Effective pretreatment of lignocellulosic co-substrates using barley straw-adapted microbial consortia to enhanced biomethanation by anaerobic digestion

Comparison of microbiome community structure and dynamics during anaerobic digestion of different renewable solid wastes

This study analysed the effect of the different lignocellulose composition of two crop substrates on the structure and dynamics of bacterial communities during anaerobic digestion (AD) processes for biogas production. To this end, cereal grains and grape pomace biomasses were analysed in parallel in an experimental AD bench-scale system to define and compare their metagenomic profiles for different experimental time intervals. The bacterial community structure and dynamics during the AD process were detected and characterised using high-resolution whole metagenomic shotgun analyses. Statistical evaluation identified 15 strains as specific to two substrates. Some strains, like Clostridium isatidis, Methanothermobacter wolfeii, and Methanobacter sp. MB1 in cereal grains, and Acetomicrobium hydrogeniformans and Acetomicrobium thermoterrenum in grape pomace, were never before detected in biogas reactors. The presence of bacteria such as Acetomicrobium sp. and Petrimonas mucosa, which degrade lipids and protein-rich substrates, along with Methanosarcina sp. and Peptococcaceae bacterium 1109, which tolerate high hydrogen pressures and ammonia concentrations, suggests a complex syntrophic community in lignin-cellulose-enriched substrates. This finding could help develop new strategies for the production of a tailor-made microbial consortium to be inoculated from the beginning of the digestion process of specific lignocellulosic biomass.

The effects of different pretreatment technologies on microbial community in anaerobic digestion process: A systematic review

Here we comprehensively review the available knowledge on effects of different pretreatment technologies on microbial population and microbial dynamics in anaerobic digestion (AD) fed with different substrates and different operational parameters. To identify peer-reviewed studies published in English-language journals, a comprehensive search was performed across multiple electronic databases. The eligible studies were analyzed to extract data and information pertaining to the configuration of anaerobic reactors, operational parameters, and various pretreatment processes such as chemical, biological, enzymatic, thermal, microaerobic, and ultrasonic. The findings derived from this current review demonstrated that different chemical, biological, and physical pretreatment technologies improve the biomethane potential (BMP) and potentially affect the dominant bacteria and archaea. Moreover, although hydrogenotrophic methanogenesis are more observed due to resistance to extreme conditions, methane production follows both aceticlastic and hydrogenotrophic pathways in AD assisted with different pretreatment process. Firmicutes and Bacteroidetes phyla of bacteria were the dominant hydrolytic bacteria due to synergetic effects of different pretreatment process on solubilization and bioavailability of recalcitrant substrates. In summary, a holistic understanding on bacteria and archaea communities, along with the mechanisms of the dominant microorganisms leads to enhanced stability and overall performance of anaerobic digestion (AD) processes.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40201-024-00917-x.

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.

Biodegradation of willow sawdust by novel cellulase-producing bacterial consortium from wood-feeding termites for enhancing methane production

This study was designed to develop a cellulase-producing bacterial consortium (CBC) from wood-feeding termites that could effectively degrade willow sawdust (WSD) and consequently enhance methane production. The bacterial strains Shewanella sp. SSA-1557, Bacillus cereus SSA-1558, and Pseudomonas mosselii SSA-1568 exhibited significant cellulolytic activity. Their CBC consortium showed positive effects on cellulose bioconversion, resulting in accelerated WSD degradation. After nine days of pretreatment, the WSD had lost 63%, 50%, and 28% of its cellulose, hemicellulose, and lignin, respectively. The hydrolysis rate of treated WSD (352 mg/g) was much higher than that of untreated WSD (15.2 mg/g). The highest biogas production (66.1 NL/kg VS) with 66% methane was observed in the anaerobic digester M-2, which contained a combination of pretreated WSD and cattle dung in a 50/50 ratio. The findings will enrich knowledge for the development of cellulolytic bacterial consortia from termite guts for biological wood pretreatment in lignocellulosic anaerobic digestion biorefineries.

