SPA-1 Influent Structure Replacement & Improvements

Dec. 28, 2021

The city of Surprise, Arizona, worked with GHD and Felix Construction to complete the rehabilitation and improve long-term life cycle and performance of rapidly deteriorating wastewater facilities. Serving a population of 171,684 people within a 45-square-mile area, Surprise uses biological treatment, filtration, and disinfection processes to turn municipal, commercial, and industrial wastewater into quality recycled water. Most of that water is for agricultural irrigation, groundwater recharge, landscape irrigation, and dust control, so the city monitors effluent to ensure that it meets state and federal regulations.

A separate Water Reclamation Facility (WRF) serves each of the city’s six Special Planning Areas (SPA). The WRF for SPA-1, at the southernmost limits of Surprise, collects an average of approximately 18 mgd of raw wastewater, at its east and west sides, from gravity sewer pipes that approach the existing influent junction structure. Originally, this meant a 30-inch diameter gravity pipeline from the north and a 48-inch diameter pipeline from the south culminated at the west junction structure prior to conveying wastewater to the headworks via a 54-inch diameter pipeline.

“The ability to bring raw wastewater into a wastewater treatment plant is critical to maintain the compliance and downward treatment of that wastewater,” said Frederick Tack, GHD project manager and senior civil engineer. “Proper configuration to promote the right hydraulic and to maintain the right quality of the wastewater is important.” 

Only nine years into the west influent structure’s service life, its gates showed excessive signs of corrosion. Composed of cast-in-place concrete with a corrosion-resistant internal coating, the state of the influent structure showed the surrounding corrosive atmosphere and environment had not been properly quantified or addressed during design and construction. The city began to doubt the viability of the influent structure’s remaining life cycle, and began to question the reliability of the facility’s capabilities.

GHD delivered the SPA-1 Influent Structure Replacement and Improvements project in two phases. The first provided the complex condition assessment, development of potential project design alternatives, and assisted the city in alternative selection. The second phase entailed moving the selected project into detailed design and assisting with permitting, bidding and construction, and commissioning. All of this required completion without disruption to service or major environmental impacts while maintaining worker safety and adequate roadway capacity.

The major goals of the project were rehabilitating and/or replacing the influent junction structure and any corresponding components to provide a greater than 30-year life, and to successfully serve the city. This meant GHD addressed each element of the project, including the assessment, through alternative evaluation, design, construction, and O&M.

To evaluate the existing condition of the influent junction structure and associated components, the team navigated significant challenges to safety and schedule availability. Because the structure’s gates were mostly inoperable, the team was unable to isolate and drain the structure to conduct an inspection.

The best option was to conduct a high-risk confined space entry given this challenge. Following extensive hydraulic monitoring and analysis to determine low flow conditions, as well as thorough planning for duration, safety, and rescue scenarios, the team identified the ideal time for the inspection. Inspection was approximately 3:00 a.m. to 4:00 a.m. during weekdays, preceded by hours of atmospheric ventilation to create a safe atmosphere for entry. An inspection team was mobilized to complete the internal structural inspections inside the 30-foot-deep structure during open flow.

GHD’s inspection results revealed that the lower interior of the structure had experienced significant deterioration. CCTV inspections revealed deterioration of the influent pipelines and manholes feeding the junction structure, which pointed to the need for comprehensive rehabilitation to not only the junction structure, but its component gravity sewer pipelines and manholes.

The chosen solution was to rebuild the junction box with a new configuration, as well as full pipe and manhole replacement. To replace the 90-degree bend through which the flow entered the juncture, GHD designed a 96-inch diameter manhole to accommodate various influent pipe sizes, elevations, and materials that are able to properly combine flows while maintaining minimum self-cleansing velocities.

A solution was provided by Amorock that eliminates corrosion potential via a polymer concrete incorporating resin as the binding agent, along with sand, aggregate, and Fiber Reinforced Polymer (FRP) rebar, to form a polymer concrete structure.

Construction for GHD’s design required the complete bypass of the influent raw wastewater flow to the junction structure, pipeline, and manholes. As with any project requiring such an extensive bypass, the team was planning primarily against the risk of sanitary sewer overflow (SSO), which could create severe impacts for the health of the community. To address this risk, GHD and contractor Felix Construction Company collaborated to quantify the level of risk, cost, and schedule, all to provide the right level of confidence in the city prior to executing.

To minimize the potential for SSOs, Felix Construction installed smart covers on two manholes in the collection system to continuously monitor flow levels. Additionally, the temporary bypass system was tested for five consecutive days, which passed without error or issue, to provide the city with the assurance that the system would perform, as well as the consistency of the monitoring. Finally, a 24/7 pump watch was implemented for the entire duration of the bypass (approximately 45 days).

This project required excavating a big hole, one wide and deep enough to demolish the existing structure and then to prepare the foundations and bedding for the new pipe, manholes, and junction structure. The team and Felix Construction had to offset the time and effort to complete that first step — as well as to build then test the temporary bypass — with improved efficiency during the installation, testing, and commissioning of the new project components. 

The initial Guaranteed Maximum Price provided by Felix Construction was just shy of $1.16 million, which included contingencies and allowances for project variable costs and unknown or unanticipated conditions. At the conclusion of the project, the final construction cost was a little less than $1.06 million, netting a return of $100,518 to the City. When combined with the effort from GHD, the overall project was planned, designed, and delivered for less than $1.2 million, and was $72,350 less than the budgetary quote developed during Phase 1.

These decisions regarding materials, configuration, and construction methodology resulted in a system with more resilient performance, longer life cycle and less maintenance. 

“The city of Surprise, as the owner, definitely went above and beyond to both promote and deliver a level of resiliency that met their needs, both for today and in the future for them and their citizens to be able to sustain wastewater treatment,” Tack added.

Project Year: 2021-12-21Contractor: Felix Construction Co.Designers: GHD Inc.Owner: City of Surprise, ArizonaLocation: Surprise, ArizonaCost: $1,198,150Size: 3,060 gpm