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Generation and Dispersion of Airborne Microorganisms from Composting Facilities
0957–5820/03/$23.50+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 81, Part B, May 2003
www.ingentaselect.com=titles=09575820.htm
GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS FROM COMPOSTING FACILITIES M. A. SA´NCHEZ-MONEDERO and E. I. STENTIFORD School of Civil Engineering, University of Leeds, UK
B
ioaerosol release from organic waste treatment plants has become an important public concern because of the potential health impacts on workers and local residents. Airborne microorganisms were monitored at seven different composting facilities with different operating systems and treating a range of organic wastes. Aspergillus fumigatus and mesophilic bacteria were used as the principal monitoring parameters. The composting plants all showed levels of both airborne microorganisms in the 103 to 105 cfu m 3 range in the operating area, making it advisable for the staff to use protective masks. The highest bioaerosol release occurred in the composting plant treating green waste using turned windrows, The main operational factors affecting airborne microorganism dispersion are discussed as well as the systems proposed for dispersion control. Bioaerosol release was reduced at enclosed systems through the use of bio lters, and the dispersion depended on meteorological conditions. In all cases the airborne levels 300 m downwind were similar to those measured at background locations. Keywords: bioaerosols; Aspergillus fumigatus; mesophilic bacteria; composting.
microbial constituents are released from the processes where the movement of material is involved, mainly during fresh waste delivery, shredding, compost pile turning and compost screening. Aspergillus fumigatus (AF) has been the most widely studied bioaerosol because of its abundance in the composting pile and its potential negative effects on human health (Olver, 1994). Millner et al. (1994) reviewed data on the main bioaerosols generated at composting facilities and their potential effects on human health. In the case of AF these included: invasive aspergillosis, allergenic bronchopulmonary aspergillosis, acute allergic alveolitis, asthma induced by aspergillosis, aspergillus sinusitis and different allergies. The Environment Agency is currently trying to regulate the impacts of bioaerosols and has proposed measures to control them (Environment Agency, 2001). The UK Composting Association has drawn together work on bioaerosol generation and dispersion from composting plants and proposed a procedure for bioaerosol monitoring in the UK. This protocol is based on the monitoring of two airborne groups, AF and total mesophilic bacteria, at different upwind and downwind locations at the composting plants (Gilbert and Ward, 1999). By studying bioaerosol generation and the factors in uencing dispersion such as meteorology, site operation and site location, we can help to establish a safety boundary around the composting plants. This is based on the point at which the concentration of bioaerosols is not signi cantly different from the background levels. This safety boundary will assist in site location for new composting plants and help to modify the operational procedures of existing plants to reduce their environmental impact.
INTRODUCTION Composting is one of the most widely used methods for the bio-treatment of organic waste in the UK. According to Slater and Frederickson (2001) the amount of organic material being composted increased around 23% from 1998 to 1999, and is expected to continue to rise in the future as a consequence of the application of the recent European legislation. The European Land ll Directive (EC, 1999) encourages the diversion of the biodegradable municipal waste away from land lls to other alternative treatments, such as composting, that ensure the recycling and recovery of energy from the organic wastes. Slater and Frederickson (2001) found that most of the composting plants operating in the UK were treating green wastes in windrows in the open air and there are only a few enclosed systems, treating mostly food waste. They emphasized the important role that composting could play in the UK in meeting the obligations of the Land ll Directive. This would involve not only small-scale green waste composting plants, but also facilities for recovering the organic fraction of the municipal solid waste, as happens in other European countries. Routine operations taking place at composting plants are the source of potential environmental impacts related to factors such as odour, bioaerosols, noise and dust. The release of microorganisms as bioaerosols currently represents one of the main health concerns from a public viewpoint. This applies not only to the workers at the plant but also to people from the local residential areas because of the potential risk to health associated with the inhalation of these bioaerosols. The airborne microorganisms and 166
GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS
The main aim of this work was to monitor the amount of AF and total mesophilic bacteria at different composting plants to identity the factors affecting bioaerosol pro les. The effect of mitigation measures such as bio lters, in the case of enclosed systems, and air dispersion in open facilities was also studied. MATERIALS AND METHODS Composting Sites Description Seven commercial composting plants were chosen for the study based on their different operational characteristics. Their main features, which include type of waste treated, throughput and composting system are shown in Table 1. Plants I–VI, referred to in Table 1, used enclosed systems. In composting plants I and II, the bioxidative phase took place in static piles with forced aeration in closed biocells. The exhaust air from the top of the biocells was vented through a bio lter. In composting plants III, IV, V and VI, the bioxidative phase, and also the maturation phase in plant III, took place in industrial warehouse buildings with forced aeration and occasional turning. The warehouses operated under negative pressure with the air being extracted from the top and vented through the bio lter in plants III, IV and V, whereas in plant VI the air was passed through a scrubber and then through the bio lter. Full description of the bio lter systems has been detailed previously (Sa´nchezMonedero and Stentiford, 2002). Plant VII windrowed green waste on an outdoor concrete pad. The shredded green wastes were composted in trapezoidal cross sectional windrows (approximately 2.5 m height) and were turned using a loading shovel once a week. The meteorological conditions during monitoring at each site are shown in Table 2.
