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Performance

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PerformancePerfor

    Performance

ChineseJournalofChemicalEngineering,16(3)441445(2008)

    ;PerformanceofNanofiltrationandReverseOsmosisMembranesin

    ;MetalEffluentTreatment

    ;LI,

    ;UFeini(刘飞妮),zHANGGuoliang(张国亮)L,MENGQin(孟琴)andZHANGHongzi

    ;(章宏梓)

    ;.

    ;1CollegeofBiologicalandEnvironmentalEngineering,ZhejiangUniversityofTechnology,Hangzhou310014,China

    ;CollegeofMaterialsScienceandChemicalEngineering,ZhejiangUniversity,Hangzhou310027,China

    ;AbstractTheperformanceofdifferentnanofiltration(NF1andreverseosmosis(RO1membraneswasstudiedin

    ;treatingthetoxicmetale

    uentfrommetallurgicalindusvy.Thecharacteristicsandfiltrationbehavioroftheproc

    ;essesincludingthewastewaterflux,saltrejectionandionrejectionversusoperatingpressurewereevaluated.Then

    ;thewastewaterfluxofR0membranewascomparedwiththeoreticalcalculationusingmassansfermodels.and

    ;goodconsistencywasobserved.Itwasfoundthatahighreiectionratemorethan95%ofmetalionsandalow

    ;CheroicalOxygenDemand(COD1valueof10mgL

    inpermeatecouldbeachievedusingtheR0composite

    ;membrane,whiletheNFrejectionofthesaltcouldbeupt078.9%andtheCODvalueinthepermeatewas35mgL.

    ;TheresultsshowedthattheproductwaterbybothNFandROdesalinationsarisfiedtheStateReutilizationQualifi

    ;cation.butNFwouldbemoresuitableforlargescaleindustrialpractice.whichofferedsig

    nificantlyhigherperme

    ;atefluxatlowoperatingpressure.

    ;Keywordsnanofiltration,reverseosmosis,metaeffuent,reuse

    ;1INTRoDUCTIoN

    ;MetallurgyPollutioninChinabecomesmoreand

    ;moreseriousinrecent10yearsastheproductionof

    ;metallurgicalindustryjumpsupowingtothefiendof

    ;priceintheinternationalmarket.Badweathermay

    ;floodun.completelytreatedwastewaterdischarged

    ;intoriverbyaccident.A1thoughvariousconventional

    ;methodssuchascoagulation.precipitationandad

    ;sorptionwereused,theyUSUallyfailedtoreducethe ;concentrationofdifferentionssuchassulfatesand ;heavymetalseffectivelytotheregulatorylevelsre

    ;quired,whichwerethemaincomponentsofeff1uents ;andmajorcausesofwaterandsoilpollution.Low ;concentrationof,ionsofzinc,chromiumandcad

    ;mium,etc.inwatercouldaffectthegrowthofliving ;organisms[3].Therefore,itisagreatchallengeto ;mitigateoreliminatewastewaterpollutioncontaining ;dangerousheavymetals.

    ;Membranetechnologyhasbecomeincreasingly ;promisinginremovingheavymetalsfromwastewater ;andimprovingwaterrecoveryrateduetoitshigheffi

    ;ciencyandlowcost48.OdaisandMoussa[91inves

    ;tigatedtheremovalofheavymetalsfromsynthetic ;wastewatercontainingcopperandcadmiumionsby ;reverseosmosis(RO)andnanofiltrationfNF1.The

     ;resultsshowedthattheRO/NFwascapableofreduc

    ;ingtheheavymetalconcentrationintheeffluent ;streamtolocalstandardsforindustrialwastewater ;discharge.Ozakieta1.[10]studiedtheperformanceof ;anaromaticpolyamide(ES201ultralowpressurere

    ;verseosmosismembraneforseparatingdivalentfCu. ;Ni)andhexavalent(Cr)ions.Experimentalresults ;showedtherejectionincreasedwithfeedpressureand ;pH.Chaieta1.[11]andQineta1.[12]reportedthat ;largerscaleoperationsintreatingindustrialwastewa

