City Of Columbus Southerly Wastewater Treatment Plant

FACILITY BASIC INFORMATION

• Plant Name: Southerly Wastewater Treatment Plant (SWWTP)
• Location: 6977 South High Street, Lockbourne, Franklin County, Ohio
• Operating Authority: City of Columbus, Department of Public Utilities (Division of Sewerage and Drainage)
• Design Capacity: 114 MGD (Average Daily Flow), 330 MGD (Secondary Peak), 440 MGD (Wet Weather Peak with CEPT)
• Current Average Flow: ~114 MGD
• Population Served: Approx. 750,000 (Part of the Greater Columbus 1.2M system)
• Service Area: Southern Columbus, Grove City, Obetz, Reynoldsburg, Grandview Heights, and surrounding townships
• Receiving Water Body: Scioto River
• NPDES Permit Number: 4PF00000
• Year Commissioned: 1967 (Major expansions in 1970s, 2010s)

TARGET AUDIENCE

• Municipal consulting engineers evaluating wet weather treatment strategies

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• Wastewater treatment plant operators and superintendents
• Equipment vendors researching thermal oxidation and CEPT specifications
• Environmental regulators and compliance officers (Ohio EPA)
• Engineering firms pursuing capital improvement projects

1. INTRODUCTION

The Southerly Wastewater Treatment Plant (SWWTP) is the largest of two treatment facilities serving the Columbus metropolitan area and stands as a critical component of Central Ohio’s hydrological infrastructure. Operated by the City of Columbus Division of Sewerage and Drainage (DOSD), the facility treats an average of 114 million gallons per day (MGD) and serves a population exceeding 700,000 residents. The plant operates in tandem with the Jackson Pike Wastewater Treatment Plant to manage the region’s wastewater needs.

Located on a sprawling campus near Lockbourne, Ohio, Southerly has gained industry recognition for its aggressive management of wet weather flows through Chemically Enhanced Primary Treatment (CEPT) and its commitment to thermal reduction of biosolids via Fluidized Bed Incineration (FBI). Following the “Wet Weather Management Plan” and significant capital investment under the city’s consent decree, Southerly has evolved into a high-rate treatment facility capable of protecting the Scioto River from combined sewer overflows (CSOs) during extreme precipitation events.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The SWWTP services the southern half of Franklin County and utilizes a massive collection system interceptor network. The service area encompasses the southern, eastern, and western portions of the City of Columbus, as well as suburban municipalities including Grove City, Reynoldsburg, Obetz, and Grandview Heights. The collection system is a hybrid of separated sanitary sewers and legacy combined sewers, necessitating robust peak flow management strategies at the plant headworks.

B. Operational Capacity

The facility is designed with a tiered capacity structure to address the region’s flashy hydrology:

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• Design Average Flow: 114 MGD
• Secondary Treatment Peak: 330 MGD
• Total Wet Weather Capacity: 440 MGD (utilizing CEPT bypass)

In the last decade, the plant has seen a steady increase in base flow due to regional population growth, while peak flow events have become more manageable due to the completion of the Olentangy-Scioto-Interceptor-Sewer Augmentation Relief Sewer (OARS) tunnel, which terminates at the plant.

C. Discharge & Compliance

Treated effluent is discharged into the Scioto River, a major tributary of the Ohio River. The discharge is regulated under National Pollutant Discharge Elimination System (NPDES) permit #4PF00000 administered by the Ohio EPA. The plant is subject to strict limits regarding Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, and E. coli. As part of the Mississippi River basin, nutrient monitoring (Phosphorus and Nitrogen) is also a critical component of the plant’s compliance regime to mitigate hypoxia in the Gulf of Mexico.

3. TREATMENT PROCESS

The Southerly WWTP utilizes a conventional activated sludge process augmented by high-rate wet weather treatment technologies and advanced solids thermal oxidation.

A. PRELIMINARY TREATMENT

The headworks facility was significantly upgraded to accommodate the OARS deep tunnel pump station. Raw wastewater enters via the interceptor system and passes through coarse bar screens (mechanically cleaned) to remove large debris. Following screening, flow enters aerated grit chambers where inorganic solids (sand, gravel, coffee grounds) settle out. The grit is washed, dewatered, and disposed of at a sanitary landfill. The new screening facility is designed to handle extreme hydraulic surges associated with tunnel dewatering.

