Optimisation of Chemical Dosing, Coagulation, Flocculation and Clarification

Practical Guide to the Optimisation of Chemical Dosing, Coagulation, Flocculation and Clarification

CO SE ND ON ITI

ED

Bruce Murray and Peter Mosse

Publications Details First Edition, February 2008.

Second Edition, February 2010.

Second Edition, reprinted September 2015.

Other WIOA Publications Practical Guides to Water Treatment.  Practical Guide to Understanding and Managing Surface Water Catchments  Practical Guide to the Optimisation of Chemical Dosing, Coagulation, Flocculation and Clarification  Practical Guide to the Operation and Optimisation of Media Filters  Practical Guide to the Operation and Optimisation of Chlorine and Chloramine Disinfection  Practical Guide to the Operation and Optimisation of Distribution Systems Practical Guides to Wastewater.  Practical Guide to Odour Control In Sewage Transport Systems  Practical Guide to the Operation and Optimisation of Activated Sludge Wastewater Treatment Plants

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

Foreword The provision of water services to communities in developed countries has been the single most important development in improving public health at the population level. It has been much more significant than any other factor, notwithstanding the improvements in medical science over the past 100 years. It is also true that lapses in the operation and maintenance of water and wastewater treatment facilities and the associated distribution networks have resulted in illness and deaths in affluent societies. If one looks at significant water quality incidents that occur in developed countries, the problems are usually due to the lack of adequate professionalism on the part of management, regulators and/or operational staff, rather than any basic inadequacy of the infrastructure involved.

People in affluent societies do not expect to die, or be made ill, from using public water supply systems and this is quite a reasonable expectation. Water quality management is a vital and important role. People with primary responsibilities for water quality management, whether at the “coal face” or at a senior or executive level should appreciate the importance of this function. Complacency, inattention and distraction from the core tasks are dangerous features in water utilities. One often hears comments like “I would like to pay more attention to filter backwashing operations but I just don’t have the time”. Unless one wants to be a central player in a water incident, then we need to ensure that the right priorities are applied.

Water quality guidelines have developed significantly over the past few decades in Australia and elsewhere.

The emphasis in the past has been on compliance with tables of numbers that defined the quality of water that had to be produced for distribution - ultimately to the customer’s tap. In the early days, compliance was more or less optional for many of the water authorities, due to the powerful positions the utilities often held in the government bureaucracy. The strengthening of regulatory systems and the use of operating licenses and stronger legislation has improved matters substantially in this regard. The Australian Drinking Water Guidelines (ADWG) now includes a Water Quality Management Framework that sets out a risk-based process of proactively managing water quality from its source through to the customer tap. It involves a thorough understanding of each system and the active management of a number of interventions that ensure that the outcome will always be optimal and safe. Compliance should be automatic. It is a logical and straightforward process, but, in some areas of the system, there can be a lot involved in sorting out the detail.

It is with this area of detail in mind that the authors of this series have decided to produce this excellent set of practical guides that will assist operators with responsibilities for key control points in water operations (eg filtration, distribution). The national water quality guidelines, (ADWG in Australia), regulatory changes, licenses and audits all tend to confront water utilities from the top down. These guides are intended to be helpful at the operational “coal face”.

Collectively, all of these relatively recent additions to the water quality management landscape should help to ensure that there are no adverse public health outcomes from the use of public water supplies in Australia.

I commend the authors of this series for their excellent initiative and I trust that the series will be helpful in ensuring that we get the very best level of performance and reliability out of our water treatment facilities.

Professor Don Bursill CEO, Cooperative Research Centre for Water Quality and Treatment (1995-2005) Chair of the Coordinating Committee for Review of the ADWG (1996-2004) Chair of the NHMRC Water Quality Advisory Committee (2007-)

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Acknowledgements The authors wish to thank the following people and organisations.

Peter Gebbie, SMEC Australia P/L, for critical review of the manuscript and his major contribution to Chapters 3 and 6; which includes material previously presented at WIOA conferences in Victoria and Queensland, and published in WIOA’s WaterWorks and the AWA journal Water.

The NHMRC for permission to include material from Chapter 8 of the Australian Drinking Water Guidelines.

Phil Dack, Russell Mack, Steve Wilson and City Water Technology staff for critical discussion and thoughtful review of the manuscript; particular thanks to Myo Min Cho for the drawings.

Water Industry Operators Association of Australia (WIOA) CEO, George Wall, for proof reading and managing the publication of this guide.

The sponsors, whose logos are displayed on the front cover, for funding the publication of this guide.

The development of this resource reflects WIOA’s commitment to improving the skills of operations staff in the water industry.

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General Introduction This series of Practical Guides to Water Treatment focuses on each of the major steps in the process from catchment to tap, namely 1.

2.

3.

4.

5.

Catchment Coagulation and clarification Filtration Disinfection Distribution

Each guide provides practical advice regarding the management and operation of each of the critical control points identified by utilities as they implement the Australian Drinking Water Guidelines Framework for the Management of Drinking Water Quality (ADWG 2004), the Safe Drinking Water Act 2003 in Victoria, or a HACCP program.

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 along the lines of:  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.

Finally, the Australian Drinking Water Guidelines are concerned “both with safety from a health point of view and with the aesthetic quality of drinking water”. These guides deal with both of these aspects; not just with safety aspects. They are about getting the absolute best out of the system, so as to deliver the absolute best quality drinking water to the consumer.

Chapter 1 is common to all of the guides and provides a general background to the concepts of hazards, risks and control points developed in the Australian Drinking Water Guidelines.

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 Foreword Acknowledgements General Introduction Chapter 1

Hazards, Risks And Control Points • • • • • • • •

Chapter 2

Coagulation, Flocculation And Clarification • •

• •

Chapter 3

The Importance Of Coagulation, Flocculation And Clarification ..................................... 15 Coagulation ................................................................................................................... 16 • Mixing ................................................................................................................ 17 • Coagulation Or “Dosed Water” pH .................................................................... 19 • Alkalinity ............................................................................................................ 20 Enhanced Coagulation ................................................................................................... 21 Flocculation .................................................................................................................... 23

Coagulants And Polymers •

• •

•

4

Hazards And Drinking Water ..........................................................................................7 Pathogens Are The Main Hazards In Drinking Water .....................................................7 Other Hazards ..............................................................................................................10 Control Points .............................................................................................................. 11 Critical Limits And Target Criteria .................................................................................12 Corrective Actions, Incident And Emergency Management Plans ................................13 Monitoring ....................................................................................................................14 Responsibilities ............................................................................................................14

Coagulants ..................................................................................................................... 26 • Basicity .................................................................................................................... 26 • Specific Gravity ........................................................................................................ 26 • Aluminium Sulphate (Alum) ..................................................................................... 27 • Aluminium Chlorohydrate (ACH).............................................................................. 28 • Polyaluminium Chloride (PACl) ................................................................................ 28 • Ferric Chloride ......................................................................................................... 28 • Ferric Sulfate ........................................................................................................... 28 • Sodium Aluminate .................................................................................................... 29 • PolyDADMAC .......................................................................................................... 29 Which Coagulant To Use................................................................................................ 32 Coagulation, Flocculation And Filtration Aids................................................................. 34 • Which Polymer ......................................................................................................... 35 • Polymer Dose Rates ................................................................................................ 35 • Making Up Polymers ................................................................................................ 36 • Dosing...................................................................................................................... 37 Chemical Dosing Sequences ......................................................................................... 38

Chapter 4

Chemical Quality Assurance • • • •

Chapter 5

Risks Associated With Drinking Water Chemicals ......................................................... 39 • Managing Risks ................................................................................................. 40 Specifications For Drinking Water Treatment Chemicals ............................................... 42 Contaminant Levels ....................................................................................................... 42 Maximum Dose Rates .................................................................................................... 45

Jar Testing •

•

• •

Chapter 6

Jar Testing ...................................................................................................................... 46 • Jar Test Equipment............................................................................................ 48 • Stock Jar Testing Solutions ............................................................................... 48 • Basic Jar Testing Procedure.............................................................................. 50 Modifications Of The Basic Jar Test ............................................................................... 52 • Enhanced Coagulation ...................................................................................... 52 • Direct And Contact Filtration ............................................................................. 52 • Dissolved Air Flotation (DAF) ............................................................................ 52 • Powdered Activated Carbon (PAC) ................................................................... 53 Procedure For pH Correction For Jar Testing................................................................. 53 Manganese Jar Testing ................................................................................................. 54

Chemical Dosing Calculations • •

• •

Chapter 7

Step 1. Calculate The Concentration Of The Active Ingredient ..................................... 55 Step 2. Calculate The Dose Rate In The Plant .............................................................. 56 • Liquid Chemicals ............................................................................................... 57 • An Alternative Method For Expressing Liquid Dose Rates................... 57 Actual Dose Rates .......................................................................................................... 58 Polymeric Coagulants .................................................................................................... 59

Clarification • •

• • • • •

Clarifiers ......................................................................................................................... 60 Settling Clarifiers ............................................................................................................ 61 • Sedimentation Tanks And Settling Clarifiers...................................................... 61 • Shallow Depth Settling Clarifiers (High Rate Settlers)....................................... 63 • Upflow Clarifiers ................................................................................................ 64 • Adsorption Clarifiers .......................................................................................... 67 • Dissolved Air Flotation....................................................................................... 68 Clarifier Loading Rates ................................................................................................... 71 Sludge Management And Removal ................................................................................ 71 • Sludge Blow Down Or Desludge ....................................................................... 72 Re-Establishment Of Sludge Blanket In An Upflow Clarifier .......................................... 73 Comparison Of Clarifier Performance ............................................................................ 74 Clarifier Operation And Trouble Shooting....................................................................... 75 • Operational Variables ........................................................................................ 75 • Trouble Shooting Guide .................................................................................... 75

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Chapter 8

Optimisation of Coagulation, Flocculation and Clarification • • • • • • • • • • • • • • • • • •

Chapter 9

Operating Criteria And Performance Parameters For Coagulation And Flocculation..... 78 Operating Criteria And Performance Parameters For Clarification ................................ 78 1. Optimise Coagulation ................................................................................................. 79 2. Ensure Coagulant Is Added After Any Return Streams .............................................. 80 3. Optimise Coagulant Dosing Point............................................................................... 80 4. Monitor Coagulation pH.............................................................................................. 81 5. Optimise Flocculation ................................................................................................. 81 6. Determine Coagulation And Flocculation Times ........................................................ 82 7. Minimise The Effect Of Weather And Temperature .................................................... 82 8. Operate Flocculation Systems Continuously .............................................................. 84 9. Observe And Monitor Flocculation And Clarifier Operation ....................................... 84 10. Monitor Sludge Blanket Depth .................................................................................. 85 11. Control Algae ............................................................................................................ 86 12. Keep Flocculation Tanks And Clarifiers Clean.......................................................... 87 13. Check And Maintain DAF Distribution Systems ....................................................... 88 14. Monitor Residual Coagulant Levels .......................................................................... 88 15. Record And Use Past Experience (Look Up Tables) ................................................ 89 16. Other Methods For Controlling Coagulant Dose ...................................................... 89

Other Chemical Processes •

•

Chapter 10

1.Water Softening .......................................................................................................... 91 • Lime Soda (Chemical) Softening....................................................................... 91 • Mixing ................................................................................................................ 93 • Softening And Conventional Clarification .......................................................... 94 • Solids Management........................................................................................... 95 • Lime Crystallisation ........................................................................................... 95 • Sequestering ..................................................................................................... 96 2.Removal Of Manganese By Potassium Permanganate ............................................. 96 • pH And Alkalinity ............................................................................................... 97 • Temperature ...................................................................................................... 97 • NOM And Sulfide............................................................................................... 97 • Reaction Time ................................................................................................... 98 • Mixing Of KMnO4 ............................................................................................... 98 • Dosing Control................................................................................................... 98

Closing Words ....................................................................................................... 99

References ........................................................................................................................................... 99 Appendices Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Appendix 6 6

Mixing ........................................................................................................................... 100 Some Polymers Used In Water Treatment ................................................................... 103 Example Product Specifications For Alum ................................................................... 106 Example Jar Test Record Sheet ................................................................................... 107 An Alternative Method For Expressing Alum Concentrations ....................................... 108 Floc Size Chart ............................................................................................................. 108

Chapter 1 Hazards, Risks And Control Points The latest edition of the Australian Drinking Water Guidelines (ADWG 2004) includes a new section entitled “Framework for the Management of Drinking Water Quality.” The Framework is based on the recognition that the best way to protect public health and ensure high quality drinking water is to use a preventive management approach that covers all steps in the production of drinking water from catchment to tap.

This new approach is quite different to the old approach that relied to a large extent on monitoring the final water using more and more tests with lower and lower limits. Often samples were only taken once per fortnight, or worse, once per month. The major problem with this approach was that if unsatisfactory water had been allowed into the distribution system, it was much too late to do anything constructive by the time the test results came back. In addition, by sampling so infrequently, one really couldn’t be sure what had happened in the time between samples. The new Framework recommends thoroughly monitoring the individual steps in the overall process. By ensuring that each step is operating correctly, the high quality of the final product can be assured at all times.

Water utilities now have an opportunity to establish an appropriate priority for the entire process of water treatment from catchment to tap, and, by fully adopting and implementing the 12 elements of the Framework, they can develop an excellent supporting management system: one that truly focuses on activities in the field.

Hazards And Drinking Water The major aim in operating a water supply system is to ensure the provision of safe water, water that does not make the consumer ill. To do this, hazards to public health must be identified and steps put in place to reduce the risk posed by the hazards.

A hazard may be thought of as any potential danger to consumers. Hazards associated with the delivery of water include:    

Microbiological pathogens e.g. bacteria, viruses and protozoa.

Chemical contaminants e.g. algal toxins, heavy metals or pesticides.

Turbidity.

Natural Organic Matter (NOM).

While not a direct threat to human health, both turbidity and NOM have been shown to be associated with an increased risk of microbial contamination of water.

A hazardous event is any occurrence that introduces hazards to, or fails to remove hazards from the water supply. A good example is a mains power failure to a chlorination site leading to a loss of chlorination and therefore disinfection.

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

The best way to manage a hazardous event is to anticipate what might go wrong and to develop a failsafe system. For example, at a disinfection site, an Uninterruptible Power Supply (UPS) or standby generator could ensure power supply and maintain disinfection during a mains power failure.

Pathogens Are The Main Hazards In Drinking Water The public is generally most concerned about chemicals in water, but in fact the big outbreaks of waterborne disease have arisen due to microbial pathogens. So why have pathogens generally been more of a problem than chemicals? Table 1.1 provides some clues.

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Chapter 1 – Hazards, Risks And Control Points Table 1.1.

Comparison of pathogens and chemicals as risk factors for waterborne disease.

Risk Factor

Pathogens

Chemicals

Time to effect

As little as a few minutes of pathogens getting into the treated water can lead to a waterborne disease outbreak and their impact is immediate, and will be evident within days of contamination.

In most cases, chemicals in water don’t cause significant harm unless consumed for many years.

Detection

Pathogens are difficult to detect using existing monitoring techniques, and testing takes time. The results normally don’t come back until after people have drunk the water.

Chemicals are generally easier to detect, with a wider range of online instrumentation available, and because in most cases the effect is longer term, it’s possible to respond before harm is done.

Harmful levels

Even swallowing one pathogenic organism can make someone ill, so very low numbers in water can have an effect. Low levels can occur without any other signs. Pathogens don’t smell, taste or have colour.

Pathogens come from natural sources.

These sources are very difficult to control. Grazing animals and wildlife can carry pathogens to water. Septic tank run off can contaminate raw water supplies.

Often chemicals can be detected through taste, odour or appearance once harmful levels are reached. The generally low levels in water require long exposure to cause illness.

Pathogens have evolved to be transmitted via food, water, air and direct contact from person to person or animal to person.

Most chemicals are not designed to cause harm in water. So most chemicals are not particularly harmful to humans at levels found in water. They are not able to cross infect from one person to another.

Source

Design

Harmful chemicals generally arise from human activities. Their use is controlled to prevent discharge to water.

There are hundreds of waterborne pathogens. Each has a slightly different combination of sources, treatment sensitivity and effect on health. But there are simple rules that can be used to understand and manage these risks. The way to simplify this is as follows:  First understand that there are three main groups of pathogens.  Then get to know the most problematic pathogen in each group.  Finally, understand how to manage the risks from the most problematic organisms.

The idea is that if the more problematic organisms are under control, risks from the hundreds of other waterborne pathogens should also be largely under control.

The three major groups of pathogens of concern are listed below, along with the most problematic reference pathogens from a water supply perspective:  Enteric (intestinal) bacteria o Campylobacter, pathogenic E. coli O157 H7 and Salmonella  Enteric protozoan parasites o Cryptosporidium and Giardia  Enteric viruses o Enterovirus, Rotavirus, Adenovirus and Hepatitis A virus.

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Chapter 1 – Hazards, Risks And Control Points If these pathogens are transmitted in water they can cause very serious widespread illness in public water supplies.

The protozoan amoebae Naegleria, while not enteric, has also been associated with water borne amoebic meningitis that can rapidly result in death. For this organism to infect it needs to enter the body through the nose. Naegleria is generally only a problem in warmer waters above 18oC. In South Australia in the 1970’s and 80’s these organisms were often detected in systems with long above ground pipelines where water temperatures often reach above 30oC. Chloramination of these systems in the early 1980’s has eliminated the problem. In Western Australia where there are also long, above ground pipelines, similar problems have been experienced.

Some of the important features of the outbreaks caused by pathogens can be readily explained by understanding the key features of the main pathogen groups (Table 1.2). Table 1.3 provides some examples of water borne disease outbreaks in the Western World.

Table 1.2.

Key features of pathogens responsible for waterborne disease in humans.

Feature

Bacteria

Protozoa

Viruses

Is sewage a common source?

Yes

Yes

Yes

Is stock manure a common source?

Yes

Yes

No

Are bird droppings a common source?

Yes (Particularly Campylobacter)

No

No

Are wildlife faeces a common source?

Yes (Particularly Salmonella)

Possibly

No

What are the principal source controls?

Control all sources of faeces

Control all sources of manure and sewage

Control sources of human sewage

Is media filtration an effective control?

Yes

Yes

Limited

No

Yes

Is primary disinfection using chlorination an effective control?

Yes

Is maintaining a residual of chlorine in the distribution system an effective control?

Limited (kills bacteria in droppings in water tanks but overwhelmed by large contamination events)

No

No (usually not enough to kill pathogens from sewer ingresses)

Are alternative disinfection techniques such as UV or ozone effective controls?

Yes

Yes

Yes

Is microfiltration an effective control?

Yes

Yes

Limited

Is reverse osmosis an effective control?

Yes

Yes

Yes

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Chapter 1 – Hazards, Risks And Control Points Table 1.3.

Examples of recent waterborne disease outbreaks in the western world.

City

Popn

Year

Pathogens

Source

Illness

Deaths

Gideon Missouri USA

1,100

1993

Salmonella

Bird faeces

650

7

Milwaukee Wisconsin USA

600,000

1993

Cryptosporidium

Sewage

403,000

100

Sydney Australia

~4 million

1998

Cryptosporidium

Manure Sewage

0

0

Walkerton Ontario Canada

4,800

2000

E. coli O157 H7 Campylobacter

Manure

2,300

7

Galway Ireland

72,000

2007

Cryptosporidium

Sewage

300

0

Nokia Finland

32,000

2007

Campylobacter and Salmonella

Treated sewage

6,500

2

23

0

27,000

0

Pitsford United Kingdom

260,000

2008

Cryptosporidium

Single rabbit carcass in a wash water tank

Ostersund Sweden

60,000

2010

Cryptosporidium

Sewage

You have to search high and low for similar events due to chemicals in a water supply system!

So how can pathogens be controlled in a water supply? The simple answer is to prevent them getting into the water supply in the first place. Even though this is practically impossible, every effort must be made to manage those activities that can lead to pollution of water. Particular attention needs to be focused on those activities that may result in pollution with human sewage since human pathogens are the most infective to other humans. Contamination with faecal pathogens from birds and other mammals also needs to be controlled since these can and do cause illness. However, even with the best-protected catchments, pathogens will gain access. The catchment barrier alone is not sufficient to control pathogens, therefore a ‘multiple barrier’ approach is needed to manage pathogens in a water supply. Thus for micro-organisms to finally reach and infect a consumer they must pass unscathed through each of these barriers in sufficient numbers to be able to infect a human being.

Other Hazards Having established that the major risks to public health are from pathogens, it is important to note that risks can also arise from chemicals (particularly over the long term) and algal contaminants in water.

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Chapter 1 – Hazards, Risks And Control Points There are three major areas of chemical contamination that have caused waterborne illnesses.  The use of contaminated groundwater, particularly associated with arsenic.  The introduction of excess or incorrect water treatment chemicals including alum and fluoride.  The backflow of hazardous substances into water distribution systems.

In large surface water catchments with limited industrial or agricultural development, risks to health from chemicals are rare. Where there is considerable industrial or agricultural activity, chemical contamination is possible. Arsenic in water is not common in Australia but does occur. There are a number of small supplies where the source water is drawn from rock strata or sands with naturally occurring arsenic or where the source water is affected by run off from old mine sites.

Contamination from incorrect chemical dosing and backflow is much more common even in large cities, with several documented cases occurring within the last 10 years or so.

Waterborne incidents associated with algae were historically associated more with the warmer climates of Queensland or with degraded river systems with irregular flows. However, as drought in Australia continues, many water storages further south have had to deal with recurring algal blooms. Indeed there are an increasing number of instances in the southern states of Australia where algal blooms have occurred even during winter. Problems arising from algae present in source water include:  Toxins that may be introduced to the water by algae and cyanobacteria (blue green algae).  Taste and odour chemicals can be introduced to the water by cyanobacteria or other species of algae (e.g. diatoms).  Taste and odour chemicals can be introduced to the water by soil bacteria and fungi.  Some species of algae can overload water treatment plants and cause blockage of filters.

These possible sources of contamination need to be considered and managed appropriately when they occur. Their occurrence needs to be anticipated and preventive strategies put in place.

Experience has shown that problems with algae can arise sporadically, often without warning, and the problem can then abate just as quickly. In practice, the management of risks from algae involves case-bycase assessment, based on proactive source water management and the reactive treatment responses.

For example, different toxins may require the use of different types of activated carbon, and some may be treatable by other means such as ozone or chlorine. There are also a number of national and statewide response protocols for the management of algal blooms.

Finally, the reaction of chlorine based disinfectants with organic matter in water can result in the formation of Disinfection By Products (DBP’s) including Trihalomethanes (THM), Haloacetic Acids (HAA) and NDMA. These can have an impact on public health, however if the levels are below those stipulated in ADWG (2004), the risk is considered very low. DBP’s are considered in more detail in Mosse, Braden and Hourigan (2009), “Practical Guide to the Operation and Optimisation of Chlorine and Chloramine Disinfection”.

Control Points Within a water supply system there are several control points. A control point can be considered to be a step in a process where, if the operators do their job well, the chances of increasing the quality and consistency of the final water is increased. Alternatively, if a poor job is done at the control point then there is a greater probability that the final water quality will be poorer.

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Chapter 1 – Hazards, Risks And Control Points Figure 1.1 represents a generalised water supply system subjected to conventional water treatment. The general diagram can be applied to almost any water supply system with the possible addition or deletion of a few process steps.

Figure 1.1. A generalised water supply system.

\

The most important control points in a water supply system are:         

Catchment Raw water extraction Chemical dosing Coagulation Flocculation Clarification Filtration Disinfection Distribution.

Clearly where the 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 “out of control” (Figure 1.2). By setting numerical limits, the operator can make this assessment and take whatever action is necessary to maintain control at the control point.

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Chapter 1 – Hazards, Risks And Control Points The ADWG (2004) puts 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.

1. 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.

2. Critical limits. 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 health risk. Such events should result in immediate notification of the appropriate health regulator and the activation of incident and emergency management plans.

Corrective Actions, Incident And Emergency Management Plans 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 water quality exceeds specification or an unacceptable health risk occurs (i.e. exceeding the critical limit). Once the critical limit is exceeded, incident and emergency plans need to be activated (Figure 1.2).

Figure 1.2. Summary of the relationship between target criteria and critical limits and the actions taken when these are exceeded.

Critical Limit

Plant shutdown or other action to prevent supply to consumer.

Possible Boil Water Notice

Emergency Management

Turbidity (NTU)

0.3

Local operational response triggered

0.2 Alarm Limit

Incident Management

Target Criteria

Corrective Action

0.1

In Control

Heading Out Of Control

Out Of Control

Time

13

Chapter 1 – Hazards, Risks And Control Points

Monitoring The testing and observations that are made to determine whether the process is achieving the limits defined for that control point is process monitoring.

Process monitoring describes the tests and observations taken at a particular control point to determine the compliance or non-compliance with the limits defined for that particular point.

Verifying that all the control points in a process are within the recommended range ensures that the final product will be within the specific limits. Testing of the final product quality is 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 water has been distributed. It is a bit like saying “I’ll tell you tomorrow if the water you drank yesterday was safe to drink”. Process monitoring of control points allows the operator to detect problems before they can have an impact on final water quality.

Operators and management should work together to firstly set target criteria and critical limits, and then to establish appropriate monitoring frequencies for all the tests. Ideally, this will allow the identification of any trends that indicate a drift away from satisfactory operation (see Figure 1.2). The frequency of monitoring needs to be set based on the rate of change for the 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!

Responsibilities Water Utilities have a responsibility to:  Produce safe drinking water at all times.  Produce the best quality water possible from the existing system.

Operators have a responsibility to ensure that:  Their actions do not compromise the production of safe drinking water at any time.  They actively provide feedback to management when they become aware of shortcomings in the treatment sequence that may contribute to the production of unsafe drinking water.  They are alert to changes in the system that they operate.  They act to produce water that is aesthetically pleasing.

So, how can operators fulfil these responsibilities?

By ensuring that each step in the process from catchment to tap is monitored effectively and optimised.

14

Chapter 2 Coagulation, Flocculation And Clarification The Importance of Coagulation, Flocculation And Clarification Historically, clear water was considered to be clean. Early civilizations were concerned with the aesthetic appeal of water, and blamed poor taste and odour on “visible cloudiness”, or turbidity in the water. Therefore, the early focus of water treatment was on straining and filtration, to remove particles and improve the taste.

Ancient Greek and Sanskrit writings from approximately 4000BC have described methods such as charcoal filtration, exposure to sunlight, boiling and straining as methods of purifying drinking water. The Hippocrates Sleeve, developed by the “Father of Modern Medicine” around 500BC, involved simply straining rainwater through a cloth bag to remove any impurities.

A variety of coagulants were also used in different societies as early as 2000BC to aid the process of producing clear water. Early substances included almonds, toasted biscuits, powdered ginger, cornmeal and crushed oyster shells. The Egyptians are known to have used alum for water treatment as early as 1500BC. After the fall of the Roman Empire and the Dark Ages, it was not until the early 1700s that domestic water filters, consisting of wool, sponge or charcoal, started to be used in Europe. In 1804 the first municipal water treatment plant was built (slow sand filter) in Scotland and by the 1890s large sand filters were being constructed in America to protect public health. Subsequently coagulation and sedimentation with rapid filtration was found to be more effective, leading us towards the many variations of modern Water Treatment Plants (WTPs).

Coagulation and clarification have thus become two of the major process steps in water treatment and the rate of settlement of particles is a significant factor in their design. Table 2.1 (adapted from Hrudey and Hrudey (2004)), provides a comparison of the size and behaviour of those particles in water.

Table 2.1. Size and settling properties of particles in water.

Particle

Diameter (mm)

Diameter (µm)

Time to Settle

Gravel

10

10,000

1 second

Coarse Sand

1

1000

10 seconds

Fine Sand

0.1

100

2 minutes

Silt & Protozoa

0.01

10

2 hours

Bacteria

0.001

1

1 week

Colour Colloids

0.0001

0.1

years

Molecules

0.000001

0.001

never?

