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ELSEVIER Desalination 110 (1997) 49-58D E S L I N T I O N

M e m b r a n e p r et re a tm e n t o f r ev e r s e o s m o s i sL o n g t e r m e x p e r i e n c e o n d i f f i c u l t w a t e r sBruce Durham

M em cor Ltd. , De rby Road, W irksworth, Derbyshire D E4 4BG, UK.Tel.: +44-1629-823811, Fax : +44-1629-825169Received 18 February 1997; accepted 24 February 1997

A b s t r a c tThe shortage of fresh water supplies is one of the main threats to sustainable development. Membranetechnologies such as reverse osmosis (RO) have been used for over twenty years to treat sea water (SWRO) andwaste waters. The applications have been limited due to the sensitivity of RO membranes to fouling and theability of convent ional pretreatment technologies. Pretreatment represents a real problem in the SWRO process.It represents 22.97% of the total product water costs at the Doha Reverse Osmosis Plant producing 4546 m3/d[1]. The development of backwashing continuous microfiltration (CMF) membranes enables RO to desalinatevariable feed waters including turbid waste waters. Completed projects include the desalination and reuse of

brackish secondary sewage and eutrophic surface water for industry and irrigation. These installations haveresulted in a 40% reduction in the sizing of the RO plant installed, a 60-90% reduction in chemical used andeffluent costs for the conventional deionisers, as well as solving local environmental issues through thereduction in potable water use and waste discharge.K e y w o r d s : Membranes; Pretrealment; Reverse osmosis

I . I n t r o d u c t i o nThe global water crisis affects over 50countries and yet water is seldom reused. The

concept of installing Kidneys for Cities waspromoted as a Best Practice sustainablesolution at the UN Habitat II conference inIstanbul in 1996. Long term operatingexperience proves the viability of micro-filtration pretreatment of RO for desalinationof poor quality waters that are variable in

loading, contain large amounts of suspendedsolids including algae and plankton blooms.

The global demand for potable water is outof balance and expanding at a frighteningrate especially when you consider that 50% ofthe world's population live in cities, only 10%of potable water is used for drinking and theworld's population is forecast to double in thenext 50 years.Through the development over the last 15years of 'enabling' technologies the use of

Presented at the International Symposium on Pretreatment of Feedwater for Reverse Osmosis Desalination Plants,March 31 - April 2, 1997, Kuwait0011-9164/97/$17.00 Copyright 1997 Published by Elsevier Science B.V. All rights reserved.P l l SO011-9164(97)00083-0

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50 B. Durham / Desalination 110 1997) 49- 58poor quality water resources is now not onlypossible but has also been widelydemonstrated to be reliable and safe as well aseconomically justified. Memcor has been atthe forefront of such developments and hasproven the ability of continuous micro-filtration in the recovery of primary andsecondary sewage, industrial waste waters andsurface waters with turbidities rising to 700NTU for direct use or for further treatmentwith reverse osmosis, nanofiltration, ionexchange and other downstream processes.Extensive use of poor quality waterresources (such as contaminated/turbid watersand sewage), hitherto considered impractical,is rapidly becoming a necessity in order tosecure resources for the future. The treatmentof such water resources for potable supplies,for aquifer recharge, and for direct use inindustry offers the benefits of cost savingsthrough avoiding the need to installunnecessary new potable and sewerageinfrastructure. In addition, planning cyclescan be significantly shortened if marginalwater sources or sewage are used as a waterfeedstock for water capacity expansions.Many t radi t ional water t reatmenttechnologies are designed to operate understeady-state conditions. Rapid changes insuspended or dissolved solids loading, achange in the nature of the solids from fineinorganic particles to algae create havocmaking the design engineers and operatorsjob very difficult. Sources containing theseproblems, such as open sea water intakes, arecomplex and variable waters to treat, and canalso include lake, river or waste waters. Thepretreatment of RO systems is perhaps themost critical part of a system design whichwhen incorrect has resulted in terminalfailure of RO membr anes from irreversiblefou l ing . Microf i l t ra t ion p re t rea tmentproduces a stable feed water for RO. Thereare many systems installed that haveovercome the challenge of maintainingproduction and being able to reliably treathigh and variable dirt loads at a competitivecost.

