The assessment of airborne microorganisms in large-scale composting facilities and their immediate surroundings

The number of airborne microorganisms in the area of large-scale composting facilities with different composting techniques (A: open facility using the intensive decomposition process [4000 t/year]. B: closed facility with compost containers [7000 t/year], C: closed facility with table-pile compositing and automatic turning equipment [22 000 t/year]) was investigated using impactor sampling systems (Andersen samplers). All counts carried out inside the closed facilities, especially during the turning process, showed values of >5.0 × 105 CFU/m3 for viable bacteria and moulds with a proportion ofAspergillus fumigatus of up to 64%. Depending on the type of facility, different median values were determined inside the plant area. Counts were highest in the immediate area around the biofilter outside of Facility C (1.7 × 104 CFU/m3 for bacteria and 9.5 × 103 CFU/m3 for moulds). In view of the high load of ambient airborne microorganisms inside the composting facilities, adequate occupational health measures are urgently required. Counts determining the hazard to neighbourhood residents at distances of between 150 and 2000 m showed, depending on the facility, annual median values of 170–330 CFU/m3 for bacteria, 75–340 CFU/m3 for moulds, and 15–52 CFU/m3 forA. fumigatus. Higher individual counts — up to 3 × 103 CFU/m3 for moulds and up to 350 CFU/m3 forA. fumigatus — were found as a result of specific climatic influences, (e.g. winds) and activities as well poor operation. Given the high proportion ofA. fumigatus in the exhaust air, this mould can serve as an indicator for the evaluation of the health risk. However, the maximum values found in the present study, may also be caused by other events in rural areas, (e.g. agricultural activities). With regard to neighbourhood residents, odour complaints are more important than pollution by microorganisms.