The effect of the order of reagent addition on the settling rate of aluminium hydroxide in the AI(III)-Na₂CO₃ system

The effects of several factors on the settling rate of aluminium hydroxide were investigated during chemical coagulation using aluminium salts. Experimental variables were pH, aluminium (III) concentration and the order of addition of reagents. Experiments were carried out at pH 5-8 and rapid settling was achieved when aluminium (III) solutions were added to Na₂CO₃ solutions near neutral pH, close to the minimum solubility pH of A1(OH)₃. For a narrow range of total A1 concentration where A1(III) species were supersaturated with respect to the solid phase, A1(III)-added-to-carbonate type mixtures yielded a higher settling rate than mixtures obtained by the reverse order of reagent addition. The results were interpreted by comparing the rates of formation of polymer and solid (amorphous A1(OH)₃) phases. It was concluded that A1(III) coagulants should be added to water containing natural or artificially incorporated carbonate alkalinity for rapid settling of A1(OH)₃ flocs.