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Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas
Journal of Bioscience and Bioengineering VOL. xxx No. xxx, xxx, xxxx www.elsevier.com/locate/jbiosc
Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas Chenghu Yan, Zhihui Wang, Xia Wu, Shumei Wen, Jing Yu, and Wei Cong* State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Received 29 October 2019; accepted 8 December 2019 Available online xxx
In order to reduce the flow resistance of the thin-film flat plat photobioreactor (FPPBR) and make it more suitable for mass microalgae cultivation, the channel diameter was modified to 0.06 m and the thin-film FPPBR consisted of 10 parallel shunt-wound channels. A thin-film FPPBR system with 100 modified FPPBRs was constructed and used for Chlorella sp. cultivation in desertification areas (Ordos, China) from July to September of 2018. The pressure drop of the modified FPPBR system decreased significantly and the microalgae showed much higher productivity. The pressure drop was about 11.8 kPa when the liquid velocity was 0.238 m sL1. The final biomass concentration and area productivity reached 2.01 g LL1 and 49.79 g mL2 dayL1 respectively, and the yearly productivity of Chlorella sp. was estimated to be about 15.24 t haL2 yearL1. The results demonstrated that high productivity of Chlorella sp. could be achieved in the improved FPPBR system in desertification areas and the improved FPPBR system was feasible for mass cultivation of microalgae in the commercial application. Ó 2019, The Society for Biotechnology, Japan. All rights reserved. [Key words: Outdoor cultivation; Flat plate photobioreactor; Mass production; Chlorella sp.; Baffle]
Microalgae are photosynthetic microorganisms from either marine or fresh water environments, which have been used as feedstock for high value compounds and biofuels production since they are able to transform carbon dioxide into cellular biomolecules such as biofuels, antioxidant compounds, nutraceuticals, animal/ fish feeds, and fertilizers via photosynthesis (1e4). Thus, the demand for algae biomass is increasing in recent years. For mass production of microalgae, efficient photobioreactors with low energy input and utilizing maximal solar radiation are required (5,6). Up to now, a large number of photobioreactors with different volumes and shapes have been developed (7e10). However, only a few of them have been used for commercial microalgae cultivation. In our previous study, a novel thin-film FPPBR mounted with inclined baffles was developed and used for Chlorella sp. cultivation (11,12). The biomass productivity of Chlorella sp. in the novel FPPBR was 25.2% higher than the FPPBR without baffles, and the biomass productivity of Chlorella sp. and Scenedesmus dimorphus in the photobioreactor system reached 11 g m2 day1 and 21.9 g m2 day1. However, the flow resistance of microalgae suspension in the photobioreactor is still high, which increases the pressure of the photobioreactor and the risk of being damaged, thus the phobioreactor system can only be operated in low flow velocity, resulting in the decrease of flashing light effect and productivity. Thus, the photobioreactor system remain needs to be improved. The configuration of the thin-film FPPBR is similar to Z-type flat plate manifolds, which had demonstrated that the flow rate uniformity and the pressure drop of Z-type manifolds could be
* Corresponding author. Tel.: þ86 10 82627060; fax: þ86 10 82627066. E-mail address: [email protected] (W. Cong).
improved by increasing the diameter of each shunt-wound channel (13). Moreover, culture efficiency is also related to channel diameter, for maximum production in a given area, the suggested channel diameter is about 0.06 m (14,15). Therefore, in order to make the thin-film flat plat photobioreactor (FPPBR) more suitable for commercial microalgae cultivation, the channel diameter was modified to 0.06 m and the thin-film FPPBR consisted of 10 parallel shunt-wound channels. An improved thinfilm FPPBR system with 100 FPPBRs was built up and used for Chlorella sp. cultivation at Engebei (Ordos, China), close to the Kubuqi Desert. The microalgae cultivation experiments were performed for three months (July to September 2018), and the relationships between biomass productivities and environmental conditions were investigated.
MATERIALS AND METHODS Configuration of the thin-film FPPBR and photobioreactor system In order to reduce the flow resistance of the photobioreactor and the pressure drop of the photobioreactor system, the thin-film FPPBR reported in our previous work was modified (12), and a photobioreactor system was set up based on the modified FPPBR, which are shown in Figs. 1 and 2. The unmodified thin-film FPPBR is shown in Fig. 1C, which is 2.0 m length and 0.96 m width, consisting of sixteen shunt-wound channels, the length of each channel is 1.44 m. In each channel, the inclined baffles are attached to both films, which are perpendicular to the film surface (12). The modified thin-film FPPBR is shown in Fig. 1A and B, which is 2.0 m length and 1.0 m width, consisting of 10 parallel shunt-wound channels. The baffles are mounted with an inclined angle a ¼ 30 shown in Fig. 1, and the dimension of the inclined baffles is length (l) ¼ 60 mm, height (h) ¼ 8 mm, width (w) ¼ 2.5 mm. In the modified photobioreactor system, an axial-flow pump is used to drive microalgae suspension, an earth pond of 10 m3 is built as medium storage tank, the
1389-1723/$ e see front matter Ó 2019, The Society for Biotechnology, Japan. All rights reserved. https://doi.org/10.1016/j.jbiosc.2019.12.007
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
2
YAN ET AL.
