Enhancement of anaerobic methanogenesis at a short hydraulic retention time via bioelectrochemical enrichment of hydrogenotrophic methanogens

Optimizing bioelectromethanosynthesis of CO2 and membrane fouling mitigation in MECs via in-situ biogas recirculation

The CO2 bioelectromethanosynthesis via two-chamber microbial electrolysis cell (MEC) holds tremendous potential to solve the energy crisis and mitigate the greenhouse gas emissions. However, the membrane fouling is still a big challenge for CO2 bioelectromethanosynthesis owing to the poor proton diffusion across membrane and high inter-resistance. In this study, a new MEC bioreactor with biogas recirculation unit was designed in the cathode chamber to enhance secondary-dissolution of CO2 while mitigating the contaminant adhesion on membrane surface. Biogas recirculation improved CO2 re-dissolution, reduced concentration polarization, and facilitated the proton transmembrane diffusion. This resulted in a remarkable increase in the cathodic methane production rate from 0.4 mL/L·d to 8.5 mL/L·d. A robust syntrophic relationship between anodic organic-degrading bacteria (Firmicutes 5.29%, Bacteroidetes 25.90%, and Proteobacteria 6.08%) and cathodic methane-producing archaea (Methanobacterium 65.58%) enabled simultaneous organic degradation, high CO2 bioelectromethanosynthesis, and renewable energy storage.

Responses of syntrophic microbial communities and their interactions with polystyrene nanoplastics in a microbial electrolysis cell

Microbial electrochemical technologies are promising for simultaneous energy recovery and wastewater treatment. Although the inhibitory effects of emerging pollutants, particularly micro/nanoplastics (MPs/NPs), on conventional wastewater systems have been extensively studied, the current understanding of their impact on microbial electrochemical systems is still quite limited. Microplastics are plastic particles ranging from 1 μm to 5 mm. However, nanoplastics are smaller plastic particles ranging from 1 to 100 nm. Due to their smaller size and greater surface area, they can penetrate deeper into biofilm structures and cell membranes, potentially disrupting their integrity and leading to changes in biofilm composition and function. This study first reports the impact of polystyrene nanoplastics (PsNPs) on syntrophic anode microbial communities in a microbial electrolysis cell. Low concentrations of PsNPs (50 and 250 μg/L) had a minimal impact on current density and hydrogen production. However, 500 μg/L of PsNPs decreased the maximum current density and specific hydrogen production rate by ∼43 % and ∼48 %, respectively. Exposure to PsNPs increased extracellular polymeric substance (EPS) levels, with a higher ratio of carbohydrates to proteins, suggesting a potential defense mechanism through EPS secretion. The downregulation of genes associated with extracellular electron transfer was observed at 500 μg/L of PsNPs. Furthermore, the detrimental impact of 500 μg/L PsNPs on the microbiome was evident from the decrease in 16S rRNA gene copies, microbial diversity, richness, and relative abundances of key electroactive and fermentative bacteria. For the first time, this study presents the inhibitory threshold of any NPs on syntrophic electroactive biofilms within a microbial electrochemical system.

Towards bio-utilization and energy recovery potential exploration of membrane foulant from membrane bioreactor by using microbial fuel cell-centered technology

The utilization of membrane foulant is expected to push forward the developments of membrane bioreactor (MBR). In this study, the combination of microbial fuel cell (MFC) with bio-electrochemical enhanced hydrolysis process was proposed, and three systems were conducted to utilize the membrane foulant and simultaneously harvest electricity. Polysaccharides (PS), proteins (PN) and humic acid (HA) concentration variations and the fluorescent compound changes in different chambers revealed the biodegradability of membrane foulant. Optimized HRT improved the hydrolysis of membrane foulant while allowing MFC to utilize the biodegradable components efficiently. MFC-MFC system had the highest voltage and satisfactory effluent quality at HRT of 1 d. Microbial community structure analysis indicated that Proteobacteria, Planctomycetes and Bacteroidetes were the majority phyla and network analysis further revealed that Proteobacteria played a key role in membrane foulant utilization. This study suggests that MFC hybrid systems has potential application for synchronous membrane foulant reuse and energy recovery.

Genome-resolved correlation mapping links microbial community structure to metabolic interactions driving methane production from wastewater

Anaerobic digestion of municipal mixed sludge produces methane that can be converted into renewable natural gas. To improve economics of this microbial mediated process, metabolic interactions catalyzing biomass conversion to energy need to be identified. Here, we present a two-year time series associating microbial metabolism and physicochemistry in a full-scale wastewater treatment plant. By creating a co-occurrence network with thousands of time-resolved microbial populations from over 100 samples spanning four operating configurations, known and novel microbial consortia with potential to drive methane production were identified. Interactions between these populations were further resolved in relation to specific process configurations by mapping metagenome assembled genomes and cognate gene expression data onto the network. Prominent interactions included transcriptionally active Methanolinea methanogens and syntrophic benzoate oxidizing Syntrophorhabdus, as well as a Methanoregulaceae population and putative syntrophic acetate oxidizing bacteria affiliated with Bateroidetes (Tenuifilaceae) expressing the glycine cleavage bypass of the Wood-Ljungdahl pathway.

Graphene oxide addition to anaerobic digestion of waste activated sludge: Impact on methane production and removal of emerging contaminants

The effect of graphene oxide on the anaerobic digestion of waste activated sludge was investigated at two graphene oxide concentrations (0.025 and 0.075 g graphene oxide per g volatile solids) using biochemical methane potential tests. The occurrence of 36 pharmaceuticals was monitored in the solid and liquid phases before and after the anaerobic treatment. The addition of graphene oxide improved the removal of most pharmaceuticals detected, even those that are considered persistent to biological degradation, such as azithromycin, carbamazepine, and diclofenac. No significant differences were observed in the final specific methane production without graphene oxide and with the lowest graphene oxide concentration, yet the highest graphene oxide concentration partially inhibited methane production. The relative abundance of antibiotic resistance genes was not affected by the graphene oxide addition. Finally, significant changes in the microbial community including bacteria and archaea were detected with graphene oxide addition.

