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

The effects of several factors on the settling rate of aluminium hydroxide were investigated during chemical coagulation using aluminium salts. Experi mental variables were pH, aluminium (III) concentration and the order of ad dition of reagents. Experiments were carried out at pH 5-8 and rapid settli ng was achieved when aluminium (III) solutions were added to Na2CO3 solutio ns near neutral pH, close to the minimum solubility pH of Al(OH)(3). For a narrow range of total Al concentration where Al(III) species were supersatu rated with respect to the solid phase, Al(III)-added-to-carbonate type mixt ures yielded a higher settling rate than mixtures obtained by the reverse o rder of reagent addition. The results were interpreted by comparing the rat es of formation of polymer and solid (amorphous Al(OH)(3)) phases. It was c oncluded that Al(III) coagulants should be added to water containing natura l or artificially incorporated carbonate alkalinity for rapid settling of A l(OH)(3) flocs.