Orange Water And Sewer Authority Mason Farm Wastewater Treatment Plant Chapel Hill

FACILITY BASIC INFORMATION

Plant Name: Mason Farm Wastewater Treatment Plant (WWTP)

Location: 170 Old Mason Farm Road, Chapel Hill, Orange County, North Carolina

Operating Authority: Orange Water and Sewer Authority (OWASA)

Design Capacity: 14.5 MGD (Million Gallons per Day)

Current Average Flow: ~7.5 – 8.5 MGD

Population Served: ~85,000 (Residents and Student Population)

Service Area: Town of Chapel Hill, Town of Carrboro, University of North Carolina at Chapel Hill

Receiving Water Body: Morgan Creek (tributary to Jordan Lake)

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NPDES Permit Number: NC0025241

Year Commissioned: Original construction late 1940s; Major expansions in 1970s, 2007, and 2017

1. INTRODUCTION

The Mason Farm Wastewater Treatment Plant (WWTP) serves as the cornerstone of wastewater infrastructure for the southern Orange County region, providing critical sanitation services to Chapel Hill, Carrboro, and the University of North Carolina (UNC) flagship campus. Operated by the Orange Water and Sewer Authority (OWASA), this 14.5 MGD facility is a prime example of biological nutrient removal (BNR) implementation in a highly regulated watershed.

Located immediately adjacent to the North Carolina Botanical Garden and discharging into the nutrient-sensitive Jordan Lake watershed, the Mason Farm facility operates under some of the strictest effluent limits in the Southeast United States. Beyond conventional treatment, the facility is distinguished by its pioneering Reclaimed Water (RCW) system, which recycles treated effluent for non-potable uses at UNC-Chapel Hill, and its commitment to energy neutrality through biogas cogeneration. The plant represents a model of sustainable utility management, balancing urban growth pressures with aggressive environmental stewardship.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The facility serves a unique demographic mix comprising the permanent residents of Chapel Hill and Carrboro, alongside the fluctuating student population of UNC-Chapel Hill. The collection system spans approximately 345 miles of gravity sewer mains and force mains, supported by 21 pump stations. The service area is characterized by a high variability in diurnal flow patterns typical of university towns, requiring robust hydraulic flexibility within the plant’s design.

B. Operational Capacity

While the plant is permitted for a hydraulic design capacity of 14.5 MGD, the current average daily flow typically ranges between 7.5 and 8.5 MGD. This places the facility at approximately 55% capacity utilization, providing significant buffer for peak wet weather flows and future regional growth. Historical flow trends have remained relatively flat despite population growth, a testament to OWASA’s aggressive inflow and infiltration (I/I) reduction programs and water conservation initiatives. The plant is designed to handle peak hourly flows exceeding 40 MGD during extreme storm events.

C. Discharge & Compliance

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Treated effluent is discharged into Morgan Creek, a tributary of B. Everett Jordan Lake. Because Jordan Lake is a regional water supply and recreational asset designated as “Nutrient Sensitive Waters” (NSW) by the North Carolina Department of Environmental Quality (NCDEQ), the Mason Farm WWTP must adhere to stringent nutrient limits. The facility consistently achieves high-level compliance, frequently earning the “Peak Performance Award” from the National Association of Clean Water Agencies (NACWA).

3. TREATMENT PROCESS

The Mason Farm WWTP utilizes a sophisticated treatment train designed to achieve Biological Nutrient Removal (BNR) to meet total nitrogen and phosphorus limits.

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through mechanical bar screens (6mm spacing) to remove large debris, rags, and plastics. Following screening, the flow enters vortex grit removal systems where sand, gravel, and heavy particulates settle out via centrifugal force. The screenings and grit are washed, compacted, and transported to a landfill. Odor control at the headworks is critical due to the facility’s proximity to public nature trails; this is managed via chemical scrubbers.

B. Primary Treatment

Flow proceeds to three primary clarifiers. These rectangular basins reduce the velocity of the wastewater, allowing settleable solids to drop to the bottom as primary sludge and floatables (grease/oils) to be skimmed from the surface. Approximately 60% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD) are removed at this stage. The primary sludge is pumped directly to the solids handling train.

