Des Moines Metropolitan Wastewater Reclamation Authority Treatment Plant
1. INTRODUCTION
The Des Moines Metropolitan Wastewater Reclamation Authority (WRA) Water Reclamation Facility (WRF) stands as the largest and most complex wastewater treatment infrastructure in the state of Iowa. Located on the banks of the Des Moines River, this facility serves as the centralized treatment hub for the Des Moines metropolitan area, processing an average of 75 million gallons per day (MGD) from a service area comprising 17 participating communities, counties, and sewer districts.
While the physical plant has roots dating back to 1940, the modern facility represents a marvel of regional cooperation and engineering innovation. Operated by the City of Des Moines under contract for the WRA, the plant has garnered national attention for its pioneering Renewable Natural Gas (RNG) production facility, which converts biogas into pipeline-quality natural gas. Facing stringent nutrient reduction targets under the Iowa Nutrient Reduction Strategy, the WRA WRF exemplifies the transition from simple waste processing to a sophisticated resource recovery center, balancing high-volume hydraulic management with sustainable energy generation.
2. FACILITY OVERVIEW
A. Service Area & Coverage
The WRA serves a rapidly growing metropolitan area covering portions of Polk, Warren, and Dallas counties. The unique governance structure involves the WRA Board, comprising representatives from 17 entities including the cities of Des Moines, Ankeny, West Des Moines, Urbandale, Johnston, Clive, and others. The collection system funnels wastewater through major interceptors to the Vandalia Road facility. The service area includes a diverse mix of residential zones, commercial districts, and significant industrial contributors, including food processing and agricultural industries that contribute high-strength waste streams.
B. Operational Capacity
The facility is designed to handle significant flow variations, a critical requirement given the region’s hydrology and combined sewer systems in older parts of Des Moines.
- Average Dry Weather Flow: 80 MGD
- Peak Wet Weather Flow: Up to 260 MGD (following equalization)
- Wet Weather Treatment: The facility utilizes flow equalization basins (FEB) to manage storm surges, capable of storing excess flow during peak events to prevent bypasses.
C. Discharge & Compliance
Treated effluent is discharged into the Des Moines River, a major tributary of the Mississippi River. The facility operates under a National Pollutant Discharge Elimination System (NPDES) permit issued by the Iowa Department of Natural Resources (IDNR). Compliance is strictly monitored for Carbonaceous Biochemical Oxygen Demand (CBOD), Total Suspended Solids (TSS), Ammonia-Nitrogen, and E. coli. As a major contributor to the Mississippi watershed, the plant is also a focal point for the Iowa Nutrient Reduction Strategy, aiming to reduce Nitrogen and Phosphorus loads contributing to the Gulf of Mexico hypoxia zone.
3. TREATMENT PROCESS
The WRA WRF utilizes a conventional activated sludge process augmented with advanced solids handling and biological nutrient reduction capabilities.
A. PRELIMINARY TREATMENT
Raw influent enters the headworks where it passes through coarse and fine bar screens to remove large debris, rags, and plastics. Following screening, flow velocity is reduced in aerated grit chambers, allowing heavier inorganic materials (sand, gravel, coffee grounds) to settle while keeping organic matter in suspension. The removed grit and screenings are washed, compacted, and landfilled. This stage also includes odor control systems utilizing chemical scrubbers to mitigate H2S emissions from the raw sewage.
B. PRIMARY TREATMENT
Wastewater flows into multiple large primary clarifiers. These rectangular tanks reduce flow velocity, allowing settleable solids to drop to the bottom as primary sludge, while grease and oils float to the surface for skimming.
- Removal Efficiency: Typically removes 30-40% of BOD and 50-70% of TSS.
- Primary Sludge: Pumped directly to the anaerobic digesters.
C. SECONDARY TREATMENT
The biological core of the plant consists of an Activated Sludge system.
- Aeration Basins: The plant operates multiple parallel aeration trains. Microorganisms (mixed liquor) consume dissolved organic matter. The basins utilize fine-bubble diffusion to maximize oxygen transfer efficiency.
- Nitrification: The system is operated to achieve nitrification, converting toxic ammonia into nitrates. This is critical for meeting river water quality standards.