Response of electroactive biofilms from real wastewater to metal ion shock in bioelectrochemical systems

The electrochemical activity of bioelectrochemical systems (BESs) was proven to be dependent on the stability of electroactive biofilms (EABs), but the response of EABs based on real wastewater to external disturbances is not fully known. Herein, we used real wastewater (beer brewery wastewater) as a substrate for culturing EABs and found that current generation, biomass, redox activity and extracellular polymeric substances (EPS) content in those EABs were lower as compared to EABs cultured with synthetic wastewaters (acetate and glucose). However, the EABs from the beer brewery wastewater showed moderate anti-shock resistance capability. The proteins and humic acid in loosely bound EPS (LB-EPS) exhibited a positive linear relationship with current recovery after Ag⁺ shock, indicating the importance of LB-EPS for protecting the EABs. Fluorescence and Fourier transform infrared spectroscopy integrated with two-dimensional correlation spectroscopy verified that the spectra of the protein-like region of LB-EPS changed considerably under the interference of Ag⁺ concentration and the CO group of humic acid or proteins was mainly responsible for binding with Ag⁺ to attenuate its toxicity to the EABs. This is the first study revealing the underlying molecular mechanism of EABs cultured with real wastewater against external heavy metal shock and provides useful insights into enhancing the application of BESs in future water treatment.

Bio-Based Processes for Material and Energy Production from Waste Streams under Acidic Conditions

The revolutionary transformation from petrol-based production to bio-based production is becoming urgent in line with the rapid industrialization, depleting resources, and deterioration of the ecosystem. Bio-based production from waste-streams is offering a sustainable and environmentally friendly solution. It offers several advantages, such as a longer operation period, less competition for microorganisms, higher efficiency, and finally, lower process costs. In the current study, several bio-based products (organic acids, biomethane, biohydrogen, and metal leachates) produced under acidic conditions are reviewed regarding their microbial pathways, processes, and operational conditions. Furthermore, the limitations both in the production process and in the scale-up are evaluated with future recommendations.

An integrated anaerobic digestion and microbial electrolysis system for the enhancement of methane production from organic waste: Fundamentals, innovative design and scale-up deliberation

In the foreseeable future, renewable energy generation from electromethanogenesis to be more cost-effective energy. Electromethanogenesis system is a recent and efficient CO₂ to methane technology to upgrade biogas to 100% methane for power generation. And this can be attained through by integrating anaerobic digestion with microbial electrolysis system. Microbial electrolysis system can able to support carbon reduction on cathode and oxidation on anode by CO₂ capture thereby provides more CH₄ production from an integrated anaerobic digestion system. Scale-up the recent advance technique of microbial electrolysis system in the anaerobic digestion process for 100% methane production for power generation is need of the hour. The overall objective of this review is to facilitate the recent technology of microbial electrolysis system in the anaerobic digestion process. At first, the function of electromethanogenesis system and innovative integrated design method are outlined. Secondly, different external parameters such as applied voltage, operating temperature, pH etc are examined for the significance on process optimization. Eventually, electrode selections, electrode spacing, surface chemistry and surface area are critically reviewed for the scale-up considerations of integration process.

Enhanced anaerobic digestion of swine manure via a coupled microbial electrolysis cell

Microbial electrolysis cell coupled anaerobic digestion (MEC-AD) is a new technology in energy recovery and waste treatment, which could be used to recycle swine manure. Here, different applied voltage effects were studied using MEC-AD with swine manure as a substrate. The maximum cumulative biogas and methane yields, both occurring with 0.9 V, were 547.3 mL/g total solid (TS) and 347.7 mL/g TS, respectively. The increased energy can counterbalance the electrical input. First order, logistic, gompertz, and back-propagation artificial neural network (BP-ANN) models were used to study cumulative biogas and methane yields. The BP-ANN model was superior to the other three models. The maximum degradation rate of hemicellulose, cellulose, and lignin was 60.97%, 48.59%, and 31.59% at 0.9 V, respectively. The BP-ANN model establishes a model for cumulative biogas and methane yields using MEC-AD. Thus, MEC-AD enhanced biogas and methane production and accelerated substrate degradation at a suitable voltage.

Critical insights into psychrophilic anaerobic digestion: Novel strategies for improving biogas production

Anaerobic digestion (AD) under psychrophilic temperature has only recently garnered deserved attention. In major parts of Europe, USA, Canada and Australia, climatic conditions are more suited for psychrophilic (<20 ℃) rather than mesophilic (35 - 37 ℃) and thermophilic (55 - 60 ℃) AD. Low temperature has adverse effects on important cellular processes which may render the cell biology inactive. Moreover, cold climate can also alter the physical and chemical properties of wastewater, thereby reducing the availability of substrate to microbes. Hence, the use of low temperature acclimated microbial biomass could overcome thermodynamic constraints and carry out flexible structural and conformational changes to proteins, membrane lipid composition, expression of cold-adapted enzymes through genotypic and phenotypic variations. Reduction in organic loading rate is beneficial to methane production under low temperatures. Moreover, modification in the design of existing reactors and the use of hybrid reactors have already demonstrated improved methane generation in the lab-scale. This review also discusses some novel strategies such as direct interspecies electron transfer (DIET), co-digestion of substrate, bioaugmentation, and bioelectrochemical system assisted AD which present promising prospects. While DIET can facilitate syntrophic electron exchange in diverse microbes, the addition of organic-rich co-substrate can help in maintaining suitable C/N ratio in the anaerobic digester which subsequently can enhance methane generation. Bioaugmentation with psychrophilic strains could reduce start-up time and ensure daily stable performance for wastewater treatment facilities at low temperatures. In addition to the technical discussion, the economic assessment and future outlook on psychrophilic AD are also highlighted.

