Odour Control In Sewage Transport Systems

Practical Guide To Odour Control In Sewage Transport Systems

Phillip Dack and Gary Finke

The Authors Phillip Dack is currently Director of Sulfide Control Pty Ltd. Gary Finke is currently Managing Director of Aromatrix Australia Pty Ltd.

Acknowledgements This practical guide is the first in a new series of publications produced by the Water Industry Operators Association of Australia (WIOA) and represents the considerable commitment of WIOA to improving the skills of operational staff in the water industry. WIOA has previously produced a set of Practical Guides to Water Treatment. The series includes:

• • • • •

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

Collectively, these guides assist WIOA to meet an organisational goal of accumulating and disseminating information on the design, control, operation and maintenance of water and wastewater systems. The authors wish to particularly thank Peter Mosse from Hydrological and George Wall from the WIOA for editorial input and critical review of this practical guide.

The authors also wish to thank Steven Nash from Goulburn Valley Water and Philippe Cussinet from Environmental Group Limited (EGL) for critical discussion and thoughtful review of the manuscript.

WIOA would like to thank the sponsoring partner organisations:

• • • • • • • • •

Australian Water Association (AWA) Government Skills Australia (GSA) International Centre for Excellence in Water Resources Management (ICE WaRM) National Water Commission (NWC) Water Directorate, New South Wales Pipes Wagga Wagga Queensland Water Directorate (qldwater) Water Industry Training Association Queensland (WITA) Water Services Association of Australia (WSAA)

Publications Details First Edition, September 2011.

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

General Introduction The Water Industry Operators Association of Australia (WIOA) has produced a successful series of Practical Guides to Water Treatment. These cover key control points from the catchment all the way through to the customers tap.

In this new series of Practical Guides produced by WIOA, the key elements of the provision and operation of sewerage treatment systems are covered. The series includes: 1. Management of odours in sewage transport systems.

2. The operation and optimisation of lagoon treatment systems.

3. The operation and optimisation of activated sludge treatment systems.

4. The operation of reclaimed water systems.

An underlying objective for 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: o That’s a good idea. o We should be doing that. o 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.

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 or subsequent editions 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.

1

Contents General Introduction Chapter 1 Background • • • • •

The History Of Sewage Transport Systems And Odours................................................................5 The Cost Of Ignoring Odour And Corrosion....................................................................................7 Current Industry Knowledge............................................................................................................8 STS Definition.................................................................................................................................8 How STS Operators Should Use This Guide..................................................................................8

Chapter 2 Odour And Odour Chemistry

• What Are The Common Odorous Compounds In Sewage?...........................................................9 • Odour Versus Hydrogen Sulfide....................................................................................................10 • Safety And Sewer Gas..................................................................................................................10 • Gas Concentrations And Conversion Of Units..............................................................................11 • The Generation Of Hydrogen Sulfide In STS’s.............................................................................12 • How Much Hydrogen Sulfide Will Be Released From Sewage As A Gas?...................................13 • Estimating Worst Case Gas Levels...............................................................................................14 1. General Method For Odorants In Sewage At 250C................................................................14 2. Method For Hydrogen Sulfide At Different Temperatures.......................................................15 • Estimation Of Sulfide Generation In Pipelines..............................................................................16 • Trade Wastes................................................................................................................................17 1. Trade Waste Quality...............................................................................................................17 2. Trade Waste Flow Management.............................................................................................18 • Hydrogen Sulfide And Corrosion...................................................................................................19

Chapter 3 Measuring Odorous Compounds Odour And Gas Flows

• Liquid Phase Measurements Including Safety Aspects................................................................20 Safety Aspects......................................................................................................................20 Sampling...............................................................................................................................21 On Site Liquid Phase Oxidation Reduction Potential (ORP) Measurements........................21 On Site Liquid Phase Soluble Sulfide Measurements..........................................................21 Laboratory Based, Liquid Phase Soluble Sulfide Measurements.........................................22 Continuous Liquid Phase Measurements.............................................................................22 • Gas Phase Measurements Including Safety Aspects...................................................................24 Safety Aspects......................................................................................................................24 Laboratory Based Measurement Of Odour And Odour Related Compounds.......................24 On Site Gas Phase Measurements .....................................................................................24 • Gas Flow Rate Testing And Estimation Of Downwind Odour Levels............................................26 • Smoke / Leak Testing ...................................................................................................................27 • Site Sampling And Testing Kit Checklist For Operators ...............................................................28

2

Chapter 4 Sewage Transport System Components And Odour Issues

• STS Components..........................................................................................................................29 Household Boundary Traps..................................................................................................29 Gravity Reticulation Collection Mains...................................................................................30 Pump Stations And Wet Wells..............................................................................................30 Rising Mains.........................................................................................................................31 Manholes..............................................................................................................................31 Hydraulic Drops ...................................................................................................................32 Sewage Treatment Plant Inlet Works....................................................................................32 Vents ....................................................................................................................................32 Ventilation Fans ...................................................................................................................33 • Principles Of Ventilation................................................................................................................34 Septicity And Venting............................................................................................................34 Ventilation Air Flow Rates.....................................................................................................34 • Gravity Mains And Reticulation Pipework Venting........................................................................34 • Pump Station Venting....................................................................................................................35 • Part Full Branch And Trunk Sewer Venting...................................................................................36 • Air Valves Versus Vent Stacks......................................................................................................36 • Pressure Sewers...........................................................................................................................37 • Vacuum Sewers............................................................................................................................37

Chapter 5 Targets For Achieving Low Odour

• Targets For Sewage Quality Including Trade Wastes...................................................................38 • Targets For Vents And Ambient Air...............................................................................................40 EPA Odour Legislation And Standards.................................................................................40

Chapter 6 Control Of Sulfide Levels In Sewage

• General Principles.........................................................................................................................41 • Treatments That Prevent Sulfide Formation.................................................................................41 Air And Oxygen.....................................................................................................................41 Chlorine................................................................................................................................43 Nitrates.................................................................................................................................43 Nitrites...................................................................................................................................45 Anthraquinone......................................................................................................................45 Bacterial Mixtures.................................................................................................................45 Nutrient or Bacterial Stimulant Mixtures................................................................................45 Alkalis...................................................................................................................................46 Other Biocides......................................................................................................................46 • Treatments To Prevent Sulfides Leaving The Liquid Phase..........................................................46 Caustic Soda (Sodium Hydroxide) Solution..........................................................................46 Lime......................................................................................................................................47 Magnesium Hydroxide (MHL)...............................................................................................47 • Treatments To Remove Existing Sulfides......................................................................................47 Air And Oxygen.....................................................................................................................47 Hydrogen Peroxide...............................................................................................................48 Chlorine................................................................................................................................48 Nitrates.................................................................................................................................48 Nitrites...................................................................................................................................48 Ferric Chloride And Ferric Sulfate.........................................................................................49 Ferrous Chloride, Ferrous Sulfate and Pickle Liquors (Ferrous Salts)..................................49 Others...................................................................................................................................50 • Removing Other Odorous Compounds From Sewage..................................................................50 • Chemical Dosing: Dosing Point and Control Type .......................................................................50 • Comparison Of Liquid Dosing Options For Sulfide Control...........................................................52 3

Chapter 7 Treating Odorous Gases

• General Treatment System Principles...........................................................................................55 • The Four Main Systems Commonly Used.....................................................................................60 1. Activated Carbon Adsorbers...................................................................................................60 2. Biofilters..................................................................................................................................63 3. Wet Chemical Scrubbers........................................................................................................68 4. Biotrickling Filters...................................................................................................................71 • Summary Of Typical Design And Operational Characteristics......................................................73 • Other Gas Treatment Systems......................................................................................................77 • Cost Comparisons.........................................................................................................................78 • Basic Operational Checks For Gas Treatment Systems...............................................................79

Chapter 8 Solving Odour Problems - A Practical Procedure

• Self Management Of Odour Control..............................................................................................81 • Define The Problem - Steps 1 to 6................................................................................................81 Step 1. Gather System Information.......................................................................................81 Step 2. Determine If The Problem Is Real ...........................................................................82 Step 3. Identify Possible STS Site(s) Responsible...............................................................82 Step 4. Carry Out Quantitative On Site Tests.......................................................................83 Step 5. Predict Downwind Concentrations Of Odorant For Existing And Required

Situations..................................................................................................................83 Step 6. Produce A Problem Definition Statement ................................................................83 • Determine And Implement The Best Solution - Steps 7 to 9.........................................................84 Step 7. Decide If There Are Any Appropriate Non-Treatment Related Solutions.................84 Step 8. Consider Sewage And/Or Gas Treatment Options, And Choose Best System(s)....85 Step 9. Install And Confirm Performance..............................................................................86

Chapter 9 Closing Words Appendices

Appendix 1. Case Study – Steps In Solving An Odour Problem In An STS...........................................................88 Appendix 2. Table Of Molecular Weights................................................................................................................92 Appendix 3. Sewage Odours Information Sheet.....................................................................................................93 Appendix 4. Odour Complaints Record Sheet........................................................................................................94

References ......................................................................................................................................... 95

4

Chapter 1 Background The History Of Sewage Transport Systems And Odours Human beings find sewage, and anything to do with it, unpleasant. The hunter gatherers were continually on the move and didn’t really face the problems associated with the accumulation of human waste. With the advent of agriculture, humans started “living” in fixed locations. Accumulation of wastes became a significant limiting factor in the development of early civilisations. The early history of human civilisations is linked closely to the history of sanitation and public health.

In most early civilisations, waste, including human waste was simply thrown into the streets. As towns and cities developed, clay was periodically thrown onto the top of the waste. Over time, the level of the street was raised to such an extent in some cases that stairs had to be cut down to allow entry to the houses.

There were many early attempts to improve this unsanitary situation. Clay pipes were used by some Babylonians to convey sewage as early as 4000BC. In most cases the waste was conveyed to cesspits.

Cesspits have been used widely throughout human civilisation. A cesspit is a cylindrical pit dug in the earth, approximately 1 metre in diameter and 2 to 3 metres deep. The cesspits were often lined with loose fitting bricks or stones to encourage liquids to seep into the soil and to leave the solids to collect and decay. With continued use, the solids built up and progressively reduced the ability of liquids to seep out of the cesspit.

Overflows were inevitable. Some cesspits were emptied but many were simply allowed to overflow.

In Ancient Greece (300BC to 500AD) in the city of Athens, public latrines drained into sewers and transferred the waste to collection basins outside the city. Brick lined conduits then transported wastewater for reuse on agricultural lands.

In Rome by 100AD, a sewer system of interconnected pipes existed and there were some direct connections to homes, however the practice of disposing of waste directly to the street persisted. Eventually the waste, via either route, was transferred to the Tiber River. Potable water supplied via the famous roman aqueduct system was periodically used to clean the filth from the streets. Rivers and smaller waterways formed the basis of many early waste “disposal” systems. The rivers simply “carried the waste away”.

With the collapse of the Roman Empire, sanitation practices regressed. Sewage was again commonly thrown out the window onto streets and was eventually washed into nearby waterways, often via open drains. The practice of throwing waste from upstairs windows is also thought to be the reason why in polite society, ladies walked on the house side of the pavement. The unlucky male escort would end up wearing the ejected waste!!!

By the 1700’s in London and Paris, cesspits were commonly situated under the houses. Excrement accumulated. It is hard to imagine living in the presence of the stench from such a pit. Not surprisingly, the poorer people lived at the lower levels in the houses and the wealthier higher up. There are accounts of families dying during the night from the effects of H2S (hydrogen sulfide) emitted from these overfull cesspools. The cesspools overflowed to the street and into shallow wells and cisterns used to store drinking water. Disease was rife.

With the advent of the industrial age, some industries such as paper making and dyeing started to use human wastes as part of the process. In cities, attempts were made to regularly clean the streets but this did little to improve the unsavoury conditions.

It was not until 1775 when Alexander Cummings developed the water seal for toilets that some odours were prevented from entering the house. The introduction of the water closet and connection to the sewage system resulted in wastes being removed from the home thereby alleviating the need to use cesspits.

5

Chapter 1 – The History Of Sewerage Transport Systems And Odours However, in London this simply transferred the problem to the Thames River, itself becoming a large cesspool.

Western sanitation was slow to recover from the horrors of the Middle Ages. In Paris, a series of cholera epidemics in the 1830s began the reawakening. A complex system of new sewers (called “Les egouts”) was constructed and became the pride of Paris. Construction of this system started in 1850, and by 1870 over 500 km of new sewers were either in service or under construction. By 1930, the entire system (a “combined” storm water and sewage system) was finished: “One sewer for each street”.

In an early attempt to control sewage odour, lime or chloride of lime was thrown into the crude transport systems with limited success. Londoners thought that diseases such as cholera were caused by a miasma (Greek language for “a noxious vapour”), a noxious form of “bad air”, and that “all smell was disease”.

The role of water in the transmission of disease had been implicated as early as 1848 when Dr John Snow hypothesised that cholera was transmitted in water contaminated by human sewage and William Budd in 1856 extended this to the transmission of typhoid fever.

During an investigation of an explosive outbreak of cholera in the Soho district of London, Snow traced the source to a specific pump station. Based on his work, Snow convinced the authorities responsible for the provision of water to remove the pump handle. In subsequent investigations a link was found between a nearby cesspit and the shallow well supplying the pump station in question.

The years 1858 and 1859 in London were known as “the years of the big stink”. The stink arose as a result of the decay of wastes from three million people discharged to the Thames River. In the Thames there were periods when no flow occurred and at other times flow reversal carried the old stinking sewage back toward the middle of London!! Finally the stench interrupted the parliament of the day. Something had to be done.

Joseph Bazalgette finally overcame the politics of the day and the firm belief in miasma as the cause of the disease, to design and install the first major ovoid brick sewer system by around 1865. As a testament to his engineering skills Bazalgette was able to construct the sewer system consisting of 1100 miles of sewers in three huge sections in north London and three similar sections south of the Thames river. The project was initiated to eliminate foul odours, which he substantially achieved. An unintended consequence of his work was to minimise cross contamination of water supplies by sewage and thus help greatly reduce the incidence of cholera and other diseases.

Experience with operating these early sewer systems showed the need to allow sewer systems to “breath” air in and out, to avoid vacuum or pressure build up as sewage levels rose or fell. Such pressure changes caused pipeline failures and breaching of water seals which were installed to prevent odorous gases from passing. Installation of vents and venting, often through manholes, began. The art of proper venting of sewers also involved the control of the location and magnitude of associated sewer odours.

As populations grew, conveyance systems grew in size and length, and as sewage retention times grew, so did the associated odours.

Whilst gravity sewers were used initially, pumped systems with associated inflow storages became necessary, and odour problems then arose from pump stations and other downstream facilities.

In Australia, sewage was also originally discharged to rivers or streams in some places and the history of development of the transport systems is similar to that which occurred in Europe.

In the mid 1800’s in Melbourne, sewage originally flowed into the Yarra River and this caused a lot of problems. There were many typhoid epidemics at that time resulting in lots of sickness and eventually Melbourne became known as “Smellbourne”. In 1897 things improved.

6

Chapter 1 – The History Of Sewerage Transport Systems And Odours Melbourne’s sewage and associated odour was transported to Werribee, thirty kilometres away, and odour control systems began to be implemented on the transport system. In Sydney, the first planned sewerage system was built in 1857. It included five pipes discharging raw sewage into Sydney Harbour. By 1875 these pipes were causing a nuisance with banks of “filthy and putrid mud” occurring at Darling Harbour and other places. By 1888 the first attempts at sewage treatment had begun at the Botany Sewage Farm, with sewage released into Botany Bay. Transfer of sewage to the ocean via ocean outfalls eventually occurred, together with implementation of odour control on the transport system and at the treatment plants.

While modern sewage transport systems now allow for basic odour control, the owners and operators of such systems are often still faced with difficult practical decisions involving how best to control sewage odours.

This guide will help to address this.

The Cost Of Ignoring Odour And Corrosion Major costs can be incurred by the owners of sewage transport systems if odour and H2S based corrosion of the asset is ignored. These costs can be both financial and non financial and may include: • • • • •

the souring of relationships with customers the cost of dealing with odour complaints operator illness and poor public health system failures which may lead to large environmental damage costs the cost of remedial works including: o replacement of corroded assets e.g. manholes as shown in Figure 1.1 o hiring consultants to assess and recommend solutions to the problem o installation and commissioning of odour control equipment.

Accurate estimates of the financial cost of odour and corrosion in Australian sewage transport systems are difficult to find, however it is likely that hundreds of millions of dollars have been spent by Australian water utilities in this area over the last twenty years.

Figure 1.1. An example of a severely corroded manhole resulting from H2S attack.

7

Chapter 1 – The History Of Sewerage Transport Systems And Odours

Current Industry Knowledge The Australian water industry has made a major contribution to the understanding of the mechanism of odour generation and corrosion and ways of preventing them. A 1989 publication by the Melbourne and Metropolitan Board of Works provides an excellent reference. Recently a number of universities, water utilities and industry partners across Australia have combined to develop national standards and procedures for cost-effective methods to manage and minimise concrete corrosion and odour emission from sewers.

This is part of a five-year, $19 million national program, partly funded by the Australian Research Council.

STS Definition For the purposes of this Guide, a Sewage Transport System (STS) is defined as that part of the system from where sewage leaves a household or trade waste system, up to and including the Sewage Treatment Plant (STP) inlet works, (but does not include the treatment plant itself).

How To Use This Guide STS operators who have been called out to respond to an odour complaint from the public, are often placed initially in a situation where it is difficult to determine how, when, where and why the odour is occurring, and what can be done about it. The complainant on these occasions may be upset, and their frustration is often taken out on the operator.

To handle such situations, the operator needs to be properly trained and equipped with some basic knowledge and procedures.

This Guide presents a logical, step by step procedure to help resolve odour complaints. The procedure is provided in Chapter 8 after the background science and engineering is provided throughout Chapters 2 to 7. Some operators may wish to turn directly to Chapter 8, but it is recommended that the information in Chapters 2 to 7 be well understood prior to putting Chapter 8 into practice.

8

Chapter 2 Odour And Odour Chemistry Solving odour problems in a Sewage Transport System (STS) is best achieved by understanding the hydraulics of the STS and the underlying chemistry of odours. The system layout and hydraulics should be understood first, before investigations into odour and odour chemistry are undertaken.

What Are The Common Odorous Compounds In Sewage?

Gases found in STS’s contain a variety of compounds which can cause odours. Some of the more important compounds and the nature of the odours are shown in Table 2.1.

Table 2.1.

Common odorous compounds found in sewage.

Compound

Comments Regarding Odour.

Typical Levels

Hydrogen sulfide (H2S).

The most common cause of sewage odours. H2S has a typical “rotten egg gas” smell.

Formed under anaerobic conditions within the sewage transport system from sulfate present in the sewage. pH has a marked effect on whether it stays in solution in the sewage, or is released into the gas space above. It is termed an “acid gas” and is typically released as the pH is lowered below pH 8.5. (See Figure 2.1).

0 to a few hundred ppm in sewage gas.

Other reduced sulfur compounds e.g. methyl mercaptan, ethyl mercaptan.

Typically a decayed cabbage or garlic odour.

Generally caused under anaerobic conditions within the sewage transport system.

0 to 100 ppb in sewage gas.

Ammonia.

Trace nitrogen compounds e.g. indole, skatole.

Other amines e.g. methyl amine, ethyl amine, n-butyl amine.

Various aldehydes e.g. formaldehyde, acetaldehyde, butyraldehyde.

Found in untreated sewage. Ammonia has a pungent odour and is mainly formed by the breakdown of the urea in urine. Ammonia is released to the gas phase at low concentrations at normal sewage pH levels. It has a relatively high odour threshold (around 100 times higher than for H2S). Not normally a major odour source.

Usually present in very small amounts. These have a typical “faecal” odour. Generally formed under anaerobic conditions within the sewerage system.

Have a “fishy” or ammonia type smell. Generally formed under anaerobic conditions within the sewerage system.

0 to 50 ppm in sewage gas.

0 to 1 ppb in liquid sewage.

0 to 50 ppb in sewage gas.

May have an acrid or fruity odour. Generally formed under anaerobic conditions within the sewerage system.

0 to 100 ppb in sewage gas.

Various ketones e.g. acetone, butanone.

May have a fruity odour. Generally formed under anaerobic conditions within the sewerage system.

Trace.

Other Volatile Organic Compounds (VOC’s).

Will exhibit a variety of odours.

Trace.

9

Chapter 2 – Odour And Odour Chemistry

Odour Versus Hydrogen Sulfide One of the early decisions faced by anyone with sewage odour problems is to decide whether the problem is caused by the usually predominant odorant hydrogen sulfide (H2S), or whether other compounds need to be considered.

If it is known that other specific compounds are relevant, it may be useful to consider them individually along with H2S. The presence of particular types of trade wastes in the sewage may give some clues as to the presence of other compounds. For example, if effluent from a pulp and paper mill was present, then it is very likely that elevated levels of mercaptans and disulfides might be contributing to the odour in addition to H2S. In this situation, measurements of these compounds should be made by specialised laboratory personnel. Where there is evidence that H2S is not the main odorant, and it is difficult to identify what the other compounds might be, then a general measure of “odour” may be used.

General odour measurements are based on “odour units” or “OU” and are determined by a process known as dynamic olfactometry. In dynamic olfactometry an “odour panel” is used to assess the odour of a gas mixture. An “odour panel” consists of a group of people with a “normal” sense of smell, who are subjected to a sample of the odorous gas which has been progressively diluted to the point where 50% of the panel members cannot smell it any more (known as the “dilution to threshold” method). This is given the value of one OU. If for example the original sample needed to be diluted 500 to one to get to the odour threshold, the original sample would have a concentration of 500 OU. Australian Standard AS/NZI 4323.3 (The Determination Of Odour Concentration By Dynamic Olfactometry) provides specific guidance on how to set up odour panels. The setting up of panels and interpretation of the results is complex and requires a good understanding of the shortcomings and limitations of the procedure.

This testing is best carried out by laboratories certified by NATA (National Association Of Testing Authorities, Australia) for this procedure.

Safety And Sewer Gases All STS operators must pay serious attention to the fact that any sewer gas may be hazardous. It may contain: • methane which is explosive at certain concentrations • volatile chemicals such as petrol fumes which can also be explosive • insufficient oxygen to support human life • biological hazards such as bacteria and viruses in water droplets in the gas • toxic gases such as hydrogen sulfide.

