Hydrogen-driven microbial biogas upgrading: Advances, challenges and solutions

An integrated review on the role of different biocatalysts, process parameters, bioreactor technologies and data-driven predictive models for upgrading biogas

As energy consumption and waste generation from human activities continue to rise, the technology of anaerobic digestion (AD), which converts waste into bioenergy, has gained popularity. Biogas produced from AD commonly contains 60 % CH4, 40 % CO2 and a minor fraction of impurities. Currently, several anaerobic reactors have been designed to upgrade the biogas with biomethane content above 90 %. This review summarizes the current trends in the biological upgradation of biogas from a bio-circular economy perspective to achieve sustainable energy goals. Examples of applications reporting the latest advancements in treating industrial effluents using high-rate anaerobic reactors have been mentioned. The integrated anaerobic-aerobic hybrid reactor offers a solution to the limitations of traditional methods in treating diverse effluents. A special focus on biological upgradation techniques such as in-situ, ex-situ, and hybrid mechanisms have been briefed. The key advantage of hybrid upgradation is its ability to address the pH rise during in-situ process. Additionally, the applications of artificial neural networks and optimization to upgrade biogas production have been discussed. The review concludes with future research directives with emphasis on the economic viability of the approaches.

Mildly acidic pH boosts up CO2 conversion to isobutyrate in H2 driven gas fermentation system

As a greenhouse gas, massive carbon dioxide (CO2) has been generated due to organic matter degradation in wastewater treatment processes. Microbial gas fermentation offers a promising approach to capture CO2 and generate various valuable chemicals. However, limited studies have achieved branched or medium-chain fatty acids production via gas fermentation. This study reported the production of isobutyrate and hexanoate by feeding H2 and CO2 into membrane biofilm reactors (MBfRs). The gas fermentation product in the reactor with neutral pH (pH of 7) was dominated by acetate (accounting for 90 % of the product spectrum), whereas a mildly acidic pH (pH of 6) resulted in isobutyrate and hexanoate as the dominant products, with a selectivity of 57 % and 42 %, respectively. Notably, a remarkably high concentration of isobutyrate (266 mmol C/L) was produced in the reactor with pH of 6. Subsequent batch test results suggest that the isobutyrate production in this study is coupled with acetogenesis and ethanol-driven chain elongation processes, rather than via methanol-driven chain elongation reported previously. High-throughput 16S rRNA gene amplicon sequencing revealed that the microbial community under neutral pH was dominated by acetate-producing homoacetogens Acetobacterium. By contrast, a mildly acidic pH promoted the community shifting towards chain elongation microorganisms, dominated by Clostridium sensu stricto 12, Oscillibacter and Caproiciproducens. Collectively, this study demonstrates the significant role of mildly acidic pH in boosting up bioisomerization and chain elongation in gas fermentation systems, thus triggering isobutyrate and hexanoate production. The findings highlight gas fermentation as a new green alternative route for generating highly valuable isobutyrate and hexanoate.

In-Situ CeO2/CuO Heterojunction Electrocatalyst for CO2 Reduction to Ethylene

CeO2/CuO heterojunction composite catalysts were synthesized using a one-step method, achieving the introduction of Ce species on nanoscale copper oxide (CuO) particles during the hydrothermal process. CeO2 is primarily encapsulated the auxiliary catalyst CuO in the form of nanoparticles. On one hand, this protects the nanostructure of the substrate from damage and prevents the agglomeration of CuO nanoparticles. On the other hand, the bimetallic synergistic effect between Ce and Cu effectively improves the conductivity and catalytic activity of the catalyst, significantly enhancing the selectivity of the catalyst for electrochemical reduction of CO2 to C2H4, while effectively suppressing the competing hydrogen evolution reaction (HER). By regulating the amount of CeO2 introducing, a series of CeO2/CuO composite catalysts were designed. The results showed that the 15 % CeO2/CuO catalyst exhibited the best selectivity and catalytic activity for C2H4. At a low overpotential of -1.2 V, the 15 % CeO2/CuO catalyst demonstrated a current density of 14.2 mA cm-2 and achieved a Faradaic efficiency for ethylene as high as 65.78 %, which is 2.85 times the current density (j=4.98 mA cm-2) and 3.27 times the Faradaic efficiency for ethylene (FEC2H4=20.13 %) of the undoped catalyst at the same potential. This work provides a feasible basis for achieving efficient CO2RR to C2 products, and even multi-carbon products.

