Microbial Electrosynthesis and Anaerobic Fermentation: An Economic Evaluation for Acetic Acid Production from CO2 and CO

Construction of CoSe2/WO3@Si Multiheterojunctions for Efficient Photoelectrochemical Water Splitting

Developing efficient photoanodes for photoelectrochemical (PEC) water splitting is crucial for solar-to-hydrogen energy conversion. Monocrystalline silicon, as a photoelectrode material, has limitations of high surface reflectivity, easy formation of oxide passivation, and instability in aqueous solutions. Herein, flower cluster CoSe2 and lamellar WO3 obtained via the solvothermal method are coated onto the surface of textured silicon by chemical bath deposition to prepare a multiheterojunction structured photoanode. The as-prepared CoSe2/WO3@Si-9 photoelectrode exhibits a desirable photocurrent of 10.1 mA cm-2 at 1.23 VRHE under simulated solar irradiation (AM 1.5G, 100 mW cm-2) in comparison to WO3@Si (0.49 mA cm-2) and CoSe2@Si (1.56 mA cm-2) and excellent stability over 10 h. The improved PEC hydrogen evolution performance comes from the synergistic effect of the multiple heterojunctions of CoSe2/WO3@Si composites. The synergistic effect can improve the separation efficiency of photogenerated electron-hole pairs while maintaining strong redox capability. The CoSe2/WO3@Si-9 photoanode exhibits a high photocurrent density and stability, making it a promising candidate for practical applications.

Microbial electrosynthesis technology for CO2 mitigation, biomethane production, and ex-situ biogas upgrading

Currently, global annual CO2 emissions from fossil fuel consumption are extremely high, surpassing tens of billions of tons, yet our capacity to capture and utilize CO2 remains below a small fraction of the amount generated. Microbial electrosynthesis (MES) systems, an integration of microbial metabolism with electrochemistry, have emerged as a highly efficient and promising bio-based carbon-capture-and-utilization technology over other conventional techniques. MES is a unique technology for lowering the atmospheric CO2 as well as CO2 in the biogas, and also simultaneously convert them to renewable bioenergy, such as biomethane. As such, MES techniques could be applied for biogas upgrading to generate high purity biomethane, which has the potential to meet natural gas standards. This article offers a detailed overview and assessment of the latest advancements in MES for biomethane production and biogas upgrading, in terms of selecting optimal methane production pathways and associated electron transfer processes, different electrode materials and types, inoculum sources and microbial communities, ion-exchange membrane, externally applied energy level, operating temperature and pH, mode of operation, CO2 delivery method, selection of inorganic carbon source and its concentration, start-up time, and system pressure. It also highlights the current MES challenges associated with upscaling, design and configuration, long-term stability, energy demand, techno-economics, achieving net negative carbon emission, and other operational issues. Moreover, we provide a summary of current and future opportunities to integrate MES with other unique biosystems, such as methanotrophic bioreactors, and incorporate quorum sensing, 3D printing, and machine learning to further develop MES as a better biomethane-producer and biogas upgrading technique.

Design and operation of pilot-scale microbial electrosynthesis for the production of acetic acid from biogas with economic and environmental assessment

The embryonic technology of microbial electrosynthesis (MES) possesses the potential to alleviate global CO2 concentration with concomitant recovery of valuables. However, due to the significant bottlenecks of inferior yield of valuables and higher capital cost, its potential has not been fully realized at a larger scale till date. With the aim of bridging this lacuna, a first of its kind pilot-scale MES (PSMES) was designed and operated to yield acetic acid from biogas. The PSMES was able to produce 70.55 g m-2.day of acetic acid in its extraction chamber with the coulombic efficiency of 77.8 % for an imposed cathode potential of -1.0 V vs. standard hydrogen electrode. Moreover, life cycle assessment (LCA) and economic analysis of the PSMES was also conducted to elucidate the economic and environmental feasibility of the same. From the LCA and economic analysis of the PSMES, it was inferred that acrylic sheet and carbon felt used during the fabrication of PSMES were the major culprit in terms of both environmental and economic sustainability and thus should be replaced with greener but cost-effective materials. Therefore, these results would guide the budding scholars in designing more economical and environment friendly scaled-up MES, thus paving towards the commercialization of this ingenious technology.

