A comprehensive study on membrane fouling in submerged membrane bioreactors operated under different

Available online at

SeparationandPuri cationTechnology

59 (2008) 91–100

Acomprehensivestudyonmembranefoulinginsubmergedmembrane

bioreactorsoperatedunderdifferentaerationintensities

FangangMeng ,FenglinYang,BaoqiangShi,HanminZhang

KeyLaboratoryofIndustrialEcologyandEnvironmentalEngineering,MOE,SchoolofEnvironmentalandBiologicalScienceandTechnology,

DalianUniversityofTechnology,Dalian116024,PRChina

Received8December2006;receivedinrevisedform22May2007;accepted29May2007

Abstract

Inthispaper,membranefoulinginthreeparallelMBRsoperatedunderdifferentaerationintensities(150,400and800L/h)wasstudiedtohaveabetterunderstandingofthemembranefoulingmechanism.Theimpactofaerationonmembranefoulingwasinterpretedfromtwoaspects:evolutionofbiomasscharacteristicsandformationmechanismofthecakelayer.Theresultsshowedthateithersmallorlargeaerationintensityhadanegativein uenceonmembranepermeability.Thelargeaerationintensityresultedinaseverebreakupofsludge ocs,andpromotedthereleaseofcolloidsandsolutesfromthemicrobial ocstothebulksolution.Thesludgesupernatantwouldbecomeheterogeneousastheaerationintensityincreased.AstheMBRoperatedunderhighaerationintensityof800L/h,colloidsandsolutesbecamethemajorfoulants.Inaddition,thebacktransportmechanismofmembranefoulantsinthethreeMBRswasdifferentfromeachother.Aerationhadapositiveeffectoncakelayerremoval,butporeblockingbecamesevereasaerationintensityincreasedto800L/h.Themaincomponentsoforganicmattersinthemembranefoulantswereidenti edasproteins,polysaccharidematerialsandlipidsbytheFouriertransforminfraredspectroscopy(FTIR).© 2007 Published by Elsevier B.V.

Keywords:Membranebioreactor;Membranefouling;Aeration;Sludgecharacteristics;Cakelayer

1.Introduction

Membranebioreactorisabiologicalwastewatertreatmentprocessthatusesmembranetoreplacethegravitationalsettlingoftheconventionalactivatedsludgeprocessforthesolid–liquidseparationofsludgesuspension[1].MBRs,inwhichbiomassisstrictlyseparatedbyamembrane,offerseveraladvantagesovertheconventionalactivatedsludgeprocess,includingahigherbiomassconcentration,reducedfootprint,lowsludgeproduc-tionandbetterpermeatequality[2].AmajorobstaclefortheapplicationofMBRsistherapiddeclineofthepermeation uxasaresultofmembranefouling[3–8].

The rstgenerationofMBRswassidestreamorcross- owsystemswiththemembranemoduleplacesinarecirculationloopexternaltothebioreactor.Theuseofrecirculationloopsleadstoincreasedenergycosts.Inaddition,thehighshearstressesinthetubesandrecirculationpumpscancontributetothedestructionofbio ocsandthishasbeenlinkedtoaloss

Correspondingauthor.

E-mailaddress:fgmeng80@(F.Meng).

ofbiologicalactivity[9].Toovercometheselimits,thesub-mergedMBRsweredevelopedandpopularlyusedinwastewatertreatment[10].InasubmergedMBR,shearstressiscreatedbyaeration,whichnotonlyprovidesoxygentothebiomass,butalsomaintainsthesolidsinsuspensionandscoursthemembranesurfacetoalleviatemembranefouling.Thenormalprocessofaerationcanbeusedtogenerateashearstressonthemem-branesurfacewithoutrequiringarecirculationpump.But,ithasbeenfoundthatmorethan80%energyconsumptionwasforaeration[11].Uedaetal.[12,13]examinedtheeffectofaera-tiononcakeremovalandsuctionpressureusingapilot-scalesubmergedMBRandconcludedthataerationwasasigni -cantfactorgoverningthe ltrationconditions.Previousworks[14,15]alsoshowedthatthecake-removingef ciencyofaera-tiondidnotincreaseproportionallywiththeincreaseintheair owrateandthattheair owratehadanoptimumvaluefromthecake-removingpointofview.

Ahighaerationratecertainlycanreducesludgeattachmenttothemembrane,butitalsohassigni cantin uenceonthebiomasscharacteristics.Mostofthepreviousliteraturesfocusedoninvestigatingthein uenceofaerationintensitiesonmem-branepermeabilityandbiomasscharacteristics,butthereislittle

1383-5866/$–seefrontmatter© 2007 Published by Elsevier B.V.doi:10.1016/j.seppur.2007.05.040

92F.Mengetal./SeparationandPuri cationTechnology 59 (2008) 91–100

informationonhowtospecifytheimpactsofaerationintensityontheformationofmembranefoulants.Infact,thereshouldbeadirectrelationbetweenaerationintensitiesandtheformationofmembranefoulants.Theaerationintensityisexpectedtohaveaverycomplexin uenceonMBRperformance.

