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Efficient hydrogen production in biocathode-driven microbial electrolysis cells (MECs) can be obtained by electroautotrophic microbes with the ability to regenerate biocatalytic activity. Inoculation and enrichment of a pure defined culture in the cathode are critical for high performance in hydrogen production. Zero-gap MECs with anion exchange membranes (AEMs) enhance hydrogen production due to low internal resistance and better pH balance. Engineered microbes and optimized microbe–electrode interactions can further increase MEC performance and accelerate its commercialization. Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe–electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production. [ABSTRACT FROM AUTHOR]

This study explores the possibility of utilising electrochemically assisted anaerobic digestion supplemented with carbon-based materials to stimulate methanogenesis. Two different carbonaceous materials—commercial activated carbon (AC), and pyrolysed argan (PA, derived from argan shells)—were employed as supplements, with cheese whey (CW) being used as the substrate. Methane production slightly increased in the electrochemically assisted digesters, potentially translating into a 2–4% increase in the output of industrial digesters. In addition, reactors supplemented with PA also exhibited better production rates (496–508 L·kgVS−1), although there was no observed improvement in the quantity of biogas at the end of the biodegradability experiment. In contrast, when commercial AC was used as the supplement, the start-up phase was accelerated (5 days), although methane productivity decreased (273–352 L·kgVS−1). These observations were supported by microbiological analyses, demonstrating that the reactors with the poorest performance (those supplemented with AC) experienced the most significant decrease in both archaeal and bacterial populations. [ABSTRACT FROM AUTHOR]

The development of net-zero emission fuels is a priority area of modern research due to the imminent reduction of fossil fuel reserves and environmental problems caused by their combustion. One of the promising fuels is hydrogen, which has a high heat of combustion and is eco-friendly, forms water as the only byproduct. Recently, methods of hydrogen production by microorganisms, which use directly the solar energy or utilize the organic waste during fermentation, have been intensively developed and applied. In this review, the basic principles of the main light-dependent (biophotolysis, photofermentation) and light-independent (dark fermentation and microbial electrolysis) methods of biological hydrogen production are discussed. Particular attention is paid to the advantages and disadvantages of these methods, the possibility of combining them into a single system, as well as various strategies for improving biohydrogen production aimed at transition from laboratory research to full-scale application. [ABSTRACT FROM AUTHOR]

In this paper, we present an updated review on the mathematical modeling of different biological methods of hydrogen production. The presented mathematical modeling and methods range from inception to the current state-of-the-art developments in hydrogen production using biological methods. A comparative study was performed along with indications for future research and shortcomings of earlier research. This review will be helpful for all researchers working on different methods of hydrogen production. However, we only covered biological methods such as biophotolysis, fermentation and microbial electrolysis cells, and this list is not exhaustive of all other methods of hydrogen production.

Sludge lysis liquid is a highly concentrated solution containing macromolecular organic matter produced during thermal hydrolysis of sludge. Anaerobic digestion is commonly used to achieve degradation conversion and resource recovery. However, during the thermal conversion process, the presence of refractory organic matter such as Melad reaction products can result in low anaerobic conversion efficiency and methane yield. To address the issue, this study proposes a method for the intensive treatment and resource recovery of sludge lysis liquid using a microbial electrolysis-assisted anaerobic digestion (ME-AD) system. The study aimed to investigate the degradation efficacy of macromolecular organic matter and the conversion efficiency of short-chain fatty acids (SCFAs). The results of the study showed that the direct treatment of raw lysis liquid with ME-AD improved the anaerobic degradation efficiency compared to the usual treatment, which involved a 5-fold dilution of lysis liquid. The COD removal rate was 40.2% in the renewal batch cycle, which was 15.6% higher than the conventional anaerobic conversion. Additionally, the complex organic matter in the intensively degraded sludge lysis liquid was converted to SCFAs, which were produced at a maximum concentration of 40.0×10^5 mg/L during the fifth day of intensive anaerobic fermentation. Among the SCFAs, acetic acid had the highest percentage at 68.1%. This study provides a new approach to address the treatment of sludgelys is liquid and utilization of sludge resources.

In this paper, we present an updated review on the mathematical modeling of different biological methods of hydrogen production. The presented mathematical modeling and methods range from inception to the current state-of-the-art developments in hydrogen production using biological methods. A comparative study was performed along with indications for future research and shortcomings of earlier research. This review will be helpful for all researchers working on different methods of hydrogen production. However, we only covered biological methods such as biophotolysis, fermentation and microbial electrolysis cells, and this list is not exhaustive of all other methods of hydrogen production. [ABSTRACT FROM AUTHOR]

