Single stage photofermentative hydrogen production from glucose: An attractive alternative to two stage photofermentation or co-culture approaches

Photosynthetic bacteria have been extensively investigated for biohydrogen production due to their high intrinsic substrate conversion efficiency. Many studies have examined different aspects of photo fermentative hydrogen production using various volatile organic fatty acids under nitrogen limited conditions, and in some cases nearly stoichiometric hydrogen yields have been obtained. In addition, there has been great interest in using photosynthetic bacteria to increase the yields of dark fermentation of glucose through either two stage or co-culture approaches. Although these processes can achieve yields of about 7 mol of H₂ per mole of glucose, there have many drawbacks. Thus, we have begun the systematic investigation of a simple one stage system for the conversion of glucose to hydrogen through photofermentation by Rhodobacter capsulatus. Yields of about 3 mol of H₂ per moles of glucose have been obtained, which represents a yield of 25% yield. Thus improvement is needed and can be sought through a variety of means, including. process optimization and gene inactivation. These approaches could allow the development of a single stage process for the complete stoichiometric conversion of glucose, or glucose containing wastes, to hydrogen with a minimal lag phase and relative insensitivity to inhibition by fixed nitrogen. This would present an attractive simple alternative to either two stage or co-culture fermentations for the complete conversion of carbohydrate substrates to hydrogen.     

Integrated biological hydrogen production

Biological systems offer a variety of ways by which to generate renewable energy. Among them, unicellular green algae have the ability to capture the visible portion of sunlight and store the energy as hydrogen (H₂). They hold promise in generating a renewable fuel from nature's most plentiful resources, sunlight and water. Anoxygenic photosynthetic bacteria have the ability of capturing the near infrared emission of sunlight to produce hydrogen while consuming small organic acids. Dark anaerobic fermentative bacteria consume carbohydrates, thus generating H₂ and small organic acids. Whereas efforts are under way to develop each of these individual systems, little effort has been undertaken to combine and integrate these various processes for increased efficiency and greater yields. This work addresses the development of an integrated biological hydrogen production process based on unicellular green algae, which are driven by the visible portion of the solar spectrum, coupled with purple photosynthetic bacteria, which are driven by the near infrared portion of the spectrum. Specific methods have been tested for the cocultivation and production of H₂ by the two different biological systems. Thus, a two-dimensional integration of photobiological H₂ production has been achieved, resulting in better solar irradiance utilization (visible and infrared) and integration of nutrient utilization for the cost-effective production of substantial amounts of hydrogen gas. Approaches are discussed for the cocultivation and coproduction of hydrogen in green algae and purple photosynthetic bacteria entailing broad utilization of the solar spectrum. The possibility to improve efficiency even further is discussed, with dark anaerobic fermentations of the photosynthetic biomass, enhancing the H₂ production process and providing a recursive link in the system to regenerate some of the original nutrients.     

Maximizing biohydrogen production from water hyacinth by coupling dark fermentation and electrohydrogenesis

Biohydrogen production via dark fermentation has shown immense potential for simultaneous energy generation and waste remediation. However, the low substrate conversion rates limit its practical feasibility. Therefore, the present work attempts to develop a single chamber microbial electrolysis cell (MEC) as an additional means for biohydrogen production. Different organic substrates including simple sugars and volatile fatty acids were demonstrated as potential substrates for H₂ production in MEC. The use of water hyacinth as sole substrate for H₂ production was examined. Furthermore, the feasibility of using MEC for second stage energy recovery after dark fermentation was explored. The two-stage process exhibited improved performance as compared to single stage MEC process with overall hydrogen yield of 67.69 L H₂/kg COD_consumed, COD removal of 70.33% and energy recovery of 46%. These results suggest that coupled dark fermentation-MEC process can be a promising means for obtaining high yield biohydrogen from water hyacinth.     