Inthispaper,threesubmergedMBRsunderdifferentaera-tionintensitieswereoperatedforabout50daystoinvestigatethein uencemechanismofaerationintensityonmembranefouling.Inthewholetests,themembranepermeate uxwasmeasuredtostudythemembranefoulingbehaviorunderdiffer-entaerationintensities.Thesludgeparticlesizedistributionsofsludgesuspensionandsludgesupernatant,solubleCOD,col-loidalCODandEPSwereanalyzedtocharacterizetheeffectofaerationintensitiesonbiomasscharacteristics.Thefoulinglayerformationmechanismwasexaminedtodescribethedepo-sition/adsorptionmechanismofmembranefoulants.Thefoulingcakelayeronthemembranesurfacewasanalyzedbasedonresistanceanalysis.Thedepositionofbiopolymersonthemem-branesurfacewascharacterizedbyFouriertransforminfraredspectroscopy(FTIR).2.Materialsandmethods2.1.OperationofMBRs

AsshowninFig.1,theexperimentalsystembasicallycon-sistedofthreeactivatedsludgebioreactor(MBR-A,MBR-BandMBR-C).Ineachbioreactoramembranemodulewassub-merged.Theeffectivevolumeofthebioreactorwas12L.EachMBRwasasubmergedhollow bermembranemodulemadeofpolyethylenethathadatotalareaof0.1m2andanormalporesizeof0.1 m(DAIKI,Japan).PriortotheMBRsoperation,thesludgewasacclimatizedbyotherMBRs.TheMLSSconcentra-tionofeachactivatedsludgesuspensionwasadjustedtoabout6000mg/Lwithwaterpriortothemembrane ltration.Thecom-positionofthefeedwaterwasasfollows:sucrose(300mg/L),urea(78mg/L)anddipotassiumhydrogenphosphate(38mg/L)wereusedasthemainfeedforactivatedsludge,andcalciumchloridewasappliedasmineral.SodiumbicarbonatewasusedasabuffertoadjustthemixedliquorpHtoabout7.0.Thetemper-atureofthemixedliquorwascontrolledat25.0 Cwithelectricheaters.Thehydraulicretentiontime(HRT)rangedfrom10to12h,thesludgeretentiontime(SRT)wassetat30days.The

aerationintensitiesforMBR-A,MBR-BandMBR-Cwere150,400and800L/h,respectively.Theaveragedissolvedoxygenconcentrationsofthesethreeaerationintensitieswere3.21,4.76and6.50mg/L,respectively.

TheMBRcanbeoperatedintwomodes[16]:constant uxandconstanttransmembranepressure(TMP).WithrespecttotherealoperationofMBRsforwastewatertreatment,constant uxispreferabletoconstantTMP.ThemodeofconstantTMPissuitableforthestudyofmembranefoulingbecauseitcanprovidemoreinformationonmembranefouling.EventhoughconstantTMPisnotpreferable,therearestillmanyreportsabouttheapplicationofconstantTMPforlong-termwastewatertreatment[17–20].Inalltheseliteratures,theMBRswereoperatedwithagravitational ltrationmode,whichgeneratedaconstantTMP.Thegravitational ltrationmodehadlowenergyconsumptionandwascost-effectivetobuild[18].Themajorobjectiveofourworkistoanalyzethemembranefoulingbehavior,butnotthetreatmentperformanceofMBRsystems.Therefore,thethreeparallelMBRswerealsooperatedwithgravitational ltrationmodeorconstantTMPmode.

ThroughouttheoperationofthethreeMBRs,themembranemodulewasdrivencontinuouslywithaconstantlow-pressure,TMP=3.97kPa,whichwasinducedbyawaterheaddrop( Z=40cm).InconstantTMPmode,themembrane uxwilldeclineduringmembrane ltrationasaresultofmembranefoul-ing.Whenthe uxwassmallerthan6L/m2h,themembranemodulesweretakenoutandcleanedby ushingwithtapwatertoremovethefoulingcakeonthemembranesurface.Thus,theextentofmembranefoulingdegreecouldbeexpressedbythefrequencyof ushing.2.2.EPSanalysis

TheextractionofboundEPSwasbasedonacationionexchangeresin(CER,Dowex-Naform)method[21]:300mLsludgesuspensionwastakenandcentrifugedat2000×gfor15minat4 C.Thesludgepelletswereresuspendedtotheiroriginalvolumeusingabufferconsistingof2mMNa3PO4,4mMNaH2PO4,9mMNaCland1mMKClatpH7.Then,thesludgewastransferredtoanextractionbeakerwithbaf esandtheCER(80g/g-MLSS)added.Thesuspensionwasstirredfortheselectedstirringintensity(900rpm)andextractiontime(1.5h)at4 C.TheselectedEPSwasharvestedby

centrifu-

Fig.1.Schematicofthesubmergedmembranebioreactors:1,feedtank;2,balancebox;3,bioreactor;4,membranemodule;5,electricheater;6,airpump;7,air owmeter.

