Insights into the suppression of filamentous bulking by quorum quenching in the activated sludge process with low dissolved oxygen: Endogenous quorum quenching mediated multi-enzymatic degradation of AHLs and carbon-flux redistribution.
Low dissolved oxygen (DO) operation can substantially reduce aeration energy consumption in activated sludge systems but often induces filamentous sludge bulking dominated by Sphaerotilus natans (S. natans). This study isolated endogenous quorum quenching (QQ) bacteria from activated sludge and evaluated their potential to control low-DO bulking. Three endogenous QQ strains were newly isolated, among which Pseudochrobactrum sp. (P. sp.) showed excellent environmental adaptability and broad-spectrum N-acyl homoserine lactone degradation through multi-enzyme synergy, quenching C6-HSL, C8-HSL, and C14-HSL by 93.1%, 94.2%, and 93.0% within 24 h, respectively. Crude QQ enzymes inhibited the filamentous differentiation of S. natans, reduced peak C8-HSL concentration by 47%, and weakened ATP levels and electron transport chain activity. 13C metabolic flux analysis showed that QQ redirected carbon flux from biomass synthesis (via the glyoxylate shunt) toward energy metabolism (via the tricarboxylic acid cycle). However, the decreased ATP level and electron transport chain activity indicate that the enhanced TCA cycle flux may represent a compensatory metabolic adjustment under energy stress, ultimately leading to an inefficient energy-cycling state. Meanwhile, carbon skeleton loss restricted the supply of biosynthetic precursors and consequently inhibited filament elongation. P. sp. was then immobilized in optimized gel beads (P-gel) and applied in sequencing batch reactors (SBRs). Under low-DO operation, the SBR supplemented with P-gel effectively suppressed sludge bulking, prevented biomass loss, and sustained stable nutrients removal by regulating physicochemical properties of activated sludge. This study provides an environmentally friendly in-situ strategy for controlling S. natans-type bulking in low-DO activated sludge systems.