Effects of Zn(Ⅱ) on the simultaneous nitrification and denitrification(SND) process: Performance and microbial community

Simultaneous nitrification and denitrification（SND） is considered an attractive alternative to traditionally biological nitrogen removal technology.Knowing the effects of heavy metals on the SND process is essential for engineering.In this study,the responses of SND performance to Zn（Ⅱ） exposure were investigated in a biofilm reactor.The results indicated that Zn（Ⅱ） at low concentration(≤2 mg·L-1) had negligible effects on the removal of nitrogen and COD in the SND process compared to that without Zn（Ⅱ）,while the removal of ammonium and COD was strongly inhibited with an increasing in the concentration of Zn（Ⅱ） at 5 or 10 mg·L-1.Large amounts of extracellular polymeric substance（EPS）,especially protein（PN）,were secreted to protect microorganisms from the increasing Zn（Ⅱ） damage.High-throughput sequencing analysis indicated that Zn（Ⅱ） exposure could significantly reduce the microbial diversity and change the structure of microbial community.The RDA analysis further confirmed that Azoarcus-Thauera-cluster was the dominant genus in response to low exposure of Zn（Ⅱ） from 1 to2 mg·L-1,while the genus Klebsiella and Enterobacter indicated their adaptability to the presence of elevated Zn（Ⅱ）.According to PICRUSt,the abundance of key genes encoding ammonia monooxygenase（EC:1.14.99.39） was obviously reduced after exposure to Zn（Ⅱ）,suggesting that the influence of Zn（Ⅱ） on nitrification was greater than that of denitrification,leading to a decrease in ammonium removal of SND system.This study provides a theoretical foundation for understanding the influence of Zn（Ⅱ） on the SND process in a biofilm system,which should be a source of great concern.