May 24 - Journal of the Air & Waste Management Association
Bioaerosol release from composting plants is a cause of concern because of the potential health impacts on site workers and local residents. A one-year monitoring was undertaken in a typical composting plant treating green wastes by windrowing in the open. Aspergillus fumigatus spores and mesophilic bacteria were used as monitoring parameters and were collected in a six-stage Andersen sampler impactor from the air at different locations and during different operational activities. Background concentrations of both microorganisms were generally below 1000 colony-forming units m^sup - 3^ when no vigorous activity was taking place. Shredding of fresh green wastes, pile turning, and screening of mature compost were identified as the activities generating the highest amounts of both bioaerosols 40 m downwind of the composting pad. These air concentrations were ~2 log units higher than background levels. Screening of mature compost generated lower amounts of A. fumigatus than the other two activities (an average of 1 log unit higher than background levels). Workers were identified as the main potential receptors of high bioaerosol concentrations in areas close to the composting pad, whereas no major risk for local residents was expected because the concentrations recorded at distances of 200 and 300 m downwind of the operational area were not significantly different from background levels.
Composting is a waste management method used around the world for stabilizing
organic waste and making it safe to use, from a pathogenic microorganism point
of view. The method is based on the aerobic degradation of organic matter and
uses a range of biological processes to give a final product that can be used
safely for different purposes, such as agriculture, horticulture, landscaping,
and as landfill cover. 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 is
currently causing concerns related to potential health impacts. This is true not
only for the workers at a plant but also for local residents as a result of
inhalation of these bioaerosols.
The bioaerosols generated at composting plants are mainly airborne
microorganisms and microbial constituents, which are released during processes
that involve vigorous movement of material, mainly during fresh waste delivery,
shredding, compost pile turning, and compost screening. Millner et al.1
presented an excellent summary on the state of the art on bioaerosols generated
at composting facilities and their potential effects. Their work focused on the
impact of airborne Aspergillus fumigatus spores on human health, which 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 because the bioaerosols are released directly into the
surrounding area without any pretreatment such as biofilters or bioscrubbers, as
occurs in enclosed systems.2,3 Slater and Frederickson4 recently surveyed the UK
composting industry and showed that ~833,000 tonnes of municipal and
nonmunicipal wastes were composted in 1999. Most of the composting plants
operating in the United Kingdom are treating source- separated green wastes by
windrowing in the open. The green wastes are diverted from landfills and are
usually treated in the open air by windrowing on concrete pads, which are often
set up at the same landfill sites. The landfill site will potentially use the
compost produced as either a daily or final cover for the landfilled materials.
Because of the characteristics of these composting sites and the distant
location of landfills with respect to residential areas, potential sensitive
receptors for the bioaerosols are expected to be the site workers rather than
local residents.
The UK Composting Association, after some research on bioaerosol generation
and dispersion, proposed a standard procedure for bioaerosol monitoring at
composting facilities that has wide acceptance in the United Kingdom. This
protocol5 is based on the monitoring of two airborne microorganisms, A.
fumigatus and total mesophilic bacteria, by impaction at different up- and
downwind locations at composting plants. The aim of this work was to monitor the
amount of A. fumigatus and total mesophilic bacteria generated at a typical
green waste composting plant over 1 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 Site Description and Sampling Locations
The composting took place at a full-scale composting plant in the North of
England, located at a landfill facility. The plant throughput was ~10,000 tonnes
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 3 2 m (length width height) over an 18-week period on a
concrete pad in the open air. The windrows were turned by a loading shovel once
a week for the first 10 weeks, after which the material was allowed to mature
for a period of 8 weeks with no further turning. Each turn of the heaps 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 concreted 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). 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 that were used are described in the following paragraphs
(Figure 1). Up- and downwind sites were located according to the different wind
directions during different sampling dates.
Background Locations (U1, U2, and U3). These represented upwind sites where
the airborne microorganism concentrations were likely to be unaffected by the
on-site plant operations. The sampling points were located either 25 m (U1 and
U3) or 40 m (U2) away from the operational activities.
Figure 1. Site map and location of the sampling points U, upwind; D,
downwind. Map not to scale.
Downwind Locations (D1-D6). These corresponded to the airborne microorganism
concentration at locations downwind from the operational activities taking place
on site. Dl 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 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.
Air Sampling and Microbiological Analysis
Airborne microorganism concentrations were monitored for a 12- month period.
