City Of Phoenix Agua Fria Water Reclamation Facility

1. INTRODUCTION

The 91st Avenue Multi-Cities Wastewater Treatment Plant stands as the largest wastewater treatment facility in the Southwest United States and a critical cornerstone of Arizona’s water management infrastructure. Operated by the City of Phoenix on behalf of the Sub-Regional Operating Group (SROG)—a coalition comprising Phoenix, Glendale, Mesa, Scottsdale, and Tempe—this facility manages the wastewater for nearly 2.5 million residents in the metro area. With a rated capacity of 204.5 million gallons per day (MGD), the plant is a marvel of hydraulic engineering and biological processing.

Situated near the confluence of the Salt and Agua Fria Rivers, the facility is not only a center for sanitation but a hub for sustainability innovation. It feeds the renowned Tres Rios Environmental Restoration Project, creating vital riparian habitat, and hosts one of the nation’s largest renewable natural gas (RNG) facilities, converting biogas into green energy. As water scarcity challenges the arid Southwest, the 91st Avenue Plant’s role in reclamation, recharge, and environmental stewardship defines the standard for large-scale municipal operations.

2. FACILITY OVERVIEW

A. Service Area & Organization

The facility operates under a unique Joint Exercise of Powers Agreement known as the Sub-Regional Operating Group (SROG). While the City of Phoenix serves as the operating agent, the plant accepts influent from five major municipalities across Maricopa County. The collection system feeding the plant includes the 90-inch Salt River Outfall (SRO) interceptor, one of the largest gravity sewers in the state. The service area encompasses a dense mix of residential zones, heavy industrial corridors, and commercial districts, requiring robust influent monitoring programs to manage industrial pretreatment compliance.

B. Operational Capacity

The plant is designed to treat an annual average daily flow of 204.5 MGD. Historically, flows have stabilized around 130–150 MGD due to aggressive water conservation efforts across the valley, despite population growth. The facility is designed to handle significant peak hydraulic surges, particularly during Arizona’s monsoon season, with peak wet weather capacity exceeding 300 MGD. Capacity utilization currently sits near 70%, providing a strategic buffer for future regional expansion.

C. Discharge & Compliance

Treated effluent is discharged under Arizona Pollutant Discharge Elimination System (AZPDES) permit AZ0020524. The primary discharge point is the Tres Rios Wetlands, a constructed wetland system that further polishes the effluent before it enters the Salt River channel, just upstream of the Agua Fria River. A portion of the effluent is also diverted to the Palo Verde Nuclear Generating Station for cooling water, representing one of the world’s largest uses of reclaimed water for power generation.

3. TREATMENT PROCESS

The 91st Avenue WWTP utilizes a unified liquid stream process that has been evolved over decades of expansion. The plant is physically divided into multiple parallel “plants” (Train 1 through Train 4/5) allowing for maintenance redundancy and flow management.

A. PRELIMINARY TREATMENT

Influent enters via the massive interceptor system into the headworks complex.

• Screening: Mechanically cleaned bar screens (typically 3/8-inch to 1/2-inch spacing) remove rags, plastics, and large debris to protect downstream pumps.
• Grit Removal: Aerated grit chambers decrease the velocity of the wastewater, allowing inorganic sands and gravel to settle while keeping organic matter in suspension.
• Odor Control: The headworks utilizes extensive biotrickling filters and chemical scrubbers to mitigate H2S emissions, a critical requirement given the facility’s proximity to residential developments in Tolleson.

B. PRIMARY TREATMENT

Flow enters rectangular primary sedimentation basins. Here, physical settling removes approximately 60-70% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).

• Configuration: Multiple batteries of rectangular clarifiers with chain-and-flight sludge collectors.
• Sludge Handling: Primary sludge and scum are pumped directly to the anaerobic digester complex.

C. SECONDARY TREATMENT

The biological heart of the facility is the Activated Sludge process, configured for Nitrification/Denitrification (NDN).

• Process Type: Single-stage activated sludge with anoxic and aerobic zones to facilitate biological nutrient removal (specifically nitrogen).
• Aeration: Deep tanks utilize fine-bubble diffusion systems powered by large centrifugal blowers to maintain dissolved oxygen levels for nitrification (conversion of ammonia to nitrate).
• Clarification: Mixed liquor flows to secondary clarifiers (circular) where biological floc settles. The clear supernatant overflows the weirs as secondary effluent.
• RAS/WAS: Return Activated Sludge is recycled to the aeration basins; Waste Activated Sludge is thickened (via Dissolved Air Flotation or gravity belts) and sent to digestion.

D. TERTIARY / ADVANCED POLISHING (Tres Rios)

While the hardscape plant concludes at disinfection, the Tres Rios Wetlands act as a nature-based tertiary step. The wetlands reduce residual nutrients, stabilize pH, and reduce chlorine residuals before the water interacts with the riparian ecosystem.