Culture adaptation for enhanced biogas production from birch wood applying stable carbon isotope analysis to monitor changes in the microbial community

Birch wood is a potential feedstock for biogas production in Northern Europe; however, the lignocellulosic matrix is recalcitrant preventing efficient conversion to methane. To improve digestibility, birch wood was thermally pre-treated using steam explosion at 220 °C for 10 min. The steam-exploded birch wood (SEBW) was co-digested with cow manure for a period of 120 days in continuously fed CSTRs where the microbial community adapted to the SEBW feedstock. Changes in the microbial community were tracked by stable carbon isotopes- and 16S r RNA analyses. The results showed that the adapted microbial culture could increase methane production up to 365 mL/g VS day, which is higher than previously reported methane production from pre-treated SEBW. This study also revealed that the microbial adaptation significantly increased the tolerance of the microbial community against the inhibitors furfural and HMF which were formed during pre-treatment of birch. The results of the microbial analysis indicated that the relative amount of cellulosic hydrolytic microorganisms (e.g. Actinobacteriota and Fibrobacterota) increased and replaced syntrophic acetate bacteria (e.g. Cloacimonadota, Dethiobacteraceae, and Syntrophomonadaceae) as a function of time. Moreover, the stable carbon isotope analysis indicated that the acetoclastic pathway became the main route for methane production after long-term adaptation. The shift in methane production pathway and change in microbial community shows that for anaerobic digestion of SEBW, the hydrolysis step is important. Although acetoclastic methanogens became dominant after 120 days, a potential route for methane production could also be a direct electron transfer among Sedimentibacter and methanogen archaea.

Effect of photocatalysis on the physicochemical properties of liquid digestate

Antibiotic residues pose a risk to the agricultural application of liquid digestate. In our previous study, photocatalysis was employed to degrade the antibiotics in liquid digestate and observed that the removal efficiency of TC, OTC, and CTC was up to 94.99%, 88.92%, and 95.52%, respectively, at the optimal experimental level, demonstrating the feasibility of this technology. In this study, the liquid digestate after photocatalysis was analyzed to evaluate the effect of photocatalysis on the nutrients, phytotoxicity, and bacterial community of liquid digestate. The results showed that photocatalysis had little effect on the major nutrients TN, TP, and TK in liquid digestate. However, photocatalysis could cause an increase in tryptophan substances as well as soluble microbial by-products and a decrease in humic acid substances in the liquid digestate. The toxicity of liquid digestate after photocatalysis exhibited an increasing trend followed by a decreasing trend, and the liquid digestate after photocatalysis for 2 h had a promoting effect on seed germination and root growth. The richness, diversity, and evenness of bacterial communities in liquid digestate were decreased as a result of photocatalysis. The dominant species in the liquid digestate was dramatically changed by photocatalysis, and the antibiotic concentration also had a major effect on the dominant species in the liquid digestate after photocatalysis. After photocatalysis for 2 h, the dominant species in the liquid digestate changed from Firmicutes to Proteobacteria.

Pretreatments of lignocellulosic and algal biomasses for sustainable biohydrogen production: Recent progress, carbon neutrality, and circular economy

Lignocellulosic and algal biomasses are known to be vital feedstocks to establish a green hydrogen supply chain toward achieving a carbon-neutral society. However, one of the most pressing issues to be addressed is the low digestibility of these biomasses in biorefinery processes, such as dark fermentation, to produce green hydrogen. To date, various pretreatment approaches, such as physical, chemical, and biological methods, have been examined to enhance feedstock digestibility. However, neither systematic reviews of pretreatment to promote biohydrogen production in dark fermentation nor economic feasibility analyses have been conducted. Thus, this study offers a comprehensive review of current biomass pretreatment methods to promote biohydrogen production in dark fermentation. In addition, this review has provided comparative analyses of the technological and economic feasibility of existing pretreatment techniques and discussed the prospects of the pretreatments from the standpoint of carbon neutrality and circular economy.

Exploring agricultural waste biomass for energy, food and feed production and pollution mitigation: A review

Globally agricultural production system generates a huge amount of solid waste. Improper agri-waste management causes environmental pollution which resulted in economic losses and human health-related problems. Hence, there is an urgent need to design and develop eco-friendly, cost-effective, and socially acceptable agri-waste management technologies. Agri-waste has high energy conversion efficiency as compared to fossil fuel-based energy generation materials. Agri-waste can potentially be exploited for the production of second-generation biofuels. However, composted agri-waste can be an alternative to energy-intensive chemical fertilizers in organic production systems. Furthermore, value-added agri-waste can be a potential feedstock for livestock and industrial products. But comprehensive information concerning agri-waste management is lacking in the literature. Therefore, the present study reviewed the latest advancements in efficient agri-waste management technologies. This latest review will help the researchers and policy planners to formulate environmentally robust residue management practices for achieving a green economy in the agricultural production sector.