Relevance of extracellular electron uptake mechanisms for electromethanogenesis applications

Electromethanogenesis has emerged as a biological branch of Power-to-X technologies that implements methanogenic microorganisms, as an alternative to chemical Power-to-X, to convert electrical power from renewable sources, and CO2 into methane. Unlike biomethanation processes where CO2 is converted via exogenously added hydrogen, electromethanogenesis occurs in a bioelectrochemical set-up that combines electrodes and microorganisms. Thereby, mixed, or pure methanogenic cultures catalyze the reduction of CO2 to methane via reducing equivalents supplied by a cathode. Recent advances in electromethanogenesis have been driven by interdisciplinary research at the intersection of microbiology, electrochemistry, and engineering. Integrating the knowledge acquired from these areas is essential to address the specific challenges presented by this relatively young biotechnology, which include electron transfer limitations, low energy and product efficiencies, and reactor design to enable upscaling. This review approaches electromethanogenesis from a multidisciplinary perspective, putting emphasis on the extracellular electron uptake mechanisms that methanogens use to obtain energy from cathodes, since understanding these mechanisms is key to optimize the electrochemical conditions for the development of these systems. This work summarizes the direct and indirect extracellular electron uptake mechanisms that have been elucidated to date in methanogens, along with the ones that remain unsolved. As the study of microbial corrosion, a similar bioelectrochemical process with Fe0 as electron source, has contributed to elucidate different mechanisms on how methanogens use solid electron donors, insights from both fields, biocorrosion and electromethanogenesis, are combined. Based on the repertoire of mechanisms and their potential to convert CO2 to methane, we conclude that for future applications, electromethanogenesis should focus on the indirect mechanism with H2 as intermediary. By summarizing and linking the general aspects and challenges of this process, we hope that this review serves as a guide for researchers working on electromethanogenesis in different areas of expertise to overcome the current limitations and continue with the optimization of this promising interdisciplinary technology.

Dominant factors analyses and challenges of anaerobic digestion under cold environments

With the development of fermentation technology and the improvement of efficiency, anaerobic digestion (AD) has been playing an increasingly primary role in waste treatment and resource recovery. Temperature is undoubtedly the most important factor because it shapes microbial habitats, changes the composition of the microbial community structure, and even affects the expression of related functional genes. More than half of the biosphere is in a long-term or seasonal low-temperature environment (<20 °C), which makes psychrophilic AD have broad application prospects. Therefore, this review discusses the influencing factors and enhancement strategies of psychrophilic AD, which may provide a corresponding reference for future research on low-temperature fermentation. First, the occurrence of AD has been discussed. Then, the adaptation of microorganisms to the low-temperature environment was analyzed. Moreover, the challenges of psychrophilic AD have been reviewed. Meanwhile, the strategies for improving psychrophilic AD are presented. Further, from technology to application, the current situation of psychrophilic AD in pilot-scale tests is described. Finally, the economic and environmental feasibility of psychrophilic AD has been highlighted. In summary, psychrophilic AD is technically feasible, while economic analysis shows that the output benefits cannot fully cover the input costs, and the large-scale practical application of psychrophilic AD is still in its infancy. More research should focus on how to improve fermentation efficiency and reduce the investment cost of psychrophilic AD.

Advances in cathode designs and reactor configurations of microbial electrosynthesis systems to facilitate gas electro-fermentation

In gas fermentation, a range of chemolithoautotrophs fix single-carbon (C1) gases (CO₂ and CO) when H₂ or other reductants are available. Microbial electrosynthesis (MES) enables CO₂ reduction by generating H₂ or reducing equivalents with the sole input of renewable electricity. A combined approach as gas electro-fermentation is attractive for the sustainable production of biofuels and biochemicals utilizing C1 gases. Various platform compounds such as acetate, butyrate, caproate, ethanol, butanol and bioplastics can be produced. However, technological challenges pertaining to the microbe-material interactions such as poor gas-liquid mass transfer, low biomass and biofilm coverage on cathode, low productivities still exist. We are presenting a review on latest developments in MES focusing on the configuration and design of cathodes that can address the challenges and support the gas electro-fermentation. Overall, the opportunities for advancing CO and CO₂-based biochemicals and biofuels production in MES with suitable cathode/reactor design are prospected.

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.

A comparative analysis of biopolymer production by microbial and bioelectrochemical technologies

Production of biopolymers from renewable carbon sources provides a path towards a circular economy. This review compares several existing and emerging approaches for polyhydroxyalkanoate (PHA) production from soluble organic and gaseous carbon sources and considers technologies based on pure and mixed microbial cultures. While bioplastics are most often produced from soluble sources of organic carbon, the use of carbon dioxide (CO₂) as the carbon source for PHA production is emerging as a sustainable approach that combines CO₂ sequestration with the production of a value-added product. Techno-economic analysis suggests that the emerging approach of CO₂ conversion to carboxylic acids by microbial electrosynthesis followed by microbial PHA production could lead to a novel cost-efficient technology for production of green biopolymers.