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Table 2. Meteorological conditions during the sampling. Site Enclosed systems Site I Site II Site III Site IV Site V Site VI Open system Site VI—no activity Site VI—shredding Site VI—turning Site VI—screening
Temperature ( C)
Cloud cover
Wind speed (m s 1)
18 19 22 22 20 21
Sunny Sunny Sunny Sunny Sunny Sunny
0–1 0–2 0–1 1–2 0–2 0–2
15.0
0–4
13.0
Cloud breaking Overcast
16.7
Overcast
0–3
12.8
Sunny
0–7
0–2
unaffected by the plant operations on site. The inlet of the air sampler was at 1.8 m above the ground. Operational area In the enclosed system sites, this location corresponded to the airborne microorganism concentration measured inside the composting hall during normal operations. In all cases, the inlet of the air sampler was 1.8 m above the ground. However, for plant VI, where it was not possible to gain access for safety reasons the sample was taken in the piping system connecting the fans to the scrubber. At plant VII, the open system based on using windrows, the air sample was taken 40 m downwind from the windrow being turned. The inlet of the air sampler was at 1.8 m above the ground. Air Sampling and Microbiological Analysis
Sampling Locations The sampling points which were chosen at each site and are presented in this paper related to background levels, levels in operational areas and levels downwind. Background This corresponded to the airborne microorganism concentration at a location upwind from the sites which was Table 1. Main characteristics of the seven composting plants studied. Composting Type of plant waste
Throughput (tonnes year 1) Composting system
I
BW, SS, GW 40,000
II
GW, FW
III IV V VI VII
BW, GW BW, SS, GW SS, GW, FW MSW GW
1750 30,000 30,000 28,000 76,000 20,000
Static pile, forced aeration in biocell Static pile, forced aeration in biocell Forced aeration and turning Forced aeration and turning Forced aeration and turning Forced aeration and turning Windrows, turned pile system
BW, biowastes—organic fraction of source separated municipal solid wastes; FW, food wastes; GW, green wastes; MSW, municipal solid wastes; SS, sewage sludge.
Trans IChemE, Vol 81, Part B, May 2003
A six-stage viable impactor Andersen sampler (Andersen Instrument Inc., Georgia, USA) was used to collect the samples on site. The air was drawn through the sampler with a pump working at a constant ow of 26 l min¡1 (calibrated in the laboratory). The sampling time was typically one minute. For every sample, the sampler was lled with six 9 cm plastic Petri dishes containing the agar medium. Once the required volume of air had been drawn through (depending on estimated concentrations of bioaerosols), the plates were covered and incubated. Three replicates were taken at every sampling point. After each sample, the sampler was sterilized by washing with a solution of either methanol or ethanol 70%. Aspergillus fumigatus detection and quanti cation were carried out according to the method of Fisher et al. (1998). The agar medium was prepared using 20 g l¡1 of malt extract agar and 15 g l¡1 of bacteriological agar. Two antibiotics were added (streptomycin, 50 mg l¡1 and novobiocin, 10 mg l¡1) to suppress bacterial development. The plates used for the sampling were incubated at 40¯ C for 48 h, and then the green-grey colonies with columnar conidial heads, characteristic of AF, were counted as representative of AF spores. Mesophilic bacteria detection and quanti cation were carried out according to the method used by Lacey and Williamson (1995) and incorporated into the Composting
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Association protocol (Gilbert and Ward, 1999). The agar medium was prepared using 14 g l¡1 of nutrient agar and 10 g l¡1 of bacteriological agar. The antibiotic cicloheximide (100 mg l¡1 dissolved in less than 2 ml of acetone) was added after autoclaving when the temperature had fallen to approximately 47¯ C. The plates used for the sampling were incubated at 37¯ C for 48 h, and then the white round-shaped colonies were counted. The positive-hole correction was used to adjust colony counts (Macher, 1989). The results were calculated as the geometric mean of the three replicates and were expressed as colony forming units per cubic metre of air (cfu m¡3). The detection limit was <102 cfu m¡3. RESULTS AND DISCUSSION Bioaerosol Generation Bioaerosol generation at composting facilities is regulated by different factors such as the activity taking place at the composting plant and the material being treated. Figure 1 shows the Aspergillus fumigatus and Figure 2 the mesophilic bacteria concentration, for the background and operational areas at seven plants. The error bars represent the standard deviation of the experimental results. Sites I–VI
(enclosed systems) were treating a range of organic materials and site VII was an open facility which was sampled under different operational conditions. In all cases the background levels for both kind of microorganisms, which were assumed to be unaffected by the composting operations taking place, were below 103 cfu m¡3. These were in the expected range found by other authors monitoring similar systems (Millner et al., 1994; Slater et al., 2001). These background concentration levels are similar to those found for AF generated by a range of agricultural activities and slightly higher than the levels expected in quiescent natural environments or indoors where the concentrations would generally be lower than 175 cfu m¡3 (Millner et al., 1994). There is no de nitive dose–response information available for the effect of AF on the health of workers. Malmros et al. (1992) suggested that the amount of total bacteria should not be over 5000 or 10,000 cfu m¡3 for an 8 h working day. Concentrations above these threshold limit values were the cause of occupational disease, mainly due to eye and throat irritation, cough at night, chills and fever (Sigsgaard et al., 1990). Based on these gures, these background levels would not represent any health impact for workers, as long as they do not have a depressed immune system or speci c breathing problem.
Figure 1. Concentrations of Aspergillus fumigatus recorded at different sites (composting plant numbers as in Tables 1 and 2). Error bars represent the standard deviation of the experimental results.
Figure 2. Concentrations of mesophilic bacteria at different sites (composting plant numbers as in Tables 1 and 2). Error bars represent the standard deviation of the experimental results.
