FLUX ENHANCEMENT BY A CONTINUOUS TANGENTIAL GAS-FLOW IN ULTRAFILTRATION HOLLOW FIBERS FOR DRINKING-WATER PRODUCTION - EFFECTS OF SLUG FLOW ON CAKE STRUCTURE

A new process is proposed to reduce particulate membrane fouling by in jecting air into the feed stream, creating a gas/liquid two-phase flow on the membrane surface. The injected air is supposed to form air slu gs inside the hollow fibres; these slugs create high wall shear stress es and flow instabilities. These phenomena may prevent filtered partic les from settling on the membrane surface, and so enhance the ultrafil tration flux. Experiments were carried out with clay suspensions in ho llow fibre membranes. A range of air velocities and particle concentra tions was examined. The air injection process led to an increase in th e permeate flux, depending on the liquid velocity and transmembrane pr essure, for all the various concentrations studied. For specific condi tions, the flux can be increased by 155% using a critical gas velocity . Above this critical value, the flux is no longer enhanced. The air i njection clearly modifies the cake structure, and seems to expand the cake. Evolutions of cake thickness, porosity and specific resistance w ith gas velocity are given and analysed.