外文翻译饮用水处理通过用金属膜制成的凝固微滤来去除天然有机物质大学论文文档格式.docx
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外文翻译饮用水处理通过用金属膜制成的凝固微滤来去除天然有机物质大学论文文档格式.docx
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ToroveLeiknes∗,HallvardØ
degaard,Hå
vardMyklebust
DepartmentofHydraulicandEnvironmentalEngineering,NTNU,NorwegianUniversityofScienceandTechnology,S.P.Andersensvei5,N-7491Trondheim,Norway
Abstract
DrinkingwatersourcesinNorwayarecharacterizedbyhighconcentrationsofnaturalorganicmatter(NOM),lowpH,lowalkalinityandlowturbidity.TheremovalofNOMisthereforeinmanycasesageneralrequirementinproducingpotablewater.DrinkingwatertreatmentplantsarecommonlydesignedwithcoagulationdirectfiltrationorwithNFspiralwoundmembraneprocesses.Thisstudyhasinvestigatedthefeasibilityandpotentialofusinginorganicmetalmicrofiltrationmembranesinasubmergedmembraneconfigurationwithcoagulationpre-treatmentfordrinkingwaterproduction.Variationsinoperatingmodesandconditionsweretested,fromdead-endoperationtosemisequencingbatchoperationusingairscouringandbackwashingcyclesformembranecleaningandfoulingcontrol.Fluxesaround180LMHattransmembranepressuresbelow0.3barwhereachievedoverproductioncyclesinexcessof50h.Treatmentefficienciesingeneralshowed>
95%colourremoval,∼85%UVremoval,65–75%TOCremovaland<
0.2NTUturbidityandnon-detectablesuspendedsolidsinthepermeate.TheinitialresultsshowthatMFmetalmembranesisaninterestingalternativetosandfiltrationincoagulation/directfiltrationfortreatingdrinkingwater.
Keywords:
Naturalorganicmatter、Coagulation–microfiltration、Metalmembranes
1.Introduction
About90%ofNorwegiandrinkingwatersuppliesarefromsurfacewatersources,generallyfromlakeswhichtypicallyhaveverylowturbidity,alkalinityandhardnessbuthighcolourresultingfromnaturalorganicmatter(NOM).OneofthemajorproblemsofusingsurfacewatersourcesinnorthernclimatesishighcontentofNOMandtotalorganiccarbon(TOC).RemovalofNOMisrequiredsincecolouredwaterisunattractivetoconsumers,resultsincolouringofclothesduringwashing,cancauseodorandtaste,increasescorrosionandbiofilmgrowthinthedistributionnetwork,andisaprecursortotheformationofdisinfectionby-products(DBP)whenwaterisdisinfected.HalogenatedcompoundsresultingfromchlorinationofdrinkingwatercontainingconcentrationsofNOMhasbeenamajorconcernsincetheirdiscoveryintheearly1970sassomeofthechlorinationby-productsarecarcinogenic.DrinkingwatersourcesinNorwayarecommonlydescribedashavinghighcolour,lowpHandlowalkalinitywheretypicalvaluesaregiveninTable1.TheremovalofNOMisthereforeamajortreatmentrequirementintheproductionofpotablewaterwhereconcentrationstypicallyinthe30–80mg/LPttruecolourrangearereducedtolessthan10mg/LPt.
ThemostcommondrinkingwatertreatmentplantdesignsinNorwayarebasedoncoagulationanddirectfiltrationornanofiltration(NF)membranefiltrationprocesses[13].Coagulationdirectfiltrationplants(enhancedcoagulation)arestillthedominanttreatmentplantdesignoption.
Inthelast10–15yearsmembraneprocessesbasedonnanofiltration(NF)usingspiralwouldmoduleconfigurationshavebeensuccess-fullyusedinNorwayforremovingNOM,andapproximately100membraneplantsareinoperationtoday.TheNFmembraneplantsarecommonlydesignedtooperatewithaconstantfluxof∼17Lm−2h−1(LMH)atatransmembranepressure(TMP)of3–6barwithawaterrecoveryof∼70%.SomeofthedisadvantagesoftheNFspiralwoundmembranesystemsusedarearelativelylowrecovery,highenergyconsumptionsduetotheoperatingpressures,andfoulingbytheNOMandsub-micronparticulatesresultingintheneedforadailycleaningprocedureinadditiontotheperiodicmaintenancecleaningprocedures[13,14].Inarecentsurveyaboutexperienceswithdifferenttreatmentplanttypes,boththeoperatorsandownersoftreatmentplantsusingmembranesweregenerallyverysatisfiedwithusingmembranetechnology.However,thesurveyalsoindicatedaninterestinalternativemembranetreatmentplantdesignsthatweremoreenergyefficientandwhichcouldreducethenecessarycleaningfrequencybyefficientfoulingcontrol.Twoapproachescanbefollowedtoachievethis;
usingvariouspre-treatmentoptionsoftherawwaterpriortotheNFfiltrationunitsorbyusingdifferenttypesofmembranes,membranemodulesandoperatingoptions.Studiesusingmicrofiltration(MF)andultrafiltration(UF)membranesaswellasalternativemembranemoduledesigns(hollowfibercross-flowmodulesandsubmergedmodules)combinedwithpre-treatmentbycoagulationtoreduceandcontrolfoulinghavebeenreported[1,4–7,9–11].Theyalldemonstratetheadvantagesandben-efitsofcombiningcoagulationpre-treatmentwithmembranefiltrationwhenUFandMFmembranesareused.
