Lactate wastewater dark fermentation: The effect of temperature and initial pH on biohydrogen production and microbial community

Biohydrogen production using dark fermentation (hydrolysis and acidogenesis) is one of the ways to recover energy from lactate wastewater from the food-processing industry, which has high organic matter. Dark fermentation can be affected by the temperature, pH and the microbial community structure. This study investigated the effects of temperature and initial pH on the biohydrogen production and the microbial community from a lactate wastewater using dark fermentation. Biohydrogen production was successful only at lower temperature levels (35 and 45 °C) and initial pH 6.5, 7.5 and 8.5. The highest hydrogen yield (0.85 mol H₂/mol lactate consumed) was achieved at 45 °C and initial pH 8.5. The COD reduction achieved by fermenting the lactate wastewater at 35 °C ranged between 21 and 30% with the maximum COD reduction at pH 8.5, and at 45 °C, the COD reduction ranged between 12 and 21%, with the maximum at pH 7.5. At 35 °C, the lactate degradation ranged between 54 and 95%, while at 45 °C, it ranged between 77 and 99.8%. 16S rRNA sequencing revealed that at 35 °C, bacteria from the Clostridium genera were the most abundant at the end of the fermentation in the reactors that produced hydrogen, while at 45 °C Sporanaerobacter, Clostridium and Pseudomonas were the most abundant.     

Study of factors involved in the behavior of biofilms formed by biohydrogen-producing microflora identified by molecular biology using dairy wastewater

Batch tests were carried out to investigate the production of H₂ considering the effects of: substrate concentration in a range of 3–25 g-COD/L; Initial pH: from 4 to 7 and 11 and temperatures of: 20, 35, 45 and 55 °C. The optimal substrate was 25 g-COD/L, with a reduction of COD of 73% and a yield of H₂ of 5.95 mM/gCOD; and the optimal initial pH was 11.0, with a 70% of COD reduction and a H₂ yield of 4236 mM/gCOD. The optimum temperature for pH = 11 was 35 °C, with a COD reduction of 69.8% and H₂ yield of 6.3 mM/gCOD. Escherichia, Acinetobacter, Alcaligenes, Brevibacterium, Clostridium and Mycobacterium were isolated from pretreated inoculum samples and identified by 16S rDNA sequencing. The results suggest that biofilm reactors developed on a natural support such as Opuntia imbricata have good potential for hydrogen production from dairy wastewater.     

Effect of storage time and temperature on hydrogen fermentation of food waste

Practically, before being fed to the treatment plant, food waste (FW) is stored for up to a week in a storage tank under ambient temperature condition, which would have an impact on the bioenergy yield. In the present work, FW was stored at different temperatures (5 °C, 20 °C, and 35 °C) for 0 d, 1 d, and 2 d, and it was used as a feedstock for mesophilic H₂ fermentation. H₂ production curves were divided by three groups, finally attaining 1.7–1.8 mol H₂/mol hexose_added, 1.4–1.5 mol H₂/mol hexose_added, and 1.2 mol H₂/mol hexose_added, achieved from the (fresh, FW stored at 5 °C), (FW stored at 20 °C, and 35 °C for 1 d), and (FW stored at 35 °C for 2 d), respectively. The different performance was attributed to the growth of indigenous lactic acid bacteria such as Lactobacillus and Weissella during storage under high temperature condition. In addition, it was found that the activity of homoacetogenic reaction (R17, 4H₂ + CO₂ → Acetate) calculated by establishing metabolic flux balance was different depending on the H₂ production performance. The flux of R17 ranged 0.03–0.06 under low H₂ yield achieved conditions, while it increased to 0.10–0.17 those showing low H₂ yields.     

Effect of changing temperature on anaerobic hydrogen production and microbial community composition in an open-mixed culture bioreactor

The temperature effect (37–65 °C) on H₂ production from glucose in an open-mixed culture bioreactor using an enrichment culture from a hot spring was studied. The dynamics of microbial communities was investigated by polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). At 45 and 60 °C the H₂ production was the highest i.e. 1.71 and 0.85 mol H₂/mol glucose, respectively. No H₂ was produced at temperatures 50 and 55 °C. At 37–45 °C, H₂ production was produced by butyrate type fermentation while fermentation mechanism changed to ethanol type at 60 °C. Clostridium species were dominant at 37–45 °C while at 50–55 °C and 60 °C the culture was dominated by Bacillus coagulans and Thermoanaerobacterium, respectively. In the presence of B. Coagulans the metabolism was directed to lactate production. The results show that the mixed culture had two optima for H₂ production and that the microbial communities and metabolic patterns promptly changed according to changing temperatures.     

