Investigation of the links between mass transfer conditions, dissolved hydrogen concentration and biohydrogen production by the pure strain Clostridium butyricum CWBI1009

Metabolic engineering of Caldicellulosiruptor bescii for hydrogen production

Hydrogen is an alternative fuel for transportation vehicles because it is clean, sustainable, and highly flammable. However, the production of hydrogen from lignocellulosic biomass by microorganisms presents challenges. This microbial process involves multiple complex steps, including thermal, chemical, and mechanical treatment of biomass to remove hemicellulose and lignin, as well as enzymatic hydrolysis to solubilize the plant cell walls. These steps not only incur costs but also result in the production of toxic hydrolysates, which inhibit microbial growth. A hyper-thermophilic bacterium of Caldicellulosiruptor bescii can produce hydrogen by decomposing and fermenting plant biomass without the need for conventional pretreatment. It is considered as a consolidated bioprocessing (CBP) microorganism. This review summarizes the basic scientific knowledge and hydrogen-producing capacity of C. bescii. Its genetic system and metabolic engineering strategies to improve hydrogen production are also discussed. KEY POINTS: • Hydrogen is an alternative and eco-friendly fuel. • Caldicellulosiruptor bescii produces hydrogen with a high yield in nature. • Metabolic engineering can make C. bescii to improve hydrogen production.

Production of biological hydrogen from Quinoa residue using dark fermentation and estimation of its microbial diversity

Although they are one of the world's environmental problems, agricultural wastes or residues are carbohydrate-rich and low-cost, so they are used as raw materials for the manufacture of biohydrogen (bio-H2). Among biological hydrogen manufacture methods, the dark fermentation method is suitable for processing waste or residues. In this regard, no study has been found in the literature on determining the potential of biological hydrogen manufacture from quinoa residue by the dark fermentation method. This work was carried out in a dark room at 36 ± 1 °C under different operating conditions in anaerobic batch bio-reactors fed with thermally pretreated anaerobic mixed bacteria + raw quinoa or quinoa extract liquid + nutrients. In the study, gas analyses were performed and biohydrogen production was detected in all the bio-reactors. Besides, taxonomic content analyses and organic acid analyses were executed. Maximum bio-H2 production was found as follows: at pH 4.5, 14,543.10-4 mL in the bio-reactor fed with 1.00 g quinoa/L and 1880.10-4 mL in the bio-reactor fed with 0.50 g quinoa extract/L, and at pH 4.0, 61,537.10-4 mL in the bio-reactor fed with 1.00 g quinoa/L and 1511.10-4 mL in the bio-reactor fed with 0.75 g quinoa extract/L. In the bio-reactors fed with raw quinoa residue, Clostridium butyricum and Hathewaya histolytica were detected as the most dominant bacteria at pH 4.5 and 4.0, respectively, whereas in the bio-reactors fed with quinoa extract liquid, Fonticella tunisiensis were detected as the most dominant bacteria at both pH 4.5 and pH 4.0.

A novel electrolytic gas lift reactor for efficient microbial electrosynthesis of acetate from carbon dioxide

The low current density impedes the practical application of microbial electrosynthesis for CO2 fixation. Engineering the reactor design is an effective way to increase the current density, especially for H2-mediated microbial electrosynthesis reactors. The electrolytic bubble column microbial electrosynthesis reactor has shown great potential for scaling up, but the mixing and gas mass transfer still need to be enhanced to further increase the current density. Here, we introduced an inner draft tube to the bubble column to tackle the problem. The addition of draft tube resulted in a 76.6% increase in the volumetric mass transfer coefficient (kLa) of H2, a 40% increase in the maximum current density (337 A/m2) and a 72% increase in average acetate production rate (3.1 g/L/d). The computational fluid dynamics simulations showed that the addition of draft tube enhanced mixing efficiency by enabling a more ordered cyclic flow pattern and a more uniform gas/liquid distribution. These results indicate that the electro-bubble column reactor with draft tube holds great potential for industrial implementation.

