Influence of applied voltage on the performance of bioelectrochemical anaerobic digestion of sewage sludge and planktonic microbial communities at ambient temperature

Using a non-precious metal catalyst for long-term enhancement of methane production in a zero-gap microbial electrosynthesis cell

Microbial electrosynthesis (MES) cells exploit the ability of microbes to convert CO2 into valuable chemical products such as methane and acetate, but high rates of chemical production may need to be mediated by hydrogen and thus require a catalyst for the hydrogen evolution reaction (HER). To avoid the usage of precious metal catalysts and examine the impact of the catalyst on the rate of methane generation by microbes on the electrode, we used a carbon felt cathode coated with NiMo/C and compared performance to a bare carbon felt or a Pt/C-deposited cathode. A zero-gap configuration containing a cation exchange membrane was developed to produce a low internal resistance, limit pH changes, and enhance direct transport of H2 to microorganisms on the biocathode. At a fixed cathode potential of -1 V vs Ag/AgCl, the NiMo/C biocathode enabled a current density of 23 ± 4 A/m2 and a high methane production rate of 4.7 ± 1.0 L/L-d. This performance was comparable to that using a precious metal catalyst (Pt/C, 23 ± 6 A/m2, 5.4 ± 2.8 L/L-d), and 3-5 times higher than plain carbon cathodes (8 ± 3 A/m2, 1.0 ± 0.4 L/L-d). The NiMo/C biocathode was operated for over 120 days without observable decay or severe cathode catalyst leaching, reaching an average columbic efficiency of 53 ± 9 % based on methane production under steady state conditions. Analysis of microbial community on the biocathode revealed the dominance of the hydrogenotrophic genus Methanobacterium (∼40 %), with no significant difference found for biocathodes with different materials. These results demonstrated that HER catalysts improved rates of methane generation through facilitating hydrogen gas evolution to an attached biofilm, and that the long-term enhancement of methane production in MES was feasible using a non-precious metal catalyst and a zero-gap cell design.

Electrocatalytic coupled biofilter for treating cyclohexanone-containing wastewater: Degradation, mechanism and optimization

Electrocatalytic coupled biofilter (EBF) technology organically integrates the characteristics of electrochemistry and microbial redox, providing ideas for effectively improving biological treatment performance. In this study, an EBF system was developed for enhanced degradation of cyclohexanone in contaminated water. Experimental results show that the system can effectively remove cyclohexanone in contaminated water. Under the optimal parameters, the removal rates of cyclohexanone, TP, NH4+-N and TN were 97.61 ± 1.31%, 76.31 ± 1.67%, 94.14 ± 2.13% and 95.87 ± 1.01% respectively. Degradation kinetics studies found that electrolysis, adsorption, and biodegradation pathways play a major role in the degradation of cyclohexanone. Microbial community analysis indicates that voltage can affect the structure of the microbial community, with the dominant genera shifting from Acidovorax (0 V) to Brevundimonas (0.7 V). Additionally, Acidovorax, Cupriavidus, Ralstonia, and Hydrogenophaga have high abundance in the biofilm and can effectively metabolize cyclohexanone and its intermediates, facilitating the removal of cyclohexanone. In summary, this research can guide the development and construction of highly stable EBF systems and is expected to be used for advanced treatment of industrial wastewater containing cyclohexanone.

Bio-electrochemical degradation of carbamazepine (CBZ): A comprehensive study on effectiveness, degradation pathway, and toxicological assessment

Recent attention on the detrimental effects of pharmaceutically active compounds (PhACs) in natural water has spurred researchers to develop advanced wastewater treatment methods. Carbamazepine (CBZ), a widely recognized anticonvulsant, has often been a primary focus in numerous studies due to its prevalence and resistance to breaking down. This study aims to explore the effectiveness of a bio-electrochemical system in breaking down CBZ in polluted water and to assess the potential harmful effects of the treated wastewater. The results revealed bio-electro degradation process demonstrated a collaborative effect, achieving the highest CBZ degradation compared to electrodegradation and biodegradation techniques. Notably, a maximum CBZ degradation efficiency of 92.01% was attained using the bio-electrochemical system under specific conditions: Initial CBZ concentration of 60 mg/L, pH level at 7, 0.5% (v/v) inoculum dose, and an applied potential of 10 mV. The degradation pathway established by identifying intermediate products via High-Performance Liquid Chromatography-Mass Spectrometry, revealed the complete breakdown of CBZ without any toxic intermediates or end products. This finding was further validated through in vitro and in vivo toxicity assays, confirming the absence of harmful remnants after the degradation process.

Comparison of anaerobic digestion of starch- and petro-based bioplastic under hydrogen-rich conditions

To identify an economically viable waste management system for bioplastics, thermoplastic starch (TPS) and poly(butylene adipate-co-terephthalate) (PBAT) were anaerobically digested under hydrogen (H2)/carbon dioxide (CO2) and nitrogen (N2) gas-purged conditions to compare methane (CH4) production and biodegradation. Regardless of the type of bioplastics, CH4 production was consistently higher with H2/CO2 than with N2. The highest amount of CH4 was produced at 307.74 mL CH4/g volatile solids when TPS digested with H2/CO2. A stepwise increased in CH4 yield was observed, with a nominal initial increment followed by accelerated methanogenesis conversion as H2 was depleted. This may be attributed to a substantial shift in the microbial structure from hydrogenotrophic methanogen (Methanobacteriales and Methanomicrobiales) to heterotrophs (Spirochaetia). In contrast, no significant change was observed with PBAT, regardless of the type of purged gas. TPS was broken down into numerous derivatives, including volatile fatty acids. TPS produced more byproducts with H2/CO2 (i.e., 430) than with N2 (i.e., 320). In contrast, differential scanning calorimetry analysis on PBAT revealed an increase in crystallinity from 10.20 % to 12.31 % and 11.36 % in the H2/CO2- and N2-purged conditions, respectively, after 65 days of testing. PBAT surface modifications were characterized via Fourier transform infrared spectroscopy and scanning electron microscopy. The results suggest that the addition of H2/CO2 can enhance the CH4 yield and increase the breakdown rate of TPS more than that of PBAT. This study provides novel insights into the CH4 production potential of two bioplastics with different biodegradabilities in H2/CO2-mediated anaerobic digestion systems.

