Polyaluminum chloride (PACl) is a commonly used coagulant for water treatment. One mode of action of PACl [(AlO 4 Al 12 (OH) 24 (H 2 O) 12 ) 7þ ] is reported to be through the formation of polymeric Al-oxyhydroxide precipitate aggregates that bridge between colloids. Although many studies have considered the effectiveness of PACl under a spectrum of conditions for influent suspensions, little is known about the effect of precipitate aggregate size on the subsequent formation of flocs that can be readily removed by sedimentation. Self-aggregation of polymeric Al-oxyhydroxide precipitate and the rate at which this occurs may be used to guide the designs used for the mixing of coagulant with the raw water. In this research, a novel kinetic aggregation model was developed and validated to give the precipitate aggregate size before mixing with a colloidal suspension and entering a flocculator. Granular PACl, a form commonly marketed and used in the developing world, was used for experiments. Efficacy of polymeric Al-oxyhydroxide precipitates of varying aggregate size in flocculation and sedimentation of a kaolin suspension was experimentally observed. These data, in combination with modeling results and geometric analysis, show that under the experimental conditions tested in this research, polymeric Al-oxyhydroxide self-aggregation consistently lowers attachment efficiency of the colloidal suspension, reduces the effectiveness of the flocculator, and reduces turbidity removal. Generally, minimization of aggregate size and maximum turbidity removal is best accomplished by immediate, rapid, and efficient mixing of PACl with the influent water. The relationship between residual turbidity and PACl dose is consistent with results from a geometric adhesive model of coagulation in which surface coverage of colloids by polymeric Al-oxyhydroxide aggregates governs the effectiveness of collisions.
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