Photo-fermentative hydrogen production from crop residue: A mini review

Biohydrogen from waste feedstocks: An energy opportunity for decarbonization in developing countries

In developing economies, the decarbonization of energy sector has become a global priority for sustainable and cleaner energy system. Biohydrogen production from renewable sources of waste biomass is a good source of energy incentive that reduces the pollution. Biohydrogen has a high calorific value and emits no emissions, producing both energy security and environmental sustainability. Biohydrogen production technologies have become one of the main renewable sources of energy. The present paper entails the role of biohydrogen recovered from waste biomasses like agricultural waste (AW), organic fraction of municipal solid waste (OFMSW), food processing industrial waste (FPIW), and sewage sludge (SS) as a promising solution. The main sources of increasing yield percentage of biohydrogen generation from waste feedstock using different technologies, and process parameters are also emphasized in this review. The production paths for biohydrogen are presented in this review article, and because of advancements in R and D, biohydrogen has gained viability as a biofuel for the future and discusses potential applications in power generation, transportation, and industrial processes, emphasizing the versatility and potential for integration into existing energy infrastructure. The investigation of different biochemical technologies and methods for producing biohydrogen, including anaerobic digestion (AD), dark fermentation (DF), photo fermentation (PF), and integrated dark-photo fermentation (IDPF), has been overviewed. This analysis also discusses future research, investment, and sustainable energy options transitioning towards a low-carbon future, as well as potential problems, economic impediments, and policy-related issues with the deployment of biohydrogen in emerging nations.

Biological fermentation pilot-scale systems and evaluation for commercial viability towards sustainable biohydrogen production

Featuring high caloric value, clean-burning, and renewability, hydrogen is a fuel believed to be able to change energy structure worldwide. Biohydrogen production technologies effectively utilize waste biomass resources and produce high-purity hydrogen. Improvements have been made in the biohydrogen production process in recent years. However, there is a lack of operational data and sustainability analysis from pilot plants to provide a reference for commercial operations. In this report, based on spectrum coupling, thermal effect, and multiphase flow properties of hydrogen production, continuous pilot-scale biohydrogen production systems (dark and photo-fermentation) are established as a research subject. Then, pilot-scale hydrogen production systems are assessed in terms of sustainability. The system being evaluated, consumes 171,530 MJ of energy and emits 9.37 t of CO2 eq when producing 1 t H2, and has a payback period of 6.86 years. Our analysis also suggests future pathways towards effective biohydrogen production technology development and real-world implementation.

Towards industrial biological hydrogen production: a review

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

Bio-hydrogen-producing Potential Evaluation and Capacity Enhancement from Tobacco Processing Leftovers by Photo-fermentation Under Diverse Initial pH

The use of tobacco growing and processing residues for bio-hydrogen production is an effective exploration to broaden the source of bio-hydrogen production raw materials and realize waste recycling. In this study, bio-hydrogen-producing potential was evaluated and the effect of diverse initial pH on hydrogen production performance was investigated. The cumulative hydrogen yield (CHY) and the properties of fermentation liquid were monitored. The modified Gompertz model was adopted to analyze the kinetic characteristics of photo-fermentation bio-hydrogen production process. Results showed that CHY increased firstly and then decreased with the increase of initial pH. Highest CHY and hydrogen production rate of appeared at the initial pH of 8, which were 257.7 mL and 6.15 mL/h, respectively. The acidic initial pH was found to severely limit the bio-hydrogen production capacity. The correlation coefficients (R2) of hydrogen production kinetics parameters were all greater than 0.99, meaning that the fitting effect was good. The main metabolites of bacteria in the system were acetic acid, butyric acid, and ethanol, and the consumption of acetic acid was promoted with the increase of initial pH.

Sustainable biohydrogen production from lignocellulosic biomass sources - metabolic pathways, production enhancement, and challenges

Hydrogen production from biological processes has been hailed as a promising strategy for generating sustainable energy. Fermentative hydrogen production processes such as dark and photofermentation are considered more sustainable and economical than other biological methods such as biophotolysis. However, these methods have constraints such as low hydrogen yield and conversion efficiency, so practical implementations still need to be made. The present review provides an assessment and feasibility of producing biohydrogen through dark and photofermentation techniques utilizing various lignocellulosic biomass wastes as substrates. Furthermore, this review includes information about the strategies to increase the productivity rate of biohydrogen in an eco-friendly and sustainable manner, like integration of dark and photofermentation techniques, pretreatment of biomass, genetic modification of microorganisms, and application of nanoadditives.

