Numerical Investigation On N₂O Emission From Constructed Wetland Based On Multiphysics Coupling

In recent years,the problem of sewage disposal in my country’s rural areas has been widely concerned by the society.As a low-cost,easy-to-operate decentralized sewage treatment method,constructed wetland has been widely used in rural areas abroad.In the operating system of constructed wetlands,the generation of greenhouse gases is a problem that cannot be ignored.However,due to the limitations of monitoring,sampling,and analysis in the experimental process,there are relatively few studies on the mechanism of greenhouse gas generation in constructed wetlands.Numerical simulation can make up for the technical limitations of monitoring and sampling in the constructed wetland experiment.At the same time,it can explore the phenomena that cannot or are difficult to observe in the experiment,and realize the condensing and improvement of the theory of experimental results.In view of this,this study uses numerical simulation technology to study the sewage treatment process of constructed wetland accompanied by N₂O production.Specifically,this study established a multi-field coupling model including 3 denitrification mechanisms and 3 biological N₂O production mechanisms.The model was validated under the saturated flow and unsaturated flow constructed wetland operation mode,and expanded Theoretical discussion on the influence of conventional constructed wetland operating parameters on the system.The main conclusions of this paper are as follows:（1）By comparing the simulation results of the model constructed by this research with the actual operation results of saturated flow and unsaturated flow constructed wetland,the total nitrogen removal rate,effluent ammonia nitrogen concentration,nitrate nitrogen concentration,etc.were compared in detail.After comparison,it is found that the model simulation values are in good agreement with the experimental data,which indicates that the multi-field coupling model constructed in this research is applicable and effective.（2）Under saturated flow operation mode,the main research is to study the influence of influent organic load（C/N）,dissolved oxygen（DO）and aeration method on sewage treatment effect and N₂O production.The factor that has the greatest impact on the concentration of nitrate nitrogen in the effluent is the aeration time interval,followed by the influent C/N.Increasing the aeration time interval or increasing the C/N will cause the nitrification process to be limited.For example,the C/N is increased from 2.When it reaches 10,the total nitrogen removal rate is reduced from 95%to about 80%.In terms of N₂O emissions,the most influential operating parameter is the influent C/N,followed by the aeration time interval.When the influent C/N is increased from 2 to 10,the N₂O production ratio will increase from 0.15%to 0.45%.（3）Under the unsaturated flow constructed wetland mode,the main research is to study the effects of daily total hydraulic load（HLR）,influent organic load（C/N）and daily influent frequency（DF）on sewage treatment effect and N₂O production.influences.Increasing the total hydraulic load,organic load or water inlet frequency in the unsaturated flow operation mode will promote the decrease of NH₄⁺-N and COD effluent concentration,resulting in the increase of NO₃^--N effluent concentration.In terms of N₂O production,increasing the C/N in the inlet water will inhibit the emission of N₂O,and increasing the hydraulic load or increasing the frequency of the inlet water will result in an increase in the emission of N₂O.