Combined Sewer Overflows

Separated System


Combined System

Combined sewer systems are wastewater collection systems designed to carry sanitary sewage (consisting of domestic, commercial, and industrial wastewater) and stormwater in a single piping system to a treatment facility. In periods of rainfall or snowmelt, total wastewater flows can exceed the capacity of the sewer collection systems and/or treatment facilities. When this occurs, the combined sewer system is designed to overflow directly to nearby streams, lakes, and harbors, discharging untreated sewage and stormwater. These overflows are called combined sewer overflows (CSOs) and can cause significant water quality problems.

CSO control is a vital part of the statewide effort to reduce and control stormwater discharges. CSO reduction programs are in place in 11 cities in Washington. Ecology estimated that, in 1988, the average volume of untreated CSOs discharged to the state waters was 3.3 billion gallons of untreated discharges per year. Since then, Washington has made progress with a reduction of CSOs to less than one billion gallons in 2009.

Contaminants in CSOs can include pathogens, oxygen consuming pollutants, solids, nutrients, toxics, and floatable matter – all of which can harm the health of people, fish and wildlife. CSOs can contribute to shellfish harvesting restrictions, impairment of the aquatic habitat, and aesthetic degradation due to unsightly floating materials associated with raw sewage.

Eleven of our cities have combined sewage and storm collection systems. A number of these communities have been successful in controlling and reducing their CSOs completely and the remaining communities continue to make significant progress in CSO control. Without controls over combined sewer overflows, we risk sending large quantities of raw sewage into Puget Sound and our freshwater lakes, streams and rivers. Our strategy directing these cities to treat and reduce their overflows follows is similar to EPA’s national policy. Our state’s CSO strategy targets high priority pollution sources and protects public health. It is a key to restoring Puget Sound. Strategies for controlling CSOs include separation, storage or treatment of flows. More recently, green stormwater infrastructure (GSI) has been used alone or in concert with other control strategies as a cost effective approach for some CSO reduction projects. We are open to using many different tools, including a variety of stormwater control strategies, to ensure the CSOs do not contribute to violations of the water quality standards.

Additional Information

Focus on Combined Sewer Overflows

An Overview: Combined Sewer Overflows in King County (Feb. 2011)

Ecology Statute Chapter 90.48.480 RCW and Regulation Chapter 173-245 WAC

EPA CSO Information

CSO control summary (Feb. 2011)

CSO Communities in Washington

NPDES Permittees with CSO Outfalls CSO Outfalls
Anacortes 3
Bellingham 2
Bremerton 15
Everett 13
King County- West Point Treatment Service Area 34
LOTT (Olympia) 1
Mount Vernon 2
Port Angeles 4
Seattle Public Utilities – City of Seattle 92
Snohomish 2
Spokane 22


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Last updated November 2015