## 3 Results
### Reactors' Operation Overview
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Operating Points**} & \multicolumn{3}{c}{**Mean Values COD Removal Efficiency**} & \multicolumn{3}{c}{**Mean Values of Methane Yield**} & \multicolumn{3}{c}{**Mean Values of Methane Productivity**} & \multicolumn{3}{c}{**Mean Values BOD\({}_{\text{S}}\) Removal Efficiency**} \\  & & **(\%)** & & \multicolumn{3}{c}{**(\(\text{{C}_{\text{CH}}}\)/\(\text{{R}_{\text{SWS}}}\))**} & \multicolumn{3}{c}{**(\(\text{{C}_{\text{CH}}}\)/\(\text{{L}_{\text{med}}}\)/\(\text{{d}}\))**} & \multicolumn{3}{c}{**(\(\text{{C}_{\text{3}}}\))**} \\ \cline{2-11}  & **R1** & **R2** & **R3** & **R1** & **R2** & **R3** & **R1** & **R2** & **R3** & **R1** & **R2** & **R3** \\ \hline Operating Point (15 d) & 52.7 & 49 & 59.5 & 265.7 & 295.6 & 217.6 & 0.23 & 0.15 & 0.29 & 88.13 & 84.98 & 26.84 \\ Operating Point (20 d) & 53.6 & 58.6 & 76.2 & 180.2 & 261.6 & 382.5 & 0.41 & 0.21 & 0.83 & 68.27 & 83.64 & 85.34 \\ Operating Point (20 d) & Operating & 45.3 & 82.5 & 63.7 & 189.2 & 395.8 & 317 & 0.64 & 0.46 & 0.96 & 90.16 & 95.22 & 97.69 \\ Operating Point (45 d) & 54.2 & 73.9 & 53.8 & 61.1 & 158 & 158.8 & 0.19 & 0.42 & 0.6 & 73.03 & 90.93 & 81.23 \\ Operating Point (5 d) & 53.2 & 77.3 & 55.7 & 78.1 & 200.3 & 193.9 & 0.34 & 0.91 & 0.79 & 78.21 & 91.09 & **27.47** \\ Operating Point (6 d) & 42.6 & 64.1 & 43.7 & 41.4 & 27.6 & 89.4 & 0.61 & 1.2 & 1.21 & 84.56 & 92.27 & 84.56 \\ \hline \hline \end{tabular} The colors are ordered red, yellow, and green from lower to higher values. It is the reader.  
\end{table} 
Table 3: The averaged values by operation point for each reactor. The maximum value of \(\eta_{\text{COD}}\) removal efficiency observed in reactor 1 was much smaller than in reactors 2 and 3. The same trend was observed with MY. The COD removal efficiency has been interpreted as a degree of reaction completion [19; 41; 49]. Thus, a strong relationship between COD removal and reaction completion was expected. Meanwhile, a similar relationship should have been found between MY and \(\eta_{\text{BOD5}}\); however, due to fluctuations in the BOD\({}_{5}\)/COD, no clear visual correlation could be established. Furthermore, the drop in the MYs of all three reactors at HRT 5 d was linked with the lack of organic dry matter (ODM) or VOA in the feed, as previously shown in Figure 4.  
The MYs in reactors 2 and 3 were also significantly higher than those in reactor 1. The maximum MY of reactors 2 and 3 were similar at 7 and 10 d, respectively. The improvement observed thanks to the addition of a third substrate to the PM+CWM mixture was probably related to a higher C/N ratio. The C/N ratio balance in feedstocks was significant for the stable operation of AD. Substrates with high C/N ratios have a poor buffering capacity; therefore, nitrogen will be consumed rapidly by methanogens to meet their protein requirements. This results in low methane production and produces excess VOAs during fermentation. In typical feedstocks with a low C/N ratio, nitrogen has been found to accumulate in the form of ammonia, which inhibits the methanogens and prevents methane production [4; 51]. The lowest recommended limit for C/N is 20 [11]; thus, a value of 15 was sufficient for our purpose [13]. The values of C/N in Table 1 are between 15 and 20 for the three-substrate mixtures. Meanwhile, the value was <15 for the two-substrate mixture. This supported our finding that the three-substrate mixtures were more stable and produced more methane. Nonetheless, [PERSON] et al. [52] have affirmed that substrates with a C/N ratio <10 can support a stable process; however, they require a multistage system to avoid reactor overloading. The maximum values observed for the COD and BOD\({}_{5}\) removal efficiencies, as well as the MY for reactors 2 and 3, were at HRTs of either 10 or 7 d. The same operation interval was observed by [PERSON] et al. [19] and regarded as the optimal operation interval for EGSB reactors.