Gas controlled hydrogen fermentation

Dark-fermentative biohydrogen production from vegetable residue using wine lees as novel inoculum

This work studied wine lees as a novel source of microorganisms for biohydrogen production from vegetable residue (VR). Green tomatoes (WLGT), bell peppers (WLBP), green beans (WLGB), zucchini (WLZ), peas (WLP), and (WLE) eggplants were used as a substrate for dark fermentation, which was conducted in batch assays at 37 °C for 60 d. The cumulative hydrogen yield was approximately 350, 344, 319, 314, 302, 170, and 149 mL H2/g VS in WLZ, WLE, WLRT, WLGT, WLP, WLGB, and WLBP, respectively. A total volatile fatty acid (VFA) accumulation of about 2059 - 4995 mg HAc/L accompanied the bio-H2 production. From day 61 to day 147, dark-fermentative digestate was subjected to an anaerobic digestion batch process under mesophilic conditions to allow the bioconversion of VFAs into renewable methane, as confirmed by a Pearson correlation value of 0.778, final VFA concentrations ≤ 131 mg HAc/L and key functional genes (e.g., K00925). Clostridium_sensu_stricto_12 and Caproiciproducens genera accounted for 44 - 78 % of relative abundance after the dark fermentation stage. The taxonomic classification also revealed a presence of Methanosaeta archaea comprised between 45 and 98 % after the two-stage anaerobic fermentation. Finally, a rough energy comparison was performed to evaluate the feasibility of the bioprocess by including practical implications and limitations.

Highly selective acetate production from wine lees through acidogenic fermentation

Among winery wastes wine lees have a high, unexplored potential for the production of carboxylic acids and more particularly acetate. In fact, they have a high ethanol and low carbohydrate content which can make thermodynamically feasible the oxidation of ethanol to acetate. In this study, the potential of wine lees for anaerobic acetate production was assessed in batch conditions, 37 °C and pH 5.5. White wine lees (WWL) and red wine lees (RWL) were fermented with and without inoculum, and RWL were also co-fermented with waste activated sludge at 20, 40, 70 and 100 gCOD/L. Endogenous microbiome had the same fermentation performances than the external inoculum in WWL, while it led to almost no carboxylates production in RWL, where the community was dominated by the H2-producer Klebsiella (81.6%). Overall, acetate always represented the majority of carboxylates (58-72% on COD basis). H2 production was low (0.31-6.97 mL H2/g bCODin), thus enabling anaerobic ethanol oxidation to acetate (ΔG = -26.6/-7.4 kJ/mol). In co-fermentation, at 70 and 100 gCOD/L caproate (10.0-16.0%) and heptanoate (1.6-5.4%) appeared, alongside a microbiome enriched in lactate-producers (up to 24.5%). Overall, the high acetate selectivity obtained is promising for biorefinery process coupling.

Conversion of wine lees and waste activated sludge into caproate and heptanoate: Thermodynamic and microbiological insights

In this study, wine lees and waste activated sludge (WAS) were co-fermented for the first time in a 4:1 ratio (COD basis) at 20, 40, 70 and 100 gCOD/L, in batch at 37 °C and pH 7.0. The substrates were successfully converted to caproate (C6) and heptanoate (C7) with a high selectivity (40.2 % at 40 gCOD/L). The rapidly-growing chain-elongating microbiome was enriched inClostridiaceaeandOscillospiraceae, representing together 3.4-8.8 % of the community. Substrate concentrations higher than 40 gCOD/L negatively affected C6 and C7 selectivities and yields, probably due to microbial inhibition by high ethanol concentrations (15.82-22.93 g/L). At 70 and 100 gCOD/L, chain elongation shifted from ethanol-based to lactate-based, with a microbiome enriched in the lactic acid bacteriaRoseburia intestinalis(27.3 %) andEnterococcus hirae(13.8 %). The partial pressure of H2(pH2) was identified by thermodynamic analysis as a fundamental parameter for controlling ethanol oxidation and improving C6 and C7 selectivities.

The effect of reduced pressure and glucose concentration on hydrogen and volatile fatty acid yield: The role of homoacetogenesis

In this study, the influence of headspace pressure (HP; 20-100 kPa) and organic loading rate (OLR; 10-30 g/L) on H₂ and volatile fatty acid (VFA) yield were investigated. The experiments were carried out in the semi-continuous mode, the main products in VFAs were ethanol and butyrate, which accounted for more than 75%. More than 79% generated H₂ was consumed through homoacetogenesis pathway when HP was 100 kPa, and lowing HP could effectively promote the accumulation of H₂ (increased by at least 2 times). Even though consumed H₂ through homoacetogenesis was related to OLR and HP, the lower HP was more likely to reduce this part H₂ consumption, especially under 10 g/L condition. As for acid production rate, both OLR and HP have a significant effect (p < 0.05). Maximum acid production rate was 489.86 mg-COD/g-COD_degrade·d^-1when OLR was 20 g/L and HP was 40 kPa.

