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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

    ;ityof2600ps?cm1.rn1eci(culatingconcentrateflux ;wasabout16000L?m?h17butthepermeateflux ;(<5OL?m?h)wasextremelylow.Itwasobserved ;thatfluxobviouslyincreasedwiththeoperatingpres

    ;sure.Duetotheconcenationpolarizationorfouling. ;therelationshipbetweenfluxandpressurewasnot ;linearalthoughsaltrejectionwasalmostconstant.It ;indicatedthattheincreaseinoperatingpressurecould ;enhancetheonvmgtorceandthenovercamethemem

    ;braneresistance.Butpressureshouldnotbetoohigh ;becausemembraneporosityandPoresizedecreaseddue ;tomembranecompac~on.Comparingthemembranes ;pressure/MPa

    ;Figure3Effectofoperatingpressureonmembranesper- ;mEatefluxandsaltrejection

    ;o1flux;flux;girejection;2#rejection

    ;used.2廿R0membraneshowedthehighestr_eiectionof ;97%butare!ativelylowflux(29.3L.m-z.h_1,.The ;reiectionof1NFmembranewasonly78.9%whilea ;l1ighfluxof47.4L.m-2.hwasobtmned.

    ;Therelationshipbetweenoperatingpressureand ;r~ectionofS0,c1,Mg,Na,Cazwereillus

    ;tratedinFigs.4(a)and4(b)usingsameindustrial ;wastewaterasfeed.Fig.4(a1showedthatthe2RO ;membranegavethehighestrejectionforal1anions. ;wl1iletheNFmembranegavethelowestreiectionfor ;chloridebuthighreiectionforsulfate.Fig.4Co)com

    ;paredthecationrejectionsbetweentwotypesof ;membranes.Thesodiumionpermeatedthroughthe ;NFmembranewasalsothemost.Thereiectionof ;saltsbytheNFmembraneismorecomplicatedthan ;reverseosmosisanddependsonbothmolecularsize ;andDonnanexclusioneffects15,16].Besideshigh

    ;permeateflux.theNFmembraneusedinourexperi

    ;mentshadagoodselectivitybetweensinglevalent ;anddivalentions.TheNaClreiectionwasverylow,

;butther~ectionofS0,M,andCazwerevery

    ;high.ThesecharacteristicsoftenmadeNFmembranes ;moredistinguishedthanROmembranes,especiallyin ;highhardnesswatertreatment.

    ;.

    ;

    ;?

    ;

    ;pressure/MPa

    ;(a)Rejectionofanions

    ;o1sulfate;?1chloride;2sulfate:_2chloride

    ;.

    ;

    ;.

    ;

    ;.

    ;pressure/MPa

    ;(b)Rejectionofcations

    ;O1magnesium;1sodium;?1calcium;

    ;?magnesium;_sodium;A2*calcium

    ;F

    ;.

    ;igu

    ;.

    ;re

    ;.

    ;4Therelationshipsofoperatingpressureandre- ;jecuoninmepermeate

    ;Figure5showedthefluxbehaviorasafunction ;oftime.Astheoperatingtimeelapsed,thetotalresis- ;tancesincreasedandthefluxdecreasedfrom35.2to ;26.6L.m-2.hafter6h.Fluxdeclinecou1dbecaused ;byseveralfactors,suchasconcentrationpolarization, ;/u0]0o2u?

    ;N

    ;g._?-1x蛊蜀矗(1

    ;

    ;Chin.J.Chem.Eng.,Vo1.16,No.3,June2008 ;Figure5Fluxbehaviorasafunctionoftime ;(R0;industrialwastewater,2600ps?cm;pressure=1.4MPa)

    ;gellayerformationandpluggingofthepores[8,17,18]. ;AUthesefactorsinducedadditionalresistancesonthe ;feedsidetothetransportacrossthemembrane ;4.2Theoreticalpredictionofflux

;ThevariousresistancesdepictedinFig.1con

    ;tributewithdifferentextenttothetotalresistances.It ;isconsideredthatonlythemembraneresistanceRMis ;involvedintheidealcase.Theresistanceofconcen

    ;ationpolarizationisinducedbytheaccumulationof ;retainedmoleculesnearthemembranesurface.This ;resultsinahighlyconcentratedlayernearthemem- ;brane.Whenthemembraneretainsthesolutestoa ;veryhighextent.therewillformagellayerwhich ;addsthegellayerresistance,R19].

