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Aims: In previous studies, it was demonstrated that co-culturing Clostridium pasteurianum and Geobacter sulfurreducens triggers a metabolic shift in the former during glycerol fermentation. This shift, attributed to interspecies electron transfer and the exchange of other molecules, enhances the production of 1,3-propanediol at the expense of the butanol pathway. The aim of this investigation is to examine the impact of fumarate, a soluble compound usually used as an electron acceptor for G. sulfurreducens, in the metabolic shift previously described in C. pasteurianum., Methods and Results: Experiments were conducted by adding along with glycerol, acetate, and different quantities of fumarate in co-cultures of G. sulfurreducens and C. pasteurianum. A metabolic shift was exhibited in all the co-culture conditions. This shift was more pronounced at higher fumarate concentrations. Additionally, we observed G. sulfurreducens growing even in the absence of fumarate and utilizing small amounts of this compound as an electron donor rather than an electron acceptor in the co-cultures with high fumarate addition., Conclusions: This study provided evidence that interspecies electron transfer continues to occur in the presence of a soluble electron acceptor, and the metabolic shift can be enhanced by promoting the growth of G. sulfurreducens., (© The Author(s) 2024. Published by Oxford University Press on behalf of Applied Microbiology International.)

Hydrogen-producing mixed cultures were subjected to a 48-h downward or upward temperature fluctuation from 55 to 35 or 75 °C. Hydrogen production was monitored during the fluctuations and for three consecutive batch cultivations at 55 °C to evaluate the impact of temperature fluctuations and bioaugmentation with synthetic mixed culture of known H 2 producers either during or after the fluctuation. Without augmentation, H 2 production was significantly reduced during the downward temperature fluctuation and no H 2 was produced during the upward fluctuation. H 2 production improved significantly during temperature fluctuation when bioaugmentation was applied to cultures exposed to downward or upward temperatures. However, when bioaugmentation was applied after the fluctuation, i.e., when the cultures were returned to 55 °C, the H 2 yields obtained were between 1.6 and 5% higher than when bioaugmentation was applied during the fluctuation. Thus, the results indicate the usefulness of bioaugmentation in process recovery, especially if bioaugmentation time is optimised.

This study evaluated the influence of the co-substrate proportions (0-60% of cardboard in dry basis) and the initial total solid contents (20-40%) on the batch fermentation performance. Maximum hydrogen yields were obtained when mono-fermenting food waste at high solids contents (89mlH 2 ·gVS -1 ). The hydrogen yields were lower when increasing the proportions of cardboard. The lower hydrogen yields at higher proportions of cardboard were translated into higher yields of caproic acid (up to 70.1gCOD·kgCOD bio ), produced by consumption of acetic acid and hydrogen. The highest substrate conversions were achieved at low proportions of cardboard, indicating a stabilization effect due to higher buffering capacities in co-fermentation. Clostridiales were predominant in all operational conditions. This study opens up new possibilities for using the cardboard proportions for controlling the production of high added-value products in dry co-fermentation of food waste.-1 ), produced by consumption of acetic acid and hydrogen. The highest substrate conversions were achieved at low proportions of cardboard, indicating a stabilization effect due to higher buffering capacities in co-fermentation. Clostridiales were predominant in all operational conditions. This study opens up new possibilities for using the cardboard proportions for controlling the production of high added-value products in dry co-fermentation of food waste., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Among the various biological routes for H 2 production, dark fermentation is considered the most practically applicable owing to its capability to degrade organic wastes and high H 2 production rate. Food waste (FW) has high carbohydrate content and easily hydrolysable in nature, exhibiting higher H 2 production potential than that of other organic wastes. In this review article, first, the current status of H 2 production from FW by dark fermentation and the strategies applied for enhanced performance are briefly summarized. Then, the technical and economic limitations of dark fermentation of FW are thoroughly discussed. Economic assessment revealed that the economic feasibility of H 2 production from FW by dark fermentation is questionable. Current efforts to further increase H 2 yield and waste removal efficiency are also introduced. Finally, future perspectives along with possible routes converting dark fermentation effluent to valuable fuels and chemicals are discussed., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Biodegradable wastes produced seasonally need an upstream storage, because of the requirement for a constant feeding of anaerobic digesters. In the present article, the potential of co-ensiling biodegradable agro-industrial waste (sugar beet leaves) and lignocellulosic agricultural residue (wheat straw) to obtain a mixture with low soluble sugar content was evaluated for long-term storage prior to anaerobic digestion. The aim is to store agro-industrial waste while pretreating lignocellulosic biomass. The dynamics of co-ensiling was evaluated in vacuum-packed bags at lab-scale during 180 days. Characterization of the reaction by-products and microbial communities showed a succession of metabolic pathways. Even though the low initial sugars content was not sufficient to lower the pH under 4.5 and avoid undesirable fermentations, the methane potential was not substantially impacted all along the experiment. No lignocellulosic damages were observed during the silage process. Overall, it was shown that co-ensiling was effective to store highly fermentable fresh waste evenly with low sugar content and offers new promising possibilities for constant long-term supply of industrial anaerobic digesters., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Production of biohydrogen and related metabolic by-products was investigated in Solid State Dark Fermentation (SSDF) of food waste (FW) and wheat straw (WS). The effect of the total solids (TS) content and H 2 partial pressure (pp H2 ), two of the main operating factors of SSDF, were investigated. Batch tests with FW at 10, 15, 20, 25 and 30% TS showed considerable effects of the TS on metabolites distribution. H 2 production was strongly inhibited for TS contents higher than 15% with a concomitant accumulation of lactic acid and a decrease in substrate conversion. Varying the pp H2 had no significant effect on the conversion products and overall degradation of FW and WS, suggesting that pp H2 was not the main limiting factor in SSDF. This study showed that the conversion of complex substrates by SSDF depends on the substrate type and is limited by the TS content., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Interspecies electron transfer is a common way to couple metabolic energy balances between different species in mixed culture consortia. Direct interspecies electron transfer (DIET) mechanism has been recently characterised with Geobacter species which couple the electron balance with other species through physical contacts. Using this mechanism could be an efficient and cost-effective way to directly control redox balances in co-culture fermentation. The present study deals with a co-culture of Geobacter sulfurreducens and Clostridium pasteurianum during glycerol fermentation. As a result, it was shown that Geobacter sulfurreducens was able to grow using Clostridium pasteurianum as sole electron acceptor. C. pasteurianum metabolic pattern was significantly altered towards improved 1,3-propanediol and butyrate production (+37% and +38% resp.) at the expense of butanol and ethanol production (-16% and -20% resp.). This metabolic shift was clearly induced by a small electron uptake that represented less than 0.6% of the electrons consumed by C. pasteurianum. A non-linear relationship was found between G. sulfurreducens growth (i.e the electrons transferred between the two species) and the changes in C. pasteurianum metabolite distribution. This study opens up new possibilities for controlling and increasing specificity in mixed culture fermentation.

