Mount Pleasant Waterworks And Sewer Commission Wastewater Treatment Plant

1. INTRODUCTION

The Center Street Wastewater Treatment Plant (CSWWTP) serves as the flagship treatment facility for Mount Pleasant Waterworks (MPW), a utility managing water and wastewater resources for one of South Carolina’s fastest-growing municipalities. Located strategically near the Charleston Harbor, the facility operates under strict environmental mandates to protect the sensitive estuarine ecosystem of the Lowcountry.

Commissioned originally to handle lower flows, the plant has undergone significant phased expansions to reach its current design capacity of 9.2 million gallons per day (MGD). MPW distinguishes itself through the implementation of advanced biological nutrient removal (BNR) processes and a forward-thinking solids handling program that produces Class A Exceptional Quality (EQ) biosolids. As the Town of Mount Pleasant faces the dual pressures of rapid urbanization and climate resilience, the Center Street facility represents a critical intersection of civil engineering infrastructure and environmental stewardship.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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The facility serves the Town of Mount Pleasant, located in Charleston County, east of the Cooper River. The service area is characterized by low-lying coastal topography, requiring an extensive network of collection infrastructure. The system supports a primarily residential customer base, alongside a growing commercial sector and light industry. The collection system feeding the plant includes over 170 pump stations, necessitated by the flat terrain and high water table typical of the South Carolina coast.

B. Operational Capacity

The Center Street WWTP is permitted for a flow of 9.2 MGD. Historically, the plant operates at approximately 60-65% of its hydraulic capacity, treating an average of 5.5 to 6.0 MGD. However, the facility is designed to handle significant peak hydraulic flows associated with wet weather events, a critical design factor given the region’s susceptibility to tropical storms and king tides. Future planning anticipates flow increases correlating with regional population projections, which predict continued growth in the East Cooper corridor.

C. Discharge & Compliance

Effluent is discharged into the Charleston Harbor via a submerged outfall. The harbor is a nutrient-sensitive water body, subject to Total Maximum Daily Loads (TMDLs) regarding dissolved oxygen and nutrient enrichment. Consequently, the plant operates under a stringent National Pollutant Discharge Elimination System (NPDES) permit issued by the South Carolina Department of Health and Environmental Control (SC DHEC). The facility consistently achieves high compliance rates, particularly regarding ammonia-nitrogen and biochemical oxygen demand (BOD) limits.

3. TREATMENT PROCESS

The Center Street WWTP utilizes an advanced activated sludge process configured for biological nutrient removal. The treatment train is designed to reduce organics, solids, and nutrients prior to disinfection and discharge.

A. Preliminary Treatment

Raw wastewater enters the headworks facility where it undergoes physical screening.

• Screening: Mechanically cleaned bar screens remove large debris, rags, and non-flushables to protect downstream pumping and aeration equipment.
• Grit Removal: A vortex grit removal system separates inorganic solids (sand, gravel, coffee grounds) through centrifugal force.
• Odor Control: The headworks is enclosed and ventilated to chemical scrubbers and/or biological filters to mitigate odors, a critical requirement given the plant’s proximity to residential zones.

B. Secondary Treatment (Biological)

The core of the treatment process utilizes an A2/O (Anaerobic/Anoxic/Oxic) configuration or similar phased oxidation ditch technology to achieve Biological Nutrient Removal (BNR).

• Anaerobic Zone: Selectors promote the growth of phosphorus-accumulating organisms (PAOs) to facilitate biological phosphorus removal.

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• Anoxic Zone: Nitrate-rich internal recycle flows mix with influent carbon sources to achieve denitrification, converting nitrates into nitrogen gas.
• Aeration (Oxic) Zone: Fine bubble diffusion systems provide oxygen for BOD oxidation and nitrification (conversion of ammonia to nitrate).
• Secondary Clarification: Large circular clarifiers allow biological floc to settle. Return Activated Sludge (RAS) is pumped back to the bioreactors, while Waste Activated Sludge (WAS) is diverted to solids handling.

C. Tertiary Treatment

To meet stringent effluent limits for suspended solids and phosphorus, the plant utilizes tertiary filtration.

• Filtration: Deep bed sand filters or cloth media filters polish the effluent, reducing turbidity and particulate-bound nutrients to very low levels.

D. Disinfection

Following filtration, the effluent undergoes disinfection to neutralize pathogens.

• UV Disinfection: The facility utilizes high-intensity Ultraviolet (UV) light banks. This method eliminates the need for chlorination/dechlorination chemicals, enhancing safety and preventing the formation of disinfection byproducts (DBPs) in the receiving water.

E. Solids Handling (Biosolids)

MPW has invested heavily in advanced solids processing to reduce disposal volume and create a beneficial product.

• Thickening/Dewatering: WAS is thickened and dewatered using centrifuges to increase solids content.
• Thermal Drying: The facility operates a thermal drying system. This process evaporates water from the dewatered cake, producing a pelletized, pathogen-free product.
• Class A Biosolids: The final product meets EPA Class A Exceptional Quality (EQ) standards, allowing it to be distributed as a fertilizer or soil amendment rather than landfilled.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Center Street site is compact and bounded by marshland and residential development, presenting site layout challenges. The architecture emphasizes low profiles and visual screening. Critical infrastructure is elevated to protect against storm surge and tidal flooding, reflecting modern resilient design standards.

B. Energy Systems

Wastewater treatment is energy-intensive. The facility utilizes Variable Frequency Drives (VFDs) on major pumps and aeration blowers to match energy consumption with biological demand. The thermal dryer typically utilizes natural gas, with heat recovery loops to maximize thermal efficiency.

