Operation and Optimisation of Chlorine and Chloramine Disinfection

Practical Guide to the Operation and Optimisation of Chlorine and Chloramine Disinfection

Pe t e r M o s s e , D e re k B r a d e n a n d To n y H o u r i g a n

First Published August 2009 – Reprinted July 2016

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

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

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

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

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

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

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

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

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Acknowledgements Special thanks are extended to Frank Pleavin from Prominent Fluid Controls, Jason Mullins from North East Water, Stephen Wilson from the Water Industry Training Centre and George Wall from WIOA for critical review of this practical guide.

Thanks also to the sponsors, whose logos are displayed on the front cover, for funding the publication of this practical guide.

Thanks to WIOA for their commitment to improving the skills of operations staff in the water industry and for supporting the development of this series of practical guides.

Publication Details First Edition August 2009

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

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

2.

3.

4.

5.

Catchment Chemical dosing, coagulation, flocculation and clarification Media Filtration Distribution Disinfection

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

An underlying objective of each of the guides is that on each and every page operational staff should be able to make mental notes to themselves along the lines of: • • •

That’s a good idea.

We should be doing that.

We can start on that immediately.

The guides are not about identifying costly solutions but simply identifying ways to get the best out of what is available at any particular time. The aim is to provide suggestions and hints that can be implemented easily, cheaply and immediately in the field.

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

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

Feedback Requested These guides have been written to help operators perform their jobs effectively and to help operators understand the vitally important roles they fulfil within the water industry. The authors would be pleased to receive any feedback on this guide. Suggestions for further information or areas where additional explanation would have aided understanding can all help to make any second edition stronger and more useful for operators in our industry. Any suggestions can be directed by e mail to the Water Industry Operators Association of Australia (WIOA) at info@wioa.org.au or by post to 22 Wyndham Street, Shepparton, Victoria, 3632.

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Contents Foreword Acknowledgements General Introduction Chapter 1

• • • • • • •

Chapter 2 Chapter 3

•

• •

•

• • •

Chapter 4

4

Hazards, Risks and Control Points....................................................................8

Hazards And Drinking Water...........................................................................................8 Pathogens Are The Main Hazards In Drinking Water......................................................8 Other Hazards............................................................................................................... 11 Control Points................................................................................................................ 11 Critical Limits And Target Criteria..................................................................................12 Corrective Actions ...........................................................................................13 Monitoring.....................................................................................................................13 Responsibilities.............................................................................................................14

The Importance Of Disinfection .......................................................................15

A Brief History Of Disinfection.......................................................................................16

Disinfection: The Final Barrier..........................................................................18 The Mechanism Of Chlorine Based Disinfection...........................................................18 The Process Of Chlorine Based Disinfection.................................................................18 Log Removal....................................................................................................18 Contact Time....................................................................................................19 Pathogen Sensitivity To Disinfectants...............................................................19 Effect Of Temperature......................................................................................20 Effect Of Turbidity.............................................................................................20 The Chemistry Of Chlorine Disinfection........................................................................21 Chlorine Demand ............................................................................................22 Breakpoint Chlorination....................................................................................22 Chlorine Residuals...........................................................................................23 Chlorine Decay.................................................................................................24 The Chemistry Of Chloramine Disinfection...................................................................24 Disinfection By Products...............................................................................................25 Comparison Of Disinfectants.........................................................................................26

Practical Operational Issues..............................................................................28 • Safety .........................................................................................................................28

Chlorine Gas....................................................................................................28

Sodium Hypochlorite........................................................................................29

Ammonia..........................................................................................................29

Calcium Hypochlorite.......................................................................................29 • Available Chlorine.........................................................................................................29 • Effect Of Chlorine Products On Water...........................................................................30 • Sodium Hypochlorite Decay..........................................................................................30 • Calcium Hypochlorite Decay.........................................................................................31 • Chlorine Demand And Chlorine Residual Decay...........................................................31 • Contact Structures And Mixing......................................................................................31 • Detention Time..............................................................................................................35

• Minimum Daily Ct..........................................................................................................35 • Backup Systems...........................................................................................................35

DisinFection Failure.........................................................................................36 • Discharge Of Chlorinated Or Chloraminated Water.......................................................37

Chapter 5

Disinfectant Dosing Calculations.....................................................................38

• Calculation Of Dose Rate..............................................................................................38

 Gas............................................................................................................38  Hypo..........................................................................................................39 • Verification Of Long Term Dose.....................................................................................39 • Calculation Of Changed Dose.......................................................................................40

Chapter 6

Chlorine Gas Disinfection Systems.................................................................42 • Types Of Vacuum Chlorinators......................................................................................42

Differential Pressure Regulated Chlorinators...................................................42 Sonic Chlorinators............................................................................................42

• The Gas Chlorination System.......................................................................................44

Gas Storage Vessels........................................................................................44

Venturi..............................................................................................................45

Vacuum Monitor And Alarms............................................................................45

 High Alarm.................................................................................................46

 Low Alarm..................................................................................................46

Control Device..................................................................................................46

Vacuum Regulators..........................................................................................47

Vent Pipe..........................................................................................................49

Automatic Changeover Systems......................................................................50

Weigh (Load) Scales........................................................................................51

Chlorine Leak Detectors...................................................................................52

Automatic Drum Or Cylinder Shutdown Systems.............................................52

Chlorine Analysers...........................................................................................53 • Trouble Shooting...........................................................................................................54

Chapter 7

Liquid Sodium Hypochlorite Disinfection Systems...................................55

• The Sodium Hypochlorite Dosing System.....................................................................55

Sodium Hypochlorite Storage...........................................................................55 Removal Of Sodium Hypochlorite....................................................................56

Prevention Of Gassing (Vapour Lock)..............................................................57

 1. Solenoid Valve Method..........................................................................57

 2. Vented Chamber Method.......................................................................59

 3. Constant Bleed Bypass Method.............................................................59  4. Bubble Diversion....................................................................................60

Metering Pumps...............................................................................................61

 Solenoid Pumps.........................................................................................61

 Motor Driven Pumps..................................................................................61

 Peristaltic Pumps.......................................................................................62  Metering Pumps And Disinfection..............................................................63

 Materials....................................................................................................64

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Contents

Back Pressure Valves......................................................................................65 Pressure Relief Valves.....................................................................................65

Injection Valves And Lances............................................................................66

 Installation..................................................................................................67 • Sodium Hypochlorite Generation Systems....................................................................68

Electrochemical Activation ..............................................................................68

Chapter 8

Chapter 9

Chloramine Disinfection Systems....................................................................69

• Chloramine Chemistry...................................................................................................70 • Control Of Monochloramine Production........................................................................70

Ratio Of Chlorine To Ammonia.........................................................................70

Chloramine Toxicity And Public Relations........................................................72 • Sequence Of Addition...................................................................................................72 • Formation Of Chloramines............................................................................................74

Chlorine Gas With Ammonia Gas.....................................................................74 Chlorine Gas With Liquid Ammonia..................................................................74

Sodium Hypochlorite With Liquid Ammonia......................................................74 • Nitrification ...................................................................................................................75 • Management Of Chloraminated Systems......................................................................76

Controlling And Measuring The Addition Of Disinfectant.......................78 • Controlling The Dose....................................................................................................78

Manual Dosing.................................................................................................78

Flow Paced Dosing..........................................................................................78

Residual Trim Dosing.......................................................................................79

Flow Paced Residual Trim................................................................................79 • Process Monitoring.......................................................................................................81 • Measuring The Chlorine Residual.................................................................................82

The DPD Test...................................................................................................82

Limitations Of The DPD Method.......................................................................83 • Field Analysis................................................................................................................84

Colorimeters And Comparators........................................................................84 • Online Analysers...........................................................................................................85

Free Chlorine Analysers...................................................................................86

pH Correction Of The Chlorine Reading...........................................................88

What Does The Measured Chlorine Residual Really Mean?............................88

Total Chlorine Analysers...................................................................................89 Calibration........................................................................................................89

• Measuring Chloramine Residuals..................................................................................90 • Mass Balance Checks...................................................................................................91 • Secondary Chlorine Dosing And Residual Maintenance In Distribution Systems..........93

Chapter 10 Operation And Optimisation Of Disinfection Systems..............................94 1.

2.

3.

4.

5.

6

Review Existing Water Quality Data..............................................................................94 Determine The Available Contact Time.........................................................................94 Assess The Quality Of The Mixing And Contact Structure............................................95 Determine The Target Ct...............................................................................................95 Determine The Practical Achievable Ct.........................................................................95

6. Minimise DBP Formation...............................................................................................96 7. Consider The Length And Complexity Of The Distribution System...............................96 8. Optimise The Disinfection System.................................................................................97 Gas Chlorination Systems................................................................................97

 Check For Chlorine Gas Leaks..............................................................97  Check For Vacuum Leaks (Drawdown Test)...........................................97  Check The Full Capacity Of The System................................................98  Check The Vent......................................................................................99  Check The Chlorine Leak Detector........................................................99

Liquid Chlorination Systems.............................................................................99  Ensure Pipe Work And Metering Pumps Have Been Designed And

Installed To Minimise Gassing................................................................99  Size The Pumps To Match “Current” Peak Demand.............................100  Check Dosing Lines Regularly.............................................................100 9. Check And Calibrate The Chlorine Analyser...............................................................100 10. Ensure Failsafe Dosing...............................................................................................101 11. Monitor Raw And Treated Water Quality.....................................................................102 12. Visit The Site Regularly...............................................................................................102 13. Maintain And Calibrate All Disinfection Equipment......................................................102 14. Report On Achievement Of Disinfection Targets.........................................................103 15. Don’t Assume, Know!..................................................................................................103

Chapter 11 Closing Words ......................................................................................................104 References ......................................................................................................................................105 Appendix 1 Simple Method For The Measurement Of Chlorine Demand...................106

7

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

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

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

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

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

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

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

Turbidity.

Natural Organic Matter (NOM).

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

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

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

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

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

8

Chapter 1 – Hazards, Risks And Control Points Table 1.1. Comparison of pathogens and chemicals as risk factors for waterborne disease.

Risk Factor

Pathogens

Chemicals

Time to effect

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

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

Detection

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

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

Harmful levels

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

Pathogens come from natural sources.

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

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

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

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

Source

Design

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

There are hundreds of waterborne pathogens. Each has a slightly different combination of sources, treatment sensitivity and effect on health. But there are simple rules that can be used to understand and manage these risks. The way to simplify this is as follows: • • •

First understand that there are three main groups of pathogens.

Then get to know the most problematic pathogen in each group.

Finally, understand how to manage the risks from the most problematic organisms.

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

The three major groups of pathogens of concern are listed below, along with the most problematic reference pathogens from a water supply perspective: • • •

Enteric (intestinal) bacteria o Campylobacter, pathogenic E. coli O157 H7 and Salmonella Enteric protozoan parasites o Cryptosporidium and Giardia Enteric viruses o Enterovirus, Rotavirus, Adenovirus and Hepatitis A virus.

9

Chapter 1 – Hazards, Risks And Control Points

If these pathogens are transmitted in water they can cause very serious widespread illness in public water supplies.

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

Some of the important features of the outbreaks caused by pathogens can be readily explained by understanding the key features of the main pathogen groups. Table 1.2 summarises these features.

Table 1.2. Key features of pathogens responsible for waterborne disease in humans.

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Feature

Bacteria

Protozoa

Viruses

Is sewage a common source?

Yes

Yes

Yes

Is stock manure a common source?

Yes

Yes

No

Are bird droppings a common source?

Yes (Particularly Campylobacter)

No

No

Are wildlife faeces a common source?

Yes (Particularly Salmonella)

Possibly

No

What are the principal source controls?

Control all sources of faeces including birds and vermin

Control all sources of manure and sewage

Control sources of human sewage

Is media filtration an effective control?

Yes

Yes

Limited

Is primary disinfection using chlorination an effective control?

Yes

No

Yes

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

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

No

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

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

Yes

Yes

Yes

Is microfiltration an effective control?

Yes

Yes

Limited

Is reverse osmosis an effective control?

Yes

Yes

Yes

Chapter 1 – Hazards, Risks And Control Points

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

There are three major areas of chemical contamination that have caused waterborne incidents.

• • •

The use of contaminated source water, particularly with arsenic and fluoride.

The introduction of excess or incorrect water treatment chemicals (including alum and fluoride).

The backflow of hazardous substances into water distribution systems.

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

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

Waterborne incidents associated with algae were historically associated more with the warmer climates of Queensland or degraded rivers with irregular flows. However, as drought in Australia continues, many water storages further south have had to deal with recurring algal blooms. Indeed there are an increasing number of instances in the southern States of Australia where algal blooms have occurred even during winter. Algae present in source water can cause a number of problems.

• • • •

Toxins may be introduced to the water by cyanobacteria (blue-green algae).

Taste and odour chemicals can be introduced to the water by cyanobacteria or other species of algae (e.g. diatoms).

Taste and odour chemicals can be introduced by soil bacteria and fungii.

Some species of algae can overload water treatment plants and cause blockage of filters.

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

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

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

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

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

Figure 1.1. A generalised water supply system.

Water Treatment and Distribution

Coagulant

Chlorine

Catchment Raw water Screening

Mixing

Flocculation

Clarifier

Filtration

Disinfection

Sludge lagoon Sludge to reuse Distribution Storage

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

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

Clearly where the step is absent it cannot be a control point for that system.

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

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Chapter 1 – Hazards, Risks And Control Points The ADWG 2004 put it this way, “A control point has several operational requirements including: • • •

Operational parameters that can be measured and for which critical limits can be set to define the operational effectiveness of the activity (e.g. chlorine residuals for disinfection).

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

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

Two different target levels are often set: 1. Target criteria. The target criteria represent the day-to-day operational limits. Any deviation of performance from the target should be regarded as a trend towards loss of control of the process and should result in appropriate actions being taken to resolve the problem and restore control. How to retain the process within the target limits would normally be described in well-written operational procedures.

2. Critical limit. This is the level that must not be exceeded. Deviation from a critical limit indicates loss of control of the process and should be regarded as representing a potentially unacceptable health risk. If by exceeding the limit, public health might be impacted, then the health regulator should be notified. Where there is uncertainty concerning that health risk, then this should be identified and an appropriate response agreed with the health regulator in advance of the event occurring e.g. a short-term failure of chlorination.

Corrective Actions Target criteria are always more stringent than the critical limits. By setting target criteria that are more stringent than critical limits, corrective actions can be taken before the water quality exceeds specification or an unacceptable health risk occurs (i.e. exceeding the critical limit).

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

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

Verifying that all the control points in a process are within the recommended range ensures that the final product will be within the specified limits. Testing of the final product quality is verification monitoring and simply confirms that the total system is working. Quality control should not rely on verification monitoring alone. Process monitoring of control points allows the operator to detect problems before they can have an impact on final water quality.

Operators and management should work together to firstly set target criteria and critical limits, and then to establish appropriate monitoring frequencies for all the tests. Ideally, this will allow the identification of any trends that indicate a drift away from satisfactory operation. The frequency of monitoring needs to be set based on the rate of change for the control measure. There is no point in having a control point and testing once per month if the parameter being measured can vary significantly from day to day!

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Chapter 1 – Hazards, Risks And Control Points

Responsibilities Water utilities have a responsibility to: • •

Produce safe drinking water at all times.

Produce the best quality water possible from the existing system.

Operators have a responsibility to ensure that: • • • •

Their actions do not compromise the production of safe drinking water at any time.

They actively provide feedback to management when they become aware of shortcomings in the treatment sequence that may contribute to the production of unsafe drinking water.

They are alert to changes in the system that they operate.

They act to produce water that is aesthetically pleasing.

So, how can operators fulfil these responsibilities?

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

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Chapter 2 The Importance Of Disinfection The major aim of a water supply system is to provide safe drinking water to consumers. What does safe mean? Perhaps our customers can guide us here. Whenever a water quality incident arises, the first question asked is “Is the water safe to drink?” What do they mean by safe? The answer is simple, “Will it make them ill?” Even if the water is dirty or smells bad, their first concern is safety.

In general discussions with consumers about water, their concerns often centre on chemicals in the water. In particular, chlorine or some other suspected chemical that may be added to the water are the main concerns raised. This concern is often misplaced since it is the presence of pathogens that is the major concern. All the big outbreaks of waterborne disease in the western world have arisen due to microbial pathogens. Table 2.1 provides a few examples.

Table 2.1. Examples of recent waterborne disease outbreaks in the western world.

City

Popn

Year

Pathogens

Source

Illness

Deaths

Gideon Missouri USA

1100

1993

Salmonella

Bird faeces

600

~7

Milwaukee Wisconsin USA

500,000

1993

Cryptosporidium

Sewage

403,000

~100

Sydney Australia

~4 million

1998

Cryptosporidium

Cattle in catchment?

Sewage

Not determined but the potential ? was high

Walkerton Ontario Canada

4800

2000

E. coli O157 H7 Campylobacter

Manure

2300

7?

Galway Ireland

72,000

2007

Cryptosporidium

Sewage

> 200

?

Nokia Finland

30,000

2007

Campylobacter and Salmonella

Treated sewage

>1000

?

Cryptosporidium

Single rabbit carcass in disinfection contact tank

22

?

Pitsford UK

250,000

2008

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

15

Chapter 2 – The Importance Of Disinfection In the conventional water treatment processes of coagulation and flocculation, pathogens are caught up in the floc and removed from the water by sedimentation in a clarifier or sedimentation tank or flotation by Dissolved Air Flotation (DAF). Any small amounts of floc left over are removed in well-operated granular media filters. Any pathogens that make it through the clarification and filtration steps can then theoretically be controlled by the use of chemicals designed to kill the pathogens. Simple isn’t it. The big problem is that in a modern water supply system using the most common disinfectants, some pathogens are not killed by the disinfectants. Bacteria and most viruses are killed by chlorine or chloramines, HOWEVER Cryptosporidium and to a lesser extent Giardia are not. So the only treatment barrier to Cryptosporidium and Giardia in a conventional treatment system is the filter. Where the filter barrier is insufficient on its own, another barrier is required to remove Cryptosporidium and Giardia. UV light or Ozone disinfection are commonly used to provide this extra barrier. The number of microorganisms penetrating the treatment barriers (coagulation, flocculation, settling, filtration and disinfection) depends to a large extent on the raw water source. To ensure that the numbers are reduced as much as possible, it is essential to have both filtration and disinfection barriers.

Historically the water industry has relied on the absence of E. coli (i.e. zero E. coli) to indicate that the water is safe to drink. However, with the emergence of the protozoan pathogens Cryptosporidium and Giardia it is essential to realize that it is possible to have water with zero (0) E. coli and still have unsafe drinking water because of the presence of those protozoans due to poorly performing or even absence of filters.

Passage of water through all these barriers results in water that is disinfected. This does not mean that there are no microorganisms in the water but rather that the numbers of pathogens are reduced to such an extent that the risk of water borne disease is very low. Disinfection differs from the process of sterilisation where all living microorganisms are killed. Boiling water is an example of a sterilisation process. Sterilisation of drinking water is not possible or desirable since our immune systems seem to work better if they are in fact exposed to microorganisms throughout our life. However, it is important to remember that if the barriers in a water supply system fail there is always one more barrier that can be lowered; a Boiled Water alert. By boiling the water, the consumer is protected from any pathogen that may be in the water.

This practical guide is about disinfection but do not ever forget that the complete control of all pathogens requires as many barriers as possible.

A Brief History of Disinfection The work of scientists such as Pasteur, Lister and Koch in the late 1800’s led to the germ theory of disease.

The theory suggested that certain diseases were caused by a single species of microorganism.

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

In 1854 for the second time cholera struck London. At the time, two companies supplied water to the city, one drawing water from upstream and one from downstream of London. During his investigation of the explosive outbreak of cholera in the Soho district of London, Snow traced the source to a specific pump, the now famous Broad Street pump. Based on his work, Snow convinced the authorities responsible for the provision of water to remove the pump handle. As a result, the cholera epidemic was contained. In subsequent investigations, a link was found between a nearby cesspool (used for the disposal of “waste and excrement”) and the shallow well supplying the Broad Street pump, thereby proving the link between sewage, water and disease. Another important aspect of the outbreak was that people from the area around the ‘safe well’ actually chose to get their water from the Broad St pump because it tasted better. Good tasting water is no guarantee of safety!

16

Chapter 2 – The Importance Of Disinfection

By 1883 the cause of cholera had been identified as an intestinal bacteria that was excreted in the faeces of humans. The cause of infection then was the unintentional ingestion of faeces, the so-called faecal oral route. The recognition of this link between faeces and oral contamination heralded in the modern age of personal hygiene and sanitation, albeit very slowly.

With the understanding of the nature of many diseases, and the possible waterborne origins of some, attention focussed on ways to disinfect the water.

Ozone was first used in Holland to disinfect water in 1893.

In the first example of distribution system management, bleach solution was added as a temporary measure to disinfect water pipes in the town of Maidstone in England in 1897. This occurred after an outbreak of typhoid fever.

In the late 1890’s and early 1900’s many disinfection systems were developed and slowly installed on drinking water supply systems. The first permanent chlorination system was installed in Belgium in 1902 and also became widespread in Britain in the early 1900’s. Chlorination was installed instead of filtration in Jersey City in the US and heralded in the period of “disinfection only” treatment. Disinfection only was attractive since its impact on public health was dramatic (Figure 2.1) and could be installed at a far lower cost than treatment systems involving coagulation, sedimentation and filtration.

The era of disinfection had arrived!

Figure 2.1. Death rates from typhoid fever in the USA. Figure 1-1

Death Rates for Typhoid Fever in the USA

Rates per 100,000 Population

Chlorination begins 24

16

8

1900

1910

1920

1930

1940

1950

1960

17

Chapter 3 Disinfection: The Final Barrier A number of different processes are available to kill or remove pathogens in water. These include: • • •

Oxidising chemicals such as chlorine, chloramines, chlorine dioxide, bromine and ozone.

UV light.

Membranes.

In this practical guide only chlorine and chloramines will be considered.

The Mechanism Of Chlorine Based Disinfection Chlorine, and to a lesser extent chloramines, are strong oxidants and are able to oxidise many of the important molecules inside pathogen cells.

Amino acids (the building blocks of proteins) once oxidised alter the function of the protein and in particular the way that the protein folds up. Incorrect folding means that the protein no longer functions the way it should. Many proteins in their functional form are actually made up of separate protein chains. Chlorine and chloramines can disrupt some of these links thus rendering the protein ineffective. Without functioning proteins the cell rapidly dies.

Chlorine and chloramine also alter the structure of the building blocks of the DNA and, as with proteins, once this structure is altered the DNA no longer operates.

There is also some evidence that chlorine directly damages the cell membrane. Once the cell membrane is damaged the cell ceases to function.

Thus chlorine and chloramines interfere with the ability of microorganisms to live and to reproduce. The net result is that the cells are mortally wounded, killed or inactivated (i.e. cannot reproduce) by the disinfection process and therefore can no longer infect other organisms including human beings.

The Process Of Chlorine Based Disinfection Log Removal Disinfectants kill microorganisms, though the extent to which they do this varies. The effectiveness of a disinfectant at killing microorganisms depends on: • • •

The concentration of the disinfectant.

The time that the micro-organisms are exposed to the disinfectant.

The temperature and pH.

The efficiency of removal of microorganisms from the water is described in terms of log removal. This can be a little confusing but can easily be related to the percentage of organisms removed. This relationship is summarised in Table 3.1.

