Possible effects of air sparging for nanofiltration of salted solutions

Among the different ways to improve the flux in ultrafiltration or microfiltration, air sparging recently appeared as an interesting process for different applications. Air sparging consists in injecting air in the concentrate compartment during filtration time. A gas/liquid two-phase flow is then obtained at the membrane surface. This study focused on the use of air sparging in nanofiltration membranes. Experiments were performed with a tab-scale flat sheet module. Previous results showed that air sparging allows flux enhancement in the case of liquid/liquid suspensions and liquid/solid suspensions, for which the main limiting phenomenon is particle deposit. The objective here was to characterise the effects of gas sparging for a salt (CaCl2)/water solution when mass transfer is limited by concentration polarisation or osmotic effects. At high concentrations, air sparging, has, no effect on the permeate flux nor on CaCl2 retention factor. This was explained by the fact that, for CaCl2 solutions until 10 g.L-1, and even for a low liquid velocity, the transfer limitation is due to osmotic effects and no polarisation concentration occurs. In that case, no benefice of turbulence increase can be expected.