A system and method for oxygenating an aerobic sludge digester is provided. The disclosed embodiments include an aerobic sludge digester tank or vessel having a mechanically agitated contactor disposed therein, an oxygen injection subsystem adapted to inject oxygen into the digester tank proximate the mechanically agitated contactor. The embodiments of the mechanically agitated contactor include an agitator or impeller disposed within the draft tube which makes the draft tube the primary site for the gas-liquid mixing. In particular, the agitator or impeller is adapted to create gas bubbles having an average diameter between about 0.3 mm and 3.0 mm which are then dispersed into the aerobic digester. The mass transfer efficiency associated with the present system and method is enhanced from the combined effect of oxygen dissolution into the sludge and greater bubble residence time.