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Odorous composting gas abatement and microbial community diversity in a biotrickling filter
International Biodeterioration & Biodegradation 82 (2013) 73e80
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Odorous composting gas abatement and microbial community diversity in a biotrickling filter Niantao Xue a, b, Qunhui Wang a, c, *, Juan Wang a, Jianhua Wang a, Xiaohong Sun d a
School of Civil and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China School of Environment, Tsinghua University, Haidian District, Beijing 100084, PR China c Key Laboratory of Educational Ministry for High Efficient Mining and Safety in Metal Mine, University of Science and Technology Beijing, Beijing 100083, PR China d Beijing Agricultural Biotechnology Centre, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100089, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2012 Received in revised form 2 March 2013 Accepted 3 March 2013 Available online 9 April 2013
This study aimed to remove complex odorous gas produced from composting using a biotrickling filter and to observe the temporal and special distributions of bacteria, fungi, and actinomycetes. The removal efficiencies of the total volatile organic compounds (TVOC) were 26.1% and 81.5% before and after inoculation of volatile organic compounds (VOC)-degrading microbes, respectively. Especially trimethylamine was 100% degraded. In the first and second composting period, the odor reduction efficiencies showed average values of 86.2% and 94.5%, respectively. The total average of the bacteria in the biofilm was 2.06 109 CFU/g TS, which was 22.2% higher than that of the control (the culture of microbes prior to the inoculation of VOC-degrading microbes). The bacteria may have played a predominant role in odor removal. The total average of the fungi in the biofilm was 9.64 106 CFU/g TS, which was only 6.40% of the control. The total average of the actinomycetes in the biofilm was 5.10 105 CFU/g TS, which was 5.63 times higher than that of the control. Findings from this study showed that usage of a biotrickling filter is a promising process for the treatment of complex odorous gas. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biotrickling filter Composting Odor Volatile organic compounds (VOC) Bacteria Fungi Actinomycetes
1. Introduction Odorous gas emitted from composting facilities has complicated components, which necessitate efficient, environment-friendly, and cost-effective treatments. The main components (i.e., pollutants) of the odorous gas are (1) nitrogen-containing compounds, such as NH3 (Komilis and Ham, 2006) and N2O (Fukumoto et al., 2003); (2) sulfur-containing compounds, such as H2S; (3) volatile organic compounds (VOC) (Akdeniz et al., 2010); and (4) bioaerosols (i.e., principally airborne microorganisms and microbial constituents released from composting processes where movement of material is involved) (Sanchez-Monedero et al., 2003). In addition, the components and their concentrations exhibit significant changes at different composting stages. The physicochemical properties of odorous gas are diversified. Easily and adversely biodegradable matters coexist, and some are
* Corresponding author. School of Civil and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China. Tel./fax: þ86 10 62332778. E-mail addresses: [email protected] (N. Xue), [email protected] (Q. Wang). 0964-8305/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ibiod.2013.03.003
either hydrophilic or hydrophobic. Hydrophilic substances have diverse solubilities in water. Hydrophobic substances are not readily available to microorganisms, and thus inadequate for use in biological treatments (Hassan and Sorial, 2010). These properties demonstrate that effective treatment of odorous gas is difficult. In the last few decades, emission control of VOC and other odorous pollutants has become a crucial issue owing to their adverse effects to humans, animals, and the environment. Most VOC are toxic and carcinogenic substances; thus, loss of these substances to the ambient air may have an adverse impact on air quality and endanger public health (Yoon and Park, 2002). Anthropogenic activities will influence the conversion of natural VOC into condensable vapors to generate natural aerosols and thus, further affect climate (O’Dowd et al., 2002). Therefore, it is very important to develop effective technologies to remove these compounds to preserve human health and the environment. Trimethylamine (TMA), one of VOC is a malodorous aliphatic amine frequently identified in gaseous emissions of multiple industrial and agricultural processes. Compared with ammonia, TMA can be perceived and detected at greater distances because of its characteristics, including persistent intensive odor and very low odor detection thresholds (Goldstein, 2002). TMA poses serious
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ecological and environmental issues and is a strong environmental pollutant. Health effects associated with inhalation of TMA include irritation of the respiratory tract, eyes, and skin. Consequently, the sensitive determination of TMA in atmospheric and human work environments is of great importance (Cháfer-Pericás et al., 2004). Thus, TMA and other VOC disposal through usage of a biotrickling filter (BTF) was studied in this paper. Biological processes for odor treatment, including bioscrubbers, biofilters, and biotrickling filters, are promising odor abatement technologies that take advantage of the ability of microorganisms to remove substrates from odorous organic compounds (Canovai et al., 2004). Their development owes its increasing popularity to two of its advantages: (1) It is operated at ambient temperatures (15e30 C); (2) It does not produce toxic by-products (Delhoménie et al., 2002). In a biotrickling filter, waste air streams pass through a packed bed of synthetic inert material where particular microbes are immobilized to form a thin aqueous layer (biofilm) (Zilli et al., 2007). Biofilters are more popular than biotrickling filters in the treatment of odorous composting gas. The latter are more complex and more expensive than biofilters but are usually more effective, especially in the treatment of compounds that are difficult to degrade or those that generate acidic by-products, such as H2S (Cox and Deshusses, 2000). Biotrickling filters have seldom been used to treat wastecomposting gas. The reason biotrickling filters are preferred over biofilters is that they contain trickling liquid which helps avoid dryness of the packing material and allows removal of metabolites produced during degradation which can then be recycled. Smits et al. (1995) applied a biotrickling filter to treat ammonia and odor from a composting facility. The biological elimination capacity was 4 g NH3/(m3$h), and the odor removal efficiency was 50% for odor loads as high as 5 o.u./(m3$s). Pei et al. (2008) revealed that a constant TVOC removal efficiency, an odor concentration above 70% and a maximum elimination capacity of 130 g/(m3$h) can be achieved. Mao et al. (2006) found that biotrickling filters have better deodorization capability for odor from food waste-composting plants than the biofilter and the chemical scrubber with deodorization efficiencies measured according to odor concentrations of 82%, 59%, and 45%. It is expected to improve biotrickling filter removal efficiency of the odor concentration. Although the performance of biotrickling filters depends on the type of microorganisms present, reports on the microbial community in biotrickling filters remain scarce. Inside the biofilm of biotrickling filters or biofilters, biodegradation is mediated by mixed cultures of bacteria, fungi, actinomycetes, and algae, all thriving in a complex ecosystem. Secondary pollutant degraders and predators, such as protozoa, metazoan, and other higher organisms, are also included. Investigating changes in the microbes in biotrickling filter will promote research on the microbial degradation mechanism, optimization of design and operation of biotrickling filters. This study aims: (1) to gain an insight into how a biotrickling filter effectively eliminates complicated composting gas; (2) to determine the removal efficiency of biotrickling filter for odor concentration; and (3) to find out the spatial and temporal distribution of microbial community in biotrickling filter. 2. Materials and methods 2.1. Equipment The composting reactor schematic representation and the biotrickling filter setup are shown in Fig. 1. The upper part of the filter was the biotrickling section, with a working volume of 5.0 L. A perforated plexiglass plate, which served as gas and liquid
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Fig. 1. Composting reactor schematic representation and the biotrickling filter setup. (1) Air compressor; (2) time relay; (3) composting air inlet pipe; (4) perforated plate; (5) composting sampling port; (6) heating belt; (7) automatic temperature controller; (8) composting reactor; (9) composting exhaust gas sampling port; (10) BTF air inlet pipe; (11) perforated plate; (12) trickling liquid holding tank used as scrubber section; (13) biotrickling section of BTF with packing; (14) BTF; (15) liquid distributor; (16) micro pump; (17) voltage- and current-steady power supply; (18) time relay; (19) gas sampling port; (20) air exhaust; (21) trickling liquid pipe.
distributor, was placed at the bottom of the biotrickling section. Packing material was supported on the plexiglass plate. About 1.8 L of trickling liquid in the holding tank was fed by a pump to the top of the BTF. It trickled through the packing material to the liquid distributor. The inlet gas pipe of the biotrickling filter was placed under the trickling liquid, which formed the scrubber section, to employ the absorption capacity of trickling liquid. The properties of the packing material, the operation of the composting bioreactor, and other specifications were given by Xue et al. (2010). 2.2. Gas sampling and analysis Sampling ports were set on top of the composting bioreactor and on each section of the biotrickling filter. Gas samples were collected from the inlet and outlet streams using a gas sampler (Model QS-1S, Beijing Municipal Institute of Labor Protection, China). Trimethylamine was transferred into an aqueous solution and then analyzed using the picric acid spectrophotometric method (SBPCI, 1999). Hydrogen sulfide was determined using gasdetection tubes made by the Beijing Municipal Institute of Labor Protection, China. TVOC was analyzed using gas chromatography (Perkin Elmer clarus600Gc-Turbomatrix ATD650, column: Elite624 30 m*320 mm, detector: FID) at the Center for Test of Environmental Quality, Tsinghua University, China (EBSEPA, 2003). Odor concentration (without unit) was measured through olfactometry, in accordance with the triangle odor bag method (SEPA, 1993). Ammonia was determined according to Nessler’s reagent colorimetric method (SEPA, 1993) (SEPA etc. are abbreviations of corresponding organisms who issue the methods). 2.3. Enrichment and screening of VOC-degrading microbes The selective inorganic salt medium consisted of the following (per liter): NaCl 1.0 g, MgSO4$7H2O 0.7 g, NH4C1 1.0 g, KCl 0.7 g, KH2PO4 2.0 g, Na2HPO4 3.0 g, and pH 7.0. A trace element solution was added after autoclaving. The trace element consisted of the following (per liter): CaCl2 0.2 mg, FeCl3$6H2O 0.5 mg, CuSO4 0.005 mg, MnCl2$4H2O 0.005 mg, and ZnSO4$7H2O, 0.1 mg (Wang and Shao, 2006). The microbial enrichment medium consisted of 1 g of peptone þ1 L of selective inorganic salt medium.
