Regrowth ability and coagulation behavior by second dose: Breakage during the initial flocculation phase

Abstract

Breakage and regrowth of flocs has been studied for several years, however little attention was paid to the regrowth ability of broken micro-flocs which are mainly formed during the initial flocculation stage. In this paper, a continuous optical monitoring method was employed to investigate the kinetics of flocs formation, and especially more evident flocs properties (strength and recovery factor) were also explored. The results showed that breakage at the beginning of flocculation stage can improve flocs regrowth to greater sizes when the coagulation was initially dominated by charge neutralization at a predetermined dosage. Furthermore, second dose of different coagulants during this breakage process improved kaolin particles removal efficiency to varying degrees. Separately, the second dose of Al2(SO4)3 continuously enlarged the floc size with the increasing dosage, contrarily that of PACl reduced the floc size. Based on the analysis of flocs properties, this phenomenon can be well explained by the difference between coagulation mechanisms under the second dose of Al2(SO4)3 and PACl respectively: newly formed hydroxides of Al2(SO4)3 enhanced broken flocs regrowth to larger sizes and entrapped more particles (lower residual turbidity) by sweep flocculation; that of PACl formed compact flocs due to stronger electric density of Al hydroxides. It was worth noting that turbidity removal performance was even better when the overall dosage by second dose only counted for ∼70% of one-time dose. Importantly, this study aimed at providing a feasible economic dose strategy for the real coagulation-flocculation application.