Zeta potential characterization of the effects of fouling and cleaning of hollow fibre forwards osmosis membrane modules

Commercial Aquaporin Inside® hollow fibre forward osmosis (HFFO) membranes embedded in lab-scale filtration modules were employed to study their fouling propensity during forward osmosis (FO) up-concentration of a fermentation broth resulting from vaccine production. The fouling and subsequent cleaning processes were analysed in terms of the lumen side active layer zeta potential of the HFFO membranes. The zeta potential indicated significant adsorption of contaminants from the feed solution on the membrane's active surface. Various cleaning conditions: alkaline, alkaline + surfactant, complexation and acidic (hydrochloric and citric), were used to investigate the potential enhancement of contaminant removal compared to a simple rinse with deionized water. Despite a full recovery of the membrane performance after cleaning, the zeta potential measurements indicated contaminants remaining on the membrane surface. The extent of this residual contamination depends strongly on the applied cleaning conditions. Rinsing with deionized water and alkaline cleaning (with or without surfactant) results in a shift in the fouled membrane isoelectric point (IEP) with a concomitant shift of the zeta potential towards the IEP and zeta potential of the pristine membrane. However, cleaning with either the complexation agent or under acidic conditions exhibits a more negative zeta potential of the fouled HFFO membranes compared to the pristine membrane, indicating an additional effect superimposed on the removal of organic contaminants.