Responses of applied voltages on the archaea microbial distribution in sludge digestion

As the development of urban population led to the increase of domestic water consumption, consequently the generation of surplus sludge (SS) produced increasingly during sewage treatment processes. In order to enhance the SS resource utilization efficiency, an electricity-assisted anaerobic digestion (EAAD) system was employed to examine the alterations in the digestion broth and the characteristics of gas production. Additionally, the response of applied voltages on the distribution of archaeal community near various electrodes within the sludge was explored. The results revealed that the application of high voltages exceeding 3.0 V hindered the CH4 production but stimulated the CO2 generation. Subsequently, both CH4 and CO2 production were impeded by the applied voltages. Furthermore, the increased voltages significantly decreased the abundance of Methanomicrobia, Methanosaeta, and Methanosarcina, which were crucial determinants of CH4 content in biogas. Notably, the excessively high voltages intensities caused the AD process to halt and even inactivate the microbial flora. Interestingly, the distribution characteristics of archaeal community were influenced not only by the voltages intensity but also exhibited variations between the anode and cathode regions. Moreover, as the applied voltage intensified, the discrepancy of responses between the cathode and anode regions became more pronounced, offering novel theoretical and technical foundations for the advancement of electricity-assisted with AD technology.

Microbial fingerprints of methanation in a hybrid electric-biological anaerobic digestion

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.

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

Cathodic biofilms - A prerequisite for microbial electrosynthesis

Cathodic biofilms have an important role in CO₂ bio-reduction to carboxylic acids and biofuels in microbial electrosynthesis (MES) cells. However, robust and resilient electroactive biofilms for an efficient CO₂ conversion are difficult to achieve. In this review, the fundamentals of cathodic biofilm formation, including energy conservation, electron transfer and development of catalytic biofilms, are presented. In addition, strategies for improving cathodic biofilm formation, such as the selection of electrode and carrier materials, cell design and operational conditions, are described. The knowledge gaps are individuated, and possible solutions are proposed to achieve stable and productive biofilms in MES cathodes.

Microbial electrochemical approaches of carbon dioxide utilization for biogas upgrading

Microbial electrochemical approach is an emerging technology for biogas upgrading through carbon dioxide (CO₂) reduction and biomethane (or value-added products) production. There is limited literature critically reviewing the latest scientific developments on the bioelectrochemical system (BES) based biogas upgrading technologies, including CO₂ reduction efficiency, methane (CH₄) yields, reactor operating conditions, and electrode materials tested in the BES reactor. This review analyzes the reported performance and identifies crucial parameters considered for future optimization, which is currently missing. Further, the performances of BES approach of biogas upgrading under various operating settings in particular fed-batch, continuous mode in connection to the microbial dynamics and cathode materials have been thoroughly scrutinized and discussed. Additionally, other versatile application options associated with BES based biogas upgrading, such as resource recovery, are presented. Three-dimensional electrode materials have shown superior performance in supplying the electrons for the reduction of CO₂ to CH₄. Most of the studies on the biogas upgrading process conclude hydrogen (H₂) mediated electron transfer mechanism in BES biogas upgrading.

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.

Advances in technological control of greenhouse gas emissions from wastewater in the context of circular economy

This review paper aims to identify the main sources of carbon dioxide (CO₂) emissions from wastewater treatment plants (WWTPs) and highlights the technologies developed for CO₂ capture in this milieu. CO₂ is emitted in all the operational units of conventional WWTPs and even after the disposal of treated effluents and sludges. CO₂ emissions from wastewater can be captured or mitigated by several technologies such as the production of biochar from sludge, the application of constructed wetlands (CWs), the treatment of wastewater in microbial electrochemical processes (microbial electrosynthesis, MES; microbial electrolytic carbon capture, MECC; in microbial carbon capture, MCC), and via microalgal cultivation. Sludge-to-biochar and CW systems showed a high cost-effectiveness in the capture of CO₂, while MES, MECC, MCC technologies, and microalgal cultivation offered efficient capture of CO₂ with associate production of value-added by-products. At the state-of-the-art, these technologies, utilized for carbon capture and utilization from wastewater, require more research for further configuration, development and cost-effectiveness. Moreover, the integration of these technologies has a potential internal rate of return (IRR) that could equate the operation or provide additional revenue to wastewater management. In the context of circular economy, these carbon capture technologies will pave the way for new sustainable concepts of WWTPs, as an essential element for the mitigation of climate change fostering the transition to a decarbonised economy.

