Biogas potential from co-fermentation of food leftovers and lignocellulosic biomass at mesophilic temperatures

Every year, large amounts of food leftovers are thrown away in catering establishments and households. Industry and agriculture produce lignocellulosic residues, including paper dust and willow biomass, which cause environmental problems if not properly disposed of. The aim of this study is to investigate the biogas and biomethane yields of these biomasses during anaerobic co-fermentation under mesophilic conditions. Biogas yields were determined by co-fermentation of food (hospital canteen, cafeteria, and household) residues and lignocellulosic (paper dust and shredded willow) biomass in a number of 0.72 L bioreactors. All bioreactors were divided into groups having the same content in reactors within each group to ensure the reliability of the results. Groups of bioreactors used for anaerobic fermentation were inoculums (0.5 L) only, inoculums with individual biomass, and inoculums with mixture of two or more biomass. Bioreactors were placed in three different thermostats with 16 bioreactors in each thermostat. Single fill batch anaerobic fermentation (AF) process was provided at 28, 33 and 38 °C. Individual reactor groups were equipped with graphite electrodes connected to DC voltage of 0.7 V. The biogas released in the bioreactor was collected into a gas bag outside the reactor. AF was maintained until gas emission ceased. The highest biogas yield in the AF process was obtained from the bioreactors at a temperature of 38 °C and the lowest at a temperature of 28 °C. Co-fermentation of biomass increased biogas and methane yields compared to AF treatment of individual biomasses. Exposure to the electric field decreased the methane yield. The energy balance on the AF process with the application of the electric field should be calculated by considering also the energy of hydrogen released from substrates with electrodes installed.