Boosting methane production and raw waste activated sludge treatment in a microbial electrolysis cell-anaerobic digestion (MEC-AD) system: The effect of organic loading rate

This study deals with the effect of different organic loading rates (OLRs) on the organic load removal and the productivity of methane, in a microbial electrolysis cell-anaerobic digestion (MEC-AD) system treating raw waste activated sludge (WAS). For comparison, two identical reactors, a control (AD) and a MEC-AD reactor were constructed. Both reactors operated for 131 days, during which different Organic Loading Rates (OLRs) were examined; 1.1, 1.7 and 2.9 gCOD/(L*d). The results showed that the MEC-AD reactor performed better, relative to the AD reactor, at high OLRs. Specifically, the additional total particulate carbon and Kjeldahl nitrogen removal reached 12% and 13%, respectively, at an OLR of 1.7 gCOD/(L*d), while they reached 19% and 13%, respectively, at an OLR of 2.9 gCOD/(L*d). Moreover, the biogas production and the methane content increased by 30% and 6%, respectively, at an OLR of 1.7 gCOD/(L*d) and by 32% and 5%, respectively, at an OLR of 2.9 gCOD/(L*d). The electrochemical measurements indicated that the power output increased from 5 to 30 mW/m2 when the OLR increased from 1.1 to 2.9 gCOD/(L*d). Overall, the results showed that the MEC-AD accelerated and enhanced the WAS treatment, boosting methane productivity.

Impact of dewatering inorganic coagulants on anaerobic digestion treating food waste leachate

Anaerobic digestion of food waste leachate (FWL) provides a viable solution for waste treatment and energy production. Returning solids from digested sludge to the reactor maintains a high microbial concentration and enhances digestion efficiency. However, this requires coagulants because the digestate has low dewaterability. This study analyzed methane production and microbial communities using biochemical methane potential tests for inorganic coagulants (AlCl3, Al2(SO4)3, FeCl3, and Fe2(SO4)3) in FWL treatment. Cumulative methane production was the highest in the control and decreased in the order of Fe2(SO4)3, AlCl3, FeCl3, and Al2(SO4)3. Iron ions inhibited H2S production while aluminum ions increased it compared to the control group. Despite the absence of significant changes in microbial communities following coagulant injection, a substantial increase in damaged cells was observed. These findings highlight the intricate repercussions of coagulant introduction in anaerobic digestion, emphasizing notable alterations in methane production dynamics and the integrity of microbial cells.

Environmental biotechnology and the involving biological process using graphene-based biocompatible material

Biotechnology is a promising approach to environmental remediation but requires improvement in efficiency and convenience. The improvement of biotechnology has been illustrated with the help of biocompatible materials as biocarrier for environmental remediations. Recently, graphene-based materials (GBMs) have become promising materials in environmental biotechnology. To better illustrate the principle and mechanisms of GBM application in biotechnology, the comprehension of the biological response of microorganisms and enzymes when facing the GBMs is needed. The review illustrated distinct GBM-microbe/enzyme composites by providing the GBM-microbe/enzyme interaction and the determining factors. There are diverse GBM modifications for distinct biotechnology applications. Each of these methods and applications depends on the physicochemical properties of GBMs. The applications of these composites were mainly categorized as pollutant adsorption, anaerobic digestion, microbial fuel cells, and organics degradation. Where information was available, the strategies and mechanisms of GBMs in improving application efficacies were also demonstrated. In addition, the biological response, from microbial community changes, extracellular polymeric substances changes to biological pathway alteration, may become important in the application of these composites. Furthermore, we also discuss challenges facing the environmental application of GBMs, considering their fate and toxicity in the ecosystem, and offer potential solutions. This research significantly enhances our comprehension of the fundamental principles, underlying mechanisms, and biological pathways for the in-situ utilization of GBMs.

Internal driving mechanism of microbial community and metabolic pathway for psychrophilic anaerobic digestion by microbial electrolysis cell

The system that microbial electrolysis cell coupled anaerobic digestion (termed MEC-AD) with metal organic framework-modified cathode was operated under different voltage levels (0-1.2 V) at 20 °C. The maximum methane yield increased to 0.23 ± 0.01 LCH₄ g^-1COD at 0.9 V, with 28% improvement compared to 0 V (0.18 ± 0.01 LCH₄ g^-1COD). Moreover, total volatile fatty acid and propionate accumulation decreased by 32% and 15% at 0.9 V, indicating the system has potential to alleviate acidity suppression. Acidogens and electroactive microorganisms was clearly enriched with increasing applied voltage. Specifically, the abundance of Smithella increased, which could degrade propionate to acetate. Methanosaeta was dominant, accounting for ca. 40.1%∼55.1% of the archaea community at 0.3-1.2 V. Furthermore, the system reinforced psychrophilic methanogenesis by activating important enzymes involved in related metabolism pathways. Overall, this study provides perspective on the future practical application for the regulation of psychrophilic AD in electrochemically integrated bioreactors.

Magnetite-based nanoparticles and nanocomposites for recovery of overloaded anaerobic digesters

The effect of magnetite nanoparticles and nanocomposites (magnetite nanoparticles impregnated into graphene oxide) supplement on the recovery of overloaded laboratory batch anaerobic reactors was assessed using two types of starting inoculum: anaerobic granular sludge (GS) and flocculent sludge (FS). Both nanomaterials recovered methane production at a dose of 0.27 g/L within 40 days in GS. Four doses of magnetite nanoparticles from 0.075 to 1 g/L recovered the process in FS systems between 30 and 50 days relaying on the dose. The presence of nanomaterials helped to reverse the effect of volatile fatty acids inhibition and enabled microbial communities to recover but also favoured the development of certain microorganisms over others. In GS reactors, the methanogenic population changed from being mostly acetoclastic (Methanothrix soehngenii) to being dominated by hydrogenotrophic species (Methanobacterium beijingense). Nanomaterial amendment may serve as a preventative measure or provide an effective remedial solution for system recovery following overloading.

Influence of applied voltage on bioelectrochemical enhancement of biomethanation for low-rank coal and microbial community distribution

The performance of peat biomethanation was investigated in bioelectrochemical anaerobic digestion at different applied voltages, and compared to conventional anaerobic digestion. The methane yield was stabilized at 16 mL/g peat in the conventional anaerobic digestion. However, in the bioelectrochemical anaerobic digestion, the methane yield was significantly increased to 264 mL/g peat at the applied voltage of 4 V, followed by 1 V, 2 V, 0.5 V and 0 V. The bioelectrochemical system could enrich more electroactive microorganisms on the electrode, as well as in the bulk solution, and further improve the direct interspecies electron transfer for methane production. The 16S rRNA analysis showed a significant increase in the abundance of specific microorganisms in the bulk solution, including Firmicutes phylum and Proteobacteria phylum, in addition to a gradual increase in acetoclastic methanogenesis with an increase in applied voltage. These results provide a solution to turn low-rank coal into a new alternative energy.

