Biofiltration of Air Contaminated by Styrene: Effect of Nitrogen Supply

Chemistry, The University of New South Wales, Sydney, NSW 2052,. Australia. Environ. .... and CCO2,s is the concentration of carbon dioxide in the gas...
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Environ. Sci. Technol. 2000, 34, 1764-1771

Biofiltration of Air Contaminated by Styrene: Effect of Nitrogen Supply, Gas Flow Rate, and Inlet Concentration HASNAA JORIO, LOUISE BIBEAU, AND M I C H EÅ L E H E I T Z * De´partement de Ge´nie Chimique, Faculte´ de Ge´nie, Universite´ de Sherbrooke, Sherbrooke, Quebec, J1K 2R1 Canada

The biofiltration process is a promising technology for the treatment of dilute styrene emissions in air (less than 1 g‚m-3). The efficiency of this process is however strongly dependent upon various operational parameters such as the filter bed characteristics, nutrient supplies, input contaminant concentrations, and gas flow rates (gas residence times). The biofiltration of air containing styrene vapors was therefore investigated, employing a novel biomass filter material, in two identical but separate laboratory scale biofiltration units (units 1 and 2), both biofilters being initially inoculated with a microbial consortium. Each biofilter was irrigated with a nutrient solution supplying nitrogen in one of two forms; i.e., mainly as ammonia for unit 1 and exclusively as nitrate for unit 2. The experimental results have revealed that greater styrene elimination rates (up to 141 g‚m-3‚h-1) are achieved in the biofilter supplied with ammonia as the major nitrogen source in comparison to the lesser elimination performance (up to 50 g‚m-3‚h-1) obtained with the nitrate provided biofilter. However, in achieving the high styrene removal rates in the ammonia supplied biofilter, the excess of biomass accumulates on the filtering pellets and causes progressive clogging of the filter media. Furthermore, the effectiveness of nitrate supply as the sole nitrogen nutrient form, on reducing or controlling the biomass accumulation in the filter media in comparison to ammonia, could not be satisfactorilly demonstrated because the two biofilters operated with very different styrene elimination capacities. The monitoring of the carbon dioxide concentration profile through both biofilters revealed that the ratio of carbon dioxide produced to the styrene removed was approximately 3/1, which confirms the complete biodegradation of removed styrene, given that some of the organic carbon consumed is also used for the microbial growth. The effects of the most important design parameters, namely styrene input concentrations and gas flow rates, were investigated for each nutrient solution.

Introduction Styrene is among the most important widely produced of the aromatic hydrocarbons, and it is used in many chemical industries such as in the production of polystyrene, buta* Corresponding author phone: (819)821-8000 ext: 2827; fax: (819)821-7955; e-mail: [email protected]. Presently on sabbatical leave at School of Chemical Engineering & Industrial Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia. 1764

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diene-styrene latex, styrene copolymers, unsaturated polyester resins, and rubber. In 1990, the annual Canadian production of styrene was 718 kt (1). The end product uses of styrene and its transformation products in Canada include tires, plastics, waxes, paints and varnishes, adhesives, metal cleaning agents, and glass fiber products. Styrene may be released into the environment during any step in the handling of styrene or its derivative products, i.e., in production, storage, transport, or utilization. The atmosphere is thus the eventual major sink for styrene losses and wastes. As reported to the Toxic Release Inventory by certain types of U.S. industries, of the total 14.9 kt of styrene released in 1992 to the environment in the United States, some 14.7 kt were released into the atmosphere (2). Styrene emissions in Canada in 1990, resulting of industrial activity, totalled 0.22% of annual styrene production, i.e., 1.6 kt/year, including some 1 kt/year emitted during the production of unsaturated polyester (1). Besides the known industrial releases, styrene is also generated in smaller quantities, from other sources such as natural microbial and fungal metabolisms, cigarette smoke, automobile exhausts, and the pyrolysis and cracking of petroleum and its derivatives (2, 3). Even though styrene is not believed to contribute significantly to the destruction of the stratospheric ozone or to global warming, it has nevertheless been listed as among the 189 hazardous and toxic atmospheric contaminants under CAAA (1990) because of its contribution to tropospheric ozone formation and its direct danger to and impact on human health (1, 2). Consequently, industries involved in generating potential styrene emissions are now subject to increasingly stringent regulations, and the development of viable and effective emissions control measures has became a necessity. Among the various emerging air pollution control technologies, biofiltration is an attractive option for the treatment of such volatile organic compounds (VOCs) emissions. The biofiltration process has been widely and efficiently applied during recent decades for the treatment of air streams contaminated by VOCs at low concentrations. The treatments for atmospheric styrene emissions, using conventional technologies such as incineration or adsorption, although efficient and reliable, are relatively expensive and energy consuming, especially when the styrene concentrations in these effluents falls below 1 g‚m-3, which, indeed, is well in the range of feasibility for the biofiltration process (95 >80