Operationalization of a microbial electrolysis cell: The interaction of the primary factors for energy storage efficiency

Microbial electrolysis cells have attracted attention as a method to enhance anaerobic digesters' performance. However, optimization of individual factors is not directly transferrable among systems as many are intimately linked and influenced by the system design, influent, and inoculum. To avoid this, here the effects and interactions between the relative electrode size, hydraulic retention time (HRT), and voltage imposed have been explored within a pair of otherwise identical reactors. Methane production has a positive correlation with the applied voltage, reaching 12.9 mL_CH4 L^-1h^-1 with 10 days HRT and 1000 mV, also achieving 35% energy storage efficiency, despite the higher electrical input. Shorter HRTs led to bacterial washouts, reducing the methane production below 10 mL_CH4 L^-1h^-1. Contour plots were constructed to relate the energy storage efficiency with operational conditions changes. These highlighted the benefits of using a relatively larger cathode than anode for improving energy storage efficiency.

An intermittent power supply scheme to minimize electrical energy input in a microbial electrolysis cell assisted anaerobic digester

From the perspective of energy saving in the operation of microbial electrolysis cell assisted anaerobic digester (MEC-AD), this study focused on developing an intermittent power supply scheme. The applied potential was switched off for 12 and 6 hours/day during the operation of a laboratory-scale MEC-AD system fed with glucose. The results from the operation under continuous applied potential served as the control. The overall biomethane generation and net energy income from the process were unaffected when the applied potential turned off for 6 hours/day. Both quantitative and qualitative analyses of microbial communities suggested that a balanced microbiome could be maintained under short-term switching-off the applied potential. However, performance substantially deteriorated when the applied potential turned off for 12 hours/day. Overall, the results of this study suggest that MEC-AD operation does not need a continuous power supply, and higher energy efficiency can be effectively achieved by intermittently powering the reactor.

Impact of decreasing hydraulic retention times on the specific affinity of methanogens and their community structures in an anaerobic membrane bioreactor process treating low strength wastewater

Maximum specific growth rate (μ_max) and substrate saturation constant (K_s) are widely used in determining the growth of microorganisms. The ratio (μ_max/K_s), also referred to as specific affinity, a_A⁰, is a better parameter to assess the advantage in competition for substrates by bridging microbial growth and the kinetics of enzymatic substrate uptake, but is not well studied. This study investigated the effect of hydraulic retention time (HRT) on the a_A⁰ of anaerobic sludge from an anaerobic membrane bioreactor (AnMBR), associated microbial communities and the overall wastewater treatment performance. The AnMBR was fed with acetate wastewater (~500 mg COD/L) and operated at fixed solids retention time (45 d) while HRT continued to decrease. There was no significant difference in K_s (ranging from 170 to 243 mg COD/L) at different HRTs. However, a_A⁰ increased from (4.0 ± 0.2) × 10^-4 to (4.9 ± 0.2) × 10^-4 and to (6.5 ± 0.1) × 10^-4 L/mg COD/d as HRT decreased from 24 h to 12 h and further to 6 h, respectively. This was accompanied by the increase in acetoclastic methanogens (mainly Methanosaeta) from 3.85 × 10¹⁰, 8.82 × 10¹⁰ to 1.05 × 10¹¹ cells/L, respectively. The fraction of Methanosaeta in the anaerobic biomass increased from 33.67% to 61.08% as HRT decreased from 24 h to 6 h. Correspondingly, effluent quality was improved, as evidenced from the COD concentrations of 32 ± 6, 21 ± 4, and 13 ± 5 mg/L at the HRTs of 24 h, 12 h, and 6 h, respectively. The results confirm that microorganisms are able to adapt to growth conditions by adjusting their kinetic properties and suggest that short HRTs in the AnMBR favor the growth and accumulation of Methanosaeta with high specific affinity likely because they can compete for acetate at low concentrations by increasing substrate uptake rate and thus specific microbial growth rate.

Fe3O4 accelerates tetracycline degradation during anaerobic digestion: Synergistic role of adsorption and microbial metabolism

Antibiotics contaminants, for example, tetracycline (TC) in the environment have attracted extensive attention around the world, and appropriate treatments for such contaminants are urgently required. In this study, five groups of anaerobic reactors supplemented with different amounts of Fe₃O₄ were operated periodically to investigate their performance on TC removal. The results showed that Fe₃O₄ effectively promoted TC removal. Compared with the control reactor, the TC removal efficiency was increased by 7.3% when co-digested with glucose, and increased by 40.4% when mono TC was digested in reactors with 5.0 g/L Fe₃O₄. Further analysis indicated that the probable mechanism of Fe₃O₄ promoting TC removal was through TC being adsorbed from the liquid onto Fe₃O₄, making TC more available for microbes to be biodegraded. Microbial community analysis indicated that the bacteria (Klebsiella, Pseudomonas, and Escherichia) related to TC removal were enriched, which meant more pathways for TC removal were available following the addition of Fe₃O₄. In addition, in the Fe₃O₄-supplemented reactors, syntrophic metabolism (between Desulfovibrio and Methanobacterium, Azonexus and Methanobacterium) were possibly established, which played an important role in improving TC removal and CH₄ production. The electron transport system data further confirmed these results. The functional gene classification for Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis demonstrated that the dominant functions enhanced by Fe₃O₄ supplementation was microbial metabolic activities.

Towards the practical application of bioelectrochemical anaerobic digestion (BEAD): Insights into electrode materials, reactor configurations, and process designs

Anaerobic digestion (AD) is one of the most widely adopted bioenergy recovery technologies globally. Despite the wide adoption, AD has been challenged by the unstable performances caused by imbalanced substrate and/or electron availability among different reaction steps. Bioelectrochemical anaerobic digestion (BEAD) is a promising concept that has demonstrated potential for balancing the electron transfer rates and enhancing the methane yield in AD during shocks. While great progress has been made, a wide range of, and sometimes inconsistent engineering and technical strategies were attempted to improve BEAD. To consolidate past efforts and guide future development, a comprehensive review of the fundamental bioprocesses in BEAD is provided herein, followed by a critical evaluation of the engineering and technical optimizations attempted thus far. Further, a few novel directions and strategies that can enhance the performance and practicality of BEAD are proposed for future research to consider. This review and outlook aim to provide a fundamental understanding of BEAD and inspire new research ideas in AD and BEAD in a mechanism-informed fashion.

