ULTRAVIOLET DISINFECTION OF REVERSE OSMOSIS PRODUCT WATER FOR GROUNDWATER REPLENISHMENT IN CALIFORNIA

The Orange County Water District operates a reclamation facility in Fountain Valley, California. The Groundwater Replenishment (GWR) Project will reclaim secondary effluent now discharged to the Pacific Ocean. Post-secondary treatment will include microfiltration, reverse osmosis, and ultraviolet (UV) disinfection prior to injection or percolation into the groundwater aquifer.The GWR Project will produce high-quality treated effluent for direct replenishment of a major groundwater basin used to supply up to 75 percent of the water consumed by more than 2 million people. A portion of the reclaimed water will be injected near the Pacific coast to prevent saltwater intrusion, while the remainder will be pumped 14 miles off site to spreading basins for aquifer recharge, diverting up to 100 mgd from ocean discharge.Information is provided on the evaluation and design of UV disinfection equipment under California Department of Health Services (DHS) Title 22 requirements. The impacts of the discovery of a trace contaminant, N-nitrosodimethylamine (NDMA), in the secondary effluent on the design of the disinfection facilities will be discussed. Selection of the optimum UV lamp technology and configuration is discussed and the basic system layout presented.Important design issues included selection of the disinfection UV dose, the energy dose (EEO) for NDMA photolysis, and the UV transmittance value. Related issues such as redundancy requirements, numbers of lamp banks and parallel trains, emergency power provisions, lamp cleaning, and related support facilities is presented. Information will be presented on how the selected UV system fits into the overall GWR Project, including the microfiltration/reverse osmosis processes, effluent pumping, and product water distribution facilities.The paper discusses the unique design requirements for the use of UV light to destroy NDMA by photolysis, and how this can be accomplished during the disinfection process. Data is presented on pilot testing of NDMA destruction. Cost information and design criteria for the various alternatives are presented in the supporting data. A process schematic is also provided showing how UV disinfection will be installed to achieve disinfection to meet the new NWRI guidelines, and, at the same time, provide destruction of NDMA to meet DHS requirements.