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Lake Oswego Interceptor Sewer Project

LOIS Project

The Lake Oswego Interceptor Sewer Project is complete, right on time and under budget. The project was a large undertaking.  Thank you to everyone in Lake Oswego for supporting this project.  An overview of the project is below.  If you would like to find out more information about the project or browse past news articles please visit the LOIS website found at the bottom of the page.

LOIS Overview: What, Why, Cost and Funding

The Lake Oswego Interceptor Sewer (LOIS) is a project that replaced the 20,000-foot interceptor pipe that forms the backbone of the City’s wastewater collection system and is in Oswego Lake. Wastewater from roughly 75% of Lake Oswego households and businesses flows through the interceptor pipe to Portland’s Tryon Creek Wastewater Treatment Facility. (Note - An interceptor sewer line or pipe is a large sewer line that controls the flow of sewage to the treatment plant. It collects the sewage from main and trunk sewer pipes and carries it to the treatment plant.)

Replacement of the existing interceptor was necessary for two main reasons:

  1. It was too small.
    • The City’s sewer system serves an area larger than originally envisioned when the first interceptor was built in the 1960’s.
    • The aging collection system also allowed too much rainwater into the sewer during sustained, heavy rains, which caused the system to back up and flow out of manholes at various locations. These spills were considered illegal discharges to the water of the state and also violated Federal law. A Mutual Agreement and Order (MAO) was executed between the City of Lake Oswego and the Oregon Department of Environmental Quality (ODEQ) in early 2007 to address overflows and plan for a long term solution to ending them. The MAO required the City to meet schedules and conditions in preparation for replacement of the interceptor sewer line.
  2. It was structurally at risk.
    • The system’s timber and steel pile supports and hardware were corroding and were at risk of collapse. Analysis showed the potential for numerous failures in the interceptor system during an earthquake. A failure would allow millions of gallons of untreated wastewater to flow into the lake and incoming lake water would overwhelm the wastewater treatment plant.

The cost for the project was approximately $95 million—below the estimated $124 million. City Council voted in April 2009 to finance the LOIS project through revenue bonds. Revenue bonds are repaid through utility bills.