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Biodegradation kinetic behaviors of n-butyl alcohol and sec-butyl alcohol in a composite bead biofilter
Process Biochemistry 44 (2009) 593–596
Contents lists available at ScienceDirect
Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Short communication
Biodegradation kinetic behaviors of n-butyl alcohol and sec-butyl alcohol in a composite bead biofilter Wu-Chung Chan *, Yu-Zhang Lai Civil Engineering Department, Chung-Hua University, Hsinchu 30067, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 January 2008 Received in revised form 18 February 2009 Accepted 20 February 2009
Biodegradation kinetic behaviors of n-butyl alcohol and sec-butyl alcohol in a composite bead biofilter were investigated. The microbial growth rate of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The microbial growth rate was inhibited at higher inlet concentration, and the inhibitive effect in the concentration range of 50–150 ppm was more pronounced than that in the concentration range of 150–300 ppm. The degree of inhibitive effect for nbutyl alcohol was more sensitive than that for sec-butyl alcohol in the concentration range of 50– 150 ppm. The zero-order kinetic with the diffusion rate limitation could be regarded as the most adequate biochemical reaction model. For the biochemical reaction process, the biochemical reaction rate coefficient of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The biochemical reaction rate coefficient was decreased with increasing inlet concentration. The inhibitive effect for sec-butyl alcohol was more pronounced than that for n-butyl alcohol. The factor of the chemical structure of compound was more predominant in the microbial growth and biochemical reaction processes. The maximum elimination capacity of n-butyl alcohol and sec-butyl alcohol were 55.7 and 20.9 g C h1 m3 bed volume, respectively. The primary alcohol was easily biodegraded by the microbial. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: n-butyl alcohol Sec-butyl alcohol Microbial growth rate Biochemical reaction rate Composite bead biofilter
1. Introduction The removal of volatile organic compounds (VOCs) from a polluted air stream using a biological process is highly efficient and has low installation and operation/maintenance costs. Biofiltration technology offers environmental advantages: it does not generate undesirable by-products by converting many organic and inorganic compounds into harmless oxidation products (e.g., water and carbon dioxide). Biofiltration involves the passage of a polluted air stream through a packed bed containing microorganisms immobilized within a biofilm attached to the bed-packing material. Contaminants are transferred to the interface between the gas and biofilm and are subsequently absorbed into the biofilm. Contaminants are then used as carbon and/or energy sources for the microorganisms within the biofilm. The solid filter material provides a nutrient source and matrix for the attachment of microorganisms in the biofiltration process. Therefore, the filter material property is an important factor in obtaining optimal pollutant removal. The optimal filter material should have the following characteristics: high moisture holding capacity, porosity, available nutrients, and pH buffer capacity [1]. A spherical
* Corresponding author. Tel.: +886 3 5186725; fax: +886 3 5372188. E-mail address: [email protected] (W.-C. Chan). 1359-5113/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2009.02.015
polyvinyl alcohol (PVA)/peat/KNO3/GAC composite bead was prepared and proved to be suitable as a filter material in the biofiltration process in our previous works [2–4]. N–butyl alcohol, iso-butyl alcohol and sec-butyl alcohol are volatile organic compound, which widely used industry chemicals. Large volumes of these compounds are released into the atmosphere during manufacturing process every year, endangering the air quality and public health [5,6]. Some reports concerned the biodegradation of n-butyl alcohol, iso-butyl alcohol and sec-butyl alcohol. A laboratory trickle-bed reactor was developed to determine the respiratory activity of microorganism immobilized on granular clay, polyamide beads, or sintered Styrofoam for the elimination of n-butanol from waste gas [7]. It was found that the volumetric