Start-up and the effect of gaseous ammonia additions on a biofilter for the elimination of toluene vapors

Biotechnological techniques, including biofilters and biotrickling filters are increasingly used to treat air polluted with VOCs (Volatile Organic Compounds). In this work, the start-up, the effect of the gaseous ammonia addition on the toluene removal rate, and the problems of the heat accumulation on the performance of a laboratory scale biofilter were studied. The packing material was sterilized peat enriched with a mineral medium and inoculated with an adapted consortium (two yeast and five bacteria). Start-up showed a short adaptation period and an increased toluene elimination capacity (EC) up to a maximum of 190 g/m3/h. This was related to increased CO2 outlet concentration and temperature gradients between the packed bed and the inlet (Tm-Tin). These events were associated with the growth of the microbial population. The biofilter EC decreased thereafter, to attain a steady state of 8 g/m3/h. At this point, gaseous ammonia was added. EC increased up to 80 g/m3/h, with simultaneous increases on the CO2 concentration and (Tm-Tin). Two weeks after the ammonia addition, the new steady state was 30 g/m3/h. In a second ammonia addition, the maximum EC attained was 40 g/m3/h, and the biofilter was in steady state at 25 g/m3/h. Carbon, heat, and water balances were made through 88 d of biofilter operation. Emitted CO2 was about 44.5% of the theoretical value relative to the total toluene oxidation, but accumulated carbon was found as biomass, easily biodegradable material, and carbonates. Heat and water balances showed strong variations depending on EC. For 88 d the total metabolic heat was -181.2 x 10(3) Kcal/m3, and water evaporation was found to be 56.5 kg/m3. Evidence of nitrogen limitation, drying, and heterogeneities were found in this study.     

Influence of mixing and water addition on the removal rate of toluene vapors in a biofilter

The effects of successive mixing (homogenization) of packing material (peat), with or without water addition, on the removal of toluene vapors in a biofilter were studied. Over a period of 50 days, an increase in the Elimination Capacity (EC) of approximately 240% was obtained by successive mixing and water additions. After each mixing, a high EC of toluene was maintained only for a short period of 3-4 days. After this time, decreased biofilter performance was observed, probably associated with the development of dried and/or clogged zones. In the long-term experiments, an attenuation of the EC recovery was observed after successive mixing. In this case, an increase of 110% over 4 months of experiment was obtained. The global reduction of EC over time could be explained by the colonization of the biofilter by filamentous fungi which was facilitated by the mixing of the packing material. The most frequently observed fungi were identified as Scedosporium sp. and Cladosporium sp.     

Biotreatment of a gas-phase volatile mixture from fibreglass and composite manufacturing industries

Acetone, toluene and styrene (ATS) are representative air pollutants emanating during the production process in fibreglass and composite manufacturing industries. In this study, the performance of a steady-state biofilter inoculated with the fungus Sporothrix variecibatus was tested at different empty bed residence times (EBRTs), and at different inlet concentrations of ATS, corresponding to total pollutant loading rates ranging from 30 to 490 gm(-3)hour(-1). Styrene was somewhat better removed (47-100%) in the biofilter than acetone (34-100%) and toluene (42-100%), with maximum elimination capacities (EC(max)) of 108, 72 and 144 gm(-3)hour(-1), for ATS, respectively. Besides, it was observed that, although increasing the concentration of ATS decreased their removal, the presence of toluene also decreased the EC(max) of both acetone and toluene in the ternary mixture. During transient operations, the biofilter was subjected to intermittent shutdown and re-start operations where the gas-phase pollutant flow was stopped for either 5 or 16d. It was observed that, for longer shutdown periods (16d), the biofilter required nearly 8-10d to reach similar removal patterns to those observed before the shutdown phase. Batch biodegradation tests were conducted, using Sporothrix-like microorganisms present in the leachate of the biofilter, with a mixture of ATS as the sole carbon source. Complete removal of ATS was observed within the test period of 168 hours. Styrene was degraded faster, with a specific substrate utilization rate of 0.9 mg styrenemg biomass(-1)hour(-1), followed by toluene (0.6) and acetone (0.44). The effectiveness of the biofilter to reach high total EC (321.3 gm(-3)hour(-1)), and withstand transient operations shows the robustness of this fungal-bioreactor and its suitability to handle emissions from a fibreglass and composite manufacturing industry.     

