Biogas production from microalgae grown in wastewater: Effect of microwave pretreatment Fabiana Passos, Maria Solé, Joan García, 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 highlights " Microwave irradiation enhanced the disintegration and digestibility of microalgae. " Algal biomass solubilisation increased by 800% with microwave pretreatment. " The main parameter influencing biomass solubilisation was the applied specific energy. " Increased biogas production rate (27–75%) and yield (12–78%) with pretreated biomass. " Linear correlation between microalgae solubilisation and biogas yield. article info Article history: Received 2 October 2012 Received in revised form 14 February 2013 Accepted 17 February 2013 Keywords: Anaerobic digestion Biofuel High rate algal pond Methane Renewable energy abstract The aim of this study was to evaluate the effect of microwave pretreatment on the solubilisation and anaerobic digestion of microalgae–bacterial biomass cultivated in high rate algal ponds for wastewater treatment. The microwave pretreatment comprised three specific energies (21,800, 43,600 and 65,400 kJ/kg TS), combining three output power values with different exposure times. Response surface analysis showed that the main parameter influencing biomass solubilisation was the applied specific energy. Indeed, a similar solubilisation increase was obtained for the same specific energy, regardless of the output power and exposure time (280–350% for 21,800 kJ/kg TS, 580–610% for 43,600 kJ/kg TS and 730–800% for 65,400 kJ/kg TS). In biochemical methane potential tests, the initial biogas production rate (27–75% increase) and final biogas yield (12–78% increase) were higher with pretreated biomass. A linear correlation was found between biomass solubilisation and biogas yield. It can be concluded that microwave irradiation enhanced the disintegration and digestibility of microalgae. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction During the last decade, there has been a growing interest in investigating the energy potential of biofuels obtained from micro- algae cultures [1]. The high lipid content of microalgae makes them an alternative to terrestrial energy crops for biodiesel production. However, microalgae cultures and energy production are at an ini- tial research phase. According to the literature, the cultivation of microalgae to produce biofuels has a number of requirements that limit its current implementation at industrial scale [2]. To make it economically feasible, massive biomass production and energy generation technologies must be addressed. The cultivation of certain specific strains of microalgae is not viable in economic and environmental terms, since freshwater and fertilizers are needed. In contrast, if microalgal biomass is grown as a by-product of high rate algal ponds (HRAPs) operated for wastewater treatment, the economic and ecological footprint are more realistic if used at large-scale [3,4]. High rate ponds are shallow, open raceway ponds, with continued mixing provided by paddle-wheels. This system works by a symbiosis between het- erotrophic bacteria, which oxidize organic matter contained in wastewater, and the phytoplankton, which by photosynthesis con- sumes the CO 2 derived from organic matter mineralization. Micro- algae–bacterial biomass grown in this environment assimilates nutrients and subsequently, its separation from the final effluent eliminates nutrients from the wastewater [5]. Anaerobic digestion of microalgae was first studied in the 1950s [6,7]. These authors used microalgal biomass from HRAP, pointing out biomass separation from the liquor as a major limitation of the process. Up to date, the literature on microalgae digestion is very limited. The review by González-Fernández et al. [1] reports a spe- cific methane production of 0.1–0.5 L CH 4 /g VS, with 60–80% CH 4 in biogas, depending on process temperature (15–52 °C) and 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.02.042 ⇑ Corresponding author. Tel.: +34 934016463; fax: +34 934017357. E-mail address: ivet.ferrer@upc.edu (I. Ferrer). Applied Energy 108 (2013) 168–175 Contents lists available at SciVerse ScienceDirect Applied Energy journal homepage: www.elsevier.com/locate/apenergy