Emerging technologies for the pretreatment of lignocellulosic biomass

Transformation of lignocellulose to starch-like carbohydrates by organic acid-catalyzed pretreatment and biological detoxification

Corn dry milling provides a mature model for lignocellulose biorefinery process. To copy this technical success, a crucial step is to transform lignocellulose into starch-like carbohydrates (SLC), similar to milled corn grain and in a similar fashion to corn dry milling. The transformation process should be zero wastewater generation and sufficient fermentable sugar conservation; the product should be in solid particle form, free of toxic residues, and high enzymatic hydrolysis yield and fermentability. Here we designed and verified a SLC transformation process by (i) biodegradable oxalic acid-catalyzed pretreatment, and (ii) simultaneous biodegradation of inhibitors and oxalic acid catalyst. The oxalic acid catalyst was effective on disrupting the lignocellulose structure and also biodegradable at low pH value. The biodetoxification fungus Paecilomyces variotii FN89 was capable of degrading the furan/phenolic aldehydes and oxalic acid simultaneously and ultimately, while the fermentable sugars were well preserved. The obtained SLC from wheat straw and corn stover were similar to dry milled corn meal in terms of morphological properties, fermentable sugar contents, enzymatic hydrolysis yield, elemental contents, and free of inhibitors and acid catalyst. The bioconversion of starch-like wheat straw and corn stover produced 78.5 and 75.3 g/L of ethanol (9.9% and 9.5%, v/v) with the yield of 0.47 and 0.45 g ethanol/g cellulose/xylose, respectively, compared with 78.7 g/L (10.0%, v/v) from corn meal and the yield of 0.48 g ethanol/g starch. Mass balances suggest that the ethanol yield, wastewater generation, and elemental recycling of the SLC from lignocellulose were essentially the same as those of corn meal.

Effect of acclimatized paddy soil microorganisms using swine wastewater on degradation of rice straw

Rice straw (RS) is one of abundant agricultural waste for biogas production in China. However, the low carbon-methane conversion rate limits its wide application due to the low degradation rate of RS during fermentation. This study investigated the effect of acclimatized paddy soil microorganisms using swine wastewater on degradation of RS before anaerobic digestion. The total organic carbon, reducing sugar and NH₄⁺-N content of paddy soil + RS + swine wastewater (PRS) (653.50 mg/L) was higher than that of other groups after 19 days. The carboxymethyl cellulose activity (4.01 IU), cellulose/lignin ratio (5.25) and the degradation rate of lignin (51.96%) in PRS were higher than those of other groups. The Firmicutes (21.02%), Chloroflexi (12.48%), Proteobacteria (20.92%), and Bacteroidetes (25.78%) were the main fermentation phyla in PRS during acclimatization. These results indicated that the acclimatized paddy soil microorganisms using swine wastewater (SW) could degrade RS more efficiently.

Bio-Delignification of Green Waste (GW) in Co-Digestion with the Organic Fraction of Municipal Solid Waste (OFMSW) to Enhance Biogas Production

The organic fraction of municipal solid waste (OFMSW) is recognized as a suitable substrate for the anaerobic digestion (AD) process and is currently considered a mature technology. A promising strategy to enhance biogas yield and productivity is the co-digestion of OFMSW with other organic biomass, such as green waste (GW), a mixture of leaves, grass, and woody materials originated from private yards and public greenspace management. The main limitation to the use of GW for biogas production is the high percentage of the lignocellulosic fraction, which makes necessary a pretreatment of delignification to dissolve the recalcitrant structure. In this study, a new strategy of sustainable bio-delignification using the white-rot fungi Bjerkandera adusta (BA) in comparison with other chemical pretreatments were investigated. Untreated and treated GW were, respectively, submitted to anaerobic co-digestion with OFMSW. AD processes were carried out in a lab-scale plant for 30 days in thermophilic conditions (55 °C). Biogas cumulative production was increased by about 100% in the case of treated GW compared with that of just OFMSW, from 145 to 289 Nm3 CH4/ton SV, and productivity almost doubled from 145 to 283 Nm3/ton FM * day. The measured average methane content values in the cumulative biogas were 55% from OFMSW and 54% from GW. Moreover, over 95% of the biogas was produced in 20 days, showing the potential opportunity to reduce the AD time.

Evaluation of Crystalline Cellulose of Corn Straw through Different Pretreatments Via X-Ray Diffraction, Scanning Electron Microscopy and Infrared Spectroscopy

Waste recycling is beneficial not only for the environment but also for the economy and the society at large. Corn stalks, the most abundant crop waste, are processed with oven drying, steaming, complex enzymatic hydrolysis (cellulase, xylanase, β-glucanase and pectinase), and fermentation (by Candida utilis and Pachysolen tannophilus), were analyzed via X-ray diffraction, Scanning Electron Microscopy (SEM) and Infrared Spectroscopy (IR). The results indicated that thermophilic digestion destroyed glycosidic bonds and fibrous crystal have a significant effect on the degradation of the corn stalks, while complex enzyme hydrolysis only slightly degraded fibrous crystal. Fermentation did not significantly reduce fibrous crystal. Therefore, our research suggested that thermophilic digestion is the appropriate way to increase the saccharification rate and feed yield of corn stalks.

