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Cultivation of cyanobacterium in various types of photobioreactors for biological CO2 fixation
T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.
471
Cultivation of cyanobacterium in various types of photobioreactors for biological COe fixation In Soo Suh a, Chan Beum Park a, Jung-Kuk Han a, Sun Bok Lee
a,b,c
aDepartment of Chemical Engineering and bSchool of Environmental Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea cResearch Institute of Industrial Science & Technology, Pohang, 790-784, Korea
Synechococcus sp. PCC6301 was cultivated in various types of enclosed photobioreactors for biological CO2 fixation. In order to maximize the photosynthetic efficiency of the cell, major culture conditions such as inlet CO2 concentration, incident light intensity, aeration rate, temperature, and agitation speed were optimized. To investigate the effects of bioreactor configuration on cell growth and CO2 fixation rate, we constructed stirred-tank photobioreactor (ST-PBR), florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OFPBR) and florescent-lamp/optical-fiber photobioreactor (FLOF-PBR). FL-PBR was found to be most energy efficient, whereas the highest CO2 fixation rate was obtained in the FLOF-PBR system. It is expected that the latter type of photobioreactor is useful for high-density cultivation of photosynethetic microorganisms and biological COe fixation in a large scale.
1. I N T R O D U C T I O N Carbon dioxide (COe), the natural product of fossil-fuel combustion, has been recognized as the main factor for the global warming. Among the various approaches to utilize the CO2 gas, there has been a considerable interest in biological CO2 fixation as a clean and energy-efficient technique [1]. In this research, we studied the cultivation of a cyanobacterium in various types of photobioreactors for biological fixation and utilization of COe. As a model photoautotrophic microorganism, Synechococcus sp. PCC6301 was chosen due to its higher growth rate, simple nutrient requirement, and a great deal of biochemical and genetic information. In order to maximize the photosynthetic efficiency of this unicellular cyanobacterium, major culture conditions were optimized. To examine the effects of bioreactor configuration on cell growth and CO2 fixation, five different types of photobioreactors were constructed and their efficiencies were compared in terms of the cell growth rate, CO2 fixation rate, and
472 energy efficiencies. The results presented in this work indicate t h a t FLOF-PBR, a combined form of externally illuminating photobioreactor (FL-PBR) and internally illuminating photobioreactor (OF-PBR), is suitable for efficient biological CO2 fixation in a large scale.
2. MATERIALS AND METHODS 2.1. S t r a i n and c u l t u r e c o n d i t i o n s Synechococcus sp. PCC6301 (ATCC27144, Anacystis nidulans) was cultivated in B G l l [2]. The cells grown on the agar were inoculated into 300 mL of the sterilized medium in a 500 mL bottle, and these cells were incubated by bubbling with the prehumidified air (0.03% CO2) at 30~ and 40 ~mol/m2/sec. The 7-dayold stock cultures were used as the inocula (10%) for all experiments. In the case of photobioreactor operation, cells were grown at 30~ using 0.03% CO2. 2.2. A n a l y t i c a l m e t h o d s In order to determine the cell concentration, optical density was measured at 600nm using a spectrophotometer (Milton Roy). The dry cell weight was then deduced from the calibration curve between the optical density and dry cell weight. Average light intensities at the surface of photobioreactors or optical fibers were determined with a quantum meter (LI-COR, LI-190SA). The CO2 concentration was measured with a gas chromatograph (Young-In, 680D), equipped with a packed column (Porapak N) and TCD detector. The flow rates of air and CO2 were precisely controlled by using mass flow controllers (Brooks Model 5850E). The elemental composition of the cells was determined by an elemental analyzer (LECO, CHNS-932). 2.3. P h o t o b i o r e a c t o r s y s t e m Based on the agitation and illumination methods, five types of enclosed photobioreactors were constructed. Working volumes, surface-to-volume ratios (S/V), and light intensities at the surface of photobioreactor or optical fibers are shown in Table 1. For mixing of culture broth, internal mechanical agitator was used in a stirred-tank photobioreactor (ST-PBR), while in bubble-column type photobioreactors gas spargers were used. Based on the light illuminating methods, photobioreactors could be divided into florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OF-PBR), and florescent-lamp/opticalfiber photobioreactor (FLOF-PBR). FL-PBR was externally illuminated with eight 20 W florescent lamps. OF-PBR-A was internally illuminated with 200 light-diffusing optical fibers connected to a 150W metal-halide lamp. In the case of OF-PBR-B, 486 light-diffusing optical fibers and a 400W lamp was used to enhance the light intensity. FLOF-PBR, a combined configuration of FL-PBR and OF-PBR-B, was illuminated with florescent lamps and optical fibers simultaneously. For efficient light emission, optical fibers were pretreated by scrubbing with sand papers.