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In a previous determination, styrene, m-xylene, and chloroform were the primary refractory components in the exhaust of composting gas. In the present study, 100 mL selective inorganic salt medium, 5 mL styrene, m-xylene, and chloroform were transferred into two 250 mL aerobic oscillation flasks. Ten mL mixed liquor from the Qinghe Wastewater Treatment Plant and the Coking Plant of the Shougang Group, respectively, were added as initial microbial consortia. The culture was performed in a shaking incubator at 30 C and 150 rpm for a 2-day culture period. At the end of each culture period, 10 mL cultured suspension and 10, 20, and 30 mL of styrene, m-xylene, and chloroform, respectively, were added into 100 mL fresh medium for the next period. After four culture cycles, VOC-degrading microbes that could degrade VOC were enriched in the microbial enrichment medium. 2.4. Biomass determination, separation, and count of bacteria, fungi, and actinomycetes Separation and count of bacteria, fungi, and actinomycetes was performed according to the conventional dilution plate count described by Chen and Zhang (2006). Three packing spheres at the top, medium, and bottom, respectively, were removed from the biotrickling filter. The biomass on each of the spheres was peeled off into three sterilized beakers, and then diluted to 200 mL as initial consortia. The initial consortia were diluted to the appropriate multiples by ten-fold dilution method to make microbe suspension. Take 0.1 mL of microbe suspension and inoculated on the surface of solid medium in petri dishes. The dishes were put into culture incubator under appropriate temperature. Three replicates were performed for all samples. The other biomass was evaporated to dryness on a water bath and then dried in a 103 Ce 105 C dry box. Incandesce in a 600 C muffle furnace was used to determine the total solids (TS) and the volatile solids (VS). The trickling liquid was diluted to the appropriate multiples by ten-fold dilution method for separation, and count of bacteria, fungi, and actinomycetes. Bacterial culture medium (beef extract, peptone, agar medium): beef extract 3.0 g, peptone 5.0 g, agar 18.0 g, sterile distilled water 1000 mL, pH 7.0 to 7.2. Petri dishes were culture in the 32e34 C incubator for 3 days. The fungal culture (Matin medium): peptone 5.0 g, glucose 10 g, K2HPO4 1.0 g, MgSO4$7H2O 0.5 g, agar 15 g, 1% rose bengal solution of 3.3 mL, water 1000 mL, normal pH value. When the medium was used, 0.3 mL of 1% streptomycin solution was added to each 100 mL medium. Petri dishes were put in the 30 C incubator for 5e7 days. Actinomycetes culture medium (modified Gao I medium): soluble starch 20.0 g, K2HPO4 0.5 g, FeSO4$7H2O 0.01 g, KNO3 1.0 g, MgSO4$7H2O 0.5 g, NaCl 0.5 g, agar 18.0 g, distilled water 1000 mL, pH 7.2e7.4. When the melted medium was used, K2Cr2O7 solution was added to inhibit the growth of bacteria and mold. One mL of 3% K2Cr2O7 solution was added to each 300 mL medium. Petri dishes were put in the 28 C incubator for 7e14 days. 2.5. Experimental process Before the VOC-degrading microbes were inoculated, odorous gas from the composting reactor entered the biotrickling filter primarily to culture the microbes in the packing material. Bacteria, fungi, and actinomycetes were counted and considered as the control. Two hundred mL of microbial enrichment media containing VOC-degrading microbes were mixed with the trickling liquid, and the microbes gradually in the biofilm of the biotrickling filter. Then, the biotrickling filter was used to treat the exhaust gas two composting periods. During the two periods, tap water and municipal sewage from an office building in the University of
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Science and Technology Beijing, respectively, served as the trickling liquid. After 1 day, 50 mL trickling liquid was collected for water quality analysis, and 50 mLe60 mL of tap water and municipal sewage, respectively, were made up every day. Apart from NaOH, which regulated the pH values, no nutrient was added during the entire experiment. 3. Results and discussion 3.1. Removal of VOC and other pollutants before and after the inoculation of VOC-degrading microbes The removal of VOC and other pollutants before and after VOC-degrading microbes inoculation is shown in Table 1. The inoculation of VOC-degrading microbes notably enhanced the abatement of VOC. After inoculation, the removal efficiencies of toluene, ethyl acetate, and other VOC in the scrubber section decreased. However, those of benzene, ethylenzene, p (m)-xylene, styrene, and o-xylene improved. Removal efficiencies of VOC in the biotrickling filter exhibited similar trends. The total removal efficiencies of almost all VOC, except for styrene, were increased. The total removal efficiencies of TVOC increased from 26.1% to 81.5%. Prior to the inoculation of VOC-degrading microbes, TVOC removal efficiencies in the scrubber section and in the biotrickling section were 58.0% and 76.0%, respectively. These results indicate that TVOC was removed in the scrubber section, and new VOC was produced after the exhaust gas passed through the biotrickling section. After the inoculation of VOC-degrading microbes, TVOC removal efficiencies in the scrubber and in the biotrickling section were dramatically enhanced by the microbes to 56.7% and 57.3%, respectively, with a total removal efficiency of 81.5%. This result demonstrated that the microbes could metabolize VOC in the influent. Hence, inoculating the VOC-degrading microbes necessary. TMA, which is readily soluble in water, was completely removed in the scrubber and in the biotrickling section, probably due to its easy biodegradation in the biofilm. Kim et al. (2005) claimed that hydrophilic substances are easily degraded, whereas the degradation of hydrophobic substances is hindered until biological cultures produce sufficient RNA or enzyme/protein to use these substances. However, Tsai et al. (2008) noted that TMA is difficult to biodegrade because microbes cannot easily break the molecule. Some microbes can probably degrade TMA in the biotrickling filter effectively. Ammonia, which accounted for 90.4% and 94.5% of the total pollutants in the two tests, respectively, was efficiently eliminated. Hydrogen sulfide was not detected because an aerobic, rather than an anaerobic, composting process was employed in this study. 3.2. Reduction in odor concentration when tap water or municipal sewage served as trickling liquid The presence of trickling liquid is the central characteristic that distinguishes the biotrickling filter from the biofilter. In this study, trickling liquid was the water trickled over the bed in the biotrickling filter driven by the continuous recirculation of the liquid in the holding tank. At certain periods, make-up water, tap water or municipal sewage, was added to the reactor to compensate for trickling liquid losses due to evaporation, sampling, purging, and so on. Trickling liquid provided a convenient means to control pH, salt, or metabolite concentration, and supplemented nutrients to the biomass. The additional nutrients, pH control, and larger gas/liquid interfacial area resulted in substantially higher removal efficiencies than that of biofilters (Mpanias and Baltzis, 1998). Odor concentration is a key integrated air quality indicator in the verification of odor emissions. To date, few investigations have
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Table 1 Removal of VOC and other pollutants before and after the inoculation of VOC-degrading microbes. Gas
Inoculation
Influent (mg/m3)
Effluent in scrubber section (mg/m3)
Effluent in biotrickling section (mg/m3)
REa in scrubber section (%)
RE in biotrickling section (%)
Overall RE in biotrickling filter (%)
Benzene
Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After
0.006 0.054 0.071 0.132 0.016 0.050 0.012 0.037 0.024 0.069 0.010 0.021 0.013 0.054 0.005 0 9.120 8.091 9.277 8.508 2.308 10.6 0 0 109 331
0.010 0.036 0.043 0.081 0.023 0.121 0.012 0.022 0.029 0.036 0.012 0.011 0.017 0.015 0 0 3.749 3.361 3.895 3.683 0 0.018 0 0 8.85 13.9
0.006 0.035 0.055 0.024 0.013 0.027 0.010 0.018 0.023 0.038 0.010 0.023 0.013 0.011 0.055 0 6.674 1.398 6.859 1.574 0 0 0 0 1.88 8.67
66.7 33.3 39.4 38.6 43.8 142 0 40.5 20.8 47.8 20.0 47.6 30.8 72.2 100 / 58.9 58.5 58.0 56.7 100 99.8 / / 91.9 95.8
40.0 2.78 34.9 70.4 43.5 77.7 16.7 18.2 20.7 5.56 16.7 109 23.5 26.7 / / 78.0 58.4 76.1 57.3 / 100 / / 78.8 37.6
0 35.2 22.5 81.8 18.8 46.0 16.7 51.4 4.17 44.9 0 9.52 0 79.6 1000 / 26.8 82.7 26.1 81.5 100 100 / / 98.3 97.4
Toluene Ethyl acetate Ethylenzene P (m)-xylene Styrene o-xylene Undecane Other VOC TVOC Trimethylamine Hydrogen sulfide Ammonia
RE in scrubber section (%) ¼ (Influent Effluent in scrubber section)/Influent 100. RE in biotrickling section (%) ¼ (Effluent in scrubber section Effluent in biotrickling section)/Effluent in scrubber section 100. Total RE in biotrickling filter (%) ¼ (Influent Effluent in biotrickling section)/Influent 100. a RE: Removal Efficiency.