The sizes and characteristics of particles have important implications for the way we need to treat the water to remove them. The simplest example of this is the practice of storing water in reservoirs or lakes prior to treatment. The still water allows the larger particles to settle out and thus partially clarify the water. The smaller colloidal particles however stay suspended, and settling alone cannot be relied upon to remove these from the water. An alternative method is required.

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Chapter 2 – Coagulation, Flocculation And Clarification In modern water treatment it is generally the turbidity and colour that determine the way the water is treated.

Other factors such as Natural Organic Matter (NOM), Dissolved Organic Carbon (DOC), iron and manganese may also need to be considered and it may also be necessary to remove calcium and magnesium ions to soften the water.

Coagulation Most of the particles in water including microbes (bacteria, viruses and protozoans), and inorganic solids have a negative charge on the surface. Some dissolved organic material may also be negatively charged.

The fine, negatively charged colloidal particles in water tend to repel each other and remain in a stable suspension making them impossible to remove by gravity settling. To destabilize the colloids, the negative charge must be neutralised. If sufficient positively charged molecules (cations) are added to the water to equalise the number of negative charges on each and every charged particle then the repulsive force will no longer exist. The modified particles will be neutrally charged and able to approach each other and stick together or agglomerate as microscopic “floc”. This process is called charge neutralisation (or adsorption destabilisation) coagulation. With gentle mixing, the destabilised particles continue to clump together to form larger visible floc in the process of flocculation. Gravity settling, flotation and/or filtration can then remove the floc.

Without coagulation, the fine particles, including pathogens, would simply pass straight through a granular media filter. Clearly then coagulation is the important first step in the process of clarifying water to remove particles and colloidal materials and thereby produce a water that is safe to drink.

In water treatment, charge neutralisation is usually carried out by adding traditional coagulants such as alum or ferric salts to provide positively charged metal ions Al3+ or Fe3+. The metal coagulants react very rapidly (< 1 second) with alkalinity in water (hydrolysis) to form a range of positively charged metal complexes such as aluminium hydroxide (AlOH 2+) or iron hydroxide (FeOH 2+). These complexes adsorb the negatively charged colloid particles and cause neutralisation (destabilisation) of the electrical charge resulting in the formation of small floc particles almost instantaneously.

At higher pHs and/ or higher doses of metal salt coagulants, the coagulants themselves form aluminium or ferric floc that, during mixing, collide with and enmesh colloidal particles, producing larger floc that thereby trap and “sweep” the colloidal particles from the water. This is known as sweep coagulation.

The amount of coagulant added depends largely on the turbidity, colour, temperature, pH and alkalinity of the raw water. Figure 2.1 and 2.2 show two very different types of raw water that would require quite different treatment.

Figure 2.1. A coastal river in a forested area of southeastern Australia with very low turbidity, high colour water.

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Chapter 2 – Coagulation, Flocculation And Clarification Figure 2.2. High turbidity, low colour water in a river after a rain event.

Good coagulation requires that all coagulant chemicals be mixed very quickly and thoroughly with the water and with sufficient energy so that the particles and the coagulant molecules collide rapidly and often. This is particularly important for the metal coagulants where the initial chemical reactions of adsorption-destabilisation occur in less than 1 second. Sweep coagulation reactions are a little slower and may take up to a minute.

Clearly rapid mixing is very important. Many problems with water treatment result from poor mixing right at the start of the process.

Mixing Two alternative types of mixing may be employed to achieve suitable conditions for coagulation.

Rapid mixing usually involves fairly vigorous mixing for 30 to 60 seconds. Rapid mixing is normally employed in the conventional treatment process, causing sweep coagulation to occur which results in large floc that are suitable for removal in gravity settling processes, prior to filtration. Rapid mixing may be achieved by adding coagulant upstream of pipe bends, valves or flow meters, basically anything that creates turbulence, but preferably well designed static or mechanical mixers.

Flash mixing is intense mixing that occurs over a few seconds with the aid of high-energy mixers. This may occur in special tanks or within pipe work (Figure 2.3 and Figure 2.4). It is normally used in direct (or contact) filtration plants to create smaller floc that are more suited to filter media penetration and subsequent removal within the bed.

Flash mixing requires a higher mixing energy than rapid mixing in order to instantaneously mix the coagulant uniformly with the raw water. This leads to the rapid formation of microscopic floc that quickly grow to form pin-floc. In this way the floc formed is of uniform, very small size that is ideally suited to capture by filtration through granular media filters.

The amount of mixing energy applied to the water and the contact mixing time distinguishes rapid and flash mixing.

The mixing energy can be measured as a mean velocity gradient (G). The calculation of G and its use in the operation of WTPs is considered in Appendix 1.

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Chapter 2 – Coagulation, Flocculation And Clarification Dilution Water The mixing of coagulant is significantly improved by the use of dilution (make up or carrier) water. Typically dilution ratios of one part of chemical to between 10 and 20 parts of dilution water should be used. If the coagulant is diluted further than this, hydrolysis or formation of aluminium or iron hydroxide floc may occur.

This can cause blockages and can reduce the efficiency of the coagulation process thereby necessitating higher coagulant doses. A simple test to determine if hydrolysis is occuring is to dilute the coagulant in a beaker or jar to the level proposed and to look for any cloudiness in the water. The cloudy appearance indicates hydrolysis and a lower dilution should be applied.

Use of dilution water achieves significant premixing that ensures better mixing of the chemical once it is finally added to the mainstream flow.

The dilution water source should be chosen carefully and may be different for different chemicals. Chlorinated water reacts with polymers, reducing their efficiency and therefore should not be used for polymer dosing.

Raw water commences reacting with the coagulant prior to being mixed with the main stream. In general, treated unchlorinated water is the best source for dilution water.

The coagulant PASS (Poly Aluminium Silico-Sulphate) is one of the few coagulants that cannot be diluted because it forms floc even with very clean water.

Mixing Systems Mixing systems found in WTPs include: • Pumped injection: A pumped side stream entrains the dosed chemical and delivers it via nozzles, jets or cones generally directed against the flow of water in a pipe.

• In-line static: Pipe sections containing fixed geometric shapes such as baffles or vanes to encourage mixing (Figure 2.3). These are often proprietary units.

• In-line mechanical: A pipe section containing a propeller/mixer system driven by a motor situated outside the pipe (Figure 2.4).

• Mechanical mixer: A tank or channel with one or more mechanical mixers designed to produce vigorous mixing (Figure 2.5).

• Hydraulic Jump: Modifications in a flow path such as weirs, Parshall flumes or venturi meters designed primarily to measure flows but also provide large-scale turbulence that may be used for flash mixing.

• Pipe Grid: A grid of pipes and associated spray nozzles within a channel or pipe make up the grid mesh (A variant of pressure jets).

Figure 2.3. Diagram of an inline static mixing system

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Chapter 2 – Coagulation, Flocculation And Clarification Figure 2.4. An inline mixing system

Figure 2.5. A rapid mixer at a large WTP

These mixing systems may be employed for either flash mixing or rapid mixing depending on the configuration and the amount of energy imparted to the water. Flash mixers often employ shrouded turbine or inline static mixers (Figure 2.4). Pumped injection and the various types of mechanical mixers can be used in both flash and rapid mixing applications.

Coagulant, either full strength or mixed with make up water, is pumped under pressure through one of these mixing systems.

Care should be taken in determining the coagulant dosing point in relation to the mixer. For adsorptiondestabilisation (flash mixing) for direct filtration, the coagulant should be added as close to the point of maximum turbulence of the mixer as possible to ensure that the chemical is mixed evenly with all the water at the same time. For sweep coagulation (rapid mixing), the coagulant is usually added at the start of the mixing tank preferably via a diffuser to spread the coagulant across the flow path. In in-line static mixers the coagulant should be added directly into the static mixer at the start of the first element.

Coagulation Or “Dosed Water” pH Each coagulant has a narrow range of pH where it functions best. For aluminium sulfate (alum), the optimum coagulation or “dosed water” pH is usually within the range 5.8 and 6.5. pHs in the lower range around 5.8 to 6.2 usually result in better colour and organics removal. At pHs lower than this, problems with residual aluminium will occur due to the increased solubility of aluminium at the low pHs, however in a process known as Enhanced Coagulation (see below), pH as low as 5.5 is used for coagulation.

In contrast, the usable pH band for iron coagulants is broader. Ferric salts normally coagulate effectively in the pH range 5 to 8 and in some cases even as high as 10. They are therefore useful for treating water at a higher pH than is possible with alum. This can be useful for improved oxidation of metals such as iron (Fe) and manganese (Mn) prior to coagulation and also results in less concrete corrosion and less need for post dosing of alkali.

Additional information on coagulants is provided in Chapter 3.

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Chapter 2 – Coagulation, Flocculation And Clarification Alkalinity Alkalinity is a measure of the water’s capacity to neutralise acid. Ions present in water that contribute to the ability to neutralise acid include carbonate (CO32-), bicarbonate (HCO3-) and hydroxide (OH-) ions. Alkalinity is expressed as ‘mg/L as CaCO3’, which is a measure of the actual ions as an equivalent amount of CaCO3, based on molecular weights. The total alkalinity is the sum of the carbonate, bicarbonate and hydroxide alkalinities, and the amount of each form inturn depends on the pH of the water.

Buffered water with a high alkalinity (e.g. >100 mg/L as CaCO3) has a strong resistance to change in pH when acid or alkali is added. Conversely, waters low in alkalinity (<20 mg/L as CaCO3) will react with a large change in pH if acid or alkali is added. Low alkalinity water may therefore be very unstable in terms of maintaining the pH to the desired level for effective coagulation and flocculation.

Coagulants react with alkalinity in the water to form hydroxide-based floc. For example, when alum is added to raw water, alkalinity is consumed and the pH of the dosed water drops. This is because alum reacts with water to form aluminium hydroxide (Al(OH)3 ) and sulfuric acid (H2SO4). The sulfuric acid lowers the pH and uses up some of the alkalinity in the water. For the aluminium hydroxide to continue to form, sufficient alkalinity needs to be present to consume all the sulfuric acid formed. If the raw water does not contain enough alkalinity then additional alkalinity needs to be provided by adding alkali prior to the addition of alum, otherwise the pH can become too low for effective coagulation. Under these conditions, poor floc formation can lead to elevated turbidity and aluminium levels (and possibly elevated colour) in the dosed water. Postfiltration flocculation may also occur when excess dissolved aluminium passes through the filters and reacts with added alkalinity for final pH adjustment or alkalinity leached from the walls of cement-lined pipes. This floc then contributes to the build up of sediment in distribution systems and can contribute to dirty water complaints.

The theoretical alkalinity consumption by common coagulant chemicals is shown in Table 2.2.

Table 2.2. The amount of alkalinity consumed by common coagulants Coagulant

Alkalinity Consumed (mg CaCO3/mg pure chemical)

Alum (Al2(SO4)3.18 H20

0.45

Alum (Al2(SO4)3.14 H20

0.51

Ferric chloride (FeCl3.6H2O)

0.56

Ferric chloride (FeCl3)

0.93

Ferric sulphate (Fe2(SO4)3)

0.75

Aluminium chlorohydrate (ACH) (Al2(OH)5Cl)

0.29

If the pH of the dosed water falls outside the range in which coagulation will occur, other chemicals such as lime, soda ash or acid have to be dosed in advance to correct its pH. So in low alkalinity waters using alum coagulation, dosing of alkali is commonly required, however in high alkalinity water, hydrochloric or sulfuric acid may need to be dosed to reduce the pH.

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Chapter 2 – Coagulation, Flocculation And Clarification Alkalinity is usually added in the form of lime, soda ash or caustic soda. Sulfuric acid is the most common acid used for pH adjustment of high alkalinity water. In some instances, rather than have a Class 8 Dangerous Good (sulfuric acid) on site, high alum doses can be employed to decrease the pH to the optimum whilst at the same time providing metal coagulant, although this can lead to high aluminium residuals.

In general, optimum coagulation doses should be evaluated by jar testing (see Chapter 5) or pilot plant testing.

There are however a number of long-hand and spreadsheet methods of calculating dosed water parameters (e.g. Gebbie 2001, or Rothberg, Tamborine, Windsor model spreadsheet available from AWA bookshop).

There are also some instruments available that assist in determining coagulant dose (see Chapter 8).

One of the benefits of using alternative coagulants such as aluminium chlorohydrate (ACH) or polyaluminium chloride (PACl) is that they do not use up as much alkalinity in the coagulation process and so the pH will be less affected. This is apparent from Table 2.2 where the lower impact of ACH on alkalinity consumption is evident. (This is explained further in Chapter 3) Alkalinity Contribution By Alkalis The theoretical amount of alkalinity contributed by the main alkali chemicals used in water treatment is shown in Table 2.3.

Table 2.3. Alkalinity contributed per mg of pure product.

Chemical Agent

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

Enhanced Coagulation Enhanced coagulation refers to the practice of dosing a coagulant, typically alum, at high concentrations and low pH, to increase the removal of Natural Organic Matter (NOM) in the coagulation process.

NOM is a complex mixture of organic compounds originating primarily from decaying plant material (humus) on top of, and in the soil. NOM contributes to colour, increases the required coagulant and disinfectant dose for treatment and contributes to the formation of Disinfection By Products (DBPs). NOM can be partly measured as colour, Dissolved Organic Carbon (DOC), Total Organic Carbon (TOC) and absorbance of UV light.

NOM also provides organic material as food for the growth and stabilisation of biofilms. Biofilms often contribute to dirty water, taste and odour and chlorine demand. The increased chlorine demand requires water utilities to increase the dose of chlorine to meet disinfection and microbiological requirements. Consumers generally do not like the taste and odour of chlorine, so water utilities should do everything possible to limit the doses necessary to achieve disinfection. By removing NOM, dosing costs can be reduced, chlorine doses lowered and distribution system management costs reduced, and the consumer receives a more palatable product!

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Chapter 2 – Coagulation, Flocculation And Clarification Conventional coagulation and flocculation is very good at removing turbidity, and some colour and NOM, however large amounts of NOM are left behind. To understand what is possible with respect to removing NOM, it is necessary to understand a bit more about what makes up NOM. The amount of NOM in water and the type of compounds making up NOM vary from water to water and will also vary from time to time. The changes can be rapid, for example, after a storm or when rain falls after a long period of drought, or may occur more slowly as land use practices change with, for example, growth or removal of forests.

A lot of work has been done in Australia at the Cooperative Research Centre for Water Quality and Treatment (CRCWQT) to understand the nature of NOM. NOM consists of two broad groups of compounds known as humic and fulvic acids. (These can be further separated into 4 different groups based on a laboratory method of fractionation). The humic acids tend to be large molecules that are difficult to dissolve in water while the fulvic acids are smaller molecules that dissolve more easily. They can be removed to a greater or lesser extent by different methods.

Enhanced coagulation is only effective at removing the high molecular weight, poorly soluble humic substances. It is not effective at removing the lower molecular weight, very soluble fulvic substances. These generally require removal with activated carbon; either powdered or granular (PAC or GAC) or through advanced oxidation using ozone.

Whilst there is still debate about the exact nature of the mechanisms involved, it does appear that at low pH (ideally 5.3-5.7, but realistically up to 6.0), direct neutralisation of negatively charged organic compounds and colloids by the aluminium Al3+ ion takes place, thereby allowing the organic molecules to become incorporated in the floc. Depending on the type of NOM, enhanced coagulation can remove between 20% and 60% of NOM. It is important to note that enhanced coagulation results in the production of more floc and sludge than in normal coagulation, thereby increasing residuals (sludge) handling costs.

Alkalinity has an adverse effect on NOM removal when using enhanced coagulation. NOM removal is generally more difficult in higher alkalinity waters. Coagulating at a low pH to increase NOM removal will require careful attention to the final pH adjustment to ensure the treated water is not corrosive and does not contain high levels of soluble aluminium. In most situations, post treatment alkalinity adjustment is necessary.

So how much NOM can be removed at your plant? You can either get some samples of water tested at the Australian Water Quality Centre (AWQC) or other suitably equipped laboratories to determine the relative amount of NOM that can be removed by enhanced coagulation, or you can carry out some enhanced coagulation jar testing. This can be done as a simple modification of the jar test procedure described in Chapter 5.

If the jar testing results look significant, >15% more removal than normal coagulation, it is worth considering implementing the altered dosing regime in the plant. If the NOM removal is improved by >25% it is certainly worth doing. Remember, we have a responsibility to produce the best quality water possible from our systems.

It is worth noting that in the USA it is a formal requirement to remove NOM. The USEPA actually regulates how much NOM must be removed from water with particular NOM levels and alkalinities.

There is no doubt that enhanced coagulation leads to increased chemical consumption, final pH correction and sludge disposal costs, however this must be weighed up against reduced chlorination and maintenance costs, reduced DBP’s, and reduced health risks and consumer complaints.

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Chapter 2 – Coagulation, Flocculation And Clarification A complete cost benefit analysis may result in a significant positive outcome. These overall savings come from reduced disinfectant dose, reduced dirty water complaints, reduced taste and odour complaints and increased compliance with DBP targets.

While measuring DOC is the best way to measure NOM, it does require that samples be sent off to a laboratory and then waiting for the results to come back. An alternative is to use a UV Vis spectrophotometer to measure the absorbance of UV light at a wavelength of 254nm (UV254). UV254 is a convenient measure of DOC and therefore NOM. This allows the operator to get the results immediately by measuring UV254. The sample with the lowest UV254 has the lowest DOC and therefore NOM. Enhanced coagulation can be practiced at the majority of WTPs however there are a number of consequences that must be considered.

•

•

•

•

•

At low pH, the reactions responsible for the removal of manganese (Mn) will be slowed down. Mn removal may still occur, but longer contact times and possibly higher doses of oxidant may be required. If removal is compromised, a balance between minimum NOM and minimum Mn levels may need to be found.

Depending on the final pH, increased levels of soluble Al may be present in the final treated water.

This needs to be managed and again a compromise between maximum NOM removal and minimum Al residual may be necessary.

Enhanced coagulation will increase the volume of sludge produced. One possibility worth considering to reduce the volume of sludge produced is to use a slightly lower dose of coagulant and use acid to lower the pH to the optimum level. This can be evaluated by jar testing. Simply increase the alum dose and if necessary add some sulfuric acid to achieve a pH of between 5.5 and 6.0 and see what results you get in reducing DOC. It may be possible to achieve the same DOC removal with a slightly lower alum dose combined with some acid rather than just a high alum dose. This would then result in lower sludge production. If the jar testing results are favourable the altered dosing regime can be tested in the plant. (Acid dosing will almost certainly be required if enhanced coagulation is being investigated with ACH or polyaluminium chloride).

Enhanced coagulation may increase the risk of corrosion of concrete structures in the WTP. The concrete structures may need to be protected against the low pH, low alkalinity water. Ideally the concrete should be high strength, corrosion resistant concrete to start with. Concrete can also be protected by the use of corrosion-resistant epoxy and other “sealing” coatings. The effect of the low pH on the corrosion of metal components in the WTP should also be considered.

If the pH and alkalinity of the final product water resulting from enhanced coagulation is not adjusted, there may be an increased risk of corrosion in the distribution system. The water should therefore be stabilised prior to leaving the WTP.

Flocculation Once the water has been coagulated and the particles destabilised (charges neutralised), gentle mixing allows the small microscopic floc to collide and aggregate leading to the formation of larger and larger floc.

As the floc grows it becomes more fragile. Therefore it is important to reduce the mixing energy during flocculation to prevent damage to the floc.

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Chapter 2 – Coagulation, Flocculation And Clarification A number of different systems may be used in WTPs to provide gentle mixing conditions and the necessary time for the floc to grow. These include: •

• •

•

Hydraulic Contact Flocculation: Low energy hydraulic mixing over short periods of time (typically 2 to 5 minutes) results in the formation of very fine floc suitable for maximum bed penetration in deep bed media filters (contact filtration). Hydraulic contact flocculation is the simplest form of flocculation since it doesn’t involve the use of mechanical equipment.

Baffled Channel: Simple chambers with either “over under” baffle systems or “serpentine” (S flow) systems that rely on hydraulic mixing to provide the energy for floc formation.

Mechanical Mixing: Slow turning propeller or hydrofoil blades on a vertical shaft (Figure 2.6), or paddles on a horizontal shaft (Figure 2.7) designed to keep the coagulated water moving slowly.

Both types operate satisfactorily however vertical flocculators are generally easier to maintain. The type of mixing system determines the shapes of the flocculation basins. Horizontal systems usually have rectangular basins while vertical systems have square basins.

Reactor Clarifiers: Sludge blanket type clarifiers that incorporate flocculation in a central zone.

Coagulated water is introduced to the flocculation zone and passes toward the bottom and out under some sort of baffle or skirt. The floc then settles to form a sludge blanket. In a modification of this, in a solids contact clarifier, a mechanical system is added at the bottom of the central flocculation zone to draw some of the settled sludge back into the flocculation zone to provide seed and an increased number of floc particles to accelerate the floc formation. The new floc therefore comes into contact with preformed and settled floc (more information is provided in Chapter 7).

Figure 2.6. Flocculation tank with vertical shaft mixing systems

Figure 2.7. A flocculation tank with a horizontal mixing system.

The best flocculation is usually achieved in tapered mixing systems. These typically consist of three tanks separated by baffles with differing mixing energies in each. The lowest mixing energy is in the last basin. The tapered mixing energies are important to prevent damage to the larger more fragile floc. The mixing energy can be reduced in a number of ways. Mixers with different sized motors can be used. Alternatively the size or number of the mixer blades can be reduced. In horizontal systems the same rotation rate can be used but with 4, 3 and then 2 paddles attached to the shaft.

Tapered mixing provides the greatest flexibility of operation. Ideally the mixers have variable speed drives, enabling the operator to fine tune the operation of each basin to optimise floc formation under differing conditions.

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Chapter 2 – Coagulation, Flocculation And Clarification The optimum size and type of floc depends on the treatment process following the flocculation stage: • • •

Large, settlable floc (1.5mm to 5mm) is ideal for the conventional treatment processes where most of the solids are removed by gravity settling in a clarifier.

Smaller, ‘pin’ floc (<0.3mm) is the target in contact or direct filtration, to give optimum penetration of the filter beds and thus produce longer run times.

Dissolved Air Flotation (DAF) requires medium sized, lighter floc (0.5mm to 1mm) that bind with fine bubbles easily.

Thus to produce large settlable floc, a larger, tapered flocculation basin is required. In a typical threecompartment flocculator (with 5 to 10 minutes contact time in each) the mixing energy provided in each compartment would decrease by about half. If the floc is light and fragile then lower speeds may be required to prevent floc shear. Polymer dosing can also be used to strengthen the floc at this point. For pin floc however, a 2 to 5 minute contact basin without any mechanical mixing may be sufficient (contact filtration) or some mechanical mixing may still be required with some waters (direct filtration).

Mechanical or hydraulic flocculators are normally used to provide between 2 minutes and 45 minutes flocculation. While it is important to ensure that there is sufficient time to grow the required quality of floc, it is also important to realise that excessive floc time can reduce the quality of floc. Operation of the flocculation stage therefore requires constant monitoring by the operators at a WTP. Too often the flocculation step is regarded as being fixed and unable to be “fine tuned”. This is considered further in Chapter 8.

Water temperature has a significant effect on flocculation. Floc forms more slowly at lower temperatures therefore longer flocculation time and/or greater mixing energy is generally required to treat water at lower temperatures. The critical transition temperature between slow and faster coagulation/ flocculation is around 10oC to 12oC. While hydraulic contact flocculation without mixers may be sufficient for waters that are usually above 12oC, dedicated mixers and longer times are definitely required for water below 12oC. Thus much of Queensland needs short flocculation times and low mixing energy while in Victoria and Tasmania, as much as 40 minutes flocculation time with higher flocculation mixing energies are often required.

The transition from winter to summer in some locations may also mean a transition from smaller to larger floc. Operators should carefully monitor and report on their floc size and make adjustments as required to optimise the performance of the WTP. If not properly managed, the larger floc size of summer may cause problems in a direct filtration plant or conversely the smaller floc of winter may be a problem in a settling basin. Thus ideally different flocculation times may be provided through different production rates (often reduced flows are required in winter allowing longer contact times naturally) and/or through bringing on or taking off available flocculation basins. Mixing energy adjustments may also be required. Furthermore, dosing of a polymer during winter months may also help in developing larger floc sizes. (Polymer dosing is considered further in Chapter 3.)

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Chapter 3 Coagulants And Polymers Coagulants Various coagulant chemicals may be used in water treatment. Typically they are aluminium or iron salts.

Polymers may also be employed to strengthen or assist in binding the floc together to form larger or stronger floc. Traditionally alum (aluminium sulfate) has been used as the primary coagulant. More recently polymerised aluminium salts such as polyaluminium chloride (PACl) and aluminium chlorohydrate (ACH) have been found to be successful in many applications. The iron salts ferric chloride, ferric sulfate and polymerised ferric sulfate (also known as hydroxylated ferric sulfate), have also been used successfully.

Many Water Treatment Plants (WTPs) use a secondary coagulant. Secondary coagulants provide partial replacement of the primary coagulant and have been found to: • • • •

Reduce the amount of primary coagulant used.

Reduce the amount of floc formed thereby allowing an increased hydraulic loading of clarifiers and/ or filters.

Reduce the amount of sludge produced.

Strengthen the floc.

PolyDADMAC (polydiallyldimethylammonium chloride) is the most common example of a secondary coagulant.

Other polymers, typically polyacrylamides, may be used to strengthen the floc as it forms or improve the flocculation process by increasing the rate of growth of floc and the floc size achieved. These are commonly called flocculation aids. These polymers can be used as coagulation or filtration aids for sludge thickening and dewatering (see below).

In describing the chemical properties of coagulants, manufacturers use a number of specific terms. There are two terms in particular that need to be understood.

Basicity Basicity is a quantitative indication of how many hydroxyl ions (OH) are included in the structure of a hydroxylated or prepolymerised polyaluminium or polyferric coagulant. The higher the basicity of a coagulant, the lower the impact it will have on raw water pH. For example, ACH has the formula Al2(OH)5Cl. There are five OH ions in its structure giving it a high basicity. Typically ACH will have a basicity of about 83%, indicating that it will have less impact on dosed water pH than PACl, which only has three OH ions in its structure and a typical basicity of 50-55%. Alum has no OH ions in its structure and hence has zero basicity and will therefore have the greatest effect on pH and require the most alkalinity in the water.

Specific Gravity (SG) This is the unit weight of a liquid chemical relative to water at the same temperature. Strictly speaking, SG has no units but you may find it stated on laboratory reports or chemical supply documents as “g/mL” or as “kg/L”.

For a given chemical, there is a direct relationship between its strength and its SG, and indeed chemical manufacturers use SG to measure the strength of a chemical during the manufacturing process. As an example, liquid alum (43 to 52% w/w) typically has a SG of 1.28 to 1.34 at 20oC, whilst liquid caustic soda (46% w/w) has a SG of 1.48 at 20oC. (An example of the SG and strength of one manufacturer’s alum solutions is provided in Appendix 3.)

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Chapter 3 – Coagulants And Polymers The commonly used primary coagulant chemicals and their chemical properties are described below and summarised in Table 3.1.

In the descriptions that follow, and in Table 3.1, various figures are quoted. A quick look at different suppliers product sheets, or at different textbooks, reveals a wide range of figures for optimum pH ranges, chemical formulae and % compositions. Similarly, asking others in the water industry about the properties of coagulants can quickly lead to confusion.

In what follows we have tried to provide the best information available, and in the case of recommended pH ranges for coagulant use we have tried to steer a middle course and indicate, where our knowledge allows us, particular conditions that fall outside the quoted ranges.