J. BIOSCI. BIOENG.,
FIG. 1. Schematic diagram and photographs of the thin-film flat plate photobioreactor (FPPBR). (A) Schematic diagram of the improved photobioreactor: 1, inlet; 2, outlet; 3, nether baffle; 4, upper baffle; 5, flow channel partition; 6, flow channel; 7, inlet cavity; 8, outlet cavity. (B) Improved photobioreactor; (C) photograph of unmodified photobioreactor.
pond is then lined with a 0.15 mm thick polyethylene films, and a polyethylene film is used to cover the pond in order to counter adverse weather conditions such as raining and freezing temperature. A gas sparger is installed at the inlet of the first photobioreactor, pure CO2 is injected into the photobioreactor directly during microalgae cultivation. Dissolved oxygen and the temperature are controlled using
similar methods reported in our previous research (12). Dissolved oxygen was regulated by changing the rate of air flow entering the medium tank using suitable valves and flowmeter through a gas sparger located at the bottom of the medium tank. A temperature control system with an electromagnetic valve was used to keep the maximum culture temperature lower than 35 C, When the temperature of the
FIG. 2. Schematic diagram (A) and photographs (B, C) of the pilot scale thin-film flat plate photobioreactor (FPPBR) system. Components: 1, medium storage tank; 2, axial pump; 3, photosynthetic reaction unit; 4, gas sparger (for O2 removal); 5, gas sparger (for CO2 supply); 6, temperature control system.
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
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TABLE 1. The pressure drop between the inlet and outlet of the photobioreactor system with respect to liquid velocity in the channels. Frequency (Hz) 34.90 36.45 38.50 40.10 42.05 44.95
Pump capacity (m3 h1)
Inlet pressure (kPa)
Outlet pressure (kPa)
Pressure drop (Pa)
Velocity (m s1)
11.310.27 11.940.12 14.360.17 20.193.07 21.180.26 25.021.02
8.20.16 9.10.13 8.80.15 10.40.31 10.80.17 11.80.11
0 0 0 0 0 0
1643.2 1822.6 1763.0 2086.2 2163.4 2362.2
0.1190.030 0.1230.029 0.1500.017 0.1820.023 0.2110.026 0.2380.029
Data are shown as mean SD, n ¼ 5. microalgae suspension exceeded 30 C, the electromagnetic valve opened and cooling was accomplished by sprinkling the surface of the photobioreactor with natural ground water. The improved photobioreactor system covers an area of about 250 m2, the photosynthetic reaction unit comprises 100 horizontal improved FPPBRs, which is arranged into 2 rows, as shown in Fig. 2. The working volume is about 13 m3 and the illuminated surface area is 119 m2. The pressure drop and liquid velocity of the thin-film FPPBR system The pressure drop between the inlet and outlet of the improved photobioreactor system at different liquid velocity in the photobioreactor was measured. The pressure drop was calculated by measuring the height difference of water column between the inlet and outlet of the FPPBR system, the average values and standard deviations were calculated for five measurements. The liquid velocity in the photobioreactor was controlled using a frequency converter and measured by solid particle tracing method, the average values and standard deviations of liquid velocity were calculated based on the measurement of five particles. Cultivation experiment and analytical methods The improved thin-film FPPBR system was located at Engebei (Ordos, China), batch culture of Chlorella sp. was carried out during the summer and autumn months (July to September) of 2018. The microalgae were incubated for 5e8 days, a part of resulted culture was harvested, and a part of remaining culture was used as the seed culture for next batch. The strain Chlorella sp. FACHB-1514 was obtained from Institute of Hydrobiology, Chinese Academy of Sciences. Chlorella sp. was cultivated in BG 11 medium, the nutrients of which had been reported in our previous research (12). Natural ground water was filtered by PP cotton filter element with bore diameter of 1 mm, and was treated with sodium hypochlorite (1 mL L1) and sodium thiosulphate. The biomass concentration was calculated by optical density at 680 nm (OD680) of microalgae suspension to monitor the microalgae growth of the photobioreactor system according to our previous work (12), the average values and standard deviations of biomass concentration were calculated for three measurements. The proportional coefficients between biomass concentration and optical density were 0.29 with correlation coefficients of 0.999 for Chlorella sp. During the cultivation, the OD680 value was measured four times a day at 8:00, 11:00, 15:00 and 19:00, respectively, and the proportional coefficients were measured every half month. The area productivity of microalgae Y can be calculated by Y ¼ ½ðX2 X1 Þ = ðt2 t1 ÞV = A
(1)
where X1 and X2 are biomass concentration of microalgae at time t1 and t2, respectively. V is the working volume and A is the illuminated surface area of the thin-film FPPBR.
