Breakage and Re-formation of Alum Flocs

Abstract

The formation and breakup of alum flocs during cycled-shear conditions have been investigated using the conventional jar-test procedure and by continuous optical monitoring. Using the experimentally determined optimum dosage of aluminum sulfate, the breakage of flocs was followed at different stirring speeds, corresponding to average shear rates (G) of about 180 and 520 s^-1. Most of the breakage occurred within a few seconds of increasing the shear rate. After each breakage the stirring rate was reduced to the original value of 50 rpm (G ≈ 23 s^-1) to allow the flocs to regrow. It was found that only limited regrowth of flocs occurred, and that the floc size after each regrowth phase was lower than the previous value, indicating a significant irreversibility of the floc breakup process. Residual turbidity increased after each breakage and regrowth cycle, indicating a progressive reduction in sedimentation rate of the regrown flocs, corresponding to the continuous monitoring results. The effect of rapid stirring time was examined at the highest shear rate (520 s^-1). Flocs were exposed to this rapid stirring rate for different durations (5-300 s). Floc breakage and recovery factors were calculated, and both were found to decrease with increased breakage time. It was also found that the floc size tends to a nearly constant value after an initial abrupt fragmentation, with only a very slow size reduction over several minutes.