Properties of the cake layer formed during crossflow 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 with suspensions of silica particles with a narrow size distribution. The 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 TMP. Upon decreasing the TMP, the cake-layer thickness either decreased (reversible cake), or stayed constant (irreversible cake). It was shown that irreversible cakes are formed when the silica particles have a relatively low charge, whereas reversible cakes are formed when the silica particles have a relatively high charge. The occurrence of irreversible cakes is unexpected, since approaching silica particles are reported to always repel each other. The irreversibility of the cakes was explained by the assumption that bridging between the particles can occur, 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 forces of the silica particles to the permeate flux through the cake layer. A detailed description of the interaction forces of silica particles was given in order to feed this model with accurate parameters. A reversibility index was introduced which quantifies the amount of reversibility. Model calculations of the reversibility index were in excellent agreement with measurements. (C) 1998 Elsevier Science B.V.