2 . M e m c o r c o n t i n u o u s m i c r o f i l t r a t i o n(CMF)CMF is a barrier filtration technologywhich provides high quality treated water andprimary disinfection via an integrity-testablemicroporous membrane.There are two key elements to the thirdgeneration CMF process - the 0.2 ~tm poresize membranes and Memcor's patented airbackwash system. The membranes are hollowfibre with an internal diameter ofapproximately 0.3 mm. They are assembledinto serviceable sub-modules each containingup to 20,000 metres of fibre length. Filtrationtakes place from outside to inside and in dead

end mode ( direct filtration ). This, coupledwith the unique air backwashing process,offers a number of benefits:- Very high feed solids can beaccommodated with no effect on thesystem performance other than anautomatic change in the frequency of thebackwash.- As a result, a consistent filtrate flowrate andquality is continuously sustained, with anautomatic process, irrespective of thequality of the feed water (SDI

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B. Durham /Desalin ation 110 1997) 49-58 51the membrane, preventing grow-throughand hence avoiding the need forprechlorination of the backwash liquorwhich is normally a prerequisite ofconventional liquid backwashing syste ms.The Memcor (air) backwash was the mostsingularly effective means of controllingfouling [2] during the Orange CountyWater District, Water Factory 21 membranepretreatment evaluation from June 93 toOctober 95.The CMF process offers a built-in fully-automatic integrity testing process with theability to measure bacterial log reductionvalue to a sensitivity of 6 logs. This integrity

test utilises the existing air backwash facilityand the membrane bubble point and is afeature unique to microfiltration i.e. theprocess cannot be used with ultrafiltrationbecause of the very high pressure whichwould be requir ed for the test. It is a highlyreliable Quality Assurance measure for theCMF plant and provides security and peace ofmind for operators of large scale plant such asRO pretreatment. It must be r ememb ered thatparticle counting is a less reliable method ofchecking system integrity and correspondingfiltrate quality and should only be treated as afinal Quality Control check of filtrate quality.Recent independent studies demonstrated thatthe integrity test detected a 0.6 mm holeintentionally punched in one of 22,400 fibres(20 m 2) [3]. This level of sensitivity is muchhigher than particle size counting.Following the automatic membrane test,individual membrane sub-modules detected ashaving a degree of integrity loss can be easilyisolated using valves built into the CMF Blockmanifold system. Isolated modules can thenbe repaired and returned to service thusdramatically extending the service life, whichis now 5 years, depending upon theapplication. Existing operating plant using theMemcor M10 system have alreadydemonstrated a life in excess of 5 years onhighly flashy surface waters.Modules are manifolded into CMF Units of90 modules and these Units are combined

into trains to meet the overall flowrequirements which range from smallindustrial plants producing 20 m3/d to largemunicipal plants.The CMF system is fully automatic, has selfdiagnostic data logging for unmanned remoteoperation and Memcor has developed ahighly sophisticated design, as well as processoptimisation tools to ensure that the plantcontinues to operate at maximum efficiencyand provide high quality performancethroughout its service life.Cleaning is an automatic in-line processand requires very low usage of chemicalssince cleaning chemicals are automatically re-used 5-20 t imes (depending uponapplication).The following projects highlight some ofthe operating experiences on largeinstallations.

3 . E r a r i n g P o w e r S t a t i o n p r o j e c tThe sewage reuse project at Eraring is agood example of two major utilities, PacificPower and Hunter Water Corporation,working together to the benefit of the

environment and the community on a costeffect ive project.Eraring Power Station is one of the largestin Australia. This 4 660 MW coal firedstation provides 25% of the power to the stateof New South Wales. It was construc ted in1980 at a cost of A$ 2 billion. The powerstation has a current life of 30 years and, untilrecently, consumed up to 4 megalitres per day(mlpd), 168 m3/h of potable water.The power station is located on the shore ofLake Macquarie, a tidal saline lake near thecoast and approximately one hour fromSydney. This is an area of rapid development.Hunter Water Corporation (HWC) are thewater and waste water utility supplying thepower station. The 4 mlpd of potable waterneeded by the power station is equivalent to 1years growth in the potable water demand tothe local community and is equivalent to thewater supply to 25,000 homes. HWC were

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5 2 B. Durha m/De salin ation 110 1997) 49-5 8E d g e w o r t h