Therefore, no person should ever enter any confined space within an STS without first taking appropriate precautions to account for these hazards. Australian Standard AS2865-2009 (Confined Spaces) defines the minimum requirements. In addition, individual water utilities may have their own specific standard operating procedures and requirements.

The main toxic gas of concern in STS’s is usually H2S. Normal domestic sewage has the potential to generate high levels of H2S under the right conditions, and the presence of certain trade wastes may make the situation even worse.

Hydrogen sulfide is puzzling though. At low concentrations, humans can smell it easily, and the odour threshold varies considerably between individuals. The Victorian EPA defines the threshold as 0.0001 ppm (0.1 ppb). At higher concentrations, humans rapidly lose the ability to even smell it. Operators must therefore not rely on their sense of smell to protect them.

10

Chapter 2 – Odour And Odour Chemistry As a guide, concentrations of H2S: • above around 10 ppm may pose a risk to human health • at around 50 ppm individuals may completely lose their ability to smell H2S • at around 100 ppm, headache, nausea, conjunctivitis and respiratory tract irritation and even death may occur.

In Australia, there are standards for workers exposed to toxic gases such as H2S (NOHSC: 1003 (1995)).

These standards require careful reading and interpretation and it is advisable to obtain assistance for this from qualified occupational health and safety personnel.

The H2S exposure standard consists of two parts; the Short Term Exposure Limit (STEL), and the TimeWeighted Average (TWA).

The STEL means a 15 minute time weighted exposure which should not be exceeded at any time during a working day, even if the eight-hour time weighted average is within the TWA exposure standard. Exposures at the STEL should not be longer than 15 minutes and should not be repeated more than four times per day.

There should be at least 60 minutes between successive exposures at the STEL. The STEL for H2S is 15 ppm.

Time-Weighted Average (TWA) means the average concentration of a particular gas when calculated over a normal eight-hour working day, for a five-day working week. The TWA for H2S is 10 ppm.

There are three important conclusions arising from these numbers.

1. As a practical guide, no-one should ever work in an environment where more than 10ppm of H2S exists, even for short periods of a few seconds.

2. H2S levels may not be high just because it can be smelled, however it must always be assumed to be high unless testing confirms otherwise.

3. Testing of H2S must be carried out to eliminate uncertainty. Methods of testing are described in Chapter 3.

Gas Concentrations And Conversion Of Units The concentration of H2S and other gases can be expressed in two ways: • •

volume / volume (parts per million e.g. litres of H2S per million litres of air), or mass per volume (mg/m3; e.g. milligrams of H2S per cubic metre of air).

It is often necessary to be able to convert between the two.

A useful formula for this is: mg/m3 of any gas = (ppmv x MW x P) (62.3626(T+273.15)) where

ppmv

= volumetric parts per million of the odorant

MW

= molecular weight of the odorant gas (see Appendix 1)

P

= pressure of the gas (mm mercury)

T

= temperature of the gas (0C)

11

Chapter 2 – Odour And Odour Chemistry An example of a conversion is provided below.

Example – To convert 10 ppm of hydrogen sulfide to mg/m3 ppmv = 10 MW = 34 P = 760 mm mercury (atmospheric pressure) T = 150C mg/m3 H2S = (ppmv x MW x P) / (62.3626(T+273.15)) = 10 x 34 x 760 / 62.3626(15+273.15) = 14.4

The Generation Of Hydrogen Sulfide In STS’s As sewage passes along an STS, it may change from a fresh aerobic condition to an aged anaerobic condition.

If a sample of fresh sewage is left to sit for a few hours, the bacteria present will consume the oxygen producing anoxic conditions (no oxygen but possibly some nitrate or nitrite). If the oxygen is not replenished, “anaerobic” conditions will be created as a result of the removal of nitrate and nitrite. Under these conditions, anaerobic bacteria will become more common. Some of these, the so called sulfate reducing bacteria, have the ability to convert sulfate into sulfide if sufficient carbon based “food” is present. In a sewer, there is generally plenty of this “food” as measured by BOD or COD. Once the sewage has aged and has become anaerobic, there is a potential to generate large amounts of sulfides and this is the main mechanism by which odours are formed in STS’s.

The typical characteristics of fresh, anoxic and anaerobic sewage are summarised in Table 2.2.

Table 2.2.

Typical characteristics for sewage of different ages.

Parameter Dissolved (soluble) sulfide (mg/L). pH. Temperature (0C).

Dissolved oxygen (mg/L).

Oxidation reduction potential – ORP (mV).

Sulfate – (mg/L).

Biochemical oxygen demand – BOD (mg/L).

Chemical oxygen demand – COD (mg/L).

Ammonia (mg/L).

Sulfate reducing bacteria numbers.

Butanone.

12

Aerobic/ “Fresh” sewage 0 6.5 to 9 5 to 35 0 to 5 +50 to +200 20 to 100 100 to 400 250 to 1000 10 to 50 very low 2.8

Anoxic sewage 0 6.5 to 9 5 to 35 0 +50 to -75 20 to 100 100 to 400 250 to 1000 10 to 50 increasing

Anaerobic/ septic sewage 0.1 to 40 6.5 to 9 5 to 35 0 -75 to -350 < Anoxic level < Anoxic level < Anoxic level 10 to 50 very high

Chapter 2 – Odour And Odour Chemistry

How Much H2S Will Be Released From Sewage As A Gas? Laboratory testing of aged sewage would show that sulfide is present as either: • •

insoluble sulfide, such as metallic sulfides e.g. ferric sulfide which would not contribute to odours, or soluble sulfide, which depending on the form it is in, may or may not contribute to odours.

If sewage contains soluble sulfide, it will be present as either dissolved molecular H2S that could be released, or as HS- or S2- which will remain in solution.

Whether or not H2S gas portion will be released from the liquid sewage depends on the following factors: • • • • •

the pH of the sewage the degree of turbulence present the level of ventilation in the STS the temperature of the sewage the level of dissolved salts.

The most important factors are generally pH, temperature and turbulence.

Figure 2.1 illustrates the large effect of pH on the maximum amount of soluble sulfide as H2S that might be released to the gas phase above the liquid.

Figure 2.1. The effect of pH on H2S levels in sewage before release to the gas phase above.

For example, if the pH of the sewage is 6, at 250C, the fraction of soluble sulfide which will be available for release as H2S is 0.9 (or 90%). Whereas if the pH is 8, the fraction is around 0.09 or only 9% of it is available for release. Therefore at pH 6, there is the potential for a lot more H2S to be released into the gases above the sewage than at higher pH levels.

13

Chapter 2 – Odour And Odour Chemistry Figure 2.1 also shows that for constant pH, at higher temperatures, a lower proportion of the soluble sulfide exists in the liquid sewage as H2S. However, higher temperatures will also encourage the H2S to leave the liquid sewage more readily and the overall effect of higher temperatures is to cause higher H2S levels in the gas above the sewage as shown in Table 2.4.

Turbulence is also very important. In general, the greater the turbulence of the sewage flow, the more H2S will be released as a gas. This is a bit like shaking up a bottle of lemonade. The carbon dioxide in the lemonade comes rapidly out of solution as bubbles if the bottle is shaken vigorously.

The final concentration of gaseous H2S will also depend on: • •

whether the gas is contained in a confined space such as a wet well or manhole whether it has enough time in the confined space to build up to its theoretical maximum or “equilibrium” level.

Estimating Worst Case Gas Levels To establish how bad the odour problem might be and the health and safety risk, the approximate worst case concentration of H2S or other odorant which may end up in the gas above the sewage can be determined.

Two methods are provided. The first is a general method for most odorants in sewage at 250C. The second is for H2S in sewage at different temperatures.

1.

General Method For Odorants In Sewage At 250C.

The following formula may be used for estimating the worst case odorant concentration: P = H x X x 18 MW where P = X =

H =

MW = Table 2.3.

the concentration of the odorant in the gas (in ppm) the amount of odorant in the sewage available for release, corrected for pH (see Figure 2.1) (in mg/L) a constant for each odorant (Henry’s Law constant or “H”), see in Table 2.3 (in atmospheres per mole fraction) Note that H values may be different at different temperatures molecular weight of the odorant gas (see Appendix 2)

Henry’s Law constants (H) for some odorous compounds at 250C. Compound Hydrogen sulfide.

Methyl mercaptan.

Ammonia.

Ethyl mercaptan.

Dimethyl sulfide.

Dimethyl disulfide.

Methylamine.

Dimethylamine.

Acetaldehyde.

Acetone.

Butanone.

14

H (atmospheres / mole fraction) 563 200 0.843 200 110 63 0.55 1.3 5.88 1.9 2.8

Chapter 2 – Odour And Odour Chemistry Example – Calculate the worst case gas concentration for H2S in sewage at 250C. If we have sewage at 250C, at a pH of 8.0, and total soluble sulfide of 10mg/L: Step 1. From Figure 2.1, determine what fraction of sulfide is available for release as a gas at a pH of 8.0 = approximately 0.09 Step 2. Calculate X the concentration of H2S available for release = 10 x 0.09 = 0.9mg/L Step 3. Obtain the molecular weight MW for H2S (from Appendix 2) = 34 Step 4. Obtain H for H2S (from Table 2.3) = 563 Step 5. Calculate the worst case H2S concentration P = H x X x 18

MW

= 563 x 0.9 x 18 / 34 = 268 ppm

2.

Method For Hydrogen Sulfide At Different Temperatures

The H2S in gas above sewage at different temperatures can also be calculated. Table 2.4 provides the worst case concentration of H2S above sewage containing 1mg/L of soluble sulfide at different temperatures. The effect of pH must also be taken into account as demonstrated previously using Figure 2.1. First the amount of H2S available for release is calculated and then the data from Table 2.4 applied.

Table 2.4.

H2S concentrations above sewage at different temperatures (USEPA 1974).

Temperature 0 C

Worst case concentration of H2S above sewage containing 1 mg/L of soluble sulfide. (ppm)

Temperature 0 C

Worst case concentration of H2S above sewage containing 1 mg/L of soluble sulfide. (ppm)

10 11 12 13 14 15 16 17 18 19 20

208 214 221 227 234 241 248 255 262 269 276

21 22 23 24 25 26 27 28 29 30 35

284 291 299 307 315 323 331 340 348 356 401

Example – Calculate the worst case gas concentration for H2S at 10OC. If we have sewage at 100C, at a pH of 7.0, and total soluble sulfide of 10mg/L: Step 1. From Figure 2.1, determine the fraction of sulfide available for release as a gas at a pH of 7.0 = approximately 0.62.

Step 2. Calculate X the concentration of H2S available for release = 10 x 0.62 = 6.2mg/L. Step 3. From Table 2.4, obtain the worst case concentration of H2S possible at 100C per mg/L of soluble sulfide = 208ppm Step 4. Calculate the worst case H2S concentration = 208 x 6.2 = 1290 ppm.

15

Chapter 2 – Odour And Odour Chemistry

Estimation Of Sulfide Generation In Pipelines There are a number of sophisticated models available that can predict the increase in sulfide levels as sewage travels down a pipeline. The models predict sulfide levels based on the characteristics of the particular STS and the quality of the sewage present. The latest models can be used to predict concentrations of sulfides, and how they might change with time. They can also predict how successful various chemical treatments may be. These models should be used if possible to properly identify the effects of daily flow variations as well as variations in the nature of the sewage. Access to these models is usually through the private consulting firms, however there are some very practical initial estimation methods that can be used quite effectively where the sophisticated models mentioned are not available.

The main quality factors which affect the rate of sulfide production in sewage pipelines are the amount of available food for the bacteria to use, as measured by BOD or COD, and temperature. Temperature affects the rate of biological sulfide production significantly. Between 100C and 300C, biological activity roughly doubles for every 100C rise.

For continuously flowing full pipes, most of the sulfide is generated by the bacterial wall slimes. The rate of sulfide production R can be estimated by first calculating G, the rate of production of sulfide from the wall slimes (in grams per square metre per hour). This ignores the small amount of sulfide produced within the sewage itself. where

G = 0.0013 x BOD5 x 1.07(T-20) BOD5 = the BOD measured at the start of the pipe in mg/L T = sewage temperature in 0C. Note that 1.07(T-20) can be calculated using an Excel spreadsheet command which for example would be “+power(1.07,3)” if the temperature was 230C.

Then assuming the sulfide concentration is zero at the start, the predicted concentration C (mg/L) of sulfide at the end of the pipe section can be calculated: C = where R = A =

Q =

R/Q G x inside surface area of the pipe wall = G x A inside area of the pipe wall = π D x L, where D is the inside diameter, and L is the pipe length in metres the sewage flow rate in cubic metres per hour

Example – Estimate the sulfide produced in a continuously flowing full pipeline.

We have a 1000 m long (L) section of pipe with internal diameter D of 0.225 m, flow rate Q of 80 m3/hr, sewage temperature of 230C, and BOD5 of 250 mg/L at the start.

Step 1. Calculate T-20 = 23-20 = 3 Step 2. Calculate G = 0.0013 x 250 x 1.073 = 0.0013 x 250 x 1.07 x 1.07 x 1.07 = 0.398 g/m2/hr Step 3. Calculate A = π x 0.225 x 1000 = 706.85 m2 Step 4. Calculate R = 0.398 x 706.85 = 281.3 grams per hour Step 5. Calculate C = R/Q = 281.3 / 80 = 3.5 mg/L of sulfide at the end of the 1000m length of pipe.

Before accepting the number calculated, care should be taken to check that sufficient sulfate is available for the predicted sulfide level to occur. This is determined by assuming that the maximum possible level of sulfide cannot exceed approximately one third of the available sulfate level.

16

Chapter 2 – Odour And Odour Chemistry So for example, if there is 30 milligrams per litre of sulfate present at the start of the pipeline, only 10 mg/L of sulfide can be generated. Sulfate can be measured by a laboratory, or using a field sulfate kit.

MMBW (1989) describes simple methods for estimating pipeline sulfide levels in other situations such as intermittently flowing rising mains, and gravity sewers.

Trade Wastes The nature and level of sewage odours can be significantly affected by trade waste inputs to the system.

1.

Trade Waste Quality

Odours can be due to direct input of odorous compounds or to odours formed from biochemical reactions between the sewage and the trade waste within the STS. Table 2.5 lists some common odorous compounds found in different trade wastes.

Table 2.5.

Some examples of odorous compounds found in trade wastes.

Industry

Possible odorous compounds

Pulp and paper.

H2S, mercaptans, organic sulfides.

Abattoir, rendering works, meat processing.

A range of compounds including fatty acids, mercaptans, sulfides, esters, aldehydes, alcohols, ammonia, and amines.

Dairy/milk processing.

Low levels of sulfur compounds from the amino acids present in proteins.

Specifically, the amino acid cysteine is converted to hydrogen sulfide and methionine becomes methyl mercaptan.

Tanning.

Sulfides, tanning chemicals, and solvents.

Egg processing.

H2S.

Petrochemical plants /service stations.

Hydrocarbons, oils, hydrogen sulfide and other sulfides.

Food.

Fat / grease.

Photographic.

Ammonia.

Comment

Eggs contain moderate levels of sulfur.

Additional safety concerns exist for combustible/ explosive hydrocarbons.

Not many discharges left today.

Odour levels in an STS can be affected by other trade waste parameters such as temperature and suspended solids. Table 2.6 summarises the impact of these parameters on odours.

17

Chapter 2 – Odour And Odour Chemistry Table 2.6.

Some examples of some trade waste parameters that can affect STS odour levels.

Industry

Compound/parameter

Comment

Food processing.

Elevated temperature, sulfate, BOD (especially BOD in readily available form for bacterial use).

May cause additional H2S emissions.

Any.

Excessive settleable solids.

May cause additional H2S emissions.

Any.

Elevated temperature.

May cause additional H2S emissions.

Any.

Elevated sulfate.

May cause additional H2S emissions.

Any.

Elevated BOD (especially BOD in readily available form for bacterial use).

May cause additional H2S emissions.

Any.

Low pH.

May cause additional H2S emissions.

Any.

Very high pH.

May increase ammonia emissions.

Elevated sulfate.

May cause additional H2S emissions.

Elevated sulfate.

May cause additional H2S emissions.

Sulfate, thiosulfate, sulfite.

May cause additional H2S emissions.

Any where seawater may be present.

Any where ground water is used.

Any which contain oxidised sulfur species.

2.

Trade Waste Flow Management

Substantial odour reduction in an STS may be possible if trade waste flows can be managed. a) The Trade Waste Agreement Acceptance of trade wastes into an STS is usually only allowed through a site specific licence or agreement which spells out specific quality, quantity and other discharge conditions. b) Testing Trade waste flows should be tested for quality conformance prior to allowing discharge.

1) For smaller intermittent flows, this can be achieved by storing the effluent, and the producer carrying out quality tests on the “batch” prior to it being discharged.

2) For continuous flows, and particularly if the flows are large, on-line instrumentation in conjunction with a “computerised controller” should be considered. It has been common practice to measure temperature, pH, electrical conductivity and dissolved oxygen levels in the past. Parameters such as soluble sulfide, COD/BOD and other odour related parameters can now be measured using on-line spectrophotometers.

Flows can be automatically shut off by the controller if they are outside agreement limits. c) Flow Storage Storage of trade waste flows of variable quality can be undertaken to average out concentrations. Storage times should be checked to ensure that sulfide generation does not result in increased sulfide levels due to the storage duration.

18

Chapter 2 – Odour And Odour Chemistry d) Timing of Discharges.

For industries that operate on a fixed day shift or part of day basis, it may be possible to require that discharges only occur at certain times such as: • • •

during the day time; in many places, night time wind speeds and atmospheric conditions can work against the proper dispersion of odorous gas discharges during morning and afternoon peak flow periods in the STS; dilution of the trade waste flow may assist in lowering odour generation in the STS between 7am and 11pm; outside these hours, retention time in the STS is likely to be long and to result in the flow sitting in the STS overnight allowing increased generation of sulfide and odour.

These suggestions may not be appropriate to every trade waste discharge and they should be assessed on a case by case basis.

Hydrogen Sulfide And Corrosion Whilst the focus of this guide is principally on odour control and how to resolve odour complaints, some information is also provided on H2S based corrosion of STS components. If an odour problem exists in an STS, then corrosion is likely to be occurring at the same time somewhere within the system. Sulfide based corrosion can take two forms.

• •

Corrosion of copper pipes and electrical switchboard components due to direct reaction of H2S gas with the copper to form black copper sulfides, or Corrosion of concrete and metallic assets by the sulfuric acid that can be formed from H2S. This is the type responsible for most of the failures seen in concrete or cement based pipelines and manholes.

The mechanism for corrosion of pipelines and manholes is summarised in Figure 2.2.

Figure 2.2. The H2S corrosion process.

Hydrogen sulfide is generated anaerobically within the sewage and submerged wall slimes.

Some of the hydrogen sulfide enters the gas space above the sewage and is absorbed into moisture on the pipe wall.

On moist nonsubmerged surfaces, bacteria (Thiobacillus) in the presence of oxygen convert the sulfide into sulfuric acid. The acid then attacks the concrete.

19

Chapter 3 Measuring Odorous Compounds, Odour And Gas Flows To solve odour problems, it is important to know something about the extent of the problem and how it might vary over time. Such measurements provide data which can be used to help explain the situation and also convince the managers and owners of the STS that the problem is real and requires action.

Chapter 2 described some methods for estimating levels of H2S and other odorants however, it is better if actual measurements of parameters can be made. In particular, the following measurements should be made: • levels of known odorants in the liquid phase of the sewage • pH, ORP and temperature of the liquid phase • levels of known odorants in the air around a particular STS site, and in the gas phase above the sewage within the STS • flow rates of odorous gas being discharged from the STS.

Liquid Phase Measurements Including Safety Aspects Safety Aspects Prior to taking liquid samples from an STS, the safety of the sampler should be considered.

Sampling equipment should be used that allows samples to be taken from outside any confined space such as a manhole (see Figure 3.1), or wet well and prevents the sampler from falling into the STS. Gas safety procedures described in Chapter 2 and as required by the individual water utility should be followed.

Appropriate personal protective equipment should be worn. As a minimum, this should include safety glasses, PVC gloves and gas mist breathing canisters, if sewage vapours are present.

Figure 3.1.

20

Example of equipment used for sewage sampling from a manhole.

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows Sampling Reliable measurements start with careful sampling. Whether taking a sample of raw sewage for onsite measurements or for laboratory measurements of hydrogen sulfide (H2S), it is really important to sample the sewage carefully. Take the sample gently and avoid mixing or agitating it in any way. Any agitation will encourage the H2S to leave the liquid and give a false low value. The sample should not be allowed to stand for any appreciable time since this will allow any oxygen present in the sample or from the atmosphere to reduce the levels of H2S.

On Site Liquid Phase Oxidation Reduction Potential (ORP) Measurements One of the easiest and most useful initial measurements is the ORP of the sewage or trade waste. Table 2.2 provides data that allows an operator to decide very quickly whether the sewage is in a state which could cause odours. Where investigation is being undertaken as a result of a complaint, care must be taken to sample the sewage at the a similar time as the odour complaint has occurred. The ORP probe is simply immersed into a sample of sewage taken from the STS and the result is read after waiting for a steady reading.

As with pH meters, regular checks of the ORP meter accuracy should be made (before each use) using standard solutions provided by the supplier. Probes should be kept clean and stored according to the supplier’s recommendations.

On Site Liquid Phase Soluble Sulfide Measurements Good information can be obtained about H2S levels and the potential for odour using on site testing. While the results may not be as accurate as laboratory based measurements, the advantage is that the results are available immediately. The following steps should be carried out quickly.

The first step is to filter out any insoluble sulfides and any other solids. Filtering raw sewage is not easy due to the very high solids content. In the first instance, try filtering the sewage using a hand held portable filter device (Figure 3.2) with 0.45 or 0.2 micron filter paper. The filter may need to have a coarse glass fibre pre-filter (available from laboratory suppliers) placed on top of the finer filter paper to improve the filtration rate. Use the hand pump slowly, just enough to get flow through the filter paper and nothing more.

Figure 3.2. Portable filtering equipment.

21

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows The filtered liquid should not have any bubbles in it. If the filter clogs too quickly, the sample may need to be dosed quickly with alum and the sample settled prior to filtration. As a guide, add 1 mL of commercial alum solution (available from most water treatment plants) to around 1L of sample. Speed and gentle handling is required here to ensure that excessive H2S is not lost from the sample.

