Interspecies Hydrogen Transfer Research Articles

Comparing the effects of magnetite-mediated direct interspecies electron transfer with biogas mixing-driven interspecies hydrogen transfer on anaerobic digestion

Although the promotive effect of direct interspecies electron transfer (DIET) on methane production has been well-documented, the practical applicability of DIET in different scenarios have not yet been systematically studied. This study compared the effects of magnetite-mediated DIET with conventional biogas mixing-driven interspecies hydrogen transfer (IHT) on anaerobic digestion (AD) of swine manure (SM). Compared with control, magnetite supplementation, biogas circulation, and their integration enhanced the CH4 yield by 19.3%, 25.9%, and 26.2%, respectively. Magnetite mainly enriched DIET-related syntrophic bacteria (Anaerolineae and Synergistia) and methanogens (Methanosarcina) to accelerate acidification and establish DIET, while biogas circulation mainly enriched hydrolytic bacteria (Clostridia) and hydrogenotrophic methanogens (Methanolinea and Methanobacterium) to promote hydrolysis and accelerate IHT. Coupling magnetite addition with biogas circulation led to the enrichment of the above six microorganisms to different extents. The effectiveness of the strategies for lowering the H2 pressure followed: magnetite + biogas circulation ≈ biogas circulation > magnetite. Under stress-free environment, the enhancement effect of magnetite-induced DIET was not even as pronounced as biogas circulation—a simple and common mixing strategy in commercial AD plants, and the promotion effect of magnetite was insignificant in the well-mixed digesters. In short, the magnetite-mediated DIET is not always effective in improving AD of SM.

Biochar-mediated synergistic promotion of DIET and IHT: Kinetic and thermodynamic insights into propionate two-step syntrophic methanogenesis

Biochar can effectively enhance the thermophilic anaerobic degradation of propionate. Nevertheless, previous studies frequently ascribed this enhancement to direct interspecies electron transfer (DIET) without considering the underlying kinetics and thermodynamics involved. Additionally, quantitative evaluation of biochar’s impact on interspecies electron transfer (IET) has been limited. In this study, we developed and optimized models for analyzing thermodynamics and electron transfer flux to elucidate the mechanism by which biochar enhances the two-step syntrophic methanogenesis of propionate. Findings revealed that biochar primarily facilitated DIET (1.78 × 10−4 A), leveraging its redox capacity. Additionally, it synergistically enhanced interspecies hydrogen transfer (4.08 × 10−10 A), significantly improving IET efficiency. This synergistic effect led to accelerated oxidative degradation rates of propionate (39.2 %) and intermediate acetate (13.5 %), expanding the thermodynamic windows for their syntrophic methanogenesis (221 % and 103 %, respectively). Accordingly, biochar increased the maximum rate of propionate methanation (71.9 %) and reduced the lag phase time (51.1 %). Moreover, biochar promoted the specific growth rates of acetogenic bacteria and methanogens while significantly increasing the abundance of Pelotomaculum, Methanoculleus, and Methanosarcina. These findings provide new insights and theoretical support for the use of biochar in promoting anaerobic digestion and facilitating IET during the process.

Enhancing kitchen waste anaerobic digestion by recycled aluminum industry waste: Alkali treatment and potential electron transfer mechanism

Kitchen waste has become a significant environmental concern worldwide. Red mud, an industrial solid waste characterized by its porous structure and high iron oxide content, exhibits potential as a conductive conduit in the direct interspecies electron transfer (DIET) pathway. In this study, red mud pretreatment of kitchen waste was conducted to enhance anaerobic digestion (AD). The effect and mechanism of enhancing kitchen waste AD by red mud pretreatment was studied for the first time. Results indicated that red mud pretreatment could accelerate the hydrolysis of organic compounds and alleviate the acid inhibition. Consequently, the methane production rate was increased from 260.7 mL/g VS to 402.3 mL/g VS, i.e., up to 54.3%. And volatile solids removal efficiency was increased from 53.9% to 73.1%, i.e., up to 35.6%. Furthermore, microbial community analysis revealed the enrichment of electrochemically active bacteria (Synergistaceae), acetogenic bacteria (Syntrophomonadaceae) and methanogens (Methanobacterium and Methanosarcina), which played essential roles in interspecies hydrogen transfer (IHT) and DIET. This study showed that red mud pretreatment has the potential to enhance kitchen waste AD which contributes to carbon mitigation. It also threw the potential new insights to the relative mechanism.

