Officials in Slayton had three reasons to look at a new aeration system for the city’s wastewater treatment lagoons.

The first reason was the cost of electricity. The second was the expectation that phosphorus reduction might be added as a permit requirement. The third was the aeration effectiveness was sometimes hindered by weather.

“The old aerators drew from a pipe in the lagoon and sprayed it back on the surface,” says Josh Malchow, city clerk/administrator in Slayton (population 2,000) in southwest Minnesota. “That’s what they are supposed to do, but they were not very effective.”

If the wind was blowing at the pumps, for example, the aerated water would stay at that end of the lagoon instead of mixing throughout the pond. In winter, the lagoons would freeze, slowing down the natural treatment process.

The treatment facility needed an aeration system that could also be used to distribute chemicals into the pond, should that become necessary. In the end, city leaders settled on a mixing solution system from Ixom Watercare that includes two solar-powered units.

Two-lagoon system

The Slayton wastewater treatment facility consists of two lagoons: a 28-acre primary pond with two aerators and a 13-acre secondary pond with one aerator. Effluent is discharged from the secondary pond once in spring and once in fall to Beaver Creek, a Des Moines River tributary. The facility (250,000 to 350,000 gpd average) serves about 1,100 households.

To improve aeration, the city chose two floating SolarBee mixers in the primary pond to replace the old shore-based aerators, and a floating GridBee mixer on the secondary pond, where the old shoreline-mounted aerator remains but is used only occasionally.

The SolarBee mixers operate entirely on solar power. Batteries store power during the day so that the units can operate around the clock. The GridBee mixer is connected to electrical power, but it uses far less energy than the old aerator. A tank with a pump can be attached to the GridBee mixer for feeding chemicals into the pond if necessary.

Electricity savings

The new mixers significantly reduced electricity costs for aeration. “We were spending roughly $12,000 a year to power the three surface aerators, and that doesn’t even include the mechanical issues and parts,” says Weston Mahon, public works director. “That’s just for the electricity.

“Now we’re spending probably $30 a month. Basically, from electricity savings alone, the two solar mixers and the shore-based mixer will pay for themselves in about eight years.” The mixers also made the ponds more efficient.

For one thing, the pond surfaces don’t completely freeze the way they did in the past. Since the mixers were installed in November 2020, there has always been some open water. There has also been a slight decrease in duckweed on the secondary pond.

“Before we put in the new mixers, we’d sometimes get to the point where we had a little too much duckweed,” Mahon says. “On days when there’s no wind, it covers a lot of that secondary pond and the sunlight can’t get to the vegetation on the bottom.

“We definitely noticed an improvement on that. It’s been small, but last year, especially on still days, the duckweed was not covering the entire secondary pond, whereas it did in previous years. Is that from the aerators? Well, that’s the only thing that’s really changed out there.”

Odor reduction

A bonus effect is less odor. When the ponds froze, the spring thaw often brought some odor problems. Now, with the ponds always at least partially open, the odor problems have diminished. “We definitely did not have a year-round issue with odor,” Mahon says. “It was really just the spring thaw. That wasn’t why we were seeking new mixers, but it has been a positive.”

The old aerators, installed in 2004 had maintenance issues. “They were just in terrible shape,” Mahon says. “We were lucky to have one going at a time the last couple years. We were constantly buying parts and spending days at a time with new seal kits or bearings.”

Mahon’s research led him to conclude that spending more for a different type of equipment would be more cost-effective than simply replacing the old aerators. “The initial purchase is expensive, but the return on investment, assuming you have no issues, is so quick.

“With some of the other aerators we looked at, we felt like we would have to deal with a lot of the same maintenance issues we’re already experiencing,” Mahon says. “Then when we looked at the electricity savings and the effectiveness, it became a bit of a no-brainer to spend a little bit more money up front.”

Help with the cost

A rebate for solar mixers from the local power company, Nobles Cooperative Electric, helped offset the cost. The mixers work by slowly drawing water from below the surface up to the top. The depth of the intake can be adjusted.

“The impeller is actually underwater,” Mahon says. “It brings water to the top and then it disperses. You can watch it. You can see how far it goes. It brings water from below that doesn’t ever get much sunlight back to the surface.”

In the past Slayton occasionally had problems with dead spots in the ponds, but since the new mixers were installed, there haven’t been any. What the city does have are smaller electricity bills and more efficient lagoons.