BioCycle September 2005, Vol. 46, No. 9, p. 50
Study identifies factors influencing bioaerosol generation and dispersion – useful data for establishing a safety boundary around composting plants and making modifications to operational procedures to reduce environmental impact.
Miguel A. Sánchez-Monedero, Edward I. Stentiford and Sari T. Urpilainen

NORMAL operations taking place at composting plants can be the source of potential environmental impacts related to odors, bioaerosols, noise and dust. The release of microorganisms in the form of bioaerosols has been a focus of study for many years as far as the potential health impacts are concerned. This is not only for the workers at the plant but also for local residents as a result of the inhalation of these bioaerosols.
Bioaerosols generated at composting plants are mainly airborne microorganisms and microbial constituents, which are released from the processes where the vigorous movement of material is involved, mainly during fresh waste delivery, shredding, compost pile turning and compost screening. (Millner et al. 1994) prepared an excellent state of the art summary on bioaerosols generated at composting facilities and their potential effects. Their work focused on the impact of airborne Aspergillus fumigatus spores on human health that included: invasive aspergillosis, allergenic bronchopulmonary aspergillosis, acute allergic alveolitis, asthma induced by aspergillosis, aspergillus sinusitis and different allergies.
The release of bioaerosols is particularly relevant for composting plants operating in the open as the bioaerosols are released directly into the surrounding area without any pretreatment such as biofilters or bioscrubbers, as happens in enclosed systems. According to the last survey of the composting industry in the United Kingdom (UK), performed by the UK Composting Association, about 1.97 million metric tons of materials were composted by 325 processors in the 2003-2004 period. Most composting plants operating in the UK are treating source separated green wastes by windrowing on concrete pads in the open air. These sites often are established at landfills (diverting green waste from disposal). The landfill site will potentially use the compost produced as either daily or final cover. Due to the characteristics of these composting sites and the distant location of landfills with respect to residential areas, the potential sensitive receptors for the bioaerosols are expected to be the site workers rather than local residents.
The UK Composting Association, following some research on bio-aerosol generation and dispersion, proposed a standard procedure for bioaerosol monitoring at composting facilities that has wide acceptance in the UK. This protocol (Gilbert, E.J., et al., 1999) is based on the monitoring of two airborne microorganisms (Aspergillus fumigatus and total mesophilic bacteria) by impaction at different upwind and downwind locations at composting plants. The aim of our research study was to monitor the amount of Aspergillus fumigatus and total mesophilic bacteria generated at a typical green waste composting plant over one year of normal operation. The intention was to determine the main activities generating bioaerosols and the levels to which site workers were exposed during normal plant activities.
EXPERIMENTAL METHODS
Composting took place at a full-scale plant in the North of England, located at a landfill. Plant throughput was about 10,000 metric tons per year of source separated green wastes from different municipalities in the surrounding area. The shredded feedstock was composted in trapezoidal cross section windrows 25 m by 3 m by 2 m (length x width x height) over a 18 week period on a concrete pad in the open air. The windrows were turned using a front-end loader once a week for the first 10 weeks and then the material was allowed to mature for a period of eight weeks with no further turning. Each turn moved the composting piles along the length of the composting pad to the opposite end of the site, where screening took place. After screening, the compost was temporarily stored at the northern edge of the concrete area for subsequent use as landfill cover.
The composting pad was bounded on the south edge by the access road to the landfill (Figure 1, page 50). A pedestrian footpath ran along the northern and eastern edges to which the public had free access (although it was not heavily used). The western edge was bounded by an open area of rough ground, which was the property of the landfill operators who could control access to it.
The sampling points (Figure 1) are described in the following paragraphs. Upwind and downwind sites were located according to the different wind directions during different sampling dates. Background locations (U1, U2 and U3) represented upwind sites where the airborne microorganism concentrations were likely to be unaffected by the plant operations on site. Sampling points were located either 25 m (U1 and U3) or 40 m (U2) away from the operational activities.
Downwind locations (D1, D2, D3, D4, D5 and D6) corresponded to the airborne microorganism concentration at locations downwind from the operational activities taking place on site. D1 and D6 were located 40 m downwind, along the footpath on the northern and eastern edges of the site; D4 was located 25 m downwind, south to the main access road to the landfill; D2 and D3 were located 300 m and 200 m downwind on the northern edge of the site, respectively; and D5 was 200 m downwind, behind the main access road to the landfill on the southern edge of the site.
