Abstract The City of San Diego in California, United States, (City) is developing new water sources to serve its arid region. Water repurification, in which reclaimed water receives additional advanced water treatment (AWT) prior to its discharge to a potable water supply reservoir, is one of the encouraging alternatives being implemented by the City to reduce the region's reliance on less dependable imported water. The City adopted the reverse osmosis (RO) process as the foundation for the AWT because RO has been shown to accomplish the best overall removal of organics, trace metals and total dissolved solids. In addition, RO has the potential for removal of all classes of pathogens. The California Department of Health Services (DHS) issued a conditional approval of the San Diego Water Repurification project in August, 1994. Several of the comments in the DHS conditional approval letter addressed the disinfection strategy and the reliability of the membrane processes in the AWT train. In response to the DHS comments, the City of San Diego initiated a major pilot testing program to evaluate the performance of various prequalified pretreatment and RO membranes. This program was initiated in 1995 and is still ongoing. Pursuant to the work performed at the Aqua2000 Research Center the DHS issued a letter on March 4, 1998, approving the Water Repurification System. The results of these studies demonstrated RO is a very effective and reliable process for water repurification. Minimal membrane fouling was observed for all of the polyamide RO membranes employed in the study. Significant contaminant rejection was achieved by all RO membrane purifying the reclaimed water to meet and exceed drinking water standards. Wide range of virus rejection was observed for the RO membranes which was dependent on the RO membrane type/manufacturer. The system consistently produces a product water that exceeds all drinking water standards.