Operation and Optimisation of Microfiltration & Ultrafiltration Membrane Processes

Practical Guide to the Operation & Optimisation of Microfiltration & Ultrafiltration Membrane Processes

K at h y No r t h co tt, G re g Cre e k a n d Pe t e r M o s s e

The Authors Dr Kathy Northcott is the R&D Manager - Veolia Australia & New Zealand, and a specialist in water treatment, wastewater treatment and water recycling processes.

Greg Creek is a senior training officer with Water Industry Training Consultants.

Dr Peter Mosse provides specialist technical support in water and wastewater technology.

Acknowledgements The authors wish to thank Konrad Mueller and Colin Ellett from Veolia ANZ; Stephen Wilson from Water Industry Training Consultants; and Rick Maffescioni from Aqua Manage for their critical review of the manuscript.

Thanks also to Veolia ANZ, Melbourne Water, Dunedin City Council, Barwon Water, DuPont Water Solutions and Victoria University for providing some of the photos appearing in this publication.

The authors acknowledge the contribution of all the sponsors who have provided funding for the publication of this book.

• New South Wales Water Directorate • Queensland Water Directorate (qldwater) • TRILITY • Victorian Water Industry Association (VicWater) • Water Services Association of Australia (WSAA) Particular thanks are extended to the Water Industry Operators Association (WIOA) for their commitment to improving the skills of operations staff in the water industry and for coordinating the development of this resource as part of the process. The efforts of WIOA MD, George Wall, in proof reading the final document and for organising the layout and publication of this book have been invaluable.

Publications Details First Edition, May 2021.

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 or Wastewater Treatment.

Other WIOA Publications Practical Guides to Water Treatment.

• Practical Guide to Understanding and Managing Surface Water Catchments.

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

• Practical Guide to the Operation and Optimisation of Media Filters.

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

• Practical Guide to the Operation and Optimisation of Distribution Systems.

Practical Guides to Wastewater Treatment.

• Practical Guide to Odour Control in Sewage Transport Systems.

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

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General Introduction The WIOA 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.

6.

Catchment.

Chemical dosing, coagulation, flocculation and clarification.

Media Filtration.

Membranes.

Disinfection.

Distribution.

Each guide provides practical advice regarding the management and operation of each of the critical control points identified by Utilities as they implement the Australian Drinking Water Guidelines Framework for the Management of Drinking Water Quality (ADWG 2011), state based legislation such as the Safe Drinking Water Act 2003 in Victoria, or a HACCP or ISO program.

An underlying objective of each of the guides is that all the way through, 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 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, in order to deliver the absolute best quality drinking water to the consumer.

Feedback Requested These guides have been written to help operators peform 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 future editions stronger and more useful for operators in our industry. Any suggestions can be directed by e-mail to the Water Industry Operators Association of Australia (WIOA) at info@wioa.org.au or by post to PO Box 6012, Shepparton, Victoria, 3632.

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Contents 1. Hazards, Risks and Control Points................................................................................. 4

• Hazards and Drinking Water............................................................................................................. 4 • Pathogens are the Main Hazards in Drinking Water ........................................................................ 5 • Other Hazards................................................................................................................................... 8 • Control Points.................................................................................................................................... 9 • Critical Limits and Target Criteria.................................................................................................... 10 • Corrective Actions........................................................................................................................... 11 • Responsibilities............................................................................................................................... 12

2. Membranes and Public Health...................................................................................... 13 • Operating Criteria and Performance Parameters............................................................................ 15

3. Introduction to Membranes........................................................................................... 17

• Membranes and Water Treatment................................................................................................... 17 • Basic Membrane Terminology......................................................................................................... 17 • Membrane Filtration Processes....................................................................................................... 19 • How Membranes Work.................................................................................................................... 20 • Membrane Fouling.......................................................................................................................... 21 • Membrane Materials........................................................................................................................ 21 ◦ Hydrophobicity............................................................................................................................ 22 ◦ Oxidant Tolerance....................................................................................................................... 23 • Flow Configurations......................................................................................................................... 23 • Types of Membrane Systems.......................................................................................................... 26 • Membrane Pre-treatment................................................................................................................ 30 • Membrane Resistance ................................................................................................................... 31 • Flux................................................................................................................................................. 31 ◦ Normalised Flux (Correction for Viscosity).................................................................................. 32 • Transmembrane Pressure and Membrane Permeability................................................................. 33

4. Optimisation of Membrane Filtration Systems ........................................................... 34

• 1. Know Your Process..................................................................................................................... 34 • 2. Know Your Plant ......................................................................................................................... 34 • 3. Feedwater Quality and Pre-treatment......................................................................................... 35 ◦ Prefiltration.................................................................................................................................. 35 ◦ Feedwater Coagulation............................................................................................................... 37 ◦ Use of Polymers......................................................................................................................... 38 • 4. Membrane Integrity..................................................................................................................... 38 • 5. Membrane Fouling...................................................................................................................... 39 • 6. Flux............................................................................................................................................. 40 • 7. Backwashing and Chemical Cleaning......................................................................................... 40 • 8. Inspections of Membrane Elements............................................................................................ 40 • 9. Calibrate Gauges, Meters and Associated Instrumentation........................................................ 42 • 10. Reduce the Risks Associated with Washwater Recycling ........................................................ 43

5. Membrane Cleaning....................................................................................................... 44

• Backwashing of Hollow Fibre Membranes...................................................................................... 44 • Backwashing of Immersed Membranes.......................................................................................... 46 • Chemical Cleaning.......................................................................................................................... 47 ◦ 1. Chemically Enhanced Backwash (CEB) ................................................................................ 47 ◦ 2. Chemical Clean-in-Place (CIP)............................................................................................... 47 • Monitoring the Effectiveness of Cleaning........................................................................................ 50 • Management of Chemical Sludge................................................................................................... 51

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6. Membrane Maintenance................................................................................................. 52

• Record Keeping............................................................................................................................... 56 • Using Maintenance Statistics.......................................................................................................... 56 • Storage and Preservation of Membranes........................................................................................ 57 • Membrane Autopsies...................................................................................................................... 58 • Trouble Shooting............................................................................................................................. 63

7. Membrane Replacements.............................................................................................. 64

• When to Change out Membranes................................................................................................... 64 • Preparation for Changing out Membranes...................................................................................... 65 • Decommissioning and Dismantling ................................................................................................ 67 • Replacement of Membrane System Components........................................................................... 68 • Recommissioning the Membrane System....................................................................................... 69

8. Nanofiltration and Reverse Osmosis............................................................................ 70

• Comparison of NF and RO to MF and UF....................................................................................... 70 ◦ Sizes of Particles Removed........................................................................................................ 70 ◦ Operating Pressures................................................................................................................... 70 ◦ How NF and RO Membranes Work............................................................................................ 71 ◦ Diffusion and Osmosis................................................................................................................ 71 ◦ Osmotic Pressure and Reverse Osmosis .................................................................................. 72 ◦ Actual Membrane Performance.................................................................................................. 72 • The RO System. Membrane Elements, Stages and Arrays ........................................................... 73 ◦ Stages and Recoveries............................................................................................................... 74 ◦ Importance of Pre-Treatment...................................................................................................... 76 ◦ Silt Density Index (SDI)............................................................................................................... 77 ◦ Integrity Testing for RO Membranes........................................................................................... 78 ◦ Post Treatment........................................................................................................................... 79 ◦ Causes of Membrane Failure..................................................................................................... 80 ◦ Energy Recovery ....................................................................................................................... 81 ◦ Concentrate Management.......................................................................................................... 81 ◦ Pressure Vessel Safety ............................................................................................................. 82 • Terminology .................................................................................................................................... 82

9. Closing Words................................................................................................................ 83

• Further Reading.............................................................................................................................. 83

Case Study in Membrane Troubleshooting...................................................................... 84

• 1. Impacts of Raw Water Quality Changes on Microfiltration Plants .............................................. 84 ◦ Monitoring Membrane Performance........................................................................................... 86 ◦ Membrane Cleaning Trials.......................................................................................................... 87 ◦ Review of Membrane Condition and Performance..................................................................... 88 ◦ Follow-up Autopsies................................................................................................................... 88 ◦ Conclusions................................................................................................................................ 89 • 2. Importance of Pre-Treatment...................................................................................................... 90

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Chapter 1 Hazards, Risks and Control Points The Australian Drinking Water Guidelines (ADWG 2011) includes a section entitled “Framework for the Management of Drinking Water Quality.” The Framework is based on the recognition that the best way to protect public health and ensure high quality drinking water is to use a preventive management approach that covers all steps in the production of drinking water from catchment to tap.

This is quite different to the old approach that relied 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 Framework recommends thoroughly monitoring the individual steps in the overall process. By ensuring that each step is operating correctly, the high quality and safety of the final product can be assured at all times.

Water Utilities 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, meaning 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, aluminium, heavy metals or pesticides.

Turbidity.

Natural Organic Matter (NOM).

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 chlorinator leading to a loss of chlorination and therefore, disinfection.

The best way to manage an identified hazard is to eliminate it. If this is not possible, 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.

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Chapter 1 - Hazards, Risks and Control Points

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

Table 1.1.

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

Risk Factor Time to effect

Detection

Harmful levels

Source

Design

Pathogens As little as a few minutes of pathogens getting into the treated water can lead to illness when the water is consumed. Their impact is immediate, and will be evident within days of contamination.

Pathogens are difficult to detect. They have no taste, odour or colour and are too small for us to see. Results are normally not available until after people have consumed the water.

Even swallowing only a few pathogenic organisms can make someone ill, meaning very low numbers in water can have an effect. Low levels can occur without any other signs. Pathogens don’t have a 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.

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

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

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

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.

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

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.

There are hundreds of waterborne pathogens. Each has a slightly different combination of sources, treatment sensitivity and effect on health. 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 that present a risk to the safety of drinking water.

• 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.

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Chapter 1 - Hazards, Risks and Control Points 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. ◦ Campylobacter, pathogenic E. coli O157H7 and Salmonella.

• Enteric (intestinal) protozoan parasites. ◦ Cryptosporidium and Giardia.

• Enteric viruses. ◦ Enterovirus, Rotavirus, Adenovirus and Hepatitis A virus.

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

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 water 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 illness in humans.

Feature Is sewage a common source?

Is livestock manure a common source?

Are bird droppings a common source?

Are wildlife faeces a common source?

What are the principal source controls?

Is media filtration an effective control?

Is primary disinfection using chlorination an effective control?

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

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

Is micro/ultra-filtration an effective control?

Is reverse osmosis an effective control?

Bacteria Protozoa Viruses Yes Yes Yes Yes Yes No Yes No No Yes Possibly No Control all sources Control all sources Control all sources of faeces of faeces of faeces Yes Yes Limited Yes

No

Yes

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

Yes

Yes

Yes

Yes

Yes

Limited

Yes

Yes

Yes

The vast majority of outbreaks of waterborne illness in the western developed world have arisen due to microbial pathogens. Table 1.3 provides some examples.

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Chapter 1 - Hazards, Risks and Control Points Table 1.3.

Examples of waterborne illnesses in the western developed world.

City Milwaukee Wisconsin USA

Population Date Hazard

Gideon Missouri USA Sydney Australia Walkerton Ontario Canada Galway Ireland Nokia Finland

500,000

1,100

~4M

4,800

Source

Ill/Dead

1993 Cryptosporidium

Sewage.

Poor WTP performance.

403,000/54

1993 Salmonella

Extensive flushing, poor tank integrity, open hatch, roof in poor repair, bird faeces.

Poor chlorine residual control.

~600/7

Possibly Storm, sewage, poor WTP 1998 Cryptosporidium operation. and Giardia E. coli O157 H7 Bores, storm, cow manure, poor 2000 chlorination. Falsification of Campylobacter records.

72,000

2007 Cryptosporidium Sewage discharge to lake.

32,000

2007

Campylobacter and Salmonella

Not determined but the potential was high 2,300/7 >200/0

450 kL treated sewage entered the distribution system.

6,500/2

Detected in 7/13 storages.

0 Several people hospitalised but link to water not proven.

Port Douglas Australia

4,000

2008 E. coli

Ostersund Sweden

59,000

2010 Cryptosporidium

Roma Australia

7,100

2011 Salmonella

1.25 M

Norovirus and various 2015 other faecal pathogens

Water and sewer pipes cracked during work. Sewage entered water mains. Poor flushing.

12,000/0

2015 Lead

Change of raw water source.

Failure to provide corrosion protection.

Falsification of records.

Gross mismanagement by multiple government agencies.

6,000 – 12,000 children exposed to drinking water with high levels of lead. Impact on health to be determined.

2016 Campylobacter

Undisinfected bore water.

Heavy rain. Sheep faeces washed into local waterways and entered 8,500/4 the shallow aquifer thought to be secure. Possible ingress through bore head as well.

Prague Czech Republic

Flint USA

Havelock North New Zealand

100,000

14,000

Cross connection between sewer pipe and storm water pipe.

Backflow from underground storage tanks contaminated by frogs.

27,000 26 ill, 3 hospitalised

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Chapter 1 - Hazards, Risks and Control Points There are now too many examples of people becoming ill or dying as a result of them trusting the drinking water supplied to them.

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 activities that can lead to pollution of water with animal, and in particular, human faeces. However, even with the best protected catchments, pathogens will gain access. The catchment barrier is therefore not sufficient to control pathogens, therefore a “multiple barrier” approach is needed to manage pathogens in a water supply. For microorganisms to finally reach and infect a consumer, they must pass unscathed through each of the barriers in sufficient numbers to be able to infect a human being. Membranes represent a very significant physical barrier to pathogens. This feature is considered in more detail in Chapter 2.

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 five major areas of chemical contamination that have caused waterborne illness.

• • • • •

The use of contaminated ground water, particularly associated with arsenic and nitrate.

The presence of naturally occurring elements in the water source, particularly iron and manganese.

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

The backflow of hazardous substances into water distribution systems.

Contaminants resulting from corrosion, particularly from old lead pipes.

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. Similarly, in parts of outback Australia, the presence of radionuclides in groundwater used for drinking water supplies is also a cause for concern.

Contamination from incorrect chemical dosing and backflow is becoming more common with several well documented cases.

Waterborne incidents associated with algae were historically associated more with the warmer climates of Queensland or degraded river systems with irregular flows. However, as the climate in Australia continues to change, many water storages further south have had to deal with recurring algal blooms. Indeed, there are an increasing number of instances in the southern states of Australia where algal blooms have occurred even during winter. Problems arising from algae present in source water include: • Toxins 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 organisms such as diatoms.

• Taste and odour chemicals can be introduced to the water by soil bacteria and fungi.

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

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

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Chapter 1 - Hazards, Risks and Control Points 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. Ultrafiltration membranes are effective at removing whole algal cells however large numbers of algae will foul the membranes without prior filtration. Nanofiltration and reverse osmosis have been shown to be effective for removal of a range of dissolved algal toxins and taste and odour chemicals.

Finally, the reaction of chlorine based disinfectants with organic matter in water can result in the formation of Disinfection Byproducts (DBP’s) including Trihalomethanes (THM), Haloacetic Acids (HAA) and Nitrosodimethylamine (NDMA). These can have an impact on public health, however if the levels are below those stipulated in ADWG (2011), the risk is considered very low. DBP’s are considered in more detail in Mosse, Braden and Hourigan (2009), “Practical Guide to the Operation and Optimisation of Chlorine and Chloramine Disinfection”.

Control Points Within a water supply system 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.

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. Membranes can take the place of the clarifier and filtration steps and even in some cases the coagulation and flocculation steps.

Figure 1.1.

A generalised water supply system.

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Chapter 1 - Hazards, Risks and Control Points The most important control points in a water supply system are: • • • • • • • • •

Catchment.

Raw water extraction.

Chemical dosing.

Coagulation.

Flocculation.

Clarification.

Filtration (either granular media or membranes).

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 (Figure 1.2).

Figure 1.2.

Summary of the relationship between target and critical limits and the actions taken when these are not achieved.

The ADWG 2011 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. turbidity for membrane filtration).

• 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.

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Chapter 1 - Hazards, Risks and Control Points Two different target levels are often set: 1. Target criteria.

Target limits 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.

In practice, this is now interpreted slightly differently by many water Utilities as the Target, Alert Limit and Critical Limit, the so-called traffic light concept (see Figure 1.2).

Corrective Actions Target limits are always more stringent than the critical limits. By setting target limits 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).

Once the critical limit is exceeded, incident and emergency plans need to be activated (Figure 1.2). There is also an opportunity after control has been re-established, to determine what went wrong and why, and to put in place actions or changes that will prevent the incident happening again. This process is very similar to analysis of near risks that are quite common in workplace health and safety.

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

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

Verifying that all the control points in a process are within the recommended range ensures that the final product will be within the specific limits. Testing of the final product quality is verification monitoring and simply confirms that the total system is working. Quality control should not rely on verification monitoring alone. 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 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 of the measured parameters. 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 Membranes and Public Health The major aim of a water supply system is to provide safe drinking water to consumers.

Pathogens are the major hazard in the operation of a water supply system. Therefore, management and removal of pathogens is the primary objective for the operation of any water treatment plant. Microbiologists quantify the removal of pathogens as a Log Removal Value (LRV). This can seem confusing at first, however it is relatively simple. Table 2.1 provides a summary of LRV compared to percentage removal of microorganisms. Box 2.1 provides an example of how the value can be calculated from the results of a filtration challenge test.

Summary of the relationship between percentage removal and log removal value.

Table 2.1.

Percent Removal 90 99 99.9 99.99 99.999 99.9999

Log Removal Value 1 2 3 4 5 6

Box 2.1. Calculation of Log Reduction Value (LRV) To establish the LRV of a particular filter, whether a membrane or media filter, inactivated pathogens or surrogate particles can be used. A known concentration is added to the feed, and the final filtrate is analysed for the pathogens or particles.

LRV = Log10 (Cinf/Ceff) Where Cinf = the concentration of particles or pathogens in the feed (influent) Ceff = the concentration of particles or pathogens in the filtrate (effluent) Example Samples of feed and filtrate were collected at a membrane filtration plant and tested for coliform bacteria. The influent concentration was 3.7 x 106 CFU/mL and the effluent concentration was 2 CFU/mL. LRV = Log10 (3.7 x106 / 2) = 6.3 In microfiltration and ultrafiltration membrane treatment processes, the primary mechanism of pathogen removal is size exclusion where anything larger than the nominal membrane pore size is physically prevented from passing through the membrane. As a targeted technology for pathogen removal, the use of microfiltration and ultrafiltration membranes makes a lot of sense. Membrane filtration has the benefit of being a complete physical barrier, with the ability to more reliably remove smaller pathogens than media filtration. Of course this is only true if the integrity of the membrane is preserved, meaning that there are no leaks in membrane element seals or damage to the membrane such as pin-holes, splits or tears.

