Inoculum production from pig slurry for potential use in agricultural biogas plants

Biogas production may benefit from the use of specialized microbial consortia (inocula) to increase biogas yields in anaerobic digesters. For this study, a specialized inoculum was obtained from raw pig slurry by applying an innovative procedure. The aims of this research were: i) to compare the performance of this inoculum in the anaerobic digestion (AD) of four agricultural feedstocks with that of digestates commonly used for bioaugmentation of agricultural biogas plants; ii) to assess the duration of the methanogenic activity of this inoculum. The nutrient content of the residual liquid from inoculum production for its potential re-use as fertilizer was also assessed. Specific methanogenic activity and specific methane yield of the inoculum from pig slurry were higher (+28% and + 39% on average) than those of the two digestates. The inoculum successfully maintained its AD performance, almost unchanged, for a prolonged storage period (up to one year). The residual clarified liquid from inoculum preparation contains nutrients and mineral salts at very low concentrations. A possible way to recycle it could be fertigation. Inoculum production from pig slurry represents an additional possibility of valorisation of animal waste, from the storage tank to land spreading, in a context of biorefinery.