Nanocelluloses: Sources, Pretreatment, Isolations, Modification, and Its Application as the Drug Carriers

The 'Back-to-nature' concept has currently been adopted intensively in various industries, especially the pharmaceutical industry. In the past few decades, the overuse of synthetic chemicals has caused severe damage to the environment and ecosystem. One class of natural materials developed to substitute artificial chemicals in the pharmaceutical industries is the natural polymers, including cellulose and its derivatives. The development of nanocelluloses as nanocarriers in drug delivery systems has reached an advanced stage. Cellulose nanofiber (CNF), nanocrystal cellulose (NCC), and bacterial nanocellulose (BC) are the most common nanocellulose used as nanocarriers in drug delivery systems. Modification and functionalization using various processes and chemicals have been carried out to increase the adsorption and drug delivery performance of nanocellulose. Nanocellulose may be attached to the drug by physical interaction or chemical functionalization for covalent drug binding. Current development of nanocarrier formulations such as surfactant nanocellulose, ultra-lightweight porous materials, hydrogel, polyelectrolytes, and inorganic hybridizations has advanced to enable the construction of stimuli-responsive and specific recognition characteristics. Thus, an opportunity has emerged to develop a new generation of nanocellulose-based carriers that can modulate the drug conveyance for diverse drug characteristics. This review provides insights into selecting appropriate nanocellulose-based hybrid materials and the available modification routes to achieve satisfactory carrier performance and briefly discusses the essential criteria to achieve high-quality nanocellulose.

Advances in Pretreatment of Straw Biomass for Sugar Production

Straw biomass is an inexpensive, sustainable, and abundant renewable feedstock for the production of valuable chemicals and biofuels, which can surmount the main drawbacks such as greenhouse gas emission and environmental pollution, aroused from the consumption of fossil fuels. It is rich in organic content but is not sufficient for extensive applications because of its natural recalcitrance. Therefore, suitable pretreatment is a prerequisite for the efficient production of fermentable sugars by enzymatic hydrolysis. Here, we provide an overview of various pretreatment methods to effectively separate the major components such as hemicellulose, cellulose, and lignin and enhance the accessibility and susceptibility of every single component. This review outlines the diverse approaches (e.g., chemical, physical, biological, and combined treatments) for the excellent conversion of straw biomass to fermentable sugars, summarizes the benefits and drawbacks of each pretreatment method, and proposes some investigation prospects for the future pretreatments.

A Review on Bacterial Contribution to Lignocellulose Breakdown into Useful Bio-Products

Discovering novel bacterial strains might be the link to unlocking the value in lignocellulosic bio-refinery as we strive to find alternative and cleaner sources of energy. Bacteria display promise in lignocellulolytic breakdown because of their innate ability to adapt and grow under both optimum and extreme conditions. This versatility of bacterial strains is being harnessed, with qualities like adapting to various temperature, aero tolerance, and nutrient availability driving the use of bacteria in bio-refinery studies. Their flexible nature holds exciting promise in biotechnology, but despite recent pointers to a greener edge in the pretreatment of lignocellulose biomass and lignocellulose-driven bioconversion to value-added products, the cost of adoption and subsequent scaling up industrially still pose challenges to their adoption. However, recent studies have seen the use of co-culture, co-digestion, and bioengineering to overcome identified setbacks to using bacterial strains to breakdown lignocellulose into its major polymers and then to useful products ranging from ethanol, enzymes, biodiesel, bioflocculants, and many others. In this review, research on bacteria involved in lignocellulose breakdown is reviewed and summarized to provide background for further research. Future perspectives are explored as bacteria have a role to play in the adoption of greener energy alternatives using lignocellulosic biomass.

Combined adaptive evolution and transcriptomic profiles reveal aromatic aldehydes tolerance mechanisms in Yarrowia lipolytica

Yarrowia lipolytica is an efficient oleaginous yeast, whereas its activity is typically reduced by inhibitors present in lignocellulosic hydrolysate. Understanding the response mechanism of Y. lipolytica to hydrolysate inhibitors and developing inhibitor tolerant strains are vital to lignocellulose valorization by this promising species. In this study, through adaptive laboratory evolution on three representative aromatic aldehyde inhibitors, evolved strains were obtained. Fermentation phenotype suggested that aromatic aldehydes conversion was one main reason for high tolerance of adapted strains. Transcriptome profiling analysis and reverse metabolic engineering confirmed that overexpressing the aldehyde ketone reductase gene YALI0_B07117g and aldehyde dehydrogenase gene YALI0_B01298g effectively converted aromatic aldehyde to corresponding alcohols and acids. The potential degradation pathways for aromatic aldehyde inhibitors in Y. lipolytica XYL+ were then discussed. This study provided insights to the aromatic aldehyde degradation in Y. lipolytica and a reliable basis for the development of aromatic aldehyde tolerant strains.

Trametes versicolor in lignocellulose-based bioeconomy: State of the art, challenges and opportunities

Although Trametes versicolor is one of the most investigated white-rot fungi, the industrial application of this fungus and its metabolites is still far from reaching its full potential. This review aims to highlight the opportunities and challenges for the industrial use of T. versicolor according to the principles of circular bioeconomy. The use of this fungus can contribute significantly to the success of efforts to valorize lignocellulosic waste biomass and industrial lignocellulosic byproducts. Various techniques of T. versicolor cultivation for enzyme production, food and feed production, wastewater treatment, and biofuel production are listed and critically evaluated, highlighting bottlenecks and future perspectives. Applications of T. versicolor crude laccase extracts in wastewater treatment, removal of lignin from lignocellulose, and in various biotransformations are analyzed separately.