Effect of gas atmosphere on hydrogen production in microbial electrolysis cells

Inert gas is often used in the deoxygenation of microbial electrolysis cells (MECs) to maintain growth and viability of anaerobes. However, the effects of the gas atmosphere on hydrogen production and microbial community of MECs are often neglected. Here, the performances and biofilm microbiomes of MECs pre-sparged with different gases were compared. MECs pre-sparged with argon gas (Ar) yielded more hydrogen (3.73 ± 0.13 mol-H₂/mol-acetate) and a higher hydrogen production rate (2.99 ± 0.17 L-H₂/L-reactor-day) than MECs pre-sparged with N₂ (3.41 ± 0.13 mol-H₂/mol-acetate and 2.27 ± 0.28 L-H₂/L-reactor-day, respectively). Microbiome analysis indicated that the relative abundance of Geobacter increased from 59.25% to 77.79% when the gas atmosphere in MECs shifted from N₂ to Ar. Hydrogen production may have been catalyzed by nitrogenase from Geobacter and photosynthetic bacteria in MECs pre-sparged with Ar. These findings suggested that the gas atmosphere substantially influences the microbiome of anode biofilms and Ar sparging is most effective for enhancing hydrogen production in MECs.

Enhancement of dark fermentative H2 production by gas separation membranes: A review

Biohydrogen production via dark fermentation is currently the most developed method considering its practical readiness for scale-up. However, technological issues to be resolved are still identifiable and should be of concern, particularly in terms of internal mass transfer. If sufficient liquid-to-gas H₂ mass transfer rates are not ensured, serious problems associated with the recovery of biohydrogen and consequent inhibition of the process can occur. Therefore, the continuous and effective removal of H₂ gas is required, which can be performed using gas separation membranes. In this review, we aim to analyze the literature experiences and knowledge regarding mass transfer enhancement approaches and show how membranes may contribute to this task by simultaneously processing the internal (headspace) gas, consisting mainly of H₂ and CO₂. Promising strategies related to biogas recirculation and integrated schemes using membranes will be presented and discussed to detect potential future research directions for improving biohydrogen technology.

Effect of total solid content and pretreatment on the production of lactic acid from mixed culture dark fermentation of food waste

Food waste landfilling causes environmental degradation, and this work assesses a sustainable food valorization technique. In this study, food waste is converted into lactic acid in a batch assembly by dark fermentation without pH control and without the addition of external inoculum at 37 °C. The effect of total solid (TS), enzymatic and aeration pretreatment was investigated on liquid products concentration and product yield. The maximum possible TS content was 34% of enzymatic pretreated waste, and showed the highest lactic acid concentration of 52 g/L, with a lactic acid selectivity of 0.6 g_lactic/g_totalacids. The results indicated that aeration pretreatment does not significantly improve product concentration or yield. Non-pretreated waste in a 29% TS system showed a lactic acid concentration of 31 g/L. The results showed that enzymatic pretreated waste at TS of 34% results in the highest production of lactic acid.

Hydrogen and carbon dioxide mixed culture fermentation in a hollow-fiber membrane biofilm reactor at 25 °C

There have been no reports of H₂ and CO₂ mixed-culture fermentation (MCF) at 25 °C in a hollow-fiber membrane biofilm reactor (HfMBR). In this study, H₂ and CO₂ MCF were conducted in an HfMBR at 25 °C producing metabolites including acetate, ethanol, butyrate, and caproate. Compared to pure culture fermentation (i.e., Clostridium carboxidivorans P7), the MCF in HfMBR at 25 °C produced a higher concentration of caproate in this study (3.4 g/L in batch 1 and 5.7 g/L in batch 2). The dominant genera were Clostridium_sensu_stricto_12 and Prevotella_7. The caproate was more likely formed from the pathway of acetate and ethanol rather than via butyrate and ethanol. Since caproate is more valuable than acetate and low temperature fermentation consumes less energy, this process of H₂ and CO₂ MCF at 25 °C is appropriate for industrial application.

