Enhanced co-production of biohydrogen and algal lipids from agricultural biomass residues in long-term operation

Production of biological hydrogen from Quinoa residue using dark fermentation and estimation of its microbial diversity

Although they are one of the world's environmental problems, agricultural wastes or residues are carbohydrate-rich and low-cost, so they are used as raw materials for the manufacture of biohydrogen (bio-H2). Among biological hydrogen manufacture methods, the dark fermentation method is suitable for processing waste or residues. In this regard, no study has been found in the literature on determining the potential of biological hydrogen manufacture from quinoa residue by the dark fermentation method. This work was carried out in a dark room at 36 ± 1 °C under different operating conditions in anaerobic batch bio-reactors fed with thermally pretreated anaerobic mixed bacteria + raw quinoa or quinoa extract liquid + nutrients. In the study, gas analyses were performed and biohydrogen production was detected in all the bio-reactors. Besides, taxonomic content analyses and organic acid analyses were executed. Maximum bio-H2 production was found as follows: at pH 4.5, 14,543.10-4 mL in the bio-reactor fed with 1.00 g quinoa/L and 1880.10-4 mL in the bio-reactor fed with 0.50 g quinoa extract/L, and at pH 4.0, 61,537.10-4 mL in the bio-reactor fed with 1.00 g quinoa/L and 1511.10-4 mL in the bio-reactor fed with 0.75 g quinoa extract/L. In the bio-reactors fed with raw quinoa residue, Clostridium butyricum and Hathewaya histolytica were detected as the most dominant bacteria at pH 4.5 and 4.0, respectively, whereas in the bio-reactors fed with quinoa extract liquid, Fonticella tunisiensis were detected as the most dominant bacteria at both pH 4.5 and pH 4.0.

Microbial co-cultures for biochemicals production from lignocellulosic biomass: A review

Global reliance on fossil oil should shift to cleaner alternatives to get a decarbonized society. One option to achieve this ambitious goal is the use of biochemicals produced from lignocellulosic biomass (LCB). The inherent low biodegradability of LCB and the inhibitory compounds that might be released during pretreatment are two main challenges for LCB valorization. At microbiological level, constraints are mostly linked to the need for axenic cultures and the preference for certain carbon sources (i.e., glucose). To cope with these issues, this review focuses on efficient LCB conversion via the sugar platform as well as an innovative carboxylate platform taking advantage of the co-cultivation of microorganisms. This review discusses novel trends in the use of microbial communities and co-cultures aiming at different bioproducts co-generation in single reactors as well as in sequential bioprocess combination. The outlook and further perspectives of these alternatives have been outlined for future successful development.

Anaerobic fermentation of organic solid waste: Recent updates in substrates, products, and the process with multiple products co-production

The effective conversion and recycling of organic solid waste contribute to the resolution of widespread issues such as global environmental pollution, energy scarcity and resource depletion. The anaerobic fermentation technology provides for the effective treatment of organic solid waste and the generation of various products. The analysis, which is based on bibliometrics, concentrates on the valorisation of affordable and easily accessible raw materials with high organic matter content as well as the production of clean energy substances and high value-added platform products. The processing and application status of fermentation raw materials such as waste activated sludge, food waste, microalgae and crude glycerol are investigated. To analyse the status of the preparation and engineering applications of the products, the fermentation products biohydrogen, VFAs, biogas, ethanol, succinic acid, lactic acid, and butanol are employed as representatives. Simultaneously, the anaerobic biorefinery process with multiple product co-production is sorted out. Product co-production can reduce waste discharge, enhance resource recovery efficiency, and serve as a model for improving anaerobic fermentation economics.

