Collection System Modifications
The existing collection system near Broadway and Black River Lane will be updated so that wet weather flow is diverted into the tunnel. These updates include construction of a screenings-overflow chamber on an existing 60-inch interceptor sewer that will control the flow to the screenings facility.
The screenings facility will have a combined capacity of 120 million gallons per day (mgd) and include two mechanically raked screens with 0.25-inch openings to filter out debris. Once wastewater is screened, it travels into the tunnel drop shaft before entering the tunnel for holding. The screenings facility, a low-profile structure that will blend into the surrounding waterfront development area, will also include a publicly accessible observation deck with scenic views of the lake and river.
Tunnel Drop Shaft
Screened wastewater will then travel through two pipes in a vertical vortex drop system that minimizes the release of hydrogen sulfide and dissipates energy during the 110-foot drop. This system has a 140 mgd capacity. The shaft will serve a dual purpose of being an access point for tunnel maintenance, personnel and equipment during construction and future maintenance.
The tunnel will extend about 5,600 feet from the drop shaft near the intersection of First Street and Broadway (across from City Hall) to the pump station near the Black River Wharf boat launch. A tunnel boring machine will make a 23-foot diameter tunnel in shale at depths ranging between 110 to 160 feet. After completion of the tunnel, cast-in-place concrete liner will be placed along the entire length of the tunnel, providing a 19-foot diameter finished tunnel having a storage capacity of 12 million gallons.
Once the Black River Wastewater Treatment Plant has available secondary treatment capacity, the stored flow will be pumped from the tunnel up to the collection system at the surface on the pump station site. A submersible pump station with a self-cleaning trench-type wet well will provide a tunnel dewatering capacity of 11.5 mgd. The pump station site includes a control building to house the electrical and control equipment for the pumps. During construction, the pump station shaft will serve as an access point for tunnel maintenance, personnel and equipment.