Simulated elephant colon for cellulose extraction from sugarcane bagasse: An effective pretreatment to reduce chemical use

Sugarcane bagasse (SCB) is an abundant by-product from sugar production and promising biomass for cellulose extraction. Simulated elephant colon pretreatment (SEP) to reduce chemical use in cellulose extraction from SCB was investigated using elephant dung as fermentation inoculum. The 16S rRNA gene sequences showed microorganisms in elephant dung that corresponded to metabolites during pretreatment. Organic acid accumulation in the fermentation broth was confirmed by the presence of lactic, acetic, propionic and butyric acids. Lignin peroxidase, manganese peroxidase and xylanase detected during the pretreatment enhanced lignin removal. The SEP fiber showed increased cellulose content, while lignin content decreased with reduced bleaching time from 7 to 5 h and high whiteness and crystallinity indices. Lignin removal was also confirmed by Fourier transform infrared spectroscopy. Scanning electron microscopy revealed increasing internal surface area through opening up the fiber structure. SEP offered an efficient and promising approach for cellulose fiber extraction with reduced use of chemicals for the bleaching process.

Endophytes in Lignin Valorization: A Novel Approach

Lignin, one of the essential components of lignocellulosic biomass, comprises an abundant renewable aromatic resource on the planet earth. Although 15%––40% of lignocellulose pertains to lignin, its annual valorization rate is less than 2% which raises the concern to harness and/or develop effective technologies for its valorization. The basic hindrance lies in the structural heterogeneity, complexity, and stability of lignin that collectively makes it difficult to depolymerize and yield common products. Recently, microbial delignification, an eco-friendly and cheaper technique, has attracted the attention due to the diverse metabolisms of microbes that can channelize multiple lignin-based products into specific target compounds. Also, endophytes, a fascinating group of microbes residing asymptomatically within the plant tissues, exhibit marvellous lignin deconstruction potential. Apart from novel sources for potent and stable ligninases, endophytes share immense ability of depolymerizing lignin into desired valuable products. Despite their efficacy, ligninolytic studies on endophytes are meagre with incomplete understanding of the pathways involved at the molecular level. In the recent years, improvement of thermochemical methods has received much attention, however, we lagged in exploring the novel microbial groups for their delignification efficiency and optimization of this ability. This review summarizes the currently available knowledge about endophytic delignification potential with special emphasis on underlying mechanism of biological funnelling for the production of valuable products. It also highlights the recent advancements in developing the most intriguing methods to depolymerize lignin. Comparative account of thermochemical and biological techniques is accentuated with special emphasis on biological/microbial degradation. Exploring potent biological agents for delignification and focussing on the basic challenges in enhancing lignin valorization and overcoming them could make this renewable resource a promising tool to accomplish Sustainable Development Goals (SDG’s) which are supposed to be achieved by 2030.

A novel mechanocatalytical reaction system driven by fluid shear force for the mild and rapid pretreatment of lignocellulosic biomass

Pretreatment is the initial stage of lignocellulosic biorefinery process, but is limited by the time-consuming processes, harsh conditions and/or undesirable products. Herein, a mild (<60 °C) and highly efficient pretreatment strategy is developed. The novel mechanocatalytical reaction system driven by fluid shear force helps to exfoliate cellulose from lignocellulose, and the heat generated by the shear process can be used to precipitate and recover the dissolved cellulose from the precooled NaOH/urea solution. The regenerated cellulose shows satisfying crystal structure (cellulose II), significantly decreased crystallinity and nearly tripled enzymolysis glucose yield. Almost 90% of lignin and hemicellulose could be rapidly separated. The separated lignin shows a nearly native structure with 64% β-O-4 linkage, which is even higher than the ball-milling lignin (60%). This research provides a theoretical guidance for the mild pretreatment of lignocellulosic biomass, which will push the application of mechanocatalytical reaction system in biorefinery processes on a large scale.

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).

Impact of substrate digestibility on microbial community stability in methanogenic digestors: The mechanism and solution

This study investigated the temporal dynamics of digestion efficiency and community stability in digesters fed with waste activated sludge (WAS), straw (STR-AD), food waste (FW-AD) and mixture of straw-and-food waste (STR-FW-AD). Results showed that carbon removals of recalcitrant substrates (i.e., 48.2 ± 3.9% in WAS-AD and 57.8 ± 4.9% in STR-AD) were lower than that of labile substrates (i.e., 70.7 ± 4.0% in FW-AD). Nonetheless, carbon removal of recalcitrant substrates was largely improved through co-digestion (70.3 ± 3.2% in STR-FW-AD). In contrast to monopoly communities (e.g., the highly enriched Paludibacter) fed with the labile substrates, recalcitrant substrates supported highly diverse communities. Accordingly, the medians of negative/positive cohesions of communities in WAS-AD, STR-AD, STR-FW-AD and FW-AD decreased from 0.86 to 0.63, suggesting their decreasing community stability. Microbial source tracking analyses showed the major contribution of the STR-AD community to the co-digestion community. This study provided unprecedented mechanistic insight into stability improvement of substrate co-digestion on the methanogenic digestion microbiome.

