Chattanooga Moccasin Bend Wastewater Treatment Plant

FACILITY BASIC INFORMATION

• Plant Name: Moccasin Bend Wastewater Treatment Plant (MBWWTP)
• Location: 455 Moccasin Bend Road, Chattanooga, Hamilton County, Tennessee
• Operating Authority: City of Chattanooga, Waste Resources Division
• Design Capacity: 140 MGD (Hydraulic); ~78 MGD (Secondary Biological)
• Current Average Flow: ~60-70 MGD
• Population Served: ~400,000 residents
• Service Area: 75+ square miles (Chattanooga, Red Bank, East Ridge, Lookout Mountain, Collegedale, and parts of North Georgia)
• Receiving Water Body: Tennessee River
• NPDES Permit Number: TN0024210
• Year Commissioned: 1961 (Major expansion 1970s)

1. INTRODUCTION

The Moccasin Bend Wastewater Treatment Plant (MBWWTP) serves as the cornerstone of the Interceptor Sewer System (ISS) for the greater Chattanooga region. As the largest wastewater treatment facility in Southeast Tennessee, it processes an average of 65 million gallons of wastewater daily for a regional population exceeding 400,000 across multiple municipalities and two states. Strategically located on the Moccasin Bend peninsula, the plant discharges treated effluent into the Tennessee River, a critical waterway for commerce, recreation, and aquatic biodiversity.

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Originally commissioned in 1961 and significantly expanded in the 1970s, MBWWTP is currently in the midst of a historic transformation driven by the “Clear Chattanooga” consent decree program. The facility is notable for its utilization of a High Purity Oxygen (HPO) activated sludge process—a distinct technological choice enabling high-rate treatment within a compact footprint. With recent capital investments exceeding $250 million dedicated to solids handling modernization, wet weather equalization, and reliability improvements, Moccasin Bend is transitioning into a modern resource recovery facility producing Class A EQ biosolids.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The MBWWTP is a regional hub, accepting flow not only from the City of Chattanooga but also from surrounding jurisdictions including Red Bank, East Ridge, Soddy-Daisy, Lookout Mountain, and portions of Walker and Catoosa Counties in North Georgia. The collection system feeding the plant is massive, comprising approximately 1,200 miles of sewer lines, roughly 85 pump stations, and several large interceptors that route gravity flow to the peninsula. The service area is characterized by a mix of residential zones and significant industrial corridors, necessitating a robust Industrial Pretreatment Program.

B. Operational Capacity

The facility operates with a permitted hydraulic design capacity of 140 MGD. However, the secondary biological treatment capacity is rated lower, historically around 78 MGD. During wet weather events, flows exceeding secondary capacity are often routed through auxiliary treatment trains or equalization basins to prevent biomass washout. The plant experiences significant peaking factors due to Infiltration and Inflow (I/I) in the older parts of the collection system, with peak instantaneous flows challenging the hydraulic grade line of the headworks.

C. Discharge & Compliance

Treated effluent is discharged via a submerged diffuser system into the Tennessee River (Nickajack Reservoir). The discharge is regulated under NPDES Permit TN0024210, enforced by the Tennessee Department of Environment and Conservation (TDEC). Compliance is critical due to the river’s designation for recreational use and domestic water supply downstream. The plant is currently operating under a Consent Decree (CD) entered with the EPA and TDEC to eliminate Sanitary Sewer Overflows (SSOs) and bypasses, driving much of the current operational strategy.

3. TREATMENT PROCESS

The Moccasin Bend WWTP utilizes a High Purity Oxygen (HPO) activated sludge process, a distinct engineering choice compared to conventional aeration. The treatment train is designed to handle high organic loading within a limited site footprint.

A. PRELIMINARY TREATMENT

Raw wastewater enters the plant via large interceptors where it undergoes preliminary treatment to protect downstream equipment.

• Screening: The headworks utilizes mechanical coarse bar screens (typically 1/2 to 3/4 inch spacing) to remove rags, plastics, and large debris.
• Grit Removal: Following screening, flow enters aerated grit chambers where velocity is reduced, allowing inorganic sands and gravel to settle while keeping organic solids in suspension.

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• Wet Weather Equalization: To manage peak flows, the plant utilizes large equalization (EQ) basins (recently expanded) to store excess influent during storm events, returning it to the head of the plant when capacity becomes available.

