水处理专业英语阅读4--Operations-of-Pretreatment

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,andconstantlevelshort-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,dependingonwhetherthesewersystemisacombinedtypeoraseparatetype,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.Flowequalizationisusuallyachievedbyconstructinglargebasinsthatcollectandstorethewastewaterflowandfromwhichthewastewaterispumpedtothetreatmentplantataconstantrate.Thesebasinsarenormallylocatedneartheheadendofthetreatmentworks,preferablydownstreamofpretreatmentfacilitiessuchasbarscreens,comminutors,andgritchambers.Adequateaerationandmixingmustbeprovidedtopreventodorsandsolidsdeposition.Therequiredvolumeofanequalizationbasinisestimatedfromamassbalanceoftheflowintothetreatmentplantwiththeaverageflowtheplantisdesignedtotreat.Thetheoreticalbasisisthesameasthatusedtosizereservoirs.Withthescreeni