428 Awardee Talk: Can the Ruminal Microbiome Ever Answer All the Questions That Nutritionists have?

Abstract The rumen microbial consortium has long been viewed as a “black box”, where we recognize what goes in, and most of what comes out, but the ruminal catabolic and anabolic activity remains shrouded in uncertainty. Microbiologists from the 1940s through the early 2000s identified microbes responsible for ruminal feedstuff degradation, highlighted animal-important fermentation endproducts (such as volatile fatty acids, VFA), and examined how microbes interacted with each other (e.g., interspecies hydrogen transfer), and with the host animal. However, our knowledge was dependent on our ability to grow anaerobic microbes in pure or mixed cultures, but it became apparent that our limiting factor was that we could culture <10% of the ruminal microbes. In the past 15 years, there has been a proliferation of new techniques to identify what is occurring in the rumen of live ruminants such as, culturable and unculturable microorganism populations, microbial biochemical pathways and important metabolic intermediates, potential interactions between microbes, and biologically relevant endproducts that impact host animal physiology, health, and production efficiency. As Next Generation Sequencing (NGS) has become increasingly affordable, there has been a surge in research examining the “microbiome” with a resultant influx of massive datasets. However, without the ability to connect microbial populations to biochemical degradative pathways, endproducts, microbial interactions or animal production parameters, this information is of limited impact in ruminant nutrition. Ruminant nutrition is on a precipice where we can finally fully integrate microbiology with metabolomics and animal performance and nutrition. Advancements in ruminal microbiology will allow us to fully grasp the functional nature of the microbial ecosystem and where it fits within ruminant nutrition, which will improve animal productivity and ruminant-derived protein food production sustainability.

Analysing the mechanism of food waste anaerobic digestion enhanced by iron oxide in a continuous two-stage process

The food waste (FW) digestion performance can be enhanced by introducing iron oxide (IO) into digesters. However, the role of IO in continuous two-stage digesters in enhancing the FW anaerobic digestion remains unclear. In this study, the effect of IO on the bioenergy recovery from a two-stage digestion process was investigated. The bioenergy recovery was significantly increased by up to 208.43 % with IO addition. The activities of dehydrogenase, α-amylase, and protease increase by 0.82–1.44, 7.24–14.56 and 7.97–20.45 times, respectively, as compared with that of the blank. With IO addition, the metabolic pathway in hydrolytic–acidogenic (HA) reactor shifted from lactic acid fermentation to butyric fermentation, which promoted stable methane production in methanogenic (MG) reactor. The activity of coenzyme F420 increased by 19.19–39.01 times, indicating that IO facilitated FW digestion by promoting hydrogenotrophic methanogenesis. The enhancement in the enzyme activity was attributable to the Fe2+ generated by dissimilatory iron reduction. According to the microbial analysis, IO enhanced interspecies hydrogen transfer between Methanobacterium and Syntrophomonas. Furthermore, IO improved direct interspecies electron transfer between Geobacter sulfurreducens and Methanosarcina. The effluent recirculation strategy greatly facilitated the hydrolysis and acidification of FW, which was critical for improving the two-stage process performance.

Facilitated CO biomethanation by exogenous materials via inducing specific methanogenic pathways

