## 3 Results and Discussion
### Removal Efficiencies of Solids and COD
#### 3.3.1 Total and Volatile Solids  
Figures 4 and 5 show the average concentrations of TS and VS, respectively, in the influent and effluent of the bioreactor, as well as their removal efficiencies. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multirow{2}{*}{**Units**} & \multicolumn{3}{c}{**DF Assays**} \\ \cline{3-5}  & & **HRT4** & **HRT5** & **HRT6** \\ \hline \multirow{2}{*}{\(\text{V}_{\text{bioreactor}}\) (working volume)} & \multirow{2}{*}{L} & \multirow{2}{*}{2.1} & \multirow{2}{*}{2.1} & \multirow{2}{*}{2.1} \\  & & & & \\ \(\text{F}_{\text{influent}}\) & \(\text{L.d}^{-1}\) & 0.53 & 0.42 & 0.35 \\ \(\text{OLR}_{\text{TS,influent}}\) & \(\text{g TS.L}^{-1}\text{.d}^{-1}\) & 25.9 & 20.6 & 12.6 \\ \(\text{OLR}_{\text{VS,influent}}\) & \(\text{g VS.L}^{-1}\text{.d}^{-1}\) & 23.6 & 18.0 & 10.6 \\ Influent pH & & \(4.7\pm 0.2\) & \(4.6\pm 0.2\) & \(4.6\pm 0.1\) \\ Effluent pH & & \(5.0\pm 0.2\) & \(5.2\pm 0.1\) & \(5.1\pm 0.7\) \\ Influent conductivity & & \(21\pm 1\) & \(20\pm 2\) & \(18\pm 2\) \\ Effluent conductivity & & \(25\pm 2\) & \(25\pm 1\) & \(22\pm 2\) \\ Redox potential inside the bioreactor & & \(-47\pm 38\) & \(-56\pm 26\) & \(-110\pm 12\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average values (\(n=10\) samples) of pH, redox potential, and conductivity in DF assays. In the HRT4 and HRT5 assays, the TS concentrations in the influent were similar, corresponding to values of 104 and 103 g TS.L\({}^{-1}\), respectively. The HRT6 assay showed an average concentration in the influent of 75.5 g TS.L\({}^{-1}\), which was significantly lower than for the other assays. In turn, the average concentrations of VS in the influent during the HRT4, HRT5, and HRT6 were 94.4, 89.8, and 63.9 g VS.L\({}^{-1}\), respectively. The HRT5 assay achieved TS and VS removal efficiencies (53.5 and 56.2%, respectively) higher than those obtained for HRT4 (48.8 and 55.3%, respectively) and HRT6 (45.6 and 49.9%, respectively). According to [PERSON] et al. [28], during the DF of OFMSW under thermophilic conditions, the removal efficiencies of TS and VS of 29.5 and 38.0%, respectively, were recorded with an HRT of 5 d and an OLR of 16.3 g VS.L\({}^{-1}\).d\({}^{-1}\), which demonstrates the better performance of the anaerobic consortium tested in the present work on the degradation of TS and VS. This is of particular relevance because these authors tested their biological system under thermophilic conditions which should theoretically present higher degradation rates of solids than in mesophilic systems, as tested in the present work. In another study by [PERSON] et al. [29], the DF of OFMSW was tested under mesophilic conditions with an HRT of 2 d and an OLR of 17.6 g VS.L\({}^{-1}\).d\({}^{-1}\). A VS removal efficiency of 29.06 \(\pm\) 2.76% was achieved. The removal efficiency of VS, with an HRT of 5 d, obtained in the present work, was again higher than the value reported by these authors. Only the values reported by [PERSON] et al. [30] are similar to the removal efficiencies obtained in the present work. These authors simulated food wastes by using a synthetic substrate under thermophilic conditions with an OLR of 11.4 g VS.L\({}^{-1}\).d\({}^{-1}\) and an HRT of 2 d. They reported a VS removal efficiency of 53.2 \(\pm\) 6.5%. Figure 4: Average concentrations (\(n=3\) samples per HRT) of total solids in the influent (TS influent) and effluent (TS effluent) of the bioreactor, and respective removal efficiencies (error bars: standard deviation) (concentrations: on the left \(y\) axis; removal efficiencies: on the right \(y\) axis). Figure 5: Average concentrations (\(n=3\) samples per HRT) of volatile solids in the influent (VS influent) and effluent (VS effluent) of the bioreactor, and respective removal efficiencies (error bars: standard deviation) (concentrations: on the left \(y\) axis; removal efficiencies: on the right \(y\) axis).