S T P

E R R I N GP O W E RST TION

C R E E KS T P

TorontoS T P

.~

L a k e M a c q u a r i e

T o o c e a nd i s c h a r g e

i= B e l m o n tE x t e n d e d

O c e a n O u t f a l l

Hunter Sewage Project Sewer wes t side of Lake M acquar ie Bui ld a16,000 EP STP at Dora Creek Ouffal l under Lake Macqu arie to BelmontReuse project w i l l Delay exten ded ouffall for 15 years Save Hunter Wa ter Corp and StateGov ernme nt 5 mi l lion

Fig. 1. Pacific Power Sewage Reuse project.16,000 EP STP = 16,000 equivalent populationsewage treatment plant.

Table 1Issue Options, solution and benefit s

Potable water shortage resulting in occasionalwater supply problemsDischarge of sewage into Lake Macquariecausing nutrient enrichment

Rising water and chemical costs for PacificPower boiler feed at 168 m3/la (potable watercosts A$ 900,000/y)

Build a new dam and increase the size of the distribution systemor reduce potable water demandBuild Dora Creek sewage works and either feed effluent to thepower station or feed a new 11.4 km pipeline to connect tothe ocean ouffall and expand potable supply at a combinedcost of over A$ 5 M.Low cost purification of sewage (including under stormflow conditions) (saving of A$ 800,000/y by 2001)

p l a n n in g t h e c o n s t r u c t i o n o f a n e w s e w a g ep i p e l i n e 1 1 . 4 k m l o n g t o c o n n e c t t o t h ee x i s t i n g p i p e l i n e d i s c h a r g i n g t o t h e o c e a n a tB e l m o n t . T h e o p p o r t u n i t y t o r e u s e s e w a g ea n d p o s t p o n e t h i s i n v e s t m e n t w a s c o n s i d e r e db y P a c i f i c P o w e r .

T h e p r o j e c t c o n s i s t e d o f c o n n e c t i n g t h e o u t le tf r o m t h e D o r a C r e e k s e w a g e t r e a tm e n t w o r k st o a n e w m e m b r a n e t r e a t m e n t s y s t e mi n c o r p o r a t in g m i c r o f i l t ra t i o n a n d r e v e r s eo s m o s i s . T h e t r e a te d s e w a g e i s t h e n u s e d

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B. Du r h a m / De s a l i n a t i o n 1 1 0 1 9 9 7 ) 4 9 - 5 8 5 3instead of potable water for boiler feed,cooling water, dust suppression and fly ashhandling. This solution resulted in majorsavings to all concerned including theenvironment and the local community.A number of issues arose that led to theconception of this innovative project (Table1).Pacific Power, with support from HunterWater Corporation undertook an environ-mental assessment and cost benefit analysis.Pacific Power estimated an initial saving ofA$ 700,000 per annum from reduced potablewater use. The study justif ied a project costof A$ 4.2 million including detailed design,manufacture, supply and civils with a paybackperiod for Pacific Power Company of 7-8years. This in turn allowed Hunter Water todefer the following projects:

m

11.4 km sewage pipeline at A$ 2.4 m.Expanded potable distribution system at A$2.6 m.This innovative approach produced awinning solution for Pacific Power, HunterWater, the environment and the localcommunity.This project enabled the Power Station tomove to zero discharge. The existing potablewater treatment plant consumed over A$160,000 of sulphuric acid and sodiumhydroxide each year with an ever increasingcost of regeneration of the deionisers beingused to remove all the dissolved salts frompotable water to protect the boilers.Pacific Power found a solution that couldmeet the following criteria:

- 'Clean' solution- Simple process- Economical ly attractive- Almost chemical free- Modular design for future expansion- Fully automatic, self diagnostic controls- Small footprint- Proven reliable technology- Local technical support.

