Biomethanation Efficiency Research Articles

Pilot-scale in-situ biomethanation of sewage sludge: Effects of gas recirculation method

The methanation efficiency and operational stability of a 2 m3 pilot-scale in-situ biomethanation reactor were investigated using on-site sewage sludge as the substrate, at a wastewater treatment plant. In parallel, a laboratory-scale study was conducted. Hydrogen conversion efficiencies of 96.7 and 97.5 %, and average methane contents of 84.2 and 83.2 % were obtained, for the laboratory and pilot experiments, respectively. The pilot-scale digester was operated at various conditions for 137 d, of which the last 30 d were stable with a high biomethanation efficiency and an average pH of 8.2. Gas recirculation increased the hydrogen conversion efficiency. When hydrogen injection and gas recirculation were applied separately, a 96 % lower gas recirculation rate was needed to achieve the same hydrogen conversion efficiency, compared to a mixture of hydrogen injection and gas recirculation in the same line. These findings may facilitate the selection of suitable gas recirculation concepts for practical biomethanation applications.

Efficiency assessment of underground biomethanation with hydrogen and carbon dioxide in depleted gas reservoirs: A biogeochemical simulation

Underground biomethanation, which converts hydrogen and carbon dioxide to methane with the catalysis of methanogens in geological formations, has great potential for carbon dioxide utilization and sequestration, renewable natural gas production, and large-scale energy storage. However, the efficient conversion of hydrogen and carbon dioxide in a complex reservoir environment has not been explored. To address this issue, a novel biogeochemical model is developed for underground biomethanation that considers reservoir environment factors (e.g. pH, temperature, and salinity) and integrated into PHREEQC software. The biogeochemical model is validated with a laboratory experiment and utilized to investigate the effects of reservoir parameters and injection parameters on biomethanation efficiency in depleted gas reservoirs. Results show that the biomethanation efficiency is 94.2% after 360 days in both sandstone and carbonate reservoirs. Underground biomethanation can be completed in 30 days if initial biomass and optimum specific growth rate increase and decay rate decreases. Additionally, the optimal ratio of injected hydrogen and carbon dioxide for biomethanation is greater than 4:1 and increases with total pressure if it is above 70 atm. To improve the biomethanation efficiency, this study suggests utilizing geothermal energy and pre-injecting highly active methanogens cultured on the ground before mixed gas injection.

Facilitated CO biomethanation by exogenous materials via inducing specific methanogenic pathways

Syngas biomethanation is a promising route for syngas (mainly CO, CO2, H2 and CH4) upgrading from pyrolysis/gasification of recalcitrant waste. However, its great difficulty stems from the toxic inhibition and low conversion efficiency of CO. This study aimed to enhance the CO biomethanation efficiency by supplying different conductive materials (including braunite, magnetite, biochar-A, biochar-B, and granular activated carbon). The results showed that the reactors supplied with magnetite had the best promotion on methane production, followed by braunite addition, with an increase of 96.7% and 88.0% compared to the Control respectively. With added carbon-based materials, the methane production in the Biochar-A reactors were increased by only 56%, which in the Biochar-B and GAC reactors were reduced by 17% and 30%. Magnetite presented strong redox property and electronic shuttle function, electrons can be shuttled rapidly between Fe (II) and Fe (III), which was found promoted methanogenesis by facilitating direct interspecies electron transfer between Brooklawnia spp. and Methanosarcina spp. Additionally, lots of oxygen cavities on the magnetite surfaces were capable of CO adsorption, facilitated the exposure of CO to microorganisms and thereby utilization of CO increased. On the other side, braunite was an iron-based mineral, may promote the microbial CO metabolic and thus facilitate the carboxydotrophic hydrogenogenesis process, and thus facilitating interspecies hydrogen transfer during hydrogenotrophic methanogenesis. Correspondingly, obligate hydrogenotrophic methanogen Methanobacterium spp., Methanoculleus spp., Methanospirillum spp. were all found increased in Braunite reactors. Among which, the dominant Methanobacterium spp. (accounted for 30%) performed strong methanogenic activity with braunite even in hydrogen-deficient situations.

