Anaerobic digestion of lignocellulosic biomasses pretreated with Ceriporiopsis subvermispora

Pre-treatment with Trichoderma viride: Towards a better understanding of its consequences for anaerobic digestion

Understanding and optimising biological pre-treatment strategies for enhanced bio-methane production is a central aspect in second-generation biofuel research. In this regard, the application of fungi for pre-treatment seems highly promising; however, understanding the mode of action is crucial. Here, we show how aerobic pre-treatment of crystalline cellulose with the cellulolytic Trichoderma viride affects substrate degradability during mesophilic, anaerobic digestion. It could be demonstrated that fungal pre-treatment resulted in a slightly reduced substrate mass. Nevertheless, no significant impact on the overall methane yield was found during batch fermentation. Short chain organic acids accumulation, thus, overall degradation dynamics including methane production kinetics were affected by the pre-treatment as shown by Gompertz modelling. Finally, 16S rRNA amplicon sequencing followed by ANCOM-BC resulted in up to 53 operative taxonomic units including fermentative, syntrophic and methanogenic taxa, whereby their relative abundances were significantly affected by fungal pre-treatment depending on the duration of the pre-treatment. The results demonstrated the impact of soft rot fungal pre-treatment of cellulose on subsequent anaerobic cellulose hydrolysis as well as on methanogenic activity. To the best of our knowledge, this is the first study to investigate the direct causal effects of pre-treatment with T. viride on basic but crucial anaerobic digestion parameters in a highly standardised approach.

Fungal Ligninolytic Enzymes and Their Application in Biomass Lignin Pretreatment

Lignocellulosic biomass is a significant source of sustainable fuel and high-value chemical production. However, due to the complex cross-linked three-dimensional network structure, lignin is highly rigid to degradation. In natural environments, the degradation is performed by wood-rotting fungi. The process is slow, and thus, the use of lignin degradation by fungi has not been regarded as a feasible technology in the industrial lignocellulose treatment. Fungi produce a wide variety of ligninolytic enzymes that can be directly introduced in industrial processing of lignocellulose. Within this study, screening of ligninolytic enzyme production using decolorization of ABTS and Azure B dyes was performed for 10 fungal strains with potentially high enzyme production abilities. In addition to standard screening methods, media containing lignin and hay biomass as carbon sources were used to determine the change in enzyme production depending on the substrate. All selected fungi demonstrated the ability to adapt to a carbon source limitation; however, four strains indicated the ability to secrete ligninolytic enzymes in all experimental conditions-Irpex lacteus, Pleurotus dryinus, Bjerkandera adusta, and Trametes versicolor-respectively displayed a 100%, 82.7%, 82.7%, and 55% oxidation of ABTS on lignin-containing media and 100%, 87.9%, 78%, and 70% oxidation of ABTS on hay-containing media after 168 h of incubation. As a result, the most potent strains of fungi were selected to produce lignocellulose-degrading enzymes and to demonstrate their potential application in biological lignocellulose pretreatment.

Dual strategy for bioconversion of elephant grass biomass into fermentable sugars using Trichoderma reesei towards bioethanol production

In this study, biodelignification and enzymatic hydrolysis of elephant grass were performed by recombinant and native strain of Trichoderma reesei, respectively. Initially, rT. reesei displaying Lip8H and MnP1 gene was used for biodelignification with NiO nanoparticles. Saccharification was performed by combining hydrolytic enzyme produced with NiO nanoparticles. Elephant grass hydrolysate was used for bioethanol production using Kluyveromyces marxianus. Maximum lignolytic enzyme production was obtained with 15 µg/L of NiO nanoparticles and initial pH of 5 at 32 °C. Subsequently, about 54% of lignin degradation was achieved after 192 h. Hydrolytic enzymes showed elevated enzyme activity and resulted in 84.52 ± 3.5 g/L of total reducing sugar at 15 µg/mL NiO NPs. About 14.65 ± 1.75 g/L of ethanol was produced using K. marxianus after 24 h. Thus, dual strategy employed for conversion of elephant grass biomass into fermentable sugar and subsequent biofuel production could become potential platform for commercialization.

