Optimizing nutrient supply in a rotatory-switching biofilter for toluene vapor treatment

The influence of nutrient conditions on the degradation of toluene vapor in a rotatory-switching biofilter (RSB) was investigated. The biofilter consists of four segments connected in series, each with a packing layer made of polyvinyl formal. The influent airstreams including toluene vapors were passed through segments 1-3 as up-flow with a toluene concentration of 0.9-1.2 g m(-3) and with an empty-bed retention time of 26-52 sec. Nutrient solutions were fed to all packed segments once a day by means of immersion. The nutrient solution was used repeatedly and replenished by the addition of (NH4)2SO4. The result at 155 days showed nitrogen depletion was particularly obvious and the lack of nitrogen affected toluene removal. By adding 161 g of nitrogen solution per volumetric cubic meter of reactor, toluene removal efficiency was immediately increased to greater than 99%. With long-term biofilter operation, 21%-32% of ammonium was utilized for nitrification because of the growth of nitrifying bacteria such as Nitrosomonas sp. Based on the carbon-nitrogen balance, the daily nitrogen demand for toluene removal was estimated 2.1 g day(-1) at a toluene load of 70 g m(-3) hr(-1).     

A hybrid biological process of indoor air treatment for toluene removal

Bioprocesses, such as biofiltration, are commonly used to treat industrial effluents containing volatile organic compounds (VOCs) at low concentrations. Nevertheless, the use of biofiltration for indoor air pollution (IAP) treatment requires adjustments depending on specific indoor environments. Therefore, this study focuses on the convenience of a hybrid biological process for IAP treatment. A biofiltration reactor using a green waste compost was combined with an adsorption column filled with activated carbon (AC). This system treated a toluene-micropolluted effluent (concentration between 17 and 52 µg/m³), exhibiting concentration peaks close to 733 µg/m³ for a few hours per day. High removal efficiency was obtained despite changes in toluene inlet load (from 4.2 × 10^?3 to 0.20 g/m³/hr), which proves the hybrid system’s effectiveness. In fact, during unexpected concentration changes, the efficiency of the biofilter is greatly decreased, but the adsorption column maintains the high efficiency of the entire process (removal efficiency [RE] close to 100%). Moreover, the adsorption column after biofiltration is able to deal with the problem of the emission of particles and/or microorganisms from the biofilter.

ImplicationsIndoor air pollution is nowadays recognized as a major environmental and health issue. This original study investigates the performance of a hybrid biological process combining a biofilter and an adsorption column for removal of indoor VOCs, specifically toluene.     

Fungal biofilters for toluene biofiltration: evaluation of the performance with four packing materials under different operating conditions

Packing materials play a key role in the performance of bioreactors for waste gas treatment and particularly in biofilter applications. In this work, the performance of four differently packed biofilters operated in parallel for the treatment of relatively high inlet concentration of toluene was studied. The reactors were compared for determining the suitability of coconut fiber, digested sludge compost from a waste water treatment plant, peat and pine leaves as packing materials for biofiltration of toluene. A deep characterisation of materials was carried out. Biological activity and packing capabilities related to toluene removal were determined throughout 240 days of operation under different conditions of nutrients addition and watering regime. Also, biofilters recovering after a short shutdown was investigated. Nutrient addition resulted in improved removal efficiencies (RE) and elimination capacities (EC) of biofilters reaching maximum ECs between 75 and 95 g m(-3)h(-1) of toluene. In the first 80 days, the pH decreased progressively within the reactors, causing a population change from bacteria to fungi, which were the predominant decontaminant microorganisms thereafter. All reactors were found to recover the RE rapidly after a 5 days shutdown and, in a maximum of 7 days, all reactors had been completely recuperated. These results point out that fungal biofilters are a suitable choice to treat high loads of toluene. In general, coconut fiber and compost biofilters exhibited a better performance in terms of elimination capacity and long-term stability.     

