The presence of hydrogenotrophic methanogens in the inoculum improves methane gas production in microbial electrolysis cells

Two-Stage and One-Stage Anaerobic Co-digestion of Vinasse and Spent Brewer Yeast Cells for Biohydrogen and Methane Production

This study aimed to evaluate the two-stage and one-stage anaerobic co-digestion of vinasse and spent brewer yeast cells (SBY) for biohydrogen and methane production. Optimization of the vinasse-to-SBY ratio and fly ash concentration of the two-stage and one-stage production processes was investigated. In the two-stage process, the vinasse-to-SBY ratio and fly ash concentration were optimized, and the leftover effluent was used for methane production. The optimum conditions for biohydrogen production were a vinasse-to-SBY ratio of 7:3% v/w and fly ash concentration of 0.4% w/v, in which the maximum hydrogen yield was 43.7 ml-H2/g-VSadded. In contrast, a vinasse-to-SBY ratio of 10:0% v/w and fly ash concentration of 0.2% w/v were considered optimal for methane production, and resulted in a maximum methane yield of 214.6 ml-CH4/g-VSadded. For the one-stage process, a vinasse-to-SBY ratio of 10:0% v/w and fly ash concentration of 0.1% w/v were considered optimal, and resulted in a maximum methane yield of 243.6 ml-CH4/g-VSadded. In the two-stage process, the energy yield from hydrogen (0.05-0.47 kJ/g-VSadded) was 0.62%-11.78%, and the major fraction was approximately 88.22%-99.38% gain from methane (3.19-7.73 kJ/g-VSadded). For the one-stage process, the total energy yield distribution ranged from 4.20 to 8.77 kJ/g-VSadded.

Microbiome Diversity of Anaerobic Digesters Is Enhanced by Microaeration and Low Frequency Sound

Biogas is produced by a consortium of bacteria and archaea. We studied how the microbiome of poultry litter digestate was affected by time and treatments that enhanced biogas production. The microbiome was analyzed at six, 23, and 42 weeks of incubation. Starting at week seven, the digesters underwent four treatments: control, microaeration with 6 mL air L-1 digestate per day, treatment with a 1000 Hz sine wave, or treatment with the sound wave and microaeration. Both microaeration and sound enhanced biogas production relative to the control, while their combination was not as effective as microaeration alone. At week six, over 80% of the microbiome of the four digesters was composed of the three phyla Actinobacteria, Proteobacteria, and Firmicutes, with less than 10% Euryarchaeota and Bacteroidetes. At week 23, the digester microbiomes were more diverse with the phyla Spirochaetes, Synergistetes, and Verrucomicrobia increasing in proportion and the abundance of Actinobacteria decreasing. At week 42, Firmicutes, Bacteroidetes, Euryarchaeota, and Actinobacteria were the most dominant phyla, comprising 27.8%, 21.4%, 17.6%, and 12.3% of the microbiome. Other than the relative proportions of Firmicutes being increased and proportions of Bacteroidetes being decreased by the treatments, no systematic shifts in the microbiomes were observed due to treatment. Rather, microbial diversity was enhanced relative to the control. Given that both air and sound treatment increased biogas production, it is likely that they improved poultry litter breakdown to promote microbial growth.

Start-up strategies of electromethanogenic reactors for methane production from cattle manure

This study qualitatively assessed the impacts of different start-up strategies on the performance of methane (CH₄) 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 CH₄ production during the test period, when reactors were fed with CM. Higher or similar CH₄ 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 CH₄ 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.

Characterization of Enrichment Cultures of Anammox, Nitrifying and Denitrifying Bacteria Obtained from a Cold, Heavily Nitrogen-Polluted Aquifer

Anammox bacteria related to Candidatus Scalindua were recently discovered in a cold (7.5 °C) aquifer near sludge repositories containing solid wastes of uranium and processed polymetallic concentrate. Groundwater has a very high level of nitrate and ammonia pollution (up to 10 and 0.5 g/L, respectively) and a very low content of organic carbon (2.5 mg/L). To assess the potential for bioremediation of polluted groundwater in situ, enrichment cultures of anammox, nitrifying, and denitrifying bacteria were obtained and analyzed. Fed-batch enrichment of anammox bacteria was not successful. Stable removal of ammonium and nitrite (up to 100%) was achieved in a continuous-flow reactor packed with a nonwoven fabric at 15 °C, and enrichment in anammox bacteria was confirmed by FISH and qPCR assays. The relatively low total N removal efficiency (up to 55%) was due to nonstoichiometric nitrate buildup. This phenomenon can be explained by a shift in the metabolism of anammox bacteria towards the production of more nitrates and less N₂ at low temperatures compared to the canonical stoichiometry. In addition, the too high an estimate of specific anammox activity suggests that N cycle microbial groups other than anammox bacteria may have contributed significantly to N removal. Stable nitrite production was observed in the denitrifying enrichment culture, while no "conventional" nitrifiers were found in the corresponding enrichment cultures. Xanthomonadaceae was a common taxon for all microbial communities, indicating its exclusive role in this ecosystem. This study opens up new knowledge about the metabolic capabilities of N cycle bacteria and potential approaches for sustainable bioremediation of heavily N-polluted cold ecosystems.

Microbial Electrosynthesis Inoculated with Anaerobic Granular Sludge and Carbon Cloth Electrodes Functionalized with Copper Nanoparticles for Conversion of CO2 to CH4

Microbial electrosynthesis (MES) can sustainably convert CO₂ to products and significant research is currently being conducted towards this end, mainly in laboratory-scale studies. The high-cost ion exchange membrane, however, is one of the main reasons hindering the industrialization of MES. This study investigates the conversion of CO₂ (as a sole external carbon source) to CH₄ using membraneless MES inoculated with anaerobic granular sludge. Three types of electrodes were tested: carbon cloth (CC) and CC functionalized with Cu NPs, where Cu NPs were deposited for 15 and 45 min, respectively. During the MES experiment, which lasted for 144 days (six cycles), methane was consistently higher in the serum bottles with CC electrodes and applied voltage. The highest CH₄ (around 46%) was found in the second cycle after 16 days. The system’s performance declined during the following cycles; nevertheless, the CH₄ composition was twice as high compared to the serum bottles without voltage. The MES with Cu NPs functionalized CC electrodes had a higher performance than the MES with plain CC electrodes. Microbial profile analysis showed that the Methanobacterium was the most dominant genus in all samples and it was found in higher abundance on the cathodes, followed by the anodes, and then in the suspended biomass. The genus Geobacter was identified only on the anodes regarding relative bacterial abundance at around 6–10%. Desulfovibrio was the most dominant genus in the cathodes; however, its relative abundance was significantly higher for the cathodes with Cu NPs.