Product gas biomethanation with inoculum enrichment and grinding

The use of cheap product gas from biomass air gasification to produce methane via anaerobic digestion is a novel and potential pathway for the large-scale production of biomass-based substitute natural gas (BioSNG). In this experimental work, the product gas biomethanation (PGB) was studied with respect to the biosludge enrichment and inoculum partial grinding as well as the mesophilic and thermophilic conditions. The results show that the biosludge enrichment can effectively stop methanogenesis inhibition from the product gas, particularly CO, thus increase the biomethanation reaction rate and shorten the reaction start-up time. The inoculum partial grinding treatment can clearly change the microorganism composition and effectively reduce the diversity of microorganisms in the mixed bacterium system for the mesophilic biomethanation, thereby improving the product gas biomethanation efficiency, which is limited for the thermophilic biomethanation.

Enhanced carbon dioxide biomethanation with hydrogen using anaerobic granular sludge and metal–organic frameworks: Microbial community response and energy metabolism analysis

In this work, metal–organic frameworks (MOFs) were prepared to evaluate its impact on carbon dioxide (CO2) biomethanization during anaerobic degradation (AD). The results showed that MOFs significantly improved the CO2 biomethanation efficiency, especially in the AD reactors using a concentration of 1.0 g/L MOFs. Furthermore, MOFs promoted direct interspecific electron transfer and alleviated the hydrogen competition of bacteria. Meanwhile, hydrogenotrophic methanogens were enriched in the AD reactors with MOFs. After the addition of MOFs, there was 3.28 times and 3.41 times increase in the abundance of metabolic functions related to methanogenesis by CO2 reduction with hydrogen and dark hydrogen oxidation, respectively. There was an increased abundance of all genes that encode the key enzymes used in methane metabolism. However, functional genes involved in nitrate reduction had their expressions inhibited. The work may offer a contribution to helping the industry achieve the carbon capture and utilization policy.

Assessing the syngas biomethanation in anaerobic sludge digestion under different syngas loading rates and homogenisation

Gasification offers digestate conversion into syngas, which can be metabolised to CH4 by a mixed anaerobic consortium. However, low gas-liquid mass transfer and CO presence can limit further scale-up. The present study assessed syngas biomethanation (55% H2, 45% CO) in a thermophilic lab-scale continuous anaerobic digester processing sewage sludge. Different syngas loading rates and mechanical stirring compared to pneumatic mixing were investigated. Results showed that anaerobic culture acclimation for CO utilisation is unnecessary. Moreover, the mechanical stirring replacement for the pneumatic system enhanced the gas-liquid mass transfer rate. Hence, the biological conversion of CO and H2 was improved by 30%. Nevertheless, increased pH to 8.6 resulted in acetic acid accumulation (>7.5 g/L). These findings improve syngas biomethanation efficiency and suggest including a proper pH control before further scale-up.

Ex-situ biogas upgrading in thermophilic trickle bed reactors packed with micro-porous packing materials

Two thermophilic trickle bed reactors (TBRs) were packed with different packing densities with polyurethane foam (PUF) and their performance under different retention times were evaluated during ex-situ biogas upgrading process. The results showed that the TBR more tightly packed i.e. containing more layers of PUF achieved higher H2 utilization efficiency (>99%) and thus, higher methane content (>95%) in the output gas. The tightly packed micro-porous PUF enhanced biofilm immobilization, gas-liquid mass transfer and biomethanation efficiency. Moreover, applying a continuous high-rate nutrient trickling could lead to liquid overflow resulting in formation of non-homogenous biofilm and severe deduction of biomethanation efficiency. High-throughput 16S rRNA gene sequencing revealed that the liquid media were predominated by hydrogenotrophic methanogens. Moreover, members of Peptococcaceae family and uncultured members of Clostridia class were identified as the most abundant species in the biofilm. The proliferation of hydrogenotrophic methanogens together with syntrophic bacteria showed that H2 addition resulted in altering the microbial community in biogas upgrading process.

