Bioelectrohydrogenesis and inhibition of methanogenic activity in microbial electrolysis cells - A review

Optimizing electrochemically active microorganisms as a key player in the bioelectrochemical system: Identification methods and pathways to large-scale implementation

The rapid global economic growth driven by industrialization and population expansion has resulted in significant issues, including reliance on fossil fuels, energy scarcity, water crises, and environmental emissions. To address these issues, bioelectrochemical systems (BES) have emerged as a dual-purpose solution, harnessing electrochemical processes and the capabilities of electrochemically active microorganisms (EAM) to simultaneously recover energy and treat wastewater. This review examines critical performance factors in BES, including inoculum selection, pretreatment methods, electrodes, and operational conditions. Further, authors explore innovative approaches to suppress methanogens and simultaneously enhance the EAM in mixed cultures. Additionally, advanced techniques for detecting EAM are discussed. The rapid detection of EAM facilitates the selection of suitable inoculum sources and optimization of enrichment strategies in BESs. This optimization is essential for facilitating the successful scaling up of BES applications, contributing substantially to the realization of clean energy and sustainable wastewater treatment. This analysis introduces a novel viewpoint by amalgamating contemporary research on the selective enrichment of EAM in mixed cultures. It encompasses identification and detection techniques, along with methodologies tailored for the selective enrichment of EAM, geared explicitly toward upscaling applications in BES.

Overcoming hydrogen loss in single-chamber microbial electrolysis cells by urine amendment

The effective hydrogen production in single-chamber microbial electrolysis cells (MECs) has been seriously challenged by various hydrogen consumers resulting in substantial hydrogen loss. In previous studies, the total ammonia nitrogen (TAN) has been used to inhibit certain hydrogen-consuming microorganisms to enhance hydrogen production in fermentation. In this study, we explored the feasibility of using source-separated urine to overcome hydrogen loss in the MEC, with the primary component responsible being TAN generated via urea hydrolysis. Experimental results revealed that the optimal TAN concentration ranged from 1.17 g N/L to 1.75 g N/L. Within this range, the hydrogen production rate substantially improved from less than 100 L/(m3·d) up to 520 L/(m3·d), and cathode recovery efficiency and energy recovery efficiency were greatly enhanced, with the hydrogen percentage achieved over 95 % of the total gas volume, while maintaining uninterrupted electroactivity in the anode. Compared to using chemically added TAN, using source separated urine as the source of ammonia also showed the effect of overcoming hydrogen loss but with lower Coulombic efficiency due to the complex organic components. Pre-adaptation of the reactor with urea enhanced hydrogen production by nearly 60 %. This study demonstrated the effectiveness of TAN and urine in suppressing hydrogen loss, and the results are highly relevant to MECs treating real wastewater with high TAN concentrations, particularly human fecal and urine wastewater.

Comparative review on microbial electrochemical technologies for resource recovery from wastewater towards circular economy and carbon neutrality

Newly arising concepts such as the circular economy and carbon neutrality motivate resource recovery from wastewater. This paper reviews and discusses state-of-the-art microbial electrochemical technologies (METs), specifically microbial fuel cells (MFCs), microbial electrolysis cells (MECs) and microbial recycling cells (MRCs), which enable energy generation and nutrient recovery from wastewater. Mechanisms, key factors, applications, and limitations are compared and discussed. METs are effective in energy conversion, demonstrating advantages, drawbacks and future potential as specific scenarios. MECs and MRCs exhibited greater potential for simultaneous nutrient recovery, and MRCs offer the best scaling-up potential and efficient mineral recovery. Research on METs should be more concerned with lifespan of materials, secondary pollutants reduction and scaled-up benchmark systems. More up-scaled application cases are expected for cost structures comparison and life cycle assessment of METs. This review could direct the follow-up research, development and successful implementation of METs for resource recovery from wastewater.