Syngas biomethanation is a promising route for syngas (mainly CO, CO2, H2 and CH4) upgrading from pyrolysis/gasification of recalcitrant waste. However, its great difficulty stems from the toxic inhibition and low conversion efficiency of CO. This study aimed to enhance the CO biomethanation efficiency by supplying different conductive materials (including braunite, magnetite, biochar-A, biochar-B, and granular activated carbon). The results showed that the reactors supplied with magnetite had the best promotion on methane production, followed by braunite addition, with an increase of 96.7% and 88.0% compared to the Control respectively. With added carbon-based materials, the methane production in the Biochar-A reactors were increased by only 56%, which in the Biochar-B and GAC reactors were reduced by 17% and 30%. Magnetite presented strong redox property and electronic shuttle function, electrons can be shuttled rapidly between Fe (II) and Fe (III), which was found promoted methanogenesis by facilitating direct interspecies electron transfer between Brooklawnia spp. and Methanosarcina spp. Additionally, lots of oxygen cavities on the magnetite surfaces were capable of CO adsorption, facilitated the exposure of CO to microorganisms and thereby utilization of CO increased. On the other side, braunite was an iron-based mineral, may promote the microbial CO metabolic and thus facilitate the carboxydotrophic hydrogenogenesis process, and thus facilitating interspecies hydrogen transfer during hydrogenotrophic methanogenesis. Correspondingly, obligate hydrogenotrophic methanogen Methanobacterium spp., Methanoculleus spp., Methanospirillum spp. were all found increased in Braunite reactors. Among which, the dominant Methanobacterium spp. (accounted for 30%) performed strong methanogenic activity with braunite even in hydrogen-deficient situations.

A large cathode surface area promotes electromethanogenesis at a proper external voltage in a single coaxial microbial electrolysis cell

Microbial electrolysis cell coupled with anaerobic digestion (MEC-AD) is currently encountering constraints on electromethanogenesis. The electrode configuration modification can be a simple yet efficient way to improve electromethanogenesis. This study evaluated two coaxial electrode configurations (large anode and small cathode: A10C1; small anode and large cathode: A1C10) using carbon felt as the electrode material. At an external voltage of 1.7 V, CH4 content was found exclusively higher in A1C10 (11 % and 13 % higher for acetate-fed and cow manure-fed, respectively) than that of the control reactors. Consequently, CH4 production was 13 % and 29 % higher in acetate-fed and CM-fed A1C10, respectively. The strengthened electromethanogenesis was attributed to the enrichment of interspecies hydrogen transfer microbes (i.e., Mesotoga and Bathyarchaeia). The coaxial configuration with a large cathode surface area demonstrated a viable stereotype in MEC-AD for improved waste treatment and energy recovery.

Metabolic effects of Fe0 on simultaneously eliminating excessive acidification and upgrading biogas in mesophilic or thermophilic anaerobic reactor

Excessive acidification and low methane (CH4) percentage are technical challenges restricting anaerobic digestion (AD) of kitchen waste, especially under high organic loading rate (OLR) conditions. Although the contribution of Fe0 powder to excessive acidification elimination and CH4 yield improvement in mesophilic AD (MAD) has been extensively reported, its impact on the thermophilic AD (TAD) process with more frequent excessive acidification problem is rarely studied. Moreover, the difference in microbial interactions and metabolic pathways between MAD and TAD reactors under Fe0 regulation was still unclear. In this study, we compared the performance of Fe0 dosing on excessive acidification elimination and biogas upgrading quality in both MAD and TAD reactors at different OLRs. The results showed that compared with excessive acidification or low CH4 percentage (less than 70%) in control reactors without Fe0 addition, the CH4 yield of MAD and TAD reactors with addition of Fe0 at OLR of 35 gVS/L were 470.88 mLCH4/gVS and 270.40 mLCH4/gVS, with higher CH4 composition of 81.32%–82.23% and 83.68%–88.48% respectively. Microbial diversity analysis showed a sharp distinction between MAD and TAD reactors with addition of Fe0. Analyses of co-occurrence networks and metabolic pathway presented hydrogenotrophic methanogenic pathway, in addition to acetoclastic methanogenesis, was dominant in MAD reactors as results of enhanced interspecies hydrogen transfer between Methanoculleus and some hydrogen producers, especially under high OLR condition. In contrast, syntrophilic acetate oxidization - hydrogenotrophic methanogenesis, which dominated by Methanothermobacter and Alkaliphilus or Pelotomaculum, was the sole methanogenic pathway in TAD reactors. The results will shed lights on the regulation mechanisms of Fe0 in AD of kitchen waste under mesophilic and thermophilic conditions, offering new insight into development of more efficient waste to energy conversion technologies.

