## 3 Results and Discussion
### BMP--Co-Digestion Scenarios  
Finally, the effect of different co-digestion scenarios between the two straw samples (WS and SE) and DDGS or RM was evaluated. Specifically, WS and SE samples were co-digested with DDGS and RM in three different ratios based on the VS content of the four AD feedstocks (50:50, 70:0 and 90:0 _w/w_). The C/N ratio was calculated for all co-digestion scenarios based on the composition of each feedstock and is presented in Table 2 along with the ratio of each feedstock used in each co-digestion trial. As can be seen in Figure 3A-D, the 50:50 co-digestion scenario offered higher, but not statistically significant, methane yields compared to the rest of the trials (70:30 and 90:10 _w/w_) for both WS and SE co-digested either with DDGS or RM. With regards to the effect of the type of co-substrate on methane production, no statistically significant differences were found in methane production of WS co-digested with either DDGS or RM (Figure 3A,B). Similarly, for SE, a co-digestion with DDGS at 50:50 (_w/w_) resulted in 377 mL CH\({}_{4}\)/g VS, while for the same ratio, the co-digestion of SE with RM offered comparable biomethane yields of 373 mL CH\({}_{4}\)/g VS. However, in SE trials with less DDGS or RM (ratio 90:10), the produced methane yields were significantly higher in DDGS co-digestions compared to RM co-digestions (Figure 3C,D), while a similar C/N ratio (35 _w/w_) was applied in both cases. According to the results from the mono-digestion BMP experiment, DDGS offered slightly higher methane yields as a sole substrate compared to the RM. This higher biogas potential of DDGS compared to RM can also explain the increased yields offered by SE co-digested with DDGS compared to SE-RM at the 90:10 ratio. It is also worth mentioning that regardless of the improvement that steam explosion can offer to the digestion of lignocellulosic biomass when WS and SE are co-digested with RM/DDGS, the biomethane potential for the two feedstocks is very similar (Figure 3A-D). As an example, the highest methane production for WS was achieved when co-digested with RM at 50:50 ratio (375 mL CH\({}_{4}\)/g VS). Similarly, when SE was co-digested with DDGS at 50:50, the biomethane outcome reached values \begin{table}
\begin{tabular}{c c c c c c} \hline  & **WS** & **SE** & **DDGS** & **RM** & **C/N** \\  & **(\% _w/w_)** & **(\% _w/w_)** & **(\% _w/w_)** & **(\% _w/w_)** & **(\% _w/w_)** \\ \hline WS-DDGS & 50 & - & 50 & - & 21 \\ WS-DDGS & 70 & - & 30 & - & 26 \\ WS-DDGS & 90 & - & 10 & - & 30 \\ WS-RM & 50 & - & - & 50 & 19 \\ WS-RM & 70 & - & - & 30 & 24 \\ WS-RM & 90 & - & - & 10 & 30 \\ SE-DDGS & - & 50 & 50 & - & 15 \\ SE-DDGS & - & 70 & 30 & - & 24 \\ SE-DDGS & - & 90 & 10 & - & 35 \\ SE-RM & - & 50 & - & 50 & 15 \\ SE-RM & - & 70 & - & 30 & 25 \\ SE-RM & - & 90 & - & 10 & 35 \\ \hline \end{tabular}
\end{table} 
Table 2: Co-digestion scenarios and carbon to nitrogen (C/N) ratio calculated for all biochemical methane potential (BMP) trials. close to 377 mL CH\({}_{4}\)/g VS. DDGS and RM performed similarly when used as co-substrates in AD, with DDGS showing slightly higher biomethane potential compared to RM. Furthermore, to evaluate the effect of co-digestion on the gas production solely from WS and SE straw, methane concentrations recorded in mono-digestion trials of DDGS and RM were subtracted from the cumulative final production of the six co-digestion scenarios (Figure 4). Despite the notable increase in gas production in DDGS and RM co-digestion at 50:50, the produced yields from SE seemed to be significantly increased when lower amounts of either DDGS or RM were added to the system (Figure 4). This observation could be explained by the differences in the C/N ratio between the different BMP trials (Table 2). The C/N ratio was calculated within the range of 12 to 15 (_w/w_) in all cases where a 50:50 digestion was applied (Table 2). At the same time, the C/N ratio reached values higher than 35 (_w/w_) when SE was digested either with RM or DDGS at a 90:10 ratio (Table 2).