Reaggregation of flocs in coagulation-cross-flow microfiltration

Abstract

Cake layer in cross-flow microfiltration can be reduced by coagulation of particles. This is because enlarging particle size by coagulation increases shear induced diffusivity and the back transport of rejected particles. In addition, it is known that flocs can be broken when they are passing through a pump. In light of these facts, this study aims to experimentally look at the floc-breaking phenomenon being related to cake layer reduction. Kaolin is used as particles and aluminum sulfate as a coagulant for the experiment. The results of particle size analyses show that the aggregated particles in feed are completely disaggregated by the pump but reaggregated in the membrane module. This suggests that the reaggregation of particles is critical to cake reduction and flux enhancement. This study proves that most of it occurs around the movable cake layer on membrane where G value is small. Charge neutralization of particles is also proven a dominant coagulation mechanism for reaggregation around the movable cake layer over sweep flocculation. These results suggest that rapid mixing and charge neutralization of particles are more important in reducing cake layer than flocculation. In this reason, in-line mixer is expected to work better with coagulation-cross-flow microfiltration than conventional coagulation tank and this study proved it.