Evaluation of gas and carbon transport in a methanogenic bioelectrochemical system (BES)

Advances in bioelectrochemical systems for bio-products recovery

Bioelectrochemical systems (BES) have emerged as a sustainable and highly promising technology that has garnered significant attention from researchers worldwide. These systems provide an efficient platform for the removal and recovery of valuable products from wastewater, with minimal or no net energy loss. Among the various types of BES, microbial fuel cells (MFCs) are a notable example, utilizing microbial biocatalytic activities to generate electrical energy through the degradation of organic matter. Other BES variants include microbial desalination cells (MDCs), microbial electrolysis cells (MECs), microbial electrosynthesis cells (MXCs), microbial solar cells (MSCs), and more. BESs have demonstrated remarkable potential in the recovery of diverse products such as hydrogen, methane, volatile fatty acids, precious nutrients, and metals. Recent advancements in scaling up BESs have facilitated a more realistic assessment of their net energy recovery and resource yield in real-world applications. This comprehensive review focuses on the practical applications of BESs, from laboratory-scale developments to their potential for industrial commercialization. Specifically, it highlights successful examples of value-added product recovery achieved through various BES configurations. Additionally, this review critically evaluates the limitations of BESs and provides suggestions to enhance their performance at a larger scale, enabling effective implementation in real-world scenarios. By providing a thorough analysis of the current state of BES technology, this review aims to emphasize the tremendous potential of these systems for sustainable wastewater treatment and resource recovery. It underscores the significance of bridging the gap between laboratory-scale achievements and industrial implementation, paving the way for a more sustainable and resource-efficient future.

Insights into the Electron Transfer Behaviors of a Biocathode Regulated by Cathode Potentials in Microbial Electrosynthesis Cells for Biogas Upgrading

Bioelectrochemical-based biogas upgrading is a promising technology for the storage of renewable energy and reduction of the global greenhouse gas emissions. Understanding the electron transfer behavior between the electrodes and biofilm is crucial for the development of this technology. Herein, the electron transfer pathway of the biofilm and its catalytic capability that responded to the cathode potential during the electromethanogenesis process were investigated. The result suggested that the dominant electron transfer pathway shifted from a direct (DET) to indirect (IDET) way when decreasing the cathode potential from -0.8 V (Bio-0.8 V) to -1.0 V (Bio-1.0 V) referred to Ag/AgCl. More IDET-related redox substances and high content of hydrogenotrophic methanogens (91.9%) were observed at Bio-1.0 V, while more DET-related redox substances and methanogens (82.3%) were detected at Bio-0.8 V. H₂, as an important electron mediator, contributed to the electromethanogenesis up to 72.9% of total CH₄ yield at Bio-1.0 V but only ∼17.3% at Bio-0.8 V. Much higher biogas upgrading performance in terms of CH₄ production rate, final CH₄ content, and carbon conversion rate was obtained with Bio-1.0 V. This study provides insight into the electron transfer pathway in the mixed culture constructed biofilm for biogas upgrading.

Hydrogen sulfide affects the performance of a methanogenic bioelectrochemical system used for biogas upgrading

Methanogenic bioelectrochemical systems (BESs) can convert carbon dioxide (CO₂) to methane (CH₄) and may be used for anaerobic digester biogas upgrading. However, the effect of hydrogen sulfide (H₂S), a common biogas component, on BES performance is unknown. Thus, the objective of this study was to assess the effect of H₂S addition to the cathode headspace on BES performance at a range of initial gas-phase H₂S concentrations (0-6% v/v), as well as its effect on the anode and cathode microbial communities. As the initial cathode headspace H₂S increased from 0 to 2% (v/v), biocathodic CH₄ production increased by two-fold to 3.56 ± 0.36 mmol/L-d, due to dissolved H₂S transport from the cathode to the anode where H₂S was oxidized. Elemental sulfur and sulfate were H₂S oxidation products detected in the anode. Above 3% initial cathode headspace H₂S, biocathodic CH₄ production declined due to inhibition. A phylotype most closely related to Methanobrevibacter arboriphilus dominated the cathode archaeal communities. In the sulfide-amended BES, a phylotype similar to the exoelectrogen Ochrobactrum anthropi was enriched in both the anode and cathode, whereas phylotypes related to sulfate-reducing and sulfur oxidizing Bacteria were detected in the bioanode. Thus, sulfide transport and oxidation in the anode play an important role in methanogenic BESs treating sulfide-bearing biogas.