4 . P r o c e s s d e s i g nPacific Power selected a Memcormembrane treatment system using CMF aspretreatment to reverse osmosis. After theplanning and approval stages the project took36 weeks to start up operation after the orderwas placed.This 2-stage membrane system has been inoperation from March 1995 producing 63m3/h with a planned expansion to 168 m3/h.Effluent from Dora Creek SewageTreatment plant flows under gravity from a8,000 m 3 elevated tank through a 500 mm(20 inches) LDPE pipeline, to the suction of3 100% centrifugal pumps (37 kW). The

pumps deliver the feedwater via a single in-line motorised self cleaning strainer (0.5 mmapertures) to the two Memcor 90M10CCMF units.SDI of water is consistently reduced to 1.5allowing RO membranes to be operated atappro ximat ely 40% highe r flux than ispossible with a pretreatment based on theconventional lime coagulation/sedimentation/filtration process.Filtrate from the CMF units is dosed withsodium hypochlorite, for downstream controlof biological growth. Sulphuric acid is alsodosed to reduce pH and minimise hydrolysisof the cellulose acetate RO membranes.Microfiltered water is drawn from the storagetank by 3 x 50% centrifugal pumps (15 kW),dosed with antiscalant and passed through a 5micron Filterite disposable cartridge guardfilter for feed to the RO plant.Two x 50% multistage centrifugal pumps(150 kW) drive the RO plant. The ROsystem comprises two trains (2 x 50%), eachwith two stages - the first stage comprising 6RO pressure vessels and the second stagecomprisin g 3 (6:3 array). Each pressurevessel houses seven RO membrane elements.The membrane elements themselves are 8.5 indiameter x 40 inches long cellulose acetatemembranes rated at 98% salt rejection. Saltsand organics are rejected by the ROmembrane, allowing only water to passthrough. Permeate (treated water) which is

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54 B. Durham /Desalina tion 110 1997) 49-58virtually free of all salts and micro-organisms,is piped to a degasser tower to increase pH byflashing off CO2 and is then stored in a 60 m 3treated water tank.The treated water has a very low totaldissolved solids (TDS) and is fedpreferentially to the demineraliser plant toreduce the operating cost by using lessregenerant chemicals, with the balance of thewater used for non-potable water applicationsaround the power plant.The RO concentrate (reject or brine) passes,via the demineraliser plant wastewater sump,to the station ash dam. Backwash from theCMF plant is sent to Pacific Power's on-sitewastewater treatment facility and thenrecycled back to the feed receiving tank.

5 . P la nt spec i f i ca t io nsTable 2Plant specifications

6 . C M F o p e r a ti n g p a r a m e t e r s

Table 3CMF operating parametersCapacityInfluent source

Integrity test

Backwash interval

Clean in place

Backwash volume

Operating: 3.5 mlpd (0.9 mgpd)Peak: 5.2 mlpd (1.35 mgpd)Secondary effluent from a cyclicactivated sludge processPeak 50/50 BOD5/solids(20/30 mg/1 average)(sea water ingress)Temperature 15 to 28CAutomatic every 24 hours ataverage pressure decay of0.27 psi/min15 to 60 minutes self regulatingdependent upon raw water turbidityand temperatureDesign: 14 days. Actual every1000 h operation in normalconditionsApproximately 10% of feedvolume (recovered and reused)

MicrofiltrationCMF membraneCMF recoveryReverse osmosis

RO membrane

RO recoveryCMF backwash systemInfluent pumpsRO booster pumpsHigh pressure pumpsControl system

2 x 90M10C(90 modules per unit)Polypropylene, 0.2 micron,hollow fibre90%6:3 array (i.e. vessels/train)2 x 50% TrainsOutput 2.5 mlpd upgradableto 3.7Cellulose acetate,7 off 8.5 inch x 40 inchelements per vessel80%Compressed air at 7 bar3 x 100% centrifugal 37 kW3 x 50% centrifugal 15 kW2 x 50% multistagecentrifugal 150 kWSupervisory SCADA withtwo slave PLC's and dedicatedcontrollers for the RO andCMF sections of the plant

7 . R O o p e r a t in g p e r f or m a n c eThe projected treated water quality from afeed salinity of up to 721 mg/1 is shown inTable 4.

Table 4RO permeate quality (in mg/1)Parameter Year 0 Year I Year 2 Year 3Chloride 17 20 25 3 2Sulphate 1.2 1.43 1.8 2.2Silica 1.4 1.8 2.1 2.6Sodium 9.5 11.4 14.3 17.8Magnesium 0.2 0.2 0.2 0.3Calcium 0.3 0.3 0.4 0.5

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HunterWater Corp

SecondaL, ysewage

f r o m DoraC r e e kSTP

Solidsd isposa l

B. Durham Desalination I10 1997) 49-58P o t a b l e w a t e rre~alned for

showers anddnnklng

P l an t p r oc ess w a t e rAuxll ia~ cool ing.washdown, f ire ' jser~ces, astlplant, etc.

