Active Mixers Bring Peace of Mind to Minnesota Utility

Submersible tank mixer solves ice problem in potable water storage tanks and provides year-round benefits.

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Ice buildup in Rochester’s potable water storage tanks used to be a common problem during Minnesota’s prolonged winters. The ice chipped away at paint on the interior walls, causing extra maintenance and budget concerns for Cary Johnson, water operations manager for Rochester Public Utilities (RPU).

It was a problem that could only increase. Home to the Mayo Clinic, Rochester is a fast-growing community: the utility receives about 600 new accounts a year. Currently about 20 water storage tanks with a combined capacity of 16.5 million gallons serve a population of 112,000.

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GridBee® GS-12 submersible tank mixers provide municipalities with a low-cost solution for meeting water quality standards and preventing ice damage.

Until a few years ago, Johnson and RPU dealt with the problem of ice buildup in their reservoirs the same way utilities in many other northern and cold-climate towns do: by exercising the tanks. In theory, water movement from frequent drawdowns and fills helps prevent ice from forming on the water surface and as warmer water enters with each fill cycle, colder water is offset in the tank. Despite the exercising strategy, “we still experienced quite a bit of ice buildup in the larger reservoirs,” said Johnson. Because the ice damaged the paint on the interior walls, storage tanks required repainting about every 10 years instead of every 15, at an average cost of $125,000 per tank.

Searching for a better alternative than exercising the tanks, Johnson learned that GridBee® GS submersible electric mixers from Medora Corporation (Dickinson, N.D.) might provide a cost-saving solution. The mixer deploys on the bottom of the tank and creates a constant floor-to-surface flow ensuring new incoming water is properly mixed throughout the water column. New water contains essential BTUs (heat) which is then actively mixed throughout the entire tank, keeping ice at bay and water temperature and water age as uniform as possible.

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Cary Johnson, water operations manager, Rochester Public Utilities.

Johnson installed the first mixer in one of the larger reservoirs. “We experienced virtually no ice buildup in the Bandel Reservoir after installing the mixer,” he said. Since then, Johnson has added mixers to most of the existing tanks and as new tanks go online. Since ice damage is no longer the problem it once was, their interior tank painting schedule has extended by three years or more.

The mixers provide benefits year-around as well. They ensure uniform distribution of disinfectant, minimize thermal stratification and prevent short-circuiting and stagnant water conditions. Their unique circulation pattern washes across the floor and sides of the entire tank where biofilms are most likely to accumulate. This circulation constantly replaces beneficial disinfectant, killing the bacteria in the most critical parts of the tank.

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GridBee mixers helped solve the ice buildup problem in Rochester’s water storage tanks.

“The mixers make my job easier and give me peace of mind,” said Johnson. “We had to stay on top of exercising the tanks, and that’s not an issue any longer.” And extending the painting schedules is a relief for the utility’s budget.

To learn more about Medora Corporation or GridBee® GS submersible electric mixers, visit

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