RESULTS AND DISCUSSION The pressure drop between the inlet and outlet of the photobioreactor system The pressure drop between the inlet and outlet of the photobioreactor system with respect to liquid velocity is plotted in Table 1. Although the pressure drop increased as liquid velocity increased, the values were low, even at the highest velocity 0.238 m s1, the pressure drop was about 11.8 kPa, while that of the photobioreactor system reported in our previous research was 21 kPa when the liquid velocity was 0.22 m s1. The results demonstrated that the pressure of the FPPBR in the improved photobioreactor system was much decreased, which made it possible to cultivate microalgae at a high velocity for longer time. Outdoor cultivation of microalgae in the improved thin-film FPPBR system To validate the scalability and performance of the improved thin-film FPPBR system, batch cultures were performed from July to September of 2018. The results are shown in Fig. 3. The microalgae showed steady growth, and the final biomass concentration and area productivity reached 2.01 g L1 and 49.79 g m2 day1. Several new industrial flat panels made in transparent plastics have been reported, such as the airlift flat panel system with internal baffles (Subitec Company, Stuttgart, Germany) (16) and ProviApt photobioreactor (Proviron Industries, Hemiksem, Belgium) (17) , while their productivities were about 15 g m2 day1, and 31.79 g m2 day1 for Chlorella vulgaris and Nannochloropsis oceanica cultivation, respectively. The results demonstrated that production of Chlorella sp. can be carried out outdoors and high productivity could be achieved in the improved thin-film FPPBR. One reason is the optimization of the channel diameter, which has been reported that when the channel diameter is 0.06 m, the maximum productivity in a given area could be achieved (14,15). On the other hand, with the
FIG. 3. Batch culture of Chlorella sp. in thin-film flat plate photobioreactor (FPPBR) system. (A) Biomass concentration; (B) area productivity of Chlorella sp. Data are shown as mean SD, n ¼ 3.
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
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decrease of the pressure drop of the photobioreactor system, the microalgae suspension flows in a high velocity in the FPPBR, the higher frequency of the flashing light effect was produced, which may enable microalgae growth rate (11). The effect of seasonal variations of environmental conditions such as temperature and solar radiation on Chlorella sp. cultivation was investigated. The weather and temperature variation during the cultivation were obtained from a Weather forecast web (http:// tianqi.2345.com) as shown in Fig. 4. The daily solar radiation was divided into 5 levels according to the weather forecast, and each level has different value: sunny as 500 , sunny interval as 4, cloudy as 3, cloudy to overcast as 2, rainy as 100. The averaged light intensity of the levels 4 and 5, levels 2 and 3, and level 1were about 1100 mmol m2 s1, 550 mmol m2 s1, and lower than 220 mol m2 s1, respectively. In the same batch, the productivity was highest when the solar radiation level was higher than level 4, followed by the levels 3 and 2, while the productivity was near to 0 when the solar radiation level was 1. The results showed that the major factor influencing biomass productivity is the light condition at similar temperature conditions, and a higher productivity could be achieved with higher light intensity. The similar results have been also identified in previous research because green algae seem to prefer high light intensity (18,19). The daily maximum and minimum air and microalgae suspension temperatures during the cultivation experiments are shown in Fig. 5. During the cultivation, the photobioreactor system was cooled intermittently at noon. It can be observed that the temperature of microalgae suspension followed the same tendency with the ambient air temperature, and was lower than 35 C, which demonstrated the microalgae suspension temperature could be controlled by sprinkling water to the surface of the photobioreactor. As shown in Figs. 3 and 5, the productivity was highest in the first two batches, followed by the third and fourth batches, and the productivity was sharply decreased in the fifth batch due to the poor temperature lower than 5 C. The results demonstrated that Chlorella sp. cultivation at outdoors could be carried out over a wide range of temperatures even the culture temperature at night was about 5 C, and the optimal air temperature at night was above 15 C (20).
