Effect of different steam explosion conditions on methane potential and enzymatic saccharification of birch

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Biogas Production from Steam-Exploded Maize Stover: Results from Continuous Anaerobic Tank Bioreactor Tests

Steam explosion pretreatment of lignocellulosic biomass presents a promising technology for agricultural residues before anaerobic degradation. This study aimed to assess biogas production in continuously stirred tank reactors using steam-exploded maize stover mono-digestion. The continuous digestion tests were carried out in four fermenters with a capacity of 150 L under mesophilic and thermophilic conditions. Maize stover was pretreated at 173 °C for 15 min. Four different organic loading rates (OLR) were tested, the biogas and methane production rate was monitored, and parameters such as dry matter (DM), volatile solids (VS), pH, and C:N were analyzed. The results of the tests showed that using steam-exploded maize stover in a continuous system over the range of an OLR from 1.0 to 3.5 kg VS m–3 d–1 is feasible with nitrogen as an additive only. The maximum methane yield, 637 LN m–3 d–1, was measured under thermophilic conditions with an OLR of 3.5 kg VS m–3 d–1. The trend of an increased gas production rate with an increasing OLR was observed over the range of the applied OLRs, although the average gas yield in the thermophilic mode was higher than it was in the mesophilic one.

Stimulating biogas production from steam-exploded birch wood using Fenton reaction and fungal pretreatment

Delignification of steam-exploded birch wood (SEBW) was stimulated using a pretreatment method including Fenton reaction (FR) and fungi. SEBW was employed as a substrate to optimize the Fe(III) and Fe(II) dosage in FR. Maximum iron-binding to SEBW was obtained at pH 3.5. FR pretreatment increased biological methane yields from 257 mL/g vS in control to 383 and 352 mL/ g vS in samples with 0.5 mM Fe(II) and 1.0 mM Fe(III), respectively. Further enzymatic pretreatment using a commercial cellulase cocktail clearly improved methane production rate but only increased the final methane yields by 2-9 %. Finally, pretreatments with the fungi Pleurotus ostreatus (PO) and Lentinula edodes (LE), alone or in combination with FR, were carried out. SEBW pretreated with only LE and samples pretreated with PO and1 mM Fe(III) + H₂O₂ increased the methane production yield to 420 and 419 mL/g vS respectively. These pretreatments delignified SEBW up to 25 %.

Recent Advances in the Bioconversion of Waste Straw Biomass with Steam Explosion Technique: A Comprehensive Review

Waste straw biomass is an abundant renewable bioresource raw material on Earth. Its stubborn wooden cellulose structure limits straw lignocellulose bioconversion into value-added products (e.g., biofuel, chemicals, and agricultural products). Compared to physicochemical and other preprocessing techniques, the steam explosion method, as a kind of hydrothermal method, was considered as a practical, eco-friendly, and cost-effective method to overcome the above-mentioned barriers during straw lignocellulose bioconversion. Steam explosion pretreatment of straw lignocellulose can effectively improve the conversion efficiency of producing biofuels and value-added chemicals and is expected to replace fossil fuels and partially replace traditional chemical fertilizers. Although the principles of steam explosion destruction of lignocellulosic structures for bioconversion to liquid fuels and producing solid biofuel were well known, applications of steam explosion in productions of value-added chemicals, organic fertilizers, biogas, etc. were less identified. Therefore, this review provides insights into advanced methods of utilizing steam explosion for straw biomass conversion as well as their corresponding processes and mechanisms. Finally, the current limitations and prospects of straw biomass conversion with steam explosion technology were elucidated.

Biotechnological interventions in food waste treatment for obtaining value-added compounds to combat pollution

Over the last few decades, the globe is facing tremendous effects due to the unnecessary piling of municipal solid waste among which food waste holds a greater portion. This practice not only affects the environment in terms of generating greenhouse gas emissions but when left dumped in landfills will also trigger poverty and malnutrition. This review focuses on the global trend in food waste management strategies involved in the effective utilization of food waste to produce various value-added products in a microbiology aspect, thereby diminishing the negative impacts caused by the unnecessary side effects of non-renewable energy sources. The review also detailed the efficiency of microorganisms in the production of various bio-energies as well. Further, recent attempts to the exploitation of genetically modified microorganisms in producing value-added products were enlisted. This also attempted to address food waste valorization techniques, the combined applications of various processes for an enhanced yield of different compounds, and addressed various challenges. Further, the current challenges involved in various processes and the effective measures to tackle them in the future have been addressed. Thus, the present review has successfully addressed the circular bio-economy in food waste valorization.

Residual Forest Biomass in Pinus Stands: Accumulation and Biogas Production Potential

Lignocellulosic biomass is an abundant resource that can be valorized for the production of bioenergy. However, studies aiming to quantify the amount of biogas production potential per km forest road are scarce in the literature. In this study, fresh pine needles, pine needle litter, pine branches, and pine bark were digested in batch reactors under mesophilic conditions after a grinding/milling pre-treatment. All samples were collected from a low-altitude Mediterranean Pinus forest (North Greece) adjacent to a category G forest road with a gentle slope. The methane yield of fresh pine needles was between 115 and 164 NmL g−1 volatile solids (VS), depending on the Pinus tree size. Pine needle litter produced a significantly lower methane yield (between 58 and 77 NmL g−1 VS), followed by pine bark (85 NmL g−1 VS) and pine branches (138 NmL g−1 VS). Considering the quantity of pine needle litter accumulated on adjacent forest roads (600 ± 200 g m−2), it was possible to calculate the biomethane production potential per km of forest road (up to 500 Nm3 km−1) if the biomass collected was disposed of at an anaerobic digestion facility. The results of the study demonstrate that residual forest biomass represents an additional resource for bioenergy production. Moreover, harvesting residual forest biomass can decrease the incidence of devastating summer forest fires and their disastrous consequences for the environment, the economy, and the local populations.

Organosolv pretreated birch sawdust for the production of green hydrogen and renewable chemicals in an integrated biorefinery approach

Sustainable production of fuels and chemicals is the most important way to reduce the carbon footprint in the environment. Forest based abundant lignocellulosic biomass as a renewable feedstock can be an attractive source of biofuels and biochemicals. This study evaluated the production of hydrogen (H₂) along with platform chemicals from an organosol pretreated birch sawdust (SD). Acidogenic fermentation (AF) of pretreated SD resulted in production of green H₂ (121.4 mL/gVS) along with short (17.8 g/L) and medium (2.64 g/L) chain carboxylic acids. Further integration of AF with anaerobic digestion (AD) in a biorefinery framework offered production of biomethane (bioCH₄: 246 mL/gVS) from the leftover SD from AF. Integration of bioH₂ with bioCH₄ at different time interval of digestion showed 8-14 L biohythane formation ran with a H₂ fraction of 1.6-0.3 H₂/(H₂ + CH₄) documenting energy content of 8-9.08 kJ/gVS.

