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

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.

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.

Applying metabolic flux analysis to hydrogen fermentation using a metabolic network constructed for anaerobic mixed cultures

In this study, a mixed-cultural metabolic network for anaerobic digestion that included the concept of a "universal bacterium" was constructed, and metabolic flux analysis (MFA) applying this network was conducted to evaluate the flow of electrons and materials during H2 fermentation under various conditions. The MFA results from two H2 fermenters feeding glucose with (GP) or without (GA) the addition of peptone suggest that hydraulic retention time (HRT) presents a significant impact on hydrogen production, and the reversed trends could be observed at HRTs below and above 4 h. From the MFA results of lactate/acetate-fed H2 fermenter, the highest flux of H2 production is associated with more significant acetate consumption and the following pathways toward the anaplerotic reactions cycle that produces NADH. The occurrence of acetogenesis in the H2 fermenters using various types of bioethanol-fermented residues (BEFRs) was also identified according to the MFA results. By analyzing the MFA results of all 49 sets of data from H2 fermenters via Pearson's correlation, it was revealed that the flux of H2 production positively correlates to the reduction of ferredoxin with pyruvate oxidation, acetate formation, and acetate emission when lactate was produced in the system. On the contrary, negative relationships were found between the flux of H2 production and these three fluxes. The extended application of MFA provides additional information, including the fluxes between intracellular metabolites, and the information has the potential to be used in decision-making systems during the future operation of anaerobic processes by connecting operational parameters.

Advances in the catalyzed photo-fermentative biohydrogen production through photo nanocatalysts with the potential of selectivity, and customization

Photo nanocatalyst have shownpromise in a variety of fields, including biohydrogen production where their catalytic efficiency is related to size, surface-to-volume ratio, and increasing the number of atoms on the surface. They can harvest solar light to create electron-hole pairs which is the key mechanism to define its catalytic efficiency, thus requiring suitable excitation wavelength, band energy, and crystal imperfections. In this review, a discussion on the role of photo nanocatalysts to catalyze biohydrogen production has been carried out. Photo nanocatalysts feature a large bandgap, andhigh defect concentration, thus having the ability to be tuned for their characteristics. Customization of the photo nanocatalyst has been addressed. Mechanism of the photo nanocatalysts in catalyzing biohydrogen has been discussed. Limiting factors of photo nanocatalysts were highlighted and several recommendations have been made to enhance the effective utilization of these photo nanocatalysts to enhance photo-fermentative biohydrogen production from biomass wastes.

Recovery of agricultural waste biomass: A path for circular bioeconomy

Circular bio-economy is a significant approach to resolving global issues elevated by environmental pollution. The generation of bioenergy and biomaterials can withstand the energy-environment connection as well as substitute petroleum-based materials as the feed stock production, thereby contributing to a cleaner and low-carbon-safe environment. Open discarding of waste is a major cause of environmental pollution in developing and under developed countries. Agricultural bio-wastes are obtained through various biological sources and industrial processing, signifying a typical renewable source of energy with ample nutrients and readily biodegradable organic substances. These waste materials are competent to decompose under aerobic and anaerobic conditions. The projected global population, urbanization, economic development, and changing production and consumption behavior result in bounteous bio-waste production. These bio-wastes mainly contain starch, cellulose, protein, hemicellulose, and lipids, which can operate as low-cost raw materials to develop new value-added products. Thus, this review discussed specifically the agricultural waste and valorization processes used to convert this waste into value-added products (biofuel, enzymes, antibiotics, ethanol and single cell protein). These value added products are used in the supply chain and enhance the overall performance of agriculture waste management, execution of circular bio-economy has attained significant importance and it explains a closed-loop system in which the potential resources remain in the loop, allowing them to be sustained into a new value.

Estimating the potential of biohydrogen production and carbon neutralization contribution from crop straw

In order to achieve the carbon neutrality goal set by Chinese government, the potential contribution of hydrogen production from crop residues by microbial fermentation technology and Greenhouse gas (GHGs) reduction have been studied. Firstly, the annual yield of crop straw was estimated according to crop yield and grass grain ratio, and then the grey model GM (1, 1) was applied to predict the crop residues resources available for hydrogen production in various provinces in China in 2021. The results showed that the maximum resource of straw being available for hydrogen production is about 4.54 × 10⁸ t, corresponding to 1.31 × 10¹¹ m³ of hydrogen, the energy carried by the obtained hydrogen was 73 % and 1.15 times than the energy of national civil natural gas consumption and energy of transportation gasoline consumption, respectively. The potential reduction of greenhouse gas emission was 2.42 × 10⁸ t/a CO₂-eq, representing 2.4 % of GHGs emissions.

