Floc formation and growth during coagulation removing humic acid: Effect of stirring condition

Abstract

Number of studies have explored the mechanism of humic acid (HA) removal by coagulation, while studies on the formation and growth mechanism of HA-floc are still limited. In this study, a series of dynamically monitored experiments were carried out and floc characterization techniques were applied to investigate HA-floc growth mechanisms and properties under different stirring conditions. First, HA was removed in the early stages and varied insignificantly during floc growth, and the stirring condition did not affect the removal of HA. Moreover, HA hindered the effective collision, resulting in a slower floc growth rate and a lower fractal dimension. Although stirring condition did not markedly affect the HA removal performance, it had a remarkable effect on floc growth and structure. For example, gentle stirring resulted in larger floc d50 and pore size. Intense stirring accelerated the breakage or aging of flocs, resulting in a reduction in the final floc size at equilibrium. In particular, fast stirring in the early stage of coagulation advanced floc growth, but decreased the zeta potential and active sites of flocs, and reduced the capacity of the primary aggregates to grow into larger flocs. This study provides information on the floc formation and growth mechanism and the properties of flocs formed under different stirring conditions, which are helpful for optimizing the coagulation process on organic matter removal and guide the processes utilizing flocs.