UV Disinfection Facility Treats Stormwater Runoff

Moonlight Beach in Encinitas, CA, has had a history of beach contamination warnings due to unsafe bacteria levels in its waters.

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By David L. McCarty

Moonlight Beach in Encinitas, CA, has had a history of beach contamination warnings due to unsafe bacteria levels in its waters. Like many other California beaches, the beach has fallen victim to pollution from urban runoff. Last year however, the City of Encinitas took decisive steps to halt the pollution with the installation of an urban runoff treatment system which incorporates Ultra Violet (UV) technology.

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Water flows through two Aquionics UV units arranged in series for treatment. Each unit contains six low pressure 160W UV lamps.
Click here to enlarge image

In 2000, an estimated 2.7 million visitors enjoyed Moonlight Beach. The tourists who visit the area contribute more than $45 million a year to the local economy. The decrease in water quality over the past two decades has threatened not only the health of the environment but that of local businesses as well.

Over the past several years, Moonlight Beach has struggled with unhealthy levels of indicator bacteria and other pollutants. An environmental group, Heal the Bay, consistently assigned Moonlight Beach a failing grade of "F" on its water quality. During 2000, there were 93 beach postings advising of bacteria levels exceeding state requirements. In May of 2001, Heal the Bay ranked the area as the dirtiest of 40 beaches in North County California.

The source of pollution was water from Cottonwood Creek, which drains a four-square-mile urbanized watershed. Approximately 20% of runoff from the City of Encinitas is channeled to the creek. The pollution is exacerbated by the fact that much of the creek runs through underground pipes where moist conditions provide an ideal breading ground for bacteria.

In the late 1990s, the City of Encinitas adopted several measures to improve the water quality of Cottonwood Creek. Sources of contamination were identified and area businesses and residents fined for runoff violations and improper storm drain usage. Education and enforcement helped curb pollution. In addition, street sweeping and routine storm drain facility cleanings were implemented. Over the course of two years, these and other measures positively impacted the water quality, reducing the amount of detergents, turbidity, and bacteria found in the creek and its ocean outlet. However, bacteria levels still continued to be high enough to warrant advisory postings at the beach. To further reduce the number of postings, the city concluded that it would need to take the next step of actively treating the runoff of dry weather flows.

Treatment Facility Options

Enlisting the help of PBS&J, an environmental engineering firm, the city examined several options for an urban runoff treatment facility at Moonlight Beach. Disinfection systems considered were chlorine, ozone, and UV.

A set of criteria was developed against which the potential solutions were judged. The system chosen would need to treat an anticipated 150 gpm of water during the dry-weather season (approximately eight months of the year). In addition, 85% of the water would have to be withdrawn, treated, and returned to the creek while the remaining 15% bypassed the system. The untreated water would provide biological continuity, giving downstream organisms the nutrients needed to survive.

The system would have to fit within a footprint of 9 x 24 ft (the site of the existing Moonlight sewage pump station) and not emit noise or air pollution that would disturb nearby residents. In addition, the city wanted an option that left no detectable disinfection residue in the treated water which could be harmful to bathers or aquatic life. Cost of equipment, installation and maintenance were also key considerations.

The treatment goal was for the system to achieve a level of less than 20 colony forming units (cfus) enterococcus bacteria per 100 ml of water. Accounting for the 15% untreated bypass and potential contamination sources between the facility and the outlet, this level would ensure that bacterial levels in the receiving water met State Assembly Bill 411 requirements. The state requirement calls for a 30-day mean total coliform count of less than 1000 cfus of bacteria per 100 ml of water.

The system eventually adopted was a low pressure UV treatment system. With the addition of basket strainers and sand filters, the system would easily handle flows with increased turbidity and suspended solids. This compact set of equipment would also fit well in the required footprint. Purchase and installation costs were comparatively low. In addition, operation and maintenance costs would be low as UV systems consume relatively little power and typically only require the replacement of UV lamps once a year. Finally, UV disinfection is safe, quiet, and leaves no chemicals or other by-products in the treated water.

System Design

PBS&J designed the diversion intake, pump station, filtration and UV disinfection system for the city. Falcon Construction of Vista, CA, and Clear Creek Systems of Bakersfield, CA, were selected to install the treatment equipment. Construction management services were provided by Brady and Associates of San Diego, CA. For the UV portion of the system, Proline equipment from Aquionics was selected.

At the facility, the treatment begins as creek water is pumped through the basket and multimedia filters to remove large debris. The water then flows through two UV units arranged in series. Each unit contains six low pressure 160W UV lamps. The series arrangement was used in order to deliver the correct UV dose needed to meet the city's discharge requirements given the flow rate and quality of the incoming creek water.

The UV control equipment includes direct UV monitoring of actual dose, a flow meter and variable power control. Self-cleaning quartz sleeves keep the lamps free of organic deposits, resulting in more consistent system performance with very little maintenance. After treatment, the water is returned to Cottonwood Creek.

The simple design of the facility led to a short construction period which began in June 2002 and was completed in late August of the same year. The total construction cost for the facility was $438,000 and was funded through a Clean Beach Initiative grant administered by the State Water Resources Control Board. The facility began operating in September 2002.

Early Results

Due to the onset of the wet weather season in late November, the facility was closed for the season after just three months of operation. While operating experience was limited, the preliminary results are encouraging. A significant reduction in bacteria counts was seen from the influent to the effluent during the months of operation. Sampling performed by the city found that bacteria levels were approximately 2 total coliform bacteria per 100 ml of water, keeping receiving water bacteria levels well within the state requirements.

In addition, the UV monitoring equipment helped the city detect an upstream watershed violation because of a spike in the influent turbidity. City officials were able to locate and correct the problem.

About the Author David L. McCarty is president of Aquionics Inc. and a specialist in UV light technologies for wastewater, water, surface and air purification processes. He holds B.S. degrees in microbiology and chemistry from Ohio State University. Prior to joining Aquionics, McCarty was Chief Microbiologist for the City of Columbus, OH, water & wastewater division. For more information, he can be reached at daveM@aquionics.com. Special Thanks to Kathy Weldon, City of Encinitas Stormwater Program Manager, and James Rasmus of PBS&J for their assistance with this article.

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