Biochar regulates enzymes activity and interspecies electron transfer to promote bioenergy recovery from a continuous two-stage food waste anaerobic digestion process

The influence of biochar on the performance of food waste anaerobic digestion in a continuous two-stage process was examined. Biochar was added to a methanogenic (MG) reactor, with 50% of its effluent recirculated to a hydrolytic–acidogenic (HA) reactor. The addition of biochar in the MG reactor simultaneously enhanced the performance of both reactors, and the energy recovery efficiency increased by 25.01–70.99%. Biochar improved the activities of protease and α-amylase in the HA and MG reactors by 1.08–2.23 and 0.46–1.21, and 13.65–15.76 and 8.73–13.64 times, respectively. The enhancement of these hydrolytic enzymes' activity in the HA reactor was correlated to the effluent recirculation from the MG reactor, contributing to the improved hydrolysis of food waste. Furthermore, the activity of coenzyme F420 in the MG reactor increased by 11.47%, 18.81%, and 27.03%, respectively, indicating that biochar can promote hydrogenotrophic methanogenesis. Microbial analysis revealed that the presence of biochar promoted interspecies hydrogen transfer between Syntrophomonas and hydrogenotrophic methanogens. Additionally, biochar triggered direct interspecies electron transfer between Geobacter sulfurreducens and acetotrophic methanogens, enabling effective methanogenesis.

Current advances and challenges for direct interspecies electron transfer in anaerobic digestion of waste activated sludge

To realize energy self-sufficiency demand is a key component of carbon–neutral goal of wastewater treatment plants (WWTPs). Waste activated sludge (WAS), as the by-product of WWTPs, is produced in large quantities. Considering that WAS is a collection of organics, anaerobic digestion (AD) is a promising technology for methane recovery from WAS. However, conventional interspecies hydrogen transfer is vulnerable to environment changes, which can lead to instability or failure of AD. The recent finding of direct interspecies electron transfer (DIET) provides an alternative strategy to promote both the methane production potential and rate of WAS through an efficient and stable electron transfer method. This review highlights the current advances in relevant AD-based DIET for methane production from WAS, including the pathways and advantages for establishing DIET in the AD of WAS, and concludes with the current challenges in theory and application. The three main strategies for establishing DIET are conductive e-pili, conductive materials, and external voltage. These strategies not only present the advantages of shortening the lag phase and promoting the potential and rate of methane production but can also enhance intermediate degradation and provide buffering effects to inhibitors. Additionally, the role and function of microbial community structures can achieve directional regulation. Based on recent findings, theory shortages include unlocking syntrophic partners, disclosing cellular structure features, and exploring sophisticated models, while sustainable pretreatment strategies, applicable technical equipments, and comprehensive assessments may potentially limit DIET in engineering applications. This review aims to provide a comprehensive platform for establishing DIET in the AD of WAS and the carbon–neutral goal of WWTPs.

A novel green composite conductive material enhancing anaerobic digestion of waste activated sludge via improving electron transfer and metabolic activity

Anaerobic digestion (AD) of waste activated sludge (WAS) is usually limited by the low generation efficiency of methane. The addition of composite conductive materials (CMs) is a promising strategy to enhance AD performance. In this study, a new green magnetic-straw-based biochar (MSBC) was synthesised by a simple ball-milling/carbonisation method, and its effects on AD performance of sludge were investigated. Experimental results showed that the as-synthesised MSBC had an intrinsic graphene-oxide-like structure, with Fe species serving as electroactive sites; these characteristics translate into a high electron transfer (ET) capability. After adding MSBC, the volatile fatty acid production and methane yield were significantly increased by 14.13% and 45.36%, respectively. Analysis of the changes in the ET system activities, hydrogenase activities, Cyt-C concentrations and the electron transfer capacity of the sludge sample with and without the MSBC revealed that the MSBC enhanced intracellular ET and changed the extracellular ET pathway from indirect interspecies hydrogen transfer to direct interspecies electron transfer (DIET), which would be responsible for increasing methane production and proportion in the biogas. However, further analyses of key enzyme activities and the microbial community indicated that the MSBC reinforces the methanogenesis pathway by creating a favourable environment (i.e., by enhancing hydrolysis-acidification and DIET-based CO2 reduction) for acetoclastic methanogens. These findings, however, are expected to provide an important reference for developing CMs application in AD.