With respect to suitable pH ranges for coagulants, rather than relying on published figures or word of mouth, the best solution is to use jar tests to assess the practicality of using a particular pH in your system. As well as measuring turbidity, colour and pH of the jar test product, you should also make sure that soluble Al is measured, since at the higher and lower pHs the level of soluble Al can be high and may lead to you exceeding the ADWG guidelines for aluminium. Soluble Fe and Mn levels may also need to be measured if these are a problem in your system.

The different chemical formulae and % compositions only become important when making calculations and it really won’t make that much difference to the end result when it is transferred to the plant. Where possible use the actual figures from your supplier’s product sheets. If these are not available use the average of the figures provided in Table 3.1. Alternatively the SG can be determined accurately by using a hydrometer. Hydrometers reading to 3 decimal places are available for well under $100 and can be found by searching the web. (Be careful though most advertised hydrometers are only accurate to 2 decimal places so you will need to search carefully to find ones to 3 decimal places.) Accurate tables of SG, alum strength as % alum are available and can be used to determine the % alum accurately. The tables are usually provided for one temperature only but there is usually another table providing temperature corrections. (An example of the SG and strength of one manufacturers alum solutions is provided in Appendix 3.)

Examples of calculations using this information are provided in Chapter 6.

Aluminium Sulphate (Alum) Alum is the workhorse of the water industry. In the dry form (filter alum) it consists of a number of different hydrates (molecules with water included) of which Al2(SO4)3.14H2O, Al2(SO4)3.16H2O and Al2(SO4)3.18H2O are the most common. A common method used in the chemical industry to measure alum strength involves a measurement of Al2O3 rather than Al2(SO4)3. This may seem confusing (and it is) however it reflects the long history of the use and manufacture of the chemical. As a consequence the liquid alum most commonly supplied is specified as containing 7.5-8.4% (w/w) Al2O3 (or 43-52% filter alum). It would be far better for the industry to standardise on actual aluminium (Al) concentrations in the same way as occurs for iron salts, it would make alum calculations much easier, however, for the moment this is not the case.

Alum is acidic and is corrosive to most metals. A 1% alum solution has a pH of about 3.4. When it is added to raw water, alkalinity is consumed and the pH of the dosed water will drop.

Alum is effective over a pH range of 5.5 to 7.7 but is typically used between 5.8 and 6.5. When used in enhanced coagulation mode the coagulation pH may be as low as 5.5.

27

Chapter 3 – Coagulants And Polymers The aluminium hydroxide formed in the reaction of alum with water is a jellylike floc and traps the neutralised colloidal particles in the process of sweep coagulation.

Alum is available in dry granular form (15.3% Al2O3), but is not commonly used these days.

Aluminium Chlorohydrate (ACH) Aluminium chlorohydrate (Al2(OH)5Cl), is a highly hydroxylated or prepolymerised aluminium-based coagulant. It has five OH molecules included in its structure and as a consequence has a very high basicity.

ACH produces far less acid in water than alum because it is more hydrolysed and thus requires less pH correction with alkali. ACH is most commonly supplied as a liquid that is clear to slightly cloudy, colourless to pale yellow liquid, with a faint odour.

ACH is typically used over the pH range of 6 to 8. It may also work outside this range, however at lower pHs high aluminium residuals are possible, and at the higher pHs, the DOC/NOM levels may be elevated and contribute to DBP formation.

Compared to alum, it produces a relatively robust floc though not always as good as alum (especially with poor quality raw water) and generally requires a lower dose and is effective over a broader pH range.

Polyaluminium Chloride (PACl) Polyaluminium chloride or PACl, (Al2(OH)3Cl3) is sometimes referred to as “PAC”. This abbreviation is now more usually reserved for Powdered Activated Carbon.

PACl is hydroxylated aluminium-based coagulant like ACH, but with a lower basicity than ACH, and hence has more impact on raw water pH when used as a coagulant but less impact than alum. PACl is most commonly supplied as a liquid that is a pale to slightly cloudy liquid, which is colourless to pale yellow and has a faint odour.

Like ACH it produces a relatively robust floc at lower doses than alum. It is also effective over a wider pH range than alum (See ACH).

Ferric Chloride Ferric chloride is most commonly supplied as a liquid that is dark red brown in colour, acidic and highly corrosive to most metals. A 1% solution of ferric chloride has a pH of 3–4.

Ferric chloride reacts with alkalinity in the water to form iron hydroxide Fe(OH)3. Iron floc is generally large and settles rapidly though it may be weaker than alum floc.

Ferric chloride is used particularly when a broad coagulation pH range is required. It is an effective coagulant and is typically used in water treatment over the pH range of 5 to 8.5. It has been used successfully outside this range, however iron residuals, DOC/NOM removal and possible DBP formation need to be monitored carefully.

When added to water, ferric chloride consumes more alkalinity than alum (see Table 2.2 in Chapter 2).

Ferric Sulfate Ferric sulfate is generally supplied as a liquid that is dark red brown in colour. It is not as corrosive as ferric chloride.

28

Chapter 3 – Coagulants And Polymers

Like ferric chloride, ferric sulphate is used particularly when a broad coagulation pH range is required. It is an effective coagulant and is typically used in water treatment over the pH range of 5 to 8.5. It has been used successfully outside this range, however iron residuals, DOC/NOM removal and possible DBP formation need to be monitored carefully.

Sodium Aluminate An advantage of sodium aluminate is that the chemical provides both aluminium and alkali. However, its use as a coagulant in water treatment is limited by cost and by its chemical properties, which make it more difficult to handle than alum or other metal salts. It is not commonly used, but has been used successfully in several small WTPs. PolyDiAllylDiMethylAmmonium Chloride (PolyDADMAC) PolyDADMAC is a cationic (positively charged) polyelectrolyte with a medium molecular weight and a high charge density. PolyDADMAC can be used as a primary coagulant or as a secondary coagulant to partially replace inorganic metal salts.

A small dose may significantly reduce the amount of inorganic salt required (thus reducing floc volume and sludge production and improving filter run times).

If polyDADMAC is used as a secondary coagulant, best results are usually obtained when it is added about 15 to 20 seconds after the primary coagulant. In some WTPs, the polyDADMAC is dosed at the same time or occasionally before the coagulant, however performance is likely to be improved by altering the dosing point to add the polyDADMAC after the coagulant. Jar testing can be used to confirm this.

PolyDADMAC is commonly used in conjunction with alum, ferric salts and ACH and pre-blended products are available from several chemical companies. However since polyDADMAC generally gives better results when dosed after the primary coagulant, it is preferable to use separate dosing systems. Separate dosing systems also allow the ratio between the primary coagulant and polyDADMAC to be modified easily.

When used as a secondary coagulant, together with an inorganic salt, concentrations are usually 0.2-1 mg/L (as polyDADMAC). When dosed alone, concentrations are typically 1-5 mg/L, however concentrations up to 10 mg/L can be used provided that the concentration of the residual monomer does not exceed 0.2 mg/L in the clarified water and there is no blinding of any filters used in the water treatment process. The dose of polyDADMAC should not exceed 10 mg/L. Jar testing is required to determine the dose of polyDADMAC. PolyDADMAC is not pH sensitive and is chlorine resistant. PolyDADMAC is most effective for the removal of particulate material and less effective for colour and NOM removal. It is used particularly in direct and contact filtration processes, where the objective of coagulation is to produce small, high-density floc.

When considering polymer concentrations and dose rates, it is important to realise that liquid polymers are not 100% active. For example, commercially available PolyDADMAC is supplied as aqueous solutions of between 20% and 40% (w/w). The higher % solutions are viscous and difficult to pump. The lower concentration polyDADMACs are generally preferred for this reason but are more expensive. The chemical is usually dosed as supplied and diluted with carrier water to less than 10% strength.

29

30

Polyaluminium chloride (PACl) Al2(OH)3Cl3

PAC-10 MEGAPAC-10

10-11% Al2O3 or 20-23% w/w PACl 10.5% w/w Cl

23-24% Al2O3 approx 50% w/w ACH 7.9-8.5% w/w Cl

Aluminium chlorohydrate (ACH) Al2(OH)5Cl.3H2O

PAC-23 MEGAPAC-23 ALCHLOR-23 PROFLOC-23

Typical Analysis

Aluminium sulphate 7.5-8.4% Al2O3 Al2(SO4)3.14 - 18H2O or 43-52% w/w Al2(SO4)3.14-18H2O

Name and Formula

1.18-1.22

1.32-1.35

1.28-1.34

SG (20oC)

Common coagulants used in the water industry in Australia

Alum

Common Name

Table 3.1.

50%

83-84%

Basicity (%)

Typically 6 to 8

Typically 6 to 8

Generally 5.8 to 6.5

5.5 to 7.7

pH Range

Used instead of alum where raw water has low alkalinity and high to moderate true colour.

More alkalinity consumed than ACH but requires less alkalinity adjustment than most coagulants because it has a medium basicity.

Very little alkalinity consumed because of its high basicity thus less alkalinity adjustment required.

Particularly suited for use in low alkalinity raw water with high to moderate colour.

Commonly used for coagulation before membrane filtration.

Most common coagulant used in water treatment.

Forms strong robust floc in most waters.

Application

Chapter 3 – Coagulants And Polymers

A 41-43% solution of 14.5% iron.

20% to 40% w/v

Ferric Chloride, Ferric Chloride Profloc F FeCl3.6H2O

PolyDADMAC

(C8H16NCl)n

Aqueous solution of 39– 45% w/w ferric sulfate (11–12.5% Fe)

Ferric Sulphate, Ferric Sulphate Megaclear 12 Fe2(SO4)3

1-1.1

1.45

1.5-1.6

1.45-1.6

Polyferric sulphate Fe2(OH)0.6(SO4)2.7 Also known as hydroxylated ferric sulphate

Polymerised ferric sulphate or PFS

12.2% w/w Fe or 43.7% w/w Fe2(SO4)3

18-21% w/w Al2O3 , 1.4-1.6 or 29-35% w/w Na2Al2O4 8-13% w/w free NaOH

NaAl(OH)4 or (Na2Al2O4)

Sodium aluminate

1.32-1.36

SG (20oC)

9.8% Al2O3 or 5.2% w/w Al

Typical Analysis

Polyaluminium Silicosulphate Al2(OH)3.24Si0.1(SO4)1.58

Name and Formula

PASS®

Common Name

Table 3.1. (cont.)

5-15%

54%

Basicity (%)

Used especially when a broader coagulation pH range is required.

Medium to high levels of manganese.

Used particularly when a broader coagulation pH range is required.

Has low levels of Mn.

Has low levels of Mn.

Strong alkaline coagulant. Can work on highly coloured water that has low alkalinity.

It can also be used in combination with alum to control alkalinity and pH.

Cannot be diluted with water because it forms floc immediately.

Used at some direct filtration WTPs.

Application

Usually used as secondary coagulant in direct filtration plants.

Not sensitive to Can be used as a primary coagulant in place of metal salts but low NOM and colour removal. pH.

5 to 8.5

5 to 8.5

5 to 8.5

pH Range

Chapter 3 – Coagulants And Polymers

31

Chapter 3 – Coagulants And Polymers Which Coagulant To Use? Alum has traditionally been the coagulant of choice because of its ability to adequately treat a wide variety of water types, its lower cost and general availability. Alum generally produces a good, reliable, robust floc across a range of water types and qualities.

There are however some disadvantages. Alum, like other metal salts, consumes alkalinity and lowers pH. It also forms hydroxide floc that contributes significantly to the quantity of sludge produced. This in turn results in increased costs for sludge management. Typically alum (or other metal salt) contributes between 30% and 50 % of the sludge volume. The amount of sludge produced can be estimated.

Estimation of Sludge Production There are many formulas available to estimate sludge formation however they all factor in the alum dose in a significant way. The following formula is one example.

Sludge production = 1 x turbidity + 0.1 x colour + 0.47 x alum dose (as mg/L Al2(SO4)3.18H2O) + 1 x total metals + 1 x polymer dose (as mg/L dry polymer) + 1 x lime impurities added

The equation shows that after turbidity, the biggest contribution to sludge production is the alum dose. However note that the turbidity ratio, which is 1 in the above equation can vary between 0.5 and 2 depending on the type of particles.

For coloured, low turbidity, low pH and low alkalinity surface waters typical of much of Eastern Australia, alum use usually requires pre-treatment with lime, soda ash or caustic soda to ensure that the optimum coagulation pH is achieved. This is especially important during periods of low alkalinity, poor quality water sometimes experienced during and after storm events.

Low alkalinity waters make the use of ACH or PACl, with their associated higher basicity attractive. It is often possible to coagulate using ACH without the need to dose any alkali for pH correction. (This may be particularly useful at small WTPs.) Simply replacing alum with ACH with no other chemical addition can result in superior performance. It may also result in lower production of sludge. This is not the case with all waters however and jar testing, pilot plant testing or plant trials should be carried out to determine the best overall coagulation process for both normal and poor quality water.

The ACH dose required is usually significantly less than that required for alum but ACH is much more expensive than alum. Overall there is usually an increase in the cost of treatment chemicals when changing over to ACH, however the lower sludge production, minimisation of the need to dose both pre and post treatment alkali, and reduced operational costs can still combine to make the use of ACH financially attractive.

Iron-based coagulants, such as ferric chloride, ferric sulphate and polymerised ferric sulphate, are used in a number of WTPs in Australia, particularly in NSW. They tend be more expensive than alum on an equivalent kg per metal dosed basis. They consume more alkalinity than alum, and hence tend to depress pH of the dosed water more dramatically. However they are effective over a wider pH range and are particularly useful when coagulating at high pH in association with iron and manganese oxidation.

Generally ferric salts produce a larger floc however these can be more fragile and in some cases may need to be strengthened by dosing polyDADMAC or other polymers.

Ferric salts produce an iron based sludge that can be more readily reused as a soil conditioner than alum based sludges. Common practice at present is to dispose of water treatment residuals to land fill, however land fill is becoming scarce and companies are having to rethink their disposal practices.

32

Chapter 3 – Coagulants And Polymers Water treatment sludges can be composted with green waste to produce useful soil conditioners. Reuse of aluminium based sludges needs to be considered carefully since in acid soils the aluminium can be released and can be toxic to some species of plant. However its use in more alkaline soils can be beneficial. There is no similar toxicity with iron sludges. With both types of sludge there can be issues with the binding of phosphorus, however this can usually be managed with the help of an agronomist.

Some ferric salt products contain significant quantities of manganese, and should be checked with the manufacturer prior to use. Usually ferric sulphate and PFS are lower in manganese concentration (see Table 4.2 in Chapter 4).

Ferric chloride is extremely corrosive to most metals. Ferric salts produce highly visible blood/rust-coloured stains when there are chemical spills and leaks.

Ferric salts were favoured for a time when aluminium in drinking water was thought to be linked to Alzheimer’s disease, however this is no longer considered to be the case.

The main advantages and disadvantages of the common coagulants are summarised in Table 3.2.

Table 3.2. Advantages and disadvantages of the main coagulants.

Advantages

Disadvantages

Alum

Low cost.

Effective in low turbidity water due to formation of Al(OH)3 floc and sweep coagulation.

Readily available.

Robust, stable floc produced.

Generates large volumes of sludge.

Consumes alkalinity and lowers pH. Increase in residual Al if pH too low.

Corrosive material.

Relatively narrow pH range.

Iron salts

Readily available.

Wide coagulation pH range.

Sludge is more readily reused as a soil conditioner.

Residual iron sometimes preferred over residual aluminium to consumers.

Large, weaker floc.

Generates large volumes of sludge.

Consumes alkalinity and lowers pH. Increase in residual Fe if pH too low.

Corrosive material.

May introduce manganese.

ACH and PACl

Lower dose.

Lower volume of sludge.

Consumes less alkalinity.

Effective over wide pH range.

Higher cost.

Not effective in all waters.

PolyDADMAC

Forms strong stable floc especially as a secondary coagulant.

Low sludge production. May be considered in high turbidity water.

Does not affect pH or alkalinity of the water.

No Al or Fe residuals.

Relatively non hazardous to use.

More expensive.

Overdosing may blind filter media.

Floc may be weak if dosed alone.

Not very effective for colour and NOM removal.

33

Chapter 3 – Coagulants And Polymers

Coagulation, Flocculation And Filtration Aids The processes of coagulation, flocculation and filtration can often be assisted by the use of organic (carbon containing) polymers (“poly”). Polymers are very large molecules that are actually chains of many smaller molecules known as monomers, rather like beads on a necklace. The majority of polymers used in the water industry are synthetic, however there are also a number of natural polymers in use. For example chitosan is produced from crab or lobster shells and TannflocTM is produced from plant material.

Polymers vary in the size, length and branching of the molecules (molecular weight) and the charge on those molecules. The net charge may be positive (cationic polymers), negative (anionic polymers) or no charge (non ionic polymers). They may also have a high charge density or a low charge density. Organic polymers with a charge are also referred to as polyelectrolytes.

Polymers act as bridging agents binding floc together when part of a polymer adsorbs to a floc particle and another part of the same polymer adsorbs to another floc particle.

Polymers may be used in a number of different ways in a WTP. •

•

•

• •

A coagulation aid is dosed in conjunction with or immediately after the coagulant to strengthen the floc as it is formed, but there is no reduction in primary coagulant dose. This is not commonly done these days because polyDADMAC (used as a secondary coagulant) has largely taken over this role.

A flocculation aid is dosed at the end of or just after coagulation to assist in floc agglomeration.

This is the most common use of polymer. The dosing point can be varied depending on the process or multiple dosing points can be provided. Polymer is also commonly dosed prior to the dissolved air flotation (DAF) chimney to assist in the binding of fine bubbles and floc. It is also dosed in the ActifloTM process to bind the fine microsand with the floc.

A filtration or filter aid is dosed prior to filtration to condition the media and make it more “sticky” to the floc and to bind floc to the media. Filter aids are generally anionic or non ionic with a high molecular weight and a low to zero charge density. Filter aids are generally added just as the water enters each filter inlet or the settled water channel distributing water to the filters. By using the settled water channel a single dosing pump can be used rather than one for each filter and the process is therefore easier to control as long as the filter aid is well mixed prior to reaching the first filter.

A thickening aid is dosed to waste wash water to improve settling. These are usually high molecular weight polymers with a high charge density.

A dewatering aid is dosed into centrifuges or other devices to concentrate and dewater sludge.

These definitions appear very specific, however in practice most WTPs are designed with multiple dosing points, and both the polymer dosing point and dose can be manipulated to achieve the best result. In some situations, polymer will even be overdosed at one location to serve more than one role such as a flocculation aid carrying over onto the filters and thereby acting as a filter aid. How a polymer works to aid flocculation is shown in Figure 3.1.

Figure 3.1. Action of polymer enmeshing floc particles to form larger floc.

+

Floc particles 34

Polymer chain

Agglomerated floc

Chapter 3 – Coagulants And Polymers There are many different polymers in use in the water industry. Not all polymers are approved for use in the production of potable water. Interestingly there are some polymers that are approved for use in potable water where the monomeric unit is in fact harmful to health while the polymeric form is not. These include polyacrylamides and polyamines. Therefore it is important to be aware that any residual monomer may present a risk to health and needs to be monitored. The ADWG (2004) states that “based on health considerations the concentration of acrylamide in drinking water should not exceed 0.0002 mg/L and the concentration of epichlorohydrin should not exceed 0.0005mg/L.” Both of these chemicals are monomers and thus to some extent it is the level of residual monomer that determines the maximum doses of a polymer.

Which Polymer?

There are a bewildering array of terms relating to polymer use, polymer names and codes in use in the industry. Appendix 2 provides a list of many of the common polymers used in the water industry in Australia.

Polymer manufacturers often change names of the products and the names used say little about the nature of the polymer. The Appendix is likely to become quickly outdated but will be of assistance for many polymers.

Polymers are also available from manufacturers other than those listed in the Appendix.

The choice available in polymers is large and there is no simple solution as to which polymer to use in a particular situation. However the following points may be useful: • •

• • • • •

Cationic, anionic or nonionic polymers (almost always polyacrylamides) may be used for any of the purposes listed above to assist in strengthening and binding particles together.

Raw water turbidity, colloids and colour are usually negatively charged. For this reason primary and secondary coagulants are positively charged. However once charge neutralisation has occurred during coagulation, non-ionic polymers are usually best for use as flocculation aids.

Filter aids are often non-ionic or anionic.

Polymers used in sludge thickening and dewatering are usually of different charge and higher molecular weight than any used earlier in the process.

Jar testing a range of polymers as coagulation or flocculation aids indicates which polymers are most compatible with the floc produced.

Jar testing a range of polymers for sludge thickening is also worthwhile.

The benefit of filter aids can only accurately be determined by pilot testing or by plant trials.

Polymer Dose Rates Care should be taken in dosing polymer. Under dosing of polymers usually produces very little effect.

Overdosing of polymers can produce dramatic water quality improvements but can cause sludge blankets to sit too low in the clarifier and can bind up filters. Even when the polymer is dosed as a flocculation aid, excess polymer will carry over into the filters and act as a filter aid. Excess polymer can rapidly overload the filters leading most commonly to filter mud balls and cracked surfaces in the media.

WTPs that dose polymer upstream of the filters must have air scouring as part of their backwash cycle in order to be able to clean the filter beds properly.

Table 3.3 provides recommended dose rates for polymers used at different points in the treatment process.

35

Chapter 3 – Coagulants And Polymers Table 3.3. Recommended dose rates for polymers according to their use in the process.

Normally Acceptable Dose Range (mg/L)

Maximum Dose (mg/L)

Coagulation or Flocculation Aid

0.05 – 0.15

0.2

Filter Aid

0.01 – 0.05

0.1

Polymer Function

When calculating dose rates it is important to realise that liquid polymers are not 100% active. The actual % active ingredient needs to be determined by reference to the manufacturers product sheet and this should be factored in to any calculations (see Chapter 6).

Making Up Polymers Polymers used in the water industry are most commonly provided as dry powder, dissolved in water (solutions) or suspended in water (emulsions).

Dry powder polymers need to be activated. The polyacrylamide powder is added to water to achieve a solution concentration of between 0.05% and 0.3%. More dilute batch solutions are satisfactory as long as the dosing pumps have sufficient capacity. Polymer make-up is best undertaken in a proprietary batch unit.

(Figure 3.2) Figure 3.2. Two examples of commercially available “poly” batching units.

The powder must be added in an even flow and dispersed into the water to prevent the formation of large sticky lumps. Even if the mixing is good, fish eyes (a polymer gel around the white powder trapped inside) may result.

Individual polymer grains need to be independently wetted. A wetting device is required, which in its most basic form consists of a funnel continuously fed with polymer during the batching cycle, and unchlorinated service water fed tangentially near the base of the funnel so that the polymer and water mix smoothly and evenly.

After wetting, the polymer should be aged to allow the long chain molecules to unravel and become most efficient. This ageing process generally requires between one and four hours of gentle mixing.

Batches can go off if not used quickly enough. Batch sizes should be adjusted to provide up to three days supply and no longer, to limit polymer ageing. With polyacrylamides, a fishy smell around the batching equipment is a sure sign of “off” poly.

36

Chapter 3 – Coagulants And Polymers Make-up and dilution water for polymers should be unchlorinated filtered water because most polymers are not fully resistant to chlorine and lose some of their efficiency in contact with chlorine.

Tanks and dosing lines should be kept thoroughly cleaned to ensure satisfactory poly make-up and delivery.

Floors should be kept very clean because polymer is very slippery and may contribute to falls.

Dosing The actual polymer dosing point needs to be considered carefully. Polymers are delicate long chain molecules and can be damaged if handled roughly during mixing or thereafter. For example, dosing filter aid into a channel and then having the water pass into the filters via a weir providing significant turbulence can cause break up the polymer and floc and result in poor water quality.

In general the more dilute the solution the better it mixes, but there are restrictions on tank size and in some cases adverse reactions can occur at low concentrations. Polymers can be dosed directly as low concentration solutions but it is preferable that they are also dosed with dilution water. Carrier or dilution water allows for partial mixing prior to dosing and transports the dosed chemical to the dosing point faster (depending on pipe sizing), reducing response times and allows flushing of the lines on shutdown. A 10:1 dilution is commonly used.

Polymer function is aided by ensuring the polymer solution is spread evenly through the water being dosed.

Dosing of chemicals may be via single point discharge pipe into a mixer but generally the more dispersed the dose, i.e. the more and smaller the holes, the better the mixing at the application point. Dosing may be via injection quills/spears or sparges across a pipe with multiple discharge holes or nozzles or via a diffuser pipe across a channel (Figure 3.3).

Figure 3.3. A simple delivery system for a filter aid polymer

The sparge or diffuser system should be readily accessible and able to be removed easily to allow for cleaning or periodic replacement necessary to prevent blockages. Inclusion of an isolation valve is useful to allow removal of the dispersal system for cleaning when the pipe work is pressurised.

Polymer function is further aided by the provision of some mixing. This may be either mechanical or hydraulic.

Simple hydraulic mixing can be achieved by pointing the chemical discharge upstream so that the water flow helps mix the chemical as it turns it around and also achieves some flow variation around the diffuser/ sparge itself.

37

Chapter 3 – Coagulants And Polymers Polyacrylamide dosing points depend on their function. The dosing point should be selected so that some mixing is provided (via pipe bends, flow meters, channels or weirs) and that even distribution of polymer is achieved. Coagulation aid polymers are usually dosed with the coagulant. Flocculation aid polymers may be added after the metal coagulant or may be dosed just upstream of the flocculation zone or indeed anywhere in between. Filter aid polymers should usually be dosed as close to the filters as possible such that there is no significant mixing of the floc and polymer. Dosing should occur after any filter inlet valves, weirs or baffles.

The ideal WTP has multiple dosing points for polymer to provide operational flexibility.

The dosing sequence (except for filter aid) should be tested, optimised and verified periodically through testing the chemical sequence in jar tests.

Chemical Dosing Sequences The sequence of addition of chemicals for coagulation and flocculation is often very important and is best determined in a jar test. However it is possible to provide some general advice: •

• • • • • •

38

Dosing of an oxidant (chlorine, potassium permanganate, ozone) for iron and manganese removal should occur first. Where potassium permanganate is dosed, alkali may need to be dosed prior to this to establish the optimum pH for manganese oxidation.

PAC dosing, if required, should occur second but only after sufficient time (5-10 minutes) is allowed for any oxidation of iron and manganese.

Dosing of alkali for pH adjustment.

Primary coagulant dosing should occur next (with a coagulant aid if required).

Cationic polyDADMAC added as a secondary coagulant should be added 15 to 20 seconds after the primary coagulant (or coagulant aid).

Flocculation aid should be dosed just after the formation of pin floc and prior to flocculation.

Filter aids should be added prior to the filters, although care should be taken to separate polymer dosing and pre-filtration chlorination if it is also employed.

Chapter 4

Chemical Quality Assurance The following case study provides a background for the need for a chemical supply quality assurance program.

“The incident occurred when an inexperienced delivery contractor dumped 20 tonnes of concentrated aluminium sulfate solution into the wrong tank at the unmanned Lowermoor (UK) water treatment plant.

This resulted in the distribution of contaminated water to a large area of north Cornwall. The event has often been referred to as the Camelford incident in reference to the town closest to the water treatment works. The affected area was estimated to have a resident population of about 12,000 people with an additional 8,000 summer visitors being present at the time of the incident, however not all people in the area would have consumed the contaminated water.