Once a clear sample has been obtained, the soluble sulfide level can be measured using a colorimetric sulfide kit, or a portable spectrophotometer (Figure 3.3). These are widely available from companies such as Merck or Hach. Gastech also sells a glass “indicator tube” which can be placed directly in the sample to provide an approximate reading.

Figure 3.3.

Portable spectrophotometer being used for sewage soluble sulfide measurements.

Laboratory Based, Liquid Phase Soluble Sulfide Measurements For laboratory measurement, it is better if the insoluble sulfides and other solids can be separated from the soluble sulfide using the filtration procedure as described above, prior to preserving the sample and subsequent laboratory analysis.

The sample can then be preserved with zinc acetate. This is usually already added to the sample bottle provided by the laboratory. If for some reason the sample has not been filtered, the result may need to be quoted as “acid soluble total sulfide” rather than “soluble sulfide”.

Researchers at the University Of Queensland have recently developed a new method of sulfide analysis which is likely to be suitable where very accurate analysis of sulfides is required.

Continuous Liquid Phase Measurements There is no doubt that continuous real time measurements of sewage sulfide levels, are very useful in helping solve odour problems in STS’s.

The variation in conditions within the STS over a 24 hour period can be quite large, and indeed within each hour of the day. This means that a single liquid phase sample is very likely to give a poor picture of what is happening with respect to odours over a period of several hours or more.

22

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows There are two main proven systems available for obtaining continuous soluble sulfide results.

1. In-situ Spectrophotometric Systems Such As The S::CAN The S::CAN system will provide accurate continuous readings of the HS- ion, and if used in conjunction with a pH meter, will measure total soluble sulfide. The S::CAN spectrophotometer probe can be placed directly into the sewage in open channels at sewage treatment plant inlet works. For sampling from pipelines, a solenoid based sampling system has been developed to take samples out to the probe located outside the pipe. The instrument measures the absorbance of light at different wavelengths, as the sewage passes through a slit in the probe. Soluble sulfide concentrations are then calculated by the S::CAN using the absorbance and pH readings. The spectrophotometer sample slit is regularly cleaned by pulses of compressed air.

Figure 3.4 shows a stainless steel S::CAN probe (which holds the slit unit) installed at the inlet works to a sewage treatment plant, for continuous measurement of inlet soluble sulfide levels. The unit has been lifted out above the sewage for inspection.

Figure 3.4.

S::CAN unit installed at the inlet to a sewage treatment plant. The location of the narrow sampling slit is shown with an arrow.

➞ 2. Purgeable H2S Systems Purgeable H2S Systems (PHS) have been on the market for many years. They provide a measurement of “worst case H2S” levels by continuously passing a stream of air at a known flow rate through a purge column which has sewage passing through it. The air passing through the sewage effectively strips the H2S out of the liquid phase into the moving gas phase. The air containing the stripped H2S is then passed through an industrial gas conditioning system to remove moisture. The dried sample is then passed through a gas monitoring system and a result displayed. The PHS reading can be regarded as a “worst case H2S odour potential”. The systems take into account the effect of pH on the potential for H2S to enter the gas phase.

23

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows

Gas Phase Measurements Including Safety Aspects Safety Aspects As with liquid sampling, equipment should be used which negates the need to enter confined spaces to collect gas samples. Gas safety procedures described in Chapter 2 should be followed in all cases, and appropriate personal protective equipment, including safety glasses, PVC gloves and gas mist breathing canisters, if sewage vapours are present, should be used.

Laboratory Based Measurement Of Odour And Odour Related Compounds The general odour level of gases containing a single odorant or a mixture of odorants, can be measured using the procedure described in Chapter 2 (Australian Standard AS/NZI 4323.3).

Numerous individual odorous compounds usually contribute to the general odour of sewage (see Table 2.1). These compounds can each be measured using a variety of techniques such as adsorption tubes, gas chromatography/mass spectroscopy or other laboratory methods. Gas phase measurements are best carried out by NATA certified laboratories.

On Site Gas Phase Measurements Surrogate Odour Measurements There are a few instruments available for onsite measurement of “odour” and/or odour related parameters such as the Nasel Ranger® system and electronic or “E-Nose” systems.

The Nasal Ranger® field olfactometer, is a portable odour detecting and measuring device which determines odour units or “Dilution-to-Threshold” values (see Chapter 2) for air surrounding an STS. It is a hand held device which draws in odorous air, and dilutes it with various amounts of carbon filtered air (which has had the odorous compounds removed) prior to the user deciding if odour can still be detected.

The Nasal Ranger® can be used relatively easily on STS’s to determine the following: • • • •

to verify an odour complaint to regularly monitor site odours to compare the effects of different operating practices on odour levels to verify predictions from odour modelling.

Electronic nose devices use an array of sensors that are calibrated to respond to particular odorants or a mixture of odorants. They can be used to obtain continuous readings. Electronic nose devices are still mainly used by researchers and require well trained operators. It is likely that after more experience has been gained with them, simpler and more user friendly units will become available for use in the field.

Gas Phase Hydrogen Sulfide Measurements Gaseous H2S can be measured using electronic instruments (Figure 3.5, and on the cover photograph) that contain small electrochemical cells or gold film based sensors. These instruments provide accurate H2S readings if: • • •

24

they are regularly and correctly calibrated (before and after each use) they are not exposed to high H2S levels for too long (eg: operating them at levels of 200 ppm for more than a few days or as prescribed by the manufacturer) they are not exposed to excessive humidity or temperature or the presence of other interfering compounds (refer to the individual manufacturers instructions for more details).

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows Paper tape analysers such as the Honeywell SPM MDA H2S analyser uses an optical sensor to determine the degree of darkening of lead acetate impregnated paper when exposed to the gas stream. These analysers are very accurate down to ppb levels and are generally employed at the discharge from a gas treatment facility. They also only have limited ranges available such as 0 to 90 ppb, 50 to 1500 ppb and 1.1 to 30 ppm.

Modern instruments usually contain a data logger for providing historical data. Units such as the OdaLog® (ThermoFisher) also allow remote data interrogation and control using a laptop computer. Note that for the logged data to be useful, the date and time must be correctly set on the logger before commencement of logging.

Before placing continuous monitoring instruments on site, operators should take into consideration the likely water levels within an STS and whether flooding as a result of rainfall events might inundate any of the equipment being used. To avoid this, locate equipment above the high water mark, or perhaps mount it on floating pontoons if this is feasible.

Figure 3.5. Examples of continuous high and low range H2S monitors.

In some cases, such as inside wet wells where moisture levels in the gas are high enough to condense out, or if excessive particulate matter exists, gases may need to be filtered and dried using commercially available equipment, prior to measurement with the gas analyser.

There are also non-instrumental systems commercially available which can give single sample readings.

These include wet chemical absorption methods and gas detector tubes (Figure 3.6) Figure 3.6.

Examples of H2S gas detector tubes and sampling pump.

25

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows Table 3.1 provides a summary of the most common situations where H2S measurements are required for STS odour control purposes and some equipment available to measure H2S. Table 3.1.

Common methods for onsite H2S measurement.

Measurement situation

Ambient air (e.g. inside or downwind from a sewage pump station).

Operator personal protection measurements.

STS confined space (e.g. inside a manhole, wet well, or tank).

Inlet to a gas treatment system (e.g. biofilter, carbon filter).

Outlet from a gas treatment system (e.g. biofilter, carbon filter).

Typical range requirement (ppm H2S)

Commonly used monitors or system.

0 to 2

Jerome 631-X or 651(Arizona Instruments).

OdaLog® Low Range (ThermoFisher).

MDA Chemcassette (Honeywell).

Dräger /Gastech tubes.

Dräger Chemlogic.

0 to 50ppm (preferred)

Refer to suppliers of personal gas monitors eg: ThermoFisher, Jaybro, Wormald. A combined personal H2S, explosive gas, carbon monoxide, and oxygen level monitor should be considered.

0 to 200

OdaLog® High Range (ThermoFisher).

Polytron Remote Head (Dräger).

Jerome 860 (Arizona Instruments).

Dräger /Gastech tubes.

0 to 200

OdaLog® High Range (ThermoFisher).

Polytron Remote Head (Dräger).

Jerome 860 (Arizona Instruments).

Dräger /Gastech tubes.

0 to 2

Jerome 631-X (Arizona Instruments).

OdaLog® Low Range (ThermoFisher).

Dräger /Gastech tubes.

MDA Chemcassette (Honeywell).

Gas Flow Rate Testing And Estimation Of Downwind Odour Levels The measurement of gas flow rates from vents in STS’s is very important since it is used in conjunction with the actual gas concentration measurements to determine the emission rates of odorants. The emission rate can then be used in standard air pollution models to determine downwind levels of odour or specific gases.

Whilst the modelling exercise is best carried out by experienced practitioners, the collection of the gas flow rate data can be carried out by operators of the STS. Hand held hot wire or vane anemometers can be placed into ducts or vents or preferably at the discharge of the vent for this purpose. These devices are simple to use and provide a fairly good indication of the flow rate.

However, with regards to hot wire anemometers, readings at various points within the duct may be required to provide accurate readings (see AS4323.1). Alternatively, pitot tubes together with a hand held manometer can also be used with readings also taken at multiple locations within the duct. Pitot tubes provide the most accurate means of flow measurement compared to hot wire and vane anemometers although their use is more complicated.

26

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows When using these instruments, the instructions of the manufacturer should be followed closely. Figure 3.7 shows a vane anemometer and temporary ducting being used to take measurements of gas flows from an STS wet well.

Note that fan ratings cannot usually be used to obtain accurate gas flow rates due to pressure drop variability throughout the extraction and treatment system.

➞

Figure 3.7. Measuring gas flows with a vane anemometer (red arrow).

Smoke / Leak Testing Smoke generating equipment (Figure 3.8) can be very useful to help find where odorous gases are going once they have left the STS, and to find where gas leaks are occurring, for example from wet well and manhole covers. They can also help prove the interconnection paths of buried pipework when this information is not available via drawings or by inspection. This equipment can be hired or purchased.

Figure 3.8.

Smoke generating equipment (left) being used to check pipework connectivity and manhole cover seals (right).

27

Chapter 3 – Measuring Odorous Compounds, Odour And Gas Flows

Site Sampling And Testing Kit Checklist For Operators Operators who are responsible for the investigation of odour complaints from an STS should have the following basic set of sampling and testing equipment available: • •

log book to record complaint information safety equipment o PVC gloves and safety glasses o mist masks o manhole sampling cover or fall grating o orange witches hats to place as markers around the sampling site o alcohol wipes for personal hygiene o a combined personal H2S, explosive gas, carbon monoxide, and oxygen level monitor • sampling equipment o manhole and cover lifters o sturdy sample bucket and rope o sample bottles from an accredited laboratory for soluble sulfide and other odorants if required • testing equipment o pH, ORP and temperature meter for sewage testing o a recently calibrated, 0 to 200 ppm or 0 to 1000 ppm, H2S gas testing meter for placing into the STS o a hand held anemometer to measure gas flow rates from the STS o sample filtration equipment and either a liquid sulfide test kit or portable spectrophotometer set up for measuring sewage soluble sulfides o a recently calibrated, low range hydrogen sulfide monitor, or a Nasel Ranger® type device for estimating odour levels in the air around the STS o smoke testing equipment.

28

Chapter 4 Sewage Transport System Components And Odour Issues STS Components Water utilities and councils in each State specify the particular requirements for components of STS’s. The Sewerage Code of Australia (WSA 02-2002) is widely used with local specific variations and covers the issue of odours from particular STS components to some extent.

A simplified diagram of a typical conventional town STS is provided in Figure 4.1.

Figure 4.1.

Diagram of a simplified STS.

Manhole Boundary Trap

Gravity Reticulation Mains

Trade Wastes

Branch / Main Sewers

Vent

Gravity Main Rising Main

Wet Well Pump Station

Hydraulic Drop At Inlet To Pump Station

Treatment Plant Inlet Works

Wet Well

The different parts of the STS can be associated with odours as discussed below.

Household Boundary Traps Individual houses have vents fitted above roof level to expel gases from the house sewage pipework and downstream gravity reticulation system. At this point, sewage usually has a “fresh” odour and is adequately dispersed by the house vent.

Many, but not all, properties also have a boundary trap (Figure 4.2) at the property boundary. This acts as an inspection point on the sewerage system, and also stops sewer odours from the downstream sewer pipes reaching the property.

29

Chapter 4 – Sewage Transport System Components And Odour Issues Figure 4.2. Household boundary trap schematic.

Vent

From House

Ground Level

Property Boundary

A gas check manhole (refer to WSA02-2002) is sometimes fitted in the branch or main sewer downstream of the property. This also helps prevent odorous gases passing back into household sewer pipes.

Gravity Reticulation Collection Mains Once sewage leaves the household property boundary, it enters the STS. At this point, the system is referred to as a “gravity reticulation sewer” because the flows occur via gravity. The pipes are normally only part full and have some air present in them. For a short time, the air present helps to keep the sewage from becoming septic. Once the oxygen is depleted, or the fresh sewage mixes with older stale sewage, anaerobic conditions will lead to the generation of odours.

Smaller gravity sewers are normally fitted with inspection manholes. Vents are provided to allow air to enter the system and sewer gases to leave, as liquid levels rise and fall. This helps prevent pressure fluctuations.

Pressure fluctuations need to be avoided since they could cause pipeline failures, or dewatering of boundary traps and S-traps within households. Vents also help to disperse odours.

One problem with the transfer of sewage in gravity systems is the tendency for solids to settle out and the sewers to block. Sedimentation will cause odours to be generated within the settled solids. For these reasons gravity sewers should be designed with minimum grades to achieve a minimum flow velocity at all times. Sometimes this is not possible due to the topography or types of ground. Where there is insufficient grade possible, for example in flat coastal areas, periodic flushing or scouring of pipes may be required. It is interesting to note that with the onset of drought and water saving behaviour, sluggish flow in sewers and resulting odours have become more of a problem.

Pump Stations And Wet Wells A gravity sewer can only transport the sewage for the extent that the hydraulic grade will allow. Once there is insufficient fall in a gravity sewer or it is necessary to transport the sewage over a hill, the sewage must be lifted using a pump.

Typically, sewage from a particular area is collected in a wet well pump station at the end of a series of gravity sewers. The accumulated sewage is then periodically pumped out of the wet well. Depending on the topography, there may be many pump stations required to transport the sewage to its final destination at the Sewage Treatment Plant (STP).

30

Chapter 4 – Sewage Transport System Components And Odour Issues The wet well is an important feature of a pump station since it allows for variations in inflows. However the presence of a wet well means that at low flows, the sewage will be held in the wet well for longer. This increases the likelihood that H2S odours will be generated in the pump station. There may therefore, be a conflict in designing wet wells to be able to cope with peak flows in the first instance or preventing formation of H2S at low flows. Designers do not always have the option of optimising wet well sizes from an odour control point of view since the primary design criteria is for safe handling of peak flows.

One method to alleviate this problem in larger pump stations is to run the wet well at low, constant levels using multiple or variable speed pumps. This may help minimise retention times in the pump station and also minimise the rise and fall in sewage level which can cause odours due to the “piston effect”.

Under normal stop/start level control in a wet well, air enters the well to replace the water as the level is pumped down. As the wet well then fills, air is expelled and any odorous gases generated in the well will be pushed out. By minimising level changes in the pump station, the piston effect and the release of odorous gas can also be reduced.

When choosing the operating level for a fixed level wet well, care must be taken to prevent surcharging of the pump station inlet gravity pipework otherwise sedimentation and blockages might occur. Where the wet well level cannot be kept constant, other odour control methods may need to be used. Common solutions include installation of well ventilation systems, wet well cleaning systems, chemical dosing, or treatment of the ventilated gas stream.

Rising Mains The pipe leaving the pump station which carries sewage from the pump station to some elevated point downstream is called the rising main. Unlike gravity mains, rising mains run full. As a consequence wall slimes completely line the inside of the pipe resulting in increased potential to form sulfides and to generate odours. Rising mains are often associated with odour. Sewage can become septic quickly and sulfides can build up rapidly. This can occur to a much greater extent than in gravity mains due to the increased amount of biofilm and the absence of oxygen.

Odorous gases can be released at the end of the rising main through vents or air valves and the septic sewage can travel further downstream and cause subsequent odour problems.

For new rising mains, it may be possible to lower their odour generating capacities during normal dry weather flow. This can be achieved by reducing their diameters and retention times, and allowing for some wet weather storage at the pump station instead of installing a larger main initially.

New housing developments can sometimes be constructed slowly and there may also be advantages for odour control if twin rising mains are installed to allow for shorter retention times in the early days.

Manholes Manholes are provided in both gravity and rising mains to allow for inspection and other maintenance tasks.

Manholes at the high point of a rising main are connected to a vent stack or air valve, to allow any built up gas to be released. Odours may also be released at this point.

This is also a point where corrosion can be quite marked due to the release of H2S and subsequent formation of sulfuric acid.

31

Chapter 4 – Sewage Transport System Components And Odour Issues Hydraulic Drops Hydraulic drops are points where there is a change in water level. These may occur in an STS: • • • •

at the inlet to wet wells at pipe junctions, bends, T-pieces at manholes in open channel sections at the inlet to treatment plants.

Generally, the greater the turbulence in the flowing sewage, the greater the amount of odorous gas entering the gas phase. Therefore any drop in hydraulic level could result in the release of odorous gases. The bigger the hydraulic drop, the greater the turbulence and the greater risk of increased odour generation.

Sewage Treatment Plant Inlet Works Unless the STS is short, delivery of sewage to a Sewage Treatment Plant (STP) is likely to bring with it a number of odorous compounds including sulfides at elevated levels. In many cities, urban sprawl has seen the buffer areas around STP’s reduced significantly with housing developments now very close to the STP. Consequently odours from the inlet works can be a significant problem. It is now common practice to cover and ventilate the inlet works, with odorous gases treated prior to discharge to the atmosphere.

It is considered good practice to cover all the following inlet works areas including: • • • • • • •

wet wells coarse and fine bar screens grit chambers grit and screenings hoppers/bins open channels drop structures such as weirs pump stations.

Other areas which may also need to be covered include: • • • • • •

flow balancing tanks primary sedimentation tanks secondary treatment anoxic zones secondary treatment anaerobic zones biosolids/sludge treatment areas biosolids/sludge storage bins.

Vents Ventilation is an essential requirement of all STS systems. Vents are provided at a number of different points in the system including: • • • • • • •

32

at individual houses at roof level and at boundary traps along gravity mains on wet well covers on pump station buildings at the high points in rising mains in pipe sections which flow full in the covers over inlet works at sewage treatment plants.

Chapter 4 – Sewage Transport System Components And Odour Issues Vents serve a number of purposes: • • • • • • • •

to allow for pressure changes that may occur in the system to allow fresh air to enter pump station buildings for safety reasons during normal and maintenance operations to release gases built up inside pipelines so that hydraulic capacity is not reduced to allow fresh air to enter part full pipelines and pump station wet wells to assist with odour and corrosion control by keeping walls as dry as possible to allow fresh air to enter pump station wet wells as water levels drop to remove and disperse odorous gases to acceptable levels to allow air to enter part full pipelines to help keep the sewage fresh through the addition of oxygen to provide transport to gas treatment systems.

Some common types of vents include ground level vent pipes with mesh covers, vent stacks which let gas in or out above ground level, and natural draft vents which are not associated with a fan. Forced draft vents are associated with a fan which either extracts gas out of the STS, as for induced air systems (see Figure 4.4), or blows it into the system. Air valves located on pipelines are usually configured to let gas in and out but prevent sewage from escaping.

Ventilation Fans Ventilation systems, including fans, must be made from non corrosive materials. Fans used for moving sewage gas are made from fibreglass, polypropylene, PVC, and stainless steel to prevent corrosion. Care must be taken to ensure that metallic fans and equipment which may produce sparks are not used in situations where explosive gas mixtures are, or may be present.

To avoid noise complaints and protect operators from hearing damage, fan noise levels must not exceed local noise standards set by the EPA and health authorities. In the absence of specific local standards, fan noise should not exceed 70 decibels adjacent to the fan. Lower levels may be required to meet EPA requirements.

Noise consultants should be employed to assess each situation.

Figure 4.3. Example of a wet well induced air fan.

33

Chapter 4 – Sewage Transport System Components And Odour Issues

Principles Of Ventilation A well designed ventilation system can go a long way to minimising odour problems, however a poorly designed system may worsen an existing problem. Simple improvements to an old or poorly designed ventilation system can often provide immediate reduction in odour problems at localised sites. Proper ventilation of long sewage pipelines can however, be more difficult to achieve.

The Sewerage Code of Australia (WSA - 2002) provides guidance on the design of ventilation systems for parts of an STS. MMBW (1989) also covers design of both natural and forced ventilation systems for major sewers.

The following general principles and practices apply to venting of STS’s.

Septicity And Venting If septic sewage exists, low level, natural draft ventilation systems will probably not be adequate for preventing odour problems and a fan based system will be required.

Ventilation Air Flow Rates Air flow rate requirements will vary according to the reason(s) that the air is being added. Determination of the appropriate amount of air to add in each situation is not easy and requires both a good understanding of the processes involved, as well as a degree of personal judgement.

•

For corrosion control purposes, to keep wall surfaces as dry as possible, air must be added in sufficient quantities to keep the relative humidity within the STS below 85% (MMBW 1989). This becomes difficult if the STS is located in a warm, humid place where the outside air itself is too “wet”.

•

For safety reasons, sufficient air must be added to prevent the occurrence of explosive gas mixtures if elevated levels of methane, hydrocarbons, or other explosive gases are present. Note that addition of a ventilation system should not be relied on to guarantee that explosions will not occur (MMBW 1989). Also, simply adding air to dilute unsafe levels of H2S (or other toxic gases) to allow access by operators is not appropriate and full confined space entry procedures are still required.

•

For odour control reasons, dilution air can be added to lower the concentration of H2S and other odorous gases. This may be required in some cases to allow the use of particular types of gas treatment systems such as biofilters where for example, H2S is best kept below 20ppm.

Whilst each situation needs to be assessed individually, it is common practice to add large volumes of ventilation air to a wet well, or STP covered inlet works area. This should be at rates which meet 12 to 15 air changes per hour, for example if a wet well volume is 75 m3, the ventilation rate might be 12 times this or 900m3/hr. While it seems attractive to use large volumes of dilution air, one down side might be that the associated gas treatment system will also be large and costly.

