Reduction potential of microbial, odour and ammonia emissions from a pig facility by biofilters

International Journal of Hygiene and Environmental Health

Int. J. Hyg. Environ. Health 203, 335-345 (2001) © Urban & Fischer Verlag http://www.urbanfischer.de/journals/intjhyg

Reduction potential of microbial, odour and ammonia emissions from a pig facility by biofilters Wolfram Martens1, Milos Martinec2, Rebeca Zapirain1, Marcus Stark1, Eberhard Hartung2, Urban Palmgren3 1 2 3

Institute for Environmental and Animal Hygiene, University of Hohenheim, Germany Institute of Agricultural Engineering, University of Hohenheim, Germany Pegasus Labor, Düsseldorf, Germany

Received September 7, 2000 · Accepted November 10, 2000

Abstract The intention of this study was the determination of the potential to reduce specific microbial bioaerosol (cultivable bacteria and fungi, total cell counts of microbes, airborne endotoxins and microbial volatile organic compounds, MVOC), odour and ammonia emissions from a pig facility by biofilters. Five identical biofilter units in half technical scale were filled with different filter materials (Biochips, coconut-peat, wood-bark, pellets+bark and compost) and connected in parallel to a piggery. The results showed obvious differences between the filter materials. Numbers of airborne cultivable bacteria were decreased by ca. 70 to 95 % and the total counts of bacterial cells from ca. 25 to () 90 %. The total amount of fungal cells was reduced by at least 60 %, although the percentage of cultivable moulds in the air after passing the filters was sometimes higher than before. Airborne endotoxins and MVOC were effectively reduced by all filter materials to at least 90 %. Regarding odour, the average reduction was between 40 and 83 %, whereas only one of the filters proved to be capable of slightly reducing the ammonia emissions. No relationships between odour/ ammonia and microbial bioaerosols with regard to the reduction efficiency of the different filter materials or the total load of the emitted air could be established. A tendency could be shown, that biofilters best capable to reduce odour emitted slightly more airborne bacteria, both cultivable and total cell counts. Key words: bioaerosol – airborne – microorganisms – endotoxin – MVOC – odour – ammonia – biofilter – reduction

Introduction Biofiltration of waste air is a commonly used technique to reduce emissions of a wide range of organic and inorganic compounds from several different industrial plants. Laboratory-scale and practical experiments have been designed to investigate the emissions such as odours and gases like ammonia as well as different vol-

atile inorganic compounds (VIC) and volatile organic compounds (VOC) and their reduction by biofilters (Kennes and Thalasso, 1998; VDI 3477, 1991). In agriculture biofilters are used to reduce odour emissions from livestock facilities located close to residential areas as claimed by national and international laws and regulations (Pearson et al., 1992; Phillips et al., 1995). By this technique the odour loads can be reduced to

Corresponding author: Dr. Wolfram Martens, Institute of Environmental and Animal Hygiene, University of Hohenheim, Garbenstraße 30, D-70599 Stuttgart, Phone: +49 711 459 2448, Fax: +49 711 459 2431, E-mail: [email protected] 1438-4639/01/203/4-335 $ 15.00/0

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about 40 to 80 % and ammonia emissions to about –40 to 90 % (Mannebeck, 1995; Hartung et al., 1997; Hopp, 1998). Biofilters are biological systems inhabited by several bacteria and fungi, and thereby sources of emissions of microorganisms (Ottengraf and Konings, 1991), biological toxins and metabolites. Additionally, several processing and production sites, for instance in the field of agriculture and waste treatment, are well known as major sources of microbial bioaerosol emissions. In spite of arising public discussion about such emissions and possible health risks of exposed people (Herr et al., 1999), only few data are available regarding such hazards and their possible reduction by biofilters (Ottengraf and Konings, 1991; Kummer et al., 1999). Still less is known about possible relationships between the efficiency to reduce gas, VOC or odour and the in- or decreased emission of microbial bioaerosols. To investigate the emissions and reduction efficiency, five identical biofilter units, each filled with different filter materials, were connected to a pig facility within a pilot study. To determine the aerial loads of the inflated air as well as the reduction efficiency for odour, ammonia and for cultivable bacteria and moulds in comparison, their concentrations were measured before and behind each filter. In addition the concentrations of total microbial cells (living and dead, bacteria and fungi) and airborne endotoxins were determined, as these parameters are known to indicate occupational health risks in highly polluted environments (Malmberg et al., 1985, 1986; Smid et al., 1992; Vogelzang et al., 1998). The amount and reduction of MVOC was measured in parallel, because they are discussed as possible markers for microbial bioaerosol air pollution.

Materials and methods Experiments were carried out on four days in early summer and, in addition, on one day two months later.

Fig. 1. Experimental setup.