Coupling Fe3O4 and micro-hydrogen for enhancing the anaerobic bio-conversion of phenol: Methanogenesis pathway and interspecies electron transfer

Low anaerobic biological conversion rate is a challenge in the anaerobic biological treatment of phenol wastewater. The dominant syntrophic acetate oxidation and hydrogenotrophic methanogenesis (SAO-HM) pathway is expected to solve the problem. However, there are raw studies on artificially simulating a dominant SAO-HM pathway for enhancing the anaerobic bio-conversion of phenol. In this study, Fe3O4 and micro-hydrogen (Fe3O4/H2) was used to strengthen the anaerobic bio-conversion of phenol by simulating the dominant SAO-HM pathway. The results suggested Fe3O4 and H2 had a coupling promotion on the anaerobic bio-conversion of phenol and Fe3O4/H2 increased the relative abundance of Syntrophus (29.35%), Syntrophorhabdu (3.27%), Clostridium (3.01%) and Methanobacterium (65.74%), indicating Fe3O4/H2 formed a dominant SAO-HM pathway. However, Fe3O4/H2 obviously reduced the conductivity of sludge, the extracellular protein and the functional gens pilA, pilB and OmcS, implying direct interspecies electron transfer (DIET) generated by Fe3O4 was interfered by micro hydrogen in extracellular electron transport process which reduced the efficiency of extracellular electron transport. This study remind that extracellular electron transport cannot be ignored in the dominate SAO-HM pathway for the anaerobic bio-conversion of phenol.

Simultaneous Biogas Upgrading and Valuable Chemical Production Using Homoacetogens in a Membrane Biofilm Reactor

Biogas produced from anaerobic digestion usually contains impurities, particularly with a high content of CO2 (15-60%), thus decreasing its caloric value and limiting its application as an energy source. H2-driven biogas upgrading using homoacetogens is a promising approach for upgrading biogas to biomethane and converting CO2 to acetate simultaneously. Herein, we developed a novel membrane biofilm reactor (MBfR) with H2 and biogas separately supplied via bubbleless hollow fiber membranes. The gas-permeable hollow fibers of the MBfR enabled high H2 and CO2 utilization efficiencies (∼98% and ∼97%, respectively) and achieved concurrent biomethane (∼94%) and acetate (∼450 mg/L/d) production. High-throughput 16S rRNA gene amplicon sequencing suggested that enriched microbial communities were dominated by Acetobacterium (38-48% relative abundance). In addition, reverse transcription quantitative PCR of the functional marker gene formyltetrahydrofolate synthetase showed that its expression level increased with increasing H2 and CO2 utilization efficiencies. These results indicate that Acetobacterium plays a key role in CO2 to acetate conversion. These findings are expected to facilitate energy-positive wastewater treatment and contribute to the development of a new solution to biogas upgrading.

Reverse electron transfer: Novel anaerobic methanogenesis pathway regulated through exogenous CO2 synergized with biochar

Acid accumulation and carbon emission are two major challenges in anaerobic digestion. Syntrophic consortia can employ reverse electron transfer (RET) to facilitate thermodynamically unfavorable redox reactions during acetogenesis. However, the potential mechanisms and regulatory methods of RET remain unclear. This study examines the regulatory mechanisms by which exogenous CO2 affects RET and demonstrates that biochar maximizes CO2 solubility at 25.8 mmol/L to enhance effects further. CO2 synergized with biochar significantly increases cumulative methane production and propionate degradation rate. From the bioenergetic perspective, CO2 decreases energy level to a maximum of -87 kJ/mol, strengthening the thermodynamic viability. The underlying mechanism can be attributed to RET promotion, as indicated by increased formate dehydrogenase and enrichment of H2/formate-producing bacteria with their partner Methanospirillum hungatei. Moreover, the 5 % 13CH4 and methane contribution result show that CO2 accomplishes directed methanogenesis. Overall, this investigation riches the roles of CO2 and biochar in AD surrounding RET.

