Search

Microalgae biomass can be a feedstock for anaerobic digestion, but the recalcitrance of the cell wall limits fully exploiting this source of organic matter. Alkaline pretreatments modify the structure of the cell wall facilitating bacterial attack and solubilizing the organic matter content. It is unknown whether alkaline hydrogen peroxide can help to improve biogas production from microalgae biomass compared to other alkaline pretreatments. This work focused on methane production at alkaline conditions (pH 9) with pretreated microalgal biomass (Scenedesmus obtusiusculus). Alkaline hydrogen peroxide (AHP) was used for the thermo-alkaline pretreatment and compared with pretreatments using NaOH or CaO. The pretreatment with 15% CaO solubilized the highest carbohydrates (22.1%) and proteins (18.3%), while the microalgal biomass pretreated with 4 M NaOH or 1.5% AHP attained the highest yields of methane (227.1 mL and 208.4 mL CH4/g VS, respectively), improving the methane potential between 34 and 46%, and producing biogas with methane content up to 91%. A preliminary techno-economic and energy analysis showed energy savings of 40% with 4 M NaOH and 31% with 1.5% AHP; therefore, AHP could be considered a suitable option for the pretreatment of microalgal biomass.

The addition of external redox mediators (e.g., humic substances) as a strategy to enhance the production of H2 and carboxylic acids through mixed-culture dark fermentation was investigated. Pahokee Peat and Leonardite humic substances along the anthraquinone-2,6-disulfonate (AQDS) humic model compound were evaluated. The use of Pahokee peat into dark fermentation assays reached an H2 potential of 618.2 ± 28.9 mL H2/L, equivalent to an increase of 8% compared with the control experiment. In contrast, the addition of Leonardite (in its basal redox state) as a redox mediator increased the H2 potential by up to 14.5% and achieved 655.8 mL H2/L-d, which was higher than the control's H2 production of 572.6 mL H2/L-d. Moreover, the use of chemically reduced Leonardite led to the highest H2 potential (1158.6 mL H2/L-d) which was about 1.8-fold the H2 potential found in the control. Overall, experimental and production-modeling results suggest that external redox mediators not only served as an additional source of electrons for dark-fermentative pathways but also as electron acceptors. Electrons from redox mediators were likely transported through membrane proteins and could be used to reduce NAD+ and Fdox. Results indicated that electrons were preferably channeled through the butyrate pathway when reduced substances were applied.

What if it was mentioned that there are good microbes that can eat garbage? There may be reports stating that microbes are bad and cause diseases. However, there are also good microbes that can eat the organic garbage (like banana peels and old vegetables) generated in houses or restaurants. These microbes can use garbage to produce energy, in the form of hydrogen, which can be used to fuel our cars. The process of turning organic garbage into energy is called dark fermentation. During dark fermentation, other helpful compounds are also generated, which can be used to make foods, medicines, beverages, and other useful things. This article discusses the dark fermentation process and the products obtained along the way.

Microbial communities involved in hydrogen (H2) production from enzymatic hydrolysates of agave bagasse were analyzed through 16S rRNA sequencing. Two types of reactor configurations and four different enzymatic hydrolysates were evaluated. Trickling bed reactors led to highly-diverse microbial communities, but low volumetric H2 production rates (VHPR, maximum: 5.8 L H2/L-d). On the contrary, well-controlled environments of continuous stirred-tank reactors favored the establishment of low diverse microbial communities composed by Clostridium-Sporolactobacillus leading to high-performance H2-production (VHPR maximum: 13 L H2/L-d). Cellulase-Viscozyme and Celluclast-Viscozyme hydrolysates led to the co-dominance of Clostridium and Sporolactobacillus, possibly due to the presence of xylose and hemicellulose-derived carbohydrates. Cellulase hydrolysates were linked to communities dominated by Clostridium, while maintaining low abundance of Sporolactobacillus. Stonezyme hydrolysates favored microbial communities co-dominated by Clostridium-Lachnoclostridium-Leuconostoc. Moreover, contrary to the prevailing theory, it was demonstrated that H2 production performance was inversely related to microbial diversity.

Sulfate-reducing microbial communities remain a suitable option for the remediation of acid mine drainage using several types of carrier materials and appropriate reactor configurations. However, acetate prevails as a product derived from the incomplete oxidation of most organic substrates by sulfate reducers, limiting the efficiency of the whole process. An established sulfate-reducing consortium, able to degrade acetate at initial acidic pH (3.0), was used to develop biofilms over granular activated carbon (GAC), glass beads, and zeolite as carrier materials. In batch assays using glycerol, biofilms successfully formed on zeolite, glass beads, and GAC with sulfide production rates of 0.32, 0.26, and 0.14 mmol H2S/L\textperiodcenteredd, respectively, but only with glass beads and zeolite, acetate was degraded completely. The planktonic and biofilm communities were determined by the 16S rRNA gene analysis to evaluate the microbial selectivity of the carrier materials. In total, 46 OTUs (family level) composed the microbial communities. Ruminococcaceae and Clostridiaceae families were present in zeolite and glass beads, whereas Peptococcaceae was mostly enriched on zeolite and Desulfovibrionaceae on glass beads. The most abundant sulfate reducer in the biofilm of zeolite was Desulfotomaculum sp., while Desulfatirhabdium sp. abounded in the planktonic community. With glass beads, Desulfovibrio sp. dominated the biofilm and the planktonic communities. Our results indicate that both materials (glass beads and zeolite) selected different key sulfate-reducing microorganisms able to oxidize glycerol completely at initial acidic pH, which is relevant for a future application of the consortium in continuous bioreactors to treat acidic streams., Netherlands Ministry of Education, Culture, and Science and the Netherlands Science Foundation through SIAM Gravitation grant 024.002.002, info:eu-repo/semantics/publishedVersion

A novel strategy to discontinuously increase the biomass concentration in a continuous stirred-tank reactor was evaluated to enhance the performance of dark fermentation. Different concentrations of biomass were evaluated at organic loading rates (OLR) ranging from 90 to 160 g lactose/L-d with a hydraulic retention time (HRT) of 6 h. The study revealed that the discontinuous increase of biomass enhanced the hydrogen (H2) production rates and carboxylic acids concentrations by 19–25% and 8–23%, respectively. In particular, a maximum H2 production rate of 30.8 L H2/L-d with carboxylic acids concentration of 20 g/L was reached at an OLR of 138 g lactose/L-d with a biomass concentration of 15 g volatile suspended solids/L. The analysis of microbial communities showed the co-dominance of Clostridium and lactic acid bacteria. Overall, the discontinuous increase of biomass was an effective strategy to improve the performance of suspended-biomass reactors operated at high OLR and low HRT.

