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Now posted: 2008 CSO Control Plan Update, June 2008

View real-time CSO notification.

Learn more about the Puget Sound Beach CSO Projects at North Beach, South Magnolia and West Seattle.

Contents

This Page: Introduction

What are CSOs?
Why do we have CSOs?
Where are King County CSOs?

Controlling CSOs
   How do we control CSOs?
   Transferring flow
   Increasing capacity
   Constructing local CSO treatment
     facilities
   Reducing stormwater entering the
     system

Reduction and Cost
   Requirements for CSO control
   How much have CSOs been reduced?
   What are CSO Control costs?

The CSO Program:
   CSO Projects
   Notification and Posting
   Sediment Management
   Water Quality Assessment
   Annual Report
   Program Updates
   Regional Wastewater Services Plan

Being a Good Neighbor

Program Library

Glossary of Terms

A King County
Earth Legacy Program

King County's Combined Sewer Overflow Control Program

Thank you for visiting the Web site for King County's Combined Sewer Overflow (CSO) Control Program Web site. Here, we share information on what a CSO is, why they occur, and where they are found in the King County system. And we explain what we're doing to control them.

King Street CSO in Seattle, WA
Undercurrents, 2003 -- Plaza view at the Denny Way Regulator Outfall, Myrtle Edwards Park, Seattle, Washington.
Artist: Laura Haddad

By reducing combined sewer overflows into local water bodies, King County is protecting public health and the environment.

What are CSOs?

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.

Why do we have CSOs?

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

Where are King County CSOs?

View a map of CSO locations.

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).

More on CSOs:
Controlling CSOs
Reduction and Cost
The CSO Program
Being a Good Neighbor
Library
Glossary of Terms

If you have questions about the CSO Control Program, contact Karen Huber.

Department of Natural Resources and Parks
Wastewater Treatment Division
201 S. Jackson St., Suite 512
Seattle, WA 98104-3855
Phone: 206-684-1246
Fax: 206-684-2057

Updated: Nov. 30, 2007

Duwamish Waterway Programs

Wastewater (NPDES) Permits

Regional Wastewater Services Plan

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