Multiple hydrolyses of rice straw by domesticated paddy soil microbes for methane production via liquid anaerobic digestion

The aim of this study was to investigate the hydrolysis of rice straw (RS) using domesticated paddy soil microbes (DPSMs) with swine wastewater (SW) as the nitrogen source and the multiple hydrolyses for CH₄ production via liquid anaerobic digestion (L-AD). Three hydrolyses of RS with a 45% inoculation ratio (IR) under the conditions of a carbon/nitrogen ratio (C/N ratio) of 40, temperature of 37 °C, inoculum/substrate ratio (I/S ratio) of 2:1, and immersion depth of 6.0 cm were optimal, attaining maximum volatile fatty acids (VFAs) after five days, possibly owing to the synergistic effect of aerobic microbes (Firmicutes and Actinomycetes) and anaerobic microbes (Bacteroidetes and Acidobacteria). After three hydrolyses, the degradation rates of hemicellulose, cellulose, and lignin in RS were 88.45%, 83.19% and 70.09%, respectively. The accumulative CH₄ production reached 462.11 mL/g VS after three hydrolyses, and its curve fitted well with the modified Gompertz model (R² > 0.984).

Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod

The production of biofuels (biogas, ethanol, methanol, biodiesel, and solid fuels, etc.), beginning with cocoa pod husk (CPH), is a way for obtaining a final product from the use of the principal waste product of the cocoa industry. However, there are limitations to the bioconversion of the material due to its structural components (cellulose, hemicellulose, and lignin). Currently, CPH pretreatment methods are considered a good approach towards the improvement of both the degradation process and the production of biogas or ethanol. The present document aims to set out the different methods for pretreating lignocellulosic material, which are: physical (grinding and extrusion, among others); chemical (acids and alkaline); thermochemical (pyrolysis); ionic liquid (salts); and biological (microorganism) to improve biofuel production. The use of CPH as a substrate in bioconversion processes is a viable and promising option, despite the limitations of each pretreatment method.

Sequential fungal pretreatment of unsterilized Miscanthus: changes in composition, cellulose digestibility and microbial communities

A sequential fungal pretreatment of Miscanthus × giganteus was conducted by mixing unsterilized Miscanthus with material previously colonized with the white-rot fungus Ceriporiopsis subvermispora. For three generations, each generation started with inoculation by mixing unsterilized fresh Miscanthus with end material from the previous generation and ended after 28 days of incubation at 28 °C. After the first generation, the cellulose digestibility of the material doubled, compared to that of the unsterilized Miscanthus, but the second and third generations showed no enhancements in cellulose digestibility. Furthermore, high degradation of Miscanthus structural carbohydrates occurred during the first generation. A microbial community study showed that, even though the fungal community of the material previously colonized by C. subvermispora was composed mainly of this fungus (> 99%), by the first generation its relative abundance was down to only 9%, and other microbes had prevailed. Additionally, changes in the bacterial community occurred that might be associated with unwanted cellulose degradation in the system. This reiterates the necessity of feedstock microbial load reduction for the stability and reproducibility of fungal pretreatment of lignocellulosic biomass. KEY POINTS: • Sequential fungal pretreatment of unsterilized Miscanthus was unsuccessful. • Feedstock changes with white-rot fungi favored the growth of other microorganisms. • Feedstock microbial reduction is necessary for pretreatment with C. subvermispora.

Microbe assisted depolymerization of lignin rich waste and its conversion to gaseous biofuel

Biomethanation potential of lignin rich residue (LRR) obtained from lignocellulosic ethanol fermentation was evaluated after subjecting to microbe assisted pretreatment using selectively enriched lignin depolymerizing consortia (LDC). The efficiency of LDC in lignin depolymerization was established using alkali and dealkali lignins (AL and DL) along with LRR as feedstocks. Microbial growth on media having lignin as sole carbon source, activity of lignin depolymerizing enzymes, viz., lignin peroxidase and laccase, ability of culture to decolorize the lignin mimicking dyes like methylene blue and ramezol brilliant blue, were considered to confirm the efficiency of enriched mixed culture. Microbial treatment using LDC showed significant positive impact on lignin breakdown irrespective of the substrate (LRR, 46.33%; AL, 31.37%; DL, 34.20%). The hydrolysate of LRR obtained from microbial pretreatment showed higher biogas yield (424 ml/g VS) owing to the efficiency of lignin depolymerization and availability of readily available biodegradable components in residual lignin from previous processing. Depolymerization of commercial lignins also produced a good amount of biogas (302-324 ml/g VS) after pretreatment with LDC. Overall, an additional energy conversion efficiency of about 11.75 kJ/g VS was obtained by valorizing the residual lignin through integrating biomethanation technology to ethanol fermentation. Outcome of this study indicated the feasibility of using lignin rich residue generated from the second generation cellulosic bioethanol plants as a potential feedstock to meet the current gaseous fuel demands. This integration also helps in closing the biomass based biorefinery loop and also promotes the circular economy.