F.Mengetal./SeparationandPuri cationTechnology 59 (2008) 91–10093

gationofasampleoftheCER/sludgesuspensionfor1minat12,000×ginordertoremovetheCER.Thesupernatantwascentrifugedtwicefor15minat12,000×gat4 Cinordertoremoveremaining occomponents.BoundEPSwasobtainedafter lteringthesupernatantthrougha0.22 mmembrane lter.TheboundEPSwasnormalizedasthesumofcarbohydrateandprotein,whichwereanalyzedusingphenol/sulfuric-acidmethodandfolinmethod[22],respectively.

2.3.SolubleCODandcolloidalCODanalysis

SolubleCODandcolloidalCODweremeasuredinordertodeterminewhichcomponentsinthesludgesuspensionweremainlyresponsibleforthe uxreduction[23].SupernatantCODwasdeterminedaftercentrifugingthemixedliquorfor2minat3000×g.SolubleCODwasobtainedafter lteringthesuper-natantthrougha0.22 mmembrane lter.ThecolloidalCODwasobtainedbysubtractingthesolubleCODfromthesuper-natantCOD.2.4.FTIRanalysis

Thefouledmembranemodulewastakenoutfromthebiore-actorand ushedwithpurewaterastheoperationoftheMBRswasterminated.About200mLwashedliquidwastakenandplacedinadryerat105 Cfor24htoobtaindryfoulants.AFTIRspectrometer(EQUINOX55,Bruker,Germany)wasusedtocharacterizethemajorfunctionalgroupsoforganicmattersinmembranefoulants.KBrpelletscontaining0.50%(drypowder)ofthesamplewaspreparedandexaminedintheFTIRspec-trophotometer.Thespectrumwascalculatedfromtheaverage of256scansoverthewavenumberrangingfrom4000to400cm1ataresolutionof4cm 1.2.5.Particlesizeanalysis

Thesludgeparticlesizedistributionsofsludgesuspensionweredeterminedbyfocusedbeamre ectancemeasurement(FBRM)(ModelM400L,Lasentec,Redmond,USA).Thepar-ticlesizedistributionofthesludgesupernatantwasmeasuredusingaMarlverncounter(Zeta100,UK).2.6.Evaluationof ltrationresistance

Membraneresistancewasevaluatedbytheresistance-in-seriesmodelasfollows:Rt=Rm+Rp+Rc=

TMP(1)

Theexperimentalproceduretogeteachresistancevaluewasasfollows[24–26]:(1)theresistanceofmembrane(Rm)wasesti-matedbymeasuringthewater uxofde-ionized(DI)water;(2)thetotalresistance(Rt)wasevaluatedbythe nal uxofsludgewastewatermicro ltration;(3)themembranesurfacewasthen ushedwithwaterandcleanedwithaspongetoremovethefoulingcakelayer.Afterthat,theDIwater uxwasmeasured

againtogettheresistanceofRm+Rp.Theporeblockingresis-tance(Rp)wascalculatedfromsteps(1)and(3),andthecakeresistance(Rc)obtainedfrom(2)and(3).

Cakeresistanceisrelatedsigni cantlytocakespeci cresis-tanceandcakemass:Rc=αmc

(2)

wheremcisthedrycakemassandαisthespeci cresistanceperunitcakemass,whichvarieswiththebulkmatrixpropertiesandTMP.2.7.Others

Dissolvedoxygenconcentration(DO)wasmeasuredbyaDOmeter(55/12FT,YSICorporation,USA).Themixedliquidofsuspendedsolids(MLSS)concentrationwasevaluatedbyStandardMethods[27].3.Resultsanddiscussion

3.1.Behaviorofmembranepermeation

Theevolutionofpermeate uxduringthemembrane ltra-tionofactivatedsludgeispresentedinFig.2a.Itisimportanttonotethathigheraerationintensityresultedinalowerfoulingrateintheinitial ltrationtime.Itiswellknownthatcakelayeronthemembranesurfaceisthemainfactorthatresultsinmem-branefouling,andtherearetwooppositeactionsthatregulatetherateofcakelayerformation:permeationdrag,whichisgen-eratedbypermeate ux,increasedwithoperationTMP,andbacktransport,consistedofBrowniandiffusion,inertialliftandshear-induceddiffusion[28].Thehigheraerationintensityinducedahighershearforce,andremovedthefoulingcakelayerfromthemembranesurface.Therefore,thefoulingratedecreasedwithincreasingaerationintensitiesintheinitial ltrationtime.Obvi-ously,thecurveofFig.2acanberoughlyseparatedintotwophases:from0thto400thhourasphaseI,from400thto1100thhourasphaseII.InphaseI,themembranefoulinginMBR-AandMBR-Bwasmoresevere,indicatingthattheaerationintensitymayhavesomenegativeimpactsonbiomasscharacteristics.InphaseII,themembranepermeationofthethreeMBRsreachedarelativelysteadyvalue,suggestingthatthesludgesuspensionineachMBRhadbeenacclimatized.

