Chapter 14: Treatment processes, filtration and adsorption

14.4 Membrane filtration

14.4.1 Introduction

This section aims to provide the reader with a general understanding of the issues related to membrane filtration for drinking-water treatment in New Zealand, and covers:

the history and current status of the technology

the fundamentals of microfiltration (MF) and ultrafiltration (UF) for drinking-water applications. Nanofiltration and reverse osmosis (NF and RO) are discussed briefly

the fundamentals of membrane filtration operations for drinking-water quality management.

The application of membrane filtration for drinking-water applications has increased markedly in recent years, with a membrane option considered for most water treatment applications. The increase in uptake has been driven by a number of factors, from lowering unit capital and operating costs, to the emergence of low-pressure membrane technology (reducing power demands), and a greater emphasis on correct pretreatment selection. In addition, advantages offered by advanced materials and low footprint designs have given membrane options additional weight when compared with more traditional treatment approaches.
These recent developments have been aided by the emergence of further, legitimate evidence supporting membrane filtration as a secure means to eliminate pathogenic organisms from the water supply, in particular, the protozoal species Cryptosporidium and Giardia.
For the Drinking-water Standards for New Zealand 2005, revised 2008 (DWSNZ), membrane systems may attain log credits in accordance with the system validation. So far most MF plants in New Zealand have been assigned 4 protozoal log credits, however, in special circumstances, this may be as high as 5, or possibly even higher.

14.4.2 Current experience in New Zealand and overseas

As at April 2005 five membrane plants exist in New Zealand for drinking-water treatment. These vary in type and capacity. All are of the UF or MF genre and have been designed and installed since 1999. A number of upgrades and projects are ongoing that may include membrane technology. At present two principal suppliers cover the market in New Zealand; additional suppliers will enter the market in the future.
Overseas, the number of plants adopting membrane technology is increasing, particularly in US, Europe and Asia. In the UK, for example, membranes have been adopted at various water treatment plants to meet the Cryptosporidium Regulations (1999). The largest membrane plant in Europe was commissioned in 2001, the Clay Lane WTP (160 ML/d). In 2003, Invercannie WTP (80 ML/d) in Scotland was commissioned to meet Scottish Executive Cryptosporidium Regulations.
An interesting application of membrane filtration at the Méry-sur-Oise water treatment plant in Paris was described by Cotte et al (2005). They replaced coagulation, filtration, ozonation, biological granular activated carbon filtration with clarification, ozonation, biological dual media filtration, cartridge filtration, nanofiltration and UV disinfection. Dissolved organic carbon in the distribution system fell from about 1.8 mg/L to 0.75 mg/L, and biodegradable dissolved organic carbon fell from about 0.6 mg/L to 0.1 mg/L. This enabled the chlorine dose to be cut back, which combined with the lower organic matter, reduced the trihalomethane content from about 0.2 mg/L to 0.008 mg/L; bacterial numbers were significantly lower too.

14.4.3 Fundamentals of membrane filtration

There are four principal classes of membrane filtration that apply to drinking-water treatment:

microfiltration (MF)

ultrafiltration (UF)

nanofiltration (NF)

reverse osmosis (RO).
Of these, MF and UF are most commonly specified for drinking-water applications, with the five existing plants in New Zealand comprising these technologies. RO has been used in industry and household supplies.

Microfiltration and ultrafiltration

MF and UF are characterised by their ability to remove suspended or colloidal particles via a sieving mechanism based on the size of the membrane pores in the membrane, relative to that of the particulate matter. Pretreatment (mainly coagulation, with or without sedimentation) is needed to remove colour and very fine particles. MF and UF are often collectively called low pressure membrane processes. They operate in the 3–50 psi range, or -3–12 psi if using a vacuum system.
Each membrane has a distribution of pores, which will vary according to the membrane material and manufacturing process. There are two ways to represent pore size:

