Article pubs.acs.org/est
Microalgae Conversion to Biogas: Thermal Pretreatment Contribution on Net Energy Production Fabiana Passos† and Ivet Ferrer*,† †
GEMMA, Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya·BarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034, Barcelona, Spain ABSTRACT: Microalgal biomass harvested from wastewater treatment high rate algal ponds may be valorised through anaerobic digestion producing biogas. However, microalgae anaerobic biodegradability is limited by their complex cell wall structure. Thus, pretreatment techniques are being investigated to improve microalgae methane yield. In the current study, thermal pretreatment at relatively low temperatures of 75−95 °C was effective at enhancing microalgae anaerobic biodegradability; increasing the methane yield by 70% in respect to nonpretreated biomass. Microscopic images showed how the pretreatment damaged microalgae cells, enhancing subsequent anaerobic digestion. Indeed, digestate images showed how after pretreatment only species with resistant cell walls, such as diatoms, continued to be present. Energy balances based on lab-scale reactors performance at 20 days HRT, shifted from neutral to positive (energy gain around 2.7 GJ/d) after thermal pretreatment. In contrast with electricity consuming pretreatment methods, such as microwave irradiation, thermal pretreatment of microalgae seems to be scalable.
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INTRODUCTION
The microalgae-bacterial biomass produced in such systems may be valorised through anaerobic digestion producing biogas. This process is already well-known for sewage sludge treatment in conventional WWTP. Nevertheless, the anaerobic digestion of microalgal biomass has shown a slow biodegradability, reaching methane yields of 0.05−0.15 L CH4/g VS when reactors are operated at HRT below 20 days.5 These values are low in respect to other organic substrates, such as starch and sugar crops (e.g., corn 0.18−0.41 L CH4/g VS and potatoes 0.43 L CH4/g VS),6 or primary sludge (0.31 L CH4/g VS).7 Indeed, microalgae methane yield is more similar to waste activated sludge (0.13−0.14 L CH4/g VS).8 Pretreatment techniques have been investigated to enhance biomass hydrolysis rate and to increase both bioavailability and biodegradability of macromolecules for anaerobic digestion. In this way, molecules that cannot be degraded inside microalgae complex cell wall, after pretreatment are more readily digested and converted to methane. Pretreatment methods, such as microwave, ultrasound and thermal hydrolysis have already been proved efficient in batch and continuous reactors, increasing between 60 and 108% microalgae methane yield.9−11 However, methods requiring electricity may not be viable in full-scale facilities, at least when biomass is not previously dewatered.11 Indeed, pretreatments with low electricity input should be prioritized. Thermal pretreatment at relatively low temperatures (2 GJ/d) and energy ratios (>1)
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AUTHOR INFORMATION
Corresponding Author
*Phone: +34 934016463; fax: +34 934017357; e-mail: ivet. [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financially supported by the Spanish Ministry of Economy and Competitiveness (BIOALGAS Project, CTM2010-17846). Fabiana Passos appreciates her PhD scholarship funded by the Coordination for the Improvement of Higher Level Personal (CAPES) from the Brazilian Ministry of Education. We acknowledge Mariona Hernández-Mariné from the University of Barcelona and Joan Garciá from the Universitat Politècnica de Catalunya for the valuable help on microalgae microscopic images and characterisation. 7177
dx.doi.org/10.1021/es500982v | Environ. Sci. Technol. 2014, 48, 7171−7178
Environmental Science & Technology
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Article
(22) Chen, P. H.; Oswald, W. J. Thermochemical treatment for algal fermentation. Environ. Int. 1998, 24, 889−897. (23) Alzate, M. E.; Muñoz, R.; Rogalla, F.; Fdz-Polanco, F.; PérezElvira, S. I. Biochemical methane potential of microalgae: Influence of substrate to inoculum ratio, biomass concentration and pretreatment. Bioresour. Technol. 2012, 123, 488−494. (24) Sumper, M.; Brunner, E. Learning from diatoms: Nature’s tools for the production of nanostructured silica. Adv. Funct. Mater. 2006, 16, 17−26.
