Seed Pretreatment for Increased Hydrogen Production Using Mixed-Culture Systems with Advantages over Pure-Culture Systems

Biological Hydrogen Production from Biowaste Using Dark Fermentation, Storage and Transportation

Hydrogen is widely considered as the fuel of the future. Due to the challenges present during hydrogen production using conventional processes and technologies, additional methods must be considered, like the use of microorganisms. One of the most promising technologies is dark fermentation, a process where microorganisms are utilized to produce hydrogen from biomass. The paper provides a comprehensive overview of the biological processes of hydrogen production, specifically emphasizing the dark fermentation process. This kind of fermentation involves bacteria, such as Clostridium and Enterobacterium, to produce hydrogen from organic waste. Synthetic microbial consortia are also discussed for hydrogen production from different types of biomasses, including lignocellulosic biomass, which includes all biomass composed of lignin and (hemi)cellulose, sugar-rich waste waters, and others. The use of genetic engineering to improve the fermentation properties of selected microorganisms is also considered. Finally, the paper covers the important aspect of hydrogen management, including storage, transport, and economics.

Development of Digested Sludge-Assimilating and Biohydrogen-Yielding Microflorae

Digested sludge (DS) is a waste product of anaerobic digestion that is produced during the biodegradation of excess sludge. It cannot be used as a substrate for further biogas production owing to its recalcitrant nature. In the present study, we used a heat treatment technique to convert DABYS microflora (DABYS = digested sludge-assimilating and biogas-yielding soil microflora), which degraded DS and produced methane gas, to a microflora that could produce hydrogen gas from DS. Heat treatment at 80 and 100 °C inactivated the methanogens that consume hydrogen for methane production but did not affect the thermotolerant bacteria. We developed three microflorae (DABYS-A80, DABYS-A100, and DABYS-80B) to exclusively produce hydrogen gas. They included representatives from the anaerobic eubacterial families Clostridiaceae and Enterobacteriaceae. Pseudomonas sp. was also present in DABYS-A80 and DABYS-A100. It is thought that bacteria in the Enterobacteriaceae family or Pseudomonas genus survive heat treatment because they are embedded in microgranules. Enzymatic analysis suggested that the microflorae hydrolyzed DS using cellulase, chitinase, and protease. Under optimum culture conditions, DABYS-A80, -A100, and B-100 produced gas yields of 8.0, 7.1, and 2.6 mL, respectively, from 1.0 g of dried DS.

Valorization of biomass through gasification for green hydrogen generation: A comprehensive review

Green and sustainable hydrogen from biomass gasification processes is one of the promising ways to alternate fossil fuels-based hydrogen production. First off, an overview of green hydrogen generation from biomass gasification processes is presented and the corresponding possible gasification reactions and the effect of respective experimental criteria are explained in detail. In addition, a comprehensive explanation of the catalytic effect on tar reduction and hydrogen generation via catalytic gasification is presented regarding the functional mechanisms of various types of catalysts. Furthermore, the commercialization aspects, the associated technical challenges and barriers, and the prospects of a biomass gasification process for green hydrogen generation are discussed. Finally, this comprehensive review provides the related advancements, challenges, and great insight of biomass gasification for the green hydrogen generation to realize a sustainable hydrogen society via biomass valorization.

Prospective review on bioelectrochemical systems for wastewater treatment: Achievements, hindrances and role in sustainable environment

Bioelectrochemical systems (BESs) are a relatively new arena for producing bioelectricity, desalinating sea water, and treating industrial effluents by removing organic matter. Microbial electrochemical technologies (METs) are promising for obtaining value-added products during simultaneous remediation of pollutants from wastewater. The search for more affordable desalination technology has led to the development of microbial desalination cells (MDCs). MDC combines the operation of microbial fuel cells (MFC) with electrodialysis for water desalination and energy generation. It has received notable interest of researchers in desalination and wastewater treatment because of low energy requirement and eco-friendly nature. Firstly, this article provides a brief overview of MDC technology. Secondly, factors affecting functioning of MDC and its applications have been accentuated. Additionally, challenges and future outlook on the development of this technology have been delineated. State-of-the-art information provided in this review would expand the scope of interdisciplinary and translational research.

q-PCR Methodology for Monitoring the Thermophilic Hydrogen Producers Enriched from Elephant Dung

