Bioaerosols are conglomerates of biological particles such as bacterial and fungal propagules and are produced in sewers and sewage treatment plants through evaporation and turbulence. In order to evaluate the hazard to employees in wastewater treatment plants, airborne microorganisms were measured at two different sites in the sewage systems and in the grit chamber of a treatment plant. Two additional samples were taken during high-pressure cleaning in the relief sewer. Outdoor air samples served as background values. Airborne microorganisms were collected using the impaction method with the MAS-100® and the impingement method with the SKC Biosampler®. The concentrations of coliform bacteria as well as the fungal species Aspergillus fumigatus were determined in addition to mesophilic bacteria counts (cfu/m³). The highest concentrations of mesophilic bacteria were found in the encased grit chamber. Coliform bacteria were found infrequently only in the aerosol of the sewage systems; A. fumigatus was detected at all sampling sites both indoors as well as outdoors. During high-pressure cleaning, total bacteria concentrations reached up to 4.0 × 104 cfu/m3, coliforms up to 3.0 × 103 cfu/m3. These results show that personnel protective measures should be recommended to decrease the exposure risk to biological particles.