nominal, the average pore size

absolute, the maximum pore size.
MF membranes are generally considered to have a pore range of 0.1–0.2 m (nominally 0.1 m), although there are exceptions, with some MF membranes marketed with pore sizes up to 10 m. Without pretreatment, MF membranes will remove most protozoa, many bacteria, but very few viruses.
For UF, pore sizes generally range from 0.01–0.05 m (nominally 0.01 m) or less. With UF, classification in terms of pore size becomes inappropriate, due to the other mechanisms/ phenomena that take place at the membrane surface. In terms of pore size, the lower cut off for a UF membrane is approximately 0.005 m. Without pretreatment, UF membranes will remove probably all protozoa, most bacteria, and many viruses (consistently greater than 3 log removals).
Some UF membranes are categorised in terms of their molecular weight cut-off (MWCO) rather than a particular pore size. The concept of MWCO, expressed in Daltons (a unit of mass) is a measure of the removal characteristic of a membrane in terms of atomic weight (or mass) rather than size. Therefore, UF membranes with a specified MWCO are presumed to act as a barrier to compounds or molecules with a molecular weight exceeding the MWCO.
Typical MWCO levels for UF membranes range from 10,000 to 500,000 Daltons, with most UF membranes for drinking-water treatment at around 100,000 MWCO. Note that these are large molecules. UF membranes for drinking-water treatment are also characterised according to pore size with respect to microbial and particulate removal capability.
A key distinction when considering MF or UF technology for a particular application is whether to select a pressure or submerged configuration. The use of hollow-fibre membranes is normally selected, and this brief explanation assumes the use of this type of membrane. Hollow-fibres are bundled longitudinally and either encased in a pressure vessel or submerged in a basin, or cell.
Modules are contained in housings, or pressure vessels. Operating pressures for such systems vary from 20–280 kPa. Most applications require designated feed pumps to generate the required operating pressure, although some may be operated under gravity if sufficient head can be developed.
Most systems are referred to as dead-end in as much as all contamination material is trapped on the membrane surface. This is as opposed to generating a continuous reject stream.
While all hollow-fibre systems require pressure as the fundamental driving force, a submerged (or vacuum) driven system is distinguished by its use of negative pressure and is significantly different in terms of design and configuration. Unlike pressure systems, where each membrane module incorporates a pressure vessel, submerged systems use hollow-fibre modules that are driven under vacuum, immersed in an open tank or cell. While the ends are fixed, the lengths of the hollow-fibres are exposed to the feed water in the cell and move freely.
Due to the feed water being contained in an open tank, the outside of the fibres cannot be pressurised above the static head in the cell. Therefore a vacuum, approximately -20 to -90 kPa, is induced at the inside of the fibre walls, where it is filtered outside-in to the lumen (the centre or bore of a hollow-fibre membrane). By design, submerged systems cannot be operated via gravity alone (a common misconception), or in an inside-out mode of filtration. In some circumstances they may be operated by siphon.
Figure 14.2 shows a typical schematic of a submerged membrane system. In this arrangement the vacuum is supplied from the filtrate pump.
Figure 14.2: Typical submerged membrane system