J. BIOSCI. BIOENG., Desert. The time with daily lowest temperature higher than 5 C lasted for 160 days from the middle of April to late September, which is suitable to cultivate Chlorella sp. in the thin-film FPPBR outdoors. The variation of solar radiation and temperature at Engebei from the middle of April to late September of 2018 is obtained from a Weather forecast web (http://tianqi.2345.com), which is shown in Fig. 4. The daily lowest temperature from midApril to mid-May and September is lower than 15 C including about 7 days even lower than 5 C. Low productivity was observed in this period, such as batch 3e5, which is mainly caused by low temperature. The days with solar radiation at levels 4 and 5, levels 2 and 3, and level 1 in this period were about 30 days, 19 days and 4 days, respectively. The average productivities with solar radiation at levels 4 and 5 and levels 2and 3 in this period were estimated using the productivities of batch 3e5, which were about 24.51 g m2 day1 and 8.97 g m2 day1. The optimal cultivating time was from the midMay to late August, the productivity is better because temperature and solar radiation are more favorable, such as batch 1 and 2. In this period, the time with solar radiation at levels 4 and 5, levels 2 and 3, and level 1 were 47, 45 and 16 days, respectively. The average productivity with solar radiation at levels 4 and 5 and levels 2 and 3 in this period were 36.32 g m2 day1 and 13.01 g m2 day1. Therefore, the yearround productivity of Chlorella sp. in the thin-film FPPBR would be about 3.20 kg m2 year1, the year-round productivity of Chlorella sp. in the thin-film FPPBR would be about 15.24 t ha2 year1, which is much higher than that in our previous research (12). The results demonstrated that it is promising for the economic and sustainable production of microalgae using the photobioreactor system. In summary, this work demonstrated the feasibility of Chlorella sp. mass cultivation in the improved thin-film photobioreactor system in Ordos, Inner Mongolia. The pressure drop of the FPPBR in the improved photobioreactor system was reduced significantly, the highest final biomass concentration and area productivity reached 2.01 g L1 and 49.79 g m2 day1, respectively. The year-
Analysis of the yearly productivities of Chlorella sp. in the pilot scale thin-film FPPBR system The thin-film FPPBR system was located at Engebei (Ordos, China), close to the Kubuqi
FIG. 4. Environmental parameters of Engebei (Ordos, China) from April 16, 2018 to September 30, 2018. Minimum temperature, the lowest temperature in a day; maximum temperature, the highest temperature in a day; solar radiation, the level of solar radiation in a day (n ¼ one replicate).
FIG. 5. Daily temperature variations of air and microalgae suspension in the photobioreactor system during the cultivation experiment at 6:00 and 14:00. TA at 6:00, air temperature at 6:00; TA at 14:00, air temperature at 14:00; TL at 6:00, microalgae suspension temperature at 6:00; TL at 14:00, microalgae suspension temperature at 14:00 (n ¼ one replicate).
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
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round productivity of Chlorella sp. was estimated to be about 15.24 t ha2 year1 in Ordos. The results demonstrated that the photobioreactor system was potential for mass cultivation of microalgae in the commercial application.
ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21706260), and the National Key R&D Program of China (2016YFC0500904).
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8. Carvalho, A., Meireles, L., and Malcata, F.: Microalgal reactors: a review of enclosed system designs and performances, Biotechnol. Prog., 22, 1490e1506 (2006). 9. Posten, C.: Design principles of photo-bioreactors for cultivation of microalgae, Eng. Life Sci., 9, 165e177 (2009). 10. Gupta, P., Lee, S., and Choi, H.: A mini review: photobioreactors for large scale algal cultivation, World J. Microbiol. Biotechnol., 31, 1409e1417 (2015). 11. Zhang, Q., Wu, X., Xue, S., Wang, Z., Yan, C., and Cong, W.: Hydrodynamic characteristics and microalgae cultivation in a novel flat-plate photobioreactor, Biotechnol. Prog., 29, 127e134 (2013). 12. Yan, C., Zhang, Q., Xue, S., Sun, Z., Wu, X., Wang, Z., Lu, Y., and Cong, W.: A novel low-cost thin-film flat plate photobioreactor for microalgae cultivation, Biotechnol. Bioprocess Eng., 21, 103e109 (2016). 13. Karvounis, P., Koubogiannis, D., Hontzopoulos, E., and Hatziapostolou, A.: Numerical and experimental study of flow characteristics in solar collector manifolds, Energies, 12, 1431e1448 (2019). 14. Bahadar, A. and BilalKhan, M.: Progress in energy from microalgae: a review, Renew. Sust. Energ. Rev., 27, 128e148 (2013). 15. Molina, E., Fernández, J., Acién, F., and Chisti, Y.: Tubular photobioreactor design for algal cultures, J. Biotechnol., 92, 113e131 (2001). 16. Degen, J., Uebele, A., Retze, A., Schmid-Staiger, U., and Trosch, W.: A novel airlift photobioreactor with baffles for improved light utilization through the flashing light effect, J. Biotechnol., 92, 89e94 (2001). 17. Norskera, N., Michielsb, M., Slegersc, P., Swinkelsd, G., Barbosae, M., and Wijffelse, R.: Productivity of Nannochloropsis oceanica in an industrial closely spaced flat panel photobioreactor, Algal Res., 43, 101632 (2019). 18. He, Q., Yang, H., Xu, L., Xia, L., and Hu, C.: Sufficient utilization of natural fluctuating light intensity is an effective approach of promoting lipid productivity in oleaginous microalgal cultivation outdoors, Bioresour. Technol., 180, 79e87 (2015). 19. Lan, S., Wu, L., Zhang, D., and Hu, C.: Effects of light and temperature on open cultivation of desert cyanobacterium Microcoleus vaginatus, Bioresour. Technol., 182, 144e150 (2015). 20. Maia, R., Bernard, O., Gonçalves, A., Bensalem, S., and Lopes, F.: Influence of temperature on Chlorella vulgaris growth and mortality rates in a photobioreactor, Algal Res., 18, 352e359 (2016).
