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1、IELTSADVANTAGETableCompletionQuestionsStrategyPracticeHowsewageistreatedThetreatmentofsewageconsistsoftheremovalofcontaminantsfrommunicipalwastewater,whichcontainsmainlyhouseholdsewageplussomeindustrialwastewater.Physical,chemical,andbiologicalprocessesareusedtoremovecontaminantsandproducetreatedwas
2、tewater(ortreatedeffluent)thatissafeenoughforreleaseintotheenvironment.Aby-productofsewagetreatmentisasemi-solidwasteorslurry,calledsewagesludge.Thesludgehastoundergofurthertreatmentbeforebeingsuitablefordisposalorapplicationtoland.Pretreatmentremovesallmaterialsthatcanbeeasilycollectedfromtherawsew
3、agebeforetheydamageorclogthepumpsandsewagelinesofprimarytreatmentclarifiers.Theinfluentinthesewagewaterpassesthroughabarscreentoremovealllargeobjectscarriedinthesewagestream.Thisismostcommonlydonewithanautomatedmechanicallyrakedbarscreeninmodernplantsservinglargepopulations,whileinsmallerorlessmoder
4、nplants,amanuallycleanedscreenmaybeused.Therakingactionofamechanicalbarscreenistypicallypacedaccordingtotheaccumulationofobjectsonthebarscreenandtheflowrate.Thesolidsarecollectedandlaterdisposedofinalandfill,orincinerated.Barscreensormeshscreensofvaryingsizesmaybeusedtooptimizesolidsremoval.Ifgrosss
5、olidsarenotremoved,theybecomejammedinpipesandmovingpartsofthetreatmentplantandcancausesubstantialdamageandinefficiencyintheprocess.Intheprimarysedimentationstage,sewageflowsthroughlargetanks,commonlycalledprimarysedimentationtanks.Thetanksareusedtosettlesludge,whilegreaseandoilsrisetothesurfaceandar
6、eskimmedoff.Primarysettlingtanksareusuallyequippedwithmechanically-drivenscrapersthatcontinuallydrivethecollectedsludgetowardsahopperinthebaseofthetank,whereitispumpedtosludgetreatmentfacilities.Secondarytreatmentisdesignedtosubstantiallydegradethebiologicalcontentofthesewagewhichisderivedfromhumanw
7、aste,foodwaste,soapsanddetergent.Themajorityofmunicipalplantstreatthesettledsewageliquorusingaerobicbiologicalprocesses.Bacteriaandprotozoaareusedtoconsumebiodegradablesolubleorganiccontaminants(sugars,fats,organicshort-chaincarbonmolecules,andsoon)andbindmanyofthelesssolublecomponentsintofloc.Secon
8、darytreatmentsystemsareclassifiedasfixed-filmorsuspended-growthsystems.Fixed-filmorattachedgrowthsystemsincludetricklingfilters,constructedaanTgeTableCompletionQuestionsStrategyPracticewetlands,bio-towers,androtatingbiologicalcontactors,wherethebiomassgrowsonmediaandthesewagepassesoveritssurface.The
9、fixed-filmprinciplehasfurtherdevelopedintoMovingBedReactorsandIntegratedFixed-FilmActivatedSludgeprocesses.Theformersystemtypicallyrequiresasmallerfootprintthansuspended-growthsystems.Suspended-growthsystemsincludeactivatedsludge,wherethebiomassismixedwiththesewageandcanbeoperatedinasmallerspacethan
10、tricklingfiltersthattreatthesameamountofwater.However,fixed-filmsystemsaremoreabletocopewithdrasticchangesintheamountofbiologicalmaterialandcanprovidehigherremovalratesfororganicmaterialandsuspendedsolidsthansuspendedgrowthsystems.Somesecondarytreatmentmethodsincludeasecondaryclarifiertosettleoutand
11、separatebiologicalflocorfiltermaterialgrowninthesecondarytreatmentbioreactor.Thepurposeoftertiarytreatmentistoprovideafinaltreatmentstagetofurtherimprovetheeffluentqualitybeforeitisdischargedintothereceivingenvironment(sea,river,lake,wetlands,ground,andsoon).Morethanonetertiarytreatmentprocessmaybeu
12、sedatanytreatmentplant,suchasfiltrationandsettlinginlagoonsandponds.Disinfectionisalwaysthefinalstageoftertiarytreatment.Biologicalnutrientremoval(BNR)isregardedbysomeasatypeofsecondarytreatmentprocess,andbyothersasatertiary(oradvanced)treatmentprocess.Wastewatermaycontainhighlevelsofthenutrientsnit
13、rogenandphosphorus.Excessivereleaseofthesesubstancesintotheenvironmentcanleadtoabuildupofnutrients,calledeutrophication,whichcaninturnencouragetheovergrowthofweeds,algae,andcyanobacteria(blue-greenalgae).Thismaycauseanalgalbloom,arapidgrowthinthepopulationofalgae,inwhichthealgaenumbersareunsustainab
14、leandeventuallymostofthemdie.Thedecompositionofthealgaebybacteriausesupsomuchoftheoxygeninthewaterthatmostoralloftheanimalsdie,whichcreatesmoreorganicmatterforthebacteriatodecompose.Inadditiontocausingdeoxygenation,somealgalspeciesproducetoxinsthatcontaminatedrinkingwatersupplies.DifferentBNRprocess
15、esarerequiredtoremovenitrogenandphosphorus.Nitrogenisremovedthroughthebiologicaloxidationofnitrogenfromammoniatonitrate(nitrification),followedbydenitrification,thereductionofnitratetonitrogengas.Nitrogengasisreleasedintotheatmosphereandthusremovedfromthewater.Phosphorusremovalcanalsobeachievedbyche
16、micalprecipitation,usuallywithsaltsofiron(e.g.ferricchloride),aluminum(e.g.alum),orlime.ThismayleadtoaanTgeTableCompletionQuestionsStrategyPracticeexcessivesludgeproductionashydroxidesprecipitateandtheaddedchemicalscanbeexpensive.Chemicalphosphorusremovalrequiressignificantlysmallerequipmentfootprintthanbiologicalremoval,iseasiertooperateandisoftenmorereliablethanbiologicalphosphorusremoval.Anothermethodforphosphorusremovalistousegranularlaterite.Sourc