respiration rate was correlated to the volumetric degradation rate, and the volumetric respiration rate recovered more slowly than did the volumetric degradation rate after interruption of VOCs supply. The volumetric degradation rate was 0.020–0.024 mmol min1 L1 packed bed. A polysulfone membrance module was used to remove n-butanol from air in a countercurrent, continuous flow bioreactor [8]. Biokinetic parameters for n-butanol utilization were determined to be a maximum specific utilization rate equal to 4.3 d1 and a half saturation constant equal to 8.9 mg L1. Anaerobic hybrid reactor was used to treat synthetic pharmaceutical wastewaters containing organic solvents such as isopropyl alcohol, isobutyl alcohol, sec-butyl
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alcohol and tert-butyl alcohol [9] It was found that the position of the –OH group affected the rate of microbial biodegradation and the order of biodegradation rate was isopropyl alcohol > isobutyl alcohol > sec-butyl alcohol > tert-butyl alcohol. Recently, we had obtained that the process for degradation of VOCs in a composite bead biofilter could be divided into lag, log growth and maximum stationary three phases, and the log growth and maximum stationary phases were important for controlling the removal efficiency of biofilter [2–4]. Therefore, studies the kinetic of log growth and maximum stationary phases was very important for operating and design on the biofiltration. However, details of the kinetic of such biodegradation process in biofilter are scant in the relevant literature. This article investigates the biodegradation kinetic behaviors of n-butanol and sec-butanol in a spherical PVA/peat/KNO3/GAC composite bead biofilter. The effect of inlet concentration and isomeric compound on the microbial growth rate and biochemical reaction rate are also studied. 2. Materials and methods Peat (industrial grade from KekkilaOyj, Tuusula, Finland) was dried at 105 8C before use. It has a dry density of 90 kg m3, a pH of 5.5, a pore volume of 96%, and an organic substance content of 91%. Boric acid, sodium monobasic phosphate, sodium dibasic phosphate, potassium nitrate, n-butyl alcohol and sec-butyl alcohol (extra pure grade from Union Chemical, Hsinchu, Taiwan) were used as received. Poly(vinyl alcohol) (PVA) powder (industrial grade from Chung Chun Petrochemical, Hsinchu, Taiwan) and granular activated carbon (GAC) (industrial grade from Taipei Chemical, Hsinchu, Taiwan) were also used as received. The procedures for preparing PVA/peat/GAC/KNO3 composite beads and the apparatus and operation of the biofilter system were described in our previous work [2–4]. N-butyl alcohol and sec-butyl alcohol were used as VOCs. Before packing, the filter material was immersed in 0.384 M KNO3 aqueous solution to adsorb KNO3 and to reach equilibrium (approximately 12 h). The bead moisture content was humidified to more than 1.5 g water g1 dry composite bead and the seeding was performed with activated sludge obtained from the wastewater treatment plant in Hsinchu Science-Based Industry Park. As the maximum stationary phase had maintained more than 3 days, according to the variations of VOCs removal efficiency with operation time, the biofilter operating was stopped. Then, repacking new filter material and following the operation procedures described as above to carry out another desired inlet concentration experiment. The desired inlet VOCs concentration in this study was 50, 100, 150, 200, 250 and 300 ppm. The gas flow rate was maintained at 0.102 m3 h1 for all experiments and consequently the empty bed residence time (EBRT) of the exit air steam at first, second, third and fourth section biofilter column was 7, 14, 21 and 28 s, respectively. The composition and concentration of VOCs in the inlet and exit air streams of each section was monitored by on-line auto-sampling and analyzed using gas chromatography (GC) (Model GC-8A from Shimadzu, Tokyo, Japan). There was no any product detected except CO2 and water in the exit air stream. The VOCs removal efficiency was calculated by the difference of the VOCs concentration between the inlet and exit air streams.
Fig. 1. The variation of VOCs removal efficiency with operation time (t) for the biofilter at the inlet concentration 150 ppm: (&) n-butyl alcohol and (~) sec-butyl alcohol.
