Environ. Sci. Technol. 2003, 37, 4299-4303
Biofiltration at Composting Facilities: Effectiveness for Bioaerosol Control MIGUEL A. SANCHEZ-MONEDERO* AND EDWARD I. STENTIFORD School of Civil Engineering, University of Leeds, Woodhouse Lane, LS2 9JT Leeds, United Kingdom CLAUDIO MONDINI Istituto Sperimentale per la Nutrizione delle Piante, Sezione di Gorizia, Via Trieste 23, 34170 Gorizia, Italy
Biofiltration was evaluated as a method to control the airborne microorganisms released at composting facilities. Seven commercial composting plants were selected for this study because of their different operating conditions and biofilter designs. In all plants, the biofilters were originally designed for odor control. The concentrations of both Aspergillus fumigatus and mesophilic bacteria were measured in the air stream before and after passing through the biofilters and compared with the background concentrations in the surrounding area. Results showed that biofiltration achieved an average reduction greater than 90% and 39% in the concentrations of A. fumigatus and mesophilic bacteria, respectively. In all the plants, the airborne A. fumigatus concentration after the biofilter was lower than 1.2 × 103 cfu m-3, independent of the inlet concentration, whereas the mesophilic bacteria concentration was dependent on the inlet concentration. The different behaviors of the two microorganism groups were thought to be due to the different aerodynamic characteristics of the particles that affected the capture by impact in the biofilter bed. The fungus, whose spores had a maximum of diameter size distribution between 2.1 and 3.3 µm, were more effectively captured in the biofilter than the bacteria, which had diameters mainly between 1.1 and 2.1 µm.
Introduction Biofiltration has become a widely accepted method for odor control at waste treatment plants because it is an environmentally friendly method with relatively low installation and maintenance costs. Several papers have recently reviewed the performance and operation of biofilters for waste gas treatment (1-3). Biofiltration involves using fans to force the contaminated air through a mass of porous and wet organic material populated with a microbial biomass capable of degrading the odorous pollutants carried by the air. When these organic pollutants are in contact with the biofilter, different physicochemical and microbiological processes take place. The main process involved is the absorption of the pollutants by the liquid layer surrounding the solid particles, making the pollutants available to the microorganisms for aerobic biodegradation. This biological air treatment method has been particularly successful for the removal of low * Corresponding author phone: +44 1133432267; fax: +44 1133432265; e-mail:
[email protected]. 10.1021/es0202309 CCC: $25.00 Published on Web 08/14/2003
2003 American Chemical Society
concentrations of chemical compounds such as VOCs, ammonia, and hydrogen sulfide from large-volume air streams, which commonly occur at composting facilities. The bioaerosols generated at composting plants are principally airborne microorganisms and microbial constituents that are released from the processes where the movement of material is involved. At composting plants, the main activities concerned are delivery of fresh waste, shredding, compost pile turning, and compost screening. The effect of bioaerosols on the health of plant operators and local residents has seen considerable research interest in recent years and is a key issue especially when a new composting site is being planned (4-6). Recently, environmental impact and performance studies of the composting plants in the U.K. have included the monitoring of Aspergillus fumigatus and mesophilic bacteria. These organisms have been selected by the Composting Association in the U.K., following studies elsewhere, as appropriate indicators for bioaerosol studies of composting sites. These microorganisms can be very easily carried by the wind to distances ranging from a few hundred meters to several kilometers (6, 7). In the case of the opportunistic pathogen A. fumigatus, the small size of its spores (around 2 µm) means that they can enter and lodge easily in the lungs if inhaled, becoming a potential cause of infections in immunodepressed people. Several guidelines have been proposed for bioaerosol control at composting plants including the use of biofilters as a possible way of limiting their impact (4). There is little published data on the evaluation of biofilter effectiveness for aerosol removal, and much of the work has been done at bench scale and in agricultural facilities, particularly piggeries (8-10). Ottengraf and Konings (11) studied the emission of microorganisms from biofilters, and proposed a theoretical model to describe the rate of microbial emission. They contended that the mechanisms involved were different from those responsible for the odor removal. Their model was based on two simultaneous mechanisms: the capture of bioaerosols due to impingement on the packed bed of particles, and the emission of the microorganisms from the wet biolayer surrounding the bed particles. According to this theoretical model, the aerosol removal efficiency was affected by the gas velocity through the biofilter and would also be affected by the particle size of both the bed medium and the bioraerosols impacting on the biofilter. The aim of this work was to evaluate the effectiveness of the biofilter systems, installed at seven different full-scale composting facilities, for A. fumigatus and mesophilic bacteria reduction. To achieve this aim, both microorganism groups were measured in the air stream before and after passing through the biofilters, and these were compared with the respective background levels in the surrounding area. The measurements were performed with a six-stage viable sampler impactor (Andersen sampler) to obtain the bioaerosol distribution for each microorganism group.
Materials and Methods Composting Sites and Biofilter Description. Seven commercial composting plants were chosen for the study on the basis of their different operational parameters. Their main characteristics, which include the type of waste treated, tons treated per year, composting system, and bioreactor capacity are shown in Table 1. In composting plants I and VII, the bioxidative phase took place in static piles with forced aeration in closed biocells. The exhaust air from the top of the biocells was blown directly through the biofilter. The bioxidative phase in composting plants II-V, and the VOL. 37, NO. 18, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Main Characteristics of the Seven Composting Plants Studieda composting plant
type of waste
throughput (wet tons year-1)
composting system
bioreactor capacity (m3)
I II III IV V VI VII
BW, SS, GW BW, SS, GW BW, GW MSW SS, GW, FW SS, GW GW, FW
40000 30000 30000 76000 28000 5000 1750
static pile, forced aeration in the biocell forced aeration and turning forced aeration and turning forced aeration and turning forced aeration and turning static pile, forced aeration by suction static pile, forced aeration in the biocell
300 6,900 6,000 na 3500 150 168
a BW ) biowastes (organic fraction of source-separated municipal solid wastes), FW ) food wastes, GW ) green wastes, MSW ) municipal solid wastes, SS ) sewage sludge. na ) not available.
