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CSOs in King County

Relief points, called combined sewer overflows (CSOs), are built into sewer systems that carry sewage and stormwater together. These pipes can fill up and overflow on rainy days. CSOs release untreated sewage and stormwater into rivers, lakes and Puget Sound. They prevent sewer backups into homes and streets.

The water released by CSOs is 10 percent sewage and 90 percent stormwater. CSOs may be harmful to people and animals living in the water because they release chemicals and disease-causing germs. Since 1979, King County has reduced its overflows by 90 percent and succeeded in keeping more than 2.3 billion gallons of sewage and stormwater out of local waterways.

King County's program to control CSOs is called Protecting Our Waters.

In Seattle, older parts of King County's wastewater system use a single set of pipes, called “combined sewers” to carry both sewage and rain that runs off streets and buildings. This is true in many cities around the country . Newer cities in King County have separate systems for stormwater and wastewater.

Most of the time, this polluted water in the combined system goes to a wastewater treatment plant. But in heavy rains, the pipes can fill up and overflow into rivers, lakes or Puget Sound. CSOs are built into the system to release the polluted water and prevent sewer backups into homes and streets.

Learn about why building combined sewers made sense in the past.

King County CSOs are located in within the Seattle city limits. CSOs can overflow into Puget Sound, the Duwamish Waterway, Elliott Bay, the Lake Washington Ship Canal and Lake Washington.

King County and the City of Seattle are each responsible for specific CSO relief points. This map shows the CSOs located in Seattle.

King County and the City of Seattle are each responsible for specific CSO relief points within the city limits. The history of the sewer system led to this shared responsibility. In the 1950s, voters created Metro to clean up the region’s waterways. Metro built a regional sewer system and took over operation of some of the City of Seattle's system. In 1994, King County assumed authority of Metro. King County is now responsible for treating wastewater for 34 local jurisdictions and agencies, including the City of Seattle.

Each CSO provides a drainage relief point for specific neighborhoods. King County manages the CSOs that serve areas that are greater than 1000 acres. The City of Seattle manages the CSOs that serve smaller areas. King County manages 38 CSOs and Seattle manages about 90.

Learn more about King County and Seattle’s success controlling CSOs.

Consent Decree

A consent decree is a legal agreement that settles a complaint. In King County’s case, the complaint is that uncontrolled overflows from its combined sewer system (CSO) violate the federal Clean Water Act (CWA). The county has been implementing its long-term program to get the remaining CSO points into compliance with state and federal standards and does not admit liability for any of the alleged violations. Nonetheless, the County agreed to resolve the complaint to avoid the cost of further litigation.

The consent decree 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 resolve alleged violations of the law.

A consent decree provides King County certainty regarding regulatory requirements, provides adequate time to reach compliance with EPA and Ecology guidelines, and avoids litigation.

EPA’s Enforcement Unit has been evaluating utilities nationwide to compel them to bring their Combined Sewer Overflow (CSO) programs in line with federal requirements outlined in the 1994 CSO Control Policy. EPA believes that all utilities with large and complex CSO systems should be regulated under a consent decree, which provides longer compliance schedules than other mechanisms.

Ecology, which has the legal authority to enforce the CWA within Washington state, had been taking a different approach through existing regulatory permits to ensure compliance is achieved. King County began its CSO control program in the 1970s, and issued its first CSO control plan in 1988.

The County has been working with EPA and successfully demonstrated that its CSO control plan, amended most recently by the King County Council in 2012, meets federal requirements. Rather than risk long and costly litigation, King County has chosen to negotiate a settlement in which that Plan amendment is brought under a consent decree.

King County negotiated the terms of the current consent decree which was approved and accepted by EPA, DOJ, and Ecology. The signed document was then lodged in federal court, and a judge approved it on July 3, 2013.

Yes. The control standard King County will meet was developed by Washington State Department of Ecology before federal policies existed.

