## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion
Six different substrates were used and \({}^{13}\)C CP-MAS NMR spectra, analyzed by principal component analysis (PCA), revealed in most cases an enrichment of protein structures with respect to carbohydrates, implying a preferential degradation of the carbohydrates over proteins and/or increase in microbial biomass upon AD of agricultural and food wastes. Moreover, the composition of digestate was found to be mainly determined by that of the feedstock. A study was carried out by [PERSON] et al. [133] to compare the impact of initial feedstock properties on the stabilization rate by AD and composting and soil amendment characteristics of the final products. Green and food wastes were considered as the main components of municipal organic waste. \({}^{13}\)C CP-MAS NMR spectroscopy demonstrated that both treatments resulted in a reduction in aliphatic chain content, while an opposite trend was observed for aromatic moieties. In fact, the aromaticity increased for all treated samples and was higher for the composted materials (Table 7). The composted samples always showed a greater change than samples produced by AD. The carboxyl groups were reduced in all the samples, except for the samples with a C/N ratio of 34 from AD. Moreover, data suggested that for the same feedstock composting gave more stable and humified products than AD. The evolution of insoluble organic matter upon AD of sewage sludge and swine manure, pretreated with a low temperature hydrothermal (HT) treatment at 125 or 225 \({}^{\circ}\)C, was investigated by [PERSON] et al. [134] using \({}^{13}\)C CP-MAS NMR experiments in a study aimed at investigating the effect of the HT treatment on the efficiency of AD in producing biogas. The HT pretreatment enhanced the release of organics in sludge and improved the degradation of organics in manure during the AD process. After both HT and AD treatments of sludge samples, more O-alkyl C was converted into carboxyl, carbonyl, and soluble organics. For manure samples, the combined processes converted soluble organics and O-alkyl C partly into carboxyl and carbonyl and partly into alkyl C (Table 6). HT pretreatment was found to enhance biogas production for sewage sludge, while a minimal effect of HT on gas production was observed for swine manure. The ability of AD to transform organic matter was also compared to those of aerobic (composting) and mixed aerobic-anaerobic processes. [PERSON] et al. [124] applied composting in two stages and a mixed process in three phases (aerobic hydrolysis, gas production under mesophilic anaerobic conditions at 30 \({}^{\circ}\)C, aerobic maturation) to CM and PM and compared the OM composition of the products to those of the corresponding digestates (see Figure 7). For CM, compost and the product of mixed aerobic-anaerobic treatment showed a higher relative content of aromatic and carboxyl functional groups with respect to digestate, with a corresponding lower content of alkyl groups, indicating a lower degradation of lignin and the generation of carboxyl groups in oxidative degradation. For PM, the mixed process gave only a slight increment in the aromatic and carboxyl/carbonyl structures, yielding a low value of the aromaticity index when compared to the stabilized samples obtained by the other processes. Differences and similarities of OM composition among digestates, composts and digested slurries, taken as references for organic amendments and fertilizers, respectively, were also investigated by [PERSON] et al. [126]. Digested slurries and digestates showed similar composition, whereas higher O-alkyl C and aromatic C contents and lower alkyl C content were found in composts with respect to the other organic matrices studied (Table 6). These results were ascribed to both the different biological process (aerobic vs. anaerobic) that compost has undergone and the use of lignocellulosic materials in the starting mixture as bulking agents, which determine higher lignin-like and O-alkyl C concentrations in composts.