## 3 Results and Discussion
### Semi-Continous Assays
The presence of electrodes and various conductive materials in R1 and R2 likely facilitated different electron transfer pathways, enhancing methanogenic efficiency and resulting in greater methane output [38]. These metabolic pathways are affected by the voltage applied to the cell and could be the cause of the differences found between R1 and R2, as observed by [PERSON] et al. [39]; other references [18] also indicate that in bath mode, an increase until 4 V could duplicate the methane production in batch tests. Similarly, [PERSON] et al. [40] highlighted the influence of the inoculum on system performance under specific voltage conditions, emphasizing the interaction between microbial communities and the applied electrical field. These findings suggest that increasing the voltage applied to R2 could further enhance its efficiency. Although this adjustment may lead to higher energy consumption compared to the conventional R3 digester, the increase could be offset by greater biogas production and improved carbon sequestration. In assessing the feasibility of the different setups used in these experiments, it is also important to consider biogas quality (i.e., the methane content of the biogas (Figure 6)). All three digesters exhibited an alignment with HRT changes. R2 generally showed a higher methane content, with two exceptions: from days 32 to 39, when R2 and R3 displayed  
Figure 5: Methane production in relation to volatile solids fed over time and HRT changes for the three digesters. similar values, and from days 67 to 93, when R2 and R1 were comparable. Other studies on the integration of bioelectrochemical systems with anaerobic digestion have also found that the bioelectrochemical reactor notably improves the quality of the biogas, probably because it helps to convert the excess CO\({}_{2}\) into methane. [PERSON] et al. [41] found that methane production in wastewater treatment was increased by 10-25% and reactor stability was improved in comparison to a conventional anaerobic reactor. [PERSON] et al. [42] reported that bioelectrochemical systems significantly improve biogas quality by converting excess CO\({}_{2}\) into methane, particularly during the startup phase with different biomass inocula. Furthermore, [PERSON] et al. [43] demonstrated that integrating microbial electrolysis cells for CO\({}_{2}\) reduction into anaerobic digestion processes leads to both biogas upgrading and enhanced methane content, supporting the idea that bioelectrochemical reactors can optimize carbon conversion efficiency. The experiments performed have been conducted under controlled laboratory conditions so that the scalability to WWTP in real treatment situations would be conditioned by factors such as variations in organic load and composition of the treated substrate. Even so, it should be noted that several periods were performed in which different HRTs were tested, demonstrating the greater capacity of the combination of BES with AD for the treatment of streams with higher organic load. Furthermore, the results obtained confirm the capacity that these systems can have in mitigating complicated and/or extraordinary situations--in this case, the accumulation of VFA that could have led to the total inhibition of digestion. To conclude this section, and to put the results in perspective, it is important to highlight the potential impact of this technology on biogas production, as an average increase of 16% (Table S2) could enable Spain to reach an installed capacity of approximately 1000 MWe with the current number of digesters in wastewater treatment plants. By extrapolating this increase to include all anaerobic digesters processing other types of waste, the installed capacity could reach around 1600 MWe as of 2023 (Figure 7). It should be borne in mind that WWTPs should aim to be energy-autonomous and improvements in biogas production such as those proposed in this work favour the energy efficiency of these facilities. Figure 6: The proportion of methane in the biogas leaving the digesters as a measure of biogas quality.