Experimental set-up The waste air originated from two fully slatted compartments for fattening pigs with a forced ventilation (Stubbe, 2000). During the experiments the pig density ranged between 14 and 22 large livestock units (LU, 500kg). The waste air was diverted from the outlet air shaft of the pig facility to a distributor and forced by five radial fans through the five biofilter units (Fig. 1). The biofilters were filled with the following biofilter materials: 1. Biochips (test material from Roth GmbH, Oberteuringen, Germany) 2. a mixture of coconut fibre and fibre peat (mixture ratio 1 : 1) 3. a mixture of chopped bark and wood (from spruce, mixture ratio 1 : 1) 4. BioContact filter pellets (Krems Chemie AG, Krems/Donau, Austria) covered with bark (2 : 1) 5. biocompost (granulate,  25 mm). The samples for microbial bioaerosols, odour and NH3 were taken before (waste air inside the air distributor) and behind (purified air in the outlet shaft of each biofilter) the biofilters within an area with a defined air stream. All samples for bioaerosol analyses were collected for one hour, and samples for the odour analyses were taken in parallel. A data logging system continuously recorded the NH3-concentrations, the air flow rate in the outlet shaft of the plant and of each biofilter (determined by a calibrated fan-wheel anemometer), the air temperature and air humidity (Martinec et al., 2000). The moisture content of the filter materials was controlled by an automatic moisturizer (Zeisig 1993; Hartung et al., 1997). The working conditions of each biofilter during the experiments are summarized in Table 1. Determined parameters Cultivable microorganisms. Cultivable microorganisms were determined on all five investigation days. The samples were collected by using AGI 30 samplers with a flow rate of 12.5 l/min., each filled with 50 ml of sterile saline (0.9 % w/v NaCl). The amount of Colony Forming Fungal Units (CFFU, 25 °C, indirect method according to the German TRBA 430, Anonymous, 1997) was determined by plating out aliquots of a serial log dilution and cultivation on DG 18 agar (Ox-

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Table 1. Working conditions of five biofilters connected to a pig facility during the experiments. Waste air

F1

F2

F3

Filter surface [m2]

–

2.18

Packing height [m]

–

0.5

Filter volume load [m3/m3/h]

–

Air temperature [°C]

12.6–17.8

Air humidity [%]

69–77

Moisture content of the filter material [%]

–

518–542

531–602

9–14.7

545–639

F4

F5

445–540

538–582

12.7–16.5

14–18.4

16.1–17.4

14.5–17.3

78–99

77–99

77–99

70–82

68–99

44–55

65–70

65–70

20–30

25–35

oid), as well as the Colony Forming Bacterial Units (CFBU, 30 °C) by cultivation on CASO agar (Oxoid). Total count of airborne microbial cells: The samples to determine the total counts of microbial cells were collected with an air flow rate of 1 l/min by using sterile one-time cartridges fitted with a polycarbonate membrane, pore size 0,45 µm (Camnea method, Palmgren et al., 1986). The determination and quantification of bacterial and fungal cells (Total Bacterial Cells = TBC; Total Fungal Cells = TFC) was done by staining with acridine orange and fluorescence microscopical analysis. Airborne endotoxins. To determine the total inhalable fraction of airborne endotoxins (according to DIN EN 481), the samples were collected by using the PGP-GSP (person carried pump – total inhalable dust sampling) system (Böhm et al., 1998) with an air flow rate of 3.5 l/min. on endotoxin-free fibre glass filters. Further sample treatment was done according to the BIA proposal 9450 for a standard procedure (BIA, 1997) without adding Tween 20 to the samples. The endotoxin content of the samples was determined by use of a commercial KQCL (kinetic quantitative chromogenic Limuluslysate, Endosafe) assay in Endotoxin Units (EU). MVOC. To determine the MVOC (only one day), the air was sucked through a specially controlled batch of activated charcoal (Anasorb 747), at an air flow rate of 0.5 l/min. The MVOC were desorbed with methylene chloride and analyzed by gas chromatography / mass spectrometry technique in selected ion monitoring mode, according to Wessén and Schoeps (1996). The amounts of 3-methylfuran, dimethyl-disulfide, 2-heptanone, 3-methyl-1-butanol, 3-octanone, 1-octen-3-ol, 2-pentanol, 2-hexanone, isobutanol and 1-butanol were determined. Results were calculated as the total amount of MVOC and the specific amount of the individual substances. Odour and ammonia concentrations. The odour units (OU) of each odour sample were analyzed with an olfactometer TO7 (Ecoma) corresponding to the draft version of CEN (1997). The ammonia concentration was continuously monitored by the Binos® gas analyser (NDIR-spectrometer, range 0–100 ppm). A logging system controlled the temporal sequence of the samples (sampling points) for the gas analyser using a measuring point change-over switch. Within 20 minutes the gas concentrations in the waste and the purified air were determined at each biofilter.