Evaluation on hydrothermal gasification of waste tires based on chemical equilibrium analysis

Massive amounts of waste tires are produced globally, which brings great challenges to the disposal and recycling of used tires. Hydrothermal gasification is a promising option for recycling waste tires. The hydrothermal gasification of waste tires was evaluated based on the chemical equilibrium analysis along with the response surface methodology (RSM) in terms of subcritical temperature range (250–300 °C), transition temperature range (350–400 °C), supercritical temperature range (550–600 °C), supercritical pressure (22.5–30.5 MPa) and feedstock concentration (5–20 wt%). CH₄ yield at 350 °C reached a maximum, 41.575 mmol/g. H₂ yield increased from 0.0283 to 53.602 mmol/g with increasing the temperature from 250 °C to 600 °C. CH₄ yield at the supercritical temperature increased with lifting the feedstock concentration, while H₂ yield decreased. The optimal parameters regarding maximum H₂ and CH₄ yields in the subcritical temperature range were 300 °C, 22.5 MPa and 12.5 wt%, respectively, while they in the supercritical temperature range were 550 °C, 30.5 MPa and 5.4 wt%, respectively. RSM was more suitable for predicting H₂ yield in the hydrothermal gasification of waste tires at subcritical and supercritical temperature ranges, but it was available for predicting CH₄ yield in three temperature ranges. This study can provide basic data for the hydrothermal treatment of waste tires.     

Turning glycerol surplus into renewable syngas through glycerol steam reforming over a sol-gel Ni–Mo2C-Al2O3 catalyst

In this work, a sol-gel Ni–Mo₂C–Al₂O₃ catalyst is employed for the first time in the glycerol steam reforming for syngas production. Catalyst stability and activity are investigated in the temperature range of 550 °C–700 °C and time on stream up to 30 h. As reaction temperature increases, from 550 °C to 700 °C, H₂ yield boosts from 22% to 60%. The stability test, carried out at milder conditions (600 °C and Gas-Hourly Space-Velocity (GHSV) of 50,000 mL h⁻¹.g_cat⁻¹), shows high catalyst stability, up to 30 h, with final conversion, H₂ yield, and H₂/CO ratio of 95%, 53% and 1.95, respectively. Both virgin and spent catalysts have been characterized by a multitude of techniques, e.g., Atomic-Absorption spectroscopy, Raman spectroscopy, N₂-adsorption-desorption, and Transmission Electron Microscopy (TEM), among others. Regarding the spent catalysts, carbon deposits’ morphology becomes more graphitic as the reaction temperature increases, and the total coke formation is mitigated by increasing reaction temperature and lowering GHSV.     

Thermophilic versus mesophilic dark fermentation in xylose-fed fluidised bed reactors: Biohydrogen production and active microbial community

Dark fermentative biohydrogen production in a thermophilic, xylose-fed (50 mM) fluidised bed reactor (FBR) was evaluated in the temperature range 55–70 °C with 5-degree increments and compared with a mesophilic FBR operated constantly at 37 °C. A significantly higher (p = 0.05) H₂ yield was obtained in the thermophilic FBR, which stabilised at about 1.2 mol H₂ mol^?1 xylose (36% of the theoretical maximum) at 55 and 70 °C, and at 0.8 mol H₂ mol^?1 xylose at 60 and 65 °C, compared to the mesophilic FBR (0.5 mol H₂ mol^?1 xylose). High-throughput sequencing of the reverse-transcribed 16S rRNA, done for the first time on biohydrogen producing reactors, indicated that Thermoanaerobacterium was the prevalent active microorganism in the thermophilic FBR, regardless of the operating temperature. The active microbial community in the mesophilic FBR was mainly composed of Clostridium and Ruminiclostridium at 37 °C. Thermophilic dark fermentation was shown to be suitable for treatment of high temperature, xylose-containing wastewaters, as it resulted in a higher energy output compared to the mesophilic counterpart.     

