Family Sphingomonadaceae as the key executor of triclosan degradation in both nitrification and denitrification systems

The usage of triclosan (TCS) has increased with the COVID-19 virus outbreak, causing more TCS are released into wastewater treatment systems. However, the difference in TCS removal pathway and TCS degrading bacteria between nitrification and denitrification systems was still unknown. In this study, batch tests of TCS biodegradation mechanism and DNA stable isotope probing (DNA-SIP) technique were applied to decrypt the different TCS removal pathway and the corresponding degrading taxon between two nitrification and two denitrification systems. The main TCS degradation pathway in both nitrification and denitrification systems were the metabolism of heterotrophic bacteria, only a little TCS was degraded by the co-metabolism of heterotrophic or nitrifying bacteria, and higher NH4+-N or NO3--N concentration contributed to more TCS degradation. Moreover, denitrification systems had stronger TCS removal capacity (0.11 and 0.65 mg TCS/g SS) than nitrification systems (0.83 and 1.12 mg TCS/g SS). DNA-SIP assay further revealed that active TCS degrading bacteria in both systems belonged to Sphingomonadaceae family. Furthermore, the oligotype TATGCC, TAATCA and GCCCCG of Sphingomonadaceae played important roles in degrading TCS in both systems. Moreover, reactor performance and mixed liquor suspended solids might play important roles in shaping the ecotypes of Sphingomonadaceae, which caused the difference in degrading TCS between nitrification and denitrification systems. This lab-scale research might provide meaningful opportunities for evaluating the scale-up applications, and the TCS degrading bacteria identified in present study might be recommended to be used as bioaugmentation strains in practical engineering.