Investigationandmitigationofscalingduringcleaninplace(CIP)processinlowpressuremembranestreatingsurfacewater

Abstract

정밀여과(Microfiltration,MF)및한외여과(Ultrafiltration,UF)를포함한저압막여과(Low-pressuremembrane)는음용수및다목적여과수생산을위해지표수(surfacewater)를처리하는수처리여과기술중하나이다.하지만저압막여과수처리시스템은여과시발생하는막오염(Membranefouling)으로인해여과효율및분리막교체주기가감소될수있어주기적인세척이필요하다.저압막여과시스템의지속적인여과수생산을보장하기위해서다양한여과세정조건및방법이연구되고있다.연구문헌에따르면,분리막의다양한물리적및화학적세정방법이논의되고있으나막오염원과분리막세정의상호작용으로발생하는MF/UF막의노후화현상에대한연구는부족한상황이다.MF/UF막을물리적으로세정할경우,여과효율이초기상태로돌아갈것이라예상하지만남아있는오염원으로인해일정수준까지만회복이되고화학적세정을통해비가역적오염원(Irreversiblefouling)을제거하여여과효율을초기상태로되돌릴수있지만이또한막이구조적으로손상되는단점이있다.본연구에서는MF/UF막의구조적손상을일으키는오염원도출및오염원과분리막세정의상호작용을통한MF/UF막의구조적변화에대하여연구되었으며(Chapter3),여과효율(Filtrationefficiency),여과플럭스감소(Filtrationfluxdecline),막의투수성(Membranepermeability),막의특성(Membranecharacterization)및막의완결성(Membraneintegrity)을포함한다양한결과가막오염물질에대해어떠한영향을미치는지에대해서도세부적으로연구되었다.또한,비가역적오염을완화시키는MF/UF전처리방법을최적화하여경제적으로효율적인방식으로분리막의수명을연장시키는것과관련된연구가진행되었으며(Chapter4),복합인공원수기반의여과를통해다양한막오염모델및R2값을통해비교평가하였다(Chapter5).본연구에서는저압막의완결성손실을완화시키기위해다양한실험조건을토대로분리막과오염원의상호작용을규명하고이를완화시키는고도제어시스템을제안하고자한다(Chapter6).저압막여과시스템의화학세정(Cleaninplace,CIP)중발생하는스케일링(Scaling)을유발하는오염원을규명하기위하여폴리비닐리덴플루오라이드(Polyvinylidenefluoride,PVDF)막이사용되었으며(Chapter3),분리막의화학세정은Deionizedwater(DIwater)및여과수에서제조된용액을활용하여연구를진행하였다.막의특성결과로는알루미늄(Al)이화학세정동안분리막의무기염축적의주요원인으로밝혀졌으며,망간(Mn)과같이존재할때플럭스회수율이93.2%에서85.69%(CIPw/DI),81.92%(CIPw/permeatewater)로더욱악화되는것을확인하였다.또한,금속이온의전하중화현상으로인한분자간복잡한형태를이루어이의상호작용에너지도증가하였고특히,알루미늄은분자복합체와분리막표면사이의정전기적반발력을감소시켜결과적으로오염물의축적을유도하였다.따라서,금속이온농도의증가(유입수혹은화학세정)는AB상호작용을유발하고비극성특성은Al>Mn>Fe순으로분리막의친수성이감소하는것을증명하였다.여과과정에서다양한종류의알루미늄(particulate(Alc),mononuclear(Ala),andpolymer(Alb))을분석한결과고분자알루미늄이화학세정물질과알칼리상호작용하고알칼리조건에서Humicacid와응집및흡착으로인해비가역적오염원을발생시키는것을확인하였다.상대적으로더높은Ala제거효율은알칼리조건에서Ala가Alb로가수분해되는증거를제공하며,그때문에Alb종은분리막표면에침착된채로남아있어CIP(PW)세척에서회복불가능한비가역적오염원형성의원인으로판단된다.비가역적오염원을통해발생하는스케일링의경향을완화시키기위해Chapter4에서그방법을제시하였다.pH8.5,전산화처리1.5mg.L-1에서5.4%의최소비가역적오염과88.8%의플럭스회복율을확인하였으며,막의특성및가역적오염분석을통해전산화처리용액이화학세정동안스케일링을덜발생시켜더낮은접촉각증가와안정적인계면에너지결과를통해분리막의계면특성이적게감소됨을확인하였다.전산화처리는Humicacid의카르복실기를감소시키고브리징(bridging)에사용되는결합부위의수가적어오염물축적이감소되었다.그외에도알칼리조건에서Al(OH)3로변형된반응성Al종(Alb)는음전하를띈Humicacid와함께흡착되어분자간반발력이증가하여분리막오염의가능성이감소하였다.초기및화학세정에의해처리된분리막에대한복합인공원수의여과성능을평가하기위해다양한분리막오염모델에대한R2및비교평가가수행되었다(Chapter5).비교평가를통해Hermia와Bowen방정식모델이분리막완전차단모델(Completeblockingmodel)이주를이룬것을통계적으로확인하였으며,각모델이실험조건별로구체적결과를나타내었다.하지만,막오염과정에서무기오염물을포함한복합인공원수를해석하기위해서는Hermia방정식모델을통해해결해야한다.전처리최적조건(Chapter4)이다양한여과방법(정압제어및일정플럭스)에서연속식으로평가되었으며,분리막의노후화및완결성평가와관련된연구는Chapter6에서진행하였다.Chapter6일정플럭스연속식여과에서잔류무기오염물질과화학세정물질의상호작용으로인해발생하는분리막의탈불소화및산화작용으로인해분리막의기공이심각한손상과구조변화를나타내었다.일정플럭스여과방식은SDI(Siltdensityindex)에의해검증된바와같이정압제어보다막오염가능성이전산화처리유무와상관없이더컸으며,전산화처리에노출된분리막은일정플럭스여과방식및정압제어방식에서초기분리막플럭스의79%및83%의회복율을보여주었다.분리막오염의경향은정압제어및일정플럭스여과방식에서전산화처리를활용할때14.54%와14.78%를보여주었다.개선Hermiamodel의막오염분석은케이크층오염형성이전산화처리유무에상관없이정압및일정플럭스여과에주를이루었고모두완전차단(Completeblocking),중간차단(Intermediateblocking)그리고표준차단(Standardblocking)이그뒤를있는오염메커니즘을보여주었다.분리막의완결성평가는두가지방법(정압제어및일정플럭스여과)으로전산화처리에노출되지않았을때,분리막의압력손실이더큰것으로확인하였고압력감소는허용한계보다더높았음을확인하였다.분리막손상의유형을검토한결과분리막의완결성손상이긁힘현상으로확인되었고전산화처리가없는복합인공원수에서분리막이여과및화학세정에노출되었을때구조적으로분리막표면손상이더욱심한것으로확인되었다.