Itcanbeseenthatthepermeate uxofMBR-AandMBR-Cdeclinedrapidlyafter10h ltration.ForMBR-A,itmayberesultedfromtheformationoffoulingcakelayerduetotheloweraerationintensity.TherewasahigheraerationintensityinMBR-C,butthepermeate uxalsodeclinedrapidly.Thepermeate uxofMBR-Breachedasteadyvalueafter100h ltration,suggestingthattheformationofafoulinglayerthatisactingasa“dynamicmembrane”withlowerpermeabilitythantheoriginal ltrationmembrane[29].Itshowsthattherateofparticleconvectiontowardsthemembranesurfaceisbalancedbytherateofbacktransport.Therefore,forMBR-B,severemembranefoulingshouldnotoccurwithrespecttotime.Assoonasadynamicmembraneformed,themembranefoulantssuch

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InthewholeoperationofthethreeMBRs,thefoulingdegreeofthethreemembranemodulesmainlyresultedfromtwofactors:differenceofshearforceandchangeofbiomasscharac-teristics.Intheinitial4h,thechangeofbiomasscharacteristicscouldbeignored,andthemembranepermeate uxwasbasi-callyin uencedbytheshearforce.Therefore,thedirectimpactofaerationintensitiesonmembranefoulingcouldbeobtainedbycomparingthepermeate uxofthethreeMBRsintheinitial4h,thedataareshowninFig.2b.Theaerationintensityhasapositiveeffectonmembranepermeability,suggestingthattheshearforcegeneratedbyairbubblescaneffectivelyremovethefoulantsdepositedonthemembranesurface.

Besidestheaerationintensityeffectonmembranefoulants,itmayhavesomeeffectonbiomasscharacteristicsalsosinceMBRsystemincludeslivingmicroorganismsandtheirmetabo-lites.Thebiomasscharacteristicswouldinturnhavesigni cantimpactsonmembranefoulingduringmembrane ltrationofsludgesuspension[31–34].Inordertoinvestigatethein u-enceofbiomasscharacteristicsonmembranefouling,short-termmembrane ltrationtestswereperformedassoonasthelong-termexperimentwasterminated.Intheshort-term ltrationtests,thesameaerationintensity,150L/h,wasadoptedforMBR-A,MBR-BandMBR-Cinordertoexcludetheeffectofshearforceonmembrane ux.Thepermeate uxofMBR-AandMBR-Bdecreasedslowlyandhadasimilardecreasetendency,Fig.2c.FromFig.2c,itcanbeseenthatMBR-Chadadramaticmembranefoulingproblem.Theseresultsshowedthattoohighaerationintensityaffectedthebiomasscharacteristicsde nitely,whichmayleadtotoomuchreleaseofEPSandthebreakageofsludge ocs.

3.2.Evolutionofbiomasscharacteristics

3.2.1.Particlesizedistributionsofsludgesuspension

Thesludgeparticlesizewasmeasuredafterthesludgesus-pensionhadbeenacclimatized.TheparticlesizedistributionsmeasuredbytheFBRMsystemcouldrevealthesizedistribu-tionsofthesludgesuspensionparticlesintheMBRs.StatisticresultsofthesludgeparticledistributionaresummarizedinTable1.Itcanbeeasilyseenthatthesizeofthesludgepar-ticlesvariedinarangeof3–450 m,andmorethan70%ofthesludgeparticleshadasizerangingfrom10to100 m.Thepeakpoints,indicatingthelargestparticlesizedistribution,andthemeansizeinthepro lesweregivenintheorderofMBR-A>MBR-B>MBR-C.Themeanparticlesizewascalculatedonthebasisofnumberofparticles.

Itwasreportedthatthesludgeparticlesthathaveasizesmallerthan50 mwouldaffectthemembranepermeationsig-

parisonofmembranepermeate ux:(a)evolutionofthemembranepermeate uxinthelong-term ltrationtests,(b)evolutionofmembraneper-meate uxintheinitial4hshort-term ltrationand(c)short-termmembrane ltrationtestsunderthesameaerationintensity(150L/h)afterthelong-termtests.

asEPS,solubleorganics,colloidalparticlesandsoon,couldberejectedorbiodegradedbythedynamicmembranecomposedoflivingmicroorganisms[30].Thus,thefoulantshavefewerchancestodepositonthemembranesurface.

