Northeast Ohio Regional Sewer District Southerly Wastewater Treatment Plant Cleveland

Introduction

The Southerly Wastewater Treatment Center (Southerly) stands as the largest of the three wastewater treatment facilities operated by the Northeast Ohio Regional Sewer District (NEORSD). Located in the Village of Cuyahoga Heights, this critical infrastructure asset provides advanced wastewater treatment for a service area covering more than 600,000 residents across the Greater Cleveland area. Commissioned originally in 1928 and substantially expanded over the decades, Southerly is a cornerstone of the region’s environmental health.

With a design average flow of 125 million gallons per day (MGD) and a wet-weather peak capacity of 735 MGD, Southerly plays a pivotal role in managing combined sewer overflows (CSOs) through the District’s “Project Clean Lake” initiative. The facility is renowned in the industry for its state-of-the-art Renewable Energy Facility (REF), which utilizes fluidized bed incineration to process biosolids and generate electricity, setting a national benchmark for sustainable utility management.

Facility Overview

Service Area & Coverage

Southerly serves the largest geographic area of NEORSD’s facilities, covering approximately 64 square miles. The service basin encompasses the southern and eastern suburbs of Cleveland, as well as portions of the city itself. The collection system feeding Southerly includes a complex network of interceptors, including the Southerly Interceptor and the Mill Creek Tunnel system. The demographics of the service area are mixed, comprising dense residential zones, significant commercial corridors, and heavy industrial sectors, necessitating a robust treatment train capable of handling variable influent characteristics.

Operational Capacity

The facility operates as an advanced secondary treatment plant with tertiary filtration capabilities. Its hydraulic profile is designed to handle extreme variations due to the region’s combined sewer system:

• Design Average Daily Flow: 125 MGD
• Secondary Treatment Capacity: Up to 400 MGD
• Peak Hydraulic Capacity: 735 MGD (utilizing primary treatment and disinfection for storm flows)

In recent years, the average daily flow has hovered between 100 and 120 MGD. The plant serves as the termination point for major deep tunnel projects designed to capture combined sewage, requiring high-capacity pump stations to lift flow from substantial depths for treatment.

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Discharge & Compliance

Treated effluent is discharged into the Cuyahoga River, a designated American Heritage River. The plant operates under an Ohio EPA National Pollutant Discharge Elimination System (NPDES) permit, adhering to strict limits on Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, Phosphorus, and E. coli. Southerly’s performance has been instrumental in the biological resurgence of the Cuyahoga River, contributing to the return of fish species that had vanished from the ecosystem decades ago.

Treatment Process Technical Description

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through mechanical bar screens (0.75-inch opening) to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit tanks. These tanks reduce the velocity of the water, allowing inorganic solids like sand, gravel, and coffee grounds to settle while keeping organic matter in suspension. The removed grit is washed and dewatered before landfill disposal. This stage also includes significant odor control infrastructure, utilizing chemical scrubbers to mitigate H₂S emissions.

B. Primary Treatment

The screened and degritted wastewater flows into large circular primary settling tanks. Here, gravity separation occurs; heavier solids settle to the bottom as primary sludge, while grease and oils float to the surface for skimming. Southerly’s primary treatment is designed to remove approximately 60-70% of suspended solids and 30-40% of BOD. During extreme wet weather events exceeding secondary capacity, excess flow may receive primary treatment and disinfection before blending or discharge, per permit allowances.

C. Two-Stage Biological Treatment

Southerly utilizes a distinct two-stage biological treatment process, differing from the conventional single-stage activated sludge systems found at many municipal plants. This configuration provides process stability and high removal efficiencies.

• First Stage (Carbon Oxidation): The first stage consists of aeration tanks and intermediate settling tanks. The primary objective here is the removal of carbonaceous BOD. High-rate activated sludge is employed, utilizing fine bubble diffusion to supply oxygen to microorganisms that consume organic matter.
• Second Stage (Nitrification): Effluent from the first stage proceeds to the second stage aeration tanks. Here, a specific population of autotrophic bacteria (Nitrosomonas and Nitrobacter) converts ammonia-nitrogen into nitrate (nitrification). This stage is critical for meeting toxicity limits for the Cuyahoga River. Following aeration, the mixed liquor settles in final clarifiers.

D. Tertiary Treatment

To ensure consistent compliance with strict effluent limits, particularly regarding solids and phosphorus, the plant employs tertiary gravity sand filters. These dual-media filters polish the secondary effluent, removing remaining suspended solids and particulate-bound pollutants. This stage is essential for maintaining water quality during low-flow river conditions.

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E. Disinfection

Disinfection is achieved using liquid sodium hypochlorite (bleach) to inactivate pathogenic organisms. Due to the sensitivity of the aquatic life in the receiving Cuyahoga River, the effluent must be dechlorinated using sodium bisulfite prior to final discharge. This chlorination/dechlorination process is typically required during the recreation season (May through October).

F. Solids Handling & The Renewable Energy Facility (REF)

Southerly is home to the Renewable Energy Facility (REF), a cutting-edge biosolids management center. The process flow includes:

• Thickening: Primary and waste activated sludge are thickened using gravity thickeners and centrifuges.
• Dewatering: High-solids centrifuges dewater the sludge to form a “cake” suitable for thermal processing.
• Incineration: The facility utilizes three Fluidized Bed Incinerators (FBI). Unlike older multiple-hearth furnaces, FBIs offer complete combustion with lower emissions.
• Energy Recovery: Waste heat from the incineration process is captured in recovery boilers to generate steam. This steam drives a turbine generator, producing up to 25% of the plant’s electrical needs, significantly offsetting operational costs and carbon footprint.

