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Biochar has been used to enhance methane generation from anaerobic digestion through establishing direct interspecific electron transfer between microorganisms. However, the microbial communication is still inadequate, thereby limiting further methane production improvement contributed by biochar. This study investigated the roles of quorum-sensing molecules, acylated homoserine lactone (AHL), in anaerobic digestion of waste activated sludge aided by biochar. Results showed that the co-addition of separated biochar and AHL achieved best methane production performance, with the maximal methane yield of 154.7 mL/g volatile suspended solids, which increased by 51.9%, 47.2%, 17.9%, and 39.4% respectively compared to that of control, AHL-loaded biochar, sole AHL, and sole biochar groups. The reason was that the co-addition of separated biochar and AHL promoted the stages of hydrolysis and acidification, promoting the conversion of organic matters and short-chain fatty acids, and optimizing the accumulation of acetate acid. Moreover, the methanogenesis stage also performed best among experimental groups. Correspondingly, the highest activities of electron transfer and coenzyme F420 were obtained, with increase ratios of 33.2% and 27.2% respectively compared to that of control. Furthermore, biochar did more significant effects on the evolution of microbial communities than AHL, and the direct interspecific electron transfer between fermentative bacteria and methanogens were possibly promoted., 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 © 2024 Elsevier Ltd. All rights reserved.)

Biochar produced from bio-wastes has been widely used to promote the performance of anaerobic digestion. Waste activated sludge (WAS) is considered as a kind of popular precursor for biochar preparation, but the abundant resources in WAS were neglected previously. In this study, the roles of biochar prepared from raw, pretreated, and fermented sludge on anaerobic digestion were investigated. That is, parts of carbon sources and nutrients like polysaccharides, proteins, and phosphorus were firstly recovered after sludge pretreatment or fermentation, and then the sludge residuals were used as raw material to prepare biochar. The methane yield improved by 22.1% with adding the biochar (AK-BC) prepared by sludge residual obtained from alkaline pretreatment. Mechanism study suggested that the characteristics of AK-BC like specific surface area and defect levels were updated. Then, the conversion performance of intermediate metabolites and electro-activities of extracellular polymeric substances were up-regulated. As a result, the activity of electron transfer was increased with the presence of AK-BC, with increase ratio of 21.4%. In addition, the electroactive microorganisms like Anaerolineaceae and Methanosaeta were enriched with the presence of AK-BC, and the potential direct interspecies electron transfer was possibly established. Moreover, both aceticlastic and CO 2 -reducing methanogenesis pathways were improved by up-regulating related enzymes. Therefore, the proposed strategy can not only obtain preferred biochar but also recover abundant resources like carbon source, nutrients, and bioenergy., 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 © 2024 Elsevier Inc. All rights reserved.)

Biomethane as a sustainable, alternative, and carbon-neutral renewable energy source to fossil fuels is highly needed to alleviate the global energy crisis and climate change. The conventional anaerobic digestion (AD) process for biomethane production from waste(water) streams has been widely employed while struggling with a low production rate, low biogas qualities, and frequent instability. The electric-biologically hybrid microbial electrochemical anaerobic digestion system (MEC-AD) prospects more stable and robust biomethane generation, which facilitates complex organic substrates degradation and mediates functional microbial populations by giving a small input power (commonly voltages < 1.0 V), mainly enhancing the communication between electroactive microorganisms and (electro)methanogens. Despite numerous bioreactor tests and studies that have been conducted, based on the MEC-AD systems, the integrated microbial fingerprints, and cooperation, accelerating substrate degradation, and biomethane production, have not been fully summarized. Herein, we present a comprehensive review of this novel developing biotechnology, beginning with the principles of MEC-AD. First, we examine the fundamentals, configurations, classifications, and influential factors of the whole system's performances (reactor types, applied voltages, temperatures, conductive materials, etc.,). Second, extracellular electron transfer either between diverse microbes or between microbes and electrodes for enhanced biomethane production are analyzed. Third, we further conclude (electro)methanogenesis, and microbial interactions, and construct ecological networks of microbial consortia in MEC-AD. Finally, future development and perspectives on MEC-AD for biomethane production are proposed., 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. Published by Elsevier Ltd.)