Long-term performance of peat biofilters treating ethyl acetate, toluene, and its mixture in air

Three laboratory-scale peat biofilters were operated at 90 s empty bed residence time (EBRT) for over a year. Biodegradation of ethyl acetate, toluene, or a 1:1 mixture were investigated. In first stage, inlet concentration was progressively increased from 0.4 to 4.5 g/m(3). The maximum elimination capacity (EC) found for ethyl acetate was 190 gC/m(3).h, and it was not affected by toluene. The maximum EC found for toluene as a sole contaminant was 150 gC/m(3).h, but the presence of ethyl acetate decreased the toluene maximum EC to 80 gC/m(3).h. From respirometry monitoring, values of 3.19 g CO(2)/gC and 3.06 g CO(2)/gC for pure ethyl acetate and pure toluene, respectively, were found, with overall yield coefficients of 0.13 g dry biomass produced per gram ethyl acetate consumed and 0.28 g dry biomass produced per gram toluene consumed. CO(2) production in the 1:1 mixture was successfully simulated. Dynamics of living and dead cells were monitored in four sections of the biofilters. Concentrations ranged between 2.6 x 10(9) and 3.0 x 10(10) cells per gram-dry peat for total bacteria, and 2.4 x 10(9)-1.9 x 10(10) cells per gram-dry peat for living bacteria. At high loads loss of bacterial density in the inlet zones, and increase in the dead cells percentages up to 60% was observed. In second stage, long-term performance at an inlet concentration of 1.5 g/m(3) was evaluated to show the process feasibility. Good agreement with previous data was obtained in terms of EC and CO(2) production. Restoration of living cells proportion was also observed.     

Determination of local elimination capacities and moisturecontents in different biofilters treating toluene and xylene

The removal of toluene and xylene from an artificial waste gaswas investigated in two laboratory scale biofilters filled withmixtures of peat, bark and wood. The packed beds differed in themixture of materials used, so that peat and then bark was thedominant constituent. The biofilters were operated in an upflowmode. Both biofilters showed relatively high removal efficienciesfor both pollutants (74–98%). The evaluation of the localelimination capacities in the peat-loaded biofilter revealed thatthe major part of pollutants was degraded in the middle layer.In this biofilter, larger differences in theremoval rates along the bed height were also observed. In thebiofilter with bark as dominant material, the major part ofpollutants was degraded at the inlet of the bed and also at arelative height of 0.7. Moisture contents of 71–80% and 65–78%were found for the biofilter with peat and bark respectively. Whenthe regular pouring of nutrient solution through the bed wasinterrupted for 1 month, a decrease in efficiency was observed inthe biofilter with bark, whilst the efficiency in the biofilter withpeat remained the same.     

Construction of a bacterial consortium for the biofiltration of benzene,toluene and xylene emissions

On equal parts of benzene, toluene and p-xylene (BTX), a stable bacterial consortium was enriched for removal of BTX vapours from air. As demonstrated by gas chromatographic monitoring, this consortium removed all three BTX components but was able to grow only on benzene and/or toluene. A Pseudomonas putida strain, PPO1, isolated from this consortium behaved in an identical manner. When immobilized on a porous peat/perlite column, both the consortium and the PPO1 isolated removed all three BTX components from metered air streams. However, due to the accumulation of products from the incompletely metabolized p-xylene, the removal rates were unsatisfactory and declined further with time. P. putida ATCC 33015 bearing the TOL plasmid was capable of growing on toluene, on para- and on meta- xylene isomers, but not on benzene. When the PPO1 and ATCC 33015 strains were immobilized, in equal parts, on peat/perlite columns a much improved and sustainable removal of all three BTX components was observed at the rate of 40–50 g/h. m3 filter bed. Due to the dominance of the ring-hydroxylating pathways over the TOL pathway, the classical enrichment approach did not result in a consortium capable of the sustained removal of all BTX components. However, a rationally formulated consortium consisting of members with complementary metabolic abilities was capable of this task and should be of use both in industrial emission control and in soil venting operations.     

