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[Display omitted] • Using acetate is not advantageous for improving performance of manure-fed MECs. • Cross-feeding lowers methane production performance during electromethanogenesis. • To produce similar CH 4 from manure as acetate, two times greater sCOD is required. • Model exoelectrogen Geobacter dominated the anodes of all manure-fed MECs. • Hydrogenotrophic Methanoculleus is abundant on the cathodes of manure-fed MECs. This study qualitatively assessed the impacts of different start-up strategies on the performance of methane (CH 4) production from cattle manure (CM) in electromethanogenic reactors. Single chamber MECs were operated with an applied voltage of 0.7 V and the impact of electrode acclimatization with a simple substrate, acetate (ACE) vs a complex waste, CM, was compared. Upon biofilm formation on the sole carbon source (ACE or CM), several MECs (ACE_CM and CM_ACE) were subjected to cross-feeding (switching substrate to CM or ACE) during the test period to evaluate the impact of the primary substrate. Even though there was twice as much peak current density via feeding ACE during biofilm formation, this did not translate into higher CH 4 production during the test period, when reactors were fed with CM. Higher or similar CH 4 production was recorded in CM_CM reactors compared to ACE_CM at various soluble chemical oxygen demand (sCOD) concentrations. Additionally, feeding ACE as primary substrate did not significantly impact either COD removals or coulombic efficiencies. On the other hand, the use of anaerobic digester (AD) seed as an inoculum in CM-fed MECs (CM_CM), relative to no inoculum added MECs (Blank), increased the initial CH 4 production rate by 45% and reduced the start-up time by 20%. In CM-fed MECs, Geobacter dominated bacterial communities of bioanodes and hydrogenotrophic methanogen Methanoculleus dominated archaeal communities of biocathodes. Community cluster analysis revealed the significance of primary substrate in shaping electrode biofilm; thus, it should be carefully selected for successful start-up of electromethanogenic reactors treating wastes. [ABSTRACT FROM AUTHOR]

[Display omitted] • Plate reactor design enables high-rate microbial electrosynthesis (MES). • High-rate electromethanogenesis obtained by M. maripaludis and mixed culture. • MES of acetate by T. kivui on par with glucose fermentation rates. • Acetate production rates by MES approach commercial gas fermentation rates. • Improved electron donor utilization by MES compared to classical biotechnology. Microbial electrosynthesis (MES) converts (renewable) electrical energy into CO 2 -derived chemicals including fuels. To achieve commercial viability of this process, improvements in production rate, energy efficiency, and product titer are imperative. Employing a compact plate reactor with zero gap anode configuration and NiMo-plated reticulated vitreous carbon cathodes substantially improved electrosynthesis rates of methane and acetic acid. Electromethanogenesis rates exceeded 10 L L–1 catholyte d–1 using an undefined mixed culture. Continuous thermophilic MES by Thermoanaerobacter kivui produced acetic acid at a rate of up to 3.5 g L−1 catholyte h−1 at a titer of 14 g/L , surpassing continuous gas fermentation without biomass retention and on par with glucose fermentation by T. kivui in chemostats. Coulombic efficiencies reached 80 %–90 % and energy efficiencies up to 30 % for acetate and methane production. The performance of this plate reactor demonstrates that MES can deliver production rates that are competitive with those of established biotechnologies. [ABSTRACT FROM AUTHOR]

Dark fermentation of cheese whey (CW) for the production of biohydrogen generates an acidic effluent, containing high concentrations of volatile fatty acids, which needs to be further treated before disposal and possibly further valorised. This study develops a dual-chamber Microbial Electrolysis Cell (MEC) that achieves simultaneously the reduction of the organic content of this effluent to environmentally acceptable levels, along with bio-electrochemical reduction of CO 2 to CH 4. The MEC was operated for 140 days and the effect of the following conditions on the MEC performance was examined: (a) the feed concentration of the acidic fermentate (in the range 6–81 g COD /L), (b) the conductivity of the feed modified via KCl addition (range 2–22 mS/cm), (c) the MEC operation mode (with or without catholyte renewal) and (d) the solids content (modified via CW filtration prior to its use). The results showed that high COD removal (>95 %) was achieved in all cases, along with a CH 4 production of up to 1.1 mmol/g CODconsumed. The best performance of the cell was obtained for a feed COD concentration of ∼30 g COD /L and a feed conductivity of ∼15 mS/cm; these conditions resulted in a COD removal exceeding 99 %, a CH 4 production of 1.1 mmol CH4 /g CODconsumed and a net energy production of 15.8 % compared to the energy demand of the system. The electrochemical study of the system revealed that higher and lower feed COD concentrations were characterized by higher internal resistances. The results indicate that the MEC can be exploited for further treatment and valorization of a high-strength effluent along with the production of CH 4 with an energy surplus, as an efficient waste-to-energy technology. [Display omitted] • High organic load removal was achieved in all cases (>95 % COD removal). • Energy gain reached up to 199 % relative to the expenditure due to applied potential. • Optimal performance was obtained at moderate conditions (30 g COD /L, 15 mS/cm). • Catholyte renewal enables treatment of the undiluted (63 g COD /L) waste. • Fermentative bacteria contained in the acidified cheese whey inhibit MEC operation. [ABSTRACT FROM AUTHOR]

Microbial electrochemical system (MES) for enhancing methane production has gained significant interest during the recent years, but the practical applications of MES are still far away due to several limitations such as low efficiency of cathodic electrochemical kinetics. In this study, novel porous reduced-graphene oxide/hollow titania (rGO/TiO 2) was successfully synthesized to be used as cathode catalyst for promoting electrochemical reduction of CO 2 to methane. The MES operation with rGO/TiO 2 catalyst exhibited 15.4% higher methane yield (0.383 ± 0.01 LCH 4 /gCOD) and 13.4% higher production rate (152.38 mL/L.d) compared to control MES with bare carbon cloth cathode. The MES-rGO/TiO 2 produced around 33% higher in total Coulomb at 3837.9 ± 351.5C compared to the pristine cathode at 2887.92 ± 254.6C. Substrate degradation and volatile fatty acids conversion were significantly improved in the presence of rGO/TiO 2 catalyst. By using cyclic voltammetry and electrochemical impedance spectroscopy analysis, rGO/TiO 2 was proved to ease the electron transfer efficiency of working cathode for the conversion of electron to methane. The results suggest that porous rGO/TiO 2 can be a promising cathode catalyst to upgrade the performance of a scalable methane-producing MES-AD system. • Porous rGO and hollow TiO 2 was successfully synthesized and applied as catalyst of MES cathode. • Catalyzed MES enhanced 20% and 33% methane accumulation and coulomb generation, respectively. • rGO/TiO 2 eased the decomposition of substrate and VFAs for a better methane production rate. • rGO/TiO 2 enhanced catalytic activity and lowered charge transfer resistance of working cathode. [ABSTRACT FROM AUTHOR]