Sludge lysate is an unavoidable and refractory liquid produced from the waste activated sludge hydrothermal pyrolysis, which contains plenty of hazardous refractory organic compounds and value-added organic resources. Here, the proof of concept for an integrated strategy that couples technically compatible pretreatment to microbial electrolysis assisted AD (ME-AD) system is investigated for sludge lysate treatment and resource recovery. The pretreatment process shows a positive effectiveness on the ME-AD by reducing the organic load and inhibitory matters, which promote the residual refractory organic compounds (Maillard reaction products and humic acid-like substances) and carbon sources further biodegradation and bio-transformation. Combining membrane separation with ME-AD increased not only both the yield and purity of methane to 268.76 mL CH 4 /g COD and 98%, respectively, but also the recovery of 70.0~82.4% crude proteins (9.1 ± 0.5 g/L) from sludge lysate. Alternatively, the alkaline precipitation combined with ME-AD enhanced the recovery efficiency of short-chain fatty acids (SCFAs). The visible decreasing in the unpleasant color of the effluents was observed, implying that the degradation of harmful refractory organic was almost eliminated in sludge lysate. This strategy is worthy to be developed in WWTP for sludge lysate treatment with considerable bio-resources recovery and refractory organics removal., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Hydrogen-entangled electron transfer has been verified as an important extracellular pathway of sharing reducing equivalents to regulate biofilm activities within a diversely anaerobic environment, especially in microbial electrosynthesis systems. However, with a lack of useful methods for in situ hydrogen detection in cathodic biofilms, the role of hydrogen involvement in electron transfer is still debatable. Here, a cathodic biofilm was constructed in CH 4 -produced microbial electrosynthesis reactors, in which the hydrogen evolution dynamic was analyzed to confirm the presence of hydrogen-associated electron transfer near the cathode within a micrometer scale. Fluorescent in situ hybridization images indicated that a colocalized community of archaea and bacteria developed within a 58.10-μm-thick biofilm at the cathode, suggesting that the hydrogen gradient detected by the microsensor was consumed by the collaboration of bacteria and archaea. Coupling of a microsensor and cyclic voltammetry test further provided semiquantitative results of the hydrogen-associated contribution to methane generation (around 21.20% ± 1.57% at a potential of -0.5 V to -0.69 V). This finding provides deep insight into the mechanism of electron transfer in biofilm on conductive materials. IMPORTANCE Electron transfer from an electrode to biofilm is of great interest to the fields of microbial electrochemical technology, bioremediation, and methanogenesis. It has a promising potential application to boost more value-added products or pollutant degradation. Importantly, the ability of microbes to obtain electrons from electrodes and utilize them brings new insight into direct interspecies electron transfer during methanogenesis. Previous studies verified the direct pathway of electron transfer from the electrode to a pure-culture bacterium, but it was rarely reported how the methanogenic biofilm of mixed cultures shares electrons by a hydrogen-associated or hydrogen-free pathway. In the current study, a combination method of microsensor and cyclic voltammetry successfully semiquantified the role of hydrogen in electron transfer from an electrode to methanogenic biofilm., (Copyright © 2020 American Society for Microbiology.)

Anaerobic digestion (AD) of organics is a challenging task under high-strength sulfate (SO 4 2- -reducing bacteria (SRB) causes low methane (CH 4 2- -reducing bacteria (SRB) causes low methane (CH 4 ) production. This study investigated the feasibility of alleviating sulfide inhibition and enhancing CH 4 production by using an anaerobic reactor with built-in microbial electrolysis cell (MEC), namely ME-AD reactor. Compared to AD reactor, unionized H 2 S in the ME-AD reactor was sufficiently converted into ionized HS - due to the weak alkaline condition created via cathodic H 2 production, which relieved the toxicity of unionized H 2 S to methanogenesis. Correspondingly, the CH 4 production in the ME-AD system was 1.56 times higher than that in the AD reactor with alkaline-pH control and 3.03 times higher than that in the AD reactors (no external voltage and no electrodes) without alkaline-pH control. MEC increased the amount of substrates available for CH 4 -producing bacteria (MPB) to generate more CH 4 . Microbial community analysis indicated that hydrogentrophic MPB (e.g. Methanosphaera) and acetotrophic MPB (e.g. Methanosaeta) participated in the two major pathways of CH 4 formation were successfully enriched in the cathode biofilm and suspended sludge of the ME-AD system. Economic revenue from increased CH 4 production totally covered the cost of input electricity. Integration of MEC with AD could be an attractive technology to alleviate sulfide inhibition and enhance CH 4 production from AD of organics under SO 4 2- -rich condition., 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 © 2020 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The underlying dynamics of microbial (bacteria and archaea) communities ecologically responding to an applied potential are critical to achieving