Value addition through biohydrogen production and integrated processes from hydrothermal pretreatment of lignocellulosic biomass

Bioenergy production is the most sought-after topics at the crunch of energy demand, climate change and waste generation. In view of this, lignocellulosic biomass (LCB) rich in complex organic content has the potential to produce bioenergy in several forms following the pretreatment. Hydrothermal pretreatment that employs high temperatures and pressures is gaining momentum for organics recovery from LCB which can attain value-addition. Diverse bioprocesses such as dark fermentation, anaerobic digestion etc. can be utilized following the pretreatment of LCB which can result in biohydrogen and biomethane production. Besides, integration approaches for LCB utilization that enhance process efficiency and additional products such as biohythane production as well as application of solid residue obtained after LCB pretreatment were discussed. Importance of hydrothermal pretreatment as one of the suitable strategies for LCB utilization is emphasized suggesting its future potential in large scale energy recovery.

Effect of physical and thermal pretreatment of lignocellulosic biomass on biohydrogen production by thermochemical route: A critical review

Energy demands and immense environmental degradation have extorted for production of low-carbon and carbon-neutral fuels. Abundantly available lignocellulosic biomass is second-generation feedstock which has potential to produce biofuels. Among all biofuels, biohydrogen is carbon neutral and sustainable biofuel which can be produced by thermochemical conversion routes mainly gasification. However, there are still numerous unsolved challenges related to physicochemical properties of lignocellulosic biomass. To tackle these issues, physical, chemical and thermal pretreatment methods can be employed to improve these properties and further strengthen usability of biomass for biohydrogen production. Pelletization, torrefaction and hydrothermal carbonization pretreatment have shown significant results for treating biomass and biohydrogen enhancement. This study reviews physical and thermal pretreatment and its effect on biohydrogen yield. Framework of techno-economic analysis of processes is provided for examining feasibility of required pretreatments. This sustainable approach will help to reduce emissions and promote concept of bioenergy with carbon capture and storage.

Recent progress and challenges in biotechnological valorization of lignocellulosic materials: Towards sustainable biofuels and platform chemicals synthesis

Lignocellulosic materials (LCM) have garnered attention as feedstocks for second-generation biofuels and platform chemicals. With an estimated annual production of nearly 200 billion tons, LCM represent an abundant source of clean, renewable, and sustainable carbon that can be funneled to numerous biofuels and platform chemicals by sustainable microbial bioprocessing. However, the low bioavailability of LCM due to the recalcitrant nature of plant cell components, the complexity and compositional heterogeneity of LCM monomers, and the limited metabolic flexibility of wild-type product-forming microorganisms to simultaneously utilize various LCM monomers are major roadblocks. Several innovative strategies have been proposed recently to counter these issues and expedite the widespread commercialization of biorefineries using LCM as feedstocks. Herein, we critically summarize the recent advances in the biological valorization of LCM to value-added products. The review focuses on the progress achieved in the development of strategies that boost efficiency indicators such as yield and selectivity, minimize carbon losses via integrated biorefinery concepts, facilitate carbon co-metabolism and carbon-flux redirection towards targeted products using recently engineered microorganisms, and address specific product-related challenges, to provide perspectives on future research needs and developments. The strategies and views presented here could guide future studies in developing feasible and economically sustainable LCM-based biorefineries as a crucial node in achieving carbon neutrality.

