**4. Conclusions**

This study clearly demonstrated that excellent SMX removal e fficiency among di fferent SMX concentrations was obtained during the treatment in the CWs. The concentration of SMX in the bottom layer was higher compared with that in the surface layer. Good removal e fficiencies for SMX were observed using the systems. A degradation mechanism of SMX was proposed. The relative abundances of *sul* genes showed an obvious increase with the increase of SMX content in the bottom and middle layers. The concentration of *sul* genes in the bottom layer was shown to be higher than that in the middle layer; the surface layer presented the lowest concentration. The relative abundance of *sul* genes exhibited an increase, which tended to be stable among the treatment duration. *Proteobacteria* was the dominant phylum in the CWs.

**Author Contributions:** Methodology, Y.-X.L.; validation, J.-J.Z. and S.L.; investigation, S.Z.; writing—original draft preparation, S.Z.; writing—review and editing, H.-L.S.; funding acquisition, H.-L.S. and X.-L.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Key Research and Development Program of China (2019YFD1100205), National Major Science and Technology Projects of China (2017ZX07202004), and the National Natural Science Foundation of China (41571476).

**Acknowledgments:** Hai-Liang Song would like to acknowledge the Qing Lan Project of Jiangsu Province.

**Conflicts of Interest:** The authors declare no conflict of interest.
