COMBINED SEWER OVERFLOWS IN SEATTLE
The West Point Treatment plant in the Magnolia neighborhood of Seattle provides wastewater treatment for approximately 1.5 million citizens from Seattle and surrounding cities and counties. During times of heavy use and rainfall, the combined sewer system can exceed capacity. As a way of preventing flooding or wastewater back-up into buildings, 86 combined sewer outfalls are built into the system that discharge untreated wastewater into either Lake Washington, Lake Union or the Puget Sound. Analyzing observed rainfall data and recorded overflow data from a four-year study period (2012 through 2015) we discuss how rainfall patterns and combined sewer overflows vary in Seattle.
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Seattle averages 36 inches of rain per year, but variations occur year to year. Over our study period, observed rainfall in the City ranged from 28 inches up to 47 inches. The graph below shows observed rainfall per month over this four-year period.
Many factors affect if a particular outfall will overflow during a rainfall event. Basin size, pipe size, land use cover, amount of rainfall, length of storm and intensity of the storm all come into play. Typically, combined sewer overflows happen during the wet season (October through April).
During our study period:
1) 27 combined sewer outfalls did not discharge a single time.
2) On an annual basis the worst offender, outfall number 152, was responsible for between 24% to 36% of the total discharge volume.
Select a CSO outfall on the map to the right to see how many gallons it discharges on an annual basis.
During our study period:
1) 27 combined sewer outfalls did not discharge a single time.
2) On an annual basis the worst offender, outfall number 152, was responsible for between 24% to 36% of the total discharge volume.
Select a CSO outfall on the map to the right to see how many gallons it discharges on an annual basis.
Outfall 152: Seattle's Worst Offender
The image to the right shows the location of outfall 152, located in the Ballard neighborhood of Seattle. In 2015, over 36 million gallons of untreated wastewater discharged here. The outfall discharged for a cumulative of 714 hours (close to 30 days) over the course of 34 CSO events. The graph below shows the date and discharge volume for each of these events. |
What is being done about CSOs?
Seattle has entered into a consent decree with the Environmental Protection Agency, Department of Justice, and the Washington State Department of Ecology to reduce sanitary sewer overflows and combined sewer. The City has committed to spend approximately $500 million on capital improvements between 2010 and 2025 to reduce CSOs to protect Seattle waterways and a quality environment. When completed, Seattle will have achieved greater than 95% reduction in all of its CSOs.
Seattle has entered into a consent decree with the Environmental Protection Agency, Department of Justice, and the Washington State Department of Ecology to reduce sanitary sewer overflows and combined sewer. The City has committed to spend approximately $500 million on capital improvements between 2010 and 2025 to reduce CSOs to protect Seattle waterways and a quality environment. When completed, Seattle will have achieved greater than 95% reduction in all of its CSOs.
References/Notes:
1) Combined sewer overflow volume and precipitation data obtained from City of Seattle Public Utilities department.
1) Combined sewer overflow volume and precipitation data obtained from City of Seattle Public Utilities department.