Green hydrogen production through consolidated bioprocessing of lignocellulosic biomass using nanobiotechnology approach

The main objective of this study was to develop a sustainable process for hydrogen production by implementing nanotechnology in combination with consolidated bioprocessing (CBP) approach from lignocellulosic biomass (LCB). Peroxidase mimicking CeFe₃O₄ nanoparticles (NPs, 4.0 g/L) were applied for degradation of lignin from raw corn cob (CC) biomass for generation of cellulose-hemicellulose fractions amenable towards Clostridium cellulovorans during fermentation process. NP-treated biomass exhibited 43.26 % lignin removal from raw CC which was further employed for hydrogen fermentation by C. cellulovorans through CBP method. The strain yielded maximum 78.45 mL of cumulative hydrogen with hydrogen production rate of 1.55 mL/h using NP-treated CC. To the best of our knowledge, this is the first study on enhanced hydrogen production using NP-treated CC biomass in single pot fermentation which can prove to be a simpler, easier, and more economical process.

Effect of Fe0 particle size on buffering characteristics and biohydrogen production in high-load photo fermentation system of corn stover

The aim of this work was to study the effects of Fe0 particle sizes (700 nm, 100 nm and 50 nm) addition on biohydrogen production, liquid culture characteristics and photosynthetic bacterial respond in the high-load photo fermentation system of corn stover within the concentration range of 200-1500 mg/L. Results showed that Fe0 with particle size of 700 nm had a better promotion effect on hydrogen production than 100 nm and 50 nm. The highest hydrogen yield of 74.32 ± 3.48 mL/g TS and hydrogen production rate of 3.31 ± 0.11 mL/g·h TS corn stover were obtained at 1000 and 1500 mg/L Fe0-700 nm, which were significantly increased by 92.88 % and 133.88 % compared with the control group. Further analysis indicated that Fe0 addition effectively alleviated pH drop, enhanced nitrogenase activity, promoted cell growth, and accelerated the consumption of acetic acid and butyric acid.

Lignin removal, reducing sugar yield and photo-fermentative biohydrogen production capability of corn stover: Effects of different pretreatments

The effects of different pretreatment methods, including hydrothermal, acid, alkali, acid-heat and alkali-heat on lignin removal, reducing sugar (RS) yield and photo-fermentative biohydrogen production (PFHP) capability of corn stover (CS) were studied. NaOH-heat pretreatment was the most effective for lignin removal from CS, and the lignin removal rate reached 77%. All the studied pretreatment methods improved the total RS yield of CS, and the highest total RS yield (46.1 g/100 g raw material (RM)) was obtained from 2% NaOH-heat pretreated CS. 2% NaOH pretreatment realized the best PFHP of CS, which increased the hydrogen yield (HY), maximal hydrogen production rate (HPR) and highest hydrogen content (HC) by 31.9%, 50.9% and 20.1% respectively, and shortened hydrogen production lag time (HPLT) by 58.8% over that of untreated CS. However, NaOH-heat and 4% NaOH pretreatment weakened the PFHP capability of CS.

Significance of Hydrogen as Economic and Environmentally Friendly Fuel

The major demand of energy in today’s world is fulfilled by the fossil fuels which are not renewable in nature and can no longer be used once exhausted. In the beginning of the 21st century, the limitation of the fossil fuels, continually growing energy demand, and growing impact of green-house gas emissions on the environment were identified as the major challenges with current energy infrastructure all over the world. The energy obtained from fossil fuel is cheap due to its established infrastructure; however, these possess serious issues, as mentioned above, and cause bad environmental impact. Therefore, renewable energy resources are looked to as contenders which may fulfil most energy requirements. Among them, hydrogen is considered as the most environmentally friendly fuel. Hydrogen is clean, sustainable fuel and it has promise as a future energy carrier. It also has the ability to substitute the present energy infrastructure which is based on fossil fuel. This is seen and projected as a solution for the above-mentioned problems including rise in global temperature and environmental degradation. Environmental and economic aspects are the important factors to be considered to establish hydrogen infrastructure. This article describes the various aspects of hydrogen including production, storage, and applications with a focus on fuel cell based electric vehicles. Their environmental as well as economic aspects are also discussed herein.

