A series of slag-based subsurface flow constructed wetlands with Phragmites australis and Calamagrostis
angustifolia were established to analyze the nitrogen and chemical oxygen demand (COD) removal effi-
ciency under different influent C/N ratios. With an increase of C/N ratio, the removal efficiency of total
nitrogen (TN), nitrate-nitrogen (NO3
−-N) and COD increased, and the removal efficiency was greatest at
a C/N ratio of 5. The input of a carbon source promoted the activity of denitrifying microorganisms, however,
the degradation of organic matter consumed the dissolved oxygen in the system. Consequently, the
nitrification of ammonium nitrogen (NH4
+-N) was inhibited, which led to the decrease in NH4
+-N removal
efficiency. The change of oxidation–reduction potential (ORP) with retention time indicated that the dissolved
oxygen concentrations decreased sharply at the beginning of the operation and then increased
gradually in all treatments. The recovery of dissolved oxygen in high C/N ratio treatments lagged behind
the middle and low C/N ratio treatments.