Electro-fermentation (EF) is a novel process that consists of electrochemically controlling microbial fermentative metabolism with electrodes. The electrodes can act as either electron sinks or sources that allow unbalanced fermentation. They can also modify the medium by changing the redox balance. Such electrochemical control exerts significant effects not only on microbial metabolism and cellular regulation but also on interspecies interactions and the selection of bacterial populations in mixed microbial cultures. In this paper we propose some basics and principles to better define the EF concept within the field of bioelectrochemistry. We also explore the up-to-date strategies to put EF into practice and propose hypothetical mechanisms that could explain the first EF results reported in the literature., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Solid-state fermentation is a potential technology for developing lignocellulosic biomass-based biorefineries. This work dealt with solid-state fermentation for carboxylates production from corn stover, as building blocks for a lignocellulosic feedstock-based biorefinery. The effect of extrusion pretreatment, together with the action of a microbial consortia and hydrolytic enzymes as biotic triggers, was investigated on corn stover conversion, microbial metabolic pathways, and populations. The extrusion caused changes in the physical and morphological characteristics, without altering the biochemical composition of the corn stover. Extrusion also led to remarkable differences in the composition of the indigenous microbial population of the substrate. Consequently, it affected the structure of community developed after fermentation and the substrate conversion yield, which increased by 118% (from 23 ± 4 gCOD/kgVSi obtained with raw substrate to 51 ± 1 gCOD/kgVSi with extruded corn stover) with regard to self-fermentation experiments. The use of activated sludge as inoculum further increased the total substrate conversion into carboxylates, up to 60 ± 2 gCOD/kgVSi, and shaped the microbial communities (mainly composed of bacteria from the Clostridia and Bacteroidia classes) with subsequent homogenization of the fermentation pathways. The addition of hydrolytic enzymes into the reactors further increased the corn stover conversion, leading to a maximum yield of 142 ± 1 gCOD/kgVSi. Thus, extrusion pretreatment combined with the use of an inoculum and enzyme addition increased by 506% corn stover conversion into carboxylates. Beside biomass pretreatment, the results of this study indicated that biotic factor greatly impacted solid-state fermentation by shaping the microbial communities and related metabolic pathways. [ABSTRACT FROM AUTHOR]

Food waste (FW) management by biological process is more attractive and eco-friendly approach than thermo-chemical conversion or landfilling. However, FW composition and physico-chemical and biological characteristics affect the overall biological process in terms of product yield and degradation rate. To overcome this major bottle-neck, the pretreatment of FW is proposed. Therefore this review aims to provide a comprehensive summary of the importance of pretreatment of FW with respect to FW management by anaerobic digestion (AD) and dark fermentation (DF). It also reviews the existing knowledge gaps and future research perspectives for better integration of FW pretreatments for AD and DF, which should include (i) the preservation of carbon mass through freeze and thaw, or drying; and (ii) improve the carbon accessibility through particle size reduction and thermal pretreatments for high-rate bioenergy recovery. [ABSTRACT FROM AUTHOR]