Organosolv Fractionation of Birch Sawdust: Establishing a Lignin-First Biorefinery

The use of residual biomass for bioconversions makes it possible to decrease the output of fossil-based chemicals and pursue a greener economy. While the use of lignocellulosic material as sustainable feedstock has been tried at pilot scale, industrial production is not yet economically feasible, requiring further technology and feedstock optimization. The aim of this study was to examine the feasibility of replacing woodchips with residual sawdust in biorefinery applications. Woodchips can be used in value-added processes such as paper pulp production, whereas sawdust is currently used mainly for combustion. The main advantages of sawdust are its large supply and a particle size sufficiently small for the pretreatment process. Whereas, the main challenge is the higher complexity of the lignocellulosic biomass, as it can contain small amounts of bark and cambium. Here, we studied the fractionation of birch sawdust by organosolv pretreatment at two different temperatures and for two different durations. We evaluated the efficiency of fractionation into the three main fractions: lignin, cellulose, and hemicellulose. The cellulose content in pretreated biomass was as high as 69.2%, which was nearly double the amount in untreated biomass. The obtained lignin was of high purity, with a maximum 4.5% of contaminating sugars. Subsequent evaluation of the susceptibility of pretreated solids to enzymatic saccharification revealed glucose yields ranging from 75% to 90% after 48 h but reaching 100.0% under the best conditions. In summary, birch sawdust can be successfully utilized as a feedstock for organosolv fractionation and replace woodchips to simplify and lower the costs of biorefinery processes.

Woody biomass as a potential feedstock for fermentative gaseous biofuel production

Biogas and biohydrogen are compatible gaseous biofuels that can be blended with natural gas for reticulated fuel supply to reduce greenhouse gas emissions. Sustainably grown woody biomass is emerging as a potential feedstock in the production of biofuels. Woody biomass is widely available, uses non-arable land for plantation, does not require synthetic fertilisers to grow and acts as a carbon sink. The cellulose and hemicellulose fractions of wood are renewable sources of sugars that can be used for fermentative production of gaseous biofuels. However, widespread use of wood as a gaseous biofuel feedstock is constrained due to the recalcitrant nature of wood to enzymatic hydrolysis. Pretreatment makes cellulose and hemicellulose accessible to microbial enzymes to produce fermentable sugars. Here we review wood composition, its structure and different pretreatment techniques in the context of their effects on deconstruction of wood to improve hydrolysis and fermentative gaseous fuel production. The anaerobic digestion of pretreated wood for biogas and dark fermentation for biohydrogen production are discussed with reference to gas yields. Key advancements in lab-scale research are described for pretreatments and for pure, co- and mixed culture fermentations. Limitations to yield improvements are identified and future perspectives and prospects of gaseous biofuel production from woody biomass are discussed, with reference to new developments in engineered biocatalysts and process integration.

Liquefaction of lignocellulosic biomass for methane production: A review

Hydrothermal pretreatment (HTP) (Hot water extraction (HWE) and steam pretreatment) and pyrolysis have the potential to liquefy lignocellulosic biomass. HTP produces hydrolysate, consisting mainly of solubilized hemicellulose, while pyrolysis produces aqueous pyrolysis liquid (APL). The liquid products, either as main products or by-product, can be used as anaerobic digestion (AD) feeds, overcoming shortcomings of solid-state AD (SS-AD). This paper reviews HWE, steam pretreatment, and pyrolysis pretreatment methods used to liquefy lignocellulosic biomass, AD of liquefied products, effects of inhibition from intermediate by-products such as furan and phenolic compounds, and pretreatment tuning to increase methane yield. HTP, focusing on methane production, produces less inhibitory compounds when carried out at moderate temperatures. APL is a challenging feed for AD due to its complexity, including various inhibitory substances. Pre-treatment of biomass before pyrolysis, adaptation of microorganism to inhibitors, and additives, such as biochar, may help the AD cultures cope with inhibitors in APL.

A novel hybrid organosolv-steam explosion pretreatment and fractionation method delivers solids with superior thermophilic digestibility to methane

Rising environmental concerns and the imminent depletion of fossil resources have sparked a strong interest towards the production of renewable energy such as biomethane. Inclusion of alternative feedstock's such as lignocellulosic biomass could further expand the production of biomethane. The present study evaluated the potential of a novel hybrid organosolv-steam explosion fractionation for delivering highly digestible pretreated solids from birch and spruce woodchips. The highest methane production yield was 176.5 mLCH₄ gVS^-1 for spruce and 327.2 mL CH₄ gVS^-1 for birch. High methane production rates of 1.0-6.3 mL min^-1 (spruce) and 6.0-35.5 mL min^-1 (birch) were obtained, leading to a rapid digestion, with 92% of total methane from spruce being generated in 80 h and 95% of that from birch in 120 h. These results demonstrate the elevated potential of the novel method to fractionate spruce and birch biomass and deliver cellulose-rich pretreated solids with superior digestibility.

Steam Explosion Pretreatment Changes Ruminal Fermentation in vitro of Corn Stover by Shifting Archaeal and Bacterial Community Structure

Steam explosion is an environment-friendly pretreatment method to improve the subsequent hydrolysis process of lignocellulosic biomass. Steam explosion pretreatment improved ruminal fermentation and changed fermentation pattern of corn stover during ruminal fermentation in vitro. The study gave a comprehensive insight into how stream explosion pretreatment shifted archaeal and bacterial community structure to change ruminal fermentation in vitro of corn stover. Results showed that steam explosion pretreatment dramatically improved the apparent disappearance of dry matter (DM), neutral detergent fiber (NDF), and acid detergent fiber (ADF). Steam explosion pretreatment significantly increased the molar proportion of propionate and decreased the ratio of acetate to propionate. At archaeal level, steam explosion pretreatment significantly increased the relative abundance of Methanobrevibacter, which can effectively remove metabolic hydrogen to keep the fermentation continuing. At bacterial level, the shift in fermentation was achieved by increasing the relative abundance of cellulolytic bacteria and propionate-related bacteria, including Spirochaetes, Elusimicrobia, Fibrobacteres, Prevotella, Treponema, Ruminococcus, and Fibrobacter.