Sampling frequency was adapted to operational and meteorological conditions, and
no samples were taken during the winter. A six-stage Andersen viable impactor
sampler was used to collect the samples on site. The air was drawn through the
sampler with a pump working at a constant flow of 26 L min^sup -1^ (calibrated
in the laboratory). The inlet of the air sampler was 1.8 m above the ground, and
the sampling time was 1 min. For every sample, the sampler was filled with six
9-cm plastic Petri dishes containing the agar medium. Once the required air had
been drawn through, the plates were covered and incubated. Two or three
replicate sets of plates were taken at every sampling point. After each sample,
the sampler was sterilized by washing with an ethanol solution.
Detection and enumeration of A. fumigatus were carried out according to the
method of Fischer et al.:6 the agar medium was prepared with 20gL^sup -1^ of
malt extract agar and 15 g L^sup -1^ of bacteriological agar. To suppress
bacterial development, two antibiotics, streptomycin at 50 mg L^sup -1^ and
novobiocin at 10 mg L^sup -1^, were added after autoclaving when the temperature
had fallen to ~47 C. The plates used for the sampling were incubated at 40 C for
48 hr, and then the green colonies were counted as indicating the numbers of A.
fumigatus spores captured on the plate.
Detection and quantification of mesophilic bacteria were carried out
according to the method used by Lacey and Williams,7 incorporated into the UK
Composting Association protocol:5 The agar medium was prepared with 14 g L^sup
-1^ of nutrient agar and 10 g L^sup -1^ of bacteriological agar. The antibiotic
cycloheximide (100 mgL^sup -1^ dissolved in less than 2 mL of acetone) was added
after autoclaving when the temperature had fallen to ~47 C. The plates used for
th\e sampling were incubated at 37 C for 48 hr, and then the white, round-shaped
colonies were counted as being mesophilic bacteria.
The positive-hole correction was used to adjust colony counts.8 The results
were calculated as the geometric mean of the replicates and were expressed as
colonyforming units per cubic meter of air (cfu m^sup -3^). The detection limit
was <10^sup 2^ cfu m^sup -3^.
Meteorological Conditions
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 (model 471, Dwyer instruments
Inc.). Wind direction was taken from the meteorological station located on the
roof of the site office building (500 m from the composting pad).
Statistical Analysis
Experimental data were subjected to analysis of variance (ANOVA) procedure (SPSS
11) to determine the effect of seasonal variation, dispersion, and operational
activities on airborne microorganism concentration. ANOVA was performed for A.
fumigatus and mesophilic bacteria after logarithmic transformation of their
concentrations. Multiple mean separations were performed with Duncan's multiple
range test at P < 0.05.
RESULTS AND DISCUSSION
Bioaerosol Monitoring
The concentration of A. fumigatus and mesophilic bacteria at different upwind
and downwind locations around the composting plant under different operational
conditions during 1 year of monitoring are shown in Tables 1 and 2. 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 10^sup 3^ cfu m^sup -3^. These concentrations represented the
background levels for both microorganisms at the composting site, unaffected by
the operational activities. The background concentrations were within the
expected range usually found for A. fumigatus during normal agricultural
activities and were higher than the levels measured in other open environments
or in indoor air.1 The cause of these somewhat enhanced background levels was
other operations, external to the composting site, possibly related to the
normal activities taking place on and around the adjacent landfill site. A.
fumigatus and mesophilic bacteria concentrations recorded at downwind locations
when no vigorous activity was taking place on the composting site were not
different from the background levels. However, vigorous activities such as green
waste shredding, mature compost screening, and pile turning generated similar
increases 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
10^sup 2^ to >2.9 10^sup 5^ cfu m^sup -3^ at downwind location Dl (40 m
downwind) and from 1.5 10^sup 2^ to 2.9 10^sup 3^ cfu m^sup -3^ at downwind
location D3 (300 m downwind).
The airborne concentrations at 25 and 40 m downwind were strongly affected by
the composting activities, which typically caused an increase up to 2
logarithmic units for both microorganisms as a consequence of the vigorous
movement of material. These results were in the range 10^sup 3^-10^sup 6^ cfu
m^sup -3^, which is in agreement with the results of other authors working with
similar conditions.6,9-12
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 (Figure 2). The airborne concentrations at these
downwind locations were occasionally slightly above the background levels, but
never exceeded 2.9 10^sup 3^ cfu m^sup -3^. These occasional high levels at
relatively long downwind distances could be attributable 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.13
Table 1. Concentrations of airborne microorganisms and meteorological
conditions at different sampling locations from autumn 2001 to spring 2002.
