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Most King County offices will be closed on Thursday and Friday, Nov. 22-23, 2018, for the Thanksgiving holiday.  
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Project update

September 4, 2018

King County’s Coal Creek Sewer Upgrade Project team is currently taking inventory of trees within the project area. The effort will help establish a tree protection plan and a replacement plan for any trees that need to be removed during construction.  Crews will be in the area on September 5-6 at a minimum; 1-2 extra days may be needed to complete this work.

tree tagsWorkers will use tags similar to ones pictured above.

View the project update for more details.

Coal Creek Natural Area

Watch for project updates as we make progress designing an upgrade to this important sewer pipeline. Once completed, this project and the new section of Coal Creek Trunk Sewer will provide service for decades to come.

View project library for past project updates.


Contact us

Contact Monica Van der Vieren community services lead at:


Project description

King County Wastewater Treatment Division (WTD) is planning to upgrade a regional sewer line that serves Bellevue and Newcastle. The existing pipeline was built between 1966 and 1991 in an area that looked different from today, with a smaller population. The pipeline is nearing its maximum capacity and needs to be upgraded to meet the growing needs of your community. When complete, the new sewer will provide safe and reliable service for another 50 years or more.

Protecting the Coal Creek Natural Area

The existing Coal Creek Trunk is approximately 2.5 miles long and primarily follows the banks of Coal Creek in the Coal Creek Natural Area between Newcastle and I-405. King County is evaluating options to move the pipe away from the Creek to protect this sensitive environmental area.

project location

Next steps

Additional information about the project area is needed before King County can finalize the route for the sewer upgrade. Over the next year, King County will study soils, groundwater, sensitive environmental areas and areas of cultural significance in the project area to confirm that the sewer can be built along the preferred route. We expect to finalize the route by late 2018.


  • Alternatives analysis: 2016 - 2017
  • Design: 2018 - 2020
  • Construction: 2021 – 2024

Where does your wastewater go?

The Coal Creek sewer line serves the City of Bellevue. The water you use in your home or business travels down the drain to the City’s sewer system. King County collects that wastewater from the City of Bellevue and routes it to South Treatment Plant in Renton.

STPaerial_600South Treatment Plant