Gravity Mains And Reticulation Pipework Venting The Sewerage Code Of Australia (WSA 02-2002) suggests vents are installed upstream of siphons (sections of pipe which flow full under roads or other structures), and where a rising main discharges into a gravity sewer. Whilst it is rare to find odour control devices on such vents, there may be a need at some sites to install them due to high concentration discharges and/or frequency of venting.

34

Chapter 4 – Sewage Transport System Components And Odour Issues

Pump Station Venting Pump stations are often sources of odour, particularly if they are at the downstream end of a pump station network in flat areas.

To handle the air entering and leaving a wet well type pump station, the following points should be considered.

•

The well access covers must be in good condition, and properly sealed to the roof with rubber strips.

•

Small ground level vents on the top of wet wells may only be appropriate if sewage is fresh at all times, and the well is outside any building.

•

If septic sewage exists, it is prudent to consider a mechanical air ventilation system. Specifically, an induced air system where fresh air is drawn into the well through an above ground inlet vent, and drawn out of the well by a fan prior to discharge through a tall vent, (Figure 4.4) with or without treatment of the odorous gases should be considered. This type of layout minimises gas entry into the pump station building under normal operating conditions as well as during power failures.

•

To minimise leaks, wet wells should be kept under negative pressures of at least -10Pa by the induced air fan.

Figure 4.4. Typical pump station induced air ventilation system.

Vent Stack

Outside Fresh Air From Vent Above Roof Line Pump Station Building

Gas Treatment Damper 2 (Closed) Fan Damper 1

Wet Well

35

Chapter 4 – Sewage Transport System Components And Odour Issues

Part Full Branch And Trunk Sewer Venting According to WSA (02-2002), natural draft vents should be installed at specific intervals on these larger sewers. MMBW (1989) suggests that these vents help to minimise corrosion and odours as sewage can be kept “fresher” by the addition of air which helps to oxygenate the sewage and limit the formation of anaerobic conditions.

The principal is to: a) Install alternating short and tall vents at specific intervals, which takes advantage of the fact that wind speeds are greater higher up. The higher wind velocity above the taller vent causes air to be drawn in through the short vent and out through the tall one. b) Take advantage of the temperature of the sewage gas which may be warmer than the outside air. If this is the case, it will want to rise up the taller vent more rapidly than the shorter vent, creating a greater “natural draft or chimney effect” in the tall vent. This also causes outside air to be drawn in through the short vent and out through the tall one.

In practice this system may not work reliably at all times due mainly to lack of wind or low sewage gas temperatures. It is also worth noting that fitting of wind driven rotary cowls is not very effective at all times.

Again, it must be noted that if septic sewage exists, fan based sytems should be considered and further assessment is required to determine whether gas treatment is warranted.

Air Valves Versus Vent Stacks Wherever possible, and if the hydraulic gradeline permits, a tall corrosion proof vent stack would be preferable to an air valve, as air valves require regular maintenance to prevent blockages and generally discharge gases at ground level only.

In a rising main or pipeline flowing full, it is usual that an air valve or elevated vent stack (Figure 4.5) is fitted at high points in the line to prevent build up of gases which might lower the hydraulic efficiency of the pipeline.

Figure 4.5. Pipeline airvalve (left) and tall vent (right).

36

Chapter 4 – Sewage Transport System Components And Odour Issues

Pressure Sewers A pressure sewerage system is different to a conventional STS. It consists of a sewage collection tank at each house, and a pump which is contained within the tank. Sewage from the house or building is pumped from the collection tank to a pressurised pipe network in the street and then on to a treatment plant.

These systems have become popular in recent years and in a number of cases, odour problems have been associated with them. Typically, odours occur at air release points along the pressurised pipe network, or at the end of the system where the sewage enters the treatment plant. The pipe system runs full all the time, thus creating a stable biofilm which will produce sulfide in a similar manner to a conventional rising main.

Odour control needs to be considered during the design stage of a pressure sewerage system. Consideration needs to be given to: • • •

gas treatment at air release points or at the entry to the sewage treatment plant, or chemical dosing at a number of points in the early stages of the network, or chemical dosing in the final common main before the treatment plant.

Vacuum Sewers A vacuum sewerage system is also different in some respects to a conventional STS. A group of houses is connected to a collection sump that has sewage entering by gravity flow. Once 40 to 50 litres builds up in the sump, an automatic valve opens and a vacuum on the downstream side draws the sewage out of the sump into the vacuum sewer pipe. The sewage is then conveyed to a centralised vacuum collection tank, and subsequently pumped to a treatment plant or conventional STS. Vacuum STS’s are commonly used when the terrain is very flat, when the area is environmentally sensitive and spills must not occur. Vacuum sewers are particularly useful if the ground is rocky or difficult to sewer using conventional STS’s.

Many vacuum sewer systems exist in Australia (eg: Sanctuary Lakes in Melbourne, and Hindmarsh Island in South Australia) and they are often found at marinas or subdivisions close to rivers and the coast.

From an odour control viewpoint there is usually sufficient retention time for generation of H2S to occur prior to the vacuum collection tank. The air discharged from the vacuum pumps on the vacuum collection tank usually requires treatment to remove odours. Most vacuum STS’s in Australia employ biofilters to treat vacuum pump gas discharges and in some cases, odour control is required downstream of the vacuum system using liquid phase or gas treatment systems.

37

Chapter 5 Targets For Achieving Low Odour Targets For Sewage Quality Including Trade Wastes Hydrogen sulfide (H2S) poses multiple risks to water utilities. Sewer workers risk illness and injury. In addition, H2S causes corrosion and reduces the life of the asset. Release of H2S and other odorous gases results in poor public relations and community perceptions of the inability to operate a system. The community expects an STS to be odour free. Indeed, the community never really thinks about it until something goes wrong and there is an odour event.

The major objective of a water utility should therefore be to operate an STS without odour release or at least if there is a release, to minimise its impact. To assist in this process, it is possible to set targets for certain parameters in an STS that will help to minimise odour generation.

Suggested targets for sewage and trade waste quality relevant to odour generation and corrosion are provided in Table 5.1. Additional targets for STS design and operation are provided in Table 5.2. These are a general guide only as site specific limitations may prevent their implementation.

Table 5.1.

Suggested sewage and trade waste quality targets.

Parameter

Suggested target for any sewage within the STS

Suggested target for trade wastes at the input point

Comment

pH.

8 to 8.5.

8 to 8.5.

Reduces release of H2S and ammonia to the gas phase.

Temperature (0C).

As low as possible.

380C maximum.

Sulfide generation rates increase with temperature between 7 and 15% for each 10C rise (MMBW 1989).

BOD (mg/L).

As low as possible, consistent with downstream sewage treatment plant requirements.

400 maximum.

As low as possible, consistent with downstream sewage treatment plant requirements.

BOD would ideally need to be lowered to less than 2mg/L to prevent any sulfide generation in the STS. In most cases this is not possible.

0.5 minimum.

0.5 minimum.

+50 minimum.

+50 minimum.

Dissolved Oxygen (mg/L).

Oxidation Reduction Potential (mV).

Soluble sulfide (mg/L).

38

0 maximum.

1 maximum.

Total oxidisable sulfur As low as possible. including sulfate, sulfite, and thiosulfate 100 maximum. (mg/L).

100 maximum.

Ammonia (mg/L).

50 maximum.

50 maximum.

Note that if 1mg/L exists at a pH of 8 and temperature of 250C, the gaseous H2S concentration could be around 28 ppm in confined spaces.

Chapter 5 – Targets For Achieving Low Odour The STS is a control point and targets can be set for the operation of the STS to minimise the risk of odour production.

Table 5.2.

Other STS operational targets and critical limits.

Parameter

Suggested target for within the STS

Minimum slopes for gravity sewers.

Refer to WSA 02-2002.

Minimum velocities for wall slime control in sewers.

1.0 m/s at least once per day for a few hours.

Refer also to MMBW (1989) for more detail.

Turbulence.

Wet well level variation.

Wet well cleanliness (fats, grease & sediment).

Suggested critical limit

Comment or action required to meet critical limit Wall slimes are extremely robust and complete removal by velocity alone is not possible. Note that “pigging” of mains will only give very short term reductions in wall slime thickness and odour.

Minimise.

Minimise.

Maximise.

Mechanical type with tall vents preferred. Vent height determined by Venting of STS components modelling, but 12m height where septic sewage is desirable, and at least 3m higher than adjacent exists. buildings and trees.

Minimum vent gas velocity of 8m/s.

Odour complaints.

None.

Maximum vent H2S or odour level.

1. To meet ambient air targets (see below).

2. One odour unit at the property boundary.

Maximum pipeline and system H2S to minimise corrosion.

< 5ppm.

Maximum H2S concentration for human exposure.

Must minimise.

Must meet local EPA licence and emission limits (see below).

10 ppm (see Chapter 2 for details).

Check adequacy of venting and gas treatment system and initiate problem solving procedure if necessary (Chapter 8).

Actual level requires full technical assessment for each site.

Reference must be made also to local Health and Safety requirements.

39

Chapter 5 – Targets For Achieving Low Odour

Targets For Vents And Ambient Air EPA Odour Legislation And Standards In Australia, allowable levels of odour, associated gas concentrations and total amounts that can be discharged are usually defined by the local state government Environment Protection Authority (EPA), or equivalent government department. STS operators should check the specific rules and limits which apply in their State, since each will be different.

As an example, in Victoria, the EPA administers the requirements of the Environment Protection Act and the associated State Environment Protection Policies (SEPP) (e.g. Air Quality Management and Ambient Air Quality).

In relation to odours and odorous compounds, the Air Quality Management SEPP sets out the following: 1. It defines Air Quality Indicators as Class 1, 2, 3, and Unclassified. General odour comes under the Unclassified indicators. Hydrogen sulfide and several odorous mercaptans are listed as Class 2 indicators.

2. It defines design criteria for limiting odour and release of odorous compounds from new emissions or modifications to existing emissions.

For example for H2S the maximum allowable downwind 3 minute level is 0.00014 mg/m3 (0.0001ppm).

For general odour the limit for new sources is 1 odour unit which applies at the property boundary and beyond.

3. It specifies the modelling procedures to be used to assess compliance with the design criteria. The approved model is called Ausplume, and must be used unless a good technical argument for an alternative is presented to the Victorian EPA. 4. It specifies emission limits from stationary sources such as vents eg: •

for H2S from an existing source, a limit of 7.5mg/m3 (5.4ppm) applies

•

for H2S from a new source in specific Air Quality Control Regions, a limit of 5mg/m3 (3.6ppm) applies.

Note that compliance with the specific emission limits above will not necessarily create an odour free situation since the proper dispersion of the gases is also required.

40

Chapter 6 Control Of Sulfide Levels In Sewage General Principles Since hydrogen sulfide (H2S) is often the major odorant in sewage, controlling sulfide will in most cases control odour. There are a number of options for treating sewage to reduce odour. They fall into four main groups: • • • •

those that prevent the formation of sulfide those that keep sulfide in solution those that remove existing sulfide those that treat non sulfide based odorants.

Preventing the problem occurring in the first place is intuitively the best way to proceed.

Treatments That Prevent Sulfide Formation Providing full secondary sewage treatment in the early stages of the STS could be considered but is often not possible. This approach was however adopted recently at the Gippsland Water Factory in Victoria, where a new secondary treatment plant was built at the top end of the STS. One of the main aims of the plant is to lower the BOD which is available to sulfide forming bacteria. In this way the treated sewage will not generate sulfide as it travels along the downstream sections of the STS. BOD levels in such situations need to be lowered to less than 1 mg/L. Once untreated sewage enters the STS, sulfide formation can be prevented in a number of ways.

1.

2.

3.

4.

5.

Provide an oxygen source.

Maintain a sufficiently high redox potential.

Modify the bio-organism type and behaviour.

Kill the bacteria present.

Produce conditions under which sulfate reducing bacteria will not grow.

Air And Oxygen If enough air or oxygen can be dissolved into sewage, it will prevent the conversion of sulfate to sulfide by the sulfate reducing bacteria.

Air is usually not used since it cannot produce sufficiently high levels of dissolved oxygen. In addition the undissolved air can cause partial blockages and reduce sewage flow in the pipelines. Pure oxygen systems are therefore preferred for delivering oxygen and are one of the most commonly used odour control systems.

When oxygen is provided, a number of biochemical changes take place. Firstly, the dissolved oxygen levels increase, thereby encouraging aerobic and facultative organisms to grow and consume it. As a result of this change, oxygen consumption rates can become very high. However, at some point downstream of the oxygen dissolver, oxygen is consumed and after a lag period, sulfate reducing bacteria re-grow and sulfide production begins again.

A single dissolver may not be sufficient to keep sewage in a fresh state in a long pipeline and a series of dissolvers may be required. Design of such systems involves calculations of the rate of oxygen depletion and sulfide generation and how much oxygen can be dissolved at particular points in the pipeline.

41

Chapter 6 – Control Of Sulfide Levels In Sewage Dissolving oxygen or air is not as simple as placing a gas supply through a tube into a pipeline and turning it on. To dissolve the oxygen, minimum levels of mixing energy and gas liquid contact are required. The pipeline pressure must also be high enough to support the required dissolved oxygen level. Engineering calculations are required to confirm that a dissolver will work at a particular site and importantly, an appropriate oxygen dissolver must then be used to meet the criteria determined by the calculations.

Care must be taken to choose a type of oxygen dissolver which dissolves the injected oxygen with as close to 100% efficiency as possible. If the oxygen is not fully dissolved, it may build up in downstream pipework and cause reductions in flow. At one extreme, coarse bubble air diffusion dissolvers may only dissolve a few percent of the oxygen in the injected air, whereas at the other extreme, a well designed multiple u-tube oxygen dissolver (Figure 6.1) may dissolve close to 100% of the injected oxygen.

A multiple u-tube dissolver consists of multiple loops of sewer pipe through which the total sewage flow is usually passed. An oxygen rich gas space exists at the top of each loop. As the sewage travels over the top of each loop, it falls down the other side in a waterfall. In the waterfall, oxygen is entrained in the sewage and most of it is dissolved. Once the sewage leaves the last loop, it travels along a short horizontal section of pipe (phase separator) where any undissolved gas bubbles rise up and are returned to the gas inlet point on top of the first loop through a gas return line. A small head loss occurs across the dissolver and this is needed to dissolve the oxygen.

Oxygen dissolvers are usually supplied with oxygen from a liquid oxygen storage tank and evaporator.

Figure 6.1. A multiple u-tube oxygen dissolver.

Gas Return

Loop No. 1

Liquid Oxygen Tank

Sewage Inflow

A smaller stainless steel sidestream dissolver (Figure 6.2) which is dissolving oxygen into a part of the main sewer flow and pumping it back in to the main flow. This smaller type of dissolver is sometimes used to minimise installation costs.

42

Chapter 6 – Control Of Sulfide Levels In Sewage Figure 6.2. A stainless steel sidestream oxygen dissolver.

Chlorine Chlorine gas or sodium hypochlorite can be used as a disinfectant to control and kill the sulfate reducing bacteria in pipelines. If added to fresh sewage, it will also kill other bacteria and oxidise any organic matter in the sewage. This results in the formation of disinfection by products. For this reason, chlorine treatment is not commonly used.

Nitrates Nitrates are chemical combinations of nitrogen and oxygen. Chemically it is written as NO3- . While there is no free oxygen available, the oxygen bound to the nitrogen can in fact be used by bacteria instead of oxygen.

Sodium and calcium nitrate solutions are the most commonly used forms, with ferric nitrate being used occasionally.

When sufficient nitrate is present, bacteria prefer to consume it rather than sulfate, and sulfide production is therefore prevented. The process results in the production of small amounts of nitrogen gas and a slight increase in pH. One advantage of nitrate over oxygen is that the oxygen it contains is not consumed as fast as pure oxygen in fresh sewage, and therefore its preventive effect may last longer in an STS. Note that a nitrate residual always needs to be maintained in the pipe to prevent re-generation of sulfides.

Unfortunately, if the nitrate residual drops to zero, higher than expected rates of sulfide generation can result than if nitrate hadn’t been added in the first place.

Note that a nitrate residual always needs to be maintained in the pipe to prevent re-generation of sulfides.

Once nitrate disappears, the sulfide producing bacteria can re-grow very quickly and produce high sulfide concentrations.

43

Chapter 6 – Control Of Sulfide Levels In Sewage Calcium nitrate is widely used for odour control and is readily available in Australia as a solution of around 50% by weight. It is a relatively safe chemical to handle and store.

Nitrate can also be readily used in low pressure STS’s, unlike oxygen which requires pressure to achieve sufficient concentrations.

Storage and metering pump systems for the dosing of nitrates are very simple and dosing control can be varied throughout the day to suit a particular STS by using a simple dosing controller (Figure 6.3).

Figure 6.3. A calcium nitrate solution storage and dosing system installed inside a sewage pump station.

One of the problems associated with dosing calcium salts into sewers is that calcium carbonate scale can build up on the walls of the pipes. Over time this can limit the diameter of the pipe and lead to reduced flows, blockages and higher pumping costs. This situation may occur if: • • • • • • •

calcium levels are too high sewage alkalinity is abnormally high dosing controls are not adequate (flow paced dosing at least must be used to limit peaks in calcium concentration ) other chemicals such as magnesium and phosphorus are present at elevated levels high alkalinity or calcium trade wastes are involved pH levels are high (the higher the pH the more likely calcium precipitation will occur) minimum sewage velocities are not adequate.

Due to the complex chemistry involved, a check of the likelihood of precipitation should be carried out by an experienced chemist both before and during use of calcium based chemicals.

44

Chapter 6 – Control Of Sulfide Levels In Sewage Nitrites Nitrite is chemically similar to nitrate and like nitrate has oxygen bound into the molecule. Chemically it is written as NO2- . Therefore nitrite can also be used to keep sewage odour free.

However, nitrite can also provide a biocidal effect. Slimes on the walls of STS pipes that contain most of the sulfide producing bacteria in a pipeline can be controlled when nitrite is dosed under the right conditions.

Compared with nitrate solutions, the doses required can be significantly lower. Nitrite systems can be turned off for part of the time due to the residual biocidal effect.

Storage and dosing systems are also very simple, and dosing control can be varied throughout the day to suit a particular STS. Sodium nitrite solutions are available at around 40% by weight, and are regarded as dangerous goods although they are very similar in nature to nitrate based solutions. Nitrite based control has only recently been used in Australian STS’s.

Anthraquinone Anthraquinone slurries have been used with partial success in STS’s overseas to stop bacteria from converting sulfate to sulfide. Anthraquinoine acts by inhibiting microbial respiration.

Once added to sewage, some of the particles are caught in the wall slimes where they exert their effect for some time. Dosing must be carried out regularly to maintain the effect.

Storage and dosing systems are more complex than for nitrate or nitrite since they need to manage the anthroquinone slurry where the particles tend to settle out. Anthraquinone is not widely used in Australia.

Bacterial Mixtures A variety of bacterial mixtures are marketed for controlling odours in STS’s. These are said to act by providing sufficient “friendly” bacteria to outnumber the undesirable species and establish a stable microbial ecosystem.

The aim is for the added bacteria to outgrow the sulfate reducers and thereby control sulfide production.

These mixtures have been used in Australia, and there are those who anecdotally attest to their benefit.

Ideally, prospective users should carry out scientifically controlled trials of such products to determine if they are capable of preventing significant sulfide generation in a particular STS. In such a trial, it is suggested that measurements should be taken of the following parameters for at least two weeks before, during and after use of the mixtures: •

pH, soluble sulfide, ORP and temperature in the sewage either continuously or by analysing several grab samples taken every thirty minutes over the peak odour time of each day

•

continuous H2S levels in gases from confined spaces within the STS such as a manhole near where the odour problem is occurring.

Careful comparison of the data should then be carried out.

Nutrient or Bacterial Stimulant Mixtures Another set of products for limiting formation of sulfide are those that claim to “stimulate friendly bacteria” by adding unknown “natural” stimulants rather than directly adding friendly bacteria.

Again, these should be scientifically evaluated to see if they work to prevent significant sulfide generation in a particular STS.

45

Chapter 6 – Control Of Sulfide Levels In Sewage Alkalis If the pH of sewage is raised to very high levels (generally above pH 10) a biocidal effect can be seen. Hence, if sufficient alkali such as sodium hydroxide or lime is added, sulfide generation will be slowed.

This is not usually a practical option since large quantities are required and the resulting high pH sewage is not desirable in a downstream STP. Slug doses of alkali have been tried but the biocidal effect on the wall slimes after a slug dose is short lived and the bacteria recover quickly.

Other Biocides A variety of biocides have been trialled to prevent formation of H2S, including chemicals such as dichlorobenzene, however most of these are either too powerful and difficult to control, are unacceptable in downstream STPs, or produce undesirable by-products.

Treatments To Prevent Sulfides Leaving The Liquid Phase Raising the pH of sewage can have a dramatic effect on stopping H2S from leaving the liquid sewage and entering the gas above it. In practice, the pH needs to be at least 8.5 to keep most of the sulfide in solution (Figure 2.1).

While in principle, this treatment method is simple, it is often difficult to determine and maintain the necessary doses. Sewage varies from one site to another, particularly where there are significant trade waste inputs to the sewer. Therefore, it is important that a site specific assessment of the quantity of alkali required is undertaken.

There are three alkalis commonly used to control odour in an STS: • • •

caustic soda lime magnesium hydroxide (MHL).

Each of these will be considered below. The method of action is the same in each case. By increasing the pH, the proportion of hydrogen sulfide as HS- or S2- is increased (Refer to Figure 2.1).

Caustic Soda (Sodium Hydroxide) Solution Caustic soda is the most “powerful” of the three alkalis considered here. A small quantity of caustic can produce a large increase in the pH. It is easy to achieve pH levels over 10, however this effect is relatively short lived when compared to the other alkalis discussed below. Careful dosing is necessary to ensure accidental overdosing does not occur. Very high pHs reaching a treatment plant can have a rapid and inhibitory effect on the plant biomass.

Caustic soda is usually supplied as a 30% or 46% solution. A 30% solution is better for storage and handling since it is unlikely to “freeze” or solidify under most Australian ambient temperatures. At low temperatures 46% caustic can solidify and block pumps and dosing lines.

Caustic soda is quite hazardous and must never be allowed to contact the eyes, skin or, mucous membranes inside the mouth or nose. It must only be handled by competent, trained personnel using appropriate PPE including full face masks, aprons and gloves as a minimum.

