1、Alberta Water&Wastewater Operators Association22th Annual Seminar March 11-14,1997-1-Various pesticides,organiccompounds and metals,cyanide,dioxins and furans,fluoride,nitrilotriacetic acid(NTA),totaldissolved solids,trichloroacetic acid,trihalomethanes(THMs).How to Select a Chemical Coagulant and F
2、locculant.Anthony S.Greville.Easy Treat Environmental.AbstractIn many water treatment processes theselection of the chemical regime is ofcritical importance.The mechanicalequipment will remove watercontaminants to a reasonable level,but tomeet the increasingly stringent Federaland Provincial licensi
3、ng requirementschemical coagulation,flocculation,anddisinfection are necessary.This paperwill address several topics that will helpthe water treatment plant operator selectthe most appropriate chemical treatmentprogramme for the needs of thecommunity that the plant services.Why do we Chemically Trea
4、t Water?Water is essential for life as we presentlyknow it and in North America we havebecome accustomed to receiving goodquality water at a reasonable cost(onworld wide terms Canadian drinking wateris provided at an extremely low cost).Intodays increasingly complex society thedemands of the consume
5、r,the medicaland scientific communities,and thereforethe Municipal,Provincial and Federalregulators,have caused the qualityguidelines for safe drinking water to bereviewed.In many cases what wasconsidered acceptable by all segments ofsociety just a decade ago would now bethought of as unsafe.In an e
6、ra ofchanging regulations and guidelines it isdifficult to define what is considered to begood drinking water,but reference tothe Guidelines for Canadian DrinkingWater Quality(Sixth Edition)as well asthe Provincial Licensing Authority willallow for a decision to be made.At thepresent time it can be
7、anticipated thatchanges will be made with respect to thefollowing parameters 1;Once the community at large has madethe decision as to the delivered quality ofthe finished water that will be enjoyed bythe consumer,the practicalconsiderations have to addressed.Alberta Water&Wastewater Operators Associ
8、ation22th Annual Seminar March 11-14,1997-2-Invariably mechanical means will beemployed to help achieve the treatmentobjectives,but since no mechanicalprocess is 100%efficient,chemicalenhancement will often be necessary.Another reason for appraising the utility ofchemical treatment is when capital c
9、ostsare being balanced against operatingcosts.Either the size/expense of a pieceof equipment can be reduced if efficiencycan be increased by implementingchemical treatment,or else physicalspace limitations do not allow for theinstallation of a large process additionand therefore optimization of exis
10、tingequipment has to be considered first.Chemicals typically find utility in theremoval of suspended,colloidal anddissolved solids from water,includingcalcium and magnesium hardness,mineral turbidity,organic colour and otherorganic substances,and undesirablemicrobiological species that can causeheal
11、th concerns in humans.The fourbroad categories of chemicals used arelime for precipitation softening,coagulants and flocculants for theremoval of suspended and colloidalsolids,powdered activated carbon fortaste and odour,and disinfectants for theremoval of pathogens.Selection of Chemical Species.The
12、re are three fundamental variables inwater treatment,all three of which willhave a significant influence on the type ofchemical that could be usefully employedin a particular application.The threevariables are;1)Raw Water Quality.2)Process Equipment.3)Treatment Objectives.These three variables can b
13、e furthercategorized as shown in the table below;Raw Water QualityProcess EquipmentTreatment ObjectivesAlkalinitySettling LagoonPotable ApplicationPartial SofteningFull SofteningpHDirect FiltrationTurbiditySedimentation+FiltrationColourSolids Contact ClarifierIndustrial ApplicationGeneral UseIon Exc
14、hangeTemperatureDissolved Air FlotationHardnessMixing IntensityTaste and OdourSludge DisposalIf the above variables are reviewed priorto embarking on the coagulant andflocculant selection process aconsiderable amount of time andAlberta Water&Wastewater Operators Association22th Annual Seminar March
15、11-14,1997-3-needless effort can be saved.Anunderstanding of how the variables effectwater chemistry will allow the operator tomake sensible pre-screening decisionsand let him/her focus on optimizing theprocess to achieve the treatment goals.Raw Water QualityClearly the quality of the raw water andt
16、he contaminant classification,has tohave a significant impact on the type ofchemicals used for liquid-solidsseparation.There are however severalfactors to consider;1)The amount of alkalinity present inthe water may eliminate somecoagulants from consideration.2)The amount of turbidity present mayonly determine the amount ofcoagulant that may be required.One also has to be aware of how the rawwater quality will change as a function ofthe time of the year.AlkalinityAlkalinity is of critical importa