Dissolving pulp production: Cellulases and xylanases for the enhancement of cellulose accessibility and reactivity

Dissolving pulps are high-grade cellulose pulps that have minimum amount of non-cellulosic impurities. Dissolving pulps are the basic source for the manufacturing of several cellulosic products such as viscose, lyocell, cellulose acetates, cellulose nitrates, carboxymethyl-cellulose, etc. Dissolving pulps are mainly manufactured by pre-hydrolysis kraft and acid sulphite pulping. A high reactivity of dissolving pulps is desirable for its eco-friendly utilization for several purposes. Several approaches including mechanical, chemical, ultrasonic, and enzymatic treatments have been employed for the improvement of pulp reactivity. This review mainly focussed on pulp reactivity improvement through enzymatic approaches. Cellulases and xylanase have been proved effective for the improvement of pulp reactivity of dissolving pulp from different sources. The different combinations of cellulase, xylanase, and mechanical refining have been tested and found more effective rather than the single one.

Cellulose nanocrystals: Pretreatments, preparation strategies, and surface functionalization

Cellulose nanocrystals (CNCs) have attracted great interest from researchers from academic and industrial areas because of their interesting structural features and unique physicochemical properties, such as magnificent mechanical strength, high surface area, and many hydroxyl groups for chemical modification, low density, and biodegradability. CNCs are an outstanding contender for applications in assorted fields comprehensive of, e.g., biomedical, electronic gadgets, water purifications, nanocomposites, membranes. Additionally, a persistent progression is going on in the extraction and surface modification of cellulose nanocrystals to fulfill the expanding need of producers to fabricate cellulose nanocrystals-based materials. In this review, the foundation of nanocellulose that emerged from lignocellulosic biomass and recent development in extraction/preparation of cellulose nanocrystals and different types of cellulose nanocrystal surface modification techniques are summed up. The different sorts of cellulose modification reactions that have been discussed are acetylation, oxidations, esterifications, etherifications, ion-pair formation, hydrogen bonding, silanization, nucleophilic substitution reactions, and so forth. The mechanisms of surface functionalization reactions are also introduced and considered concerning the impact on the reactions. Moreover, the primary association of cellulose and different forms of nanocellulose has likewise been examined for beginners in this field.

Cellulose Recovery from Agri-Food Residues by Effective Cavitational Treatments

Residual biomass from agri-food production chain and forestry are available in huge amounts for further valorisation processes. Delignification is usually the crucial step in the production of biofuels by fermentation as well as in the conversion of cellulose into high added-value compounds. High-intensity ultrasound (US) and hydrodynamic cavitation (HC) have been widely exploited as effective pretreatment techniques for biomass conversion and in particular for cellulose recovery. Due to their peculiar mechanisms, cavitational treatments promote an effective lignocellulosic matrix dismantling with delignification at low temperature (35–50 °C). Cavitation also promotes cellulose decrystallization due to a partial depolymerization. The aim of this review is to highlight recent advances in US and HC-assisted delignification and further cellulose recovery and valorisation.

Efficient high-solids enzymatic hydrolysis of corncobs by an acidic pretreatment and a fed-batch feeding mode

Corncob is an abundant and renewable resource that could be enzymatically hydrolyzed to fermentable sugar. A major impediment in corncob utilization is the low hydrolysis efficiency at high-solids content. This study attempted different pretreatment methods and fed-batch modes to achieve a 25% solids content hydrolysis with high yields. Natural corncobs were compared with acid-treated and acid-alkali-treated corncobs in terms of kinetics parameters, conversion rate and glucose titer. By feeding in batches, a "low amount and high frequency" mode (10%-3%-3%-3%-3%-3%, every 5 h) was confirmed to be optimal for a 25% high-solids hydrolysis system with a cellulase loading of 12 mg/g (7.3 FPU/g), resulted with an 84.4% glucose yield at 96 h. Our results demonstrated that combination of both optimized pretreatment method and fed-batch mode were a favored process model for high-solids hydrolysis of lignocellulose, boosting cellulose hydrolysis efficiency and sugar yields on an industrial scale.

Ultrasound-assisted biomass valorization to industrial interesting products: state-of-the-art, perspectives and challenges

Nowadays, the application of ultrasound (US) energy for assisting the lignocellulosic biomass and waste materials conversion into value-added products has dramatically increased. In this sense, this review covers theoretical aspects, promising applications, challenges and perspectives about US and its use for biomass treatment. The combination of US energy with a suitable reaction time, temperature and solvent contributes to the destruction of recalcitrant lignin structure, allowing the products to be used in thermochemical and biological process. The main mechanisms related to US propagation and impact on the fragmentation of lignocellulosic materials, selectivity, and yield of conversion treatments are discussed. Moreover, the synergistic effects between US and alternative green solvents with the perspective of industrial applications are investigated. The present survey analysed the last ten years of literature, studying challenges and perspectives of US application in biorefinery. We were aiming to highlight value-added products and some new areas of research.