Biological bromate reduction coupled with in situ gas fermentation in H2/CO2-based membrane biofilm reactor

Bromate, a carcinogenic contaminant generated in water disinfection, presents a pressing environmental concern. While biological bromate reduction is an effective remediation approach, its implementation often necessitates the addition of organics, incurring high operational costs. This study demonstrated the efficient biological bromate reduction using H2/CO2 mixture as the feedstock. A membrane biofilm reactor (MBfR) was used for the efficient delivery of gases. Long-term reactor operation showed a high-level bromate removal efficiency of above 95 %, yielding harmless bromide as the final product. Corresponding to the short hydraulic retention time of 0.25 d, a high bromate removal rate of 4 mg Br/L/d was achieved. During the long-term operation, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed, which can be regulated by controlling the gas flow. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms underpinning the efficient bromate removal, showing that the microbial bromate reduction was primarily driven by the VFAs produced from in situ gas fermentation. Microbial community analysis showed an increased abundance of Bacteroidota group from 4.0 % to 18.5 %, which is capable of performing syngas fermentation, and the presence of heterotrophic denitrifiers (e.g., Thauera and Brachymonas), which are known to perform bromate reduction. Together these results for the first time demonstrated the feasibility of using H2/CO2 mixture for bromate removal coupled with in situ VFAs production. The findings can facilitate the development of cost-effective strategies for groundwater and drinking water remediation.

In-situ methane enrichment in anaerobic digestion of food waste slurry by nano zero-valent iron: Long-term performance and microbial community succession

In this work, nano zero-valent iron (nZVI) was added to a lab-scale continuous stirring tank reactor (CSTR) for food waste slurry treatment, and the effect of dosing rate and dosage of nZVI were attempted to be changed. The results showed that anaerobic digestion (AD) efficiency and biomethanation stability were optimum under the daily dosing and dosage of 0.48 g/gTCOD. The average daily methane (CH4) yield reached 495.38 mL/gTCOD, which was 43.65% higher than that at control stage, and the maximum CH4 content reached 95%. However, under single dosing rate conditions, high nZVI concentrations caused microbial cell rupture and loosely bound extracellular polymeric substances (LB-EPS) precipitation degradation. The daily dosing rate promoted the hydrogenotrophic methanogenesis pathway, and the activity of coenzyme F420 increased by 400.29%. The microbial analysis indicated that daily addition of nZVI could promote the growth of acid-producing bacteria (Firmicutes and Bacteroidetes) and methanogens (Methanothrix).

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.

Integrated biogas upgrading and medium-chain fatty acids production for more efficient resource recovery

The conversion of carbon dioxide (CO2) from biogas into medium-chain fatty acids (MCFAs) represents an eco-friendly resource recovery approach to reduce dependence on fossil fuels and combat global climate change. This study presented the novel concept of integrated resource recovery by coupling biogas upgrading and MCFAs production for the first time. Initially, the impact of different initial ethanol concentrations on chain elongation was examined, determining that an ethanol concentration of 160 mmol/L maximized MCFAs yield at 45.7 mmol/L. Subsequently, using this optimal ethanol supply, the integrated strategy was implemented by connecting two bioreactors in series and maintaining continuous operation for 160-day. The results were noteworthy: upgraded bio-methane purity reached 97.6 %, MCFAs production rate and carbon-flow distribution reached 24.5 mmol/L d-1 and 76.1 %, respectively. In summary, these promising outcomes pioneer a resource recovery approach, enabling the high-value utilization of biogas and the conversion of CO2 into valuable bio-chemicals.

