City Of Duluth Western Lake Superior Sanitary District

TARGET AUDIENCE

• Municipal consulting engineers evaluating co-incineration technologies
• Wastewater treatment plant operators and managers
• Environmental regulators focusing on Great Lakes water quality
• Industrial pretreatment coordinators
• Energy managers interested in CHP and waste-to-energy systems

1. INTRODUCTION

The Western Lake Superior Sanitary District (WLSSD) Regional Treatment Plant represents a unique convergence of wastewater treatment and solid waste management within the United States infrastructure landscape. Located in Duluth, Minnesota, on the banks of the St. Louis River, this facility serves as the environmental guardian for the western tip of Lake Superior—the largest freshwater lake in the world by surface area. Commissioned in 1978, WLSSD was created by the Minnesota Legislature to address severe pollution problems in the St. Louis River basin, consolidating numerous failing municipal plants and industrial outfalls into a single, advanced regional system.

What distinguishes WLSSD technically is its integrated approach to waste. It is one of the few facilities in North America that co-incinerates sewage sludge with Refuse Derived Fuel (RDF) processed from municipal solid waste to generate process steam and electricity. Treating an average of 35-40 million gallons daily (MGD) with a significant industrial loading component, the facility utilizes high-purity oxygen activated sludge treatment to handle high-strength waste in cold climate conditions. With ongoing investments in energy recovery and nutrient reduction, WLSSD remains a model of holistic environmental engineering.

2. FACILITY OVERVIEW

A. Service Area & Coverage

WLSSD serves a massive 530-square-mile region encompassing the cities of Duluth, Cloquet, Hermantown, Proctor, Carlton, Scanlon, Thomson, and Wrenshall, along with nine surrounding townships. The service area is characterized by challenging topography—Duluth is built on a steep hillside—necessitating a complex collection system. The distinct aspect of the WLSSD service area is its heavy industrial component; the district treats wastewater from major industrial users, including the Sappi pulp and paper mill in Cloquet, which contributes significant organic loading to the plant via dedicated interceptors.

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B. Operational Capacity

The facility is designed for an average wet weather flow of 43 MGD, though it routinely handles dry weather flows closer to 30 MGD. However, the system is engineered to handle extreme peaking factors due to Inflow and Infiltration (I/I) inherent in the region’s aging collection infrastructure and rocky geology. Peak instantaneous flows can exceed 120 MGD during spring thaw and heavy storm events. The plant’s hydraulic profile and pumping systems are robustly designed to manage these rapid fluctuations to prevent combined sewer overflows (CSOs).

C. Discharge & Compliance

Treated effluent is discharged into the St. Louis River Estuary, a sensitive ecosystem that serves as the primary tributary to Lake Superior. The facility operates under NPDES Permit MN0049786, issued by the Minnesota Pollution Control Agency (MPCA). Due to the status of Lake Superior as an Outstanding Resource Value Water (ORVW), the plant faces stringent limits on mercury, phosphorus, and conventional pollutants. The facility has historically maintained an excellent compliance record, playing a pivotal role in the delisting of the St. Louis River as a severe “Area of Concern.”

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

The headworks utilizes mechanical bar screens to remove large debris, followed by grit removal systems. Given the combined nature of older parts of the collection system and the grit accumulation from winter road sanding in Duluth, grit removal is a critical protection step for downstream pumps. The headworks is enclosed and ventilated to chemical scrubbers for odor control, utilizing sodium hypochlorite and caustic soda to neutralize hydrogen sulfide and other odorous compounds.

B. PRIMARY TREATMENT

Flow proceeds to primary clarification, where heavy solids settle and floatables are skimmed. The primary clarifiers are circular units designed to reduce Total Suspended Solids (TSS) and Biochemical Oxygen Demand (BOD) significantly before the biological stage. Primary sludge is pumped directly to the solids processing train for thickening and incineration.

C. SECONDARY TREATMENT (High-Purity Oxygen)

The core of WLSSD’s liquid treatment is a High-Purity Oxygen (HPO) Activated Sludge process. Unlike conventional aeration that uses ambient air, WLSSD utilizes enclosed reactor basins (UNOX style) where high-purity oxygen is introduced.
Why Pure Oxygen? This technology was selected to handle the high-strength industrial waste (particularly from the paper industry) and to maintain high biological activity during Minnesota’s frigid winters. The high partial pressure of oxygen accelerates the transfer rate, allowing for a smaller footprint and the ability to treat higher BOD loadings compared to conventional air systems. The oxygen is generated on-site via a Cryogenic Oxygen Plant (and backup VPSA systems), which separates oxygen from air through distillation.