Measurement value calculation: Resulting values were, as a basis, calculated as unit/m3 of air. In addition they were expressed as emitted loads of single emitting LU (500 kg) per h (unit/LU*h), based on the actual number and weight of the animals and the total volume stream of the facility. The emission loads of odour and NH3 were calculated and expressed as unit/LU*s and g/LU*h, respectively. To get the percentage of the reduction of the emitted loads (Reduction Efficiency = RE%), the reduction efficiency of each filter was calculated as RE =

waste – Cpurified   C––––––––––––– –   100  Cwaste  

[%]

Results The total amount of emitted cultivable bacteria from the pig facility turned out to be low, with concentrations between 7.4  103 and 4.9 105 CFBU/m3, and an average of 1.1  105 (Tab. 2). The emissions were further decreased by 12.7 to 99.9 % in all biofilters (Tab. 5). All filters reached a maximum reduction of more than 99 %. Coconut fibre/peat, chopped wood/ bark and pellets/bark showed a relatively constant (SD  10 %) and good reduction efficiency of nearly 90 % and more (av.). Compared to these, the compost and the Biochip filters were less efficient with an average of 83 and 70 %, respectively, due to a higher variation of the reduction level (SD 23/32 %). For the Biochips, the concentration was only reduced by 13 % as a minimum. On average the filters emitted loads between 105 and 106 CFBU/LU*h (Tab. 3). The mould emissions of the pig facility were remarkably low (Tab. 3), never exceeding concentrations of 103 CFFU/m3 (av. 2.2  102). Compared to this, the bark/wood, the pellet/wood and the compost filter materials emitted more cultivable fungi in most cases, sometimes leading to calculated negative reduction efficiencies with a maximum of –409 % (pellet/wood, Tab. 5). Nevertheless, none of the concentrations emitted by the filters exceeded 103 CFFU/m3, and two of them, the Biochips and the coconut fibre/peat mixture,

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Table 2. Concentrations of different microbial bioaerosol parameters and odour/NH3 in the waste of a pig facility station an behind different connected biofilters. Parameter

Filters

N=

waste air

F1

F2

F3

F4

F5

5

5

5

5

4

4

colony forming bacterial units CFBU/m3

Av. Med. Min. Max.

1.1  105 1.1 104 7.4 103 4.9 105

 3.9  103  3.9 103  1.1 103  6.8 103

 2.6  103  1.2 103  6.9 102  8.3 103

 8.5  102  5.6 102  2.6 102  2.4 103

 1.2  103*  1.1 103*  3.3 102  2.5 103

 2.0  103*  7.9 102*  3.3 102  6.2 103

total bacterial cells TBC/m3

Av. Med. Min. Max.

4.1  106 2.3 106 1.3 106 1.3 107

 1.7  106  1.3 106  1.7 105  4.7 106

 8.2  105  1.7 105  8.7 104  2.8 106

 3.1  105  1.7 105  8.7 104  9.6 105

 7.1  105**  4.4 105**  8.7 104  1.6 106

 6.2  105*  3.9 105*  8.7 104  1.6 106

colony forming fungal units CFFU/m3

Av. Med. Min. Max.

2.2  102 1.3 102 6.6 101 6.2 102

 3.3  101  3.3 101  n. d.  6.6 101

 3.7  101  3.3 101  n. d.  1.2 102

 2.5  102  3.3 101  n. d.  7.5 102

 5.2  102  5.2 102*  2.0 102  8.2 102

 1.8  102*  1.6 102*  1.3 102  2.6 102

total fungal cells TFC/m3

Av. Med. Min. Max.

5.4  105 4.4 105 1.7 105 1.2 106

 8.7  104  8.7 104  8.7 104  8.7 104

 1.2  105  8.7 104  8.7 104  2.6 104

 8.7  104  8.7 104  8.7 104  8.7 104

 8.7  104**  8.7 104**  8.7 104  8.7 104

 8.7  104*  8.7 104*  8.7 104  8.7 104

airborne endotoxins EU/m3

Av. Med. Min. Max.

792.5 594.5 303.7 1744.5

19.9 17.6 8.6 30.6

11.9 6.7 4.8 33.0

7.6 7.6 2.9 12.9

32.1* 34.7* 6.7 52.4

42.4* 20.9* 2.4 125.2

MVOC µg/m3

total

13.4***

1.0***

0.8***

1.7***

0.5***

0.9***

Odour OU/m3

Av. Med. Min. Max.

1714 1600 770 3100

206* 195* 63 370

381 360 76 720

490 300 150 1300

1075* 890* 320 2200

396** 200** 98 890

NH3 ppm

Av. Med. Min. Max.