18

Chapter 3 – Disinfection: The Final Barrier Table 3.1. The relationship between the percentage of microorganisms removed from the water and the log removal. % Removed Log Removal 90 1 99 2 99.9 3 99.99 4 While it is not a formal requirement in this country, it is interesting to note that in the USA, the USEPA specifies the log removals required for WTPs. The total log removal/inactivation currently required by the USEPA (LT2ESWTR) is a minimum of 4 log for viruses, a minimum of 3 log for Giardia cysts and up to 5.5 log for Cryptosporidium depending on the number of Cryptosporidium in the raw water. A number of Australian utilities have adopted the USEPA requirements in the absence of an Australian Standard. In the national Capital, a large UV disinfection system is being considered for a large WTP to achieve approximately log 5.5 removal of Cryptosporidium.

Contact Time Disinfection requires microorganisms to be exposed to a given concentration of disinfectant, for a certain time, at a particular temperature. The higher the concentration of disinfectant, the shorter the exposure time necessary to inactivate or kill the organisms. This concentration-time relationship is referred to as Ct. The Ct value therefore defines the effectiveness of disinfection. Ct is calculated by simply multiplying the free chlorine residual concentration in mg/L, by the time in minutes that the chlorine is in the water. This is usually the time before the water reaches the first customer. For example if the residual is maintained at 2 mg/L for 8 minutes, the Ct is 16 mg.min/L.

Pathogen Sensitivity To Disinfectants Pathogens respond differently to disinfectants. Some are killed easily, others not. In general, bacteria are killed most easily while the protozoans Cryptosporidium and Giardia are quite difficult to kill. Viruses fit in the middle between bacteria and protozoans. The concept of Ct is useful in recording this different sensitivity.

Table 3.2 provides some examples of Ct values for chlorine to kill a range of different microorganisms.

Table 3.2. Comparison of Ct values (mg.min/L) necessary to achieve inactivation of E. coli and Cryptosporidium with free chlorine (data from Hrudey and Hrudey 2004).

Pathogen E. coli Cryptosporidium

99 % (2 Log) 0.032 – 0.05 7,200

99.99% (4 Log) 0.067 – 0.090

The data clearly shows that while E. coli is readily killed by chlorine (small Ct value), Cryptosporidium is not as is evident by the large Ct value.

19

Chapter 3 – Disinfection: The Final Barrier Disinfectants differ in their ability to inactivate pathogens. Once again the concept of Ct is useful to illustrate this. The relative efficiency of chloramine and chlorine for disinfection is shown in Table 3.3.

Table 3.3. Comparison of Ct values (mg.min/L) necessary to achieve 99.9% (3 log) inactivation of Giardia and viruses with free chlorine and chloramine (data from Hrudey and Hrudey 2004).

Pathogen Giardia Viruses

Chloramine (10oC) 3 Log removal 1,850 1,070

Chlorine (10oC) 3 Log removal 73-292 4-33

The significantly higher Ct values for chloramines are very clear, indicating the relatively poor disinfecting power of chloramines. This means that if chloramination is being used for primary disinfection, a much longer contact time, or much higher concentration, or both, is required to ensure a sufficient kill is achieved. This then translates to the need for larger disinfection tanks or basins. However, if chloramination precedes long distribution systems, such large tanks or basins may not be required since the contact times in the pipe can be very high.

Effect Of Temperature Temperature also has an impact on the efficiency of disinfection. Table 3.4 provides an example of the effect of temperature on Ct (chlorine) requirements for Giardia and viruses.

Table 3.4. Comparison of Ct values (mg.min/L) necessary to achieve 99.9% (3 log) inactivation of Giardia and viruses with free chlorine (data from Hrudey and Hrudey 2004).

Pathogen Giardia Viruses

1oC 137-552 9-66

10oC 73-292 4-33

20oC 36-146 2-16

Clearly, as the temperature decreases, higher Ct values are necessary to achieve the same degree of removal. The decrease in disinfection efficiency at lower temperatures is greater for chloramines than chlorine. Note also that like Cryptosporidium, Giardia is more resistant to disinfection than viruses.

Effect Of Turbidity The effectiveness of any disinfectant is reduced whenever the turbidity of the water is increased. Turbidity particles can shield any microorganisms present in the water from the action of disinfectants. Therefore a higher Ct is required to achieve the same disinfection effectiveness in water with a higher turbidity.

Higher levels of turbidity are also often associated with higher levels of Natural Organic Matter (NOM) both contributing to an increased demand for chlorine and formation of higher levels of Disinfection By Products (DBPs).

The ADWG (2004) emphasises the role of turbidity in the disinfection process and states “a turbidity of less than 1 NTU is desirable at the time of disinfection”. By reducing turbidity below 1 NTU, the efficiency of disinfection can be increased and disinfectant dose decreased. Therefore every effort should be made to ensure the lowest possible turbidity prior to disinfection.

20

Chapter 3 – Disinfection: The Final Barrier

The Chemistry Of Chlorine Disinfection Chlorine is generally added to water as chlorine gas or sodium hypochlorite solution and in some cases, calcium hypochlorite tablets.

Chlorine gas, sodium hypochlorite and calcium hypochlorite all react with water to produce two main – chemicals, hypochlorous acid (HOCl) and the hypochlorite ion (OCl ).

For chlorine gas (Cl2) the chemical reactions can be summarised as Cl2 + H2O  HOCl + H+ + Cl – HOCl  H+ + OCl

–

(Note the H+ produced. This explains why when chlorine gas is added to water the pH is lowered and the water becomes more acidic) For sodium hypochlorite (NaOCl) the chemical reactions are a little different. The sodium hypochlorite first separates to form sodium ions and hypochlorite ions in water.

NaOCl  Na+ + OCl

–

The hypochlorite ion then reacts with water to form hypochlorous acid.

OCl– + H2O  HOCl + OH

–

(Note the OH- produced. This explains why when sodium hypochlorite is added to water the pH is increased or becomes more alkaline) For calcium hypochlorite the reactions are a little more complicated but the outcome is the same, hypochlorous acid and hypochlorite ions are formed.

Ca(OCl)2 + 2 H2O  2 HOCl + Ca(OH)2 HOCl  H+ + OCl

–

(Note the Ca(OH)2 produced. This also causes the pH of the water to increase or become more alkaline) The relative amount of hypochlorous acid and hypochlorite ion formed depends on the pH of the water. At lower pHs (acid conditions), hypochlorous acid dominates while at higher pH (alkaline conditions) hypochlorite ion dominates. This is vitally important since the hypochlorous acid is up to 100 times more efficient at disinfecting water than the hypochlorite ions.

The relationship between hypochlorous acid and hypochlorite ion at different pH values is shown in Figure 3.1. The graph shows that at pH 7 roughly 70% of the available chlorine is in the form of hypochlorous acid whereas at pH 8 only 20% is in that form. Clearly at higher pH values the ability of chlorine to disinfect is greatly reduced.

Therefore in the range acceptable for drinking water pH, 6.5 to 8.5 (ADWG 2004), chlorine disinfection occurs best at the lower end. Thus for disinfection purposes it is better for operators to target their final water with a lower pH. Care needs to be taken however, because the lower pH can also encourage corrosion.

Under no circumstance should disinfection be compromised.

21

Chapter 3 – Disinfection: The Final Barrier Figure 3.1. Graph showing the relationship of HOCl and OCl to pH. % HOCl and %OCl- versus pH

120.0

% HOCl or % OCl-

100.0

80.0 %HOCL %OCL-

60.0

40.0

20.0

0.0 5

5.5

6

6.5

7

pH

7.5

8

8.5

9

The relationship to pH also affects the distribution system. In concrete pipes, slow leaching of cement from the pipe progressively increases the pH of the water. As a result, the disinfection efficiency decreases markedly due to the decreased proportion of hypochlorous acid. This needs to be considered when managing a distribution system. Persistent coliforms or high heterotrophic plate counts at the end of a distribution system, even in the presence of a chlorine residual, may simply be caused by a high pH and consequently essentially no hypochlorous acid present to kill the organisms.

Chlorine Demand When chlorine is added to water it immediately reacts with a number of chemicals in the water. These include: • Manganese.

• Iron.

• Ammonia.

• Hydrogen sulphide.

• NOM. In reacting with these, the chlorine is consumed. These chemicals are therefore said to exert a chlorine demand. Only once this chlorine demand is satisfied does free chlorine appear in the water and become available to disinfect the water.

When chlorine reacts with ammonia and NOM in water, DBPs are formed. These DBPs are not able to disinfect. DBPs are considered later in this chapter.

Breakpoint Chlorination The point at which free chlorine first starts to appear during the process of adding chlorine is referred to as the chlorine break point. As chlorine is added to water a series of chemical reactions occur. The effect of these reactions on the level of chlorine in the water is shown in Figure 3.2. At first, a small amount of chlorine reacts with iron and manganese in the water and is converted to chloride. There is no measurable level of chlorine in the water at this stage.

22

Chapter 3 – Disinfection: The Final Barrier As more chlorine is added, some of it reacts more slowly with ammonia and other organic material in the water. During this period chloramines and other chlorinated organic chemicals are formed. These persist in the water, can be measured and do provide some disinfection. As still more chlorine is added, the chloramines and chlorinated organic chemicals are broken down and the level of measurable chlorine in the water actually decreases. Further addition of chlorine beyond the point where all these reactions have ceased results in free chlorine remaining in solution. The point at which free chlorine first starts to appear during the process of adding chlorine is referred to as the chlorine break point. This free chlorine is able to react with anything that may subsequently appear in the water including microorganisms.

Figure 3.2. Typical chlorine breakpoint graph.

C

B

Reducing compounds eg Fe & Mn destroy chlorine

Formation of chloroorganics and chloramines (by products)

D

Chlororganics and chloramines partly destroyed

Breakpoint

Chlorine residual

A

Free available residual formed

Combined residual Chlorine that is added The process of breakpoint chlorination therefore refers to the addition of sufficient chlorine to meet all the chlorine demand, leaving free chlorine in the water to further react with microorganisms as necessary. Once all the demand has been satisfied, the chlorine can be measured as free available chlorine. It is important to realise that to achieve a free chlorine residual of say 0.8 mg/L much more than 0.8 mg/L of chlorine will need to be added to the water. However once the break point is reached, further addition of chlorine, for example 1mg/L, will result in a 1 mg/L increase in the free chlorine residual.

Most operators have probably heard of breakpoint chlorination and many may have seen the graph shown in Figure 3.2. A simple exercise is to actually generate this curve by adding chlorine in small amounts (say 0.2 mg/L) at a time to water and measure both free and total chlorine. Plot the results on graph paper. The process of breakpoint chlorination will become very clear.

Chlorine Residuals Hypochlorous acid in water, either as the acid itself or as the hypochlorite ion is commonly known as free residual chlorine. Chlorine reacted with other chemicals (in particular with ammonia to form chloramines) but still able to act as a disinfectant is referred to as combined residual chlorine. The sum of the free residual chlorine and combined residual chlorine is known as the total residual chlorine.

Free residual chlorine + Combined residual chlorine = Total residual chlorine

23

Chapter 3 – Disinfection: The Final Barrier Past the breakpoint, free residual chlorine is maximised and combined residual chlorine is at its minimum due to the breakdown of the combined residual in the presence of free residual chlorine. Therefore taste and odour due to combined residual chlorine is at its minimum past the breakpoint, however taste and odour due to free residual chlorine is more apparent.

Chlorine Decay As water with a free chlorine residual passes through pipes in a distribution system, the level of free chlorine decreases. This decrease in the free chlorine residual is referred to as chlorine decay. The amount of decay depends on the: • • • • •

Length of the system.

Cleanliness of system (biofilms, corroding fittings).

Disinfectant demand of the water.

Sediments.

Temperature.

To ensure that disinfected water reaches the consumer, it is important that an adequate free chlorine residual is maintained in the distribution system to control minor recontamination of the water from pipe breaks or backflow of water from houses into the mains or poor maintenance practices. This could be due to the free chlorine residual typically maintained in distribution systems is generally sufficient to maintain the quality of the water and control biofilms to some extent. The residual however is not enough to re disinfect the water if there are breaks in the distribution system (either accidental or deliberate) and fresh pathogens, particularly viruses and protozoans, contaminate the water. In these situations, full disinfection with the appropriate Ct is required (see Mosse and Deere 2009).

The Chemistry Of Chloramine Disinfection A modification of the chlorine disinfection process is the use of chloramines for disinfection. Chlorine can react with ammonia (NH3) to form a group of chemicals called chloramines. These include monochloramine (NH2Cl), dichloramine (NHCl2) and trichloramine or nitrogen trichloride (NCl3) as summarised in the reactions below.

Monochloramine

NH3 + HOCl  NH2Cl + H2O

Dichloramine

NH2Cl + HOCl  NHCl2 + H2O

Trichloramine (Nitrogen trichloride)

NHCl2 + HOCl  NCl3 + H2O

Monochloramine has approximately 1/150th the efficiency of chlorine as a disinfectant, while dichloramine has approximately 1/80th the efficiency of chlorine as a disinfectant.

If the efficiency of chloramines is so much lower than chlorine, the natural question is why would they be used in the first place. The answer is that chloramines, while less efficient than chlorine, have certain advantages that ensure their continued use. They do not form DBPs to the same extent as chlorine and they also persist for longer in the distribution system. Chloramine disinfection is covered in more detail in Chapter 8.

24

Chapter 3 – Disinfection: The Final Barrier

Disinfection By Products Disinfection by products form when the disinfectant interacts chemically or physically with other chemical compounds in the water. In the vast majority of cases it is the interaction of the disinfectant with NOM that results in the formation of DBPs. The interaction of chlorine with NOM, results in the formation of a number of DBPs. The most important of these are the Trihalomethanes (THMs) and Haloacetic acids (HAAs). Chloral Hydrate is also formed as a by-product during chlorination. Table 3.5 lists the common DBPs and the ADWG (2004) health limits.

Table 3.5. Common DBPs and their ADWG health limits.

DBP Total THMs* Individual THMs* Trichloromethane (chloroform) Bromodichloromethane Dibromochloromethane Tribromomethane (bromoform) HAA Chloroacetic acid Dichloroacetic acid Trichloroacetic acid Other Chloral Hydrate 2 Chlorophenol 2,4 Dichlorophenol 2,4,6 Trichlorophenol

Limit (µg/L) 250 250 250 250 250 150 100 100 20 300 200 20

* Neither the sum of all the THM concentrations or the concentration of an individual THM may exceed 250 µg/L.

THMs usually make up about 30% to 50% of the DBPs after chlorination and may contain both chlorinated and brominated forms. The relative proportion of brominated to chlorinated THMs depends on the levels of bromide in the raw water. Where the levels of bromide are higher in the raw water, higher proportions of brominated THMs are produced.

The amount of THMs formed depends on: • • • •

•

The concentration of chlorine. The higher the concentration of chlorine used, the greater the formation of THMs. The time that the water is exposed to the chlorine residual. Longer exposure times can result in higher concentrations of THMs. The concentration of THM precursors (NOM). Higher NOM levels lead to higher concentrations of THMs. The number of times the water is rechlorinated. Secondary chlorination is used to boost chlorine residuals in long distribution systems operated with chlorine. Chlorine may also be added to chloraminated systems to manage nitrification. Each time additional chlorine is added, the concentrations of THMs may increase.

The extent of biofilm formation in the distribution system. Dirty systems will result in higher DBP formation than clean systems.

25

Chapter 3 – Disinfection: The Final Barrier Over the years, DBPs have been linked to cancers and premature births. The literature is confusing but most recent research seems to indicate that the links are very weak indeed. Suffice to say that if the levels of DBPs are less than the health limits stated in the ADWG, the risk is extremely low. The consequences of not disinfecting can be much more serious as the Walkerton (Canada) event in 2000 showed. In a township of 4800 people, 2300 persons became ill and 7 died. The reason for this was that for long periods of time the water was not chlorinated. Just prior to the outbreak, heavy rain washed water contaminated with cow manure into the main well in use at the time. E. coli O157 H7 present in the cow manure was therefore allowed into the distribution system without any disinfection. Walkerton must serve as a clear reminder that disinfection must never be compromised.

Where the necessary disinfectant dose results in unacceptable levels of DBPs, steps must be taken to alter the chemical composition of the water prior to disinfection so that the DBP levels are reduced or alternative disinfectants used. The most effective way to do this is to reduce the NOM content of the water. Some NOM is removed during normal conventional water treatment however increased amounts of NOM can be removed by: • • • •

Enhanced coagulation using high doses of alum at low coagulation pHs. Ozone BAC. MIEX resins.

UV/H2O2.

Where this fails, the only option may be to use chloramine disinfection, however this is not always as easy as it sounds (See Chapter 8).

Until recently it was thought that the use of chloramine for disinfection did not result in the formation of DBPs. This is true for THMs and HAAs, however recent studies have found N-nitrosodimethylamine (NDMA) to be formed after chloramination. NDMA has been linked to bladder cancer.

Regardless of the disinfection system used and the nature of the DBPs formed, primary disinfection must not be compromised.

Comparison Of Disinfectants Chlorine and chloramine differ in many ways and have various advantages and disadvantages (see Table 3.6). So, when should the different disinfection systems be used?

Chlorine is suitable for use in relatively short distribution systems and where the water to be disinfected has a low NOM content. Where the water to be disinfected has a high NOM content or the distribution system is long, chloramination should be considered.

The use of chloramine presents a number of unique problems.

•

•

26

Chloramines are poisonous to kidney dialysis patients and also to aquarium fish. Therefore utilities planning to implement chloramination for the first time, or converting a chlorinated system to a chloraminated system, must consult widely with the community prior to implementation. This is to ensure these groups are aware of the change and have taken the necessary steps to protect their own health or the health of any aquarium fish.

The conversion to chloramination may result in a period of increased dirty water and increased taste and odour problems, however these generally decrease and result in fewer taste and odour complaints than experienced with chlorine.

Chapter 3 – Disinfection: The Final Barrier Table 3.6. Comparison of chlorine and chloramine as disinfectants.

Equipment Reliability Relative complexity of technology Safety concerns Control bacteria Control viruses Control Cryptosporidium Control of Giardia Control amoebae (N Fowleri) Control regrowth in biofilms # DBPs Residual Formed Persistent Residual Contact time (Ct) required pH Dependent Taste and odour Cost Operation and maintenance Installation

Chlorine Good Simple Yes Good Moderate No Moderate Yes Moderate Yes Yes Moderate (<5 days) Moderate Yes Yes Low Easy Easy

Chloramine Good Simple to moderate Yes Good Poor No Poor Yes Good Fewer Yes Long (>5 days) Longer Yes Low Moderate More complex More complex

# Particularly in distribution systems, chlorine can fail to control coliform bacteria in biofilms whereas chloramine is better able to control any coliforms present. One of the features of the application of chloramine is a decrease in coliform numbers in the distribution system. This is generally thought to be due to the fact that chloramines penetrate the biofilm more completely and provide better control especially in longer distribution systems where water detention times are high.

The use of chlorine or chloramine will also be dependent on the levels necessary to effect disinfection. Table 3.7 summarises the ADWG aesthetic and health limits for chlorine and chloramines and provides guidance as to the maximum levels of the different disinfectants.

Table 3.7. ADWG (2004) limits for chlorine and chloramine disinfectants.

Type Chlorinated systems Chloraminated systems

Measure Free Chlorine (mg/L) Total Chlorine (mg/L)

Aesthetic limit # 0.2 to 0.6 0.5

Health Limit 5 4.1*

The ADWG (2004) do not specify aesthetic limits for chlorine or monochloramine but note that chlorine has an odour threshold of about 0.6 mg/L but may be detected at 0.2 mg/L by sensitive individuals. Monochloramine has an odour threshold of 0.5 mg/L. * 4.1 mg/L total chlorine is equivalent to 3 mg/L monochloramine.

#

Disinfectant levels anywhere near the health limit will result in poor quality water with unpleasant taste and odour, particularly for chlorine systems. Since it is our responsibility to supply safe water but also to provide water that is pleasing to drink, water utilities should do everything possible to keep residuals below levels that lead to significant numbers of taste and odour complaints.

27

Chapter 4 Practical Operational Issues Safety Chlorine Gas Chlorine gas is poisonous at low concentrations. Care must therefore be taken when working with chlorine gas. Gas chlorination installations must be designed to conform to Australian Standard AS 2927:2001 The storage and handling of liquefied chlorine gas. AS2927 is a document capable of some interpretation and each utility should ensure that all chlorine sites have been inspected and approved by the relevant State authority.

All operators must be specifically trained in the use of chlorine gas and equipment. Regular refresher training is also required. The chlorine gas manufacturers run safe handling courses and a number of Registered Training Organisations (RTOs) offer a range of disinfection courses. The National Water Package (NWP) has a unit of competency specifically for working safely with liquefied chlorine gas.

Chlorine gas is yellow green in colour with a strong smell of bleach. The smell is also sometimes described as being sweet. Gaseous chlorine causes irritation of the respiratory system and mucous membranes of the eyes and nose. The degree of irritation depends on the concentration of the chlorine gas and the person’s exposure time.

Chlorine is much heavier than air and will tend to stay “near the floor” and “flow along the floor”. If chlorine gas can be seen, it is extremely dangerous. If liquid chlorine can be seen it is even more dangerous since liquid chlorine rapidly “boils” and forms enough gas to occupy a volume approximately 450 times the volume of the liquid. For this reason any leak where chlorine gas or chlorine liquid is visible should be managed by emergency services personnel. Heat should never be applied to a chorine container and water should never be applied to a leaking or burning container.

All sites with chlorine gas should have: • • • • • •

Clearly visible signs as required by State and National regulations.

A wind direction indicator to allow staff, nearby residents and emergency services to know which way the leaked gas will disperse.

Eye washes and emergency showers in close proximity to the site but not where exposure to the gas is likely in the event of a leak.

5% ammonia solution for the detection of minor leaks.

Leak detectors that activate a local alarm, flashing light and also a SCADA alarm. If an alarm is activated, or if chlorine is smelt, then it must be assumed that there is a leak and appropriate procedures (see Chapter 10) followed for its identification and correction.

Self-Contained Breathing Apparatus (SCBA) should be used at all times when new cylinders or drums are connected or when adjustments are made to the connections to the existing cylinders or drums. Changeover and emergency drills should be practiced regularly. The SCBA should not be located in the chlorine storage room. It should be located at least 10 metres away from the chlorine installation in a readily accessible location upwind of the most prevalent wind direction.

A good solution is to have an SCBA at the entrance gate to the site where the wind direction indicator and leak alarm flashing light are clearly visible.

Since many chlorine dosing sites are now fully contained within personnel proof fences, it is recommended that there be emergency exit gates on each side of the site to allow escape in the event that the site becomes unsafe due to a chlorine leak.

28

Chapter 4 – Practical Operation Issues Due to the considerable risk associated with a chlorine leak, consideration should be given to conducting scenario training with emergency services personnel so that all parties involved in the management of an incident from operators to police, emergency services and even media and PR representatives know what their roles are and can practice them together.

Sodium Hypochlorite Compared to chlorine gas, liquid sodium hypochlorite is relatively safe. In fact sodium hypochlorite is commonly used around the home as household bleach. However it can still cause damage to the eyes and mucous membranes of the nose and upper respiratory passages. Appropriate personal protective equipment (PPE) should always be used when working with sodium hypochlorite. This includes a minimum of safety glasses and gloves. A long waterproof apron is also appropriate to prevent damage to clothes.

Safety showers and eyewashes must be present at all disinfection sites.

Many sodium hypochlorite facilities have a definite chlorine smell and this is regarded as normal and just ignored. A well designed and operated liquid facility should not smell, when there is a smell, care must be taken to protect the operator.

Ammonia Use of chloramines requires the addition of chlorine and ammonia. Ammonia can be added as a gas or liquid in the same way as chlorine. Similar safe work procedures are required when handling ammonia due to it being poisonous.