3. Results and discussion The variation of VOCs removal efficiency with operation time for two compounds is shown in Fig. 1 (only the inlet concentration of 150 ppm is shown because the data for the other concentration were visually similar). It was found that the variation of VOCs removal efficiency with operation time appeared in three phases: lag phase (phase I), log growth phase (phase II) and maximum stationary phase (phase III). We had obtained that the VOCs were actually removed by the microbial biodegradation in the log growth phase and maximum stationary phase [2–4]. 3.1. Microbial growth process In the log growth phase (phase II), the microbial growth rate increased exponentially and was represented by the following equation [10] dX ¼ kg X dt
(1)
where X is the number of viable cells per unit volume, kg is the microbial growth rate and t is the operation time. The amount of contaminant degraded was proportional to the amount of viable cell produced in the biofilter because the kinetic of contaminant degradation is closely related to the kinetics of microbial growth [1]. Therefore, the concentration of VOCs in the exit stream (C) was inversely proportional to the number of viable cells per unit volume in the bed, and Eq. (1) can be converted into dC ¼ kg C dt Integration of Eq. (2) yields C ¼ kg t ln C0
(2)
(3)
where C0 is the concentration of VOCs in the inlet air stream. A plot of ln(C/C0) versus t should correspond to a straight line and kg can be determined. Therefore, the microbial growth rate kg of two compounds at various inlet concentrations was calculated from the data in phase II and Eq. (3). The variations of kg with inlet concentration C0 for two compounds are shown in Fig. 2. It was found that the kg value of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The water solubility of sec-butyl alcohol (125 g L1 H2O) was greater than that of n-butyl alcohol (77 g L1 H2O), so the amount of sec-butyl alcohol dissolved in the biofilm was greater than that of n-butyl alcohol. This phenomenon would lead more microorganisms participating in the sec-butyl alcohol biodegradation activity. N-butyl alcohol was a primary alcohol compound and sec-butyl alcohol was a secondary alcohol compound, so the result indicated that the microbial growth rate of the primary alcohol compound was greater than that of the secondary alcohol compound. The result was closely corresponding to the result reported that the maximum specific growth rate of 1-propanol (0.1039 h1) was greater than that of isopropanol (0.035 h1) [11]. Thus, the factor of the chemical structure of compound was more predominant than that of the water solubility of compound in the microbial growth process. The kg value decreased with increasing inlet concentration in the inlet concentration range of 50–300 ppm. An increase in the inlet concentration generally would enhance the transfer rate of the VOCs from the gas phase to the biofilm lead to more microorganisms participating in the biodegradation activity (enhancing effect). However, high concentrations of some recalcitrant VOCs may produce inhibitive effects on the metabolic activity
W.-C. Chan, Y.-Z. Lai / Process Biochemistry 44 (2009) 593–596
595
Therefore, Eq. (6) can be rewritten as 1
Fig. 2. The variations of kg with inlet concentration (C0) for two compounds: (&) nbutyl alcohol and (~) sec-butyl alcohol.
of the microbial population [12]. Therefore, the result indicated that the inhibitive effect predominated and the microbial growth rate was inhibited at higher inlet concentration. The result was closely corresponding to the result reported that the maximum specific growth rate of 1-propanol decreased from 0.1039– 0.0715 h1 as the initial concentration increased from 1% (v/v) to 2% (v/v) [11]. The inhibitive effect in the low concentration range (from 50 to 150 ppm) was more pronounced than that in the high concentration range (from 150 to 300 ppm), and the degree of inhibitive effect for n-butyl alcohol was more sensitive than that for sec-butyl alcohol in the low concentration range. 3.2. Biochemical reaction process In the maximum stationary phase, the population of viable cells was at a relatively constant value. A biofilter modeling that described steady-state elimination of individual carbon energy substrates was proposed by Ottengraf. Three basic situations of Ottengraf’s model was first-order kinetics, zero-order kinetics with reaction limitation and zero-order kinetics with diffusion limitation [1]. The corresponding equations expressed the rates of biochemical reaction for each situation as follows: First-order kinetic C ln ¼ k1 u C0
(4)
C C0
1=2
¼ kd u
(7)
where kd is the rate coefficient of zero-order kinetic with diffusion limitation. The substrate utilization rate by microbial was generally expressed by the Michaeilis–Menten relationship. Under the state of microbial population does not change with time, three possible situations may be encountered in a biochemical reaction system [13]: Situation (1) if the substrate concentration was very low (Ks C0), the reaction rate expression could be simplified to firstorder kinetic; Situation (2) if the substrate concentration was very high (Ks C0), the reaction rate expression could be simplified to zero-order kinetic; Situation (3) if the substrate concentration C0 was comparable with Ks, the reaction rate expression could not be simplified and it was followed fractional-order kinetic, and the Ottengraf’s diffusion limiting model was found to be the most approximate expression. In order to verify the biochemical reaction kinetic model, assume there was a plug airflow in the biofilter column and the following equation