Response of Methanogen Communities to the Elevation of Cathode Potentials in Bioelectrochemical Reactors Amended with Magnetite

Electromethanogenesis refers to the process whereby methanogens utilize current for the reduction of CO₂ to CH₄. Setting low cathode potentials is essential for this process. In this study, we tested if magnetite, an iron oxide mineral widespread in the environment, can facilitate the adaptation of methanogen communities to the elevation of cathode potentials in electrochemical reactors. Two-chamber electrochemical reactors were constructed with inoculants obtained from paddy field soil. We elevated cathode potentials stepwise from the initial -0.6 V versus the standard hydrogen electrode (SHE) to -0.5 V and then to -0.4 V over the 130 days of acclimation. Only weak current consumption and CH₄ production were observed in the bioreactors without magnetite. However, significant current consumption and CH₄ production were recorded in the magnetite bioreactors. The robustness of electroactivity of the magnetite bioreactors was not affected by the elevation of cathode potentials from -0.6 V to -0.4 V. However, the current consumption and CH₄ production were halted in the bioreactors without magnetite when the cathode potentials were elevated to -0.4 V. Methanogens related to Methanospirillum were enriched on the cathode surfaces of magnetite bioreactors at -0.4 V, while Methanosarcina relatively dominated in the bioreactors without magnetite. Methanobacterium also increased in the magnetite bioreactors but stayed off electrodes at -0.4 V. Apparently, the magnetite greatly facilitates the development of biocathodes, and it appears that with the aid of magnetite, Methanospirillum spp. can adapt to the high cathode potentials, performing efficient electromethanogenesis. IMPORTANCE Converting CO₂ to CH₄ through bioelectrochemistry is a promising approach to the development of green energy biotechnology. This process, however, requires low cathode potentials, which entails a cost. In this study, we tested if magnetite, a conductive iron mineral, can facilitate the adaptation of methanogens to the elevation of cathode potentials. In two-chamber reactors constructed by using inoculants obtained from paddy field soil, biocathodes developed robustly in the presence of magnetite, whereas only weak activities in CH₄ production and current consumption were observed in the bioreactors without magnetite. The elevation of cathode potentials did not affect the robustness of electroactivity of the magnetite bioreactors over the 130 days of acclimation. Methanospirillum strains were identified as the key methanogens associated with the cathode surfaces during the operation at high potentials. The findings reported in this study shed new light on the adaptation of methanogen communities to the elevated cathode potentials in the presence of magnetite.

Towards the practical application of bioelectrochemical anaerobic digestion (BEAD): Insights into electrode materials, reactor configurations, and process designs

Anaerobic digestion (AD) is one of the most widely adopted bioenergy recovery technologies globally. Despite the wide adoption, AD has been challenged by the unstable performances caused by imbalanced substrate and/or electron availability among different reaction steps. Bioelectrochemical anaerobic digestion (BEAD) is a promising concept that has demonstrated potential for balancing the electron transfer rates and enhancing the methane yield in AD during shocks. While great progress has been made, a wide range of, and sometimes inconsistent engineering and technical strategies were attempted to improve BEAD. To consolidate past efforts and guide future development, a comprehensive review of the fundamental bioprocesses in BEAD is provided herein, followed by a critical evaluation of the engineering and technical optimizations attempted thus far. Further, a few novel directions and strategies that can enhance the performance and practicality of BEAD are proposed for future research to consider. This review and outlook aim to provide a fundamental understanding of BEAD and inspire new research ideas in AD and BEAD in a mechanism-informed fashion.

Ammonium Recovery and Biogas Upgrading in a Tubular Micro-Pilot Microbial Electrolysis Cell (MEC)

Here, a 12-liter tubular microbial electrolysis cell (MEC) was developed as a post treatment unit for simultaneous biogas upgrading and ammonium recovery from the liquid effluent of an anaerobic digestion process. The MEC configuration adopted a cation exchange membrane to separate the inner anodic chamber and the external cathodic chamber, which were filled with graphite granules. The cathodic chamber performed the CO2 removal through the bioelectromethanogenesis reaction and alkalinity generation while the anodic oxidation of a synthetic fermentate partially sustained the energy demand of the process. Three different nitrogen load rates (73, 365, and 2229 mg N/Ld) were applied to the inner anodic chamber to test the performances of the whole process in terms of COD (Chemical Oxygen Demand) removal, CO2 removal, and nitrogen recovery. By maintaining the organic load rate at 2.55 g COD/Ld and the anodic chamber polarization at +0.2 V vs. SHE (Standard Hydrogen Electrode), the increase of the nitrogen load rate promoted the ammonium migration and recovery, i.e., the percentage of current counterbalanced by the ammonium migration increased from 1% to 100% by increasing the nitrogen load rate by 30-fold. The CO2 removal slightly increased during the three periods, and permitted the removal of 65% of the influent CO2, which corresponded to an average removal of 2.2 g CO2/Ld. During the operation with the higher nitrogen load rate, the MEC energy consumption, which was simultaneously used for the different operations, was lower than the selected benchmark technologies, i.e., 0.47 kW/N·m3 for CO2 removal and 0.88 kW·h/kg COD for COD oxidation were consumed by the MEC while the ammonium nitrogen recovery consumed 2.3 kW·h/kg N.

