Site-related airborne biological hazard and seasonal variations in two wastewater treatment plants

Results of a study conducted to assess the degree of airborne bacterial contamination generated by two wastewater treatment plants (WWTP) with different treatment systems and evaluate the dispersion of potential pathogens, have been described. Aerosols samples were collected in summer and winter with an agar impact sampler from several plant sites. External upwind and downwind controls were also examined. Total colony-forming counts of mesophilic and thermophilic bacteria, actinomycetes and streptomycetes, Gram-negatives, coliforms and sulfite-reducers were determined. Selective media were used in order to detect pathogenic bacteria. The lowest concentrations of mesophilic and thermophilic bacteria were 8 and 28 CFU/m3 in plants A and B respectively, the highest >40 000 CFU/m3 in both plants. Strains of Escherichia coli, Clostridium perfringens, Staphylococcus aureus and Enterococcus spp. were isolated in some sites of the two plants. Salmonella spp., Yersinia enterocolitica and Legionella spp. were never detected. The activities involving nebulization and mechanical aeration of wastewaters and the sewage inflows have proved to be of greatest potential risk. In both plants, we found a statistically significant dependence of bacterial contamination on the season for many of the analyzed parameters but a clear seasonal trend could not be observed.