Given the current issues with global warming and rising greenhouse gas emissions, biohydrogen is a viable alternative fuel option. Technologies to produce biohydrogen include photo fermentation, dark fermentation, direct and indirect bio-photolysis, and two-stage fermentation. Biological hydrogen generation is a green and promising technique with mild reaction conditions and low energy consumption compared to thermochemical and electrochemical hydrogen generation. To optimize hydrogen gas output using this method, the activity of hydrogen-consuming bacteria should be restricted during the production stages of hydrogen and acetate to prevent or limit hydrogen consumption. Raw material costs, poor hydrogen evolution rates, and large-scale output are the main limitations in biological hydrogen generation systems. Organic wastes would be the most preferred target feedstock for hydrogen fermentation, aside from biodegradable wastes, due to their high amount and simultaneous waste treatment advantage. This study examined the three primary methods for converting waste into bio-hydrogen: microbial electrolysis cell, thermochemical gasification, and biological fermentation, from both a technological and environmental standpoint. The effectiveness and applicability of these bioprocesses in terms of aspects influencing processes and their constraints are discussed. Alternative options for improving process efficiency, like microbial electrolysis, bio-augmentation, and multiple process integration, are also considered for industrial-level applications. Biohydrogen generation might be further enhanced by optimization of operating conditions and adding vital nutrients and nanoparticles. Cost reduction and durability enhancement are the most significant hindrances to fuel-cell commercialization. This review summarizes the biohydrogen production pathways, the impact of used organic waste sources, and bacteria. The work also addresses the essential factors, benefits, and challenges. • Biophoton-driven biohydrogen production strategies are reviewed. • Pathways for biohydrogen production are discussed with key examples. • Role of various bioreactors and impact of nanoparticles on biohydrogen production. [ABSTRACT FROM AUTHOR]

AbstractIntroduction: Hydrogen gas has great potential as a renewable energy source. Although hydrogen gas is obtained from fossil fuels, there is a demand for its production by chemical methods and electrolysis of water, and its extraction from oil sands is currently being studied. Its biological production is one of the cleanest types of hydrogen fuel production due to the consumption of greenhouse gases such as carbon dioxide and/or aerobic and anaerobic fermentation of agricultural wastes, and it can be considered as a solution for simultaneous production of the fuel and elimination of environmental pollutions.Materials and Methods: The present study briefly explains the hydrogen-producing microorganisms, the mechanisms, and the effective enzymes in their production. For this purpose, authoritative articles published between 2006 and 2021 and information in the Scopus database have been used to draw graphs and write this review article. In addition, some limitations and the ways to overcome them for increasing biohydrogen production are described in detail.Results: The results show that carbon dioxide fixation, fermentation, nitrogen fixation, aerobic and anaerobic photosynthesis are some ways of biological production of this fuel.Discussion and Conclusion: Important elements in biological production are the effective enzymes in these reactions and the possibility of using these enzymes in the continuous production of gas, and the purification of hydrogen from other gases. The stabilization of these enzymes under their conditions is of particular importance for the mass production of this gas. Purification of bio-produced hydrogen gas is essential for consumption as fuel, and some technologies, including nanomembranes such as polysulfonate, are very helpful in purifying this gas from other gases such as oxygen, nitrogen, ammonium, hydrogen sulfide, and oxygen.

• Pilot MEC was integrated with HTL to treat wastewater and produce H2. • Developed effective strategies for fast startup, methanogenesis and acetogenesis inhibition. • Sufficient H2 was produced to meet biocrude upgrading demand. The global shift toward net-zero emissions necessitates resource recovery from wet waste. In this study, we demonstrate the first feasibility of combining pilot-scale microbial electrolytic cells (MECs) with hydrothermal liquefaction (HTL) for simultaneous post-hydrothermal liquefaction wastewater (PHW) treatment and efficient hydrogen (H₂) production to meet biocrude upgrading requirements. Long-term single reactor operation revealed that fixed anode potential enabled rapid startup, and low catholyte pH and high salinity were effective in suppression of cathodic methanogenesis and acetogenesis – resulting in high current density of 16.6 A m −2 and 9.3 A m −2 when feeding synthetic wastewater and PHW respectively. Additionally, the anode biofilm exhibited spatial variations in response to local environmental conditions. Onsite parallel or serial operations of multiple MECs showed good performance using actual PHW with a record-high H 2 production rate of 0.5 L L R day−1 for MEC over 10 liters scale, and the optimal chemical oxygen demand (COD)-to-H 2 yield reached 0.127 kg-H 2 per kg-COD, supporting a self-sufficient, closed-loop upgrade to jet fuel. [Display omitted] [ABSTRACT FROM AUTHOR]

Microbial electrochemical technologies now enable microbial electrosynthesis (MES) of organic compounds using microbial electrolysis cells handling waste organic materials. An electrolytic cell with an MES cathode may generate soluble organic molecules at a higher market price than biomethane, thereby satisfying both economic and environmental goals. However, the long-term viability of bioanode activity might become a major concern. In this work, a 15-L MES reactor was designed with specific electrode configurations. An electrochemical model was established to assess the feasibility and possible performance of the design, considering the aging of the bioanode. The reactor was then constructed and tested for performance as well as a bioanode regeneration assay. Biowaste from an industrial deconditioning platform was used as a substrate for bioanode. The chemical oxygen demand (COD) removal rate in the anodic chamber reached 0.83 g day−1 L−1 of anolyte. Acetate was produced with a rate of 0.53 g day−1 L−1 of catholyte, reaching a maximum concentration of 8.3 g L−1. A potential difference (from 0.6 to 1.2 V) was applied between the bioanode and biocathode independent of reference electrodes. The active biocathode was dominated by members of the genus Pseudomonas, rarely reported so far for MES activity. [ABSTRACT FROM AUTHOR]