been conducted on odor concentration during a composting process, especially the control of odor concentration biological processes. In this study, the odor concentrations of the gas entering and leaving the biotrickling filter was measured and is shown in Fig. 2. During the first period, inlet odor concentrations increased from 750 to 9000. The odor reduction efficiencies of the biotrickling section ranged from 42.9% to 91.7%, with an average efficiency of 86.2%. During the second period, odor reduction was more stable, ranging from 87.5% to 98.3% with an average efficiency of 94.5%. The inlet concentrations during the second period were higher than those during the first period. In general, odor reduction was improved during the second period. Chen et al. (2009), Lau and Cheng (2007) found that the odor concentrations of untreated barn air varied from 8553 1006 OU/m3 to 12171 1575 OU/m3, and that the average odor removal efficiency of a biofilter system was 95 3%.
long-term stable operation of biotrickling filters. Various methods have been evaluated to control biomass accumulation and to prevent clogging. Examples are reduction of biomass growth and removal of excess biomass. The former includes nutrient limitation, addition of growth inhibitors, and use of predators that prey on other microbes the latter includes chemical washes, backwashing, periodic, and stirring of the packed filter. Reduction of biomass growth usually causes decreases in the removal efficiency. In this study, the nutrients available for the microbes were limited. However, the pollutant removal efficiencies were never reduced, as microbes readily prefer biodegradable substrates. When the substrates were not sufficient for growth, the microbes were forced to metabolize other substrates. The inlet pollutants were thus subjected to degradation. 3.4. Microbial community diversity in the biofilm and in the trickling liquid
3.3. Changes in biomass of the biofilm The changes in TS and VS/TS are presented in Fig. 3. The biomass contents were related to the size of the packing spheres, such that sometimes the contents exhibited significant differences. The fewer quantity of the end biomass compared with the initial quantity may reflect the evolution of the biomass to some extent, owing to the insufficient nutrients for the microbes. The average VS/TS increased from 0.586 to 0.627 and finally decreased to 0.541, indicating that the inorganic matter build-up trend. Besides to the nutrients for the microbes, ammonia, VOCs, microbial enrichment medium, and other components in the inlet gas, no other nutrients was added to the trickling liquid. The inlet components were probably insufficient for the microbes, leading to a small decrease in biomass. Excessive biomass accumulation, especially clogging, can reduce pollutant removal and can increase pressure drop, flow channeling, and investment cost. These factors are major hindrances to the
The treatment of gaseous pollutants using biotrickling filters involves a series of complex physicochemical and biological phenomena, such as gaseliquid followed by liquid-biofilm or direct gas-biofilm mass transfer of the pollutant, pollutant diffusion within the biofilm, and pollutant biodegradation in the biofilm (Popat and Deshusses, 2010). Fig. 4 shows the changes in the bacteria in the trickling liquid. The error bars of the microbes in the biofilm are the standard error of averages of the microbes at the top, medium, and bottom of the biotrickling filter, revealing changes in the scope of microbe distribution along the vertical direction of the biotrickling filter. The error bars of the microbes in the trickling liquid are the standard error of sample duplicates for analysis. Bacteria normally show a rapid substrate uptake and growth. Under favorable conditions, they can become the dominant consortia, although fungi may be also present (Dorado et al., 2008).
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Fig. 3. Changes in biomass of the biofilm.
Scrubber section Biotricking section Tatal
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Time/ d Fig. 2. Reduction in odor concentration when tap water or municipal sewage served as trickling liquid.
Bacteria can effectively remove hydrophilic compounds. Hydrophobic and recalcitrant compounds, such as aromatic compounds, alkenes, and alkanes, are poorly absorbed by the bacterial biofilm because of their low solubility in water. However, some researchers think that some hydrophobic and recalcitrant compounds can be removed by bacteria (Romantschuk et al., 2000). The changes in the bacteria in the biofilm exhibited notable variations (Fig. 4). During days 2e8, the odor concentrations were much higher than that of day 1, indicating the abundance of nutrients for bacteria that thrived to 4.2 109 CFU/g TS. During days 14e20, the bacteria count also decreased because of the decline in odor concentration. Bacterial distribution is related to odor concentration (pollutant load), transfer by trickling liquid, biofilm performance, and bacteria characters. Wu et al. (2008) claimed that the bacterial count along the packing material in a biotrickling filter that treated styrene-polluted gaseous streams depended on both styrene load and gas residence time. The average total bacterial was 2.06 109 CFU/g TS, which was 22.2% higher than that of the control in the present study. On the other hand, ammonia accounted for more than 90% of the total pollutants. In our previous work, the results of polymerase chain reaction-denaturing gradient gel electrophoresis revealed that ammonia was effectively removed by the ammonia-oxidizing bacteria Nitrosospira and the nitriteoxidizing bacteria Nitrococcus mobilis in the biotrickling filter (Xue et al., 2011)Hence, bacteria may have played a dominant role in the removal of complex gas.
In most of the other biotrickling filters, the air inlet pipes are above the trickling-liquid level, resulting in high pollutant concentrations at the air inlet area of the biotrickling filter. In the present study, however, the air inlet pipe was below the tricklingliquid level, and most pollutants were absorbed. Thus, the pollutant concentrations at the outlet area of the biotrickling filter were high, affecting the microbial distribution in the biofilm. Fungi can degrade substrates in nature all the time, without resorting to extreme conditions. On the other hand, they themselves degrade under extreme environmental conditions relative to pH, low water content, and limited nutrient (Kennes and Veiga, 2004). Furthermore, the aerial mycelia of fungi, which are in direct contact with the gas, remove hydrophobic compounds faster than flat aqueous bacterial biofilm surfaces. Fungi generally grow slower than bacteria, but they are capable of degrading a wide variety of contaminants and of withstanding more adverse conditions (Van Groenestijn et al., 2001). As a disadvantage, the release of fungal spores to the environment may occur in cases of severe drying (Dorado et al., 2008). A previous study suggested that a fungal vapor phase bioreactor containing a strain of dimorphic black yeast, Exophiala lecanii-corni, could be used to treat a toluenecontaminated gas stream with a maximum elimination capacity of 270 g/(m3$h) in short-term tests, a value which is 2e7 times greater than the toluene elimination capacity typically reported for bacterial systems (Woertz et al., 2001). 1.0E+10
1.0E+07 Tap water as trickling liquid
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Fungi in biofilm/ (CFU/g TS)
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Fig. 5. Changes in the fungi in the biotrickling filter.
Fig. 6. Changes in the actinomycetes in the biotrickling filter.
