King County is Protecting Our Waters
Controlling combined sewer overflows
Protecting our Waters is King County’s program to prevent pollution caused by excess stormwater in the sewer system on rainy days.
Combined sewer overflows (CSOs) are relief points in sewer systems that carry sewage and stormwater in the same pipe. When heavy rains fill the pipes, CSOs release sewage and stormwater into rivers, lakes, or Puget Sound. They prevent sewage backups into homes and businesses. But, they can harm people and animals living in the water because they carry chemicals and germs.
CSOs in King County exist only in older Seattle neighborhoods, where one set of pipes carries both sewage and stormwater. CSOs release 10 percent sewage and 90 percent stormwater. Most of the time, this water goes to a wastewater treatment plant.
King County and Seattle are working to control CSOs. A controlled CSO overflows no more than one time each year on long-term average. This is a Washington State standard.
Since the 1960s King County has greatly reduced its CSOs’ volumes - from approximately 20 to 30 billion gallons per year to around 600 million gallons per year.