Influenceofcolloidalfoulingonrejectionof

JournalofMembraneScience244(2004)215–226InfluenceofcolloidalfoulingonrejectionoftraceorganiccontaminantsbyreverseosmosisHowY.Nga,MenachemElimelechb,∗aCentreforWaterResearch,DepartmentofCivilEngineering,NationalUniversityofSingapore,BlockE1A#07-03,No.1EngineeringDrive2,S117576,SingaporebDepartmentofChemicalEngineering,EnvironmentalEngineeringProgram,YaleUniversity,P.O.Box208286,NewHaven,CT06520-8286,USAReceived15January2004;receivedinrevisedform16June2004;accepted23June2004Availableonline11September2004AbstractAsystematicinvestigationontheinfluenceofcolloidalfoulingonremovaloftraceorganics(steroidhormones)andinertorganicsbyreverseosmosis(RO)membranesisreported.Resultsoflaboratory-scalecrossflowmembranefiltrationexperimentsshowedthatcolloidalfoulingcausedamarkeddecreaseinrejectionofinertorganicmoleculeswithmolecularweightsmallerthanabout100g/mol.Whiletheremovalofinertorganicswasmainlyduetosteric(size)exclusion,adsorptionanddiffusionacrossthemembranepolymerplayedaprominentroleintheremovalofthesteroidhormonesbytheROmembrane.Duringmembranefouling,rejectionoftheinertorganicsandsaltdeclinedtoaminimum,butthenimprovedslowlyandeventuallystabilizedatafixedvalue.Hormonerejection,ontheotherhand,decreasedrapidlyinitially,andthengraduallysloweddownregardlessoffouling.Thisrejectionbehaviorisassociatedwiththedecreaseinfeedhormoneconcentrationduetoadsorptionandtheincreaseinpermeateconcentrationduetodiffusionacrossthemembraneskinlayer.Thedeclineinhormonerejectionwithtimewasmuchmoreseverewhencolloidalfoulingtookplace.Thesoluterejectionbehaviorismarkedlyinfluencedbythecake-enhancedconcentrationpolarization,wherebythecolloidalcakelayerhindersbackdiffusionofsolutesfromthemembranesurfacetothebulksolution.Thebuildupofsolutesatthemembranesurfaceresultsinahighersoluteconcentrationgradientacrossthemembraneand,thus,agreatersolutetransportthroughthemembraneandalowerobservedsoluterejection.Therateofdeclineinpermeatefluxandsolute(hormones,inertorganics,andsalt)rejectioncorrelatedinverselywithchannelwallshearrate.©2004ElsevierB.V.Allrightsreserved.Keywords:Reverseosmosis;Colloidalfouling;Soluterejection;Fluxdecline;Traceorganiccontaminants;Hormones;Concentrationpolarization;Cake-enhancedconcentrationpolarization1.IntroductionFoulingisaninevitablephenomenoninreverseosmosis(RO)systemsusedinvariouswaterqualitycontrolapplica-tions.Membranefoulingresultsinseveraldeleteriouseffects,includingadecreaseinwaterproductionbecauseofagrad-ualdeclineinflux,anincreaseinappliedpressurerequiredforaconstantrateofwaterproduction,agradualmembranedegradationwhichresultsinashortermembranelife,andadecreaseinpermeatequality.∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](M.Elimelech).Numerousstudieshavebeenreportedoncolloidalfoul-ingwithemphasisonpermeatefluxdecline[1–7].CohenandProbstein[3]investigatedthefoulingrateofcellu-loseacetateROmembranesbycolloidalferrichydroxideindeionizedwater.Theirworkdemonstratedalinearrela-tionshipbetweenpermeatefluxandfoulantlayerthicknessduringtheinitialstagesoffouling.Inarecentstudy,Hoeketal.[7]foundthatfluxdeclineincolloidalfoulingofROandNFmembranesisprimarilydueto“cake-enhancedos-moticpressure”.Theysuggestedthataseverefluxdeclinewasobservedbecausethecolloidalcakelayerlimitsbackdiffusionofsaltionsfromthemembranesurfacetothebulksolution,thussignificantlyelevatingsaltconcentration0376-7388/$–seefrontmatter©2004ElsevierB.V.Allrightsreserved.doi:10.1016/j.memsci.2004.06.054216H.Y.Ng,M.Elimelech/JournalofMembraneScience244(2004)215–226atthemembranesurfaceandthetrans-membraneosmoticpressure.Tominimizefouling,pretreatmentisfrequentlyappliedwiththegoalofeliminatingfoulantsfromthefeedwater.However,pretreatedfeedwatermaystillcontaindissolvedorganiccompoundsandsmallcolloidalparticlessuchasclay,silica,andorganicmatterwhichcontributetoROmembranefouling[8].Sugaharaetal.[1]andWinfield[2]havedeter-minedthatlargerparticlesdonotcontributesignificantlytofoulingandthatfoulingiscontrolledbysmallcolloidalpar-ticles.Winfield[2]foundthatdissolvedcolloidalmaterialsinsecondarywastewatereffluents,butnotlargesuspendedparticles(5
m),contributedsignificantlytoROmembranefouling.Sugaharaetal.[1]concludedthatparticlessmallerthan0.45
m,includingtruecolloidsanddissolvedsolids,contributethemosttoROmembranefouling.Membranerejectionoforganiccompoundshasbeenex-tensivelystudiedinrecentyears[9–15].Therejectionofor-ganicsdependsonmembranepropertiessuchasporesize,membranematerial,andmembranecharge;solutecharacter-isticssuchasmolecularsize,charge,andpolarity;andfeedsolutionchemistry.WiththeincreaseusageofsecondarywastewatereffluentasasourcewaterforadvancedwaterreusebyROseparation,theeffectivenessofROinreject-ingtraceorganiccompoundsbecomesratherimportant.Re-movaloftraceorganicsinreclaimedwaterisamajorconcernsinceitisnowrecognizedthattraceorganics–suchashor-moneswhichhavethegreatestendocrinedisruptingpotencyamongallendocrinedisruptingchemicals–causeadversehealtheffectstohumansandanimals[16–22].Anumberofstudieshaveconcludedthatmembranefoul-ing