Ammonia removal from wastewater was successfully achieved by simultaneous nitrification and denitrification (SND) in a double-chamber microbial electrolysis cell (MEC). The MEC operations at different applied voltages (0.7 to 1.5 V) and initial ammonia concentrations (30 to 150 mg/L) were conducted in order to evaluate their effects on MEC performance in batch mode. The maximum nitrification efficiency of 96.8% was obtained in the anode at 1.5 V, followed by 94.11% at 1.0 V and 87.05% at 0.7. At 1.5 V, the initial ammonia concentration considerably affected the nitrification rate, and the highest nitrification rate constant of 0.1601/h was determined from a first-order linear regression at 30 mg/L ammonium nitrogen. The overall total nitrogen removal efficiency was noted to be 85% via the SND in the MEC operated at an initial ammonium concentration of 50 mg/L and an applied cell voltage of 1.5 V. The MEC operation in continuous mode could remove ammonia (50 mg/L) in a series of anode and cathode chambers at the nitrogen removal rate of 170 g-N/m3.d at an HRT of 15. This study suggests that a standalone dual-chamber MEC can efficiently remove ammonia via the SND process without needing additional organic substrate and aeration, which makes this system viable for field applications. [ABSTRACT FROM AUTHOR]

In microbial electrolysis cells (MECs), microbial communities catalyze conversions between dissolved organic compounds, electrical energy, and energy carriers such as hydrogen and methane. Bacteria and archaea, which catalyze reactions on the anode and cathode of MECs, interact with phages; however, phage communities have previously not been examined in MECs. In this study, we used metagenomic sequencing to study prokaryotes and phages in nine MECs. A total of 852 prokaryotic draft genomes representing 278 species, and 1476 phage contigs representing 873 phage species were assembled. Among high quality prokaryotic genomes (>95% completion), 55% carried a prophage, and the three Desulfobacterota spp. that dominated the anode communities all carried prophages. Geobacter anodireducens , one of the bacteria dominating the anode communities, carried a CRISPR spacer showing evidence of a previous infection by a Peduoviridae phage present in the liquid of some MECs. Methanobacteriaceae spp. and an Acetobacterium sp., which dominated the cathodes, had several associations with Straboviridae spp. The results of this study show that phage communities in MECs are diverse and interact with functional microorganisms on both the anode and cathode., Competing Interests: The authors declare no conflict of interest., (© The Author(s) 2024. Published by Oxford University Press on behalf of the International Society for Microbial Ecology.)

Microbial electrolysis cells (MECs) are devices that employ electroactive bacteria to perform extracellular electron transfer, enabling hydrogen generation from biodegradable substrates. In our previous work, we developed and analyzed a differential-algebraic equation (DAE) model for MECs. The model resembles a chemostat or continuous stirred tank reactor (CSTR). It consists of ordinary differential equations for concentrations of substrate, microorganisms, and an extracellular mediator involved in electron transfer. There is also an algebraic constraint for electric current and hydrogen production. Our goal is to determine the outcome of competition between methanogenic archaea and electroactive bacteria, because only the latter contribute to electric current and the resulting hydrogen production. We investigate asymptotic stability in two industrially relevant versions of the model. An important aspect of many chemostat models is the principle of competitive exclusion. This states that only microbes which grow at the lowest substrate concentration will survive as t → ∞. We show that if methanogens can grow at the lowest substrate concentration, then the equilibrium corresponding to competitive exclusion by methanogens is globally asymptotically stable. The analogous result for electroactive bacteria is not necessarily true. In fact we show that local asymptotic stability of competitive exclusion by electroactive bacteria is not guaranteed, even in a simplified version of the model. In this case, even if electroactive bacteria can grow at the lowest substrate concentration, a few additional conditions are required to guarantee local asymptotic stability. We provide numerical simulations supporting these arguments. Our results suggest operating conditions that are most conducive to success of electroactive bacteria and the resulting current and hydrogen production in MECs. This will help identify when producing methane or electricity and hydrogen is favored.

In this study, EIS data collected from three electrode half-cell configurations was used to qualitatively identify and quantitatively determine the responses of ohmic, kinetic, and mass transfer impedances to buffer concentration, flow rate, and applied potential in an MEC consisting of a bioanode and an abiotic nickel-mesh cathode separated by a microporous membrane. EIS measurements were collected during startup and growth (including an abiotic run) at closed circuit and open circuit conditions to accurately match portions of the EIS spectra with the corresponding physical processes and to quantify kinetic changes as the biofilm matured. Once the MEC reached a target current density of 10 A/m2, a multifactorial experimental design formulated as a Taguchi array was executed to assess the impact of flow rate, buffer concentration, and applied voltage on EIS and performance response variables. Multivariate analysis was conducted to ascertain the relative importance of the independent variables and identify any correlations between process conditions and system response. The liquid flow through the anode was found to be strongly correlated with the impedance parameters and the MEC performance, while applied voltage influenced them to a lesser degree. The results are important from an industrial application perspective and provide insights into parameters important for process optimization.