biofilter enriched by

Exophiala jeanselmei Exophiala jeanselmei activated sludge Nocardia sp styrene degrading microorganisms

In this present study, the biofiltration of styrene vapors was investigated using a novel filter material, consisting of small pellets of preconditioned biomass. Two separate but identical biofiltration units, unit 1 and unit 2, were operated each using a different nutrient solution for the filter bed irrigation. The nutrient solution A which supplied nitrogen mainly in the form of NH4+ to the biofilter of unit 1 had already been used in previous biofiltration studies, the results of which have been published by our research group: this nutrient solution has been proven to favor biofilter performance for toluene and xylene emissions (20, 21). However, occasional excess biomass accumulations were encountered in these studies and were successfully handled by simply applying greater but controlled flows of irrigation solution to the filter material, which permitted the drainage and removal of the excess biomass. Nutrient solution B, used for the biofilter of unit 2, supplied nitrogen exclusively as KNO3. Nutrient solutions A and B were both prepared with equal masses of phosphorus nutrient (KH2PO4), the nitrogen content of both solutions being maintained in identical mass ratios to the phosphorus content. This present study was set up (a) to assess the effectiveness of supplying nitrogen exclusively as nitrate, in comparison to the substantially ammonia nutrient, in controlling biomass production within the filter media without adversely affecting the biofilter performance, and (b) to determine the effects due to varying the inlet styrene concentration and gas flow rate on the overall process performance, for both biofilter units. The trends of variation of the quasi-steady-state biofilter performance due to these air stream characteristics could provide valuable information for the design of biofilters.

Experimental Set-Up and Procedure Biofiltration Units. A schematic of one of the biofiltration units employed for this study is shown in Figure 1. Each biofilter consisted of a vertical, three section cylinder, with an inner diameter of 0.15 m and a total height of 1.5 m. Small pellets, composed essentially of a preconditioned biomass, were used as filter material, in both units, each biofilter being filled with the pellets to a total height of 1 m, evenly divided into three identical sections. Each biofilter cylinder was made of rigid and transparent Plexiglas, allowing observation of the biofilm layer gradual development and evolution on the surface of the filtering pellets. The filter material contained in each section was piled on a layer of inert Plexiglas beads, supported by a sieve plate, to ensure homogeneous distribution of the rising gas flow throughout the cross section of the filter bed. The inflowing gas was supplied tangentially at the base of the column, at a height of 0.15 m under the first section of the filter bed, which permits uniform distribution of the gas flow over the filter chamber before it contacted the filter material. To generate the styrene vapor contamined air stream, the input flow of laboratory compressed air was divided into two streams, the major stream being directed through a separate water column for humidification to a VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Biofiltration unit. level of >95% relative humidity and the minor stream being saturated with styrene vapors by passage through a sealed “bubbler” vessel, containing liquid styrene. These two streams were then mixed before feeding to the biofilter column base. Flow rates of the major and minor fractions of the input air stream were metered by means of previously calibrated gas flowmeters in order to obtain the desired styrene inlet concentrations and overall gas flow rates through the biofilter. The humidification applied to the major air stream was necessary in order to avoid the effects of filter bed drying by the air throughput. To maintain adequate moisture content of the filtering pellets and to supply the microbial population with additional nutrients, the filter bed was irrigated daily, 1 L of the selected nutrient solution being evenly distributed over the top of each of the three sections of the filter bed. Inoculum and Nutrients Solutions. At the filter bed startup, inoculation was effected by introducing a specific activated consortium of microbial species. This inoculum was prepared from a consortium supplied by GSI Environnement Inc., Sherbrooke, Que´bec, Canada (Environmental biotechnology microbial line products, EVB-110). The consortium consisted essentially of specific microbial aerobic and facultative anaerobic species. The nutrient solution A used for irrigation of the biofilter of unit 1 consisted of an aqueous solution of KH2PO4, NH4NO3, (NH4)2SO4, NH4HCO3, and some trace level nutrients. The nutrient solution B was an aqueous solution of KH2PO4, KNO3, and the same trace nutrients. The trace nutrients employed were ZnCl2, FeCl3, CuCl2, MnCl2, Na2B4O7, (NH4)6Mo7O24, and CoNO3. Analytical Methods. The filtering media temperature was measured daily at the midlevel of each section of each filter bed by means of three “T” type thermocouples connected in turn to a digital temperature display (Omega DP 465). Three gas sampling ports were installed at the exit of each filter section and served also as measurement points for pressure drop determinations by means of a differential pressure manometer (Air Flow Developments, Canada LTD, 1766

