Ind. Eng. Chem. Res. 1997, 36, 4719-4725
4719
Biofiltration of Air Polluted with Toluene under Steady-State Conditions: Experimental Observations Karim Kiared,†,‡ Bruno Fundenberger,‡ Ryszard Brzezinski,‡ Guy Viel,§ and Miche` le Heitz*,‡ De´ partement de Ge´ nie Chimique, Faculte´ des Sciences Applique´ es et De´ partement de Biologie, Faculte´ des Sciences, Universite´ de Sherbrooke, 2500 Boul. Universite´ , Sherbrooke, Que´ bec J1K 2R1, Canada, and Valoraction Inc., 855 rue Pe´ pin, Sherbrooke, Que´ bec J1L 2P8, Canada
In this study, we describe the removal of toluene vapors in a pilot scale biofilter. Biofiltration tests have been performed in a column fed upward with contaminated air at ambient conditions. The column was packed with a mixture of conditioned biomass and structuring agent on which a mixed microbial population of four selected strains was immobilized and then formed a biolayer. The biofilter was operated under various inlet-airstream toluene concentrations and flow rates of the contaminated airstream. Based on the present measurements, the biofilter proved effective in removing toluene at rates up to 165 g‚h-1‚m-3 of packing. The effect of some design and operation parameters (concentration of nutrients solution, presence of xylene, gas flow rate, pressure drop, temperature, etc.) are reported. Introduction Biological air treatment is an emerging environmental control technology which is becoming increasingly used in situations where odorous or vapors of volatile organic compounds (VOCs) are present in polluted air at low or moderate concentrations, e.g., in the range of 0-6 g.m-3 (Leson, 1991). When compared to other control technologies, such as catalytic oxidation, incineration, and adsorption, biofiltration processes are much more economical and efficient. On the other hand, and in contrast with the more recent biological processes, the conventional methods (a) do not lead to destruction of the pollutants and cause secondary water, air, or soil pollution which needs further treatments and (b) consume more energy. The biofiltration process is one of the three-phase reactor configurations commonly used in biological air treatment. The micro-organisms that biodegrade VOCs are immobilized in a fixed bed of solid particles acting as filtering media, around which a biolayer (biofilm) is formed. Biomass, peat, compost, activated carbon, or other porous media that are capable of adsorbing gaseous compounds and supporting biological growth are generally used as filtering materials. In the biofilter, waste gases are purified during their passage through the biological filter. Contaminants diffuse through the wet biofilm and are subsequently degraded aerobically to carbon dioxide (CO2) and water or are used as a carbon source for microbial growth. In contrast with the trickling filter which operates with a continuous water phase (Diks and Ottengraf, 1991), the biofilter is periodically fed with a nutrient solution to sustain the micro-organisms and to maintain an optimal moisture level (from 50 to 70%) in the filtering material, and to remove excess growth of biomass. A further improvement with the object of preventing dryness from occurring in the packing consists of saturating the input gases in a separate humidification apparatus before they enter the reactor. * Author to whom all correspondence should be addressed. E-mail:
[email protected]. Fax: (819)821-7955. † E Ä cole Polytechnique, C. P. 6079, Station “Centre-Ville”, Montre´al, Que´bec H3C 3A7, Canada. ‡ Universite ´ de Sherbrooke. § Valoraction Inc. S0888-5885(97)00147-4 CCC: $14.00
Our biofiltration research has focused on expanding the range of applications for the biofiltration technology to treatments conducted on the pilot scale of high concentrations of VOCs. Several types of biological attachment media have been tested at both laboratory and pilot scale by our group (Bibeau et al., 1997; Kiared et al., 1997; Jorio et al., 1997) to establish the feasibility of developing an engineered system. One of our studies (Kiared et al., 1997) was performed with the aim of removing VOCs present in a print shop air exhaust at the pilot scale with peat balls (Rothenbu¨hler, 1995) which offer an optimal exchange surface and facilitate gaseous diffusion. Indeed, the rigid structure of the peat balls permitted a filter bed height of 2-5 m to be constructed. The biofiltration