the goal of enhancing bioenergy recovery but are not sufficiently understood. We built a MEC-AD mode that increased methane production rate by several times (max. 3.8 times) during the startup period compared to control AD, changed the absence or presence of external voltage to provide the pre-, dur-, and post- samples for microbial analysis. From a time and spatially dependent community analysis of electrode-respiring bacteria and methanogens, the corresponding Geobacter developed under the influence of external voltage, pairing with methanogens in the anodic and cathodic biofilm to generate methane. Additionally, at the cathode, the Alkaliphilus (basophilic bacteria) also correspondingly shifted alongside the change of external voltage. The mcrA sequencing confirmed a change in the dominant microbe from acetoclastic (mostly Methanosarcina mazei LYC) to hydrogenotrophic methanogens (mostly basophilic Methanobacterium alcaliphilum) at the cathode with 0.8 V voltage. Overall, the external voltage not only enriched the functional microbes including electrogens and methanogens but also indirectly shifted the composition of the bacterial and archaeal community via disturbing the pH condition. The predictive functional profiling indicated that the cathodic methanogenesis principally followed the metabolism pathway of the hydrogenotrophic methanogens, suggesting the F420 co-enzyme could be the key mediate for electron transfer. All data suggested that the electric stimulation would change and maintain the micro-environmental conditions to shift the bacterial/archaeal community., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Recently, a new oxygenic pathway has been proposed based on the disproportionation of NO with putative NO dismutase (Nod). In addition to a new process in nitrogen cycling, this process provides ecological advantages for the degradation of substrates in anaerobic conditions, which is of great significance for wastewater treatment. However, the Nod distribution in aquatic environments is rarely investigated. In this study, we obtained the nod genes with an abundance of 2.38 ± 0.96 × 10 5 copies per gram of dry soil from the Zoige wetland and aligned the molecular characteristics in the corresponding Nod sequences. These Nod sequences were not only found existing in NC10 bacteria, but were also found forming some other clusters with Nod sequences from a WWTP reactor or contaminated aquifers. Moreover, a new subcluster in the aquifer-similar cluster was even dominant in the Zoige wetland and was named the Z-aquifer subcluster. Additionally, soils from the Zoige wetland showed a high potential rate (10.97 ± 1.42 nmol of CO 2 per gram of dry soil per day) for nitrite-dependent anaerobic methane oxidation (N-DAMO) with low abundance of NC10 bacteria, which may suggest a potential activity of Nod in other clusters when considering the dominance of the Z-aquifer subcluster Nod. In conclusion, we verified the occurrence of Nod in an alpine wetland for the first time and found a new subcluster to be dominant in the Zoige wetland. Moreover, this new subcluster of Nod may even be active in the N-DAMO process in this alpine wetland, which needs further study to confirm.

Cathodic methanogenesis is a promising method for accelerating and stabilising bioenergy recovery in anaerobic processes. The change in composition of microbial (especially methanogenic) communities in response to an applied potential-and especially the associated pH gradient-is critical for achieving this goal, but is not well understood in cathodic biofilms. We found here that the pH-polarised region in the 2 mm surrounding the cathode ranged from 6.9 to 10.1, as determined using a pH microsensor; this substantially affected methane production rate as well as microbial community structure. Miseq sequencing data of a highly conserved region of the mcrA gene revealed a dramatic variation in alpha diversity of methanogens concentrated in electrode biofilms under the applied potential, and confirmed that the dominant microbes at the cathode were hydrogenotrophic methanogens (mostly basophilic Methanobacterium alcaliphilum). These results indicate that regional pH variation in the microenvironment surrounding the electrode is an ecological niche enriched with Methanobacterium., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Methane production was tested in membrane-less microbial electrolysis cells (MECs) under closed-circuit (R CC ) and open-circuit (R OC ) conditions, using glucose as a substrate, to understand the regulatory effects of bioelectrochemistry in anaerobic digestion systems. A dynamic model was built to simulate methane productions and microbial dynamics of functional populations, which were colonized in groups R CC and R OC during the start-up stage. The experiment results showed significantly greater methane production in R CC than R OC , the average methane production of R CC was 0.131 m 3 /m 3 /d, which was 1.4 times higher than that of R OC (0.055 m 3 /m 3 /d). The simulation results revealed that bioelectrochemistry had a significant influence on the abundance of microorganisms involved in acidogenesis and methanogenesis. The abundance of glucose-uptaking microorganisms was 87% of the total biomass in R OC without applied voltage, which was 20% higher than that in R CC (67%) when external voltages were applied between the anode and cathode. The abundance of hydrogenotrophic methanogens in R CC was 6% higher than that in R OC . The simulation results were verified through 16S rDNA high-throughput sequencing analysis. An electron balance analysis revealed that alteration of the acidogenesis type led to more acetate and hydrogen production from glucose fermentation, compared with the situation without bioelectrochemistry. An additional pathway from acetate to hydrogen was introduced by bioelectrolysis. These two factors resulted in significant enhancement of methane production in R CC . Bioelectrolysis process directly contributed to 26% of the total methane production after the start-up stage. When the applied voltages were cut down or decreased, R CC could maintain considerable methane productions, because the microbial communities and electron transfer pathways were already formed. Starting-up with high voltage, but operating under low voltage, could be an energy-favorable strategy for accelerating biogas production in bioelectro-anaerobic bioreactors., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Anaerobic digestion (AD) represents a potential way to achieve energy recovery from waste organics. In this study, a novel bioelectrochemically-assisted anaerobic reactor is assembled by two AD systems separated by anion exchange membrane, with the cathode placing in the inside cylinder (cathodic AD) and the anode on the outside cylinder (anodic AD). In cathodic AD, average methane production rate goes up to 0.070 mL CH4/mL reactor/day, which is 2.59 times higher than AD control reactor (0.027 m(3) CH4/m(3)/d). And COD removal is increased ∼15% over AD control. When changing to sludge fermentation liquid, methane production rate has been further increased to 0.247 mL CH4/mL reactor/day (increased by 51.53% comparing with AD control). Energy recovery efficiency presents profitable gains, and economic revenue from increased methane totally self-cover the cost of input electricity. The study indicates that cathodic AD could cost-effectively enhance methane production rate and degradation of glucose and fermentative liquid., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Anaerobic digestion of waste-activated sludge (WAS) towards biogas recovery is constrained by the limited hydrolysis and inhibited acetogenesis steps that hinder subsequent energy recovery. This study employed Fe(VI)/S(IV) oxidation to enhance the WAS solubilization and coupled it with the syntrophic interaction of hydrogen-producing acetogen (HPA) and sulfate-reducing bacteria (SRB) to stimulate the successive procedure towards methane production. Results showed that the dosage ratio of HPA-SRB to WAS (H-S-W) with 1:1:50 outperformed with the highest methane production potential (11.63 ± 1.87 mL CH4/(g VSS·d). Meanwhile, the efficient and sequential process from acetogenesis to methanogenesis stimulated by HPA-SRB was evidenced by a significant decrease of 30.2% in the acetate concentration. The microbial community structure further manifested the crucial role of HPA-SRB with increased abundance of Desulfobulbus (2.07%), Syntrophomonas (1.24%) and Smithella (1.63%), which stimulated acetophilic methanogen boost with Methanobacterium dominating with 77.51% in H-S-W100. Furthermore, the positive syntrophic relationships among HPA-SRB and acetophilic methanogens towards methane production were confirmed via molecular ecological network and canonical correspondence analysis. This study highlighted the syntrophic cooperation of the mixed consortia of HPA and SRB on methane production based on Fe(VI)/S(IV) pretreatment and provided the theoretical and technical basis for the potential implementation of novel methanogenesis technology for WAS treatment. [ABSTRACT FROM AUTHOR]

Challenges are always proposed when pursuing more energy and resource recovery from waste activated sludge via the anaerobic digestion (AD) process. Recently, microbial electrolysis has been integrated with AD and has been proven to enhance sludge conversion and system stability. This study investigated the effect of organic load on fermentation and biogas production in a bioelectrolysis-assisted AD reactor. Four different organic loads of alkaline waste sludge from 6 g/L to 14 g/L were investigated for their effects on the methanogenesis rate, substrate metabolism, electrochemical performance, and contribution. The results showed that the integrated system had a stronger tolerance to organic loads than the traditional anaerobic system. When the sludge concentration reached 14 g/L, the methanogenic rate, total methane yield, and SS removal rate significantly increased, reaching 47.1 mL/d, 96.2 mL/gVSS, and 71.6%, which were 1.27, 2.08, and 1.28 times those of the control, respectively. A high organic load was beneficial to the overall methanogenic rate but prolonged the fermentation period. Under a low organic load, the energy efficiency of the system deteriorated because the power loss increased and the electrochemical contribution rate was less than 50%. This result suggests that the integrated system can work and improve the overall energy yield from a high organic load of wasted sludge digestion. [ABSTRACT FROM AUTHOR]

