## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion
Since the chemical composition is a major controlling factor for the biodegradability and stability of pretreated waste, \({}^{13}\)C CP-MAS NMR spectroscopy is particularly useful in this context. [PERSON] et al. [130] applied this technique, in combination with chemolytic analyses and determination of the respiration rate, to investigate the stabilization of MSW in composting and AD pretreatments used before landfilling. High correlations were found between carbohydrate content, integral of the O-alkyl C region in \({}^{13}\)C NMR spectra and respiration rate, indicating that MSW stabilization is mainly due to carbohydrate degradation during pretreatments and that carbohydrates are the most important components of MSW as far as emission potential in landfills is concerned. Proteins, aromatic C, and the _A/OA_ ratio were found to be correlated with the respiration rate, but only exhibited a low correlation coefficient. A detailed description of OM changes upon different aerobic, anaerobic, and combined processes was reported, following the trends of integral intensities of different functional groups. The concentration of plastics in the final products was also highlighted by the trend of the intensities of specific signals in the \({}^{13}\)C CP-MAS spectra. A dependence of digestate composition and stability on the composition of the corresponding waste was reported by several authors [124; 125; 127]. Feedstocks with different compositions were considered in Ref. [127] in order to verify possible chemical differences among the corresponding digestates and finally ascertain if AD leads to stabilized final products regardless of the initial composition of biomass. In particular, very different trends of variations of C types were observed using pig slurry as ingestate with respect to feedstocks made of mixtures of energetic crops, agro-industrial residues and beef cattle slurry. Alkyl C content was found to decrease in the pig slurry digestion, whereas it increased in all the other cases. Moreover, aromatic C slightly decreased for pig slurry but increased for the other feedstocks. Digestate from pig slurry showed a distinct aliphatic nature related to the much higher content of aliphatic structures in pig slurry with respect to other organic matrices. In conclusion, this study highlighted that alkyl C and O-alkyl C contents in digestates are correlated with those in feedstocks. Feedstock degradation resulted in a reduction in O-alkyl C and an increase in alkyl C, whereas the increase in aromatic C observed for some digestates was not statistically significant. In all cases, concentration of more recalcitrant aliphatic molecules occurred during AD, regardless of feedstock composition. [PERSON] et al. [131] reported the feedstock and digestate compositions derived from \({}^{13}\)C CP-MAS NMR spectra analysis in a study where the performance of giant cane as a new energy crop in biogas production was compared to that of corn, taken as an example of a traditional energy crop. Digestate was obtained in a continuously stirred lab scale reactor after mixing crops with pig slurry. As shown in Table 6, a decreasing intensity in the spectral region of O-alkyl C was found for both giant cane and corn mixtures upon digestion, indicating that AD mainly proceeds by degradation of carbohydrate-like molecules; correspondingly, a relative increase in intensity was observed for the alkyl C region where carbons from recalcitrant aliphatic structures resonate. The degradation of carbohydrates was more marked for corn compared to giant cane mixtures because the former contained more easily degradable starch while the latter contained fibers mainly made of cellulose. [PERSON] et al. [132] investigated the molecular properties of particulate organic matter (POM) in feedstock and digestate samples of seven full-scale AD plants present in Sweden for the production of biogas (three agricultural waste and four food waste digesters).