Even though the $170 million Greenfield Water Reclamation Plant Phase III expansion in Gilbert, Arizona, required 4.5 miles of new underground piping, 249,000 cubic yards of excavation, 92,100 cubic yards of backfill, and multiple new technologies, the most critical focus was simply maintaining plant operations throughout construction.

“This plant is the primary means of treating wastewater in the area,” said Pat Payne, Project Director for Construction Manager at Risk (CMAR) McCarthy Building Companies, Inc., of Phoenix. “As an end-of-the-line plant, it’s imperative that plant operations be maintained. We very thoroughly vetted and planned all of our outages, and we didn’t have any unplanned outages.”

Significant growth in the Phoenix-area municipalities served by the plant – the City of Mesa, Town of Gilbert, and Town of Queen Creek – stretched the plant near capacity when the project started. The urgency only grew through the three-year construction.

“We’ve been seeing a lot of increase in the concentrations of certain constituents in the wastewater,” explained Russ Wachter, P.E., Principal in Charge for Design Engineer Carollo Engineers of Phoenix. “A lot of that is tied to water conservation and changes in the plumbing code. Although the flows may be less, the concentrations of waste are a little higher. The concentrations continued to increase throughout the project’s planning, design, and construction – to the point where we really pushed the full limits of the existing facility’s capacity.”

Construction for the plant expansion started in November 2017 and reached substantial completion in August. The project increased the plant’s treatment capacity by 14 million gallons per day (MGD), bringing total plant capacity to 30 MGD for liquids and 38 MGD for solids on an annual average flow basis. The work also included alternative technologies to improve reliability and lower operating costs.

Maintained and operated by the City of Mesa, the plant originally opened in 2007, constructed by McCarthy in a joint venture with Sundt Construction of Tempe, Arizona.

During the expansion work, in order to prevent disruptions to operations, “We needed to make sure that all of the new utilities and structures we constructed would fit in, under, and around this sea of existing utilities and yard piping that was already onsite,” Payne said.

To achieve that, McCarthy utilized in-house virtual design and construction (VDC) modeling. They started with as-built drawings, then conducted extensive investigations onsite to confirm and update the model.

“We wanted to vet and understand as much as we could before we performed the work,” Payne said. “We imported survey data into our model. If there was a critical tie-in underground, we’d pothole and locate it to make sure we understood what was there. Imagine repeatedly putting in that effort throughout the site.”

During the pre-construction phase, McCarthy and the design engineers identified more than 100 key outages, or maintenance of plant operations (MOPOs), as they referred to them.

“Starting in pre-construction and throughout the construction process, the team took the opportunity to really dive into the existing electrical and utility systems and refine our plan step-by-step in terms of how we’d perform the MOPOs,” Payne said. “We planned most of them out to the minute.”

As part of that process, McCarthy led weekly MOPO meetings with as many as 25 key stakeholders contributing ideas. In addition to the CMAR and design engineering teams, “The plant’s staff worked closely with the contractor to make sure they knew the constraints of the operation,” said Joseph Schroeder, P.E., Project Manager for the City of Mesa. “The plant was able to make changes to accommodate the contractor, and the contractor was able to make changes to accommodate the plant.”

Each MOPO varied in duration. “Some constraints were prescripted to us in our contract, but as we built relationships with all the team members, we could really work together to make sure we had enough time to do our work without impacting plant operations,” Payne said. “Sometimes we shut down a system for 30 minutes; other times we were able to shut down a system for weeks at a time. For some MOPOs, we fit in multiple operations so we didn’t need to come back and shut down the same system multiple times throughout the job.”

With extensive excavations needed throughout the 61-acre site, McCarthy utilized detailed modeling and soil analysis.

“During preconstruction, we planned out for the owner’s benefit how we’d bring costs down by minimizing shoring,” Payne said. “If we had the logistical space to open up an excavation, we sloped it back. However, that meant losing laydown space and setting up the crane further back from the structure. There are always a lot of different trade-offs to evaluate.”

Some areas were tighter than others. “Sometimes we’d build a new structure pinned between two existing structures,” Payne said. “For the new aeration basins, though, we had more space because the original master plan accounted for those. The key was understanding the existing utilities around the perimeter. If we sloped back that excavation and exposed existing utilities, we needed to support them during construction.”

In some areas, “The contractor had to build deep structures close to existing deep structures,” Schroeder added. “They had to dig down adjacent to the structure without disturbing it.”

For one of the excavations, crews temporarily relocated existing electrical controls and wiring. “If they had dug the excavation without doing that, the existing treatment process wouldn’t have been able to keep on,” Schroeder said.

To further reduce costs and address operational issues, the project included replacing, repairing, and rehabilitating deteriorating equipment.

For instance, “With the programmable logic controllers put in 12 years ago, you can imagine what kinds of challenges they provided with new software upgrades,” Schroeder said. “The sequencing of the work to replace those was pretty intense.”

Aeration basins also underwent improvements. “The plant was originally designed with coarse bubble diffusers,” Wachter said. “We converted those to fine bubble diffusers, which offer a significant power cost savings.”

In addition, “Because the concentrations in the wastewater had increased significantly since the original design criteria, we made some other modifications to the aeration basins,” Wachter said. “By adding some additional baffling and walls to provide a little more zone compartmentalization, and increasing some of the internal piping, we were able to retrofit the existing basins to add more capacity.”

McCarthy constructed two new basins with the baffling and upsized equipment. “Once they started those up, they took the two existing basins down and made the modifications,” Wachter added. “If we followed the design criteria we had the first time, we would’ve needed to build three more basins. The changes to the existing basins allowed us to get by with four total basins instead of five.”

McCarthy also implemented alternative filtration and disinfection technologies designed to provide operational cost savings. The net result of the project allows the facility to handle the growing treatment capacities needed by the communities it serves.