Reusing H2S-exhausted carbon as packing material for odor biofiltration

Exhausted carbon coming from the H2S adsorption process is a big environmental problem in Wastewater Treatment Plants. In this study, reusing exhausted carbon as a carrier of sulfide-oxidizing bacteria in lab-scale biofilters was evaluated. The exhausted carbons from different heights of the adsorption bed have different exhaustion extents, i.e. characteristics in terms of sulfur content, pH and porosity. Therefore, four biofilters were packed separately with exhausted carbon from top, middle, bottom of H2S adsorption bed, and a mixture of the three, to investigate the suitability for further H2S biofiltration. The results showed a quick startup in these biofilters (approximately 80 h). The numbers of sulfide-oxidizing bacteria immobilized on activated carbon were approximately 4.8, 9.2 and 14 x 108 CFU g-1 top, middle and bottom carbon after the 240-h operation, respectively. In addition, the biofilters demonstrated a rapid recovery to the original removal efficiency (RE) within 2 h after the H2S spike loadings. After a 110-h shutdown, the RE was rapidly recovered for all the biofilters within 5 h, with a shorter time (1 h) observed for the bottom carbon biofilter. The H2S removal mechanism of these biofilters was studied through a full analysis of sulfur products in both liquid (recycling medium) and activated carbon, and variable characterization of activated carbon before and after biofiltration. This study shows that the exhausted carbon-based biofilter is a feasible and economical alternative to conventional odor biofiltration.     

Backfitting Electrostatic Precipitators: Gas Velocity and Reliability Considerations

Abstract

The kinetic behavior of the toluene biofiltration process was investigated in this research. Toluene was used as a model compound for less water-soluble gas pollutants. The limiting factor in the overall toluene biofiltration process was determined by analyzing the effectiveness factor of the biofilm along the biofilter. Experiments were conducted in three laboratory-scale biofilters packed with mixtures of chaff/compost, D.E. (diatomaceous earth)/compost and GAC (granular activated carbon)/compost, respectively. A mathematical model previously proposed was verified in this study as being applicable to these biofilters packed with different filter materials. Both the experimental and theoretical results confirmed that the biodegradation rate along the biofilter followed the zero order, fractional order to first order kinetics as toluene concentration decreased. Moreover, at higher toluene concentration, biodegradation rate and mass flux of toluene were lower near the bottom of the biofilter due to substrate inhibition. Analysis of the effectiveness factor indicated that biofiltration of a less soluble compound such as toluene should not be operated at high gas flow rates (low gas residence times) due to the mass transfer limitation of such a system. At an approximate constant inlet toluene concentration of 0.9 g/m³, the toluene removal efficiency in these three biofilters would drop below 90% when the gas residence time decreased to 2.5, 2.5, and 2.0 min, respectively.     

Biofilter treatment of volatile organic compound emissions from reformulated paint: complex mixtures, intermittent operation, and startup

Two biofilters were operated to treat a waste gas stream intended to simulate off-gases generated during the manufacture of reformulated paint. The model waste gas stream consisted of a five-component solvent mixture containing acetone (450 ppm(v)), methyl ethyl ketone (12 ppm(v)), toluene (29 ppm(v)), ethylbenzene (10 ppm(v)), and p-xylene (10 ppm(v)). The two biofilters, identical in construction and packed with a polyurethane foam support medium, were inoculated with an enrichment culture derived from compost and then subjected to different loading conditions during the startup phase of operation. One biofilter was subjected to intermittent loading conditions with contaminants supplied only 8 hr/day to simulate loading conditions expected at facilities where manufacturing operations are discontinuous. The other biofilter was subjected to continuous contaminant loading during the initial start period, and then was switched to intermittent loading conditions. Experimental results demonstrate that both startup strategies can ultimately achieve high contaminant removal efficiency (>99%) at a target contaminant mass loading rate of 80.3 g m(-3) hr(-1) and an empty bed residence time of 59 sec. The biofilter subjected to intermittent loading conditions at startup, however, took considerably longer to reach high performance. In both biofilters, ketone components (acetone and methyl ethyl ketone) were, more rapidly degraded than aromatic hydrocarbons (toluene, ethylbenzene, and p-xylene). Scanning electron microscopy and plate count data revealed that fungi, as well as bacteria, populated the biofilters.     

Removal of low concentrations of carbon tetrachloride in compost-based biofilters operated under methanogenic conditions.