A Review of Biohydrogen Production from Saccharina japonica

Saccharina japonica (known as Laminaria japonica or Phaeophyta japonica), one of the largest macroalgae, has been recognized as food and medicine for a long time in some Asian countries, such as China, South Korea, Japan, etc. In recent years, S. japonica has also been considered the most promising third-generation biofuel feedstock to replace fossil fuels, contributing to solving the challenges people face regarding energy and the environment. In particular, S. japonica-derived biohydrogen (H2) is expected to be a major fuel source in the future because of its clean, high-yield, and sustainable properties. Therefore, this review focuses on recent advances in bio-H2 production from S. japonica. The cutting-edge biological technologies with suitable operating parameters to enhance S. japonica’s bio-H2 production efficiency are reviewed based on the Scopus database. In addition, guidelines for future developments in this field are discussed.

Boosting hydrogen production from fermentation effluent of biomass wastes in cylindrical single-chamber microbial electrolysis cell

Microbial electrolysis cells (MECs) are considered as green technologies for H₂ production with simultaneously wastewater treatment. Low H₂ recovery and production rate are two key bottlenecks of MECs fed with real H₂ fermentation effluent of biomass wastes. This work established a 1 L cylindrical single chamber MEC and enriched electroactive anodic biofilms from cow dung compost to overcome the bottlenecks. These MEC components (platinum-coated cylindrical titanium mesh cathode and cylindrical graphite felt anode) were arranged in a concentric configuration. A high H₂ production rate of 6.26 ± 0.23 L L^-1 day^-1 with H₂ yield of 5.75 ± 0.16 L H₂ L^-1 fermentation effluent was achieved at 0.8 V. The degradation of acetate and butyrate (main components of H₂ fermentation effluent) could reach to 95.3 ± 2.1% and 78.4 ± 3.6%, respectively. The microbial community analysis for anode showed the abundance of Geobacter (22.6%), Syntrophomonas (8.7%), and Dysgonomonas (6.3%) which are responsible to complex substrate oxidation, electrical current generation, and H₂ production. These results prove the feasibility of cylindrical single-chamber MEC for high H₂ production rate and high acetate and butyrate removal from H₂ fermentation effluent.

Suppressing Methane Production to Boost High-Purity Hydrogen Production in Microbial Electrolysis Cells

Hydrogen gas (H₂) is an attractive fuel carrier due to its high specific enthalpy; moreover, it is a clean source of energy because in the combustion reaction with oxygen (O₂) it produces water as the only byproduct. The microbial electrolysis cell (MEC) is a promising technology for producing H₂ from simple or complex organics present in wastewater and solid wastes. Methanogens and non-archaeal methane (CH₄)-producing microorganisms (NAMPMs) often grow in the MECs and lead to rapid conversion of produced H₂ to CH₄. Moreover, non-archaeal methane production (NAMP) catalyzed by nitrogenase of photosynthetic bacteria was always overlooked. Thus, suppression of CH₄ production is required to enhance H₂ yield and production rate. This review comprehensively addresses the principles and current state-of-the-art technologies for suppressing methanogenesis and NAMP in MECs. Noteworthy, specific strategies aimed at the inhibition of methanogenic enzymes and nitrogenase could be a more direct approach than physical and chemical strategies for repressing the growth of methanogenic archaea. In-depth studies on the multiomics of CH₄ metabolism can possibly provide insights into sustainable and efficient approaches for suppressing metabolic pathways of methanogenesis and NAMP. The main objective of this review is to highlight key concepts, directions, and challenges related to boosting H₂ generation by suppressing CH₄ production in MECs. Finally, perspectives are briefly outlined to guide and advance the future direction of MECs for production of high-purity H₂ based on genetic and metabolic engineering and on the interspecific interactions.

