New insight into filamentous sludge bulking: Potential role of AHL-mediated quorum sensing in deteriorating sludge floc stability and structure.

The microcosmic mechanisms underlying filamentous bulking remain unclear. The role of extracellular polymeric substances (EPS) governed by quorum sensing (QS) in deteriorating sludge floc stability and structure during filamentous bulking and the feasibility of using quorum quenching (QQ) to maintain sludge floc stability and structure and sludge settling were investigated in this study. The results indicated that the concentration of C6HSL increased from 22.08±3.22 ng/g VSS to 81.42±5.98 ng/g VSS during filamentous bulking. The filamentous bacteria gradually evolved the hdtS gene related to the synthesis of C6HSL with increases in the population density. Triggered QS by filamentous bacteria proliferation induced variation in the composition and structure of EPS within the sludge flocs. The proteins (PN) content of the EPS increased evidently from 40.06 ± 2.41 mg/g VSS to 110.32 ± 4.32 mg/g VSS, and the polysaccharides (PS) content slightly increased during filamentous bulking. The upregulated proteins in the EPS led to a decrease in the relative hydrophobicity of the sludge and an increase in negative surface charge. The α-helix/(ß-sheet+random coil) ratio evidently increased from 0.76 to 0.99 during filamentous bulking, revealing that the proteins were tightly structured, which prevented the exposure of inner hydrophobic groups. The total energy of the interaction (WT) between bacteria increased during sludge bulking, which resulted in the weakening of sludge aggregation. Variation in the physicochemical properties of EPS induced by QS in the filamentous bacteria markedly restrained adhesion between the filamentous bacteria and floc-forming bacteria. The production of PN in the EPS and the expression of the hdtS gene were inhibited by vanillin, which served as a QS inhibitor. The WT between bacteria with 50 mg/L of vanillin basically did not change. Filamentous bulking was significantly inhibited by the addition of vanillin. Therefore, QQ is a potential strategy for the prevention and control of filamentous bulking. This study provides new information regarding the microcosmic mechanisms of filamentous bulking.