Renewable biohydrogen production from Clostridium sp. LQ25 using different forms of ferric as electron acceptor

Clostridium sp. LQ25 was cultured in different forms of ferric (ferric citrate and ferric hydroxide) as electron acceptors to investigate growth, ferric reduction, hydrogen production, fermentation products and fermentation process. The growth of the strain LQ25 detected by protein was 82.8 ± 2.1 mg/L and 73.5 ± 1.7 mg/L using ferric citrate and ferric hydroxide as electron acceptors, which was 33.3 % and 18.4 % higher than without ferric, respectively. The accumulation concentration of Fe(II) was 9.0 ± 0.6 mg/L and 5.0 ± 0.2 mg/L when using ferric citrate and ferric hydroxide as electron acceptors, and ferric citrate was 1.8-fold higher than ferric hydroxide, which indicated that the ability of ferric reduction was higher using ferric citrate as electron acceptor. The hydrogen production of strain LQ25 was 238.0 ± 1.0 mmol/mol glucose and 113.0 ± 1.3 mmol/mol glucose under condition of ferric citrate and ferric hydroxide as electron acceptors, which was 2.6 and 1.2-fold higher than without ferric, respectively. The growth and hydrogen production of strain LQ25 was promoted by using ferric as electron acceptor, while the fermentation type of strain did not change and was always butyrate type. The differential expression of the genes of strain LQ25 was significant when using ferric as electron acceptor, mainly in NADH and PFL pathway. This study provided preliminary evidence for hydrogen production by Clostridium sp. LQ25 in the presence of electron acceptor.

Evaluating Bio-Hydrogen Production Potential and Energy Conversion Efficiency from Glucose and Xylose under Diverse Concentrations

Lignocellulose bioconversion to hydrogen has been proposed as a promising solution to augment the fossil fuel dominated energy market. However, little is known about the effects of the substrate concentration supplied on hydrogen production. Herein, the hydrogen producing bacteria Thermoanaerobacter thermosaccharolyticum W16 feeding with respective glucose, xylose, and glucose and xylose mixture (glucose–xylose) at different concentrations was evaluated, to study whether substrate concentration could impact the lignocellulose bioconversion to hydrogen and the associated kinetics. An average bio-hydrogen yield of 1.40 ± 0.23 mol H2·mol−1 substrate was obtained at an average substrate concentration of 60.89 mM. The maximum bio-hydrogen production rate of 0.25 and 0.24 mol H2·mol−1 substrate h−1 was achieved at a substrate concentration of 27.75 mM glucose and 30.82 mM glucose–xylose, respectively, while the value reached the high point of 0.08 mol H2·mol−1 xylose·h−1 at 66.61 mM xylose. Upon further energy conversion efficiency (ESE) analysis, a substrate of 10 g·L−1 (amounting to 55.51 mM glucose, 66.61 mM xylose or 60.55 mM glucose–xylose) provided the maximum ESE of 15.3 ± 0.3%, which was 15.3% higher than that obtained at a substrate concentration of 5 g·L−1 (amounting to 27.75 mM glucose, 33.30 mM xylose or 30.28 mM glucose–xylose). The findings could be helpful to provide effective support for the future development of efficient and sustainable lignocellulosic bio-hydrogen production.

Sustainable production of biohydrogen from algae biomass: Critical review on pretreatment methods, mechanism and challenges

The production of chemicals and energy from sustainable biomass with an important objective decreasing carbon impressions has recently become one of the key areas of attention. Algae biomass have been recognized and researched as a potential renewable biomass of biohydrogen production attributed to their limited multiplying time, fast growing qualities and ability of lipid accumulation. This review additionally envelops various key perspectives such as composition and properties of algae biomass and pretreatment strategies such as physical, chemical and biological methods adopted for the algae biomass. This review is mainly focused on pretreatment strategies which have been developed to enhance biohydrogen production. The present review deals with methods and mechanism, enzymes involved and factors influencing on biohydrogen production which help to grasp various bottlenecks, challenges and constraints. Finally, the significant progressions and economical perspective on improving biohydrogen yield because of the expansion of co-substrates and the current trends are examined.

A comprehensive review on the biological conversion of lignocellulosic biomass into hydrogen: Pretreatment strategy, technology advances and perspectives

The globe has dependent on energy generation and utilization for many years; conversely, ecological concerns constrained the world to view hydrogen as an alternative for economic development. Lignocellulosic biomass is broadly accessible as a low-cost renewable feedstock and nonreactive nature; it has received a lot of consideration as a global energy source and the most attractive alternative to replace fossil natural substances for energy production. Pretreatment of lignocellulosic biomass is essential to advance its fragmentation and lower the lignin content for sustainable energy generation. This review's goal is to provide the different pretreatment strategies for enlarging the solubility and surface area of lignocellulosic biomass. The biological conversion of lignocellulosic biomass to hydrogen was reviewed and operational conditions and enhancing methods were discussed. This review summarizes the working conditions, parameters, yield percentages, techno-economic analysis, challenges, and future recommendations on the direct conversion of biomass to hydrogen.