Correlation of biological activity and reactor performance in biofiltration of toluene with the fungus Paecilomyces variotii CBS115145

A biofiltration system inoculated with the mold Paecilomyces variotii CBS115145 showed a toluene elimination capacity (EC) of around 250 g/m3 of biofilter/h, which was higher than the values usually reported for bacteria. P. variotii assimilated m- and p-cresols but not the o isomer. Initial toluene hydroxylation occurred both on the methyl group and through the p-cresol pathway. These results were corroborated by detecting benzyl alcohol, benzaldehyde, and p-cresol as volatile intermediates. In liquid cultures with toluene as a substrate, the activity of toluene oxygenase (TO) was 5.6 nmol of O2/min/mg of biomass, and that of benzyl alcohol dehydrogenase was 16.2 nmol of NADH/min/mg of protein. Toluene biodegradation determined from the TO activity in the biofilter depended on the biomass distribution and the substrate concentration. The specific enzymatic activity decreased from 6.3 to 1.9 nmol of O2/min/mg of biomass along the reactor. Good agreement was found between the EC calculated from the TO activity and the EC measured on the biofilter. The results were confirmed by short-time biofiltration experiments. Average EC measured in different biofiltration experiments and EC calculated from the TO activity showed a linear relation, suggesting that in the biofilters, EC was limited by biological reaction. As the enzymatic activities of P. variotii were similar to those reported for bacteria, the high performance of the fungal biofilters can possibly be explained by the increased transfer of the hydrophobic compounds, including oxygen, from the gas phase to the mycelia, overcoming the transfer problems associated with the flat bacterial biofilms.     

Degradation of BTEX compounds in liquid media and in peat biofilters

A mixed culture, enriched from Sphagnum peat moss, contaminated with gasoline vapours, degraded individual and mixed components of BTEX (benzene, toluene, ethylbenzene, xylene). Complete degradation of radiolabelled toluene by the mixed culture was observed in mineralisation studies. Individual isolates from a mixed culture containingPseudomonas maltophilia, P. testosteroni andP. putida biotype A exhibited contrasting BTEX degradation patterns. WhileP. putida biotype A degraded all of the BTEX compounds,P. maltophilia andP. testosteroni, appeared unable to degrade benzene and xylenes, respectively. When the peat, inoculated with the mixed culture, was used as a biofilter (6.2 cm diameter ×93 cm length) for degradation of toluene and ethylbenzene vapours, percentage removal efficiencies were 99 and 85, respectively. When the capacity of the biofilter to degrade a combination of BTEX compounds was evaluated, percentage removal efficiencies for toluene, ethylbenzene,p-xylene,o-xylene and benzene were 99, 85, 82, 80 and 78, respectively. The importance of using the mixed culture as an inoculum in the biofilter was established and also the relationship between contaminated vapour flow rate and percentage removal efficiency.     

