2024剑桥雅思讲义Table_Completion_Questions_Strategy_Practice.docx

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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