The changes in the fungi in the biotrickling filter are demonstrated in Fig. 5. Unlike that of bacteria, the fungal distributions in the biofilm exhibited a continuous decline. The fungal count was dramatically reduced during day 2e14, and then remained stable afterward. The average total fungal was 9.64 106 CFU/g TS, which was only 6.40% of the control. On the other hand, the fungal counts were irrelevant to the inlet odor concentration. Hence, fungi may not have been effective in odor removal compared with bacteria. Few reports are available on fungi or actinomycetes in biotrickling filters. The studies on relative to biological treatment technologies have focused on biofilters because of the low moisture content or the possible acid accumulation in biofilters. A perlite biofilter inoculated with he newly isolated fungus, Sporothrix variecibatus, was used for the biofiltration of gas phase styrene with styrene-loading rates of between 50 and 845 g/(m3$h) and styrene removal of 65% (Rene et al., 2010). A bench-scale combined bioreactor consisting of two zones, one containing bacterial suspension and the other packed with material for attached growth of fungi, was employed to treat VOC with different water solubilities. The average elimination capacity of xylene, with its three isomers at steady state, was 62 g/(m3$h), and the total removal rate was >90%, which was 24.0% in the first zone and 67.6% in the second (Li and Liu, 2006). In this study, the pH was in the range of 5.5e7.7, favoring bacterial growth. If the pH was controlled below 5.5, the fungi will be more active. The actinomycete species are well-known saprophytic bacteria that decompose organic matter, especially polymers, such as lignocellulose, starch, and chitin in soil (Crawford et al., 1993). Furthermore, they can produce an array of secondary metabolites, many of which have antibacterial or antifungal properties. They are widely distributed, especially in environments with low water content, good permeability, and rich organic content, as well as those that are neutral to weak alkaline. In the field of environmental engineering, actinomycetes are known to biotransform a broad variety of substrates, including pesticide (Fuentes et al., 2010), aliphatic and aromatic hydrocarbons, such as trichloroethylene (Lee et al., 2000). The changes in the actinomycetes in the biotrickling filter are demonstrated in Fig. 6. During the first period, the actinomycete count reached the peak value on day 8, and then gradually decreased in accordance with the change in odor concentration, indicating that actinomycetes could metabolize some of the odorous gases. When municipal sewage was added into the biotrickling filter the second period, the nutrient in the municipal sewage promoted the growth of
actinomycetes, leading to increase in actinomycete count. However, the subsequent reduction in actinomycete count in day 32 may have been due to the inhibitive substances in municipal sewage because actinomycetes were sensitive to living environments. The average total actinomycete was 5.10 105 CFU/g TS, which was 5.63 times higher than that of the average of the control. Hence, actinomycetes may degrade some odorous gases. Nevertheless, the average total actinomycete was only 0.25% of the average. The activity of actinomycetes evidently declined at the end of the experiment, showing that they are not as important as bacteria in pollutant abatement. Some reports have previously discussed the function of actinomycetes in the composting process (Yang et al., 2009). However, only a few articles have referred to actinomycetes in biological processes for waste gas treatment, most reports have mainly focused on biofilters. A new species of the genus Actinomadura nitritigenes sp. nov. from four nitrite-producing strains have been isolated from experimental biofilters supplied with ammonia (Lipski and Altendorf, 1995). In another work, seven strains capable of oxidizing methyl sulfides have been isolated from experimental biofilters filled with tree-bark compost. The results of 16S rDNA analyzes have revealed two new species: Pseudonocardia asaccharolytica sp. nov. and Pseudonocardia sulfidoxydans sp. nov. (Reichert et al., 1998). The results of statistical analysis on amounts of bacteria, fungi, and actinomycetes are given in Table 2. TMA, ammonia, and odor concentration were considerably reduced in the scrubber section. The trickling liquid can reduce pollutants through absorption and biodegradation. The percentages of bacteria and fungi in the trickling liquid were only 0.13% and Table 2 Statistical analysis of the amounts of bacteria, fungi, and actinomycetes in the biofilm and in the trickling liquid.
Average (CFU/g TS) Maximum (CFU/g TS) Minimum (CFU/g TS) Standard deviation Total (CFU) Total (CFU) Percentage (%) Percentage (%)
Location
Bacteria
Fungi
Actinomycetes
Biofilm Biofilm
2.06 109 4.20 109
9.64 106 4.62 107
5.10 105 1.69 106
Biofilm
5.57 1.20 4.46 5.67
Biofilm Trickling liquid Biofilm Trickling liquid
99.9 0.13
108 109 1011 108
7.40 1.56 2.08 1.14 99.4 0.55
105 107 109 107
8.38 5.44 1.10 2.15 83.7 16.3
104 105 108 107
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0.55%, respectively. The average value of the actinomycetes were much less than that those of in bacteria. Later in the experiment, the average of actinomycetes in the trickling liquid approached 0, implying that biodegradation using suspended microbes in a trickling liquid could be ignored, and that pollutant abatement in the scrubber section was primarily through of absorption. These results from the conclusion of Cox et al. (2000) that biodegradation in the trickling liquid significantly contributed 21% of the overall elimination capacity, although the amount of suspended biomass was only 1% of the amount of the attached biomass. The pollutants absorbed in the trickling liquid were transferred to the biofilm when the trickling liquid passed through the packing material. At the same time, biodegradation was performed by the attached microbes in the biofilm. The pollutant concentration in the trickling liquid was reduced, so the trickling liquid in the scrubber section could absorb pollutants again. Pollutant abatement in the biotrickling filter depended on the continuous recycling of absorption, transfer, and biodegradation. Without biodegradation in the biofilm, pollutants can accumulate in the trickling liquid, so no sustainable abatement in the biotrickling filter occurs. The odor treatment in this study was thus dependent on the absorption of the trickling liquid and the biodegradation of the biofilm, and was radically dependent on biodegradation. 4. Conclusions This study focused on removing odorous gas produced from composting process using a biotrickling filter. Bacteria may play a predominant role in odor removal. Fungi may not be as effective in odor removal compared with bacteria. Actinomycetes may metabolize some odorous gas, but they were not as important as bacteria in pollutant abatement. The odor treatment in this study was dependent on the absorption by the trickling liquid and the biodegradation by the biofilm, and was radically dependent on biodegradation. Biotrickling filters are promising alternatives in the treatment of complex odorous gases. Acknowledgments This study was supported by the National Environmental Protection Public Welfare Science and Technology Research Program of China (201109024). References Akdeniz, N., Koziel, J.A., Ahn, H., Glanville, T.D., Crawford, B.P., Raman, D.R., 2010. Laboratory scale evaluation of volatile organic compound emissions as indication of swine carcass degradation inside biosecure composting units. Bioresource Technology 101, 71e78. Canovai, A., Valentini, F., Manetti, E., Zagaroli, M., 2004. Odor control in composting plants: results from full-scale experiences. Journal of Environmental Science and Health Part A 39, 927e937. Cháfer-Pericás, C., Herráez-Hernández, R., Campíns-Falc, P., 2004. Selective determination of trimethylamine in air by liquid chromatography using solid phase extraction cartridges for sampling. Journal of Chromatography A 1042, 219e 223. Chen, S., Zhang, L., 2006. Microbiology Research Techniques. Science Press, Beijing, China (in Chinese). Chen, L., Hoff, S., Lingshuang, C., Koziel, J., Zelle, B., 2009. Evaluation of wood chipbased biofilters to reduce odor, hydrogen sulfide, and ammonia from swine barn ventilation air. Journal of the Air & Waste Management Association 59, 520e530. Cox, H.H.J., Deshusses, M.A., 2000. Combined removal of H2S and toluene in a single-stage biotrickling filter. The 93rd Annual Meeting and Exhibition of the Air and Waste Management Association, Pittsburgh, PA, Salt lake City, Utah. Cox, H.H.J., Nguyen, T.T., Deshusses, M.A., 2000. Toluene degradation in the recycle liquid of biotrickling filters for air pollution control. Applied Microbiology and Biotechnology 54, 133e137. Crawford, D.L., Lynch, J.M., Whipps, J.M., Ousley, M.A., 1993. Isolation and characterization of actinomycete antagonists of a fungal root pathogen. Applied and Environmental Microbiology 59, 3899e3905.
