City Of Columbia Wastewater Treatment Plant

1. INTRODUCTION

The Metropolitan Wastewater Treatment Plant (Metro WWTP) serves as the cornerstone of wastewater infrastructure for the Midlands region of South Carolina. Operated by Columbia Water, this regional facility treats wastewater for approximately 60 wastewater customers spanning the City of Columbia and portions of Richland and Lexington Counties. With a permitted design capacity of 60 MGD, Metro WWTP stands as one of the largest treatment facilities in the state.

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Since the launch of the “Clean Water 2020” initiative—a comprehensive program driven by an EPA Consent Decree to upgrade the city’s sanitary sewer system—the Metro plant has seen significant capital investment aimed at modernization, energy efficiency, and flood resilience. Located strategically near the confluence of the Saluda and Broad Rivers where they form the Congaree, the plant plays a pivotal role in protecting the water quality of the Congaree River, a vital natural resource for the region. Through the implementation of Combined Heat and Power (CHP) technology and advanced solids handling, Metro WWTP is transitioning from a traditional waste disposal facility to a resource recovery center.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Metro WWTP services a sprawling, geographically diverse area encompassing approximately 300 square miles. The collection system feeding the plant is massive, consisting of over 1,000 miles of gravity sewer lines, 50+ miles of force mains, and approximately 60 pump stations. The demographics served include a mix of dense urban residential zones, the University of South Carolina campus, state government complexes, and rapidly growing suburban commercial corridors in the Midlands.

B. Operational Capacity

The facility is permitted for a hydraulic capacity of 60 MGD. Current average daily flows typically range between 32 and 38 MGD, indicating a capacity utilization of approximately 60%, providing ample headroom for projected regional growth over the next decade. The plant is designed to handle significant peak wet weather flows, a critical necessity given the region’s susceptibility to tropical storms and heavy rainfall events. Historical trends show a steady increase in flow consistent with the urbanization of Richland County.

C. Discharge & Compliance

Treated effluent is discharged into the Congaree River. The discharge is regulated under NPDES Permit No. SC0020940, administered by the South Carolina Department of Health and Environmental Control (SCDHEC). The Congaree River has specific Total Maximum Daily Load (TMDL) concerns regarding dissolved oxygen and phosphorus, placing strict operational requirements on the facility’s nutrient reduction and oxygenation capabilities prior to discharge.

3. TREATMENT PROCESS

The Metro WWTP utilizes a conventional activated sludge process with anaerobic digestion for solids handling. The liquid treatment train is designed for high-efficiency removal of BOD and TSS.

A. PRELIMINARY TREATMENT

Influent wastewater enters the headworks facility where it undergoes mechanical screening to remove large debris, rags, and plastics.

• Screening: Mechanically cleaned bar screens remove coarse solids to protect downstream pumps.
• Grit Removal: Following screening, flow enters aerated grit chambers where velocity is controlled to allow inorganic sands and grit to settle while keeping organic material in suspension.

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• Flow Monitoring: Parshall flumes are utilized for influent flow metering, integrated into the plant’s SCADA system for real-time pacing of chemical feeds.

B. PRIMARY TREATMENT

Flow proceeds to the primary clarification stage. The plant utilizes multiple rectangular primary clarifiers. These basins reduce the velocity of the wastewater, allowing settleable solids to drop to the bottom as primary sludge, while oil and grease are skimmed from the surface.

• Removal Efficiency: Typically removes 50-60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).
• Sludge Handling: Primary sludge is pumped directly to the gravity thickeners before digestion.

C. SECONDARY TREATMENT

The biological core of the Metro WWTP is the activated sludge process.

• Aeration Basins: The plant utilizes large rectangular aeration basins equipped with fine-bubble diffused aeration systems. Blowers provide the necessary oxygen to maintain an aerobic environment for microorganisms to consume organic matter.
• Process Control: Operators maintain specific Dissolved Oxygen (DO) setpoints and Mixed Liquor Suspended Solids (MLSS) concentrations to optimize biological activity.
• Secondary Clarification: Mixed liquor flows to circular secondary clarifiers where biological floc settles. The clear supernatant overflows the weirs as secondary effluent.
• RAS/WAS: A portion of the settled sludge is returned to the aeration basins (Return Activated Sludge) to maintain the biological population, while excess growth is wasted (Waste Activated Sludge) to the solids handling train.

D. DISINFECTION

Historically utilizing chlorine gas, the facility has undergone upgrades to safer liquid chemical alternatives.

• Chlorination: Sodium hypochlorite is dosed into the chlorine contact chambers to destroy pathogenic bacteria and viruses.
• Dechlorination: Prior to discharge into the Congaree River, sodium bisulfite is added to remove residual chlorine, preventing toxicity to aquatic life in the receiving water body.
• Post-Aeration: If necessary to meet permit limits for Dissolved Oxygen, the effluent undergoes post-aeration before final discharge.

E. SOLIDS HANDLING

The Metro WWTP is notable for its robust solids handling and energy recovery systems.

• Thickening: Primary sludge is gravity thickened, while Waste Activated Sludge (WAS) is thickened via Dissolved Air Flotation (DAF) or rotary drum thickeners.
• Anaerobic Digestion: Thickened sludge is stabilized in mesophilic anaerobic digesters. This process reduces volatile solids, destroys pathogens, and produces methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-performance centrifuges to produce a cake with 20-25% solids content.
• Disposal: The resulting Class B biosolids are beneficially reused through land application on permitted agricultural fields, recycling nutrients back to the soil.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Metro WWTP occupies a large industrial footprint along the riverbank. Key structures include the Administration Building (housing SCADA control and engineering offices), the Maintenance Complex, and a state-certified water quality laboratory. The site is protected by an extensive flood protection berm system, which was tested and subsequently reinforced following the historic 1,000-year flood event of October 2015.

