Municipal wastewater and activated sludge from a sewage treatment plant were introduced into two lab. bioreactors with a fine bubble diffuser or with a micro bubble diffuser. The aeration process was carried out for 24 h. The dissolved O-2 concn., pH and temp. were monitored. The COD and BOD values as well as NO3, NH4 and PO4 concns. were detd. The oxygenation of sewage treated in the reactor with a micro bubble diffuser was significantly more efficient.