13.8 14.4 8.4 17.5

12.4 12.3 8.7 14.8

14.3 14.6 9.5 20.2

14.2 13.8 8.9 18.2

16.0* 16.9* 9.5 20.6

15.2* 16.1* 9.6 18.8

n. d. = not detected; * = (N = 4); ** = (N = 3), *** = (N = 1) ;  = influenced by detection limit. F1 = Biochips, F2 = coconut fibre/peat mixture, F3 = chopped bark and wood, F4 = BioContact filter pellets covered with bark, F5 = crude compost.

reached an average level of emission concentrations of  102 (Tab. 2). The average loads ranged between 2  104 and 3  105 CFFU/LU*h (Tab. 3). Compared to the “cultivable” microorganisms, the amount of total microbial cells emitted by the facility was higher, for the bacteria ca. 100 times and for fungal cells ca. 3000 times. The emissions proved to be relatively constant with concentrations of 1.3  106 to 1.3  107 TBC/m3 and 1.7  105 to 1.6  106 TFC/m3 (Tab. 2). The analyses of the biofilter emissions, e. g. of the fungal cells, were often prevented by reaching the lower detection limit of the CAMNEA method (Palmgren et al., 1986). Nevertheles, on average a reduction of aerial total microbial cells of the plant’s waste air

could be established. For the bacterial cells, the reduction maxima of all filter materials were  90 %, although the Biochip, coconut fibre/peat and compost materials emitted sometimes more of them than were led in. In contrast to that, the pellet/bark and the bark/wood mixtures showed a constant (SD  5 %) reduction of the aerial load of about 90 % and more. For the fungal cells the assessment was heavily harmed by problems of the methodical detection limit, but the reduction was always at least  49 % for all filter materials, reaching maxima of over 90 % (Tab. 5). The plant’s waste air concentrations of the plant for airborne endotoxins of about 800 EU/m3 (average, max.  1,700, Tab. 2) were reduced remarkably by all

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339

Table 3. Emission rates (units per livestock unit and time) in the waste air and purified air behind different biofilters connected to a pig facility regarding different microbial bioaerosol parameters and odour/ammonia. Parameter

Filters

N=

waste air

F1

F2

F3

F4

F5

5

5

5

5

4

4

colony forming bacterial units CFBU/(LU*h)

Av. Min. Max.

3.4  107 2.6 106 1.5 108

 1.0  106  2.0 105  2.5 106

 4.1  105  2.7 105  5.2 105

 1.7  105  9.8 104  2.3 105

 4.9  105*  1.2 105  9.1 105

 9.4  105*  1.3 105  3.0 106

total bacterial cells TBC/(LU*h)

Av. Min. Max.

1.3  109 4.5 108 3.6 109

 5.3  108  6.3 107  1.1 109

 2.8  108  3.1 107  8.8 108

 1.4  108  4.7 107  3.6 108

 2.9  108**  3.2 107  6.8 108

 2.6  108*  4.1 107  6.3 108

colony forming fungal units CFFU/(LU*h)

Av. Min. Max.

6.6  105 1.8 105 1.9 106

 1.9  104  n.d.  3.8 104

 2.2  104  n.d.  6.9 104

 1.6  105  n.d.  4.9 105

 3.0  105*  9.6 104  4.8 105

 1.1  105*  7.7 104  1.6 105

total fungal cells TFC/(LU*h)

Av. Min. Max.

2.0  108 4.6 107 5.1 108

 3.0  107  2.1 107  4.3 107

 4.9  107  2.7 107  1.2 108

 3.5  107  2.9 107  4.7 107

 3.6  107**  3.1 107  4.2 107

 3.7  107*  3.1 107  4.2 107

airborne endotoxins EU/(LU*h)

Av. Min. Max.

6.4  107 1.2 107 2.0 108

 1.9  105  4.0 104  4.1 105

 8.8  104  4.6 104  1.5 105

 8.3  104  1.7 104  1.4 105

 4.0  105*  7.6 104  7.4 105

 5.8  105*  3.7 104  1.7 106

MVOC µg/(LU*h) total

4.200***

246***

254***

557***

197***

322***

Odour OU/(LU*s) Av. Min. Max.

159.3 77.0 299.8

21.1* 6.5 49.4

39.9 8.1 78.5

56.0 15.5 138.9

121.1* 32.4 260.1

52.0** 9.6 120.3

ammonia g NH3/(LU*h)

3.39 1.87 4.67

3.09 1.45 4.16

3.90 2.11 5.52

4.04 2.07 4.81

4.59* 2.46 6.35

4.70* 2.41 6.25

Av. Min. Max.

n. d. = not detected; * = (N = 4); ** = (N = 3), *** = (N = 1) ;  = influenced by detection limit. F1 = Biochips, F2 = coconut fibre/peat mixture, F3 = chopped bark and wood, F4 = BioContact filter pellets covered with bark, F5 = crude compost.

Table 4. Concentrations of MVOC in the waste air of a pig facility and behind different connected biofilters, in µg/m3. Parameter

Filters waste air

F1

F2

F3

F4

F5

3-Methylfuran Dimethyl-disulfide 2-Heptanone 3-Methyl-1-butanol 3-Octanone 1-Octen-3-ol 2-Pentanol 2-Hexanone Isobutanol 1-Butanol

 0,050  0,560  0,110  1,310  0,500  1,690  0,035  0,140  1,690  7,320

 0,050  0,810  0,026  n. d.  0,100  n. d.  n. d.  0,096  n. d.  n. d.