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Chapter 2 - Membranes and Public Health This contrasts with the operation of granular media filters where pathogens are removed mainly by the attachment of floc particles to the surface of the filter media grains throughout the depth of the filter bed. In media filtration, particles can slip through the void spaces between the media and some particles will always get through. If there are more particles in the feed water it is likely there will be more in the filtrate. Under excessive load (flow, solids), media filters have a higher risk of turbidity and therefore potential pathogen breakthrough.

By comparison, membrane filters will reliably reject all particles bigger than the nominal pore size. If the feed water is dirtier, then provided membrane integrity is maintained, the filtrate water coming out will still be of the same high quality. The trade off for this is a greater risk of membrane fouling and a deterioration in filtration capacity over time.

In 2015, the Water Services Association of Australia (WSAA) published the Manual for the Application of Health-Based Treatment Targets for Drinking Water Safety. Health Based Targets (HBT) have been defined for use in the water industry to quantify the level of treatment a particular source water requires in order to ensure that the water is adequately treated to protect public health. The Manual provides a comparison of default LRV values for a range of filtration technologies under a specified range of operating conditions (Table 2.1).

As can be seen in the table, direct filtration and conventional treatment have higher LRVs for protozoa compared to microfiltration, but only when operating with an individual filter turbidity ≤ 0.15 NTU for 95% of month and not > 0.3 NTU for ≥ 15 consecutive minutes. However, both microfiltration and ultrafiltration provide higher LRV for bacteria. Ultrafiltration has the highest LRV for viruses. An additional benefit of membranes is that as long as membrane integrity is maintained, the same LRVs can be achieved regardless of adverse source water quality (e.g. elevated turbidity) and process conditions.

Table 2.1.

Log Reduction Values (LRVs for typical filtration processes. (From WSAA HBT Manual 2015).

Log Reduction Value

Process

Virus

Process critical limits

Direct Filtration

2.5

1.0

1.0

Individual filter turbidity ≤ 0.3 NTU for 95% of month and not > 0.5 NTU for ≥ 15

Direct Filtration

3.5

1.0

1.0

Individual filter turbidity ≤ 0.15 NTU for 95% of month and not > 0.3 NTU for ≥ 15 consecutive minutes

Conventional Treatment

3.0

2.0

2.0

Individual filter turbidity ≤ 0.3 NTU for 95% of month and not > 0.5 NTU for ≥ 15 consecutive minutes Combined filtrate turbidity < 0.2 NTU for 95% of the month and not > 0.5 NTU for 15 consecutive minutes.

Conventional Treatment

4.0

2.0

2.0

Individual filter turbidity ≤ 0.15 NTU for 95% of month and not > 0.3 NTU for ≥ 15 consecutive minutes

1.0

Individual filter turbidity ≤ 0.1 NTU for 95% of month and not > 0.15 NTU for ≥ 15 consecutive minutes.

Membrane Integrity Test (MIT) to manufacturer’s specification for the required LRV and/or certified to a recognised standard.

2.5

Individual filter turbidity ≤ 0.1 NTU for 95% of month and not > 0.15 NTU for ≥ 15 consecutive minutes.

Membrane Integrity Test (MIT) to manufacturer’s specification for the required LRV and/or certified to a recognised standard.

MicroFiltration (MF)

UltraFiltration (UF) 14

Protozoa Bacteria

3.0

3.0

3.0

3.0

Chapter 2 - Membranes and Public Health As a critical pathogen barrier, membrane processes must be optimised and operated to ensure that the safety, as well as the quality of water is maintained as it passes through the treatment process and into the distribution system. Simple, practical target criteria and critical limits can be set for membrane filtration and appropriate monitoring undertaken to check that operations are within specification.

Operating Criteria and Performance Parameters In the case of membrane filtration, integrity of the membranes is the most critical requirement to ensure adequate removal of pathogens. Tables 2.2 and 2.3 provide some useful targets and critical limits for the management of both health related and aesthetic characteristics of the water in membrane filtration. The limits in the table are not meant to be prescriptive but rather provide an indication of the types of parameters that can be measured and controlled.

Each water Utility should consider their own requirements, however the targets in the table will assist most Utilities to manage their systems well and optimise the quality of water supplied to consumers. Once decided by the Utility, these target and critical limits should be documented, for example in the operations manuals for the treatment process. Treatment plant operators should have copies of these operating criteria readily available and should be trained, regularly assessed and certified as being competent to operate the process to meet the targets.

Table 2.2.

Some typical target criteria and critical limits for membrane processes.

Parameter

Target

Critical Limits

Turbidity (NTU)

<0.1

0.15

Particle count (2-5 µm) particles/mL

<10

30

Log Reduction Value (LRV)

4.5

4

The Normalised water measured permeability NWP should (NWP) be within 20% of the design (L/m2-h-bar) NWP.

Comment WSAA HBT manual recommends individual filter turbidity ≤ 0.1 NTU for 95% of month and not > 0.15 NTU for ≥ 15 consecutive minutes.

The count is generally averaged over a period of time (e.g. 30 seconds) to account for anomalies in the particle readings.

As per the manufacturer’s specification for the required LRV and certified to a recognised standard.

For example, a Pressure Decay Test (PDT) from a particular membrane supplier may have been validated to less than 5kPa/min pressure loss, being equivalent to 4 LRV for protozoa and bacteria.

NWP is determined under standard pressure and temperature conditions.

NWP = Total permeate flow/total membrane area TMP The NWP values should be compared to initial levels and trended over time.

15

Chapter 2 - Membranes and Public Health Table 2.3.

Some additional operational criteria for treatment processes, to protect membrane performance and integrity.

Parameter

Target

Critical Limits

Feed turbidity (NTU)

<30

200

Coagulant dose (mg/L)

<5

15

Temperature (oC)

12-25

40

Oils and Greases

Undetectable

Oxidant exposure (e.g. chlorine) Total Organic Carbon (mg/L)

pH

Variable (see manufacturer’s specifications) <10 average

40

6-9

>2 and <10

Comment High raw water turbidity over an extended period may result in excessive and possibly irreversible fouling.

Typically Aluminium Chlorohydrate (ACH) is most commonly used for coagulation prior to membrane treatment.

The targets provided are for ACH use with PVDF membranes.

High levels of inorganic coagulant can cause irreversible fouling and possible damage to the membranes.

Temperatures below 12oC generally limit the filtered water flow. Temperatures above 40oC may damage membranes and affect membrane integrity.

Oils and greases may cause irreversible fouling and potentially damage the membrane.

Sensitivity to oxidants such as chlorine and chloramine varies between different membrane materials.

Membranes generally do not tolerate high organics for extended periods of time. Such extended exposure may cause irreversible fouling.

Material dependant.

Example pH tolerance provided is for PVDF membranes.

PP membranes can tolerate a greater range of pH.

Regardless of the impact of treatment and distribution, treated water will retain some of the characteristics of the raw water. Typically, these include hardness and subtle differences in taste and odour that simply reflect the nature of the catchment and storage conditions. These differences are usually noticed when someone moves from one town or city to another. Such differences from supply to supply cannot easily be controlled, nor is it desirable unless the characteristics cause general consumer dissatisfaction. Such acceptable characteristics of the raw water are not considered in this practical guide.

On the other hand, if the source water contains high levels of undesirable taste and odour compounds, such as MIB and geosmin, or algal toxins, additional pre-treatment steps such as powdered activated carbon need to be considered.

16

Chapter 3 Introduction to Membranes Membranes and Water Treatment Membranes are often considered to be newcomers to water treatment particularly in countries such as Australia, however membranes have been used in industrial applications for decades.

The first synthetic membranes were developed in the late 1940’s and early 1950’s. The earliest of these was probably cellulose acetate. Indeed, the first trials on desalination of sea water were carried out in 1949 using cellulose acetate membranes.

As the plastics industry developed, many new and different polymers were used to manufacture different membranes with different properties. Ceramics were also used.

An Australian company Memcor was a major player in development of continuous microfiltration (CMF) technology in the late 1980’s with the first large scale WTP built at Meredith in Victoria in 1992 using this technology.

In 1997, the first reverse osmosis desalination plant was opened in Eilat in Israel. In 2012, the Ashkelon desalination plant was producing 15,000 to 16,000 kL of drinking water from sea water every hour, supplying 15 percent of Israel’s annual water supply.

There was also a parallel development of microfiltration and ultrafiltration membranes for use in activated sludge treatment processes in the wastewater industry. These plants are known as membrane bioreactors (MBR).

Basic Membrane Terminology The separation that occurs in membrane processes and some of the terminology associated with membrane use are portrayed in Figure 3.1. A liquid source (the feed) flows into a membrane system with the feed to the inside of the hollow fibre. This is called inside-out flow. If the feed is to the outside of the fibre, the flow path is called outside-in flow.

A percentage of the feed passes through the membrane as the filtered flow, called permeate or filtrate. The term filtrate is more commonly used for microfiltration and ultrafiltration processes, although some membrane suppliers will call it permeate. Permeate is a more common term in reverse osmosis and nanofiltration.

Depending on the type of membranes and flow mode used, various membrane systems will produce a side stream, which is called the retentate. Simply put, the retentate is that part of the feed that does not pass through the membrane, while the permeate is that part of the feed that does pass through the membrane.

In the earliest applications in science (e.g. concentration of protein samples) and industry (concentration of pharmaceuticals and milk processing) it is the retentate that is important and is the product that is kept (Figure 3.2).

17

Chapter 3 - Introduction to Membranes

FEED

RETENATE

PERMEATE Figure 3.1.

Basic membrane separation.

KEEP

REJECT

Figure 3.2.

Earliest use of membranes where the retentate was retained for use and the permeate rejected.

In water treatment, particles (silt, clay and pathogens) and chemicals, including salt, need to be removed from the water. Therefore, the membranes are used in the opposite way to that shown in Figure 3.2. The permeate is kept as the purified water while the retentate is rejected (Figure 3.3).

REJECT

KEEP

Figure 3.3.

Use of membranes in the water industry, the permeate is retained for use and the retentate is rejected.

In the water industry, the retentate is the waste stream from the membrane and is called the concentrate or reject.

Another concept related to membrane filtration is that of recovery. Recovery is the percentage of feed water that becomes filtered water, taking into account the waste streams (reject and/or backwash). That is, recovery is the percentage (%) of feed that ends up as product water.

18

Chapter 3 - Introduction to Membranes

Membrane Filtration Processes The water industry uses a combination of microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), and nanofiltration (NF), all of which are based on the same principle of using pressure to push water through a membrane. The smaller the contaminant removed the more pressure (and energy) is required to push the water through the membrane, so it is important to select the appropriate membrane for a given application.

Membrane processes are broadly classified according to the size of the particles and chemicals removed during filtration. Figure 3.4 diagrammatically shows the differences between microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).

Microfiltration <0.1 µm

Ultrafiltration 0.1 - 0.01 µm

Nanofiltration 0.01 - 0.001 µm

Low molecular weight compounds

Suspended particles

Viruses

Microbes

Proteins

Collids, turbidity

Large organics and oil emulsions

Figure 3.4.

Reverse Osmosis <0.001 µm

Ions

Types of membrane filtration.

The pore size of a membrane determines the classification as either microfiltration, ultrafiltration and to a certain extent nanofiltration or reverse osmosis membranes. However, because NF and RO membranes don’t really have discrete pores, reference to pore size in these membranes isn’t appropriate. Rather, the concept of molecular weight cut off (MWCO) is used. The molecular weight of a chemical is related to the size of a molecule. For example, effective removal of all pathogens requires a membrane with a MWCO of <10,000.

Table 3.1 shows the pore size, MWCO and operating pressures for the four different membrane processes.

Table 3.1

Typical pore sizes, MWCO and operating pressures for different membrane processes.

Process Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis

Pore size 0.1 µm 0.01 µm 0.001 µm Not applicable

MWCO Not applicable >1000 300-10,000 300

Operating Pressure 40-150 kPa 100-700 kPa 500-1,800 kPa 1,400-7,000 kPa

Note that the pore sizes and MWCO values are indicative only.

19

Chapter 3 - Introduction to Membranes

How Membranes Work Not all membranes remove contaminants from water in the same way. The four types of membranes discussed earlier in this chapter (MF, UF, NF & RO) are usually grouped into two categories based on how they work.

MF and UF membranes which are the main focus of this Practical Guide, have pores that pass from one side of the membrane to the other and are referred to as Permeable Membranes. These membranes act like a sieve or screen and the dominant removal mechanism is straining. If particles in the water are larger than the pore opening, they cannot pass through the membrane and are filtered out. This is illustrated in Figure 3.5.

This process is also referred to as surface filtration because it occurs mainly on the surface of the membrane, with few or no particles expected to enter the pores.

Particles

Membrane

Pores

Figure 3.5.

Diagram of a membrane showing particles that are larger than the pore size being removed by straining.

There are some other secondary mechanisms which further improve the performance of these membranes, the main ones being cake filtration and adsorption. When water is forced through membranes, the particles that are too big to fit through the pores build up on the surface and this material is called a filter cake. Cake material can itself act as a filter and remove some of the smaller particles that would otherwise pass through the pores (Figure 3.6).

In summary, MF and UF are size exclusion processes where the particles are removed by straining.

Microfiltration is effective for removing turbidity and other particles, as well as bacteria, protozoa and some algal species. UF can remove algae, colloids, viruses and some dissolved organics molecules. However, MF and UF processes do not remove dissolved salts, dissolved organics, or macromolecules that may be associated with colour or taste and odour compounds in the water. As a result of the physical removal that occurs in MF/UF membranes, particle breakthrough will not occur unless there is damage to the membranes.

This differs from granular media filtration where some particle breakthrough occurs at the start of a filter run (filter ripening) and at the end of a filter run.

RO and NF membranes don’t have distinct pores through the membrane and are referred to as Semipermeable Membranes. Some NF membranes are a cross-over between permeable and semi-permeable membranes. Semi-permeable membranes work by using more complex solubility and diffusion mechanisms.

A semipermeable membrane provides a barrier for particles (as MF and UF membranes do), as well as a larger range of chemical molecules, but allows water molecules to preferentially move through the membrane.

However, unlike MF and UF membranes, NF and RO are not designed for bulk removal of particles and some form of prefiltration is always recommended. RO and NF processes are discussed briefly in Chapter 8.

20

Chapter 3 - Introduction to Membranes

Membrane Fouling MF and UF membranes will strain out particles which are larger than the pore size on their filtering surfaces.

However, some particles that are smaller than the pore size are also trapped. This trapping occurs on the cake that has formed on the surface of the membrane and further acts to build up the cake on the surface (Figure 3.6).

Smaller particles retained

Particles

Cake of larger particles

Membrane

Pores

Pores

Figure 3.6.

Diagrammatic representation of blocking of pores by particles with a larger size than the pores themselves and the subsequent filtering of materials on the surface layer leading to the formation of filter cake.

At the same time, another mechanism is in operation. Some chemicals and in particular dissolved organic matter can penetrate the pores and adsorb to the walls of the pores thereby reducing the pore size (Figure 3.7).

Adsorption is caused by forces of attraction between surfaces, either particle-to-particle, or particle-tomembrane. Adsorption can result in very small particles entering and sticking to the surface of the pores. This improves the membranes removal efficiency for these smaller particles but they are more difficult to remove requiring more aggressive cleaning methods and are a cause of membrane fouling.

Membrane

Particles adsorb to pore walls

Figure 3.7.

Diagram showing adsorption of dissolved organic material onto the walls of the pores.

Over time, membrane fouling increases and membrane filtration capacity decreases.

Membrane Materials Membranes can be manufactured from a range of polymer, ceramic and sometimes sintered metal materials.

They can be of various thickness with either a homogeneous or heterogeneous structure.

Homogeneous membranes are ones which are made of a single material. They can have either uniform pore sizes (symmetric membranes) or a variation in pore sizes (asymmetric membranes). For example, an asymmetric membrane may have a small pore size filtering layer supported by a larger pore size draining layer that helps reduce the intrinsic resistance of the membrane (Figure 3.8). Heterogeneous membranes are ones that are composed of different materials, so called composite membranes.

21

Chapter 3 - Introduction to Membranes

Figure 3.8.

High power magnification of an asymmetric membrane showing the thin surface layer with small pores and the underlying draining and support layer with large pores.

MF membranes are typically homogeneous symmetrical membranes while UF membranes are usually asymmetrical membranes, that is, they have an open porous layer and a more dense “skin” which does much of the work of filtration.

The material of construction determines the chemical compatibility, suitability for different feedwater types and resistance to fouling and damage of the different membrane types.

The material that makes up a membrane will have an impact on the size and distribution of pores, which in turn will determine the size of particles that can be filtered. Figure 3.9 shows the microscopic membrane structure of three different membrane materials, PVDF microfiltration, polysulfone ultrafiltration and anodised alumina microfiltration. All three materials have very different pore structures and therefore will display different filtration characteristics relative to each other.

PVDF Microfiltration Figure 3.9.

Polysulfone Ultrafiltration

Anodized Alumina Microfiltration

Examples of pore structure for three different membrane materials. Images sourced from Veolia MF/UF training manual.

Hydrophobicity Membranes can be hydrophobic or hydrophilic. These terms refer to whether the membrane material attracts or repels water. Hydrophobic surfaces repel water. Think of rain falling on a newly washed and polished car bonnet. The water beads and often runs off.

Hydrophilic surfaces attract water. Again, think of a car bonnet which hasn’t been polished recently. The rain droplets tend to flatten on the bonnet, don’t bead and don’t run off as quickly.

Long term experience with membranes indicates that hydrophilic membrane materials tend to foul less, but show decreased chemical, thermal and mechanical stability. It is the hydrophobic compounds in water such as natural organic matter (NOM), colloids, clays, silicates and ferric hydroxides that foul membranes, as they are more attracted to the hydrophobic surface of the membrane than to water molecules.

22

Chapter 3 - Introduction to Membranes Oxidant Tolerance The main oxidant of concern in the water industry is chlorine. It can be used for chemical cleaning of membranes, but only if the membrane material has an oxidation tolerance. The oxidant tolerance of membrane materials is generally provided by membrane suppliers, quoted as hours of exposure to oxidants at parts per million concentration (ppm hours). Different materials show significant differences in stability toward oxidants. This is summarised along with other characteristics of different membrane materials in Table 3.2.

Table 3.2.

Typical characteristics of different membrane materials.

Material Polyvinylidene Fluoride (PVDF) Polypropylene (PP) Polyethersulfone (PES) Polysulfone (PS)

Type MF UF MF UF UF

Hydrophobicity Modified hydrophilic Slightly hydrophobic Very hydrophilic Modified hydrophilic

Tolerance to Oxidants

Fouling resistance

Very high

Excellent

Low

Acceptable

High

Very Good

Moderate

Good

PVDF and PP are commonly used in the water industry, particularly in drinking water applications. Care needs to be taken with PP membranes, to ensure they are not exposed to chlorine.