46

Chapter 6 – Control Of Sulfide Levels In Sewage Lime Until recently, lime has been available only in powder forms either as quicklime (calcium oxide) which is quite hazardous, or in its hydrated form where some water has been added to reduce reactivity and make it safer to handle, known as hydrated or slaked lime (calcium hydroxide).

Recently, lime slurries have become available and are being used more widely. The problem with using slurries is that the particles can settle out. Care must be taken to choose a slurry which will not settle out too quickly during storage, or alternately store it in properly designed agitated tanks.

Like caustic soda, when added to sewage, hydrated lime will have a quick effect on pH but it will also provide a degree of residual buffering. High pH’s above 10 are also possible.

As described previously for calcium nitrate, any excess calcium may lead to build up of calcium carbonate (scale) on pipe walls. Therefore, great care is required to control dosing. For this reason, and due to the difficulty of handling, lime has not been used widely for control of pH in STS’s.

Magnesium Hydroxide (MHL) MHL is another alkali that is dosed as a slurry. Unlike caustic soda and lime, magnesium hydroxide is slow to cause an increase in pH, however like lime, it provides good residual pH control because the fine undissolved particles in the slurry continue to dissolve as it passes down the pipeline.

MHL will only increase the pH to around 9.5. This has one advantage in that it prevents the excessive pH’s possible with caustic soda. Some MHL slurries contain lime and may allow higher pH’s to occur.

There are a number of MHL products available. Care must be taken to use slurries that have been properly manufactured and are not just mixtures of magnesium oxide and water. The magnesium oxide that is one of the raw materials of the product must have been fully hydrated to form the alkali, and the insoluble solids in the slurry must not settle out too quickly. Laboratory tests can be conducted to determine the level of alkali present and settling times. The most commonly used product available in Australia is a 60% solids slurry.

Although it has a reasonably long non-settling period, some users opt to store it in purpose built agitated tanks. Attention must also be paid to keeping pumps and transfer lines free from settled solids.

MHL slurries are regarded as being relatively safe to store and handle.

Treatments To Remove Existing Sulfides Air And Oxygen If dissolved oxygen is present in sewage, it will tend to react quickly with any soluble sulfides present to form sulfate and a small amount of other species including sulfur. There is also some removal of sulfide by aerobic bacteria.

Since high dissolved oxygen levels are required, oxygen rather than air is the preferred method to achieve this.

Using oxygen to remove existing sulfides is the most efficient way to use oxygen. The sewage should not contain high levels of bacteria that might compete for the oxygen as prior to the addition of the oxygen, the sewage would have been anaerobic. Reference to the previous section should be made regarding use of dissolved oxygen and air.

Design of oxygen dissolving systems for this type of use involves calculation of the rate of oxygen utilisation by sulfide oxidation, and how much oxygen can be dissolved at the particular point in the pipeline.

47

Chapter 6 – Control Of Sulfide Levels In Sewage Hydrogen Peroxide Hydrogen peroxide is a strong oxidant capable of reducing sulfide levels to zero, however it is not used widely in STS’s in Australia partly because it is a Class 5.1 dangerous good, requiring specialised knowledge and safety procedures to store and handle. It can be supplied at different strengths but is normally used as a stable 50% w/w solution.

Hydrogen peroxide does however have many advantages. Hydrogen peroxide: • • • • •

can oxidise a variety of odorants can be used as a source of oxygen to increase dissolved oxygen levels breaks down to products of oxygen and water can be dosed into low pressure and gravity mains can be dosed using small liquid dosing pumps.

Dosing systems for hydrogen peroxide must provide pressure relief due to the inevitable breakdown of liquid peroxide into gaseous oxygen.

When dosed into sewage containing sulfides, the doses required, the reaction products produced and the reaction times will vary largely according to the prevailing pH. This is shown in Table 6.1.

Table 6.1. pH range just below 7 7 to 9 above 9.2

The effect of pH on sulfide oxidation by hydrogen peroxide.

Main reaction products Relative dose required Relative reaction speed sulfur low slow sulfate medium to high fast sulfate medium moderate

Hydrogen peroxide is often used to treat a variety of odorous trade wastes.

Chlorine Chlorine gas or sodium hypochlorite can be used to oxidise sulfides quickly and to create low levels in an STS. Under acidic conditions, sulfate is likely to be formed and under alkaline conditions, sulfur is more likely to be formed.

The main disadvantages of using chlorine for sulfide removal are its potential to form unwanted byproducts, and the difficulty in handling. It is not widely used in Australia for sulfide removal in STS’s.

Nitrates Dosing nitrate into STS’s for sulfide removal is possible although it is not widely used in Australia, with most dosing applications being designed to keep sewage fresh. The process is thought to occur by bio-chemical oxidation and is relatively slow when compared to oxygen and other oxidants. To be effective, it relies on the growth of sulfide tolerant strains of bacteria, which in the presence of nitrate under anaerobic conditions can oxidise sulfide to sulfate.

Nitrites Nitrite solutions also have the ability to remove sulfides from sewage under anaerobic conditions. They are not widely used in Australia.

Nitrite is a strong oxidiser, particularly at lower pH’s, and will convert sulfides to oxidised species including sulfur very rapidly. It is important to carefully match the nitrite doses to the sulfide levels to avoid over dosing.

48

Chapter 6 – Control Of Sulfide Levels In Sewage Ferric Chloride And Ferric Sulfate Addition of ferric salts (see ferrous salts also) is a very effective means of quickly and relatively safely removing soluble sulfides from sewage. Ferric salt addition is commonly used in Australia for odour control.

The active component is the ferric ion which reacts readily with soluble sulfides to produce insoluble black iron sulfides that are non odorous. The fine iron sulfides will readily settle out in the primary treatment sections of an STP. There are a number of other benefits to using ferric salts: • • • •

they will raise the ORP of the sewage which may help slow down sulfide generation they can be added at the start or end of an STS pipeline and perform best at pH’s above 7 due to the density of the iron sulfide particles, they may improve the settling in the early stages of a sewage treatment plant any excess iron will also help to reduce phosphorous levels.

However, there are also some disadvantages: • • • •

ferric salts are acidic, are classed as dangerous goods, and require proper storage, handling and safety procedures they cause intense staining of concrete if spilled ferric chloride solutions will add additional chloride to sewage, which may need to be considered ferric sulfate solutions will add additional sulfate to sewage, however this is often not a problem as there is usually plenty of sulfate already contained in the sewage to be treated.

Ferric chloride is normally available as a solution containing around 14.5% iron, whilst ferric sulfate solutions typically contain around 12.5% iron by weight. Both contain a few percent of free acid which will lower the pH of the sewage.

One variation of ferric sulfate is PFS® or Poly Ferric Sulfate which will perform similarly to ferric sulfate but which, having no free acid, will not depress the pH of the sewage as much.

Dose requirements can be estimated, however it is best to conduct dosing trials on site to ascertain doses due to the possibility of iron hydroxide and other minor reaction products being formed.

Ferrous Chloride, Ferrous Sulfate and Pickle Liquors (Ferrous Salts) Iron can also be dosed as ferrous salts. Ferrous chloride and ferrous sulfate solutions can be used to quickly remove soluble sulfides and are used widely in Australia when available (eg: some Sydney Water sites).

Pickle liquors are a byproduct of the rust removal process in the steel industry. They contain ferrous salts and are used in the same way as manufactured ferrous products. When using pickle liquors, care must be taken to ensure that free acid levels, trace metal and other contaminants are not too high. Pickle liquors may have lower iron content and show significant product quality variations.

The iron sulfides that are formed during ferrous salt addition are slightly different to those formed during ferric salt addition, but again they are relatively insoluble and appear as fine black solids. Note that: • • • •

the most insoluble iron sulfide is formed by adding a mixed iron salt containing two thirds ferric and one third ferrous iron ferrous salts are best used at pH’s above 7 to minimise final sulfide concentrations ferrous salts have a much lower redox potential, and will not raise the ORP of sewage as much as ferric salts can to maximise sulfide removal efficiency, ferrous salts should be added so that a small residual iron content is always present.

49

Chapter 6 – Control Of Sulfide Levels In Sewage Ferrous chloride solutions are available with around 12% iron, whilst ferrous sulfate solutions typically contain around 7% iron by weight.

Others Ozone and potassium permanganate can oxidise H2S in sewage but are not widely used for this purpose in STSs. Ozone is used to oxidise H2S in the gas phase (see Chapter 7) in odour control systems.

Removing Other Odorous Compounds From Sewage For non sulfide based odorants, a case by case assessment is required to determine the best treatment system.

Some general comments are provided below.

Mercaptans Some mercaptans can easily be oxidised with oxygen and other stronger oxidants. Care must be taken to assess what the reaction products will be and ensure that they are not more of a problem than the particular mercaptan present initially.

Again, if oxidants are added, their effect on any downstream sewage treatment plant must be assessed.

Ammonia The effect of high pH on ammonia is opposite to that of H2S. While high pH will prevent release of H2S, it will increase the release of ammonia. To control ammonia, a pH of <8.5 is desirable. Controlling pH to between 8 and 8.5 minimises H2S and ammonia release if both are present (Table 5.1).

Chemical oxidation of ammonia is not practical in STS’s and biological nitrification within the sewage treatment plant is the usual means of removal. However, there can be some nitrification within STS pipelines if dissolved oxygen levels can be maintained either by installation of oxygen dissolvers or by encouraging good natural ventilation.

Trade waste streams containing high ammonia levels should be pre-treated to remove ammonia. This is usually done by stripping the ammonia out of the liquid phase at high pH followed by ammonia removal in an acid scrubber.

Trace organic compounds Phenol, aldehydes and organic amines can be oxidised by hydrogen peroxide or Fentons reagent.

Chemical Dosing: Dosing Point And Control Type The correct location for dosing sulfide control chemicals and the method of controlling the dose are vitally important to ensure that good odour control is achieved and over or under dosing does not occur. They are also important to help minimise the costs of controlling odour and minimising undesirable corrosion or scale build up.

50

Chapter 6 – Control Of Sulfide Levels In Sewage Dosing Point for Pipeline Odour Control a) To keep sewage fresh throughout the pipeline (if no sulfide is present at the start).

In general, the start of a pipeline is the preferred location for dosing so that odours along the whole of the pipe can potentially be controlled. However, the type of chemical and the length of the system can pose some limitations. For short systems, single dosing stations at the start may be possible. For long systems and for chemicals which are consumed quickly such as oxygen and hydrogen peroxide, multiple dosing points along the pipeline may be needed to achieve full treatment along its length. Nitrates and nitrites can provide cost effective control throughout short and long pipelines with minimal dosing stations, when fresh sewage is dosed at the start of the pipeline. b) To control odours and corrosion throughout the pipeline (if sewage contains sulfide at the start).

It takes time for sulfides to be removed by some chemicals. For short pipelines (retention times of only a few hours) with sewage containing sulfides at the start, the chemical used must be capable of removing sulfides relatively quickly. In this situation, dosing of oxygen, hydrogen peroxide, alkalis, and iron salts could be appropriate if dosing occurs only at the start of the system.

For longer pipelines, these same chemicals can be used, but if sulfide levels are high, then multiple dosing points may be required after the initial dosing. Slower acting chemicals such as nitrates or nitrites can also work well if sulfides can be tolerated in the first few hours of sewage travelling down the pipeline. c) To control odours only at the end of the pipeline, (if sulfides can be tolerated upstream).

If odour control is only required at the end of the pipeline system, then some of the faster reacting chemicals such as oxygen, hydrogen peroxide, alkalis, and iron salts could be dosed close to the end only. d) Dosing into the wet well or rising main?

Another issue which arises is whether a chemical should be dosed at the start of a rising main into the wet well or into the main itself. Obviously if odour control is required within the pump station and wet well, then dosing of a fast acting oxidant or an iron salt into the wet well should be considered. If odour control is not required in the wet well itself, then other slower acting chemicals such as calcium nitrate can be used and should be dosed into the rising main. Dosing into a wet well can result in unnecessary consumption of some chemicals such as calcium nitrate. There may also be some additional flotation of fats, greases and solids, if nitrogen gas is produced or pH changes occur.

Dosing directly into the rising main requires a proper corrosion proof tapping saddle, injection valve, non return valve and injection lance to be installed onto the main. The dosing pump selected must be capable of pumping against the maximum pressure in the pipeline at the point of injection. Direct dosing to the main ensures that good mixing of the chemical can be achieved and the control of the dosing pump can easily be linked to the sewage pump controls.

Dosing Control Type Simple Flow Paced Control Chemicals should always be dosed to achieve a properly controlled concentration. Simple flow paced control where dosing only occurs when the sewage pumps are operating and the metering pump is controlled by a flow signal from a flow meter in the pipeline is the best way to achieve this and avoid under or over dosing.

51

Chapter 6 – Control Of Sulfide Levels In Sewage Allowing For Variations In Dose Requirements At Different Times Many of the factors associated with the generation of odour such as temperature, BOD and retention time vary during each hour of the day, each day of the week, and also from season to season. It is therefore necessary to allow for these variations in dosing requirements using intelligent, time based dosing systems.

If trade wastes are involved, then allowance should also be made in dosing timing requirements for variations in trade waste flow and quality.

Feedback And Feedforward Control Dosing systems for liquid phase odour control are now being set up with up to date feedforward or feedback control systems (Figure 6.4).

In simple terms, if enough information is known about an STS, how odours are forming and what doses of chemical are needed at particular times, a mathematical model of the whole process can be developed and a feed forward method of dosing can be used. Real time flow rates, and upstream quality parameters such as COD and soluble sulfide can be fed into the model and appropriate chemical dosing rates predicted.

Feedback controls can also be used where a downstream controlled parameter such as soluble sulfide can be continuously measured (perhaps by an S::CAN system) and fed back to the upstream dosing pump controller which decides whether to increase or decrease the chemical dose. Feedback control in long pipelines can be difficult to set up due to the variable time lag between changing the dose rate and measuring its downstream effect.

Figure 6.4.

Schematic diagram of feedback and feedforward type dosing systems.

Comparison Of Liquid Dosing Options For Sulfide Control Table 6.2 provides a summarised comparison of the main methods for the control of sulfide in the most common pipeline odour control situation ie: a rising main. Three completely different dosing scenarios are considered, and the doses provided in the table are an approximate guide only. Actual doses at a particular site would need to be determined by trial or by modelling for each particular STS. The requirements for downstream treatment plants have not been considered and relative operating costs will depend heavily on delivered costs of chemicals at particular sites.

52

6

6

Lime Slurry (45% w/w Ca(OH)2).

(MHL) (60% w/w).

Sodium nitrite solution (40% w/w).

6

0 throughout.

Calcium nitrate solution (50% w/w).

Sodium hydroxide solution (30% w/w).

0 throughout.

Oxygen.

1. Keep sewage fresh by dosing at the start of the STS. (For a 250mm rising main with a 3hr retention time. Sewage at 200 C, BOD=250mg/L).

2. Keep 6mg/L of sulfide in solution by pH control.

(Adjusting sewage with pH of 7 to around 8.5. Assumes dosing at start of pipe).

0 throughout.

Treatment method

Likely treated sewage sulfide level (mg/L)

125

210

350

125

145

65

Approx. daily ave. concentration requirement (mg/L)

Relatively Easy.

Difficult.

High.

Requires careful handling.

Low.

Easy.

Easy.

Easy.

Easy.

Relative operator ease of use

High.

Requires careful handling.

Low.

Low.

Low.

Hazard level for normal use

Comparison of the main liquid dosing options for sulfide control in a rising main.

Comparison scenario

Table 6.2.

Low.

Low.

Low.

Low.

Low.

High.

Relative capital cost

Medium.

Medium.

Medium.

Medium.

Medium.

Low.

Relative operating cost

Equivalent theoretical alkali dose to MHL.

Equivalent theoretical alkali dose to MHL.

A single dosing station at start is required.

A single dosing station at start is required.

Based on calculated oxygen usage rates (MMBW 1989). Calculations show that more than one dissolver is needed to dissolve the required amount of oxygen.

Comments

Chapter 6 – Control Of Sulfide Levels In Sewage

53

54

136

60

Near 0 only at the end of the pipe.

Near 0 at the end of the pipe.

Near 0 at the end of the pipe.

0.1 Minimum at end of pipeline only (higher along the way).

0.1 Minimum at end of pipeline only (higher along the way).

Oxygen.

Hydrogen peroxide (50% w/w).

Calcium nitrate solution (50% w/w).

Sodium nitrite solution (40% w/w).

Ferric iron salts e.g. ferric chloride (14.5% iron w/w).

Ferrous iron salts e.g. ferrous chloride (12% iron w/w).

3. Remove 1mg/L of existing sulfide,at start of pipeline.

(For a 300mm rising main with a 4.5hr retention time. Sewage at 260 C, pH=7,BOD=50mg/L).

80

240

20

60

Mostly 0 throughout.

Treatment method

Approx. daily ave. concentration requirement (mg/L)

Comparison scenario

Likely treated sewage sulfide level (mg/L)

Medium.

Requires careful handling.

Medium.

Requires careful handling.

Low.

Relatively Easy.

Relatively Easy.

Easy.

Easy.

Relatively Easy.

High.

Requires careful handling.

Low.

Easy.

Relative operator ease of use

Low.

Hazard level for normal use

Low.

Low.

Low.

Low.

Medium.

High.

Relative capital cost

Low.

Medium.

Medium.

Medium.

Low.

Low.

Relative operating cost

Calculated using 20% more than theoretical doses for pH 7 sewage, sulfide generation along the pipe, and dosing at end of pipeline only. No odour control along the pipeline.

Calculated using 20% more than theoretical dose for pH 7 sewage, sulfide generation along the pipe, and dosing at end of pipeline only. No odour control along the pipeline.

Based on 300mm rising main tests.

Single dosing station at start.

Based on 300mm rising main tests.

Single dosing station at start.

Calculated using 1.5mg pure peroxide per mg sulfide, allowing for sulfide generation along the pipe, and dosing at end of pipeline only. No odour control along the pipeline.

Based on calculated oxygen usage rates (MMBW 1989). Calculations show that more than one dissolver is needed to dissolve the required amount of oxygen.

Comments

Chapter 6 – Control Of Sulfide Levels In Sewage

Chapter 7 Treating Odorous Gases General Treatment System Principles Where hydrogen sulfide (H2S) or other odorous compounds have been released to the atmosphere in an STS, there are a number of options to treat and reduce their levels. The treatments fall into two main types; those that involve passing the gases through a column filled with some sort of media or packing material and those that do not.

Those that use packing material include: • • • • •

activated carbon adsorbers biofilters biotrickling filters chemical scrubbers (gas purifiers) activated alumina systems.

Those that do not use packing material include: • • • • •

thermal oxidation systems ozonation systems ionisation systems UV (ultraviolet) irradiation systems odour counteractants.

The choice of which treatment system is best for a particular situation will be site specific and will depend on a number of factors not the least of which includes the required final treated odorant level. Chapter 8 provides a process for identifying whether a treatment system is required and how to choose the best system.

Table 7.1 provides a general comparison of the main gas treatment options available. Further details and information on the four most common systems are provided after the table and in Table 7.3.

55

56

In-ground biofilters.

Gas is passed through a vessel filled with porous activated carbon which traps the odorant.

Activated carbon adsorption.

Microrganisms in the bed break down the odorant.

Gases are passed through a soil, compost or mixed media bed installed in the ground.

Activated carbon can be impregnated with chemicals such as sodium hydroxide or potassium iodide, to improve the adsorption capacity for specific compounds (e.g.

H2S).

Description

H2S, mercaptans, ammonia, phenol, formaldehyde, methanol, xylene, acetates, ketones.

Organic compounds adsorb more strongly than inorganic compounds.

Large polar molecules (those with positive and negative charges on them), tend to adsorb more strongly than small non-charged molecules.

H2S, mercaptans, and a large range of volatile organic compounds.

Some odorant(s) treated

Low to medium concentration gases. Low gas flow rates, otherwise large systems are required.

Where land is available away from houses.

At remote unattended sites.

Ideal for intermittent loads.

Where low odour levels are required (i.e. < 500 OU as per AS4323.3).

Moderate.

High.

Low concentration gases.

Typically used as either the primary treatment method or after other treatment systems such as biofilters, and biotrickling filters.

Frequency of use

Applications

A comparison of the main methods available to treat STS gases.

Technique

Table 7.1.

Controlling the moisture content and pH of the media bed can be difficult.

Low maintenance.

Effective.

Low to moderate O&M costs.

No chemicals.

Large space required.

Applicable only to relatively dilute air streams in order to ensure long carbon life.

Disadvantages

Simple.

Effective.

Few moving parts.

Simple.

Advantages

Low to moderate capital cost.

Cost-effectiveness depends on carbon replacement interval or regeneration frequency.

Cost factors

Chapter 7 – Treating Odorous Gases

Gas is passed through a vessel usually filled with an inert plastic or rock packing that has microorganisms growing on it.

Biotrickling filters.

Wet chemical scrubbers.

As for in-ground biofilters but the media is in an above ground vessel, and has higher treatment capacity for the same media volume.

High rate enclosed biofilters.

Gas is passed through a vessel usually filled with an inert packing. Liquid is recirculated over the packing and odorants removed by reaction with the chemicals in the scrubbing liquid.

Liquid is recirculated over the packing, and the microorganisms break down the odorant.

Description

Technique

Spent hazardous chemical to manage.

High chemical consumption.

Effective and reliable.

Long track record.

Used at some larger STS sites (eg: Melbourne Water’s Hoppers Crossing PS).

For very high and variable gas flows and concentrations.

Low.

Moderate to high capital and O&M costs.

Small footprint.

Complex equipment and control systems.

Generally not suited to highly varying loads. Typically requires secondary treatment step such as activated carbon to achieve low odour concentrations.

Relatively large volume of liquid waste to dispose.

H2S, mercaptans, ammonia.

Effective for high loads.

Low O&M costs.

Little or no chemicals.

Smaller footprint than biofilters.

Moderate capital cost.

Higher capital costs than in-ground biofilters.

Disadvantages

Low hazard level.

High.

Smaller footprint and higher treatment capacity for the same media volume, than inground biofilters.

No chemicals.

Low maintenance.

Advantages

For higher gas flows and high outlet concentrations at pump stations and near to houses.

Low to moderate O&M costs.

Moderate capital cost.

Cost factors

H2S, mercaptans, ammonia, phenol, formaldehyde, methanol, xylene, acetates, ketones, carbon disulfide, organic acids, alcohols.

Moderate.

Frequency of use

Around pump stations and houses.

