High-efficient nitrogen removal and its microbiological mechanism of a novel carbon self-sufficient constructed wetland

Abstract The low denitrification efficiency in constructed wetlands (CWs) was a crucial problem for treating the carbon-deficient wastewater. To further improve the N removal from the tailwater of wastewater treatment plant, the plant fermentation liquid (PFL, derived from wetland plant litter in our CWs) as a supplementary carbon source was used in horizontal subsurface-flow CW amended with Fe-modified biochar (FeB-HSCW) to increase the influent C/N ratio and drive the denitrification. The N removal performance of FeB-HSCW was investigated under various influent C/N ratios for achieving high-efficient N removal. The excellent removal rates of COD (89.35%), NO3--N (97.48%), TN (93.76%), NH4+-N (>16%) and NO2--N (>98%) were obtained in FeB-HSCW at the C/N of 9.0. When the large amount of PFL was added into the systems, the amount of 16S rRNA genes from bacteria and anammox bacteria, as well as of denitrification and anammox functional genes (narG, napA, nirS, nirK, qnorB, cnorB, nosZ-I, nosZ-II and hzsA) were statistically greater in FeB-HSCW than those in other two systems (C-HSCW, without biochar and FeB; B-HSCW, amended with biochar). However, the abundances of nitrification genes (amoA of AOA, amoA of AOB and nxrA) were the lowest in FeB-HSCW after adding the PFL. High-throughput analysis for 16S rRNA gene revealed that the PFL coupled with FeB remarkably optimized the structure of bacterial community. The FeB-HSCW added with sufficient PFL was enriched with the highest abundances of norank_p_Saccharibacteria, Bacillus, Geobacter, Pseudolabrys, Mizugakiibacter, Bradyrhizobium and norank_f_ Planctomycetaceae which were capable of autotrophic and/or heterotrophic denitrification as well as anammox, whereas the distribution of nitrifiers (Nitrospira) was the lowest. Overall, this work illustrated that the PFL coupled with FeB-HSCWwas a feasible and effective carbon self-sufficient HSCW for the high-efficient N removal from wastewater with low C/N ratio mainly via the autotrophic and heterotrophic denitrification and anammox.