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Wastewater treatment plants have two persistent financial and energetic drains, the carbon dioxide content of biogas, which limits its commercial sale, and the presence of trace organics in the wastewater effluent, which damages the aquatic ecosystem, like the Great Barrier Reef. Biogas is a renewable methane resource that is underutilized due to the variable CO2 content (~40%). Biogas is energy intensive to purify and limited by the economy of scale (> 8.85 GJ/h) to large‐scale purification methods, thus small‐scale processes require development. Electrocatalytic microbes native to wastewater have been shown to convert CO2 to CH4 and acetate, however complete conversion of the CO2 content to CH4 is energy intensive. Here we show a low power bioelectrochemical fuel cell design to purify biogas to pipeline quality methane (98%), manufacture methane and/or acetate, and remove trace organics, using HCO3− as the transport charge carrier from dissolved CO2 from the biogas through an anion exchange membrane. This decreased the power required to separate CO2 from methane in biogas on a molar basis, resulting in a net energy recovery similar to current industrial systems. Magnesium anode use resulted in an energy positive system. Tests evaluated the influence of cathode potential on the current density, HCO3− ion flux and the rates and efficiencies of methane production, resulting in optimization at −0.7 V versus standard hydrogen electrode (SHE). A techno‐economic analysis modeled a positive return on investment for scaled‐up production to purify small biogas streams that are otherwise financially unrecoverable. Carbon sequestration by production of methane, acetate and solid fertilizers demonstrated profitable and energy efficient waste‐to‐resource conversion. Methane is manufactured from carbon dioxide simultaneous to biogas purification for carbon neutral energy production in a biological fuel cell during the final process step of wastewater treatment. Lab tests demonstrate proof of concept during wastewater treatment. The techno‐economic analysis (financial and manufacturing model) demonstrates industrial scale operation can make a profit at manufacturing renewable methane and/or acetate. [ABSTRACT FROM AUTHOR]

Industrial applications of microbial electrochemical systems will require regular maintenance shutdowns, involving inspections and component replacements to extend the lifespan of the system. Here, we examined the impact of such shutdowns on the performance of three electrode materials (i. e., platinized titanium, graphite, and nickel) as cathodes in a microbial electrochemical system that would be used for electromethanogenesis in power‐to‐gas applications. We focused on methane (CH4) production from hydrogen (H2) and carbon dioxide (CO2) using Methanothermobacter thermautotrophicus. We showed that the platinized titanium cathode resulted in high volumetric CH4 production rates and Coulombic efficiencies. Using a graphite cathode would be more cost‐effective than using the platinized titanium cathode in microbial electrochemical systems, but showed an inferior performance. The microbial electrochemical system with the nickel cathode showed improvements compared to the graphite cathode. Additionally, this system with a nickel cathode demonstrated the fastest recovery during a shutdown experiment compared to the other two cathodes. Fluctuations in pH and nickel concentrations in the catholyte during power interruptions affected CH4 production recovery in the system with the nickel cathode. This research enhances understanding of the integration of biological and electrochemical processes in microbial electrochemical systems, providing insights into electrode selection and operating strategies for effective and sustainable CH4 production. [ABSTRACT FROM AUTHOR]

Abstract Industrial applications of microbial electrochemical systems will require regular maintenance shutdowns, involving inspections and component replacements to extend the lifespan of the system. Here, we examined the impact of such shutdowns on the performance of three electrode materials (i. e., platinized titanium, graphite, and nickel) as cathodes in a microbial electrochemical system that would be used for electromethanogenesis in power‐to‐gas applications. We focused on methane (CH4) production from hydrogen (H2) and carbon dioxide (CO2) using Methanothermobacter thermautotrophicus. We showed that the platinized titanium cathode resulted in high volumetric CH4 production rates and Coulombic efficiencies. Using a graphite cathode would be more cost‐effective than using the platinized titanium cathode in microbial electrochemical systems, but showed an inferior performance. The microbial electrochemical system with the nickel cathode showed improvements compared to the graphite cathode. Additionally, this system with a nickel cathode demonstrated the fastest recovery during a shutdown experiment compared to the other two cathodes. Fluctuations in pH and nickel concentrations in the catholyte during power interruptions affected CH4 production recovery in the system with the nickel cathode. This research enhances understanding of the integration of biological and electrochemical processes in microbial electrochemical systems, providing insights into electrode selection and operating strategies for effective and sustainable CH4 production.