During the study, although the sampling dates covered a 12-month period, the
data did not show any variations that could have been attributed to seasonal
variations (Figure 3). Meteorological conditions, particularly wind speed and
direction, were the main factors governing airborne dispersion from the
composting pad. Sudden changes in meteorological conditions did in some cases
produce a high SD in the concentration for both microorganisms over the sampling
period. These practical difficulties when monitoring open facilities are a
common occurrence and, although reflecting 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.14- 16
Assessment of Bioaerosol Sources
Shown in Figure 4 are the annual average A. fumigatus and mesophilic bacteria
concentrations at the potential sensitive receptor location (D1) 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 increases in airborne concentrations, up to 2 logarithmic units higher
than background levels.
Screening of mature compost and the movement of mature compost (piling and
truck loading) caused an intermediate effect, and there was a larger increase in
levels of mesophilic bacteria than in A. 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 A.
fumigatus levels were significantly lower than in other activities and were
similar to background levels. In the case of A. fumigatus, this effect may have
been a result of the sanitization achieved during the composting process.6 On
the basis of the assumption that the sanitization effect would have had an
impact on A. fumigatus, we would have expected a decrease in the concentration
of this microorganism if the composting process was performed effectively (e.g.,
keeping high temperatures, frequent turning of material, and avoiding mixing
with fresh materials). Piling and loading of mature compost did not involve
vigorous movement of material and occurred at a relatively low frequency
compared with other activities, such as shredding or screening, that required
continuous movement of material.
Table 2. Concentrations of airborne microorganisms and meteorological
conditions at different sampling locations during summer and autumn 2002.
Assessment of Potential Risk for Site Workers
There is no dose-response information available for the effect of A.
fumigatus on the health of workers, but it has been proposed that that the
amount of total bacteria should not exceed 5 10^sup 3^ or 1 10^sup 4^ cfu m^sup
-3^ for an 8-hr working day.17 On this basis, the background levels registered
during the monitoring (10^sup 2^- 10^sup 3^ cfu m^sup -3^ should not have any
health impacts for plant operators as long as they do not have established
immunodeficiencies or breathing problems. In the case of mesophilic bacteria,
the concentrations recorded 40 m downwind were higher than the range proposed in
the literature, indicating a greater potential risk.17 The concentrations of A.
fumigatus measured 40 m downwind were at levels that other authors have reported
previously to be the cause of bronchitis and gastrointestinal complaints from
staff during waste collection.18 As a minimum requirement at these levels,
Kiviranta et al.19 recommended the use of personal protective equipment for
plant operators. On the other hand, Browne et al.20 did not find an association
between the incidences of allergy and asthma and A. fumigatus spore levels near
a grass and leaf composting plant. Although the dose response for A. fumigatus
exposure has not been established, the levels recorded at locations 40 m
downwind from the composting activities would make it advisable for site staff
working inside the composting plant or those using the internal pedestrian
access to wear appropriate masks. It would also be advisable to temporarily
interrupt any vigorous activity related to composting whenever the installation
was used by staff or visitors not wearing the appropriated breathing masks.
For the site in question, the levels of airborne microorganisms at the site
boundaries were very little different from background concentrations.
Consequently, as far as A. fumigatus and mesophilic bacteria are concerned, the
local residents would not be considered to be at risk from infections related to
the composting operation.
CONCLUSIONS
The background levels for A. fumigatus and mesophilic bacteria varied within
the range from <10^sup 2^ up to 10^sup 3^ cfu m^sup - 3^. 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. Vigorous activities such as shredding, turning, and screening were
identified as the major sources of bioaerosol generation and release and caused
increases in both A. 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. Meteorological conditions were thought to be the main factors
affecting airborne dispersion from the composting pad. The high levels recorded
on the operating area when vigorous activities were taking place suggested that
it would be advisable for staff working in the area to have appropriate
respiratory protection equipment.
ACKNOWLEDGMENTS
This research was supported through a European Community Marie Curie
Fellowship. The authors are solely responsible for the information communicated,
and the European Commission is not responsible for any views or results
expressed.
IMPLICATIONS
Identification of the factors influencing bioaerosol generation and
dispersion, such as site operation and meteorology, can help to establish a
safety boundary around composting plants. 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.
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Miguel A. Snchez-Monedero, Edward I. Stentiford, and Sari T. Urpilainen
School of Civil Engineering, The University of Leeds, Leeds, UK
About the Authors
Miguel A. Snchez-Monedero, Edward I. Stentiford, and Sari T. Urpilainen are
affiliated with the School of Civil Engineering, The University of Leeds, Leeds
LS2 9JT, UK. Address correspondence to: Miguel A. Snchez-Monedero, CEBAS-CSIC,
Campus Universitario de Espinado, Murcia 30100, Spain; e-mail: monedero@cebas.csic.es.
Copyright Air and Waste Management Association May 2005