A mini review on effect of nano particles of Fe in the anaerobic digestion of waste activated sludge

Anaerobic digestion is one of the most preferred methods for sludge treatment in wastewater treatment plants. Though it is widely used because of its potential to generate renewable energy (biogas), there are some limitations in its efficient operation. The solubilization of sludge is identified as a very slow rate reaction and because of that higher volume of sludge digester is required for sludge treatment and is been identified as one of the main problems. This problem has been addressed by various pre-treatment methods. Nano materials-based pre-treatment was also attempted to enhance the solubilization of sludge and ultimately to improve the bio-methanation efficiency of sludge digesters. Recent studies have shown that nano particles of metallic oxides have aided in enhancing the methane generation and nano particles of iron oxides and hydroxides have shown a positive effect on the methane generation rate. This mini-review aims to summarize the recent advancements in nano particles of iron oxides and hydroxides as an additive, elucidate the various effects of parameters affecting the sludge treatment and thereby enhance the bio-methanation. This article is also supported with a preliminary study to examine the effect of granulated nanoscale oxyhydroxides of Fe (GNOF) in the sludge solubilization rate and overall effect on the sludge digestibility of a lab scale anaerobic reactor fed with waste activated sludge. The study was conducted for hydraulic retention time (HRT) of 40 days and 20 days. The treated samples were analyzed for pH, volatile fatty acids (VFA), alkalinity, and SCOD. The results showed a positive impact of GNOF in the sludge solubilization and subsequent anaerobic digestion. The work presented in this review would help to advance the knowledge and ultimately helps in improving the bio-methanation efficiency of anaerobic digesters.

Pilot-scale biomethanation in a trickle bed reactor: Process performance and microbiome functional reconstruction

Biogas upgrading is an emerging technology offering unique opportunities for further exploitation of biomethane as fuel for vehicles or direct injection into the gas grid, expanding the conventional use of biogas for combined heat and electricity generation. Up to date, most of the studies exploring the potential of biological carbon dioxide hydrogenation was performed at laboratory scale systems, hampering the evaluation of the process under real environmental conditions. The current work demonstrates the performance of a pilot trickle bed reactor that was fed with real biogas as CO2 source under progressively increased gas provision rates. Additionally, the study is supported by a genome-centric metagenomic analysis to gain deep insights into the microbiome of the reactor. A maximum methane content of 98.5% was achieved at a gas retention time of 5 h. Stand-by periods in which no influent gas was provided in the reactor did not lead to fatal deterioration of the overall process, as the biomethanation efficiency was recovered after a certain period of time. Samples obtained from three different layers of the packing material, the liquid phase of the reactor and the inoculum demonstrated a distinct clustering of microbial members. The provision of the nutrient media from the top layer led to the enrichment of specific bacteria, such as Clostridiaceae DTU-pt_113, whose genome profile contains Veg-family genes, which are known to be associated with biofilm formation. Similarly, the injection of influent gases from the bottom of the reactor favoured the proliferation of hydrogenotrophic methanogens, solely belonging to family Methanobacteriaceae.

Comparative Evaluation of Two Packing Materials (Glass Pipe and Ceramic Ball) for Hydrogenothrophic Biomethanation (BHM) of CO2

Hydrogenothrophic biomethanation of CO2 is an attractive alternative method for either to increase biogas production or upgrading biogas in biogas plants as a carbon capture and utilization (CCU) technology. The use of various packaging materials providing efficient cell immobilization can increase hydrogenotrophic biomethanation efficiency. For this purpose, the effect of two packing materials (ceramic ball and glass pipe) on hydrogenotrophic biomethanation efficiency were investigated. It was found that glass pipe outcompeted ceramic ball in terms of both H2 utilisation efficiency (55% for ceramic ball and 85% for the glass pipe), high CH4 content in the headspace (65% for ceramic ball and 78% for the glass pipe) and methane formation rates (MFR: 3.9 and 4.8 m3CH4/m3reactor/day for ceramic ball and glass pipe, respectively).

Carbon monoxide conversion and syngas biomethanation mediated by different microbial consortia

Syngas biomethanation is an attractive process for extending application of gasification products. In the present study, anaerobic sludges from three methanogenic reactors feeding cattle manure (CS), sewage sludge (SS) and gaseous H2/CO2 (GS) were used to investigate the effect of microbial consortia composition on syngas biomethanation. The results showed that CS presented the highest CO consumption rate due to its highest relative abundance of CO consuming bacteria. The CO was mainly converted to acetate, and syntrophic acetate oxidization (SAO) bacteria converted acetate to H2/CO2 for hydrogenotrophic methanogenesis in CS and SS. However, acetate was accumulated in GS for lacking acetoclastic methanogens and SAO bacteria, leading to lower biomethanation efficiency. Additionally, adding stoichiometric H2 could convert CO and CO2 to nearly pure methane, while, the CO consumption rate declined in H2 added systems. The results present novel insights into microbial consortia on CO conversion and syngas biomethanation.