Mechanism and industrial application feasibility analysis on microwave-assisted rapid synthesis of amino-carboxyl functionalized cellulose for enhanced heavy metal removal

The study presented the successful microwave-assisted (MW-assisted) preparation of a novel adsorbent derived from rice straw (RS_MW-AC) and explored its adsorption performance toward heavy metal ions from water. The RS_MW-AC was rapidly synthesized through pretreatment and one step grafting via the MW-assisted approach. The quantitative predictive correlations between target performance of RS_MW-AC and process parameters were obtained through the response surface methodology (RSM). Meanwhile, the optimal preparation process conditions were determined: NaOH solution concentration, 20%; MW irradiation temperature for pretreatment, 100 and 150 °C; MW irradiation time for pretreatment and grafting, 10 and 60 min; EDTAD-RS mass ratio, 3. The RS_MW-AC showed a good adsorption of different heavy metal ions from water (152.39, 55.46, 52.91, 35.60 and 20.11 mg g^-1 for Pb(Ⅱ), Mn(Ⅱ), Cd(Ⅱ), Cu(Ⅱ) and Ni(Ⅱ), respectively). The adsorption behaviors followed the Langmuir model and pseudo second-order kinetics model with a highly significant correlation. Also of note was that amino and carboxyl groups were successfully introduced on the rice straw based on characterization results. Furthermore, preparation mechanism was explored to reveal reasons why microwave irradiation could accelerate the preparation of the adsorbent; its adsorption process was dominated by electrostatic attraction and chelation. Finally, the study made the industrial application feasibility analysis of MW-assisted approach used for pretreatment and graft reaction of agro-waste biomass.

Interspecific evolutionary relationships of alpha-glucuronidase in the genus Aspergillus

The increased availability and production of lignocellulosic agroindustrial wastes has originated proposals for their use as raw material to obtain biofuels (ethanol and biodiesel) or derived products. However, for biomass generated from lignocellulosic residues to be successfully degraded, in most cases it requires a physical (thermal), chemical, or enzymatic pretreatment before the application of microbial or enzymatic fermentation technologies (biocatalysis). In the context of enzymatic technologies, fungi have demonstrated to produce enzymes capable of degrading polysaccharides like cellulose, hemicelluloses and pectin. Because of this ability for degrading lignocellulosic material, researchers are making efforts to isolate and identify fungal enzymes that could have a better activity for the degradation of plant cell walls and agroindustrial biomass. We performed an in silico analysis of alpha-glucoronidase in 82 accessions of the genus Aspergillus. The constructed dendrograms of amino acid sequences defined the formation of 6 groups (I, II, III, IV, V, and VI), which demonstrates the high diversity of the enzyme. Despite this ample divergence between enzyme groups, our 3D structure modeling showed both conservation and differences in amino acid residues participating in enzyme-substrate binding, which indicates the possibility that some enzymes are functionally specialized for the specific degradation of a substrate depending on the genetics of each species in the genus and the condition of the habitat where they evolved. The identification of alpha-glucuronidase isoenzymes would allow future use of genetic engineering and biocatalysis technologies aimed at specific production of the enzyme for its use in biotransformation.

Photocatalytic hydrogen evolution from biomass conversion

Biomass has incredible potential as an alternative to fossil fuels for energy production that is sustainable for the future of humanity. Hydrogen evolution from photocatalytic biomass conversion not only produces valuable carbon-free energy in the form of molecular hydrogen but also provides an avenue of production for industrially relevant biomass products. This photocatalytic conversion can be realized with efficient, sustainable reaction materials (biomass) and inexhaustible sunlight as the only energy inputs. Reported herein is a general strategy and mechanism for photocatalytic hydrogen evolution from biomass and biomass-derived substrates (including ethanol, glycerol, formic acid, glucose, and polysaccharides). Recent advancements in the synthesis and fundamental physical/mechanistic studies of novel photocatalysts for hydrogen evolution from biomass conversion are summarized. Also summarized are recent advancements in hydrogen evolution efficiency regarding biomass and biomass-derived substrates. Special emphasis is given to methods that utilize unprocessed biomass as a substrate or synthetic photocatalyst material, as the development of such will incur greater benefits towards a sustainable route for the evolution of hydrogen and production of chemical feedstocks.

Advances in Valorization of Lignocellulosic Biomass towards Energy Generation

The booming demand for energy across the world, especially for petroleum-based fuels, has led to the search for a long-term solution as a perfect source of sustainable energy. Lignocellulosic biomass resolves this obstacle as it is a readily available, inexpensive, and renewable fuel source that fulfills the criteria of sustainability. Valorization of lignocellulosic biomass and its components into value-added products maximizes the energy output and promotes the approach of lignocellulosic biorefinery. However, disruption of the recalcitrant structure of lignocellulosic biomass (LCB) via pretreatment technologies is costly and power-/heat-consuming. Therefore, devising an effective pretreatment method is a challenge. Likewise, the thermochemical and biological lignocellulosic conversion poses problems of efficiency, operational costs, and energy consumption. The advent of integrated technologies would probably resolve this problem. However, it is yet to be explored how to make it applicable at a commercial scale. This article will concisely review basic concepts of lignocellulosic composition and the routes opted by them to produce bioenergy. Moreover, it will also discuss the pros and cons of the pretreatment and conversion methods of lignocellulosic biomass. This critical analysis will bring to light the solutions for efficient and cost-effective conversion of lignocellulosic biomass that would pave the way for the development of sustainable energy systems.