Advances in biohydrogen production processes: An approach towards commercialization

Biological H₂ production has an edge over its chemical counterpart mainly because it is environmentally benign. Despite having simpler technology, higher evolution rate of H₂ and the wide spectrum of substrate utilization, the major deterrent of anaerobic dark fermentation process stems from its lower achievable yields. Theoretically, the maximum H₂ yield is 4 mol H₂/mol glucose when glucose is completely metabolized to acetate or acetone in the anaerobic process. But it is somewhat difficult to achieve the complete degradation of glucose to carbon dioxide and H₂ through anaerobic dark fermentation. Moreover, this yield appears too low to be economically viable as an alternative to the existing chemical or electrochemical processes of hydrogen generation. Intensive research studies have already been carried out on the advancement of these processes, such as the development of genetically modified microorganism, improvement of the reactor designs, use of different solid matrices for the immobilization of whole cells, development of two-stage processes, and higher H₂ production rates. Maximum H₂ yield is found to be 5.1 mol H₂/mol glucose. However, major bottlenecks for the commercialization of these processes are lower H₂ yield and rate of H₂ production. Competent microbial cultures are required to handle waste materials efficiently, which are usually complex in nature. This will serve dual purposes: clean energy generation and bioremediation. Scale-up studies on fermentative H₂ production processes have been done successfully. Pilot plant trials of the photo-fermentation processes require more attention. Use of cheaper raw materials and efficient biological H₂ production processes will surely make them more competitive with the conventional H₂ generation processes in near future.     

Hydrogen production from cellulose in a two-stage process combining fermentation and electrohydrogenesis

A two-stage dark-fermentation and electrohydrogenesis process was used to convert the recalcitrant lignocellulosic materials into hydrogen gas at high yields and rates. Fermentation using Clostridium thermocellum produced 1.67 mol H₂/mol-glucose at a rate of 0.25 L H₂/L-d with a corn stover lignocellulose feed, and 1.64 mol H₂/mol-glucose and 1.65 L H₂/L-d with a cellobiose feed. The lignocelluose and cellobiose fermentation effluent consisted primarily of: acetic, lactic, succinic, and formic acids and ethanol. An additional 800 ± 290 mL H₂/g-COD was produced from a synthetic effluent with a wastewater inoculum (fermentation effluent inoculum; FEI) by electrohydrogensis using microbial electrolysis cells (MECs). Hydrogen yields were increased to 980 ± 110 mL H₂/g-COD with the synthetic effluent by combining in the inoculum samples from multiple microbial fuel cells (MFCs) each pre-acclimated to a single substrate (single substrate inocula; SSI). Hydrogen yields and production rates with SSI and the actual fermentation effluents were 980 ± 110 mL/g-COD and 1.11 ± 0.13 L/L-d (synthetic); 900 ± 140 mL/g-COD and 0.96 ± 0.16 L/L-d (cellobiose); and 750 ± 180 mL/g-COD and 1.00 ± 0.19 L/L-d (lignocellulose). A maximum hydrogen production rate of 1.11 ± 0.13 L H₂/L reactor/d was produced with synthetic effluent. Energy efficiencies based on electricity needed for the MEC using SSI were 270 ± 20% for the synthetic effluent, 230 ± 50% for lignocellulose effluent and 220 ± 30% for the cellobiose effluent. COD removals were ∼90% for the synthetic effluents, and 70–85% based on VFA removal (65% COD removal) with the cellobiose and lignocellulose effluent. The overall hydrogen yield was 9.95 mol-H₂/mol-glucose for the cellobiose. These results show that pre-acclimation of MFCs to single substrates improves performance with a complex mixture of substrates, and that high hydrogen yields and gas production rates can be achieved using a two-stage fermentation and MEC process.     

Enhanced H2 production from corn stalk by integrating dark fermentation and single chamber microbial electrolysis cells with double anode arrangement

The low conversion efficiency of substrate is one of the main bottlenecks in dark fermentation for bio-H₂ production. Herein, an enhanced H₂ yield from corn stalk was achieved by integrating dark fermentation and single chamber microbial electrolysis cells (MECs). In the dark fermentation stage, a H₂ yield of 129.8 mL H₂/g-corn stalk and an average H₂ production rate of 1.73 m³/m³ d were recorded at 20 g/L of corn stalk and initial pH 7.0. The effluent from dark fermentation was diluted and further employed as feedstock to generate H₂ by MECs. A H₂ yield of 257.3 mL H₂/g-corn stalk, an HPR of 3.43 ± 0.12 m³/m³ d and an energy efficiency of 166 ± 10% were obtained with the effluent COD of 3995.5 mg/L under 0.8 V applied voltage. During MECs operation stage, about 90 ± 2% of acetate was converted to H₂ and the corresponding COD removal reached 44 ± 2% in MECs. Overall, the H₂ yield can reach 387.1 mL H₂/g-corn stalk by integrating dark fermentation and MECs, which had nearly tripled as against that of dark fermentation.     

