## 3 Results and Discussion  
### First Experimental Cycle  
The overall efficiency of the PABR throughout the whole experimental cycle is presented in Table 3. The HRT reduction from 10 d to 1 d affected the biogas yield, as well as the COD removal achieved. Furthermore, the experimental phase with the lowest HRT showed the highest amount of biogas (1d HRT--44.3 L/d) and biomethane production (1d HRT--26.5 L/d). On the other hand, the average COD removal rate during the HRT 1 d phase is reduced to 69.4%. The highest COD removal occurred during the fourth experimental phase (HRT 3 days) where the COD consumption of the process reached 89%. As shown in Table 3, during all experimental phases the PABR effluent had significant deviations in COD concentrations. As shown in Figure 2, the daily biogas production increased as the HRT was reduced from 10 days to 6 days and remained at the same daily production levels when the HRT took the values of 4, 3 and 2 d. During the last experimental phase, the average daily biogas increased to 44.3 L/d. The biomethane production is calculated by the concentration of methane in the daily generated biogas. Generally, it is concluded that as the HRT is lowered, the methane concentration of biogas increases, with the exception of the last experimental phase, as shown in Figure 3. In Figure 3, the daily concentration of methane in biogas is presented. It is clear that as the HRT is lowered, the methane content of the biogas increases. Figure 2: Daily biogas and biomethane production of PABR. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**First Experimental Cycle** & \multicolumn{6}{c}{**Experimental Phases**} \\ \hline
**Characteristics** & **1 st** & **2 nd** & **3 rd** & **4 th** & **5 th** & **6 th** \\ \hline Biogas Production (L/d) & 25.6 \(\pm\) 4.2 & 33.8 \(\pm\) 3.7 & 37.0 \(\pm\) 3.2 & 35.6 \(\pm\) 2.5 & 32.3 \(\pm\) 4.9 & 44.3 \(\pm\) 1.8 \\ Biomethane Production (L/d) & 6.9 \(\pm\) 3.4 & 15.2 \(\pm\) 5.4 & 18.7 \(\pm\) 3.7 & 18.5 \(\pm\) 5.8 & 21.1 \(\pm\) 4.2 & 26.5 \(\pm\) 1.6 \\ CH\({}_{4}\) Biogas content (\%) & 27.0 \(\pm\) 10.7 & 44.5 \(\pm\) 13.4 & 50.9 \(\pm\) 10.5 & 54.3 \(\pm\) 17.6 & 66.5 \(\pm\) 8.4 & 59.6 \(\pm\) 0.6 \\ sCOD Feedstock (g/L) & 9.11 \(\pm\) 0.75 & 5.74 \(\pm\) 0.23 & 3.78 \(\pm\) 0.59 & 2.71 \(\pm\) 0.29 & 1.81 \(\pm\) 0.16 & 1.06 \(\pm\) 0.06 \\ sCOD Effluent (g/L) & 1.86 \(\pm\) 1.20 & 0.94 \(\pm\) 0.94 & 0.50 \(\pm\) 0.91 & 0.28 \(\pm\) 0.12 & 0.25 \(\pm\) 0.15 & 0.37 \(\pm\) 0.12 \\ Average COD removal (\%) & 79.5 & 83.7 & 86.9 & 89.6 & 85.30 & 64.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average daily results of the first experimental cycle phases. Regarding the behavior of the process in removing the organic content of the waste, two aspects are examined. First, the COD removal is examined between the feedstock and the effluent of the reactor, and second, the concentration distribution of the COD between the four compartments of the PABR. Figure 4 shows the variation of sCOD with time during the consecutive six phases of the PABR switching mode operation. In all cases, the process shows significant sCOD reduction. The only experimental phase presenting significant sCOD at the effluent is the first experimental phase (HRT 10 d). This happens due to the time required for acclimation to the new feedstock type. The inoculum used for the PABR startup was obtained from a CSTR reactor operating in higher HRTs and with excess activated sludge as feedstock. By observing the COD reduction of the last experimental phase (HRT 1 d), where the synthetic waste resembles as much as possible the characteristics of MWW, it is concluded that although the effluent is not yet directly disposable due to its COD concentration, a significant reduction is achieved without consuming the equivalent energy needed for aeration in biological oxidation. Figure 5 shows the average sCOD concentration and also the variation in sCOD in the four compartments of the PABR.