给水排水专业英语课件(李田)20

1Unit20AdvancedWastewaterTreatment污水深度处理2Althoughsecondarytreatmentprocesses,whencoupledwithdisinfection,mayremoveover85percentoftheBODandsuspendedsolidsandnearlyallpathogens,onlyminorremovalofsomepollutants,suchasnitrogen,phosphorus,solubleCOD,andheavymetals,isachieved.Insomecircumstances,thesepollutantsmaybeofmajorconcern.3Inthesecases,processescapableofremovingpollutantsnotadequatelyremovedbysecondarytreatmentareusedinwhatiscalledtertiarywastewatertreatment(theseprocesseshaveoftenbeencalledadvancedwastewatertreatment,orAWTforshort).ThefollowingsectionsdescribeavailableAWTprocesses.Inadditiontosolvingtoughpollutionproblems,theseprocessesimprovetheeffluentqualitytothepointthatitisadequateformanyreusepurposes,andmayconvertwhatwasoriginallyawastewaterintoavaluableresourcetoogoodtothrowaway.[1]4PhosphorusRemovalAllthepolyphosphates(molecularlydehydratedphosphates)graduallyhydrolyzeinaqueoussolutionandrevertto(heorthaform(PO43-)fromwhichtheywerederived.[2]Phosphorusistypicallyfoundasmono-hydrogenphosphate(HPO42-)inwastewater.Theremovalofphosphorustopreventorreduceeutrophicationistypicallyaccomplishedbychemicalprecipitationusingoneofthreecompounds.5Theprecipitationreactionsforeachareshownbelow.UsingferricchlorideUsingalumUsinglime6YoushouldnotethatferricchlorideandalumreducethepHwhilelimeincreasesit.Theeffectiverangs:ofpHforalumandferricchlorideisbetween5.5and7.0.Ifthereisnotenoughnaturallyoccurringalkalil1ity[3]tobufferthesystemtothisrange,thenlimemustbeaddedtocounteracttheformationofH+.7Theprecipitationofphosphorusrequiresareactionbasinandasettlingtanktoremovetheprecipitate.Whenferricchlorideandalumareused,thechemicalsmaybeaddeddirectlytotheaerationtankintheactivatedsludgesystem.Thus,theaerationtankservesasareactionbasin.Theprecipitateisthenremovedinthesecondaryclarifier.8ThisisnotpossiblewithlimesincethehighpHrequiredtoformtheprecipitateisdetrimentaltotheactivatedsludgeorganisms.Insomewastewatertreatmentplant,theFeCl3(oralum)isaddedbeforethewastewaterenterstheprimarysedimentationtank.Thisimprovestheefficiencyoftheprimarytank,butdeprivethebiologicalprocessesofneedednutrients.9NitrogenControlNitrogeninanysolubleform(NH3,NH4+,NO2-,andNO3-,butnotN2gas)isanutrientandmayneedtoberemovedfromwastewatertohelpcontrolalgaegrowthinthereceivingbody.Inaddition,nitrogenintheformofammoniaexertsanoxygendemandandcanbetoxictofish.Removalofnitrogencanbeaccomplishedeitherbiologicallyorchemically.Thebiologicalprocessiscallednitrification/denitrification.Thechemicalprocessiscalledammoniastripping.10Nitrificationl/DenitrificationThenaturalnitrificationprocesscanbeforcedtooccurintheactivatedsludgesystembymaintainingacelldetentiontimeof15daysormore.[4]Thenitrificationstepisexpressedinchemicaltermsasfollows:Ofcourse,bacteriamustbepresenttocausethereactiontooccur.11Thisstepsatisfiestheoxygendemandoftheammoniumion.Ifthenitrogenlevelisnotofconcernforthereceivingbody,thewastewatercanbedischargedaftersettling.12Ifnitrogenisofconcern,thenitrificationstepmustbefollowedbyanoxicdenitrificationbybacteria:Asindicatedbythechemicalreaction,organicmatterisrequiredfordenitrification.Organicmatterservesasanenergysourceforthebacteria.Theorganicsmaybeobtainedfromwithinoroutsidethecell.13Inmultistagenitrogenremovalsystems,becausetheconcentrationofBOD5intheflowtothedenitrificationprocessisusuallyquitelow,asupplementalorganiccarbonsourceisrequiredforrapiddenitrification.(BOD5concentrationislowbecausethewastewaterpreviouslyhasundergonecarbonaceousBODremovalandthenitrificationprocess.)Theorganicmattermaybeeitherraw,settledsewageorasyntheticmaterialsuchasmethanol(CH3OH).Raw,settledsewagemayadverselyaffecttheeffluentqualitybyincreasingtheBOD5andammoniacontent.14AmmoniaStrippingNitrogenintheformofammoniacanberemovedchemicallyfromwaterbyraisingthepHtoconverttheammoniumionintoammonia,whichcanthenbestrippedfromthewaterbypassinglargequantitiesofairthroughthewater.Theprocesshasnoeffectonnitrate,sotheactivatedsludgeprocessmustbeoperatedatashortcelldetentiontimetopreventnitrification.Theammoniastrippingreactionis15Thehydroxideisusuallysuppliedbyaddinglime.ThelimealsoreactswithCO2,intheairandwatertoformacalciumcarbonatescale,whichmustberemovedperiodically.Lowtemperaturescauseproblemswithicingandreducedstrippingability.Thereducedstrippingabilityiscausedbytheincreasedsolubilityofammoniaincoldwater.16Besideschemicalprecipitationofphosphorous,biologicalnitrification-denitrification,therearefiltrationprocesssimilartothatusedinwatertreatmentplants,carbonadsorptionandbiologicalphosphorousremoval,thosearealsoimportantcomponentsofadvancedwastewatertreatment.17Thanks！