Application of Reeds as Carbon Source for Enhancing Denitrification of Low C/N Micro-Polluted Water in Vertical-Flow Constructed Wetland

Constructed wetlands have been applied to micro-polluted rivers and lakes. However, they often show poor nitrogen removal efficiency due to insufficient carbon sources for complete denitrification in the waters. In this study, a vertical-flow wetland system was built, in which reeds as a carbon source were added in the middle layer of the substrate. Thereby, the effect of the reed carbon source on denitrification of micro-polluted rivers and lakes with a low C/N ratio in the wetland and the denitrification mechanism were studied. The results showed that the concentrations of NH₄⁺-N, NO₃⁻-N and NO₂⁻-N in the effluent of the constructed wetland were reduced to 0.17–0.35, 0.20–0.49 and 0.01–0.02 mg/L after adding the reed carbon source, and the removal efficiencies of the system for NH₄⁺-N and NO₃⁻-N reached 93.84% and 84.69%, respectively. The abundances of nirK, nirS, hzo and nrfA genes in the wetland substrate increased by 95.51%, 54.96%, 52.89% and 731.95%, respectively, which was considered to be related to the enhanced denitrification, anammox and dissimilatory nitrate reduction to ammonium of the wetland system. Reed planting promoted the increased abundances of amoA and nxrB genes, which might play a positive role in enhancing nitrification in wetland systems. The result of this study may provide a theoretical basis for the ecological restoration of low C/N micro-polluted water bodies.