High overpotential and soaring prices of the cathode electrode are the bottlenecks for the development of microbial electrolysis technology for hydrogen production. In this study, a novel one-step electrodeposition method has been attempted to fabricate electrodeposited cathodes in situ growth of Ni–Co–S, Ni–S, Co–S catalyst on nickel foam (NF) to reduce the overpotential of electrodes. Finally, a uniform nanosheet with a high specific surface area and more active sites is formed on the NF surface, resulting in a lower overpotential than plain NF. At 0.8 V, the Co–S/NF cathode produces a favorable 42% increase in hydrogen yield (0.68 m3·m−3·d−1), 40% upsurge in current density (10.6 mA/cm3) and 39% rise of cathodic recovery rate (58.0 ± 3.2%) than bare NF, followed by Ni–Co–S/NF and Ni–S/NF cathode. All the electrodeposited electrodes demonstrate enhanced current density and reduced electron losses, thereby achieving efficient hydrogen production. These innovative varieties of electrodes are highly advantageous as they are relatively inexpensive and easy to manufacture with great potential in reducing costs and further real time application in large scale. • The overpotential of nickel foam is greatly reduced by Ni/Co/S electrodeposition. • High hydrogen yield is achieved by Co–S/NF electrode in neutral condition. • Electrodeposited electrodes show high current density and low electron losses in MEC. [ABSTRACT FROM AUTHOR]

[Display omitted] • Acetate-fed MECs demonstrated the highest H 2 yield, followed by lactate-fed MECs. • Ethanol-fed MECs had the lowest coulombic efficiency but highest H 2 production rate. • Butyrate-fed MECs showed the slowest start-up and reaction, followed by propionate. • The mixture used as a model DFE was efficiently utilized to produce H 2 in MECs. • DFE composition impacted the development of the active anodic microbial community. This study comparatively investigated the exoelectrogenic utilization and hydrogen conversion of major dark fermentation products (acetate, propionate, butyrate, lactate, and ethanol) from organic wastes in dual-chamber microbial electrolysis cells (MECs) alongside their mixture as a simulated dark fermentation effluent (DFE). Acetate-fed MECs showed the highest hydrogen yield (1,465 mL/g chemical oxygen demand), near the theoretical maximum yield, with the highest coulombic efficiency (105%) and maximum current density (7.9 A/m2), followed by lactate-fed, propionate-fed, butyrate-fed, mixture-fed, and ethanol-fed MECs. Meanwhile, the highest hydrogen production rate (514 mL/L anolyte∙d) was observed in ethanol-fed MECs despite their lower coulombic efficiency. Butyrate was the least favored substrate, followed by propionate, leading to significantly delayed startup and reaction. The active anodic microbial community structure varied considerably among the MECs utilizing different substrates, particularly between Geobacter and Acetobacterium dominance. The results highlight the substantial effect of the DFE composition on its utilization and current-producing bioanode development. [ABSTRACT FROM AUTHOR]

• Continuous Operation at 20 g/L-d resulted in H 2 productivity of 7.9 ± 0.4 L/L-d. • Fed-batch operation at 2.5 g/L resulted in substantial loss of performance. • High concentration resulted in selective advantages of a specific Geobacter sp. • Buildup of propionic acid and loss of electrons increased substantially at 2.5 g/L. • Continuous operation was able to better mitigate these effects. Microbial electrolysis cells have potential to be integrated into biorefineries for waste valorization in the form of renewable hydrogen, a reagent needed for bio-oil upgrading. However, reaching high productivities and sustaining them during long-term operation requires a better understanding of the impact of process conditions on the biocatalyst and overall performance. The impact of feeding regime and different concentration levels in continuous vs. fed-batch delivery of a switchgrass-derived substrate were comparatively investigated. At an organic loading of 20 g/L-d, continuous operation resulted in a stable average H 2 productivity of 7.9 ± 0.4 L/L-d over a 3-week period, the highest reported value for continuous hydrogen production from complex streams via microbial electrolysis. Corresponding fed-batch operation yielded 46% lower productivity on average, which dropped further to <1 L/L-d after two-weeks of operation. Principle component analysis (PCA) of the data linked changes in exoelectrogen population to the decrease in performance at high concentrations during fed-batch operation. Accumulation of propionic acid was linked to electron diversion to undefined sinks. The differences in working concentration between the two modes of feeding resulted in different selective pressures, leading to an increase in fermenting microbes such as Firmicutes , and emergence of a more tolerant Geobacter sp. at high substrate concentrations. These insights increase the understanding of how different microbial families change with process conditions, and their impact on performance. This study highlights how process design can improve microbial community management and sustain long-term performance, which is key for commercial applications of microbial electrolysis systems. [ABSTRACT FROM AUTHOR]