Co-Ensiling of Wheat Straw as an Alternative Pre-Treatment to Chemical, Hydrothermal and Mechanical Methods for Methane Production

Wheat straw without pre-treatment is only converted to methane to a low degree during anaerobic digestion for fuel production due to its low hydrolysis. Current pre-treatment technologies are challenged by high expenses to energy or chemical agents. We examined the low-tech co-ensiling pre-treatment as an alternative pre-treatment of wheat straw, and compared the results with hydrothermal, chemical and mechanical pre-treatment methods. The effects of co-ensiling duration and the mixing ratio between straw and sugar beet root on the methane yields, surface morphology and chemical composition were examined. It was found that co-ensiling could improve production of methane by 34.7%, while a combined hydrothermal and chemical pre-treatment could increase the production of methane by 25.4%. The study demonstrated that the effect of co-ensiling could overlap with hydrothermal and chemical pre-treatment by having similar effects to increase lignocellulosic hydrolysis and improve methane production.

Review on pulp and paper activated sludge pretreatment, inhibitory effects and detoxification strategies for biovalorization

Biovalorization of pulp and paper activated sludge to value-added products could be an effective alternative to traditional sludge management methods, which tend to pose serious environmental issues. Since pulp and paper activated sludge consists of microbial biomass, cellulose, hemicellulose and lignin and thus, could be subjected to different hydrolysis methods to solubilize sludge solids and release simple sugars to form value-added products by the microbial fermentation process. Hence, different sludge hydrolysis methods have been summarized in this review paper. However, hydrolysis of lignocellulosic materials generates variety of toxic compounds during hydrolysis and causes detrimental effects. Therefore, different toxic compounds and their impact on microorganisms, cellulolytic enzymes and fermentation process have been discussed in detail and recent strategies to counteract the problems of inhibitors have also been briefly explained.

Pretreatment and fermentation of lignocellulosic biomass: reaction mechanisms and process engineering

At a time of rapid depletion of oil resources, global food shortages and solid waste problems, it is imperative to encourage research into the use of appropriate pre-treatment techniques using regenerative raw materials such as lignocellulosic biomass.

Effect of power ultrasound and Fenton reagents on the biomethane potential from steam-exploded birchwood

The global demand for non-fossil energy sources is increasing rapidly. As a result, biogas presents a suitable alternative; however, first generation biofuels (e.g., sugar cane) potentially impact food crops globally. Second generation biofuels based on lignocellulose-based biomass are being used more frequently as they do not impact food crops. Furthermore, in Northern Europe, there is a significant interest in utilizing birchwood and paper mill waste for biogas production due to its high availability. The utilization of birchwood for biogas has significantly improved in recent years with the improvement of required pretreatment processes. To date, the most effective and economically feasible pretreatment in an industrial context is the steam explosion of lignocellulose-based biomass. Despite this, there is potential for releasing more digestible components from this biomass by efficiently degrading the lignocellulose components. This research presents another pretreatment that can be applied to steam-exploded wood based on ultrasonication and Fenton reagents. It was observed that by treating the steam exploded birchwood with ultrasonication and mild concentrations of Fenton reagents, an increase in the rate of biogas production was achievable. This would allow the increase in biogas yield of a continuously feed industrial anaerobic digester without increasing the size of the reactor.

Steam Explosion Conditions Highly Influence the Biogas Yield of Rice Straw

Straws are agricultural residues that can be used to produce biomethane by anaerobic digestion. The methane yield of rice straw is lower than other straws. Steam explosion was investigated as a pretreatment to increase methane production. Pretreatment conditions with varying reaction times (12–30 min) and maximum temperatures (162–240 °C) were applied. The pretreated material was characterized for its composition and thermal and morphological properties. When the steam explosion was performed with a moderate severity parameter of S₀ = 4.1 min, the methane yield was increased by 32% compared to untreated rice straw. This study shows that a harsher pretreatment at S₀ > 4.3 min causes a drastic reduction of methane yield because inert condensation products are formed from hemicelluloses.

Steam Explosion Treatment of Byproduct Feedstuffs for Potential Use as Ruminant Feed

Simple Summary

Feedstuffs, such as cassava alcohol residue or potato starchy residue, have not been traditionally used as ruminant feeds for commercial production due to high fiber content and low digestibility. Steam explosion treatment can break down lignocellulosic structural components with no (or few) chemical agents, is environmentally friendly, and uses little energy, which offers great potential for large-scale application. We studied the effect of steam explosion treatment on the chemical composition, in vitro digestibility, energy value, Cornell Net Carbohydrate and Protein System (CNCPS) composition of five byproduct feedstuffs, to provide a theoretical basis for the development and utilization of the steam explosion technique to produce ruminant feedstuff. The data revealed that steam explosion treatment improved the nutritive value and in vitro dry matter digestibility of cassava alcohol residues, distillers’ grains, rapeseed meal and potato starchy residue. Therefore, steam explosion treatment offers the potential to improve the suitability of these feedstuffs.

Abstract

Although many byproducts of milling industries have potential as a ruminant feed, they have not been widely used due to their low nutritive value, especially high-fiber content and difficult processing techniques. Steam explosion can increase the degradation of hemicellulose, cellulose and lignin and make byproduct feedstuffs more suitable as ruminant feed. Five byproduct feedstuffs: cassava alcohol residue (CAR), distillers’ grains (DG), cottonseed meal (CM), rapeseed meal (RM) and potato starchy residues (PSR), were steam-exploded using five different processing parameters and the effects on the chemical composition, in vitro digestibility, energy value, and Cornell Net Carbohydrate and Protein System composition were assessed in order to provide a theoretical basis for the technique’s development and utilization for ruminant feed production. In this study, after steam-explosion treatment, the nutritive value and in vitro dry matter digestibility (IVDMD) of CAR, DG, RM and PSR were improved (p < 0.05), while there was no effect on nutritive value of CM (p > 0.05). Specifically, steam explosion treatment decreased the contents of neutral detergent fiber, acid detergent fiber, available cell wall, and slowly degraded protein, and increased the total digestible nutrients, digestible energy, metabolic energy, net energy for maintenance, and net energy for gain, sugar, non-structural carbohydrate and IVDMD. Therefore, steam-explosion treatment offers the potential to improve the suitability of byproduct feedstuffs as ruminant feed.