High-rate microbial electrosynthesis using a zero-gap flow cell and vapor-fed anode design

Microbial electrosynthesis (MES) cells use renewable energy to convert carbon dioxide into valuable chemical products such as methane and acetate, but chemical production rates are low and pH changes can adversely impact biocathodes. To overcome these limitations, an MES reactor was designed with a zero-gap electrode configuration with a cation exchange membrane (CEM) to achieve a low internal resistance, and a vapor-fed electrode to minimize pH changes. Liquid catholyte was pumped through a carbon felt cathode inoculated with anaerobic digester sludge, with humidified N₂ gas flowing over the abiotic anode (Ti or C with a Pt catalyst) to drive water splitting. The ohmic resistance was 2.4 ± 0.5 mΩ m², substantially lower than previous bioelectrochemical systems (20-25 mΩ m²), and the catholyte pH remained near-neutral (6.6-7.2). The MES produced a high methane production rate of 2.9 ± 1.2 L/L-d (748 mmol/m²-d, 17.4 A/m²; Ti/Pt anode) at a relatively low applied voltage of 3.1 V. In addition, acetate was produced at a rate of 940 ± 250 mmol/m²-d with 180 ± 30 mmol/m²-d for propionate. The biocathode microbial community was dominated by the methanogens of the genus Methanobrevibacter, and the acetogen of the genus Clostridium sensu stricto 1. These results demonstrate the utility of this zero-gap cell and vapor-fed anode design for increasing rates of methane and chemical production in MES.

Coupling of bioelectrochemical toluene oxidation and trichloroethene reductive dechlorination for single-stage treatment of groundwater containing multiple contaminants

Bioremediation of groundwater contaminated by a mixture of aromatic hydrocarbons and chlorinated solvents is typically challenged because these contaminants are degraded via distinctive oxidative and reductive pathways, thus requiring different amendments and redox conditions. Here, we provided the proof-of-concept of a single-stage treatment of synthetic groundwater containing toluene and trichloroethene (TCE) in a tubular bioelectrochemical reactor, known as a "bioelectric well". Toluene was degraded by a microbial bioanode (up to 150 μmol L^-1 d^-1) with a polarized graphite anode (+0.2 V vs. SHE) serving as the terminal electron acceptor. The electric current deriving from microbially-driven toluene oxidation resulted in (abiotic) hydrogen production (at a stainless-steel cathode), which sustained the reductive dechlorination of TCE to less-chlorinated intermediates (i.e., cis-DCE, VC, and ETH), at a maximum rate of 500 μeq L^-1 d^-1, in the bulk of the reactor. A phylogenetic and functional gene-based analysis of the "bioelectric well" confirmed the establishment of a microbiome harboring the metabolic potential for anaerobic toluene oxidation and TCE reductive dechlorination. However, Toluene degradation and current generation were found to be rate-limited by external mass transport phenomena, thus indicating the existing potential for further process optimization.

Microbial electrosynthesis of methane and acetate—comparison of pure and mixed cultures

Abstract

The electrochemical process of microbial electrosynthesis (MES) is used to drive the metabolism of electroactive microorganisms for the production of valuable chemicals and fuels. MES combines the advantages of electrochemistry, engineering, and microbiology and offers alternative production processes based on renewable raw materials and regenerative energies. In addition to the reactor concept and electrode design, the biocatalysts used have a significant influence on the performance of MES. Thus, pure and mixed cultures can be used as biocatalysts. By using mixed cultures, interactions between organisms, such as the direct interspecies electron transfer (DIET) or syntrophic interactions, influence the performance in terms of productivity and the product range of MES. This review focuses on the comparison of pure and mixed cultures in microbial electrosynthesis. The performance indicators, such as productivities and coulombic efficiencies (CEs), for both procedural methods are discussed. Typical products in MES are methane and acetate, therefore these processes are the focus of this review. In general, most studies used mixed cultures as biocatalyst, as more advanced performance of mixed cultures has been seen for both products. When comparing pure and mixed cultures in equivalent experimental setups a 3-fold higher methane and a nearly 2-fold higher acetate production rate can be achieved in mixed cultures. However, studies of pure culture MES for methane production have shown some improvement through reactor optimization and operational mode reaching similar performance indicators as mixed culture MES. Overall, the review gives an overview of the advantages and disadvantages of using pure or mixed cultures in MES.

Key points

• Undefined mixed cultures dominate as inoculums for the MES of methane and acetate, which comprise a high potential of improvement

• Under similar conditions, mixed cultures outperform pure cultures in MES

• Understanding the role of single species in mixed culture MES is essential for future industrial applications

Microbial electrolysis cell coupled with anaerobic granular sludge: A novel technology for real bilge water treatment

In the current study, treatment of undiluted real bilge water (BW) and the production of methane was examined for the first time using a membraneless single chamber Microbial Electrolysis Cell (MEC) with Anaerobic Granular Sludge (AGS) for its biodegradation. Initially, Anaerobic Toxicity Assays (ATAs) were used to evaluate the effect of undiluted real BW on the methanogenic activity of AGS. According to the results, BW shown higher impact to acetoclastics compared to hydrogenotrophic methanogens which proved to be more tolerant. However, dilution of BW caused lower inhibition allowing BW biodegradation. Maximum methane production (142.2 ± 4.8 mL) was observed at 50% of BW. Operation of MEC coupled with AGS, seemed to be very promising technology for BW treatment. During 80 days of operation in increasing levels of BW, R2 (1 V) reactor resulted in better performance than AGS alone. Exposure of AGS to gradual increase of BW content revealed that CH₄ production was possible and reached 51% in five days even after feeding with 90% of BW using simple commercial iron electrodes. Successful chemical oxygen demand (sCOD) removal (up to 70%) was observed after gradual biomass acclimatization. Among the different monitored volatile fatty acids (VFAs), acetic and valeric acids were the most frequently detected compounds with concentrations up to 2.79 and 1.81 g L^-1, respectively. The recalcitrant nature of BW did not allow the MEC-AD (anaerobic digester) to balance the consumed energy. Microbial profile analysis confirmed the existence of several methanogenic microorganisms of which Desulfovibrio and Methanobacterium presented significantly higher abundance in the cathodes compared to anodes and AGS.

