PROPERTIES OF THE CAKE LAYER FORMED DURING CROSS-FLOW MICROFILTRATION

The conditions necessary for the formation of a reversible cake layer during crossflow microfiltration were studied both experimentally and theoretically. Crossflow microfiltration experiments were performed wi th suspensions of silica particles with a narrow size distribution. Th e steady-state flux was first measured at a low transmembrane pressure (TMP), then at increased TMP, and again at the original low TMP. The cake-layer thickness was measured indirectly using a light absorbance technique. The thickness of the cake layer increased with increased TM P. Upon decreasing the TMP, the cake-layer thickness either decreased (reversible cake), or stayed constant (irreversible cake). It was show n that irreversible cakes are formed when the silica particles have a relatively low charge, whereas reversible cakes are formed when the si lica particles have a relatively high charge. The occurrence of irreve rsible cakes is unexpected, since approaching silica particles are rep orted to always repel each other. The irreversibility of the cakes was explained by the assumption that bridging between the particles can o ccur, causing the interparticle interaction to be attractive when the particles retreat. To explain the reversibility results quantitatively , a model was developed which links the physicochemical interaction fo rces of the silica particles to the permeate flux through the cake lay er. A detailed description of the interaction forces of silica particl es was given in order to feed this model with accurate parameters. A r eversibility index was introduced which quantifies the amount of rever sibility. Model calculations of the reversibility index were in excell ent agreement with measurements. (C) 1998 Elsevier Science B.V.