Contribution analysis of different electron transfer pathways to methane production in anaerobic digestion coupled with bioelectrochemical system

The contribution analysis of different electron transfer pathways to CH₄ production was investigated in bioelectrochemical anaerobic digestion (BEAD). It demonstrates that the indirect interspecies electron transfer (IIET) pathway and the direct interspecies electron transfer (DIET) pathways contributed to 41.7 % and 58.3 % of the CH₄ production in the BEAD reactor, respectively. The DIET pathway was further divided into DIET via electrode (eDIET) and biological DIET (bDIET) in the bulk solution, and contributed 11.1 % and 47.2 % of CH₄ production, respectively. This indicates that the dominant electron transfer pathway for CH₄ production is from the bulk solution, rather than on the polarized electrode. The electroactive microorganisms were well enriched in the bulk solution by the electric field generated between anode and cathode. The enriched electroactive microorganisms significantly improved the CH₄ production in the bulk solution through the bDIET pathway.

Enhanced methane production in anaerobic digestion: A critical review on regulation based on electron transfer

Anaerobic digestion (AD) is a potential bioprocess for waste biomass utilization and energy conservation. Various iron/carbon-based CMs (e.g., magnetite, biochar, granular activated carbon (GAC), graphite and zero valent iron (ZVI)) have been supplemented in anaerobic digestors to improve AD performance. Generally, the supplementation of CMs has shown to improve methane production, shorten lag phase and alleviate environmental stress because they could serve as electron conduits and promote direct interspecies electron transfer (DIET). However, the CMs dosage varied greatly in previous studies and CMs wash out remains a challenge for its application in full-scale plants. Future work is recommended to standardize the CMs dosage and recover/reuse the CMs. Moreover, additional evidence is required to verify the electrotrophs involved in DIET.

A multifaceted screening of applied voltages for electro-assisted anaerobic digestion of blackwater: Significance of temperature, hydrolysis/acidogenesis, electrode corrosion, and energy efficiencies

A microbial electrolysis cell-assisted anaerobic digester (MEC-AD) was operated with vacuum toilet blackwater at different applied voltages (0-1.6 V) at room temperature (R₂₀). A parallel MEC-AD was operated at 35 °C (R₃₅) to provide a kinetics index at mesophilic temperature. Both reactors failed at 1.6 V due to the alkaline pH created by anodic corrosion. In R₂₀, the best performance was observed at 1.2 V, with methane yield, COD removal, hydrolysis and acidogenesis efficiency increased by 59.9%, 27.0%, 52.0%, and 44.9%, respectively, compared to those of 0 V. Enrichment of hydrolytic and syntrophic bacteria (e.g., Clostridium, Bacteroidales, Sedimentibacter, Syntrophomonas) and increased abundance of genes encoding complex organics (e.g., proteins, carbohydrates, lipids) metabolism in R₂₀ at 1.2 V corresponded to the enhanced hydrolysis/acidogenesis processes. R₂₀ at 1.2 V generated 1.16 times more net energy than R₃₅ at the optimum voltage for methane yield (0.8 V), indicating ambient temperature operation of MEC-AD systems would be a more sustainable strategy.

Effect of tetracycline on bio-electrochemically assisted anaerobic methanogenic systems: Process performance, microbial community structure, and functional genes

Bio-electrochemically assisted anaerobic methanogenic systems (An-BES) are highly effective in wastewater treatment for methane production and degradation of toxic compounds. However, information on the treatment of antibiotic-bearing wastewater in An-BES is still very limited. This study therefore investigated the effect of tetracycline (TC) on the performance, microbial community, as well as functional and antibiotic resistance genes of An-BES. TC at 1 and 5 mg/L inhibited methane production by less than 4.8% compared to the TC-free control. At 10 mg/L TC, application of 0.5 and 1.0 V decreased methane production by 14 and 9.6%, respectively. Under the effect of 1-10 mg/L TC, application of 1.0 V resulted in a decrease of current from 42.3 to 2.8 mA. TC was mainly removed by adsorption; its removal extent increased by 19.5 and 32.9% with application of 0.5 and 1.0 V, respectively. At 1.0 V, current output was not recovered with the addition of granular activated carbon, which completely removed TC by adsorption. Metagenomic analysis showed that propionate oxidizing bacteria and methanogens were more abundant in electrode biofilms than in suspended culture. Antibiotic resistance genes (ARGs) were less abundant in biofilms than in suspended culture, regardless of whether voltage was applied or not. Application of 1.0 V resulted in the enrichment of Geobacter in the anode and Methanobacterium in the cathode. TC inhibited exoelectrogens, propionate oxidizing bacteria, and the methylmalonyl CoA pathway, leading to a decrease of current output, COD consumption, and methane production. These findings deepen our understanding of the inhibitory effect of TC in An-BES towards efficient bioenergy recovery from antibiotic-bearing wastewater, as well as the response of functional microorganisms to TC in such systems.

Polydimethyldiallylammonium chloride induces oxidative stress in anaerobic digestion of waste activated sludge

The effects and key mechanisms of polydimethyldiallylammonium chloride on anaerobic digestion of waste activated sludge were investigated. Polydimethyldiallylammonium chloride at 38.1 g/kg total solids substantially reduced cumulative methane production from 138.2 ± 5.5 to 49.4 ± 5.0 L CH₄/kg volatile solids added, a reduction of 64.3 ± 0.2%. The quaternary ammonium groups on polydimethyldiallylammonium chloride agglomerated sludge flocs by neutralizing negatively charged amino groups in in extracellular polymeric substances, which hindered the release of organic matter. Quaternary ammonium groups induce oxidative stress by inducing the production of reactive oxygen species, thereby inhibiting the activity of anaerobic digestive enzymes. In addition, quaternary amine groups reduced the abundance of hydrolyzing bacteria, acidifying bacteria, and acetylotrophic methanogens. Oxidative stress could be an underappreciated mechanism that quaternary ammonium groups deteriorate anaerobic digestion, which could be transformative for understanding the potential risks of quaternary ammonium cationic flocculants in biological sludge treatment.

