Citizens Energy Group Southport Advanced Wastewater Treatment Plant Indianapolis

The Southport Advanced Wastewater Treatment Plant (AWT) serves as a cornerstone of central Indiana’s environmental infrastructure, operating as one of two primary treatment facilities for the City of Indianapolis. Managed by Citizens Energy Group since the utility transfer in 2011, this facility treats an average of 60 to 80 million gallons per day (MGD) with a design capacity of 125 MGD, serving the southern metropolitan area of Indianapolis and surrounding communities.

Distinguished by its complex tertiary treatment train involving Nitrification Towers and rapid sand filtration, Southport AWT achieves exceptionally high effluent quality required for discharge into the West Fork White River. The facility has recently gained national engineering prominence as the terminus for the DigIndy Tunnel System. Following nearly $2 billion in regional combined sewer overflow (CSO) infrastructure investments, Southport is now home to one of the largest deep tunnel pump stations in the Midwest, capable of lifting stored CSO flow from 250 feet underground for full treatment, marking a historic shift in the region’s water resource management.

Facility Overview

A. Service Area & Coverage

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The Southport AWT operates in conjunction with the Belmont AWT to serve the greater Indianapolis area. While the collection systems are interconnected, Southport primarily services the southern portion of Marion County, encompassing residential suburbs, commercial corridors along I-65 and I-465, and significant industrial zones. The collection system feeding the plant includes a complex network of sanitary sewers and combined sewers dating back to the early 20th century. The service area is characterized by a mix of dense urban environments and rapidly growing suburban districts, necessitating a robust hydraulic loading strategy.

B. Operational Capacity

The plant is designed for substantial hydraulic variability. While the average daily flow hovers between 60-80 MGD, the plant’s peak hydraulic capacity is critical for wet weather management. The facility is rated for:

• Average Design Flow: 125 MGD
• Peak Secondary Treatment Capacity: 180 MGD
• Peak Hydraulic Capacity (with wet weather handling): Exceeding 250 MGD

Historical flow trends indicate a steady baseline with extreme spikes during precipitation events, typical of combined sewer systems. The recent commissioning of the DigIndy tunnel system has altered the flow profile, allowing the plant to treat stored volume over time rather than managing instantaneous storm surges alone.

C. Discharge & Compliance

Treated effluent is discharged into the West Fork White River. Given the river’s low flow during summer months and its classification for recreational use, the Indiana Department of Environmental Management (IDEM) imposes strict limitations on the facility. The NPDES permit (IN0023183) mandates rigorous control of Ammonia-Nitrogen (NH3-N), Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), and E. coli. The plant consistently achieves high compliance rates, often discharging water cleaner than the river itself.

Treatment Process

The Southport AWT utilizes a multi-stage advanced treatment train designed to handle high organic loads and meet stringent nutrient limits. The process flow moves from preliminary screening through high-purity oxygen activated sludge, followed by tertiary nitrification and filtration.

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, flow enters aerated grit chambers where velocity is reduced to allow inorganic solids (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The grit is washed, dewatered, and disposed of in a sanitary landfill. Odor control at the headworks is managed through chemical scrubbers to mitigate nuisance issues for nearby residents.

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B. Primary Treatment

Flow proceeds to rectangular primary clarifiers. Here, hydraulic velocity is minimized to facilitate the sedimentation of heavy organic solids and the flotation of grease and oils (scum). Scrapers collect the primary sludge from the bottom and scum from the surface. Approximately 50-60% of suspended solids and 30-40% of BOD are removed in this stage. Primary sludge is pumped to gravity thickeners before being transferred via an inter-plant pipeline to the Belmont AWT for thermal processing.

C. Secondary Treatment (High Purity Oxygen)

The biological heart of Southport is the High Purity Oxygen (HPO) Activated Sludge system. Unlike conventional aeration which uses ambient air, the HPO system utilizes cryogenic oxygen generation plants on-site to inject >90% pure oxygen into covered aeration basins. This system allows for:

• Higher mixed liquor suspended solids (MLSS) concentrations.
• Reduced retention times compared to conventional aeration.
• Greater ability to handle shock loads common in industrial influent.

Following the HPO basins, the mixed liquor flows to secondary clarifiers where biological floc settles out. The clear supernatant proceeds to tertiary treatment, while settled biomass is returned as Return Activated Sludge (RAS) or wasted as Waste Activated Sludge (WAS).

D. Tertiary Treatment (Nitrification & Filtration)

To meet strict ammonia limits, Southport employs Nitrification Towers (Trickling Filters). These large vertical structures contain plastic media that support the growth of nitrifying bacteria (Nitrosomonas and Nitrobacter). Effluent from secondary clarifiers is pumped over the media, converting toxic ammonia into nitrate. This is followed by Deep Bed Sand Filters (gravity filtration), which polish the effluent by removing remaining fine suspended solids and particulate-associated BOD.

E. Disinfection

The polished effluent undergoes disinfection using sodium hypochlorite (chlorination) to eliminate pathogenic organisms. To protect aquatic life in the White River, the water is then dechlorinated using sodium bisulfite prior to final discharge at the outfall. This process ensures E. coli limits are met without releasing residual chlorine.

F. Solids Handling

Southport AWT operates as part of a regional solids management strategy. While thickening occurs on-site (using gravity belts or rotary drums for WAS), the facility does not process solids to final disposal. Instead, thickened sludge is pumped via dual inter-plant force mains to the Belmont AWT. There, it joins Belmont’s solids stream for dewatering and thermal oxidation (incineration), recovering energy and reducing volume to inert ash.