A cheap but efficient electrode material is required to explore and apply to microbial electrolysis cell (MEC) with high hydrogen evolution reaction (HER) efficiency and low over-potential loss. Pt coating carbon cloth (Pt/CC) was one of the most efficient catalyst for hydrogen production in current lab research, but it is difficult to be applied in practice because of expensive cost and week strength from the base material (carbon cloth). Thus a cheap and effective supporting base material is worth to evaluate on hydrogen recovery and loss to methane for the MEC future application. In this study, nickel foam (NF) was used as an alternative to expensive carbon cloth, and NF coated with Pt (Pt/NF) was applied and evaluated through catalytic performance, hydrogen production efficiency and economic assessment in comparison with Pt/CC. The Pt/NF showed a competitive HER performance to Pt/CC. The highest hydrogen yield was reached 0.71 ± 0.03 m 3 /m 3 ·d by Pt/NF under 0.8 V, which exceeded 6%, 10% over Pt/CC and NF, respectively. The energy efficiency relative to the electrical energy input was 127% for Pt/NF and 123%, 110% for Pt/CC and NF, respectively. For fifteen cycles, the methane content of Pt/NF got the lowest due to its higher hydrogen evolution activity. The economic analysis showed a 56% reduction when using Pt/NF as supporting base in place of carbon cloth to achieve similar performance. The linear sweep voltammetry (LSV) showed the possibility to further reduce input voltage in a long term operation. [ABSTRACT FROM AUTHOR]

Methane production rate (MPR) in waste activated sludge (WAS) digestion processes is typically limited by the initial steps of complex organic matter degradation, leading to a limited MPR due to sludge fermentation speed of solid particles. In this study, a novel microbial electrolysis AD reactor (ME-AD) was used to accelerate methane production for energy recovery from WAS. Carbon bioconversion was accelerated by ME producing H 2 at the cathode. MPR was enhanced to 91.8 gCH 4 /m 3 reactor/d in the microbial electrolysis ME-AD reactor, thus improving the rate by 3 times compared to control conditions (30.6 gCH 4 /m 3 reactor/d in AD). The methane production yield reached 116.2 mg/g VSS in the ME-AD reactor. According to balance calculation on electron transfer and methane yield, the increased methane production was mostly dependent on electron contribution through the ME system. Thus, the use of the novel ME-AD reactor allowed to significantly enhance carbon degradation and methane production from WAS. [ABSTRACT FROM AUTHOR]

Abstract: Methane production occurs during hydrogen gas generation in microbial electrolysis cells (MECs), particularly when single chamber systems are used which do not keep gases, generated at the cathode, separate from the anode. Few studies have examined the factors contributing to methane gas generation or the main pathway in MECs. It is shown here that methane generation is primarily associated with current generation and hydrogenotrophic methanogenesis and not substrate (acetate). Little methane gas was generated in the initial reaction time (<12h) in a fed batch MEC when acetate concentrations were high. Most methane was produced at the end of a batch cycle when hydrogen and carbon dioxide gases were present at the greatest concentrations. Increasing the cycle time from 24 to 72h resulted in complete consumption of hydrogen gas in the headspace (applied voltage of 0.7V) with methane production. High applied voltages reduced methane production. Little methane (<4%) accumulated in the gas phase at an applied voltage of 0.6–0.9V over a typical 24h cycle. However, when the applied voltage was decreased to 0.4V, there was a greater production of methane than hydrogen gas due to low current densities and long cycle times. The lack of significant hydrogen production from acetate was also supported by Coulombic efficiencies that were all around 90%, indicating electron flow was not altered by changes in methane production. These results demonstrate that methane production in single chamber MECs is primarily associated with current generation and hydrogen gas production, and not acetoclastic methanogenesis. Methane generation will therefore be difficult to control in mixed culture MECs that produce high concentrations of hydrogen gas. By keeping cycle times short, and using higher applied voltages (≥0.6V), it is possible to reduce methane gas concentrations (<4%) but not eliminate methanogenesis in MECs. [Copyright &y& Elsevier]

• The simulation results of the dynamic distribution of tracer were compared with the results of the RTD experiment, to verify the reliability of the model settings, especially the Non-rigid carbon brush.. • Cathode space ratio based on hydrodynamics character affected electrogenesis capacity. • Electron balance was analyzed on different electrode assembly. • The considerable organic removal, methane recovery, and hydrodynamics characteristics were linked to cathode space ratio. Compared with common anaerobic digestion, microbial electrolysis has been proven feasibly to accelerate biodegradation and methanogenesis with the advantages of effective electron flow regulation. However, its actual application and scale-up required a full understanding and further investigation on electrode size and distribution. For making full use of the space of the integrated reactor and improve methane recovery, an effective interior configuration was significant. In this work, three types of reactors with different cathode spatial distributions, that is, different cathode space ratios (ratio of cathode surface area to reaction region volume), were studied to form a good flow pattern for obtaining high methane production. Tracer experiments and numerical simulation were employed simultaneously for understanding the hydrodynamics characters of the interior flow field. The results showed that by increasing the cathode space ratio to 1.33 cm2/cm3 and 2 cm2/cm3, respectively, better flow patterns with the residence time of 1.336 times and 1.363 times of theoretical hydraulic retention time could be obtained. The stacked structure of nickel meshes was beneficial to prolong the contact time of contaminant and improve the mass transfer. Increasing the cathode space ratio could also enhance the electrochemical performance. Considering the organic removal, methane recovery, electrons generation, and material consumption, the recommended cathode space ratio was 1.33 cm2/cm3. With this structure, COD removal efficiency reached 93.2 ± 1.9% and 94.1 ± 1.5%, methane production rate reached 332.0 and 334.8 mL CH 4 /L reactor/day, and methane yield was 171.3 and 246.4 mL CH 4 /g COD under the HRT of 24 h and 36 h, respectively. