Biofiltration for mitigation of methane emission from animal husbandry

Removal of methane from exhaust air of animal houses and manure storage has a large potential for the reduction of greenhouse gas emissions from animal husbandry. The aim of this study was to design a biofilter for methane removal at a full-scale livestock production facility. Air from the headspace of a covered 6 m3 liquid manure storage (air flow: 0.75-8.5 m3 m(-3) h(-1); CH4: 500-5500 mg m(-3)) was treated in an experimental biofilter (160 L). The filterbed, a mixture of compost and perlite in a 40:60 (v/v) ratio, was inoculated with activated sludge that had shown a good methane oxidation rate as compared to pure cultures in preceding laboratory tests. Methane removal up to 85% could be achieved in the experimental biofilter. The methane removal (g m(-3) h(-1)) appeared to be proportional to the concentration (g m(-3)) with k = 2.5 h(-1). Relatively low methane concentrations and high air flows, as reported for the exhaust air of animal houses, would require very large biofilter sizes. Extrapolation of the results showed that treatment of air from a 1000 m3 liquid manure storage with a methane concentration of 22 g m(-3) would require a 20 m3 biofilter for a desired emission reduction of 50%. The costs for such a biofilter are USD 26 per t of CO2 equiv reduction.     

NO(x) removal from simulated flue gas by chemical absorption-biological reduction integrated approach in a biofilter

A chemical absorption-biological reduction integrated approach, which combines the advantages of both the chemical and biological technologies, is employed to achieve the removal of nitrogen monoxide (NO) from the simulated flue gas. The biological reduction of NO to nitrogen gas (N2) and regeneration of the absorbent Fe(II)EDTA (EDTA:ethylenediaminetetraacetate) take place under thermophilic conditions (50 +/- 0.5 degrees C). The performance of a laboratory-scale biofilter was investigated for treating NO(x) gas in this study. Shock loading studies were performed to ascertain the response of the biofilter to fluctuations of inlet loading rates (0.48 approximately 28.68 g NO m(-3) h(-1)). A maximum elimination capacity (18.78 g NO m(-3) h(-1)) was achieved at a loading rate of 28.68 g NO m(-3) h(-1) and maintained 5 h operation at the steady state. Additionally, the effect of certain gaseous compounds (e.g., O2 and SO2) on the NO removal was also investigated. A mathematical model was developed to describe the system performance. The model has been able to predict experimental results for different inlet NO concentrations. In summary, both theoretical prediction and experimental investigation confirm that biofilter can achieve high removal rate for NO in high inlet concentrations under both steady and transient states.     

Efficient Total Nitrogen Removal in an Ammonia Gas Biofilter through High-Rate OLAND

Ammonia gas is conventionally treated in nitrifying biofilters; however, addition of organic carbon to perform post-denitrification is required to obtain total nitrogen removal. Oxygen-limited autotrophic nitrification/denitrification (OLAND), applied in full-scale for wastewater treatment, can offer a cost-effective alternative for gas treatment. In this study, the OLAND application thus was broadened toward ammonia loaded gaseous streams. A down flow, oxygen-saturated biofilter (height of 1.5 m; diameter of 0.11 m) was fed with an ammonia gas stream (248 ± 10 ppmv) at a loading rate of 0.86 ± 0.04 kg N m(-3) biofilter d(-1) and an empty bed residence time of 14 s. After 45 days of operation a stable nitrogen removal rate of 0.67 ± 0.06 kg N m(-3) biofilter d(-1), an ammonia removal efficiency of 99%, a removal of 75-80% of the total nitrogen, and negligible NO/N(2)O productions were obtained at water flow rates of 1.3 ± 0.4 m(3) m(-2) biofilter section d(-1). Profile measurements revealed that 91% of the total nitrogen activity was taking place in the top 36% of the filter. This study demonstrated for the first time highly effective and sustainable autotrophic ammonia removal in a gas biofilter and therefore shows the appealing potential of the OLAND process to treat ammonia containing gaseous streams.     

