A two-stage bioprocess for hydrogen and methane production from rice straw bioethanol residues

Recent advances in swine wastewater treatment technologies for resource recovery: A comprehensive review

Swine wastewater (SW), characterized by highly complex organic and nutrient substances, poses serious impacts on aquatic environment and public health. Furthermore, SW harbors valuable resources that possess substantial economic potential. As such, SW treatment technologies place increased emphasis on resource recycling, while progressively advancing towards energy saving, sustainability, and circular economy principles. This review comprehensively encapsulates the state-of-the-art knowledge for treating SW, including conventional (i.e., constructed wetlands, air stripping and aerobic system) and resource-utilization-based (i.e., anaerobic digestion, membrane separation, anaerobic ammonium oxidation, microbial fuel cells, and microalgal-based system) technologies. Furthermore, this research also elaborates the key factors influencing the SW treatment performance, such as pH, temperature, dissolved oxygen, hydraulic retention time and organic loading rate. The potentials for reutilizing energy, biomass and digestate produced during the SW treatment processes are also summarized. Moreover, the obstacles associated with full-scale implementation, long-term treatment, energy-efficient design, and nutrient recovery of various resource-utilization-based SW treatment technologies are emphasized. In addition, future research prospective, such as prioritization of process optimization, in-depth exploration of microbial mechanisms, enhancement of energy conversion efficiency, and integration of diverse technologies, are highlighted to expand engineering applications and establish a sustainable SW treatment system.

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.

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.

Microalgal Hydrogen Production in Relation to Other Biomass-Based Technologies—A Review

Hydrogen is an environmentally friendly biofuel which, if widely used, could reduce atmospheric carbon dioxide emissions. The main barrier to the widespread use of hydrogen for power generation is the lack of technologically feasible and—more importantly—cost-effective methods of production and storage. So far, hydrogen has been produced using thermochemical methods (such as gasification, pyrolysis or water electrolysis) and biological methods (most of which involve anaerobic digestion and photofermentation), with conventional fuels, waste or dedicated crop biomass used as a feedstock. Microalgae possess very high photosynthetic efficiency, can rapidly build biomass, and possess other beneficial properties, which is why they are considered to be one of the strongest contenders among biohydrogen production technologies. This review gives an account of present knowledge on microalgal hydrogen production and compares it with the other available biofuel production technologies.

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.

Recent advancement on biological technologies and strategies for resource recovery from swine wastewater

Swine wastewater is categorized as one of the agricultural wastewater with high contents of organics and nutrients including nitrogen and phosphorus, which may lead to eutrophication in the environment. Insufficient technologies to remove those nutrients could lead to environmental problems after discharge. Several physical and chemical methods have been applied to treat the swine wastewater, but biological treatments are considered as the promising methods due to the cost effectiveness and performance efficiency along with the production of valuable products and bioenergies. This review summarizes the characteristics of swine wastewaters in the beginning, and briefly describes the current issues on the treatments of swine wastewaters. Several biological techniques, such as anaerobic digestion, A/O process, microbial fuel cells, and microalgae cultivations, and their future aspects will be addressed. Finally, the potentials to reutilize biomass produced during the treatment processes are also presented under the consideration of circular economy.

Metabolic flux network analysis of hydrogen production from crude glycerol by Clostridium pasteurianum

The present study has attempted to get insight into ultrasound induced enhancement in biohydrogen production from glycerol fermentation using metabolic flux analysis (MFA). A pseudo steady state metabolic flux network model was constructed and analyzed using experimentally measured glycerol uptake rate and fluxes of four metabolites, viz. acetate, butyrate, succinate and 1,3-PDO. Glycerol consumption increased by ∼50% under sonication. Biohydrogen yield showed marked rise of ∼40% with application of ultrasound. Butyrate and 1,3-PDO were the major products of glycerol metabolism. Sonication had major influence on carbon fluxes at the acetyl-CoA node. MFA results revealed enhanced flux towards butyrate under sonication, which was manifested in higher butyrate to acetate (B/A) ratio in products and greater H₂ generation. Biohydrogen production was also a microbial growth associated process. Finally, two theoretical alternatives for further enhancement of biohydrogen production were assessed with MFA, viz. enhancement of glycerol uptake and blocking of butyrate pathway.

