Cobb County South Cobb Water Reclamation Facility

Facility Basic Information

• Plant Name: South Cobb Water Reclamation Facility (WRF)
• Location: 4400 connecticut Place, Austell, GA 30106
• Operating Authority: Cobb County Water System (CCWS)
• Design Capacity: 44 MGD (Million Gallons per Day)
• Current Average Flow: ~30-34 MGD
• Service Area: South Cobb Basin (Austell, Mableton, Powder Springs, parts of Smyrna)
• Receiving Water Body: Chattahoochee River
• NPDES Permit Number: GA0024341
• Year Commissioned: 1964 (Major expansions in 1990s, 2000s)

1. Introduction

The South Cobb Water Reclamation Facility (WRF) serves as the cornerstone of wastewater infrastructure for the southern basin of Cobb County, Georgia. As the largest treatment facility operated by the Cobb County Water System (CCWS), this 44-MGD plant plays a pivotal role in protecting the water quality of the Chattahoochee River, a critical water source for the metropolitan Atlanta region. Located in Austell, the facility processes wastewater from a rapidly developing service area encompassing heavy industrial zones, commercial districts, and dense residential communities.

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Distinguished by its integration with the massive South Cobb Tunnel system—one of the most significant hydraulic engineering projects in the state’s history—the South Cobb WRF represents a model of modern wet-weather management and advanced nutrient removal. With a permitted capacity of 44 MGD and peak hydraulic capabilities exceeding 100 MGD, the plant utilizes advanced biological treatment and tertiary filtration to meet stringent effluent limits, consistently earning Gold Awards from the Georgia Association of Water Professionals (GAWP) for permit compliance.

2. Facility Overview

A. Service Area & Coverage

The South Cobb WRF services the South Cobb Basin, which includes the municipalities of Austell, Powder Springs, Mableton, and portions of Smyrna and unincorporated Cobb County. The collection system is characterized by complex topography requiring significant pumping infrastructure. The basin drains naturally toward the Chattahoochee River, necessitating a robust interception system. The service area is a mix of mature suburban residential developments and significant industrial corridors along the railway lines and the I-20 corridor.

B. Operational Capacity

The facility is permitted for a maximum monthly average flow of 44 MGD. While the plant treats an average daily flow ranging between 30 and 34 MGD, it is engineered to handle substantial diurnal peaks and wet-weather events. A critical component of the facility’s capacity management is the South Cobb Tunnel Lift Station (SCTLS), located on-site, which can pump over 100 MGD from the deep rock tunnel storage system directly into the headworks, effectively leveling influent flow during storm events and eliminating sanitary sewer overflows (SSOs) in the basin.

C. Discharge & Compliance

Treated effluent is discharged into the Chattahoochee River. Due to the river’s status as a primary drinking water source and recreational asset downstream, the Georgia Environmental Protection Division (EPD) enforces stringent NPDES permit limits. The facility is subject to strict limitations on Total Phosphorus (TP), Total Nitrogen, Carbonaceous Biochemical Oxygen Demand (CBOD), and Total Suspended Solids (TSS). The plant operates under a “zero-tolerance” philosophy regarding permit violations, utilizing redundancy in critical systems to ensure consistent compliance.

3. Treatment Process

The South Cobb WRF employs an advanced biological treatment train designed to achieve high-level nutrient removal and tertiary filtration. The process flow is linear, utilizing gravity flow where possible after the initial lift.

A. Preliminary Treatment

Raw wastewater enters the facility primarily via the South Cobb Tunnel Lift Station and surface interceptors. The headworks facility is equipped with mechanical coarse and fine bar screens to remove large debris, rags, and plastics. Following screening, flow enters vortex grit removal chambers where inorganic solids (sand, gravel, eggshells) are settled out to protect downstream pumps and prevent accumulation in the digesters. The headworks is fully enclosed with negative pressure odor control scrubbers.

B. Primary Treatment

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Flow proceeds to primary clarifiers, which are large rectangular basins equipped with chain-and-flight sludge collectors. Here, heavy organic solids settle to the bottom as primary sludge, while fats, oils, and grease (FOG) are skimmed from the surface. This physical separation step reduces the organic loading (BOD) on the secondary biological system by approximately 30-35%.

C. Secondary Treatment (Biological Nutrient Removal)

The core of the treatment process is the activated sludge system configured for Biological Nutrient Removal (BNR). The aeration basins utilize a multi-stage configuration (Anaerobic/Anoxic/Oxic) to facilitate:

• Phosphorus Removal: Anaerobic zones encourage the growth of Phosphate Accumulating Organisms (PAOs) for biological phosphorus uptake.
• Nitrification/Denitrification: Anoxic and oxic zones allow for the conversion of ammonia to nitrate (nitrification) and nitrate to nitrogen gas (denitrification).

Fine bubble diffusers provide oxygen in the aerobic zones, controlled by dissolved oxygen (DO) sensors linked to the blower control system to optimize energy efficiency. The mixed liquor then flows to secondary clarifiers where biological floc settles, returning Activated Sludge (RAS) to the front of the aeration basins.

D. Tertiary Treatment

To meet strict effluent limits (particularly for Phosphorus < 0.3 mg/L and TSS), the plant utilizes tertiary deep-bed sand filters. These filters physically entrap remaining suspended solids and particulate phosphorus that escaped secondary clarification. The filters undergo periodic backwashing based on head loss or time intervals, with backwash water returned to the head of the plant.

