Secondarysettlingtanksareimportantinwastewatertreatmentplants.Theycomeaftertheaerationtanks.Themainjobistoseparatesolidsfromwater.Microorganismsandparticlesclumptogether.Theseclumpsarecalledactivatedsludge.Mixedliquorcomesfromtheaerationtank.Itcontainstheseclumps.Themixedliquorentersthesecondaryclarifier.Theflowslowsdowndramatically.Heavysludgeflocssinktothebottom.Clearwaterstaysontop.

Gravitydoestheseparationwork.Sludgesettlesbecauseitisdenserthanwater.Thesettledsludgeformsalayeratthetankbottom.Thislayeriscalledthesludgeblanket.Theclearwaterontopiscalledsupernatant.Weirscollectthisclearwater.Thewaterflowsovertheweirs.Itleavesthetank.Thiswaterisnowmuchcleaner.Itoftengoesforfurthertreatmentordischarge.

Thesettledsludgemustberemoved.Scrapersatthebottommoveslowly.Theypushthesludgetowardacollectionpit.Apumpremovessomesludge.Thissludgeiscalledreturnactivatedsludge.Itgoesbacktotheaerationtank.Thisreturniscrucial.Itkeepstherightamountofmicroorganismsintheaerationtank.Themicroorganismsneedtoeatthewasteinthesewage.Withoutreturningsludge,theprocessfails.

Sometimes,toomuchsludgebuildsup.Thesystemmakesmoresludgethanitneeds.Thisextrasludgeiscalledwasteactivatedsludge.Itispumpedoutofthesystem.Itgoestootherprocessesforhandling.Thickeninganddigestionarecommonnextsteps.Thegoalistomanagethesolids.

Manyfactorsaffectthesecondarysettlingtank.Themostimportantfactorissludgequality.Goodsludgesettlesfastandformsatightblanket.Poorsludgedoesnotsettlewell.Itmayfloatorstayinthewater.Thiscausesproblems.Solidscanescapewiththecleanwater.Thismakesthefinalwaterqualitypoor.

Sludgevolumeindexisacommonmeasure.Ittellsushowwellthesludgesettles.Alowindexisgood.Itmeansthesludgeisdenseandsettlesquickly.Ahighindexisbad.Thesludgeislightandfluffy.Itsettlesslowly.Thisconditioniscalledbulkingsludge.Filamentousbacteriaoftencausebulking.Thesebacteriagrowinlongstrings.Theypreventthesludgefromcompacting.

Dissolvedoxygenlevelsmatter.Toolittleoxygenintheaerationtankcausesproblems.Themicroorganismsgetstressed.Theydonotformgoodflocs.Toomuchfoodcanalsobeaproblem.Ifthefood-to-microorganismratioistoohigh,thesludgeisyoung.Youngsludgedoesnotsettlewell.Ithassmall,lightflocs.Iftheratioistoolow,thesludgeisold.Oldsludgemaybreakapart.Italsosettlespoorly.

Thetankdesignisimportant.Mosttanksarecircular.Somearerectangular.Waterflowsintothecenter.Itspreadsout.Thetankiswideandnotdeep.Thisgivesalargeareaforsettling.Depthisalsoimportant.Adeepertankholdsmoresludge.Butitneedsmoretimetofill.Surfaceareaiskey.Alargersurfacearealetswatermoveslower.Slowerwaterallowsbettersettling.Theoverflowrateisakeynumber.Itistheflowdividedbythesurfacearea.Alowoverflowrateisbetterforsettling.

Temperatureplaysarole.Coldwateristhickerthanwarmwater.Particlessettleslowerincoldwater.Inwinter,tanksmaynotworkaswell.Operatorsmustaccountforthis.Theymayslowdowntheflow.Theymaykeepmoresludgeinthesystem.

Windcanbeaproblemforopentanks.Itcreateswavesonthewatersurface.Wavescandisturbthesettledsludge.Scumandfloatingmaterialneedremoval.Skimmersmovearoundthesurface.Theypushfloatingstufftoascumtrough.

Theunderflowrateisimportanttoo.Thisistherateofsludgeremoval.Ifsludgeispumpedouttoofast,theblanketrises.Solidsmightgetsuckedout.Ifpumpedtooslow,theblanketgetstoothick.Itcantakeuptoomuchtankspace.Thisleaveslessroomforclearwater.Solidsmightoverflowtheweirs.Operatorswatchtheblanketlevel.Theyadjustthepumpstokeepitsteady.

Secondarysettlingisaphysicalprocess.Butitdependsonbiology.Theaerationtankcreatesthebiologicalconditions.Goodbiologymakesgoodsettling.Operatorstestthesludgeoften.Theylookatitinajar.Theymeasurehowfastitsettles.Thissimpletestgivesgoodinformation.Theyalsolookunderamicroscope.Theycheckforfilamentousbacteria.Theyseethetypesofmicroorganismspresent.

Chemicaladditionissometimesused.Polymerscanhelpsludgeclumptogether.Chlorineorperoxidecankillfilamentousbacteria.Theseareshort-termfixes.Thebestsolutionistofixthebiologyintheaerationtank.Adjusttheair.Adjustthefood.Adjusttheamountofreturnedsludge.

Thegoalisalwayscleareffluent.Effluentistheleavingwater.Itmustbelowinsolids.Itmustbelowinorganicmatter.Environmentalrulesarestrict.Plantsmustmeettheserules.Thesecondaryclarifieristhelaststepforsolidsremoval.Itprotectstheriverorlakewherethewatergoes.

Problemsintheclarifierareeasytosee.Cloudywaterleavingisasign.Arisingsludgeblanketisasign.Foamonthesurfaceisasign.Operatorsusethesesigns.Theymakechangestofixtheprocess.Itisaconstantbalancingact.Theincomingsewagechangesallthetime.Theweatherchanges.Theprocessmustadapt.

Researchonsecondarysettlingcontinues.Scientistsbuildcomputermodels.Thesemodelssimulateflowandsettling.Theyhelpdesignbettertanks.Theyhelppredictproblems.Newequipmentistested.Betterscrapers.Bettersensors.Sensorscanmeasuresludgeblanketdepthautomatically.Theycancontrolthepumps.Thismakestheprocessmorestable.

Thebasicidearemainsthesame.Letgravitydothework.Createconditionsforgoodsettling.Removecleanwaterfromthetop.Removethickenedsludgefromthebottom.Returnwhatisneeded.Wastewhatisextra.Itisasimpleidea.Butinpractice,itrequiresskillandattention.Awell-runningsecondaryclarifierisakeytocleanwater.Itisaquiet,essentialpartofthetreatmentplant.Itturnsamurkymixtureintoclearwater.Itcapturesthesolidsforfurtherprocessing.Thisprocessprotectspublichealth.Itprotectstheenvironment.