City Of Tempe Water Reclamation Facility

1. INTRODUCTION

The Tempe Water Reclamation Plant (TWRP) serves as the cornerstone of wastewater management and water resource recovery for the City of Tempe, Arizona. Situated in an arid desert environment, the facility transcends the traditional definition of a wastewater treatment plant, functioning as a critical production facility for reclaimed water—a vital resource for the region’s economic and ecological stability. Operated by the City of Tempe Municipal Utilities Department, the plant treats an average of 36.5 million gallons daily (MGD) with a peak design capacity of 52 MGD.

TWRP is distinct among Southwestern facilities for its integral role in maintaining Tempe Town Lake, an engineered urban lake that serves as a focal point for regional economic development and flood control. The facility utilizes advanced Biological Nutrient Removal (BNR) processes to produce Class A+ reclaimed water, meeting strict Arizona Department of Environmental Quality (ADEQ) standards. With a robust bio-energy program that co-digests fats, oils, and grease (FOG) to generate renewable power, TWRP represents a model of sustainable infrastructure, balancing hydraulic capacity with resource recovery.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The TWRP serves the entire City of Tempe and the neighboring Town of Guadalupe, covering a service area of approximately 40 square miles. The collection system serving the plant comprises over 530 miles of sewer lines and 16 lift stations. The demographics of the service area are dense and mixed-use, including the campus of Arizona State University, significant industrial corridors, and mature residential neighborhoods. This results in a diurnal flow pattern heavily influenced by university schedules and commercial activity.

B. Operational Capacity

While originally commissioned with a significantly lower capacity in the 1960s, successive expansions have brought the TWRP to a rated design capacity of 52 MGD.

• Design Capacity: 52.0 MGD
• Average Daily Flow: 36.5 MGD
• Peak Hydraulic Capacity: ~80 MGD

Flow trends have stabilized in recent years despite population growth, attributed to aggressive water conservation mandates and low-flow fixture retrofits within the service area. The plant currently operates at approximately 70% of its hydraulic design capacity, providing a safety buffer for future vertical growth within the landlocked city limits.

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C. Discharge & Compliance

In the arid Southwest, discharge is synonymous with reuse. TWRP operates under an Arizona Pollutant Discharge Elimination System (AZPDES) permit. The effluent management strategy is tripartite:

1. Tempe Town Lake: A significant portion of effluent is discharged into the Salt River channel to maintain water levels in Tempe Town Lake.
2. Direct Reuse: Water is piped to local golf courses, parks, and industrial users (Class A+ Reclaimed Water).
3. Aquifer Recharge: Excess flow is diverted to recharge basins to replenish the groundwater table.

The facility consistently achieves compliance with stringent nutrient limits, particularly for Total Nitrogen (TN), which is critical to preventing eutrophication in the Town Lake.

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

Influent wastewater enters the headworks where it undergoes aggressive physical screening to protect downstream equipment. The facility utilizes automated mechanical bar screens (typically 6mm to 10mm spacing) to remove rags and large debris. Following screening, flow passes through vortex grit removal systems where inorganic solids (sand, coffee grounds, eggshells) settle out via centrifugal force. The grit is washed, dewatered, and disposed of at a landfill. High-capacity odor control scrubbers (chemical wet scrubbers and carbon adsorption) continuously treat foul air extracted from the headworks building.

B. PRIMARY TREATMENT

Flow proceeds to primary clarifiers (sedimentation tanks). TWRP utilizes rectangular clarifiers equipped with chain-and-flight sludge collectors. These tanks reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge, while grease and floatables are skimmed from the surface. This stage typically removes 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD), significantly reducing the organic load on the biological stage.

C. SECONDARY TREATMENT (Biological Nutrient Removal)

The core of TWRP’s treatment is the activated sludge process configured for Biological Nutrient Removal (BNR).

