Air sparging for flux enhancement in nanofiltration membranes: application to O/W stabilised and non-stabilised emulsions

The aim of this study was to enhance the flux during nanofiltration of droplet suspensions in water, using air sparging, which consists of injecting air directly into the feed stream during filtration. This process was tested in crossflow with flat sheet nanofiltration membranes for two kinds of feeds: stabilised oil-in-water macroemulsions and non-stabilised oil-in-water emulsions. The module was operated vertically and air and liquid were injected in cocurrent flow. It was first shown that injecting air, even at high gas velocities, does not modify the permeability to pure water. In both cases (for stabilised and non-stabilised oil-in-water emulsions), a significant flux enhancement was observed with air sparging, due to the ability of air bubbles for disrupting the oil layer over the membrane surface. The permeate flux (Fp) was increased by a factor 2.4 at 4.5 bar with a 10% non-stabilised emulsion by injecting air at a 1 m s−1 gas velocity. With a 4% stabilised macroemulsion, air sparging makes it possible to reach the water flux (FW). Influence of the transmembrane pressure (TMP), of gas velocity and of the percentage of oil in the emulsion was investigated and discussed. Some first explanations on how air sparging can help in enhancing the flux are given.