B. PRIMARY TREATMENT

Flow proceeds to rectangular primary clarifiers. These basins reduce the velocity of the wastewater to allow settleable solids to drop to the bottom as sludge, while fats, oils, and grease (FOG) float to the surface for skimming.

• Efficiency: Primary treatment typically removes 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).
• Chemical Enhancement: During high flow events, Chemically Enhanced Primary Treatment (CEPT) utilizing metal salts and polymers may be employed to increase settling velocity and removal rates.

C. SECONDARY TREATMENT (High Purity Oxygen)

This is the technological core of the MBWWTP. Unlike conventional plants that blow ambient air (21% oxygen) into basins, Moccasin Bend uses a High Purity Oxygen (HPO) system.

• Oxygen Generation: The facility maintains an on-site Cryogenic Oxygen Plant (and backup VSA systems) to generate ~90%+ pure oxygen.
• UNOX System: The biological reactors are covered, multi-stage tanks. Pure oxygen is injected into the headspace and mechanically mixed into the mixed liquor.
• Advantages: The high partial pressure of oxygen drives mass transfer, allowing the plant to maintain a higher Mixed Liquor Suspended Solids (MLSS) concentration and handle higher organic loading rates in a smaller volume than conventional air systems.
• Secondary Clarification: Mixed liquor flows to secondary clarifiers where the biological floc settles. Return Activated Sludge (RAS) is sent back to the HPO tanks, while clean supernatant overflows to disinfection.

D. DISINFECTION

Historically a chlorine gas facility, the plant has modernized its disinfection approach to ensure safety and compliance.

• Chlorination: Sodium hypochlorite (bleach) is used for pathogen inactivation.
• Dechlorination: Before discharge to the Tennessee River, sodium bisulfite is added to remove residual chlorine, preventing toxicity to aquatic life.

E. SOLIDS HANDLING (Biosolids)

The solids handling train has recently undergone a complete overhaul ($68M project).

• Thickening: Primary and Waste Activated Sludge (WAS) are thickened using gravity thickeners and centrifuges to reduce water content.
• Dewatering: High-performance centrifuges further dewater the sludge cake.
• Thermal Drying: The facility now utilizes an Andritz EcoDry thermal drying system. This drum drying process uses heat to evaporate remaining moisture, heating the biosolids to temperatures that kill pathogens.
• Final Product: The result is a Class A Exceptional Quality (EQ) dried pellet product, marketed as a fertilizer/soil amendment, diverting significant tonnage from landfills.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The MBWWTP occupies a significant footprint on the northern tip of the Moccasin Bend peninsula, an area of archaeological significance. The site layout is constrained by the river on three sides and the Moccasin Bend National Archeological District. The plant includes an Administration Building, extensive maintenance shops, the Cryogenic plant structure, and the massive new Thermal Drying facility which dominates the vertical profile of the site.

B. Energy Systems

The HPO process is energy-intensive due to the requirements of oxygen generation and mechanical mixing. The plant is a major power consumer, often exceeding 5 MW of demand. Energy efficiency initiatives have focused on Variable Frequency Drives (VFDs) for large pumps and upgrading the efficiency of the oxygen generation compressors. The new thermal drying facility uses natural gas but reduces the carbon footprint associated with hauling wet sludge to landfills.

C. Odor Control

Due to the plant’s proximity to downtown Chattanooga (directly across the river) and the North Shore residential areas, odor control is a high priority. The HPO process inherently reduces odors by covering aeration basins, preventing the stripping of volatile compounds into the atmosphere. Additionally, the headworks and solids handling facilities are equipped with chemical scrubbers and carbon absorption systems to treat foul air before release.

5. RECENT UPGRADES & MAJOR PROJECTS

Under the “Clear Chattanooga” consent decree, the city has executed a massive Capital Improvement Plan (CIP).

Current/Upcoming Projects (2024-2027): Future planning includes upgrades to the secondary clarifiers, further headworks screening improvements, and continued sewer rehabilitation upstream to reduce I/I, thereby reclaiming hydraulic capacity at the plant.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

MBWWTP operates under NPDES Permit TN0024210. Key effluent limitations typically include:

• CBOD5: Monthly average limits ~25 mg/L (seasonal variations apply).
• TSS: Monthly average ~30 mg/L.
• E. coli: Strict seasonal limits (usually 126 CFU/100ml monthly geometric mean) for recreational water protection.
• Dissolved Oxygen: Minimum effluent DO requirements to protect aquatic life.