Flow Configurations In hollow fibre membranes which are the most common form of membrane for MF and UF applications in water treatment, the flow pattern can be from “inside to outside” or from “outside to inside” (Figure 3.11) and is specific to the manufacturer.

Inside-Out Flow

Outside-In Flow

Figure 3.11. Illustration of inside-out and outside-in flow configurations for membrane fibres.

When operated in an Inside-Out mode, the feed water enters the inside of the membrane fibre and passes through the fibre wall to produce filtrate on the outer side. For these membranes, tighter pre-screening is required as fouling on the inside of the hollow fibre can be more difficult to remove. Solids collected on the inside of the fibre can cause fibre blockages so more strictly controlled or more frequent backwashing, chemically enhanced backwashing and chemical clean-in-place (CIP) is often required. However, there is some evidence that the greater velocity inside on the feed side of the fibre provides for better flushing of the membrane during backwash.

When operated in the Outside-In mode, the feed water enters the housing (for pressurised membranes), passes through the fibres from the outer wall through to the inside of the fibre. In this configuration there is less risk of fibre plugging where a particle completely blocks a fibre so the UF modules may cope better with feed water containing more solids or other fouling materials. In addition, it allows the use of air scour for fouling control and needs a lower volume for backwash compared to Inside-Out fibres.

23

Chapter 3 - Introduction to Membranes Most secondary treatment wastewater applications use an outside-in flow configuration with membrane sheets or tubes hanging submerged in a tank of mixed liquor (Figure 3.12). A vacuum is applied to the inside of the membrane and water is “sucked” through the membrane material. These systems are often called membrane bioreactors. For tertiary stage recycled water treatment, membrane process configurations can either be submerged membranes or pressurised membranes. There are probably an equal number of each of these in the water industry.

Figure 3.12. Submerged outside-in membranes in a recycled water treatment plant.

Flows in MF and UF applications can also be described as being dead-end or cross-flow filtration (Figure 3.13).

Feed

Product

Feed

Product

Recirculate to feed

Product

Feed

Figure 3.13. Comparison of flow paths in hollow fibre membranes. Inside-Out Dead End (left), Inside-Out Cross Flow (centre) and Outside-In (right).

24

Chapter 3 - Introduction to Membranes In dead-end filtration, all of the feedwater passes through the membrane, therefore the feedwater flow is equivalent to the filtrate flow during filtration. There is no reject stream. All feed water is collected on the filtrate side, so there is 100% recovery of the water during filtration. Particles too large to pass through the membrane pores are either trapped on the membrane surface producing a “filter cake” or stuck inside the pore channels leading to a gradual increase in the transmembrane pressure (TMP) and a decline in permeability. At some point the system is taken off-line and the membranes are cleaned hydraulically by backwashing and/or chemically using chemical enhanced backwash or chemical CIP. Any flow losses in these systems are associated with the backwashing and chemical CIP to remove fouling.

Dead-end filtration is often selected because the net energy loss and water wastage is less than compared to cross-flow filtration. This is because all the energy is used to push water through the membrane. However, dead end filtration is only really suited to a low solids, high quality raw water feed.

Cross-flow filtration (Figure 3.13 centre) gets its name because the majority of the feed flow travels tangentially across the surface of the membrane and a percentage of the feedwater that passes across the membrane becomes a reject stream, whilst the remainder passes through the membrane to produce filtrate, or in the case of NF and RO membranes, permeate. The shear force generated on the membrane surface by the tangential flow (parallel to the membrane) potentially reduces the fouling rate as it continuously flushes the contaminants away from the fibre. The benefit of cross-flow filtration is the ability to operate the membranes at higher flows, increasing flow turbulence and reducing the amount of fouling deposited on the membrane surface (Figure 3.14).

Dead End Filtration

Filter cake Membrane Resistance due to cake formation

Cross Flow Filtration

Time

Filter cake Membrane

Figure 3.14. Comparison of cake formation and membrane resistance in dead end and cross flow filtration.

The disadvantage is that cross flow filtration systems tend to have a lower recovery compared to dead end filtration, that is, less filtrate production. Cross flow filtration is often used for high fouling water to avoid excessive build-up of contaminants on the membrane surface. A further disadvantage of the cross-flow mode of operation is higher energy consumption due to the high flows necessary to create enough flow velocity, and also the cost of the larger feed pump for flow recirculation to achieve the desired cross-flow velocity.

Cross flow filtration is the configuration seen in NF and RO applications. It is used to manage a phenomenon called concentration polarisation caused by increasing salt and colloid concentrations at the surface of the membrane, which can have severe impacts on membrane permeability, filtration capacity and contaminant rejection.

25

Chapter 3 - Introduction to Membranes

Types of Membrane Systems MF and UF membranes can be configured as hollow fibre, tubular or flat sheet. Figure 3.15 provides an example of the most common membrane types in use.

Figure 3.15. Flat sheet (left) and hollow fibre (right) membranes.

For MF and UF applications in drinking water treatment in Australia and New Zealand, hollow fibre membranes are most commonly used. The fibres are generally cylindrical with a hollow lumen. Hundreds to thousands of hollow fibres are bundled to form a module (Figure 3.16 and 3.17).

Figure 3.16. An opened module showing the individual hollow fibres.

Figure 3.17. The individual membrane fibres in a module are embedded in a “resin pot” which secures the end of the fibres and provides a water tight seal.

26

Chapter 3 - Introduction to Membranes

Figure 3.18. An individual membrane module (left), and a skid (right).

Flat sheet membranes such as plate and frame (Figure 3.19) and spiral wound (Figure 3.20) are a series of alternate layers of membrane sheets with feed and filtrate spacers in between.

Figure 3.19. An example of a flat sheet membrane bioreactor.

Used with permission from Kubota Corporation (www.kubota.com).

Figure 3.20. Diagram of a spiral wound membrane. (Wagner, 2001).

27

Chapter 3 - Introduction to Membranes Flat sheet membranes can be used in membrane bioreactors (MBR) treating sewage. Spiral wound membranes are most commonly seen in NF and RO applications, either brackish or seawater desalination or in recycled water treatment. Further details on NF and RO are covered in Chapter 8.

Tubular membranes can be constructed from polymers, sintered metal or ceramic materials (Figure 3.21).

The tubular membrane materials are essentially self-supporting, requiring less reinforcing to maintain their shape. The tubes can be quite large, big enough to roll a ball through in the biggest examples. They are not common in Australia, however they are popular in Scotland for the treatment of highly coloured water.

Figure 3.21. An example of a tubular membrane system. Used with permission from Koch Separations Solutions Inc. (www.kochseparation.com).

Depending on the type of driving force, MF/UF modules can be categorised as either pressurised or vacuum driven. In the pressurised form, the membrane modules are placed inside pressure vessels, or “housings” (with vertical or horizontal orientation), and the modules are grouped to form skids (Figure 3.22). Typical operating pressure of pressurised UF systems can be up to 2500 kPa, but are typically of the order of 100700 kPa in drinking water applications.

Figure 3.22. Two pressurised membrane system skids.

28

Chapter 3 - Introduction to Membranes Vacuum driven modules are typically submerged in a feed tank with no membrane housing (Figures 3.23 and 3.24), and can only operate at low suction pressures, typically below 80 kPa (vacuum pressure). Due to this limitation in the maximum pressure that can be applied, they typically operate at a lower flux than pressurised systems and are more sensitive to water temperature fluctuations. Also, access for module installation, cleaning, fibre repair, isolation or maintenance is more laborious in submerged systems than in pressurised systems. However, submerged membrane systems do have the benefit of being well suited to large volume treatment processes, as well as having slightly lower energy requirements relative to pressurised systems.

Figure 3.23. Example of a small submerged membrane system being opened for inspection (left) and the exposed membranes (right).

Figure 3.24. Example of a larger submerged membrane system.

29

Chapter 3 - Introduction to Membranes

Membrane Pre-treatment Many membrane applications will require some form of pre-treatment unless the source water is consistently low in suspended solids, organics, iron and manganese, algae and scaling compounds such as calcium and silica.

Pre-treatment prior to membranes is necessary for two main reasons.

1. To protect the membranes.

While membrane strength has improved with the use of different polymers, the membranes can be damaged. Pre-treatment is therefore necessary to protect the membranes from physical damage and also to prevent excessive fouling. Membranes can be damaged by grit or fragments of shells, as well as silica particles and even diatoms.

Pores may be blocked by high solids loads. This may be inorganic fouling by dissolved metals and mineral salts, in particular iron and manganese, or organic fouling by dissolved organic carbon (mainly the hydrophobic fraction). Dosed polymer can also cause premature and/or irreversible fouling.

Such damage, blockages or contamination of the membranes can lead to reduced performance and a shortened life.

Excessive fouling can substantially increase the weight of the membrane. Individual fibres can sag and stretch, or even break under the weight of the fouling layer.

2. To improve membrane performance for poor quality raw water.

Addition of coagulants, typically aluminium chlorohydrate (ACH), can improve removal of turbidity and some other particulates, as well as remove colour.

Other pre-treatments include: 1. pH adjustment to ensure the feed water is in the range suitable for the particular type of membrane.

2. Oxidation and removal of iron and manganese typically using potassium permanganate or Chlorine (but only if the membrane material is stable to chlorine).

3. Powdered activated carbon (PAC) to improve the removal of organics. Typically PAC would be removed in a presettling stage prior to membrane filtration to reduce the risk of excessive fouling and damage to the membrane from PAC particles.

Pre-treatment is covered in more detail in Chapter 4.

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Chapter 3 - Introduction to Membranes

Membrane Resistance Membrane Resistance is the tendency of a membrane to resist flow across the membrane. The two main contributors to resistance are the intrinsic resistance of the membrane itself, as well as the resistance produced by the formation of a fouling layer (Figure 3.6). While the resistance of the membrane material remains essentially constant, the resistance due to the fouling layer varies considerably. The Total Membrane Resistance (RT) is the sum of these two separate resistances.

RT = RI + RF Where RT =Total Resistance RI = Intrinsic Membrane Resistance RF= Resistance due to Fouling Resistance due to fouling is also made up of separate parts.

R F = RC + RA + RP Where RC = Resistance due to cake formation on the surface of the membrane. This is mostly reversible since the cake is generally removed during backwashing.

RA = Resistance due to adsorption of particles to the membrane material lining the pores (Figure 3.7) is generally not removed or reduced during backwashing. Rather, it requires chemical cleaning either as Chemical Enhanced Backwash (CEB) or more importantly, Clean-in-Place (CIP). Cleaning of membranes is covered in Chapter 5.

RP = Resistance due to permanent fouling. Despite backwashing and chemical cleans, some material remains on the membrane surface or lining the pores. RP is therefore considered as being irreversible.

While this is theoretical, it is important to relate it to the mechanisms of fouling as explained above.

Flux Membrane flux is the flowrate of water per unit surface area of membrane. The typical units are litres per hour, per square metre (L/hr/m2). Note that this is equivalent to the filtration rate in media filters which is expressed as m3/m2/hr. This relationship can be summarised in the following equation.

Flux (J) = Flow Rate (Q) / Surface Area (SA) An example calculation is shown in Box 3.1.

Box 3.1. Calculation of flux A membrane filtration process is producing filtrate at 1000 m3/hr and the combined modules have a total membrane surface area of 2000 m2. What is the flux?

100,000 m3/hr = 100,000 L/hr So, Flux = 100,000 L/hr / 2,000 m2 = 50 L/hr/m2

31

Chapter 3 - Introduction to Membranes Flux is influenced by a number of parameters.

1. Pressure.

Flux increases with increasing pressure used to force the fluid across the membrane, the so called Transmembrane Pressure (TMP). This is only true within the limits defined by the manufacturer. At higher pressures the membrane can become compacted and the flux become reduced resulting in permanent damage to the membrane.

2. Total Membrane Resistance.

Flux decreases with increasing total resistance.

3. Viscosity/Temperature.

Flux decreases with increasing viscosity.

The relationship between these parameters is summarised below.

• As TMP increases, the flux increases.

• As viscosity increases, the flux decreases.

• As the total membrane resistance increases, the flux decreases.

Normalised Flux (Correction for Viscosity) Even though in the water industry we are only dealing with water, the viscosity of water can change significantly.

Table 3.4 provides the ratio of water viscosity at a given temperature to the viscosity of water at 20oC. Table 3.4.

Ratio of water viscosity at a given temperature to the viscosity at 20oC. Temperature (oC)

Viscosity Ratio

Correction Factor

5

1.52

0.66

10

1.31

0.76

15

1.14

0.88

20

1.00

1.00

25

0.890

1.12

30

0.798

1.25

This means that at 25oC the viscosity is only 89% of the viscosity at 20oC, i.e. it is less viscous at 25oC. And importantly, at lower temperatures the viscosity increases significantly.

To compare membrane flows and fluxes, they need to be standardised to 20oC. This is called Normalised Flux. For MF and UF membranes the reference temperature is 20oC (Boxes 3.2 and 3.3).

Box 3.2. Correction of flows for temperature A membrane process operating on a surface water supply has a design flow rate at 20oC of 500 m3/day. Average summer and winter water temperatures recorded at the plant are 25oC and 10oC respectively. What is the expected flowrate during summer and winter?

Summer flow = Flow at 20oC x correction factor for 25oC = 500 kL/day x 1.12 = 560 kL/day Winter flow = Flow at 20oC x correction factor for 10oC = 500 kL/day x 0.76 = 380 kL/day

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Chapter 3 - Introduction to Membranes

Box 3.3. Normalisation of flux A membrane plant operating with a water temperature of 25oC has a measured flux of 90 L/m2.hr.

What is the normalised flux?

From Table 3.4, the viscosity ratio at 25oC is 0.89.

Normalised flux for the membranes = 90 x 0.89 = 80.1 L/m2.hr Membrane flux is an important parameter in membrane design and in operations management. A major determining factor for the capacity and configuration of a membrane process is often the desired membrane flux.

Transmembrane Pressure and Membrane Permeability The Transmembrane Pressure (TMP) is the pressure difference (driving pressure) across the membrane.

TMP = Feed Pressure – Filtrate Pressure (often expressed in kilopascals, kPa).

Pressurised membrane systems will have a positive TMP across the membrane, as the driving force is a feed pump pushing the water through the membrane. Submerged membrane systems have downstream vacuum pumps, therefore the TMP is generally represented as a negative number.

Membrane Permeability is the tendency of a membrane to allow water to pass through. The permeability is affected by the membrane material, porous characteristics, presence of fouling and the water temperature.

Mathematically, Permeability is Flux divided by TMP and corrected for viscosity. It should be noted that permeability and resistance are effectively the inverse of each other.

TMP and flux are intimately connected. Figure 3.24 shows how the rate of fouling, and hence the rate of change of TMP, is significantly affected by the membrane flux. The higher the flux across the membrane, the more particles the membranes comes into contact with, which results in increased fouling and hence higher TMP.

Figure 3.25. The trans-membrane pressure recovery and gradual TMP increase over a number of backwash cycles.

Diagnostically, at a given temperature, increasing TMP indicates fouling and a need for backwash or chemical cleaning. A decrease in TMP indicates a loss of membrane integrity and requires inspection and possible pinning of fibres or fixing of leaks. The rate of fouling, and therefore the rate of change of TMP, is significantly affected by the membrane flux. The more the membrane fouls, the faster the increase in TMP. 33

Chapter 4 Optimisation of Membrane Filtration Systems To ensure continuous production of safe drinking water, it is important to actively manage and monitor membrane filters. Often membrane processes are operated the way they always have been, without any real knowledge of how the existing operational parameters were set. Quite often this information is simply passed down from operator to operator So, how should membrane filters be run?

Get to know your membranes intimately and be alert to changes. The following sections should assist you in “getting to know” your membranes. They are numbered to provide a logical sequence to the process.

With an appropriate awareness of what can be done, there is considerable scope to improve the operation of membrane processes throughout Australia.

1. Know Your Process Prior to commencing a program to optimise the operation of membrane filters, it is important to collect all the design information and specifications for the process: • Find all the control parameters and how they might have changed since commissioning.

• Whilst the actual process configuration doesn’t generally change, the control parameters often do.

In particular, parameters such as backwash sequences and times, chemical recirculation and soak times for Clean-in-Places (CIP) may have changed.

• Find out what the design filtration, air scour and backwash rates are.

• Find out the materials of construction, operational constraints (e.g. pH, temperature and pressure limitations), chemical compatibilities and typical trans-membrane pressure (TMP) drop for new membranes.

• Locate the as-built drawings of the plant. Determine the configuration of air scour, backwash water and chemical CIP pipework. It is good practice to follow all the pipework from the drawings, identify all the valves and instrumentation and ensure they are all clearly labelled and that the labels agree with those on the SCADA system. Without a clearly labelled plant, it is difficult to provide training for new operators, prepare Standard Operating Procedures and can lead to inappropriate actions being taken during an incident or emergency. All too often, new plants are not completely labelled.

All the design parameters should be provided in the Operational and Maintenance manuals. If not, the information should be obtained from the manufacturers; however this can be difficult at times.

2. Know Your Plant Does the operating plant sound right? Membrane plants include supporting equipment like pumps, valves, and pressure and flow transmitters. The electro-mechanical nature of pumps and valves directly relate to the flows and pressures within the system and associated cycles. The backwash cycle on a membrane plant utilises the majority of these automated valves. With backwash cycles that are typically performed every 20-60 minutes, the same noises should occur. If something doesn’t sound right with the plant, then there will almost certainly be an issue with the supporting equipment.

Check the operation of the plant against what is described in the manual. In particular, check the operation of all the automated air and backwash water valves before, during and after a membrane air scour and backwash to ensure their full and complete operation. This will usually require two operators. While one operator is watching the steps on the SCADA and calling them out, the other operator can check the changing status of the valves and pumps in the plant.

Knowing how to do such a complete check can also be very useful for troubleshooting any backwash problems that may arise as if they are not corrected, excessive membrane fouling can occur.

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Chapter 4 - Optimisation of Membrane Filtration Systems Automated cleaning cycles are often incorporated in membrane plant design. Whilst this is good in theory, it should be determined whether or not the clean is required and whether it is done using the correct chemicals and the correct concentration of chemicals. Best practice is to incorporate a semi-automated cleaning cycle, where the operator initiates the clean and manually checks and confirms the solution type and strength. More importantly, the membrane performance data should be thoroughly checked and compared before and after each clean cycle.

In some situations monitoring and process changes are supposedly done by the membrane company remote from the site, with the operators knowing little about the more detailed aspects of running the plant. They may not be aware that changes have been made. This is not a good way to run membranes!

3. Feedwater Quality and Pre-treatment Routine monitoring of source water is critical to identify problems before they affect membrane performance.