This study aims to create a quantitative polymerase chain reaction (q-PCR) methodology for monitoring the hydrogen-producing mixed cultures enriched from elephant dung using alpha-cellulose as a carbon source through five generations of repetitive sub-culture. The enriched thermophilic mixed cultures from the fifth cultivation cycle gave the highest hydrogen yield of 170.3 mL H2/g cellulose and were used to generate hydrogen from sawdust. Clostridium sp. and Thermoanaerobacterium sp. were the dominant bacteria in thermophilic mixed cultures with high hydrogen yield, according to polymerase chain reaction-denatured gradient gel electrophoresis (PCR-DGGE). q-PCR primers Chis150F and ClostIR, TherF and TherR, and BacdF and BacdR were developed to amplify the 16S rRNA genes of Clostridium sp., Thermoanaerobacterium sp., and Bacillus sp., respectively, for the quantification of hydrogen-producing bacteria in biohydrogen fermentation. Similar q-PCR analysis of Clostridium sp., Thermoanaerobacterium sp., and Bacillus sp. 16S rRNA gene amplification during hydrogen production from cellulose and sawdust revealed increasing gene copy number with time. The molecular approaches developed in this study can be used to monitor microbial communities in hydrogen fermentation processes efficiently.

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.

Microbiomes of biohydrogen production from dark fermentation of industrial wastes: current trends, advanced tools and future outlook

Biohydrogen production through dark fermentation is very attractive as a solution to help mitigate the effects of climate change, via cleaner bioenergy production. Dark fermentation is a process where organic substrates are converted into bioenergy, driven by a complex community of microorganisms of different functional guilds. Understanding of the microbiomes underpinning the fermentation of organic matter and conversion to hydrogen, and the interactions among various distinct trophic groups during the process, is critical in order to assist in the process optimisations. Research in biohydrogen production via dark fermentation is currently advancing rapidly, and various microbiology and molecular biology tools have been used to investigate the microbiomes. We reviewed here the different systems used and the production capacity, together with the diversity of the microbiomes used in the dark fermentation of industrial wastes, with a special emphasis on palm oil mill effluent (POME). The current challenges associated with biohydrogen production were also included. Then, we summarised and discussed the different molecular biology tools employed to investigate the intricacy of the microbial ecology associated with biohydrogen production. Finally, we included a section on the future outlook of how microbiome-based technologies and knowledge can be used effectively in biohydrogen production systems, in order to maximise the production output.

An Overview of Recent Advancements in Microbial Polyhydroxyalkanoates (PHA) Production from Dark Fermentation Acidogenic Effluents: A Path to an Integrated Bio-Refinery

Global energy consumption has been increasing in tandem with economic growth motivating researchers to focus on renewable energy sources. Dark fermentative hydrogen synthesis utilizing various biomass resources is a promising, less costly, and less energy-intensive bioprocess relative to other biohydrogen production routes. The generated acidogenic dark fermentative effluent [e.g., volatile fatty acids (VFAs)] has potential as a reliable and sustainable carbon substrate for polyhydroxyalkanoate (PHA) synthesis. PHA, an important alternative to petrochemical based polymers has attracted interest recently, owing to its biodegradability and biocompatibility. This review illustrates methods for the conversion of acidogenic effluents (VFAs), such as acetate, butyrate, propionate, lactate, valerate, and mixtures of VFAs, into the value-added compound PHA. In addition, the review provides a comprehensive update on research progress of VFAs to PHA conversion and related enhancement techniques including optimization of operational parameters, fermentation strategies, and genetic engineering approaches. Finally, potential bottlenecks and future directions for the conversion of VFAs to PHA are outlined. This review offers insights to researchers on an integrated biorefinery route for sustainable and cost-effective bioplastics production.

Bio-Hydrogen Production Using Landfill Leachate Considering Different Photo-Fermentation Processes

Recently, it has become imperative to find new sustainable and renewable sources of energy, in order to avoid dependence on non-renewable traditional energy resources. This would help to overcome the depleting of natural resources for energy production. Hydrogen gas production using biological processes is one of the most attractive solutions in this regard, due to its high energy content and ecofriendly nature. Production of hydrogen using single photo-fermentation process and landfill leachate as substrate was carried out in this paper, by utilizing batch bio-reactor and anaerobic conditions. The pH value and temperature, play an essential role in a bio-hydrogen production process. Thus, in this study, the pH values considered were 6, 6.5, and 7.2, respectively, at a controlled temperature of 37 ± 1°C. This study investigated various schemes that have the possibility of producing hydrogen using; landfill leachate alone, with leachate and addition of inoculum such as sewage sludge, and with substrate such as sucrose and glucose. All experiments were conducted with and without mixing, for effective comparative study. Heat and pH pretreatment were applied in each experiment with the objectives of decreasing the activities of methane-producing bacteria and enhancing the activities of hydrogen-producing bacteria. The hydraulic retention time used in this study was 48 h, in order to obtain optimal performance of the schemes employed. Analysis of liquid leachate was performed for each experiment, and based on the obtained results, the maximum yield of hydrogen produced was 5,754 ml H₂/L, with a medium pH scale of 6.0, fermentation temperature of 37 ± 1°C and constant mixing speed of 100 rpm.

