Air sparging has shown to be very efficient for flux enhancement in different membrane processes and for different membrane geometries. This study focuses on the application of air sparging to nanofiltration flat sheet membranes and aims to understand which hydrodynamic parameter is responsible for flux enhancement. The two-phase flow in a flat sheet cell was characterised: local wall shear stresses were measured using an electrochemical method. The flux enhancement during nanofiltration of clay suspensions was measured in the same cell and relationships could be established between this flux enhancement and some hydrodynamic parameters including the wall shear stresses.