COAGULATION OF SUBMICRON COLLOIDS IN WATER-TREATMENT BY INCORPORATION INTO ALUMINUM HYDROXIDE FLOC

In a series of laboratory experiments it was observed that the rate and stoichiometry of aluminum hydroxide precipitation depended on pH and Al dose, and on HCO3-, SO4(2-), and natural organic matter concentrations. Incorporation of hydrous aluminium oxide colloids into the precipitate followed either of two paths. At pH 7.5 a loose floc formed; colloid removal involved subsequent aggregation of the colloids with the floc. At pH 6.5, the precipitate coated the seed particle surface; removal involved rapid self-coagulation of the colloids. Removal was greater-than-or-equal-to 99.9% at both pH's. At pH 5.5, colloid removals were insignificant. Results suggest that heterogeneous nucleation and growth of precipitate at pH 6.5 gives destabilized particles and smaller, compact floc. The homogeneous pathway involving precipitation and subsequent particle aggregation was less efficient. The heterogeneous pathway had a lower precipitate OH/Al ratio and a lower electrophoretic mobility.