Vine Basin CSO Control

What & Why

Sewer pipes in Seattle carry sewage (wastewater) away from homes and buildings for treatment at King County’s treatment plants before discharging into Puget Sound. In some neighborhoods, like Belltown (Vine Basin), the same sewer pipes also carry untreated rain water (stormwater) from roofs, drains and streets. During heavy rains, if the amount of sewage and stormwater exceeds the sewer system capacity, the excess flows overflow into nearby water bodies through an outfall pipe. These overflows can harm fish, wildlife and swimmers. This is called a combined sewer overflow (CSO).

The Washington State Department of Ecology (Ecology) requires SPU to limit the number of combined sewer overflows at each of its outfall pipes to no more than one per year on average. Outfall 69, located at the base of Vine Street, does not currently meet this requirement.

The Vine Basin CSO Control project will make improvements to the combined wastewater and stormwater system in the Vine Basin so the frequency of overflows at the outfall meets Ecology’s requirements.

What’s happening now?

The project is currently in the early planning phase. Between now and summer 2019, the project team will be analyzing things like partnership opportunities with King County, options for managing combined sewer flows and location of other underground utilities. This analysis will help SPU decide what type of improvements best meet our triple bottom line analysis that evaluates social, environmental and financial goals and feasibility. Improvements may include upsized pipes, underground storage, and/or green infrastructure like natural drainage systems.

Project goals

To improve water quality in the region and meet the commitments made to Ecology and detailed in the Plan to Protect Seattle’s Waterways.