PURPOSE:To control aeration power for dissolving oxygen by a method wherein a flow amount of a gas containing 50% or more of oxygen is measured as well as sewage and an activated sludge are supplied to a lid covered type aeration tank and an oxygen concn. in a waste gas is adjusted to 21% or more. CONSTITUTION:To each stage of an aeration tank 5 partitioned into multiple stages, usually 3-4 stages, an aerating machine 4 is provided and, by said aerating machine 4, a gaseous phase oxygen enriched gas is dissolved in a liquid. Oxygen is supplid by controlling an oxygen supply valve 7 so as to keep pressure in the tank constant by a pressure control device 9. At this time, the oxygen supply valve 7 is opened in order to keep the pressure constant and new oxygen enriched gas is automatically supplied and, thereby, the oxygen enriched gas corresponding to an oxygen demand amount in the tank is supplied. Therefore, if a supplied oxygen flow meter 8 is provided and an oxygen supply amount is measured, an oxygen demand amount in the tank can be known.
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