Trans IChemE, Vol 81, Part B, May 2003
GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS The concentration of bioaerosols released at the composting facilities during normal operation was generally two logarithmic units higher than the background as a consequence of the movement of the material and the forced aeration. The green waste composting in the open air site (site VII) was the site that released the highest amount of both microorganisms, ranging between 3.8 £ 103 and 9.8 £ 104 cfu m¡3 for AF and between 2.3 £ 104 and 1.1 £ 105 cfu m¡3 for mesophilic bacteria measured 40 m downwind from the windrows. These concentrations were similar to those recorded inside the composting halls at sites I–VI, which ranged from 4.7 £ 103 to 2.2 £ 105 and 2.3 £ 103 to 1.9 £ 105 cfu m¡3 for AF and mesophilic bacteria, respectively. These concentrations were in agreement with the expected range (between 103 and 106 cfu m¡3), typically found by other researchers near to composting piles in the open air and running with different operational conditions (Millner et al., 1980; Fischer et al., 1998; Maricou et al., 1998). The amount of bioaerosol released in all the composting plants suggests that it is advisable for operators working in the immediate vicinity of the composting mass to use protective equipment such as face masks to avoid negative health impacts. The concentration of mesophilic bacteria in particular were higher than the range suggested by Sigsgaard et al. (1990) for unprotected working. The amount of AF within the buildings on the enclosed sites were at levels that other authors have previously reported to have been the cause bronchitis and gastrointestinal complaints from the staff at composting plants (Nielsen et al., 1997). In the case of the open site the levels were dependent on the type of operation being carried out and the operators should be made aware of the occasions when some form of protection is necessary. The differences in the air concentrations of both microorganisms found at the facilities monitored indicated that the airborne release was not only affected by the amount of microorganisms present in the waste, but also by the different operational processes which were carried out at the facility. A key issue is the level of these organisms which is present within the material itself, since this will affect the amount released when the material is agitated or aerated. Millner et al. (1977) reported the highest concentration of AF in green wastes and woodchip varied from 103 to 107 cfu g¡1, whereas for sewage sludge the range was from 102 to 103 cfu g¡1. Fisher et al. (1998) measured AF concentrations in a mixture of green waste and kitchen waste varying from 105 to 106 cfu g¡1. The high amount of AF in the green waste and the turning of the material caused the highest concentration of the airborne microorganisms around site VII (green waste composting in the open). Whilst we would expect similar wastes to develop similar concentrations of AF, the release rates from plants processing the same wastes can be very different. For example, in this study composting plants treating similar kinds of biowastes (sites I and IV were treating a mixture of biowastes, sewage sludge and green wastes) released very different amounts of AF spores. The other plants released a similar amount of airborne microorganisms, ranging from 4.7 £ 103 to 1.2 £ 104 cfu m¡3, which was probably related more to the forced aeration and turning which took place than the material being treated. The plant treating only MSW (site VI) released a very low amount of AF, compared Trans IChemE, Vol 81, Part B, May 2003
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with the amount of mesophilic bacteria released. This type of behaviour was previously reported by Tolvanen et al. (1998), who found low levels of AF at some of the operation processes at a biowaste processing plant. In the case of site VII, it was possible to study the effect of the activity taking place at the composting plant on the bioaerosol release. Shredding, turning and screening increased the air concentration of both airborne microorganisms 40 m downwind by 1 or 2 logarithmic units, compared with the amount measured when there was no mechanical agitation of the material. Screening released lower amounts of bioaerosols than the other two operations studied. The high standard deviation for both airborne microorganisms measured at the open composting plant made it dif cult to draw any clear conclusions. The sampling was strongly affected by any sudden changes in the meteorological conditions such as wind speed and direction. These practical dif culties associated with monitoring open facilities have been reported by other workers (Gilbert et al., 2002). Bioaerosol Dispersion The extent of bioaerosol dispersion should be different depending on the composting system being used. The exhaust air from enclosed systems is typically treated using bio lters or bioscrubbers which reduce bioaerosol levels before the air is released to the atmosphere. In the case of an open system it is necessary to rely on natural air dispersion to dilute the amount of airbornes being spread in the surroundings. In both cases, guidelines have been proposed to mitigate bioaerosol generation in order to reduce the impact on workers and the amount of bioaerosols released to the air around the plant. Epstein et al. (2001) found a signi cant decrease in dust and bioaerosol generation after making simple operational modi cations such as increasing the moisture content of the mixtures, installing dust collection systems, using sweepers and water vehicles to control dust on roadways and installing adequate ventilation in buildings. In the case of open systems, the mitigation guidelines generally include factors such as design, site topography and landscape to abate transport of aerosols downwind (Millner et al., 1994). The effectiveness of bio lters for bioaerosol removal has been recently suggested at composting plants (Sa´nchezMonedero and Stentiford, 2002) and also on farms (Martens et al., 2001). All the six enclosed composting sites studied were tted with either a bio lter or a combination of a bio lter and a bioscrubber to treat the exhaust air. In all cases the amount of AF measured at the surface of the bio lter did not exceed 103 cfu m¡3 regardless of the incoming concentration. This meant that bio lters typically removed more than 90% of the incoming bioaerosols. However the mesophilic bacteria removal was not so effective and in some cases, only 40% of the incoming bioaerosols were removed (Sa´nchez-Monedero et al., in preparation). As part of this study the level of both microorganisms was measured at 300 m downwind during shredding and turning and at 200 m downwind during screening, at the composting pad treating green wastes by windrows (site VII). The results indicated that the levels measured 300 m downwind from the composting pad did not show any signi cant difference compared to the background levels measured upwind. However, the levels measured 200 m downwind