The incident began at about 5pm on Wednesday 6 July 1988 when the delivery truck driver mistakenly deposited the aluminium sulfate solution into the chlorine contact tank at the treatment works. Within about three hours, the local water company, the South West Water Authority (SWWA), began to receive complaints from customers that their tap water tasted unpleasant and felt sticky to the touch. When SWWA investigated, they initially attributed the low pH and high aluminium sulfate content to a problem with lime dosing at the plant, and at about 11pm on 6 July they began flushing mains to remove the acidic water from the distribution system. The true source of the problem remained undiscovered until sometime on the morning of Friday 8 July when it was noticed that the level in the aluminium sulfate storage tank was low despite a delivery having been recorded on Wednesday. This failure to correctly diagnose the problem in the first few hours of the incident resulted in the entire load of aluminium sulfate being drawn into the distribution system during the intervening two days.

Over the months following the incident a number of residents of the area complained of symptoms including skin rashes, arthritic pains, sore throats, loss of memory and general exhaustion. These complaints were investigated by health authorities and the government appointed the Lowermoor Incident Health Advisory Group to report on the health effects of the incident. In two reports, delivered in 1989 and 1991, the Advisory Group concluded that there was no convincing evidence that harmful accumulation of aluminium had occurred, nor that there was a greater prevalence of ill health due to the toxic effects of the contaminated water.”

Source: http://www.waterquality.crc.org.au/hsarch/HS37a.htm This case study is an important and real demonstration of the potential problems WTP operators may face.

Risks Associated With Drinking Water Chemicals A cornerstone of the management of drinking water quality is the analysis of hazards and the management of risk.

The intentional addition of chemicals to water intended for drinking purposes carries with it a potential risk.

This may result from any of the following: • • • •

The chemical itself.

Overdosing of the chemical.

Contaminants in the chemical arising from the manufacturing process or the raw materials used.

By-products formed through the use of the chemical.

39

Chapter 4 – Chemical Quality Assurance A complete water quality management program needs to recognise any potential risks from use of drinking water treatment chemicals and include strategies to manage them appropriately. This risk must be minimised by the implementation of a fully developed quality assurance system for the production, supply and use of water treatment chemicals.

Contamination of chemicals can be minimised by the use of good manufacturing practices that use quality control and quality assurance programs to maximise product purity. The purity of chemicals used in Australia for the treatment of drinking water supplies will vary depending on the manufacturing process. Specific contaminants may occur in specific treatment chemicals due to the manufacturing process. The impact of any impurities on treated water quality depends on the levels of the impurity and the amount of chemical used in the treatment process. Where impurities are present these will be added to the water in proportion to the amount of chemical dosed.

Managing Risks The first step in managing the risk associated with the use of drinking water treatment chemicals is to ensure that the chemicals supplied meet a minimum standard. Figure 4.1 shows an actual example of a chemical supplied to a WTP on an adhoc basis with no chemical quality assurance.

Figure 4.1. What is really in the bag???

Water utilities should formally specify the strength of the active ingredient and acceptable contaminant levels in each drinking water treatment chemical. However, this in itself will not adequately control the risk.

The contractual requirement should be supported by batch-testing data provided by the supplier from an independent accredited laboratory, and random testing carried out by the water utility itself. Chemicals should not be accepted for delivery unless a batch analysis certificate has been obtained and checked by the water utility staff.

Chemical suppliers should be evaluated and selected on their ability to supply products in accordance with required specifications. Formal accreditation of the manufacturing facility by an independent accreditation agency such as the International Organisation for Standardisation (ISO) or National Sanitation Foundation (NSF) International provides a further level of risk management. Such accreditation should include random site visits to the manufacturing facilities by the relevant regulatory agency and, if warranted, the water utility.

Documented procedures for the control of chemicals, including purchasing, verification, handling and storage should be established to assure the quality of the chemical at the point of application. Responsibilities for testing and quality assurance of chemicals (supplier, purchaser or both) should be clearly defined in purchase contracts.

40

Chapter 4 – Chemical Quality Assurance A second and important step in a quality assurance system for chemical addition to drinking water is to ensure that the correct chemical, of the correct quality, at the correct strength, is delivered into the correct storage vessel, at the correct site at the correct time. This is necessary to: • • •

Ensure that the correct chemical is used in drinking water treatment.

Ensure that cross contamination of storages does not occur.

Help to ensure the health and well being of staff and contractors during the delivery and dosing process.

Broadly, the objective of the quality assurance system is to manage all the steps associated with the supply, storage, use and handling of water treatment chemicals that could adversely impact upon the health and wellbeing of staff, contractors and consumers.

An effective quality assurance system consists of: • • • •

A standard operating procedure for the delivery and receipt of chemicals at each delivery site.

An identity check directly linking the delivery driver to the chemical company.

The provision of details of all delivery sites (eg digital photographs and/or detailed maps, with the locations of fill points, storage vessels and safety equipment clearly identified).

The clear identification and labelling of chemical storage vessels, filling points and delivery pipe work at all sites (locks on filling points are desirable) (Figures 4.2 and 4.3).

Figure 4.2.

• • • •

A chemical delivery point prior to labelling

Figure 4.3.

The same chemical delivery point after labelling

A requirement that chemicals should only be delivered when an appropriate water utility staff member is present.

A delivery driver induction system for each site, with each driver inducted onto each site and appropriate records kept.

A check that a batch analysis certificate has been received by the water utility before any delivery.

An appropriate monitoring program, to ensure compliance of chemicals with specifications.

41

Chapter 4 – Chemical Quality Assurance •

• •

A formal checklist for the dispatch and delivery process, including: • Verification of the order.

• A gross visual check on the product before it leaves the production site.

• A second gross visual check by the water utility representative at the delivery site before unloading.

• A check by both parties that the delivery vessel is being connected to the correct storage vessel.

• A check that appropriate personal protective equipment is being worn, and that relevant health and safety requirements are being addressed.

• Filing of the completed checklists and records being kept for an appropriate period.

A system to ensure that any spillage associated with the delivery process is contained and does not escape to the environment.

An emergency procedure in the event of possible systems failure or human error.

The combination of a chemical quality assurance system and a delivery and storage quality assurance system can significantly reduce risks to all stakeholders.

Specifications For Drinking Water Treatment Chemicals The preparation of specifications for a chemical supply contract can be a daunting, time consuming and difficult task. To simplify the process for water utility staff, the ADWG (2004) Chapter 8 provides an example specification. This has been reproduced here (Table 4.1). The specification is for the supply and delivery of liquid Aluminium Sulfate. The specification includes details on the Al2O3 content required, product clarity, solids content and pH as well as specific impurity limits. The specification also details some delivery and acceptance criteria.

Contaminant Levels The ADWG (2004) provide a table (Table 8.4 of ADWG) listing recommended maximum impurity concentrations for some commonly used drinking water treatment chemicals when used at particular dose rates. They also provide an example of how to calculate recommended maximum impurity concentrations.

At high doses of treatment chemicals, the final treated water should be regularly tested for contaminants to ensure that the maximum contaminant levels have not been exceeded. The ADWG (2004) provide health limits for all contaminants of concern to public health and should be consulted to determine whether the contaminant levels exceed those health limits. Clearly if they do, the use of the chemical responsible for the contaminant should be stopped and an alternative with lower impurities sourced.

42

Chapter 4 – Chemical Quality Assurance

Table 4.1 Example specification for the supply and delivery of liquid alum This specification is for the supply and delivery of liquid alum (Al2(SO4)3.18H2O). This specification is based on the American Water Works Association Standard for Aluminium sulfate – liquid, ground or lump (ANSI/AWWA B403-93) and the Water Chemicals Codex (NRC, 1982).

REQUIREMENTS Material Safety Data Sheets (MSDS) The successful Tenderer must supply a current MSDS with a review date not exceeding five (5) years. The MSDS must, as a minimum, comply with the requirements of the National Occupational Health and Safety Council (NOHSC) MSDS Guidelines. Whilst the NOHSC MSDS format is preferred, alternative formats exceeding the level of information required by NOHSC MSDS Guidelines are acceptable.

Clarity Liquid alum shall be of such clarity as to permit the reading of flow measuring devices without difficulty.

Content of Aluminium The water soluble aluminium content of liquid alum is expected to be greater than or equal to 4.23% of Al, or to fall within the range of 7.5 to 8.0 % as Al2O3.

Suspended Solids In liquid alum, it is expected that the level of suspended solids is below 0.2%. pH The pH of liquid alum is expected to fall within the range of 2.3 to 2.8 pH units.

Specific Impurity Limits It is expected that the total water-soluble iron (expressed as Fe2O3) content of liquid alum shall be no more than 0.35%.

The level of heavy metal contamination of the liquid alum shall be such that compliance with the Recommended Maximum Impurity Content (RMIC) values from the Water Chemicals Codex (NRC, 1982) is achieved. The RMICs are given below based on a maximum dose rate of 150 mg/L. RMIC values of heavy metals in Alum Impurity Concentration (mg/kg) 30 Arsenic Cadmium 7 30 Chromium Lead 30 Mercury 1 Selenium 7 30 Silver

43

Chapter 4 – Chemical Quality Assurance Table 4.1. Example specification for the supply and delivery of liquid alum (cont.)

VERIFICATION Quality Assurance The supplier is expected to posses a Quality System that facilitates the tracking of product from raw material to delivery. The Quality System may be audited to verify the correctness of information relating to the purchased product. In addition, the product may be sampled to verify the quality of the supplied product.

Sampling If the product is to be sampled at the point of delivery, the sampling procedure outlined in the American Water Works Association Standard for Aluminium Sulfate – Liquid, Ground, or Lump (ANSI/AWWA B403-93) will apply.

Nonconforming Product If the delivered aluminium sulfate does not meet the requirements of this specification, a notice of nonconformance will be issued within ten working days of the receipt of the goods.

A nonconformance will also be issued if deficiencies are detected during any audit of the supplier’s Quality System.

DELIVERY Liquid Marking, packaging and shipping of aluminium sulfate shall comply with AS 3780-1994 The Storage and Handling of Corrosive Substances, and current Federal, State, and local regulations.

The carrying vessel shall be in a suitable condition for hauling liquid alum and shall not contain any substances that might affect the use or usefulness of the liquid alum in treating potable water or in treating wastewater.

Contamination Bulk or semi-bulk containers shall be carefully inspected prior to loading of the chemical by the supplier to ensure no contaminating material exists.

The supplier must have a system in place to ensure that liquid alum is not contaminated by any other product.

This may involve implementing a specific cleaning regime between loads or the dedication of tankers or containers to only one type of product.

Certificate of Weight Bulk shipment weight certificates may be inspected at any time.

Affidavit of Compliance An affidavit of compliance from the manufacturer or supplier to the effect that the aluminium sulfate complies with all applicable requirements of this specification is required with each delivery. A certified analysis of the aluminium sulfate is also required prior to acceptance of a delivery.

Documentation A copy of the order, the delivery docket, and the affidavit of compliance and/or the record of certified analysis will accompany the delivery of aluminium sulfate. This documentation shall be left in an appropriate location at the delivery point.

The contaminant levels of chemicals supplied by different manufacturers can vary considerably. Table 4.2 provides information relating to ferric salts available from three main manufacturers. The table is based on information provided by the manufacturers.

44

Chapter 4 – Chemical Quality Assurance There are significant differences in most of the contaminants. Notably some of the manufactured iron salts contain significant levels of manganese, enough to compromise most treated water contaminant concentration targets; others contain low levels of manganese. For example, a WTP dosing say 50 mg/L of the Company C product would add 0.06 mg/L of Mn to the final water; well above the usual target of 0.02 mg/L for manganese necessary to minimise dirty water complaints.

Table 4.2. A comparison of ferric chloride and ferric sulfate quality comparisons

Parameter Specific gravity @ 20 C Acid Content (% w/w) Ferric salt anhydrous (% w/w) Ferric iron (% w/w) Ferrous iron (% w/w) Manganese (mg/kg) Nickel (mg/kg) Zinc (mg/kg) Titanium (mg/kg) Phosphate (mg/kg) Chromium (mg/kg) Cobalt (mg/kg) Copper (mg/kg) Arsenic (mg/kg) Mercury (mg/kg) Cadmium (mg/kg) Lead (mg/kg) Selenium (mg/kg) Fluoride (mg/kg)

Ferric Chloride Company A

Ferric Sulfate Company B

Ferric Sulfate Company C

1.45 <2 >42 >14.5 <0.5 <400 <80 <50

1.58 2.1 43 to 45 12.0 to 12.5 0 250 14 3

1.50

<100

25 2 18 <1 <1 <0.05 <1 <1

<100 <10 <1 <2 <20 <5 <100

42 11.2 <0.2 1,200 55 53 41 30 8 5 3 1.55 0.23 <1 <1

There is clearly a need to test treatment chemicals and the final treated water regularly!

Maximum Dose Rates If high doses of coagulant are planned, it would be advisable to check the contaminant levels in the treatment chemical and calculate the resultant contaminant levels in the treated water and compare them with those health limits listed in the ADWG (2004). Examples of the calculations are provided in Chapter 8 of the ADWG (2004).

Many WTPs in Australia occasionally need to treat particularly dirty water that may require very high alum doses (up to 200mg/L), or other coagulants. Operators of such plants must be alert to changes in raw water quality that will require very high doses and ensure extra monitoring is in place to test for contaminant levels in the final water.

45

Chapter 5 Jar Testing Jar Testing Jar testing (Figure 5.1) is perhaps one of the most important tests required at a Water Treatment Plant (WTP) and all operators should aim to be proficient at carrying out these tests. Jar testing is generally used to determine how much and which coagulant(s) and aids to dose for a particular raw water, at a particular time. Other parameters such as mixing rate, contact times, floc formation times, settling rates and sludge production may also be examined.

Figure 5.1. Jar testing being carried out in a well set up WTP laboratory.

The chemicals and doses required will vary according to the overall process operating (conventional treatment, dissolved air flotation and filtration, contact or direct filtration or membrane filtration), the raw water quality and temperature. Jar testing allows a range of different chemical doses to be tested and compared in a laboratory. Once determined, the optimum dose can be applied to the plant and fine-tuned.

Jar testing should be carried out regularly during routine operation but in particular when the raw water source is changed or when any significant variation in raw or treated water quality is detected (Figure 5.2).

46

Chapter 5 – Jar Testing

Figure 5.2. Changing raw water quality requires operators to be alert and in most cases carry out lots of jar tests.

With experience, operators can learn how water quality and other factors influence the treatment process.

For example, higher levels of colour require higher alum doses, and as raw water turbidity increases, higher cationic polyDADMAC and higher polyacrylamide doses may be required. Lower temperatures will require either higher chemical doses or longer contact time throughout the process, as reaction times are slower.

The basic jar test can be modified to incorporate this specific knowledge.

To achieve the best results from the jar test, the test should be set up to simulate the plant operating conditions as closely as possible. Similar initial mixing energies, flocculation energies and contact times should be used.

For example if the plant does not have an efficient static or flash mix system then using a high rpm for the flash mix stage may give misleading results. In this situation the initial mixing should be done at a lower rpm to mimic poorer initial mixing in the plant.

Ideally, a series of jar tests should be performed, optimising the dose of one chemical at a time. The general approach is to perform tests in the following order: • • •

•

Optimise coagulant dose with the coagulant alone.

Optimise alkali/acid dose to achieve best pH of coagulation (if required).

Optimise cationic polyDADMAC (secondary coagulant) dose with coagulant and alkali/acid (if required) – although polyDADMAC dosing usually results in a significant reduction in coagulant dose.

Optimise polyacrylamide (flocculation aid) dose with optimised coagulant and alkali/acid doses.

Note that filter aid doses are not generally determined by jar testing since the performance and dose of filter aid are mainly dependent on the characteristics of the filter media.

Results of jar tests should be recorded on jar testing sheets and filed. A sample jar testing record sheet is provided in Appendix 4.

The following sections describe what equipment is required and how to conduct a jar test.

47

Chapter 5 – Jar Testing Jar Test Equipment Typical jar testing equipment includes: •

•

• • • • • •

Jar testing apparatus with adjustable speed mixing paddles.

• A six jar system is preferable to a four jar system because it allows extra combinations of chemicals to be tested quickly.

• Older style jar testers with dial up knobs are often easier to use than the newer digital versions where the up and down arrows for paddle speed adjustment can be difficult to adjust quickly and accurately.

1 L beakers (many one litre beakers can really only be used with about 900 mL in them. Try to buy beakers that can actually hold 1 litre without overflowing, but if you can’t the difference is relatively insignificant.) Some prefer square jars to circular jars.

1L plastic measuring cylinders.

1 mL, 2.5 mL and 5 mL plastic syringes (A few 20 mL syringes are also useful for sampling water from the jars).

Adjustable micropipette(s) (0-1 mL and 1-5 mL are useful).

Whatman No.1 filter paper circles (~ 10 cm diameter).

6 x 500 mL plastic Erlenmeyer flasks and 6 x medium sized plastic funnels.

Stock solutions of the chemicals to be tested.

Stock Jar Testing Solutions Typical stock solutions and recipes for coagulant solutions for jar testing are provided in Table 5.1. Ensure that all solutions are clearly labelled with the concentration, date made up and the initials of the person who made up the solution.

NB: For alum jar testing the way you make up the stock solution depends on how the concentration of alum is expressed at your WTP. It may be in mg/L of the active ingredient, for example Al2(SO4)3.18 H2O or it may be in mg/L of supplied liquid. The table includes recipes for both.

New jar test solutions should be made up whenever a new batch of chemical is received. However before throwing out the old solution it is worth conducting a jar test using both the old and new stock solution to make sure the new chemical is essentially the same as the old one. Otherwise jar test solutions should be renewed as indicated by the shelf life in Table 5.1.

Solutions for the adjustment of pH are also necessary for jar testing. Where possible, it is better to use the pH adjustment chemicals that are actually used in the WTP. So for adjusting pH, make up a 1% solution of alkali or acid. Take 1 mL of the concentrated solution and add 99 mL of deionised water and mix well.

For lime, weigh out 1g of lime and add 100 mL of deionised water. Place this on a magnetic stirrer and keep it continually mixed. Label the bottles. Use reagent grade products or the products supplied by your water treatment chemical supply company. Some cheap alkali and acid preparations have high levels of contaminants and may give a poor indication of final water quality particularly if the jar tests are being used to evaluate Fe and Mn removal.

Try the solution in some jars of raw water. If it takes too many drops to adjust the pH, make up a slightly stronger batch. Alternatively, if the pH adjustment is too sensitive make up a more dilute solution.

48

X mL concentrated solution made up to 1 L where X= 1 g supplied polyDADMAC liquid into 1 L deionised water.

1% w/v = 10g/L of active filter alum (Al2(SO4)3.14-18 H2O

0.1% w/v = 1g/L of supplied polyDADMAC.

0.01% w/v = 0.1 g/L of supplied polyacrylamide.

Alum (where the units are mg/L of active ingredient)

Cationic polyDADMAC

Cationic/ Nonionic/ Anionic polyacrylamide

1 mL of this stock 0.1% solution in 1 L of raw water gives dose of 1 mg/L of liquid polymer. (If the actual concentration of active ingredient is required this can be calculated using the solution strength and the SG. (See chapter 6)) 1 mL of this 0.01% solution in 1 L of raw water gives dose of 0.1 mg/L.

Disperse powder in ~30 mL methylated spirits. Add water and stir until the stock solution is homogeneous; approximately 2-4 hours.

1 mL of this stock 1% solution dosed into 1 L of raw water in a jar gives a dose of 10 mg/L of active alum.

1 mL of this stock 1% solution dosed into 1 L of raw water in a jar gives a dose of 10 mg/L of supplied liquid alum.

Jar Test concentration

Mix well.

This solution can be quite hard to make up because of the high viscosity of the supplied liquid.

Mix well.

Mix well.

Mixing

1 week

2 weeks

1 month

1 month

Shelf Life

For pH adjustment, use stock solutions of 10% soda ash or 2-5% sulfuric or hydrochloric acid.

* When using PAC or potassium permanganate in jar tests, make up stock solutions of 1%, 0.1% or 0.01% depending on the required doses in your jars. 0.1% is a good starting point.

p 53). Glass pipettes should be clean. If adjustable pipettes are used these should be calibrated. Simply measure 1 mL of deionised/distilled water and weigh it on the balance. If it doesn’t weigh 1.000gm the pipette needs to be calibrated or alternatively just adjust the volume until the volume delivered reads 1.000gm on the balance.

# The recipes require the measurement of accurate volumes or weights of coagulants. Most WTP labs have pipettes but may not have balances. Cheap good quality balances are available (see

First make up a 0.1% stock with 1g of powder into 1 L deionised water.

Then add 100 mL of this stock solution to 1 L deionised water to make 0.01% solution.

1000 SG x %w/w conc solution strength

10 g liquid alum into 1 L deionised water

1% w/v = 10g/L of supplied liquid.

Alum (where the units are mg/L of supplied liquid)

Recipe#

Typical stock solution strength*

Chemical

Table 5.1. Typical stock solutions for jar testing.

Chapter 5 – Jar Testing

49

Chapter 5 – Jar Testing If it becomes important to know the actual amount of alkali or acid added, the concentration of the active ingredient can be calculated using the specific gravity and % concentration of the concentrated acid or alkali.

This information would only really be important for designers or for careful evaluation of dosing costs of different acids or alkalis.

Basic Jar Testing Procedure •

Determine the coagulant doses to be tested: • If the jar test is being carried out to fine tune the plant, use the current coagulant dose and step up or down in 2 mg/L steps.

• If the jar test is being carried out as a result of a change in raw water quality, use the current coagulant dose and step up or down in 5 mg/L steps. Table 5.2 provides an example.

Table 5.2. Examples of coagulant doses (mg/L) relative to a hypothetical current plant dose of 35 mg/L.

•

•

•

• • • • •

50

Jar Test Purpose

Jar 1

Jar 2

Jar 3

Jar 4

Jar 5

Jar 6

Fine tuning

31

33

35

37

39

41

Worsening water quality

30

35

40

45

50

55

Unknown Water

10

20

30

40

50

60

Determine the volume (mL) of coagulant required to be added to the jars and enter these on the jar test sheet.

• Volume required (mL/L) = dose/10 for a 1% solution into a 1 L beaker. e.g. 15 mg/L divided by 10 = 1.5 mL of stock jar test solution.

Collect a fresh sample (approximately 20L) of raw water. In general the raw water must be collected from a site prior to the addition of any chemicals and must be representative of the water going into the plant. However in the situation where other chemicals such as potassium permanganate and or PAC are dosed prior to alum, this water should also be tested and the results of both tests compared.

Analyse the raw water for: • Turbidity.

• True colour (filter through 0.45 micron membrane filter then measure colour).

• Alkalinity (if the alkalinity is < 25 mg/L as CaCO3, lime, caustic or soda ash will need to be added to bring the alkalinity up to at least 25 mg/L).

• pH. • Iron and manganese may also be tested, depending on the purpose of the test.

Fill the 1 L beakers (‘jars’) with 1000 mL, (or 900 mL depending on the beakers) of raw water and place these under the jar testing stirrers.

Lower the paddles into the beakers and set the stirrer to slow stir at approximately 30-35 rpm.

If alkali or acid is needed to achieve the optimum pH, dose the required chemical and mix for 15 seconds.

Fill syringes with the required volume of stock solution and place in front of the correct jar. (Alternatively, adjustable micropipettes may be used.) Place the syringes in the order they are to be added.

Increase the stirring rate to that required for the rapid mix (between 130 rpm and 250 rpm depending on the mixing system in use in the plant and the upper limit of the jar tester. For example, if the coagulant is simply injected into a pipe with no turbulent mixing use 130 rpm, if there is an in-line mixer or other rapid mix system then 250 rpm is better).

Chapter 5 – Jar Testing

• • •

•

• •

•

•

•

•

• • •

Add each chemical as quickly as possible below the surface of the water and in the same order they would be dosed in the plant, with the same contact time. Start the timer.

Rapid mix for 30 seconds (or the same mixing time as in the WTP).

If cationic polyDADMAC is to be used, dose this polymer approximately 15 seconds after alum and then continue to mix for another 15 seconds. (Remember, with polyDADMAC, smaller alum doses will be required compared to alum alone.) If your plant doses polyDADMAC before or with the coagulant, the polyDADMAC can be dosed in the same order to mimic what happens in the WTP. All these different options can be compared using the jar tests.

If nonionic polyacrylamide is to be used, dose this polymer and mix for 10 seconds. These polymers are often added just as the first pin floc are detected in the jars. This may be after the end of the rapid mix stage.

Slow the stirring rate to approximately 30-35 rpm and mix for the same period of time as provided for flocculation in the WTP. During this period floc will form if the dose rate is correct or nearly correct.

Watch floc growth during this time and record floc sizes on a jar testing record sheet at 5 minute intervals. Note also the clarity of the water in the space between the floc. Floc of uniform size in otherwise clear water is the target.

At the end of the flocculation period remove the paddles and allow floc to settle, typically for 30 minutes. Note the settling rate of the floc and whether the water is clear or there are straggler or pin floc left behind. The settling rate can be calculated by timing 90% of the flocc passing between two marked levels and converting to m/hr.

Draw water samples off from the top of each beaker. Filter this water through a Whatman No.1 filter paper into an Erlenmeyer flask. The filter paper should be folded in quarters, opened and placed in the top of a funnel placed in one of the flasks. If filter paper is not available, cotton wool plugs or even 1 or 2 layers of paper towel may work satisfactorily but watch for breakthrough.

Measure the true colour and turbidity of the water and record the results. (Ideally, for the filtered water, true colour should be < 5 HU and turbidity less than 0.3 NTU – although turbidity in jar tests is often higher than will actually occur in a WTP with media filters). Aluminium, iron, manganese and alkalinity may also be measured depending on the focus of the test.

• Soluble aluminium residuals should also be measured for the optimised process. If the soluble aluminium residual is high (>0.1 mg/L) then the coagulation pH needs to be raised and/or the alum dose is too high. If total aluminium is high but soluble aluminium is low then the coagulant aid dose should be increased.

Determine the “ best” jar based on the measurements and observations.

• If the water is under dosed, the water will be cloudy with little or no floc and essentially no settling.

• If the coagulant is overdosed, dense floc may be formed but may not settle well and tends to be very fragile. A gentle shake of the jars after the samples have been taken may help show this.

Trial the “best” jar dose in the plant. Make minor adjustments on the plant to get the absolute best dose.

Repeat the above steps with different doses if necessary to confirm or narrow down the optimum doses for each chemical.

Repeat at different pH to determine optimum coagulant pH (see below).

51

Chapter 5 – Jar Testing

Modifications Of The Basic Jar Test Enhanced Coagulation The process of enhanced coagulation was discussed in Chapter 2. The basic jar test can be modified to investigate how enhanced coagulation might work in a WTP. Set up the jar test in the same way as that described above, and then simply add alum or other coagulants in 10 mg/L steps up to quite high doses. In low alkalinity water (< 60 mg/L as CaCO3) you should be able to get the pH down to around 5.5. At higher alkalinity (>100 mg/L CaCO3) it may only be possible to get the pH down to about 6. Several jar tests may need to be carried out one after the other to cover the full range of concentrations. At the completion of the tests note the dose and measure and record the pH, turbidity and colour for each dose. Take samples of the supernatant from each jar and filter through 0.45 µm filters.

Send the samples to a laboratory for DOC, UV254 and soluble aluminium (Al) analysis. Alternatively UV254 and soluble Al may be measured on site if the equipment is available.

When the results come back, have a look at the data and note the conditions that resulted in the greatest removal of DOC without associated high soluble Al levels.

If the pH for any of the jars was less than 5.3, repeat the jar tests but add sufficient alkali to keep the pH between 5.3 and 5.8 in those jars. In high alkalinity waters the pH may not have been lowered very much.

Repeat the jar tests adding small volumes of 1% hydrochloric or sulfuric acid to bring the pH down to at least 6. Again sample the settled water and filter it through 0.45 µm filters and send the samples off for analysis.

Once the results return, look for the best removal. The conditions can then be tested in the plant but keep in mind the consequences of enhanced coagulation discussed in Chapter 2.