Generally, membrane systems can tolerate excursions in feed water quality for short periods of time, as shown by the maximum recommended values in Table 2.3. If the feed water quality is consistently outside the design feedwater quality, membrane suppliers will recommend a process investigation and optimisation exercise to improve pre-treatment. In some cases a significant change in source water characteristics may require pilot studies and pre-treatment upgrades to ensure sustainable membrane performance.

Pre-treatment is necessary to protect membranes, enhance treatment or make treatment possible in the case of poor raw water quality. Therefore, it is essential to make sure pre-treatment is actually working as intended and routinely optimised to ensure that under/over-dosing is not occurring. It is not a set and forget process.

There are two main types of pre-treatment, pre-filtration and feedwater coagulation.

Prefiltration Pre-filters are most commonly used to protect membranes and are designed to remove larger (>100 µm) particles as well as abrasive particles. The actual size of particles that need to be removed depends on the type of membrane system.

Hollow fibre, inside out filtration is more susceptible to plugging than outside in filtration and therefore finer pre-filters are used for inside out filtration.

A range of different pre filtration systems are shown in Figures 4.1 to 4.4.

Figure 4.1.

Coarse particle filters upstream of a membrane plant.

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Chapter 4 - Optimisation of Membrane Filtration Systems

Figure 4.2.

Fibre filters and media filters prior to an RO plant (left) and spent fibre cartridges (right).

Figure 4.3.

A drum screen for a large MF system.

UF RO

Figure 4.4.

36

UF membrane prefiltration prior to an RO plant.

Chapter 4 - Optimisation of Membrane Filtration Systems Feedwater Coagulation Addition of coagulants, typically aluminium chlorohydrate (ACH), can improve removal of colour, turbidity and some other particulates. The ACH may be used prior to the membranes themselves or the system may have a full flocculation and clarification system prior to the membranes (Figure 4.5).

Figure 4.5.

Flocculation tank and clarifier upstream of a membrane plant.

Feedwater coagulation is a relatively common practice for membrane treatment. The formation of floc assists in the removal of higher levels of turbidity as well as some dissolved organic and inorganic compounds that may otherwise pass through the membranes. Ideally, flocculation should be optimised to form a “pin” floc.

These have a particle size larger than the nominal pore size of the membrane and are only just visible to the observer (around 50 micron, or 0.05 mm). Larger floc tends to be too “sticky” and causes fouling that is difficult to remove through backwashing.

Selection of the most appropriate coagulants and flocculants are important in membrane operations.

Inorganic coagulants such as aluminium sulphate, ferric sulphate and ferric chloride are not as common in membrane applications as for conventional media filtration. These coagulants increase the risk of inorganic fouling of the membranes.

Alternatives such as aluminium chlorohydrate (ACH) and polyaluminium chloride (PACl) generally produce a good floc suitable for membrane filtration. Typically, only low doses are required, resulting in reduced solids loading compared to other coagulants. Membrane suppliers will provide recommendations on the best coagulants for their products, as well as the design and maximum dose rates. Table 2.3 in Chapter 2 provides some general guidance on ACH dose rates.

While jar testing is used widely in setting and adjusting coagulant doses in conventional water treatment, a different approach is used for membrane plants. As pin floc is all that is required for a membrane plant, dose rates are simply adjusted to achieve a true colour target < 5 PtCo units. As MF/UF membranes have pores <0.2 µm, operation of the plant simulates laboratory testing of true colour.

Where duty and standby pumps are installed, drop down tests should be performed on both pumps to ensure they both deliver the same dose. Also check that when one pump fails, the standby pump is actually called into service and functions.

As pH can be critical for coagulation, it is essential that there is an online pH meter monitoring the dosed water pH and that the meter is calibrated regularly. Weekly calibration is recommended. It is also important to visually inspect mixing tanks for correct operation of the mixers and also the flocculation tanks for correct floc size.

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Chapter 4 - Optimisation of Membrane Filtration Systems Be alert for changes in the raw water. In particular, increases in turbidity after heavy rain but also increasing salt concentrations and organics concentration during periods of high evaporation and extended drought.

Also, be on the lookout also for increases in the number of algae in the source water and possibly changes in the types of algae that may have a direct effect on the membranes. Some diatoms have a silica based outer “shell” that can damage and foul membranes.

Use of Polymers From time to time when the feedwater contains high levels of total organic carbon (TOC) and or colour, it may be tempting to use organic polymer coagulation and flocculation aids to enhance primary coagulation upstream of membranes. Membrane suppliers will generally advise against the use of polymer flocculants due to the high risk of fouling. If it becomes absolutely necessary to use polymer flocculants, the following general rules apply.

• Consult with the membrane supplier for their advice and recommendations for polymer type and maximum levels.

• Use of any polymer flocculants as pre-treatment in membrane plants without a clarification step is inadvisable.

• Keep the polymer dose as low as possible, and ensure there is a high ratio of inorganic coagulant to polymer being dosed.

• Use of a polymer with a charge that is opposite to that of the membrane poses a higher risk of irreversible fouling.

• Manage the pH of CIPs to obtain the optimum cleaning efficiency and recovery of membranes. This can be judged by the reduction in the TMP after cleaning.

Where pH adjustment of the raw water is required either to optimise coagulation or to adjust the pH of the feedwater so that it is in the range suitable for a particular membrane type, consideration must be given to the levels of calcium in the raw water. For example, addition of lime may increase calcium levels above the maximum recommended. In this situation another alkali such as soda ash or caustic should be used to adjust the pH. Other pre-treatments include: • Oxidation and removal of iron and manganese typically using potassium permanganate or prechlorination (but only if the membrane material is resistant to oxidants).

• Powdered activated carbon (PAC) for improved organics removal.

Prior to using any chemical in pre-treatment it is important to ensure that the chemical(s) is compatible with the membrane material. If in doubt, consult the membrane supplier.

Appendix 2 provides a case study showing the importance of pre-treatment.

4. Membrane Integrity Online turbidity monitoring and Pressure Decay Testing (PDT) or Membrane Integrity Testing (MIT) are the two fundamental methods of monitoring membrane integrity.

Trends of daily PDTs and calculated Log Removal Values (LRVs), and turbidity, are effective ways to determine if there is a problem with any gradual deterioration of membrane integrity or acute damage to the membranes.

Turbidity trends should be analysed statistically to ensure that the target set in the WSAA Manual for the Application of Health-Based Treatment Targets For Drinking Water Safety of “Individual filter turbidity ≤ 0.1 NTU for 95% of month and not > 0.15 NTU for ≥ 15 consecutive minutes” is achieved.

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Chapter 4 - Optimisation of Membrane Filtration Systems PDT derived LRV values should be assessed against the manufacturer’s specification for the required LRV and/or a recognised standard.

Gradual increases in the PDT and associated decreases in LRV suggest gradual deterioration in the condition of the membrane. In contrast, filtrate turbidity spikes, coupled with poor PDTs suggest actual membrane damage.

The plant should be optimised and operated to achieve the turbidity and LRV targets at all times.

5. Membrane Fouling The simplest way to monitor the rate of fouling of membranes is to measure and trend the Transmembrane Pressure (TMP). TMP measurements can be used as an upper-limiting trigger value for backwashing membranes, as well as to calculate membrane resistance.

In pressurised hollow fibre systems, the TMP will be a positive number, whereas the TMP will be a negative number in submerged membrane systems where a vacuum is applied as the driving force for filtration.

TMP is usually monitored continuously and displayed as a trend in the plant SCADA. If it is not provided, the manufacturer should be approached to modify the SCADA system to include the trend.

An example of the build-up of TMP over a period of a month of operation is shown in Figure 4.6. The impact of backwashes and CIP on TMP is shown in Figure 4.7.

Figure 4.6.

Example of a TMP measurement for pressurised hollow fibre membranes.

Figure 4.7.

Schematic trend showing gradual increases in Total Membrane Resistance (which is proportional to TMP) and the effect of backwashing and CIP. Over time there is a gradual increase in the permanent fouling.

39

Chapter 4 - Optimisation of Membrane Filtration Systems Any sustained increase in TMP or excessive TMP indicates fouling and a need for backwash or chemical cleaning (Figure 4.7). A decrease in TMP indicates a loss of membrane integrity and requires inspection and pinning of fibres (covered in more detail in Chapter 6) or less commonly, a sudden increase in feed water temperature.

6. Flux Flux (L/m2/hr) changes during a filter run. The only way to increase the flux is to increase the driving force, i.e. the pressure in a pressurised system, or the vacuum pressure in a vacuum system.

For a given membrane plant, there is usually a flux range specified to: • Minimise the rate of fouling.

• Optimise energy efficiency by operating at as low TMP as possible.

• Reduce the frequency of backwashing and chemical CIP. Therefore, flux should be monitored routinely and compared with design values. This can either be done manually or can be incorporated into the PLC. For the flux to be incorporated in the PLC, the feed water temperature must be monitored continuously during operation. This is because flux at a given TMP is strongly affected by the feed water temperature due to the changes in the viscosity of the water (see Chapter 3). It is the reason the flow rate may need to be decreased at lower temperatures.

If the PLC does not display the flux directly, plant operators should regularly calculate and trend the flux as described in Chapter 3. Manual measurements of temperature may be necessary to allow this.

7. Backwashing and Chemical Cleaning Effective backwashing and chemical cleaning are an essential part of ensuring ongoing operation of membranes. These are covered in detail in Chapter 5.

8. Inspections of Membrane Elements Without adequate maintenance and checking, membranes can deteriorate.

Trends of daily PDTs and/or calculated LRVs are an effective way to determine if there is a problem with gradual deterioration of membrane integrity. In contrast, filtrate turbidity spikes, coupled with poor PDTs suggest actual membrane damage.

A leak test is used to identify individual membrane elements that have damaged membrane fibres or resin pot. Leak tests are conducted as part of maintenance and involve either visual or auditory detection of leaks as bubbles pass through holes and tears in the fibres. This is in contrast to a PDT, which is an automated process that occurs periodically during normal operations. Both tests assess the integrity of the membrane and identify if maintenance is needed.

The leak test involves pressurising the inside of the element fibres, so that only a broken or damaged fibre will allow the air to escape. Once the fibres are pressurised, identification of leaking membranes is done either by visual inspection for submerged membranes (Figure 4.8), or by bubble tube or sonic testing for pressurised membranes (Figure 4.9).

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Chapter 4 - Optimisation of Membrane Filtration Systems

Figure 4.8.

An example of a small bubble leak during pressure decay testing of submerged membranes.

Figure 4.9.

Sonic testing of a pressurised membrane module.

Membrane element testing and repair, including pinning of fibres, should be conducted as part of normal plant maintenance activities. This is covered in detail in Chapter 6.

Membranes should not be left out of water or with air applied for more than a couple of hours as the fibre is likely to “dry out” and in most cases will not return to a permeable state.

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Chapter 4 - Optimisation of Membrane Filtration Systems

9. Calibrate Gauges, Meters and Associated Instrumentation Pressure gauges, flow meters and turbidity meters are all vitally important for the operation of membrane filters. The information provided by these instruments must be accurate and reliable. High quality instruments should be installed and calibrated regularly according to the manufacturer’s instructions.

Pressure gauges are required to accurately monitor the TMP. As these are the main method of determining the rate of fouling and effectiveness of recovery after backwashing and chemical cleans, they need to be calibrated to accurately pick up a change in TMP of a few kPa. For example, depending on the feedwater type, the backwash cycle for a membrane process may be set up to ensure that the change in TMP be controlled to within as little as 5-10 kPa. In order to do this, pressure gauges must be accurate to around +/- 1 kPa. Turbidity meters are a critical component of membrane integrity monitoring. Operational best practice requires turbidity meters to reliably measure turbidity values well below 0.1 NTU. Therefore turbidity meters should be installed to minimise the risk of false readings as a result of entrained air bubbles. Manufacturers will offer debubbler assemblies specifically designed for their turbidity analyser products and it is recommended that these be installed in accordance with the manufacturer specifications.

Routine inspections of turbidity analysers should include checking, flushing and replacement of sample lines to minimise the risk of false readings as a result of solids build-up and biofilm growth in the sample lines which periodically become resuspended or slough off.

Regular checks of the flow rate through the meters should also be carried out to ensure the flow is within specification. Zero or inconsistent flows compromise the quality of the data. Introduction of an air gap in the discharge line makes the assessment of flow through the meter easy to observe (Figure 4.10).

Figure 4.10. An analyser wet rack showing multiple turbidity meters where the flow through the meters can be easily seen (yellow arrow). There is no flow through one of the meters (red arrow).

The analysers should also be installed to prevent overheating of the instruments. Shaded or air conditioned cabinets may be necessary.

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Chapter 4 - Optimisation of Membrane Filtration Systems

10. Reduce the Risks Associated with Washwater Recycling Supernatant from settling basins or thickeners, underflow from drying beds and centrate from centrifuges are commonly recycled back into the treatment plant process. The recycling of waste streams increases the risk that pathogens or other contaminants removed by filtration will be concentrated in the recycle return. For example, there are cases where both the number of pathogens and also the concentration of taste and odour and algal toxin compounds have been demonstrated to increase as a result of backwash water recycling Depending on the membrane type, and the integrity of the membrane, these compounds can pass through membranes and potentially enter the drinking water supply. While recycling is a good idea from a watersaving and sustainability perspective, the risk from recycled water contamination should also be considered.

There are several options to reduce the risk of recycling contaminants from the supernatant return.

• Dispose of the supernatant to waste. Supernatant may be disposed of to the sewer, or to reuse schemes if suitable reuse options exist. During drought conditions, there is pressure to recycle water within a WTP. Wasting of supernatant to sewer is not necessarily “wasting” the water. Where reuse schemes are in place for treated effluent from the Sewage Treatment Plant (STP), the flow from the WTP is simply reused along with water from that STP. This system of management is preferable to returning the supernatant to the head of the plant with the associated risk of increasing the load of contaminants to be managed.

• Optimise the wastewater handling system to produce a supernatant with the best water quality characteristics.

• Return supernatant water continuously in proportion to the plant flow. Supernatant should be returned to the head of the plant and upstream of any coagulant dosing at no more than 10% and preferably less than 5% of the plant inflow.

• Fully treat the supernatant before returning it to the head of the works. This typically involves the addition of coagulants and possibly media filtration to remove any contamination that may be present.

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Chapter 5 Membrane Cleaning There are a number of different methods used to clean membranes. These may occur singly or in various combinations.

• • • •

Backwash (also referred to as Flux Maintenance or Reverse Filtration).

Air scour/agitation (also referred to as an Air Scrub).

Chemically enhanced backwash (CEB) (also referred to as a Maintenance Wash or Clean).

Chemical Clean-in-Place (CIP) (also referred to as Enhanced Flux Maintenance or Recovery Cleans).

Backwashing of Hollow Fibre Membranes Backwashing is a periodic reversal of flow to a membrane to remove particles from the membrane surface.

Hollow fibre membranes, whether of inside-out or outside-in configuration or immersed fibres, can be backwashed. A backwash typically occurs automatically every 20 to 60 minutes and lasts from 2 to 3 minutes.

It is most often triggered by time, but can also be triggered by a performance measure such as filtration time, volumetric throughput, transmembrane pressure (TMP) (which is equivalent to headloss triggering a backwash in a conventional WTP with media filters), and/or a decline in flux.

A backwash may also incorporate an air scour to assist in removal of solids, as well as a cross-flow flush to transport the dislodged solids out of the membrane housing (Figure 5.1).

Feedwater

Water flush Air

Water

Water

Water

Air

Air

Permeate

Water

Fitration Figure 5.1.

Backwash

Air

Air Scour

Diagrammatic representation of normal filtration mode and common backwash modes.

Backwashing intervals are normally set according to the manufacturer’s recommendations, however sitespecific considerations such as the feed water quality may necessitate different settings. For example, a poorer quality feed water may require more frequent backwashes. The frequency of backwashing is a balance of optimising membrane fluxes by removal of fouling while minimising downtime and washwater volume created by backwashing.

A backwash of hollow fibre membranes generally has the following process steps.

1. Partial drain down of feedwater from the membrane cell or housing.

2. Air scour to dislodge and break-up solids accumulated on the surface.

3. Flow reversal and filtrate flushing of the membrane to push the dislodged solids away.

4. Refill of the membrane cell or housing to further flush, then displace the air in the system with water and return to filtration mode.

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Chapter 5 - Membrane Cleaning In an ideal situation, the backwash restores the TMP to a baseline level, equivalent to a clean membrane.

However most membranes exhibit a gradual increase in the TMP with successive backwash cycles (Figure 5.1). The gradual increase is due to the accumulation of fouling materials that cannot be removed by the backwash process alone. Some, but not all, of these foulants can be removed by using a Chemically Enhanced Backwash (CEB) or Chemical Clean-in-Place (CIP). Chemical cleans are considered in detail below.

Figure 5.2.

Schematic trend showing gradual increases in total membrane resistance (which is proportional to TMP) and the effect of backwashing and CIP. Over time there is a gradual increase in the permanent fouling.

When troubleshooting a backwash, the following questions should be considered.

• • • • •

Does the backwash cycle sound normal?

What is the TMP recovery after the backwash?

What is the membrane resistance and recovery after the backwash?

What is the membrane permeability before and after the backwash?

Is there visual evidence of solids build-up in a membrane bundle during membrane pinning (see Chapter 6).

If problems are identified during a backwash troubleshooting investigation, the following corrective steps can be taken.

• Review and adjust the backwash frequency to attempt to achieve the baseline TMP, membrane resistance and permeability.

• Closely examine all available trends to identify system problems, in particular faults with valve operation, possible leaks, or problems with instrumentation.

For example, Figure 5.3 shows pressures and flows across the membrane system during each step of the backwash cycle for a plant taken two months apart. The trends indicate there is no air pressure to the system during the backwash cycle (top red rectangle in Figure 5.3). Inspection out in the plant may identify an air valve that fails to open upon initiation of the air scour. Once rectified, the trends can again be reviewed to confirm that the backwash is proceeding correctly and that the identified problem valve is operating correctly and there is air pressure during the backwash cycle (bottom red rectangle in Figure 5.3).

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Chapter 5 - Membrane Cleaning

Figure 5.3.

Two backwash trends taken two months apart showing that the membrane has not pressurised, indicating failure of the process air (top). The bottom image shows the backwash after process air has been reinstated.

Backwashing of Immersed Membranes Flat sheet immersed membranes cannot be backwashed in the same way as hollow fibre membranes mainly because the higher back pressure would damage the membrane sheet. These systems are cleaned by introducing air below the level of the membranes and allowing the rapidly rising bubbles to agitate the membranes and release any accumulated cake from their surface. This is followed by draining or flushing the contents of the backwash liquor to waste. In some cases with immersed membrane systems, the air scour/ agitation process operates continuously.

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Chapter 5 - Membrane Cleaning

Chemical Cleaning Chemically Enhanced Backwash (CEB) and Chemical Clean-in-Place (CIP) are often both used routinely to remove the build-up of fouling from membranes that is not removed by backwashing.