Research was performed to demonstrate the removal of carbon tetrachloride (CT) using compost biofilters operated under methanogenic conditions. Biofilters were operated at an empty-bed residence time of 2.8 minutes using nitrogen as the atmosphere. Hydrogen and carbon dioxide were supplied as an electron donor and carbon source, respectively, during acclimation of the bed medium microbes. Once methanogenesis was demonstrated, CT flow to the biofilter was established. Biofilters were operated over a CT concentration range from 20 to 700 ppbv for 6 months. Bed medium microbes were able to remove up to 75% of the inlet CT. At excessively high CT concentrations (> 500 ppmv), methane production and hydrogen utilization by the bed medium microbes appeared to be inhibited. CT removal by the biofilter decreased when the hydrogen supply was removed from the biofilter inlet, indicating that hydrogen acted as the electron donor for reductive dechlorination. The removal efficiency and relatively low empty bed residence times demonstrated by these laboratory-scale biofilters indicate that anaerobic biofiltration of CT may be a feasible full-scale process.     

Degradation of toluene,xylene, and trimethylbenzene vapors by biofiltration: a comparison

This paper presents a comparative study of the biodegradation of three aromatic volatile compounds in a compost-based biofilter: toluene, xylene, and 1,2,4-trimethylbenzene, used in the course of this work for the first time in the field of biofiltration. Hence, three identical biofiltration units have been operated at the laboratory scale. During the experiments, nitrogen (as urea) was supplied at various concentrations to each reactor, via irrigated nutrient solutions. A comparative analysis of the results showed that the biodegradability scale followed the degree of substitution around the aromatic ring: toluene > xylene > trimethylbenzene, with 95, 80, and 70% maximum conversions, respectively. In addition, and despite the different removal levels achieved in the three bioreactors, it was established that from a reaction viewpoint, the degradation of the three compounds seemed to follow similar metabolic pathways involving methylcatechol isomers. Finally, by varying the nitrogen input concentrations in the three reactors, three degradation regimes have been highlighted: an N-limitation regime and an N-optimum regime, common to the three solvents, and an N-excess regime, favorable to the colonization of the filter beds by nitrifying species, which particularly affected the xylene and trimethylbenzene biodegradation.     

Biofilter Treatment of Volatile Organic Compound Emissions from Reformulated Paint: Complex Mixtures,Intermittent Operation,and Startup

Abstract

Two biofilters were operated to treat a waste gas stream intended to simulate off-gases generated during the manufacture of reformulated paint. The model waste gas stream consisted of a five-component solvent mixture containing acetone (450 ppm_v), methyl ethyl ketone (12 ppm_v), toluene (29 ppm_v), ethylbenzene (10 ppm_v), and p-xylene (10 ppm_v). The two biofilters, identical in construction and packed with a polyurethane foam support medium, were inoculated with an enrichment culture derived from compost and then subjected to different loading conditions during the startup phase of operation. One biofilter was subjected to intermittent loading conditions with contaminants supplied only 8 hr/day to simulate loading conditions expected at facilities where manufacturing operations are discontinuous. The other biofilter was subjected to continuous contaminant loading during the initial start period, and then was switched to intermittent loading conditions. Experimental results demonstrate that both startup strategies can ultimately achieve high contaminant removal efficiency (>99%) at a target contaminant mass loading rate of 80.3 g m^?3 hr^?1 and an empty bed residence time of 59 sec. The biofilter subjected to intermittent loading conditions at startup, however, took considerably longer to reach high performance. In both biofilters, ketone components (acetone and methyl ethyl ketone) were more rapidly degraded than aromatic hydrocarbons (toluene, ethylbenzene, and p-xylene). Scanning electron microscopy and plate count data revealed that fungi, as well as bacteria, populated the biofilters.     

Removal of Low Concentrations of Carbon Tetrachloride in Compost-Based Biofilters Operated under Methanogenic Conditions

ABSTRACT

Research was performed to demonstrate the removal of carbon tetrachloride (CT) using compost biofilters operated under methanogenic conditions. Biofilters were operated at an empty-bed residence time of 2.8 minutes using nitrogen as the atmosphere. Hydrogen and carbon dioxide were supplied as an electron donor and carbon source, respectively, during acclimation of the bed medium microbes. Once methanogenesis was demonstrated, CT flow to the biofilter was established. Biofilters were operated over a CT concentration range from 20 to 700 ppbv for 6 months. Bed medium microbes were able to remove up to 75% of the inlet CT. At excessively high CT concentrations (>500 ppmv), methane production and hydrogen utilization by the bed medium microbes appeared to be inhibited. CT removal by the biofilter decreased when the hydrogen supply was removed from the biofilter inlet, indicating that hydrogen acted as the electron donor for reductive dechlorination. The removal efficiency and relatively low empty bed residence times demonstrated by these laboratory-scale biofilters indicate that anaerobic biofiltration of CT may be a feasible full-scale process.     