Methanogenesis from Mineral Carbonates, a Potential Indicator for Life on Mars

Priorities for the exploration of Mars involve the identification and observation of biosignatures that indicate the existence of life on the planet. The atmosphere and composition of the sediments on Mars suggest suitability for anaerobic chemolithotrophic metabolism. Carbonates are often considered as morphological biosignatures, such as stromatolites, but have not been considered as potential electron acceptors. Within the present study, hydrogenotrophic methanogen enrichments were generated from sediments that had received significant quantities of lime from industrial processes (lime kiln/steel production). These enrichments were then supplemented with calcium carbonate powder or marble chips as a sole source of carbon. These microcosms saw a release of inorganic carbon into the liquid phase, which was subsequently removed, resulting in the generation of methane, with 0.37 ± 0.09 mmoles of methane observed in the steel sediment enrichments supplemented with calcium carbonate powder. The steel sediment microcosms and lime sediments with carbonate powder enrichments were dominated by Methanobacterium sp., whilst the lime/marble enrichments were more diverse, containing varying proportions of Methanomassiliicoccus, Methanoculleus and Methanosarcina sp. In all microcosm experiments, acetic acid was detected in the liquid phase. Our results indicate that chemolithotrophic methanogenesis should be considered when determining biosignatures for life on Mars.

Reduced overpotential of methane-producing biocathodes: Effect of current and electrode storage capacity

Cathode overpotential is a key factor in the energy efficiency of bioelectrochemical systems. In this study the aim is to demonstrate the role of applied current density and electrode storage capacity on cathode overpotential. To do so, eight reactors using capacitive granular activated carbon as cathode material were operated. Four reactors were controlled at -5 A m^-2 and four at -10 A m^-2. Additionally, to evaluate the electrode storage capacity, weekly charge/discharge tests were conducted for half of the reactors at each applied current density. Results show that cathode potential as high as -0.50 V vs. Ag/AgCl can be reached. Furthermore, the resulting low cathode overpotential is both dependent on applied current density and employment (or not) of charge/discharge tests: reactors at -10 A m^-2 without charge/discharge regimes did not result in increasing cathode potential whereas reactors at -5 A m^-2 and at -10 A m^-2 with charge/discharge regimes did.

Overview of pretreatment technologies on vegetable, fruit and flower market wastes disintegration and bioenergy potential: Indian scenario

Disposal of segregated organic fractions of centralized wholesale market wastes (i.e. vegetable, fruit and flower markets waste) in dumpsites/landfills are not only a serious issue but also underutilizes the huge potency of these organic wastes. Anaerobic digestion (AD) is a promising technology for converting organic wastes into methane, as a carbon-neutral alternative to conventional fuels. The major challenges related to the AD process are poor biodegradation of wastes and buffering capacity within the anaerobic digester that lowers the biogas yield. To accelerate biodegradation and to enhance the process efficacy of anaerobic digestion, several pretreatment technologies (mechanical, thermal, biological, chemical and combined pre-treatments) for organic wastes prior to the AD process were developed. This review article presents a comprehensive analysis of research updates in pretreatment techniques for vegetable, fruit and flower markets wastes for enhancing biogas yields during the AD process. The technological aspects of the pretreatment process are described and their efficiency comparison with the resultant process yields and environmental benefits are also discussed. The challenges and technical issues associated with each pretreatment and future research directions for overcoming the field implementation issues are also proposed.

Enhancement of methane production from waste activated sludge using hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) process - A review

Hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) was proved to increase methane productivity and methane yield of waste activated sludge (WAS) by establishing direct interspecies electron transfer method and enriching functional microorganisms. This review first summarized the pretreatment methods of WAS for MEC-AD and then reviewed the reactor configurations, operation parameters, and the economic benefit of MEC-AD. Furthermore, the enhancement mechanisms of MEC-AD were reviewed based on the analysis of thermodynamics and microbial community. It was found that the decrease of hydrogen partial pressure due to the hydrogenotrophic methanogens enriched in cathodic biofilm and direct interspecies electron transfer between exoelectrogens and anode were the core mechanisms for improving acidogenesis, acetogenesis, and methanogenesis. Finally, the potentially technological issues that need to be addressed to increase energy efficiency in large-scale MEC-AD processes were discussed.

Concentration-dependent effects of nickel doping on activated carbon biocathodes

In microbial electrosynthesis (MES), microorganisms grow on a cathode electrode as biofilm, or in the catholyte as planktonic biomass, and utilize CO2 for their growth and metabolism. Modification of the...

Abiotic-biotic hybrid for CO2 biomethanation: From electrochemical to photochemical process

Converting CO₂ into sustainable fuels (e.g., CH₄) has great significance to solve carbon emission and energy crisis. Generally, CO₂ methanation needs abundant of energy input to overcome the eight-electron-transfer barrier. Abiotic-biotic hybrid system represents one of the cutting-edge technologies that use renewable electric/solar energy to realize eight-electron-transfer CO₂ biomethanation. However, the incompatible abiotic-biotic hybrid can result in low efficiency of electron transfer and CO₂ biomethanation. Herein, we present the comprehensive review to highlight how to design abiotic-biotic hybrid for electric/solar-driven CO₂ biomethanation. We primarily introduce the CO₂ biomethanation mechanism, and further summarize state-of-the-art electrochemical and photochemical CO₂ biomethanation in hybrid systems. We also propose excellent synthetic biology strategies, which are useful to design tunable methanogenic microorganisms or enzymes when cooperating with electrode/semiconductor in hybrid systems. This review provides theoretical guidance of abiotic-biotic hybrid and also shows the bright future of sustainable fuel production in the form of CO₂ biomethanation.

Microbial electrolysis cells for return flow: Simultaneous nitrogen and carbon removal

The concentration of solids in secondary sludge before anaerobic digestion in a wastewater treatment plant, bring about the production of a return flow, which contains high concentrations of all the common pollutant parameters. This return flow could unfavourably affect the performance of the processes and effluent quality of the waterline. Here, we report the utilisation of three similar microbial electrolysis cells reactors that performs simultaneous carbon and nitrogen removal to reduce the impact of the return flow in the plant. The result of the batch-fed (72 h) experiment showed COD and total nitrogen removal efficiencies that reached 90% and 80%, respectively, supporting the premise that return flows are suitable substrates for a bioelectrochemical treatment. The three reactors followed similar trends, showing good replicability and confirming the potential of MECs as a feasible technology for return flow treatment. Furthermore, when cathodic conversion efficiency was higher than 80%, the pure hydrogen production allows to recover the electric energy consumption, indicating that the system could be theoretically energy neutral.