B. Compliance History

The facility is central to the 2013 Federal Consent Decree regarding SSOs. While the plant generally meets effluent concentration limits for BOD and TSS, the primary regulatory challenge has been managing hydraulic volume during wet weather to prevent bypasses. The “Clear Chattanooga” program has successfully reduced the volume and frequency of overflows significantly since implementation.

7. OPERATIONAL EXCELLENCE

A. Staffing

The plant is staffed 24/7 by a team of licensed wastewater operators, industrial mechanics, electricians, and instrumentation specialists. Tennessee Grade IV Wastewater Treatment certification is required for shift supervisors due to the complexity of the HPO process. The City also maintains a robust laboratory on-site for process control and compliance testing.

B. Technology & Innovation

Chattanooga has integrated advanced SCADA (Supervisory Control and Data Acquisition) systems to monitor the complex variables of the Cryogenic plant and biological reactors. The shift to Thermal Drying places MBWWTP among a select group of facilities in the Southeast leveraging heat drying for beneficial reuse.

8. CHALLENGES & FUTURE PLANNING

A. Wet Weather Management

The primary engineering challenge remains Infiltration and Inflow (I/I). During heavy rains, the Tennessee River valley geology and aging clay pipes result in massive flow spikes. While plant capacity is 140 MGD, flows can surge rapidly. Future planning leans heavily on “sewer rehabilitation” rather than just plant expansion—fixing the pipes to keep rain out is more cost-effective than building larger tanks to treat rain.

B. Aging Infrastructure

While the thermal dryer is new, the concrete structures of the aeration basins and clarifiers date back to the 1970s. Concrete rehabilitation and mechanical equipment replacement (scrapers, drives, valves) are constant line items in the capital budget.

C. Nutrient Regulations

Like many dischargers to major river systems, Chattanooga monitors upcoming regulatory trends regarding nutrient limits (Total Nitrogen and Phosphorus). While not currently under strict nutrient caps compared to Chesapeake Bay facilities, the plant monitors these levels for future readiness.

9. COMMUNITY & REGIONAL IMPACT

The MBWWTP is vital to Chattanooga’s “Gig City” economy. By ensuring reliable wastewater capacity, the plant supports industrial growth, including major manufacturing facilities in the region. Environmentally, the plant’s performance is directly tied to the health of the Tennessee River, which hosts the Ironman Chattanooga triathlon and serves as a major recreational boating hub. The move to Class A biosolids also supports regional agriculture by providing a nutrient-rich, pathogen-free soil amendment.

10. TECHNICAL SPECIFICATIONS SUMMARY

12. FAQ

Technical Questions

1. What is the distinction between hydraulic capacity and biological capacity at MBWWTP?
The plant is hydraulically rated for 140 MGD, meaning pumps and pipes can move this volume. However, the secondary biological system (HPO basins) is rated for approximately 78 MGD. Flows exceeding this during wet weather must be managed via equalization or blending protocols as permitted.

2. Why does Moccasin Bend use High Purity Oxygen?
HPO was selected in the 1970s expansion to treat high organic loads in a small footprint. Pure oxygen allows for higher Mixed Liquor Suspended Solids (MLSS) concentrations and faster reaction rates than conventional air aeration.

3. How is the oxygen generated on-site?
The facility uses a Cryogenic Air Separation plant, which cools air to liquefaction temperatures to separate oxygen from nitrogen. Backup systems utilize Vacuum Swing Adsorption (VSA) technology.

4. Does the plant generate energy from biosolids?
Currently, the plant does not use anaerobic digestion for biogas generation (co-gen). Instead, it focuses on volume reduction and product creation via thermal drying using natural gas.

Public Interest Questions

5. Does the plant smell?
While wastewater treatment inherently generates odors, MBWWTP uses covered aeration basins (unique to the HPO process) and chemical scrubbers to treat foul air. Occasional odors may occur during maintenance or extreme weather, but significant investments have been made to minimize off-site impact.

6. Is the discharge safe for the Tennessee River?
Yes. The effluent is treated to meet strict EPA and TDEC standards. It is disinfected to kill bacteria and dechlorinated before release to ensure it is safe for aquatic life and downstream users.