Anaerobic digestion of extracts from steam exploded Agave tequilana bagasse

Anaerobic digestion (AD) in the beverage industry is a proven treatment technology. But adding dissolved organic matter to AD can increase the on-site output of renewable energy. In the tequila industry such waste-derived organic matter can be obtained from semisolid agave bagasse submitted to steam explosion as pretreatment. Vapor at pressure <1.0 MPa is commonly available so that steam explosion can be integrated into extant production schemes. This study investigates the efficiency of agave bagasse hydrolyzation via steam explosion (applying severity factors between 2.4 and 3.7 with 0.98 MPa maximum pressure) as well as the efficiency of the bio-conversion in anaerobic batch assays. The best steam explosion yield was 14.3 ± 0.1 g_COD 100 g^-1 (0.98 MPa vapor pressure during 22 min followed by fast pressure release). The average biochemical methane potential (BMP) was 290 mL_N g_COD^-1 with 74% of the biogas released within seven days.

Methylation of the N-terminal histidine protects a lytic polysaccharide monooxygenase from auto-oxidative inactivation

The catalytically crucial N-terminal histidine (His1) of fungal lytic polysaccharide monooxygenases (LPMOs) is post-translationally modified to carry a methylation. The functional role of this methylation remains unknown. We have carried out an in-depth functional comparison of two variants of a family AA9 LPMO from Thermoascus aurantiacus (TaLPMO9A), one with, and one without the methylation on His1. Various activity assays showed that the two enzyme variants are identical in terms of substrate preferences, cleavage specificities and the ability to activate molecular oxygen. During the course of this work, new functional features of TaLPMO9A were discovered, in particular the ability to cleave xyloglucan, and these features were identical for both variants. Using a variety of techniques, we further found that methylation has minimal effects on the pK_a of His1, the affinity for copper and the redox potential of bound copper. The two LPMOs did, however, show clear differences in their resistance against oxidative damage. Studies with added hydrogen peroxide confirmed recent claims that low concentrations of H₂ O₂ boost LPMO activity, whereas excess H₂ O₂ leads to LPMO inactivation. The methylated variant of TaLPMO9A, produced in Aspergillus oryzae, was more resistant to excess H₂ O₂ and showed better process performance when using conditions that promote generation of reactive-oxygen species. LPMOs need to protect themselves from reactive oxygen species generated in their active sites and this study shows that methylation of the fully conserved N-terminal histidine provides such protection.

A Review of the Role of Critical Parameters in the Design and Operation of Biogas Production Plants

Many operating parameters, individually or together, may influence the performance of anaerobic digestion towards biogas or digestate yield and quality maximization. The most preferred method of optimizing an anaerobic digestion plant often relies on how carefully the crucial parameters, such as pH, temperature, organic loading rate, hydraulic retention time, and pressure, are chosen. There is a large amount of literature available on optimization of anaerobic digestion; however, given the continued development and implementation of innovative technologies, together with the introduction of increasingly complex systems, it is necessary to update present knowledge on process parameters and their role on operational ranges and flexibilities in real-life anaerobic digestion system. Accordingly, the present review discusses the importance of the selection of operational parameters in existing technologies and their impact on biogas yield. Notably, the four broad areas of feedstock utilization (substrate, inoculum, codigestion and pretreatment), process condition (pH, temperature, pressure, and reactor design), reactor control (HRT and OLR) and inhibition (Ammonia and VFAs) are covered in this review. In addition, particular emphasis is placed on the most recent innovations that have been or may be implemented in current or future biogas plants.

Enhanced enzymatic hydrolysis and methane production from rubber wood waste using steam explosion

Rubber wood waste (RW) requires due to its recalcitrance a pretreatment step before efficient biochemical conversion is possible. Non chemical steam explosion pretreatment was adopted to enhance enzymatic hydrolysis and anaerobic digestion with severity from 2.70 to 4.35. RW treated at severity 4.35 (214 °C for 10 min) gave the highest 83.9 L CH₄/kgVS effectiveness in anaerobic digestibility together with 45.2% hydrolysability in terms of glucan conversion. The intense pretreatment decreased particle size and degraded most of the hemicellulose, resulting in increased specific surface and better access for enzymes to cellulose. Additionally, the energy yield of steam exploded RW was enhanced by combined enzymatic hydrolysis with anaerobic digestion, in comparison to enzymatic hydrolysis or anaerobic digestion alone. This allowed for an efficient steam explosion pretreatment with co-production of sugar and methane. This study provides a technical approach for efficient biofuel production from RW after steam explosion pretreatment. Valorization of lignin-rich residue generated from the integrated process may increase value of RW, but assessing this requires further study.

Enhancing Lignocellulosic Biomass Hydrolysis by Hydrothermal Pretreatment, Extraction of Surface Lignin, Wet Milling and Production of Cellulolytic Enzymes

Acetone and ethanol extraction of lignin deposits from the surface of hydrothermally (liquid hot water) pretreated beech wood biomass alleviates the lignin inhibitory effects during enzymatic hydrolysis of cellulose to glucose and boosts the enzymatic digestibility to high values (≈70 %). Characterization of the extracted lignins (FTIR, pyrolysis/GC-MS, differential thermogravimetry, gel permeation chromatography) indicated high purity, low molecular weight, and features that suggest that it consists mainly of fragments of the native wood lignin partially depolymerized and recondensed on the biomass surface during the hydrothermal pretreatment. The pyrolysis products of the extracted surface lignins suggest their high potential as a feedstock for the production of high added value phenolic compounds. When the enzymatic hydrolysis of the pretreated and extracted biomass solids was assisted by mild wet milling, near complete cellulose digestibility (≥95 %) could be achieved. In the context of the biorefinery and whole-biomass valorization concept, it was also shown that the hydrothermally (hemicellulose-deficient) pretreated and delignified biomass solids could be also successfully used for the production of crude cellulase from Trichoderma reesei cultures, providing a simple and low-cost method for the complementary production of cellulases by utilizing fractions of the integrated hydrolysis process.

The use of lytic polysaccharide monooxygenases in anaerobic digestion of lignocellulosic materials

BACKGROUND

The recent discovery that LPMOs can work under anaerobic conditions when supplied with low amounts H₂O₂ opens the possibility of using LPMOs as enzyme aids in biogas reactors to increase methane yields from lignocellulosic materials. We have explored this possibility by studying anaerobic digestion of various lignocellulosic materials: Avicel, milled spruce and birch wood, and a lignin-rich hydrolysis residue from steam-exploded birch. The digestions were added LPMOs and various cellulolytic enzyme cocktails and were carried out with or without addition of H₂O₂.

