Microalgae as source of energy have generated an enormous interest in the last decades. Microalgae seem to be the most feasible option to obtain renewable liquid fuels due to high growth rates, CO2 fixation capability and large accumulation of oil compared to other crop plants. The bottleneck of this technology is anyway represented by the costs of the process, both from the economic and energetic points of view.
In order to reduce the energetic costs and to make microalgae cultivation more attractive, the possibility of exploiting the energetic content of microalgal biomass residues after oil extraction by means of anaerobic digestion to produce biogas was studied. Two microalgal species, Scenedesmus obliquus and Chlorella protothecoides, selected for their high oil contents and fast growth rates, were tested for biogas production, before and after the oil extraction. Oil extraction was carried out by Soxhlet method, using a mixture of methanol and chloroform as the solvent. Biochemical Methane Potential (BMP) tests were carried out to evaluate biogas production capacity from microalgae and degradability rates. Two different kinds of inocula were used to compare the specific hydrolytic capacities and to assess the most suitable one to maximize the biogas conversion of microalgae. The digestion tests were performed at controlled temperature of 37 °C, in batch reactors. Production of biogas and the proportion of CO2 and CH4 content were measured. The results are discussed in view of feasible industrial application.