City Of Huntsville Aldridge Creek Wastewater Treatment Plant

Location: Huntsville, Madison County, Alabama
Operating Authority: City of Huntsville Water Pollution Control (WPC)

1. Introduction

The Aldridge Creek Wastewater Treatment Plant (WWTP) is a critical infrastructure asset for the City of Huntsville, Alabama, serving the rapidly developing southern corridor of the city. While smaller than the city’s Spring Branch facility, Aldridge Creek is strategically vital, managing wastewater for a distinct sewershed defined by the region’s topography. Operated by the City of Huntsville Water Pollution Control Department, this 9.6-MGD facility exemplifies the effective use of oxidation ditch technology to achieve high-quality effluent standards.

Situated near the Tennessee River, the plant plays a pivotal role in protecting the water quality of one of the Southeast’s most important waterways. Following significant capital investments, including a transition from chlorine gas to ultraviolet (UV) disinfection, the facility represents a modern approach to municipal wastewater treatment, balancing operational efficiency with strict environmental stewardship required by the Alabama Department of Environmental Management (ADEM).

2. Facility Overview

A. Service Area & Coverage

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The Aldridge Creek WWTP serves the South Huntsville area, a region characterized by established residential neighborhoods, growing commercial districts along South Memorial Parkway, and increasing density. The service area is geologically complex, sitting atop Karst topography (limestone), which presents unique challenges regarding groundwater infiltration.

• Geographic Scope: Bounded roughly by the Tennessee River to the south and the ridge of Huntsville Mountain to the east.
• Collection System: The plant is fed by a network of gravity mains and strategic lift stations designed to navigate the rolling terrain of the Tennessee Valley.

B. Operational Capacity

The plant is permitted for a design flow of 9.6 Million Gallons per Day (MGD). While average dry weather flows typically range between 5 and 6 MGD, the facility experiences significant hydraulic peaking during wet weather events due to Inflow and Infiltration (I/I) common in the region’s aging clay pipes and limestone geology. The hydraulic operational mode allows for peak handling up to approximately 24 MGD to prevent sanitary sewer overflows (SSOs).

C. Discharge & Compliance

Treated effluent is discharged into Aldridge Creek at Mile 6.3, which flows immediately into the Tennessee River. Under NPDES Permit AL0055743, the facility adheres to strict limits on BOD, TSS, Ammonia-Nitrogen, and E. Coli. The transition to UV disinfection has eliminated the production of chlorinated disinfection byproducts (DBPs), enhancing the ecological safety of the discharge into the receiving stream.

3. Treatment Process

The Aldridge Creek WWTP utilizes an extended aeration activated sludge process, specifically employing oxidation ditch technology. This process is renowned for its stability, ease of operation, and ability to handle shock loads.

A. Preliminary Treatment

Raw wastewater enters the headworks facility where it undergoes physical screening and grit removal to protect downstream mechanical equipment.

• Screening: Mechanical bar screens remove rags, plastics, and large debris. Debris is washed, compacted, and disposed of in a landfill.
• Grit Removal: A vortex grit chamber system utilizes centrifugal force to separate inorganic solids (sand, gravel, coffee grounds) from the organic waste stream.

B. Secondary Treatment (Biological)

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The core of the treatment process occurs in the oxidation ditches. The facility utilizes a looped reactor configuration (Orbal-style or similar concentric ditch design).

• Oxidation Ditches: The plant employs mechanical rotors (brush aerators) or disc aerators to provide oxygen and maintain channel velocity. This extended aeration process provides a long Hydraulic Retention Time (HRT) and high Solids Retention Time (SRT), facilitating not only carbonaceous BOD removal but also nitrification (conversion of ammonia to nitrate).
• Process Control: Dissolved Oxygen (DO) probes automate the rotor speeds or immersion depths to optimize energy consumption and biological activity.

C. Secondary Clarification

Mixed liquor from the oxidation ditches flows to circular secondary clarifiers. Here, the biological floc settles by gravity to the bottom, forming a sludge blanket.

• Clarifiers: Center-feed, peripheral overflow circular clarifiers.
• RAS/WAS: Settled biomass is returned to the oxidation ditches as Return Activated Sludge (RAS) to maintain the biological population. A portion is wasted as Waste Activated Sludge (WAS) to control the sludge age and inventory.

D. Disinfection

Historically a chlorine gas facility, Aldridge Creek was upgraded to Ultraviolet (UV) disinfection.

• Technology: Open-channel, low-pressure, high-intensity UV lamp banks.
• Mechanism: UV light penetrates the cell walls of pathogens (bacteria and viruses), disrupting their DNA/RNA and preventing replication.
• Benefit: This eliminates the need for hazardous gas storage on-site and the subsequent dechlorination step using sulfur dioxide.

E. Post-Aeration & Discharge

Before entering the receiving water, the effluent undergoes cascade post-aeration to ensure the Dissolved Oxygen (DO) levels meet the permit minimum (typically > 5.0 mg/L) to support aquatic life in Aldridge Creek.

F. Solids Handling

Solids management is a critical operational cost center.

• Stabilization: Waste Activated Sludge is stabilized in aerobic digesters. The extended aeration nature of the main process means the sludge is already partially stabilized, but digestion further reduces volatile solids and vector attraction.
• Dewatering: Digested sludge is thickened and dewatered, typically using belt filter presses or screw presses, to produce a “cake” with higher solids content.
• Disposal: The dewatered biosolids are transported to a municipal landfill for final disposal.