Agro-Food Residues and Bioethanol Potential: A Study for a Specific Area

Bioethanol obtained from agro-food wastes could contribute to decrease the dependency on fossil resources, reduce the impact of fossil fuels on the environment, and mitigate the food versus fuel debate. This study is aimed to investigate the availability of residual inexpensive agro-food biomasses that could feed a second-generation bioethanol plant located in a specific area of North Eastern Italy. After the identification of all crops in the area, more than 40 agro-food residues were analyzed for their availability and compositions in terms of water, polysaccharides, and sugars potentially convertible into bioethanol. 574,166 Mg of residual wet lignocellulosic biomass corresponding to 297,325 Mg of dry material were found available for bioethanol conversion. The most promising substrates were wheat straw and vine shoots. Based on the chemical composition of residues, the potential attainable ethanol was determined. Theoretical potential ethanol production was estimated at nearly 72,000 Mg per year. This quantity extensively exceeds the minimum yearly capacity of a sustainable bioethanol plant previously identified as around 50,000 Mg of ethanol. Taken together, these results demonstrate that, in the analyzed area, agro-food residues are available in an amount that could sustain bioethanol production in a specific and restricted district. Techno-economical evaluations are in progress to assess the actual feasibility of installing a second generation bioethanol production plant in the area of interest.

Reduction of particulate matter and volatile organic compounds in biorefineries: A state-of-the-art review

A biorefinery is an efficient approach to generate multiple bio-products from biomass. With the increasing demand for bioenergy and bio-products, biorefineries are essential industrial platforms that provide needed demand while significantly reducing greenhouse gas emissions. A biorefinery consists of various conversion technologies where particulate matter (PM) and volatile organic compounds (VOCs) are emitted. The released PM and VOCs pose detrimental health and environmental risks for society. Moreover, the projected rise of global bioenergy demand may lead to an increase in PM and VOCs from biorefineries. With the use of cleaner technologies and approaches, PM and VOCs can be avoided in biorefineries. The study presents the landscape of the research field through a bibliometric review of emissions from a biorefinery. A comprehensive review of works on the reduction of PM and VOCs in a biorefinery is outlined. The study includes a perspective of cleaner technologies and approaches utilized in biorefineries to mitigate these hazardous materials. The results reveal that the employment of life cycle assessment, safety assessment, and green chemistry processes can significantly reduce PM and VOC emissions as well as the consumption of hazardous substances in the biorefinery.

A review on the environment-friendly emerging techniques for pretreatment of lignocellulosic biomass: Mechanistic insight and advancements

The process of pretreatment is considered as an indispensable unit operation in the field of lignocellulosic conversion. The traditional pretreatment operations of lignocellulosic biomass are observed as inefficient to meet the demand for an industrial adaptation. In view of that, numerous investigations are reported on various conventional pretreatment methods but very limited information's are available on the advanced technologies. The present review article provides an exclusive discussion on various emerging and environment-friendly pretreatment methods applied on a number of different feedstock materials. Further, an insight on the reaction mechanism involved with each of the technologies such as microwave, ultrasound, deep eutectic solvent, irradiation, and high force assisted pretreatment methods are elucidated for an effective valorization of lignocellulosic biomass. Hence, in a single article, the readers of this paper will get to know all important aspects of the emerging pretreatment techniques of lignocellulosic biomass including the advancements, and the mechanistic insight which will be highly beneficial towards the selection of an efficient pretreatment method for large scale of commercial implementation in a lignocellulosic biorefinery.

Food waste valorization: Energy production using novel integrated systems

Management of food waste (FW) is a global challenge due to increasing population and economic activities. Presently, landfill and incineration are the keyways of FW management, while economical and environmental sustainability have been an issue. Therefore, the biological processes have been investigated for resource and energy recovery from FW. However, these biological approaches have certain drawbacks and cannot be a complete solution for FW management. Therefore, this review aims to offer a detailed and complete analysis of current available technologies to achieve environmental and economical sustainability. In this context, zero solid waste discharge for resource and energy recovery has been put into view. Corresponding to which several innovative technologies using integrated biological methods for resource and energy recovery from FW have been elucidated.

Pulsed electric field: An emerging pretreatment technology in a biogas production

This review focuses on current status of pulsed electric field (PEF) technology and its implementation in biogas production. First, basic principles of PEF and a schematic overview of typical PEF processing system were provided. Thereafter, lab- and pilot-scale PEF pretreatments of sludge with subsequent anaerobic digestion (AD) were provided. Furthermore, PEF technology, as an emerging technology for the lignocellulose (LC) pretreatment in biogas production which is still predominantly used at lab-scale, was outlined. Eventually, conclusion together with future perspectives and challenges were outlined.

Transforming lignocellulosic biomass into biofuels enabled by ionic liquid pretreatment

Processes that can convert lignocellulosic biomass into biofuels and chemicals are particularly attractive considering renewability and minimal environmental impact. Ionic liquids (ILs) have been used as novel solvents in the process development in that they can effectively deconstruct recalcitrant lignocellulosic biomass for high sugar yield and lignin recovery. From cellulose-dissolving ILs to choline-based and protic acidic ILs, extensive research in this field has been done, driven by the promising future of IL pretreatment. Meanwhile, shortcomings and technological hurdles are ascertained during research and developments. It is necessary to present a general overview of recent developments and challenges in this field. In this review paper, three aspects of advances in IL pretreatment are critically analyzed: biocompatible ILs, protic acidic ILs and combinatory pretreatments.