The increasing food waste production calls for developing efficient technologies for its treatment. Anaerobic processes provide an effective waste valorization. The influence of the initial substrate load on the performance of batch dry anaerobic co-digestion reactors treating food waste and cardboard was investigated. The load was varied by modifying the substrate to inoculum ratio (S/X), the total solids content and the co-digestion proportions. The results showed that the S/X was a crucial parameter. Within the tested values (0.25, 1 and 4 g VS·g VS −1 ), only the reactors working at 0.25 produced methane. Methanosarcina was the main archaea, indicating its importance for efficient methanogenesis. Acidogenic fermentation was predominant at higher S/X, producing hydrogen and other metabolites. Higher substrate conversions (≤48%) and hydrogen yields (≤62 mL·g VS −1 ) were achieved at low loads. This study suggests that different value-added compounds can be produced in dry conditions, with the initial substrate load as easy-to-control operational parameter. [ABSTRACT FROM AUTHOR]

The impact on dark fermentation of seven carbohydrates as model substrates of lignocellulosic fractions (glucose, cellobiose, microcrystalline cellulose, arabinose, xylose, xylan and wheat straw) was investigated. Metabolic patterns and bacterial communities were characterized at the end of batch tests inoculated with manure digestate. It was found that hydrogen production was linked to the sugar type (pentose or hexose) and the degree of polymerisation. Hexoses produced less hydrogen, with a specific selection of lactate-producing bacterial community structures. Maximal hydrogen production was five times higher on pentose-based substrates, with specific bacterial community structures producing acetate and butyrate as main metabolites. Low hydrogen amounts accumulated from complex sugars (cellulose, xylan and wheat straw). A relatively high proportion of the reads was affiliated to Ruminococcaceae suggesting an efficient hydrolytic activity. Knowing that the bacterial community structure is very specific to a particular substrate offers new possibilities to design more efficient H 2 -producing biological systems. [ABSTRACT FROM AUTHOR]

Background: Glycerol is currently an over-produced chemical that can be used as substrate for the production of high value products such as 1,3-propanediol (1,3-PDO) in fermentation processes. The aim of this study was to investigate the effect of initial pH on a batch mixed culture fermentation of glycerol, considering both the bacterial community composition and the fermentation patterns. Results: For pH values between 5 and 9, 1,3-PDO production yields ranged from 0.52 ± 0.01 to 0.64 ± 0.00 mol1,3-PDOmol-1 glycerol, with the highest values obtained at pH 7 and 8. An Enterobacteriaceae member closely related to Citrobacter freundii was strongly enriched at all pH values. Within the less dominant bacterial species, two different microbial community structures were found, one at acid pH values and another at neutral to basic pH values. Conclusions: 1,3-PDO production was improved at pH values over 7. It was anti-correlated with lactate and ethanol production but positively correlated with acetate production. No direct correlation between 1,3-PDO production and a specific family of bacteria was found, suggesting functional redundancies in the microbial community. However, 1,3-PDO production yield remained high over the range of pH studied and was comparable to the best obtained in the same conditions in the literature. [ABSTRACT FROM AUTHOR]

Fixed-biomass reactors present the main advantage of increasing the hydrogen production rates by supporting high organic loading rates. However, the use of complex natural inoculum often precludes a long-term operation through the rapid emergence of methanogens. In this study, an Anaerobic Sequencing Batch Biofilm Reactor (ASBBR) was inoculated with a simplified H 2 -producing culture to evaluate its influence on robustness and more particularly on methanogenic activity. An ASBBR was operated over 6 months with unsterile lactose-rich effluent and under Hydraulic Retention Time ranging from 1 to 34 h. Hydrogen performances in terms of productivity and yields ranged from 0.23 to 519 mmole H2 L −1 d −1 and 0.01 to 7.11 mole H2 mole lactose −1 , respectively. No significant methane production was observed all along the experimental procedure, showing that inoculating with a simplified and highly enriched preculture could increase substantially the robustness of the process. Specific preparation of the inoculum may represent a solution to sustain long-term operation of biofilm-based reactors. [ABSTRACT FROM AUTHOR]

Currently, the use of alternative renewable energies is broadly supported in many countries, some of which are seriously evaluating the possibility of using hydrogen as an alternative fuel in their power systems. Hydrogen production by biological processes, such as dark fermentation, is a very promising alternative. However, this process has only been studied on the laboratory scale, and there is limited experience at the pilot scale. The main reasons of non-scaling hydrogen production by dark fermentation at large scale are unpurified hydrogen production, stability of the bioprocesses, as well as their low conversion yields joined at the formation of byproducts. Improvement of energetic yields of dark fermentation requires a better knowledge of the microorganisms involved in the mixed culture and their possible interactions, as well as the use of appropriate substrates and strategies, such as solid-state fermentation, the purification of hydrogen and the coupling of dark fermentation with other biological processes as anaerobic digestion. The present work offers an overview of the current knowledge dealing with H-production by dark fermentation and its integration into a concept of an environmental biorefinery. Several key points are addressed, such as the benefits of using local waste as substrates, the new solid-state fermentation processes, the coupling of hydrogen purification with the production process, the association of the H-producing process with other biological processes, such as anaerobic digestion towards biohythane production (H/CH). Information about pilot plant experiments was added to illustrate the feasibility of producing fermentative hydrogen and methane from organic waste at a pilot scale, as developed at Feng Chia University (Taiwan). [ABSTRACT FROM AUTHOR]