REFERENCES
(1) Oswald, W. J.; Golueke, C. G. Biological transformation of solar energy. Adv. Appl. Microbiol. 1960, 2, 223−262. (2) García, J.; Green, B. F.; Lundquist, T.; Mujeriego, R.; HernándezMariné, M.; Oswald, W. J. Long term diurnal variations in contaminant removal in high rate ponds treating urban wastewater. Bioresour. Technol. 2006, 97 (14), 1709−1715. (3) Clarens, A. F.; Resurreccion, E. P.; White, M. A.; Colosi, L. M. Environmental life cycle comparison of alge and other bioenergy feedstocks. Environ. Sci. Technol. 2010, 44, 1813−1819. (4) Menger-Krug, E.; Niederste-Hollenberg, J.; Hillenbrand, T.; Hiess, H. Integration of microalgae systems at municipal wastewater treatment plants: Implication for energy and emission balances. Environ. Sci. Technol. 2012, 46, 11505−11514. (5) González-Fernández, C.; Sialve, B.; Bernet, N.; Steyer, J. P. Impact of microalgae characteristics on their conversion to biofuel. Part II: Focus on biomethane production. Biofuels, Bioprod. Biorefin. 2011, 6, 205−218. (6) Frigon, J. C.; Guiot, S. R. Biomethane production from starch and lignocellulosic crops: A comparative review. Biofuels, Bioprod. Biorefin. 2010, 4, 447−458. (7) Kepp, U.; Solheim, O. E. Thermodynamical Assessment of the Digestion Process. In 5th European Biosolids and Organic Residuals Conference, Wakefield, UK, 2000. (8) Bougrier, C.; Delgenès, J. P.; Carrère, H. Combination of thermal treatments and anaerobic digestion to reduce sewage sludge quantity and improve biogas yield. Process Saf. Environ. Prot. 2006, 84, 280− 284. (9) González-Fernández, C.; Sialve, B.; Bernet, N.; Steyer, J. P. Thermal pretreatment to improve methane production of Scenedesmus biomass. Biomass Bioenerg. 2012, 40, 105−111. (10) Schwede, S.; Rehman, Z.-U.; Gerber, M.; Theiss, C.; Span, R. Effects of thermal pretreatment on anaerobic digestion of Nannocloropsis salina biomass. Bioresour. Technol. 2013, 143, 505−511. (11) Passos, F.; Hernández-Mariné, M.; García, J.; Ferrer, I. Longterm anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment. Water Res. 2014, 49 (1), 351−359. (12) Passos, F.; Solé, M.; García, J.; Ferrer, I. Impact of low temperature pretreatment on the anaerobic digestion of microalgal biomass. Bioresour. Technol. 2013, 138, 79−86. (13) APHA-AWWA-WPCF. Standard Methods for the Examination of Water and Wastewater. 20th ed.; Washington, 1999. (14) Palmer, C. M. Algas en los Abastecimientos de Agua. Manual Ilustrado Acerca de la Identificación, Importancia y Control de las Algas en los Abastecimientos de Agua.; Editiorial Interamericana: Mexico, 1692. (15) Bourrelly, P. Les Algues d’eau Douce. Tome I: Les Algues Vertes.; Édition N. Boubée & Cie: Paris, 1966. (16) Ferrer, I.; Serrano, E.; Ponsá, S.; Vázquez, F.; Font, X. Enhancement of thermophilic anaerobic digestion by 70 °C pretreatment: Energy considerations. J. Resid. Sci. Technol. 2009, 59, 1777−1784. (17) Metcalf & Eddy, Tchobanoglous, G.; Burton, F. L.; Stensel, H. D. Wastewater Engineering, Treatment and Reuse, 4th ed.; McGraw Hill Education, 2003. (18) Lu, J.; Gavala, H. N.; Skiadas, I. V.; Mladenovska, Z.; Ahring, B. K. Improving anaerobic sewage sludge digestion by implementation of a hyperthermophilic prehydrolisis step. J. Environ. Manag. 2008, 88, 881−889. (19) Mussgnug, J. H.; Klassen, V.; Schlüter, A.; Kruse, O. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotechnol. 2010, 150, 51−56. (20) Molinuevo-Salces, B.; Gómez, X.; Morán, A.; García-González, M. C. Anaerobic co-digestion of livestock and vegetable processing wastes: Fibre degradation and digestate stability. Waste Manag. 2013, 33 (6), 1332−1338. (21) Ras, M.; Lardon, L.; Sialve, B.; Bernet, N.; Steyer, J. P. Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris. Bioresour. Technol. 2011, 102, 200−206. 7178
dx.doi.org/10.1021/es500982v | Environ. Sci. Technol. 2014, 48, 7171−7178
Microalgae Conversion to Biogas: Thermal Pretreatment Contribution on Net Energy Production Fabiana Passos† and Ivet Ferrer*,† †
GEMMA, Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Politècnica de Catalunya·BarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034, Barcelona, Spain ABSTRACT: Microalgal biomass harvested from wastewater treatment high rate algal ponds may be valorised through anaerobic digestion producing biogas. However, microalgae anaerobic biodegradability is limited by their complex cell wall structure. Thus, pretreatment techniques are being investigated to improve microalgae methane yield. In the current study, thermal pretreatment at relatively low temperatures of 75−95 °C was effective at enhancing microalgae anaerobic biodegradability; increasing the methane yield by 70% in respect to nonpretreated biomass. Microscopic images showed how the pretreatment damaged microalgae cells, enhancing subsequent anaerobic digestion. Indeed, digestate images showed how after pretreatment only species with resistant cell walls, such as diatoms, continued to be present. Energy balances based on lab-scale reactors performance at 20 days HRT, shifted from neutral to positive (energy gain around 2.7 GJ/d) after thermal pretreatment. In contrast with electricity consuming pretreatment methods, such as microwave irradiation, thermal pretreatment of microalgae seems to be scalable.