Nanofiltration and reverse osmosis

NF and RO comprise a class of membrane processes that provide a higher degree of removal of contaminants compared with MF/UF. For example, NF systems can remove particles as small as 0.001 to 0.002 m (microns). Although NF and RO can remove nearly all bacteria and viruses, they are specified less frequently in drinking-water applications, mainly due to the much greater pressure requirements, eg, 800–1000 psi (say 5500 to 6900 kPa or kN/m²), or even higher for RO where up to 10,000 kPa can be needed in desalination plants.
Removal of viruses by RO membranes may vary significantly and is a function of the membrane itself as well as its condition and the integrity of the entire system, including seals. Removals ranging from 2.7 to more than 6.8 logs, depending on the type of RO membrane, have been reported at bench scale using MS2 bacteriophage as the model virus, and the selection of membranes is an important factor in determining virus removal. Although RO constitutes an excellent barrier to micro-organisms, the maintenance of that barrier depends on the integrity of the system. Breaches of integrity in the membranes or the O-rings could lead to the passage of pathogens into the process water and must be monitored by integrity testing. Effective methods to measure the integrity of RO membranes should be used to achieve target removals. Currently, conductivity measurements are used, but the sensitivity limits their application to about 2 logs of removal. As bacteria have been shown to traverse through membrane defects, membranes cannot be considered as completely effective for disinfection and are commonly succeeded by a disinfection step WHO (2011a).
Nanofiltration is a high-pressure membrane process that has been used traditionally as a softening process to remove hardness ions. Generally, NF membranes reject divalent ions (eg, Mg²⁺, Ca²⁺), but pass monovalent ions (eg, Na⁺, Cl^-). Recently, NF has been used more extensively for removal of DBP precursors and colour. Although NF processes remove nearly all turbidity in feed water, they cannot be used for turbidity removal in the same manner as MF and UF due to the smaller pore sizes. Smaller pore size makes NF membranes more prone to fouling. The application of NF for surface waters is generally not accomplished without extensive pretreatment for particle removal.

NF/RO remove some dissolved contaminants, as represented by measurements of total dissolved solids (TDS) or conductivity (S/cm, or mS/m). The typical range of MWCO is less than 100 Daltons for RO membranes, and between 200 and 1000 for NF membranes. RO is used sometimes for removal of chemical determinands not removed by existing processes. For example, it can be used to remove arsenic. Some point-of-use (POU) units use NF or RO.

As the majority of drinking-water applications involve MF/UF, these NF and RO technologies are not elaborated upon in the remainder of this section.

14.4.4 Membrane selection

The task of selection of the appropriate membrane will involve consideration of:

design targets, final water quality, guarantees

commercial, capital (CAPEX), operating (OPEX) and whole-life costs (WLC)

site-specific conditions, any process limiting criteria.

One of the major issues to resolve in drinking-water applications is whether to use MF or UF. An outline of their characteristics was given earlier in section 14.4.
A key issue is virus rejection. Although the current DWSNZ do not impose a requirement for viruses, should the water supplier wish to have a higher level of security against virus infiltration using membrane technology, UF should be considered. Future editions of the DWSNZ are likely to address viruses. However, it should be emphasised that not all UF membranes are capable of virus rejection. Studies undertaken in the US in the mid 1990s demonstrated that membranes with a MWCO of 500,000 Daltons were less efficient at virus removal than another with a MWCO of 100,000. The designer should refer to the UF membrane manufacturer for data on their specific virus sized challenge data. It is not advisable to rely on membranes alone to provide primary virus protection; post-chlorination is recommended.

The decision to go with MF or UF technology can be marginal in some cases. In essence a higher degree of separation is returned by UF yet these can be counterweighted with risks. For all applications a risk assessment and cost/benefit analysis should be developed as an aid to the selection process.

Membrane materials

Material properties directly impact performance. The main properties are shown in Table 14.1. Features such as porosity, pore size and shape, are surface roughness are important. Membrane materials can be manufactured in different geometrical configurations, which are then incorporated into a membrane module. Commercially available configurations include hollow fibre, spiral wound, tubular and plate-and-frame. Hollow fibre membranes are the most common form used in community water supplies. The most common membrane materials encountered in drinking-water treatment are:

polypropylene (PP)

polyvinylidene fluoride (PVDF)

polyethersulphone (PES).

