水处理专业英语阅读4OperationsofPretreatment 6页

4OperationsofPretreatmentSeveraldevicesandstructuresareplacedupstreamoftheprimarytreatmentoperationtoprovideprotectiontothewastewatertreatmentplant(WWTP)equipment.ThesedevicesandstructuresareclassifiedaspretreatmentbecausetheyhavelittleeffectinreducingBOD5.BarRacksTypically,thefirstdeviceencounteredbythewastewaterenteringtheplantisabarrack.Theprimarypurposeoftherackistoremovetrashandlargeobjectsthatwoulddamageorfoulpumps,valves,andothermechanicalequipment.Rags,logs,andotherobjectsthatfindtheirwayintothesewerareremovedfromthewastewaterontheracks.InmodernWWTPs,theracksarecleanedmechanically.Thesolidmaterialisstoredinahopperandremovedtoasanitarylandfillatregularintervals.Barracksplacedupstreamofgritchambersandpumparesizedtohaveanopenspaceof50to150mmbetweeneachbar.Racksplaceddownstreamofgritchambersnormallyhaveopeningsof20to50mm,with25mmbeingacommonlyacceptedvalue.Channelapproachvelocitiesareheldbetween0.4and0.9m/s.Twochannelsareprovidedtoallowonetobetakenoutofserviceforcleaningandrepair.GritChambersInertdensematerialsuchassand,brokenglass,silt,andpebblesiscalledgrit.Ifthesematerialsarenotremovedfromthewastewater,theyabradepumpsandothermechanicaldevices,causingunduewear.Inaddition,theyhaveatendencytosettleincornersandbends,reducingflowcapacityand,ultimately,cloggingpipesandchannels.Therearethreebasictypesofgritremovaldevices:velocitycontrolled,aerated,andconstant\nlevelshort-termsedimentationbasins.Wewilldiscussonlythefirsttwo,sincetheyarethemostcommon.Velocitycontrolled.Thistypeofgritchamber,alsoknownasahorizontalflowgritchamber,canbeanalyzedbymeansoftheclassicallawsofsedimentationfordiscrete,non-flocculatingparticles.Stokes’lawmaybeusedfortheanalysisanddesignofhorizontalflowgritchambersifthehorizontalliquidvelocityismaintainedatabout0.3m/s.Liquidvelocitycontrolisachievedbyplacingaspeciallydesignedweirattheendofthechannel.Aminimumoftwochannelsmustbeemployedsothatonecanheoutofservicewithoutshuttingdownthetreatmentplant.Cleaningmaybeeitherbymechanicaldevicesorbyhand.Mechanicalcleaningisfavoredforplantshavingaverageflowsover0.04m3/s.Theoreticaldetentiontimesaresetataboutoneminuteforaverageflows.Washingfacilitiesarenormallyprovidedtoremoveorganicmaterialfromthegrit.Aeratedgritchambers.Thespiralrolloftheaeratedgritchamberliquid“drives”thegritintoahopperwhichislocatedundertheairdiffuserassembly.Theshearingactionoftheairbubblesissupposedtostriptheinertgritofmuchoftheorganicmaterialthatadherestoitssurface.Aeratedgritchamberperformanceisafunctionoftherollvelocityanddetentiontime.Therollvelocityiscontrolledbyadjustingtheairfeedrate.Nominalairflowvaluesareintherangeof0.15to0.45cubicmetersperminuteofairpermeteroftanklength[m3/(min•m)J.Liquiddetentiontimesareusuallysettobeaboutthreeminutesatmaximumflow.Length-to-widthratiosrangefrom2：5to5：1withdepthsontheorderof2to5m.Gritaccumulationinthechambervariesgreatly,dependingonwhetherthesewersystemisa\ncombinedtypeoraseparatetype,andontheefficiencyofthechamber.Forcombinedsystems,90m3ofgritpermillioncubicmetersofsewage(m3J105m3)isnotuncommon.Inseparatesystemsyoumightexpectsomethinglessthan30m3/lOtm3.Normallythegritisburiedinasanitarylandfill.ComminutorsDevicesthatareusedtomaceratewastewatersolid(rags,paper,plastic,andothermaterials)byrevolvingcuttingbarsarecalledcomminutors.Thesedevicesareplaceddownstreamofthegritchamberstoprotectthecuttingbarsfromabrasion,Theyareusedasareplacementforthedownstreambarrackbutmustbeinstalledwithahand-cleanedrackinparallelincasetheyfail.EqualizationFlowequalizationisnotatreatmentprocess