Dynamics of sustainable grazing fauna and effect on performance of gas biofilter

The inherent operational problems of biofilters such as a pressure drop increase and nutrient limitations were managed in a toluene-removing gas biofilter with a sustainable grazing fauna consisting of micrometazoa and ciliate protozoa. Dynamic populations of predatory nematodes (Caenorhabditis sp.), rotifers (Philodina sp.), tardigrades (Echiniscus sp.) and fly larvae represented the micrometazoa community in the filter bed. Colpoda inflata, Euplotes harpa and Acineria sp. constituted the grazing ciliate community. The spatiotemporal distribution and abundance of the grazing fauna depends on physicochemical conditions and interspecies interactions in the biofilter. Of the micro metazoa, Caenorhabditis and Philodina tolerated wide concentration ranges for toluene (0.75-2.63 g m(-3)) and CO(2) (0.92-6.08 g m(-3)) and maintained stable populations of 3.4-4.7 x 10(3) and 5.8-7.65 x 10(4) g medium(-1), respectively. The grazing fauna supported a stable toluene-degrading bacterial community composed of four Pseudomonas spp. Under a maximum toluene load of 120.72 g m(-3) h(-1), at steady-state conditions 80% toluene removal was achieved in the biofilter. Of the grazing organisms, owing to their reproductive cycle and feeding behaviour, fly larvae were not suited for application in the biofilter. Meanwhile, organisms such as nematodes, rotifers and ciliates capable of tolerating a wide pollutant concentration range and maintaining a sustainable population are ideal candidates for application in biofilter technology.     

The biofiltration of indoor air: air flux and temperature influences the removal of toluene, ethylbenzene, and xylene

An alternative approach to maintaining indoor air quality may be the biofiltration of air circulated within the space. A biofilter with living botanical matter as the packing medium reduced concentrations of toluene, ethylbenzene, and o-xylene concurrently present at parts per billion (volume) in indoor air. The greatest reduction in concentrations per pass was under the slowest influent air flux (0.025 m s(-1)); however, the maximum amount removed per unit time occurred under the most rapid flux (0.2 m s(-1)). There was little difference between the different compounds with removal capacities of between 1.3 and 2.4 micromol m(-3) biofilter s(-1) (between 0.5 and 0.9 g m(-3) biofilter h(-1)) depending on influent flux and temperature. Contrary to biofilters subjected to higher influent concentrations, the optimal temperatures for removal by this biofilter decreased to less than 20 degrees C at the most rapid flux for all three compounds. Microbial activity was decreased at these cooler temperatures suggesting the biofilter was not microbially limited but rather was limited by the availability of substrate. The cooler temperatures allowed greater partitioning of the VOCs into the water column which had a greater impact on removal than its reduction in microbial activity.     

Biofiltration of methyl tert-butyl ether vapors by cometabolism with pentane: modeling and experimental approach

Degradation of methyl tert-butyl ether (MTBE) vapors by cometabolism with pentane using a culture of pentane-oxidizing bacteria (Pseudomonas aeruginosa) was studied in a 2.4-L biofilter packed with vermiculite, an inert mineral support. Experimental pentane elimination capacity (EC) of approximately 12 g m(-3) h(-1) was obtained for an empty bed residence time (EBRT) of 1.1 h and inlet concentration of 18.6 g m(-3). For these experimental conditions, EC of MTBE between 0.3 and 1.8 g m(-3) h(-1) were measured with inlet MTBE concentration ranging from 1.1 to 12.3 g m(-3). The process was modeled with general mass balance equations that consider a kinetic model describing cross-competitive inhibition between MTBE (cosubstrate) and pentane (substrate). The experimental data of pentane and MTBE removal efficiencies were compared to the theoretical predictions of the model. The predicted pentane and MTBE concentration profiles agreed with the experimental data for steady-state operation. Inhibition by MTBE of the pentane EC was demonstrated. Increasing the inlet pentane concentration improved the EC of MTBE but did not significantly change the EC of pentane. MTBE degradation rates obtained in this study were much lower than those using consortia or pure strains that can mineralize MTBE. Nevertheless, the system can be improved by increasing the active biomass.     

High nitrogen removal performance at moderately low temperature utilizing anaerobic ammonium oxidation reactions

