Flux enhancement by a continuous tangential gas flow in ultrafiltration hollow fibres for drinking water production: Effects of slug flow on cake structure

Abstract

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