Competition of two highly specialized and efficient acetoclastic electroactive bacteria for acetate in biofilm anode of microbial electrolysis cell

Maintaining functional stability of microbial electrolysis cell (MEC) treating wastewater depends on maintaining functional redundancy of efficient electroactive bacteria (EAB) on the anode biofilm. Therefore, investigating whether efficient EAB competing for the same resources (electron donor and acceptor) co-exist at the anode biofilm is key for the successful application of MEC for wastewater treatment. Here, we compare the electrochemical and kinetic properties of two efficient acetoclastic EAB, Geobacter sulfurreducens (GS) and Desulfuromonas acetexigens (DA), grown as monoculture in MECs fed with acetate. Additionally, we monitor the evolution of DA and GS in co-culture MECs fed with acetate or domestic wastewater using fluorescent in situ hybridization. The apparent Monod kinetic parameters reveal that DA possesses higher jmax (10.7 ± 0.4 A/m2) and lower KS, app (2 ± 0.15 mM) compared to GS biofilms (jmax: 9.6 ± 0.2 A/m2 and KS, app: 2.9 ± 0.2 mM). Further, more donor electrons are diverted to the anode for respiration in DA compared to GS. In acetate-fed co-culture MECs, DA (98% abundance) outcompete GS for anode-dependent growth. In contrast, both EAB co-exist (DA: 55 ± 2%; GS: 24 ± 1.1%) in wastewater-fed co-culture MECs despite the advantage of DA over GS based on kinetic parameters alone. The co-existence of efficient acetoclastic EAB with high current density in MECs fed with wastewater is significant in the context of functional redundancy to maintain stable performance. Our findings also provide insight to future studies on bioaugmentation of wastewater-fed MECs with efficient EAB to enhance performance.

Enrichment of salt-tolerant CO2-fixing communities in microbial electrosynthesis systems using porous ceramic hollow tube wrapped with carbon cloth as cathode and for CO2 supply

Microbial inocula from marine origins are less explored for CO₂ reduction in microbial electrosynthesis (MES) system, although effective CO₂-fixing communities in marine environments are well-documented. We explored natural saline habitats, mainly salt marsh (SM) and mangrove (M) sediments, as potential inoculum sources for enriching salt-tolerant CO₂ reducing community using two enrichment strategies: H₂:CO₂ (80:20) enrichment in serum vials and enrichment in cathode chamber of MES reactors operated at -1.0 V vs. Ag/AgCl. Porous ceramic hollow tube wrapped with carbon cloth was used as cathode and for direct CO₂ delivery to CO₂ reducing communities growing on the cathode surface. Methanogenesis was dominant in both the M- and SM-seeded MES and the methanogenic Archaea Methanococcus was the most dominant genus. Methane production was slightly higher in the SM-seeded MES (4.9 ± 1.7 mmol) compared to the M-seeded MES (3.8 ± 1.1 mmol). In contrast, acetate production was almost two times higher in the M-seeded MES (3.1 ± 0.9 mmol) than SM-seeded MES (1.5 ± 1.3 mmol). The high relative abundance of the genus Acetobacterium in the M-seeded serum vials correlates with the high acetate production obtained. The different enrichment strategies affected the community composition, though the communities in MES reactors and serum vials were performing similar functions (methanogenesis and acetogenesis). Despite similar operating conditions, the microbial community composition of M-seeded serum vials and MES reactors differed from the SM-seeded serum vials and MES reactors, supporting the importance of inoculum source in the evolution of CO₂-reducing microbial communities.

Production of high-calorific biogas from food waste by integrating two approaches: Autogenerative high-pressure and hydrogen injection

Auto-generative high pressure digestion (AHPD) and hydrogen-injecting digestion (HID) have been introduced to directly produce high CH₄-content biogas from anaerobic digester. However, each approach has its own technical difficulties (pH changes), and practical issues (high cost of H₂) to obtain > 90% CH₄ containing biogas, particularly, from the high-strength waste like food waste (FW). To overcome this problem, in this study, AHPD and HID were integrated, which can offset each drawback but maximize its benefit. Substrate concentration of FW tested here was 200 g COD/L, the highest ever applied in AHPD and HID studies. At first, the reactor was operated by elevating the autogenerative pressure from 1 to 3, 5, and 7 bar without H₂ injection. With the pressure increase, the CH₄ content in the biogas gradually increased from 52.4% at 1 bar to 77.4% at 7 bar. However, a drop of CH₄ production yield (MPY) was observed at 7 bar, due to the pH drop down to 6.7 by excess CO₂ dissolution. At further operation, H₂ injection began at 5 bar, with increasing its amount. The injection was effective to increase the CH₄ content to 82.8%, 87.2%, and 90.6% at 0.09, 0.13, and 0.18 L H₂/g COD_FW.fed of H₂ injection amount, respectively. At 0.25 L H₂/g COD_FW.fed, there was a further increase of CH₄ content to 92.1%, but the MPY was dropped with pH increase to 8.7 with residual H₂ being detected (4% in the biogas). Microbial community analysis showed the increased abundance of piezo-tolerant microbe with pressure increase, and direct interspecies electron transfer contributors after H₂ injection. In conclusion, the integration of two approaches enabled to directly produce high calorific biogas (90% > CH₄, 180 MJ/m³ biogas) from high-strength FW at the lowest requirement of H₂ (0.18 L H₂/g COD_FW.fed) ever reported.

Characterization and significance of extracellular polymeric substances, reactive oxygen species, and extracellular electron transfer in methanogenic biocathode

The microbial electrolysis cell assisted anaerobic digestion holds great promises over conventional anaerobic digestion. This article reports an experimental investigation of extracellular polymeric substances (EPS), reactive oxygen species (ROS), and the expression of genes associated with extracellular electron transfer (EET) in methanogenic biocathodes. The MEC-AD systems were examined using two cathode materials: carbon fibers and stainless-steel mesh. A higher abundance of hydrogenotrophic Methanobacterium sp. and homoacetogenic Acetobacterium sp. appeared to play a major role in superior methanogenesis from stainless steel biocathode than carbon fibers. Moreover, the higher secretion of EPS accompanied by the lower ROS level in stainless steel biocathode indicated that higher EPS perhaps protected cells from harsh metabolic conditions (possibly unfavorable local pH) induced by faster catalysis of hydrogen evolution reaction. In contrast, EET-associated gene expression patterns were comparable in both biocathodes. Thus, these results indicated hydrogenotrophic methanogenesis is the key mechanism, while cathodic EET has a trivial role in distinguishing performances between two cathode electrodes. These results provide new insights into the efficient methanogenic biocathode development.

Emerging trends in microbial fuel cell diversification-Critical analysis

Global need for transformation from fossil-based to bio-based economy is constantly emerging for the production of low-carbon/renewable energy/products. Microbial fuel cell (MFC) catalysed by bio-electrochemical process gained significant attention initially for its unique potential to generate energy. Diversification of MFC is an emerging trend in the context of prioritising/enhancing product output while exploring the mechanism specificity of individual processes. Bioelectrochemical treatment system (BET), microbial electrosynthesis system (MES), bioelectrochemical system (BES), electro-fermentation (EF), microbial desalination cell (MDC), microbial electrolysis cell (MEC) and electro-methanogenesis (EM) are the diversified MFC systems that are being researched actively. Owing to its broad diversification, MFC domain is increasing its potential credibility as a platform technology. Microbial catalyzed electrochemical reactions are the key which directly/indirectly are proportionally linked to electrometabolic activity of microorganisms towards final anticipated output. This review intends to holistically document the mechanisms, applications and current trends of MFC diversifications towards multi-faced applications.

