Why are King County and Seattle addressing combined sewer overflows (CSOs)?
CSOs are discharges of untreated sewage combined with stormwater that is 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. As part of King County’s National Pollutant Discharge Elimination System (NPDES) permit, State of Washington standards require controls to reduce CSOs to an average of no more than one per year per location based on a long term average. Both the County and the City of Seattle are working to reduce CSOs to that standard. More about King County’s CSO Control Program.