The objective of this work was to comparatively evaluate the production of biohydrogen (bio-H2) from tequila vinasses at optimized mesophilic and thermophilic conditions and to elucidate the main metabolic routes involved. Optimal temperatures of 35 °C and 55 °C, and pH of 5.5 maximized the bio-H2 production rates, 25.5 ± 0.01 NmL h−1 and 169.9 ± 8.9 NmL h−1 in the mesophilic and thermophilic regimens, respectively. During the operation of anaerobic sequencing batch reactors, the thermophilic process allowed a volumetric bio-H2 production rate of 519 ± 13 NmL-H2 L−1 d−1 equivalent to 750 ± 19 NmL-H2 Lvinasse−1, while the mesophilic one 448 ± 23 NmL-H2 L−1 d−1 and 647 ± 33 NmL-H2 Lvinasse−1, respectively. Furthermore, the gas produced under thermophilic conditions showed high hydrogen content (86.5%). Finally, formate degradation and glucose fermentation to acetic and butyric acids were the main metabolic routes involved in bio-H2 production under thermophilic conditions, while at mesophilic conditions, the lactate and formate degradation pathways governed.

Optimum conditions for the enzymatic hydrolysis of non-pretreated agave bagasse using two binary mixtures of commercial enzymatic preparations with cellulase and hemicellulase activities (Celluclast 1.5 L/Viscozyme L and Cellic® Ctec2/Cellic® Htec2) were determined. The optimization was carried out by using the response surface methodology (RSM). Two response variables were analyzed, hydrolysate reducing sugar concentration (RS; mg/mL) and saccharification yield (SY; mg RS/g bagasse). Higher RS concentration (27.58 ± 3.68 mg/mL) and SY (312.54 ± 46.89 mg RS/g bagasse) were obtained with the mixture Celluclast 1.5 L and Viscozyme L, even when compared with lignocellulosic biomass specialized enzyme mixture of Cellic® Ctec2 and Cellic® Htec2. The RS concentration in the hydrolysate and the SY obtained under optimal conditions for both enzymatic mixtures were higher when compared with previous results. In the case of the mixture Cellic® Ctec2 and Cellic® Htec2, an increase of 5.4- and 2.2-times for the RS concentration and SY were respectively observed, while in the case of the mixture Celluclast 1.5 L and Viscozyme L, an increase of 2- and 1.6-times were detected. The degree of synergy (DS) for the enzymatic hydrolysis was 1.52 (with SY as response variable) and 1.65 (with RS concentration as response variable) for the mixture of Celluclast 1.5 L and Viscozyme L, which indicates the synergism between the enzymes present in this binary mixture.

Hydrolysis can be considered as the bottleneck for biofuel production from microalgae due to the recalcitrance of this type of biomass. Thermochemical pretreatment is an effective and widely used process to solubilize different types of biomass. Nevertheless, the variability of the cell wall composition, among different microalgae species, hinders the development of a suitable pretreatment. In this work, the most relevant factors for thermochemical pretreatment to solubilize organic matter from microalgal biomass (Scenedesmus obtusiusculus) were analyzed and identified. The aim was to obtain hydrolysates, rich in soluble organic matter, that allow attaining higher hydrogen and methane yields than with raw microalgae, using a factorial design to identify the significant factors of the thermochemical pretreatment. The results showed that temperature (100 °C) and HCl concentration (3%) were the most significant factors to achieve carbohydrates solubilization close to 100%. Further, reaction time (1.7 h) and biomass concentration (30 g total solids/L) were also important to achieve up to 60% chemical oxygen demand (COD) solubilization. Overall, the microalgal hydrolysates produced 48 NmL H2/g VS and 296 NmL CH4/g VS, which is 1.7- and 1.3-fold higher hydrogen and methane yields, respectively, than raw microalgae. These results are meaningful because high soluble carbohydrates and COD would allow the application of high organic loading rates in continuous reactors for biofuel production using renewable biomass such as microalgae.

International audience; 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.

In the present work, agave bagasse enzymatic hydrolysates obtained with newly locally-available commercial enzymatic preparations were explored for their corresponding hydrogen and methane production potential in batch mode. The major levels in chemical oxygen demand and total carbohydrates were provided by enzymatic hydrolysates made with Zymapect and Stonezyme, respectively. Batch experiments demonstrated that Celluclast 1.5L achieves the maximum hydrogen productivity (1.88 L H2/L), from 1.6 to 2.0-fold higher than other alternatives, whereas Zymapect attains the highest methane productivity (1.32 L CH4/L), with high specific yield reached by both Stonezyme and Zymapect (162 and 163 L CH4/kg bagasse), from 1.7 to 2.0-fold higher than other options. Finally, a preliminary techno-economic analysis allowed to elucidate that the cheapest alternatives for hydrogen and methane production at batch scale are Celluclast 1.5L and Stonezyme, respectively. Overall, the present analysis could serve as groundwork for the selection of the best enzymatic alternatives for hydrogen and methane production.

One of the bottlenecks of the hydrogen production by dark fermentation is the low yields obtained because of the homoacetogenesis persistence, a metabolic pathway where H2 and CO2 are consumed to produce acetate. The central reactions of H2 production and homoacetogenesis are catalyzed by enzyme hydrogenase and the formyltetrahydrofolate synthetase, respectively. In this work, genes encoding for the formyltetrahydrofolate synthetase (fthfs) and hydrogenase (hydA) were used to investigate the diversity of homoacetogens as well as their phylogenetic relationships through quantitative PCR (qPCR) and next-generation amplicon sequencing. A total of 70 samples from 19 different H2-producing bioreactors with different configurations and operating conditions were analyzed. Quantification through qPCR showed that the abundance of fthfs and hydA was strongly associated with the type of substrate, organic loading rate, and H2 production performance. In particular, fthfs sequencing revealed that homoacetogens diversity was low with one or two dominant homoacetogens in each sample. Clostridium carboxivorans was detected in the reactors fed with agave hydrolisates; Acetobacterium woodii dominated in systems fed with glucose; Blautia coccoides and unclassified Sporoanaerobacter species were present in reactors fed with cheese whey; finally, Eubacterium limosum and Selenomonas sp. were co-dominant in reactors fed with glycerol. Altogether, quantification and sequencing analysis revealed that the occurrence of homoacetogenesis could take place due to (1) metabolic changes of H2-producing bacteria towards homoacetogenesis or (2) the displacement of H2-producing bacteria by homoacetogens. Overall, it was demonstrated that the fthfs gene was a suitable marker to investigate homoacetogens in H2-producing reactors. • qPCR and sequencing analysis revealed two homoacetogenesis phenomena. • fthfs gene was a suitable marker to investigate homoacetogens in H2 reactors.