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas Chenghu Yan, Zhihui Wang, Xia Wu, Shumei Wen, Jing Yu, and Wei Cong* State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China Received 29 October 2019; accepted 8 December 2019 Available online xxx
In order to reduce the flow resistance of the thin-film flat plat photobioreactor (FPPBR) and make it more suitable for mass microalgae cultivation, the channel diameter was modified to 0.06 m and the thin-film FPPBR consisted of 10 parallel shunt-wound channels. A thin-film FPPBR system with 100 modified FPPBRs was constructed and used for Chlorella sp. cultivation in desertification areas (Ordos, China) from July to September of 2018. The pressure drop of the modified FPPBR system decreased significantly and the microalgae showed much higher productivity. The pressure drop was about 11.8 kPa when the liquid velocity was 0.238 m sL1. The final biomass concentration and area productivity reached 2.01 g LL1 and 49.79 g mL2 dayL1 respectively, and the yearly productivity of Chlorella sp. was estimated to be about 15.24 t haL2 yearL1. The results demonstrated that high productivity of Chlorella sp. could be achieved in the improved FPPBR system in desertification areas and the improved FPPBR system was feasible for mass cultivation of microalgae in the commercial application. Ó 2019, The Society for Biotechnology, Japan. All rights reserved. [Key words: Outdoor cultivation; Flat plate photobioreactor; Mass production; Chlorella sp.; Baffle]
Microalgae are photosynthetic microorganisms from either marine or fresh water environments, which have been used as feedstock for high value compounds and biofuels production since they are able to transform carbon dioxide into cellular biomolecules such as biofuels, antioxidant compounds, nutraceuticals, animal/ fish feeds, and fertilizers via photosynthesis (1e4). Thus, the demand for algae biomass is increasing in recent years. For mass production of microalgae, efficient photobioreactors with low energy input and utilizing maximal solar radiation are required (5,6). Up to now, a large number of photobioreactors with different volumes and shapes have been developed (7e10). However, only a few of them have been used for commercial microalgae cultivation. In our previous study, a novel thin-film FPPBR mounted with inclined baffles was developed and used for Chlorella sp. cultivation (11,12). The biomass productivity of Chlorella sp. in the novel FPPBR was 25.2% higher than the FPPBR without baffles, and the biomass productivity of Chlorella sp. and Scenedesmus dimorphus in the photobioreactor system reached 11 g m2 day1 and 21.9 g m2 day1. However, the flow resistance of microalgae suspension in the photobioreactor is still high, which increases the pressure of the photobioreactor and the risk of being damaged, thus the phobioreactor system can only be operated in low flow velocity, resulting in the decrease of flashing light effect and productivity. Thus, the photobioreactor system remain needs to be improved. The configuration of the thin-film FPPBR is similar to Z-type flat plate manifolds, which had demonstrated that the flow rate uniformity and the pressure drop of Z-type manifolds could be
* Corresponding author. Tel.: þ86 10 82627060; fax: þ86 10 82627066. E-mail address: [email protected] (W. Cong).
improved by increasing the diameter of each shunt-wound channel (13). Moreover, culture efficiency is also related to channel diameter, for maximum production in a given area, the suggested channel diameter is about 0.06 m (14,15). Therefore, in order to make the thin-film flat plat photobioreactor (FPPBR) more suitable for commercial microalgae cultivation, the channel diameter was modified to 0.06 m and the thin-film FPPBR consisted of 10 parallel shunt-wound channels. An improved thinfilm FPPBR system with 100 FPPBRs was built up and used for Chlorella sp. cultivation at Engebei (Ordos, China), close to the Kubuqi Desert. The microalgae cultivation experiments were performed for three months (July to September 2018), and the relationships between biomass productivities and environmental conditions were investigated.
