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CSO regulations and requirements
King County must control combined sewer overflows (CSOs) by 2030.
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 in homes, business, or the sewage system. We will always need CSOs in our combined sewer system to protect people and property, but they need to be controlled because they carry chemicals and germs into local water bodies.
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 plan that addresses changing conditions and that the region can afford.
Learn more about the consent decree
The consent decree, signed in 2013, is a written agreement between King County, the Washington state Department of Ecology (Ecology), the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Justice (DOJ) that outlines the planned actions to bring King County’s CSO program into compliance with the federal Clean Water Act (CWA).
In 2019, King County asked to begin negotiations to modify the agreement because conditions had changed since the consent decree was approved.
The 2013 consent decree requires King County to control all its CSOs by:
- Building the specific CSO projects outlined in the 2012 CSO Long Term Control Plan
- Completing the projects by the end of 2030
- Monitoring and making adjustments to ensure CSOs remain controlled.
“Control” is defined in the Washington State rule and consent decree as no more than one untreated discharge per outfall per year on a 20-year average.
Yes. A “controlled” CSO can overflow no more than one time each year, based on a long-term average. This limit is set by Ecology and King County’s own policies. This is as stringent or more stringent than the federal requirements.
EPA has agreed that the County’s CSO control efforts may move forward based on the Washington State standard.
The 2013 consent decree includes 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 requested that the Department of Justice, EPA, and Ecology begin discussions to modify the consent decree.
Here is what has changed since we signed the consent decree in 2013:
- New modeling of climate change and other conditions suggests that the list of projects contained in the consent decree may not effectively control CSOs in the future.
- The cost of controlling the remaining CSOs has increased substantially. Climate change is affecting the amount of water we need to pump and treat. Property in our growing region is more expensive and difficult to find.
- We have to plan for other significant expenses – in addition to CSO control.
- The Washington State Department of Ecology has increased King County’s requirements for removing nutrients from treated wastewater.
- Our aging wastewater system needs updates and maintenance to continue to serve the region. The urgency for these investments has increased.
- There are limits to how many large projects a region can do at one time. Building large infrastructure projects requires tons of materials like concrete and steel and many trained staff. When multiple projects (wastewater, bridges, transit, etc.) are competing for the same resources, it drives the costs up and affects schedules and our ability to do the projects.
King County wants to work with our regulators to create a path forward that allows King County to get the greatest CSO reduction at the earliest possible date. In 2019, we asked to discuss:
- Updates to the required CSO control projects and programs to make sure they effectively control the remaining CSOs.
- A new schedule for CSO control projects that:
- Fits a comprehensive approach to water quality investments and that the region can complete, given logistic realities
- Maintains rate affordability.
- Flexibility to use adaptive management approaches as population increases, climate changes, infrastructure ages, and regulatory requirements evolve. This would allow us to make investments in the most effective improvements in any given time period and to pursue more collaboration with the City of Seattle.