Pima County Roger Road Wastewater Reclamation Facility Tucson

1. INTRODUCTION

The Agua Nueva Wastewater Reclamation Facility (WRF) represents a pivotal shift in water management for the arid Southwest. Replacing the aging Roger Road WRF—which had operated since the 1950s using older trickling filter technology—Agua Nueva was commissioned in 2013 as part of Pima County’s historic $720 million Regional Optimization Master Plan (ROMP). Situated near the confluence of the Rillito and Santa Cruz Rivers, this 32-MGD facility serves the metropolitan core of Tucson.

Designed to meet strict Environmental Protection Agency (EPA) mandates regarding nutrient loading, the facility utilizes a compact 5-stage Bardenpho process to deliver Class A+ reclaimed water. Its operation is critical not only for municipal sanitation but for the restoration of the Santa Cruz River, where high-quality effluent supports riparian habitats and groundwater recharge in an effluent-dependent watercourse. As a Design-Build-Operate (DBO) project, Agua Nueva stands as a case study in public-private partnership efficacy and modern engineering footprint optimization.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The facility serves a dense urban catchment area encompassing midtown Tucson and the central business districts. The collection system feeding Agua Nueva relies heavily on gravity mains that follow the natural slope of the Tucson basin toward the Santa Cruz River. The facility is strategically located to intercept flows that historically overloaded the collection system, acting as a sister plant to the Tres Rios WRF (formerly Ina Road) located further downstream. The service area is largely built-out, meaning flow characteristics are stable, though diurnal peaking factors remain a critical operational consideration.

B. Operational Capacity

Agua Nueva is rated for an Annual Average Daily Flow (AADF) of 32.0 million gallons per day (MGD). The hydraulic peak capacity is significantly higher, designed to handle up to 64 MGD during wet weather events, which can be intense during the Arizona monsoon season. Since commissioning, the plant typically treats between 26 and 29 MGD, operating at roughly 85% of its design capacity. This utilization rate allows for operational flexibility and maintenance rotation of the process trains.

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C. Discharge & Compliance

Treated effluent is discharged into the Santa Cruz River via a specialized outfall structure. Unlike coastal plants, Agua Nueva’s discharge constitutes the majority of the river’s flow during dry seasons, creating a unique “effluent-dependent” ecosystem. The facility operates under an Arizona Pollutant Discharge Elimination System (AZPDES) permit with stringent limits on Total Nitrogen (TN) and Ammonia (NH3-N) to prevent eutrophication and toxicity in the river channel. The effluent quality consistently meets Arizona Department of Environmental Quality (ADEQ) Class A+ Reclaimed Water standards.

3. TREATMENT PROCESS

The Agua Nueva WRF utilizes a highly automated, biological nutrient removal (BNR) process designed to achieve low nitrogen concentrations within a compact physical footprint.

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through 6mm mechanical bar screens to remove large debris. Following screening, the flow enters vortex grit chambers. These units utilize centrifugal force to separate inorganic sands and gravels (grit) from the organic waste stream. The removed grit and screenings are washed, compacted, and transported to a landfill. Odor control is paramount at the headworks, utilizing biotrickling filters to scrub hydrogen sulfide (H2S) from the air.

B. Secondary Treatment (Biological Nutrient Removal)

The core of Agua Nueva’s treatment is the 5-Stage Bardenpho Process. This activated sludge configuration is specifically engineered for nitrogen and phosphorus removal without heavy chemical reliance.

• Anaerobic Zone: Selects for phosphorus-accumulating organisms (PAOs) to facilitate biological phosphorus removal.
• First Anoxic Zone: Nitrate produced in the aerobic zone is recycled here, where bacteria use the oxygen attached to nitrates to consume BOD, releasing nitrogen gas (denitrification).
• Aerobic Zone: Ammonia is converted to nitrate (nitrification) via aeration. This zone utilizes high-efficiency turbo blowers and fine-bubble diffusers to maximize oxygen transfer efficiency.
• Second Anoxic Zone: Removes remaining nitrates created in the aerobic stage to meet strict Total Nitrogen limits.
• Reaeration Zone: Strips nitrogen gas bubbles attached to sludge flocs to improve settling in the clarifiers and adds dissolved oxygen to the effluent.

Following the biological reactors, the mixed liquor flows to secondary clarifiers where solids settle out. Return Activated Sludge (RAS) is pumped back to the anaerobic zone, while Waste Activated Sludge (WAS) is removed for solids processing.

C. Tertiary Treatment

To achieve Class A+ standards, secondary effluent undergoes tertiary filtration using cloth media disk filters. These units provide a physical barrier to remove remaining suspended solids (TSS) and turbidity. The cloth media filtration system was selected for its small footprint and low backwash water requirements compared to traditional sand filters.

D. Disinfection

The filtered water is disinfected using chlorination (sodium hypochlorite) in contact basins to ensure pathogen inactivation. Prior to discharge into the sensitive aquatic environment of the Santa Cruz River, the water undergoes dechlorination using sodium bisulfite to neutralize residual chlorine, which would otherwise be toxic to aquatic life.