Anaerobic digestion of corn straw pretreated by ultrasonic combined with aerobic hydrolysis

Corn straw (CS) was pretreated by ultrasonic combined aerobic with biogas slurry as medium for anaerobic digestion (AD), that strengthened the degradation efficiency CS, varied in the composition of digestion slurry, thereby the methane production was increased. Central combinatorial design (CCD) test was used to treat CS at ultrasonic power (200, 400, and 600 W), time (10, 20, and 30 min) and AD for 25 days, at 37 ± 1℃. According to data showed that the pH and volatile fatty acids (VFAs) affected methane production directly. With an ultrasonic power 309 W, time 26 min, it reached the maximum content of VFAs with 16.24 g/L, the cumulative methane production achieved the highest with 198.56 mL/g VS, which was 46.73% higher than unpretreated raw material as CK. Ultrasonic-aerobic hydrolysis pretreatment can obtain higher VFAs and methane production content in a short period of time that is great significance to biogas engineering.

Improvement of empty palm fruit bunches biodegradability and biogas production by integrating the straw mushroom cultivation as a pretreatment in the solid-state anaerobic digestion

Empty fruit bunches (EFB) have low biodegradability and restrict their commercial utilization in biogas plants. Integration of straw mushroom (Volvariella volvacea) cultivation as a function of bio-pretreatment on EFB to improve biodegradability and methane production by solid-state anaerobic digestion (SS-AD) was investigated. The mushroom yield was 47.3 kg·tonne^-1 EFB with remaining weight in spent mushroom-EFB (S-mEFB) of 82%. The cellulose, hemicellulose, and lignin of EFB were degraded by 3.3%, 21.3%, and 17.6%, respectively, with an increased surface area of S-mEFB. The biodegradability of S-mEFB (62.7%) was 2 times higher than raw EFB (33.5%) with the highest methane yield and production of 281 mL CH₄·g^-1 VS and 50.6 m³·tonne^-1 S-mEFB, respectively. The co-digestion of S-mEFB with 5% v/w POME had highest methane yield of 405 mL CH₄·g^-1 VS with biodegradability of 90.8%. Integrating straw mushroom cultivation with SS-AD is a promising strategy for achieving an environmentally friendly and economically feasible process.

A novel enrichment approach for anaerobic digestion of lignocellulosic biomass: Process performance enhancement through an inoculum habitat selection

Inocula enrichment was performed using an innovative habitat-based selection approach to improve wheat straw (WS) anaerobic digestion (AD) efficiency. The procedure was carried out by sequentially re-inoculating the primary microbial community seven times in subsequent anaerobic reactors containing untreated WS. Re-inocula were performed at different re-inoculum times (24, 48, and 96 h) by moving a porous support mimicking a rumen structure from one batch to the next (S-tests) or re-inoculating only the culture medium (C-tests). Highest H₂ production yields were observed after four and five re-inocula (0.08 ± 0.02 NmL h^-1 gVS^-1 and 0.09 ± 0.02 NmL h^-1 gVS^-1) for S-24 and S-48, respectively. For S-96, higher CH₄ yields were observed after the start-up test and sixth re-inoculum (0.05 ± 0.003 NmL h^-1 gVS^-1 and 0.04 ± 0.005 NmL h^-1 gVS^-1, respectively). Accordingly, S-96 showed the highest active Archaea component (7%). C-test microbial communities were dominated by fermenting, hydrogen-producing bacteria and showed lower microbial community diversity than S-tests.