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model 4u.5). The extracted gas samples were analyzed for their styrene content by means of an online total hydrocarbon analyzer (model FIA-220, Horiba), equipped with a flame ionization detector. The gas phase at each sampling level in the biofilter was also analyzed for its carbon dioxide concentration, using a CO2 analyzer (Ultramat 22P, Siemens) equipped with a continuous sampling pump. The gas analysis equipment was calibrated daily ahead of measurements of filter bed gas styrene and carbon dioxide concentration. Clean air and air prepared with various known styrene and carbon dioxide concentrations served as standards.

Results and Discussion Tests of biofiltration of styrene vapor emissions were conducted over a continuous period of 65 days on biofiltration unit 1, using nutrient solution A (NH4+) for the filter bed irrigation. Biofiltration unit 2 using solution B (NO3-) for filter bed irrigation was initially run for a continuous period of 51 days and then shut down for a period of 55 days, followed by a further continuous operating period of 85 days (days 107-191). During these operations, various conditions of gas flow rate and input styrene concentration were examined in both units. In selecting the tests conditions, special attention was given to the determination of the optimum ranges of gas flow rate and input styrene concentration permitting removal efficiencies of 85% or greater to be achieved for both biofiltration units. This efficiency standard was chosen so that the process would be in accord with the environmental law applied to the technologies (excluding the incineration process) for the treatment of organic compound emissions that is imposed in Que´bec, Canada (22). Each set of operating conditions employed (inlet concentration and gas flow rate) was maintained constant for at least 3 days in order to ensure the reproducibility of the biofilter performance under the applied conditions. Biofiltration performance is discussed in terms of styrene inlet load IL (g‚m-3‚h-1), removal efficiency X, elimination capacity EC (g‚m-3‚h-1), and mass of carbon dioxide pro-

TABLE 2. Experimental Results Obtained at Various Gas Flow Rates and at an Approximately Constant Inlet Styrene Concentration in the Biofilter Irrigated with Solution A (NH4+) gas flow rate inlet concn (m3‚h-1) (g‚m-3) 1.0 1.5 2.0 2.5 3.0

0.88 ( 0.04 0.92 ( 0.05 0.88 ( 0.04 1.01 ( 0.08 0.97 ( 0.05

removal effic (%)

inlet load (g‚m-3‚h-1)

elimination capacity (g‚m-3‚h-1)

97.1 ( 1.5 57.0 ( 2.0 55.3 ( 2.9 91.3 ( 2.3 90.0 ( 4.8 81.8 ( 5.0 85.2 ( 4.7 114.3 ( 4.0 97.3 ( 6.2 86.0 ( 7.0 164.4 ( 12.4 141.3 ( 15.0 50.6 ( 11.7 188.2 ( 10.0 94.9 ( 21.3

