Like the Agua Nueva facility that won the Best Project award in this category, the Tres Rios Water Treatment Plant improvements and upgrades are also part of Pima County's $720-million Regional Optimization Plan. Tres Rios expanded the plant's capacity to 50 million gallons per day from 37.5 mgpd. Close collaboration between the construction team and plant operations staff was integral in managing more than 150 process tie-ins and three plant shutdowns.
"The plant was 40 years old when we got there so there were a lot of unknown conditions," says Duane Frerker, project manager with MWH Constructors. "But the team we had out there handled it very well."
This fast-tracked project included two new digesters, a blower building, primary clarifier, two aeration tanks, five secondary clarifiers, three centrifuges and a gravity belt thickener. A new five-stage Bardenpho system processes 25 mgpd to biologically remove not only organic matter but also nitrogen and phosphorous from the wastewater stream.
The project team excavated 280,000 cu yd of material. The deepest pit descended 65 ft for the plant drain pump station, which was completed through a combination of open cut excavation and shoring.
Detailed sequencing, scheduling and contingency planning of construction activities allowed the excavation to be completed without delay, even when 3,000-year-old artifacts were discovered during excavation of the digesters.
"The entire plant was considered an archeological site, so before [we] could start digging in any of the new areas, [experts] had to come in and clear those sites," says Frerker. Coordination with archeologists and researchers led to the recovery of more than 70,000 artifacts that were donated to the Arizona State Museum, according to the project team.
The project was delivered early and under budget while maintaining existing plant operations.
Tres Rios Water Reclamation Facility
General Contractor MWH Constructors Inc., Broomfield, Colo.
Owner/Developer Pima County, Tucson
Lead Design Firm CH2M HILL, Tempe, Ariz.