Changes in Bacterial Aerosols with Height above Aeration Tanks

This paper describes the results of a study conducted to determine the fate of bacterial aerosol emissions from aeration tanks of an activated sludge treatment plant relative to the height above the liquid surface. Bacterial aerosols were collected using Andersen six-stage viable samplers positioned at heights ranging from 7.6 to 318 cm above the aeration tank liquid surface. Samples were analyzed for heterotrophic plate count (HPC), total coliform (TC), fecal coliform (FC), and fecal streptococci (FS). A statistical analysis of the data indicates that HPC and TC concentrations decreased exponentially with height above the aeration tank liquid surface. Fecal coliform and FS concentrations were low or below detection limits ( <4 cfu/m 3 ) in the collected samples and showed no decrease with height. This study demonstrates that a significant decline in number of aerosolized bacteria (leaving the surface of an aeration tank) occurs within the air column directly above the aeration tank liquid surface. Also, bacteria of potential public health concern, such as FC and FS, often reach nondetectable levels ( <4 cfu/m 3 ) within one meter above the liquid surface of these aeration tanks.