How does the polymerisation of PAC solutions relate to coagulation performance?
When aluminium or iron salts act as coagulants, they undergo a series of reactions with water (termed hydrolysis), ultimately forming metal hydroxides, which settle out, resulting in removal of suspended solids. When a non-polymerised aluminium salt such as alum sulphate is applied to the water to be treated, the operative chemical species are formed in situ, during or after mixing, by reaction with water (which is called hydrolysis). The hydrated Al3+ cation is not involved (and neither is the sulphate ion). The operative chemical species are believed to be various polymeric forms of aluminium, similar in fact to the species present in solutions of PAC. For this reason, PAC solutions are sometimes termed “pre-hydrolysed” aluminium salts. The fact that the operative coagulant species are already partially formed in PAC solutions has a lot to do with their superior coagulation properties. Coagulation and flocculation processes In water treatment, the purpose of coagulation is to remove
