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Chemical oxygen demand, nitrogen and phosphorus removal by subsurface wetlands with Phragmites vegetation in different models

To improve the removal efficiency of subsurface wetlands vegetated mainly by Phragmites, pilot‐scale gravel‐based wetlands were used to treat sewage characterized by chemical oxygen demand (COD), total nitrogen (TN) and total phosphorus (TP) pollution. For Phragmites vegetation, COD, TP and TN removal loads of wetland vegetation with Phragmites australis–Typha angustata–Scirpus validus as main species reached 0.517 g m−2 d−1, 0.277 g P m−2 d−1 and 0.023 g N m−2 d−1. The COD removal loads in pilot‐scale and medium‐scale (260 m2 in area) wetlands with Phragmites‐monoculture vegetation were 0.62–0.64 g m−2 d−1, while that of P. australis–T. angustata–S. validus wetland reached 0.974 g m−2 d−1. Thus, the preferable poly‐culture model for Phragmites wetland vegetation was P. australis, T. angustata, S. validus and Zizania latifolia with stem density ratio of 8:1:5:1. After harvest, nitrogen and phosphorus standing stocks of wetland vegetations ranged only 2.2–9.93 g N m−2 and 5.39–13.5 g P m−2, respectively, as both the above ground biomass and the nitrogen and phosphorus contents of the wetland vegetation harvested in late autumn were low.