Particle deposition during membrane filtration of colloids: Transition between concentration polarization and cake formation

The transition from concentration polarization to cake formation has been studied for the membrane filtration of colloidal silica by imposing flux and observing the system response. A critical flux (J(crit)) has been measured, below which transmembrane pressure drop, Delta P, is stable for increasing and decreasing flux. The flux-pressure profiles for operations below J(crit) show little (for MF) or negligible (for UF) hysteresis. Above J(crit) the pressure has a period of instability for increasing and decreasing flux, and there is significant hysteresis. It appears that once J(crit) is exceeded, the colloids in the 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 Delta 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.