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WaterResearch36(2002)1018–1024ApplicationofamembranebioreactorfortreatingexplosivesprocesswastewaterKyung-DukZoha,MichaelK.Stenstromb,*aDepartmentofEnvironmentalHealth,GraduateSchoolofPublicHealth,SeoulNationalUniversity,Seoul,SouthKoreabDepartmentofCivilandEnvironmentalEngineering,UniversityofCalifornia,LosAngeles,CA90095,USAReceived1August2000;accepted2May2001AbstractAbench-scaleanoxicmembranebioreactor(MBR)system,consistingofabioreactorcoupledtoaceramiccross-flowultrafiltrationmodule,wasevaluatedtotreatasyntheticwastewatercontainingalkalinehydrolysisbyproducts(hydrolysates)ofRDX.Thewastewaterwasformulatedthesameashydrolysiswastewaterandconsistedofacetate,formateandformaldehydeascarbonsourcesandnitriteandnitrateelectronacceptors.TheMBRsystemremoved80–90%ofthecarbonsources,andapproximately90%ofthestoichiometricamountofnitrate,60%ofnitrite.Thereactorwasalsooperatedoverarangeoftransmembranepressure,temperature,suspendedsolidsconcentration,andorganicloadingratetomaximizetreatmentefficiencyandpermeateflux.Increasingthetransmembranepressureandtemperaturedidnotimprovefluxsignificantly.Increasingmixedliquorvolatilesuspendedsolids(MLVSS)concentrationinthebioreactordecreasedthepermeatefluxsignificantly.Themaximumvolumetricorganicloadingratewas0.72kgCOD/m3/day.Themaximumfood-to-massratiowas0.50kgN/kgMLVSS/dayand1.82kgCOD/kgMLVSS/day.Membranepermeatewasclearandessentiallyfreeofbacteria,asindicatedbyheterotrophicplatecount.Permeatefluxrangedbetween0.15and2.0m3/m2dayandwasmaintainedbyroutinebackwashingeverythreedays.Backwashingwithtapwatercontainingchlorinebleacheveryfourthorfifthbackwashingwasabletorestoremembranefluxtoitsoriginalvalue.r2002ElsevierScienceLtd.Allrightsreserved.Keywords:Membranebioreactor(MBR);Hexahydro-1,3,5-trinitro-1,3,5-triazine(RDX);Hydrolysates;Ceramictubularmembrane;Acetate;Formate;Formaldehyde;Nitrate;Nitrite;Ultrafiltration1.IntroductionTheendofthecoldwarcreatedaworldwidesurplusofbothconventionalandnuclearweapons.TheUnitedStatesandothercountriesaredestroyinglargequan-titiesofweapons,whichcreatestheneedforsafeandreliabledisposaltechnologiesforenergeticmaterialsandwastewatersassociatedwiththeirprocessing.Amongthehighexplosivesthataremanufacturedandusedinweapons,RDX(Hexahydro-1,3,5-trinitro-1,3,5-triazine)isoneofthemostcommon,andisclassifiedasapossiblehumancarcinogen[1].PreviousworkinourlaboratoryoverthepastfouryearshasshownthatRDXcanbereadilydestroyedbyalkalinehydrolysisinsolutionoronthesurfaceofactivatedcarbon[2].Thehydrolysisprocessproducesconcentratedwastewaterscontainingacetate,formalde-hyde,formate,ammoniaandnitrite.Theresultingwastewaterrequiresadditionaltreatment,whichcanbeaccomplishedinapacked-bedreactor.Usingnitriteastheelectronacceptor[3],over90%ofeachorganiccarbonsourcewasremoved,andtheresultscloselymatchthestoichiometrypredictedbyempiricalredoxequations.*Correspondingauthor.Tel.:+1-310-825-1408;fax:+1-310-206-5476.E-mailaddress:stenstro@seas.ucla.edu(M.K.Stenstrom).0043-1354/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.PII:S0043-1354(01)00284-6Thisinvestigationextendsthepreviousworkusingamembranebioreactor.Membranetechnologyhasat-tractedmuchattentionfromscientistsandengineersinrecentyearsasanewseparationprocessinwaterandwastewatertreatment.Theseseparationprocesseshavebeenappliedtotreatmunicipalandindustrialwaste-waters.Combiningmembraneswithbiologicaltreat-mentisanattractivetechniqueandhasresultedinanewconcept:themembranebioreactor(MBR)[4].Therecentdevelopmentofanewgenerationofmoreproductiveandlessexpensiveultrafiltration(UF)andmicrofiltra-tion(MF)membranesmakethispossible.IntheMBR,theentirebiomassisconfinedwithinthesystem,providingbothexactcontroloftheresidencetimeforthemicroorganismsinthereactor(solidsretentiontime)andthedisinfectionoftheeffluent.Theobjectiveofthisstudyistoinvestigatetheperformanceofabench-scalecontinuousflowanoxicMBRsystem,consistingofabiologicaldenitrificationreactorcoupledtoaceramicultrafiltrationmembranemodule,anditseffectivenessinthedegradationofRDXhydrolysate.2.Methodology2.1.DenitrifyingcultureThereactorwasstartedusinganinoculumfromapacked-bedreactortreatingsynthetichydrolysateovera2-yearperiod[3].Theinitialcultureforthepacked-bedreactorwasobtainedfromananaerobicdigesterattheHyperionWastewaterTreatmentPlantinElSegundo,California.Theseculturesweredilutedwithoxygen-freephosphatebuffer,filtered,andincubatedfor3weeksinaminimalmediumcontaining0.2g/LNH4Cl,0.1g/LMgCl2H2O,0.04g/LCaCl22H2O,and0.02g/LNa2SO3.Thesecultureswerealsosupple-mentedwithRDXhydrolysatesandtracemineralsolution.ThepHwasmaintainedbetween7.0and7.5,whichistheoptimumrangefordenitrification,byaddingsmallamountsof0.5MH3PO4dailytocompensatepHincreasefromthedenitrificationandtoprovidephosphorusasanutrient.2.2.FeedcompositionThefeedsolutionwaspreparedusingtheratioof1.6MNO2,1.5MHCOO,0.1MCH3COO,and1.1MHCHO,whichisthesameasRDXhydrolysates[2].Nitrate(NO3)wasaddedtobalancetheelectronacceptoranddonortoallowallcarbonandnitrogentoberemoved.Usingthisapproach,Zohetal.[3]foundthatover90%ofeachorganiccarbonsourcecouldbere