Calcium Hypochlorite Calcium hypochlorite is usually supplied as white granules or tablets. Typically the tablets are used in floating pool dispensers located in tanks or reservoirs to provide some additional chlorine.

Calcium hypochlorite must be stored dry and must not be exposed to heat. It must be stored by itself and well away from any material that can burn. Spontaneous fires have resulted from inappropriately stored calcium hypochlorite.

Safety gloves and eye protection must always be worn when handling calcium hypochlorite.

Available Chlorine Although chlorine gas is poorly soluble in water, it is considered that 100% of the gas is available to dissolve.

Historically other sources of chlorine, or more correctly hypochlorous acid in water, were compared to the availability of chlorine from chlorine gas. The amount of available chlorine was determined by reacting the chlorine product with an acidic iodine solution, and measuring the amount of iodide produced. The amount of iodide produced is directly proportional to the amount of chlorine available. A summary of the main disinfection products and the available chlorine is provided in Table 4.1.

29

Chapter 4 – Practical Operation Issues Table 4.1. Summary of available chlorine for the major chlorine products.

Product Chlorine gas Sodium hypochlorite Calcium hypochlorite

Available Chlorine (%) 100 12 to12.5 60 to 65

Note that sodium hypochlorite solutions can also be supplied as 5% and 3% solutions.

The concept of available chlorine has some limitations but does provide information on the relative disinfecting power of the different available chlorine based disinfectants.

Effect Of Chlorine Products On Water Use of chlorine gas results in the water becoming more acidic. In contrast, use of sodium hypochlorite (or indeed calcium hypochlorite tablets) increases the pH of the water (See Chapter 3). Operators should be aware of this and the effect of pH on disinfection efficiency when optimising the water treatment process at a plant. Due to the different characteristics of the raw water, the effect of the different chlorine disinfectants on pH can be used to advantage. Thus in a low alkalinity, low pH water, sodium hypochlorite in theory would be the disinfectant of choice. In contrast, in water with a higher pH, gas chlorine can help to reduce the pH. While these considerations are useful, the practical application of the disinfectant may limit the application.

For example a water supply system might be too small for gas chlorination, alternatively it may be too big for a liquid system to be economical.

The impact of the different types of chlorine on the pH of the water must be remembered when considering changing from one type of chlorine to another. For instance, many gas systems require alkali to be added after dosing to return the pH of the final water to acceptable levels. If there is a change to sodium hypochlorite, the alkali might no longer be required or perhaps in lesser amounts.

Sodium Hypochlorite Decay Sodium hypochlorite solutions are unstable and will start breaking down as soon as they are manufactured.

The amount of available chlorine therefore reduces over time. The rate of decay is more noticeable in the higher concentration solutions and less so in lower concentration solutions.

Sodium hypochlorite decay can occur in two ways. In the presence of light or heat, sodium hypochlorite breaks down to form sodium chlorate. In the presence of impurities such as nickel, copper and iron, it breaks down to form oxygen gas.

Sodium hypochlorite is commonly supplied in Australia as a 12.5 % (equivalent to125 g/L) solution of chlorine.

This strength rapidly decays requiring increased volumes to be dosed to compensate for the decay. In many situations, particularly smaller dosing installations, a 3% or 5% solution of sodium hypochlorite may be a better alternative because the chlorine decays much more slowly. The problem of compensating for the decay can be best managed by using a residual trim dosing system (see Chapter 9).

Decay is increased at higher temperatures and by exposure to sunlight. For this reason, sodium hypochlorite tanks should be fully enclosed but vented and protected from direct sunlight by shading. Air-conditioning can also be used to minimise decay. Decay will also be increased if metals such as stainless steel are in contact with the sodium hypochlorite.

30

Chapter 4 – Practical Operation Issues Where sodium hypochlorite is to be used, care should be taken to carefully select the size of the tank so that there is sufficient storage for normal operation but not excessive storage that simply allows decay to occur.

Similarly, metering pump sizes need to be selected carefully for the dose required. This in turn depends on the stock concentration of sodium hypochlorite and the flow of water.

If sodium hypochlorite is diluted with tap water this can also cause decay due to contaminants such as iron and manganese present in the water. Ideally distilled water should be used however this is not always possible. Therefore every effort should be made to use water with very low metal levels.

Calcium Hypochlorite Decay Calcium hypochlorite will lose about 3% to 5% available chlorine per year. If the product is stored in a damp location or if the product is exposed to humid air, the decay rate will be increased. In general, store only sufficient calcium hypochlorite for a sensible period (say 3 months). It is better to reorder stock on a fairly regular basis rather than storing large amounts of it.

Chlorine Demand And Chlorine Residual Decay In practice, chlorine is often added to water and the dose dialled up and down until things “seem” to be OK. While this may be satisfactory sometimes, it is worthwhile knowing more about the chlorine demand of the treated water to be disinfected. The chlorine demand can be measured by simple tests. Chlorine demand is “the difference between the added chlorine dose and the residual chlorine concentration measured after a prescribed contact time, temperature and pH” (Standard Methods 2004). A simplified chlorine demand test based on this method is provided in Appendix A. By regularly measuring the chlorine demand, changes in raw water quality that may have an impact on treatment and/or disinfection can be tracked. Changes in chlorine demand can explain the need for changes in chlorine dose. Knowing the chlorine demand in advance allows better control of chlorine dosing rather than just leaving it to guess work. Regular measurement of chlorine demand in conjunction with the measurement of Dissolved Organic Carbon (DOC) or UV absorbance at 254nm will allow the operator to better understand the changing nature of the water and be better able to proactively adjust chlorine dosing rates. In this way operators can help to avoid overdosing and its associated consumer complaints and also avoid under dosing and the associated risk of exposing consumers to an increased risk from pathogens.

Contact Structures And Mixing The concept of the Ct value requires that all the water to be disinfected be exposed to the disinfectant for the necessary time at all times, including during peak flows. To achieve this, the water must be thoroughly mixed with the disinfectant and then held in “contact” for the necessary time to achieve the Ct. Mixing can be achieved by combining injection lances (Figure 4.1) with inline static mixers (Figure 4.2) or mechanical mixers (Figure 4.3).

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Chapter 4 – Practical Operation Issues Figure 4.1. An injection lance.

Figure 4.2. Examples of two types of inline static mixers.

Figure 4.3. An inline mechanical mixer.

Many systems simply rely on the length of pipe or the presence of valves or pipe bends to achieve mixing.

Most seem to work reliably, however, where bends and valves are absent or the distance to the contact tank short, purpose built mixers should be installed.

Achieving the required contact time after mixing requires a carefully designed contact tank or sufficient pipe length before the first customer. Short-circuiting of the contact tank must not be allowed to occur. A good contact structure requires purpose built baffles to ensure that the water flows as a unit through the tank.

Basins or tanks with a single inlet pipe and a single outlet pipe are prone to short-circuiting and represent the worst alternative. Baffles can be retrofitted to an existing basin or tank by use of divers and careful placement of flexible walls.

32

Chapter 4 – Practical Operation Issues The nature of contact structures can be described as: • • • •

Poor. Significant short-circuiting occurs with poor utilisation of the contactor volume. Typically the inlet and outlet pipes are close together and there are no intra-basin baffles.

Average. Some attempt to use baffles to limit short-circuiting. This could be a simple baffle plate placed in front of the inlet and/or outlet structure. There may be some crude intra-basin baffles.

Excellent. Purpose built contactors with baffles or walls arranged to prevent short-circuiting by forming long narrow flow channels. Separation of inlet and outlet pipes in both the vertical and horizontal planes. An excellent contactor is typically one with serpentine flow channels.

Plug Flow. The disinfected water undergoes no dilution or further mixing in the contact tank system. All elements in the mixture have the same residence time. This is best approximated by a long tubular reactor. i.e. pipeline flow.

Figure 4.4. Baffle systems that provide excellent contact between the disinfectant and the water.

Simpler systems can be retrofitted by just hanging vertical curtains that alternate with each other to provide a serpentine flow channel for the water (Figure 4.5).

Figure 4.5. Hanging curtain baffles provide a flow channel for improved contact.

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Chapter 4 – Practical Operation Issues In tanks with simple inlet and outlet pipes with or without bell mouths, short-circuiting is a real problem.

Some improvement can be achieved by extending the inlet and outlet pipes and getting them as far away as possible from each other both in a vertical and horizontal plane (Figure 4.6).

Figure 4.6. Improved mixing can be achieved by modifying inlet and outlet structures.

Purpose built nozzles (Figure 4.7) can be retrofitted to provide directional flow and mixing in treated water storages.

Figure 4.7. An example of a directional nozzle that can be retrofitted to an inlet to a tank to provide better mixing and therefore better contact.

Where “contact tanks” are large and demand low, thermal stratification may occur and enhance chlorine decay. In these situations the tank contents must be kept mixed by the installation of a mixing system.

Storage levels should also be changed throughout the year to prevent long detention times. 24 hours storage should be adequate security in most circumstances. During periods of high demand, for example fire fighting or hot weather, the storage level can be increased. While large storages are attractive to some, the fact is that if the water does not turn over regularly, water quality deteriorates, DBP’s increase and chlorine residual management becomes more difficult.

34

Chapter 4 – Practical Operation Issues

Detention Time Detention times are often calculated by simply dividing the total volume of a storage by the flow without any consideration of how the flow occurs through the storage. Such calculations very much overestimate the detention time unless the storage is purpose built and well baffled. Short-circuiting results in poor control of Ct. The best way to determine Ct is to measure it. A pulse of high strength chlorine can be used to measure how long it takes for water to pass through the storage. A pulse of calcium or fluoride can also be used for this purpose. Increase the strength of the chlorine dose (double it) for several minutes and then measure the chlorine residual in the outgoing water. When the chlorine residual goes up, the water with the high dose chlorine must have passed through the storage. Keep measuring the residual. It is interesting to see the total range of time for the water to pass through. In poorly baffled storages the spread of time can be quite large.

Detention time should be measured at different times of the year so that the Ct can be fairly accurately determined during periods of differing demands.

Minimum Daily Ct Since Ct is such an important determinant of effective disinfection, it is essential that it be monitored and recorded. Once a target Ct has been established, the achievement of this objective needs to be monitored closely. Remember, the disinfection and filter control points are the two most important control points for the control of pathogens.

One option is to report on the daily minimum Ct. This can be determined from SCADA trends of chlorine residual and flow. If the daily minimum Ct is less than the target objective set in a risk management plan, then that is cause for some concern. If the minimum daily Ct is zero then undisinfected water has left the disinfection site. Such a situation is unacceptable and should be reported to the Health Regulator and steps put in place to ensure that it cannot happen again.

Backup Systems Since disinfection is an important barrier to pathogens, it is absolutely essential that no water passes into the distribution system without having been disinfected. This requires knowledge of how disinfection failure might occur and what should be done about it.

Whatever the cause of the loss of disinfection, systems must be established to ensure that if there is a disinfection failure that no undisinfected water is allowed to enter the distribution system. The simplest option is to immediately stop the flow when disinfection failure is detected.

Control valves should fail closed in the event of a power failure and the absence of an alternative power source. Where water flow cannot be stopped, the changeover system must be totally failsafe.

Whatever the system, it must be tested and verified regularly and records to that effect retained. Indeed if flow cannot be stopped under any circumstance, consideration should be given to the installation of two back up metering pumps or venturis and two or even three chlorine analysers.

Table 4.2 provides a summary of some possible causes and actions to prevent undisinfected water entering the distribution system.

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Chapter 4 – Practical Operation Issues Table 4.2. Types of disinfection failures and suitable preventive measures.

Cause Power failure

Preventive measure If possible have control valves close on loss of power to prevent flow of undisinfected water.

Install an Uninterruptible Power Supply (UPS).

Install a back up generator.

Loss of dosing chemical (gas)

Install vacuum monitors and alarms.

Install load scales on chlorine gas cylinders and drums.

Set up alarms for low and low low levels.

Prevent water transfer if there is no chlorine residual e.g. high lift pump or valve shut down in response to low chlorine alarm.

Inspect regularly.

Install automatic cylinder changeover.

Test alarms and changeover regularly as part of a preventive maintenance program.

Loss of dosing chemical (liquid)

Install flow monitors on metering pumps.

Install level sensors on storages of all liquid dosing chemicals.

Set up alarms for low and low low levels.

Prevent water transfer if there is no chlorine residual e.g. high lift pump or valve shut down in response to low chlorine alarm.

Consider automatic reordering with chemical supply company based on telemetered levels.

Inspect tanks regularly.

Metering pump failure

Monitor metering pump flow.

Ensure duty standby pumps are in place and duty is changed regularly.

Ensure regular preventive maintenance is carried out.

Prevent water transfer if there is no chlorine residual e.g. high lift pump or valve shut down in response to low chlorine alarm.

Disinfection Failure In the event that a failure of primary disinfection occurs, the response must be rapid and effective. Responses must be planned well in advance and practiced. Incident and emergency responses will only work well if they are well documented and practiced.

The type of response can be based on the degree of risk. If it is known or strongly suspected that consumers are receiving undisinfected water, the response must be immediate. If there is any uncertainty at all as to whether the water is disinfected or not, the response must also be immediate. A delayed response is only acceptable if there is absolute certainty that no consumers are or will be receiving undisinfected water. If in doubt, assume the worst and respond accordingly.

The only really acceptable response in this situation is to notify the Health Regulator and issue an immediate Boil Water notice. Boiling water is the ultimate barrier to pathogens. All pathogens will be killed if water is boiled for several minutes.

36

Chapter 4 – Practical Operation Issues Identification of the affected area and the notification of the Boil Water notice can be difficult, however systems must be developed to allow a prompt response to occur.

To limit the duration of the Boil Water notice, flushing is essential. A chlorine residual must be restored as soon as possible throughout the entire system, including all dead end mains. Samples for microbiological testing must also be taken to confirm the system is pathogen free as part of the recovery from such an event.

If a high level of chlorine occurs (>5 mg/L) the public must be notified not to use the water. The distribution system needs to be flushed immediately and normal levels of chlorine re established as soon as possible.

Discharge Of Chlorinated Or Chloraminated Water Many operations in the water industry require the discharge of water that contains a chlorine or chloramine residual. Water needs to be discharged from tanks and mains during draining, flushing, cleaning and repairs.

This discharged water has the potential to harm the environment. Chlorine, chloramine and ammonia are poisonous to aquatic life. Highly turbid water discharged to a waterway may result in settling of sediment and possible “choking” of animals living on the bottom. The sediment can also reduce light penetration prior to it settling.

Before any water is discharged to the environment, an assessment of its impact on the environment needs to be carried out. In most situations the EPA needs to be notified and in most States there are restrictions imposed on the discharge of water with disinfection residuals present. Prior to any discharge, the EPA should be contacted to find out what the requirements are in your State. Typically a permit is required from the EPA where a discharge is to a waterway (including dry water courses) and where the water contains chlorine, chloramine or ammonia. The permit is also likely to state the maximum residual allowed to be discharged and may stipulate the maximum volume. In some cases a zero residual may be required. Where this is the case, dechlorination can be carried out using sodium metabisulfite, sulfur dioxide, sodium sulfite or sodium thiosulfite and a portable metering pump. As a guide, sodium metabisulfite dosed at a concentration of approximately 3:1 to 3.5:1 metabisulfite to free chlorine will adequately dechlorinate water. Crystalline sodium metabisulfite can be obtained and stored and made into a solution of known concentration when required.

Based on the chlorine residual in the water the amount of sodium metabisulfite solution can be calculated and delivered. A modification of this method is to make up a saturated solution of sodium metabisulfite and to dose the liquid phase into the water to be dechlorinated. The dose is simply adjusted to give zero chlorine residual just downstream of the dosing point. For either method the chlorine residual should be measured very regularly during the discharge to ensure complete dechlorination.

Another alternative to manage the residual prior to the water being discharged to a waterway is to run the water over the ground for several hundred metres. This will be more effective if the water flows through grass.

If this method is used it is important to ensure that no erosion occurs as a result of the water flowing over the ground. This method also requires regular monitoring of the chlorine residual prior to the water entering the waterway. Where an ammonia residual is present, aeration of the water will remove it. Aeration can be achieved by spraying the water into the air. The more alkaline the water the easier the removal will be. Once again, testing must be carried out to verify that the method is working satisfactorily.

Any spills of chlorine or ammonia to the environment should be reported.

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Chapter 5 Disinfectant Dosing Calculations Calculations are an important part of water treatment. While it is possible to make adjustments to dosing by just “twiddling dials”, it is far better to have an understanding of the underlying chemistry and to be able to make simple calculations. Disinfection is one area where calculations are particularly relevant. The purpose of this chapter is to provide examples of the main types of calculations necessary for the operation of disinfection facilities.

Concentrations of disinfectants are commonly expressed as mg/L or ppm (parts per million) weight/volume.

Chlorine is most commonly supplied as either a gas or a solution of sodium hypochlorite (hypo). When gas is used, all the gas is available as chlorine. This is not the case with sodium hypochlorite that is commonly supplied as a 12.5%, 5% or 3% solution. Only a small part of the total volume in these solutions is available as chlorine. The characteristics of these hypo solutions are provided in Table 5.1.

Table 5.1. Characteristics of typical sodium hypochlorite solutions used at disinfection plants.

Solution 12.5% 5% 3%

Available Chlorine 125 g/L 50 g/L 30 g/L

Specific Gravity @ 25oC 1.17-1.18 1.070-1.090 1.040-1.060

12.5% hypo solutions are normally purchased for disinfection of water supply systems. Sodium hypochlorite has a limited shelf life. The concentration of chlorine will reduce with time and so will the effectiveness of disinfection using the aged solutions unless a residual trim control system is used (see Chapter 9). Where hypo is stored in small containers for periodic use, e.g. for dosing mains or tanks any old chemicals should be disposed of and replaced with new chemicals at least once per year.

The actual concentration of stock sodium hypochlorite solutions is best measured by titration. Simple test kits are available for this purpose. While there are tables of SG (specific gravity) available for solutions of different strengths, use of SG to determine the strength of a solution or the rate of decay is not reliable. Calculations may be done in a number of different ways all ending up with the correct answer. In the examples below one method of doing the calculations is shown. Feel free to use a different method provided you get the same answer.

Calculation Of Dose Rate Step 1. Calculate The Concentration Of The Active Ingredient For chlorine gas the concentration of active ingredient is 100 % or 1000 g/kg.

For a 12.5% hypo solution the concentration of chlorine is 125 g/L, while for a 5% hypo solution the concentration of chlorine is 50 g/L. If the stock solution might have decayed, the actual concentration should be measured by titration.

Step 2. Calculate The Dose Rate In The Plant

Gas If 1 kg of gas is added to 1 ML of water that has no chlorine demand (doesn’t actually happen due to the presence of dissolved organics in the water) the dosage rate of chlorine will be 1 mg/L. 38

Chapter 5 – Disinfectant Dosing Calculations In reality the chlorine demand in natural raw waters will result in a total chlorine concentration of less than 1 mg/L and a free chlorine concentration less than that.

More commonly a gas chlorine feed rate will be known. For example, at a gas site the feed might be 5 kg/day into a flow of 4 ML/d. This corresponds to a dose of 5 kg/4 ML = 5000 g/ 4,000,000 L = 5,000,000 mg/4,000,000 L = 1.25 mg/L

Hypo To calculate the dose rate the following information is required: • Flow rate in L/s.

• Metering pump rate in mL/s.

• Batch strength in g/L (from Step 1).

The metering pump rate should be determined from a volumetric draw down test. The dose rate can be calculated using the following formula.

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

Flow rate (L/s) Example 1. In a small plant the draw down for 12.5% hypo was measured as 100 mL in 65 seconds. This is equal to 1.54 mL/sec. Plant flow from the plant flow meter was 120 L/s. Therefore Chemical dose (mg/L Cl2) = 1.54 x 125 120 = 1.60 mg/L Note that these calculated doses only provide the dosing rate. In the case of chlorine they will not be equal to the free chlorine residual (in fact nowhere near the free chlorine residual). The free chlorine residual will be very much less due to the chlorine demand exerted by the water. The question may well be asked why then calculate the dose rate? Knowing the dose rate is useful for two reasons. Firstly it will allow a check on the long-term dose rate and use of chemical. Secondly it will allow an estimate of the chlorine demand by knowing the difference between the dose in mg/L and the free chlorine residual in mg/L.

Verification Of Long Term Dose As described in Chapter 3, it is useful to check the dose rate by periodically checking the mass balance.

Example 1. At a gas chlorine site the monthly records show that 950 kg of chlorine gas was used and that the total flow through the disinfection site was 740 ML. The dose therefore was 950 kg/740 ML = 1.28 kg/ ML = 1,280,000 mg/ 1,000,000 L = 1.28 mg/L This figure can then be compared with the residual analyser data for the period. Chlorine demand will reduce the figure.

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Chapter 5 – Disinfectant Dosing Calculations Based on the chlorine analyser data it may be possible to estimate the chlorine demand. This would allow future calculations to be more meaningful. Alternatively, samples can be submitted to an accredited laboratory for the determination of chlorine demand. These figures can then be incorporated in the calculations.

Any major discrepancy in use for approximately the same water quality should be immediately investigated as an under dosing or over dosing may have occurred.

Example 2. At a hypo site 12.5% hypo is used. Monthly records show 210 L hypo was used for a total volume of 22 ML The concentration of the active ingredient for the 12.5% hypo is 125 g/L. Therefore the total amount of chlorine added is

= 125g/L x 210 L = 26,250 g Therefore the average dose for the month was

= 26,250 g/ 22 ML

= 26,250,000 mg/ 22,000,000 L = 1.19 mg/L Again this figure can be compared with data from the residual analyser for the period.

By measuring the chlorine demand periodically during the month, the operator can then do a sensibility check to see whether the total amount dosed relates well to the residual experienced during the month. Any major discrepancy should be investigated immediately.

Calculation Of Changed Dose Perhaps the most common requirement at a disinfection site is to change the dose. For example the measured chlorine residual might be 1.2 mg/L free chlorine. The required free chlorine might be 1.5 mg/L. Sure the rotameter or the dosing pump can be “twiddled” to achieve the necessary increase by trial and error, however it is better to calculate how much extra needs to be added.

The following example is for a gas chlorine site Example 1. At a gas chlorinator site the residual analyser reads 0.25 mg/L. The required residual is 1.2 mg/L. The current chlorine feed rate is 45 kg/d and the flow is 1300 L/s. What does the feed rate need to be increased to?

Note: With all calculations it is important to ensure you use the same units.

Convert flow to ML/d 1300 L/s = 1300 x 60 x 60 x 24/1,000,000 ML/d = 112 ML/d Current dosing is 45 kg/d into 112 ML/d = 45 kg/112 ML each day = 45,000,000 mg/ 112,000,000 L = 0.40 mg/L 40

Chapter 5 – Disinfectant Dosing Calculations Since the measured free residual is 0.25 mg/L, the chlorine demand must be = 0.40 – 0.25 = 0.15 mg/L Therefore the required dose is the chlorine demand plus the required chlorine residual. = 0.15 + 1.2 mg/L = 1.35 mg/L and the plant flow stays the same!

Since the plant flow hasn’t changed, the site requires 1300 L/s x 1.35 mg/L = 1755 mg/s = 1755 x 60 x 60 x 24 mg/d = 1755 x 60 x 60 x 24/1,000,000 kg/d = 152 kg/d Therefore the dosing needs to increase from 45 kg/d to 152 kg/d to raise the free chlorine residual to the required 1.2 mg/L. Getting the dose right by twiddling the dials would be quite difficult!

Similar calculations can be made for a sodium hypochlorite dosing changes.

Note: Whenever a dose is changed, the new dose must be checked by monitoring the chlorine residual leaving the plant. In addition, a check using a volumetric draw down test for hypo or a weight check for gas chlorine is also required.