MATERIALS AND METHODS Configuration of the thin-film FPPBR and photobioreactor system In order to reduce the flow resistance of the photobioreactor and the pressure drop of the photobioreactor system, the thin-film FPPBR reported in our previous work was modified (12), and a photobioreactor system was set up based on the modified FPPBR, which are shown in Figs. 1 and 2. The unmodified thin-film FPPBR is shown in Fig. 1C, which is 2.0 m length and 0.96 m width, consisting of sixteen shunt-wound channels, the length of each channel is 1.44 m. In each channel, the inclined baffles are attached to both films, which are perpendicular to the film surface (12). The modified thin-film FPPBR is shown in Fig. 1A and B, which is 2.0 m length and 1.0 m width, consisting of 10 parallel shunt-wound channels. The baffles are mounted with an inclined angle a ¼ 30 shown in Fig. 1, and the dimension of the inclined baffles is length (l) ¼ 60 mm, height (h) ¼ 8 mm, width (w) ¼ 2.5 mm. In the modified photobioreactor system, an axial-flow pump is used to drive microalgae suspension, an earth pond of 10 m3 is built as medium storage tank, the
1389-1723/$ e see front matter Ó 2019, The Society for Biotechnology, Japan. All rights reserved. https://doi.org/10.1016/j.jbiosc.2019.12.007
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
2
YAN ET AL.
J. BIOSCI. BIOENG.,
FIG. 1. Schematic diagram and photographs of the thin-film flat plate photobioreactor (FPPBR). (A) Schematic diagram of the improved photobioreactor: 1, inlet; 2, outlet; 3, nether baffle; 4, upper baffle; 5, flow channel partition; 6, flow channel; 7, inlet cavity; 8, outlet cavity. (B) Improved photobioreactor; (C) photograph of unmodified photobioreactor.
pond is then lined with a 0.15 mm thick polyethylene films, and a polyethylene film is used to cover the pond in order to counter adverse weather conditions such as raining and freezing temperature. A gas sparger is installed at the inlet of the first photobioreactor, pure CO2 is injected into the photobioreactor directly during microalgae cultivation. Dissolved oxygen and the temperature are controlled using
similar methods reported in our previous research (12). Dissolved oxygen was regulated by changing the rate of air flow entering the medium tank using suitable valves and flowmeter through a gas sparger located at the bottom of the medium tank. A temperature control system with an electromagnetic valve was used to keep the maximum culture temperature lower than 35 C, When the temperature of the
FIG. 2. Schematic diagram (A) and photographs (B, C) of the pilot scale thin-film flat plate photobioreactor (FPPBR) system. Components: 1, medium storage tank; 2, axial pump; 3, photosynthetic reaction unit; 4, gas sparger (for O2 removal); 5, gas sparger (for CO2 supply); 6, temperature control system.