Recent advances in biogas production using Agro-Industrial Waste: A comprehensive review outlook of Techno-Economic analysis

Agrowaste sources can be utilized to produce biogas by anaerobic digestion reaction. Fossil fuels have damaged the environment, while the biogas rectifies the issues related to the environment and climate change problems. Techno-economic analysis of biogas production is followed by nutrient recycling, reducing the greenhouse gas level, biorefinery purpose, and global warming effect. In addition, biogas production is mediated by different metabolic reactions, the usage of different microorganisms, purification process, upgrading process and removal of CO₂ from the gas mixture techniques. This review focuses on pre-treatment, usage of waste, production methods and application besides summarizing recent advancements in biogas production. Economical, technical, environmental properties and factors affecting biogas production as well as the future perspective of bioenergy are highlighted in the review. Among all agro-industrial wastes, sugarcane straw produced 94% of the biogas. In the future, to overcome all the problems related to biogas production and modify the production process.

Microbial electrolysis cells for the production of biohydrogen in dark fermentation - A review

To assess biohydrogen for future green energy, this review revisited dark fermentation and microbial electrolysis cells (MECs). Hydrogen evolution rate in mesophilic dark fermentation is as high as 192 m³ H₂/m³-d, however hydrogen yield is limited. MECs are ideal for improving hydrogen yield from carboxylate accumulated from dark fermentation, whereas hydrogen production rate is too slow in MECs. Hence, improving anode kinetic is very important for realizing MEC biohydrogen. Intracellular electron transfer (IET) and extracellular electron transfer (EET) can limit current density in MECs, which is proportional to hydrogen evolution rate. EET does not limit current density once electrically conductive biofilms are formed on anodes, potentially producing 300 A/m². Hence, IET kinetics mainly govern current density in MECs. Among parameters associated with IET kinetic, population of anode-respiring bacteria in anode biofilms, biofilm density of active microorganisms, biofilm thickness, and alkalinity are critical for current density.

Biohydrogen production and purification: Focusing on bioelectrochemical systems

Innovative technologies on green hydrogen production become significant as the hydrogen economy has grown globally. Biohydrogen is one of green hydrogen production methods, and microbial electrochemical cells (MECs) can be key to biohydrogen provision. However, MECs are immature for biohydrogen technology due to several limitations including extracellular electron transfer (EET) engineering. Fundamental understanding of EET also needs more works to accelerate MEC commercialization. Interestingly, studies on biohydrogen gas purification are limited although biohydrogen gas mixture requires complex purification for use. To facilitate an MEC-based biohydrogen technology as the green hydrogen supply this review discussed EET kinetics, engineering of EET and direct interspecies electron transfer associated with hydrogen yield and the application of advanced molecular biology for improving EET kinetics. Finally, this article reviewed biohydrogen purification technologies to better understand purification and use appropriate for biohydrogen, focusing on membrane separation.

Valorization of spent mushroom substrate for low-carbon biofuel production: Recent advances and developments

Due to the nutritional values and functional bioactivities of mushrooms, the global market value of the edible mushroom industry has been growing steadily. However, the production of 1 kg of fresh mushroom generates about 5 kg of wet byproducts (known as spent mushroom substrate; SMS). This necessitates proper waste management to mitigate potential environmental threats. Embracing the "waste-to-fuel" concept, SMS as lignocellulosic waste can serve as cheap and abundant feedstock for the production of a variety of biofuels, including biogas, biohydrogen, bioethanol, bio-oil, and solid-biofuels. Mushroom cultivation serves as efficient biological pretreatment for biofuel production, promoting biofuel yield and improving the overall economy. Therefore, integrated mushroom cultivation and biofuel production can simultaneously satisfy the rapidly rising food and energy demand. The article systematically reviewed the recycling and re-utilization of SMS in sustainable biofuel production, discussing the possible challenges and proposing future directions for the green development of the mushroom industry.