Changes in syntrophic microbial communities, EPS matrix, and gene-expression patterns in biofilm anode in response to silver nanoparticles exposure

Understanding the toxic effect of silver nanoparticles (AgNPs) on various biological wastewater treatment systems is of significant interest to researchers. In recent years, microbial electrochemical technologies have opened up new opportunities for bioenergy and chemicals production from organic wastewater. However, the effects of AgNPs on microbial electrochemical systems are yet to be understood fully. Notably, no studies have investigated the impact of AgNPs on a microbial electrochemical system fed with a complex fermentable substrate. Here, we investigated the impact of AgNPs (50 mg/L) exposure to a biofilm anode in a microbial electrolysis cell (MEC) fed with glucose. The volumetric current density was 29 ± 2.0 A/m³ before the AgNPs exposure, which decreased to 20 ± 2.2 A/m³ after AgNPs exposure. The biofilms produced more extracellular polymeric substances (EPS) to cope with the AgNPs exposure, while carbohydrate to protein ratio in EPS considerably increased from 0.4 to 0.7. Scanning electron microscope (SEM) imaging also confirmed the marked excretion of EPS, forming a thick layer covering the anode biofilms after AgNPs injection. Transmission electron microscope (TEM) imaging showed that AgNPs still penetrated some microbial cells, which could explain the deterioration of MEC performance after AgNPs exposure. The relative expression level of the quorum signalling gene (LuxR) increased by 30%. Microbial community analyses suggested that various fermentative bacterial species (e.g., Bacteroides, Synergistaceae_vadinCA02, Dysgonomonas, etc.) were susceptible to AgNPs toxicity, which led to the disruption of their syntrophic partnership with electroactive bacteria. The abundance of some specific electroactive bacteria (e.g., Geobacter species) also decreased. Moreover, decreased relative expressions of various extracellular electron transfer associated genes (omcB, omcC, omcE, omcZ, omcS, and pilA) were observed. However, the members of family Enterobacteriaceae, known to perform a dual function of fermentation and anodic respiration, became dominant after biofilm anode exposed to AgNPs. Thus, EPS extraction provided partial protection against AgNPs exposure.

Enhanced two-phase anaerobic digestion of waste-activated sludge by combining magnetite and zero-valent iron

Previous studies have found that magnetite can promote the hydrolysis-acidification but inhibit the methanogenesis, while zero-valent iron (ZVI) only promoted the methanogenesis. Therefore, a new two-phase anaerobic digestion model, in which magnetite was added to the first phase, and ZVI was added to the second phase, was proposed to promote both hydrolysis-acidification and methanogenesis and avoid magnetite inhibition. The results showed that in the new model, methane production was improved by 10.2% and 18.1% and chemical oxygen demand (COD) removal was improved by 7.9% and 10.9% compared with reactors that included only magnetite and only ZVI, respectively. In the new model reactors, inhibition of methanogenesis by magnetite was avoided compared with that of the magnetite-only reactors, and hydrolysis efficiency was improved via dissimilatory iron reduction (DIR) compared with that of ZVI-only reactors. The data on volatile fatty acids (VFAs), coenzyme F420 and electron transfer system (ETS) further confirmed these conclusions.

Perspectives on Cultivation Strategies of Archaea

Archaea have been recognized as a major domain of life since the 1970s and occupy a key position in the tree of life. Recent advances in culture-independent approaches have greatly accelerated the research son Archaea. However, many hypotheses concerning the diversity, physiology, and evolution of archaea are waiting to be confirmed by culture-base experiments. Consequently, archaeal isolates are in great demand. On the other hand, traditional approaches of archaeal cultivation are rarely successful and require urgent improvement. Here, we review the current practices and applicable microbial cultivation techniques, to inform on potential strategies that could improve archaeal cultivation in the future. We first summarize the current knowledge on archaeal diversity, with an emphasis on cultivated and uncultivated lineages pertinent to future research. Possible causes for the low success rate of the current cultivation practices are then discussed to propose future improvements. Finally, innovative insights for archaeal cultivation are described, including (1) medium refinement for selective cultivation based on the genetic and transcriptional information; (2) consideration of the up-to-date archaeal culturing skills; and (3) application of multiple cultivation techniques, such as co-culture, direct interspecies electron transfer (DIET), single-cell isolation, high-throughput culturing (HTC), and simulation of the natural habitat. Improved cultivation efforts should allow successful isolation of as yet uncultured archaea, contributing to the much-needed physiological investigation of archaea.

Effect of a side-stream voltage supplied by sludge recirculation to an anaerobic digestion reactor

This study showed that side stream voltage supplied by sludge recirculation from an auxiliary bio-electrochemical anaerobic digestion (ABEAD) reactor appears to have a similar effect as main stream voltage supply to an anaerobic digestion (AD) reactor. The increased sludge recirculation rate enhanced the operation stability at a high OLR. H₂-producing bacterial community was improved in bio-electrochemical anaerobic digestion (BEAD) and ABEAD reactors and was increased with increase in sludge recirculation rate. Despite the dominance of hydrogenotrophic methanogens in all reactors, high operational performances of BEAD and ABEAD reactors supports the results of H₂-producing bacteria increase in those reactors. The ABEAD reactors having 1/7 of the capacity of the main AD reactor showed possibility of integration of BEAD technology into new and existing facilities economically. The findings of this study would provide useful information for approaching the commercialization of BEAD and suggest direction of further research for practical applications.

