STORMWATER FLOW SEPARATION

In a combined sewer system, stormwater and wastewater travel through the same system of pipes to treatment plants. During large storms, the combined system can become overwhelmed and overflow, impacting water quality. Stormwater flow separation removes the stormwater from the combined system by installing a new storm drain system to capture and carry stormwater directly to bodies of water like lakes and the Puget Sound. Additionally, separated stormwater runoff can be treated before it reaches water bodies to protect natural ecosystems.

An infographic visual 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.