1. Chemically Enhanced Backwash (CEB) A Chemically Enhanced Backwash (CEB) can also be referred to as a maintenance clean. It is essentially a backwash with the addition of cleaning chemicals to remove some organics and scale, as well as solids from the membrane surface. Removal of fouling materials enhances membrane flux and extends the interval between CIPs. This increases the efficiency of operation as CIP requires a membrane unit to be taken offline for an extended period during which time production drops to zero for the elements offline.

As with backwashing, the goal of a CEB is to quickly restore the TMP of the system to its baseline clean level.

A chemical clean is generally required when backwashing fails to adequately restore system productivity (i.e. increased flux and reduced TMP) and reaches a point of diminishing returns. That is, ever more frequent backwashes for a small improvement each time.

Although some membrane systems, such as those with an inside to out flow configuration, can use CEBs routinely to maintain membrane performance.

A CEB for hollow fibre membranes generally has the following process steps.

• • • • • • • • •

Conduct a standard backwash.

Partial drain-down.

Fill with cleaning solution.

Recirculation and / or soak.

Repeat the backwash to flush out the cleaning chemical.

Drain-down.

Refill with feed water.

Feed flush to further flush out the cleaning chemical.

Return to filtration mode.

Sodium hypochlorite is typically added if the membrane material is compatible with chlorine. Addition of chlorine enhances the removal of solids, possibly enhances pathogen inactivation, controls biofouling and improves the overall effectiveness of the backwash. A chlorine CEB may be added at a frequency ranging from every backwash to once each day.

Some manufacturers recommend CEBs that involve the use of chemicals other than chlorine. These include acids, bases, surfactants, or other proprietary chemicals.

The effectiveness of a CEB can be determined by reviewing the standard process parameters of flows, flux, turbidity, TMP and permeability. If a CEB does not result in improvements in these parameters, alternative chemicals or modified cycle times can be trialled to bring about improvement, or otherwise a CIP implemented.

2. Chemical Clean-in-Place (CIP) A CIP is a more thorough clean involving backwashing, recirculating a chemical solution and soaking the membranes in the chemical solution for a period of time. Depending on the combination of chemicals used, and the recirculation and soak settings, a CIP may take anywhere between 4 and 24 hours.

CIP can be triggered automatically by hours of filtration, volume filtered, or according to a specified increase in TMP. They can also be triggered by the operator.

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Chapter 5 - Membrane Cleaning Membrane CIPs involve a series of chemical recirculation and soak stages. Normally at the beginning of a CIP, the “clean” (filtrate) side of the membrane is held in contact with the CIP chemical first. Some membrane systems make it possible to adjust the timers so that either the feed or filtrate sides are in contact with chemical for longer periods. This helps with targeting specific kinds of fouling, in particular surface fouling versus fouling that has penetrated the fibre.

Badly fouled membranes may need a longer time in the soak step. It may be possible to halt the CIP in the membrane soak step. This allows for a long soak of the membranes either overnight or for a period of weeks. Alternatively the CIP sequence can be repeated several times, however the CIP chemical strength and quality will be worse after every repeat as fouling is removed from the membranes. Typically 3-5 repeats is sufficient.

A cleaning solution with a low pH is often used to target inorganic fouling, as many inorganic fouling compounds are at least partly soluble in acid solutions. Citric acid, which is a weak acid, is commonly used.

Stronger acids such as hydrochloric acid or sulfuric acid can also be used. Hydrofluoric acid may be used in cases of fouling from silica but hydrofluoric acid is extremely hazardous and must be used with great caution.

Every membrane material has limits for pH, and these limits change depending upon the temperature of the cleaning solution. Membrane suppliers will have specifications regarding exposure limits. An example of pH limits is provided in Table 2.3 in Chapter 2.

High pH cleans are used with certain membrane materials to target organic fouling. Typically, sodium hydroxide (caustic soda) is used for a high pH clean. It is also common to include additives such as hydrogen peroxide, sodium hypochlorite or surfactants such as sodium lauryl sulfate to enhance removal of organic materials from the membranes.

Chelating agents have multiple ionic sites in their molecular structure. Their negative sites act as electrically charged “claws” which bind to the positive ions, for example, metals such as iron, aluminium, manganese in the inorganic fouling layer thereby breaking down the fouling layer. Two common chelating agents are citric acid and Ethylene Diamine Tetracetic Acid (EDTA). Citric acid is used for iron removal, and EDTA can remove calcium and magnesium. Chelating agents are used as a low dose additive in cleaning solutions at a range of pH levels.

The main cleaning products and their target for foulants are summarised in Table 5.1.

Table 5.1.

Common membrane cleaning chemicals and their targets.

Category Acids

Bases Oxidants/disinfectants Surfactants/Detergents

Chemicals Commonly Used Citric acid Hydrochloric acid Sulfuric acid Caustic Soda (sodium hydroxide) Chlorine (typically Sodium hypochlorite) Hydrogen peroxide Large range of surfactants, including proprietary mixtures

Typical Target Inorganics Scale Organics Organics Biofilms Organics Inert particles

Chemical CIPs should remove almost all foulants on the membranes. This can be verified by monitoring the TMP and/or resistance before and after a CIP and between CIPs.

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Chapter 5 - Membrane Cleaning Some general advice on chemical cleaning is provided below.

• Check the Workplace Substance Register and Safety Data Sheets for safety requirements, chemical compatibility and necessary PPE when handling the various chemicals.

• Ensure that all the cleaning chemicals are of a high quality and batched according to the manufacturer’s recommendations.

• Check and calibrate chemical dosing pumps routinely and conduct drop tests on pumps to ensure correct dose rates are used.

• Check any chemical storage trends to confirm that levels in the storage tank are actually dropping and there is sufficient chemical to continue operation of the membranes, along with the necessary cleaning.

• Ensure all chemicals and additives are dosed in the correct concentrations.

• Caustic and acid cleans of MF and UF filters often have an additive, such as EDTA or wetting agent to enhance removal of organics. ◦ EDTA is used for binding material dissolved off the membrane to prevent it from re-fouling the membrane.

• Peroxide is good for enhanced organics removal in caustic cleans, but will dry membranes out if used too often or at high concentrations.

• Various surfactants (detergents), many of them proprietary chemical mixtures, can be added to caustic for rewetting of the membranes.

• Citric acid is good for manganese and iron removal.

• Either hydrochloric acid or sulfuric acid can be used as a stronger acid clean, with sulfuric acid often being chosen on the basis of chemical cost.

• Sodium lauryl sulfate (a surfactant) can also be used as a milder and safer chemical to address silica fouling, however it can cause extreme foaming if not added correctly.

• Other specialised cleaners include phosphoric acid, oxalic acid or sodium bisulfite. Seek the advice of the membrane supplier before using these.

• When making up a batch of chemical for CIP, ensure the make-up water is low in silica (<5mg/L), otherwise scale formation may occur on the membranes during the CIP clean. Filtered water should always be used for preparing CIP solutions.

• Warm water is more effective for CIP cleans typically around 35oC, but no more than 25oC for hypochlorite cleans. Therefore, improvements in chemical cleaning can be achieved by carefully increasing temperatures but staying below the maximum temperature specified by the membrane manufacturer.

• pH is critically important. ◦ For caustic cleans, aim for pH >12 but remember to check the pH limit of the particular membrane. ◦ For acid cleans, aim for pH 2-3, but remember to check the pH limit of the particular membrane.

• Specialised cleaners should only be used after extensive cleaning studies have been conducted by membrane suppliers and should only take place under the guidance of the supplier.

Poor management of CIPs results in poor removal of foulants, consistently high resistances, and the potential to permanently damage fibres and compromises membrane integrity.

After CIP, the membranes must be rinsed before returning to service. It is important that all residual chemical be rinsed off the membranes at this stage. An indication of insufficient rinsing may be out of specification pH in the filtrate. Grab samples of the filtrate should be taken periodically after the CIP rinse for testing. Usually, testing for pH is sufficient but occasionally, more sophisticated laboratory testing might be needed to test for residual cleaning chemical.

As chemical cleaning produce a lot of waste that ultimately requires disposal, it is not uncommon to reuse the CIP solution. Some CIP systems allow chemical batches to be re-used with adjustment of pH and/or top-up of additives.

49

Chapter 5 - Membrane Cleaning The question is how many times should CIP batches be re-used? There is no easy answer and it depends on the extent and nature of the fouling removed. There are some useful checks that can be made.

• Review the recovery of TMP and membrane resistance after cleaning. If this is deteriorating it may indicate the cleaning chemicals need replacing.

• Take grab samples of the used cleaning chemicals and visually inspect them for solids and colour. If a CIP batch contains a little to no solids and is a light colour, it is possible it may be reused.

Monitoring the Effectiveness of Cleaning Membrane TMP and resistance will increase gradually over a period of weeks during normal operations.

The combination of CEB and CIPs should be capable of removing almost all of the fouling material on the membranes. This can be verified by monitoring the TMP and/or resistance before and after chemical cleans (Figure 5.4). An acceptable CIP recovery should be recommended by the membrane supplier.

8 7

Resistance

6 Skid Number 2101 2201 2301

5 4 3 2 1 0 30 Mar 09 Apr

Figure 5.4.

19 Apr 29 Apr 09 May 19 May 29 May 8 Jun

18 Jun 28 Jun

A three month trend showing changes in membrane resistance and recovery after CIP of three skids. The CIP for skid numbered 2201 (mauve) is shown with an arrow.

A more advanced method of monitoring the effectives of chemical cleans is to monitor membrane performance as the change in resistance (delta R), as function of the volume of water filtered (delta V) between CIP cleans. This is plotted over the lifetime of the membranes, to identify long term trends in increasing resistance as a result of changing feedwater quality or problems in CIP management. Increases in this value indicate more serious fouling resulting in less water produced between cleans. An example is provided in Figure 5.5.

(delta R is the change in resistance between CIP’s and delta V is the change in volume between CIP’s)

Ratio delta R / delta V

0.1

50

Unit 2 Unit 6

Unit 3 Unit 7

Unit 4 Unit 8

0.08

0.06

0.04

0.02

0 Sep 02

Figure 5.5.

Unit 1 Unit 5

May 04

Jan 06

Aug 07

Apr 09

Long term monitoring of change in resistance relative to volumes of water filtered between CIPs.

Chapter 5 - Membrane Cleaning The variation shown at the start of the trend is when the new membranes were settling down. The disturbance in 2007 was due to a change in source water and re optimising operation of the plant. The variation after April 2009 reflects the end of the life of the membranes, partly due to increased permanent fouling.

Management of Chemical Sludge MF and UF treatment plants produce sludges from their backwash and chemical cleaning operations consisting of suspended solids, protozoa, bacteria, algae and possibly viruses that have been separated from the feedwater. There is also a mixture of chemicals, usually with a high or low pH, that have been used in the cleaning processes.

The quantity of sludge produced depends on several factors including the amount of water treated, the turbidity of the raw water and the types and quantities of chemicals used in cleaning. Typically wastewater from MF and UF plants is around 2% to 15% of the volume of the feedwater depending on the efficiency of the water treatment and sludge treatment facilities. Of this 95% to 99% is from the routine backwash wastewater. The remainder is due to chemical cleaning of the membranes.

Wastes contained in the backwash water will be concentrated from 7 to 50 times compared to feed water concentrations, with suspended solids typically between 10 - 1,000 mg/L and Giardia and Cryptosporidium between 10 - 1,000 orgs/L Chemical cleaning can significantly alter the composition of the waste stream. Chemical cleaning wastes typically consist of: • • • • •

SS up to 500 mg/L. TOC 10 - 30 times feed water concentrations.

BOD up to 5,000 - 10,000 mg/L (if citric acid has been used). pH 2 - 14.

Surfactants at low concentrations.

The chemicals in the sludge will often be the controlling factor determining whether the waste can be used beneficially or whether disposal is the only option.

MF and UF membrane filtration processes do have the advantage that they provide a physical barrier for particles down to very small sizes and generally only require low or zero addition of coagulants to form a floc. Therefore, compared to conventional WTP’s, the volume of sludge is much less. Careful operation of all chemical dosing facilities and optimisation of backwash and chemical cleaning processes will further minimise sludge production.

Another advantage of not adding alum (the most common coagulant) is that some of the undesirable characteristics of alum sludges for agricultural reuse can be avoided. Reuse opportunities, including agricultural reuse as a soil conditioner should be fully explored for these sludges.

Water Treatment Plant sludges may be discharged to a wastewater treatment plant where a sewer is available and such discharge is approved by the relevant authority. This allows the water treatment sludge to be used in any sludge reuse scheme at the wastewater treatment facility.

As discharge of sludges to waterways is likely to cause pollution, it is not normally acceptable to Regulators.

Current Guidelines discourage the application of liquid sludges to land, so any likely disposal must be preceded by dewatering and, in some cases drying.

As a last resort, disposal of dewatered water treatment sludges may be permitted at approved landfill sites, subject to EPA licence conditions. Disposal however, does not reflect the true value of the sludge as a resource and is not consistent with current waste minimisation principles and policies.

51

Chapter Chapter 6 5 - Membrane Cleaning

Membrane Maintenance Testing and repair of membrane systems should be conducted on a routine basis to maintain the integrity of the membranes, thereby preventing breakthrough of turbidity and pathogenic microorganisms. All associated equipment should also be maintained to ensure safe and efficient operation.

Table 6.1 outlines typical inspection and maintenance tasks required to maintain optimal membrane performance and prevent damage due to wear or corrosion.

Table 6.1.

A general preventive maintenance schedule for membrane systems.

Task

Daily

Complete daily log sheets

4

Conduct routine lab testing on feedwater and filtrate

4

Inspect for leaks or damage

4

Check and calibrate analysers for feed and filtrate water quality

Weekly

Monthly

Quarterly

4

4 4

Perform chemical CIP to remove organic/inorganic fouling

4 And as required

Check and maintain pumps, blowers and fans

4 And as required

Conduct membrane leak testing, pinning and patching

4 And as required

Back up PLC system

4 And when changes are made to the program

Inspect cells and skids for damage or corrosion (as necessary)

Annually

4

Check backwash air and water flows and pressures

Inspect pipework and valves for damage or corrosion (as necessary)

Six Monthly

4

4

4

Replace PLC batteries every 2-4 years Replace UPS every 5 years

Maintenance should also be scheduled based on the results of routine monitoring of pressure decay tests (PDT) and the associated log removal values (LRV). It is important to be consistently reviewing trends of PDTs or LRVs to determine if there is an emerging problem with the membranes, such as a gradual deterioration in membrane integrity.

52

Chapter 6 - Membrane Maintenance Membrane inspections and repair activities are labour intensive requiring planning and appropriate resourcing to perform effectively. For submerged membrane systems, 3-4 maintenance staff are recommended to carry out the testing and repairs, whereas 2-3 staff should be sufficient for maintenance of pressurised systems.

Some membrane suppliers offer membrane systems where in-situ testing and repair can be conducted. In this case, the membrane elements are generally not removed from their housing and visual inspections are less likely to be done as part of the maintenance routine.

Where modules need to be removed, it is necessary to ensure that all equipment for removal and testing of the membranes is available and has been serviced. It is important to carry out a thorough risk assessment and ensure the appropriate isolation procedures are adhered too. Membrane inspections require a large amount of manual handling, so it is important that there is easy and safe access to lift and carry membranes and associated equipment. There needs to be a large enough cleared area for placement of modules or racks, and space for membrane testing, dismantling and reassembly.

Once removed, a full assessment of membrane condition needs to be undertaken including: • Inspection of the membrane modules for cracks or other obvious faults (Figure 6.1 and 6.2).

• Inspection of the membrane bundle for obvious broken fibres, damage to the screen around the membrane bundle or to the O-rings.

• Inspection of the membrane potting for cracks, or other visible damage or defects.

Note that membranes should not be exposed to air or the cell drained for more than a couple of hours.

Figure 6.1.

Visual inspection of membrane modules on a submerged membrane rack.

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Chapter 6 - Membrane Maintenance

Figure 6.2.

Example of a cracked housing due to ageing of plastic components and possible cross-threading during maintenance.

• Measurement of the diameter of the membrane modules for evidence of swelling.

• Comparison of the weight of the removed membrane with that of a new membrane, to assess for solids accumulation. Standard bathroom scales are suitable. (Figure 6.3).

Figure 6.3.

Weighing the old membranes can be useful to determine the amount of solids that have accumulated within the bundle.

• Checking for any significant solids build-up in the fibre bundle (Figure 6.4) or on the screen.

In submerged membranes, opening the cassette readily reveals any fouling (Figure 6.4)

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Chapter 6 - Membrane Maintenance

Figure 6.4.

Example of fouled membranes in a submerged membrane system.

Membrane modules identified as being in need of repair can then be placed one at a time in a membrane testing unit, or in the case of in-situ membrane repair, into a “test mode”. For hollow fibre membranes, the top of the membrane module is covered by a small volume of water and air is applied to the module. This allows streams of bubbles to be seen coming from faulty fibres which can then be sealed with pins (Figure 6.5). If the damage is in more than one fibre (i.e. a small area) then a patch can sometimes be fitted. All O-rings are then checked, lubricated and the membrane is returned to service.

Figure 6.5.

The leaking fibre is situated where the small bubble is (Left). Individual fibre pinning (Right).

Once repaired, another leak test is generally conducted to make sure that the correct membrane was repaired.

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Chapter 6 - Membrane Maintenance

Record Keeping Record keeping is an essential part of maintenance. Maintenance activities should always be recorded and stored either in a computerised maintenance management system (CMMS), a dedicated membrane maintenance database or at the very least, a spreadsheet.

A critical part of membrane management is the tracking of individual membrane modules throughout their service life. This can be done by keeping records of the unique serial numbers of each membrane. At a minimum, the following information is needed in the membrane record-keeping system: • • • • •

Unique membrane serial number.

Module/element position in the cell or skid.

Number of inspections and whether the membrane required pinning and if so, how many pins.

Date when the maintenance was undertaken.

Visual observations of membrane condition. Good qualitative observations are crucial for warranty claims and justifying replacements. For example, “The membrane was dry and could not recover, most of the membrane face dry” is an example of a good observation. Just recording “Membrane beyond repair” is not such a good observation.

The first observation indicates what might be wrong with the membrane(s). If a number of similar observations are noted, then this may trigger further investigations relating to why the membranes are becoming dry. The second observation does not provide sufficient information to be able to commence investigations into the cause of membrane failure.

Other observations that can be useful to include are cracked head pieces, fouling and the type and extent of fouling on the membranes and any damaged threads on the housings.

Using Maintenance Statistics Membrane maintenance statistics and trending is very useful to keep track of membrane performance and for predicting the end of the service life of the membrane. The rate of change of inspections and pinning activities may provide an indication of whether defects are due to manufacture, feedwater quality, or ageing of the membranes. The increase in pinning or patching can be used for planning for a future membrane replacement. Figure 6.6 shows examples of maintenance statistics for individual modules.

