The article presents the results of research over microorganisms (psychrophilic and mesophilic bacteria and microscopic fungi) found in wastewater in denitrification and nitrification chambers and specifies the proportion of these microorganisms in bioaerosol at various levels above wastewater level (20, 50 and 100 cm). In the denitrification chamber (anoxic) in 1 cm3 of sewage there were on average 30.35 · 106 CFU of mesophilic bacteria, 72.88 · 106 CFU of psychrophilic bacteria, and 37.3 · 105 CFU of microscopic fungi. In the nitrification chamber, where the oxygen concentration ranged from 0.37 to 2.32 mg O2·dm−3 of wastewater, the number of microorganisms was lower. In 1 cm3 of wastewater there were on average 20.2 · 106 CFU of mesophilic bacteria, 51.76 · 106 CFU of psychrophilic bacteria, and 15.22 · 105 CFU of microscopic fungi. In sewage bioaerosols above these chambers, higher numbers of psychrophilic bacteria than mesophilic ones and microscopic fungi were reported. At the same time differences in the number of microorganisms at different heights above the surface of wastewater could be observed in bioaerosol, as well as between the chambers of the bioreactor. It was found that most frequently the amount of microorganisms decreased with height. The percentage emission ratio (ER) of microorganisms in bioaerosols coming from wastewater accounted for only a fraction of a percent and ranged from 1.13 · 10−8 % (microscopic fungi over the denitrification chamber) to 24.53 · 10−9 % (psychrophilic bacteria over the denitrification chamber). It was found that the process of mixing, aeration of wastewater, have an effect on the emission of microorganisms.