Industrial wastewater to biohydrogen: Possibilities towards successful biorefinery route

Investigation of the agricultural reuse potential of urban wastewater and other resources derived by using membrane bioreactor technology within the circular economy framework

The European Union, as delineated in Regulation (EU) 2020/741, sets forth minimum criteria for the reuse of wastewater. Directive 86/278/CEE sets the regulations for the reuse of sewage sludge in agriculture. This study aimed to investigate the treated water derived from a pilot plant situated in Granada, Spain, that utilizes membrane bioreactor technology to process real urban wastewater with the quality standards necessary for agricultural reuse. Additionally, the study evaluated the utilization potential of other resources generated during wastewater treatment, including biogas and biostabilized sludge. The pilot plant incorporated a membrane bioreactor featuring four ultrafiltration membranes operating continuously alongside a sludge treatment line operating in batch mode. The pilot plant operated during four cycles, each with distinct hydraulic retention times (6 h and 12 h) and variable mixed liquor-suspended solids concentrations (ranging from 2688 mg L-1 to 7542 mg L-1). During these cycles, the plant was doped with increasing concentrations of emerging contamination compounds (diclofenac, ibuprofen, and erythromycin) to test their effect on the resources derived from the treatment. Subsequently, a tertiary treatment involving an advanced oxidation process was applied to the different water lines, which left the wastewater treatment plant for a period of 30 min and utilized varying concentrations of oxidant. The results indicate that the effluent obtained meets the required quality standards for agricultural use. Therefore, there is potential to use this waste as a resource, which is in line with the principles of the circular economy. Furthermore, the other resources generated during the treatment process, such as the biogas produced during the digestion process and the biostabilized sludge, have the potential to be used as resources according to the circular economy indicators.

Towards planetary boundary sustainability of food processing wastewater, by resource recovery & emission reduction: A process system engineering perspective

Meeting the needs of a growing population calls for a change from linear production systems that exacerbate the depletion of finite natural resources and the emission of environmental pollutants. These linear production systems have resulted in the human-driven perturbation of the Earth's natural biogeochemical cycles and the transgression of environmentally safe operating limits. One solution that can help alleviate the environmental issues associated both with resource stress and harmful emissions is resource recovery from waste. In this review, we address the recovery of resources from food and beverage processing wastewater (FPWW), which offers a synergistic solution to some of the environmental issues with traditional food production. Research on resource recovery from FPWW typically focuses on technologies to recover specific resources without considering integrative process systems to recover multiple resources while simultaneously satisfying regulations on final effluent quality. Process Systems Engineering (PSE) offers methodologies able to address this holistic process design problem, including modelling the trade-offs between competing objectives. Optimisation of FPWW treatment and resource recovery has significant scope to reduce the environmental impacts of food production systems. There is significant potential to recover carbon, nitrogen, and phosphorus resources while respecting effluent quality limits, even when the significant uncertainties inherent to wastewater systems are considered. This review article gives an overview of the environmental challenges we face, discussed within the framework of the planetary boundary, and highlights the impacts caused by the agri-food sector. This paper also presents a comprehensive review of the characteristics of FPWW and available technologies to recover carbon and nutrient resources from wastewater streams with a particular focus on bioprocesses. PSE research and modelling advances are discussed in this review. Based on this discussion, we conclude the article with future research directions.

Microalgae to bioenergy production: Recent advances, influencing parameters, utilization of wastewater - A critical review

Fossil fuel limitations and their influence on climate change through atmospheric greenhouse gas emissions have made the excessive use of fossil fuels widely recognized as unsustainable. The high lipid content, carbon-neutral nature and potential as a biofuel source have made microalgae a subject of global study. Microalgae are a promising supply of biomass for third-generation biofuels production since they are renewable. They have the potential to produce significant amounts of biofuel and are considered a sustainable alternative to non-renewable energy sources. Microalgae are currently incapable to synthesize algal biofuel on an extensive basis in a sustainable manner, despite their significance in the global production of biofuels. Wastewater contains nutrients (both organic and inorganic) which is essential for the development of microalgae. Microalgae and wastewater can be combined to remediate waste effectively. Wastewater of various kinds such as industrial, agricultural, domestic, and municipal can be used as a substrate for microalgal growth. This process helps reduce carbon dioxide emissions and makes the production of biofuels more cost-effective. This critical review provides a detailed analysis of the utilization of wastewater as a growth medium for microalgal - biofuel production. The review also highlights potential future strategies to improve the commercial production of biofuels from microalgae.

