| Using soil infiltration technology to treat domestic sewage has the advantages of low cost,easy management and good treatment effect,which is one of the measures to treat domestic sewage in rural and remote areas of China.However,there are still some shortcomings,such as low nitrogen removal efficiency,easy blockage and small hydraulic load,which limit its application and popularization.Based on the current mainstream technology of soil infiltration for sewage treatment,this study proposes to enhance the removal effect of pollutants by strengthening and improving artificial infiltration soil layer.According to the low content of soil organic matter(10.9g/kg)and the average concentration of TN,NH4+-N in sewage(42.47 mg/L and 33.49 mg/L,respectively),the improvement experiment of artificial infiltration soil layer was carried out.Three inorganic substrates(zeolite,expanded vermiculite and activated carbon)were selected for isothermal adsorption and desorption tests,and two organic substrates(peat and mushroom residue)were added for isothermal adsorption tests with different particle sizes.Soil column simulation test was carried out by using inorganic matrix and organic matrix with preferred particle size to analyze pollutant removal rate.The optimum proportion of artificial infiltration soil layer suitable for Jiuquan area was obtained,and the pilot test and monitoring were carried out.The conclusions are as follows:(1)in the isothermal adsorption experiment with zeolite,vermiculite and activated carbon as the research objects and soil and gravel as the control,the theoretical maximum adsorption capacity of ammonia nitrogen by the five substrates from high to low is vermiculite > zeolite > activated carbon >soil > > gravel,and the desorption rate is gravel > soil > vermiculite > activated carbon > zeolite.The maximum theoretical adsorption capacity of vermiculite for ammonia nitrogen is 1.85 mg/g and the desorption rate is 42.65%,which is the best treatment for wastewater with high ammonia nitrogen adsorption.(2)Isothermal adsorption experiments were carried out on peat and mushroom residue with different particle sizes.It was found that the maximum adsorption capacity of ammonia nitrogen by peat with 1mm particle size was 1.9012mg/g,which was significantly higher than that by mushroom residue1.1514mg/g,but there was no significant difference in TP adsorption capacity between them,which were 0.413mg/g and 0.433mg/g,respectively.Therefore,mushroom residue can be used as adsorption matrix for high phosphorus wastewater instead of peat,and peat with 1mm particle size is preferred as organic improvement matrix for high ammonia nitrogen wastewater.It is also the optimal treatment for soil column simulation test.(3)By comparing the physical properties of soil columns with different volume ratios of peat,it was found that with the increase of peat addition,soil porosity,saturated water content and field water holding capacity increased significantly,while soil density and penetration rate decreased significantly.After adding peat,the organic matter and total nitrogen in soil increased significantly.However,the treatment efficiency of ammonia nitrogen and total phosphorus in domestic sewage increased at first and then decreased.Adding 15% peat with the particle size of 1mm by volume has the best effect,which can not only ensure the highest ammonia nitrogen removal rate of sewage,but also reduce the soil penetration rate to a certain extent.(4)The pilot-scale test was carried out on the optimized results of indoor test,and the removal rate of pollutants in the system was continuously monitored for three years.It was found that the average removal rate of ammonia nitrogen in the system with improved substrate could reach 96.2%,and the CK treatment of backfill soil increased by 13.4%,which further verified that the removal efficiency of ammonia nitrogen in the system could be improved by adding peat and vermiculite.The average removal rates of total phosphorus and CODcr by matrix system are 92.5% and 87.1%,which are 19.3%and 7.9% higher than CK,respectively,and have good pollutant removal effect.The water quality in the observation wells meets the requirements of Grade B in the Emission Standard for Pollutants from Urban Sewage Treatment Plants(GB18918-2002).The above research results provide theoretical support for the selection of artificial infiltration soil layer materials and the improvement of technology in the treatment technology of soil infiltration sewage in arid areas of Gansu Province,and have certain popularization significance. |
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