Infrastructure & Facilities

Physical Plant

The Southerly site spans approximately 280 acres in Cuyahoga Heights. The complex includes administrative offices, a fully certified water quality laboratory, extensive maintenance shops, and the massive REF structure. The layout is bifurcated by the Cuyahoga River and a railroad corridor, requiring internal bridges and careful logistical planning for utility piping.

Energy Systems

Energy management is a core focus at Southerly. The plant consumes approximately 90-100 million kWh annually. The REF is the centerpiece of the energy strategy, capable of generating approximately 2 to 4 MW of electricity depending on loading conditions. Additionally, the facility has implemented VFDs (Variable Frequency Drives) on major aeration blowers and pumping systems to optimize consumption.

Tunnel Dewatering Pump Station (TDPS)

A critical piece of infrastructure located at the site is the Tunnel Dewatering Pump Station. This facility lifts flow from the massive underground storage tunnels (constructed as part of Project Clean Lake) roughly 200 feet up to the headworks for treatment. The TDPS has a capacity of 160 MGD, allowing the District to dewater tunnels rapidly between storm events to regain storage capacity.

Recent Upgrades & Major Projects

Renewable Energy Facility (REF) – $160 Million (Completed 2014)

• Scope: Demolition of aging multiple-hearth incinerators and construction of a new facility housing three fluidized bed incinerators, waste heat boilers, and a steam turbine generator.
• Drivers: New MACT (Maximum Achievable Control Technology) air emission standards and the need for energy efficiency.
• Outcome: The project reduced natural gas consumption/costs significantly and reduced air emissions (mercury, particulate matter) well below federal limits.

Southerly First Stage Aeration Improvements – ~$50 Million (2018-2021)

• Scope: Rehabilitation of the first-stage aeration tanks, including concrete repair, replacement of aeration grids with high-efficiency fine bubble diffusers, and upgrades to blower control systems.
• Drivers: Aging infrastructure (50+ years old) and energy optimization.
• Technical Highlights: Installation of automated dissolved oxygen (DO) control logic to minimize air delivery based on real-time loading.

Southerly Second Stage Settling Tank Improvements – ~$25 Million (2017-2020)

• Scope: Rehabilitation of the secondary clarifier mechanisms, weirs, and sludge collection systems specific to the nitrification stage.
• Results: Improved solids capture and reliability of the nitrification process.

Current/Upcoming: Chemically Enhanced Primary Treatment (CEPT)

As part of ongoing wet weather management strategies, the District is optimizing primary treatment performance during peak flow events. This involves the addition of coagulants and polymers to increase settling velocities, allowing the plant to treat higher hydraulic loads through the primary batteries without washing out solids.

Regulatory Compliance & Environmental Performance

Permit Requirements

Southerly operates under NPDES Permit No. 3PA00002*MD (subject to renewal cycles). Key parameters include:

• CBOD5: Monthly average limits typically < 10 mg/L (Summer).
• Ammonia-Nitrogen: Strictly limited (often < 1.5 mg/L in summer) to prevent toxicity to aquatic life.
• Phosphorus: 1.0 mg/L monthly average (Great Lakes basin standard).
• Mercury: Operated under a variance requiring pollutant minimization plans due to low-level detection limits.

Project Clean Lake (Consent Decree)

NEORSD is under a federal Consent Decree to reduce CSOs from 4.5 billion gallons annually to under 500 million gallons by 2036. Southerly is the treatment terminus for much of the gray infrastructure (tunnels) built under this $3 billion program. The plant’s ability to ramp up flows rapidly and treat stored tunnel water is the linchpin of this compliance strategy.

Technical Specifications Summary

Frequently Asked Questions

Technical Questions

1. What distinguishes the biological treatment at Southerly?

Southerly uses a two-stage activated sludge system. The first stage focuses on carbonaceous BOD removal, while the second stage is dedicated to nitrification (ammonia removal). This separation allows for optimized control of different bacterial populations.

2. How does the Renewable Energy Facility (REF) benefit the plant?

The REF incinerates biosolids in fluidized bed incinerators. The heat generated creates steam, which drives a turbine to produce up to 25% of the plant’s electricity, significantly lowering energy costs and reducing landfill waste.

3. What happens during wet weather events exceeding 400 MGD?

Flows up to 400 MGD receive full secondary and tertiary treatment. Flows exceeding this, up to 735 MGD, receive primary treatment and disinfection before being blended with fully treated effluent or discharged, in accordance with the NPDES permit wet weather protocols.

4. Does Southerly perform nutrient removal?

Yes. The plant is designed for nitrification (ammonia removal) and achieves phosphorus removal through biological uptake and chemical precipitation (typically utilizing ferric chloride or alum) to meet the 1.0 mg/L limit.

Public Interest Questions

5. Why is the Southerly plant important for the Cuyahoga River?

The plant treats wastewater that would otherwise pollute the river. Its high-quality effluent has been a primary factor in the river’s recovery from its heavily polluted industrial past to a waterway that now supports diverse fish populations and recreation.

6. Can I tour the facility?

Yes, NEORSD frequently hosts open houses and scheduled tours for educational groups, engineering students, and the public. Check the NEORSD website for the “Clean Water Fest” or tour request forms.

7. How are odors controlled at such a large facility?

The District uses extensive odor control technologies, including chemical scrubbers and carbon adsorption systems, particularly at the headworks and solids handling buildings, to minimize impact on the neighboring village of Cuyahoga Heights.