Effect of Woody Biomass Gasification Process Conditions on the Composition of the Producer Gas

Using woody biomass in thermochemical gasification can be a viable alternative for producing renewable energy. The type of biomass and the process parameters influence the producer gas composition and quality. This paper presents research on the composition of the producer gas from the gasification of three woody biomass species: spruce, alder, and pine. The experiments were conducted in a drop-tube reactor at temperatures of 750, 850, and 950 °C, using air as the gasifying agent, with equivalence ratios of 0.38 and 0.19. Gas chromatography with a thermal conductivity detector was used to determine the composition of the producer gas, while the production of total organic compounds was detected using Fourier-transform infrared spectroscopy. All three wood species exhibited very similar producer gas composition. The highest concentration of combustible gases was recorded at 950 °C, with an average of 4.1, 20.5, and 4.6 vol% for H2, CO, and CH4, respectively, and a LHV ranging from 4.3–5.1 MJ/m3. The results were in accordance with other gasification studies of woody species. Higher temperatures enhanced the composition of the producer gas by promoting endothermic and exothermic gasification reactions, increasing gas production while lowering solid and tar yields. The highest concentrations of combustible gases were observed with an equivalence ratio of 0.38. Continuous TOC measurement allowed understanding the evolution of the gasification process and the relation between a higher production of TOC and CO as the gasification temperature raised.

Regulation of volatile fatty acid accumulation from waste: Effect of inoculum pretreatment

The study investigates the implications of waste feedstock, inoculum origin, and pretreatment on volatile fatty acids accumulation (VFA). The acidogenic fermentation of the feedstocks, rice mill effluent (RME), and brewery effluent (BE) was studied using untreated and pretreated (cyclic heat-acid shock) brewery anaerobic sludge as inoculum. The pretreatment was successful in refining and stabilizing VFA production from the feedstocks. The fermentation of RME with pretreated sludge had an enhanced acetate yield of 0.37 ± 0.02 mgCOD/mgCOD, even to odd ratio of 20.97 ± 0.08 mg/mg and the highest butyrate yield of 0.39 ± 0.01 mgCOD/mgCOD compared to untreated system. The pretreated system had stability in COD and pH profile, while VFA content depends on the origin of inoculum. Pretreatment inhibited the carbon sinks and augmented acetate-butyrate type metabolism with stable performance. The fermentation of RME by pretreated sludge produced a higher even-numbered VFAs and enhanced even to odd ratio in comparison with fermentation of BE, thereby affecting polymer composition and property. PRACTITIONER POINTS: The pretreated system had stable acidification, chemical oxygen demand, and pH profile. The pretreated system had higher acetate and butyrate yield compared to the untreated system. Rice mill effluent acidified with pretreated sludge had the highest even to odd ratio, 20.97 mg/mg. The even to odd ratio for acidification of brewery effluent was insignificant. Pretreatment, the origin of sludge, and the effluent had a regulatory effect on acidification.

A shift from anaerobic digestion to dark fermentation in glycol ethylene fermentation

Anaerobic digestion of aqueous glycol ethylene was tested. The process lasted two cycles of 7 days, but after the second cycle, high hydrogen production occurred shift to dark fermentation. The biogas production lasted 14 days, obtaining peak values of hydrogen, and then rapidly stopped. In investigations, the following were checked: dependence of hydrogen, methane and hydrogen sulphide in the process. Mixtures of water with glycol ethylene mass ratio from 0.6 to 0.85 were substrates in experiments. The highest methane production was for water ethylene 0.7 ratio 2.85 L of methane with a yield of 178 mL of methane/g VSS (volatile suspended solids) of glycol ethylene. The optimal ratio of water and glycol ethylene was 0.85 25.5 mL of hydrogen (giving yield 1.71 mL of hydrogen/g VSS of glycol ethylene) and 1.71 mL of hydrogen sulphide emission for a 0.6 ratio. Popular polymer industry wastes, glycol ethylene, can be utilised by anaerobic digestion.