• Process Configuration: The plant employs a modified process (often variations of the Modified Ludzack-Ettinger or MLE process) involving anoxic and aerobic zones.
• Nitrification/Denitrification: In the aerobic zones, ammonia is converted to nitrate (nitrification). Internal recycle pumps return nitrate-rich mixed liquor to the anoxic zones, where bacteria strip the oxygen from the nitrate molecules, releasing nitrogen gas (denitrification). This is essential for meeting the TN limit of < 10 mg/L required for release into the Salt River channel.
• Aeration: Fine bubble diffusion systems provide oxygen transfer efficiency, driven by high-efficiency turbo blowers.
• Secondary Clarification: Mixed liquor flows to secondary clarifiers where biological floc settles. Return Activated Sludge (RAS) is pumped back to the aeration basins, while Waste Activated Sludge (WAS) is sent to solids handling.

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D. TERTIARY TREATMENT

To achieve Class A+ reclaimed water standards, secondary effluent undergoes tertiary filtration. The facility utilizes deep-bed denitrifying sand filters or cloth media disk filters (depending on the specific treatment train active). These filters polish the water, reducing turbidity to < 2 NTU, ensuring the effectiveness of downstream disinfection.

E. DISINFECTION

Unlike many facilities switching solely to UV, TWRP maintains robust chlorination capabilities due to the specific pathogen requirements for body-contact recreation in Tempe Town Lake.

• Method: Chlorination using sodium hypochlorite.
• Dechlorination: Before discharge to the Salt River/Town Lake ecosystem, the water is dechlorinated using sodium bisulfite to prevent toxicity to aquatic life.

F. SOLIDS HANDLING & BIOGAS

TWRP is a leader in resource recovery through its solids handling train:

• Thickening: WAS is thickened using Dissolved Air Flotation (DAF) or rotary drum thickeners.
• Anaerobic Digestion: Primary sludge and thickened WAS are stabilized in mesophilic anaerobic digesters.
• FOG Co-Digestion: The plant accepts Fats, Oils, and Grease (FOG) from local restaurants directly into the digesters. This high-strength organic waste boosts biogas production significantly.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges, producing a cake of approximately 20-25% solids.
• Disposal: Biosolids are hauled for land application (agriculture) or landfill cover.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The TWRP occupies a substantial footprint north of Rio Salado Parkway. The site layout is constrained by the Salt River bed to the north and urban development to the south. The architecture is strictly industrial, dominated by the digester complexes and aeration basins. Recent additions include modernized administration and SCADA control centers.

B. Energy Systems & Co-Generation

One of TWRP’s defining technical features is its energy independence strategy.

• Combined Heat and Power (CHP): The facility utilizes internal combustion engines fueled by methane gas produced in the anaerobic digesters.
• Energy Output: These generators provide a significant portion (often 30-40%) of the plant’s electrical demand.
• Heat Recovery: Waste heat from the engines is captured to maintain optimal temperatures in the anaerobic digesters, creating a closed-loop energy cycle.

C. Odor Control

Given the plant’s proximity to the “Tempe Marketplace” retail center and the Loop 202 freeway, odor control is paramount. The facility employs a multi-stage approach including biotrickling filters for H2S removal at the headworks and chemical scrubbers at the solids handling complex. The entire headworks and primary clarifier areas are covered to capture and treat fugitive emissions.

5. RECENT UPGRADES & MAJOR PROJECTS

The City of Tempe utilizes a robust Capital Improvement Program (CIP) to maintain TWRP. Below are key recent and ongoing projects.

Solids Handling Rehabilitation (2020-2023)

Project Scope: Replacement of aging centrifuges and rehabilitation of digester covers. The project also included improvements to the sludge transfer pumps and polymer injection systems.
Drivers: Aging infrastructure (end of useful life) and the need for drier cake solids to reduce hauling costs.
Status: Completed.

Plant Interceptor Rehabilitation (2022-2025)

Budget: ~$15 Million (Est.)
Scope: Cured-In-Place Pipe (CIPP) lining and rehabilitation of the major interceptors entering the plant. While not “inside” the fence line, this is critical for plant influent integrity.
Drivers: Prevention of inflow/infiltration and protection against hydrogen sulfide corrosion in concrete pipes.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under AZPDES Permit No. AZ0020478. Key effluent limitations typically include:

• Total Nitrogen: < 10 mg/L (Annual Average)
• E. coli: Non-detect in 4 of 7 weekly samples (for Class A+)
• Turbidity: < 2 NTU (24-hour average)
• BOD/TSS: < 10 mg/L (30-day average for reuse applications)

B. Compliance History

TWRP maintains an exemplary compliance record with ADEQ. The facility has received multiple Gold and Silver awards from the Arizona Water Association for safety and operational excellence. Occasional challenges have arisen regarding foaming in aeration basins (nocardia) or minor excursions during heavy monsoon events, but no significant consent decrees are currently active against the facility.

