Abstract: Excessive nitrogen and phosphorus affect water quality and pose a threat to urban water safety. Purifying river water and enhancing the ecological functions of rivers are essential for restoring the ecological integrity of urban watersheds. To address ecological issues such as water pollution in urban areas, projects for wetland water quality purification have been initiated. In this study, we analyzed the seasonal variations in nitrogen and phosphorus removal efficiency of a large wetland water purification project in northern China. The results showed that the project effectively removed nitrogen and phosphorus from the influent during the summer (wet season). However, in spring and winter (normal and dry seasons), low temperatures and substantial plant residues and bird excretion led to lower removal efficiency. Overall, the wetland project demonstrated a positive effect on nitrogen removal across different seasons, with the three areas-pre-sedimentation ecological ponds, subsurface flow constructed wetland areas, and aquatic plant ponds-working synergistically and complementarily. To enhance nitrogen and phosphorus removal during the less effective seasons, it is recommended to implement management measures such as plant harvesting, membrane insulation, adding carbon sources, and extending hydraulic residence time.

Key words: northern river, water quality purification, engineering application, constructed wetland, pond