Durham Water Management North Durham Water Reclamation Facility

The North Durham Water Reclamation Facility (NDWRF) serves as a critical infrastructure asset for the City of Durham, North Carolina, operating within one of the most environmentally sensitive watersheds in the state. With a permitted design capacity of 20 million gallons per day (MGD), the facility provides wastewater treatment for residential, commercial, and industrial users in the northern tier of Durham County. Operated by the City of Durham Department of Water Management, the plant is distinguished by its deployment of advanced Bio-Denipho phased isolation ditch technology to meet stringent nutrient limits required for the protection of Falls Lake, a primary drinking water reservoir for the Raleigh-Durham metropolitan area.

Commissioned in its current major configuration in 1994 and subsequently expanded, the NDWRF represents a model of regulatory compliance in a nutrient-limited environment. The facility plays a pivotal role in the region’s hydrological cycle, treating wastewater to near-drinking water standards before discharging into Ellerbe Creek. Through continuous capital improvement and process optimization, the plant ensures the protection of public health while supporting the rapid population growth of the Research Triangle region.

Facility Overview

A. Service Area & Coverage

The NDWRF services the northern portion of the City of Durham, a high-growth area characterized by a mix of dense urban residential zones, commercial corridors along I-85 and US-70, and significant industrial sectors. The collection system feeding the plant includes major interceptors tracking the Ellerbe Creek basin. As Durham acts as a hub for medicine and research, the influent profile includes domestic wastewater mixed with specific commercial discharges, requiring robust industrial pretreatment monitoring.

B. Operational Capacity

• Design Capacity: 20.0 MGD
• Current Average Daily Flow: Approximately 10–12 MGD
• Peak Hydraulic Capacity: ~40 MGD

While currently operating at approximately 50-60% of its hydraulic design capacity, the biological loading is carefully managed to optimize the nutrient removal processes. The facility is designed to handle significant wet-weather flow variations typical of the Piedmont region.

C. Discharge & Compliance

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The facility discharges treated effluent into Ellerbe Creek, a tributary of the Neuse River Basin that flows directly into Falls Lake. Because Falls Lake is a nutrient-sensitive water body classified as a drinking water source, the NDWRF operates under an extremely strict NPDES permit.

Critical Compliance Drivers:

• The Falls Lake Rules: A state nutrient management strategy requiring aggressive reductions in Total Nitrogen (TN) and Total Phosphorus (TP).
• Neuse River Basin Requirements: Basin-wide nitrogen limits.

Treatment Process: Technical Description

The North Durham WRF utilizes a sophisticated treatment train centered on the Krüger Bio-Denipho process, designed specifically for high-efficiency Biological Nutrient Removal (BNR).

A. Preliminary Treatment

Raw wastewater enters the headworks where it undergoes physical separation to protect downstream equipment:

• Screening: Mechanical step screens remove large debris, rags, and plastics. Screenings are washed, compacted, and disposed of in landfills.
• Grit Removal: Vortex grit chambers separate inorganic solids (sand, gravel, coffee grounds) through centrifugal force.
• Flow Equalization: While the plant has robust hydraulic capacity, equalization basins are utilized during peak wet weather events to stabilize loading to the biological reactors.

B. Primary Treatment

Following headworks, flow is directed to primary clarifiers. These circular tanks reduce the velocity of the wastewater, allowing settleable organic solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. This stage typically removes 30-50% of Total Suspended Solids (TSS) and 25-35% of Biochemical Oxygen Demand (BOD), reducing the load on the secondary system.

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C. Secondary Treatment (Bio-Denipho Process)

The core of the NDWRF is the Bio-Denipho oxidation ditch system. Unlike conventional aeration basins, this phased isolation ditch technology utilizes alternating aerobic and anoxic phases within the same reactor volume to achieve nitrification and denitrification.

• Process Configuration: The system consists of interconnected oxidation ditches equipped with variable speed rotors or brush aerators and submerged mixers.
• Phased Operation: Programmable Logic Controllers (PLCs) cycle the ditches through specific operational modes (aerobic for nitrification, anoxic for denitrification) based on real-time oxidation-reduction potential (ORP) and dissolved oxygen (DO) sensors.
• Biological Phosphorus Removal: An anaerobic selector zone precedes the ditches, conditioning the biomass to release phosphorus, which is then luxuriously taken up by Phosphate Accumulating Organisms (PAOs) in the aerobic zones.
• Clarification: Mixed liquor flows to secondary clarifiers where the biological floc settles. Return Activated Sludge (RAS) is pumped back to the anaerobic selector, while Waste Activated Sludge (WAS) is sent to solids handling.

D. Tertiary Treatment

To meet the ultra-low nutrient limits for Falls Lake (often requiring TN < 3.0 mg/L and TP < 0.1 mg/L), the facility employs Deep Bed Denitrification Filters.

• Filtration Mechanism: Downflow sand filters remove remaining suspended solids.
• Denitrification: A carbon source (typically methanol or glycerin) is added to the filter influent. Heterotrophic bacteria residing in the deep filter bed convert remaining nitrate (NO3) to nitrogen gas (N2) under anoxic conditions.
• Chemical Trimming: Alum or ferric salts may be added upstream of filters to precipitate any remaining soluble phosphorus.

E. Disinfection

The facility utilizes Ultraviolet (UV) Disinfection. This chemical-free process passes effluent through channels containing banks of UV lamps. The UV light disrupts the DNA of pathogenic microorganisms (bacteria, viruses, protozoa), preventing replication. Following disinfection, the water is re-aerated to raise Dissolved Oxygen levels before discharge into Ellerbe Creek.