Effects of dissimilatory iron reduction on acetate production from the anaerobic fermentation of waste activated sludge under alkaline conditions

Anaerobic digestion of waste activated sludge (WAS) to produce acetate has recently attracted growing interest, while the slow hydrolytic acidification of sludge and the consumption of acetate by methanogens both decrease the yield of acetate. In this study, Fe₃O₄ was added to a WAS anaerobic digester under alkaline conditions (pH = 10). The concentration of short-chain fatty acids (SCFA) during WAS anaerobic fermentation was found to be affected positively by Fe₃O₄. The maximal SCFA production of the Fe₃O₄-added digester was 3619.4 mg/L, while the maximal SCFA production in the control was 2899.7 mg/L. The increase of SCFA with Fe₃O₄ was mainly resulted from the increase in acetate accumulation (accounting for 90%), because Fe₃O₄ stimulated dissimilatory iron reduction (DIR) that participated in the decomposition of complex organics and the transformation of pronionate and butyrate into acetate. Further investigation showed that each step of hydrolytic-acidification process was promoted except the homoacetogenesis. The activity of enzymes and abundance of microbes relevant to hydrolysis and acidification were in agreement with the above results.

Enhancing hydrogenotrophic activities by zero-valent iron addition as an effective method to improve sulfadiazine removal during anaerobic digestion of swine manure

In this study, the feasibility of Fe⁰ addition for driving sulfadiazine (SDZ) removal during anaerobic digestion of swine manure (SM) was tested. Compared with the Fe⁰-free digesters spiked with 200 mg/L SDZ (R_SDZ), treatments with 5.0 g/L Fe⁰ (R_SDZ/Fe⁰) significantly accelerated and optimized the acidification process by enriching Clostridia and Bacteroidia (key members responsible for VFAs/H₂ production), providing more readily available substrates for methanogenesis. Furthermore, Fe⁰ increased the overall abundance of hydrogenotrophic methanogens, specifically toxicant resistant Methanoculleus and Methanosphaera spp. were selectively enriched, helping achieve a 36.9% higher CH₄ yield and a 26.4% greater total solids removal efficiency. A positive correlation between the solid content and the SDZ concentration adsorbed in SM was observed. The addition of Fe⁰ increased the distribution of SDZ in liquid and facilitated its removal through the enhanced biodegradation and physicochemical processes. Overall, the total SDZ removal efficiency was improved by 86.8% with Fe⁰.

Progress towards catalyzing electro-methanogenesis in anaerobic digestion process: Fundamentals, process optimization, design and scale-up considerations

Electro-methanogenesis represents an emerging bio-methane production pathway that can be achieved through integrating microbial electrolysis cell (MEC) with conventional anaerobic digester (AD). Since 2009, a significant number of publications have reported superior methane productivity and kinetics from MEC-AD integrated systems. The overall objective of this review is to communicate the recent advances towards promoting electro-methanogenesis in the anaerobic digestion process. Firstly, the electro-methanogenesis pathways and functional roles of key microbial members are summarized. Secondly, various extrinsic process parameters, such as applied voltage/potential, pH, and temperature are discussed with emphasis on process optimization. Moreover, available methods for the inoculation and start-up of MEC-AD process are critically reviewed. Finally, system design and scale-up considerations, such as the selection of electrode materials, surface area and surface chemistry of electrode materials, and electrode spacing are summarized.

Contribution analysis of methane production from food waste in bulk solution and on bio-electrode in a bio-electrochemical anaerobic digestion reactor

Quantitative evaluation of methane production either in bulk sludge or biofilm on electrodes was performed in a bio-electrochemical anaerobic digestion (BEAD) reactor with a lower electrode surface area/reactor working volume (A/V) ratio (7.0 m²/m³). Methane production by electrochemical reaction was also evaluated in the BEAD reactor with a biofilm-free electrode under the same conditions as in other experimental sets. The contributions of bulk sludge, biofilms on the electrodes, and electrochemical reactions in the BEAD reactor, on methane production, were 70.2%, 29.8%, and 0%, respectively. The principal methane-producing reactions occurred in the bulk sludge facilitated by H₂-dependent methylotrophic and hydrogenotrophic methanogens. Hydrogenotrophic methanogenesis was also the main methane-producing reaction in the biofilms attached to the bio-electrodes. Quantitative analysis of methane production (29.8%) in the biofilm revealed that bio-electrochemical processes involving H₂ and direct bio-electrochemical methane production contributed 8.7% and less than 0.1%, respectively. Interestingly, biochemical processes (21.1%) contributed the most to the overall production of methane in the biofilm. Bulk sludge contributed more to methane production than the biofilm, but the methane production per unit mass of volatile solid on the electrodes was about 1.6-times higher than that of bulk sludge. Methane was not produced in the BEAD reactor with biofilm-free electrodes. Therefore, formation and maintenance of biofilms on the electrodes are essential for improved methane production in BEAD reactors.

Efficient methane production from petrochemical wastewater in a single membrane-less microbial electrolysis cell: the effect of the operational parameters in batch and continuous mode on bioenergy recovery

The main objective of this study is to evaluate the treatment and simultaneous production of methane from low-strength petrochemical wastewater by single membrane-less microbial electrolysis cells. To achieve this objective, the influence of variables such as applied voltage, operation mode, and hydraulic retention time (HRT) on the performance of the MEC system was investigated over a period of 110 days. According to the obtained results, the maximum COD removal efficiency in the batch mode was higher than which in the continuous mode (i.e. 85.9% vs 75.3%). However, the maximum methane production in the continuous mode was almost 1.6 times higher than which in the batch mode. The results show, COD removal, methane content, and methane production in both operation modes, were enhanced as applied voltage increased from 0.6 to 0.8-1 V. The proportion of methane, methane production rate, and COD removal were increased as HRT decreased from 72 to 48 h, while these values were decreased as the HRT decreased from 48 to 12 h. In continues mode, the energy efficiency had a range of 94.7% to 97.9% with an average of 96.6% in phase III, which almost recovered all of the electrical energy input into the system. These results suggest that single membrane-less microbial electrolysis cell is a promising process in order to the treatment of low-strength wastewater and methane production.