Simultaneous biogas upgrading and medium-chain fatty acids production using a dual membrane biofilm reactor

Utilizing H2-assisted ex-situ biogas upgrading and acetate recovery holds great promise for achieving high value utilization of biogas. However, it faces a significant challenge due to acetate's high solubility and limited economic value. To address this challenge, we propose an innovative strategy for simultaneous upgrading of biogas and the production of medium-chain fatty acids (MCFAs). A series of batch tests evaluated the strategy's efficiency under varying initial gas ratios (v/v) of H2, CH4, CO2, along with varying ethanol concentrations. The results identified the optimal conditions as initial gas ratios of 3H2:3CH4:2CO2 and an ethanol concentration of 241.2 mmol L-1, leading to maximum CH4 purity (97.2 %), MCFAs yield (54.2 ± 2.1 mmol L-1), and MCFAs carbon-flow distribution (62.3 %). Additionally, an analysis of the microbial community's response to varying conditions highlighted the crucial roles played by microorganisms such as Clostridium, Proteiniphilum, Sporanaerobacter, and Bacteroides in synergistically assimilating H2 and CO2 for MCFAs production. Furthermore, a 160-day continuous operation using a dual-membrane aerated biofilm reactor (dMBfR) was conducted. Remarkable achievements were made at a hydraulic retention time of 2 days, including an upgraded CH4 content of 96.4 ± 0.3 %, ethanol utilization ratio (URethanol) of 95.7 %, MCFAs production rate of 28.8 ± 0.3 mmol L-1 d-1, and MCFAs carbon-flow distribution of 70 ± 0.8 %. This enhancement is proved to be an efficient in biogas upgrading and MCFAs production. These results lay the foundation for maximizing the value of biogas, reducing CO2 emissions, and providing valuable insights into resource recovery.

Strain-resolved metagenomics approaches applied to biogas upgrading

Genetic heterogeneity is a common trait in microbial populations, caused by de novo mutations and changes in variant frequencies over time. Microbes can thus differ genetically within the same species and acquire different phenotypes. For instance, performance and stability of anaerobic reactors are linked to the composition of the microbiome involved in the digestion process and to the environmental parameters imposing selective pressure on the metagenome, shaping its evolution. Changes at the strain level have the potential to determine variations in microbial functions, and their characterization could provide new insight into ecological and evolutionary processes driving anaerobic digestion. In this work, single nucleotide variant dynamics were studied in two time-course biogas upgrading experiments, testing alternative carbon sources and the response to exogenous hydrogen addition. A cumulative total of 76,229 and 64,289 high-confidence single nucleotide variants were discerned in the experiments related to carbon substrate availability and hydrogen addition, respectively. By combining complementary bioinformatic approaches, the study reconstructed the precise strain count-two for both hydrogenotrophic archaea-and tracked their abundance over time, while also characterizing tens of genes under strong selection. Results in the dominant archaea revealed the presence of nearly 100 variants within genes encoding enzymes involved in hydrogenotrophic methanogenesis. In the bacterial counterparts, 119 mutations were identified across 23 genes associated with the Wood-Ljungdahl pathway, suggesting a possible impact on the syntrophic acetate-oxidation process. Strain replacement events took place in both experiments, confirming the trends suggested by the variants trajectories and providing a comprehensive understanding of the biogas upgrading microbiome at the strain level. Overall, this resolution level allowed us to reveal fine-scale evolutionary mechanisms, functional dynamics, and strain-level metabolic variation that could contribute to the selection of key species actively involved in the carbon dioxide fixation process.

Prospect of denitrifying anaerobic methane oxidation (DAMO) application on wastewater treatment and biogas recycling utilization

Old-fashioned wastewater treatments for nitrogen depend on heterotrophic denitrification process. It would utilize extra organic carbon source as electron donors when the C/N of domestic wastewater was too low to ensure heterotrophic denitrification process. It would lead to non-compliance with carbon reduction targets and impose an economic burden on wastewater treatment. Denitrifying anaerobic methane oxidation (DAMO), which could utilize methane serving as electron donors to replace traditional organic carbon (methanol or sodium acetate), supplies a novel approach for wastewater treatment. As the primary component of biogas, methane is an inexpensive carbon source. With anaerobic digestion becoming increasingly popular for sludge reduction in wastewater treatment plants (WWTPs), efficient biogas utilization through DAMO can offer an environmentally friendly option for in-situ biogas recycling. Here, we reviewed the metabolic principle and relevant research for DAMO and biogas recycling utilization, outlining the prospect of employing DAMO for wastewater treatment and biogas recycling utilization in WWTPs. The application of DAMO provides a new focal point for enhancing efficiency and sustainability in WWTPs.

