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King County uses different tools to keep more sewage and stormwater out of the environment. Not every tool is right for every situation. Before King County builds a facility, a project team thinks about which tools will work best. To find the right way to serve an area, King County has to answer these questions:

  • What tools will be the most reliable?
  • How can the County’s existing pipes and pumps help solve the problem?
  • Can natural drainage solutions like rain gardens or cisterns help?
  • What will keep the facility size small?
  • If there is a need to buy property, is property already available?
  • How can costs be kept low?
  • How will the facility fit in with the neighborhood?

There are three major ways to prevent untreated sewage and stormwater from overflowing from combined sewer overflows (CSOs) during heavy storms:

Storage: Hold more polluted water to avoid overflows

Protecting Our Waters uses underground storage tanks and tunnels to store water during storms. When heavy rains fill the sewer pipes, regulator stations send the water into storage until the storm is over.

When there is room in the pipes, the water travels to a regional treatment plant. Operators can also sometimes make adjustments during storms and use existing pipes to hold more water. This is called “system optimization.” King County is building CSO storage projects in North Beach, Magnolia, West Seattle, and the Rainier Valley.

One example of this method is at the Henderson/MLK CSO Treatment Facility, which stores excess water in an underground tunnel 15 feet in diameter.

Treatment: Clean & disinfect polluted water

Wet weather treatment stations run only on rainy days. Stations provide primary treatment by removing solids from the water through screening, allowing the water to settle, and disinfecting the water.

Wet weather treatment stations protect the waters in the neighborhoods where they are located. They may include storage tanks to hold water during smaller storms. King County operates four wet weather treatment stations and is building one in Georgetown.

Erika Peterson
Water Quality Project Manager

206-477-5525