Impact factors and novel strategies for improving biohydrogen production in microbial electrolysis cells

Microbial electrolysis cell (MEC) system is an environmentally friendly method for clean biohydrogen production from a wide range of biowastes owing to low greenhouse gas emissions. This approach has relatively higher yields and lower energy costs for biohydrogen production compared to conventional biological technologies and direct water electrolysis, respectively. However, biohydrogen production efficiency and operating costs of MEC still need further optimization to realize its large-scale application.This paper provides a unique review of impact factors influencing biohydrogen production in MECs, such as microorganisms and electrodes. Novel strategies, including inhibition of methanogens, development of novel cathode catalyst, advanced reactor design and integrated systems, to enhance low-cost biohydrogen production, are discussed based on recent publications in terms of their opportunities, bottlenecks and future directions. In addition, the current challenges, and effective future perspectives towards the practical application of MECs are described in this review.

A multi-perspective review on microbial electrochemical technologies for food waste valorization

The worldwide generation of food waste (FW) has been increasing enormously due to the growing food industry and population. However, FW contains a large amount of biodegradable organics that can be converted to clean energy, which can potentially minimize the utilization of fossil fuels. Conventional biowaste valorization technologies, such as anaerobic digestion and composting, have been adopted for FW management for recovering useful biogas and compost. However, they are often limited by high capital and operation costs, low recovery efficiency, slow process kinetics, and system instability. On the other hand, microbial electrochemical technologies (METs) have been highly promising for efficiently harvesting bioenergy and high value-added products from FW. Hence, this article critically reviews up-to-date studies on applying various METs regarding their value-added products recovery efficiencies from FW. Moreover, this review lists existing challenges, ways to optimize the system performance and provides perspectives on future research needs.

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.

Onset Investigation on Dynamic Change of Biohythane Generation and Microbial Structure in Dual-chamber versus Single-chamber Microbial Electrolysis Cells

Biohythane is alternative fuel to replace fossil fuel for car combustion, and biohythane generation could be potential pathway for energy recovery from wastewater treatment. Microbial electrolysis cell (MEC) is electrochemical technique to convert waste to methane and hydrogen gas for biohythane generation, but the feasibility and stability of MEC needs further investigation to assure sustainable energy recovery. System configuration is paramount factor for electrochemical reaction and mass transfer, and this study was to investigate the configuration impact (single vs dual chamber) of MEC for biohythane generation rate and stability. This study showed that dual-chamber MEC could separate methane and hydrogen gas production in the anode and cathode, and combined both together to produce biohythane. To reduce ohmic resistance for higher current, cation exchange membrane (CEM) was removed from dual-chamber to single-chamber MEC. However, free hydrogen diffusion was allowed in the single chamber since CEM was removed. The diffused hydrogen and substrate towards the cathode would favor the methanogen growth, and thus the hydrogen was consumed to reduce the biohythane generation and energy recovery efficiency (i.e., 7.5 × 10^-3 reduced to 5.7 × 10^-3 kWh kg^-1 degraded COD day^-1 after converting dual-chamber to single-chamber MEC). Absolute abundance of methanogen in single-chamber MEC was greatly boosted, as Methanosarcina and Methanobacteriale on the anode surface, increased by 132% and 243%, respectively, while the original dual-chamber MEC could maintain Geobacter growth for high current generation. This is the keystone study to demonstrate the importance of dual-chamber MEC for the feasibility and stability for the biohythane generation, building up the foundation to use electrochemical device to convert the organic waste to the alternative biohythane.

Hydrogen production from lignocellulosic hydrolysate in an up-scaled microbial electrolysis cell with stacked bio-electrodes

Hydrogen production from renewable resources via microbial electrolysis cells (MECs) is a promising approach for sustainable energy production. Yet high hydrogen yield from real feedstocks has not been demonstrated in up-scaled MECs. In this study, a 10-L single chamber MEC with a high electrode surface area to volume ratio (66 m²/m³) was constructed and electroactive cathodic biofilms were enriched for hydrogen evolution reaction. A high hydrogen yield of 91% was achieved using lignocellulosic hydrolysate with a hydrogen production rate of 0.71 L/L/D at an organic loading rate of 0.4 g/D. The anodic and cathodic microbial communities, with Enterococcus spp. as the known electroactive bacteria, were capable of achieving current densities of 13.7 A/m² and 16.5 A/m², respectively. A machine learning algorithm was used to investigate the correlation between community data and electrochemical performance, and the critical genera on determining current density were identified.