Airborne microorganism concentration was monitored for a 12 month period. Sampling frequency was adapted to operational and meteorological conditions; no samples were taken during the winter. A six stage Andersen viable impactor sampler was used to collect the samples on site. The inlet of the air sampler was at 1.8 m above the ground and the sampling time was one minute. Petri dishes containing the agar medium were in the sampler. Once the required air had been drawn through, the plates were covered and incubated. Aspergillus fumigatus detection and enumeration were carried out according to the method of Fisher et al. (1998). Mesophilic bacteria detection and quantification were carried out according to the method used by Lacey and Williamson (Sánchez-Monedero, M.A.; et al., 2003) and incorporated into the UK Composting Association protocol. The results were calculated as the geometric mean of the replicates and were expressed as colony forming units per cubic meter of air (cfu m3). The detection limit was
The meteorological conditions corresponded to the average recorded during the monitoring time at each sampling location. Wind speed and ambient temperature were recorded with a digital thermo-anemometer. Wind direction was taken from the meteorological station located on the roof of the site office building (500 m away from the composting pad).
Monitoring in the open air is exposed to the weather conditions. During the monitoring event there may be sudden changes in wind speed and direction that directly affect the sampling. Error figures (in brackets in Tables 1 and 2) give an idea of the experimental variation of the results due to the weather conditions (according to the statistical analysis of the data). In some cases errors are quite low (small differences between duplicates), but occasionally they may be rather large (duplicates rather different because of the variable weather conditions). It is expected that the variation between duplicates or triplicates of experimental results are much larger than laboratory scale experiments (under controlled conditions).
Experimental data were subjected to analysis of variance procedure (SPSS 11.0) to determine the effect of seasonal variation, dispersion and operational activities on airborne microorganism concentration. Analysis of variance (ANOVA) was performed for Aspergillus fumigatus and mesophilic bacteria after logarithmic transformation of their concentrations. Multiple mean separations were performed using Duncan’s multiple range test at P < 0.05.
MONITORING RESULTS
Tables 1 and 2 show the concentration of Aspergillus fumigatus and mesophilic bacteria at different upwind and downwind locations around the composting plant, under different operational conditions during one year of monitoring. The concentration of both microorganisms measured at upwind locations remained within the same range for the whole monitoring period, varying from less than 102 up to 103 cfu m3. These concentrations represented the background levels for both microorganisms at the composting site, unaffected by the operational activities. They were within the expected range usually found for Aspergillus fumigatus during normal agricultural activities and are higher than the levels measured in other open environments or in indoor air (Millner et al., 1994). The cause of these somewhat enhanced background levels was due to other operations, external to the composting site, possibly related to the normal activities taking place on and around the adjacent landfill site.
Aspergillus fumigatus and mesophilic bacteria concentrations recorded at downwind locations, when no vigorous activity was taking place on the composting site, were no different from the background levels. However, vigorous activities such as green waste shredding, mature compost screening and pile turning generated a similar increase in the concentrations of both airborne microorganisms at downwind locations. The concentrations recorded during these operational activities at the potential sensitive receptor locations varied over a wide range, from 1.5×102 to higher than 2.9×105 cfu m3 at downwind location D1 (40 m downwind) and from 1.5×102 to 2.9×103 cfu m3 at downwind location D3 (300 m downwind).
The airborne concentrations at 25 and 40 m downwind were strongly affected by the composting activities that typically caused an increase up to two logarithmic units for both microorganisms as a consequence of the vigorous movement of material. These results were in the range, between 103 to 106 cfu m3, which is in agreement with the results of other authors working in with similar conditions (Fischer, J.L. et al., 1998; Millner, P.D., et al., 1980; Sánchez-Monedero, M.A., et al., 2003).
The amounts recorded at downwind locations D2, D3 and D5 (200 and 300 m downwind) during vigorous activity were similar to background levels reflecting the good air dispersion. The amounts of the airborne concentrations at these downwind locations were occasionally slightly above the background levels, but never exceeded 2.9×103 cfu m3. These occasional high levels at relatively long downwind distances could be due either to the meteorological conditions, a key factor affecting dispersion, or to different sources of bioaerosols other than the composting operation (adjacent landfill site).
The importance of assessing when levels reach background values relates to the fact that this has often been used as the minimum distance open composting plants need to be from sensitive receptors. For example, the UK Environment Agency is currently using 250 m as the minimum separation distance for these plants, which is in line with the results from this work.
During the study, even though the sampling dates covered a 12-month period, the data did not show any variations that could have been attributed to seasonal changes. Meteorological conditions, particularly wind speed and direction, were the main factors governing the airborne dispersion from the composting pad. Sudden changes in meteorological conditions did in some cases produce a high standard deviation in the concentration for both microorganisms over the sampling period. These practical difficulties when monitoring open facilities are a common occurrence. While they reflect the true variation under those conditions, they can limit the validity of some of the conclusions drawn from the experimental results. Similar comments and observations have been made by other workers (Reinthaler, F.F. et al., 1997; Recer, G.M, et al., 2001; Gilbert, E.J., et al., 2002).
