City Of Rochester Water Reclamation Plant

A Technical Review of Minnesota’s Premier High Purity Oxygen Wastewater Facility

FACILITY BASIC INFORMATION

Plant Name: City of Rochester Water Reclamation Plant (WRP)

Location: 301 37th Street NW, Rochester, Olmsted County, Minnesota 55901

Operating Authority: City of Rochester Public Works Department

Design Capacity: 21.7 MGD (Average Wet Weather), 48.2 MGD (Peak Hourly)

Current Average Flow: ~13.0 MGD

Population Served: ~122,000 residents

Service Area: City of Rochester and immediate surrounding townships

Receiving Water Body: Zumbro River (South Fork)

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NPDES Permit Number: MN0024970

Original Commissioning: 1957 (Major HPO conversion in 1980s)

1. INTRODUCTION

The City of Rochester Water Reclamation Plant (WRP) serves as the environmental backbone for one of the Midwest’s fastest-growing economic hubs, anchored by the world-renowned Mayo Clinic. As a Class A facility, the plant treats an average of 13 million gallons per day (MGD) of wastewater, safeguarding the water quality of the South Fork of the Zumbro River. Distinguished by its utilization of a High Purity Oxygen (HPO) activated sludge process—a rarity among mid-sized municipalities—the facility achieves high-rate treatment within a compact physical footprint.

Operated by the City of Rochester Public Works Department, the WRP plays a critical role in the “Destination Medical Center” (DMC) initiative, a multi-billion dollar economic development strategy that relies on robust utility infrastructure. With recent investments exceeding $25 million in anaerobic digestion and energy recovery, the facility stands as a model of resource recovery in a cold-weather climate, consistently meeting stringent MPCA (Minnesota Pollution Control Agency) standards for phosphorus and nitrogen removal.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The WRP services the City of Rochester and limited portions of the surrounding townships in Olmsted County. The collection system encompasses over 500 miles of sanitary sewer mains and 16 lift stations. The influent profile is unique due to the significant contribution from the medical sector and hospitality industries supporting the Mayo Clinic, though the waste stream remains predominantly domestic in strength.

B. Operational Capacity

The plant is designed to handle substantial hydraulic variability:

• Design Average Wet Weather Flow: 21.7 MGD
• Peak Hourly Wet Weather Flow: 48.2 MGD
• Current Capacity Utilization: Approx. 60%

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This excess capacity places the City in a strong position to accommodate projected population growth over the next 20 years without requiring immediate expansion of biological tankage.

C. Discharge & Compliance

Treated effluent is discharged into the South Fork of the Zumbro River. The river is classified as a 2B water body (suitable for aquatic life and recreation). The plant operates under strict NPDES permit limits, particularly concerning nutrients, to protect the downstream Lake Pepin watershed. The facility maintains a stellar compliance record, frequently receiving the MPCA Certificate of Commendation for perfect permit adherence.

3. TREATMENT PROCESS

The Rochester WRP utilizes a physical-chemical-biological treatment train. The core differentiator of this facility is the secondary treatment phase, which utilizes High Purity Oxygen rather than conventional atmospheric aeration.

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through mechanical bar screens to remove large debris. Following screening, the flow enters vortex grit removal systems. These systems use centrifugal force to settle out inorganic solids (sand, gravel, eggshells) that could cause abrasive wear on downstream pumps. The headworks building is fully enclosed with dedicated chemical scrubbers for odor control.

B. Primary Treatment

The plant utilizes rectangular primary clarifiers equipped with chain-and-flight mechanisms. Here, approximately 60-70% of Total Suspended Solids (TSS) and 30-40% of Biochemical Oxygen Demand (BOD) are removed via gravity settling. Ferric chloride is added at this stage to precipitate phosphorus, a critical step for meeting the Zumbro River nutrient limits. Primary sludge is pumped directly to the anaerobic digesters.

C. Secondary Treatment: High Purity Oxygen (HPO)

Unlike conventional activated sludge plants that blow ambient air (21% oxygen) into basins, the Rochester WRP uses an enclosed reactor High Purity Oxygen (HPO) system.

• Oxygen Generation: The plant generates onsite oxygen (90%+ purity) using a Vacuum Swing Adsorption (VSA) system (which replaced the older cryogenic plant).
• The Process: Pure oxygen and settled wastewater are introduced into covered, multi-stage aeration tanks. Mechanical surface aerators facilitate the transfer of oxygen into the mixed liquor.
• Engineering Advantage: The high partial pressure of oxygen drives dissolution rates much higher than conventional air, allowing the plant to maintain a higher Mixed Liquor Suspended Solids (MLSS) concentration. This results in a significantly reduced hydraulic retention time (HRT) and a smaller physical footprint for the aeration basins.
• Secondary Clarification: Mixed liquor flows to circular final clarifiers where the biological floc settles. Return Activated Sludge (RAS) is recycled to the HPO tanks, while Waste Activated Sludge (WAS) is thickened via Dissolved Air Flotation (DAF) before digestion.

D. Disinfection

The plant utilizes Ultraviolet (UV) Disinfection. The UV system consists of banks of low-pressure, high-output lamps submerged in the effluent channel. This physical disinfection method scrambles the DNA of pathogens (fecal coliform/E. coli), preventing reproduction without adding chemical byproducts to the Zumbro River. This system operates seasonally (April through October) as mandated by the MPCA.