Membranesindrinkingwatertreatmentarecommonlybasedonspiralwoundsystemsorcross-flowhollowfiber/tubularsystems.Thesemembraneprocessesarepres-suredrivenmembranemodulesandmountedindifferentarraydesignstooptimizetheprocess.Energycostsrequiredtopressurizethemembranevesselsandmaintainhighenoughfluidcross-flowvelocitiesoftenisasubstantialcomponentofthesesystems.Submergedmembranedesignsofferanewapproachbothtothemembranemoduledesignandlowpressureoperatingconditionswhichcanbebeneficialfortheoverallenergyrequirementstooperateaprocess.Thesubmergedmembraneprocessdesigncombinedwithcoagulationpre-treatmentwaschosenforthisstudyasanalternativetreatmentprocesstocoagulationdirect-flirtationfortheremovalofNOM.Inorganicmetalmembraneswerealsochosenasthemembraneisbothchemicallyandphysicallyrobust,allowingforalternativecleaningstrategiesforfoulingcontrolcomparedtowhatisfeasiblewithpolymericmembranes.
TheobjectiveofthisstudyhasbeentoinvestigatethefeasibilityandpotentialofinorganicMFmetalmembranescombinedwithcoagulationforthetreatmentofdrinkingwaterfromhighlycolouredsurfacewater.Alowpressuresub-mergedmembranemoduleconfigurationwaschosencombinedwiththecoagulationpre-treatment.Themetalmem-braneshavebeensuppliedbyHitachiMetalsLtd.,Japan.
2.Experimental
2.1.Productionofrawwater
Alltheexperimentsinthisstudywereconductedwithfeedwaterhavingacolourof50mg/LPtatpH7whichistypicalandrepresentativeforNorwegianrawwatersources.ThefeedwatertothemembranereactorwaspreparedusingaNOMconcentratefromafull-scaleionexchangetreatmentplantbymixingtheconcentrateintotapwatertomakeupthedesiredcomposition.Analysisofthereconstructedwatershowedthatthefeedwaterisrepresentativeofthenaturalwatersource.Reconstructedfeedwaterwaschosenforthestudytomaintainthesameinitialconditionsforallexperimentsconductedsuchthattheperformanceoftheprocessundervaryingoperatingconditionscouldbeevaluatedandcompared.Hydrochloricacid(HCl)wasusedforpHadjustmentandcontroltoensureoptimalpHof6.3±
0.2forthecoagulationstep.Thereconstructedrawwaterwithacolourof50mg/LPthadaDOCconcentrationof6.1±
0.25mg/LCandaUV254-absorbanceof31.1±
1.1m−1.
Thecoagulantusedwasapolyaluminiumchloride(PAX-16),aqueoussolutionfromKemiraChemicalsAS.Preliminarycoagulationtestswerefirstconductedinjar-teststofindtheoptimumpHandcoagulantdosagenecessarytoremovetheNOM.Dosagesof2,3,4and5mg/LAlweretestedattheoptimalpHof6.3±
0.1todeterminethecolourremoval.Resultsrevealedthataspecificaluminiumdosageof5mg/LAlremoved94%oftruecolour,87%ofUV-absorbingcompounds,and71%ofDOC[9].Theremovalofcolourdidnotincreasemuchfromadosageof4–5mg/LAl,however,theremovalofDOCdidin-creaseaswellastheZetapotentialoftheparticlesformed.Adosageof5mg/LAlwasthereforechosenasthepreferredcoagulantamount.TheZetapotentialoftheparticlesformedincreasedfromaround−22to+5mVwithincreasingdosage.ThisincreaseintheZetapotentialwasalsoconsideredbeneficialcomparedtothelowerdosageswhichgavenegativeZetapotentialvalues.However,theaverageZetapotentialoftheparticlesinthemembranereactorweremeasuredtobearound−7.75±
4.19mV.Thelowervaluefoundmaybeduedifferentconditionsinthemembranereactorcomparedtothejar-testssuchaseffectsofsludgeconcentration,hydraulicandflocculationconditions,how-ever,thevaluemeasuredisclosetoaneutralchargewhichisbeneficialfortheaggregateformation.Allexperimentswerethereforeconductedwithacoagulantdoseof5mg/LAlproducingafeedwatertothemembranereactorwithapHofaround6.3±
0.2.
Flocculationofthefeedwaterwasdoneusingapipeflocculatortomaintainarapiddevelopmentofthemicro-flocs.Thepipeflocculatorwasdesignedwithahydraulicretentiontime(HRT)of30sandahydraulicgradientGof400s−1.Thesuspendedsolidsconcentrationinthefeedwateraftercoagulation/flocculationwithacoagulantdoseof5mg/LAlwasaround25mg/LSS.
2.2.Membranemodulespecification
ThemetalmembranesprovidedbyHitachiMetalLtd.aremadeassheets.Eachsheetisconstructedbysinteringmetalpowderinasupportlayertoformthemembrane.Thenominalporesizeofthemembranehasbeencharacterizedusingboththebubblepointmethodandaparticlesizeexclusionanalysis.Thesemethodsdeterminedthemembranehavinganominalporesizeof0.95and0.2m,respectively.Assuchthemembranecanbeclassifiedasan“open”microfiltrationmembrane,h
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