Enhanced carboxylic acids production by decreasing hydrogen partial pressure during acidogenic fermentation of glucose

Enhancing methane production in two-phase anaerobic digestion of perishable organic waste: Mini-review on acidogenic fermentation pathways and regulatory strategies

Two-phase anaerobic digestion is a highly effective approach for efficient reduction and resource recovery of perishable organic waste. Within this technological framework, organic wastes undergo multiple metabolic pathways during the acidogenic phase, which is classified into ethanol, butyrate, propionate, lactate, and mixed acid fermentation depending on the acidification end products. The nature of these acidification products critically influences the performance of the subsequent methanogenic phase. Strategic regulation of operational parameters during the acidogenic phase fosters the enrichment of specific microbial communities and establishment of dominant consortia, which enable the production of the targeted acidification end-products. This review provides a comprehensive analysis of the metabolic characteristics and regulatory strategies associated with various acidogenic fermentation types and methanogenic properties of different acidification products. The findings presented here are crucial for enhancing the stability and methanogenic efficiency of anaerobic digestion systems that process perishable organic waste.

The influence of carbon dioxide on fermentation products, microbial community, and functional gene in food waste fermentation with uncontrol pH

Food waste is a major problem faced by human beings. Acidogenic fermentation is an effective and feasible technology for resource recovery from food waste. The mixture of volatile fatty acids (VFAs) hinders the utilization of fermentation products. In this study, we constructed fermentation reactors for food waste treatment. The operation period was separated to three stages: Stage 1 (from day 1-102), Stage 2 (from day 103-208), and Stage 3 (from day 209-304). CO2 was sparged to the reactors to promote the acetate enrichment at Stage 3. Bioinformatics analysis were performed to analyze the microbial community, genes, and pathways. Results showed that the highest average concentration of acetate was 6044 mg-COD/L (R1) and 5000 mg-COD/L (R2) at Stage 3, which was corresponded to the stage with highest acetate ratio (63% and 66% in R1 and R2). But the highest total VFAs concentration was 39424 mg-COD/L at Stage 2. Aeriscardovia belonging to Actinobacteria had an average relative abundance of 85.7% after CO2 sparging. Compared with Stage 1 and Stage 2, the number of down-regulated genes and pathways at Stage 3 were much higher than the number of up-regulated genes and pathways. The significant down-regulated genes were wcaB and ttrC, and the significant down-regulated pathways were pyruvate fermentation to acetone and acetyl-CoA fermentation to butanoate II pathway. This study demonstrated that CO2 can promote the acetate enrichment during food waste fermentation. The main mechanism was enriching acetate fermentation microorganisms and inhibiting the interfere genes and pathways.

Evaluation of short-circuited electrodes in combination with dark fermentation for promoting biohydrogen production process

Short-circuited electrodes, in combination with dark fermentation, were evaluated in a biohydrogen production process. The system is based on an innovative design of a non-compartmented electromicrobial bioreactor with a conductive tubular membrane as cathode and a graphite felt as anode. In particular, the electrode specialization occurred when the bioreactor was inoculated with manure as the whole medium and when a vacuum was applied in the tubular membrane, for allowing continuous extraction of gaseous species (H2, CH4, CO2) from the bioreactor. This specialization of the electrodes as anode and cathode was further confirmed by microbial ecology analysis of biofilms and by cyclic voltammetry measurements. In these experimental conditions, the potential of the electrochemical system (short-circuited electrodes) reached values as low as -320 mV vs. SHE, associated with a significant bioH2 production. Moreover, a higher bioH2 production occurred and a potential of the electrochemical system as low as -429 mV vs SHE was temporarily observed, when additional heat treatments of the whole manure were applied in order to remove methanogen microorganisms (i.e., hydrogen consumers). In the bioreactor, the higher production of bioH2 would be promoted by electrofermentation from the current flow observed between short-circuited anode and cathode.