RESULTS

In several cases, enzyme addition had a beneficial effect on methane production, which was partly due to components present in the enzyme preparations. It was possible to detect LPMO activity during the initial phases of the anaerobic digestions of Avicel, and in some cases LPMO activity could be correlated with improved methane production from lignocellulosic materials. However, a positive effect on methane production was only seen when LPMOs were added together with cellulases, and never upon addition of LPMOs only. Generally, the experimental outcomes showed substrate-dependent variations in process efficiency and the importance of LPMOs and added H₂O₂. These differences could relate to variations in the type and content of lignin, which again will affect the activity of the LPMO, the fate of the added H₂O₂ and the generation of potentially damaging reactive-oxygen species. The observed effects showed that the interplay between cellulases and LPMOs is important for the overall efficiency of the process.

CONCLUSION

This study shows that it may be possible to harness the power of LPMOs in anaerobic digestion processes and improve biogas production, but also highlight the complexity of the reaction systems at hand. One complicating factor was that the enzymes themselves and other organic components in the enzyme preparations acted as substrates for biogas production, meaning that good control reactions were essential to detect effects caused by enzyme activity. As also observed during regular aerobic enzymatic digestion of lignocellulosic biomass, the type and contents of lignin in the substrates likely plays a major role in determining the impact of LPMOs and of cellulolytic enzymes in general. More work is needed to unravel the interplay between LPMOs, O₂, H₂O₂, and the multitude of redox-active components found in anaerobic bioreactors degrading lignocellulosic substrates.

Enhancement of sugar recovery and ethanol production from wheat straw through alkaline pre-extraction followed by steam pretreatment

To improve sugar recovery and ethanol production from wheat straw, a sequential two-stage pretreatment process combining alkaline pre-extraction and acid catalyzed steam treatment was investigated. The results showed that alkaline pre-extraction using 8% (w/w) sodium hydroxide at 80 °C for 90 min followed by steam pretreatment with 3% (w/w) sulfur dioxide at 151 °C for 16 min was sufficient to prepare a substrate that could be efficiently hydrolyzed at high solid loadings. Moreover, alkaline pre-extraction reduced the process severity of steam pretreatment and decreased the generation of inhibitory compounds. During enzymatic hydrolysis, increasing solid loading decreased the yield of monomeric sugars. Enzymatic hydrolysis at 25% (w/v) solid loading, the yields of approximately 80% of glucose and 65% of xylose could be reached with an enzyme dosage of 25 mg protein/g glucan. Following fermentation of hydrolysate with sugar concentration of approximately 120 g/L, an ethanol concentration of 54.5 g/L was achieved.

Characterization of Pseudo-Lignin from Steam Exploded Birch

There is a growing interest in a more wholesome utilization of biomass as the need for greener chemistry and non-mineral oil-based products increases. Lignin is the largest renewable resource for aromatic chemicals, which is found in all types of lignocellulosic biomass. Steam-explosion of lignocellulosic biomass is a useful pretreatment technique to make the polymeric material more available for processing. However, this heat-based pretreatment is known to result in the formation of pseudo-lignin, a lignin-like polymer made from carbohydrate degradation products. In this work, we have analyzed steam-exploded birch with a varying severity factor (3.1-5.0) by pyrolysis-gas chromatography-mass spectrometry, 2D-NMR, and Fourier transform infrared spectroscopy. The main results reveal a consumption of acetic acid at higher temperatures, with the increase of furan components in the pyrolyzate. The IR and NMR spectral data support these results, and there is a reason to believe that the conditions for humin formation are accomplished under steam explosion. Pseudo-lignin seems to be a humin-like compound.

Post-anaerobic digestion thermal hydrolysis of sewage sludge and food waste: Effect on methane yields, dewaterability and solids reduction

Post-anaerobic digestion (PAD) treatment technologies have been suggested for anaerobic digestion (AD) to improve process efficiency and assure hygenization of organic waste. Because AD reduces the amount of organic waste, PAD can be applied to a much smaller volume of waste compared to pre-digestion treatment, thereby improving efficiency. In this study, dewatered digestate cakes from two different AD plants were thermally hydrolyzed and dewatered, and the liquid fraction was recirculated to a semi-continuous AD reactor. The thermal hydrolysis was more efficient in relation to methane yields and extent of dewaterability for the cake from a plant treating waste activated sludge, than the cake from a plant treating source separated food waste (SSFW). Temperatures above 165 °C yielded the best results. Post-treatment improved volumetric methane yields by 7% and the COD-reduction increased from 68% to 74% in a mesophilic (37 °C) semi-continuous system despite lowering the solid retention time (from 17 to 14 days) compared to a conventional system with pre-treatment of feed substrates at 70 °C. Results from thermogravimetric analysis showed an expected increase in maximum TS content of dewatered digestate cake from 34% up to 46% for the SSFW digestate cake, and from 17% up to 43% in the sludge digestate cake, after the PAD thermal hydrolysis process (PAD-THP). The increased dewatering alone accounts for a reduction in wet mass of cake leaving the plant of 60% in the case of sludge digestate cake. Additionaly, the increased VS-reduction will contribute to further reduce the mass of wet cake.

Synergistic effects of anaerobic co-digestion of whey, manure and fish ensilage

Biogas production potential of the three feedstocks fish ensilage, manure and whey was evaluated using biochemical methane potential (BMP) tests. Since anaerobic digestion of single substrates may be inefficient due to imbalances in the carbon-nitrogen ratio, degree of biodegradability and/or due to lack of nutrients needed by the microbial community, co-digestion of these substrates was also assessed, revealing synergistic effects and a particularly good effect of combining manure with fish ensilage. In this latter case, methane yields were up to 84% higher than the weighted average of the methane yields obtained with the individual substrates. The type of substrate was the dominating cause of variation in methane production rates and yields.

Enhancing methane production from lignocellulosic biomass by combined steam-explosion pretreatment and bioaugmentation with cellulolytic bacterium Caldicellulosiruptor bescii

BACKGROUND

Biogas production from lignocellulosic biomass is generally considered to be challenging due to the recalcitrant nature of this biomass. In this study, the recalcitrance of birch was reduced by applying steam-explosion (SE) pretreatment (210 °C and 10 min). Moreover, bioaugmentation with the cellulolytic bacterium Caldicellulosiruptor bescii was applied to possibly enhance the methane production from steam-exploded birch in an anaerobic digestion (AD) process under thermophilic conditions (62 °C).

