Wastewater Collection System & Treatment Facility Upgrades Design

Confederated Tribes of the Chehalis Reservation, OR


Century West was responsible for designing and providing construction management of sewer system improvements for the Chehalis Tribes Reservation. The project involved creating a new septic tank effluent (STEP) collection system with individual simplex pump stations to serve 21 existing residential homes and five (5) non-residential tribal government complex buildings. Approximately 5,300 lf of 3-inch to 4-inch pipe and 3,000 lf of 1 1/4-inch to 2-inch pipe were required to complete the new PVC piping system. In addition, two 100 gpm package duplex pump stations were included in the system to accommodate other uses.

The existing MBR treatment plant, which served the community, including the Lucky Eagle Casino, was evaluated. The current membrane system was deemed capable of treating flows for the next 20 years without requiring expansion. Therefore, the wastewater treatment facility improvements involved the installation of a duplex pump station, complete replacement of all gravity and force main piping outside the building, electrical control upgrades, replacement of the blower system for overflow ponds, new valve vault, oil/water separator, installation of new influent and effluent composite samplers, turbidimeter, and other miscellaneous improvements to the treatment plant.

During construction, the Tribe requested an additional $288,000 in improvements due to available funding, which increased the contract to $2,541,000. These improvements included rehabilitating the interior of Pump Station No. 2, coating both Anoxide Tanks and one Sludge Storage Tank, and replacing the existing flash mixers with larger units.

When completed in June 2022, the project’s final cost was $2,500,000. This was $41,000 below the approved contract due to various cost-saving measures that reduced contractor scope where possible when originally planned improvements were no longer necessary.

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