Figure 6.6.

Bar graph showing the number of inspections and pinned membranes for a membrane system with seven pressurised membrane skids.

Manufacturers may have recommendations for the maximum acceptable number of pins. For example, a manufacturer may recommend that no more than 30 fibres per module should be pinned in a 12 month period. More than this may be a trigger for a membrane module to be discarded and replaced.

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Chapter 6 - Membrane Maintenance

Storage and Preservation of Membranes Many WTP’s have periods when demand is significantly less than the plant design output. These conditions may arise as a result of a regular seasonal change in demand or, increasingly in Australia, due to drought and associated water restrictions. Options available to reduce plant capacity include reducing flows to all membrane filters, or when demand is significantly reduced, taking a membrane skid offline.

Other situations where membrane systems may be taken offline may include during major maintenance activities such as recoating of submerged membrane cells. In this situation, membranes need to be removed and stored appropriately, ready for reinstallation.

The simplest option to reduce filtration capacity is to slow the feed to the plant while keeping the same number of membrane skids operational. Slowing the filtration rate does result in long plant run times, which may be a problem if energy consumption and power costs are an issue.

Most membrane plants also have the capacity for the operator to select the number of membrane skids operating at any given time, and the ability to cycle the skids through filtration runs so that no membranes are left offline for extended periods. Significant reduction of inflow is therefore unlikely to have any adverse effect on the operation of the plant or water quality.

Chemical dosing may become limiting if the feed flow is reduced too much. The turn down of chemical metering pumps, either pre or post-treatment, may not be sufficient to ensure accurate dosing of chemicals at the lowest flows. The options are to install a smaller chemical dosing pump or to increase the flow back to a rate where the size of the pump is appropriate.

If the chemical dosing pump is running at the bottom end of its capacity, it is important to ensure that mixing is still adequate.

The typical shutdown recommendations for a MF/UF membrane skid are provided in Table 6.2.

Table 6.2.

Summary of shutdown requirements for a MF/UF membrane system.

Storage Duration 0 to 24 hours 24 to 48 hours

>2 days to 7 days

>7 days to 90 days >90 days

Recommended Steps Air scour and chemical free backwash.

Close all valves to seal the system.

Check that the valves are sealed and that the membranes are still in contact with water.

The membrane modules should never be allowed to dry out. Dry membrane fibres will irreversibly lose flux.

Typically, membrane filters can be shut down for 48 hours without adding preservative or taking precautions for microbiological fouling.

In addition to the above steps, the filter should be adequately protected against biological growth. Addition of chemical preservative is generally necessary.

Manufacturers will provide information on the recommended storage and preservation procedures for shutdowns longer than 48 hours.

The filter should be protected against temperature extremes.

The plant should be flushed daily or operated in accordance with the recommendations of the manufacturer.

Renew preservative every three months.

Consult the manufacturer.

57

Chapter 6 - Membrane Maintenance If the membrane filter will be out of service for a long time, or the skid requires major upgrade or maintenance, the modules can be removed and stored. Prior to removal, the membranes should be air scoured and backwashed.

Once the membranes are removed, they should be placed either in a membrane bath or appropriate membrane storage housings and submerged in a membrane preservative solution in accordance with the manufacturer’s instructions.

Prior to returning the stored membranes to service, they should undergo a number of rinses to ensure removal of all residual preservative solution. Backwashing is also recommended to remove any deposits that may have accumulated on the membranes. A membrane integrity test will identify any possible deterioration of membrane fibres. The reinstalled membranes can then be returned to service, ensuring they are carefully monitored for any fouling and integrity issues during the initial filtration period.

Membrane Autopsies Where fouling is persistent or there is a sustained increase in the TMP despite chemical cleans and backwashing, or the membranes are not achieving the design life stated by the manufacturer, closer inspection of the membrane may be necessary to try to determine the cause of the problem(s).

A membrane autopsy is a destructive analytical test requiring removal of a module from a skid and opening up the module to expose the membranes (Figure 6.7) thereby allowing closer inspection of the membranes.

Figure 6.7.

A membrane module being opened for inspection of the membranes.

Samples of the membranes can then be removed for further chemical and microscopic analysis. Individual fibres can easily be removed, and spiral wound elements can be unwound to fully expose the surface of the membranes to allow samples to be cut out for further detailed analysis (Figure 6.8).

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Chapter 6 - Membrane Maintenance

Figure 6.8.

Samples of membrane have been cut from this spiral wound membrane for chemical and microscopic analysis.

A large number of tests can be performed on the membrane and the membrane fouling material to determine the nature and extent of the fouling. The main types of tests are described below.

A visual inspection as the name suggests, is a complete visual inspection of the entire membrane element and associated structures. Potting at the top and bottom of the module is closely examined and foreign material sampled for relevant analysis. The membrane element casing is cut open and removed. The internal fibres are examined and photographed (Figure 6.9).

BOTTOM

Figure 6.9.

TOP

Exposed membrane fibres showing differences in fouling between the top and bottom of the element.

The fibres are cut away from the potting and the ends of the fibres are examined. Samples of the fibres from representative locations within the element are taken for further analysis. The fibres may be cut open along the axial length and any visual fouling inside the fibres documented. Magnified images of the internal, external and cross sections of the fibres are often taken. Pot hardness tests may also be performed if deemed necessary.

59

Chapter 6 - Membrane Maintenance The Loss on Ignition (LOI) test is used to determine the amount of organic versus inorganic material in the fouling layer (cake). Samples of fibres are placed into water and agitated to remove surface fouling. The fibres are removed from the solution and the solution containing the fouling deposits is evaporated at 110oC and weighed. The remaining dry deposit is then ignited at 550oC and weighed again. Ignition at 550oC burns off all organic material leaving only inorganic material. The differences in weight can be used to determine the amount of inorganic and organic fouling. However, the test presupposes that the agitation removes all the fouling which it may not.

It is preferable to have a greater proportion of organic fouling relative to inorganic fouling since deposits of inorganic materials such as silica, clays, iron, manganese and aluminium can be more difficult to remove via chemical CIP compared to organics. Therefore, they are often associated with irreversible fouling that can result in eventual membrane failure.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) is used to characterise the inorganic content of the fouling material. Sample preparation involves placing a known length of membrane cut into small pieces in a 5% w/w nitric acid solution to dissolve the foulants. The solution is filtered through a 0.45 µm filter and then analysed for a range of metals such as aluminium (Al), barium (Ba), boron (B), cadmium, (Cd), calcium (Ca), chromium (Cr), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), nickel (Ni), phosphorus (P), potassium (K), sodium (Na), sulfur (S), silicon (Si) and zinc (Zn).

Total Organic Carbon (TOC) is used to measure the amount of organic fouling that is readily removed from the membranes (i.e. reversible fouling). Generally TOC is determined by testing the surface deposits removed from a membrane, through high temperature combustion of samples at 680oC. Total organic carbon is the difference between the total carbon and inorganic carbon. Inorganic carbon is measured by acid digestion.

Scanning Electron Microscopy (SEM) allows for visual identification of the membrane surface characteristics and the nature of any fouling (Figure 6.10 and 6.11).

Damage caused by a small, sharp particle

Figure 6.10. A scanning electron microscope image of a membrane that has been damaged by a small sharp particle.

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Chapter 6 - Membrane Maintenance

A

B

C

D

Figure 6.11. SEM pictures of different types of membrane fouling. A: biofouling, B: organics fouling, C: inorganic fouling, and D: colloidal fouling.

Energy Dispersive X-ray Spectroscopy (EDS) uses electron beams to excite the atoms of the membrane and the fouling materials. A detector captures the X-rays emitted. The energy of the X-rays can be used to determine the elements present on the membrane surface (Figure 6.12). This allows for a combined visual and chemical interpretation of the nature of the fouling material.

Figure 6.12. SEM and EDS analysis indicating aluminium silicate fouling on the surface of a membrane fibre.

61

Chapter 6 - Membrane Maintenance Fourier Transform Infrared (FTIR) spectroscopy is a technique that is used to identify the surface characteristics of the membrane material. Every chemical bond has a characteristic infrared absorption spectra which allows identification of fragments and functional groups in the material. FTIR testing is used to identify damaged membranes.

Fibre Tensile Testing takes a known length of membrane fibre, clamped at each end and stretches it at a constant rate. The force required to elongate the membrane is recorded until the membrane breaks. The parameters such as force at break and extension at break can be calculated from the force versus elongation graph to give information on the physical properties of the membrane. This technique is generally used if membrane integrity problems are suspected.

In the case of hollow fibre modules, the construction materials of the module are also important for the overall integrity of a membrane. Inspection and chemical analysis of the housing, resin and plastic supports used within the module can be assessed during the autopsy process.

Cleaning studies can be carried out in the laboratory to check the efficacy of different cleaning chemicals.

Fibres removed from the module can be soaked in different cleaning solutions and once cleaned, the flux can be determined with a porometer to measure the effect of a particular cleaning chemical.

Autopsies are typically carried out at the following times.

• As part of an annual maintenance program to track the performance and ageing of a membrane system. This is important to allow proactive planning of membrane replacements.

• After a significant water quality incident or extreme weather event to gauge the impact on membrane integrity and in particular, the extent of fouling.

• To troubleshoot performance problems detected during routine operation, such as failed membrane integrity tests, increased TMP/resistance and failure to recover TMP/resistance after backwashing or CIP. Based on the findings of a membrane autopsy, modifications can be made to how the plant is operated and in particular any additional pre-treatment that may be necessary (See Appendix 2).

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Chapter 6 - Membrane Maintenance

Trouble Shooting Some common membrane plant problems and suggested actions are summarised in Table 6.4.

Table 6.4.

MF/UF trouble shooting guide.

Condition Unit fails integrity test

Possible Actions 1. Repeat the test and check the readings.

2. Check the membranes are fully wetted. Dryness of fibres can be picked up in visual inspections.

3. Locate and isolate any modules with flaws or leaking seals and conduct a new integrity test.

4. Determine the nature of the failure.

5. Repair the failed modules and return them to service.

6. Conduct a new integrity test.

7. Carry out a CIP cycle.

High filtered 1. Clean and calibrate the online turbidity meter. water turbidity 2. Check the online reading with a calibrated bench turbidity meter (not a hand held meter).

3. Conduct a membrane integrity test.

4. Inspect and repair problem membranes.

High rate 1. Check the pressures applied during all operating cycles including backwashing are of fibre within specification. breakage 2. Check for water hammer.

3. Check for possible exposure to chemicals, for example chlorine.

4. Check for exposure to high temperatures.

5. Check the quality of CEB and CIP chemicals.

6. Consider membrane age. Whilst membranes age differently, membranes older than five years will start to see increased rates of breakage.

7. Organise for the manufacturer to conduct tests on a sample module.

8. Check fibre strength and allowable elongation for old membranes.

9. Check that fibres are maintained in a wet environment.

10. Possible warranty claim for manufacturing defect.

Interval 1. Allow for the water temperature. TMPs can be higher at a lower temperature if flux is between CIPs the same. too short 2. Is the flux too high.

3. Check for any changes in raw water quality.

4. Check the operation of pre-treatment and assess the potential for carryover of particles.

5. Review process settings, pressures, flows, chemical doses, and pH. 6. Evaluate backwash pressures and valve timing (see Chapter 5, Figure 5.3 Backwash Snapshot).

7. Evaluate CIP procedures and the quality of the CIP chemicals.

CIP does 1. Check resistance adjusted for water temperature. not return 2. Change cleaning methods. For example, higher temperatures, longer soak times, or unit to clean stronger chemicals. Seek assistance from manufacturer. baseline 3. Try an extended soak, for example overnight. values 4. Try changing the CIP order, for example acid before caustic/hypo.

5. Check the pH during CIP against the expected pH. 63

Chapter 7 Membrane Replacements When to Change out Membranes Membrans have a finite life. A pressurised, hollow fibre, PVDF ultrafiltration membrane treating a low to medium fouling feed water and operated within normal recommended flux ranges should have a service life of around five to ten years. Some of the newer and more robust membrane materials such as ceramics, have a reported service life of as much as 30 years.

Membrane suppliers may provide some useful tools and indicators for predicting the life expectancy of membranes, however, the actual service life of a membrane is determined by feed water quality, membrane type, manufacture, plus the maintenance and operational regimes.

There are numerous reasons that can trigger the decision to change out membranes.

• Consistent failing of PDTs indicating poor membrane integrity.

• Consistently high TMPs/resistances that do not adequately recover after CIP indicating irreversible fouling.

• Membrane ageing. A gradual trend of deteriorating integrity and/or increasing resistance associated with increased maintenance requirements and damaged potting.

• Results of membrane autopsies indicating fibre brittleness and damage to membrane materials.

• Maintenance data indicating a significant increase in the number of membranes inspected, pinned or replaced due to damage.

Except where there has been a major failure, determining when a membrane may need replacing involves ongoing assessment of monitoring data and possibly some testing. Typically, this includes one or more of the following.

• Evaluation of long term SCADA trends. ◦ Membrane PDT and LRV calculations. ◦ Membrane TMP or resistance. ◦ Membrane fluxes and permeability data. ◦ Energy consumption relative to volume production.

• Backwash and CIP results. ◦ TMP/resistance before and after cleans. ◦ Percentage recoveries from backwash and CIP. ◦ Rate of change of resistance between CIPs. • Maintenance statistics including numbers of inspections, pins, patches and membrane replacements.

• Recommendations of the manufacturer based on the number of pins.

• Visual inspections.

• Membrane autopsies and testing. ◦ Analysis of foulants (SEM, EDS, ICP, TOC), membrane surface characterisation (FTIR), fibre strength and elongation at break testing.

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Chapter 7 - Membrane Replacements

Preparation for Changing out Membranes Preparation for a complete membrane change-out is important. Depending on the size of the membrane plant and how many cells or skids are to be changed out, there may be many membranes that need to be ordered. For large orders, depending on the membrane supplier and the membrane type, be prepared for as much as a four to six month lead-time on supply of the membranes.

Site preparation is essential. Ensure that all equipment for removal of the membranes is available and has been serviced. Ensure there is sufficient space for storage and access to the newly supplied membranes.

Membrane replacements require a large amount of manual handling, so ergonomic considerations should be taken into account. This includes access/egress to lift and carry membranes and equipment and having tables of a suitable size and height for removal, dismantling and reassembly activities.

When scheduling the membrane change-out, ensure there are sufficient staff to undertake the work safely and efficiently. Depending on the size of the membrane cell or skid to be changed out, 6 or 7 people are recommended, 2 either side of the membrane skid to remove and reinstall membrane modules and a further 2-3 people set up away from the skid, pulling apart the membrane housing, clips and end caps, checking for faults or damage, and disposing of old membranes (Figure 7.1). They will then reassemble the membrane module ready for reinstallation.

Figure 7.1.

Preparation for the membrane change-out is crucial. Ensure there is plenty of space to dismantle and reassemble membrane modules.

If the work is not expected to occur immediately, membranes should be stored out of the sun and weather and away from extreme heat and cold. If stored unopened in their original packaging, in a suitable cool, dark storage area, many membrane types can be stored for several months up to a year.

Plans should also be made for the disposal of the old membranes and any predominantly plastic parts that are replaced. Additionally, there will be wooden pallets, cardboard boxes, plastic bags, used tins and tubes of silicon grease and rags requiring disposal. Organise separate skips for rubbish and any materials that may be able to be recycled (Figure 7.2).

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Chapter 7 - Membrane Replacements

Figure 7.2.

Membrane change-outs produce a large amount of waste.

When considering options for disposal, the following questions should be considered.

Is it possible to keep any of the used membranes and return them to service?

Used membranes that are still considered to be in serviceable condition may be kept for interim membrane replacements on other skids. If membranes are being kept, they need to be stored and preserved appropriately.

Ensure that there is a suitable storage bath set up for the membranes and the correct preservation chemicals on hand ready to put the membranes into storage as soon as they are removed from the skid.

What disposal options are available?

The most common disposal option is to landfill. However, there may be other more sustainable options.

• Recycling. Recovery of the various polymer materials for manufacture of other products.

• Alternative uses for membranes such as taking old membranes and using them for lower quality water treatment purposes, for example suspended solids removal in wastewater treatment for Class B and C recycling.

• Donate them to science. There are various universities around Australia that do membrane research and analysis. Many of these universities are happy to accept old membranes for teaching and research purposes (generally not a whole skid though!).

Is it worth sending a few membranes away for testing/autopsy?

Particularly in the case where membranes are being changed out due to a serious process upset or major water quality incident or event, it can be extremely useful to have a few of the old membranes tested to identify the cause of failure. This information is then available to proactively monitor and troubleshoot the new membranes to prevent a repeat of events that caused the initial membrane failures. Contact the testing laboratory several weeks in advance of the scheduled membrane change-out to arrange for membrane autopsies to be undertaken.

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Chapter 7 - Membrane Replacements

Decommissioning and Dismantling The process for decommissioning a membrane skid in preparation for a membrane change-out is summarised below.

• • • •

Take the skid offline via SCADA or PLC. Carry out a thorough risk assessment and follow all isolation procedures.

Drain down the membrane skid and ensure all the water is drained from the housings and pipework.

Ensure all the systems are de-energised (water, electricity, compressed air).

Plan out the membrane removal and replacement carefully. For some pressurised systems arranged in skids, the order in which membranes are removed and replaced is crucial to prevent any stress being placed on the top and bottom of the membrane skid frame and support structures.

The membranes provide some level of structural support and stiffening to the skid. It is a good practice to work from different corners and ends of the skid when removing the membranes (Figure 7.3). This ensures an even weight distribution across the skid as membranes are removed and replaced.

Never pull all the membranes out from one end, or one side, at the same time.

Figure 7.3.

To prevent excessive stress being placed on the membrane frame and blocks when removing membranes, work from opposite corners and replace the membranes as you go.

When removing the used membranes, it is good practice to inspect them for evidence of fouling, discoloration and damage to the fibres or potting. It is also a good idea to see if it is possible to determine how the membrane may have been fouled or damaged. For example, if the potting appears swollen, this may be due to use of incorrect cleaning chemicals or wrongly made up cleaning chemicals. Excessive solids or discoloration at the top or bottom of the membrane may be an indication of problems with delivery of air or water during the backwash. There may be evidence of damage to fibres associated with hard particles such as sand or diatoms entering the membranes.

Weighing the old membrane (see Figure 6.3) is also useful as an indication of the fouling potential of the feedwater and the possible impact on the new membranes. Checking the weights also gives an indication of possible problems with the backwashing or CIP cleans that need to be followed up.

Check all the parts in addition to the membrane, including the cups, clips, and O-rings. Discard and replace any components with obvious evidence of brittleness, cracking or damage (Figure 7.4). Check the threads on the housings and replace the housings where obvious cross-threading and damage has occurred.

