Electrokinetic leakage as a tool to probe internal fouling in MF and UF membranes

Tangential electrokinetic measurements are widely used to characterize membrane fouling as the membrane zeta potential is partly governed by the presence of foulant materials on its surface. However, in the case of porous materials as micro- (MF) and ultrafiltration (UF) membranes, a part of the streaming current flows through the porosity of the membrane during measurements. This electrokinetic leakage, is directly impacted by the presence of foulant materials inside the membrane porosity. Hence, this paper investigates for the first time the possibility of using electrokinetic leakage as a probe for detecting internal fouling, taking lipid fouling as example. Firstly, a lab-scale methodology combining "upside-down" fouling experiments with electrokinetic measurements demonstrated that the intensity of the electrokinetic leakage was related to the presence of internal fouling. Secondly, the concept was applied to the pilot-scale MF and UF of an oil-in-water emulsion under various transmembrane pressures (TMP). A significant impact of the TMP on the internal fouling of a MF PES membrane was highlighted, whereas almost no impact of the TMP was noticed in the case of an UF PAN membrane. The developed methodology using the quantification of the electrokinetic leakage phenomenon allows distinguishing the contributions of internal and external (surface) fouling. These findings offer new application of tangential electrokinetic measurements to gain more insight into the characterization of membrane fouling.