Highlights • Electroactive characteristics of thermophiles. • Harnessing thermophiles for biohydrogen production in microbial electrolysis cells. • Electron transfer mechanisms in thermophiles. • Advantages of using thermophiles for microbial electrolysis. Abstract 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • The optimal applied voltage was 1.1 V for MAB to treat N-rich saline wastewater. • Candidatus Scalindua was enriched and strengthened by properly applied voltage. • Highest ammonium oxidation rate was 55 g/m3•d by marine anammox without nitrite. • Anodic anammox and anodic nitrification were integrated for marine anammox process. Currently, ammonium removal from saline wastewater still poses a challenge to conventional biological nitrogen removal process. Herein, the conversion of NH 4 +-N to N 2 was successfully driven by marine anammox bacteria (MAB) in a single chamber microbial electrolytic cell (MEC) using NH 4 +-N as the sole substrate. Compared with no ammonium removal in the abiotic reactor with applied voltage (0–1.5 V), the maximum ammonium and total nitrogen removal rate (ARR and TNRR) of 55 and 50 g/m3•d were obtained at the voltage of 1.1 V, which were 78 % and 76 % higher than that of 0.5 V, respectively. At the voltage ≥ 1.1 V, the accumulation of NO 2 –-N and NO 3 –-N was observed in the MEC, and the nitrifying bacteria Nitrosmonas and Nitrospira were detected on the anode surface with the relative abundances of 0.84 % and 0.21 %, respectively. It was proved that the production of NO 2 –-N and NO 3 –-N was an electrochemical biological process by electroactive nitrifying bacteria. Candidatus Scalindua was the main functional genus for ammonium removal at anode biofilm with the relative abundance of 19.27 %. Therefore, ammonium was successfully removed by integrating anodic anammox and anodic nitrification. Applied voltage increased the microbial diversities at anode surface and promoted the formation of electrode biofilm. These findings indicated a promising saline wastewater treatment by employing salt-tolerant MAB for ammonium removal. [ABSTRACT FROM AUTHOR]

Valorization of waste streams is becoming increasingly important to improve resource recovery and economics of bioprocesses for the production of fuels. The pyrolysis process produces a significant portion of the biomass as an aqueous waste stream, called bio-oil aqueous phase (BOAP), which cannot be effectively converted into fuel. In this report, we detail the separation and utilization of this stream for the production of electrons, hydrogen, and chemicals, which can supplement fuel production improving economics of the biorefinery. Separation methods including physical separation via centrifugal separator, chemical separation via pH manipulation, and electrochemical separation via capacitive deionization are discussed. Bioelectrochemical systems (BES) including microbial fuel cells (MFCs), microbial electrolysis cells (MECs), and electro-fermentation processes are reviewed for their potential to generate current, hydrogen, and chemicals from BOAP. Recent developments in MECs using complex waste streams and electro-active biocatalyst enrichment have resulted in advancement of the technology toward performance metrics closer to commercial requirements. Current densities above 10 A/m2 have been reported using BOAP, which suggest further work to demonstrate the technology at pilot scale should be undertaken. The research on electro-fermentation is revealing potential to generate alcohols, diols, medium chain fatty acids, esters, etc. using electrode-based electrons. The ability to derive electrons and chemical building blocks from waste streams illustrate the advancement of the BES technology and potential to push the frontiers of bioenergy generation one step further toward development of a circular bioeconomy.

Fermentation effluents from organic wastes contain simple organic acids and ethanol, which are good electron sources for exoelectrogenic bacteria, and hence are considered a promising substrate for hydrogen production in microbial electrolysis cells (MECs). These fermentation products have different mechanisms and thermodynamics for their anaerobic oxidation, and therefore the composition of fermentation effluent significantly influences MEC performance. This study examined the microbial electrolysis of a synthetic fermentation effluent (containing acetate, propionate, butyrate, lactate, and ethanol) in two-chamber MECs fitted with either a proton exchange membrane (PEM) or an anion exchange membrane (AEM), with a focus on the utilization preference between the electron sources present in the effluent. Throughout the eight cycles of repeated batch operation with an applied voltage of 0.8 V, the AEM-MECs consistently outperformed the PEM-MECs in terms of organic removal, current generation, and hydrogen production. The highest hydrogen yield achieved for AEM-MECs was 1.26 L/g chemical oxygen demand (COD) fed (approximately 90% of the theoretical maximum), which was nearly double the yield for PEM-MECs (0.68 L/g COD fed). The superior performance of AEM-MECs was attributed to the greater pH imbalance and more acidic anodic pH in PEM-MECs (5.5-6.0), disrupting anodic respiration. Although butyrate is more thermodynamically favorable than propionate for anaerobic oxidation, butyrate was the least favored electron source, followed by propionate, in both AEM- and PEM-MECs, while ethanol and lactate were completely consumed. Further research is needed to better comprehend the preferences for different electron sources in fermentation effluents and enhance their microbial electrolysis.

Neutralization of the bio-oil pH has been shown to generate a neutralized bio-oil aqueous phase (NBOAP) that includes most of the acidic components and a neutralized bio-oil organic phase (NBOOP) that includes hydrophobic organics, such as phenols. NBOOP can be used for fuel production, while NBOAP can be fed to microbial electrolysis cells (MECs) for hydrogen production. After pH neutralization, some organic acidic components remain in NBOOP. This work is focused on capturing acidic compounds from NBOOP through water extraction and electrosorption, and demonstrating hydrogen production via MECs. Capacitive deionization (CDI) is proven effective in capturing ions from NBOOP-contacted water and NBOAP via electrosorption. Captured acidic compounds enable the MEC application to effectively produce renewable hydrogen. Chemical oxygen demand (COD) removal of 49.2%, 61.5%, and 60.8% for 2, 4, and 10 g/L-anode/day loading were observed, corresponding to a total COD degradation of 0.19 g/L, 0.79 g/L, and 1.3 g/L, respectively. A maximum hydrogen productivity of 4.3 L-H 2 /L-anode/day was obtained. Major compounds in the water phase such as fatty acids, sugar derivatives, furanic and phenolic compounds were converted to hydrogen with an efficiency of 80–90%. This approach may lead the entire biomass pyrolysis process to be an overall carbon-neutral process. [ABSTRACT FROM AUTHOR]

