Organic matter characterization during the anaerobic digestion of different biomasses by means of CPMAS 13C NMR spectroscopy

The aim of this work was to characterize ingestates and their corresponding digestates obtained in two full-scale biogas production plants processing a) mixtures of organic wastes in co-digestion, and b) pig slurry in order to assess the organic matter transformation during anaerobic digestion by means of chemical analysis and 13CPMAS-NMR spectroscopy. Results proved that digestates obtained by different organic substrates exhibited significant chemical differences related to the different initial composition of substrates. We proposed the use of the aliphaticity index in order to highlight the different chemical nature of ingestates and their corresponding digestates. In order to verify whether the AD process leads to stabilized final products regardless the initial composition of biomass in view of a possible agronomical use of digestate, a comparison of CPMAS 13C NMR data of a number of ingestates and digestates available in literature was carried out. Results indicated that most of the aromatic structures present in the substrate tend to degrade during the process and that anaerobic digestion proceeds through preferential degradation of carbohydrates such as cellulose and hemicellulose and, as a consequence, concentration of more chemically recalcitrant aliphatic molecules occurs.

► We studied anaerobic digestion by means of chemical analysis and 13CPMAS-NMR spectroscopy.
► Significant chemical differences in digestates were highlighted.
► We proposed the use of the aliphaticity index in order to differentiate digestates.
►  Most of the aromatic structures tend to degrade.
► Carbohydrates are degraded and recalcitrant aliphatic molecules concentrate.