Valorization of microalgal biomass for biohydrogen generation: A review

Third generation biomass, i.e. microalgae, has emerged as a promising alternative to first and second generation biomass for biohydrogen production. However, its utilization is still low at present, due to several reasons including the strong and rigidity of the microalgal cell wall that limit the hydrolysis efficiency during dark fermentation (DF) and photofermentation (PF) processes. To improve the utilization efficiency of microalgal biomass, it is crucial that important aspects related to the production of the biomass and the following processes are elaborated. Thus, this article provides detailed overview of algal strains, cultivation, and harvesting. It also presents recent research and detailed information on microalgal biomass pretreatment, and biohydrogen production through DF, PF, and co-digestion of microalgal biomass with organic materials. Furthermore, factors affecting fermentation processes performance and the use of molecular techniques in biohydrogen production are presented. This review also discusses challenges and future prospects towards biohydrogen production from microalgal biomass.

Biorenewable hydrogen production through biomass gasification: A review and future prospects

Hydrogen is recognized as one of the cleanest energy carriers, which can be produced from renewable biomass as a promising feedstock to achieve sustainable bioeconomy. Thermochemical technologies (e.g., gasification and pyrolysis) are the main routes for hydrogen production from biomass. Although biomass gasification, including steam gasification and supercritical water gasification, shows a high potential in field-scale applications, the selectivity and efficiency of hydrogen production need improvement to secure cost-effective industrial applications with high atom economy. This article reviews the two main-stream biomass-to-hydrogen technologies and discusses the significance of operating conditions and considerations in the catalytic system design. Challenges and prospects of hydrogen production via biomass gasification are explored to advise on the critical information gaps that require future investigations.

Waste-to-energy nexus for circular economy and environmental protection: Recent trends in hydrogen energy

The energy demand has increased exponentially worldwide owing to the continuously growing population and urbanization. The conventional fossil fuels are unable to satiate this requirement causing price inflation and significant environmental damage due to unrestrained emission of greenhouse gases. The focus now has shifted towards alternative, economical, renewable and green sources of energy such as hydrogen to deal with this bottle-neck. Hydrogen is a clean energy-source having high energy content (122 kJ/g). Recently, biological methods for the hydrogen production have attracted much attention because traditional methods are expensive, energy-exhaustive and not eco-friendly. The employment of biological methods promises utilization of waste or low-value materials for producing energy and building waste-to-energy nexus. Around 94% of the waste is discarded precariously in India and waste generation is growing at an alarming rate of 1.3% per year. The "waste-to-energy" techniques follow 'Reuse, Reduce, Recycle, Recovery and Reclamation' system solving three subjects at once; waste-management, energy-demand and environmental concern. Moreover, these methods have easy operability, cost-effectiveness and they help to shift from linear to circular model of economy for sustainable development. Biological processing of waste materials like agricultural discard (lignocellulosic biomass), food-waste and industrial discharge can be used for biohydrogen production. Dark and photo fermentation are the chief biological processes for the transformation of organic substrates to hydrogen. Dark fermentation is the acidogenic fermentation of carbohydrate-rich materials without light and oxygen. Clostridia, Enterobacter and Bacillus spp. are appropriate heterotrophic bacteria for dark fermentation. Various pretreatment methods like heat treatment, acid or base treatment, ultrasonication, aeration, electroporation, etc., can be applied on inoculums to increase H₂ producing bacteria eventually improving the hydrogen yield. However, only around 33% of COD in organic materials is transformed to H₂ by this method. Photofermentation by the photosynthetic non-sulfur bacteria (PNS) converts organic substrate to H₂ and CO₂ in the presence of nitrogenase enzyme in ammonium-limited and anoxygenic conditions. Rhodobacter or Rhodopseudomonas strains have been widely examined in this regard. But these methods are only able to produce H₂ with a poor yield. Combining dark and photofermentation is a noteworthy alternative for procuring enhanced hydrogen yields. Two-stage sequential method utilizes volatile fatty acids accumulated as byproducts after dark fermentation (in the first stage) for photofermentation by suitable bacteria (in the second stage). A proper investigation of the dark fermenter effluents is required before using them as a substrate for photo-fermentation. In a single-stage dark and photofermentation, co-culture of anaerobic and PNS bacteria in a single reactor is carried out for obtaining improved yield. The single stage system is comparatively inexpensive and less laborious; moreover, a limited requirement for an intermediate dilution stage is necessary. Economic analysis of hydrogen production showed that H₂ production by the present methods, save pyrolysis, is reasonably higher than the conventional approaches of fuel production. Probable routes to make H₂ production more cost-effective are reducing the cost of photobioreactor, installing proper storage system, etc. A constructive effort in the area of research and development of biological approaches of H₂ production technologies is vital. The commercial viability of biohydrogen production is imperative for accomplishment of circular economy system and sustainable development.

