Removal Of Ammonia Nitrogen Pollution From Rainfall By Artificial Unsaturated Zones

In order to solve the problem of ammonia nitrogen pollution in rainfall,based on the physicochemical adsorption principle,ammonia nitrogen adsorption experiments were carried out in the laboratory under static and dynamic conditions,and the transport and adsorption of ammonia nitrogen were simulated with the software HYDRUS-1D on the basis of the experiments.Eight kinds of materials,including zeolite,ceramsite,fine sand,activated carbon,medical stone,volcanic rock,emery and quartz sand,were screened,and three static tests of adsorption kinetics,adsorption isotherm and adsorption thermodynamics were carried out.The results show that the maximum theoretical saturation adsorption capacity of zeolite,ceramsite and fine sand is 2.031mg/g,0.998mg/g and 0.413mg/g,respectively.Langmuir and Freundlich models can be used to fit the adsorption of ammonia nitrogen for all materials,and Langmuir model is slightly better than Freundlich model(except zeolite).The adsorption of ammonia-nitrogen by zeolite,ceramsite,fine sand,activated carbon and volcanic rock conforms to the quasi-second-order kinetic equation,and chemical adsorption is the main process.The adsorption process of each material is heat absorption,and the adsorption of zeolite,ceramsite,fine sand and activated carbon can be carried out spontaneously.Based on the three media of zeolite,ceramsite and fine sand determined by the static test,the dynamic adsorption,desorption and re-adsorption tests were carried out to simulate the ammonia nitrogen infiltration in artificial unsaturated zone.Among the three groups of test columns with mass ratio of zeolite,ceramsite and fine sand of 1:1:1,2:1:2 and 1:1:2,the dynamic saturation adsorption capacity of the test column with mass ratio of 2:1:2 was the highest,which was 2.673mg/g.The desorption rates of the three test columns ranged from 30% to 50%,lower than their regeneration rates.The desorption rate of the medium column with a mass ratio of 2:1:2 was the least(31.32%),and the regeneration rate of the medium column with a mass ratio of 1:1:1was the highest(64.56%).With the help of HYDRUS-1D software,the numerical simulation of the column test was built.It was found that the concentration of ammonia nitrogen in the column decreased with the increase of depth and increased with the increase of time.The sensitivity analysis of the parameters showed that the distribution coefficient had the greatest effect on the transport results of ammonia nitrogen,followed by the mass transfer coefficient,and the vertical dispersion had the least effect.Finally,based on the identified numerical model,the ammonia removal effect of the artificial unsaturated zone was predicted under rainfall conditions in Hefei.When the 5cm thick artificial unsaturated zone was designed to adsorb ammonia nitrogen in the rainfall,the artificial unsaturated zone with the mass ratio of zeolite,ceramic and fine sand of 1:1:2had the best adsorption effect.After 3 years of operation,the concentration of ammonia nitrogen decreased from 10mg/L of influent to 0.86mg/L of effluent.The removal rate was 91.4%.The medium material selected in this study has low cost,simple technology and easy operation,which provides a reference for green areas or parking lots and other places to absorb materials.