## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion
The modifications of OM occurring during AD were found to determine a higher degree of biological stability of digestate, measured for instance through the cumulative oxygen uptake rate and anaerobic potential biogas production [125], with respect to the starting mixture. [PERSON] et al. [128] applied \({}^{13}\)C CP-MAS NMR to study organic matter in feedstock (mixture of household waste (50%), slaughterhouse waste (25%)) and industrial waste (25%)) and digestate from a full-scale AD plant in Sweden, as well as in fractions of feedstock and digestate obtained by sequential extraction (DOM; extractable soluble from particulate organic matter, SPOM; readily extractable organic matter, REOM; slowly extractable organic matter, SEOM; poorly extractable organic matter, PEOM). The study intended to evaluate OM composition and trace element bioaccessibility in the different fractions of feedstock and digestate. A higher contribution of aliphatic, aromatic and carbonyl C resonances, arising from lipid and/or protein-like structures, was observed in the spectra of digestate solid residues compared to feedstock solid residues, while carbohydrate signals had higher intensities in the feedstock spectra. NMR analysis of solid pellets after each step of the sequential extraction showed a similar distribution of C among different organic groups in both feedstock and digestate samples. Only a slight reduction in carbohydrates was observed in SEOM solid residue in the feedstock. On the other hand, a reduction in carbohydrates was found in solid residues of feedstock and digestate after PEOM extraction. As far as trace elements are concerned, a much lower bioaccessibility was found for all elements in digestates with respect to feedstocks. [PERSON] et al. [129] performed thermophilic high-solid anaerobic digestion (THSAD) of a mixture of municipal sewage sludge (85%), sewage sludge from the agro-food industry (5%) and liquid fraction of food waste from separate collection (10%) in a plant located in Lombardia region (Italy) to obtain fertilizers, besides biogas for electrical and thermal energy production. The process was monitored for 42 months and three digestate samples were collected at almost one year intervals and investigated by \({}^{13}\)C CP-MAS NMR experiments. A quite uniform composition of digestate in time was found, with spectra dominated by signals from polysaccharides and proteins (54.8 \(\pm\) 10% of total area), followed by hydrocarbons (29.2 \(\pm\) 6.7%), and low amounts of aromatic carbons (7.6 \(\pm\) 2.5%) from lignin and lignin-derived molecules, and carbonyl carbons (8.4 \(\pm\) 2.9%) from amino acid moieties and lipids. When compared to digestates of different origin, the spectroscopic characteristics of the investigated digestates resembled those originating from animal manure [126; 127], because of the similar presence of fecal material and vegetal matter. On the contrary, they showed a lower content of aliphatic structures than digestates from the organic fraction of MSW [125; 126], due to the lower content of vegetable waste and oil/fat from food in the feedstock. Interestingly, \({}^{13}\)C CP-MAS NMR spectra of the investigated digestates resembled those of composts [126], apart from the lower content of aromatic C and higher content of alkyl C fractions due to biomass origin (fecal origin material in AD vs. lignocellulosic material in composting). Considering the total organic carbon content and biological stability, determined through substrate oxygen rate uptake and potential biogas production tests, NMR data also allowed to state that thermophilic digestate from sewage sludge is a valuable product to be used as amendment in soil. Stability is also important, particularly when digestate has to be disposed in landfills. In fact, anaerobic digestion of waste, especially MSW, is used as pretreatment before landfilling to reduce odorous emissions. In this case, parameters are needed that describe the stability of the treatment product with respect to further biodegradation in the landfill.