Charge reversal effect on blanket in full-scale floc blanket clarifier

Operators at waterworks in which flat-bottom floc blanket clarifiers are used, are advised to increase coagulant dose when the turbidity of clarified water is high, to stabilize the blanket and yield quality drinking water. This work showed that, even when the raw water is sufficiently alkaline, the blanket responded anomalously to the increased dose of polyaluminum chloride (PACl), which was used herein as a coagulant. Restated, rather than being stabilized, the blanket was destabilized by the high dose of PACl. This "anomalous" behavior of the blanket is caused by a temporary drop in local pH at the injection port at the bottom of the blanket, which is caused by poor PACl dispersion, and the subsequent charge reversal of the constituent particles and the decline in the blanket stability. A step increase in PACl dose may result in complete blanket loss. ? 2006 ASCE.