Emission level, particle size and exposure risks of airborne bacteria from the oxidation ditch for seven months observation

The emission of airborne bacteria and their human exposure risks were studied in a wastewater treatment plant (WWTP). Air samples were collected by an Andersen Cascade Impactor at different distances from the rotor disc aerators (RDAs) equipped in the oxidation ditch in seven months. Spatial and seasonal variations in the concentrations of airborne bacteria emissions were detected. The highest concentration was observed near the RDAs (835 +/- 91 CFU/m(3) to 8916 +/- 155 CFU/m(3)) during each sampling process, with the concentration decreasing by 76.70% and 79.91% as sampling distance and height increased, respectively. Most of the airborne bacteria were coarse particles that exceeded 4.7 mu m in size. The dominant bacteria were Bacillus sp., Lysinibacillus sp., and Sphingomonas sp. The spatial distribution of airborne bacteria visualized by contour maps indicated potential health impact of wastewater treatment processes on biological pollution within the WWTP and surrounding areas. The deposition fraction of airborne bacteria that could penetrate the lung through inhalation were used to assess the human exposure risks. Hazard quotients ranged from 1.66 x 10 (-2) to 1.06. There were cumulative risks to on-site workers and nearby residents posed by chronic exposure. The airborne microorganisms disseminated by the RDAs should be reduced.