Potable water mixing prevents ice buildup
During the winter months, average daytime highs in Rochester, Minn., hover in the mid-20s (°F). Such prolonged cold has taken an annual toll on the town’s aboveground potable water storage tanks. Ice buildup sometimes measured as much as 2 ft thick inside the larger tanks, chipping away at paint on the interior walls and causing extra maintenance and expense.
Searching for a better alternative than simply exercising the tanks, Rochester Public Utilities Water Operations Manager Cary Johnson learned that GridBee GS-12 submersible electric mixers from Medora Corp.  of Dickinson, N.D., might provide the solution.
Tank Exercise Falls Short
Rochester Public Utilities (RPU), a division of the city of Rochester, is Minnesota’s largest municipal utility, serving 48,000 electric customers and 38,000 water customers. Rochester draws its water from the Jordan sandstone formation, 400 ft below, via 32 groundwater wells and pumps, and stores it in aboveground reservoirs located throughout the city. One of the larger reservoirs, the Bandel Reservoir, has a total capacity of 2.25 million gal and measures 45 ft high and 95 ft in diameter.
Until recently, Johnson and RPU dealt with the problem of ice buildup in the reservoirs in the same way as utilities in many other northern towns: by exercising the tanks.
“We start and stop our pumps based on the water level in the tanks,” Johnson said. “In the winter months, we let the water level drop down farther than in the summer before calling for a well to start and fill the tank. In the summer, we may run at a 5-ft range for starting and stopping the pumps, and in the winter, that range may be 10 ft.”
The water movement from frequent drawdowns and fills helps prevent ice from forming on the water surface, and the warmer water that enters with each fill cycle offsets the colder water in the tank.
Despite exercising the tanks, “we still experienced quite a bit of ice buildup in the larger 2.5 million-gal reservoir,” Johnson said. Because the ice damaged the paint on the interior walls, RPU moved its painting schedule up from 15 years to 10 years.
“We looked at other tank mixers, and decided—based for the most part on price, and ease of installation—that we would try the GS-12 mixer,” Johnson said.
Year-round Benefits of Mixing
Already known for its SolarBee solar-powered mixers, Medora Corp.  introduced the GridBee GS-12 submersible electric mixer in 2011 as a low-cost solution for sites where 110 volts AC is available. The mixer, 36 in. long and 11.5 in. in diameter, sits on the bottom of the tank and creates a constant flow, taking new incoming water off the floor and mixing it throughout the water column.
“We experienced virtually no ice buildup in the Bandel Reservoir after installation of the GS-12 mixer,” Johnson said. “For comparison, we checked for icing conditions in another of our large water storage reservoirs and found we did still have icing in that tank.”
While GridBee mixers help municipalities solve problems of ice buildup, the year-round benefits of mixing water in potable water storage tanks are to ensure uniform distribution of disinfectant, minimize thermal stratification and prevent short-circuiting and stagnant water conditions. GridBee mixers pull water right off the tank floor for the most efficient mixing of important boundary layers. The constant flow of water in effect scrubs the floor and sides of the entire tank, constantly replacing disinfectant and killing the bacteria in the most critical parts of the tank.
Due to the success in preventing ice buildup in the Bandel Reservoir, Rochester has budgeted for two more GS-12 mixers to be installed in 2013. The city will also conduct studies documenting changes in thermal stratification, short-circuiting and prevention of stagnation. By solving the ice buildup problem in the winter as well as mixing problems year-round, GridBee mixers are helping Rochester allocate its utility budget as cost-effectively as possible.
Pat Schnaidt is vice president of marketing at Medora Corp. Schnaidt can be reached at [email protected]  or 970.484.3437.