The South Sewer Interceptor Project is a 6-mile long sewer interceptor constructed in challenging groundwater conditions. The project allowed for abandonment of a historically non-compliant lagoon treatment system and opened up new land for development. The project is located in West Richland, Washington.

The Issues: The South Waste Water Treatment Plant (WWTP) for the City of West Richland had a history of failing to comply with the National Pollutant Discharge Elimination System (NPDES) discharge limitations. The City was faced with upgrading two treatment facilities (North and South WWTP) or the option of moving all flow to a single treatment facility.

How we addressed the issues: This project consisted of constructing a 6-mile long sanitary sewer interceptor for the City of West Richland. The completion of construction of the South Sewer Interceptor was coordinated with the timeline for decommissioning of the South WWTP, as required by the NPDES Permit. Portions of the interceptor are over 25-feet in depth and were installed in groundwater conditions requiring substantial dewatering. The PVC sanitary sewer pipe ranged in size from 8-inch to 24-inch diameter. The work included deep trench excavation, dewatering, removal of existing sanitary sewer pipe, demolition of an existing lift station, constructing sanitary sewer manholes and an irrigation canal under crossing. The work also involved over 70,000 cubic yards of roadway excavation and embankment construction for the future extension of Keene Road.

The Benefits: The new interceptor allowed for the abandonment of the City’s South Sewer Lagoon and opened up 2,500 acres to new development. The South Sewer Interceptor routes all flow away from the former South WWTP to the North WWTP. Not only did the project allow for the decommissioning of the South WWTP, but it also opened up an additional 2,500 acres for residential, commercial, and industrial development.

 Unique and Memorable:

  • Coordination of the decommissioning of the South WWTP, which included removal of 450 dry tons of biosolids, removing and salvaging of equipment, where possible, sampling, and coordination of the filling of the lagoon with excess cut material from a nearby roadway project.
  • Hydraulic modeling for optimization of the size of the sewer interceptor.
  • Projecting the effects of increased flows on the North Wastewater Treatment Plant (WWTP).
  • Environmental issues included eliminating two low-quality wetlands and the enhancement of nearby existing wetlands.