Microorganisms and Enzymes Used in the Biological Pretreatment of the Substrate to Enhance Biogas Production: A Review

The pretreatment of lignocellulosic biomass (LC biomass) prior to the anaerobic digestion (AD) process is a mandatory step to improve feedstock biodegradability and biogas production. An important potential is provided by lignocellulosic materials since lignocellulose represents a major source for biogas production, thus contributing to the environmental sustainability. The main limitation of LC biomass for use is its resistant structure. Lately, biological pretreatment (BP) gained popularity because they are eco-friendly methods that do not require chemical or energy input. A large number of bacteria and fungi possess great ability to convert high molecular weight compounds from the substrate into lower mass compounds due to the synthesis of microbial extracellular enzymes. Microbial strains isolated from various sources are used singly or in combination to break down the recalcitrant polymeric structures and thus increase biogasgeneration. Enzymatic treatment of LC biomass depends mainly on enzymes like hemicellulases and cellulases generated by microorganisms. The articles main purpose is to provide an overview regarding the enzymatic/biological pretreatment as one of the most potent techniques for enhancing biogas production.

Evaluation of methane production and energy conversion from corn stalk using furfural wastewater pretreatment for whole slurry anaerobic co-digestion

In this study, corn stalk (CS) was pretreated with furfural wastewater (FWW) for whole slurry anaerobic digestion (AD), which increased the degradability of CS components, changed the parameters in pretreatment slurry and improved the biochemical methane potential (BMP). The ultimate goal was to optimize the time and temperature for FWW pretreatment and evaluate whether FWW pretreatment is feasible from BMP and energy conversion. The results of path analysis suggested that lignocellulosic degradability (LD) was the main factor affecting methane production with the comprehensive decision of 0.7006. The highest BMP (166.34 mL/g VS) was achieved by the pretreatment at 35 °C for 6 days, which was 70.36% higher than that of control check (CK) (97.64 mL/g VS) and the optimal pretreatment condition was predicted at 40.69 °C for 6.49 days by response surface methodology (RSM). The net residual value (NRV) for the pretreatment of 35 °C and 6 days was the highest (0.6201), which was the most appropriate condition for AD in real application.

Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H₂S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

Insight into Pretreatment Methods of Lignocellulosic Biomass to Increase Biogas Yield: Current State, Challenges, and Opportunities

Lignocellulosic biomass is recalcitrant due to its heterogeneous structure, which is one of the major limitations for its use as a feedstock for methane production. Although different pretreatment methods are being used, intermediaries formed are known to show adverse effect on microorganisms involved in methane formation. This review, apart from highlighting the efficiency and limitations of the different pretreatment methods from engineering, chemical, and biochemical point of views, will discuss the strategies to increase the carbon recovery in the form of methane by way of amending pretreatments to lower inhibitory effects on microbial groups and by optimizing process conditions.

Furfural wastewater pretreatment of corn stalk for whole slurry anaerobic co-digestion to improve methane production

Previous studies showed that excellent anaerobic digestion performance could be achieved using acid pretreatment, whereas the development of acid pretreatment was limited by high cost of acid consumption and severe operation. The aim of this study consisted in expanding the possibilities of low-cost acid pretreatment method for anaerobic digestion. For this, the feasibility of substituting conventional acid pretreatment with furfural wastewater was verified, and the whole slurry anaerobic digestion was performed to improve the production of methane. The furfural wastewater was used to pretreat crop stalk at different ambient temperatures (20, 35, 50°C) for different time periods (0, 3, 6, 9days). Subsequently, all treated and untreated crop stalk were digested at 35°C for 25days. According to experimental data showed that the dissimilar degradability of compositions for crop stalk was due to furfural wastewater pretreatment, and the reducing sugar content, volatile fatty acid content, pH during pretreatment phase, and their initial maximum & minimum values in anaerobic digestion phase were changed, which made a significant difference in methane production. The highest total methane production of anaerobic digestion (196.68mL/g VS) was achieved by the treatment at 35°C for 6days, which was 59.28% higher than untreated crop stalk (123.48mL/g VS). On the whole, the results showed that furfural wastewater pretreatment followed by the whole slurry anaerobic co-digestion was feasible and could contribute to application value for anaerobic digestion industry while providing an effective way for the treatment of furfural wastewater.