A consolidated review of commercial-scale high-value products from lignocellulosic biomass

For decades, lignocellulosic biomass has been introduced to the public as the most important raw material for the environmentally and economically sustainable production of high-valued bioproducts by microorganisms. However, due to the strong recalcitrant structure, the lignocellulosic materials have major limitations to obtain fermentable sugars for transformation into value-added products, e.g., bioethanol, biobutanol, biohydrogen, etc. In this review, we analyzed the recent trends in bioenergy production from pretreated lignocellulose, with special attention to the new strategies for overcoming pretreatment barriers. In addition, persistent challenges in developing for low-cost advanced processing technologies are also pointed out, illustrating new approaches to addressing the global energy crisis and climate change caused by the use of fossil fuels. The insights given in this study will enable a better understanding of current processes and facilitate further development on lignocellulosic bioenergy production.

Enhanced photo-fermentative biohydrogen production from biowastes: An overview

Clean energy like hydrogen can be a promising strategy to solve problems of global warming. Photo-fermentation (PF) is an attractive technology for producing biohydrogen from various biowastes cost-effectively and environmentally friendly. However, challenges of low light conversion efficiency and small yields of biohydrogen production still limit its application. Thus, advanced strategies have been investigated to enhance photo-fermentative biohydrogen production. This review discusses advanced technologies that show positive outcomes in improving biohydrogen production by PF, including the following. Firstly, genetic engineering enhances light transfer efficiency, change the activity of enzymes, and improves the content of ATP, ammonium and antibiotic tolerance of photosynthetic bacteria. Secondly, immobilization technology is refined. Thirdly, nanotechnology makes great strides as a scientific technique and fourthly, integration of dark and photo-fermentation technology is possible. Some suggestions for further studies to achieve high levels of efficiency of photo-fermentative biohydrogen production are mentioned in this paper.

Novel strategies towards efficient molecular biohydrogen production by dark fermentative mechanism: present progress and future perspective

In the scenario of alarming increase in greenhouse and toxic gas emissions from the burning of conventional fuels, it is high time that the population drifts towards alternative fuel usage to obviate pollution. Hydrogen is an environment-friendly biofuel with high energy content. Several production methods exist to produce hydrogen, but the least energy intensive processes are the fermentative biohydrogen techniques. Dark fermentative biohydrogen production (DFBHP) is a value-added, less energy-consuming process to generate biohydrogen. In this process, biohydrogen can be produced from sugars as well as complex substrates that are generally considered as organic waste. Yet, the process is constrained by many factors such as low hydrogen yield, incomplete conversion of substrates, accumulation of volatile fatty acids which lead to the drop of the system pH resulting in hindered growth and hydrogen production by the bacteria. To circumvent these drawbacks, researchers have come up with several strategies that improve the yield of DFBHP process. These strategies can be classified as preliminary methodologies concerned with the process optimization and the latter that deals with pretreatment of substrate and seed sludge, bioaugmentation, co-culture of bacteria, supplementation of additives, bioreactor design considerations, metabolic engineering, nanotechnology, immobilization of bacteria, etc. This review sums up some of the improvement techniques that profoundly enhance the biohydrogen productivity in a DFBHP process.

Enhanced Fermentative Hydrogen Production from Food Waste in Continuous Reactor after Butyric Acid Treatment

End-product accumulation during dark fermentation leads to process instability and hydrogen production inhibition. To overcome this constraint, microbial community adaptation to butyric acid can induce acid tolerance and thus enhance the hydrogen yields; however, adaptation and selection of appropriate microbial communities remains uncertain when dealing with complex substrates in a continuous fermentation mode. To address this question, a reactor fed in continuous mode with food waste (organic loading rate of 60 gVS·L·d−1; 12 h hydraulic retention time) was first stressed for 48 h with increasing concentrations of butyric acid (up to 8.7 g·L−1). Performances were compared with a control reactor (unstressed) for 13 days. During 6 days in a steady-state, the pre-stressed reactor produced 2.2 ± 0.2 LH2·L·d−1, which was 48% higher than in the control reactor (1.5 ± 0.2 LH2·L·d−1). The pretreatment also affected the metabolites’ distribution. The pre-stressed reactor presented a higher production of butyric acid (+44%) achieving up to 3.8 ± 0.3 g·L−1, a lower production of lactic acid (−56%), and an enhancement of substrate conversion (+9%). The performance improvement was attributed to the promotion of Clostridium guangxiense, a hydrogen -producer, with a relative abundance increasing from 22% in the unstressed reactor to 52% in the stressed reactor.