Anaerobic acidification membrane bioreactor operating at acidic condition for treating concentrated municipal wastewater: Performance and implication

To address the low-carbon treatment requirements for municipal wastewater, a novel anaerobic acidification membrane bioreactor (AAMBR) was developed for recovering organic matter in terms of volatile fatty acids (VFAs). While the AAMBR successfully generated VFAs from municipal wastewater through forward osmosis (FO) membrane concentration, its operation was limited to a single pH value of 10.0. Here, performance of the AAMBR operating at acidic condition was evaluated and compared with that at alkaline condition. The findings revealed that the AAMBR with pH 5.0 efficiently transformed organic matter into acetic acid, propionic acid, and butyric acid, resulting in a VFAs yield of 0.48 g/g-CODfeed. In comparison with the AAMBR at pH 10.0, this study achieved a similar VFAs yield, a lower fouling tendency, a lower loss of nutrients and a lower controlling cost. In conclusion, this study demonstrated that a pH of 5.0 is optimal for the AAMBR treating municipal wastewater.

Effect of Lipid Type on the Acidogenic Performance of Food Waste

Due to its high lipid content and intricate constitution, food waste poses a considerable challenge for biotreatment. This research aims to investigate the potential influence of diverse lipid species on anaerobic fermentation, induced by the varying dietary patterns observed in distinct regions. The investigation involved incorporating 5% (w/w) of beef tallow, mutton fat, soybean oil, peanut oil, and rapeseed oil, separately, into simulated food waste, and subjected it to batch mode acidogenic fermentation. The inclusion of unsaturated fatty acids resulted in a redirection of the metabolic pathway from the lactic acid type to the ethanol, acetic acid, and butyric acid types. The succession of the acidogenic metabolic pathway was highly correlated with the lipid types; beef tallow, mutton fat, soybean oil, and peanut oil delayed the metabolic process by 1, 2, 3, and 8 d, respectively, whereas rapeseed oil accelerated it by 2 d. The lipids contained within the food waste did not facilitate the buildup of soluble substances, resulting in a decrease of 14.0~59.7%. Notwithstanding, valeric acid was exclusively generated during the beef tallow and peanut oil treatments, whereas the production of lactic acid in peanut oil showed a 35.9% increase in comparison to the control.

Highly selective butyric acid production by coupled acidogenesis and ion substitution electrodialysis

Selective production of carboxylic acids (CAs) from mixed culture fermentation remains a difficult task in organic waste valorization. Herein, we developed a facile and sustainable carbon loop strategy to regulate the fermentation micro-environment and steer acidogenesis towards selective butyric acid production. This new ion substitution electrodialysis-anaerobic membrane bioreactor (ISED-AnMBR) integrated system demonstrated a high butyric acid production at 11.19 g/L with a mass fraction of 76.05%. In comparison, only 1.04 g/L with a mass fraction of 30.56% was observed in the uncoupled control reactor. The carbon recovery reached a maximum of 96.09% with the assistance of ISED. Inorganic carbon assimilation was believed to be an important contributor, which was verified by ¹³C isotopic tracing. Microbial community structure shows the dominance of Clostridia (80.16%) in the unique micro-environment (e.g., pH 4.80-5.50) controlled by ISED, which is believed beneficial to the growth of such fermentative bacteria with main products of butyric acid and acetic acid. In addition, the emergence of chain elongators such as Clostridium sensu stricto 12 was observed to have a great influence on butyric acid production. This work provides a new approach to generate tailored longer chain carboxylic acids from organic waste with high titer thus contributing to a circular economy.

Organic waste recycling for carbon smart circular bioeconomy and sustainable development: A review

The development of sustainable and low carbon impact processes for a suitable management of waste and by-products coming from different factors of the industrial value chain like agricultural, forestry and food processing industries. Implementing this will helps to avoid the negative environmental impact and global warming. The application of the circular bioeconomy (CB) and the circular economic models have been shown to be a great opportunity for facing the waste and by-products issues by bringing sustainable processing systems which allow to the value chains be more responsible and resilient. In addition, biorefinery approach coupled to CB context could offer different solution and insights to conquer the current challenges related to decrease the fossil fuel dependency as well as increase efficiency of resource recovery and processing cost of the industrial residues. It is worth to remark the important role that the biotechnological processes such as fermentative, digestive and enzymatic conversions play for an effective waste management and carbon neutrality.

