Nitrogen removal performance and microbiological characteristics for the landfill leachate treatment in a three-stage vertical flow constructed wetlands system

Landfill leachate (LL) has the characteristics of high NH4+-N and low C/N ratio, increasing the cost and difficulty of efficient treatment. Constructed wetlands system (CWs) has proven to be feasible for the treatment of LL. A constructed three-stage series of vertical flow CWs (termed CW1, CW2, and CW3) based on effluent recirculation (T-VF) was used to treat LL, and nitrogen removal effect and microbiological characteristics was investigated. The results show that the nitrogen removal efficiency showed a increased trend from CW1 to CW3. The average removal efficiencies of TN, NH3+-N, and NO3−-N in T-VF were 91.43%, 94.19%, and 98.11%, respectively, which showed good nitrogen removal performance. Furthermore, application of the combined CW1, CW2 and CW3 increased diversity of substrate bacterial communities and decreased the relative abundances of the dominant nitrifying(e.g. Nitrosomonas, Nitrobacter) and heterotrophic denitrifying bacteria (e.g. Thauera). The relative abundances of autotrophic denitrification (e.g. Thiobacillus) increased. Pathway of nitrogen metabolism and functional genes analysis showed that the abundance of enzymes associated with the denitrification and dissimilatory nitrate reduction increased and that the abundance of enzymes associated with the nitrification decreased. Application of the combined CW1, CW2 and CW3 could be respectively formed corresponding nitrogen removal microbial community, influent quality and pathways of nitrogen to enhance the nitrogen removal effect of T-VF to treat LL.