Biogas has long been used as a household cooking fuel in many tropical counties, and it has the potential to be a significant energy source beyond household cooking fuel. In this study, we describe the use of low electrical energy input in an anaerobic digestion process using a microbial electrochemical cell (MEC) to promote methane content in biogas at 18, 28, and 37 °C. Although the maximum amount of biogas production was at 37 °C (25 cm3), biogas could be effectively produced at lower temperatures, i.e., 18 (13 cm3) and 28 °C (19 cm3), with an external 2 V power input. The biogas production of 13 cm3 obtained at 18 °C was ~65-fold higher than the biogas produced without an external power supply (0.2 cm3). This was further enhanced by 23% using carbon-nanotubes-treated (CNT) graphite electrodes. This suggests that the MEC can be operated at as low as 18 °C and still produce significant amounts of biogas. The share of CH4 in biogas produced in the controls was 30%, whereas the biogas produced in an MEC had 80% CH4. The MEC effectively reduced COD to 42%, whereas it consumed 98% of reducing sugars. Accordingly, it is a suitable method for waste/manure treatment. Molecular characterization using 16s rRNA sequencing confirmed the presence of methanogenic bacteria, viz., Serratia liquefaciens and Zoballella taiwanensis, in the inoculum used for the fermentation. Consistent with recent studies, we believe that electromethanogenesis will play a significant role in the production of value-added products and improve the management of waste by converting it to energy.

Biogas has long been used as a household cooking fuel in many tropical counties, and it has the potential to be a significant energy source beyond household cooking fuel. In this study, we describe the use of low electrical energy input in an anaerobic digestion process using a microbial electrochemical cell (MEC) to promote methane content in biogas at 18, 28, and 37 °C. Although the maximum amount of biogas production was at 37 °C (25 cm3), biogas could be effectively produced at lower temperatures, i.e., 18 (13 cm3) and 28 °C (19 cm3), with an external 2 V power input. The biogas production of 13 cm3 obtained at 18 °C was ~65-fold higher than the biogas produced without an external power supply (0.2 cm3). This was further enhanced by 23% using carbon-nanotubes-treated (CNT) graphite electrodes. This suggests that the MEC can be operated at as low as 18 °C and still produce significant amounts of biogas. The share of CH4 in biogas produced in the controls was 30%, whereas the biogas produced in an MEC had 80% CH4. The MEC effectively reduced COD to 42%, whereas it consumed 98% of reducing sugars. Accordingly, it is a suitable method for waste/manure treatment. Molecular characterization using 16s rRNA sequencing confirmed the presence of methanogenic bacteria, viz., Serratia liquefaciens and Zoballella taiwanensis, in the inoculum used for the fermentation. Consistent with recent studies, we believe that electromethanogenesis will play a significant role in the production of value-added products and improve the management of waste by converting it to energy. [ABSTRACT FROM AUTHOR]

Electromethanogenesis is a process of microbial electrosynthesis (MES) in which electroactive microorganisms reduce carbon dioxide (CO2) to produce methane (CH4), using a cathode as an electron donor. The efficiency of this reaction is greatly determined by the establishment of a robust microbial community on the biocathodes, which eventually affects the global performance of the bioreactor. Moreover, the development of the biofilm depends on several characteristics of the electrodes, more specifically their material and geometry. Since electrode geometry is a crucial parameter, this study aims at evaluating the sole influence of the electrode shape by installing carbon‐based electrodes with two different constructions (brush and carbon felt) of biocathodes in an electromethanogenic reactor for CO2 capture. The overall performance of the reactors showed coulombic efficiencies around 100%, with high‐quality biogas reaching methane concentrations above 90%. The results reveal that the electrode geometry affects the individual biocathode performance, and the carbon brush showed a bigger contribution to current generation and electrical capacitance, exhibiting higher peak hydrogen production compared to the carbon felt, which could be reflected in higher CO2 capture and methane generation. Both geometries showed a greater proliferation of archaea over bacteria (between 53 and 85%), which was more significant on the brush than on the carbon felt. Archaea community was dominated by Methanobacterium in carbon felt electrodes and codominated with Methanobrevibacter in brush electrodes, while bacteria analyses showed a very similar community for both geometries. © 2022 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