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
VOL. xxx, xxxx
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3
TABLE 1. The pressure drop between the inlet and outlet of the photobioreactor system with respect to liquid velocity in the channels. Frequency (Hz) 34.90 36.45 38.50 40.10 42.05 44.95
Pump capacity (m3 h1)
Inlet pressure (kPa)
Outlet pressure (kPa)
Pressure drop (Pa)
Velocity (m s1)
11.310.27 11.940.12 14.360.17 20.193.07 21.180.26 25.021.02
8.20.16 9.10.13 8.80.15 10.40.31 10.80.17 11.80.11
0 0 0 0 0 0
1643.2 1822.6 1763.0 2086.2 2163.4 2362.2
0.1190.030 0.1230.029 0.1500.017 0.1820.023 0.2110.026 0.2380.029
Data are shown as mean SD, n ¼ 5. microalgae suspension exceeded 30 C, the electromagnetic valve opened and cooling was accomplished by sprinkling the surface of the photobioreactor with natural ground water. The improved photobioreactor system covers an area of about 250 m2, the photosynthetic reaction unit comprises 100 horizontal improved FPPBRs, which is arranged into 2 rows, as shown in Fig. 2. The working volume is about 13 m3 and the illuminated surface area is 119 m2. The pressure drop and liquid velocity of the thin-film FPPBR system The pressure drop between the inlet and outlet of the improved photobioreactor system at different liquid velocity in the photobioreactor was measured. The pressure drop was calculated by measuring the height difference of water column between the inlet and outlet of the FPPBR system, the average values and standard deviations were calculated for five measurements. The liquid velocity in the photobioreactor was controlled using a frequency converter and measured by solid particle tracing method, the average values and standard deviations of liquid velocity were calculated based on the measurement of five particles. Cultivation experiment and analytical methods The improved thin-film FPPBR system was located at Engebei (Ordos, China), batch culture of Chlorella sp. was carried out during the summer and autumn months (July to September) of 2018. The microalgae were incubated for 5e8 days, a part of resulted culture was harvested, and a part of remaining culture was used as the seed culture for next batch. The strain Chlorella sp. FACHB-1514 was obtained from Institute of Hydrobiology, Chinese Academy of Sciences. Chlorella sp. was cultivated in BG 11 medium, the nutrients of which had been reported in our previous research (12). Natural ground water was filtered by PP cotton filter element with bore diameter of 1 mm, and was treated with sodium hypochlorite (1 mL L1) and sodium thiosulphate. The biomass concentration was calculated by optical density at 680 nm (OD680) of microalgae suspension to monitor the microalgae growth of the photobioreactor system according to our previous work (12), the average values and standard deviations of biomass concentration were calculated for three measurements. The proportional coefficients between biomass concentration and optical density were 0.29 with correlation coefficients of 0.999 for Chlorella sp. During the cultivation, the OD680 value was measured four times a day at 8:00, 11:00, 15:00 and 19:00, respectively, and the proportional coefficients were measured every half month. The area productivity of microalgae Y can be calculated by Y ¼ ½ðX2 X1 Þ = ðt2 t1 ÞV = A
(1)
where X1 and X2 are biomass concentration of microalgae at time t1 and t2, respectively. V is the working volume and A is the illuminated surface area of the thin-film FPPBR.
RESULTS AND DISCUSSION The pressure drop between the inlet and outlet of the photobioreactor system The pressure drop between the inlet and outlet of the photobioreactor system with respect to liquid velocity is plotted in Table 1. Although the pressure drop increased as liquid velocity increased, the values were low, even at the highest velocity 0.238 m s1, the pressure drop was about 11.8 kPa, while that of the photobioreactor system reported in our previous research was 21 kPa when the liquid velocity was 0.22 m s1. The results demonstrated that the pressure of the FPPBR in the improved photobioreactor system was much decreased, which made it possible to cultivate microalgae at a high velocity for longer time. Outdoor cultivation of microalgae in the improved thin-film FPPBR system To validate the scalability and performance of the improved thin-film FPPBR system, batch cultures were performed from July to September of 2018. The results are shown in Fig. 3. The microalgae showed steady growth, and the final biomass concentration and area productivity reached 2.01 g L1 and 49.79 g m2 day1. Several new industrial flat panels made in transparent plastics have been reported, such as the airlift flat panel system with internal baffles (Subitec Company, Stuttgart, Germany) (16) and ProviApt photobioreactor (Proviron Industries, Hemiksem, Belgium) (17) , while their productivities were about 15 g m2 day1, and 31.79 g m2 day1 for Chlorella vulgaris and Nannochloropsis oceanica cultivation, respectively. The results demonstrated that production of Chlorella sp. can be carried out outdoors and high productivity could be achieved in the improved thin-film FPPBR. One reason is the optimization of the channel diameter, which has been reported that when the channel diameter is 0.06 m, the maximum productivity in a given area could be achieved (14,15). On the other hand, with the
FIG. 3. Batch culture of Chlorella sp. in thin-film flat plate photobioreactor (FPPBR) system. (A) Biomass concentration; (B) area productivity of Chlorella sp. Data are shown as mean SD, n ¼ 3.
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
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YAN ET AL.
