Evansville Wastewater Treatment Plant

FACILITY BASIC INFORMATION

• Plant Name: Evansville East Wastewater Treatment Plant (East WWTP)
• Location: 1301 South Waterworks Road, Evansville, IN 47713
• Operating Authority: Evansville Water and Sewer Utility (EWSU)
• Design Capacity (Average): 22.5 MGD
• Peak Design Flow: 35.0 MGD (Secondary Treatment); >50 MGD (Wet Weather Capacity)
• Population Served: ~117,000 (City of Evansville Total)
• Service Area: City of Evansville (Eastern Basin), Vanderburgh County
• Receiving Water Body: Ohio River
• NPDES Permit Number: IN0020109
• Year Commissioned: 1956 (Major expansions in 1972, 1990s, 2018-Present)

1. INTRODUCTION

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The Evansville East Wastewater Treatment Plant (East WWTP) serves as the cornerstone of wastewater infrastructure for the Evansville Water and Sewer Utility (EWSU), the largest municipality in Southern Indiana. Situated along the banks of the Ohio River, this facility is critical not only for public health in Vanderburgh County but also for the water quality of the Ohio River Basin. While the utility operates two plants (East and West), the East WWTP handles the majority of the metro area’s flow, with an average design capacity of 22.5 million gallons per day (MGD).

The facility is currently the focal point of “Renew Evansville,” a massive, federally mandated infrastructure improvement program valued at over $729 million. Driven by a Consent Decree with the EPA to address Combined Sewer Overflows (CSOs), the plant is undergoing a transformation from a conventional secondary treatment facility into a highly sophisticated wet-weather management hub. This transition involves integrating high-rate treatment technologies and massive storage solutions, making it a case study in modernizing aging combined sewer systems.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The East WWTP serves the eastern drainage basin of Evansville, which encompasses the dense, historic downtown core, industrial zones, and expanding suburban residential areas. The collection system is a hybrid of separate sanitary sewers and aging combined sewers (stormwater and wastewater in one pipe). The system includes hundreds of miles of gravity lines and force mains, supported by a network of over 90 lift stations throughout the utility’s jurisdiction.

B. Operational Capacity

The facility operates with a design average daily flow of 22.5 MGD. However, the critical engineering constraint for this facility is hydraulic peaking due to the combined sewer system. During heavy precipitation events, influent rates surge dramatically. Historically, this led to CSOs; however, recent upgrades aim to capture and treat peak flows exceeding 40 MGD through primary and advanced high-rate treatment trains before discharging.

C. Discharge & Compliance

Treated effluent is discharged into the Ohio River via a submerged outfall. The plant operates under an NPDES permit issued by the Indiana Department of Environmental Management (IDEM). Compliance is monitored strictly for Carbonaceous Biochemical Oxygen Demand (CBOD5), Total Suspended Solids (TSS), Ammonia-Nitrogen, and E. coli. As a major discharger to the Ohio River, the plant also adheres to standards set by ORSANCO (Ohio River Valley Water Sanitation Commission).

3. TREATMENT PROCESS

The Evansville East WWTP utilizes a conventional activated sludge process, recently augmented with advanced wet-weather handling capabilities. The treatment train is designed to handle variable loading associated with the combined collection system.

A. PRELIMINARY TREATMENT

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Raw wastewater enters the facility primarily via the massive Sunrise Pump Station. Preliminary treatment consists of:

• Coarse Screening: Mechanically cleaned bar screens remove large debris, rags, and plastics to protect downstream pumps.
• Grit Removal: Vortex grit chambers utilize centrifugal force to separate heavy inorganic solids (sand, gravel, coffee grounds) from the organic waste stream. This is critical for preventing abrasion in the sludge pumps and volume loss in the digesters.

B. PRIMARY TREATMENT

Flow enters rectangular primary clarifiers. These tanks reduce flow velocity, allowing settleable organic solids to drop to the bottom as primary sludge, while fats, oils, and grease (FOG) float to the surface for skimming.

• Efficiency: Typically achieves 30-40% removal of BOD and 50-60% removal of TSS.
• Wet Weather Mode: During peak storm events, flow exceeding the biological capacity may be chemically enhanced in the primaries or diverted to auxiliary storage/treatment to prevent washout of the biological mass.