7. OPERATIONAL EXCELLENCE

A. Staffing

The plant is staffed 24/7 by a team of approximately 40-50 personnel, including operations, maintenance, instrumentation/control technicians, and laboratory staff. Lead operators are generally required to hold ADEQ Grade 4 Wastewater Treatment certifications, the highest level in the state.

B. Technology & Innovation

Tempe has pioneered the “Grease Cooperative” model. Rather than penalizing restaurants for grease trap issues, the utility facilitates the collection of grease to fuel the plant’s digesters. This circular economy approach is studied by municipalities nationwide as a method to turn a waste liability into an energy asset.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

PFAS Management: Like all wastewater utilities, TWRP is monitoring emerging regulations regarding Per- and Polyfluoroalkyl Substances (PFAS). While current technologies capture some precursors, future regulations may require advanced treatment (RO or GAC) which would necessitate massive capital investment.

Salinity: As water is reused and cycled through the system, Total Dissolved Solids (TDS) increase. High salinity in reclaimed water can be problematic for golf course turf and cooling towers.

B. Future Planning

The City’s Integrated Water Resources Management Plan focuses on “One Water.” Future plans involve maximizing the recharge credits obtained from TWRP effluent. There are long-term discussions about potential Direct Potable Reuse (DPR) capabilities, although this would likely involve a satellite advanced purification facility rather than a retrofit of the existing TWRP secondary process.

9. COMMUNITY & REGIONAL IMPACT

The economic impact of TWRP is uniquely visible. Without the consistent 8-10 MGD flow of high-quality treated effluent, Tempe Town Lake would evaporate. The lake generates an estimated $1.5 billion in regional economic impact through development, tourism, and events. Therefore, the reliability of TWRP is directly tied to the financial health of the city’s commercial downtown.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

Kyrene Water Reclamation Facility: A satellite facility located in south Tempe. It is capable of treating approximately 4-9 MGD but has often been placed in standby mode or operated at lower flows depending on regional water demands and the specific needs of the Kyrene Generating Station (which uses the effluent for cooling).

12. FAQ SECTION

Technical/Professional Questions

1. What is the specific BNR configuration at TWRP?
TWRP generally utilizes a Modified Ludzack-Ettinger (MLE) process configuration, utilizing internal recycle rates to achieve denitrification in anoxic zones prior to aerobic zones.

2. Does TWRP generate its own power?
Yes. Through the co-digestion of sewage sludge and fats, oils, and grease (FOG), the plant produces biogas which fuels internal combustion engines, offsetting roughly 30-40% of the plant’s electrical load.

3. What are the Total Nitrogen discharge limits?
The facility generally aims for a Total Nitrogen (TN) limit of < 10 mg/L to comply with AZPDES requirements and maintain water quality in the Town Lake.

4. How are biosolids disposed of?
Biosolids are processed to Class B standards (typically) via anaerobic digestion and centrifuge dewatering, then hauled for land application on non-food crop agricultural land.

Public Interest Questions

5. Is the water in Tempe Town Lake safe to touch?
Yes. The water entering the lake is treated to Class A+ standards, meaning it is safe for incidental contact (kayaking, paddleboarding), though swimming is generally restricted for safety/management reasons rather than water quality.

6. Does the plant smell?
While wastewater treatment inherently involves odors, TWRP uses advanced chemical scrubbers and biofilters to scrub air from the headworks and solids buildings. Occasional odors may occur during atmospheric inversions or maintenance events.

7. Where does the water go after treatment?
It is recycled three ways: filling Tempe Town Lake, irrigating local golf courses/parks, or recharged into the underground aquifer for future use.