RESULTS

Overall, the combined SE and bioaugmentation enhanced the methane yield up to 140% compared to untreated birch, while SE alone contributed to the major share of methane enhancement by 118%. The best methane improvement of 140% on day 50 was observed in bottles fed with pretreated birch and bioaugmentation with lower dosages of C. bescii (2 and 5% of inoculum volume). The maximum methane production rate also increased from 4-mL CH₄/g VS (volatile solids)/day for untreated birch to 9-14-mL CH₄/g VS/day for steam-exploded birch with applied bioaugmentation. Bioaugmentation was particularly effective for increasing the initial methane production rate of the pretreated birch yielding 21-44% more methane than the pretreated birch without applied bioaugmentation. The extent of solubilization of the organic matter was increased by more than twofold when combined SE pretreatment and bioaugmentation was used in comparison with the methane production from untreated birch. The beneficial effects of SE and bioaugmentation on methane yield indicated that biomass recalcitrance and hydrolysis step are the limiting factors for efficient AD of lignocellulosic biomass. Microbial community analysis by 16S rRNA amplicon sequencing showed that the microbial community composition was altered by the pretreatment and bioaugmentation processes. Notably, the enhanced methane production by pretreatment and bioaugmentation was well correlated with the increase in abundance of key bacterial and archaeal communities, particularly the hydrolytic bacterium Caldicoprobacter, several members of syntrophic acetate oxidizing bacteria and the hydrogenotrophic Methanothermobacter.

CONCLUSION

Our findings demonstrate the potential of combined SE and bioaugmentation for enhancing methane production from lignocellulosic biomass.

Microbial biogas production from hydrolysis lignin: insight into lignin structural changes

BACKGROUND

The emerging cellulosic bioethanol industry will generate huge amounts of lignin-rich residues that may be converted into biogas by anaerobic digestion (AD) to increase the output of energy carriers from the biorefinery plants. The carbohydrates fraction of lignocellulosic biomass is degradable, whereas the lignin fraction is generally considered difficult to degrade during AD. The objective of this study was to investigate the feasibility of biogas production by AD from hydrolysis lignin (HL), prepared by steam explosion (SE) and enzymatic saccharification of birch. A novel nylon bag technique together with two-dimensional nuclear magnetic resonance spectroscopy, pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS), and Fourier transform infrared (FTIR) spectroscopy was used to identify recalcitrant and degradable structures in the lignin during AD.

RESULTS

The HL had a lignin content of 80% which included pseudo-lignin and condensed-lignin structures resulting from the SE pretreatment. The obtained methane yield from HL was almost twofold higher than the theoretical methane from the carbohydrate fraction alone, indicating that part of the lignin was converted to methane. Characterization of the undegradable material after AD revealed a substantial loss of signals characteristic for carbohydrates and lignin-carbohydrate complexes (LCC), indicating conversion of these chemical components to methane during AD. The β-O-4' linkage and resinol were not modified as such in AD, but major change was seen for the S/G ratio from 5.8 to 2.6, phenylcoumaran from 4.9 to 1.0%, and pseudo-lignin and condensed-lignin were clearly degraded. Scanning electron microscopy and simultaneous thermal analysis measurements demonstrated changes in morphology and thermal properties following SE pretreatment and AD. Our results showed that carbohydrate, LCC, pseudo-lignin, and condensed-lignin degradation had contributed to methane production. The energy yield for the combined ethanol production and biogas production was 8.1 MJ fuel per kg DM of substrate (4.9 MJ/kg from ethanol and 3.2 MJ/kg from methane).

CONCLUSION

This study shows the benefit of using a novel bag technique together with advanced analytical techniques to investigate the degradation mechanisms of lignin during AD, and also points to a possible application of HL produced in cellulosic bioethanol plants.

Corn stover for biogas production: Effect of steam explosion pretreatment on the gas yields and on the biodegradation kinetics of the primary structural compounds

This study evaluated the effect of steam explosion on the chemical composition and biomethane potential of corn stover using temperatures ranging between 140 and 220°C and pretreatment times ranging between 2 and 15min. Biodegradation kinetics during the anaerobic digestion of untreated and corn stover, pretreated at two different intensities, 140°C for 5min and 180°C for 5min, were studied in tandem. Results showed that pretreatment at 160°C for 2min improved the methane yield by 22%. Harsher pretreatment conditions led to lower hemicellulose contents and methane yields, as well as higher lignin contents, which may be due to the formation of pseudo-lignin. The biodegradation kinetics trial demonstrated that steam explosion enhances the degradation of structural carbohydrates and acid insoluble lignin.

Quantitative Metaproteomics Highlight the Metabolic Contributions of Uncultured Phylotypes in a Thermophilic Anaerobic Digester

In this study, we used multiple meta-omic approaches to characterize the microbial community and the active metabolic pathways of a stable industrial biogas reactor with food waste as the dominant feedstock, operating at thermophilic temperatures (60°C) and elevated levels of free ammonia (367 mg/liter NH3-N). The microbial community was strongly dominated (76% of all 16S rRNA amplicon sequences) by populations closely related to the proteolytic bacterium Coprothermobacter proteolyticus. Multiple Coprothermobacter-affiliated strains were detected, introducing an additional level of complexity seldom explored in biogas studies. Genome reconstructions provided metabolic insight into the microbes that performed biomass deconstruction and fermentation, including the deeply branching phyla Dictyoglomi and Planctomycetes and the candidate phylum "Atribacteria" These biomass degraders were complemented by a synergistic network of microorganisms that convert key fermentation intermediates (fatty acids) via syntrophic interactions with hydrogenotrophic methanogens to ultimately produce methane. Interpretation of the proteomics data also suggested activity of a Methanosaeta phylotype acclimatized to high ammonia levels. In particular, we report multiple novel phylotypes proposed as syntrophic acetate oxidizers, which also exert expression of enzymes needed for both the Wood-Ljungdahl pathway and β-oxidation of fatty acids to acetyl coenzyme A. Such an arrangement differs from known syntrophic oxidizing bacteria and presents an interesting hypothesis for future studies. Collectively, these findings provide increased insight into active metabolic roles of uncultured phylotypes and presents new synergistic relationships, both of which may contribute to the stability of the biogas reactor.

