Effect of Shear on the Strength of Polymer-Induced Flocs

Micromechanics was used to show that the rupture strength of polymer-induced flocs varied with the hydrodynamic conditions at which the flocs were formed. A maximum floc strength at an intermediate shear rate was observed. The overall performance of the polymeric flocculants, as determined by two independent methods, showed that the conditions for optimal flocculation did not coincide with those for maximum floc strength. The amount of flocculation was explained in terms of the competing effects of the particle collision frequency and the destructive hydrodynamic forces. The contributing forces to the floc strength are, however, more likely to be densification of the floc by shear and the weakening of reattachment strengths. The evidence presented here may be useful for explaining flocculation data that depart from the constant yield stress theory. Thus, the notion of the floc strength varying with shear rate may offer an alternative to multilevel floc structure models in the description of flocculation kinetics. Copyright 1997 Academic Press. Copyright 1997Academic Press