Influence of colloidal fouling on rejection of

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)rejectioncorrelatedinverselywithchann