Effect of high pressure thermal pretreatment on Chlorella vulgaris biomass: Organic matter solubilisation and biochemical methane potential Lara Mendez a , Ahmed Mahdy a,b , Marie Demuez a , Mercedes Ballesteros a,c , Cristina González-Fernández a,⇑ a IMDEA Energy, Avda. Ramón de la Sagra 3, 28935 Móstoles, Madrid, Spain b Department of Agricultural Microbiology, Zagazig University, 44519 Sharkia, Egypt c CIEMAT, Avda Complutense, 28040 Madrid, Spain highlights  Efficiency of anaerobic digestion is hampered due to microalgae cell wall.  Organic matter solubilisation and improved biodegradability was achieved by all thermal pretreatments.  Thermal pretreatments enhanced hydrolysis rate constant and thus faster biogas production.  Carbohydrates solubilised provided a fairly close estimation for methane production.  Best case scenario was achieved by subjecting biomass to 160 °C which resulted in 64% methane yield enhancement. article info Article history: Received 24 June 2013 Received in revised form 6 September 2013 Accepted 10 September 2013 Available online 24 September 2013 Keywords: Chlorella vulgaris Microalgae Anaerobic digestion Methane Pretreatment abstract This study investigates the effect of high pressure thermal hydrolysis on organic matter solubilisation and biogas production from Chlorella vulgaris biomass. Microalgae biomass was subjected to three temperatures, namely 140, 160, and 180 °C and two heating times (10 and 20 min). Results showed that carbohydrates release prevailed over proteins. Carbohydrates were solubilised concomitantly with increasing temperatures. According to the infrared spectra and monomeric sugars determined in the pretreated medium, temperatures applied clearly affected the solubilisation of structural carbohydrates of the microalgae cell wall. Likewise, thermal pretreatment provided enhanced methane production with regard to the raw algal biomass. Enhanced hydrolysis rate constant supported faster biogas production. Regardless the heating time employed, increasing temperatures depicted increasing methane production. Even thought, organic matter solubilisation was greater at 180 °C, the anaerobic biodegradability did not show the same trend. This fact was ascribed to the formation of reaction products that hampered meth- ane production. Best case scenario was achieved by subjecting biomass to 160 °C which resulted in 64% methane yield enhancement. Ó 2013 Published by Elsevier Ltd. 1. Introduction Photosynthetic microorganisms, including cyanobacteria and microalgae, are nowadays studied as potential feedstock for next- generation biofuels. This kind of feedstock not only provides bio- mass to produce energy but also present several advantages. More specifically, they contribute to reduce CO 2 emission by carbon up- take taking place during photosynthesis and remove nutrients from wastewater [1–3]. In this manner, algae are emerging as one of the most promising sustainable biomass sources. In addition to biomass for bioenergy production, this type of feedstock can be cultivated for feed, food and value added-products [4]. Coupling wastewater treatment and energy production by means of microalgae biomass would decrease biomass production cost [5]. Sturm and Lamer [6] achieved a positive net energy ratio when evaluation microalgae biomass production coupled with nutrient removal from open ponds fed with wastewater. In this manner, it seems rather logical that bioener- gy research mainly focuses on microalgae strains commonly found in wastewater treatment plants. Even though some cyanobacteria are also studied lately [7], microalgae strains more frequents include Chlorella sp., Scenedesmus sp., Oocystis sp. and Chlamydomonas sp. [2,8,9]. 0016-2361/$ - see front matter Ó 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.fuel.2013.09.032 ⇑ Corresponding author. Tel.: +34 917371127. E-mail address: cristina.gonzalez@imdea.org (C. González-Fernández). Fuel 117 (2014) 674–679 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel