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DepartmentofCivilandEnvironmentalEngineeringSungllyunllwanUniversity,300,Chunchun-dong,Jangan-gu,Suwon-Si440-746,RepuhlicofKorea

Keywords:

A2/OreactorMBRNutrientremovalTMP

ABSTRACT

Inthepresentstudy,anadvancedsewagetreatmentprocesshasbeendevelopedbyincorporatingexcesssludgereductionandphosphorousrecoveryinanA2O-MBRprocess.TheA2O-MBRreactorwasoperatedatafluxof77LMHoveraperiodof270days.Thedesignedfluxwasincreasedstepwiseoveraperiodoftwoweeks.ThereactorwasoperatedattwodifferentMLSSrange.ThermochemicaldigestionofsludgewascarriedoutatafixedpH（11）andtemperature（75℃）for25%CODsolubilisation.Thereleasedpbospborouswasrecoveredbyprecipitationprocessandtheorganicswassentbacktoanoxictank.ThesludgedigestiondidnothaveanyimpactonCODandTPremovalefficiencyofthereactor.Duringthe270daysofreactoroperation,theMBRmaintainedrelativelyconstanttransmembranepressure.Theresultsbasedonthestudyindicatedthattheproposedprocessconfigurationhaspotentialtoreducetheexcesssludgeproductionaswellasitdidn'

tdetonatedthetreatedwaterquality.

@2008ElsevierLtd.

1.Introduction

Excesssludgereductionandnutrientsremovalarethetwoimportantproblemsassociatedwithwastewatertreatmentplant.MBRprocesshasbeenknownasaprocesswithrelativelyhighdecayrateandlesssludgeproductionduetomuchlongersludgeageinthereactor（Wenetal.,2004）.SludgeproductioninMBRisreducedby28-68%,dependingonthesludgeageused（Xiaetal.,

2008）.However,minimizingthesludgeproductionbyincreasingsludgeageislimitedduetothepotentialadverseeffectofhighMLSSconcentrationsonmembrane（Yoonetal.,2004）.Thisprob-

lemcanbesolvedbyintroducingsludgedisintegrationtechniqueinMBR（Youngetal.,2007）.Sludgedisintegrationtechniqueshavebeenreportedtoenhancethebiodegradabilityofexcesssludge（VlyssidesandKarlis,2004）.Inoverall,thebasisforsludgereductionprocessesiseffectivecombinationofthemethodsforsludgedisintegrationandbiodegradationoftreatedsludge.Advancesinsludgedisintegrationtechniquesofferafewpromisingoptionsincludingultrasound（Guoetal.,2008）,pulsepower（Choietal.,2006）,ozone（Weemaesetal.,2000）,thermal（Kimetal.,2003）,alkaline（Lietal.,2008）acid（Kimetal.,2003）andthermochemical

（VlyssidesandKarlis,2004）.Amongthevariousdisintegrationtechniques,thermochemicalwasreportedtobesimpleandcosteffective（WeemaesandVerstraete,1998）.Inthermal-chemicalhy-

drolysis,alkalisodiumhydroxidewasfoundtobethemosteffectiveagentininducingcelllysis（Rockeretal.,1999）.

Conventionally,thenutrientremovalwascarriedoutinanA2Oprocess.Ithasadvantageofachieving,nutrientremovalalongwithorganiccompoundoxidationinasinglesludgeconfigurationusinglinkedreactorsinseries（Tchobanoglousetal.,2003）.Thephosphoroesremovalhappensbysubjectingphosphorousaccumulatingorganisms（PAO）bacteriaunderaerobicandanaerobicconditions（AkinandUgurlu,2004）.TheseoperatingproceduresenhancepredominancePAO,whichareabletouptakephosphorousinexcess.Duringthesludgepretreatmentprocessestheboundphosphorouswassolubilisedanditincreasesthephosphorousconcentrationintheeffluentstream（Nishimura,2001）.So,itisnecessarytoremovethesolubilisedphosphorusbeforeitentersintomainstream.Besides,thereisagrowingdemandforthesustainablephosphorousresourcesintheindustrializedworld.Inmanydevelopedcountries,researchesarecurrentlyunderwaytorecoverthephosphoroesboundinthesludge'

sofenhancedbiologicalphosphorusremovalsystem（EBPR）.Thereleasedphosphorouscanberecoveredinusableproductsusingcalciumsaltsprecipitationmethod.Keepingthisfactinmind,inthepresentstudy,anewadvancedwastewatertreatmentprocessisdevelopedbyintegratingthreeprocesses,whichare:

（a）thermochemicalpretreatmentinMBRforexcesssludgereduction（b）A2Oprocessforbiologicalnutrientremoval（c）Precoverythroughcalciumsaltprecipitation.Theexperimentaldataobtainedwerethenusedtoevaluatetheperformanceofthisintegratedsystem.

2.Methods

2.1.Wastewater

Thesyntheticdomesticwastewaterwasusedastheexperimentalinfluent.Itwasbasicallycomposedofamixedcarbonsource,macronutrients（NandP）,analkalinitycontrol（NaHCO3）andamicroelementsolution.Thecompositioncontained（L-'

）210mgglucose,200mgNH4C1,220mgNaHCO3,22一34mgKH2PO4,microelementsolution（0.19mgMnCl24H20,0.0018mgZnCl22H2O,0.022mgCuCl22H2O,5.6mgMgSO47H2O,0.88mgFeCl36H2O,1.3mgCaCl2·

2H2O）.Thesyntheticwastewaterwaspreparedthreetimesaweekwithconcentrationsof210±

1.5mg/Lchemicaloxygendemand（COD）,40±

1mg/Ltotalnitrogen（TN）and5.5mg/Ltotalphosphorus（TP）.