Evaluation of Trickle Bed Air Biofilter Performance as a Function of Inlet VOC Concentration and Loading,and Biomass Control

ABSTRACT

The 1990 Amendments to the Clean Air Act have stimulated strong interest in the use of biofiltration for the economical, engineered control of volatile organic compounds (VOCs) in effluent air streams. Trickle bed air biofilters (TBABs) are especially applicable for treating VOCs at high loadings. For long-term, stable operation of highly loaded TBABs, removal of excess accumulated bio-mass is essential. Our previous research demonstrated that suitable biomass control for TBABs was achievable by periodic backwashing of the biofilter medium. Backwashing was performed by fluidizing the pelletized biological attachment medium with warm water to about a 40% bed expansion. This paper presents an evaluation of the impact of backwashing on the performance of four such TBABs highly loaded with toluene. The inlet VOC concentrations studied were 250 and 500 ppmv toluene, and the loadings were 4.1 and 6.2 kg COD/m³ day (55 and 83 g toluene/m³ hr). Loading is defined as kg of chemical oxygen demand per cubic meter of medium per day. Performance deterioration at the higher loading was apparently due to a reduction of the specific surface of the attached biofilm resulting from the accumulation of excess biomass. For a toluene loading of 4.1 kg COD/m³ day, it was demonstrated that the long-term performance of biofilters with either inlet concentration could be maintained at over 99.9% VOC removal by employing a backwashing strategy consisting of a frequency of every other day and a duration of 1 hr.     

Exceedances of the National Ambient Air Quality Standard for Ozone Occurring at a “Pristine” Area Site

Abstract

This paper presents a comparative study of the biodegradation of three aromatic volatile compounds in a compost-based biofilter: toluene, xylene, and 1,2,4-trimethyl-benzene, used in the course of this work for the first time in the field of biofiltration. Hence, three identical biofiltration units have been operated at the laboratory scale. During the experiments, nitrogen (as urea) was supplied at various concentrations to each reactor, via irrigated nutrient solutions. A comparative analysis of the results showed that the biodegradability scale followed the degree of substitution around the aromatic ring: toluene > xylene > trimethylbenzene, with 95, 80, and 70% maximum conversions, respectively. In addition, and despite the different removal levels achieved in the three biore-actors, it was established that from a reaction viewpoint, the degradation of the three compounds seemed to follow similar metabolic pathways involving methylcatechol isomers. Finally, by varying the nitrogen input concentrations in the three reactors, three degradation regimes have been highlighted: an N-limitation regime and an N-optimum regime, common to the three solvents, and an N-excess regime, favorable to the colonization of the filter beds by nitrifying species, which particularly affected the xylene and trimethylbenzene biodegradation.     

Effects of nitrogen and oxygen on biofilter performance

Three laboratory-scale biofilters packed with inert material were used to study the nitrogen and oxygen requirements for biofiltration of methanol. Mixtures of methanol with inorganic nitrogen (NH3 or NO3) at nitrogen-to-carbon (N:C) ratios ranging from 0.015 to 0.4 were employed to reveal nitrogen effects on biofiltration. In the oxygen study, mixtures of air and oxygen at different oxygen contents were used. At low nitrogen levels, the removal rate increased with increasing N:C ratio for both NH3 and NO3. However, at high concentrations, NH3 had an inhibitory effect on biodegradation while the removal rate reached a plateau at high NO3 concentrations. Biofiltration with 63% oxygen in the inlet gas stream increased the maximum removal rate from 120 to 145 g/m3/hr after 3 days in comparison with biofiltration with air. However, a further increase in oxygen content up to 80% did not lead to a further improvement in biofilter performance, suggesting that both oxygen and biofilm thickness can be the relevant factors limiting biofilter performance and creating the plateau in removal rates at high loadings.     