was derived from the Michaelis–Menten equation [10] ðC 0 CÞ u ¼ Vm Ks (8) lnðC 0 =CÞ lnðC 0 =CÞ where Ks is half-saturation constant and Vm is maximum reaction rate. A plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) should correspond to a straight line, and Ks and Vm can be determined. The plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) for two compounds is shown Fig. 3. The calculated Ks for n-butyl alcohol and sec-butyl alcohol were 3.09 and 3.41 ppm, respectively. The calculated Vm for n-butyl alcohol and sec-butyl alcohol were 4.42 and 2.87 g C h1 kg1 packed material, respectively. The ratio values of C0/Ks for n-butyl alcohol and sec-butyl alcohol were 16.18–97.09 and 14.66–87.98, respectively. The results indicated that the relationship of C0 and Ks was not corresponding to situation 1 or situation 2, and it was corresponding to situation 3 for two compounds. Therefore, the concentration C0 was comparable with Ks, and zero-order kinetic with diffusion limitation was regarded as the most adequate biochemical reaction kinetic model in this study. The kd value of two compounds at various inlet concentrations was calculated from the data in phase III and Eq. (7). The kd value is inversely proportional to the square root of C0 according to the definition of kd described in the earlier section of 1=2 this paper. So the variation of kd with inlet concentration C0 for
Zero-order kinetic with reaction limitation C 0 C ¼ k0 u
(5)
Zero-order kinetic with diffusion limitation " C ¼ C0 1 u
a k0 De 2mC 0 d
1=2 #2
(6)
where a is the interfacial area per unit volume, De is the effective diffusion coefficient, m is the distribution coefficient of the component, u is the empty bed residence time (EBRT), d is the biofilm thickness, and k1 and k0 are the rate coefficients of firstorder kinetic and zero-order with reaction limitation, respectively. However, for convenience of use Ottengraf’s model, it is necessary to define a new parameter, kd = (ak0De/2mC0d)1/2. It can be seen that kd is a function of the operating conditions of the biofilter system, and kd is constant under steady-state conditions [13].
Fig. 3. Plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) for two compounds: (&) n-butyl alcohol and (~) sec-butyl alcohol.
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596
tion in this concentration range. The slope of the linearized profiles in this concentration range for n-butyl alcohol and sec-butyl alcohol were 0.170 and 0.258 s1ppm1/2, respectively. The result indicated that the inhibitive effect, resulting from increased inlet concentration, for sec-butyl alcohol was more sensitive than that for n-butyl alcohol. The maximum elimination capacity of n-butyl alcohol and sec-butyl alcohol were 55.7 and 20.9 g C h1 m3 bed volume, respectively. References
1=2
Fig. 4. The variations of kd with inlet concentration (C0 n-butyl alcohol and (~) sec-butyl alcohol.
) for two compounds: (&)
two compounds is shown in Fig. 4. The kd value of n-butyl alcohol 1=2 was greater than that of sec-butyl alcohol in the C0 value range of 0.14–0.058 ppm (equals inlet concentration C0 range of 50– 300 ppm). The result indicated that the biochemical reaction rate of n-butyl alcohol was faster than that of sec-butyl alcohol in this concentration range. Therefore, the factor of the chemical structure of compound also predominated in the biochemical reaction process and the primary alcohol was easier biodegraded by the microbial. The kd value decreased with increasing inlet concentration in this concentration range for both compounds. The result indicated that the inhibitive effect also predominated and the biochemical reaction rate was inhibited at higher inlet concentra-
[1] Deviney JS, Deshusses MA, Webster TS. Biofiltration for air pollution control. New York: Lewis Publishers; 1999. [2] Chan WC, Lin ZY. A process to prepare a synthetic filter material containing nutrients for biofilteration. Bioresour Technol 2006;97:1927–33. [3] Chan WC, Xiao HZ. Removal ethyl acetate by composite bead biofilter. Chung Hua J Sci Eng 2006;4:53–62. [4] Chan WC, Peng KH. Biodegradation of methyl ethyl ketone and methyl isopropyl ketone in a composite bead biofilter. Eng Life Sci 2008;8:167–74. [5] Chemicals in the Environment: 1-Butanol. Washington, DC: U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics; 1994. [6] NIOSH Pocket Guide to Chemical Hazards. National Institute of Occupational Safety and Health (NIOSH). Washington, DC: Government Printing Office; 1997. [7] Heinze U, Friedrich CG. Respiratory activity of biofilms: measurement and its significance for the elimination of n-butanol from waste gas. Appl Microbiol Biotechnol 1977;48:411–6. [8] Fitch MW, England E, Zhang B. 1-butanol removal from a contaminated airstream under continuous and diurnal loading conditions. J Air Waste Manage Assoc 2002;52:1288–97. [9] Henry MP, Donlon BA, Lens PN, Colleran LEM. Use of anaerobic hybrid reactors for treatment of synthetic pharmaceutical wastewaters containing organic solvents. J Chem Tech Biotechnol 1996;66:251–64. [10] Valsaraj KT. Elements of environmental engineering: thermodynamics and kinetics. New York: Lewis Publishers; 1995. [11] Bustard MT, McEvoy EM, Goodwin JAS, Burgess JG, Wright PC. Biodegradation of propanol and isopropanol by a mixed microbial consortium. Appl Microbial Biotechnol 2000;54:424–31. [12] Leson G, Winer AM. Biofiltration: an innovative air pollution control technology for VOC emission. J Air Waste Manage Assoc 1991;41:1045–54. [13] Yang Y, Allen ER. Biofiltration control of hydrogen sulfide. 2. Kinetics, biofilter performance, and maintenance. J Air Waste Manage Assoc 1994;44:1315–21.
