Reverse osmosis of milk with thin-film composite membranes

Abstract

The effect of operating parameters (transmembrane pressure, flow rate, temperature and feed concentration) on the performance of a spiral-wound and a tubular thin-film composite reverse osmosis (RO) membrane during the concentration of milk was studied. No permeation was observed until the applied transmembrane pressure exceeded the osmotic pressure of the feed (0.70–0.76 MPa or 100–110 psi). With all modules, flux increased linearly with applied transmembrane pressure up to about 2.1–2.8 MPa. Flux then became asymptotic and decreased at much higher pressures. The pressure at which maximum flux was obtained was higher with higher flow rate. In comparison to cellulose acetate, the composite membranes gave higher flux and better rejection of salts and sugars; the difference in permeate quality was greater at higher concentration factors. Flux declined with feed concentration, as expected for an osmotic pressure-limited system. Under otherwise equivalent operating conditions, higher flux could be obtained with the tubular unit but at the expense of higher energy consumption. Substantial energy savings are possible by incorporating RO in certain milk evaporation and dehydration plants, depending on the design of the evaporation system.