Polysulfone and cellulose acetate have been used too.
Table 14.1: Properties of typical membrane materials

	Unit	Polyethersulphone (PES)	Polyvinylidene fluoride (PVDF)	Polypropylene (PP)
Hydrophobicity (water hating)	–	Medium	Medium	High
pH range	–	1 to 13	2 to 10	1 to 13
Chlorine tolerance	–	Good	Excellent	None
Temperature tolerance	C	High	High	High

14.4.5 Membrane plant operations

The operation modes of a MF/UF system comprise:

service: time which the system is online and generating filtrate

backwash: time which the membrane requires washing to remove entrapped particles and solids. Wastewater is produced during this operation. Operation restores clean head, although not completely

clean in place (CIP): time which the membrane system requires chemicals applied to eliminate foulants not removed by backwashing. For example, natural organic matter or micro-organisms or biofilms adsorbed on the membrane; excess cationic polyelectrolytes need to be controlled carefully too. CIP restores permeability and resistance, although not completely due to some irreversible fouling

offline or out of service: time which backwashing or CIP is taking place, or membrane integrity testing/maintenance procedures are being carried out. Some membrane systems remain in place while back-pulsing.

Service mode

During service the membranes are pressurised, either by positive pressure or vacuum, and generate filtrate. The normal mode of operation is to maintain the flux by increasing the pressure as the filter blocks. In the constant pressure mode, the filtrate output drops as the pores block. Typically the membrane system shall be monitored for the following during the service mode:

filtrate turbidity or particle count*

filtrate flow measurement (for measuring plant recovery)¹*

transmembrane pressure² (TMP)*

cell level (protects membranes)

filtrate temperature (computed in TMP)

filtrate pump speed/frequency (if VSD operated).

* Key operating parameter.

Depending on the degree of automatic control, during this mode the membrane plant requires little operator attention other than observation of key operating parameters. Normally a PLC based control system is provided, with an operator interface to observe key operating variables.
The service period should be of the order 98–99 percent of the operating day, accounting for regular backwashing.
Should the water quality deteriorate, for example in the case of a membrane plant fed from surface water during a flood event, or a significant decrease in temperature, this should be detected by the upstream instrumentation. This should then be communicated to the operations staff in the form of automatic alarms or flags to adjust the process conditions to maintain performance.
Submerged membrane systems are capable of handling high swings in solids loading or turbidity. The level and duration of such events will impact the permeability of the membrane. Membrane plants fed from surface waters may experience sudden changes in influent raw water quality, particularly in terms of colour, turbidity, organic material and metals. A change in pH may also be experienced depending on the nature of the catchment geology. Should the raw water quality be outside the design criteria of the system, the system may still cope as long as the appropriate operator actions are undertaken. These actions may comprise one or more of the following:

visual checking of membrane outer surface/colouring. This is straightforward in submerged systems as membranes are exposed

checking raw water quality and adjusting chemical conditioning if necessary to suit conditions

checking upstream water quality instrumentation, ensuring still within calibration etc

checking correct coagulant and/or coagulant aid dose rate. Dose rates should be established at commissioning/performance testing across the range in flows and qualities

checking the coagulation pH is optimised

checking hydraulic loading (flow) to membrane units, verification that flux is within design limits. The flux rate will reduce as fouling increases and the membrane manufacturer’s minimum flux should be noted

checking backwash and air scour flows and pressures are set correctly

checking clean washwater quality/volume to ensure the correct quality and quantity of water is being used. Poor quality washwater should not be used.
Regularity of backwashing and/or CIP may increase temporarily to remove the additional contaminants from the membrane.

Backwashing and clean-in-place (CIP)

During filtration mode, particles and materials that are too large to pass through the membrane pores stay in the raw water or stick to the surface of the membrane. The latter process is known as fouling. Fouling progresses as the membrane system progresses in the service mode and results in decreasing permeability. MF/UF membrane systems typically employ three separate cleaning strategies to alleviate membrane fouling, and these can be automated:

air scour

backwash (water)

