Nanofiltration of the permeate from the ultrafiltration of skim milk allows for the concentration of the valuable lactose and calcium content, with the concurrent removal of unwanted monovalent salts. In this work, bench scale experiments are used to identify the extent and nature of the membrane fouling that can occur during such an operation. It is shown that the fouling of fresh permeate is substantially different to that of simulated solutions, indicating that the small quantities of native protein present in the permeate play an influential role in the fouling process. With fresh permeate, fouling occurs most rapidly at high temperatures (50 degrees C) and high pH (8.9) due to the bulk precipitation of calcium phosphate as a fouling cake. However, protein was also identified in the fouling layer. Conversely, at lower operating temperatures (10 degrees C) and pH (5.5) a less permeable and more strongly bound scale forms. (C) 2008 Elsevier B.V. All rights reserved.