Recovery of ammonia from wastewater attracts increasing attentions as a sustainable way of nitrogen treatment. Here a novel gas-diffusion electrode membrane with negative-pressure extraction was developed in bioelectrochemical systems, achieving the highest ammonia recovery rate of 178 gNH 3 –N/(m2·d) with a low energy consumption (2.41 kW h kgNH 3 –N−1). The conjugate acid–base conversion of ammonium consumed OH− near the cathode, enhanced current density by decreasing overpotential which in turn facilitated the ammonia recovery. With an optimized thickness of 0.5 mm, the membrane electrode achieved ammonia removal and recovery efficiencies of 83% and 77% at 0.03 MPa, and the performance was highly repeatable after 20 recovery cycles. Modeling prediction exhibited a better performance under a higher vacuum, where the back-diffusion effect can be effectively mitigated. The membrane electrode can be fabricated at a low cost of 13.5 $ m−2, showing a potential usage in ammonia recovery from wastewater. [Display omitted] • The negative-pressure gas diffusion electrode is effective for ammonia recovery. • The optimal thickness of membrane electrode was 0.5 mm. • The highest ammonia recovery rate was 178 gNH 3 –N/(m2·d) with a low energy need. • It achieved ammonia removal and recovery efficiencies of 83% and 77% at 0.03 MPa. • High vacuum setting can enhance ammonia recovery and reduce back-diffusion effect. [ABSTRACT FROM AUTHOR]

Hydrogen production from cassava starch processing wastewater by two-stage thermophilic dark fermentation and microbial electrolysis was investigated. Single-chamber membrane-free microbial electrolysis cell with applied voltage of 0.6 V was optimum for hydrogen production with hydrogen yield of 245 ml H 2 gCOD −1 . Continuous microbial electrolysis reactor has a maximum hydrogen yield of 182 ml H 2 gCOD −1 at HRT of 48 h with energy recovery efficiency of 217%. The continuous integrated two-stage dark fermentation and microbial electrolysis have hydrogen yield of 465 ml H 2 gCOD −1 with 2 times hydrogen yield improving compared with a single stage and a maximum COD removal of 58% was achieved. Dominated bacteria at an anode of microbial electrolysis cell were exoelectrogens belong to Brevibacillus sp. Caloranaerobacter sp. and Geobacillus sp. Continuous two-stage thermophilic dark hydrogen fermentation and microbial electrolysis were efficient processes for hydrogen production from cassava starch processing wastewater. [ABSTRACT FROM AUTHOR]

This study reports that wastewater can be used to capture and store CO 2 directly from ambient air and produce energy. The proof-of-concept system consisted of an ion exchange resin column that captures and concentrates ambient CO 2 using a moisture-driven cycle. The concentrated CO 2 was then transferred into a microbial electrochemical carbon capture (MECC) reactor for carbon sequestration and hydrogen production. Data from an average batch cycle showed that approximately 8 mmol/L CO 2 was captured in the MECC cathode when 0.14 g/L COD was removed in the anode. With 90% hydrogen conversion efficiency, the energy intensity and CO 2 absorption from the process could be 11.3 kJ/gCOD and 0.49 gCO 2 /gCOD respectively. If the proposed process is applied, over 68 million tons of atmospheric CO 2 can be captured yearly during wastewater treatment in the US, which equates to significant economic values if CO 2 taxes were to be implemented more widely. [ABSTRACT FROM AUTHOR]

Combining the advantages of biological components (e.g., reaction specificity, self-replication) and electrochemical techniques in bioelectrochemical systems offers the opportunity to develop novel efficient and sustainable processes for the production of a number of valuable products. The choice of electrode material has a great impact on the performance of bioelectrochemical systems. In addition to the redox process at the electrodes, interactions of biocatalysts with electrodes (e.g., enzyme denaturation or biofouling) need to be considered. In recent years, gas diffusion electrodes (GDEs) have proved to be very attractive electrodes for bioelectrochemical purposes. GDEs are porous electrodes, that posses a large three-phase boundary surface. At this interface, a solid catalyst supports the electrochemical reaction between gaseous and liquid phase. This mini-review discusses the application of GDEs in microbial and enzymatic fuel cells, for microbial electrolysis, in biosensors and for electroenzymatic synthesis reactions. [ABSTRACT FROM AUTHOR]