Thermodynamic and kinetic properties of calcium hydride

Calcium hydride has shown great potential as a hydrogen storage material and as a thermochemical energy storage material. To date, its high operating temperature (above 800 °C) has not only hindered its opportunity for technological application but also prevented detailed determination of its thermodynamics of hydrogen sorption. In addition, calcium metal suffers from high volatility, high corrosivity from Ca (and CaH₂), slow kinetics of hydrogen sorption, and the solubility of Ca in CaH₂. In this work, a literature review of the wide-ranging thermodynamic properties of CaH₂ is provided along with a detailed experimental investigation into the thermodynamic properties of molten and solid CaH₂. The thermodynamic values of hydrogen release from both molten and solid CaH₂ were determined as ΔH_des (molten CaH₂) = 216 ± 10 kJ mol⁻¹.H₂, ΔS_des (molten CaH₂) = 177 ± 9 J K⁻¹ mol⁻¹.H_2, which equates to a 1 bar hydrogen equilibrium temperature for molten CaH₂ of 947 ± 65 °C. Similarly, in the solid-state: ΔH_des (solid CaH₂) = 172 ± 12 kJ mol⁻¹.H₂, ΔS_des (solid CaH₂) = 144 ± 10 J K⁻¹ mol⁻¹.H₂. Moreover, the activation energy of hydrogen release from CaH₂ was also calculated using DSC analysis as E_a = 203 ± 12 kJ mol⁻¹. This study provides the first thermodynamics for the Ca–H system in over 60 years, providing more accurate data on this emerging energy storage material.     

Hydrogen production by sequential dark and photofermentation using wet biomass hydrolysate of Spirulina platensis: Response surface methodological approach

Microalgae and cyanobacteria can be used as a potential biomass to produce hydrogen from stored glycogen and starch through fermentation and photofermentation. In this study, the potential of algal biomass i.e. Spirulina platensis hydrolysate as a substrate for sequential fermentative (I-stage) and photo-fermentative (II-stage) biohydrogen production was evaluated. Response Surface Methodology (RSM) was employed to find the optimum photofermentation conditions. From the preliminary optimization experiments, it was found that the significantly affecting factors for H₂ production were pH, dilution fold (D.F.) of fermentate and Fe(II) sulfate concentration during photofermentation (second stage). In the present study, 1% (w/v) Spirulina platensis hydrolyzate produced 23.06 ± 3.63 mmol of H₂ with yield of 1.92 ± 0.20 mmol H₂/g COD reduced. In the second stage experiment 1510 ± 35 mL/l hydrogen was produced using inoculum volume-20.0% (v/v) and inoculum age-48 h of co-culture of Rhodobacter sphaeroides NMBL-01 and Bacillus firmus NMBL-03 under conditions pH-5.95, D.F. of dark fermentate-20.30 folds, Fe(II) sulfate concentration-0.412 μM, temperature-32±2 °C and light intensity-2.5 klux.     

A meta-analysis of research trends on hydrogen production via dark fermentation

This work provides a meta-analysis of the state-of-the-art research on H₂ and value-added products production from biomass, via Dark Fermentation (DF) between 2015 and 2019. The meta-analysis data clusters are created considering inputs (i.e., feedstocks, and microorganisms used in DF), process conditions (i.e., feedstock pretreatments, and temperature, pH, working volume, substrate concentration in DF), yield and productivity of H₂ and the most common by-products (i.e., acetic, lactic, butyric, propionic acids and ethanol). Agricultural and green residues were the most common feedstock (36.5%), followed by Aquatic biomass (29.8%). Pretreated feedstocks and mixed cultures were employed in 72% and 79% of the studies, respectively. The meta-analysis relates H₂ high productivity to 6 ≤ pH ≤ 6.8 and 35 °C ≤ T ≤ 37 °C and H₂ high yield to 5.5 ≤ pH ≤ 7.5 under mesophilic conditions. The paper elaborates on the production strategies tested at the laboratory scale for each of the DF-products mentioned above, highlighting the pros and cons towards improving yield and productivity and discussing what are the challenges to integrating DF in large-scale biorefining schemes for industrial production of H₂ and value-added products.     

