给水排水专业英语课件18

1Unit18BiologicalFilters生物过滤2Trickling-filtersystemsarecommonlyusedforsecondarytreatmentofmunicipalwastewater.Primaryeffluentissprayedonabedofcrushedrock,orothermedia,coatedwithbiologicalfilms.Thebiologicalslimelayerconsistsofbacteria,protozoa,andfungi.Sludgeworms,filter-flylarvae,rotifers,andotherhigheranimalsfrequentlyfindtheenvironmentsuitableforgrowth.3Thesurfaceofthebedmaysupportalgalgrowthwhentemperatureandsunlightconditionsareoptimum.Thelowerportionofadeepfilterfrequentlysupportspopulationsofnitrifyingbacteria.Althoughclassifiedasanaerobictreatmentdevice,themicrobialfilmonthefiltermediumisaerobictoadepthofonly0.1~0.2mm.4Thezonenexttothemediumisanaerobic.Asthewastewaterflowsoverthemicrobialfilm,thesolubleorganicsarerapidlymetabolizedandthecolloidalorganicsadsorbedontothesurface.Microorganismsnearthesurfaceofthebed,wherefoodconcentrationishigh,areinarapidgrowthphase,whilethelowerzoneofabedisinastateofstarvation.Overalloperationofatricklingfiltermaybeconsideredintheendogenousgrowthphase.5Dissolvedoxygenextractedfromtheliquidlayerisreplenishedbyre-oxygenationfromthesurroundingair.Undesirableanaerobicconditionscanbecreatedinatricklingfilterbyinhibitingaerationofthebed,.Pluggingoftheairpassageswithexcessmicrobialgrowth,asaresultoforganicoverload,cancreateanaerobiosisandfoulodors.Generally,tricklingfilterstreatingdomesticwasteoperaterelativelyodorfree:however,certaintypes(e.g.,food-processingwastewaters)havecausedfoulodors.6TricklingFilterAcutawayviewofastone-filledtricklingfilterisshowninFig.3-6.Majorcomponentsarethefiltermedia,underdrainsystem,androtarydistributor.Thefiltermediaprovideasurfaceforbiologicalgrowthandvoidsforpassageofliquidandair.Themostcommonmediainexistingfiltersarecrushedrock,slag,orfieldstone,becausethesematerialsaredurable,insoluble,andresistanttospalling.7Theunderdrainsystemcarriesawaytheeffluentandpermitscirculationofairthroughthebed.Theneedforfreepassageofaircontrolsthesizeofopeningsintheunderdrain.Arotarydistributorprovidesauniformhydraulicloadonthefiltersurface.Themostprevalentkindisdrivenbythereactionofthewastewaterflowingoutofthedistributornozzles.Thisgenerallyrequiresaminimumpressureheadof24inchesmeasuredfromthecenterofarms.8Normally,tricklingfiltersareprecededbyprimarytreatment.Thosereceivingunsettledwastes,aswellasothersusedforpretreatmentofsettledstrongwastespriortosubsequentaeration,arereferredtoasroughingfilters.Becauseoftheheavyorganicloadingonroughingbed,theirdesignmustconsidertheproblemofbedplugging.Thestandardstone-filledfiltermustbeprecededbyasedimentationtankequippedwithascum-collectiondevice.9Thishighpercentageofvoidvolumeinmanufacturedmediapermitstheirapplicationanduseinroughingfilter.Inahigh-ratetricklingfilter,rawwastewaterisdilutedwithrecirculationflowsothatithaspassedthroughthefiltermorethanonce.Atypicalhigh-ratefilterdesignisillustratedinfig.3-7.Areturnlineformthefinalclarifierservesadualfunction,asasludgereturnandarecirculationline.Thecombinationflow(Q+QR)throughthetricklingfilterisalwayssufficientlygreattoturnthedistributor,andadosingsiphonisnotneeded.Ahigh-ratefilterisdosedcontinuously.10BiologicalFilterwithSyntheticMediaTheprimaryreasonsforthepopularityofstone-mediatricklingfiltersaretheirsimplicity,lowoperatingcost,andproductionofawastesludgethatiseasytohandle.Nevertheless,whentreatinghigh-strengthorcoldwastewater,single-statefiltersconnotprovidesufficientorganicremovaltomeettheeffluentstandardof30mg/LofBOD,andinthecaseoftwo-stagesystemsthefirstunitunderhighloadingmayhaveproblemsofbedpluggingandemissionoffoulodors.Severalformsofmanufacturedmediahavebeendevelopedtoovercomethesedisadvantagesofcrushedrockbyprovidingahighspecificsurfacewithacorrespondinglargepercentageofvoidvolume.11Thispermitssubstantialbiologicalgrowthwithoutinhibitingpassageofairthroughthebed.Auniformmediumalsoallowsevenloadingdistribution,andlightweightpackingpermitstheconstructionofdeeperbedswiththeabilitytohandlehigh-strengthwastes.12DescriptionofPlastic-mediaFiltersThetwocommonkindsofplasticmediaarevertical-sheetpackingandrandompacking.Thevertical-sheetpacking,madefrompolyvinylchloride(PVC),ismanufacturedinthemodulesusually2ftwide,2fthigh,and4ftlong.Typicalconstructionisofcorrugatedsheetsbondedbetweenalternateflatsheetstopreventclearverticalopenings;wastewaterpassingdownthroughthepackingisthusdistributedoverthesurfacesofthemedia.Thespecificsurfacevariesfrom90~140m2/m3dependingonthemanufacturerandthevoidspaceisabout95%.13Thelightweightofplasticmodulesallowsself-supportingstacksintowers20ft.ormoreindepth.Theunderdrainsystemconsistsofasupportingblockwithdraintroughsconnectedtoaneffluentchannel.Wastewaterpumpedtothetopofthebedisspreadeitherbyarotarydistributormountedonacentralinfluentpipeorbyfixednozzlesattachedtopipingthatisstructurallysupportedbybeamsandcolumns.14Deepbiologicalfiltersfortreatingmunicipalwastewateraregenerallysingle-stateunitsfollowingprimarysedimentation,althoughtwostagesareinstalledwhenprocessingstrongwastewaterresultingfrom