Applications

H2S, mercaptans, ammonia, phenol, formaldehyde, methanol, xylene, acetates, ketones.

Some odorant(s) treated

Chapter 7 – Treating Odorous Gases

57

58

Gas is passed through a vessel filled with activated alumina impregnated with potassium permanganate.

Activated alumina adsorption.

Gas passed through a fuel fired vessel with air or oxygen at temperatures sufficient to oxidise the odorant.

Gas is passed through a vessel where it mixes with ozone. Odorants are oxidised into less odorous compounds.

Positively and negatively charged oxygen molecules are created and mixed with STS gas.

Thermal oxidation.

Ozonation.

Ionisation (of air).

Odorant is adsorbed onto the media, or reacts with it.

Description

Technique

Trade waste streams or others with high gas concentrations.

Wet wells.

Wet wells and pump station buildings containing odorous gases.

H2S, mercaptans, phenols, amines and a range of other compounds.

H2S, ammonia and other volatile organic compounds.

Similar to carbon systems.

Applications

H2S, mercaptans, and a broad range of other compounds.

H2S, aldehydes and other specific compounds.

Some odorant(s) treated

Very Low.

Low.

Very Low.

Low.

Frequency of use

Low capital and O&M costs.

Low capital and O&M costs.

Very high capital and O&M (energy) costs.

Cost-effectiveness depends on media replacement frequency.

Cost factors

Low capital cost.

(Tube type ozone generators).

Low capital cost.

Effective for wide spectrum of odours and volatile organic compounds.

Effective.

Few moving parts.

Simple.

Advantages

Limited application and performance data is available.

Trials are recommended before use.

Limited application.

Respiratory and other health effects can occur due to exposure to unreacted ozone.

Only economical for high-strength, difficult to treat gas streams.

Applicable to relatively dilute air streams in order to ensure long media life.

Disadvantages

Chapter 7 – Treating Odorous Gases

UV light reacts with moist air to produce highly reactive ions which oxidise components of STS gas.

The odorous gas has a “masking agent” or other vapourised chemical mixed with it.

Ultra Violet Irradiation.

Odour counteractants.

The resulting mixture may appear to be less odorous.

Description

Technique Very Low.

Low.

STP open inlet works.

General “odour”, mercaptans and ammonia.

Frequency of use

Wet wells.

Applications

H2S, ammonia, mercaptans and other compounds.

Some odorant(s) treated

Cost dependent on chemical usage.

Low capital and O&M costs.

Cost factors

Quickly implemented.

Low capital cost.

Low capital cost.

Advantages

Trails recommended before use in all cases.

Detailed performance data lacking.

Counteractants can become source of odour nuisance.

Reliability questionable.

Provide limited odour reduction.

Trials are recommended before use.

Limited application and performance data is available.

Disadvantages

Chapter 7 – Treating Odorous Gases

59

Chapter 7 – Treating Odorous Gases

The Four Main Systems Commonly Used 1.

Activated Carbon Adsorbers

Configuration The most common activated carbon adsorbers or “filters” for odour control applications in an STS are the horizontal bed type where air flow can be directed up or down through the carbon. For large air flows, a dual bed system (Figure 7.1) is used so that pressure losses are minimised by lowering the velocities through the beds. Gas inflow is directed into the middle of the unit and then splits between the top and bottom beds. This enables the flow rate to be doubled without change to the vessel diameter.

Figure 7.1. A typical dual bed activated carbon installation. Air is fed into the units on the right hand side and the treated air out from the top of the vessels.

Carbon Type Activated carbon can be produced from different carbon sources such as coal, wood, peat and coconut shells. When heated to very high temperatures with steam, the carbon chars and opens out somewhat like popcorn creating a large number of small (micropores) to medium sized (mesopores) pores. The pore surface area generated is enormous and typically ranges from 300–1500 square metres per gram. Generally, the greater the surface area, the greater the capacity of the carbon to adsorb odorants. Figure 7.2 shows an example of some granular activated carbon (GAC) pellets.

Figure 7.2. Granular activated carbon pellets.

60

Chapter 7 – Treating Odorous Gases Activated carbon can be modified by impregnating it with chemicals such as sodium hydroxide, potassium hydroxide and potassium iodide. The presence of these chemicals in the activated carbon promotes a chemical reaction with odorants such as H2S and methyl mercaptan thereby allowing the odorant to adhere more strongly to the carbon and increase its adsorption capacity.

Some impregnated carbons undergo an exothermic reaction in the presence of oxygen and can generate heat. This has lead to smoldering and spontaneous combustion in some cases. Although combustion of carbon is unlikely to occur, it is good practice to include a temperature sensor and alarm on the activated carbon unit. The alarm point is typically set around 700C. Gas Flow Rate, Velocity And Pressure Drop Consideration of the gas flow rate and pressure drop is very important when operating an activated carbon system. The system will usually be designed to allow a minimum flow rate of gas to pass through it (e.g. a certain minimum flow rate might be required to properly ventilate an upstream wet well). As gases pass through a bed of fresh carbon there is a drop in the pressure. The size and shape of the granules, degree of compaction of the bed, together with the flow rate will determine the pressure drop through the media. The pressure drop in a bed containing fresh carbon can be determined by pilot testing or using information from the supplier.

Over time, as odorants, moisture or other contaminants are adsorbed into the pores of the carbon, the pressure drop through the carbon bed may increase, with a possible decrease in the flowrate. The pressure drop or flow rate needs to be monitored regularly to ensure that the required minimum gas flow rate is still being achieved.

Adsorption Capacity The life of a carbon bed needs to be estimated before a system is put into place. Some idea of the adsorption capacity of an individual compound onto a particular carbon can be determined from data provided by the supplier. The adsorption capacity of a fresh carbon bed depends on the inlet odorant concentration, the carbon surface area, whether it has had other chemicals added, gas moisture level and the presence or absence of other contaminants. For a particular carbon, adsorption capacity is usually expressed in either: • •

grams of odorant (eg H2S) per volume (eg : millilitre) of carbon, or grams of odorant per 100 grams of carbon (%w/w).

Unimpregnated carbons can have H2S adsorption capacities of around 0.02 g/mL while sodium hydroxide impregnated carbons may be as high as 0.14 g/mL. Note that a volume of carbon can be converted to a weight if the “bulk density” is known for example if a carbon has a bulk density of 0.583 g/mL (= 583 kg/m3 ), then 1 L of carbon would weigh 583 grams.

As there are usually a number of compounds to be removed, an accurate carbon capacity is best determined by laboratory or pilot plant testing with the actual moist gas mixture.

How Much Time Is Required?

Adsorption of an odorant onto activated carbon takes time. Different compounds take different amounts of time.

The Empty Bed Residence Time (EBRT) forms the basis of the design for many carbon filter applications.

The EBRT is based on the volume of the filter without anything in it and calculates how long the gas will take to move through that volume at a particular flow rate. It can be calculated by dividing the volume by the flow rate.

61

Chapter 7 – Treating Odorous Gases EBRT (seconds) = Volume of the filter housing containing the packing (m3)

Air flow rate (m3/sec)

To achieve high removal efficiencies (e.g. in excess of 90-99%) of most odorants, the EBRT should be at least 3.0 seconds. Empty bed residence times greater than 3.0 seconds may be required if multiple gases need to be removed, longer carbon life is required, or if gas concentrations are high. Provided sufficient residence time has been provided, outlet odour concentrations below the level of detection can be achieved (i.e. < 50-100 OU as per AS 4323.3).

How Much Activated Carbon is Needed?

The volume of carbon required can be estimated based on the suppliers stated adsorption capacity for particular pollutants, the required life of the carbon and the mass flow rate of odorant. An example calculation for H2S is provided below.

Example – Determining Activated Carbon Needs Carbon type: Airflow rate V: H2S concentration in gas to be treated C: Required design life: Molecular weight MW of H2S: Volume Factor MV: H2S carbon adsorption capacity, CC: Carbon bulk density D:

Impregnated carbon (sodium hydroxide) 2,000 m3 / hr (0.555 m3 / sec) 10 ppm 1 year 34 grams per mole (from Appendix 2) 0.0224 m3 / mol (constant for most gases) 20% w / w (from supplier) 583 kg / m3

1. Calculate H2S inlet rate (kg/yr) =

=

=

V x (C / 106) x (MW) x (1 / MV) x (1 / 103) 2,000 m3 / hr x (10 ppm / 106) x (34) x (1 / 0.0224m3 / mol) x (1 / 103 g / kg) 0.030 kg/hr = 266 kg / year.

2. Calculate mass of carbon required to last one year:

= 266 kg / ( CC / 100) = 266 / (20 / 100) = 1330 kg

3. Calculate Volume of carbon required using suppliers density factor:

= 1330 kg / 583 kg / m3 = 2.28 m3

4. Check the EBRT:

= volume of carbon / V = 2.28 m3 / 0.555 m3 / sec = 4.1 sec

Where there is more than one odorant, the calculation needs to be repeated for each one and the individual carbon quantities calculated and added together. The final step is to check that the total EBRT is greater than 3.0 seconds. If not, then the volume of carbon needs to be increased.

Carbon Regeneration Once an odorant adsorbs to activated carbon it generally remains attached. This means that over time there are less and less sites available for adsorption of new odorants. The activated carbon becomes less effective and finally inactive!

The main disadvantage of activated carbon is therefore that when subjected to high loadings, the carbon becomes exhausted rapidly, requiring frequent replacement or regeneration. This is required when the outlet odorant concentration exceeds the maximum allowable level that has been set for the system.

62

Chapter 7 – Treating Odorous Gases Once activated carbon can no longer achieve the required outlet concentration (i.e. has become saturated), it will need to be regenerated, reactivated or disposed.

While regeneration of carbon is possible, it is not generally undertaken in Australia because of the large amount required to make it cost effective. It is also worth noting that after regeneration, the carbon only recovers around 10 to 30% of its effectiveness.

Carbon Disposal Although activated carbon itself is inert and does not present any disposal problems, once it becomes loaded with odorants care must be taken with its disposal. At the present time most disposal of spent activated carbon is to landfill.

In the case of carbon used to treat H2S and other sewage gases, acidic by-products build up in the carbon and these may leach out after being disposed to landfills. For this reason, industrial waste sites rather than municipal landfill sites are generally used. An acid leachate test may be required by councils before the disposal method or location is finalised.

2.

Biofilters

Configuration The use of soil bed and compost filters to manage odours can be traced back to the 1950s and 1960s in Europe and the USA. These early biofilters were fairly simple in-ground designs or open concrete wall structures (MMBW 1989). The soil or compost was kept moist and the odorous gas was simply passed through the bed.

In their simplest form, biofilters have an inlet fan, humidifier (optional), pre-filter (optional), air distribution piping, media, spray header or soaker hose supplying water, liner and drainage, as shown in Figure 7.3.

Over the years, a greater understanding has been obtained of the main treatment mechanisms and the operating conditions over which effective treatment performance can be achieved. Biofilters now come in many configurations, from vast in-ground or open vessel filters to fully enclosed high-rate biofilters (Figure 7.4).

Figure 7.3. Schematic of a typical in-ground biofilter system.

63

Chapter 7 – Treating Odorous Gases Figure 7.4. Examples of an open biofilter (left) and a closed biofilter system (right).

Microorganisms Biofilters rely on microorganisms (mainly bacteria and fungi) to break down the odorous gases. Biofilms (thin layers of microorganisms growing on some sort of support material) are maintained in a moist environment.

The odorous gases dissolve in the liquid of the biofilm and then become available to the microorganisms as a source of food to provide energy for growth and reproduction.

When H2S is present, the microorganisms convert it to sulfuric acid. The acid can build up and lower the removal capacity of the filter if neutralising agents are not present, or if the acid cannot be washed out of the media.

Media The type of media used in a biofilter is one of the most important considerations in achieving effective treatment. The media is necessary to support and promote growth of biofilms. Two broad types of media are used; bio-active organic materials and inert materials.

Organic materials such as compost, wood chips, bark, rice hulls and peat have the ability to support a large population of micro-organisms and therefore have higher loading capacities. Indeed organic media may have their own natural microbial population. Organic media however, has a disadvantage in that it is susceptible to decay and compaction over time resulting in a relatively low bed life of around 2–5 years. To overcome this problem inert media has also been used.

Inert media such as perlite, lava rock, activated carbon, glass, ceramic, and polystyrene do not suffer the same degree of degradation as organic media and typically exhibit a longer bed life. These materials have practically no nutrients or micro-organism populations associated with them and generally also support a lower microbial population resulting in lower loading capacities. When used on their own they require inoculation with nutrients and micro-organisms in the form of specially prepared microbial and nutrient mixtures. Activated sludge contains a wide variety of bacteria capable of degrading a multitude of compounds and may be used. However, inoculation with specific cultures, can achieve very high rates and substantially reduce acclimatisation periods.

Inert media have also been used in conjunction with organic media to provide support, stability, and increased porosity of the bed and prevent compaction.

Soils can also be used in biofilters. Soils are classified as sand, clay or loam. Loam is the most suitable for biofiltration because of its high porosity which in turn allows high moisture retention and air circulation (Figure 7.5). Soil may be used on its own or mixed with other organic media to enhance performance. The top surface of soil bed filters is typically covered with grass or other vegetation.

64

Chapter 7 – Treating Odorous Gases Different media will vary in their ability to eliminate odour. If the elimination capacity of the media is exceeded, then breakthrough of odorants will occur. While elimination capacities are reported in the literature, the best way to determine the capacity of media in a specific system is to run pilot plant trials. These would best be carried out in conjunction with a specialist practitioner.

For soil based media, the inorganic mineral content is generally much higher than that of organic media, and therefore they tend to support a lower microbial population. This, together with the relatively low permeability of soil means that EBRT’s are generally much higher, and lower surface loading rates are required.

Figure 7.5. Textural differences between loam (left) and sandy (right) soil types.

Biofilters can also use media with a variety of mixed organic and inorganic components such as rice hulls mixed with compost and lime chips. Above ground high rate enclosed biofilters often use such media to provide better odour removal capability or improved operational life.

Removal Efficiencies Closed, high rate, organic media filters can achieve up to 99% removal of H2S and 95% odour reduction with EBRTs from 30–60 s. Removal efficiencies of 85%, 75% and 90% have also been reported for mercaptans, carbonyl sulfide and dimethyl sulfide, respectively. Removal efficiencies of 98–99% have been reported for easily biodegradable compounds such as aldehydes, organic acids, sulfur dioxide, and nitrous oxides. High efficiencies have also been reported for a number of other compounds: toluene, butanol, methyl acetate, ethyl acetate, butyl acetate, methanol, methylethyl ketone, methane, propane and isobutane.

Soil bed filters can achieve up to 98% removal for H2S and 95% for odour.

Biofilter Sizing As for most gas treatment systems, biofilters are sized to ensure sufficient time exists for the odorants to be removed. For STS gases containing H2S, inlet levels for a biofilter should preferably be kept below 20ppm.

For design, minimum EBRT’s are used and values vary depending on the type of biofilter and media. For inground open biofilters on STS’s, EBRT’s of 30–120 seconds are common, with a usual value of 60 seconds.

For closed, engineered biofilters utilising efficient media, EBRT’s of 30–45 seconds are common.

A simple procedure for determining the size required for a biofilter, based on the media suppliers media characteristics, is provided in the next box.

65

Chapter 7 – Treating Odorous Gases Example – Biofilter Design < 0.1 ppm H2S In The Outlet From The Filter Media type: Media loading rate LR: Media H2S removal capacity RC: Media depth H: Airflow rate Q: Hydrogen sulfide (H2S) concentration C:

Commercial efficient biofilter media = 150 m3 / m2 / hr from media supplier = 99% (see Table 7.3 or as suggested by media supplier) = 1.5m (see Table 7.3 or as suggested by media supplier) = 2,000 m3 / hr (0.555 m3 / sec) = 9 ppm

1. Calculate the biofilter minimum allowable surface area SA:

= Q / LR = 2000 / 150 = 13.3 m2

2. Calculate the biofilter diameter D:

=

(4 x SA / 3.1415) = 4.11 m

3. Calculate the media volume required V: = SA x H = 13.3 x 1.5 = 20 m3 Check the EBRT is satisfactory:

= V / Q = 20 m3 / 0.555 m3 / sec = 36 sec (exceeds 3.0 secs so design is OK)

Final check of outlet H2S concentration: = C - (C x RC) / 100 = 9 - (9 x 99) / 100

= 0.09 ppm (meets design requirement) The other parameter which is used for design of biofilters is “loading rate” which needs to be kept within certain ranges so that the gas velocity and associated pressure drop are not excessive. Loading rates are given in m3/m2/hr (cubic metres of gas per square metre of media per hour). For open biofilters, loading rates for soil media are typically 6–60 m3/m2/hr, and for organic/compost media, typically 35–90 m3/m2/hr. For closed, high rate biofilters with more efficient media, rates of 100–350 m3/(m2/hr can apply.

Operational Considerations Once a biofilter is operating well and the required type of micro-organisms have been established, it is important to control the conditions in the media bed to maintain treatment effectiveness. Large changes in conditions that effect the biological process should be avoided.

Temperature Temperature has a large impact on the efficiency of operation of biofilters. As a general rule, biofilters operate best at between 50C and 450C, and in this range, for every 100C increase, the biological activity is generally increased by a factor of two.

Biodegradation reactions carried out by the microorganisms generate heat and this will increase the bed temperature (typically by between 2–100C). If the bed temperatures are too high, they need to be reduced.

This can be achieved by adding additional cooler ambient air, or by passing the inlet air through water sprays to humidify and cool the gas stream. Humidification prior to the biofilter can significantly increase biofiltration costs.

Moisture Moisture control is critical because microbial degradation of pollutants does not occur in a dry environment.

Drying out of the bed can also lead to shrinkage, and results in air flow channeling thereby reducing the efficiency of the biofilter. Organic media are generally hydrophobic and, once dry, are difficult to re-wet, while inert media is easier to rewet and less prone to shrinkage and swelling.

The use of covered biofilters is preferred since the moisture level in the media can be controlled to ensure that it is neither too high nor too low.

66

Chapter 7 – Treating Odorous Gases The media moisture content (see Table 7.3) for a biofilter is typically measured using moisture probes placed in the media or by laboratory analysis of samples taken from the bed.

To help maintain the correct bed moisture levels, the incoming air should be saturated (>99% relative humidity) so that the air passing through the biofilter does not absorb water from the media. In practice, achievement of 99% relative humidity may be difficult in some climates. Even with pre-humidification, through the use of an upstream spray chamber, supplementary water may have to be added by using sprinklers on the surface of the media or soaker hoses within the media itself. Water needs to be added carefully. If the flow rate is too high, nutrients and the microorganisms themselves may be washed out of the bed. If the volume of water applied is too high it may cause swelling of organic media and therefore reduce the porosity of the media. This in turn will lower the odorant removal capacity of the filter. In worst cases, the bed can become anaerobic and in fact generate odour rather than remove it.

Water spray requirements for biofilters need to be reviewed and adjusted over the four seasons of the year.

As a rough guide, for closed biofilters without pre-humidification, water sprays should initially be set up to spray water droplets over the whole surface of the media once per day. The duration should be adjusted to allow wetting of the media with minimal outflow in the biofilter drainage system after the sprays are turned off.

Air Flow Distribution Biofilters can be designed with air flowing either up or down through the media. In both cases, efficient operation requires uniform loading rates across the entire media surface. This is necessary to ensure sufficient contact time between pollutants and the media.

For downward flow designs, even air flow distribution is usually achieved by maintaining a suitable height between the media surface and the roof of the enclosure.

For up-flow designs such as in-ground biofilters, some sort of distribution network is provided below the media. This may take the form of perforated plastic piping surrounded by coarse gravel or a plenum floor design with mesh over the top (Figure 7.6).

Smoke tests can be used to assess whether the air flow distribution is even or not. Smoke is introduced with the inflowing air and the pattern of distribution is observed as it leaves the media.

Installation of a gas flow rate meter, or periodic checks of gas flow rate are recommended to ensure that flows are kept within design requirements.

Figure 7.6. Examples of a piped air distribution system (left) and a plenum arrangement (right).

67

Chapter 7 – Treating Odorous Gases pH Microbial activity occurs best over a fairly limited range of pH, typically between 4 and 8. Since biodegradation of sulfur and nitrogen based compounds results in the formation of acidic products such as sulfuric and nitric acids, the pH will decrease and inhibit most bacteria. A few bacteria such as Thiobacillus are able to grow under very acidic conditions between pH 1 to 4. This is in fact an advantage if H2S is the predominant pollutant to be removed, however where a mixture of odorous compounds require treatment, then biodegradation of the other compunds may be limited in such acidic conditions. The acid conditions may also lead to accelerated degradation of the biofilter media itself.

In general, biofilter beds should be operated at neutral pH. Compost tends to have pH <5, while the pH of some activated carbons occasionally used in biofilters can have a pH >9. If these materials are to be used they need to be neutralised first. This can be done by mixing lime, crushed limestone or sea-shells with an acidic media, or pre-washing an alkaline media with a weak acid to lower the pH within a bed.

Any excess moisture draining from the filter will tend to be quite acidic and require treatment prior to disposal.

For this reason most soil bed or compost biofilters are designed with clay or HDPE liners to allow easy collection of the drainage water.

3.

Wet Chemical Scrubbers

Configuration In wet chemical scrubbers the gas to be treated is brought into close contact with a chemical scrubbing solution. The odorants in the gas dissolve into the scrubbing solution where they are converted to odourless compounds. Figure 7.7 shows a typical schematic of a chemical scrubbing system.

Figure 7.7. Schematic of a typical chemical scrubber system (with optional Odorgard® catalyst system shown).

Optional ®

68

Chapter 7 – Treating Odorous Gases The scrubbing solution is recirculated continuously from a sump at the bottom of the scrubber to spray headers located above the packing. The packing is designed to maximise the air to liquid surface area for interaction between the scrubbing solution and the gas stream. Figure 7.8 shows an example of some common packing types.

Figure 7.8. Examples of packing types (Tellerettes® on left and Q-Pac ® on right).

For the removal of H2S and other sulfur based compounds, a mixture of sodium hypochlorite (NaOCl) and sodium hydroxide (NaOH) is typically used. In this case, the H2S is completely converted to sodium chloride, sodium sulfate and water.

For the removal of alkaline compounds such as ammonia, scrubbing with an acid such as sulfuric acid (H2SO4) is generally employed. The ammonia is converted to ammonium sulfate.