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Chapter 6 Chlorine Gas Disinfection Systems Chlorine gas was first discovered in 1774 by Swedish chemist Karl W. Scheele and was named “dephlogisticated muriatic acid”. Many years later in 1810, Humphry Davey gave it the name chlorine, derived from the Greek word chloros that roughly translates as “green” or “greenish yellow”. This latter name is clearly in reference to the colour of the gas.

Chlorine gas has been widely used in the water and wastewater industry since the early 1900’s.

Since chlorine gas is poorly soluble in water, any system using gas must provide excellent mixing of the water and gas. The first chlorinators were so called “pressure” chlorinators. Chlorine gas was delivered under pressure and was usually injected into the water or wastewater via some type of diffuser. In contrast, modern gas chlorination systems use a vacuum to “drag” or “suck” the gas into a carrier water system and then deliver this “chlorine solution” into the water or wastewater. Modern chlorinators also have sophisticated controllers that finely control the dose of chlorine.

Only vacuum systems will be considered further in this guide. A typical vacuum gas chlorination system is shown in Figure 6.1.

Types of Vacuum Chlorinators There are two types of vacuum chlorinators used in the industry. These are differential pressure regulated chlorinators and sonic chlorinators (see Figure 6.2).

Differential Pressure Regulated Chlorinators Differential pressure regulated chlorinators use a regulating valve (see Figure 6.2) to ensure that the pressure drop across the control device (V notch or control valve) is constant at all times. This ensures that the set flow rate on the rotameter stays constant even when minor variations in vacuum created by the venturi are present. Differential pressure regulated chlorinators are available in capacities up to 200 kg/hr.

Sonic Chlorinators A sonic chlorinator draws its name from the fact that if the vacuum created is high enough, the rate of gas flow can be raised to the speed of sound (hence the name “sonic”). Once this is achieved, there is no need for a differential pressure-regulating valve as the gas flow is now no longer a function of the pressure drop across the control device as is the case with a differential pressure regulator type. To achieve this “sonic” condition, a much higher vacuum is required and this requires more energy. The requirement to achieve the higher vacuum limits the maximum capacity of sonic chlorinators to 40 kg/hr.

Sonic chlorinators are best suited to small disinfection systems (< 40 kg/hr) with continuous operation. They are not as suited to systems that operate intermittently. This is because the vacuum generated at each start up may vary somewhat and consequently cause slight variations in dose. This results in a lower repeatability of dose than that provided by a differential pressure regulated chlorinator.

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Chapter 6 – Chlorine Gas Disinfection Systems Figure 6.1. A typical gas chlorination system.

Control device

Water Flow

Vacuum Monitor

Leak Detector

Automatic change over Venturi

Gas Storage Vessels Vacuum Regulators

Booster Pump

Sensor

Scale beds Vent lines to outside building or scrubber

Figure 6.2. Comparison of differential and sonic regulated chlorination systems.

Differential Regulator

Vacuum Regulator

Flow

Flow

Vacuum Regulator

Orifice

Orifice

Injector Rotameter

Chlorine Cylinder

Injector Rotameter

Chlorine Cylinder

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Chapter 6 – Chlorine Gas Disinfection Systems

The Gas Chlorination System In a gas chlorination system (Figure 6.1), a vacuum created by a venturi is transferred down a line, through a control device and into the vacuum regulator. This causes the vacuum regulator to open, allowing the chlorine gas from the drum or cylinder to flow through to the venturi. The control device controls the flow rate, either by manual adjustment or automatically by a remote process control signal that may come from a flow meter, chlorine residual analyser or plant PLC. Each of the chlorination system components will be considered below.

Gas Storage Vessels Chlorine gas is generally supplied in 70 kg cylinders or 920 kg drums (more commonly referred to as one tonne drums). Cylinders are deployed in a vertical position with the valve at the top, while the drums are deployed in a horizontal position with the openings at one end. Cylinders should be secured at all times to prevent them falling over.

The use of cylinders or drums in a chlorination system depends on the required gas flow. Drums and cylinders have a maximum “draw off” rate. This is the maximum amount of gas that can be drawn from the drum or cylinder at a given ambient temperature before the liquid inside the vessel becomes chilled (Table 6.1).

Table 6.1. Available draw off rates (kg/hr) of cylinders and drums (Orica 2006).

Temperature oC 5 10 15 20 25

920 kg drum 1.6 3.0 5.0 7.0 9.0

70 kg cylinder 0.4 0.7 1.2 1.7 2.3

Clearly, at higher temperatures more gas is available. A more important consideration however is that the amount of chlorine available decreases dramatically at lower temperatures. This is an important consideration in the design of a gas system. The system must be sized so that sufficient chlorine is available at the minimum expected temperature.

To ensure a constant draw off rate, the ambient temperature of the chlorine storage room should be controlled.

This in turn maintains the temperature of the drum or cylinder and hence controls the draw off rate. Under no circumstances should heat be applied directly to the drum or cylinder to try to increase the draw off rate.

If the draw off rate is exceeded, frosting can occur on the outside of the drum or cylinder. This may be visible as frost during the day or simply show up as a wet patch on the floor. The problem with exceeding the draw off rate is that the delivery of chlorine, and therefore the efficiency of disinfection, will be reduced. Exceeding the draw off rate can also damage the dosing equipment.

Even if the draw off rate from a cylinder is sufficient for the disinfection system, drums are sometimes used instead of cylinders to reduce the number of cylinder changeovers that an operator has to perform. This may be useful for remote plants or plants where a permanent operator is not in attendance.

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Chapter 6 – Chlorine Gas Disinfection Systems Venturi The venturi (Figure 6.3), otherwise called an injector or ejector is the most important component in a gas chlorination system because it creates the vacuum. To put it simply “if there is no vacuum there will be no gas flow”. In a venturi system, a liquid, in this case water, passing along a pipe section is forced through a narrower section of pipe that then broadens out again. As the water flows through the narrow section, the velocity of flow increases. This increase of velocity is associated with a drop in pressure. This low pressure can then be used via a regulator to open an inlet valve and to suck chlorine gas into the passing stream of water.

All venturis have a maximum vacuum that can be produced. This is typically around – 28 inches of mercury however it does vary from unit to unit. Although inches of mercury is not a metric unit, many manufacturers of chlorination equipment still use this unit. It is equivalent to approximately – 95 kPa. The actual vacuum developed by the venturi does vary between different manufacturers and different models.

Figure 6.3. Diagram of a venturi.

Water Nozzle

Water Fluid

Outlet

Diffuser Throat Chlorine Gas

Most gas chloination systems also include a booster pump (Figure 6.1) to ensure sufficient pressure and flow is available for the correct operation of the venturi.

Vacuum Monitor And Alarms Vacuum monitors are placed between the venturi and the control device (Figure 6.1) to give a constant reading of the vacuum in the system. Figure 6.4 shows a typical vacuum monitor and alarm system.

Figure 6.4. A vacuum monitor and alarm installed at a disinfection site. The readout is displayed on the wall.

To the left of the monitor is an automatic control device and to the right a manually operated control device.

45

Chapter 6 – Chlorine Gas Disinfection Systems The reading may be displayed locally in the “chlorine” room as well as remotely on the plant control system.

Continuous monitoring of the vacuum provides an early warning of developing problems with the vacuum and hence possible disinfection failure. This warning is available much earlier than waiting for the chlorine residual analyser to show loss of residual and can therefore help to prevent serious loss of disinfection.

Vacuum alarm systems have adjustable high and low alarms that can be monitored locally or remotely via a SCADA system.

High Alarm A high vacuum in the system can mean that: 1. The drum or cylinder is closed or empty, and a high vacuum has been created as the venturi continues to operate.

2. There is a blockage in the system between the vacuum monitor and the vacuum regulator (see Figure 6.1). This could be in the control device, stand alone change over device or the vacuum regulator. A blockage in any of these devices would appear the same as a closed valve or empty drum with the venturi continuing to create vacuum until it has reached its maximum vacuum level.

Low Alarm A low vacuum can be created by: 1. A total failure in the venturi. This could be due to increased pressure at the point of injection or a mechanical failure of the venturi itself. A worn or broken nozzle, cracked body or fittings, or a failure of the diaphragm or seal would all impact on the integrity of the venturi and its ability to create a vacuum.

2. Failure of the service water line or booster pump.

3. A broken vacuum line between the vacuum monitor and the venturi (Refer to Figure 6.1).

Control Device The control device also referred to as “the chlorinator”, “rate valve”, “V notch”, “dosing controller” or “metering orifice” is used to set the desired flow of gas into the system. The control device varies the level of vacuum delivered to the vacuum regulator and therefore the extent of valve opening and the delivery of chlorine.

The exact valve style varies between manufacturers, however the principle remains the same. By moving a “tapered plug” in or out of an orifice, the degree of opening of the orifice will change and therefore the extent of the vacuum applied to the diaphragm will change. For example with a V notch, the rod is a constant diameter and the V notch is tapered (Figure 6.5). As the V notch plug moves up or down, the opening for the chlorine gas to flow through increases or decreases thereby allowing more or less gas to flow.

Control devices can be operated manually or automatically (Figure 6.6). In a manually operated system, an operator adjusts a knob on the control device to increase or decrease the flow of chlorine. Automated systems can respond to electronic signals from a flow meter (flow paced) or a residual analyser (residual trim) or a combination of both (flow paced residual trim) (see Chapter 9) thereby removing the need for continual adjustment by the operator.

46

Chapter 6 – Chlorine Gas Disinfection Systems Figure 6.5. Detail of a V notch.

Gas Flow “Out”

V Notch Plug

Variable Open space for Gas Gas Flow “In”

These devices range from stand alone auto control valves incorporating an electronic control box and variable orifice, to a servomotor turning the adjustment knob on a conventional manual control device.

Figure 6.6. Examples of manual (far left) and two different automatic controls (centre and right).

Vacuum Regulators Vacuum regulators (Figure 6.7) have an inlet valve that is held closed by a spring. The spring also assists in the quick return of the valve to the closed position whenever vacuum is lost from the system. Once vacuum has been built up by the venturi such that the vacuum is greater than the force applied by the spring, the flexible diaphragm is sucked back triggering the inlet valve to open and allowing gas to flow from the drum or cylinder. If the vacuum is removed from the system by the breakage of a line or failure of the venturi, then the regulator will shut via the spring and stop any gas from leaving the drum or cylinder.

47

Chapter 6 – Chlorine Gas Disinfection Systems Figure 6.7. Diagram of a typical vacuum regulator.

Vacuum draws gas through the vacuum regulator

Chlorine Gas Vent Flexible Diaphragm

“Regulation” valve.

Vacuum overcomes spring tension and lifts the valve off the seat and allows gas to pass through

Vacuum regulators may be stand alone units or they may have the control device combined within the regulator (Figure 6.8).

Figure 6.8. An integrated regulator, control device and rotameter.

Vacuum regulators are commonly mounted directly to the chlorine drum or cylinder (Figure 6.9) .

Figure 6.9. Example of drum mounted automatic changeover vacuum regulators.

48

Chapter 6 – Chlorine Gas Disinfection Systems Liquid chlorine must never pass through a regulator. During transport of drums, liquid chlorine may enter the gas dip tube (Figure 6.10). For this reason, regulators mounted directly on drums should incorporate an integral drip leg to catch liquid chlorine that may have trapped in the gas dip tube during transport and could enter the system when the drum is first turned on. An integral heater in the drip leg slowly warms the liquid, returning it back to a gas state for use in the system.

With chlorine cylinders that are transported in a vertical position, it is unlikely that liquid chlorine will leave the cylinder when it is first opened. For this reason an integral drip leg is not required, however, after transport, the cylinder should be secured in a vertical position and not connected to the system for 24 hours to allow any liquid to drain away from the gas outlet valve.

Regulators may also be mounted as part of a manifold system. In these situations drip leg assemblies may also be required, however, depending on the design of the system, alternative structures such as a “drip pot” (Figure 6.11) may be used to perform the same function.

Figure 6.10. Diagram showing the arrangement of the dip tubes in a one tonne drum.

Gas CI2

Chlorine Gas Valve

Integral Drip Leg Dip Tubes

Chlorine Liquid Valve

Liquid CI2

Figure 6.11. Liquid chlorine trap or “drip” pot.

Vent Pipe All vacuum systems have a vent pipe that is run to atmosphere so that in the case of a pressure build up in the system, the vent allows the pressurized gas to escape (see Figure 6.1).

49

Chapter 6 – Chlorine Gas Disinfection Systems This is an important feature of all vacuum chlorination systems and protects against rupture of pipes, fittings and equipment that are not designed to withstand pressure. The vent should be situated according to the Australian Standard AS 2927.

Carbon filters for the purpose of adsorbing chlorine are now available. These are being used in some gas chlorine systems and vary from small canister style filters (Figure 6.12) to large complex filters. The carbon has a limited capacity and must be regenerated otherwise chlorine breakthrough may occur.

Automatic Changeover Systems Automatic changeover systems allow changeover to a standby drum or cylinder when the duty drum or cylinder becomes empty. The automatic changeover mechanism can be built into the vacuum regulator (integral system) or be a stand alone system. Examples of both systems are shown in Figure 6.12.

Figure 6.12. Cylinder mounted integral changeover regulators on the left and a stand alone changeover device on the right.

Changeover device

Carbon filter

In stand-alone systems, the changeover device is connected between the two vacuum regulators and works on a “flip flop” principle. The device consists of a plunger and two diaphragms and depending on the position of the plunger, gas is drawn from either of two ports. This is shown diagrammatically in Figure 6.13.

50

Chapter 6 – Chlorine Gas Disinfection Systems Figure 6.13. Diagram of a stand alone change over device.

Gas flow direction being drawn through under vacuum

The plunger is activated to the opposite position whenever a drum or cylinder runs out creating a high vacuum within the entire system. This high vacuum builds up behind one of the diaphragms in the changeover device and activates the change of supply. Stand alone changeover devices usually have a visual indication of which drum or cylinder is being used and also send a signal to the plant PLC so the operator can see on the screen which drum is in duty mode. It can also be set up to alert the operator that a change over has occurred and that the empty drum or cylinder will need to be changed.

The integral changeover vacuum regulators are widely used in Australia and are available from several major manufacturers of gas chlorinators.

Weigh (Load) Scales Weigh or load scales (Figure 6.14) are used to provide an indication of the remaining weight in a drum or cylinder. They may be either electronic or mechanical. Mechanical scales are only available for cylinders.

They consist of a base with the weigh platform built into it and a mast with an analogue indicator at the top Figure 6.14. Examples of electronic load scales for a one tonne drum (left) and cylinders (right).

51

Chapter 6 – Chlorine Gas Disinfection Systems Some scales come complete with a support bar positioned approximately halfway up the mast to steady the cylinder and also have chains attached to them to secure the cylinder in place. They are available to suit either one or two cylinders.

Electronic scales are available for drums or cylinders. In the case of drum scales, they consist of large rectangular weigh beds that the drum actually sits on (Figure 6.14). They have an electronic display for each cylinder or drum and may include low weight alarms. These alarms can register locally or remotely via a SCADA system. Some electronic scales also provide totalisation of gas consumed over a period of time.

To ensure that disinfection is not compromised by an unexpected loss of chlorine, load scales must be accurate. To ensure the accuracy of the scales they need to be calibrated regularly by qualified technicians.

Chlorine Leak Detectors Since chlorine is a highly poisonous gas, it is important that any leak is detected promptly and action taken to control the leak. Chlorine leak detectors are used to detect and alarm the presence of chlorine in a room.

Leak detectors can have multiple sensors in different locations and can provide local and remote readout and alarms. Since chlorine gas is heavier than air, the sensors should be located near the floor. A suitable position is about 300 mm from the floor. Modern detectors also have features such as self-test modes that can be programmed to perform a self-test at regular intervals and activate an alarm if the sensor does not pass its test. While the self-test is useful, regular manual testing of sensors is still essential to ensure they will work when needed. Australian Standard AS 2927 requires leak detectors to be tested weekly for correct operation (see Chapter 10).

All chlorine leak detectors on the market come with high and low alarms. Based on the safe exposure level for chlorine of 1ppm for an 8-hour day, a suitable level to set for the low-level alarm is 1 ppm. Once the low level alarm is triggered, a flashing warning light should be activated to alert anyone planning on entering the area that there is chlorine present. A suitable level for the high level alarm is 3 ppm. Once this is triggered, the system should alert the duty operator via the SCADA system and may also activate an automated drum closure depending on the actual set up of the system.

Automatic Drum Or Cylinder Shutdown Systems These devices are attached to the drum or cylinder valve directly and are operated via an alarm that is generated by the chlorine leak detector. In the event of a chlorine leak, the leak detector will send a signal to the shutdown system, which will automatically shut the valves on the drum or cylinder. The most common type of shutdown system uses compressed air to drive ratchets that are attached to the drum or cylinder valve. An alternative to this system is one that uses electrically driven actuators or motors that attach directly to the drum or cylinder valve (Figure 6.15).

Figure 6.15. Example of an air driven actuator (left) and an electric actuator (right).

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Chapter 6 – Chlorine Gas Disinfection Systems Figure 6.16 shows an automatic shut down system on a chlorine drum.

Figure 6.16. A drum system with integral changeover vacuum regulator and automatic shutdown system (red arrow).

Both types of shutdown systems include a local control panel that can be used to initiate a test shutdown sequence and will also send remote alarms to the plant control system to alert personnel of the shutdown.

These systems are an extremely valuable safety feature for any chlorine system that does not have the vacuum regulators mounted directly on the drum or cylinder. In such systems, all pipe work between the drum or cylinder and the vacuum regulator will be under the full pressure of the drum or cylinder. If there were a failure at a joint or in the pipe work there would be a risk of the entire contents of the connected storage vessel escaping to atmosphere. In a system where the vacuum regulator is mounted directly on the drum or cylinder valve, it is very rare that the gasket sealing the joint will fail once in use and this is the only point where there could be a pressure leak to atmosphere.

Shutdown systems should be capable of operating for a minimum of 4 hours in the event of a power failure.

These systems shall only be used to shut the containers and shall not be capable of reopening the container.

They need to be removed prior to reopening the container.

In air-operated systems a low pressure alarm is also required.

Chlorine Analysers Chlorine analysers should be integral part of any gas chlorination system. They should also be an integral part of a sodium hypochlorite dosing system. For this reason chlorine analysers are considered in detail in Chapter 9.

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Chapter 6 – Chlorine Gas Disinfection Systems

Trouble Shooting Problem

Symptoms

Possible Cause

Corrective Actions

No chlorine dosing

Float or ball sitting on the bottom of the rotameter.

Booster pump or venturi failure.

Check carrier water system.

Vacuum leak between the control device and venturi.

Check for vacuum at venturi and control device.

Process signal failure.

The control device and incoming process signal must be checked by an instrument technician.

Blockage in gas supply.

If duty/standby system, change duty.

Empty drum or cylinder.

Check drum or cylinder.

Check weigh scales.

Vacuum leak between control device and gas supply allowing air to be drawn into the system.

Shut drum or cylinder with system running and perform vacuum leak test (see Chapter 10).

Dirty, failed or fouled seat in vacuum regulator

Clean or replace the regulator inlet filter and clean or replace the seat in the vacuum regulator.

Chlorinator failure.

Check chlorination system (see Chapter 10).

Residual analyser failure.

Check analyser calibration, sample lines and output signal.

High chlorine demand in the water

Review raw water quality and treated water quality.

Submit samples for DOC testing.

Float or ball showing gas flow. (May also be accompanied by a milky white build up on the rotameter) Gas detected at vent

No or low chlorine residual reported by system.

Residual analyser showing low chlorine residual.

Perform chlorine demand test (see Appendix).

Duty/standby system failed to change over

54

Faulty change over device.

(This could be stand alone or an integral part of the vacuum regulator)

Check/Service change over device or vacuum regulator.

System unable to generate sufficient vacuum to trigger change over.

Perform vacuum leak test.

Check for vacuum at venturi.

Chapter 7 Liquid Sodium Hypochlorite Disinfection Systems The first record of a liquid disinfectant being used in the water industry was in 1897 when “bleach solution” was used to disinfect distribution mains in England following an outbreak of typhoid fever, however it wasn’t until the 1930s that the use of sodium hypochlorite became widespread in the USA and UK. Sodium hypochlorite (hypo) is produced by reacting chlorine gas and caustic soda.

The Sodium Hypochlorite Dosing System Sodium hypochlorite dosing systems, no matter how different or complex they appear to be, are all based on the same principal and are in fact quite similar. A small but precise amount of sodium hypochlorite is removed from a storage container, and added to water in such a way that the sodium hypochlorite becomes evenly mixed and achieves the necessary final concentration of chlorine.

The differences that do exist generally relate to differing philosophies on how best to: • • •

Remove the chemical from the storage container.

Transfer the chemical.

Add or mix the chemical.

A typical sodium hypochlorite dosing system consists of a sodium hypochlorite storage tank, a metering pump, backpressure valve, pressure relief valve and injection valve.

Each of the system components are considered below.

Sodium Hypochlorite Storage Sodium hypochlorite solutions can be stored in High Density Polyethylene (HDPE) or Poly Vinyl Chloride (PVC) containers. In our experience, HDPE is the preferred material giving longer, reliable service. Other things to consider in the design of a sodium hypochlorite storage include: • •

• •

Use of black materials that have higher UV stability and are therefore better suited for outdoor use.

Provision of bunding to retain the liquid in the event of a tank failure. Ideally the bund should provide sufficient volume to contain the full volume of the tank. Also if the height of the bund wall is lower than the tank, splash curtains (Figure 7.1) should be used to deflect any leaks from the tank into the bund. While these requirements may not be stipulated in regulations, they assist in trouble free operation of a chemical storage.

Provision of venting for the storage containers. The size of the vent needs to be 2.5 times the area of the inlet connection.

Provision of an overflow. The overflow should be the same size as the vent. The vent and overflow can be combined. The overflow should run into the bund.

55

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Figure 7.1. An example of ceiling to floor splash curtains in place surrounding a chemical storage tank.

Specific legal requirements are likely to vary somewhat from site to site, therefore local dangerous goods regulations should be consulted.

Removal Of Sodium Hypochlorite In removing the chemical from the storage container there are two fundamentally different systems, suction lift and flooded suction. In suction lift (Figure 7.2) the chemical is delivered to the transferring pump by suction generated by the pump itself, while in flooded suction (Figure 7.3), the liquid chemical is supplied to the pump by gravity.

Figure 7.2. Suction Lift

Figure 7.3.

Flooded Suction

Pipe or Tank

Pipe or Tank Withdrawable Injection Valve Back Pressure Valve

Mix

Transfer

Pump

Mix

Withdrawable Injection Valve Back Pressure Valve

Transfer

Chemical

Chemical Remove Pump

Remove

Both flooded suction and suction lift systems will provide excellent performance when installed correctly. One difference between the systems is that flooded suction installations require more fittings on the suction side of the metering pump e.g. vent tube, sight glass and isolation valves. These extra fittings have an associated risk of developing leaks. Suction lift systems require the chemical tank to be positioned below the metering pump, which is not always convenient. Suction lift is impractical in a situation where large storage tanks are used since it is impractical to have the pump at the top of a tall tank.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems In both systems, changing levels of sodium hypochlorite in the storage tank has an impact on dosing.

1. Liquid in the tank applies a low pressure to the suction of the metering pump. This pressure reduces as the chemical level becomes lower. This difference in pressure, all be it a small amount, can result in changes to the accuracy of dosing.

2. The changing levels in the storage also result in a change to the suction lift distance. In particular as the chemical level drops the increased distance will reduce the ability of the pump to lift the chemical. In most metering pumps the lift is limited to 1.5 to 2 metres. Depending upon the metering pump design, this difference in suction lift, all be it a small amount, can result in changes to the accuracy of dosing.