Microbial composition and nitrogen removal pathways in a novel sequencing batch reactor integrated with semi-fixed biofilm carrier: evidence from a pilot study for low- and high-strength sewage treatment

The sequencing batch reactor (SBR) activated sludge process is a well-established technology for sewage treatment. One of the drawbacks of SBRs, however, total nitrogen (TN) removals is insufficient. By means of introducing four improvements, including semi-fixed biofilm carrier, sludge elevation mixing and change for the mode of influent and effluent, compliant standard for TN discharge was obtained in this novel SBR configuration during low- and high-strength sewage load. To illustrate the microbial compositions and functions of the attached biofilm on semi-fixed carrier and the suspended aggregates, as well as the nitrogen removal pathway, high throughput 16S rRNA gene amplicon sequencing, PICRUSt2 algorithm, and KEGG database were applied. The results revealed that (i) the microbial communities from suspended aggregates and biofilm samples were significantly different from each other; (ii) during low-strength sewage loads, TN removal was mainly by nitrification-denitrification. The suspended aggregates was responsible for denitrification, while the biofilm was focused on ammonium oxidation; (iii) during high-strength sewage loads, function of nitrate reductase from suspended aggregates was faded, and anammox and N assimilation by biofilm became dominant. Meanwhile, TN removal referring to the formation of L-glutamine via assimilation was the main pathway.

Effect of an Electromagnetic Field on Anaerobic Digestion: Comparing an Electromagnetic System (ES), a Microbial Electrolysis System (MEC), and a Control with No External Force

This study examined the application of an electromagnetic field to anaerobic digestion by using an electromagnetic system (ES), a microbial electrolysis cell (MEC), and a control with no external force. The experimental work was performed by carrying out biochemical methane potential (BMP) tests using 1 L biodigesters. The bioelectrochemical digesters were supplied with 0.4 V for 30 days at 40 °C. The electromagnetic field of the ES was generated by coiling copper wire to form a solenoid in the BMP system, whereas the MEC consisted of zinc and copper electrodes inside the BMP system. The best performing system was the MEC, with a yield of 292.6 mL CH4/g chemical oxygen demand removed (CODremoved), methane content of 86%, a maximum current density of 23.3 mA/m2, a coulombic efficiency of 110.4%, and an electrical conductivity of 180 µS/cm. Above 75% removal of total suspended solids (TSS), total organic carbon (TOC), phosphate, and ammonia nitrogen (NH3-N) was also recorded. However, a longer exposure (>8 days) to higher magnetic intensity (6.24 mT) on the ES reduced its overall performance. In terms of energy, the MEC produced the greatest annual energy profit (327.0 ZAR/kWh or 23.36 USD/kWh). The application of an electromagnetic field in anaerobic digestion, especially a MEC, has the potential to maximize the methane production and the degradability of the wastewater organic content.

Enhancement of methane production from waste activated sludge using hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) process - A review

Hybrid microbial electrolysis cells-anaerobic digestion (MEC-AD) was proved to increase methane productivity and methane yield of waste activated sludge (WAS) by establishing direct interspecies electron transfer method and enriching functional microorganisms. This review first summarized the pretreatment methods of WAS for MEC-AD and then reviewed the reactor configurations, operation parameters, and the economic benefit of MEC-AD. Furthermore, the enhancement mechanisms of MEC-AD were reviewed based on the analysis of thermodynamics and microbial community. It was found that the decrease of hydrogen partial pressure due to the hydrogenotrophic methanogens enriched in cathodic biofilm and direct interspecies electron transfer between exoelectrogens and anode were the core mechanisms for improving acidogenesis, acetogenesis, and methanogenesis. Finally, the potentially technological issues that need to be addressed to increase energy efficiency in large-scale MEC-AD processes were discussed.

Deciphering the key operational factors and microbial features associated with volatile fatty acids production during paper wastes and sewage sludge co-fermentation

This work explored the feasibility of paper waste (PW)/sewage sludge (SS) co-fermentation for volatile fatty acids (VFAs) production, and disclosed its correlation with the key operational parameters (i.e., pH and PW/SS ratio). The results indicated that the maximal VFAs was 251.55 mg COD/g TSS at optimal conditions, which was approximately 10-folds of sole SS fermentation. PW feeding contributed to the bioavailable substrates and C/N balance during co-fermentation process. The pH exhibited evident impacts on organics solubilization/hydrolysis, in which acidic pH was more beneficial for carbohydrates metabolism while alkaline pH was better for proteins. Under optimal operational conditions, the metabolic functions associated with VFAs production (i.e., substrate membrane transport, intracellular metabolism and VFAs biosynthesis) were up-regulated. Moreover, functional microorganisms (i.e., Saccharofermentans and Bacteroides) responsible for VFAs generation were enriched. This work provided an innovative approach to recovery valuable products from biowastes, and in-depth understandings of microbial features in PW/SS co-fermentation systems.

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.

Synergistic Effect of Magnetite and Bioelectrochemical Systems on Anaerobic Digestion

Conventionally, the anaerobic digestion of industrial effluent to biogas constitutes less than 65% methane, which warrants its potential methanation to mitigate carbon dioxide and other anthropogenic gas emissions. The performance of the anaerobic digestion process can be enhanced by improving biochemical activities. The aim of this study was to examine the synergistic effect of the magnetite and bioelectrochemical systems (BES) on anaerobic digestion by comparing four digesters, namely a microbial fuel cell (MFC), microbial electrolysis cell (MEC), MEC with 1 g of magnetite nanoparticles (MECM), and a control digester with only sewage sludge (500 mL) and inoculum (300 mL). The MFC digester was equipped with zinc and copper electrodes including a 100 Ω resistor, whereas the MEC was supplied with 0.4 V on the electrodes. The MECM digester performed better as it improved microbial activity, increased the content of methane (by 43% compared to 41% of the control), and reduced contaminants (carbon oxygen demand, phosphates, colour, turbidity, total suspended solids, and total organic carbon) by more than 81.9%. Current density (jmax = 25.0 mA/m²) and electrical conductivity (275 µS/cm) were also high. The prospects of combining magnetite and bioelectrochemical systems seem very promising as they showed a great possibility for use in bioelectrochemical methane generation and wastewater treatment.

Electrostatic Fields Promote Methanogenesis More than Polarized Bioelectrodes in Anaerobic Reactors with Conductive Materials

Direct interspecies electron transfer (DIET) is a breakthrough that can surpass the limitations of anaerobic digestion. Conductive materials and polarized bioelectrodes are known to induce DIET for methane production but are still challenging to apply at a field scale. Herein, compared to polarized bioelectrodes, electrostatic fields that promote DIET were investigated in an anaerobic reactor with conductive materials. As a conductive material, activated carbon enriched its surface with electroactive microorganisms to induce DIET (cDIET). cDIET improved the methane yield to 254.6 mL/g COD_r, compared to the control. However, polarized bioelectrodes induced electrode-mediated DIET and biological DIET (bDIET), in addition to cDIET, improving the methane yield to 310.7 mL/g COD_r. Electrostatic fields selectively promoted bDIET and cDIET for further methane production compared to the polarized bioelectrodes. As the contribution of DIET increased, the methane yield increased, and the substrate residue decreased, resulting in a significant improvement in methane production.

