Toluene Removal by Biofiltration - American Chemical Society

Ind. Eng. Chem. Res. 2001, 40, 5405-5414

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Toluene Removal by Biofiltration: Influence of the Nitrogen Concentration on Operational Parameters Marie-Caroline Delhome´ nie, Louise Bibeau, Julie Gendron, Ryszard Brzezinski, and Miche` le Heitz* Department of Chemical Engineering, Engineering Faculty, Universite´ de Sherbrooke, 2500 Boulevard de l’Universite´ , Sherbrooke, Que´ bec, Canada J1K 2R1

The study presented in this paper dealt with the operation of a laboratory-scale upflow biofilter, packed with compost-based filter material. The airborne contaminant studied was toluene, maintained at a constant inlet concentration of 1.7 g‚m-3. The input air was conveyed upward through the filter column at a flow rate of 1 m3‚h-1. The objective of this work was the study of the impact of increasing concentrations of nitrogen contained in the nutrients solution and, hence, the establishment of a new correlation between this parameter and the overall degradation performance. Depending on the nitrogen concentration employed, two biodegradation regimes have been identified. Over the optimal range of nitrogen concentrations [2.0-8.0 g of N‚L-1], the maximum level of elimination capacity achieved was =100 g‚m-3‚h-1. This value is in line with theoretical considerations that suggest that an optimal nitrogen concentration of =2.6 g of N‚L-1 is required to achieve the same performance (100 g‚m-3‚h-1). Introduction One of today’s important environmental problems is recognized as the increasing presence of volatile organic compounds (VOCs) in the atmosphere.1 Many techniques have already been utilized for the elimination of airborne VOCs, including the processes of incineration, condensation, adsorption, absorption, membrane separation, and plasma technologies.2 One of the drawbacks of most of these technologies is that they only transfer VOCs from the gas phase to another phase that either requires the use of further disposal treatments and/or is expensive to operate, especially when the target VOCs are present in the air at low concentrations.3 Over the last 20 years, biological purification of waste gases has become an important alternative to many of the conventional methods, especially for odor abatement associated with readily biodegradable compounds. Three types of configurations are typically employed in biological reactors used for air treatment: the biofilter, the bioscrubber, and the biotrickling filter.4,5 Biofiltration, especially when concentrations of VOCs are low, has proved to be an effective method for eliminating VOCs from gaseous effluents and has thus gained in popularity in this application.6,7 Why has this technology become so attractive? As noted by Bohn,8 biofiltration is, in fact, a technological application of the natural processes of atmospheric purification that operate at ambient pressure and temperature and generally produce no secondary pollutants.9 A biofilter is, in fact, a three-phase biocatalytic oxidizer and is essentially made from a support medium on which microorganisms are able to grow when optimal conditions are provided. Thus, VOCs are induced to diffuse from the gaseous phase through the wet biofilm and are consequently catabolized aerobically to carbon dioxide, water, and biomass. Even though biofilters are becoming increasingly popular, much research is still * Author to whom correspondence should be addressed. Tel: (819) 821-8000, ext 2827. Fax: (819) 821-7955. E-mail: [email protected].

needed to define the conditions under which they function optimally. Since the beginning of the 1990s, Universite´ de Sherbrooke researchers have developed an interesting biofilter process that is aimed at reducing the VOCs (aromatic compounds and/or alcohols) present in atmospheric effluents produced by some smaller scale industries. Excellent performance, noncompactable filtering materials, based on commercial peat, have been fabricated into filter beds, which, when combined with selected microorganisms, have resulted in high levels of conversion and elimination capacity.10 This paper reports the results obtained from a laboratory up-scale biofilter, employing the indigenous microflora of mature compost. If it can be shown that compost is at least as efficient as peat for its use in biofilters, compost-based biofiltration may become an important way to exploit this biomass product whose industrial/ commercial production and availability is expected to increase appreciably in the future.11-17 It is a generally accepted point that, in order to sustain the microorganisms and to maintain optimal moisture levels in the filter bed, a nutrient solution is required to be periodically added to biofilters. Most authors have used inorganic forms of nutrients;18-22 a few studies only deal with the use of organic nutrient forms.23-25 Urea is particularly useful in this context as it can act not only as a nitrogen source but also as a pH buffer, through the generation of carbonate ions.23 A further objective of our study has, therefore, been to determine the influence of the nitrogen concentration (as urea) contained in the nutrient solution and to subsequently relate the optimal experimental nitrogen concentration to a predictive theoretical model. Experimental Setup and Procedure The main component of the laboratory upflow biofilter is the 1-m tall Plexiglas cylindrical column, having a cross-sectional area of 180 cm2 (Figure 1). The column is filled with the packing of prepared beads, spread equally over three identical column sections. The com-