Contents lists available at ScienceDirect
Process Biochemistry journal homepage: www.elsevier.com/locate/procbio
Short communication
Biodegradation kinetic behaviors of n-butyl alcohol and sec-butyl alcohol in a composite bead biofilter Wu-Chung Chan *, Yu-Zhang Lai Civil Engineering Department, Chung-Hua University, Hsinchu 30067, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 January 2008 Received in revised form 18 February 2009 Accepted 20 February 2009
Biodegradation kinetic behaviors of n-butyl alcohol and sec-butyl alcohol in a composite bead biofilter were investigated. The microbial growth rate of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The microbial growth rate was inhibited at higher inlet concentration, and the inhibitive effect in the concentration range of 50–150 ppm was more pronounced than that in the concentration range of 150–300 ppm. The degree of inhibitive effect for nbutyl alcohol was more sensitive than that for sec-butyl alcohol in the concentration range of 50– 150 ppm. The zero-order kinetic with the diffusion rate limitation could be regarded as the most adequate biochemical reaction model. For the biochemical reaction process, the biochemical reaction rate coefficient of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The biochemical reaction rate coefficient was decreased with increasing inlet concentration. The inhibitive effect for sec-butyl alcohol was more pronounced than that for n-butyl alcohol. The factor of the chemical structure of compound was more predominant in the microbial growth and biochemical reaction processes. The maximum elimination capacity of n-butyl alcohol and sec-butyl alcohol were 55.7 and 20.9 g C h1 m3 bed volume, respectively. The primary alcohol was easily biodegraded by the microbial. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: n-butyl alcohol Sec-butyl alcohol Microbial growth rate Biochemical reaction rate Composite bead biofilter
1. Introduction The removal of volatile organic compounds (VOCs) from a polluted air stream using a biological process is highly efficient and has low installation and operation/maintenance costs. Biofiltration technology offers environmental advantages: it does not generate undesirable by-products by converting many organic and inorganic compounds into harmless oxidation products (e.g., water and carbon dioxide). Biofiltration involves the passage of a polluted air stream through a packed bed containing microorganisms immobilized within a biofilm attached to the bed-packing material. Contaminants are transferred to the interface between the gas and biofilm and are subsequently absorbed into the biofilm. Contaminants are then used as carbon and/or energy sources for the microorganisms within the biofilm. The solid filter material provides a nutrient source and matrix for the attachment of microorganisms in the biofiltration process. Therefore, the filter material property is an important factor in obtaining optimal pollutant removal. The optimal filter material should have the following characteristics: high moisture holding capacity, porosity, available nutrients, and pH buffer capacity [1]. A spherical
* Corresponding author. Tel.: +886 3 5186725; fax: +886 3 5372188. E-mail address: [email protected] (W.-C. Chan). 1359-5113/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2009.02.015
polyvinyl alcohol (PVA)/peat/KNO3/GAC composite bead was prepared and proved to be suitable as a filter material in the biofiltration process in our previous works [2–4]. N–butyl alcohol, iso-butyl alcohol and sec-butyl alcohol are volatile organic compound, which widely used industry chemicals. Large volumes of these compounds are released into the atmosphere during manufacturing process every year, endangering the air quality and public health [5,6]. Some reports concerned the biodegradation of n-butyl alcohol, iso-butyl alcohol and sec-butyl alcohol. A laboratory trickle-bed reactor was developed to determine the respiratory activity of microorganism immobilized on granular clay, polyamide beads, or sintered Styrofoam for the elimination of n-butanol from waste gas [7]. It was found that the volumetric respiration rate was correlated to the volumetric degradation rate, and the volumetric respiration rate recovered more slowly than did the volumetric degradation rate after interruption of VOCs supply. The volumetric degradation rate was 0.020–0.024 mmol min1 L1 packed bed. A polysulfone membrance module was used to remove n-butanol from air in a countercurrent, continuous flow bioreactor [8]. Biokinetic parameters for n-butanol utilization were determined to be a maximum specific utilization rate equal to 4.3 d1 and a half saturation constant equal to 8.9 mg L1. Anaerobic hybrid reactor was used to treat synthetic pharmaceutical wastewaters containing organic solvents such as isopropyl alcohol, isobutyl alcohol, sec-butyl