Biogas upgrading, CO2 valorisation and economic revaluation of bioelectrochemical systems through anodic chlorine production in the framework of wastewater treatment plants

Biogas production in wastewater treatment plants (WWTPs) plays a decisive role in the reduction of CO₂ emissions and energy needs in the context of the water-energy nexus. The biogas obtained from sewage sludge digestion can be converted into biomethane by the use of biogas upgrading technologies. In this regard, an innovative water scrubbing based technology, known as ABAD Bioenergy® is presented and considered in this work. The effluents resulting from this system consist of biomethane and treated wastewater with a high CO₂ concentration. Therefore, the study explores the feasibility of using this CO₂-containing effluent in the cathode of a bioelectrochemical system (BES) for the transformation of CO₂ into methane. Techno-economic assessment of the process is presented, including the valorisation of anode reactions through the production of chlorine compounds. Finally, the potential impacts of applying this technology in a WWTP operated by FCC Aqualia are (i) increasing biomethane production by 17.4%, (ii) decreasing CO₂ content by 42.8% and (iii) producing over 60 ppm of chlorine compounds to disinfect all the treated wastewater of the plant.

Methanothrix enhances biogas upgrading in microbial electrolysis cell via direct electron transfer

Bioelectrochemical conversion of CO₂ to CH₄ is a promising way to increase the calorific value of biogas produced during anaerobic digestion. There are two groups of methanogens enriched in these systems, hydrogenotrophs and acetoclastic methanogens that can also directly accept electrons from an electrode or another microorganism. In this study, a microbial electrolysis cell (MEC) poised at -500 mV (vs. SHE) was operated for biogas upgrading. Methane content in the biogas increased from 71% to >90%, and 8.2% of the CO₂ was converted to methane. Methanothrix, an acetoclastic methanogen that can participate in direct electron transfer (DET), and Azonexus, an acetate-oxidizing electrogen, were enriched on the cathode. Transcriptomics revealed that Methanothrix on the cathode were using the CO₂ reduction pathway, while Methanothrix in the bulk sludge were using the acetate decarboxylation pathway for production of methane. These results show that stimulation of DET in MEC enhances biogas-upgrading processes.

Evidence of Spatial Homogeneity in an Electromethanogenic Cathodic Microbial Community

Microbial electrosynthesis (MES) has been gaining considerable interest as the next step in the evolution of microbial electrochemical technologies. Understanding the niche biocathode environment and microbial community is critical for further developing this technology as the biocathode is key to product formation and efficiency. MES is generally operated to enrich a specific functional group (e.g., methanogens or homoacetogens) from a mixed-culture inoculum. However, due to differences in H₂ and CO₂ availability across the cathode surface, competition and syntrophy may lead to overall variability and significant beta-diversity within and between replicate reactors, which can affect performance reproducibility. Therefore, this study aimed to investigate the distribution and potential spatial variability of the microbial communities in MES methanogenic biocathodes. Triplicate methanogenic biocathodes were enriched in microbial electrolysis cells for 5 months at an applied voltage of 0.7 V. They were then transferred to triplicate dual-chambered MES reactors and operated at -1.0 V vs. Ag/AgCl for six batches. At the end of the experiment, triplicate samples were taken at different positions (top, center, bottom) from each biocathode for a total of nine samples for total biomass protein analysis and 16S rRNA gene amplicon sequencing. Microbial community analyses showed that the biocathodes were highly enriched with methanogens, especially the hydrogenotrophic methanogen family Methanobacteriaceae, Methanobacterium sp., and the mixotrophic Methanosarcina sp., with an overall core community representing > 97% of sequence reads in all samples. There was no statistically significant spatial variability (p > 0.05) observed in the distribution of these communities within and between the reactors. These results suggest deterministic community assembly and indicate the reproducibility of electromethanogenic biocathode communities, with implications for larger-scale reactors.

[NiFe]-hydrogenases are constitutively expressed in an enriched Methanobacterium sp. population during electromethanogenesis

Electromethanogenesis is the bioreduction of carbon dioxide (CO₂) to methane (CH₄) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F₄₂₀-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp.

