Study On The Removal For Nitrogen And Phosphorus In Subsurface Flow Constructed Wetland With Different Particle Size Substrate

To study the removal effect for nitrogen（N）and phosphorus（P）pollutants with different particle sizes and gradation of zeolite substrates under the subsurface flow constructed wetland system.Five parallel vertical subsurface flow constructed wetland systems were built,filled by zeolite substrates with fine particle size of 2-4mm,medium particle size of 4-8mm,coarse particle size of 8-16mm,progressively-sized with particle size increase from top to bottom and anti-sized with particle size decrease from top to bottom of three layer,respectively.And two horizontal subsurface flow constructed wetland systems were filled by zeolite substrates with progressively-sized and anti-sized,respectively.And the removal effects at different depths of each constructed wetlands were analyzed.The influences of hydraulic retention time（HRT）and carbon source on the removal ability for N and P pollutions were explored,and the changes of the porosity,p H,TDS in constructed wetlands with were observed.And an ecological dynamics model was established to simulate the removal process for N.The main conclusions were as follows.（1）During the experimental period,the removal efficiency for nitrate nitrogen（NO₃^--N）,ammonia nitrogen（NH₄⁺-N）,total nitrogen（TN）and total phosphorus（TP）in five vertical subsurface flow constructed wetlands were arranged as fine particles size>anti-sized>medium particle size>progressively-sized>coarse particle size.Overall,in the three layers of the upper,middle and lower substrates for these five vertical subsurface flow constructed wetlands,the upper substrate contributed the most to the removal of NO₃^--N,NH₄⁺-N and TN,while the lower substrate contributed the most to the removal of TP.（2）When HRT increased by 1 time,the removal rate of NO₃^--N in these five vertical subsurface flow constructed wetlands decreased,while it increased for NH₄⁺-N and TN.And the change trend of TN removal rate was inconsistent for each constructed wetland.The removal rate of NO₃^--N in the horizontal subsurface flow wetland with progressively-sized substrate decreased,which increased NH₄⁺-N,TN and TP.The removal rates of NO₃^--N and TN in the horizontal subsurface flow wetland with anti-sized substrate decreased,with the increased removal rates for NH₄⁺-N and TP.When the HRT was ths same for the five vertical subsurface flow constructed wetland,the removal rates of NO₃^--N,NH₄⁺-N,TN,and TP with carbon source were higher than that in the control group without carbon source.（3）The porosity in five vertical subsurface flow constructed wetlands gradually decreased,and the porosity in the upper layer was significantly lower than that in the middle and lower layers.And the p H in these five vertical subsurface flow constructed wetlands gradually increased,but the changes of TDS were inconsistent.The actual HRTs were arranged as anti-sized>progressively-sized>fine particle size>medium particle size>coarse particle size.So the actual HRTs in the vertical subsurface flow constructed wetlands with heterogeneous substrates were greater than that with homogeneous substrates.（4）The ecological dynamics model was established,and the software STELLA was used to simulate the removal effect of nitrogen pollutants in the vertical subsurface flow constructed wetland system with fine particle sized substrate.Compared with the observed data,it was found that the model simulation data could reflect the nitrogen pollutants concentrations in the effluent of the constructed wetland.The R²for the simulated and observed NO₃^--N,NH₄⁺-N and Org-N concentrations were 0.78,0.59,0.97,and the corresponding NSE were 0.893,0.909,0.896,respectively.This indicated that the simulated NO₃^--N,NH₄⁺-N and Org-N concentrations were close to the observed NO₃^--N,NH₄⁺-N and Org-N concentrations in effluent.Therefore,the ecological dynamics model established in this study successfully predict the nitrogen removal process in the vertical subsurface flow constructed wetland system.However,the constant empirical parameters without value changes in real wetland were used in the model,which is an further issue needed to be done in future.