Sulfate-reducing microbial communities remain a suitable option for the remediation of acid mine drainage using several types of carrier materials and appropriate reactor configurations. However, acetate prevails as a product derived from the incomplete oxidation of most organic substrates by sulfate reducers, limiting the efficiency of the whole process. An established sulfate-reducing consortium, able to degrade acetate at initial acidic pH (3.0), was used to develop biofilms over granular activated carbon (GAC), glass beads, and zeolite as carrier materials. In batch assays using glycerol, biofilms successfully formed on zeolite, glass beads, and GAC with sulfide production rates of 0.32, 0.26, and 0.14 mmol H

Fermentation effluents are exploited in microbial electrolysis cells (MECs) for hydrogen production, but they may include ethanol and other compounds that are detrimental to MEC performance. Ethanol can disrupt microbial viability, and lactate makes MEC feeding more complex. The present work aimed to assess the effects of lactate and ethanol on hydrogen and electric current production in MECs to determine the corresponding inhibitory concentrations. A two-chamber MEC was inoculated with compost leachate and fed with lactate-ethanol mixtures at four ratios (total 2.5 g COD L−1), and a volatile fatty acid mixture was used as a base substrate to achieve anolyte at a final concentration of 3.5 g COD L−1. The MEC operated at 1.0 V for 21 days, and its performance was evaluated for COD removal, cathodic and energy efficiency. The inhibitory ethanol concentration was 0.8 g COD L−1 (0.38 g L−1), and lactate reduced the current density lag phase. The start of hydrogen production required 218 ± 35C in the MEC system utilized. This work shows that ethanol content exerts an inhibitory effect on current and hydrogen production; moreover, a relationship between accumulated charge and accumulated hydrogen volume was found suitable to characterize the process.

Agave tequilana bagasse is a suitable lignocellulosic residue for energy production. However, the presence of lignin and the heterogeneous structure of hemicellulose may hinder the availability of polysaccharides. In this work, the pretreatment of A. tequilana bagasse with alkaline hydrogen peroxide (AHP) followed by enzymatic saccharification with hemicellulases and cellulases was assessed for the removal of lignin and extraction of fermentable sugars, respectively. Results of the AHP pretreatment indicated that it is possible to attain up to 97% delignification and recover 88% of cellulose and hemicellulose after only 1.5 h of treatment. Regarding the saccharification process, the total sugar yield and productivity were both increased by 2-fold using an enzymatic mixture (cellulases + hemicellulases) compared to single enzyme hydrolysis (cellulases), evidencing synergism. Further evaluation of the hydrolyzates as substrate for hydrogen and methane production, resulted in yields 1.5 and 3.6-times (215.14 ± 13 L H2 and 393.4 ± 13 L CH4 per kg bagasse, respectively) superior to those obtained with hydrolyzates of non-pretreated bagasse processed with a single enzyme. Overall, using AHP pretreatment and subsequent hydrolysis with enzymatic mixtures improves the saccharification of A. tequilana bagasse enhancing the production of hydrogen and methane.

In the pursuit of renewable sources for biogas generation, microalgae biomass constitutes a promising alternative. In this work, thermally pretreated Scenedesmus obtusiusculus was evaluated as source of biomass for methane production, in systems operated under batch and semi-batch regime. In batch experiments, methane production yields ranging from 109.3 to 192.7 mL CH4/g VSadded were observed with thermally-pretreated (98 °C, 6 h) microalgae biomass at concentrations between 1 and 10 g tCOD/L. On the other hand, the system operated under semi-batch mode for 50 days attained a volumetric methane production rate of 1.15 L CH4/L-d with a methane yield of 177.9 mL CH4/g VSadded at an organic loading rate of 10 g tCOD/L-d and a reaction time of 12 h. These results demonstrate that methane production from microalgae biomass in a semi-batch reactor is possible at low reaction time (12 h) and high organic loading rate using a moderate-temperature pretreatment for the microalgae biomass.

Continuous H2 and CH4 production in a two-stage process to increase energy recovery from agave bagasse enzymatic-hydrolysate was studied. In the first stage, the effect of organic loading rate (OLR) and stirring speed on volumetric hydrogen production rate (VHPR) was evaluated in a continuous stirred tank reactor (CSTR); by controlling the homoacetogenesis with the agitation speed and maintaining an OLR of 44 g COD/L-d, it was possible to reach a VHPR of 6 L H2/L-d, equivalent to 1.34 kJ/g bagasse. In the second stage, the effluent from CSTR was used as substrate to feed a UASB reactor for CH4 production. Volumetric methane production rate (VMPR) of 6.4 L CH4/L-d was achieved with a high OLR (20 g COD/L-d) and short hydraulic retention time (HRT, 14 h), producing 225 mL CH4/g-bagasse equivalent to 7.88 kJ/g bagasse. The two-stage continuous process significantly increased energy conversion efficiency (56%) compared to one-stage hydrogen production (8.2%).

Methane production from an enzymatic agave bagasse hydrolysate was evaluated in a continuous stirred tank reactor (CSTR) and in an upflow anaerobic sludge blanket reactor (UASB). The effect of different organic loading rates (OLRs) and hydraulic retention times (HRTs) on volumetric methane production rate (VMPR) was tested. To hydrolyze the bagasse, Cellulase 50XL, a food-grade enzyme, allowed a saccharification yield of lignocellulosic biomass similar to Celluclast 1.5L, which is used to treat lignocellulosic substrates. The UASB reactor operated at larger OLR (e.g., 20 g CODs/L-d) and shorter HRT (e.g., 30 h) than the CSTR (e.g., OLR of 7.3 g CODs/L-d and HRT of 66 h). The UASB achieved a VMPR of 4.5 ± 0.1 L CH4/L-d and COD removal of 94% while the CSTR had a VMPR of 1.92 ± 0.03 L CH4/L-d and COD removal of 88%. The better performance of the UASB than the CSTR can be explained by the high solid retention time (SRT) and the microbial community composition, which was less diverse than the CSTR’s community. Hydrogenotrophic methanogens were abundant in the UASB while acetoclastic methanogens dominated in the CSTR. The high VMPR obtained by the UASB reactor and the use of Cellulase 50XL can be advantageous features for future industrial application.