E. DISINFECTION

The facility employs chlorination for pathogen reduction using sodium hypochlorite. Following sufficient contact time in serpentine basins, the effluent is dechlorinated using sodium bisulfite to prevent toxicity to aquatic life in the Tres Rios ecosystem and the river channel.

F. SOLIDS HANDLING & RNG

The 91st Avenue plant is an industry leader in solids processing.

• Digestion: Primary and thickened waste activated sludge undergo mesophilic anaerobic digestion in a massive battery of digesters (over 20 digesters on-site).
• Biogas Reuse: In partnership with Ameresco, the “91st Ave RNG Project” processes raw biogas. This facility, capable of processing 3,250 SCFM, scrubs impurities to pipeline-quality natural gas standards.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges.
• Disposal: The resulting Class B biosolids are largely hauled for land application on non-food crops (cotton/alfalfa) in Arizona, closing the nutrient loop.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans nearly 600 acres (excluding the wetlands). The industrial scale is evident in the redundancy of systems; for example, the plant operates multiple distinct “batteries” of treatment trains, allowing operators to take entire sections offline for rehabilitation without impacting total capacity.

B. Energy Systems & Renewables

The plant is a heavy energy consumer, particularly for aeration blowers and influent pumping. However, the Renewable Natural Gas (RNG) facility is the defining energy feature. Commissioned in 2019, this facility reduces carbon emissions by nearly 45,000 tons per year. The gas is injected into the commercial grid, generating revenue and offset credits for the SROG cities.

C. Odor Control

Given the high ambient temperatures in Phoenix (exceeding 110°F in summer), wastewater septicity is a major challenge. The facility employs a multi-tiered odor control strategy involving vapor-phase treatment (scrubbers) at turbulent flow points and liquid-phase treatment (iron salts) in the collection system to precipitate sulfides before they reach the plant.

5. RECENT UPGRADES & MAJOR PROJECTS

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

The facility operates under strict scrutiny regarding nitrogen discharge. The AZPDES permit sets limits on Total Nitrogen to prevent eutrophication in downstream waterways. The plant consistently achieves high removal efficiencies:

• BOD Removal: >98%
• TSS Removal: >98%
• Ammonia-N: Consistently < 1.0 mg/L

The facility plays a pivotal role in the “Tres Rios” habitat, home to over 150 species of birds and mammals. The quality of the effluent directly supports this ecosystem, making process reliability an ecological necessity, not just a regulatory one.

7. OPERATIONAL EXCELLENCE

The City of Phoenix employs a highly credentialed workforce, with many operators holding Grade 4 ADEQ certifications (the highest level). The facility emphasizes “Total Water Management,” integrating laboratory data (from the on-site compliance lab) with operations. The plant has received numerous Peak Performance Awards from NACWA (National Association of Clean Water Agencies) for permit compliance.

8. CHALLENGES & FUTURE PLANNING

A. Aging Infrastructure

Portions of the plant date back to 1958. Managing concrete corrosion in headworks and digesters due to H2S attack is a constant maintenance priority. A comprehensive asset management plan drives the capital improvement schedule.

B. Changing Flows

While population grows, per-capita water use is dropping. This results in “stronger” wastewater (higher concentrations of BOD/TSS per gallon). The plant must adapt aeration and biological loading strategies to treat this more concentrated influent without overwhelming the biology.

C. Future Expansion

Master planning anticipates eventual expansion to 230+ MGD as the West Valley continues to urbanize. Future technologies under consideration include advanced membrane filtration if discharge regulations tighten regarding emerging contaminants (PFAS).

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical Questions

1. What is the hydraulic retention time (HRT) of the aeration basins?
The HRT varies by flow train and influent rates but typically ranges between 4 to 6 hours to ensure complete nitrification.

2. How does the plant handle industrial loads?
The City of Phoenix maintains a strict Industrial Pretreatment Program (IPP). Industries must permit significant discharges, and the plant monitors influent closely for shock loads that could inhibit biological activity.

3. Is there nutrient recovery?
Currently, the plant practices nutrient *removal* (Nitrogen) via biology. Phosphorus is largely captured in the biosolids, which are land-applied, returning nutrients to agricultural soil.

Public Interest Questions

4. Does the plant smell?
While wastewater treatment has inherent odors, the City invests millions annually in chemical scrubbers and covers to contain odors. The plant is surrounded by agricultural and industrial land, providing a buffer, though nearby residential growth has increased the focus on odor control.

5. Can I visit the plant?
The City of Phoenix occasionally offers tours for educational groups and industry professionals. The adjacent Tres Rios Wetlands requires a permit for entry but is a popular site for birdwatching.

6. Is this water drinkable?
The effluent is treated to a high non-potable standard (suitable for agriculture, cooling, and environmental flow). It is not currently plumbed for direct potable reuse, though it is legally Class A+ or B+ reclaimed water depending on the specific process train and end-use.