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13BiologicalWastewaterTreatmentApplicationofmicrobialdegradationandremovalofundesirableconstituentsinindustrialandmunicipalwastesisnotanewconcept.Itiscommonlyusedprocessforgeneralwastewatertreatmentactivitiesandhasbeenformanyyears.Astheawarenessofchemicalcontaminationoftheenvironment,muchresearchonbiologicaldegradationoftoxicchemicalshasoccurred.Amongtherangeoftreatmenttechnologies,biologicaldegradationranksamongthemosteffective.Itsmanagementandapplicationisenhancedbythepotentialtoapplybiologicaltreatmentinsequencewithotherchemicalandthermalprocesses.ActivatedsludgeTheactivatedsludgeprocessisstypicaltypeofsuspendedgrowthbiologicaltreatmentsystemandprobablythemostwidelyusedbiologicalprocessforthetreatmentoforganicandindustrialwastewaters.However,itcanonlytreataqueousorganicwastestreamshavinglessthan1%suspendedsolidcontent,andcannottolerateshockloadingofconcentratedorganics.Therefore,thewastestreamenteringthisprocesswillusuallyhavepassedthroughapretreatmentprocesswhichincludesaclarifier(primaryclarifier)andanequalizationbasin.Theprimaryclarifierisusedforremovalofgrit,oilyandfattymaterialandgrosssolidmaterial,whiletheequalizationbasinisusedtodampenwastewaterflowvariationsandtoprovidemoreuniformorganicloadingtotheactivatedsludgesystem.Activatedsludgeprocessareusedtotreatmunicipalandindustrialwastessincetheyareversatile,flexible,andcanbeusedtoproduceaneffluentofdesiredqualitybyvaryingprocessparameters.Theprocesswasso-namedbecauseitproducesanactivemassofmicroorganismscapableofaerobicallystabilizingawaste.Manyversionsofthebasicprocessexistbutallarefundamentallysimilar.2Thetermactivatedsludgeisappliedtoboththeprocessandtothebiologicalsolidsinthetreatmentunit.Themixedliquorsuspendedsolidsoractivatedsludgecontainsavarietyofheterotrophicmicroorganismssuchasbacteria,protozoa,fungi,andlargermicroorganisms.Thepredominanceofaparticularmicrobialspeciesdependsuponthewastethatistreatedandthewayinwhichtheprocessisoperated.Theactivatedsludgeprocessiscurrentlythemostwidelyusedbiologicaltreatmentprocess.Thisispartlytheresultofthefactthatrecirculationofthebiomass,whichisanintegralpartoftheprocess,allowsmicroorganismstoadapttochangesinwastewatercompositionwitharelativelyshortacclimationtimeandalsoallowsagreaterdegreeofcontrolovertheacclimatedbacterialpopulation.Anactivatedsludgesystemconsistsofanequalizationbasin,asettlingtank,anaerationbasin,aclarifier,andasludgerecycleline.Wastewaterishomogenizedinanequalizedbasintoreducevariationsinthefeed,whichmaycauseprocessupsetsofthemicroorganismsanddiminishtreatmentefficiency.Settleablesolidsarethenremovedinasettlingtank.Next,wastewaterentersanaerationbasin,whereanaerobicbacterialpopulationismaintainedinsuspensionandoxygen,aswellasnutrients,areprovide.Thecontentsofthebasinarereferredtoasthemixedliquor.Oxygenissuppliedtotheaerationbasinbymechanicalordiffusedaeration,whichalsoaidsinkeepingthemicrobialpopulationinsuspension.Themixedliquoriscontinuouslydischargedfromtheaerationbasinintoaclarifier,wherethebiomassisseparatedfromthetreatedwastewater.Aportionofthebiomassisrecycledtotheaerationbasintomaintainanoptimumconcentrationofacclimatedmicroorganismsintheaerationbasin.Theremainderoftheseparatedbiomassisdischargedor“wasted”,Thebiomassmaybefurtherdewateredonsludgedryingbedsorbysludgefiltrationtodisposal.Theclarifiedeffluentisdischarged.3Therecycledbiomassisreferredtoasactivatedsludge.Theterm“activated”isusedbecausethebiomasscontainslivingandacclimatedmicroorganismsthatmetabolizeandassimilateorganicmaterialatahigherratewhenreturnedtotheaerationbasin.Thisoccursbecauseofthelowfood-to-microorganismratiointhesludgefromtheclarifier.Forthetreatmentofindustrialwastewater,supplementalnutrientsourcesareoftenneededtoprovidesufficientnitrogenandphosphorus.Inmostcases,nitrogenisaddedasammoniaandphosphorusasphosphoricacid.AproperpHrange(6to8)andssufficientdissolvedoxygenconcentration(aminimumofIto2mg/L)mustalsobemaintainedintheaerationbasintosupportahealthyandactivesystem.Theaerationbasinhydraulicretentiontime(HRT)andsludgeresidencetime(SRT)areimportantoperationalfactors.HRTisdefinedastheratioofthevolumeofaerationtanktotheinfluentliquidflowrate,andSRTisthetotalamountofsludgeinthesystemdividedbytherateofsludgeleavingthesystemaswaste.Sufficienttimemustbeprovidedtoallowthebacteriatoassimilatetheorganicmaterialinthewastewater.TheHRTisusuallyfrom6to24hoursandSRTisfrom4to10daysfortheactivatedsludgeprocess.Theoptimumoperatingtemperatureisintherangeof25°Cto32°C.Althoughorganismspresentinactivatedsludgesystemsrangefromvirusestomulticellularorganisms,thepredominantandmostactiveareheterotrophic,andtolesserextent,autotrophicbacteria,whicharebothaggregatedinthesludgeflocsanddispersedintheliquid.Heterotrophicbacteriautilizeorganicmaterialasasourceofcarbonandenergy,whileautotrophicbacteriagenerallydependontheoxidationofmineralcompoundsforenergyrequirementsandutilizecarbondioxideasacarbonsource.Thesebacteriaarecapableofperforminghydrolysisandoxidationreactions.Complexhydrocarbonsareoxidizedtolowermolecularweightsbyoxygenase