A comprehensive review on current status and future perspectives of microbial volatile fatty acids production as platform chemicals

Volatile fatty acids (VFA), the secondary metabolite of microbial fermentation, are used in a wide range of industries for production of commercially valuable chemicals. In this review, the fermentative production of VFAs by both pure as well mixed microbial cultures is highlighted along with the strategies for enhancing the VFA production through innovations in existing approaches. Role of conventionally applied tools for the optimization of operational parameters such as pH, temperature, retention time, organic loading rate, and headspace pressure has been discussed. Furthermore, a comparative assessment of above strategies on VFA production has been done with alternate developments such as co-fermentation, substrate pre-treatment, and in situ removal from fermented broth. The review also highlights the applications of different bioreactor geometries in the optimum production of VFAs and how metagenomic tools could provide a detailed insight into the microbial communities and their functional attributes that could be subjected to metabolic engineering for the efficient production of VFAs.

Electro-fermentation: Sustainable bioproductions steered by electricity

The market of biobased products obtainable via fermentation processes is steadily increasing over the past few years, driven by the need to create a decarbonized economy. To date, industrial fermentation (IF) employs either pure or mixed microbial cultures (MMC) whereby the type of the microbial catalysts and the used feedstock affect metabolic pathways and, in turn, the type of product(s) generated. In many cases, especially when dealing with MMC, the economic viability of IF is hindered by factors such as the low attained product titer and selectivity, which ultimately challenge the downstream recovery and purification steps. In this context, electro-fermentation (EF) represents an innovative approach, based on the use of a polarized electrode interface to trigger changes in the rate, yield, titer or product distribution deriving from traditional fermentation processes. In principle, the electrode in EF can act as an electron acceptor (i.e., anodic electro-fermentation, AEF) or donor (i.e., cathodic electro-fermentation, CEF), or simply as a mean to control the oxidation-reduction potential of the fermentation broth. However, the molecular and biochemical basis underlying the EF process are still largely unknown. This review paper provides a comprehensive overview of recent literature studies including both AEF and CEF examples with either pure or mixed microbial cultures. A critical analysis of biochemical, microbiological, and engineering aspects which presently hamper the transition of the EF technology from the laboratory to the market is also presented.

Two-phase anaerobic digestion of food waste: Effect of semi-continuous feeding on acidogenesis and methane production

In present investigation, effect of diverting acidogenic off-gas from leached bed reactor (LBR) to up-flow anaerobic sludge blanket (UASB) reactor during semi-continuous food waste (FW) anaerobic digestion was evaluated. In test LBR headspace pressure (3.3 psi) was maintained with intermittent headspace gas transfer into UASB. In control, same headspace pressure was maintained without gas transfer. The semi-continuous FW addition affected the characteristics and production of leachate in control and test LBR. The cumulative COD, total soluble products and methane yields were 1.26, 1.37 and 3 times higher in the test LBR than the control. The acetate and methane yields from test LBR were 697.8 g·kgVSadded-1 and 167.55 mL·gCOD-1feeding. Acidogenic gas transfer maintained low partial pressure of hydrogen and the hydrogen to carbon-di-oxide ratio in the headspace of LBR, which were thermodynamically favorable for microbial metabolism and concomitant high-rate production of acetate-rich volatile fatty acid and methane-rich biogas from FW.