Characterization of compost biofiltration system degrading dichloromethane

The effects of acclimatization of microbial populations, compound concentration, and media pH on the biodegradation of low concentration dichloromethane emissions in biofiltration systems was evaluated. Greater than 98% removal efficiency was achieved for dichloromethane at superficial velocities from 1 to 1.5 m(3)/m(3). min (reactor residence times of 1 and 0.7 min, respectively) and inlet concentrations of 3 and 50 ppm Although acclimatization of microbial populations to toluene occurred within 2 weeks of operation start-up, initial dichloromethane acclimatization took place over a period of 10 weeks. This period was shortened to 10 days when a laboratory grown consortium of dichloromethane degrading organism, isolated from a previously acclimatized column, was introduced into fresh biofilter media. The mixed culture consisted to 12 members, which together were able to degrade dichloromethane at concentrations up to 500 mg/L. Only one member of the consortium was able to degrade dichloromethane were sustained for more than 4 months in a biofilter column receiving an inlet gas stream with 3 ppm(v) of dichloromethane acidification of the column and resulting decline in performance occurred when a 50-ppm(v) inlet concentration was used. A biofilm model incorporating first order biodegradation kinetics provided a good fit to observed concentration profiles, and may prove to be a useful tool for designing biofiltration systems for low concentration VOC emissions. (c) 1994 John Wiley & Sons, Inc.     

Removal of dimethyl disulfide by the peat seeded with night soil sludge

The removal characteristics of dimethyl disulfide (DMDS) with a fibrous peat biofilter were studied. The peat itself did not remove DMDS. The peat inoculated with aerobically-digested night soil sludge as a source of microorganisms showed an efficient removal of DMDS with the maximum removal rate, 0.68 g-S·kg-dry peat⁻¹·d⁻¹ and the saturation constant, 1 ppm. The removal rate of DMDS by the biofilter decreased when pH was below 5.5. The number of microorganisms isolated on thiosulfate-agar plates (pH 7) remarkably increased in DMDS-acclimated peat. Similar removal characteristics and the change in microflora were observed in methanethiol (MT)- and dimethyl sulfide (DMS)-acclimated peat. These results indicated that some chemolithotrophic and non-acidophilic sulfur-oxidizing microorganisms such as Thiobacilli, originating from night soil sludge, were responsible for degradation of these organosulfur compounds in the peat biofilter.     

Biofiltration of ethylbenzene vapours: influence of the packing material

In order to investigate suitable packing materials, a soil amendment composed of granular high mineralized peat (35% organic content) locally available has been evaluated as carrier material for biofiltration of volatile organic compounds in air by comparison with a fibrous peat (95% organic content). Both supports were tested to eliminate ethylbenzene from air streams in laboratory-scale reactors inoculated with a two-month conditioned culture. In pseudo-steady state operation, experiments at various ethylbenzene inlet loads (ILs) were carried out. Maximum elimination capacity of about 120 g m(-3) h(-1) for an IL of 135 g m(-3) h(-1) was obtained for the fibrous peat. The soil amendment reactor achieved a maximum elimination capacity of about 45 g m(-3) h(-1) for an inlet load of 55 g m(-3) h(-1). Ottengraf-van den Oever model was applied to the prediction of the performance of both biofilters. The influence of gas flow rate was also studied: the fibrous peat reactor kept near complete removal efficiency for empty bed residence times greater than 1 min. For the soil amendment reactor, an empty bed residence time greater than 2 min was needed to achieve adequate removal efficiency. Concentration profiles along the biofilter were also compared: elimination occurred in the whole fibrous peat biofilter, while in the soil amendment reactor the biodegradation only occurred in the first 65% part of the biofilter. Results indicated that soil amendment material, previously selected to increase the organic content, would have potential application as biofilter carrier to treat moderate VOC inlet loads.     

Correlation of Biological Activity and Reactor Performance in Biofiltration of Toluene with the Fungus Paecilomyces variotii CBS115145

