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.
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dx.doi.org/10.1021/es500982v | Environ. Sci. Technol. 2014, 48, 7171−7178
