The effect of additional coagulant on the re-growth of alum–kaolin flocs

Abstract

The process of floc formation, breakage and re-growth by aluminum sulfate (alum) with kaolin suspension was investigated by continuous optical monitoring. This paper focuses on the re-growth ability of broken flocs after an additional low dosage of alum, added during floc breakage. When coagulated by charge neutralization at pH 5, the broken flocs could re-grow almost fully without additional coagulant dosage. In this case an additional low dosage of alum during floc breakage lowered re-growth ability of broken flocs, because of charge reversal and restabilization of the clay particles. However, without additional coagulant dosage, broken flocs showed incomplete re-growth ability at pH 7, where an amorphous hydroxide precipitate was formed. In this case, additional dosage of coagulant led to re-grown flocs that were larger than those before breakage. It is likely that, under these conditions, new precipitate coats the surface of the broken flocs so that stronger bonds are formed between floc fragments. It seems that surface characteristics of flocs and not just their zeta potential are important in influencing the re-growth of flocs.