Agave bagasse (AB) is the main solid waste generated by the tequila industry in Mexico, which is an environmental concern due to its considerable volume of production (377,000 Ton in 2016). AB is a lignocellulosic biomass that has been considered as a potential feedstock for different industrial uses in the framework of lignocellulosic biorefinery concept. The lignocellulosic biomass is a complex structure constituted by cellulose, hemicellulose and lignin. Therefore, for a complete waste revalorization, different processing steps would be required. In this work, the scientific advances toward the AB biorefinery composed by three sequential stages: pretreatment, treatment and biofuels production were reviewed. Moreover, the by-products of the process could also be recovered and used for the synthesis of value-added products. This integrative approach of AB in the conceptualized biorefinery generates positive impacts on environment as well as on local and regional economies.

The aurocyanide complex recovery by precipitation with zinc decreases when gold concentration drops to − 1 . Therefore, a technology capable of recovering gold from very diluted solution is necessary. Adsorption and desorption experiments were carried out with real solutions in batch and in continuous processes to determine the gold adsorption capacity and rate onto two activated carbons (AC). The results showed that AC F400 and CSC removed 86 and 81 μg g − 1 , respectively, at pH 11 and C e of 10 μg L − 1 in about 4 h. Whereas desorption test by NaOH and CN − showed a gold recovery of up to 86%. These experiments were correlated to the textural and chemical properties of adsorbents and to the chemical composition of samples from a heap leach process. AC is a highly efficient and selective adsorbent for gold cyanide solutions when gold is present in very low concentration ( − 1 ).

The start-up of microbial fuel cells (MFCs) requires biofilm development onto electrodes and acclimation of the microbial community to specific substrates. This process is time-consuming and shows low effectivity when complex substrates are utilized. The varying proportions and complex mixtures of volatile fatty acids (VFAs) in bioprocess effluents makes it difficult to predict the power output of VFA-based MFCs when this technology is coupled to a previous bioprocess. In this work, the effect of the degree of reduction of single VFAs on MFC potential was investigated, and a novel acclimation procedure involving the feeding of gradually more complex VFA mixtures is proposed. The power output obtained with butyric acid (6.1 ± 2 mW m−2) was higher than that obtained with propionic acid (5.0 ± 1 mW m−2), as expected based on their degree of reduction. The MFC potential generated by an acetic acid-fed MFC reached 60 mV, but an acetic-butyric acid mixture reduced the cell potential by one order of magnitude. The subsequent addition of propionic acid to the acetic-butyric mixture increased the MFC potential from 1.9 ± 0.1 mV to 35 ± 10 mV in sequential feeding cycles. Consequently, the feeding sequence of acetic, acetic-butyric, and acetic-butyric-propionic acids is proposed as an efficient strategy for the acclimation of MFCs to complex substrates.

The effect of two different inoculum pretreatments, thermal and cell wash-out (A1 and A2, respectively) on the performance of anaerobic fluidized bed reactors for hydrogen production was determined. The reactors were operated for 112 days under the same operational conditions using glucose as substrate at increasing organic loading rates and decreasing hydraulic retention times. Both treatments were effective avoiding methanogenesis. Reactor A2 showed better performance and stability than reactor A1 in each one of the different operational conditions. Cell wash-out treatment produced higher hydrogen volumetric production rates and yields than thermal treatment (7 L H2/L-d, 3.5 mol H2/mol hexose, respectively). DGGE analysis revealed that the microbial communities developed were affected by the inoculum treatment. Organisms from the genera Clostridium and Lactobacillus predominated in both reactors, with their relative abundances linked to hydrogen production. Resilience was observed in both reactors after a period of starvation.

Lignocellulosic biomass is a promising alternative energy source, which after pretreatment can be efficiently used as feedstock for biological production of second generation biofuels. Hydrogen is considered an ideal biofuel and its production through dark fermentation has been recognized as a sustainable process. However, more studies with continuous systems are needed for its application at industrial scale. In this study, enzymatic hydrolysate of Agave tequilana bagasse was used for long-term continuous hydrogen production in both, a continuous stirred tank reactor (CSTR) and a trickling bed reactor (TBR), which were operated up to 87 days under different organic loading rates (OLR) ranging from 17 to 60 g COD/L-d. Volumetric hydrogen production rate (VHPR) and hydrogen molar yield (HMY) in CSTR displayed an inverse correlation with maximum values of 2.53 L H 2 /L-d and 1.35 mol H 2 /mol substrate, attained at OLR 52.2 and 40.2 g COD/L-d, respectively. In contrast, increasing OLR up to 52.9 g COD/L-d simultaneously enhanced VHPR and HMY in TBR, attaining values of 3.45 L H 2 /L-d and 1.53 mol H 2 /mol substrate, respectively. Acetate and butyrate were the main metabolites in both reactors, while lactate and propionate were detected in minor concentrations. Metabolites distribution, electron balances and hydrogen production trends obtained from both reactors suggest that CSTR may be more susceptible to inhibition by hydrogen accumulation than TBR. Apparent hydrogen consumption and susceptibility to high solids load were found to limit further OLR increments in CSTR and TBR, respectively.

Agave bagasse is the lignocellulosic residue of the tequila industry and a potential feedstock for hydrogen production (H2) via dark fermentation after proper saccharification. Highly specialized enzymes for hydrolysis of lignocellulosic biomass are costly, hindering their application in continuous processes. This work investigated the use of a generic cellulase (Cellulase 50 XL), to prepare the hydrolysates from agave bagasse, either alone or combined with a highly specialized hemicellulase (Viscozyme L). The combination of Cellulase 50 XL and Viscozyme L increased the release of both total carbohydrates and chemical oxygen demand (COD) in 28.5% and 22.2%, respectively compared with Cellulase 50 XL. Conversely, the maximum volumetric H2 production rate (VHPR, 9.9 L H2/L-d) reached with the Cellulase 50 XL hydrolysates in continuous stirred tank reactors at an organic loading rate of 100 g COD/L-d, is the highest VHPR reported so far with enzymatic agave bagasse hydrolysates obtained with non-specialized enzyme. The estimated cost of H2 production using Cellulase 50 XL at lab-scale was 0.7 USD/L H2. From a systematic comparison of H2 productivities and lab-scale costs with similar studies it was concluded that: i) the composition, and not only the concentration of total carbohydrates plays a critical role in the performance of H2 production, ii) Cellulase 50 XL was one of the best cost-effective alternatives despite being designed for food sectors and not for the hydrolysis of lignocellulosic biomass. Overall, considering H2 productivities and cost of H2 production, Cellulase 50 XL hydrolysates resulted an appropriate option for cost-effective continuous H2 production.