"Self-degradation" of 2-chlorophenol in a sequential cathode-anode cascade mode bioelectrochemical system

A sequential cathode-anode cascade mode bioelectrochemical system (BES) was designed and developed to achieve the "self-degradation" of 2-chlorophenol (2-CP). With the cooperation of cathode and anode, the electrons supplied for the cathode 2-CP dechlorination come from its own dechlorinated product in the anode, phenol. Separate degradation experiments of cathode 2-CP and anode phenol were firstly conducted. The optimum concentration ratio of anode acetate to phenolic compound (3.66/1.56) and the phenolic compound degradation ability of BES were investigated. With the formation of the bioanode able to degrade phenol, the sequential cathode-anode cascade mode BES was further developed, where 2-CP could achieve sequential dechlorination and ring-cleavage degradation. When applied voltage was 0.6 V and cathode influent pH was 7, 1.56 mM 2-CP reached 80.15% cathode dechlorination efficiency and 58.91% total cathode-anode phenolic compounds degradation efficiency. The bioanodes played a decisive role in BES. Different operating conditions would affect the overall performance of BES by changing the electrochemical activity and microbial community structure of the bioanodes. This study demonstrated the feasibility of the sequential cathode-anode cascade mode BES to degrade 2-CP wastewater and provided perspectives for the cooperation of cathode and anode, aiming to explore more potential of BES in wastewater treatment field.

Simultaneous nitrogen removal and methane production from Taihu blue algae against ammonia inhibition using integrated bioelectrochemical system (BES)

Simultaneous nitrogen removal and methane production using an integrated bioelectrochemical system (BES) during the anaerobic digestion (AD) process of Taihu blue algae were investigated. Upon an applied voltage of 0.4 V and total solids (TS) ratio of blue algae to anaerobic sludge as 1:1, the highest methanogenesis potential as 69.12 mL/g VS could be obtained, attaining 18.7 times of the TS ratio group of 3:1. Moreover, methane production of the integrated BES group reached 3.18 times of the AD group using conical flask, even with the same TS ratio (1:1) and initial ammonia nitrogen concentration (1000 mg NH₄⁺-N/L). Apart from the bettered electrochemical performance, bio-augmented microbial genus responsible for acetoclastic methanogenesis, power generation, resisting to hostile circumstance, co-existence with hydrogenotrophic methanogens could all be enriched. Therefore, integrated BES with appropriate TS ratio under applied voltage might help offset both the ammonia and electrical stress, thereby to maintain enhanced biomethanation performance.

Biological Inoculant of Salt-Tolerant Bacteria for Plant Growth Stimulation under Different Saline Soil Conditions

Using salt-tolerant bacteria to protect plants from salt stress is a promising microbiological treatment strategy for saline-alkali soil improvement. Here, we conducted research on the growthpromoting effect of Brevibacterium frigoritolerans on wheat under salt stress, which has rarely been addressed before. The synergistic effect of B. frigoritolerans combined with representative salttolerant bacteria Bacillus velezensis and Bacillus thuringiensis to promote the development of wheat under salt stress was also further studied. Our approach involved two steps: investigation of the plant growth-promoting traits of each strain at six salt stress levels (0, 2, 4, 6, 8, and 10%); examination of the effects of the strains (single or in combination) inoculated on wheat in different salt stress conditions (0, 50, 100, 200, 300, and 400 mM). The experiment of plant growth-promoting traits indicated that among three strains, B. frigoritolerans had the most potential for promoting wheat parameters. In single-strain inoculation, B. frigoritolerans showed the best performance of plant growth promotion. Moreover, a pot experiment proved that the plant growth-promoting potential of co-inoculation with three strains on wheat is better than single-strain inoculation under salt stress condition. Up to now, this is the first report suggesting that B. frigoritolerans has the potential to promote wheat growth under salt stress, especially combined with B. velezensis and B. thuringiensis.

Explore the difference between the single-chamber and dual-chamber microbial electrosynthesis for biogas production performance

Microbial electrosynthesis (MES) is an advanced technology for efficient treatment of organic wastewater and recovery of new energy, with the advantages and disadvantages of single-chamber and dual-chamber MES reactors being less understood. Therefore, we explored the effects of single-chamber and dual-chamber structures on the methane production performance and microbial community structure of MES. Results indicated that methane concentration and current density of single-chamber MES were higher than those of dual-chamber MES, and the system stability was better, while chemical oxygen demand (COD) removal rate and cumulative methane production were not significantly different. Analysis of microbial community structure showed the abundance of acidogens and H₂-producing bacteria was higher in single-chamber MES, while fermentation bacteria and methanogens was lower. The abundance of methanogens of dual-chamber MES (21.74-24.70%) was superior to the single-chamber MES (8.23-10.10%). Moreover, in dual-chamber MES, methane was produced primarily through acetoclastic methanogenic pathway, while in single-chamber MES cathode, methane production was mainly by hydrogenotrophic methanogenic pathway. Information provided will be useful to select suitable reactors and optimize reaction design.

A high-rate anaerobic biofilm reactor for biomethane recovery from source-separated blackwater at ambient temperature

Anaerobic bioreactors for source-separated blackwater are mostly operated at low organic loading rates (OLRs) due to low biodegradability and the potential of ammonia inhibition. In this study, an anaerobic biofilm reactor having conductive carbon fibers as the media was investigated for the high-rate treatment of blackwater collected from vacuum toilets. The bioreactor was operated at different OLRs ranged from 0.77 to 3.01 g COD/L-d in four stages for a total operating period of ~ 250 days. With the increase of OLRs, the specific methane production rate increased from 105.3 to 304.6 ml/L-d with high methane content in biogas (75.5%-83%). The maximum methane yield was achieved at hydraulic retention time (HRT) of 15 days. Highest organics and suspended solids removal (80%-83%) were achieved at 20-days HRT, while increased OLRs resulted in diminished removal efficiencies. The state variables, including pH, total ammonia nitrogen, short-chain volatile fatty acids, and soluble chemical oxygen demand, indicated the system had a great capability to withstand the high OLRs. Microbial community analysis revealed that the high performance might be attributed to direct interspecies electron transfer (DIET) facilitated by potentially electroactive bacteria (e.g., Syntrophomonas, Clostridium) and electrotrophic archaea (e.g., Methanosaeta and Methanosarcina species) enriched on the carbon fibers. PRACTITIONER POINTS: An anaerobic biofilm reactor was investigated for biomethane recovery from source-separated blackwater. Conductive carbon fibers were utilized as the media to stimulate enrichment of potentially electroactive methanogenic communities. The bioreactor was operated at ambient temperature for over 250 days. High methane production rate and high-quality biogas were achieved at OLRs ranged from 0.77 to 3.01 g COD/L-d. Microbial community analysis suggested direct interspecies electron transfer (DIET) between specific electroactive bacteria and electrotrophic archaea.