Tolerance of photo-fermentative biohydrogen production system amended with biochar and nanoscale zero-valent iron to acidic environment

Anaerobic fermentation system is easy to become acidic due to the generation of small molecular acids, which will affect the metabolism of bacteria. Therefore, it is necessary to improve the acid resistance of system. In this work, the tolerance of photo-fermentative biohydrogen production system amended with biochar, nanoscale zero-valent iron (nZVI) and biochar + nZVI to acidic environment was studied. Results showed that additives improved the stability and performance of the photo fermentation. The best increment of biohydrogen from 0 to 286.83 ± 2.77 mL was obtained by adding biochar and nZVI together at the original pH of 4.5. The additive reduced the oxidation-reduction potential and promoted the consumption of acetate and butyrate. At initial pH of 5, 6 and 7, the highest biohydrogen yield of 361.02 ± 10.11, 419.36 ± 10.70 and 382.67 ± 25.08 mL was obtained by adding nZVI, respectively, representing 42%-44.45% increase compared with the control group under the same conditions.

Study of the interrelationship between nano-TiO2 addition and photo-fermentative bio-hydrogen production of corn straw

This study explored the interrelationship between nano-TiO₂ addition and photo-fermentative hydrogen production (PFHP) of corn straw. The maximum cumulative hydrogen volume (CHV) was up to 688.8 mL under the optimal photo-fermentative process conditions with nano-TiO₂ addition of 300 mg/L. Initial pH and interaction between substrate concentration and light intensity had highly significant effects on PFHP of corn straw with nano-TiO₂ addition. With the improvement of CHV, nano-TiO₂ addition decreased the optimal initial pH and substrate concentration for PFHP of corn straw. Moreover, nano-TiO₂ addition promoted the metabolism of butyric acid and acetic acid by photosynthetic bacteria HAU-M1, and significantly reduced the total concentration of intermediate byproducts during hydrogen production to a low level of 1.6-2.5 g/L, thus making the CHV, maximum hydrogen production rate (HPR) and average hydrogen content (HC) increased by 32.6%, 27.9% and 8.3% respectively over the control without nano-TiO₂ addition.

Integrated biohydrogen production via lignocellulosic waste: Opportunity, challenges & future prospects

Hydrogen production through biological route is the cleanest, renewable and potential way to sustainable energy generation. Productions of hydrogen via dark and photo fermentations are considered to be more sustainable and economical approach over numerous existing biological modes. Nevertheless, both the biological modes suffer from certain limitations like low yield and production rate, and because of these practical implementations are still far away. Therefore, the present review provides an assessment and feasibility of integrated biohydrogen production strategy by combining dark and photo-fermentation as an advanced biochemical processing while using lignocellulosics biomass to improve and accelerate the biohydrogen production technology in a sustainable manner. This review also evaluates practical viability of the integrated approach for biohydrogen production along with the analysis of the key factors which significantly influence to elevate this technology on commercial ground with the implementation of various environment friendly and innovative approaches.

Experimental study on optimization of initial pH for photo-fermentation bio-hydrogen under different enzymatic hydrolysis of chlorella vulgaris

In the paper, Use Chlorella as raw material, HAU-M1 Photosynthetic bacteria (PSB) as hydrogen-producing bacteria, the influence of initial pH on bio-hydrogen by photosynthetic organisms from Chlorella vulgaris with diverse enzyme addition was studied. The results showed that when using cellulase as hydrolase, the optimum initial pH was 7.0 and highest bio-hydrogen was 25.99 mL/g dry cell weight. Using neutral protease as hydrolase, the optimum initial pH was 8.0 and highest bio-hydrogen was 16.47 mL/g dry cell weight. Using mixed enzyme of cellulase and protease as hydrolase, the optimal initial pH was 7.0 and highest bio-hydrogen was 27.43 mL/g dry cell weight. The bio-hydrogen from Chlorella after mixed enzymatic hydrolysis is better than that of single enzymatic hydrolysis, we think the mixed enzymatic hydrolysis of cellulase and protease was superior to the single enzymatic hydrolysis of the two enzymes, which provides a scientific reference and low-cost bio-hydrogen technology by microalgae.

Food Waste: A Promising Source of Sustainable Biohydrogen Fuel

Annually, approximately 1.3 billion tons of food is lost worldwide, accounting for one-third of annual food production. Therefore, turning food waste into energy is of enormous environmental significance because of its sustainable nature. Nutrients and organic acids present in food waste can be used to produce (bio)products such as biohydrogen through biological processes. However, our understanding of the production of biohydrogen from food waste through photofermentation and dark fermentation is still restricted. This comprehensive study aims to review the potential of food waste for biohydrogen production using microbial mediators, including a brief overview of process parameters that affect the (bio)hydrogen production pathway.