Biopolymers production from renewable carbon sources.

A short review of graphene in the microbial electrosynthesis of biochemicals from carbon dioxide

Microbial electrosynthesis (MES) is a potential energy transformation technology for the reduction of the greenhouse gas carbon oxide (CO2) into commercial chemicals.

Wastewater-powered high-value chemical synthesis in a hybrid bioelectrochemical system

A microbial electrochemical system could potentially be applied as a biosynthesis platform by extracting wastewater energy while converting it to value-added chemicals. However, the unfavorable thermodynamics and sluggish kinetics of in vivo whole-cell cathodic catalysis largely limit product diversity and value. Herein, we convert the in vivo cathodic reaction to in vitro enzymatic catalysis and develop a microbe-enzyme hybrid bioelectrochemical system (BES), where microbes release the electricity from wastewater (anode) to power enzymatic catalysis (cathode). Three representative examples for the synthesis of pharmaceutically relevant compounds, including halofunctionalized oleic acid based on a cascade reaction, (4-chlorophenyl)-(pyridin-2-yl)-methanol based on electrochemical cofactor regeneration, and l-3,4-dihydroxyphenylalanine based on electrochemical reduction, were demonstrated. According to the techno-economic analysis, this system could deliver high system profit, opening an avenue to a potentially viable wastewater-to-profit process while shedding scientific light on hybrid BES mechanisms toward a sustainable reuse of wastewater.

Enhanced Volatile Fatty Acid Production from Oil Palm Empty Fruit Bunch through Acidogenic Fermentation—A Novel Resource Recovery Strategy for Oil Palm Empty Fruit Bunch

The glucan-rich fraction, hemicellulosic compounds-rich fraction, and a mixture of both fractions obtained from organosolv pretreatment of oil palm empty fruit bunch (OPEFB) were used as substrates to produce volatile fatty acids (VFAs) in acidogenic fermentation. In this study, the effects of medium adjustment (carbon to nitrogen ratio and trace elements supplementation) and methanogenesis inhibition (through the addition of 2-bromoethanesulfonate or by heat shock) to enhance VFAs yield were investigated. The highest VFA yield was 0.50 ± 0.00 g VFAs/g volatile solid (VS), which was obtained when methanogens were inhibited by heat shock and cultivated in a mixture of glucan-rich and hemicellulosic compounds-rich fractions. Under these conditions, the fermentation produced acetic acid as the only VFA. Based on the results, the mass balance of the whole process (from pretreatment and fermentation) showed the possibility to obtain 30.4 kg acetic acid and 20.3 kg lignin with a 70% purity from 100 kg OPEFB.

Emerging bioelectrochemical technologies for biogas production and upgrading in cascading circular bioenergy systems

Biomethane is suggested as an advanced biofuel for the hard-to-abate sectors such as heavy transport. However, future systems that optimize the resource and production of biomethane have yet to be definitively defined. This paper assesses the opportunity of integrating anaerobic digestion (AD) with three emerging bioelectrochemical technologies in a circular cascading bioeconomy, including for power-to-gas AD (P2G-AD), microbial electrolysis cell AD (MEC-AD), and AD microbial electrosynthesis (AD-MES). The mass and energy flow of the three bioelectrochemical systems are compared with the conventional AD amine scrubber system depending on the availability of renewable electricity. An energy balance assessment indicates that P2G-AD, MEC-AD, and AD-MES circular cascading bioelectrochemical systems gain positive energy outputs by using electricity that would have been curtailed or constrained (equivalent to a primary energy factor of zero). This analysis of technological innovation, aids in the design of future cascading circular biosystems to produce sustainable advanced biofuels.

Bioproducts generation from carboxylate platforms by the non-conventional yeast Yarrowia lipolytica

In recent years, there has been a growing interest in the use of renewable sources for bio-based production aiming at developing sustainable and feasible approaches towards a circular economy. Among these renewable sources, organic wastes (OWs) can be anaerobically digested to generate carboxylates like volatile fatty acids (VFAs), lactic acid, and longer-chain fatty acids that are regarded as novel building blocks for the synthesis of value-added compounds by yeasts. This review discusses on the processes that can be used to create valuable molecules from OW-derived VFAs; the pathways employed by the oleaginous yeast Yarrowia lipolytica to directly metabolize such molecules; and the relationship between OW composition, anaerobic digestion, and VFA profiles. The review also summarizes the current knowledge about VFA toxicity, the pathways by which VFAs are metabolized and the metabolic engineering strategies that can be employed in Y. lipolytica to produce value-added biobased compounds from VFAs.