- Secondary Clarifiers: Mixed liquor flows to circular secondary clarifiers where biological floc settles out. The clear supernatant overflows the weirs as treated effluent.
- RAS/WAS: Return Activated Sludge (RAS) is recycled to the aeration basins to maintain biomass. Waste Activated Sludge (WAS) is removed to control mean cell residence time (MCRT) and sent to thickening.
D. DISINFECTION
The WRA utilizes a Ultraviolet (UV) Disinfection system. Historically a chlorination facility, the switch to UV eliminated the safety hazards of storing chlorine gas and the need for dechlorination chemicals. The UV system damages the genetic material of pathogens (E. coli, bacteria, viruses), rendering them unable to reproduce. Disinfection is typically required during the recreation season (March 15 – November 15).
E. SOLIDS HANDLING & RNG PRODUCTION
The solids handling train is the facility’s technological centerpiece.
- Thickening: WAS is thickened using dissolved air flotation (DAF) or rotary drum thickeners before digestion.
- Anaerobic Digestion: The plant operates a complex of thermophilic and mesophilic anaerobic digesters. Primary sludge, thickened WAS, and hauled-in high-strength waste (FOG and industrial food waste) are digested to reduce volatile solids.
- Biogas Utilization: The digestion process produces methane-rich biogas. The WRA operates a scrubbing system that upgrades this biogas to pipeline-quality Renewable Natural Gas (RNG). This gas is injected into the commercial natural gas grid, generating significant revenue through Renewable Identification Numbers (RINs).
- Dewatering & Disposal: Digested sludge is dewatered using high-speed centrifuges. The resulting biosolids are land-applied to agricultural fields as a soil conditioner, recycling nutrients back to the land.
4. INFRASTRUCTURE & FACILITIES
A. Physical Plant
The site spans extensive acreage along the Des Moines River levee system. Key structures include the Administration Building (housing SCADA control and engineering offices), a fully accredited environmental laboratory, and the massive digester complex which dominates the skyline. The site is protected by a flood wall designed to withstand 500-year flood events, a critical addition following the floods of 1993 and 2008.
B. Energy Systems & RNG
The facility is a net energy producer in terms of equivalency. The Biogas Upgrading Facility allows the WRA to inject renewable gas directly into the MidAmerican Energy pipeline.
- Production: The system can produce over 1,000 MMBtu of RNG per day.
- High Strength Waste (HSW): The WRA actively solicits hauled-in waste from industrial partners to boost gas production, serving as a regional disposal solution for food processors.
5. RECENT UPGRADES & MAJOR PROJECTS
Biogas to Renewable Natural Gas (RNG) Project – ~$20 Million (Commissioned 2020/2021)
Project Scope: Construction of a biogas scrubbing and conditioning facility to convert digester gas into pipeline-quality natural gas.
Drivers: The previous cogeneration engines were reaching end-of-life. An economic analysis determined that producing RNG and selling credits (RINs) under the Federal Renewable Fuel Standard offered a superior ROI compared to generating electricity onsite.
Technical Highlights:
- Installation of membrane separation technology to remove CO2 and impurities.
- Construction of a high-strength waste receiving station to increase volatile solids loading.
- Grid injection point integration with the local utility.
Results: The project generates millions of dollars in annual revenue for the WRA, offsetting operational costs for member communities and reducing the facility’s carbon footprint.
Flood Mitigation Improvements (Ongoing/Multi-phase)
Project Scope: Strengthening perimeter protection and stormwater pumping capabilities.
Context: Located in a floodplain, the facility is critical infrastructure. Following the historic floods of 2008, the WRA invested heavily in armoring the facility against river levels that exceeded previous 100-year estimates.
Upcoming Projects (2024-2027)
- Nutrient Reduction Retrofits: Continued optimization of aeration basins to maximize biological phosphorus and nitrogen removal without massive chemical addition.
- Sewer Interceptor Rehabilitation: Lining and replacement of major trunk lines to reduce Inflow and Infiltration (I/I).
