City Of Mesa Northwest Water Reclamation Plant

TARGET AUDIENCE

• Municipal consulting engineers evaluating advanced BNR facilities
• Wastewater treatment plant operators and asset managers
• Regulatory compliance officers (ADEQ/EPA)
• Engineering firms pursuing capital improvement projects in arid environments
• Water resource planners interested in water exchange and recharge credits

1. INTRODUCTION

The Northwest Water Reclamation Plant (NWWRP) is a cornerstone of the City of Mesa’s centralized water management strategy. As an 18-MGD advanced water reclamation facility, it serves the densely populated northwest corridor of Mesa, Arizona. Operated by the City of Mesa Water Resources Department, the facility is distinct in its operational philosophy: it functions not merely as a disposal mechanism, but as a production factory for Class A+ reclaimed water, which is vital for the region’s long-term water portfolio.

Following a major expansion completed in 2018, the NWWRP utilizes advanced Biological Nutrient Removal (BNR) and tertiary filtration to meet strict Aquifer Protection Permit (APP) standards. The facility plays a critical role in the city’s unique water exchange agreement with the Gila River Indian Community (GRIC), where high-quality effluent is delivered for agricultural use in exchange for potable water credits, demonstrating a model of sustainable water resource management in the arid Southwest.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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The NWWRP services the northwest quadrant of Mesa, an area characterized by mature residential neighborhoods, significant commercial retail centers (including Mesa Riverview), and the spring training facilities for the Chicago Cubs. The collection system feeding the plant relies on a network of gravity mains and strategic lift stations designed to divert flow away from the joint-owned Greenfield Water Reclamation Plant, thereby balancing the hydraulic load across the city’s infrastructure.

B. Operational Capacity

The facility currently operates with a rated liquid stream capacity of 18.0 Million Gallons per Day (MGD).

• Design Capacity: 18.0 MGD
• Peak Hydraulic Capacity: ~36.0 MGD (Peak Hour)
• Solids Handling Capacity: Designed to accommodate regional solids loading

Historically, the plant operated at an 8 MGD capacity until the Phase III expansion (completed circa 2018) more than doubled its throughput to accommodate infill growth and regional diversion strategies. The site master plan allows for an ultimate build-out capacity exceeding 30 MGD should future density require it.

C. Discharge & Reuse Strategy

Unlike facilities discharging into flowing rivers, the NWWRP operates under a “Zero Liquid Discharge” philosophy regarding surface waters of the US. The treated effluent is a valuable commodity utilized in three primary ways:

1. GRIC Exchange: Piped to the Gila River Indian Community for irrigation, generating Long-Term Storage Credits (LTSCs) for Mesa.
2. Aquifer Recharge: Discharged to the Northwest Recharge Project (recharge basins) to replenish the East Salt River Valley sub-basin aquifer.
3. Direct Reuse: Used for landscape irrigation at local parks (Riverview Park) and the Sloan Park stadium complex.

3. TREATMENT PROCESS

The NWWRP employs a multi-stage advanced treatment train designed to achieve Class A+ Reclaimed Water standards, specifically targeting nitrogen reduction and low turbidity.

A. Preliminary Treatment

Raw wastewater enters the headworks where it undergoes physical separation. The system utilizes automated mechanical bar screens to remove large debris, followed by vortex grit removal systems to extract inorganic sands and gravel. This stage is critical for protecting downstream pumps and protecting the integrity of the biosolids.

B. Secondary Treatment (Biological Nutrient Removal)

The core of the liquid stream treatment is the activated sludge process utilizing Biological Nutrient Removal (BNR). The aeration basins are configured to facilitate nitrification and denitrification.

• Configuration: Modified Ludzack-Ettinger (MLE) or A2O process configurations are typical for this region to maximize nitrogen removal.
• Aeration: Fine bubble retained media diffusers provide oxygen transfer, controlled by DO sensors to optimize blower energy consumption.
• Clarification: The mixed liquor flows to secondary clarifiers (circular configuration) where solids settle via gravity. A portion of the sludge is returned (RAS) to the anoxic zones to maintain biomass, while excess sludge (WAS) is wasted to solids handling.

C. Tertiary Treatment

To meet Arizona’s Class A+ standards (turbidity < 2 NTU average), the secondary effluent undergoes tertiary filtration. The facility utilizes cloth media disk filters or similar deep-bed filtration technologies. This stage ensures the removal of fine suspended solids and any remaining particulate-bound pathogens.

D. Disinfection

Disinfection is achieved via Ultraviolet (UV) Irradiation. UV is the preferred method for recharge applications as it avoids the formation of Trihalomethanes (THMs) and other disinfection byproducts associated with chlorination. The UV system is designed with redundancy to ensure pathogen inactivation even during peak flow events.

E. Solids Handling

The NWWRP serves as a regional solids processing hub.

• Thickening: Waste Activated Sludge (WAS) is thickened using Dissolved Air Flotation (DAF) or rotary drum thickeners prior to digestion.
• Digestion: The facility employs mesophilic anaerobic digesters. These digesters reduce volatile solids, stabilizing the sludge and producing methane gas.
• Dewatering: Digested sludge is dewatered using high-performance centrifuges, producing a cake typically in the 20-25% solids range.
• Disposal: The dewatered Class B biosolids are hauled off-site for land application on non-food crops (cotton/alfalfa) or to landfills.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site is compactly designed within an urban interface. Major structures include the headworks building, BNR basins, secondary clarifiers, a tertiary filter complex, and a comprehensive solids handling building. The architecture utilizes block walls and landscaping buffers to blend with the surrounding commercial/residential district.

