Micro-aeration has been shown to be favorable for anaerobic digestion (AD) process. However, the underlying mechanisms are not fully revealed. In this study, the effects of micro-aeration on AD process were investigated, the underlying mechanisms were also explored by comprehensively analysis of the substrate degradation, intermediate products, electron-transfer capacity, key enzymes and microbial communities. Results showed micro-aeration could benefit the AD process, and the beneficial effects were oxygen intensity-dependent. Maximum methane yield (262.0 ± 6.0 mL/(g·VS)) was achieved at micro-aeration of 0.2 mL/(g·VS·day), which was 7.8% higher than that of the control (without micro-aeration). The volatile fatty acids concentration, cellulase activity, soluble humic acids (SHAs) concentration and substrate degradation rate were all enhanced under micro-aeration condition. Moreover, micro-aeration could regulate the bacterial and archaeal communities: bacterial diversities and relative abundances of facultative and acetate-oxidizing bacteria (e.g., Firmicutes, Synergistota and Spirochaetota) increased, leading to higher substrate utilization rate; archaeal diversities decreased while oxygen-tolerating methanogens (e.g., Methanobacterium) became dominant to promote the methanogenesis. AD system with micro-aeration showed higher electron-transfer capacity and better conductive environment due to the increased SHAs concentration. The increased relative abundance of direct-interspecies-electron-transfer (DIET)-associated microbes could be favorable to DIET-based syntrophic methanogenesis which promotes AD under micro-aeration.

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