Reducing Overflows and Pollution Into the Duwamish River

Much like other areas, older parts of King County’s sewer system use the same set of pipes to carry both sewage and stormwater to a treatment plant. During storms, the pipes can fill with stormwater that runs off roofs, driveways and streets. When the system is overwhelmed, it is designed to overflow.

To keep the sewer system working and to prevent sewer backups, the excess water and sewage are released into our local water bodies through combined sewer overflows. However, CSOs pose a risk to public health and the environment. Over the past several years, King County has been planning a project to reduce CSOs from the South Park drainage basin into the Duwamish River. The requirement is to reduce CSOs to no more than one overflow per year, per outfall on average.

In 2025, the County will begin building a new 1.25-million-gallon underground storage facility to hold stormwater and wastewater (combined sewage) that currently overflows into the Duwamish River during large storms. The combined sewage in the storage facility will then be conveyed to the West Point Treatment Plant for treatment before it is released into Puget Sound.

The project team is now focused on finalizing the design and preparing for construction. HDR is providing engineering and related services, and MIG has been providing landscape design. The above-ground facility building will be approximately 3,500 square feet and is being designed in close collaboration with the surrounding community, including members of the Coast Salish tribes, whose ancestral lands are where the facility will be built.

Incorporating this feedback, the new building will include several sustainable design elements, such as solar panels on the roof, and will be built with climate-friendly concrete. Its surrounding landscape will include rain gardens, denser trees and habitat features that support wildlife.