IMPORTANCE

Biogas production through anaerobic digestion of organic waste provides an attractive source of renewable energy and a sustainable waste management strategy. A comprehensive understanding of the microbial community that drives anaerobic digesters is essential to ensure stable and efficient energy production. Here, we characterize the intricate microbial networks and metabolic pathways in a thermophilic biogas reactor. We discuss the impact of frequently encountered microbial populations as well as the metabolism of newly discovered novel phylotypes that seem to play distinct roles within key microbial stages of anaerobic digestion in this stable high-temperature system. In particular, we draft a metabolic scenario whereby multiple uncultured syntrophic acetate-oxidizing bacteria are capable of syntrophically oxidizing acetate as well as longer-chain fatty acids (via the β-oxidation and Wood-Ljundahl pathways) to hydrogen and carbon dioxide, which methanogens subsequently convert to methane.

LPMOs in cellulase mixtures affect fermentation strategies for lactic acid production from lignocellulosic biomass

Enzymatic catalysis plays a key role in the conversion of lignocellulosic biomass to fuels and chemicals such as lactic acid. In the last decade, the efficiency of commercial cellulase cocktails has increased significantly, in part due to the inclusion of lytic polysaccharide monooxygenases (LPMOs). However, the LPMOs' need for molecular oxygen to break down cellulose demands reinvestigations of process conditions. In this study, we evaluate the efficiency of lactic acid production from steam-exploded birch using an LPMO-containing cellulase cocktail in combination with lactic acid bacteria, investigating both separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). While the SSF set up generally has been considered to be more efficient because it avoids sugar accumulation which may inhibit the cellulases, the SHF set up in our study yielded 26-32% more lactic acid than the SSF. This was mainly due to competition for oxygen between LPMOs and the fermenting organisms in the SSF process, which resulted in reduced LPMO activity and thus less efficient saccharification of the lignocellulosic substrate. By means of aeration it was possible to activate the LPMOs in the SSF, but less lactic acid was produced due to a shift in metabolic pathways toward production of acetic acid. Overall, this study shows that lactic acid can be produced efficiently from lignocellulosic biomass, but that the use of LPMO-containing cellulase cocktails in fermentation processes demands re-thinking of traditional process set ups due to the requirement of oxygen in the saccharification step. Biotechnol. Bioeng. 2017;114: 552-559. © 2016 Wiley Periodicals, Inc.

Influence of steam explosion pretreatment on the anaerobic digestion of vinegar residue

Vinegar residue is the by-product in the vinegar production process. The large amount of vinegar residue has caused a serious environmental problem owing to its acidity and corrosiveness. Anaerobic digestion is an effective way to convert agricultural waste into bioenergy, and a previous study showed that vinegar residue could be treated by anaerobic digestion but still had room to improve digestion efficiency. In this study, steam explosion at pressure of 0.8, 1.2, and 1.5 MPa and residence time of 5, 10, 15, and 20 min were used to pretreat vinegar residue to improve methane production, respectively. Scanning electron microscopy and X-ray diffraction analyses were applied to validate structural changes of vinegar residue after steam explosion. Results showed that steam explosion pretreatment could destroy the structure of lignocellulose by removing the hemicellulose and lignin, and improve the methane yield effectively. Steam explosion-treated vinegar residue at 0.8 MPa for 5 min produced the highest methane yield of 153.58 mL gVS (-1), which was 27.65% (significant, α < 0.05) more than untreated vinegar residue (120.31 mL gVS (-1)). The analyses of pH, total ammonia-nitrogen, total alkalinity, and volatile fatty acids showed that steam explosion did not influence the stability of anaerobic digestion. This study suggested that steam explosion pretreatment on vinegar residue might be a promising approach and it is worth further study to improve the efficiency of vinegar residue waste utilisation.

Sequential parametric optimization of methane production from different sources of forest raw material

The increase in environmental problems and the shortage of fossil fuels have led to the need for action in the development of sustainable and renewable fuels. Methane is produced through anaerobic digestion of organic materials and is a biofuel with very promising characteristics. The success in using methane as a biofuel has resulted in the operation of several commercial-scale plants and the need to exploit novel materials to be used. Forest biomass can serve as an excellent candidate for use as raw material for anaerobic digestion. During this work, both hardwood and softwood species-which are representative of the forests of Sweden-were used for the production of methane. Initially, when untreated forest materials were used for the anaerobic digestion, the yields obtained were very low, even with the addition of enzymes, reaching a maximum of only 40 mL CH4/g VS when birch was used. When hydrothermal pretreatment was applied, the enzymatic digestibility improved up to 6.7 times relative to that without pretreatment, and the yield of methane reached up to 254 mL CH4/g VS. Then the effect of chemical/enzymatic detoxification was examined, where laccase treatment improved the methane yield from the more harshly pretreated materials while it had no effect on the more mildly pretreated material. Finally, addition of cellulolytic enzymes during the digestion improved the methane yields from spruce and pine, whereas for birch separate saccharification was more beneficial. To achieve high yields in spruce 30 filter paper units (FPU)/g was necessary, whereas 15 FPU/g was enough when pine and birch were used. During this work, the highest methane yields obtained from pine and birch were 179.9 mL CH4/g VS and 304.8 mL CH4/g VS, respectively. For mildly and severely pretreated spruce, the methane yields reached 259.4 mL CH4/g VS and 276.3 mL CH4/g VS, respectively. We have shown that forest material can serve as raw material for efficient production of methane. The initially low yields from the untreated materials were significantly improved by the introduction of a hydrothermal pretreatment. Moreover, enzymatic detoxification was beneficial, but mainly for severely pretreated materials. Finally, enzymatic saccharification increased the methane yields even further.

The effect of a combined biological and thermo-mechanical pretreatment of wheat straw on energy yields in coupled ethanol and methane generation

Ethanol and biogas are energy carriers that could contribute to a future energy system independent of fossil fuels. Straw is a favorable bioenergy substrate as it does not compete with food or feed production. As straw is very resistant to microbial degradation, it requires a pretreatment to insure efficient conversion to ethanol and/or methane. This study investigates the effect of combining biological pretreatment and steam explosion on ethanol and methane yields in order to improve the coupled generation process. Results show that the temperature of the steam explosion pretreatment has a particularly strong effect on possible ethanol yields, whereas combination with the biological pretreatment showed no difference in overall energy yield. The highest overall energy output was found to be 10.86 MJ kg VS(-1) using a combined biological and steam explosion pretreatment at a temperature of 200°C.