Table1

StatisticresultsofsludgeparticlesizedistributionsinMBR-A(150L/h),MBR-B(400L/h)andMBR-C(800L/h)

Sludgeparticlesize( m)Mean

MBR-AMBR-BMBR-C

484130

Range3–4643–3983–398

Peak937454

Particlesizedistribution(%)<10 m0.1260.2260.826

10–50 m19.50426.68947.258

50–100 m49.60049.73639.531

100–200 m29.31622.11910.373

>200 m1.4561.2032.001

F.Mengetal./SeparationandPuri cationTechnology 59 (2008) 91–10095

ni cantly[35].Wenotedthatonly20%ofthesludgeparticleshaveasizesmallerthan50 minMBR-A,whereasmorethan48%ofthesludgeparticlesdistributedasizerangefrom0to50 minMBR-C(Table1).InMBR-C,thehighaerationinten-sitywasthemainfactorcausingtheformationofsmallparticles.FromTable1,italsocanbeseenthattherewere0.826%parti-clesthathaveasizesmallerthan10 minMBR-C.However,therewereonly0.126%and0.226%particlesthathaveasizesmallerthan10 minMBR-AandMBR-B.Althoughthehigheraerationintensitycouldinduceaneffectivebacktransport,thesmallparticlesinsludgesuspensionhadastrongtendencytodepositonthemembranesurface.Thehigheraerationintensitygeneratedastrongershearstress,andthenresultedinasevere ocbreakage.Thebreakageofthesludge ocscertainlyduetoerosionstrengthsortoaruptureofthenetworkofpolysaccha-ride brilswhichisthesupportofthedifferentcompoundsandparticularlythecells[36].

3.2.2.Particlesizedistributionsofsludgesupernatant

After30minsettlement,thesupernatantinthesludgesus-pensionwassampledanditsparticlesizedistributionwasmeasured.WithrespecttothesupernatantinMBR-A,therewasasharppeakat60nm(Fig.3a),suggestingthatmost

ofthesmallparticlesorsolutesinthesupernatantdistributedinthesizeof60nm.Thisresultalsoindicatesthatitwasarelativehomogeneoussystem.Withrespecttothesludgesuper-natantinMBR-B,thereweretwosigni cantpeaks:200and800nm,suggestingthereweretwoclassesofparticlesormacro-moleculesolutesinthissupernatant.InFig.3c,therewerethreesigni cantpeakswhichdistributedat:150,700and6000nm,respectively.ThisresultsuggeststhatthesupernatantinMBR-CwasmoreheterogeneousthanthoseinMBR-AandMBR-B.Theheterogeneoussystemcanresultinacomplexmembranefoulingduetothecomplexinteractionbetweentheseparticlesandsolutes.Asynergisticfoulingbehaviorwasfoundduringmembrane ltrationofcolloidalmaterialsanddissolvedmat-ters[37].Thesynergisticfoulingbehaviorisattributedtothehinderedbackdiffusionoffoulantscausedbytheinteractionsbetweenorganicandcolloidalfoulants,whichresultinfasterandmoresubstantialfoulantdepositiononthemembranesurface[37].

Thepeaksat60–800nmwereduetothepresenceofcol-loidsandsolutes,whichcausedbythereleaseofboundEPSfromsludge octosludgesuspension.But,thepeakat6000nmindicatedthepresenceofnon-settleablecellsor ocfragmentsinthesludgesuspension,whichfurthersuggeststhatthe

high

Fig.3.ParticlesizedistributionsofthesludgesupernatantinthethreeMBRs:(a)MBR-Awithanaerationintensityof150L/h,(b)MBR-Bwithanaerationintensityof400L/hand(c)MBR-Cwithanaerationintensityof800L/h.

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Fig.4.Evolutionof(a)EPSconcentration,(b)colloidalCODand(c)solubleCODduringlong-termmembrane ltrationtests.

aerationintensitycouldleadtoseverebreakageofthesludge ocs.

3.2.3.ChangeofEPS,colloidalCODandsolubleCOD

Inthiswork,thesumoftotalproteinsandcarbohydrateswasconsideredtorepresentthetotalamountofEPSbecausethesearethedominantcomponentstypicallyfoundinextractedEPS[38].ResultsfromsomerecentstudiesindicatethatmainlyproteinandcarbohydrateintheEPScontributetothedeclineofthepermeate ux[39,40].ItwasthusexpectedthatthequantityofEPSwouldcorrelatetomembranefouling.TheEPSconcentrationofMBR-AwassmallerthanthatofMBR-BandMBR-C,indicatingthatthehigheraerationintensitycausedthereleaseoftoomuchEPS(Fig.4a).Duringthewholeexperiment,theEPSconcentrationsofMBR-BandMBR-C

increaseddramatically,thendecreasedandreachedsteadyval-ues.

