Divided by the waters of Puget Sound, the city of Bremerton, Washington, sought a cost-effective alternative to piping combined sewer overflow (CSO) to the existing wastewater treatment plant across town. Dry weather wastewater flows in East Bremerton are 1 million gallons per day (mgd), but may be as much as 30 mgd during wet weather events. The solution: a new technology—combining high-rate clarification (HRC) with ultraviolet (UV) disinfection—implemented on a fast-track construction schedule, saved the city more than $30 million over constructing storage and 7 miles of relief sewers.
HRC—a physical-chemical process (ballasted flocculation)—involves the fixing of sludge floc to ballast, and separates solids by centrifugal action. Disinfection by UV avoids handling toxic chemicals in the neighborhood. The system is stable, operating over a wide range of conditions and flow variations at optimal efficiency without equalization. HRC removal efficiencies far exceed conventional clarification systems—solids removal is achieved at more than 10 times the normal clarifier loading rate.
Designed to treat 20-mgd peak flows, Bremerton's plant is the nation's first HRC/UV CSO treatment facility, and first in the Puget Sound region to surpass CSO treatment standards and meet the state's disinfection standards. The permitted facility treats East Bremerton's CSOs, lowering the city's total maximum daily load to Puget Sound and eliminating unpermitted discharges.
City operators activate the CSO facility using real-time collection system flows and a supervisory control and data acquisition system at the onset of wet weather.
The HRC process::
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Produces high-quality effluent
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Achieves consistent effluent turbidity of less than 5 nephelometric
turbidity units
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Removes 85-95 percent of total suspended solids, 65 percent biological oxygen demand, and 85-90 percent total phosphorus
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Requires only a 15-minute start up
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Processes at a surface overflow rate of 55 gallons per minute
per square foot
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Fits in a site footprint 1/10th the size of conventional treatment
Providing a low-cost, space-efficient, and effective solution, Bremerton's application of state-of-the-art HRC and UV technology serves as an example for other communities with similar overflow problems. Completed in December 2001, the facility brought the city into compliance with regulatory measures, and aided state health department efforts to improve the water quality of Puget Sound.