UV/AOP A Key Part of the Groundwater Replenishment System

In 1933 the Orange County Water District (OCWD) was formed in Fountain Valley, California to maintain the quality and quantity of Orange County's groundwater basin. Orange County's groundwater basin (the Basin) currently provides potable water for over 2.3 million residents of North and Central Orange County. The Basin's primary water supply is obtained from the Santa Ana River, which is recharged into the Basin. Supplemental sources are imported from the Colorado River and State Water Project via the Metropolitan Water District (MWD). To address the issue of seawater intrusion, in 1976 OCWD began recycling Orange County Sanitation District (OCSD) wastewater and injecting it along the Talbert Gap via a series of injection wells to form a hydraulic barrier. The treatment facility was known as Water Factory 21 (WF–21) and was the first municipal scale wastewater reclamation plant to use membrane technology (in the form of reverse osmosis).The Groundwater Replenishment System (GWRS) was developed and is an indirect potable reuse project that produces 70 mgd with an ultimate capacity of 130 mgd for groundwater recharge and groundwater basin protection from seawater intrusion. The heart of the GWRS is the 70 mgd Advanced Water Purification Facility (AWPF) treatment plant. The GWRS is the largest indirect potable reuse project in the United States and one of the largest in the world. The GWRS has been in service since January 2008.The AWPF treats secondary effluent, currently discharged by OCSD into the ocean, using microfiltration (MF), reverse osmosis (RO), and advanced oxidation (ultraviolet light treatment with hydrogen peroxide). As an indirect potable reuse project the California Department of Public Health (CDPH) requires stringent product water quality, especially in regards to total nitrogen and total organic carbon. CDPH requirements call for total nitrogen of less than 5 mg/L and total organic carbon of less than 0.5 mg/L. These stringent standards drive the treatment facility toward the use of advanced membrane technology. The AWPF product water is equally distributed to the seawater intrusion barrier and recharge basins.The advanced oxidation process (AOP) is a key component of the treatment process due to the presence of N-nitrosodimethylamine or NDMA in the feed water to the AWPF. NDMA is a regulated contaminant in the State of California and the AWPF must meet an effluent limit of 10 parts per trillion. The AOP system is designed have a 1.2 log removal of NDMA and a 0.5 log removal of 1,4 dioxane. In addition, the AOP serves as a additional treatment barrier against any other low molecular weight organics that pass through the RO process. The OCWD AOP system is the largest of its kind in the world and as such there are many unique operational issues that have been encountered. This paper will attempt to detail OCWD's experience with their AOP system to serves as a guide to other agencies contemplating the use of this technology.