Combined sewer overflows

Working to protect water quality

With our oversight, local governments that own and operate combined sewer systems in their communities work to reduce overflows and protect water quality. Federal and state regulations require each system to reduce overflows to control water pollution.

Seven of the state’s 11 communities with combined sewer systems have controlled their overflows and the rest are making progress. Find more information about a specific area below.

How do we reduce overflows?

A Washington regulation (chapter 173-245 WAC) requires communities to reduce the frequency of untreated combined sewer overflows. They are required to reduce overflow incidents to no more than an average of one discharge per year, per outfall.

Overflows are a problem

Without controls over combined sewer overflows (CSOs), large quantities of raw sewage can be released into lakes, rivers, and Puget Sound. Contaminants in CSOs can include pathogens, oxygen-consuming pollutants, solids, nutrients, and toxics — all of which can harm people, fish, and wildlife.

Our role in preventing CSOs

We target high priority pollution sources to protect public health. We use permits — and legal orders — to implement state and federal requirements for communities to control their overflows. The permits and orders often include schedules for when communities must complete control projects. 

If communities violate the conditions in their permits or orders, they may face penalties or other enforcement.

Community plans to control CSOs

We review and approve planning and engineering documents to ensure proposed wastewater projects comply with water quality laws. Our oversight of CSO communities includes reviewing the plans they develop to control their CSOs. Those plans can propose a variety of approaches to achieve control.

To control CSOs, they may:

• Separate sewer and stormwater collection.
• Construct large tunnels and tanks to store and then treat combined flows after storm events.
• Provide facilities that operate during high flows.
  • These facilities typically provide primary treatment, disinfection, and screening to remove large solids.
  • They must meet technology-based and water quality-based pollution limits in permits.
• Implement a green stormwater infrastructure. 
  • They can create roadside rain gardens, permeable pavement, and downspout-to-ground disconnects that reduce or eliminate the flow of stormwater into the combined sewer.

Communities with combined sewer overflows

Largest CSOs in the state: King County and Seattle

King County and the City of Seattle own and operate the two largest combined sewer systems in the state. The systems operated by them contain nearly 70 percent of the permitted CSO outfalls in the state.

Both local governments control different portions of a complex, interconnected system. Since CSO controls put in by one agency can impact the other’s system, both agencies must work together to solve shared problems. Given the size and complexity of the projects both agencies must complete, we recognize that it will take both agencies more time to control all of their outfalls.

Agreement to control overflows by 2025, 2030

We joined with EPA to negotiate separate federal agreements (consent decrees) with King County and the City of Seattle in July 2013. In those agreements, both King County and Seattle are scheduled to have their overflows mostly controlled by 2025.

The agreements generally:

• Set schedules for completing CSO control projects.
• Set requirements for sanitary sewer overflows.
• Set requirements for compliance with the county’s and city’s National Pollutant Discharge Elimination System (NPDES) permits.
• Set stipulated penalties for violations of the consent decrees and for certain NPDES permit violations.

A link to each agreement (called a consent decree) can be found in the summary table above.

Seattle prioritizing stormwater projects

Seattle’s agreement allowed them to defer completing projects to control six of their CSO outfalls until 2030 in exchange for completing high-priority stormwater projects by 2025. The city showed that the stormwater projects would provide a greater environmental benefit to the region than the CSO projects. 

Where does the money go?

Stipulated penalties are set in the consent decrees for violating the agreements. Penalties are based on the type of violation. These stipulated penalties are paid to both us and EPA.

We receive half of each penalty and deposit the money into the state’s Coastal Protection Fund. This fund pays for local projects throughout the state that improve water quality or enhance fish and wildlife habitat.  

King County stipulated penalties

King County’s consent decree requires them to complete all of the control projects identified in their 2012 CSO Control Plan by 2030. For more information about a specific penalty, use the link to the demand letter in the table below.

City of Seattle stipulated penalties

Seattle developed a plan in 2015 and is required to control all of its outfalls by 2025 except for 6 outfalls that will be controlled by 2030. For more information about a specific penalty, use the link to the demand letter in the table below.