Metropolitan Nashville Central Wastewater Treatment Plant

1. INTRODUCTION

The Central Wastewater Treatment Plant (CWWTP) is the cornerstone of the Metropolitan Government of Nashville and Davidson County’s wastewater infrastructure. As the largest of three wastewater treatment facilities operated by Metro Water Services (MWS), CWWTP handles the majority of the region’s sanitary flow, including the critical combined sewer system (CSS) serving the historic downtown core. The facility is situated on the banks of the Cumberland River, directly north of the central business district.

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Operating under the “Clean Water Nashville” overflow abatement program, the facility has recently undergone historic transformation. With a peak hydraulic capacity of nearly 440 MGD, the plant utilizes advanced headworks, optimized aeration strategies, and a state-of-the-art Ultraviolet (UV) disinfection system. Most notably, the facility is currently commissioning a landmark Biosolids Master Plan project featuring thermal hydrolysis technology, positioning Nashville as a national leader in energy-neutral wastewater treatment and sustainable resource recovery.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The CWWTP serves the highly urbanized core of Nashville, covering approximately 85 square miles of the Metro Water Services service area. This includes the entire Combined Sewer System (CSS) zone, which spans approximately 15 square miles of downtown Nashville. The collection system feeding CWWTP is a complex network of gravity mains and major force mains, heavily influenced by the region’s topography and limestone geology. The facility supports a diverse demographic mix, handling flows from dense residential neighborhoods, the central business district, major sports venues (Nissan Stadium), and significant industrial corridors.

B. Operational Capacity

Designed to handle the extreme flow variations characteristic of combined sewer systems, the CWWTP demonstrates significant hydraulic flexibility.

• Average Daily Flow: Approximately 135 MGD.
• Peak Hydraulic Capacity: 440 MGD (following recent optimization).
• Wet Weather Operations: During heavy rainfall events, the plant operates in conjunction with upstream equalization basins and detention facilities to manage surges from the combined system, aiming to minimize Combined Sewer Overflows (CSOs) into the Cumberland River.

C. Discharge & Compliance

Treated effluent is discharged into the Cumberland River via a submerged multi-port diffuser outfall. The facility operates under National Pollutant Discharge Elimination System (NPDES) Permit No. TN0020575, administered by the Tennessee Department of Environment and Conservation (TDEC). The plant is also a focal point of a Consent Decree between Metro Nashville, the EPA, and the Department of Justice, which mandates specific milestones for eliminating sanitary sewer overflows and reducing combined sewer overflows.

3. TREATMENT PROCESS

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The Central Wastewater Treatment Plant utilizes a conventional activated sludge process that has been heavily modified for wet-weather flow management and energy efficiency. The treatment train is designed to ensure compliance with strict effluent limits for BOD, TSS, and E. coli.

A. PRELIMINARY TREATMENT (Headworks)

Due to the combined sewer system, the headworks is robustly designed to handle large debris and variable flows.

• Screening: The facility utilizes multiple mechanically cleaned coarse bar screens followed by fine screens to protect downstream equipment. Recent upgrades included the installation of heavy-duty multi-rake screens to handle “first flush” debris.
• Grit Removal: Vortex grit removal systems separate inorganic solids (sand, gravel, cinders) which are washed and dewatered before landfill disposal. Efficient grit removal is critical here to protect the thermal hydrolysis pumps in the solids train.

B. PRIMARY TREATMENT

Flow enters rectangular primary clarifiers where gravity settling removes approximately 60-70% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).

• Configuration: Multiple batteries of rectangular tanks with chain-and-flight sludge collectors.
• Scum Removal: Automated scum skimmers remove floating grease and oils, which are pumped to the digester complex.
• Chemically Enhanced Primary Treatment (CEPT): During peak wet weather events, the facility has the capability to utilize polymer or coagulant addition to increase settling velocity and removal rates, protecting the secondary biology from washout.

C. SECONDARY TREATMENT

The biological heart of the plant is an Activated Sludge system configured for carbonaceous BOD removal.

• Aeration Basins: The plant utilizes deep-tank aeration basins equipped with fine-bubble diffusion systems.
• Optimization: A major “Optimization Project” replaced aging coarse bubble diffusers with high-efficiency fine bubble membranes and installed turbo blowers. This significantly reduced energy consumption and improved oxygen transfer efficiency (OTE).
• Secondary Clarifiers: Mixed liquor flows to a battery of circular secondary clarifiers. Rapid sludge removal mechanisms ensure return activated sludge (RAS) is fresh, preventing denitrification in the clarifiers which could lead to rising sludge.

D. DISINFECTION

In a major shift from historical chlorination (gas/bleach), the CWWTP converted to Ultraviolet (UV) Disinfection.

• Technology: High-intensity, low-pressure UV lamp banks arranged in open channels.
• Capacity: The UV system is sized to treat the full peak hydraulic flow of 440 MGD.
• Benefits: Elimination of chlorine byproducts (THMs/HAAs) in the effluent and removal of hazardous bulk chemical storage (chlorine gas/hypochlorite) from the site, improving safety for the surrounding urban developments.

E. SOLIDS HANDLING (Advanced)

The solids handling facility is the most technologically advanced portion of the plant, currently finalizing a transition to Thermal Hydrolysis.