Groundwater denitrification using a continuous flow mode hybrid system combining a hydrogenotrophic biofilter and an electrooxidation cell

An attached-growth continuous flow hydrogenotrophic denitrification system was investigated for groundwater treatment. Two bench-scale packed-bed reactors were used in series, without external pH adjustment or carbon source addition, while inorganic carbonate salts already contained in the groundwater were the sole carbon source used by the denitrifying bacteria. The hydrogen was produced by water electrolysis using renewable energy sources thus minimizing resource-draining factors of the treatment process. The biofilter was subjected to a combination of three groundwater retention times (13.5, 27 and 54 min, corresponding to 20, 10 and 5 mL min^-1 inlet water flow rates) and two hydrogen flow values (10 and 20 mL min^-1) to evaluate its efficiency under different operating parameters. In all cases, significant nitrate percentage removals were achieved, ranged between 64.1% and 100%. The treatment process appears to slow down with lower retention times and H₂ flow rate values, although residual nitrate concentrations were always in the range of 0-5.1 mg L^-1, values below the maximum permitted limit of 11.3 mg L^-1. In cases where nitrite accumulation was detected, a continuous flow electrochemical oxidation process with three different current density values (5.0, 7.5 and 10.0 mA cm^-2) was examined as a post-treatment step aiming to completely remove the toxic nitrite anions. Finally, an advanced mathematical model of the attached growth hydrogenotrophic denitrification process was developed to predict concentrations of all the substrates examined in the bio-filter (nitrate, nitrite, inorganic carbon and hydrogen).

Enhancement of acetate production in hydrogen-mediated microbial electrosynthesis reactors by addition of silica nanoparticles

Microbial electrosynthesis (MES) is a promising technology for CO2 fixation and electrical energy storage. Currently, the low current density of MES limits its practical application. The H2-mediated and non-biofilm-driven MES could work under higher current density, but it is difficult to achieve high coulombic efficiency (CE) due to low H2 solubility and poor mass transfer. Here, we proposed to enhance the hydrogen mass transfer by adding silica nanoparticles to the reactor. At pH 7, 35 ℃ and 39 A·m- 2 current density, with the addition of 0.3wt% silica nanoparticles, the volumetric mass transfer coefficient (kLa) of H2 in the reactor increased by 32.4% (from 0.37 h- 1 to 0.49 h- 1), thereby increasing the acetate production rate and CE of the reactor by 69.8% and 69.2%, respectively. The titer of acetate in the reactor with silica nanoparticles (18.5 g·L- 1) was 56.9% higher than that of the reactor without silica nanoparticles (11.8 g·L- 1). Moreover, the average acetate production rate of the reactor with silica nanoparticles was up to 2.14 g·L- 1·d- 1 in the stable increment phase, which was much higher than the other reported reactors. These results demonstrated that the addition of silica nanoparticles is an effective approach to enhancing the performance of H2-mediated MES reactors.

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

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

High rate continuous biohydrogen production by hyperthermophilic Thermotoga neapolitana

This study focused on continuous-flow hydrogen production by Thermotoga neapolitana at a hydraulic retention time (HRT) decreasing from 24 to 5 h. At each HRT reduction, the hydrogen yield (HY) immediately dropped, but recovered during prolonged cultivation at constant HRT. The final HY in each operating period decreased from 3.4 (±0.1) to 2.0 (±0.0) mol H₂/mol glucose when reducing the HRT from 24 to 7 h. Simultaneously, the hydrogen production rate (HPR) and the liquid phase hydrogen concentration (H_2aq) increased from 82 (±1) to 192 (±4) mL/L/h and from 9.1 (±0.3) to 15.6 (±0.7) mL/L, respectively. Additionally, the effluent glucose concentration increased from 2.1 (±0.1) to above 10 mM. Recirculating H₂-rich biogas prevented the supersaturation of H_2aq reaching a value of 9.3 (±0.7) mL/L, resulting in complete glucose consumption and the highest HPR of 277 mL/L/h at an HRT of 5 h.

Continuous hydrogen and methane production from Agave tequilana bagasse hydrolysate by sequential process to maximize energy recovery efficiency

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%).

