Insights into the Effect of Sludge Retention Times on System Performance, Microbial Structure and Quorum Sensing in an Activated Sludge Bioreactor

Effects of sludge retention times (SRTs) on system performance, microbial structure and quorum sensing in an activated sludge bioreactor were investigated. SRTs significantly (ANOVA, p < 0.05) affected the total inorganic nitrogen (TIN) removal. Significant (Tukey's HSD, p < 0.05) higher TIN removal efficiency was achieved at the SRT of 5 days (72.3%) than at SRTs of 15 (71.3%) and 25 (67.7%) days, respectively. This could be partially explained by the effect of SRT on the sludge properties, abundance of denitrifying bacterium and genes, and quorum sensing (QS) signals (e.g., N-acyl-L-homoserine lactones (AHLs)). Specifically, a short SRT resulted in a high protein content in tightly bound extracellular polymeric substances (TEPS), which could be an indicator for compact sludge structure. Furthermore, denitrifying bacterium including Candidatus Competibacter, Thauera and Zoogloea and denitrifying genes including napAB, nirS, norB and nosZ were enriched at a short SRT. Moreover, N-decanoyl-L-homoserine lactone and N-3-(oxododecanoyl)-homoserine lactone were the dominating AHLs in the water, EPS and biomass phases of activated sludge and a particular high content of these AHLs was found in sludge at a short SRT. Based on the metagenomic results, functional microorganisms carried both nitrogen metabolism genes and AHLs synthesis genes, allowing their interactions with other autotrophs or heterotrophs through nitrogen conversion and QS signal exchange. Identification of QS and its possible roles in activated sludge systems under different operational conditions may provide potential strategies through QS regulation for enhancing system performance and microbial aggregation capability.

Automated summary

The effects of sludge retention times (SRTs) on system performance, microbial structure, and quorum sensing were investigated in an activated sludge bioreactor. Short SRTs resulted in higher removal efficiency of inorganic nitrogen (TIN) and enrichment of specific bacteria and genes. Additionally, an abundance of quorum sensing signals was found in the sludge at short SRTs.