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).

The infographic visual on this page shows a depiction of a combined sewer system below a residential home. As rain falls from the sky and into the roof gutter and drain, the runoff travels into a storm drain above ground, reaching a drainage pipe below ground. Below the home, wastewater from toilets and sinks travels through the property owner’s side sewer pipe to a combined sewage and stormwater pipe headed for a treatment plant. During heavy rain an overflow pipe takes an untreated mix of combined sewage and stormwater to an outfall pipe, creating a combined sewer overflow into a natural water body like the ocean.