 0,050  0,680  traces  n. d.  0,057  n. d.  n. d.  0,070  n. d.  n. d.

 0,050  1,330  n. d.  n. d.  0,043  0,044  n. d.  0,022  n. d.  0,260

 0,050  0,410  0,018  n. d.  0,092  n. d.  n. d.  0,022  n. d.  n. d.

 0,050  0,530  n. d.  n. d.  0,018  n. d.  n. d.  0,011  0,060  0,290

total

13,360

 1,040

 0,810

 1,700

 0,540

 0,910

n. d. = not detected. F1 = Biochips, F2 = coconut fibre/peat mixture, F3 = chopped bark and wood, F4 = BioContact filter pellets covered with bark, F5 = crude compost.

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Table 5. Reduction efficiency (red. %) of different biofilters for different microbial bioaerosol parameters and the odour/ammonia emissions of a pig fadility. Parameter

Filters

N=

F1

F2

F3

F4

F5

5

5

5

4

4

colony forming bacterial units red.%

Av. SD Min. Max.

69.5 31.6 12.7 99.1

88.1 8.2 74.5 99.7

94.9 3.2 92.0 99.9

92.4* 7.0 80.9 99.5

82.5* 22.8 43.4 99.8

total bacterial cells red.%

Av. SD Min. Max.

25.2 69.0 –95.8 92.6

() 71.1 () 43.9 () –16.7 () 96.2

() 91.9 () 3.6 () 86.9 () 96.4

() 87.6** () 4.8** () 81.7 () 93.3

() 65.3* () 51.1 (–23.1 () 96.2

colony forming fungal units red.%

Av. SD Min. Max.

69.3 18.6 50.0 100.0

74.7 22.7 48.6 100.0

–72.1 –210.7 –475.0 –100.0

–408.6* – 443.0 –1150.0 –15.8

–99.0* –141.6 –300.0 – 79.0

total fungal cells red.%

Av. SD Min. Max.

() 71.6 () 19.1 () 48.8 () 92.8

() 68.8 () 16.6 () 48.8 () 87.6

() 71.6 () 19.1 () 48.8 () 92.8

() 61.7** () 18.3** () 48.8 () 87.6

() 66.4* () 17.7 () 48.8 () 87.6

airborne endotoxins red.%

Av. SD Min. Max.

96.2 2.6 90.5 99.2

98.4 0.9 96.9 99.5

98.6 1.0 96.9 99.7

93.1* 5.5 83.8 97.8

88.1* 17.0 58.8 99.3

92***

94***

87***

96***

93***

MVOC red.% odour red.%

Av. SD Min. Max.

83.5* 11.9* 63.0 91.8

78.8 8.0 65.7 90.1

72.1 15.3 50.0 90.5

–40.3* –32.8 –4.8 –80.0

75.2** 14.8** 57.6 93.9

ammonia red.%

Av. SD Min. Max.

–8.4 –8.6 –3.6 –19.7

–5.3 –24.4 –40.3 –35.5

–3.5 –14.6 –26.4 –19.7

–18.8* –14.3 –43.1 –6.3

–13.6* –11.5 –30.6 – 1.7

Av. = average; SD = standard deviation; Min. = minimum; Max. = maximum. n. d. = not detected; * = (N = 4); ** = (N = 3), *** = (N = 1); () () = influenced by detection limit. F1 = Biochips, F2 = coconut fibre/peat mixture, F3 = chopped bark and wood, F4 = BioContact filter pellets covered with bark, F5 = crude compost.

tested filter materials. Four of the filters showed an average reduction of  90 %. E. g. the Biochips, the coconut fibre/peat and the bark/wood mixture reached a very constant emission reduction (SD 0.9 to 2.6 %) with maximal clearances of over 99 % (Tab. 5). For the compost filter, also reaching this maximum clearance, the calculated average value (only 88 %) was influenced by the result of a single measurement, as an endotoxin concentration of 125 EU/m3 was determined for the outgoing air. The MVOC emitted by the facility were only tested once (Tab. 4). They mainly consisted of 1-butanol, isobutanol, 3-methyl-1-butanol and 1-octen-3-ol, and were remarkably reduced (87 to 96 %) by all biofilter

materials (Tab. 5). Only in case of dimethyl-disulfide, which was found in a concentration of 0.56 µg/m3, most of the filters (exception the pellet/bark mixture) emitted slightly higher concentrations (max. 1.7 µg/m3). The odour concentrations in the waste air varied between 770 and 3.100 OU/m3, leading to odour emission factors between 77 and 300 OU/LU*s. The concentrations in the purified air varied from 63 to 1.300 OU/m3, and the emission factors ranged between 8 and 260 OU/LU*s (Tab. 2 and 3). The results showed obvious differences in the reduction efficiency between the different filter materials (Table 5). Four of the filters reduced the odour concentrations by  70 %, excelled by the Biochips (approximately 83 %), and relatively

