Analysis of separation and fouling behaviour during nanofiltration of dairy ultrafiltration permeates

Dairy ultrafiltration permeate (UFP) has been filtered using polyamide nanofiltration (NF) membranes in order to separate potentially valuable calcium and lactose from unwanted monovalent salts. An investigation into both the fouling and separation behaviour has been carried out to gather useful information for the optimal selection of operating conditions that maximise efficiency and yield. Supersaturated calcium phosphate in the feed resulted in severe flux decline during filtration at high temperatures and pH due to salt precipitation. Investigation into the ion separation behaviour using sub-saturated model solutions containing KCl and CaCl2 showed that the addition of the trivalent anions citrate and phosphate changed the pH dependent separation of K+ and Ca2+. For the multicomponent system, maximum calcium rejection and minimum potassium rejection occurred at high pH, whereas the opposite was true for calcium in the simpler systems. A possible explanation for this separation behaviour has been proposed which incorporates the membrane charge, represented by the zeta potential, and the speciation of calcium with these anions.