## 4 Conclusions  
This study evaluates the impact of adding conductive materials and integrating biogeochemical systems on the efficiency of anaerobic digestion for a mixture of primary and secondary sludge from an urban wastewater treatment plant (WWTP). The findings provide valuable insights for designing and optimizing more efficient and sustainable anaerobic digestion systems and advancing waste treatment and energy production technologies.  
The initial biodegradability tests indicated that adding a conductive filler to the anaerobic digester facilitates a faster start-up phase. However, in subsequent cycles, differences in methane production became negligible as the inoculum adapted. In the semi-pilot plant experiments (R1), it is deduced that electron transfer is favoured through these porous spherical fillings, also improving substrate degradation and supporting methanogenic microorganisms. The integration of an external bioelectrochemical cell (R2) enhances anaerobic digestion, improving substrate degradation and biogas quality while favouring system stability.  
However, the efficiency and specific benefits of these technologies depend on the process conditions and the type of conductive material used, and larger-scale testing and evaluation of energy consumption and long-term performance of the materials used should be carried out in the future. The application of these technologies on an industrial scale requires a detailed assessment and optimisation of the operating conditions to maximise the benefits and ensure the sustainability of the process, supported by modelling and automation.  
**Supplementary Materials:** The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app15010143/s1](https://www.mdpi.com/article/10.3390/app15010143/s1)]([https://www.mdpi.com/article/10.3390/app15010143/s1](https://www.mdpi.com/article/10.3390/app15010143/s1)), Figure S1: PLA pellets used for biodegradability experiments; PLA/Graphene (left) and PLA/Carbon Black (right); Figure S2: Flowchart of the digesters, from left to right (R1, R2 and R3). R1 includes PLA fillers, R2 an external bioelectrochemical cell and R3 is a normal digester as a control; Figure S3: Analysis parameters during digesters operation: (A) Redox potential and (B) pH; Table S1: Initial ratio in litres of inoculum and substrate used in each digester; Table S2: Average methane production per reactor.  
**Author Contributions:** Conceptualization, A.E., R.M. and A.M.; methodology, A.M.-G., A.V.G.-T. and R.M.; validation, A.M. and A.E.; formal analysis, A.M.-G. and A.E.; investigation, A.M.-G., A.V.G.-T. and A.E.; resources, A.M.-G., A.V.G.-T. and R.M.; data curation, A.M.-G. and A.V.G.-T.; writing--original draft preparation, A.M.-G., A.E. and A.M.; writing--review and editing, A.M.-G., A.E. and A.M.; supervision, A.E. and R.M.; project administration, A.M. and R.M.; funding acquisition, A.M. and A.E. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Ministerio de Ciencia e Innovacion (Spain Government), project ref: PID2020-115948 RB-I00 funded by MCIN/AEI/10.13039/501100011033. We would also like to thank the staff of the WWTP in Leon (Spain).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.