Airborne dissemination of bacteria from sewage treatment plants

During the last 5 yr, the air of sewage treatment plants was examined bacteriologically under different climatic conditions. Methods of examination were sedimentation on nutrient agar, membrane filtration and the use of an impinger. At each sampling site wind velocity and humidity of air were measured. In addition to colony counts, the presence of fecal indicators (E. coli, Clostridia), Salmonella, non-agglutinable cholera vibrios and Pseudomonas aeruginosa was determined. The results were as follows: The formation of mist above the sewage in aeration tanks was preferably observed during the winter season. Correspondingly, the bacteriological results were less favourable in winter-time; trickling filters are also capable of spreading bacteria into ambient air; E. coli could be found up to a distance of 300 m downwind from the aeration tanks, Salmonella up to a distance of 10 m; higher wind velocity resulted in a higher number of positive findings, sunshine diminished it, covering of the aeration tanks clearly resulted in a lower number of positive findings, parallel examinations performed at other sampling sites far from sewage treatment plants, such as underground stations, shops, or sports facilities revealed that the bacterial level at such sites was much lower than in the air of sewage treatment plants. Quantitative studies (Salmonella was taken as an example) revealed that the dissemination by air was low in relation to the Salmonella content of the sewage itself, so that the infectious dose was hardly attained by direct ingestion. Nevertheless, hygienic measures, such as hand washing, use of protecting clothes and avoidance of contact with foods in the open air should be performed in sewage treatment plants so that the infectious dose also cannot be indirectly reached. Beyond this, the possibility cannot be ruled out that conditions are quite different for other pathogens having much lower infectious doses.