In both cases the effect is small and can easily be compensated for by using residual trim dosing systems (see Chapter 9).

In general, the differences between equipment from different suppliers has minimal impact on the end results. There are however, some very important issues quite specific to the handling of sodium hypochlorite that MUST be considered at the design stage and when making choices on equipment. The main one of these is gassing and the subsequent vapour lock that can result.

Prevention of Gassing (Vapour Lock) The biggest problem associated with dosing sodium hypochlorite is the tendency for it to breakdown over time. There are two pathways for the breakdown of sodium hypochlorite. Heat and light result in the formation of sodium chlorate. The presence of minor impurities in the sodium hypochlorite causes the formation of oxygen gas. These bubbles then accumulate in areas of the metering pump suction line. This can occur in both suction lift and flooded suction systems. The accumulation of bubbles compress and expand and act like a spring within the pump head. The end result is that the liquid chemical is prevented from passing through.

This process is often referred to as vapour lock or gassing. The problem with gassing and vapour lock is that disinfection is compromised or fails. This must not be allowed to occur and every effort must be made to prevent vapour lock. In general terms, gassing of sodium hypochlorite cannot be avoided. It is therefore important that the bubbles are managed appropriately. Since this is so important it is worth describing how bubbles can be managed.

The problem is most noticeable where the metering pump capacities are “below” 5 to 6 L/hr. Above this, the flows through the pump are often great enough to pass the bubble on. However, this too has limitations that are governed by the “line pressure” the pump is being used to pump into. The higher the pressure, the greater the difficulty to pass the bubble from the suction to the discharge side of the pump.

Manufacturers of metering pumps have addressed the problem of gassing in different ways and each has merit. In general, flooded suction systems rely on some mechanism to release gas after or while it is being formed whereas suction lift systems tend to prevent the gas reaching the pump head. Some examples of degassing methods are presented below.

1. Solenoid Valve Method A vent point controlled by a solenoid valve is added to the downstream side of the metering pump and is regularly opened for fixed intervals of time (Figure 7.4 and 7.5). The vent may be situated on the individual pump heads or a short distance from the pump and able to be operated to service both a duty and standby pump (Figure 7.5). When the vent is closed, the chemical is passed through to the injection point.

57

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems A timer attached to the top of the solenoid operates at a pre-set duration, (typically 15-20 minutes), and opens the solenoid for several seconds. This releases pressure in the discharge area and allows any bubbles formed in the suction area to pass through the pump. The chemical and bubbles are returned to the chemical storage tank during this brief period.

If the system has flow monitoring and a fault relay, this can be used to trigger a degassing procedure when gassing occurs. The procedure can be run a number of times before sending a full failure message. Modern pumps have this system built in.

The solenoid valve is carefully matched to operate against the line pressure in use. Solenoid degassing can be applied to either suction lift or flooded suction applications.

Figure 7.4. Diagram of a solenoid degassing system Back Pressure Valve Withdrawable Dosing Lance

One-Way Valve

“Timed” to open solenoid

GPO

Figure 7.5. A timed solenoid degas system connected to duty standby metering pumps.

Timed to open Solenoid Valve Back Pressure Valve

“Standby” Metering Pump

Chemical Tank

58

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems 2. Vented Chamber Method In this system, a separate “vented” chamber in the pump head is first filled with hypo. Any bubbles that have formed are vented at this point. The metering pump then draws hypo from the “bubble-free” chamber and on into the discharge (Figure 7.6).

Vented chamber systems are very efficient but can only apply to suction lift systems.

Figure 7.6. Diagram of a vented chamber system.

Principle of Operation The pre-lifting chamber A is filled with the dosing medium via the suction valve B The calibration tube C is filled from the pre-lifting chamber A

Unused dosing medium flows back into the tank via the overflow line D

The dosage flows from the calibation tube C to the pressure valve E via the small dosing diaphragm.

Note V Shut-off valve V must be open!

3. Constant Bleed Bypass Method In a constant bleed bypass system, the metering pump is fitted with a permanent “bypass” arrangement in the discharge side of the pump (Figure 7.7). This continually removes bubbles that may form by providing a discharge line (i.e. a bypass) that is vented to atmosphere.

This method is very efficient, and primarily used on flooded suction systems.

Figure 7.7. Constant bleed bypass system.

Process Flow Stream Injection spear Pressurised discharge

Metering Pump

Bypass passage at atmosphere

Bypass valve Vent Pump Tube chamber

Chemical Storage Vessel

Inclined ports

59

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems The submersible metering pump head is at the lowest point of the fluid. Air bubbles that might be drawn into the pump rise up the inclined ports away from the pump and into the vent tube. The liquid on the other hand gravitates down the inclined port to the pump.

A bypass valve situated on the outlet of the pump provides a continuous small volume bleed of chemical and any air bubbles that might have formed through the bypass and into the vent tube. Any liquid drains back to the chemical storage while the gas passes to atmosphere. The operation of the bypass may be adjusted manually.

The constant bleed bypass method of degassing should not be used in conjunction with flow-paced control of chlorine dosing. This is because the bleed is constant regardless of the pump speed and results in the pump delivery being nonlinear. It should only be used in conjunction with residual trim control (see Chapter 9) The methods described above relate to the dosing pump. Other preventive measures can be employed to prevent gassing. These involve the configuration of the suction pipework. Some of those known to have excellent benefits have come from operator initiatives and “outside of the square” thinking. Two examples are provided below. Both are based on providing ways for the bubbles formed to be diverted away from the suction point of the metering pump head. Both are applicable to flooded suction configurations.

4. Bubble Diversion All pipework and fittings are assembled to provide the shortest distance between the chemical storage tank and the metering pump suction valve (Figure 7.8). A clear PVC sight tube is used to serve three functions: 1. Visual measurement of tank contents (Isolation Valve Open).

2. Metering pump flow rate (Isolation Valve Closed).

3. Venting of bubbles prior to metering pump suction valve entry. (Note that the tubing between the bottom of the vented sight gauge and the metering pump suction valve is kept short, and tapers upward towards the sight gauge).

Figure 7.8. A bubble diversion system.

Vented Sight Gauge

Bubbles are diverted Sodium Hypochlorite

Metering Pump

Isolation Valve

Strainer

Suction Valve Tubing length kept short

In a modification of this basic system, the flexible tubing between the bottom of the vented sight gauge and the metering pump suction valve (shown in black in Figure 7.8) is replaced with a purpose built PVC fitting as shown in Figure 7.9.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Figure 7.9. Purpose built bubble diversion system with gas entrapment and vent path shown in blue.

To metering pump suction valve

To metering pump suction valve

From chemical supply and vented sight gauge

From chemical supply and vented sight gauge

“Bubbles” forming A special PVC “Y” tee fitted in place of the short tubing shown in Fig 7.8

Metering Pumps There are a number of different types of metering pumps available to transfer the sodium hypochlorite from the storage vessel to the dosing point.

Solenoid Pumps These pumps rely on the motion provided by the “momentary” powering of an electric solenoid, similar to one found in a car starter motor. This short burst or “pulse” of energy moves fluid “out” of a chamber through a one-way discharge valve. Then, when the power burst is off, the reverse motion causes fluid to be drawn into the same chamber through a separate one-way suction valve. The distance that the solenoid shaft is allowed to travel, coupled with the frequency, determines the amount of chemical pumped.

The frequency of “pulses” can be manually set by a dial or push button on the pump. External control of this pulse can also be applied by another controlling instrument.

Motor Driven Pumps In principle, a motor driven pump transfers fluid in the same way as the solenoid type via a chamber fitted with a suction and discharge valve. However, the force to pump the fluid is generated by rotating a motor shaft against a cam (Figure 7.10). The point at which the cam is in contact with the shaft, and the rate of rotation of the shaft determines the amount of fluid that passes through the chamber.

61

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Figure 7.10. Diagram showing the operation of a motor driven pump.

Closed

Opened

Cam & Motor shaft

Cam & Motor shaft

Diaphragm

Diaphragm Opened

Closed

SUCTION STROKE

DISCHARGE STROKE

Motor driven pumps can be controlled in two ways, variable speed or pulsed control. In variable speed control, the speed of the motor and therefore the stroke rate is varied. In pulsed control, the motor is turned on and off at variable rates to deliver different amounts of chemical.

In a recent development, “digital” metering pumps have been released. These are characterised by having a very rapid suction and slow delivery. In this way the chemical being dosed can be added into the stream of water almost continuously (see Figure 7.13). Such delivery is preferred for disinfection where all water needs to see the full chemical dose at all times. Digital pumps may be either motor driven or solenoid driven.

Peristaltic Pumps Peristaltic pumps are quite different to the other types of pumps described above in that they pass liquid through a single section of tubing that is stretched around a cam located on the end of a motor shaft (Figure 7.11). The liquid is squeezed forward by the pressure generated by the rotating cam points.

Figure 7.11. Diagram showing the operation of a peristaltic pump.

Trapped fluid is forced to move through the tube over the cam points.

Replaceable tube Motor direction

INLET

62

OUTLET

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems The rotation causes the entrapment of the fluid in the tube as the cam squeezes the tube closed. Further rotation of the cam moves the liquid forward to the discharge side. No valves are required. The speed of the pump motor, coupled with the tubing diameter determines the amount of fluid transferred. Peristaltic pumps are less susceptible to gassing and vapour lock.

Peristaltic pumps have been traditionally limited to low-pressure applications of around 2–3 Bar; however, new developments have resulted in pumps capable of greater than 8 Bar. They are commonly used in laboratories but less commonly in sodium hypochlorite dosing. They are finding increased use in lime dosing systems. They do however offer a significant advantage, the flow of disinfectant is continuous whereas in the other types described above the flow of disinfectant is discontinuous. The implications of this for disinfection are discussed further below.

In ALL cases, regardless of the pump type being used, a very important but frequently overlooked consideration is the pressure at the point of injection. A dosing pump must be able to constantly exceed this to ensure dosing occurs at all times. Higher pump rates at higher pressures will require a physically larger pump. Most of the smaller metering pumps available can deliver up to 12 L/hr at 10 Bar (100 m head), which generally suits most disinfection applications.

Metering Pumps And Disinfection A chlorine dosing system needs to be designed to provide the necessary dose at both the peak and minimum flows experienced. This can create problems since at the minimum flow rates experienced, the dosing system, may be oversized and not be able to be turned down sufficiently. Metering pump design has advanced considerably and the “turndown” available on modern metering pumps is much better than was available on older pumps. Where this is not sufficient, plans need to be prepared to cope with different demands.

Possibilities include: 1. Replace the over sized pump with suitable sized modern pumps.

2. Install a duty-standby-assist system. This involves installing three equal sized pumps. For maximum demand, two pumps operate. At intermediate and low demand, one pump operates. The third pump is always available as standby should one pump fail.

3. The stock concentration of sodium hypochlorite can be reduced during the low demand periods. In this way the same pump can be used. Because of the difference in strength, a higher volume needs to be dosed to achieve the target objective for chlorine residual. Any dilution of stock chlorine must be made with chlorinated tap water.

In a gas system, as long as the vacuum is maintained, dosing is continuous. In contrast, a pumped system is to a greater or lesser extent discontinuous. The degree of discontinuity is worst where a large pump is operating at low frequencies. In these circumstances, the addition of sodium hypochlorite will occur into some water and then not into the next “package” of water. This is not a problem so long as efficient mixing occurs, however, where this is not the case, there is a potential to compromise disinfection. It is important to try to minimise the discontinuity and make dosing as continuous as possible.

Metering pump design has advanced significantly with pumps now able to provide an even flow of disinfectant into the flow of water with only a very short period while the pump head refills. This is shown diagrammatically in Figure 7.12 and 7.13.

63

Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Figure 7.12. Modern metering pumps deliver approximately continuous dosing to the water due to the very short suction and long stroke.

Dosing in a pipe system

System pressure P [bar]

Figure 7.13. Comparison of the output from different metering pumps highlighting the almost continuous flow from the digital pump. (➀ solenoid, ➁ motor and ➂ digital.)

1

2 3

0

Time t [s] Materials Suitable pump and tubing materials must also be considered. Pump suppliers have chemical vs material resistance charts available on request. The most common mistake made is to use polyethylene fittings for direct contact with sodium hypochlorite. This material is commonly found in hardware shops in sizes that appear to suit some dosing plant pipe configurations. The temptation is to use these because of their convenience and cost. Be assured that they will not last.

Polyethylene fittings vary in density and are not always labelled to indicate this. Low-density polyethylene (LDPE) is typically used with garden drip feed systems. High-density polyethylene (HDPE) is typically used on domestic plastic plumbing. Depending upon the pressure and temperature that the fitting is exposed to, eventually these fittings will split, snap or develop pinholes when placed in contact with sodium hypochlorite solution. PVC is the preferred material for pipes and fitings that will come into contact with sodium hypochlorite.

Having said this, HDPE tubing is supplied by all metering pump suppliers for dosing purposes, even with sodium hypochlorite. They all recommend the regular replacement of this tubing at 12-month intervals. When sourcing tubing from these suppliers, ask for PVC alternatives. Use of HDPE, although cheaper at first, often brings with it increased maintenance requirements.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Backpressure Valves Backpressure valves should be fitted in the discharge line of the metering pump. They perform two important functions: 1.

2.

They provide a constant pressure on the discharge side of the metering pump. This constant pressure ensures reliable and repeatable operation of the pump resulting in increased accuracy and repeatability of the chemical dosing. This is particularly important in systems that have a variable pressure at the point of injection into the main.

They are an effective anti-siphon device and can be used in systems where the inlet pressure to the metering pump is greater than the pressure at the point of injection. This is often the case in a system where the injection point is lower than the metering pump and or the chemical tank.

Back pressure valves will also stop a siphon that is generated by a vacuum being created in the discharge line. The vacuum will actually suck the diaphragm down onto the seat harder thereby stopping the vacuum passing through to the metering pump and dragging chemical into the system.

The valves are not complicated and consist of a spring loaded diaphragm (Figure 7.14) that can only be lifted by the metering pump developing sufficient pressure to overcome the tension of the spring. Once the diaphragm is lifted, chemical is able to flow. The set pressure of the valve can be adjusted to suit each application by adjusting the spring tension but a pressure gauge must be present in the system so adjustments can be measured.

Figure 7.14. Diagram of a backpressure valve.

Pressure Relief Valves All metering pumps have a maximum operating pressure. To prevent damage to the pump and discharge lines, there needs to be a device in the system to release excess pressure. Pressure relief valves operate on the same principle as backpressure valves but have three ports instead of two, namely inlet, outlet and overflow. When the spring pressure is exceeded, flow is directed out of the overflow port thereby preventing any further build up of pressure. The spring pressure is set to the maximum recommended by the pump manufacturer. Pressure relief valves are installed in the metering pump discharge line immediately after the pump with nothing else in between.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems The overflow port can be connected back to the chemical tank or in some cases piped back into the suction side of the metering pump. A pressure gauge must be included to allow accurate setting of the device.

Typical scenarios where a pressure relief valve will be activated include: 1. A blockage at the point of injection causing continual build up of pressure in the line.

2. A closed valve anywhere in the discharge line.

3. A higher than expected system (mains) pressure.

Recent developments in metering pump design now give the option for the pressure relief function to be performed by the metering pump itself. This is achieved by monitoring the pressure in the metering pump head at all times via an integral flow or pressure sensor. This device will also report on low pressure that could indicate a broken discharge line between the metering pump and the point of injection and can be used as a very effective proof of delivery system. In this system the maximum operating pressure is set in the metering pump electronics and if this set point is reached, then the pump will stop and then perform four to six restarts to try and clean the blockage. If the high pressure is still present then the pump will stop and activate alarms.

Injection Valves And Lances In any dosing system, mixing is essential. Chemicals need to be mixed very quickly and thoroughly with the water. The objective is to achieve an even concentration and maximum contact with any microbial pathogens that might be present in the water. Many problems in water treatment result from poor mixing and disinfection is no exception.

Most dosing of disinfectant occurs into a pipe. Direct dosing to the side of the pipe through a simple welded fitting does not provide satisfactory dosing. Effective dosing and mixing requires the use of injection valves and withdrawable lances (or spears) (Figure 7.15, 7.16 and 4.1) to introduce the disinfectant into the mainstream flow. Injection lances come in a range of sizes to suit a wide range of pipe sizes and also for direct injection into tanks (Figure 7.16). Even better mixing requires the use of inline static mixers (Figure 4.2) or mechanical mixers (Figure 4.3).

The inclusion of a one-way valve as shown in Figure 7.15, provides a small backpressure to assist in reducing run on dosing and minimises the possibility of water running back down to the chemical storage tank in the case of a leaky valve and low flows. It also reduces the possibility of bleed of chemical into the water at very low dosing rates or when the pumps have stopped. This can cause the water in the pipeline to have spikes of high then low chlorine levels following a shut down.

Injection valves are available from each of the metering pump manufacturers, however the sizes and threaded parts are generally NOT compatible with products of alternate manufacturers. This is an unfortunate situation and requires careful replacement of parts. For example a decision to replace a metering pump at an existing site will usually also require this valve and other components to be changed because of incompatibility between different makes of metering pump.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems Figure 7.15. Diagram showing the installation of a typical withdrawable injection lance.

Tank or Pipe Line

Isolating Valve Closed

d

Closed

Insert

One-Way Valve

Safety Chain

Tank or Pipe Line

Isolating Valve Open

Open

d

Open

One-Way Valve

One-Way Valve

Figure 7.16. Two injection valve assemblies. One is inserted into the pipe, the other longer one (leaning against the pipe) is for use in delivering chlorine directly into the side of a tank.

Installation Installation of the withdrawable injection valve and lance requires that there is a suitable flange welded to the pipe or tank with an attached isolating valve. The procedure is described below. Refer to Figure 7.15 to aid understanding.

• • •

The length of the lance (L) needs to be determined prior to the insertion. The length should be such that the injected chemical will enter the water stream ‘away’ from the pipe wall when fully inserted.

With the ball valve at the end of the withdrawable injection valve in the closed position, the end of the valve (d) is screwed into the closed isolating valve located on the water main.

The isolating valve on the water main is then opened.

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Chapter 7 – Liquid Sodium Hypochlorite Disinfection Systems • •

The valve at the end of the withdrawable injection valve is opened. Water is prevented from leaking by a one-way valve at the top of the valve assembly and also by O rings along the lance shaft. A safety chain prevents the lance from being fully withdrawn.

The lance is pushed through and into the main. Coupling at the one-way valve secures the lance from slipping out.

Sodium Hypochlorite Generation Systems Sodium hypochlorite can be manufactured on site using a sodium hypochlorite generator. These typically produce chlorine-based solutions ranging from 0.02 to 2% w/vol chlorine. They all involve passing a saturated brine solution (table salt mixed with water), through an electrolytic cell while an electric current is applied to the electrolytic plates of the cell. The low strength sodium hypochlorite solution formed can then be immediately transferred to the dosing point in the way described above. Onsite generation offers significant safety benefits due to the fact that transport and handling of high strength sodium hypochlorite is avoided.

Sizing for the chlorine generator requires careful planning. Regular maintenance is also important as calcification by-products can lead to frequent blockages and plant failures. Regular descaling of the electrolytic cell is required. Desacaling is carried out using an acid solution.

Electrochemical Activation Electrochemical Activation for the production of liquid disinfectant is new in Australia although it is used quite widely in the northern hemisphere by large water utilities. The system is currently being trialled in Australia and offers some potential significant advantages particularly in the area of OHS and operational simplicity.

The system produces approximately 99% hypochlorous acid at pH 7.

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Chapter 8 Chloramine Disinfection Systems Mixtures of chlorine and ammonia have been used to disinfect water since the 1920’s. Chloramines were used widely in the USA in the 1930’s, however the number of systems using chloramine disinfection fell during World War II due to the shortage of ammonia. Since the war there has been a steady increase in the number of systems using chloramines with a rapid increase in the last decade. The main reason for this increase is the fact that chloramines produce lower levels of trihalomethanes (THMs). In contrast to the Australian Drinking Water Guidelines (ADWG 2004) where the limit for THMs is 250 µg/L, in the USA, total THMs are restricted to 80 µg/L and in Canada the limit is 100 µg/L. Many water utilities cannot achieve these limits using chlorine for disinfection and have turned to chloramines to meet this regulatory requirement.

While the limit for THMs is much higher in Australia than in other parts of the world, the need to achieve even these limits is proving difficult for many Australian water utilities and consequently there are an increasing number of systems being converted to chloramine disinfection.

Chloramines are also often used in long distribution systems where a free chlorine residual would disappear well before reaching the extremities of the system. Chloramine residuals persist for longer and therefore penetrate further into long distribution systems. Figure 8.1 shows the persistence of chloramine residuals through very long distribution systems operated by SA Water.

Figure 8.1. Persistence of chloramine residuals through SA Water systems (courtesy SA Water).

Chloramine Decays 7

6

Monochloramine (mg/L)

5

4

3

2

1

0 0

200 Summit SP 9208

400 Summit SP 1104

Hours TBK SP 2149

600 TBK SP 7052

800 SR SP 2139

1000 Loxton SP 2016

1200 Morgan SP 2137

There is also some evidence that chloramines are better able to control biofilms than chlorine. One other advantage of chloramines is that the production of monochloramine, the preferred form for disinfection, is favoured at higher pH’s and chloraminated distributions systems are best operated at slightly higher pH’s.

This is in contrast to chlorinated systems that are better operated at low pH’s. Therefore, in a water supply with high alkalinity and high pH, chlorine disinfection is likely to be compromised. However, these same conditions are likely to favour chloramine use as long as the necessary Ct can be achieved prior to the first consumer. Once again, as water utilities strive to better manage their distribution systems, many towns and cities are being converted from chlorine to chloramine.

Brisbane in Queensland, Ballarat, Maryborough, Wodonga and Lance Creek in Victoria and Whyalla and Port Augusta in South Australia have been operated using chloramine disinfection for many years.

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Chapter 8 – Chloramine Disinfection Systems

Chloramine Chemistry As described in Chapter 3, chloramines are produced by reacting chlorine and ammonia.

Monochloramine

NH3 + HOCl  NH2Cl + H2O

Dichloramine

NH2Cl + HOCl  NHCl2 + H2O

Trichloramine (Nitrogen trichloride)

NHCl2 + HOCl  NCl3 + H2O

While monochloramine is acceptable to consumers in water, dichloramine and trichloramine impart an unpleasant taste and odour to the water and are not acceptable. Therefore wherever chloramination is used for disinfection, it is vitally important that the formation of dichloramine and trichloramine is prevented, or at least minimised. A suitable practical target is for the monochloramine to be 90-95% of the total chlorine. If it is less than this, experience has shown that there is sufficient dichloramine present to produce unacceptable taste and odour.

Control Of Monochloramine Production Ratio Of Chlorine To Ammonia The main factors affecting the formation of the different types of chloramines are the Cl2 : NH3 (chlorine to ammonia) ratio and the pH. Temperature has a minor effect.

At a pH of 8.2 to 8.4 and a Cl2 : NH3 ratio of 4.2:1, monochloramine forms preferentially and rapidly. Under these conditions, over 99% of combined chlorine is monochloramine. As the Cl2 : NH3 ratio increases above 4.2:1, dichloramines are formed. As the ratio increases still further trichloramines are formed. Some dichloramines can however be formed at the lower Cl2 : NH3 ratio. Trichloramine does not appear to exist in significant quantities if the pH of the water is above 5 (Wilson, 1995).