In this study, the effect of total solid content (TS) on thermophilic hydrogen production from wheat straw was investigated. Six TS contents ranging from wet to dry conditions (10-34% TS) were tested in batch tests. A decrease of H2 yields was observed and three statistical groups were distinguished according to the TS content: wet conditions (10% and 14% TS) with 15.3 ± 1.6 NmlH2 gTS-1, intermediate conditions (19% TS) with 6.4 ± 1.0 NmlH2 gTS-1 and dry conditions (25-34% TS) with 3.4 ± 0.8 NmlH2 gTS-1. Such a decrease in biohydrogen yields was related to a metabolic shift with an accumulation of lactic acid under dry conditions. Concomitantly, a microbial population shift was observed with a dominance of species related to the class Clostridia under wet conditions, and a co-dominance of members of Bacilli, Clostridia classes and Bacteroidetes phylum under dry conditions. [ABSTRACT FROM AUTHOR]

Abstract: Dark fermentation is an intermediate microbial process occurring along the anaerobic biodegradation of organic matter. Saline effluents are rarely treated anaerobically since they are strongly inhibited by high salt concentrations. This study deals with the characterization of microbial communities producing hydrogen under moderate halophilic conditions. A series of batch experiments was performed under anaerobic conditions, with glucose as substrate (5 g L−1) and under increasing NaCl concentrations ranging from 9 to 75 gNaCl L−1. A saline sediment of a lagoon collecting salt factory wastewaters was used as inoculum. Interestingly, a gradual increase of the biohydrogen production yield according to NaCl concentration was observed with the highest value obtained for the highest NaCl concentration, i.e. 75 gNaCl L−1, suggesting a natural adaptation of the sediment inoculum to salt. This work reports for the first time the ability of mixed culture to produce hydrogen in moderate halophilic environment. In addition, maximum hydrogen consumption rates decreased while NaCl concentration increased. A gradual shift of the bacterial community structure, concomitant to metabolic changes, was observed with increasing NaCl concentrations, with the emergence of bacteria belonging to Vibrionaceae as dominant bacteria for the highest salinities. [Copyright &y& Elsevier]

Abstract: Solid organic waste represents an abundant, cheap, and available source of biodegradable substrates not yet exploited to produce biohydrogen by dark fermentation. The impact of the composition of solid organic waste on microbial metabolic pathways and subsequently on biohydrogen production, has not been clearly elucidated. The aim of this study is to determine the compositional features of different substrates that influence bioH2 production. For this, we measured Biological hydrogen potentials (BHP) on 26 different substrates and performed a multivariate statistical analysis of the experimental data using a partial least square regression. The results showed that the BHP values correlated well with the initial carbohydrate content measured after mild hydrolysis. A predictive model explaining more than 89% of the experimental variability was then built to predict the maximal biohydrogen yield with a high accuracy and for a large spectrum of organic waste. An explicative model showed that only carbohydrates, butyrate and lactate concentrations were significant variables explaining more than 98% of biohydrogen yield variability. Interestingly, an interaction term between carbohydrates and lactate concentrations was required to explain microbial pathways producing hydrogen. [Copyright &y& Elsevier]

Production of energy from lignocellulosic biomass or residues is receiving ever-increasing interest. Among the different processes, dark fermentation for producing biohydrogen and anaerobic digestion for producing biomethane present considerable advantages. However, they are limited by the accessibility of holocelluloses that are embedded in the lignin network. The authors propose a review of works on the conversion of biomass into biohydrogen and biomethane with the comprehensive description of (a) biomass composition and features that may impact on its anaerobic conversion and (b) the impact of different kinds of pretreatment on these features and on the performance of biohydrogen and methane production. [ABSTRACT FROM PUBLISHER]

Abstract: The degradation of the natural environment and the energy crisis are two vital issues for sustainable development worldwide. Hydrogen is considered as one of the most promising candidates as a substitute for fossil fuels. In this context, biological processes are considered as the most environmentally friendly alternatives for satisfying future hydrogen demands. In particular, biohydrogen production from agricultural waste is very advantageous since agri-wastes are abundant, cheap, renewable and highly biodegradable. Considering that such wastes are complex substrates and can be degraded biologically by complex microbial ecosystems, the present paper focuses on dark fermentation as a key technology for producing hydrogen from crop residues, livestock waste and food waste. In this review, recent findings on biohydrogen production from agricultural wastes by dark fermentation are reported. Key operational parameters such as pH, partial pressure, temperature and microbial actors are discussed to facilitate further research in this domain. [ABSTRACT FROM AUTHOR]