Enhancement of pH values stability and photo-fermentation biohydrogen production by phosphate buffer

The main aim of this study was to investigate the effects of the initial pH values of the buffer on photo-fermentation biohydrogen production. Hydrogen production and the kinetics of it under different initial pH values were analyzed. Effects of initial pH values on reducing sugar consumption, hydrogen production rate, and byproduct production were evaluated at initial pH values of 5–7. The results showed that initial pH values of phosphate buffer had a significant effect on biohydrogen production via photo-fermentation. With the initial pH value of phosphate buffer at 6.0, the cumulative hydrogen production reached its maximum, 569.6 mL. The maximum hydrogen production rate was 23.96 mL/h at the initial pH value of 6.5. With the initial pH values at 5.0 and 7.5, the maximum hydrogen production rates were becoming lower, only 5.59 mL/h and 5.42 mL/h, respectively. And with the increase in pH values, the peak period of hydrogen production was gradually delayed, indicating that the alkaline environment had a negative effect on the ability of photosynthetic bacteria. This study revealed the influence of phosphate buffer initial pH values on the biohydrogen production via photo-fermentation and aimed to provide a scientific reference for further improving the theory and technology for biohydrogen production from biomass.

Statistical optimization of simultaneous saccharification fermentative hydrogen production from corn stover

Corn stovers are rich in carbohydrates and can be used by anaerobic bacteria to produce hydrogen by fermentation. In the present study, using hydrogen production as the main experimental index, the effect of different influential factors on hydrogen production from corn stover saccharification and fermentation was studied, using the response surface method BBD model. The significance of interactions between different influential factors on hydrogen production by simultaneous saccharification and fermentation of corn stover material were investigated and optimized. Results showed that there were several factors affecting simultaneous saccharification fermentative hydrogen production from corn stover, including substrate concentration, inoculation amount, pH value and enzyme concentration. In linear terms, substrate concentration had the greatest influence on hydrogen production by anaerobic simultaneous saccharification and fermentation. In terms of multi-factor interactions, the interaction between pH and enzyme concentration was the most significant. The optimal hydrogen production conditions established from the BBD model were as follows: substrate concentration of 25 mg/mL, inoculation amount proportion of 32.62%, initial pH value of 6.50 and enzyme concentration of 172.08 mg/g, resulting in the maximum hydrogen production of 55.29 mL/g TS. The actual maximum hydrogen production reached 56.66 mL/g TS, with these experimental results consistent with the predicted value established from equation fitting. This study provides a reference for hydrogen production by anaerobic synchronous saccharification fermentation using corn stover as substrate and lays a foundation and provides technical support for the industrialization of biological hydrogen production using corn stover as substrate.

The adherence-associated Fdp fasciclin I domain protein of the biohydrogen producer Rhodobacter sphaeroides is regulated by the global Prr pathway

Expression of fdp, encoding a fasciclin I domain protein important for adherence in the hydrogen-producing bacterium Rhodobacter sphaeroides, was investigated under a range of conditions to gain insights into optimization of adherence for immobilization strategies suitable for H₂ production. The fdp promoter was linked to a lacZ reporter and expressed in wild type and in PRRB and PRRA mutant strains of the Prr regulatory pathway. Expression was significantly negatively regulated by Prr under all conditions of aerobiosis tested including anaerobic conditions (required for H₂ production), and aerobically regardless of growth phase, growth medium complexity or composition, carbon source, heat and cold shock and dark/light conditions. Negative fdp regulation by Prr was reflected in cellular levels of translated Fdp protein. Since Prr is required directly for nitrogenase expression, we propose optimization of Fdp-based adherence in R. sphaeroides for immobilized biohydrogen production by inactivation of the PrrA binding site(s) upstream of fdp.

Enhanced buffer capacity of fermentation broth and biohydrogen production from corn stalk with Na2HPO4/NaH2PO4

The remarkable buffer capacity of buffer solution can significantly improve the biohydrogen production yield and energy conversion efficiency. In the present study, the effect of buffer solution Na₂HPO₄/NaH₂PO₄ on buffer capacity of fermentation broth and photo-fermentation biohydrogen production (PFHP) was studied. Gas characteristics, fermentation broth properties, and kinetic parameters in PFHP were investigated. With the increase in pH values (5-7) of buffer solution Na₂HPO₄/NaH₂PO₄, firstly hydrogen yield increased and then decreased. Maximum energy conversion efficiency 9.84%, hydrogen yield 132.69 mL/g corn stalk, and hydrogen content 53.88% were achieved at pH value of 6. The results of one-way ANOVA showed that pH values of fermentation broth and cumulative hydrogen production were strongly affected by pH values of buffer solution. Buffer solution Na₂HPO₄/NaH₂PO₄ retarded the decrease of pH value of photo-fermentation broth, and significantly improved the PFHP.