Integration of Membrane Contactors and Bioelectrochemical Systems for CO2 Conversion to CH4

Anaerobic digestion of sewage sludge produces large amounts of CO2 which contribute to global CO2 emissions. Capture and conversion of CO2 into valuable products is a novel way to reduce CO2 emissions and valorize it. Membrane contactors can be used for CO2 capture in liquid media, while bioelectrochemical systems (BES) can valorize dissolved CO2 converting it to CH4, through electromethanogenesis (EMG). At the same time, EMG process, which requires electricity to drive the conversion, can be utilized to store electrical energy (eventually coming from renewables surplus) as methane. The study aims integrating the two technologies at a laboratory scale, using for the first time real wastewater as CO2 capture medium. Five replicate EMG-BES cells were built and operated individually at 0.7 V. They were fed with both synthetic and real wastewater, saturated with CO2 by membrane contactors. In a subsequent experimental step, four EMG-BES cells were electrical stacked in series while one was kept as reference. CH4 production reached 4.6 L CH4 m−2 d−1, in line with available literature data, at a specific energy consumption of 16–18 kWh m−3 CH4 (65% energy efficiency). Organic matter was removed from wastewater at approximately 80% efficiency. CO2 conversion efficiency was limited (0.3–3.7%), depending on the amount of CO2 injected in wastewater. Even though achieved performances are not yet competitive with other mature methanation technologies, key knowledge was gained on the integrated operation of membrane contactors and EMG-BES cells, setting the base for upscaling and future implementation of the technology.

Design, Operation, Modeling and Grid Integration of Power-to-Gas Bioelectrochemical Systems

This paper deals with the design, operation, modeling, and grid integration of bioelectrochemical systems (BES) for power-to-gas application, through an electromethanogenesis process. The paper objective is to show that BES-based power-to-gas energy storage is feasible on a large scale, showing a first approximation that goes from the BES design and operation to the electrical grid integration. It is the first study attempting to cover all aspects of a BES-based power-to-gas technology, on authors’ knowledge. Designed BES reactors were based on a modular architecture, suitable for a future scaling-up. They were operated in steady state for eight months, and continuously monitored in terms of power consumption, water treatment, and biomethane production, in order to obtain data for the following modeling activity. A black box linear model of the BES was computed by using least-square methods, and validated through comparison with collected experimental data. Afterwards, a BES stack was simulated through several series and parallel connections of reactors, in order to obtain higher power consumption and test the grid integration of a real application system. The renewable energy surplus and energy price variability were evaluated for the grid integration of the BES stack. The BES stack was then simulated as energy storage system during low energy price periods, and tested experimentally with a real time system.

Methanogens: biochemical background and biotechnological applications

Since fossil sources for fuel and platform chemicals will become limited in the near future, it is important to develop new concepts for energy supply and production of basic reagents for chemical industry. One alternative to crude oil and fossil natural gas could be the biological conversion of CO2 or small organic molecules to methane via methanogenic archaea. This process has been known from biogas plants, but recently, new insights into the methanogenic metabolism, technical optimizations and new technology combinations were gained, which would allow moving beyond the mere conversion of biomass. In biogas plants, steps have been undertaken to increase yield and purity of the biogas, such as addition of hydrogen or metal granulate. Furthermore, the integration of electrodes led to the development of microbial electrosynthesis (MES). The idea behind this technique is to use CO2 and electrical power to generate methane via the microbial metabolism. This review summarizes the biochemical and metabolic background of methanogenesis as well as the latest technical applications of methanogens. As a result, it shall give a sufficient overview over the topic to both, biologists and engineers handling biological or bioelectrochemical methanogenesis.

Evaluation of key parameters on simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode

Bioelectrochemical systems (BESs) are being studied as an alternative technology for the treatment of several kinds of wastewaters with a lack of electron donor such as high-strength sulfate wastewaters. This study evaluates different parameters that influence the simultaneous sulfate reduction and sulfide oxidation in an autotrophic biocathode: ion-exchange membrane (IEM), cathodic pH and cathode potential. Two different membranes were studied to evaluate sulfate and sulfide adsorption and diffusion from the cathode to the anode, observing that a cation-exchange membrane (CEM) widely decreased these effects. Three different cathode pH (5.5, 7 and 8.5) were studied in a long-term operation observing that pH = 7 was the optimal for sulfate removal, achieving reduction rates around 150 mg S-SO₄^2- L^-1 d^-1. Microbial community analysis of the cathode biofilm demonstrated a high abundance of sulfate-reducing bacteria (SRB, 67% at pH 7, 60% at pH 8.5 and 42% at pH 5.5), mainly Desulfovibrio sp. at pH 5.5 and 7 and Desulfonatronum sp. at pH 8.5. The cathode potential also was studied from -0.7 to -1.2 V vs. SHE achieving sulfate removal rates higher than 700 mg S-SO₄^2- L^-1 d^-1 at cathode potentials from -1.0 to -1.2 V vs. SHE. Also, the highest cathodic recovery and the highest sulfur species imbalance were observed at a cathode potential of -1.0 V vs. SHE, which indicated a higher elemental sulfur production.

