Continuous biomethanation of flue gas-carbon dioxide using bio-integrated carbon capture and utilization

Biomethanation of carbon dioxide (CO2) from flue gas is a potential enabler of the green transition, particularly when integrated with the power-to-gas chain. However, challenges arise in achieving synthetic natural gas quality when utilizing CO2 from diluted carbon sources, and the high costs of CO2 separation using amine-based solutions make large-scale implementation unfeasible. We propose an innovative continuous biomethanation system that integrates carbon capture and CO2 stripping through microbial utilization, eliminating expenses with the stripper. Stable continuous biomethane production (83-92 % methane purity) was achieved from flue gas-CO2 using a biocompatible aqueous n-methyldiethanolamine (MDEA) solution (50 mmol/L) under mesophilic and hydrogen-limiting conditions. MDEA was found to be recalcitrant to biodegradation and could be reused after regeneration. Demonstrating the microbial ability to simultaneously strip and convert the captured CO2 and regenerate MDEA provides a new pathway for valorization of flue gas CO2.

The Selenoproteome as a Dynamic Response Mechanism to Oxidative Stress in Hydrogenotrophic Methanogenic Communities

Methanogenesis is a critical process in the carbon cycle that is applied industrially in anaerobic digestion and biogas production. While naturally occurring in diverse environments, methanogenesis requires anaerobic and reduced conditions, although varying degrees of oxygen tolerance have been described. Microaeration is suggested as the next step to increase methane production and improve hydrolysis in digestion processes; therefore, a deeper understanding of the methanogenic response to oxygen stress is needed. To explore the drivers of oxygen tolerance in methanogenesis, two parallel enrichments were performed under the addition of H2/CO2 in an environment without reducing agents and in a redox-buffered environment by adding redox mediator 9,10-anthraquinone-2,7-disulfonate disodium. The cellular response to oxidative conditions is mapped using proteomic analysis. The resulting community showed remarkable tolerance to high-redox environments and was unperturbed in its methane production. Next to the expression of pathways to mitigate reactive oxygen species, the higher redox potential environment showed an increased presence of selenocysteine and selenium-associated pathways. By including sulfur-to-selenium mass shifts in a proteomic database search, we provide the first evidence of the dynamic and large-scale incorporation of selenocysteine as a response to oxidative stress in hydrogenotrophic methanogenesis and the presence of a dynamic selenoproteome.

Reducing greenhouse gas emissions from pig slurry by acidification with organic and inorganic acids

Methane (CH₄) emission from pig slurry is a large contributor to the climate footprint of livestock production. Acidification of excreta from livestock animals with sulfuric acid, reduce CH₄ emission and is practiced at many Danish farms. Possible interaction effects with other acidic agents or management practices (e.g. frequent slurry removal and residual slurry acidification) have not been fully investigated. Here we assessed the effect of pig slurry acidification with a range of organic and inorganic acids with respect to their CH₄ inhibitor potential in several batch experiments (BS). After careful selection of promising CH₄ inhibitors, three continuous headspace experiments (CHS) were carried out to simulate management of manure in pig houses. In BS experiments, more than <99% CH₄ reduction was observed with HNO₃ treatment to pH 5.5. Treatments with HNO₃, H₂SO₄, and H₃PO₄ reduced CH₄ production more than acetic acid and other organic acids when acidified to the same initial pH of 5.5. Synergistic effects were not observed when mixing inorganic and organic acids as otherwise proposed in the literature, which was attributed to the high amount of acetic acid in the slurry to start with. In the CHS experiments, HNO₃ treatment reduced CH₄ more than H₂SO₄, but increased nitrous oxide (N₂O) emission, particularly when the acidification target pH was above 6, suggesting considerable denitrification activity. Due to increased N₂O emission from HNO₃ treatments, HNO₃ reduced total CO₂-eq by 67%, whereas H₂SO₄ reduced CO₂-eq by 91.5% compared to untreated slurry. In experiments with daily slurry addition, weekly slurry removal, and residual acidification, HNO₃ and H₂SO₄ treatments reduced CO₂-eq by 27% and 48%, respectively (not significant). More cycles of residual acidification are recommended in future research. The study provides solid evidence that HNO₃ treatment is not suitable for reducing CO₂-eq and H₂SO₄ should be the preferred acidic agent for slurry acidification.