High rates of nitrogen removal from wastewater have been reported using anammox bacteria at temperatures around 37 degrees C, but not at moderately low temperatures. In this study, nitrogen removal performance of an anaerobic biological filtrated (ABF) reactor, filled with porous polyester nonwoven fabric carriers as a fixed bed for anammox bacteria, was tested at 37 degrees C and at moderately low temperature (20-22 degrees C). To attain higher nitrogen removal performance, effects of influent nitrogen concentrations and hydraulic retention time (HRT) on nitrogen removal rates were investigated. Nitrogen removal rate increased with influent ammonium and nitrite concentrations, resulting in a removal rate of 3.3 kg-N/m(3)/d on day 32 for an HRT of 180 min at 37 degrees C. However, influent nitrite concentrations greater than 280 mg/l inhibited anammox activity. Therefore, the influent nitrite concentration was adjusted to be below 280 mg/l, and high-loading tests were performed for a shorter HRT. As a result, a nitrogen conversion rate of 11.5 kg-N/m(3)/d was achieved. Moreover, to evaluate long-term anammox activity at moderately low temperatures, ABF reactors were operated for 446 d. Anammox activity could be maintained at 20-22 degrees C, and stable nitrogen removal performance was observed. Furthermore, high nitrogen conversion rate of 8.1 kg-N/m(3)/d was attained. These results clearly show that an appropriate nitrite concentration in the influent and a shorter HRT resulted in high nitrogen conversion rates. The nitrogen removal performance we obtained at moderately low temperatures will open the door for application of anammox processes to many types of industrial wastewater treatment.     

Low temperature catalytic oxidation of hydrogen sulfide and methanethiol using wood and coal fly ash

The feasibility of reusing waste material as an inexpensive catalyst to remove sulfur compounds from gaseous waste streams has been demonstrated. Wood and coal fly ash were demonstrated to catalytically oxidize H2S and methanethiol (CH3SH) at low temperatures (23-25 degrees C). Wood ash had a significantly higher surface area compared to coal ash (44.9 vs 7.7 m2/g), resulting in a higher initial H2S removal rate (0.16 vs 0.018 mg/g/min) under similar conditions. Elemental sulfur was determined to be the end product of H2S oxidation, since X-ray diffraction analysis indicated the presence of crystalline sulfur. Catalytic decay occurred apparently due to surface deposition of sulfur and a subsequent decline in surface area (44.9-1.4 m2/g) during the reaction of H2S with the ash. Methanethiol was stoichiometrically converted to dimethyl disulfide ((CH3)2S2) without significant catalytic decay. Catalytic decay was reduced and H2S conversion increased (10% at 1.8 days vs 94% at 4.2 days) when H2S loading was decreased to levels typical of many environmental applications (500 ppmv inlet and 1.43 mg/min vs 60 ppmv, 0.09 mg/ min). Catalyst regeneration using hot water (85 degrees C) washing was possible, but only increased fractional conversion from 0.2 to 0.6 and the initial reaction rate to 50% of the original H2S oxidation activity.     

Removal of Pseudomonas putida biofilm and associated extracellular polymeric substances from stainless steel by alkali cleaning

Alkali (NaOH)-based compounds are commonly used in the food industry to clean food contact surfaces. However, little information is available on the ability of alkali and alkali-based cleaning compounds to remove extracellular polymeric substances (EPS) produced by biofilm bacteria. The objectives of this study were to determine the temperature and NaOH concentration necessary to remove biofilm EPS from stainless steel under turbulent flow conditions (clean-in-place simulation) and to determine the ability of a commercial alkaline cleaner to remove biofilm EPS from stainless steel when applied under static conditions without heat. Biofilms were produced by growing Pseudomonas putida on stainless steel for 72 h at 25 degrees C in a 1:10 dilution of Trypticase soy broth. The biofilms were treated using NaOH at concentrations of 1.28 to 6.0% and temperatures ranging from 66 to 70 degrees C. Other biofilms were treated with commercial alkaline cleaner at 25 or 4 degrees C for 1 to 30 min. Removal of EPS was determined by direct microscopic observation of samples stained with fluorescent-labeled peanut agglutinin lectin. Treatment with 1.2% NaOH at 66 degrees C for 3 min was insufficient to remove biofilm EPS. A minimum of 2.5% NaOH at 66 degrees C and 2.0% NaOH at 68 degrees C for 3 min were both effective for EPS removal. Commercial alkaline cleaner removed over 99% of biofilm EPS within 1 min at 4 and 25 degrees C under static conditions. Selection of appropriated cleaning agent formulation and use at recommended concentrations and temperatures is critical for removal of biofilm EPS from stainless steel.     