E. Solids Handling

Agua Nueva operates as a “satellite” solids facility in terms of final processing.

1. Thickening: WAS is thickened on-site using Dissolved Air Flotation (DAF) units to reduce volume.
2. Transfer: The thickened sludge is not digested at Agua Nueva. Instead, it is pumped via an interconnect pipeline to the Tres Rios WRF (formerly Ina Road).
3. Processing: At Tres Rios, the solids undergo anaerobic digestion, dewatering, and final disposition (land application or compost). This regional approach centralizes biogas production and reduces capital redundancy.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Agua Nueva facility is notable for its density. While the previous Roger Road plant occupied nearly 90 acres with expansive trickling filters, the modern Agua Nueva plant was constructed on an adjacent parcel of approximately 27 acres. The site features a centralized Operations and Administration building, housing a state-of-the-art control room and compliance laboratory.

B. Energy Systems

Energy efficiency was a primary design criterion. The plant utilizes high-speed turbo blowers with air-foil bearings for the aeration basins, which consume 30-40% less energy than conventional centrifugal blowers. The facility is integrated into Pima County’s energy management program, though it does not host significant on-site generation (biogas generation occurs at the centralized Tres Rios facility).

C. Odor Control

Given the facility’s proximity to residential zones and the I-10 corridor, odor control was a critical driver for the replacement of the old plant. Agua Nueva employs a dual-stage odor control strategy:

• Containment: Headworks and primary process areas are covered or enclosed.
• Treatment: Foul air is extracted and treated through biotrickling filters (biological scrubbers) followed by activated carbon polishing units. This achieves >99% H2S removal, effectively eliminating nuisance odors.

5. RECENT UPGRADES & MAJOR PROJECTS

Current Optimization (2020-Present)

Recent minor capital projects have focused on:

• SCADA Security: Enhancing cybersecurity protocols for industrial control systems.
• Perimeter Protection: Upgrading physical security measures.
• Asset Management: Implementation of predictive maintenance sensors on major rotating assemblies.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Under NPDES Permit AZ0026107, Agua Nueva is held to some of the strictest standards in the state due to the effluent-dependent nature of the receiving water. Key parameters include:

• Total Nitrogen: < 10 mg/L (Annual Average)
• Ammonia (NH3-N): Variable based on pH/Temp, typically < 2.0 mg/L
• E. coli: Non-detect in 4 out of 7 samples (Class A+ standard)
• Turbidity: < 2 NTU (24-hour average)

B. Compliance History

Since commissioning, Agua Nueva has maintained an exemplary compliance record. The facility was specifically designed to address the Notice of Violation (NOV) issues associated with the old Roger Road plant. The modern facility has consistently received Gold and Platinum awards from the National Association of Clean Water Agencies (NACWA) for peak performance.

C. Environmental Stewardship

The high-quality effluent from Agua Nueva is the primary water source for the Santa Cruz River Heritage Project. This initiative has successfully restored perennial flow to a stretch of the river through downtown Tucson, reviving native cottonwood-willow habitats and attracting diverse bird species, including the endangered Gila Topminnow.

7. CHALLENGES & FUTURE PLANNING

A. Emerging Contaminants (PFAS)

Like all wastewater facilities, Agua Nueva faces the challenge of Per- and Polyfluoroalkyl Substances (PFAS). While current regulations do not yet mandate removal at the WRF level in Arizona, Pima County is actively monitoring influent and effluent levels and participating in research regarding the fate of PFAS through the Bardenpho process.

B. Water Reuse Integration

Tucson Water (the potable water utility) utilizes the treated effluent for the Reclaimed Water System. Future planning involves potentially tighter integration between the WRF effluent and Advanced Water Purification (AWP) facilities for direct potable reuse (DPR) scenarios, should state regulations evolve to permit it.

C. Climate Resilience

Flash flooding in the Santa Cruz basin poses a risk to outfall structures. Future capital planning includes hardening the river bank and outfall infrastructure to withstand increasingly intense monsoon storm events associated with climate change.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ SECTION

Technical Questions

Q: Is Agua Nueva the same as the Roger Road plant?
A: Agua Nueva is the direct replacement for the Roger Road WRF. They are located on adjacent sites, but the Roger Road plant (trickling filters) was decommissioned and demolished upon the startup of Agua Nueva.

Q: Does the facility generate biogas?
A: No. Agua Nueva thickens sludge and sends it via pipeline to the Tres Rios WRF for anaerobic digestion. Biogas generation and utilization occur at Tres Rios.

Q: What is the hydraulic retention time (HRT)?
A: The HRT in the aeration basins is approximately 10-12 hours, depending on flow rates, to ensure complete nitrification and denitrification.

Public Interest Questions

Q: Does the plant smell?
A: Unlike the old facility, Agua Nueva utilizes advanced biotrickling filters and carbon scrubbers, resulting in negligible odors at the fence line.

Q: Is the water discharged into the river safe?
A: Yes. The water is Class A+ reclaimed water, meaning it is safe for incidental human contact (wading, fishing) and fully supports wildlife, though it is not treated for direct drinking.