Ex-situ biomethanation for CO2 valorization: State of the art, recent advances, challenges, and future prospective

Ex-situ biomethanation is an emerging technology that facilitates the use of surplus renewable electricity and valorizes carbon dioxide (CO2) for biomethane production by hydrogenotrophic methanogens. This review offers an up-to-date overview of the current state of ex-situ biomethanation and thoroughly analyzes key operational parameters affecting hydrogen (H2) gas-liquid mass transfer and biomethanation performance, along with an in-depth discussion of the technical challenges. To the best of our knowledge, this is the first review article to discuss microbial community structure in liquid and biofilm phases and their responses after exposure to H2 starvation during ex-situ biomethanation. In addition, future research in areas such as reactor configuration and optimization of operational parameters for improving the H2 mass transfer rate, inhibiting opportunistic homoacetogens, integration of membrane technology, and use of conductive packing material is recommended to overcome challenges and improve the efficiency of ex-situ biomethanation. Furthermore, this review presents a techno-economic analysis for the future development and facilitation of industrial implementation. The insights presented in this review will offer useful information to identify state-of-the-art research trends and realize the full potential of this emerging technology for CO2 utilization and biomethane production.

Integrated urban water management by coupling iron salt production and application with biogas upgrading

Integrated urban water management is a well-accepted concept for managing urban water. It requires efficient and integrated technological solutions that enable system-wide gains via a whole-of-system approach. Here, we create a solid link between the manufacturing of an iron salt, its application in an urban water system, and high-quality bioenergy recovery from wastewater. An iron-oxidising electrochemical cell is used to remove CO2 (also H2S and NH3) from biogas, thus achieving biogas upgrading, and simultaneously producing FeCO3. The subsequent dose of the electrochemically produced FeCO3 to wastewater and sludge removes sulfide and phosphate, and enhances sludge settleability and dewaterability, with comparable or superior performance compared to the imported and hazardous iron salts it substitutes (FeCl2, and FeCl3). The process enables water utilities to establish a self-reliant and more secure supply chain to meet its demand for iron salts, at lower economic and environmental costs, and simultaneously achieve recovery of high-quality bioenergy.

Impact of electrochemically generated iron on the performance of an anaerobic wastewater treatment process

Anaerobic treatment of domestic wastewater has the advantages of lower biomass yield, lower energy demand and higher energy recover over the conventional aerobic treatment process. However, the anaerobic process has the inherent issues of excessive phosphate and sulfide in effluent and superfluous H₂S and CO₂ in biogas. An electrochemical method allowing for in-situ generation of Fe²⁺ in the anode and hydroxide ion (OH^-) and H₂ in the cathode was proposed to overcome the challenges simultaneously. The effect of electrochemically generated iron (e‑iron) on the performance of anaerobic wastewater treatment process was explored with four different dosages in this work. The results showed that compared to control, the experimental system displayed an increase of 13.4-28.4 % in COD removal efficiency, 12.0-21.3 % in CH₄ production rate, 79.8-98.5 % in dissolved sulfide reduction, 26.0-96.0 % in phosphate removal efficiency, depending on the e‑iron dosage between 40 and 200 mg Fe/L. Dosing of the e‑iron significantly upgraded the quality of produced biogas, showing a much lower CO₂ and H₂S contents in biogas in experimental reactor than that in control reactor. The results thus demonstrated that e‑iron can significantly improve the performance of anaerobic wastewater treatment process, bringing multiple benefits with the increase of its dosage regarding effluent and biogas quality.

