The role of mixing conditions on floc growth, breakage and re-growth

Abstract

This work aims to evaluate the effect of different rapid mixing times and slow stirring speeds on coagulation and floc properties, using aluminum sulfate as coagulant, under conditions where significant precipitation of an amorphous hydroxide precipitate occurs. The growth, breakage and re-growth of flocs were followed by a continuous monitoring technique, in order to explore the underlying mechanisms. Floc size distributions were derived from microscopy and image analysis. The speed of rapid mixing during and after coagulant addition was kept constant, but the duration was varied. Increasing the rapid mix time led to a decrease in the final floc size. Another important parameter is the slow stirring speed during floc growth. As expected, the steady-state floc size decreased with increasing slow stirring rate. Despite these effects, floc size after breakage at high shear and after re-growth at low shear were found to be very little influenced by shear conditions during the initial floc growth. As previously found, broken flocs did not fully re-grow after breakage, probably as a result of a change in floc surface properties arising from rupture of bonds within the hydroxide precipitate.