The research was carried out in a conventional sewage treatment plant equipped with completely mixed reactors, located in Olsztyn. The main problem was that it was impossible to simultaneously achieve high treatment efficiency and appropriate sedimentation of the activated sludge. The observations were made at different levels of aeration intensity, controlled by the depth of the mechanical surface aerator submersion. The study confirmed a positive effect of the intensive aeration on the process efficiency and the structure of microfauna and a negative effect on the sedimentation of the activated sludge. The increase of the dissolved oxygen concentration from 1-2 mg?? dm-3 to 3,0-3,3 mg?? dm-3 was accompanied by a 10% growth of COD reduction (from 72-75% to 80-89%), 2-fold increase of ciliates species diversity (from 4-7 to 8-12 taxa), including predators, and the rise of the SVI value from 73 to 441 ml?? g-1. It was recorded that a higher aeration rate results in the flocs lightness, as a consequence of air saturation, and the development of filamentous microorganisms, mainly Microthrix parvicella and Type 1851. The double mechanism of sedimentation disturbances was indicated by the decline of the SVI value to about 200 ml?? g-1 recorded after emerging of aerators irrespective of the same number of filaments. Foam was thought to be the primary proliferation area of low F:M filamentous microorganisms and it appeared as a result of excessive turbidity in the reactors. Foam contained a large amount of M. parvicella and Type 1851. Foam removal by means of water stream at the initial stage of its formation seems to be the most effective method to prevent the bulking of the activated sludge.