City Of Iowa City Wastewater Treatment Plant

1. INTRODUCTION

The Iowa City Wastewater Treatment Plant represents a critical cornerstone of environmental infrastructure for the Johnson County region, serving the City of Iowa City, the municipality of University Heights, and the massive educational and medical complex of the University of Iowa. Following a strategic consolidation initiative completed in 2014, the city decommissioned its aging North Plant and centralized all treatment operations at the expanded South Napoleon Street facility. Today, this 24.2 MGD plant operates as a sophisticated Biological Nutrient Removal (BNR) facility, positioning Iowa City as a leader in the Iowa Nutrient Reduction Strategy.

Operated by the City of Iowa City Public Works Department, the facility manages complex influent characteristics driven by a fluctuating university population and significant commercial discharge. With over $65 million invested in major capital improvements over the last decade, including the installation of a Modified Bardenpho process and advanced UV disinfection, the plant consistently achieves high-efficiency removal of nitrogen and phosphorus while generating renewable energy through anaerobic digestion and biogas utilization.

2. FACILITY OVERVIEW

A. Service Area & Coverage

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The facility serves a distinct urban service area encompassing approximately 26 square miles. The collection system supports a permanent population of approximately 75,000, which swells significantly to over 85,000 when the University of Iowa is in session. The service area is characterized by a mix of medium-density residential zones, the dense downtown commercial district, and institutional facilities including the University of Iowa Hospitals & Clinics.

B. Operational Capacity

Following the major consolidation project, the plant features a design average wet weather flow (AWWF) of 24.2 MGD and a peak hourly flow capacity of approximately 64 MGD. Current average daily flows hover between 9.5 and 11.0 MGD, providing the municipality with adequate reserve capacity for projected growth through 2035. The hydraulic profile of the plant was specifically engineered to manage the high infiltration and inflow (I/I) rates common in the region’s river valley topography during spring thaw and heavy storm events.

C. Discharge & Compliance

Treated effluent is discharged directly into the Iowa River. The facility operates under a rigorous National Pollutant Discharge Elimination System (NPDES) permit issued by the Iowa Department of Natural Resources (IDNR). As a Class A1 primary contact recreational water body, the Iowa River requires strict compliance regarding E. coli bacteria levels during the recreational season (March 15 – November 15), as well as ammonia-nitrogen and dissolved oxygen limits year-round.

3. TREATMENT PROCESS

The Iowa City WWTP utilizes an advanced activated sludge process configured for Biological Nutrient Removal (BNR). The treatment train is designed to address both carbonaceous biochemical oxygen demand (CBOD) and nutrient loads (Nitrogen and Phosphorus).

A. Preliminary Treatment

Raw wastewater enters the headworks facility where it passes through mechanical bar screens to remove large debris, rags, and plastics. Following screening, flow enters vortex grit removal systems where inorganic solids (sand, gravel, coffee grounds) are settled out using centrifugal force. The grit is washed, dewatered, and disposed of at the landfill. The headworks is enclosed and equipped with a chemical scrubber system for odor control.

B. Primary Treatment

Flow proceeds to primary clarification, consisting of circular clarifiers. These tanks reduce the velocity of the wastewater, allowing settleable organic solids to drop to the bottom as primary sludge, while grease and oil float to the surface for skimming. Primary treatment removes approximately 30-35% of BOD and 50-60% of Total Suspended Solids (TSS), significantly reducing the organic load on the secondary biological stage.

C. Secondary Treatment (BNR)

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The core of the facility is the **Modified Bardenpho** process, a 5-stage activated sludge configuration designed specifically for biological nitrogen and phosphorus removal. This system eliminates the need for chemical addition for phosphorus removal in most operating conditions.

• Anaerobic Zone: Promotes the growth of Phosphorus Accumulating Organisms (PAOs), causing them to release stored phosphorus to condition them for uptake in the aerobic zone.
• Anoxic Zone: Nitrate-rich mixed liquor is recycled here to mix with carbon-rich influent, facilitating denitrification (conversion of nitrate to nitrogen gas).
• Aerobic Zone: Fine bubble diffusion provides oxygen for nitrification (ammonia to nitrate) and luxury phosphorus uptake by PAOs.
• Secondary Clarification: Mixed liquor flows to large circular secondary clarifiers where biological floc settles. Activated sludge is returned (RAS) to the anaerobic zone, while clear supernatant flows to disinfection.

D. Disinfection

The facility utilizes high-intensity **Ultraviolet (UV) Disinfection**. The effluent passes through channel-mounted UV banks which disrupt the DNA of pathogenic organisms (bacteria and viruses), rendering them unable to reproduce. This method avoids the creation of chlorinated disinfection byproducts and eliminates the need for dechlorination chemicals prior to river discharge.

F. Solids Handling

Solids management is a critical energy-generation component of the plant:

• Thickening: Primary sludge is gravity thickened, while Waste Activated Sludge (WAS) is thickened using gravity belt thickeners or dissolved air flotation (DAF).
• Anaerobic Digestion: Thickened sludge is pumped to anaerobic digesters operated at mesophilic temperatures (95°F – 98°F). This process stabilizes the solids, reduces volatile content, and generates methane-rich biogas.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges or belt filter presses to achieve a cake solid suitable for land application.
• Disposal: The Class B biosolids are land-applied to local agricultural fields as a nutrient-rich soil amendment, closing the nutrient loop.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The site spans nearly 30 acres adjacent to the Iowa River. The architectural design of the newer administration and headworks buildings emphasizes functionality and flood resilience. All critical electrical gear and backup power generation systems are elevated above the 500-year flood plain elevation, a direct response to the historic floods of 2008.