Two-stage anaerobic digestion (AD) is a promising method of converting organic waste into clean and sustainable energy. This work aimed to study the efficiency of a two-stage AD system with the production of hydrogen at the first acidogenic stage and methane in an electromethanogenic reactor. Two types of biocathodes based on carbon cloth were used in the electromethanogenic reactor: with and without retention of biomass on the electrode surface. A high applied voltage continuously supplied to the pair of electrodes was automatically maintained at 2.5 V. Hydrogen yield (0.11 NL/g volatile solids (VS) init or 2 mol H 2 /mol hexose) and hydrogen content in biogas (52%) were relatively high, even despite the extremely low pH (∼4.0). Improved retention of biomass on the surface of the carbon cloth-based biocathode led to greater stability of the AD process (no water electrolysis was observed) and a significant increase in the efficiency of electromethanogenesis, including the methane yield (by 40.5% up to 0.39 NL/g VS init), volumetric methane production rate (by 38.8% up to 1.16 NL/L/d) and current density (by 233% up to 0.56 A/m2). • T= 57.5 °C and pH ∼4 completely suppressed methanogenesis in the acidogenic reactor. • 2 mol H2/mol glucose during continuous dark fermentation of OF-MSW is achieved. • Better biomass attachment to the biocathode increased the current density by 233%. • Increase in CH4 yield by 40.5% and volumetric CH4 production rate by 38.8%. • A physical barrier is an effective tool to retain biomass on the biocathode surface. [ABSTRACT FROM AUTHOR]

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO 2 into methane. Unlike biomethanation processes where CO 2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO 2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO 2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H 2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology. • Electromethanogenesis is a promising technology being optimized at the lab scale. • Interdisciplinary work is required to overcome the system limitations for upscaling. • New mechanisms on electron uptake have been elucidated in methanogens. • Electrochemical parameters must suit the methanogenic electron uptake mechanisms. • Non-electrotrophic methanogens are key biocatalysts for this technology. [ABSTRACT FROM AUTHOR]

This study aimed to propose a novel approach for simultaneous CO 2 abatement and energy storage in microbial electrolysis cells (MECs). The methane (CH 4) production from CO 2 was achieved in a graphite granule–filled cathodic compartment of an MEC separated by a modified nylon cloth. Using the modified nylon cloth in MEC significantly reduced membrane internal resistance compared with the control where a commercial proton exchange membrane (PEM) was used (PEM: 1746 ± 80 TΩ/cm; nylon cloth: 251 ± 8 TΩ/cm). Consequently, CH 4 production realized 72.9 ± 24.4 mL/(L ∙ d) with an average energy efficiency of 121 ± 39% compared with halted electromethanogenesis in PEM-equipped MEC. Metagenomics affirmed the predominance of hydrogenotrophic methanogens Methanocorpusculum and Methanobacterium. Moreover, H 2 -mediated methanogenesis contributed significantly to electromethanogenesis in nylon-MEC. This study demonstrated that the bioelectrochemical conversion of CO 2 into CH 4 in a nylon-MEC could be viable for CO 2 sequestration combined with energy storage (power to gas). • Microbial electrolysis cells (MECs) with different separators were used for CH 4 yield. • Modified nylon cloth surpassed proton exchange membrane in electromethanogenesis. • Nylon cloth-equipped MECs obtained prominent energy efficiency. • Hydrogenotrophic Methanocorpusculum and Methanobacterium were dominant methanogens. • H 2 -mediated methanogenesis contributed significantly to electromethanogenesis. [ABSTRACT FROM AUTHOR]

Power-to-Gas technologies are high-capacity systems, which can be used to optimise the use of renewable energy to meet energy demands. Among them, Power-to-Methane (PtM) stands out due to the ease of storage and use of the produced methane. However, its total storing efficiency (Power-to-Methane-to-Power) is likely still low as there are various losses. This paper presents an overview of PtM improvement through the utilisation of low-temperature waste heat produced upon the two steps by converting power to gas, namely electrolysis and methanation. An organic Rankine cycle, Stirling engine, and thermoelectric generator were chosen as promising technologies that could be implemented to recover the waste heat and improve the PtM system. Several experimental studies of each technology were reviewed. Moreover, some technological and economic surveys are described and discussed. At the end of the article, the study discussed the direction of further research and development for waste heat recovery in the system to increase the competitiveness of the PtM system and its efficiency. • Some possible Power-to-Methane configurations are described. • Power-to-Methane offers high energy density and long-term capabilities. • An improvement of Power-to-Methane based on waste heat recovery is discussed. • Selected promising waste heat recoveries below 100.0 °C are described and compared. • Several techno-economic parameters for assessing the improvement are described. [ABSTRACT FROM AUTHOR]

Hydrothermal carbonization (HTC) is a biomass conversion process that generates a CO 2 -rich gaseous phase that is commonly released directly into the atmosphere. Microbial electromethanogeneis (EM) can potentially use this off-gas to convert the residual CO 2 into CH 4, thus avoiding GHG emissions while adding extra value to the overall bioprocess. In the present work, the HTC gas phase was fed to two mixed-culture biocathodes (replicates) polarized at −1.0V vs. Ag/AgCl. Compared to pure CO 2 , HTC gas had a marked negative effect on the process, decreasing current density by 61%, while maximum CH₄ yield contracted up to 50%. HTC also had an unequal impact on the cathodic microbial communities, with the methanogenic hydrogenotrophic archaea Methanobacteriaceae experiencing the largest decline. Despite that, the present study demonstrates that HTC can be used in EM as a raw material to produce a biogas with a methane content of up to 70%. • Electromethanogenesis can convert hydrothermal carbonization off-gas into methane. • HTC off-gas had a marked negative effect on the process in comparison to pure CO 2. • qPCR analyses revealed a larger impact on methanogenic communities. • The presence of CO in the HTC off-gas would explain the poorer performance. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Design of bubble column reactor for various types of bioelectrochemical processes. • First use of a highly flexible bubble column reactor in bioelectrochemistry. • Proof of principle was carried out with two different process. • Reactor characterization was done allowing better comparison with other BES. • Suggestion of scale-up parameters for bioelectrochemical systems. Abstract In times of energy revolution, bioelectrochemistry is a growing field of research, either for the generation of electrical energy from organic substrates or the use of electrical energy to produce various products. By now, this technology is on the turning point from lab scale to industrial applications. Unfortunately, there is still a lack of well characterized, scalable reactor systems that are capable of hosting different bioelectrochemical processes, linking lab scale research to industrial application. In this paper, we introduce a two-chamber bioelectrochemical bubble-column reactor (one liter working volume), which can be used as microbial fuel cell as well as for microbial electrosynthesis and is especially advantageous for processes with gaseous substrates. It is designed flexible in terms of electrode material and area, membrane material and area, and capable of hosting continuous processes. It is a promising replacement of lab-scale H-cells for wider screening possibilities with regard to industrial applications. We characterized the reactor by giving key values such as k L a and gas hold up, and suggest scale-up parameters. These are, for example, dimensionless numbers like Reynolds and Wagner number and different ratios that should be kept constant during scale-up. Therefore, this paper can be a guideline for the development and scale-up of bioelectrochemical systems. [ABSTRACT FROM AUTHOR]

Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO 2 . To optimize electromethanogenesis process, this study physically prepared and examined several carbon-based cathode materials: carbon stick (CS), CS twined by Ti wire (CS-Ti) or covered with carbon fiber (CS-CF), graphite felt (CS-GF) and carbon cloth (CS-CC). CS-GF electrode had constantly stable methane production (75.8 mL/L/d at −0.9 V vs. Ag/AgCl) while CS-CC showed a suppressed performance over time caused by the desposition of inorganic shell. Electrode material properties affected biofilms growth, cell-electrode contact behaviors and electron exchange. Methane formation with CS-CC biocathode was H 2 -concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H 2 route. This study re-interpreted the application potentials of carbon-based materials in CO 2 electroreduction and electrofuel recovery, providing valuable guidance for materials’ selection. [ABSTRACT FROM AUTHOR]

The integration of anaerobic digestion (AD) and a microbial electrolysis cell (MEC) with an electromethanogenic biocathode is proposed to increase the stability and robustness of the AD process against organic and nitrogen overloads; to keep the effluent quality; to recover ammonium; and to upgrade the biogas. A thermophilic lab-scale AD, fed with pig slurry, was connected in series with the bioanode compartment of a two-chambered MEC. In turn, the biocathode of the MEC was poised at −800 mV vs Standard Hydrogen Electrode and fed with CO 2 to increase the methane production of the system. After doubling its organic and nitrogen loading rate, the AD operation became stable thanks to the connection of a recirculation loop with the MEC effluent. Ammonium removal in the anode compartment of the MEC achieved 14.46 g N-NH 4 + m −2 d −1 , while obtaining on average 79 L CH 4 m −3 d −1 through the conversion of CO 2 in the cathode compartment. The microbial analysis showed that methylotrophic Methanossiliicoccaceae family ( Methanomassiliicoccus genus) was the most abundant among the metabolically active archaea in the AD during the inhibited state; while, on the cathode, Methanobacteriaceae family ( Methanobrevibacter and Methanobacterium genus) shared dominance with Methanomassiliicoccaceae and Methanotrichaceae families ( Methanomassiliicoccus and Methanothrix genus, respectively). [ABSTRACT FROM AUTHOR]

Electromethanogenesis is a form of electrobiofuel production through a microbial electrolysis cell (MEC) where methane (CH 4 ) is directly produced from an electrical current and carbon dioxide (CO 2 ) using a cathode. With the aim of maximizing methanogenesis in an MEC, this study utilized granular activated carbon (GAC) and a transition metal catalyst to fabricate nickel (Ni) nanoparticle (NP)-loaded GAC (Ni NP/GAC) composites and incorporated these into MECs. In this set-up, GAC acted as the main electrical conduit for direct interspecies electron transfer (DIET) between exoelectrogens and methanogenic electrotrophs, and the Ni NPs served as a catalyst to further improve microbe-to-GAC electron transfer. The Ni NP/GAC-composites were prepared using two different methods (microwave irradiation and solution plasma ionization). The Ni NPs were determined to be well doped on the GAC surface according to a field emission scanning electron microscope (FE-SEM) and energy-dispersive X-ray (EDX) spectroscopy analysis. Adding GAC into MECs improved CH 4 production. The Ni NP/GAC composites prepared by solution plasma ionization showed the highest CH 4 production (20.7 ml), followed by the Ni NP/GAC composite prepared by microwave irradiation (19.6 ml), bare GAC (15.6 ml), and GAC-free control (9.6 ml). In the methanogenic MECs, 40.6% of CH 4 was produced from an electrode reaction (i.e., reduction of CO 2 to CH 4 ), and the remaining 59.4% was generated by nonelectrode reactions. [ABSTRACT FROM AUTHOR]

Functioning biocathodes are essential for electromethanogenesis. This study investigated the development of a biocathode from non-acclimated anaerobic sludge in an electromethanogenesis cell at a cathode potential of −0.7 V (vs. standard hydrogen electrode) over four cycles of repeated batch operations. The CO 2 -to-CH 4 conversion rate increased (to 97.7%) while the length of the lag phase decreased as the number of cycles increased, suggesting that a functioning biocathode developed during the repeated subculturing cycles. CO 2 -resupply test results suggested that the biocathode catalyzed the formation of CH 4 via both direct and indirect (H 2 -mediated) electron transfer mechanisms. The biocathode archaeal community was dominated by the genus Methanobacterium , and most archaeal sequences (>89%) were affiliated with Methanobacterium palustre . The bacterial community was dominated by putative electroactive bacteria, with Arcobacter , which is rarely observed in biocathodes, forming the largest population. These electroactive bacteria were likely involved in electron transfer between the cathode and the methanogens. [ABSTRACT FROM AUTHOR]

