Disentangling microbial niche balance and intermediates' trade-offs for anaerobic digestion stability and regulation.

Anaerobic digestion (AD) is a key technology for converting organic matters to methane-rich biogas. However, nutrient imbalance can destabilize the whole digestion. To realize stable operation of AD and improve its efficiency, this work considers a new strategy to control the intermediate concentrations of poor AD under nutrient stress. For this purpose, long-term digestion under different nutrient conditions was investigated. Results showed that the feedstock with a low C/N ratio (= 6) caused VFA accumulation (2072 ± 632 mg/L), leading to the inhibition of methane production. Employing a substrate with a higher C/N ratio (= 11) and/or adding NHHCO (200 mg NH-N/L) could alleviate the VFA inhibition, but excessive dosage of NHHCO would induce ammonia inhibition. Through the established digestion balance between free ammonia nitrogen (FAN) between 0 and 25 mg/L, volatile fatty acid (VFA) 510-2100 mg/L, and alkalinity (ALK) 3300-7800 mg/L, an efficient methane yield of 150-250 mL/g VS was achieved and stable operation of AD under nutrient stress (low C/N ratio) was realized. Metabolic reconstruction between Euryarchaeota sp. MAG162, Methanosarcina mazei MAG53 and Mesotoga infera MAG119 highlighted that microbial niche balance was developed as a result of digestion balance, which is beneficial for stable operation of AD. These findings improved our understanding of the interaction mechanism between intermediates and microbial niches for stability control in AD.