combined air scour and backwash

clean-in-place (CIP).
In the context of a submerged system, air scour is employed continuously or intermittently in each tank. Blowers supply air back to the membrane tank. The bubbles physically agitate the membrane fibres and help to both displace debris that has collected on the membrane surface and to keep the water in the tank mixed.
On a regular basis (can be as frequently as every 15 minutes or even longer than 60 minutes), a backwash is conducted in which filtered water is pumped back through the membranes (inside out) to clean the membrane pores. This action is brief, normally lasting less than one minute. Membrane manufacturers may opt to use a low-concentration chemical solution, such as a dilute chlorine solution, in the backwash water to assist in the cleaning of the membrane pores. This may be favoured to arrest development of organic fouling. The backwash action can be provided by the permeate pumps on some multiple-unit systems through automatic valve switching.
Membranes require periodic chemical cleaning to remove fouling materials that are not displaced by backwashing. The term for the cleaning process is clean-in-place (CIP), since the membrane system(s) are shut down and the membranes are not removed from their locations. During CIP, the membranes are subject to intimate chemical contact through a series of operations. The operations may vary in their degree of automation depending on the nature of the system design. The different chemicals that may be used are proprietary agents, acids, alkalis, oxidants, chlorine and detergents, depending on the composition of the membrane fibres and the nature of the foulant. The membrane manufacturer’s guidance should be followed.
The volume of chemical wastewater for CIP operations can be of the order 3 percent of the raw water flow rate during periods of high organic or metal salts loading on the membranes. The required frequency of this CIP will vary based on the raw water characteristics, operating flux³ and the particular membrane material. A frequency of approximately once per month or longer should be attainable. The chemical cleaning solution in the tanks may be reusable if its strength does not deteriorate too significantly, but it is normally neutralised and discarded to waste.
An example of a CIP record sheet applicable to a MF system is presented in Table 14.4.

Types of integrity testing

Integrity testing is required to ensure continuous and repeatable security that the membrane system is performing within its specifications. Integrity testing is also discussed in Chapter 8: Protozoa Compliance, sections 8.4.3.5, 8.6.2.3 and 8.6.2.4.
At present there are two classes of integrity testing:

direct integrity testing (DIT): a physical test applied to a membrane unit in order to identify and/or isolate integrity breaches

continuous indirect integrity monitoring (CIIM): monitoring some aspect of filtrate water quality (ie, turbidity, particle counts) that is indicative of the removal of particulate matter at a frequency of at least once every 15 minutes.
Currently there are two general types of DIT that are commercially available for use with membrane filtration plants:

pressure-based tests (MF/UF)

molecular marker-based tests (NF/RO).
The test used for a particular system depends upon the type of membrane filtration, target organism(s) and test sensitivity. The DWSNZ specify the compliance requirements within section 5.11 Membrane filtration – treatment compliance criteria, which should be referred to.
In addition to complying with the DWSNZ, the DIT method must be compatible with the particular membrane system. The membrane supplier should confirm whether the system is compliant or non-compliant.
Pressure (vacuum decay) tests are compatible with all the various types of membrane filtration that qualify under DWSNZ. The equipment required to conduct these tests is typically supplied with the proprietary membrane system. However, some types of DIT may not be available from a particular supplier. The test selection may also take account of site or system specific factors. For further details refer to the Membrane Filtration Guidance Manual (USEPA 2005) and the LT2ESWTR Final Rule (USEPA 2006). Membrane filtration is also discussed in Chapter 14 of the review draft LT2ESWTR Toolbox Guidance Manual (USEPA 2009).

Control limits and transgression action guidance

The DWSNZ recommend upper control limits (UCL) set at, as a general rule, two-thirds of the appropriate compliance criterion, or MAV. For example, a membrane filtration system validated and certified to provide 4 log removal of protozoa requires an outlet (filtrate) turbidity of 0.10 NTU. Thus the control limit may be set at 0.07 NTU (and the outlet turbidity must always be less than the inlet turbidity). Turbidimeters must be specified to ensure resolution to this level. The water supplier may elect to specify upper and lower control limits (LCL) to give additional performance control and foresight of deteriorating performance.
Should the control limit be met for a period of 15 minutes, and assuming the backwash TMP trigger has not been met, it is recommended a backwash be initiated manually on the offending units. Should a CIIM instrument be located on each unit’s filtrate, the offending unit should be backwashed. If a common instrument is used, each unit that the instrument monitors should be backwashed in a controlled sequence. Should the filtrate turbidity remain in excess of the control limit after the backwash, the unit must be taken out of service for direct integrity testing. This is preventive action as a transgression has not occurred (ie, not greater than 0.10 NTU). Note turbidity can exceed 0.10 NTU, but pass DIT.