Modification of corn stover for improving biodegradability and anaerobic digestion performance by Ceriporiopsis subvermispora

Ceriporiopsis subvermispora was used to modify corn stover for improving the biodegradability and biomethane yield. Corn stover was incubated with C. subvermispora for 5-90 days then anaerobically digested. It was found that the corn stover modified for 15 days achieved the highest biomethane yield of 235 mL·g^-1 VS, which was an increase of 15.2% over that of the non-modified one. The mechanism analyses indicated that the improvement resulted from the combined roles of degradation selectivity, destruction of lignocellulosic structures, and linkages. The analyses showed that C. subvermispora has a high relative selectivity of lignin degradation. The structure of the lignin and the linkages among lignin and hemicellulose and cellulose were broken obviously by acetyl group removal, and the enzymatic hydrolysis of cellulose was increased by 35.61%. The finding indicated that C. subvermispora modification is one of the effective methods for enhancing biomethane yield of corn stover.

Co-pretreatment of wheat straw by potassium hydroxide and calcium hydroxide: Methane production, economics, and energy potential analysis

To improve the methane production of wheat straw (WS), the mono-pretreatment (MP) and co-pretreatment (CP) of WS with KOH and Ca(OH)₂ were conducted in this study. The results showed that the MP with KOH presented better effects than the MP with Ca(OH)₂. However, the CP with 2% KOH combined with 1% Ca(OH)₂ displayed similar effects to those of the MP with 3% KOH, obtaining the cumulative methane yield of 239.8 mL g_VS^-1 and an improved biodegradability from 56.37% of raw WS to 66.10%. Methane production and kinetic analyses suggested that 2% KOH combined with 1% Ca(OH)₂ was the ideal condition of alkaline pretreatment for anaerobic digestion of WS. The mechanism for the improvement in methane production was clearly described by biochemical component, scanning electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy analyses. Moreover, preliminary economics and energy potential analyses also confirmed that alkaline co-pretreatment was a reasonable method, which not only gave important guidance for future utilization of WS waste but also showed useful reference for the efficient pretreatment of other lignocellulosic wastes.

Anaerobic co-digestion of food waste/excess sludge: substrates - products transformation and role of NADH as an indicator

The process of anaerobic co-digestion is vital importance to resource recovery from organic solid wastes such as food waste and municipal sludge. However, its application is hindered by the limited understanding on the complex substrates-products transformation reactions and mechanisms therein. In this study, food waste (FW) and excess sludge (ES) from municipal wastewater treatment were mixed at various ratios (ES/FW 5:0, 4:1, 2:1, 1:1, 1:2, 1:4, w/w), and the co-digestion process was studied in a batch test. The consumption of substrates including soluble proteins and carbohydrates, the variation in the intermediates such as various volatile fatty acids, and the production of hydrogen and methane gases were monitored. The results suggested that 4:1 was likely the optimal ratio where substrates were consumed and biogas generated efficiently, whereas 1:2 and 1:4 caused severe inhibition. Fermentation of ES alone produced mainly acetic and propionic acid, while the addition of FW led to butyric acid type fermentation. Intermediates in the fermentation liquid were tentatively identified, and the levels of NADH quantified using 3D-excitation/emission fluorescence spectrometry. One class of the intermediates, tryptophan-like proteins were correlated to the butyric acid accumulation in ES/FW mixtures, and NADH level was proposed as an indicator of VFAs production activities.

Solid-state anaerobic digestion of wheat straw: Impact of S/I ratio and pilot-scale fungal pretreatment