Photoreforming driven by indium hydroxide/oxide nano-objects

In(OH)₃/In₂O₃ nano-objects (plates and tubes) were prepared under controlled mild reaction conditions by simple basic thermal hydrolysis of 1-(2-methoxy-ethyl)-3-methyl imidazolium tetrachloro-indate (BMIOMe·InCl₄) ionic liquid (IL). As the reaction temperature increased from 10 °C to 80 °C, there was not only an increase in concentration of In₂O₃ but the growth of nano-tubes (NTs) also increased. At 10 °C, 30 °C, 50 °C, and 80 °C, 25.2 ± 7 nm sized nanoplates (NPls), small nanotubes (26.6 ± 6 nm in length and 8 ± 2 nm in width), NTs with a length of 25.9 ± 6 nm, and larger nanotubes (49.2 ± 18 nm) were obtained, respectively. The surface concentration of In(OH)₃ decreased with the augmentation of the reaction temperature (from 100% at 10 °C to 5.1% at 80 °C). The nano-objects displayed a band gap of 5.4 eV, which is in the range expected for photo-catalytic activity to generate hydrogen from alcoholic solutions. Remarkably, In(OH)₃/In₂O₃ catalyst prepared at 30 °C revealed 3.72 μmol h⁻¹ of rate of hydrogen with apparent quantum yield of 16.3%, yielding a total H₂ productivity of 57.6 μmol (5.76 mmol/g) after 32 h. This is the most active semiconductor photocatalysts reported so far of bare indium-based. The presence of In₂O₃ inside the nano-tubes is also important for the photacatalytic activity of the In(OH)₃ based semiconductor. The presence of the IL layer not only acts as a template to control the structure of the nano-objects, but its presence also induces H₂ generation.     

Supercritical water gasification of sewage sludge and combined cycle for H2 and power production – A thermodynamic study

An integrated system of supercritical water gasification (SCWG) and combined cycle has been developed for H₂ production and power generation. Sewage sludge and lignite coal were selected as raw material in this simulation. The effects of feed concentration (10–30 wt%) and lignite coal addition (0–50 wt%) on syngas yield and H₂ yield were also investigated in the temperature range of 500 °C–700 °C. Several heat exchangers were considered in the proposed integrated system to minimize energy loss. High pressure syngas was expanded by using turbo-expander to produce electricity, resulting in the improvement of the total efficiency. The results showed that the minimum feed concentrations of 14.25 wt%, 18.75 wt%, and 25.50 wt% were required to achieve self-sufficient energy at 500 °C, 600 °C, and 700 °C, respectively. However, the lower feed concentration and higher temperature were preferable for syngas production. The highest syngas and H₂ yield were obtained at 700 °C and 10 wt% feed concentration. The SCWG could produce 178.08 kg syngas from 100 kg feed and 9.06 kg H₂ were obtained after H₂ separation. The total power generation from turbo-expander and combined cycle module was 48.37 kW. By combining SCWG and combined cycle, the total efficiency could reach 63.48%. It worth mentioning that the addition of lignite coal could help reduce the minimum feed concentration to achieve autothermal condition, but did not have significant improvement on H₂ production.     

Dark fermentative hydrogen production from xylose by a hot spring enrichment culture

Dark fermentative hydrogen production by a hot spring culture was studied from different sugars in batch assays and from xylose in continuous stirred tank reactor (CSTR) with on-line pH control. Batch assays yielded hydrogen in following order: xylose > arabinose > ribose > glucose. The highest hydrogen yield in batch assays was 0.71 mol H₂/mol xylose. In CSTR the highest H₂ yield and production rate at 45 °C were 1.97 mol H₂/mol xylose and 7.3 mmol H₂/h/L, respectively, and at 37 °C, 1.18 mol H₂/mol xylose and 1.7 mmol H₂/h/L, respectively. At 45 °C, microbial community consisted of only two bacterial strains affiliated to Clostridium acetobutulyticum and Citrobacter freundii, whereas at 37 °C six Clostridial species were detected. In summary hydrogen yield by hot spring culture was higher with pentoses than hexoses. The highest H₂ production rate and yield and thus, the most efficient hydrogen producing bacteria were obtained at suboptimal temperature of 45 °C for both mesophiles and thermophiles.     