Setting the direct integrity test (DIT) test limit

The DIT is the principal means to assess membrane integrity where protozoal removal is the prime function of the membrane system. Test parameters and results can be linked to the particular treatment objectives to give a quantifiable and objective assessment of system performance.
Resolution, sensitivity and frequency needs for DIT are specified in the DWSNZ. For example, the resolution must be that a 3 m particle (equivalent to the smallest Cryptosporidium oocyst) generates a test response. Thus, for pressure based DITs, the applied pressure must be great enough to overcome the capillary static forces that hold water in a breach of 3 m in diameter in a fully wetted membrane thereby allowing air to escape (through a Cryptosporidium sized hole) and thus allowing the loss of air to be detected. A similar concept is applied to marker based tests.

Sensitivity is defined by the particular system performance validation and characteristics of the membrane system itself. The sensitivity must exceed that required to achieve the log credit. For example, 4 log credits must prove repeated 99.99 percent removal of protozoan species tested for. No generic limits can therefore be set.

The DIT limit must be defined and certified before the system enters service. As a minimum the DIT must quote the test threshold specified by the membrane manufacturer. The DIT must be carried out once every 24 hours of operating time.

Interpreting continuous indirect integrity monitoring (CIIM) results

CIIM results are intended to provide an indication of system integrity between direct integrity test applications. The results are compared with the control limit that represents a potential integrity breach.
Caution should be noted as false negative and false positive results are possible with these methods. For example, false positive results may be created by the use of an air scour of the membrane surface as part of the backwash sequence and this may create artificially high results after the unit is returned to service. This may be overcome by running to waste until this known condition is resolved. In practice this may be a period of minutes per unit. Once the performance stabilises below the control limit, the unit is returned to feed forward service. This is more easily accommodated in multiple unit systems than systems with fewer units.
False negative results that arise may be more common than false positives when using CIIM. For example, turbidimeters are less sensitive than monitoring techniques used for direct integrity testing. This may be overcome by using more sensitive instrumentation, eg, laser turbidimetry, although this is more costly. The water supplier could increase the number of CIIM instruments. Each option should be evaluated for each application.

Membrane repair/replacement

This applies to any component of the membrane unit that may allow an integrity breach should it fail, not just the membrane itself. The purpose of repairing the membrane is to prevent integrity breaches that may lead to performance transgressions.
The repair should take place whenever an integrity breach is detected, using DIT or CIIM methods. The source of the integrity breach, for example broken membrane fibre(s), should be located and repaired. The validation of the repair must be by a subsequent DIT, meeting the DIT limit for the system, to prove unit integrity has been restored before the membrane is returned to service.
As outlined previously, depending on the control limit set, integrity breaches leading to membrane repair/DIT may not necessarily represent a transgression. Proactive maintenance can therefore credit the system with maintaining compliance. The repair itself may take the form, for example, of pinning the fibre hole in each end of the membrane module. Thus two pins are inserted per fibre. Should a membrane module be subject to multiple repairs the operator may elect to replace the module itself, or insert a new module immediately to return the unit to service. Here, the repair may take place at the operator’s leisure. This may be more cost-effective in terms minimising plant outage.
The DWSNZ require direct integrity testing in accordance with section 5.11.1.

