Fractionation of dairy wastewater into lactose-enriched and protein-enriched streams using ultrafiltration membrane technique was examined. Three membranes of MWCO of 3, 5 and 10 kDa of regenerated cellulose material were used to determine the efficiency of the process. The performance was determined under various processing conditions that include the operating temperature and transmembrane pressure across the membrane and the concentration of lactose in the feed solution. The experiments were initially conducted with a made-up solution of (i) pure lactose and then (ii) lactose solution with protein. Finally the experiments were performed with dairy wastewater supplied by a local dairy processing plant. It was found that the 3, 5 and 10 kDa membranes provided 70-80%, 90-95% and 100% recovery of lactose in permeate, respectively from made-up solution of pure lactose. The 10 kDa membrane results showed a 100% recovery of lactose from wastewater sample. At temperatures higher than the room temperature the lactose recovery was higher by a minimum of 5% and a maximum of 20% for 3 kDa membrane. An increase in transmembrane pressure increased the permeate flux which increased the percent of lactose being recovered. The addition of protein to the made-up lactose solution or using waster sample as feed did not decrease the percent of lactose being recovered even at transmembrane pressure of I bar. The 10 kDa membrane showed approximately 60-95% rejection of protein, the percentage being low at lower transmembrane pressures (less than 2.0 bars) and high at higher transmembrane pressures (greater than 2.5 bar). Other experiments conducted with wastewater showed that the permeate flux decreased slightly over a period of 25 min showing that fouling was not significant. In conclusion the most effective membrane to produce fractions: one with lactose-enriched (as permeate) and protein-enriched (retentate) from made-up solution as well as from dairy wastewater is 10 kDa. (c) 2006 Elsevier B.V. All rights reserved.