The purpose of the study was to investigate the change in membrane resistance in relation to operating pressure for clean RO and NF membranes during filtration of a bioreactor effluent. Experiments were conducted using a SEPA CFII membrane element cell and effluent from a bioreactor with high-ionic content. Flux-pressure profiles of two RO and one NF membranes were analyzed and compared with those for filtration of deionized water. The flux-pressure profile increased linearly with increasing pressure for all three membranes during filtration of deionized water. However, during filtration of the bioreactor effluent, the rate of increase in flux decreased after the pressure reached a critical level with both RO membranes. The changes in flux and membrane resistance were analyzed in relation to the operating pressure which was correlated to transmembrane pressure using logarithmic and second order polynomial correlations. Experimental results showed that a second order polynomial function described the flux-pressure profile more adequately than the linear correlation. The use of the flux ratio (i.e., flux with bioreactor effluent to flux with deionized water) allowed a basis for comparison of change in membrane resistance with transmembrane pressure for different membranes. The critical pressure which resulted in the minimum membrane resistance was determined by the change in the membrane resistance expressed in dimension-less form as the ratio of the observed membrane resistance with bioreactor effluent and deionized water. (c) 2008 Elsevier B.V. All rights reserved.