FIGURE 2. Operating conditions (gas flow rate and inlet styrene concentration) versus time; styrene removal efficiency at various heights along the filter bed versus time for the biofilter irrigated with nutrient solution A (NH4+): [ gas flow rate; ] inlet concentration; 0 height 0.33 m; × height 0.66 m; and 2 height 1 m (exit). duced per unit volume of filter media and time, PCO2 (g‚m-3‚h-1), which are evaluated using the following equations:

Cg0 - Cgs Q ; ‚C ; X ) V g0 Cg0 Q PCO2 ) ‚(CCO2,s - CCO2,0) V

IL )

EC )

Q ‚(C - Cgs); V g0

where Q is the gas flow rate (m3‚h-1); V is the volume of the filter bed (m3); Cg0 is the inlet concentration of the pollutant in the gas phase (g‚m-3); Cgs is the concentration of the pollutant in the gas phase at the exit (g‚m-3); CCO2,0 is the inlet concentration of carbon dioxide in the gas phase (g‚m-3); and CCO2,s is the concentration of carbon dioxide in the gas phase at the exit (g‚m-3). Experiments Performed and Removal Efficiencies of Biofilters. Biofiltration Unit 1. Figure 2 presents the operating conditions (gas flow rate and styrene inlet concentrations) and the removal efficiency profile versus time for the biofilter irrigated with solution A (NH4+). At start-up, the biofilter was run at an inlet styrene concentration of 0.5 g‚m-3 and a gas flow rate of 1 m3‚h-1, corresponding to an empty bed gas residence time of 64 s. The overall removal efficiency attained 40% at day 5 and by day 7 had increased to more than 97%, corresponding to an elimination capacity of 32 g‚m-3‚h-1 for an inlet load of 33 g‚m-3‚h-1. Increasing the inlet styrene concentration to around 0.97 g‚m-3 at day 8 led to removal efficiencies of at least 97% being achieved and maintained between days 8 and 12. During this period, elimination capacities of around 55 g‚m-3‚h-1 were reached for inlet loads of around 57 g‚m-3‚h-1. Since this biofilter developed excellent removal performance in only a few days after startup, experiments were programmed for performance at lower empty bed gas residence times, i.e., at higher gas flow rates (up to 3 m3‚h-1) and at a constant inlet styrene concentration of around 0.9 g‚m-3. The experimental results, summarized in Table 2, shows that average removal efficiencies of at least 85% were achieved at inlet styrene concentrations around 1.0 g‚m-3 and gas flow rates up to 2.5 m3‚h-1, corresponding to an empty bed residence time as little as 25 s. However, continuous accumulation of excess biomass on the filter pellets was observed and produced a gradual increase in the pressure drop through the filter bed, which rose to a maximum of 14 cm of H2O by day 50 (Figure 3), accompanied by the progressive clogging of the filter media throughout the

