Determination of the Main Factors Behind the Improved Biogas Yield of Co-Digested Brewery s Spent Grain with Walnuts from Timber Trees

Agro-industrial by-products are generated in large quantities every year, and the adoption of environmentally safe management strategies for these materials is of growing importance. Brewers’ Spent Grain (BSG), the main by-product of the brewing industry, was used as substrate for biogas production through Anaerobic Digestion (AD). BSG was digested both as a single substrate and in co-digestion with walnut residues (WN) derived from timber trees. This lignocellulosic biomass is a by-product of wood production and therefore does not compete with food resources. The selection of walnut residues was based on their chemical composition, which allows improvement of the overall carbon-to-nitrogen (C/N) ratio of the feedstock. Anaerobic digestion experiments were conducted in batch lab-scale bioreactors, using an inoculum-to-substrate ratio of 3:1 on a dry weight basis. The results, normalized per unit of Volatile Solids (VS) showed that the co-digestion treatment achieved a higher biogas yield than the mono-digestion of the individual biomasses, with a production of 9.6*10-3 Nm3/gVS, compared to 7.4*10-3 Nm3/gVS for BSG alone and 4.5*10-3 Nm3/gVS for WN alone. The present study aims to investigate the properties of each biomass which led to the improved production detected during co-digestion tests. Key biomass parameters, influencing AD performances, were explored, such as C/N ratio, fiber composition, and morphological structure of the organic material.