Protecting our Waters by Controlling Combined Sewer Overflows
Important pollution control program nearing the finish line
Protecting Our Waters is King County’s program to prevent pollution caused by excess stormwater in the sewer system on rainy days.
The older parts of King County's wastewater system use a single set of pipes to carry both sewage and rain running off streets and buildings. Most of the time, this polluted water goes to a wastewater treatment plant. But in heavy rains, the pipes can overflow into rivers, lakes, or Puget Sound. Overflow points called “combined sewer overflows” or CSOs are built into the system. CSOs 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 carry 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.
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