2010ESTPartialnitrificationresponseofnitriteoxidiz

PartialNitrification/DenitrificationCanBeAttributedtotheSlowResponseofNitriteOxidizingBacteriatoPeriodicAnoxicDisturbancesM.KORNAROS,*,†S.N.DOKIANAKIS,§ANDG.LYBERATOS†,‡DepartmentofChemicalEngineering,UniversityofPatras,1KaratheodoriStreet,GR26500Patras,Greece,InstituteofChemicalEngineeringandHighTemperatureChemicalProcesses,GR26504Patras,Greece,andTechnologicalEducationalInstituteofCrete,SchoolofAgriculturalTechnology,Stavromenos,GR71004HeraklionCrete,GreeceReceivedFebruary18,2010.RevisedmanuscriptreceivedJune2,2010.AcceptedJune8,2010.Thisworkaimstoassessandmodelthebehaviorofbothammonium(AOB)andnitrite(NOB)oxidizingbacteriaduringthetransitionfromcompletelyanoxictoaerobicconditions.AnenhancedaerobicallygrownculturecontainingAOBandNOBwassubjectedtoanoxicconditionsofvaryingdurationsfrom1.5to12hbeforeitsexposuretoaerobicconditions.Experimentswerecarriedoutinbothcontinuouslystirredtankreactor(CSTR)andbatchtypereactors.AlthoughtheAOBdidnotexhibitanyimpactintheirperformancefollowingtheanoxicdisturbance,theNOBwereseriouslyinhibitedpresentingaperiodofreducedgrowthrate,whichwasproportionaltothedurationofthedisturbance.Thisfindingprovesthepreviouslypostulatedmechanism(NOBinhibitionunderperiodicaerobic/anoxicoperation)forachievingnitrogenremovalviathepartialnitrification/denitrification(PND)processasdemonstratedinlab-andpilot-scaleoperatingconditions.AmathematicalmodelwasdevelopedtodescribewithsufficientaccuracytheperformanceofAOBandNOBunderaerobic,anoxic,andtransientconditionsinbothCSTRandbatchtypesystems.ThemodelisabletodescribetheinhibitoryeffectofanoxicexposuretoNOBbyassumingenzymedeactivation(underanoxicconditions)andreactivation(adjustmentoftheNOBenzymaticmechanism)underaerobicconditions.ThepresentedkineticmodelisquitesimpleandgeneralandthereforemaybeusedforpredictingtheperformanceofmixedgrowthbiologicalsystemsoperatingviathePNDprocess.1.IntroductionThemostwidelyusedprocessforbiologicalammoniumremovalisthebiologicalnitrification-denitrification(ND)systemthatconvertsammoniumintomolecularnitrogen.Moreprecisely,inthenitrificationprocess,ammoniumisconvertedtonitritebyammonium-oxidizingbacteria(AOB)andnitriteisoxidizedtonitratebynitrite-oxidizingbacteria(NOB):Denitrificationisafive-stepprocessinwhichnitrateisreducedtonitriteandfinallytomolecularnitrogen:Duringthepastdecadeseveralnewnitrogenremovalprocesseshavebeendevelopedtotreatmunicipalandindustrialwastewaterswithmediumorhighammoniumload.Specialattentionhasbeenfocusedonpartialnitrification-denitrification(PND)(1–3),eitherasastand-aloneprocessorcoupledwithAnammox(4),suchasOLAND(5),CANON(6),andotherprocesses.Inthestand-alonePNDprocess,ammoniumoxidationtakesplaceuptothelevelofnitrite(viaAOB)andthennitriteisdenitrifiedbydenitrifyingbacteria(DB)tomolecularnitrogen:ComparedtoconventionalN-removal,PNDpresentssignificantadvantages,asittheoreticallysavesapproximately25%ofelectronacceptor(oxygen)fornitrification,40%ofelectrondonor(organiccarbon)fordenitrification,andachievesalowersludgeproduction(1–3,7).Inaddition,ithasbeenreported(7,8)thatnitritereductionenhancesthedenitrificationrateby63%witha33-35%lowersludgeproductioninnitrificationprocessand55%indenitrificationprocess.Moreover,PNDcancontributetothereductionofCO2emissionsby20%duetothedenitrificationfromnitriteinsteadofnitrate(8).Manyresearchershavereportednumerousimportantfactorsthatmakeitpossibletoac-complishpartialnitrification-denitrification,suchasdis-solvedoxygen(DO)concentration,pH,temperature,freeammonia(FA)concentration,andfreenitrousacid(FNA)concentration(9–15).Amongthevariousmethodsthathavebeenproposedforpartialnitrification,theexploitationofthetime-lagexhibitedbyNOBunderalternatingaerobic/anoxicconditionsisbyfarpreferable,asitdoesnotrequireanyadditionofchemicalsorextremegrowthconditions(e.g.,temperature),butasimplemanipulationoftheoperatingconditions.Thekeypoint,accordingtothisstrategy,istoenhancethenitritificationprocess,whileinhibitingorsuppressing,atthesametime,thenitratificationprocesstoobtainamixedliquorbiomassenrichedinAOBandpoorinNOB.DependingonthedifferenceintheeffectivegrowthratesofAOBandNOB,theeffluentofthesystemcanrangefrombasicallynitritetoamixtureofammoniumandnitriteandalsoamixtureofnitriteandnitratewithlowammoniaconcentration(16,17).Ifthenitratificationprocessisinhibitedtoanextentthatthemini-mumSRTrequiredforNOBgrowthishigherthanthecurrentSRTofthesystem,thenNOBwillbewashedoutandtheeffluentwillnotcontainnitrate.Ourresearchteamhasdemonstrated(3)thatusingafrequentenoughswitchingbetweenaerobicandanoxicconditionsinasequencingbatchreactor(SBR)andanaerobictoanoxicphaseratiospecifictothetreatedwastewater,itispossibletoachievePND,i.e.,partiallyoxidizeammoniatonitrite(aerobically)andthensubsequentlyreducenitritetonitrogengas(underanoxicconditions),withoutproducing*Correspondingauthorphone:[phone];fax:[phone];e-mail:[e-mail].†UniversityofPatras.§TechnologicalEducationalInstituteofCrete.‡InstituteofChemicalEngineeringandHighTemperatureChemicalProcesses.NH4+98AOBNO2-(ammoniumoxidation)NO2-98NOBNO3-(nitriteo