Volatile Fatty Acid Production from Food Waste Leachate Using Enriched Bacterial Culture and Soil Bacteria as Co-Digester

The production of volatile fatty acids (VFAs) from waste stream has been recently getting attention as a cost-effective and environmentally friendly approach in mechanical–biological treatment plants. This is the first study to explore the use of a functional bacterium, AM5 isolated from forest soil, which is capable of enhancing the production of VFAs in the presence of soil bacteria as a co-digester in non-strict anaerobic fermentation processes of food waste leachates. Batch laboratory-scale trials were conducted under thermophilic conditions at 55 °C and different pH values ranging from approximately 5 to 11, as well as under uncontrolled pH for 15 days. Total solid content (TS) and volatile solid content (VS) were observed with 58.42% and 65.17% removal, respectively. An effluent with a VFA concentration of up to 33,849 mg/L (2365.57 mg/g VS; 2244.45 mg/g chemical oxygen demand (COD)-VFA VS; 1249 mg/g VSremoved) was obtained at pH 10.5 on the second day of the batch culture. The pH resulted in a significant effect on VFA concentration and composition at various values. Additionally, all types of VFAs were produced under pH no-adjustment (approximately 5) and at pH 10.5. This research might lead to interesting questions and ideas for further studies on the complex metabolic pathways of microbial communities in the mixture of a soil solution and food waste leachate.

Potential Valorization of Organic Waste Streams to Valuable Organic Acids through Microbial Conversion: A South African Case Study

The notion of a “biobased economy” in the context of a developing country such as South Africa (SA) necessitates the development of technologies that utilize sustainable feedstocks, have simple and robust operations, are feasible at small scale and produce a variety of valuable bioproducts, thus fitting the biorefinery concept. This case study focuses on the microbial production of higher-value products from selected organic waste streams abundant in the South African agricultural sector using microbes adapted to utilize different parts of biomass waste streams. A ruminant-based carboxylate platform based on mixed or undefined anaerobic co-cultures of rumen microorganisms can convert the carbohydrate polymers in the lignocellulosic part of organic waste streams to carboxylic acids that can be upgraded to biofuels or green chemicals. Furthermore, yeast and fungi can convert the simpler carbohydrates (such as the sugars and malic acid in grape and apple pomace) to ethanol and high-value carboxylic acids, such as lactic, fumaric, succinic and citric acid. This review will discuss the combinational use of the ruminal carboxylate platform and native or recombinant yeasts to valorize biomass waste streams through the production of higher-value organic acids with various applications.

Biocrude Oil from Algal Bloom Microalgae: A Novel Integration of Biological and Thermochemical Techniques

Algal bloom microalgae are abundant in polluted water systems, but their biocrude oil production potential via hydrothermal liquefaction (HTL) is limited. This study proposed a novel process that combined biological (dark fermentation) and thermochemical (HTL) techniques aimed at changing the feedstock characteristics to be more suitable for thermochemical conversion, herein named integrated dark fermentation-hydrothermal liquefaction (DF-HTL). DF-HTL conversion of algae significantly enhanced the biocrude oil yield (wt %), carbon content (mol), energy content (MJ), and energy conversion ratios by 9.8, 29.7, 40.0, and 61.0%, respectively, in comparison to the control. Furthermore, DF-HTL processing significantly decreased the aqueous byproduct yield (wt %), carbon content (mol), nitrogen content (mol), and ammonia content (mol) by 19.0, 38.4, 25.0, and 13.2%, respectively, in comparison to the control. Therefore, DF-HTL reduced the environmental impact associated with disposing of the wastewater byproduct. However, DF-HTL also augmented the nitrogen content (mol) of the biocrude oil by 42.2% in comparison to the control. The benefits of DF-HTL were attributed to the increased acid content, the incorporation of H₂ as a processing gas, and the enhancement of the Maillard reaction, which shifted the distribution of reaction products from the aqueous phase to the biocrude oil phase. This article provides insights into the efficacy of a novel integrated biological-thermochemical processing method with distinct environmental and energetic advantages over conventional HTL that heightens the biocrude oil yield for feedstocks with a high carbohydrate and a high protein content.