of airstreams containing toluene or BTEX components (benzene, toluene, ethylbenzene, and xylene) has been previously studied by others. Biofilters, containing peat or other natural medias (e.g., compost) have been shown to have limited elimination capacities for these compounds. The work done by Weber et al. in 1995 (cited by Smith et al., 1996) reported a removal efficiency of 25 % and an elimination capacity of 44 g‚h-1‚m-3 for a toluene loading of 175 g‚h-1‚m-3. Morales et al. (1994) reported achieving a maximum elimination capacity of 25 g‚h-1‚m-3 (of toluene) for several inlet air concentrations used. The research performed by Severin et al. (1993) reported, possibly, the most successful applications attaining toluene removal efficiencies of 84% for loadings of toluene that corresponded to a maximum elimination capacity of 55 g‚h-1‚m-3 (of toluene). Others (Ottengraf et al., 1983; Leson and Winer, 1991; Liu et al., 1994, Tahraoui et al., 1994) have reported maximum elimination capacities not exceeding 60 g‚h-1‚m-3. Without doubt, the results obtained by Smith et al. (1996) are the most spectacular. These authors developed two biomass control strategies for extending the stable operation of highly efficient biofilters at substantial toluene loadings. The strategies developed were focused on avoiding biofilter failure by preventing bed plugging due to accumulating biomass. The method is indeed somewhat arduous to conduct industrially as the backwashing of the biofilter was performed by means of medium fluidization. However, a sustained toluene © 1997 American Chemical Society
4720 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
removal efficiency of over 99% was achieved by this means. Our objective was to meet or surpass their performances at this level, with the biofilters designed and constructed at Universite´ de Sherbrooke. To achieve this goal, our efforts were concentrated in choosing an optimal filtering material to avoid all problems related to bed compaction and/or biomass accumulation. This was done by submitting the conditionned biomass to various tests at different stages (adsorption tests, rigidity test, etc.) at the laboratory scale (see Bibeau et al., 1997) and also by selecting a suitable synthetic agent that can be mixed with the biomass inside the column in order to ensure the long-term stability of the bed in avoiding failure by compaction and fissuring. A complementary research study was undertaken on the selection of bacterial strains capable of degrading the target pollutant (i.e., toluene). This paper reports the results of our research group’s effort on the development of a pilot scale biofilter utilizing strains of the microflorae, immobilized on the conditioned biomas, for the removal of toluene present in the airstream at relatively high concentrations. The purpose of this work is to present impressive results on toluene biofiltration by highlighting the effects of some design and operation parameters such as nutrient solution concentration, inhibiting effects, gas flow rate, and temperature and pressure drop. The production of CO2 and the total bacterial counts are also mentioned in this work. Indeed, the data presented serve not only to extend our knowledge of this relatively new technology but also to become a basis for biofilter modeling, design, and scale-up. Experimental Setup and Procedure Figure 1 shows the main components of the biofiltration facility. The experimental setup consisted essentially of a single-stage, packed 0.3 mm i.d. and 3 m high rigid Plexiglas column fed at the base with contaminated air at ambient temperature and pressure and an inlet air wetting column. The biofilter was filled to a height of over 2 m with conditioned biomass mixed with a synthetic agent. The biomass and synthetic agent are introduced carefully into the column to avoid excessive initial compacting and fissuring of the bed. A small fraction of air saturated with the vapor from the solvent was mixed with a larger humid air flow (air humidified to more than 93% of relative humidity in the humidification column) prior to entering the biofilter. Flow rates of main airstream and air loaded with VOC were controlled by valves and monitored by previously calibrated rotameters in order to maintain the desired VOC inlet concentrations and air residence times within the column. The contaminated air stream is