The ability of a gas dissolving device to improve the DO condition of water is evaluated using the oxygen transfer rate. Usually the oxygen transfer rate is measured in mixing-type treatment, where air bubbles are released in the water tank. This method, however, has some disadvantages, such as that it cannot be used for compressed-type gas dissolving devices and that it requires the evaluation of the overall gas transfer coefficient. The present study proposes a convenient method of evaluating the oxygen transfer rate which is applicable to gas dissolving devices of both compressed and pressure-free types, applicable to water with an arbitrary DO concentration, and does not require the evaluation of the overall gas transfer coefficient. Evaluation of DO improvement is done using the equivalent DO increment, devised to eliminate the influence of the DO concentration of the water before treatment. The value of the oxygen transfer rate is represented as the value at a temperature of 20 degrees Celsius. Laboratory experiments to check the method have shown that the proposed method of measurement is valid.
Oxygen deficiency in closed waters seems to induce serious problems. The authors have been trying to solve the problem utilizing liquid membranes of bubbles cluster formed by aeration in the h-shaped pipe located near the water surface, which enables to improve DO condition in the water at high efficiency and low energy cost. In this paper experimental results are given for DO improvement of the above system. The oxygen transfer rate N (20) and the efficiency E P defined as the transfer rate per unit work rate required for aeration are examined for various values of the aeration depth H A , flow rate of supplied air Q g , and the height of the pipe H U . As the results, both N (20) and E P take high values when H U is small and H A is large. Also there is some optimum Q g for a given diameter of the pipe.
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