Hydrogen-driven microbial biogas upgrading: Advances, challenges and solutions

As a clean and renewable energy, biogas is an important alternative to fossil fuels. However, the high carbon dioxide (CO₂) content in biogas limits its value as a fuel. 'Biogas upgrading' is an advanced process which removes CO₂ from biogas, thereby converting biogas to biomethane, which has a higher commercial value. Microbial technologies offer a sustainable and cost-effective way to upgrade biogas, removing CO₂ using hydrogen (H₂) as electron donor, generated by surplus electricity from renewable wind or solar energy. Hydrogenotrophic methanogens can be applied to convert CO₂ with H₂ to methane (CH₄), or alternatively, homoacetogens can convert both CO₂ and H₂ into value-added chemicals. Here, we comprehensively review the current state of biogas generation and utilization, and describe the advances in biological, H₂-dependent biogas upgrading technologies, with particular attention to key challenges associated with the processes, e.g., metabolic limitations, low H₂ transfer rate, and finite CO₂ conversion rate. We also highlight several new strategies for overcoming technical barriers to achieve efficient CO₂ conversion, including process optimization to eliminate metabolic limitation, novel reactor designs to improve H₂ transfer rate and utilization efficiency, and employing advanced genetic engineering tools to generate more efficient microorganisms. The insights offered in this review will promote further exploration into microbial, H₂-driven biogas upgrading, towards addressing the global energy crisis and climate change associated with use of fossil fuels.

Supply chain optimization framework for CO2 capture, utilization, and storage in Germany

In this study, the carbon capture, utilization, and sequestration (CCUS) supply chain network with real geographic locations of sources and sinks, and different CO2-based products for Germany is proposed here for the first time, because not yet investigated in the literature. The CCUS network is a large-scale comprehensive model which is used to meet the mandated target of CO2 emission reduction at different levels with a maximum profit. The novel CCUS infrastructure includes various stationary sources, capture processes, transportation modes, and sequestration and utilization sites. The results suggest that it is possible to reduce current CO2 emissions by 40–80% in Germany with the total annual costs ranging from 519.34 to 1372.03 billion euro while generating 681.55 to 1880.98 billion euro of revenue annually as a result of producing CO2-based chemical products including methanol, dimethyl ether, formic acid, acetic acid, urea, and polypropylene carbonate. Overall, the optimal CCUS systems achieve economic profits of 999.62–1568.17 euro per ton of CO2 captured and utilized. The CCUS model may be critical in aiding decision-makers to ascertain investment strategies for designing CCUS infrastructures.

Microbial electrosynthesis: is it sustainable for bioproduction of acetic acid?

Microbial electrosynthesis (MES) is an innovative technology for electricity driven microbial reduction of carbon dioxide (CO2) to useful multi-carbon compounds. This study assesses the cradle-to-gate environmental burdens associated with acetic acid (AA) production via MES using graphene functionalized carbon felt cathode. The analysis shows that, though the environmental impact for the production of the functionalized cathode is substantially higher when compared to carbon felt with no modification, the improved productivity of the process helps in reducing the overall impact. It is also shown that, while energy used for extraction of AA is the key environmental hotspot, ion-exchange membrane and reactor medium (catholyte & anolyte) are other important contributors. A sensitivity analysis, describing four different scenarios, considering either continuous or fed-batch operation, is also described. Results show that even if MES productivity can be theoretically increased to match the highest space time yield reported for acetogenic bacteria in a continuous gas fermenter (148 g L-1 d-1), the environmental impact of AA produced using MES systems would still be significantly higher than that produced using a fossil-based process. Use of fed-batch operation and renewable (solar) energy sources do help in reducing the impact, however, the low production rates and overall high energy requirement makes large-scale implementation of such systems impractical. The analysis suggests a minimum threshold production rate of 4100 g m-2 d-1, that needs to be achieved, before MES could be seen as a sustainable alternative to fossil-based AA production.

From Acetate to Bio-Based Products: Underexploited Potential for Industrial Biotechnology

Currently, most biotechnological products are based on microbial conversion of carbohydrate substrates that are predominantly generated from sugar- or starch-containing plants. However, direct competitive uses of these feedstocks in the food and feed industry represent a dilemma, so using alternative carbon sources has become increasingly important in industrial biotechnology. A promising alternative carbon source that may be generated in substantial amounts from lignocellulosic biomass and C1 gases is acetate. This review discusses the underexploited potential of acetate to become a next-generation platform substrate in future industrial biotechnology and summarizes alternative sources and routes for acetate production. Furthermore, biotechnological aspects of microbial acetate utilization and the state of the art of biotechnological acetate conversion into value-added bioproducts are highlighted.

