New technology for aeration of Lake Winona


From: Cal Fremling and Bruce Fuller

On August 28, 1973, about 75 sportsmen, townspeople, and students voluntarily assisted the Lake Winona Committee in installing six Helixor aerators in Lake Winona’s deepest holes. Compressed air was supplied via submerged hoses to the 10-foot tall Helixors by three 7.5 HP on-land piston compressors. The aerators were essential to prevent disastrous winter kills like the ones of 1965 and 1969.

The Helixors prevented winter kills for 34 years, but they had some disadvantages. They were expensive to run. Fishermen accidentally uncoupled air hoses with their anchors, necessitating many repairs by volunteer scuba divers. If the units were turned off purposely or by an electrical outage, they were difficult to restart because airlines had to be laboriously purged of lake water. If the malfunction happened in winter, water could back up in the airlines all the way to the shore where they could freeze solid. The units created large, hazardous areas of open water that had to be posted annually with warning signs. Mallards and Canada geese loved the open water and the food provided by park visitors, but the birds fouled the bike path and adjacent park areas.

The Helixors were not used during the winter of 2008-2009, allowing the lake to freeze over and enabling the troublesome waterfowl to relocate successfully to areas of open water in the river and nearby backwaters. The decommissioned Helixors still stand in the lake’s deepest holes. They are inert (polyethylene, stainless steel, concrete) and serve as fish attractors.

On March 6, 2007, Bruce Fuller (Director of Park Maintenance), Keith Nelson (Assistant City Manager for Public Works), and Cal Fremling (retired Winona State University biology professor), drove to Eagan, Minn., to inspect a new type of aeration unit that had prevented winter kills on two shallow lakes for several years. Further investigation showed that the aging Helixors should be replaced with floating Neptune Aquaculture Compressed Aspirating Aerators from American Aerators of Monticello, Minn. Three of the units were purchased in January 2008, at a total cost of $11,825.

The novel aerators inject air into lake water via compression and aspiration. A 2 HP electric motor drives an above-water turbofan that injects compressed air into a tubular rotating shaft under the water. A high-tech hollow-hub propeller on the end of the hollow shaft aspirates additional atmospheric air into the turbulent prop wash and breaks the air into very fine bubbles which maximize oxygen transfer. The prop wash is directed forward away from the aerator, creating a small area of open water. Neptunes, like Helixors, are widely used to aerate sewage lagoons.

The advantages of the new system are many. Each Neptune unit uses less than half the electricity of an equivalent Helixor unit. With the exception of the unit at Huff Street, Neptunes needn’t run continuously in winter. They can be employed only when necessary. A strong man can lift an entire unit, floats and all - and all repairs can be done above water! The Neptune’s PVC floats are filled with marine foam and can be left in the lake year-round.

Virtually all hardware is stainless steel. The Neptune machinery and electrical lines are stored in the steel sheds that formerly housed the Helixor compressors. All of the electrical hookups necessary for the Neptunes are already in place in the old Helixor compressor sheds that are now used to store Neptunes. Maintenance and repairs have been minimal and easily performed.

One Neptune aerator is strategically located beneath the Huff Street fishing pier where it can be easily observed. It will operate continuously all winter, adding oxygen to the water that flows continually through the culvert from the west lake into the east lake. The Neptune will create open water around the pier, thus preventing powerful ice pushes from damaging it.

Another Neptune will be activated if oxygen levels in the west lake become dangerously low in late winter. The west lake is very vulnerable to kills because it is so shallow and because of its highly organic sediments and intense weed growth. Dissolved oxygen monitoring during late winter in the west lake will determine if aeration is necessary.

By law, aerators cannot be turned on until the ice is safe enough for workers to place warning signs in a rectangular pattern around the area that will be opened by the aerators.

The massive dredging project completed in the east lake in 2001 removed about 1,598,000 cubic yards of sand and about 580,000 cubic yards of organic-rich muck, increasing the volume of that lake by almost 70%, and making winter kills there unlikely. But just in case, a third Neptune would be employed at the Helixor site near Mankato Avenue.


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