Secondary effluent purification by a large-scale multi-stage surface-flow constructed wetland: A case study in northern China

Assessment of treatment performance in the large-scale constructed wetland (CW) for secondary effluent purification remains limited. The aim of this case study was to therefore to investigate the long-term treatment capacity of organics and ammonium pollutants in a large-scale multi-stage surface-flow (SF) CW fed with secondary effluents from the wastewater treatment plants (WWTPs) in northern China. The results for two-and-half-year study period indicated that the water quality parameters including chemical oxygen demand (COD) and ammonium (NH₄⁺-N) met the Chinese Grade III of Environmental Quality Standards. The mass reductions of COD and NH₄⁺-N were 53% (4032 kg ha^-1 y^-1) and 72% (511 kg ha^-1 y^-1), respectively. However, there was a significant positive correlation between influent loads and treatment performance. The optimal loading of 2.5 g m^-2 d^-1 for COD and 0.3 g m^-2 d^-1 for NH₄⁺-N could be recommended for designing the sustainable large-scale multi-stage SF CW wastewater treatments.