The implementation of a computer model to simulate reverse osmosis in systems with several components is described. The model was based on the Spiegler-Kedem theory of reverse osmosis. The results of a pilot-scale investigation of the use of nanofiltration to recover sodium chloride from waste brine from the regeneration of anion exchange resin were used in this study. This effluent contains around 50 g/l of sodium chloride and 5 g/l (as total carbon) of organic colourants, many of which are negatively charged. When using the SelRO(TM) MPT-30 or MPT-31 membrane at an operating pressure of 3 MPa, temperatures between 45 degrees C and 60 degrees C, and linear feed flow rates around 1.6 m/s, the retention of sodium chloride was found to be between 0% and 20%, while the organic compounds had an overall retention of 80% to 97%. The experimental results were used to obtain a set of parameters relating to the system, which were then used to generate design data for a full-scale plant. Suitable module arrangements were proposed and the system performance for each case was predicted.