C. SECONDARY TREATMENT

The biological stage utilizes a Conventional Activated Sludge (CAS) system.

• Aeration Basins: Mechanical surface aerators and/or diffused air systems provide oxygen to microorganisms that consume dissolved organic matter. The system operates to maintain a specific Food-to-Microorganism (F/M) ratio.
• Secondary Clarifiers: Mixed liquor flows to circular secondary clarifiers where the biological floc settles.
• RAS/WAS: Settled sludge is returned to the aeration basins (Return Activated Sludge) to maintain the biological population. Excess growth is removed as Waste Activated Sludge (WAS).

D. DISINFECTION

Before discharge into the Ohio River, the effluent undergoes chlorination to neutralize pathogens (specifically E. coli). Following sufficient contact time in the chlorine contact basins, the water is dechlorinated using sulfur dioxide or sodium bisulfite to prevent toxicity to aquatic life in the river.

E. SOLIDS HANDLING

EWSU employs a robust solids handling regime:

• Thickening: Primary and waste activated sludges are co-thickened, often utilizing gravity belt thickeners or rotary drum thickeners.
• Anaerobic Digestion: Thickened sludge is stabilized in anaerobic digesters. This process reduces volatile solids, mitigates odors, and generates methane gas.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges to produce a cake suitable for disposal.
• Disposal: The dewatered biosolids are typically transported to a sanitary landfill.

F. PROCESS CONTROL

The facility utilizes a centralized SCADA (Supervisory Control and Data Acquisition) system. This allows operators to monitor dissolved oxygen levels, pump status, and tank levels in real-time. The construction of the new Sunrise Pump Station included significant upgrades to the fiber-optic network and telemetry controlling the headworks.

4. INFRASTRUCTURE & FACILITIES

A. Sunrise Pump Station

Completed recently as a cornerstone of the Renew Evansville project, the Sunrise Pump Station is one of the largest of its kind in the region. With a pumping capacity of approximately 105 MGD, it serves as the “heart” of the East Plant’s hydraulic profile, lifting wastewater from the deep tunnel interceptors to the treatment plant elevation.

B. Energy Systems

The plant is a significant energy consumer, primarily due to aeration and pumping requirements. EWSU has implemented energy recovery where feasible, utilizing methane gas generated in the anaerobic digesters to heat the digesters themselves, reducing reliance on natural gas. Variable Frequency Drives (VFDs) are standard on major pumps to optimize energy usage based on flow demand.

C. Odor Control

Given the plant’s proximity to residential areas and the Ohio Riverfront, odor control is a priority. The headworks and primary clarifier areas utilize chemical scrubbers and/or activated carbon filters to capture Hydrogen Sulfide (H₂S) and other odorous compounds before air is released.

5. RECENT UPGRADES & MAJOR PROJECTS

The “Renew Evansville” plan drives capital investment at the facility. This 25-year plan is designed to comply with the Clean Water Act and significantly reduce CSOs.

SUNRISE PUMP STATION REPLACEMENT (2015-2018)

• Project Budget: ~$30 Million
• Scope: Construction of a new 105 MGD pump station to replace a 1950s-era facility.
• Technical Highlights: Installation of massive submersible pumps capable of handling dry and wet weather flows; advanced screening facility integration; improved hydraulic grade line for the plant.
• Outcome: Eliminated a major bottleneck in the collection system, allowing more wet weather flow to reach the plant for treatment rather than overflowing at satellite CSO points.

BEE SLOUGH WET WEATHER TREATMENT FACILITY (Ongoing/Planned)

• Project Scope: Converting the existing Bee Slough (an open-air equalization lagoon) into a sophisticated wet weather treatment facility.
• Budget: Estimated >$80 Million
• Technical Approach: The project involves constructing a Chemically Enhanced High-Rate Clarification (CEHRC) system or similar high-rate technology alongside massive storage capacity.
• Goal: To capture the “first flush” of stormwater, store it, and treat it, drastically reducing the volume of untreated overflow entering the Ohio River.

