Northwest Outfall Sewer Siphon


Located near the 18th tee of the Silverbell Golf Course, the Northwest Outfall Sewer Siphon was not operating well and started giving off a foul odor in the area. Originally built in the 1960s, the aging siphon system is a critical part of Pima County’s sewer infrastructure. The system comprises an inlet structure south of the Santa Cruz River, two large-diameter siphon pipes (30- and 39-inch pipes) that convey wastewater beneath the river, and an outlet structure north of the river. Nearly 20 million gallons of wastewater flow through this system each day. After decades of use, the system was experiencing excessive turbulence, corrosion, strong odors, and solids deposition.

Dibble designed new siphon inlet and outlet structures. Our design improved hydraulic performance, increased velocity in the siphon pipes, used corrosion-resistant materials, and improved access for future maintenance.

The 18th tee of the Silverbell Golf Course, near the Northwest Outfall Sewer Siphon inlet (under construction)

Our team communicated effectively with Pima County throughout the design process. In early meetings, Dibble brought 3D-printed models of potential designs for the inlet structure, allowing the County to identify the model that best met their needs. The team then used the preferred 3D model to develop plan details and collaborate with the manufacturer for the siphon structure.

During construction, Dibble worked with the Construction Manager at Risk to develop a feasible wastewater bypass across the Santa Cruz River. The team proposed constructing a rented 270-foot temporary modular steel bridge for the wastewater bypass, which was a fast, cost-effective, and environmentally responsible solution. The inlet and outlet structures were connected to the existing siphon pipes using fiber-reinforced polymer (FRP) internal joints, and cured-in-place pipe (CIPP) lining was used to fully rehabilitate the system.

The completed temporary bridge
Wastewater bypass piping on the temporary bridge

Through collaboration with numerous stakeholders, communication with the community, and innovative design, Dibble contributed to an award-winning project that restored the siphon system’s operation while protecting the environment and community.

Lowering the massive inlet structure into place (photo by Hunter Contracting Co.)
Installing the FRP internal joint


Pima County

Tucson, AZ


Project Lifecycle