Removal of hydrogen sulfide by sulfate-resistant Acidithiobacillus thiooxidans AZ11

Toxic H2S gas is an important industrial pollutant that is applied to biofiltration. Here, we examined the effects of factors such as inlet concentration and space velocity on the removal efficiency of a bacterial strain capable of tolerating high sulfate concentrations and low pH conditions. We examined three strains of Acidithiobacillus thiooxidans known to have sulfur-oxidizing activity, and identified strain AZ11 as having the highest tolerance for sulfate. A. thiooxidans AZ11 could grow at pH 0.2 in the presence of 74 g l(-1) sulfate, the final oxidation product of elemental sulfur, in the culture broth. Under these conditions, the specific sulfur oxidation rate was 2.9 g-S g-DCW (dry cell weight)(-1) d(-1). The maximum specific sulfur oxidation rate of A. thiooxidans AZ11 was 21.2 g-S g-DCW(-1) d(-1), which was observed in the presence of 4.2 g-SO4(2-) l(-1) and pH 1.5, in the culture medium. To test the effects of various factors on biofiltration by this strain, A. thiooxidans AZ11 was inoculated into a porous ceramic biofilter. First, a maximum inlet loading of 670 g-S m(-3) h(-1) was applied with a constant space velocity (SV) of 200 h(-1) (residence time, 18 s) and the inlet concentration of H2S was experimentally increased from 200 ppmv to 2200 ppmv. Under these conditions, less than 0.1 ppmv H2S was detected at the biofilter outlet. When the inlet H2S was maintained at a constant concentration of 200 ppmv and the SV was increased from 200 h(-1) to 400 h(-1) (residence time, 9 s), an H2S removal of 99.9% was obtained. However, H2S removal efficiencies decreased to 98% and 94% when the SV was set to 500 h(-1) (residence time, 7.2 s) and 600 h(-1) (residence time, 6 s), respectively. The critical elimination capacity guaranteeing 96% removal of the inlet H2S was determined to be 160 g-S m(-3) h(-1) at a space velocity of 600 h(-1). Collectively, these findings show for the first time that a sulfur oxidizing bacterium has a high sulfate tolerance and a high sulfur oxidizing activity below pH 1.     

Construction of a low-pressure microwave plasma reactor and its application in the treatment of volatile organic compounds

Volatile organic compounds (VOCs) containing gases exhausted from industrial processes are highly toxic to human health; therefore, their safe elimination is extremely important. This paper describes how a novel prototype microwave plasma reactor was sequentially improved. The design concepts for each modification are also carefully described. For testing the reactor's performance, ethanol was selected as the target VOC. The decomposing and removal efficiency (DRE) value was used to evaluate the reactor's performance. The final stage of the revamping reactor was evaluated to meet the requirements of both stability and long term, high volume operation. When studying the VOC treatment operations, oxygen, air, and water vapor were used as carrier gases or additives to improve the efficiency of the DRE value and to facilitate the ethanol treatment. A DRE value higher than 99% was obtained under these optimal conditions: an ethanol vapor flow rate of 1730 cm3/min (67 mbar, 150 degrees C), power input higher than 1.5 kW, a frequency of 200 Hz, an air flow rate of 100 cm3/min (4 atm, 25 degrees C), and a liquid water addition rate higher than 0.21 mL/min (1 atm, 25 degrees C). With these conditions, the initial concentration of ethanol vapor mixed with the carrier gas was about 7-10%. Because of high processing efficiency and capacity of this system, it could be a potential alternative tool for treating industrial VOCs.     

Perchlorate reduction by autotrophic bacteria attached to zerovalent iron in a flow-through reactor

Biological reduction of perchlorate by autotrophic microorganisms attached to zerovalent iron (ZVI) was studied in flow-through columns. The effects of pH, flow rate, and influent perchlorate and nitrate concentrations on perchlorate reduction were investigated. Excellent perchlorate removal performance (> or = 99%) was achieved at empty bed residence times (EBRTs) ranging from 0.3 to 63 h and an influent perchlorate concentration of 40-600 microg L(-1). At the longest liquid residence times, when the influent pH was above 7.5, a significant increase of the effluent pH was observed (pH > 10.0), which led to a decrease of perchlorate removal. Experiments at short residence times revealed that the ZVI column inoculated with local soil (Colton, CA) containing a mixed culture of denitrifiers exhibited much better performance than the columns inoculated with Dechloromonas sp. HZ for reduction of both perchlorate and nitrate. As the flow rate was varied between 2 and 50 mL min(-1), corresponding to empty bed contact times of 0.15-3.8 h, a maximum perchlorate elimination capacity of 3.0 +/- 0.7 g m(-3) h(-1) was obtained in a soil-inoculated column. At an EBRT of 0.3 h and an influent perchlorate concentration of 30 microg L(-1), breakthrough (> 6 ppb) of perchlorate in the effluent did not occur until the nitrate concentration in the influent was 1500 times (molar) greater than that of perchlorate. The mass of microorganisms attached on the solid ZVI/sand was found to be 3 orders of magnitude greater than that in the pore liquid, indicating that perchlorate was primarily reduced by bacteria attached to ZVI. Overall, the process appears to be a promising alternative for perchlorate remediation.     

