Combined sewer system

Combined sewer systems are the oldest type in Seattle, with infrastructure ranging from about 70 to 100 years in age. Combined sewers convey wastewater from the sinks, showers, toilets, washing machines and dishwashers of households and businesses, combined with stormwater that runs off of rooftops, lawns, streets, parking lots, and sidewalks.

Wastewater and stormwater travel together to treatment plants through the same sewer pipe. When too much water enters the pipes the system overflows into waterbodies, in an event called a combined sewer overflow (CSO).