decrease of the pressure drop of the photobioreactor system, the microalgae suspension flows in a high velocity in the FPPBR, the higher frequency of the flashing light effect was produced, which may enable microalgae growth rate (11). The effect of seasonal variations of environmental conditions such as temperature and solar radiation on Chlorella sp. cultivation was investigated. The weather and temperature variation during the cultivation were obtained from a Weather forecast web (http:// tianqi.2345.com) as shown in Fig. 4. The daily solar radiation was divided into 5 levels according to the weather forecast, and each level has different value: sunny as 500 , sunny interval as 4, cloudy as 3, cloudy to overcast as 2, rainy as 100. The averaged light intensity of the levels 4 and 5, levels 2 and 3, and level 1were about 1100 mmol m2 s1, 550 mmol m2 s1, and lower than 220 mol m2 s1, respectively. In the same batch, the productivity was highest when the solar radiation level was higher than level 4, followed by the levels 3 and 2, while the productivity was near to 0 when the solar radiation level was 1. The results showed that the major factor influencing biomass productivity is the light condition at similar temperature conditions, and a higher productivity could be achieved with higher light intensity. The similar results have been also identified in previous research because green algae seem to prefer high light intensity (18,19). The daily maximum and minimum air and microalgae suspension temperatures during the cultivation experiments are shown in Fig. 5. During the cultivation, the photobioreactor system was cooled intermittently at noon. It can be observed that the temperature of microalgae suspension followed the same tendency with the ambient air temperature, and was lower than 35 C, which demonstrated the microalgae suspension temperature could be controlled by sprinkling water to the surface of the photobioreactor. As shown in Figs. 3 and 5, the productivity was highest in the first two batches, followed by the third and fourth batches, and the productivity was sharply decreased in the fifth batch due to the poor temperature lower than 5 C. The results demonstrated that Chlorella sp. cultivation at outdoors could be carried out over a wide range of temperatures even the culture temperature at night was about 5 C, and the optimal air temperature at night was above 15 C (20).
J. BIOSCI. BIOENG., Desert. The time with daily lowest temperature higher than 5 C lasted for 160 days from the middle of April to late September, which is suitable to cultivate Chlorella sp. in the thin-film FPPBR outdoors. The variation of solar radiation and temperature at Engebei from the middle of April to late September of 2018 is obtained from a Weather forecast web (http://tianqi.2345.com), which is shown in Fig. 4. The daily lowest temperature from midApril to mid-May and September is lower than 15 C including about 7 days even lower than 5 C. Low productivity was observed in this period, such as batch 3e5, which is mainly caused by low temperature. The days with solar radiation at levels 4 and 5, levels 2 and 3, and level 1 in this period were about 30 days, 19 days and 4 days, respectively. The average productivities with solar radiation at levels 4 and 5 and levels 2and 3 in this period were estimated using the productivities of batch 3e5, which were about 24.51 g m2 day1 and 8.97 g m2 day1. The optimal cultivating time was from the midMay to late August, the productivity is better because temperature and solar radiation are more favorable, such as batch 1 and 2. In this period, the time with solar radiation at levels 4 and 5, levels 2 and 3, and level 1 were 47, 45 and 16 days, respectively. The average productivity with solar radiation at levels 4 and 5 and levels 2 and 3 in this period were 36.32 g m2 day1 and 13.01 g m2 day1. Therefore, the yearround productivity of Chlorella sp. in the thin-film FPPBR would be about 3.20 kg m2 year1, the year-round productivity of Chlorella sp. in the thin-film FPPBR would be about 15.24 t ha2 year1, which is much higher than that in our previous research (12). The results demonstrated that it is promising for the economic and sustainable production of microalgae using the photobioreactor system. In summary, this work demonstrated the feasibility of Chlorella sp. mass cultivation in the improved thin-film photobioreactor system in Ordos, Inner Mongolia. The pressure drop of the FPPBR in the improved photobioreactor system was reduced significantly, the highest final biomass concentration and area productivity reached 2.01 g L1 and 49.79 g m2 day1, respectively. The year-
Analysis of the yearly productivities of Chlorella sp. in the pilot scale thin-film FPPBR system The thin-film FPPBR system was located at Engebei (Ordos, China), close to the Kubuqi
FIG. 4. Environmental parameters of Engebei (Ordos, China) from April 16, 2018 to September 30, 2018. Minimum temperature, the lowest temperature in a day; maximum temperature, the highest temperature in a day; solar radiation, the level of solar radiation in a day (n ¼ one replicate).
FIG. 5. Daily temperature variations of air and microalgae suspension in the photobioreactor system during the cultivation experiment at 6:00 and 14:00. TA at 6:00, air temperature at 6:00; TA at 14:00, air temperature at 14:00; TL at 6:00, microalgae suspension temperature at 6:00; TL at 14:00, microalgae suspension temperature at 14:00 (n ¼ one replicate).
Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007
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round productivity of Chlorella sp. was estimated to be about 15.24 t ha2 year1 in Ordos. The results demonstrated that the photobioreactor system was potential for mass cultivation of microalgae in the commercial application.
ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21706260), and the National Key R&D Program of China (2016YFC0500904).
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Please cite this article as: Yan, C et al., Outdoor cultivation of Chlorella sp. in an improved thin-film flat-plate photobioreactor in desertification areas, J. Biosci. Bioeng., https://doi.org/10.1016/j.jbiosc.2019.12.007