## 3 Results and Discussion
### Production and Composition of Biogas
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Units**} & \multicolumn{4}{c}{**DF Assays**} \\ \cline{3-5}  & & **HRT4** & **HRT5** & **HRT6** \\ \hline HRT & d & 4 & 5 & 6 \\ n (total days of operation) & d & 38.4 & 30.3 & 18.8 \\ OLRVS\({}_{\text{in}}\) & g VS.L\({}^{-1}\).d\({}^{-1}\) & 23.6 & 18.0 & 10.6 \\ Vbiogas accumulated & L & 64.5 & 62.4 & 15.0 \\ H\({}_{2}\) & \(\%\)\(\circ\)/\(\
u\) & 25 & 32 & 5 \\ CO\({}_{2}\) & \(\%\)\(\circ\)/\(\
u\) & 75 & 68 & 77 \\ CH\({}_{4}\) & \(\%\)\(\circ\)/\(\
u\) & \textless{}0.1 & \textless{}0.1 & 18 \\ O\({}_{2}\) & \(\%\)\(\circ\)/\(\
u\) & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 \\ N\({}_{2}\) & \(\%\)\(\circ\)/\(\
u\) & \textless{}0.1 & \textless{}0.1 & \textless{}0.1 \\ \(V_{bioH2}\) accumulated & L H\({}_{2}\) & 16.13 & 19.97 & 0.75 \\ \(F_{\text{hiuf2}}\) & L H\({}_{2}\).d\({}^{-1}\) & 0.42 & 0.66 & 0.04 \\ VHP rate & L H\({}_{2}\).L\({}^{-1}\)\({}_{\text{bioreactor}}\).d\({}^{-1}\) & 0.20 & 0.32 & 0.02 \\ \(\eta_{bioH2}\) & L H\({}_{2}\).kg\({}^{-1}\) VS\({}_{\text{influent}}\) & 8.48 & 18.2 & 1.64 \\ \(H_{u,act}\) & kJ.kg\({}^{-1}\) VS\({}_{\text{influent}}\) & 123 & 175 & 17.3 \\ Thermal energy in biogas & kWh.t\({}^{-1}\) VS\({}_{\text{influent}}\) & 34.2 & 48.6 & 4.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Production and composition of biogas, bioH\({}_{2}\) volumetric production (VHP) rate, bioH\({}_{2}\) yield (\(\eta_{bioH2}\)), calorite value (\(H_{u,act}\)), and thermal energy in biogas. \(24-60\) g COD.L\({}^{-1}\).d\({}^{-1}\) supplied by glucose; HRT of \(2-10\) h; pH 5.7; different pre-treatments of the inoculum were tested. The best results (7 L H\({}_{2}\).L\({}^{-1}\).d\({}^{-1}\), 3.5 mol H\({}_{2}\).mol\({}^{-1}\) glucose) were obtained with the highest load (60 g COD.L\({}^{-1}\).d\({}^{-1}\)) and an HRT of 6 h. A lower HRT caused a decrease in the bioH\({}_{2}\) production rate, possibly caused by the cell washout from the bioreactor. The HRT of 5 d (OLR of 18.0 g VS.L\({}^{-1}\).d\({}^{-1}\)) showed the best results in the present study: an H\({}_{2}\) content of 32% \(v/v\); \(\eta_{bioH2}\): 18.2 L H\({}_{2}\).kg\({}^{-1}\) VS\({}_{\text{influent}}\); and VHP rate of 0.32 L H\({}_{2}\).L\({}^{-1}\)\({}_{\text{bioreactor}}\).d\({}^{-1}\). The biogas calorific value was 175 kJ.kg\({}^{-1}\) VS\({}_{\text{influent}}\), corresponding to a thermal energy potential of 48.6 kWh.t\({}^{-1}\) VS\({}_{\text{influent}}\). Among the three experimental trials, the HRT of 5 d and an OLR of 18.0 g VS.L\({}^{-1}\).d\({}^{-1}\) provided the optimal conditions tested for the DF of a real OFMSW feedstock under mesophilic conditions and at a pH of approximately 5. Table 7 compares the main results of the present work with some of the data available in the literature. The studies carried out under similar conditions to those of the present work (type of substrate, OLR, and HRT) but under a thermophilic regime produced better results in terms of bioH\({}_{2}\) production rate and yields [29, 36], except when an HRT of 6.6 d was used [35]. The thermophilic regime can be more interesting for bioH\({}_{2}\) production, provided that heat is available in the plant to heat the bioreactor and an ulterior high added-value valorization process of the VFAs is performed, aiming to increase the financial return of the plant. Additionally, considering the data obtained in the present work, it is possible to conclude that HRT values higher than 6 d seem not to be advantageous for the production of bioH\({}_{2}\) in the DF of OFMSW under both mesophilic and thermophilic conditions. The tests performed in bioreactors with bigger volumes revealed that although the percentages of bioH\({}_{2}\) are similar to that obtained in the present study, they achieve much higher bioH\({}_{2}\) production rates and yields, showing that scaling up this process may contribute to its sustainability with a negligible loss of productivity.