Low-Viscosity Ether-Functionalized Ionic Liquids as Solvents for the Enhancement of Lignocellulosic Biomass Dissolution

Due to the substantial usage of fossil fuels, the utilization of lignocellulosic biomass as renewable sources for fuels and chemical production has been widely explored. The dissolution of lignocellulosic biomass in proper solvents is vital prior to the extraction of its important constituents, and ionic liquids (ILs) have been found to be efficient solvents for biomass dissolution. However, the high viscosity of ILs limits the dissolution process. Therefore, with the aim to enhance the dissolution of lignocellulosic biomass, a series of new ether-functionalized ILs with low viscosity values were synthesized and characterized. Their properties, such as density, viscosity and thermal stability, were analyzed and discussed in comparison with a common commercial IL, namely 1-butyl-3-methylimidazolium chloride (BMIMCl). The presence of the ether group in the new ILs reduces the viscosity of the ILs to some appreciable extent in comparison to BMIMCl. 1-2(methoxyethyl)-3-methylimidazolium chloride (MOE-MImCl), which possesses the lowest viscosity value among the other ether-functionalized ILs, demonstrates an ability to be a powerful solvent in the application of biomass dissolution via the sonication method. In addition, an optimization study employing response surface methodology (RSM) was carried out in order to obtain the optimum conditions for maximum dissolution of biomass in the solvents. Results suggested that the maximum biomass dissolution can be achieved by using 3 weight% of initial biomass loading with 40% amplitude of sonication at 32.23 min of sonication period.

Influence of Coniferous Wood Conditioning by Pulsed Electric Field on Its Combustion Heat Characteristics

The article presents the impact of sawdust conditioning from coniferous trees with a pulsed electric field (PEF) on the characteristics of the combustion heat. The study used four variants of electric field parameters, i.e., (I) 300 discharges with an electric field intensity of 25 kV·cm−1, (II) 150 discharges with an electric field intensity of 25 kV·cm−1, (III) 300 discharges with an electric field intensity of 30 kV·cm−1, (IV) 150 discharges with an electric field intensity of 30 kV·cm−1. It should be noted that the interval between successive discharges was 10 sec and it was constant for each variant of the experiment. A change in the characteristics of the heat of combustion was found under the influence of a pulsed electric field on sawdust from coniferous trees in each period of the measurement. The highest temperatures were achieved by the interaction of a pulsed electric field with a discharge pulse of 30 kV·cm−1 and the number of discharge cycles amounting to 300, whereas significantly lower temperature values were recorded with the discharge electric field intensity of 25 kV·cm−1. It was found that the longest times from the moment of ignition to reaching the maximum temperature were obtained for the material after the influence of PEF with an electric field intensity of 30 kV·cm−1 and the number of pulses 300 (IV combination), while the shortest time of the analyzed interval was stated for the first combination of the experiment (electric field intensity 25 kV·cm−1 and a number of pulses 150). The use of a pulsed electric field allows modeling the combustion time of a homogeneous material, which enables the improvement of the process control possibilities, and is also important information for the design of devices intended for biomass combustion.

Lignin valorization by bacterial genus Pseudomonas: State-of-the-art review and prospects

The most prominent aromatic feedstock on Earth is lignin, however, lignin valorization is still an underrated subject. The principal preparatory strategies for lignin valorization are fragmentation and depolymerization which help in the production of fuels and chemicals. Owing to lignin's structural heterogeneity, these strategies result in product generation which requires tedious separation and purification to extract target products. The bacterial genus Pseudomonas has been dominant for its lignin valorization potency, owing to a robust enzymatic machinery that is used to funnel variable lignin derivatives into certain target products such as polyhydroxyalkanotes (PHAs) and cis, cis-muconic acid (MA). In this review, the potential of genus Pseudomonas in lignin valorization is critically reviewed along with the advanced genetic techniques and tools to ease the use of lignin/lignin-model compounds for the synthesis of bioproducts. This review also highlights the research gaps in lignin biovalorization and discuss the challenges and possibilities for future research.

Hydrogen production from lignocellulosic hydrolysate in an up-scaled microbial electrolysis cell with stacked bio-electrodes

Hydrogen production from renewable resources via microbial electrolysis cells (MECs) is a promising approach for sustainable energy production. Yet high hydrogen yield from real feedstocks has not been demonstrated in up-scaled MECs. In this study, a 10-L single chamber MEC with a high electrode surface area to volume ratio (66 m²/m³) was constructed and electroactive cathodic biofilms were enriched for hydrogen evolution reaction. A high hydrogen yield of 91% was achieved using lignocellulosic hydrolysate with a hydrogen production rate of 0.71 L/L/D at an organic loading rate of 0.4 g/D. The anodic and cathodic microbial communities, with Enterococcus spp. as the known electroactive bacteria, were capable of achieving current densities of 13.7 A/m² and 16.5 A/m², respectively. A machine learning algorithm was used to investigate the correlation between community data and electrochemical performance, and the critical genera on determining current density were identified.