Direct And Contact Filtration The basic jar test can also be modified for a direct or contact filtration plant in the following way.

Set up the jar test in the same way as that described above. Rapid mix the alkali and coagulants at maximum speed and then add any polymers at reduced speed.

To simulate contact filtration, reduce the stirrer speed to approximately 20 rpm and mix for a contact time the same as that used in the WTP process. For direct filtration, reduce stirrer speed to 50 rpm and mix for a time the same as the time provided for flocculation in the WTP. Detention time for the flocculation process is critical in both of these types of plants. The minimum detention time recommended ranges from 1 to 5 minutes for contact filtration and 5 to 20 minutes for direct filtration systems compared with 15 to 45 minutes for conventional clarification/ filtration. Therefore the jar test should reflect these differences.

In both cases the aim is to achieve an even “cloud” of robust pin floc throughout the jar.

As a guide, a direct filtration process usually requires approximately 40% less coagulant than a conventional process. This can be demonstrated in the jar tests.

Dissolved Air Flotation (DAF) Since the DAF process involves flotation it is sometimes necessary to conduct jar tests for DAF systems using flotation rather than gravity settling.

There are a number of proprietary DAF jar testing systems available. Alternatively if a feed line of the airsaturated water can be provided to the WTP laboratory, a simple modification of the basic jar test can be set up.

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Chapter 5 – Jar Testing Start by setting the jars up as described in the basic jar test procedure above; however only fill the jars to 900 mL (or 800 mL depending on the size). Add the coagulants (remember to adjust the dose accordingly for the smaller volume in the jar) and rapid mix and slow mix in the normal way. At the end of the slow mix period, introduce a hose to the bottom of the jar and add 100 mL of the air-saturated water to the jar. Watch the response and judge the suitability of the floc for use in a DAF system. Sample the subnatant water (the clear water beneath the floated sludge) and analyse it for whichever parameters are of interest.

Powdered Activated Carbon (PAC) PAC is used in water treatment for the control of taste and odour, removal of algal metabolites and toxins, reduction of DOC and in some situations for the removal of herbicides and pesticides. While PAC is not directly involved in the coagulation process, the PAC does become part of the floc and is removed during clarification and filtration. Jar testing can be used to evaluate different types of PAC, the impact of PAC dosing on floc formation and the effectiveness of PAC for the required removal process.

The basic jar test is simply modified by adding the necessary dose of PAC to each beaker. PAC is difficult to suspend in the water so very vigorous mixing will be required for quite some time. Because of this it is best to add PAC first. This is appropriate since in most dosing systems PAC is dosed prior to any coagulants.

Since the doses of PAC are generally quite low, around 5 to 20 mg/L, an accurate sensitive balance is required to weigh out the PAC. Small balances are readily available from a number of different suppliers for well under $100. (Enter “pocket scales” into Google, or other search engine, and take your pick). Simply weigh out the required amount onto a piece of aluminium foil or filter paper. Set the jar tester paddles rotating at a high rate. The rate should be high enough to cause a vortex around the shaft of the paddle. Add the pre weighed PAC into the vortex. Once the PAC is wetted, the rest of the jar test can be carried out as described above. If less than 10 mg is required for a particular jar it may be better to produce a concentrated slurry (say 1%), keep it vigorously mixed and dose the PAC using a pipette. You may prefer to use this latter method for all PAC jar testing.

Evaluating the performance of the PAC depends on why it is being dosed. If it is for control of taste and odour, the operator can smell the products of the jar test and a decision made immediately. If tasting of the product is required, remember that the product in the jar is not disinfected so there may be some risk associated with tasting the product directly. To control this risk, filter the water through a 0.45 µm filter (available from most laboratory supply companies) and then taste it.

If the test is being conducted to evaluate removal of algal toxins or pesticides or herbicides, a sample of the product water from each of the jars must be sent to an analytical laboratory for analysis. It will be some days before a conclusion can be drawn regarding which dose is best, but regardless of this it can still be done on a scientific basis rather than a simple guess.

If the test is being done to evaluate removal of DOC there are two possibilities. The effectiveness of the removal can be evaluated by measuring the UV absorbance at 254nm on site (if there is a UV spectrophotometer available) or the sample again needs to be sent off to a lab for analysis. Many high schools and most universities would have a UV spectrophotometer so it may be possible to borrow it for the measurements.

This would allow a quick result rather than having to wait until the test results come back.

Procedure For pH Correction For Jar Testing Correction of pH may require acid or alkali depending on the nature of the water. Most commonly it requires alkali – lime, caustic or soda ash.

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Chapter 5 – Jar Testing This method requires the use of one 1L beaker only.

• • • • • • • • • •

Use the same coagulant doses decided for the jar test.

Fill one 1L beaker (‘jar’) with 1000 mL (or 900ml depending on the beakers) of raw water and place these under the stirrer and set the stirrer to slow stir at 30-35 rpm.

Set up a pH meter to constantly monitor the pH in the jars using a retort stand and clamp.

Set up a burette (10 or 20 mL) with a clamp and a retort stand.

Fill the burette with the required pH correction solution.

Add the lowest coagulant dose to the first jar.

Titrate the pH correction solution slowly until the set pH is reached. Record this volume. Do not zero the burette.

To the same beaker, add more coagulant to reach the second volume of coagulant.

Continue titration until the set pH is reached. Record the total amount titrated on the record sheet.

Repeat the above steps for each coagulant dose.

The amount of pH correction solution that needs to be added to compensate for each coagulant dose can then be calculated, by adding up the volumes. This process can be repeated for different pHs. An example is provided in Table 5.3.

Table 5.3. Example of a titration to determine the amount of pH solution necessary for each dose of coagulant.

Coagulant Dose (mg/L)

10

20

30

40

Volume of pH solution added (mL)

1.0

1.1

1.3

1.6

Total volume of pH solution added (mL)

1.0

2.1

3.4

5.0

So, based on the Table, 3.4 mL of pH correction solution would need to be added to the jar with 30 mg/L of coagulant and 2.1 mL to the jar with 20 mg/L of coagulant.

The jar test can then be carried out with the correct amount of alkali and coagulant until the optimum pH and coagulant dose are determined.

Manganese Jar Testing Many Australian WTPs have problems with manganese (Mn). One of the best ways of removing Mn is by dosing potassium permanganate. Jar testing is the best way to determine the correct dose.

First measure the soluble Mn in the raw water. Set up the six (or 4) jars with the ratio of potassium permanganate to soluble Mn of 2:1 in the first jar. That is, if the raw water soluble Mn is 0.15 mg/L, start the dose at 0.3 mg/L potassium permanganate. Set up higher ratios in the other jars up to 6:1 in the last jar.

Rapid mix for about 20 seconds and then slow the mixing to about 35 rpm for 5 to 10 minutes depending on the contact time available in your plant. Now complete a normal coagulant jar test using the current plant dose of the coagulant added to each of the permanganate dosed jars. After settling, filter the supernatant in the normal way and measure the soluble Mn. If the results don’t make sense, you may need to filter the sample through a 0.2um filter. The oxidised Mn is very small and can sometimes pass through Whatman No 1 paper. The lowest Mn shows the best dose.

Be aware though that the actual dose may need to be varied slightly in the plant to achieve the best results.

This can only really be done by trial and error. A useful in-plant test is to sample water from the filter outlet. If it is pink, the potassium permanganate is being overdosed. If it is clear, add a drop of potassium permanganate jar test solution. If it goes pink and stays pink the dose is about right. If it goes brown, then the dose in the plant is still a bit low.

Note that the oxidation of Mn by potassium permanganate is best at pH 8 to 8.5. You can jar test this but really you are limited by the pH of your raw water unless you can change this by alkali dosing.

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Chapter 6 Chemical Dosing Calculations Chemical concentrations are commonly expressed as mg/L or ppm (parts per million) weight/volume.

Unfortunately, in the water industry concentrations of the coagulants are expressed in many different ways. For example for alum, the units may be mg/L of crystalline filter alum Al2(SO4)3.14.3 H2O, or mg/L Al2(SO4)3.18H2O, mg/L Al2(SO4)3 or mg/L Al2O3 or mg/L of aluminium or ppmV (parts per million by volume) liquid alum. Clearly there is room for confusion and it is very important that when doses of chemicals are calculated or quoted, operators understand the basis of how the doses were derived.

Chemicals used in the water industry are supplied either as dry powder, crystals or granules or concentrated liquid solutions. The necessary calculations vary a little depending on the type.

Solution strengths based on dry chemicals are easy. The weight of dry chemical added to a volume of water is calculated. This involves knowing the batch strength and the plant flow and the dose is expressed as mg/L. However for chemicals supplied as a liquid, dose rate can be expressed two ways; either as a weight per volume (mg/L or ppm) or as a volume per volume (ppm(V)). The calculation is a little more difficult since the composition of the liquid may vary with respect to specific gravity and % active constituent.

Step 1. Calculate The Concentration Of The Active Ingredient For dry chemicals such as polyacrylamide polymers, potassium permanganate and in some places dry powder or granular alum, a batch is usually made up by adding a known weight to a known volume in a tank. Concentrations may be expressed as % or g/L. The relationship between these can be summarised as follows.

• 10% = 10 g/100 mL • 1% = 1 g/100 mL • 0.1% = 0.1 g/100 mL So for example, a batch of potassium permanganate may be made up by adding 50 kg of potassium permanganate to 2000L. This is equivalent to 50 x 1000 g/ 2000 L = 25 g/L or 25,000 mg/L or 2.5% For liquid chemicals, the concentration of the active ingredient can also be calculated by referring to the suppliers product sheet. Specific gravity and solution strengths will often vary if different products are used and must be checked for each calculation.

From the sheet find the active ingredient, the SG and the % w/w of the active ingredient. Often the product sheet is not specific enough and will quote the figures as a range, e.g. for alum the SG is quoted as 1.30 to 1.32 and the % w/w active ingredient is quoted as approximately 48% Al2(SO4)3.14H2O. If accuracy is important, ask the supplier for a Certificate of Analysis. Often this will be supplied with the delivery of the chemical. This should have a single figure for the batch delivered. Alternatively the SG can be determined accurately by using a hydrometer. Hydrometers reading to 3 decimal places are available for well under $100. These are a little harder to find than hydrometers reading to 2 decimal places but do exist.

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Chapter 6 – Chemical Dosing Calculations Accurate tables of SG and alum strength as % alum are available from suppliers and can be used to determine the % alum accurately (Appendix 3). The tables are usually provided for one temperature only but there is often another table providing temperature corrections. Alternatively use the approximate value and the average of the range figures; the result will generally be close enough.

From these figures calculate the concentration of the active ingredient in g/L (w/v) from equation (1).

Chemical Strength (g/L) = 10 x % w/w x SG.

(1)

So for the alum described above the specific gravity can be taken as 1.31 and the % Al2(SO4)3.14H2O as 43%. The concentration of Al2(SO4)3.14H2O in the supplied coagulant is = 10 x 43 x 1.31 = 563 g/L For ACH a suppliers Certificate of Analysis sheet may list the SG as 1.33 and the %ACH as 40.2%. The concentration of ACH in the supplied coagulant is therefore = 10 x 40.2 x 1.33 = 535 g/L

Step 2. Calculate The Dose Rate In The Plant To calculate the dose rate the following information is required • Flow rate in L/s • Dosing pump rate in mL/s • Batch strength in g/L (from Step 1) The dosing pump rate should be determined from a “draw-down” or “drop” test. The dosing pump needs to be connected to a suitably sized draw-down tube (Figure 6.1). The size will vary from plant to plant however, it should be such that it is possible to measure the amount accurately over say a 15 to 20 second interval and the liquid moves a reasonable distance say 100mm or more in that period. If the tube is too small the drop will have to be measured over a period of time that is too short to be accurate, if it is too large the shift in the level of the liquid will be too little in a reasonable time for it to be accurate.

It is well worth setting up a correct sized tube to allow accurate determination of the volume of coagulant (or any other chemical) dosed. The dose rate can be calculated using equation (2).

Chemical dose (mg/L) = Dosing pump rate (mL/s) x Batch strength(g/L) Flow rate (L/s)

(2)

Example 1. In a small plant the draw down for KMnO4 was measured as 300mL in 30seconds. This is equal to 10mL/s. Batch strength as calculated above is 25 g/L. Plant flow from the plant flow meter was 130 L/s.

Therefore from equation (2): Chemical dose (mg/L KMnO4))

56

= 10 x 25 130 = 1.9 mg/L KMnO4

Chapter 6 – Chemical Dosing Calculations Example 2. In the case of alum at a larger plant the draw down was 2L in 33 seconds. This is equivalent to: 2 x 1000mL/33 s = 60.6 mL/s The plant flow rate was 450 L/s and the alum batch strength calculated in Step 1 was 563 g/L Chemical dose (mg/L Al2(SO4)3.14H2O) = 60.6 x 563 450 = 76 mg/L Al2(SO4)3.14H2O Alternatively, if you need to calculate the dosing pump rate this can be calculated by rearrangement of equation (2). Let’s say based on a jar test that you want to decrease the dose rate from 76 to 50 mg/L Al2(SO4)3.14H2O after conducting a jar test.

Dosing pump rate (mL/s) = Chemical dose (mg/L) x Flow rate (L/s) Batch strength (g/L) So dosing pump rate (mL/s)

= 50 x 450 563 = 40 mL/s

This equates to 2 L in 50 seconds. Somewhere close to 50 seconds will be near enough if you have to change the dose rate manually. Alternatively if the alum dosing is PLC controlled and the calibration is accurate, and current, simply entering the value of 50 mg/L Al2(SO4)3.14H2O or the dosing pump rate of 40 mL/s will decrease the dose. Chemical dosing pumps should be set using calibration curves and the calibration of the dosing pumps must be checked regularly. Any manual change of the pump stroke in particular will throw out the calibration badly such that changes made on the PLC will no longer result in the correct dose in the plant.

Most plant operations manuals or PLC manuals will describe how to recalibrate the system. If in doubt, ring the pump manufacturer for help.

Whenever a dose is changed the new dose concentration must be checked using a “draw-down” (“drop”) test.

Liquid Chemicals In the above example Al2(SO4)3.14H2O was used as the basis for the calculation of the alum dose rate. As explained in Chapter 3, alum can exist in different states of hydration. So if for example your alum is described as Al2(SO4)3.18H2O you can simply calculate the dose of alum in your plant as mg/L Al2(SO4)3.18H2O in the same way shown above.

If there is a need to express it as mg/L Al2(SO4)3.14H2O, the value for mg/L Al2(SO4)3.18H2O is simply multiplied by 1.12 (This is simply the ratio between the molecular weights for the two different forms of alum) An Alternative Method Of Expressing Dose Rates Another method of expressing a liquid chemical dose rate used by some water utilities, is to express the dose as ppmV, that is as volume/volume (v/v). This method is generally not recommended because it may cause confusion when comparing with other WTPs, the vast majority of which use mg/L (w/v).

57

Chapter 6 – Chemical Dosing Calculations It is explained in Appendix 5 to assist those who use it and others who may wish to use it since the ability to compare dose rates is often more important to designers and consultants rather than operators themselves.

One advantage of this method is that calulations are simpler, and going from the jar test to the plant is very easy.

Actual Dose Rates Careful operation of a WTP requires regular checks of the dose of all chemicals. It is risky to rely on the figures presented on a SCADA screen without regular checks. Checking of actual dose rates requires a “draw-down” or calibration tube assembly (Figure 6.1) and a stopwatch. Valves are switched to allow dosing to occur from the calibrated draw-down tube.

Figure 6.1. A commercially available calibrated draw-down tube.

If the tube is not calibrated or the calibration is unclear, the volume marks can be determined in the following way. Measure the internal diameter of the tube and calculate the radius by dividing by 2. The height of the tube corresponding to a certain volume can be calculated using equation (3).

Height = Volume/ r2

(3)

So, for a tube with an internal diameter of 44 mm, the radius is 22 mm. For a tube this size, 500 mL might be a suitable interval to mark on the tube.

Height (mm) = Volume (mL)x 1000 3.14 x r (mm) x r (mm) = 500 x 1000/ 3.14 x 22 x 22 = 329 mm You can then use a hacksaw blade to cut marks each 329 mm and then a felt tipped pen or white out to mark inside the cut (Figure 6.2).

58

Chapter 6 – Chemical Dosing Calculations Figure 6.2. A “home made” calibrated draw-down tube.

Polymeric Coagulants With the increased use of chemicals such as PACl and ACH and PFSTM, the dose rate can still be calculated in the way described above. However, there can be no real comparisons to alum doses based on these calculations. If a water utility has committed to these coagulants and is not concerned with price or dose comparisons then these dosing calculations will be sufficient. However, as soon as comparisons with other coagulants are required the dose needs to be expressed in a different way.

This can be done by calculating the dose for all aluminium-based coagulants in terms of Al2O3. All suppliers sheets include information relating to the % Al2O3 of the product (see Table 4.1 in Chapter 4). For example alum is typically 7.5 to 8% Al2O3 and ACH is typically 23-24% Al2O3. The dose rate is then calculated as described above in Step 2.

The use of Al2O3 is historical and relates to the chemical manufacturing industry. Another way is to express the dose as mg/L of aluminium. After all it is the aluminium ion that does the work. To convert % Al2O3 to % Al, multiply by 0.53. Thus ACH at 23.5% w/w Al2O3 is also 23.5 x 0.53 = 12.5% w/w Al. If you would like to start using this to make real comparison of doses of different aluminium containing coagulants it can be calculated from the % Al2O3 information provided by manufacturers. All manufacturers of aluminium based coagulants express the strength of the coagulant based on the % Al2O3. Therefore the % Al2O3 can be found on the manufacturers product sheet and the dose rate of Al calculated using equation (4).

Dose rate mg/L (Al) = Dose rate mg/L (Al2O3) x 0.53 (4) For iron-based coagulants including PFSTM the % w/w of Fe content is usually stated on the product sheet.

Therefore it is very easy to base the dose rates of iron coagulants on the mg/L Fe. There is no doubt that it would be better for all to express the dose of all metal based coagulants on the dose of the metal ion itself. Unfortunately history leads us into confusing territory with Al coagulants!

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Chapter 7 Clarification Conventional water treatment processes include clarification as the first stage of removal of floc and suspended solids. Clarification removes the majority of solid material prior to the filters thereby preventing overloading of the filters. Generally a separate clarification step makes the Water Treatment Plant (WTP) less sensitive to rapid changes in raw water quality than either contact or direct filtration because of the long detention times provided in the clarification process. Typically >90% of the floc is removed in the clarifiers prior to filtration, although this can vary significantly depending on the load on the clarifier and how well it is performing. A well operated clarifier will produce settled water with 2-5 mg/L of suspended solids or 1-3 NTU turbidity.

One of two physical processes may be used in the clarification step. In sedimentation, gravity acts to settle the floc to the bottom of a “tank”. In flotation, the buoyancy associated with air bubbles is used to float the floc to the surface of a “tank”.

Sedimentation is more suitable for waters with high suspended solids and turbidity. Dissolved Air Flotation (DAF) is more suitable for light floc usually associated with high colour and lower turbidity. It is also the preferred process when algae may be present in the raw water. The fine air bubbles gently float the algae without damaging the cells that may contain toxins or taste and odour chemicals.

Sedimentation or settling of floc is achieved by decreasing the velocity of the water to below that at which the entrained floc can be carried by the water. A number of design variations have been developed to improve the clarification process such as employing a sludge blanket, recirculating sludge or adding lots of plates or tubes to reduce settling distances. In flotation, fine diffused bubbles are injected into the flocculated water and floc is floated and skimmed off rather than settled.

Clarifiers Many types of clarifiers may be employed and they fall into several categories depending on whether the floc is allowed to settle or float, or to be adsorbed and filtered through a sludge blanket. The main clarifier types are: • • • • •

Settling.

Upflow sludge blanket.

Solids contact or sludge recirculation.

Adsorption.

Dissolved air flotation (DAF).

There are also several proprietary clarifier designs that provide enhanced settling such as ActifloTM where microsand is dosed and bound in with the floc, or SiroflocTM where magnetite is bound with the floc. In each case the binding to the sand or magnetite achieves more rapid settling due to the increased density of the modified floc particle. The microsand or magnetite is then separated out from the floc and recycled.

Clarifiers are designed with particular surface loading (flow) rates and detention times. These vary considerably and will be considered in the sections below.

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Chapter 7 – Clarification

Settling Clarifiers Sedimentation Tanks And Settling Clarifiers Sedimentation tanks and clarifiers are typically designed to provide a detention time of around four hours at peak flow rates. They may be rectangular or circular and are traditionally used in large WTPs in Australia.

A simple rectangular sedimentation tank is like a swimming pool with floc settling occurring as the water flows evenly from one end of the tank to the other (Figure 7.1, 7.2 and 7.3). The tank typically has a flat floor sloping back towards sludge collection hoppers. The settled sludge is usually collected by a horizontal sludge rake situated on a travelling bridge that scrapes the sludge in the opposite direction to the flow of the water, back to the sludge collection hopper at the inlet end of the tank. Sludge in the hopper may be withdrawn periodically from bottom hoppers via gravity blowdown or sludge draw-off pumps (Figure 7.2).

Figure 7.1. A rectangular sedimentation tank with a travelling bridge

Figure 7.2. Diagram of a rectangular sedimentation tank with a shallow sludge hopper and gravity sludge removal.

Chemical addition and coagulation Flocculation

Treated water outlet

Sludge scrapper Drain for sludge

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Chapter 7 – Clarification Figure 7.3. A rectangular upflow sedimentation tank/clarifier with suspended cones (not visible) for sludge removal.

Other systems of sludge removal may also be used such as suspended cones (Figure 7.3 and 7.4).

Figure 7.4. A suspended cone in a rectangular clarifier.

Circular sedimentation tanks may be centre or peripheral feeding. Centre-feeding tanks have influent water fed to a central well, and clarified water flows outwards to where it is collected in effluent launders around the perimeter of the circular tank. Peripheral-feeding tanks have influent water flowing from peripheral inlets towards the centre of the tank, where clarified water is collected in a central well. Sludge collects either in suspended inverted cones (Figure 7.4) or on the bottom of the tank where it is removed via sludge scrapers to a central sump (Figure 7.5). Sludge is periodically removed from the sump or cone by gravity blow down or sludge draw off pumps.

Rectangular settling basins are suitable for most applications and in particular, large capacity plants with high concentrations of particulates in the raw water. Horizontal flow rectangular basins tolerate shock loads better than other clarifier designs and are easy to operate with low maintenance costs, however separate flocculation facilities are required.

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Chapter 7 – Clarification Figure 7.5. An empty clarifier showing the sludge rake system at the bottom.

Shallow Depth Settling Clarifiers (High Rate Settlers) These clarifiers incorporate either lamella plates or settling tubes.

In these clarifiers, plates or tubes are constructed at various angles. The underlying theory is that by introducing these angled tubes and plates, the distance for the floc to settle is greatly decreased. As a result the time for the floc particle to settle onto the surface is significantly reduced compared to a deeper sedimentation tank or clarifier. They can therefore be operated at higher rates, although on projected plate or adjusted surface area the settling rates are similar to sedimentation basins (see Table 7.1).

Lamella plate settler clarifiers are the most compact type, consisting of a series of parallel, closely spaced sloping plates stacked in compact rectangular tanks. The plates are typically sloped at around 60o and may be provided in various configurations with either flat or corrugated plates. The plates may be configured with the flow (co-current) so that the floc settles onto the plates and rolls or slides down the plate with the water flow; or against the flow (counter-current) so that the accumulated floc slides against the flow.

Tube settlers are another variation and are small sized inclined tubes with hexagonal cross section manufactured in plastic blocks (Figure 7.6). They provide both co and counter current tube slopes in the one block.

Figure 7.6. Tube settlers visible below the surface of the water in a clarifier.

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Chapter 7 – Clarification If the angle of the plate or tube is < 45o, the solids that settle on the plates stay there and must be periodically removed. This is usually done by periodically lowering the level of water in the tank slowly. This causes the “captured” sludge to slough off the plate and settle to the bottom of the tank from where it can be removed by a sludge draw off system. If however the angle is >45o, solids don’t usually accumulate on the tubes or plates, but rather slide or roll slowly off the plates or tubes and accumulate at the bottom of the tank from where they can be removed.

In practice not all the sludge is removed from the plates or tubes and therefore they all require periodic cleaning.

Lamella plates or tube settlers are often added to sedimentation tanks or clarifiers in plant upgrades to allow increased flow through the plant.

Upflow Clarifiers In Upflow or Sludge Blanket Clarifiers (Figure 7.7 and 7.8) the settling sludge forms a sludge blanket in the up flowing stream. Upflow clarifiers are generally suitable for most sizes of WTPs, preferably with relatively constant raw water flow and quality.

Figure 7.7. Diagram of an upflow sludge blanket clarifier with sludge recirculation.

Chemical addition

Coagulant mixing and flocculation

Clear water zone

Influent

Sludge zone

Apron

Mixing impeller Sludge discharge

Figure 7.8. A sludge blanket clarifier

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Chapter 7 – Clarification Gravity causes settling of the sludge while the force of the water flowing upward lifts the particles. By carefully designing the flow such that the upward force is similar to the downward force, a sludge blanket will form above the floor of the clarifier. Once the sludge blanket has formed, new floc particles will collide with those already in the floc blanket enhancing their removal from the flocculated water. A central flocculation zone is usually provided within the clarifier, but less flocculation time is required than in conventional settling.

Several types of upflow clarifier are available including those with flat and hopper bottom tanks, with normal or pulsing inflow and with various sludge draw off techniques.

Compared to conventional rectangular sedimentation tanks, clarification is more efficient and sludge removal is generally easier in circular clarifiers. However, upflow clarifiers need to be carefully operated to avoid short-circuiting and exposure to high hydraulic shock loads. Raw water temperature changes (see Chapter 8) and motor malfunctions can have a significant impact on sludge blanket clarifiers.

Several days are required to develop a sludge blanket, thus the clarifier always needs to be kept operating.

If flow stops for any reason, the sludge blanket will settle to the floor and will need to be re-established once the flow starts up again.

Solids Contact Clarifiers or Sludge Recirculation Clarifiers are a variant of an upflow sludge blanket clarifier and combine the processes of coagulation, flocculation and sedimentation in the one structure and include sludge recirculation. They are generally circular with the coagulated water introduced through a central well where flocculation occurs before discharge beneath an apron (Figures 7.7, 7.9 and 7.10) that is situated below the suspended sludge blanket. The floc is subjected to gravitational settling as well as adsorption-filtration by the sludge blanket as the water passes upwards through it. Solids contact clarifiers include mechanical flocculation inside the middle compartment. Sludge is recirculated into the flocculation zone from the sludge blanket or settling sludge zone to provide material for high levels of particle collisions and further rapid flocculation. The clarified supernatant rises and is collected in radial collection pipes (Figure 7.11). Sludge is periodically drawn from the bottom of the clarifier (see Figure 7.7). This process is often used in water softening applications.

Figure 7.9. An empty clarifier showing the central flocculation zone.

Figure 7.10. A partially empty clarifier showing the central flocculation zone.

65

Chapter 7 – Clarification Figure 7.11. An upflow solids contact clarifier.

The coagulated water is visible in the central well and the sludge blanket below the clarified water in the right of the figure. The centrally located collection launder is also visible.

There are various proprietary versions of these clarifiers.

In upflow sludge recirculation clarifiers (see Figure 7.7), impeller speed in or below the flocculation zone is an important variable and is used to vary the sludge blanket level as well as the slurry concentration in the centre of the clarifier. The optimum impeller speed is the speed at which slurry concentration is relatively uniform over all mixing zones.

