Investigation on the factors determining permeate pH in reverse osmosis membrane processes

The dominant factors on determination of permeate pH in the reverse osmosis (RO) membrane process have been systematically investigated to further our understanding of the separation mechanisms of the RO process. The surface of a commercial RO membrane was modified to neutralize the surface charge, and the effect of surface properties including charge on both the pH shift and individual ion removal rate during the filtration was studied. Then, the pH shift of aqueous solution during the filtration was evaluated throughout a broad range of feed pH, from pH 3 to 12, with or without carbonate species in the feed water. The permeate pH values were predicted on the basis of the ion rejection of the RO membrane, charge balance of the aqueous solution, and dynamic equilibrium in the dissociation of water. The predicted pH values were then compared with both experimental data acquired in flat sheet filtration tests in a laboratory and pH data obtained from a commercially operating RO plant. The results support the hypothesis that the ion rejection of the RO membrane is one of main factor to determine the degree of shift in pH and the critical pH is influenced by the feed water characteristics.