C M F u n i t e Chlor ineReceiv ingtank

I STP i CMF backwash

To boi lers

Exis t lngdomes t i cw a t e r r i nm a i nRO unit , ,. n : Oegaeeer~ . : ~ . :

. i,i el .

I I I 1 " w a t e r

D RO reject P r e f e r e n c et o aah dam g iven to deminp lant a sr ec la imedwater lower inT D S

55

Fig. 2. Eraring waterreclamation plant.

8 . O v e r a l l p l a n t p e r f o r m a n c eTable 5Overall plant performanceParameter Influent EffluentSuspended solids, mg/l 30 to 50 < 1Turb idity, NTU approx. 50

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5 6 B. Durham /Desal inati on 110 1997) 49-5 8- All effluents from the system are recyc ledor used for dust suppression.

A similar system treating algae-ladensurface water for the boiler feed to a PowerStation is installed near Budapest, Hungary.This installation also demonstrates the benefitsof enabling microfiltration technology .

- fully automatic operation- g u a r a n t e e d treated water quality withvarying and poor raw water quality- two stage membrane process (CMF & RO)- significantly increased deioniser capacitywith subsequent reduction in ch em ic alregenerant costs.

9 . V ~ r t e s i P o w e r P l a n t C o . B o k o d P o w e rS t a t i o n 2 3 5 M W O r o sz l~ in y H u n g a r yAlgae laden brackis h lake water is adifficult water to treat for high pressure boilerfeed compared with valuable potable water.

Once again, RO could not easily be applied totreat this water source for use in the powerstation and CMF was used to enable theapplication o f RO.Over the last ten years the quality of thecooling lake has deteriorated significantly.The dissolved salt content has increased to6,000 mg/1 total dissolved solids (TDS) andthe algae content during bloom periods hasincreased to 225 x 106 counts per litre andtotal suspended solids levels up to 100 mg/l.Due to these changes the chemical usage ofthe existing deioniser at the power plantincreased dramatically, the period betweenregeneration became very short and theoperation became troublesome. This resultedin significant labour and maintenance costs tocontinuously achieve the required waterquality.The objective of the project was to selectthe best technical solution that would lowerthe operating costs, produce a reliable andsignificantly higher water quality to protectthe existing ion exchange plant from theproblems caused by the variable feed waterquality. They also neede d to reduce labourdemand and provide a more convenientoperation. Reverse osmosis with micro-filtration pretreatment was selected as offeringthe best and most economically viabletechnical solution with the following benefits:- low chemical consumption- low power consumpt ion

1 0. C M F s y s t e mThe system design is similar to the Eraringproject and includes self-cleaning screens toremove large suspended solids, at the inlet toa raw water feed tank. Two 36M10C

Continuous Microfiltration (CMF) units, withduty/standby feed pumps, valves, instrumentsand membrane modules, produce a flow rateof 44 m3/h in duty/standby mode or 88 m3/hin parallel mode. The filtrate produced has asilt density index (SDI) of less than 3 withparticle removal down to 0.2 mic ron in size.The system includes an automaticcompressed air backwash with sequence every30 minutes to remove the suspended solids,and is cleaned using a clean-in-place (CIP)system.The backwash frequency is variable and isdetermined automatically depending on theraw water quality. The CMF product water isfree of bacteria and suspended solids. This isan important benefit in comparison with theconventional pretreatme nt systems thatremove suspended sol ids down toapproximat ely 20 g m (micron) and do notreduce the bacterial loading onto the ROplant.The CMF system produces a continuoussupply of high quality product water. The

Bokod power plant uses Memcor's latest highcapacity M10C submodules which have a lowenergy requirement through operating indead end mode at 1.5 bar operatin gpressure. This comp act design gives thebenefits of low operating and capital costs,particularly in large flow applications.The control panel contains a PLC fittedwith a data panel graphical user interface to

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B. Durham /Desalin ation 110 1997) 49-58 57allow the plant operators to collect and reviewplant data and control the system.

