A Low-Powered Electric Mixer Saves Big Money for a Vermont Lagoon Treatment Plant

A low-powered electric mixer replaces some aerators in a Vermont treatment plant lagoon and delivers significant savings on energy

A Low-Powered Electric Mixer Saves Big Money for a Vermont Lagoon Treatment Plant

A Medora Corporation - SolarBee / GridBee mixer floats on the large lagoon at the Waterbury plant.

A simple change in equipment produced big savings on electricity for the Waterbury (Vermont) Wastewater Treatment Plant.

The plant was using surface aerators to provide mixing and aeration for its three lagoons, which hold 12 million gallons. The addition of a Medora Corporation - SolarBee / GridBee low-power subsurface mixer in the largest lagoon enabled the plant team to shut down some of the surface aerators and still get sufficient mixing and aeration.

The plant’s eggbeater-style and fountain-style surface devices provide good aeration but are not efficient at mixing the large lagoon, and their 10 hp motors use substantial power. On the other hand, the low-power subsurface mixer motor is just 0.1 hp and draws only about 100 watts, but it creates a slow upwelling in the lagoon that provides good mixing of wastewater and bacteria.

The impact on the plant’s electricity bill was immediate, says Peter Krolczyk, plant superintendent: “The cost-savings to us have been close to 40 percent.” That along with an incentive from Efficiency Vermont meant the first subsurface mixer, also called a circulator, paid for itself in its first year.

Data first

Waterbury operators did their own data collection to be sure mixing in the lagoon was adequate. “Nobody had done a really thorough job of collecting data, for example on dissolved oxygen or BOD, that could conclusively tell us that we were getting good mixing, so we actually did that ourselves,” Krolczyk says. “It convinced us that we were getting good mixing and that it was not harming the process. It was actually enhancing it.”

Krolczyk originally looked at a solar version of the same type of mixer, but his research convinced him that the grid-connected type would be more economical. Waterbury has since added two more GridBee units, a second one in the 6 million-gallon large lagoon, and another for one of the facility’s two 3 million-gallon lagoons.

“We reduced our aerators from 17 to nine,” Krolczyk says. “We’re thinking about getting another circulator for our third lagoon. We’ve had very good success, and they are very low-maintenance. Even during the winter, these units don’t miss a beat.” The plant team reduces costs for its second and third mixers by doing the installation in-house and getting a reconstituted frame.

Upgrade for phosphorus

While reducing electricity demand, Waterbury has been making process changes to meet a new requirement for phosphorous discharge in the Lake Champlain watershed. “We had to drop the phosphorus in our effluent from 3 to 7 ppm down to 0.8, and our new limit is going to be 0.2,” Krolczyk says. “The whole plant, except for the lagoon system, got a complete upgrade.”

Before the upgrade, Waterbury had a secondary lagoon system with chlorine disinfection. The new process adds a ballasted floc along with an anhydrous polymer and a coagulant to drop the phosphorus and solids out of the effluent. “We have a very enhanced process now,” Krolczyk says. “We’re able to get to any nutrient limit the EPA or the state wants us to meet.”

The new process added a number of motors and other equipment: “The phosphorous treatment adds a lot of electrical load. Without the upgrade, we’d really be low with our electrical consumption.”

After the storm

The Waterbury plant has had one other adjustment that cuts electrical consumption: It shuts down some operations several days a week. That’s possible because of reduced flows that are a lingering effect from Tropical Storm Irene in 2011. “We received a lot of damage in our little town,” Krolczyk says. “The whole town was flooded. We lost our main pump station. We came within a couple of feet of our entire lagoon system washing out. We had surcharging from the river coming up our discharge pipe. It was an absolute disaster.”

With help from the Federal Emergency Management Agency, the treatment plant has recovered, but the town has not. The plant has a design capacity of 0.51 mgd and a storage capacity of 12 million gallons. The typical flow had been about 0.28 mgd, but lately it is down to 0.15 mgd. “We lost a number of homes. We lost a number of businesses,” Krolczyk says. A state office building was destroyed and rebuilt smaller than before; now it houses about 1,000 employees instead of 2,000.

“We’ve recovered from an equipment point of view, but from an economic point of view, we still haven’t seen full recovery yet,” Krolczyk says. “Our budget since Irene has really been affected by the loss of the influent.” Flows are low enough that the plant can partially shut down for part of the week: “The mixing and the aerating have to continue 24/7. What we turn off is the actual throughput. We just store it in the lagoons and don’t allow it to run through the treatment plant.”

Typically, the plant operates Monday, Tuesday, Wednesday and part of Thursday, and then the operations beyond mixing and aeration are shut down until the next Monday. The operators tried to run the plant at 100 to 120 gpm, but the results weren’t satisfactory.

“It doesn’t really perform well unless it’s running at about 350 to 400 gpm. We just don’t have the flows,” Krolczyk says. “We monitor the levels of the lagoons. When they reach 12.5 feet, we turn the flow on. When the lagoons drop about a foot, we turn the flow off.” Operating that way saves on chemicals and labor, as well as electricity.

Rewards for savings

Other treatment plants may have opportunities to save energy on mixing and aeration just like the Waterbury (Vermont) Wastewater Treatment Plant did. The project was supported by Efficiency Vermont, a statewide energy efficiency program funded through surcharges on utility bills. Similar programs exist in many states, and many offer services and incentives for wastewater treatment.

Low-power mixing technology is one of the tools Efficiency Vermont has found to be effective in some circumstances. “We tried to spread the word about how it worked, and Waterbury thought it would be a good fit for their system,” says Michael Socks, an Efficiency Vermont engineer who worked on the Waterbury project.

“The operator loved not maintaining all those extra aerators out there. The system itself has to be a good match for what these mixers can do. Nationwide, I’m guessing there might be quite a few that would fit in the same situation as Waterbury had.”

Between 2012 and 2017, Efficiency Vermont completed projects at 58 wastewater treatment plants. “Identifying opportunities to reduce costs and helping operators project the cost for doing something a little bit different will not only reduce energy costs, but also in a lot of cases improve the quality of the treatment,” Socks says.

Sources of information about energy efficiency programs available for wastewater treatment plants in other locations can include local utilities, the state energy office, or regional energy efficiency organizations.


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