Dark fermentation and microalgae cultivation coupled systems: Outlook and challenges

The implementation of a sustainable bio-based economy is considered a top priority today. There is no doubt about the necessity to produce renewable bioenergy and bio-sourced chemicals to replace fossil-derived compounds. Under this scenario, strong efforts have been devoted to efficiently use organic waste as feedstock for biohydrogen production via dark fermentation. However, the technoeconomic viability of this process needs to be enhanced by the valorization of the residual streams generated. The use of dark fermentation effluents as low-cost carbon source for microalgae cultivation arises as an innovative approach for bioproducts generation (e.g., biodiesel, bioactive compounds, pigments) that maximizes the carbon recovery. In a biorefinery context, after value-added product extraction, the spent microalgae biomass can be further valorised as feedstock for biohydrogen production. This integrated process would play a key role in the transition towards a circular economy. This review covers recent advances in microalgal cultivation on dark fermentation effluents (DFE). BioH₂ via dark fermentation processes and the involved metabolic pathways are detailed with a special focus on the main aspects affecting the effluent composition. Interesting traits of microalgae and current approaches to solve the challenges associated to the integration of dark fermentation and microalgae cultivation are also discussed.

Biohydrogen production using algae: Potentiality, economics and challenges

The biohydrogen production from algal biomass could ensure hydrogen's sustainability as a fuel option at the industrial level. However, some bottlenecks still need to be overcome to achieve the process's economic feasibility. This review article highlights the potential of algal biomasses for producing hydrogen with a detailed explanation of various mechanisms and enzymes involved in the production processes. Further, it discusses the impact of various experimental parameters on biohydrogen production. This article also analyses the significant challenges confronted during the overall biohydrogen production process and comprehends the recent strategies adopted to enhance hydrogen productivity. Furthermore, it gives a perception of the economic sustenance of the process. Moreover, this review elucidates the future scope of this technology and delineates the approaches to ensure the viability of hydrogen production.

Waste biorefinery towards a sustainable circular bioeconomy: a solution to global issues

Global issues such as environmental problems and food security are currently of concern to all of us. Circular bioeconomy is a promising approach towards resolving these global issues. The production of bioenergy and biomaterials can sustain the energy-environment nexus as well as substitute the devoid of petroleum as the production feedstock, thereby contributing to a cleaner and low carbon environment. In addition, assimilation of waste into bioprocesses for the production of useful products and metabolites lead towards a sustainable circular bioeconomy. This review aims to highlight the waste biorefinery as a sustainable bio-based circular economy, and, therefore, promoting a greener environment. Several case studies on the bioprocesses utilising waste for biopolymers and bio-lipids production as well as bioprocesses incorporated with wastewater treatment are well discussed. The strategy of waste biorefinery integrated with circular bioeconomy in the perspectives of unravelling the global issues can help to tackle carbon management and greenhouse gas emissions. A waste biorefinery-circular bioeconomy strategy represents a low carbon economy by reducing greenhouse gases footprint, and holds great prospects for a sustainable and greener world.

Flocculation performance and mechanism of fungal pellets on harvesting of microalgal biomass

In this study, a bioflocculation method assisted by fungal pellets was developed for highly efficient microalgae harvesting. Effects of critical parameters, including flocculation type, temperature, rotation speed and initial pH, on the bioflocculation of fungal Aspergillus niger for microalgae Scenedesmus sp. were investigated. Results showed that the maximum flocculation efficiency of 99.4% was obtained when the fungal pellets were inoculated in the algal solution at the initial pH of 8.0, temperature of 30 °C and rotation speed of 160 rpm for 48 h in BG-11 medium. Furthermore, microscopy examination, scanning electron microscopy, Fourier transform infrared spectroscopy, Zeta potential measurement and three-dimensional excitation emission matrix fluorescence spectroscopy were used to explore the mechanism of bioflocculation process. It was found that the interaction of fungi and microalgae was related to the surface functional groups of fungal pellets. This study provides a interpretation of conceivable mechanism for microalgal bioflocculation by fungal pellets.

LPMO AfAA9_B and Cellobiohydrolase AfCel6A from A. fumigatus Boost Enzymatic Saccharification Activity of Cellulase Cocktail