10.1021/ie0011270 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/18/2001

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Figure 1. Illustration of the experimental setup of the laboratory-scale biofilter.

post beads are composed of 90% (w/w) mature compost, provided by Outarde Environnement, Montreál, Que´bec, Canada, and 10% (w/w) of a proprietary organic binder and are prepared with a mean diameter of 0.8 cm. The nutrients supporting the bacterial activity are delivered by means of a multicomponent solution, periodically pumped onto the top of each stage of the filter bed; this also maintains the desired bed moisture content. No artificial bacterial inoculation of compost filter is practiced, thus fully exploiting the indigenous microflora of the filter bed. To generate the polluted toluene air stream, a small fraction of the total air throughput is saturated with the VOC and is then mixed with the main air stream, previously humidified (relative humidity >95%), before entering the biofilter column. Sampling points are distributed along the column, which enables sampling of the filter bed content to undertake bacterial counts and measurements of the concentrations of the VOC and CO2, of the temperature and the pressure drop at the different levels of the filter. Gas analyses were performed by means of a total hydrocarbon analyzer (Horiba model FIA-220) equipped with a flame ionization detector and a carbon dioxide analyzer (Siemens, Ultramat 22P). Both analyzers were calibrated daily prior to measurements, using standards having different pollutant concentrations. The temperature inside the biofilter was recorded at three measurement points by thermocouples. The pressure drop through the biofilter was measured by means of a differential manometer (Air Flow Developments, Canada, Ltd.). The density of the microbial populations was followed by determination of the total and toluene-specific bacterial counts according to the Most Probable Number (MPN) method adapted to 96-well microplates. Compost beads were sampled from the biofilter bed, and extracts were prepared by mechanical vortexing in a buffer containing 2% NaCl and 0.1% sodium pyrophosphate followed by low speed centrifugation (1000g for 5 min). The toluene-specific counts were obtained by incubation of appropriately diluted compost bead extracts in a

medium consisting of a commercial saline solution (Bu¨shnell-Haas, Difco; BH) completed with toluene as the sole carbon source. Toluene was supplied as a vapor in a toluene-saturated air chamber. The total microbial count was obtained by incubation of the same compost bead extracts in a medium containing BH salts and completed with tryptone (2.5 g‚L-1), yeast extract (1.25 g‚L-1), and glucose (0.5 g‚L-1). Incubations were carried out at 25 °C for 1 week (total microbial counts) or 2 weeks (toluene-specific counts). Positive wells were detected by reduction of iodonitrotetrazolium violet according to the method described by Wrenn and Venosa.26 Biofilter irrigation was effected twice, daily, with 2 L of the nutrient solution made up as follows: MgSO4‚ 7H2O, 0.020 g‚L-1; FeCl3‚6H2O, 0.005 g‚L-1; CaCl2 anhydride, 0.002 g‚L-1; NaCl, 2.0 g‚L-1; Na2HPO4 anhydride, 0.45 g‚L-1; K2SO4 anhydride, 0.036 g‚L-1; distilled water, 1 L. The nitrogen concentration (as urea) was varied between 0.0 and 8.0 g of N‚L-1, with the latter nitrogen input concentration (8.0 g of N‚L-1) being very high for such an operation. Above such a nitrogen input concentration, nutrient solution usage would generally not be economically interesting for industrial applications of the biofilters. The toluene inlet load was fixed at 105 g‚m-3‚h-1, corresponding to an airflow rate of 1 m3‚h-1 and an inlet concentration of 1.7 g‚m-3. The biofilter was operated during a 104-day period. The operational conditions are summarized in Table 1, and the various experimental periods of irrigation solution usage are set out in Table 2. The measurement results are expressed in terms of the following parameters:

Inlet load: IL ) CinQ/V

(1)

