Influence of velocity gradient and rapid mixing time on flocs formed by polysilica iron (PSI) and polyaluminum chloride (PACl)

The aim of this investigation was to evaluate the effect of different velocity gradient and rapid mixing time on coagulation and floc properties, using polysilica iron (PSI) and polyaluminum chloride (PACl) with different rapid mixing speeds and times. The growth, breakage, and re-growth of flocs were monitored by a particle-size analyzer during the rapid mixing, in order to determine the underlying mechanisms. For PSI and PACl, three different zones were found to depending on the G value in the coagulation process. At low G values (G < 546 s^-1 for PSI flocs and G < 390 s^-1 for PACl flocs), the aggregation dominated over breakup. At mean G values (G = 546 s^-1 for PSI flocs and G = 390 s^-1 for PACl flocs), floc formation was maximized with the largest flocs produced at the lowest residual turbidity. For large G values (G > 546 s^-1 for PSI flocs and G > 390 s^-1 for PACl flocs), breakup dominated over floc formation. Broken flocs did not fully re-grow after breakage, probably as a result of a change in the floc surface properties arising from the rupture of bonds within the hydroxide precipitate. A rapid mixing time of 60 s led to maximum floc formation. In addition, PSI flocs were more durable to high than PACl.