Phase 2 began in summer 2024 and is currently about 64% complete. As the larger and more complex portion of the expansion, this phase includes the 24-mgd capacity increase along with the addition of sand-ballasted flocculation, ozone generation and six new filters. This segment also enhances solids handling, adds chemical systems and introduces a sodium hypochlorite generation disinfection process to supplement existing operations.

Crews are currently focused on installing process mechanical and electrical systems while coordinating with product vendors to prepare for start-up and commissioning, which is scheduled to run from October through the end of the year.

Matthew Schmidt, project manager for McCarthy’s national water team, says commissioning these new additions will involve 40 maintenance of plant operations designed to test and verify equipment functionality. To ensure total readiness, the team is closely coordinating manufacturer’s certificate of installation and manufacturer’s certificate of operation requirements with vendors.

Six new granular activated carbon filters with SST underdrain systems have been installed to remove suspended solids and dissolved organic compounds while supporting biological treatment. The ballasted flocculation system was selected over a conventional clarifier due to its smaller footprint and faster treatment time. The ozone generation system, supplied by Veolia Water Technologies, provides oxidation to reduce organics and required complex integration during installation, Schmidt says.

He notes that one of Phase 2’s most challenging aspects has been yard piping and shoring in highly congested underground conditions, particularly after the scope expanded to six filters from four.

“McCarthy’s civil team was able to address the challenge by employing extensive 3D mapping and laser scanning of the site, soft digging and potholing, paired with creative shoring solutions and careful planning to safely tie in the new facility to existing infrastructure,” Schmidt says. “The integration of new structures with existing ones has involved highly detailed, advanced planning and day-to-day coordination with the owner to ensure that the complex tie-ins and modifications are successfully executed.”

The tie-in of the six new filters to the existing system is scheduled for a fall 2026 shutdown window. While full plant shutdowns are typically planned for the spring and fall, their execution depends heavily on regional rainfall and municipal water demands. Lower-than-average regional rainfall allowed the team to collaborate with the city of Mesa to resequence the schedule and entirely avoid a fall shutdown. Instead, crews used short-duration, targeted windows to install the CO2 isolation valves and replace the nearby Desert Sage water system valve while the plant remained live, pushing the more complex filter tie-ins to the spring.

Schmidt says the team is proactively planning staffing and sequencing to limit risk during this critical period. Instead of loading all filter media at once, the team developed a strategy to load the material one filter at a time because the plant can only handle a limited amount of water until the system is fully operational.

“This strategy not only accommodates delivery schedules, but the adjustment allows the project to maintain progress and is preventing excessive water from being lost,” Schmidt says. “With this approach, each filter is tested and monitored to ensure it functions correctly while allowing the plant to continue running and serving the community.”

Mike Caruso, associate vice president and project director for Black & Veatch, says this solution was developed during the initial design phase and is “another result of the close coordination and trust that was established on the project between all of the partners.”

Oscar Munos, project director for McCarthy’s water team, says efforts are aimed at limiting risk through early labor forecasting and specialized presite training—including preparation at the Chandler Innovation Center—so crews arrive job-ready.

“They’re also leaning on detailed scheduling and sequencing to level labor loads, prioritize critical path work and avoid idle time, supported by prefabrication to reduce onsite labor hours and improve predictability ahead of the final shutdown window,” Munos says.

From a CMAR control perspective, McCarthy’s self-perform workforce has been vital to maintaining the schedule during Phase 2, company officials say.

“Self-perform has been critical because it gives the team tighter control over quality, schedule and resource allocation. This project benefited from McCarthy’s self-perform teams in civil, concrete and mechanical work because of the increased flexibility these teams afforded,” Munos says. “When the scope was modified, shutdown requirements shifted [and] equipment deliveries changed, [and] when field conditions adjusted, the project was able to be nimble and quickly adjust because those performing the work were McCarthy employees.”

This internal flexibility also solved logistical and spatial constraints on the limited site. With no space available to store 20,000 cu yd of excavated dirt from the reservoir, the traditional approach would have required hauling the material off site and purchasing new backfill later. Instead, McCarthy collaborated with Mesa’s Parks, Recreation and Community Facilities division to temporarily extend the project fence line into an adjacent city park. Utilizing this space to stockpile the original material eliminated the need for new material procurement, reduced logistical coordination by several months and generated major cost savings for the city.

Munos adds that the consistency and quality provided by McCarthy’s self-perform teams offer the owner additional value and scheduling efficiencies that keep the project moving forward—control that proved just as vital when managing early financial uncertainties on the project.

A four-month gap between vendor pricing and the notice to proceed introduced potential cost increases, presenting an early test for the project’s CMAR delivery method. Under a traditional delivery model, such a delay often triggers price escalations or contract renegotiations, but the collaborative nature of CMAR allowed McCarthy to leverage its early involvement and strong supplier relationships to maintain original pricing.

“Although there was an approval gap, our team maintained proactive, transparent timeline communications with our trade partners and ultimately secured buyout savings on the project for the owner,” Munos says. “Additionally, thanks to our ongoing communication and collaboration with the city, the team was able to create a phased integration schedule that prioritized existing equipment and accommodated equipment delivery changes as a result of the gap.”

With commissioning scheduled to begin in October and continue through year-end, the project remains on track for completion in 2027. Once finished, the expanded Signal Butte Water Treatment Plant will provide Mesa with increased treatment capacity, improved system redundancy and a more sustainable water supply to support long-term growth.

Other members of the project team include Tempe Mechanical (HVAC and mechanical systems), Industrial Power Solutions (electrical), Suncoast Post Tension (concrete reinforcing), Veolia Water Technologies (ozone system) and De Nora Holdings (sodium hypochlorite generation).