Insight into sludge bulking in a full-scale wastewater treatment plant: Quorum sensing, microbial community, and metabolic characteristics

Sludge bulking has always been a persistent problem in wastewater treatment plants (WWTPs) using activated sludge process due to the lack of understanding of its microscopic mechanism. The aim of this study was to investigate the sludge bulking and microbial community behavior in a full-scale WWTP from the perspective of quorum sensing, microbial community, and metagenomic characteristics. This study found that the excessive growth of Candidatus Microthrix was the primary case of sludge bulking occurred from January to March. This overgrowth also resulted in changes of microbial diversity and microbial community structure. Quorum sensing, mediated by N-acyl-L-homoserine lactones (AHLs), played a role in regulating sludge bulking, particularly C6-HSL, while EPS production was mediated by 3OC8-HSL. In addition, the EPS-producing bacteria, AHLs-producing bacteria, and quorum quenching bacteria were also detected in this WWTP. The metagenomic results also showed that bacterial intercellular communication was enhanced to control sludge bulking. The "Two-component system" was responsible for bacterial communication and cooperation to regulate the filamentous bacteria growth, and it was activated by "Oxidative phosphorylation". Meanwhile, "Bacterial secretion system", "Valine, leucine and isoleucine biosynthesis", "Lysine biosynthesis", as well as "Phenylalanine, tyrosine and tryptophan biosynthesis" were related to EPS production. These results provide new insights into the microscopic mechanisms of filamentous sludge bulking in full-scale WWTPs, which will contribute to a better understanding of the mechanisms of sludge bulking.