2.2.A2/O-MBR

TheworkingvolumeoftheA2/O-MBRwas83.4L.Abafflewasplacedinsidethereactortodivideitintoanaerobic（8.4L）anoxic（25L）andaerobicbasin（50L）.Thesyntheticwastewaterwasfeedintothereactorataflowrateof8.4L/h（Q）usingafeedpump.Aliquidlevelsensor,plantedinaerobicbasinofA2O-MBRcontrolledtheflowofinfluent.TheHRTofanaerobic,anoxicandaerobicbasinswere1,3and6h,respectively.Inordertofacilitatenutrientremoval,thereactorwasprovidedwithtwointernalrecycle（1R）.IRl（Q=1）connectsanoxicandanaerobicandIR2（Q=3）wasbetweenaerobicandanoxic.Anaerobicandanoxicbasinswereprovidedwithlowspeedmixertokeepthemixedliquidsuspendedsolids（MLSS）insuspension.Intheaerobiczone,diffuserswereusedtogenerateairbubblesforoxidationoforganicsandammonia.Dissolvedoxygen（DO）concentrationintheaerobicbasinwasmaintainedat3.5mg/1andwasmonitoredcontinuouslythroughonlineDOmeter.Thesolidliquidseparationhappensin

aerobicbasinwiththehelpoffiveflatsheetmembraneshavingaporesizeof0.23pm.Theareaofeachmembranewas0.1m2.Theywereconnectedtogetherbyacommontube.Aperistalticpump

wasconnectedinthecommontubetogeneratesuctionpressure.Inthecommontubeprovisionwasmadetoaccommodatepressuregaugetomeasuretransmembranepressure（TMP）duringsuction.Thesuctionpumpwasoperatedinsequenceoftiming,whichconsistsof10minswitchon,and2minswitchoff.

2.3.Thermochemicaldigestionofsludge

MixedliquorfromaerobicbasinofMBRwaswithdrawnattheratioof1.5%ofQ/dayandsubjectedtothermochemicaldigestion.ThermochemicaldigestionwascarriedoutatafixedpHof11（NaOH）andtemperatureof75℃for3h.Afterthermochemicaldigestionthesupernatantandsludgewereseparated.Thethermo-chemicallydigestedsludgewasamenabletofurtheranaerobicbio-degradation（VlyssidesandKarlis,2004）,soitwassenttotheanaerobicbasinoftheMBR

2.4.Phosphorusrecovery

Limewasusedasaprecipitanttorecoverthephosphorousinthesupernatant.Aftertherecoveryofprecipitantthecontentwassentbacktoanoxictankasacarbonsourceandalkalinitysupelementfordenitrification.

2.5.Chemicalanalysis

COD,MLSS,TP,TNoftherawandtreatedwastewaterwereanalyzedfollowingmethodsdetailedin（APHA,2003）.Theinfluentandeffluentammoniaconcentrationwasmeasuredusinganion-selectiveelectrode（TheretoOrion,Model:

95一12）.Nitrateinthesamplewasanalyzedusingcadmiumreductionmethod（APHA,2003）.

3.Resultsanddiscussion

Fig.1presentsdataofMLSSandyieldobservedduringtheoperationalperiodofthereactor.OneoftheadvantagesofMBRreactorwasitcanbeoperatedinhighMLSSconcentration.ThereactorwasseededwithEBPRsludgefromtheKiheung,sewagetreatmentplant,Korea.ThereactorwasstartupwiththeMLSSconcentrationof5700mg/L.Itstartstoincreasesteadilywithincreaseinperiodofreactoroperationandreachedavalueof8100mg/Londay38.Fromthenonwards,MLSSconcentrationwasmaintainedintherangeof7500mg/LbywithdrawingexcesssludgeproducedandcalledrunI.Theobservedyields（Yobs）forexperimentswithoutsludgedigestion（runI）andwithsludgedigestionwerecalculatedandgiveninFig.1.TheYobsforrunIwasfoundtobe0.12gMLSS/gCOD.Itwascomparativelylowerthanavalueof0.4gMLSS/gCODreportedfortheconventionalactivatedsludgeprocesses（Tchoba-noglousetal.,2003）.ThedifferenceinobservedyieldofthesetwosystemsisattributedtotheirworkingMLSSconcentration.AthighMLSSconcentrationtheyieldobservedwasfoundtobelow（Visva-nathanetal.,2000）.AsaresultofthatMBRgeneratedlesssludge.ThepresentlyusedMLSSranges（7.5一10.5g/L）areselectedonthebasisoftherecommendationbyRosenbergeretal.（2002）.Intheirstudy,theyreportedthatthegeneraltrendofMLSSincreaseonfoulinginmunicipalapplicationsseemstoresultinnoimpactatmediumMLSSconcentrations（7一12g/L）.

Thethermochemicalsludgedigestionwasstartedonday70bywithdrawingsludgeattheratioof1.5％Q/day.Thesludgedigestionperiodwasdividedintotwophasesnamely,runII（day70-139）andrunIII（day140-210）.DuringrunII,theMLSSconcentrationinMBRwasmaintainedaround7500mg/LandforrunIIIitwasmaintainedaround10500mg/L.Bothofthesetworuns（IIandIII）demonstratetheroleofsludgedisintegrationincontrollingtheexcesssludgeproduction.TheYobsforrunIIandIIIwerefoundtobe0.03gMLSS/gCOD,respectively.Itaccountsfor58%and75%ofsludgereductionwhencomparedtorunI.TheobservedyieldforrunIIIwasfoundtobelowerthanrunII.Thisisduet