Comparison of mesophilic and thermophilic anaerobic hydrogen production by hot spring enrichment culture

Anaerobic hydrogen producing mesophilic and thermophilic cultures were enriched and studied from an intermediate temperature (45 °C) hot spring sample. H₂ production yields at 37 °C and 55 °C were highest at the initial pH of 6.5 and 7.5, respectively. Optimum glucose, iron and nickel concentrations were 9 g/l, 25 mg/l and 25 mg/l both at 37 °C and 55 °C, respectively. The highest H₂ yields at 37 °C and 55 °C were 1.8 and 1.0 mol H₂/mol glucose, respectively, with the optimal pH, glucose concentration and iron addition. Hydrogen production from glucose at 55 °C and 37 °C was associated with ethanol- and acetate–butyrate type fermentations, respectively. Bacterial composition was analyzed by 16S rRNA gene-targeted denaturing gradient gel electrophoresis (DGGE). Clostridium species dominated at both temperatures and the microbial diversity decreased with increasing temperature. At 55 °C, Clostridium ramosum was the dominant organism.     

Reactor start-up strategy as key for high and stable hydrogen production from cheese whey thermophilic dark fermentation

Using the right start-up strategy can be vital for successful hydrogen production from thermophilic dark fermentation (55 °C), but it needs to be affordable. Hence, three start-up strategies modifying only influent concentration and temperature were assessed in a reactor fed with cheese whey: (i) high temperature (55 °C) and a high organic loading rate (OLR_A - 15 kgCOD m^?3 d^?1) right at the beginning of the operation; (ii) slowly increasing temperature up to 55 °C using a high OLR_A and (iii) slowly increasing temperature and OLR_A up to the desired condition. Strategy (iii) increased hydrogen productivity in 39% compared to the others. The combination of high temperature and low pH thermodynamically favored H₂ producing routes. Synergy between Thermoanaerobacterium and Clostridium might have boosted hydrogen production. Three reactors of 41 m³ each would be needed to treat 3.4 × 10³ m³ year^?1 of whey (small-size dairy industry) and the energy produced could reach 14 MWh month^?1.     

Fermentative hydrogen production from cheese whey with in-line,concentration gradient-driven butyric acid extraction

Hydrogen (H₂) generation from cheese whey with simultaneous production and extraction of volatile fatty acids (VFAs) was studied in UASB reactors at two temperatures (20 and 35 °C) and pH values (5.0 and 4.5). The extraction module, installed through a recirculation loop, was a silicone tube coil submerged in water, which allows concentration-driven extraction of undissociated VFAs. Operating conditions were selected as a compromise for the recovery of both H₂ and VFAs. Batch experiments showed a higher yield (0.9 mol H₂ mol⁻¹ glucose_eq.) at 35 °C and pH 5.0, regardless of the presence of the extraction module, whereas lower yields were obtained at pH 4.5 and 20 °C (0.5 and 0.3 mol H₂ mol⁻¹ glucose_eq., respectively). VFAs crossed the silicone membrane, with a strong preference for butyric over propionic and acetic acid due to its higher hydrophobicity. Sugars, lactic acid and nutrients were retained, resulting in an extracted solution of up to 2.5 g L⁻¹ butyric acid with more than 90% purity. Continuous experiment confirmed those results, with production rates up to 2.0 L H₂ L⁻¹ d⁻¹ and butyric acid extraction both in-line (from the UASB recirculation) and off-line (from the UASB effluent). In-line VFA extraction can reduce the operating costs of fermentation, facilitating downstream processing for the recovery of marketable VFAs without affecting the H₂ production.     

Catalytic supercritical water gasification of aqueous phase directly derived from microalgae hydrothermal liquefaction