[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]

Electromethanogenesis is an interesting next-generation technology to produce methane from CO2 and electricity by using methanogens. Iron-corroding methanogens might be of special interest for that application due to their natural ability for electron uptake. Methanococcus maripaludis Mic1c10 and KA1 were tested in bioelectrochemical systems. Strain Mic1c10 showed a 120% higher current density and an 84% higher methane production rate (16.2 mmol m−2 d−2) than the non-corrosive strain Methanococcus maripaludis S2, which was identified earlier as the best methane producer under the same experimental conditions. Interestingly, strain KA1 also showed a 265% higher current density than strain S2. Deposits at the cathodes were detected and analyzed, which were not described earlier. A comparative genome analysis between the corrosive methanogen and the S2 strain enables new insights into proteins that are involved in enhanced electron transfer. [ABSTRACT FROM AUTHOR]

Bioelectrochemically improved anaerobic digestion (AD-BES) represents an upgrading strategy for existing biogas plants, consisting of the integration of bioelectrodes within the AD reactor. For this study, a series of laboratory-scale AD-BES reactors were operated, valorising agricultural digestates through the production of biogas. The reactors were inoculated and started-up with three different digestates, leading to significant differences in the microbial community developed on the bioelectrodes. After the start-up was completed, the AD-BES were all fed with a unique digestate, to evaluate the stability of the bioelectrodes' biofilm performances against variations of the organic feedstock. In terms of methane (CH 4 ) production rate, the presence of bioelectrodes allowed between 25 and 82% improvement, compared with control AD reactors. The application of an optimal voltage of 0.3 V resulted in an additional 40% improvement in CH 4 production rate, but only when the biofilm was previously acclimated to the fed digestate. Comprehensive microbial characterization revealed that fed digestate significantly influences the composition and homogenization of microbial communities within AD-BES reactors, with applied voltage showing only a secondary effect. Even when reactors were transitioned to a uniform digestate feeding, resulting in closely similar microbial profiles, variations in CH 4 production persisted, underscoring the lasting impact of initial microbial conditioning. A critical observation was the differentiation in archaeal colonization on bioelectrodes at 0.3 V, the voltage yielding the highest CH 4 conversion. These insights suggest that while the microbial community structure depends on fed digestate, operational efficiency and methanogenic potential are intricately linked to both initial microbial establishment and the specific electrochemical conditions applied to AD-BES reactors., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Anaerobic digestion faces numerous challenges, including high CO 2 content in biogas and volatile fatty acids (such as propionate) accumulation in digestate. To address these issues, an up-flow dual-chamber electrocatalytic anaerobic bioreactor (UF-DC-EAB) was developed to enhance propionate degradation through microbial symbiosis while improving biogas quality via CO 2 electromethanogenesis. Under the extreme conditions with propionate as the primary carbon source at 6-h HRT, the UF-DC-EAB achieved a propionate removal efficiency of 72.1 ± 9.4 % and a faradaic efficiency of 25.5 ± 5.1 %. Microbial community analysis revealed an enrichment of acetoclastic methanogens (Methanosarcinales, 5.4 %) and syntrophic propionate-oxidizing bacteria (Syntrophobacterales, 13.9 %) in the anode, which facilitated propionate degradation. In the cathode, hydrogenotrophic methanogens (Methanobacterium, 13.6 %) and electroactive bacteria (Geobacter, 6.2 %) were predominant, further promoting CO 2 electromethanogenesis and biogas upgrading. Co-occurrence network and structural equation modeling indicated that the electrocatalytic regulation roused the intrinsic capability of the microbial community to oxidize propionate and provoked the occurrence of direct interspecies electron transfer (DIET) among the enriched functional microorganisms, by regulating the synthesis of key molecules like F 420 and cytochrome c in response to propionate-induced changes. The DIET-mediated syntropy increased the net energy output by 212.5 %. This study presents a novel electrochemical system combining CO 2 electromethanogenesis with propionate-rich digestate degradation, offering an efficient approach for anaerobic post-treatment., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier Ltd.)