    ;terbyROsystemswerepossibleandeffective. ;Thereappearedfewworkreportedonreuseof ;metallurgicalwastewaterbyNF/R0membrane.espe

    ;ciallyonthecharacteristicsofmasstransferofthe ;process,whichisveryimportantformembraneoper

    ;atingandfoulingpreventing.Thispaperpresentsa ;laboratorystudywithsyntheticwastewaterandindus

    ;talwastewatertoexaminethefeasibilityof ;large--scaleindustrialpracticeandtrytosetupatheo-- ;reticalwayforpredictionofmembraneperformance. ;2THEoRY

    ;Membraneperformancereferstothewaterflux ;andtherejectionofaparticularpollutant.Thesalt ;rejectionRisdefinedby

    ;R:

    ;flCpI×100%(1)LcM.J

    ;wherecpisthepermeateconcentration,andCMthesalt

    ;concentrationatthemembranewal1.However,CMis ;difficulttoobtainsothatitisoftenreplacedbyCbthe ;feedconcentration.Therefore,thesaltrejectionisalso ;definedas

    ;=

    ;.%

    ;Thepermeatefluxintheunitofm3?m-2.his

    ;thevolumetricrateofflowthroughunitmembrane ;area,expressedby

    ;w=

    ;V(3)

    ;Thewaterfluxbasedonthediffusiontheoryis ;Received20070924,accepted20080320.

    ;SuppoSedbytheNationalNaturalScienceFoundationofChina(20476096,20776133),Z

    heiiangProvincialBureauofSci

    ;ence&Technology(2005C33040)andBureauofEducation(KeyDisciplineofEnvir

    onmental.Engineering56310503014).

    ;T0whomcorrespondenceshouldbeaddressed.E.mail:guoliangz@z{ut.edu.ca

    ;

    ;Chin.J.Chem.Eng.,Vo1.16,No.3,June2008 ;primarilydependentontheoperatingpressurediffer

    ;enceandtheosmoticpressuredifferenceacrossthe ;membraneascalculatedby

    ;Jw=Kw(?p?)(4)

    ;?P=-1

    ;2

    ;(Pf+Pr)

    ;whereApisthepressuredifferenceacrossthemem

    ;brane,A7/7theosmoticpressuredifferenceacrossthe ;membrane(?=),andosmoticpressure

    ;..ThesubscriptsCPindicespredictfeed, ;plplngandpermeate,respectively.Kwiswaterper

    ;meabilitycoefficient.Sincethepermeatestreamusu

    ;allyhasaverylowsaltcontent.itsosmoticpressure ;canbeneglected.Inaddition,permeatenormally ;leavesamembranemoduleatnearatmosphericpres

    ;suresothattheoperatingpressuredifferenceisthe ;feedpressure.Consequently,thenetoperatingpres

    ;suredifferencecanbesimplifiedto[13

    ;Jw=(Pp一乃)(6)

    ;ThewaterpermeabilitycoefficientKwisex

    ;pressedintermsofthetworesistancesinseriesas ;follows:

    ;Kw1’7

;whereRM,RDarethemembraneanddynamicresis

    ;tances.respectively.Thedynamicresistanceisprimar

    ;ilyformedbygelatinandconcentrationpolarization. ;Overviewofconcentrationgradientatthesurfaceof ;membraneandvariousresistancesisshowninFig.1. ;SymbolsRandRmeantheresistancescausedby ;concentrationpolarizationandgelatinaccumulating ;onthemembranesurface.Tosimplifycalculation,R ;andR.weresunnedandrepresentedbydynamicre

    ;sistance,RD,atacertainpressureandtime. ;membranebulkoffeed

    ;/

    ;.

    ;fl

    ;cbCp

    ;L

    ;RcRMRg

    ;Figure1Overviewofconcentrationgradientandvarious ;resistances

    ;Thewaterpermeabilitycoefficientofmembrane. ;K,wasobtaineddirectlyfromtheslopeofwater ;fluxversuspressurefromdistilledwaterexperiments ;(usingEq.(4)withATrtakentobezero)[141and ;(8)Infact,Kwimpliesfluxincrementwithtrans

    ;membranepressureincreasing.Tocalculatethedif- ;ferentialoffluxfunctionexpressedbypressure,we ;canfinallydeduceresistancesandgetKwinthesyn

    ;theticwastewaterexperiment.