Enhancement of styrene removal efficiency in biofilter by mixed cultures of Pseudomonas sp. SR-5

The styrene-degrading bacterium Pseudomonas sp. SR-5 exhibited a high styrene removability in a biofilter. However, the styrene removal efficiency (RE) of SR-5 decreased with time. We carried out styrene gas removal in a biofilter inoculated with mixed cultures of SR-5 and other microorganisms to determine the possibility of obtaining an enhanced RE for a long period. The following three inocula were carried out: (i) styrene-degrading bacteria, strains 1 and 3, (ii) a benzoic acid-degrading bacterium Raoultella sp. A, and (iii) wastewater from a chemical company dealing with styrene. These biofilters with mixed SR-5 showed an enhanced RE compared with those with a single culture of SR-5. The complete styrene elimination capacities for ensuring 100% styrene removal in those mixed cultures were 151, 108 and 124 g/m(3)/h, compared with a single culture of SR-5.     

Performance of a styrene-degrading biofilter inoculated with Pseudomonas sp. SR-5

Styrene removal was studied for 3 months in a laboratory-scale biofilter packed with a mixed packing material of peat and ceramic at a ratio of 1 to 1 on a dry-weight basis and inoculated with Pseudomonas sp. SR-5. More than 90% removal efficiency (RE) was attained at 1-140 g/m3/h styrene loads under nitrogen-source limitation. When RE decreased to 70% after 30 d with an increase in styrene load, readdition of SR-5 and washing of the filter packing material restored the RE to more than 90% by maintaining the population of SR-5 at 1-10% of the total cell number. The maximum elimination capacity (EC) by kinetic analysis was estimated to be 290 g/m3/h. High conversion of the removed styrene carbon to CO2, and significantly small production of cell mass from the removed carbon were confirmed.     

HPLC-ELSD法同时测定连翘叶中齐墩果酸和熊果酸

目的：建立高效液相色谱-蒸发光散射检测法(high performance liquid chromatography -evaporative light scattering detector,HPLC-ELSD)法同时测定连翘叶中齐墩果酸和熊果酸。方法：色谱柱：Waters Symmetry C18(4.6mm×250mm,5μm);柱温25℃,流动相：甲醇-0.4%冰醋酸溶液(93:7,V/V),流速0.4mL/min;蒸发光散射检测器检测条件：漂移管温度80℃,气体压力25psi。结果：齐墩果酸在0.107～2.136μg范围内线性关系良好(r=0.9991),熊果酸在0.179～3.584μg范围内线性关系良好(r=0.9993);齐墩果酸和熊果酸的回收率分别为98.67%和98.56%。结论：此方法简便、准确,重现性良好,为评价连翘叶的质量提供可靠的分析方法。     

Efficacy of novel organic acid and hypochlorite treatments for eliminating Escherichia coli O157:H7 from alfalfa seeds prior to sprouting

