A microbial risk assessment was carried out for the rotating-brush aerator used during summer in a wastewater treatment plant (WWTP) of Xi’an, China. Bacterial aerosols were first collected by an Andersen cascade impactor at selected sampling sites near the rotating-brush aerator. The concentrations of airborne bacteria were used to obtain microbial emission rate by back calculation. The downwind concentrations of airborne bacteria were then calculated by a modified Gaussian dispersion model accounting for environmental impact and microorganism decay. Subsequently, the exposure parameters suitable for Chinese people were incorporated into a risk assessment model to evaluate non-carcinogenic risks of airborne mesophilic bacteria to sewage workers and surrounding residents. The results indicate that both mean bacterial concentrations at ground level and the exposure hazard quotient decrease rapidly with downwind distance. The exposure hazard quotient by inhalation route is over 105 times more than by dermal contact route for both children and adults, suggesting that inhalation route is the major exposure pathway of microbial aerosol intake for surrounding people. Although the present model gives acceptable low risk values at various downwind distances, it is worth noting that health risks of microbial aerosols associated with rotating-brush aeration for children are generally much more than those for adults.