Redox Potential Heterogeneity in Fixed-Bed Electrodes Leads to Microbial Stratification and Inhomogeneous Performance

Bed electrodes provide high electrode area-to-volume ratios represent a promising configuration for transferring bioelectrochemical systems close to industrial applications. Nevertheless, the intrinsic electrical resistance leads to poor polarization behavior. Therefore, the distribution of Geobacter spp. and their electrochemical performance within exemplary fixed-bed electrodes are investigated. A minimally invasive sampling system allows characterization of granules from different spatial locations of bed electrodes. Cyclic voltammetry of single granules (n=63) demonstrates that the major share of electroactivity (134.3 mA L-1 ) is achieved by approximately 10 % of the bed volume, specifically that being close to the current collector. Nevertheless, analysis of the microbial community reveals that Geobacter spp. dominated all sampled granules. These findings clearly demonstrate the need for engineered bed electrodes to improve electron exchange between microorganisms and granules.

Bioelectrical Methane Production with an Ammonium Oxidative Reaction under the No Organic Substance Condition

The present study investigated bioelectrical methane production from CO₂ without organic substances. Even though microbial methane production has been reported at relatively high electric voltages, the amount of voltage required and the organisms contributing to the process currently remain unknown. Methane production using a biocathode was investigated in a microbial electrolysis cell coupled with an NH₄⁺ oxidative reaction at an anode coated with platinum powder under a wide range of applied voltages and anaerobic conditions. A microbial community analysis revealed that methane production simultaneously occurred with biological denitrification at the biocathode. During denitrification, NO₃^– was produced by chemical NH₄⁺ oxidation at the anode and was provided to the biocathode chamber. H₂ was produced at the biocathode by the hydrogen-producing bacteria Petrimonas through the acceptance of electrons and protons. The H₂ produced was biologically consumed by hydrogenotrophic methanogens of Methanobacterium and Methanobrevibacter with CO₂ uptake and by hydrogenotrophic denitrifiers of Azonexus. This microbial community suggests that methane is indirectly produced without the use of electrons by methanogens. Furthermore, bioelectrical methane production occurred under experimental conditions even at a very low voltage of 0.05‍ ‍V coupled with NH₄⁺ oxidation, which was thermodynamically feasible.

Efficient resource valorization by co-digestion of food and vegetable waste using three stage integrated bioprocess

During two-stage (Acidogenesis-Methanogenesis) process, solid organics and gaseous by-products are usually left unused. To increase resource recovery efficiency, a three stage process (Hydrolysis/Acidogenesis-Methanogenesis-Composting) was designed. Initially, co-digestion of food waste (FW) and vegetable waste (VW) was carried out in Leach Bed Reactor (LBR) for hydrolysis and acidogenesis, followed by airlift reactor (ALR) for methanogenesis for 21 days using two different feed stocks [2:3 FW:VW~FVW; FW alone]. Off gas from LBR was diverted to ALR to enhance methane recovery. Results depicted that volatile fatty acids (VFA) and biohydrogen production was more for FW fed system, while methane production was higher in FVW fed system. Three different functional zones in three separate chambers significantly accelerated organic removal rate while gas diversion increased overall methane recovery. In third stage, residual solid organic matter from LBR was subjected to aerobic composting and compost with N (%): 2.90 & 2.76; C/N ratio: 18.2 & 20.8 for FVW and FW was recovered. The three-stage process has advantages of zero waste generation and overall process stability, accounting for resource efficient circular loop.

Bioelectrochemical CO2 Reduction to Methane: MES Integration in Biogas Production Processes

Anaerobic digestion (AD) is a widely used technique to treat organic waste and produce biogas. This article presents a practical approach to increase biogas yield of an AD system using a microbial electrosynthesis system (MES). The biocathode in MES reduces carbon dioxide with the supplied electrons and protons (H+) to form methane. We demonstrate that the MES is able to produce biogas with over 90% methane when fed with reject water obtained from a local wastewater treatment plant. The optimised cathode potential was observed in the range of −0.70 V to −0.60 V and optimised feed pH was around 7.0. With autoclaved feed, these conditions allowed methane yields of about 9.05 mmol/L(reactor)-day. A control experiment was then carried out to make a comparison between open circuit and MES methanogenesis. The highest methane yield of about 22.1 mmol/L(reactor)-day was obtained during MES operation that performed 10–15% better than the open circuit mode of operation. We suggest and describe an integrated AD-MES system, by installing MES in the reject water loop, as a novel approach to improve the efficiency and productivity of existing waste/wastewater treatment plants.

Startup performance of microbial electrolysis cell assisted anaerobic digester (MEC-AD) with pre-acclimated activated carbon

The feasibility of using pre-acclimated activated carbon to start up microbial electrolysis cell assisted anaerobic digester (MEC-AD) has been testified in this study. Two identical lab-scale digesters were separately packed with granular activated carbon (GAC) and powered activated carbon (PAC), which were initially acclimated as anaerobic digester and then transferred to MEC-AD. When a voltage of 0.5 V was applied, increased methane generation and substrate removal rates were observed. Hydrogenotrophic methanogens predominated in both digesters before and after transition, indicating that the pre-cultured microbial community on carbon materials could provide necessary microbiome favorable for starting up MECs. Although a low abundance of Geobacter was detected in inoculum, a rapid propagation could be realized when reactors were subjected to the electro-stimulation. The abundance of Methanosarcina closely attached to PAC was four times than that of GAC, which might be partially contributed to the improved resilience of anaerobic digester subjected to electro-stimulation.

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Pre-acclimated PAC/GAC are favorable for starting up MEC-AD.

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Methane yield was increased by ~30% when transferring AD to MEC-AD.

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Abundance of electroactive bacteria on pre-enriched PAC was higher than GAC.

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The rapid propagation of Geobacter was found in MEC-AD.