E. Disinfection

The South Cobb WRF utilizes high-intensity Ultraviolet (UV) disinfection. The effluent flows through UV channels where lamps disrupt the DNA of pathogenic microorganisms (bacteria and viruses), rendering them unable to reproduce. This method eliminates the need for chlorination and subsequent dechlorination, removing the risk of residual chemicals entering the Chattahoochee River. Following UV treatment, the water undergoes post-aeration to ensure permitted Dissolved Oxygen levels are met prior to discharge.

F. Solids Handling

Primary sludge and Waste Activated Sludge (WAS) are co-settled or thickened separately (using Gravity Belt Thickeners or Rotary Drum Thickeners). The thickened sludge is stabilized via anaerobic digestion. The facility utilizes centrifuges for final dewatering to produce a “cake” product. Dewatered biosolids are typically transported off-site to a landfill or a third-party soil amendment facility, depending on current county contracts.

4. Infrastructure & Facilities

A. The South Cobb Tunnel & Lift Station

The most distinctive infrastructure feature associated with the WRF is the South Cobb Tunnel. Completed in the mid-2000s, this 27,000-foot-long, 24-foot-diameter deep rock tunnel serves as both a conveyance mechanism and an equalization tank. It can store up to 90 million gallons of wastewater during heavy rain events. The associated Lift Station at the plant site is a massive engineering feat, featuring a 200-foot deep vertical shaft housing high-capacity pumps capable of lifting wastewater from the tunnel to the surface treatment units.

B. Energy & Power Systems

The facility is a major energy consumer, primarily driven by the aeration blowers and the massive pumps at the tunnel lift station. The plant is equipped with dual-feed power supply from the utility grid and extensive emergency standby generation capacity (diesel generators) to maintain full operational capability during power outages, ensuring no untreated discharge occurs during storm-related blackouts.

C. Odor Control

Given the proximity to Six Flags Over Georgia and residential areas, odor control is a priority. The facility utilizes multi-stage chemical scrubbers (wet scrubbers) and activated carbon units at high-generation points, including the headworks, primary clarifiers, and sludge processing buildings. The South Cobb Tunnel also features a sophisticated ventilation and odor control system to manage air displaced during rapid filling events.

5. Recent Upgrades & Major Projects

6. Regulatory Compliance & Environmental Performance

The South Cobb WRF operates under NPDES Permit No. GA0024341 issued by the Georgia EPD. The facility is subject to “Metro District” requirements, which impose some of the strictest discharge limits in the Southeast due to the Chattahoochee River’s flow constraints and downstream usage.

• Phosphorus: The plant must typically meet a monthly average of 0.3 mg/L or lower, necessitating rigorous chemical trimming (alum/ferric) alongside biological removal.
• Fecal Coliform: Strict limits (typically 200 MPN/100 mL monthly geometric mean), achieved reliably via the UV disinfection system.
• Awards: The facility is a frequent recipient of the GAWP Gold and Platinum Awards, signifying years of operation without a single permit violation.

7. Challenges & Future Planning

A. Infrastructure Aging

While the tunnel is relatively new, much of the surface treatment plant infrastructure dates to the expansions in the 1990s. Concrete degradation in headworks and clarifiers, along with obsolescence of control systems (SCADA), requires continuous capital reinvestment.

B. Changing Waste Characteristics

Like many municipal plants, South Cobb faces challenges from “flushable” wipes causing pump clogging (ragging) and shifting influent loads due to industrial dischargers in the Austell area. CCWS maintains an active Industrial Pretreatment Program to monitor and regulate these inputs.

C. Future Capacity

Cobb County continues to grow. Long-term master planning evaluates the potential need to expand the South Cobb WRF beyond 44 MGD or to divert flow to other basins. However, the current capacity combined with the tunnel storage provides a robust buffer for the immediate future (5-10 years).

8. Technical Specifications Summary

9. FAQ Section

Technical Questions

Q: What is the peak flow capacity of the South Cobb WRF?
A: While the permitted treatment capacity is 44 MGD, the plant can process significantly higher hydraulic peaks (exceeding 100 MGD) for short durations, supported by the massive flow equalization provided by the South Cobb Tunnel.

Q: Does the facility generate energy from biogas?
A: The facility utilizes anaerobic digestion which produces biogas. Historically, this gas has been used for process heating (digester heating), though excess is typically flared. Detailed cogeneration (CHP) status varies based on current operational contracts.

Q: How is the tunnel lift station configured?
A: The South Cobb Tunnel Lift Station is a deep-shaft pumping system. It is designed to dewater the 24-foot diameter tunnel, lifting wastewater approx. 200 feet vertically to the headworks using high-horsepower centrifugal pumps.

Public Interest Questions

Q: Does the plant smell?
A: Wastewater treatment naturally generates odors, but South Cobb WRF employs advanced chemical scrubbers and carbon filters at key sources (headworks, solids handling) to minimize off-site impact. The buffer zone around the plant helps mitigate community nuisance.

Q: Is the water released into the river clean?
A: Yes. The effluent discharged is often cleaner than the river water itself regarding suspended solids and organic matter. It meets strict state and federal standards for aquatic life and human health safety.