Elimination capacity: EC ) ∆CQ/V Production of CO2: PCO2 ) ∆CCO2Q/V

(2) (3)

Removal effiency: X ) 100 ∆C/Cin

(4)

with Cin ) inlet VOC concentration (g‚m-3), Q ) airflow

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Figure 2. Evolution of the removal efficiency and the bacterial counts (total and toluene-specific) as a function of time. Table 1. Design and Operating Parameters of the Biofilter filter bed composition microorganisms mean diameter of pellets porosity of the filter bed column diameter height of the filter bed pollutant pollutant concentration flow rate inlet load residence time irrigation

compost- based indigenous 8 mm 52% 15.25 cm 3 × 33 cm toluene 500 ppm 1 m3‚h-1 105 g‚m-3‚h-1 65 s 2 L‚day-1

Table 2. Nutrients Solution Nitrogen Concentrationa period

nitrogen concn (g of N‚L-1)

A B C D E

0.0 1.2 2.0 2.5 3.0

period

nitrogen concn (g of N‚L-1)

F G H I

4.0 5.0 7.0 8.0

a The airflow rate and the inlet concentration of toluene were maintained at 1 m3‚h-1 and 1.7 g‚m-3‚h-1, respectively.

rate (m3‚h-1), V ) bed volume (m3), ∆C ) VOC concentration difference between the inlet and the outlet (g‚m-3), and ∆CCO2 ) CO2 concentration difference between the outlet and the inlet (g‚m-3). Results and Discussion Biofilter Performance and Bacterial Counts. Toluene degradation is mainly promoted by the micro-

organisms present on the filter bed material. They use the VOC as a source both of carbon and of energy for sustaining the various requirements of their metabolism.27 The evolution of the total and the toluene-specific microbial populations, as a function of time, is presented in Figure 2. The evolution of the total removal efficiency versus the elapsed time is also reported in Figure 2. The total and toluene-specific bacterial counts follow the same pattern. At the beginning of period A, a startup peak for all of the parameters studied is observed (second day of biofilter operation): the total counts rise from 5 × 107 to 2.5 × 108 cfu‚(g of dry mass)-1 (colonyforming unit) and the toluene-specific counts from 3.7 × 106 to 2.5 × 108 cfu‚(g of dry mass)-1, which implies that the total population on this start-up day predominately contained the toluene-degrading species. On the same day, the total removal efficiency increased from 40 to 75%, an increase closely linked to the very significant growth of the toluene-specific bacteria. Nitrogen was not distributed on the filter beds during the period A. Because both the bacterial counts and the removal efficiency fell away steeply on the third day, it is reasonable to assume that the sudden initial surge in the bacterial growth and the degradation capacity were most probably due to the rapid consumption, shortly followed by exhaustion, of the compost’s intrinsic nutrients (N, P, K, S, etc.), with their availability becoming the growth-limiting factor after the start-up peak. During this period, a peak value for the CO2 production rate, rising from 80 to 140 g‚m-3‚h-1, was also noted, confirming that a biodegradation process rather than an adsorption phenomenon was taking place.

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Figure 3. Evolution of the elimination capacity and the production of CO2 as a function of the nitrogen concentration in nutrients solution.

Following the initial peak of activity, bacterial counts increased exponentially until the middle of period B, with the total counts rising from 6.6 × 107 to 1.5 × 109 cfu‚(g of dry mass)-1 and the toluene-specific counts even more is, from 2.7 × 104 to 1 × 108 cfu‚(g of dry mass)-1. Most of the microbial growth occurred within the first 10 days. This period corresponds to the lapse of time required for the adaptation of degrading microorganisms to their new environment. Note that the toluene-degrading microorganisms are selected among the species initially indigenous to compost. Thus, these observations also reveal that biofiltration of toluene on a compost-based bed does not require additional inoculation. During the further biofilter operational periods, C-I, both classes of bacterial counts remained relatively stable, at values around those reached at the end of period B, with the toluene-degrading microbial population representing about 10% of the total bacterial populations. This ratio remained roughly constant over the operations period (80 days), which indicates that an equilibrium condition had become established among the various microorganisms established in the biofilm. However, the removal efficiency, as presented in Figure 2, does not seem to evolve in the same manner as the bacterial counts. Indeed, as the input nitrogen concentration was incremented from one period to the next (ranging from 0 up to 2.0 g of N‚L-1), the removal efficiency also increased stepwise, from 20% (period A, without additional nitrogen) to 95% (period C with a 2.0 g of N‚L-1 solution). So, for this range of input nitrogen concentrations, the nitrogen appears to act as a limiting factor in the biodegradation of toluene: the greater the nitrogen concentration, the greater the rate