Heterologous Hydrogenase Overproduction Systems for Biotechnology-An Overview

Hydrogenases are complex metalloenzymes, showing tremendous potential as H2-converting redox catalysts for application in light-driven H2 production, enzymatic fuel cells and H2-driven cofactor regeneration. They catalyze the reversible oxidation of hydrogen into protons and electrons. The apo-enzymes are not active unless they are modified by a complicated post-translational maturation process that is responsible for the assembly and incorporation of the complex metal center. The catalytic center is usually easily inactivated by oxidation, and the separation and purification of the active protein is challenging. The understanding of the catalytic mechanisms progresses slowly, since the purification of the enzymes from their native hosts is often difficult, and in some case impossible. Over the past decades, only a limited number of studies report the homologous or heterologous production of high yields of hydrogenase. In this review, we emphasize recent discoveries that have greatly improved our understanding of microbial hydrogenases. We compare various heterologous hydrogenase production systems as well as in vitro hydrogenase maturation systems and discuss their perspectives for enhanced biohydrogen production. Additionally, activities of hydrogenases isolated from either recombinant organisms or in vivo/in vitro maturation approaches were systematically compared, and future perspectives for this research area are discussed.

Organic waste biorefineries: Looking towards implementation

The concept of biorefinery expands the possibilities to extract value from organic matter in form of either bespoke crops or organic waste. The viability of biorefinery schemes depends on the recovery of higher-value chemicals with potential for a wide distribution and an untapped marketability. The feasibility of biorefining organic waste is enhanced by the fact that the biorefinery will typically receive a waste management fee for accepting organic waste. The development and implementation of waste biorefinery concepts can open up a wide array of possibilities to shift waste management towards higher sustainability. However, barriers encompassing environmental, technical, economic, logistic, social and legislative aspects need to be overcome. For instance, waste biorefineries are likely to be complex systems due to the variability, heterogeneity and low purity of waste materials as opposed to dedicated biomasses. This article discusses the drivers that can make the biorefinery concept applicable to waste management and the possibilities for its development to full scale. Technological, strategic and market constraints affect the successful implementations of these systems. Fluctuations in waste characteristics, the level of contamination in the organic waste fraction, the proximity of the organic waste resource, the markets for the biorefinery products, the potential for integration with other industrial processes and disposal of final residues are all critical aspects requiring detailed analysis. Furthermore, interventions from policy makers are necessary to foster sustainable bio-based solutions for waste management.

Evaluation of biohydrogen yield potential and electron balance in the photo-fermentation process with different initial pH from starch agricultural leftover

Starch agricultural leftover is a potential substrate for photosynthetic bacteria to produce hydrogen. In this work, the effect of initial pH on photo-fermentation biohydrogen production process and electron balance were investigated. The modified Gompertz model was adopted, and hydrogen yield, sugar consumption and metabolite evolution were monitored with initial pH varying from 5 to 9. Results showed that hydrogen was produced mainly through acetic acid and butyric acid metabolism pathways when starch taken as carbon source. It was found that the maximum hydrogen yield (642 ± 22 mL) and highest hydrogen production rate (77.78 mL/(L·h)) were obtained at initial pH of 7. 37.65% substrates electrons diverted towards hydrogen. Lower hydrogen yield, hydrogen production rate and lower electrons diversion were obtained at other initial pH levels. Also, the lag phase time was 0.04 h when pH was 7, which was significantly lower than other levels.