If the impeller speed is too low, some of the slurry will settle at the bottom of the clarifier and the concentration in the primary and secondary mixing zones and the sludge blanket will be low and not uniform. High impeller speeds can push slurry out from the mixing zones and raise the sludge blanket. Optimum impeller speed will ensure uniform slurry concentration in each zone and provide maximum solids contact for coagulation (see also Chapter 8).

The Pulsator or Super Pulsator (Figure 7.12) is a sludge blanket type clarifier that may also include settling plates or tubes. A vacuum system draws coagulated water up in the central well of the clarifier. When the water reaches a preset height, the vacuum is broken and the water falls rapidly back down and is directed out into the clarifier. This causes the sludge blanket to expand and the coagulated water forced up into the sludge blanket very efficiently bringing the newly formed floc in contact with the sludge in the blanket. This modification allows the clarifier to be operated at a higher loading rate (See Table 7.1). It is a proprietary process offered by Degremont Suez.

66

Chapter 7 – Clarification Figure 7.12. A Superpulsator. The central structure houses the vacuum apparatus.

Adsorption Clarifier A Roughing Filter, First Stage Filter, Primary Filter or Adsorption Clarifier (Figure 7.13) may be situated before the conventional media filter to clarify the water in a two-stage filtration process. The roughing filter contains media that may be coarse sand, pumice, gravel or plastic media.

These filters can be configured for downflow or upflow operation with upflow the more usual arrangement.

The filter is cleaned via backwashing to waste. Adsorption clarifiers can be operated at quite high loading rates (See Table 7.1)

Figure 7.13. An upflow adsorption clarifier. The mesh used to retain the buoyant media is visible beneath the surface of the water.

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Chapter 7 – Clarification Dissolved Air Flotation Dissolved Air Flotation (DAF) is a flotation process primarily used for clarification but may also be used for thickening suspended solids. It is suitable for coloured, generally lower turbidity waters, and where significant algal problems occur or may be expected. Flexibility of operation has allowed the process to be used successfully with quite high turbidity water.

Figure 7.14. Diagram of a DAF system.

Chemical addition & coagulation Flocculation

Chimney

Float Collector

Sludge

Raw water Floatation Tank

Pressure reducing valve

Subnatant to filters Compressor

Saturator Recycling Pump

Figure 7.15. A DAF saturator

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Figure 7.16. DAF Air release or distribution valves (coloured red)

Chapter 7 – Clarification Figure 7.17. Fine bubbles in a DAFF system. Note the even distribution of the bubbles across the DAFF system. A saturator is also visible in the background.

In the DAF system (Figure 7.14), some filtered or clarified water is recirculated via pumps and mixed with air in a saturator (Figure 7.15) under pressure. The recirculated air-water stream is then returned via distribution valves (Figure 7.16) to mix with the flocculated water, typically in a chimney type arrangement (see Figure 7.14). The stream of recirculated, aerated water passes through a series of valves that causes the release of microscopic air bubbles (Figure 7.17) that attach themselves to the floc and float the floc to the top of the water (Figure 7.18). Flotation of the solids may occur in a separate tank prior to the filters, or combined with the filters (in-filter DAF or DAFF). This latter arrangement reduces construction costs. While DAFF works well it does require the filtration rate to be similar to the clarification loading rate.

DAF units may be circular or rectangular in design. In circular systems, air saturated water passes to a central circular chamber where fine bubbles develop and float the floc to the surface for removal by an intermittent float/wash overflow system via a radial launder. In rectangular systems, the floated sludge/flocc is removed periodically (typically two hours) by a scraper and beach system (Figure 7.19), or by raising the level of the water so that the water overflows a weir carrying the sludge with it (Figure 7.20).

Figure 7.18. A DAF system with sludge forming a blanket on the surface of the water.

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Chapter 7 – Clarification Figure 7.19. Sludge rollers at the end of a DAFF system

Figure 7.20. Floated sludge removal by hydraulic overflow.

DAF units require slightly lower chemical doses than other clarification options but are able to produce a higher solids concentration in the float thereby reducing sludge volumes and sludge management costs.

DAF plants do however have higher power and operating and maintenance costs than most sedimentation options.

DAF plants are less sensitive to changes in raw water quality since their operation can be modified. They will also handle higher raw water turbidity and colour better than direct or contact filtration.

DAF systems typically operate with a recycle ratio of between 8% and 12% of the inflow. Saturator pressure is typically between 550 kPa and 600 kPa. DAF plants should be provided with a fully enclosed roof to protect the DAF float from being broken up by rain and the adverse effect of wind on sludge removal (see Figure 8.1 in Chapter 8)

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Chapter 7 – Clarification

Clarifier Loading Rates Surface or clarifier loading rates are used to design and compare the performance of clarifiers. The surface loading rate is defined as the flow rate through the clarifier (m3/h) divided by the surface area (m2) available for clarification. Thus surface loading rates are measured in units of m3/m2/h, which is simplified to m/h.

Typical loading rates for various types of clarifiers are presented in Table 7.1.

Table 7.1. Typical surface loading rates for different clarification systems.

Clarifier Type Sedimentation basin

Typical Surface Loading Rate (m/h) 0.7 – 1.5

Shallow depth settlers with lamella plates or tube settlers, (based on clarifier area)

4–8

Upflow/Solids contact Adsorption DAF

1.5 – 4 8 – 30 6 – 12

For DAF, the recycle stream (between 8 and 12 % of the flow) should be excluded from the calculation.

The most suitable loading rate for a clarifier will depend on the characteristics of the water being treated.

Dosing flocculation aid polymers, or adding plate or tube settlers can improve loading rates.

Sludge Management And Removal Clarifiers typically have a sloping or hopper bottom where sludge is allowed to collect. This collected sludge is periodically removed via a desludge system. Desludging may be controlled manually or automatically.

Where desludging occurs automatically, both the frequency of desludging and the period of desludge can be set. Typically, desludging occurs every ½ to 1 hour for a period of 1 to 5 minutes. Where the raw water has a higher solids load, the clarification system requires desludging more often and for longer time each desludge.

Clarifiers with sludge blankets sometimes have concentration hoppers that are emptied periodically. They are positioned in the clarifier in such a way that they do not affect the level of the sludge blanket when they are emptied. There may also be suspended cones that allow the rising sludge blanket to flow over the edges of the upturned cone (see Figure 7.4). Valve systems provide draw off from the bottom of the cone.

Clarifiers may be fitted with mechanical sludge scrapers. The main types are travelling bridge scrapers (Figure 7.21), floating-bridge siphon systems, bottom scraping systems, and flight and chain scrapers. Note that travelling bridge and floating bridge systems are not suitable for use in clarifiers fitted with plate or tube settlers. In DAF units the sludge may be removed by periodically raising the level in the tank and the flow over a weir carries the sludge with it (Figure 7.20). Alternatively sludge rollers (Figure 7.19) may be used to draw the sludge off. The use of sludge rollers results in a higher solids content and therefore reduces the load on downstream residuals handling systems.

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Chapter 7 – Clarification Figure 7.21. Travelling bridge scraper

Some DAF systems have been fitted with blowers to blow the sludge along the surface of the tank (Figure 7.22).

In these systems, the water level is raised slightly so that it is just level with or slightly over the weir, and the blower blows the sludge over the weir. Use of the blower system has resulted in a thicker sludge and significant savings in residuals handling costs.

Figure 7.22. Comparison of DAF float removal with (left) and without (right) air blowers operating after 90 seconds of float. (From Williams 2005)

Providing suitable access to equipment must always be considered in the design of these mechanisms.

Sludge Blowdown Or Desludge Regular sludge removal is required for efficient clarifier operation and to prevent • Resuspension of the sludge.

• Reduced tank or hopper capacity.

• Anaerobic conditions that can lead to sludge floating to the top of the clarifier and resulting in tastes and odours and increased levels of iron and manganese in the water.

The frequency and period for automatic desludging is best determined by trial and error. The objective is to minimise sludge accumulation in the clarifier while also minimising the amount of cleaner water drawn off to the sludge handling system (in other words maximising sludge concentration).

The concentration of sludge in the blowdown should average between 0.2% (2000 mg/L) and 2% (20,000 mg/L) solids but will start off at high concentration and then drop off in concentration during the removal period. The operator must also watch out for sludge hold-up and “rat-holing”. Sludge has a greater tendency to hold up in the sludge hopper the older the sludge is, especially if the raw water has high colour and turbidity, and conditions are warm. “Rat-holing” occurs when sludge holds-up and only the sludge immediately above the draw-off pipe flows out and clean water from above passes through the pipe or “rat-hole” in the sludge.

Larger draw off pipes or sludge agitation through water or air jets may be required in this case.

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Chapter 7 – Clarification All clarifiers being equal, it would be expected that each one has similar blowdown requirements. Dissimilar blowdown requirements imply that each clarifier is generating different quantities of sludge. Because short sharp desludge operation is disruptive to clarifier operation and performance, it is better to draw out sludge at lower rates for longer periods if possible.

When desludging clarifiers that use sludge blankets, the process should be carefully monitored to ensure that the sludge blanket is not lost, as it can take several days to build up a new sludge blanket. Sludge blanket levels and slurry concentration in solids contact clarifiers should be inspected regularly to maintain efficient operation of the clarifier (see Chapter 8). White tiles, Secchi disks or other visual aids can be used to determine the level of the sludge blanket. Portable and on line sludge blanket detectors are available that provide a visual or audible signal when the detector is lowered into the clarifier and the blanket level is reached. The on line instruments can also be trended on a SCADA screen. Whatever system is used, the information can be used to better control flow rates and polymer doses in order to maintain the blanket within a set range. Alarms may also be provided for elevated sludge blanket that may result in carry over of sludge on to the filters.

Re-establishment Of Sludge Blanket In An Upflow Clarifier Re-establishing a sludge blanket after it has been lost due to coagulation problems, or poor desludging, or after a clarifier has been offline is always difficult. Prior to bringing a clarifier back on line, check the operation of scrapers and arms with sufficient water in the clarifier to cover these structures.

Where there is an operational clarifier on site, sludge may be transferred from the operational clarifier to the clarifier being recommissioned. The clarifier blowdown system can be used or alternatively, a submersible pump and lay flat hose can be used. The amount of sludge required from another clarifier should be estimated based on the normal sludge blanket depth and measured concentration in that blanket. The sludge should be transferred slowly and blended in as gradually as possible. Once the sludge is transferred, fill the clarifier as carefully as possible to minimise surges. Slowly open the inlet penstock. Use a light polymer dose on initial start-up and then gradually increase the polymer to the normal operating dose. The clarifier should be brought online slowly and at very low rates initially and then slowly brought up to speed with optimised polymer dosing.

Where there is no operational clarifier on site, or where sludge transfer is not preferred, start the clarifier at very low flow rates and wait for the sludge blanket to form. This may take several hours or days, during which time the clarified water quality will be poor. There will be significant carry over to the filters which must be monitored carefully to ensure breakthrough does not occur and the filters do not become blinded with the higher level of solids. Use a light polymer dose initially and then gradually increase polymer. If polymer is not usually used at the plant, temporary dosing may help establish the sludge blanket Record the process details employed each time a clarifier is brought back online. Also record any observations that might help to make the task easier next time. These records could then be developed into an optimised procedure.

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Chapter 7 – Clarification

Comparison Of Clarifier Performance A comparison of clarifier performance and suitability for well-coagulated and flocculated water is summarised in Table 7.2.

Table 7.2. Comparison of clarifier performance and suitability for use with different types of source water.

Sedimentation

Upflow/ Solids Contact

Plate/ Tube Adsorption Settler Clarifier

High Turbidity Raw Water

Very good

Excellent

Very good

Medium levels of turbidity only

Medium levels of turbidity only

Organics /Colour Raw Water

Poor

Good if some turbidity

Poor

Good

Excellent

Algae Removal

Poor

Poor Medium

Poor

Medium

Excellent

Taste and odour generation potential

Low

Possible

Possible

Possible

Negligible

Sensitivity to Temperature Changes

High

Medium to High

Medium to Low

Negligible

Negligible

Sensitivity to Raw Water Quality Change

Medium

Low

Low

Medium to High

Medium to High

Surface rating (m/h)

0.7 - 1.5

1.5 - 4

4-8

8 – 30

6 – 12

DAF

Note that the performances quoted refers to well coagulated water. Where coagulation is poor all clarifier function is compromised.

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Chapter 7 – Clarification

Clarifier Operation And Troubleshooting Operational Variables There are many operational variables that can affect the performance of a clarifier. The main ones are summarised in Table 7.3.

Table 7.3. Factors affecting clarifier performance (the actions are further described in Chapter 8).

Category

Parameter

Impact

Action

Flow Rate

Flow

Disruptive to floc settling

Better plant performance at low flow rate.

Slowly ramp flow when increasing plant flows.

Spread flows evenly between clarifiers.

Raw water quality

Raw water turbidity, colour, pH, alkalinity

Variation in raw water quality makes optimisation difficult.

Use jar testing to optimise for coagulant dose.

Continually monitor clarified water quality.

Raw water temperature

Temperature

Extended reaction time delays floc formation.

Variation causes short circuit flows and floc upwelling.

Take raw water from deep water or main river current where possible.

Shade or cover the clarifier if upwelling is significant

Coagulation

Coagulant dose, polymer dose

Uniformity and size of floc.

Quality of supernatant.

Monitor coagulation parameters.

Monitor clarified water quality (Tb, colour) Jar test.

Flocculation

Floc size

Poor clarification with incorrect or varied floc.

Monitor clarified water quality (Tb, colour) Monitor floc size Optimise flocculation detention time.

Optimise flocculation mixing.

Clarification

Impeller speed

Sludge blanket level and supernatant quality

Monitor clarified water quality (Tb, colour) Optimise for sludge volume and supernatant quality (see Chapter 8).

Sludge blanket detectors may also be employed.

DAF recycle rate

Float efficiency

Optimise for solids load.

Monitor subnatant (floated water) quality (Tb, colour).

Optimise recycle ratio and saturator pressure (see Chapter 8).

Sludge removal

Blow down frequency

Solids accumulation

Monitor sludge blanket level Optimise to balance solids in clarifier.

Maximise sludge blowdown concentration.

Environment

Wind and rain

Can turn water over leading to floc boil ups or disrupt DAF float and removal.

Protect or enclose clarifier.

Sun

Differentially warms tank walls and floc blanket leading to boil ups

Protect or enclose clarifier.

Shroud/flaps adjustment

Blow down duration

Trouble Shooting Guide A list of common problems with clarifiers and possible remedies is provided in Table 7.4

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76

High soluble iron or manganese in treated water.

Tastes and odours.

Sludge is not being removed from sludge hoppers.

Floating sludge.

Floc boil-up.

Poor supernatant quality.

Visible biofilm accumulation

Metals and/or taste and odours in the final water.

Sludge accumulation, hold-up

Short circuiting, currents

Algae/slime build up and blockages

Seasonal variation in algal populations.

Infrequent draining and cleaning of the clarifier.

Sludge releasing nutrients.

Poor distribution of inflow.

Physical effect of water temperature, sun or wind.

Flow increases.

Aged sludge.

Biological growth.

Rat-holing.

Warm weather.

Old sludge.

Inadequate sludge removal.

Excess polymer.

Excessive sludge removal.

In sludge recirculation clarifiers the impeller speed may be too high or too low.

Poor supernatant quality

Low or poor sludge blanket

Possible Cause

Floc difficult to settle.

Poor coagulation.

Poor supernatant or subnatant Poor flocculation. quality.

Symptoms

Poor floc formation

Problem

Table 7.4. Clarifier troubleshooting guide.

Drain and clean the clarifiers regularly.

Dose chlorine periodically but not continually.

Desludge more frequently.

Reduce the polymer dose.Cover the clarifiers with a roof or shade cloth.

Adjust flow distribution if possible or install baffles.

Check water temperature changes and investigate.

Cover the clarifiers with a roof or shade cloth.

Ramp flows slowly.

Increase the polymer dose.

Agitate sludge with water or air lances.

Clean out sludge hoppers.

Increase sludge blowdown frequency and/or duration.

Reduce solids detention time (but watch sludge blanket level).

Optimise clarifier blowdown settings.

Clean the clarifier.

Temporarily dose chlorine or alkali.

Optimise polymer dose.

Optimise clarifier blowdown settings.

Optimise impeller speed.

Check coagulation with jar testing.

Test alternative coagulants.

Consider use of a polymer.

Consider alternative injection point/s for coagulant.

Check and optimise flocculation detention time and mixing energy.

Corrective Actions

Chapter 7 – Clarification

Poor subnatant quality.

Poor float.

Poor filter performance in DAFF systems.

Float hung up on walls.

Excessive water to remove float.

Dirty filter media in DAFF systems.

Poor DAF float removal

Symptoms

Poor DAF float

Problem

Table 7.4. Clarifier troubleshooting guide (cont.)

Optimise floc size (B to C on floc chart in Appendix 6).

Use polymer to improve floc binding.

Check and optimise saturator pressure and recycle ratio.

Adjust distribution values to provide even distribution of fine bubbles.

Enclose and protect DAF. Install cutting sprays to free the float from the wall.

Repair and aim the cutting sprays.

Install blower to blow float to skimmer.

Increase frequency of float removal.

Increase level of water above the weir during float but beware this may cause increased turbidity spikes (see Chapter 8).

Wind preventing float removal.

Float adhering to walls and old sludge sloughing off onto filters.

Corrective Actions

Inadequate dissolved air quantity.

Poor binding of dissolved air and floc.

Poor distribution of bubbles.

Presence of large bubbles.

Possible Cause

Chapter 7 – Clarification

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Chapter 8 Optimisation Of Coagulation, Flocculation And Clarification Coagulation and flocculation, and clarification are key control points in the process of water treatment. As explained in Chapter 1, for each control point there need to be target criteria and critical limits set so that it is possible to determine whether the control point is in control, heading out of control or isout of control. Once these target criteria have been set, process monitoring is used to monitor the status of the control point.

Operating Criteria And Performance Parameters For Coagulation And Flocculation The coagulation control point requires careful attention to alkalinity and coagulation pH. Each Water Treatment Plant (WTP) needs its own set of target criteria. It is not possible in a practical guide such as this to anticipate all raw water and coagulant combinations and provide guidance for all WTPs. Most operators will know what the optimum pH is for coagulation in their plant and how that may change in the face of changing raw water conditions. This information should be documented for each WTP in the same way as is done in the next section on clarification. Reference to the information in Chapter 3 and in Table 3.1 will assist in setting these targets. Once documented, the coagulation step needs to be monitored to ensure that the targets are being achieved. For the coagulation step, the easiest way to do this is to install an on line pH meter in the coagulated water “channel” and trend this on a SCADA system and set appropriate alarm limits.

The same applies to the flocculation control point. This is very much plant specific with specific mixing energies and detention times applicable. At a minimum, targets should be set for detention times in the flocculation tanks. As described in Chapter 2, these times may need to be varied for different water temperatures. In some plants it may also be necessary to alter the mixing energy. (Appendix 1 provides some information on mixing energies that could be applied here if you wish). This information should be documented and target criteria presented in a table like the one below (Table 8.1) for clarification.

Operating Criteria And Performance Parameters For Clarification For the clarification step it is possible to provide generic target criteria and critical limits since the overall outcome of the coagulation, flocculation and clarification steps should be the same for all conventional WTPs regardless of the different coagulants, raw water quality, mixing and flocculation processes.

A well-run coagulation, flocculation and clarification system should be able to produce water of the quality summarised in Table 8.1. The table includes two targets for turbidity depending on the typical turbidity of the incoming raw water.

Table 8.1. Some target criteria and critical limits for clarified water. The two targets for turbidity relate to raw water with different turbidity.

Parameter Target Critical Comment Limit

#

Turbidity (NTU)

<1

3

WTA# objective for clarified water turbidity <1 95% of the time for raw water turbidity <=10 NTU.

Turbidity (NTU)

<2

3

WTA objective for clarified water turbidity <2 95% of the time for raw water turbidity >10 NTU.

True Colour (Hazen)

<10

15

ADWG* limit is 15 HU.

Alkalinity (mg/L as CaCO3 )

>40

< 25

Necessary for efficient coagulation. Helps reduces corrosion.

Soluble coagulant metal (Al or Fe) mg/L

0.05

0.1

(WTA) Water Treatment Alliance

*(ADWG) Australian Drinking Water Guidelines 2004

With a well-run DAF system it is often possible to produce water with turbidity around 0.3 NTU.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification With a well run clarifier system it is often possible to produce water with a turbidity around 0.5 NTU. With very poor quality water, clarified water turbidity may exceed 3 NTU which puts more load on the filters.

Once the target criteria and critical limits have been set and adequate process monitoring is in place, the process of coagulation and flocculation and clarification can be optimised. This needs to be done in a series of steps outlined below.

1. Optimise Coagulation For optimum performance, clarification systems require uniform, robust floc in the optimum size range. For gravity settling systems, large (C to D on the floc chart in Appendix 6), dense floc is best. For DAF systems, medium (B to C on the floc chart in Appendix 6), light floc is best. Direct filtration systems require small robust floc (A or A– on the floc chart in Appendix 6).

The size and nature of the floc can be affected by both the coagulation reactions and the flocculation step.

The first step is to ensure that coagulation itself is optimised.

Coagulation can be optimised by extensive use of jar testing followed by testing of the doses in the plant. Jar testing should be used to thoroughly test the existing coagulant(s) but also to trial alternative or additional chemicals including polymers. Many plants that run satisfactorily on alum could be improved considerably by the addition of small amounts of polymer as a flocculation aid.

Jar testing takes time and operators and managers should ensure that time is available for such optimisation work. General jar tests of the type described in Chapter 5 can be used productively, however it is well worth taking the time to customise the jar test so that it closely mimics what happens at the particular WTP. 1. Try to calculate the rapid mix time. How long does it take for water to flow through the “mixing section” of the plant at peak, average and minimum flows? Use these times in the jar test for that particular plant.

2. Calculate the flocculation time. How long does it take for the water to flow through the flocculation section of the plant at peak, average and minimum flows? Use these times in the jar test for that particular plant.

3. Some attempt should be made to match the energies of mixing and flocculation. Appendix 1 provides some information on mixing energies that can be applied here, and Chapter 5 gives some suggestions on using different rpm of the paddles for different mixing systems. It may not be possible to match the energy of mixing with many jar testers. Older jar testers only went to around 120 rpm while newer jar testers go up to 250 rpm to better mimic actual mixing energies. With both types of jar testers it is probably worth using the maximum rpm if the plant has good mixing or reducing the rpm somewhat if mixing in the plant is “average”! (see Chapter 5). Even if the initial mixing energies cannot be matched it is well worth trying to work out the flocculation mixing energy and setting the jar test to approximately equal the average flocculation energy, since this effects the formation of the floc considerably.

4. If the plant uses DAF for primary solids removal, it is worth setting up a system to allow the use of saturated water in the jar test as described in Chapter 5.

This sounds like a lot of work but remember, it only needs to be done once and the information documented for all who follow.

Coagulation is very much affected by the nature of the raw water. For example, water that is highly coloured with high levels of NOM may coagulate better at low pH and with higher doses of primary coagulant, in particular alum. Water with very low turbidity, particularly if it is combined with high colour, is difficult to treat.

It may be necessary to artificially introduce some turbidity.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification This can be done by adding bentonite (a very fine clay). Doses up to 15 mg/L may be required. Once again a jar test is a convenient way to determine the benefit of adding turbidity.

In the past, some higher turbidity water from the sludge tanks or sludge lagoons was returned to the head of the plant to assist with coagulation of low turbidity waters and to save water. With the understanding of Cryptosporidium and Giardia as significant risks to human health, this practice should only be continued in conjunction with a careful risk assessment, since the residuals water may contain increased levels of these protozoa previously removed in the clarification and filtration steps. Additional treatment may be required, or selective return of wastewater (protozoa are likely to be more concentrated in clarifier blowdown water than filter backwash water) practiced to avoid adding more protozoa to the incoming water. If residuals handling water is recycled, the recycle stream must be returned prior to the introduction of any coagulation chemicals and be returned as continuously as possible (see below).

Jar testing should be conducted regularly, however particular attention should be paid during changes in raw water quality. This may occur as a result of rainfall in the catchment, drought, changed raw water sources or algal growth. Anticipate change and check that the coagulant dose is optimised at all times.

Actual coagulant doses should be checked regularly by conducting draw down checks (see Chapter 6) and comparing the calculated values with jar tests or those on the PLC.

2. Ensure Coagulant Is Added After Any Return Streams In many plants, clarified supernatant from residuals processing (sludge and backwash water) is returned to the head of the plant as part of water conservation initiatives.

As described above, the impact of such a return on filtered water turbidity and therefore risk to public health must be carefully evaluated (see Mosse and Murray (2009) Practical Guide to the Operation and Optimisation of Media Filters).

Where supernatant streams are returned to the head of the plant, they should be continuous and occur at a rate no more than about 5% of the plant inflow. Stop/start operation of supernatant returns makes it extremely difficult to optimise coagulation because of the changing nature of the water when the supernatant stream is mixed in with the raw water. It is essential that the return stream occurs before coagulant is added, otherwise the initial dosing conditions will be changed by the returning water, thereby compromising coagulation.

3. Optimise Coagulant Dosing Point The actual dosing point for coagulant addition in a WTP has a huge impact on the success or otherwise of coagulation. Where there are no specialist mixing structures such as in line or mechanical mixing systems, it is important to ensure that the dosing point is upstream of valves, flow meters or pipe bends. These structures provide turbulence that helps the coagulant make contact with the particles in the water.

Additional mixing can be provided by partially closing any valves. This creates turbulence with minimal effect on flows.

Ideally multiple dosing points should be set up so that the effect of moving the dosing point can be evaluated.

Don’t be afraid to move outside the WTP property boundary to get a nice long mixing time. Dosing lines can easily be run in conduits underground.

Make sure dosing points are clearly labelled.

Polymer is commonly dosed at the end of the coagulation zone or into the flocculation zone prior to the clarifier to assist in the formation of a suitable dense robust floc. Polymer may be necessary to ensure good settling or to help bind fine air bubbles and floc in DAF systems.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification As with primary coagulant dosing it is well worth establishing multiple dosing points for polymer addition as well. These can be tested in combination with jar tests and the best dosing point selected. Dosing of polymer at different points can also be tested using a 20 L container of polymer and a portable dosing pump. Pump suppliers or chemical suppliers could help set this up.

Where there are multiple clarifiers, individual metering pumps for delivery of polymer to each clarifier is preferred to accurately control the different doses to each clarifier.

The importance of ensuring that there is adequate mixing intensity and retention time after chemical addition in a WTP cannot be over emphasised.

4. Monitor Coagulation pH Since coagulation pH can be critical for optimum formation of floc, the coagulation pH needs to be monitored carefully. This is best done with an on line pH meter. The value should be trended on the plant SCADA system to allow close investigation of the trends and comparison with other trended parameters such as raw, clarified and even filtered water turbidity.

Where on line meters are not available, portable pH meters can be taken out and the electrode placed into the flow. Alternatively, a grab sample can be taken just after the addition of coagulant and taken back to a bench pH meter. Coagulation pH should be measured and recorded every day and more frequently when conditions are changing.

5. Optimise Flocculation Floc with different properties is required for different systems (see Section 1 of this chapter). Once jar testing has established that an appropriate floc can be formed, it is necessary to set this up in the WTP. Flocculation time and mixing energy have to be optimised to produce the best floc size for the conditions (including water temperature) and the type of clarifier.

Mixing must be controlled to minimise damage to the growing floc. If floc in the plant isn’t forming in the same way as the jar tests suggest, try modifying the mixing energy. If the floc is smaller, try reducing the mixing speed in the last tank or possibly take one tank off line. Reduce or increase the flow rate to see what happens. Make small changes say +/-10% adjustment each time and observe the effect over several days.

Make one change at a time so that the effect of each change can be measured. Making many changes at the one time makes it difficult to work out which change had what effect.