Substrate profiling and tolerance testing of Halomonas TD01 suggest its potential application in sustainable manufacturing of chemicals

Halomonas TD01, which can grow under non-sterile and continuous processes at high pH and high salt concentrations, is a robust platform for PHA production from glucose. For extending other low-cost sustainable substrates and increasing the potential application in other value-added products, a better understanding of substrates utilization and chemicals tolerance is necessary. In this study, the substrate profiling of TD01 was analyzed via Biolog. Phenotype microarray results demonstrated that TD01 has a wide-ranging substrate spectrum and can utilize 140 of the 190 test compounds. Some cheap, abundant carbon sources, such as sodium acetate, glycerol, ethanol and lactate can well support the growth of TD01 in shake-flask, and are therefore suggested to be its alternative low-cost substrates for chemicals production in future. Furthermore, the tolerance of TD01 to various chemicals was tested. The results showed that the tolerability of TD01 to high concentrations of organic acid salts is prominent. When adding 75 g/L sodium acetate, 100 g/L succinic acid and 100 g/L itaconic acid in the medium, the growth rate reduced 56.14%, 52.63% and 47.37%, respectively. All these results highlight TD01 as a promising, next generation industrial workhorse in chemicals biomanufacturing from cheap organic acid salts.

Bioelectrochemical systems for a circular bioeconomy

In the past two decades, bioelectrochemical systems (BESs) have received a considerable attention as a way of transforming wastewater directly to electricity and chemicals. Since BESs are capable of energy and chemical production by removing wastewater, these systems are considered promising sustainable waste-to-energy/chemical platforms and parts of circular bioeconomy. For the estimation of practical applicability of BESs in the circular bioeconomy, economic assessment of these systems is reviewed in this work. This estimation is necessary to decide whether BESs can be further developed for commercialization or there is any limiting factor for making the systems commercially viable in circular bioeconomy. This review also presents current developments of BESs, providing a critical review of the current status and challenges of techno-economic analysis for BESs. The results highlight the key factors to suggest the future research directions to make BESs economically available as a part of promising circular bioeconomy.

Review—Microbial Electrosynthesis: A Way Towards The Production of Electro-Commodities Through Carbon Sequestration with Microbes as Biocatalysts

There has been a considerable increment in the atmospheric CO2 concentration, which has majorly contributed to the problem of global warming. This issue can be extenuated by effectively developing microbial electrosynthesis (MES) for the sequestration of CO2 with the concurrent production of biochemical and biofuels. Though the MES technology is in its infancy, it has exhibited enormous potential for sustainable mitigation of CO2 and bioelectrosynthesis of multi-carbon organic compounds. The problem of storage of excess renewable electrical energy by conventional means can also be alleviated by employing MES, which stores it in the form of C–C bonds of chemicals. This review focuses on the various aspects of MES and recent developments made in this field to overcome its bottlenecks, such as the lower yield of organic compounds, separation of products of higher chain organic compounds, etc. In particular, the microbial catalysts and cathode materials employed in MES have also been emphasized. Keeping in mind the potential of this innovative technology, researchers should focus on improving the yield of MES by developing novel low-cost cathode materials and discovering efficient and robust micro-organisms, which would be a significant step forward towards the further advancement of this technology.

Sustainable hydrogen-rich syngas from steam reforming of bio-based acetic acid over ZnO and CeO2–ZnO supported Ni-based catalysts

Sustainable hydrogen-rich syngas from steam reforming (SR) of bio-based acetic acid over ZnO and CeO₂–ZnO supported Ni-based catalysts was studied by means of a bench-scale fixed-bed unit combined with NDIR/TCD techniques. The effects of Ni/ZnO catalysts with different nickel loadings (5–15%), temperature (T = 500–900 °C), steam to carbon molar ratio (S/C = 1–5) and weight hourly space velocity (WHSV = 3–7 h⁻¹) on SR of acetic acid were explored. In addition, the influence of CeO₂ addition on the catalytic performance was assessed to investigate the improvement effect of Ce as a promoter on the catalytic activity. As the nickel loading increased from 5 to 15%, the H₂ yield increased significantly from 31.0 to 51.0% with a growth rate of 64.5%, while the CO yield first decreased from 31.6 to 27.7% and then increased to 35.7%. Between 500 and 900 °C, the yields of H₂ and CO first increased and then decreased, corresponding to the peak yields of 51.0% and 35.7% at 800 °C, respectively. S/C gave a similar trend of H₂ yield to the T, while the CO yield continued to decrease with increasing S/C from 1 to 5. The H₂ yield gradually decreased from 54.1 to 28.7% as the WHSV increased, while the peak value of CO yield was 35.7%, corresponding to WHSV = 5. The addition of 25 wt% CeO₂ to the Ni/ZnO catalyst with a nickel loading of 15% improved the H₂ yield from 51.0 to 74.0% when reforming acetic acid under the optimal operating conditions of T = 800 °C, S/C = 3 and WHSV = 5 h⁻¹. The CO yield was reduced from 35.7 to 33.2%, and the corresponding H₂/CO ratio increased from 2.9 to 4.5. The excellent catalyst stability was obtained in the SR of acetic acid using Ni/CeO₂–ZnO catalyst. H₂ yield was reduced from 76.0 to 73.5% with a decrease of 3.4%, while CO yield increased from 32.1 to 41.3% with a growth rate of 28.7% within 15–360 minutes.