The later federal policy included two different approaches for CSO control: presumptive and demonstrative. Under the presumptive approach, targets such as four to six untreated discharges per year on average are allowed in the CSO system. Under the demonstrative approach, computer modeling or other tools are used to predict the impacts of CSOs on water bodies leading to the development of more waterbody specific control targets.

In recent years King County has worked with both federal Environmental Protection Agency (EPA) and Washington Department of Ecology to demonstrate that both approaches result in equivalent levels of control for the County CSO system. As a result, EPA has agreed that the County’s CSO control efforts may move forward based on the Washington State approach.

Yes. There was a 30-day public comment period after the consent decree was lodged in federal court.

Since King County has had a plan for CSO control for decades, this consent decree acknowledges and accepts the County’s current plan, making it an “implement only” decree. This decree also allows but does not require King County to prepare an integrated plan whereby the CSO long-term control plan can be integrated with other water quality improvement projects to achieve greater environmental benefit than if only the CSO projects were done.

The proposed decree is consistent with the CSO plan approved by King County Council in September 2012 as it requires King County complete the proposed CSO projects by the end of 2030. Control is defined in the State rule and Consent Decree as no more than one untreated discharge per outfall per year on a 20-year average.

Yes, there are up-front penalties called “civil penalties” as well as stipulated penalties that are prescribed for any future violations. The decree assesses King County a civil penalty of $400,000 that would be divided equally between EPA and Ecology.

Future violations may be subject to penalties depending on federal and state enforcement policies and discretion. Stipulated penalties may range between $2,000 per event to $10,000 annum, depending on the type of violation.

No. In fact, King County is one of many CSO agencies in the nation to have a consent decree. Nearly 800 cities across the country manage CSOs and need to be under some form of regulatory enforcement.

In the Pacific Northwest, Portland, Oregon, has completed a CSO control program. Portland has been under a state-stipulated order to control overflows to the Willamette River and other waterways and invested more than $1.4 billion on CSO facilities, mostly deep, large storage tunnels.

The City of Seattle is also negotiating a consent decree. Other smaller Washington State CSO agencies with uncontrolled CSOs are being enforced under state administrative orders.

The consent decree allows for, but doesn’t require, King County to prepare an “integrated plan” for CSO control projects. King County will be evaluating whether the integrated plan approach might be incorporated into its long-term CSO control plan.

The integrated plan will be based on new and existing scientific information and analysis that may allow King County to change sequencing and scheduling of projects to invest ratepayers’ dollars to maximize water quality benefits. The integrated plan requires EPA, Ecology and Court approval to be implemented.

The CSO beach projects (Barton, Murray, North Beach, and South Magnolia) were already included in the County’s approved long-term CSO control plan and the consent decree has not changed the schedule or scope of the work. The beach projects are needed to reduce the discharge of contaminants contained in stormwater and sewage where people typically recreate.

The Ballard Siphon replacement project, while it has the benefit of controlling the Ballard CSO, was a necessary refurbishment of a vital part of the King County system.

No. Costs for King County’s long-term CSO control plan, approved in 1999 and amended by the King County Council in September 2012, will not be greater than what the Council has approved. Any penalties associated with the consent decree would be additional.

While CSOs are more of a concern in their immediate discharge area, reducing any pollution that eventually may make its way to Puget Sound remains a priority. The decree requires the County to implement CSO controls which will reduce the volume of direct discharges of untreated CSOs to waters that reach Puget Sound.

Yes. Green stormwater infrastructure (GSI), also called natural drainage, has been evaluated for all remaining CSO control sites. This analysis showed GSI to be a possible option for four of the control projects.

More extensive analysis for GSI was identified in the 2012 long-term CSO control plan amendment for these four projects, which will ascertain if GSI can provide effective CSO reduction as part of these projects. GSI is the approach being implemented to control the Barton CSO.


Many natural drainage solutions can be used to address CSOs. King County is applying the lessons learned from other green infrastructure installations across the country and here in Seattle to assure they provide the safe, functional systems that manage and reduce stormwater inputs to the sewer system.

As individual projects are designed, more specific information will be available at project websites in the future.

Erika Peterson
Water Quality Project Manager

206-477-5525