## 5 \({}^{13}\)C Solid-State NMR Applications to Anaerobic Digestion
This study also demonstrated that macromolecular composition of OM, as determined by \({}^{13}\)C CP-MAS NMR spectra analysis, is not a suitable approach to determine recalcitrance/biological stability of OM in digestate by comparison to compost. In fact, digestates with macromolecular compositions different from that of composts showed high biological stability, similar to that of composts, as measured by the anaerobic potential biogas production. Digestates were also compared to composts and products of mixed aerobic-anaerobic processes in Ref. [130]. However, a direct comparison of OM composition for the products of the different processes was not possible because of considerable differences among the initial compositions of substrates. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Ref. \({}^{*}\)** & **Feedback \({}^{*}\)** & **Sample \({}^{*}\)** & **Alky1C** & **O-Alky1C** & **Aromatic C** & **Chebyshev1C** \\ \hline
[129] & Sewage sludge & D & 292 \(\pm\) 67 & 548.4 \(\pm\) 10 & 7.63 \(\pm\) 2.5 & 8.97 \(\pm\) 2.9 \\ \hline \multirow{9}{*}{[134]} & Stulge & PS & 44.4 & 28.9 & 0.5 & 26.1 \\  & D & 38.7 & 19.0 & 4.5 & 37.7 \\  & Stulge hydrochloric 125 \({}^{*}\)C & PS & 41.8 & 18.0 & 0.4 & 39.8 \\  & Stulge hydrochloric 225 \({}^{*}\)C & D & 38.7 & 10.0 & 3.9 & 47.4 \\  & Stulge hydrochloric 225 \({}^{*}\)C & PS & 43.8 & 10.1 & 2.9 & 43.2 \\  & Stulge & PS & 44.5 & 21.4 & 6.5 & 27.8 \\  & Stulge & PS & 41.1 & 20.9 & 2.4 & 25.6 \\  & Stulge & PS & 32.7 & 44.6 & 4.9 & 17.5 \\  & Stulge & PS & 38.4 & 6.8 & 20.9 \\  & Stulge & PS & 58.4 & 16.6 & 8.2 & 16.8 \\  & D & 45.7 & 14.1 & 17.4 & 22.8 \\ \hline
[128] & Organic fraction of household W/SW/industrial W (O25\(\times\)25) & PS & 25 & 60 & 8 & 7 \\ \hline \multirow{9}{*}{[132]} & Stulch/cenoks/storage/com single & PS & 8-16 & 70-75 & 5-13 & 4.7 \\  & D & 1-15 & 70-75 & 5-16 & 5-4 \\  & Peak-D & PS & 14-15 & 64-65 & 16-5 & 6-5 \\  & Census/sager bets/cor/com single & PS & 8-8 & 9 & 9 & 2 \\  & D & 17-23 & 55-42 & 15 & 7-8 \\  & Peak-D & PS & 16-27 & 55-45 & 9-12 & 7-8 \\  & Pog slurry/CM/IBM/deep filter/cenoks & PS & 16-27 & 55-46 & 9-12 & 7-8 \\  & D & 19-20 & 57-56 & 15-16 & 7 \\  & MW/SW/food industry W & PS & 22 & 69 & 8 & 7 \\  & Peak-D & 37 & 38 & 13 & 12 \\  & MW/vergetables/food package/SW/food industry & PS & 38-39 & 42-45 & 8-9 & 10 \\  & W/SM/CSS & D & 30-35 & 47-49 & 11-13 & 8-9 \\  & RW/W & PS & 19-22 & 61-44 & 8-90 & 7-8 \\  & RW/D & 34-37 & 35-42 & 12-14 & 11-12 \\  & RW/D & PS & 36-18 & 54-49 & 10-11 & 4-7 \\  & MW & PS & 21-22 & 52-54 & 17-18 & 7-8 \\ \hline \multirow{9}{*}{[133]} & Stulge & PS & 21.19 & 63.8 & 7.49 & 7.22 \\  & D & 16-74 & 63.05 & 14.27 & 5.93 \\  & RS & 15.5 & 67.44 & 11.43 & 5.63 \\  & C & 14.88 & 70.37 & 12.14 & 2.61 \\  & D & 12.19 & 67.79 & 14.49 & 5.53 \\  & RS & 12.35 & 69.12 & 14.05 & 4.48 \\  & C & 10.27 & 71.11 & 14.97 & 3.65 \\  & D & 12.24 & 65.17 & 13.59 & 6.59 \\ \hline \end{tabular}
\end{table} 
Table 6: Relative areas (% of total area) of the different regions of \({}^{12}\)C CP-MAS NMR spectra for digestate samples and corresponding feedstocks reported in the literature.