Harnessing Nanomaterials for Enhanced Biohydrogen Generation from Wastewater

Biohydrogen is considered a green fuel due to its eco-friendly nature since it only produces water and energy on combustion. However, their lower yield and production rate is one of the foremost challenges that need an instant sustainable approach. The use of nanotechnology is a potential approach for the enhanced generation of biohydrogen, owing to the significant characteristics of the nanomaterials such as greater specificity, high surface-area-to-volume ratio, better reactivity and dispersibility, enhanced catalytic activity, superb selectivity, greater electron transfer, and better anaerobic microbiota activity. This article explores the recent trends and innovations in the production of biohydrogen from wastewater through the applications of different nanomaterials. The potential of various nanomaterials employed for biohydrogen production from wastewater is evaluated and the impacts of important parameters such as the concentration and size of the nanomaterials, temperature, and pH on the production and yield of biohydrogen are explained in detail. Several pathways involved in the mechanistic approach of biohydrogen generation from wastewater are critically assessed. Lastly, numerous technological challenges are highlighted and recommendations regarding future research are also provided.

Biofuel production for circular bioeconomy: Present scenario and future scope

In recent years, biofuel production has attracted considerable attention, especially given the increasing worldwide demand for energy and emissions of greenhouse gases that threaten this planet. In this case, one possible solution is to convert biomass into green and sustainable biofuel, which can enhance the bioeconomy and contribute to sustainable economic development goals. Due to being in large quantities and containing high organic content, various biomass sources such as food waste, textile waste, microalgal waste, agricultural waste and sewage sludge have gained significant attention for biofuel production. Also, biofuel production technologies, including thermochemical processing, anaerobic digestion, fermentation and bioelectrochemical systems, have been extensively reported, which can achieve waste valorization through producing biofuels and re-utilizing wastes. Nevertheless, the commercial feasibility of biofuel production is still being determined, and it is unclear whether biofuel can compete equally with other existing fuels in the market. The concept of a circular economy in biofuel production can promote the environmentally friendly and sustainable valorization of biomass waste. This review comprehensively discusses the state-of-the-art production of biofuel from various biomass sources and the bioeconomy perspectives associated with it. Biofuel production is evaluated within the framework of the bioeconomy. Further perspectives on possible integration approaches to maximizing waste utilization for biofuel production are discussed, and what this could mean for the circular economy. More research related to pretreatment and machine learning of biofuel production should be conducted to optimize the biofuel production process, increase the biofuel yield and make the biofuel prices competitive.

Strategies to enhance biohydrogen production from microalgae: A comprehensive review

Microalgae represent a promising renewable feedstock for the sustainable production of biohydrogen. Their high growth rates and ability to fix carbon utilizing just sunlight, water, and nutrients make them well-suited for this application. Recent advancements have focused on improving microalgal hydrogen yields and cultivation methods. This review aims to summarize recent developments in microalgal cultivation techniques and genetic engineering strategies for enhanced biohydrogen production. Specific areas of focus include novel microalgal species selection, immobilization methods, integrated hybrid systems, and metabolic engineering. Studies related to microalgal strain selection, cultivation methods, metabolic engineering, and genetic manipulations were compiled and analyzed. Promising microalgal species with high hydrogen production capabilities such as Synechocystis sp., Anabaena variabilis, and Chlamydomonas reinhardtii have been identified. Immobilization techniques like encapsulation in alginate and integration with dark fermentation have led to improved hydrogen yields. Metabolic engineering through modulation of hydrogenase activity and photosynthetic pathways shows potential for enhanced biohydrogen productivity. Considerable progress has been made in developing microalgal systems for biohydrogen. However, challenges around process optimization and scale-up remain. Future work involving metabolic modeling, photobioreactor design, and genetic engineering of electron transfer pathways could help realize the full potential of this renewable technology.

