Skip to main content
King County logo

King County is required by state and federal law to control its CSOs at the earliest possible date.

What is a “controlled” CSO?

A “controlled” CSO can overflow no more than one time each year, based on a long-term average. This limit is set by the Washington State Department of Ecology and King County’s own policies.   

CSOs are important safety relief valves that prevent sewer backups and flooding. We will always need CSOs in our combined sewer system to protect people and property, but they release chemicals and germs into the environment.

State and federal requirements

King County’s CSOs are regulated through the West Point Treatment Plant’s National Pollutant Discharge Elimination System (NPDES) permit. The U.S. Environmental Protection Agency (EPA) has delegated management of NPDES permits in Washington State to the Washington State Department of Ecology (Ecology).

King County meets the Nine Minimum Controls that the U.S. Environmental Protection Agency (EPA) codified as part of their CSO Control Policy.

Federal “consent decree” for Protecting Our Waters

In 2013 King County signed an agreement with the Environmental Protection Agency (EPA) and the Washington State Department of Ecology (Ecology). This agreement, called a “consent decree” included a list of specific projects and specific dates for completion. We cannot make changes to this list without modifying the consent decree. In 2019, King County asked to begin negotiations to modify the agreement.

King County is committed to controlling CSOs and has a legal obligation to meet the requirements. We want to work with federal and state governments to update our agreement and make a realistic compliance plan that addresses changing conditions and that the region can afford.

State and Federal resources

Janice Johnson
CSO Control Program

206-477-5624