Improved hydrogen production in the single-chamber microbial electrolysis cell with inhibition of methanogenesis under alkaline conditions

The aim of this study was to investigate hydrogen production enhanced by methanogenesis inhibition in the single-chamber microbial electrolysis cell (MEC) under alkaline conditions. With 50 mM bicarbonate buffer and 1 g L⁻¹ acetate, the MEC was tested at pH = 8.5, 9.5, 10.5, and 11.2, respectively, within 124 d operation. Effective methanogenesis inhibition in the MEC increased with pH from 8.5 to 11.2. At pH 11.2, Methanobacteriaceae reached the lowest absolute quantity (i.e., biomass and mcrA gene copy number of methanogens) within the microbial community in the cathodic biofilm among the pH values. Under the alkaline conditions, a hydrogen percentage of 85–90% and a methane percentage < 15% were achieved within 25 cycles (50 d) of operation. The maximum current density in the MEC reached 83.7 ± 1.5 A m⁻³ with the average electrical recovery of 171 ± 18% and overall energy recovery of 72 ± 3%. The excellent performance of the MEC at pH = 11.2 was attributed to the low abundance of methanogens within the cathodic biofilm (2.23 ± 0.46 copy per cm²), low cathodic biomass (0.12 ± 0.01 mg protein per g), and low anode potential (−0.228 mV vs. saturated calomel electrode). Results from this study should be valuable to expand applications of the MEC with methanogenesis inhibition in alkaline wastewater treatment.

Effective methanogenesis inhibition was achieved in a single-chamber MEC at pH 11.2 with the H₂ percentage of 85–90% for 50 days.

Co-digestion of food waste and sewage sludge for methane production: Current status and perspective

Food waste (FW) is a valuable resource which requires sustainable management avenues to reduce the hazardous environmental impacts and add-value for better economy. Anaerobic digestion (AD) is still reliable, cost-effective technology for waste management. Conventional AD was originally designed for sewer sludge digestion, is not effective for FW due to mainly high organics and volatile fatty acid (VFA) accumulation, hence better technical aptitudes and biochemical inputs are required for optimal biogas production. Besides, to overcome these challenges, FW co-digestion with complementary organic waste e.g. sewage sludge (SS) mixed which complement each other for better process design. The main aim of this article is to summarize the recent updates and review different holistic approaches for efficient anaerobic co-digestion (AcoD) of FW and SS to provide a comprehensive review on the topic. Moreover, to demonstrate the status and perspectives of AcoD at present scenario for Hong Kong and rest of the world.

A comprehensive comparison of five different carbon-based cathode materials in CO2 electromethanogenesis: Long-term performance, cell-electrode contact behaviors and extracellular electron transfer pathways

Each carbon-based material, due to the discrepancy in critical properties, has distinct capability to enrich electroactive microbes able to electrosynthesize methane from CO₂. 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 H₂-concnetration dependent; CS-GF cathode possessed high antifouling properties and extensive space, enriching the microorganisms capable of catalyzing electromethanogenesis through more efficient non-H₂ route. This study re-interpreted the application potentials of carbon-based materials in CO₂ electroreduction and electrofuel recovery, providing valuable guidance for materials' selection.

Electron Fluxes in Biocathode Bioelectrochemical Systems Performing Dechlorination of Chlorinated Aliphatic Hydrocarbons

Bioelectrochemical systems (BESs) are regarded as a promising approach for the enhanced dechlorination of chlorinated aliphatic hydrocarbons (CAHs). However, the electron distribution and transfer considering dechlorination, methanogenesis, and other bioprocesses in these systems are little understood. This study investigated the electron fluxes in biocathode BES performing dechlorination of three typical CAHs, 1,1,2,2-tetrachloroethene (PCE), 1,1,2-trichloroethene (TCE) and 1,2-dichloroethane (1,2-DCA). Anaerobic sludge was inoculated to cathode and biocathode was acclimated by the direct acclimation and selection. The constructed biocathode at −0.26 V had significantly higher dechlorination efficiency (E_24h > 99.0%) than the opened circuit (E_24h of 17.2–27.5%) and abiotic cathode (E_24h of 5.5–10.8%), respectively. Cyclic voltammetry analysis demonstrated the enhanced cathodic current and the positive shift of onset potential in the cathodic biofilm. Under autotrophic conditions with electrons from the cathode as sole energy source (columbic efficiencies of 80.4–90.0%) and bicarbonate as sole carbon source, CAHs dechlorination efficiencies were still maintained at 85.0 ± 2.0%, 91.4 ± 1.8%, and 84.9 ± 3.1% for PCE, TCE, and 1,2-DCA, respectively. Cis-1,2-dichloroethene was the final product for PCE and TCE, while 1,2-DCA went through a different dechlorination pathway with the non-toxic ethene as the final metabolite. Methane was the main by-product of the heterotrophic biocathode, and methane production could be enhanced to some extent by electrochemical stimulation. The various electron fluxes originating from the cathode and oxidation of organic substrates might be responsible for the enhanced CAHs dechlorination, while methane generation and bacterial growth would probably reduce the fraction of electrons provided for CAH dechlorination. The study deals with the dechlorination and competitive bioprocesses in CAH-dechlorinating biocathodes with a focus on electron fluxes.

Bioelectrochemically assisted anaerobic digestion system for biogas upgrading and enhanced methane production

The objective of this study was to evaluate the effect of biofilm and external voltage on the performance and microbial community composition of batch-fed, combined anaerobic digestion-bioelectrochemical cell (AD-BEC) systems under different operational conditions. A dextrin/peptone mixture was fed at a range of organic loading rates (0.34 to 1.37g COD/L-d). The hybrid system with both suspended biomass and biofilm without any external potential application achieved a substantially higher initial soluble COD consumption (53.7±2.3% vs. 39.7±3.7) and methane (CH₄) production (331 vs. 225mL) within one day of feeding than the conventional AD system (suspended biomass only). Compared to the conventional AD system, the hybrid systems had higher resilience to shock organic loadings. A range of external potential (0.5 to 2.0V vs. Ag/AgCl) was applied to AD-BEC reactors, developed with two different start-up procedures. A potential of 2.0V resulted in water electrolysis leading to a higher CH₄ production rate (105 vs. 84mL/L-d) and biogas CH₄ content (88.5±1.4 vs. 64.5±1.9%) in the AD-BEC reactor (closed vs. open circuit condition, respectively). Application of external potential enriched putative exoelectrogens at the anode biofilm and hydrogenotrophic methanogens at the cathode biofilm, which may have contributed to the observed enhanced CH₄ production in the AD-BEC system. A phylotype related to Methanobacterium formicicum, a hydrogenotrophic methanogen, dominated the archaeal community in the AD-BEC cathode biofilm.