Hydrogen from sewage sludge: Production methods, influencing factors, challenges, and prospects

The rising global population and rapid industrialization have frequently resulted in a significant escalation in energy requirements. Hydrogen, renowned for its eco-friendly and renewable characteristics, has garnered substantial interest as a fuel alternative to address the energy needs currently fulfilled by fossil fuels. Embracing such energy substitutes holds pivotal importance in advancing environmental sustainability, aiding in the reduction of greenhouse gas emissions - the primary catalysts of global warming and climate fluctuations. This study elucidates recent trends in sewage sludge (SS)-derived hydrogen through diverse production pathways and critically evaluates the impact of varying parameters on hydrogen yield. Furthermore, a detailed analysis of the breakdown of the hydrogen generation process from SS is provided, along with an assessment of its economic dimensions. The review culminates by illuminating key obstacles in the adoption of this innovative technology, accompanied by practical recommendations to surmount these challenges. This comprehensive analysis is expected to attract considerable interest from stakeholders within the hydrogen production domain, fostering substantial engagement.

Comparative analysis of hydrogen production and bacterial communities in mesophilic and thermophilic consortia using multiple inoculum sources

This study investigates the hydrogen (H2) production performance and bacterial communities in mesophilic (37 °C) and thermophilic (50 °C) H2-producing consortia derived from different inoculum sources and utilizing food waste as a substrate. This study found notable variations in H2 production characteristics among these consortia. Among the mesophilic consortia (MC), the W-MC obtained with wetland (W) as the inoculum source exhibited the highest hydrogen production (3900 mL·L-1 and 117 mL·L-1·h-1), while among the thermophilic consortia (TC), the FP-TC obtained with forest puddle sediment (FP) as the inoculum source showed the highest performance (2112 mL·L-1 and 127 mL·L-1·h-1). This study reveals that the choice of inoculum source plays a crucial role in determining hydrogen production efficiency. Furthermore, the bacterial community analysis demonstrated varying microbial diversity and richness in different inoculum sources. Clostridium, a well-known H2-producing bacterium, was found in both mesophilic and thermophilic consortia and showed a positive correlation with H2 production. Other bacteria, such as Sporanaerobacter, Caproiciproducens, and Caldibacillus, also exhibited significant correlations with H2 production, suggesting their potential roles in the process. The study highlights the complex interactions between bacterial communities and hydrogen production performance, shedding light on the critical factors influencing this renewable energy source. Overall, this study contributes to our understanding of the microbial ecology and the factors affecting hydrogen production in different temperature conditions, which can have practical implications for optimizing biohydrogen production processes using organic waste substrates.

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.

Molten salt strategy to activate biochar for enhancing biohydrogen production

Generally, dark fermentation (DF) of hydrogen (H₂) synthesis has low H₂ production from industrial-scale plant. In this study, campus greening wastes-ginkgo leaves were used to produce molten salt-modified biochar (MSBC) and nitrogen (N₂)-atmosphere BC (NBC) in molten salt and N₂ environment at 800 °C, respectively. MSBC showed excellent properties including high specific surface area and electron transfer ability. After supplementation with MSBC, H₂ yield rose by 32.4% compared to the control group without carbon material. Electrochemical analysis revealed MSBC improved the electrochemical properties of sludge. Furthermore, MSBC optimized the microbial community structure and increased the relative abundance of dominant microbes, thus promoting H₂ production. This work is provide the deep understanding of two carbons that play vital roles in increasing microbial biomass, supplementing trace element and favoring electron transfer in DF reactions. Salt recovery achieved 93.57% in molten salt carbonization, which has sustainability compared with N₂-atmosphere pyrolysis.