B. Odor Control

Given the plant’s proximity to the sophisticated Mesa Riverview shopping district and Sloan Park, odor control is a “mission-critical” operation. The facility utilizes a multi-tiered approach:

• Headworks & Primary: Covered channels and basins with foul air extraction.
• Treatment: Biotrickling filters and chemical scrubbers (wet scrubbers using hypochlorite/caustic) treat hydrogen sulfide and organic odors before releasing air to the atmosphere.

C. Energy Systems

The plant is a significant energy consumer, primarily due to aeration blowers. Energy efficiency measures include VFDs on major pumps and turbo blowers for aeration. Biogas produced in the anaerobic digesters is captured and currently flared, though feasibility studies for Cogeneration (Combined Heat and Power) are periodically reviewed to offset grid dependency.

5. RECENT UPGRADES & MAJOR PROJECTS

NWWRP Phase III Expansion (Completed ~2018)

Project Value: Approx. $100 – $130 Million

Project Scope:
This massive Capital Improvement Project (CIP) was required to meet the projected growth of Northwest Mesa and provide redundancy for the City’s wastewater collection system.

Key Contractors:

• Construction Manager at Risk (CMAR): McCarthy Building Companies
• Design Engineer: Brown and Caldwell

Technical Highlights:

• Capacity Increase: Expanded liquid stream capacity from 8 MGD to 18 MGD.
• New Infrastructure: Construction of new aeration basins, secondary clarifiers, and a new solids handling facility.
• Process Improvements: Installation of new cloth media filters for tertiary treatment and upgrades to the UV disinfection system.
• Odor Control: Significant expansion of odor control scrubbers to protect the neighboring Chicago Cubs spring training facility.

Results Achieved: The project was delivered on time and allowed the City to divert flows that were previously overwhelming the sanitary sewer lines leading to the Greenfield plant. It secured the water exchange capabilities with the Native American communities by ensuring consistent Class A+ effluent production.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The NWWRP operates under strict oversight from the Arizona Department of Environmental Quality (ADEQ).

• Aquifer Protection Permit (APP): Regulates the discharge to the vadose zone to ensure groundwater quality is protected. Key limits include Total Nitrogen (< 10 mg/L) and limits on enteric viruses/bacteria.
• AZPDES: While primarily a zero-discharge facility to surface waters, permits are maintained for emergency operational flexibility.
• Reclaimed Water Permit: Mandates Class A+ quality (Turbidity < 2 NTU; Fecal Coliform: None detectable in 4 of 7 samples).

B. Compliance History

The facility maintains a high level of compliance. The plant has received Gold and Silver awards from the Arizona Water Association for operational safety and compliance. Regular reporting ensures transparency regarding nitrogen loading to the aquifer.

7. CHALLENGES & FUTURE PLANNING

A. Current Challenges

Urban Encroachment: As the Riverview area continues to develop with high-density housing and retail, the buffer zone around the plant shrinks, making odor control and noise mitigation increasingly difficult and expensive.

Aging Infrastructure: While the Phase III expansion is new, the original 1990s components of the plant (Phase I/II infrastructure) require ongoing rehabilitation and asset management.

B. Future Planning

The City of Mesa’s Master Plan anticipates potential future expansion of the NWWRP to an ultimate capacity of 30-36 MGD. This timeline is dictated by the rate of infill densification in Northwest Mesa. Future capital projects will likely focus on:

• Biogas-to-energy conversion systems.
• Advanced water purification pilot testing (Direct Potable Reuse readiness).
• Replacement of aging electrical switchgear.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. RELATED FACILITIES

The NWWRP functions as part of an integrated regional system:

• Greenfield Water Reclamation Plant: A large regional facility (jointly owned by Mesa, Gilbert, and Queen Creek) that handles the majority of Mesa’s flow.
• Southeast Water Reclamation Plant: Serves the fast-growing southeast corridor near the Phoenix-Mesa Gateway Airport.
• Northwest Recharge Project: The physical basins located near the plant where treated effluent is infiltrated into the aquifer.

10. FAQ SECTION

Technical Questions

Q: What is the specific nutrient removal target for NWWRP?
A: The facility targets Total Nitrogen levels below 10 mg/L to comply with Aquifer Protection Permit standards and prevent groundwater nitrate contamination.

Q: Does NWWRP generate energy from biosolids?
A: Currently, the facility stabilizes solids via anaerobic digestion and flares the resulting biogas. Cogeneration is not currently the primary energy source but is studied for future phases.

Q: Is the plant capable of Direct Potable Reuse (DPR)?
A: Not currently. While the effluent is high quality (Class A+), it would require additional treatment (RO/AOP) to meet potable standards. However, Mesa is a partner in regional “One Water” initiatives exploring this for the future.

Public Interest Questions

Q: Does the plant smell?
A: The plant utilizes advanced chemical scrubbers and biofilters to scrub air from the treatment process. While occasional odors may occur during maintenance, the system is designed to contain odors within the fence line.

Q: Where does the water go?
A: 100% of the water is reused. It is either sent to the Gila River Indian Community for agriculture, used to water local parks, or put back underground (recharged) to save for future use.