Hydrogen production from macroalgae by simultaneous dark fermentation and microbial electrolysis cell

Hydrogen production from Saccharina Japonica by simultaneous dark fermentation (DF) and microbial electrolysis cell (MEC), called sDFMEC, was studied. In the novel sDFMEC process, substrates were converted to H₂ and volatile fatty acids (VFAs) by DF in the bulk phase, and VFAs are simultaneously oxidized by the exoelectrogens in the microbial film on anode electrode with further production of H₂ at the cathode. The sDFMEC process was compared with DF and a combined process of DF and MEC in series (DF-MEC) in terms of H₂ production. The overall H₂ production from S. Japonica in sDFMEC process was higher (438.7 ± 13.3 mL/g-TS), than DF (54.6 ± 0.8 mL/g-TS) and DF-MEC (403.5 ± 7.9 mL/g-TS) process, respectively, which is approximately 3-times higher than those reported in the literature.

Methanogenesis inhibitors used in bio-electrochemical systems: A review revealing reality to decide future direction and applications

Microbial fuel cell (MFC) is a robust technology capable of treating real wastewaters by utilizing mixed anaerobic microbiota as inoculum for producing electricity from oxidation of the biodegradable matters. However, these mixed microbiota comprises of both electroactive microorganisms (EAM) and substrate/electron scavenging microorganisms such as methanogens. Hence, in order to maximize bioelectricity from MFC, different physio-chemical techniques have been applied in past investigations to suppress activity of methanogens. Interestingly, recent investigations exhibit that methanogens can produce electricity in MFC and possess the cellular machinery like cytochrome c and Type IV pili to perform extracellular electron transfer (EET) in the presence of suitable electron acceptors. Hence, in this review, in-depth analysis of versatile behaviour of methanogens in both MFC and natural anaerobic conditions with different inhibition techniques is explored. This review also discusses the future research directions based on the latest scientific evidence on role of methanogens for EET in MFC.

Suppressing methanogens and enriching electrogens in bioelectrochemical systems

Suppression of methanogens is considered as one of the main challenges in achieving the practical application of several types of bioelectrochemical system (BES). Feasibility of mixed culture as an inoculum in BES is mainly restricted by methanogenic population. Methanogens compete with electrogens (in bioanodes) or acetogens (in biocathodes) for substrate which results in diminishing Coulombic efficiency. Selection of particular inoculum pretreatment method affects the microbial diversity in anodic/cathodic microenvironments and hence the performance of BES. This review discusses various physical, chemical and biological pretreatment methods for suppressing the growth of methanogens. Selective microbial enrichment in anodic/cathodic biofilm can be promoted with bioaugmentation and/or applied external potential approach to harvest maximum Coulombs from the substrate. For field application of BES, physical pretreatment methods can be proposed with intermittent addition of chemical inhibitors and conversion of methane to electricity in order to make the process inexpensive along with recovering the maximum energy.

Thermophiles for biohydrogen production in microbial electrolytic cells

Thermophiles are promising options to use as electrocatalysts for bioelectrochemical applications including microbial electrolysis. They possess several interesting characteristics such as ability to catalyze a broad range of substrates at better rates and over a broad range of operating conditions, and better electrocatalysis/electrogenic activity over mesophiles. However, a very limited number of investigations have been carried out to explore the microbial reactions/pathways and the molecular mechanisms that contribute to better electrocatalysis/electrolysis in thermophiles. Here, we review the electroactive characteristics of thermophiles, their electron transfer mechanisms, and molecular insights behind the choice of thermophiles for bioelectrochemical/electrolytic processes.