Epitomizing biohydrogen production from microbes: Critical challenges vs opportunities

Hydrogen is a clean and green biofuel choice for the future because it is carbon-free, non-toxic, and has high energy conversion efficiency. In exploiting hydrogen as the main energy, guidelines for implementing the hydrogen economy and roadmaps for the developments of hydrogen technology have been released by several countries. Besides, this review also unveils various hydrogen storage methods and applications of hydrogen in transportation industry. Biohydrogen productions from microbes, namely, fermentative bacteria, photosynthetic bacteria, cyanobacteria, and green microalgae, via biological metabolisms have received significant interests off late due to its sustainability and environmentally friendly potentials. Accordingly, the review is as well outlining the biohydrogen production processes by various microbes. Furthermore, several factors such as light intensity, pH, temperature and addition of supplementary nutrients to enhance the microbial biohydrogen production are highlighted at their respective optimum conditions. Despite the advantages, the amounts of biohydrogen being produced by microbes are still insufficient to be a competitive energy source in the market. In addition, several major obstacles have also directly hampered the commercialization effors of biohydrogen. Thus, this review uncovers the constraints of biohydrogen production from microbes such as microalgae and offers solutions associated with recent strategies to overcome the setbacks via genetic engineering, pretreatments of biomass, and introduction of nanoparticles as well as oxygen scavengers. The opportunities of exploiting microalgae as a suastainable source of biohydrogen production and the plausibility to produce biohydrogen from biowastes are accentuated. Lastly, this review addresses the future perspectives of biological methods to ensure the sustainability and economy viability of biohydrogen production.

Biohydrogen in a circular bioeconomy: A critical review

Hydrogen produced from biomass feedstocks is considered an effective solution in moving toward a decarbonized economy. Biohydrogen is a clean energy source that has gained global attention for adoption as it promises to mitigate climate change and human environmental damage. Through the circular economy framework, sustainable biohydrogen production with other bioproducts while addressing issues such as waste management is possible. This study presents a comprehensive review of the various biomass feedstocks and processing technologies associated with biohydrogen generation, as well as the possible integration of existing industries into a circular bioeconomy framework. The currently standing challenges and future perspectives are also discussed.

Biosolids: The Trojan horse or the beautiful Helen for soil fertilization?

The simultaneous requirement to manage resources and wastes in more rational way has meant that many communities worldwide have begun to search for long-term alternative solutions. Reuse and recovery of biosolids is considered to be a constant solution of circular sustainability, as waste disposal without further reuse background like fertilizer is no longer an alternative to be promoted. There have been developed many treatment methods over the years for the stabilization and sanitization of biosolids. However, the literature concludes that none of them is fully integrated by meeting all the basic criteria. Each method has its Achilles heel, and the appropriateness of the method lies in what is the goal each time. There are conventional methods with positive reciprocity in terms of sustainability, reuse indicators and technological maturity, but have high risk of microorganisms' reappearance. New advanced sustainable technologies, such as cold plasma, need to be further studied to apply on a large scale. The reuse of biosolids as construction materials is also discussed in the context of circular economy. Biosolids reuse and management legislation frame need to be revised, as a directive adopted 30 years ago does not fully meet communities' current needs.

Valorisation of residues from municipal wastewater sieving through anaerobic (co-)digestion with biological sludge

The Circular and Green Economy principles is inspiring new approaches to municipal wastewater treatment plants (MWWTPs) design and operation. Recently, an ever-growing interest is devoted to exploring the alternatives for switching the WWTPs from being able to 'simply' removing contaminants from water to biorefinery-like plants where energy and material can be recovered. In this perspective, both wastewater and residues from process can be valorised for recovering nutrients (N and P), producing value added products (i.e. biopolymers), energy vectors and biofuels (i.e. bio-H₂, bio-CH₄ and bioethanol). As an additional benefit, changing the approach for WWTPs design and operation will decrease the overall amount of landfilled residues. In this context, the present research is aimed at evaluating the CH₄ production potential of MWW screening units' residues. While such a stream is typically landfilled, the expected progressive increase of biodegradable matter content due to the ban on single-use plastic along with the boost of bioplastics makes the investigation of different biochemical valorisation routes more and more interesting from an environmental and economical perspective. Thus, a full-scale data collection campaign was performed to gain information on screening residues amount and properties and to analyse the relationship with influent flowrate. The most relevant residue properties were measured, and lab-scale tests were carried out to evaluate the bio-CH₄ potential.