B. PRIMARY TREATMENT & CEPT

Primary treatment consists of rectangular primary settling tanks equipped with chain-and-flight sludge collectors. A defining feature of Southerly is its Chemically Enhanced Primary Treatment (CEPT) capability. During high-flow wet weather events (flows exceeding 330 MGD), ferric chloride and anionic polymers are injected prior to the primary clarifiers. This coagulation and flocculation process significantly increases settling velocities, allowing the plant to treat up to 110 MGD of excess flow through the primaries while bypassing the biological secondary stage. This “blend” is then disinfected and discharged, preventing raw sewage overflows while protecting the biomass in the secondary system from washout.

C. SECONDARY TREATMENT

The biological treatment stage utilizes a conventional Activated Sludge process. The facility operates multiple parallel aeration basins equipped with fine-bubble diffused aeration systems to maximize oxygen transfer efficiency. The mixed liquor then flows to circular secondary clarifiers where biological floc settles. Return Activated Sludge (RAS) is pumped back to the aeration basins to maintain the microbial population, while Waste Activated Sludge (WAS) is sent to solids handling. The secondary system is designed to achieve high removal rates of CBOD and Ammonia-Nitrogen (nitrification).

D. DISINFECTION

Effluent from the secondary clarifiers (and CEPT flow during wet weather) is disinfected using sodium hypochlorite (liquid chlorine) in chlorine contact tanks. To protect aquatic life in the Scioto River, the effluent is dechlorinated using sodium bisulfite prior to final discharge. The disinfection season typically runs from May through October, though permit requirements dictate specific operational windows.

E. SOLIDS HANDLING & INCINERATION

Columbus Southerly is a regional leader in thermal solids reduction. The process train includes:

• Thickening: Gravity thickening for primary sludge and centrifugation or gravity belt thickening for WAS.
• Dewatering: High-solids centrifuges dewater the blended sludge to achieve the necessary cake dryness (typically 20-25% solids) for autogenous combustion.
• Thermal Oxidation: The plant operates Fluidized Bed Incinerators (FBI). Unlike older multiple-hearth furnaces, FBIs offer superior combustion efficiency, lower emissions, and better odor control. The incineration process reduces the volume of solids by approximately 90-95%, resulting in an inert ash that is disposed of in a sanitary landfill.

Waste heat from the incinerators is recovered to generate steam, which is used for building heat and process requirements, contributing to the facility’s energy efficiency.

F. PROCESS CONTROL

The entire facility is monitored and controlled via a comprehensive SCADA system. Operators in the Central Control Room utilize HMI screens to monitor dissolved oxygen levels, tank levels, pump status, and chemical feed rates in real-time. The plant maintains a fully accredited on-site laboratory to perform daily compliance testing and process control analysis.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Southerly WWTP occupies a massive industrial footprint south of I-270. The site includes the New Headworks Building, Screen building, Grit removal facilities, Primary and Secondary galleries, the Solids Processing Complex, and the OARS Pump Station shaft. The architecture is strictly industrial, focused on durability and ease of maintenance.

B. Energy Systems

Energy management is a priority given the high power draw of aeration blowers and influent pumps. The facility utilizes Variable Frequency Drives (VFDs) on major rotating equipment to match energy consumption with hydraulic load. The centerpiece of the energy strategy is the waste heat recovery system attached to the Fluidized Bed Incinerators, which offsets natural gas consumption for heating purposes during winter months.

C. Odor Control

Given the proximity to growing residential areas and the “Com-Til” composting facility (located nearby but distinct), odor control is paramount. The plant utilizes dual-stage chemical scrubbers and biofilters, particularly at the headworks and solids handling buildings, to treat foul air before release. The FBI technology also significantly reduces odors compared to historical incineration methods.

5. RECENT UPGRADES & MAJOR PROJECTS

The City of Columbus has invested heavily in Southerly to meet Consent Decree requirements related to sanitary sewer overflows (SSOs) and combined sewer overflows (CSOs).

New Headworks and OARS Interface (2010s)

• Project Scope: Construction of a new 440 MGD screening and grit removal facility to accept flow from the Olentangy-Scioto-Interceptor-Sewer Augmentation Relief Sewer (OARS).
• Budget: Part of the larger $370 million OARS investment.
• Technical Highlights: The OARS tunnel is 20 feet in diameter and 170 feet deep. The project required massive deep-lift pumps to elevate flow from the tunnel shaft to the plant’s surface-level treatment train.
• Result: Drastic reduction in CSOs into the Scioto River downtown.