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from the screening operation (5.4 £ 102 and ¡3 4 1.6 £ 10 cfu m for AF and mesophilic bacteria respectively) showed that the bioaerosol concentration was still higher than background levels indicating that this distance (200 m) was not suf cient on this site to reduce the microorganisms to background levels. These results were in agreement with those found by the Composting Association in the UK, and a boundary distance of at least 250 m is currently being recommended by the Environment Agency to ensure that the composting plants will not have any adverse impacts on the health of people living in the area (Environment Agency, 2001). However, other authors have measured airborne concentrations higher than the background at 500 m or more from composting sites (Millner et al., 1977, 1994). The strong effect of the meteorological conditions on bioaerosol dispersion from composting plants makes it dif cult to establish an accurate distance where there is no enhanced adverse health impact for people. Mathematical modelling is often employed at the design stage but these models have limitations due to the complexity of the process. CONCLUSIONS Open composting plants and those treating the organic wastes in biocells released amounts of Aspergillus fumigatus to the air that could be a risk for staff working in the vicinity. The airborne release appeared to be related to both the amount of microorganisms in the wastes, the design of the site and to the operational procedures used in the composting plants. The highest amounts of bioaerosols were generated at composting plants treating green waste and mechanical agitation released higher amounts of bioaerosols than forced aeration. The airborne microorganism levels measured 300 m downwind did not show any difference to the background levels. REFERENCES EC, 1999, Council directive on the land ll of waste (1999=13=EC), Of c J E C, 16 July: L 182=1. Environment Agency, 2001, Technical guidance on composting operations, Draft for external consultation, October 2001. Available at: www.environment-agency.gov.uk=commondata=105385=compostin.pdf. Accessed 4 August 2002. Epstein, E., Wu, N., Youngberg, C. and Croteau, G., 2001, Controlling dust and bioaerosols at a biosolids composting facility, BioCycle, 42(4): 50–54. Fischer, J.L., Beffa, T., Lyon, P.F. and Aragno, M., 1998, Aspergillus fumigatus in windrow composting: effect of turning frequency, Waste Mgmt Res, 16(4): 320–329. Gilbert, E.J. and Ward, C.W., 1999, Standardised Protocol for the Sampling and Enumeration of Airborne Micro-organisms at Composting Facilities (The Composting Association, Coventry). Gilbert, E.J., Kelsey, A., Karnon, J.D., Swan, J.R.M. and Crook, B., 2002, Preliminary results of monitoring the release of bioaerosols from
composting facilities in the UK: interpretation, modelling and appraisal of mitigation measures, in Proceedings of the International Symposium Composting and Compost Utilization, Ohio, USA, pp 1094–1105. Lacey, J. and Williams, P.A.M., 1995, Airborne microorganisms associated with RDF res at Castle Bromwich, Report No. CWM=110=93, Department of the Environment. Macher, J.M., 1989, Positive-hole correction of multiple-jet impactors for collecting viable microorganisms, Am Ind Hyg Assoc J, 50(11): 561–568. Malmros, P., Sigsgaard, T. and Bach, B., 1992, Occupational health problems due to garbage sorting, Waste Mgmt Res, 10: 227–234. Maricou, H., Verstraete, W. and Mesuere, K., 1998, Hygienic aspects of biowaste composting: airborne microbial concentrations as a function of feedstock, operation and season, Waste Mgmt Res, 16(4): 304–311. Martens, W., Martinec, M., Zapirain,R., Stark, M., Hartung, E. and Palmgre, U., 2001, Reduction potential of microbial, odour and ammonia emissions from a pig facility by bio lters, Int J Hyg Environ Health, 203: 335–345. Millner, P.D., Bassett, D.A. and Marsh, P.B., 1980, Dispersal of Aspergillus fumigatus from sewage sludge compost piles subjected to mechanical agitation in open air, Appl Environ Microbiol, 39(5): 1000–1009. Millner, P.D., Marsh, P.B., Snowden, R.B. and Parr, J.F., 1977, Occurrence of Aspergillus fumigatus during composting of sewage sludge, Appl Environ Microbiol, 34(6): 765–772. Millner, P.D., Olenchock, S.A., Epstein, E., Rylander, R., Haines, J., Walker, J., Ooi, B.L., Horne, E. and Maritato, M., 1994, Bioaerosols associated with composting facilities, Compost Sci Util, 2(4): 6–57. Nielsen, E.M., Breum, N.O., Nielsen, B.H., Wurtz, H., Poulsen, O.M. and Midtgaard, U., 1997, Bioaerosol exposure in waste collection: a comparative study on the signi cance of collection equipment, type of waste and seasonal variation, Ann Occup Hyg, 41(3): 325–344. Olver Jr., W.M., 1994, The Aspergillus fumigatus problem, Compost Sci Util, 2(3): 27–31. Sa´nchez-Monedero, M.A. and Stentiford, E.I., 2002, Aspergillus fumigatus control at composting plants through the use of bio lters, in Proceedings of the International Symposium Composting and Compost Utilization, Ohio, USA, pp 1106–1114. Sigsgaard, T., Bach, B. and Malmros, P., 1990, Respiratory impairment among workers in a garbage-handling plant, Am J Ind Med, 17: 92–93. Slater, R.A. and Frederickson, J., 2001, Composting municipal waste in the UK: some lessons from Europe, Resour Conserv Recycl, 32: 359–374. Slater, R.A., Frederickson, J. and Gilbert, E.J., 2001, The State of Composting—1999. Results of the Composting Association’s Survey of UK Composting Facilities and Collection Systems in 1999 (The Composting Association, Wellingborough). Tolvanen, O.K., Hanninen, K.I., Veijanen, A. and Villberg, K., 1998, Occupational hygiene in biowaste composting, Waste Mgmt Res, 16(6): 525–540.
ACKNOWLEDGEMENT This research was supported through a European Community Marie Curie Fellowship. The authors are solely responsible for information communicated and the European Commission is not responsible for any view or results expressed.
ADDRESS Correspondence concerning this paper should be addressed to Professor E. I. Stentiford, School of Civil Engineering, Leeds University, Woodhouse Lane, LS2 9JT Leeds, UK. E-mail: [email protected]. The manuscript was received 2 September 2002 and accepted for publication after revision 3 April 2003.