|Low–pressuremembrane(LPM)filtration,includingmicrofiltration(MF)andultrafiltration(UF),isapromisingtechnologyforthetreatmentofsurfacewaterfordrinkingandotherpurposes.Despitetheadvantagesoverconventionalsystem,LPMsystemsaresusceptibletomembranefouling,whichcanreducethemembranelifespanandthusrequirescleaningtorecovermembraneperformance.VariousconfigurationsandoperationalsequenceshavebeendevelopedtoensurethesustainableprovisionofcleanwaterthroughLPMssystem.Intheliterature,variousperiodicphysicaland/orchemicalapproachestothecleaningofLPMshavebeenreported,whereaslittledataisavailableontheagingofMF/UFmembranesresultingfromtheinteractionbetweenthefoulantsandthecleaningagent.Periodicphysicalcleaningofthemembraneisexpectedtoreturnthemembranetoitsoriginalperformancecapacity,butitonlyrecoverstoacertainlevelbecausetheremainingfoulantscauseirreversiblefouling.Chemicalcleaningcanthenbeemployedtorecoverthemembranefromthisirreversiblefoulingwhereas,intheprocess,itcancauseirrecoverabledamagetothemembrane.InthisdissertationthefoulantsresponsibleforirrecoverabledamagetoMF/UFmembranesareanalyzed,andconsequently,theirinteractionwithcleaningagentsandotherfoulantsisdescribed(chapter3).Theimpactofthesefoulantsonvariousmembraneparameters,includingfiltrationefficiency,fluxdecline,permeability,membranecharacterization,andmembraneintegrityarealsodiscussedindetail.Inaddition,mitigationoptionsbyoptimizingthepretreatmentmethodologyarealsodiscussedwithregardtoincreasetheoperationallifespanofamembraneinacost-effectivemannerbyovercomingirreversiblefouling(chapter4).Furthermore,aninvestigationhasbeenmadeforcomparativeevaluationofR2andparticipationequationfordifferentfoulingmodelstoelucidatethefiltrationtheoryforcomplexfeedsolutionmixture(chapter5).Ultimately,thisresearchsuggestsanadvancedcontrolsystembasedonmembrane−foulantinteractionsundertheimpactofvariousoperationalparameterstomitigatetheintegritylossofmembranes(chapter6). TheidentificationoffoulantsresponsibleforscalingduringchemicalcleaningofaLPMsystemwascarriedoutbyusingpolyvinylidenefluoride(PVDF)membraneinbatchscaleexperiments(Chapter3).Chemicalcleaning(cleaninplace(CIP))ofthemembranewasconductedbyusingsolutionspreparedindeionizedwater(DI)andpermeatewater(PW).ThemembranecharacterizationresultsconfirmedthatAluminum(Al)wasthemajorcontributortotheaccumulationofinorganicsaltsonthemembraneduringchemicalcleaninganditsrolewasmoresevereinthepresenceofManganese(Mn)bydecreasingthefluxrecoveriesfrom93.2%to85.69%and81.92%,forthepristinetoCIP(DI)andCIP(PW)cleanedmembrane.Moreover,theincreaseininteractionenergieswerealsoobservedduetointermolecularcomplexformation(withdeprotonatedcarboxylgroup)bythechargeneutralizationpropertiesofmetalionsparticularlyAl,whichdecreasestheelectrostaticrepulsiveforcesbetweencomplexandmembranesurface,whichconsequentlyresponsibleforthedepositionoffoulants.Thus,theincreaseinmetalionconcentration(eitherinfeedsolutionorinCIP(PW))resultedinincreaseintheABinteraction.Moreover,thenonpolarnatureofthemembraneincreasesintheorderofAl>Mn>Feexposedmembrane,whichprovidesthesignificantevidenceforthedecreaseinhydrophilicityofthemembrane.TheanalysisofdifferentspeciesofAl(particulate(Alc),mononuclear(Ala),andpolymer(Alb))duringthefiltrationprocessconfirmedthatpolymericAl(Alb)interactwithCIPchemicalsandresponsibleforscalingowingtosweepcoagulationandadsorptionwithhumicacid(HA)inalkalineconditionsoftheenvironment.Moreover,thecomparativehigherAlaremovalefficiencyprovidestheevidenceofhydrolysisofAlatoAlbinalkalineenvironment.Owingtothat,Albspeciesremaindepositedonmembranesurfaceandconsideredtoberesponsiblefortheformationofirrecoverablescales,particularlyinCIP(PW)cleaning. Toovercomethescalingpropagationalterationoffeedwaterbypreoxidationissuggestedtobeoptimummethodology(chapter4).Theminimumirreversiblefoulingof5.4%withhighestfluxrecoveryof88.8%ofinitialvaluewasobservedat1.5mg.L-1preoxidationdoseatpH8.5.Membranecharacterizationandfoulingreversibilityanalysisconfirmedthatfeedsolutionwithpreoxidationproducedlessscalingduringchemicalcleaningandshowssignificantlystableinterfacialenergieswithlowerincreaseincontactangleanddecreaseininterfacialpropertiesofthemembrane.Moreover,thepreoxidationreducesthecarboxylicgroupoftheHAbyalterationofthepropertiesandasmallernumberofbindingsiteswereavailableforthebridging,resultstoloweraggregation.Inadditiontothat,reactiveAlspecie(Alb)transformedintoAl(OH)3inalkalineconditionssorbwiththenegativechargedHA,withincreasestheintermolecularrepulsionthatwouldalsoberesponsibleforlowerthefoulingpotential. AninvestigationhasbeencarriedoutforcomparativeevaluationofR2andparticipationequationfordifferentfoulingmodelstoelucidatethefiltrationtheoryforcomplexfeedsolutionmixtureforpristineandCIPtreatedmembranes(chapter5).TheresultsindicatedthatHermiaandBowen’smodelshowsbetterstatisticalcorrespondencewithconfirmingthecompleteblockingdominance.Inaddition,italsoverifiesthateachdevelopedmodelisparticularaboutitsexperimentconditions.Nevertheless,theroleofcomplexmixtureincludinginorganicfoulantsinthefoulingprocessneedstobeaddressedwiththeHermia’sfoulingmodelforfutureprospectsandlong-termanalysis. Theimpactofoptimizedpretreatmentconditions(discussedinchapter4)onmembranedegradationwereevaluatedforcontinuousscaleunderdifferentfiltrationmode(constantpressure(CP)andconstantflux(CF))andresultsrelatedtoageingandintegrityevaluationwerediscussed(chapter6).Resultsshowsseveredamagetothemembraneporesandstructurevariationwiththeuseofuntreatedfeedwater,particularlyunderCFmode,duetothedefluorinationandoxidationofthemembranearisingfromtheactionoftheresidualinorganicfoulantsandcleaningchemicals.CFmodealsohadgreaterfoulingpotentialthanCPmodewithorwithoutchemicallyenhancedpretreated(CEPT)asillustratedbythesiltdensityindex(SDI).MembranesexposedtoCEPTfeedwaterexhibitedarecoveryof79%and83%oftheinitialmembranefluxunderCFandCPmodes.Thefoulingpropensityofthemembraneswas14.54%and14.78%whenusingCEPTfeedwaterunderCPandCFmodesforcontinuousanalysis,respectively.ModifiedHermia’sfoulingmodelanalysisalsoconfirmedthatcakelayerformationwasthedominantfoulingmechanismfollowedbycomplete,intermediate,andstandardblockingforbothCFandCPfiltrationwithorwithoutCEPTfeedwater.TheintegrityevaluationofthemembranerevealedamoresignificantlossinpressureforwithoutCEPTexposedmembraneinbothoperationalmodes.Thepressuredecayratesweremuchhigherthantherecommendedlimits.Areviewofmembranedamagetypeconfirmedthattheintegritybreachesofmembraneswereduetoscratches.StructuralanalysisofthemembranesconfirmedthatthesemembraneshavescratchesontheirsurfacesduetothesevereeffectsofchemicalexposureandtheinteractionofcomplexfoulantsinthefeedwaterparticularlyforthemembraneexposedtowithoutCEPTfeedwater.