BIOLOGICAL AERATED FILTERS (BAF) (Planning/Construction Phase)

• Scope: Implementation of BAF technology to treat dilute wet weather flows.
• Technical Highlights: BAF systems use a submerged media bed that acts as both a biological reactor and a filter. This technology was selected for its small footprint and ability to handle rapid fluctuations in hydraulic loading—ideal for CSO management.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. NPDES & Consent Decree

The East WWTP operates under NPDES Permit No. IN0020109. The facility is subject to a Federal Consent Decree lodged in 2016, requiring EWSU to increase the capacity of its sewer system and treatment plants to minimize CSOs. The decree mandates a Long-Term Control Plan (LTCP) which dictates the schedule for the projects listed above.

B. Effluent Limits

Strict limits are enforced for:

• CBOD5: Monthly average limits typically ~25 mg/L (summer).
• TSS: Monthly average limits typically ~30 mg/L.
• Ammonia: Seasonal limits to prevent toxicity.
• Phosphorus: Monitoring requirements are increasing statewide to address nutrient loading in the Mississippi River basin.

C. Compliance History

EWSU has maintained a strong record of compliance regarding dry-weather treatment. The primary compliance challenges have historically been related to wet-weather bypasses (CSOs), which the current capital projects are directly addressing.

7. OPERATIONAL EXCELLENCE

The East WWTP is staffed 24/7 by a team of state-certified wastewater operators, maintenance mechanics, and instrumentation technicians. Indiana classifies wastewater plants I through IV; the East Plant is a Class IV facility (highest complexity).

• Laboratory: The facility houses a certified laboratory that performs daily analysis for process control and regulatory reporting.
• Safety: Rigorous confined space entry, lockout/tagout, and chemical handling protocols are enforced, particularly given the use of chlorine gas and sulfur dioxide.

8. CHALLENGES & FUTURE PLANNING

A. Combined Sewer Overflows (CSOs)

The defining challenge for Evansville is the legacy combined sewer system. Managing the hydraulic shock of storm events without washing out the biological solids inventory requires complex flow equalization and diversion strategies.

B. Aging Infrastructure

Much of the collection system dates back to the early 20th century. Infiltration and Inflow (I/I) from decaying pipes exacerbate the hydraulic load on the plant. The “Renew Evansville” program includes significant funds for sewer lining and rehabilitation.

C. Nutrient Regulations

Future regulatory cycles from IDEM and the EPA will likely lower limits on Total Nitrogen and Total Phosphorus. EWSU engineers are currently evaluating process modifications (such as anoxic zones) to facilitate Biological Nutrient Removal (BNR) in anticipation of these permits.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. RELATED FACILITIES

West Wastewater Treatment Plant: Located on the west side of the city, this smaller sister facility treats approximately 10 MGD average flow. While smaller, it operates under similar regulatory requirements (NPDES IN0020117) and utilizes similar treatment processes. The two plants work in concert to manage the region’s watershed.

Lift Stations: The topography of Evansville requires over 90 lift stations to convey wastewater to the East and West plants. Maintaining these stations is a critical component of the utility’s preventative maintenance program.

11. FAQ SECTION

Technical Questions

1. What is the peak flow capacity of the East WWTP?
While the secondary treatment capacity is rated near 35 MGD, the Sunrise Pump Station can convey over 100 MGD. Flows exceeding secondary capacity are currently diverted or stored, but future BAF projects will treat these high flows.

2. Does the plant perform biological nutrient removal (BNR)?
Currently, the plant is designed primarily for BOD and TSS removal with ammonia nitrification. Full BNR for Total Nitrogen and Phosphorus is not yet a permit requirement but is being factored into long-term master planning.

3. How are biosolids handled?
Solids are anaerobically digested, dewatered via centrifuge, and typically disposed of in sanitary landfills.

4. What is “Renew Evansville”?
It is the utility’s long-term control plan to comply with the Clean Water Act, involving $729 million in upgrades to sewers and treatment plants to reduce Combined Sewer Overflows.

Public Interest Questions

5. Why do sewer rates keep increasing in Evansville?
Rates are increasing to fund the federally mandated “Renew Evansville” projects. These are legally required infrastructure upgrades necessary to protect the Ohio River and comply with the EPA Consent Decree.

6. Does the plant smell?
While wastewater treatment naturally generates odors, the East WWTP employs chemical scrubbers and carbon filtration at key process points (headworks, sludge handling) to minimize impact on the surrounding community.

7. Where does the water go after treatment?
The treated, disinfected effluent is discharged into the Ohio River, meeting strict water quality standards to ensure it is safe for aquatic life and downstream users.