Chemical scrubbers can be vertical counter current or horizontal cross flow design (Figure 7.9). In the vertical countercurrent arrangement, the gas flows in the opposite direction to the scrubbing liquid. The vertical counter current type is generally more efficient, less prone to leakage around the packing and most cost effective overall, however, they can be relatively tall. Where height restrictions apply, horizontal types may be more practical. Multiple vertical scrubbers in series can also be used where height restrictions apply or where multiple chemical streams are required for treatment (e.g. acid scrubbing followed by NaOCl scrubbing with a final NaOH wash to remove entrained chlorine).

Horizontal units do offer a number of benefits such as lower pressure drop, lower recirculation rates and they are less susceptible to blockages.

Figure 7.9. Vertical (left) and horizontal (right) chemical scrubbing units.

69

Chapter 7 – Treating Odorous Gases Spray nozzles or weir troughs are used to distribute the scrubbing liquid evenly over the surface of the packing (Figure 7.10). Sprays achieve a more even coverage however, there is a potential for blockage of the spray nozzles.

Weir troughs work by distributing the scrubbing liquid into a series of troughs which overflow and allow the liquid to pass over the packing material. While these are less likely to block than sprays, they need to be well designed and assembled to ensure complete coverage of liquid over the packing.

Figure 7.10. Example of weir trough type (left), and spray nozzles (right).

Distribution of the scrubbing liquid also generates a mist in the scrubber. The liquid in the mist needs to be recaptured and returned to the scrubber. Mist eliminators (otherwise known as entrainment separators) are situated above the sprays (Figure 7.7) or weirs to achieve this. Two examples are shown in Figure 7.11.

Figure 7.11. Mesh type entrainment separator (left), and Chevron blade type (right).

To improve the removal of non-H2S based pollutants such as mercaptans, dimethyl sulfide, dimethyl disulfide and carbon disulfide, a catalyst may be employed within the scrubber recirculation line (e.g. Odorgard® process) of hypochlorite scrubbing systems (see Figure 7.7). The treatment process must be carefully controlled to prevent the formation of elemental sulfur which can collect on the catalyst and limit its effectiveness.

In the process of scrubbing, some of the chemical in the scrubbing liquid is used up and salts are produced. This will limit the performance efficiency of the scrubber. Therefore it is necessary to continuously or periodically waste (blow down) a small proportion of the scrubbing solution. Wasting is normally set up to limit the concentration of sodium chloride (NaCl) and sodium sulfate (Na2SO4) formed during the scrubbing process to <5%. Waste scrubbing solutions are normally suitable to be disposed of to sewer but local regulations should be checked.

70

Chapter 7 – Treating Odorous Gases 4.

Biotrickling Filters

Configuration Wet chemical scrubbers have largely been replaced by bioscrubbers and biotrickling filters in recent years as the main method for treating gas streams with high concentrations of H2S (>20 ppm) and odour. Biotrickling filters are similar to biofilters except that biotrickling filters have a recirculating liquid stream and usually require an external source of nutrients.

For STS’s, fixed film biotrickling filters (Figure 7.12) use microorganisms to degrade the odorants in gases passed through the vessel. A fixed packing with a large surface area is provided for the microorganisms to grow on. Water is continuously circulated over the packing. Odorous gases dissolve in the recirculating water and are transferred to the biofilm where they are converted to odourless compounds. The recirculating liquid also helps take away waste products.

Figure 7.12. Diagrammatic representation of a fixed film biotrickling filter.

As with wet chemical scrubbers there are both vertical and horizontal systems. Figure 7.13 shows examples of a horizontal and vertical flow biotrickling filter.

Figure 7.13. Examples of horizontal flow (left) and vertical flow biotrickling filter installations.

Gas Flow

Gas Flow

71

Chapter 7 – Treating Odorous Gases Packing Types A number of packing materials have been used in biotrickling filters such as lava rock, pall rings (a variety of ring shaped structures used originally in the petrochemical industry), structured plastic or stainless steel packings, activated carbon based packings and open-pore polyurethane foam (Figure 7.14). Whilst lava rock has been used in many installations, recent use of polyurethane foam has shown excellent performance which is due to the relatively high surface area (approx. 600 m2/m3) and low pressure drop compared to other packings. One disadvantage of foam however, is that it may compact over time.

Figure 7.14.

Examples of biotrickling filter packing: polyurethane foam cubes (by M+W Zander) and structured plastic packing (by Lantec).

Microorganisms Since the packing media is inert, it initially needs to be inoculated with pollutant-specific micro-organisms to encourage the growth of an efficient biofilm. Activated sludge has a wide variety of bacteria capable of degrading a diverse range of compounds and it is generally sufficient for easily biodegradable compounds or waste gas mixtures. However, selection of the inoculum becomes increasingly important for compounds that are harder to degrade. This may be achieved through the use of pure cultures, either micro-organism samples that are enriched in the laboratory, or samples from other biotrickling filters treating similar wastes.

Use of the appropriate inoculum for the pollutants to be degraded rather than simply allowing micro-organisms to adapt from an activated sludge could dramatically decrease the time it takes for high removal efficiencies to be achieved. Operating experience for biotrickling filters at several sewerage treatment plants has shown that acclimatisation of the bacteria inoculated onto the packing usually takes 2–6 weeks.

Bacteria in the biofilm are living organisms and need to be looked after. They require oxygen and need to be protected from high concentrations of the pollutants, and also any toxic byproducts. This can be managed by ensuring the recirculation rate of the liquid phases is high enough to deliver sufficient oxygen and remove any toxic byproducts.

If the recirculation rate is too low, oxygen levels may decrease and toxic byproducts may accumulate. If the rate is too high, the thickness of the water film over the biofilm may increase and lead to reduced performance.

Nutrients While the pollutant is generally the source of energy for the bacteria, the bacteria also require small amounts of nitrogen, sulfur, phosphorus, potassium and magnesium, together with other trace elements. Therefore for biotrickling filters, an external nutrient supply is required in order to provide these elements in excess and ensure effective biodegradation.

72

Chapter 7 – Treating Odorous Gases Nutrient dosing is typically achieved through the use of a small storage tank and dosing pump, where potable water is used as the water source for the biotrickling filter. Secondary treated sewage which contains sufficient nutrients can also be used as the main water and nutrient source. pH The pH can have a marked effect on the removal of odorants. Control of pH is easily achieved by using a pH control system for the recirculating liquid by adding acids or alkalis. Reaction processes that result in the formation of acids or alkali therefore do not create the same problems as in biofilters where pH control throughout the bed is more difficult. In addition, removal of specific compounds may require different pH levels. Removal of VOC’s is usually carried out at neutral pH whereas H2S removal can be carried out at a lower range of pH between 1–6. Where reduction of a number of compounds is required to achieve high odour removal rates, a compromise may need to be found with respect to the normal operating pH. Alternatively, multiple packing sections (either combined or separate), each operating at a different pH, may be necessary.

As with wet chemical scrubbers, regular wasting (blowdown) of the liquid is necessary to control reaction by-products, replenish fresh water supplies and control pH. Since the pH of the wasted liquid may be low, disposal should be carefully considered. The volumes are normally quite small and are disposed of to sewer, but local regulations should be checked.

Removal Efficiencies Biotrickling filters achieve very high removal efficiencies for hydrogen sulfide (>99%), and mercaptans (>75%). Reasonable efficiencies are also achieved for other sulfur based organic compounds. Relatively high odour removal efficiencies can also be achieved (>90%) depending on the characteristics of the inlet gas stream.

Table 7.2 provides some examples of removal efficiencies for H2S and odour at some full-scale biotrickling filter installations at STS pump stations.

Table 7.2. Summary of removal efficiencies recorded for full scale biotrickling filter installations.

Application

Inlet concentration/range

Average removal efficiency (%)

19,000 OU

90

8ppm

99.5

23,000 OU

90

11ppm

99.8

400 to 500ppm

>99.8

200 to 500ppm

>99.5

1. Pump Station, Queensland.

Odour.

Hydrogen sulfide.

2. Pump station, Queensland.

Odour.

Hydrogen sulfide.

3. Pump Station, Florida, USA .

Hydrogen sulfide.

4. Pump Station, USA. Hydrogen sulfide.

Summary Of Typical Design And Operational Characteristics Table 7.3 provides a summary for the four treatment systems most commonly used on STS’s for removal of odorants in gases.

73

74

>100 m3/hr.

High rate: >99% for H2S. Organic/compost biofilter 99% for H2S, 98% for odour.

Soil bed biofilters: 98% for H2S;

>20 m3/hr.

>99% for H2S.

(depends on contact times).

>98% for odour.

Inlet gas concentrations.

Gas flow rates treated.

Activated carbon granules or pellets. Sometimes impregnated with other additives such as NaOH or KMnO4.

High surface area, high adsorption capacity, low pressure drop.

Depends mainly on volume of carbon and inlet odorant level.

Usually to approved landfill.

Media /packing type.

Desirable media characteristics.

Media replacement.

Media disposal.

Usually to approved landfill.

90 to 95% for odour.

Various mixtures of soil, compost, peat, heather, bark, wood chips, rice hulls plus additives such as limestone chips.

Media able to support a diverse population of micro-organisms.

Has good load bearing capacity and will not compress. Exhibits a low pressure drop. Provides for pH buffering, and exhibits limited biodegradation.

2 to 5 years. Shorter times when exit gas odorant level too high, or clogging causes low flows.

Average H2S level <20 ppm for soil and organic/compost systems. Can handle some variation in inlet loads.

Average H2S level <10 ppm for long bed life. Good for variable inlet loads.

Typical performance.

Biofilters

Activated carbon

Parameter

Usually to approved landfill.

Usually to approved landfill.

Generally 5 to 10 years.

Open, corrosion proof.

Open, inert media not likely to compact compared to media in biofilters.

>10 years. Clogging should not be an issue.

Tellerettes, pall rings, ceramics, other structured plastics.

90 to 95% for odour.

>99% for H2S.

>100 m3/hr.

For most levels found in STS’s, and for high levels above 500ppm. Good for variable inlet loads.

Wet chemical scrubbers

Lava rock, pall rings, structured plastic or stainless steel, activated carbon, and open-pore proprietary polyurethane foam.

90 to 95% for odour.

>99% for H2S.

>100 m3/hr.

Average H2S level <500 ppm.

1,000 to 50,000 OU. Not suitable for highly variable inlet loads.

Biotrickling filters (BTF’s)

Table 7.3. Summary of typical design and operational information for the four most common gas treatment systems.

Chapter 7 – Treating Odorous Gases

Upflow or downflow. In ground or above ground vessel.

Required. Important to take away excess water.

>3 seconds.

Usually 0.9 m.

Upflow or downflow.

Drainage of treatment “vessel”.

Contact time (EBRT).

Bed depth .

Usually not a factor.

Not required.

Not required.

Nutrient dosing.

pH control system / buffering.

May clog the media.

Required otherwise carbon pore blockage occurs.

Not required, but may grow on carbon and enhance performance.

Oxygen.

Microrganisms.

Particulates pre-treatment.

Single or dual bed.

Usually 0.5 to 1.5 m.

0.6 to 0.9 kPa/m of bed.

Pressure Drop (when new).

Bed configuration.

Best between 5 to 45°C. Variation must be minimised to protect microorganisms present..

T > 40°C limits adsorption capacity.

Generally not a problem with sewer applications.

Gas Temperature requirements.

Required.

Required. Neutral pH needed for most organics but some systems treating H2S can operate at lower pH’s of 1 to 6.

Not normally required.

Neutral pH should be aimed for but is not as easy to achieve as for BTF’s. For H2S, alkali in the form of lime chips may may need to be added to the media prior to installing.

Required via air.

Required. Odorant must be continually fed to keep desirable microbes alive.

Required. Odorant must be continually fed to keep desirable microbes alive.

Required via air.

Not required.

Desirable.

Horizontal or vertical.

Usually required.

Not required.

Not required.

Not required.

Not required.

Single or multiple vessels, usually with gas and liquid in countercurrent flow.

Single or multiple vessels.

1 to 3 seconds.

Usually 1.5 to 3.0 m.

10 to 15 seconds, (hence BTF’s are often smaller than biofilters).

30 seconds to 8 minutes (large range depending on media type).

Blowdown or wasting required to take away built up reaction products.

0.6 to 1.5 kPa.

Depends on specific situation.

Not an issue with STS gases.

Wet chemical scrubbers

Usually 1 to 5 m.

Blowdown or wasting required to take away built up reaction products.

0.3 to 0.5 kPa/m of bed.

Best between 5 to 45°C. Variation must be minimised to protect microorganisms present.

Biotrickling filters (BTF’s)

Required. Important to take away excess water and reaction products.

Up to 50 kPa for soil bed filters.

0.1 to 0.5 kPa/m of bed.

Biofilters

Activated carbon

Parameter

Chapter 7 – Treating Odorous Gases

75

76

Must have moisture addition system. Pre- humidifier and water sprays suggested. Best if media contains 20 to 40% moisture content, and is never allowed to dry out.

Essential.

Care required when handling dry media. Bacteria present in wet media.

Good for low attendance sites.

Simple operation.

Slight earthy odour in exit gas.

Acclimatisation period required, but less than a BTF. Lower gas flows and higher pressure drops than for a biotrickling filter. Media replacement required.

Pre-treatment required to remove moisture which might clog carbon pores. Prefilter; fabric type for low humidity gas streams, plastic mesh pad for high humidity conditions.

Dilute high relative humidity gas with low humidity air. Increasing gas temperature by 3 to 5°C will help lower the relative humidity of the gas.

Not applicable.

Essential.

Care required when handling dry media. Dust can be explosive and an irritant.

Good for unattended or low attendance sites.

Frequent media replacement if odorant levels are high. Higher temperatures can cause release of odorants from the carbon. Water vapour, particulates and grease may blind carbon therefore pretreatment is necessary.

Inlet air and media moisture control.

Liquid Recycling.

Good air flow distribution.

Safety.

Operability.

Limitations.

Not used.

Biofilters

Activated carbon

Parameter

Acclimatisation period required at startup, typically 2 to 6 weeks before full performance is achieved.

Safety is more of an issue.

Multiple stages needed if many odorants present.

More complex than other systems due to additional equipment and operating and control procedures. Requires more frequent site attendance.

More complicated start up and operation than biofilters, but more controllable treatment results.

Relatively low site attendance.

Requires careful attention with chemical handling.

Essential.

Required.

Not required.

Wet chemical scrubbers

Bacteria present in circulating liquid.

Essential.

Required.

Not required.

Biotrickling filters (BTF’s)

Chapter 7 – Treating Odorous Gases

Chapter 7 – Treating Odorous Gases

Other Gas Treatment Systems There are other gas treatment systems but these are not common in Australia. Additional comments are provided below on a variety of systems.

Activated Alumina Activated alumina systems have many similarities to activated carbon systems. They must be kept dry as the additive can be easily washed from the alumina.

Thermal Oxidisers/Flares Thermal oxidation is the process which uses a fuel such as natural gas burning in air at high temperature (usually above 8000C) to oxidise odorous compounds present. Thermal oxidisers achieve removal efficiencies of around 99% for a broad range of odorous compounds and can be operated on a continuous or intermittent basis. They are particularly useful for high concentration gas streams that may have significantly varying gas loads and compositions and which contain compounds that are unsuitable for absorption systems or biological degradation. Examples could include high concentrations of aldehydes or ketones.

Examples of both open flame and enclosed flare systems are provided in Figure 7.15. The capital and operating costs for treating STS gases is high and therefore this treatment method is not commonly used on STS’s.

Figure 7.15.

Simple ‘open flame’ gas flare (left) and enclosed ground flare (right).

Ozonation Ozone has a relatively long history of treating odours from municipal and industrial facilities overseas although its use in Australia has been limited. Ozone has been proven effective in the treatment of compounds such as H2S, mercaptans, amines and phenols.

In modern systems, air is passed through an ozone generator and then pumped into areas requiring treatment (e.g. pump station wet well) allowing the ozone to mix and oxidise the pollutant gas stream.

Ozone is hazardous and care must be taken around any site using ozone to ensure that personnel are not exposed to unsafe levels.

77

Chapter 7 – Treating Odorous Gases Ionisation Air is treated through special ion tubes and strong electromagnetic fields to produce positively and negatively charged oxygen molecules that treat H2S, ammonia, Volatile Organic Compounds (VOCs) and other toxic gases.

These systems have been used in some European countries for the past twenty years, mostly in treatment of air in large commercial buildings. This technology has been applied to sewerage treatment systems within Australia to a limited degree with mixed success.

Ultra Violet Irradiation In the presence of UV light, oxygen and water vapour molecules break down to form hydroxyl free radicals.

These short-lived free radicals then oxidise odorous molecules such as H2S, ammonia and mercaptans to simple non odorous compounds such as carbon dioxide and water with the associated production of oxygen, trace ozone, nitrogen, sulfur, chlorine and weak mineral acids.

This technology has also been applied to sewerage treatment systems within Australia to a limited degree.

Odour Counteractants Two main types of odour counteractants are commercially available: •

masking agents, which by the nature of their own odour, mask the STS odour when the two are mixed, and

•

“diffuse” vapour treatment systems which involve spraying fine vapour droplets of essential oils or other chemicals into the odorous air, with the odorous compounds absorbing into the chemical droplets and being removed.

Neither system uses a conventional closed vessel such as those used in a wet scrubber.

Masking agents have been used extensively over many years to disguise the odours from sewerage treatment facilities and sewage transport systems. This is because they can be implemented immediately once a nuisance has been identified, and have very low equipment set-up costs and relatively low ongoing chemical supply costs. However, they provide limited odour reduction and cannot be relied upon to prevent off-site odour nuisance. In some cases, they themselves can become an odour nuisance due to the nature of the chemical or due to excessive dosing levels. These chemicals should be avoided if possible as they do not provide an effective means of odour abatement.

For diffuse vapour treatment systems, the efficiency of the removal process, the final form and fate of the odorant, and the overall odour removal capability is unknown due to a lack of published performance data.

Pilot performance testing should be carried out on any system being considered.

Cost Comparisons The overall capital and operating costs for biological treatment and activated carbon adsorption are generally low compared with other proven treatment technologies such as wet chemical scrubbing and thermal combustion (see Table 7.1).

78

Chapter 7 – Treating Odorous Gases

Basic Operational Checks For Gas Treatment Systems Some minimum suggestions for checking of faults which might effect performance for each of the four main gas treatment systems are provided in Table 7.4.

Table 7.4.

Operational checks for gas treatment systems.

System

1. Activated Carbon.

Check w=weekly, Fan is operating, drain moisture and ensure grease/particulate pretreatment filters are clean.

Possible Remedial Action Required

w

• re-start system • replace or wash out grease/particle filter

m

• replace grease/particulate filters • clean or adjust dampers

m

• replace media • full system check

w

• re-start system • replace or clean grease/particulate filters

m

• replace media • adjust dampers • adjust water sprays/timing and flow

Media moisture level and drainage pH against specification.

3m

• adjust water sprays/timing and flow • replace media if pH outside specification

Inlet and outlet H2S levels (and odour levels if specified) against specification.

m

• replace media • full system check

Gas flow and pressure drop against range specification.

2. Biofilter.

Suggested Frequency m=monthly

Inlet and outlet H2S levels (and odour levels if specified) against specification.

Fan is operating and pre-humidifier and water sprays are controlling to correct moisture levels in the media (inspect).

Gas flow and pressure drop against range specification.

79

Chapter 7 – Treating Odorous Gases System

3. Biotrickling Filter.

4. Chemical Scrubber.

80

Check w=weekly, Fan is operating, recirculation and dosing pumps running, water level, and spray operation against specification.

Gas flow, pressure drop, liquid flow and pH/ conductivity against range specification.

Suggested Frequency m=monthly

Possible Remedial Action Required

w

• re-start system • wash out pump strainers

m

• adjust dampers • check nutrient supply • check/replace media

3m

• calibrate pH probes

m

• full system check

w

• re-start system • wash out pump strainers • unblock sprays

m

• adjust dampers • adjust/clean water sprays/ flow

m

• replace anode or cathode if required

Check pH/ORP meters against specification.

m

• calibrate pH/ORB probes

Inlet and outlet H2S levels (and odour levels if specified) against specification.

m

• full system check

Check pH reading, check nutrient and blowdown system.

Inlet and outlet H2S levels (and odour levels if specified) against specification.

Fan is operating, recirculation and dosing pumps running, water and chemical level, and spray operation against specification.

Gas flow and pressure drop against range specification.

Check electrolyser (if fitted) volts and amps against specification.

Chapter 8 Solving Odour Problems – A Practical Procedure Self Management Of Odour Control Operators of STS’s who are confronted with odour control problems could easily evaluate the problem themselves using the procedure suggested in this chapter. Often the operator who knows the characteristics of the system will have the best chance to evaluate the causes and assist with the identification of possible solutions. Remember that odour problems in STS’s are common to most systems across Australia, and the solutions may not be difficult to implement.

A summary of the suggested step by step process for resolving odour issues, from the time of receiving an odour complaint is provided in Figure 8.1.

Figure 8.1. A step by step process for resolving odour issues.

A. DEFINE THE PROBLEM FIRST 1. Know your system layout, flows, and quality

2. Is the problem real?

3. Identify possible STS site(s) responsible

4. Carry out quantitative on-site tests.

5. Predict downwind concentrations for existing and required situation.

6. Produce problem definition statement and required treatment level.

B. DETERMINE AND IMPLEMENT THE BEST SOLUTION

7. Are there any appropriate nontreatment solutions available?

8. Define and assess gas and sewage treatment solutions available, carry out a cost benefit and suitability analysis and choose treatment system(s).

9. Install the best system(s) conduct proof of performance tests, and obtain feedback.

Define The Problem – Steps 1 to 6 Step 1.

Gather System Information

Prior to commencing any odour assessment work, the details of the STS in question must be collected.

These should include: • • • • • •

plans of the sewer network details of the rising main, including pipe diameters, lengths and retention times details of pump stations and wet wells average daily flows for the pipework in question sewage quality data if available, including pH, temperature and soluble sulfide levels details of any major trade wastes (flows and quality).

81

Chapter 8 – Solving Odour Problems – A Practical Procedure Step 2.

Determine If The Problem Is Real

Since odour is to some extent subjective, the next step is to see if the problem is real. Don’t assume anything at this stage.

When meeting with complainants or their neighbours for the first time, it might be useful to hand out some general information about sewerage systems and their possible odours (see Appendix 3; Sewage Odours Information Sheet).