Fermentative hydrogen production in an up-flow anaerobic biofilm reactor inoculated with a co-culture of Clostridium acetobutylicum and Desulfovibrio vulgaris

Dark fermentation systems often show low H₂ yields and unstable H₂ production, as the result of the variability of microbial dynamics and metabolic pathways. Recent batch investigations have demonstrated that an artificial consortium of two anaerobic bacteria, Clostridium acetobutylicum and Desulfovibrio vulgaris Hildenborough, may redirect metabolic fluxes and improve H₂ yields. This study aimed at evaluating the scale-up from batch to continuous H₂ production in an up-flow anaerobic packed-bed reactor (APBR) continuously fed with a glucose-medium. The effects of various parameters, including void hydraulic retention time (HRTv), pH, and alkalinity, on H₂ production performances and metabolic pathways were investigated. The results demonstrated that a stable H₂ production was reached after 3-4days of operation. H₂ production rates increased significantly with decreasing HRTv from 4 to 2h. Instead, H₂ yields remained almost stable despite the change in HRTv, indicating that the decrease in HRTv did not affect the global metabolism.

Recent insights into the cell immobilization technology applied for dark fermentative hydrogen production

The contribution and insights of the immobilization technology in the recent years with regards to the generation of (bio)hydrogen via dark fermentation have been reviewed. The types of immobilization practices, such as entrapment, encapsulation and adsorption, are discussed. Materials and carriers used for cell immobilization are also comprehensively surveyed. New development of nano-based immobilization and nano-materials has been highlighted pertaining to the specific subject of this review. The microorganisms and the type of carbon sources applied in the dark hydrogen fermentation are also discussed and summarized. In addition, the essential components of process operation and reactor configuration using immobilized microbial cultures in the design of varieties of bioreactors (such as fixed bed reactor, CSTR and UASB) are spotlighted. Finally, suggestions and future directions of this field are provided to assist the development of efficient, economical and sustainable hydrogen production technologies.

Stoichiometry evaluation of biohydrogen production from various carbohydrates

In this paper, biochemical hydrogen potential (BHP) tests were conducted to investigate H₂ production from different substrate with acid-treated anaerobic digested sludge at the mesophilic range. The sludge was collected from an anaerobic digester and was subjected to sulfuric acid pretreatments at pH 3 for 24 h. The effects of substrate type (glucose, fructose, and sucrose as carbon source) were investigated in batch experiments. Results showed that substrate degradation rate for all of the substrates was up 95 % and the electron equivalent balance showed good closure for glucose and sucrose. Batch experiments showed that the maximum molar hydrogen yield with glucose, fructose, and sucrose was 3.27, 3.16, and 6.46 mol H₂/mol of substrate. The maximum cumulative biohydrogen production was 1552, 1487, and 1366 mL and maximum hydrogen production rate was 308, 279, and 275 mL/h for glucose, sucrose, and fructose, respectively. The experimental results suggest that the formation of hydrogen associates with the main aqueous products, i.e., acetate butyrate.

Comparative evaluation of the hydrogen production by mixed consortium, synthetic co-culture and pure culture using distillery effluent

Wastewater comprises of various carbon sources. So, the use of microbial consortium may improve the hydrogen production and organic reduction. The present study deals with biohydrogen production by acidogenic mixed consortia (AMC), synthetic co-culture (Klebsiella pneumoniae IIT-BT 08 and Citrobacter freundii IIT-BT L139) and pure culture using distillery effluent (DE). Higher hydrogen yield was observed in case of AMC (9.17 mol/kg CODreduced) as compared to the synthetic co-culture and pure culture. PCR-DGGE analysis indicated that the consortium was predominated by species closely affiliated to Clostridium sp. The average hydrogen production rate was 267 mL/Lh. The maximum hydrogen production rate (Rm), hydrogen production potential (P) and lag time (λ) by AMC using DE were 507.2 mL/Lh, 3729 m/L and 2.04 h, respectively. Maximum gaseous energy recovery by AMC was found to be higher by 21.9% and 45.4% than that of using co-culture and pure culture respectively.