594
W.-C. Chan, Y.-Z. Lai / Process Biochemistry 44 (2009) 593–596
alcohol and tert-butyl alcohol [9] It was found that the position of the –OH group affected the rate of microbial biodegradation and the order of biodegradation rate was isopropyl alcohol > isobutyl alcohol > sec-butyl alcohol > tert-butyl alcohol. Recently, we had obtained that the process for degradation of VOCs in a composite bead biofilter could be divided into lag, log growth and maximum stationary three phases, and the log growth and maximum stationary phases were important for controlling the removal efficiency of biofilter [2–4]. Therefore, studies the kinetic of log growth and maximum stationary phases was very important for operating and design on the biofiltration. However, details of the kinetic of such biodegradation process in biofilter are scant in the relevant literature. This article investigates the biodegradation kinetic behaviors of n-butanol and sec-butanol in a spherical PVA/peat/KNO3/GAC composite bead biofilter. The effect of inlet concentration and isomeric compound on the microbial growth rate and biochemical reaction rate are also studied. 2. Materials and methods Peat (industrial grade from KekkilaOyj, Tuusula, Finland) was dried at 105 8C before use. It has a dry density of 90 kg m3, a pH of 5.5, a pore volume of 96%, and an organic substance content of 91%. Boric acid, sodium monobasic phosphate, sodium dibasic phosphate, potassium nitrate, n-butyl alcohol and sec-butyl alcohol (extra pure grade from Union Chemical, Hsinchu, Taiwan) were used as received. Poly(vinyl alcohol) (PVA) powder (industrial grade from Chung Chun Petrochemical, Hsinchu, Taiwan) and granular activated carbon (GAC) (industrial grade from Taipei Chemical, Hsinchu, Taiwan) were also used as received. The procedures for preparing PVA/peat/GAC/KNO3 composite beads and the apparatus and operation of the biofilter system were described in our previous work [2–4]. N-butyl alcohol and sec-butyl alcohol were used as VOCs. Before packing, the filter material was immersed in 0.384 M KNO3 aqueous solution to adsorb KNO3 and to reach equilibrium (approximately 12 h). The bead moisture content was humidified to more than 1.5 g water g1 dry composite bead and the seeding was performed with activated sludge obtained from the wastewater treatment plant in Hsinchu Science-Based Industry Park. As the maximum stationary phase had maintained more than 3 days, according to the variations of VOCs removal efficiency with operation time, the biofilter operating was stopped. Then, repacking new filter material and following the operation procedures described as above to carry out another desired inlet concentration experiment. The desired inlet VOCs concentration in this study was 50, 100, 150, 200, 250 and 300 ppm. The gas flow rate was maintained at 0.102 m3 h1 for all experiments and consequently the empty bed residence time (EBRT) of the exit air steam at first, second, third and fourth section biofilter column was 7, 14, 21 and 28 s, respectively. The composition and concentration of VOCs in the inlet and exit air streams of each section was monitored by on-line auto-sampling and analyzed using gas chromatography (GC) (Model GC-8A from Shimadzu, Tokyo, Japan). There was no any product detected except CO2 and water in the exit air stream. The VOCs removal efficiency was calculated by the difference of the VOCs concentration between the inlet and exit air streams.
Fig. 1. The variation of VOCs removal efficiency with operation time (t) for the biofilter at the inlet concentration 150 ppm: (&) n-butyl alcohol and (~) sec-butyl alcohol.
