Like many Iowa municipalities, the City of De Soto requires an upgrade to their lagoon system to meet more stringent ammonia effluent requirements in coming years. In this article, we’ll review the conditions that led De Soto to choose to pilot Triplepoint’s NitrOx™ Process for lagoon ammonia removal, outline why NitrOx was the most attractive option, describe the pilot setup, and reveal the results of the pilot so far.
Iowa lagoon ammonia removal pilot location
The City of De Soto, Iowa, is located in the center of the state, south of I-80 and west of West Des Moines, just 15 miles from Winterset, the birthplace and boyhood home of John Wayne. Its population of 1,050 is spread over 1.5 square miles.
De Soto’s domestic wastewater is treated with a three-cell aerated lagoon system located southeast of town. Its two aerated cells and quiescent cell treat an average daily dry weather flow of 0.11 mgd, with a maximum wet weather flow of 0.78 mgd. With a total surface area of 4.24 acres, a storage volume of almost 10 million gallons, and a 42 day retention time, the lagoon system had been meeting the city’s effluent requirements for CBOD and TSS. The treated wastewater discharges into Bulger Creek, a recreational waterway upstream from rivers on the state’s impaired waters list due in part to high ammonia levels.
The Iowa Department of Natural Resources is implementing new ammonia standards to protect the state’s waterbodies, which puts a burden on smaller municipalities that can ill afford a significant capital expense and an increase in operation and maintenance costs. Although De Soto’s lagoon system has been providing sufficient ammonia treatment in warmer months, the city would be out of compliance in the winter, unable to achieve the new ammonia limits of below 5 mg/L.
De Soto, IA, is very cold during winter when temperatures tend to be in the 20s, with average January highs of 31°F and lows of 11°F. Winter lagoon water temperatures average 32.9°F. Water temperature that low means nitrification has virtually stopped: the optimal temperature for nitrification is 30°C/86°F, considerably warmer.
De Soto needed a cost-effective way to achieve the new ammonia limits in the winter, ideally by upgrading their current system, which was otherwise providing adequate treatment. How could De Soto leverage the existing infrastructure to meet the new ammonia limits without increasing the burden on taxpayers with high construction and energy costs?
De Soto chooses NitrOx for lagoon nitrification
De Soto chose to be the first municipality in Iowa to pilot Triplepoint’s NitrOx system because:
- It is based on proven technologies—with decades of research studies and thousands of successful installations around the world.
- It is 2/3 the cost of available alternatives. Capital costs are lower because it integrates into the existing process rather than replacing it and only takes up a small footprint thereby negating the need for additional land.
- It would preserve the ease of operation and low maintenance that De Soto’s existing lagoon system provides. It is simple to operate, automated, self-cleaning, and flexible.
- Treatment would not be disrupted, because the lagoons wouldn’t need to be taken off-line for installation.
- It offers controlled, reliable nitrification: Because influent temperature is thermally regulated only when needed, energy costs are minimized.
Purpose of the pilot study
From the pilot study, De Soto and Triplepoint would be able to
- Verify the year-round nitrification capabilities of NitrOx
- Demonstrate that temperature optimization improves nitrification
- Confirm existing research that demonstrates nitrification can occur at 4–5°C (39–41°F)
- Generate data that could be extrapolated to predict the success of a full-scale installation at De Soto and fine-tune the design
The set up
The process began with the installation of the pilot unit in March 2014. It was placed between the second and third cells of the lagoon system, with an influent pump in the manhole between the two cells and on the Cell 2 side of a weir. This is due to the need for BOD levels to be between 20–30 mg/L before nitrifying bacteria will begin to nitrify. Effluent from the insulated pilot crate flows out of the effluent pipe into Cell 3 at a rate of no more than 0.5 gallons per minute.
The pilot unit consists of a galvanized steel box with 2” of insulation to simulate both the insulation of the ground and the floating cover of a full NitrOx system. Inside the box are two 150 gallon tanks with PVC coarse-bubble aeration grids at the bottom, designed to create a complete mix aeration system. Each tank is provided air via a ¾ hp piston-type compressor and is filled with 50% attached growth media that is suspended in the water column, constantly moving around in the complete mix environment. Influent water is pumped into Tank 1 via a small pump averaging a rate of 0.5 gallons per minute. Each tank is equipped with a 1000 watt aquarium tank heater with temperature sensors and a digital controller that targets 41 degrees Fahrenheit during the winter months. After four hours of retention time, flows from Tank 1 enter Tank 2, then, after another 4 hours of retention, enter the effluent pipe. For testing purposes, influent is pulled from the pump line going into the pilot and a sample tap is used to pull effluent from Tank #2.
The testing parameters chosen for the pilot conform to the Iowa Department of Natural Resources’ standards for lagoon systems. All samples are grab samples: The influent sample is taken from the bleed line of the influent pump; the effluent sample is taken from the effluent pipe directed toward Cell 3. Influent and effluent CBOD, soluble BOD, pH, TSS, and ammonia are tested weekly and samples are sent to an independent lab for analysis.
Year-round nitrification with NitrOx
Cost-effective lagoon ammonia removal, especially in winter, is becoming a concern for municipalities in Iowa—and elsewhere—as permits start to require nitrification. The success of the De Soto pilot demonstrates that NitrOx can cost-effectively achieve near-complete lagoon ammonia removal in cold winter temperatures.