Greater Peoria Sanitary District Wastewater Treatment Plant

1. Introduction

The Greater Peoria Sanitary District (GPSD) Wastewater Treatment Plant represents a cornerstone of critical infrastructure in Central Illinois, providing advanced wastewater treatment and resource recovery for the City of Peoria and surrounding municipalities. Commissioned originally in 1931 and continuously modernized, the facility operates with a design average flow of 37 million gallons per day (MGD) and a peak hydraulic capacity of 154 MGD to manage significant wet-weather events common to the river valley.

Distinguished by its aggressive approach to energy neutrality and sustainability, the GPSD facility is not merely a disposal site but a resource recovery center. It features one of the region’s most robust biogas-to-energy programs, utilizing local high-strength organic waste to fuel cogeneration engines that power a significant portion of plant operations. Serving a population equivalent of approximately 135,000 residents, the plant plays a vital role in protecting the water quality of the Illinois River while setting industry benchmarks for biosolids management and energy efficiency.

2. Facility Overview

A. Service Area & Coverage

GPSD serves a comprehensive 55-square-mile service area encompassing the City of Peoria, the Village of Peoria Heights, the Village of Bartonville, West Peoria, and unincorporated areas of Peoria County. The collection system is a complex network comprising over 570 miles of sanitary sewers ranging from 6 inches to 84 inches in diameter. The district maintains extensive interceptor sewers that convey flow by gravity along the riverfront, supplemented by strategic lift stations to manage topography.

B. Operational Capacity

The facility is designed to handle extreme flow variations due to the combined sewer systems (CSS) present in older parts of the service area.

• Design Average Flow: 37 MGD
• Design Maximum Flow: 60 MGD (Secondary Treatment Capacity)
• Peak Hydraulic Capacity: 154 MGD (utilizing excess flow facilities)
• Average Daily Flow: ~23-26 MGD (Dry Weather)

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Historically, the plant has demonstrated high resilience during flood stages of the Illinois River, employing flood protection levees and pump stations to maintain operations even when river levels exceed discharge elevations.

C. Discharge & Compliance

Treated effluent is discharged directly into the Illinois River via multiple outfalls. The facility operates under National Pollutant Discharge Elimination System (NPDES) Permit No. IL0023914, administered by the Illinois Environmental Protection Agency (IEPA). The discharge is monitored strictly for Carbonaceous Biochemical Oxygen Demand (CBOD5), Total Suspended Solids (TSS), Ammonia Nitrogen, and Fecal Coliform during disinfection seasons.

3. Treatment Process

The GPSD facility utilizes a conventional activated sludge process supplemented by specialized wet-weather treatment trains and an advanced anaerobic digestion complex.

A. Preliminary Treatment

Raw wastewater enters the headworks where it passes through mechanically cleaned bar screens to remove large debris, rags, and plastics. Following screening, the flow enters aerated grit chambers where velocity is reduced to allow inorganic solids (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The grit is classified, washed, and dewatered for landfill disposal. Odor control at the headworks is managed via chemical scrubbers and biofilters to mitigate H2S emissions.

B. Primary Treatment

Flow is distributed to primary clarifiers where approximately 60-70% of suspended solids and 30-40% of BOD are removed via gravity settling. The settled solids (primary sludge) are pumped to the anaerobic digesters, while floating grease and scum are skimmed from the surface. During high-flow events exceeding secondary capacity, excess flow is routed through primary treatment and disinfection before discharge to minimize untreated CSO releases.

C. Secondary Treatment (Activated Sludge)

The biological core of the plant consists of aeration tanks utilizing a plug-flow activated sludge configuration.

• Aeration: Compressed air is introduced via fine-bubble diffusers to provide oxygen for microorganisms that metabolize dissolved organics.
• Process Control: The system is operated to achieve nitrification (conversion of ammonia to nitrate) to meet strict ammonia limits.
• Secondary Clarification: Mixed liquor flows to secondary clarifiers where biological floc settles. Activated sludge is returned (RAS) to the aeration tanks to maintain biomass, while excess biomass (WAS) is wasted to the solids handling train.

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D. Tertiary Filtration & Disinfection

While the plant does not use traditional sand filters for dry weather flow, it utilizes Ultraviolet (UV) disinfection. The effluent passes through UV channels where high-intensity light alters the DNA of pathogenic bacteria (fecal coliform), rendering them unable to reproduce. This eliminates the need for chlorination and subsequent dechlorination, removing residual toxicity risks to the Illinois River aquatic life.

F. Solids Handling & Resource Recovery

GPSD is a leader in solids processing, transforming waste into energy and soil amendments.

• Thickening: Waste Activated Sludge is thickened using gravity belt thickeners or dissolved air flotation (DAF) prior to digestion.
• Anaerobic Digestion: Thickened sludge and primary sludge are blended and fed into anaerobic digesters. These tanks are heated and mixed, reducing volatile solids and producing methane-rich biogas.
• Co-Digestion: GPSD accepts High Strength Organic Waste (HSOW) from industrial and commercial sources (brewery waste, food processing byproducts). This feedstock significantly boosts biogas production.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges or belt filter presses.
• Biosolids Disposal: The resulting Class B biosolids are land-applied to local farmland as a nutrient-rich fertilizer, closing the nutrient loop.

