THE (RELATIVE) INSIGNIFICANCE OF G IN FLOCCULATION

The kinetics of flocculation of heterodisperse suspensions like those in water treatment plants are usually described by the Smoluchowski equation, which incorporates collision frequency functions for particle collisions by Brownian motion, fluid shear, and differential sedimentation. These collision-frequency functions are based on a rectilinear view of collisions, i.e., one that ignores short-range forces and changes in fluid motion as particles approach one another. With this rectilinear approach, the velocity gradient, G, is the principal design parameter for flocculation units. A curvilinear approach, i.e., one that accounts for short-range effects in particle collisions, is presented as a set of corrections to the rectilinear collision frequency functions for each mechanism. The primary ramification of this curvilinear, heterodisperse approach is that G is found to be not nearly so important. Previous experimental work in which the role of G has been examined is reviewed in light of this finding.