Correction for ‘The environmental biorefinery: state-of-the-art on the production of hydrogen and value-added biomolecules in mixed-culture fermentation’ by Roman Moscoviz et al., Green Chem., 2018, 20, 3159–3179. [ABSTRACT FROM AUTHOR]

Abstract Electro-fermentation has been recently proposed as a new operational mode of bioprocess control using polarized electrodes. This paper aims to evaluate how polarized electrodes are affecting microbial metabolic fermentative pathways, with a special focus on how the bacterial populations are affected during hydrogen production by dark fermentation. Four different potentials were applied on the working electrode in batch electro-fermentation tests operated with mixed culture and using glucose as a substrate. Two different metabolic behaviours for H 2 production were observed in electro-fermentation. The first one led to a higher H 2 production compared to conventional fermentation with a strong selection of Clostridium sp. The second behaviour led to lower H 2 production along with ethanol, and strongly correlated with the selection of Escherichia and Enterobacter genera. However, the effect of the applied potential on population selection was mostly non-linear and no simple relationship was found between these two parameters. Overall, electro-fermentation process has shown its potential as a new type of control for mixed-culture bioprocesses with significant effects of polarized electrodes on glucose fermentation. Highlights • Electro-fermentation increased hydrogen yields while decreasing lactate production. • Well known hydrogen-producing bacteria were selected during electro-fermentation. • Small amount of current was sufficient to trigger population selection. [ABSTRACT FROM AUTHOR]

The objective of this study was to assess the influence of the hydraulic retention time (HRT) and the pH on the fermentative hydrogen production from a food industry waste (FIW), in a continuous stirred tank bioreactor. Thus, hydrogen production was investigated for HRTs of 12, 8, 6 and 4 h and the results showed a long and stable reactor operation with high hydrogen content in the gas phase and high hydrogen production rates. The optimal HRT was found to be in the range between 6 and 12 h, corresponding to hydrogen yields of 96.27 ± 3.36 and 101.75 ± 3.71 L H 2 /kg FIW for 12 and 6 h, respectively. In the sequel, the effect of the pH (in the range 5–5.9) was investigated, by changing the buffer solution (BS) components, while the HRT value was maintained at 12 h. As anticipated, the results showed that the operating pH had a significant influence on hydrogen production rates and yields. Characterization of the microbial community was performed at various pH values, giving thus a deeper insight to the well-established hydrogen production process from FIW, since the possible biochemical pathways followed by the microbial consortium, under different operational conditions were elucidated. [ABSTRACT FROM AUTHOR]

The aim of this work is to evaluate biohydrogen production from agro-industrial wastewaters and by-products, by combining dark fermentation and microbial electrolysis in a two-step cascade process. Such coupling of both technologies constitutes a technological building block within a concept of environmental biorefinery where sustainable production of renewable energy is expected. Six different wastewaters and industrial by-products coming from cheese, fruit juice, paper, sugar, fruit processing and spirits factories were evaluated for the feasibility of hydrogen production in a two-step process. The overall hydrogen production when coupling dark fermentation and microbial electrolysis was increased up to 13 times when compared to fermentation alone, achieving a maximum overall hydrogen yield of 1608.6 ± 266.2 mLH 2 /gCOD consumed and a maximum of 78.5 ± 5.7% of COD removal. These results show that dark fermentation coupled with microbial electrolysis is a highly promising option to maximize the conversion of agro-industrial wastewaters and by-products into bio-hydrogen. [ABSTRACT FROM AUTHOR]

[Display omitted] • Importance of pH adjustment for efficient food waste fermentation into lactic acid. • High lactic acid concentration (68 g/L), yield (0.38 g/gTS) and selectivity (77%). • Successful replication fermentation assays using depackaging pulp at pilot scale. • Efficient fermentation residues valorization into biomethane by anaerobic digestion. In this study, the production of lactic acid from food waste in industrially relevant conditions was investigated. Laboratory assays were first performed in batch conditions to determine the suitable operational parameters for an efficient lactic acid production. The use of compost as inoculum, the regulation of temperature at 35 °C and pH at 5 enhanced the development of Lactobacillus sp. resulting in the production of 70 g/L of lactic acid with a selectivity of 89% over the other carboxylic acids. Those parameters were then applied at pilot scale in successive fed-batch fermentations. The subsequent high concentration (68 g/L), yield (0.38 g/gTS) and selectivity (77%) in lactic acid demonstrated the applicability of the process. To integrate the process into a complete value chain, fermentation residues were then converted into biogas through anaerobic digestion. Lastly, the experiment was successfully replicated using commercial and municipal waste collected in France. [ABSTRACT FROM AUTHOR]

