Metropolitan Sewer District Mill Creek Wastewater Treatment Plant Cincinnati

1. INTRODUCTION

The Mill Creek Wastewater Treatment Plant stands as the flagship facility of the Metropolitan Sewer District of Greater Cincinnati (MSDGC), treating approximately 60% of the district’s total wastewater volume. Located near the confluence of Mill Creek and the Ohio River, this critical infrastructure asset serves a massive sewershed encompassing 218 square miles, including the City of Cincinnati and 20 surrounding municipalities. As the largest wastewater treatment plant in the region, Mill Creek plays a pivotal role in protecting the water quality of the Ohio River, a vital waterway that serves as a drinking water source for millions downstream.

Originally commissioned in 1959, the facility has evolved from primary treatment to a sophisticated secondary treatment complex with advanced solids handling and wet weather management capabilities. Recent capital investments exceeding $300 million have modernized the plant, introducing Fluidized Bed Incineration (FBI) for solids management and a specialized High Rate Treatment (HRT) facility to mitigate Combined Sewer Overflows (CSOs). Today, the plant exemplifies the balance between aging infrastructure rehabilitation and the adoption of cutting-edge technologies to meet stringent EPA Consent Decree requirements.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Mill Creek plant serves the “Mill Creek Basin,” the most urbanized and industrialized portion of Hamilton County. The service area includes the central business district of Cincinnati, major industrial corridors along I-75, and dense residential suburbs. The collection system feeding Mill Creek is a complex network of combined and separate sewers, necessitating robust wet weather management strategies. The plant receives flow via large-diameter interceptors, most notably the Mill Creek Interceptor.

B. Operational Capacity

The facility operates with a dry weather design capacity of 120 Million Gallons per Day (MGD). However, due to the prevalence of combined sewers in the collection system, hydraulic loading spikes dramatically during precipitation events. The plant’s total hydraulic capacity has been engineered to handle peak wet weather flows exceeding 360 MGD through a combination of the main secondary treatment train and the auxiliary High Rate Treatment (HRT) facility. Historical data indicates an average daily flow hovering near 100 MGD, making it one of the highest-volume plants in the Midwest.

C. Discharge & Compliance

Treated effluent is discharged directly into the Ohio River. The plant operates under an Ohio EPA NPDES permit (1PM00001) with strict limits on Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, and E. coli. The facility is also a focal point of MSDGC’s “Project Groundwork,” a multi-billion dollar initiative to reduce CSOs and improve water quality in compliance with a federal Consent Decree.

3. TREATMENT PROCESS

The Mill Creek WWTP utilizes a conventional activated sludge process for its main treatment train, supplemented by specialized systems for wet weather flows and advanced thermal solids processing.

A. PRELIMINARY TREATMENT

Raw wastewater enters the headworks where it passes through coarse bar screens (mechanically cleaned) to remove large debris, rags, and plastics. Following screening, flow enters aerated grit chambers. These tanks reduce the velocity of the influent, allowing heavier inorganic materials (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit is washed and dewatered before being hauled to a landfill. Odor control at the headworks is a critical operational parameter due to the plant’s proximity to downtown Cincinnati.

B. PRIMARY TREATMENT

Flow proceeds to rectangular primary settling tanks. Here, gravity separation removes approximately 60-70% of Suspended Solids and 30-40% of BOD. Flights (scrapers) move settled sludge to hoppers at the tank inlet, while surface skimmers remove fats, oils, and grease (FOG). The primary sludge is pumped directly to the solids handling complex.

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

C. SECONDARY TREATMENT

The biological heart of the plant is the Conventional Activated Sludge system. The plant utilizes multiple aeration basins equipped with fine-bubble diffusers to maximize oxygen transfer efficiency. Microorganisms consume dissolved organic matter and convert ammonia to nitrate (nitrification) depending on seasonal permit limits.

Following aeration, the mixed liquor flows to secondary clarifiers. These large tanks allow the biological floc to settle, separating the treated water from the biomass. A portion of the settled solids is returned to the aeration tanks as Return Activated Sludge (RAS) to maintain the biological population, while excess biomass is removed as Waste Activated Sludge (WAS).

D. WET WEATHER STRATEGY (High Rate Treatment)

To manage excess flows during storm events, Mill Creek utilizes a dedicated High Rate Treatment (HRT) facility. When influent flow exceeds the capacity of the biological secondary treatment train, excess primary effluent is diverted to the HRT system. This system utilizes chemically enhanced settling and/or high-rate clarification technologies to provide primary-equivalent (or better) treatment for stormwater-diluted flows, followed by disinfection, significantly reducing untreated CSOs to the Ohio River.