Solid State Anaerobic Digestion (SSAD) of fungal pretreated wheat straw was evaluated in a leach bed reactor. During a first experiment, the effect of Substrate/Inoculum (S/I) ratios on the start-up phase was investigated. High S/I increased methane productivity but also raised the risk of reactor failure due to Volatile Fatty Acid (VFA) accumulation. With S/I ratios between 1.2 and 3.6 (Volatile Solid (VS) basis), the SSAD start-up using wheat straw was successful. Moreover, reactors were able to recover from acidification when the Total VFA/alkalinity ratio was lower than 2 gHAc_eq/gCaCO₃, with VFA concentrations lower than 10 g/L and a pH close to 5.5. The conventional threshold of 0.6 gHAc_eq/gCaCO₃ for stable wet AD is therefore not adapted to SSAD. During a second experiment, after the wheat straw was submitted to a fungal pretreatment in a non-sterile pilot-scale reactor, it was digested with an S/I ratio of 2.8-2.9. Under batch SSAD conditions, the biodegradability of pretreated wheat straw was slightly improved in comparison to the control (254 versus 215 NmL/g VS, respectively). Considering mass losses occurring during the pretreatment step, suboptimal pretreatment conditions caused a slightly lower methane production (161 versus 171 NmL/gTS_initial after 60-days anaerobic digestion). Nevertheless, pretreatment improved the start-up phase with lower acidification relative to controls. It would be particularly beneficial to improve the methane production in reactors with short reaction times.

Improvement of ethanol and biogas production from sugarcane bagasse using sodium alkaline pretreatments

Sugarcane bagasse was pretreated with sodium carbonate, sodium sulfite, and sodium acetate in concentrations of 0.5 M and 0.25 M, as well as hydrothermal pretreatment, to break down its structural recalcitrance and improve biogas and ethanol production. The pretreatments were conducted at 100, 140, and 180 °C for 1 h. The highest biogas and ethanol production was observed for sugarcane bagasse pretreated with 0.5 M sodium carbonate solution at 140 °C, which was 239 ± 20 Nml CH₄/g VS, and 7.27 ± 0.70 g/l, respectively, containing gasoline equivalents of 164.2 ± 14.3 l/ton of raw bagasse and 147.8 ± 14.2 l/ton of raw bagasse, respectively. The highest gasoline equivalent was obtained for biogas production from the substrate pretreated with 0.5 M sodium sulfite solution at 100 °C (190.2 ± 2.1 l/ton of raw bagasse). In comparison to sodium carbonate and sodium sulfite, sodium acetate had less effect on biofuel production and was comparable with hydrothermal pretreatment. In contradiction to sodium acetate pretreated bagasse, in which increased pretreatment temperature intensified biofuel production, a reduction of biofuel production was observed for sodium carbonate and sodium sulfite pretreatment when temperature was increased from 140 to 180 °C. Besides considerable amounts of biofuel production at the best conditions obtained, over 762 and 543 kilotons of equivalent CO₂ can be reduced annually in Iran by biogas and ethanol production from sugarcane, respectively.

Emerging technologies for the pretreatment of lignocellulosic biomass

Pretreatment of lignocellulosic biomass to overcome its intrinsic recalcitrant nature prior to the production of valuable chemicals has been studied for nearly 200 years. Research has targeted eco-friendly, economical and time-effective solutions, together with a simplified large-scale operational approach. Commonly used pretreatment methods, such as chemical, physico-chemical and biological techniques are still insufficient to meet optimal industrial production requirements in a sustainable way. Recently, advances in applied chemistry approaches conducted under extreme and non-classical conditions has led to possible commercial solutions in the marketplace (e.g. High hydrostatic pressure, High pressure homogenizer, Microwave, Ultrasound technologies). These new industrial technologies are promising candidates as sustainable green pretreatment solutions for lignocellulosic biomass utilization in a large scale biorefinery. This article reviews the application of selected emerging technologies such as ionizing and non-ionizing radiation, pulsed electrical field, ultrasound and high pressure as promising technologies in the valorization of lignocellulosic biomass.

Biological Pretreatment Strategies for Second-Generation Lignocellulosic Resources to Enhance Biogas Production

With regard to social and environmental sustainability, second-generation biofuel and biogas production from lignocellulosic material provides considerable potential, since lignocellulose represents an inexhaustible, ubiquitous natural resource, and is therefore one important step towards independence from fossil fuel combustion. However, the highly heterogeneous structure and recalcitrant nature of lignocellulose restricts its commercial utilization in biogas plants. Improvements therefore rely on effective pretreatment methods to overcome structural impediments, thus facilitating the accessibility and digestibility of (ligno)cellulosic substrates during anaerobic digestion. While chemical and physical pretreatment strategies exhibit inherent drawbacks including the formation of inhibitory products, biological pretreatment is increasingly being advocated as an environmentally friendly process with low energy input, low disposal costs, and milder operating conditions. Nevertheless, the promising potential of biological pretreatment techniques is not yet fully exploited. Hence, we intended to provide a detailed insight into currently applied pretreatment techniques, with a special focus on biological ones for downstream processing of lignocellulosic biomass in anaerobic digestion.

