Biomethane enhancement from corn straw using anaerobic digestion by-products as pretreatment agents: A highly effective and green strategy

Effect of hydrothermal-acid pretreatment on methane yield and microbial community in anaerobic digestion of rice straw

Hydrothermal pretreatment has been proposed to enhance straw methane yield during anaerobic digestion recently. However, the combined effect of hydrothermal and organic acid pretreatment (HTOAP) needs further investigation. This study identified optimal pretreatment at 120 °C with 3 % acetic acid for 24 h by orthogonal design method. The HTOAP increased the reducing sugar content by destroying the lignocellulosic structure. A 79 % increment of methane production after HTOAP was observed compared to the untreated group. Microbial analysis showed that HTOAP enriched the relative abundance of lignocellulose-degraders, such as W5053, Thermanaerovibrio, Caldicoprobacter, as well as the syntrophic acetate oxidizing bacteria Syntrophaceticus. Moreover, Methanobacterium conducted hydrogenotrophic methanogenesis dominantly. Furthermore, the potential function analysis showed that HTOAP stimulated the expression of key enzymes in the hydrogenotrophic pathway, including carbon-monoxide dehydrogenase (EC 1.2.7.4) and coenzyme F420 hydrogenase (EC 1.12.98.1). This investigation illustrated the potential of HTOAP of rice straw to facilitate methane production.

Integrated dark-fermentation-microbial electrosynthesis for efficient wastewater treatment and bioenergy recovery

High ammonia concentration in wastewater can hinder methane production rate in anaerobic digestion (AD)-microbial electrosynthesis systems (ADMES). To address this issue, a dual-chamber reactor was fabricated using an anion exchange membrane (AEM) to separate the dark-fermentation (DF) and ADMES process, preventing ammonia migration from the DF chamber to the ADMES chamber. As a result, the DF-ADMES achieved a high methane yield based on chemical oxygen demand (COD) of 0.35 L CH4/gCOD compared to control operation AD (0.23 L CH4/gCOD) and ADMES (0.30 L CH4/gCOD). Additionally, hydrogen could be recovered from the DF chamber which improved the energy efficiency of the DF-ADMES reactor (91.7 %) as compared to control AD (53.4 %) and ADMES (71.9 %). Thus, a dual-chamber DF-ADMES with an AEM separator could be a feasible design for scalable treatment of high nitrogen-containing wastewater and high bioenergy recovery.

Two-step anaerobic digestion of rice straw with nanobubble water

Lignocellulose usually requires pretreatment to improve biogas production. To enhance lignocellulose biodegradability and improve anaerobic digestion (AD) efficiency, different types (N₂, CO₂, and O₂) of nanobubble water (NW) were applied in this study as soaking agent and AD accelerant to increase the biogas yield of rice straw. The results showed that the cumulative methane yields of treating with NW in two-step AD increased by 11.0%-21.4% compared with untreated straw. The maximum cumulative methane yield was 313.9±1.7 mL/gVS in straw treated with CO₂-NW as soaking agent and AD accelerant (P_CO2-M_CO2). The application of CO₂-NW and O₂-NW as AD accelerants increased bacterial diversity and relative abundance of Methanosaeta. This study suggested that using NW could enhance soaking pretreatment and methane production of rice straw in two-step AD; however, combined treatment with inoculum and NW or microbubble water in the pretreatment needs to compare in future.

Critical review of biochemical pathways to transformation of waste and biomass into bioenergy

In recent years, biofuel or biogas have become the primary source of bio-energy, providing an alternative to conventionally used energy that can meet the growing energy demand for people all over the world while reducing greenhouse gas emissions. Enzyme hydrolysis in bioethanol production is a critical step in obtaining sugars fermented during the final fermentation process. More efficient enzymes are being researched to provide a more cost-effective technique during enzymatic hydrolysis. The exploitation of microbial catabolic biochemical reactions to produce electric energy can be used for complex renewable biomasses and organic wastes in microbial fuel cells. In hydrolysis methods, a variety of diverse enzyme strategies are used to promote efficient bioethanol production from various lignocellulosic biomasses like agricultural wastes, wood feedstocks, and sea algae. This paper investigates the most recent enzyme hydrolysis pathways, microbial fermentation, microbial fuel cells, and anaerobic digestion in the manufacture of bioethanol/bioenergy from lignocellulose biomass.