Over the past decades, biofuels from microalgae have gained increasing attention. In the present work, methane production from mono-digestion of thermochemical microalgal hydrolysate and co-digestion with cheese whey was evaluated in an anaerobic sequencing batch reactor. The reactor was fed with 5 g total chemical oxygen demand (TCOD)/L and operated either at alkaline or neutral pH. The average methane content in the biogas was higher at alkaline pH (83 %) than at neutral pH (61 %), whereas the methane yield from co-digestion was similar at alkaline (233 N mL/g TCOD) and neutral pH (226 N mL/g TCOD), being 1.8-fold higher than the mono-digestion of the microalgal hydrolysate. After increasing stepwise the organic load up to 12 g TCOD/L, the methane production rate reached peaks around 1000 N mL/L-d, being between 200−800 mL/L-d in those stages with lower organic loads. The methane production from the particulate fraction of the microalgal hydrolysate and its co-digestion with cheese whey demonstrated that the particulate fraction was indeed used to produce methane, but co-digestion did not improve methane yield. The results showed that microalgal hydrolysate could be efficiently co-digested with a carbon-rich substrate enhancing methane production.

Most of the fermentative hydrogen production studies based on mixed cultures have shown enrichment of the microbial community by means of a heat treatment. This heat treatment enrichment strategy selects for Clostridum spp., an efficient hydrogen producer; however, other bacteria that may contribute to the systems performance could be excluded. Another enrichment strategy based on high dilution rates selects different taxonomic groups, which may affect hydrogen production and the system stability. In this work, two enrichment strategies were evaluated, heat shock and cell wash-out, for hydrogen production and the system stability in continuous stirred reactors. The enriched communities were then inoculated in packed bed reactors and operated up to 70 days. Both strategies selected hydrogen producing bacteria, mainly Clostridium spp. The highest hydrogen production rate (6.01 L H 2 /L-d), molar yield (1.29 mol H 2 /mol glucose consumed ), and stability were achieved by the wash-out procedure; this high performance was attributed to facultative bacteria like Lactobacillus and Lactococcus . Furthermore, there was a transition within the community (along the operation time in the reactor with cell wash-out inoculum) and a selection for methanogenic activity (due to the long solids retention time).

The intensive use of groundwater by the gold mining industry is an important sustainability concern, especially in arid and semiarid regions where groundwater is a scarce resource. Alternatives, such as water reuse, treatment and recycling, have been implemented to overcome this issue. The potential use of reclaimed wastewater in gold heap leaching, without decreasing the process efficiency, has not yet been considered. Hence, this review focuses on the physicochemical, biological, and chemical features (organic matter, microbial loads, metal ions, and anions) of reclaimed wastewater that may limit its use in gold heap leaching.

Hydrogen production by fermentation with granular anaerobic biomass has been regarded as one alternative to obtain clean energy. However critical aspects such as process conditions to obtain hydrogenogenic granules have been scarcely investigated. In this work two inoculum pretreatments and different upward liquid velocities (ULV) in expanded granular sludge bed (EGSB) reactors were applied to determine the influence of these parameters on the formation, structure and specific hydrogenogenic activity (SHA) of the granules. Heat pretreated inoculum produced granules with greater manipulation resistance compared to those produced with wash-out pretreatment. Furthermore, the increase of ULV (2.5–4.5 m/h) generated bigger granules with a granule size distribution with a trend to a normal distribution and higher protein to carbohydrate ratios. Mass balances showed that propionate was the main metabolite, nonetheless, its production decreased substantially as the ULV applied increased, indicating a selective wash-out of propionate-producing bacteria. In the opposite way, SHA values were higher as the ULV applied increased as a result of selective enrichment and better mass transfer conditions. The whole results indicated that the inoculum pretreatment and hydrodynamic conditions play a key role in the formation, structure and in the biological properties of hydrogen-producing granules in EGSB reactors.

In this research, the performance of two thermophilic inocula of different origin on continuous hydrogen production from an enzymatic hydrolysate of agave bagasse were compared; one of them was obtained from a thermophilic reactor and the second one was taken from a mesophilic reactor and acclimated to thermophilic conditions. The acclimation process in one-step quickly established a high-performance hydrogen producing community, obtaining a volumetric hydrogen production rate of 3811 ± 19 mL H2/L-d with an hydrogen yield of 121 L H2/kg bagasse compared to 1473 ± 6 mL H2/L-d and 26.6 L H2/kg obtained with the thermophilic-origin inoculum. The differences in the performance of both inocula were closely linked to the profile of volatile fatty acids produced, the homoacetogenic pathway and the microbial community, the latter being the determining factor. The use of mesophilic-origin inoculum acclimated to thermophilic conditions can significantly improve the hydrogen production from lignocellulosic bagasse.

Continuous hydrogen (H2) production from individual (Stonezyme, IH) and binary (Celluclast-Viscozyme, BH) enzymatic hydrolysates of agave bagasse was evaluated in continuous stirred-tank reactors (CSTR) and trickling bed reactors (TBR). The volumetric H2 production rates (VHPR) in CSTR were 13 and 2.25 L H2/L-d with BH and IH, respectively. Meanwhile, VHPR of 5.76 and 2.0 L H2/L-d were obtained in the TBR configuration using BH and IH, respectively. Differences on VHPR between reactors could be explained by substrate availability, which is intrinsic to the growth mode of each reactor configuration; while differences of VHPR between hydrolysates were possibly related to the composition of enzymatic hydrolysates. Furthermore, homoacetogenesis was strongly influenced by H2 and substrate transfer conditions. Considering VHPR, H2 yields, and costs of hydrolysis, hydrogen production from binary hydrolysates of agave bagasse was identified as the most promising alternative evaluated with scale-up potential for the production of energy biofuels.

International audience; Hydrogen (H-2) produced by dark fermentation is an alternative to fulfill the requirements of the transportation sector and to be a complementary source in the forthcoming electricity grid. However, the dark fermentative H-2 production is limited by the accumulation of H-2 in the fermentation broth. In continuous stirred-tank reactors (CSTR), such phenomenon is associated with poor mass transfer conditions. Nevertheless, this parameter has been scarcely considered to enhance H-2 production. In this research, the effect of the H-2 mass transfer conditions on the productivity and efficiency of continuous H-2 production was evaluated using a series of CSTR operated at H-2 mass transfer coefficients (kLa) ranging from 1.04 to 4.23 1/h. The results showed that volumetric H-2 production rate (VHPR) and H-2 yield increased 74 and 78%, respectively, due to enhanced mass transfer conditions. This behavior was driven by 53% decrease of the dissolved H-2 concentration. More specifically, the maximum VHPR of 7.66 L/L-d with a H-2 yield of 1.1 mol H-2/mol hexose was obtained at a k(L)a= 4.23 1/h. Furthermore, 16S-DGGE analysis and sequencing revealed that Clostridium and Lactobacillus were the dominant bacterial genera in continuous operation. In particular, Clostridium increased its occurrence at kLa of 2.72-4.23 1/h as a response to lower dissolved H-2 concentrations. The novelty of this work relies on the demonstration that mass transfer conditions control not only the H-2 accumulation and reactor performance (VHPR and H-2 yield), but they also influence the metabolic pathways and the composition of the microbial community.

