Metropolitan Council Environmental Services Metro Wastewater Treatment Plant St Paul

1. INTRODUCTION

The Metropolitan Wastewater Treatment Plant, commonly referred to as the “Metro Plant,” serves as the backbone of the Twin Cities’ water infrastructure and stands as one of the largest wastewater treatment facilities in the United States. Located on the banks of the Mississippi River in St. Paul, this facility processes approximately 170 million gallons of wastewater daily—roughly 70% of the wastewater generated in the seven-county metropolitan area.

Operated by Metropolitan Council Environmental Services (MCES), the Metro Plant is a marvel of large-scale environmental engineering. While the original facility dates back to the New Deal era of 1938, the current plant utilizes advanced secondary treatment, sophisticated biological nutrient removal, and one of the nation’s most advanced fluidized bed incineration systems for solids handling. With a peak wet weather flow capacity exceeding 700 MGD, the plant plays a critical role in protecting the water quality of the Mississippi River, serving as a model for regional wastewater consolidation and resource recovery.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Metro Plant serves a massive regional catchment area comprising 66 communities, including the core cities of Minneapolis and St. Paul, as well as surrounding inner-ring suburbs. The collection system feeding the plant is extensive, involving hundreds of miles of regional interceptors that gravity-feed or pump wastewater to the Pig’s Eye Lake location. The demographics served are diverse, including dense residential zones, significant commercial districts, and heavy industrial contributors, which necessitates a robust industrial pretreatment program to protect biological processes.

B. Operational Capacity

The facility is designed to handle significant hydraulic variations typical of the Upper Midwest’s climate (snowmelt and thunderstorms).

• Average Daily Flow: Approx. 170 MGD
• Design Average Wet Weather Flow: 214 MGD
• Peak Hourly Capacity: >700 MGD

Read moreA Comparison Between Aerobic And Anaerobic Wastewater Treatment Technology

Utilization currently hovers around 80% of average design capacity, allowing headroom for regional growth. However, infiltration and inflow (I/I) during spring thaw events can push hydraulic loading near peak limits, necessitating rigorous flow management strategies.

C. Discharge & Compliance

Treated effluent is discharged directly into the Mississippi River. The plant operates under a National Pollutant Discharge Elimination System (NPDES) permit issued by the Minnesota Pollution Control Agency (MPCA). The discharge point is critical as it lies upstream of Lake Pepin, a natural widening of the river sensitive to eutrophication. Consequently, the Metro Plant operates under strict phosphorus limits to prevent algae blooms and hypoxia downstream.

3. TREATMENT PROCESS

The Metro Plant employs an advanced secondary treatment process with targeted nutrient removal. The treatment train is designed for high-volume reliability and consists of the following stages:

A. PRELIMINARY TREATMENT

Raw wastewater enters the facility through massive interceptors deep underground.

• Screening: Mechanical bar screens remove large debris (rags, wood, plastics) to protect downstream pumps.
• Grit Removal: Aerated grit chambers reduce the velocity of the flow, allowing heavy inorganic materials like sand, gravel, and eggshells to settle out.
• Odor Control: Given the plant’s proximity to downtown St. Paul, headworks air is captured and treated through chemical scrubbers and activated carbon filters.

B. PRIMARY TREATMENT

The flow enters large rectangular primary settling tanks. Here, gravity allows settleable solids to drop to the bottom as sludge, while fats, oils, and grease (FOG) float to the surface for skimming.

• Efficiency: The primary stage removes approximately 60-70% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD).
• Sludge Handling: Primary sludge is pumped directly to the solids handling train for thickening.

C. SECONDARY TREATMENT (High Purity Oxygen)

The Metro Plant utilizes a High Purity Oxygen (HPO) activated sludge process, which is distinct from conventional aeration.

• Reactors: Covered aeration basins are used where pure oxygen is generated on-site (via Cryogenic Air Separation) and injected into the wastewater. This allows for a higher biomass concentration (MLSS) and smaller tank footprint compared to conventional air systems.
• Biological Process: Microorganisms consume the remaining organic matter. The covered tanks also aid significantly in odor containment.
• Secondary Clarifiers: Following aeration, the mixed liquor flows to secondary clarifiers where the biological floc settles. Clean water flows over the weirs, while settled biomass is returned (RAS) or wasted (WAS).

D. ADVANCED/NUTRIENT REMOVAL

To address river eutrophication, the plant employs Biological Phosphorus Removal (Bio-P). By creating specific anaerobic zones within the treatment train, phosphorus-accumulating organisms (PAOs) are selected, which uptake large amounts of phosphorus in the aerobic zones. This significantly reduces the need for chemical precipitation (alum/ferric), although chemical backup systems exist for peak trimming.

E. DISINFECTION

Disinfection is achieved through chlorination, followed by dechlorination to protect aquatic life in the Mississippi River.

• Method: Liquid sodium hypochlorite is added to the effluent in contact channels.
• Dechlorination: Sodium bisulfite is added prior to the outfall to neutralize residual chlorine.
• Seasonality: Disinfection is typically required from April through October to protect recreational users of the river.

F. SOLIDS HANDLING (Thermal Oxidation)

The Metro Plant is a leader in thermal processing of biosolids.

