Assessmentof Coagulants for Water Treatment

ReportNo WSAA 41


May 1992




Theaim of the project was to produce an independent assessment of coagulants,coagulant aids and flocculant aids available in Australia for the treatment ofdrinking waters.


Theterms coagulation and flocculation are often confused and interchanged.Coagulation involves the chemical destabilisation of particles, whereasflocculation involves the physical transportation of destabilised particlesresulting in particle collision and floc formation.


Suppliersof coagulation and flocculation products provided samples for laboratoryevaluation together with technical and operational information. Each productwas categorised according to its chemical structure.


Theeffectiveness of each product was based on performance in laboratory tests inthree district raw water types, medium colour/medium turbidity, high colour/lowturbidity and low colour/high turbidity. The optimum dose of a particularproduct was the minimum dose required to achieve the following treated waterquality objectives.


Turbidity           :           <0.50 NTU


Colour              :           <10 HU


Aluminium        :           <0.2 mg/L


Iron                  :           <0.3 mg/L


Manganese       :           <0.1 mg/L


Theassessment of products was divided into three sections, coagulants,coagulant aids and flocculant aids.


Effectivecoagulants treat waters on their own, ie they are cationic species thatcan destabilise particles in water. Both inorganic and organic coagulants wereassessed and compared with aluminium sulphate or alum, the most widely usedcoagulant.


Ofthe inorganic coagulants tested, only polyaluminium chloride (PACI), ferricsulphate and ferric chloride were possible alternatives to alum. Metal iondoses required to satisfy treated water objectives compared with that for alum.However, aluminium chloride, sodium aluminate and ferrous sulphate were notrecommended as alternative coagulants to alum because they required high metalion levels to satisfy treated water objectives and pH correction for effectivecoagulation.


Thefour different types of organic coagulants tested, polydadmac, polyamine,epi/dma and polyacrylamide polymers performed best in low colour/high turbiditywaters. They showed limited ability to remove colour but were very effectivefor turbidity removal. Therefore, unless the raw water was low in colour, thefour polymer types were not considered viable alternatives to alum inconventional water treatment.


However,commercially available inorganic salt-polydadmac polymer blends, whichsatisfied all treated water objectives, were recommended as possible alumalternatives.


Coagulantaids are designed to partially replace coagulants, in particular inorganiccoagulants by assisting in the coagulation process. All liquid cationicpolymers tested, polydadmacs, polyamines and epi/dmas performed well as partialreplacements for alum in all water types. Their use at doses of up to 1.0 mgactive polymer/L enabled the required alum dose to be reduced to as low as 60%of the optimum level. However, only polydadmac polymers were recommended ascoagulant aids as polyamines and epi/dmas are not approved for use inAustralia. Cationic polyacrylamides were effective coagulant aids only in lowcoloured waters.


Non-ionicand anionic polyacrylamides, anionic latex emulsions and activated silica,which did not satisfy treated water objectives, were not recommended ascoagulant aids.


Thereis a distinct difference between coagulant aids and flocculant aids.Coagulant aids assist in particle destabilisation by charge neutralisation andtherefore assist in colour and turbidity removal. Flocculant aids, however, canonly assist in the physical process of flocculation. They reduce turbidity byinter-particle bridging but do not affect the removal of colour.


Theuse of polymers as flocculant aids offer a number of benefits which are notapparent in small scale laboratory testing. For example, polymers increase therate of flocculation, produce larger, denser floc that settles faster andstrengthen the floc which helps improve filtration. They enable a greatervolume of water to be treated in a given plant size. Their use is alsoimportant during periods when flocculation becomes difficult because of waterquality changes or because of the effect of low temperatures.


If  turbidity removal is the major criterion bywhich flocculant aids are judged, then cationic polyacrylamides and activatedsilica were more effective than non-ionic and anionic polyacrylamides andanionic latex emulsions. However, as their true benefits are measured infiltration plant performance, no product should be discounted without fullscale assessment.


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