Degremont水处理手册(第7版)-5

Degremont水处理手册(第7版)-510FLOCCULATION-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-Settli