General operations guidance

The operators should generate plant logs based on the plant operation and maintenance manuals (O&Ms). An example of a monitoring datasheet is presented in Table 14.3. In addition to data recorded online, for example by a SCADA system, this provides essential evidence of operations and plant performance and enables operators to become more knowledgeable of the features of the membrane plant.
The operator should review operational data regularly, such as the TMP, flow rates and the outlet turbidity. The operational staff should aim to detect any anomalies in the data and investigate them. This may lead to early detection of a problem. DIT data should be reviewed continually.
After a CIP, the operational staff should calculate the permeability, record the TMP immediately before and after in order to evaluate the magnitude of fouling that is removed, to determine how frequently cleaning is required and to estimate the long-term impact on membrane life. This information should be recorded in the operation log. Supplier consultation may be required should abnormal results be observed.
The outer colouring of the membrane fibres may be observed on submerged MF systems, however, little is visible on pressure systems. Should membrane change be required prematurely, the membrane supplier may offer biopsy services to help establish the nature of the failure. The end user should note the membrane supplier’s warranty conditions as these are generally non-negotiable.
The calibration of any online instruments, for example turbidimeters, pH meters, should be performed on a weekly basis. If particle counters are used, calibrate in line with manufacturer’s recommendations.
Other operational checks should be conducted following the requirements of the validation, the DWSNZ and the manufacturer’s instructions.
Table 14.2: Typical design/operating criteria for MF/UF systems (guidance only)

Criteria	Typical range for drinking-water application using MF/UF technology
Flux rate – flow per unit of membrane filter area (recovery)	@ <2 NTU feed 50–90 L/m^2/h (94–98%) @ 2–10 NTU feed 40–60 L/m^2/h (92–94%) @ >10 NTU feed <50 L/m^2/h (90% min)
Recovery	95–98%
Flow control, maximum rate of change per minute	1.5–5%
Backwash and chemical clean-in-place (CIP) intervals	Backwash: 15–40 minutes CIP: 30–40 days
Membrane life	>5 years

Membrane plant hydraulics

The membrane system should meet the hydraulic characteristics and constraints of the site and/or interfacing plant. This applies to both gravity (submerged) and pressure systems.
Where submerged systems are being designed, caution should be taken with respect to upstream buffering and lags at varied design flows. A hydraulic model should be constructed across the design envelope, for example, to size channels correctly and to determine if overflows are necessary.

Pressure systems are pumped and this design should default to the membrane supplier. Generally it is recommended that head is broken (filtrate side) from the membrane system, as opposed to feeding the next stage process directly. This avoids possible interference with the downstream process in terms of pressure variance or loss.

The operation sequencing of membrane systems should be considered carefully in terms of operation dynamics as this can be complex. The modes of operation, for example filtration, backwash, CIP, must be balanced to ensure operation risk is low. It is normal to provide standby membrane plant to securely meet the nominal design output. For example, for a submerged system, two standby cells may accompany an array of six operating cells.
Table 14.3: Data log and check sheet

Unit model			Site
Backwash timer setting
Date
Time
Hours run
TMP^(1) (kPa)
Temperature (°C)
pH
Feed turbidity (NTU)
Filtrate turbidity (NTU)
Feed FFI (x 10^12 m^-2)
Filtrate flow (L/h)
Feed flow totaliser (m^3)
Filtrate flow totaliser (m^3)

1 TMP = feed pressure – filtrate pressure.

Daily pressure decay test

Perform approximately five minutes after backwash.

Filtrate pressure
At 0 minutes	________ kPa
At 2 minutes	________ kPa
At 4 minutes	________ kPa

Table 14.4: Chemical cleaning log sheet – provided by membrane supplier

Chemical cleaning report
Client		Location
Membrane model		Application
Today’s date		Date of last cleaning

Parameters	Before cleaning	After cleaning
Flow rate
Backwash interval
TMP
Feed temperature
pH
TMP rise between backwash

Chemical	% conc	pH	Conductivity	Temperature

Cleaning scheme	Duration (minutes)
Recirculation
Aeration
Soak

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