Lake Destratification System Evaluation
Tech Talk 101
Over the last 35 years at Pentair AES, we have seen a lot of lake aeration companies come and go. One thing they had in common was that they all exaggerated their performance. It is no different today! We see others with superior claims who have not measured or don’t know how to measure their performance. It's not easy, but following is the background you will need to evaluate manufacturers' claims.
Lakes have a very low BOD to volume ratio, as opposed to aquaculture or wastewater. This difference makes the standard aeration techniques ineffective or impractical. Realizing this problem early on, we developed the air-driven, unconfined, destratification technique, which is very efficient at moving bottom water to the surface. The goal of our technique is to move enough water to keep the lake bottom above 5 mg/l dissolved oxygen, in keeping with the Clean Water Act of 1972.
During our research and development, we naturally began with a draft tube because of its "chimney effect," high efficiency and lack of a requirement for a sophisticated air diffuser (for an explanation of how ducted airlifts work, see "Airlift Notes" Tech Talk). The draft tubes worked very well; however, we had to discontinue their use for most lake destratification jobs because of their high capital, installation and maintenance costs. We then developed the synergistic airlifts as the best nonducted or unconfined airlifts. We estimate their efficiency at 90 percent.
The performance of systems can be quantified, either by measuring the time required to destratify a large lake-like impoundment or by taking direct measurements of their flow rate. You can use either a dye or a flow grid (along with a diver and underwater camera) for in situ measurements. The goal is to identify and measure the uprising column of water's minimum diameter and speed, then estimate and subtract losses due to eddy currents (dye will provide a good visual).
Unfortunately, you cannot use a draft tube measuring device (A), as the confining tube will increase the velocity and flow rate ("chimney effect"). It will also cause all of the tested devices that have the same airflow rate to have the same water flow rate, making comparisons impossible. That would be like comparing the rise rate of free floating helium balloons to that of a chimney full of helium balloons.
The method we used in our R&D, and the one that we feel is the most accurate, is the flow grid, as illustrated by B. It can be used for diffused air, propeller or venturi type destratification devices. It will require a wire rack with 2" x 2" grid, with 12" long negatively buoyant ribbons attached at each intersection and adjustable legs. It is important that this be done in a large lake-like impoundment with very clear water (much of our testing was done in clear ocean waters). The flow rack should be fixed at the minimum column diameter in the up-welling stream, measurements recorded, then flows calculated by the diameter of the current and rise rate. Subtract losses from eddy currents and express results in gallons per minute per kilowatt hour.
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