• Thickening/Dewatering: Sludge is thickened via gravity belts and centrifuges to increase solids content.
• Incineration: The facility operates state-of-the-art Fluidized Bed Incinerators (FBI). Unlike older multiple-hearth furnaces, FBIs use a bed of hot sand suspended by air jets to instantly combust biosolids.
• Energy Recovery: The incineration process includes heat recovery steam generators (HRSG) that power steam turbines, generating electricity to offset plant energy demands.
• Ash Management: The remaining inert ash is landfilled, though research into ash reuse (phosphorus recovery or construction aggregate) is ongoing.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans approximately 156 acres on the Pig’s Eye Lake peninsula. The campus includes extensive maintenance shops, a comprehensive analytical laboratory certified by the Minnesota Department of Health, and the dedicated Solids Management Building (SMB) which houses the incineration complex.

B. Energy Systems

The Metro Plant is an energy-intensive facility but employs aggressive recovery strategies.

• Steam Turbines: Waste heat from the fluidized bed incinerators generates steam that drives turbines, providing a significant portion of the plant’s electrical and heating needs.
• Efficiency: Variable Frequency Drives (VFDs) are standard on major pumps and aeration blowers to match energy use with hydraulic load.

C. Odor Control

MCES maintains a “good neighbor” policy with aggressive odor control. The plant utilizes a combination of biofilters (organic media) and chemical wet scrubbers to treat foul air extracted from the headworks, primary tanks, and solids processing areas.

5. RECENT UPGRADES & MAJOR PROJECTS

Metro Plant Rehabilitation & Facilities Improvements

Budget: Ongoing annual CIP allocation ($10M – $20M annually)

Scope: Continuous rehabilitation of the vast network of tunnels, concrete tanks, and conveyance systems, some of which date back to the 1930s and 1960s. Recent work includes the rehabilitation of the double-barrel interceptors entering the plant and upgrades to the primary settling tank mechanisms.

Phosphorus Recovery Feasibility

While not a full construction project yet, MCES is actively researching technologies to harvest phosphorus from the incineration ash or liquid stream, turning a waste product into a valuable fertilizer commodity.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The plant operates under NPDES Permit MN0029815. Key parameters include:

• CBOD5: < 25 mg/L (Monthly Avg)
• TSS: < 30 mg/L (Monthly Avg)
• Phosphorus: Strict limits to manage river eutrophication (typically < 1.0 mg/L effluent targets).

B. Compliance History

The Metro Plant consistently maintains a high level of compliance. It has received multiple Platinum and Gold Peak Performance Awards from the National Association of Clean Water Agencies (NACWA), recognizing facilities with near-perfect permit compliance records over consecutive years.

7. OPERATIONAL EXCELLENCE

Staffing: The facility is staffed 24/7 by a team of over 200 professionals, including licensed wastewater operators (Class A and B), skilled tradespeople (millwrights, electricians), process engineers, and laboratory scientists.

Automation: A robust SCADA system monitors thousands of data points, allowing for real-time adjustments to chemical dosing, return sludge rates, and air handling. This automation is crucial for managing the energy-intensive Cryogenic Oxygen plant.

8. CHALLENGES & FUTURE PLANNING

A. PFAS and Emerging Contaminants

Like all major WWTPs, the Metro Plant is facing the challenge of Per- and Polyfluoroalkyl Substances (PFAS). While the plant is not a generator of PFAS, it receives them from industrial and domestic sources. MCES is currently conducting extensive monitoring and source-reduction strategies upstream to prevent these chemicals from entering the wastewater stream.

B. Climate Resilience

Situated in the floodplain of the Mississippi River, the plant is hardened against flooding. However, changing climate patterns predict more intense precipitation events, increasing the risk of wet-weather inflows exceeding hydraulic capacity. Long-term planning involves reinforcing flood berms and expanding wet-weather equalization capabilities.

C. Aging Infrastructure

With core components approaching 80 years of age, the “rehab vs. replace” analysis is constant. The Met Council’s Capital Improvement Program (CIP) allocates hundreds of millions over the next decade specifically for asset preservation at the Metro Plant.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical/Professional Questions

1. Does the Metro Plant use anaerobic digestion?
No. The Metro Plant utilizes thermal oxidation (Fluidized Bed Incineration) rather than anaerobic digestion. This reduces the volume of solids by over 95%, resulting in sterile ash.

2. How is oxygen supplied to the aeration basins?
Oxygen is generated on-site using Cryogenic Air Separation plants. This high-purity oxygen is fed into covered aeration tanks, increasing the transfer efficiency and reaction rate compared to atmospheric air.

3. What is the strategy for peak wet weather flows?
The plant utilizes robust hydraulic routing and step-feed capabilities. In extreme events, the plant can engage high-capacity pumping to ensure hydraulic conveyance through the plant to prevent backup in the interceptor system.

Public Interest Questions

4. Does the plant smell?
While wastewater treatment has inherent odors, MCES invests heavily in odor control. Headworks and sludge processing areas are enclosed, and foul air is scrubbed before release. Most residents in St. Paul do not experience odors from the plant.

5. Can I tour the facility?
Yes, MCES offers tours for educational groups, engineering students, and professional organizations. Requests must be made in advance through the Metropolitan Council website.