City Of Raleigh Neuse River Resource Recovery Facility
1. INTRODUCTION
The Neuse River Resource Recovery Facility (NRRRF) stands as the cornerstone of wastewater infrastructure for the rapidly growing Research Triangle region of North Carolina. Operated by Raleigh Water, this facility is the largest wastewater treatment plant in the region, with a permitted capacity of 75 million gallons per day (MGD). Renamed from “Wastewater Treatment Plant” to “Resource Recovery Facility,” the site represents a paradigm shift in municipal utility management, viewing waste streams as opportunities for energy generation and nutrient recovery rather than simple disposal liabilities.
Discharging into the nutrient-sensitive Neuse River, the facility operates under some of the most stringent nitrogen and phosphorus limits in the southeastern United States. To meet these challenges, NRRRF employs advanced biological nutrient removal (BNR) and tertiary filtration. Most notably, the facility recently commissioned a landmark Bioenergy Recovery Project, incorporating thermal hydrolysis technology to produce renewable natural gas (RNG) and Class A biosolids, positioning Raleigh as a national leader in sustainability and circular economy infrastructure.
2. FACILITY OVERVIEW
A. Service Area & Coverage
The NRRRF serves the bulk of the City of Raleigh and six surrounding merged municipalities: Garner, Knightdale, Rolesville, Wake Forest, Wendell, and Zebulon. This consolidation, known as the “Merger Authorities,” allows for regionalized treatment efficiency. The service area covers a significant portion of Wake County, characterized by a mix of dense urban residential, rapid suburban expansion, and a growing biotechnology and industrial sector. The collection system feeding the plant includes over 2,500 miles of pipeline and over 100 lift stations.
B. Operational Capacity
Originally constructed in the late 1970s with a capacity of 30 MGD, the plant has undergone successive expansions to keep pace with the region’s population explosion. The current NPDES permit allows for 75 MGD, with hydraulic structures designed to handle peak wet weather flows significantly higher (approx. 225 MGD). While current average daily flows hover between 52 and 55 MGD, the facility is master-planned to expand to 90 MGD or greater in future phases to accommodate projected growth through 2040.
C. Discharge & Compliance
The facility discharges treated effluent into the Neuse River, a designated Nutrient Sensitive Water (NSW) which flows into the Albemarle-Pamlico Sound, the second-largest estuary in the United States. Due to historical fish kills and algae blooms in the estuary, the facility operates under a stringent Total Nitrogen (TN) allocation. The plant consistently achieves effluent quality well below permitted limits, often discharging water cleaner than the river itself. The facility has received the NACWA (National Association of Clean Water Agencies) Platinum Peak Performance Award for over 20 consecutive years of perfect compliance.
3. TREATMENT PROCESS
The NRRRF utilizes an advanced tertiary treatment train designed specifically for high-level nutrient removal. The process flow is categorized as follows:
A. Preliminary Treatment
Raw wastewater enters the headworks where it passes through coarse bar screens to remove large debris. Following coarse screening, the flow is lifted via influent pump stations to fine screens (typically 6mm perforated plate or similar) to protect downstream equipment. Grit removal is accomplished using vortex grit chambers, which use centrifugal force to separate heavy inorganic solids (sand, gravel) from the organic waste stream. This protects pumps and prevents volume loss in the bioreactors.
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). Floating grease and scum are skimmed from the surface. The primary sludge is pumped directly to the solids handling train (gravity thickeners), serving as a carbon-rich feedstock for anaerobic digestion.
C. Secondary Treatment (Biological Nutrient Removal)
The core of the treatment process is the Biological Nutrient Removal (BNR) system. The facility utilizes a multi-stage activated sludge process (typically a 5-stage Bardenpho or modified VIP configuration) to remove nitrogen and phosphorus biologically:
- Anaerobic Zone: Promotes the growth of Phosphate Accumulating Organisms (PAOs) for biological phosphorus removal.
- Anoxic Zone: Facilitates denitrification, where bacteria convert nitrates into harmless nitrogen gas, stripping nitrogen from the water.
- Aerobic Zone: Provides oxygen for nitrification (converting ammonia to nitrate) and BOD reduction.
Secondary clarification follows the aeration basins, separating the biomass from the treated water. Return Activated Sludge (RAS) is recycled to the front of the BNR basins, while Waste Activated Sludge (WAS) is sent to solids handling.
D. Tertiary Treatment
To meet strict effluent limits, secondary effluent undergoes tertiary filtration. The facility employs deep bed sand filters (denitrification filters). Methanol or an alternative carbon source can be added prior to filtration to drive further denitrification within the filter media, ensuring Total Nitrogen limits are met even during cold weather or high loading events.
E. Disinfection
Historically a chlorine gas facility, the NRRRF converted to Ultraviolet (UV) Disinfection. The UV system inactivates pathogens (bacteria and viruses) by disrupting their DNA without adding chemicals to the water, eliminating the formation of disinfection byproducts and the need for dechlorination chemicals (sulfur dioxide or bisulfite) prior to river discharge.
F. Solids Handling & Bioenergy Recovery
This is the most technologically advanced section of the facility following the recent Bioenergy Recovery Project (BRP):
- Thickening: Primary sludge is gravity thickened; WAS is thickened using Rotary Drum Thickeners (RDT).
- Thermal Hydrolysis Process (THP): The facility utilizes the Cambi THP system. Sludge is heated under pressure (approx. 165°C/6 bar) prior to digestion. This lyses cells, reduces viscosity, and increases volatile solids reduction.
- Anaerobic Digestion: Four mesophilic digesters process the hydrolyzed sludge. Because of THP, the digesters operate at double the conventional loading rate.
- Dewatering: Digested sludge is dewatered using high-speed centrifuges.