Electric Field-Driven Direct Interspecies Electron Transfer for Bioelectrochemical Methane Production from Fermentable and Non-Fermentable Substrates

The bioelectrochemical methane production from acetate as a non-fermentable substrate, glucose as a fermentable substrate, and their mixture were investigated in an anaerobic sequential batch reactor exposed to an electric field. The electric field enriched the bulk solution with exoelectrogenic bacteria (EEB) and electrotrophic methanogenic archaea, and promoted direct interspecies electron transfer (DIET) for methane production. However, bioelectrochemical methane production was dependent on the substrate characteristics. For acetate as the substrate, the main electron transfer pathway for methane production was DIET, which significantly improved methane yield up to 305.1 mL/g chemical oxygen demand removed (CODr), 77.3% higher than that in control without the electric field. For glucose, substrate competition between EEB and fermenting bacteria reduced the contribution of DIET to methane production, resulting in the methane yield of 288.0 mL/g CODr, slightly lower than that of acetate. In the mixture of acetate and glucose, the contribution of DIET to methane production was less than that of the single substrate, acetate or glucose, due to the increase in the electron equivalent for microbial growth. The findings provide a better understanding of electron transfer pathways, biomass growth, and electron transfer losses depending on the properties of substrates in bioelectrochemical methane production.

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.

In-situ membrane fouling control by electrooxidation and microbial community in membrane electro-bioreactor treating aquaculture seawater

Ammonia and nitrite in aquaculture recirculating seawater need to be strictly controlled to avoid deleterious effects on aquatic organisms. However, traditional biological approach can hardly meet the standard due to the short hydraulic retention time (HRT) and nitrite accumulation. A Membrane Electro-Bioreactor (MEBR) was developed for ammonia removal enhancement and in-situ electrochemical membrane fouling mitigation. The fouling mechanism was first found to proceed via the standard filtration model. The flux decrease was mainly caused by an internal pore clogging phenomenon. Membrane fouling resistance was enhanced by increasing anode potential from 0 to 1.4 V vs. SCE (Saturated Calomel Electrode). The ammonia removal rate in the MEBR was above 95% (HRT: 2 h, after day-13) and membrane fouling was mitigated that operation duration was extended by 71.4%. Higher total proportion of Proteobacteria, Bacteroidetes, Planctomycetes and Actinobacteria was obtained in the MEBR, suggesting higher nitrification and nitrogen removal potentials.

Influence of electrostatic field and conductive material on the direct interspecies electron transfer for methane production

The influence of electrostatic field on the direct interspecies electron transfer (DIET) pathways for methane production was investigated in a batch bioelectrochemical anaerobic digester (BEAD). The ultimate methane production and methane yield in the BEAD reactor saturated to 925 ± 29 mL/L and 309.9 ± 9.6 mL CH₄/g COD, respectively, which were much higher than 616 ± 3 mL/L and 205.4 ± 205.4 mL CH₄/g COD in the anaerobic digester (AD). In the cyclic voltammogram (CV) for bulk solution, the oxidation peak current was 0.52 mA in the BEAD reactor, which was higher than 0.24 mA of AD reactor. This shows that the oxidizing ability of microorganisms was greatly improved in the BEAD reactor. Anaerolineaceae, a well-known electroactive bacterial family, was well enriched in the BEAD reactor. It indicates that the electrostatic field can enrich the electroactive bacteria and activate the DIET pathways for methane production. Moreover, the conductive material (activated carbon) further improved the performance of BEAD reactor, implies that the conductivities of bulk solution is one of the important parameters for the DIET pathways.

In-situ mineral CO2 sequestration in a methane producing microbial electrolysis cell treating sludge hydrolysate

Microbial electrolysis cell (MEC) has excellent CH₄ production performance, however, CO₂ still remains in the produced biogas at high content. For achieving in-situ CO₂ sequestration and thus upgrading biogas, mineral carbonation was integrated into a MEC treating sludge hydrolysate. With 19 g/L wollastonite addition, in-situ mineral CO₂ sequestration was achieved by formation of calcite precipitates. CH₄ content in the biogas was increased by 5.1 % and reached 95.9 %, with CH₄ production improved by 16.9 %. In addition, the removals of polysaccharide, protein, and chemical oxygen demand (COD) of the MEC were increased by 4.4 %, 6.7 %, and 8.4 %, respectively. The generated precipitates rarely accumulated on bio-cathode, and did not significantly affect the morphology of cathode biofilm. However, integrating mineral carbonation resulted in a higher relative abundance of Methanosarcina on anode and slightly decreased the ratio of Methanobacterium to Methanosaeta on cathode, which should be noticed. In conclusion, integrating mineral carbonation is an attractive way to improve the performance of MEC by achieving in-situ CO₂ sequestration, accompanied with CH₄ production enhancement.

Enhanced removal of bisphenol A from contaminated soil by coupling Bacillus subtilis HV-3 with electrochemical system

Exposure to endocrine disruptors interferes with the synthesis, release, transport and metabolic activities of hormones, thus impairing human health significantly. Bisphenol A (BpA), an endocrine disruptor, commonly released into the environment by industrial activities and needs immediate attention. This study aims at investigating the process and prospects of deploying bio-electrochemical systems (BES) for the removal of BpA from artificially contaminated soil using Bacillus subtilis HV-3. The BES was setup with desired operating conditions: initial concentration of BpA (80-150 mg/L), pH (3-11) and applied potential voltage (0.6-1.4 V). Under optimized conditions (initial BpA concentration, 100 mg/L; pH 7; and applied voltage 1.0 V), close to 98% degradation of BpA was achieved. The intermediates produced during degradation were analysed using High performance liquid chromatography-Mass spectrometry and the possible degradation pathway was elucidated. Phytotoxicity studies in the remediated soil with Phaseolus mungo confirmed the environmental applicability of the BES system.

Electrodes bioaugmentation promotes the removal of antibiotics from concentrated sludge in microbial electrolysis cells

Microbial electrolysis cells (MECs) had a potential to improve antibiotics removal from wastewater. However, research on antibiotics removal from concentrated sludge using MECs is still very limited. In this study, antibiotics removal and microbial responses in MECs treating concentrated sludge under different applied voltages (0.3 V-1.5 V) were investigated. Results showed that antibiotics removal efficiencies at 0.6 V and 1.0 V were 16.7%-26.6% higher than other applied voltages. The applied voltages had no obvious effects on the viability, activity and composition of microorganisms in the suspended sludge even up to 1.5 V. Bioelectrodes exhibited higher bioelectrocatalytic activity and denser microbial aggregation at 0.6 V and 1.0 V, under which higher antibiotics removal was also achieved. The enhanced removal of antibiotics at the optimal applied voltages was mainly contributed by the bioaugmentation of electrodes, but was irrelative with the electrochemical reaction and the microbial responses in suspended sludge.