Trans IChemE, Vol 81, Part B, May 2003
www.ingentaselect.com=titles=09575820.htm
GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS FROM COMPOSTING FACILITIES M. A. SA´NCHEZ-MONEDERO and E. I. STENTIFORD School of Civil Engineering, University of Leeds, UK
B
ioaerosol release from organic waste treatment plants has become an important public concern because of the potential health impacts on workers and local residents. Airborne microorganisms were monitored at seven different composting facilities with different operating systems and treating a range of organic wastes. Aspergillus fumigatus and mesophilic bacteria were used as the principal monitoring parameters. The composting plants all showed levels of both airborne microorganisms in the 103 to 105 cfu m 3 range in the operating area, making it advisable for the staff to use protective masks. The highest bioaerosol release occurred in the composting plant treating green waste using turned windrows, The main operational factors affecting airborne microorganism dispersion are discussed as well as the systems proposed for dispersion control. Bioaerosol release was reduced at enclosed systems through the use of bio lters, and the dispersion depended on meteorological conditions. In all cases the airborne levels 300 m downwind were similar to those measured at background locations. Keywords: bioaerosols; Aspergillus fumigatus; mesophilic bacteria; composting.
microbial constituents are released from the processes where the movement of material is involved, mainly during fresh waste delivery, shredding, compost pile turning and compost screening. Aspergillus fumigatus (AF) has been the most widely studied bioaerosol because of its abundance in the composting pile and its potential negative effects on human health (Olver, 1994). Millner et al. (1994) reviewed data on the main bioaerosols generated at composting facilities and their potential effects on human health. In the case of AF these included: invasive aspergillosis, allergenic bronchopulmonary aspergillosis, acute allergic alveolitis, asthma induced by aspergillosis, aspergillus sinusitis and different allergies. The Environment Agency is currently trying to regulate the impacts of bioaerosols and has proposed measures to control them (Environment Agency, 2001). The UK Composting Association has drawn together work on bioaerosol generation and dispersion from composting plants and proposed a procedure for bioaerosol monitoring in the UK. This protocol is based on the monitoring of two airborne groups, AF and total mesophilic bacteria, at different upwind and downwind locations at the composting plants (Gilbert and Ward, 1999). By studying bioaerosol generation and the factors in uencing dispersion such as meteorology, site operation and site location, we can help to establish a safety boundary around the composting plants. This is based on the point at which the concentration of bioaerosols is not signi cantly different from the background levels. This safety boundary will assist in site location for new composting plants and help to modify the operational procedures of existing plants to reduce their environmental impact.
INTRODUCTION Composting is one of the most widely used methods for the bio-treatment of organic waste in the UK. According to Slater and Frederickson (2001) the amount of organic material being composted increased around 23% from 1998 to 1999, and is expected to continue to rise in the future as a consequence of the application of the recent European legislation. The European Land ll Directive (EC, 1999) encourages the diversion of the biodegradable municipal waste away from land lls to other alternative treatments, such as composting, that ensure the recycling and recovery of energy from the organic wastes. Slater and Frederickson (2001) found that most of the composting plants operating in the UK were treating green wastes in windrows in the open air and there are only a few enclosed systems, treating mostly food waste. They emphasized the important role that composting could play in the UK in meeting the obligations of the Land ll Directive. This would involve not only small-scale green waste composting plants, but also facilities for recovering the organic fraction of the municipal solid waste, as happens in other European countries. Routine operations taking place at composting plants are the source of potential environmental impacts related to factors such as odour, bioaerosols, noise and dust. The release of microorganisms as bioaerosols currently represents one of the main health concerns from a public viewpoint. This applies not only to the workers at the plant but also to people from the local residential areas because of the potential risk to health associated with the inhalation of these bioaerosols. The airborne microorganisms and 166
GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS
The main aim of this work was to monitor the amount of AF and total mesophilic bacteria at different composting plants to identity the factors affecting bioaerosol pro les. The effect of mitigation measures such as bio lters, in the case of enclosed systems, and air dispersion in open facilities was also studied. MATERIALS AND METHODS Composting Sites Description Seven commercial composting plants were chosen for the study based on their different operational characteristics. Their main features, which include type of waste treated, throughput and composting system are shown in Table 1. Plants I–VI, referred to in Table 1, used enclosed systems. In composting plants I and II, the bioxidative phase took place in static piles with forced aeration in closed biocells. The exhaust air from the top of the biocells was vented through a bio lter. In composting plants III, IV, V and VI, the bioxidative phase, and also the maturation phase in plant III, took place in industrial warehouse buildings with forced aeration and occasional turning. The warehouses operated under negative pressure with the air being extracted from the top and vented through the bio lter in plants III, IV and V, whereas in plant VI the air was passed through a scrubber and then through the bio lter. Full description of the bio lter systems has been detailed previously (Sa´nchezMonedero and Stentiford, 2002). Plant VII windrowed green waste on an outdoor concrete pad. The shredded green wastes were composted in trapezoidal cross sectional windrows (approximately 2.5 m height) and were turned using a loading shovel once a week. The meteorological conditions during monitoring at each site are shown in Table 2.
167
Table 2. Meteorological conditions during the sampling. Site Enclosed systems Site I Site II Site III Site IV Site V Site VI Open system Site VI—no activity Site VI—shredding Site VI—turning Site VI—screening
Temperature ( C)
Cloud cover
Wind speed (m s 1)
18 19 22 22 20 21
Sunny Sunny Sunny Sunny Sunny Sunny
0–1 0–2 0–1 1–2 0–2 0–2
15.0
0–4
13.0
Cloud breaking Overcast
16.7
Overcast
0–3
12.8
Sunny
0–7
0–2
unaffected by the plant operations on site. The inlet of the air sampler was at 1.8 m above the ground. Operational area In the enclosed system sites, this location corresponded to the airborne microorganism concentration measured inside the composting hall during normal operations. In all cases, the inlet of the air sampler was 1.8 m above the ground. However, for plant VI, where it was not possible to gain access for safety reasons the sample was taken in the piping system connecting the fans to the scrubber. At plant VII, the open system based on using windrows, the air sample was taken 40 m downwind from the windrow being turned. The inlet of the air sampler was at 1.8 m above the ground. Air Sampling and Microbiological Analysis
Sampling Locations The sampling points which were chosen at each site and are presented in this paper related to background levels, levels in operational areas and levels downwind. Background This corresponded to the airborne microorganism concentration at a location upwind from the sites which was Table 1. Main characteristics of the seven composting plants studied. Composting Type of plant waste
Throughput (tonnes year 1) Composting system
I
BW, SS, GW 40,000
II
GW, FW
III IV V VI VII
BW, GW BW, SS, GW SS, GW, FW MSW GW
1750 30,000 30,000 28,000 76,000 20,000
Static pile, forced aeration in biocell Static pile, forced aeration in biocell Forced aeration and turning Forced aeration and turning Forced aeration and turning Forced aeration and turning Windrows, turned pile system
BW, biowastes—organic fraction of source separated municipal solid wastes; FW, food wastes; GW, green wastes; MSW, municipal solid wastes; SS, sewage sludge.
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A six-stage viable impactor Andersen sampler (Andersen Instrument Inc., Georgia, USA) was used to collect the samples on site. The air was drawn through the sampler with a pump working at a constant ow of 26 l min¡1 (calibrated in the laboratory). The sampling time was typically one minute. For every sample, the sampler was lled with six 9 cm plastic Petri dishes containing the agar medium. Once the required volume of air had been drawn through (depending on estimated concentrations of bioaerosols), the plates were covered and incubated. Three replicates were taken at every sampling point. After each sample, the sampler was sterilized by washing with a solution of either methanol or ethanol 70%. Aspergillus fumigatus detection and quanti cation were carried out according to the method of Fisher et al. (1998). The agar medium was prepared using 20 g l¡1 of malt extract agar and 15 g l¡1 of bacteriological agar. Two antibiotics were added (streptomycin, 50 mg l¡1 and novobiocin, 10 mg l¡1) to suppress bacterial development. The plates used for the sampling were incubated at 40¯ C for 48 h, and then the green-grey colonies with columnar conidial heads, characteristic of AF, were counted as representative of AF spores. Mesophilic bacteria detection and quanti cation were carried out according to the method used by Lacey and Williamson (1995) and incorporated into the Composting
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Association protocol (Gilbert and Ward, 1999). The agar medium was prepared using 14 g l¡1 of nutrient agar and 10 g l¡1 of bacteriological agar. The antibiotic cicloheximide (100 mg l¡1 dissolved in less than 2 ml of acetone) was added after autoclaving when the temperature had fallen to approximately 47¯ C. The plates used for the sampling were incubated at 37¯ C for 48 h, and then the white round-shaped colonies were counted. The positive-hole correction was used to adjust colony counts (Macher, 1989). The results were calculated as the geometric mean of the three replicates and were expressed as colony forming units per cubic metre of air (cfu m¡3). The detection limit was <102 cfu m¡3. RESULTS AND DISCUSSION Bioaerosol Generation Bioaerosol generation at composting facilities is regulated by different factors such as the activity taking place at the composting plant and the material being treated. Figure 1 shows the Aspergillus fumigatus and Figure 2 the mesophilic bacteria concentration, for the background and operational areas at seven plants. The error bars represent the standard deviation of the experimental results. Sites I–VI
(enclosed systems) were treating a range of organic materials and site VII was an open facility which was sampled under different operational conditions. In all cases the background levels for both kind of microorganisms, which were assumed to be unaffected by the composting operations taking place, were below 103 cfu m¡3. These were in the expected range found by other authors monitoring similar systems (Millner et al., 1994; Slater et al., 2001). These background concentration levels are similar to those found for AF generated by a range of agricultural activities and slightly higher than the levels expected in quiescent natural environments or indoors where the concentrations would generally be lower than 175 cfu m¡3 (Millner et al., 1994). There is no de nitive dose–response information available for the effect of AF on the health of workers. Malmros et al. (1992) suggested that the amount of total bacteria should not be over 5000 or 10,000 cfu m¡3 for an 8 h working day. Concentrations above these threshold limit values were the cause of occupational disease, mainly due to eye and throat irritation, cough at night, chills and fever (Sigsgaard et al., 1990). Based on these gures, these background levels would not represent any health impact for workers, as long as they do not have a depressed immune system or speci c breathing problem.
Figure 1. Concentrations of Aspergillus fumigatus recorded at different sites (composting plant numbers as in Tables 1 and 2). Error bars represent the standard deviation of the experimental results.
Figure 2. Concentrations of mesophilic bacteria at different sites (composting plant numbers as in Tables 1 and 2). Error bars represent the standard deviation of the experimental results.