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Higher CH 4 yield and economic revenue achieved with intermittent electrostimulations. • Intermittent electro field regulated interspecies electron transfer in bulk solution. • Suspended microbes made considerable contributions to methane recovery. • More complex interactions in extracellular electron transfer bacteria with periodic on/off electrical field. Lately, extracellular electron transfer (EET) is widely disclosed on the surface of the bioelectrodes, and conductive (bio)carriers involved in anaerobic biodegradation/biosynthesis. By electrostimulation, microbial consortia colonize the electrodes and accelerate substrate (waste/wastewater) metabolization on the bioanode or biosynthesize value-added products (methane, acetate, etc.) on the biocathode. However, the connections and contributions of planktonic microbial communities have not been effectually understood. Herein, electromethanogenesis were comprehensively investigated in response to different driving-force modes: intermittent electric field applied by manual on-off or natural solar power and continuous electrical field. Intermittent modes implied preferable electron transfer efficiency, higher methane yield and energy recovery efficiencies from wastewater by the microbes in the bulk solutions. Microbial community analysis revealed that less electroactive microorganisms and acetotrophic methanogens in the bulk solutions were accommodated under the intermittent modes than the continuous electrical field, whereas more fermentative bacteria and hydrogenotrophic methanogens evolved in the intermittent driving modes, implying that the interspecies electron transfer both on and away from the electrodes were favorably regulated. Redundancy and network analysis proved that more complicated ecological interactions were shown in the bulk solutions with the periodic on/off of electrical field. These results hinted that the electrostimulation effectively regulated EET bacteria, even in the bulk solutions, while more efficient electron flow to methane through interspecies electron transfer was developed during the intermittent driving regulation. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Methane recovery from bio-wastes is one promising renewable energy to relieve the energy crisis faced around the world. Zeolite is one of attractive exogenous additives for promoting methane production from anaerobic digestion due to its potentials in regulating microorganism enrichment and anaerobic environment. However, the poor electron mediated ability limited its positive effects. Hence, this study hypothesized that the electron mediated ability of zeolite would be improved by loading magnetite, thereby generating more positive impacts on the performance of sludge anaerobic digestion. Compared to control group, magnetite modified zeolite (MZ) effectively improved methane yield by 37.4%. The reason was that the electron mediated ability and adsorption capacity of zeolite were improved by loading magnetite, facilitating the degradation of refractory organics and the conversion of volatile fatty acids. Moreover, the activities of cytochrome c and F420 increased by 33.3% and 29.8% with MZ additive, contributing to stimulating efficient electron transfer process. In addition, the electro-active genera were enriched, such as Bacteroidetes_vadinHA17 , Methanobacterium , and Methanosaeta , and the direct interspecies electron transfer was possibly established for methanogenesis with the help of MZ. This study will provide valuable references for the application of zeolite in anaerobic digestion. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Immobilized CB and NF electrode can increase methane yield by 38%-94%. • Applying no voltage to the electrodes is more conducive to methane production. • Syntrophic methane production was enhanced by immobilized electrode. • Voltage weakens anode methanogenesis but strengthens cathode DIET/IHT methanogenesis. Introduction of voltage or conductive materials have been proven to be effective strategies for enhancing the stability and efficiency of sludge digestion for methane recovery. This experiment simulated a two-phase anaerobic system, consisting of an acid-producing phase with alkali pretreatment and a methane-producing phase with immobilized electrodes and voltage enhancement. Various effective and comparative experiments were conducted to investigate the crucial roles of immobilized electrodes like carbon brush (CB) and nickel foam (NF), electroactive biofilm and voltage during the sludge methane-producing phase. The experimental results indicate that directly application of immobilized CB and NF electrode to promote sludge anaerobic