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 activat ed sludge treatment plant relative to the height above the liquid surf ace. Bacterial aerosols were collected using Andersen six-stage viable samplers positioned at heights ranging from 7.6 to 318 cm above the a eration tank liquid surface. Samples were analyzed for heterotrophic p late count (HPC), total coliform (TC), fecal coliform (FC), and fecal streptococci (FS). A statistical analysis of the data indicates that H PC and TC concentrations decreased exponentially with height above the aeration tank liquid surface. Fecal coliform and FS concentrations we re low or below detection limits (<4 cfu/m(3)) in the collected sample s and showed no decrease with height. This study demonstrates that a s ignificant decline in number of aerosolized bacteria (leaving the surf ace of an aeration tank) occurs within the air column directly above t he aeration tank liquid surface. Also, bacteria of potential public he alth concern, such as FC and FS, often reach nondetectable levels (<4 cfw/m(3)) within one meter above the liquid surface of these aeration tanks.