- Product: The result is a Class A “Exceptional Quality” biosolid that is pathogen-free and safe for unrestricted land application.
4. RECENT UPGRADES & MAJOR PROJECTS
Neuse River Bioenergy Recovery Project – $150+ Million (2018-2023)
- Project Scope: Construction of a new solids handling facility centered around Thermal Hydrolysis Process (THP), new anaerobic digesters, and gas cleaning/upgrading equipment.
- Funding Sources: A combination of low-interest loans from the State Revolving Fund (SRF) and revenue bonds.
- Key Contractors:
- Design: Hazen and Sawyer / Black & Veatch (Joint Venture)
- Construction Manager at Risk: Crowder Construction
- THP Supplier: Cambi
- Technical Highlights: This project made NRRRF one of only a handful of facilities in the U.S. using CAMBI THP. It included a biogas upgrading system to convert methane into Renewable Natural Gas (RNG).
- Results Achieved:
- Reduction of biosolids volume by roughly 50%, significantly cutting hauling costs.
- Production of Class A biosolids for public distribution.
- Generation of RNG injected into the PSNC Energy pipeline, creating a new revenue stream and offsetting the plant’s carbon footprint.
75 MGD Plant Expansion – (Ongoing/Phased)
- Project Scope: Hydraulic expansion to increase permitted capacity from 60 MGD to 75 MGD.
- Key Elements: Upgrades to influent pumps, additional aeration basin capacity, and filter complex enhancements to manage peak wet weather flows.
- Drivers: Rapid population growth in Eastern Wake County requiring increased hydraulic headroom.
5. INFRASTRUCTURE & ENERGY
Physical Plant
The site spans several hundred acres, incorporating extensive buffer zones to minimize impact on neighbors. The Administration and Operations Building is a LEED Silver certified structure, housing a state-of-the-art SCADA control room and an environmental laboratory certified for compliance testing.
Energy Systems & Sustainability
The NRRRF is a net energy producer regarding its solids processing. The biogas generated (approx. 600-800 scfm) is scrubbed of impurities (H2S, CO2, siloxanes) and injected into the natural gas grid as Renewable Natural Gas (RNG). This allows the City to claim renewable energy credits (RINs). Additionally, the facility utilizes extensive Variable Frequency Drives (VFDs) on large blowers and pumps to optimize electrical consumption.
Odor Control
Odor control is a critical component of the BRP. The facility utilizes centralized odor control systems, primarily bio-trickling filters and activated carbon scrubbers, treating foul air from the headworks, primary clarifiers, and solids handling building. The closed-loop nature of the THP process also significantly reduces odors compared to conventional digestion.
6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE
Permit Requirements
Operating under NPDES Permit NC0029033, the facility faces limits largely driven by the Neuse River Basin Nutrient Management Strategy.
- Total Nitrogen (TN): Subject to an annual mass loading limit. The facility typically targets effluent TN < 3.0 mg/L.
- Total Phosphorus (TP): Target effluent < 1.0 mg/L (often achieving < 0.5 mg/L).
- Toxicity: Routine Whole Effluent Toxicity (WET) testing required.
Compliance History
Raleigh Water maintains an exemplary compliance record. The NRRRF is a recipient of the NACWA Platinum Peak Performance Award, recognizing facilities with 100% compliance with NPDES permits over a period of five consecutive years or more. The facility serves as a model for nutrient management in estuaries.
7. TECHNICAL SPECIFICATIONS SUMMARY
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Tertiary Treatment with BNR |
| Design Capacity | 75 MGD |
| Peak Hydraulic Capacity | ~225 MGD |
| Secondary Treatment | Activated Sludge (BNR – 5-stage Bardenpho type) |
| Filtration | Deep Bed Sand Filters (Denitrifying) |
| Disinfection | Ultraviolet (UV) Irradiation |
| Solids Processing | Cambi Thermal Hydrolysis + Mesophilic Anaerobic Digestion |
| Biosolids Class | Class A (Exceptional Quality) |
| Energy Recovery | RNG Injection to Grid (Biogas Upgrading) |
| Service Population | ~600,000 |
| Receiving Water | Neuse River (Nutrient Sensitive Water) |
| Operating Authority | City of Raleigh Public Utilities (Raleigh Water) |
8. FAQ
1. What is the difference between Class A and Class B biosolids at this facility?
Prior to the Bioenergy Recovery Project, the plant produced Class B biosolids, which have strict restrictions on land application. With the new Thermal Hydrolysis process, the plant now produces Class A biosolids. These are pathogen-free and can be distributed to the public, used for landscaping, or applied to agricultural land without site access restrictions.
2. Does the Neuse River facility produce its own electricity?
While the facility has backup diesel generators for emergencies, its primary energy focus is producing Renewable Natural Gas (RNG). Rather than burning gas in engines to make electricity on-site (Cogeneration), Raleigh Water determined it was more economically and environmentally beneficial to clean the gas and inject it into the commercial natural gas pipeline for use as vehicle fuel or heating.
3. How does the plant remove Nitrogen?
Nitrogen is removed biologically. Bacteria in the “anoxic” zones of the treatment tanks use the oxygen attached to Nitrate (NO3) molecules to breathe, releasing pure Nitrogen gas (N2) into the atmosphere. The deep bed sand filters provide a final “polishing” step to ensure limits are met.
4. Is the plant affected by PFAS regulations?
Like all major utilities in the US, Raleigh Water is monitoring emerging regulations regarding PFAS (forever chemicals). The City is actively sampling influent and effluent and participating in state-level studies to understand the prevalence of PFAS in the Neuse River basin.
5. Can the public tour the facility?
Yes, Raleigh Water offers tours of the Neuse River Resource Recovery Facility for educational groups, universities, and professional organizations. These must be scheduled in advance through the Raleigh Water public education coordinator.