digestion achieves better performance than additional voltage supply. The electrodes can increase methane production by 38% to 94%, whereas under additional voltage supply, methane production only increases by 22%–53% compared to the control. Through the analysis of key transformations in organic substrates and changes in microbial community structure, the inherent advantages of electrode materials in promoting the syntrophic methanogenesis processes are confirmed. Compared to suspended sludge, the types and relative abundance of typical syntrophic methane-producing microorganisms, Methanosarcina , and acetogenic bacteria, Syntrophomonas on the electrode surface significantly increase by 8.5% and 1.7%. This process is notably superior to the methane generation process dominated by hydrogenotrophic methanogensis after applying voltage. The results indicate that CB and NF are suitable conductive materials to promote anaerobic methane recovery, and their effectiveness surpasses that of applying voltage. These research outcomes provide new insights for the development of novel anaerobic digestion enhancement systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • Citric acid promoted hydrogen corrosion of zero valent iron for methane production. • Zero valent iron and citric acid increased methane production potential by 63.8%. • Hydrogen accumulation with zero valent iron and citric acid were the highest. • Ferrous ions increased to 2.32 times higher than that with solo zero valent iron. • Hydrogenotrophic methanogens enriched while acetotrophic methanogens reduced. Anaerobic digestion of waste activated sludge (WAS) for biofuel recovery is one key step for developing energy-self-sufficient and carbon–neutral wastewater treatment plants. Although zero valent iron (ZVI) is an alternative additive to promote methane production from WAS, low hydrogen corrosion of ZVI is one important limitation. This study explored whether methane production potential could be further enhanced while using citric acid (CA) to accelerate hydrogen corrosion of ZVI. Results showed that the maximum methane production reached 92.77 mL/g volatile suspended solid (VSS) with ZVI of 30 mM and CA of 6 mM, which increased by 63.8% compared to that with solo ZVI. Mechanism analysis demonstrated that the proposed strategy could accelerate the solubilization of WAS, increase the accumulation of short-chain fatty acids (SCFAs), and optimize the composition of SCFAs. Meanwhile, both the efficiencies of hydrogen yield and production rate obtained with ZVI and CA were the highest. The accumulation of hydrogen with ZVI and CA reached 5 mL/g VSS during the 9-hour period, which was 2.42 times higher than that with solo ZVI, and the hourly hydrogen production rate was 2.47 mL/g VSS−1h−1, which was higher than those with either solo CA or ZVI. Thus, the effectiveness of 1 + 1＞2 was obtained. Correspondingly, the concentration of ferrous ion with ZVI and CA increased to 2.32 times higher than that with solo ZVI. In addition, the microbial community analysis indicated that hydrogenotrophic methanogens were predominated with ZVI and CA. These findings may provide some new solutions for promoting the role of nano-ZVI in enhancing methane production from anaerobic digestion. [ABSTRACT FROM AUTHOR]

Integrated microbial electrolysis with anaerobic digestion is proved to be an effective way to improve methanogenesis efficiency of waste activated sludge (WAS). WAS requires pretreatment for efficient improvement of acidification or methanogenesis efficiency, but excessive acidification may inhibit the methanogenesis. In order to balance these two stages, a method for efficient WAS hydrolysis and methanogenesis has been proposed in this study by high-alkaline pretreatment integrated with microbial electrolysis system. The effects of pretreatment methods and voltage on the normal temperature digestion of WAS have also been further investigated with emphasis on the effects of voltage and substrate metabolism. The results show that compared to low-alkaline pretreatment (pH = 10), high-alkaline pretreatment (pH > 14) can double the SCOD release and promote the VFAs accumulation to 5657 ± 392 mg COD/L, but inhibit the methanogenesis process. Microbial electrolysis can alleviate this inhibition effectively through the rapid consumption of VFAs and speeding up of the methanogenesis process. The optimal methane yield of the integrated system is 120.4 ± 8.4 mL/g VSS at the voltage of 0.5 V. Enzyme activities, high-throughput and gene function prediction analysis reveal that the cathode and anode maintain the activity of methanogens under high substrate concentrations. Voltage positively responded to improved methane yield from 0.3 to 0.8 V, but higher than 1.1 V is found to be unfavorable for cathodic methanogenesis and results in additional power loss. These findings provide a perspective idea for rapid and maximum biogas recovery from WAS. • Microbial electrolysis contributed the methanogenesis in high-alkaline pretreated sludge. • 0.5 V was the optimal and economical for accelerating methanogenesis. • High voltage could reduce the enrichment and activity of cathodic methanogens. • Anodic microorganisms facilitated the syntrophic methanogenesis process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Recent advances for establishing DIET in AD of WAS were systematically reviewed. • How to establish potential DIET in AD of WAS was analyzed and discussed. • The potential advantages of AD-based on DIET were summarized and discussed. • Challenges and perspectives for establishing DIET in AD of WAS were proposed. 