of toluene degradation. From period C to period I, the value of X remained stable, on the order of 95%, while the nitrogen concentration was progressively increased from 2.0 to 8.0 g of N‚L-1. During this 80-day period, the biofilter performance was neither adversely affected nor improved by the nitrogen additions, indicating that the microbial activity had achieved its maximum rate of degradation. In an examination of Figure 2, an optimal range of the nitrogen concentration can be deduced from the experimental results. In the present case, the optimal biofilter performance, i.e., where the removal efficiencies are greater than 90% or the elimination capacities are superior to 90 g‚m-3‚h-1, was actually obtained within the range (2.0-8.0 g of N‚L-1) of nitrogen concentrations employed. In the context of eventual applications of the process, it should be noted that the maximum value of X, determined as 90%, is greater than the standards for efficiency presently required by the MEF, Que´bec, Canada, for allowable VOC releases from industrial air pollution control equipment. Moreover, it appears from Figure 2 that the plots for changes in the removal efficiency and the microbial counts with time are quite different. In fact, it was expected that the plotted data for microbial counts would have paralleled, step by step, the removal efficiency plot, as a function of the nitrogen level. In this case, the microbial growth (multiplication) of the toluenespecific microorganisms due to increasing concentrations of nitrogen (i.e., microbial growth proportional to nutrient availability) would have explained the observed rise in the removal efficiency. In the case of the experiments presented here, it appears that the multi-


Figure 4. Evolution of the production of CO2 as a function of the elimination capacity.

plication is not directly linked to the concentration of nitrogen supplied. So, the increased biofilter performance does not seem to be related to the number of microorganisms present but is instead related to their reaction activity. Indeed, irrespective of the concentration of nitrogen, the microorganism counts remained essentially constant (microflora equilibrium is reached), whereas their degrading activity, quantified by the removal efficiency, evolved with the nitrogen level. Thus, in the biofilter, the amount of nutrient nitrogen appears to have been employed (a) to maintain the equilibrium status of both the total and the specific microflora populations and (b) to promote the catalytic activity of the toluene-specific species. Correlation between the Elimination Capacity and the Nitrogen Concentration. (a) Elimination Capacity and Production of Carbon Dioxide. Figure 3 presents the influence of the nitrogen concentration on the biodegradation parameters, i.e., elimination capacity (EC) and production of carbon dioxide (PCO2). Two regimes of biodegradation can be identified in this figure. Indeed, for nitrogen concentrations between 0 and 2.0 g of N‚L-1, both the EC and PCO2 values depend on the nitrogen level in a linear fashion, increasing from 20 to 95 g‚m-3‚h-1 and from 70 to 220 g‚m-3‚h-1, respectively. Over this nitrogen concentration range, the toluene biodegradation reaction is limited by the nitrogen supply (N limitation region). At higher nitrogen concentrations, EC and PCO2 reach the maximum levels of 100 and 250 g‚m-3‚h-1, respectively, and they remain at these optimal values over the range of concentrations (2.0-8.0 g of N‚L-1). This domain is the N saturation region and corresponds to the maximum degradation activity of the microflora. The height of the respective

graphical plateau probably depends on other parameters (chemical, biological, and/or physical) that limit the biodegradation rate. (b) Theoretical Considerations. Analysis of Figure 3 shows that the maximum performance achieved by the biofilter correlates with an elimination capacity of around 100 g‚m-3‚h-1, with this value being attained when the nitrogen supply was maintained at between 2.0 and 8.0 g of N‚L-1. From a theoretical viewpoint, the optimal nitrogen quantity required to obtain the elimination capacity of 100 g‚m-3‚h-1 can be calculated as follows: The molar quantity of carbon (QC), converted per day in the biofilter, being related to an elimination capacity of 100 g of toluene‚m-3‚h-1 ≡ 7.60 mol of C‚m-3‚h-1, is given by