473 T a b l e 1. Comparison of photobioreactor configuration and performance
Reactor type
Volume S/V Light intensity Growth rate CO2 fixation rate c Energy input d (L) (m 1) (}~mol/m2/sec) (g/L/day) (g/L/day) (W/g/L/day)
ST-PBR FL-PBR OF-PBR-A OF-PBR-B FLOF-PBR
4.0 2.5 2.5 2.5 2.5
24 75 273 557 632
100 a 100 a 5b 20 b 100 a + 205
0.25 0.43 0.07 0.38 0.53
0.43 0.75 0.12 0.66 0.92
279 213 1250 606 609
a Incident light intensity measured at the surface of photobioreactor. b Incident light intensity measured at the surface of optical fibers. c CO2 fixation rate calculated from the growth rate and carbon content of the cell. dEnergy required for illumination at the CO2 fixation rate of 1.0 gCOJL/day.
3. R E S U L T S A N D D I S C U S S I O N
Among the various photobioreactor systems, we excluded the outdoor open system such as pond-type and raceway-type cultivator. Although these types of photobioreactors are relatively less expensive and easy to operate, there are several disadvantages such as low growth rate, large space requirement, limited light availability, and difficulties in control and optimization [3]. In this research, we designed five types of enclosed photobioreactors and compared their energy efficiencies (Table 1). It appears that energy-efficient photobioreactor system is necessary for large scale CO2 fixation since high energy consumption for illumination and agitation could lead to additional CO2 gas release from a fossilfuel power plant. From the preliminary experiments, it was found that most cell growth (>70%) occurred in the linear growth phase during batch cultivation. Optimal inlet CO2 concentration for the cell used in this work was found to be in the range of 0.54.0% although the lag periods were increased with increasing CO2 concentration. The linear growth rates were increased with increasing the light intensity, followed by light saturation at 180 ~mol/m2/sec. The optimal agitation speed and aeration rate were found to be 200 rpm and 2.5 L/min at 0.03% CO2 and 30~ for ST-PBR and FL-PBR, respectively. In the case of ST-PBR, high electric power might be required for the agitation of culture broth. Since bubble-column type photobioreactors provide energyefficient mixing of culture broth and simple bioreactor configuration ease to scale-up, we designed several types of bubble-column type photobioreactors. Whereas the CO2 fixation rate in the ST-PBR was 0.43 g COJL/day, the CO2 fixation rate in the FL-PBR under the same incident light intensity (100 ~mol/m2/sec) was improved to 0.75 g COJL/day (see Table 1). Under optimal culture conditions in FL-PBR, the CO2 fixation rates reached a
474 maximum value of 4.33 g COJL/day in an exponential growth phase, which is equivalent to 16.5% conversion of input CO2 to biomass, and then decreased steadily with time in the linear growth phase. The incident light intensities were exponentially decreased inside the reactor with the specific absorption coefficient of 2.16 L/g/cm in the Beer-Lambert's law. Thus FL-PBR has a problem of light transfer limitation in large-scale photoautotrophic cultivation. The average CO2 fixation rate was calculated to be 0.87 gCOJL/day and the carbon content of the cell was 47.4%. If the above data are employed for estimating the reactor volume required for the biological t r e a t m e n t of CO2 gas from a typical 150 MW t h e r m a l power plant (assuming the CO2 emission rate of 130 tonCOJh), the required culture volume is calculated to be as much as 6• s liters. The OF-PBR system, which employs light-diffusing optical fibers for light distribution, has been known to provide a higher illuminating surface area per culture volume [4]. Since the light intensity from light-diffusing optical fibers can be changed depending on the light source and the number of optical fibers, we compared the effect of light intensities on reactor performance. As shown in Table 1, the CO2 fixation rate was enhanced from 0.12 to 0.66 gCOJL/day with increasing the light intensities. From the viewpoint of energy efficiency, however, OF-PBR system was not an ideal photobioreactor: OF-PBR requires at least 2.8 