Ž . Journal of Membrane Science 125 1997 109–122 Particle deposition during membrane filtration of colloids: transition between concentration polarization and cake formation V. Chen a, ) , A.G. Fane a , S. Madaeni a , I.G. Wenten b a UNESCO Centre for Membrane Science and Technology, School of Chemical Engineering and Industrial Chemistry, UniÕersity of New South Wales, Sydney, N.S.W. 2052, Australia b Centre for Membrane Science and Technology, Institute of Technology Bandung, Jurusan Tecknick Kimia ITB, Bandung-40132, Indonesia Received 31 January 1996; revised 10 June 1996; accepted 19 June 1996 Abstract The transition from concentration polarization to cake formation has been studied for the membrane filtration of colloidal Ž . silica by imposing flux and observing the system response. A critical flux J has been measured, below which crit transmembrane pressure drop, D P, is stable for increasing and decreasing flux. The flux–pressure profiles for operations Ž . Ž . below J show little for MF or negligible for UF hysteresis. Above J the pressure has a period of instability for crit crit increasing and decreasing flux, and there is significant hysteresis. It appears that once J is exceeded, the colloids in the crit polarized layer form a consolidated cake structure that is slow to depolarize and which reduces the flux. Evidence for cake deposition was obtained from electron micrographs. The depolarization can be increased by crossflow, by washing, and increasing pH. It was observed that the slow incrementation of flux to a given high value can result in significantly lower D P than the direct application of that flux. These differences are ascribed to formation of a stagnant, highly concentrated layer near the membrane surface due to consolidation and aggregation of solute resulting from very rapid flux increases. Keywords: Concentration polarization; Fouling; Ultrafiltration; Critical flux; Microfiltration 1. Introduction In 1970 a classic paper by Michaels and cowork- wx ers 1 provided the first comprehensive analysis of Ž . concentration polarization in ultrafiltration UF . This paper highlighted the difference between concentra- tion polarization in microsolute reverse osmosis and macrosolute and colloidal UF, and introduced the term ‘‘gel-polarization’’ to describe the pressure-in- dependent, ‘‘limiting flux’’ region in UF. The film model, based on a solute balance in the boundary ) Corresponding author. layer where convective transport is equated to diffu- sive back-transport, was also developed in this paper for ‘‘gel polarized’’ UF. This model was shown to explain qualitatively the observed effects of concen- tration, fluid flow regime, and system geometry, and pointed to ways of optimizing UF operation. The gel-polarization model has been challenged w x by proponents for the osmotic model 2,3 and the wx classical particle filtration model 4 . A subsequent wx paper by Michaels and coworkers 5 tested these models for unstirred UF of albumin protein and found for this system more support for the classical filtration model. However, this paper also showed 0376-7388r97r$17.00 Copyright q 1997 Published by Elsevier Science B.V. All rights reserved. Ž . PII S0376-7388 96 00187-1