C. Secondary Treatment (BNR)

The biological treatment stage is the heart of the facility, utilizing a modified activated sludge process designed for nutrient removal. The system consists of aeration basins configured for the Modified Ludzack-Ettinger (MLE) or similar BNR process configurations involving anaerobic, anoxic, and aerobic zones.

• Phosphorus Removal: Anaerobic zones encourage the growth of Phosphate Accumulating Organisms (PAOs), promoting biological phosphorus uptake. Ferric sulfate is added as a chemical backup to precipitate remaining phosphorus to meet limits as low as 0.5 mg/L or lower.
• Nitrogen Removal: Anoxic zones facilitate denitrification, converting nitrate to nitrogen gas. Internal recycle pumps return nitrate-rich mixed liquor from the aerobic zones to the anoxic zones.
• Clarification: Four circular secondary clarifiers separate the biomass from the treated water. The Return Activated Sludge (RAS) is sent back to the aeration basins, while Waste Activated Sludge (WAS) is pumped to thickening.

D. Tertiary Treatment

To ensure consistent compliance with low TSS and nutrient limits, secondary effluent passes through tertiary disk filters (cloth media filtration). These filters capture fine suspended solids that may escape the secondary clarifiers, significantly reducing the particulate-bound phosphorus and BOD in the final effluent.

E. Disinfection

The facility utilizes Ultraviolet (UV) Disinfection. The flow passes through channels containing banks of low-pressure, high-intensity UV lamps. This method disrupts the DNA of pathogens (bacteria and viruses) without introducing chemical byproducts into Morgan Creek. Following UV treatment, the water is aerated to raise Dissolved Oxygen (DO) levels before discharge.

F. Water Reclamation (Sidestream)

A portion of the secondary effluent is diverted to a specialized Water Reclamation Facility (WRF) located on-site. This system treats water to “Type 2” reclaimed water standards using additional filtration and high-level chlorination. This water is pumped to the UNC campus for cooling tower makeup water and irrigation, offsetting millions of gallons of potable water demand annually.

G. Solids Handling

Primary sludge and thickened WAS are blended and stabilized in anaerobic digesters. The digestion process is mesophilic (operated at approx. 98°F). The digested sludge is dewatered using high-performance centrifuges. The resulting Class B biosolids are transported off-site for beneficial reuse through land application on agricultural fields in Orange and surrounding counties.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site occupies a footprint bordered by sensitive ecological zones. The layout is compact, maximizing the use of existing hydraulic profiles. The administration building houses the SCADA central control room and a fully certified environmental laboratory capable of performing regulatory compliance testing for BOD, TSS, ammonia, and fecal coliform.

B. Energy Systems & Sustainability

Mason Farm is a leader in energy recovery. The anaerobic digesters produce biogas (methane), which is captured and treated. This gas fuels a Combined Heat and Power (CHP) system. The CHP unit generates electricity to power facility equipment and produces heat which is looped back to maintain the temperature of the digesters. This cogeneration system significantly reduces the plant’s reliance on the electrical grid and lowers its carbon footprint.

C. Odor Control

Given the proximity to the NC Botanical Garden and the Mason Farm Biological Reserve, odor control is a top priority. The facility employs covers on primary clarifiers and sludge holding tanks, extracting foul air to biotrickling filters and chemical scrubbers designed to remove hydrogen sulfide and mercaptans before releasing air to the atmosphere.

5. RECENT UPGRADES & MAJOR PROJECTS

Nutrient Reduction and Reliability Improvements (NRRI)

Timeline: Completed circa 2017
Project Scope: This comprehensive upgrade was driven by the need to meet tighter Jordan Lake nutrient rules and replace aging assets. Key components included:

• Installation of new tertiary disk filters to replace aging sand filters.
• Upgrades to the intermediate pump station.
• Modifications to aeration basins to optimize BNR performance.
• Replacement of the UV disinfection system with more energy-efficient technology.

Impact: The NRRI project ensured the facility could reliably meet Total Nitrogen (TN) limits of roughly 3.0 mg/L (seasonal/annual avg basis) and Total Phosphorus limits while reducing energy consumption per gallon treated.

