often above 5 mgN/L) but the total nitrogen was < 10 mgN/L. The plant was also using large quantities of alum to chemically remove phosphorus. A spare reactor tank had been installed (Figure 15.9) as it was thought that these tanks could be aerated to provide increased stability of operation. Unfortunately the instability was still apparent.

Figure 15.9.

Schematic showing the plant configuration.

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Case Study 2: Nutrient Removal Optimisation (continued) A model was developed and appeared to be well calibrated given the match of effluent quality parameters, solids produced and aeration input. The model correctly predicted the variability in ammonia and total nitrogen observed in the plant.

The model was used to test the following options: 1. Changing the position of the feed to, and return from, the side stream tanks.

2. Altering the RAS rates.

3. Changing the control of the system.

4. Running the side stream tank with air off.

5. Running the side stream tank based on ammonia feedback control.

6. Reducing the alum dose for P removal.

The results of the different model runs are summarised in Table 15.2.

Table 15.2.

Summary of the results of the different modelling runs.

Power kW

Alum L/d

NH3 mgN/L

TN mgN/L

TP mg/L

Existing

37

300

2-5

5 - 15

0.5

Altering feed positions to and from the main reactor

37

300

2-5

5 - 15

0.5

Altering RAS rates

37

300

2-5

5 - 15

0.5

Anoxic side stream tanks

30

100

0-2

3-6

0.5

Ammonia control with anoxic side stream tanks

25

75

0-2

3-6

0.5

Actual plant results after implementation

-

100

0 - 0.2

0-1

0 - 0.2

Change Modelled

Changing the position of the flows to and from the side stream tanks and altering the RAS rate had very little effect on the instability.

Turning the aeration off in the side stream tanks showed a marked difference in process stability in that the system was able to consistently meet licence limits with the added bonus of using significantly less power and alum.

The ammonia based control used data from an online ammonia probe in the channel leading from the oxidation ditch to the clarifier to control the aeration in the side stream tanks and reactor.

The model findings were implemented successfully and a full plant upgrade was avoided with only the cost of a modelling investigation and an online ammonia analyser and associated PLC control system. The actual effluent quality and power and alum consumption is shown in Table 15.2.

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Chapter 16 Closing Words Production of high quality effluent from a Sewage Treatment Plant (STP) requires time, diligence, attention to detail and effective process monitoring.

This practical Guide provides a sound background to the operation and optimisation of continuous and intermittent activated sludge plants. Activated sludge is fundamentally a biological process and as such shows much of the variation typical of biological systems. They are not “set and forget”, but rather require operators to develop a feel for a plant. Combined with thorough process monitoring, this allows the operator to make appropriate changes and maintain optimised operation.

The advice provided here might seem to some like more work, BUT it IS the work of operating an activated sludge plant to adequately protect both public and environmental health. Time simply must be found.

Managers need to fully support operators and facilitate the implementation of the tasks. In many cases it can result in operational savings and avoid unnecessary capital expenditure.

Think about it. A well run STP means peace of mind that the environment is being fully protected and impact on public health is minimised.

Take the first step now!

Good luck!

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Appendix A Method to Determine Flocculated Filtered COD and Readily Biodegradable COD 1. Dose 1 L of wastewater with 10 mL of undiluted aluminium sulphate (alum). This can be obtained from most Water Treatment Plants (WTP).

2. Stir the mixture rapidly (~200 rpm) for 2 minutes (rapid mix phase) and then slowly, at the lowest rpm that will keep the floc in suspension without settling, for 30 minutes (flocculation phase). A jar tester from a WTP can be used for this or alternatively a magnetic stirrer.

3. Allow the sample to settle, and immediately draw a 50 mL sample from the “clear” supernatant and filter it through a glass fibre filter (Whatman’s GF/C).

4. Filter the filtrate (from the GF/C filter) through 0.45 μm filter paper and determine the COD of this filtrate. o This will be the total soluble COD of the influent which is the sum of the soluble (or readily) biodegradable COD and the soluble unbiodegradable COD. 5. Repeat steps 1 to 4 but using final effluent from the treatment plant. The results of this test will be a measure of the soluble unbiodegradable COD in the effluent, which is the same as the soluble unbiodegradable COD in the influent. This fraction of COD is not changed by the treatment process because it is unbiodegradable and passes through the treatment plant. The RBCOD is found by subtracting this value from the total soluble COD measured in Point 4 above.

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Disclaimer This document does not purport to be a comprehensive statement on the operation or optimisation of activated sludge processes but rather aims to create awareness of what is possible, what might be done and indeed what has been done.

The authors, reviewers and organisations supporting the production of this guide are not responsible for the outcomes of any actions taken on the basis of information in this document and: •• Disclaim any and all liability for any errors and omissions of information in this guide.

•• Disclaim any and all liability to any person in respect of anything and the consequences of anything done or omitted to be done by a person in reliance upon the whole or any part of this document.

If expert advice is required, the services of a competent professional should be sought.

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Water Industry Operators Association of Australia (WIOA) PO Box 6012, Shepparton 3632 22 Wyndham Street, Shepparton 3630 Phone: 03 5821 6744 Fax: 03 5821 6033

www.wioa.org.au