Comparison of various post-treatments for recovering methane from agricultural digestate

At full scale biogas plants, a large amount of digestate, which still contains a residual methane potential, is produced daily. Problems related to digestate storage and its use (i.e., biogas losses, the high cost of digestate transportation and limitations imposed by the European Nitrate Directive on its use as soil amendment) have attracted great attention among researcher to find solutions to take advantage of its residual methane potential. Thus, the aim of this study was to evaluate the methane production from digestate (DIG) and solid separated digestate (SS-DIG) and the feasibility of applying different kinds of post-treatments (i.e., thermal, thermo-chemical and enzymatic) in order to enhance their methane recovery. Results revealed that the methane recovery from digestate and solid separated digestate is feasible, considering their residual methane yields (70 NmL CH4/g VS and 90 NmL CH4/g VS, respectively). Thermal and alkaline post-treatments did not have a beneficial effect in enhancing methane potentials, while enzymatic post-treatment resulted in an increase of methane yield of 13% and 51% for SS-DIG and DIG samples, respectively. Finally, digestate recirculation permitted to obtain an extra electrical production (up to 4818 kWhel/day), which could represent an extra economical income to farmers.