Using low-cost porous materials to increase biogas production: A case study in Extremadura (Spain)

Abstract This study presents the effect of certain porous materials on the production of biogas from the anaerobic co-digestion of a mix of sheep manure (20 wt.%) and cheese whey (80 wt.%). These porous materials are custom-made from recycling agricultural and forestry wastes - almond shells, walnut shells, kenaf fibre, and charcoal. The manufacturing cost is therefore low. They are crushed and sieved, and then fed into the bioreactor to form a fixed bed in order to make a greater surface available for bacterial adhesion, and thus increase the microbial population in the reaction medium. The ultimate objective is to increase biogas production, i.e., to maximize the energy yielded by the codigestion of this mixture. The conclusion drawn from the results of the study was that the greatest long-term (for reaction times over 168 days) biogas production is obtained when using charcoal as the porous material. There was a 27.82% increase in methane (CH4) production compared with the biomethanization of the mixture without any porous material in the reactor (control experiment), and a 50% reduction in chemical oxygen demand (COD). The study of the economic viability of an industrial plant, based on previous results, showed that the period of return on investment (PRI), net present value (NPV), and internal rate of return (IRR) economic parameters are highly favourable. In particular, with the use of charcoal, a PRI of 8 years and an IRR of 10% are achieved.