Real-field acidogenic effluents rich in short chain carboxylic acids/volatile fatty acids (VFA) were obtained from biohydrogen (H 2 ) producing reactor operated with spent wash as substrate. A single chambered microbial electrolysis cell (MEC) was designed to electro ferment the effluents towards additional H 2 production with simultaneous treatment using acid pretreated biocatalyst. The effect of VFA concentration (4000 mg/l and 8000 mg/l) on biohydrogen production with simultaneous remediation was studied at 0.2 V and 0.6 V applied potential along with closed circuit (CC) and control operations. Maximum cumulative H 2 production (CHP) and hydrogen production rate (HPR) of 39.35 ml and 0.057 mmol/h was observed at 0.6 V. VFA utilization as substrate was high at 0.6 V (68%) followed by 0.2 V (45%), CC (39%) and control (19%). Dehydrogenase activity, pH profiles, low redox slopes ( β a and β c ) and polarization resistance ( R p ) at 0.6 V corroborated well with the H 2 production at both the VFA concentrations operated. Additional H 2 recovery utilizing VFA rich effluents demonstrates the sustainability of this integrated approach through electro fermentation in a single chamber MEC. [ABSTRACT FROM AUTHOR]

The production of reduced value-added chemicals and fuels using microorganisms as cheap cathodic electrocatalysts is recently attracting considerable attention. A robust and sustainable production is, however, still greatly hampered by a poor understanding of electron transfer mechanisms to microorganisms and the lack of strategies to improve and manipulate thereof. Here, we investigated the use of electrically-conductive magnetite (Fe 3 O 4 ) nanoparticles to improve the electrocatalytic activity of a H 2 -producing Desulfovibrio paquesii biocathode. Microbial biocathodes supplemented with a suspension of nanoparticles displayed increased H 2 production rates and enhanced stability compared to unamended ones. Cyclic voltammetry confirmed that Faradaic currents involved in microbially-catalyzed H 2 evolution were enhanced by the addition of the nanoparticles. Possibly, nanoparticles improve the extracellular electron path to the microorganisms by creating composite networks comprising of mineral particles and microbial cells. [ABSTRACT FROM AUTHOR]

Highlights: [•] Acid and base solutions produced from renewable organic matter. [•] Desalinated water produced at the same time. [•] Acid solutions used to accelerate dissolution of the mineral serpentine. [•] Magnesium/calcium carbonates formed that sequestered CO2. [ABSTRACT FROM AUTHOR]

The microbial electrolysis cell coupled the two-phase anaerobic digestion (MEC-TPAD) was developed for simultaneous recovery of bio-sulfur and bio-methane from sulfate-rich wastewater. In acidogenic phase, the produced sulfides were efficiently converted into bio-sulfur via anodic bio-oxidation, with a maximum recovery of 59 ± 5.5 %. The anode coupled acidogenesis produced more volatile fatty acids which were benefit for the subsequent methanogenesis. The cathode in methanogenic phase created a suitable pH condition and enhanced the methanogenesis. Correspondingly, the maximum bio-methane yield in MEC-TPAD was 2 times higher than that in TPAD. Microbial communities revealed that major functional consortia capable of sulfides oxidation (e.g. Alcaligenes) in anode biofilm, hydrogenotrophic methanogenesis (e.g. Methanobacterium) in cathode biofilm, and acetotrophic methanogenesis (e.g. Methanosaeta) in methanogenic sludge were enriched. Economic benefit could totally cover the cost of input electric energy. This work opens an appealing avenue for recovering nutrient and energy from wastewater., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

To reduce membrane scaling, effectively desalinate seawater, and recover magnesium, acid and alkali from the desalination process, a novel five-chamber bioelectrochemical system (BES) was developed in this study. This development was based on a four-chamber BES proposed recently, called microbial electrolysis desalination and chemical-production cell (MEDCC). Results showed that the desalination efficiency of seawater in the five-chamber BES was two times of that in the MEDCC. Removal efficiencies of Na+, Mg2+, and Ca2+ within 18h using the system were 65±2%, 100±0%, and 80±2%, respectively, which were 20%, 66%, and 36% higher than those in the MEDCC. With the form of Mg(OH)2 precipitation, 73% of the total magnesium in solutions was recovered from the cathodic surface. Although the removal efficiencies of Mg2+ and Ca2+ in the five-chamber BES were higher, the Mg2+ and Ca2+ scaling found on the membrane surface was only 38.5% and 18.5% of that in MEDCC, respectively. With the different removal mechanism of Mg2+/Ca2+ ions, the membrane scaling problem was better resolved in the five-chamber BES than the other desalination BESs. With reduction of membrane scaling, the production of alkali, acid, and magnesium, the five-chamber BES should be a promising way to realize an integrated utilization of seawater. [ABSTRACT FROM AUTHOR]

Abstract: Variation in the dehydrogenase (DH) activity and its simultaneous influence on hydrogen (H2) production, substrate degradation rate (SDR) and volatile fatty acid (VFA) generation was investigated with respect to varying poised potential in single chambered membrane-less microbial electrolysis cell (MEC) using anaerobic consortia as biocatalyst. Poised potential showed significant influence on H2 production and DH activity. Maximum H2 production was observed at 1.0V whereas the control system showed least H2 production among the experimental variations studied. DH activity was observed maximum at 0.6V followed by 0.8, 0.9 and 1.0V, suggests the influence of poised potential on the microbial metabolism. Almost complete degradation of substrate was observed in all the experimental conditions studied irrespective of the applied potential. Experimental data was also analysed employing multiple regression analysis and 3D-surface plots to find out the best theoretical poised potential for maximum H2 production and DH activity. [Copyright &y& Elsevier]