Abstract: Dilute-acid pretreatments are commonly used to solubilize holocelluloses of lignocellulosic materials and represent a promising route to enhance biohydrogen production by dark fermentation. Besides the soluble sugars released, furan derivatives, such as furfural and 5-HMF, as well as phenolic compounds can accumulate in dilute-acid hydrolyzates and that may affect fermentative microbial populations. In this study, biohydrogen production from glucose (5 g VS L− 1) in batch tests was investigated in presence of increasing volumes (0% – control, 3.75%, 7.5%, 15% and 35% (v/v)) of dilute acid hydrolyzate generated from sunflower stalks (170 °C, 1 h, 4 g HCl/100 gTS). A sharp decrease of the hydrogen yield was observed from 2.04 mol H2 mol− 1 eq. hexose initial in the control to 0 mol H2 mol− 1 eq. hexose initial for volumes higher than 15% of added hydrolyzate. Although acetate and butyrate were the main end-products found in the control, ethanol and lactate accumulated accordingly with the increasing addition of hydrolyzate. A clear shift of dominant microbial populations from Clostridium sp. to Sporolactobacillus sp. was concomitantly observed, suggesting a specific inhibition of the biohydrogen-producing bacteria by adding increasing volumes of hydrolyzates. [Copyright &y& Elsevier]

Abstract: Wheat straw is an abundant agricultural residue which can be used as raw material to produce hydrogen (H2), a promising alternative energy carrier, at a low cost. Bioconversion of lignocellulosic biomass to produce H2 usually involves three main operations: pretreatment, hydrolysis and fermentation. In this study, the efficiency of exogenous enzyme addition on fermentative H2 production from wheat straw was evaluated using mixed-cultures in two experimental systems: a one-stage system (direct enzyme addition) and a two-stage system (enzymatic hydrolysis prior to dark fermentation). H2 production from untreated wheat straw ranged from 5.18 to 10.52 mL-H2 g-VS−1. Whatever the experimental enzyme addition procedure, a two-fold increase in H2 production yields ranging from 11.06 to 19.63 mL-H2 g-VS−1 was observed after enzymatic treatment of the wheat straw. The high variability in H2 yields in the two step process was explained by the consumption of free sugars by indigenous wheat straw microorganisms during enzymatic hydrolysis. The direct addition of exogenous enzymes in the one-stage dark fermentation stage proved to be the best way of significantly improving H2 production from lignocellulosic biomass. Finally, the optimal dose of enzyme mixture added to the wheat straw was evaluated between 1 and 5 mg-protein g-raw wheat straw−1. [Copyright &y& Elsevier]

Hydrogen production by dark fermentation is an emerging technology of increasing interest due to its renewable feature. Recent scientific advances have well investigated the operational conditions to produce hydrogen through the valorization of several wastes or wastewaters. However, the development of standardized protocols to accurately assess the biohydrogen potential (BHP) is of crucial importance. This work is the first interlaboratory and international effort to validate a protocol estimating hydrogen potential using batch tests, using glucose as individual model substrate. The repeatability of the hydrogen potential (HP) increased with variations of the proposed protocol: reducing substrate concentration, increasing the buffer capacity, and using an automatic device. The interlaboratory variation of the HP was reduced from 32 to 12%, demonstrating the reproducibility and robustness of the proposed protocol. Recommendations to run BHP tests were formulated in terms of i) repeatability and reproducibility of results, ii) criteria for results validation and acceptance, iii) workload of the proposed protocols. • The standardized protocol for bioH 2 potential was internationally validated. • Substrate content and buffer capacity affect the repeatability and reproducibility. • H 2 potential fits the acceptance criteria of ≥14% of interlaboratory variation. • Either the manual or automatic protocol fits the acceptance criteria. [ABSTRACT FROM AUTHOR]

Hydrogen is a promising alternative of clean energy carrier which can be biologically produced from glycerol-rich waste an abundant and economic source of substrate. However, continuous hydrogen-producing systems still need to be improved and in particular by manipulating the only few available operating conditions. The aim of this study was to investigate the effect of the two main operational parameters, ie. pH and hydraulic retention time (HRT) on hydrogen yields and microbial community structures. For that, a continuous stirred tank reactor (CSTR) was first inoculated with an enriched mixed microflora and was then fed with glycerol. A strong influence of these two operational parameters was shown on hydrogen yields where the maximum yield (0.58 ± 0.13 mol H2 mol −1 glycerol ) was achieved at pH 5.5 - HRT 12 h and was 28.5 times higher than the minimum (0.02 ± 0.02 mol H2 mol −1 glycerol ) obtained at pH 5.0 - HRT 14 h. Changes in most dominant microbial populations were mainly influenced by the pH. Interestingly, HRT parameter related to changes in the metabolic patterns and influenced the composition of subdominant microorganisms, suggesting they might have a key role in changing the ability of the consortium and/or the activity of dominant microorganisms to produce hydrogen. [ABSTRACT FROM AUTHOR]

