A general layout for two-module feed-forward RO systems which incorporates most functional arrangements has been developed. A simple mathematical model, suitable for all candidate arrangements, has been developed to simulate the systems under a wide range of operating conditions. The model has been employed to study the concentration polarization on membrane walls. The effects of feed specifications, operating conditions and membrane dimensions on the concentration at the wall of each membrane have been examined. Based on the magnitude of wall concentration and the range in which it changes, the arrangements experiencing lower values are pointed out The results indicated that the value of wall concentration is almost twice the value of feed concentration. The wall concentration increases as the feed concentration and feed pressure increase. This increase is more pronounced in retentate reprocessing arrangements. The wall concentration significantly decreases as the feed temperature increases within the limits recommended for module operation. For module 1, an average decrease in wall concentration of about 23% of its original value takes place when the feed temperature is increased from 25 to 50 degrees C The corresponding decrease for module 2 is about 21%. These reductions allow about 7% increase in the effective pressure. (C) 2014 Elsevier B.V. All rights reserved.