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synergisms weren't formed with co-additive of BC and NM. • The methane yield decreased by 24.7 and 38.3% compared to that in BC and NM. • The direct interspecies electron transfer contributed by NM was reduced by BC. • The NM adsorbed on BC were converted to nano-hematite. • The electron-active microorganisms were reduced at suspended and attached stages. To realize energy-self-sufficient operation of wastewater treatment plants is a key step for carbon–neutral goal, and conductive materials have been widely used to build direct interspecies electron transfer (DIET) to promote methane production from waste activated sludge (WAS). However, the role and significance of co-additive of biochar (BC) and nano-magnetite (NM) have not been reported, and whether synergistic effects can be formed or not is still unknown. This study investigated the effects of co-additive of BC and NM by stepwise and simultaneous strategies on anaerobic digestion. Non-synergistic rather than synergistic effects were observed. The methane production reduced to 122.4 mL/g volatile suspended solids in simultaneous co-additive strategy, which was 24.7 and 38.3% lower than that in sole BC and NM, respectively. Differently, methane productions in stepwise additive strategies were higher than that in sole BC but lower than that in sole NM. Mechanism studies indicated that although NM accelerated WAS solubilization and hydrolysis, BC adsorbed soluble organics, reducing available substrates for methanogens at suspended stage. Also, extracellular polymeric substances were decreased, weakening electrochemical functions of redox-active shuttles. Besides, the DIET contributed by NM was reduced by the adsorption of BC to NM, and the NM adsorbed on BC were converted to nano-hematite, which presented inhibitions to methanogenesis. Moreover, the electron-active microorganisms, such as Clostridium and Methanobacterium , were all reduced at suspended and attached stages with the presence of BCs, limiting potential contributions of DIET on methane production improvement. This work may provide some new insights for additive strategies of BC and NM for methane production enhancement from WAS. [ABSTRACT FROM AUTHOR]

• High-efficiency methanogenic biocathode formed on layered nickel foam cathodes. • The maximum methane production rate of MEC-AD increased by 45% with biocathode. • The biocathode regulates the methanogens distribution in sludge and electrodes. • Biocathode plays a key role in the enrichment of methanogens. Towards the regulation and enhancement of inter-species electron transfer in sludge anaerobic digestion system, microbial electrolysis technology has become one of the effective ways to accelerate both fermentation and methanogenesis. In this study, the reactor performances and microbial activities related to biocathode formation are evaluated when the role of biocathode is regulated by series of layered cathodes. The results show the abundance of the cathodic methanogens decreased when enlarges the cathode area due to the lower current density. The biocathode evolution is directly related to the spatial methane distribution, which can further determine 25% increase of methane production rate compared to control without biocathode. Ultimately, the maximum methane production yield of 145.79 mL·d−1 is achieved by the optimal cathode area with a current density of 5.3 mA/cm3. The spatially methanogens distribution in suspended sludge and electrodes regulated by the layered cathodes is regarded to be the key to increase methanogenesis. [ABSTRACT FROM AUTHOR]

Methane production enhanced by electrode respiring microorganisms is a recently developing technology for bioelectrolysis assisted anaerobic digestion. The direct interspecies electron transfer between electrodes and microbe has been recognized as the core mechanism for methanogenesis in this coupled system. However, the detailed mechanisms of methane electrosynthesis on cathodes need in-depth investigation. This work is to evaluate the effects of hydrogen evolution capacity of cathode on methane production and biocathode community that developed on commonly used cathode materials (copper, nickel and stainless steel). Integrated reactors with nickel cathode exhibited a maximum methane yield of 59.2 mL CH 4 /gVSS with a higher current density of 9 A/m2 compared to copper electrode and stainless-steel electrode. About 40% increase of methane production rate is achieved in the coupled reactors with nickel electrode compared to traditional anaerobic digestion technique. Analysis of microbial community structure reveals that the bioelectrolysis cathode persuades the whole system towards the hydrogenotrophic methanogenic pathway. A methanogenic system derived from Methanobacterium is formed on metal electrodes, which simultaneously utilizes both electrons and hydrogen to produce methane. However, higher hydrogen evolution reaction capacity leads to higher electrochemical contribution for methane synthesis, which results in an improved conversion efficiency. • High efficiency CO 2 -reducing biocathodes can formed on metal electrodes. • Higher HER capacity of cathode means higher contribution for methanogenesis. • Electrodes stimulate the sludge towards hydrogenotrophic methanogenic pathway. • The cathode community structure can be affected by the material properties. [ABSTRACT FROM AUTHOR]