Study On Optimization Of Nitrogen And Phosphorus Utilization In Economic Subsurface Flow Constructed Wetlands

As the advanced treatment process of combination of biological and ecological system,the purpose of subsurface flow constructed wetlands is to remove nitrogen and phosphorus,improve effluent quality,nitrogen and phosphorus resource utilization by the ecosystem with plants in the wetlands.For the purpose of increasing the utilization rate of nitrogen and phosphorus in wastewater and achieving better economic benefits,vegetable plants are used to replace the general landscape plants in traditional wetlands.The removal of nitrogen and phosphorus in sewage by wetlands is mainly through the nitrification-denitrification by microorganism and adsorption and precipitation of the wetland substrate,while little proportion by plants absorption.This research structured constructed wetlands with different economic plants,several measures were presented to improve contribution of absorption of plants on the nitrogen and phosphorus removal in wetlands for the purpose of increase resource utilization ratio of nitrogen and phosphorus.Based on the analysis of treatment effect of different plant configurations during growth cycle and impact of operating conditions on removal efficiency of nitrogen and phosphorus in wetlands,and utilize the first order reaction kinetics model to predict effect of nitrogen and phosphorus removal,the main conclusions are as follows:(1)From the results of whole year,effect of nitrogen and phosphorus removal in constructed wetlands with different plant configuration seasonal change is significant.In summer and autumn,effect of nitrogen and phosphorus removal of each wetland is best in July to September,under the condition of hydraulic load of 0.24 m3/(m2·d),except short time during early plants growing,concentration of NH4+-N、TN、TP in effluent of wetlands could basically attain the primary A standard of urban sewage treatment plant pollutant discharge standard(GB 18918-2002).Compared with summer and autumn,effect of nitrogen and phosphorus removal significantly reduce and appear a certain degree of volatility in winter and spring,the average concentration of NH4+-N of effluent could attain the primary A standard,while of TN and TP in most wetlands could attain primary B standard.There is a certain effect on wetland protection with winter greenhouse,while after a sharp rise in temperature in March,disparity of treatment effect inside and outside of greenhouse is narrowed.(2)The effect of the depth of saturation line on the removal efficiency of NH4+-N、TN、TP of each wetland are different with plant kinds.Under the condition of low saturation line(20cm),there is a faster rate of reoxygenation for water and high dissolved oxygen concentration in the wetland,which is propitious to removal of NH4+-N,while as a result of relatively short hydraulic retention time and destruction of anoxic environment,removal rate of TN is not high.Removal rate of TN increases significantly with increasing the depth of saturation line to 30cm while decreases under condition of high saturation line,the reason is that the lack of dissolved oxygen limited microbial nitrification.Removal rate of TP increases with increasing the depth of saturation line,as a result of effective contact time is prolonged which is more conducive to the adsorption of substrate under the precondition of the same hydraulic load.(3)Either end inlet mode with "the same end sustained inlet for 5 days-suspended for 2 days-switched inlet end" is used in either end inlet wetlands,compared with single end inlet wetlands,removal efficiency of NH4+-N、TN、TP are slightly lower in either end inlet wetlands,while the disparity is not obvious especially in summer and autumn,effluent could attain primary A standard,but plants growth is significantly different in wetlands with two kinds of inlet modes,the plant height、root length and harvest per unit area are improved in either end inlet wetlands,which indicate either end inlet is effective to improve growth differences of plants before and after in wetlands because of nutritional imbalance.(4)Intermittent operation mode with "instantaneous inflow-running 48h-instantaneous emptying-idle 6h" is used in discontinuous wetlands,compared with continuous wetlands,removal efficiency of NH4+-N、TN、TP are higher in discontinuous wetlands,because intermittent operation could effectively improve the internal dissolved oxygen environment which is beneficial to nitrogen removal,and during idle time substrate reoxygenation is favorable for the adsorption of phosphorus on the substrate.(5)Analysis the growth status of wetland plants and the content of total nitrogen、total phosphorus after harvest and calculate total nitrogen and phosphorus removal in different test period in wetlands,then investigate contribution of plant absorption in nitrogen and phosphorus removal in wetlands.The results indicate that leaves have the highest content of nitrogen and phosphorus for each kind of plants,in summer and autumn plants accumulate nitrogen and phosphorus most is spinach and lettuce least,while cress most and coriander least in winter and spring.In summer and autumn contribution of plants absorption to removal of TN、TP in wetlands is 9.74%and 21.85%while 6.56%and 21.62%in winter and spring.(6)Make a simulation and prediction on nitrogen and phosphorus removal process of wetlands with first order reaction dynamic model,with spinach wetland and cress wetland as the research object,compared with predicted and measured value of concentration of TN、TP in effluent,the RRMSE is close to zero value indicates that first order reaction dynamic model is suitable for simulation on wetland pollutant removal.While fitting of Arrhenius equation on area removal rate constant K in nitrogen and phosphorus removal process appear deviation as existence of interference factor,which cause the linear correlation coefficient is not high,while value of K increases with temperature increment indicates that temperature has an important influence on nitrogen and phosphorus removal in wetlands.The results indicate that,it is effective for improving wetland treatment efficiency and plants growth to raise the contribution of plant absorption of nitrogen and phosphorus removal in wetland by controlling the depth of saturation line and changing inlet mode and operation mode of wetlands.