Biological-Wastewater-Treatment

1BiologicalWastewaterTreatmentThisisabriefsummaryofthevarioustechniquesthathavebeendevelopedtotreatwastewaterbybiologicalmeans.Theyaccomplishwhatisgenerallycalledsecondarytreatment.Purpose:Theideabehindallbiologicalmethodsofwastewatertreatmentistointroducecontactwithbacteria(cells),whichfeedontheorganicmaterialsinthewastewater,therebyreducingitsBODcontent.Inotherwords,thepurposeofbiologicaltreatmentisBODreduction.Typically,wastewaterentersthetreatmentplantwithaBODhigherthan200mg/L,butprimarysettlinghasalreadyreducedittoabout150mg/Lbythetimeitentersthebiologicalcomponentofthesystem.ItneedstoexitwithaBODcontentnohigherthanabout20-30mg/L,sothatafterdilutioninthenearbyreceivingwaterbody(river,lake),theBODislessthan2-3mg/L.Thus,thebiologicaltreatmentneedstoaccomplisha6-folddecreaseinBOD.Principle:Simplebacteria(cells)eattheorganicmaterialpresentinthewastewater.Throughtheirmetabolism,theorganicmaterialistransformedintocellularmass,whichisnolongerinsolutionbutcanbeprecipitatedatthebottomofasettlingtankorretainedasslimeonsolidsurfacesorvegetationinthesystem.Thewaterexitingthesystemisthenmuchclearerthanitenteredit.Akeyfactoristheoperationofanybiologicalsystemisanadequatesupplyofoxygen.Indeed,cellsneednotonlyorganicmaterialasfoodbutalsooxygentobreathe,justlikehumans.Withoutanadequatesupplyofoxygen,thebiologicaldegradationofthewasteissloweddown,therebyrequiringalongerresidencytimeofthewaterinthesystem.Foragivenflowrateofwatertobetreated,thistranslatesintoasystemwithalargervolumeandthustakingmorespace.Advantages:Likeallbiologicalsystems,operationtakesplaceatambienttemperature.Thereisnoneedtoheatorcoolthewater,whichsavesonenergyconsumption.Becausewastewatertreatmentoperationstakemuchspace,theyarelocatedoutdoor,andthisimpliesthatthesystemmustbeabletooperateatseasonallyvaryingtemperatures.Cellscomeinamixofmanytypes,andaccommodationtoatemperaturechangeissimplyaccomplishedbyselfadaptationofthecellpopulation.2Similarly,achangeincompositionoftheorganicmaterial(duetopeople’schangingactivities)leadstoaspontaneouschangeincellpopulation,withthetypesbestsuitedtodigestthenewmaterialgrowinginlargernumbersthanothercelltypes.3Typesofequipmentforbiologicaltreatment:Therearetwobroadtypesofbiologicalwastewatertreatment:thosethatincludemechanicalmeanstocreatecontactbetweenwastewater,cellsandoxygen,andthosethandon’t.a)Withmechanicalmeans:1.Activatedsludge:Thisisthemostcommontype.Itconsistsinasetoftwobasins.Inthefirst,airispumpedthroughperforatedpipesatthebottomofthebasin,airrisesthroughthewaterintheformofmanysmallbubbles.Thesebubblesaccomplishtwothings:theyprovideoxygenformtheairtothewaterandcreatehighlyturbulentconditionsthatfavorintimatecontactbetweencells,theorganicmaterialinthewaterandoxygen.Thesecondbasinisasettlingtank,wherewaterflowismadetobeveryquietsothatthecellularmaterialmayberemovedbygravitationalsettling.Someofthecellmaterialcollectedatthebottomiscapturedandfedbackintothefirstbasintoseedtheprocess.Therestistreatedanaerobically(=withoutoxygen)untilitistransformedintoacompost-typematerial(likesoil).Thecostofanactivated-sludgesystemischieflyduetotheenergyrequiredtopumpairathighpressureatthebottomoftheaeratortank(toovercomethehydrostaticpressureofthewater).Anotherdisadvantageisthattheoperationisaccomplishedintwoseparatebasins,therebyoccupyingasubstantialamountofrealestate.42.Tricklingfilter:Atricklingfilterconsistsinabedoffist-sizerocksoverwhichthewastewaterisgentlysprayedbyarotatingarm.Slime(fungi,algae)developsontherocksurface,growingbyinterceptingorganicmaterialfromthewaterasittricklesdown.Sincethewaterlayerpassingovertherocksmakesthinsheets,thereisgoodcontactwithairandcellsareeffectivelyoxygenated.Wormsandinsectslivinginthis“ecosystem”alsocontributetoremovaloforganicmaterialfromthewater.Theslimeperiodicallyslidesofftherocksandiscollectedatthebottomofthesystem,whereitisremoved.Waterneedstobetrickledseveraltimesovertherocksbeforeitissufficientlycleaned.Multiplesprayingalsoprovidesawaytokeepthebiologicalslimesfromdryingoutinhoursoflow-flowconditions(ex.atnight).Plasticnetsaregraduallyreplacingrocksinnewerversionsofthissystem,providingmoresurfaceareapervolume,therebyreducingthesizeoftheequipment.3.Biologicalcontactor:Thisisessentiallyavariationonthetricklingfilter,withthedifferencebeingthatsolidmaterialonwhichslimegrowsisbroughttothewaterratherthanwaterbeingbroughttoit.Rotatingdisksalternateexposurebetweenairandwater.b)Withoutmechanicalmeans:Thewastewaterismadetoflowbygravitythroughaspeciallyconstructedwetland.There,thewaterisbroughtintoclosecontactwithvegetation(ex.reeds),whichactsasa5biologicalfiltertothewater.Theorganicmaterialinthewastewaterisusedasnutrientbytheplants.Oxygensupplyispassivelyaccomplishedbysurfaceaeration(contactwithoxygenoftheatmosphere).Sincewaterflowisslowinsuchsystem,togiveampletimeforthebiologicalactivitytotakeplace,thereisalmostnoturbulenceinthewaterandreaerationisweak.Comparedtomechanicalsystems,constructedwetlandsoccupyfarmorerealestate,buttheymaybeaestheticallypleasing,especiallyiftheyarewellintegratedi