Achieving low concentrations of chromium in drinking water by nanofiltration: membrane performance and selection

This study evaluates nanofiltration as a feasible process to reach low concentrations of chromium in drinking water and provides means for the selection of the most suitable membrane based on the specific treatment needs. Chromium removal is concerning since new stringent limits (10 mu g/L) for hexavalent Cr concentration in potable water were recently adopted in various countries. Three commercial nanofiltration membranes were tested against this threshold value: two membranes made of semi-aromatic polyamide and the third having a sulfonated polyethersulfone asymmetric film as the selective layer. The rejection observed as a function of chemical composition in the feed solution suggests that electrostatic effect is an important mechanism of chromium(VI) removal for the membranes with higher surface charge and lower film density. The performance of such membranes is strongly affected by the presence of salts, especially divalent cations, which reduce both Cr(VI) rejection and the permeate flux. The removal of Cr(VI) by denser membranes is dominated by solution-diffusion and is not influenced by feed ionic strength. The exposure of membranes to high chromium concentrations and to hypochlorite, typically employed as an oxidizing agent in water treatment plants, was also investigated. An analysis of the operational membrane life is thus discussed, based on the loss in performance due to active film degradation. All three membranes showed adequate rejection of chromium from tap and well water of diverse chemical composition, suggesting that nanofiltration is an effective process to remove chromium for the production of safe drinking water. However, membranes with different properties should be adopted depending on specific feed water composition and on the productivity required from the system. A final analysis is presented to help with the choice of the most suitable nanofiltration membrane based on initial and target Cr(VI) concentration in feed and product water, respectively.