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.