Combined sewer overflows

Working to protect water quality

With our oversight, local governments that own and operate combined sewer systems work to reduce overflows and control water pollution. Federal and state regulations (chapter 173-245 WAC) require each system to reduce overflows to no more than an average of one discharge per year, per outfall. Eight of the state’s 11 communities with combined sewer systems have controlled their overflows and the rest are making progress. 

Without control 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 use permits — and legal orders — to implement the requirments 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.

Also, we review and approve the plans and engineering documents that communities develop to ensure their proposed wastewater projects meet water quality laws. Those plans can propose a variety of approaches to achieve control.

To control CSOs, communities 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.
• Use green stormwater infrastructure, such as create roadside rain gardens, permeable pavement, and downspout-to-ground disconnect programs that reduce or eliminate the flow of stormwater into the combined sewer.

Communities with combined sewer overflows

Wastewater discharge permits for the following 11 communities conditionally authorize occasional discharges of untreated combined sewage. Please use the links in the “PARIS summary page” column to access the current permit and fact sheet from our permitting database along with available annual reports.

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% 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.

Original Agreement

In 2013, we joined with EPA to negotiate separate federal agreements (consent decrees) with King County and the City of Seattle. In those agreements, both King County and Seattle were scheduled to have their overflows mostly controlled by 2030.

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.