The nitrogen removal performance and microbial communities in a pilot multistage trickling constructed wetland for purifying polluted groundwater

A pilot-scale multistage trickling constructed wetland (TCW) system was constructed from a first stage TCW filled with zeolite (TCW-A), a second stage TCW filled with lava (TCW-B), and a third stage TCW (TCW-C) for purifying groundwater polluted by NH4+-N, and the NH4+-N removal characteristics with different plants [i.e., Scirpus validus (TCWs) and Canna indica (TCWc)] at various hydraulic retention times (HRTs, 3 d and 1 d) were investigated. Efficient and stable N removals were observed in both systems at different HRTs, while the NH4+-N removal and the accumulation amount of NO3--N decreased significantly as the HRT decreased from 3 to 1 d. A higher total inorganic nitrogen (TIN) removal was observed in the multistage TCWc compared with that in TCWs. The results of microbial analysis showed that phototrophic organisms (i.e., Bacillariophyta and Streptophyta) could contribute to the higher NO3--N removal obtained in the multi-stage TCWc. Furthermore, the co-existence of phototrophic organisms, nitrifying bacteria (i.e., unclassified Xanthomonadaceae and Nitrospira), and denitrifying bacteria (e.g., Bradyrhizobium) has resulted in the multi-stage TCW system capable of substantially removing NH4+-N and TIN from contaminated groundwater without adding organic carbon.