Methanogenic archaea play an important role in the global carbon cycle, as they catalyze the final step in anaerobic biomass decomposition to generate methane. This physiological trait has attracted much attention for the production of biogenic methane with methanogenic archaea as an alternative to fossil natural gas. Considerable progress has been made on the bioprocessing aspects for this purpose, and bioelectrochemical systems are considered to further optimize cost-effectiveness. Genetic tools for mesophilic methanogenic archaea are long available; however, more sophisticated methodology is still in its infancy. Thus, there is a requirement to develop further genetic tools. Moreover, biotechnologically relevant species are becoming genetically accessible only recently. However, the production of value-added products, such as isoprene, with methanogenic archaea has been demonstrated at low levels. In this perspective article, we discuss some of the recent developments on bioprocessing and genetic engineering strategies and provide a brief perspective on the biotechnological prospects of methanogenic archaea.

Carbon capture and utilization has been proposed as one strategy to combat global warming. Microbial electrolysis cells (MECs) combine the biological conversion of carbon dioxide (CO2) with the formation of valuable products such as methane. This study was motivated by the surprising gap in current knowledge about the utilization of real exhaust gas as a CO2 source for methane production in a fully biocatalyzed MEC. Therefore, two steel mill off-gases differing in composition were tested in a two-chamber MEC, consisting of an organic substrate-oxidizing bioanode and a methane-producing biocathode, by applying a constant anode potential. The methane production rate in the MEC decreased immediately when steel mill off-gas was tested, which likely inhibited anaerobic methanogens in the presence of oxygen. However, methanogenesis was still ongoing even though at lower methane production rates than with pure CO2. Subsequently, pure CO2 was studied for methanation, and the cathodic biofilm successfully recovered from inhibition reaching a methane production rate of 10.8 L m−2d−1. Metagenomic analysis revealed Geobacter as the dominant genus forming the anodic organic substrate-oxidizing biofilms, whereas Methanobacterium was most abundant at the cathodic methane-producing biofilms.

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]

BACKGROUND Biogas can be upgraded to biomethane, which can be used as vehicle fuel and natural gas substitute. Bioelectrochemical biogas upgrade is an innovative alternative to energy‐consuming physicochemical processes and bio‐upgrade methods which require H2 supply. Bioelectrochemical biogas upgrade is conducted by methanogenic microorganisms that convert CO2 into CH4, in the biocathode of a bioelectrochemical system (BES), using electric current as energy source. The aim of the present work was to study the efficiency of an H‐type BES in the conversion of CO2 into CH4, by applying different potentials at the electromethanogenic biocathode. RESULTS: The H‐type BES was operated in a three‐electrode configuration (working: graphite rod; counter: Pt/Ti; reference: Ag/AgCl) with a potentiostat, which set the biocathode's potential initially at −0.7 V versus a standard hydrogen electrode (SHE) and monitored the current demand. Based on cyclic voltammetry runs, a highly electroactive methanogenic biocathode was developed in a short time. The methane production rate (MPR) at a cathode potential of −0.7 V versus SHE was 31.1 mmol m−2 d−1, with an electron capture efficiency of 77.6%. The efficiency of the BES was reduced by applying a potential of −0.5 V versus SHE at the biocathode, resulting in negligible CH4 production. The BES achieved its maximum performance at a potential of −0.9 V versus SHE with a MPR of 53.8 mmol m−2 d−1 and an electron capture efficiency of 86%. The CO2 consumption rate achieved was 0.8 mmol d−1. CONCLUSIONS: The H‐type BES achieved an effective biolectrochemically driven methane production, while the biocathode electroactive behavior was evaluated during the whole operation of the system. © 2021 Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

