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1、PrinciplesofWastewaterTreatmentEffluentRequirementsTheprimaryobjectiveofwastewatertreatmentistoremoveormodifythosecontaminantsdetrimentaltohumanhealthorthewater,land,andairenvironment.Landdisposal,evaporationfromponds,anddeep-wellinjectionareoccasionaloptions,butusuallytheonlypracticaloutletsforthe
2、disposaloftreated(oruntreated)wastewaterarestreams,rivers,lakes,andoceans.Toprotectthesewaterresources,thedischargeofpollutantsintothemmustbecontrolled.ThisisdoneinNorthAmericabysettingeffluentrequirementsforBOD,SS,andfecalcoliforms.Secondarytreatmentisnormallynecessarytomeettheserequirements.Inthe
3、UnitedStates,forexample,BOD5orSSvaluesintheeffluentmustnotexceedanaverageof30mg/L.Themaximumlimitforfecalcoliformsis200/100mL(geometricmean).Afurtherstipulationisthatnotlessthan85percentremovalofBODandSSmustbeprovided.Canadianrequirementsaresimilar,butsomeprovincesuse15mg/LasthemaximumforeffluentBO
4、D5andSS.Thisimpliesagreaterthan90percentremovalefficiencyinthetreatmentof“normal”municipalwastewaterwithaBOD5of200mg/L.Wherereceivingwatershavelimitedassimilationcapacity,containexcessivenutrients,provideessentialwateruse,orsupportvaluableaquaticlife,thenmorestringenteffluentrequirementsarewarrante
5、d.Suchsituationsentaildetailedinvestigationstoevaluatetheneedforadditionalwastewatertreatment.TreatmentProcessesThesuspended,colloidal,anddissolvedcontaminants(bothorganicandinorganic)inwastewatermayberemovedphysically,convertedbiologically,orchangedchemically.PhysicalprocessesGravitysettlingisthem
6、ostcommonphysicalprocessforremovingsuspendedsolidsfromwastewater.Itisemployedin⚫Removinggrit(definedassandparticlesof0.2mmdiameterorgreater)⚫Clarifyingrawsewageandconcentratingthesettledsolids(calledraworprimarysludge)⚫Clarifyingbiologicalsuspensionsandconcentratingthesettledfloc(calledbiological,a
7、ctivated,orsecondarysludge)⚫Gravitythickeningofprimaryorsecondarysludges.BiologicalprocessesMostoftheorganicconstituentsinwastewatercanserveasfood(substrate)toprovideenergyformicrobialgrowth.Thisistheprinci
