Frequently asked questions (FAQs)

CSOs are discharges of untreated sewage and stormwater released directly into marine waters, lakes and rivers during heavy rainfall, when the sewers have reached their capacity.

Although the sewage in CSOs is greatly diluted by stormwater, both CSOs and stormwater may be harmful to public health and aquatic life because they carry chemicals and disease-causing pathogens.

View animation: Controlling Combined Sewer Overflows

From the late 1800s through the 1940s, engineers designed combined sewers (sewers that carry sewage and stormwater runoff in a single pipe) to convey sewage, horse manure, street and rooftop runoff, and garbage from city streets to the nearest receiving body of water.

 
Older combined system (pre-1950s)

Around the 1950s, most sewer systems were built as separated systems (sewage in one pipe; stormwater in another pipe). In the late 1950s, treating wastewater became the standard. Interceptor pipes were built to transport all wastewater (from either combined or separated systems) to treatment plants.

 
Typical modern combined sewer system
(post-1950s)

 
Typical separated sewer system

Combined sewers exist in many parts of older cities around the country (external link), including Seattle. During heavy or long storms, the volume of the stormwater runoff may become too much for the combined sewers to handle. To protect treatment plants and avoid sewer backups into homes, businesses and streets, combined sewers sometimes overflow into Puget Sound, the Duwamish Waterway, Elliott Bay, the Lake Washington Ship Canal and Lake Washington.

Many of these CSOs have been reduced. Both King County and the City of Seattle manage CSOs within Seattle. King County's Wastewater Treatment Division manages 38 locations and Seattle Public Utilities manages about 90 locations (see map).

King County and the City of Seattle share management of the CSOs based on the size of the drainage basin served by each CSO outfall. The county manages CSOs from basins greater than 1,000 acres.

The City of Seattle's combined areas make up about 1/3 of the city's total service area, another 1/3 is partially separated, and 1/3 is separated. Typically, a partially separated area differs from a combined area, in that, the drainage from streets is separated and sent to a storm sewer. Because of the varying amount of separation in the city's system, King County operates CSO control facilities throughout the city's service area. The City of Seattle makes up about 21% of King County's total service area.