D. DISINFECTION

Following secondary clarification, the effluent undergoes chlorination using sodium hypochlorite for pathogen inactivation. The plant utilizes a serpentine contact basin to ensure adequate contact time (CT). Prior to discharge into the sensitive St. Louis River, the effluent is dechlorinated using sodium bisulfite to protect aquatic life from chlorine toxicity.

E. SOLIDS HANDLING & INCINERATION (The “Energy” Loop)

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This is the facility’s most technically distinct unit process. WLSSD operates as a waste-to-energy facility.
1. Thickening & Dewatering: Primary and waste activated sludge (WAS) are thickened (gravity belts/centrifuges) and dewatered to maximize solids content before combustion.
2. Refuse Derived Fuel (RDF): The district operates a solid waste processing facility that shreds municipal solid waste (garbage), separates metals for recycling, and creates RDF.
3. Fluidized Bed Incinerators: The dewatered biosolids are co-incinerated with the RDF in two fluidized bed incinerators. The silica sand bed, suspended by high-velocity air, ensures uniform combustion at temperatures around 1,400°F. This process destroys pathogens and organic pollutants (including PFAS precursors) effectively.
4. Ash Management: The resulting inert ash is captured via electrostatic precipitators and Venturi scrubbers and disposed of in an industrial landfill.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans over 80 acres in the Lincoln Park neighborhood of Duluth. The complex integrates the wastewater treatment plant, the solid waste transfer station, the RDF processing facility, and the household hazardous waste facility. The architecture is functional and industrial, dominated by the incinerator stack and the cryogenic oxygen production towers.

B. Energy Systems (CHP)

WLSSD is a leader in energy recovery. The heat generated from the fluidized bed incinerators is captured in waste heat recovery boilers to produce high-pressure steam. This steam drives turbines to generate electricity (Combined Heat and Power – CHP), offsetting a significant portion of the plant’s electrical demand. Additionally, steam is used for building heat and process heating, crucial during Duluth winters where temperatures can drop to -30°F.

C. Odor Control

Given the proximity to I-35 and residential areas, odor control is paramount. The facility uses a multi-tiered approach involving chemical scrubbers (wet scrubbers) for high-intensity sources like the headworks and sludge processing areas, and activated carbon polishing. Negative pressure is maintained in key process buildings to prevent fugitive emissions.

5. RECENT UPGRADES & MAJOR PROJECTS

Combined Heat and Power (CHP) Energy Project – $23 Million (2016-2018)

• Project Scope: Replacement of aging steam turbines and switchgear to maximize energy recovery capabilities from the incinerator train.
• Funding: Financed through a combination of WLSSD capital reserves and state bonding.
• Technical Highlights: Installation of new backpressure steam turbines designed to optimize electricity generation while still providing low-pressure steam for process heating.
• Results: The project allows WLSSD to produce approximately 20-30% of its own electrical needs and nearly all of its heating needs, significantly insulating the district from fluctuating energy market costs.

Oxygen Supply System Upgrade – (2020-2022)

• Project Scope: Rehabilitation and modernization of the oxygen generation infrastructure.
• Drivers: The original cryogenic plant was nearing end-of-life. Reliable oxygen supply is critical for the HPO process; without it, biological treatment fails.
• Technical Highlights: Upgrades included new compressors and cold box rehabilitation to improve energy efficiency of oxygen separation.

Upcoming: Clarifier Rehabilitation & Nutrient Optimization (2024-2027)

• Scope: Structural rehabilitation of secondary clarifiers and potential retrofits to enhance biological nutrient removal (BNR) capabilities in anticipation of stricter future phosphorus limits for the Lake Superior basin.
• Projected Budget: Estimated $15-$20 million range.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

Under NPDES Permit MN0049786, WLSSD adheres to strict effluent limits. Key parameters typically include:

• CBOD5: Strict monthly average limits (typically < 25 mg/L).
• TSS: < 30 mg/L monthly average.
• Phosphorus: 1.0 mg/L limit (Lake Superior Basin standard).
• Mercury: Extremely low limits due to bioaccumulation concerns in the Great Lakes; WLSSD has a specialized mercury minimization plan.