Toward applications of bio-electrochemical systems in industrial processes and extreme environments, electromethanogenesis under high-pressure conditions was examined. Stainless-steel single-chamber reactors specifically designed to examine bio-electrochemical reactions under pressurized conditions were inoculated with thermophilic microorganisms originated from an oilfield formation water. The reactors were incubated at 5 MPa, 55°C in fed-batch operational mode with an applied voltage of 0.7 V. In the first few fed-batch cycles, hydrogen was mainly produced. After the third cycle, however, the reactors produced only methane simultaneously with current generation. The methane-production rate of the reactors showed an applied-voltage dependence and increased from 34.9 to 168.4 mmol m −2 day −1 with an increase in the applied voltage from 0.4 to 0.9 V. The efficiency of capturing electrons in the produced methane on average exceeded 70% with the applied voltage of 0.4 V or higher. Cyclic voltammetry further confirmed abilities of the bioelectrodes to catalyze electrochemical reactions at 5 MPa. Performance of the electromethanogenesis system was not altered under lower pressure conditions (1.2 and 2.5 MPa). An exoelectrogenic bacterium affiliated with the genus Thermincola and a methanogen belonging to the genus Methanothermobacter were detected as the dominant species in the bioanode and biocathode microbiotas, respectively. Thus, our results indicated that electromethanogenesis systems could be developed and operated under highly-pressurized conditions, suggesting that applications of the bio-electrochemical system in high-pressure environments (including high-temperature subsurface reservoirs) can be technically feasible. [ABSTRACT FROM AUTHOR]

The dispersion of granules in upflow anaerobic sludge blanket (UASB) reactor represents a critical technical issue in methanolic wastewater treatment. In this study, the potentials of coupling a microbial electrolysis cell (MEC) into an UASB reactor for improving methanolic wastewater biodegradation, long-term process stability and biomethane recovery were evaluated. The results indicated that coupling a MEC system was capable of improving the overall performance of UASB reactor for methanolic wastewater treatment. The combined system maintained the comparatively higher methane yield and COD removal efficiency over the single UASB process through the entire process, with the methane production at the steady-state conditions approaching 1504.7 ± 92.2 mL-CH 4 L − 1 -reactor d − 1 , around 10.1% higher than the control UASB (i.e. 1366.4 ± 71.0 mL-CH 4 L − 1 -reactor d − 1 ). The further characterizations verified that the input of external power source could stimulate the metabolic activity of microbes and reinforced the EPS secretion. The produced EPS interacted with Fe 2+/3+ liberated during anodic corrosion of iron electrode to create a gel-like three-dimensional [-Fe-EPS-] n matrix, which promoted cell-cell cohesion and maintained the structural integrity of granules. Further observations via SEM and FISH analysis demonstrated that the use of bioelectrochemical stimulation promoted the growth and proliferation of microorganisms, which diversified the degradation routes of methanol, convert the wasted CO 2 into methane and accordingly increased the process stability and methane productivity. [ABSTRACT FROM AUTHOR]

Electromethanogenesis (EM) is a reaction in a bio-electrochemical system (BES) in which microorganisms catalyse the reduction of CO 2 to methane by electric current at the cathode. We have proposed a novel application of EM combined with CO 2 Capture and Storage (CCS), in which geologically-sequestrated CO 2 is reduced to methane using electricity from variable renewables. To develop this technology, further investigation and establishment of the mathematical model are indispensable. In this study, experimental and mathematical analyses were conducted to develop mathematical model. A mathematical model for EM is developed based on BES models constructed in previous studies. As a result of mathematical analysis, it is suggested that EM reaction is dominated by the direct pathway at high cathode potential, and the methane production rate is proportional to the electrical current generated in the system. To calculate methane production rate via the indirect pathway requires measurement of microorganism concentration, and it will enable completion of mathematical model. Moreover, to examine the practical application of EM, a scale-up experiment was conducted in fed-batch mode using a 3 L volume reactor. As electrode material, carbonized coconut shells were chosen due to their large surface area, good biocompatibility, and good chemical stability with relatively low cost. Thermophilic microorganisms originated from a subsurface formation (a depleted petroleum reservoir) were inoculated in the reactor and incubated at 55 °C with applied voltages of 1.0 V or 0.7 V. The maximum CH 4 production rate was 340 mmol m -3 day -1 with current-capture efficiency of >30%. A rises of current generation were observed every time after the medium was exchanged, which indicates the microbial activity is responsible for current generation and CH 4 production. The maximum CH 4 production rates in 1st, 2nd and 3rd cycle were 108, 179 and 340 mmol m -3 day -1 respectively, which shows a growth of performance in microbial catalytic ability. These results suggest that it is feasible to scale up the EM system and, however, further studies are required to commercialize the system as a CCS technology. [ABSTRACT FROM AUTHOR]

• Electromethanogenesis is resilient to power supply interruptions. • Power interruptions with no H 2 supply had a large impact on methanogens. • Power interruptions with no H 2 supply induced large variability. • Power interruptions with H 2 supply limited the metabolism of electrotrophs. • Shifts in microbial populations do not correlate methane production rates. The need to accommodate power fluctuations intrinsic to high-renewable systems will demand in the future the implementation of large quantities of energy storage capacity. Microbial electromethanogenesis (EM) can potentially absorb the excess of renewable energy and store it as CH₄. However, it is still unknown how power fluctuations impact on the performance of EM systems. In this study, power gaps from 24 to 96 h were applied to two 0.5 L EM reactors to assess the effect of power interruptions on current density, methane production and current conversion efficiency. In addition, the cathodes were operated with and without external H₂ supplementation during the power-off periods to analyse how power outages affect the two main metabolic stages of the EM (i.e.: the hydrogenic and methanogenic steps). Methane production rates kept around 1000 mL per m2 of electrode and per day regardless of the duration of the power interruptions and of the supplementation of hydrogen. Interestingly, current density increased in the absence of hydrogen (averaged current density during hydrogen supplementation was 0.36 A·m−2, increasing up to 0.58 A·m−2 without hydrogen). However current was less efficiently used in the production of methane with no hydrogen supplementation. Nevertheless, when the electrical power was restored after the power interruption experiments, performance parameters were similar to those observed before. These results indicate that EM is resilient to power fluctuations, which reinforces the opportunity of using EM as a technology for renewable energy storage. [ABSTRACT FROM AUTHOR]

The use of liquid fraction of pressed municipal solid waste (LPW) for hydrogen production was evaluated via electrohydrogenesis in a single-chamber microbial electrolysis cell (MEC). The highest hydrogen production (0.38 ± 0.09 m 3 m −3 d −1 and 30.94 ± 7.03 mmol g −1 COD added ) was achieved at an applied voltage of 3.0 V and pH 5.5, increasing by 2.17-fold than those done at the same voltage without pH adjustment (pH 7.0). Electrohydrogenesis was accomplished by anodic oxidation of fermentative end-products (i.e. acetate, as well as propionate and butyrate after their acetification), with overall hydrogen recovery of 49.5 ± 11.3% of COD added . These results affirm for the first time that electrohydrogenesis can be a noteworthy alternative for hydrogen recovery from LPW and simultaneous organics removal. Electrohydrogenesis efficiency of this system has potential to improve provided that electron recycling, electromethanogenesis and deposition of non-conductive aggregates on cathode surface, etc. are effectively controlled. [ABSTRACT FROM AUTHOR]