A comprehensive review on the framework to valorise lignocellulosic biomass as biorefinery feedstocks

An effective pretreatment is the first step to enhance the digestibility of lignocellulosic biomass - a source of renewable, eco-friendly and energy-dense materials - for biofuel and biochemical productions. This review aims to provide a comprehensive assessment on the advantages and disadvantages of lignocellulosic pretreatment techniques, which have been studied at the lab-, pilot- and full-scale levels. Biological pretreatment is environmentally friendly but time consuming (i.e. 15-40 days). Chemical pretreatment is effective in breaking down lignocellulose and increasing sugar yield (e.g. 4 to 10-fold improvement) but entails chemical cost and expensive reactors. Whereas the combination of physical and chemical (i.e. physicochemical) pretreatment is energy intensive (e.g. energy production can only compensate 80% of the input energy) despite offering good process efficiency (i.e. > 100% increase in product yield). Demonstrations of pretreatment techniques (e.g. acid, alkaline, and hydrothermal) in pilot-scale have reported 50-80% hemicellulose solubilisation and enhanced sugar yields. The feasibility of these pilot and full-scale plants has been supported by government subsidies to encourage biofuel consumption (e.g. tax credits and mandates). Due to the variability in their mechanisms and characteristics, no superior pretreatment has been identified. The main challenge lies in the capability to achieve a positive energy balance and great economic viability with minimal environmental impacts i.e. the energy or product output significantly surpasses the energy and monetary input. Enhancement of the current pretreatment techno-economic efficiency (e.g. higher product yield, chemical recycling, and by-products conversion to increase environmental sustainability) and the integration of pretreatment methods to effectively treat a range of biomass will be the steppingstone for commercial lignocellulosic biorefineries.

Cellulosic Ethanol: Improving Cost Efficiency by Coupling Semi-Continuous Fermentation and Simultaneous Saccharification Strategies

A novel approach to improve ethanol production from sugarcane bagasse is proposed. Biomass was pretreated with sodium hydroxide, sulfuric, oxalic, and maleic acids (1% w/v) at different temperatures (130–170 °C) and times (10–30 min). The pretreatment with NaOH at 160 °C for 20 min was found to be the most efficient for further enzymatic saccharification. A semi-continuous fermentation system coupled with a simultaneous saccharification and fermentation strategy was used, attaining fermented liquor every 24 h. The amount of enzymes needed for saccharification was optimized, as well as the production time and ethanol concentration. The process occurred with near to complete depletion of glucose, obtaining ethanol concentrations ranging from 8.36 to 10.79% (v/v). The whole system, at bench scale, showed stability over 30 days, and ease of management and control. This strategy may improve cost efficiency in the production of cellulosic ethanol at industrial scale.

Liquid Hot Water Pretreatment and Enzymatic Hydrolysis as a Valorization Route of Italian Green Pepper Waste to Delivery Free Sugars

In this work, liquid hot water pretreatment (autohydrolysis) was used to improve enzymatic hydrolysis of a commonly consumed vegetable waste in Spain, Italian green pepper, to finally produce fermentable sugars. Firstly, the effect of temperature and contact time on sugar recovery during pretreatment (in insoluble solid and liquid fraction) was studied in detail. Then, enzymatic hydrolysis using commercial cellulase was performed with the insoluble solid resulting from pretreatment. The objective was to compare results with and without pretreatment. The results showed that the pretreatment step was effective to facilitate the sugars release in enzymatic hydrolysis, increasing the global sugar yield. This was especially notable when pretreatment was carried out at 180 °C for 40 min for glucose yields. In these conditions a global glucose yield of 61.02% was obtained. In addition, very low concentrations of phenolic compounds (ranging from 69.12 to 82.24 mg/L) were found in the liquid fraction from enzymatic hydrolysis, decreasing the possibility of fermentation inhibition produced by these components. Results showed that Italian green pepper is an interesting feedstock to obtain free sugars and prevent the enormous quantity of this food waste discarded annually.

Impact of centrifugation treatment on enzymatic hydrolysis of cellulose and xylan in poplar fibers with high lignin content

Centrifugation is very common in the production and treatment of lignocellulose for applications like pretreatment for enzymatic hydrolysis, but it is not certain whether it affects applications of lignocellulose and almost no one realizes this problem. This study investigated the effects of centrifugation on the characteristics and enzymatic hydrolysis of poplar fibers with high lignin content. The results showed that centrifugation inhibited the enzymatic hydrolysis of fiber, but fiber characteristics and enzymatic digestibility fluctuated with increasing centrifugation time. Centrifugation for about 15 min had the least effect on fiber properties while centrifugation for 30 min had the least effect on enzymatic hydrolysis. The water retention value was closely related to the enzymatic digestibility, but the pore characteristics and crystallinity index could not reflect the enzymatic accessibility of the fiber. This article will provide useful references for the enzymatic hydrolysis of lignocellulose and other high-value applications to improve production efficiency furtherly.

State of the art of straw treatment technology: Challenges and solutions forward

Straw as an agricultural byproduct has been recognized as a potential resource. However, open-field straw burning is still the main mean in many regions of the world, which causes the wasting of resource and air pollution. Recently, many technologies have been developed for energy and resource recovery from straw, of which the biological approach has attracted growing interests because of its economically viable and eco-friendly nature. However, pretreatment of straw prior to biological processes is essential, and largely determines the process feasibility, economic viability and environmental sustainability. Thus, this review attempts to offer a critical and holistic analysis of current straw pretreatment technologies and management practices. Specifically, an integrated biological processes coupled with microbial degradation and enzymatic hydrolysis was proposed, and its potential benefits, limitations and challenges associated with future large-scale straw treatment were also elaborated, together with the perspectives and directions forward.