times more light energy t h a n FL-PBR when compared at the same CO2 fixation rate. It was also observed that the optical fibers interfered the mixing of culture broth and that cells attached to the surface of optical fibers reduced the light irradiance as the cells grew up. To compensate the limitation of FL-PBR and OF-PBR, we designed the FLOFPBR system, a combined form of externally illuminating photobioreactor (FLPBR) and internally illuminating photobioreactor (OF-PBR), and examined the efficiency of CO2 fixation rate and light energy utilization. As can be seen from Table 1, the highest CO2 fixation rate (0.92 gCOJL/day) was obtained in FLOFPBR among the photobioreactor systems employed in this work. This again indicates that a higher CO2 fixation rate can be achieved if more light is provided to the cell inside the reactor. In order to establish a more energy-efficient photobioreactor system, therefore, more efficient light transmission and light distribution methods need be developed. The results presented in this work implies that the use of FLOF-PBR may be an alternative way for high-density cultivation of photosynethetic microorganisms and biological CO2 fixation in a large scale.
REFERENCES 1. 2. 3. 4.
S.B. Lee, C.B. Park, and I.S. Suh, Chem. Ind. Technol. (Korea), 13, (1995), 347. M.M. Allen, J. Phycol. 4 (1968) 1. M.R. Tredici and R. Materassi, J. Appl. Phycol. 4 (1992) 221. H. Takano, H. Takeyama, N. Nakamura, K. Sode, J. G. Burgess, E. Manabe, M. Hirano, and T. Matsunaga, Appl. Biochem. Biotech. 34/35 (1992) 449.
471
Cultivation of cyanobacterium in various types of photobioreactors for biological COe fixation In Soo Suh a, Chan Beum Park a, Jung-Kuk Han a, Sun Bok Lee
a,b,c
aDepartment of Chemical Engineering and bSchool of Environmental Engineering, Pohang University of Science and Technology, Pohang, 790-784, Korea cResearch Institute of Industrial Science & Technology, Pohang, 790-784, Korea
Synechococcus sp. PCC6301 was cultivated in various types of enclosed photobioreactors for biological CO2 fixation. In order to maximize the photosynthetic efficiency of the cell, major culture conditions such as inlet CO2 concentration, incident light intensity, aeration rate, temperature, and agitation speed were optimized. To investigate the effects of bioreactor configuration on cell growth and CO2 fixation rate, we constructed stirred-tank photobioreactor (ST-PBR), florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OFPBR) and florescent-lamp/optical-fiber photobioreactor (FLOF-PBR). FL-PBR was found to be most energy efficient, whereas the highest CO2 fixation rate was obtained in the FLOF-PBR system. It is expected that the latter type of photobioreactor is useful for high-density cultivation of photosynethetic microorganisms and biological COe fixation in a large scale.
1. I N T R O D U C T I O N Carbon dioxide (COe), the natural product of fossil-fuel combustion, has been recognized as the main factor for the global warming. Among the various approaches to utilize the CO2 gas, there has been a considerable interest in biological CO2 fixation as a clean and energy-efficient technique [1]. In this research, we studied the cultivation of a cyanobacterium in various types of photobioreactors for biological fixation and utilization of COe. As a model photoautotrophic microorganism, Synechococcus sp. PCC6301 was chosen due to its higher growth rate, simple nutrient requirement, and a great deal of biochemical and genetic information. In order to maximize the photosynthetic efficiency of this unicellular cyanobacterium, major culture conditions were optimized. To examine the effects of bioreactor configuration on cell growth and CO2 fixation, five different types of photobioreactors were constructed and their efficiencies were compared in terms of the cell growth rate, CO2 fixation rate, and
472 energy efficiencies. The results presented in this work indicate t h a t FLOF-PBR, a combined form of externally illuminating photobioreactor (FL-PBR) and internally illuminating photobioreactor (OF-PBR), is suitable for efficient biological CO2 fixation in a large scale.
