Evaluation of the environmental impact of microbial aerosols generated by wastewater treatment plants utilizing different aeration systems

Using three sampler devices (SAS, Andersen Six-Stages and All Glass Impinge r), the environmental impact of bacterial and fungal aerosols generated by municipal wastewater treatment plants operating with different methods of s ludge oxygenation were evaluated. The highest microbial concentrations were recovered above the tanks (2247 cfu m(-3)) and in downwind positions (1425 cfu m(-3)), where a linear correlation (P < 0.05) was found between the qu antity of sewage treated and the entities of microbial aerosol dispersion. Moreover, an exponential increase (P < 0.05) in the bacteria recovered from the air occurred at increasing times of treatment. However, after long-ter m plant operation, high bacterial and fungal concentrations were found in a lmost all of the sites around the plant. Coliforms, enterococci, Escherichi a coli and staphylococci were almost always recovered in downwind positions . Considerable fractions (20-40%) of sampled bacteria were able to penetrat e the final stages of the Andersen apparatus and thus, are likely to be abl e to penetrate the lungs. The plant operating with a fine bubble diffused a ir system instead was found to generate rather low concentrations of bacter ia and fungi; moreover, staphylococci and indicator micro-organisms were al most absent. Finally, salmonellae, Shigella, Pseudomonas aeruginosa and Aer omonas spp, were not detected in either of the plants. The results indicate a remarkable dispersion of airborne bacteria and fungi from tanks in which oxygen is supplied via a mechanical agitation of sludge, and suggest the n eed to convert them to diffused aeration systems which pose a lesser hazard for human health.