4. Infrastructure & Facilities

A. Energy Systems & Cogeneration

A defining feature of the GPSD infrastructure is its Combined Heat and Power (CHP) system. The facility utilizes biogas produced in the digesters to fuel multiple Caterpillar internal combustion engines/generators.

• Power Generation: The cogeneration system supplies a substantial percentage (often 40-60%) of the plant’s electrical demand.
• Heat Recovery: Waste heat from the engine jackets and exhaust is captured to heat the anaerobic digesters and facility buildings, maximizing thermal efficiency.

B. Physical Plant

The site encompasses over 75 acres along the Illinois River. The facility includes dedicated maintenance workshops, a fully equipped administration building, and an NELAP-accredited laboratory responsible for process control and compliance testing. The site is protected by a levee system designed to withstand historic flood levels of the Illinois River.

5. Recent Upgrades & Major Projects

Electrical Distribution & Biogas Enhancement (2018-2023)

Scope: Replacement of aging electrical switchgear and upgrades to the biogas conditioning system.
Drivers: Increasing reliability and maximizing the efficiency of the cogeneration engines. The project included the installation of new gas cleaning skids to remove siloxanes and moisture from biogas, protecting the generator engines and extending maintenance intervals.

Headworks Improvements

Recent capital projects have focused on the replacement of mechanical bar screens and the rehabilitation of grit removal systems. These upgrades were critical to protecting downstream pumps and sludge processing equipment from ragging and abrasive wear, particularly given the older nature of the collection system.

6. Regulatory Compliance & Environmental Performance

A. Permit Requirements

The facility operates under a stringent NPDES permit that dictates effluent quality. Key parameters include:

• CBOD5: Monthly average limit of 25 mg/L.
• TSS: Monthly average limit of 30 mg/L.
• Ammonia Nitrogen: Seasonal limits, typically tighter in summer (e.g., 1.5 mg/L) to prevent toxicity to aquatic life.
• Phosphorus: Illinois is aggressively targeting nutrient reduction. GPSD is currently monitoring phosphorus and developing feasibility studies for future removal limits (0.5 mg/L or 1.0 mg/L targets).

B. Compliance History

GPSD maintains a strong compliance record with the IEPA. The facility has successfully navigated the challenges of the Illinois River Nutrient Loss Reduction Strategy, proactively optimizing operations to reduce nutrient loading even ahead of strict regulatory mandates.

7. Operational Excellence

A. Staffing & Expertise

The facility is staffed by a team of approximately 50-60 professionals, including Illinois Class 1 Certified Wastewater Operators, maintenance mechanics, electricians, laboratory technicians, and administrative staff. The District emphasizes continuous professional development to maintain licensure and technical proficiency.

B. Technology & Innovation

GPSD utilizes a comprehensive SCADA (Supervisory Control and Data Acquisition) system for real-time monitoring of all process variables. The District has historically partnered with research institutions, such as the University of Illinois, to pilot innovative technologies. Notable past initiatives include the “Bio-Crude” pilot project, which explored hydrothermal liquefaction (HTL) to convert wet wastewater biosolids into crude oil, demonstrating the District’s commitment to cutting-edge research.

8. Challenges & Future Planning

A. Nutrient Removal Standards

The most significant future challenge is the impending imposition of strict Total Phosphorus limits by the IEPA. Achieving these low limits (potential for 0.5 mg/L) will likely require significant capital investment in chemical precipitation systems (Ferric/Alum) or biological phosphorus removal (Bio-P) retrofits to the existing aeration basins.

B. Aging Infrastructure

With original structures dating back to 1931, asset management is a constant priority. The District maintains a capital improvement plan focused on the systematic rehabilitation of concrete structures, pipe galleries, and electrical systems that are reaching the end of their useful life.

C. Climate Resilience

Located on the banks of the Illinois River, the facility must account for increasing flood frequencies and intensities. Future planning includes reinforcing flood protection measures and ensuring critical electrical gear is elevated or hardened against potential inundation.

10. Technical Specifications Summary

12. FAQ Section

Technical Questions

1. Does GPSD perform biological nutrient removal (BNR)?
Currently, the plant operates for ammonia removal (nitrification). While it does not yet have a full mandate for Total Nitrogen or Biological Phosphorus removal, the District is conducting feasibility studies to retrofit the plant for future phosphorus limits required by the Illinois Nutrient Loss Reduction Strategy.

2. What engines are used for cogeneration?
The facility utilizes Caterpillar internal combustion engines modified for biogas use. This leverages the local proximity to Caterpillar’s headquarters and support network.

3. How are wet weather flows managed?
Flows exceeding secondary capacity are routed through the “Excess Flow” train, which provides primary settling and disinfection before blending with secondary effluent or discharging, in compliance with the NPDES permit provisions for CSOs.

Public Interest Questions

4. What is the flame sometimes seen at the plant?
The flame is the waste gas flare. When the cogeneration engines are under maintenance or biogas production exceeds engine capacity, the excess gas is safely burned off to prevent methane release into the atmosphere.

5. Does the plant smell?
Wastewater treatment inherently involves odors, but GPSD employs advanced odor control technologies, particularly at the headworks and sludge processing areas, to capture and scrub air before it is released. The District actively monitors and manages odors to minimize impact on neighbors.