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Chapter 7 - Membrane Replacements When the membranes are removed from the skids, inspect the top and bottom end caps for evidence of cracking (Figure 7.4).

Figure 7.4.

Inspect all components of the membrane modules for signs of damage and replace as required.

Replacement of Membrane System Components When installing the new membrane into the housing, ensure all new O-rings are used on the membrane module. Good quality silicone grease approved for use in water treatment should be used to lubricate all the O-rings and ends of the membranes. This allows for easy reassembly and reduces the risk of jamming of the components and twisting of the O-rings.

A critical part of membrane replacement is tracking individual membrane modules for their service life. This can be done by keeping records of the unique serial numbers of each membrane. The membrane will arrive packaged for transport with an information slip attached to the packaging. It is a good practice to tape the information slip to the outside of the membrane housing during reassembly. When the membrane is installed back into the skid, the location in the skid should be written on the information slip. The slips should then collected and used to record the serial number and the installed location in the membrane database or CMMS (Figure 7.5).

Figure 7.5.

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Each membrane will have a unique serial number. These slips are supplied with the membranes and have been marked with the location of each installed membrane.

Chapter 7 - Membrane Replacements

Recommissioning the Membrane System When putting the membrane skid back into service, the following steps should be followed.

• Reinstate all systems (power, water and compressed air).

• Conduct a leak test on the membrane skid to identify any possible issues that may have arisen during installation.

• If any leaks are identified, remove those membranes and inspect and rectify any problems. These may be due to debris from installation trapped under the membrane, twisted O-rings, or clips that have slipped.

• Repeat the leak test until no further issues are found.

Once installed and recommissioned, a pressure decay test should be conducted to check the overall integrity of the membrane skid. This should be followed with a CIP clean as well as rinses. In the case of polypropylene membranes this would be a caustic clean. In the case of PVDF membranes use a sodium hypochlorite clean.

These cleans will remove any residual preservation chemicals that may be on the membranes, as well as assist in “wetting-up” the membrane, ready to be put back into service.

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Chapter 8 Nanofiltration and Reverse Osmosis Reverse Osmosis (RO) was developed in the 1960’s as a way of producing potable water from seawater.

Early RO units had relatively thick membranes and required very high pressures to operate. Applications for RO membranes have increased to enable the treatment of a variety of source water with differing salt concentrations.

Nanofiltration (NF) was developed in the late 1970’s. It has a number of applications including removal of colour and trihalomethane precursors, and for softening water through the removal of calcium and magnesium.

In Australia a number of large RO plants have been constructed to remove salt from seawater to augment water supplies for large coastal cities. A number of much smaller plants have also been installed to secure water supplies for small inland, island and isolated communities. In the early 2010’s, several NF plants have been installed for small communities in Tasmania where the prime objective is the removal of colour.

Comparison of NF and RO to MF and UF All membranes are similar in that they require a pressure difference across the membrane to force the water through. All membranes remove a range of particles from the feedwater.

The major differences between NF and RO, and MF and UF are discussed below.

Sizes of Particles Removed NF and RO membranes remove soluble substances, for example colour compounds and salts that will pass through MF and UF membranes. This was shown diagrammatically in Figure 3.4.

RO membranes exclude almost everything except water, however there are some molecules that are smaller than water, and therefore can also pass through RO membranes. A key example of this is boron, which can pass through RO processes at desalination plants.

Boron is usually present at low concentrations in Australian surface water (< 0.1 mg/L), however, seawater can contain significantly higher concentrations typically 4 to 5.5 mg/L (proportional to salt concentrations).

The ADWG value for boron, based on health considerations (gastrointestinal, skin and nervous system disturbances) is 4 mg/L but the WHO guideline value is only 0.5 mg/L and the EU guideline is 1.0 mg/L. Therefore, consideration must be given to the removal of boron for RO treatment plants producing drinking water from seawater.

Boron can also result in some agricultural problems if not managed appropriately. Boron is a nutrient that is only required by plants in small amounts. At higher levels it becomes toxic. The difference between levels required for plant growth and levels that are toxic is small. Therefore, water containing boron that is to be used for watering plants requires careful management.

Operating Pressures The operating pressures required for NF, and particularly RO membranes, are much higher than for MF and UF membranes (Table 8.1).

Table 8.1

Typical operating pressures for different membrane types.

Process Microfiltration Ultrafiltration Nanofiltration Reverse Osmosis

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Operating Pressure 40 - 150 kPa 100 - 700 kPa 500 - 1800 kPa 1400 - 7000 kPa

Chapter 8 - Nanofiltration and Reverse Osmosis While there have been developments in membrane design that have reduced operating pressures in recent years, the pressures required for these membranes remain high and are a major contributing factor to the operating costs for RO and NF plants.

How NF and RO Membranes Work Diffusion and Osmosis A membrane is a selective (semi permeable) barrier that allows some molecules to pass, and prevents others. This selectivity is often based on the size of the molecule, but can also depend on things like charge.

Natural membranes surround all cells, from bacteria to human cells, and have been studied for decades.

The passive movement of molecules through a membrane occurs via diffusion, with molecules moving from an area of high concentration to an area of low concentration. The end state typically moves toward a balance with the same concentration on either side of the membrane. Diffusion is generally limited to small, uncharged molecules, such as salts and small organic molecules.

Water is another small molecule that can pass through membranes. Osmosis refers specifically to the passage of water through a membrane, and was first described in 1748. In this case, the water moves from a low salt concentration to an area of high salt concentration. Whilst this may seem counterintuitive to the principle of diffusion described above, think about it as the water moving from an area of high water concentration (low salt) to an area of low water concentrations (high salt). In biological systems, this typically means that water moves into cells.

A simple experiment using synthetic membranes to demonstrate both osmosis and diffusion is shown in Figure 8.1.

5 minutes

Figure 8.1.

45 minutes

Demonstration of osmosis and diffusion. MW is the molecular weight of the chemical in the tube. The yellow line shows the starting level for the coloured liquids in the membrane tubes.

The left hand purple membrane tube in both photographs contains a low molecular weight (MW=158) small molecule. The right hand dark blue membrane tube contains a high molecular weight (MW 2000) large molecule. For comparison, water has a low molecular weight (MW 18).

At the start of the experiment, the level of the coloured liquid in both of the membrane tubes was level with the top level of distilled water in the measuring cylinders.

In both the 5 minutes and 45 minutes photograph, the levels inside the coloured membrane tubes have risen.

This is due to the passage of water into the tubes (osmosis) from an area with a low concentration of salts (distilled water) to an area of a high concentration of salts in the coloured tubes.

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Chapter 8 - Nanofiltration and Reverse Osmosis At the same time, the purple chemical (low molecular weight small molecule) has moved out of the coloured membrane tube and into the distilled water. This is clearly seen at 5 minutes and continues to 45 minutes.

In contrast, the high molecular weight chemical (the blue one) cannot pass through the semi permeable membrane and is trapped in the synthetic membrane tubing. None of the blue chemical is seen in the distilled water at 5 minutes or at 45 minutes. It is trapped inside the membrane tube.

Osmotic Pressure and Reverse Osmosis Where water and a salt solution such as seawater are separated by a semipermeable membrane, water is able to diffuse through this membrane but not salts. In this situation water will diffuse through the membrane to dilute the salt water on the other side in an attempt to attain the same concentration of salts in the solutions on both sides of the membrane. The diffusion of water will continue until the water level rises to such an extent that the extra hydrostatic pressure on the concentrated side of the membrane is large enough to prevent any further diffusion of water. This pressure is defined as the osmotic pressure of the solution.

If a pressure higher than the osmotic pressure is applied to the salt water side of the membrane, it will cause a reversal of the osmotic flow, and push water back through the membrane to the pure water side. This phenomenon is called reverse osmosis (RO) and is summarised in Figure 8.2.

OSMOTIC EQUILIBRIUM

REVERSE OSMOSIS Pressure Pressure greater than the osmotic pressure is applied

Osmotic pressure

Water flows in reverse direction Pure water

Figure. 8.2

Salt water

Pure water

Salt water

Osmotic flow through a semipermeable membrane.

On the left side of Figure 8.2, water has flowed from the pure water side to the salt water side and the level of the liquid has increased exactly as shown in Figure 8.1. On the right hand side of Figure 8.2, increasing the pressure on the raised liquid on the salt water side, results in pure water being squeezed back in the opposite direction. That is why the process is called reverse osmosis.

For RO membranes, the pressure required to force the water through the membrane has got to be high enough to not only overcome the intrinsic resistance of the membrane but also to cause a reversal of the osmotic flow, which explains why the pressures required for RO membranes are very high.

Actual Membrane Performance A semipermeable membrane doesn’t provide a complete barrier for particles above a certain size as MF and UF membranes do, but allows some materials, in particular water, to move more easily through the membrane than other materials such as salt. Therefore, there is not a complete rejection of undesirable materials but rather a range of rejection percentages. The percentage rejection of salts in the case of RO depends on a number of variables including the type of membrane material, contaminant concentration and the pressure differential across the membrane.

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Chapter 8 - Nanofiltration and Reverse Osmosis

The RO System. Membrane Elements, Stages and Arrays RO membrane elements, or cartridges as they are sometimes called, typically consist of spiral wound membrane sheets and spacers (Figure 8.3).

Figure 8.3.

Diagram of a spiral wound membrane. (Wagner, 2001).

The individual elements are connected in series inside a pressure vessel (Figure 8.4 and 8.5).

Feed Permeate Plug Concentrate

Figure. 8.4.

Six individual membrane spiral-wound elements are housed in a single pressure vessel with their central permeate tubes connected using o-rings.

Figure 8.5.

Removal of a spiral wound element from an RO pressure vessel (left) and a view inside the pressure vessel after removal of one of the elements (right).

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Chapter 8 - Nanofiltration and Reverse Osmosis Pressure vessels are usually connected in parallel to make up a stage. RO plants can be single stage, two stage and three stage. Figure 8.6 shows three, single stage RO plants on a small island in the Torres Strait.

Figure 8.6.

A small single stage RO plant on a Torres Strait island.

Stages and Recoveries Membrane stages are connected in series to make up membrane arrays (Figure 8.7). The number of stages is dependent upon the feed water salt concentration, salt rejection required and the product recovery percentage.

STAGE 1 Elements

STAGE 2

STAGE 3

Pressure Vessel

OF

OC

OP OF Feedwater Flow Rate Figure 8.7.

OC

Concentrate Flow Rate

OP Permeate Flow Rate

A three stage RO plant made up of arrays linked in series. Each array is made up of elements and pressure housings.

There is a decreasing number of pressure vessels in each succeeding stage as illustrated in Figure 8.7.

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Chapter 8 - Nanofiltration and Reverse Osmosis Note that it is usually the concentrate that is further treated in later stages. This increases the overall water recovery percentage (Figure 8.8). Typically, recovery across each stage is 50%, so in a single stage plant, 50% of the feedwater becomes filtrate. In a two-stage plant the total recovery is 75% (50% from the first stage and 50% of 50% = 25% in the second stage) as summarised in Figure 8.8 and Table 8.2.

50% Recovery

75% Total Recovery

Single Stage

Two Stage (2:1 Array) 50% Recovery

F

C

Pump

50% Recovery

C

F

C

Pump P

P

87.5% Total Recovery Three Stage with booster pump (3:2:1 Array) 50% Recovery

50% Recovery

50% Recovery

F

C Booster Pump

Pump

P

Figure 8.8.

Diagram showing single stage, two stage and three stage RO configurations and the percent recoveries possible (F= feedwater; C= concentrate; P= permeate).

Flow Direction

HIGH PRESSURE SIDE

Figure 8.9.

Semipermeable membrane

Water Molecule

Salt Ions

LOW PRESSURE SIDE

Diagrammatic representation of the RO process and concentration of salt ions on the upstream surface of the membrane leading to scaling.

A diagrammatic representation of the RO process is shown in Figure 8.9.

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Chapter 8 - Nanofiltration and Reverse Osmosis The higher hydrostatic pressure on the left hand side of the membrane forces the water to move to the right hand side. However, it also causes the salt molecules to “congregate” against the membrane which can result in production of scale which is very difficult to remove. Note also that two salt ions have managed to get through the membrane and pass to the other side. This is simply a result of the chemistry and dynamic nature of the membrane material.

Table 8.2.

Single Stage Two Stage Three Stage

Summary of water recoveries in RO plants with a different number of stages.

Recovery from 1st stage 50% 50% 50%

Recovery from 2nd stage

Recovery from 3rd stage

50% of 50% = 25% 50% of 50% = 25%

50% of 25% = 12.5%

Total Recovery 50% 75% 87.5%

In practice, the interconnection between the stages is quite difficult to determine. Figure 8.10 shows two multistage plants where it is hard to determine which pressure vessels belong to which stage.

Figure 8.10. Example of small and midsize multi stage RO plants. The stages are not clearly identified and the plumbing can be confusing.

Importance of Pre-Treatment As with MF and UF membranes, the basic layout of NF and RO plants consist of various pre-treatment steps followed by the membrane plants, then various post-treatment steps. With NF and particularly RO membranes, the quality of feed water required from pre-treatment processes and the number of posttreatment processes is higher than that required for MF and UF membranes. Often MF and UF are used as pre-treatment for NF and RO systems.

Pre-treatment processes, discussed in Chapter 3, are required to remove down to protect NF and RO membranes from particulate fouling and mechanical damage. The membrane supplier should be consulted regarding the necessary pre-treatment option.

For RO membranes, additional pre-treatment is required to protect these membranes from scale fouling.

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Chapter 8 - Nanofiltration and Reverse Osmosis As the feedwater passes through the RO and some NF membranes, salts are retained on the feed side of the membrane causing the water to become increasingly salty. This may result in the water becoming saturated with certain salts causing the salts to precipitate onto the membrane surface. The resulting scale formation can severely reduce the flow through the membrane and can result in irreversible damage. High concentrations of calcium, strontium, barium, carbonates and sulfates can all cause scale formation to occur.

There are several methods to control scale formation and precipitation on RO and NF membranes.

• Addition of acid to adjust the pH to 5.5 - 6.0. This minimises the hydrolysis of the membrane and scaling by converting carbonates to bicarbonates. Particular care must be taken to stay within the acceptable pH range for the membrane material.

• Addition of antiscalant chemicals to interfere with salt crystal formation. For example, polymers and sodium hexametaphosphate.

• Ion exchange or lime softening to reduce divalent metal ions such as calcium and magnesium in the feed water.

• Aeration or chemical oxidation to precipitate out iron and manganese prior to membrane filtration.

Membrane materials must be able to stand oxidation with air.

Disinfection may also be required to prevent microbiological growth which can physically block membranes and attack cellulose acetate materials.

Silt Density Index (SDI) The Silt Density Index (SDI) is a measure of the rate of fouling of a membrane due to colloids and particles in the feedwater. It is only used for RO and NF applications.

The SDI is based on the measurement of the time it takes to collect 500 mL of water filtered through a standard 47 mm diameter, 0.45 µm filter at a constant pressure of 30 psi (Figure 8.11).

Figure 8.11. Equipment used to measure SDI. The arrow shows where the sample membrane is inserted.

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Chapter 8 - Nanofiltration and Reverse Osmosis This measurement is taken twice, the first (ti) immediately at the start of filtering, and the second (tf) after 15 minutes of continuous filtering. The temperature must remain constant during the test. The time for the second sample is always longer than the time taken for the first sample because of clogging of the membrane. The SDI is then calculated as follows: SDI = 100(1- ti/tf) T An example is provided in Box 8.1.

Box 8.1. Calculation of SDI At the start of the SDI test it took 30 seconds (ti) to collect the first 500 mL of filtrate. After 15 minutes of continuous filtering, it took 48 seconds (tf) to collect the second 500 mL of filtrate. ti = 30 sec tf = 48 sec T = 15 mins Therefore SDI = 100(1- 30/48) 15

= 100(1- 0.625)

15

= 37.5

15

= 2.5

Pre-treatment processes need to be operated to produce feedwater that is suitable for the RO membranes.

Manufacturers typically specify that the SDI should be less than 5 for spiral wound membranes and less than 3 for hollow fibre membranes to prevent premature fouling. SDIs greater than these suggested upper limits will result in reduced life of the membranes unless the existing pre-treatment can be improved or enhanced by the addition of extra pre-treatment steps.

The SDI should be measured regularly, as often as once each week and up to three times daily if required.

The frequency depends on the operation of the RO process and the source of the raw water and variation in quality.

Integrity Testing for RO Membranes Any leaks in the membranes or associated pipework could result in pathogens being able to cross the membrane barrier.

Various membrane integrity tests have been discussed in Chapter 3, however these are not commonly used for NF and RO. The most common test for leaks in an RO system is based on electrical conductivity (EC). Plants can be fitted with an online EC meter and trends observed for any sustained increase in EC.

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Chapter 8 - Nanofiltration and Reverse Osmosis There is an additional method called probing also based on EC. Probing involves taking conductivity readings at various locations along the permeate tube using a sampling probe or spear. The probe allows a sample of the permeate to be collected at a range of locations within the pressure vessel. Usually samples are taken at the feed end, centre and concentrate end. This procedure can locate damaged brine seals or membranes that are the cause of the leak which results in elevated conductivity readings.

One of the most common causes of elevated conductivity is damage to O rings used in connections between membrane elements. Causes of damage to O rings include: • • • •

Use of incompatible materials.

Movement in membrane elements, particularly during the commissioning of new membranes.

Flattening of rings as they age.

Cuts and abrasions of O-rings incurred during installation or removal of membrane elements.

Suspect elements can be removed from the pressure vessels (see Figure 8.5) and transferred to a test rig for further testing (Figure 8.12).

Figure 8.12. Test rig for testing individual RO spiral wound elements.

The test rig is set up to allow pressure integrity testing of the membrane element and to determine the degree of fouling.

Post Treatment Many of the pre-treatment steps necessary for NF and RO cause problems with the final filtered water.

• The use of acid to control scaling as part of pre-treatment can result in low pH, high dissolved carbon dioxide water that is highly corrosive in the distribution system.

• If the feedwater is sourced from groundwater, dissolved hydrogen sulfide gas can also be present that can cause the water to have a foul odour.

• The loss of alkalinity during pre-treatment results in an unstable water with very little pH buffering capacity that can also be corrosive.

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Chapter 8 - Nanofiltration and Reverse Osmosis There are a variety of post treatment steps used to overcome these problems.

• Aeration, usually in a packed tower, strips carbon dioxide and hydrogen sulfide from the water and adds dissolved oxygen to the filtered water.

• Stabilisation, using sodium hydroxide, to raise the pH of the water and convert carbonic acid to bicarbonate to increase its pH buffering capacity (bicarbonate alkalinity). Sodium hydroxide added before aeration will convert carbonic acid to bicarbonate which will not be lost during aeration. This method of effectively recovering the alkalinity in the water can only be used if there are low hydrogen sulfide levels in the water.