Microbial succession analysis reveals the significance of restoring functional microorganisms during rescue of failed anaerobic digesters by bioaugmentation of nano-biochar-amended digestate

Nano-biochar application was investigated for anaerobic digestion of orange peel waste. The application for methane production focused on the optimization of biochar feedstock, rescue of failed digesters, and microbial succession analysis. It showed that sewage sludge (SS) derived biochar had the highest performance enhancement among the different feedstocks, which could be ascribed to the improvement of electron transfer, interspecies hydrogen transfer, and supply of trace elements. Subsequently, nano SS biochar-amended digestate was evaluated for rescuing failed digesters, and the experimental results indicated its positive roles through gradual bioaugmentation operation. The dynamic analysis of microbial succession indicated the successful application was through the mechanism of restoring partially the functional microbial communities. The major reconstruction of functional microorganisms included bacteria phyla Hydrogenispora (24.5%) and Defluviitoga (18.8%) as well as methanogenic genera of Methanosarcina (41.5%) and Methanobacterium (27.3%). These findings would contribute to rescuing failed anaerobic digesters by bioaugmentation with biochar-amended digestate.

Insights into the effects of micro and nanoscale Fe0 on elimination of excessive acidification during anaerobic digestion of the organic fraction of municipal solid waste: Similarities and differences in reactor performance and syntrophic metabolism

Fe0 above the micro size (zero valent iron; ZVI) can effectively eliminate or inhibit excessive acidification during anaerobic digestion of the organic fraction of municipal solid waste, particularly at high organic loading rates. However, the role of nanoscale ZVI (NZVI) in anaerobic digestion remains controversial. In this study, the effect of NZVI at different dosages (0.4, 0.5, 0.6, 0.7, 0.8 g/g volatile solids [VS]) on the anaerobic digestion stability of the organic fraction of municipal solid waste was investigated and compared with the performance of ZVI, as were its effects on microbial syntrophism. Similarly to ZVI, NZVI recovered methane production from excessive acidification conditions, but the optimal NZVI dosage (0.4 g/gVS) was far lower than that for ZVI (0.8 g/gVS), and eventually the methane yield reached 324 mL/gVS. Microbial structure changes coupled with substance transformation revealed that NZVI enhanced the metabolism of hydrogenotrophic methanogens for interspecies hydrogen transfer and acetotrophic methanogens.

Exploring the response patterns of strong-flavor baijiu brewing microecosystem to fortified Daqu under different pit ages

The fermentation of strong-flavor baijiu depends largely on the evolution of the microbial community that originated from Daqu and pit mud (PM). Applying fortified Daqu (FD) has been proven to be an effective strategy to improve the quality and yield of baijiu. However, it is unclear what the effects of FD on the liquor brewing microecosystem under different pit ages because of the temporal heterogeneity of the PM community. Taking 2-year (new) and 40-year (aged) pits as the objects, the influence of FD on the metabolic profile, physicochemical parameters, and community diversity in Zaopei and PM was investigated by polyphase detecting approaches. Present results showed that the metabolic profiles of Zaopei were significantly improved by FD, whereas those of PM were mainly dependent on pit age. Aspergillus, Caproiciproducens, and Methanosarcina were more abundant in the aged pit, while Kazachstania, Lactobacillus, and Sphingomonas dominated in the new pit, whether in Zaopei or in PM. The interaction relationships among the communities were also altered by FD, and the co-occurrence network, especially the increased links between archaea and bacteria in the new pit. Notably, this interaction in the aged pit distinctly affected the hexanoic acid content based on the Mantel test. The results of PICRUSt2 analysis inferred that FD perhaps improved the interspecies hydrogen transfer in the new pit and increased the carbon flow of hexanoic acid production during chain elongation in the aged pit. These results provide new insights into the production of high-quality strong-flavor baijiu and the aging of PM.