Ithasbeenobservedthatcolloidalparticlesinthesludgesus-pensionhaveparticularimpactonmembranefouling[41,42].Thesolubleproductscanbereadilydepositedontothemem-branesurfacesbypermeationdrag,andnotreadilydetachedbyshearforceduetoitslowbacktransportvelocity[43].Theevo-lutionofthecolloidalCODofsludgesuspensionisgiveninFig.4b.Wenotethatatthebeginningofthetest,thecolloidalCODofthethreeMBRsincreaseddramatically,andhadhighvaluesfrom100to300h.EventhoughthecolloidalCODofthethreeMBRsdecreasedslowlyandreachedasteadyvalueafterabout400h,thecolloidalCODofMBR-CandMBR-BhavehighervaluesthanthatofMBR-A.Inaddition,asitcanbeseenfromFig.4c,thereisasimilarchangingtendencyofsolubleCODforMBR-A,MBR-BandMBR-C.

Theseresultsindicatethattheintensiveshearstressledto ocbreakageandcausedanincreaseofcolloidalparticlesandsolutesinsludgesuspension.Becausethecolloidalparticleandsoluteshavesmallersize,theycouldresultinaseveremembranefouling.FromFigs.2aand4,wecanseethatEPS,colloidalCODandsolubleCODmayhavesigni cantrelationwithmembranepermeation,thatiswhythepermeatebehaviorofthethreeMBRscouldbeseparatedintotwophases(Fig.2a).

Moreover,theDOconcentrationinducedbyaerationwouldhavesomeeffectonbiomasscharacteristics.Inactivatedsludgeprocess,iftheDOconcentrationistoolow(<2.0mg/L),itcanresultinsludgebulkingbecauseoftheovergrowthof lamen-tousbacteria.But,theDOconcentrationsofthethreeMBRswerechangedfrom3.2to4.76mg/L,andto6.50mg/L,respec-tively.ItindicatesthattherewereenoughDOinthethreeMBRs;therefore,thein uenceofDOconcentrationonsludgepropertycanbeignored.Additionally,theMLSSconcentrations(datawasnotshown)ofthethreeMBRshadlittlechangeinthewholelong-term ltrationtests,sotheimpactofMLSSconcentrationonmembranefoulingalsocanbeignored.3.3.Analysisofmembranefoulants

3.3.1.Deposition/adsorptionmechanismofmembranefoulants

Asthelong-termoperationwasterminated,thethreemem-branemodulesweretakenoutfromthebioreactorsand ushedbypurewater.Thesuspendedsolids(SS),colloidalCOD(CODc)andsolubleCOD(CODs)ofthewashedliquidwereevaluatedtoquantifythefoulantsthataccumulatedonthemem-branesurface(Table2).Atthesametime,thecomponentsofthesludgesuspensionineachMBRweremeasuredtointer-pretthedeposition/adsorptionmechanismofmembranefoulants(Table2).Evidently,themembranefoulantsandsludgesuspen-sionconsistedofsludgeparticles,colloidalparticlesandsolutes.ItcanbeseenthattheSS,whichmainlyconsistedofsludge ocs,decreasedsigni cantlywithincreasingaerationintensity,indi-catingthattheshearstressinducedbyaerationhasgreateffectonthedepositionoflargeparticles.Astheaerationintensityincreasedfrom150to400L/h,therelativecontentofcolloidalparticlesdecreasedfrom8.05%to5.38%,however,therelative

F.Mengetal./SeparationandPuri cationTechnology 59 (2008) 91–100

Table2

Analysisresultsofthemembranefoulants

Componentsofmembranefoulants(%)SS(g/m2)

MBR-AMBR-BMBR-C

34.6(87.31)22.6(88.11)7.9(69.66)

CODc(g/m2)3.19(8.05)1.38(5.38)1.11(9.79)

CODs(g/m2)1.84(4.64)1.67(6.51)2.33(20.55)

Totalfoulants(g/m2)39.6325.6511.34

Componentsofsludgesuspension(%)SS(g/L)6.24(99.54)6.43(99.44)6.59(99.23)

CODc(g/L)0.015(0.24)0.019(0.29)0.027(0.41)

CODs(g/L)0.014(0.22)0.017(0.26)0.024(0.36)

97

Total(g/L)6.276.476.64

contentofsolubleproductsincreasedfrom4.64%to6.51%.Italsocanbeseenthattherelativecontentofcolloidalparti-clesandsolutesonmembranesurfaceweremuchhigherthantheirrelativecontentinthesludgesuspension,con rmingthatthecolloidsandsoluteshaveastrongertendencytodepositontothemembranesurfacethanthesludge ocs.Table2alsodepictsthatinMBR-CthecontributionofcolloidalparticlesandsolutestothetotalfoulantswaslargerthanthatinMBR-AandMBR-B,implyingthatastheMBRoperatedunderhighaera-tionintensity,colloidalparticlesandsolubleproductsmaybethemajorfoulantstomembranefouling.Accordingtoprevi-ousliteratures[43–45],ithasbeenshownthatthecolloidsanddissolvedorganicmatterareresponsibleformembranefoul-ingduringmembrane ltrationofsludgesuspension.Therefore,thecolloidsandsolutesinsludgesuspensionshouldbecon-trolledduringthelong-termoperationofMBRsinordertoobtainpreferablemembranepermeation.