Sustainable production of algae-bacteria granular consortia based biological hydrogen: New insights

Microbes recycling nutrient and detoxifying ecosystems are capable to fulfil the future energy need by producing biohydrogen by due to the coupling of autotrophic and heterotrophic microbes. In granules microbes mutualy exchanging nutrients and electrons for hydrogen production. The consortial biohydrogen production depend upon constituent microbes, their interdependence, competition for resources, and other operating parameters while remediating a waste material in nature or bioreactor. The present review deals with development of granular algae-bacteria consortia, hydrogen yield in coculture, important enzymes and possible engineering for improved hydrogen production.

Enhancement effect of defoamer additives on photo-fermentation biohydrogen production process

Foaming is a key issue should be solved in the process of photo-fermentation biohydrogen production (PFHP), since it has negative influence on the hydrogen yield potential, especially when taken straw as substrate. Appropriate foam control measures must be considered for industrialization. Hence, in this work, foam height and biohydrogen yield were selected as index, the effect of defoamer addition on PFHP was investigated. The defoamer has no negative effect on bacterial growth. In the addition range of 0-1 mL/L, the higher addition amount, indicates better foam control effect. The maximum foam height could be reduced by 55% and the foam existence time by 36 h. The reduction of foam was beneficial to biohydrogen production, and the highest cumulative hydrogen yield was increased 23% at the addition level of 0.125 mL/L.

Advanced strategies for enhancing dark fermentative biohydrogen production from biowaste towards sustainable environment

As a clean energy carrier, hydrogen is a promising alternative to fossil fuel so as the global growing energy demand can be met. Currently, producing hydrogen from biowastes through fermentation has attracted much attention due to its multiple advantages of biowastes management and valuable energy generation. Nevertheless, conventional dark fermentation (DF) processes are still hindered by the low biohydrogen yields and challenges of biohydrogen purification, which limit their commercialization. In recent years, researchers have focused on various advanced strategies for enhancing biohydrogen yields, such as screening of super hydrogen-producing bacteria, genetic engineering, cell immobilization, nanomaterials utilization, bioreactors modification, and combination of different processes. This paper critically reviews by discussing the above stated technologies employed in DF, respectively, to improve biohydrogen generation and stating challenges and future perspectives on biowaste-based biohydrogen production.

A review on biological recycling in agricultural waste-based biohydrogen production: Recent developments

Hydrogen has become a research highlight by virtue of its clean energy production technology and high energy content. The technology of biohydrogen production from biological waste via fermentation has lower costs, provides environment-friendly methods regarding energy balance, and creates a pathway for sustainable utilization of massive agricultural waste. However, biohydrogen production is generally limited by lower productivity. Many studies have been conducted aimed at improving biohydrogen production efficiency. Hence, this review is intended to describe improving routes for biohydrogen production from agricultural waste and highlights recent advances in these approaches. In addition, the critical factors affecting biohydrogen production, including the pretreatment method, substrate resource, fermentation conditions, and bioreactor design, were also comprehensively discussed along with challenges and future prospects.

Resource recovery from lignocellulosic wastes via biological technologies: Advancements and prospects

Lignocellulosic wastes were recently considered as biomass resources, however, its conversion to valuable products is still immature although researchers have put lots of effort into this issue. This article reviews the key challenges of the biorefinery utilizing lignocellulosic materials and recent developments to conquer those obstacles. Available biological techniques and processes, from the pretreatments of cellulosic materials to the valorization processes, were emphasized. Biological pretreatments, including hydrolysis using microbial consortia, fungi, enzymes, engineered bacterial/fungal strains, and co-culture systems, could enhance the release of reducing sugar. Resources recovery, including biogases, ethanol, butanol, PHA, etc., from lignocellulosic materials were also discussed, while the influences of composition of lignocellulosic materials and pretreatment options, applications of co-culture system, and integrated treatments with other wastes, were described. In the review, co-culture system and metabolic engineering are emphasized as the promising biological technologies, while perspectives are provided for their future developments.