contacted only by glass, Teflon, and the Plexiglass of the biofilter. Reactor gas and solids analyses were performed by means of several sampling and measurement access ports located at three different levels along the biofilter. Other measurement points were also placed along the column for occasional measurement of temperature and pressure profiles through the column. Gas analyses were performed by means of a total hydrocarbon analyzer (Olson Horiba model 9050) equipped with a flame ionization detector and a Hewlett-Packard, model 5890, gas chromatograph coupled to a mass spectrometer (GC/ MS). The two pieces of equipment were provided with a sampling loop permitting automatic and repeated sampling of the polluted air from the biofilter. Inlet and outlet air samples from the biofilter were collected and
diverted directly, via Teflon tubes, to the gas analysis equipment. Both analyzers were calibrated daily prior to each column measurement (levels 1, 2, and 3) with clean air and with standards having different pollutant concentrations. The temperature inside the biofilter was measured at three measurement points by thermocouples in order to use the variation of temperature as a diagnostic of the microbial population activity. The pressure drop through the biofilter was measured at three measurement points by means of a differential pressure meter (air flow developments, Canada, Ltd). The bed voidage (bed voidage (%) ) void volume/filter bed volume × 100) of the filter bed was measured. The void volume is obtained by completing with distilled water the volume of a 1000 mL cylinder already filled with the porous filter bed. The cylinder wall was tapped lightly to free air bubbles, thus ensuring precise measurements. The filter bed volume is already known. The microbial population density was followed by determining the total microbial counts in samples of the filtering material regularly withdrawn through the sampling ports. The nutrient solution that was used for the biofilter humidification consisted of the following: 500 mL of a solution, prepared using distilled water, consisting of Na2HPO4 (30 g/L), KH2PO4 (15 g/L), NH4Cl (5 g/L), KNO3 (2.5 g/L), MgSO4 (1 mol/L) (5 mL), CaCl2 (0.01 mol/L) (50 mL), and NaCl (5% w/v) to which was added 5 mL of a micronutrient solution. The nutrients were added periodically on the top of the biofilter. Biofiltration of air contamined by toluene was maintained in the pilot scale biofilter for a period of 10 months but we report here only the results of a 2 month period. The input air flow rate through the biofilter was in the range of 4-10 m3‚h-1 corresponding to a residence time, based on the porosity of the bed, from 127 to 50 s. Concentrations of the contaminant ranged between 0.8 and 8 g‚m-3. The biofilter design and operating parameters are summarized in Table 1. At start-up, the filter bed was inoculated with a solution containing four specific microorganisms, namely Pseudomonas putida (ATCC 31483), Pseudomonas putida biotype A (ATCC 39213), Rhodococcus sp. (ATCC 21499), and Arthrobacter paraffineus (ATCC 15590). The inoculum was poured onto the packing from the top, collected at the base, and recycled several times in order to ensure homogeneous distribution of the microbial population inside the filter bed. Experiments were performed immediately after column inoculation. The lag phase took 6 days. This phenomena associated with process start-up (lag phase period) cannot easily be described, so only steady-state operations are in the scope of the present study. In order to give favorable life support conditions for the microflorae and to remove the excess of biomass produced during the biodegradation operation, a nutrient solution was occasionnally pumped through the biofilter. The moisture content of the filtering material was occasionally measured, by a weight difference in samples before and after drying at 110 °C, and adjusted to remain in the range of 50-70% by controlling the humidification frequency. Results and Discussions The experimental observations on pilot scale biofilter behavior when toluene is removed from polluted air under ambient conditions are now presented. However, some experiments with xylene with or without toluene are presented to illustrate the xylene inhibition effect
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4721
Figure 1. Schematic of the experimental setup.