ASSESSMENT OF BIOAEROSOL SOURCES
Figure 2 shows the annual average of Aspergillus fumigatus and mesophilic bacteria concentrations on the potential sensitive receptor location (Downwind 1) adjacent to the composting site under different operational activities. Airborne concentrations 40 m downwind when no activity was taking place and when the footpath was upwind did not differ from background levels. Shredding and turning produced the largest increase of airborne concentrations up to two logarithmic units higher than background levels.
Screening of mature compost and the movement of mature compost (piling and truck loading) caused an intermediate effect; there was a larger increase in mesophilic bacteria levels than in Aspergillus fumigatus. The amount of mesophilic bacteria generated in these less vigorous operations was not significantly different from the amount generated by the other activities. However, the Aspergillus fumigatus levels were significantly lower than in other activities, and similar to those of the background levels. In the case of Aspergillus fumigatus, this effect may have been due to the sanitization achieved during the composting process (Fischer, J.L., et al., 1998). (Pile temperature was over 70°C over most of the composting period.) Based on the assumption that the sanitization effect would have had an impact on the Aspergillus fumigatus, we would have expected a decrease in the concentration of this microorganism if the composting process was performed effectively (keeping high temperatures, frequent turning of material, avoiding mixing with fresh materials, etc). Piling and loading of mature compost did not involve vigorous movement of material and had relatively low levels of Aspergillus fumigatus when compared to the other activities such as shredding or screening that required continuous movement of material.
ASSESSMENT OF POTENTIAL RISK FOR SITE WORKERS
There is no dose-response in-formation available for the effect of Aspergillus fumigatus on the health of workers, but it has been proposed that the amount of total bacteria should not be over 5×103 or 104 cfu m3 for an eight-hour working day (Sigsgaard, T., et al., 1990). On this basis, the background levels registered during the monitoring (from 102 to 103 cfu m3) should not have any health impact for plant operators as long as they do not have established immunodeficiency or breathing problems. In the case of meso-philic bacteria, the concentrations recorded 40 m downwind were higher than the range proposed in the literature indicating a greater potential risk. The concentration of Aspergillus fumigatus measured 40 m downwind were at levels that other authors have previously reported to be the cause of bronchitis and gastrointestinal complaints from the staff during waste collection (Nielsen, E.M., et al., 1997). As a minimum requirement at these levels, Kiviranta et al. (1999) recommended the use of personal protective equipment for plant operators. Even if the dose-response for Aspergillus fumigatus exposure has not been established, the levels recorded at locations 40 m downwind from the composting activities would make it advisable for appropriate masks to be worn by site staff working inside the composting plant or by those using the internal pedestrian access. It would also be advisable to temporarily interrupt any vigorous activity related to composting whenever the installation was used by staff or visitors not using the appropriated breathing masks.
For the site in question, the levels of airborne microorganisms at the site boundaries differed very little from background concentrations. Consequently, as far as Aspergillus fumigatus and mesophilic bacteria are concerned, the local residents would not be considered to be at risk from infection related to the composting operation.
Miguel Sánchez-Monedero is with CEBAS-CSIC at the Campus Universitario de Espinardo in Murcia, Spain (monedero@cebas.cslc.es). Edward Stentiford and Sari T. Urpilainen are in the School of Civil Engineering, The University of Leeds, LS2 9JT Leeds, UK. This article is based on a paper originally published in the Journal of the Air & Waste Management Association in Pittsburgh, Pennsylvania and reprinted with the association’s permission.
ACKNOWLEDGEMENTS
This research was supported through a European Community Marie Curie Fellowship. Disclaimer: The authors are solely responsible for information communicated and the European Commission is not responsible for any view or results expressed.
REFERENCES
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RESEARCH FINDING HIGHLIGHTS
THE BACKGROUND levels for Aspergillus fumigatus and mesophilic bacteria varied within the range from less than 102 up to 103 cfu m3.
o The concentrations measured at locations downwind, potentially considered as sensible receptors, when no vigorous activity was taking place were no different from the background levels.
o Vigorous activities such as shredding, turning and screening were identified as the major sources of bioaerosol generation and release and caused an increase in both Aspergillus fumigatus and mesophilic bacteria concentrations on the adjacent footpath up to 2 log units higher than background levels. The amounts measured 300 m downwind of the operational activities did not differ from the background levels.
o Meteorological conditions were thought to be the main factors affecting airborne dispersion from the composting pad.
o The high levels recorded In the operating area when vigorous activities were taking place suggested that it would be advisable for staff in that area to have appropriate respiratory protection equipment.