A rebuild of a wastewater treatment train at the 91st Avenue plant, lasting about three years and three months, is finally done. The City of Phoenix Water Services Department and McCarthy Building Companies completed a $73.5 million rehabilitation of Plant 2B at the 91st Avenue Wastewater Treatment Plant, renewing equipment that dates back to the 1980s and keeps wastewater moving for five cities.
The project, finished in September, focused on what Hazen and Sawyer describes as the 36-million-gallon-per-day Plant 2B treatment train, one of eight at the sprawling facility. The 91st Avenue plant is owned by the Sub-Regional Operating Group, a partnership of Phoenix, Glendale, Tempe, Mesa and Scottsdale, and is operated by Phoenix on the group's behalf, according to AZ Big Media. Hazen and Sawyer served as project design engineer, providing detailed planning and design work, while McCarthy acted as construction manager at risk.
What Actually Got Rebuilt
Crews removed and replaced equipment in four primary sedimentation basins, each 140 feet in diameter, as part of the overhaul, per the outlet's reporting. The work also covered structural and concrete rehabilitation in the aeration basins, new coarse- and fine-bubble aeration diffuser systems, and rehabilitation of the primary sludge and return/waste activated sludge pump stations.
Beyond the basins themselves, the project touched nearly every corner of the treatment train. Improvements extended to secondary sedimentation basins, effluent distribution structures and related treatment equipment, along with upgrades to buried piping, electrical equipment and process control systems. The reuse water pump station and chlorination facility also received repairs, the report notes.
A New Biofilter Replaces An Old Chemical Scrubber
One of the more visible changes involves odor control. The rehabilitation swapped the existing chemical scrubber for a new biofilter, which the project's engineering source describes as a system replacing the combined headworks chemical odor scrubber with a 60,000-standard-cubic-feet-per-minute unit, per Hazen and Sawyer. The new biofilter contains four in-ground treatment cells built with specialized filtration media, air-distribution piping, fans and supporting equipment.
Trade partners on the job included OCM Inc., the Truesdell Corporation, Penington Painting Company and Industrial Power Solutions, according to the AZ Big Media account. The entire effort required careful coordination so treatment operations could continue while essential equipment was replaced and rehabilitated, since the plant could not simply shut down during construction.
Why Phoenix Says The Upgrade Matters
Phoenix Water Services Director Brandy Kelso said investment in existing infrastructure is essential for reliable wastewater service, framing the Plant 2B work as part of a broader push to extend infrastructure's useful life, improve operational reliability and provide greater maintenance flexibility. The five partner cities in the Sub-Regional Operating Group collaborate to maintain regional wastewater infrastructure for residents and businesses, the outlet reports, and the rehabilitated train is meant to support preservation of treatment capacity, regulatory compliance and reliable service going forward.
The 91st Avenue plant produces Class B+ effluent, most of which is sent to the Palo Verde Nuclear Generating Station for reuse, according to Hazen and Sawyer. That reuse arrangement underscores just how much is riding on the plant's reliability beyond the five cities it directly serves.
Separately, a Phoenix solicitation for engineering services for aeration-system rehabilitation elsewhere at the 91st Avenue plant had a proposal deadline of October 16, 2026, according to a bid listing from SledAtlas. It is not yet clear whether that solicitation has led to an award or construction. The city says it maintains five water treatment plants overall, with the oldest built in the early 1950s and the newest completed in 2007, though older plants have been renovated and modernized over time, according to the City of Phoenix.