QC ) 24(EC)V ) 3.10 mol of C‚day-1

(5)

According to a previous experimental microkinetic study,28 the mean yield of biomass produced during the conversion of toluene is 0.60 mol of CBiomass‚(mol of Ctoluene)-1. The quantity of carbon (QCB) needed to form the biomass is therefore QCB ) 1.85 mol of CBiomass‚day-1. A generic formula for the biomass composition has been proposed as CH1.8O0.5N0.2,29 from which it appears that the 5 mol of carbon required to form biomass need to be combined with 1 mol of nitrogen extracted from the nutrient solution. Taking into consideration the point that the nitrogen introduced into the biofilter is only involved in microbial metabolism, the required quantity of nitrogen (QN) becomes 0.04 mol of N‚day-1. Two liters of nutrient solution are introduced into the biofilter each day; therefore, the required solution

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Figure 5. Evolution of the total removal efficiency and the removal efficiency of each biofilter stage as a function of the nitrogen concentration in nutrients solution. The values presented are mean values per period.

nitrogen concentration (CN) to be added is 0.19 mol of N‚L-1‚day-1 or 2.60 g of N‚L-1‚day-1, equivalent to 0.12 g of N‚(g of toluene)-1. To validate this evaluation, Figure 3 also presents the elimination capacity as a function of the theoretically required nitrogen concentrations, with the model estimates and the experimental data being compared in this figure. The maximum elimination capacity value experimentally obtained by these tests, i.e., EC ) 100 g‚m-3‚h-1 for CN g 2.0 g of N‚L-1, is thus in satisfactory agreement with the estimated value [0.12 g of N‚(g of toluene)-1], taking into consideration the assumptions involved in the evaluation. It is also to be noted that the maximum elimination capacity value so obtained, 100 g‚m-3‚h-1, is among the largest values recorded in the literature for compostbased, not previously inoculated biofilters when operated for toluene elimination. This excellent result is probably linked to the nature of the filtering material that provides interesting physical characteristics, such as a relatively high porosity (surface area for the gas/ biofilm exchanges), a low specific weight (ca. 510 kg‚m-3), and a high mechanical strength that resists bed compaction phenomena, with all of these qualities thus being favorable for maintaining good aeration of the filter bed. Other factors that contribute to the high elimination capacity values are the strict maintenance of the moisture content of the bed [50-60% (w/w)] and the periodically supplied nutrient solution regime, which sustains the continuous biodegradation of toluene in providing the microflora with their basic nutrient requirements.

Degradation Reaction Balance. A complete balanced chemical oxidation reaction for toluene can be written as follows:

C7H8 + 9O2 f 7CO2 + 4H2O

(6)

Thus, the stoichiometric mass ratio of the carbon dioxide produced to the toluene removed is 3.3. Figure 4 presents the evolution of carbon dioxide production versus the elimination capacity for the biofilter during the whole experimental period. The data points obtained are aligned, and the equation for the linear interpolation (least-squares method) of these points is PCO2 ) 2.3EC + 41.9. The slope of the linear regression, therefore, represents the experimental mass ratio for the carbon dioxide produced to the toluene removed; this is 2.3, a value that is lower than the value estimated from eq 6. The difference between these values is due mainly to the formation of biomass that has been ignored in the reaction balance. The experimental data indicate that, on a mass basis, 30% of the toluene introduced into the bioreactor is utilized as the carbon source for the biomass formation, while the remainder is mineralized in carbon dioxide. A further factor affecting the theoretical mass ratio is that the carbon dioxide is also involved in aqueous equilibria distributions that take place in the biofilm. The value for the PCO2 at nil EC is 41.9 g‚m-3‚h-1. This PCO2 value is an endogenous respiration term and corresponds to the CO2 released by those microbial populations, other than the specific toluene-degrading bacteria, i.e., those that do not utilize the toluene as a metabolic source.


Figure 6. Evolution of the production of CO2 as a function of the nitrogen concentration in nutrients solution. The values presented are mean values per period.