While a pH of 8.2 to 8.4 is desirable for preferential formation of monochloramine, in practice these conditions are seldom met. The amount of monochloramine actually produced is a compromise between the pH and the Cl2 : NH3 ratio. As long as the pH is > 7, well over 90% of the combined chlorine will be monochloramine. In contrast at pH 6, dichloramine will make up approximately 20% of the combined chlorine. For these reasons, pH between 7 and 9 is suitable for chloramination with a pH of 8.0-8.5 being best. At pHs below 8.4 the proportion of monochloramine can be increased by decreasing the ratio of Cl2 : NH3 to as low as 2.4:1 (Table 8.1) (i.e. this is the same as increasing the amount of ammonia available for each chlorine), however this will tend to increase the level of free ammonia at the completion of the reaction. Free ammonia encourages nitrification. The free ammonia should be < 0.2 mg/L and preferably < 0.1 mg/L.

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Chapter 8 – Chloramine Disinfection Systems Table 8.1. Chlorine to ammonia ratios necessary to achieve 90% monochloramine at different pH values (Wilson 1995). pH 7.2 7.6 8.0 8.4

Cl2 : NH3 2.4 : 1 3.3 : 1 3.7 : 1 4.2 : 1

The actual ratio is dependent on the type of water, temperature and pH. As long as the monochloramine is >90% of the combined chlorine, taste and odour will be minimised, but it is well worth trying to get at least 95% monochloramine to make sure of this. The only way to find out which ratio to use is to trial it and measure the levels of combined chlorine, monochloramine, dichloramine and ammonia (see Chapter 9). While this can be done using trial and error at the primary disinfection site, it is preferable to do some bench scale testing in a laboratory. Most commercial water testing laboratories can provide this sort of testing. By setting up experimental mixtures of chlorine and ammonia and measuring the levels of the different chloramines and ammonia they can provide a recommended starting point. This reduces a lot of trial and error. Once the recommended levels are implemented at the disinfection site, the conditions will still need to be fine tuned to maximise the production of monochloramine and minimise the free ammonia levels.

Having established the best conditions it is important to realise that the ratio may need to be changed during the year or even from year to year. The levels of monochloramine, dichloramine and ammonia must be monitored very regularly and appropriate changes made to the dosing. If the monochloramine proportion goes up and the ammonia level is OK…don’t do anything except smile. If the monochloramine goes down and the proportion of dichloramine goes up, try: 1. Increasing the pH (if that is an option), or 2. Decreasing the Cl2 : NH3 ratio.

A word of warning, many references on chloramination refer to the Cl2 : NH3 ratio. Unfortunately the ratio can be expressed in two different ways, Cl2 : NH3 or Cl2 : NH3-N. The numbers will vary somewhat. Since most water system operators are familiar with the concept of concentration expressed as mg/L of added disinfectant, the ratios used in this manual will be based on the Cl2 : NH3 ratio. So where a ratio of 4.2:1 is referred to, this equates to dosing of 4.2 mg/L chlorine and 1 mg/L of ammonia. Equally important is the fact that it is 4.2 mg/L of free chlorine, i.e. after the chlorine demand has been satisfied. So in reality more than 4.2 mg/L of chlorine will need to be added.

The set up of chloramine dosing systems varies, some can be quite complex and some result in poor control by virtue of them being too simple to allow changes in ratios that become necessary at different times.

Perhaps the best system to use is a modification of the flow paced residual trim control system outlined in Chapter 9. A total chlorine analyser and a free chlorine analyser are used to measure the total and free chlorine residuals immediately downstream of the dosing points for the chlorine and ammonia. The analysers must be situated at a point where full mixing has occurred. The control system allows the operator to set a total chlorine residual. The controller, with two outputs, one for chlorine and one for ammonia adjusts the chlorine dose to maintain the set point. The ammonia is then dosed to achieve a minimum free chlorine residual. Some utilities have also been experimenting with the use of ammonia analysers to help control chloramine dosing.

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Chapter 8 – Chloramine Disinfection Systems Where monochloramine is used as a primary disinfectant, concentrations of monochloramine typically range from 1.5 to 2.5 mg/L (ADWG 2004). Based on health considerations, the concentration of monochloramine should not exceed 3 mg/L (ADWG 2004).

Chloramine Toxicity And Public Relations While there are some advanatges to using chloramine for disinfection, there are also some disadvantages.

Chloramines are toxic to patients requiring kidney dialysis and to aquarium fish. Both these problems can be solved. Aquarium shops stock chemical treatment and filter kits. For kidney dialysis patients, the water is filtered through activated carbon filters to remove the chloramines from the water prior to entry into the dialysis machines.

Before installing a chloramination system on a non disinfected supply, or before converting a chlorinated system to a chloraminated system, it is essential that good quality information is provided to all households and individuals that will be affected by the change. This information should explain why the change is being made, when it will be made and what to expect. Consumers are likely to experience a short period of off taste (but usually very few odour problems) after which the disinfectant should be almost undetectable. Reports from many consumers where chloramination has replaced a chlorine system is that they much prefer it to chlorine disinfection. This presupposes that the dosing of chlorine and ammonia is well controlled and that the levels of dichloramines are as low as possible. During the changeover period there may also be some dirty water complaints.

Before commissioning a chloramination system it is well worth ensuring that the distribution system has been cleaned and as much biofilm removed as possible using either air scouring or swabbing. These processes are described in more detail in Mosse and Deere (2009).

While it may seem dishonest, there may be some advantage to delaying the official start up of the system by a week or so. This allows the separation of imagined complaints from real complaints. Unfortunately, there will be a large number of complaints that are not based on fact. On the many occasions when start up has been delayed unintentionally, there have been a significant number of complaints on and immediately after the advertised date. Whether this approach is ethical is a decision for each utility, however with the utilities responsibility to provide aesthetically pleasing water to consumers, it may be necessary to find a way to record actual experiences. This will allow further fine-tuning of the system and attention provided to the solving of actual problems rather than imagined ones.

Sequence Of Addition In practice, chloramines can be produced in different ways. The ammonia may be added to the water before the addition of chlorine (pre ammoniation) (Figure 8.2) or after the addition of chlorine (post ammoniation).

Since chloramination requires much longer contact times than chlorine to achieve the necessary Ct for inactivation of pathogens, it is often not feasible to add ammonia and chlorine simultaneously to form chloramines immediately because the disinfection process would not be completed prior to the water reaching the first consumer. One way to overcome this limitation of chloramination is to use chlorine first to bring about the primary disinfection. Once this is achieved, the ammonia is added to form the chloramines that will then be available to provide residual protection throughout long distribution systems. However, if chloramines are being used to reduce the formation of DBPs then prechlorination will not prevent them being formed since the chlorine that is added first quickly reacts with the other chemical compounds in the water. Use of chloramine to limit the formation of DBPs will only work if the ammonia is added first or the chlorine and ammonia are added simultaneously.

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Chapter 8 – Chloramine Disinfection Systems Figure 8.2.

An example of dosing points where the ammonia is added just before the chlorine (pre ammoniation).

Table 8.2 provides a summary of the advantages and disadvantages of pre and post ammoniation.

Table 8.2. Comparison of pre and post ammoniation.

Process Pre ammoniation

Advantages Reduced formation of DBPs.

Disadvantages Requires long Ct (contact times).

Reduced taste and odour due to chlorine Reduced disinfection efficiency. products.

Reduced taste and odour if phenol is present in the raw water.

Post ammoniation

More efficient disinfection initially, based on chlorine disinfection.

Increased formation of DBPs (may be still OK provided that the ammonia is added within minutes of the chlorine)#.

Shorter contact times required.

Formation of taste and odour due to chlorine products. # The time between the addition of chlorine and ammonia can be adjusted to keep DBPs below the target level. In one published study a post ammoniation sequence with about 2 minutes of free chlorine contact time prior to ammonia addition resulted in a 50% reduction in THMs compared to chlorine only disinfection.

While the chloramine residual lasts longer and therefore penetrates further into a distribution system, chloramine, like chlorine, undergoes breakdown or decay (Chapter 3). Even in pure water, the residuals will decrease. Where there are compounds in the water, or materials or surfaces of pipes that exert a chlorine demand, the decay rate will be even higher. Decay will also be accelerated by increased temperature. This effect of temperature needs to be kept in mind in managing systems at different times of the year and different parts of the country.

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Chapter 8 – Chloramine Disinfection Systems While chloramination reduces the formation of THM’s, it can result in the production of another DBP called Nitrosodimethylamine (NDMA).

A recent study found that the level of NDMA could be reduced by prechlorinating for a period of 2 hours prior to the addition of ammonia. Based on this, where chloramination is used, the ammonia should be added as long as possible after the chlorine to reduce the levels of NDMA formed. However, note that under this regime more chlorine based DBPs will form. The final chloramination system used must be based on a compromise between the following factors: 1. Achievement of the necessary Ct prior to the first consumer.

2. Minimisation of chlorine DBPs. 3. Minimisation of NDMA.

Formation Of Chloramines In chloramination systems, both ammonia and chlorine can be dosed as a gas or liquid.

Chlorine Gas With Ammonia Gas This type of system tends to be found on larger supplies where the turn over of ammonia approaches 18 kg per day. For smaller systems it is better to use liquid ammonia.

Chlorine Gas With Liquid Ammonia This type of system is generally found on medium to larger size plants where liquid ammonia volumes are below about 2,000 L per month. Figure 8.3 shows an example of a medium sized liquid ammonia dosing site.

Figure 8.3. A liquid ammonia dosing site.

Sodium Hypochlorite With Liquid Ammonia This type of system is generally found on small installations where chemical requirements are low. Typically chlorine requirements would be less than 3 kg per day.

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Chapter 8 – Chloramine Disinfection Systems Wherever liquid ammonia is used it is important to ensure that the storage volume is turned over regularly and that degradation of the ammonia is prevented as much as possible. Liquid ammonia degrades rapidly if venting is poorly designed. Each storage tank requires a vent pipe that incorporates a water trap to prevent direct venting to atmosphere (Figure 8.4).

Figure 8.4.

Vent points for each of the two liquid ammonia dosing systems shown in Figure 8.3. The cylinders marked A and B are the water traps.

Since monochloramine formation is sensitive to pH, it is important to consider the effect that addition of the chlorine and ammonia has on pH. Ammonia, either as a gas or liquid will increase the pH. Chlorine gas will decrease the pH while sodium hypochlorite will increase the pH. Addition of chlorine gas and ammonia gas is likely to have no net effect on the pH, however addition of liquid sodium hypochlorite and ammonia will result in an increase in pH. The effect on pH will only be noticed in low alkalinity waters. Waters with a high alkalinity will tend to buffer the changes, however a net increase in the pH of low alkalinity water would be advantageous for chloramine formation.

Nitrification The presence of free ammonia in water encourages the growth of nitrifying bacteria (nitrifiers). These bacteria, convert ammonia to nitrite and nitrate in a process known as nitrification. While the bacteria themselves and the presence of low concentrations of nitrite (<3 mg/L) and nitrate (<50 mg/L) (ADWG 2004) do not pose a risk to consumers, the action of the nitrifiers reduces the levels of monochloramine below the level where disinfection is maintained in the system. Under the right conditions, nitrifying bacteria can multiply and rapidly deplete the chlorine residuals. This loss of residual typically starts at the ends of the system and moves progressively back towards the treatment plant, however nitrification may also start right up at the head of the system. Therefore the control of free ammonia becomes essential to limit the growth of nitrifying bacteria.

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Chapter 8 – Chloramine Disinfection Systems

Management of Chloraminated Systems A brief summary of the main features of operating a chloraminated distribution system is summarised below.

More detail is provided in Mosse and Deere (2009).

The first important point to make is that operating a chloraminated system requires a lot of careful monitoring.

The purpose of this monitoring is twofold, firstly to achieve and maintain suitable residual levels, and secondly to prevent and manage nitrification. The necessary monitoring includes: • • • • •

Free and total Cl2.

Free ammonia.

Monochloramine and dichloramine/trichloramine.

Nitrite and nitrate. pH and possibly dissolved oxygen.

While nitrification only becomes a problem in the distribution system, it is essential that the primary disinfection site be managed so as to keep the free ammonia levels to an absolute minimum as the water enters the distribution system.

The key to controlling nitrification in distribution systems is to maintain the levels of free ammonia <0.2 mg/L and preferably <0.1 mg/L at all times and at all points throughout the distribution system. Ammonia is the main source of energy (food) for nitrifying bacteria and keeping it low keeps nitrifier numbers down. Ammonia levels and the resultant nitrification can be controlled by: • • • • • •

Reducing detention times in the distribution system as much as possible by lowering tank levels or taking tanks off line during low demand periods.

Maintaining a clean system by unidirectional flushing, air scouring or swabbing.

Maintaining the total Cl2 >1.5 mg/L. Nitrification usually only becomes a problem when the residual drops below 1.0 mg/L. Maintaining monochloramine 90% to 95% of total chlorine.

Maintaining low nitrate (NO3) and nitrite (NO2) levels.

Increasing the pH so that it is >7.8 and preferably >8.0. At high pHs, the decay of chloramines is reduced and therefore the chloramine will penetrate further into the distribution system.

If ammonia levels rise, the distribution system can be “topped” up with chlorine to maintain the Cl2 : NH3 ratio between 3.5:1 to 4:1. Chlorine can be topped up by the direct addition of sodium hypochlorite to tanks or treated water storages or by using tablets in floating dispensers. In either case it is important to ensure that the added chorine is very well mixed. Liquid hypochlorite must not just be poured into the tank through a hatch. One way to get some mixing is to lower the level of the tank to say 30%, add the disinfectant and then refill it. Alternatively a mechanical mixer or submersible pump can be used. A better way is to dose sodium hypochlorite into a main immediately upstream of the tank. Tablet dispensers are best placed near the inlet to the tank to assist dissolving the tablets and mixing the chlorine through the tank.

While tablet dispensers are used, there are limitations with this method. The main one is the difficulty in controlling the dose. This can result in the formation of dichloramines and trichloramines. Also if all the ammonia is used up, the chlorine then reacts with the chloramines to effectively reduce the chloramine levels and replace a chloramine residual with a free chlorine residual. Controlled dosing of sodium hypochlorite and full mixing is the preferred option when the ratio needs to be adjusted in the distribution system.

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Chapter 8 – Chloramine Disinfection Systems Failure to monitor or take action when it becomes obvious that nitrification may be occurring is likely to result in a system being “infected” with high levels of nitrifying bacteria. If this occurs, it is difficult and expensive to restore the stability of the residuals in the system. Systems are most vulnerable to nitrification when the flow rates are low (i.e. longer detention times) and the temperature of the water is high. Above ground pipelines are particularly vulnerable in spring when water temperatures rise in the pipeline but demand is still low.

This combination results in older and warmer water – the ideal incubator for nitrifying bacteria. In the hotter weather the water is warmer but residuals will generally be higher due to the shorter detention times in the system, however during drought, high level water restrictions may in fact increase the detention time in the system.

Decreasing chlorine residuals, and increasing levels of nitrite and nitrate indicate the onset of nitrification.

Decreasing levels of dissolved oxygen (DO) and decrease in pH may also indicate the onset of nitrification.

If the problem is detected early enough, minor adjustments can often be made to restore performance.

However if monitoring is insufficient, or the changes not recognised, there will be no early indication of the onset of nitrification and the possible emergence of a serious problem. Without regular monitoring it is not uncommon for nitrifying bacteria to contaminate a significant part of the system before it is too late to implement effective control measures. In such cases the first indication of nitrification is the rapid loss of residuals throughout much of the distribution system. Where, despite all the best management practices, nitrification becomes established, the only option is to superchlorinate the system. (Nitrifying bacteria are readily inactivated by free chlorine!)

The best way to achieve a free chlorine residual is to shut down the ammonia dosing at the treatment plant and revert to break point chlorination. The change to chlorination will be noticed by consumers and result in an increase in taste and odour complaints unless the consumers are notified in advance. An essential part of planning and implementing such a chlorination program is to inform customers of the need for chlorination, the expected duration of the change and the likely changes to the taste and odour of the water.

Typically the chlorine dose needs to be increased to achieve a free chlorine residual at the disinfection point.

The target level varies considerably between different utilities. In part it depends on the length and complexity of the system, but also on what the utility believes is acceptable to consumers or on past experience in managing nitrification events. Levels between 1.5 mg/L and 5 mg/L are used with the higher levels generally used in the longer more complex systems. Once the target residual is achieved, the water needs to be moved into the system as quickly as possible. This can be assisted by reducing storage levels to an absolute minimum at the start of the process and by flushing dead end mains.

If the distribution system is long, and the penetration of chlorine is insufficient to reach the ends, additional mobile booster chlorinators may be set up well into the system. The best time to carry out a nitrification control program is during periods of higher flows. The higher flows will assist the movement of the chlorine front through the network and reduce the need to waste water by flushing. Once a free chlorine residual is established throughout the system, it should be maintained for at least one day prior to reversion back to chloramination. The whole process may take several weeks to complete.

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Chapter 9 Controlling And Measuring The Addition Of Disinfectant Controlling The Dose Regardless of whether chlorine gas or sodium hypochlorite is used, the methods used to control the dose are the same. The four main methods are described below.

Manual Dosing As the name implies, this system has no automated control device. The operator makes adjustments based on manual measurement of chlorine residuals. If the residual is low, the dose is increased, if the residual is high, the dose is decreased. While it is possible to simply “twiddle the dial” up or down, it is better to do a rough calculation to work out how much more or less is required and make the adjustment based on the calculation (see Chapter 5). The problem with twiddling is that it has a tendency to lead to more twiddling.

Flow Paced Dosing In a flow-paced system, the signal from a flow meter is used to control the dose (Figure 9.1).

Figure 9.1. Schematic showing flow paced control.

Flow Signal

Metering Pump

Flow Meter

Check Valve

As the flow increases or decreases, the control device matches the changes and will maintain a constant chlorine dose. As long as the chlorine demand of the water does not change this will mean a constant residual will be maintained. However, a flow-paced system cannot respond automatically to changes in water quality, therefore it also requires some form of manual adjustment to make allowance for any changes that do occur.

Flow paced control is best suited to systems with variable flow but constant water quality.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Residual Trim Dosing In a residual trim system, the signal from an online chlorine analyser situated downstream of the dosing point is used to control the dose. If the measured residual is less than the set point, the dose is increased. If the residual is greater than the set point, the dose is decreased. The system is sometimes referred to as set point control with the required “set point” being entered into a PLC, controller or analyser.

Reliable operation of the system is absolutely dependent on the chlorine being very thoroughly mixed prior to the point where the sample is taken for the analyser.

Since a residual trim system has no way of compensating for a change in flow, residual trim control is best suited to systems with a fixed flow and variable water quality.

Flow Paced Residual Trim Where both flow and quality can vary, flow paced residual trim is the preferred method of control. By combining signals from a flow meter and a chlorine residual analyser (Figure 9.2), the system is able to maintain a constant residual in response to changes in flow and changes in water quality and in the case of a hypo dosing system, changes in the strength of the hypo. Flow paced residual trim can also be referred to as feed forward, cascade, combined or compound loop. A well-tuned system can maintain the residual with great accuracy.

This form of control is the most effective for management of disinfection, however the set up, tuning, programming and diagnosis of the control loops is a specialist field and needs to be performed by trained technicians. Once the set up is completed, a plant operator can be trained to operate the system reliably. It is important to realise that control loops may need to be reset or retuned at some stage and that these changes need to be made by a specialist technician.

Figure 9.2. Schematic showing flow paced residual trim control.

Chlorine Analyser

Controller Flow Signal Chlorine Signal Control Signal

Chlorine Sensor

Metering Pump

Flow Meter

Check Valve

One important application of flow paced residual trim control is maintaining a constant chlorine level in water leaving a storage (or contact) tank, where the primary dosing has occurred upstream of the tank.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Due to differences in the amount of water used during a 24-hour period, the time that water is held in storages will vary. For example, during the night, chlorine residuals may decay to quite low levels in the storages. The first customers to use the water will therefore receive water with a low chlorine residual. As more people rise and start to shower and use the toilet, water consumption rises rapidly, detention time decreases and the water received by the customers will have a higher chlorine level. Customers may therefore experience water in the morning with a very slight chlorine smell or with a noticeable chlorine smell or taste depending on which water they actually receive. They may even notice it changing. Figure 9.3 shows variation in the free chlorine residuals over a 13-hour period measured at a customer’s tap. The data was obtained by manually taking samples every hour. Although instrumentation is available to obtain this data continuously, it is important to realise that if you don’t have the fancy equipment you can still get that all important data.

Figure 9.3.

Changes in free and total chlorine at a site over a 13 hr period show why consumers experience changes in chlorine taste and odour.

0.7 0.6

mg/L

0.5 0.4 0.3 0.2 0.1 0 6:00 AM

7:55 AM

9:50 AM

11:45 AM Free Chlorine

1:40 PM

3:36 PM

5:31 PM

7:26 PM

Total Chlorine

Typically, consumers ask why the water utility increases the chlorine dose in the mornings. In most cases the utility has not done anything to the dose rate. The changes are the result of differential chlorine decay in response to different detention times and temperatures. These widely varying levels of chlorine in the storages also make it more difficult to maintain disinfection to the extremities of systems.

The best way to manage these fluctuations is to install residual trim chlorine dosing systems at the exit point from all major tanks and from all secondary or booster disinfection sites. A suitable set point, say 0.6 mg/L free chlorine is entered into the PLC and the system then measures the free residual in the water a short distance from the tank and supplies just enough chlorine to make sure the water leaving the tank has reached the required set point. In this way, variations in chlorine levels can be levelled out. These systems work particularly well and can even be modified to control small chlorine booster sites in the distribution system.

The sample point for the residual analyser should be situated after complete and thorough mixing has occurred. In the absence of a static or mechanical mixer, mixing is generally accepted to have occurred in a distance of 10 to 20 pipe diameters from the dosing point. In practice it is sometimes difficult to achieve this distance for a flow paced residual trim system.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant In practice good control has been obtained in control systems where the sample has been taken as little as one pipe diameter from the dosing point. Complete mixing is unlikely to have occurred at that point, however conditions inside the pipe appear to be constant enough to allow useful feedback information to closely control the dosing. The actual set point used will often need to be different for the purposes of control than that actually achieved further down the pipe. For example at a small residual trim secondary dosing site where the control system measures the incoming residual and doses sodium hypochlorite to achieve a constant residual leaving the site, the set point might be 0.6 mg/L but the residual measured 20 metres from the site might be around 0.4 mg/L. By measuring the actual residual some distance from the dosing point the set point can be established to consistently provide the required residual even though full mixing has not occurred at the sample point for the analyser.

Process Monitoring Disinfection is a classic control point (see Chapter 1). If it is not managed well, water quality can deteriorate.

There are parameters that can be monitored, and online monitoring is possible.

Since chlorine residuals can change rapidly in response to changing water quality and failure of dosing systems, chlorine residuals (particularly at primary disinfection sites) need to be monitored continuously.

Chlorine analysers should be situated as close as possible to the sampling point to minimise the delay associated with getting the sample to an analyser. It is better to send the electrical signal over the longer distance than the water sample. By minimising the sampling delay, the accuracy of the measurements is improved and the interval between loss of disinfection and cessation of flow or automatic changeover minimised. In this way, the likelihood of any undisinfected water entering the system is minimised.

Manual samples for calibration or analyser checks should be taken as close to the dosing point as possible and preferably from the same flow passing into the online analyser.

All disinfection facilities should have chlorine residual alarms established on the basis of the required Ct and the chlorine residual levels needed to ensure adequate penetration of chlorine into the distribution system.

Three alarm levels are appropriate: • • •

Low. Low-low.

High.

These allow early detection of a loss of disinfection and possible developing health risk, and allow for different response levels to be established. A low level alarm indicates a developing failure of the disinfection barrier. A low-low alarm indicates the barrier has failed and requires immediate action. The high level alarm is primarily in place to prevent overdosing and the associated unacceptable chlorinous taste and odours. High dosing will not compromise the primary disinfection process however it may lead to exceeding the ADWG health limits for chlorine.