Variation of hydrogen-rich syngas from steam reforming of bio-based acetic acid over Ni/ZnO and Ni/CeO₂–ZnO catalysts was assessed.

Kinetic Study on Heterotrophic Growth of Acetobacterium woodii on Lignocellulosic Substrates for Acetic Acid Production

Extensive research has been done on examining the autotrophic growth of Acetobacterium woodii with gaseous substrates (hydrogen and carbon dioxide) to produce acetic acid. However, only limited work has been performed on the heterotrophic growth of A. woodii using pure sugars or lignocellulosic feedstocks-derived sugars as substrates. In this study, we examine the growth kinetics and acetic acid production of A. woodii on glucose and xylose. While good growth was observed with glucose as substrate, no significant growth was obtained on xylose. Kinetic studies were performed in batch culture using different concentrations of glucose, ranging from 5 g/L to 40 g/L. The highest acetate production of 6.919 g/L with a product yield of 0.76 g acetic acid/g glucose was observed with 10 g/L glucose as initial substrate concentration. When testing A. woodii on corn stover hydrolysate (CSH) and wheat straw hydrolysate (WSH) formed after pretreatment and enzymatic hydrolysis, we found that A. woodii showed acetic acid production of 7.64 g/L and a product yield of 0.70 g acetic acid/g of glucose on WSH, while the acetic acid production was 7.83 g/L with a product yield of 0.65 g acetic acid/g of glucose on CSH. These results clearly demonstrate that A. woodii performed similarly on pure substrates and hydrolysates, and that the processes were not inhibited by the heterogenous components present in the lignocellulosic feedstock hydrolysates.

Enhanced butyric acid production using mixed biomass of brown algae and rice straw by Clostridium tyrobutyricum ATCC25755

A brown alga Saccharina japonica and rice straw are attractive feedstock for microbial butyric acid production. However, inefficient fermentation of mannitol (a dominant component in S. japonica) and toxicity of inhibitors in lignocellulosic hydrolysate are limitations. This study demonstrated that mixed biomass with S. japonica and rice straw was effective in butyric acid production over those restrictions. Mannitol was consumed only when acetic acid was present. Notably, acetic acid was not produced but consumed along with mannitol. By mixing S. japonica and rice straw to take advantage of glucose and acetic acid in rice straw, Clostridium tyrobutyricum effectively consumed mannitol by utilizing acetic acid in hydrolysate and acetic acid derived from glucose with the enhanced butyric acid production. Furthermore, cell growth was restored owing to the decreased inhibitor concentration. The results demonstrate the potential of butyric acid production from mixed biomass of macroalgae/lignocellulose overcoming the drawbacks of single biomass.

Carbon dioxide and organic waste valorization by microbial electrosynthesis and electro-fermentation

Carbon-rich waste materials (solid, liquid, or gaseous) are largely considered to be a burden on society due to the large capital and energy costs for their treatment and disposal. However, solid and liquid organic wastes have inherent energy and value, and similar as waste CO₂ gas they can be reused to produce value-added chemicals and materials. There has been a paradigm shift towards developing a closed loop, biorefinery approach for the valorization of these wastes into value-added products, and such an approach enables a more carbon-efficient and circular economy. This review quantitatively analyzes the state-of-the-art of the emerging microbial electrochemical technology (MET) platform and provides critical perspectives on research advancement and technology development. The review offers side-by-side comparison between microbial electrosynthesis (MES) and electro-fermentation (EF) processes in terms of principles, key performance metrics, data analysis, and microorganisms. The study also summarizes all the processes and products that have been developed using MES and EF to date for organic waste and CO₂ valorization. It finally identifies the technological and economic potentials and challenges on future system development.

