Countermeasure for prevention of reservoir eutrophication by controlling thermo-solutal convection

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An in-situ experiment on hypolimnic aeration has been carried out in a eutrophic reservoir. Micro air bubbles were discharged into the anaerobic hypolimnion to supply oxygen and improve water quality. The system has been in operation since March in 1999. The aerator unit was so designed that the hypolimnic water is aerated without disturbing thermal stratification. At the beginning of the experiment, however, because of unsuitable design of nozzle structure, the aerator generated unexpectedly large bubbles and promoted vertical mixing. Although the hypolimnic water was aerated due to aerobic water entrained from the epilimnion, this was not the hypolimnic water aeration what we expected. The aerator structure was then modified to prevent macro-bubble generation and the hypolimnic water was successfully aerated without destratification. After all, in the course of the experiment the reservoir has experienced three different dissolved oxygen regimes, i.e. (1) no aeration, (2) aeration with vertical mixing, (3) aeration with little vertical mixing. The present study is to discuss how the dissolved oxygen affects hypolimnic water quality by comparing the field data from the three different DO regimes. The results provide useful information in designing an aeration system as a countermeasure against hypolimnic water eutrophication.