While measurement of the chlorine residual levels is the ultimate measure of the functioning of the dosing system, another parameter can be measured to provide earlier warning of the impending loss of disinfection.

Failure of vacuum in a gas disinfection system, and failure of flow from a metering pump provide early warning of loss of disinfection. For this reason vacuum in a gas system and metering pump flow in a liquid system should be continuously monitored, and targets and critical limits set for these parameters. Appropriate alarms and alarm notification for these parameters should also be established.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant If vacuum fails completely, or flow stops from a metering pump, the system should automatically change to standby equipment. If standby equipment fails, water flow to the distribution system should stop.

Monitoring data (including SCADA trends) should be reviewed frequently to ensure that the target Ct (or free residual) is attained at all times. Review of the monitoring should be carried out by those primarily responsible for the operation of the site but also by supervisors or managers to provide a second level of scrutiny. Remember, this is not about seeing if someone has done something incorrectly but rather to ensure, that as a team, disinfection is operating correctly and public health is being adequately protected.

Measuring The Chlorine Residual The concentration of chlorine in water can be measured in a number of different ways. The different types of chlorine namely the free residual, combined residual and total residual can also be measured. Because of the simplicity and reliability of the DPD test, it is the benchmark test in the water industry and is also the basis of calibration of all online instruments.

For a chlorinated system, free and total chlorine are the most important measurements. For a chloraminated system, total chlorine and monochloramine are the most important measurements.

The DPD Test The Diethyl Phenylene Diamine (DPD) test measures chlorine as hypochlorous acid only. DPD reacts with hypochlorous acid to form a pink colour. The intensity of the pink colour is proportional to the concentration of chlorine in the form of hypochlorous acid.

While the DPD test is a general test and the same general principles apply, there are subtle differences in the way the DPD 1 test has been developed by different manufacturers. It is very important that the manufacturers specific instructions are followed carefully. Incorrect readings may otherwise result.

There are 4 DPD tests available.

1. DPD 1 The DPD 1 test measures the free chlorine residual at pH 6.2 (buffers to achieve this pH are included in the DPD 1 tablets or powders). At this pH, 98% of the OCl- is converted back to HOCl (see Figure 3.1).

Therefore the DPD 1 test measures as the sum of HOCl and OCl- as hypochlorous acid.

The DPD 1 test should be performed as quickly as possible and the residual measured immediately after the tablet or powder is dissolved. If the sample is allowed to stand, the test sample will develop a darker colour and therefore result in a false high reading.

2. DPD 2 Combination of the DPD 1 and DPD 2 tests allows the concentration of monochloramine to be measured.

The DPD 2 tablets or powders contain a small amount of potassium iodide that reacts with monochloramine to produce iodine that in turn reacts with the DPD to produce the pink colour.

The DPD 2 tablet or powder is added to the DPD 1 test as soon as the free chlorine has been measured.

The concentration of monochloramine is the difference between the two values. In many photometers, the instrument carries out the calculation.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant 3. DPD 3 The DPD 3 test used in conjunction with the DPD 1 and DPD 2 tests allows the concentration of dichloramine (and any small amount of nitrogen trichloride) to be measured. The DPD 3 tablets or powder contain an excess of potassium iodide that reacts with all chloramines and liberates iodine that in turn reacts with the DPD to produce the pink colour.

The DPD 3 tablet or powder is added after the DPD 1 and DPD 2 tablet or powder. If the result from the free chlorine (DPD 1) and monochloramine test (DPD 2) is subtracted from the DPD 3 result, the concentration of di and any tri chloramine will be given.

4. DPD 4 The DPD 4 test is really a combination of the DPD 1 and DPD 3 tests i.e. free chlorine + combined chlorine. The DPD 4 tablet or powder is simply added to a sample of water to give the total chlorine.

The DPD tests are summarised in Table 9.1.

Table 9.1. Summary of the DPD tests and what they can be used to measure.

Free chlorine Monochloramine Other chloramines (di and tri) Total chloramines Total chlorine Total chlorine

DPD 1 DPD 2 - DPD 1 DPD3 – (DPD 2 + DPD 1) DPD 3 – DPD 1 DPD 1 + DPD 2 + DPD 3 DPD 4

Limitations Of The DPD Method 1. The total chlorine measured with the DPD test will only measure those chlorine compounds that react with DPD. There will be small amounts of chlorine compounds such as chloroform and other DBP’s that do not react with DPD. It will only be a small amount but explains why if you add 1 mg of chlorine to the water the total chlorine test will usually be less than 1 mg/L. 2. If the chlorine level is very high (>10 mg/L), the free chlorine test may be deceptive because initially a dark pink colour develops and then disappears giving a false impression of a low residual. This is known as chlorine bleaching. If a high residual is expected, repeat the test using a sample diluted with chlorine free water.

3. High concentrations of monochloramine interfere with free chlorine determination. If free chlorine is being measured in a chloraminated system where there are high levels of monochloramine, the pink colour can develop very quickly and incorrectly give the impression of the presence of free chlorine.

For this reason free chlorine must be measured as soon after dissolving the DPD 1 tablet or powder as possible.

Modifications of the DPD method have been developed to minimise this problem.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant

Field Analysis Measurement of chlorine residuals in the field provide a rapid assessment of whether the conditions necessary to ensure disinfection are being maintained. Portable field systems allow many more sites to be checked than are possible with online analysers. Online analysers are essential for monitoring the primary disinfection that is in place to control pathogens. Field analysers allow checks of the online equipment and assessment of residuals throughout a distribution system.

By far the most common method of measuring free and total chlorine residuals in the field is by the use of the DPD colorimetric method. This involves the use of DPD tablets or powders. The DPD tablets or powders are usually provided in small foil packets and contain the active DPD ingredient and a pH buffer to bring the sample to pH 6.2.

There are many different brands of equipment, however the methods for determining the chlorine residuals are very similar. The individual equipment manuals should be consulted and followed carefully. Glass vials should be kept clean and discarded if they are stained or scratched.

The DPD test has an error of between 5% and 10%. Specific details of the accuracy of determination can usually be found in the manuals for the individual instruments. If the instrument is not well maintained and the instructions are not followed precisely, the error could be substantially greater than this.

Colorimeters And Comparators Comparators (Figures 9.4 and 9.5) were once common for the measurement of chlorine residuals. They relied on comparing the colour in the sample after the addition of DPD with some coloured glass reference discs. The accuracy of determination depended on the ability of the operator to match the colours.

Figure 9.4. Comparator and colour disc for the measurement of chlorine residuals.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Figure 9.5. The operator rotates the colour disc until the two colours visible in the comparator are matched.

While some comparators may still be in use, the ready availability of portable chlorine colorimeters (Figure 9.6) provides a more reliable way of measuring chlorine residuals by overcoming the limitations of using comparators where differences in vision (particularly colour blindness) can lead to inaccuracies of measurement.

Figure 9.6.

Examples of two portable colorimeters for the measurement of chlorine residuals. The system on the right includes calibration tubes.

Online Analysers There are many types of chlorine analysers available, however the function of each is the same: to provide an instantaneous reading of the chlorine levels in the water being dosed. Chlorine analysers are an essential part of a chlorination system since they provide the key process monitoring for the disinfection control point.

Only by monitoring the chlorine levels continuously can the operator know whether the disinfection control point is in control, is heading out of control, or is out of control.

In their basic form, online chlorine analysers simply monitor chlorine residuals. More advanced forms are also available that include a control function for the addition of chlorine.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant The different types of online analysers are described below.

Free Chlorine Analysers Amperometric (galvanic) analysers (Figure 9.7) use the amperometric or galvanic principle to measure the free chlorine residual. Two electrodes of dissimilar metals, typically platinum and copper, are placed in the water sample. If chlorine is present, a small electrical current is established between the electrodes.

The amount of current is proportional to the amount of chlorine in the water. The current is converted to a measurement of free chlorine in ppm or mg/L. Figure 9.7. An example of an amperometric chlorine analyser.

Unlike the other methods described below, amperometric analysers do not generally require the addition of any indicator chemicals or pH buffers. The method is robust but is limited in its ability to read chlorine accurately at pH values higher than 8. To overcome this problem and allow the determination of chlorine levels regardless of pH, some amperometric analysers incorporate the addition of a pH 4 buffer or CO2 into the sample water line or measuring cell to reduce the pH. Three electrode potentiostatic analysers are similar to amperometric analysers but use three electrodes (Figure 9.8), a measuring (usually platinum or gold), counter (copper) and reference electrode (a glass probe similar to a pH electrode) to give a more reliable and stable reading of the chlorine in the sample.

Figure 9.8. An example of a 3-electrode potentiostatic analyser.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Unlike the amperometric analysers where the voltage between the electrodes varies somewhat, in the potentiostatic system the voltage is kept constant (hence the name potentiostatic) and therefore the current generated between the measuring and counter electrode can be measured more accurately. Measurement accuracy at low values is increased due to the high sensitivity of the cell.

Probe style analysers essentially use the amperometric method. Two dissimilar electrodes are combined together in a housing that is capped with a membrane and the housing filled with an acidic gel solution that reduces the pH of the sample before it is read by the electrodes (Figure 9.9).

Figure 9.9. An example of a probe style analyser

The combination of the electrodes and the acidic gel enable the probe to read the free chlorine at pH levels as high as 10. The membrane cap and gel need periodic replacement to ensure stability of reading. The interval for replacement will vary depending on the quality of water that is being measured but most manufacturers would recommend 12 monthly replacement. More frequent replacement is required if the electrodes are used in water with low alkalinity and low ionic strength. Continuous use in water with a high pH may also reduce the life of the gel cap slightly.

DPD Colorimetric analysers (Figure 9.10) determine the chlorine residual in exactly the same way as the familiar handheld field chlorine analysers. The necessary chemicals, including pH buffers are added via an integral pump system. The solutions are mixed and time allowed for the pink colour to develop. The intensity of the colour is proportional to the amount of chlorine in the water. The chemical solutions have a limited shelf life and this is clearly printed on the bottles. Any chemicals past their use by date should be discarded.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Figure 9.10. A colorimetric analyser. Note the bottles of chemicals in the base of the unit.

This type of system is not capable of analysing chlorine residuals in a truly continuous way, but rather samples the water as a series of “grab” samples. This is because of the time delay necessary for the colour to develop. The sample time can be set and this varies between manufacturers, however the shortest interval from sample to sample is approximately 4 minutes. It is very important to be aware of this delay in reading, especially when considering using this type of unit for the control of a chlorination system. Good control requires rapid feedback. Colorimetric systems cannot provide this.

pH Correction Of The Chlorine Reading As described in Chapter 3, the relative proportions of hypochlorous acid and hypochlorite ion depend on the pH. Therefore measuring the chlorine residual at different pHs presents problems. The DPD method that is the basis of all practical measurements of chlorine in the water industry only measures hypochlorous acid.

Therefore the method involves converting any hypochlorite ion that may be present to hypochlorous acid before adding the DPD. In this way all the chlorine in the water is measured as hypochlorous acid. Most modern chlorine analysers also measure the amount of chlorine as hypochlorous acid. They do this by pH correction (and in most cases temperature correction as well). This can be done in one of two ways.

1. The sample is buffered to pH 6.2 using a low pH buffer, thereby converting any hypochlorite ion to hypochlorous acid. The resultant hypochlorous acid is then measured and reported as free chlorine in mg/L or ppm.

2. The analyser includes separate pH and temperature probes. The analyser measures the chlorine residual at the pH of the sample, along with the pH and temperature. The analyser then uses all the data to calculate the chlorine residual at pH 6.2, i.e. the same conditions that the DPD 1 test uses.

This method eliminates the need for the addition of liquid pH buffer or CO2. The pH and temperature readings are also displayed with the chlorine residual.

What Does The Measured Chlorine Residual Really Mean?

All chlorine analysers provide a measure of the amount of hypochlorous acid present at pH 6.2. That is, exactly the same as the DPD 1 test. But what happens if the water sample was at pH 8.0 or above. This is quite common in distribution systems with concrete pipes. While the analyser is telling you that there may be say 0.6 mg/L free chlorine, a quick look at Figure 3.1 shows that in fact only 20% of it is in the form of hypochlorous acid. Now remember that hypochlorous acid is the only form that is active against microbial pathogens. The real chlorine residual in terms of disinfection is therefore only 0.12 mg/L. Suddenly the residual doesn’t look very good at all.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant The effect of pH and how the chlorine residual is actually measured must be understood and considered when target objectives are set for disinfection Ct and minimum chlorine residual in the distribution system.

Total Chlorine Analysers Total chlorine analysers are available in all of the styles described in the preceeding pages and incorporate the addition of potassium iodide to the sample, either impregnated in the gel in the case of a probe style unit or via liquid addition in the case of amperometric/galvanic units. Potassium iodide simply reacts with any chloramines present to release iodine that is then measured in the normal way.

Calibration Since accurate measurement of chlorine residuals is so important for the production of safe drinking water, it is essential that all online residual analysers be calibrated regularly. A suitable interval is one week, however if there is evidence of the instrument drifting, the calibration may be needed more frequently.

Since everything is related to the DPD 1 test, the DPD 1 test is used to calibrate the online analysers.

Instructions on how to calibrate each type of analyser will be described in the manuals provided with the instrument. These instructions should be followed carefully. Typically they involve checking the residual with a portable chlorine analyser and making appropriate adjustments to the online analyser. This of course presupposes that the portable unit is accurate.

Since portable hand held chlorine analysers are used to calibrate online analysers, it is essential that they be accurate. All handheld instruments and measuring devices that are used in the field must be checked against a reference and/or calibrated regularly. Gel standards are available for most portable chlorine analysers (Figure 9.11).

Figure 9.11. Gel chlorine standards.

They are quite expensive and have a limited shelf life, HOWEVER there is really no alternative, chlorine residuals must be measured accurately. The instruments can also be submitted to an independent laboratory for certification of calibration to NATA standards. Manufacturers manuals should be consulted for the correct way to calibrate the instruments.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Since the disinfection control point is so important in the production of safe drinking water, at really critical disinfection sites, consideration should be given to double validation of chlorine residuals. In catchments that are heavily impacted by human activity, including intense animal husbandry and the presence of septic tanks close to the surface water source, the risk of pathogens is much higher than in catchments composed of pristine forest. In the impacted catchments, extra preventive measures and associated monitoring and checking is appropriate. Double validation of chlorine residuals involves using two chlorine residual analysers side by side. This minimises the consequences of instrumental error. Be warned though that you will need to learn to tolerate minor differences in readings and not drive yourself mad trying to get them exactly the same.

It’s the big differences that indicate something has gone wrong.

Measuring Chloramine Residuals The same principles and equipment described above for measuring chlorine residuals can be applied to the measurement of chloramine residuals.

While monochloramine is the predominant chlorine compound in a chloraminated system, for routine monitoring it is convenient to monitor chlorine residuals since these can be readily measured in the field using portable chlorine analysers.

Many utilities simply measure total chlorine in the distribution system and operate the system to achieve a constant total chlorine residual. Other utilities measure total and free chlorine and subtract the free chlorine from the total chlorine to give the combined chlorine residual. Care should be taken in measuring free chlorine in a chloraminated system because in the presence of monochloramine, the DPD 1 test gives a false positive result (see page 83). The free chlorine should be essentially zero in a chloraminated system unless the water is very cold (2 to 3o C) in which case a free residual may persist for up to 5 minutes. Free chlorine close to the dosing site indicates that the ammonia dosing has failed.

In chloraminated systems, the measured combined chlorine residual is often assumed to be equal to the amount of monochloramine. This assumption is based on operating the system with: • • •

A suitable Cl2 : NH3 ratio.

A suitable pH. No taste and odour due to dichloramine or trichloramine.

These methods of measuring monochloramine, combined with measurement of free ammonia, are generally satisfactory for monitoring the distribution system. However monitoring of the primary chloramine disinfection site requires the measurement of both monochloramine and dichloramine to check that the chloramine formation always remains under control. To fully optimise the process of chloramination, monochloramine, dichloramine, total chlorine and ammonia need to be closely monitored. Daily monitoring is recommended.

Chloramine species can be measured using a titration system. Methods for determination of mono and dichloramine are available in Standard Methods (2004). As well as being used in laboratories the method can be adapted for use in the field. Indeed SA Water make up specific titrator units for use in the field (Figure 9.12) for this purpose.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant Figure 9.12. A custom built titrator developed by SA Water for use in the field for the determination of chloramines (courtesy of SA Water).

Note that as an extra check, some utilities also occasionally monitor monochloramine and dichloramine from samples taken in the distribution system.

There are also a number of online instruments available for measurement of monochloramine and ammonia.

These are finding increasing use at primary chloramine disinfection sites to monitor and control dosing.

Mass Balance Checks A useful secondary check of the chlorine dose is to carry out a chlorine mass balance at a suitable interval.

This interval may vary, with large sites carrying it out once per day and smaller sites perhaps once per week. The check is done by determining the mass or volume of chlorine used over the period (either kg in of chlorine gas or litres of sodium hypochlorite) and the volume of water passing through the site for the same period. These figures can then be used to calculate an average chlorine dose and perhaps an approximate free chlorine residual. If the chlorine demand is also known, the free chlorine residual can be calculated more accurately (see Chapter 5).

For chlorine gas, rotameter readings should be checked occasionally by comparing the dose measured by the rotameter with the dose over the same period using the change in weight of the cylinder or drum and the volume of water treated (see Chapter 5 for an example calculation).

For liquid sodium hypochlorite dosing it is essential that a volumetric drawdown assembly (Figure 9.13) be included at the dosing site. The amount dosed per unit time can be calculated by knowing the metering pump dose rate and the plant flow. Most drawdown assemblies include a calibrated tube, however in some older facilities the calibration is not clear. In some cases the tube is just clear plastic with no marks at all. In these cases the tube needs to be calibrated.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant This can be done by measuring the internal diameter and calculating the height of the tube corresponding to a certain volume using the following formula.

Height (mm) = Volume (mL)x 1000 3.14 x r (mm) x r (mm) You can then use a hacksaw blade to cut multiple marks corresponding to this height and then a felt tipped pen or white out to mark inside the cut (Figure 9.13) Figure 9.13. A “home made” volumetric draw down tube.

Even if the assembly has a calibrated tube, the calibration should be checked using the same calculation above so that you know it is correct. It is important to have confidence in the calculation of dose rates.

The draw down tube needs to be sized appropriately for the size of the dosing pump and the size of the dosing facility. The size will vary from plant to plant, however it should be such that it is possible to measure the amount accurately over say a 60 second interval and the liquid moves approximately 60% of the length of the calibration tube over that period. If the tube is too small, the drop will have to be measured over a period of time that is too short to be accurate, if it is too large the change in the level of the liquid will be too small for the measurement to be accurate. It is well worth setting up a correct sized tube to allow accurate determination of the volume of hypo dosed.

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Chapter 9 – Controlling And Measuring The Addition Of Disinfectant

Secondary Chlorine Dosing And Residual Maintenance In Distribution Systems Once satisfactory primary disinfection has been achieved, usually at a WTP, it is important to manage the chlorine or chloramine residual levels throughout the distribution system.

While the residual levels are not sufficient to provide a primary kill in the event of a significant recontamination event, the maintenance of a residual helps to maintain biofilms under control and provides some security against minor contamination events. Whenever a significant recontamination event occurs e.g. a burst main where trench water has entered the water supply main, the section involved must be isolated and disinfected to achieve a Ct for example of 300 mg.min/L. If major contamination is suspected a Boil Water notice must be posted for all consumers downstream of the break. This is necessary since it is not possible to control Cryptosporidium with local dosing. Boiling water is the final barrier for all pathogens. All pathogens are killed by boiling water for several minutes.

More information on the management of chlorine residuals in distribution systems is available in Mosse and Deere (2009).

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Chapter 10 Operation And Optimisation Of Disinfection Systems Even though most disinfection systems are well established, it is worth considering the optimisation of a disinfection system right from basic first principles. In particular, if upgrades to disinfection systems are planned, following the steps outlined here will be very valuable. Simply upgrading what is there may not be the best alternative.

Similarly, as the pressure to comply with regulations and guidelines intensifies, disinfection systems will need to be reviewed. In particular, compliance with Disinfection By Products (DBP) limits will find many utilities considering disinfection options other than chlorine. The principles presented here will be valuable in assessing what is required.

The following suggestions to optimise a system are presented in a logical sequence of steps and are numbered accordingly.

1. Review Existing Water Quality Data The nature of the water to be disinfected has a large impact on the design and operation of a disinfection system.

Where there is an existing disinfected water supply system there will be some monitoring data available.

There are very few water sources in Australia that have not been used for the supply of water at some stage.

The exception may be some ground water sources that are now being tapped as a result of drought. In these cases, the only option is to resort to some laboratory work and determine the chlorine demand, DOC levels and THM formation potential. Where data is available, the following parameters should be reviewed carefully: • • • •

THMs. HAAs. Other DBPs e.g. chloral hydrate.

DOC.

There should also be some monitoring of chlorine residuals throughout the distribution system. The relative levels of free and total chlorine can be useful in assessing the chlorine demand of the water and the cleanliness of the distribution system (Mosse and Deere 2009).

The complaints database should also be reviewed. Are there problems with chlorinous taste and odours? Are there problems with other taste and odours?

This background review provides information on whether existing DBP levels present a problem, whether the DOC levels are likely to cause excessive chlorine demand making management of chlorine residuals in a distribution system very difficult and whether there are taste and odour problems that may be attributed to disinfection.

2. Determine The Available Contact Time The time available for disinfection prior to the first customer is a critical parameter in any system. Disinfection must be completed before the water reaches the first customer. The time should be calculated at peak flows and be based on all the volume available in tanks, storages and pipes prior to the first customer.

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Chapter 10 – Operation And Optimisation Of Disinfection Systems In the first instance a simple calculation made by dividing the volume by the peak flow will be sufficient. This may need to be revised and allowance made for short-circuiting if the available time becomes limiting for disinfection. Chapter 4 (page 35) provides some suggestions on how to actually measure detention times in storages.

3. Assess The Quality Of The Mixing And Contact Structure Mixing and contact are essential for good disinfection. An honest assessment of the mixing and contact structure needs to be made. With a modern gas chlorination system, mixing is generally excellent. With liquid dosing systems consideration needs to be given to how the mixing is carried out. The following factors need to be considered.

• • • •

Is carrier water used?

Is an injection lance used?

Is there an inline mechanical or static mixer?

Is there turbulence generated through valves or flow meters or bends in the pipe?

Similarly with the contact structure: • •

Is it a purpose built contact tank with baffles?

Is it a simple in out system with no baffles and poorly directed inlet and outlet pipes?

If mixing and contact is considered to be poor then some adjustment of the Ct should be considered in an attempt to counter this. Preferably, good mixing and good contact should be established by the installation of inline static mixers or mechanical mixers, and the installation of baffles in the contact structure (see Chapter 4). Remember, the two key control points for the management of pathogens in conventional water treatment are media filtration and disinfection. These MUST be optimised and run well. The assumption that no-one has become ill and therefore nothing needs to be done is not a sound basis for the management of pathogen risk.

4. Determine The Target Ct Based on the catchment risk assessment and knowledge of the pathogen load in the raw water, consult Ct tables and determine the required Ct. For example, do you want to have sufficient chlorine Ct for management of Giardia (up to 300) or is a single barrier, namely media filtration, for Cryptosporidium or Giardia sufficient.

There is no absolute guideline for Ct. Each utility must decide on an appropriate Ct for each supply based on risk assessments and acceptable risk reductions.