Expanding the product spectrum of value added chemicals in microbial electrosynthesis through integrated process design-A review

Microbial electrosynthesis (MES) is a novel microbial electrochemical technology proposed for chemicals production with the storage of sustainable energy. However, the practical application of MES is currently restricted by the limited low market value of products in one-step conversion process, mostly acetate. A theme that is pervasive throughout this review is the challenges associated with the expanded product spectrum. Several recent research efforts to improve acetate production, using novel reactor configuration, renewable power supply, and various 3-D cathode are summarized. The importance of genetic modification, two-step hybrid process, as well as input substrates other than CO₂ are highlighted in this review as the future research paths for higher value chemicals production. At last, how to integrate MES with existing biochemicals processes is proposed. Definitely, more studies are encouraged to evaluate the overall performances and economic efficiency of these integrated process designs to make MES more competitive.

Salinity-gradient energy driven microbial electrosynthesis of value-added chemicals from CO2 reduction

Biological conversion of CO₂ to value-added chemicals and biofuels has emerged as an attractive strategy to address the energy and environmental concerns caused by the over-reliance on fossil fuels. In this study, an innovative microbial reverse-electrodialysis electrolysis cell (MREC), which combines the strengths of reverse electrodialysis (RED) and microbial electrosynthesis technology platforms, was developed to achieve efficient CO₂-to-value chemicals bioconversion by using the salinity gradient energy as driven energy sources. In the MREC, maximum acetate and ethanol concentrations of 477.5 ± 33.2 and 46.2 ± 8.2 mg L^-1 were obtained at the cathode, catalyzed by Sporomusa ovata with production rates of 165.79 ± 11.52 and 25.11 ± 4.46 mmol m^-2 d^-1, respectively. Electron balance analysis indicates that 94.4 ± 3.9% of the electrons derived from wastewater and salinity gradient were recovered in acetate and ethanol. This work for the first time proved the potential of innovative MREC configuration has the potential as an efficient technology platform for simultaneous CO₂ capture and electrosynthesis of valuable chemicals.

Social Hotspot Analysis and Trade Policy Implications of the Use of Bioelectrochemical Systems for Resource Recovery from Wastewater

Bioelectrochemical systems (BESs) have been catalogued as a technological solution to three pressing global challenges: environmental pollution, resource scarcity, and freshwater scarcity. This study explores the social risks along the supply chain of requisite components of BESs for two functionalities: (i) copper recovery from spent lees and (ii) formic acid production via CO2 reduction, based on the UK’s trade policy. The methodology employed in this study is based on the UNEP/SETAC guidelines for social life-cycle assessment (S-LCA) of products. Relevant trade data from UN COMTRADE database and generic social data from New Earth’s social hotspot database were compiled for the S-LCA. The results revealed that about 75% of the components are imported from the European Union. However, the social risks were found to vary regardless of the magnitude or country of imports. “Labour and Decent Work” was identified as the most critical impact category across all countries of imports, while the import of copper showed relatively higher risk than other components. The study concludes that BESs are a promising sustainable technology for resource recovery from wastewater. Nevertheless, it is recommended that further research efforts should concentrate on stakeholder engagement in order to fully grasp the potential social risks.

An overview of microbial biogas enrichment

Biogas upgrading technologies have received widespread attention recently and are researched extensively. Microbial biogas upgrading (biomethanation) relies on the microbial performance in enriched H₂ and CO₂ environments. In this review, recent developments and applications of CH₄ enrichment in microbial methanation processes are systematically reviewed. During biological methanation, either H₂ can be injected directly inside the anaerobic digester to enrich CH₄ by a consortium of mixed microbial species or H₂ can be injected into a separate bioreactor, where CO₂ contained in biogas is coupled with H₂ and converted to CH₄, or a combination hereof. The available microbial technologies based on hydrogen-mediated CH₄ enrichment, in particular ex-situ, in-situ and bioelectrochemical, are compared and discussed. Moreover, gas-liquid mass transfer limitations, and dynamics of bacteria-archaea interactions shift after H₂ injection are thoroughly discussed. Finally, the summary of existing demonstration, pilot plants and commercial CH₄ enrichment plants based on microbial biomethanation are critically reviewed.