Corn silage fungal-based solid-state pretreatment for enhanced biogas production in anaerobic co-digestion with cow manure

The objective of this research was to use white-rot fungus Trametes versicolor for corn silage pretreatment and to investigate the effect of pretreatment on biogas productivity. Semi-continuous pilot-scale experiment, comprised of two experimental phases, was carried out. In the first phase, operational conditions of the full-scale biogas plant were reproduced at pilot-scale. In that phase, the reactor was daily fed with the mixture of cow manure, digestate from industrial postfermentor, corn grits and ensiled corn silage, and the average methane generation rate was 0.167 m³_CH4 kg_VS^-1. In the second phase, corn grits and ensiled corn silage were replaced with corn silage pretreated with T. versicolor, and the average methane generation rate increased up to 0.236 m³_CH4 kg_VS^-1. The results of this study suggest that application of fungal-based solid-state pretreated corn silage has positive effect on pH stability and increase the biogas productivity.

Mechanical and Alkaline Hydrothermal Treated Corn Residue Conversion in to Bioenergy and Biofertilizer: A Resource Recovery Concept

In this research fall time harvested corn residue (CR) was first mechanically pretreated to produce 5 mm chopped and <500 µm ground particles, which underwent an anaerobic digestion (AD) process to produce biomethane and biofertilizer. Another sample of CR was pretreated by an alkaline hydrothermal (HT) process using 1%, 2% and 3% NaOH to produce solid biocarbon and the resulting alkaline hydrothermal process water (AHTPW), a co-product of biocarbon, underwent fast digestion under AD conditions to produce biomethane and biofertilizer. A predetermined HT process of 240 °C for 30 min was considered and the effect of alkali content on the HT process for biocarbon and biomethane product a rate of 8.21 MJ kg−1 and 9.23 MJ kg−1 of raw CR, respectively. Among the three selected alkaline HT processes, the 1% NaOH HT process produced the highest hybrid bioenergy of 11.39 MJ kg−1 of raw CR with an overall energy recovery of 62.82% of raw CR. The AHTPW of 2% and 3% NaOH HT-treated CR did not produce considerable amount of biomethane and their biocarbons contained 3.44 MJ kg−1 and 3.27 MJ kg−1 of raw CR of bioenergy, respectively. The biomethane produced from 5 mm chopped CR, <500 µm ground CR and 1% alkaline AHTPW for 30 days retention time were of 275.38 L kg−1 volatile solid (VS), 309.59 L kg−1 VS and 278.70 L kg−1 VS, respectively, compared to non-treated CR of 144–187 L kg−1 VS. Nutrient enriched AD digestate is useable as liquid fertilizer. Biocarbon, biomethane and biofertilizer produced from the 1% alkaline HT process at 240 °C for 30 min can reduce the greenhouse gas (GHG) emissions of Ontario.

Lignocellulosic waste valorisation strategy through enzyme and biogas production

Lignocellulosic wastes are generally pre-treated to facilitate the hydrolysis stage during the anaerobic digestion process. A process consisting of solid state fermentation carried out by white rot fungi and anaerobic digestion was evaluated on corn stover to produce ligninolytic enzymes and biogas. The enzyme production was quantified every 3d for a month at 30°C, and three fungal strains and two particle sizes of waste were compared. Of the main outcomes, Pleurotus eryngii produced the highest laccase enzyme activity compared with Pleurotus ostreatus and Trametes versicolor. Furthermore, this activity was improved by 16% when copper was used as an enzyme inducer. On the other hand, most of the conditions studied showed a decrease in maximum biogas production compared with untreated waste, the addition of copper decreased biogas production by 20%. Despite the above, Pleurotus eryngii showed promising results allowing a 19% increase of biogas production and high enzyme production values.