Development of biocathode during repeated cycles of bioelectrochemical conversion of carbon dioxide to methane

Functioning biocathodes are essential for electromethanogenesis. This study investigated the development of a biocathode from non-acclimated anaerobic sludge in an electromethanogenesis cell at a cathode potential of -0.7V (vs. standard hydrogen electrode) over four cycles of repeated batch operations. The CO₂-to-CH₄ conversion rate increased (to 97.7%) while the length of the lag phase decreased as the number of cycles increased, suggesting that a functioning biocathode developed during the repeated subculturing cycles. CO₂-resupply test results suggested that the biocathode catalyzed the formation of CH₄ via both direct and indirect (H₂-mediated) electron transfer mechanisms. The biocathode archaeal community was dominated by the genus Methanobacterium, and most archaeal sequences (>89%) were affiliated with Methanobacterium palustre. The bacterial community was dominated by putative electroactive bacteria, with Arcobacter, which is rarely observed in biocathodes, forming the largest population. These electroactive bacteria were likely involved in electron transfer between the cathode and the methanogens.

Continuous micro-current stimulation to upgrade methanolic wastewater biodegradation and biomethane recovery in an upflow anaerobic sludge blanket (UASB) reactor

The dispersion of granules in upflow anaerobic sludge blanket (UASB) reactor represents a critical technical issue in methanolic wastewater treatment. In this study, the potentials of coupling a microbial electrolysis cell (MEC) into an UASB reactor for improving methanolic wastewater biodegradation, long-term process stability and biomethane recovery were evaluated. The results indicated that coupling a MEC system was capable of improving the overall performance of UASB reactor for methanolic wastewater treatment. The combined system maintained the comparatively higher methane yield and COD removal efficiency over the single UASB process through the entire process, with the methane production at the steady-state conditions approaching 1504.7 ± 92.2 mL-CH₄ L^-1-reactor d^-1, around 10.1% higher than the control UASB (i.e. 1366.4 ± 71.0 mL-CH₄ L^-1-reactor d^-1). The further characterizations verified that the input of external power source could stimulate the metabolic activity of microbes and reinforced the EPS secretion. The produced EPS interacted with Fe^2+/3+ liberated during anodic corrosion of iron electrode to create a gel-like three-dimensional [-Fe-EPS-]_n matrix, which promoted cell-cell cohesion and maintained the structural integrity of granules. Further observations via SEM and FISH analysis demonstrated that the use of bioelectrochemical stimulation promoted the growth and proliferation of microorganisms, which diversified the degradation routes of methanol, convert the wasted CO₂ into methane and accordingly increased the process stability and methane productivity.

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.

Bringing High-Rate, CO2-Based Microbial Electrosynthesis Closer to Practical Implementation through Improved Electrode Design and Operating Conditions

The enhancement of microbial electrosynthesis (MES) of acetate from CO2 to performance levels that could potentially support practical implementations of the technology must go through the optimization of key design and operating conditions. We report that higher proton availability drastically increases the acetate production rate, with pH 5.2 found to be optimal, which will likely suppress methanogenic activity without inhibitor addition. Applied cathode potential as low as -1.1 V versus SHE still achieved 99% of electron recovery in the form of acetate at a current density of around -200 A m(-2). These current densities are leading to an exceptional acetate production rate of up to 1330 g m(-2) day(-1) at pH 6.7. Using highly open macroporous reticulated vitreous carbon electrodes with macropore sizes of about 0.6 mm in diameter was found to be optimal for achieving a good balance between total surface area available for biofilm formation and effective mass transfer between the bulk liquid and the electrode and biofilm surface. Furthermore, we also successfully demonstrated the use of a synthetic biogas mixture as carbon dioxide source, yielding similarly high MES performance as pure CO2. This would allow this process to be used effectively for both biogas quality improvement and conversion of the available CO2 to acetate.

Anode hydrodynamics in bioelectrochemical systems

This study assesses the hydrodynamics in the anode compartment of a bioelectrochemical system (BES) when using different electrode materials (graphite rod, granular graphite, stainless steel mesh or graphite plate).