Impact of operational mode on the relationship between quorum sensing and sludge bulking

Suboptimal reactor conditions are pivotal in triggering sludge bulking, where quorum sensing (QS) mediates the deterioration of sludge settleability through regulating the physiological behavior of specific microorganisms. The effects of two operational modes (R1: anoxic/aerobic; R2: fully aerobic) on the performance, QS, and microbial community of the sequencing batch reactors were investigated using a prolonged feeding strategy. Both modes induced severe sludge bulking; however, the anoxic condition in R1 delayed bulking onset by approximately 10 days relative to R2. QS analysis indicated that the increase in C4-homoserine lactone (-HSL) and the decrease in C12-HSL in R1, as well as the increase in C6-HSL and the decrease in C10-HSL in R2, were significantly correlated with the sludge volume index. Functional gene dynamics revealed that the upregulation of autoinducer receptor genes coupled with the downregulation of degradation enzyme genes synergistically enhanced QS signal accumulation. Pearson's correlation analysis demonstrated that these acyl-HSLs regulated secretion of extracellular polymeric substances, particularly polysaccharides, by mediating the metabolic activities of dominant genera such as Thiothrix and Zoogloea ; N-acyl-L-HSL concentrations were subject to fine-tuned feedback regulation within the microbial community, which varied with operational mode. The continuous dynamic interplay between QS and the microbial community drove changes in core system performance indicators, including pollutant removal efficiency, sludge settling characteristics, and extracellular polymeric substance secretion patterns. These findings provide a theoretical foundation for understanding the microscopic mechanisms underlying filamentous sludge bulking. • Continuous low substrate concentration is a key filamentous bulking trigger. • Dominant R1 (C4/C12-HSL) and R2 (C6/C10-HSL) signals correlate with SVI. • Thiothrix and Zoogloea abundance shifts respond to different signaling molecules. • C4/C6/C10-HSL mediates Zoogloea in regulating EPS to influence settling. • Differential QS gene regulation links operational mode to bulking phenotypes.