2. MATERIALS AND METHODS 2.1. S t r a i n and c u l t u r e c o n d i t i o n s Synechococcus sp. PCC6301 (ATCC27144, Anacystis nidulans) was cultivated in B G l l [2]. The cells grown on the agar were inoculated into 300 mL of the sterilized medium in a 500 mL bottle, and these cells were incubated by bubbling with the prehumidified air (0.03% CO2) at 30~ and 40 ~mol/m2/sec. The 7-dayold stock cultures were used as the inocula (10%) for all experiments. In the case of photobioreactor operation, cells were grown at 30~ using 0.03% CO2. 2.2. A n a l y t i c a l m e t h o d s In order to determine the cell concentration, optical density was measured at 600nm using a spectrophotometer (Milton Roy). The dry cell weight was then deduced from the calibration curve between the optical density and dry cell weight. Average light intensities at the surface of photobioreactors or optical fibers were determined with a quantum meter (LI-COR, LI-190SA). The CO2 concentration was measured with a gas chromatograph (Young-In, 680D), equipped with a packed column (Porapak N) and TCD detector. The flow rates of air and CO2 were precisely controlled by using mass flow controllers (Brooks Model 5850E). The elemental composition of the cells was determined by an elemental analyzer (LECO, CHNS-932). 2.3. P h o t o b i o r e a c t o r s y s t e m Based on the agitation and illumination methods, five types of enclosed photobioreactors were constructed. Working volumes, surface-to-volume ratios (S/V), and light intensities at the surface of photobioreactor or optical fibers are shown in Table 1. For mixing of culture broth, internal mechanical agitator was used in a stirred-tank photobioreactor (ST-PBR), while in bubble-column type photobioreactors gas spargers were used. Based on the light illuminating methods, photobioreactors could be divided into florescent-lamp photobioreactor (FL-PBR), optical-fiber photobioreactor (OF-PBR), and florescent-lamp/opticalfiber photobioreactor (FLOF-PBR). FL-PBR was externally illuminated with eight 20 W florescent lamps. OF-PBR-A was internally illuminated with 200 light-diffusing optical fibers connected to a 150W metal-halide lamp. In the case of OF-PBR-B, 486 light-diffusing optical fibers and a 400W lamp was used to enhance the light intensity. FLOF-PBR, a combined configuration of FL-PBR and OF-PBR-B, was illuminated with florescent lamps and optical fibers simultaneously. For efficient light emission, optical fibers were pretreated by scrubbing with sand papers.
473 T a b l e 1. Comparison of photobioreactor configuration and performance
Reactor type
Volume S/V Light intensity Growth rate CO2 fixation rate c Energy input d (L) (m 1) (}~mol/m2/sec) (g/L/day) (g/L/day) (W/g/L/day)
ST-PBR FL-PBR OF-PBR-A OF-PBR-B FLOF-PBR
4.0 2.5 2.5 2.5 2.5
24 75 273 557 632
100 a 100 a 5b 20 b 100 a + 205
0.25 0.43 0.07 0.38 0.53
0.43 0.75 0.12 0.66 0.92
279 213 1250 606 609
a Incident light intensity measured at the surface of photobioreactor. b Incident light intensity measured at the surface of optical fibers. c CO2 fixation rate calculated from the growth rate and carbon content of the cell. dEnergy required for illumination at the CO2 fixation rate of 1.0 gCOJL/day.