on the toluene biodegradation rate. The results obtained for toluene are presented in two ways. First, the measured or calculated parameters are presented versus time to show the evolution of the biofilter function and the effect of certain interventions performed on the biofilter such as backwashing and changes of nutrient concentrations. Secondly, the elimination capacity (EC,
calculated by eq 1 in Table 2) and removal efficiency (X, calculated by eq 2 in Table 2) are presented as a function of the inlet load (IL, calculated by eq 3 in Table 2) of the pollutant where effects of certain operating parameters such as gas flow rate, nutrients concentration, pressure drop, and temperature are shown. The microbial population was also measured at the midcol-
4722 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Design and Operating Parameters parameters
values
filter material: conditioned biomass (diameter) height of the packing diameter of the column gas flow rate temperature pressure drop VOC concentration in waste gases toluene relative humidity of air
3.5 mm 2m 30 cm 4-10 m3‚h-1 25-36 °C 0-2 cm of H2O/m 0.8-8 g‚m-3 >93%
Table 2. Elimination Capacity, Removal Efficiency, and Inlet Load elimination capacity removal efficiency inlet load
Qg(Cg,in - Cg,out) V Cg,in - Cg,out X) Cg,in QgCg,in IL ) V EC )
(1) (2) (3)
umn level in terms of total bacterial count, for diagnosing their growth rate within the filter bed. These experimental results provide valuable information leading to better understanding of the biofiltration process. 1. Effect of Nutrients Solution Concentration. The effect of the nutrients solution concentration ratio (N/P) on the biofilter elimination capacity for toluene was first examined. Figures 2a and 2b present the evolution of the elimination capacity and inlet load (Figure 2a) and the removal efficiency (Figure 2b) versus time during the whole test period at two N/P ratios, a low ratio A and a high ratio B. By taking a constant value of the gas flow rate equal to 6.5 m3‚h-1, increasing the N/P ratio from A to B leads to a net improvement of the removal efficiency from 20 to 80% and the elimination capacity from 25 to 125 g‚h-1‚m-3 at an inlet toluene load of around 200 g‚h-1‚m-3. After a period of 10 days, after the day 50, corresponding to the adaptation to the novel experimental conditions, the removal efficiency and the elimination capacity decreased to 45% and to 100 g‚h-1‚m-3, respectively and then increased again, reaching the stable values of 90% and 165 g‚h-1‚m-3. A more revealing presentation of the estimated elimination capacity is its variation versus inlet load as shown in Figure 2c. At a lower N/P ratio (e.g., A) the toluene elimination capacity remains constant at around 25 g‚h-1‚m-3. At a higher ratio, N/P ) B, the toluene elimination capacity is an increasing function of the inlet load, indicating the diffusion limitation of toluene into the biofilm in the process. The maximum toluene elimination capacity of 165 g‚h-1‚m-3 is reached beyond an inlet load of 200 g‚h-1‚m-3. In this region the EC is independent of the toluene inlet load, and the process becomes exclusively limited by the biodegradation reaction. 2. Effect of Total Gas Flow Rate. The input gas flow rate is one of the important hydrodynamic parameters in the process because it governs the flow regime inside the bed and quantifies the amount of polluted gas to be treated per unit of time, as well as the masstransfer rates between phases. The variation in gas flow rate is studied to quantify the dynamic response of the system, the response is directly related to the physical properties of the filter bed (as the adsoption/ desorption capacity) and to the biochemical reaction induced by the micro-organisms. The effect of gas residence time on toluene consumption was investigated
Figure 2. (a) Toluene elimination capacity and inlet load versus time: Qg ) 6.5 m3‚h-1; (b) toluene conversion versus time; (c) toluene elimination capacity versus inlet load at two different N/P ratios.
at different gas flow rates spanning the range of 4-10 m3‚h-1 and corresponding to a gas residence time of 50 and 20 s. The inlet toluene concentration in air was maintained constant at 2.63 g‚m-3. Figure 3 presents the elimination capacity and removal efficiency versus the gas flow rate. As shown on this figure, increasing the gas flow rate has an inverse effect on the elimination capacity and removal efficiency. Increases in the gas flow rate (times a factor of 2.5) leads to decreased contact time between polluted air and the immobilized microflorae and consequently lowered removal efficiency of 93-78%. Note that a significant variation in the gas flow rate leads to a relatively low variation in the removal efficiency, indicating the absence of gas to biofilm mass-transfer limitations. The ability of the micro-organisms to intercept effectively the VOC flowing
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4723
Figure 3. Effect of air flow rate on toluene conversion and elimination capacity.
Figure 5. (a) Temperature recording of three bed stages versus time; (b) effect of temperature in the medium stage on toluene elimination capacity.
Figure 4. (a) Inhibition effect of xylene on toluene elimination capacity; (b) elimination capacity of toluene and xylene versus inlet load.