Steam explosion pretreatment of wheat straw to improve methane yields: investigation of the degradation kinetics of structural compounds during anaerobic digestion

Wheat straw can serve as a low-cost substrate for energy production without competing with food or feed production. This study investigated the effect of steam explosion pretreatment on the biological methane potential and the degradation kinetics of wheat straw during anaerobic digestion. It was observed that the biological methane potential of the non steam exploded, ground wheat straw (276 l(N) kg VS(-1)) did not significantly differ from the best steam explosion treated sample (286 l(N) kg VS(-1)) which was achieved at a pretreatment temperature of 140°C and a retention time of 60 min. Nevertheless degradation speed was improved by the pretreatment. Furthermore it was observed that compounds resulting from chemical reactions during the pretreatment and classified as pseudo-lignin were also degraded during the anaerobic batch experiments. Based on the rumen simulation technique, a model was developed to characterise the degradation process.

Study on loading coefficient in steam explosion process of corn stalk

The object of this work was to evaluate the effect of loading coefficient on steam explosion process and efficacy of corn stalk. Loading coefficient's relation with loading pattern and material property was first revealed, then its effect on transfer process and pretreatment efficacy of steam explosion was assessed by established models and enzymatic hydrolysis tests, respectively, in order to propose its optimization strategy for improving the process economy. Results showed that loading coefficient was mainly determined by loading pattern, moisture content and chip size. Both compact loading pattern and low moisture content improved the energy efficiency of steam explosion pretreatment and overall sugar yield of pretreated materials, indicating that they are desirable to improve the process economy. Pretreatment of small chip size showed opposite effects in pretreatment energy efficiency and enzymatic hydrolysis performance, thus its optimization should be balanced in investigated aspects according to further techno-economical evaluation.

Harnessing the potential of LPMO-containing cellulase cocktails poses new demands on processing conditions

BACKGROUND

The emerging bioeconomy depends on improved methods for processing of lignocellulosic biomass to fuels and chemicals. Saccharification of lignocellulose to fermentable sugars is a key step in this regard where enzymatic catalysis plays an important role and is a major cost driver. Traditionally, enzyme cocktails for the conversion of cellulose to fermentable sugars mainly consisted of hydrolytic cellulases. However, the recent discovery of lytic polysaccharide monooxygenases (LPMOs), which cleave cellulose using molecular oxygen and an electron donor, has provided new tools for biomass saccharification.

RESULTS

Current commercial enzyme cocktails contain LPMOs, which, considering the unique properties of these enzymes, may change optimal processing conditions. Here, we show that such modern cellulase cocktails release up to 60 % more glucose from a pretreated lignocellulosic substrate under aerobic conditions compared to anaerobic conditions. This higher yield correlates with the accumulation of oxidized products, which is a signature of LPMO activity. Spiking traditional cellulase cocktails with LPMOs led to increased saccharification yields, but only under aerobic conditions. LPMO activity on pure cellulose depended on the addition of an external electron donor, whereas this was not required for LPMO activity on lignocellulose.

CONCLUSIONS

In this study, we demonstrate a direct correlation between saccharification yield and LPMO activity of commercial enzyme cocktails. Importantly, we show that the LPMO contribution to overall efficiency may be large if process conditions are adapted to the key determinants of LPMO activity, namely the presence of electron donors and molecular oxygen. Thus, the advent of LPMOs has a great potential, but requires rethinking of industrial bioprocessing procedures.

Evaluation of dried sweet sorghum stalks as raw material for methane production

The potential of utilizing dried sweet sorghum stalks as raw material for anaerobic digestion has been evaluated. Two different treatments were tested, a mild thermal and an enzymatic, alone or in combination. Thermal pretreatment was found to decrease the methane yields, whereas one-step enzymatic treatment resulted in a significant increase of 15.1% comparing to the untreated sweet sorghum. Subsequently, in order to increase the total methane production, the combined effect of enzyme load and I/S on methane yields from sweet sorghum was evaluated by employing response surface methodology. The obtained model showed that the maximum methane yield that could be achieved is 296 mL CH₄/g VS at I/S ratio of 0.35 with the addition of 11.12 FPU/g sweet sorghum.

Steam explosion pretreatment for enhancing biogas production of late harvested hay

Grasslands are often abandoned due to lack of profitability. Extensively cultivating grassland for utilization in a biogas-based biorefinery concept could mend this problem. Efficient bioconversion of this lignocellulosic biomass requires a pretreatment step. In this study the effect of different steam explosion conditions on hay digestibility have been investigated. Increasing severity in the pretreatment induced degradation of the hemicellulose, which at the same time led to the production of inhibitors and formation of pseudo-lignin. Enzymatic hydrolysis showed that the maximum glucose yields were obtained under pretreatment at 220 °C for 15 min, while higher xylose yields were obtained at 175 °C for 10 min. Pretreatment of hay by steam explosion enhanced 15.9% the methane yield in comparison to the untreated hay. Results indicate that hay can be effectively converted to methane after steam explosion pretreatment.

Assessment and regression analysis on instant catapult steam explosion pretreatment of corn stover

Instant catapult steam explosion (ICSE) offers enormous physical force on lignocellulosic biomass due to its extremely short depressure duration. In this article, the response surface methodology was applied to optimize the effect of working parameters including pressure, maintaining time and mass loading on the crystallinity index and glucose yield of the pretreated corn stover. It was found that the pressure was of essential importance, which determined the physical force that led to the morphological changes without significant chemical reactions, and on the other hand the maintaining time mainly contributed to the thermo-chemical reactions. Furthermore, the pretreated biomass was assessed by scanning electron microscope, X-ray diffraction and Fourier transform infrared spectra to understand mechanisms underlying the ICSE pretreatment.

Anaerobic co-digestion of steam-treated Quercus serrata chips and sewage sludge under mesophilic and thermophilic conditions

The biodegradation of Quercus serrata chips was evaluated by anaerobic digestion under various steam explosion conditions. In continuous experiments, untreated chips (W₀) and chips steam-treated at less than 1.0 MPa (W₁) and 2.0 MPa (W₄) were co-digested with sewage sludge (S₁ and S₂) taken from two different wastewater treatment plants. The apparent methane yield of W₁ and W₄ co-digested with S₁ (thermophilic) was 261 dm(3)/kgVS (volatile solids) and 248 dm(3)/kgVS, respectively. The apparent methane yield of W₄ co-digested with S₂ was 258 dm(3)/kgVS (mesophilic) and 271 dm(3)/kgVS (thermophilic). Methane production was inhibited by W₀ due to components released during hydrolysis. The methane conversion ratio of pretreated chips obtained in batch experiments varied from 40.5% to 53.8% (mesophilic) and from 49.0% to 63.7% (thermophilic). The methane conversion ratio increased with decreasing acid-soluble lignin content in the chips.