Bioelectrochemical Methane Production from Food Waste in Anaerobic Digestion Using a Carbon-Modified Copper Foam Electrode

Anaerobic bioelectrochemical digestion (ABD) is widely used for treating wastewater and recovering energy. The electrode is the key point for ABD system, which was sparsely studied with food waste. In this study, a carbon-modified copper foam was fabricated with copper foam and multiple wall carbon nanotubes (MWCNT) through electrophoretic deposition and screen-printing methods. The carbon-modified copper foam electrode was investigated in an ABD reactor for food waste. The features of bioelectrochemical methane production, process stability, and electrochemical characterization were evaluated in the ABD reactor, and were compared to the control reactor without equipping electrode. The ultimate methane production reached 338.1 mL CH4/L in the ABD reactor, which was significantly higher than the 181.0 mL CH4/L of the control reactor. The methane produced from the electrode was 137.8 mL CH4/L, which was up to 40.8% of total methane production in the ABD reactor. It was attributed to the electroactive bacteria that were enriched and activated by the carbon-modified copper foam electrode, further activating the direct interspecies electron transfer (DIET) pathways for methane production. The cyclic voltammetry (CV) analysis showed higher redox peaks, which is one of the pieces of evidence for the enrichment of electroactive bacteria. The carbon-modified copper foam electrode has the advantages of both carbon and metal materials, and demonstrated a high possibility for use in bioelectrochemical methane production for food waste.

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.

Individual and combined effects of magnetite addition and external voltage application on anaerobic digestion of dairy wastewater

Direct interspecies electron transfer (DIET) between exoelectrogenic fatty acid oxidizers and electrotrophic methanogens plays an important role in keeping the overall anaerobic digestion (AD) process well-balanced. This study examined the individual and combined effects of two different DIET-promoting strategies, i.e., magnetite addition (20 mM Fe) and external voltage application (0.6 V), in continuous digesters treating dairy wastewater. Although the strategies were both effective in enhancing the process performance and stability, adding magnetite had a much greater stimulatory effect. External voltage contributed little to the methane yield, and the digester with magnetite addition alone achieved stable performance, comparable to that of the digester where both strategies were combined, at short hydraulic retention times (down to 7.5 days). Diverse (putative) electroactive microorganisms were significantly enriched under DIET-promoting conditions, particularly with magnetite addition. The overall results suggest that magnetite addition could effectively enhance AD performance and stability by promoting DIET-based electro-syntrophic microbial interactions.

Contribution of Yeast Extract, Activated Carbon, and an Electrostatic Field to Interspecies Electron Transfer for the Bioelectrochemical Conversion of Coal to Methane

The bioelectrochemical conversion of coal to methane was investigated in an anaerobic batch reactor containing yeast extract and activated carbon. In anaerobic degradation of coal, yeast extract was a good stimulant for the growth of anaerobic microorganisms, and activated carbon played a positive role. An electrostatic field of 0.67 V/cm significantly improved methane production from coal by promoting direct and mediated interspecies electron transfers between exoelectrogenic bacteria and electrotrophic methanogenic archaea. However, the accumulation of coal degradation intermediates gradually repressed the conversion of coal to methane, and the methane yield of coal was only 31.2 mL/g lignite, indicating that the intermediates were not completely converted to methane. By supplementing yeast extract and seed sludge into the anaerobic reactor, the intermediate residue could be further converted to methane under an electrostatic field of 0.67 V/cm, and the total methane yield of coal increased to 98.0 mL/g lignite. The repression of the intermediates to the conversion of coal to methane was a kind of irreversible substrate inhibition. The irreversible substrate inhibition in the conversion of coal to methane could be attenuated under the electrostatic field of 0.67 V/cm by ensuring sufficient biomass through biostimulation or bioaugmentation.

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.

Effect of applied voltage and temperature on methane production and microbial community in microbial electrochemical anaerobic digestion systems treating swine manure

Microbial electrochemical technology (MET) that can harvest electricity/valuable materials and enhance the efficiency of conventional biological processes through the redox reaction of organic/inorganic compounds has attracted considerable attention. MET-based anaerobic digestion (AD) systems treating swine manure were operated at different applied voltages (0.1, 0.3, 0.5, 0.7, and 0.9 V) and temperatures (25, 35, and 45 °C). Among the MET-based AD systems with different applied voltages at 35 °C, M4 at 0.7 V showed the highest methane production (2.96 m3-CH4/m3) and methane yield (0.64 m3-CH4/kg-VS). The methane production and yield increased with increasing temperature at an applied voltage of 0.7 V. Nevertheless, the MET-based AD systems (LM at 25 °C and 0.7V) showed competitive AD performance (2.33 m3-CH4/m3 and 0.53 m3-CH4/VS) compared with the conventional AD system (35 °C). The microbial community was affected by the applied voltage and temperature, and hydrogenotrophic methanogens such as M. flavescens, M. hungatei, and M. thermautotrophicus were mainly responsible for methane production in MET-based AD systems. Therefore, the methane production can be enhanced by an applied voltage or by direct interspecies electron transfer because M. flavescens and M. thermautotrophicus were especially predominant in cathode of MET-based AD systems. The MET-based AD systems can help enhance biogas production from swine manure with no significant change in methane content. Furthermore, MET-based AD systems will be a promising AD system through low material development and the optimal operation.

Electroactive microorganisms enriched from activated sludge remove nitrogen in bioelectrochemical reactor

The bioelectrochemical anaerobic nitrogen removal was demonstrated in an anaerobic batch reactor equipped with a pair of polarized bioelectrodes. The bioelectrochemical reactor was operated in sequential batch mode after inoculating activated sludge and polarizing the electrode to 0.6 V. The medium contains ammonium, nitrite, alkalinity and trace minerals, but no organic carbon source. By the repetitive sequential operation, simultaneous removals of ammonium, nitrite and alkalinity were improved, and the electrochemical activity of the bulk sludge was confirmed from the redox peaks of the cyclic voltammogram. This indicates that ammonia oxidizing exoelectrogens (AOE) and denitritating electrotrophs (DNE) were enriched more in the bulk solution. Biogas production that mainly consisted of nitrogen was observed from the bioelectrochemical reactor, and the minor components in the biogas were methane and carbon dioxide. This demonstrates that AOE use nitrite as an electron acceptor to oxidize ammonia. The requirements of nitrite and alkalinity for the removal of ammonia nitrogen are around 0.72 mg NO₂-N/mg NH₄-N and 1.73 mg as CaCO₃/mg NH₄-N, respectively, and nitrate was not produced as a by-product. The bacterial groups involved in the bioelectrochemical nitrogen removal are electroactive autotrophs and can be enriched from activated sludge by polarized electrode. This bioelectrochemical ammonia oxidation is a novel approach recommended for treatment of nitrogen-rich wastewater.