Electroactive microorganisms in bulk solution contribute significantly to methane production in bioelectrochemical anaerobic reactor

The role of anaerobic microorganisms suspended in the bulk solution on methane production was investigated in a bioelectrochemical anaerobic reactor with the electrode polarized at 0.5 V. The electron transfer from substrate to methane and hydrogen were 25% and 7.5%, respectively, in the absence of the anaerobic microorganisms in the bulk solution. As the anaerobic microorganisms increased to 4400 mg/L, the electrons transferred to methane increased to 83.3% but decreased to 0.3% in hydrogen. The electroactive microorganisms (EAM), including exoelectrogens and electrotrophs, enriched in the bulk solution that confirmed by the redox peaks in the cyclic voltammogram was proportional to the anaerobic microorganism. The methane yield based on COD removal was dependent on the anaerobic microorganisms in the bulk solution rather than on the bioelectrode surface. The EAM suspended in the bulk solution are enriched by the polarized electrode, and significantly improve methane production in bioelectrochemical anaerobic reactor.

Heat-Treated Stainless Steel Felt as a New Cathode Material in a Methane-Producing Bioelectrochemical System

Methane-producing bioelectrochemical systems (BESs) are a promising technology to convert renewable surplus electricity into the form of storable methane. One of the key challenges for this technology is the search for suitable cathode materials with improved biocompatibility and low cost. Here, we study heat-treated stainless steel felt (HSSF) for its performance as biocathode. The HSSF had superior electrocatalytic properties for hydrogen evolution compared to untreated stainless steel felt (SSF) and graphite felt (GF), leading to a faster start-up of the biocathodes. At cathode potentials of -1.3 and -1.1 V, the methane production rates for HSSF biocathodes were higher than the SSF, while its performance was similar to GF biocathodes at -1.1 V and lower than GF at -1.3 V. The HSSF biocathodes had a current-to-methane efficiency of 60.8% and energy efficiency of 21.9% at -1.3 V. HSSF is an alternative cathode material with similar performance compared to graphite felt, suited for application in methane-producing BESs.

Changes in Glucose Fermentation Pathways as a Response to the Free Ammonia Concentration in Microbial Electrolysis Cells

When a mixed-culture microbial electrolysis cell (MEC) is fed with a fermentable substrate, such as glucose, a significant fraction of the substrate's electrons ends up as methane (CH₄) through hydrogenotrophic methanogenesis, an outcome that is undesired. Here, we show that free ammonia-nitrogen (FAN, which is NH₃) altered the glucose fermentation pathways in batch MECs, minimizing the production of H₂, the "fuel" for hydrogenotrophic methanogens. Consequently, the Coulombic efficiency (CE) increased: 57% for 0.02 g of FAN/L of fed-MEC, compared to 76% for 0.18 g of FAN/L of fed-MECs and 62% for 0.37 g of FAN/L of fed-MECs. Increasing the FAN concentration was associated with the accumulation of higher organic acids (e.g., lactate, iso-butyrate, and propionate), which was accompanied by increasing relative abundances of phylotypes that are most closely related to anode respiration (Geobacteraceae), lactic-acid production (Lactobacillales), and syntrophic acetate oxidation (Clostridiaceae). Thus, the microbial community established syntrophic relationships among glucose fermenters, acetogens, and anode-respiring bacteria (ARB). The archaeal population of the MEC fed 0.02 g FAN/L was dominated by Methanobacterium, but 0.18 and 0.37 g FAN/L led to Methanobrevibacter becoming the most abundant species. Our results provide insight into a way to decrease CH₄ production and increase CE using FAN to control the fermentation step, instead of inhibiting methanogens using expensive or toxic chemical inhibitors, such as 2-bromoethanesulfonic acid.

Freestanding and flexible graphene papers as bioelectrochemical cathode for selective and efficient CO2 conversion

During microbial electrosynthesis (MES) driven CO₂ reduction, cathode plays a vital role by donating electrons to microbe. Here, we exploited the advantage of reduced graphene oxide (RGO) paper as novel cathode material to enhance electron transfer between the cathode and microbe, which in turn facilitated CO₂ reduction. The acetate production rate of Sporomusa ovata-driven MES reactors was 168.5 ± 22.4 mmol m⁻² d⁻¹ with RGO paper cathodes poised at −690 mV versus standard hydrogen electrode. This rate was approximately 8 fold faster than for carbon paper electrodes of the same dimension. The current density with RGO paper cathodes of 2580 ± 540 mA m⁻² was increased 7 fold compared to carbon paper cathodes. This also corresponded to a better cathodic current response on their cyclic voltammetric curves. The coulombic efficiency for the electrons conversion into acetate was 90.7 ± 9.3% with RGO paper cathodes and 83.8 ± 4.2% with carbon paper cathodes, respectively. Furthermore, more intensive cell attachment was observed on RGO paper electrodes than on carbon paper electrodes with confocal laser scanning microscopy and scanning electron microscopy. These results highlight the potential of RGO paper as a promising cathode for MES from CO₂.

Temporal Microbial Community Dynamics in Microbial Electrolysis Cells - Influence of Acetate and Propionate Concentration

Microbial electrolysis cells (MECs) are widely considered as a next generation wastewater treatment system. However, fundamental insight on the temporal dynamics of microbial communities associated with MEC performance under different organic types with varied loading concentrations is still unknown, nevertheless this knowledge is essential for optimizing this technology for real-scale applications. Here, the temporal dynamics of anodic microbial communities associated with MEC performance was examined at low (0.5 g COD/L) and high (4 g COD/L) concentrations of acetate or propionate, which are important intermediates of fermentation of municipal wastewaters and sludge. The results showed that acetate-fed reactors exhibited higher performance in terms of maximum current density (I: 4.25 ± 0.23 A/m²), coulombic efficiency (CE: 95 ± 8%), and substrate degradation rate (98.8 ± 1.2%) than propionate-fed reactors (I: 2.7 ± 0.28 A/m²; CE: 68 ± 9.5%; substrate degradation rate: 84 ± 13%) irrespective of the concentrations tested. Despite of the repeated sampling of the anodic biofilm over time, the high-concentration reactors demonstrated lower and stable performance in terms of current density (I: 1.1 ± 0.14 to 4.2 ± 0.21 A/m²), coulombic efficiency (CE: 44 ± 4.1 to 103 ± 7.2%) and substrate degradation rate (64.9 ± 6.3 to 99.7 ± 0.5%), while the low-concentration reactors produced higher and dynamic performance (I: 1.1 ± 0.12 to 4.6 ± 0.1 A/m²; CE: 52 ± 2.5 to 105 ± 2.7%; substrate degradation rate: 87.2 ± 0.2 to 99.9 ± 0.06%) with the different substrates tested. Correlating reactor's performance with temporal dynamics of microbial communities showed that relatively similar anodic microbial community composition but with varying relative abundances was observed in all the reactors despite differences in the substrate and concentrations tested. Particularly, Geobacter was the predominant bacteria on the anode biofilm of all MECs over time suggesting its possible role in maintaining functional stability of MECs fed with low and high concentrations of acetate and propionate. Taken together, these results provide new insights on the microbial community dynamics and its correlation to performance in MECs fed with different concentrations of acetate and propionate, which are important volatile fatty acids in wastewater.