Abstract: In this paper we explored the use of an electrochemical quartz crystal microbalance (QCM) to follow the development of electrochemically active biofilms on electrodes. With this technique it should be possible to monitor simultaneously the increase in biomass and the current generated by the electrogenic bacteria in the biofilm. We monitored the adsorption and the subsequent growth of bacteria that are used in microbial electrolysis cells, on a gold electrode (anode). After attachment it took about 3h for the bacteria to start to grow and develop a biofilm. Although the current was still relatively low, there is a clear correlation with the increase in biomass. The method is promising for the further investigation of the development of biofilms on electrodes (bioelectrodes). [ABSTRACT FROM AUTHOR]

Abstract: Microbial electrolysis cells (MECs) could be integrated with dark fermentative hydrogen production to increase the overall system yield of hydrogen. The influence of catholyte pH on hydrogen production from MECs and associated parameters such as electrode potentials (vs Ag/AgCl), COD reduction, current density and quantity of acid needed to control pH in the cathode of an MEC were investigated. Acetate (10 mM, HRT 9 h, 24 °C, pH 7) was used as the substrate in a two chamber MEC operated at 600 mV and 850 mV applied voltage. The effect of catholyte pH on current density was more significant at an applied voltage of 600 mV than at 850 mV. The highest hydrogen production rate was obtained at 850 mV, pH 5 amounting to 200 cm3 stp/lanode/day (coulombic efficiency 60%, cathodic hydrogen recovery 45%, H2 yield 1.1 mol/mol acetate converted and a COD reduction of 30.5%). Within the range (18.5–49.4 °C) of temperatures tested, 30 °C was found to be optimal for hydrogen production in the system tested, with the performance of the reactor being reduced at higher temperatures. These results show that an optimum temperature (approximately 30 °C) exists for MEC and that lower pH in the cathode chamber improves hydrogen production and may be needed if potentials applied to MECs are to be minimised. [ABSTRACT FROM AUTHOR]

Hydrothermal liquefaction is a promising technology for microalgae-based biofuel production. However, hydrothermal liquefaction's aqueous wastes have little established reuse, and contain significant fractions of toxic ammoniacal nitrogen. Careful reuse of this waste can assure microalgae-based biofuels are produced with less environmental impact and larger energy efficiency. Microbial electrolysis cells were investigated to valorize this waste product by converting the leftover organics into hydrogen and remove ammonia. Waste hydrothermal liquefaction aqueous phase from two microalgal strains, Tetraselmis sp. and Chlorella sp. were used as feedstocks for hydrogen production in microbial electrolysis cells. Chlorella and Tetraselmis aqueous phase-fed microbial electrolysis cells reach an average current density of 5.1 ± 0.19 A/m2 and 3.8 ± 0.08 A/m2. Compound removal rates and mass removal percentages were also investigated for each feedstock. Acetic acid, propionic acid, ethanol, and glycerol were effectively removed from the aqueous byproduct. Further, microbial electrolysis cells separated up to 34.3% of ammoniacal nitrogen present in the aqueous phase. Charge transfer analysis indicated that proton transfer, not ammonium transfer, contributed to the majority of the hydrogen production in the cathode. Finally, the microbial electrolysis cell effluent was reused to grow the same microalgal strains, leading to the development of a circular biofuel production system. Microalgae regrowth studies using microbial electrolysis cell effluent showed nearly complete removal of total organic carbon, but significantly less removal of total nitrogen. Tetraselmis sp. growth occurred with the Tetraselmis -derived MEC effluent, however, the control medium without effluent produced the most growth. These findings support the possibility of a circular biofuel framework using MECs, but additional constraints, including the removal of inorganic contaminants, are necessary to realize the circular processes. [Display omitted] • Microbial electrolysis produced hydrogen using two microalgal waste effluents. • Up to 34.4% of ammoniacal nitrogen was separated to the reactor cathode. • Reactor effluent was used to regrow microalgae strains and remove remaining organics. [ABSTRACT FROM AUTHOR]

Waste sludge lysate was produced by dehydration after pyrolysis of waste activated sludge. In addition to dominant components such as protein, polysaccharide, and volatile fatty acids (VFAs), it also contained melanoidins, which produced from Maillard reaction. The inclusion of melanoidins will lead to poor biological degradation in conventional anaerobic digestion (AD). While microbial electrolysis cell (MEC) was proved an enhanced degradation of complex organic matter for hydrogen production. The results showed that under high concentration conditions, conventional AD caused the accumulation of propionic acid and slowed down the use of acetic acid, but MEC overcame the defects and increased the chemical oxygen demand (COD) removal efficiency by 40.33%, and achieved average hydrogen production rate (0.15 ± 0.05 L L−1 day−1), which was 79 times that of AD system (0.0019 ± 0.0009 L L−1 day−1). Therefore, MEC can enhanced biodegradation of the waste sludge lysate for high hydrogen production. Unlabelled Image • MEC achieved higher COD removal than AD in high concentration sludge lysate. • MEC overcame the limit of AD on VFAs utilization to degrade sludge lysate. • H 2 production was enhanced significantly in MEC than AD. [ABSTRACT FROM AUTHOR]