City Of Davenport Wastewater Treatment Plant

FACILITY BASIC INFORMATION

Plant Name: Davenport Water Pollution Control Plant (WPC)

Location: 2606 South Concord Street, Davenport, Scott County, Iowa

Operating Authority: City of Davenport Public Works

Design Capacity: 26.0 MGD (Average Dry Weather) / 60+ MGD (Peak Hydraulic)

Current Average Flow: ~22-24 MGD

Population Served: ~140,000 (Regional Service Area)

Service Area: City of Davenport, City of Bettendorf, City of Riverdale, City of Panorama Park

Receiving Water Body: Mississippi River

NPDES Permit Number: IA0044415

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Year Commissioned: 1938 (Original Primary), 1973 (Secondary Upgrade)

1. INTRODUCTION

The Davenport Water Pollution Control (WPC) Plant serves as the cornerstone of wastewater infrastructure for the Iowa side of the Quad Cities metropolitan area. Strategically located along the banks of the Mississippi River, this regional facility operates under a joint-use agreement to treat wastewater for the cities of Davenport, Bettendorf, Riverdale, and Panorama Park. With a rated design capacity of 26 million gallons per day (MGD) and wet-weather peak handling capabilities exceeding 100 MGD, the plant is a critical environmental safeguard for the Mississippi River watershed.

Originally commissioned in the late 1930s and significantly expanded in the 1970s and 1990s, the facility is distinguished not only by its treatment efficacy but also by its pioneering biosolids management program, which produces the nationally recognized “Earth Cycle” compost. Facing the unique engineering challenges of a riverfront location, the WPC Plant has recently become a case study in climate resiliency, implementing substantial flood mitigation infrastructure following historic Mississippi River levels in 2019. Today, the facility combines conventional activated sludge treatment with innovative resource recovery to meet stringent NPDES compliance standards.

2. FACILITY OVERVIEW

A. Service Area & Coverage

The Davenport WPC Plant operates as a regional hub. While owned and operated by the City of Davenport, it functions through intergovernmental agreements to serve a broader metropolitan population of approximately 140,000 residents. The collection system is vast, comprising over 400 miles of sanitary sewer mains within Davenport alone, augmented by the connecting interceptors from Bettendorf (to the east) and industrial corridors in Riverdale. The service area includes a diverse mix of residential zones, commercial districts, and significant industrial contributors, requiring a robust treatment process capable of handling variable loading characteristics.

B. Operational Capacity

The facility is designed for an Average Dry Weather Flow (ADWF) of 26.0 MGD. However, due to the age of the collection system and the facility’s river valley location, the plant experiences significant peaking during wet weather events. The hydraulic capacity allows for peak flows exceeding 60 MGD through secondary treatment, with excess flows managed through equalization and wet-weather operational protocols. Historical flow trends indicate an average daily influent ranging between 22 and 24 MGD. Future planning focuses less on dry-weather capacity expansion and more on wet-weather flow management and Inflow/Infiltration (I/I) reduction.

C. Discharge & Compliance

Treated effluent is discharged directly into the Mississippi River via a submerged outfall. The facility operates under NPDES Permit No. IA0044415, issued by the Iowa Department of Natural Resources (IDNR). Compliance parameters are strict regarding Carbonaceous Biochemical Oxygen Demand (CBOD5), Total Suspended Solids (TSS), pH, and E. coli bacteria (during recreational seasons). The plant consistently achieves high removal efficiencies, typically exceeding 95% removal for BOD and TSS, thereby protecting the water quality of one of North America’s most vital commercial and ecological waterways.

3. TREATMENT PROCESS

A. PRELIMINARY TREATMENT

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Influent wastewater enters the headworks facility where it undergoes rigorous preliminary treatment to protect downstream mechanical equipment. The process begins with **mechanical bar screens** (stepped screens) that remove large debris, rags, and plastics. Following screening, the flow enters **aerated grit chambers**. These chambers reduce the velocity of the water, introducing air to keep organic matter suspended while allowing heavier inorganic materials (sand, gravel, coffee grounds) to settle. The grit is removed via screw conveyors, washed, and disposed of in a sanitary landfill. Odor control at the headworks is managed through chemical scrubbers to mitigate nuisance odors for nearby recreational areas.

B. PRIMARY TREATMENT

Flow proceeds to the **primary clarifiers**, which are large rectangular sedimentation tanks. Here, the hydraulic velocity is minimized to allow settleable organic solids to drop to the bottom of the tank as primary sludge, while grease and oils float to the surface for skimming. The primary treatment stage is critical for reducing the organic load (BOD) on the secondary biological system by approximately 30-35% and suspended solids by 50-60%. Primary sludge is pumped directly to the solids handling train for thickening and digestion.

