Occurrence and seasonal variation of airborne Gram negative bacteria in a sewage treatment plant

A study was carried out to determine the microbial density and the seasonal variation of airborne Gram negative bacteria in a sewage treatment plant. Sampling was made at 16 sites and the settle plate technique was used. Of t he 201 samples examined, 43.2% revealed fecal coliforms (mean value = 14 cf u/p/h), 53% Pseudomonas spp. (mean value = 11 cfu/p/h), 46.5% Shigella spp. (mean value = 13 cfu/p/h), 3% Legionella spp. (mean value = 2cfu/p/h) and 2% Salmonella (mean value <1 cfu/p/h). 72% of the samples contained "other" Gram negative bacteria such as Aeromonas hydrophila, Serratia marcescens, Enterobacter cloacae and others. With the exception of Legionella spp. and Salmonella, all other bacteria were more frequent and numerous in the Octob er-March period, when temperatures were lower and humidity higher. Although the oxidation tanks were covered overall contamination was nevertheless hi gh, thus presenting a potential health risk for plant workers.