Study On The Effect Of Nitrogen Removal In Vertical Subsurface Flow Constructed Wetland And Its System Dynamics Model

In the agricultural production process of China,the amount of chemical fertilizer is large and the utilization rate is low.Under the influence of rainfall,sedimentation and runoff,the residual chemical fertilizer enters the water body,causing serious water pollution and great harm.Due to the disadvantages of high maintenance cost,complex construction and low efficiency of nitrogen removal,the traditional wastewater treatment technology has been gradually replaced by new ecological purification technologies such as constructed wetlands and ecological floating beds.In order to optimize the design scheme of constructed wetland,two field experiments were conducted in this study to explore the influence of matrix and hydraulic retention time(HRT)on the effect of vertical subsurface flow constructed wetland,with nitrogen pollutants in water,so as to provide field experimental data and practical reference for the optimization of constructed wetland design.On this basis,the nitrogen removal mechanism of constructed wetlands was analyzed,and a simulation and prediction model of nitrogen cycle was constructed by using the system dynamics model which is good at dealing with nonlinear and complex problems,which will provide reference for the development and application of the model.The main conclusions for this study are as following:(1)Vertical subsurface flow constructed wetland and ecological floating bed can effectively remove various nitrogen pollutants in polluted water.The nitrate removal rate of constructed wetlands with vertical subsurface flow of volcanic rocks and zeolite can reach more than 80%.The vertical subsurface flow constructed wetland of zeolite matrix has strong comprehensive treatment ability,especially suitable for ammonia nitrogen removal.Matrix is the key factor to reduce ammonia concentration in constructed wetlands.The nitrate-nitrogen removal rate of ecological floating bed can reach more than 70%,with better effect,but its stability is not as good as that ofzeolite and volcanic artificial wetlands with vertical underflow.(2)The removal rates of nitrate and ammonia nitrogen in the vertical subsurface flow constructed wetland treatment of zeolite matrix increased firstly and then decreased with the HRT.The best removal rates were all 2d for HRT.Poor inlet water quality is likely to cause device blockage,so it is suitable to add filtration devices at the inlet of wetlands or use constructed wetlands as secondary and tertiary purification devices in combination with other devices.The HRT has a significant effect on nitrate removal in the vertical subsurface flow constructed wetland treatment of zeolite matrix,while the effect on ammonia removal is less.(3)The system dynamics software named Vensim can be used to establish a numerical model for the process of denitrification of vertical subsurface flow constructed wetlands.The simulated effluent nitrate,ammonia and organic nitrogen concentrations were consistent with the measured concentrations,with the Nash efficiency coefficient(NSE)between 0.7 and 0.8.The main reasons for the errors are insufficient data,the application conditions of empirical parameters and the influence of external factors on wetlands not reflected in the model.System dynamics has a broad application prospect in artificial wetland modeling,but it needs further study and cooperation from different fields before the model can be applied in practical projects.