Simultaneous biogas and biogas slurry production from co-digestion of pig manure and corn straw: Performance optimization and microbial community shift

Anaerobic co-digestion (AcoD) is proved as an effective approach to solving a bottleneck problem of the low biogas yield in agricultural biomass waste treatment with anaerobic digestion (AD) technology. The present study investigated the effect of C/N radio, organic loading rate (OLR) and total solids (TS) contents on reactor performance in AcoD of pig manure and corn straw for simultaneous biogas and biogas slurry production. It was found that the highest biogas production was obtained at C/N ratio of 25, while the best biogas slurry performance was achieved at C/N ratio of 35. And high OLR and TS resulted in good performances in both biogas production and biogas slurry. At last, the microbial community analysis suggested that Bacteroidetes played a significant role in AcoD process. Acetoclastic methanogenesis was the main pathway for methane production in the stable system. And changing operational parameters could transform and shift the microbial community.

Startup performance of microbial electrolysis cell assisted anaerobic digester (MEC-AD) with pre-acclimated activated carbon

The feasibility of using pre-acclimated activated carbon to start up microbial electrolysis cell assisted anaerobic digester (MEC-AD) has been testified in this study. Two identical lab-scale digesters were separately packed with granular activated carbon (GAC) and powered activated carbon (PAC), which were initially acclimated as anaerobic digester and then transferred to MEC-AD. When a voltage of 0.5 V was applied, increased methane generation and substrate removal rates were observed. Hydrogenotrophic methanogens predominated in both digesters before and after transition, indicating that the pre-cultured microbial community on carbon materials could provide necessary microbiome favorable for starting up MECs. Although a low abundance of Geobacter was detected in inoculum, a rapid propagation could be realized when reactors were subjected to the electro-stimulation. The abundance of Methanosarcina closely attached to PAC was four times than that of GAC, which might be partially contributed to the improved resilience of anaerobic digester subjected to electro-stimulation.

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Pre-acclimated PAC/GAC are favorable for starting up MEC-AD.

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Methane yield was increased by ~30% when transferring AD to MEC-AD.

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Abundance of electroactive bacteria on pre-enriched PAC was higher than GAC.

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The rapid propagation of Geobacter was found in MEC-AD.

Long-term evaluation of methane production in a bio-electrochemical anaerobic digestion reactor according to the organic loading rate

In this study, the effects of differing organic loading rates (OLRs) on methane production were evaluated via long-term operation of a bio-electrochemical anaerobic digestion (BEAD) reactor and an anaerobic digestion (AD) reactor. In the AD reactor, the maximum OLR was 4 kg/m³·d whereas the electro-active microbial community in bulk and on the biofilm of the BEAD reactor produced methane even at 10 kg/m³·d. The BEAD reactor rapidly removed volatile fatty acids (VFAs) and reduced H⁺ to H₂ at high OLRs, thereby preventing VFA accumulation and pH decrease. After the steady state was achieved, both the AD and BEAD reactors exhibited similar organic matter removal and methane production at a low OLR. Thus, a BEAD reactor can stably produce methane under conditions of high OLR and severe OLR fluctuation and can even shorten the stabilization period over the long term.

Enhanced anaerobic digestion of ammonia-rich swine manure by zero-valent iron: With special focus on the enhancement effect on hydrogenotrophic methanogenesis activity

Zero-valent iron (ZVI) supplementation for improving anaerobic digestion (AD) of ammonia-rich swine manure (initial ammonia-N ∼5000 mg/L) was tested. The addition of 5 g/L ZVI powder apparently accelerated the acidification process to produce more volatile fatty acids (VFAs) and optimized the fermentation type by contributing to a lower system oxidation-reduction potential (ORP) level of -181.7 to -250.0 mV favorable for ethanol-type and butyric-type fermentation during day 14-30, in comparison with that of -164.3 to -216.3 mV in the control group favorable for propionic-type. Overall, ZVI significantly decreased the proportion of propionic acid from 49.8% to 30.9% while increased the proportion of n-butyric acid from 6.8% to 18.7%. Microbial analysis revealed that fast growing and ammonia-tolerant hydrogenotrophic Methanoculleus species were enriched with ZVI, helping achieve a 54.2% higher CH₄ yield relative to control. Results from this study demonstrated the potential of ZVI addition to enhance AD of ammonia-rich animal manure.

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.

Improving the co-digestion performance of waste activated sludge and wheat straw through ratio optimization and ferroferric oxide supplementation

Low anaerobic digestion efficiency of wheat straw (WS) has been an intractable problem owing to its high C/N ratio and complex structure. In this study, co-digestion of WS and waste activated sludge (WAS) at different ratios was performed to identify conditions that would elevate the acidic pH and increase methane production. The results showed that using a 1:1 ratio of WS and WAS, methane production in the co-digester was 26.9% higher than the sum of equal WAS and WS mono-digestion. When Fe₃O₄ was added to the co-digester, the acidic pH was further relieved and the anaerobic digestion efficiency was additionally enhanced. Microbial analysis showed that the ethanol-type fermentative bacterial genus Ethanoligenens was enriched in the WAS + WS-Fe₃O₄ reactor, in which the production of propionate was notably reduced, indicating that Fe₃O₄ could prevent the accumulation of volatile fatty acids by changing the types of fermentative bacteria present and promote anaerobic digestion efficiency.

Effect of trace elements on the development of co-cultured nitrite-dependent anaerobic methane oxidation and methanogenic bacteria consortium

The aim of this work was to study the effects of key trace elements (i.e., iron, copper and molybdenum) on the development of co-cultured n-damo and methanogenic bacteria consortium, which could realize in situ CH₄ production and utilization. The results showed that rational dosage, which was 50 mg/L of Fe, 1 mg/L of Cu and 5 mg/L of Mo, significantly stimulated the removal of NO₂^-. However, the activity of microbes was noticeably inhibited at 5 mg/L of Cu and 1 mg/L of Mo. Microbial community analysis indicated that the abundances of n-damo bacteria and methanogens showed a positive response to the rational dosage. Furthermore, the expression of key functional genes was enhanced under the rational dosage condition.