Recovery of methane and acetate during ex-situ biogas upgrading via novel dual-membrane aerated biofilm reactor

Biological biogas upgrading has been well-proven to be a promising approach for renewable bioenergy recovery, but hydrogen (H₂)-assisted ex-situ biogas upgrading is hindered by a large solubility discrepancy between H₂ and carbon dioxide (CO₂). This study established a new dual-membrane aerated biofilm reactor (dMBfR) to improve the upgrading efficiency. Results showed that dMBfR operated at 1.25 atm H₂ partial pressure, 1.5 atm biogas partial pressure, and 1.0 d hydraulic retention time could significantly improve the efficiency. The maximum methane purity of 97.6%, acetate production rate of 34.5 mmol L^-1d^-1, and H₂ and CO₂ utilization ratios of 96.5% and 96.3% were achieved. Further analysis showed that the improved performances of biogas upgrading and acetate recovery were positively correlated with the total abundances of functional microorganisms. Taken together, these results suggest that the dMBfR, which facilitates the precise CO₂ and H₂ supply, is an ideal approach for efficient biological biogas upgrading.

Novel Use of a Ferric Salt to Enhance Mainstream Nitrogen Removal from Anaerobically Pretreated Wastewater

This study aims to demonstrate a new technology roadmap to support the ongoing paradigm shift in wastewater management from pollutant removal to resource recovery. This is achieved by developing a novel use of an iron salt (i.e., FeCl₃) in an integrated anaerobic wastewater treatment and mainstream anammox process. FeCl₃ was chosen to be dosed in a proposed sidestream unit rather than in a primary settler or a mainstream reactor. This causes acidification of returned activated sludge and enables stable suppression of nitrite-oxidizing bacterial activity and excess sludge reduction. A laboratory-scale system, which comprised an anaerobic baffled reactor, a continuous-flow anoxic-aerobic (A/O) reactor, and a secondary settler, was designed to treat real domestic wastewater, with the performance of the system comprehensively monitored under a steady-state condition. The experimental assessments showed that the system had good effluent quality, with total nitrogen and phosphorus concentrations of 12.6 ± 1.3 mg N/L and 0.34 ± 0.05 mg P/L, respectively. It efficiently retained phosphorus in excess sludge (0.18 ± 0.03 g P/g dry sludge), suggesting its potential for further recovery. About half of influent organic carbon was recovered in the form of bioenergy (i.e., methane). This together with low energy consumption revealed that the system could produce a net energy of about 0.11 kWh/m³-wastewater, assessed by an energy balance analysis.

Elucidation of microbial interactions, dynamics, and keystone microbes in high pressure anaerobic digestion

High-pressure anaerobic digestion (HPAD) is a promising technology for producing biogas enriched with high methane content in a single-step process. To enhance HPAD performance, a comprehensive understanding of microbial community dynamics and their interactions is essential. For this, mesophilic batch high-pressurized anaerobic reactors were operated under 3 bars (H3) and 6 bars (H6). The experimental results showed that the effect of high-pressure (up to 6 bar) on acidification was negligible while methanogenesis was significantly delayed. Microbial analysis showed the predominance of Defluviitoga affiliated with the phylum Thermotogae and the reduction of Thiopseudomonas under high-pressure conditions. In addition, the microbial cluster pattern in H3 and H6 was significantly different compared to the CR, indicating a clear shift in microbial community structure. Moreover, Methanobacterium, Methanomicrobiaceae, Alkaliphilus, and Petrimonas were strongly correlated in network analysis, and they could be identified as keystone microbes in the HPAD reactor.

Synergistic effect of hydrogen and nanoscale zero-valent iron on ex-situ biogas upgrading and acetate recovery

Hydrogen (H₂) assisted ex-situ biogas upgrading and liquid chemicals production can augment the fossil fuel-dominated energy market, and alleviate CO₂-induced global warming. Recent investigations confirmed that nanoscale zero-valent iron (nZVI) enabled the enhancement of anaerobic digestion for biogas production. However, little is known about the effect of nZVI on the downstream ex-situ biogas upgrading. Herein, different levels (0 mg L^-1, 100 mg L^-1, 200 mg L^-1, 500 mg L^-1, 1000 mg L^-1, 2000 mg L^-1) of nZVI were added for H₂-assisted ex-situ biogas upgrading, to study whether nZVI could impact the biomethane purity and acetate yield for the first time. Results showed that all tested nZVI levels were favorable for biogas upgrading in the presence of H₂, the highest biomethane content (94.1 %, v/v), the CO₂ utilization ratio (95.9 %), and acetate yield (19.4 mmol L^-1) were achieved at 500 mg L^-1 nZVI, respectively. Further analysis indicated that increased biogas upgrading efficiency was related to an increase in extracellular polymeric substances, which ensures the microbial activity and stability of the ex-situ biogas upgrading. Microbial community characterization showed that the Petrimonas, Romboutsia, Acidaminococcus, and Clostridium predominated the microbiome during biogas upgrading at 500 mg L^-1 nZVI with H₂ supply. These results suggested that nZVI and H₂ contributed jointly to promoting the bioconversion of CO₂ in biogas to acetate. The findings could be helpful for paving a new way for efficient simultaneous ex-situ biogas upgrading and liquid chemicals recovery.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Bioconversion of H2 and CO2 from dark fermentation to methane: Effect of operating conditions on methane concentration