Biofilter Removal Efficiency and the Production of Carbon Dioxide on a Stage-by-Stage Basis. Figures 5 and 6 represent respectively the evolution of the removal efficiency and the carbon dioxide production by individual biofilter stage (the values are averaged over each experimental period) as a function of the nitrogen concentration. It appears that, irrespective of the nitrogen concentration, the PCO2 and X values for each of the three stages are the same. In addition, the equivalence of the degradation capacity for each stage is clearly revealed over all periods and leads us to suppose that the three column sections act similarly during the biodegradation of toluene. This has also been referred to in the course of a microkinetics batch study for the lower and upper filter stages during which the Monod kinetic constants were found to be similar.28 Moreover, the fact that the similarity between the different levels in the biofilter is found for each nitrogen concentration demonstrates that this equivalence does not result from the amount of nitrogen supplied, but rather it is due to the protocol followed for the nutrients solution distribution (i.e., each stage is irrigated individually with the same amount of solution per day). Indeed, given that the amount of toluene removed can be correlated to the nitrogen concentration, X and PCO2 of equal volumes of filter bed are expected to be the same for equal nitrogen increments. This is shown by the equivalent values of X and PCO2 for the different stages of the biofilter for all of the irrigation periods. Thus, the lower efficiency and the released CO2 from each stage are about 7% and 35 g‚m-3‚h-1, respectively, without nitrogen supply, and the individual stage

maximum X and PCO2 values obtained during these experiments are 30% and 90 g‚m-3‚h-1, respectively, for nitrogen concentrations between 2.0 and 8.0 g of N‚L-1. The conclusion derived from the stage-by-stage study is that the chosen mode of bed irrigation seems to be well adapted to a modular unit because it permits the maintenance of equivalent removal capacities for all filter stages, independently of their respective position along the column. Temperature. Figure 7 presents the evolution of temperature at the center of each biofilter’s section as a function of the nitrogen concentration. In Figure 7, the three plots for temperature versus the level of nitrogen are seen to be parallel, and it also appears that during the entire trial the temperature gradient between two adjacent stages is essentially constant, i.e., on the order of 1-3 °C. Further, it appears from Figure 7 that the evolution of each temperature value as a function of the nitrogen concentration follows the same pattern as the evolution of the elimination capacity versus the nitrogen concentration. Assuming that the incoming air temperature and that of the laboratory are maintained constant during the biofilter trial, the sole source of heat generation throughout the filtering material is the exothermic toluene biological decomposition. Thus, up to nitrogen additions of 2.0 g of N‚L-1, bed temperatures rose from 23, 25, and 26 °C to 26, 28.5, and 30 °C in the lower, middle, and upper filter bed stages, respectively, because of the increasing intensity of the nitrogen-dependent microbial activity. For the period of N addition of 2.0-8.0 g of N‚L-1, temperatures reached their maximum levels and became stabilized (26, 29, and 31

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Figure 7. Evolution of temperature as a function of the nitrogen concentration in nutrients solution.

°C in the three successive sections), indicating that the microorganisms achieved their optimum level of activity (N saturation). The results of the temperature study are in agreement with the variations in the X, EC, and PCO2 values as analyzed earlier. This points to the fact that the toluene removal during the biofiltration operations clearly results from biological oxidation and that the toluene-specific microflora are in effect very sensitive to the nitrogen influence. Pressure Drop and Bacterial Counts. The parallel evolutions for the pressure drop through the bed and the bacterial counts (both total and toluene-specific) versus time are set out in Figure 8. The pressure drop remained stable during periods A and B, with values steady at around 0.1 cm of H2O‚m-1, which is quite a low value for a biofiltration system. During period C, the pressure drop increased to 1.8 cm of H2O‚m-1, still a low value for a biofiltration system. Indeed, values for operating biofilter bed pressure drops quoted in the literature can commonly rise to 30 cm of H2O‚m-1.5 During the observed exponential increase in the pressure drop, the biomass accumulation became clearly visible in the bed as a yellow biofilm, expanding around and between the filter bed pellets. Interestingly, the bacterial counts did not vary over the same periods [total and toluene-specific counts remaining constant at around 1.5 × 109 and 1 × 108 cfu‚(g of dry mass)-1, respectively]. Also, during the exponential phase of the pressure drop increase, the removal efficiency remained stable at around 95% (see Figure 2), indicating that the biofilter performance was not affected by the pressure drop increase, at least to values of 1.8 cm of H2O‚m-1.