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Odorous composting gas abatement and microbial community diversity in a biotrickling filter Niantao Xue a, b, Qunhui Wang a, c, *, Juan Wang a, Jianhua Wang a, Xiaohong Sun d a
School of Civil and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China School of Environment, Tsinghua University, Haidian District, Beijing 100084, PR China c Key Laboratory of Educational Ministry for High Efficient Mining and Safety in Metal Mine, University of Science and Technology Beijing, Beijing 100083, PR China d Beijing Agricultural Biotechnology Centre, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100089, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 June 2012 Received in revised form 2 March 2013 Accepted 3 March 2013 Available online 9 April 2013
This study aimed to remove complex odorous gas produced from composting using a biotrickling filter and to observe the temporal and special distributions of bacteria, fungi, and actinomycetes. The removal efficiencies of the total volatile organic compounds (TVOC) were 26.1% and 81.5% before and after inoculation of volatile organic compounds (VOC)-degrading microbes, respectively. Especially trimethylamine was 100% degraded. In the first and second composting period, the odor reduction efficiencies showed average values of 86.2% and 94.5%, respectively. The total average of the bacteria in the biofilm was 2.06 109 CFU/g TS, which was 22.2% higher than that of the control (the culture of microbes prior to the inoculation of VOC-degrading microbes). The bacteria may have played a predominant role in odor removal. The total average of the fungi in the biofilm was 9.64 106 CFU/g TS, which was only 6.40% of the control. The total average of the actinomycetes in the biofilm was 5.10 105 CFU/g TS, which was 5.63 times higher than that of the control. Findings from this study showed that usage of a biotrickling filter is a promising process for the treatment of complex odorous gas. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Biotrickling filter Composting Odor Volatile organic compounds (VOC) Bacteria Fungi Actinomycetes
1. Introduction Odorous gas emitted from composting facilities has complicated components, which necessitate efficient, environment-friendly, and cost-effective treatments. The main components (i.e., pollutants) of the odorous gas are (1) nitrogen-containing compounds, such as NH3 (Komilis and Ham, 2006) and N2O (Fukumoto et al., 2003); (2) sulfur-containing compounds, such as H2S; (3) volatile organic compounds (VOC) (Akdeniz et al., 2010); and (4) bioaerosols (i.e., principally airborne microorganisms and microbial constituents released from composting processes where movement of material is involved) (Sanchez-Monedero et al., 2003). In addition, the components and their concentrations exhibit significant changes at different composting stages. The physicochemical properties of odorous gas are diversified. Easily and adversely biodegradable matters coexist, and some are
* Corresponding author. School of Civil and Environmental Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China. Tel./fax: þ86 10 62332778. E-mail addresses: [email protected] (N. Xue), [email protected] (Q. Wang). 0964-8305/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ibiod.2013.03.003
either hydrophilic or hydrophobic. Hydrophilic substances have diverse solubilities in water. Hydrophobic substances are not readily available to microorganisms, and thus inadequate for use in biological treatments (Hassan and Sorial, 2010). These properties demonstrate that effective treatment of odorous gas is difficult. In the last few decades, emission control of VOC and other odorous pollutants has become a crucial issue owing to their adverse effects to humans, animals, and the environment. Most VOC are toxic and carcinogenic substances; thus, loss of these substances to the ambient air may have an adverse impact on air quality and endanger public health (Yoon and Park, 2002). Anthropogenic activities will influence the conversion of natural VOC into condensable vapors to generate natural aerosols and thus, further affect climate (O’Dowd et al., 2002). Therefore, it is very important to develop effective technologies to remove these compounds to preserve human health and the environment. Trimethylamine (TMA), one of VOC is a malodorous aliphatic amine frequently identified in gaseous emissions of multiple industrial and agricultural processes. Compared with ammonia, TMA can be perceived and detected at greater distances because of its characteristics, including persistent intensive odor and very low odor detection thresholds (Goldstein, 2002). TMA poses serious
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ecological and environmental issues and is a strong environmental pollutant. Health effects associated with inhalation of TMA include irritation of the respiratory tract, eyes, and skin. Consequently, the sensitive determination of TMA in atmospheric and human work environments is of great importance (Cháfer-Pericás et al., 2004). Thus, TMA and other VOC disposal through usage of a biotrickling filter (BTF) was studied in this paper. Biological processes for odor treatment, including bioscrubbers, biofilters, and biotrickling filters, are promising odor abatement technologies that take advantage of the ability of microorganisms to remove substrates from odorous organic compounds (Canovai et al., 2004). Their development owes its increasing popularity to two of its advantages: (1) It is operated at ambient temperatures (15e30 C); (2) It does not produce toxic by-products (Delhoménie et al., 2002). In a biotrickling filter, waste air streams pass through a packed bed of synthetic inert material where particular microbes are immobilized to form a thin aqueous layer (biofilm) (Zilli et al., 2007). Biofilters are more popular than biotrickling filters in the treatment of odorous composting gas. The latter are more complex and more expensive than biofilters but are usually more effective, especially in the treatment of compounds that are difficult to degrade or those that generate acidic by-products, such as H2S (Cox and Deshusses, 2000). Biotrickling filters have seldom been used to treat wastecomposting gas. The reason biotrickling filters are preferred over biofilters is that they contain trickling liquid which helps avoid dryness of the packing material and allows removal of metabolites produced during degradation which can then be recycled. Smits et al. (1995) applied a biotrickling filter to treat ammonia and odor from a composting facility. The biological elimination capacity was 4 g NH3/(m3$h), and the odor removal efficiency was 50% for odor loads as high as 5 o.u./(m3$s). Pei et al. (2008) revealed that a constant TVOC removal efficiency, an odor concentration above 70% and a maximum elimination capacity of 130 g/(m3$h) can be achieved. Mao et al. (2006) found that biotrickling filters have better deodorization capability for odor from food waste-composting plants than the biofilter and the chemical scrubber with deodorization efficiencies measured according to odor concentrations of 82%, 59%, and 45%. It is expected to improve biotrickling filter removal efficiency of the odor concentration. Although the performance of biotrickling filters depends on the type of microorganisms present, reports on the microbial community in biotrickling filters remain scarce. Inside the biofilm of biotrickling filters or biofilters, biodegradation is mediated by mixed cultures of bacteria, fungi, actinomycetes, and algae, all thriving in a complex ecosystem. Secondary pollutant degraders and predators, such as protozoa, metazoan, and other higher organisms, are also included. Investigating changes in the microbes in biotrickling filter will promote research on the microbial degradation mechanism, optimization of design and operation of biotrickling filters. This study aims: (1) to gain an insight into how a biotrickling filter effectively eliminates complicated composting gas; (2) to determine the removal efficiency of biotrickling filter for odor concentration; and (3) to find out the spatial and temporal distribution of microbial community in biotrickling filter. 2. Materials and methods 2.1. Equipment The composting reactor schematic representation and the biotrickling filter setup are shown in Fig. 1. The upper part of the filter was the biotrickling section, with a working volume of 5.0 L. A perforated plexiglass plate, which served as gas and liquid
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21
14 15
19
10 9 7 18 5
13 6
5
17 8
16 11
2
12
5 3
4
1
Fig. 1. Composting reactor schematic representation and the biotrickling filter setup. (1) Air compressor; (2) time relay; (3) composting air inlet pipe; (4) perforated plate; (5) composting sampling port; (6) heating belt; (7) automatic temperature controller; (8) composting reactor; (9) composting exhaust gas sampling port; (10) BTF air inlet pipe; (11) perforated plate; (12) trickling liquid holding tank used as scrubber section; (13) biotrickling section of BTF with packing; (14) BTF; (15) liquid distributor; (16) micro pump; (17) voltage- and current-steady power supply; (18) time relay; (19) gas sampling port; (20) air exhaust; (21) trickling liquid pipe.
distributor, was placed at the bottom of the biotrickling section. Packing material was supported on the plexiglass plate. About 1.8 L of trickling liquid in the holding tank was fed by a pump to the top of the BTF. It trickled through the packing material to the liquid distributor. The inlet gas pipe of the biotrickling filter was placed under the trickling liquid, which formed the scrubber section, to employ the absorption capacity of trickling liquid. The properties of the packing material, the operation of the composting bioreactor, and other specifications were given by Xue et al. (2010). 2.2. Gas sampling and analysis Sampling ports were set on top of the composting bioreactor and on each section of the biotrickling filter. Gas samples were collected from the inlet and outlet streams using a gas sampler (Model QS-1S, Beijing Municipal Institute of Labor Protection, China). Trimethylamine was transferred into an aqueous solution and then analyzed using the picric acid spectrophotometric method (SBPCI, 1999). Hydrogen sulfide was determined using gasdetection tubes made by the Beijing Municipal Institute of Labor Protection, China. TVOC was analyzed using gas chromatography (Perkin Elmer clarus600Gc-Turbomatrix ATD650, column: Elite624 30 m*320 mm, detector: FID) at the Center for Test of Environmental Quality, Tsinghua University, China (EBSEPA, 2003). Odor concentration (without unit) was measured through olfactometry, in accordance with the triangle odor bag method (SEPA, 1993). Ammonia was determined according to Nessler’s reagent colorimetric method (SEPA, 1993) (SEPA etc. are abbreviations of corresponding organisms who issue the methods). 2.3. Enrichment and screening of VOC-degrading microbes The selective inorganic salt medium consisted of the following (per liter): NaCl 1.0 g, MgSO4$7H2O 0.7 g, NH4C1 1.0 g, KCl 0.7 g, KH2PO4 2.0 g, Na2HPO4 3.0 g, and pH 7.0. A trace element solution was added after autoclaving. The trace element consisted of the following (per liter): CaCl2 0.2 mg, FeCl3$6H2O 0.5 mg, CuSO4 0.005 mg, MnCl2$4H2O 0.005 mg, and ZnSO4$7H2O, 0.1 mg (Wang and Shao, 2006). The microbial enrichment medium consisted of 1 g of peptone þ1 L of selective inorganic salt medium.