TABLE 2. Main characteristics of the Biofilters at the Composting Plants Studieda composting plant
dimensions, surface × depth (m2 × m)
bed medium
moisture content (%)
age of the medium (months)
ventilation rate (m3 h-1)
gas-phase residence time (s)
superficial gas velocity (m min-1)
I II III IV V VI VII
1500 × 1.1 700 × 2.4 450 × 1.3 400 × 1.3 572 × 1.8 110 × 1.2 6.75 × 1
MC, W P MC, W peat, P MC, W MC, W MC, W
60 >80 65 na 50-60 70 50
12 18 12 36 1 12 12
165000 70000 50000 na 100000 16000 250
36 86 42 na 37 29 97
1.83 1.67 0.93 na 2.91 2.42 0.62
a
MC ) coarse fraction of mature compost, W ) woodchip, P ) pine bark and roots. na ) not available.
maturation phase in plant III took place in industrial warehouse type buildings with forced aeration and occasional turning. The warehouses had a negative air pressure environment, with the air being extracted from the top and blown directly by a fan to the biofilter in plants II, III, and V. In plant IV, the exhaust air stream was passed initially through a scrubber and then through the biofilter. In composting plant VI, the bioxidative phase used forced aeration by sucking the air from the bottom of the static pile, and it was not turned during the process. The exhaust air was blown to the biofilter after passing through a condensation chamber of 1 m3 capacity. The meteorological conditions were very similar at the seven composting plants, with temperatures in the range between 18 and 22 °C, 3/8 cloud cover in plant VI and cloudless in the rest, and with a gentle wind varying in speed from 0 to 2 m s-1. Table 2 shows the main characteristics of the biofilters including the dimensions, type of bed medium, average moisture content, age of the bed medium, maximum flow rate, and gas-phase residence time. Biofilters at composting plants I, III, and V-VII were prepared with a mixture of woodchips and the coarse fraction of mature compost. The biofilter at plant IV was prepared with a mixture of peat, pine bark, and roots. The biofilter at plant II contained only pine bark and roots. In all cases, the biofilters were watered weekly on the top to maintain the moisture content between 50% and 70%. In biofilters I-III, the top surface was occasionally mixed with a tractor to avoid compaction and clogging of the material that could lead to the formation of preferential flow paths for the air. Sampling Locations. Three sampling points were chosen at each site, which were set out as follows. Background: This corresponded to the bioaerosol concentrations at a location upwind from the sites which was unaffected by the plant operations on site. The inlet of the air sampler was 1.8 m above the ground. Before the biofilter: This corresponded to the bioaerosol concentrations measured inside the composting hall during normal operations, except for composting plant IV, where it was not possible to gain access for safety reasons. In all cases, the inlet of the air sampler was 1.8 m above the ground. In plant IV the sample 4300
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was taken from the pipe system connecting the fans to the scrubber. At composting plant VI, an air sample was also taken at the outlet of the condensation chamber. After the biofilter: This corresponded to the bioaerosol concentrations in the air sample taken 40 cm above the top surface of the biofilters. Air Sampling and Microbiological Analysis. A six-stage viable sampler impactor (Andersen Instruments Inc.) was used to collect the samples on site. The sampler design allowed us to obtain a size distribution of the collected airborne microorganisms according to their aerodynamic behavior. The air was sucked with a pump working at a constant flow rate of 26 L min-1 (measured in the laboratory). The sampling time was 1 min. For every sample, the sampler was filled with six plastic Petri dishes (diameter 9 cm) containing the agar medium. Once the required air had been drawn through, 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%. A. fumigatus detection and quantification were carried out according to the method of Fisher et al. (12). The agar medium was prepared using 20 g L-1 malt extract agar and 15 g L-1 bacteriological agar. Two antibiotics were added (streptomycin, 50 mg L-1, and novobiocin, 10 mg L-1) to suppress bacterial development, after autoclaving when the temperature had fallen to approximately 47 °C. The plates used for the sampling were incubated at 40 °C for 48 h, and then the green colonies were counted and recorded as A. fumigatus. Mesophilic bacteria detection and quantification were performed according to the method proposed by Gilbert and Ward (13). The agar medium was prepared using 14 g L-1 nutrient agar and 10 g L-1 bacteriological agar. The antibiotic cycloheximide (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 as mesophilic bacteria.
FIGURE 1. A. fumigatus concentration (cfu’s m-3) at the background locations and in the air before and after the biofilters: (gray bars) background; (black bars) before the biofilter; (white bars) after the biofilter. (Vertical bars correspond to the standard deviation; n ) 3.)
FIGURE 2. Mesophilic bacteria concentration (cfu’s m-3) at the background locations and in the air before and after the biofilters: (gray bars) background; (black bars) before the biofilter; (white bars) after the biofilter. (Vertical bars correspond to the standard deviation, n ) 3.)
The positive-hole correction was used to adjust colony counts according to Macher (14). The results were calculated as the geometric mean of the three replicates and were expressed as colony-forming units per cubic meter of air (cfu’s m-3). The detection limit was
