BACTERIAL AEROSOL EMISSION RATES FROM MUNICIPAL WASTE-WATER AERATION TANKS

In this report we describe the results of a study conducted to determi ne the rates of bacterial aerosol emission from the surfaces of the ae ration tanks of the Metropolitan Water Reclamation District of Greater Chicago John E. Egan Water Reclamation Plant. This study was accompli shed by conducting test runs in which Andersen six-stage viable sample rs were used to collect bacterial aerosol samples inside a walled towe r positioned above an aeration tank liquid surface at the John E. Egan Water Reclamation Plant. The samples were analyzed for standard plate counts (SPC), total coliforms (TC), fecal coliforms, and fecal strept ococci. Two methods of calculation were used to estimate the bacterial emission rate. The first method was a conventional stack emission rat e calculation method in which the measured air concentration of bacter ia was multiplied by the air flow rate emanating from the aeration tan ks. The second method was a more empirical method in which an attempt was made to measure all of the bacteria emanating from an isolated are a (0.37 m2) of the aeration tank surface over time. The data from six test runs were used to determine bacterial emission rates by both calc ulation methods. As determined by the conventional calculation method, the average SPC emission rate was 1.61 SPC/m2/s (range, 0.66 to 2.65 SPC/m2/s). As determined by the empirical calculation method, the aver age SPC emission rate was 2.18 SPC/m2/s (range, 1.25 to 2.66 SPC/m2/s) . For TC, the average emission rate was 0.20 TC/m2/S (range, 0.02 to 0 .40 TC/m2/S) when the conventional calculation method was used and 0.2 7 TC/m2/s (range, 0.04 to 0.53 TC/m2/s) when the empirical calculation method was used. The fecal coliforms and fecal streptococci both had emission rates that were much lower than the SPC and TC emission rates . Our estimates of bacterial source strength emission rates should be useful in any future dispersion modeling work performed at this or any other water reclamation plant in which the activated sludge process i s used.