A biofiltration system inoculated with the mold Paecilomyces variotii CBS115145 showed a toluene elimination capacity (EC) of around 250 g/m³ of biofilter/h, which was higher than the values usually reported for bacteria. P. variotii assimilated m- and p-cresols but not the o isomer. Initial toluene hydroxylation occurred both on the methyl group and through the p-cresol pathway. These results were corroborated by detecting benzyl alcohol, benzaldehyde, and p-cresol as volatile intermediates. In liquid cultures with toluene as a substrate, the activity of toluene oxygenase (TO) was 5.6 nmol of O₂/min/mg of biomass, and that of benzyl alcohol dehydrogenase was 16.2 nmol of NADH/min/mg of protein. Toluene biodegradation determined from the TO activity in the biofilter depended on the biomass distribution and the substrate concentration. The specific enzymatic activity decreased from 6.3 to 1.9 nmol of O₂/min/mg of biomass along the reactor. Good agreement was found between the EC calculated from the TO activity and the EC measured on the biofilter. The results were confirmed by short-time biofiltration experiments. Average EC measured in different biofiltration experiments and EC calculated from the TO activity showed a linear relation, suggesting that in the biofilters, EC was limited by biological reaction. As the enzymatic activities of P. variotii were similar to those reported for bacteria, the high performance of the fungal biofilters can possibly be explained by the increased transfer of the hydrophobic compounds, including oxygen, from the gas phase to the mycelia, overcoming the transfer problems associated with the flat bacterial biofilms.     

Toluene biofiltration by the fungus Scedosporium apiospermum TB1

The performance of biofilters inoculated with the fungus Scedosporium apiospermum was evaluated. This fungus was isolated from a biofilter which operated with toluene for more than 6 months. The experiments were performed in a 2.9 L reactor packed with vermiculite or with vermiculite-granular activated carbon as packing material. The initial moisture content of the support and the inlet concentration of toluene were 70% and 6 g/m3, respectively. As the pressure drop increased from 5-40 mm H2O a strong initial growth was observed. Stable operation was maintained for 20 days with a moisture content of 55% and a biomass of 33 mg biomass/g dry support. These conditions were achieved with intermittent addition of culture medium, which permitted a stable elimination capacity (EC) of 100 g/m3(reactor)h without clogging. Pressure drop across the bed and CO2 production were related to toluene elimination. Measurement of toluene, at different levels of the biofilter, showed that the system attained higher local EC (200 g/m3(r)h) at the reactor outlet. These conditions were related to local humidity conditions. When the mineral medium was added periodically before the EC decreases, EC of approximately 258 g/m3(r)h were maintained with removal efficiencies of 98%. Under these conditions the average moisture content was 60% and 41 mg biomass/g dry support was produced. No sporulation was observed. Evaluation of bacterial content and activities showed that the toluene elimination was only due to S. apiospermum catabolism.     

Biodegradation of benzo[α]pyrene,toluene, and formaldehyde from the gas phase by a consortium of Rhodococcus erythropolis and Fusarium solani

Polycyclic aromatic hydrocarbons (PAHs) and volatile organic compounds (VOCs) are important indoor contaminants. Their hydrophobic nature hinders the possibility of biological abatement using biofiltration. Our aim was to establish whether the use of a consortium of Fusarium solani and Rhodococcus erythropolis shows an improved performance (in terms of mineralization rate and extent) towards the degradation of formaldehyde, as a slightly polar VOC; toluene, as hydrophobic VOC; and benzo[α]pyrene (BaP) as PAH at low concentrations compared to a single-species biofilm in serum bottles with vermiculite as solid support to mimic a biofilter and to relate the possible improvements with the surface hydrophobicity and partition coefficient of the biomass at three different temperatures. Results showed that the hydrophobicity of the surface of the biofilms was affected by the hydrophobicity of the carbon source in F. solani but it did not change in R. erythropolis. Similarly, the partition coefficients of toluene and BaP in F. solani biomass (both as pure culture and consortium) show a reduction of up to 38 times compared to its value in water, whereas this reduction was only 1.5 times in presence of R. erythropolis. Despite that increments in the accumulated CO2 and its production rate were found when F. solani or the consortium was used, the mineralization extent of toluene was below 25%. Regarding BaP degradation, the higher CO2 production rates and percent yields were obtained when a consortium of F. solani and R. erythropolis was used, despite a pure culture of R. erythropolis exhibits poor mineralization of BaP.     

