## 3 Results and Discussion
### BMP--Comparison of WS, RM and DDGS as AD Feedstock  
In the second BMP experiment, four different lignocellulosic residues were examined solely as substrates for their anaerobic biodegradability at two different inoculum to substrate ratios (I/S) expressed as g of VS of the inoculum to g of VS of the substrate (3.5 and 1.75 (_w/w_)). The C/N ratios of WS and SE were 88 and 64, respectively, while for both DDGS and RM, it was \(\sim\)9 (_w/v_). The digestion of all four feedstock types was efficient for both tested I/S ratios, while the higher feeding rate seemed to be slightly more favourable in terms of gas yields for all examined feedstocks (Figure 2A,B). DDGS and RM had a higher biomethane potential compared to WS and SE, while no inhibition phenomena occurred as a result of their high nitrogen content. However, it is expected that in long-term digestion of DDGS and RM without a prior balancing of the C/N ratio, the AD system would not be biologically sustainable due to ammonium accumulation [21]. In the present study, the highest produced gas yields for DDGS and RM were 445 and 405 mL CH\({}_{4}\)/g VS, respectively, both at an I/S ratio of 1.75 (_w/v_) (Figure 2A,B). These results are in agreement with previous studies where DDGS and RM were evaluated as sole substrates for BMP systems [26; 36]. Specifically, with regards to the digestion of RM, the methane production was close to the maximum theoretical value for this feedstock (450 mL CH\({}_{4}\)/g VS) [21]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **\%, _w/w_ **(db)** \\ \end{tabular} & \begin{tabular}{c} **Wheat Straw** \\ **(WS)** \\ \end{tabular} & \begin{tabular}{c} **Steam-Exploded** \\ **Straw (SE)** \\ \end{tabular} & \begin{tabular}{c} **DDGS** \\ \end{tabular} & \
\begin{tabular}{c} **RM** \\ \end{tabular} &
\begin{tabular}{c} **Inoculum** \\ \end{tabular} \\ \hline Total solids (TS) & 36.06 \(\pm\) 0.52 & 21.04 \(\pm\) 0.57 & 91.46 \(\pm\) 0.29 & 92.42 \(\pm\) 0.65 & 3.37 \(\pm\) 0.13 \\ \hline Volatile solids (VS) & 33.38 \(\pm\) 0.63 & 19.37 \(\pm\) 0.53 & 86.1 \(\pm\) 0.34 & 86.39 \(\pm\) 0.68 & 2.53 \(\pm\) 0.11 \\ \hline Crude protein & 0.29 \(\pm\) 0.01 & 0.47 \(\pm\) 0.02 & 28.3 \(\pm\) 0.5 & 25.28 \(\pm\) 0.15 & n.d. \\ \hline Cellulose (glucose) & 37.41 \(\pm\) 0.77 & 47.92 \(\pm\) 0.50 & 11.1 \(\pm\) 0.4 & 20.17 \(\pm\) 2.32 & n.d. \\ \hline Hemicellulose & 27.56 \(\pm\) 0.75 & 28.23 \(\pm\) 0.12 & 20.3 \(\pm\) 1.7 & 14.03 \(\pm\) 2.03 & n.d. \\ \hline Acid insoluble lignin & 28.09 \(\pm\) 2.6 & 25.28 \(\pm\) 1.68 & n.d. & 16.08 \(\pm\) 0.15 & n.d. \\ \hline Acid soluble lignin & 2.38 \(\pm\) 0.02 & 2.05 \(\pm\) 0.11 & 2.9 \(\pm\) 0.1 & 1.9 \(\pm\) 0.1 & n.d. \\ \hline \hline \end{tabular}  
All data were produced in duplicate and the standard deviations are presented. n.d: not detected.  
\end{table} 
Table 1: Physicochemical characteristics of the feedstock and the inoculum used in the present study. The significantly lower concentration of lignin and higher presence of hemicellulose carbohydrates that are present in DDGS and RM (Table 1) can partially explain the enhanced methane production. RM and DDGS also contain other, more assimilable carbohydrates, such as pectin [37], as well as beta-glucans [20]. It is possible that due to the differences in the hemicellulose carbohydrates of the feedstocks, the microorganisms of the system had increased affinity towards pectin or beta-glucans, as opposed to arabinylans in straw, leading into enhanced biomethanation. In addition to the above, the high protein content in DDGS and RM (Table 1) can also explain the improvement in methane production from these two materials. The presence of nitrogen in AD systems not only balances the C/N ratio but also offers higher amounts of biogas per g of dry matter. The methane potential at standard temperature and pressure conditions (STP) for proteins is higher (0.496 L CH4/g VS) compared to carbohydrates (0.415 L CH4/g VS).