UV treatment reduces beach water bacteria
Encinitas, Calif., is home to one of the more popular destinations in San Diego County. Its Moonlight Beach attracted nearly 2.7 million visitors in 2000 and could have hosted more, except that for 93 days that year, the beach was closed or posted because of high bacteria counts.
Today, water quality has improved dramatically. The city has one of the most aggressive urban runoff programs in San Diego County. For years, the city implemented source identification, upstream best management practices and enforcement within the Cottonwood Creek watershed, which drains to Moonlight Beach. Although water testing conducted by the city at both the creek and ocean outlet showed a decrease in turbidity, bacteria and detergents, it wasn’t sufficient to reduce the beach postings, and the city was forced to find other ways to reduce the contaminants.
To reduce bacterial levels in the perennial, dry weather runoff from Cottonwood Creek, the city considered diverting it to the sanitary system or disinfecting it with either chlorination/dechlorination, ozone or ultraviolet processes. Eventually, the city chose to build an Urban Runoff Dry Weather Flow Treatment Facility.
“Several cities like Los Angeles are routing their stormwater into the municipal sewer system,” said Pete Silva, vice president of the Sacramento, Calif.-based California State Water Resources Control Board, at the plant’s opening in September 2002. “Encinitas’ decision to treat the contaminants is ultimately better for the local wetlands and less costly too.”
Although traditional disinfection methods (chlorine and sodium hypochlorite) often are used to treat urban stormwater, the Moonlight Beach facility uses ultraviolet (UV) light. Unlike the other alternatives considered, UV treatment does not permanently divert stream flows and does not require chemical storage.
The advantages of UV treatment were well known by the city’s engineering consultants, Miami, Florida-based PBS&J. For example, ultraviolet light leaves no residual in the treated water that can affect aquatic life or human interaction.
The engineering firm helped the city develop a preliminary design report, and later designed, bid and constructed the facility’s filtration, intake, pump station, and disinfection systems. The engineers also knew that it was important to build the plant on as small a footprint as possible and to not alter the Cottonwood Creek streambed.
The entire project was funded with a grant from the California State Water Resources Control Board’s Clean Beach Initiative. Total construction costs for the facility were $438,000, well within budgeted city and grant funds.
Throughout the first season of operation in fall 2002, the plant consistently removed 99.9 percent of the coliform bacteria. In subsequent years, the plant will operate an estimated eight months out of the year with treatable flowrates expected to average 150 gallons per minute.
Even though the plant has been operating less than a year, the Moonlight Beach Urban Runoff Treatment Facility has already accomplished the city’s goal of reducing the number of days the beach has closed. Through fall 2002, when bacteria is typically highest, the beach did not need to be closed once.
