Nitrogen removal of simulated low-polluted water of Lake Erhai buffer zone by surface-flow wetland

Aiming at the pollution problem of typical low-polluted water (produced by agricultural non-point sources) in Lake Erhai buffer zone, the typical buffer zone mostly occupied by farmland between Baihe Stream and Mei Stream in the western Lake Erhai basin were selected to monitor the surface runoff quality of the buffer zone, and to set up the surface flow pilot wetland for field study. The nitrogen removal effect for simulated low-polluted water of Lake Erhai buffer zone from September 2016 to August 2017 was studied. The results indicate that the pilot system has a high nitrogen removal effect, with annual average removal rate of total nitrogen as Nymphoides peltatum wetland (83.1%) >reed wetland (73.9%) >Hydrilla verticillata wetland (73.3%) >blank wetland (65.5%). The average annual removal rate of ammonia nitrogen was higher than 77%. Among different plant systems, Nymphoides peltatum system has the best removal effect of total nitrogen and the most stable removal rate. The removal rate and width of each system were simulated, and it reveals that the optimum width of the Nymphoides peltatum, Hydrilla verticillata and Phragmites communis wetland systems was respectively 7.04 m, 8.64 m and 6.92 m while the effluent water quality is of class Ⅲ water. The contribution rate of nitrogen removal in Nymphoides peltatum, Hydrilla verticillata and Phragmites communis wetland systems during the test is 5.88%, 3.23% and 21.12%, respectively. The total nitrogen content of reeds in wetlands is significantly higher than that of Nymphoides peltatum and Hydrilla verticillata. The contribution rate of Phragmites communis and Hydrilla verticillata to nitrogen removal in wetland shows a large seasonal difference, while the contribution rate of Nymphoides peltatum in wetland to nitrogen removal in different seasons varies little.