Agave tequilana bagasse for methane production in batch and sequencing batch reactors: Acid catalyst effect, batch optimization and stability of the semi-continuous process

Agave tequilana bagasse is the main solid waste of the tequila manufacturing and represents an environmental issue as well as a potential feedstock for biofuel production due to its lignocellulosic composition and abundance. In this contribution, this feedstock was subjected to pretreatments with HCl and H₂SO₄ for sugar recovery and methane was produced from the hydrolysates in batch and sequencing batch reactors (AnSBR). Sugar recovery was optimized by using central composite designs at different levels of temperature, acid concentration and hydrolysis time. Results showed that at optimal conditions, the HCl pretreatment induced higher sugar recoveries than the H₂SO₄ one, 0.39 vs. 0.26 g total sugars/g bagasse. Furthermore, the H₂SO₄ hydrolysate contained higher concentrations of potential inhibitory compounds (furans and acetic acid). Subsequent anaerobic batch assays demonstrated that the HCl hydrolysate is a more suitable substrate for methane production; a four-fold increase was found. A second optimization by using HCl as acid catalyst and methane production as the response variable demonstrated that softer hydrolysis conditions are required to optimize methane production as compared to sugar recovery (1.8% HCl, 119 °C and 103min vs. 1.9% HCl, 130 °C and 133min). This softer conditions were used to feed an AnSBR for 110 days and evaluate its stability at three different cycle times (5, 3 and 2 days). Results showed stable reactor performances at cycle times of 5 and 3 days, obtaining the highest methane yield and production at 3 days, 0.28 NL CH₄/g-COD and 1.04 NL CH₄/d respectively. Operation at shorter cycle times is not advised due to microbial imbalance.

Food waste enhanced anaerobic digestion of biologically pretreated yard waste: Analysis of cellulose crystallinity and microbial communities

Solid waste treatment through anaerobic digestion (AD) technology contributes to energy recycling and reuse of various solid organic wastes. However, yard waste (YW) is generally recalcitrant to AD due to the presence of high cellulose and hemicellulose content, which are difficult to be hydrolyzed. In this study, to enhance hydrolysis efficiency, YW was biologically pretreated with digested sludge and supplemented with food waste (FW) before AD process. Effects of FW supplementation on pH, SCOD, cellulose and hemicellulose content and cellulose crystallinity were examined. The optimal amount of FW supplementation was determined to be 10 wt%. An increase of 6.5-20.3% in cellulose reduction and an increase of 14.8-53.1% in hemicellulose reduction in digesters was achieved within the optimal pretreatment time of 4 days. After hydrolysis, cellulose crystallinity decreased by 23% from 71% in the control digester, which was responsible for improved biodegradability of cellulose in YW. FT-IR analysis of hydrolysis mixture confirmed that partial hydrogen bonds were destroyed in digesters with supplementation of 10 wt% FW, leading to a higher extent of degradation of the feedstock. In the batch AD of FW supplemented YW, results indicated that methane yield was 35% higher than that of the control digester without FW supplementation. Pyrosequencing analysis indicated that the abundance of bacterial genus Sphaerochaeta and Cellulosibacter in subsequent digestion were enhanced by 10- and 5-folds by 10 wt% FW supplementation, respectively, and were deemed to be responsible for the enhanced anaerobic digestion performance.

Methanol metabolism and archaeal community changes in a bioelectrochemical anaerobic digestion sequencing batch reactor with copper-coated graphite cathode

In this study, the metabolism of methanol and changes in an archaeal community were examined in a bioelectrochemical anaerobic digestion sequencing batch reactor with a copper-coated graphite cathode (BEAD-SBR_Cu). Copper-coated graphite cathode produced methanol from food waste. The BEAD-SBR_Cu showed higher methanol removal and methane production than those of the anaerobic digestion (AD)-SBR. The methane production and pH of the BEAD-SBR_Cu were stable even under a high organic loading rate (OLR). The hydrogenotrophic methanogens increased from 32.2 to 60.0%, and the hydrogen-dependent methylotrophic methanogens increased from 19.5 to 37.7% in the bulk of BEAD-SBR_Cu at high OLR. Where methanol was directly injected as a single substrate into the BEAD-SBR_Cu, the main metabolism of methane production was hydrogenotrophic methanogenesis using carbon dioxide and hydrogen released by the oxidation of methanol on the anode through bioelectrochemical reactions.

Application of a rotating impeller anode in a bioelectrochemical anaerobic digestion reactor for methane production from high-strength food waste

In this study, a practical bioelectrochemical anaerobic digestion (BEAD) reactor equipped with a rotating STS304 impeller was tested to verify its methane production performance. Methane production in the BEAD reactor was possible without accumulation of volatile fatty acids (VFAs) and decreases in pH at high organic loading rates (OLRs) up to 6 kg-COD/m³·d (COD: chemical oxygen demand). Methane production in a BEAD-O (open circuit) reactor was inhibited at OLRs above 4 kg-COD/m³·d; however, the performance could be recovered bioelectrochemically by supplying voltage. The population density of hydrogenotrophic methanogens increased to 73.3% in the BEAD-C (closed circuit) reactor, even at high OLRs, through the removal of VFAs and conversion of hydrogen to methane. The energy efficiency in the BEAD-C reactor was 85.6%, indicating that the commercialization of BEAD reactors equipped with rotating STS304 impeller electrodes is possible.