The chemostat metabolite spectra of alkaline mixed culture fermentation under mesophilic, thermophilic, and extreme-thermophilic conditions

Alkaline mixed culture fermentation (MCF) is a promising technology for reducing organic waste and producing biochemicals. However, chemostat metabolite spectra that are necessary for constructing a model and analyzing factors are seldom reported. In the present study, the effects of pH on the metabolites distribution in mesophilic (35 °C), thermophilic (55 °C), and extreme-thermophilic (70 °C) alkaline MCF were demonstrated. A chemical oxygen demand balance above 80% was achieved, and the main metabolites included acetate, ethanol, propionate, lactate, and formate. The yields of ethanol and formate increased as pH was increased from 7.5 to higher pH under mesophilic and thermophilic conditions, while the yields of acetate, lactate, and/or propionate decreased. The yields of ethanol, acetate, and formate increased under extreme-thermophilic conditions as pH was increased from 7.5 to 9.0, whereas lactate and hydrogen yields decreased. Low biomass yield under thermophilic and extreme-thermophilic conditions benefited higher metabolite production and minimized the accumulation of sludge.

Insights into Butyrate Production in a Controlled Fermentation System via Gene Predictions

Butyrate is a common fatty acid produced in important fermentative systems, such as the human/animal gut and other H₂ production systems. Despite its importance, there is little information on the partnerships between butyrate producers and other bacteria. The objective of this work was to uncover butyrate-producing microbial communities and possible metabolic routes in a controlled fermentation system aimed at butyrate production. The butyrogenic reactor was operated at 37°C and pH 5.5 with a hydraulic retention time of 31 h and a low hydrogen partial pressure (PH₂). High-throughput sequencing and metagenome functional prediction from 16S rRNA data showed that butyrate production pathways and microbial communities were different during batch (closed) and continuous-mode operation. Lactobacillaceae, Lachnospiraceae, and Enterococcaceae were the most abundant phylotypes in the closed system without PH₂ control, whereas Prevotellaceae, Ruminococcaceae, and Actinomycetaceae were the most abundant phylotypes under continuous operation at low PH₂. Putative butyrate producers identified in our system were from Prevotellaceae, Clostridiaceae, Ruminococcaceae, and Lactobacillaceae. Metagenome prediction analysis suggests that nonbutyrogenic microorganisms influenced butyrate production by generating butyrate precursors such as acetate, lactate, and succinate. 16S rRNA gene analysis suggested that, in the reactor, a partnership between identified butyrogenic microorganisms and succinate (i.e., Actinomycetaceae), acetate (i.e., Ruminococcaceae and Actinomycetaceae), and lactate producers (i.e., Ruminococcaceae and Lactobacillaceae) took place under continuous-flow operation at low PH₂. IMPORTANCE This study demonstrates how bioinformatics tools, such as metagenome functional prediction from 16S rRNA genes, can help understand biological systems and reveal microbial interactions in controlled systems (e.g., bioreactors). Results obtained from controlled systems are easier to interpret than those from human/animal studies because observed changes may be specifically attributed to the design conditions imposed on the system. Bioinformatics analysis allowed us to identify potential butyrogenic phylotypes and associated butyrate metabolism pathways when we systematically varied the PH₂ in a carefully controlled fermentation system. Our insights may be adapted to butyrate production studies in biohydrogen systems and gut models, since butyrate is a main product and a crucial fatty acid in human/animal colon health.

Critical analysis of hydrogen production from mixed culture fermentation under thermophilic condition (60 °C)

Bio-hydrogen production from mixed culture fermentation (MCF) of glucose was studied by conducting a comprehensive product measurement and detailed mass balance analysis of their contributions to the final H2 yield. The culture used in this study was enriched on glucose at 60 °C through a sequential batch operation consisting of daily glucose feeds, headspace purging and medium replacement every third day in serum bottles for over 2 years. 2-Bromoethanesulfonate (BES) was only required during the first three 3-day cycles to permanently eliminate methanogenic activity. Daily glucose feeds were fully consumed within 24 h, with a persistent H2 yield of 2.7 ± 0.1 mol H2/mol glucose, even when H2 was allowed to accumulate over the 3-day cycle. The measured H2 production exceeded by 14 % the theoretical production of H2 associated with the fermentation products, dominated by acetate and butyrate. Follow-up experiments using acetate with a (13)C-labelled methyl group showed that the excess H2 production was not due to acetate oxidation. Chemical formula analysis of the biomass showed a more reduced form of C5H11.8O2.1N1.1 suggesting that the biomass formation may even consume produced H2 from fermentation.