Renewable hydrogen production by dark-fermentation: Current status, challenges and perspectives

Global urbanization has resulted in amplified energy and material consumption with simultaneous waste generation. Current energy demand is mostly fulfilled by finite fossil reserves, which has critical impact on the environment and thus, there is a need for carbon-neutral energy. In this view, biohydrogen (bio-H₂) is considered suitable due to its potential as a green and dependable carbon-neutral energy source in the emerging 'Hydrogen Economy'. Bio-H₂ production by dark fermentation of biowaste/biomass/wastewater is gaining significant attention. However, bio-H₂production still holds critical challenges towards scale-up with reference to process limitations and economic viabilities. This review illustrates the status of dark-fermentation process in the context of process sustainability and achieving commercial success. The review also provides an insight on various process integrations for maximum resource recovery including closed loop biorefinery approach towards the accomplishment of carbon neutral H₂ production.

A comprehensive study of volatile fatty acids production from batch reactor to anaerobic sequencing batch reactor by using cheese processing wastewater

Volatile fatty acids (VFAs) has great potential for closed-loop production in dairy industries via resource recovery from waste-streams. In the current study, the transition of VFA production from batch reactor to anaerobic sequencing batch reactor (ASBR) by using cheese industry wastewater under alkali pH was evaluated with respect to seed sludge structure, microbial diversity and reactor type. The transition from the batch reactor to the ASBR demonstrated that the maximum VFA production yield (g COD/g SCOD) was comparable in two reactors (batch: 0.97; ASBR: 0.94), whereas, the dominant acid type was different (batch: 49% lactic acid; ASBR: 80% propionic acid). There was a significant correlation between the productions of butyric acid with Gracilibacteraceae and Desulfovibrionaceae; propionic acid with Desulfovibrionaceae and Synergistaceae; lactic acid with Pseudomonadaceae and Rhodocyclaceae. The high VFA production efficiency can be achieved by long term reactor operation, which enables the shift from industrial waste-streams to biorefineries.

Volatile fatty acid production from semi-synthetic milk processing wastewater under alkali pH: The pearls and pitfalls of microbial culture

Volatile fatty acids (VFA) are one of the most promising sustainable and environmentally friendly bioproduct owing to their wide usage area and high market demand. For this reason, in this study, the evaluation of VFA production from pure and mixed bacterial cultures was aimed. Three different mixed cultures with C. aceticum, C. butyricum and P. acidipropionici as pure cultures were used for inoculation of milk processing wastewater fermentation under pH 10 for 15 days. The mixed culture fermentation had the highest VFA production efficiency whereas the highest amount of acetic, butyric and propionic acid productions were obtained by C. aceticum, C. butyricum and P. acidipropionici, respectively. Also, the mixed cultures demonstrated faster pH regulation and acclimation than the pure cultures tested. Therefore, development of synthetic cultures may offer a useful approach to produce VFA mixtures with one-dominant acid type and with high production efficiency.

Volatile fatty acids production via mixed culture fermentation: Revealing the link between pH, inoculum type and bacterial composition

The aim of the study was to investigate the effects of operational parameters, inoculum type and bacterial community on mixed culture fermentation to produce one dominant acid type in the mixture of volatile fatty acids (VFA). The study was performed using three different inocula (large&small granular and slurry) with glucose under various initial pH. The VFA production efficiency reached to 0,97 (gCOD/gSCOD) by granular sludge. VFA composition was changed by initial pH: in neutral conditions, acetic acid; in acidic conditions, acetic and butyric acids, in alkali conditions butyric acid were dominated, respectively. The VFA production was positively affected by the high relative abundance of Firmicutes. On the contrary, a negative correlation was seen between VFA production and the relative abundance of Chloroflexi. The results revealed the physical sludge structure of inoculum was the key factor for production efficiency, whereas, pH was the most important parameter to affect VFA composition.

Three-ways changed in headspace air on anaerobic fermentation

Different headspace condition has a great influence on fermentation process. In this study, whey protein was used as substrate, and the headspace air was changed in three different ways (H₂ mixed N₂, H₂ mixed CO₂, N₂ with different sparging rates) to explore the effects of these three methods on products. The result showed that H₂ mixed with CO₂ is more conducive to acid production. Homoacetogenesis played a central role in fermentation process. There is a turning point in the role of hydrogen and Homoacetogenesis, which is when the partial pressure of hydrogen is 0.268 atm. In the first two conditions, the acid concentration increased with the increase of hydrogen percentage. Nitrogen sparging way is adverse to acid production, but conducive to gas production.