Purple phototrophic bacteria for resource recovery: Challenges and opportunities

Sustainable development is driving a rapid focus shift in the wastewater and organic waste treatment sectors, from a "removal and disposal" approach towards the recovery and reuse of water, energy and materials (e.g. carbon or nutrients). Purple phototrophic bacteria (PPB) are receiving increasing attention due to their capability of growing photoheterotrophically under anaerobic conditions. Using light as energy source, PPB can simultaneously assimilate carbon and nutrients at high efficiencies (with biomass yields close to unity (1 g COD_biomass·g COD_removed^-1)), facilitating the maximum recovery of these resources as different value-added products. The effective use of infrared light enables selective PPB enrichment in non-sterile conditions, without competition with other phototrophs such as microalgae if ultraviolet-visible wavelengths are filtered. This review reunites results systematically gathered from over 177 scientific articles, aiming at producing generalized conclusions. The most critical aspects of PPB-based production and valorisation processes are addressed, including: (i) the identification of the main challenges and potentials of different growth strategies, (ii) a critical analysis of the production of value-added compounds, (iii) a comparison of the different value-added products, (iv) insights into the general challenges and opportunities and (v) recommendations for future research and development towards practical implementation. To date, most of the work has not been executed under real-life conditions, relevant for full-scale application. With the savings in wastewater discharge due to removal of organics, nitrogen and phosphorus as an important economic driver, priorities must go to using PPB-enriched cultures and real waste matrices. The costs associated with artificial illumination, followed by centrifugal harvesting/dewatering and drying, are estimated to be 1.9_, 0.3-2.2 and 0.1-0.3 $·kg_{dry biomass}^-1. At present, these costs are likely to exceed revenues. Future research efforts must be carried out outdoors, using sunlight as energy source. The growth of bulk biomass on relatively clean wastewater streams (e.g. from food processing) and its utilization as a protein-rich feed (e.g. to replace fishmeal, 1.5-2.0 $·kg^-1) appears as a promising valorisation route.

Effect of alkaline pretreatment on photo-fermentative hydrogen production from giant reed: Comparison of NaOH and Ca(OH)2

Giant reed was the first time used for photo-fermentative hydrogen production with HAU-M1 bacteria. Effects of NaOH and Ca(OH)₂ pretreatments of giant reed on structural changes, enzymatic digestibility, hydrogen production, and energy conversion efficiency were evaluated. Compared to Ca(OH)₂ pretreatment, NaOH pretreatment removed more dry matter and lignin at the same loading. The highest glucose yield (44.9%) of NaOH pretreatment was 1.74-fold higher than that of Ca(OH)₂ pretreatment. 20% NaOH pretreated giant reed biomass achieved the highest hydrogen yield (98.3 mL/g TS), which was 20% and 70% higher than the highest level of Ca(OH)₂ pretreated (20% Ca(OH)₂) and untreated giant reed, respectively. Only giant reed biomass pretreated with 20% NaOH resulted in a significant (p < 0.05) increase (25%) in energy conversion efficiency.

Biohydrogen production through active saccharification and photo-fermentation from alfalfa

Studying biohydrogen production from alfalfa is of practical significance to cleaner production and biomass utilization. The performances of biohydrogen production through active/passive saccharification and photo-fermentation were compared. The effects of initial pH, substrate concentration, and cellulase loading on biohydrogen production from alfalfa by photosynthetic bacteria HAU-M1 were presented. It was found that the maximum hydrogen yield of 55.81 mL/g was achieved at initial pH of 6.90, substrate concentration of 31.23 g/mL, and cellulase loading of 0.13 g/g. Hydrogen yield of active saccharification and photo-fermentation was much higher as compare to passive saccharification and photo-fermentation. Initial pH value showed a more significant influence on photosynthetic bacteria in comparison to cellulase in active saccharification and photo-fermentation biohydrogen production. The low yield of propionic acid suggested that it was an efficient photosynthetic hydrogen production. Photo-fermentation hydrogen production from alfalfa provides a novel path for efficient utilization of alfalfa.

Enhancement of bio-hydrogen yield and pH stability in photo fermentation process using dark fermentation effluent as succedaneum

The photo fermentation hydrogen yield from dark fermentation effluents (DFEs) can be promoted by adding corn straw enzymatic hydrolysate adjusts the nutritional composition of DFEs. As compared with the control group (without enzymatic hydrolysate addition), the effect of adding enzymatic hydrolysate make H₂ yield increase from 312.54 to 1287.06 mL H₂/g TOC, and maximum hydrogen production rate increase 2.14 to 10.23 mL/h. On the other hand, buffer reagents remained in DFEs make which can replace part sodium citrate buffer to maintain pH stability in synchronized saccharification and photosynthetic fermentation process with corn straw as substrate, the best result was observed at the ration of 1:2 (33 mL DFEs, 67 mL sodium citrate buffer) with the hydrogen yield of 436.30 ± 10 mL, and which can cut down the GHG in the life cycle of hydrogen production.

