Mixing Mastery

Solar-powered mixers help a small lagoon treatment plant meet BOD and TSS limits, solve short-circuiting problems and minimize nuisance odors.
Mixing Mastery
Solar-powered mixers improve hydraulic circulation and mixing in the cells at the Iola (Kan.) Wastewater Treatment Plant.

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The 100-acre facultative lagoon system at the wastewater treatment plant in Iola, Kan., was big enough to serve the population of about 5,700 and a candy manufacturer.

However, the design used only about a third of each cell, and over time various water-quality issues accumulated, starting with short-circuiting. A lagoon retrofit in 2007 brought the system under control and in compliance without high-horsepower equipment. Instead, 11 SolarBee mixers (Medora Corporation) thoroughly agitate the cells, helping the plant meet BOD and TSS discharge limits, while preventing odors in spring when the cells turn over.

Hydraulic circulation

“Along with short circuiting, we had poor dissolved oxygen distribution and uneven TSS buildup,” says Toby Ross, wastewater superintendent. “To solve the problem, we first tried a wetland pilot plant to reduce our BOD and TSS, but it was too small and didn’t make an impact.”

The solution lay in improving the system’s efficiency by taking advantage of the large lagoon volumes. Without changing the plant’s capacity, engineers recommended adding an equalization basin in front of the lagoons to bring in diffused air and increase dissolved oxygen.

From the equalization basin, wastewater flows through the three-cell lagoon system, which comprises about 100 surface acres with an operating volume of 157 million gallons and an operating depth of about five feet. Influent and effluent piping was changed to use the entire cell. Before discharge, water flows through a final polishing cell, where a cover was added to block out sunlight and kill off algae, and baffles lengthen the hydraulic detention time.

Within the lagoon system, six solar mixers are installed in Cell 1, three in Cell 2, and two in Cell 3. The units improve hydraulic circulation and mixing in the cells, reducing short-circuiting and improving dissolved oxygen and suspended solids distribution.

Driven by the sun

“The engineers recommended the mixing units,” says Ross. “We wanted mixing in the cells, but we didn’t want the additional costs of wiring all of the mixers in. SolarBee gave us the mixing we wanted by using the sun to power the units.” The mixers provide the required mixing despite large BOD loading from the candy plant, which accounts for about half of the lagoons’ BOD.

The mixers take advantage of the way water forms thin horizontal layers in all reservoirs. They pull in water at the desired depth from all corners of the lagoon and provide efficient mixing that distributes dissolved oxygen evenly throughout the mixed depth.

The mixers also help reduce odors when the lagoons turn over twice a year. “Before the upgrade, the odor was horrible in town,” says Ross. “Because the lagoons are located south of town and the wind is usually out of the south, all the odors were blown right into town. Since the upgrade the lagoons still turn over twice a year, but the odor is minimal.”

In addition, the mixers make the superintendent’s job easier by helping the plant comply with its permit. Says Ross, “The discharge limits are 30 mg/L for BOD and 80 mg/L for TSS. We have met those limits ever since the upgrade.”

About the author

Patrick Schnaidt is vice president of marketing for Medora Corporation, a provider of sustainable reservoir water-quality improvement technologies with low energy or chemical use, based in Dickinson, N.D.  He can be reached at pat.s@medoraco.com.   


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