3. R E S U L T S A N D D I S C U S S I O N
Among the various photobioreactor systems, we excluded the outdoor open system such as pond-type and raceway-type cultivator. Although these types of photobioreactors are relatively less expensive and easy to operate, there are several disadvantages such as low growth rate, large space requirement, limited light availability, and difficulties in control and optimization [3]. In this research, we designed five types of enclosed photobioreactors and compared their energy efficiencies (Table 1). It appears that energy-efficient photobioreactor system is necessary for large scale CO2 fixation since high energy consumption for illumination and agitation could lead to additional CO2 gas release from a fossilfuel power plant. From the preliminary experiments, it was found that most cell growth (>70%) occurred in the linear growth phase during batch cultivation. Optimal inlet CO2 concentration for the cell used in this work was found to be in the range of 0.54.0% although the lag periods were increased with increasing CO2 concentration. The linear growth rates were increased with increasing the light intensity, followed by light saturation at 180 ~mol/m2/sec. The optimal agitation speed and aeration rate were found to be 200 rpm and 2.5 L/min at 0.03% CO2 and 30~ for ST-PBR and FL-PBR, respectively. In the case of ST-PBR, high electric power might be required for the agitation of culture broth. Since bubble-column type photobioreactors provide energyefficient mixing of culture broth and simple bioreactor configuration ease to scale-up, we designed several types of bubble-column type photobioreactors. Whereas the CO2 fixation rate in the ST-PBR was 0.43 g COJL/day, the CO2 fixation rate in the FL-PBR under the same incident light intensity (100 ~mol/m2/sec) was improved to 0.75 g COJL/day (see Table 1). Under optimal culture conditions in FL-PBR, the CO2 fixation rates reached a
474 maximum value of 4.33 g COJL/day in an exponential growth phase, which is equivalent to 16.5% conversion of input CO2 to biomass, and then decreased steadily with time in the linear growth phase. The incident light intensities were exponentially decreased inside the reactor with the specific absorption coefficient of 2.16 L/g/cm in the Beer-Lambert's law. Thus FL-PBR has a problem of light transfer limitation in large-scale photoautotrophic cultivation. The average CO2 fixation rate was calculated to be 0.87 gCOJL/day and the carbon content of the cell was 47.4%. If the above data are employed for estimating the reactor volume required for the biological t r e a t m e n t of CO2 gas from a typical 150 MW t h e r m a l power plant (assuming the CO2 emission rate of 130 tonCOJh), the required culture volume is calculated to be as much as 6• s liters. The OF-PBR system, which employs light-diffusing optical fibers for light distribution, has been known to provide a higher illuminating surface area per culture volume [4]. Since the light intensity from light-diffusing optical fibers can be changed depending on the light source and the number of optical fibers, we compared the effect of light intensities on reactor performance. As shown in Table 1, the CO2 fixation rate was enhanced from 0.12 to 0.66 gCOJL/day with increasing the light intensities. From the viewpoint of energy efficiency, however, OF-PBR system was not an ideal photobioreactor: OF-PBR requires at least 2.8 times more light energy t h a n FL-PBR when compared at the same CO2 fixation rate. It was also observed that the optical fibers interfered the mixing of culture broth and that cells attached to the surface of optical fibers reduced the light irradiance as the cells grew up. To compensate the limitation of FL-PBR and OF-PBR, we designed the FLOFPBR system, a combined form of externally illuminating photobioreactor (FLPBR) and internally illuminating photobioreactor (OF-PBR), and examined the efficiency of CO2 fixation rate and light energy utilization. As can be seen from Table 1, the highest CO2 fixation rate (0.92 gCOJL/day) was obtained in FLOFPBR among the photobioreactor systems employed in this work. This again indicates that a higher CO2 fixation rate can be achieved if more light is provided to the cell inside the reactor. In order to establish a more energy-efficient photobioreactor system, therefore, more efficient light transmission and light distribution methods need be developed. The results presented in this work implies that the use of FLOF-PBR may be an alternative way for high-density cultivation of photosynethetic microorganisms and biological CO2 fixation in a large scale.
REFERENCES 1. 2. 3. 4.
S.B. Lee, C.B. Park, and I.S. Suh, Chem. Ind. Technol. (Korea), 13, (1995), 347. M.M. Allen, J. Phycol. 4 (1968) 1. M.R. Tredici and R. Materassi, J. Appl. Phycol. 4 (1992) 221. H. Takano, H. Takeyama, N. Nakamura, K. Sode, J. G. Burgess, E. Manabe, M. Hirano, and T. Matsunaga, Appl. Biochem. Biotech. 34/35 (1992) 449.