Comparison of Carbon Dioxide with Anaerobic Digester Biogas as a Methanogenic Biocathode Feedstock

BES biogas upgrading studies have typically used bicarbonate or commercial gas mixtures as a biocathode substrate instead of anaerobic digester biogas. Therefore, the objective of this study was to (i) compare the performance of a methanogenic BES between CO₂-fed and biogas-fed cycles; (ii) investigate possible factors that may account for observed performance differences; and (iii) assess the performance of a biogas-fed biocathode at various applied cathode potentials. The maximum 1-d CH₄ production rate in a biogas-fed biocathode (3003 mmol/m²-d) was 350% higher than in a CO₂-fed biocathode (666 mmol/m²-d), and the biogas-fed biocathode was capable of maintaining high performance despite a variable biogas feed composition. Anode oxidation of reduced gases (e.g., CH₄ and H₂S) from biogas may theoretically contribute 4% to 35% of the total charge transfer from anode to cathode at applied cathode potentials of -0.80 to -0.55 V (vs SHE). The introduction of biogas did not significantly change the biocathode archaeal community (dominated by a Methanobrevibacter sp. phylotype), but the bacterial community shifted away from Bacteroidetes and toward Proteobacteria, which may have contributed to the improved performance of the biogas-fed system. This study shows that anaerobic digester biogas is a promising biocathode feedstock for BES biogas upgrading.

Evidence of Spatial Homogeneity in an Electromethanogenic Cathodic Microbial Community

Microbial electrosynthesis (MES) has been gaining considerable interest as the next step in the evolution of microbial electrochemical technologies. Understanding the niche biocathode environment and microbial community is critical for further developing this technology as the biocathode is key to product formation and efficiency. MES is generally operated to enrich a specific functional group (e.g., methanogens or homoacetogens) from a mixed-culture inoculum. However, due to differences in H₂ and CO₂ availability across the cathode surface, competition and syntrophy may lead to overall variability and significant beta-diversity within and between replicate reactors, which can affect performance reproducibility. Therefore, this study aimed to investigate the distribution and potential spatial variability of the microbial communities in MES methanogenic biocathodes. Triplicate methanogenic biocathodes were enriched in microbial electrolysis cells for 5 months at an applied voltage of 0.7 V. They were then transferred to triplicate dual-chambered MES reactors and operated at -1.0 V vs. Ag/AgCl for six batches. At the end of the experiment, triplicate samples were taken at different positions (top, center, bottom) from each biocathode for a total of nine samples for total biomass protein analysis and 16S rRNA gene amplicon sequencing. Microbial community analyses showed that the biocathodes were highly enriched with methanogens, especially the hydrogenotrophic methanogen family Methanobacteriaceae, Methanobacterium sp., and the mixotrophic Methanosarcina sp., with an overall core community representing > 97% of sequence reads in all samples. There was no statistically significant spatial variability (p > 0.05) observed in the distribution of these communities within and between the reactors. These results suggest deterministic community assembly and indicate the reproducibility of electromethanogenic biocathode communities, with implications for larger-scale reactors.

Reduction of AOX in pharmaceutical wastewater in the cathode chamber of bio-electrochemical reactor

A bio-electrochemical reactor (BER) operating at different cathode potentials ranging from -300 to -1000 mV (vs standard hydrogen electrode, SHE) was used to reduce adsorbable organic halogens (AOX) in pharmaceutical wastewater. Cathode polarization enriched the electron donor of the biological system. Thus, the AOX removal efficiency in the BER improved from 59.9% to 70.2%, and the AOX removal rate increased from 0.87 to 1.17 mg AOX/h when the cathode potential was reduced from -300 to -1000 mV with the addition of methyl viologen, a known redox mediator. The decrease of the cathode potential was also beneficial for methane production, and the inhibition of the methanogenic process enhanced the AOX removal. Additionally, cathode coulombic efficiency analysis demonstrated that the proportion of electrons used for AOX reduction decreases with decreasing potential, from 37.6% at -300 mV to 17.3% at -1000 mV, although the AOX removal efficiency improves.

Bioelectrochemically assisted anaerobic digestion system for biogas upgrading and enhanced methane production

The objective of this study was to evaluate the effect of biofilm and external voltage on the performance and microbial community composition of batch-fed, combined anaerobic digestion-bioelectrochemical cell (AD-BEC) systems under different operational conditions. A dextrin/peptone mixture was fed at a range of organic loading rates (0.34 to 1.37g COD/L-d). The hybrid system with both suspended biomass and biofilm without any external potential application achieved a substantially higher initial soluble COD consumption (53.7±2.3% vs. 39.7±3.7) and methane (CH₄) production (331 vs. 225mL) within one day of feeding than the conventional AD system (suspended biomass only). Compared to the conventional AD system, the hybrid systems had higher resilience to shock organic loadings. A range of external potential (0.5 to 2.0V vs. Ag/AgCl) was applied to AD-BEC reactors, developed with two different start-up procedures. A potential of 2.0V resulted in water electrolysis leading to a higher CH₄ production rate (105 vs. 84mL/L-d) and biogas CH₄ content (88.5±1.4 vs. 64.5±1.9%) in the AD-BEC reactor (closed vs. open circuit condition, respectively). Application of external potential enriched putative exoelectrogens at the anode biofilm and hydrogenotrophic methanogens at the cathode biofilm, which may have contributed to the observed enhanced CH₄ production in the AD-BEC system. A phylotype related to Methanobacterium formicicum, a hydrogenotrophic methanogen, dominated the archaeal community in the AD-BEC cathode biofilm.