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates. [ABSTRACT FROM AUTHOR]

Renewable energy recovery from submerged aquatic plants such as Egeria densa ( E. densa ) via continuous anaerobic digestion (AD) represents a bottleneck because of process instability. Here, a single-chamber membrane-free microbial electrolysis cell (MEC) equipped with a pair of Ti/RuO 2 mesh electrodes (i.e. the combined MEC-AD system) was implemented at different applied voltages (0–1.0 V) to evaluate the potential effects of bioelectrochemical stimulation on methane production and process stability of E. densa fermentation. The application of MEC effectively stabilized E. densa fermentation and upgraded overall process performance, especially solid matters removal. E. densa AD process was operated steadily throughout bioelectrochemical process without any signs of imbalance. The solubilization-removal of solid matters and methane conversion efficiency gradually increased with increasing applied voltage, with an average methane yield of approximately 248.2 ± 21.0 mL L −1 d −1 at 1.0 V. Whereas, the stability of the process became worse immediately once the external power was removed, with weaken solid matters removal along with methane output, evidencing the favorable and indispensable role in maintaining process stability. The stabilizing effect was further quantitatively demonstrated by statistical analysis using standard deviation (SD), coefficient of variance (CV) and box-plots. The syntrophic and win–win interactions between fermenting bacteria and electroactive bacteria might have contributed to the improved process stability and bioenergy recovery. [ABSTRACT FROM AUTHOR]

• A thorough overview of a microbial electrochemistry with anaerobic digestion is summarized. • Principles, configurations, and influential factors for the emerging technology are concluded. • Electron flows of electroactive bacteria and methanogens in CH 4 yield are disclosed. • Community interactions effectively regulated by electrodes are analyzed. • Microbial ecology is well understood, and more efforts are expected before the field application. Biomethane as a sustainable, alternative, and carbon-neutral renewable energy source to fossil fuels is highly needed to alleviate the global energy crisis and climate change. The conventional anaerobic digestion (AD) process for biomethane production from waste(water) streams has been widely employed while struggling with a low production rate, low biogas qualities, and frequent instability. The electric-biologically hybrid microbial electrochemical anaerobic digestion system (MEC-AD) prospects more stable and robust biomethane generation, which facilitates complex organic substrates degradation and mediates functional microbial populations by giving a small input power (commonly voltages < 1.0 V), mainly enhancing the communication between electroactive microorganisms and (electro)methanogens. Despite numerous bioreactor tests and studies that have been conducted, based on the MEC-AD systems, the integrated microbial fingerprints, and cooperation, accelerating substrate degradation, and biomethane production, have not been fully summarized. Herein, we present a comprehensive review of this novel developing biotechnology, beginning with the principles of MEC-AD. First, we examine the fundamentals, configurations, classifications, and influential factors of the whole system's performances (reactor types, applied voltages, temperatures, conductive materials, etc.,). Second, extracellular electron transfer either between diverse microbes or between microbes and electrodes for enhanced biomethane production are analyzed. Third, we further conclude (electro)methanogenesis, and microbial interactions, and construct ecological networks of microbial consortia in MEC-AD. Finally, future development and perspectives on MEC-AD for biomethane production are proposed. [Display omitted] [ABSTRACT FROM AUTHOR]

Microbial electromethanogenesis, relying on electrochemically active biofilm on biocathode to convert carbon dioxide to methane, provides a novel approach for renewable energy storage. One key factor that governs electron exchange and methane formation efficiencies is the electrode material. To promote methane production, a biocathode via modifying plain carbon stick with a layer of graphite felt (GF) (hereafter referred as “hybrid GF-biocathode”) was developed and evaluated in a two-chamber microbial electrolysis cells (MECs). Methane production with hybrid GF-biocathode reached 80.9 mL/L at the potential of −1.4 V after 24 h of incubation with coulombic efficiency of 194.4%. The tests by flushing three substrates (CO 2 , N 2 and H 2 –CO 2 [80:20]) revealed that direct electron transfer rather than intermediate H 2 contributed more to the electromethanogenesis. Cyclic voltammetry showed that GF enhanced the microbial electrocatalysis activity and reduced the cathode overpotential needed for methane production. Scanning electron microscope and fluorescence in situ hybridization analysis confirmed that the three-dimensional GF afforded the abundant space for the growth of electroactive microorganisms and promoted the electron exchange (e.g. cathode-to-cell etc.) via severing as “artificial pili”. This study reveals that GF with the open structure and high conductivity has the substantial potential to upgrade electromethanogenesis efficiency. [ABSTRACT FROM AUTHOR]

The functional and taxonomic microbial dynamics of duplicate electricity-consuming methanogenic communities were observed over a 6 months period to characterize the reproducibility, stability and recovery of electromethanogenic consortia. The highest rate of methanogenesis was 0.72 mg-CH 4 /L/day, which occurred during the third month of enrichment when multiple methanogenic phylotypes and associated Desulfovibrionaceae phylotypes were present in the electrode-associated microbial community. Results also suggest that electromethanogenic microbial communities are very sensitive to electron donor-limiting open-circuit conditions. A 45 min exposure to open-circuit conditions induced an 87% drop in volumetric methane production rates. Methanogenic performance recovered after 4 months to a maximum value of 0.30 mg-CH 4 /L/day under set potential operation (−700 mV vs Ag/AgCl); however, current consumption and biomass production was variable over time. Long-term functional and taxonomic analyses from experimental replicates provide new knowledge toward understanding how to enrich electromethanogenic communities and operate bioelectrochemical systems for stable and reproducible performance. [ABSTRACT FROM AUTHOR]

To better understand the underlying mechanisms for methane bioelectrosynthesis, a two-chamber MECs containing a carbon biocathode was developed and studied. Methane production substantially increased with increasing cathode potential. Considerable methane yield was achieved at a poised potential of −0.9 V (vs. Ag/AgCl), reaching 2.30 ± 0.34 mL after 5 h of operation with a faradaic efficiency of 24.2 ± 4.7%. Confirmatory tests done at 0.9 V by switching the type of flushed substrates (CO 2 /N 2 ) or the electrical exposure modes (ON/OFF) demonstrated that cathode serving as an electron donor was the vital driving force for methanogenesis occurring at microbe–electrode surface. Fluorescence in situ hybridization reveled Methanobacteriaceae (particularly Methanobacterium ) was the predominant methanogens, supporting the mechanisms of direct electron transfer between cell-electrode. Additionally, the analysis of scanning electron microscope confirmed that the multiple pathways of electron transfer, including direct cathode-to-cell, interspecies exchange and semi-conductive conduits all together ensured the successful electromethanogenesis process. [ABSTRACT FROM AUTHOR]

