Influence of velocity gradient and rapid mixing time on floes formed by polysilica iron (PSI) and polyaluminum chloride (PAC1)

摘要

The aim of this investigation was to evaluate the effect of different velocity gradient and rapid mixing time on coagulation and floe properties, using polysilica iron (PSI) and polyaluminum chloride (PAC1) with different rapid mixing speeds and times. The growth, breakage, and re-growth of floes were monitored by a particle-size analyzer during the rapid mixing, in order to determine the underlying mechanisms. For PSI and PAC1, three different zones were found to depending on the G value in the coagulation process. At low G values (G<546s~(-1) for PSI floes and G<390 s~(-1) for PAC1 floes), the aggregation dominated over breakup. At mean G values (G = 546 s~(-1) for PSI floes and G = 390s~(-1) for PAC1 floes), floe formation was maximized with the largest floes produced at the lowest residual turbidity. For large G values (G>546s~(-1) for PSI floes and G>390s~(-1) for PAC1 floes), breakup dominated over floe formation. Broken floes did not fully re-grow after breakage, probably as a result of a change in the floe surface properties arising from the rupture of bonds within the hydroxide precipitate. A rapid mixing time of 60s led to maximum floe formation. In addition, PSI floes were more durable to high than PAC1.