Bioelectrochemical Enhancement of Biogenic Methane Conversion of Coal

This study demonstrated the enhancement of biogenic coal conversion to methane in a bioelectrochemical anaerobic reactor with polarized electrodes. The electrode with 1.0 V polarization increased the methane yield of coal to 52.5 mL/g lignite, which is the highest value reported to the best of our knowledge. The electrode with 2.0 V polarization shortened the adaptation time for methane production from coal, although the methane yield was slightly less than that of the 1.0 V electrode. After the methane production from coal in the bioelectrochemical reactor, the hydrolysis product, soluble organic residue, was still above 3600 mg chemical oxygen demand (COD)/L. The hydrolysis product has a substrate inhibition effect and inhibited further conversion of coal to methane. The dilution of the hydrolysis product mitigates the substrate inhibition to methane production, and a 5.7-fold dilution inhibited the methane conversion rate by 50%. An additional methane yield of 55.3 mL/g lignite was obtained when the hydrolysis product was diluted 10-fold in the anaerobic toxicity test. The biogenic conversion of coal to methane was significantly improved by the polarization of the electrode in the bioelectrochemical anaerobic reactor, and the dilution of the hydrolysis product further improved the methane yield.

Polarized electrode enhances biological direct interspecies electron transfer for methane production in upflow anaerobic bioelectrochemical reactor

The influence of polarized electrodes on the methane production, which depends on the sludge concentration, was investigated in upflow anaerobic bioelectrochemical (UABE) reactor. When the polarized electrode was placed in the bottom zone with a high sludge concentration, the methane production was 5.34 L/L.d, which was 53% higher than upflow anaerobic sludge blanket (UASB) reactor. However, the methane production was reduced to 4.34 L/L.d by placing the electrode in the upper zone of the UABE reactor with lower sludge concentration. In the UABE reactor, the methane production was mainly improved by the enhanced biological direct interspecies electron transfer (bDIET) pathway, and the methane production via the electrode was a minor fraction of less than 4% of total methane production. The polarized electrodes that placed in the bottom zone with a high sludge concentration enhance the bDIET for methane production in the UABE reactor and greatly improve the methane production.

Electroactive microorganisms in bulk solution contribute significantly to methane production in bioelectrochemical anaerobic reactor

The role of anaerobic microorganisms suspended in the bulk solution on methane production was investigated in a bioelectrochemical anaerobic reactor with the electrode polarized at 0.5 V. The electron transfer from substrate to methane and hydrogen were 25% and 7.5%, respectively, in the absence of the anaerobic microorganisms in the bulk solution. As the anaerobic microorganisms increased to 4400 mg/L, the electrons transferred to methane increased to 83.3% but decreased to 0.3% in hydrogen. The electroactive microorganisms (EAM), including exoelectrogens and electrotrophs, enriched in the bulk solution that confirmed by the redox peaks in the cyclic voltammogram was proportional to the anaerobic microorganism. The methane yield based on COD removal was dependent on the anaerobic microorganisms in the bulk solution rather than on the bioelectrode surface. The EAM suspended in the bulk solution are enriched by the polarized electrode, and significantly improve methane production in bioelectrochemical anaerobic reactor.

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.

Influence of conductive material on the bioelectrochemical removal of organic matter and nitrogen from low strength wastewater

The treatment of low strength wastewater that has the level of discharge standard for wastewater treatment plant was studied using an upflow bioelectrochemical reactor with an applied voltage of 0.6 V. The direct interspecies electron transfer (DIET) between electroactive bacteria was activated in the upflow bioelectrochemical reactor, which improved the substrate affinity of bacteria. The effluent qualities in COD and ammonia nitrogen was stable at less than 3.5 mg/L and 7.46 mg/L at 1 h of hydraulic retention time, respectively. The conductive materials, including conductive sheets and conductive particles, further increased the biomass retention and the DIET by altering the abundance of dominant bacterial groups. The effluent qualities in COD and ammonia nitrogen was improved up to 1.98 mg/L and 2.65 mg/L, respectively, by the conductive sheets. The upflow bioelectrochemical reactor with conductive materials is a good tertiary treatment process for improving the quality of the final effluent discharged from wastewater treatment plant.

Bioelectrochemical enhancement of methane production from highly concentrated food waste in a combined anaerobic digester and microbial electrolysis cell

A microbial electrolysis cell (MEC) is a promising technology for enhancing biogas production from an anaerobic digestion (AD) reactor. In this study, the effects of the MEC on the rate of methane production from food waste were examined by comparing an AD reactor with an AD reactor combined with a MEC (AD+MEC). The use of the MEC accelerated methane production and stabilization via rapid organic oxidation and rapid methanogenesis. Over the total experimental period, the methane production rate and stabilization time of the AD+MEC reactor were approximately 1.7 and 4.0 times faster than those of the AD reactor. Interestingly however, at the final steady state, the methane yields of both the reactors were similar to the theoretical maximum methane yield. Based on these results, the MEC did not increase the methane yield over the theoretical value, but accelerated methane production and stabilization by bioelectrochemical reactions.

Bioelectrochemical enhancement of direct interspecies electron transfer in upflow anaerobic reactor with effluent recirculation for acidic distillery wastewater

Methane production in the upflow anaerobic bioelectrochemical reactor (UABE) treating acidic distillery wastewater was compared to the upflow anaerobic sludge blanket reactor (UASB), and the electron transfer pathways for methane production were also evaluated in the effluent recirculation. The methane productions from reactors were influenced by the low pH of influent wastewater. However, the methane production rate and yield of the UABE were 2.08L/L.d and 320mL/g COD_r, which were higher than the UASB. The effluent recirculation containing alkalinity neutralized the acidic influent and increased the upflow velocity in both reactors, and improved the direct interspecies electron transfer more in the UABE. When the effluent recirculation ratio was 3.0 in the UABE, the methane production rate and yield were reached up to 3.88L/L.d and 501.0mL/g COD_r, respectively. The UABE requires electrode installation and electrical energy for operation, but the benefits from increased methane production are much higher.