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Combined Sewer Overflow Reduction Program

Water defines Seattle.

It offers many benefits: recreational, commercial, and environmental.

But polluted stormwater threatens the quality of our creeks, lakes, rivers, and Elliott Bay.

Seattle Public Utilities is working to restore our waters – to protect property, human health, and the environment.

What is a Combined Sewer Overflow (CSO)?

In many parts of Seattle, both wastewater (sewage that goes down the drain) and stormwater (rain that washes off of rooftops, streets, and parking lots) flow together through the Combined Sewer System. But during heavy rains, the combination of stormwater (about 90% of the volume) and sewage may exceed the capacity of the combined sewer. When this happens, sewage and stormwater overflow into nearby lakes, streams or Puget Sound, creating Combined Sewer Overflows (CSOs). Seattle manages 90 CSO locations located along shorelines throughout the city.

What is Seattle doing to control CSOs?

The Clean Water Act requires that Seattle reduce CSOs. Seattle Public Utilities is developing a comprehensive Long-term Control Plan to meet that goal. Other actions include improving operations and maintenance, retrofitting the system for greater efficiency, and building storage facilities and green stormwater infrastructure where appropriate. Since 1972, CSOs have been reduced from 30 billion gallons to less than one billion gallons per year. But we must finish the job.

How can you help?

Improving water quality in our creeks, lakes, rivers, and Puget Sound begins at your home or business. Seattle Public Utilities offers many ways you can reduce polluted runoff, keep the system operating efficiently, and improve the health of our waterways.

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