C. SECONDARY TREATMENT

The core of the Davenport WPC’s treatment capability is the **Activated Sludge Process**. The plant utilizes a plug-flow aeration basin configuration equipped with **fine-bubble diffusion systems**. Large centrifugal blowers supply oxygen to the basins, sustaining a diverse culture of microorganisms (mixed liquor) that consume dissolved organic matter and convert ammonia to nitrate (nitrification). Ferric chloride is occasionally added to precipitate phosphorus.

Following aeration, the mixed liquor flows into **circular secondary clarifiers**. In these quiescent tanks, the biological floc settles out, separating the clean water from the biomass. A portion of the settled solids is returned to the aeration basins as Return Activated Sludge (RAS) to maintain the biological population, while the excess is removed as Waste Activated Sludge (WAS).

D. DISINFECTION

Historically a chlorine gas facility, the Davenport WPC transitioned to **Ultraviolet (UV) Disinfection** to eliminate the safety hazards of storing chlorine gas and to reduce chemical byproducts in the effluent. The UV system operates seasonally (typically March 15 through November 15) in accordance with IDNR requirements to protect recreational users of the Mississippi River. The UV banks are installed in open channels, where high-intensity light disrupts the DNA of pathogenic bacteria (E. coli), rendering them unable to reproduce.

E. SOLIDS HANDLING & COMPOSTING

Davenport is an industry leader in biosolids beneficiation. The solids handling process involves:

• Thickening: Primary sludge is gravity thickened, while WAS is thickened using Dissolved Air Flotation (DAF) or rotary drum thickeners.
• Anaerobic Digestion: Thickened sludge is pumped to anaerobic digesters. In these heated, oxygen-free tanks, bacteria break down the organic matter, reducing volatile solids and producing methane gas.
• Dewatering: Digested sludge is dewatered using high-solids centrifuges to create a dry “cake.”
• Composting (Earth Cycle): The dewatered biosolids are mixed with yard waste and wood chips at the on-site Davenport Compost Facility. Using an aerated static pile method, the mixture is composted at high temperatures to kill pathogens. The final product is sold commercially as “Earth Cycle” soil conditioner, achieving 100% beneficial reuse of biosolids and diverting thousands of tons of green waste from landfills annually.

4. INFRASTRUCTURE & FACILITIES

A. Physical Plant

The facility occupies a substantial industrial footprint along South Concord Street. Key structures include the Administration Building (housing SCADA control and accredited laboratories), the Headworks Building, Primary and Secondary Clarifier complexes, the Blower Building, and the massive Compost Facility sheds. The site architecture is functional and industrial, heavily modified in recent years to incorporate flood barriers.

B. Energy Systems & Biogas

The plant is a significant energy consumer, primarily due to aeration blowers and pumping systems. However, the facility recovers energy through its anaerobic digestion process. The methane gas (biogas) produced during digestion is captured and utilized in **Combined Heat and Power (CHP)** units or boilers to heat the digesters and facility buildings, offsetting natural gas purchases. Energy efficiency upgrades, such as the installation of turbo blowers and VFDs on large pumps, are continuous priority projects.

C. Flood Resiliency

Situated in the floodplain, the physical security of the plant is paramount. Following the record-breaking 2019 flood, the city has invested heavily in permanent perimeter protection. This includes reinforced earthen berms, concrete flood walls, and upgraded stormwater pumping stations designed to protect the critical treatment assets from river stages well above the historic crest.

5. RECENT UPGRADES & MAJOR PROJECTS

The City of Davenport has executed a robust Capital Improvement Plan (CIP) focusing on nutrient reduction, aging infrastructure replacement, and flood mitigation.

[COMPLETED] 1930s Interceptor Sewer Rehabilitation (2018-2020)

• Project Scope: Rehabilitation of the massive interceptor sewer that feeds the plant, which dates back to the original 1930s construction. Cured-in-place pipe (CIPP) technology was utilized to line large-diameter pipes without open-cut excavation.
• Budget: ~$15 Million
• Drivers: Structural integrity concerns and reduction of Inflow/Infiltration (I/I).
• Outcome: Extended the service life of the critical conveyance artery by 50+ years.

[COMPLETED/ONGOING] Flood Mitigation & Resiliency Phase I & II (2020-2024)

• Project Scope: Construction of permanent flood protection measures following the 2019 flood event where the river crested at 22.7 feet. This included raising access roads, installing permanent flood gates, and reinforcing perimeter berms.
• Budget: ~$4-6 Million (Phased)
• Drivers: Climate resilience and asset protection.
• Outcome: The plant can now maintain full operation during river stages that previously required emergency sandbagging and threatened plant shutdown.

