Evaluation of the methane potential of different agricultural and food processing substrates for improved biogas production in rural areas

Anaerobic Digestion is largely applied in the rural context because of its capability of stabilizing the organic matter while recovering biogas, thus renewable energy, and a renewable fertilizer, the digestate. In the present study, the methane yield of a large number of organic biomasses generated in the agro-industrial sector was evaluated by biomethane potential trials. More than 50 different organic substrates were grouped according to their chemical characteristics or their application in 7 major categories: energy crops, lignocellulosic by-products, herbaceous by-products, vegetable by-products, fruits by-products, livestock effluents and miscellaneous food processing by-products. Results demonstrated that the concentration and the nature of the organic matter is able to influence the methane production. Energy crops are among the more diffused substrates with about 200 million tons/year. Its wide adoption is justified by relative high methane yields (250–350 L CH4/kg TVS) and mainly by the rapid degradation rates with hydrolysis constant of about 0.15 d−1. By-products characterized by high content of lignocellulosic materials showed slower kinetics (0.05–0.09 d−1) and a methane production in the range of 150–400 L CH4/kg TVS, which increased with cellulose content. Livestock effluents had generally a lower methane yield (50–200 L CH4/kg TVS) as effect of the higher ammonia inhibiting compounds. Finally, food by products were characterized by a large methane production's range, 150–700 L CH4/kg TVS, due to heterogeneous nature of these substrates. Food by products rich in lipids content had the higher methane yield (400–700 L CH4/kg TVS), but showed the slowest kinetics (kh lower than 0.1 d−1). P Substrates rich in proteins and carbohydrates had lower methane yields (300–450 L CH4/kg TVS) but higher hydrolysis constants, generally upper than 0.1 d−1. Considering the global biomass generation of these substrates at European level, it was determined that their valorisation could contribute with an annual potential energy output of 2584 PJ (61.7 Mtoe), representing 5.7% of total energy consumed in EU- 28 in 2015 or 34,1% of total renewable energy in the same year.