Hydrodynamic analysis of full-scale in-situ biogas upgrading in manure digesters

The addition of hydrogen to anaerobic digesters is an emerging technique for the sustainable upgrading of biogas to biomethane with renewable electricity. However, it is critically dependent on the effective gas-liquid transfer of hydrogen, which is a sparingly soluble gas. Very little is known about the impact of liquid and gas flow and bubble size on gas-liquid transfer during H₂ injection in full-scale anaerobic digesters. A computational fluid dynamic model was developed using a two-fluid approach for non-Newtonian liquid in the open-source computational fluid dynamics (CFD) platform, OpenFOAM. The newly developed model was validated against published experimental data-sets of a gas-mixed, laboratory-scale anaerobic digester, with good agreement between the numerical and experimental velocity fields. The hydrodynamics of the full-scale in-situ biomethanation system using venturi ejectors for H₂ injection was then simulated to investigate gas-liquid dynamics, including gas-liquid mass transfer, at different operational conditions. Gas-liquid mixing is mainly controlled by the gas-plumes interaction, which promotes turbulence at the interaction zone, resulting in increasing gas bubbles mixing with the liquid and the gas-liquid interfacial area. However, beyond the plume interaction zone, the digester had flow short-circuiting and inactive zones. It was found that, due to this short-circuiting behaviour, an increase in gas flow-rate may not be an effective option in reducing inactive zones, although it can increase the gas-liquid interfacial area. Comparative analysis of the impact of gas flow and bubble size indicated that gas flow had a linear effect on both kLa and gas holdup, but that bubble size had a non-linear impact, with higher kLa values achieved at bubble sizes less than 2 mm. Comparison against measured data in the same system indicated the predicted kLa values were at the same level as measured kLa, at a bubble size of 2 mm.

Sporofaciens musculi gen. nov., sp. nov., a novel bacterium isolated from the caecum of an obese mouse

A bacterial strain, designated WCA-9-b2^T, was isolated from the caecal content of an 18-week-old obese C57BL/6NTac male mouse. According to phenotypic analyses, the isolate was rod-shaped, strictly anaerobic, spore-forming, non-motile and Gram-stain-positive, under the conditions tested. Colonies were irregular and non-pigmented. Analysis of the 16S rRNA gene sequence indicated that the isolate belonged to the order Clostridiales with Dorea longicatena ATCC 27755^T (94.9 % sequence identity), Ruminococcus gnavus ATCC 29149^T (94.8%) and Clostridium scindens ATCC 35704^T (94.3%) being the closest relatives. Whole genome sequencing showed an average nucleotide identity <74.23 %, average amino acid identity <64.52-74.67 % and percentage of conserved proteins values <50 % against the nine closest relatives (D. longicatena, Ruminococcus gnavus, C. scindens, Dorea formicigenerans, Ruminococcus lactaris, Clostridium hylemonae, Merdimonas faecis, Faecalicatena contorta and Faecalicatena fissicatena). The genome-based G+C content of genomic DNA was 44.4 mol%. The major cellular fatty acids were C_16 : 0 (24.5%), C_18 : 1 cis9 (19.8 %), C_16 : 0 DMA (11.7%), C_18 : 0 (8.4%) and C_14 : 0 (6.6%). Respiratory quinones were not detected. The predominant metabolic end products of glucose fermentation were acetate and succinate. Production of CO₂ and H₂ were detected. Based on these data, we propose that strain WCA-9-b2^T represents a novel species within a novel genus, for which the name Sporofaciens musculi gen. nov., sp. nov. is proposed. The type strain is WCA-9-b2^T (=DSM 106039^T=CECT 30156^T).

