Assessment of Coagulants for Water Treatment

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report No UWRAA 41

May 1992

SYNOPSIS

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

The terms coagulation and flocculation are often confused and interchanged. Coagulation involves the chemical destabilisation of particles, whereas flocculation involves the physical transportation of destabilised particles resulting in particle collision and floc formation.

Suppliers of coagulation and flocculation products provided samples for laboratory evaluation together with technical and operational information. Each product was categorised according to its chemical structure.

The effectiveness of each product was based on performance in laboratory tests in three district raw water types, medium colour/medium turbidity, high colour/low turbidity and low colour/high turbidity. The optimum dose of a particular product was the minimum dose required to achieve the following treated water quality objectives.

Turbidity : <0.50 NTU

Colour : <10 HU

Aluminium : <0.2 mg/L

Iron : <0.3 mg/L

Manganese : <0.1 mg/L

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

Effective coagulants treat waters on their own, ie they are cationic species that can destabilise particles in water. Both inorganic and organic coagulants were assessed and compared with aluminium sulphate or alum, the most widely used coagulant.

Of the inorganic coagulants tested, only poly aluminium chloride (PACI), ferric sulphate and ferric chloride were possible alternatives to alum. Metal ion doses required to satisfy treated water objectives compared with that for alum. However, aluminium chloride, sodium aluminate and ferrous sulphate were not recommended as alternative coagulants to alum because they required high metal ion levels to satisfy treated water objectives and pH correction for effective coagulation.

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

However, commercially available inorganic salt-polydadmac polymer blends, which satisfied all treated water objectives, were recommended as possible alum alternatives.

Coagulant aids are designed to partially replace coagulants, in particular inorganic coagulants by assisting in the coagulation process. All liquid cationic polymers tested, polydadmacs, polyamines and epi/dmas performed well as partial replacements for alum in all water types. Their use at doses of up to 1.0 mg active 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 as coagulant aids as polyamines and epi/dmas are not approved for use in Australia. Cationic polyacrylamides were effective coagulant aids only in low coloured waters.

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

There is a distinct difference between coagulant aids and flocculant aids. Coagulant aids assist in particle destabilisation by charge neutralisation and therefore assist in colour and turbidity removal. Flocculant aids, however, can only assist in the physical process of flocculation. They reduce turbidity by inter-particle bridging but do not affect the removal of colour.

The use of polymers as flocculant aids offer a number of benefits which are not apparent in small scale laboratory testing. For example, polymers increase the rate of flocculation, produce larger, denser floc that settles faster and strengthen the floc which helps improve filtration. They enable a greater volume of water to be treated in a given plant size. Their use is also important during periods when flocculation becomes difficult because of water quality changes or because of the effect of low temperatures.

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

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