给排水专业英语Unit14.docx

给排水专业英语Unit14.docx

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Unit14Activatedsludgeprocess

Theactivatedsludgeprocessisnowthemostwidelyusedbiologicalprocessfortreatmentofmunicipalandindustrialwastewaters.ActivatedsludgeprocesswasdevelopedinEnglandin1914andwassonamedbecauseitinvolvedtheproductionofanactivatedmassofmicroorganismscapableofaerobicstabilizationoftheorganiccontentofwastewater.

ThebasicactivatedsludgeprocessisillustratedbyFigure14.1.Themixtureofinfluentwastewatercommonlycomingfromprimaryclarifierandreturningactivatedsludgeisknownasmixedliquor.Inactivatedsludgeprocess,themixedliquorflowstoaerationtankinwhichthemicroorganismsresponsiblefortreatmentarekeptinsuspensionandaerated.Intheaerationtank,contacttimeisprovidedformixingandaeratinginfluentwastewater.Aerationservesatleastthreeimportantfunctions

（1）mixingthemixedliquor,

（2）keepingtheactivatedsludgeinsuspension,and（3）supplyingtheoxygentothebiochemicalreactionsnecessaryforthestabilizationofthewastewater.Oncethemixedliquorhasreceivedsufficienttreatment,excessmixedliquorisdischargeintosecondaryclarifier.Thefunctionofthesecondaryclarifieristoseparatetheactivatedsludgesolidsfromthemixedliquor.Thesesolidsrepresentthecolloidalanddissolvedsolidsthatwereoriginallypresentinthewastewater.Intheaerationunittheywereincorporatedintotheactivatedsludgefloc,whicharesettleablesolids.Theseparationofthesesolids,acriticalstepintheactivatedsludgeprocess,isaccomplishedinthesecondaryorfinalsettlingtanks.Theexcesssludgemustberemovedbeforeitlosesitsactivitybecauseofthedeathoftheaerobicorganismsresultingfromlackofoxygenatthebottomofthetank.Themostcommonpracticeistowastefromthesludgereturnline.Sometimeswastesludgereturnline.Sometimeswastesludgeisremovedbywithdrawingmixedliquordirectlyfromtheaerationtank.Thewastedmixedliquorcanthenbedischargedtoathickeningtankortotheprimarytankswherethesludgesettlesandmixeswiththerawprimarysludge.Thewastesludgeisfurtherthickenedbyfinalsedimentation,centrifuging,orflotationthickeningandthentreatedbybiologicalorchemicalmeans.

Sincetheactivatedsludgeprocesscameintocommonuse,anumberofmodificationsoftheconventionalactivatedsludgeprocesshavebeendevelopedtomeetspecifictreatmentobjectives:

（1）Complete-mixactivatedsludgeprocess:

Acompletelymixedsystemcanallowamoreuniformaerationofthewastewaterintheaerationtank.Thisprocesshasbeenappliedtohandleavarietyofwastewaterwithgreatsuccess,especiallybecausetheprocesscansustainshocksandtoxicloads.

（2）Step-aerationactivatedsludgeprocess:

Inthismodifiedsystem,influentwastewaterisintroducedatseveralpointalongtheaerationtanklength.Thisleadstoarelativelyhomogenousloaddistributionalongthelengthoftheaerationtankresultinginamoreefficientuseofdissolvedoxygen.

（3）Contact-stabilizationactivatedsludge:

Theinfluentcontactswithahighconcentrationofbiomassinasmallcontacttankofshortoftime（20to40min）.Inthiscontacttank,arapidbiosorptionoforganiccompoundsisexpectedfollowedbytheoxidationoftheorganic.Thissystemwouldneedsmallertankageandproducesmalleramountsofbiosolids.

（4）Taperedaerationprocess:

Inthebasicactivatedsludgeprocess,organicinfluentisone-pointloadedtotheheadofaerationtank,thustheoxygendemandisextremelyhighattheheadoftheaerationtankbutverylowattheexitend.

（5）Higheravailabilityofoxygenformicroorganismsleadstoimprovedtreatmentefficiencyandreducedproductionofbiosolidsandreactorvolume.

Toensurehighlevelsoftreatmentperformancewiththeactivatedsludgeprocess,severalcontrolmeasuremustbeconsideredsuchas

（1）ensuringthatthereissufficientlevelofdissolvedoxygenintheaerationtanks,

（2）adjustingtheamountofreturnactivatedsludge,and（3）controllingthewastesludge.Basicallythereareseveralparametersonecanrefertoasaguidelinetofindoutwhetherthewholeprocessisoperatingatpeakperformance.Theparameterusedmostcommonlyforcontrollingtheprocessissolidsretentiontime（SRT,theSRTrepresenttheaverageperiodoftimeduringwhichthesludgehasremainedinthesystem）.Mixedliquorsuspendedsolids（MLSS）concentrationmayalsobeusedtodeterminewhetherthesludgeishealthyorwhetheritistoooldortooyoung.MLSSconcentrationcanbecontrolledbasedontheamountofreturnactivatedsludge.Thefoodtomicroorganismratioiscommonlyusedtocharacterizeoperatingconditions.Oxygenuptakeratesarealsousedtomonitorandcontroltheactivatedsludgeprocess.Sludgevolum