Enrichment and characterization of a sulfate-reducing toluene-degrading microbial consortium by combining in situ microcosms and stable isotope probing techniques

A toluene-degrading microbial consortium was enriched directly in a BTEX-contaminated aquifer under sulfate-reducing conditions using in situ microcosms consisting of toluene-loaded activated carbon pellets. Degradation of toluene and concomitant sulfide production by the consortium was subsequently demonstrated in laboratory microcosms. The consortium was physiologically and phylogenetically characterized by isotope tracer experiments using nonlabeled toluene, [¹³C]-α-toluene or [¹³C₇]-toluene as growth substrates. Cells incubated with [¹³C]-α-toluene or [¹³C₇]-toluene incorporated 8–15 at.%¹³C and 51–57 at.%¹³C into total lipid fatty acids, respectively, indicating a lower specific incorporation of ¹³C from [¹³C₇]-toluene. In order to identify the toluene-assimilating bacteria, the incorporation of carbon from both [¹³C]-α-toluene and [¹³C₇]-toluene into rRNA was analyzed by stable isotope probing. Time and buoyant density-resolved 16S rRNA gene-based terminal restriction fragment length polymorphism profiles, combined with cloning and sequencing, revealed that an uncultured bacterium (99% sequence similarity) related to the genus Desulfocapsa was the main toluene-degrading organism in the consortium. The ratio of the respective terminal restriction fragments changed over time, indicating trophic interactions within this consortium.     

Fungal removal of gaseous hexane in biofilters packed with poly(ethylene carbonate) pine sawdust or peat composites

The removal of volatile organic compounds (VOC) in biofilters packed with organic filter beds, such as peat moss (PM) and pine sawdust (PS), frequently presents drawbacks associated to the collapse of internal structures affecting the long-term operation. Poly(ethylene ether carbonate) (PEEC) groups grafted to these organic carriers cross linked with 4,4'-methylenebis(phenylisocyanate) (MDI) permitted fiber aggregation into specific shapes and with excellent hexane sorption performance. Modified peat moss (IPM) showed very favorable characteristics for rapid microbial development. Water-holding capacity in addition to hexane adsorption almost equal to the dry samples was obtained. Pilot scale hexane biofiltration experiments were performed with the composites after inoculation with the filamentous fungus Fusarium solani. During the operation of the biofilter under non-aseptic conditions, the addition of bacterial antibiotics did not have a relevant effect on hexane removal, confirming the role of fungi in the uptake of hexane and that bacterial growth was intrinsically limited by an adequate performance of the composites. IPM biofilter had a start-up period of 8-13 days with concurrent CO(2) production of approximately 90 g m(-3) h(-1) at day 11. The final pressure drop after 70 days of operation was 5.3 mmH(2)O m(-1) reactor. For modified pine sawdust (IPS) packed biofilter, 5 days were required to develop an EC of about 100 g m(-3) h(-1) with an inlet hexane load of approximately 190 g m(-3) h(-1). Under similar conditions, 12-17 days were required to observe a significant start-up in the reference perlite biofilter to reach gradually an EC of approximately 100 g m(-3) h(-1) at day 32. Under typical biofiltration conditions, the physical-chemical properties of the modified supports maintained a minimum water activity (a(w)) of 0.925 and a pH between 4 and 5.5, which allowed the preferential fungal development and limited bacterial growth.     

Comparison of bioremediation strategies for soil impacted with petrochemical oily sludge

Different bioremediation techniques (natural attenuation, biostimulation and bioaugmentation) in contaminated soils with two oily sludge concentrations (1.5% and 6.0%) in open and closed microcosms systems were assessed during 90 days. The results showed that the highest biodegradation rates were obtained in contaminated soils with 6% in closed microcosms. Addition of microbial consortium and nutrients in different concentrations demonstrated higher biodegradation rate of total petroleum hydrocarbons (TPH) than those of the natural attenuation treatment. Soils treated in closed microcosms showed highest removal rate (84.1 ± 0.9%) when contaminated at 6% and bacterial consortium and nutrients in low amounts were added. In open microcosms, the soil contaminated at 6% using biostimulation with the highest amounts of nutrients (C:N:P of 100:10:1) presented the highest degradation rate (78.7 ± 1.3%). These results demonstrate that the application of microbial consortium and nutrients favored biodegradation of TPH present in oily sludge, indicating their potential applications for treatment of the soils impacted with this important hazardous waste.     

