Seattle completes $255 million sewage retention tunnel

Seattle Public Utilities has completed the Ship Canal Water Quality Project, a retention tunnel designed to manage excess sewage and stormwater during heavy rainfall. The structure is approximately 4.3 km long with an internal diameter of 5.7 m, capable of holding over 112 million liters of mixture.

The system is expected to reduce annual discharges into Lake Washington Ship Canal, Salmon Bay, and Lake Union by an average of more than 284 million liters. The infrastructure includes a main tunnel constructed using a Tunnel Boring Machine (TBM), smaller connecting tunnels, shafts, and a pumping station. The project, which had a contract value of $255 million, requires the completion of additional pumping and connecting infrastructure before full operation, which is anticipated in 2027.