FIGURE 3. Operating conditions (gas flow rate and inlet styrene concentration) versus time; pressure drop through the filter bed versus time for the biofilter irrigated with nutrient solution A (NH4+): [ gas flow rate; ] inlet concentration; and b pressure drop. three compartments of the filter bed, particularly the upper section. This situation was worsened by the use of excessive filter bed irrigation applied with the aim of removing the excess biomass. In fact, the excessive filter bed irrigation after day 51 had initially a beneficial effect by reducing the pressure drop through the filter bed. However the drainage of excess biomass was not then possible because the irregular shape of the filtering pellets did not permit the free passage of biomass slurry to the bottom of the biofilter for removal. Thus, in a few days after this operation, the pressure drop through the filter bed again increased, and by day 66, the input air stream was seen to be bubbling, indicating the condition of bed flooding and forcing the interruption of the experiment. Unlike other biofiltering materials employed in previous studies by our research group (7, 20, 21), the physical properties of these filtering pellets acted against the efforts made to remove the excess biomass simply by the method of excessive controlled irrigation of the filter bed. Biofiltration Unit 2. Figures 4 and 5 present the operating conditions (gas flow rate and styrene inlet concentration) and the removal efficiency profile versus time for the biofilter irrigated with solution B (NO3-) during the first and second periods of its operation, respectively. At start-up, this biofilter was run at an average styrene inlet concentration of 1.4 g‚m-3 and a gas flow rate of 1 m3‚h-1, corresponding to an empty bed gas residence time of 64 s. Under these conditions, the removal of styrene started only after 6 days of operation providing an overall removal efficiency of less than 15%. In maintaining these same operating conditions for 12 days, the overall removal efficiency rose to a stable value of around 46% corresponding to an elimination capacity of 36 g‚m-3‚h-1 for an inlet load of 79 g‚m-3‚h-1. Operations were continued between days 17 and 37, at a constant gas flow rate of 1 m3‚h-1 and various inlet concentrations, and the obtained results are summarized in Table 3. These tests revealed that VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. Experimental Results Obtained at Various Inlet Styrene Concentrations and Gas Flow Rates in the Biofilter Irrigated with Solution B (NO3-)

FIGURE 4. Operating conditions (gas flow rate and inlet styrene concentration) versus time; styrene removal efficiency at various heights along the filter bed versus time for the biofilter irrigated with nutrient solution B (NO3-) (first operating period): [ gas flow rate; ] inlet concentration; 0 height 0.33 m; × height 0.66 m; and 2 height 1 m (exit). removal efficiencies of 85% or more could not be achieved for inlet concentrations varying from 0.6 to 2.17 g‚m-3 (i.e. with inlet loads in the range 34-121 g‚m-3‚h-1). After day 37, the biofilter was operated at a gas flow rate of 2 m3‚h-1 for a further period of 12 days during which the styrene removal efficiency varied between 8% and 30% for inlet styrene concentrations ranging between 0.59 and 2.12 g‚m-3 (Table 3). At day 107, after a shut-down period of 55 days, the operation of the biofilter was restarted at a gas flow rate of 1 m3‚h-1 and an inlet styrene concentration around 1.2 g‚m-3 (Figure 5). Previous researches had shown that shut-down periods of from 2 days to 2 weeks have no significant effect on the styrene removal performance of biofilters. After such shut-down periods, biofilters recovered their original efficiency within a few hours (between 5 and 8 h) (15, 16). Also, previous research conducted in our laboratory has shown that excellent biofiltration performance for toluene removal was still obtained on restart after a shut-down period of 8 months (23). Therefore, for the present study, rapid recovery of the original biofilter efficiency after a shut-down period was anticipated. However, a regeneration period of 6 days (days 107-113, Figure 5) was found to be necessary for the biofilter to recover the removal efficiency of around 48%, a value similar to that obtained under the same operating

gas flow rate (m3‚h-1)

inlet concn (g‚m-3)

removal effic (%)

inlet load (g‚m-3‚h-1)

elimination capacity (g‚m-3‚h-1)

0.5 0.5 0.5 0.5 0.5 0.5 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.7 1.0 1.0 1.0 1.0 2.0 2.0 2.0

0.58 ( 0.05 1.21 ( 0.01 1.79 ( 0.03 2.13 ( 0.15 2.71 ( 0.02 3.33 ( 0.06 0.34 ( 0.04 0.60 ( 0.02 1.20 ( 0.01 1.37 ( 0.04 1.71 ( 0.01 2.13 ( 0.01 2.51 ( 0.04 2.89 ( 0.04 0.60 ( 0.01 1.22 ( 0.07 1.42 ( 0.05 2.17 ( 0.06 0.59 ( 0.01 1.20 ( 0.02 2.12 ( 0.03