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In a previous determination, styrene, m-xylene, and chloroform were the primary refractory components in the exhaust of composting gas. In the present study, 100 mL selective inorganic salt medium, 5 mL styrene, m-xylene, and chloroform were transferred into two 250 mL aerobic oscillation flasks. Ten mL mixed liquor from the Qinghe Wastewater Treatment Plant and the Coking Plant of the Shougang Group, respectively, were added as initial microbial consortia. The culture was performed in a shaking incubator at 30 C and 150 rpm for a 2-day culture period. At the end of each culture period, 10 mL cultured suspension and 10, 20, and 30 mL of styrene, m-xylene, and chloroform, respectively, were added into 100 mL fresh medium for the next period. After four culture cycles, VOC-degrading microbes that could degrade VOC were enriched in the microbial enrichment medium. 2.4. Biomass determination, separation, and count of bacteria, fungi, and actinomycetes Separation and count of bacteria, fungi, and actinomycetes was performed according to the conventional dilution plate count described by Chen and Zhang (2006). Three packing spheres at the top, medium, and bottom, respectively, were removed from the biotrickling filter. The biomass on each of the spheres was peeled off into three sterilized beakers, and then diluted to 200 mL as initial consortia. The initial consortia were diluted to the appropriate multiples by ten-fold dilution method to make microbe suspension. Take 0.1 mL of microbe suspension and inoculated on the surface of solid medium in petri dishes. The dishes were put into culture incubator under appropriate temperature. Three replicates were performed for all samples. The other biomass was evaporated to dryness on a water bath and then dried in a 103 Ce 105 C dry box. Incandesce in a 600 C muffle furnace was used to determine the total solids (TS) and the volatile solids (VS). The trickling liquid was diluted to the appropriate multiples by ten-fold dilution method for separation, and count of bacteria, fungi, and actinomycetes. Bacterial culture medium (beef extract, peptone, agar medium): beef extract 3.0 g, peptone 5.0 g, agar 18.0 g, sterile distilled water 1000 mL, pH 7.0 to 7.2. Petri dishes were culture in the 32e34 C incubator for 3 days. The fungal culture (Matin medium): peptone 5.0 g, glucose 10 g, K2HPO4 1.0 g, MgSO4$7H2O 0.5 g, agar 15 g, 1% rose bengal solution of 3.3 mL, water 1000 mL, normal pH value. When the medium was used, 0.3 mL of 1% streptomycin solution was added to each 100 mL medium. Petri dishes were put in the 30 C incubator for 5e7 days. Actinomycetes culture medium (modified Gao I medium): soluble starch 20.0 g, K2HPO4 0.5 g, FeSO4$7H2O 0.01 g, KNO3 1.0 g, MgSO4$7H2O 0.5 g, NaCl 0.5 g, agar 18.0 g, distilled water 1000 mL, pH 7.2e7.4. When the melted medium was used, K2Cr2O7 solution was added to inhibit the growth of bacteria and mold. One mL of 3% K2Cr2O7 solution was added to each 300 mL medium. Petri dishes were put in the 28 C incubator for 7e14 days. 2.5. Experimental process Before the VOC-degrading microbes were inoculated, odorous gas from the composting reactor entered the biotrickling filter primarily to culture the microbes in the packing material. Bacteria, fungi, and actinomycetes were counted and considered as the control. Two hundred mL of microbial enrichment media containing VOC-degrading microbes were mixed with the trickling liquid, and the microbes gradually in the biofilm of the biotrickling filter. Then, the biotrickling filter was used to treat the exhaust gas two composting periods. During the two periods, tap water and municipal sewage from an office building in the University of
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Science and Technology Beijing, respectively, served as the trickling liquid. After 1 day, 50 mL trickling liquid was collected for water quality analysis, and 50 mLe60 mL of tap water and municipal sewage, respectively, were made up every day. Apart from NaOH, which regulated the pH values, no nutrient was added during the entire experiment. 3. Results and discussion 3.1. Removal of VOC and other pollutants before and after the inoculation of VOC-degrading microbes The removal of VOC and other pollutants before and after VOC-degrading microbes inoculation is shown in Table 1. The inoculation of VOC-degrading microbes notably enhanced the abatement of VOC. After inoculation, the removal efficiencies of toluene, ethyl acetate, and other VOC in the scrubber section decreased. However, those of benzene, ethylenzene, p (m)-xylene, styrene, and o-xylene improved. Removal efficiencies of VOC in the biotrickling filter exhibited similar trends. The total removal efficiencies of almost all VOC, except for styrene, were increased. The total removal efficiencies of TVOC increased from 26.1% to 81.5%. Prior to the inoculation of VOC-degrading microbes, TVOC removal efficiencies in the scrubber section and in the biotrickling section were 58.0% and 76.0%, respectively. These results indicate that TVOC was removed in the scrubber section, and new VOC was produced after the exhaust gas passed through the biotrickling section. After the inoculation of VOC-degrading microbes, TVOC removal efficiencies in the scrubber and in the biotrickling section were dramatically enhanced by the microbes to 56.7% and 57.3%, respectively, with a total removal efficiency of 81.5%. This result demonstrated that the microbes could metabolize VOC in the influent. Hence, inoculating the VOC-degrading microbes necessary. TMA, which is readily soluble in water, was completely removed in the scrubber and in the biotrickling section, probably due to its easy biodegradation in the biofilm. Kim et al. (2005) claimed that hydrophilic substances are easily degraded, whereas the degradation of hydrophobic substances is hindered until biological cultures produce sufficient RNA or enzyme/protein to use these substances. However, Tsai et al. (2008) noted that TMA is difficult to biodegrade because microbes cannot easily break the molecule. Some microbes can probably degrade TMA in the biotrickling filter effectively. Ammonia, which accounted for 90.4% and 94.5% of the total pollutants in the two tests, respectively, was efficiently eliminated. Hydrogen sulfide was not detected because an aerobic, rather than an anaerobic, composting process was employed in this study. 3.2. Reduction in odor concentration when tap water or municipal sewage served as trickling liquid The presence of trickling liquid is the central characteristic that distinguishes the biotrickling filter from the biofilter. In this study, trickling liquid was the water trickled over the bed in the biotrickling filter driven by the continuous recirculation of the liquid in the holding tank. At certain periods, make-up water, tap water or municipal sewage, was added to the reactor to compensate for trickling liquid losses due to evaporation, sampling, purging, and so on. Trickling liquid provided a convenient means to control pH, salt, or metabolite concentration, and supplemented nutrients to the biomass. The additional nutrients, pH control, and larger gas/liquid interfacial area resulted in substantially higher removal efficiencies than that of biofilters (Mpanias and Baltzis, 1998). Odor concentration is a key integrated air quality indicator in the verification of odor emissions. To date, few investigations have
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Table 1 Removal of VOC and other pollutants before and after the inoculation of VOC-degrading microbes. Gas
Inoculation
Influent (mg/m3)
Effluent in scrubber section (mg/m3)
Effluent in biotrickling section (mg/m3)
REa in scrubber section (%)
RE in biotrickling section (%)
Overall RE in biotrickling filter (%)
Benzene
Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After Before After
0.006 0.054 0.071 0.132 0.016 0.050 0.012 0.037 0.024 0.069 0.010 0.021 0.013 0.054 0.005 0 9.120 8.091 9.277 8.508 2.308 10.6 0 0 109 331
0.010 0.036 0.043 0.081 0.023 0.121 0.012 0.022 0.029 0.036 0.012 0.011 0.017 0.015 0 0 3.749 3.361 3.895 3.683 0 0.018 0 0 8.85 13.9
0.006 0.035 0.055 0.024 0.013 0.027 0.010 0.018 0.023 0.038 0.010 0.023 0.013 0.011 0.055 0 6.674 1.398 6.859 1.574 0 0 0 0 1.88 8.67
66.7 33.3 39.4 38.6 43.8 142 0 40.5 20.8 47.8 20.0 47.6 30.8 72.2 100 / 58.9 58.5 58.0 56.7 100 99.8 / / 91.9 95.8
40.0 2.78 34.9 70.4 43.5 77.7 16.7 18.2 20.7 5.56 16.7 109 23.5 26.7 / / 78.0 58.4 76.1 57.3 / 100 / / 78.8 37.6
0 35.2 22.5 81.8 18.8 46.0 16.7 51.4 4.17 44.9 0 9.52 0 79.6 1000 / 26.8 82.7 26.1 81.5 100 100 / / 98.3 97.4
Toluene Ethyl acetate Ethylenzene P (m)-xylene Styrene o-xylene Undecane Other VOC TVOC Trimethylamine Hydrogen sulfide Ammonia
RE in scrubber section (%) ¼ (Influent Effluent in scrubber section)/Influent 100. RE in biotrickling section (%) ¼ (Effluent in scrubber section Effluent in biotrickling section)/Effluent in scrubber section 100. Total RE in biotrickling filter (%) ¼ (Influent Effluent in biotrickling section)/Influent 100. a RE: Removal Efficiency.
been conducted on odor concentration during a composting process, especially the control of odor concentration biological processes. In this study, the odor concentrations of the gas entering and leaving the biotrickling filter was measured and is shown in Fig. 2. During the first period, inlet odor concentrations increased from 750 to 9000. The odor reduction efficiencies of the biotrickling section ranged from 42.9% to 91.7%, with an average efficiency of 86.2%. During the second period, odor reduction was more stable, ranging from 87.5% to 98.3% with an average efficiency of 94.5%. The inlet concentrations during the second period were higher than those during the first period. In general, odor reduction was improved during the second period. Chen et al. (2009), Lau and Cheng (2007) found that the odor concentrations of untreated barn air varied from 8553 1006 OU/m3 to 12171 1575 OU/m3, and that the average odor removal efficiency of a biofilter system was 95 3%.