Bioaerosol and odour reduction by biofilters

constant (SD 8 to 15 %). The pellet/bark mixture reached only 40 % effectivity (av.) and a SD of 33 %, because the reduction varied between – 4.8 and 80 %. Regarding the ammionia concentrations, hardly any differences could be detected comparing the waste and the purified air (12.4 to 16 ppm, Tab. 2). The ammonia emission loads in the waste and purified air varied between 1.45 and 6.35 g NH3/LU*h (Tab. 3). The filters changed the emissions between –43.1 and 35.5 % (Tab. 5). On average, only the Biochips reached a slight ammonia reduction of the waste air of ca. 8.4 %. Whereas the coconut fibre/peat and bark/wood mixtures showed only slightly higher average emissions, the pellet/wood mixture and the compost always released more NH3 than was in the waste air, leading to additional emissions of ca. 19 % or 14 %, respectively. Regarding all data, none of the filter materials turned out to be the best, concerning the reduction of all emissions investigated. As shown in Fig. 2, comparing the average reduction efficiencies of the five filters for microbial bioaerosols as well as for odours and NH3, no clear positive or negative relationship between the parameters could be discovered. No influences of the bioaerosol waste air load and the resulting emissions of the biofilters were detectable. The three

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filter materials Biochips, coconut fibre/peat mixture and compost turned out to be best to reduce the odour emissions. Nevertheless, they emitted the highest concentrations of both cultivable and total bacteria, leading to a negative ranking of the filters regarding odour clearance efficiencies and those of bacterial emissions.

Discussion In contrast to several studies on odour and VOC emission reduction, there is only few data available on microbial bioaerosol clearance efficiency and emissions of biofilters, especially in the field of agriculture. It might be stressed here, that also the results of this study do not allow generalizing conclusions. At first, the examined pig facility represented an experimental unit in a good hygienic state. For that reason, the measured bioaerosol concentrations and emitted loads (units/ LU*time) have to be relativized or modified, as it is well known, that under “normal” practical conditions other situations may occur, leading to higher concentrations of microbial air pollutants inside of piggeries (Clark et al., 1983; Cormier et al., 1990; Crook et al., 1991; Heederik et al., 1991; Dutkiewitz et al., 1994;

Fig. 2. Comparison of the clearance efficiency of different biofilters connected to a pig fattening station, regarding bioaerosol, odour and ammonia reduction. F1 = Biochips™, F2 = coconut fibre/peat mixture, F3 = chopped bark and wood, F4 = BioContact filter pellets covered with bark, F5 = crude compost, error balk = standard failure.

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Seedorf et al., 1998; Radon et al., 1999) and by this to higher loadings for biofilters and perhaps resulting higher emissions. At second, all biofilters of this study were treated carefully and were in an optimal status during the investigations, e. g. with regard to the water content, which is of great importance for a biofilter function (Auria et al.,1998; Kennes and Thalasso, 1998). No drying out effects occurred, leading to breaches in the filter material and by that to short cut streams as well as a reduced efficiency. At next, the time period of the investigations was only short, and for that reason aspects of seasonal influences, aging effects of the filter materials etc. could only be investigated marginally. This was done by an additional measuring day approximately 2 months after the first set, which was carried out within of only a few days. Within these two months no changes of the overall filter status, regarding function and emissions, could be observed. Due to technical reasons it was also not possible to collect all samples before and behind all filters in a strict parallelism of time. This would have been useful to evaluate the emissions and reduction efficiencies by comparing the filter inlet and outlet concentrations. Variations of the particle concentrations of the waste air due to animal activities may have influenced the measuring values. Nevertheless, despite these restrictions some conclusions may be drawn. As mentioned, the concentrations of ca. 105 CFBU/m3 (av.) of cultivable bacteria detected in the waste air were within the same magnitude as found in other pig housings (Clark et al., 1983; Cormier et al., 1990; Crook et al., 1991; Heederik et al., 1991; Seedorf et al., 1998), although also higher concentrations have been reported (Crook et al., 1991; Dutkiewitz et al., 1994; Radon et al., 1999). These emissions were always reduced by the biofilters, on average between 69 and 95 % (12.7 to 99.9 %), thus leading to only low average filter emissions of 105 to 106 CFBU/ LU*h. This is plainly below the emitted average loads, which have been reported for other pig facilities without waste air cleaning by biofilters (appr. 5  107 CFBU/ LU*h, Seedorf et al. 1998). Such low dose emissions are in correspondence to the findings of Ottengraf and Konings (1991), who investigated the microbial bioaerosol emissions of six full-scale biofilters with defined working conditions and installed in different industrial (nonagricultural) fields over a long period of time. Although not always investigated, the filter inlet concentrations varied within 9.3  102 and  2  104 CFBU/m3, whereas the filter outlet concentrations were determined as 1.0  103 to 9.3  103 CFBU/m3. The emissions of three biofilters installed at biological waste treatment plants ranged within the same average magnitude (Fanta et al., 1999), although peak concentrations of 4.5  104 CFBU/m3 were also determined. Even higher outlet