Cool temperature performance of a wheat straw biofilter for treating dairy wastewater

A wheat straw biofilter was evaluated for attenuating pollutants in dairy (milkhouse and milking parlor) wastewater. During the 14-day study, the biofilter was operated in a sequential aerobic-anaerobic mode in a temperature range of 8-14 degrees C. While the biofilter was very effective (89% removal) in attenuating total suspended solids and moderately effective (76% removal) in attenuating oil and grease, its effectiveness in attenuating chemical oxygen demand was low (37% removal). The biofilter was ineffective in attenuating nitrate, while its effectiveness in attenuating ammonium (20% removal) and total Kjeldahl nitrogen (15% removal) was low. The biofilter was not effective in attenuating ortho-phosphate, total phosphorus, and fecal coliform. Though microbial degradation accounted for some pollutant removal, filtration seemed to be the primary mechanism. Lower temperature of operation and high oil and grease concentration (that reduced nutrient transfer to the biofilm) decreased microbial activity, reducing pollutant attenuation. Biofilter performance could be enhanced by using residual heat in the wastewater to raise the operating temperature of the biofilter and by removing oil and grease prior to applying the wastewater to the biofilter.     

Degradation Kinetics of Biofilter Media Treating Reduced Sulfur Odors and VOCs

ABSTRACT

A lab-scale study was conducted to determine the rate and extent of decomposition of three biofilter media materials—compost, hog fuel, and a mixture of the two in 1:1 ratio—used in biofiltration applied to removal of reduced sulfur odorous compounds from pulp mill air emissions. The rate of carbon mineralization, as a measure of biofilter media degradation, was determined by monitoring respiratory CO₂ evolution and measuring the changes in carbon and nitrogen fractions of the biofilter materials over a period of 127 days. Both ambient air and air containing reduced sulfur (RS) compounds were used, and the results were compared. After 127 days of incubation with ambient air, about 17% of the media carbon was evolved as CO₂ from compost as compared to 6 and 12% from hog fuel and the mixture, respectively. The decomposition showed sequential breakdown of carbon moieties, and three distinct stages were observed for each of the biofilter media. First-order rate kinetics were used to describe the decomposition stages. Decomposition rates in the initial stages were at least twice those of the following stages. Carbon mineralization showed close dependence on the C/N ratio of the biofilter material. Media decomposition was enhanced in the presence of RS gases as a result of increased bioactivity by sulfur-oxidizing bacteria and other microorganisms, thus reducing the media half-life by more than 50%. At higher concentrations of RS gases, the CO₂ evolution rates were proportionally lower than those at the low concentrations because of the limited acid buffering capacity of the biofilter materials.     

Comparison of different packing materials for the biofiltration of air toxics

Four different biofilter packing materials (two porous ceramics, perlite, and open pore polyurethane foam) were compared for the removal of toluene vapors. The focus was on evaluating performance at relatively short gas retention time (13.5 and 27 sec). The reactors were initially operated as biotrickling filters with continuous feeding and trickling of a nutrient solution. After significant plugging of the biotrickling filter beds with biomass was observed, the operation mode was switched to biofiltration with only periodic supply of mineral nutrients. This resulted in stable conditions, which allowed detailed investigations over > 6 months. The reactor packed with cattle bone Porcelite (CBP), a ceramic material containing some macronutrients and micronutrients, exhibited the highest performance. The critical load (i.e., load at which 95% removal occurred) was 29 g m(-3) hr(-1) at a gas retention time of 13.5 sec and 66 g m(-3) hr(-1) at a gas retention time of 27 sec. After the long-term experiment, the packing materials were taken from the reactors and examined. The reactors were divided into three sections, top, middle, and bottom, to determine whether spatial differentiation of biomass occurred. The assays included a double-staining technique to count total and live microorganisms and determination of moisture, protein, and dry weight contents. Microbial community analysis was also conducted by denaturing gradient gel electrophoresis. The results showed that most reactors had a significant fraction of inactive biomass. Comparatively, the CBP biofilter held significantly higher densities of active biomass, which may be the reason for the higher toluene removal performance. The analyses suggest that favorable material properties and the nutrients slowly released by the CBP provided better environmental conditions for the process culture.     

Microbial degradation of livestock-generated ammonia using biofilters at typical ambient temperatures

The purpose of this research was to neutralize livestock-generated ammonia by using biofilters packed with inexpensive inorganic and organic packing material combined with multicultural microbial load at typical ambient temperatures. Peat and inorganic supporting materials were used as biofiltration matrix packed in a perfusion column through which gas was transfused. Results show the ammonia removal significantly fell in between 99 and 100% when ammonia concentration of 200 ppmv was used at different gas flow rates ranged from 0.030 to 0.060 m3 h(-1) at a fluctuating room temperature of 27.5 +/- 4.5 C (Mean +/- SD). Under these conditions, the emission concentration of ammonia that is liberated after biofiltration is less than 1 ppmv (0.707 mg m(-3)) over the period of our study, suggesting the usage of low-cost biofiltration systems for long-term function is effective at wider ranges of temperature fluctuations. The maximum (100%) ammonia removal efficiency was obtained in this biofilter was having an elimination capacity of 2.217 g m(-3) h(-1). This biofilter had high nitrification efficiencies and hence controlled ammonia levels with the reduced backpressure. The response of this biofilter to shut down and start up operation showed that the biofilm has a superior stability.     