F. Solids Handling

Sludge processing is a critical component of operations:

• Thickening: Primary sludge and WAS are thickened, often using gravity belt thickeners or rotary drum thickeners.
• Stabilization: The thickened sludge undergoes aerobic digestion to reduce volatile solids and pathogen content.
• Dewatering: Digested biosolids are dewatered using belt filter presses or centrifuges to produce a “cake” material.
• Disposal: The Class B biosolids are typically hauled for land application on permitted agricultural fields, recycling nutrients back to the soil, or transported to a composting facility.

Infrastructure & Facilities

A. Power & Reliability

Given the critical nature of wastewater treatment, NDWRF is equipped with comprehensive backup power systems. Large-capacity diesel generators ensure that headworks, biological treatment, and disinfection processes remain operational during grid outages, which is vital during hurricane season in North Carolina.

B. Odor Control

The facility utilizes chemical scrubbers and biofilters at key odor-generation points, particularly the headworks and solids handling buildings. This is essential due to the plant’s location near public amenities, including the Museum of Life and Science.

C. Automation

A centralized SCADA (Supervisory Control and Data Acquisition) system monitors thousands of data points across the plant. This allows operators to visualize dissolved oxygen profiles in the Bio-Denipho ditches, manage chemical dosing for phosphorus removal, and automate filter backwash cycles.

Recent Upgrades & Major Projects

The City of Durham maintains a robust Capital Improvement Program (CIP) for NDWRF to ensure asset preservation and regulatory compliance.

Notable Recent & Ongoing Projects:

1. Electrical & Standby Power Improvements

Scope: Replacement of aging switchgear and installation of new, higher-capacity emergency generators to cover the full plant load.

Drivers: Increasing grid reliability concerns and the need to maintain full nitrification/denitrification during power loss to prevent permit violations.

2. Aeration System Optimization

Scope: Upgrades to the aeration rotors and blower systems within the Bio-Denipho ditches.

Benefit: Significant reduction in energy consumption (aeration typically accounts for 50-60% of plant energy use) and tighter control of Dissolved Oxygen setpoints.

3. Phosphorus Reduction Upgrades

Scope: Enhancements to chemical feed systems (Alum/Ferric) and control logic to ensure compliance with the tightening Falls Lake nutrient strategy.

Context: As the “Stage II” rules for Falls Lake approach, the facility must ensure it can meet technology-limit phosphorus concentrations consistently.

Regulatory Compliance & Environmental Performance

The NDWRF operates under NPDES Permit NC0023841. The facility is a high-performing asset, consistently earning Platinum or Gold Awards from the National Association of Clean Water Agencies (NACWA) for peak performance and permit compliance.

Key Permit Parameters (Typical Targets):

• CBOD5: < 5.0 mg/L (Summer)
• TSS: < 30 mg/L
• Ammonia Nitrogen (NH3-N): < 1.0 mg/L (Summer)
• Total Nitrogen: Mass-load based limits (Annual allocation)
• Total Phosphorus: Mass-load based limits (Annual allocation)

The facility is heavily monitored for “Total Nitrogen” and “Total Phosphorus” due to the eutrophication issues in Falls Lake. The advanced BNR process allows NDWRF to frequently discharge effluent with TN levels below 3 mg/L, significantly better than conventional secondary treatment (which averages 15-20 mg/L TN).

Challenges & Future Planning

1. The Falls Lake Rules

The most significant engineering challenge for NDWRF is the multi-stage nutrient management strategy for Falls Lake. Future regulatory phases may require even lower nutrient concentrations, pushing the limits of current biological treatment technology and potentially requiring membrane filtration or advanced oxidation in the future.

2. Emerging Contaminants (PFAS)

Like many NC utilities, Durham is closely monitoring per- and polyfluoroalkyl substances (PFAS). While primarily a concern for drinking water, wastewater plants are conduits for these “forever chemicals.” Future regulations may require investigating source control or effluent polishing technologies.

3. Regional Growth

Durham County is one of the fastest-growing regions in the US. The Department of Water Management is continuously updating its hydraulic modeling to ensure the collection system and the NDWRF headworks can accommodate increased flows from new developments in the northern basin.

Technical Specifications Summary

FAQ

Technical Questions

1. What specific BNR technology does NDWRF use?
The facility uses the Krüger Bio-Denipho process, which utilizes oxidation ditches with phased cycles of aeration and mixing to create alternating aerobic and anoxic conditions for nitrification and denitrification.

2. How does the plant manage peak wet weather flows?
NDWRF utilizes on-site flow equalization basins to store excess hydraulic surge during storm events. This stored volume is returned to the headworks once flows subside to prevent washing out the biological mass.

3. Is there a phosphorus limit?
Yes. Due to the Falls Lake Nutrient Management Strategy, the plant operates under strict mass loading limits for Total Phosphorus, often requiring effluent concentrations below 0.5 mg/L, achieved through biological uptake and chemical polishing.

4. Does the facility generate energy?
Currently, NDWRF uses aerobic digestion, which does not produce methane for energy recovery (unlike anaerobic digestion). The focus is on energy efficiency via equipment optimization rather than energy generation.

General Public Questions

5. Where does the treated water go?
The treated water is discharged into Ellerbe Creek, which flows into Falls Lake. Because Falls Lake is a water source for Raleigh, the treatment standards are exceptionally high.

6. Does the plant smell?
While all wastewater plants have the potential for odors, NDWRF employs chemical scrubbers and biofilters to scrub air from the headworks and sludge processing areas to minimize impact on the surrounding community.

7. How many people does it serve?
The facility serves the population of North Durham. While exact population figures fluctuate, the 20 MGD capacity is designed to handle the equivalent of over 100,000 residents plus commercial/industrial load.

8. Can I tour the facility?
The City of Durham Department of Water Management occasionally offers tours for educational groups and industry professionals. Interested parties should contact the department’s public affairs office directly.