E. DISINFECTION

The combined effluent (from both the secondary train and HRT) undergoes disinfection using liquid sodium hypochlorite (chlorination). To protect aquatic life in the Ohio River, the effluent is subsequently dechlorinated using sodium bisulfite before final discharge through the submerged river outfall.

F. SOLIDS HANDLING & ENERGY RECOVERY

Mill Creek is distinct for its advanced thermal processing capabilities. Primary sludge and WAS are thickened (using gravity thickeners and centrifuges respectively) and dewatered via high-solids centrifuges.

The dewatered cake is processed in three Fluidized Bed Incinerators (FBI). These units replaced aging Multiple Hearth Incinerators in a major capital project. The FBIs offer superior combustion efficiency and lower emissions. Heat recovery steam generators (HRSG) capture waste heat from the incinerators to drive a steam turbine, generating up to 4.6 MW of electricity to offset plant power consumption. Ash from the incineration process is collected and disposed of in a sanitary landfill.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Mill Creek site is an industrial campus characterized by large-scale concrete hydraulic structures and the distinctive stack of the incineration complex. Key structures include the Pump Building, Grit Building, Sludge Processing Building, and the Administration/Laboratory complex. The site is geographically constrained by Gest Street, the Mill Creek channel, and existing rail lines, requiring vertical integration of processes.

B. Energy Systems

MSDGC has prioritized energy neutrality at Mill Creek. The centerpiece is the Waste-to-Energy (WTE) system attached to the Fluidized Bed Incinerators. The 4.6 MW steam turbine generator utilizes steam produced from the combustion of sewage sludge. This system not only reduces the volume of waste by 90% but also provides a significant portion of the plant’s electrical load, reducing reliance on the grid and lowering carbon footprint.

C. Odor Control

Given the plant’s location near the Queensgate neighborhood and downtown Cincinnati, odor control is paramount. The facility utilizes a combination of chemical wet scrubbers (typically multi-stage packed towers using bleach and caustic) and biological treatment systems to treat foul air extracted from the headworks, primary clarifiers, and sludge processing areas.

5. RECENT UPGRADES & MAJOR PROJECTS

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Operating under NPDES Permit 1PM00001, Mill Creek is subject to rigorous monitoring. Key effluent limits typically include:

• CBOD5: Weekly/Monthly average limits (approx. 15-25 mg/L).
• TSS: Weekly/Monthly average limits (approx. 20-30 mg/L).
• E. Coli: Seasonal limits (May-Oct) for recreational water safety.
• Whole Effluent Toxicity (WET): Annual testing to ensure effluent does not harm aquatic life.

B. Compliance History

Mill Creek operates under a global Consent Decree involving the DOJ, EPA, and State of Ohio. This legal framework mandates specific projects (like the HRT facility and sewer separation) to address historical CSOs. The plant generally maintains strong compliance with dry weather parameters. Occasional exceedances are typically associated with extreme wet weather events that overwhelm regional hydraulic capacity.

C. Environmental Stewardship

The switch to Fluidized Bed Incineration reduced the facility’s air emissions considerably, lowering particulate matter and heavy metal output compared to the old hearths. Additionally, the waste-to-energy component qualifies the plant as a green energy producer under Ohio standards.

7. OPERATIONAL EXCELLENCE

A. Staffing

The facility is staffed 24/7 by a team of Ohio EPA certified wastewater operators, maintenance mechanics, electricians, and instrumentation specialists. MSDGC maintains a robust training program, encouraging staff to obtain Class III and Class IV wastewater certifications.

B. Technology & Innovation

Mill Creek utilizes a sophisticated SCADA (Supervisory Control and Data Acquisition) system for real-time process monitoring. The District has also implemented Real-Time Control (RTC) in the collection system, using sensor data to optimize storage in large interceptors and reduce the peak surge reaching the plant headworks.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

Wet Weather Management: Despite the HRT upgrades, the sheer volume of combined sewage during intense storms remains the primary operational challenge. Managing the hydraulic shock to the biological system requires precise operator intervention.

Aging Infrastructure: While major processes have been upgraded, underlying conveyance infrastructure and concrete tankage from the mid-20th century require continuous rehabilitation and asset management investment.

B. Future Planning

MSDGC’s capital planning focuses on “Wet Weather Phase 2” projects under the Consent Decree. For Mill Creek, this involves further optimizing the collection system to bring more flow to the plant for treatment rather than allowing overflows in the system. Future considerations include potential nutrient removal upgrades (Nitrogen/Phosphorus) should the Ohio River Valley Water Sanitation Commission (ORSANCO) or EPA mandate stricter nutrient limits for the Gulf of Mexico hypoxia zone reduction.

10. TECHNICAL SPECIFICATIONS SUMMARY

12. FAQ SECTION