A review on the applications of microbial electrolysis cells in anaerobic digestion

Anaerobic digestion (AD) has been widely used for biogas or biofuel generation from waste treatment. Because a low production rate and instability of AD occur frequently, various technologies have been applied to improvement of AD. Microbial electrolysis cells (MECs), an emerging technology, can convert organic matter into hydrogen, methane, and other value-added products. Recent studies showed that application of MEC to AD (MEC-AD) can accelerate degradation of a substrate (including recalcitrant compounds) and alter AD microbial community by enriching exoelectrogens and methanogens thus increasing biogas production. With stable microbial communities established, improvement of MEC-AD for methane production was achieved. MEC-AD process can be monitored in real-time by detecting electric signals, which linearly correlate with substrate concentrations. This review attempts to evaluate interactions among the decomposition of substrates, MEC-AD system, and the microbial community. This analysis should provide useful insights into the improvement of methane production and the performance of MEC-AD.

Electrochemical biotechnologies minimizing the required electrode assemblies

Microbial electrochemical systems (MESs) are expected to be put into practical use as an environmental technology that can support a future environmentally friendly society. However, conventional MESs present a challenge of inevitably increasing initial investment, mainly due to requirements for a large numbers of electrode assemblies. In this review, we introduce electrochemical biotechnologies that are under development and can minimize the required electrode assemblies. The novel biotechnologies, called electro-fermentation and indirect electro-stimulation, can drive specific microbial metabolism by electrochemically controlling intercellular and extracellular redox states, respectively. Other technologies, namely electric syntrophy and microbial photo-electrosynthesis, obviate the need for electrode assemblies, instead stimulating targeted reactions by using conductive particles to create new metabolic electron flows.

Continuous micro-current stimulation to upgrade methanolic wastewater biodegradation and biomethane recovery in an upflow anaerobic sludge blanket (UASB) reactor

The dispersion of granules in upflow anaerobic sludge blanket (UASB) reactor represents a critical technical issue in methanolic wastewater treatment. In this study, the potentials of coupling a microbial electrolysis cell (MEC) into an UASB reactor for improving methanolic wastewater biodegradation, long-term process stability and biomethane recovery were evaluated. The results indicated that coupling a MEC system was capable of improving the overall performance of UASB reactor for methanolic wastewater treatment. The combined system maintained the comparatively higher methane yield and COD removal efficiency over the single UASB process through the entire process, with the methane production at the steady-state conditions approaching 1504.7 ± 92.2 mL-CH₄ L^-1-reactor d^-1, around 10.1% higher than the control UASB (i.e. 1366.4 ± 71.0 mL-CH₄ L^-1-reactor d^-1). The further characterizations verified that the input of external power source could stimulate the metabolic activity of microbes and reinforced the EPS secretion. The produced EPS interacted with Fe^2+/3+ liberated during anodic corrosion of iron electrode to create a gel-like three-dimensional [-Fe-EPS-]_n matrix, which promoted cell-cell cohesion and maintained the structural integrity of granules. Further observations via SEM and FISH analysis demonstrated that the use of bioelectrochemical stimulation promoted the growth and proliferation of microorganisms, which diversified the degradation routes of methanol, convert the wasted CO₂ into methane and accordingly increased the process stability and methane productivity.

Methanogenic Biocathode Microbial Community Development and the Role of Bacteria

The cathode microbial community of a methanogenic bioelectrochemical system (BES) is key to the efficient conversion of carbon dioxide (CO₂) to methane (CH₄) with application to biogas upgrading. The objective of this study was to compare the performance and microbial community composition of a biocathode inoculated with a mixed methanogenic (MM) culture to a biocathode inoculated with an enriched hydrogenotrophic methanogenic (EHM) culture, developed from the MM culture following pre-enrichment with H₂ and CO₂ as the only externally supplied electron donor and carbon source, respectively. Using an adjacent Ag/AgCl reference electrode, biocathode potential was poised at -0.8 V (versus SHE) using a potentiostat, with the bioanode acting as the counter electrode. When normalized to cathode biofilm biomass, the methane production in the MM- and EHM-biocathode was 0.153 ± 0.010 and 0.586 ± 0.029 mmol CH₄/mg biomass-day, respectively. This study showed that H₂/CO₂ pre-enriched inoculum enhanced biocathode CH₄ production, although the archaeal communities in both biocathodes converged primarily (86-100%) on a phylotype closely related to Methanobrevibacter arboriphilus. The bacterial community of the MM-biocathode was similar to that of the MM inoculum but was enriched in Spirochaetes and other nonexoelectrogenic, fermentative Bacteria. In contrast, the EHM-biocathode bacterial community was enriched in Proteobacteria, exoelectrogens, and putative producers of electron shuttle mediators. Similar biomass levels were detected in the MM- and EHM-biocathodes. Thus, although the archaeal communities were similar in the two biocathodes, the difference in bacterial community composition was likely responsible for the 3.8-fold larger CH₄ production rate observed in the EHM-biocathode. Roles for abundant OTUs identified in the biofilm and inoculum cultures were highlighted on the basis of previous reports.

Enhanced treatment of Fischer-Tropsch (F-T) wastewater using the up-flow anaerobic sludge blanket coupled with bioelectrochemical system: Effect of electric field

The coupling of bioelectrochemical system (BES) with an up-flow anaerobic sludge blanket (UASB) was established for enhanced Fischer-Tropsch (F-T) wastewater treatment while the UASB (control group) was operated in parallel. The presence of electric field could offer system a more reductive micro-environment that lower the ORP values and maintain the appropriate pH range, resulting in the higher chemical oxygen demand (COD) removal efficiency and methane production for BES-UASB (86.8% and 2.31±0.1L/(L·d)) while those values in control group were 72.1% and 1.77±0.08L/(L·d). In addition, the coupled system could promote sludge granulation to perform a positive effect on maintaining stability of pollutants removal. The high-throughput 16S rRNA gene pyrosequencing in this study further confirmed that the promoting direct interspecies electron transfer (DIET) between Geobacter and Methanosarcina might be established in BES-UASB to improve the syntrophic degradation of propionate and butyrate, finally facilitated completely methane production.

On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.