Inthemembrane ltrationprocessunderaconstantTMP,the ltrationprocesscanbedividedintothefollowingtwostages:accumulationstageofundetachablecakelayer,andasecondstagewhereaccumulationanddetachmentofcakelayerreachequilibriumstage[46].InertialliftisthedominantmechanismforlargeparticlesandhighshearrateswhereasBrowniandif-fusionisthedominantforsmallparticlesandlowshearrates[47,48].However,shear-induceddiffusionseemstobethemostimportantoneforintermediateparticlesizesandshearrates[47,48].FromTable2,itcanbeseenthatastheaerationintensityincreasedfrom150to400L/h,thedepositionofcolloidsonthemembranesurfacedecreasedfrom3.19to1.3g/m2,however,thedepositionofsoluteshadlittlevariation.ItisclearthattheBrowniandiffusionwasthemainbacktransportmechanismforMBR-A,butBrowniandiffusionandshear-induceddiffusioncoexistedinMBR-B.Astheaerationintensityincreasedfrom400to800L/hinMBR-C,theconcentrationofthelargeparti-clesdecreaseddramatically.Thedataobtainedfromthecurrentinvestigation,togetherwithpreviousworkintheliterature,con- rmthatthebacktransportmechanismforMBR-CconsistedofBrowniandiffusion,shear-induceddiffusionandinertiallift.

Table3

Analysisresultsof ltrationresistance

Items(%)Rm(1011m 1)

MBR-AMBR-BMBR-C

1.05(6.02)1.05(10.76)1.05(9.44)

Rp(1011m 1)2.45(14.06)2.71(27.77)4.18(37.59)

FromTable2,itcanbeseenthatthesolubleproductsandcolloidalparticleshadagreatcontributiontothecakelayerastheMBRoperatedunderhighshearforcecondition.Itindicatesthatlargeaerationintensitycaninducetheformationofanon-porouscakelayer.Thisisthereasonwhythepermeate uxofMBR-CdecreasedmoreabruptlythanthatofMBR-AandMBR-B.InMBR-A,thelowershearforcecouldnotremovethefoulinglayereffectively,andhenceresultedintheformationofathickerfoulinglayer(seeTable3)onthemembranesurfacewhichwouldincreasethemembrane ltrationresistancestrongly.

3.3.2.Evaluationof ltrationresistance

Toexaminethefoulingtendencies,cakemass,speci ccakeresistanceandeachresistancetermwereanalyzed(Table3).DuringtheoperationofMBRs,sludge ocs,colloidsandsolutesdepositedonthemembranesurface,thecakeresistancebecamethedominantresistance.Thecontributionofcakeresistancetototalresistancehadarangefrom52.97%to79.40%.Table3alsoshowsthatthecakeresistanceinMBR-AwasmorethantwotimesofthoseinMBR-BandMBR-C,indicatingthataera-tionhadgreatimpactsontheremovalofcakelayer.Furthermore,theporeblockingresistanceincreasedwithincreasingaerationintensity,especiallyforMBR-C.ThisresultsuggeststhatthereoccurredasevereirreversiblefoulinginMBR-C.Thisresult,togetherwithTable2,showsthatunderhighaerationinten-sitythedepositionandadsorptionofcolloidsandsolutesonmembranewouldresultinsevereporeblockingorirreversiblefouling.

Theeffectofaerationoncakemassandspeci cresistanceisshowninTable3.Thecakemassdecreaseddramaticallyasaer-ationintensityincreased,however,thespeci ccakeresistanceincreasedde nitelyastheaerationintensityincreasedfrom400to800L/h.Itindicatesthatthedepositionofcolloidsandsolutesonmembranesurfacewouldformadensecakelayer.AccordingtoCarmanKozenyequation,thespeci cresistanceissigni -cantlyin uencedbysludgeparticlesizeandcakeporosity.InMBR-C,thecolloidsandsolutesweresigni cantcontributorstothefoulantsthatdepositedonthemembranesurface,soitcouldformadensecakelayer.

Rc(1011m 1)13.84(79.40)6.00(61.48)5.89(52.97)

Rt(1011m 1)17.439.7611.12

mc(g/m2)39.6325.6511.34

α(1010m/g)3.492.345.19

98F.Mengetal./SeparationandPuri cationTechnology

59 (2008) 91–100

Fig.5.FTIRspectraofthemembranefoulants:(a)MBR-A,(b)MBR-Band(c)MBR-C.

