I n California, where water has long been recognized as a scarce and valuable resource, the Orange County (Calif.) Water District (OCWD) has demonstrated commitment to advanced water purification and reuse for more than three decades. Completion of the initial expansion of OCWD’s groundwater replenishment system (GWRS) early in 2015 made the world’s largest advanced water purification system for potable reuse even bigger. OCWD’s original GWRS, completed in 2008, has an advanced water purification facility (AWPF) that purifies secondary effluent through microfiltration, reverse osmosis (RO), and advanced oxidation using ultraviolet (UV) light and hydrogen peroxide. Purified water from the AWPF is injected into groundwater aquifers to minimize seawater intrusion and replenish local groundwater supplies. The initial expansion of the GWRS provided an opportunity to enhance AWPF processes while expanding production capacity from 70 to 100 mgd. Enhancements focused on energy-recovery devices on the new RO units, secondary effluent flow equalization, and alternative treated water quality targets, as well as new technologies to enhance the reliability and flexibility of posttreatment operations. Evaluation and enhancement of post-treatment stabilization processes at the AWPF were especially important for system reliability and flexibility. A RECENT EXPANSION OF