The impact of spacer configurations (i.e. cavity, zigzag and submerged) and mesh length on the alleviation of concentration polarization and the enhancement of permeate flux in the crossflow reverse osmosis membrane channels was investigated. In this study, the wall concentration and permeate flux were directly determined from the numerical solutions of the fully coupled governing equations of momentum and mass transfer in the feed channel. It was demonstrated that the average permeate flux could be significantly enhanced by the spacers, especially those with zigzag configuration. Simulations showed that the zigzag configuration was the most effective one to alleviate concentration polarization and to enhance permeate flux while the submerged configuration is the least. It was further found that an optimum mesh length (corresponding to the maximum permeate flux enhancement) existed for cavity and zigzag configurations and the optimum mesh length decreased with increasing salinity of the feed water. The results suggested that different mesh length should be used in membrane modules for feed waters of different salinities to obtain the maximum permeate flux enhancement. (c) 2006 Elsevier B.V. All rights reserved.