5. Determine The Practical Achievable Ct This requires consideration of what is an acceptable residual level at the first customer tap. Most system managers would have some idea of what level of free chlorine residual is acceptable to the majority of the consumers. Allowing for chlorine decay in the system, it should be possible to estimate what concentration of chlorine at the primary dosing point will result in the acceptable free chlorine residual level. If this can’t be estimated, then set up some laboratory trials. Chlorinate the water and allow the chlorinated sample to stand for the equivalent of the detention time in the system and then measure the residual. Repeat this several times and at a number of different temperatures. This will allow you to roughly determine the highest dose acceptable in your system. As an added check, smell and taste the water.

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Chapter 10 – Operation And Optimisation Of Disinfection Systems Would you accept it? (And take off the rose coloured glasses and forget you work for the utility, would you really drink it?) If not, then it should not be acceptable to deliver that water to the consumers.

By multiplying the available contact time by the maximum acceptable chlorine residual the practical achievable Ct can be calculated. Where this is less than the target Ct, some additional disinfection is required. Usually ozone or UV is used to provide improved disinfection. Where the achievable Ct is greater than the target Ct, there is scope to use a lower initial dose thereby further limiting formation of DBPs and the generation of chlorinous taste or odour. If consideration is being given to using chloramine disinfection to limit formation of DBPs, or to achieve better penetration into the distributions system or better control of biofilms, the same calculations can be made to determine whether in fact there is a sufficient Ct for chloramine disinfection.

6. Minimise DBP Formation Where DBPs are identified as a problem (see Step 1 above), one option is to convert to chloramine disinfection.

If this is proposed, then the target Ct and achievable Ct need to be recalculated. Chloramination is only a viable option if the achievable Ct is equal to or greater than the target Ct. If the achievable Ct is less than the target Ct, then chloramination may still be possible by using post ammoniation chloramination (see Chapter 8). By only allowing a short period for the chlorine to act prior to the addition of ammonia, it may be possible to achieve adequate pathogen control and limit DBP formation.

This strategy is only acceptable if the necessary Ct can be achieved. Testing out various options in the laboratory can save a lot of time and answer a lot of questions. Most water quality laboratories would be able to assist with these sorts of tests.

7. Consider The Length And Complexity Of The Distribution System As described in Chapter 8, chloramine residuals persist for much longer than chlorine residuals.

If hydraulic models are available, calculate the water age at the furthest end of the distribution system. Carry out some chlorine decay testing. Dose some water from the treatment plant with chlorine until the usual free chlorine residual is reached. Cover the water so that there is little or no air space above the water. Leave it for the longest water age calculated from the hydraulic model. Even if a hydraulic model is not available, the chances are that some of the field staff that have managed the system for a long time will have a good idea how long it takes for the water to reach the ends of the distribution system. At the end of the time, see what residual is left. This will provide a best-case estimate assuming clean mains. If the residual is greater than or equal to the minimum residual target for the distribution system then chlorine disinfection is possible. If not, then booster chlorination will be required (or chloramination introduced) to achieve the required residual at the end of the distribution system.

Alternatively, based on experience running an existing system, it will be well known whether chlorine residuals can be maintained to the end of the system. Indeed many systems will have one or more booster disinfection sites. The problem with booster disinfection is that every time water is rechlorinated, DBP levels increase. If as a result of this, DBP levels are greater than the target levels then one option is to convert to chloramination. However, prior to making this decision it is important to ensure that the system, including the distribution system has been optimised and DBP formation minimised.

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Chapter 10 – Operation And Optimisation Of Disinfection Systems Prior to making the decision to change, it is important to ensure that: • •

•

The system is clean.

Detention time in storages is at the minimum acceptable to ensure security of supply. Each utility needs to make its own decision on this, however there is no doubt that many storages are run at too high a level to ensure best water quality. The two requirements are often conflicting and a compromise needs to be found.

DOC removal at the WTP is maximised. This may be achieved by the use of enhanced coagulation and possibly also powdered activated carbon (PAC).

Alternatively, it might just be easier to convert to chloramination as long as there is sufficient Ct available at the primary dosing point. Running booster chlorination sites increases demands on the operational team.

Chloramination may be a simpler option although it requires more monitoring.

8. Optimise The Disinfection System Gas Chlorination Systems Check For Chlorine Gas Leaks In a system with the vacuum regulators mounted directly to the drum or cylinder, there is only one point that is under full pressure and that is the point where the vacuum regulator connects to the drum or cylinder. This point is sealed by either a lead or fibre gasket. It is extremely rare for this to leak whilst in operation. If it is going to leak then it will normally occur at the time that the drum or cylinder is changed and the seal replaced.

To check for possible pressure leaks on any part of the system use a 5% solution of aqueous ammonia in a plastic “puffer” bottle. Allow plenty of space on top of the ammonia liquid to allow the ammonia vapours to accumulate and allow the “puffing” action of the bottle to gently blow ammonia fumes around the various parts of the system. Chlorine gas will appear as a smoke like cloud once it comes into contact with the ammonia. If a leak is detected, remove the vacuum regulator and replace the seal. Make sure appropriate PPE is used.

Check For Vacuum Leaks (Drawdown Test) A leak in a vacuum system will simply allow atmospheric air to be drawn into the system. The rotameter may still show the required flow but some of it may be due to air being drawn into the system via the leak. It is therefore important to be sure there are no leaks in the system.

Since vacuum systems are under negative pressure, any leak will not be detected using the ammonia puff method since no chlorine actually leaks out. To test for vacuum leaks: 1. Ensure that the cylinders or drums are turned off. It is important to realise that during this test there will be no disinfection occurring. The test can be conducted quite quickly, however, if the water flow to the distribution system cannot be stopped during the test, there is a risk to consumers from the undisinfected water. Each utility should assess the risks associated with this and if considered unacceptable, devise methods to allow the test to be done.

2. Start the carrier water system.

3. Ensure that vacuum is being created by the venturi. This can be confirmed by checking the vacuum monitor if installed, or by removing the vacuum line from the venturi and placing your thumb over the vacuum outlet to confirm the presence of vacuum.

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Chapter 10 – Operation And Optimisation Of Disinfection Systems a. Observe the float or ball in the rotameter on your control device. If there is no leak in the system, the float should rise when vacuum is applied and slowly fall until it sits on the bottom of the rotameter as it would if you were out of gas. The rate that the ball falls depends on the size of the system. In a big system with a large rotameter the ball will fall rapidly, whereas in a small system it may take a long time for the ball to fall. b. If there is a leak, usually the float or ball will not settle to the bottom of the rotameter.

This will be the case if the leak is between the rotameter and the drum or cylinders.

If a leak is detected, (refer to Figure 6.1): • • • • • •

Remove the vacuum line from the vacuum regulator and cover with your finger. If the float or ball now settles, the leak is in the vacuum regulator.

If the float or ball does not settle the leak is further along the line. Reattach the vacuum line to the vacuum regulator.

Trace the vacuum line to the next equipment connection. This might be the rotameter or a remote change over device.

Remove the vacuum line at the next connection and cover the connection on this device. Has the float or ball now settled? If it has then the leak is in the actual length of vacuum line that runs between the vacuum regulator and the device.

Replace the line.

If the float or ball still has not settled then move to the other side of the device that you are now at and remove the vacuum line and repeat the above steps.

Continue this process of elimination until you reach the rotameter or controller. Once the leaky section is identified, that section can be replaced and retested.

Check The Full Capacity Of The System Even though a gas chlorine delivery system may operate at the low to mid range of its capability, it is important to know that the system can operate at the full design flow.

If the system is controlled automatically then it is necessary to disengage the automatic controller drive or servo and manually wind the system up to maximum setting. Refer to the equipment manufacturers manual on how to do this. Remember that this will actually deliver a slug dose of chlorine into the system so the test needs to be as brief as possible. If the system cannot reach the maximum flow rate, check the following: • • • •

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What is the designed maximum flow rate? Keep in mind that some systems were installed with intention of never reaching this flow.

Is the “draw off” rate of the drum or cylinder being exceeded (see Table 6.1) at the current ambient temperature?

Is the carrier water system or booster pump system operating correctly?

Has the pressure at the point of injection changed? This could be checked by looking at the pressure in the main either before or after the point of injection. Most plants have some form of pressure monitoring in the main or at least points where a pressure gauge can be attached.

Chapter 10 – Operation And Optimisation Of Disinfection Systems Check The Vent To check if the vacuum regulator is venting to atmosphere: • • •

Locate the outlet of the vent pipe and using a puffer bottle check for any presence of chlorine whilst the system is shutdown. Ensure that the cylinder or drums are open but that the system is not operating.

If gas is detected, the vacuum regulator should be serviced.

Perform visual inspection of the vent line along the entire run from the vacuum regulator right through to the point where it terminates outside the building. If there is any sign of fatigue or failure of the pipe, replace it immediately.

The only cause of a gas leaking out the vent is a failure of the main seat in the vacuum regulator. This can be caused by a build-up of deposits on the seat face leaving it unable to seal, or it could be a total by-pass of the seat caused by failure of the sealing arrangement. Either way the regulator must be serviced or replaced.

Check The Chlorine Leak Detector The chlorine leak detector should be tested on a regular basis (AS 2927 requires weekly testing) regardless of any internal self-test modes that it may have. To perform a sensor test independent of the internal selftest mode, hold a weak source of chlorine near the air sample inlet. AS 2927 suggests a suitable source of chlorine can be obtained by using domestic bleach to which a small amount of dilute acid (such as vinegar) has been added. Try 50 mL of “hypo” in a 250 mL bottle and a few drops of vinegar. Hold the solution under the sensor and check that the unit responds to the chlorine.

There are also commercial units available that deliver a known amount of gas into the sensor for testing.

Clearly if the sensor fails to respond it should be replaced urgently. Check with your supplier as to whether they hold stock or whether you need to hold stock.

All sensors have a limited life expectancy and this is reduced proportionally by the amount and level of exposure to gas. Therefore prior exposure to gas will reduce the life of the sensor.

Liquid Chlorination Systems Ensure Pipe Work And Metering Pumps Have Been Designed And Installed To Minimise Gassing Trapping of gas in a liquid dosing system will decrease the efficiency of disinfection and may in fact stop the flow of liquid disinfectant. The single most important thing to ensure in a liquid system is the prevention of gassing.

• Choose metering pumps that have “proven” de-gassing features and that match the pipe work configuration being considered, i.e. suction lift or flooded suction operation. Generally, flooded suction type features rely upon some mechanism to release gas after and/or while it is being formed, whereas, suction lift types tend to address the avoidance of the gas before reaching the pump head (see Chapter 7).

• Keep the length of pipe work as short as possible.

• Ensure the suction and discharge tubing “size” matches the metering pump feed rate requirements. Refer to the relevant manufacturers manual for this information.

• Keep pipe work out of direct sunlight.

• Use the lowest strength sodium hypochlorite possible. Full strength sodium hypochlorite solutions are more likely to produce bubbles than low (5% or 3%) strength solutions.

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Chapter 10 – Operation And Optimisation Of Disinfection Systems Size The Pumps To Match “Current” Peak Demands.

Sizing of metering pumps to cater for future needs may lead to inaccurate dosing, particularly at the lower end of flow rates being treated. Metering pumps that are too large are likely to lead to discontinuous dosing rather than approximate continuous dosing (see Figures 7.12 and 7.13).

Check Dosing Lines Regularly.

Sodium hypochlorite and liquid ammonia are normally dosed in carrier water. Depending on the nature of the water used as a carrier, the dosing lines for both of these chemicals can become blocked with scale (Figure 10.1).

Figure 10.1. A blocked hypo delivery fitting.

To prevent this occurring, and the associated risk of loss of disinfection, the dosing lines should be checked regularly. One strategy to avoid blockage is to keep the dosing lines short, keep them easily accessible and simply to replace them on a regular basis.

9. Check And Calibrate The Chlorine Analyser Chlorine analysers are an essential part of any chlorination system since they provide the only real process monitoring of the disinfection control point. To check that the analyser is working reliably: • • • •

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Consult the instrument manual to find the design flow rate of the water sample for the analyser.

Check that the sample line is clear of obstruction and sample water is flowing at the required flow rate.

Check that the discharge from the analyser sample cell is clear of obstruction.

Check the calibration of the instrument. This can be done by comparing the reading on the analyser to a reading from a hand held unit. Remember to always take the sample for the portable analyser from as close as possible to the measuring electrode.

Chapter 10 – Operation And Optimisation Of Disinfection Systems Since portable hand held chlorine analysers are used to calibrate online analysers, it is essential that they be accurate. All handheld instruments and measuring devices that are used in the field must be calibrated regularly (see Chapter 9).

10. Ensure Failsafe Dosing Undisinfected water must never be allowed to leave a primary disinfection site. This can be ensured with a reasonable degree of certainty in a number of ways, however if any of these fail then flow of water to consumers must be shut off until full disinfection is re established.

• •

•

• • • •

Install duty standby systems for dosing. This includes metering pumps for liquid dosing systems and vacuum systems for gas chlorination systems. Ensure automatic changeover actually occurs and there is a system to automatically notify the operator that changeover has occurred.

Have vacuum alarms for gas systems and flow monitor alarms for liquid systems. Loss of vacuum in a chlorinator or loss of flow from a metering pump is the first sign that disinfection may be failing. An increase in vacuum above the normal operating range may also indicate developing problems with the chlorination system. By responding quickly in these situations, serious risks to consumers can be avoided. Loss of chlorine delivery is not only due to low vacuum conditions. High vacuum can result from blockages. For failsafe operation, monitoring of high vacuum should also take place.

Make sure that sufficient chemical stocks are maintained. o Install ultrasonic level sensors and alarms on the chemical supply tanks, or cylinder load scales to indicate when levels are low. o Carry out regular visual inspections of chemical storage vessels. o Set up an ordering and delivery system that minimises the likelihood of running out of chemical. The simplest of these is to have a reorder alarm level on the level sensor or load cells. Another possibility is to send the information from the level sensors or load cells directly to the chemical supply company so that they can act as a secondary check on your system.

Ensure dosing lines are clear and are not obstructed. o Regularly check the internal condition of dosing lines to ensure that there has been no build up of material on the inside of the lines.

Have back up power systems or an uninterruptible power supply (UPS) sufficient to maintain primary dosing.

Have alarms on the residual analyser. Alarms should be activated whenever there is a sustained drop in residual below the target objective for the site. Alarms should also be activated and shut down the site if the chlorine residual is lower than the critical limit.

Check that all automatic systems work reliably. This check must be carried out regularly and completely. For example, it is not satisfactory to just carry out a simple electronic 4-20 mA function test of the alarm. Rather, the alarm condition should be triggered by introducing water with a low chlorine residual to the chlorine analyser and the total alarm response system checked.

This includes alarm dial outs, pager or mobile phone activation and operator and management responses.

Where any failure of disinfection is detected or is suspected, systems must be shut down immediately.

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11. Monitor Raw And Treated Water Quality Extensive monitoring of raw water is essential for the operation of any water treatment process. For the management of disinfection it is important to monitor the organic content of the raw water. Monitoring of the filtered water from the individual filters is important to verify that the filter control point is operating well but also to provide information on the likely requirements for disinfection. A number of tests can be used. These include DOC and UV254 of the raw water and turbidity, DOC or UV254 of the treated water. An increase in these in the raw water indicates an increase in the organic content of the raw water. This may have an adverse impact on coagulation and may lead to increased levels of organics in the treated water. The monitoring of the treated water will confirm this. Whenever the level of organics increases in the treated water, a number of outcomes are likely: • • • •

Chlorine demand will increase.

The dose of chlorine will need to be increased.

There may be an increase in taste and odour.

DBP levels are likely to increase.

If the turbidity increases in the treated water then the chlorine dose will need to be increased due to the shielding effect of the turbidity particles. It is more difficult for the chlorine to get to the pathogens through the small turbidity particles.

Hrudey and Hrudey (2004) go further and state “Any sudden change in chlorine demand that decreases the free chlorine residual effectively signals exposure to possibly dangerous contamination.”

Another useful alternative is to simply determine the chlorine demand on a regular basis. A simple test for this is provided in Appendix 1. Measuring the chlorine demand and plotting it on a graph will gradually construct a trend. Over months or years, repeatable patterns may be observed and help anticipate necessary changes to dosing rates. As a guide, chlorine residual should be determined weekly. This interval may need to be reduced if levels start to change significantly.

12. Visit The Site Regularly All disinfection plants must be visited regularly to undertake routine inspections and maintenance. Do all the items look and sound OK? Does the site smell OK? Use all your senses to check the site. At a minimum, weekly inspections should be carried out.

While it may be tempting to assume that the sites have been automated, the reality is that problems will arise that cannot be controlled by the automated system.

13. Maintain And Calibrate All Disinfection Equipment All equipment associated with disinfection should be maintained and calibrated according to the manufacturers specifications. One option is to enter the intervals and requirements into a computer based maintenance database so that a reminder is automatically generated when the time arrives.

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14. Report On Achievement Of Disinfection Targets Disinfection is an important control point in a water supply system. As such, target criteria and critical limits must be set (see Chapter 1). It is an essential feature of control point theory that the performance of the control point must be continually assessed. The actual performance of each primary disinfection site should be regularly compared against the target criteria and critical limits that have been set for the disinfection control point.

One useful way of to assess performance of the disinfection control point is to report: • •

Daily minimum Ct. % of time that Ct ≥ target.

Clearly if the daily minimum Ct is less than the target then water has been allowed to enter the distribution system that has not been adequately disinfected. If the daily minimum Ct is zero then undisinfected water must have entered the distribution system. This must not happen. The % compliance gives a feel of how often these problems arise and how significant they are. Reporting of performance of the disinfection control point should be made to the highest levels of management within the utility and possibly also to the Health Regulator.

15. Don’t Assume, Know!

Often assumptions are made that appropriate systems are in place, alarms work and doses are appropriate.

All to often these assumptions are not checked. A fundamental element of the ADWG Framework is to know the system. Assumptions don’t stand up to public scrutiny in the face of waterborne disease. It is important to know that everything is as it should be.

External audits by management system auditors are useful, but often overlook these critical evaluations. The onus is on the operational team to fulfil this role in an unbiased fashion.

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Chapter 11 Closing Words Provision of safe drinking water requires time, diligence, attention to detail and effective process monitoring.

Disinfection is the vital final step in the process of water treatment prior to delivering safe water to the consumer.

This practical guide has provided a sound background to the control and optimisation of chlorine and chloramine disinfection systems. Disinfection is a key control point where if we optimise the step and monitor it thoroughly, we increase the chance of producing safe drinking water. Alternatively if the step is not optimised and monitored thoroughly, the chances of producing unsafe drinking water are greatly increased.

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

Think about it? A well run WTP means peace of mind that the water delivered to consumers is safe to drink and will not result in an outbreak of waterborne disease of the types that have been described all around the developed world.

Take the first step now!

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References References ADWG (2004). Australian Drinking Water Guidelines. NHMRC, Australia.

Hrudey,S.E. and Hrudey,E.J. (2004). Safe Drinking Water. Lessons from Recent Outbreaks in Affluent Nations. IWA Publishing. London UK. Mosse, P. and Murray, B. (2006) Practical guide to the operation and optimisation of media filters. WIOA. Mosse, P. and Deere, D. (2009) Practical guide to the operation and optimisation of distribution systems.

WIOA (second edition).

Murray, B. and Mosse, P. (2008) Practical guide to the optimisation of chemical dosing, coagulation, flocculation and clarification. WIOA. Orica. (2006) Chlorine handbook. Orica Watercare. Melbourne.

Orica (2002) Sodium hypochlorite handbook. Orica Chemicals. Melbourne.

Standard Methods (2004). Standard Methods for the Analysis of Water and Wastewater. APHA Wilson, S. (1995). Chloramination. Predicting monochloramine to dichloramine ratios. Proceedings of the 58th Annual Water Industry Engineers and Operators Conference.

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Appendices

Appendix 1 A Simple Method For The Measurement Of Chlorine Demand Reagents Chlorine Stock Solution -approximate 500 mg/L Add 1.0 mL of 5% w/v sodium hypochlorite solution to 99 mL of distilled water and store in a clean container. This stock solution will deteriorate over time, so must be freshly prepared prior to use. pH buffers 4.0, 7.0 & 10.0.

Free & Total DPD reagents.

Equipment 2 x 1 L glass bottles with plastic lids. (Schott bottles are suitable) The bottles should be cleaned and thoroughly rinsed and kept aside as dedicated bottles for the chlorine demand test.

1 x 500 mL glass bottle with plastic lid. (Schott bottles are suitable) Calibrated pH meter capable of reading to at least one decimal place.

Thermometer capable of reading from +5 to +300C. Chlorine meter. (Using the DPD method)

Method 1. Rinse the bottles with the sample three times before filling. Fill two 1 L bottles and one 500 mL bottle with un-chlorinated water from the filters.

Note: Keep samples away from direct sun light by covering the bottle in Aluminium foil and store bottles in a cupboard between analyses.

2. Analyse the 500 mL sample for pH, Temperature, Turbidity, True Colour, Aluminium, Iron and Manganese, and record the results.

3. Add 3 mL of the stock chlorine solution to each of the 1 L bottles and immediately mix the solution by briskly stirring to avoid over aerating the sample. (3 mL of the stock chlorine solution added to the un-chlorinated filtered water sample should give a free chlorine concentration of approximately 1.5 mg/L.) 4. After mixing, immediately analyse for total & free chlorine and record the time and the free & total chlorine measurements for each bottle and re cap tightly.

5. Store samples in the cupboard for 10 minutes.

6. Remove a sample from the cupboard and stir (10 turns to the right, 10 turns to the left, then 10 turns to the right again) and take a 10 mL sub sample from each bottle and record the time.

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Appendix 1 – A Simple Method For The Measurement Of Chlorine Demand 7. Measure the free & total chlorine of the samples and record the results.

8. Return the bottles to the cupboard promptly when analysis is completed.

9. Repeat steps 5 through 8, twice more. Total number of readings, including the initial reading will be four (4).

10. After the final chlorine analyses, measure the test samples pH and temperature and record the results with the 30 minute test.

Calculate and record the following.

Free Chlorine Demand (mg/L) = Chlorine consumed over 30 minute contact period = Free chlorine at zero minutes – Free chlorine at 30 minutes Free Chlorine Decay (%) = Percentage chlorine lost over 30 min relative to the initial dose. = Free chlorine at zero minutes – Free chlorine at 30 minutes x 100 Free chlorine at zero minutes

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Appendix 1 – A Simple Method For The Measurement Of Chlorine Demand

Chlorine Demand Test Result Sheet Date: _____________

Time: ____________

Site: ______________________

Operator: _____________

Sample Point or Location: ________________________

Un-chlorinated filtered water, analysis: pH

Turbidity

True Colour

Iron

Aluminium

Manganese

Test sample analysis: Sample Number

Test Time (minutes)

1

Zero

2

10

3

20

4

30

Free Chlorine Demand

Free Chlorine Decay

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Free chlorine (mg/L) Bottle 1

Bottle 2

Total chlorine (mg/L) Bottle 1

Bottle 1 Bottle 2 Bottle 1 Bottle 2

Bottle 2

pH

Temperature (oC)

mg/L

%

Disclaimer This document does not claim to be a definitive statement on chlorination and chloramination, 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 afore mentioned organisations disclaim all and any liability to any person in respect of anything and the consequences of anything done or omitted to be done by a person in reliance upon the whole or any part of this document. If further expert advice is required, the services of a competent professional should be sought.

www.wioa.org.au

WILLPRINT SHEPPARTON 03 5831 6565

Water Industry Operators Association of Australia (WIOA) PO Box 6012, Shepparton 3632 Phone: 03 5821 6744 Fax: 03 5821 6033