Partially separated sewer system

Partially separated sewers convey wastewater from inside homes and businesses, and stormwater from roof gutters and downspouts and send these waters to treatment plants. All other stormwater (from places like storm drains) is conveyed in separate drainage pipes.

40% of Seattle’s sewer system is partially separated (primarily throughout the southern parts of the city, and in several northern basins).

This infographic shows how a partially separated sewer system works under a home. Rainwater flows from the roof into gutters and storm drains, while wastewater from sinks and toilets flows through a separate sewer pipe. Most of the time, wastewater is sent to a treatment plant. During heavy rain, however, some stormwater and wastewater can mix and overflow through a pipe that releases untreated water into a nearby water body.