Effect of Tubifex tubifex on the purification function of saturated vertical flow constructed wetlands for effluents with varying C/N ratios

The improvement effect of Tubifex tubifex on the pollutant removal efficiencies (REs) of vertical flow constructed wetlands (VF-CWs) treating wastewater with various C/N ratios was explored. The experiment was conducted in pilot-scale saturated VF-CWs, being added different densities of T. tubifex and fed synthetic wastewater with successive C/N ratios of 0.5, 1.5, 3.0 and 6.0. The results suggest that T. tubifex addition and the influent C/N ratio had an interactive effect, i.e., T. tubifex addition improved NOx−-N, NH4+-N, TN and COD REs by 36.7%, 56.5%, 22.6%, and 10.0%, respectively, under low C/N ratios, while high C/N ratios inhibited this improvement. Low-density T. tubifex addition significantly increased substrate dissolved oxygen (DO) by retarding excessive soil organic matter (OM) accumulation. With T. tubifex addition, an improvement in bacterial diversity, the relative abundance of N-cycle and fermentative bacteria, and N-cycle functional genes was only observed in substrates under low C/N ratios. T. tubifex can improve the purification function of saturated VF-CWs, but this strategy strongly depends on both the influent C/N ratio and density of T. tubifex addition.