## 4 Conclusions and Future Perspectives  
The effects of different conductive carbon materials on AD have been thoroughly studied in recent years, but research gaps still exist, namely on the mechanism underlying these effects, and how these materials influence different key microbial groups. The results obtained in this work show that MWCNT accelerate methane production rates both from direct methanogenic substrates and from typical syntrophic substrates. Hence, the activity of acetoclastic and hydrogenotrophic methanogens  
Figure 5: Proposed mechanism of organic matter oxidation in the presence of MWCNT-Fe (based on [35]). Three alternatives are depicted: electron flow from the organic matter oxidation by bacteria to the methanogens, via H\({}_{2}\)/formate (1), or to the carbon of the composite, and then to the methanogens (2); electron flow from the Fe\({}^{2+}\) impregnated in the MWCNT to the carbon of the composite, and then to the methanogens (3). Cyclical iron regeneration by electrons derived from the biological oxidation of organic matter is also represented (4). was enhanced in the presence of MWCNT, which might have contributed to the faster syntrophic conversions of ethanol and butyrate. Additionally, faster and more extensive conversion of natural organic matter to methane was achieved by the sediment' native microbial community in the presence of MWCNT-Fe and MWCNT. Therefore, the stimulatory effects of MWCNT is transversal to different microbial communities, and appears to assume higher relevance when the microbial activity is lower (e.g., the incubations with acetate and butyrate). Overall, by enhancing the activity of complex methanogenic communities, the incorporation of MWCNT in AD processes will favor the application of higher organic loads, as described before by [PERSON] et al. [49] during anaerobic digestion of leachate, and improve the efficiency of AD processes, speeding up slower bioprocesses. MWCNT addition to AD processes treating solid wastes, wastewaters or performing ex situ bioremediation of contaminated environmental matrixes thus represent a promising strategy for accelerating the recovery of their bioenergy value. Further studies are required to assess the environmental and economic impacts of these bioprocess, including the costs of the anaerobic treatment process and the potential revenue of the generated biogas, as well as the costs of the nanomaterials. In addition, current knowledge needs to be expanded toward pilot scale and full scale experiments, and integrated with monitoring and control strategies, targeting the control, recovery, and reuse of MWCNT in the biological processes. During the operation of a UASB reactor for AO10 reduction, [PERSON] et al. [50] showed that MWCNT were efficiently retained inside the continuous reactor. Moreover, MWCNT could be recovered at the end of the operation, thus allowing their potential reuse. In another work [35], the same authors showed that due to the magnetic character of MWCNT-Fe, these nanomaterials were easily removed from the media and successfully applied in successive cycles of AO10 decolourization, without significant loss of their catalytic performance during the cycles. This magnetic character facilitates the recovery of the material, ultimately envisioning future applications in biological treatment processes. **Supplementary Materials:** The following are available online at [[http://www.mdpi.com/2076-3417/10/22/8184/s1](http://www.mdpi.com/2076-3417/10/22/8184/s1)]([http://www.mdpi.com/2076-3417/10/22/8184/s1](http://www.mdpi.com/2076-3417/10/22/8184/s1)), Figure S1: Sediment sampling in Ribeira da Granja, Vila Nova de Gaia, Portugal, Figure S2: Linear correlation between MWCNT concentration and the specific methane production rate (SMPR) for acetate, H\({}_{2}\)/CO\({}_{2}\), ethanol, and butyrate (2 nd addition, t = 0-20 h and t > 20 h).