[Display omitted] Graphene-based biocathodes enhance the kinetics of electrochemical conversion of CO 2 into bio-methane in comparison to conventional biocathodes. It also promotes biomass development. • Graphene oxide electrode modifications improve the methanogenic processes in MES systems. • Methanobacterium genus is a dominant hydrogenotrophic methanogenic Archaea found in biocathodes. • Methane can be produced by means of CO 2 at concentrations above 95%. Microbial electrosynthesis (MES), a sub-branch of bioelectrochemical processes, takes advantage of a certain type of electroactive microorganism to produce added value products (such as methane) from carbon dioxide (CO 2). The aim of this study is to quantify the benefits of using a carbon felt electrode modified with reduced graphene-oxide (rgoCF) as a methanogenic biocathode. The current density generated by the rgoCF was almost 30% higher than in the control carbon felt electrode (CF). In addition, charge transfer and ohmic resistances were, on average, 50% lower in the rgoCF electrode. These improvements were accompanied by a larger presence of bacteria (31% larger) and archaea (18% larger) in the rgoCF electrode. The microbial communities were dominated by hydrogenotrophic methanogenic archaea (Methanobacterium) and, to a lesser extent, by a low-diversity group of bacteria in both biocathodes. Finally, it was estimated that for a CO 2 feeding rate in the range 15–30 g CO 2 per m2 of electrode per day, it is possible to produce a high-quality biogas (>95% methane concentration). [ABSTRACT FROM AUTHOR]

[Display omitted] • Most copper-based cathodes improved the microbial methanogenesis cell performance. • Electroless copper-coated cathodes showed the best CH 4 production and stability. • Copper foil did not improve performance due to its non-biocompatible surface. • Surface hydrophobicity and uniformity of Cu layer affected methane production rate. Copper can be used as a metal catalyst for abiotic CO 2 conversion into methane and organic chemicals, but it has not been examined as a catalyst for enhancing biotic methane generation in microbial methanogenesis cells (MMCs). In this study, copper-based electrodes prepared using several different techniques were compared to the performance of MMCs containing graphite block cathodes. Gas production was examined under both abiotic and biotic conditions at a fixed cathode potential of –0.9 V vs. Ag/AgCl in two-chamber electrochemical cells. All copper-based cathodes showed better methane production than plain graphite blocks except for the cathode made from copper foil which lacked a biocompatible surface. The cathode prepared by an electroless Cu deposition (electroless-Cu) method had the highest current density production of 0.6 A/m2 and methane production rate of 201 nmol/cm3/d, and its performance was stable over time. Both the electroless-Cu and electro-deposited Cu electrodes produced more current than that obtained with copper powders with a Nafion binder (Nafion-Cu), likely due to different surface characteristics such as hydrophobicity and uniformity of the copper layer. The results of this study showed that copper-based biocathodes improved methane production relative to plain graphite materials and techniques for preparing copper electrodes impact bioelectrochemical performance with the highest performance in the electroless-Cu reactors. [ABSTRACT FROM AUTHOR]

Abstract: To establish a biotechnological system to convert CO2 into methane, we are trying to develop a new CO2 bio-conversion technology based on “electromethanogenesis”, a new bio-electrolysis reaction using microbially-catalyzed electrode. In this study, we characterized bio-electrochemical properties of electromethanogenic reaction by Methanothermobacter thermautotrophicus strain AH, a thermophilic methanogen. The methanogen can electromethanogenically produce methane without exogenously-supplied hydrogen. In the reaction, the methanogen utilized molecular hydrogen, which was evolved by the abiotic electrochemical reaction, for the hydrogenotrophic methanogenesis. The current-to-methane conversion efficiency was 20% and the hydrogen evolution reaction was the rate-limiting step of the reaction. [Copyright &y& Elsevier]

Abstract: To develop a technological system to add substantial value to CCS operations, we are proposing a system to employ a new bio-electrochemical reaction, called “electromethanogenesis”, to convert geologically-stored CO2 into methane, a recycled energy source. In this study, we showed that microorganisms derived from a subsurface reservoir were electromethanogenically active. Moreover, the microbial consortium selectively enriched based on electrochemical activity had the highest electromethanogenic activity reported so far. Thus, our study indicated that, for the electromethanogenic conversion of geologically-stored CO2, recruitment of microorganisms endogenous to the reservoir was an effective strategy. [Copyright &y& Elsevier]

Electromethanogenesis is a new bio-electrochemical reaction potentially useful for energy conversion. As a first step toward its technical application, electromethanogenic reactors were built, and their bio-electrochemical properties were analyzed. Comparisons of the microbial compositions of the electromethanogenic cathode and the current-producing anode suggested an electromethanogenic pathway mediated by exoelectrogenic bacteria. [ABSTRACT FROM AUTHOR]

Abstract: Geological storage of carbon dioxide (CO2) as currently conceived is not commercially viable. To promote deployment of CO2 capture and storage (CCS), substantial value must be added to CCS operations. We have proposed a subterranean carbon plantation that involves storing CO2 in a geological reservoir, biologically converting the stored CO2 to methane in situ, and harvesting the biogenic methane as a recycled energy source. To examine the durability of methanogenic metabolism under storage reservoir conditions, the methanogenic activity of Methanothermobacter thermautotrophicus (a representative subsurface methanogen) was assessed under nutrient-limited and reduced-pH conditions in actual formation-water-based media. Moreover, to examine the possibility of electrochemically supplying the source of reducing power into the reservoir, methanogen was also incubated in absence of exogenously supplied molecular hydrogen with applied voltage. Applied-voltage-dependent methanogenesis was observed, suggesting that methanogen can utilize electrons and protons as a reducing-power source to reduce CO2 to methane. Towards practical deployment of the electromethanogenic system to utilize CCS reservoirs as energy-reserving tanks, further studies are required to enhance the bio-electromethanogenic activity and optimize well configurations. [Copyright &y& Elsevier]