Microalgae (N. chlorella) hydrothermal liquefaction (HTL) was conducted at 320 °C for 30 min to directly obtain original aqueous phase with a solvent-free separation method, and then the supercritical water gasification (SCWG) experiments of the aqueous phase were performed at 450 and 500 °C for 10 min with different catalysts (i.e., Pt-Pd/C, Ru/C, Pd/C, Na₂CO₃ and NaOH). The results show that increasing temperature from 450 to 500 °C could improve H₂ yield and TGE (total gasification efficiency), CGE (carbon gasification efficiency), HGE (hydrogen gasification efficiency), TOC (total organic carbon) removal efficiency and tar removal efficiency. The catalytic activity order in improving the H₂ yield was NaOH > Na₂CO₃ > None > Pd/C > Pt-Pd/C > Ru/C. Ru/C produced the highest CH₄ mole fraction, TGE, CGE, TOC removal efficiency and tar removal efficiency, while NaOH led to the highest H₂ mole fraction, H₂ yield and HGE at 500 °C. Increasing temperature and adding proper catalyst could remarkably improve the SCWG process above, but some N-containing compounds were difficult to be gasified. This information is valuable for guiding the treatment of the aqueous phase derived from microalgae HTL.     

Synergistic strategy for the enhancement of biohydrogen production from molasses through coculture of Lactobacillus brevis and Clostridium saccharobutylicum

This study aimed to evaluate the capacity of different inoculum sources and their bacterial diversity to generate hydrogen (H₂). The highest Simpson (0.7901) and Shannon (1.581) diversity indexes for H₂-producing bacterial isolates were estimated for sewage inocula. The maximum cumulative H₂ production (H_max) was 639.6 ± 5.49 mL/L recorded for the sewage inoculum (SS30) after 72 h. The highest H₂-producing isolates were recovered from SS30 and identified as Clostridium saccharobutylicum MH206 and Lactobacillus brevis MH223. The H_max of C. saccharobutylicum, L. brevis, and synergistic coculture was 415.00 ± 24.68, 491.67 ± 15.90, and 617.67 ± 3.93 mL/L, respectively. The optimization process showed that the H_max (1571.66 ± 33.71 mL/L) with a production rate of 58.02 mL/L/h and lag phase of 19.33 h was achieved by the synergistic coculture grown on 3% molasses at 40 °C, pH 7, and an inoculum size of 25% (v/v). This study revealed the economic feasibility of the synergistic effects of coculture on waste management and biohydrogen production technology.     

Inoculum pretreatment differentially affects the active microbial community performing mesophilic and thermophilic dark fermentation of xylose

The influence of different inoculum pretreatments (pH and temperature shocks) on mesophilic (37 °C) and thermophilic (55 °C) dark fermentative H₂ production from xylose (50 mM) and, for the first time, on the composition of the active microbial community was evaluated. At 37 °C, an acidic shock (pH 3, 24 h) resulted in the highest yield of 0.8 mol H₂ mol^?1 xylose. The H₂ and butyrate yield correlated with the relative abundance of Clostridiaceae in the mesophilic active microbial community, whereas Lactobacillaceae were the most abundant non-hydrogenic competitors according to RNA-based analysis. At 55 °C, Clostridium and Thermoanaerobacterium were linked to H₂ production, but only an alkaline shock (pH 10, 24 h) repressed lactate production, resulting in the highest yield of 1.2 mol H₂ mol^?1 xylose. This study showed that pretreatments differentially affect the structure and productivity of the active mesophilic and thermophilic microbial community developed from an inoculum.     

Biohydrogen production from thermochemically pretreated corncob using a mixed culture bioaugmented with Clostridium acetobutylicum

Bovine ruminal fluid (BRF) bioaugmented with Clostridium acetobutylicum (Clac) was assessed for hydrolyzing cellulose and produce biohydrogen (BioH₂) simultaneously from pretreated corncob in a single step, without the use of external hydrolytic biocatalysts. The corncob was pretreated using three thermochemical methods: H₂SO₄ 2%, 160 °C; NaOH 2%, 140 °C; NaOCl 2%, 140 °C; autohydrolysis: H₂O, 190 °C. Subsequently, BioH₂ production was carried out using the pretreated material with the highest digestibility applying a Taguchi experimental array to identify the optimal operating conditions. The results showed a higher glucose released from pretreated corncob with H₂SO₄ (134.7 g/L) compared to pretreated materials by autohydrolysis, NaOH and NaOCl (123 g/L, 89.8 g/L and 52.9 g/L, respectively). The mixed culture was able to hydrolyze the pretreated corncob and produce 575 mL of H₂ (at 35 °C, pH 5.5, 1:2 ratio of BRF:Clac and 5% of solids loading) equivalent to 132 L H₂/Kg of biomass.