Contents Introduction:    What are CSOs?    Why are there 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 This Page: Reduction and Cost Requirements for CSO control (at left) 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
	This graph shows CSO reductions from the many control projects King County has completed.

The CSO Control Program -- Reduction and Cost

What are the requirements for CSO control?

CSO control is required by state law and federal CSO policy. Washington State has chosen to have a less flexible approach, requiring that CSO discharges be reduced to no more than one untreated discharge per year on average at each CSO location.

The federal policy allows two different approaches for CSO control: presumptive and demonstrative. Under the presumptive approach, four to six untreated discharges per year on average are allowed at each CSO location. Under the demonstrative approach, computer modeling or other tools are used to predict the impacts of CSOs on water bodies. Depending on the results, zero or more than six untreated discharges may be allowed at each CSO location.

How much have CSOs been reduced ?

Controlling CSOs has been a priority for King County and the City of Seattle since the 1960s, when 20 billion to 30 billion gallons of untreated CSOs were discharged into area water bodies. Although CSOs still happen during heavy rainfall, the volume of discharge is far less. Because of control efforts during the past 30 years, King County CSO volume has been reduced significantly from a 1980-83 baseline of 2.3 billion gallons to about 1.5 billion gallons per year on average.

King County is committed to protecting water quality and human health. Under its Regional Wastewater Services Plan, King County has developed CSO control projects to bring the remaining uncontrolled CSOs to the Washington state requirement of one untreated discharge per year. These projects will be completed by 2030.

What are CSO control costs?

When CSO control is complete, it will have cost about $600 million. CSO control is expensive for several reasons:

• Large storms generate large volumes of CSO quickly. This requires large facilities to treat and/or contain the large volumes.
• Large facilities require more land, additional contractor work, and materials.
• Most of the projects are in heavily urbanized areas, and construction must be carried out in ways that minimize disruption and fit into the character of the community.

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

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: April 26, 2007
 

Related Information:
Duwamish Waterway Programs