Innovative integrated approach of biofuel production from agricultural wastes by anaerobic digestion and black soldier fly larvae

In the present study, a new biorefinery approach for efficient conversion of chicken manure mixed with rapeseed straw was investigated through anaerobic co-digestion though digestate recycling. The liquid digestate fraction was used for straw pretreatment, while solid fraction was utilized for rearing the black soldier fly larvae. Anaerobic digestion of raw straw resulted in biomethane yield of 144.2 L kg−1 VS, while the pretreatment enhanced it to 227.6 L kg−1 VS. Co-digestion of the pretreated straw with chicken manure at different ratios of 1:1, 1:3, and 3:1 increased the biomethane yield to 323.5, 349.6, and 262.3 L kg−1 VS, respectively, with higher biomethanation efficiency. Black soldier fly larvae were grown on different ratios of solid digestate/larva (D/L) of 0.25, 0.50, 0.75 and 1.00, where it showed higher growth and faster development by increasing the digestate ratio. In addition, lipid content significantly increased by increasing the digestate ratio, reaching the maximum values of 31.8 dw% at 1.00 D/L. Therefore, the highest fatty acid methyl esters recovery of 301.8 mg g−1 dw was recorded at 1.00 D/L, with pronounced enhanced biodiesel characteristics. The suggested integrated approach using a mixture of liquid digestate-pretreated straw to manure at a ratio of 1:3 for dual purpose of biogas and biodiesel production enhanced the gross bioenergy yield by 95.7%, 24.6%, and 38.7% over those of raw rapeseed straw, pretreated straw, or chicken manure, respectively. The present study demonstrates an innovative waste-to-energy route that will have a positive impact on the future of biofuel industry.

Integrative Effects of Sonication and Particle Size on Biomethanation of Tropical Grass Pennisetum purpureum Using Superior Diverse Inocula Cultures

Biogas from the fast growing crop, Pennisetum purpureum, has received considerable attention in Southeast Asia since wastewater and bio-waste materials are almost completely utilized. To overcome slow hydrolysis, a rate-limiting step in anaerobic digestion of lignocellulosic biomass, superior microorganism culture, size reduction, and sonication pretreatment were co-applied. In the first experiment, the selection of anaerobic microbial culture to be used in digestion, so-called inoculum, was carried out. Specific anaerobic activities for hydrolysis and methanogenesis of sludge from different sources, a slurry digester of cattle farm (CF) and a wastewater digester of rubber latex factory (RL) were assessed. Results revealed a remarkable synergistic capability in the combined sludge, adding 10% and 49% to the overall biomethanation efficiency over the individual CF and RL sludges. In the second part, interactive effects of size reduction and sonication intensity were studied. Biomethanation efficiency as methane yield increased by 62% by size and 115% by sonication variation, but when optimally combined an additional gain of 40% was recorded. The regression model generated could estimate the energy yield increase as a function of size and sonication intensity with a satisfactory statistical precision R2 of 0.945.

Biomethanation efficiency of para-grass in piggery wastewater in single stage and temperature phased anaerobic systems

Abstract Effects of para-grass (PG) addition to pig manure (PM) digester were evaluated at organic loadings 0.10–3.76 gVS/L.d by different PG mixing ratios 0–8% in mesophilic single-stage (MS) and temperature-phased anaerobic digestion (TPAD, comprising T1 and M2 reactors) systems. Results showed equivalent methane production between MS and TPAD until 4%PG mix. Highest biogas yields obtained were 271.7 and 264.0 m3/tondry for MS and TPAD, respectively. Even with intense VFAs accumulation, the acidification yield in T1 was less than M2 because of continuous conversion of VFAs to CH4. Only at higher loading (8%PG), reactor staging by temperature was justified for this co-digestion as TPAD exhibited a superior performance and lesser mass transfer impediment. Para-grass addition to PM digester shifted the domination of bacterial strains whereas achaea were steady. Higher microbial diversity and some evolving hydrolytic bacteria observed in T1 could contribute to greater system stability at high solid loading. An addition of small front-end thermophilic tank should be considered when expansion of the existing MS system is planned to process higher solid.

