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本科生毕业设计外文文献翻译ControllingsludgesettleabilityintheoxidationditchprocessK.J.HartleyABSTRACTThispaperdescribesaninvestigationaimedatdevelopinganoperatingtechniqueforcontrollingsludgesettleabilityintheoxidationditchformofthenitrificationdenitrificationactivatedsludgeprocess.Itwashypothesizedthatspecificsludgevolumeindex(SSVI)islowestatanoptimumprocessanoxicfractionandincreasesathigherandlowerfractions.Usingeffluentammonia:nitrateratioasasurrogateforanoxicfraction,itwasfoundthatasimpleempiricalmodelbasedonathreesolidsretentiontimemovingaveragenitrogenratiowasabletoreplicatethelong-termSSVIvariationsintwoindependentoxidationditchesatafull-scaleplant.Operatingdatafromasecondoxidationditchplantduringperiodswhenaprefermenterwason-oroff-lineshowedthatSSVIalsovarieswithRBCOD,greaterRBCODgivinglowerSSVI.Itwasconcludedthatbestsettleabilityoccursataboutthesameanoxicfractionaslowesteffluenttotalnitrogenconcentration,withanammonia:nitrateratioofabout1.Anoperatingruleofthumbistousedissolvedoxygencontroltomaintaineffluentammoniaandnitratenitrogenconcentrationsaboutequal.Athirdoxidationditchplantdeliberatelyoperatedinthismannerachieved15-monthmedianoperatingvaluesforSSVIof60mL/gandforeffluentammonia,nitrateandtotalN,respectively,of0.2,0.3and2.0mgN/L.KeywordsSettleability；Sludgevolumeindex；SVI；Control；Nitrogen；Oxidationditch1IntroductionTheoxidationditchprocesshasthecapabilitytoreliablymeetlowtotalnitrogenstandardsbecauseofitsspecialoperatingcharacteristics.Nitrogenremovalcanbecontinuouslyoptimizedinoperationbyvaryingtheanoxicandaerobicmassfractionstomeetshort-andlong-termchangesinwastewatercharacteristics(Hartley,1997;MinesandWoods,1994).Ithasalsobeenobservedthatsludgesettleabilityinoxidationditchesvarieswiththeoperatingmassfractions(VivianandHartley,1989).Sludgesettleabilityisoneofthemostimportantdesignandoperatingcharacteristicsoftheactivatedsludgeprocess.Itaffectsboththesizingoftankageandtheprocesscapacityactuallyachieved.Yet,nearly100yearsaftertheactivatedsludgeprocesswasinvented(ArdernandLockett,1914),positivecontrolofthisparameterremainselusive.Fourslightlydifferenttestsareinuseformeasurementofsettleability,sludgevolumeindex(SVI,employingsimplecylindersettling,Mohlman,1934),stirredSVI(sSVI,inwhichthecylinderisfittedwithastirrer,APHAetal,1998),dilutedSVI(DSVI,usingserialdilutionsofthesludge,KoopmanandCadee,1983)andSSVI3.5,orsimplySSVI(stirredspecificvolumeindex,inwhichthesludgeconcentrationisstandardizedat3.5g/L,White,1975).Thevaluesoftheseparameterscanberelatedtothesettlingpropertiesofthesludge,andtoeachother,throughpublishedstatisticalcorrelations(Daigger,1995;Ekamaetal.,1997;OzinskyandEkama,1995).Arangeoffactorsaffectssettleabilityinactivatedsludgeprocesses,includingfeedquality,solidsretentiontime(SRT),mixingcharacteristics,oxygenregime,pHandnutrientavailability(Jenkinsetal.,2004).ForNandN&Premovalprocesses,theWaterResearchGroupattheUniversityofCapeTownhasconductedacomprehensivesettleabilitystudy,whichtheypublishedinalengthyseriesofpapersandsummarisedinEkamaetal.(1996),Caseyetal.(1999)andTsaietal.(2003).Theirstudyincludedintermittentaerationformatsmimickingtheoxidationditchprocess.TheyfoundthatthelowF/Mgroupoffilaments(suchasMicrothrixparvicellaand0092)causepracticallyallbulkingproblemsinlongSRTnutrientremovalprocessesandthattheycannotbecontrolledbyanaerobic,anoxicoraerobicselectors.Whetherornottheprocessincludesbiologicalphosphorusremoval,itistheoperatingnitrogenregimethatgovernssludgesettleability.Accordingtothemodeltheydeveloped,inatwo-stageanoxic–aerobicsystemutilisingmainlyslowlybiodegradableCOD,ifdenitrificationintheprimaryanoxiczoneisincompleteandnitriteispresent,theintracellularintermediatenitricoxidepersistsandinhibitsthedenitrifyingfloc-formersintheaerobiczone.Thefilamentousorganisms,whichdonotcompetewellintheanoxiczonebecausetheycanonlydenitrifynitratetonitrite,arenotinhibitedintheaerobiczonewheretheyproliferate.IntracellularnitricoxidedoesnotaccumulatewhentheCODisreadilybiodegradable(RBCOD)soasignificantfractionofRBCODrelievesthecompetitivepressureonthefloc-formers.Tsaietal.(2003)observedthatM.parvicellagrowthcouldbestimulatedorinhibitedbyvaryingtheconcentrationofammonia(theorganism’srequirednitrogensource)tohighorlowvalues,respectively.Theythereforepostulatedthatdeteriorationofsettleabilityatlowaerobicfractioncanbeduetoincreasingammoniaconcentration(decreasingnitrificationefficiency).2TheoxidationditchFig.1illustratesthegeneraloxidationditchreactorformat.Mixedliquorisimpelledaroundacircuitbyoneormoremechanicalaerators,orbymixersinthecaseofdiffusedairaeration.Thisprovidesatypicalmixedliquor:feedflowratioof100–200.Thedissolvedoxygen(DO)concentrationjumpsupattheaeratoranddeclinesdownstream.TheDOconcentrationattheaeratorequalstheoxygentransferratedividedbythecirculatingflowrate.TheanoxicfractioniscontrolledbyvaryingtheaerationratetomaintainaDOsetpointataprobelocatedtowardsthedownstreamendoftheaerobiczone.Theanoxicfractioncanbevariedbyadjustingthesetpoint(Hartley,1997).Fig.2showstheprocess-operatingcharacterist