Application of IWA Anaerobic Digestion Model No. 1 to simulate butyric acid, propionic acid, mixed acid, and ethanol type fermentative systems using a variable acidogenic stoichiometric approach

IWA Anaerobic Digestion Model No. 1 (ADM1) is the most widely recognised and popular mathematical model for anaerobic digestion processes. However, the application of ADM1 to acidogenic fermentation is limited by its use of constant stoichiometry to describe the formation of products via carbohydrate fermentation. This study presents a modification of ADM1 using a variable acidogenic stoichiometric approach in which the hydrogen partial pressure (p_H2) and pH are used to predict and regulate the acidogenic process. The fermentation of ethanol and its kinetics were introduced into the model structure. Experimental data from mixed acid-type fermentation in a 28.4 L anaerobic baffled reactor (ABR) fed with a sucrose solution with a chemical oxygen demand of 4000 mg L^-1 were used to calibrate the model parameters. Two case studies involving continuous ethanol-type fermentation in an ABR and a continuous stirring tank reactor (CSTR) were used to validate the approach. The modified model achieved good predictions of the experimental data collected from butyric acid, propionic acid, mixed acid, and ethanol-type fermentation in the ABR and CSTR using the standard ADM1 parameter values without any parameter fitting beyond implementation of the variable acidogenic stoichiometry. The p_H2 and pH thresholds in butyric acid, propionic acid, mixed acid, and ethanol-type fermentation could be predicted using this model, which was shown to be a valid mathematical tool for the regulation of fermentation type.

Efficient resource valorization by co-digestion of food and vegetable waste using three stage integrated bioprocess

During two-stage (Acidogenesis-Methanogenesis) process, solid organics and gaseous by-products are usually left unused. To increase resource recovery efficiency, a three stage process (Hydrolysis/Acidogenesis-Methanogenesis-Composting) was designed. Initially, co-digestion of food waste (FW) and vegetable waste (VW) was carried out in Leach Bed Reactor (LBR) for hydrolysis and acidogenesis, followed by airlift reactor (ALR) for methanogenesis for 21 days using two different feed stocks [2:3 FW:VW~FVW; FW alone]. Off gas from LBR was diverted to ALR to enhance methane recovery. Results depicted that volatile fatty acids (VFA) and biohydrogen production was more for FW fed system, while methane production was higher in FVW fed system. Three different functional zones in three separate chambers significantly accelerated organic removal rate while gas diversion increased overall methane recovery. In third stage, residual solid organic matter from LBR was subjected to aerobic composting and compost with N (%): 2.90 & 2.76; C/N ratio: 18.2 & 20.8 for FVW and FW was recovered. The three-stage process has advantages of zero waste generation and overall process stability, accounting for resource efficient circular loop.

Elevated volatile fatty acids production through reuse of acidogenic off-gases during electro-fermentation

Electro-fermentation is gaining attention for its advantage in promoting product recovery and valorization of organic wastes. However, emission of by-product gases during acidogenic fermentation is one of the key reasons for reduced product recovery whereas high gas pressure in the acidogenic headspace could pose an inhibitory effect on the production of volatile fatty acids (VFAs). This study presents a novel electro-fermentation (EF) system for enhancing VFAs production by in situ reuse of anodic off-gases (mainly CO₂ and H₂) in the cathode. A total VFAs production of 0.57 g-VFAs/g-VS was achieved through reuse of acidogenic off-gases in EF system, corresponding to 48.70% increase in comparison with the treatment without off-gases reuse. Consequently, the conversion efficiency of carbon to VFAs was improved significantly by 13.92%. Acidogenic metabolic pathway in the anode shifted to mixed -acid fermentation with the succession of dominant microbes from genus of Escherichia in the seeding inocula to Bacteroides and Desulfovibrio in the anode and cathode chambers, respectively. This would provide a way to enhance VFAs recovery from organic wastes, which also contributes to reduced carbon footprint and increased environmental sustainability.