■
INTRODUCTION
The microalgae-bacterial biomass produced in such systems may be valorised through anaerobic digestion producing biogas. This process is already well-known for sewage sludge treatment in conventional WWTP. Nevertheless, the anaerobic digestion of microalgal biomass has shown a slow biodegradability, reaching methane yields of 0.05−0.15 L CH4/g VS when reactors are operated at HRT below 20 days.5 These values are low in respect to other organic substrates, such as starch and sugar crops (e.g., corn 0.18−0.41 L CH4/g VS and potatoes 0.43 L CH4/g VS),6 or primary sludge (0.31 L CH4/g VS).7 Indeed, microalgae methane yield is more similar to waste activated sludge (0.13−0.14 L CH4/g VS).8 Pretreatment techniques have been investigated to enhance biomass hydrolysis rate and to increase both bioavailability and biodegradability of macromolecules for anaerobic digestion. In this way, molecules that cannot be degraded inside microalgae complex cell wall, after pretreatment are more readily digested and converted to methane. Pretreatment methods, such as microwave, ultrasound and thermal hydrolysis have already been proved efficient in batch and continuous reactors, increasing between 60 and 108% microalgae methane yield.9−11 However, methods requiring electricity may not be viable in full-scale facilities, at least when biomass is not previously dewatered.11 Indeed, pretreatments with low electricity input should be prioritized. Thermal pretreatment at relatively low temperatures (2 GJ/d) and energy ratios (>1)
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +34 934016463; fax: +34 934017357; e-mail: ivet. [email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was financially supported by the Spanish Ministry of Economy and Competitiveness (BIOALGAS Project, CTM2010-17846). Fabiana Passos appreciates her PhD scholarship funded by the Coordination for the Improvement of Higher Level Personal (CAPES) from the Brazilian Ministry of Education. We acknowledge Mariona Hernández-Mariné from the University of Barcelona and Joan Garciá from the Universitat Politècnica de Catalunya for the valuable help on microalgae microscopic images and characterisation. 7177
dx.doi.org/10.1021/es500982v | Environ. Sci. Technol. 2014, 48, 7171−7178
Environmental Science & Technology
■
Article
(22) Chen, P. H.; Oswald, W. J. Thermochemical treatment for algal fermentation. Environ. Int. 1998, 24, 889−897. (23) Alzate, M. E.; Muñoz, R.; Rogalla, F.; Fdz-Polanco, F.; PérezElvira, S. I. Biochemical methane potential of microalgae: Influence of substrate to inoculum ratio, biomass concentration and pretreatment. Bioresour. Technol. 2012, 123, 488−494. (24) Sumper, M.; Brunner, E. Learning from diatoms: Nature’s tools for the production of nanostructured silica. Adv. Funct. Mater. 2006, 16, 17−26.