4. Infrastructure & Facilities

A. Physical Plant

The site is laid out to maximize gravity flow where possible. The campus includes the headworks building, oxidation basins, clarifiers, UV building, solids handling building, and an administration/laboratory building. The grounds are maintained to blend with the surrounding South Huntsville green spaces.

B. SCADA & Automation

The facility operates on a robust SCADA (Supervisory Control and Data Acquisition) system. This network integrates sensors from all unit processes (DO, pH, flow, tank levels) to a central control room. Operators can monitor pump statuses, adjust aeration setpoints, and track trending data for compliance reporting. The system also provides remote alarming capabilities for after-hours reliability.

C. Odor Control

Given its proximity to residential developments and the Tennessee River recreation areas, odor control is a priority. The aerobic nature of the oxidation ditch process naturally generates fewer odors than anaerobic processes. However, the headworks and solids handling areas utilize containment and air exchange systems to mitigate nuisance odors.

5. Recent Upgrades & Major Projects

UV Disinfection Conversion (Completed circa 2012-2014)

• Scope: Replacement of chlorine gas disinfection and sulfur dioxide dechlorination systems with a new UV disinfection facility.
• Driver: Increasing safety regulations regarding hazardous gas storage and stricter effluent toxicity limits.
• Impact: Eliminated the risk of toxic gas leaks and ensured consistent compliance with E. Coli limits without chemical residuals.

South Huntsville Sewer Rehabilitation (Ongoing)

• Scope: Large-scale Cured-in-Place Pipe (CIPP) lining and manhole rehabilitation throughout the Aldridge Creek basin.
• Driver: Reduction of Inflow and Infiltration (I/I).
• Impact: While not “inside the fence” of the plant, these capital projects directly impact the plant by reducing peak hydraulic loads during heavy rains, preserving treatment capacity for biological growth.

Electrical and SCADA Modernization (2018-2022)

• Scope: Upgrades to Motor Control Centers (MCCs) and integration of newer PLC hardware.
• Impact: Increased energy efficiency through the installation of Variable Frequency Drives (VFDs) on major pumps and aerators.

6. Regulatory Compliance & Environmental Performance

A. NPDES Permit Parameters

The facility operates under NPDES Permit No. AL0055743. Key discharge limitations typically include:

• BOD5 (Biochemical Oxygen Demand): Monthly Avg ~15-20 mg/L
• TSS (Total Suspended Solids): Monthly Avg ~30 mg/L
• Ammonia-Nitrogen (NH3-N): Seasonal limits (typically tighter in summer, < 2.0 mg/L)
• Dissolved Oxygen: Minimum 5.0 – 6.0 mg/L
• E. Coli: Geometric mean limits (seasonally adjusted)

B. Compliance History

The Aldridge Creek plant maintains a strong record of compliance with ADEM regulations. Occasional challenges have historically been associated with hydraulic washouts during extreme precipitation events (100-year storms), a challenge common to all facilities in the Tennessee Valley region. The move to UV disinfection has significantly stabilized bacteriological compliance.

7. Challenges & Future Planning

A. Inflow and Infiltration (I/I)

The primary engineering challenge for the Aldridge Creek system is I/I. The collection system sits on limestone rock (Karst). During heavy rains, groundwater enters the pipes through cracks or defects, causing flow spikes at the plant. Future planning focuses heavily on collection system asset management rather than just expanding plant concrete.

B. Nutrient Limits

Regulatory trends suggest future permits may include tighter limits on Total Nitrogen (TN) and Total Phosphorus (TP) to prevent eutrophication in the Tennessee River. The oxidation ditch process is well-positioned for biological nutrient removal (BNR), but future upgrades may require chemical addition facilities for phosphorus polishing.

C. Growth in South Huntsville

As South Huntsville continues to redevelop, specifically with higher-density multi-family housing, the organic loading (BOD load per gallon) to the plant may increase even if hydraulic flow remains stable. The City’s Master Plan accounts for this through process optimization.

8. Technical Specifications Summary

9. FAQ

Technical Questions

Q: Does Aldridge Creek WWTP perform Biological Nutrient Removal (BNR)?
A: Yes, the oxidation ditch process naturally facilitates nitrification and some denitrification through anoxic zones within the channel, reducing nitrogen levels. Phosphorus removal is largely incidental via solids removal unless chemical precipitation is added.

Q: How does the plant handle peak wet weather flows?
A: The plant has a hydraulic peak capacity significantly higher than its biological design capacity. Flows exceeding biological treatment capabilities may be routed through equalization basins or subjected to primary treatment and disinfection to prevent sanitary sewer overflows.

Q: What is the solids retention time (SRT) for this facility?
A: As an extended aeration facility, Aldridge Creek typically operates with a high SRT, often between 20 to 30 days, to ensure complete nitrification and sludge stabilization.

General Interest

Q: Who do I contact for odor complaints?
A: Residents should contact the City of Huntsville Water Pollution Control Department directly regarding any nuisance issues.

Q: Is the treated water safe for the river?
A: Yes. The effluent is treated to standards often cleaner than the river water itself regarding bacteria and solids, protecting aquatic life and downstream users.