Impact of microbial inoculum storage on dark fermentative H2 production

Effects of carbon source variability on enhanced Bio-hydrogen production

This study investigated how glucose, starch, and rapeseed oil, three common food waste components with diverse molecular and physicochemical characteristics, influenced hydrogen production and microbial communities in dark fermentation under varying carbon/nitrogen (C/N) ratios. The results indicated that glucose and starch groups, significantly increased hydrogen yields to 235 mL H2/gVS (C/N = 40) and 234 mL H2/gVS (C/N = 40), respectively, while rapeseed oil, with a lower yield of 30 mL H2/gVS (C/N = 20), demonstrated a negative impact. Additionally, an accumulation of propionate was observed with increasing carbon source complexity, suggesting that simpler carbon sources favored hydrogen production and bacterial growth. Conversely, lipid-based materials required rigorous pre-treatment to mitigate their inhibitory effects on hydrogen generation. Overall, this study underscores the importance of carbon source selection, especially glucose and starch, for enhancing hydrogen production and microbial growth in dark fermentation, while highlighting the challenges posed by lipid-rich substrates that require intensive pre-treatment to optimize yields.

Biomass acid pretreatment impacts on metabolic routes and bacterial composition of dark fermentation process

Complex organic matter represents a suitable substrate to produce hydrogen through dark fermentation (DF) process. To increase H2 yields, pretreatment technology is often required. The main objective of the present work was to investigate thermo-acid pretreatment impact on sugar solubilization and biotic parameters of DF of sorghum or organic fraction of municipal solid waste (OFMSW). Biochemical hydrogen potential tests were carried out without inoculum using raw or thermo-acid pretreated substrates. Results showed an improvement in sugar solubilization after thermo-acid pretreatments. Pretreatments led to similar DF performances (H2 and total metabolite production) compared to raw biomasses. Nevertheless, they were responsible for bacterial shifts from Enterobacteriales towards Clostridiales and Bacillales as well as metabolic changes from acetate towards butyrate or ethanol. The metabolic changes were attributed to the biomass pretreatment impact on indigenous bacteria as no change in the metabolic profile was observed after performing thermo-acid pretreatments on irradiated OFMSW (inactivated indigenous bacteria and inoculum addition). Consequently, acid pretreatments were inefficient to improve DF performances but led to metabolic and bacterial community changes due to their impact on indigenous bacteria.

Highly Efficient Transformation of Tar Model Compounds into Hydrogen by a Ni-Co Alloy Nanocatalyst During Tar Steam Reforming

Hydrogen (H2) production from coal and biomass gasification was considered a long-term and viable way to solve energy crises and global warming. Tar, generated as a hazardous byproduct, limited its large-scale applications by clogging and corroding gasification equipment. Although catalytic steam reforming technology was used to convert tar into H2, catalyst deactivation restricted its applicability. A novel nanocatalyst was first synthesized by the modified sol-gel method using activated biochar as the support, nickel (Ni) as the active component, and cobalt (Co) as the promoter for converting tar into H2. The results indicated that a high H2 yield of 263.84 g H2/kg TMCs (Tar Model Compounds) and TMC conversion of almost 100% were obtained over 6% Ni-4% Co/char, with more than 30% increase in hydrogen yield compared to traditional catalysts. Moreover, 6% Ni-4% Co/char exhibited excellent resistance to carbon deposition by removing the nucleation sites for graphite formation, forming stable Ni-Co alloy, and promoting the char gasification reaction; resistance to oxidation deactivation due to the high oxygen affinity of Co and reduction of the oxidized nickel by H2 and CO; resistance to sintering deactivation by strengthened interaction between Ni and Co, high specific surface area (920.61 m2/g), and high dispersion (7.3%) of Ni nanoparticles. This work provided a novel nanocatalyst with significant potential for long-term practical applications in the in situ conversion of tar into H2 during steam reforming.

Sequential dark fermentation and microbial electrolysis cells for hydrogen production: Volatile fatty acids influence and energy considerations

Hydrogen production from food waste by coupling dark fermentation (DF) and microbial electrolysis cells (MEC) was studied. Metabolic patterns in DF, their effects on MECs efficiency, and the energy output of the coupling were investigated. Mesophilic temperature and acidic pH 5.5 resulted in 72 ± 20 mL H₂/g CODin and a butyrate-enriched profile (C2/C4, 0.5-0.6) contrasting with an acetate-enriched profile (C2/C4, 1.8-1.9) and 36 ± 10 mL H₂/g CODin at pH 7. Assessment in series of the DF effluents in MECs resulted in a higher hydrogen yield (566-733 mL H₂/g CODin) and volatile fatty acids (VFAs) removal (84-95%) obtained from pH 7 effluents compared to pH 5.5 effluents (173-186 mL H₂/g CODin and 29-59%). Finally, the output energy was lower in DF at pH 7, however, these effluents retrieved the highest energy in the MEC, showing the importance of process pH and VFAs profile to balance the coupling.