Pretreatment of second and third generation feedstock for enhanced biohythane production: Challenges,recent trends and perspectives

In the context of biofuel production and achieving sustainable bioeconomy, the use of lignocellulosic and algae biomass in anaerobic fermentation processes yields biohythane that has a typical composition of 10–15% H₂, 50–55% CH₄ and 30–40% CO₂. Using organic biomass-based substrates has been shown to minimize environmental impacts due to the versatile production of high-value products under normal operating conditions that are practically achievable. However, the biohythane yield depends on different factors such as the biomass type, the organic loading rate, soluble metabolic products formed, the type of fermentation (single/dual stage) and the pretreatment strategy adopted for the biomass. Different pretreatment strategies based on physical, chemical and biological processes have been proposed in the literature. In this review, improvements in biohythane yield as a result of these pretreatment strategies, the need/effect of inoculum enrichment, the effects of pH, temperature, trace element addition and organic loading rate has been reviewed. Finally, the major developments of improving biohythane yield due to the addition of co-substrates and the current trends are discussed.     

Dark fermentation for H2 production from food waste and novel strategies for its enhancement

Hydrogen is an attractive energy vector that can be obtained through biological methods such as dark fermentation, allowing organic wastes to be used as substrates. This means an advantage compared to other methods since dark fermentation enables the revalorization of waste, such as food waste, with generation and disposal contributing to the current environmental issue. However, the complex composition of food waste and the microbial dynamics involved in dark fermentation have made it difficult to obtain maximum yields. Although several strategies have been evaluated to improve hydrogenogenic systems, the common limitations have been the economic costs and the re-emergence of unwanted microorganisms. Therefore, developing and evaluating novel strategies such as lactate-driven dark fermentation, bioaugmentation with native strains, metabolic engineering, and construction of synthetic microbiomes are innovative proposals to enhance H₂ production from food waste.     

Effect of enzyme addition on fermentative hydrogen production from wheat straw

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

Controlled oxidation-reduction potential on dark fermentative hydrogen production from glycerol: Impacts on metabolic pathways and microbial diversity of an acidogenic sludge

The oxidation-reduction potential (ORP) is an important factor in H₂ production via dark fermentation however its effect over microbial diversity in an acidogenic sludge has not, been well studied. This work studies the effect of ORP controlled by hydrogen peroxide and potassium ferricyanide on continuous hydrogen production and microbial diversity in an acidogenic sludge fed (HRT 12 h and pH 5.5) with glycerol. Results show that the more oxidizing ORP control environment (?540 mV) improves H₂ yield by 50–70% (0.31–0.51 molH₂/mol glycerol) over non-ORP control conditions. Oxidizing ORP values were shown to enrich microorganisms of the genus Clostridium, which have been linked to high H₂ yields. Therefore, controlling ORP in an acidogenic sludge was shown to directly modify microbial diversity at the genus level, and could likely to indirectly regulate metabolic function. Additionally, metabolic pathways were regulated by the kind of agent used.     

Homoacetogenesis during hydrogen production by mixed cultures dark fermentation: Unresolved challenge

This paper is a comprehensive review of H₂ consumption during anaerobic mixed culture H₂ dark fermentation with a focus on homoacetogenesis. Homoacetogenesis consumed from 11% to 43% of the H₂ yield in single and repeated batch fermentations, respectively. However, its quantification and extent during continuous fermentation are still not well understood. Models incorporating thermodynamic and kinetic controls are required to provide insight into the dynamic of homoacetogenesis during H₂ dark fermentation. Currently, no adequate method exists to eliminate homoacetogenesis because it does not depend on the culture's source, pre-treatment, substrate, type of reactor, or operation conditions. Controlling CO₂ concentrations during dark fermentation needs further investigation as a potential strategy towards controlling homoacetogenesis. Incorporating radioactive labeling technique in H₂ fermentation research could provide information on simultaneous production and consumption of H₂ during dark fermentation. Genetic studies investigating blocking H₂ consuming pathways and enhancing H₂ evolving hydrogenases are suggested towards controlling homoacetogenesis during dark fermentation.     

