Today, reverse osmosis is one of the most economical desalting techniques. It is an accepted technology in the industries with the need to separate solute from solvent. In the reverse osmosis process, one of the most expensive parts is membrane elements. In spite of 3-5 years of theoretical life considered for the membranes, their useful life reduces due to the problems like scaling, concentration polarization, colloidal fouling, biological fouling, etc. Biological fouling is one of the most important phenomenon affecting the membrane performance and its life time. A field study at water treatment plant of Fajr Petrochemical Company was carried out. The performance of biologically fouled membranes was improved by rearranging the elements in pressure vessels. By this way the conductivity of permeate and differential pressure in the pressure vessels were reduced. (C) 2010 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.