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King County Washington home

About South Interceptor Rehabilitation Project

Get information about the South Interceptor Rehabilitation Project schedule, project need, and construction process.

Project need

The South Interceptor carries wastewater flows from Renton and South King County to the South Treatment Plant in Renton, where it is cleaned and released into Puget Sound.

The South Interceptor was installed in the early 1960s. It is a big concrete pipe, measuring 72-inches and 90-inches in width. Over time, normal sewage gas corroded the pipe walls. Corrosion from sewer gas is typical of old pipes. Repairing the pipe walls will keep the system working safely and reliably for years to come.

he image shows the inside of a large underground pipe with water at the bottom and deposits along the upper surface, captured from a sewer inspection camera.

Gases from wastewater flows cause decay inside the pipe (white portions of pipe in photo).

 Maintaining sewer service during repairs

You will be able to use your sinks, toilets, and showers normally.  Here are the steps we will take to protect sewer service in your area.

  • To keep sewer services working we will use a temporary diversion pipe while repairing the pipe
  • A temporary pump will lift sewage into the temporary pipe so the underground pipe can be repaired.
  • The pumps will create some noise
  • Odor control equipment will be used to reduce smells from the work area

The image shows a rainy roadway with lane‑shift warning signs, cones, and an arrow board directing traffic through a construction area.

An example of a temporary diversion pipe

Project schedule

In April 2026. King County hired a design-builder to help King County design and rehabilitate the South Interceptor. Currently, the project team is at early design stage.

The image shows a timeline from 2023 to 2029 outlining planning, contracting, design, construction, and closeout phases of a project.

  • Planning/Contracting

    During the Planning/Contracting phase we:

    • Approved the project charter
    • Set up King County project team with subject matter experts.
    • Began project planning
    • Reviewed documents about the existing pipe and project area.
    • Hired the contractor for design and construction

  • Design

    During the design phase, we will:

    • Develop the design and construction schedule
    • Determine the location of the temporary diversion pipe which will carry wastewater during pipe rehabilitation
    • The sewer rehabilitation approach
    • Locate Staging areas
    • Acquire permits and local agency coordination
    • Identify impacts on the surrounding community
    • Continue sharing information about the project with interested community members

  • Construction

    During the construction and close out phase, we will:

    • Prepare the site for construction
    • Install and operate the temporary diversion system
    • Rehabilitate the existing South Interceptor
    • Remove the temporary diversion systems once rehabilitation activities are complete.
    • Restore and clean up from construction
    • Update neighbors and the surrounding communities about project work and progress
    • Work with community members to minimize disruptions from the project until work is complete.