at “high” velocity is also demonstrated by this experience. Inversely, increases in gas flow rate led to marked increases in the amount of toluene removed, i.e., from 80 to 144 g‚h-1‚m-3. Increases in the gas flow rate, then in the toluene inlet load, enhance the toluene transferred from gas to biofilm which in turn contributes to increased EC values, especially in the diffusion dominant region (between 0 and 200 g‚h-1‚m-3 inlet load). 3. Inhibition Effect of Xylene on the Toluene Elimination Capacity. A series of trials were conducted by varying the toluene inlet load from 25 to 280 g‚h-1‚m-3 at two xylene inlet loads (0 and 67 g‚h-1‚m-3). The N/P ratio was held constant at its optimum value (i.e., B) and the gas flow rate at 6.5 m3‚h-1‚ These runs have been fully described elsewhere (Jorio et al., 1997), so we report here only the most pertinent results. Figure 4a depicts the elimination capacity for toluene removal when used alone (IL of xylene ) 0) and mixed with xylene (IL of xylene ) 67 g‚h-1‚m-3) versus the inlet load of toluene. It is clearly observed that the
toluene elimination capacity is drastically affected by the presence of the xylene and that a decrease of the toluene EC by a factor of 2 occurs, especially at high loadings located in the reactive region, where the only process limitation is due to the biodegradation kinetics. This strong inhibition phenomenon occurs when two or more pollutants, possessing different biodegradation rates, are degraded by the same population of microorganisms. This was confirmed by a complementary trial series performed by separately measuring the toluene and xylene biodegradation rate. The experiments were conducted under similar, hydrodynamic conditions (gas flow rate, filtering material, etc.) and under a comparable inlet load range (from 0 to 280 g‚h-1‚m-3). The elimination capacities of the two pollutants are plotted against their respective inlet loads and are shown in Figure 4b. An apparent preference by the micro-organisms to degrade toluene over xylene is noted from the figure, ECmax ) 165 g‚h-1‚m-3 for toluene and ECmax ) 66 g‚h-1‚m-3 for xylene. A closer examination of past publications (Tahraoui et al., 1994 for example) reveals that xylene is more resistant than toluene and its presence in the air tends to slow down the metabolism of the biodegradation of toluene. 4. Biofilter Pressure Drop and Temperature. Biofilter temperature and pressure drop were regularly monitored for diagnosing respectively the intensity of microbial metabolism and the state of filtering material compacting and biomass accumulation. Figure 5a presents the evolution of temperature measured at three different levels in the bed (levels 1, 2, and 3) as a function of time for the test period. Figure 5a reveals fluctuations on temperature measurements, mainly produced by local variations of the interstitial gas velocity. Channeling and solids humidity content are the main parameters responsible for these variations. However, a clear trend is revealed in the relationship
4724 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 6. Effect of pressure drop through the filter bed on toluene elimination capacity.
between the temperatures measured at the three levels. The highest temperatures are found at the highest level (i.e., 3) due to the presence of more favorable bacterial growth conditions in terms of solids humidity content and nutrients concentration. Also, this region is not subject to the cooling effect of the entering air, especially after the humidification operation which lowers the polluted air temperature due to the water vaporization. The effect of increasing the N/P ratio is also observed at day 50 where increases of 5-10 °C in filter bed temperatures are recorded. The temperatures at the different levels in the bed stabilize at between 25 and 40 °C. Figure 5b shows the values for the toluene elimination capacity plotted against the temperature measured at the mid level (i.e., level 2) for a constant inlet load. Major augmentation in the toluene elimination capacity from 25 to 160 g‚h-1‚m-3 leads to a bed temperature rise from 25 to 38 °C. Figure 5b indicates that when the toluene elimination capacity reaches its maximum value of 165 g‚h-1‚m-3, further increases in bed temperature are observed. The bed temperature increases when cells are more active and in the conditions used it seems that the maximum temperature reached by the uncontrolled temperature system is 3540 °C. Another constraint is related to the limit of temperature favorable to the micro-organisms; 40 °C is a relatively safe and convenient temperature of biofilter functioning (Ottengraf, 1983). Similarly, bed pressure drop was measured daily but only its effect on toluene elimination capacity is discussed in the following. Figure 6 shows this dependence at a constant inled load of toluene. Increases in the pressure drop from 0.5 to 9.5 cm H2O/m packing, which is caused essentially by biomass accumulation through the bed provoke drastic decreases in elimination capacity from 165 to 90 g‚h-1‚m-3. Since after each filter backwashing operation, the pressure drop again met its initial value (of around 2 cm H2O/m packing) it can be concluded that accumulating biomass is exclusively responsible for increases in bed pressure drop and compaction problems are virtually nonexistent. Finally, in absence of the compaction problems of the filter bed, the control of the pressure drop is easier to ensure since it is only related to the biomass production which can be controlled by removing its excess by a regular backwashing procedure. 5. CO2 Production and Total Bacterial Count. Another method employed for monitoring biofilter performances consists of regularly measuring the CO2 concentrations in polluted air (biofilter inlet) and in the cleaned air (exit), the difference representing the carbon dioxide production during the biodegradation operation.