98.2 ( 2.0 95.1 ( 1.4 73.3 ( 0.3 73.5 ( 1.2 65.8 ( 0.7 40.8 ( 0.5 97.5 ( 2.5 90.1 ( 2.7 65.7 ( 2.1 61.8 ( 0.8 55.0 ( 0.7 48.6 ( 1.3 35.9 ( 0.6 27.9 ( 3.3 77.2 ( 0.1 46.6 ( 5.2 46.1 ( 4.6 25.6 ( 2.6 30.4 ( 0.4 16.4 ( 0.3 8.1 ( 0.7

16.2 ( 1.7 33.5 ( 0.3 49.7 ( 0.8 59.1 ( 0.5 75.3 ( 0.5 92.6 ( 1.8 14.3 ( 1.6 23.4 ( 0.9 46.5 ( 0.3 53.4 ( 1.8 66.6 ( 0.5 82.8 ( 0.3 97.6 ( 1.6 112.3 ( 1.3 33.7 ( 0.4 68.0 ( 3.4 78.8 ( 2.7 120.7 ( 3.8 65.8 ( 1.1 132.8 ( 1.9 236.0 ( 1.7

15.9 ( 1.4 31.7 ( 0.6 36.4 ( 0.6 43.4 ( 1.0 49.5 ( 0.9a 37.7 ( 1.2 13.9 ( 1.4 21.0 ( 1.1 30.6 ( 1.1 33.0 ( 0.7 36.7 ( 0.5 40.2 ( 1.2a 35.1 ( 1.2 31.4 ( 3.5 26.0 ( 0.2 31.7 ( 4.4 36.4 ( 4.2a 30.8 ( 2.2 20.0 ( 0.3 21.7 ( 0.5 19.2 ( 1.6

a

Optimal operating conditions for each considered gas flow rate.

conditions before the shut-down period (Table 3). Since removal efficiencies obtained at gas flow rates of 1 and 2 m3‚h-1 during the first operation period were not satisfactory (target removal efficiency was 85%), in this biofilter (irrigated with solution B (NO3-)), experiments using higher gas flow rates were not pursued. Hence, during the next operating period (starting at day 115), two smaller gas flows (i.e. 0.5 and 0.7 m3‚h-1) carrying various inlet styrene concentrations were subsequently examined (Figure 5). The experimental results obtained for each gas flow rate, summarized in Table 3, show that average removal efficiencies of greater than 95% were achieved at the gas flow rate of 0.5 m3‚h-1 and inlet concentration up to 1.21 g‚m-3. For the gas flow rate 0.7 m3‚h-1, average removal efficiencies of at least 90% were attained for inlet concentrations up to 0.6 g‚m-3. It is important to record that during the entire test period with this biofilter, no excess biomass accumulations were observable on the filter bed material, and the pressure drop through the filter bed rarely exceeded 1 cm H2O‚m-1.

FIGURE 5. Operating conditions (gas flow rate and inlet styrene concentration) versus time; styrene removal efficiency at various heights along the filter bed versus time for the biofilter irrigated with nutrient solution B (NO3-) (second operation period): [ gas flow rate; ] inlet concentration; 0 height 0.33 m; × height 0.66 m; and 2 height 1 m (exit). 1768

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FIGURE 6. Styrene elimination capacity versus the gas flow rate for various styrene inlet concentrations: O Cg0 ) 0.9 g‚m-3, solution A (NH4+); ] Cg0 ) 0.6 g‚m-3, solution B (NO3-); + Cg0 ) 1.2 g‚m-3, solution B (NO3-); and 4 Cg0 ) 2.1 g‚m-3, solution B (NO3-).