1 1 . R e v e r s e o s m o s i s s y s t e mThe pretreated water is fed to the lowpressure RO feed booster pump equipmentthrough two 5 ~tm polypropylene pre-flltercartridges (in stainless steel housings).Sulphuric acid and anti-scalant are thendosed, the acid is to adjust the feed water pHto 7, and the anti-scaling agent to preventscale formation on the surface of themembranes. The booster pumps supply themulti-stage centrifugal high pressure pumps,

which provide the necessary operatingpressure of 47-48 bar. The reverse osmosissystem supplied by Zenon Systems KFTTatabanya Hungary, includes two LF 26000units in parallel, each with a capacity of 26m a/h. The RO membranes are contained insix glass fibre reinforced plastic pressurevessels and each pressure vessel contains sixsea water high salt retention thin filmmembranes (SWHR).Unlike the CMF system which operates indead-end mode, the RO system operates incross-flow mode. The pretreated feed waterenters the RO pressure vessels at one end, andflows along one side of the membrane. Waterpasses through the membrane to producetreated water (permeate) and residual water(concentrate) flows out of the module. Thevolume of concentrate produced is about17-18 m3/h per unit.The RO system has a recovery rate of 60-70 per cent. This is the reco very limit atwhich the RO system can be operated tooptimise the use of anti-scaling agent.

The membranes are sea-water type,polyamide, spiral wound, high salt retentionthin film composite membranes. They have atypical TDS rejection of 99.5 per cent, and aminimum rejection 99.2 per cent at standardtest conditions of 32,000 ppm NaC1, 800 psipressure and 25C temperature. The RO

system has a common CIP tank for cleaningand sanitisation. The membranes are sanitisedevery 4-8 weeks and also following downtimeperiods of 72 hours. The sanitising andcleaning operations use low pressure and highflowrate through the pressure vessels, whilenormal process uses high pressure and lowerflowrate. The RO system and associateddosing and CIP equipment is PLC controlled.The Siemens control system incorporates aMaster Control Centre with 6 independentPLC's which provide maximum operationalsecurity and are monitored from the centralPLC.The treated water has a conductivity of 10-15 ~ts/cm. Since the start up of this plant inMay 1994 the downstream ion exchangeoperating costs have reduced by 50%.The key benefits are:- Membrane technology uses simple physicalseparation with minimal chemical con-sumption- Chemical usage has reduced by 90%- Effluent values have been reduce d pro-portionately- O p e r a t i n g and emergency maintenancelabour has been dramatically reduced- Power consumption was reduced- Payback period will be under 4 years.1 2 . C o n c l u s i o n s- 'Enabling' technologies such as ContinuousMicrofiltration are allowing reverse osmosistechnology to treat previously impracticalsource waters as microflltration allows ROfeedwater quality to be easily controlledand consistent.- The reuse of secon dary sewage can be

considered as an economic alternative andis easier to treat than eutrophic surfacewater.- Reverse osmosis capital and operating costscontinue to reduce. Microfiltration allowsthe membrane inventory of an RO plant tobe reduced by up to 30 to 40%.

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5 8 B. Durham / Desalination 110 1997) 49-58R e f e r e n c e s[ 1 ] S . E b r a h i m , S . B o u - H a m a d , M . S a f a r , a n d A . A 1-S a i r a f i , P r e t r e a t m e n t f o r s e a w a t e r r e v e r s e o s m o s i sp l a n t s u s i n g m i c r o f i l t r a t i o n s y s t e m s , I D A W o r l d

C o n g r e s s o n D e s a l i n a t i o n a n d W a t e r S c i e n c e s , A b uD h a b i , N o v e m b e r 1 9 9 5 , p p . 2 3 0 .

[ 2 ] G . L . L e s l i e , W . R . D u n i v i n , P . G a b i l l e t , S . R .C o n k l i n , W . R . M i l l s , a n d R . G . S u d a k , O r a n g eC o u n t y W a t e r D i s t r i c t , F o u n t a i n V a l l e y , C a l i f o r n i a ,S e p a r a t i o n P r o c e s s e s I n c . , V i s t a , C A .[ 3 ] S . S . A d h a m , J . G . J a c a n g e l o , a n d J . L a i n e , L o w -p r e s s u r e m e m b r a n e s : A s s e s s i n g i n t e g r i t y , J .A W W A , M a r c h 1 9 9 5 .