Removal of Selenium and Nitrate from Surface Waters Using a Subsurface Microbial Filter

Many surface waters throughout the United States exceed one or more common forms of regulated contaminants. This study describes a pilot‐scale facility that was designed to remove nitrate and selenium (selenate) from surface waters by microbial processes. In an oxygen-free environment, nitrate is reduced to nitrogen gas and selenate is reduced to an insoluble salt that precipitates onto the biofilm. Based on the results from bench-scale tests, a demonstration facility was designed and constructed adjacent to a creek in Irvine, CA. This paper describes the design, construction and lessons learned in the development of a gravel‐filled filter whose effluent will meet the current standards for nitrate and selenium in surface waters. The facility treated in approximately 194,000 gallons per day and was designed to run in a contaminant removal, recycling and back-flush mode. The benefits of this system are the consistent removal of nitrate and selenium throughout the year with a low O&M cost and the possibility of multiple uses of the land that could include both the belowhyphen;grade filter and a parking lot or recreational area at the surface. This project is an example of a local water district assuming the role of environmental steward whose projects provides multiple benefits to the community and wildlife within the watershed.