B. Energy Systems & Cogeneration

A standout feature of the Metro WWTP is its commitment to energy neutrality. The facility operates a Combined Heat and Power (CHP) system.

• Biogas Utilization: Methane gas generated in the anaerobic digesters is captured and scrubbed.
• Cogeneration: This gas fuels engine generators to produce electricity that offsets the plant’s grid consumption.
• Heat Recovery: Waste heat from the engines is captured to heat the anaerobic digesters, maintaining the mesophilic temperature range (95-98°F) required for optimal digestion, creating a closed-loop energy cycle.

5. RECENT UPGRADES & MAJOR PROJECTS

Under the banner of Clean Water 2020, the City of Columbia has invested hundreds of millions into the wastewater system. While much of this funding targets the collection system (rehabilitation of pipes and pump stations to reduce SSOs), the Metro WWTP has seen specific, high-value upgrades.

Metro WWTP Digester Complex Improvements – $20+ Million (Completed approx. 2019)

• Project Scope: Comprehensive rehabilitation of the anaerobic digesters, including new mixing systems, cover replacements, and gas handling equipment.
• Drivers: Aging infrastructure and the desire to maximize biogas production for the CHP system.
• Technical Highlights: Installation of linear motion mixers and membrane gas holders to optimize volatile solids reduction and gas storage.
• Results: Increased biogas generation reliability and improved solids stabilization.

Headworks and Grit Removal Upgrades (2018-2021)

• Project Scope: Replacement of aging mechanical bar screens and rehabilitation of grit removal mechanisms.
• Drivers: Protection of downstream process equipment and improved removal of inorganic solids.
• Funding: Financed through Revenue Bonds and State Revolving Funds (SRF).

Disinfection System Conversion

• Project Scope: Conversion from gaseous chlorine and sulfur dioxide to liquid sodium hypochlorite and sodium bisulfite.
• Drivers: Safety concerns regarding the storage of hazardous gaseous chemicals in a populated area.
• Result: Elimination of Risk Management Plan (RMP) requirements associated with chlorine gas and improved operator safety.

6. REGULATORY COMPLIANCE

A. Permit Requirements

The facility operates under NPDES Permit SC0020940. Key parameters include:

• BOD5: Monthly average limits typically < 30 mg/L (often stricter seasonally).
• TSS: Monthly average limits typically < 30 mg/L.
• Ammonia-Nitrogen: Seasonally varied limits to prevent toxicity in the Congaree River.
• Fecal Coliform: < 200/100mL monthly average.
• Whole Effluent Toxicity (WET): Chronic toxicity testing required.

B. Clean Water 2020 Consent Decree

The City of Columbia entered into a Consent Decree with the EPA and SCDHEC to address Sanitary Sewer Overflows (SSOs). While this largely impacts the collection system, the Metro WWTP plays a critical role in this compliance strategy by ensuring it has the reliability and hydraulic capacity to treat peak wet weather flows without bypassing treatment stages.

7. OPERATIONAL EXCELLENCE

Metro WWTP is staffed 24/7 by a team of over 50 professionals, including SC-certified Biological Wastewater Operators (A-level to Trainee), industrial maintenance mechanics, and laboratory technicians. The facility utilizes a robust SCADA system for automated control of aeration blowers, return sludge rates, and chemical dosing. The on-site laboratory is DHEC-certified for varying parameters, allowing for rapid process control decisions without waiting for external lab results.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• Wet Weather Flows: Inflow and Infiltration (I/I) in the older parts of the collection system can cause rapid spikes in influent flow during tropical storms.
• Fats, Oils, and Grease (FOG): A significant challenge in the collection system, requiring aggressive public education and grease trap inspection programs.

B. Future Planning

Columbia Water continually updates its Wastewater Master Plan. Future considerations for the Metro WWTP include:

• Nutrient Removal: Potential future regulatory tightening on Total Phosphorus and Total Nitrogen may require the retrofitting of BNR (Biological Nutrient Removal) zones within the existing aeration basins.
• Capacity Expansion: While currently at ~60% capacity, long-term growth projections for the Midlands suggest a need to evaluate expansion to 75 MGD or beyond in the coming decades.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical/Professional Questions

1. What is the treatment capacity of the Metro WWTP?
The plant has a permitted design capacity of 60 Million Gallons per Day (MGD).

2. Does the facility utilize Biological Nutrient Removal (BNR)?
The plant is currently configured for conventional activated sludge. While it achieves some incidental nutrient removal, it is not a dedicated BNR facility (e.g., A2O or Bardenpho). However, ammonia limits are met through nitrification in the aeration basins.

3. How are biosolids managed?
Solids are anaerobically digested, dewatered via centrifuges, and land-applied as Class B biosolids on permitted agricultural sites.

4. What energy sustainability measures are in place?
Metro WWTP utilizes a Cogeneration (CHP) system that captures methane from the digesters to generate electricity and heat for the facility.

Public Interest Questions

5. How many people does the Metro Plant serve?
It serves approximately 375,000 customers throughout Columbia, Richland County, and Lexington County.

6. Does the plant smell?
Odor control is a priority. The plant uses chemical scrubbers and covers on odor-generating processes (like the headworks and sludge holding tanks) to minimize impact on the surrounding community.

7. Is the water released into the river safe?
Yes. The treated effluent is tested daily and must meet strict standards set by SCDHEC to ensure it is safe for aquatic life in the Congaree River.