Effect of isobutanol on toluene biodegradation in nitrate amended, sulfate amended and methanogenic enrichment microcosms

Isobutanol is an alternate fuel additive that is being considered because of economic and lower emission benefits. However, future gasoline spills could result in co-contamination of isobutanol with gasoline components such as benzene, toluene, ethyl-benzene and xylene. Hence, isobutanol could affect the degradability of gasoline components thereby having an effect on contaminant plume length and half-life. In this study, the effect of isobutanol on the biodegradation of a model gasoline component (toluene) was examined in laboratory microcosms. For this, toluene and isobutanol were added to six different toluene degrading laboratory microcosms under sulfate amended, nitrate amended or methanogenic conditions. While toluene biodegradation was not greatly affected in the presence of isobutanol in five out of the six different experimental sets, toluene degradation was completely inhibited in one set of microcosms. This inhibition occurred in sulfate amended microcosms constructed with inocula from wastewater treatment plant activated sludge. Our data suggest that toluene degrading consortia are affected differently by isobutanol addition. These results indicate that, if co-contamination occurs, in some cases the in situ half-life of toluene could be significantly extended.     

Microbial response and elimination capacity in biofilters subjected to high toluene loadings

Elimination capacity (EC) is frequently used as a performance and design criterion for vapor-phase biofilters without further verification of the microbial quantity and activity. This study was conducted to investigate how biofilters respond to high pollutant loadings and ultimately how this affects the EC of the biofilter. Two identical laboratory-scale biofilters were maintained at an initial toluene loading rate of 46 g m⁻³ h⁻¹ for a period of 24 days. After the initial biofilm development stage, the loading rates were increased to 91 g m⁻³ h⁻¹ and 137 g m⁻³ h⁻¹, respectively. Following a short period of pseudo-steady state, toluene removal efficiencies rapidly declined in both biofilters, with a concurrent decline in both critical and maximum ECs. The decline was mainly due to deterioration in the biodegradation activity of the biofilm and a decline in the toluene-degrading bacterial population within the biofilm phase. The findings imply that high toluene loadings accelerated the deterioration in overall performance due to a rapid accumulation of inactive biomass. As a result, care must be used when relying on EC values for biofilter design and operational purposes, since the values do not appropriately reflect the temporal changes in biodegradation activity and active biomass quantities that can occur in biofilters subjected to high inlet loadings.     

Characterization of a biofilter treating toluene contaminated air

The removal of toluene from an experimental gas-stream was studied in an industrial biofilter filled with poplar wood bark. Toluene degradation, approximately 85% through the operating period, resulted in low levels of toluene in the off-gas effluent. For a toluene load of 6.7 g m^-3 h^-1 the elimination capacity of the biofilter was found to be 6.0 g m^-3 h^-1. Toluene removal was due to biodegradative activity of microorganisms in the filter bed; the most probable number counts of toluene degraders increased from 2.4×10² to 6.4×10⁷ MPN/g dry packing material in about seven months of air-toluene supply. The degradative capacity of a Burkholderia (Pseudomonas) cepacia strain, isolated from the biofilter material, as an example of the effectiveness of microbial toluence removal was tested in batch culture. The microorganism degraded completely 250 ppm of toluence supplied as sole carbon source in 24 hours. The high performance demonstrated for a long period and the mechanical and physico-chemical stability of the biofilter favour its use in industrial full-scale off-gas control.