City Of Minneapolis Wastewater Treatment Plant

TARGET AUDIENCE

• Municipal consulting engineers evaluating advanced solids thermal processing
• Wastewater treatment plant operators and managers
• Environmental regulators focusing on Upper Mississippi River compliance
• Energy managers interested in waste-to-energy systems
• Engineering firms pursuing regional infrastructure projects

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1. INTRODUCTION

The Metropolitan Wastewater Treatment Plant, situated on the banks of the Mississippi River in Saint Paul, acts as the hydrological kidney for the Twin Cities metropolitan area. While geographically located in Saint Paul, it is the primary treatment facility for the City of Minneapolis and 65 surrounding communities, processing approximately 70% of the wastewater generated in the seven-county metro region. Operated by Metropolitan Council Environmental Services (MCES), the Metro Plant is one of the largest wastewater treatment facilities in the Midwest, treating an average of 175 million gallons daily (MGD).

Commissioned originally in 1938 as the first dedicated treatment plant for the region, the facility has evolved from a primary treatment plant into a sophisticated resource recovery facility. It is particularly renowned in the engineering community for its advanced solids handling capabilities, specifically its use of fluidized bed incineration for energy recovery. The plant consistently achieves Platinum Peak Performance Awards from NACWA, demonstrating a commitment to protecting the Mississippi River ecosystem while serving a population of nearly 2 million.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Metro Plant serves the core urban density of Minnesota, covering the City of Minneapolis, the City of Saint Paul, and inner-ring suburbs. The service area is characterized by a dense mixture of residential, commercial, and significant industrial contributors. The collection system feeding the plant includes over 600 miles of regional interceptors owned by the Met Council, which collect flow from municipal lateral systems. The system serves a population of approximately 1.8 million people, with projections indicating steady growth requiring continued infrastructure resilience.

B. Operational Capacity

The facility is designed to handle massive hydraulic variations due to the combined sewer systems still present in parts of Minneapolis and Saint Paul, alongside separate sanitary systems in the suburbs.

• Average Dry Weather Flow: ~160 MGD
• Average Annual Flow: ~175 MGD
• Peak Hydraulic Capacity: The plant can process sustained peaks over 600 MGD during wet weather events.

Historical trends show a stabilization in flow despite population growth, attributed to successful water conservation efforts and inflow/infiltration (I/I) reduction programs implemented by MCES and member cities.

C. Discharge & Compliance

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Treated effluent is discharged into the Mississippi River. The plant’s performance is critical to the water quality of the river as it flows south toward Lake Pepin. The facility operates under a stringent NPDES permit issued by the Minnesota Pollution Control Agency (MPCA), with specific focus on phosphorus reduction to prevent eutrophication in downstream water bodies. The plant consistently removes over 99% of suspended solids and organic matter.

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

Influent wastewater enters the plant via deep tunnel interceptors. The headworks facility is designed to protect downstream equipment from debris and abrasion.

• Screening: Multiple mechanically cleaned bar screens remove large debris, rags, and plastics.
• Grit Removal: Aerated grit chambers reduce the velocity of the wastewater, allowing heavy inorganic materials like sand, gravel, and coffee grounds to settle while keeping organics suspended.
• Odor Control: The headworks and primary areas utilize chemical scrubbers and activated carbon systems to mitigate odors given the plant’s proximity to commercial and residential zones.

B. PRIMARY TREATMENT

The Metro Plant utilizes a vast array of rectangular primary settling tanks. Here, the velocity of the water is slowed significantly to allow settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming. This step removes approximately 50-60% of suspended solids and 30-40% of BOD. Primary sludge is pumped directly to the gravity thickening process.

C. SECONDARY TREATMENT

The secondary treatment train is the biological core of the facility, utilizing a variation of the Activated Sludge process. The Metro Plant historically utilized high-purity oxygen (HPO) systems but has transitioned toward advanced aeration strategies to optimize nutrient removal.

• Biological Phosphorus Removal (Bio-P): A key modification in recent decades was the retrofitting of aeration basins to include anaerobic zones. This selects for Phosphorus Accumulating Organisms (PAOs), allowing the plant to biologically remove phosphorus without excessive chemical addition, meeting strict river limits (1 mg/L or lower).
• Aeration Basins: The plant utilizes fine-bubble diffusion to supply oxygen to the microorganisms that consume organic matter.
• Secondary Clarification: Following aeration, the mixed liquor flows into secondary clarifiers. The biomass settles and is returned to the aeration tanks (RAS), while excess biomass (WAS) is removed for processing.

D. DISINFECTION

The Metro Plant utilizes seasonal disinfection (typically April through October) to protect recreational users of the Mississippi River.

• Chlorination: Liquid chlorine (sodium hypochlorite) is added to the effluent to kill pathogenic bacteria and viruses.
• Dechlorination: Before discharge, sodium bisulfite is added to neutralize any remaining chlorine residual, ensuring the effluent is non-toxic to aquatic life in the river.

E. SOLIDS HANDLING & ENERGY RECOVERY

The solids processing stream at the Metro Plant is an engineering benchmark for the industry, centered on thermal reduction and energy recovery.