B. Energy Systems

The Iowa City WWTP is a net energy producer regarding heat. The biogas generated in the anaerobic digesters is captured, scrubbed, and utilized in Combined Heat and Power (CHP) units or boilers. This gas powers the heating of the digesters themselves and provides building heat during winter months, significantly offsetting natural gas purchases. The facility also employs Variable Frequency Drives (VFDs) on all major pumps and aeration blowers to minimize electrical consumption.

C. Odor Control

To mitigate impact on the nearby recreational areas and growing residential zones, the plant employs a multi-stage odor control strategy. The headworks and primary sludge pumping stations are vented to biological soil filters or chemical scrubbers that oxidize hydrogen sulfide and other odorous compounds before air is released to the atmosphere.

5. RECENT UPGRADES & MAJOR PROJECTS

Wastewater Treatment Plant Consolidation & Expansion (2012-2014)

• Project Budget: ~$55 Million
• Scope: This landmark project involved the decommissioning of the North Wastewater Treatment Plant and the expansion of the South Plant from 10 MGD to 24.2 MGD.
• Funding: Financed primarily through the Iowa State Revolving Fund (SRF).
• Key Engineers: HDR Engineering.
• Technical Highlights: Construction of new aeration basins configured for BNR, new secondary clarifiers, a new UV disinfection facility, and a new administration building. The project successfully centralized operations, reducing O&M costs and improving effluent consistency.

Digester Complex Improvements (2018-2020)

• Project Budget: ~$8 Million
• Scope: Rehabilitation of existing digesters, cleaning of digester tanks, replacement of mixing equipment (switching to linear motion mixers or draft tube mixers for better efficiency), and upgrades to the biogas handling safety equipment.
• Drivers: Aging infrastructure and the need to maximize biogas production for energy recovery.

Future Planning: Nutrient Reduction Optimization

While the plant currently performs well, upcoming projects in the 2024-2028 CIP focus on optimizing the BNR process to meet voluntarily tighter targets under the Iowa Nutrient Reduction Strategy. This includes potential upgrades to aeration control systems (Ammonia-Based Aeration Control – ABAC) and side-stream treatment for nitrogen in centrate return flows.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The facility operates under NPDES Permit No. IA4926101. Key discharge limits include:

• CBOD5: 25 mg/L (30-day average)
• TSS: 30 mg/L (30-day average)
• E. coli: 126 org/100ml (Geometric Mean, March 15-Nov 15)
• Ammonia (N): Seasonal limits varying from roughly 2.0 mg/L (summer) to higher allowances in winter.

B. Environmental Stewardship

Iowa City is a signatory to the Iowa Nutrient Reduction Strategy. The plant was designed to achieve significant reductions in total nitrogen (TN) and total phosphorus (TP), aiming for a 66% reduction in nitrogen and 75% reduction in phosphorus from baseline levels. This proactive approach helps mitigate hypoxia in the Gulf of Mexico and improves local water quality in the Iowa River.

7. OPERATIONAL EXCELLENCE

The facility is staffed by a dedicated team of approximately 20-25 personnel, including operations, maintenance, laboratory, and administrative staff. The plant requires Grade IV wastewater certification from the Iowa DNR for senior operators, the highest level of certification in the state, reflecting the complexity of the BNR process.

The on-site laboratory is fully certified to perform all permit-required analyses (excluding acute toxicity testing), allowing for real-time process control decisions. SCADA integration allows operators to monitor dissolved oxygen profiles, return sludge rates, and influent flows remotely, ensuring 24/7 reliability.

8. CHALLENGES & FUTURE PLANNING

A. Current Challenges

River Flooding: Situated in the Iowa River floodplain, the plant faces operational risks during extreme high-water events. While the 2014 expansion hardened the facility, high river levels can impact hydraulic grade lines and require careful effluent pumping management.

Variable Loading: The University of Iowa schedule creates distinct seasonal flow and loading variations (when students leave in summer/winter), requiring operators to adjust biological mass (MLSS) inventories dynamically to maintain treatment efficiency.

B. Future Planning

The city maintains a comprehensive Wastewater Facilities Master Plan. Immediate future focuses include aging sewer main rehabilitation to reduce Inflow & Infiltration (I/I) and the evaluation of emerging contaminant technologies (PFAS) should regulations change.

9. TECHNICAL SPECIFICATIONS SUMMARY

10. FAQ SECTION

Technical Questions

Q: What is the specific nutrient removal process used?
A: The plant uses a Modified Bardenpho process, consisting of anaerobic, anoxic, and aerobic zones to achieve biological phosphorus removal and nitrification/denitrification.

Q: Does the plant use chemical precipitation for Phosphorus?
A: The design intent is Bio-P (Biological Phosphorus removal). However, the plant has chemical feed backup capabilities (Alum or Ferric) for polishing if biological uptake is inhibited.

Q: How is biogas utilized?
A: Methane produced in the digesters is captured and used in boilers to heat the digesters and facility buildings. Excess gas is flared.

Public Interest Questions

Q: Where is the North Wastewater Plant?
A: The North Plant was decommissioned in 2014. The site has been reclaimed for public use and parkland, with all flows diverted to the expanded South Plant.

Q: Does the plant treat drinking water?
A: No. The Iowa City Water Division operates a separate Water Treatment Plant on North Madison Street for drinking water. This facility specifically treats sewage and wastewater.

Q: Can I tour the facility?
A: Yes, the City of Iowa City offers tours for educational groups, engineering students, and the public by appointment through the Public Works Department.