Puget Sound Beach CSO Control Projects

King County begins planning new projects to protect Puget Sound

Click on map location for project details.

As part of its mission to protect public health and the environment, King County is responsible for conveying and treating wastewater collected by 34 local city and sewer agencies, including the City of Seattle.

Reducing combined sewer overflows (CSOs)

Like many cities around the country (external link), the older parts of King County's wastewater system carry both wastewater and stormwater to the treatment plant. When heavy rains fill the pipes, excess stormwater and sewage flow directly into local waterbodies. These discharges are called combined sewer overflows or CSOs. When this happens, about 90 percent of the overflow is stormwater and the rest is dilute sewage. CSOs help avoid sewer backups into homes and businesses and onto streets during storms.

CSOs are a public health and environmental concern because untreated wastewater and storm water may be discharged to Puget Sound during large storms. King County's goal is to reduce the number of CSOs each year, with a long-term goal of less than one untreated discharge per location per year to meet state regulations.

To protect Puget Sound and meet our goal, the county is identifying  CSO control options (PDF file) for North Beach, West Seattle, and South Magnolia. These locations are top priority because people are most likely to come in contact with water during recreational activities such as swimming.