• Thickening: Primary sludge is gravity thickened, while Waste Activated Sludge (WAS) is thickened using dissolved air flotation (DAF) or gravity belt thickeners.
• Dewatering: The blended thickened sludge is dewatered using high-speed centrifuges to create a “cake” with approximately 26-28% solids.
• Thermal Processing (Incineration): The dewatered cake is fed into Fluidized Bed Incinerators (FBIs). The plant operates multiple FBI units. These incinerators burn the biosolids at temperatures exceeding 1,400°F.
• Ash Management: The remaining ash is captured by electrostatic precipitators and baghouses. The inert ash is disposed of in landfills, reducing the volume of solids by over 90%.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Metro Plant occupies a massive footprint in the Pig’s Eye Lake industrial area. The site includes extensive maintenance shops, a fully accredited analytical laboratory, and the centralized administration for MCES operations. The layout is designed to handle flood stages of the Mississippi River, with critical infrastructure elevated or protected by levees.

B. Energy Systems: The Renewable Energy Facility (REF)

The Metro Plant is a leader in waste-to-energy. The solids incineration process is coupled with a heat recovery system.

• Steam Generation: Heat from the incinerator flue gas is captured in waste heat boilers to generate high-pressure steam.
• Power Generation: This steam drives turbines to generate electricity.
• Performance: The system generates approximately 20% to 30% of the plant’s total electrical needs, saving millions in utility costs annually. Additionally, recovered heat provides building heating during Minnesota’s harsh winters.

5. RECENT UPGRADES & MAJOR PROJECTS

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

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

• CBOD5: Strict monthly average limits (typically < 25 mg/L).
• TSS: < 30 mg/L monthly average.
• Phosphorus: The plant operates under a river-wide waste load allocation, generally requiring effluent phosphorus to be < 1.0 mg/L.
• Mercury: Due to statewide mercury TMDLs, the plant has stringent monitoring and minimization plans for mercury in both effluent and incinerator stack emissions.

B. Compliance History

The Metro Plant has an exemplary compliance record. It frequently receives the National Association of Clean Water Agencies (NACWA) Platinum Peak Performance Award, which recognizes facilities with five or more consecutive years of 100% compliance with NPDES permit limits.

7. OPERATIONAL EXCELLENCE

A. Staffing

The complex operation requires a staff of over 200 professionals, including heavy equipment operators, skilled trades (electricians, pipefitters), process engineers, and laboratory scientists. Operators are required to hold Class A wastewater certifications from the state of Minnesota due to the plant’s size and complexity.

B. Technology & Innovation

MCES utilizes a robust SCADA system for real-time monitoring. The plant is also a hub for research, frequently partnering with the University of Minnesota for studies on emerging contaminants, PFAS fate and transport, and viral surveillance (including COVID-19 wastewater epidemiology).

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

• PFAS Management: As regulations around Per- and Polyfluoroalkyl Substances tighten, the plant faces challenges regarding the fate of these chemicals in the incineration process and potential air deposition or ash management.
• Aging Infrastructure: Managing the replacement of massive interceptor sewers approaching 80-100 years of age requires significant capital investment.

B. Future Planning

The MCES 2040 Water Resources Policy Plan outlines the long-term vision. Major initiatives include increasing energy efficiency to approach energy neutrality and enhancing resource recovery (nutrients and energy). The “Metro Plant Asset Management Program” dictates a rolling schedule of rehabilitation to ensure the facility remains viable well into the 21st century.

9. COMMUNITY & REGIONAL IMPACT

The Metro Plant’s impact on the region cannot be overstated. Before the plant’s upgrades in the 1980s and the introduction of Bio-P, the Mississippi River south of the Twin Cities was often hypoxic. Today, the river supports a healthy population of game fish, bald eagles, and recreational activity. The plant enables the economic density of the Twin Cities by providing reliable, invisible sanitation services.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

The Metro Plant is the hub of a regional system. It is supported by massive lift stations and interceptors. It works in conjunction with smaller regional reclamation plants, such as the Blue Lake Wastewater Treatment Plant (Shakopee) and the Empire Wastewater Treatment Plant (Farmington), which serve the outer ring suburbs, allowing the Metro Plant to focus on the core urban flow.

12. FAQ SECTION

Technical/Professional Questions

1. Does the Metro Plant use Anaerobic Digestion?
No. Unlike many large facilities that use anaerobic digestion to produce biogas, the Metro Plant utilizes thermal destruction (Fluidized Bed Incineration) of raw sludge to recover energy via steam turbines and reduce solids volume.

2. How does the plant meet low phosphorus limits?
The plant utilizes Biological Phosphorus Removal (Bio-P) by creating anaerobic zones in the aeration basins, selecting for Phosphorus Accumulating Organisms (PAOs). Chemical addition (alum or ferric) is available as a backup/polish but is not the primary removal mechanism.

3. Is the facility under a consent decree?
Currently, the MCES system is not under a federal consent decree for CSOs/SSOs, largely due to a successful long-term separation of sewers in Minneapolis and Saint Paul over the last 30 years.

4. What is the current status of the Fourth Incinerator?
The Fourth Incinerator project is complete and the unit is operational, providing necessary redundancy for the solids handling train.

Public Interest Questions

5. Does the plant smell?
While wastewater treatment has inherent odors, the Metro Plant employs advanced chemical scrubbers and carbon filters at the headworks and solids handling buildings to minimize off-site odors. The “industrial” smell in the area is often a mix of the plant and nearby heavy industry/refineries.

6. Where does the water go?
The treated water is discharged into the Mississippi River. It is clean enough to support aquatic life, and water quality monitoring shows the river is healthier now than it has been in decades.

7. Who runs the plant?
The plant is operated by the Metropolitan Council Environmental Services (MCES), a regional policy-making body, planning agency, and provider of essential services for the Twin Cities metro area.