On the role of aluminium hydroxide in the conditioning of an alum sludge

Abstract

This study dwells on the optimum dosage of a nonionic polymer during the conditioning of an alum sludge arising from the treatment of a coloured, upland water. Dewaterability was investigated through Capillary Suction Time (CST) and Specific Resistance to Filtration (SRF) tests, together with viscosity measurements on the liquid phase of sludge. The optimum dose associated with CST and SRF was 1.5 kg/T while for the case of viscosity behaviour it was 2.0 kg/T. From polymer adsorption studies on the sludge, it was shown that the optimum doses associated with the viscosity measurements were linked to a dose at which the adsorption capacity reached a saturation value (1.8 kg/T). The SRF and CST optima were found to correspond to a state of 70-85% of the saturation coverage. Parallel adsorption tests on alum sludge and pure precipitate showed that polymer adsorption on the precipitate was the prime determinant of the adsorption characteristics of the sludge and suggested that the optimum dose was linked to the mass of precipitate present in the sludge.