FIGURE 7. Styrene elimination capacity versus the inlet load. Data obtained with various styrene inlet concentrations and gas flow rates on the identical biofilters each irrigated with nutrient solution A (NH4+) or solution B (NO3-): O nutrient solution A and + nutrient solution B. Effect of Nutrient Solution. An overall analysis of the experimental observations obtained with the two nutrient solutions (Tables 2 and 3) revealed that the ammonia supplied biofilter permitted higher elimination capacities even under the more severe operating conditions in comparison with the nitrate supplied biofilter. Figure 6 shows the calculated elimination capacity versus the gas flow rate at various inlet concentrations for both biofilters. As indicated in this figure, the elimination capacities of the solution B (NO3-) supplied biofilter at inlet concentrations of around 0.6, 1.2, and 2.1 g‚m-3 and at gas flow rates of 1 and 2 m3‚h-1 were in the range 20-32 g‚m-3‚h-1, whereas at the same gas flow rates (1 and 2 m3‚h-1) and an inlet concentration of around 0.9 g‚m-3, the elimination capacities ranged from 55 to 97 g‚m-3‚h-1 in the solution A (NH4+) supplied biofilter. Figure 7 shows the computed elimination capacities versus the styrene inlet loads for both biofilters. These data include the results of all the experiments performed on each biofilter under the employed operating conditions for styrene inlet concentration and gas flow rate at a quasi-steady state (excluding the acclimation period results). As seen in Figure 7, when the styrene inlet load was low (less than 38 g‚m-3‚h-1), the impact of the form of nitrogen supply on biofilter performance would have been difficult to detect since almost 100% removal of styrene was achieved with either nutrient solution A or B. However, for increasingly greater inlet loads, the elimination performance of the biofilter supplied with solution B (NO3-) was much less and declined in comparison with the solution A (NH4+) supplied biofilter, indicating that

FIGURE 8. Carbon dioxide produced versus the elimination capacity. Data obtained with various inlet styrene concentrations and gas flow rates on the identical biofilters each irrigated with nutrient solution A (NH4+) or solution B (NO3-): O nutrient solution A; + nutrient solution B; and s line y ) 3x. microbial styrene biodegradation activity was increasingly more intense in the biofilter supplied with solution A (NH4+). With ammonia as the nitrogen source, the maximum elimination capacity of 141 g‚m-3‚h-1 (removal efficiency of more than 85%) was obtained with an inlet load of 164 g‚m-3‚h-1 (inlet concentration around 0.9 g‚m-3) operating with an empty bed residence time as low as 25 s. For the nitrate supplied biofilter, the maximum elimination rate achieved was 50 g‚m-3‚h-1 at an inlet concentration around 2.7 g‚m-3 and an empty bed residence time of 127 s (i.e. 75 g‚m-3‚h-1 of styrene inlet load). Similar biodegradation behavior was reported by Kopchynski and Maier (24) in their study of hexane emissions treatment in two separate biotrickling filter columns, one supplied with nitrogen as nitrate and the other supplied with nitrogen as ammonia. In this study, hexane was found to be removed at rates ranging from 0 to 4.28 mg‚h-1 for the nitrate fed column and from 9.46 to 13.46 mg‚h-1 for the ammonia fed column. In our present study, since the beginning of the biofilters operation, a reduced biomass accumulation (smaller biofilm volume) was observed in the nitrate supplied biofilter, in comparison with the biofilter supplied with ammonia. However, no conclusion on the effectiveness of supplying nitrogen exclusively as nitrate in reducing the rate of biomass production could be drawn because the styrene biodegradation microbial activity in the nitrate supplied biofilter was less efficient compared with that of the ammonia supplied biofilter. The higher biomass accumulation observed with nutrient solution A (NH4+) in comparison with nutrient solution B (NO3-) not only could be related to the form of nitrogen supply but also more likely can be attributed to the greater intensity of the microbial styrene biodegradation activity, induced by solution A (NH4+). Finally, the outstanding biofiltration performance by the biofilter irrigated with solution A (NH4+) indicates that the use of this irrigation solution may be very advantageous for biofilters treating styrene emissions if adequate quality filtering material providing facilities for excess biomass drainage is used. In future studies, development and tests of alternate filter materials for styrene vapors removal using solution A (NH4+) for biofilter irrigation is recommended. Carbon Dioxide Production Analysis. The importance of monitoring the carbon dioxide profile through the biofilters in following the process performance has already been discussed elsewhere (20). Figure 8 shows the mass of carbon dioxide produced at the exit of the biofilter per units of filter media volume and time (PCO2) versus the elimination capacity EC for both units 1 and 2. The rate of carbon dioxide VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 9. Styrene elimination capacity versus the inlet load for various gas flow rates obtained in the biofilter irrigated with nutrient solution B (NO3-): O Q ) 0.5 m3‚h-1; + Q ) 0.7 m3‚h-1; and ] Q ) 1.0 m3‚h-1. production was higher with nutrient solution A (NH4+), confirming that styrene biodegradation activity is more intense with this nutrient supply. In the complete chemical oxidation of styrene to carbon dioxide and water, the ratio of carbon dioxide produced to styrene consumed should be 3.4. In the biodegradation reactions, a smaller ratio of the carbon dioxide was produced compared to the styrene removed, i.e., PCO2/EC is anticipated because some of the organic carbon is converted to biomass. From Figure 8, the experimental data are seen to lie reasonably along the line y ) 3x indicating that the ratio PCO2/EC is approximately 3 for all of the test conditions with both nutrient solutions, which confirms the complete biodegradation of removed styrene, given that some of the organic carbon consumed is also used for the microbial growth. Interestingly, the ratio PCO2/EC, which is stoichiometrically related to the ratio of biomass produced to styrene removed, is almost the same for both nitrate and ammonia fed biofilters. This similarity can be viewed as an indication that the biomass accumulation could have been similar in both biofilters if the same elimination capacity was achieved with each nutrient solution. This obversation confirms the finding that in the case of the biofilter irrigated with solution B (NO3-), the impact of nitrogen supply being exclusively in the form of nitrate on the reduction of biomass accumulation is not significant in comparison to the effect of the less efficient microbial activity found in this biofilter. Effect of Styrene Inlet Concentrations and Gas Flow Rates. As seen in Table 2, for the biofilter irrigated by solution A (NH4+), the elimination capacity EC increased as the gas flow rate Q increased from 1 up to 2.5 m3‚h-1, the inlet styrene concentration Cg0 being maintained at an approximately constant value of 0.9 g‚m-3. However, for the same Cg0, the EC decreased for the relatively greater gas flow rate of 3 m3‚h-1. Two separate phenomena may have contributed to the decrease in the EC at the gas flow rate 3 m3‚h-1: (a) insufficient residence time (21 s) for the degradation reactions to take place or (b) the bacterial clogging of the filter bed, this being observed for the biofilter irrigated with solution A (NH4+) after day 50 of operation, the period at which this particular gas flow rate was tested. Analysis of the experimental results obtained at various operating conditions in the biofilter irrigated with solution B (NO3-) shows that for a given gas flow rate, the removal efficiency decreases as styrene inlet concentration is increased (Table 3). Figure 9 presents the elimination capacity versus styrene concentration for three different gas flow rates 1770