    ;3EXPERIMNTAL

    ;SchematicdiagramsoflabscaleROlsystem

    ;usedintheexperimentsareshowninFig.2.T11efeed ;wastakenfromthefeedtankandwaspumpedintothe ;module.Thefiltrationunitusedinthesystemcon

    ;sistedoftwostacksofplateframemembranemodules,

    ;wl1ichwasplacedinseriesandhadatotaleffective ;filtratingareaof36cm.Thepressuredifferencebe

    ;tweenthefeedinletandtheoutletduringoperation ;wasnormallyfrom0.005to0.015MPa.thereforeit ;couldbeomittedincalculation.Impuritywasretained ;bythemembranetoformaconcentratedandtherest ;passesthroughthemembranebecomingpermeate ;stream.Therateofthepermeatestreamwasmeasured ;byarotameterandagaugedcylinderT11econcentrate ;wasthenmixedwithsecondaryeffiuentandretumed

;tothefeedtanktokeepthemassbalance.

;concentratereturn

    ;eate

    ;Figure2SchematicdiagramoflabscaleRO/NFsystem

    pressure;nux;T--temperature ;P_

    ;Aself-madecelluloseacetatenanofiltration ;membraner1,madebyLoebSouriraianProcess.

    ;China.withanonwovenclothbaselayerandstabi

    ;lizedrejectionrateof82%1andacommercialcom

    ;positereverseosmosismembranef2廿,BW30,Filmtec,

    ;USA.aromaticpolyamidecompositemembranewith ;stabilizedrejectionrateof99%,wereusedintheex

    ;periments.Thefeedsolutionwaspreparedusingthree ;salts.Thepuredistilledwaterwasfirstusedtomeas

    ;urethewatelpermeabilitycoefficientofmembrane, ;/GJ.Then,thesyntheticwastewaterofNa’sO4solu—

    ;tionwithamassconcentrationof0.15%wastested

     ;andaimedtoobtainKw.Finally,thesecondaryeffiu

    ;entfromawastewatertreatmentplantafterpretreat

    ;mentwasmainlyusedastherawfeed(Table11. ;Duringexperiment,differentfeedsweretestedin ;arangeofpressurebychangingtheconcen

    ;trate/permeateratiotoevaluatetheperformanceof ;membranes.Atstableoperation.theexperimentswere ;

    ;Chin.J.Chem.Eng.,Vo1.16,No.3,June2008 ;Table1Theparametersofwastewaterand ;NF/ROpermeate

    ;StreamandCOD

    ;standard/mg?L

    ;!ca2+soc1

    ;ValueRejection/mg.L/mg.L/mg.L

    ;/gs.cm|%

    ;?Pressure=1.4MPa;1flux=47.4L.m-2.h:flux=29.3 ;L?m-2.h;samplevolume=50m1.

    ;

    ;?Self-made.

    ;?Coofingwaterforreuse.

    ;carriedoutatoperatingpressureof1.4MPa.The ;samplesfromthepermeateandtheconcentratewere ;pickedupsimultaneouslyduringoperationsforanaly

    ;sis.MetallicionsweredeterminedbyanInductively ;CoupledP1asma(ICIRISIntrepid,USAandani

    ;onsweredeterminedbyaMetrohm861CompactIC

    ;ionicchromatographywithconductivitydetection. ;4RESU1SANDDISCUSSIONS

    ;4.1Filtrationbehavior

    ;Experimentswerecarriedoutwiththeindustrial ;wastewaterbyadjustingoperatingpressure(inthe ;rangeof0.6,1.0,1.4,1.8,2.2MPa).Fig.3showed

     ;ettectotoperatingpressureonpermeatefluxandsol

    ;uterejecUontorindustrialwastewaterwithconducfiv