Batch cultivation of an anaerobic consortium fed with glucose as sole carbon source showed a sharp decrease of the hydrogen productivity when volatile fatty acids (VFA) concentration exceeded 12.5 g L −1 . To avoid VFA accumulation, fermentative batch cultures were thereafter carried out with a submerged membrane anaerobic bioreactor to continuously remove hydrogen fermentation co-products, while retaining the biomass. The membrane made it possible to separate the residence times of bacterial biomass and hydraulic part. With this technology, average and maximal productivities reached 0.75 and 2.46 L H2 L −1 h −1 , corresponding to an increase of 44 and 51% in comparison to the control, respectively. By removing the VFAs from the cultivation medium, H 2 -producing pathways were favored, confirming the metabolic inhibitory effects of co-product accumulation in fermentation medium. Such hydrogen productivity is one of the highest values encountered in the literature. Readily implementable, such technology offers a good opportunity for further developing high rate hydrogen fermentation bioprocesses. [ABSTRACT FROM AUTHOR]

In the present work, the influence of hydraulic retention time (HRT) on dark fermentation metabolism was evaluated through the operation and analysis of a series of four continuous stirred tank reactors (CSTR) at four HRT ranging from 6 h to 24 h. A maximum volumetric hydrogen production rate (VHPR) of 2000 ± 149 mL/L-d corresponding to an H 2 yield of 0.86 mol H2 /mol lactose was observed at 6 h HRT. In depth analysis of metabolite profiles and microbial communities showed that low values of HRT favored the emergence of a community dominated by Clostridiaceae-Lachnospiraceae-Enterobacteriaceae , which performed metabolic pathways co-producing hydrogen. In contrast, long HRT led to the establishment of Sporolactobacillaceae-Streptococcaceae microbial community that outcompeted hydrogen producing bacteria and was responsible of lactate production. Results suggested that these two communities mutually excluded themselves and HRT can act as an operational parameter to control the microbial communities and consequently the related metabolic pathways. [ABSTRACT FROM AUTHOR]

[Display omitted] • pH 5.5 promoted hydrogen and low acetate/butyrate ratio in dark fermentation. • The use of pH 7 in dark fermentation enriched the effluent in acetate. • Acetate-rich effluent from pH 7 enhanced hydrogen yield and VFAs removal in MECs. • Butyrate consumption in MECs needs to be optimized in future studies. • Total energy output was similar regardless of the pH. 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 H 2 /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 H 2 /g CODin at pH 7. Assessment in series of the DF effluents in MECs resulted in a higher hydrogen yield (566–733 mL H 2 /g CODin) and volatile fatty acids (VFAs) removal (84–95%) obtained from pH 7 effluents compared to pH 5.5 effluents (173–186 mL H 2 /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. [ABSTRACT FROM AUTHOR]

This study aimed at maximizing the energy yields from food waste in a three-step conversion scheme coupling dark fermentation (DF), photofermentation (PF) and anaerobic digestion (AD). Continuous H 2 production was investigated over a period of nearly 200 days in a thermophilic semi-continuous DF process with no pH control. The highest H 2 yield of 121.45 ± 44.55 N L H 2 /kg VS was obtained at an organic loading rate of 2.5 kg VS/m 3 /d and a hydraulic retention time of 4 days. The DF effluents mainly contained volatile fatty acids (VFAs) and alcohols as metabolites and un-hydrolyzed solid residues. The supernatant, after separation, was used to recover H 2 in a PF using Rhodobacter sphaeroides . The solid residual fraction along with PF effluent was converted into methane by anaerobic digestion. By combining DF and PF, the H 2 yield from the food waste increased 1.75 fold. Moreover, by adding AD as a post treatment of the DF residue, the total energy yield was substantially increased to reach 5.55 MJ/kg VS food waste added, versus 3.55 MJ/kg VS food waste when applying solely AD. [ABSTRACT FROM AUTHOR]

Dark fermentation of organic biomass, i.e. agricultural residues, agro-industrial wastes and organic municipal waste is a promising technology for producing renewable biohydrogen. In spite of its potential, this technology needs further research and development to improve the biohydrogen yield by optimizing substrate utilization, microbial community enrichment and bioreactor operational parameters such as pH, temperature and H 2 partial pressure. On the other hand, the technical and economic viability of the processes need to be enhanced by the use of valuable by-products from dark fermentation, which mostly includes volatile fatty acids. This paper reviews a range of different organic biomasses and their biohydrogen potential from laboratory to pilot-scale systems. A review of the advances in H 2 yield and production rates through different seed inocula enrichment methods, bioreactor design modifications and operational conditions optimization inside the dark fermentation bioreactor is presented. The prospects of valorizing the co-produced volatile fatty acids in photofermentation and bioelectrochemical systems for further H 2 production, methane generation and other useful applications have been highlighted. A brief review on the simulation and modeling of the dark fermentation processes and their energy balance has been provided. Future prospects of solid state dark fermentation are discussed. [ABSTRACT FROM AUTHOR]

