From activation to stability: Deterministic selection and adaptive mechanisms reshape microbial community for efficient volatile fatty acids production in batch-to-continuous sludge fermentation

Anaerobic fermentation is widely regarded as a sustainable technique for treating waste activated sludge (WAS). In practical application, the batch-to-continuous operational strategy is advantageous to maximize volatile fatty acids (VFAs) production. However, it remains unclear how this strategy impacts microbial metabolic activity and succession process. In this study, a WAS fermentation system was constructed under batch mode for 12 days, followed by long-term semi-continuous operation with a short sludge retention time (SRT) of 4 days. Results showed that after the mode changed to semi-continuous, the reactor gradually gained good VFAs-producing performances, stabilized at an acidification ratio of 70 %. The ecological network structure of semi-continuous stage became more stable, characterized by higher modularity and lower betweenness centralization. Although the batch stage provided sufficient substrates, the metabolic activity of fermentative microbes was initially inhibited by the shift in fermentation mode and restored via adaptive mechanisms (two-component system and quorum sensing). The null model analysis revealed that deterministic selection, governed by the short SRT, dominated microbial community assembly. This process enriched hydrolytic and acidogenic bacteria while excluding VFAs consumers, with diffusion limitation further enhancing community stability. Overall, for batch-to-continuous anaerobic fermentation, the batch stage laid a foundation for an efficient metabolism throughout later periods, while in the semi-continuous stage, the short SRT served as an environmental filter, promoting the succession of hydrolytic and acidogenic bacteria, meanwhile excluding VFAs consumers. These findings provide a comprehensive insight into the mechanisms of community assembly in sludge fermentation, offering a theoretical support for potential community design and reactor regulation.