The main goal of this study was to assess the methane production in a biotrickling filter (BTF) using a synthetic gas mixture (H₂/CO₂: 60/40), evaluating the effect of the empty bed gas residence time (EBRT), pH, and temperature. The BTF was inoculated with acclimated granular anaerobic sludge. Three EBRT were tested: 11.6, 5.8, and 2.9 h. The decrease in EBRT (from 11.6 to 5.8 h) increased 1.3-fold the methane content (69 ± 3%) with H₂ and CO₂ removals of 100% and 24 ± 6%, respectively. The following reduction to 2.9 h showed no effect on CH₄ content. The increment of the pH had no significant effect; however, the highest CH₄ percentage (74%) was observed at a pH of 8.5. The system showed flexibility to adapt to changes in temperature without drastically diminishing CH₄ concentration. In these stages, the principal hydrogenotrophic archaea detected was Methanobacterium flexile. Soluble microbial products such as butanol, caproate, and iso-valerate were detected in all the operating stages. This study demonstrates the potential of methane generation from a dark fermentation gaseous effluent.

Overview of recent progress in exogenous hydrogen supply biogas upgrading and future perspective

With the rapid development of renewable and sustainable energy, biogas upgrading for producing high-quality biomethane as an alternative to natural gas has attracted worldwide attention. This paper comprehensively reviews the current state of biogas upgrading technologies. The advances in physicochemical, photosynthetic autotrophic, and chemical autotrophic biogas upgrading technologies are briefly described with particular attention to the key challenges. New chemical autotrophic biogas upgrading strategies, such as direct and indirect exogenous hydrogen supply, for overcoming barriers to biogas upgrading and realizing highly efficient bioconversion of carbon dioxide are summarized. For each approach to exogenous hydrogen supply for biogas upgrading, the key findings and technical limitations are summarized and critically analyzed. Finally, future developments are also discussed to provide a reference for the development of biogas upgrading technology that can address the global energy crisis and climate change issues related to the application of biogas.

Constructing Dual-Transport Pathways by Incorporating Beaded Nanofillers in Mixed Matrix Membranes for Efficient CO2 Separation

Mixed matrix membranes (MMMs) have attracted significant attention in the field of CO₂ separation because MMMs have potential to overcome an undesirable "trade-off" effect. In this study, the beaded nanofillers of ZIF-8@aminoclay (ZIF-8@AC) were synthesized using an in situ growth method, and they were doped into a Pebax MH 1657 (Pebax) matrix to fabricate MMMs for efficient CO₂ separation. The beaded structure was formed by ZIF-8 particles joined together during the process of AC coating on the ZIF-8 surface. ZIF-8@AC played a vital role in the improvement of gas separation performance. It was mainly attributed to the following reasons: First, the inherent micropores of ZIF-8 constructed the internal pathways for gas transport in the beaded nanofillers, benefiting the improvement of gas permeability. Second, the staggered AC layers constructed the external pathways for gas transport in the beaded nanofillers, increasing the tortuosity of gas transport for larger molecules and favoring the improvement of gas selectivity. Therefore, the internal and external pathways of ZIF-8@AC co-constructed the dual-transport pathways for CO₂ transport in MMMs. In addition, the abundant amino groups of the beaded nanofillers provided abounding carriers for CO₂, facilitating CO₂ transport in the dual-transport pathways. Therefore, the CO₂ separation performance of Pebax/ZIF-8@AC-1 MMMs was significantly improved. The CO₂ permeability and CO₂/CH₄ separation factor of Pebax/ZIF-8@AC-1-7 MMM were 620 ± 10 Barrer and 40 ± 0.4, which were 2.3 and 1.6 times those of a pure Pebax membrane, respectively. Furthermore, the CO₂/CH₄ separation performance of Pebax/ZIF-8@AC-1-7 MMM overcame successfully the "trade-off" effect and approached the 2019 upper bound. It is a novel strategy to design a beaded nanofiller doped into MMMs for carbon capture.