Bio-based volatile fatty acid production and recovery from waste streams: Current status and future challenges

Bio-based volatile fatty acid (VFA) production from waste-stream is getting attention due to increasing market demand and wide range usage area as well as its cost-effective and environmentally friendly approach. The aim of this paper is to give a comprehensive review of bio-based VFA production and recovery methods and to give an opinion on future research outlook. Effects of operation conditions including pH, temperature, retention time, type of substrate and mixed microbial cultures on VFA production and composition were reviewed. The recovery methods in terms of gas stripping with absorption, adsorption, solvent extraction, electrodialysis, reverse osmosis, nanofiltration, and membrane contractor of VFA were evaluated. Furthermore, strategies to enhance bio-based VFA production and recovery from waste streams, specifically, in-line VFA recovery and bioaugmentation, which are currently not used in common practice, are seen as some of the approaches to enhance bio-based VFA production.

High-calorific bio-hydrogen production under self-generated high-pressure condition

For the use of biologically produced H₂, removal of CO₂ is an indispensable process. Unlike conventional CO₂ removal methods, this study proposed a self-generated high-pressure dark fermentation (HPDF) process as a novel strategy for directly producing high-calorific bio-H₂. The pressure was automatically increased by self-generated gas, while the maximum pressure inside fermenter was restricted to 1, 3, 5, 7, and 10 bar in a batch operation. As the pressure increased from 1 to 10 bar, the H₂ content increased from 55% to 80%, whereas the H₂ yield decreased from 1.5 to 0.9 mol H₂/mol hexose_added. The highest H₂ content of 80% was obtained at both of 7 and 10 bars. Increased lactate production with increased abundance of lactic acid bacteria was observed at high-pressure. Despite the lower H₂ yields at high-pressure conditions, HPDF was found to be economically beneficial for obtaining high-calorific bio-H₂ owing to the low CO₂ removal cost.

The diversity of hydrogen-producing bacteria and methanogens within an in situ coal seam

BACKGROUND

Biogenic and biogenic-thermogenic coalbed methane (CBM) are important energy reserves for unconventional natural gas. Thus, to investigate biogenic gas formation mechanisms, a series of fresh coal samples from several representative areas of China were analyzed to detect hydrogen-producing bacteria and methanogens in an in situ coal seam. Complete microbial DNA sequences were extracted from enrichment cultures grown on coal using the Miseq high-throughput sequencing technique to study the diversity of microbial communities. The species present and differences between the dominant hydrogen-producing bacteria and methanogens in the coal seam are then considered based on environmental factors.

RESULTS

Sequences in the Archaea domain were classified into four phyla and included members from Euryarchaeota, Thaumarchaeota, Woesearchaeota, and Pacearchaeota. The Bacteria domain included members of the phyla: Firmicutes, Proteobacteria, Bacteroidetes, Actinobacteria, Acidobacteria, Verrucomicrobia, Planctomycetes, Chloroflexi, and Nitrospirae. The hydrogen-producing bacteria was dominated by the genera: Clostridium, Enterobacter, Klebsiella, Citrobacter, and Bacillus; the methanogens included the genera: Methanorix, Methanosarcina, Methanoculleus, Methanobrevibacter, Methanobacterium, Methanofollis, and Methanomassiliicoccus.

CONCLUSION

Traces of hydrogen-producing bacteria and methanogens were detected in both biogenic and non-biogenic CBM areas. The diversity and abundance of bacteria in the biogenic CBM areas are relatively higher than in the areas without biogenic CBM. The community structure and distribution characteristics depend on coal rank, trace metal elements, temperature, depth and groundwater dynamic conditions. Biogenic gas was mainly composed of hydrogen and methane, the difference and diversity were caused by microbe-specific fermentation of substrates; as well as by the environmental conditions. This discovery is a significant contribution to extreme microbiology, and thus lays the foundation for research on biogenic CBM.