DIRECT OBSERVATION OF PARTICLE DEPOSITION ON THE MEMBRANE-SURFACE DURING CROSS-FLOW MICROFILTRATION

In the crossflow microfiltration of particles, a deposit cake layer te nds to form on the membrane and this usually controls the performance of the filtration process. This paper presents observations of particl e deposition on membrane surfaces using a non-invasive, in situ, conti nuous direct observation through the membrane (DOTM) technique. The pa rticles used in the experiments were typical of microfiltration proces ses, yeast (mean diameter 5 mu m) and latex beads (3, 6.4 and 12 mu m) . The filtration tests were conducted in the imposed flux mode, so tha t the flux could be controlled at, below, or above the ''critical flux ''. Below the critical flux, the particle deposition was negligible; n ear the critical flux the particle deposition was significant; and abo ve the critical flux, particle layers were formed on the membrane surf ace. Rolling of the particles was observed during the filtration of 6. 4 mu m latex near the critical flux whereas a flowing cake layer was o bserved during the filtration of 3 mu m latex. The particle size distr ibution of the deposited particles changed with the crossflow velocity , with smaller particles deposited on the membrane at higher crossflow velocity. Comparison of the normalised flux (J/Delta P) with the memb rane area coverage by the particles revealed that for filtration of la tex particles ''flux percentage (with respect to the clean membrane)'' was marginally greater than the percentage of uncovered membrane area , whereas for filtration of yeast, the ''flux percentage'' was signifi cantly less than the uncovered area percentage due to the deposition o f smaller cell debris species. This paper demonstrates that DOTM is a powerful technique for the study of fundamentals of particle depositio n and interactions between the particles and the membrane. (C) 1998 El sevier Science B.V. All rights reserved.