Metropolitan Council Environmental Services Blue Lake Wastewater Treatment Plant Shakopee

1. INTRODUCTION

The Blue Lake Wastewater Treatment Plant (WWTP) is a cornerstone of the sanitary infrastructure serving the rapidly growing southwest region of the Twin Cities metropolitan area. Operated by Metropolitan Council Environmental Services (MCES), this advanced secondary treatment facility processes an average of 32 million gallons per day (MGD) of wastewater, protecting the water quality of the Minnesota River. Originally commissioned in 1971 and significantly expanded in subsequent decades, the plant serves over 300,000 residents across 28 communities, including Shakopee, Prior Lake, Chaska, and Eden Prairie.

Distinguished by its advanced resource recovery capabilities, Blue Lake operates a sophisticated solids handling facility that produces Class A Exceptional Quality (EQ) biosolids pellets for use as agricultural fertilizer. The facility represents a model of modern environmental engineering, balancing stringent nutrient discharge limits—specifically regarding phosphorus reduction in the Minnesota River basin—with energy-efficient operations and resource reclamation.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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The Blue Lake WWTP serves a 260-square-mile area primarily covering Scott County and portions of eastern Carver and southern Hennepin counties. This catchment area includes some of the fastest-growing municipalities in Minnesota. The collection system feeding the plant consists of a complex network of regional interceptors and lift stations that convey flow from residential, commercial, and industrial sources. Industrial contributions are monitored via a strict industrial pretreatment program to prevent process upsets.

B. Operational Capacity

While the facility is designed for a hydraulic capacity of 52 MGD, current average daily flows hover between 28 and 32 MGD. The plant is engineered to handle significant wet-weather peaking factors, with peak hourly flow capabilities exceeding 80 MGD to manage inflow and infiltration (I/I) events common in the Minnesota River valley. Historical flow trends indicate a steady upward trajectory correlating with suburban housing expansion in the southwest metro, necessitating ongoing master planning for future capacity expansions.

C. Discharge & Compliance

Treated effluent is discharged directly into the Minnesota River via a submerged outfall. The Minnesota River is a sensitive waterway with established Total Maximum Daily Loads (TMDLs) for sediment and nutrients. Consequently, the Blue Lake facility operates under a stringent National Pollutant Discharge Elimination System (NPDES) permit issued by the Minnesota Pollution Control Agency (MPCA). The plant consistently achieves high removal rates for Total Suspended Solids (TSS) and Carbonaceous Biochemical Oxygen Demand (CBOD), and maintains aggressive effluent phosphorus limits to mitigate eutrophication downstream.

3. TREATMENT PROCESS

The Blue Lake WWTP utilizes a continuous flow, activated sludge process with specialized configurations for biological nutrient removal. The treatment train is designed to ensure compliance with MPCA river standards while generating energy-rich biogas and reusable biosolids.

A. PRELIMINARY TREATMENT

Raw wastewater enters the headworks where it undergoes physical screening. Several mechanically cleaned bar screens remove large debris, rags, and plastics to protect downstream pumps. Following screening, flow enters aerated grit chambers where heavier inorganic materials (sand, gravel, coffee grounds) settle out. The grit is washed and dewatered before landfill disposal. The headworks area is enclosed and ventilated through chemical scrubbers for odor control.

B. PRIMARY TREATMENT

Flow proceeds to rectangular primary clarifiers. Here, hydraulic velocity is reduced to allow settable organic solids to drop to the bottom as primary sludge, while fats, oils, and grease (FOG) float to the surface for skimming. Primary treatment typically removes 50-60% of suspended solids and 30-35% of BOD. Primary sludge is pumped to gravity thickeners before anaerobic digestion.

C. SECONDARY TREATMENT

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The biological heart of the plant is the activated sludge system. The aeration basins are configured to support Biological Phosphorus Removal (Bio-P). This involves anaerobic zones followed by aerobic zones to select for Phosphate Accumulating Organisms (PAOs). By uptake of phosphorus into the cellular biomass, the plant significantly reduces the need for chemical precipitants (like ferric chloride), though chemical backup systems exist.
Fine bubble diffusion provides oxygen to the mixed liquor, facilitating the breakdown of dissolved organic matter and nitrification (conversion of ammonia to nitrate). Secondary clarifiers then separate the biomass from the treated water; the Return Activated Sludge (RAS) is recycled to the front of the aeration tanks, while Waste Activated Sludge (WAS) is sent to the solids processing train.

D. DISINFECTION

Clear effluent from the secondary clarifiers enters chlorine contact tanks. Liquid sodium hypochlorite is applied to deactivate pathogenic microorganisms (bacteria and viruses). Prior to discharge into the Minnesota River, the water is dechlorinated using sodium bisulfite to prevent toxicity to aquatic life. This system operates seasonally or year-round as dictated by the NPDES permit requirements for fecal coliform/E. coli.