Cellulose is the most abundant polysaccharide in lignocellulosic biomass, where it is interlinked with lignin and hemicellulose. Bioethanol can be produced from biomass. Since breaking down biomass is difficult, cellulose-active enzymes secreted by filamentous fungi play an important role in degrading recalcitrant lignocellulosic biomass. We characterized a cellobiohydrolase (AfCel6A) and lytic polysaccharide monooxygenase LPMO (AfAA9_B) from Aspergillus fumigatus after they were expressed in Pichia pastoris and purified. The biochemical parameters suggested that the enzymes were stable; the optimal temperature was ~60 °C. Further characterization revealed high turnover numbers (kcat of 147.9 s-1 and 0.64 s-1, respectively). Surprisingly, when combined, AfCel6A and AfAA9_B did not act synergistically. AfCel6A and AfAA9_B association inhibited AfCel6A activity, an outcome that needs to be further investigated. However, AfCel6A or AfAA9_B addition boosted the enzymatic saccharification activity of a cellulase cocktail and the activity of cellulase Af-EGL7. Enzymatic cocktail supplementation with AfCel6A or AfAA9_B boosted the yield of fermentable sugars from complex substrates, especially sugarcane exploded bagasse, by up to 95%. The synergism between the cellulase cocktail and AfAA9_B was enzyme- and substrate-specific, which suggests a specific enzymatic cocktail for each biomass by up to 95%. The synergism between the cellulase cocktail and AfAA9_B was enzyme- and substrate-specific, which suggests a specific enzymatic cocktail for each biomass.

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.

Enhanced microalgal growth and lipid accumulation by addition of different nanoparticles under xenon lamp illumination

In this study, the growth and lipid accumulation of Scenedesmus sp. using different nanoparticles and light sources were investigated. Xenon lamp can produce a broad illumination spectrum, and exhibited better performance than light-emitting diode. SiC and g-C₃N₄ nanoparticles improved the biomass and lipid accumulation, whereas TiO₂ and TiC nanoparticles had inhibitory influence on microalgae. Lipid production can be improved by oxidative stress produced by combination of nanoparticles and xenon lamp irradiation. At the optimal SiC nanoparticles concentration of 150 mg L^-1 and photoperiod of 6:18 h, the maximum biomass concentration and total lipid content reached 3.18 g L^-1 and 40.26%, respectively. The addition of SiC nanoparticles could promote the substrate utilization rate and induce stress condition, thereby enhancing the activity of acetyl-CoA carboxylase and lipid biosynthesis. This research shows that SiC nanoparticles addition combined with xenon lamp illumination is a promising strategy to promote microalgal growth and lipid accumulation.

Simultaneous nutrition removal and high-efficiency biomass and lipid accumulation by microalgae using anaerobic digested effluent from cattle manure combined with municipal wastewater

BACKGROUND

Microalgae as a viable biodiesel feedstock show great potential to approach the challenges of energy shortage and environment pollution, but their economic feasibility was seriously hampered by high production cost. Thus, it is in urgent need to reduce the cost of cultivation and improve the biomass and lipid production of microalgae. In this work, anaerobic digestion effluent from cattle manure combined with municipal wastewater was used as a cost-effective medium for cultivating microalgae and expected to obtain high biomass. The pretreatment of anaerobic digested effluent containing dilution rate, sterilization and nutrient optimization was investigated. Then, initial pH and light intensity for algal growth, lipid production and wastewater purification were optimized in this study.

RESULTS

Scenedesmus sp. could grow rapidly in 10% anaerobic digestion effluent from cattle manure combined with secondary sedimentation tank effluent without sterilization. Optimum nutrient additives for higher biomass were as follows: glucose 10 g/L, NaNO₃ 0.3 g/L, K₂HPO₄·3H₂O 0.01 g/L, MgSO₄·7H₂O 0.075 g/L and trace element A5 solution 1 mL/L. Biomass of 4.65 g/L and lipid productivity of 81.90 mg/L/day were achieved during 7-day cultivation accompanying over 90% of COD, NO₃ ^--N, NH₄ ⁺-N, and 79-88% of PO₄ ^3--P removal with optimized initial pH of 7.0 and light intensity of 5000 l×. The FAME profile in ADEC growth medium consisted in saturated (39.48%) and monounsaturated (60.52%) fatty acids with the 16- to 18-chain-length fatty acids constituting over 98% of total FAME.

CONCLUSIONS

This study proves the potential of anaerobic digested effluent combined with municipal wastewater for microalgae culture, and provides an effective avenue for simultaneous microalgal lipid production and treatment of two kinds of wastewater.