Performance of experimental horizontal subsurface-flow-constructed wetlands treating river water: effect of substrate, configuration, hydraulic retention time, temperature and external carbon source

The water quality of Yapu River flowing into Lake Taihu was monitored for one year and the effects of substrate, configuration, hydraulic retention time (HRT) and temperature on nitrogen removal were evaluated in five pilot-scale horizontal subsurface-flow-constructed wetlands (CWs). Additionally, the impact of adding external carbon on nitrogen removal was investigated. Yapu River was shown to be eutrophic, with a low C/N ratio (COD/ TN = 1.5) and the main pollutant was total nitrogen (TN = 2 mg/L). CWs with substrates of combined gravel-zeolite and gravel-ceramsite showed higher ammonium nitrogen (NH_4 -N) and chemical oxygen demand (COD) removal efficiencies than the CWs with gravel substrates. Trapezoidal CWs showed improved dissolved oxygen in the front when compared with the rectangular CWs, enabling complete nitrification. A longer HRT (four days) improved the removal efficiencies of organics, nitrogen, and phosphorus. CWs performed better during the warm period. Moreover, addition of 400 g cattail litter during winter led to increased influent of COD. The average removal efficiencies of NO3-N and TN were increasing up to 90.5 and 85.3%, respectively.