• Thickening: Gravity belt thickeners (GBT) and centrifuges concentrate primary and waste activated sludge.
• Thermal Hydrolysis Process (THP): The plant utilizes the CAMBI THP system. This process “pressure cooks” the sludge at high temperature and pressure before digestion.
• Anaerobic Digestion: THP-pretreated sludge is fed into high-rate anaerobic digesters. The THP step increases volatile solids reduction (VSR), maximizes biogas production, and creates Class A (pathogen-free) biosolids.
• Dewatering: High-solids centrifuges dewater the digested sludge. The resulting Class A cake is suitable for unrestricted land application or fertilizer use.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant & Resilience

The site occupies significant acreage along the Cumberland River. Following the catastrophic May 2010 floods, where the plant was inundated, MWS invested heavily in flood protection. A comprehensive flood wall system now protects the facility to an elevation exceeding the 500-year flood level plus freeboard. Critical electrical switchgear and motor control centers (MCCs) have been elevated or hardened.

B. Energy Systems & CHP

The CWWTP is designed to be an energy recovery facility.

• Biogas Utilization: The enhanced biogas production from the THP/Digester system fuels a Combined Heat and Power (CHP) cogeneration facility.
• Energy Loop: Waste heat from the CHP engines generates steam required for the Thermal Hydrolysis process, creating a closed-loop energy cycle. The electricity generated offsets a significant portion of the plant’s grid demand.

C. Odor Control

Given the plant’s location near the Germantown residential district and the Nissan Stadium, odor control is paramount. The facility utilizes a combination of biological scrubbers (biotowers) and chemical scrubbers (carbon adsorption) covering the headworks, primary clarifiers, and solids handling complex. The entire biosolids building is maintained under negative pressure.

5. RECENT UPGRADES & MAJOR PROJECTS

Biosolids Master Plan & Heat Recovery Project (2018-2024)

• Project Budget: ~$400 Million (Est.)
• Scope: Construction of a new solids processing complex featuring CAMBI Thermal Hydrolysis, new digesters, CHP cogeneration units, and a biosolids drying facility.
• Drivers: Need for Class A Biosolids, landfill diversion, and energy neutrality goals.
• Contractors: Design by Brown and Caldwell; Construction management involving heavy civil firms like Kiewit.
• Technical Highlight: This is one of the largest THP installations in North America, significantly reducing sludge volume and eliminating pathogen concerns.

Central WWTP Capacity Improvements & CSS Abatement

• Project Budget: Part of the multi-billion dollar Clean Water Nashville program.
• Scope: Hydraulic optimization of the headworks, modifications to the secondary clarifier flow distribution, and aeration upgrades.
• Results: Increased peak wet weather treatment capacity to 440 MGD, directly reducing the volume of untreated overflow discharging to the Cumberland River during storm events.

Headworks & Odor Control Improvements

• Project Scope: Replacement of aging bar screens with fine screening technology and installation of comprehensive odor control covers and scrubbers.
• Timeline: Completed in phases over the last 5-7 years.
• Impact: Dramatically reduced odor complaints from the rapidly gentrifying surrounding neighborhoods.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

Consent Decree Status

Metro Nashville entered into a Consent Decree with the U.S. EPA and the State of Tennessee in 2009. The “Clean Water Nashville” program was established to fulfill these requirements. The CWWTP upgrades are the central component of the plan to reduce Combined Sewer Overflows (CSOs). The facility has consistently met milestone deadlines for capacity expansion.

Effluent Quality

The plant consistently achieves high compliance rates for its NPDES parameters.

• CBOD5: Consistently < 10 mg/L (Monthly Avg Limit is typically 25-30 mg/L).
• TSS: Consistently < 15 mg/L.
• Disinfection: The switch to UV has ensured consistent compliance with E. coli limits without the risk of aquatic toxicity from residual chlorine.

7. CHALLENGES & FUTURE PLANNING

Growth & Densification

Nashville is one of the fastest-growing cities in the United States. The CWWTP faces the dual challenge of managing hydraulic capacity for a growing population while restricted by a fixed physical footprint in a dense urban area. Future planning relies heavily on process intensification (like THP and potentially MBR or IFAS in the future) rather than physical expansion.

Nutrient Regulations

While Tennessee has historically focused on organic removal, anticipated future regulations regarding Total Nitrogen (TN) and Total Phosphorus (TP) for the Cumberland River basin may require the retrofitting of the aeration basins for Biological Nutrient Removal (BNR), likely necessitating Bardenpho or MLE configurations.

8. TECHNICAL SPECIFICATIONS SUMMARY

9. FAQ SECTION

Technical & Engineering Questions

1. What is the peak hydraulic capacity of the Central WWTP?
Following recent hydraulic optimizations under the Clean Water Nashville program, the plant has a peak wet weather capacity of 440 MGD.

2. How does the plant manage biosolids?
The plant utilizes Thermal Hydrolysis Process (THP) technology prior to anaerobic digestion. This produces Class A biosolids and significantly increases biogas production for energy recovery.

3. Does the facility utilize chemical disinfection?
No. The facility has transitioned from chlorine gas to Ultraviolet (UV) disinfection to improve safety and eliminate disinfection byproducts.

4. Is the Central WWTP under a Consent Decree?
Yes. The facility operates under a federal Consent Decree aimed at reducing Combined Sewer Overflows (CSOs) and Sanitary Sewer Overflows (SSOs). This drives many of the current capital improvement projects.

Public Interest Questions

5. Does the plant smell?
While wastewater treatment naturally generates odors, MWS has installed extensive odor control systems, including covered headworks and biological scrubbers, to minimize impact on the surrounding downtown and Germantown neighborhoods.

6. Is the treated water safe for the river?
Yes. The effluent discharged into the Cumberland River meets or exceeds all state and federal water quality standards, protecting aquatic life and downstream users.

7. What happens to the “waste” solids?
Through the new biosolids facility, waste is converted into a sterilized, nutrient-rich soil amendment (Class A Biosolids) that can be used as fertilizer, keeping it out of landfills.