Observation And Simulation Of Nutrients Removal For Constructed Lake-side Wetlands

Lake pollution and eutrophication is a severe environmental problem in China, and non-point pollution is one of the main reasons. As an ecological purifying method, constructed wetlands are being applied to reduce pollutants in agricultural runoffs. For lakes, wetlands can be constructed between the water body and the agricultural land so that nutrients in stream flow can be filtered via physical-chemical processes in the wetland before entering the lake.The objectives of this thesis are: (1) to investigate nitrogen and phosphorus removal efficiencies of different function units of a wetland including sedimentation tank (ST),subsurface flow unit (SSF) and free water surface (FWS) unit; (2) Regression models are attempted to apply to different function units and suitability is assessed. Outcomes from this study are expected to provide guidance for wetland design and management and maintenance during wetland operation. The Maliao river wetland, one of the lake-side wetlands at the north shore of Fuxian lake, Yunnan province was selected as a study area. In-situ observations were carried out to obtain inflow and outflow nitrogen and phosphorus concentration data for each function unit. The linear regression method was employed to analyze the observation data. Regression models were established to simulate the nitrogen and phosphorus concentrations of inflow and outflow water. Reliability and suitability of the models are assessed and compared for different units and different nutrient formation.This research shows that the wetland system removes nitrogen stably and the removal rate is greater than phosphorus removal rate. For nitrogen, the removal capability from the greatest to the least follows the order of FWS unit> SSF unit> ST unit. The nitrogen removal rate in the ST unit is not apparent, while the SSF and FWS units demonstrate some removal efficiency for nitrogen,and among which, the FWS unit has a noticeable effect on nitrogen removal due to its active system of hydrophytes and microorganism function. In the SSF unit, the average removal efficiencies of NH4+- N, NO3--N and TN are 28.5%, 33.5%and 22.4%, respectively. In the FWS unit, the average removal rates for NH4+-N, NO3--N and TN are 50.9%, 40.3%and 44.4%, respectively. The ST unit can remove phosphorus more effectively than the SSF and FWS units, with an average rate of 23.4%. According to the specific chemical composition of the inflow stream water, a reasonable combination of different function units for a wetland can be helpful to achieve the best removal efficiency.Comparison of nutrients removal under different hydrological conditions shows that the ST has a stable function, while the SSF and the FWS units demonstrate a net phosphorus removal during wet seasons with a rate of 20.9% and 16.7%, respectively. In dry seasons,...