2024剑桥雅思讲义Table_Completion_Questions_Strategy_Practice.docx

IELTSADVANTAGETableCompletionQuestionsStrategyPracticeHowsewageistreatedThetreatmentofsewageconsistsoftheremovalofcontaminantsfrommunicipalwastewater,whichcontainsmainlyhouseholdsewageplussomeindustrialwastewater.Physical,chemical,andbiologicalprocessesareusedtoremovecontaminantsandproducetreatedwastewater(ortreatedeffluent)thatissafeenoughforreleaseintotheenvironment.Aby-productofsewagetreatmentisasemi-solidwasteorslurry,calledsewagesludge.Thesludgehastoundergofurthertreatmentbeforebeingsuitablefordisposalorapplicationtoland.Pretreatmentremovesallmaterialsthatcanbeeasilycollectedfromtherawsewagebeforetheydamageorclogthepumpsandsewagelinesofprimarytreatmentclarifiers.Theinfluentinthesewagewaterpassesthroughabarscreentoremovealllargeobjectscarriedinthesewagestream.Thisismostcommonlydonewithanautomatedmechanicallyrakedbarscreeninmodernplantsservinglargepopulations,whileinsmallerorlessmodernplants,amanuallycleanedscreenmaybeused.Therakingactionofamechanicalbarscreenistypicallypacedaccordingtotheaccumulationofobjectsonthebarscreenandtheflowrate.Thesolidsarecollectedandlaterdisposedofinalandfill,orincinerated.Barscreensormeshscreensofvaryingsizesmaybeusedtooptimizesolidsremoval.Ifgrosssolidsarenotremoved,theybecomejammedinpipesandmovingpartsofthetreatmentplantandcancausesubstantialdamageandinefficiencyintheprocess.Intheprimarysedimentationstage,sewageflowsthroughlargetanks,commonlycalledprimarysedimentationtanks.Thetanksareusedtosettlesludge,whilegreaseandoilsrisetothesurfaceandareskimmedoff.Primarysettlingtanksareusuallyequippedwithmechanically-drivenscrapersthatcontinuallydrivethecollectedsludgetowardsahopperinthebaseofthetank,whereitispumpedtosludgetreatmentfacilities.Secondarytreatmentisdesignedtosubstantiallydegradethebiologicalcontentofthesewagewhichisderivedfromhumanwaste,foodwaste,soapsanddetergent.Themajorityofmunicipalplantstreatthesettledsewageliquorusingaerobicbiologicalprocesses.Bacteriaandprotozoaareusedtoconsumebiodegradablesolubleorganiccontaminants(sugars,fats,organicshort-chaincarbonmolecules,andsoon)andbindmanyofthelesssolublecomponentsintofloc.Secondarytreatmentsystemsareclassifiedasfixed-filmorsuspended-growthsystems.Fixed-filmorattachedgrowthsystemsincludetricklingfilters,constructedaanTgeTableCompletionQuestionsStrategyPracticewetlands,bio-towers,androtatingbiologicalcontactors,wherethebiomassgrowsonmediaandthesewagepassesoveritssurface.Thefixed-filmprinciplehasfurtherdevelopedintoMovingBedReactorsandIntegratedFixed-FilmActivatedSludgeprocesses.Theformersystemtypicallyrequiresasmallerfootprintthansuspended-growthsystems.Suspended-growthsystemsincludeactivatedsludge,wherethebiomassismixedwiththesewageandcanbeoperatedinasmallerspacethantricklingfiltersthattreatthesameamountofwater.However,fixed-filmsystemsaremoreabletocopewithdrasticchangesintheamountofbiologicalmaterialandcanprovidehigherremovalratesfororganicmaterialandsuspendedsolidsthansuspendedgrowthsystems.Somesecondarytreatmentmethodsincludeasecondaryclarifiertosettleoutandseparatebiologicalflocorfiltermaterialgrowninthesecondarytreatmentbioreactor.Thepurposeoftertiarytreatmentistoprovideafinaltreatmentstagetofurtherimprovetheeffluentqualitybeforeitisdischargedintothereceivingenvironment(sea,river,lake,wetlands,ground,andsoon).Morethanonetertiarytreatmentprocessmaybeusedatanytreatmentplant,suchasfiltrationandsettlinginlagoonsandponds.Disinfectionisalwaysthefinalstageoftertiarytreatment.Biologicalnutrientremoval(BNR)isregardedbysomeasatypeofsecondarytreatmentprocess,andbyothersasatertiary(or"advanced")treatmentprocess.Wastewatermaycontainhighlevelsofthenutrientsnitrogenandphosphorus.Excessivereleaseofthesesubstancesintotheenvironmentcanleadtoabuildupofnutrients,calledeutrophication,whichcaninturnencouragetheovergrowthofweeds,algae,andcyanobacteria(blue-greenalgae).Thismaycauseanalgalbloom,arapidgrowthinthepopulationofalgae,inwhichthealgaenumbersareunsustainableandeventuallymostofthemdie.Thedecompositionofthealgaebybacteriausesupsomuchoftheoxygeninthewaterthatmostoralloftheanimalsdie,whichcreatesmoreorganicmatterforthebacteriatodecompose.Inadditiontocausingdeoxygenation,somealgalspeciesproducetoxinsthatcontaminatedrinkingwatersupplies.DifferentBNRprocessesarerequiredtoremovenitrogenandphosphorus.Nitrogenisremovedthroughthebiologicaloxidationofnitrogenfromammoniatonitrate(nitrification),followedbydenitrification,thereductionofnitratetonitrogengas.Nitrogengasisreleasedintotheatmosphereandthusremovedfromthewater.Phosphorusremovalcanalsobeachievedbychemicalprecipitation,usuallywithsaltsofiron(e.g.ferricchloride),aluminum(e.g.alum),orlime.ThismayleadtoaanTgeTableCompletionQuestionsStrategyPracticeexcessivesludgeproductionashydroxidesprecipitateandtheaddedchemicalscanbeexpensive.Chemicalphosphorusremovalrequiressignificantlysmallerequipmentfootprintthanbiologicalremoval,iseasiertooperateandisoftenmorereliablethanbiologicalphosphorusremoval.Anothermethodforphosphorusremovalistousegranularlaterite.Sourc