Development of different pretreatments and related technologies for efficient biomass conversion of lignocellulose

Lignocellulose, a kind of biological resource widely existing in nature, which can be transformed into value-added biochemical products through saccharification, fermentation or chemical catalysis. Pretreatments are the necessary step to increase the accessibility and digestibility of lignocellulose. This paper comprehensively reviewed different pretreatment progress of lignocellulose in recent year, including mechanical/thermal, biological, inorganic solvent, organic solvent and unconventional physical-chemical pretreatments, focusing on quantifying the influence of pretreatments on subsequent biomass conversion. In addition, related pretreatment techniques such as genetic engineering, reactor configurations, downstream process and visualization technology of pretreatment were discussed. Finally, this review presented the challenge of lignocellulose pretreatment in the future.

Exergy analysis and optimization of bio-methane production from corn stalk pretreated by compound bacteria based on genetic algorithm

An exergy equilibrium model was established to obtain the exergy efficiency under different conditions of compound bacteria pretreatment and anaerobic digestion (AD) of corn stalk. The Genetic Algorithm (GA) was applied to optimize the exergy efficiency of the combination process of the pretreatment and AD. The maximum exergy efficiency with the GA was 19.04%, corresponding to the optimal pretreatment parameters: pretreatment temperature 33.34℃, stalk particle size 0.50 mm, ventilation rate 0.88 L/min, pretreatment time 169.03 h. The optimal AD parameters were: digestion temperature 38.08℃ and stirring rate 48.04 r/min. The validation experiment exergy efficiency reached to 19.25%, which was 24.37% higher as compared to that of the non-pretreatment process. Under these optimal conditions, the energy consumption of the compound bacteria pretreatment and the time of the bio-methane production process were effectively reduced.

Agricultural wastes co-densification: A solution for seasonal feedstock storage and anaerobic digestion performance improvement

Rice straw and pig manure pellets (RPP) and sorghum straw and pig manure pellets (SPP) were used to identify their competition as the flexible feedstock of anaerobic digestion with one-year indoor storage. The results indicated the effect of time on their characteristic was tiny during storage period, such as density, calorific value, total solid, volatile solid, ratio of carbon and nitrogen, and lignocellulosic components. Biogas yields of stored RPP and SPP were 8.8% and 26.7% lower than that of fresh pig manure (PM), and 45.4% and 56.1% higher than the sum of corresponding straw and PM digestion alone, respectively. Improvements in biodegradability were observed in co-densified biomass anaerobic digestion. Net biogas yield of RPP was 24.2% higher than that of rice straw, considering volatile matter loss and biogas yield decline during densification and storage stage. Priority of manure and supplement of co-densified biomass were proposed for feedstock supply on demand.

Enhanced methane production and hydrocarbon removal from petroleum refinery sludge after Pseudomonas putida pretreatment and process scale-up

The influence of Pseudomonas putida 7525 strain on the pretreatment of petroleum refinery sludge was optimized at different dosages to maximize solubilization for improved biodegradability. Laccase-producing P. putida strain at a dosage of 10⁸ CFU/mL resulted in 249% and 121.57% increments in soluble chemical oxygen demand and volatile fatty acids production respectively as compared to untreated within 6 days of incubation. 1L biochemical methane potential test conducted for optimization of different inoculum and pretreated substrate ratios (0.3, 0.4, 0.5, 0.7 and 1.0) revealed maximum methane augmentation (62%) and volatile solids degradation (66.7%) at ratio 0.5. Scaled-up study (20L) for ratio 0.5 resulted in 57.07% total petroleum hydrocarbon, 62.98% oil and grease and 91.9% phenol removal within 50 days of digestion of pretreated PS. Kinetic modelling of cumulative methane yield indicated that modified Gompertz model showed the best fit thereby, evincing the potency of bacterial species for bioremediation of PS.

