Frequently asked questions (FAQs)
See also Consent Decree FAQs.
What is a CSO?
Relief points, called combined sewer overflows (CSOs), are built into sewer systems that carry sewage and stormwater together. These pipes can fill up and overflow on rainy days. CSOs release untreated sewage and stormwater into rivers, lakes and Puget Sound. They prevent sewer backups into homes and streets.
The water released by CSOs is 10 percent sewage and 90 percent stormwater. CSOs may be harmful to people and animals living in the water because they release chemicals and disease-causing germs. Since 1979, King County has reduced its overflows by 90 percent and succeeded in keeping more than 2.3 billion gallons of sewage and stormwater out of local waterways.
Learn more about Protecting Our Waters, King County’s program to control CSOs.
Why do we have CSOs?
The older parts of King County's wastewater system use a single set of pipes, called “combined sewers” to carry both sewage and rain that runs off streets and buildings. This is true in many cities around the country (external link). Newer cities in King County have separate systems for stormwater and wastewater.
Most of the time, this polluted water in the combined system goes to a wastewater treatment plant. But in heavy rains, the pipes can fill up and overflow into rivers, lakes or Puget Sound. CSOs are built into the system to release the polluted water and prevent sewer backups into homes and streets.
Learn more about why building combined sewers made sense in the past.
Where are King County CSOs?
King County CSOs are located in within the Seattle city limits. King County and the City of Seattle are each responsible for specific CSO relief points. CSOs can overflow into Puget Sound, the Duwamish Waterway, Elliott Bay, the Lake Washington Ship Canal and Lake Washington. This map shows King County and Seattle CSOs.
What is the relationship between King County's and the City of Seattle's management of CSOs?
King County and the City of Seattle are each responsible for specific CSO relief points within the city limits. The history of the sewer system led to this shared responsibility. In the 1950s, voters created Metro to clean up the region’s waterways. Metro built a regional sewer system and took over operation of some of the City of Seattle's system. In 1994, King County assumed authority of Metro. King County is now responsible for treating wastewater for 34 local jurisdictions and agencies, including the City of Seattle.
Each CSO provides a drainage relief point for specific neighborhoods. King County manages the CSOs that serve areas that are greater than 1000 acres. The City of Seattle manages the CSOs that serve smaller areas. King County manages 38 CSOs and Seattle manages about 90.
Learn more about King County and Seattle’s success controlling CSOs.