Hydrogen-Fueled Microbial Pathways in Biogas Upgrading Systems Revealed by Genome-Centric Metagenomics.

Biogas upgrading via carbon dioxide hydrogenation is an emerging technology for electrofuel production. The biomethanation efficiency is strongly dependent on a balanced microbial consortium, whose high- resolution characterization along with their functional potential and interactions are pivotal for process optimization. The present work is the first genome-centric metagenomic study on mesophilic and thermophilic biogas upgrading reactors aiming to define the metabolic profile of more than 200 uncultivated microbes involved in hydrogen assisted methanogenesis. The outcomes from predictive functional analyses were correlated with microbial abundance variations to clarify the effect of process parameters on the community. The operational temperature significantly influenced the microbial richness of the reactors, while the H2 addition distinctively alternated the abundance of the taxa. Two different Methanoculleus species (one mesophilic and one thermophilic) were identified as the main responsible ones for methane metabolism. Finally, it was demonstrated that the addition of H2 exerted a selective pressure on the concerted or syntrophic interactions of specific microbes functionally related to carbon fixation, propionate and butanoate metabolisms. Novel bacteria were identified as candidate syntrophic acetate oxidizers (e.g., Tepidanaerobacter sp. DTU063), while the addition of H2 favored the proliferation of potential homoacetogens (e.g., Clostridia sp. DTU183). Population genomes encoding genes of Wood-Ljungdahl pathway were mainly thermophilic, while propionate degraders were mostly identified at mesophilic conditions. Finally, putative syntrophic interactions were identified between microbes that have either versatile metabolic abilities or are obligate/facultative syntrophs.

Influence of transitional states on the microbial ecology of anaerobic digesters treating solid wastes

A better understanding of the microbial ecology of anaerobic processes during transitional states is important to achieve a long-term efficient reactor operation. Five wastes (pig manure, biodiesel residues, ethanol stillage, molasses residues, and fish canning waste) were treated in five anaerobic reactors under the same operational conditions. The influence of the type of substrate and the effect of modifying feeding composition on the microbial community structure was evaluated. The highest biomethanation efficiency was observed in reactors fed with fish canning waste, which also presented the highest active archaeal population and the most diverse microbial communities. Only two Bacteria populations could be directly related to a particular substrate: Ilyobacter with biodiesel residues and Trichococcus with molasses residues. Results showed that the time to achieve steady-state performance after these transitional states was not dependent on the substrate treated. But reactors needed more time to handle the stress conditions derived from the start-up compared to the adaptation to a new feeding. Cluster analyses showed that the type of substrate had a clear influence on the microbiology of the reactors, and that segregation was related to the reactors performance. Finally, we conclude that the previous inoculum history treating solid waste and higher values of active Archaea population are important factors to face a successful change in substrate not entailing stability failure.

Methanogenesis mediated by methylotrophic mixed culture

Enrichment of methanogenic cultures on methanol from the microbial population in the anaerobic digesters operated on agricultural wastes revealed a high rate of biomethanation efficiency. Routine maintenance of this enrichment in a minimal basal medium at room temperature resulted in maximal growth in 40–50 d, and indicated pigment production toward the end of the growth phase. The cultures grown in three different media, with different substrates under light and dark conditions, were analyzed for protein, pigment, and gaseous products, and morphological studies were carried out by light, phase-contrast, fluorescence, and electron microscopy. In different media with methanol as substrate, growth and pigment production were maximal for the light-grown cells, decreasing in the order: phototrophic (PS(m)) > mineral > basal medium. Methanation and phototrophic growth were inversely correlated under lightgrown conditions. In contrast, growth in the dark was predominently methanogenic in the decreasing order: mineral > basal > PS (m). Among other growth conditions tested, utilization of phototrophic substrates under light and dark conditions indicated the following: 1. Basal and mineral media were supportive of methanogenic growth under both light and dark conditions, although methane yields under light-grown conditions were low; 2. Among the different substrates tested, methanol-grown cells gave the highest methane yield in the dark and; 3. Phototrophic growth in PS medium with succinate, malate, and pyruvate was better than that with methanol.