H2 production by dark fermentation using mixed cultures has been studied intensively during the last two decades, and its feasibility has been demonstrated. Different substrates, operational conditions, and reactor technologies have been widely studied and there is a general agreement that the use of non-sterile fermentable substrates is required to make the process feasible for scaling up. Nonetheless, stability problems during long term operation may hinder its application at large scale. This work, written by members of the Latin American Biohydrogen Network, analyse and discuss instability causes and possible solutions in the H2 production by dark fermentation. It is concluded that instability is mostly linked to the biotic aspects of the process (i.e., changes in the microbial community composition, presence of organisms that consume hydrogen and compete for the substrate, and accumulation of fermentation products); regardless of the reactor configuration. However, some problems like excessive growth of microorganisms and methanogens presence were mostly reported in fixed bed reactors and granular sludge reactors. The novelty of this work relies on the comprehensive revision of the main causes behind the unstable and low hydrogen production and how these causes are linked to the technology used. The strategies to overcome the problems as well as the potential implications are also analysed.

The number of studies dealing with the use of activated carbons as redox mediators in biological systems has grown rapidly in recent years. However, evidence explaining the role of the surface chemistry of activated carbons in the biotransformation of recalcitrant pollutants under continuous conditions is rather meager at present. This study offers a discussion about the role of the chemistry of activated carbon fibers (ACFs) on the increased reduction of a model nitroaromatic compound 4-nitrophenol (NP), under continuous conditions and using novel UASB-packed reactors. ACFs with different physicochemical properties and different levels of redox-active groups were studied, including un-modified (AW), HNO3 treated (OX) and modified with anthraquinone-2,6-disulfonate (AQDS) ACFs. The results indicate that biofilm formation could cover up the effect of the different ACFs studied during the first days of the continuous experiment. However, with modifications in the concentration of ethanol (exogenous carbon source) the interaction bacteria-ACF surface was enhanced and resulted in increased biotransformation efficiencies of 1.47-, 1.97- and 2.11-fold for the reactors packed with AW, AQDS and OX materials, respectively, as compared with the control reactor, which lacked any carbonaceous support. We correlated the observed NP biotransformation efficiencies in the order OX > AQDS > AW with the content or redox-active groups such as carbonyl groups (quinone moieties) on the ACF surface in the order 1.3 > 1.0 > 0.78 milliequivalents/g, respectively. The present work proposes a novel treatment concept to enhance the reductive biotransformation of contaminants and provides a deep understanding on the complex subject of using activated carbons as redox mediators in biological systems.

The recent development of bioelectrochemical systems (BESs) is based on the growth of microorganisms on carbon electrode materials. Carbon felt and cloth are the most widely used electrode materials; however, less expensive alternative materials are required when BESs are used for wastewater treatment. In this work, graphite carbon foil was modified using chemical, thermal, and electrochemical oxidation methods to improve its surface characteristics, and the resulting material was compared with oxidized carbon felt. The materials were physically, chemically, and electrochemically characterized. The isotherms revealed that thermal oxidation increased the surface area of the carbon foil by 39 %, reaching a value of 26.2 m2 g−1. Electrochemical impedance spectroscopy showed that the charge transfer resistance of the untreated carbon foil was 445 Ω, which decreased to 34 Ω after electrochemical oxidation. Additionally, cyclic voltammetry revealed a 100-fold increase in current density for the foil and felt materials following electrochemical oxidation. Based on its large surface area, low charge transfer resistance, remarkable current density under polarization, and comparatively low cost, the graphite carbon foil was identified as a promising alternative that could be used for constructing bioelectrodes.

The capacity of anaerobic granular sludge to reduce Pd(II), using ethanol as electron donor, in an upflow anaerobic sludge blanket (UASB) reactor was demonstrated. Results confirmed complete reduction of Pd(II) and immobilization as Pd(0) in the granular sludge. The Pd-enriched sludge was further evaluated regarding biotransformation of two recalcitrant halogenated pollutants: 3-chloro-nitrobenzene (3-CNB) and iopromide (IOP) in batch and continuous operation in UASB reactors. The superior removal capacity of the Pd-enriched biomass when compared with the control (not exposed to Pd) was demonstrated in both cases. Results revealed 80 % of IOP removal efficiency after 100 h of incubation in batch experiments performed with Pd-enriched biomass whereas only 28 % of removal efficiency was achieved in incubations with biomass lacking Pd. The UASB reactor operated with the Pd-enriched biomass achieved 81 ± 9.5 % removal efficiency of IOP and only 61 ± 8.3 % occurred in the control reactor lacking Pd. Regarding 3-CNB, it was demonstrated that biogenic Pd(0) promoted both nitro-reduction and dehalogenation resulting in the complete conversion of 3-CNB to aniline while in the control experiment only nitro-reduction was documented. The complete biotransformation pathway of both contaminants was proposed by high-performance liquid chromatography-mass spectrometry (HPLC-MS) analysis evidencing a higher degree of nitro-reduction and dehalogenation of both contaminants in the experiments with Pd-enriched anaerobic sludge as compared with the control. A biotechnological process is proposed to recover Pd(II) from industrial streams and to immobilize it in anaerobic granular sludge. The Pd-enriched biomass is also proposed as a biocatalyst to achieve the biotransformation of recalcitrant compounds in UASB reactors.