Thermogravimetric Analysis-Fourier Transform Infrared Spectroscopy Study on the Effect of Extraction Pretreatment on the Pyrolysis Properties of Eucalyptus Wood Waste

Eucalyptus wood is one of the important hardwood resources with attractive properties of rapid growth and good quality, which are widely used for the manufacture of wood-based boards, furniture, pulp and paper, and so on. In order to explore the potential of sawdust waste from the eucalyptus wood furniture factory as a bioenergy feedstock, its pyrolysis properties after different solvent extractions were examined using thermogravimetric analysis coupled with Fourier transform infrared spectrometry. The mass ratio of extractives in eucalyptus wood sawdust by benzene-alcohol, hot water, and sodium hydroxide solution was 4.25, 9.68, and 16.11%, respectively. After extraction, the thermal decomposition process of eucalyptus wood was promoted with a higher weight loss rate, lower activation energy, and lower residue content compared to the raw sample without pretreatment, and the promotion level was positively correlated to the strength of extracting solvent. CO₂, CO, CH₄, H₂O, acids, aldehydes, aromatics, ethers, and alcohols were identified as the important intermediates in pyrolysis vapors, which can be tuned by different extraction pretreatments. In terms of typical gas products, benzene-alcohol enhanced the release of carbon dioxide, and hot water enhanced the water generation from dehydration reactions and slightly increased the production of carbon monoxide, while sodium hydroxide promoted the formation of methane at the early stage under 280 °C and later stage over 460 °C during the pyrolysis of eucalyptus wood. It is believed that the extraction pretreatment can not only obtain the bioactive extractive products but also benefit the pyrolysis process by lowering the energy barrier and tuning the composition of pyrolysis products.

Theoretical modeling and experimental validation of hydrodynamic cavitation reactor with a Venturi tube for sugarcane bagasse pretreatment

A hydrodynamic cavitation reactor with a Venturi tube was modeled through a computational fluid dynamics approach in order to evaluate the influence of pressure ratio, diameter and length of the throat zone. A cavitation reactor equipped with a Venturi tube was built in accordance with the computational modeling results. Hydrodynamic cavitation assisted alkaline pretreatment was performed to evaluate the influence of NaOH concentration (1-5%), the weight to volume percentage of solid in liquid (1-5%) and the reaction time (20-60 min.) in the lignin removal. The response surface methodology was used to optimize pretreatment parameters for maximum lignin removal. The optimal condition was 4.90% of NaOH and a solid weight percentage in liquid of 2.03% in 58.33 min, resulting in a maximum removal of 56.01% of lignin. Hydrodynamic cavitation can be easy to employ, an efficient and promissory pretreatment tool.

Bioreactor production of 2,3-butanediol by Pantoea agglomerans using soybean hull acid hydrolysate as substrate

Production of 2,3-butanediol (2,3-BD) by Pantoea agglomerans strain BL1 was investigated using soybean hull hydrolysate as substrate in batch reactors. The cultivation media consisted of a mixture of xylose, arabinose, and glucose, obtained from the hemicellulosic fraction of the soybean hull biomass. We evaluated the influence of oxygen supply, pH control, and media supplementation on the growth kinetics of the microorganism and on 2,3-BD production. P. agglomerans BL1 was able to simultaneously metabolize all three monosaccharides present in the broth, with average conversions of 75% after 48 h of cultivation. The influence of aeration conditions employed demonstrated the mixed acid pathway of 2,3-BD formation by enterobacteria. Under fully aerated conditions (2 vvm of air), up to 14.02 g L-1 of 2.3-BD in 12 h of cultivation were produced, corresponding to yields of 0.53 g g-1 and a productivity of 1.17 g L-1 h-1, the best results achieved. These results suggest the production potential of 2,3-BD by P. agglomerans BL1, which has been recently isolated from an environmental consortium. The present work proposes a solution for the usage of the hemicellulosic fraction of agroindustry biomasses, carbohydrates whose utilization are not commonly addressed in bioprocess.

Chemoenzymatic conversion of Sorghum durra stalk into furoic acid by a sequential microwave-assisted solid acid conversion and immobilized whole-cells biocatalysis

In this study, chemoenzymatic conversion of Sorghum durra stalk (SDS) into furoic acid was developed by a sequential microwave-assisted solid acid conversion and immobilized whole-cells biocatalysis method. Dry dewaxed SDS (75 g/L) was catalyzed into furfural at 57.8% yield with heterogeneous Sn-argil (2.0 wt% dosage) in n-ethyl butyrate-H₂O (1:1, v:v) biphasic system using a microwave (600 W) for 10 min at 180 °C. In this biphasic media (pH 6.5), SDS-derived furfural (125.0 mM) was biologically oxidized to furoic acid by immobilized Brevibacterium lutescens cells harboring furfural-oxidizing activity at 30 °C, and furfural was wholly transformed to furoic acid within 24 h. Finally, the recovery and reuse of the Sn-argil catalyst and immobilized biocatalysts were conducted for synthesizing furoic acid from SDS in the biphasic system. This chemoenzymatic route can be attractive for furoic acid production.

By-Products in the Malting and Brewing Industries—Re-Usage Possibilities

Beer production includes the formation of different by-products such as wastewater, spent grains, spent hops, and yeast. In addition to these well-known by-products, it is necessary to mention germ/rootlets, which also remain after the malting process. Given that a huge amount of beer is produced annually worldwide, by-products are available in large quantities throughout the year. Spent grains, spent hops, and spent yeasts are high-energy raw materials that possess a great potential for application in the branch of biotechnology, and the food industry, but these by-products are commonly used as livestock feed, disposed of in the fields, or incinerated. Breweries by-products can be utilized for microalgae production, biofuel production, extraction of proteins, polyphenolic, antioxidative substances, etc. This paper aims to address each of these by-products with an emphasis on their possible application in biotechnology and other industries.