Under the demonstrative approach, computer modeling or other tools are used to predict the impacts of CSOs on waterbodies. Depending on the results, zero or more than six untreated discharges may be allowed at each CSO location. 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 waterbodies. Although CSOs still occur during heavy rainfall, the volume of discharge is far less. As a result of control efforts over the past 30 years, King County CSO volume has been reduced significantly from a 1980-83 baseline volume of 2.3 billion gallons to approximately 1.5 billion gallons per year on average.
King County is committed to protecting water quality and human health.
Under the 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. Large facilities require more land, additional contractor work, and materials. Most of the projects are located in heavily urbanized areas and construction must be carried out in ways that minimize disruption and fit into the character of the community. For questions about the CSO Control Program, contact Kathryn White .