Effect of mixing ratio and total solids content on temperature-phased anaerobic codigestion of rice straw and pig manure: Biohythane production and microbial structure

Long-term semi-continuous experiments were carried out under three feedstock conditions to study the effects of mixing ratio and total solids (TS) content on temperature-phased anaerobic codigestion of rice straw (RS) and pig manure (PM). The results showed that biohythane only produced from the mixture with 6% TS content and its average content were 12.83 ± 1.19% (hydrogen) and 23.68 ± 1.12% (methane). Increasing mixture TS content and decreasing its RS ratio increased biohythane production and organic matter removal by creating a suitable process pH and increasing the anaerobic reaction rates. The highest biohythane production of the mixture reached 73.09 ± 3.03 ml/g VS (hydrogen) and 235.81 ± 9.30 ml/g VS (methane) at a mixing ratio of 5:1 and TS content of 6%. A variety of hydrogen-producing bacteria were found in the thermophilic reactor and Clostridium_sensu_stricto_1 played an important role. Butyric acid fermentation is the main hydrogen-producing pathway. Methanobacterium and Methanosaeta were dominant archaea in the mesophilic reactor.

Comparative life cycle assessment of biochar-based lignocellulosic biohydrogen production: Sustainability analysis and strategy optimization

Lignocellulose has been considered a potential feedstock for biohydrogen production. Recently, a novel closed-loop concept of biochar approach was developed for enhanced lignocellulosic biohydrogen production. This study therefore targets to analyze the environmental impacts of the three existing lignocellulosic biohydrogen production processes, and evaluate the environmental performance of applying biochar in each process at this early stage of technological development. The results suggest that biochar dosing shows better environmental performance for all impact categories, especially in the consolidate bioprocessing case. Electricity consumption was found to be the dominant cause of environmental impact over the life cycle, while by-products generation was also found to have an effect on the life-cycle impacts. Future research focuses on the biohydrogen production scale, the electricity generation scheme transition towards renewable and cleaner energetic systems, and recovery the by-products to the maximum extent, that will make lignocellulosic biohydrogen production more environmentally sustainable.

Biohydrogen from organic wastes as a clean and environment-friendly energy source: Production pathways, feedstock types, and future prospects

Microbial fermentation of organic matter under anaerobic conditions is currently the prominent pathway for biohydrogen production. Organic matter present in waste residues is regarded as an economic feedstock for biohydrogen production by dark and photo fermentative bacteria. Agricultural residues, fruit wastes, vegetable wastes, industrial wastewaters, and other livestock residues are some of the organic wastes most commonly used for biohydrogen production due to their higher organic content and biodegradability. Appropriate pretreatments are required to enhance the performance of biohydrogen from complex organic wastes. Biohydrogen production could also be enhanced by optimizing operation conditions and the addition of essential nutrients and nanoparticles. This review describes the pathways of biohydrogen production, discusses the effect of organic waste sources used and microbes involved on biohydrogen production, along with addressing the key parameters, advantages, and difficulties in each biohydrogen production pathway.

Biogenic enabled in-vitro synthesis of nickel cobaltite nanoparticle and its application in single stage hybrid biohydrogen production

In biomass to biofuels production technology enzyme plays a key role. Nevertheless, the high production cost of cellulase enzyme is one of the critical issues in the economical production of biofuels. Nowadays, implementation of nanomaterials as catalyst is emerging as an innovative approach for the production of sustainable energy. In this context, synthesis of nickel cobaltite nanoparticles (NiCo₂O₄ NPs) via in vitro route has been conducted using fungus Emericella variecolor NS3 meanwhile; its impact has been evaluated on improved thermal and pH stability of crude cellulase enzyme obtained from Emericella variecolor NS3. Additionally, bioconversion of alkali treated rice straw using NiCo₂O₄ NPs stabilized cellulase produced sugar hydrolyzate which is further used for H₂ production via hybrid fermentation. Total 51.7 g/L sugar hydrolyzate produced 2978 mL/L cumulative H₂ production after 336 h along with maximum rate 34.12 mL/L/h in 24 h using Bacillus subtilis PF_1 and Rhodobacter sp. employed for dark and photo-fermentation, respectively.