3. Results and discussion The variation of VOCs removal efficiency with operation time for two compounds is shown in Fig. 1 (only the inlet concentration of 150 ppm is shown because the data for the other concentration were visually similar). It was found that the variation of VOCs removal efficiency with operation time appeared in three phases: lag phase (phase I), log growth phase (phase II) and maximum stationary phase (phase III). We had obtained that the VOCs were actually removed by the microbial biodegradation in the log growth phase and maximum stationary phase [2–4]. 3.1. Microbial growth process In the log growth phase (phase II), the microbial growth rate increased exponentially and was represented by the following equation [10] dX ¼ kg X dt
(1)
where X is the number of viable cells per unit volume, kg is the microbial growth rate and t is the operation time. The amount of contaminant degraded was proportional to the amount of viable cell produced in the biofilter because the kinetic of contaminant degradation is closely related to the kinetics of microbial growth [1]. Therefore, the concentration of VOCs in the exit stream (C) was inversely proportional to the number of viable cells per unit volume in the bed, and Eq. (1) can be converted into dC ¼ kg C dt Integration of Eq. (2) yields C ¼ kg t ln C0
(2)
(3)
where C0 is the concentration of VOCs in the inlet air stream. A plot of ln(C/C0) versus t should correspond to a straight line and kg can be determined. Therefore, the microbial growth rate kg of two compounds at various inlet concentrations was calculated from the data in phase II and Eq. (3). The variations of kg with inlet concentration C0 for two compounds are shown in Fig. 2. It was found that the kg value of n-butyl alcohol was greater than that of sec-butyl alcohol in the inlet concentration range of 50–300 ppm. The water solubility of sec-butyl alcohol (125 g L1 H2O) was greater than that of n-butyl alcohol (77 g L1 H2O), so the amount of sec-butyl alcohol dissolved in the biofilm was greater than that of n-butyl alcohol. This phenomenon would lead more microorganisms participating in the sec-butyl alcohol biodegradation activity. N-butyl alcohol was a primary alcohol compound and sec-butyl alcohol was a secondary alcohol compound, so the result indicated that the microbial growth rate of the primary alcohol compound was greater than that of the secondary alcohol compound. The result was closely corresponding to the result reported that the maximum specific growth rate of 1-propanol (0.1039 h1) was greater than that of isopropanol (0.035 h1) [11]. Thus, the factor of the chemical structure of compound was more predominant than that of the water solubility of compound in the microbial growth process. The kg value decreased with increasing inlet concentration in the inlet concentration range of 50–300 ppm. An increase in the inlet concentration generally would enhance the transfer rate of the VOCs from the gas phase to the biofilm lead to more microorganisms participating in the biodegradation activity (enhancing effect). However, high concentrations of some recalcitrant VOCs may produce inhibitive effects on the metabolic activity
W.-C. Chan, Y.-Z. Lai / Process Biochemistry 44 (2009) 593–596
595
Therefore, Eq. (6) can be rewritten as 1
Fig. 2. The variations of kg with inlet concentration (C0) for two compounds: (&) nbutyl alcohol and (~) sec-butyl alcohol.