peak concentrations, up to 3.7  105 CFBU/m3, were detected by Schilling et al. (1999), who also investigated the emissions of biofilters connected to waste treatment plants, and by Seedorf and Hartung (1999) in investigations on two biofilters installed at two pig facilities. These differences may partly be due to differences in analytical methods, but may also be due to differences in the biofilters or biofilter states, as in these studies no detailed informations are given on the biofilter working and maintenance conditions. E. g. bad maintaining conditions, for instance effects of drying out and not adequate filter humidities may have an important influence on the microbial biofilter emissions and its clearance efficiencies, as mentioned before. The concentrations of airborne cultivable fungi in the waste air were remarkably constant and low, at ca. 2.2  102 and never exceeding the benchmark of 103 CFFU/m3. Thus they were within a range of concentrations, which are usually found as “background” in the normal environmental air. As it is well known that higher concentrations can be found for other swine confinements (Crook et al., 1991; Seedorf et al., 1998; Radon et al., 1999), this may be caused by the good hygienic conditions of the experimental pig housing. Also the amount of cultivable fungi emitted by the biofilters, never exceeding concentrations of 103 CFFU/m3, can be assessed as within the normal background range, although only two of the filters apparently reduced the aerial load in all cases, whereas three of them sometimes emitted higher counts of cultivable fungi than they had taken up, leading to proportional increases of the load up to 400 %. This is in correspondence to the results of Ottengraf and Konings (1991), who also found a generally low biofilter emission of cultivable moulds combined with a sometimes higher emission of the filters in cases of low spore loads of the ingoing air. In contrast to that, for biofilters installed at biological waste treatment plants slightly higher average ( 103) emission concentrations and peak values  105 CFFU/m3 have been reported (Fanta et al., 1999; Schilling et al., 1999). Up to now it cannot be evaluated whether this is due to the different kind of aerosol sources or to different filter states and maintenance conditions. Although it is of minor importance for the assessment of possible allergic and toxic emission-caused health risks, whether a microbe is capable of growing on agar plates, up to now no data have been available about biofilter emissions of total microbial cells. Within this investigation, a great discrepancy between the total aerial load (“living and dead”) emitted by the facility and its share detectable by culture based techniques could be stated. The concentrations of TBC/m3 in the plant waste air exceeded the CFBU/m3 by a factor of ca. 100, whereas on average more than 1.000

Bioaerosol and odour reduction by biofilters

times higher concentrations of TFC/m3 were detectable compared to the number of CFFU/m3. Regarding the bacteria, this relationship is coincident to the findings of Radon et al. (1999), who also investigated the amount of airborne cultivable bacteria in pig facilities in comparison to the total counts determined by the CAMNEA method. For the fungi, 10 % of the total counts could be cultivated. This difference may be due to differences in methods, as far higher average concentrations (3.8  105 CFFU/m3) of airborne cultivable fungi were detected in pig facilities by Radon et al. (1999), compared to the findings of others (Clark et al., 1983; Cormier et al., 1990; Crook et al., 1991; Seedorf et al., 1998). Radon et al. (1999) used three different nutritient media for fungal detection in their investigation. This may have included the growth of some fungi, for instance Candida, into the measurement value calculation, which cannot be detected by the media used in the other studies mentioned, and e. g. not by using the DG 18 agar as done in this investigation, following the protocol of the German standard method (Anonymous, 1997). In spite of the high detection limits of the CAMNEA method, all biofilters within this investigation proved to be capable to reduce the aerial load of total cells, sometimes to a high extent ( 90 %). The amount of emitted total fungal cells was always reduced. In contrast, regarding the bacteria, sometimes more total cells were emitted than were in the inflated air before. E. g. the Biochips, emitting also the highest concentrations of cultivable bacteria, showed the lowest reduction efficiency (25 %) with regard to the TBC/m3. A hygienic assessment of the microbial cell loads emitted by the biofilters is currently difficult if not impossible, as not enough data describing “normal” environmental concentrations is available and therefore cannot give the basis for such an assessment. In case of airborne endotoxins, “normal” background concentrations are within a range of  0.1 to 1 EU/m3. The pig facility investigated in this study emitted approximately 800 EU/m3, which is within the lower, but still normal range reported for other pig housings (Clark et al., 1983; Heederik et al., 1991; Dutkiewitz et al., 1994; Seedorf et al., 1998; Radon et al., 1999). All biofilters tested reduced this aerial load remarkably with reduction efficiencies between ca. 93 to 99 %. There was one exception, the compost biofilter, only reaching 88 % reduction (av.), as the calculation values were heavily influenced by the detection of a single peak value of 125 EU/m3 in the outgoing air. But, as detected only once, this single value must be assessed carefully, as, for instance, a contamination of the sample with gram-negative bacteria emitted by the filter cannot be excluded. Without this single value, also the compost biofilter reached an average endotoxin clearence efficien-