Two energy system analysis models: A comparison of methodologies and results

This paper presents a comparative study of two energy system analysis models both designed for the purpose of analysing electricity systems with a substantial share of fluctuating renewable energy. The first model (EnergyPLAN) has been designed for national and regional analyses. It has been used in the design of strategies for integration of wind power and other fluctuating renewable energy sources into the future energy supply. The model has been used for investigating new operation strategies and investments in flexibility in order to utilize wind power and avoid excess production. The other model (H₂RES) has been designed for simulating the integration of renewable sources and hydrogen into island energy systems. The H₂RES model can use wind, solar and hydro as renewable energy sources and diesel blocks as backup. The latest version of the H₂RES model has an integrated grid connection with the mainland. The H₂RES model was tested on the power system of Porto Santo Island, Madeira, Portugal, Corvo and Graciosa Islands, Azores Islands, Portugal and Sal Island, Cape Verde. This paper presents the results of using the two different models on the same case, the island of Mljet, Croatia. The paper compares methodologies and results with the purpose of identifying mutual benefits and improvements of both models.     

Review of Polybed pressure swing adsorption for hydrogen purification

Hydrogen (H₂) will play a key role in the future low-carbon energy society. The industrial production of hydrogen involves chemical reactions and purification steps. Pressure Swing Adsorption (PSA) is a versatile process able to produce ultrapure hydrogen (99.99+%) from various gas mixtures, resulting in the most widespread purification technology worldwide. In particular, the Polybed PSA system, having more than six beds and a complex cycle configuration, has proven to maximize H₂ recovery and H₂ throughput, exceeding 90% and 240 MMSCFD (265,000 Nm³ h^?1) per single train, respectively.This paper systematically reviews the Polybed H₂ PSA process for the first time, highlighting the latest technical advances and discussing its optimal integration in industrial clusters. A bespoke Polybed PSA process designed for simultaneous production of high-purity hydrogen and carbon dioxide (CO₂) is also reviewed in light of the recent international directives aimed to reduce CO₂ emissions and produce blue hydrogen.     

Hydrogen production through sorption enhanced steam reforming of natural gas: Thermodynamic plant assessment

A detailed and comprehensive simulation model of a H₂ production plant based on the Sorption Enhanced Reforming (SER) process of natural gas has been developed in this work. Besides thermodynamic advantages related to the shift of reforming equilibrium, SER technology features an intrinsic CO₂ capture that can be of interest in environmentally constrained economies. The model comprises natural gas treatment, H₂ and CO₂ compression, as well as H₂ purification with an adsorption unit that has been integrated within the SER process by using the off-gas for sorbent regeneration. A complete thermal integration has been also performed between the available hot gas streams in the plant, so that high pressure steam is generated and used to generate power in a steam cycle.     

Challenges for biohydrogen production via direct lignocellulose fermentation

Direct cellulose fermentation by cellulolytic anaerobic bacteria offers potential to generate renewable hydrogen (H₂) from inexpensive “waste” cellulosic feedstocks. The rates and yields of H₂ production via direct cellulose fermentation are low and must be increased significantly if this technology is to become a viable method for generating usable H₂. A much more comprehensive understanding of the relationships between gene and gene product expression, end-product synthesis patterns, and the factors that regulate carbon and electron balance, within the context of the bioreactor conditions must be achieved if we are to improve molar yields of H₂ during cellulose fermentation. Strategies to increase yields of H₂ production from cellulose include manipulation of carbon and electron flow via end-product inhibition (metabolic shift), metabolic engineering at the genetic level, synergistic co-cultures, and bioprocess engineering and bioreactor designs that maintain a neutral pH during fermentation and ensure rapid removal of H₂ and CO₂ from the aqueous phase.     

Harnessing of biohydrogen from wastewater treatment using mixed fermentative consortia: Process evaluation towards optimization