This study investigated novel two-step organic acid/hypochlorite treatments as alternatives to 20000 ppm active chlorine (from calcium hypochlorite) for eliminating Escherichia coli O157:H7 from alfalfa seeds prior to sprouting. Commercially available alfalfa seeds were inoculated with a five-strain E. coli O157:H7 mixture and dried to attain ca. 10(6) CFU/g of seeds. Seeds then underwent one of several soak treatments including: (1) 5% (v/v) lactic acid for 10 min at 42 degrees C; (2) 5% acetic acid (v/v) for 10 min at 42 degrees C; (3) 2.5% lactic acid for 10 min at 42 degrees C followed by 2000 ppm active chlorine (from calcium hypochlorite) for 15 min at 25 degrees C; (4) 5% lactic acid for 10 min at 42 degrees C followed by 2000 ppm active chlorine for 15 min at 25 degrees C; or (5) 20000 ppm active chlorine for 15 min at 25 degrees C. Each treatment reduced numbers of inoculum cells by about 6.0 log10 CFU/g as determined by plating on Sorbitol MacConkey agar (SMac). Plating on non-selective brain heart infusion agar (BHI) showed that treatments 1-4 reduced counts by 2.3-4.1 log10 CFU/g, thus indicating a large proportion of injured cells. Successive lactic acid and hypochlorite treatments (3 and 4) were more lethal than either organic acid alone (1 and 2). No surviving cells were detected on SMac or BHI following treatment with 20000 ppm active chlorine (treatment 5). Regardless of the previous treatment, E. coli O157:H7 counts increased to 10(7)-10(8) CFU/g during sprouting. Germination of seeds was not adversely affected by any of the treatments (germination > 90%). Results of this study show that: (a) non-lethal cell injury must be considered when evaluating intervention treatments against E. coli O157:H7 on alfalfa seeds; (b) reductions of 2-4 log10 CFU/g can be attained without using 20000 ppm active chlorine; (c) successive lactic acid and hypochlorite treatments have greater lethality than organic acid treatments alone; and (d) none of the treatments tested can prevent regrowth of surviving E. coli O157:H7 during sprouting.     

Cholesterol removal from homogenized milk with beta-cyclodextrin

This study was carried out to determine optimum conditions of five different factors (beta-cyclodextrin concentration, mixing temperature, mixing time, centrifugal force, and centrifugation time) in reduction of cholesterol in 3.6% fat, homogenized milk by application of beta-cyclodextrin. beta-Cyclodextrin at 0.5 to 1.5% provided 92.2 to 95.3% removal of cholesterol when mixed at 10 degrees C for 10 min. Among other factors, mixing time (5 to 20 min) did not significantly affect cholesterol removal. Removal was enhanced with increasing centrifugal forces up to 166x g (95.9%) but decreased thereafter. Various centrifugation times (5 to 20 min) did not have significant effects. Based on these results, we suggest that the optimum conditions for the process are addition of 1.5% beta-cyclodextrin, mixing temperature of 10 degrees C, 10-min mixing time, and centrifugation at 166x g for 10 min.     

三维电极床反应器去除甲苯废气的研究

介绍了用三维电极反应器去除有害废气甲苯的情况。实验研究了电流、气体停留时间等不同因素对甲苯去除率的影响。当电流为3 A、气体流量为2000 mL/min时,反应器对浓度为1300 mg/m3的甲苯废气的去除率达40%以上。     

Biofiltration and inhibitory interactions of gaseous benzene, toluene, xylene, and methyl tert-butyl ether

This study evaluated the individual and combined removal capacities of benzene, toluene, and xylene (B, T, and X) in the presence and absence of methyl tert-butyl ether (MTBE) in a polyurethane biofilter inoculated with a BTX-degrading microbial consortium, and further examined their interactive effects in various mixtures. In addition, Polymerase chain reaction-denaturing gradient gel electrophoresis and phylogenetic analysis of 16S rRNA gene sequences were used to compare the microbial community structures found in biofilters exposed to the various gases and gas mixtures. The maximum individual elimination capacities (MECs) of B, T, and X were 200, 238, and 400 g m(-3) h(-1), respectively. There was no significant elimination of MTBE alone. Addition of MTBE decreased the MECs of B,T, and X to 75, 100, and 300 g m(-3) h(-1), respectively, indicating that benzene was most strongly inhibited by MTBE. When the three gases were mixed (B + T + X), the removal capacities of individual B, T, and X were 50, 90, and 200 g m(-3) h(-1), respectively. These capacities decreased to 40, 50, and 100 g m(-3) h(-1) when MTBE was added to the mix. The MEC of the three-gas mixture (B + T + X) was 340 g m(-3) h(-1), and that of the four-gas mixture was 200 g m(-3) h(-1). Although MTBE alone was not degraded by the biofilter, it could be co-metabolically degraded in the presence of toluene, benzene, or xylene with the MECs of 34, 23, and 14 g m(-3) h(-1), respectively. The microbial community structure analysis revealed that two large groups could be distinguished based on the presence or absence of MTBE, and many of the dominant bacteria in the consortia were closely related to bacteria isolated from aromatic hydrocarbon-contaminated sites and/ or oil wastewaters. These findings provide important new insights into biofiltration and may be used to improve the rational design of biofilters for remediation of petroleum gas-contaminated airstreams according to composition types of mixed gases.