    ;Topredicttheoreticallytheflux,wecalculated ;eachofresistanceusingthemethodsillustratedin ;Section2.Tosimplifytheprocedure.2ROmem

    ;braneonlywastakenfortestingintheexperiment. ;FromFig.6thevalueofRMandRDcouldbeobtained ;bycalculation.RMisthereciprocalofthewaterper

    ;meabilitycoeicientofmembraneinthedistilledwa

    ;ter,whichiswithavalueof25.74,anditequals ;to0.039mz.h.MPa.L_..

    ;Inthesyntheticwaterex

    ;periment(SeeFig.7),wegottheexpressionofKw, ;the

    ;waterpermeabilitycoefficientofmembraneinthe ;syntheticwater,is24.49l×f??1.RDisalSO

    ;thereciprocalofKw,anditsvalueisafunctionof ;pressure,whichcaribededucedfromEq.(7)11].

    ;00.51.O1.52.O2.53.O3.5

    ;?A:r/MPa

    ;Figure6Calculationof(2R0:distilledwater,420 ;ps?cm:Na2SO4syntheticwastewater,2500Bs.cm) ;Afluxfromdistilledwaterexperiments;fluxfromsynthetic

    ;wastewaterexperiments

    ;1OO

    ;8O

    ;60

    ;40

    ;20

    ;00.51.01.52.02.53.03.5

    ;?A:r/MPa

    ;Figure7Comparisonoftheexperimentalandpredicted ;permeatefluxvalues

    ;[experiment:2R0,industrialwastewater(2600ps?cm), ;after1hoperation;model:basedonEq.(6)andKw

    ;24.491x(Ap?0?055]

;——

    ;theoretical:?experimental

    ;Here,itwassupposedthatthemembranefoulingin ;industrialwastewaterwasthesalTleasinsynthetic ;water.Suchamodelshouldbebasedonverylowest ;fouling.Theindustrialwastewaterwassecondaryef- ;fluentafteraserialofpretreatment.AndfromFig.5. ;after6hofoperation,thefluxlosswasnotbig. ;Therefore,suchassumptionwouldbesuitable. ;Sincethevalueofeveryresistancewasascer- ;tmned,thetheoreticalfluxcouldbecalculatedbased ;onEq.r41.Fig.7liststhetheoreticalfluxexcluding ;theresistanceofgelatin.R..Theeectofgellayer

    ;resistancewasnotclearinourexperiments,which ;couldbeseenfromthephenomenaoffluxdeclinein ;Fig.5.Therefore.thedeviationfromincludingR.or ;notwasnotobviOUS.TheexperimentalvalHeaccords

     ;withthetheoreticalvalHeverywellwithaveragerela

    ;tiveerrorbelow3%.

    ;4.3Feasibilityoflarge-scaleindustrialpractice ;Beingapressuredrivenmembraneprocess,the

    ;performancecanbeexpressedintermsoftheperme

    ;atefluxandtherejectionofspecificsubstanceswhich ;arerequiredtoberemoved.Thecomparativewater ;qualityofwastewaterandNF/ROpermeatewas ;summarizedinTable1.A11permeatesreachedthe ;regulatorylevelsrequired.Thefluxattenuationof ;permeateswasbelow20%foreach6hperiodofcon

    ;tinuousoperationandcouldbealmostrecovered ;through10minhighspeedfeedflushingacrossmem- ;braneunderlowpressure.ThesaltrejectionofNF ;membranecouldbeupto80%.TheNFmembrane ;wascapableoftreatingrawwastewaterofaCOD ;valueof150mg’LandreducingittoalowlevelOf

    ;35mg.L.whichsarisfiedtheStateReutilization ;Qualificationf201forreusingasindustrialcoolingwa

    ;ter.Inthemeantime.ionsrejectionof2廿R0mem-

    ;braneforindustrialwastewatereatmentcouldbeas

    ;highas97%.butwithobviouslylowerfluxrate. ;Compromisedbetweenfluxandrejection,thatNFwas ;moresuitableforlargescaleindustrialpractice.which ;heldasignificantlyhighpermeatefluxofabout47.4 ;L?m?hatalowoperatingpressureOf1.4MPa. ;N

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