Biogas upgrading with hydrogenotrophic methanogenic biofilms

Hydrogen produced from periodic excess of electrical energy may be added to biogas reactors where it is converted to CH₄ that can be utilized in the existing energy grid. The major challenge with this technology is gas-to-liquid mass transfer limitation. The microbial conversions in reactors designed for hydrogenotrophic methanogenesis were studied with microsensors for H₂, pH, and CO₂. The H₂ consumption potential was dependent on the CO₂ concentration, but could partially recover after CO₂ depletion. Reactors with 3-dimensional biofilm carrier material and a large gas headspace allowed for a methanogenic biofilm in direct contact with the gas phase. A high density of Methanoculleus sp. in the biofilm mediated a high rate of CH₄ production, and it was calculated that a reactor filled with 75% carrier material could mediate a biogas upgrading from 50 to 95% CH₄ within 24 h when an equivalent amount of H₂ was added.

Chironomus plumosus larvae increase fluxes of denitrification products and diversity of nitrate-reducing bacteria in freshwater sediment

Benthic invertebrates affect microbial processes and communities in freshwater sediment by enhancing sediment-water solute fluxes and by grazing on bacteria. Using microcosms, the effects of larvae of the widespread midge Chironomus plumosus on the efflux of denitrification products (N2O and N2+N2O) and the diversity and abundance of nitrate- and nitrous-oxide-reducing bacteria were investigated. Additionally, the diversity of actively nitrate- and nitrous-oxide-reducing bacteria was analyzed in the larval gut. The presence of larvae increased the total effluxes of N2O and N2+N2O up to 8.6- and 4.2-fold, respectively, which was mostly due to stimulation of sedimentary denitrification; incomplete denitrification in the guts accounted for up to 20% of the N2O efflux. Phylotype richness of the nitrate reductase gene narG was significantly higher in sediment with than without larvae. In the gut, 47 narG phylotypes were found expressed, which may contribute to higher phylotype richness in colonized sediment. In contrast, phylotype richness of the nitrous oxide reductase gene nosZ was unaffected by the presence of larvae and very few nosZ phylotypes were expressed in the gut. Gene abundance of neither narG, nor nosZ was different in sediments with and without larvae. Hence, C. plumosus increases activity and diversity, but not overall abundance of nitrate-reducing bacteria, probably by providing additional ecological niches in its burrow and gut.

Succession of Deferribacteres and Epsilonproteobacteria through a nitrate-treated high-temperature oil production facility

Members of Epsilonproteobacteria and Deferribacteres have been implied in nitrate-induced souring control in high-temperature oil production facilities. Here we report on their diversity and abundance in the injection and production part of a nitrate-treated, off-shore oil facility (Halfdan, Denmark) and aimed to assess their potential in souring control. Nitrate addition to deoxygenated seawater shifted the low-biomass seawater community dominated by Gammaproteobacteria closely affiliated with the genus Colwellia to a high-biomass community with significantly higher species richness. Epsilonproteobacteria accounted for less than 1% of the total bacterial community in the nitrate-amended injection water and were most likely outcompeted by putative nitrate-reducing, methylotrophic Gammaproteobacteria of the genus Methylophaga. Reservoir passage and recovery of the oil resulted in a significant change in the bacterial community. Members of the thermophilic Deferribacteres were the second major fraction of the bacterial community in the production water (~30% of the total bacterial community). They were not found in the injection water and were therefore assumed to be indigenous to the reservoir. Additional diversity analysis and targeted quantification of periplasmic nitrate reductase (napA) genes indicated that most resident Deferribacteres possessed the functional potential to contribute to nitrate reduction in the system. In sum, the dominance of nitrate-reducing Deferribacteres and the low relative abundance of Epsilonproteobacteria throughout the production facility suggested that the Deferribacteres play a major role in nitrate-induced souring control at high temperatures.