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.

Effects of temperature and total solid content on biohydrogen production from dark fermentation of rice straw: Performance and microbial community characteristics

Semi-continuous experiments were carried out in lab-scale continuous stirred tank reactors to evaluate the effects of fermentation temperature (37 ± 1 °C and 55 ± 1 °C) and total solids (TS) contents (3 %, 6 %, and 12 %) on biohydrogen production from the dark fermentations (DF) of rice straw (RS) and the total operation duration was 105 days. The experimental results show that biohydrogen production (0.46-63.60 mL/g VS_added) from the thermophilic (55 ± 1 °C) DF (TDF) was higher than the mesophilic (37 ± 1 °C) DF (MDF) (0.19-2.13 mL/g VS_added) at the three TS contents, and achieved the highest of 63.60 ± 2.98 mL/g VS_added at TS = 6 % in TDF. The pH, NH₄⁺-N and total volatile fatty acid of fermentation liquids in the TDF were all higher than those in the MDF. The high abundance of lactic acid-producing bacteria resulted in low biohydrogen produced at TS = 3 %. Under the TDF with TS = 6 %, the highest abundance of hydrolytic bacteria (Ruminiclostridium 54.24 %) led to the highest biohydrogen production. The increase of TS content from 6 % to 12 % induced degradation pathway changes from biohydrogen production to methane production. This study demonstrated that butyric acid fermentation was the main pathway to produce biohydrogen from RS in both DFs.

Algal-based system for removal of emerging pollutants from wastewater: A review

The bioremediation of emerging pollutants in wastewater via algal biotechnology has been emerging as a cost-effective and low-energy input technological solution. However, the algal bioremediation technology is still not fully developed at a commercial level. The development of different technologies and new strategies to cater specific needs have been studied. The existence of multiple emerging pollutants and the selection of microalgal species is a major concern. The rate of algal bioremediation is influenced by various factors, including accidental contaminations and operational conditions in the pilot-scale studies. Algal-bioremediation can be combined with existing treatment technologies for efficient removal of emerging pollutants from wastewater. This review mainly focuses on algal-bioremediation systems for wastewater treatment and pollutant removal, the impact of emerging pollutants in the environment, selection of potential microalgal species, mechanisms involved, and challenges in removing emerging pollutants using algal-bioremediation systems.

Bioprocesses for the recovery of bioenergy and value-added products from wastewater: A review

Wastewater and activated sludge present a major challenge worldwide. Wastewater generated from large and small-scale industries, laundries, human residential areas and other sources is emerging as a main problem in sanitation and maintenance of smart/green cities. During the last decade, different technologies and processes have been developed to recycle and purify the wastewater. Currently, identification and fundamental consideration of development of more advanced microbial-based technologies that enable wastewater treatment and simultaneous resource recovery to produce bioenergy, biofuels and other value-added compounds (organic acids, fatty acids, bioplastics, bio-pesticides, bio-surfactants and bio-flocculants etc.) became an emerging topic. In the last several decades, significant development of bioprocesses and techniques for the extraction and recovery of mentioned valuable molecules and compounds from wastewater, waste biomass or sludge has been made. This review presents different microbial-based process routes related to resource recovery and wastewater application for the production of value-added products and bioenergy. Current process limitations and insights for future research to promote more efficient and sustainable routes for this under-utilized and continually growing waste stream are also discussed.

Emerging technologies for sustainable production of biohydrogen production from microalgae: A state-of-the-art review of upstream and downstream processes

Biohydrogen (BioH₂) is considered as one of the most environmentally friendly fuels and a strong candidate to meet the future demand for a sustainable source of energy. Presently, the production of BioH₂ from photosynthetic organisms has raised a lot of hopes in the fuel industry. Moreover, microalgal-based BioH₂ synthesis not only helps to combat current global warming by capturing greenhouse gases but also plays a key role in wastewater treatment. Hence, this manuscript provides a state-of-the-art review of the upstream and downstream BioH₂ production processes. Different metabolic routes such as direct and indirect photolysis, dark fermentation, photofermentation, and microbial electrolysis are covered in detail. Upstream processes (e.g. growth techniques, growth media) also have a great impact on BioH₂ productivity and economics, which is also explored. Technical and scientific obstacles of microalgae BioH₂ systems are finally addressed, allowing the technology to become more innovative and commercial.

