Effects of copper and zinc on pollutants removal in horizontal subsurface flow constructed wetlands

A series of horizontal subsurface flow constructed wetlands with Phragmites australis were set up to evaluate the removal efficiency of nitrogen, phosphorus and chemical oxygen demand (COD) under different influent copper (Cu2+) or zinc (Zn2+) concentrations, and they were operated intermittently with a 9-d residence time (RT). Different Cu2+and Zn2+(0, 2, 4, 8, and 16.32 mg/L) levels were set up in the synthetic influent water prepared in eleven feed tanks, respectively. With an increase of Cu2+concentrations, the removal efficiency of total nitrogen (TN) and ammonium nitrogen (NH^-N) increased, and the optimal Cu2+concentration was 2 mg/L to enhance nitrogen removal. The presence of Cu2+promoted the bioactivity of anaerobic ammonia-oxidizing bacteria bioactivity. Because of Zn2+complexed by recalcitrant fractions of organic matter, the presence of significantly inhibited TN and nitrate nitrogen (NO3-N) removal, while Zn2+slightly improved COD removal in the constructed wetlands. Both of Cu2+and Zn2+did not affect total phosphorus removal in constructed wetlands. The presence of Zn2+may promote plant metabolism and an ability of roots to secrete oxygen, which increased oxidation–reduction potential and affected denitrification in constructed wetlands.