## 3 Results and Discussion
### Semi-Continous Assays  
The semi-continuous tests were conducted in 3L glass reactors (Figure 1) at three different HRTs. As expected, the destruction of VS increased with the HRT (Table 2). The best results were provided by the reactor that incorporated the external bioelectrochemical cell (R2), while the reactor that performed standard anaerobic digestion (R3) and the one containing the filling pellets (R1) displayed similar results. Although the bioelectrochemical cell may have contributed to VS destruction in R2--as the presence of an electrode enables diverse electron transfer pathways [19]--this remains uncertain. Operational challenges in digester R2, particularly issues with the pumping and recirculation system that led to digestate leakage and, most notably, to the accumulation of solids at the bottom of the reactor during the first phase of the experiment, limit the reliability of this conclusion. It is indicated that these failures are due to the difficulty of handling flow rates with suspended solids on a semi-pilot scale in continuous operation with electrodes; this situation was solved by changing the piping systems and joints; on a large scale, these problems have been solved, except for the position of the electrodes in the digester or as in this experiment in a separate container. Once the initial problems have been solved, sludge recirculation sufficiently replaces mixing in the digester and can even improve the homogeneity of the sludge mixture [37]. Figure 3: Extraction results for the different digesters at the family level. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**HRT (Days)** & **R1** & **R2** & **R3** \\ \hline
20 & 1.35 \(\pm\) 0.06 & 1.11 \(\pm\) 0.14 & 1.36 \(\pm\) 0.09 \\
15 & 1.48 \(\pm\) 0.17 & 1.44 \(\pm\) 0.13 & 1.57 \(\pm\) 0.22 \\
10 & 1.57 \(\pm\) 0.05 & 1.39 * & 1.62 \(\pm\) 0.15 \\ \hline \end{tabular} \({}^{*}\)Single result.  
VFAs accumulation, especially acetic and propionic remained relatively low (Figure 4), indicating a well-balanced digestion process, with a stable interaction between acidogenic (VFA-producing) and methanogenic (VFA-consuming) microorganisms. Only reactor R2 showed a slight accumulation (below 25 mg L\({}^{-1}\)) at HRT of 20 days, which became almost negligible even when the HRT was reduced to 10 days.  
\end{table} 
Table 2: Volatile Solids Average (%). Figure 4: Presence of acetic acid and propionic acid over time and HRT changes for the three digesters. In terms of biogas production (Figure 5), during start-up and the first hydraulic retention time of operation, digesers R1 and R3 demonstrate higher biogas output compared to R2, likely due to the lack of homogenization in the latter, as previously discussed. However, after day 60, methane production in R2 surpasses that in R3. In addition, as HRT decreases--thus increasing the organic load--methane levels in reactors R1 and R3 progressively declined, while R2 remained relatively stable, effectively buffering the increase in load. Additionally, R1 consistently shows a higher biogas yield than the R3 control, which may be attributed to the presence of porous and conductive fillers [23]. Despite minor fluctuations in biogas production due to experimental challenges, reactors R1 and R2 generally achieve higher output than R3; on average, reactor R1 produced 16% more biogas than the control reactor R3, while R2 produced 6% more (Table S2). In the case of the R2 reactor, it should also be noted that, as it does not have an agitator, there is energy saving due to the difference in the energy consumption between agitation and pumping, which in this case is much lower, bearing in mind that the cell has an associated consumption below 58 KJ in the 156 days of operation. However, the energy expenditure of the cell is insignificant compared to that consumed by the stirrer and the pump. Furthermore, between these two, the stirrer (R1 and R3) had an energy requirement of approximately twice that of the peristaltic pump (R2).