Fermentative molecular biohydrogen production from cheese whey: present prospects and future strategy

Waste-dependent fermentative routes for biohydrogen production present a possible scenario to produce hydrogen gas on a large scale in a sustainable way. Cheese whey contains a high portion of organic carbohydrate and other organic acids, which makes it a feasible substrate for biohydrogen production. In the present review, recent research progress related to fermentative technologies, which explore the potentiality of cheese whey for biohydrogen production as an effective tool on a large scale, has been analyzed systematically. In addition, application of multiple response surface methodology tools such as full factorial design, Box-Behnken model, and central composite design during fermentative biohydrogen production to study the interactive effects of different bioprocess variables for higher biohydrogen yield in batch, fed-batch, and continuous mode is also discussed. The current paper also emphasizes computational fluid dynamics-based simulation designs, by which the substrate conversion efficiency of the cheese whey-based bioprocess and temperature distribution toward the turbulent flow of reaction liquid can be enhanced. The possible future developments toward higher process efficiency are outlined.

A review on the factors influencing biohydrogen production from lactate: The key to unlocking enhanced dark fermentative processes

Dark fermentation (DF) is one of the most promising biological methods to produce bio-hydrogen and other value added bio-products from carbohydrate-rich wastes and wastewater. However, process instability and low hydrogen production yields and rates have been highlighted as the major bottlenecks preventing further development. Numerous studies have associated such concerns with the inhibitory activity of lactate-producing bacteria (LAB) against hydrogen producers. However, an increasing number of studies have also shown lactate-based metabolic pathways as the prevailing platform for hydrogen production. This opens a vast potential to develop new strategies to deal with the "Achilles heel" of DF - LAB overgrowth - while untapping high-performance DF. This review discusses the key factors influencing the lactate-driven hydrogen production, paying particular attention to substrate composition, the operating conditions, as well as the microbiota involved in the process and its potential functionality and related biochemical routes. The current limitations and future perspectives in the field are also presented.

Bio-Hydrogen Production from Wastewater: A Comparative Study of Low Energy Intensive Production Processes

Billions of litres of wastewater are produced daily from domestic and industrial areas, and whilst wastewater is often perceived as a problem, it has the potential to be viewed as a rich source for resources and energy. Wastewater contains between four and five times more energy than is required to treat it, and is a potential source of bio-hydrogen—a clean energy vector, a feedstock chemical and a fuel, widely recognised to have a role in the decarbonisation of the future energy system. This paper investigates sustainable, low-energy intensive routes for hydrogen production from wastewater, critically analysing five technologies, namely photo-fermentation, dark fermentation, photocatalysis, microbial photo electrochemical processes and microbial electrolysis cells (MECs). The paper compares key parameters influencing H2 production yield, such as pH, temperature and reactor design, summarises the state of the art in each area, and highlights the scale-up technical challenges. In addition to H2 production, these processes can be used for partial wastewater remediation, providing at least 45% reduction in chemical oxygen demand (COD), and are suitable for integration into existing wastewater treatment plants. Key advancements in lab-based research are included, highlighting the potential for each technology to contribute to the development of clean energy. Whilst there have been efforts to scale dark fermentation, electro and photo chemical technologies are still at the early stages of development (Technology Readiness Levels below 4); therefore, pilot plants and demonstrators sited at wastewater treatment facilities are needed to assess commercial viability. As such, a multidisciplinary approach is needed to overcome the current barriers to implementation, integrating expertise in engineering, chemistry and microbiology with the commercial experience of both water and energy sectors. The review concludes by highlighting MECs as a promising technology, due to excellent system modularity, good hydrogen yield (3.6–7.9 L/L/d from synthetic wastewater) and the potential to remove up to 80% COD from influent streams.