F. Solids Handling & Biosolids

The solids handling train is designed for energy recovery and volume reduction:

• Thickening: WAS is thickened using Dissolved Air Flotation (DAF) units.
• Anaerobic Digestion: Primary sludge and thickened WAS are blended and fed into mesophilic anaerobic digesters. The system operates at approximately 98°F, reducing volatile solids and producing methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-performance centrifuges, producing a cake with 20-25% solids content.
• Disposal: The resulting Class B biosolids are land-applied on permitted agricultural fields in Olmsted and surrounding counties, returning nutrients to the soil.

4. INFRASTRUCTURE & FACILITIES

A. Energy Systems & Biogas

The WRP is a net energy producer regarding heat. Biogas generated in the anaerobic digesters is captured and treated. This gas is utilized in dual-fuel boilers to heat the digesters and facility buildings, significantly offsetting natural gas purchases during Minnesota’s harsh winters. The plant focuses heavily on thermal efficiency to maximize the value of this renewable fuel.

B. Odor Control

Due to the facility’s proximity to expanding residential zones, odor control is paramount. The primary odor control strategy is the containment provided by the covered HPO aeration tanks. Additionally, the headworks and solids processing buildings are kept under negative pressure, with foul air treated via chemical wet scrubbers and activated carbon filters before release.

5. RECENT UPGRADES & MAJOR PROJECTS

Anaerobic Digester Complex Improvements (2015-2017)

• Cost: ~$12 Million
• Scope: Comprehensive rehabilitation of the existing digesters, installation of new mixing systems (linear motion mixers), and upgrades to the heat exchangers.
• Driver: Aging infrastructure and the need to improve volatile solids reduction (VSR) efficiency.
• Result: Increased biogas production and improved sludge stability for land application.

Phosphorus Removal Optimization

• Scope: Optimization of chemical feed systems (ferric chloride) and logic controls.
• Driver: Minnesota’s River Eutrophication Standards (RES) impacting the Zumbro River watershed.
• Technical Highlight: The plant achieves effluent phosphorus levels consistently below 0.8 mg/L through chemical precipitation integrated with the primary clarification process.

Current/Upcoming: Sewer Master Plan Implementation

The city is currently executing phases of its Wastewater Master Plan to address the Destination Medical Center growth. This includes upsizing major interceptors feeding the plant and evaluating the long-term conversion of the oxygen generation system to ensure reliability for the next 20 years.

6. REGULATORY COMPLIANCE

The Rochester WRP operates under NPDES Permit MN0024970. Key parameters include:

• CBOD5: Monthly average limit of 25 mg/L (Plant typically achieves < 5 mg/L).
• TSS: Monthly average limit of 30 mg/L (Plant typically achieves < 5 mg/L).
• Phosphorus: Strict mass loading limits to protect the Lake Pepin watershed.
• Ammonia: Seasonal limits to prevent toxicity to aquatic life.

The facility consistently earns the MPCA Operational Award, given to facilities that maintain perfect compliance with permit limits for extended periods.

7. TECHNICAL SPECIFICATIONS SUMMARY

8. CHALLENGES & FUTURE PLANNING

Chloride Management

Like many Minnesota municipalities, Rochester faces challenges with chloride (salt) levels in wastewater, primarily derived from residential water softeners. Because reverse osmosis is prohibitively expensive for full-plant flow, the City is focusing on source reduction strategies and high-efficiency water softener rebates rather than end-of-pipe treatment.

PFAS (Forever Chemicals)

The MPCA is actively monitoring PFAS levels statewide. Rochester is proactively monitoring influent and biosolids to understand the fate and transport of these compounds, preparing for potential future regulation regarding biosolids land application.

9. FAQ SECTION

Technical Questions

1. Why does Rochester use High Purity Oxygen (HPO) instead of conventional aeration?
HPO was selected during the 1980s expansion because it allows for a much higher biomass concentration in the aeration tanks. This enables the plant to treat high loads in a very small footprint, which was critical given the site constraints at the time.

2. How is the oxygen generated on-site?
The plant utilizes Vacuum Swing Adsorption (VSA) technology, which separates oxygen from atmospheric air using specialized molecular sieves. This provides a steady stream of >90% purity oxygen to the reactors.

3. Does the plant generate electricity from biogas?
Currently, the plant focuses on thermal energy recovery. The biogas is burned in boilers to heat the digesters and buildings. While cogeneration (CHP) has been evaluated, the economics of thermal offset currently outweigh electricity generation.

Public Interest Questions

4. Does the Mayo Clinic have its own treatment plant?
No. The Mayo Clinic discharges to the City of Rochester’s collection system. The WRP treats this flow along with the rest of the city’s residential and commercial wastewater.

5. Is the discharged water safe for the Zumbro River?
Yes. The effluent meets all state and federal standards for aquatic life safety. The UV disinfection process ensures that bacteria levels are well below limits for recreational contact during the summer season.

Note: This document is intended for informational purposes for engineering professionals. Specific operational parameters may vary based on seasonal conditions and ongoing optimization efforts. For official regulatory data, please consult the Minnesota Pollution Control Agency (MPCA).