Anaerobic co-digestion of rabbit manure and sorghum crops in a bench-scale biodigester

Abstract

Any type of biomass can be used as substrate for biogas production, but the performance of the biodigestion depends on the composition of the feed, and no direct extrapolation of the yield of the process from one substrate to another can be made. In this work, the performance of a bench-scale anaerobic biodigester of 93 L installed at ambient conditions is studied. The biodigester was set up in a region where temperature varies significantly during the year, and was operated under semi-batch conditions with non-thermal control for 16 months with a feed of rabbit manure and ground sorghum grains. To our knowledge, this is the first time the co-digestion of rabbit manure with sorghum grains is considered. To evaluate the biodigestion performance, critical operational variables (pH, temperature, biogas flowrate) were monitored, and composition of substrate, digestate and produced biogas was determined. Moreover, the following variables were quantified: (a) the theoretical methane potential, (b) the specific methane yield and (c) the degree of degradation of the substrate. A 1-D non-stationary model was formulated and validated with experimental data in order to analyze, in a theoretical form, the impact of incorporating thermal insulation to the unit. The results show that is it possible to produce biogas in a bench-scale biodigester, with a novel feed of rabbit manure and ground sorghum grains, in a region with significant temperature changes along the year. Moreover, it is shown that the 1-D model constitutes a useful tool for the design or improvement of biodigesters regarding the insulation system and the warming policies.