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GENERATION AND DISPERSION OF AIRBORNE MICROORGANISMS The concentration of bioaerosols released at the composting facilities during normal operation was generally two logarithmic units higher than the background as a consequence of the movement of the material and the forced aeration. The green waste composting in the open air site (site VII) was the site that released the highest amount of both microorganisms, ranging between 3.8 £ 103 and 9.8 £ 104 cfu m¡3 for AF and between 2.3 £ 104 and 1.1 £ 105 cfu m¡3 for mesophilic bacteria measured 40 m downwind from the windrows. These concentrations were similar to those recorded inside the composting halls at sites I–VI, which ranged from 4.7 £ 103 to 2.2 £ 105 and 2.3 £ 103 to 1.9 £ 105 cfu m¡3 for AF and mesophilic bacteria, respectively. These concentrations were in agreement with the expected range (between 103 and 106 cfu m¡3), typically found by other researchers near to composting piles in the open air and running with different operational conditions (Millner et al., 1980; Fischer et al., 1998; Maricou et al., 1998). The amount of bioaerosol released in all the composting plants suggests that it is advisable for operators working in the immediate vicinity of the composting mass to use protective equipment such as face masks to avoid negative health impacts. The concentration of mesophilic bacteria in particular were higher than the range suggested by Sigsgaard et al. (1990) for unprotected working. The amount of AF within the buildings on the enclosed sites were at levels that other authors have previously reported to have been the cause bronchitis and gastrointestinal complaints from the staff at composting plants (Nielsen et al., 1997). In the case of the open site the levels were dependent on the type of operation being carried out and the operators should be made aware of the occasions when some form of protection is necessary. The differences in the air concentrations of both microorganisms found at the facilities monitored indicated that the airborne release was not only affected by the amount of microorganisms present in the waste, but also by the different operational processes which were carried out at the facility. A key issue is the level of these organisms which is present within the material itself, since this will affect the amount released when the material is agitated or aerated. Millner et al. (1977) reported the highest concentration of AF in green wastes and woodchip varied from 103 to 107 cfu g¡1, whereas for sewage sludge the range was from 102 to 103 cfu g¡1. Fisher et al. (1998) measured AF concentrations in a mixture of green waste and kitchen waste varying from 105 to 106 cfu g¡1. The high amount of AF in the green waste and the turning of the material caused the highest concentration of the airborne microorganisms around site VII (green waste composting in the open). Whilst we would expect similar wastes to develop similar concentrations of AF, the release rates from plants processing the same wastes can be very different. For example, in this study composting plants treating similar kinds of biowastes (sites I and IV were treating a mixture of biowastes, sewage sludge and green wastes) released very different amounts of AF spores. The other plants released a similar amount of airborne microorganisms, ranging from 4.7 £ 103 to 1.2 £ 104 cfu m¡3, which was probably related more to the forced aeration and turning which took place than the material being treated. The plant treating only MSW (site VI) released a very low amount of AF, compared Trans IChemE, Vol 81, Part B, May 2003
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with the amount of mesophilic bacteria released. This type of behaviour was previously reported by Tolvanen et al. (1998), who found low levels of AF at some of the operation processes at a biowaste processing plant. In the case of site VII, it was possible to study the effect of the activity taking place at the composting plant on the bioaerosol release. Shredding, turning and screening increased the air concentration of both airborne microorganisms 40 m downwind by 1 or 2 logarithmic units, compared with the amount measured when there was no mechanical agitation of the material. Screening released lower amounts of bioaerosols than the other two operations studied. The high standard deviation for both airborne microorganisms measured at the open composting plant made it dif cult to draw any clear conclusions. The sampling was strongly affected by any sudden changes in the meteorological conditions such as wind speed and direction. These practical dif culties associated with monitoring open facilities have been reported by other workers (Gilbert et al., 2002). Bioaerosol Dispersion The extent of bioaerosol dispersion should be different depending on the composting system being used. The exhaust air from enclosed systems is typically treated using bio lters or bioscrubbers which reduce bioaerosol levels before the air is released to the atmosphere. In the case of an open system it is necessary to rely on natural air dispersion to dilute the amount of airbornes being spread in the surroundings. In both cases, guidelines have been proposed to mitigate bioaerosol generation in order to reduce the impact on workers and the amount of bioaerosols released to the air around the plant. Epstein et al. (2001) found a signi cant decrease in dust and bioaerosol generation after making simple operational modi cations such as increasing the moisture content of the mixtures, installing dust collection systems, using sweepers and water vehicles to control dust on roadways and installing adequate ventilation in buildings. In the case of open systems, the mitigation guidelines generally include factors such as design, site topography and landscape to abate transport of aerosols downwind (Millner et al., 1994). The effectiveness of bio lters for bioaerosol removal has been recently suggested at composting plants (Sa´nchezMonedero and Stentiford, 2002) and also on farms (Martens et al., 2001). All the six enclosed composting sites studied were tted with either a bio lter or a combination of a bio lter and a bioscrubber to treat the exhaust air. In all cases the amount of AF measured at the surface of the bio lter did not exceed 103 cfu m¡3 regardless of the incoming concentration. This meant that bio lters typically removed more than 90% of the incoming bioaerosols. However the mesophilic bacteria removal was not so effective and in some cases, only 40% of the incoming bioaerosols were removed (Sa´nchez-Monedero et al., in preparation). As part of this study the level of both microorganisms was measured at 300 m downwind during shredding and turning and at 200 m downwind during screening, at the composting pad treating green wastes by windrows (site VII). The results indicated that the levels measured 300 m downwind from the composting pad did not show any signi cant difference compared to the background levels measured upwind. However, the levels measured 200 m downwind