Residue cornstalk derived biochar promotes direct bio-hydrogen production from anaerobic fermentation of cornstalk

In this study, an innovative approach was proposed based on the implement of biochar derived from residue cornstalk left after anaerobic bio-hydrogen production (RCA-biochar) to improve direct bio-hydrogen production from anaerobic fermentation of cornstalk. The bio-hydrogen production potential and maximum bio-hydrogen production rate increased from 156.2 to 286.1 mL H₂/g substrate and 3.5 to 5.7 mL H₂/g substrate/h, respectively, following the added RCA-biochar increased from 2.5 to 15.0 g/L. Cornstalk chemical component analysis showed the cellulose and hemicellulose content decreased by 17.9-33.7% and 14.4-25.2%, and lignin content increased by 20.3-42.8%, respectively, after 96 h anaerobic fermentation with RCA-biochar 2.5-15.0 g/L. Further analyses revealed that RCA-biochar not only provided more specific surface area for hydrogen-producing bacteria attachment, but also promoted the cellulolytic enzyme activity, thereby resulted in increased substrate conversion to bio-hydrogen.The findings obtained in this study may provide supports for effective and sustainable lignocellulosic bio-hydrogen production in the future.

Metabolic pathways for microalgal biohydrogen production: Current progress and future prospectives

Microalgal biohydrogen (bioH₂) has attracted global interest owing to its potential carbon-free source of sustainable renewable energy. Most of previous reviews which focused on microalgal bioH₂, have shown unclear differentiation among the metabolic pathways. In this review, investigation of all different metabolic pathways for microalgal bioH₂ production along with discussion on the recent research work of last 5-years have been considered. The major factors (such as light, vital nutrients, microalgal cell density, and substrate bioavailability) are highlighted. Moreover, effect of various pretreatment approaches on the constituent's bioaccessibility is reported. Microbial electrolysis cells as a new strategy for bioH₂ production is stated. Comparison between the operation conditions of various bioreactors and economic feasibility is also emphasized. Genetic, metabolic engineering, and synthetic biology as recent technologies improved the microalgal bioH₂ production through inactivation of uptake hydrogenase (H₂ase), inhibition of the competing pathways in polysaccharide synthesis, and improving the O₂ tolerant H₂ase.

Geothermal industry waste-derived catalyst for enhanced biohydrogen production

The main aim of this work was to develop sustainable catalyst from geothermal waste by hydrothermal process for enhanced biohydrogen production. The effects of Si/Al ratio and pH neutralization on the catalyst were also investigated to provide further insight into the hydrogen production capability. Results have shown with increasing Si/Al ratio, a lower amount of catalyst was synthesized and smaller particle size was obtained. pH neutralization treatment resulted in higher conversion compared to non-neutralized ones. Meanwhile, the highest conversion of biohydrogen from ethanol through steam reforming process (95.19%) was obtained from catalyst with pH neutralization treatment and Si/Al ratio of 10. The catalyst developed in this study was concluded to be suitable for framework/supporting catalyst due to relatively low selectivity.

Sulfur, Phosphorus and Metals in the Stoichiometric Estimation of Biomethane and Biohydrogen Yields

The estimation of biomethane or biohydrogen yield is used to evaluate energy recovery during the process of the anaerobic treatment of waste and wastewater. Mathematically calculated theoretical values can also be used in biomethane or biohydrogen potential tests as reference points to calculate which fraction of substrate is decomposed, when the substrate degradation stopped and when the sample’s self-digestion begins. This study suggests expanded forms of equations for anaerobic processes leading to either biomethane or biohydrogen. The traditional equations describing the conversion of a substrate with known carbon, hydrogen, oxygen and nitrogen composition were expanded to account for the composition of sulfur (for biohydrogen yields) and phosphorus (both biohydrogen and biomethane yields). As an optional part, one metal cation was also incorporated into the chemical formula of the evaluated wastewater composition in case the compound of biodegradable interest exists as a salt. The equations derived here can be useful for researchers estimating energy recovery based on the elemental analysis of samples, such as algal biomass harvested during harmful algal blooms (HABs). Examples of biomethane and biohydrogen yield estimations from sulfur- and phosphorus-containing compounds are also provided.