Design and operation of subsurface flow dam for intercepting nitrogen and phosphorus of farmland drainage

Abstract: In order to control non-point pollution in Taihu Lake Basin, the technique of subsurface flow dam (SFD) was designed for intercepting nitrogen and phosphorus in farmland drainage . To reduce the content of nitrogen and phosphorus in farmland drainage, SFD presented the features of controlling the hydraulic retention time and the hydraulic head. Two same ponds were built in the field, including ecological pond 1 (22 m×45m×1 m) , and ecological pond 2 (25.3 m×45 m×1 m) with two SFD (24 m2) and Artificial island (100 m2). The SFD, 6m×4 m×0.8m (long×wide×high), was made of hollow brick and filled with gravel. Gravel with diameter between 5-8 cm was filled at one end of the intake, and the diameter of the gravel gradually decreased to 1-2 cm along with the direction of flow at the other end of intake. A 40 cm soil layer was paved at the top of the gravel layer. The overflow pipe with 20 cm diameter was placed at the site 50 cm from the bottom of SFD. Five species of free floating plants, emergent plants, submerged plants, revetment plants and shrub were used in this study. After planting, the units were flooded. The two types of experimental wetlands were run in parallel under identical conditions for 3a. The results showed that SFD for nitrogen and phosphorus interception was more stable in the nitrogen and phosphorus treatment and had a better treatment effect and landscape benefit, and could ensure the quality of final effluent water. Meanwhile, the level of dissolved NH4+-N, TN and PO4--P in the effluent of the constructed wetlands increased to a range of 8.3%, 7.2% and 17% by flowing though SFD, respectively. SFD for nitrogen and phosphorus interception is very efficient in the treatment of nitrogen and phosphorus for agricultural non-point source pollution, and can be one of the effective technologies for treating the non-point pollution in farmland drainage in Taihu Lake Basin.