Utilizing wastewater as a potential source for renewable energy generation through biological routes has instigated considerable interest recently due to its sustainable nature. An attempt was made in this communication to review and summarize the work carried out in our laboratory on dark fermentation process of biohydrogen (H₂) production utilizing wastewater as primary substrate under acidogenic mixed microenvironment towards optimization of dynamic process. Process was evaluated based on the nature and composition of wastewater, substrate loading rates, reactor configuration, operation mode, pH microenvironment and pretreatment procedures adopted for mixed anaerobic culture to selectively enrich acidogenic H₂ producing consortia. The fermentative conversion of the substrate to H₂ is possible by a series of complex biochemical reactions manifested by selective bacterial groups. In spite of striking advantages, the main challenge of fermentative H₂ production is that, relatively low energy from the organic source was obtained in the form of H₂. Further utilization of unutilized carbon sources present in wastewater for additional H₂ production will sustain the practical applicability of the process. In this direction, enhancing H₂ production by adapting various strategies, viz., self-immobilization of mixed consortia (onto mesoporous material and activated carbon), integration with terminal methanogenic and photo-biological processes and bioaugmentation with selectively enriched acidogenic consortia were discussed. Application of acidogenic microenvironment for in situ production of bioelectricity through wastewater treatment employing microbial fuel cell (MFC) was also presented.     

Effects of inoculum and indigenous microflora on hydrogen production from the organic fraction of municipal solid waste

Biological production of hydrogen (H₂) by dark fermentation is an exciting scientific area for the conversion of low-cost residues and waste into biofuel. The main requirement for an efficient H₂ production process is the availability of efficient microbial consortia in which H₂-utilizing and non-H₂-producing bacteria are suppressed. This study was performed to evaluate the H₂ production potentials from the organic fraction of municipal solid waste (OFMSW) with and without addition of inoculum. The results showed that hydrogen productions from OFMSW without addition of inoculum were comparable to those obtained with inoculum but a latency phase of about 6 days occurred. On the contrary, addition of inoculum resulted in higher H₂ production potentials without any latency phase. The use of a properly pre-treated inoculum confirmed to be an interesting and improvable tool to obtain high H₂ yields from organic waste. However the indigenous OFMSW microbiota showed promising hydrogen yields especially toward the development of efficient hydrogen producing microbial inoculants.     

Co‐culture strategies for increased biohydrogen production

Biological hydrogen production from organic wastes is a less expensive, less energy‐demanding, and environmental‐friendly process. Pure monoculture delivers low H₂ content and low yield; these limitations are overcome by a defined co‐culture system, which outperforms mixed cultures with increased H₂ yield. The strategies used in co‐culture systems for increasing H₂ production have been discussed in this review. The strategies include hydrolysis of a variety of complex substrates, such as cellulose, molasses, crude glycerol, and algal biomass into simple fermentable sugars for increased H₂ yield by eliminating the use of exogenous enzymes. The strategies can bring geographically distant isolated microorganisms from different sources to coexist for simultaneous utilization of substrate and end metabolites into H₂ production of 99.99% purity without the expenses of reducing agents. In the case of maximum hydrogen production using co‐culture strategies, Clostridium, Enterobacter, and photo‐fermenting bacteria in a consolidated bioprocess system will result in increased H₂ yield. A co‐culture system is more feasible to achieve theoretical H₂ yield with high conversion efficiency of organic wastes, enhance the economic viability of H₂ production, provide better effluent treatment quality, and concurrently address the limitations of H₂ production. Copyright © 2015 John Wiley & Sons, Ltd.     

Hydrogen production from rotten dates by sequential three stages fermentation

This study was devoted for H₂ production from rotten fruits of date palm (Phoenix dactylifera L.) by three fermentation stages. A facultative anaerobe, Escherichia coli EGY was used in first stage to consume O₂ and maintain strict anaerobic conditions for a second stage dark fermentative H₂ production by the strictly anaerobic Clostridium acetobutylicum ATCC 824. Subsequently, a third stage photofermentation using Rhodobacter capsulatus DSM 1710 has been conducted for the H₂ production. The maximum total H₂ yield of the three stages (7.8 mol H₂ mol⁻¹ sucrose) was obtained when 5 g L⁻¹ of sucrose was supplemented to fermentor as rotten date fruits. A maximum estimated cumulative H₂ yield of the three stages (162 LH₂ kg⁻¹ fresh rotten dates) was estimated at the (5 g L⁻¹) sucrose concentration. These results suggest that rotten dates can be efficiently used for commercial H₂ production. The described protocol did not require addition of a reducing agent or flashing with argon which both are expensive.     

Fuel cell efficiency in a combined electrochemical-thermochemical process

The thermodynamics of a H₂/Cl₂ fuel cell combined with a thermochemical hydrogen chloride oxidation stage are considered. Such a cycle is superior to the phosphoric acid H₂/O₂ systems but does not better the efficiency demonstrated in more advanced H₂/O₂ fuel cell systems.