Dark fermentation and microalgae cultivation coupled systems: Outlook and challenges

The implementation of a sustainable bio-based economy is considered a top priority today. There is no doubt about the necessity to produce renewable bioenergy and bio-sourced chemicals to replace fossil-derived compounds. Under this scenario, strong efforts have been devoted to efficiently use organic waste as feedstock for biohydrogen production via dark fermentation. However, the technoeconomic viability of this process needs to be enhanced by the valorization of the residual streams generated. The use of dark fermentation effluents as low-cost carbon source for microalgae cultivation arises as an innovative approach for bioproducts generation (e.g., biodiesel, bioactive compounds, pigments) that maximizes the carbon recovery. In a biorefinery context, after value-added product extraction, the spent microalgae biomass can be further valorised as feedstock for biohydrogen production. This integrated process would play a key role in the transition towards a circular economy. This review covers recent advances in microalgal cultivation on dark fermentation effluents (DFE). BioH₂ via dark fermentation processes and the involved metabolic pathways are detailed with a special focus on the main aspects affecting the effluent composition. Interesting traits of microalgae and current approaches to solve the challenges associated to the integration of dark fermentation and microalgae cultivation are also discussed.

Fundamentals and recent progress in bioelectrochemical system-assisted biohythane production

Biohythane, a balanced mixture of 10%-30% v/v of hydrogen and 70%-90% v/v of methane, could be the backbone of an all-purpose future energy supply. Recently, bioelectrochemical systems (BES) became a new sensation among environmental biotechnology processes with the potential to sustainably generate biohythane. Therefore, to unleash its full potential for scaling up, researchers are consistently improving microbial metabolic pathways, novel reactors, and electrode designs. This review presents a detailed analysis of recently discovered fundamental mechanisms and science and engineering intervention of different strategies to improve the biohythane composition and production rate from BES. However, several milestones are to be achieved, for instance, improving electrode kinetics using efficient catalysts, engineered microbial communities, and improved reactor configurations, for commercializing this sustainable technology. Thus, a future perspective section is included to recommend novel research lines, mainly focusing on the microbial communities and the efficient electrocatalysts, to enhance reactor performance.

Enhanced Fermentative Hydrogen Production from Food Waste in Continuous Reactor after Butyric Acid Treatment

End-product accumulation during dark fermentation leads to process instability and hydrogen production inhibition. To overcome this constraint, microbial community adaptation to butyric acid can induce acid tolerance and thus enhance the hydrogen yields; however, adaptation and selection of appropriate microbial communities remains uncertain when dealing with complex substrates in a continuous fermentation mode. To address this question, a reactor fed in continuous mode with food waste (organic loading rate of 60 gVS·L·d−1; 12 h hydraulic retention time) was first stressed for 48 h with increasing concentrations of butyric acid (up to 8.7 g·L−1). Performances were compared with a control reactor (unstressed) for 13 days. During 6 days in a steady-state, the pre-stressed reactor produced 2.2 ± 0.2 LH2·L·d−1, which was 48% higher than in the control reactor (1.5 ± 0.2 LH2·L·d−1). The pretreatment also affected the metabolites’ distribution. The pre-stressed reactor presented a higher production of butyric acid (+44%) achieving up to 3.8 ± 0.3 g·L−1, a lower production of lactic acid (−56%), and an enhancement of substrate conversion (+9%). The performance improvement was attributed to the promotion of Clostridium guangxiense, a hydrogen -producer, with a relative abundance increasing from 22% in the unstressed reactor to 52% in the stressed reactor.

Microbial Biogas Production from Pork Gelatine

This research describes the results of the anaerobic digestion of gelatine as a potential hydrogen source with heat-shocked inoculum. The concentrations of applied gelatine were of VSS (volatile suspended solids) ranging from 10 g VSS/L to 30 g VSS/L. The initial process pH was 5.5, and, depending on the concentration, reached pH values from 7.5 to 7.8 after 55 days. Although the inoculum was heat-shocked in 30 g VSS/L of collagen, the process that occurred was hydrogenotrophic anaerobic digestion. In gelatine concentrations below 30 g VSS/L, hydrogen production was dominant only during the first 5 days of the experiments. Then, there was a change from dark fermentation to hydrogenotrophic methane production. The optimal hydrogen and methane yields resulted from the concentrations of 10 g VSS/L (7.65 mL ± 0.01 mL H2/g VSS and 3.49 ± 0.01 L CH4/g VSS). Additionally, 10 g VSS/L had the lowest accumulated emission of hydrogen sulphide (10.3 ± 0.01 mL of H2S), while 30 g VSS/L (0.440 ± 0.01mL H2S/g VSS) produced the lowest yield. After a lag time, the hydrogen production and hydrogen sulphide grew with a specific ratio, depending on the concentration. The hydrogen sulphide emission and sulphur added analysis proved that hydrogen sulphide originating from biogas created by bacteria remains longer than that from a substrate.

Renewable hydrogen production from biomass and wastes (ReBioH2-2020)

Growing consumption of fossil reserves to meet the rising demand of energy has led to climate deterioration and simultaneous waste generation, urging modern society to find sustainable energy resource that can meet the growing energy demands and reduce greenhouse gas emissions and carbon footprints. In this aspect, hydrogen (H₂) is one of the most promising sustainable clean fuels that has gained significant interest in recent years. This article highlights the major research progress on biohydrogen production from renewable bioresources such as organic wastes, lignocellulosic biomass, algal biomass, and industrial wastewaters. It summarizes the research highlights of manuscripts published in the special issue (VSI: ReBioH2-2020), which contains twenty-two articles, including seven critical reviews and fifteen research articles, focusing on biotechnological and thermochemical routes for biohydrogen production from renewable feedstocks. The major findings of the research works in this special issue can be used as a road-map for sustainable renewable hydrogen production from bioresources.