Effect of Start-Up Strategies and Electrode Materials on Carbon Dioxide Reduction on Biocathodes

The enrichment of CO₂-reducing microbial biocathodes is challenging. Previous research has shown that a promising approach could be to first enrich bioanodes and then lower the potential so the electrodes are converted into biocathodes. However, the effect of such a transition on the microbial community on the electrode has not been studied. The goal of this study was thus to compare the start-up of biocathodes from preenriched anodes with direct start-up from bare electrodes and to investigate changes in microbial community composition. The effect of three electrode materials on the long-term performance of the biocathodes was also investigated. In this study, preenrichment of acetate-oxidizing bioanodes did not facilitate the start-up of biocathodes. It took about 170 days for the preenriched electrodes to generate substantial cathodic current, compared to 83 days for the bare electrodes. Graphite foil and carbon felt cathodes produced higher current at the beginning of the experiment than did graphite rods. However, all electrodes produced similar current densities at the end of the over 1-year-long study (2.5 A/m²). Methane was the only product detected during operation of the biocathodes. Acetate was the only product detected after inhibition of the methanogens. Microbial community analysis showed that Geobacter sp. dominated the bioanodes. On the biocathodes, the Geobacter sp. was succeeded by Methanobacterium spp., which made up more than 80% of the population. After inhibition of the methanogens, Acetobacterium sp. became dominant on the electrodes (40% relative abundance). The results suggested that bioelectrochemically generated H₂ acted as an electron donor for CO₂ reduction.

IMPORTANCE In microbial electrochemical systems, living microorganisms function as catalysts for reactions on the anode and/or the cathode. There is a variety of potential applications, ranging from wastewater treatment and biogas generation to production of chemicals. Systems with biocathodes could be used to reduce CO₂ to methane, acetate, or other high-value chemicals. The technique can be used to convert solar energy to chemicals. However, enriching biocathodes that are capable of CO₂ reduction is more difficult and less studied than enriching bioanodes. The effect of different start-up strategies and electrode materials on the microbial communities that are enriched on biocathodes has not been studied. The purpose of this study was to investigate two different start-up strategies and three different electrode materials for start-up and long-term operation of biocathodes capable of reducing CO₂ to valuable biochemicals.

Methanogens: biochemical background and biotechnological applications

Since fossil sources for fuel and platform chemicals will become limited in the near future, it is important to develop new concepts for energy supply and production of basic reagents for chemical industry. One alternative to crude oil and fossil natural gas could be the biological conversion of CO2 or small organic molecules to methane via methanogenic archaea. This process has been known from biogas plants, but recently, new insights into the methanogenic metabolism, technical optimizations and new technology combinations were gained, which would allow moving beyond the mere conversion of biomass. In biogas plants, steps have been undertaken to increase yield and purity of the biogas, such as addition of hydrogen or metal granulate. Furthermore, the integration of electrodes led to the development of microbial electrosynthesis (MES). The idea behind this technique is to use CO2 and electrical power to generate methane via the microbial metabolism. This review summarizes the biochemical and metabolic background of methanogenesis as well as the latest technical applications of methanogens. As a result, it shall give a sufficient overview over the topic to both, biologists and engineers handling biological or bioelectrochemical methanogenesis.

Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane

Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide (CO₂) directly to methane (CH₄), promise to be an innovative technology for anaerobic digester biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH₄ production in anaerobic digesters, has not been explored in methanogenic biocathodes. Thus, the objective of this study was to assess the effect of biocathode ZVI on BES performance at 1 and 2 g/L initial ZVI concentrations and at various cathode potentials (-0.65 to -0.80 V versus SHE). The total CH₄ produced during a 7-day feeding cycle with 1 and 2 g/L initial ZVI was 2.8- and 2.9-fold higher, respectively, than the mean CH₄ production in the four prior cycles without ZVI addition. Furthermore, CH₄ production by the ZVI-amended biocathodes remained elevated throughout three subsequent feeding cycles, despite catholyte replacement and no new ZVI addition. The fourth cycle following a single ZVI addition of 1 g/L and 2 g/L yielded 123% and 231% more total CH₄ than in the non-ZVI cycles, respectively. The higher CH₄ production could not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H₂ by hydrogenotrophic methanogens. Microbial community analysis showed that the same phylotype, most closely related to Methanobrevibacter arboriphilus, dominated the archaeal community in the ZVI-free and ZVI-amended biocathodes. However, the bacterial community experienced substantial changes following ZVI exposure, with more Proteobacteria and fewer Bacteroidetes in the ZVI-amended biocathode. Furthermore, it is likely that a redox-active precipitate formed in the ZVI-amended biocathode, which sorbed to the electrode and/or biofilm, acted as a redox mediator, and enhanced electron transfer and CH₄ production. Thus, ZVI may be used to increase biocathode CH₄ production, assist in the start-up of an electromethanogenic biocathode, and/or maintain microbial activity during voltage interruptions.

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.