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In this work microbiological air pollution at several commune sewage treatment plants (capacity up to 15,000 PE) was investigated. The bioreactors in all plants had a covered construction. The air samples were taken indoors as well as outdoors (both on the windward and leeward side) during different seasons. The samples were collected using the collision method. The presence of indicator organisms in the samples was determined according to the Polish Standards. Identifi cation of individual indicators was performed on solid selective-differentiating substrates. To verify the presence of bacteria from Salmonella, Shigella, coliforms and enterococci species, the colonies observed on the MacConkey substrate were then sifted onto SS and Endo substrates. At all facilities (with one exception) the average CFU for the total number of bacteria and fungi did not exceed 1000/m3, which is the limit set by the Polish Standards for a pollution-free atmospheric air. Bacteria and fungi concentrations, observed at windward and leeward sides of all plants, were relatively low (<100 CFU/m3 and <1000 CFU/m3, respectively) and comparable. A sewage collection point had only a slight impact on the bioaerosol emission. The concentration of microorganisms in the immediate vicinity of covered reactors (aeration chambers) was rather low and remained below the limits sets by the Polish Standards at three facilities. The CFU of individual indicators, measured in rooms accessible for the personnel, was comparable to the CFU in technological rooms. However some indicators, e.g. a number of Actinomycetes, were signifi cantly higher and reached >100 CFU/m3, which means signifi cant air pollution. Similarly, the CFU of hemolytic bacteria had nonzero values. The only place where higher concentrations of bioaerosol were found was the centrifuge room, where digested sludge was dewatered. The number of fungi stayed below the limits there, but the amount of heterotrophic and hemolytic bacteria exceeded the limits and reached the values of ~10000 CFU/m3 and 800 CFU/m3, respectively; it means that the personnel working in this area is exposed to microbiological agents. Microbiological hazards in closed facilities at sewage treatment plants 59 can be expected, however the risk to their exposure remains unknown (Anielak et al. 2015). Inhalation of air containing fungal spores can cause a series of bronchial and pulmonary diseases of allergic origin, alveolitis, asthma or rhinitis (Croft et al. 2016). The main pathogens responsible for these diseases are representatives of the genera Aspergillus, Candida, Fusarium, Trichosporon and Scedosporium. It can be assumed that about 10% of the described species of conidial fungi pose a threat to human health (Enoch et al. 2017, Richardson and Lass-Flörl 2008). Emission of bioaerosols varies with the size of the treatment plant, and may be local (no environmental pressure) or have a broader environmental impact. The risk varies with the treatment plant capacity, the type of treatment technology used, atmospheric conditions and work/operation schedule (Gotkowska-Płachta et al. 2013, Korzeniewska 2011). For example, at many small and medium plants, the rooms where subsequent stages of sewage treatment are carried out are integrated with the personnel facilities in one building. An occupational risk related to work at the sewage treatment plant is diffi cult to assess since some of bioaerosol components (bacterial endotoxins, allergens, viruses) are undetectable by standard microbiological methods or the measured concentrations of microorganisms are underestimated. The methods based on cultures on selective-differentiating substrates, although commonly used, have a number of disadvantages such as: poor reproducibility, selection of certain species against the others and inability to detect microorganisms nonculturable on microbiological media (Douwes et al. 2003). The methods completely ignore other toxic or allergenic components of bioaerosols, e.g. dead cells. In turn, other methods (e.g. fl ow cytometry, DAPI staining, FISH or combinations of these techniques) are currently too complicated and expensive to be used on a routine basis (Chen and Li 2005, Lange et al. 1997). Since most of works that have been carried out on emission of bioaerosols by sewage treatment plants focused on big objects located in larger urban agglomerations (Gotkowska-Płachta et al. 2013, Teixeira et al. 2013) and there is not much data available on small and medium installations (below 15,000 PE and 2000 m3/d) the authors focused on evaluation of the health risk resulting from emission of bioaerosols to enclosed spaces of such sewage treatment plants. All microbiological indicators provided for in Polish legislation were included. Materials and methods Characteristics of sewage treatment plants Four sewage treatment plants, located in the south of Poland, with a daily capacity below 2000 m3/d and PE below 15,000 were selected for the study. Two of them worked as fl ow-through reactors with activated sludge (facilities I and II), two other (facilities III and IV) were SBR type reactors with a cyclic operation mode. Two facilities (II and III) had biological reactors encased by a reinforced concrete ceiling with inspection openings. In facility I, the biological reactor was placed in a hall with gravitational and mechanical ventilation while in facility IV a closed “tower” type of steel biological reactor was used; the reactor was elevated 10 m above the ground level. At three wastewater treatment plants (I, II, III), technological units were located in the same buildings as personnel facilities and control rooms. One facility (IV) had closed steel reactors adjacent to the separate building, with no connection to the personnel facilities. Biological treatment is preceded by a preliminary mechanical treatment. It constitutes a compact device (screens and grit chamber) at facilities I, II and IV or a fi ne screen and a vertical grit chamber at facility III. All these units have been housed in closed and ventilated rooms. Each of the treatment plants employs an installation for supplementary chemical phosphorus precipitation with iron sulphate. Sludge produced at four plants undergoes aerobic stabilization in separate chambers and then is dewatered in fi lter presses (I, II, III) or in a centrifuge (IV). The sewage treatment plants accept mainly municipal sewage and waste from slurry tankers (except for facility II). Only facility III additionally treats wastewater from a tannery, while facility II from local slaughterhouses. Only one of the sewage treatment plants (II) does not meet the effl uent quality standards due to hydraulic and pollution overloads and is planned for expansion. The sewage treatment plants characteristic is given in Tab. 1. Table 1. Characteristic of the studied sewage treatment plants