Biogas production is an established sustainable process for simultaneous generation of renewable energy and treatment of organic wastes. The increasing interest of utilizing biogas as substitute to ...natural gas or its exploitation as transport fuel opened new avenues in the development of biogas upgrading techniques. The present work is a critical review that summarizes state-of-the-art technologies for biogas upgrading and enhancement with particular attention to the emerging biological methanation processes. The review includes comprehensive description of the main principles of various biogas upgrading methodologies, scientific and technical outcomes related to their biomethanation efficiency, challenges that have to be addressed for further development and incentives and feasibility of the upgrading concepts.
•Data related to the development of commercial biogas upgrading plants are provided.•Physicochemical technologies for biogas upgrade are reviewed.•Biological biogas upgrading technologies are extensively presented.•Incentives and feasibility of biogas upgrading are discussed.•Biomethanation efficiencies from different upgrading technologies are compared.
Anaerobic digestion (AD) of waste activated sludge (WAS) has been widely used, while it poses problems including low methane yield and production rate. Hydrochar is produced by hydrothermal ...liquefaction of biomass; however, little is known about the role of hydrochar in promoting AD of WAS. The present study showed that hydrochar increased the methane production rate by 30.8% and yield by 31.4% of hydrothermal pretreated dewatered WAS. Hydrochar increased the methane production rate and yield by enhancing the acidification and methanogenesis processes. Genomic-centric metatranscriptomics were used to identify the metabolic activities and transcriptomic response of individual metagenome-assembled genomes that were enriched by hydrochar. Although Methanosarcina sp. FDU0106 had been shown unable to used H2, it had the complete pathway for the reduction of CO2 to methane. Syntrophomonas sp. FDU0164 expressed genes for extracellular electron transfer via electrically pili, suggesting that Syntrophomonas sp. FDU0164 and Methanosarcina sp. FDU0106 were exchanging electrons via direct interspecies electron transfer. The expression of pili was decreased, indicating that hydrochar could replace its roles. Additionally, Firmicutes sp. FDU0048, Proteiniphilum sp. FDU0082, and Aminobacterium mobile FDU0089 were related to the degradation of organics, which could be related to the enhanced methane yield.
The aim of this work was to elucidate the microbial ecology in twelve mesophilic and thermophilic full-scale biogas plants using a genome-centric metagenomic approach. In this study both biogas ...plants treating manure and those treating sludge from waste water treatment plants were considered. The identification of 132 Metagenome-Assembled Genomes (MAGs) and analysis of their abundance profile in different samples allowed the identification of the most abundant core members of the anaerobic digestion microbiome. Canonical correspondence analysis was used to determine the influence of biotic and environmental factors on MAGs abundance and to investigate the methanogenic performance of the biogas plants. Prediction of the functional properties of MAGs was obtained analyzing their KEGG pathways and their carbohydrate active domains. Network analysis allowed investigation of species-species associations and shed light on syntrophic interactions between members belonging to the anaerobic digestion dark matter (phylum Fermentibacteria). By stratifying and comparing different levels of information, it was predicted that some MAGs have a crucial role in the manure-supplemented thermophilic biogas plants and it was highlighted the importance of the glycine cleavage system in complementing the “truncated” Wood-Ljungdahl pathway.
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•Metagenomic binning identified core MAGs of the anaerobic digestion microbiome.•CCA revealed influence of biotic and environmental factors on MAGs abundance.•Functional properties of core MAGs were analyzed.•Network analysis revealed the syntrophic interactions between MAGs.
In typical anaerobic digestion (AD) systems, the microbial functional assertion is hampered by synchronised versatile metabolism required for heterogeneous substrates degradation. Thus, the intricate ...methanogenic process from organic compounds remains an enigma after decades of empirical operation. In this study, simplified AD microbial communities were obtained with substrate specifications and continuous reactor operation. Genome-centric metagenomic approach was followed to holistically investigate the metabolic pathways of the AD and the microbial synergistic networks. In total, 63 metagenome assembled genomes (MAGs) were assembled from 8 metagenomes acquired in specific methanogenic niches. The metabolic pathways were reconstructed from the annotated genes and their dynamicity under experimental conditions. The results show that the methanogenic niches nourish unique metabolism beyond current knowledge acquired from cultivation-based methods. A novel glucose mineralization model without acetate formation was proposed and asserted in a pair of syntrophs: Clostridiaceae sp. and Methanoculleus thermophilus. Moreover, the catabolic pathway was elucidated in uncharacterized syntrophic acetate oxidizers, Synergistaceae spp. A remarkable evolutionary insight is the discovery that electron transport and energy conservation mechanisms impose selective pressure on syntrophic partners. Overall, the functional roles of the individual microbes tightly rely on the catabolic pathways and cannot always be physiologically defined in accordance with conventional four-step AD concept. The substrate-specific systems provided a traceable microbial community to dissecting the AD process. The genome-centric metagenomics successfully constructed genomes of microbes that have not been previously isolated and illustrated metabolic pathways that beyond the current knowledge of AD process. This study provides new perspectives to unravel the AD microbial ecology and suggests more attention should be paid on uncharacterized metabolism specifically harboured by AD microbial communities.