long-term stable operation of biotrickling filters. Various methods have been evaluated to control biomass accumulation and to prevent clogging. Examples are reduction of biomass growth and removal of excess biomass. The former includes nutrient limitation, addition of growth inhibitors, and use of predators that prey on other microbes the latter includes chemical washes, backwashing, periodic, and stirring of the packed filter. Reduction of biomass growth usually causes decreases in the removal efficiency. In this study, the nutrients available for the microbes were limited. However, the pollutant removal efficiencies were never reduced, as microbes readily prefer biodegradable substrates. When the substrates were not sufficient for growth, the microbes were forced to metabolize other substrates. The inlet pollutants were thus subjected to degradation. 3.4. Microbial community diversity in the biofilm and in the trickling liquid
3.3. Changes in biomass of the biofilm The changes in TS and VS/TS are presented in Fig. 3. The biomass contents were related to the size of the packing spheres, such that sometimes the contents exhibited significant differences. The fewer quantity of the end biomass compared with the initial quantity may reflect the evolution of the biomass to some extent, owing to the insufficient nutrients for the microbes. The average VS/TS increased from 0.586 to 0.627 and finally decreased to 0.541, indicating that the inorganic matter build-up trend. Besides to the nutrients for the microbes, ammonia, VOCs, microbial enrichment medium, and other components in the inlet gas, no other nutrients was added to the trickling liquid. The inlet components were probably insufficient for the microbes, leading to a small decrease in biomass. Excessive biomass accumulation, especially clogging, can reduce pollutant removal and can increase pressure drop, flow channeling, and investment cost. These factors are major hindrances to the
The treatment of gaseous pollutants using biotrickling filters involves a series of complex physicochemical and biological phenomena, such as gaseliquid followed by liquid-biofilm or direct gas-biofilm mass transfer of the pollutant, pollutant diffusion within the biofilm, and pollutant biodegradation in the biofilm (Popat and Deshusses, 2010). Fig. 4 shows the changes in the bacteria in the trickling liquid. The error bars of the microbes in the biofilm are the standard error of averages of the microbes at the top, medium, and bottom of the biotrickling filter, revealing changes in the scope of microbe distribution along the vertical direction of the biotrickling filter. The error bars of the microbes in the trickling liquid are the standard error of sample duplicates for analysis. Bacteria normally show a rapid substrate uptake and growth. Under favorable conditions, they can become the dominant consortia, although fungi may be also present (Dorado et al., 2008).
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2.5
(a) 14000
Tap water as trickling liquid
0.8
Sewage as trickling liquid
Tap water as trickling liquid
Composting 12000
Sewage as trickling liquid
Composting 2.0
Inlet
0.7
Control
TS
10000
VS/TS
Outlet of biotricking section
0.6
1.0
0.5
0.5
0.4
VS/TS
8000
1.5 TS/ g
Odor concentration
Outlet of scrubber section
6000
4000
2000
0
0.0 -5
0
5
10
15
20
25
30
35
40
45
50
0.3 -5
0
5
10
15
20
25
30
35
40
45
Time/ d Time/ d
(b)
Tap water as trickling liquid
110
Sewage as trickling liquid
Fig. 3. Changes in biomass of the biofilm.
Scrubber section Biotricking section Tatal
50
30
10
-10
Composting
-30 -5
0
5
10
15
20
25
30
35
40
45
50
Time/ d Fig. 2. Reduction in odor concentration when tap water or municipal sewage served as trickling liquid.
Bacteria can effectively remove hydrophilic compounds. Hydrophobic and recalcitrant compounds, such as aromatic compounds, alkenes, and alkanes, are poorly absorbed by the bacterial biofilm because of their low solubility in water. However, some researchers think that some hydrophobic and recalcitrant compounds can be removed by bacteria (Romantschuk et al., 2000). The changes in the bacteria in the biofilm exhibited notable variations (Fig. 4). During days 2e8, the odor concentrations were much higher than that of day 1, indicating the abundance of nutrients for bacteria that thrived to 4.2 109 CFU/g TS. During days 14e20, the bacteria count also decreased because of the decline in odor concentration. Bacterial distribution is related to odor concentration (pollutant load), transfer by trickling liquid, biofilm performance, and bacteria characters. Wu et al. (2008) claimed that the bacterial count along the packing material in a biotrickling filter that treated styrene-polluted gaseous streams depended on both styrene load and gas residence time. The average total bacterial was 2.06 109 CFU/g TS, which was 22.2% higher than that of the control in the present study. On the other hand, ammonia accounted for more than 90% of the total pollutants. In our previous work, the results of polymerase chain reaction-denaturing gradient gel electrophoresis revealed that ammonia was effectively removed by the ammonia-oxidizing bacteria Nitrosospira and the nitriteoxidizing bacteria Nitrococcus mobilis in the biotrickling filter (Xue et al., 2011)Hence, bacteria may have played a dominant role in the removal of complex gas.
In most of the other biotrickling filters, the air inlet pipes are above the trickling-liquid level, resulting in high pollutant concentrations at the air inlet area of the biotrickling filter. In the present study, however, the air inlet pipe was below the tricklingliquid level, and most pollutants were absorbed. Thus, the pollutant concentrations at the outlet area of the biotrickling filter were high, affecting the microbial distribution in the biofilm. Fungi can degrade substrates in nature all the time, without resorting to extreme conditions. On the other hand, they themselves degrade under extreme environmental conditions relative to pH, low water content, and limited nutrient (Kennes and Veiga, 2004). Furthermore, the aerial mycelia of fungi, which are in direct contact with the gas, remove hydrophobic compounds faster than flat aqueous bacterial biofilm surfaces. Fungi generally grow slower than bacteria, but they are capable of degrading a wide variety of contaminants and of withstanding more adverse conditions (Van Groenestijn et al., 2001). As a disadvantage, the release of fungal spores to the environment may occur in cases of severe drying (Dorado et al., 2008). A previous study suggested that a fungal vapor phase bioreactor containing a strain of dimorphic black yeast, Exophiala lecanii-corni, could be used to treat a toluenecontaminated gas stream with a maximum elimination capacity of 270 g/(m3$h) in short-term tests, a value which is 2e7 times greater than the toluene elimination capacity typically reported for bacterial systems (Woertz et al., 2001). 1.0E+10
1.0E+07 Tap water as trickling liquid
Sewage as trickling liquid
Composting
8.0E+09
Control
8.0E+06
Bacteria in biofilm Bacteria in trickling liquid
6.0E+09
6.0E+06
4.0E+09
4.0E+06
2.0E+09
2.0E+06
0.0E+00
0.0E+00 -5
0
5
10
15
20
25
30
35
40
Time/ d Fig. 4. Changes in the bacteria in the biotrickling filter.
45
Bacteria in trickling liquid/ (CFU/mL)
70
Bacteria in biofilm/ (CFU/g TS)
Reduction efficiency/ %
90
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Fungi in biofilm/ (CFU/g TS)
Fungi in biofilm
3.4E+04
Fungi in trickling liquid
2.9E+04 2.4E+04
1.4E+08 1.9E+04 1.4E+04
9.0E+07
9.0E+03 4.0E+07 4.0E+03 0
5
10
15
20
25
30
35
40
45
-1.0E+03
Actinomycetes in biofilm/ (CFU/g TS)
Tap water as trickling liquid
3.9E+04
Control
1.9E+08
-1.0E+07 -5
1.5E+05
Sewage as trickling liquid
Composting
2.4E+08
3.5E+06
4.4E+04
Tap water as trickling liquid
Fungi in trickling liquid/ (CFU/mL)
2.9E+08
3.0E+06
Sewage as trickling liquid
1.3E+05
Composting Control
2.5E+06
Actinomycetes in biofilm
2.0E+06
Actinomycetes in trickling liquid
1.1E+05 9.0E+04 7.0E+04
1.5E+06 5.0E+04 1.0E+06
3.0E+04
5.0E+05 0.0E+00 -5
Actinomycetes in trickling liquid/ (CFU/mL)
78
1.0E+04 -1.0E+04 0
5
10
15
Time/ d
20 25 Time/ d
30
35
40
45
Fig. 5. Changes in the fungi in the biotrickling filter.
Fig. 6. Changes in the actinomycetes in the biotrickling filter.