Taking the technical microbiome into the next decade

The 'microbiome' has become a buzzword. Multiple new technologies allow to gather information about microbial communities as they evolve under stable and variable environmental conditions. The challenge of the next decade will be to develop strategies to compose and manage microbiomes. Here, key aspects are considered that will be of crucial importance for future microbial technological developments. First, the need to deal not only with genotypes but also particularly with phenotypes is addressed. Microbial technologies are often highly dependent on specific core organisms to obtain the desired process outcome. Hence, it is essential to combine omics data with phenotypic information to invoke and control specific phenotypes in the microbiome. Second, the development and application of synthetic microbiomes is evaluated. The central importance of the core species is a no-brainer, but the implementation of proper satellite species is an important route to explore. Overall, for the next decade, microbiome research should no longer almost exclusively focus on its capacity to degrade and dissipate but rather on its remarkable capability to capture disordered components and upgrade them into high-value microbial products. These products can become valuable commodities in the cyclic economy, as reflected in the case of 'reversed sanitation', which is introduced here.

Life cycle, techno-economic and dynamic simulation assessment of bioelectrochemical systems: A case of formic acid synthesis

A novel framework, integrating dynamic simulation (DS), life cycle assessment (LCA) and techno-economic assessment (TEA) of a bioelectrochemical system (BES), has been developed to study for the first time wastewater treatment by removal of chemical oxygen demand (COD) by oxidation in anode and thereby harvesting electron and proton for carbon dioxide reduction reaction or reuse to produce products in cathode. Increases in initial COD and applied potential increase COD removal and production (in this case formic acid) rates. DS correlations are used in LCA and TEA for holistic performance analyses. The cost of production of HCOOH is €0.015-0.005 g^-1 for its production rate of 0.094-0.26 kg yr^-1 and a COD removal rate of 0.038-0.106 kg yr^-1. The life cycle (LC) benefits by avoiding fossil-based formic acid production (93%) and electricity for wastewater treatment (12%) outweigh LC costs of operation and assemblage of BES (-5%), giving a net 61MJkg^-1 HCOOH saving.

Rediscovering Acetate Metabolism: Its Potential Sources and Utilization for Biobased Transformation into Value-Added Chemicals

One of the great advantages of microbial fermentation is the capacity to convert various carbon compounds into value-added chemicals. In this regard, there have been many efforts to engineer microorganisms to facilitate utilization of abundant carbon sources. Recently, the potential of acetate as a feedstock has been discovered; efforts have been made to produce various biochemicals from acetate based on understanding of its metabolism. In this review, we discuss the potential sources of acetate and summarized the recent progress to improve acetate utilization with microorganisms. Furthermore, we also describe representative studies that engineered microorganisms for the production of biochemicals from acetate.

Modeling of acetate-type fermentation of sugar-containing wastewater under acidic pH conditions

In this study, a kinetic model was developed based on Anaerobic Digestion Model No. 1 to provide insights into the directed production of acetate and methane from sugar-containing wastewater under low pH conditions. The model sufficiently described the dynamics of liquid-phase and gaseous products in an anaerobic membrane bioreactor by comprehensively considering the syntrophic bioconversion steps of sucrose hydrolysis, acidogenesis, acetogenesis and methanogenesis under acidic pH conditions. The modeling results revealed a significant pH-dependency of hydrogenotrophic methanogenesis and ethanol-producing processes that govern the sucrose fermentative pathway through changing the hydrogen yield. The reaction thermodynamics of such acetate-type fermentation were evaluated, and the implications for process optimization by adjusting the hydraulic retention time were discussed. This work sheds light on the acid-stimulated acetate-type fermentation process and may lay a foundation for optimization of resource-oriented processes for treatment of food wastewater.

Balancing cellular redox metabolism in microbial electrosynthesis and electro fermentation - A chance for metabolic engineering

More and more microbes are discovered that are capable of extracellular electron transfer, a process in which they use external electrodes as electron donors or acceptors for metabolic reactions. This feature can be used to overcome cellular redox limitations and thus optimizing microbial production. The technologies, termed microbial electrosynthesis and electro-fermentation, have the potential to open novel bio-electro production platforms from sustainable energy and carbon sources. However, the performance of reported systems is currently limited by low electron transport rates between microbes and electrodes and our limited ability for targeted engineering of these systems due to remaining knowledge gaps about the underlying fundamental processes. Metabolic engineering offers many opportunities to optimize these processes, for instance by genetic engineering of pathways for electron transfer on the one hand and target product synthesis on the other hand. With this review, we summarize the status quo of knowledge and engineering attempts around chemical production in bio-electrochemical systems from a microbe perspective. Challenges associated with the introduction or enhancement of extracellular electron transfer capabilities into production hosts versus the engineering of target compound synthesis pathways in natural exoelectrogens are discussed. Recent advances of the research community in both directions are examined critically. Further, systems biology approaches, for instance using metabolic modelling, are examined for their potential to provide insight into fundamental processes and to identify targets for metabolic engineering.