In-situ biogas upgrading with H2 addition in an anaerobic membrane bioreactor (AnMBR) digesting waste activated sludge

Biological in-situ biogas upgrading is a promising approach for sustainable energy-powered technologies. This method increases the CH₄ content in biogas via hydrogenotrophic methanogenesis with an external H₂ supply. In this study, an anaerobic membrane bioreactor (AnMBR) was employed for in-situ biogas upgrading. The AnMBR was operated in semi-batch mode using waste activated sludge as the substrate. Pulsed H₂ addition into the reactor and biogas recirculation effectively increased the CH₄ content in the biogas. The addition of 4 equivalents of H₂ relative to CO₂ did not lead to appreciable biogas upgrading, although the acetate concentration increased significantly. When 11 equivalents of H₂ were introduced, the biogas was successfully upgraded, and the CH₄ content increased to 92%. The CH₄ yield and CH₄ production rate were 0.31 L/g-VS_input and 0.086 L/L/d, respectively. In this phase of the process, H₂ addition increased the acetate concentration and the pH because of CO₂ depletion. Compared with a continuously-stirred tank reactor, the AnMBR system attained higher CH₄ content, even without the addition of H₂. The longer solid retention time (100 d) in the AnMBR led to greater degradation of volatile solids. Severe membrane fouling was not observed, and the transmembrane pressure remained stable under 10 kPa for 117 d of continuous filtration without cleaning of the membrane. The AnMBR could be a promising reactor configuration to achieve in-situ biogas upgrading during sludge digestion.

A review on biogas upgrading in anaerobic digestion systems treating organic solids and wastewaters via biogas recirculation

Biogas upgrading is an essential process for efficient and safe utilization of biogas produced from anaerobic digestion (AD), a cost-effective and environmentally friendly technology for bioenergy recovery from organic wastes. Biogas recirculation in AD reactors has been recently reported as a cost-effective and promising method to enhance methane content in biogas. This review aimed to summarize the state-of-the-art of biogas recirculation-based AD systems to better understand the possible mechanisms and main factors relating to in-situ biogas upgrading. It shows that biogas recirculation in the AD reactor can not only enhance methane content via both physicochemical and biological effects, but also help establish a robust AD system with high buffering capacity for highly efficient treatment of various organic wastes. More research works are demanding for a better understanding of the mechanisms and the optimization of the whole AD system, targeting its further development for high-calorie bioenergy production.

In situ biogas upgrading via cathodic biohydrogen using mitigated ammonia nitrogen during the anaerobic digestion of Taihu blue algae in an integrated bioelectrochemical system (BES)

Biohydrogen using migrated ammonia as nitrogen source, and biogas upgrading with hydrogen collected at biocathode in an integrated bioelectrochemical system (BES) were investigated, during the anaerobic digestion of Taihu blue algae. Under an applied voltage of 0.4 V, biohydrogen (202.87 mL) reached 2.34 and 2.90 times than groups with applied voltage of 0 V and 0.8 V, respectively. Moreover, biohydrogen of the group with 1000 mg/L initial ammonia addition (524.16 mL) reached 1.53 times than that the of the control. With 0.25 bar of H₂ injected at the beginning (R1), highest methane production (286.62) mL and content (75.73%) were obtained. Comparing to other groups, not only microbial genus responsible for both aceticlastic and hydrogenotrophic methanogens of the group R1 were apparently enriched, but key enzymes related to methane production also acquired better abundances. Therefore, it's promising to conduct the ammonia alleviating, hydrogen producing and biogas upgrading simultaneously using BES.