When the 91st Avenue Wastewater Treatment Plant (WWTP) needs attention, it jumps to the top of the city’s priority list. As the largest WWTP in Arizona, the 56-year-old facility has a wide reach, so small improvements have large impacts. Located just outside Phoenix, the plant is owned by the Sub-Regional Operating Group—a cooperation between the cities of Phoenix, Scottsdale, Glendale, Mesa and Tempe—and operated by the city of Phoenix. The plant can treat 230 million gal per day.
The city identified the need for a rehabilitation program at the facility requiring dome replacement and digester rehabilitation. This program aimed to address short- and long-term capital improvement investments to aging infrastructure and improve reliability and sound fiscal planning.
GHD Inc. and PCL Construction Inc. collaborated on the rehabilitation project. Before diving in, they conducted a condition assessment, a topographic survey, 3-D scanning and mapping of the existing components. The project maintained a holistic approach. Then, they removed the existing steel dome, pipe and appurtenances. They repaired the concrete and steel structures, and designed, fabricated and constructed the new steel dome. The old digester was an above-ground concrete tank that was 90 ft in diameter and 30 ft tall with a fixed steel cover. According to Frederick Tack, treatment and consulting group manager, and senior civil engineer for GHD, it was the oldest digester and dome at the facility, so rehabilitating and replacing it extends the digester’s service life. This project is part of a larger program to replace 10 digester domes at the facility.
A program of this size is not without challenges. Initially, a structural inspection showed severe corrosion and deterioration of the existing dome’s structural steel due to exposure to the corrosive environment inside the dome, according to Tack. What’s more, the location of the digester proved problematic.
“There were a number of construction challenges, including limited access for cranes to remove the old dome, as well as additional environmental controls [that] were needed to prevent debris and material from entering the treatment process,” Tack said.
Despite these challenges, the project did not impact plant operations. Tack said the digester was placed into service immediately after construction was complete, with no issues in operation. The teams prepared design and construction documents within six months and completed the demolition, rehabilitation, and dome replacement in just more than 12 months. Digester 1 was ready for operation in November 2016.
“The aspect we are most proud of is how well the project team worked with each other and with the city’s operations and maintenance staff to complete the project on time and on budget,” Tack said.
Ultimately, Tack explained, the project demonstrates how life cycle planning and budgeting can help meet the water and wastewater treatment and reuse needs for a community.