(66a) Second Dose during the Breakage Occurred in Initial Flocculation Phase: Does It Work Positively?

Coagulation is a critical unit commonly functions as the first barrier for the removal of particulate and/or natural organic matter during the treatment of drinking water. However, flocs breakage has been found to exert negative effects on the effluent water quality. Although breakage and regrowth of flocs has been studied for several years, little attention was paid to the regrowth ability of broken micro-flocs which mainly formed during the initial flocculation stage. In this paper, we employed a continuous optical monitoring method 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 formation to greater sizes when the coagulation dominated by charge neutralization at a predetermined dosage. Furthermore, a 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 enlarged the flocs sizes with increasing dosage, contrarily that of PACl reduced the flocs sizes. Based on the flocs properties, this observation elicits the different coagulation mechanisms between Al₂(SO₄)₃ and PACl: hydroxides of Al₂(SO₄)₃ formed in larger sizes and entrapped more particles (lower residual turbidity) by sweep flocculation; while PACl formed compact flocs due to stronger charge of single Al. It is worth noting that turbidity removal performance was even better when the overall dosage by second dose only counted for ~ 70% of one-time dosage. Importantly, this study provides a feasible economical dosing strategy for coagulation treatment.