Degremont水处理手册(第7版)-5

10FLOCCULATION-SETTLINGFLOTATION1.ADDITIONOFREAGENTSTheeffectivenessoftheflocculationprocessisdirectlyaffectedbytheefficiencyofpriorcoagulation.Toensurethelatterthereagentmustbedispersedvirtuallyinstantaneouslyinacoagulationchamber,aflash-mixreactorcharacterizedbyhighpowerinputandahighvelocitygradient(seepage135).Dependingonthetechnology,theliquid'sretentiontimevariesfromafewsecondstoafewminutes.1.1.IMPELLERTYPEFLASHMIXERSThesecylindrical(steel)orrectangular(concrete)tanksarefittedwithahighspeedimpellertypemixer(Figure295).Dependingontheirdesign,retentiontimevariesfrom1upto3minutes,forthelargest.Thevelocitygradientisbetween250and1,000s-1dependingupontheapplication.Thereagentisfedintothezoneofhighestturbulence,i.e.,aboveorbelowtheimpeller,dependingontheflowdirection(upwardordownward).Forsomeapplicationsinwhichthevelocitygradientmustbeadjustableaccordingtothetemperatureoravaryingpollutionload,theimpellermixerisfittedwithavariablespeeddrive.Chap.10:Flocculation-Settling-Flotation1.2.STATICMIXERSStaticmixersareplaceddirectlyinthetreatmentlineupstreamfromtheflocculationphase.Theseunitsfeaturehighvelocitygradients(2,000to30,000s-1),andbriefretentiontimes(generallylessthanonesecond).Thepowerdissipatedisdirectlylinkedtotheirheadloss,andthequalityofthemixingisafunctionoftheirdesign.Unitefficiencydecreaseswiththeflowratetobetreated.1.2.1.TheRadialmixThisstaticmixerfeatureshydraulicradialdiffusion.Itcomprisesanozzlelocatedonthepipe'scentrelineandadiaphragmwithaperipheralopening.Standardmodelsareavailableforpipediametersfrom100to400mm.Theheadlossrangesfrom0.2to5mofWCdependingonthemodelandtheapplication.•Radialmixtype1(Figure296a)ThismixerisbestsuitedforinorganicreagentsandpolymersinjectedataflowrateqithatislowerthantheflowrateQoftheliquidtobetreated.Itisdesignedtoallowinjectionofconcentratedsolutions.0.0005%qi1%ofQThereagentispre-dilutedintheshortdosingline.•Radialmixtype2Theflowrateqiofliquidtobeinjectedishigherthanforthepreviousunit(qi0.5%ofQ).Theheadlossexceeds0.2mofWC,andisadjustedaccordingtotheapplication.Therearethreemodelsoftype2Radialmix.•Radialmix2MPrimarilyintendedforfeedingreagentsintoliquidswithahighsolidscontent.Theunitishousedinasleeveandhasanozzlewithoutanimpeller.1.Additionofreagents•Radialmix2MHSimilartotheRadialmix2M,butwithanozzleandimpeller.Thismixerisnot.suitableforfluidscontainingcoarseorfibrousparticles.•Radialmix2CThiscompactmodelishousedinanorificeplateandisfittedwithanozzleandimpeller.Itisespeciallysuitedforadditionofclearreagentsorliquids.1.2.2.TheMSCThiscyclone-typemixerissuitedforsludgeconditioningapplications(Figure297).Itconsistsofacylindricalsectionequippedwithsludgeandreagentinlets,contiguouswithaconicalsectioninwhichthespiralmotionensurescompletemixing.Thecylindricalsectionalsohastwoadjustabledeflectors.Chap.10;Flocculation-Settling-Flotation1.3.TURBACTORTheTurbactor(Figure298)isaclosedflash-mixreactorhavingnomovingparts.Itisdesignedtooperateunderpressureandconsistsoftwosections:-avigoroushydraulicmixingzone,-acontactzonewhicheliminatesshortcircuits.Theunit,ofplasticorprotectedsteelconstruction,maybefittedwithapHandrHcontrolsystem,makingitsuitableforneutralizationordetoxicationapplicationsaswellascoagulation.Fortheseapplications,theminimumretentiontimeistwominutes,andthevelocitygradientisabout600s-1foraliquidviscosityof1centipoise.2.Flocculators2.FLOCCULATORSFlocculationiscarriedoutintanksknownasflocculators,equippedwithmixers.Themixingsystem,reactorvolumeandenergydissipationdifferaccordingtothespecificapplicationorfluidinvolved.Aflocculatorischaracterizedbyitsvelocitygradient,itscontacttime,andtheextremelocalvelocitiesofthemovingelementandliquid,whichshearthefloc.Forexample,forametallichydroxidefloc,theperipheralvelocitiesofthemovingelementmustnotexceed40cm.s-1Geometryofthetank,mixingsystemandrelatedequipmentisdefinedsuchthat:-deadzonesareavoided(areasofdepositsonthebottom,forexample),-dissipatedenergyisrecoveredasturbulence(bymeansofperipheraldeflectorsincirculartanks,forexample),-shortcircuitsbetweentheliquidinletandoutletareprevented.Finally,itisimportantnottorupturetheflocasitistransferredfromtheflocculatortothesettlingzone.Dependingonthequalityofthewatertreated,thefollowingtransfervelocitiesareusedforsurfacewaterclarification:-fragilemetallichydroxideflocv=0.20m.s-1-strongmetallichydroxideflocv=0.50m.s-1Flocculatorsmaybeclassifiedintwocategories:-flocculatorswithamobilemixerunit(mechanicalflocculators),-off-setbaffleorstaticflocculators.2.1.MECHANICALFLOCCULATORS2.1.1.PaddletypeflocculatorsTherotatingunitconsistsofaseriesofpaddlesmountedonaverticalorhorizontalshaftinregularly-spaceddiametricalplanes(Figure300).Thesystemisdrivenbyareductiongearmechanismthatmayormaynotincludeavariable-speeddrive.Therotatingunitmaybedriveneitherbyachainordirectly.Directdriveeliminatesrisksofcorrosionandtheconstraintsofchainmaintenance.2.1.2.PropellertypeflocculatorsTherotatingunitconsistsofaverticalshaftpropellerwiththreeorfourblades,andisdrivendirectlybyareductiongearsystem,mostoftenequippedwith