F.Mengetal./SeparationandPuri cationTechnology 59 (2008) 91–10099

3.3.3.FTIRanalysisofmembranefoulants

Ingeneral,theFTIRtechniquecanprovidemoredetailedinformationaboutthedepositionofbiopolymersonthemem-branesurface.TheFTIRspectraofmembranefoulantsinthethreeMBRsarepresentedinFig.5.Theyaresimilarinthepro- lebutsigni cantlydifferentintheadsorptionintensity.Thespectrumshowsabroadregionofabsorptionat3400cm 1,whichisduetothestretchingoftheO–Hbondinhydroxylfunc-tionalgroups[49].Therewasabroadpeakat1100cm 1,whichisduetoC–Obondsandisassociatedwithalcohols,ethersandpolysaccharides.InMBRs,thispeakisusuallyattributedtothepresenceofpolysaccharidesorpolysaccharide-likesub-stances.Choetal.[50]attributedthispeaktopolysaccharidesorpolysaccharide-likemembranefoulants;whereasThurman[51]attributedthispeaktosilicateimpuritiesinhumicsamples.AsshowninFig.5,therearetwopeaks(1640and1550cm 1)inthespectrumwhichareuniquetotheproteinsecondarystructure,calledamidesIandII[52].TheamideIisthestretchingvibrationbandsassociatedprimarilywiththepeptidecarbonyls(CO),andtheamideIIbandsat1550cm 1isduetotheinteractionbetweentheN–HbondingandtheC–NstretchingoftheC–N–Hgroup[53].Thisresultindicatesthepresenceofproteinsinmem-branefoulants.Basedonthepeakat1380cm 1,themembranefoulantscontainedamediumamountoflipids[54].Jarusutthiraketal.observedasigni cantpeakat1720cm 1duringmembrane ltrationofwastewatertreatmentplantef uent[55].Thispeakisassociatedwithcarboxylicgroups,representingatypicalchar-acteristicsofhumicandfulvicacids.Inourwork,thispeakwasabsentindicatingthattherewasnohumicorfulvicacidsinthemembranefoulantsortheamountofhumicandfulvicacidsinthemembranefoulantscouldbeignored.Thepresenceofproteins,polysaccharidesandlipidsinmembranefoulantssug-gestsasigni cantorganicfoulingwhichmainlyresultedfromEPS.

FromFig.5,italsocanbeseenthattheabsorptioninten-sityofFTIRspectrumsinthethreeMBRswasdifferentfromeachother.Theintensitywasgivenintheorderof:MBR-C>MBR-B>MBR-A.Theabsorptionintensityre ectedtherelativeamountofbiopolymersinthetotalfoulants.Therefore,theabove-mentionedresultindicatesthattherelativeamountofbiopolymersorEPSinthetotalfoulantsalsofollowedtheorder:MBR-C>MBR-B>MBR-A.ThisresultcoincidedwiththeresultsobtainedfromSection3.3.1.4.Summary

Thispaperpresentsacomparativeandcorrelativestudyofaerationintensitiesonmembraneperformance.ThemembranefoulingmechanismsofthreeparallelMBRswereinvestigatedfromtwoaspects:analysisofsludgecharacteristicsandevalua-tionofmembranefoulants.Fromtheresultsreportedhere,thefollowingconclusionscanbedrawn:

Aerationintensityhadsigni cantimpactsonmembraneper-meation.Smallorlargeaerationintensityhadanegativein uenceonmembranepermeability.Lowaerationcouldnotremovethemembranefoulantsfrommembranesurfaceeffec-

tively.However,highaerationcouldinduceaseverebreakageofsludge ocs.

Thehighaerationintensitygeneratedastrongershearstress,andthenresultedinasevere ocbreakage.Thesmallsizeofsludgeparticlesgeneratedatthehighershearstresswasincloseassociationwiththedramaticmembranefouling.Furthermore,thehighshearconditioncouldgenerateahet-erogeneoussludgesupernatant,andresultedinacomplexmembranefouling.Thehighshearconditionscouldpromotethereleaseofcolloidalandsolublecomponentsfromthemicrobial ocstothebulksolutionduetomicrobial ocbreakageandthuscausearapidlossinmembraneperme-ability.

Themembranefoulantsconsistedofsludge ocs,colloidalparticlesandsolutes.ThecontributionofcolloidalparticlesandsolutestothemembranefoulantsbecamemoreimportantastheMBRoperatedunderlargeaerationintensity.ThethreeMBRshaddifferentbacktransportmechanismoffoulantsdeposition.Underaerationintensityof150L/h,Browniandif-fusionwasthemainbacktransportmechanismformembranefoulants,whichcouldnotremovalthecakelayereffectively.ThecakeresistanceofMBR-A(150L/h)wasmorethantwotimesofMBR-B(400L/h)andMBR-C(800L/h),indicatingthataerationhasgreatimpactsontheremovalofcakelayer.Thehighaerationintensity(800L/h)couldresultinseveremembraneporeblocking.WiththehelpofFTIRtechnique,themajorcomponentsoforganicmattersinthemembranefoulantswereidenti edasproteins,polysaccharidemattersandlipids.Acknowledgement

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