Air sparging in ultrafiltration hollow fibers: relationship between flux enhancement, cake characteristics and hydrodynamic parameters

The gas sparging process consists in injecting a gas with the feed during the filtration period. Many studies have recently demonstrated the interest of gas sparging for various membrane configurations to enhance the flux or to modify the membrane selectivity. This study focuses on gas sparging in ultrafiltration hollow fibers to prevent a particulate deposit. New experimental results on ultrafiltration of river water are introduced and demonstrate that air sparging is more interesting for waters having a high fouling ability (low critical flux). On the basis of experimental results for clay suspensions, an analysis of the influence of the two-phase flow parameters on the cake characteristics (porosity, thickness and specific resistance) is then introduced and allows to demonstrate that the notion of cake deposit is no more available with a gas/liquid flow. Hydrodynamics of the gas/liquid two-phase flow was characterized, in order to identify the parameters responsible for deposit prevention and for flux enhancement. Mixing or turbulence near the membrane surface seems to control the flux enhancement whereas the cake structure seems to be linked either to mixing or to the flow intermittence. To conclude, perspectives of future processes which could involve air sparging are discussed.