Recent progress and challenges in biotechnological valorization of lignocellulosic materials: Towards sustainable biofuels and platform chemicals synthesis

Lignocellulosic materials (LCM) have garnered attention as feedstocks for second-generation biofuels and platform chemicals. With an estimated annual production of nearly 200 billion tons, LCM represent an abundant source of clean, renewable, and sustainable carbon that can be funneled to numerous biofuels and platform chemicals by sustainable microbial bioprocessing. However, the low bioavailability of LCM due to the recalcitrant nature of plant cell components, the complexity and compositional heterogeneity of LCM monomers, and the limited metabolic flexibility of wild-type product-forming microorganisms to simultaneously utilize various LCM monomers are major roadblocks. Several innovative strategies have been proposed recently to counter these issues and expedite the widespread commercialization of biorefineries using LCM as feedstocks. Herein, we critically summarize the recent advances in the biological valorization of LCM to value-added products. The review focuses on the progress achieved in the development of strategies that boost efficiency indicators such as yield and selectivity, minimize carbon losses via integrated biorefinery concepts, facilitate carbon co-metabolism and carbon-flux redirection towards targeted products using recently engineered microorganisms, and address specific product-related challenges, to provide perspectives on future research needs and developments. The strategies and views presented here could guide future studies in developing feasible and economically sustainable LCM-based biorefineries as a crucial node in achieving carbon neutrality.

Deeper insight into the effect of salinity on the relationship of enzymatic activity, microbial community and key metabolic pathway during the anaerobic digestion of high strength organic wastewater

The threshold salt concentration to inhibit the anaerobic digestion (AD) has been intensively investigated, but its insight mechanism is not fully revealed. Therefore, this study systematically investigated the effect of salinity on acidogenesis and methanogenesis and its mechanism. Results showed that low salinity level (i.e. 0.6%) had stimulatory effect on volatile fatty acids (VFA) and methane production, while significant inhibition was observed with further increased salinity. Moreover, high salinity limited the butyric acid degradation at acidogenesis process. The decreases of enzymes (AK and PTA) activity and functional genes (ackA, pta and ACOX) expression that related to β-oxidation explained the butyric acid accumulation at high salinity levels. Microbial community analysis revealed high salinity levels significantly inhibited the proliferation of Syntrophomonas sp., which are known to be associated with butyric acid degradation. Similarly, the relative abundance of acetoclastic methanogen (Methanothrix sp.) and methylotrophic methanogen (Methanolinea sp.) significantly decreased at salinity condition.

Integrative process for rubberwood waste digestibility improvement and levulinic acid production by hydrothermal pretreatment with acid wastewater conversion process

This study aimed to develop an integrative process for converting rubberwood waste into sugars, methane, and levulinic acid. Sulfuric acid pretreatment at pH 2.5 yielded the highest glucose of 182.5 g/kg rubberwood waste. Replacing the acid solution with sulfuric acid wastewater led to 11.0% lower glucose yield than that obtained using sulfuric acid. However, the cost reduction equals the difference in revenues between sulfuric acid wastewater and sulfuric acid, resulting in similar total cost and revenue. Furthermore, thermal reactions of the process water resulted in the highest yield of levulinic acid, 17.9% at 220 °C. Meanwhile, anaerobic digestibility of enzymatic hydrolysis residue was increased using inoculum from a digester treating pig farm wastewater owing to the acetoclastic pathway. These co-products potentially returned additional revenues, accounting for 45.8% of the total revenue. These findings highlight the potential pathway for valorization of rubberwood waste via the integrated approach with acid wastewater pretreatment.

Syntrophic bacteria- and Methanosarcina-rich acclimatized microbiota with better carbohydrate metabolism enhances biomethanation of fractionated lignocellulosic biocomponents

An inadequate lignocellulolytic capacity of a conventional anaerobic digester sludge (ADS) microbiota is the bottleneck for the maximal utilization of lignocellulose in anaerobic digestion. A well-constructed microbial consortium acclimatized to lignocellulose outperformed the ADS in terms of biogas productivity when fractionated biocomponents of rice straw were used to achieve a high methane bioconversion rate. A 33.3 % higher methane yield was obtained with the acclimatized consortium (AC) compared to that of ADS control. The dominant pair-wise link between Firmicutes (18.99-40.03 %), Bacteroidota (10.94-28.75 %), and archaeal Halobacteriota (3.59-20.57 %) phyla in the AC seed digesters indicated that the keystone members of these phyla were responsible for higher methane yield. A high abundance of syntrophic bacteria such as Proteiniphilum (1.22-5.19 %), Fermentimonas (0.71-5.31 %), Syntrophomonas (0.87-3.59 %), and their syntrophic partner Methanosarcina (4.26-18.80 %) maintained the digester stability and facilitated higher substrate-to-methane conversion in the AC seed digesters. The present combined strategy will help in boosting the 'biomass-to-methane" conversion.