The changes in the fungi in the biotrickling filter are demonstrated in Fig. 5. Unlike that of bacteria, the fungal distributions in the biofilm exhibited a continuous decline. The fungal count was dramatically reduced during day 2e14, and then remained stable afterward. The average total fungal was 9.64 106 CFU/g TS, which was only 6.40% of the control. On the other hand, the fungal counts were irrelevant to the inlet odor concentration. Hence, fungi may not have been effective in odor removal compared with bacteria. Few reports are available on fungi or actinomycetes in biotrickling filters. The studies on relative to biological treatment technologies have focused on biofilters because of the low moisture content or the possible acid accumulation in biofilters. A perlite biofilter inoculated with he newly isolated fungus, Sporothrix variecibatus, was used for the biofiltration of gas phase styrene with styrene-loading rates of between 50 and 845 g/(m3$h) and styrene removal of 65% (Rene et al., 2010). A bench-scale combined bioreactor consisting of two zones, one containing bacterial suspension and the other packed with material for attached growth of fungi, was employed to treat VOC with different water solubilities. The average elimination capacity of xylene, with its three isomers at steady state, was 62 g/(m3$h), and the total removal rate was >90%, which was 24.0% in the first zone and 67.6% in the second (Li and Liu, 2006). In this study, the pH was in the range of 5.5e7.7, favoring bacterial growth. If the pH was controlled below 5.5, the fungi will be more active. The actinomycete species are well-known saprophytic bacteria that decompose organic matter, especially polymers, such as lignocellulose, starch, and chitin in soil (Crawford et al., 1993). Furthermore, they can produce an array of secondary metabolites, many of which have antibacterial or antifungal properties. They are widely distributed, especially in environments with low water content, good permeability, and rich organic content, as well as those that are neutral to weak alkaline. In the field of environmental engineering, actinomycetes are known to biotransform a broad variety of substrates, including pesticide (Fuentes et al., 2010), aliphatic and aromatic hydrocarbons, such as trichloroethylene (Lee et al., 2000). The changes in the actinomycetes in the biotrickling filter are demonstrated in Fig. 6. During the first period, the actinomycete count reached the peak value on day 8, and then gradually decreased in accordance with the change in odor concentration, indicating that actinomycetes could metabolize some of the odorous gases. When municipal sewage was added into the biotrickling filter the second period, the nutrient in the municipal sewage promoted the growth of
actinomycetes, leading to increase in actinomycete count. However, the subsequent reduction in actinomycete count in day 32 may have been due to the inhibitive substances in municipal sewage because actinomycetes were sensitive to living environments. The average total actinomycete was 5.10 105 CFU/g TS, which was 5.63 times higher than that of the average of the control. Hence, actinomycetes may degrade some odorous gases. Nevertheless, the average total actinomycete was only 0.25% of the average. The activity of actinomycetes evidently declined at the end of the experiment, showing that they are not as important as bacteria in pollutant abatement. Some reports have previously discussed the function of actinomycetes in the composting process (Yang et al., 2009). However, only a few articles have referred to actinomycetes in biological processes for waste gas treatment, most reports have mainly focused on biofilters. A new species of the genus Actinomadura nitritigenes sp. nov. from four nitrite-producing strains have been isolated from experimental biofilters supplied with ammonia (Lipski and Altendorf, 1995). In another work, seven strains capable of oxidizing methyl sulfides have been isolated from experimental biofilters filled with tree-bark compost. The results of 16S rDNA analyzes have revealed two new species: Pseudonocardia asaccharolytica sp. nov. and Pseudonocardia sulfidoxydans sp. nov. (Reichert et al., 1998). The results of statistical analysis on amounts of bacteria, fungi, and actinomycetes are given in Table 2. TMA, ammonia, and odor concentration were considerably reduced in the scrubber section. The trickling liquid can reduce pollutants through absorption and biodegradation. The percentages of bacteria and fungi in the trickling liquid were only 0.13% and Table 2 Statistical analysis of the amounts of bacteria, fungi, and actinomycetes in the biofilm and in the trickling liquid.
Average (CFU/g TS) Maximum (CFU/g TS) Minimum (CFU/g TS) Standard deviation Total (CFU) Total (CFU) Percentage (%) Percentage (%)
Location
Bacteria
Fungi
Actinomycetes
Biofilm Biofilm
2.06 109 4.20 109
9.64 106 4.62 107
5.10 105 1.69 106
Biofilm
5.57 1.20 4.46 5.67
Biofilm Trickling liquid Biofilm Trickling liquid
99.9 0.13
108 109 1011 108
7.40 1.56 2.08 1.14 99.4 0.55
105 107 109 107
8.38 5.44 1.10 2.15 83.7 16.3
104 105 108 107
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0.55%, respectively. The average value of the actinomycetes were much less than that those of in bacteria. Later in the experiment, the average of actinomycetes in the trickling liquid approached 0, implying that biodegradation using suspended microbes in a trickling liquid could be ignored, and that pollutant abatement in the scrubber section was primarily through of absorption. These results from the conclusion of Cox et al. (2000) that biodegradation in the trickling liquid significantly contributed 21% of the overall elimination capacity, although the amount of suspended biomass was only 1% of the amount of the attached biomass. The pollutants absorbed in the trickling liquid were transferred to the biofilm when the trickling liquid passed through the packing material. At the same time, biodegradation was performed by the attached microbes in the biofilm. The pollutant concentration in the trickling liquid was reduced, so the trickling liquid in the scrubber section could absorb pollutants again. Pollutant abatement in the biotrickling filter depended on the continuous recycling of absorption, transfer, and biodegradation. Without biodegradation in the biofilm, pollutants can accumulate in the trickling liquid, so no sustainable abatement in the biotrickling filter occurs. The odor treatment in this study was thus dependent on the absorption of the trickling liquid and the biodegradation of the biofilm, and was radically dependent on biodegradation. 4. Conclusions This study focused on removing odorous gas produced from composting process using a biotrickling filter. Bacteria may play a predominant role in odor removal. Fungi may not be as effective in odor removal compared with bacteria. Actinomycetes may metabolize some odorous gas, but they were not as important as bacteria in pollutant abatement. The odor treatment in this study was dependent on the absorption by the trickling liquid and the biodegradation by the biofilm, and was radically dependent on biodegradation. Biotrickling filters are promising alternatives in the treatment of complex odorous gases. Acknowledgments This study was supported by the National Environmental Protection Public Welfare Science and Technology Research Program of China (201109024). References Akdeniz, N., Koziel, J.A., Ahn, H., Glanville, T.D., Crawford, B.P., Raman, D.R., 2010. Laboratory scale evaluation of volatile organic compound emissions as indication of swine carcass degradation inside biosecure composting units. Bioresource Technology 101, 71e78. Canovai, A., Valentini, F., Manetti, E., Zagaroli, M., 2004. Odor control in composting plants: results from full-scale experiences. Journal of Environmental Science and Health Part A 39, 927e937. Cháfer-Pericás, C., Herráez-Hernández, R., Campíns-Falc, P., 2004. Selective determination of trimethylamine in air by liquid chromatography using solid phase extraction cartridges for sampling. Journal of Chromatography A 1042, 219e 223. Chen, S., Zhang, L., 2006. Microbiology Research Techniques. Science Press, Beijing, China (in Chinese). Chen, L., Hoff, S., Lingshuang, C., Koziel, J., Zelle, B., 2009. Evaluation of wood chipbased biofilters to reduce odor, hydrogen sulfide, and ammonia from swine barn ventilation air. Journal of the Air & Waste Management Association 59, 520e530. Cox, H.H.J., Deshusses, M.A., 2000. Combined removal of H2S and toluene in a single-stage biotrickling filter. The 93rd Annual Meeting and Exhibition of the Air and Waste Management Association, Pittsburgh, PA, Salt lake City, Utah. Cox, H.H.J., Nguyen, T.T., Deshusses, M.A., 2000. Toluene degradation in the recycle liquid of biotrickling filters for air pollution control. Applied Microbiology and Biotechnology 54, 133e137. Crawford, D.L., Lynch, J.M., Whipps, J.M., Ousley, M.A., 1993. Isolation and characterization of actinomycete antagonists of a fungal root pathogen. Applied and Environmental Microbiology 59, 3899e3905.
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Delhoménie, M.C., Bibeau, L., Bredin, N., Roy, S., Broussau, S., Brzezinski, R., Kugelmass, J.L., Heitz, M., 2002. Biofiltration of air contaminated with toluene on a compost-based bed. Advances in Environmental Research 6, 239e254. Dorado, A.D., Baquerizo, G., Maestre, J.P., Gamisans, X., Gabriel, D., Lafuente, J., 2008. Modeling of a bacterial and fungal biofilter applied to toluene abatement: kinetic parameters estimation and model validation. Chemical Engineering Journal 140, 52e61. Editorial Board of the State Environmental Protection Administration of the People’s Republic of China for Monitoring and Analysis Methods for Air and Waste Gas, 2003. Monitoring and Analysis Methods for Air and Waste Gas, fourth ed. China Environmental Science Press, Beijing, China (in Chinese). Fuentes, M.S., Benimeli, C.S., Cuozzo, S.A., Amoroso, M.J., 2010. 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