Research On The Selection Of Envelope Filter For Agricultural Subsurface Drain Pipe Based On Irrigation Return Water Reuse

The Ningxia Yinbei Irrigation District is characterized by low-lying terrain and poor drainage,leading to aggravated soil salinization,which severely restricts the sustainable development of agriculture,ecology,and socio-economics in the area.Subsurface drain pipe technology,as one of the effective ways to control soil salinization,has been successfully promoted and applied in the region for over 40 years.However,considering the water scarcity in the arid and semi-arid regions of Northwest China,exploring the recycling and reuse of subsurface drain pipes is a new challenge.Therefore,this thesis investigates the use of adsorptive materials with water purification functions as an envelope filter for the drain pipes,constructing a composite envelope filter composed of filter materials and geotextiles,to improve soil water and salt transport properties and enhance the efficiency of subsurface drain pipe reuse.First,a selection test was conducted using a soil column infiltration device developed by the research team,selecting the native soil from the Ningxia Yinbei Irrigation District’s subsurface drain pipe project site,combined with three adsorptive materials and one locally used non-woven geotextile,designing preliminary schemes with only geotextile wrap and geotextile combined with a single adsorptive material.Based on the initial test results,the design and test plans were further adjusted,examining the flow attenuation process,permeability coefficient changes,salt removal rate,and the duration of adsorption saturation under different envelope filter arrangements,exploring the structural form and performance of the drain pipe envelope filter,and proposing an improved plan with geotextile and mixed adsorptive filter materials.On the basis of the one-dimensional soil column infiltration selection test,a physical model test device of the seepage trench was developed,using adsorptive materials to replace traditional sand filter materials,and setting up two types of drain pipe envelope filters with only geotextile wrap and geotextile wrap plus 10 cm mixed filter materials to further simulate the impact of different envelope filter structures on soil water and salt transport and outlet water quality under winter irrigation salt leaching conditions.Finally,using the HYDRUS-2D software and combining the seepage trench model test data,the actual subsurface drain pipe project in the Yinbei Irrigation District was simulated,analyzing the effects of two simulated scenarios with only geotextile wrap and geotextile wrap plus 10 cm envelope filter on soil profile water and salt dynamics and subsurface drain pipe performance.The main research results and conclusions are as follows:(1)The soil column infiltration screening test results showed that,in terms of drainage performance,the method of laying geotextile with 10 cm thick adsorptive filter material is superior to the method of only wrapping geotextile;laying geotextile with mixed adsorptive materials is better than laying geotextile with a single adsorptive material.In terms of water purification,the removal of salt from water was significant when water-washed straw and slag were mixed and spread,with a high adsorption capacity and long saturation duration,meeting the requirements for both filtering soil drainage and water purification.(2)Monitoring the dynamic changes of soil water and salt in the subsurface drain pipe control area through the seepage trench model test showed that,in the early stage of the leaching test,water gradually infiltrated the soil,and the salt in the soil migrated vertically downward with the water flow and accumulated around the drain pipe,specifically in the 60-90 cm soil layer.During the middle stage of leaching,the saturation line water level rose gradually from bottom to top.For drain pipes with only geotextile wrap,drainage began when the saturation line water level exceeded the pipe’s axis position,whereas for drain pipes with geotextile wrap and 10 cm mixed filter materials,drainage started only after the saturation line water level reached the top of the pipe.Regarding the desalination effect,as the depth increased,the soil desalination rate of both envelope materials gradually decreased,with the overall average desalination rate being 11.45%lower for soil with only geotextile wrap compared to soil with mixed filter materials.During the subsurface drain pipe process,the average mineralization levels of water samples entering the mixed filter material drain pipe during the two leaching events were reduced by 1.89 g/L and 1.21 g/L,respectively,compared to samples with only geotextile wrap,and the subsurface drain pipe flow rate also increased significantly.In the late stage of leaching,due to the delayed effect of gravity drainage,the drain pipe stopped draining when the saturation line water level approached the bottom of the pipe,and the salt in the soil mostly presented a concave curve distribution centered around the drain pipe.(3)The HYDRUS-2D software can effectively simulate the influence of different envelope filter types on soil water and salt transport and drainage performance of the subsurface drain pipe.The results show that after laying the 10 cm envelope filter,the closer the distance to the drain pipe,the faster the drainage rate of water content in different soil layers,and the more effective the control of groundwater levels.The soil salt content exhibited a similar pattern,with the desalination phenomenon in the upper soil layer around the drain pipe being more pronounced,and the salt removal effect significantly enhanced compared to the scenario with only geotextile wrap.In terms of drainage performance,laying a 10 cm envelope filter can increase the drainage flow rate of the drain pipe and improve its drainage efficiency.