Northwest Outfall Sewer Siphon

Building bridges to solve problems.

Written by: Crystal Faucett and Keith Faucett, PE

When golfers reached the 18th tee of the Silverbell Golf Course, the round was almost over. It was their final chance to improve scores, take in the mountain views, and enjoy the brilliant Tucson sun. But something did not smell right—in fact, it smelled like sewage. Just a putt’s distance away, the aging inlet to the Northwest Outfall Sewer Siphon was not operating well, causing a foul odor to permeate the area.

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

This siphon system, originally built in the 1960s, is a critical part of Pima County’s sewer infrastructure. It includes an inlet structure south of the Santa Cruz River (within the Silverbell Golf Course), 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, it was in need of rehabilitation, so the County hired Dibble to investigate the system and identify potential solutions.

Existing siphon system

Inside the siphon inlet structure, flows from a 48-inch interceptor sewer dropped and abruptly split into the two large-diameter siphon pipes, causing excessive turbulence, corrosion, and strong odors. Average velocities in the siphon pipes were about 1.5 feet per second, when a velocity of at least 3 feet per second is needed to suspend solids. This meant that solids were depositing within the siphon pipes—the extent of which was unknown at the time. To make matters worse, the existing location and configuration of the access hatches made it difficult to maintain the system because space for equipment and workers was limited. Fortunately, these issues could all be addressed by replacing the siphon inlet and outlet structures.

Turbulent flow inside the corroded inlet structure

Innovative New Design

To improve siphon system performance and thereby mitigate odors and corrosion issues, Dibble designed new inlet and outlet structures with several goals in mind:

  • Maintain the energy gradient within the system by ensuring head loss does not exceed the available elevation difference (head).
  • Increase velocity in the siphon pipes.
  • Use corrosion-resistant materials.
  • Improve access for future maintenance.

Dibble calculated the head loss in each component of the system—the incoming and outgoing sewers, the inlet and outlet structures, and the siphon pipes. By designing the inlet and outlet structures so head loss does not exceed available head, Dibble minimized backwater effects in the system

Siphon system analysis: Equations for calculating head loss in each component of the siphon system

The new structures include lengthened flow areas to provide smoother transitions, which reduces head loss, turbulence, corrosion, and odor. A weir in the inlet structure channelizes average daily flows into a single siphon pipe, thereby maintaining energy, increasing velocity, and avoiding solids deposition. The new design uses corrosion-resistant materials to extend the service life of the structures. Both structures are lined with monolithic (seamless) fiberglass, and an HDPE stop gate in the inlet structure allows the County to direct flows as needed. To facilitate future maintenance, each structure is designed with a large access hatch for increased accessibility.

New siphon inlet structure design
New siphon outlet structure design, shown with access hatch

During the design process, effective communication was crucial to ensure the new design met the County’s goals. In early meetings, Dibble brought 3D-printed models of potential designs for the inlet structure, allowing the County to identify the model that best reflected what they were envisioning. The team then used the preferred 3D model to develop plan details and collaborate with the manufacturer for the siphon structure. This clarity allowed the team to move forward with confidence and efficiency.

Construction Challenges and Solutions

The County and Dibble worked with Hunter Contracting Co. (the Construction Manager at Risk) to develop constructable plans and a feasible wastewater bypass procedure, which was no small feat. To take the siphon system out of service, Hunter needed to install temporary wastewater bypass piping across the Santa Cruz River. This river crossing was subject to numerous strict requirements, as a sewage spill in the riverbed would be a substantial environmental problem with risks to wildlife and public health.

The team considered several options. Trenching in the dry riverbed to lay the temporary piping was not a viable option due to potential flood control implications. Drilling deep beneath the riverbed to install the temporary piping was cost-prohibitive for the project. The high cost of building a new bridge to support the temporary piping made this option impractical. Then the team proposed building a temporary bridge. The County chose to move forward with a rented 270-foot temporary modular steel bridge, which was a fast, cost-effective, and environmentally responsible solution.

The completed temporary bridge

Once the wastewater bypass was in operation, Hunter inspected the siphon pipes and found that they were 90% blocked with debris including sandbags, concrete blocks, and even furniture among the sludge. This blockage could have resulted in a sewer overflow had the project been delayed, and removing it was not easy. Workers used shovels and buckets to manually remove the debris from inside the 30-inch and 39-inch siphon pipes—a laborious process in a confined space. With monsoon season approaching, the temporary bridge and bypass piping needed to be removed as soon as possible; this required around-the-clock work to meet the short timeframe.

Debris at the siphon pipe entrance
Wastewater bypass piping on the temporary bridge

The sheer size of the new siphon inlet and outlet structures added complexity. Each finished structure weighed nearly 40 tons, and moving the structures required cranes and significant logistical coordination. These were the largest structures the manufacturer had ever cast with a continuous liner. In fact, the manufacturer modified their shop space to accommodate the structures. Connecting the new structures to the existing system was another challenge. The existing siphon pipes were encased in concrete, so connecting with a typical outer coupling was not possible. To work around this issue, the team used custom fiber-reinforced polymer (FRP) internal joints to connect to the existing pipes (similar to the FRP connection on our recent Tucson Boulevard Diversion Structure project). Later, after the siphon pipes were cleaned, the County opted to use cured-in-place pipe (CIPP) lining to fully rehabilitate the system.

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

This project had numerous stakeholders, including the Silverbell Golf Course, Tucson Parks and Recreation Department, Tucson Floodplain Administration, the Loop trail system, Tucson Transportation and Mobility, and a neighborhood HOA. Through proactive communication including progress meetings, flyers, and signs, the team kept the community informed and minimized impacts before and during construction. To ensure minimal disruption to the golf course, Hunter constructed a temporary detour golf cart path and modified tee boxes, allowing all 18 holes to stay open throughout the project.

Placing the inlet structure (photo by Hunter Contracting Co.)

With the new structures in place and the siphon pipes restored, the operation of the siphon system has drastically improved. The odor issues have been resolved, and the 18th tee is a pleasant place to golf again. This project provided an innovative, long-term solution that serves the community while protecting the environment. Effective collaboration between the project team and stakeholders greatly contributed to this achievement—a clear example of how building bridges can solve problems and ultimately lead to success.

This noteworthy project earned APWA Arizona’s 2024 Project of the Year Award in the Environment: Less than $5 Million category.

An effective team (photo by Hunter Contracting Co.)

Markets Mentioned

Services Mentioned
Infrastructure Rehabilitation, Water + Wastewater

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Keith Faucett, PE