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Combined sewer overflow regulations and requirements

King County must control combined sewer overflows (CSOs) by 2037.

What is a “controlled” CSO?

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

CSO outfalls in King County exist only in older Seattle neighborhoods, where one set of pipes carries both sewage and stormwater. CSOs are important safety relief valves that prevent sewer backups and flooding in homes, businesses, or the sewage system during heavy storms. While we rely on CSOs, they should be controlled so we can minimize polluted water from entering our local waterways for the benefit of public health and the environment.

State and federal requirements

King County’s CSOs are permitted 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 Ecology.

King County meets the Nine Minimum Controls that the EPA codified as part of their CSO Control Policy.

Federal “consent decree” for Protecting Our Waters

In 2013, King County signed an agreement with EPA and Ecology. This agreement, called a “consent decree” included a list of specific CSO projects and dates for completion.

In 2016, King County and Seattle Public Utilities coordinated on non-material modifications to their respective consent decrees to define a joint project that would control two King County outfalls and five City of Seattle outfalls. This collaboration resulted in the 2016 non-material modification. Learn more about the project: Ship Canal Water Quality project

In 2019, King County asked to begin negotiations to modify the agreement because conditions had changed since the consent decree was approved.

In 2024, King County negotiated an amended agreement to the 2013 consent decree with EPA and Ecology. The 2024 update adjusts the deadline for the remaining CSO control projects from 2030 to 2037 giving King County more time to build larger, more complex and climate-resilient projects and to better manage the additional cost burden on local sewer ratepayers. This is a fiscally responsible way to protect water quality in the Puget Sound region for decades to come as storms grow more severe.

With equity top of mind, King County also re-sequenced the remaining CSO projects to prioritize finishing work in the Duwamish.