Long-term anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment Fabiana Passos a , Mariona Herna ´ ndez-Marine ´ b , Joan Garcı´a a , Ivet Ferrer a, * a GEMMA e Group of Environmental Engineering and Microbiology, Department of Hydraulic, Maritime and Environmental Engineering, Universitat Polite `cnica de Catalunya$BarcelonaTech, c/Jordi Girona 1-3, Building D1, E-08034 Barcelona, Spain b Universitat de Barcelona, Av. Joan XXIII s/n, E-08028 Barcelona, Spain article info Article history: Received 22 July 2013 Received in revised form 4 October 2013 Accepted 7 October 2013 Available online xxx Keywords: Algae Bioenergy Biogas High rate algal pond Hydrolysis Methane abstract This paper describes the anaerobic digestion of microalgal biomass from high rate ponds in continuous anaerobic reactors for biogas production. With hydraulic retention times (HRT) of 15 and 20 days, the volatile solids (VS) removal did not exceed 30%, and the methane production rate ranged between 0.12 and 0.14 L CH 4 /L day. To improve process perfor- mance, microwave irradiation at 900 W for 3 min (specific energy w70,000 kJ/kg VS) was applied as a pretreatment step. The VS removal increased to 40 and 45% at 15 and 20 days HRT, respectively. Consequently, the methane production rate increased to 0.16 and 0.20 L CH 4 /L day at 15 and 20 days HRT, respectively. Microscopic analysis confirmed cell wall damage, although generally without lysis, after irradiating microalgal biomass. However, the energy consumption was much higher than the extra energy production of the process. Indeed, microalgal biomass should not only be thickened but also dewatered if microwave irradiation was to be applied as a pretreatment to anaerobic digestion for biogas production. ª 2013 Elsevier Ltd. All rights reserved. 1. Introduction Combining wastewater treatment and bioenergy production is already a well-known concept, developed in many full-scale facilities where sludge is digested to produce biogas and electricity. This may cover around 50% of the electricity de- mand in conventional activated sludge wastewater treatment plants (WWTP). The most energy intensive process is aeration of the biological reactor, which accounts for 55% of the total energy consumption (Metcalf and Eddy, 2003). Thus, reducing aeration requirements can contribute towards achieving en- ergy sufficient WWTP. In this sense, natural treatment sys- tems such as ponds have been developed, especially for small communities. High rate algal ponds (HRAP) are shallow race- way reactors, where microalgae and bacteria grow in symbi- osis. In these systems, organic matter is degraded by heterotrophic bacteria, which consume oxygen provided by microalgal photosynthesis and, therefore, no aeration is * Corresponding author. Tel.: þ34 934016463; fax: þ34 934017357. E-mail address: ivet.ferrer@upc.edu (I. Ferrer). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/watres water research xxx (2013) 1 e9 Please cite this article in press as: Passos, F., et al., Long-term anaerobic digestion of microalgae grown in HRAP for wastewater treatment. Effect of microwave pretreatment, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.10.013 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.10.013