Methane, Volume 3, Issue 2 (June 2024) – 3 articles

Saprotrophic fungi, key players in global carbon cycling, have been identified as methane (CH₄) sources not yet accounted for in the global CH₄ budget. This study, for the first time, explores the influence of oxygen (O₂) and temperature on CH₄ production by two fungi, Laetiporus sulphureus and Pleurotus sapidus. To explore the relationship between these parameters and fungal CH₄ formation, we examined CH₄ formation under varying O₂ levels (0 to 98%) and temperatures (17, 27, and 40 °C) during fungal growth on pine wood, beech wood, and grass under sterile conditions. Our findings show that fungal CH₄ formation strongly depends on O₂ levels. Methane formation was highest when O₂ levels exceeded 5%, whilst no CH₄ formation was observed after complete O₂ consumption. Reintroducing O₂ immediately resumed fungal CH₄ production. Methane formation normalized to O₂ consumption (CH₄__norm) showed a different pattern. L. sulphureus showed higher CH₄__norm rates with higher O₂ levels, whereas P. sapidus showed elevated rates between 0 and 5%. Temperature also significantly influenced CH₄ and CH₄__norm rates, with the highest production at 27 °C, and comparatively lower rates at 17 and 40 °C. These findings demonstrate the importance of O₂ levels and temperature in fungal CH₄ emissions, which are essential for refining CH₄ source predictions. Full article

Low and unstable digestion performance is a challenging issue for anaerobic digestion, which prompts researchers to develop new strategies. In addition to traditional approaches such as co-digestion, pre-treatment, and recirculation, some emerging strategies, namely additive processes and microaeration, have also been recognized and developed in recent years. Many studies have evaluated the effect of these strategies on digestion performance. However, their comprehensive analysis is lacking, especially regarding the mechanisms of the different strategies. This review presents a comprehensive overview of research progress on these strategies based on the latest research, considering the five main strategies listed above. Through critical thinking, a summary of their mechanism, reactor performance, and availability of these strategies is presented. The results demonstrate that the contribution of microaeration is mainly to balance the composition and activity of hydrolysis, acidogenesis, and methanogenic archaea. Recirculation and co-digestion mainly balance mass and reaction environments. Pre-treatment, such as removing lignin, reducing cellulose crystallinity, and increasing the substrate-specific surface area, makes the characteristics of the substrate more conducive to the digestion of microorganisms. The mechanism of additive strategies varies greatly depending on the type of additive, such as enhancing interspecies electron transfer through conductive materials, resisting adverse digestion conditions through functional microbial additives, and accelerating nutrient absorption by regulating the bioavailability of trace elements. Although these strategies have different mechanisms for promoting digestion performance, their ultimate effect is to allow the parameters of the reactor to reach an ideal status and then achieve a balance among the substance, microorganisms, and water in an anaerobic reactor. Full article

Psychrophilic anaerobic digestion emerges as an appealing integrated solution for the management of agricultural waste, particularly for farmers in regions where the average temperature does not exceed 26 °C, as seen in coffee cultivation. Therefore, this study seeks to assess the biomethane potential of thermochemical-treated coffee husk through psychrophilic anaerobic digestion (C3-20 °C-w/pretreatment). To examine its viability, outcomes were compared with reactors operating at both mesophilic (C1-35 °C) and psychrophilic (C2-20 °C) conditions, albeit without the use of pretreated coffee husk. The C3-20 °C-w/pretreatment test demonstrated a 36.89% increase (150.47 NmL CH₄/g VS; 161.04 NmL CH₄/g COD), while the C1-35 °C test exhibited a 24.03% increase (124.99 NmL CH₄/g VS; 133.77 NmL CH₄/g COD), both in comparison to the C2-20 °C test (94.96 NmL CH₄/g VS; 101.63 NmL CH₄/g COD). Notably, the C3-20 °C-w/pretreatment trial yielded superior outcomes, accompanied by an associated energy output of 3199.25 GWh/year, sufficient to meet the annual energy demands of 494 residences. This marks an increase of 83 and 182 million residences compared to the mesophilic and psychrophilic AD of CH without pretreatment, respectively. Full article