UV Disinfection Reduces Bacteria in Stormwater

Dec. 1, 2002
Encinitas, CA, recently held a ribbon cutting ceremony for its new Moonlight State Beach Urban Runoff Dry Weather Flow Treatment Facility.
The Moonlight Beach facility in Encinitas uses UV disinfection to treat stormwater flows.
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Encinitas, CA, recently held a ribbon cutting ceremony for its new Moonlight State Beach Urban Runoff Dry Weather Flow Treatment Facility. The facility will significantly reduce bacterial levels in the perennial, dry weather runoff from Cottonwood Creek, which drains to Moonlight Beach.

"Several cities like Los Angeles are routing their stormwater into the municipal sewer system," said Pete Silva, vice president of the California State Water Resources Control Board. "Encinitas' decision to treat the contaminants is ultimately better for the local wetlands and less costly too. It's the kind of thing, frankly, I think all the beach communities are going to have do."

The City of Encinitas has one of the most aggressive urban runoff programs in San Diego County, CA. For years the city has investigated pollutant sources and methods for reducing beach closures at Moonlight State Beach, which saw nearly 2.7 million visitors in 2000 even though it was closed or posted for public health reasons (high bacteria counts) 93 days that year.

Two years of source identification, upstream best management practices, and enforcement within the Cottonwood Creek watershed have improved water quality. However, these efforts have not reduced the number of closures or postings at Moonlight Beach. Therefore, other means of reducing contaminants were sought for implementation in tandem with the city's growing source control programs.

PBS&J was retained by the City of Encinitas to develop a preliminary design report for the Moonlight Beach Urban Runoff Treatment Facility and then to follow through with design, bid phase, and construction phase services for the intake, pump station, filtration, and disinfection systems.

Although traditional disinfection methods (chlorine and sodium hypochlorite) have been used to treat urban storm water since the late 20th century, the Moonlight Beach facility uses ultraviolet (UV) light. Unlike other alternatives considered, the UV treatment process does not permanently divert stream flows and does not require chemical storage.

"UV Disinfection offered many other advantages," said James Rasmus, P.E., DEE, project manager for PBS&J. "Ultraviolet light leaves no residual in the treated water that can affect aquatic life or human interaction."

"It was also important that the plant be built on as small a footprint as possible and be completed with no alterations to the Cottonwood Creek streambed," said Rasmus. "Costs of construction and operation were also a consideration."

The entire project was funded with grant money derived from the California State Water Resources Control Board's "Clean Beach Initiative," with PBS&J assisting the city in preparing the grant application as a major project enabler.

"PBS&J's efforts during the design phase kept the project on schedule and within the budget limits of the grant funding. PBS&J provided us with great solutions to the unconventional issues encountered on this unique project," said Kathy Weldon, stormwater program manager with the City of Encinitas and the person who was ultimately responsible for bringing the project to fruition. "We are confidant that the new UV system will result in zero closures at Moonlight Beach."

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