• Electromethanogenesis (EM) can convert pure CO 2 into high-purity biogas (∼90% CH 4). • EM performance greatly deteriorates when temperature is reduced from 30 to 15 °C. • This deterioration is attributed to methanogenic rather than hydrogenic activity. • EM can easily recover its previous performance when temperature returns to 30 °C. • Methanogenic rather than hydrogenic activity explains the low replicability of EM. In this study we aimed to understand the impact of medium–low temperatures on the two main steps that usually comprise the electromethanogenesis (EM) process: electrothrophic hydrogenesis and hydrogenothrophic methanogenesis. Results revealed that pure CO 2 could effectively be converted into a high-purity biogas (∼90:10 CH 4 /CO 2) at 30 °C. However, when temperature was reduced to 15 °C, methane richness greatly decreased (∼40:60 CH 4 /CO 2). This deterioration in performance was mostly attributed to a decline in methanogenic activity (represented mainly by Methanobacterium and Methanobrevibacter). In contrast, the hydrogenic activity (mostly Desulfomicrobium) did not suffer any significant decay. Results also seemed to indicate that methanogenesis, rather than hydrogenesis, is the main source of variability in EM. Increasing the temperature again to 30 °C restored previous performance, which highlights the resilience of EM to wide temperature fluctuations (from 30 to 15 and back 30 °C). [ABSTRACT FROM AUTHOR]

• Methanogenesis biocathodes were constructed by simulated unstable renewable energy. • Intermittent step-up voltage enhanced stability and resilience of biocathode. • Compact and extensive biofilm with nanowires interconnected was enriched. • The CH 4 production and the metabolism of biofilm were improved. • Cathodic harsh environment stimulated extracellular polymeric substances secretion. Electromethanogenesis (EMG) system could efficiently convert CO 2 to CH 4 by using excess renewable electricity. However, the fluctuation and interruption of renewable electricity will adversely affect the biocathode and therefore the CH 4 production of the EMG system. In this work, a novel biocathode acclimation strategy with intermittent step-up voltage (ISUV) was proposed to improve the stability and resilience of the EMG system against the unstable input of renewable power. Compared with the intermittent application of constant voltage (IACV), the ISUV increased the rate of CH 4 production by 11.7 times with the improvement of the stability and resilience by 56% and 500%, respectively. Morphology and microflora structure analysis revealed that the biofilm enriched with ISUV exhibited a compact microflora structure with high-density cells and nanowires interconnected. This study provided a novel effective strategy to regulate the biofilm structure and enhance the performance of the EMG system. [ABSTRACT FROM AUTHOR]

The development of an effective biocathode with high catalytic ability and dense biomass is a major challenge for the industrial applications of electromethanogenesis (EM) process. In our previous study, intact anaerobic granular sludge (AnGS) biocathode and EM hybrid system (AnGS-EM) showed superior ability and stability when treating raw biogas, but its maximum CO 2 -to-CH 4 conversion potential and the response to different operating conditions are still unknown. Herein, we optimized the performance of the AnGS-EM system and explored its maximum CH 4 production capacity. The AnGS-EM system achieved a maximum methane production rate of 202.15 L CH 4 /m2cat proj /d, which is over 3 times higher than the maximum value reported so far. Within a certain range, the methane production rate increased with the buffer concentration, applied voltage, and bicarbonate concentration. Excessive applied voltage and carbonate concentration not only led to resource waste but also inhibited methanogen performance. The AnGS biocathode could withstand oxygen exposure for 24 h, the acidic (pH of 5.5), and alkaline conditions (pH over 9). Illumina sequencing results showed that hydrogenotrophic methanogen (especially Methanobacterium) were dominant. This work using AnGS as biocathode for CH 4 synthesis offers insight into the development of scalable, efficient, and cost-effective biocathode for biofuels and value-added chemicals production. ga1 • Intact AnGS was used as biocatalyst to boost CO 2 -to-CH 4 conversion. • System achieved a highest CH 4 production rate (202.15 L/m2/d) reported so far. • The AnGS biocathode presented robustness to O 2 , acidic and alkaline disturbances. • The functional population in this system was Methanobacterium. • This study offers a scalable, efficient and cost-effective biocathode for BESs. [ABSTRACT FROM AUTHOR]

Electromethanogenesis is a promising strategy for bioconversion of CO 2 to CH 4 via direct electron transfer (DET) and/or H 2 -mediated indirect electron transfer (IET). However, specific contribution ratios of DET or H 2 -mediated IET are unclear. Here, pathway preferences were firstly calculated in the "cage" type cathode colonized by Methanosarcina barkeri. The highest CH 4 production rate of 4.4 μmol cm−2•day−1 was detected from 36 to 72 h at −1.2 V. The average ratios of H 2 -mediated IET were calculated as 59.73 ± 8.29%, 69.45 ± 20.75%, 69.55 ± 18.30% and 82.97 ± 16.13% at −0.6, −0.8, −1.0 and −1.2 V (vs SHE), respectively. While above −0.4 V, only the DET pathway was detected. The real time polymerase-chain reaction amplification at the transcriptional level of ccdA and frhB showed the similar trend for the pathway preference in CH 4 production. Results provide specific ratios and preference of CH 4 production via these two pathways, which may be used for parameter reference for industrial applications. Image 1 • New "cage" type cathode was designed to calculate pathway preference. • Ratios of CH 4 via DET are 40–18% at −0.6∼-1.2 V, the remaining are H 2 -mediated IET. • Almost no CH 4 was found via H 2 -mediated IET below −0.4 V. • The expression level of ccdA and frhB gene were assessed by newly designed primers. [ABSTRACT FROM AUTHOR]

• Electrocatalytic activity of thermophilic methanogenic consortium enhanced methane yield. • 54.7% higher methane yield with an applied external voltage. • Improved substrate utilization due to increased electron supply at electrode–electrolyte interface. • Bioelectrosynthesis regulates cell adherence and biofilm formation. This work investigated the electrocatalytic activity of a thermophilic methanogenic consortia (TMC) for developing a bioelectrosynthesis process to convert food and paper wastes to methane. Electroanalytical techniques were used to analyze the electrocatalytic activity of the TMC biofilm formed onto the electrodes. The developed electromethanogenesis process enhanced the yield of methane by 54.7% than control experiments. Scanning electron micrographs of the TMC bioelectrodes showed that the electrosynthesis process accelerates biofilm formation onto the electrodes leading to enhanced direct electron transfer reactions at electrode–electrolyte interface. This study will help in developing a novel approach for valorization of food and paper waste to biofuels. [ABSTRACT FROM AUTHOR]