of the microbial population [12]. Therefore, the result indicated that the inhibitive effect predominated and the microbial growth rate was inhibited at higher inlet concentration. The result was closely corresponding to the result reported that the maximum specific growth rate of 1-propanol decreased from 0.1039– 0.0715 h1 as the initial concentration increased from 1% (v/v) to 2% (v/v) [11]. The inhibitive effect in the low concentration range (from 50 to 150 ppm) was more pronounced than that in the high concentration range (from 150 to 300 ppm), and the degree of inhibitive effect for n-butyl alcohol was more sensitive than that for sec-butyl alcohol in the low concentration range. 3.2. Biochemical reaction process In the maximum stationary phase, the population of viable cells was at a relatively constant value. A biofilter modeling that described steady-state elimination of individual carbon energy substrates was proposed by Ottengraf. Three basic situations of Ottengraf’s model was first-order kinetics, zero-order kinetics with reaction limitation and zero-order kinetics with diffusion limitation [1]. The corresponding equations expressed the rates of biochemical reaction for each situation as follows: First-order kinetic C ln ¼ k1 u C0
(4)
C C0
1=2
¼ kd u
(7)
where kd is the rate coefficient of zero-order kinetic with diffusion limitation. The substrate utilization rate by microbial was generally expressed by the Michaeilis–Menten relationship. Under the state of microbial population does not change with time, three possible situations may be encountered in a biochemical reaction system [13]: Situation (1) if the substrate concentration was very low (Ks C0), the reaction rate expression could be simplified to firstorder kinetic; Situation (2) if the substrate concentration was very high (Ks C0), the reaction rate expression could be simplified to zero-order kinetic; Situation (3) if the substrate concentration C0 was comparable with Ks, the reaction rate expression could not be simplified and it was followed fractional-order kinetic, and the Ottengraf’s diffusion limiting model was found to be the most approximate expression. In order to verify the biochemical reaction kinetic model, assume there was a plug airflow in the biofilter column and the following equation was derived from the Michaelis–Menten equation [10] ðC 0 CÞ u ¼ Vm Ks (8) lnðC 0 =CÞ lnðC 0 =CÞ where Ks is half-saturation constant and Vm is maximum reaction rate. A plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) should correspond to a straight line, and Ks and Vm can be determined. The plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) for two compounds is shown Fig. 3. The calculated Ks for n-butyl alcohol and sec-butyl alcohol were 3.09 and 3.41 ppm, respectively. The calculated Vm for n-butyl alcohol and sec-butyl alcohol were 4.42 and 2.87 g C h1 kg1 packed material, respectively. The ratio values of C0/Ks for n-butyl alcohol and sec-butyl alcohol were 16.18–97.09 and 14.66–87.98, respectively. The results indicated that the relationship of C0 and Ks was not corresponding to situation 1 or situation 2, and it was corresponding to situation 3 for two compounds. Therefore, the concentration C0 was comparable with Ks, and zero-order kinetic with diffusion limitation was regarded as the most adequate biochemical reaction kinetic model in this study. The kd value of two compounds at various inlet concentrations was calculated from the data in phase III and Eq. (7). The kd value is inversely proportional to the square root of C0 according to the definition of kd described in the earlier section of 1=2 this paper. So the variation of kd with inlet concentration C0 for
Zero-order kinetic with reaction limitation C 0 C ¼ k0 u
(5)
Zero-order kinetic with diffusion limitation " C ¼ C0 1 u
a k0 De 2mC 0 d
1=2 #2
(6)
where a is the interfacial area per unit volume, De is the effective diffusion coefficient, m is the distribution coefficient of the component, u is the empty bed residence time (EBRT), d is the biofilm thickness, and k1 and k0 are the rate coefficients of firstorder kinetic and zero-order with reaction limitation, respectively. However, for convenience of use Ottengraf’s model, it is necessary to define a new parameter, kd = (ak0De/2mC0d)1/2. It can be seen that kd is a function of the operating conditions of the biofilter system, and kd is constant under steady-state conditions [13].
Fig. 3. Plot of (C0 C)/ln(C0/C) versus u/ln(C0/C) for two compounds: (&) n-butyl alcohol and (~) sec-butyl alcohol.
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tion in this concentration range. The slope of the linearized profiles in this concentration range for n-butyl alcohol and sec-butyl alcohol were 0.170 and 0.258 s1ppm1/2, respectively. The result indicated that the inhibitive effect, resulting from increased inlet concentration, for sec-butyl alcohol was more sensitive than that for n-butyl alcohol. The maximum elimination capacity of n-butyl alcohol and sec-butyl alcohol were 55.7 and 20.9 g C h1 m3 bed volume, respectively. References
1=2
Fig. 4. The variations of kd with inlet concentration (C0 n-butyl alcohol and (~) sec-butyl alcohol.
) for two compounds: (&)
two compounds is shown in Fig. 4. The kd value of n-butyl alcohol 1=2 was greater than that of sec-butyl alcohol in the C0 value range of 0.14–0.058 ppm (equals inlet concentration C0 range of 50– 300 ppm). The result indicated that the biochemical reaction rate of n-butyl alcohol was faster than that of sec-butyl alcohol in this concentration range. Therefore, the factor of the chemical structure of compound also predominated in the biochemical reaction process and the primary alcohol was easier biodegraded by the microbial. The kd value decreased with increasing inlet concentration in this concentration range for both compounds. The result indicated that the inhibitive effect also predominated and the biochemical reaction rate was inhibited at higher inlet concentra-
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