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tested in the biofilter irrigated with solution B (NO3-). For a given constant gas flow rate, as the styrene inlet concentration Cg0 is increased, the elimination capacity, EC, at first increases, reaching a maximum value corresponding to an optimal Cg0. Further increases in Cg0 above this optimum lead to decreases in the EC. As shown in Table 3, the optimal inlet concentration and the corresponding elimination capacity are decreasing functions of the gas flow rate Q, whereas the optimal inlet load does not vary significantly with Q. Because the optimum Cg0 is not the same for all gas flow rates, there is a middle range of inlet styrene concentrations (1.4-2.7 g‚m-3 in the present study) where the response of the biofilter to an increase in Cg0 depends on the value of the operating gas flow rate (Figure 9). As an illustration, within this range, the EC increases with increasing Cg0 for the relatively low gas flow rate (0.5 m3‚h-1) but decreases with increasing Cg0 for the relatively high gas flow rate (1 m3‚h-1). Such behavior is not predictable since one can be mislead into expecting that the biofilter response (the EC) to an increase in Cg0 in a given range would be similar for all gas flow rates. On the other hand, it is also important to note that the inlet concentration Cg0 also has an impact on the variations of the elimination capacity EC versus the gas flow rate Q. As one can see from Figure 9, at high Cg0 (>2 g‚m-3), the EC decreases with increasing Q. However, at low Cg0 (