B. Compliance History

WLSSD is a consistent recipient of the National Association of Clean Water Agencies (NACWA) Peak Performance Awards (Gold and Platinum), indicating years of 100% compliance with permit limits. The facility has successfully navigated the complexities of treating highly variable industrial loads without upsetting the biological process.

7. OPERATIONAL EXCELLENCE

A. Staffing

The facility employs approximately 100+ staff across operations, maintenance, planning, and administration. Operators utilize a high degree of automation via a robust SCADA system to manage the integration between the solid waste fuel feed and the wastewater treatment demands.

B. Technology & Innovation

WLSSD’s laboratory is state-certified and capable of running low-level detection analysis for mercury and other emerging contaminants. The district is also at the forefront of PFAS (per- and polyfluoroalkyl substances) research, actively studying the fate of these compounds through the incineration process, with data suggesting high-temperature incineration may effectively mineralize certain PFAS compounds.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

Inflow and Infiltration (I/I): This is the district’s most significant hydraulic challenge. Duluth’s aging sanitary lines, combined with illegal sump pump connections and foundation drains, cause flow spikes during rain events. WLSSD operates a comprehensive I/I reduction grant program for homeowners and municipalities.

Emerging Contaminants: As regulations around PFAS tighten, WLSSD is evaluating source control (preventing industrial PFAS from entering the plant) versus end-of-pipe treatment.

B. Future Planning

The 10-year Capital Improvement Plan (CIP) focuses on “Asset Renewal.” With much of the plant dating to 1978, the focus is on replacing concrete, electrical switchgear, and mechanical systems. The district is also evaluating long-term alternatives for biosolids management should incineration regulations change or equipment reach obsolescence.

9. COMMUNITY & REGIONAL IMPACT

WLSSD is more than a utility; it is a regional environmental hub. By managing solid waste and wastewater under one roof, it offers a “one-stop” environmental service for the region. The facility operates a popular Household Hazardous Waste (HHW) drop-off center, keeping paints, chemicals, and mercury out of the landfill and the sewer system. Economically, the plant enables the operation of major industries like the Sappi mill by providing reliable, affordable industrial wastewater treatment that would be cost-prohibitive for the mill to perform independently.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

WLSSD operates an extensive network of interceptors and pumping stations to convey flow over the hill from Hermantown and Proctor down to the riverfront plant. Key satellite facilities include:

• Scanlon Interceptor: Key conveyance for Cloquet and Sappi flows.
• Materials Recovery Facility (MRF): Located near the main plant, processing recyclables.
• Yard Waste Compost Site: Diverts organic waste from the incinerator/landfill stream.

12. FAQ SECTION

Technical Questions

1. Does WLSSD treat industrial wastewater?
Yes. A significant portion of the plant’s loading comes from industrial sources, most notably the pulp and paper industry. The High-Purity Oxygen process was specifically chosen to handle this high-BOD industrial load.

2. How does the co-incineration process work?
WLSSD processes municipal solid waste into Refuse Derived Fuel (RDF). This fluff-like fuel is blown into fluidized bed incinerators alongside dewatered sewage sludge. The RDF provides the BTU value needed to burn the wet sludge, making the process autogenous (self-sustaining) without needing significant auxiliary natural gas.

3. What is the oxygen purity used in the aeration basins?
The cryogenic plant produces oxygen at approximately 95-98% purity, which is then fed into the covered aeration basins.

4. How is ash from the incinerator handled?
The ash is scrubbed of particulates and metals, collected, and transported to a dedicated industrial landfill. It is tested regularly for toxicity (TCLP) to ensure it meets non-hazardous disposal standards.

Public Interest Questions

5. What is the “smoke” seen coming from the plant?
The visible plume from the main stack is almost entirely water vapor (steam) resulting from the wet scrubbers used to clean the exhaust air. It is most visible in cold weather.

6. Does the plant smell?
While wastewater treatment inherently involves odors, WLSSD uses advanced chemical scrubbers and activated carbon filters to treat air from the headworks and sludge processing buildings before it is released.

7. How big is the service area?
The district covers 530 square miles, making it one of the largest sanitary districts by geographic area in the state of Minnesota.

8. Is the water safe to enter the St. Louis River?
Yes. The effluent meets strict state and federal standards and is actually cleaner than the river water itself in many parameters. WLSSD’s operation has been a primary driver in the environmental recovery of the St. Louis River estuary.