Sequential hydrolysis of oat straw and hydrogen (H2) production from hydrolysates and hydrolysates constituents were investigated. One acid-enzymatic and two acid-alkaline-enzymatic procedures were assessed to measure their capability to convert hemicellulose and cellulose from oat straw to sugars. Acid hydrolysis, using HCl, resulted effective to depolymerize hemicellulose (81%) and also facilitated the activity of cellulases over remaining fiber. When alkaline hydrolysis, using KOH/NaClO2/KOH or NaOH/H2O2, were applied, the depolymerization of cellulose was only slightly increased. Total sugar recoveries ranged from 69 to 79% in the different sequential hydrolysis tested. Hence, hydrolysates from sequential acid-enzymatic hydrolysis were used as substrates for H2 production in batch assays. The enzymatic hydrolysate produced a higher H2 molar yield (2.39 mol H2/mol reducing sugars) than the acid hydrolysate (1.1 mol H2/mol reducing sugars). It was found that lower H2 production from the acid hydrolysate was partially due to a lower H2 yield from arabinose and not to measured microbial inhibitors (furfural, hydroxymethylfurfural, vanillin and syringaldehyde). Also, it was found that the commercial enzyme (Celluclast 1.5 L) was easily fermented, and greatly contributed to the H2 production. This is the first study that provides experimental evidence of H2 production from fermentation of a commercial enzyme.

BACKGROUND The aim of this study was to investigate the feasibility of utilizing a continuous-flow anaerobic granular sludge bed (UASB) reactor for the removal of palladium (II) from aqueous streams and the recovery of this precious metal as biogenic Pd(0). Anaerobic methanogenic granular sludge has been recently shown to reduce Pd(II) in the presence of a suitable electron donor leading to the formation of biomass-associated Pd(0). RESULTS Two laboratory-scale UASB reactors were fed with ethanol or acetate, in order to compare their performance in terms of Pd removal and reduction. Pd(II) was added in two periods of increasing concentration, 5 mg Pd(II) L−1 and then 15 mg Pd(II) L−1. Extensive removal of Pd (up to 99%), was attained in both reactors. X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) analysis confirmed significant differences in the content of Pd(0) in the biomass, depending on the electron donor used. The main mechanisms of Pd(II) removal when acetate and ethanol were used as electron donor were biosorption and reduction to Pd(0) by biogenic H2 released as fermentation product, respectively. CONCLUSIONS The results obtained demonstrate that it is possible to apply a continuous process for Pd reduction and recovery using a methanogenic consortium. © 2015 Society of Chemical Industry

Hydrogen production by dark fermentation is one of the most promising methods for obtaining clean energy. Inoculum pretreatments allow the selection of bacteria that have better performance in hydrogen production, because the selection of pretreatment limits the presence of some species while favoring others. In order to elucidate the inoculum pretreatment influence during the operation of two EGSB reactors, two pretreatments were assayed: heat shock and cell wash-out. Different organic loading rates (24–60 g glucose/L d) and hydraulic retention times (10–4 h) were applied to both reactors to determine population dynamics along 100 days of operation. Reactors exhibited differences in both volumetric hydrogen production rate and molar yield but with cell wash-out pretreatment showing better performance than heat shock pretreatment. Maximum molar yield (0.92 mol H 2 /mol hexose) and volumetric hydrogen production rate (4.23 L H 2 /L d) were obtained with organic loading rates of 36 g glucose/L d at HRT of 10 h in EGSB reactor inoculated with cell wash-out pretreated sludge. The microbial community of the reactors samples was analyzed by 16S rRNA genes profiles and the predominant bands were excised and their DNA sequence determined. Clostridium and representatives of Enterobacteriaceae were dominant, with a strong presence of Lactobacillus genus. The whole result indicates that the inoculum pretreatment has a strong initial effect during early stages of fermentation, after which the operating conditions have a greater impact on reactor performance.

The hydrogen (H-2) production efficiency in dark fermentation systems is strongly dependent on the occurrence of metabolic pathways derived from the selection of microbial species that either consume molecular H-2 or outcompete hydrogenogenic bacteria for the organic substrate. In this study, the effect of organic loading rate (OLR) on the H-2 production performance, the metabolic pathways, and the microbial community composition in a continuous system was evaluated. Two bacterial genera, Clostridium and Streptococcus, were dominant in the microbial community depending on the OLR applied. At low OLR (14.7-44.1 g(Lactose)/L-d), Clostridium sp. was dominant and directed the system towards the acetate-butyrate fermentation pathway, with a maximum H-2 yield of 2.14 mol(H2)/mol(Hexose) obtained at 29.4 g(Lactose)/L-d. Under such conditions, the volumetric hydrogen production rate (VHPR) was between 3.2 and 11.6 L-H2/L-d. In contrast, relatively high OLR (58.8 and 88.2 g(Lactose)/L-d) favored the dominance of Streptococcus sp. as co-dominant microorganism leading to lactate production. Under these conditions, the formate production was also stimulated serving as a strategy to dispose the surplus of reduced molecules (e.g., NADH(2) (+)), which theoretically consumed up to 5.72 L-H2/L-d. In such scenario, the VHPR was enhanced (13.7-14.5 L-H2/L-d) but the H-2 yield dropped to a minimum of 0.74 mol(H2)/mol(Hexose) at OLR = 58.8 g(Lactose)/L-d. Overall, this research brings clear evidence of the intrinsic occurrence of metabolic pathways detrimental for biohydrogen production, i.e., lactic acid fermentation and formate production, suggesting the use of low OLR as a strategy to control them.

BACKGROUND The development of carbon materials with redox functional groups on their surface is an attractive alternative for faster treatment of contaminants with electron withdrawing groups like 4-nitrophenol (4NP). In the search for better catalytic materials, this paper reports the anchorage of two anthraquinones on the surface of polyacrilonitrile activated carbon fibers (ACFs). RESULTS 2-aminoanthraquinone (AAQ) and anthraquinone-2,6-disulfonate (AQDS) were anchored on SOCl2 treated carbon materials. The results show that fixation of AAQ and AQDS on ACFs caused changes in the surface area and the pore size distribution of ACFs, but only AQDS materials (8.3% in weight) showed an improvement in the catalytic properties of the material (49%) towards the reduction of 4NP to 4-aminophenol, providing sodium sulfide as the electron donor. HPLC and spectrometric scans at the end of the reduction experiments showed that AQDS remained anchored on ACFs. CONCLUSIONS Immobilized AQDS on ACFs was shown to be an effective solid-phase redox mediator improving the reductive transformation of 4NP. Other applications for the method of immobilization of redox mediators on the surface of ACFs were reported in other fields of study such as energy storage and microbial fuel cells. © 2014 Society of Chemical Industry

Methane occurrence is a common concern in hydrogen producing reactors. This study presents the analysis of the microbial community structure during the application of operational strategies to decrease methane production, in three different up-flow anaerobic sludge blanket hydrogen-producing reactors. Cloning and denaturing gradient gel electrophoresis approach were used to establish the presence of homoacetogens, methanogens and hydrogen producers. The results showed that homoacetogenic organisms related to Blautia hydrogenotrophica and Oscillibacter valericigenes, and the hydrogen producer Enterobacter aerogenes where favored during pH decreasing strategies (5.6 to 4.5). The increment of the organic loading rate from 20 to 30 g chemical oxygen demand/L-d, selected hydrogen producers similar to Clostridium tyrobutyricum, Citrobacter freundii and E. aerogenes ; further increments caused inhibition of hydrogen production due to the high undissociated acids concentration. Methane production was inhibited completely only when the biomass of the reactor was heat treated for a second time, this strategy selected hydrogen producers capable to sporulate, but homoacetogens were also favored. In all reactors the methanogenic activity was attributed to hydrogenotrophs related to the genera Methanobrevibacter and Methanobacterium .