Start by documenting any existing odour complaints. For each odour complaint, an Odour Complaint Record Sheet similar to the example shown in Appendix 4, should be completed.

The sheet is designed to obtain information under the five headings of: • •

• • •

Who are the complainants (names and addresses)?

What issues are they complaining about? Get a description in their words. Ask them to describe the odour strength as “low”, “medium”, or “high”, and as shown in Table 2.1, try to get them to characterise the odour using terms such as: o like rotten eggs o like cabbage o pungent o fishy o fruity o other When are the odours noticed? o times of day o wind speed and direction Where are the odours occurring?

Why are the odours occurring (in the opinion of the complainants)?

Next, install a low range H2S monitor(s) (see Chapter 3) in the area(s) where complaints are being made.

Collect data for a minimum of one week or over several reported odour events, and then see if elevated H2S levels exist.

Try to correlate H2S levels and complaints, but if this is unsuccessful, or if other compounds are suspected, carry out air sampling and odour panel analysis to measure general odour as described in Chapter 3.

Step 3.

Identify Possible STS Site(s) Responsible

In defining whether the problem is real in Step 2, a number of possible sites, times of day, and weather conditions may have been identified in the STS. Monitoring for H2S or odour or specific odorants at the times already identified in Step 2 should now be carried out at these sites.

Measurements carried out at this stage do not need to be continuous or exhaustive. A simple comparison with data obtained from the general ambient air monitoring should reveal whether the odour problem is related to the particular site.

In addition to the initial monitoring results, confirm that the particular STS site(s) is the problem by: • •

82

estimating sulfide levels which should apply at the site using the procedures discussed in Chapter 2, and confirming any trade waste or trace odorous compounds (measured in a NATA accredited laboratory) which are contributing. This may involve a search of trade waste inputs to the particular STS site, and attempts to identify which one(s) are contributing the compounds found.

Chapter 8 – Solving Odour Problems – A Practical Procedure Step 4.

Carry Out Quantitative On Site Tests

Once it is apparent that the STS is the most likely cause of the odours, it is important to obtain some quantitative monitoring results for later use in helping to identify the best solution to the problem. This should include: •

•

the minimum, average and worst case levels of H2S (or odorant) in the gas and liquid phases, over a period of at least one week during normal, and preferably worst case, operation. This should involve the use of continuous monitors (for H2S), or frequent sampling programs for other identified odorants. Note that gas concentrations will depend on the ventilation rate. If a fan is to be fitted in future, then allowance for the additional dilution air must be made to the gas concentrations. The effect of the extra air can be determined by installing a temporary ventilation fan and extracting the air at a rate which is as close as possible to the final gas flow rate. the minimum, average and worst case gas flow rates. If possible, actual flow rate tests should be carried out during minimum, average and peak daily flow periods using anemometers as described in Chapter 3. For pump stations, if flow testing cannot be undertaken, an estimate of gas emission rates (“piston effect” see Chapter 4) is required. These are based on wet well filling rates. Wet well filling rates can be obtained by knowing the wet well volume between pump start and stop levels, and the time it takes to fill from pump stop start timing records. This information is still required even if a fan is to be fitted in future.

Step 5.

Predict Downwind Concentrations Of Odorant For Existing And Required Situations

Based on the data collected in the previous steps, the cause of the problem should be able to be identified as being due to: • • •

“odour” from a mixture of compounds and/or H2S and/or specific trace odorous compounds.

For the selected parameter of odour units, H2S, or other trace odorant, multiply the worst case concentration determined in Step 4, with the worst case associated existing gas flow rate, to produce a mass emission rate for input to an appropriate air emissions model as discussed in Chapter 5.

Using qualified consultants, determine the downwind concentration from the model, and compare with appropriate local ambient air quality standards (see Chapter 5).

If the emission rate is too high, the next step is to: 1.

2.

Re-assess the allowable gas flow rates (allowing for possible use of a fan).

Determine the new maximum allowable odorant mass emission rate, and minimum vent height which must be used in future.

Step 6.

Produce A Problem Definition Statement

Summarise the results of the investigations suggested above in a problem definition statement (see later worked example). Include in the statement what levels of the odorants are considered to be acceptable and what level of treatment is required.

83

Chapter 8 – Solving Odour Problems – A Practical Procedure

Determine and Implement the Best Solution – Steps 7 to 9 The best STS odour control solution must now be selected to allow the required level of treatment defined in Step 5 to be achieved. Reference should be made to Chapters 6 and 7 for detailed discussion on particular treatment methods.

Step 7.

Decide If There Are Any Appropriate Non-Treatment Related Solutions

A list of some possible ways to meet the odour reduction requirements without the need for specific treatment is provided in Table 8.1. Each possibility should be considered fully on a case by case basis.

Table 8.1.

Some possible non treatment odour control solutions.

Option

Description

Comments

1. Eliminate the odorous gas at the site.

Shut down, relocate or enclose the STS component. (e.g. Extend a rising main containing high sulfide levels to a different location if possible).

Air valves and vents should only be shut if they are not actually needed to provide a venting function, or if gas build up or corrosion will not occur as a result.

2. Reduce or eliminate direct inputs of odorous chemicals to the sewer.

Reduce or eliminate input of chemicals that may give rise to the generation of odorous compounds (e.g. sulfate).

Require trade waste customers to reduce or eliminate odorous compounds, or compounds which may be converted to odorous compounds in the STS.

3. Provide mechanical ventilation with an appropriate vent (see Step 5 for vent height determination).

Install fan and vent stack with appropriate height.

4. Convert pump station wet well ventilation systems to induced air type.

See Chapter 5.

5. Convert part full pipelines to flooded/full flow.

Install a control valve at the downstream end of the pipeline to keep the pipe full at all times so that air is not entering and leaving the pipe via air valves or vents.

Not applicable to gravity sewers but may work with pipe work downstream of a rising main if other factors such as solids transport velocities, and pipe ratings are considered.

May worsen sulfides at end of pipeline.

Lower the pump cycle volume (e.g: lower the pump cut in level).

Overnight flows often remain in the wet well for too long and will cause higher odours from sewage from the first morning pump start.

6. Reduce retention times in wet wells.

84

Could apply at older pump stations, and many existing natural draft systems.

Chapter 8 – Solving Odour Problems – A Practical Procedure Option

7. Convert pump stations to continuous pumping using variable speed drives and appropriate flow control regime.

Description

Can be considered for larger pump stations.

8. Reduce turbulence in wet wells.

Modify the wet well inlet flow structure to ensure flows from odorous inputs (eg: pressure sewers) are submerged at all times.

Eliminate weirs and use low drop structures where possible.

9. Reduce the potential for odour generation in wet wells.

Keep solids from building up on well floors and walls, by regular cleaning.

Run well levels as low as possible.

10. Reduce the average daily retention times in pipelines.

Divert flows from other parts of the STS to increase flows in the problem areas. Add flushing water if available and acceptable. Consider use of recycled water.

11. Mimimise diurnal flow variations.

Comments Switching from intermittent to continuous pumping requires careful re-design.

For intermittent stations, the sewage containing the solids is almost completely pumped out of the wet well. Additional changes to floor profiles may be required to ensure solids do not build up in the fixed pool of water required for continuous stations.

May need a modified or new inlet pipe.

Where possible ensure extra inflows do not occur during the domestic peaks. This can be done by altering pumping regimes or holding up trade waste inputs on site until suitable times.

Divert flows from other parts of the STS. Add flushing water if available and acceptable.

Step 8.

Consider Sewage And/Or Gas Treatment Options, And Choose Best System(s) Where the interventions described in the previous section are impractical or do not achieve the required outcomes, the only option is to consider a treatment system. Treatment systems involve either treating the liquid phase in an attempt to prevent or reduce odours, or treating the gas phase to remove odorous gases, or both (see Chapters 6 and 7).

85

Chapter 8 – Solving Odour Problems – A Practical Procedure Some general guidance on how to decide whether liquid or gas phase treatment, or both, are required is provided in table 8.2.

Table 8.2. Situations requiring liquid or gas phase treatment, or both.

Situation Suggested Treatment For Further Consideration Liquid Phase Gas Phase Y= “usually best option” M=”may also be required” 1. Pump station odours, if M fed from a single Y If very close to houses odorous inlet pipeline. etc.

2. Pump station odours, if fed from multiple Y odorous inlet pipelines.

3. Odour from single air Y valve or vent.

4. Odour from multiple M air valves/vents on Y If very close to houses same pipeline. etc.

5. Odours from pipeline M where corrosion is Y If very close to houses occurring. etc.

6. Odours from the inlet works of a sewage treatment plant (for a single inlet pipeline).

Y

7. Odours from the inlet works of a sewage treatment plant (for multiple inlet pipelines).

8. Odours from STS very close to houses.

May be too costly to treat multiple pipelines.

Long term corrosion costs should be considered.

M

Y

M

Comments

May be too costly to treat multiple pipelines.

Y

For liquid treatment, a short term chemical dosing trial often provides the operator of the STS with invaluable, timely information regarding doses, operability and costs. Gas phase trials are usually more time consuming and costly to set up.

The final assessment of which system(s) to choose must include such factors as initial and ongoing cost, performance, operability, safety, and site suitability.

Once the treatment system and system modifications have been decided on, advise the complainants and neighbours as to what is going to happen to improve the odour situation. In general, keep them informed as studies and work progresses.

Step 9.

Install And Confirm Performance

Once installed and commissioned, the new system must have proof of performance tests carried out, and preferably at different times of the year including during summer and winter.

Feedback from the original complainants and neighbours should also be obtained to ensure the system is performing adequately, and they are satisfied with the result!

86

Chapter 9 Closing Words Provision and operation of sewerage treatment systems to meet both environmental and public expectations requires time, diligence, attention to detail and effective process monitoring.

Adequately controlling odours is one of the steps in achieving the desired outcomes for the operation of a sewage transport system. This practical guide provides a sound background to the control of odours in sewage transport systems. If the advice provided here is implemented and incorporated into the routine work practices of the operations team, the chances of problematic odour events occurring will be greatly reduced.

An additional advantage of implementing the advice in this guide will be a saving in sewer main and sewage treatment plant inlet works maintenance and replacement costs.

The advice provided here might seem to some like more work, BUT IT IS the work of operating a sewage transport system efficiently and effectively. Time simply must be found. Managers need to fully support operators to facilitate the implementation of these tasks.

Think about it? A well run sewage transport network means peace of mind that the public will not have to endure nuisance odours at their own homes and that the life of assets will be maximised.

So why not take the first step now!

87

Appendices Appendix 1.

Case Study – Steps In Solving An Odour Problem In An STS

Background Operators of the main pump station in Mooroopna, Victoria had been receiving odour complaints from householders living nearby, as mentioned in an internal report: “A current odour control problem exists here with odour complaints having been received from the houses approximately 30 metres northeast of the pump station. Action is required to understand why this is occurring and what options are available to prevent it.”

Step 1:

Gather System Information

Information on the system was readily available via drawings, manuals, and good operator knowledge.

Sewage flows from several pipelines from different areas of the city flowed into the pump station. The pump station incorporated a wet well and pumps which started and stopped in response to high and low level signals. Both domestic sewage and some trade wastewater entered the upstream system, but no single input seemed to be an obvious problem. The pump station wet well inside the building was covered and had a six metre vent stack installed without a fan. A small carbon canister had been placed on top of the vent stack.

Step 2:

Determine If The Problem Is Real?

Who was complaining ?

To ascertain whether the problem was “real”, the operators of the facility carried out interviews with the nearby householders and provided them with forms to fill out to record basic details of the weather and odour strength whenever they noticed an odour. The results showed that odours occurred at various times and that the wind direction was usually from the direction of the nearby sewage pump station.

What were complainants concerned about ?

Complainants described “sewage” type odours which entered their homes through roof top airconditioning ducts at times.

When were odours occurring ?

Complainants stated that odours occurred at various times and that the wind direction was usually from the direction of the nearby sewage pump station.

Where were odours occurring ?

The STS operators knew that pedestrians walking past the pump station had mentioned odours around it, and the owners of downwind houses had made formal complaints.

Why did complainants think odours were occurring ?

Complainants unambiguously blamed the sewage pump station.

The STS operator concluded that the problem did seem to be real and committed to the complainants to further undertake investigation.

Step 3:

Identify Possible STS Site(s) Responsible

Only one STS site was located nearby and since there were no other industries or potential odour sources close, it was agreed to concentrate on the pump station as the likely odour source.

88

Appendix 1. Case Study – Steps In Solving An Odour Problem In An STS Step 4:

Carry Out Quantitative On Site Tests.

It was decided that since the sewage was mainly of domestic origin, and the odour type was being described as “sewage like”, initial testing would involve continuous gas phase H2S monitoring in the wet well, rather than non-continuous measurements of “odour” levels. a) Hydrogen Sulfide Results A 0 to 200ppm H2S Odalog® unit (ThermoFisher) was placed into the wet well for a week and results confirmed that significant H2S levels were present (Table A1.1).

Table A1.1 Wet well H2S monitoring results.

Time Period

Time Range

1 2 3 4

4pm to 1am 1am to 6am 6am to 10am 10am to 4pm

Maximum H2S Level (ppm) 35 5 25 10

Average H2S Level (ppm) 20 3 10 5

The results showed that: 1. The highest H2S concentrations were seen between 4pm and 1am each day.

2. H2S levels were lowest between 1am and 6am during periods of low flow into the wet well.

3. A second peak of high levels occurred between 6am and 10am during the morning peak flow period.

4. Levels were low to moderate between 10am and 4pm.

A small allowance was made for some additional H2S being produced in summer, which gave likely summer H2S concentrations of 50ppm. b) Gas Flow Rate Results A T-piece was installed into the stack, to allow an anemometer to be inserted to measure gas flow rates up the vent. Very low gas flows were observed which showed that the small carbon canister on top was partially blocked. Gases were being forced out of the wet well through the wet well air inlet duct in a direction reverse to what should have been occurring. Gases were therefore exiting the pump station at a low level and probably causing odours around the outside of the station, rather than being dispersed by the stack. The small canister was later removed to help provide a short term improvement in gas dispersion.

Since gas flow rates were difficult to measure continuously, an analysis was carried out of the natural “piston effect” (Chapter 4) which causes gas to be drawn in and expelled from the wet well, using wet well pump stop and start time data. Results were determined by dividing the fixed wet well rise and fall volume (32m3) during each pump event.

The recorded wet well fill times over a 24 hour period are summarised in Table A1.2.

For example at 0802 hrs, it took 11minutes for the 32m3 wet well to fill with sewage, therefore the gas emission rate was 32 x 60/11=174 m3/hr.

89

Appendix 1. Case Study – Steps In Solving An Odour Problem In An STS Table A1.2. Calculated wet well gas emission flowrates.

Time of day Wet well of pump fill time start (minutes) 0802 11 0810 7 0815 7 0819 7 0830 7 0845 8 0857 8 0920 9 0945 12 1545 20 1911 12 0004 43 0244 100 0536 54

Step 5:

Calculated wet well gas emission rate (m3/hr) 174 274 274 274 274 240 240 213 160 96 160 45 19 35

Predict Downwind Concentration Of Odorant For Existing And Required Situations

a) Existing Situation With assistance from outside consultants using the Ausplume model, the above worst case data was used (approximately 300m3/hr of gas coming out of the 6m vent stack at an estimated summer level of 50ppm H2S). Results showed that H2S levels when converted to equivalent odour levels, were of the order 50–100 odour units at the nearby houses under worst case conditions, which was consistent with the earlier assessment of odours being present at the houses at times. b) Required Situation An assessment by the consultants of what level of H2S was allowable was then carried out using the Ausplume model. The assessment was based on the addition of an induced air ventilation fan running at 750 m3/hr (to provide better wet well ventilation air rates and velocities) at a calculated diluted H2S concentration of 20 ppm. This resulted in the requirement for a reduction in H2S levels out of the 6 m vent, from 20 ppm down to below 0.1 ppm in order to allow the EPA requirement of 1 odour unit to be met downwind.

Step 6:

Produce Problem Definition Statement

A summarised statement for this situation was produced: “Odours due mainly to hydrogen sulfide from the sewage pump station are causing complaints at nearby houses. To prevent this in future, hydrogen sulfide concentrations need to be lowered to below 0.1 ppm (at a gas flow rate through the 6 m stack of 750 m3/hr)”.

Step 7:

Decide If There Are Any Appropriate Non-Treatment Related Solutions

After some consideration of possible options (Table 8.1), none were considered to be practical. Consultants also carried out some further “what if” calculations using Ausplume, which showed that even if an induced air fan system was installed, at other stack heights, the required level of H2S at the nearby houses, would not be met. This confirmed that some form of liquid or gas phase treatment was required.

90

Appendix 1. Case Study – Steps In Solving An Odour Problem In An STS Step 8: Consider Sewage And / Or Gas Treatment Options And Choose The Best System Due to the potential need to treat a number of the inlet pipeline flows and the very close proximity of the pump station to nearby houses (Table 8.2), it was decided that the most reliable and easily operated solution would be to treat the gas discharge from the pump station.

Given that: • • • • •

the gas would exit the wet well at up to 20 ppm H2S and be required to exit the treatment unit at below 0.1 ppm (99.5% removal) a flow rate of 750 m3/hr was required, and that limited space was available, and that minimal operator attendance was important, it was decided (based on Table 7.3) to obtain quotations for either an activated carbon system, or biofilter unit.

After considering initial and ongoing costs, performance, operability, safety, and site suitability, a high rate biofilter with a mixed organic media was chosen as the best option. This system also provided some comfort that the likely low levels of non H2S type odours would also be removed.

Step 9:

Install and Confirm Performance

The fan and biofilter were installed (Figure A1.1), and continuous H2S monitoring was carried out before and after the biofilter over two separate weekly periods, including one in summer. Results showed that the 0.1 ppm H2S maximum was being achieved.

Since installation, no odour complaints have been received over a twelve month period, local householders can no longer smell odours, and are very pleased with the results.

Figure A1.1.

Completed Biofilter.

91

Appendix 2.

92

Table Of Molecular Weights Compound

Molecular Weight

Hydrogen sulfide.

34

Methyl mercaptan.

48

Ethyl mercaptan.

62

Ammonia.

17

Methyl amine.

31

Ethyl amine.

45

Oxygen.

32

Air.

29

Nitrogen.

28

Water.

18

Appendix 3. Sewage Odours Information Sheet How Does A Sewage Transport System Work?

Sewage produced by households and industry is usually conveyed to the sewage treatment plant via the type of system shown in the diagram below.

Manhole Boundary Trap

Gravity Reticulation Mains

Trade Wastes

Branch / Main Sewers

Vent

Gravity Main Rising Main

Wet Well Pump Station

Hydraulic Drop At Inlet To Pump Station

Treatment Plant Inlet Works

Wet Well

Why Do Odours Sometimes Occur?

As sewage travels down the system, it ages via a natural process, and can become more odorous. This is mainly due to very small amounts of hydrogen sulfide or “rotten egg gas” being produced. Odorous gases can be released at vents on pipelines and pump stations, or at the inlet to the sewage treatment plant.

Odours are usually noticed more at certain times of the day or perhaps overnight when wind speeds are low, or are directing the odours towards buildings and people.

Are Odours Unhealthy?

Even though a sewage odour might exist, it is extremely unlikely to be a health hazard. The odorous components are usually present only in very low concentrations downwind from the sewerage system.

What Are Water Authorities Able To Do About Reducing Odours?

Pipelines, pump stations and other components of the sewage transport system are designed to minimise sewage odours. However, should odours occur, the Water Authority may be able to: • • • • •

make operational changes which improve the situation reduce inputs of odorous substances from industries reduce odour emissions by sealing up any leaks around facilities installing vents or increasing vent heights installing odorous gas treatment systems.

Your Water Authority would appreciate any information regarding where and when sewage odours have been occurring, and will make a genuine effort to address each situation which might occur.

93

Appendix 4. Odour Complaints Record Sheet Odour Complaints Record Sheet.

Water Authority Complaint Received Date Complainant’s Name Complainant’s Address Complainant’s Phone Number Operator’s Name Complaint Description 1. Odour Level

Low

2. Odour Description

Rotten Eggs Cabbage Sewage Pungent Fishy Fruity Other .................................................................

3. Timing

E

Wind Speed: None Light

SE

5. Location And Reason For Odours (Complainant’s Description)

94

S

Moderate

Air Temperature: Cold Moderate / Warm

Operator’s Conclusions

High

Date Odours Observed: Time Odours Noticed: Wind Direction: N NE

4. Weather Conditions At Time Of Odours

Medium

SW Strong Hot

W

NW

References Note that most USEPA references are available free on the website : http://www.epa.gov/epahome/publications.htm Other references should be available through most water authority and university libraries.

APHA / AWWA / WEF (2006). Standard Methods For The Examination Of Water And Wastewater.

AS 2865: (2009). Confined Spaces.

AS / NZS 4323.3: (2001). The Determination Of Odour Concentration By Dynamic Olfactometry.

AS 4323.1: (1995). Stationary Source Emissions. Selection Of Sampling Positions.

MMBW (1989). Hydrogen Sulfide Control Manual, Technological Standing Committee On Hydrogen Sulfide Corrosion In Sewerage Works.

NOHSC: 1003 (1995). Adopted National Exposure Standards For Atmospheric Contaminants In The Occupational Environment.

USEPA (1974). Process Design Manual For Sulfide Control In Sanitary Sewerage Systems.

USEPA (1985). Design Manual ; Odor And Corrosion Control In Sanitary Sewerage Systems and Treatment Plants.

USEPA (1992). Detection, Control, And Correction Of Hydrogen Sulfide Corrosion In Existing Wastewater Systems.

USEPA (2003). “Using Bioreactors To Control Air Pollution”, USEPA EPA-456/R-03-003.

WEF (1995). “Odour Control In Wastewater Treatment Plants”, Water And Environment Federation, Manual Of Practice No. 22.

WEF (2004). “Control of Odours And Emissions From Wastewater Treatment Plants”, Water And Environment Federation, Manual Of Practice No. 25.

WSA 04 (2005). Sewage Pumping Station Code Of Australia - V2.1.

WSA 02 (2002). Sewerage Code Of Australia V2.3.

WSA 07 (2007). Pressure Sewerage Code Of Australia V1.1.

95

Notes

96

Disclaimer This document does not purport to be a comprehensive statement on odour management in sewage transport systems 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 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 aforementioned 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

Cert no. L2/0004.2011

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