•Genome-centric metagenomics was used to unveil the anaerobic digestion process.•Traceable microbial communities were obtained with substrate specifications.•The metabolisms were dissected by stepwise simplified substrates.•A novel glucose mineralization model without acetate formation was proposed.•Electron transfer mechanisms are key factors of selecting syntrophic partners.
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•The bioaugmentation with Methanoculleus sp. culture remarkably enhanced the methane yield by 21%•The volatile fatty acids were decreased by 10% compared to the period before ...bioaugmentation.•Peptococcaceae and Tissierellales were dominating the microbiome as glucose degraders.•Syntrophaceticus was the key acetate oxidizer under the extremely high level of 13.5 NH4+-N/L.•Multiple glucose/acetate metabolism were reconstructed by metagenomic approach.
The bioaugmentation is crucial to improve the energy-efficient process for anaerobic digestion of organic wastes at high ammonia levels. Genomic insights into the intricate microbial networks at a high ammonia level remain underexplored. The present study showed that the addition of Methanoculleus sp. DTU887 remarkably enhanced the methane production yield of organic fraction of municipal solid waste by 21% and decreased the volatile fatty acids by 10% when compared to the period before bioaugmentation. Genome-centric metagenomics reports the functional contribution of microbial members during organic waste degradation under the extremely high level of 13.5 g NH4+-N/L. Specifically, metabolic reconstruction revealed that these organisms have the potential to perform fermentative and acetogenic catabolism, a process facilitated by energy conservation-related with H2/CO2 metabolism. Peptococcaceae spp. (DTU903, DTU900, and DTU895). and Tissierellales sp. DTU879 could degrade the organic waste hydrolysis product, i.e., glucose to acetate and H2. Tissierellales sp. DTU879 and Syntrophaceticus sp. DTU783 could degrade the derived acetate. The H2 scavenging Methanoculleus sp. DTU887 performs complementary metabolic reactions with Peptococcaceae spp., Tissierellales sp. and Syntrophaceticus sp., indicating syntrophic glucose and acetate degradation. This research offers the first insight that the key organisms form a syntrophy-supported food web in response to the bioaugmentation with ammonia tolerant methanogens performed in an AD system subjected to severe ammonia inhibition.
This study evaluated the process performance and determined the microbial community structure of two lab-scale thermophilic trickling biofilter reactors used for biological methanation of hydrogen ...and carbon-dioxide for a total period of 94 days. Stable and robust operation was achieved by means of a single-pass gas flow. The quality of the output gas (>97%) was comparable to the methane purity achieved by commercial biogas upgrading systems fulfilling the specifications to be used as substitute to natural gas. The reactors' methane productivity reached >1.7 LCH4/(LR·d) at hydrogen loading rate of 7.2 LH2/(LR·d). The spatial distribution of the microbial consortia localized in the liquid media and biofilm enabled us to gain a deeper understanding on how the microbiome is structured inside the trickling biofilter. Sequencing results revealed a significant predominance of Methanothermobacter sp. in the biofilm. Unknown members of the class Clostridia were highly abundant in biofilm and liquid media, while acetate utilising bacteria predominated in liquid samples.
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•Biological biogas upgrading process was evaluated in trickling biofilter reactors.•Gas input was directed in concurrent or countercurrent flow of the trickling media.•Stable and robust operation was achieved by means of a single-pass gas flow.•Methane concentration in output gas was higher than 97%.•Hydrogenotrophic methanogens were localized in biofilm.
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•Ex situ biogas upgrading to 96% CH4 is achieved in thermophilic up-flow reactors.•The totality of H2 and CO2 is converted to CH4 reaching 0.25 LCH4/LH2 CH4 yield.•Higher gas ...recirculation and diffusers’ pore size improved gas-liquid contact.•H2 selectively enhances hydrogenotrophic methanogens and syntrophic bacteria.