BACKGROUND Previous studies have reported that complexity of the substrate, type of inoculum and reactor configuration strongly affect fermentative hydrogen production. Therefore, the aim of this study was to evaluate the effect of simple (glucose, xylose and sucrose) and complex (acid and enzymatic oat straw hydrolysates) substrates on the continuous hydrogen production in a trickling bed reactor. Novel inoculum (triticale silage) and biofilm support configuration (vertically organized PET tubing) were tested. RESULTS The enzymatic hydrolysate was a suitable substrate for hydrogen production, since its hydrogen molar yield was similar to that obtained with glucose, mean values of 1.6 and 1.7 mol H2 mol−1 sugar consumed were obtained, respectively. In contrast, hydrogen was not produced from the acid hydrolysate. The highest hydrogen production rate (840 mL H2 L−1 h−1) was obtained with glucose at an organic loading rate of 160 g COD L−1 d−1. In spite of the high organic loading rate applied, clogging due to excessive biomass growth was not observed. Finally, PCR-DGGE analysis revealed that bacteria from Clostridium genus were the putative responsible for hydrogen production. CONCLUSION This work demonstrates that continuous hydrogen production is affected by complexity of the substrate. Furthermore, it also shows the feasibility of using triticale silage as inoculum in hydrogen production systems. © 2014 Society of Chemical Industry

One of the problems in fermentative hydrogen producing reactors, inoculated with pre-treated anaerobic granular sludge, is the eventual methane production by hydrogen-consuming methanogens. In this study, strategies such as reduction of pH and HRT, organic shock loads and repeated biomass heat treatment were applied to hydrogenogenic UASB reactors fed with cheese whey, that showed methane production after certain time of continuous operation (between 10 and 60 days). The reduction of pH to 4.5 not only decreased methane production but also hydrogen production. Organic shock load (from 20 to 30 g COD/L-d) was the more effective strategy to decrease the methane production rate (75%) and to increase the hydrogen production rate (172%), without stopping reactor operation. Repeated heat treatment of the granular sludge was the only strategy that inhibited completely methane production, leading to high volumetric hydrogen production rates (1.67 L H2/L-d), however this strategy required stopping reactor operation; in addition homoacetogenesis, another hydrogen-consuming pathway, was not completely inhibited. This work demonstrated that it was possible to control the methane activity in hydrogen producing reactors using operational strategies.

Bioelectrochemical systems (BESs) are based on the catalytic activity of biofilm on electrodes, or the so-called bioelectrodes, to produce electricity and other valuable products. In order to increase bioanode performance, diverse electrode materials and modification methods have been implemented; however, the factors directly affecting performance are yet unclear. In this work carbon cloth electrodes were modified by thermal, chemical, and electrochemical oxidation to enhance oxygenated surface groups, to modify the electrode texture, and consequently the electron transfer rate and biofilm adhesion. The oxidized electrodes were physically, chemically, and electrochemically characterized, then bioanodes were formed at +0.1 V vs. Ag/AgCl using domestic wastewater amended with acetate. The bioanode performance was evaluated according to the current and charge generated. The efficacy of the treatments were in the order Thermal>Electrochemical>Untreated>Chemical oxidation. The maximum current observed with untreated electrode was 0.152±0.026 mA (380±92 mA m −2 ), and it was increased by 78% and 28% with thermal and electrochemical oxidized electrodes, respectively. Moreover, the volatile solids correlated significantly with the maximum current obtained, and the electrode texture was revealed as a critical factor for increasing the bioanode performance.

Feasibility of hydrogen production from acid and enzymatic oat straw hydrolysates was evaluated in an anaerobic sequencing batch reactor at 35 °C and constant substrate concentration (5 g chemical oxygen demand/L). In a first experiment, hydrogen production was replaced by methane production. Selective pressures applied in a second experiment successfully prevented methane production. During this experiment, initial feeding with glucose/xylose, as model substrates, promoted biomass granulation. Also, the highest hydrogen molar yield (HMY, 2 mol H 2 /mol sugar consumed) and hydrogen production rate (HPR, 278 mL H 2 /L-h) were obtained with these model substrates. Gradual substitution of glucose/xylose by acid hydrolysate led to disaggregation of granules and lower HPR and HMY. When the model substrates were completely substituted by enzymatic hydrolysate, the HMY and HPR were 0.81 mol H 2 /mol sugar consumed and 29.6 mL H 2 /L-h, respectively. Molecular analysis revealed a low bacterial diversity in the stages with high hydrogen production and vice versa. Furthermore, Clostridium pasteurianum was identified as the most abundant species in stages with a high hydrogen production. Despite that feasibility of hydrogen production from hydrolysates was demonstrated, lower performance from hydrolysates than from model substrates was obtained.

Sulfate reduction is an appropriate approach for the treatment of effluents with sulfate and dissolved metals. In sulfate-reducing reactors, acetate may largely contribute to the residual organic matter, because not all sulfate reducers are able to couple the oxidation of acetate to the reduction of sulfate, limiting the treatment efficiency. In this study, we investigated the diversity of a bacterial community in the biofilm of a laboratory scale down-flow fluidized bed reactor, which was developed under sulfidogenic conditions at an influent pH between 4 and 6. The sequence analysis of the microbial community showed that the 16S rRNA gene sequence of almost 50% of the clones had a high similarity with Anaerolineaceae. At second place, 33% of the 16S rRNA phylotypes were affiliated with the sulfate-reducing bacteria Desulfobacca acetoxidans and Desulfatirhabdium butyrativorans, suggesting that acetotrophic sulfate reduction was occurring in the system. The remaining bacterial phylotypes were related to fermenting bacteria found at the advanced stage of reactor operation. The results indicate that the acetotrophic sulfate-reducing bacteria were able to remain within the biofilm, which is a significant result because few natural consortia harbor complete oxidizing sulfate-reducers, improving the acetate removal via sulfate reduction in the reactor.