Figure 7. Amount of toluene degraded and carbon dioxide produced versus toluene inlet load.
The calibration of analysis equipment GC/MS was performed in the main, using standard gases containing known CO2 concentrations. The results obtained are shown on Figure 7, illustrating both the amount of toluene degraded in the biofilter and the corresponding amount of CO2 produced by the biodegradation reaction. It is seen that the quantity of toluene consumed systematically exceeds the CO2 production by a factor of 2 for all the experiments carried out. As the biodegradation of toluene is well-known to be complete oxidation, leading to carbon dioxide, water, and metabolites, the observed deficit in CO2 production is partly explained by the accumulation of dissolved CO2 in the liquid phase surrounding the packing media. This proposal is supported by pH measurements performed on wet solids samples that revealed acidity levels at relatively low pH (around 4). However, CO2 dissolution in the biomass does not fully explain the substantial drop in the pH measurements (from 7 to 4), and this change can be the result of anaerobic zones present that are responsible for organic acids production. The temporary acidification of the medium is rapidly controlled by the buffering effect of the nutrient solution which acts to stabilize the pH conditions inside the filter bed. The solids humidity content was also measured, the humidity content varied between 60 and 70% on a dried solid basis, with a marked gradient from top (higher humidity level) to bottom (lower humidity level), and the lower levels of the filter are subject to the drying caused by the air entering at relatively low temperatures of around 18 °C. The total bacterial counts of the mixed culture used were regularly measured to monitor the proliferation of the inoculated strains inside the biofilter. The attainment of a stable bacterial population is depicted in Figure 8, the average value being 108 c.f.u./g of humid filtering media. The results are not presented here, but it is interesting to note that after a shut down period of 20 days the bacterial counts level seemed to maintain its optimum value (i.e., 108 c.f.u./g of packing), indicating excellent compatibility between the chosen filtering material and the introduced microflorae specially selected for its BTEX removal from air performance. This compatibility results from providing favorable bacterial support conditions, in terms of humidity, buffering capacity, and the necessary organic and inorganic nutrient sources required to maintain the microbial population. Concluding Remarks The pilot scale biofilter has been successfully operated for a period of over 2 months, demonstrating effective
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4725
Nomenclature c.f.u. ) colony forming units Cg ) pollutant concentration in the gaseous phase (g‚m-3 of air) IL ) inlet load of pollutant (g‚h-1‚m-3 of packing) EC ) toluene elimination capacity (g‚h-1‚m-3 of packing) N/P ) nitrogen and phosphorus ratio, N/P ( ) Qg ) gas flow rate (m3‚h-1) V ) volume of empty column (m3) X ) pollutant conversion or removal efficiency (%)
Literature Cited Figure 8. Total bacterial counts versus time with toluene as the pollutant.
and efficient removal of high levels of organic contaminants, e.g., from 0.8 to 8 g/m3 of toluene in the air. The filter unit, using a conditioned biomass mixed with a synthetic agent as the filtering media, demonstrated biodegradation rates 3 times higher than previous filters (Ottengraf et al., 1983; Leson and Winer, 1991; Liu et al., 1994; Tahraoui et al., 1994), e.g., an elimination capacity of 165 g‚m3‚h-1. The filtering media permitted removal of the excess biomass when necessary and prevented filter bed compaction. The filter pressure drop was, in general, minimal, i.e., 0-2 cm of H2O/m of packing, for most of the experimental period. The occasional removal of the biomass helped to maintain the pressure drop at this desirable low level. The filter unit, kept in a humid state and supplemented with inorganic nutrients, offers a desirable environment for biogrowth. A commercial unit, designed with these features and constructed by Valoraction, is available and has been tested at hazardous waste sites for offgas cleanup. The biofilter functioning was optimized by diagnosing its behavior under various operational conditions. The nutrient composition appeared to be optimal when the nitrogen-phosphorus ratio was N/P ) B. By changing the air input flow rates over a relatively wide range, from 4 to 10 m3‚h-1, the biodegradation maintained high-removal efficiencies, indicating that there are no gas-biofilm mass-transfer limitations within the biofilter. Tests with air polluted with a mixture of toluene and xylene demonstrated the degradation-inhibiting effect induced by the presence of xylene on the toluene biodegradation. Homogeneous and stable bacterial density populations have been quantified throughout the filter bed, suggesting the presence of satisfactory bacterial growth conditions. Acknowledgments The authors gratefully acknowledge financial support from the Ministe`re de l’Environnement et de la Faune, Que´bec, Canada and from the Natural Sciences and Engineering Research Council of Canada (NSERC). The authors express their gratitude to Dr. P. Lanigan, H. Jorio, and S. Lebrun.
