Assessing airborne biological hazard from urban wastewater treatment

The production of microbial aerosols by urban sewage treatment plants may have wide hygienic implications which call for careful evaluation: exposure to such aerosols may in fact represent a health hazard for plant workers and nearby residents alike. This paper describes the results of a study analysing the degree of microbial contamination at different levels of treatment at several plants in the City of Leghorn (Livorno, Italy). Monthly aerosol samples were collected with an agar impact sampler from January to November 1996, from different sites at an activated sludge plant, an anaerobic sludge plant and a wastewater washing station. The total bacterial and coliform counts were determined, and pathogenic enteric bacteria and viruses were determined. These same parameters were also measured in wastewater and sludge samples obtained at the same sites. The results revealed that high-grade airborne contamination existed at several of the studied sites. In particular, pathogenic enteric bacteria (Salmonella enteritidis and S. boydii) were isolated in 2% of the samples (540 l per sample), reovirus in 46% and enterovirus in 9% (1800 litres per sample in indoor environment and 3000 l outdoor), always in association with the former virus. The biological parameters measured had no evident correlation with meteorological factors such as temperature, relative humidity or wind characteristics. Viral contamination proved to be quite wide-spread and detectable even in the presence of low levels of bacterial contamination. Although virological analysis have been only qualitative, and the different volumes examined for viruses and bacteria cannot allow us to appraise with accuracy the association between these two parameters, the viral presence along with low bacteria contamination suggests more dedicated studies to address with greater accuracy the quantitative aspects of this association. However the monitoring performed allowed for a determination of the areas of greatest potential risk for plant workers, and the preventive measures most suitable to guaranteeing their safety.