6. REGULATORY COMPLIANCE & ENVIRONMENTAL PERFORMANCE
Permit Requirements: The facility operates under NPDES Permit No. IA 25-00-1-01. The permit dictates strict limits on CBOD, TSS, and E. coli, with monitoring requirements for Ammonia, Chloride, Sulfate, and Total Nitrogen/Phosphorus.
Nutrient Reduction Strategy: Iowa’s nutrient reduction strategy targets a 45% reduction in Nitrogen and Phosphorus. The WRA has committed to these goals, implementing process changes to encourage denitrification and biological phosphorus uptake. The facility consistently performs well below its permitted limits for conventional pollutants.
7. OPERATIONAL EXCELLENCE
Staffing: The facility employs over 100 staff members, including Grade IV certified operators, maintenance technicians, chemists, and engineers. The plant is staffed 24/7/365.
Technology: A robust SCADA system monitors thousands of data points, allowing for real-time automation of dissolved oxygen levels, return sludge rates, and chemical dosing. The on-site laboratory performs daily analysis to ensure process stability and regulatory compliance.
8. CHALLENGES & FUTURE PLANNING
A. Nutrient Regulations
The pressure to reduce nutrient loading to the Mississippi watershed is increasing. While current reduction strategies are voluntary or based on technology-based limits, future regulations may impose strict numeric limits on Nitrogen and Phosphorus, potentially requiring costly tertiary filtration upgrades.
B. Inflow and Infiltration (I/I)
The WRA service area includes older combined sewers. During heavy spring rains and snowmelt, hydraulic loading can spike dramatically. Managing these peak flows without washing out the biological mass in the secondary system is a constant operational challenge requiring careful management of equalization basins.
C. Growth Capacity
Suburbs like Ankeny and Waukee are among the fastest-growing in the nation. The WRA Master Plan continuously evaluates the need for hydraulic capacity expansion to accommodate this rapid residential and commercial development.
9. TECHNICAL SPECIFICATIONS SUMMARY
| Parameter | Specification |
|---|---|
| Facility Type | Advanced Secondary Treatment (Activated Sludge) |
| Design Capacity (Average) | 100 MGD |
| Peak Hydraulic Capacity | 260 MGD |
| Treatment Process | Screening, Grit Removal, Primary Clarification, Activated Sludge, Secondary Clarification, UV Disinfection |
| Biosolids Processing | Anaerobic Digestion, Centrifuge Dewatering |
| Biogas Utilization | Upgrading to Renewable Natural Gas (RNG) for Pipeline Injection |
| Nutrient Removal | Biological Nutrient Removal (Nitrification/Denitrification) |
| Population Served | ~500,000 – 600,000 |
| Member Communities | 17 Municipalities/Districts |
| Receiving Water | Des Moines River |
| Disinfection Method | Ultraviolet (UV) Radiation |
| Site Location | 3000 Vandalia Rd, Des Moines, IA |
10. FAQ SECTION
1. What is the treatment capacity of the Des Moines WRA facility?
The plant has an average design capacity of 100 MGD and can handle peak wet weather flows up to 260 MGD.
2. How does the Des Moines WRA create renewable energy?
The facility uses anaerobic digesters to break down organic waste, producing biogas. This biogas is scrubbed of impurities to create Renewable Natural Gas (RNG), which is injected into the commercial natural gas pipeline.
3. Does the facility remove nutrients like Nitrogen and Phosphorus?
Yes. The plant utilizes biological nutrient removal processes to reduce Ammonia, Nitrogen, and Phosphorus levels to support the Iowa Nutrient Reduction Strategy.
4. Who operates the WRA facility?
The facility is owned by the Wastewater Reclamation Authority (a regional entity) but is operated and maintained by the City of Des Moines Public Works Department under an operating contract.
5. What happens to the biosolids produced at the plant?
After digestion and dewatering, the biosolids are land-applied to agricultural fields in the region as a nutrient-rich soil conditioner.
6. Does the plant use chlorine for disinfection?
No. The facility switched to Ultraviolet (UV) disinfection, which is safer for operators and the aquatic environment of the Des Moines River.
7. How does the plant handle industrial waste?
The WRA has a robust Industrial Pretreatment Program and a Hauled Waste program that accepts high-strength waste (Fats, Oils, Grease) directly into the digesters to boost gas production.