REFERENCES
(1) Oswald, W. J.; Golueke, C. G. Biological transformation of solar energy. Adv. Appl. Microbiol. 1960, 2, 223−262. (2) García, J.; Green, B. F.; Lundquist, T.; Mujeriego, R.; HernándezMariné, M.; Oswald, W. J. Long term diurnal variations in contaminant removal in high rate ponds treating urban wastewater. Bioresour. Technol. 2006, 97 (14), 1709−1715. (3) Clarens, A. F.; Resurreccion, E. P.; White, M. A.; Colosi, L. M. Environmental life cycle comparison of alge and other bioenergy feedstocks. Environ. Sci. Technol. 2010, 44, 1813−1819. (4) Menger-Krug, E.; Niederste-Hollenberg, J.; Hillenbrand, T.; Hiess, H. Integration of microalgae systems at municipal wastewater treatment plants: Implication for energy and emission balances. Environ. Sci. Technol. 2012, 46, 11505−11514. (5) González-Fernández, C.; Sialve, B.; Bernet, N.; Steyer, J. P. Impact of microalgae characteristics on their conversion to biofuel. Part II: Focus on biomethane production. Biofuels, Bioprod. Biorefin. 2011, 6, 205−218. (6) Frigon, J. C.; Guiot, S. R. Biomethane production from starch and lignocellulosic crops: A comparative review. Biofuels, Bioprod. Biorefin. 2010, 4, 447−458. (7) Kepp, U.; Solheim, O. E. Thermodynamical Assessment of the Digestion Process. In 5th European Biosolids and Organic Residuals Conference, Wakefield, UK, 2000. (8) Bougrier, C.; Delgenès, J. P.; Carrère, H. Combination of thermal treatments and anaerobic digestion to reduce sewage sludge quantity and improve biogas yield. Process Saf. Environ. Prot. 2006, 84, 280− 284. (9) González-Fernández, C.; Sialve, B.; Bernet, N.; Steyer, J. P. Thermal pretreatment to improve methane production of Scenedesmus biomass. Biomass Bioenerg. 2012, 40, 105−111. (10) Schwede, S.; Rehman, Z.-U.; Gerber, M.; Theiss, C.; Span, R. Effects of thermal pretreatment on anaerobic digestion of Nannocloropsis salina biomass. Bioresour. Technol. 2013, 143, 505−511. (11) Passos, F.; Hernández-Mariné, M.; García, J.; Ferrer, I. Longterm anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment. Water Res. 2014, 49 (1), 351−359. (12) Passos, F.; Solé, M.; García, J.; Ferrer, I. Impact of low temperature pretreatment on the anaerobic digestion of microalgal biomass. Bioresour. Technol. 2013, 138, 79−86. (13) APHA-AWWA-WPCF. Standard Methods for the Examination of Water and Wastewater. 20th ed.; Washington, 1999. (14) Palmer, C. M. Algas en los Abastecimientos de Agua. Manual Ilustrado Acerca de la Identificación, Importancia y Control de las Algas en los Abastecimientos de Agua.; Editiorial Interamericana: Mexico, 1692. (15) Bourrelly, P. Les Algues d’eau Douce. Tome I: Les Algues Vertes.; Édition N. Boubée & Cie: Paris, 1966. (16) Ferrer, I.; Serrano, E.; Ponsá, S.; Vázquez, F.; Font, X. Enhancement of thermophilic anaerobic digestion by 70 °C pretreatment: Energy considerations. J. Resid. Sci. Technol. 2009, 59, 1777−1784. (17) Metcalf & Eddy, Tchobanoglous, G.; Burton, F. L.; Stensel, H. D. Wastewater Engineering, Treatment and Reuse, 4th ed.; McGraw Hill Education, 2003. (18) Lu, J.; Gavala, H. N.; Skiadas, I. V.; Mladenovska, Z.; Ahring, B. K. Improving anaerobic sewage sludge digestion by implementation of a hyperthermophilic prehydrolisis step. J. Environ. Manag. 2008, 88, 881−889. (19) Mussgnug, J. H.; Klassen, V.; Schlüter, A.; Kruse, O. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. J. Biotechnol. 2010, 150, 51−56. (20) Molinuevo-Salces, B.; Gómez, X.; Morán, A.; García-González, M. C. Anaerobic co-digestion of livestock and vegetable processing wastes: Fibre degradation and digestate stability. Waste Manag. 2013, 33 (6), 1332−1338. (21) Ras, M.; Lardon, L.; Sialve, B.; Bernet, N.; Steyer, J. P. Experimental study on a coupled process of production and anaerobic digestion of Chlorella vulgaris. Bioresour. Technol. 2011, 102, 200−206. 7178
dx.doi.org/10.1021/es500982v | Environ. Sci. Technol. 2014, 48, 7171−7178
