The off-gas method was used to study oxygen transfer efficiency in a long, narrow activated sludge basin. The basin was equipped with square, ceramic, fine pore diffuser plates arranged in a uniform longitudinal pattern at a depth of 4.3 m. Oxygen transfer efficiency, at 20°C and zero dissolved oxygen concentration, during the second and third years following diffuser cleaning, was between 15 and 17%. A uniform distribution of air to the tank was compared with a tapered air supply (more air at the inlet end and less at the outlet). The tapered air supply gave lower dissolved oxygen at the outlet and did not increase overall oxygen transfer. With a 3 to 5-fold variation in air flow, at either the inlet or outlet zone of the basin, the oxygen transfer efficiency, adjusted for zero dissolved oxygen, stayed constant.

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