• Hydrated lime may also be added to the product water to provide alkalinity and calcium hardness but due to its limited solubility, care must be taken with feeding arrangements to prevent blockages.

Ideally water should be treated so that a thin layer of calcium carbonate is formed on the pipe walls to provide a physical barrier against corrosion in the distribution system The Langelier Saturation Index, and other measures of calcium carbonate saturation, can be used to indicate the potential for corrosion (or calcium carbonate deposition) in distribution systems.

Disinfectants are also usually added after a membrane system to ensure any pathogens from membrane leaks are destroyed. The common disinfectants used include chlorine, monochloramine, ozone and UV radiation. A chlorine residual is usually required to be maintained in distribution systems to provide a barrier to contamination in the operation of the distribution system and to control the growth of biofilms.

RO and NF processes have the advantage of removing organic precursors which could form trihalomethanes (THM) when reacting with chlorine, and provide nutrients to support the growth of biofilms.

RO water can have a fairly unpleasant bland taste. To improve the palatability of the water, small amounts of calcium and magnesium, and possibly sodium salts, are added back, or the RO water is blended with water from a conventional surface water plant.

Causes of Membrane Failure Apart from fouling and O ring failure described above, there are other kinds of mechanical failure which are more prevalent in RO and NF membranes because of the high pressures involved.

Telescoping of the membrane involves movement of the outer layers of a spiral-wound module in the direction of the feed flow due to excessive pressure difference across the feed channel spacer. An example is shown in Figure 8.13.

Figure. 8.13. Mechanical failure of a RO membrane element due to telescoping.

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Chapter 8 - Nanofiltration and Reverse Osmosis The permeate tube can also collapse due to excessive pressure. This can be minimised by making the tube from stainless steel or by placing a stainless steel reinforcing spring inside the tube.

Mechanical failure of RO membranes is normally catastrophic, necessitating the replacement of the element.

Energy Recovery RO and NF are energy intensive processes due to the high pressures required to operate them. Most of this pressure is lost as the water passes through the semipermeable membrane to become product water but relatively little is lost in feed channels. Consequently, the pressure of the concentrate waste stream is nearly as high as the feed water pressure. If the pressure in the concentrate stream is dissipated by the concentrate control valve, the energy in this waste stream is wasted.

As the concentrate stream is at high pressure and the volumes discharged are relatively high, the amount of energy wasted is substantial. With increasing emphasis now being placed on energy conservation and reducing greenhouse gas production, energy recovery devices are being employed to reclaim the energy inherent in the concentrate stream that would otherwise be wasted. Several energy recovery devices are being used including turbines, Pelton wheels (Figure 8.14) and reverse running centrifugal pumps.

Figure. 8.14. Pelton Wheels used for energy recovery at a small RO desalination plant (left) and at a hydro power station (right).

Ninety percent of the energy in the concentrate stream can be recovered using these methods providing realistic pay back periods for the expenditure required to install the energy recovery equipment.

Concentrate Management Management of the concentrate stream from RO and NF treatment plants is one of the major issues currently facing the water industry. The volume of waste produced by many conventional water treatment plants is less than 5%. However, with RO and NF treatment processes the waste stream volume typically ranges from 15% to 50% and can be considerably higher, 50% to 70% for high salinity source water. Also, the high salinity of the concentrate stream makes it unsuitable for many forms of reuse. High salt levels can severely restrict discharge opportunities to either land or surface water. Concentrate streams from treatment plants using seawater as their feed water will have hypersaline salt concentrations (typically > 100,000 mg/L). These limitations are compounded if the waste stream contains concentrated levels of toxic substances such as arsenic, which may be the case for feed water sourced from groundwater or industry. Discussions should be held with environmental regulators in the early stages of planning RO and NF membrane plants to determine suitable options for any waste discharges.

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Chapter 8 - Nanofiltration and Reverse Osmosis Many RO plants discharge through long pipelines back into the salt water environment. This can result in increased salinity in the regions near the outfall and also lower dissolved oxygen levels. Where the discharge is to a high energy marine environment (waves and/or currents) it is less of a problem. However, where the discharge is to a bay with limited turnover and flushing, it can create a problem. Some RO plants are restricted on when and how much they can discharge.

Used cleaning solutions from RO and NF plants also have to be carefully managed as they can be highly acidic or basic, and contain detergents, surfactants, enzymes, chelating agents and disinfectants. In most cases, the volumes of cleaning solutions are small compared to the concentrate stream volume and are often combined with the concentrate. The effect of cleaning solutions on the concentrate stream, and the need for treatment of cleaning solutions must be considered before it is combined with the concentrate water.

Pressure Vessel Safety RO and NF membrane modules operate at substantial pressures. Operators need to be aware of the hazards associated with pressure vessels and implement appropriate control measures when operating and maintaining RO and NF membranes. The following factors should be considered and these control measures implemented.

• • • • • • •

Do not change the operational parameters of pressure equipment without appropriate authorisation.

Do not modify pressure vessels without appropriate authorisation.

All pressure vessels must be suitably anchored and restrained.

Depressurise pressure vessels before undertaking work on them.

Ensure pressure relief devices are regularly inspected and maintained.

Avoid subjecting pressure vessels to water hammer and excessive vibration.

Carefully follow the manufacture’s Operations and Maintenance and WHS instructions.

Terminology Unfortunately, the terminology used for RO and NF membranes can vary from the terminology used for the same components in MF and UF plants. An important example is the terms used for the pressure difference required to force water through the membrane. For NF and RO membranes this is usually referred to as Differential Pressure (and given the symbol sP) whereas for MF and UF membranes the term Transmembrane Pressure is used (abbreviated to TMP). Note also that there can be two or three terms used for the same thing (e.g. element, module, skid, train, rack) depending on different manufacture’s and operator’s preferences.

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Chapter 9 Closing Words Provision of safe drinking water whether from conventional water treatment plants or membrane treatment plants requires diligence, attention to detail and effective process monitoring.

This Practical Guide provides a sound background to the operation of MF and UF membrane filters. If the advice provided here is implemented and incorporated into the routine work practices of the operations team, the chances of producing safe drinking water will be greatly increased. An additional advantage of implementing the advice in this guide will be a considerable saving in membrane replacement costs and filter rebuilds when deterioration in filtration performance is finally noted but it is all too late to save the membrane.

The advice provided here might seem to some like more work, BUT it IS the work of operating membrane processes. Time simply must be found. Managers need to fully support operators in this to facilitate the implementation of these tasks.

Think about it? A well run plant 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.

Why not take the first steps now!

Further Reading AWWA (2011). Microfiltration and Ultrafiltration Membranes for Drinking Water, AWWA Manual M53.

Dow Water & Process Solutions (2016). DOW™ Ultrafiltration Product Manual.

USEPA (2005). Membrane filtration guidance manual. United States Environmental Protection Agency Office of Water, EPA 815-R-06-009, November 2005.

Wagner (2001). Membrane filtration handbook. Practical tips and hints.

Water Research Australia (WRA) (2015). Good practice guide to the operation of drinking water supply systems for the management of microbial risk.

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Appendix 1 Case Study in Membrane Troubleshooting Impacts of Raw Water Quality Changes on Microfiltration Plants After almost 13 years of drought, Central Victoria experienced a period of extremely high rainfall from mid2010 culminating in a major flood event in January 2011. Storages filled from record low levels to 100% full in a matter of months. The changes in the raw water quality resulting from these high rainfall events were dramatic.

• • • •

True colour increased from around 10-30 HU to over 200 HU. DOC increased from around 4 mg/L to over 11 mg/L. Iron increased from 0.2 mg/L to approximately 0.6 mg/L. Manganese increased from around 0.01mg/L to over 0.02 mg/L.

The changing raw water quality profiles for Sandhurst and McCay reservoirs for a seven month period between the beginning of August 2010 and the end of February 2011 are shown in Figures 1 to 4. This period includes the major rainfall events that had the most significant impact on storage levels in the Coliban Water catchment area.

Sandhurst reservoir supplies the Bendigo WTP, whilst McCay reservoir supplies the Castlemaine WTP.

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Figure 1.

True colour in raw water at Sandhurst and McCay reservoirs between August 2010 and February 2011.

Figure 2.

DOC in raw water at Sandhurst and McCay reservoirs between August 2010 and February 2011.

Appendix 1 - Impacts of Raw Water Quality Changes on Microfiltration Plants

Figure 3.

Iron in raw water at Sandhurst and McCay reservoirs between August 2010 and February 2011.

Figure 4.

Manganese in raw water for Sandhurst and McCay reservoirs between August 2010 and February 2011.

The impact on membrane performance was particularly significant at the Castlemaine WTP which runs pressurised polypropylene (PP) membranes. There was a noticeable decrease in permeability (Figure 5) and increase in membrane resistance (Figure 6) across all membrane skids.

Figure 5.

Membrane permeability for Castlemaine WTP skid 2.

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Appendix 1 - Impacts of Raw Water Quality Changes on Microfiltration Plants

Figure 6.

Membrane resistance for Castlemaine WTP skid 2.

Monitoring Membrane Performance In addition to the observed reduction in membrane performance, the operators identified significant fouling of the membranes during routine inspections and maintenance. When the problem membranes were removed, closer visual inspections identified a significant build-up of clay in the membrane bundle (Figure 7 top panel).

In response to this finding, the membranes were sent away for autopsy and full chemical and physical analysis.

2014

2013

2012

The autopsies identified that the bulk of the fouling was due to aluminium silicate (Figure 8). This was in turn most likely due to aluminium silicate-based clays being washed into the feedwater, during the 2011 floods.

Figure 7.

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Visual inspections of membranes sent away for autopsy. 2012 (top), 2013 (middle) and 2014 (bottom) showing a progressive reduction in surface fouling on the membrane fibres over the 3 years.

Appendix 1 - Impacts of Raw Water Quality Changes on Microfiltration Plants

Scanning electron microscope analysis in the 2012 autopsy report, indicating aluminium silicate fouling on surface of fibres.

Figure 8.

Membrane Cleaning Trials

9 8 7 6 5 4 3 2 1 0

Figure 9.

Alternative caustic clean (Na-Dss)

Normal caustic clean (Memclean C)

Alternative acid clean (ABF)

Alternative acid clean (citric)

Before clean Normal acid clean (sulfuric)

Resistance

Following on from the membrane autopsies, a series of bench scale CIP cleaning trials were carried out to determine whether the CIP cleaning regime was effective in removing the build-up of fouling on the membranes. Typical chemical CIP at Castlemaine is a sulfuric acid clean, followed by a caustic clean. It was clear from the trials that the standard CIP was not effectively removing the aluminium silicate fouling.

Therefore, alternative CIP cleans were tested in the laboratory (Figure 9). Based on the results, a full scale test of a combined caustic-sodium lauryl sulfate (Na-DSS) clean was performed. Figure 10 shows the outcome of the full-scale CIP trial, with significant improvement in resistance for all membrane skids.

After clean

Results of bench scale CIP cleaning trials, comparing normal CIP to alternative cleaning solutions.

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Appendix 1 - Impacts of Raw Water Quality Changes on Microfiltration Plants Castlemaine CMF-P Membranes Resistance 7 6 5 4

Resistance 3 2 1 0 28 Jan

17 Feb

09 Mar

29 Mar

18 Apr

08 May

28 May

17 Jun

07 Jul

27 Jul

16 Aug

Skid Number 2101 2501

Figure 10.

2201 2601

2301 2701

2401

Results of full scale CIP cleaning trial, using caustic Na-DSS to remove aluminium silicate fouling.

The data in Figure 10 indicates a continuous improvement in the membrane resistance after each CIP clean using Na-DSS. The most significant improvement is evident after the first two cleans in this example. Over time, continuing to use Na-DSS allows the membrane resistance to return to a base level.

Review of Membrane Condition and Performance Ongoing monitoring and maintenance at Castlemaine has indicated no long term damage to the membranes as a result of the 2011 floods and resulting build-up of fouling in the membranes. Recovery of the membranes required an intensive program of CIP cleans over time. However, the results have been very positive with membranes returning to close to their original performance prior to the floods.

Follow-up Autopsies Follow-up autopsies were performed in 2013 and again in 2014 (Figure 7). The 2013-2014 autopsy data indicated improvements in all parameters used to determine fouling levels on membranes.

• Loss on ignition, LOI (fixed and volatile).

• Inductively Coupled Plasma, ICP, for metals analysis.

• Total organic carbon, organics fouling.

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Appendix 1 - Impacts of Raw Water Quality Changes on Microfiltration Plants Table 1 shows results for core fibres within the membrane bundle, the most heavily fouled part of membrane.

The LOI has dramatically reduced between 2012 and 2014. Percentage volatile LOI has increased, indicating more of the inorganic (clay-based) fouling has been successfully removed. This is also reflected in ICP results which show significant improvements in inorganic concentrations over time. Significant improvements in organics removal have also been reported, with improved TOC results.

Table 1.

Results of membrane autopsies, 2012-2014, for fouling indicators LOI, metals by ICP analysis and organics by TOC analysis.

Year 2012

2013

2014

Location Top Middle Bottom Top Middle Bottom Top Middle Bottom

LOI mg/m2 90,574 78,022 93,124 51,144 5,748 59,707 5,227 6,818 2,955

LOI volatile % 14% 14% 13% 44% 33% 43% 48% 48% 46%

ICP mg/m2 3,145 5,333 5,741 1,688 524 1,985 356 343 397

TOC mg/m2 2,485 4,729 3,267 801 206 660 247 263 242

Conclusions Feedwater conditions and operational practices are unique to each facility. Sustainable operation of membranes requires a comprehensive system of feedwater quality and performance monitoring (integrity, resistance, flux, backwash troubleshooting), maintenance and record keeping, membrane inspections and membrane autopsies.

Acknowledgements This case study has been included in this Practical Guide with permission from Veolia Australia-New Zealand.

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Appendix 2 Case Study in Membrane Troubleshooting Importance of Pre-Treatment The Fairfield Membrane Bioreactor (MBR) Sewage Treatment Plant services suburbs on the fringes of Brisbane City. It is surrounded by sporting fields, housing and is located on a very small footprint within the suburb of Yeronga. It supplies approximately 40% of its effluent as recycled water to nearby properties.

Over 90% of the flows received at the plant are from domestic sources with minimal commercial or industrial flows. The plant itself was built in 2010 with commissioning completed in February 2011 due to the 2011 Brisbane flood impacting the plant during the testing period. The MBR replaced an activated sludge plant and clarifiers. Once the transfer to the new plant was complete, the primary sedimentation tank was converted to a wet weather overflow, and the clarifiers were demolished leaving behind the chlorine contact tanks and aerated digesters to be integrated into the MBR process.

The MBR treats up to 3 times average dry weather flows (ADWF) of 135 L/s comfortably with excess diverted to the old primary tank where it is stored and returned to the process when the wet weather event has passed. Fairfield receives all flows via gravity to an internal pump station which then lifts the influent to the inlet works setting the hydraulic gradient for the treatment plant.

The inlet works contain the pre-screening which consists of 3 mm Inclined Rotary Drum Screens (IRDS) for both the process and the bypass weir system for when flows exceed the plant capacity. All flows are then delivered to the bioreactor for biological processing. The mixed liquor is then distributed to the four membrane chambers in sequence. The membranes run in a cycle of Production, Backwash, Air Scour and Deaeration and results in the production of high quality effluent for discharge to the environment, and recycling. The membranes are air scoured and backwashed every cycle. To further maintain the viability of the membranes, there is an automatic weekly clean with sodium hypochlorite, a monthly manual citric acid backwash and once a year the membranes are taken off line one at a time for a 12 to 24 hour soak in a solution of sodium hypochlorite and caustic soda (for pH correction).

In early 2017, concerns were raised about the recovery of the transmembrane pressure (TMP) after backwashing and cleaning. The pressure drop across the membrane was becoming unmanageable.

Inspection of the membranes during the yearly clean (Figures 1 and 2) revealed that the membrane cassettes were severely fouled with rag (Figure 3).

Figure 1.

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The membrane cassette being removed.

Appendix 2 - Importance of Pre-Treatment

Figure 2.

The contaminated membrane cassette when first removed from its chamber.

Figure 3.

The fouled membranes being stripped for disposal.

The contamination of the cassettes was concerning, and an investigation was raised to determine the source and fix the problem before new membranes could be delivered. The investigations identified the inlet works and the screening to be a likely cause. Unfortunately, drum screen skirts that surround the drums were not able to be sourced in a timely manner. Therefore, these were replaced with temporary skirts manufactured in the workshop out of non-standard rubber not designed for the operating environment. Once installed, it was assumed that they had been replaced, which led to the belief that the screens were doing their job of removing the trash. In fact, the temporary skirts failed and the trash had been flowing into the MBR. This meant rags had bypassed the screens and made it through to the membrane chamber and this led to the increased deterioration of the membranes.

The correct skirts were installed, and the problem corrected. However, due to the volume that had gone through undetected, the membranes still needed to be replaced.

It did not matter how many cleans or soaks were applied to the membranes, the impact of the rags within the cassettes was just too great. The weight of the rags was pulling the membrane straws down in their guides (Figure 4) and they were getting bent, kinked and distorted, reducing their ability to produce product water from the process.

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Appendix 2 - Importance of Pre-Treatment

Figure 4.

The membrane cassette in the chamber before removal. Note that there are no membrane straws protruding above the cassette.

In early April 2018, turbidity breakthrough was observed. This was traced back initially to one membrane chamber but as time progressed, all membrane chambers became involved. As delivery date for the new membranes approached, the old membranes began to deteriorate significantly. A portion of the influent had to be tankered off site because the plant could no longer process the design capacity. Plant operation went from day work to 24/7. This led to significantly increased operational costs and the new membranes were still four weeks away.

The new membrane installation (Figure 5) went almost smoothly. The compaction within the cassettes had caused structural cracking of the cassette framework, adding to the refurbishment time.

Figure 5.

New membranes prior to installation into the cassette.

The lesson learned after this event is that screens are everything when operating an MBR STP.

Acknowledgements This case study has been included in this Practical Guide with permission from the author Bill Collie and Urban Utilities in Queensland.

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Other WIOA Publications Practical Guides to Water Treatment.

• Practical Guide to Understanding and Managing Surface Water Catchments.

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

• Practical Guide to the Operation and Optimisation of Media Filters.

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

• Practical Guide to the Operation and Optimisation of Distribution Systems.

Practical Guides to Wastewater Treatment.

• Practical Guide to Odour Control in Sewage Transport Systems.

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

Disclaimer This document does not claim to be a definitive statement on the management of membrane filter systems, but rather aims to create awareness of what is possible, what might be done and indeed what has been done.

However, the authors, reviewers and the organisations supporting the production of this Practical Guide 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.

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Water Industry Operators Association of Australia (WIOA) PO Box 6012, Shepparton 3632 Phone: 03 5821 6744 Fax: 03 5821 6033

www.wioa.org.au