Bibeau, L.; Kiared, K.; Leroux, A.; Brzezinski, R.; Viel, G.; Heitz, M. Biological Purification of Exhaust Air Containing Toluene in a Filter-Bed Reactor. Can. J. Chem. Eng. 1997, in press. Diks, R. M. M.; Ottengraf, S. P. P. Verification Studies of a Simplified Model for Removal of Dichloromethane from Waste Gases Using a Biological Trickling Filter. Bioprocess Eng. 1991, 6, 93-99. Jorio, H.; Kiared, K.; Leroux, A.; Brzezinski, R.; Viel, G.; Heitz, M. Purification of Air Polluted by Toluene and Xylene in a Pilot Scale Biofilter, Report, Department of Chemical Engineering, Universite´ de Sherbrooke: Sherbrooke, Que´bec, 1997. Kiared, K.; Wu, G.; Beerli, M.; Rothenbu¨hler, M.; Heitz, M. Application of Biofiltration to the Control of VOCs Emissions. Environ. Technol. 1997, 18, 55-63. Leson, G.; Winer, A. M. Biofiltration: an Innovative Air Pollution Control Technology for VOC Emissions. J. Air Waste Manage. Assoc. 1991, 41 (8), 1045-1053. Liu, P. K. T.; Gregg, L. R.; Sabol, H. K.; Barkley, N. Engineered Biofilter for Removing Organic Contaminants in Air. J. Air. Waste Manage. Assoc. 1994, 44, 299-303. Morales, M.; Pe´rez, F.; Auria, R.; Revah, S. Toluene Removal from Air Stream by Biofiltration. Adv. Bioprocess Eng. 1994, 405411. Ottengraf, S. P. P.; Van Den Oever, A. H. C. Kinetics of Organic Compound Removal from Waste Gases with a Biological Filter. Biotechnol. Bioeng. 1983, 12 (25), 3089-3102. Rothenbu¨hler, M.; Heitz, M.; Beerli, M.; Marcos, B. Biofiltration of Volatile Organic Emissions in Reference to Flexographic Printing Process. Water, Soil Air Pollut. 1995, 83, 37-50. Severin, B. F.; Shi, J.; Hayes, T. Destruction of Gas Industry VOCs in a Biofilter. Proceedings from the IGT 6th International Symposium on Gas, Oil, and Environmental Technology, Colorado Springs, CO, 1993; Institute of Gas Technology: Des Plaines, IL, 1993; pp 621-631. Smith, F. L.; Sorial, G. A.; Suidan, M. T.; Breen, A. W.; Bswas, P. Development of Two Biomass Strategies for Extended, Stable Operation of Highly Efficient Biofilters with High Toluene Loadings. Environ. Sci. Technol. 1996, 30, 1744-1751. Tahraoui, K.; Samson, R.; Rho, D. Biodegradation of BTX from Waste Gases in a Biofilter Reactor. Proc. Annu. Meet.sAir Waste Manage. Assoc. 1994, 87th, 2-13. Weber, F. J.; Hartmans, S. Prevention of Clogging in a Biological Trickle-Bed Reactor Removing Toluene from Contaminated Air. Biotechnol. Bioeng. 1996, 50 (1), 91-97.
Received for review February 17, 1997 Revised manuscript received June 30, 1997 Accepted June 30, 1997X IE9701478
X Abstract published in Advance ACS Abstracts, October 1, 1997.