Bioaugmented degradation of rice straw combining two novel microbial consortia and lactic acid bacteria for enhancing the methane production

Two consortia of lignocellulolytic microbes (CL and YL) were isolated from the rumen of ruminants. Their ability to facilitate the degradation of rice straw and enhance methane (CH₄) production were evaluated, both individually and combined with lactic acid bacteria (LAB). After 30 days of degradation, rice straw powders (RSPs) were observed to change in physical structure and also displayed a significant reduction in lignocellulose content. Combined application of microbial consortia with LAB efficiently improved enzymatic hydrolysis of RSPs, increasing organic acid as well as mono- and disaccharide contents. Synergistic action between microbial consortia and LAB enhanced CH₄ yield, and rice straw treated with YL + LAB had the highest CH₄ production (357.53 mL CH₄/g VS), more than fivefold of the control. The newly identified microbial consortia are capable of efficiently degrading lignocellulosic biomass. Functioning synergistically with LAB, they provide a feasible way biodegrade rice straw and enhance methane production from agricultural wastes.

Post-digestate composting shifts microbial composition and degrades antimicrobial resistance genes

Post-digestate treatments may reduce the risk linked to Antibiotic Resistant Genes (ARGs) release with digestate direct land application. Thus, this study aimed to evaluate post-digestate composting and co-composting with biogas production feedstock (maize silage, food processing waste, and poultry litter) effect on abundance of selected ARGs: erm(B), tet(K), tet(M), tet(O), and tet(S) genes. More than 80% of all ARGs were removed after 90 days of composting but removals from co-composting were lower. Bacteroidetes, Firmicutes, and Proteobacteria dominated fresh digestate, and a network analysis indicated that these were potential hosts of ARGs. The emergence of Actinobacteria (dominant), Planctomycetes, and Verrucomicrobia phyla during composting shifted the microbial composition. Moreover, canonical correspondence analysis showed trace elements explaining 90% variations in ARGs abundance. The study illustrates significance of post-digestate composting in mitigating ARGs release, and effectiveness could be linked to shift in microbial composition and trace elements release.

An efficient strategy to improve enzymatic hydrolysis of naked oat straw pretreated by Irpex lacteus

The objective of this study was aiming at developing an efficient strategy to promote enzymatic hydrolysis of naked oat straw and deciphering the potential mechanism. Irpex lacteus and Phlebia acerina were employed to inoculated on the naked oat straw for 4 weeks which the changes of fiber components, fermentation losses, lignin-degrading enzymes production pattern were determined weekly. Furthermore, the 72 h enzymatic hydrolysis of ultimately fermented naked oat straw were also evaluated. The acid detergent lignin was degraded at about 25% along with the moderate dry matter and cellulose loss which both showed selective degradation. The lignin-degrading enzymes production patterns of the two fungi were different which lignin peroxidase was not detected in Irpex lacteus treatment. In addition, the activities of cellulolytic enzymes were higher in Phlebia acerina treatment. After 72 h enzymatic hydrolysis, the reducing sugar content and hydrolysis yield pretreated by Irpex lacteus was 12.92 g/L and 69.49%, respectively. It was much higher than that in sterilized substrate and Phlebia acerina treatment. Meanwhile, the hydrolysis yields of glucose, sum of xylose and arabinose were all improved by Irpex lacteus which were 30.96% and 25.62%, respectively, and showed significant enhancements compared to control and Phlebia acerina treatment. Irpex lacteus is one of effective white rot fungi which could promote the enzymatic hydrolysis of naked oat straw obviously.

Effect of Biochar Addition on the Microbial Community and Methane Production in the Rapid Degradation Process of Corn Straw

Anaerobic digestion with corn straw faces the problems of difficult degradation, long fermentation time and acid accumulation in the high concentration of feedstocks. In order to speed up the process of methane production, corn straw treated with sodium hydroxide was used in thermophilic (50 °C) anaerobic digestion, and the effects of biochar addition on the performance of methane production and the microbial community were analyzed. The results showed that the cumulative methane production of all treatment groups reached over 75% of the theoretical methane yield in 7 days and the addition of 4% biochar increased the cumulative methane production by 6.75% compared to the control group. The addition of biochar also decreased the number of biogas and methane production peaks from 2 to 1, and had a positive effect on shortening the digestion start-up period and reducing the fluctuation of biogas production during the digestion process. The addition of 4% biochar increased the abundance of the bacterial family Peptococcaceae throughout the digestion period, promoting the hydrolysis rate of corn straw. The dominant archaeal genus Methanosarcina was significantly more abundant at the peak stage and the end of methane production with 4% biochar added compared to the control group.