Methanogenic Biocathode Microbial Community Development and the Role of Bacteria

The cathode microbial community of a methanogenic bioelectrochemical system (BES) is key to the efficient conversion of carbon dioxide (CO₂) to methane (CH₄) with application to biogas upgrading. The objective of this study was to compare the performance and microbial community composition of a biocathode inoculated with a mixed methanogenic (MM) culture to a biocathode inoculated with an enriched hydrogenotrophic methanogenic (EHM) culture, developed from the MM culture following pre-enrichment with H₂ and CO₂ as the only externally supplied electron donor and carbon source, respectively. Using an adjacent Ag/AgCl reference electrode, biocathode potential was poised at -0.8 V (versus SHE) using a potentiostat, with the bioanode acting as the counter electrode. When normalized to cathode biofilm biomass, the methane production in the MM- and EHM-biocathode was 0.153 ± 0.010 and 0.586 ± 0.029 mmol CH₄/mg biomass-day, respectively. This study showed that H₂/CO₂ pre-enriched inoculum enhanced biocathode CH₄ production, although the archaeal communities in both biocathodes converged primarily (86-100%) on a phylotype closely related to Methanobrevibacter arboriphilus. The bacterial community of the MM-biocathode was similar to that of the MM inoculum but was enriched in Spirochaetes and other nonexoelectrogenic, fermentative Bacteria. In contrast, the EHM-biocathode bacterial community was enriched in Proteobacteria, exoelectrogens, and putative producers of electron shuttle mediators. Similar biomass levels were detected in the MM- and EHM-biocathodes. Thus, although the archaeal communities were similar in the two biocathodes, the difference in bacterial community composition was likely responsible for the 3.8-fold larger CH₄ production rate observed in the EHM-biocathode. Roles for abundant OTUs identified in the biofilm and inoculum cultures were highlighted on the basis of previous reports.

On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Hybrid photosynthesis-powering biocatalysts with solar energy captured by inorganic devices

The biological reduction of CO₂ driven by sunlight via photosynthesis is a crucial process for life on earth. However, the conversion efficiency of solar energy to biomass by natural photosynthesis is low. This translates in bioproduction processes relying on natural photosynthesis that are inefficient energetically. Recently, hybrid photosynthetic technologies with the potential of significantly increasing the efficiency of solar energy conversion to products have been developed. In these systems, the reduction of CO₂ into biofuels or other chemicals of interest by biocatalysts is driven by solar energy captured with inorganic devices such as photovoltaic cells or photoelectrodes. Here, we explore hybrid photosynthesis and examine the strategies being deployed to improve this biotechnology.

External Resistances Applied to MFC Affect Core Microbiome and Swine Manure Treatment Efficiencies

Microbial fuel cells (MFCs) can be designed to combine water treatment with concomitant electricity production. Animal manure treatment has been poorly explored using MFCs, and its implementation at full-scale primarily relies on the bacterial distribution and activity within the treatment cell. This study reports the bacterial community changes at four positions within the anode of two almost identically operated MFCs fed swine manure. Changes in the microbiome structure are described according to the MFC fluid dynamics and the application of a maximum power point tracking system (MPPT) compared to a fixed resistance system (Ref-MFC). Both external resistance and cell hydrodynamics are thought to heavily influence MFC performance. The microbiome was characterised both quantitatively (qPCR) and qualitatively (454-pyrosequencing) by targeting bacterial 16S rRNA genes. The diversity of the microbial community in the MFC biofilm was reduced and differed from the influent swine manure. The adopted electric condition (MPPT vs fixed resistance) was more relevant than the fluid dynamics in shaping the MFC microbiome. MPPT control positively affected bacterial abundance and promoted the selection of putatively exoelectrogenic bacteria in the MFC core microbiome (Sedimentibacter sp. and gammaproteobacteria). These differences in the microbiome may be responsible for the two-fold increase in power production achieved by the MPPT-MFC compared to the Ref-MFC.

Bioelectrochemical enhancement of methane production in low temperature anaerobic digestion at 10 °C

Anaerobic digestion at low temperature is an attractive technology especially in moderate climates, however, low temperature results in low microbial activity and low rates of methane formation. This study investigated if bioelectrochemical systems (BESs) can enhance methane production from organic matter in low-temperature anaerobic digestion (AD). A bioelectrochemical reactor was operated with granular activated carbon as electrodes at 10 °C. Our results showed that bioelectrochemical systems can enhance CH4 yield, accelerate CH4 production rate and increase acetate removal efficiency at 10 °C. The highest CH4 yield of 31 mg CH4-COD/g VSS was achieved in the combined BES-AD system at a cathode potential of -0.90 V (Ag/AgCl), which was 5.3-6.6 times higher than that in the AD reactor at 10 °C. CH4 production rate achieved in the combined BES-AD system at 10 °C was only slightly lower than that in the AD reactor at 30 °C. The presence of an external circuit between the acetate-oxidizing bioanode and methane-producing cathode provided an alternative pathway from acetate via electrons to methane, potentially via hydrogen. This alternative pathway seems to result in higher CH4 production rates at low temperature compared with traditional methanogenesis from acetate. Integration of BES with AD could therefore be an attractive alternative strategy to enhance the performance of anaerobic digestion in cold areas.

Graphene-Coated Hollow Fiber Membrane as the Cathode in Anaerobic Electrochemical Membrane Bioreactors--Effect of Configuration and Applied Voltage on Performance and Membrane Fouling

Electrically conductive, graphene-coated, hollow-fiber porous membranes were used as cathodes in anaerobic electrochemical membrane bioreactors (AnEMBRs) operated at different applied voltages (0.7 and 0.9 V) using a new rectangular reactor configuration compared to a previous tubular design (0.7 V). The onset of biofouling was delayed and minimized in rectangular reactors operated at 0.9 V compared to those at 0.7 V due to higher rates of hydrogen production. Maximum transmembrane pressures for the rectangular reactor were only 0.10 bar (0.7 V) or 0.05 bar (0.9 V) after 56 days of operation compared to 0.46 bar (0.7 V) for the tubular reactor after 52 days. The thickness of the membrane biofouling layer was approximately 0.4 μm for rectangular reactors and 4 μm for the tubular reactor. Higher permeate quality (TSS = 0.05 mg/L) was achieved in the rectangular AnEMBR than that in the tubular AnEMBR (TSS = 17 mg/L), likely due to higher current densities that minimized the accumulation of cells in suspension. These results show that the new rectangular reactor design, which had increased rates of hydrogen production, successfully delayed the onset of cathode biofouling and improved reactor performance.