C. Odor Control

Odor control is a paramount operational objective at Center Street due to the encroaching urban density. The facility employs a multi-stage approach, including covering primary odor sources (headworks, sludge holding) and treating foul air through biotowers and activated carbon polishing units. Real-time odor monitoring helps operators adjust chemical dosing and ventilation rates proactively.

5. RECENT UPGRADES & MAJOR PROJECTS

Center Street WWTP Expansion & Upgrade (Phase 3)

• Project Scope: Increased capacity to 9.2 MGD, upgraded aeration basins for enhanced nutrient removal, and installed new secondary clarifiers.
• Project Budget: ~$35 – $40 Million (Phased)
• Funding Sources: SC State Revolving Fund (SRF) loans and Revenue Bonds.
• Technical Highlights: Implementation of advanced SCADA controls for dissolved oxygen optimization and nitrate recirculation.
• Results: Secured capacity for 20-year planning horizon and improved total nitrogen removal efficiency.

Biosolids Dryer Project

• Project Scope: Installation of a thermal drying facility to replace lime stabilization or hauling of wet cake.
• Drivers: Rising costs of landfill disposal, regulatory pressure on land application of Class B solids, and sustainability goals.
• Technical Highlights: Indirect thermal dryer capable of producing >90% dry solids pellets.
• Results: Reduction in hauling trucks by over 70%, creation of a marketable fertilizer product, and significant reduction in carbon footprint associated with transport.

Upcoming: Collection System Rehabilitation (Ongoing)

• Scope: Lining of gravity mains (CIPP) and rehabilitation of manholes to reduce Inflow and Infiltration (I&I).
• Target: Reducing peak wet weather flows arriving at the Center Street plant to preserve hydraulic capacity and prevent sanitary sewer overflows (SSOs).

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under NPDES Permit SC0021598. Key parameters include:

• BOD5: Monthly average limits typically < 10 mg/L.
• TSS: Monthly average limits typically < 15 mg/L.
• Ammonia-Nitrogen: Seasonally variable, often < 2.0 mg/L in summer.
• Dissolved Oxygen: Minimum effluent concentration required to protect harbor aquatic life.
• Fecal Coliform: Strict limits maintained via UV disinfection.

B. Compliance History

Mount Pleasant Waterworks maintains a strong record of compliance with SC DHEC regulations. The utility has received multiple “Peak Performance Awards” from the National Association of Clean Water Agencies (NACWA) for consecutive years of permit compliance without violations.

7. OPERATIONAL EXCELLENCE

MPW employs a highly credentialed staff, with lead operators holding “A” level biological wastewater certification in South Carolina. The utility emphasizes continuous training and safety.

Technology & Innovation: The plant utilizes a comprehensive SCADA system that provides real-time data on flows, tank levels, dissolved oxygen, and equipment status. This allows for remote monitoring and automated response to storm events. The laboratory is certified for immediate process control analysis, ensuring operators can make adjustments to the biological system within hours of sampling.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• Sea Level Rise & Flooding: As a coastal facility, the plant faces risks from king tides and storm surges. Saltwater intrusion into the collection system can upset biological processes.
• Aging Infrastructure: While the plant has been expanded, components of the older phases and the collection system require ongoing asset management and rehabilitation.
• Workforce: Like many utilities, MPW faces the “silver tsunami” of retiring senior operators, necessitating aggressive recruitment and apprenticeship programs.

B. Future Planning

MPW’s Master Plan focuses on resilience and “One Water” management. Future considerations include:

• Hardening of pump stations and electrical switchgear against flooding.
• Evaluation of water reuse opportunities (purple pipe) for irrigation to offset potable water demand.
• Continued aggressive I&I reduction to reclaim hydraulic capacity.

9. COMMUNITY & REGIONAL IMPACT

The Center Street WWTP is a cornerstone of economic development in Mount Pleasant. By ensuring reliable sewer capacity, the facility supports the region’s robust real estate and tourism sectors. The utility engages the public through the “Water is Life” educational campaigns and maintains transparency regarding rate structures and capital projects. Environmental stewardship efforts at the plant directly contribute to the water quality of Charleston Harbor, supporting the local shrimping, fishing, and recreation industries.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

• Rifle Range Road WWTP: A smaller, secondary facility within the MPW system (approx. 2-3 MGD capacity) that serves specific basins within the service area. It works in conjunction with Center Street to manage regional flows.
• Collection Network: A massive network of over 170 pump stations and hundreds of miles of gravity and force main piping.

12. FAQ SECTION

Technical Questions

1. What is the current treatment capacity of the Center Street WWTP?
The plant is permitted for a design capacity of 9.2 MGD.

2. Does the facility utilize chemical addition for phosphorus removal?
The plant is designed for Biological Phosphorus Removal (Bio-P), but maintains chemical feed systems (alum or ferric) as backup or for polishing to meet strict effluent limits.

3. How are biosolids handled at the site?
Biosolids are thickened, dewatered via centrifuge, and processed in a thermal dryer to create Class A pelletized fertilizer.

4. Is the plant capable of water reuse?
While the plant produces high-quality effluent, it primarily discharges to the harbor. However, future master planning includes evaluations for water reuse infrastructure.

Public Interest Questions

5. Does the treatment plant smell?
MPW utilizes extensive odor control technologies, including chemical scrubbers and biofilters, to capture and treat air from the plant. Occasional odors may occur during maintenance or extreme weather, but they are actively managed.

6. Where does the water go after it is cleaned?
The treated water (effluent) is discharged through a deep-water outfall into the Charleston Harbor, where it safely mixes with the tide.

7. Who runs the facility?
The facility is operated by Mount Pleasant Waterworks, a commission of public works established to manage the town’s water and wastewater utility.

8. Is the plant prepared for hurricanes?
Yes. The facility has backup power generation to run critical systems during grid outages and has been hardened to withstand significant storm events.