The flow path for flocculated water needs to be considered carefully and every effort must be made to avoid pumping or hydraulic drops over weirs or through valves that might shear the floc. If floc shearing is suspected, try to remove the problem by altering operational levels in the tank to reduce the vertical distance that floc falls or try constructing gentle chutes rather than vertical weirs.

Flocculators are normally designed to provide between 5 minutes and 45 minutes retention time. While it is important to ensure that there is sufficient time to grow the required quality of floc, it is also important to realise that excessive floc time can reduce the quality of floc. Again, trial increased or decreased flow rates and observe what happens. If it is possible, try taking one unit off line or bring one on line.

Water temperature has a significant effect on flocculation. Floc forms more slowly at lower temperatures, therefore longer flocculation time and/or greater mixing energy is generally required to treat water at lower temperatures. The critical transition temperature between slow and faster coagulation/ flocculation is around 10oC to 12oC. While hydraulic contact flocculation without mixers may be sufficient for waters that are usually above 12oC, dedicated mixers and longer times are definitely required for water below 10oC.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification Thus much of Queensland needs short flocculation times and low mixing energy but Victoria and Tasmania require as much as 40 minutes flocculation time with much higher mixing intensities.

The flocculation step is often regarded as a set and forget step in water treatment, however there are a number of changes that can be made as described above. The best way to evaluate these is to make the changes and carefully monitor the responses. Observe floc in the tanks. Take dip samples and place them on a jar tester watch how the floc grows compared to the floc in the plant. Carefully monitor clarified water turbidity during settlement after the end of the jar test. It is probably the best indicator of the efficiency of the coagulation, flocculation and clarification process.

6. Determine Coagulation And Flocculation Times In operating a conventional treatment system it is very useful to know how long it will take for a change to have an impact on a plant, in particular the formation of floc. Knowing this allows the operator to know when to look for changes in the nature of the floc, when there has been a dosing fault, or when there has been a change made to the dosing.

Detention times can be calculated theoretically based on the flow and the volume of the pipe or tank. The volume of the pipe can be calculated knowing the diameter and length. The volume of a tank can be calculated knowing the length, width and height. The theoretical detention time can then be calculated by dividing the volume by the flow rate. Remember to work in the same units. It is best to convert all dimensions to metres first. Volumes and flows can then be calculated in m3 and m3/min.

This is however a theoretical time and some water will leave the tank long before the calculated time. It is really a maximum detention time. A useful, more practical way to determine how long it takes for a change to be seen in a floc tank is to cause a change in the primary coagulant dose and watch how long it takes for the change to be seen at the start of the floc tank, and at the end of the floc tank. Decrease the alum dose by around 25% and time how long the appearance of the floc takes to change. Then increase the dose back to normal and measure the time again. You should then have two good measurements of the actual time at the particular raw water flow.

If the plant has pre chlorine or doses potassium permanganate for manganese control, dosing of these can be turned off and then on again after say 30 minutes to 1 hour. The chlorine residual or purple colour can be monitored to determine how long it takes to reach the start and end of the floc tanks.

7. Minimise The Effect Of Weather And Temperature Wind, rain and sun can have adverse effects on the operation of clarification systems.

• •

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Wind and rain on exposed clarifiers can set up currents that adversely affect settling sludge or DAF float (Figure 8.1).

Wind on exposed DAF systems can actually prevent sludge removal by blowing the float back to the opposite end of the tank away from the removal end.

Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification Figure 8.1. The effect of wind on the floated sludge on a DAFF system

• •

•

Rain can physically damage float collected on the surface of the DAF. Temperature variations in the incoming water during the course of the day can cause short-circuiting in a clarifier. Typically these temperature variations arise when water is drawn from a shallow river or lake bed. The problem is usually observed as welling up of sludge in upflow clarifiers when the raw water temperature changes by more than about 0.5oC per hour.

Direct sun on the side of an above ground steel clarifier (and even cement clarifiers) can cause significant thermal currents that generally lead to up welling of floc or ‘boil ups’, increasing the turbidity of the clarified water (Figure 8.2). This then increases the load on the filters and increases the likelihood of early filter breakthrough.

Figure 8.2. Upwelling of floc in the late afternoon in an above ground concrete clarifier.

• • •

Changes in surface water temperatures at night and on cold days can create a denser surface layer that sinks and displaces floc.

Sunlight also encourages biological growth and slimes that slough off and disrupt the process or create mudballs in filters.

Leaves can accumulate in tanks.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification Operators need to be alert to these possibilities and carefully observe clarifier function. This needs to be supported with regular measurements of clarified water turbidity. Ideally, online turbidity meters should installed to monitor clarified water turbidity continuously, however if these are unavailable, dip samples can be taken. Obtaining samples of floated water from DAF units requires a little ingenuity however, it can be done. Experiment with stoppered bottles lowered beneath the float and then remove the stopper. Most DAF plants have sample taps taking water from below the float but these may be blocked. Don’t allow blockages to prevent adequate monitoring of floated water quality, but do clear the blockages as well!

Individual problems, once identified, should be investigated and steps taken to reduce or prevent the problem.

Possible solutions include: • Painting the sides of steel clarifiers with reflective paint.

• Shading clarifiers.

• Covering DAF tanks and clarifiers.

• Increasing polymer dosing.

• Sourcing raw water from deep water off takes where possible.

8. Operate Flocculation Systems Continuously Many plants do not operate continuously. Where plants are operated intermittently, plant shutdown generally includes shutdown of the floc tanks. This allows formed floc in the tanks to settle. Over repeated start/stop cycles this floc material compacts and builds up on the bottom of the floc tank. Eventually, some of this old compacted sludge may lift or be lifted by the flocculators.

To prevent the build up of sludge, formation of sludge rafts and possible transfer of large lumps of sludge to the filters, flocculators should be kept running 24 hours a day even if the plant shuts down.

9. Observe And Monitor Flocculator And Clarifier Operation Floc tanks and clarifiers should be inspected regularly several times each day so that any changes are detected early. Critical parameters such as settled (or floated) water turbidity, sludge blanket depth, Settled Sludge Volumes (SSV), desludge frequency/ duration, recirculation speed and chemical doses should be determined and entered into a spreadsheet and trends graphed and analysed.

Settled (or floated) water turbidity trends should be checked several times per day and any abnormal trending investigated immediately.

The clarified water turbidity should be measured by the operators at least once per day, or better still continuously using an on line instrument.

Higher clarified water turbidity indicates deteriorating or poor clarifier performance. Any sudden increases in turbidity indicate floc carryover and immediate action should be taken to bring the control point back under control.

Increases in flow tend to lead to floc carryover and should be carefully ramped. Flow changes where the flow is decreased normally do not result in floc carryover.

There must be equal hydraulic loading on each clarifier and this should be controlled or adjusted as required.

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10. Monitor Sludge Blanket Depth In clarifiers with sludge blankets it is useful to regularly monitor the depth of the sludge blanket. Clarifiers with draw off cones tend to run at a constant level but in other clarifiers the depth of the sludge blanket may vary. There are a range of sludge blanket detectors that can be lowered into a clarifier and provide a visual or auditory signal once the sludge blanket interface is reached. By plotting this on a graph, operators can trend the behaviour of the sludge blanket and be able to identify • •

Whether the sludge blanket is rising and may create a problem.

Whether the sludge blanket has disappeared or become very thin.

This information can be used to fine-tune the desludging of the clarifier.

The levels of sludge concentration in various zones of the clarifier may be measured using a settled sludge volume (SSV) test. In this test, a sample of sludge is settled for 10 minutes (some people prefer 30 minutes) and the volume of the sludge measured as a percentage of the original volume. Thus a 500 mL sample with a settled sludge volume of 100 mL has an SSV of 20%. The SSV can be used to optimise the operation of a clarifier. Particular figures depend on the specific clarifier design and operating conditions, however, as experience is gained with a particular clarifier, the critical values can be recorded and used to assist future monitoring.

In general, try to maintain a constant SSV at a given sampling point in each clarifier.

In the flocculation zone of sludge recirculation type clarifiers, the SSV should be between 20 to 50% with a differential of no more than 5% between the top and bottom of the flocculation zone. The SSV in samples taken from about 1m below the top of the sludge blanket should be 15 – 20% less than the SSV in the bottom of the flocculation zone. The key thing here is to measure regularly and look for trends and changes and optimise performance.

In upflow sludge recirculation clarifiers, impeller speed in or below the flocculation zone is an important variable and is used to vary the sludge blanket level as well as the solids concentration in the centre of the clarifier. The optimum impeller speed is that at which the solids concentration is relatively uniform over all of the mixing zones.

If the impeller speed is too low, some of the solids will settle at the bottom of the clarifier and the concentration in the mixing zone and the sludge blanket will be low and not uniform. High impeller speeds can push solids out from the mixing zone and raise the sludge blanket. Optimum impeller speed will ensure uniform solids concentration in each zone and provide maximum solids contact for coagulation. This is demonstrated in Figure 8.3.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification Figure 8.3. Effect of impellor mixing speed on solids concentration for solids contact clarifiers.

Floc Zone Slurry Concentration

Too Slow

Optimum

Too Fast

Sludge Recirculation Clarifier

Low sludge blanket.

Most solids in bottom zone of clarifier.

Good spread of sludge floc, good sludge blanket

Sludge pushed out of floc zoneÆ poor solids contact

Assumptions: * Total mass of slurry inside the clarifier is constant * Chemical dosing correct

Impeller Speed (rpm)

11. Control Algae Algae will grow in uncovered floc tanks and clarifiers. Often algae can be seen adhering to launders in long green threads and in thick mats on clarifier and sedimentation tank walls. Thick accumulations of algae can also often be seen on and inside tube settlers to such an extent that flow through the tubes is compromised.

Apart from being unsightly, these algal growths, depending on the species, can impart taste and odour to the water. All structures involved in the treatment of water should be maintained in a clean state.

Launders should be cleaned regularly using pressure sprays and bristle brushes. Tube settlers, lamella plates, floc tanks, DAF tanks and clarifiers should be emptied periodically and the walls cleaned of algae.

Pressure hoses are useful, but care must be taken to avoid damaging plastic tube settlers if fitted. To assist in the process of cleaning, spraying of 1% sodium hypochlorite solutions can assist in removing the growth. The cleaning interval will vary according to the climate and water type and the levels of phosphorus and nitrogen.

Experience of operators will readily indicate the cleaning interval required.

While expensive, the best solution is to cover the structures (Figure 8.4). Erecting covers over DAF systems is relatively easy (Figure 8.5).

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification Figure 8.4. A fully covered DAFF plant protects the float from the effects of weather and provides an all weather 24 hour working environment for operators.

Figure 8.5. Construction of covers.

Larger clarifiers may be a bit more difficult however, the advantages extend beyond just control of algae.

Covers prevent birds accessing the surface of clarifiers and adding faecal material directly to the water.

Covers over DAF units allow better control of sludge removal free from the effects of wind and rain. The covers also protect all mechanical and electrical instrumentation and provide a 24-hour workplace for operators free from constraints of ambient light and weather (Figure 8.4).

12. Keep Flocculation Tanks And Clarifiers Clean In the operation of most clarifier systems there will be some build up of sludge on the bottom and on structures in the clarifiers. This sludge can age and become old and stale. Old compacted sludge may become anaerobic and release anaerobic gases that may float sludge to the surface in large lumps that may be carried over to the filter and contribute to the formation of mud balls.

To avoid this, all clarifiers and floc tanks should be periodically drained and thoroughly cleaned. Alternatively it may be possible to use diving teams to clean the structures without taking them off line.

Regular maintenance schedules for clarifiers should include complete drainage of the clarifier vessel for cleaning of accumulated sludge and inspection of mechanical sludge collection equipment.

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13. Check And Maintain DAF Distribution Systems An even dense distribution of fine bubbles across the width of the DAF unit is essential for optimum performance (see Figure 7.17 in Chapter 7). The distribution of bubbles should be observed regularly and if the distribution is not regular, adjustments to the dissolved air distribution valves should be made until an even pattern is established.

DAF units should be maintained at the saturator pressure specified by the manufacturer to ensure optimum performance. Air distribution nozzles should be inspected and cleaned regularly to prevent blocking by debris.

If a blockage occurs, the valve can be opened and closed several times to try and free the blockage. If this fails then all other valves can be turned off to encourage the full force of the saturated water flow to “blast” the valve clean. Once it is freed, even flow must be restored for optimum operation of the DAF unit. If a nozzle remains blocked, the tank will need to be emptied and the valve repaired or replaced.

Cutting spray nozzles around the edges of the DAF unit often become blocked. These must be serviced regularly to ensure efficient release of the float, thereby reducing the amount of water needed to remove the float. By reducing the amount of water necessary to remove the float, the load on the downstream residuals handling steps can be reduced. The nozzles also may need to be realigned to direct the spray at the interface between the sludge and the wall of the DAF tank. A filter installed in the spray water supply pipe work will reduce the spray nozzle blockages.

Where a significant change in raw water quality occurs, the saturated water recycle rate should be adjusted to ensure optimum operating conditions are maintained. High raw water turbidity or higher coagulant doses often require the recycle rate or operating air saturation pressure to be increased.

In DAFF systems, the float removal process generally involves raising the level of the water in the tank.

This results in an increased hydraulic head and causes turbidity break through in the underlying filters. The degree of breakthrough increases as the filter run time increases. This is clearly evident on filter turbidity trends (Figure 8.6).

Figure 8.6. The effect of DAF floats on filtered water turbidity (red). As run time increases, the height of the Tb spike increases. Headloss is also shown (blue).

This turbidity breakthrough needs to be minimised. The DAFF tank should be operated at as high a level as possible, generally just below the level of the weir, and the float level adjusted to the absolute minimum that will allow efficient removal of the float. In this way the increase in head pressure is minimised and turbidity (and pathogen) breakthrough is also minimised.

14. Monitor Residual Coagulant Levels Aluminium levels can be used to monitor the effectiveness of alum coagulation. The dissolved aluminium concentration should be around 0.05 mg/L in well coagulated water and preferably not more than 0.1 mg/L.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification High aluminium levels indicate: 1. Over dosing of alum leaving unreacted aluminium in the water.

2. Poor mixing of alum leading to pockets of unreacted aluminium.

3. Suboptimal coagulation pH. The cause should be identified and corrected. If the steps described above have been carried out, the problem should not arise. If it does, go back and check the dose and the mixing step carefully. Try changing the dose or shifting the dosing point.

15. Record And Use Past Experience (Look-up Tables) Over years of operation, operators build up extensive knowledge of plant operation.

This experience, if recorded in a meaningful way can lead to the construction of “look-up” tables. These tables simply represent the sum total of experience gained in operating a plant. They record information such as coagulant dose rates under different raw water conditions or changes to be made to floc tank operation during winter and summer.

Many of these “look up” tables can be incorporated into PLC programs to allow the plant to immediately adjust coagulant doses automatically when raw water quality changes.

An example of a look-up table might include some of the information shown in Table 8.2.

Table 8.2. A look-up table for coagulant dose and raw water quality.

Raw Water Turbidity (NTU)

Max Flow (ML/d)

Alum Dose (mg/L)

Poly Dose (mg/L)

< 10 10 - 20 20 – 30 30 – 40 40-50 > 50

25 ML/d 25 ML/d 15 ML/d 15 ML/d 15 ML/d Shut down plant

30 38 45 55 65 -

Zero Zero 0.3 0.5 0.5

16. Other Methods For Controlling Coagulant Dose While jar testing is a widely used as a cheap and reliable method for determining coagulant dose, there are a number of systems available to assist in this function. A few of the common ones are described below. This is a partial list only and new technologies are emerging all the time.

Streaming Current Detectors Streaming Current Detectors (SCDs) are instruments that continuously measure the electrical charge on particles in the water. If water without added coagulant is passed through the instrument, the instrument will read a negative charge because of the negative charges on the particles. As positively charged coagulant is added, the measured charge will gradually become less negative as the negative charges on the particles are neutralised. If the coagulant is overdosed, the measured charge will become more positive. This measurement can be used to control the dose of coagulant.

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Chapter 8 – Optimisation Of Coagulation, Flocculation And Clarification While individual SCDs may differ in style and configuration, the general principle for setting them up is the same.

• Set the instrument in operation according to the manufacturers instructions.

• Use a jar test to determine the lowest concentration of coagulant to give the desired quality and set the plant to run at that dose.

• Fine-tune the plant to produce the absolute best quality treated water. Note the streaming current potential.

• Increase the coagulant dose by approximately 15% and note the streaming current. Decrease the optimal dose by 15% and note the streaming current. This step is undertaken to confirm that the instrument responds to changes.

• Set the dose back to the optimum dose and set the streaming current to zero then let the instrument control the dose in the plant. As water quality changes, the potential will change and the SCD will alter the dosing to bring the current back to zero.

The use of an SCD detector has been shown to save money by ensuring that water isn’t overdosed with coagulant, by reducing sludge production and, by responding to changes in raw water quality without the operator being at the plant. In this way it can help avoid dirty water events. The SCD is only capable of dealing with minor changes in raw water quality. An operator using jar testing is still absolutely essential when significant changes in raw water quality occur.

The use of lime slurry for pre-treatment alkalinity and pH adjustment has been reported to produce erratic behaviour with an SCD at a number of WTPs. In such cases changing to soda ash solved the problem. Also moving the dosing point further upstream has been reported to aid operation of the SCD. Alternatively using a more dilute solution of lime can help.

Zeta Potential Meters are similar to SCD’s but measure the charge in a slightly different way. Like an SCD, they can be used to control coagulant dose in a similar way.

Compass™ Recently a New Zealand company has developed a feed forward control system (Compass) using a sophisticated on line spectrophotometer to measure water quality and control the dose of the primary coagulant and polymer. The system has been tested at a New Zealand WTP with impressive results. The system delivered savings in chemical dosing over that achieved by an SCD (Laidlaw 2007).

mEnCo The Cooperative Research Centre for Water Quality and Treatment have developed a software based tool (mEnCo) for the calculation of coagulant dose. The software requires entry of values for UV254, turbidity, colour and alkalinity for the raw water being treated and calculates a recommended dose for alum or ferric chloride. The tool is particularly useful for “run of river” systems where raw water quality changes, and where DOC removal needs to be maximised for maintenance of chlorine residuals or compliance with THM limits.

The product is used widely in South Australia and also in Tasmania. More information can be obtained from the Cooperative Research Centre for Water Quality and Treatment or Water Quality Research Australia.

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Chapter 9 Other Chemical Processes In different water treatment processes, many chemicals are used. This practical guide has concentrated mainly on chemical dosing associated with coagulation and flocculation. It is not the intention of the guide to cover all forms of chemical dosing in water treatment, however there are a number of chemical dosing processes that are closely aligned to conventional coagulation, flocculation and clarification. These include softening of water, stabilisation of water and the removal of manganese by potassium permanganate dosing.

These processes are considered briefly in this chapter.

1. Water Softening Hard water is not well accepted by consumers. Lathering soap is difficult and unpleasant scums form on the surface of water during cooking. Scale forms on the surface of appliances used to heat or boil hard water such as kettles and hot water systems. The scale consists of precipitated calcium carbonate, magnesium hydroxide, calcium sulphate and silica depending on the type of hardness in the raw water.

Hardness in water is caused by the slow dissolving of limestone and other rock types by rainwater containing CO2. As such, hardness is due to the presence of dissolved calcium and magnesium ions. Two types of hardness are commonly recognised, carbonate or temporary hardness and non-carbonate or permanent hardness.

•

Carbonate (‘temporary’) hardness is due to the presence of bicarbonate (HCO3-) and carbonate (CO32-) compounds of magnesium and calcium however calcium bicarbonate (Ca(HCO3)2) is the main cause. Since carbonate hardness can be removed from solution by boiling it is also referred to as ‘temporary’ hardness.

•

Non-carbonate (‘permanent’) hardness is due to the presence of chloride (Cl-) or sulphate (SO42-) compounds of magnesium and calcium. Since non-carbonate hardness remains in solution after prolonged boiling it is also referred to as permanent hardness.

Total hardness is the sum of carbonate and non-carbonate hardness.

Sometimes the terms magnesium hardness and calcium hardness are used. This simply refers to the sum total of all magnesium compounds in water or all calcium compounds in water.

Hardness is expressed as an equivalent amount of calcium carbonate (CaCO3) in mg/L. The ADWG (2004) states that total hardness should not exceed 200 mg/L as CaCO3 in order to minimise undesirable build-up of scale in hot water systems. Water is generally described as soft below 60 mg/L CaCO3, as good quality up to 200 mg/L CaCO3 with increasing scaling problems up to 500 mg/L CaCO3, and severely scaling above 500 mg/L CaCO3.

Softening is a process used to reduce the hardness of water. Chemical softening is one way to remove the ions responsible for hardness. Ion exchange, reverse osmosis and nanofiltration are other methods to soften water that do not involve the addition of chemicals and are therefore not considered here.

Lime Soda (Chemical) Softening A number of different types of chemical softening are available: • • • •

Partial lime softening.

Excess lime softening.

Lime-soda ash softening.

Caustic soda or lime plus caustic soda softening.

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Chapter 9 – Other Chemical Processes In each case the calcium and magnesium compounds are precipitated variously as calcium carbonate, magnesium hydroxide and magnesium carbonate. The precipitated compounds contribute to the formation of sludge that is usually settled out in a clarifier.

The type of softening process used depends on the type of hardness present, the degree of softening required, operational convenience and the degree to which the production of lime sludge must be reduced.

Chemical costs also need to be considered.

Each type of chemical softening will be considered briefly below.

Partial Lime Softening This process is normally used when there is a very low non-carbonate hardness level and the magnesium hardness is <30 mg/L. Partial lime softening will remove carbonate hardness associated with calcium, but will not remove magnesium hardness or non-carbonate hardness.

Lime is added to increase the pH to around 9.5 or higher.

Calcium carbonate hardness reacts with the lime (Ca(OH)2) to precipitate calcium carbonate (CaCO3).

Ca(HCO3)2 + Ca(OH)2  2CaCO3 (solid) + 2H2O Excess Lime Softening Excess lime softening will remove both magnesium and calcium carbonate hardness but will not remove non-carbonate hardness. This process is used if there is a very low level of non-carbonate hardness and magnesium hardness is >30 mg/L. An “excess” of lime is used to raise the pH to around 11.0 – 11.3.

Calcium carbonate hardness reacts with lime to precipitate calcium carbonate as described above.

Magnesium carbonate hardness reacts with lime in a two-stage process, firstly to produce calcium carbonate plus magnesium carbonate (MgCO3). The magnesium carbonate then reacts with the available excess lime to precipitate magnesium hydroxide and more calcium carbonate.

Mg(HCO3)2 + Ca (OH)2  CaCO3 (solid) + MgCO3 + 2H2O MgCO3 + Ca(OH)2  2CaCO3 (solid) + Mg(OH)2 (solid) For magnesium carbonate removal, enough calcium ion must be added in the form of lime to precipitate the carbonate ion as calcium carbonate while sufficient hydroxide ion must also be provided to react with and precipitates the magnesium.

Lime-Soda Ash Softening This process is normally used if there are significant levels of both carbonate and non-carbonate hardness in the water.

Lime dosing is used to remove the calcium carbonate hardness at pH >9.5 and/or the magnesium carbonate hardness with excess lime at pH >11 as described above. Soda ash dosing is then used to remove the noncarbonate calcium hardness, and lime plus soda ash dosing is used to remove the non-carbonate magnesium hardness.

Calcium carbonate and magnesium carbonate hardness is removed as described above.

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Chapter 9 – Other Chemical Processes Finally non-carbonate calcium hardness reacts with soda ash (Na2CO3) to produce calcium carbonate and sodium salts.

CaCl2 + Na2CO3  CaCO3 (solid) + 2NaCl CaSO4 + Na2CO3  CaCO3 (solid) + Na2SO4 Non-carbonate magnesium hardness first reacts with lime to produce magnesium hydroxide (a precipitate) plus calcium chloride/ sulphate (non-carbonate calcium hardness). The non-carbonate calcium hardness then reacts with soda ash (Na2CO3) to produce calcium carbonate (a precipitate) and sodium salts as per the equations below.

MgCl2 + Ca(OH)2  Mg(OH)2 (solid) + CaCl2 CaCl2 + Na2CO3  CaCO3 (solid) + 2NaCl MgSO4 + Ca(OH)2  Mg(OH)2 (solid) + CaSO4 CaSO4 + Na2CO3  CaCO3 (solid) + Na2SO4 Thus, the lime-soda ash process is able to remove both carbonate and non-carbonate calcium and magnesium hardness.

Caustic Soda Softening Caustic soda (NaOH) softening is able to remove both carbonate and non-carbonate hardness.

The advantages of using caustic soda instead of lime are that the chemical is easier to handle and less sludge is produced. Lime also typically contains some impurities although these are usually removed in subsequent coagulation and filtration processes. The disadvantages of using caustic soda are that caustic is significantly more expensive than lime and the chemical poses a potential hazard to operator safety and the environment in the event of a leak or spill. Caustic dosing also results in an increase to the final treated water sodium levels.

The reaction of caustic with calcium and magnesium bicarbonates produces soda ash (Na2CO3), which then in turn reacts with non-carbonate calcium and magnesium hardness to precipitate calcium carbonate and magnesium hydroxide.

2NaOH + Ca(HCO3)2 Na2CO3 + CaSO4 4NaOH + Mg(HCO3)2 2NaOH + MgSO4

 Na2CO3 + CaCO3 (solid) + H2O  Na2SO4 + CaCO3 (solid)  2Na2CO3 + Mg(OH)2 (solid) + 2H2O  Na2SO4 + Mg(OH)2 (solid)

A combination of lime softening and caustic soda softening can also be used. It is most likely to be used in situations where the capital costs of the lime delivery system are high. The reaction for removal of calcium carbonate hardness by lime and caustic soda is as follows: 2Ca(HCO3)2 + Ca(OH)2 + 2NaOH  3CaCO3 (solid) + Na2CO3 + 4H2O

Mixing Good mixing of the softening chemicals is necessary to ensure even distribution of the chemicals through the water. Lime generally requires high energy mixing to overcome the low solubility of lime in water. Soda ash has a higher solubility than lime and can be dosed with lower mixing energy. Soda ash (and caustic soda if it is used) can be dosed directly into the flocculation zone of a clarifier.

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Chapter 9 – Other Chemical Processes Flocculation times necessary for lime or lime-soda ash softening are longer than for conventional flocculation (typically 30 to 40 minutes but longer at lower temperatures). This is due to lime’s relatively slow reaction rate and the time necessary to grow the particles into solids that will settle out.

The primary goal during flocculation is to form large, settable floc that can be readily removed during the sedimentation/ clarification step. Calcium carbonate crystals tend to be heavy while magnesium hydroxide floc tends to be lighter and gelatinous.

Softening And Conventional Clarification Softening and conventional clarification may take place in the same clarifier if the raw water has relatively low turbidity and organics levels. At high levels, turbidity and NOM adversely impact on the softening reactions and the formation of calcium carbonate crystals. Alternatively, softening precipitates and floc formed by conventional coagulation and flocculation may be settled in separate clarifiers with the softening clarifier following the conventional clarifier (Figure 9.1).

Figure 9.1. A lime softening clarifier in operation.

The addition of lime and soda ash as softening agents increases the pH of the water. Thus the coagulant used in combination with chemical softening must be compatible with the high pH. Ferric salts are often suitable for this. High pH will also facilitate the oxidation reactions used to remove dissolved iron and manganese (see below) and any heavy metals in the water.

While the high pH is an advantage for softening and iron and manganese removal, there are other parts of the water treatment process where high pH presents problems. The pH must be lowered prior to filtration to prevent calcium carbonate precipitating on filter media. It is also essential that the pH be lowered prior to disinfection since high pH compromises disinfection.