Influence of phosphorus and trace metals in biofilters treating gaseous VOCs using a novel irrigation system

ABSTRACT

Although the appropriate supply of nutrients has been extensively researched, more information is required on the effects of nutrients in treating gaseous volatile organic compounds (VOCs) in biofiltration. In this study, the effects of phosphorous and trace metals on gaseous toluene and methyl ethyl ketone (MEK) removal were investigated. The transfer of nutrients from the irrigation liquid to the packed bed, and the consumption and holding amount of nutrients in the packing material were observed during biofiltration. Under conditions of 20–24 s of empty bed residence time, MEK removal was 95% or more in all conditions of the biofiltration reactors, whereas toluene removal was affected by the operating conditions of the reactors. Consumption ratio of phosphorus to carbon was from 1.7 × 10^?4 to 1.1 × 10^?3 in the steady state of VOC removal under the conditions of this study. When gaseous VOC treatment was restarted after nine days of shutdown, a significant decline in toluene removal was observed by the reactor in which phosphorus supply was approximately one fifth of the amount in another reactor. Two types of irrigation systems, soaking and spraying, were compared and soaking irrigation achieved a more even distribution of nutrients held inside the packed bed. Soaking irrigation was expected to lead to higher VOC removal capacity by this distribution effect of nutrients, but toluene removal in the reactor with this irrigation was lower than that in the reactor with spraying irrigation. One of the possible reasons for this was the inhibition of nutrients transfer in the bottom part of the reactor. The trend of transfer in all ingredients from the irrigation liquid to the packed bed was synchronized on the whole; however, this transfer relatively tended to be high in nitrate and sodium and low in ammonium and phosphate.

Implications: A major concern about using biofiltration systems to treat VOCs is the uncertainty regarding the appropriate nutrient supply to the filter bed to preserve microbial activity. This study showed that all the elements, except nitrogen, were retained sufficiently in the filter bed when a proper composition of nutrient solution was used for irrigation; however, phosphate addition may be needed when restarting a reactor from a prolonged period of shutdown. Distinct differences in the amount of transfer to the filter bed for different ingredients are probable, and may have to be taken into account when operating biofiltration reactors.     

Thermophilic Toluene Biofiltration

ABSTRACT

Thermophilic biodégradation of toluene with active compost biofilters was studied. Thermophilic conditions were maintained either by daily substrate addition (semicontinuous composting) or with a heating system (batch thermophilic composting). The semicontinuous system was designed for the treatment of cool (less than approximately 35 °C) gases under thermophilic conditions, while the extended batch approach was developed for the treatment of warmer gases. When the semicontinuous system was operated at 50 °C (after a one-day start-up period) at an average inlet concentration of 5.5 g m^-3, toluene was degraded at a rate ranging from 73 to 110 g C m^-3 hr^-1. Batch thermophilic treatment was somewhat less effective at the same inlet concentration. Semicontinuous toluene biofiltration at 60 °C was also investigated, but biodegradation rates were significantly lower than at 50 °C. In all systems, toluene biodegradation was proportional to the inlet concentration. Rates of up to 289 g C m^-3 hr^-1 (at an inlet concentration of 14.7 g m^-3) were achieved for semicontinuous and batch operation and 251 g C m^-3 hr^-1 (at an inlet concentration of 18.4 g m^-3) for batch thermophilic at 50 °C. Semicontinuous thermophilic operation at 60 °C showed a maximum rate of 119 g C m^-3 hr^-1. Active compost ther-mophilic biofiltration was found to be very effective when concentrations are high. At lower concentrations, rates were similar to those obtained with mesophilic biofiltration. Mixing, humidity, and the presence of cosubstrate were important parameters in maintaining high degradation rates. Biofiltration in the batch thermophilic mode could be useful when conventional biofiltration is ineffective due to elevated gas temperatures. Biofiltration in the semicontinuous thermophilic could reduce the biofilter size necessary for treatment of cooler gases containing high concentrations of volatile organic compounds.