[CURRENT] Nutrient Reduction Strategy Upgrades (2023-2027)

• Project Scope:Modifications to the biological treatment process to enhance biological nutrient removal (BNR). This involves retrofitting aeration basins with anoxic zones and upgraded mixing equipment to optimize nitrogen and phosphorus removal.
• Funding: Iowa State Revolving Fund (SRF) loans.
• Drivers: The Iowa Nutrient Reduction Strategy and anticipated future permit limits.
• Technical Highlights: Implementation of ABAC (Ammonia Based Aeration Control) to optimize energy usage while meeting ammonia limits.

6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE

A. Permit Requirements

The WPC Plant operates under a rigid NPDES framework. Key effluent limits typically 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)
• pH: 6.0 – 9.0 Standard Units

B. Environmental Stewardship

The City of Davenport has been recognized for its environmental leadership, particularly regarding the Earth Cycle Compost Program. By co-composting biosolids with community yard waste, the facility prevents methane generation in landfills and returns nutrients to the soil. This circular economy model has received accolades from the U.S. Composting Council and serves as a model for municipalities nationwide.

7. OPERATIONAL EXCELLENCE

The facility is staffed by a dedicated team of Grade I-IV certified wastewater operators, laboratory technicians, and maintenance mechanics. The on-site laboratory is state-certified for wastewater analysis, ensuring rapid feedback for process control decisions. SCADA systems allow for real-time monitoring of all critical pumps, motors, and biological parameters, enabling the staff to adjust aeration rates and return sludge flows dynamically based on influent loading.

8. CHALLENGES & FUTURE PLANNING

A. Climate Resilience

The primary engineering challenge for the Davenport WPC remains the Mississippi River. As extreme weather events become more frequent, managing high river stages while simultaneously handling peak wet-weather influent flows requires complex hydraulic balancing. Future planning heavily weights “hardening” the facility against flooding.

B. Aging Infrastructure

With original components dating to 1938 and major expansions in the 1970s, asset management is critical. The city utilizes a computerized maintenance management system (CMMS) to track asset lifecycle and prioritize replacements of clarifier mechanisms, electrical switchgear, and concrete structures degraded by hydrogen sulfide corrosion.

C. Nutrient Limits

Like all dischargers to the Mississippi watershed, Davenport faces pressure to reduce Nitrogen and Phosphorus contributions to the Gulf of Mexico hypoxic zone. While current upgrades address this voluntarily under the Iowa Nutrient Reduction Strategy, future regulatory cycles may mandate stricter numeric limits, requiring capital-intensive tertiary treatment additions.

10. TECHNICAL SPECIFICATIONS SUMMARY

11. FREQUENTLY ASKED QUESTIONS

Technical/Professional Questions

1. What is the treatment capacity of the Davenport WPC Plant?
The plant has an average design capacity of 26 MGD, but is engineered to handle wet weather peak flows significantly higher, utilizing storage and storm modes to manage influent surges common in river cities.

2. How does Davenport handle biosolids disposal?
Davenport utilizes a 100% beneficial reuse program. Anaerobically digested biosolids are dewatered and mixed with yard waste to produce “Earth Cycle” compost, which is sold to the public and used in municipal landscaping.

3. Is the facility equipped for nutrient removal?
The facility is currently implementing upgrades consistent with the Iowa Nutrient Reduction Strategy to optimize nitrogen and phosphorus removal through biological process modifications.

4. How has the 2019 flood impacted facility design?
The 2019 flood drove the implementation of permanent flood mitigation infrastructure, including higher perimeter berms, flood gates, and hardened pumping systems to ensure operability during extreme river stages.

Public Interest Questions

5. Can the public buy the compost produced at the plant?
Yes. The Earth Cycle compost is available for purchase by residents and commercial contractors at the Davenport Compost Facility located adjacent to the treatment plant.

6. Does the plant smell?
The plant utilizes chemical scrubbers and biofilters, particularly at the headworks and solids handling areas, to control odors. However, some odors may be detectable in the immediate industrial vicinity depending on wind conditions.

7. What happens to the water after treatment?
After treatment and seasonal disinfection, the clean water (effluent) is discharged into the Mississippi River, where it rejoins the natural water cycle.

Note: Technical specifications and operational data presented in this article are based on publicly available regulatory documents, municipal capital improvement plans, and engineering reports. For the most current real-time data or official inquiries, please contact the City of Davenport Public Works Department.