The ability of microorganisms to carry out interspecies electron transfer during the degradation of organic substances under anaerobic conditions opens up new possibilities for a controlled increase in the efficiency of the methanogenic decomposition of organic waste. This review presents the main principles of the effects of a direct electric current on the anaerobic degradation of organic substances, process parameters, changes in the composition of the microbial community, and factors affecting the optimization of the hybrid systems comprising microbial electrolysis cell (MEC) and anaerobic digester (AD), i.e., the performance of the MEC-AD system. The research in this field has been analyzed for the subsequent application of electromethanogenesis, which represents a new energy-efficient biotechnology for anaerobic wastewater treatment and organic waste digestion. [ABSTRACT FROM AUTHOR]

Anaerobic digestion is a traditional method of producing methane-containing biogas by utilizing the methanogenic conversion of organic matter like agricultural waste and animal excreta. Recently, the application of microbial electrolysis cell (MECs) technology to a traditional anaerobic digestion system has been extensively studied to find new opportunities in increasing wastewater treatability and methane yield and producing valuable chemicals. The finding that both anodic and cathodic bacteria can synthesize methane has led to the efforts of optimizing multiple aspects like microbial species, formation of biofilms, substrate sources and electrode surface for higher production of the combustible compound. MECs are very fascinating because of its ability to uptake a wide variety of raw materials including untreated wastewater (and its microbial content as biocatalysts). Extensive work in this field has established different systems of MECs for hydrogen production and biodegradation of organic compounds. This review is dedicated to explaining the operating principles and mechanism of the MECs for electromethanogenesis using different biochemical pathways. Emphasis on single- and double-chambered MECs along with reactor components is provided for a comprehensive description of the technology. Methane production using hydrogen evolution reaction and nanocatalysts has also been discussed. [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 is an interesting next-generation technology to produce methane from CO2 and electricity by using methanogens. Iron-corroding methanogens might be of special interest for that application due to their natural ability for electron uptake. Methanococcus maripaludis Mic1c10 and KA1 were tested in bioelectrochemical systems. Strain Mic1c10 showed a 120% higher current density and an 84% higher methane production rate (16.2 mmol m−2 d−2) than the non-corrosive strain Methanococcus maripaludis S2, which was identified earlier as the best methane producer under the same experimental conditions. Interestingly, strain KA1 also showed a 265% higher current density than strain S2. Deposits at the cathodes were detected and analyzed, which were not described earlier. A comparative genome analysis between the corrosive methanogen and the S2 strain enables new insights into proteins that are involved in enhanced electron transfer.

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]

• 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]

• 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]

[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]

Carbon capture and utilization has been proposed as one strategy to combat global warming. Microbial electrolysis cells (MECs) combine the biological conversion of carbon dioxide (CO 2 ) with the formation of valuable products such as methane. This study was motivated by the surprising gap in current knowledge about the utilization of real exhaust gas as a CO 2 source for methane production in a fully biocatalyzed MEC. Therefore, two steel mill off-gases differing in composition were tested in a two-chamber MEC, consisting of an organic substrate-oxidizing bioanode and a methane-producing biocathode, by applying a constant anode potential. The methane production rate in the MEC decreased immediately when steel mill off-gas was tested, which likely inhibited anaerobic methanogens in the presence of oxygen. However, methanogenesis was still ongoing even though at lower methane production rates than with pure CO 2 . Subsequently, pure CO 2 was studied for methanation, and the cathodic biofilm successfully recovered from inhibition reaching a methane production rate of 10.8 L m -2 d -1 . Metagenomic analysis revealed Geobacter as the dominant genus forming the anodic organic substrate-oxidizing biofilms, whereas Methanobacterium was most abundant at the cathodic methane-producing biofilms., Competing Interests: Authors SS, ASC, ST, and MH were employed by K1-MET GmbH. Author NK was employed by voestalpine Stahl GmbH. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Spiess, Sasiain Conde, Kucera, Novak, Thallner, Kieberger, Guebitz and Haberbauer.)

• 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]