Biological hydrogen and methane production from bagasse bioethanol fermentation residues using a two-stage bioprocess

This study investigated the recovery of H2 and CH4 from bagasse bioethanol fermentation residues (bagasse BEFR) using a two-stage bioprocess. In the hydrogen fermentation bioreactor (HFB), carbohydrate removal efficiency was maintained at 82-93% and the highest hydrogen yield was 8.24mL/gCOD at volumetric loading rate (VLR) of 80kgCOD/m(3)/day. The results indicated a positive correlation between hydrogen yield and butyrate-to-acetate ratio, which might be due to the mechanisms of lactate/acetate utilization for hydrogen production and acetogenesis occurring in the HFB. Remaining volatile fatty acids and alcohols in the HFB effluent were further utilized for methane production in methane fermentation bioreactor (MFB), in which the highest methane yield of 345.2mL/gCOD was attained at VLR of 2.5kgCOD/m(3)/day. Overall, the two-stage bioprocess achieved a maximum COD removal of 81% from bagasse BEFR, and converted 0.3% and 72.8% of COD in the forms of H2 and CH4, respectively.

Biological butanol production from microalgae-based biodiesel residues by Clostridium acetobutylicum

This study conducted batch experiments to evaluate the potential of butanol production from microalgae biodiesel residues by Clostridium acetobutylicum. The results indicated that with 90 g/L of glucose as the sole substrate the highest butanol yield of 0.2 g/g-glucose was found, but the addition of butyrate significantly enhanced the butanol yield. The highest butanol yield of 0.4 g/g-glucose was found with 60 g/L of glucose and 18 g/L of butyrate. Using microalgae biodiesel residues as substrate, C. acetobutylicum produced 3.86 g/L of butanol and achieved butanol yield of 0.13 g/g-carbohydrate via ABE fermentation, but the results indicated that approximately one third of carbohydrate was not utilized by C. acetobutylicum. Biological butanol production from microalgae biodiesel residues can be possible, but further research on fermentation strategies are required to improve production yield.

Biohydrogen production: strategies to improve process efficiency through microbial routes

The current fossil fuel-based generation of energy has led to large-scale industrial development. However, the reliance on fossil fuels leads to the significant depletion of natural resources of buried combustible geologic deposits and to negative effects on the global climate with emissions of greenhouse gases. Accordingly, enormous efforts are directed to transition from fossil fuels to nonpolluting and renewable energy sources. One potential alternative is biohydrogen (H₂), a clean energy carrier with high-energy yields; upon the combustion of H₂, H₂O is the only major by-product. In recent decades, the attractive and renewable characteristics of H₂ led us to develop a variety of biological routes for the production of H₂. Based on the mode of H₂ generation, the biological routes for H₂ production are categorized into four groups: photobiological fermentation, anaerobic fermentation, enzymatic and microbial electrolysis, and a combination of these processes. Thus, this review primarily focuses on the evaluation of the biological routes for the production of H₂. In particular, we assess the efficiency and feasibility of these bioprocesses with respect to the factors that affect operations, and we delineate the limitations. Additionally, alternative options such as bioaugmentation, multiple process integration, and microbial electrolysis to improve process efficiency are discussed to address industrial-level applications.

Induced catabolic bio-electrohydrolysis of complex food waste by regulating external resistance for enhancing acidogenic biohydrogen production

A novel bio-electrohydrolysis system (BEH) based on self-inducing electrogenic activity was designed as pretreatment device to enhance biohydrogen (H2) production efficiency from food waste. Two-stage hybrid operation with hydrolysis in the initial stage and acidogenic fermentation of the resulting hydrolysate (after hydrolysis) for H2 production in the second stage was evaluated. Application of variable external resistances viz., 10Ω, 100Ω, 1000Ω and closed circuit (CC) influenced the hydrolysis of substrate in BEH system and hydrogen production in acidogenic reactor compared to control. Pretreated substrate at 100Ω documented higher H2 production (1.05l) than 10Ω (0.93l), CC (0.91l), 1000Ω (0.88l) and control operation (0.68l). Comparatively, 10Ω documented higher substrate degradation (53.4%) followed by CC (52.42%), 100Ω (49.51%), 1000Ω (47.57%) and control (43.68%). Voltammetric profiles were in agreement with the observed bio-electrohydrolysis and H2 production efficiency.

Metabolic flux network analysis of fermentative hydrogen production: using Clostridium tyrobutyricum as an example

This study applies metabolic flux network analysis (MFA) to evaluate the metabolic flux of fermentative hydrogen production (FHP) with the use of Clostridium tyrobutyricum fed with either glucose or lactate/acetate as substrates. The MFA results suggest that hydraulic retention time (HRT) presents significant impact on hydrogen production from glucose. At HRT between 4 and 18 h, increase of HRT increased hydrogen production but decreased lactate production, while at HRT below 4 h decrease of HRT increased hydrogen production but decreased lactate production. The flux for lactate, butyrate and acetate seemed to affect H₂ production, due presumably to their impacts on the balance of NADH, ferredoxin and ATP. It is suggested that the MFA can be a useful tool to provide valuable information for optimization and design of the fermentative hydrogen production process.