Demonstrating Cost-Effective Lagoon Compliance with Cold-Weather MABR Technology

Fluence’s compact Aspiral™ M2+ unit installed at Dow City’s lagoon site enables advanced nitrogen removal in cold-weather conditions, helping the town meet updated discharge limits without building a new plant.

What Makes This Lagoon Treatment Special

• First Aspiral™ MABR system installed in Iowa.
• Enables cold-weather nitrogen removal without costly upgrades.
• Reuses existing lagoons as equalization basins—no need for a new plant.
• Adds to Fluence’s global portfolio of 300+ MABR units.
• Offers a replicable model for small towns facing ammonia and nitrogen limits.

Background

Dow City, Iowa, a rural community with fewer than 500 residents, faced major challenges adapting to stricter wastewater treatment regulations. Its facultative (seasonal discharge) lagoon system, permitted for carbonaceous biochemical oxygen demand (cBOD), total suspended solids (TSS), and pH, could no longer meet updated requirements. A new National Pollutant Discharge Elimination System (NPDES) permit from the Iowa Department of Natural Resources (IDNR) introduced year-round limits, including ammonia concentrations of 1.6–6.6 milligrams per liter (mg/L) and Escherichia coli (E. coli) levels not exceeding 126 colony-forming units per 100 milliliters (mL). Designed for continuous discharge, the permit exceeded the capabilities of the town’s aging infrastructure. To comply, Dow City is upgrading to a continuous discharge system using Aspiral™ technology, improving both regulatory compliance and environmental stewardship.

Challenges

Across the nation, rural lagoon systems are struggling to meet updated effluent discharge standards mandated by federal and state guidelines. These include:

• Ammonia reduction to safeguard aquatic ecosystems
• Disinfection to reduce E. coli concentrations for public health
• BOD control to protect downstream water quality

Dow City partnered with Fluence and ISG Inc. to install a single Aspiral™ M2+ unit, containing MABR technology designed to deliver advanced treatment within a compact footprint. The system offered a solution uniquely suited to the town’s technical and financial constraints.

Dow City's system, originally designed decades ago for organic load reduction and solids settling, could not accommodate these stricter requirements—particularly for cold-weather ammonia performance. The system needed to operate reliably during winter months with water temperatures dipping as low as 37°F, well below traditional treatment design thresholds.

Further, the town's average design flow (ADW) is 0.038 MGD, with peak wet-weather flows reaching up to 0.394 MGD. The system needed to handle daily fluctuations between 20,000 and 33,000 gallons per day, and still maintain performance during seasonal extremes.

Key Benefits

Low Energy Demand: The MABR system uses low-pressure air diffusion through membranes, eliminating the need for traditional high-energy blowers. It operates with a 1/3-horsepower blower, contributing to significant energy savings and reduced O&M costs, which are estimated at $83,400 per year.

Cold-Climate Capability: Proven operation down to 37°F, well below the typical design minimum of 50°F, makes this system ideal for Iowa’s winters. High nitrification efficiency is maintained even in cold water.

Modular and Space-Efficient: The compact Aspiral™ units integrate easily with the existing lagoon infrastructure, avoiding the need for a full plant overhaul. No downtime was expected during the switchover, as existing lagoons provided redundancy and equalization.

Built-In Total Nitrogen Removal: While not currently mandated, total nitrogen reduction is inherently part of the MABR process. Unlike conventional systems that require separate anoxic tanks and complex controls, the Aspiral™ achieves simultaneous nitrification and denitrification, reducing TN levels to below 10 mg/L with no additional capital cost. This delivers high-quality effluent while minimizing infrastructure and operational complexity.

Results

The Aspiral™ M2+ system is designed to deliver the following performance:

• Influent Quality: ~81 mg/L BOD, 140 mg/L TSS, 34 mg/L TKN
• Effluent Quality Targets:

• < 25 mg/L BOD
• < 30 mg/L TSS
• < 10 mg/L Total Nitrogen

By leveraging existing infrastructure, Dow City saved significantly on capital expenditures and ensured rapid deployment. The system is expected to take approximately six months from the time of order to startup. The land footprint is minimal, and the modular design allows a flexible layout within the lagoon boundaries.

Ready to Upgrade Your Lagoon System Without Starting from Scratch?

Learn how Fluence’s modular Aspiral™ units can help your community meet strict effluent limits — even in cold climates — without costly infrastructure overhauls. Contact our water treatment professionals.