• Magnetite/zeolite (MZ) was synthesized as novel cathode catalyst in MES. • Electrochemical analyses proved superior catalytic activity of MZ for methanation. • Current generation, substrate and VFAs removal rate were promoted in MES-MZ. • MZ catalyst facilitated electron transfer by lowering cathode overpotential losses. In this study, magnetite/zeolite (MZ) was successfully synthesized to use as a feasible and cost-effective cathode catalyst for enhancing methane generation in a microbial electrochemical system (MES). The novel MZ catalyst consists of hydrophilic zeolite cores and conductive magnetite nanoparticles for enhanced electroactive biofilm development on the cathode by facilitating micro-channels for nutrient diffusion, increased surface area, and reduced charge transfer resistance. The MES using an MZ cathode (MES-MZ) achieved a maximum methane yield of 0.38 ± 0.010 LCH 4 /gCOD, which was significantly higher than that of the control operation without a catalyst (0.33 ± 0.008 LCH 4 /gCOD). The methane production rate was increased by almost 21% from 196 mL/(L.d) in the control MES to 238 mL/(L.d) in the MES-MZ, along with an improvement in the methane percentage from 73% to 79%. In addition, the maximum current generation was recorded using the MES-MZ at 9.29 ± 0.16 mA, which was about 16% higher than that of 8.0 ± 0.13 mA observed in the control reactor and is consistent with about a 36.2% improvement of the Coulombic efficiency. The CV and EIS analyses revealed that MZ lowered the overpotential losses during the electron transfer process, and revealed a more positive cathode potential with the MES-MZ (−0.48 V vs. Ag/AgCl), which possibly suggests direct electron transfer for the dominant pathway for the conversion of carbon dioxide to methane. [ABSTRACT FROM AUTHOR]

• Bioelectrochemical systems can be used as power-to-gas technology for energy storage. • A BES prototype was long-term operated to store electric energy in the form of biomethane. • The prototype produced 4.4 L CH 4 m−2 d−1 with an energy storage efficiency of 42–47%. • Electric behavior of BES prototype was simulated to design its electric converter. • Future research increasing current density demand can lead to positive business case. The development and implementation of energy storage solutions is essential for the sustainability of renewable energy penetration in the electrical system. In this regard, power-to-gas technologies are useful for seasonal, high-capacity energy storage. Bioelectrochemical systems for electromethanogenesis (EMG-BES) represent an additional power-to-gas technology to the existing chemical and biological methanation. EMG-BES process can be retrofitted in traditional anaerobic digesters, with advantages in terms of biologic process stability and high-quality biogas production. Nowadays, there are no reported studies of scaled-up EMG-BES plants for energy storage. The present work describes the setup and operation of a medium-scale EMG-BES prototype for power-to-gas, storing energy in the form of biomethane. The prototype was built by stacking 45 EMG-BES cells, accounting for a total volume of 32 L. It was continuously fed with 10 L day−1 municipal wastewater, and it was long-term operated at different voltage and temperature ranges. A steady-state current density demand of 0.5 A m−2 was achieved at 32 °C while producing 4.4 L CH 4 m−2 d−1 and removing 70% of the initial organic matter present in wastewater. Microbial competition between electro-active bacteria and acetoclastic methanogens was observed. Energy storage efficiency was estimated around 42–47%, analyzing surplus CH 4 production obtained when applying voltage to the stack. A first order electric model was calculated, based on the results of a series of electrical characterization tests. The model may be used in the future to design the converter for EMG-BES plant connection to the electrical grid. The obtained results show that energy storage based on EMG-BES technology is possible, as well as its future potential, mixing renewable power overproduction, biomethane generation and wastewater treatment under the circular economy umbrella. [ABSTRACT FROM AUTHOR]

• Magnetite addition and external voltage were applied to promote DIET in AD process. • Individual and combined effects of the two DIET-promoting strategies were examined. • Both strategies, alone and in combination, promote DIET to enhance AD performance. • Magnetite serves as a conduit for electrical connections between DIET partners. • DIET stimulation via magnetite addition promotes economic and stable high-rate AD. Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions. [ABSTRACT FROM AUTHOR]

• Methanogenesis in MES was significantly enhanced with magnetite nanoparticles. • Magnetite nanoparticles promoted current generation, substrate and VFAs removal. • Overpotential losses of MES was decreased with magnetite nanoparticles addition. • Magnetite may act as electrical conduit for extracellular electron transfer. Performance of methane-producing microbial electrochemical systems (MESs) is highly reliant on electron transfer efficiency from electrode to microorganisms and vice versa. In this study, magnetite nanoparticles were used as electron carriers to enhance extracellular electron transfer in single chamber MESs. The MES with magnetite exhibited the highest methane yield and current generation of 0.37 ± 0.009 L CH4 /g COD and 9.6 mA, respectively among the tested reactors. The experimental data was observed to be highly consistent with modified Gompertz model results (R2 > 0.99), which also showed 74.2% and 22.1% enhanced methane production rate in MES with magnetite as compared to control AD and MES without magnetite, respectively. Cyclic voltammetry and electrochemical impedance spectroscopy analysis confirmed that magnetite enhanced catalytic activity of biofilm and lowered both solution and charge transfer resistance. Therefore, supplementing magnetite in MESs could be a strategy to develop an efficient syntrophic biomethanation in field scale applications. [ABSTRACT FROM AUTHOR]

• Bioelectrochemical methane upgrading was achieved via DET. • Biogas with >90% methane content was generated when −500 mV potential was applied. • 8.2% of the CO 2 was converted to methane under the applied potential condition. • Species from the genera Methanothrix and Azonexus dominated the cathode community. • Genes for DIET-based growth were highly transcribed by Methanothrix on cathode. Bioelectrochemical conversion of CO 2 to CH 4 is a promising way to increase the calorific value of biogas produced during anaerobic digestion. There are two groups of methanogens enriched in these systems, hydrogenotrophs and acetoclastic methanogens that can also directly accept electrons from an electrode or another microorganism. In this study, a microbial electrolysis cell (MEC) poised at −500 mV (vs. SHE) was operated for biogas upgrading. Methane content in the biogas increased from 71% to >90%, and 8.2% of the CO 2 was converted to methane. Methanothrix , an acetoclastic methanogen that can participate in direct electron transfer (DET), and Azonexus , an acetate-oxidizing electrogen, were enriched on the cathode. Transcriptomics revealed that Methanothrix on the cathode were using the CO 2 reduction pathway, while Methanothrix in the bulk sludge were using the acetate decarboxylation pathway for production of methane. These results show that stimulation of DET in MEC enhances biogas-upgrading processes. [ABSTRACT FROM AUTHOR]