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from the screening operation (5.4 £ 102 and ¡3 4 1.6 £ 10 cfu m for AF and mesophilic bacteria respectively) showed that the bioaerosol concentration was still higher than background levels indicating that this distance (200 m) was not suf cient on this site to reduce the microorganisms to background levels. These results were in agreement with those found by the Composting Association in the UK, and a boundary distance of at least 250 m is currently being recommended by the Environment Agency to ensure that the composting plants will not have any adverse impacts on the health of people living in the area (Environment Agency, 2001). However, other authors have measured airborne concentrations higher than the background at 500 m or more from composting sites (Millner et al., 1977, 1994). The strong effect of the meteorological conditions on bioaerosol dispersion from composting plants makes it dif cult to establish an accurate distance where there is no enhanced adverse health impact for people. Mathematical modelling is often employed at the design stage but these models have limitations due to the complexity of the process. CONCLUSIONS Open composting plants and those treating the organic wastes in biocells released amounts of Aspergillus fumigatus to the air that could be a risk for staff working in the vicinity. The airborne release appeared to be related to both the amount of microorganisms in the wastes, the design of the site and to the operational procedures used in the composting plants. The highest amounts of bioaerosols were generated at composting plants treating green waste and mechanical agitation released higher amounts of bioaerosols than forced aeration. The airborne microorganism levels measured 300 m downwind did not show any difference to the background levels. REFERENCES EC, 1999, Council directive on the land ll of waste (1999=13=EC), Of c J E C, 16 July: L 182=1. Environment Agency, 2001, Technical guidance on composting operations, Draft for external consultation, October 2001. Available at: www.environment-agency.gov.uk=commondata=105385=compostin.pdf. Accessed 4 August 2002. Epstein, E., Wu, N., Youngberg, C. and Croteau, G., 2001, Controlling dust and bioaerosols at a biosolids composting facility, BioCycle, 42(4): 50–54. Fischer, J.L., Beffa, T., Lyon, P.F. and Aragno, M., 1998, Aspergillus fumigatus in windrow composting: effect of turning frequency, Waste Mgmt Res, 16(4): 320–329. Gilbert, E.J. and Ward, C.W., 1999, Standardised Protocol for the Sampling and Enumeration of Airborne Micro-organisms at Composting Facilities (The Composting Association, Coventry). Gilbert, E.J., Kelsey, A., Karnon, J.D., Swan, J.R.M. and Crook, B., 2002, Preliminary results of monitoring the release of bioaerosols from
composting facilities in the UK: interpretation, modelling and appraisal of mitigation measures, in Proceedings of the International Symposium Composting and Compost Utilization, Ohio, USA, pp 1094–1105. Lacey, J. and Williams, P.A.M., 1995, Airborne microorganisms associated with RDF res at Castle Bromwich, Report No. CWM=110=93, Department of the Environment. Macher, J.M., 1989, Positive-hole correction of multiple-jet impactors for collecting viable microorganisms, Am Ind Hyg Assoc J, 50(11): 561–568. Malmros, P., Sigsgaard, T. and Bach, B., 1992, Occupational health problems due to garbage sorting, Waste Mgmt Res, 10: 227–234. Maricou, H., Verstraete, W. and Mesuere, K., 1998, Hygienic aspects of biowaste composting: airborne microbial concentrations as a function of feedstock, operation and season, Waste Mgmt Res, 16(4): 304–311. Martens, W., Martinec, M., Zapirain,R., Stark, M., Hartung, E. and Palmgre, U., 2001, Reduction potential of microbial, odour and ammonia emissions from a pig facility by bio lters, Int J Hyg Environ Health, 203: 335–345. Millner, P.D., Bassett, D.A. and Marsh, P.B., 1980, Dispersal of Aspergillus fumigatus from sewage sludge compost piles subjected to mechanical agitation in open air, Appl Environ Microbiol, 39(5): 1000–1009. Millner, P.D., Marsh, P.B., Snowden, R.B. and Parr, J.F., 1977, Occurrence of Aspergillus fumigatus during composting of sewage sludge, Appl Environ Microbiol, 34(6): 765–772. Millner, P.D., Olenchock, S.A., Epstein, E., Rylander, R., Haines, J., Walker, J., Ooi, B.L., Horne, E. and Maritato, M., 1994, Bioaerosols associated with composting facilities, Compost Sci Util, 2(4): 6–57. Nielsen, E.M., Breum, N.O., Nielsen, B.H., Wurtz, H., Poulsen, O.M. and Midtgaard, U., 1997, Bioaerosol exposure in waste collection: a comparative study on the signi cance of collection equipment, type of waste and seasonal variation, Ann Occup Hyg, 41(3): 325–344. Olver Jr., W.M., 1994, The Aspergillus fumigatus problem, Compost Sci Util, 2(3): 27–31. Sa´nchez-Monedero, M.A. and Stentiford, E.I., 2002, Aspergillus fumigatus control at composting plants through the use of bio lters, in Proceedings of the International Symposium Composting and Compost Utilization, Ohio, USA, pp 1106–1114. Sigsgaard, T., Bach, B. and Malmros, P., 1990, Respiratory impairment among workers in a garbage-handling plant, Am J Ind Med, 17: 92–93. Slater, R.A. and Frederickson, J., 2001, Composting municipal waste in the UK: some lessons from Europe, Resour Conserv Recycl, 32: 359–374. Slater, R.A., Frederickson, J. and Gilbert, E.J., 2001, The State of Composting—1999. Results of the Composting Association’s Survey of UK Composting Facilities and Collection Systems in 1999 (The Composting Association, Wellingborough). Tolvanen, O.K., Hanninen, K.I., Veijanen, A. and Villberg, K., 1998, Occupational hygiene in biowaste composting, Waste Mgmt Res, 16(6): 525–540.
ACKNOWLEDGEMENT This research was supported through a European Community Marie Curie Fellowship. The authors are solely responsible for information communicated and the European Commission is not responsible for any view or results expressed.
ADDRESS Correspondence concerning this paper should be addressed to Professor E. I. Stentiford, School of Civil Engineering, Leeds University, Woodhouse Lane, LS2 9JT Leeds, UK. E-mail: [email protected]. The manuscript was received 2 September 2002 and accepted for publication after revision 3 April 2003.
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