,butstechniquethatcanbeusedtoimprovetheeffectivenessofbothsecondaryandadvancedwastewatertreatmentprocesses.Wastewaterdoesnotflowintoamunicipalwastewatertreatmentplantataconstantrate;theflowratevariesfromhourtohour,reflectingthelivinghabitsoftheareaserved.Inmosttowns.thepatternofdailyactivitiessetsthepatternofsewageflowandstrength.Above-averagesewageflowsandstrengthoccurinmid-morning.Theconstantlychangingamountandstrengthofwastewatertobetreatedmakesefficientprocessoperationdifficult.Also,manytreatmentunitsmustbedesignedforthemaximumflowconditionencountered,whichactuallyresultsintheirbeingoversizedforaverageconditions.Thepurposeofflowequalizationistodampenthesevariationssothatthewastewatercanbetreatedatanearlyconstantflowrate.Flowequalizationcansignificantlyimprovetheperformanceofanexistingplantandincreaseitsusefulcapacity.Innewplants,flowequalizationcanreducethesizeandcostofthetreatmentunits.Flowequalizationisusuallyachievedbyconstructinglargebasinsthatcollectandstorethe\nwastewaterflowandfromwhichthewastewaterispumpedtothetreatmentplantataconstantrate.Thesebasinsarenormallylocatedneartheheadendofthetreatmentworks,preferablydownstreamofpretreatmentfacilitiessuchasbarscreens,comminutors,andgritchambers.Adequateaerationandmixingmustbeprovidedtopreventodorsandsolidsdeposition.Therequiredvolumeofanequalizationbasinisestimatedfromamassbalanceoftheflowintothetreatmentplantwiththeaverageflowtheplantisdesignedtotreat.Thetheoreticalbasisisthesameasthatusedtosizereservoirs.Withthescreeningcompletedandthegritremoved,thewastewaterstillcontainslightorganicsuspendedsolids,someofwhichcanberemovedfromthesewagebygravityinasedimentationtank.Thesetankscanberoundorrectangular,areusuallyabout3.5mdeep,andholdthewastewaterforperiodsoftwotothreehours.Themassofsettledsolidsiscalledrawsludge.Thesludgeisremovedfromthesedimentationtankbymechanicalscrapersandpumps.Floatingmaterials,suchasgreaseandoil,risetothesurfaceofthesedimentationtank,wheretheyarecollectedbyasurfaceskimmingsystemandremovedfromthetankforfurtherprocessing.Primarysedimentationbasins(primarytanks)arecharacterizedbyTypeIIflocculantsettling.TheStokesequationcannotbeusedbecausetheflocculatingparticlesarecontinuallychanginginsize,shapeand,whenwaterisentrappedinthefloc,specificgravity.ThereisnoadequatemathematicalrelationshipthatcanbeusedtodescribeTypeIIsettling.Laboratorytestswithsettlingcolumnsareusedtodevelopdesigndata.Thebehavioroftheprimarytankisanalogoustothebehaviorofclarifiersusedinwatertreatment,withafewimportantexceptions.Thesolidsconcentrationismuchlowerinaprimarytankthaninawatertreatmentplantclarifier.Thus,theoverflowratesandweirloadingrates\ndifferappreciably.Overflowratesarecommonlyintherange25to60m/d.TheGreatLakes-UpperMississippiRiverBoardofStateSanitaryEngineers(GLUMRB)recommendsthatweirloading(hydraulicflowovertheeffluentweir)ratesnotexceed120m3/dofflowpermofweirlength[m3/(d.m)]forplantswithaverageflowslessthan0.04m3/a.Forlargerflowstherecommendedrateis190m3/(d.m)Asmentionedpreviously,approximately50to60percentoftherawsewagesuspendedsolidsandasmuchas30to35percentoftherawsewageBOD5mayberemovedintheprimarytank.\n单词表upstreamN上游部门weirN堰barrack格栅池Detentiontime停留时间rackN齿条spiralA螺旋形的foulV淤塞hopperN漏斗valveN阀门airdiffuser空气扩散器RagN破布碎屑assemblyN装配，集会logN圆木shearingN切应力hopperN漏斗comminutorN粉碎机sanitarylandfill垃圾填埋场macerateV浸软gritchambers沉砂池cuttingbarsN切削条approachvelocities行进速度；过栅流速abrasionN磨损，损伤inertA惰性的迟钝的Hand-cleanedrack人工清洁格栅siltN泥沙煤粉残渣rectangularA举行的pebbleN小鹅卵石scraperN刮刀abradeV磨损greaseN油脂undueA过度的过分的skimmingN浮渣bendN弯曲specificgravity比重clogv阻塞