Abstract: A series of batch tests was conducted to investigate the effect of a CO2 sparging pretreatment on hydrogen production from fermentation of the organic fraction of municipal solid waste. Tests were carried out in a stirred bioreactor without adding inoculum or supplementation solution, at 35 °C and with a pH regulated at 5.5. The influence of CO2 flowrate (30, 100 and 500 mL/min) was evaluated. Results showed a decrease in biological hydrogen production for a sparging flowrate of , similar performances for the control experiments and for a sparging flowrate of and an increase in biohydrogen production for a sparging flowrate of . For this latter operating condition, a substantial improvement in hydrogen production performances was obtained with an increase in the hydrogen yield by more than 20% and a decrease in the chemicals consumption used to maintain the pH by more than 20%. No shift in the metabolic pathways and in the bacterial community structure was observed but positive bacterial interactions occurred and enhanced the hydrogen production. Therefore, this study suggested that the growth of hydrogen producing bacteria could be either enhanced or slowed down by a CO2 sparging pretreatment depending on the level of CO2 flowrate, the shift being close to . [Copyright &y& Elsevier]

Abstract: Dark fermentation using mixed cultures is an attractive biological process for producing hydrogen (H2) from lignocellulosic biomass at a low cost. Physicochemical pretreatment is generally used to convert lignocellulosic materials into monosaccharides. However, the processes also involved release degradation byproducts which can, in turn, inhibit microbial growth and metabolism and, hence, impact substrate conversion. In this study, the impact on H2 production of lignocellulose-derived compounds (i.e. furan derivatives, phenolic compounds and lignins) was assessed along with their effect on bacterial communities and metabolisms. Batch tests were carried out using xylose as model substrate (1.67molH2 molxylose −1 in the control test). All the putative inhibitory compounds showed a significant negative impact on H2 production performance (ranging from 0.34 to 1.39molH2 molxylose −1). The H2 yields were impacted more strongly by furan derivatives (0.40–0.51molH2 molxylose −1) than by phenolic compounds (1.28–1.39molH2 molxylose −1). Except for the batch tests supplemented with lignins, the lag phase was shorter for inhibitors having the highest molecular weight (8 days versus 22 days for the lowest MW). Variability of the lag phase was clearly related to a shift in bacterial community structure, as shown by multivariate ordination statistics. The decrease in H2 yield was associated with a decrease in the relative abundance of several H2-producing clostridial species. Interestingly, Clostridium beijerinkii was found to be more resistant to the inhibitors, making this bacterium an ideal candidate for H2 production from hydrolyzates of lignocellulosic biomass. [Copyright &y& Elsevier]

Abstract: Dark fermentation using mixed cultures is a promising biotechnology for producing hydrogen (H2) from renewable organic waste at a low cost. The impact of the characteristics of carbohydrates was evaluated on H2 production and the associated changes in clostridial populations. A series of H2-producing batch experiments was performed from mono-, di- to tri-saccharides (i.e. fructose, glucose, sucrose, maltose, cellobiose, maltotriose). Both chain length and alpha- or beta-linkage of carbohydrates impacted H2 production performance as well as the patterns of hydrogenases. The H2 yield, ranging from 1.38 to 1.84 mol-H2/mol-hexose, decreased with the increasing chain length of the carbohydrates, showing a negative effect of the hydrolysis step on H2 production efficiency. Changes in H2 yield were associated with a specialization of clostridial species, which used different metabolic routes. The rise in H2 production was associated with butyrate and acetate increases while H2 consumption was related to caproate formation. Both clostridial [FeFe]- and [NiFe]-hydrogenases were identified in cellobiose cultures by a proteomic approach. This is the first study that combines genetic and proteomic analyses focused on H2-producing bacteria under various conditions and it opens very interesting perspectives to better understand and optimize H2 production using mixed cultures. [Copyright &y& Elsevier]

Abstract: The fermentative hydrogen production occurs during the anaerobic digestion of organic matter. It is a promising technology to produce renewable energy. However, the mixed culture fermentation performances vary considerably depending on the operating conditions such as pH. We investigated the potential of a molecular CE-SSCP (capillary electrophoresis-single strand conformation polymorphism) fingerprinting method based on the hydA functional genes to better describe the bacterial community dynamics, with regards to the standard 16S rDNA-based method. A series of batch experiments was performed from sucrose at different initial pH from 4 to 6. As expected, the highest H2 production potentials (H max) and rates (R max) were obtained at the highest pH. Changes in batch performances were clearly associated with shifts in the hydA diversity and structure. In contrast, 16S rDNA-based fingerprints were less sensitive to changes in H2 production performances. The H max was related to lower hydA diversity, with Clostridium sporogenes as the major H 2 producer. Communities harboring larger hydA diversities were found in experiments with the higher R max, suggesting that species coexistence may have positive effects on H2 production. [Copyright &y& Elsevier]