Abstract: A constructed complex wetland model,consisting of a first-step vertical flow wetland(VFW),four sequential submerged macrophyte oxidation ponds and a second-step VFW,was used to treat tailwater from a small urban wastewater treatment plant.Results show that when the hydraulic loading of the model was 0.13 m3·m-2·d-1,the outflow had only CODCr and NH4+-N up to Grade Ⅲ Criteria of the Environmental Quality Standards for Surface Water(GB 3838-2002),but when ρ(TP) in the inflow satisfied Grade Ⅰ B Criteria of the standard for pollutant discharge from municipal wastewater treatment plants(GB 18918-2000),TP in the outflow also met Grade Ⅲ Criteria of the Environmental Quality Standards for Surface Water.When the hydraulic loading at the second-step VFW was adjusted to 0.06 m3·m-2·d-1,both effluent TP and TN finally met the Grade Ⅲ Criteria.It is assumed that in the model,TN was removed mainly by filtration and adsorption in first-step VFW and by denitrification in the second-step VFW,while TP was mainly removed through adsorption and sedimentation by limestone in first-step VFW.

Key words: constructed complex wetland, wastewater treatment plant, tailwater, deep treatment, nitrogen, phosphorus