Chemically Enhanced Primary Treatment (CEPT) Upgrade

• Project Scope: Retrofitting primary clarifiers and chemical feed systems to enable high-rate treatment of wet weather flows.
• Project Drivers: Consent Decree compliance to maximize wet weather treatment without expanding biological secondary treatment to unsustainable sizes.
• Technical Highlights: Installation of rapid mix and flocculation zones with precise ferric/polymer dosing controls.
• Result: Allowed the plant to treat up to 440 MGD total peak flow, securing regulatory compliance during heavy storms.

Fourth Fluidized Bed Incinerator (FBI)

• Project Scope: Installation of a new Fluidized Bed Incinerator to replace aging multiple-hearth furnaces and provide redundancy.
• Timeline: Completed circa 2017-2019.
• Technical Highlights: Advanced air pollution control systems (scrubbers, wet electrostatic precipitators, carbon polishers) to meet strict MACT (Maximum Achievable Control Technology) emission standards.
• Result: Improved reliability of solids disposal and reduced mercury and dioxin/furan emissions.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Under NPDES Permit 4PF00000, Southerly is held to strict effluent limits. The permit includes tiered limits for dry weather and wet weather operations (related to the CEPT bypass authorization). Key parameters include:

• CBOD5: Weekly average limits typically around 12-15 mg/L.
• TSS: Weekly average limits typically around 18-20 mg/L.
• Ammonia: Seasonal limits, often as low as 1.0-2.0 mg/L in summer to prevent toxicity.

B. Environmental Stewardship

The DOSD actively monitors the Scioto River’s water quality. The upgrades at Southerly have directly contributed to the revitalization of the Scioto River corridor through downtown Columbus, enabling the removal of the Main Street Dam and the restoration of natural river habitats.

7. OPERATIONAL EXCELLENCE

The facility is staffed 24/7 by a team of Ohio EPA certified wastewater operators, maintenance mechanics, electricians, and laboratory technicians. The City of Columbus DOSD encourages Class IV certification (the highest level in Ohio) for its management staff. The plant utilizes a Computerized Maintenance Management System (CMMS) to track asset lifecycle and schedule preventative maintenance, ensuring high uptime for critical pump and aeration systems.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

Aging Infrastructure: While major expansions have occurred, core components of the 1967/1970s infrastructure require ongoing rehabilitation, particularly concrete basins and conveyance channels susceptible to hydrogen sulfide corrosion.
Nutrient Regulations: Ohio is under increasing pressure to reduce nutrient loading to the Mississippi basin. Future permit cycles may mandate tighter Phosphorus limits, potentially requiring chemical addition or biological process modifications.

B. Future Planning

The City’s “Blueprint Columbus” initiative focuses on removing inflow and infiltration (I/I) from the collection system to reduce the hydraulic load on Southerly. Concurrently, the plant is planning for long-term asset renewal of the secondary aeration system and potential integration of energy recovery technologies such as cogeneration, should the economics of biogas-to-energy become favorable over incineration.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. RELATED FACILITIES

• Jackson Pike WWTP: The sister facility (68 MGD) serving the northern/central portions of Columbus.
• OARS Tunnel: The 20-foot diameter deep tunnel that conveys wet weather flows to Southerly.
• Compost Facility: Located nearby on State Route 104, this facility produces “Com-Til” soil amendment, though Southerly primarily directs solids to incineration.

11. FAQ SECTION

Technical/Professional Questions:

1. What is the CEPT capacity at Southerly?
The plant utilizes Chemically Enhanced Primary Treatment to handle flows between 330 MGD and 440 MGD. This “bypass” flow is settled and disinfected but skips the biological aeration stage.

2. Does Southerly practice Biological Nutrient Removal (BNR)?
Southerly primarily achieves nitrification (ammonia removal). While some incidental phosphorus removal occurs, it is not currently configured for high-level BNR like Enhanced Biological Phosphorus Removal (EBPR), relying instead on chemical precipitation if limits require it.

3. How are biosolids handled?
Biosolids are thickened, dewatered via centrifuges, and thermally oxidized in Fluidized Bed Incinerators. The resulting ash is landfilled.

4. Does the plant generate electricity?
The plant recovers waste heat from incineration for steam generation (heating) but does not currently utilize biogas cogeneration engines for electrical production.

Public Interest Questions:

5. What is the smoke coming from the plant?
The plume visible from Route 104 or I-270 is primarily steam (water vapor) from the scrubbing systems of the incinerators. The plant uses advanced pollution controls to remove particulates and chemicals.

6. Does the plant smell?
While wastewater treatment naturally generates odors, Southerly employs extensive chemical scrubbers and biofilters to scrub air from the headworks and solids buildings. Occasional odors may occur during maintenance or extreme weather.