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cy of 98 %. It may be a common quality of the most well maintained biofilters, that they are well capable to reduce the amount of airborne endotoxins, because biofilters are humidified systems and endotoxins are well water soluble. But this has to be further investigated. Regarding the MVOC, the reduction efficiency of the biofilters was investigated only once, and thus the data have to be assessed carefully. Nevertheless, a good clearance efficiency of ca. 90 % and more was determined. This is not surprising, because the plant emitted only a total concentration of 13.4 µg/m3, mainly consisting of 1-butanol, isobutanol, 3-methyl-1-butanol and 1-octen-3-ol , and the sufficient elimination of by far higher loads for these (and other) substances have been reported, if the biofilter technology is used to eliminate these components as main pollutants of a plant’s waste air (Kennes and Thalasso, 1998). Within this investigation, the MVOC were only regarded as possible markers for microbially caused air pollution, e.g such caused by the biofilter emissions. Dimethyldisulfide may be of importance under this aspect, as in most cases the concentrations increased by passing through the filters, thus indicating an intensified degradation of sulfuric compounds in the filter material or in-trapped particles. But this also has to be further investigated. On average, the detected odour reduction of the filters (40 to 83 %) confirm the data of previous experiments (40 to 80 %, Mannebeck, 1995; Hartung et al., 1997; Hopp, 1998), and increasing odour concentrations in the waste air were found to be the main factor (data not shown) for the odour reduction efficiency (Martinec et al. 2000). The ammonia emissions of this facility confirmed the References data (ca. 1 to 7 g/LU*h, Monteny, 1992; Groot Koerkamp et al., 1998; Stegbauer, 1999). Our ammonia reduction rates (–18.8 to 8.4 %) confirmed those of References (–40 to 90 %) (Mannebeck, 1995; Hartung et al., 1997; Hopp, 1998). The release of NH3 from filter material has been caused by a very high filter volume load (app. 550 m3/m3*h), as it is known that the ammonia reduction decreases with increasing filter volume loads (Mannebeck, 1995; Hartung et al., 1997). Thus, in long term experiments with the same materials (data not shown) an ammonia reduction of about 9 to 33 % was established with a filter volume load of 450 to 500 m3/m3*h (Martinec et al., 2000). In comparison to odour reduction biofilters are unsuitable for NH3-reduction. Regarding the whole data, no correlations between the odour and NH3 emissions and reduction efficiencies of the biofilters could be found, as has been reported before (Oldenburg, 1989). Also no clear relationships seem to exist or could be detected between the odour/NH3 emission/reduction of the biofilters com-

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pared to the emitted/reduced bioaerosol emissions. As a tendency, a negative correlation between those filters best capable to reduce odour loads and their bacterial emissions, both cultivable and total counts, could be stated. But as the differences of the filter emitted bacterial parameters varied only within a slight range, this has to be further evaluated before drawing any further conclusions. As biofilters are biological systems and thus sources for microbial emissions, their emitted microbial loads are always a sum consisting of not deposited microbial particles originating from the plant waste air and microbial particles blown off from the filter material surfaces by the throughflowing air stream. It is difficult to assess the proportional part of the microorganisms emitted by the biofilter itself. Within this investigation no relationship between the microbial load of the waste and the purified air could be detected nor has been reported elsewhere up to now, in contrast to an existing relationship between load and reduction efficiency/release of odour (Mannebeck, 1995; Hartung et al., 1997; Martinec et al., 2000). But, since a principle reduction efficiency for all tested microbial parameters of  90 % (max.), independent from the burden, could be stated for all filter materials tested within this study, it has to be assumed that their emitted microbial bioaerosol loads are prevailed by the filters own emissions. Ottengraf and Konings (1991) developed a mathematical model to predict and describe those emissions. According to this a (well maintained) biofilter, fed with a constant gas as a nutritient for the inherent microbes and operated with a constant air stream, should emit relatively constant amounts of microbial bioaerosols. Regarding the data from this investigation, this can be confirmed in general. As the first tests were performed within only a few days without major changes of the facility conditions (animal density) and as the waste air volume stream was held constant ( 10 % differences), constant conditions for the filters can be demanded. Assuming a “normal” variation of the resulting measurement values within one log caused by using biological test systems, the concentrations and the emitted loads of the biofilters regarding the cultivable bacteria and the endotoxins (the latter under exclusion of the one high measurement value mentioned, the fungi not regarded because of low concentration and detection limit uncertain) proved to be relatively constant, only slightly exceeding the factor ten between min. and max. concentrations. Lesser fitting into this were the emissions of the total bacterial cells, varying within a broader range between min. and max. than the other bioaerosol parameters. Whether the measurement values have still been influenced by concentration variations of filter passing cells originating from the plant,

or whether this parameter indicates a limit for the model under practical conditions, cannot be assessed at the moment. Acknowledgement. The authors are grateful to the Geschwister Stauder Foundation, Hohenheim, for funding this investigation.

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