After chemical softening, the pH is lowered in a process known as stabilisation or recarbonation. Carbon dioxide is used rather than a more conventional acid since it lowers the pH without affecting the alkalinity or hardness, or adding chloride or sulfate ions.

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Chapter 9 – Other Chemical Processes Solids Management A clarifier is required to remove the bulk of the calcium and magnesium precipitates before filtration. A recirculating type solids contact clarifier is best used in the lime-soda ash softening process since precipitation of calcium carbonate and magnesium hydroxide takes place more readily on previously formed particles.

Calcium/magnesium sludge is recirculated to the flocculation zone from the settled sludge. This recirculated sludge provides material for high levels of particle collisions and further rapid precipitation of the calcium carbonate and magnesium hydroxide.

Sludge is periodically drawn from the bottom of the clarifier and transported to the sludge handling system.

Waste sludges from softening processes may vary in solids content from 3 to 15% w/w solids. 5% w/w is typical.

The dried sludge produced from the lime and soda ash processes can be used commercially in agriculture due to its high lime value.

Figure 9.2. A lime softening solids sludge drying bed (right). Solids from the conventional clarifier are dried in separate drying beds (left).

Lime Crystallisation Lime crystallisation uses the same chemical reaction as conventional lime softening; precipitating calcium and bicarbonate ions as calcium carbonate. The process takes place in a high rate, upflow, fluidised media column, rather than a clarifier as for conventional lime softening. Pre-aeration of the raw water is required to remove carbon dioxide and start oxidising any iron. The process is very popular in the Netherlands where the raw water is quite hard.

The main steps of the lime crystallisation process are: •

Lime dosing and reaction at pH 9.0.

•

Calcium carbonate crystals adhere to the media grains (garnet or quartz sand) in the fluidised column.

•

Larger grains fall to the bottom and are removed and replaced by new media grains.

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Chapter 9 – Other Chemical Processes Post-softening recarbonation before filtration is still required.

The reactor column can have very high upflow rates reportedly as high as 80 m/h, saving on construction costs and ground footprint compared to those required for a clarifier.

The solid calcium carbonate pellets removed from the reaction column are easier to manage than the sludge produced in a clarifier and may also be of commercial value as a feed additive for stock and birds or road base material.

Sequestering While not a true softening process, sequestering is the use of chemicals that hinder the precipitation of calcium carbonate scale in hot water systems and electrical appliances. Agents such as sodium hexametaphosphate have this effect by keeping calcium in a soluble form.

Sequestering is a low cost solution for preventing scaling problems associated with hardness, however, the hardness is not removed and soap lathering would not be improved. It is also likely to contribute to increased waste treatment costs to remove the phosphate. If the phosphate is not removed during waste treatment, it can contribute to the growth of algae in natural waterways where the effluent is discharged.

2. Removal Of Manganese By Potassium Permanganate Iron (Fe) and manganese (Mn) are often considered together, however it is manganese that requires the closest attention during water treatment. Iron is relatively easily removed from water by simple oxidation (aeration or treatment with chlorine) and the steps taken to remove manganese will also remove iron.

The chemistry of manganese is interesting and a little complex; it can exist in several forms. Reduced manganese is soluble. This can be oxidised to form manganese dioxide (MnO2) that is not soluble and precipitates as a fine black particulate. Soluble (reduced) manganese can pass through a conventional water treatment plant directly into the distribution system. Once in the distribution system it may be taken up by manganese bacteria in biofilms, or oxidised by chlorine or bleaches and detergents used in washing clothes.

The oxidised form of manganese is capable of staining laundry fittings and clothes a blue-black colour.

Manganese biofilms may also periodically slough off and create widespread dirty water problems. Poorly controlled chlorine residuals in the distribution system increase the rate of sloughing.

The relative amounts of soluble and oxidised manganese in water varies from place to place and from time to time (Figure 9.3). Soluble manganese levels in raw water must be measured regularly to allow effective control. Soluble manganese is measured by filtering the sample through a 0.45µm filter and measuring the manganese in the filtrate. Often storage reservoirs will have different levels of manganese at different times of the year. In particular, where reservoir “turn-over” occurs, anoxic (low O2) water rich in manganese rises and may reach the level of the off take leading to higher levels of manganese in the raw water. In drought conditions, the manganese levels may also rise significantly due to the effect of evaporation.

One of the main methods to remove manganese from water is to dose the water with potassium permanganate (KMnO4) and convert the soluble manganese to insoluble manganese dioxide. The fine manganese dioxide precipitate is then caught up in the formation of floc and subsequently removed in the clarification and filtration steps.

To avoid dirty water complaints the level of manganese needs to be <0.02 mg/L and preferably < 0.01 mg/L. It is important to note that this is well below the aesthetic limit of 0.1 mg/L set in the ADWG (2004).

Theoretical chemistry tells us that each mg/L of soluble Mn2+ requires 1.92 mg/L of KMnO4, a ratio of 1.92:1.

In practice this seldom occurs because other factors interfere with the efficiency of the chemical reaction. The ratio may be as high as 8 or 9:1 but is normally around 3 to 5:1.

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Chapter 9 – Other Chemical Processes An example of variation in manganese levels in a reservoir

Figure 9.3.

2.5

Mn(sol) mg/L

2

1.5

1

0.5

6/ 08 /2 00 7 14 /1 1/ 20 07

28 /0 4/ 20 07

18 /0 1/ 20 07

2/ 07 /2 00 6 10 /1 0/ 20 06

24 /0 3/ 20 06

5/ 09 /2 00 5 14 /1 2/ 20 05

28 /0 5/ 20 05

0

The factors that affect the efficiency of removal include: • • • • •

pH and alkalinity.

Temperature.

Presence of NOM and/ or sulfide.

Reaction time.

Mixing of KMnO4.

Each of these factors is considered separately.

pH And Alkalinity The ideal pH for manganese removal with potassium permanganate is 8.5. While it can occur at lower pHs the rate of the reaction is reduced. This immediately dictates the dosing sequence necessary for manganese removal. Since the addition of metal salt coagulants lowers the pH, KMnO4 dosing should occur prior to dosing of coagulant.

The reaction of KMnO4 produces acid that consumes alkalinity; therefore in very low alkalinity waters this may limit the effectiveness of the process.1.22 mg alkalinity as CaCO3 is consumed per mg of Mn in the raw water.

As a guide the raw water alkalinity should be >25 mg/L as CaCO3.

Temperature Higher doses are generally required in summer months. In part this may be due to changes in the levels of manganese in reservoirs during summer and in the relative proportion of soluble and oxidised manganese in the raw water in summer months.

NOM And Sulfide NOM and sulfide in raw water result in a higher dosing ratio being required. These chemicals consume some of the KMnO4 thereby requiring a higher dose to compensate for its removal. It should also be noted that if both permanganate and PAC are dosed together they react and neutralise each other. Therefore PAC should only be dosed after the manganese oxidation reactions are complete.

97

Chapter 9 – Other Chemical Processes Reaction Time There is much conjecture about contact times required for manganese oxidation and it seems to depend on how organically bound-up the manganese is. Reaction time depends on the degree to which the manganese is bound to organic material. In most bore waters and surface waters with low NOM levels, the soluble inorganic manganese is rapidly oxidised generally in less than 3 minutes.

Where the manganese has been organically bound or taken up by microorganisms at the bottom of lakes or deeper regions of rivers, the contact times required are longer, typically 5 to 10 minutes. In either case the closer the pH is to 8.5, the shorter the contact time that is required.

Plant designers often use 5 minutes as a suitable starting point. If you have the luxury of deciding where to put the dosing point or being able to shift the dosing point, err on the side of increased contact time and provide at least 5 to 10 minutes.

Mixing Of KMnO4 Potassium permanganate (like all chemicals) needs to be rapidly and thoroughly mixed with all the water. If not the dosing ratio may need to be increased.

Dosing Control Dosing is a balancing act! If the KMnO4 dose is not sufficient, some of the soluble manganese will be left and this will pass directly through the filters and ultimately cause dirty water complaints. If the KMnO4 dose is too high, the excess will pass through the filters and contribute to pink water and dirty water complaints as well.

The answer is get the dosing right!

Dosing for optimum manganese removal requires diligence and lots of measurements. Raw water soluble and total manganese should be measured frequently, at least daily and possibly several times per day when levels are changing. The dose should be based on the soluble manganese levels.

The following method can be used to help set the dose.

• • • • •

•

Measure raw water soluble manganese.

Conduct a jar test to determine the theoretical dose for the plant.

Record the KMnO4 to soluble Mn ratio.

Set up the optimum dose in the plant.

Measure filtered water soluble manganese.

• If the filtered water appears pink and the soluble Mn is higher than the target (<0.02 mg/L) the dose is too high.

• If the filtered water is not pink but the soluble Mn is above the target level the dose should be gradually increased (10% per adjustment is sensible) and the soluble Mn remeasured.

This should be repeated until the minimum soluble Mn is reached.

Note the actual plant KMnO4 to Mn ratio.

Over time the records of dosing ratios may reveal interesting trends that may be useful in helping to know what dose to use. The system will not run itself; it requires operator diligence and the time necessary to carry out the testing and monitoring of the plant.

Manganese may also be removed by other oxidants (chlorine, ozone), greensand or manganese oxide (MnO2) coated filter media. These methods are not considered here.

98

Chapter 10

Closing Words Provision of safe drinking water requires time, diligence, attention to detail and effective process monitoring.

Chemical addition for the purpose of forming floc for subsequent removal during clarification and filtration is the all important first step in the process of delivering safe water to the consumer.

This practical guide provides a sound background to the control and optimisation of the coagulation and flocculation process and the operation of clarifiers. While the addition of chemical(s) and the clarification step are often regarded as “set and forget” steps in the Water Treatment Plant (WTP), they are indeed control points where if we optimise the steps and monitor them thoroughly we increase the chance of producing safe drinking water. Alternatively if the step is not optimised and monitored thoroughly, the chances of producing unsafe drinking water are greatly increased.

The advice provided here might seem to some like more work, BUT it IS the work of operating the coagulation, flocculation and clarification steps. Time simply must be found. Managers need to fully support operators in this to facilitate the implementation of these tasks.

Think about it? A well run WTP means peace of mind that the water delivered to consumers is safe to drink and will not result in an outbreak of waterborne disease of the types that have been described all around the developed world.

Take the first step now!

Good Luck!

References ADWG (2004). Australian Drinking Water Guidelines. NHMRC, Australia.

Gebbie, P. (2001) Water conditioning and stability assessment: An introduction.

Water: March 2001 pp 50-56.

Laidlow, C. (2007) Coagulant dosing control moves forward. WaterWorks: December 2007 pp 22-24.

Mosse, P. and Murray, B. (2009) Practical guide to the operation and optimisation of media filters. WIOA. Williams K. (2005) Improved float removal at a DAF. WaterWorks: June 2005. pp 48-50.

99

Appendices Appendix 1. Mixing Although not essential for understanding and operating water treatment processes, some readers might wish to understand more about mixing and how it is measured. Designers of water treatment plants (WTPs) specify the amount of mixing necessary in various parts of the treatment process. It is therefore useful for operators to understand the basic concept and some might like to employ the concept in their daily operations. The information provided here is an introduction only, but will provide sufficient background to enable use of the concept.

Good coagulation requires that all coagulant chemicals be mixed very quickly and thoroughly with the water, and with sufficient energy so that the particles and the coagulant molecules collide rapidly and often. This is particularly important for the metal coagulants where the initial chemical reactions of adsorption-destabilisation occur in less than 1 second. Sweep coagulation reactions are slower taking between 10 and 60 seconds.

Therefore for the reactions to occur rapidly and efficiently, it is important that the correct mixing is applied.

Once the initial coagulation reactions have occurred it is important that the forming floc is also continually mixed to allow the microfloc to come together and promote the formation of floc (agglomeration).

The intensity of mixing is commonly measured as a mean velocity gradient (G) with units of s-1. The velocity gradient aims to measure the differences in the velocity of the water that occur across the mixing zone. The more violent the mixing, the higher the G value. In some cases the G value will be specified in the WTP process documentation. If the G values are not available in the documentation then it can be calculated.

Note however that velocity gradient measurements and calculations are approximations only and many influencing factors are not accounted for, principally the mixer design parameters. It is also often difficult to define exactly what mixing volume is relevant.

G can be calculated using equation (A1):

G=

Where

P uμ x V

(A1)

P = power input (W) μ = dynamic viscosity of the water (Ns/m2) V = volume of the mixing chamber (m3)

This formula can be applied to both WTP and jar test mixing zones for flash and rapid mixing and flocculation.

The volume of the mixing zone must be measured, either within the beaker or mixing tank. The hard part is to determine the power input. It can be based on the power output of a motor (kW) multiplied by the motor and gearbox efficiency. The manufacturer of your mixer may be able to assist with this information.

For example a flocculation basin of 500 m3 has a flocculator drive with a power rating of 0.8 kW. The motor is new and the manufacturer claims 75% efficiency.

P = 800 x 0.75 = 600 W

100

Appendix 1 – Mixing

Some values for the dynamic viscosity of water are provided in the following Table.

Temperature (oC) 5 10 12 15 17 19 21 23 25 27

Dynamic Viscosity Ns/m2 0.001519 0.001307 0.001235 0.001139 0.001081 0.001027 0.000998 0.000932 0.000890 0.000851

So for the 500 m3 flocculation chamber referred to in the above example with a water temperature of 19oC the G value can be calculated as follows.

V = 500 m3 P = 600 W  = 0.001027 Ns/m2 Substituting these values in equation (A1) G=

P uμ x V

G = (P/ x V)0.5 G = (600/0.001027 x 500)0.5 G = (1168) 0.5 G = 34 s-1 Velocity gradient data is a very useful means of obtaining similar mixing conditions between jar testing and the treatment plant. Calibration curves are available for some jar testing equipment that give the value of the velocity gradient (G) for particular paddle speeds. These are typically given for jar testers with 2 L square jars or 2 L beakers with stators. For 1 L beakers G values would need to be worked out from first principles. For a 1 L jar, the values can be multiplied by 1.4 to give an approximate G value.

If the G values can be determined for mixing and flocculation in the plant, then these can be matched for the jar test making the jar test results a better predictor of behaviour in the plant. It is particularly useful to do this for flocculation. For the example given above a G value of 34 s-1 is equivalent to a stirrer speed of 40 rpm for a jar tester with a 7.6 cm flat paddle and a 2 L square beaker. Where there is tapered flocculation you can use the average flocculation energy, or alternatively make 2 or 3 adjustments to the rpm of the jar tester during the flocculation stage. It is often not possible to mimic the mixing energy of the rapid or flash mix stage in jar testers. The older jar testers provide nowhere near a high enough G. New jar testers that provide rpm up to approximately 300 allow closer approximation to actual mixing G values. In general the maximum rpm of the jar tester should be selected for the rapid mixing stage.

101

Appendix 1 – Mixing Some Typical G Values Rapid mixing is generally in the range of 400 to 750 s-1, while flash mixing is >=1000 s-1.

Mechanical or hydraulic flocculators are typically designed to provide between 5 and 45 minutes flocculation at velocity gradients decreasing from between 150 s-1 and 20 s-1. In colder waters, such as in Victoria and Tasmania, the longer flocculation times and higher mixing energies are required.

To produce large settlable floc, a large, tapered floc basin is required. In a typical three-compartment flocculator with 5 to 10 minutes contact time in each compartment, the mixing energy provided in each compartment would taper from around 125 s-1 to 100 s-1 to 50-75 s-1 in respective compartments. If the floc is light and fragile then lower speeds may be required to prevent floc shear. If there were a particularly heavy suspended solids load then a slightly higher mixing energy would be required.

Softening applications require around 130 – 300 s-1 depending on the % solids in the softened water. Higher solids loads require the higher mixing energies.

102

Appendix 2. Some Polymers Used In Water Treatment Name

Type

Charge

Charge Density

MW

Manufacturer Listed Use

Company

Filter aid

Ciba Ciba Ciba Ciba Ciba GE-Betz GE-Betz GE-Betz GE-Betz

Anionic Polymers LT25 LT25S LT26 LT27 LT28 AE1115 AE1117 AE1125 1115LP FLOPAM® AH 912 SH

Polyacrylamide Polyacrylamide Polyacrylamide Polyacrylamide Polyacrylamide

Anionic Anionic Anionic Anionic Anionic Anionic Anionic Anionic Polyacrylamide Anionic

Weak High Weak Very High Medium High Medium-Low Very high Weak High Low Low Low Medium High High Low High

Coagulation flocculation aid Sludge thickener

Polyacrylamide Anionic

Low

All liquid/solids separation systems

SNF

®

Polyacrylamide Anionic

Low

All liquid/solids separation systems

SNF

®

Polyacrylamide Anionic

Medium

All liquid/solids separation systems

SNF

FLOPAM AN 923 VHM Polyacrylamide Anionic

Medium

All liquid/solids separation systems

SNF

Polyacrylamide Anionic

Medium

All liquid/solids separation systems

SNF

FLOPAM AN 934 VHM Polyacrylamide Anionic

Medium

All liquid/solids separation systems

SNF

FLOPAM AN 910 SH FLOPAM AN 923 SH ® ®

FLOPAM AN 934 SH ® ®

FLOPAM EM 235

Polyacrylamide Anionic

Low

Clarification, thickening

SNF

®

FLOPAM EM 430

Polyacrylamide Anionic

Low

Clarification, thickening

SNF

®

FLOPAM EM 533

Polyacrylamide Anionic

Medium

Clarification, thickening

SNF

FLOPAM® EMA 20

Polyacrylamide Anionic

Low

Clarification, thickening

SNF

Cationic Polymers LT22 LT22S LT30 LT31

Polyacrylamide Polyacrylamide Polyacrylamide Polyacrylamide

Medium-Low High Medium-Low Low High Low

Coagulant aid Coagulant aid Secondary coagulant Secondary coagulant, direct filtration

Ciba Ciba Ciba Ciba

LT35 Magnafloc LT35

PolyDADMAC Cationic DADMAC - 40% Cationic

Magnafloc LT405

Cationic Cationic Cationic Cationic

High

Low Primary or secondary coagulant Low-Medium

Ciba Ciba

DADMAC - 20% Cationic

High

Low

Ciba

Magnafloc LT410

DADMAC - 30% Cationic

High

Low

Magnafloc LT425

DADMAC - 40% Cationic

High

Low

Magnafloc LT505

DADMAC - 20% Cationic

High

Medium

Magnafloc LT510

DADMAC - 27% Cationic

High

Medium

Primary coagulant for water and wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant for water and wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant for water and wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant for water and wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant for water and wastewater clarification, Can eliminate need for Alum, Ferric

Ciba

Ciba

Ciba

Ciba

103

Appendix 2 – Some Polymers Used In Water Treatment Name

Type

Charge

Charge Density

MW

Manufacturer Listed Use

Magnafloc LT525

DADMAC - 40% Cationic

Magnafloc LT610

Company

High

Medium

DADMAC - 27% Cationic

High

High

Magnafloc 368

DADMAC 100%

Cationic

High

Low

Primary coagulant for water and Ciba wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant for water and Ciba wastewater clarification, Can eliminate need for Alum, Ferric Primary coagulant or Clarification aid, can Ciba be used with a high molecular wt anionic.

Magnafloc 1697

DADMAC - 40% Cationic

High

Low

Process water and effluent treatment

Magnasol 500

PAC

Cationic

Low

Alum replacement, also removes colour, Ciba for wastewater and oily water

Magnasol 576

PACH

Cationic

Low

Alum replacement, also removes colour, Ciba for wastewater and oily water

Magnasol 575 / 572

PAC/DADMAC Cationic Blend

Low

Alum replacement, also removes colour, Ciba for wastewater and oily water

Magnasol 589

PACH/DADMAC Cationic Blend

Low

Alum replacement, also removes colour, Ciba for wastewater and oily water

Magnafloc 1597

Polyamine 50% Cationic

Low

Colour and Turbidity removal for Industrial Ciba Process waters Coagulant aid GE-Betz Sludge thickener GE-Betz GE-Betz GE-Betz GE-Betz GE-Betz GE-Betz GE-Betz GE-Betz Secondary coagulant. Effective for colour GE-Betz removal SNF

Cationic Polymers (cont)

CP1158 CP1160 PC1190 PC1192 PC1195 CE1168 CE2651 CE1169 CE2681 PC1190

Cationic Cationic Cationic Cationic Cationic Cationic Cationic Cationic Cationic Cationic

Strong Very High Medium-Low High High Low High Low High Low Low Low Low Medium Medium Medium Low Medium Strong Low

Quaternary polyamine Polyacrylamide Cationic

Low

®

FLOPAM FO 4190

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Low

SNF

FLOPAM FO 4190 SSH Polyacrylamide Cationic

Low

SNF

FLOPAM® FO 4140 SH FLOPAM FO 4190 SH ® ®

FLOPAM FO 4198

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Medium

SNF

FLOPAM FO 4550 BPM Polyacrylamide Cationic

Medium

SNF

High

SNF

Medium

SNF

FLOPAM FO 4240 SH FLOPAM FO 4290 SH FLOPAM FO 4498 ® ®

FLOPAM FO 4550 SH ®

Polyacrylamide Cationic

FLOPAM FO 4550 SSH Polyacrylamide Cationic

104

High

Ciba

Appendix 2 – Some Polymers Used In Water Treatment Name

Type

Charge

Charge Density

MW

Manufacturer Listed Use

Company

Cationic Polymers (cont) FLOPAM® FO 4650 SH

Polyacrylamide Cationic

High

SNF

FLOPAM FO 4650 SSH Polyacrylamide Cationic

High

SNF

Polyacrylamide Cationic

High

SNF

FLOPAM FO 4700 SSH Polyacrylamide Cationic

High

SNF

®

®

FLOPAM FO 4700 SH ® ®

Polyacrylamide Cationic

Low

SNF

®

FLOPAM EM 145 L

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Low

SNF

®

Polyacrylamide Cationic

Medium

SNF

®

Polyacrylamide Cationic

Medium

SNF

®

FLOPAM EM 445 L

Polyacrylamide Cationic

Medium

SNF

®

Polyacrylamide Cationic

Medium

SNF

®

Polyacrylamide Cationic

Medium

SNF

®

Polyacrylamide Cationic

High

SNF

®

FLOPAM EM 640 L

Polyacrylamide Cationic

High

SNF

FLOPAM® EM 640 LH

Polyacrylamide Cationic

High

SNF

FLOPAM® EM 840 BD

Polyacrylamide Cationic

High

SNF

FLOPAM® EM 840 CT

FLOPAM EM 140 CT FLOPAM EM 240 CT FLOPAM EM 340 CT FLOPAM EM 445 CT FLOPAM EM 540 CT FLOPAM EM 540 LH FLOPAM EM 640 CT

Polyacrylamide Cationic

High

SNF

®

FLOPAM EM 840 L

Polyacrylamide Cationic

High

SNF

®

Polyacrylamide Cationic

High

SNF

®

FLOPAM EM 949 L

Polyacrylamide Cationic

High

SNF

®

FLOPAM EMR 2560

Polyacrylamide Cationic

High

SNF

Nonionic Polymers LT20 Magnafloc 800HP

Polyacrylamide Non ionic Polyacrylamide Nonionic

High Very High

Filter aid For dewatering very slimey pulps eg tailings, mineral processing

Ciba Ciba

AE1115P

Alkylamide

High

Filter aid

GE-Betz

FLOPAM EM 840 LH

Non ionic -

There are a range of proprietary mixtures available from different suppliers. Some examples include Nalco Ultrion products e.g. 44560 and Allclear e.g. 325 and WTS. There are also a range of coagulants branded with names such as Allclear 300 and Allclear 200.

Such names make it very difficult for operators to know exactly what they are dosing, and, with mixtures, what the actual dose is in mg/L for the individual chemicals making up the mixture. Where these products are used, every effort should be made to get full details from the supplier.

105

Appendix 3. Example Product Specifications For Alum Specific Gravity Versus Alum Strengths As Al2O3 At 20oC SG at 20oC

Al2O3 %w/w

1.300 1.301 1.302 1.303 1.304 1.305 1.306 1.307 1.308 1.309 1.310 1.311 1.312 1.313 1.314 1.315 1.316 1.317 1.318 1.319 1.320 1.321 1.322

7.79 7.81 7.83 7.86 7.88 7.90 7.92 7.94 7.97 7.99 8.01 8.03 8.05 8.08 8.10 8.12 8.14 8.16 8.19 8.21 8.23 8.25 8.27

Source: www.omegachem.com.au The website includes the following notes • The above table should be used as a guide only.

• Physical chemical composition may vary results.

• Actual values can be ascertained via laboratory analysis.

106

Appendix 4. Example Jar Test Record Sheet Jar Test Record Sheet Jar Test No. Treatment Plant Raw Water Source Plant Operating Conditions Coagulant/dose Polymer/dose Polymer/dose Jar Test Conditions Rapid Mix Time (secs) Flocculation Time (mins) Test Objective

Raw Water Quality

Date Time Operator Coagulation pH Alkali

Rapid Mix Speed (rpm) Flocculation Speed (rpm) Turbidity (NTU)

True Colour (Hazen)

pH

Mn (total/sol) (mg/L)

Fe (total/sol) (mg/L)

Temp (˚C)

2

3

4

5

6

Alkalinity (mg/L CaCO3)

1 Chemicals (mg/L) mLs/jar (mg/L) mLs/jar Time to form pin floc (mins) Floc size (mm) at ……mins Floc size (mm) at ……mins Floc size (mm) at ……mins Description Settling rate (mm/min) Description Supernatant Quality Turbidity (NTU) Filtered Water Quality Turbidity (NTU) True Colour (Hazen) pH Al (total/sol) (mg/L) Mn (total/sol) (mg/L) Fe (total/sol) (mg/L) Comments/Conclusions

107

Appendix 5.

An Alternative Method For Expressing Alum Concentrations.

Alum doses can be expressed as v/v rather than w/v. One advantage of this is that it makes it really easy to go from a jar test to the whole plant. However it makes it difficult to compare doses from plant to plant using different units.

Some operators prefer this method because it is conceptually simpler and avoids calculations using SG and % strengths, and for operators comparison between plants is not terribly relevant.

So for example if the alum draw down for the plant was 60.6 mL/s into plant flow of 450 L/s the dose is 60.6 mL/450 L = 0.135 mL/L = 135 L/L = 135 ppm(V) So for jar testing, a 1% stock solution can be made up by adding 10 mL of liquid alum (or other coagulant) to 990 mL of deionised water. 1 mL of this stock 1% solution into a 1L jar gives 10 ppmV of supplied liquid alum. If the optimum dose in the jar test is found to be 5 mL/L then this is equivalent to 50 ppmV or 50uL/L in the plant. All that is required is to scale it up by the flow rate in the plant to get the amount that needs to be delivered by the coagulant dosing pumps.

Remember if you do need to make comparisons from plant to plant, then you should express the figure as so many ppm(V) of a certain solution strength alum (e.g. 35 ppmV of a 48%, 1.35 SG alum stock) or alternatively do the calculations and make the comparison based on mg/L (see Chapter 6).

Appendix 6.

Floc Size Chart

A floc-size chart is useful for obtaining uniform reporting of floc-size from different operators.

108

Disclaimer This document does not purport to be a comprehensive statement on the management of chemical dosing, coagulation, flocculation and clarification but rather aims to create awareness of what is possible, what might be done and indeed what has been done. However the authors, reviewers and the organisations supporting the production of this manual (including WIOA, AWA, WSAA, CRCWQT and ICEWaRM) are not responsible for the outcomes of any actions taken on the basis of information in this document, nor for any errors or omissions. The authors and the afore mentioned organisations disclaim all and any 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 further expert advice is required the services of a competent professional should be sought.

www.wioa.org.au

WILLPRINT SHEPPARTON 03 5831 6565

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