Grid columnar flat panel photobioreactor with immobilized photosynthetic bacteria for continuous photofermentative hydrogen production

A biophotoreactor with a transparent glass flat panel with polymethyl methacrylate (PMMA) grid columnar for enhanced biofilm growth with Rhodopseudomonas palustris GCA009 was developed and tested at 590 nm incident light. Continuous photofermentative hydrogen production from glucose was tested using this novel reactor. At light intensity of 210 W/m², feed substrate concentration of 56.0 mmol/L, and crossflow velocity of 1.68 × 10^-6 m/s, a maximum hydrogen production rate of 32.6 mmol/L-d (3.56 mmol/m²-h), hydrogen yield of 1.15 mol H₂/mol glucose and light conversion efficiency of 5.34% can be achieved. Since the revised grid columnar effectively enlarged the surface area of reactor and enhanced cell attachment, the present reactor design led to higher hydrogen production rates than literature works.

State of the art and challenges of biohydrogen from microalgae

Biohydrogen from microalgae has attracted extensive attention owing to its promising features of abundance, renewable and self sustainability. Unlike other well-established biofuels like biodiesel and bioethanol, biohydrogen from microalgae is still in the preliminary stage of development. Criticisms in microalgal biohydrogen centered on its practicality and sustainability. Various laboratory- and pilot-scale microalgal systems have been developed, and some research initiatives have exhibited potential for commercial application. This work provides a review of the state of the art of biohydrogen from microalgae. Discussions include metabolic pathways of light-driven transformation and dark fermentation, reactor schemes and system designs encompassing reactor configurations and light manipulation. Challenges, knowledge gaps and the future directions in metabolic limitations, economic and energy assessments, and molecular engineering are also delineated. Current scientific and engineering challenges of microalgal biohydrogen need to be addressed for technology leapfrog or breakthrough.

Direct microbial transformation of carbon dioxide to value-added chemicals: A comprehensive analysis and application potentials

Carbon dioxide storage in petroleum and other geological reservoirs is an economical option for long-term separation of this gas from the atmosphere. Other options include applications through conversion to valuable chemicals. Microalgae and plants perform direct fixation of carbon dioxide to biomass, which is then used as raw material for further microbial transformation (MT). The approach by microbial transformation can achieve reduction of carbon dioxide and production of biofuels. This review addresses the research and technological processes related to direct MT of carbon dioxide, factors affecting their efficiency in operation and the review of economic feasibility. Additionally, some commercial plants making utilization of CO₂ around the globe are also summarized along with different value-added chemicals (methane, acetate, fatty acids and alcohols) as reported in literature. Further information is also provided for a better understanding of direct CO₂ MT and its future prospects leading to a sustainable and clean environment.

Comparison of deep eutectic solvents (DES) on pretreatment of oil palm empty fruit bunch (OPEFB): Cellulose digestibility, structural and morphology changes

In this work, a novel solvent, deep eutectic solvent (DES) was applied to examine its effectiveness in pretreating OPEFB. Three types of DESs, i.e. choline chloride-lactic acid (ChCl-LA), choline chloride-urea (ChCl-U) and choline chloride-glycerol (ChCl-G) were investigated. The pretreatment performance was based on cellulose digestibility, structural and morphology changes. At molar ratio of 1:2, ChCl-LA attained the highest reducing sugars yield of 20.7%, followed by ChCl-G (20.0%) and ChCl-U (16.9%). FT-IR and SEM results further confirmed the outstanding ability of ChCl-LA due of its ability in cellulose, hemicellulose and lignin disruption, exposing its cellulose fraction to enzymatic hydrolysis. ChCl-LA is also more favorable compare to acid and alkaline solvents as it could prevent sugars loss, use of expensive corrosion resistant equipment and ease products separation.

Bio-conversion of photosynthetic bacteria from non-toxic wastewater to realize wastewater treatment and bioresource recovery: A review

Generating or recycling water and resources from wastewater other than just treating wastewater is one of the most popular trends worldwide. Photosynthetic bacteria (PSB) wastewater treatment and resource recovery technology is one of the most potential methods. PSBs are non-toxic and contain lots of value-added products that can be utilized in the agricultural and food industries. They are effective to degrade pollutants and synthesize useful biomass, thus realizing wastewater treatment, bioresource production, and eliminating waste sludge. If all the nutrients in wastewaters could be bio-converted by PSB, then pollutant reductions and economic benefits would be achieved. This review paper firstly describes and summarizes this technology, including PSBs classification, metabolism, and the market application. The feasibility, technical procedures, bioreactors, pollutant removal, and bioresource production are also summarized, compared and evaluated. Issues that concern the advantages and industrialization of this technologies at the plant scale are also discussed.