Desulfovibrio vulgaris Growth Coupled to Formate-Driven H2 Production

Formate is recognized as a superior substrate for biological H2 production by several bacteria. However, the growth of a single organism coupled to this energetic pathway has not been shown in mesophilic conditions. In the present study, a bioreactor with gas sparging was used, where we observed for the first time that H2 production from formate can be coupled with growth of the model sulfate-reducing bacterium Desulfovibrio vulgaris in the absence of sulfate or a syntrophic partner. In these conditions, D. vulgaris had a maximum growth rate of 0.078 h(-1) and a doubling time of 9 h, and the ΔG of the reaction ranged between -21 and -18 kJ mol(-1). This is the first report of a single mesophilic organism that can grow while catalyzing the oxidation of formate to H2 and bicarbonate. Furthermore, high volumetric and specific H2 production rates (125 mL L(-1) h(-1) and 2500 mL gdcw(-1) h(-1)) were achieved in a new bioreactor designed and optimized for H2 production. This high H2 production demonstrates that the nonconventional H2-producing organism D. vulgaris is a good biocatalyst for converting formate to H2.

Simultaneous production and separation of biohydrogen in mixed culture systems by continuous dark fermentation

Hydrogen production by dark fermentation is one promising technology. However, there are challenges in improving the performance and efficiency of the process. The important factors that must be considered to obtain a suitable process are the source of the inoculum and its pre-treatment, types of substrates, the reactor configurations and the hydrogen partial pressure. Furthermore, to obtain high-quality hydrogen, it is necessary to integrate an effective separation procedure that is compatible with the intrinsic characteristics of a biological process. Recent studies have suggested that a stable and robust process could be established if there was an effective selection of a mixed microbial consortium with metabolic pathways directly targeted to high hydrogen yields. Additionally, the integration of membrane technology for the extraction and separation of the hydrogen produced has advantages for the upgrading step, because this technology could play an important role in reducing the negative effect of the hydrogen partial pressure. Using this technology, it has been possible to implement a production-purification system, the 'hydrogen-extractive membrane bioreactor'. This configuration has great potential for direct applications, such as fuel cells, but studies of new membrane materials, module designs and reactor configurations are required to achieve higher separation efficiencies.

Controlled continuous bio-hydrogen production using different biogas release strategies

Dark fermentation for bio-hydrogen (bio-H2) production is an easily operated and environmentally friendly technology. However, low bio-H2 production yield has been reported as its main drawback. Two strategies have been followed in the past to improve this fact: genetic modifications and adjusting the reaction conditions. In this paper, the second one is followed to regulate the bio-H2 release from the reactor. This operating condition alters the metabolic pathways and increased the bio-H2 production twice. Gas release was forced in the continuous culture to study the equilibrium in the mass transfer between the gaseous and liquid phases. This equilibrium depends on the H2, CO2, and volatile fatty acids production. The effect of reducing the bio-H2 partial pressure (bio-H2 pp) to enhance bio-H2 production was evaluated in a 30 L continuous stirred tank reactor. Three bio-H2 release strategies were followed: uncontrolled, intermittent, and constant. In the so called uncontrolled fermentation, without bio-H2 pp control, a bio-H2 molar yield of 1.2 mol/mol glucose was obtained. A sustained low bio-H2 pp of 0.06 atm increased the bio-H2 production rate from 16.1 to 108 mL/L/h with a stable bio-H2 percentage of 55% (v/v) and a molar yield of 1.9 mol/mol glucose. Biogas release enhanced bio-H2 production because lower bio-H2 pp, CO2 concentration, and reduced volatile fatty acids accumulation prevented the associated inhibitions and bio-H2 consumption.

In situ hydrogen utilization for high fraction acetate production in mixed culture hollow-fiber membrane biofilm reactor

Syngas fermentation is a promising route for resource recovery. Acetate is an important industrial chemical product and also an attractive precursor for liquid biofuels production. This study demonstrated high fraction acetate production from syngas (H₂ and CO₂) in a hollow-fiber membrane biofilm reactor, in which the hydrogen utilizing efficiency reached 100% during the operational period. The maximum concentration of acetate in batch mode was 12.5 g/L, while the acetate concentration in continuous mode with a hydraulic retention time of 9 days was 3.6 ± 0.1 g/L. Since butyrate concentration was rather low and below 0.1 g/L, the acetate fraction was higher than 99% in both batch and continuous modes. Microbial community analysis showed that the biofilm was dominated by Clostridium spp., such as Clostridium ljungdahlii and Clostridium drakei, the percentage of which was 70.5%. This study demonstrates a potential technology for the in situ utilization of syngas and valuable chemical production.