E. SOLIDS HANDLING & RESOURCE RECOVERY

Blue Lake utilizes a distinct and advanced solids processing train:

• Thickening: Primary sludge is gravity thickened; WAS is thickened using dissolved air flotation (DAF) or centrifugal thickeners.
• Anaerobic Digestion: Thickened sludge is pumped to anaerobic digesters heated to mesophilic temperatures (~98°F). This process stabilizes the solids, reduces volatile content, and produces methane-rich biogas.
• Dewatering: Digested biosolids are dewatered using high-solids centrifuges to create a “cake.”
• Heat Drying (Pelletization): The dewatered cake is processed in thermal dryers. This creates a pathogen-free, Class A EQ biosolid pellet. These pellets are marketed as a high-value fertilizer for agriculture, reducing the volume of waste and closing the nutrient loop.

F. PROCESS CONTROL

The entire facility is monitored via a centralized SCADA (Supervisory Control and Data Acquisition) system. Dissolved Oxygen (DO) probes, ORP sensors, and online nutrient analyzers allow operators to optimize blower output and return rates in real-time, maximizing energy efficiency and treatment stability.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The Blue Lake site occupies a substantial footprint along the highway corridor. The facility includes separate structures for headworks, primary tanks, aeration basins, final clarifiers, and the distinct Solids Management Building. The site also houses extensive maintenance shops and a process control laboratory certified for compliance testing.

B. Energy Systems

Energy recovery is a priority at Blue Lake. The biogas generated during anaerobic digestion is captured, scrubbed, and utilized in on-site boilers to heat the digesters and facility buildings, offsetting natural gas purchases. The plant implements Variable Frequency Drives (VFDs) on major pumps and aeration blowers to match energy consumption with hydraulic loading.

C. Odor Control

Given the proximity to Highway 169 and commercial developments, odor control is critical. The plant utilizes a combination of chemical scrubbers and activated carbon systems, particularly focusing on the headworks and solids processing areas, to capture hydrogen sulfide and other odorous compounds before air is released.

5. RECENT UPGRADES & MAJOR PROJECTS

Solids Renewal and Improvement Project (2018-2022)

• Scope: Comprehensive overhaul of the solids processing train, including replacement of centrifuges, dryer refurbishment, and safety system upgrades.
• Budget: ~$30 Million
• Project Drivers: Aging infrastructure and the need to maintain Class A biosolids reliability.
• Technical Highlights: Installation of new high-efficiency dewatering centrifuges improved cake dryness, reducing the thermal load on the dryers and saving energy.

Interceptor Rehabilitation and Lift Station Improvements

• Scope: Rehabilitation of the regional interceptors (L-70) feeding the plant to prevent corrosion and I/I.
• Result: Extended service life of collection assets and reduced peak wet weather flows reaching the headworks.

Current/Upcoming Planning (2024-2027)

MCES is currently in the planning stages for future liquid stream capacity expansions. As the southwest metro population continues to densify, the Blue Lake plant will likely require additional aeration volume and clarification capacity to handle flows approaching the 40+ MGD range within the next decade.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under MPCA NPDES Permit MN0029157. Key effluent limits typically include:

• CBOD5: Monthly average limits (typically < 15-25 mg/L).
• TSS: Monthly average limits (typically < 30 mg/L).
• Phosphorus: Strict mass loading limits (kg/day) to address river eutrophication, often requiring effluent concentrations averaging < 1.0 mg/L or lower depending on river flow conditions.

B. Compliance History

Blue Lake WWTP has a strong record of compliance, frequently earning the National Association of Clean Water Agencies (NACWA) Peak Performance Awards (Silver and Gold) for consecutive years of permit compliance. The facility is a key contributor to the overall improvement of water quality in the lower Minnesota River over the last three decades.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. RELATED FACILITIES

Seneca Wastewater Treatment Plant: Located directly across the Minnesota River in Eagan, the Seneca plant serves the southeastern metro area and shares similar discharge characteristics into the Minnesota River.

Blue Lake Lift Stations: The plant is supported by a network of major lift stations, including the Blue Lake Lift Station and various interceptor metering stations that manage flow from Chaska and Prior Lake.

12. FREQUENTLY ASKED QUESTIONS

Technical Questions

Q: Does Blue Lake WWTP utilize chemical phosphorus removal?
A: While the plant is designed for Biological Phosphorus Removal (Bio-P) to reduce chemical costs and sludge production, it retains the capability to add ferric chloride or alum as a polishing step or backup to ensure strict permit compliance.

Q: How is the biogas utilized?
A: Methane produced in the anaerobic digesters is captured and treated to remove moisture and siloxanes, then combusted in boilers to provide process heat for the digesters and heating for facility buildings.

Q: What is the dry solids content of the final biosolids product?
A: The thermal drying process produces pellets with a solids content typically exceeding 90%, making them stable, pathogen-free, and suitable for bulk storage and agricultural application.

Public Interest Questions

Q: Does the plant smell?
A: Wastewater treatment naturally generates odors, but Blue Lake employs advanced chemical scrubbers and activated carbon filters to treat foul air from the headworks and solids processing buildings. The Council maintains a 24-hour odor complaint line to address community concerns immediately.

Q: Where does the water go after treatment?
A: The highly treated, clean effluent is discharged into the Minnesota River, where it eventually flows into the Mississippi River. The water quality often exceeds that of the river itself.