Spiral liquid circulation rate in the conventional aeration tank was investigated based upon the theoretical development of the hydrodynamics and the experimental observation. The effect of liquid circulation rate on the gas-liquid oxygen transfer and the oxygen exchange between the bubbling zone and the bubble-free zone in the aeration tank were clarified. The spiral liquid circulation rate at the liquid surface in the aeration tank was correlated as functions of the superficial gas feed rate, the diffuser depth and the bubble diameter generated from the air diffusers. The oxygen exchange resistance between the bubbling zone and the bubble-free zone was 3 - 5 % compared to the overall oxygen transfer resistance from the atmosphere to the bubble-free zone, while that in the deep aeration tank was significant. It was also clarified that the higher spiral liquid circulation rate increases the bubble ascending velocity and thus hinders the gas-liquid oxygen transfer.

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