This indicates that the observed increasing resistance to gaseous flow resulted from blocking that was almost certainly related to the accumulation of dead microbial populations and an excessive production of exopolysaccharides and not to an excessive growth/multiplication of microorganisms in the filtering medium. To avoid a decline in the performance and to keep the microflora degrading activity sustainable, the bed was gently washed with water (between periods C and D) to remove excess biofilm and to subsequently prevent nonhomogeneous aeration of the bed, which could have had adverse consequences for the biofilter efficiency. Because the biofilm accumulation (as described from period A to C) was a recurrent phenomenon (periodicity of 30 days) during the experiment and to sustain the full efficiency of the biofilter, the bed washing was repeated periodically (periods E and H). Conclusion A modular, laboratory up-scale biofilter, packed with a new type of compost-based filtering material, was operated over a 104-day period. The target air contaminant of the study was toluene, diluted at the concentration of 1.7 g‚m-3 in an air stream, flowing upward through the filter bed at 1 m3‚h-1. Analysis of the trends in the measured toluene elimination capacity, conversion, and production of carbon dioxide as a function of the nitrogen concentration has revealed the existence of two distinct degradation regimes, depending on the level of the nitrogen supply: a nitrogen limitation regime (from 0 to 2.0 g of N‚L-1) and a saturation region (between 2.0 and 8.0 g


Figure 8. Evolution of the pressure drop and the bacterial counts (total and toluene-specific) as a function of time.

of N‚L-1). Through this analysis, it was also shown that the particular biofilter studied performed as a long period functioning, highly efficient unit. Indeed, its elimination capacity reached a maximum value of 100 g‚m-3‚h-1, i.e., 95% of the input toluene was removed, over a continuous period of 80 days, when the concentrations of nitrogen contained in the nutrients solution were g2.0 g of N‚L-1. The appropriate nitrogen concentration for this task was also evaluated through theoretical considerations, based on the carbon and nitrogen component balances. This calculation suggests that the optimal nitrogen level, which is needed to achieve EC ) 100 g‚m-3‚h-1, is 2.6 g of N‚L-1 or 0.12 g of N‚(g of toluene)-1, and this value was confirmed experimentally. Further, a filter bed stage-by-stage study of the biofilter performance clearly demonstrated the practical relevance of the modular design of the column, as the three individual column sections performed in a very similar manner, due mostly to the stage-by-stage mode of filter bed irrigation. Analyses of the bacterial counts have also clearly demonstrated that the increasing efficiency of the biofilter, following incremental increases in the nitrogen levels, were mainly due to an increase in the catalytic degradation activity of the microorganisms rather than to an overpopulation of them. Finally, the same bacterial count analyses have shown that the rising value of the pressure drop through the biofilter was essentially due to an excessive accumulation of exopolysaccharides in the biofilm on the filter bed pellets.

Acknowledgment The authors thank Outarde Environnement Inc. (Montreál, Que´bec, Canada), who provided the compost, and Company E Ä cosfe´ra (Fleurimont, Que´bec, Canada), who also participated in the project. They are also indebted to the Natural Sciences and Engineering Research Council of Canada (NSERC) for their contribution to the project’s financial needs (Strategic Research Grant). Finally, they also express their gratitude to Dr. P. Lanigan for text translation and Mrs. D. Gagne´ for text preparation. Nomenclature Cin ) VOC concentration at the inlet, g‚m-3 CN ) concentration of nitrogen required, mol of N‚L-1‚day-1 ∆C ) VOC concentration difference between the inlet and the outlet, g‚m-3 ∆CCO2 ) CO2 concentration difference between the outlet and the inlet, g‚m-3 EC ) elimination capacity, g‚m-3‚h-1 IL ) inlet load, g‚m-3‚h-1 PCO2 ) production of carbon dioxide, g‚m-3‚h-1 Q ) airflow rate, m3‚h-1 QCB ) quantity of carbon needed to form biomass, mol of C‚day-1 QN ) quantity of nitrogen needed, mol of N‚day-1 QC ) quantity of carbon converted per day, mol of C‚day-1 V ) bed volume, m3 X ) removal efficiency, %

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Received for review December 30, 2000 Revised manuscript received June 25, 2001 Accepted July 7, 2001 IE0011270

Toluene Removal by Biofiltration - American Chemical Society

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