What are the requirements for CSO control?

CSO control is required by state law and federal CSO policy. Washington State developed control targets ahead of the federal policy, requiring that CSO discharges be reduced to no more than one untreated discharge per year on average at each CSO location.

The later federal policy provided for two different approaches for CSO control: presumptive and demonstrative. Under the presumptive approach, targets such as four to six untreated discharges per year on average are allowed in the CSO system. Under the demonstrative approach, computer modeling or other tools are used to predict the impacts of CSOs on water bodies leading to the development of more waterbody specific control targets.

In recent years King County has worked with both federal Environmental Protection Agency (EPA) and Washington Department of Ecology to demonstrate that both approaches result in equivalent levels of control for the County CSO system. As a result, EPA has agreed that the County’s CSO control efforts may move forward based on the Washington State approach.