Reduction of ZnO nanoparticles toxicity to methanogenic wastewater treatment by the presence of Fe3O4 nanoparticles: Focusing on nanomaterial interaction and direct interspecies electron transfer

The extensive production and wide use of ZnO and Fe3O4 nanoparticles (NPs) potentially leads to their synchronous release into the environment, and the exposure of ZnO NPs can inhibit the methanogenic process. This study investigated the effect of Fe3O4 NPs on the toxicity of ZnO NPs to methanogenesis of wastewater and the related mechanism. It was observed that the presence of 50 mg/L Fe3O4 NPs entirely mitigated the inhibition caused by 100 mg/L ZnO NPs, which had been decreased by 39.0% compared with the control group. This was because that ZnO NPs induced the accumulation of short-chain fatty acid, deterioration of extracellular polymeric substances, low activities of acetate kinase and coenzyme F420, low levels of intracellular NADH/NAD+ and ATP, and even the death of methanogenic Archaea. However, all these suppressed activities were recovered when co-presence of Fe3O4 NPs. Moreover, Fe3O4 NPs not only suppressed the levels of hydrogenase and c-type cytochrome but also enriched the direct interspecies electron transfer (DIET)-linking partners, suggesting a lower interspecies hydrogen transfer and higher DIET activities occurred. The investigations of NPs interactions demonstrated that the agglomeration of NPs, the decrease of Zn2+ and release of Fe2+ were primarily responsible for the reduced toxicity of ZnO NPs.

High Proton Conductivity of MOF-808 Promotes Methane Production in Anaerobic Digestion

Metal–organic frameworks (MOFs) are widely used in fuel cells, catalysis, and other fields. However, the influence and mechanism of the excellent properties of MOFs on biomethane production are still unclear. In this paper, MOF-808 was used to study anaerobic digestion (AD) of corn stalk, which increased biogas production and methane content by 11.06–39.82% and 10.15–14.28%, respectively. The optimal effect of biogas production was reached by adding 0.5 g/L MOF-808. The −OH of MOF-808 provided protons through protonation during AD, and then, a continuous hydrogen bond network was formed in the channels of frameworks. The hydrogen bond network led to the appearance of the Grotthuss mechanism, which made the dispersed H+ in the system to rapidly transport to the outside of the methanogen membrane along the inner surface of the sample and the nanopores, and promoted interspecies hydrogen transfer. The formation of a transmembrane proton gradient improved the hydrogen metabolism level of methanogens and promoted the energy synthesis of microorganisms. In addition, MOF-808 had a positive impact on the community and function of the microorganisms. Due to the stable structure of MOF-808, the reflux of anaerobically digested effluent is a potential way of MOF-808 recycling.

Carbon-based conductive materials enhance biomethane recovery from organic wastes: A review of the impacts on anaerobic treatment

Amongst the most important sustainable waste management strategies, anaerobic biotechnology has had a central role over the past century in the management of high-pollution load sources, such as food, agricultural and municipal wastes. During anaerobic digestion (AD), valuable by-products such as digestate and biogas are produced. Biogas (mainly composed of methane) is generated through a series of reactions between bacteria and archaea. Enhancement of AD process with higher methane yield, accelerated methane production rate, and shorter start-up time is possible via tapping into a novel methanogenic pathway discovered a decade ago. This fundamentally new concept that is a substitute to interspecies hydrogen transfer is called direct interspecies electron transfer (DIET). DIET, a thermodynamically more feasible way of electron transfer, has been proven to occur between bacteria and methanogens. It is well-documented that amendment of carbon-based conductive materials (CCMs) can stimulate DIET via serving as an electrical conduit between microorganisms. Therefore, different types of CCMs such as biochar and activated carbon have been amended to a variety of AD reactors and enhancement of process performance was reported. In this review, a comparative analysis is presented for enhancement of AD performance in relation to major CCM related factors; electrical conductivity, redox properties, particle size and dosage. Additionally, the impacts of AD operational conditions such as organic loading rate and temperature on CCM amended reactors were discussed. Further, the changes in microbial communities of CCM amended reactors were reviewed and future perspectives along with challenges for CCM application in AD have been provided.