This study evaluates the efficiency of four novel up-flow reactors for ex situ biogas upgrading converting externally provided CO2 and H2 to CH4, via hydrogenotrophic methanogenesis. The gases were injected through stainless steel diffusers combined with alumina ceramic sponge or through alumina ceramic membranes. Pore size, input gas loading and gas recirculation flow rate were modulated to optimize gas-liquid mass transfer, and thus methanation efficiency. Results showed that larger pore size diffusion devices achieved the best kinetics and output-gas quality converting all the injected H2 and CO2, up to 3.6L/LREACTOR·d H2 loading rate. Specifically, reactors’ CH4 content increased from 23 to 96% and the CH4 yield reached 0.25LCH4/LH2. High throughput 16S rRNA gene sequencing revealed predominance of bacteria belonging to Anaerobaculum genus and to uncultured order MBA08. Additionally, the massive increase of hydrogenotrophic methanogens, such as Methanothermobacter thermautotrophicus, and syntrophic bacteria demonstrates the selection-effect of H2 on community composition.
Methanogenesis, a biological process mediated by complex microbial communities, has attracted great attention due to its contribution to global warming and potential in biotechnological applications. ...The current study unveiled the core microbial methanogenic metabolisms in anaerobic vessel ecosystems by applying combined genome-centric metagenomics and metatranscriptomics. Here, we demonstrate that an enriched natural system, fueled only with acetate, could support a bacteria-dominated microbiota employing a multi-trophic methanogenic process. Moreover, significant changes, in terms of microbial structure and function, were recorded after the system was supplemented with additional H
. Methanosarcina thermophila, the predominant methanogen prior to H
addition, simultaneously performed acetoclastic, hydrogenotrophic, and methylotrophic methanogenesis. The methanogenic pattern changed after the addition of H
, which immediately stimulated Methanomicrobia-activity and was followed by a slow enrichment of Methanobacteria members. Interestingly, the essential genes involved in the Wood-Ljungdahl pathway were not expressed in bacterial members. The high expression of a glycine cleavage system indicated the activation of alternative metabolic pathways for acetate metabolism, which were reconstructed in the most abundant bacterial genomes. Moreover, as evidenced by predicted auxotrophies, we propose that specific microbes of the community were forming symbiotic relationships, thus reducing the biosynthetic burden of individual members. These results provide new information that will facilitate future microbial ecology studies of interspecies competition and symbiosis in methanogenic niches. Video abstract.
In the present study, RNA-sequencing was used to elucidate the change of anaerobic digestion metatranscriptome after long chain fatty acids (oleate) exposure. To explore the general transcriptional ...behavior of the microbiome, the analysis was first performed on shotgun reads without considering a reference metagenome. As a second step, RNA reads were aligned on the genes encoded by the microbial community, revealing the expression of more than 51 000 different transcripts. The present study is the first research which was able to dissect the transcriptional behavior at a single species level by considering the 106 microbial genomes previously identified. The exploration of the metabolic pathways confirmed the importance of Syntrophomonas species in fatty acids degradation, and also highlighted the presence of protective mechanisms toward the long chain fatty acid effects in bacteria belonging to Clostridiales, Rykenellaceae, and in species of the genera Halothermothrix and Anaerobaculum. Additionally, an interesting transcriptional activation of the chemotaxis genes was evidenced in seven species belonging to Clostridia, Halothermothrix, and Tepidanaerobacter. Surprisingly, methanogens revealed a very versatile behavior different from each other, even among similar species of the Methanoculleus genus, while a strong increase of the expression level in Methanosarcina sp. was evidenced after oleate addition.
The process stability of biogas plants is often deteriorated by the accumulation of Long Chain Fatty Acids (LCFA). The microbial community shifts due to LCFA disturbances have been poorly understood ...as the molecular techniques used were not able to identify the genome characteristics of uncultured microorganisms, and additionally, the presence of limited number of reference genomes in public databases prevented the comprehension of specific functional roles characterizing these microorganisms. The present study is the first research which deciphers by means of high throughput shotgun sequencing the dynamics of the microbial community during an inhibitory shock load induced by single pulses of unsaturated LCFA at two different concentrations (i.e. 2 g/L-reactor and 3 g/L-reactor). The metagenomic analysis showed that only the microbes associated with LCFA degradation could encode proteins related to "chemotaxis" and "flagellar assembly", which promoted the ability to move towards the LCFA sources so as to degrade them. Moreover, the syntrophic interactions found between Syntrophomonas sp. together with Methanosarcina sp. were possibly assigned to the menaquinone-electron transfer. Finally, it was proven that a previously exposed to LCFA inoculum is more efficient in the degradation process of LCFA due to the specialization of the microbial consortium.
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