Infrastructure & Facilities

A. Physical Plant

The site spans over 100 acres along Southport Road. The campus is divided into distinct process zones: the headworks and primary treatment at the north end, secondary and tertiary processes centrally located, and the new Deep Rock Tunnel Pump Station complex. The site includes fully equipped maintenance shops, an operations control center, and an on-site process laboratory for compliance testing.

B. Energy Systems

Southport is a significant energy consumer, primarily due to the cryogenic oxygen generation plant and the massive influent pumps. Energy efficiency initiatives have included the installation of Variable Frequency Drives (VFDs) on major pumps and blowers. The facility draws power from the grid but relies on extensive backup generation capabilities to maintain hydraulic control during power outages, essential for preventing sewer backups.

C. Odor Control

Given the suburban proximity, Citizens Energy Group has invested heavily in odor control. Technologies include biological scrubbers at the tunnel pump station vents and chemical scrubbers at the headworks. The enclosed nature of the HPO aeration basins also significantly reduces the release of volatile organic compounds and aerosols compared to open-air basins.

Recent Upgrades & Major Projects

The most significant engineering achievement at Southport in the last decade is its integration with the DigIndy project, a consent-decree driven initiative to reduce CSOs.

DigIndy Deep Rock Tunnel Pump Station (2014-2018)

• Project Budget: ~$80 Million (Pump Station component)
• Project Scope: Construction of a 250-foot deep pump station to lift wastewater from the 28-mile DigIndy tunnel network into the Southport headworks.
• Technical Highlights:
  • Installation of massive submersible pumps capable of emptying the 54-million-gallon storage capacity of the tunnel system.
  • Construction of a coarse screen facility located 240 feet underground to protect pumps.
  • Integration with a new 25 MGD high-rate treatment train for wet weather flows.

• Impact: The station allows the capture of billions of gallons of sewage annually that previously overflowed into area waterways, pumping it to Southport for full treatment during dry periods following storms.

Southport AWT Expansion & Modernization (Ongoing)

• Drivers: Aging infrastructure and changing influent characteristics from the tunnel system.
• Scope: Upgrades to the secondary clarifier mechanisms, rehabilitation of the cryogenic oxygen plant, and modernization of the SCADA architecture.
• Funding: Primarily financed through Revenue Bonds and State Revolving Fund (SRF) loans.

Regulatory Compliance & Environmental Performance

Permit Requirements

The facility operates under NPDES Permit No. IN0023183. Key discharge limitations include:

• CBOD5: Monthly average limits typically < 10 mg/L in summer.
• TSS: Monthly average limits typically < 15 mg/L.
• Ammonia-Nitrogen: Seasonal limits, often as low as 1.5 mg/L in summer to prevent toxicity in the White River.
• E. coli: 125 counts/100ml (Geometric Mean) during recreational season (April-October).

Compliance History

Southport AWT has maintained a strong compliance record, successfully navigating the operational challenges posed by the variable loading from the DigIndy tunnel. The facility has received Peak Performance Awards from the National Association of Clean Water Agencies (NACWA) for its adherence to permit limits.

Operational Excellence

The facility is staffed 24/7/365 by a team of state-certified wastewater operators, maintenance mechanics, instrument technicians, and laboratory analysts. Citizens Energy Group utilizes a robust SCADA system that provides real-time monitoring of thousands of data points, allowing operators to automate dissolved oxygen control, return sludge rates, and chemical dosing.

Innovation: The integration of “Tunnel Operations” into daily plant management is a unique operational aspect. Operators must balance the treatment of fresh daily influent with the controlled dewatering of the deep tunnel, optimizing plant capacity without overwhelming biological processes.

Challenges & Future Planning

Current Challenges

• Tunnel Dewatering Management: Managing high-volume returns from the DigIndy tunnel which can be septic or have different chemical characteristics than fresh sewage.
• Nutrient Regulations: Anticipating future tighter limits on Total Nitrogen and Total Phosphorus, which may require retrofitting the existing nitrification towers or adding denitrification steps.
• Aging HPO Infrastructure: The cryogenic plants and covered basins are approaching the end of their useful life, requiring capital-intensive rehabilitation.

Future Planning

Citizens Energy Group’s master planning involves continuing the asset management program to rehabilitate concrete structures and mechanical systems. A major focus over the next 5-10 years will be optimizing energy efficiency and evaluating the feasibility of biological nutrient removal (BNR) within the existing footprint.

Technical Specifications Summary

Frequently Asked Questions

Technical Questions

Q: Does Southport AWT process its own biosolids?
A: No. Southport thickens sludge on-site but pumps the liquid slurry via inter-plant force mains to the Belmont AWT, which houses the thermal oxidation (incineration) facilities for the utility.

Q: What is the role of the DigIndy tunnel at this plant?
A: Southport is the terminus for the deep rock tunnel. It houses the Deep Rock Tunnel Pump Station (DRTPS) which lifts stored combined sewage from 250 feet underground to the headworks for treatment, preventing overflows into local waterways.

Q: Why does Southport use Nitrification Towers?
A: The High Purity Oxygen system is excellent for BOD removal but less efficient for nitrification in its current configuration. The towers provide a dedicated fixed-film biological stage to convert ammonia to nitrate to meet strict river discharge limits.

Public Interest Questions

Q: Who operates the Southport plant?
A: The plant is operated by Citizens Energy Group, a public charitable trust that acquired the water and wastewater assets from the City of Indianapolis in 2011.

Q: Does the plant smell?
A: Like any wastewater facility, odors can occur, but Southport utilizes advanced chemical scrubbers and biofilters, particularly at the headworks and tunnel vents, to minimize impact on the surrounding community.