Comparison of biomethane production and digestate characterization for selected agricultural substrates in Italy

Starting from (but not limited to) their importance in the Italian context, three agricultural substrates, two of fruit origin (grape seeds and plum stones) and one of herbaceous origin (woad), were comparatively tested for both biomethane production and digestate characterization. The anaerobic digestion tests showed that grape seeds had the highest net methane production of 253.0 NmL g volatile solids (VS)−1, followed by plum stones, whose best resulting net methane production was 174.7 NmL gVS−1, and finally by woad with a net methane production of 153.1 NmL gVS−1. Interestingly, the best methane productions of the fruit substrates were obtained with different substrate to inoculum ratios (on a VS basis), 1:1 for grape seeds but 2:1 for plum stones. On the other hand, a three-month ageing of woad caused a limited reduction of methane production. The estimation of obtained degrees of conversion, carried out on a chemical oxygen demand (COD) basis for the specific tests achieving the respective best methane productions, gave values of 48%, 31%, and 33% for grape seeds, plum stones, and woad, respectively. The estimated degrees of conversion were evaluated along with the respective methane productions and substrate COD/VS ratios. The comparison of Fourier transform infrared (FT-IR) spectra and differential thermal analysis (DTA) profiles, carried out for selected digestates in pairs, revealed some distinctive differences in the relative intensities or presence and absence of particular peaks in the FT-IR spectra and in the relative intensities of the exothermic peaks or horizontal curve shifting of the DTA profiles.