Sequential dark and photo fermentation hydrogen production from hydrolyzed corn stover: A pilot test using 11 m3 reactor

Pilot tests of sequential dark and photo fermentation H₂ production were for the first time conducted in a 11 m³ reactor (3 m³ for dark and 8 m³ for photo compartments). A combined solar and light-emitting diode illumination system and a thermal controlling system was installed and tested. With dark fermentation unit maintained at pH 4.5 and 35 °C and photo fermentation unit at pH 7.0 and 30 °C, the overall biogas production rate using hydrolyzed corn stover as substrate reached 87.8 ± 3.8 m³/d with 68% H₂ content, contributed by dark unit at 7.5 m³-H₂/m³-d and by photo unit at 4.7 m³/m³-d. Large variation was noted for H₂ production rate in different compartments of the tested units, revealing the adverse effects of poor mixing, washout, and other inhomogeneity associated with large reactor operations.

Enzymatic saccharification of lignocellulosic biorefinery: Research focuses

To realize lignocellulosic biorefinery is of global interest, with enzymatic saccharification presenting an essential stage to convert polymeric sugars to mono-sugars for fermentation use. This mini-review summarizes qualitatively the research focuses discussed the review articles presented in the past 22 months and other relevant papers. The research focuses on pretreatment with improved efficiency, enhanced enzyme production with high yields and high extreme tolerance, feasible combined saccharification and fermentation processes, detailed mechanisms corresponding to the enzymatic saccharification in lignocellulosic biorefinery, and the costs are discussed.

Comparison of bio-hydrogen production yield capacity between asynchronous and simultaneous saccharification and fermentation processes from agricultural residue by mixed anaerobic cultures

Taken common agricultural residues as substrate, dark fermentation bio-hydrogen yield capacity from asynchronous saccharification and fermentation (ASF) and simultaneous saccharification and fermentation (SSF) was investigated. The highest hydrogen yield of 472.75mL was achieved with corncob using ASF. Hydrogen yield from corn straw, rice straw, corncob and sorghum stalk by SSF were 20.54%,10.31%,13.99% and 5.92% higher than ASF, respectively. The experimental data fitted well to the modified Gompertz model. SSF offered a distinct advantage over ASF with respect to reducing overall process time (60h of SSF, 108h of ASF). Meanwhile, SSF performed better than SSF with respect to shortening the lag-stage. The major metabolites of anaerobic fermentation hydrogen production by ASF and SSF were butyric acid and acetic acid.

Effect of substrate concentration on hydrogen production by photo-fermentation in the pilot-scale baffled bioreactor

Effect of substrate concentration on photo-fermentative hydrogen production was studied with a self-designed 4m³ pilot-scale baffled photo-fermentative hydrogen production reactor (BPHR). The relationships between parameters, such as hydrogen production rate (HPR, mol H₂/m³/d), hydrogen concentration, pH value, oxidation-reduction potential, biomass concentration (volatile suspended solids, VSS) and reducing sugar concentration, during the photo-fermentative hydrogen production process were investigated. The highest HPR of 202.64±8.83mol/m³/d was achieved in chamber #3 at a substrate concentration of 20g/L. Hydrogen contents were in the range of 42.19±0.94%-49.71±0.27%. HPR increased when organic loading rate was increased from 3.3 to 20g/L/d, then decreased when organic loading rate was further increased to 25g/L/d. A maximum HPR of 148.65±4.19mol/m³/d was obtained when organic loading rate was maintained at 20g/L/d during continuous bio-hydrogen production.

Potential use and the energy conversion efficiency analysis of fermentation effluents from photo and dark fermentative bio-hydrogen production

Effluent of bio-hydrogen production system also can be adopted to produce methane for further fermentation, cogeneration of hydrogen and methane will significantly improve the energy conversion efficiency. Platanus Orientalis leaves were taken as the raw material for photo- and dark-fermentation bio-hydrogen production. The resulting concentrations of acetic, butyric, and propionic acids and ethanol in the photo- and dark-fermentation effluents were 2966mg/L and 624mg/L, 422mg/L and 1624mg/L, 1365mg/L and 558mg/L, and 866mg/L and 1352mg/L, respectively. Subsequently, we calculated the energy conversion efficiency according to the organic contents of the effluents and their energy output when used as raw material for methane production. The overall energy conversion efficiencies increased by 15.17% and 22.28%, respectively, when using the effluents of photo and dark fermentation. This two-step bio-hydrogen and methane production system can significantly improve the energy conversion efficiency of anaerobic biological treatment plants.