Solids Handling & Digester Rehabilitation

Timeline: 2018-2020
Scope: Rehabilitation of the anaerobic digesters, including cover replacements and mixing system upgrades. Installation of new dewatering centrifuges to increase cake solids percentage.
Impact: Improved volatile solids reduction (VSR), increased biogas production for the CHP system, and reduced hauling costs due to drier biosolids cake.

Upcoming/Current Planning

OWASA is currently planning for the Mason Farm WWTP Bioclarifier Rehabilitation Project and ongoing headworks screening improvements. These projects are part of the long-term Capital Improvement Plan (CIP) to address asset age (some structures dating to the 1970s) and enhance resilience against climate-related wet weather events.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Operating under NPDES Permit NC0025241, the Mason Farm WWTP is subject to the Jordan Lake Nutrient Management Strategy.

• Ammonia Nitrogen (NH3-N): Seasonal limits, typically < 2.0 mg/L in summer.
• Total Phosphorus (TP): Target limits often require effluent < 0.5 mg/L or mass-loading caps.
• Total Nitrogen (TN): Annual mass loading limits that effectively restrict concentration to < 3-4 mg/L.
• TSS/BOD: Standard secondary limits (30 mg/L), though the plant typically achieves < 5 mg/L.

B. Compliance History

The facility maintains an exemplary compliance record. It has received the NACWA Platinum Award for Peak Performance, indicating consecutive years of 100% permit compliance without a single violation. This reliability is crucial given the low dilution ratio in Morgan Creek during dry months.

7. OPERATIONAL EXCELLENCE

Staffing: The facility is staffed by approximately 20-25 personnel, including NC Grade IV Biological Water Pollution Control System Operators, maintenance mechanics, laboratory analysts, and administrative staff.

Process Control: A robust SCADA system monitors thousands of data points, including Dissolved Oxygen (DO), Oxidation-Reduction Potential (ORP), and tank levels. The facility utilizes online instrumentation for ammonia and nitrate monitoring to automate blower output, optimizing energy usage for aeration.

8. CHALLENGES & FUTURE PLANNING

A. Emerging Contaminants (PFAS)

Like many utilities in the Cape Fear River Basin, OWASA is closely monitoring Per- and Polyfluoroalkyl Substances (PFAS). While Mason Farm is a discharger rather than a water source, future regulations may require wastewater plants to monitor or treat for these “forever chemicals” to protect downstream users.

B. Climate Resilience

Flash flooding in the Morgan Creek watershed poses a risk to hydraulic throughput. Future planning includes hardening electrical infrastructure and evaluating pump station capacities to handle increasingly intense precipitation events associated with climate change.

C. Biosolids Disposal

As regulations regarding land application of biosolids tighten and farmland availability decreases near urban centers, OWASA is evaluating long-term solids management strategies, potentially looking toward thermal drying or regional partnerships.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

Rogerson Drive Pump Station: The primary raw wastewater pumping station that conveys the majority of Chapel Hill’s flow to the Mason Farm headworks.

Jones Ferry Water Treatment Plant: OWASA’s drinking water facility. While separate, the management of the water cycle is integrated, with Mason Farm’s reclaimed water system offsetting demand on the Jones Ferry supply.

12. FAQ

Technical Questions

Q: Does Mason Farm WWTP perform biological phosphorus removal?
A: Yes, the facility utilizes anaerobic zones to promote Bio-P removal, supplemented by chemical addition (ferric salts) only when necessary for polishing.

Q: What is the destination of the reclaimed water?
A: The reclaimed water is pumped primarily to the UNC-Chapel Hill campus for use in chiller cooling towers and athletic field irrigation, as well as to the NC Botanical Garden.

Q: How is the facility powering its operations?
A: A significant portion of the plant’s energy demand is met through a Combined Heat and Power (CHP) system running on methane gas produced in the onsite anaerobic digesters.

Public Interest Questions

Q: Does the plant smell?
A: While wastewater treatment naturally produces odors, Mason Farm employs advanced chemical scrubbers and carbon filters to capture and neutralize odors, minimizing impact on the neighboring botanical gardens and trails.

Q: Can the public tour the facility?
A: OWASA frequently organizes tours for schools, university groups, and the general public. Interested parties should contact OWASA public affairs to schedule a visit.