Abstract Following the recent market penetration of back-washable capillary micro- and ultra-filtration membranes, their application for treating polluted fresh water and for pre-treatment of wastewater as well as polluted brackish and seawater is now widely considered. In 1998 Mekorot initiated a field testing program to investigate the operation of integrated UF-RO systems to desalt polluted brackish surface water in comparison with a system using conventional pre-treatment. The results show that UF pre-treatment yields a performance superior to that obtained by the conventional pre-treatment regarding quality and overall system availability. However, at times of very high turbidities, a high load on the UF requires reduction of the filtration rate, effecting significant water losses. Therefore, a clarifier is needed to reduce loads of the UF and to improve substantially its performance regarding filtration rate and water losses. A preliminary economic evaluation made for a case study of an integrated UF-RO 20,000m 3/d desalination plant, indicates a projected cost of about 40–45 US cent/m 3. This cost is significantly lower than the projected seawater desalination cost. Therefore, a priority should be given to this source desalination in the new water sources development plan, especially in inland locations where neither potable nor brackish groundwater sources are available. The paper presents the results of the ongoing optimization study to improve the performance of the UF and conventional RO systems, and provides a detailed economics case study.