Modelling Optimum Conditions for Reservoir Destratification Using Mechanical Mixers

This report was produced for the Urban Water Research Association of Australia, a now discontinued research program.

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Report no. UWRAA 24

April 1991

Synopsis

For the last two years three submersible mixers have been used at Myponga Reservoir in South Australia for destratification. The mixers lift cold water from near the bottom of the reservoir to the surface and offer a number of advantages over a conventional air curtain including less power; lower noise levels; and greater flexibility of operation.

This report presents the results of using a mathematical model to simulate mixer performance. The optimum position and orientation for the mixers have been determined, a number of operating scenarios examined in detail, and the performance of the mixers compared with a conventional aeration destratifier. The results are generally applicable to any reservoir.

The model has also been used to assess the effectiveness of operating a single mixer fitted with a shroud. The purpose of the shroud is to stop entrainment of warmer water as the cold water is lifted to the surface. A mixer with shroud was field tested and the data collected compared with model simulations.

The effect of the mechanical mixers on water quality was assessed by a comprehensive monitoring program. Data for nutrients, heavy metals, temperature, dissolved oxygen, chlorophyll and phytoplankton are discussed in detail. Changes in the phytoplankton community structure since 1963 have also been examined to determine if the mixers have had a major impact.

The report concludes that the model can be used to simulate the performance of the mixers but that insufficient field data was available to validate the mixer with shroud. Although the mixers have improved water quality in the reservoir they have not been an unqualified success.

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