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Seattle’s drainage and wastewater collection system is a blend of combined, partially-separated and separated systems. See map of Sewer Service Area (pdf). About two-thirds of Seattle is served by a combined or partially-separated sewer system. Completely separated systems serve the other one-third. The city conveys its wastewater to King County facilities for treatment.
During heavy rains, the volume of stormwater and sewage may exceed the capacity of the system in areas served by the older combined sewer system. That system carries wastewater from homes and businesses, as well as stormwater from streets and parking lots. A few times each year, the volume builds quickly and overwhelms the system, leading to a combined sewer overflow (CSO).
CSOs not only spill sewage into our waterways, they also spill polluted stormwater runoff that flows off rooftops, streets and other hard surfaces.
View a video about the Seattle Sewer System and why we have CSOs