Evolutionary causes and consequences of metabolic division of labour: why anaerobes do and aerobes don't

Metabolic division of the labour of organic matter decomposition into several steps carried out by different types of microbes is typical for many anoxic - but not oxic environments. An explanation of this well-known pattern is proposed based on the combination of three key insights: (i) well-studied anoxic environments are high flux environments: they are only anoxic because their high organic matter influx leads to oxygen depletion; (ii) shorter, incomplete catabolic pathways provide the capacity for higher flux, but this capacity is only advantageous in high flux environments; (iii) longer, complete catabolic pathways have energetic happy ends but only with high redox potential electron acceptors. Thus, aerobic environments favour longer pathways. Bioreactors, in contrast, are high flux environments and therefore favour division of catabolic labour even if aeration keeps them aerobic; therefore, host strains and feeding strategies must be carefully engineered to resist this pull.

Energy Sustainability Analysis (ESA) of Energy-Producing Processes: A Case Study on Distributed H2 Production

In the sustainability context, the performance of energy-producing technologies, using different energy sources, needs to be scored and compared. The selective criterion of a higher level of useful energy to feed an ever-increasing demand of energy to satisfy a wide range of endo- and exosomatic human needs seems adequate. In fact, surplus energy is able to cover energy services only after compensating for the energy expenses incurred to build and to run the technology itself. This paper proposes an energy sustainability analysis (ESA) methodology based on the internal and external energy use of a given technology, considering the entire energy trajectory from energy sources to useful energy. ESA analysis is conducted at two levels: (i) short-term, by the use of the energy sustainability index (ESI), which is the first step to establish whether the energy produced is able to cover the direct energy expenses needed to run the technology and (ii) long-term, by which all the indirect energy-quotas are considered, i.e., all the additional energy requirements of the technology, including the energy amortization quota necessary for the replacement of the technology at the end of its operative life. The long-term level of analysis is conducted by the evaluation of two indicators: the energy return per unit of energy invested (EROI) over the operative life and the energy payback-time (EPT), as the minimum lapse at which all energy expenditures for the production of materials and their construction can be repaid to society. The ESA methodology has been applied to the case study of H2 production at small-scale (10–15 kWH2) comparing three different technologies: (i) steam-methane reforming (SMR), (ii) solar-powered water electrolysis (SPWE), and (iii) two-stage anaerobic digestion (TSAD) in order to score the technologies from an energy sustainability perspective.

The Use of Acidic Hydrolysates after Furfural Production from Sugar Waste Biomass as a Fermentation Medium in the Biotechnological Production of Hydrogen

This study investigates a simultaneous processing of sugar beet pulp (SBP) for furfural, hydrogen and methane production using various pretreatment methods. In the experiments, sugar beet pulp was first subjected to thermal and thermochemical pretreatment at 140 °C. Then hydrolysates from these operations were investigated for their potential for methane and hydrogen production in batch tests. The experiments showed that thermal pretreatment of SBP resulted in the highest biogas and methane yields of 945 dm3/kg volatile solids (VS) and 374 dm3 CH4/kg VS, respectively, and a moderate hydrogen production of 113 dm3 H2/kg VS, which corresponded to a calculated energy production of 142 kWh/t; however, only low amount of furfural was obtained (1.63 g/L). Conversely, the highest furfural yield of 12 g/L was achieved via thermochemical pretreatment of SBP; however, biogas production from hydrolysate was much lower (215 dm3/kg VS) and contained only 67 dm3/kg VS of hydrogen. Meanwhile, in the experiment with lower amounts of sulfuric acid (2%) used for pretreatment, a moderate furfural production of 4 g/L was achieved with as high as 220 dm3/kg VS of hydrogen and the corresponding energy yield of 75 kWh/t.