Combined effect of zero valent iron and magnetite on semi-dry anaerobic digestion of swine manure

Combined effect of zero valent iron (ZVI) and magnetite on semi-dry anaerobic digestion of swine manure was studied. Compared with control, the addition of 5 g/L ZVI, magnetite and their mixture (1:1 wt) increased the CH4 yield by 17.6%, 22.7% and 21.9%, respectively. The three additives improved CH4 production through altering the metabolism pathways, rather than improving the solid degradation efficiency. ZVI promoted interspecies hydrogen transfer (IHT) by enriching H2-comsuming Methanolinea and acetate-oxidizing bacteria (Sedimentibacter and Clostridium). Magnetite enriched dissimilatory iron reduction bacteria (Acinetobacter) to accelerate organic hydrolysis and established direct interspecies electron transfer (DIET) by enriching Methanothrix and Methanospirillum. Key microorganisms relative to IHT (Clostridium) and DIET (Methanothrix and Methanospirillum) were simultaneously enriched with ZVI + magnetite, but they only showed an additive effect on methanogenesis, the lack of synergetic effect was attributable to the possible trade-off between IHT and DIET, or the little improvement effect of additives on substrate biodegradability.

Enhanced degradation of phenolic compounds in coal gasification wastewater by an iron‑carbon micro-electric field coupled with anaerobic co-digestion

Coal gasification wastewater contains many refractory and toxic pollutants, especially high concentrations of total phenols, which are difficult to degrade by microorganisms. The aim of our study is to explore the anaerobically enhanced degradation of coal gasification wastewater by an iron‑carbon micro-electric field coupled with anaerobic co-digestion. The optimal ratio of activated carbon to iron and the optimal dosage of co-substrate (glucose = 1500 mg/L) were investigated by batch tests. In the long-term operation of the iron‑carbon reactor, 1500 mg/L glucose was added into the influent, and carbon and iron in a ratio of 2:1 were added to the anaerobic sludge. The average effluent COD and total phenols concentrations were kept at approximately 455 and 56.3 mg/L, respectively, and removal rates of both reached 90% after treatment with the iron‑carbon micro-electric field coupled with anaerobic co-digestion in the iron‑carbon reactor. Moreover, compared with the control reactor, the methane production from the iron‑carbon reactor increased to 200 mL/day, with an increase in the methane production rate by 90%. Microbial community analysis indicated that hydrogenotrophic methanogens were enriched, and syntrophic metabolism via interspecies hydrogen transfer was enhanced. Direct interspecies electron transfer might occur between the potential electroactive bacteria Clostridium, Bacteroidetes, and Anaerolinea and the methanogens Methanosaeta, Methanobacterialies, and Methanobacterium for syntrophic metabolism through the iron‑carbon process coupled with anaerobic co-digestion.

New insights into the mechanisms underlying biochar-assisted sustained high-efficient co-digestion: Reducing thermodynamic constraints and enhancing extracellular electron transfer flux

To clarify the roles of biochar in the anaerobic co-digestion of waste activated sludge (WAS) and food waste (FW), batch tests were conducted coupled with thermodynamics, extracellular electron transfer flux and microbial community analysis. Compared with the control group, biochar significantly facilitated the co-digestion at three periods, but its sustainable facilitation was mainly in the syntrophic methanogenesis of volatile fatty acids (VFAs). The thermodynamic analysis confirmed that biochar could alleviate limitations imposed by high hydrogen partial pressure during interspecies hydrogen transfer (IHT), the thermodynamic windows was expanded 137% and 92% in the syntrophic methanogenesis of acetate and propionate, respectively. Meanwhile, due to the redox capacity of biochar (4.85 and 0.35 μmol e−/g biochar), the equivalent current of direct interspecies electron transfer (DIET) flux for syntrophic methanogenesis of acetate and propionate obtained were 1.0 × 10−4 A and 0.9 × 10−4 A, which were 108 times than that of IHT. It should be noticed that the functional microorganisms like Methanosarcina which could participate DIET were only enriched on the surface of biochar, the dominant Methanothermobacter in suspended sludge probably indicate IHT was still the main pathway for syntrophic methanogenesis. Nevertheless, the DIET triggered by the redox-active moieties on the surface of biochar and the enhanced IHT by alleviating thermodynamic restrictions, promoted the syntrophic methanogenesis synergistically.