Improving the desalination of subsurface drain pipe with envelope adsorption filter using seepage tank test

Drainage water can be recycled from subsurface pipe networks in farmland, in order to alleviate the shortage of irrigation water resources in the arid and semi-arid regions of Northwest China. The outsourcing filter material and structure of subsurface pipes play a key role in the effluent water quality. In this study, a self-developed seepage tank device was used to simulate and investigate the effects of different outsourcing filter material structures on soil water and salt transport, drainage water quality and flow rate, as well as saturated zone water quality and water level, under winter irrigation and leaching salt conditions. Adsorption materials were used instead of traditional sand filter materials. Two types of outer filter materials were tested, namely geotextile only and geotextile wrapped with 10 cm mixed filter materials. The soil water and salt dynamics were monitored from the start of irrigation to the end of drainage. The results showed that in the early stage of the leaching test, the water gradually infiltrated the soil, where the salt migrated vertically downward with the water flow, and then accumulated around the subsurface pipes in the soil layer at a depth of 60-90 cm. In the mid-stage of leaching, the water level of the saturation zone gradually rose from the bottom to the top. The subsurface pipe with geotextile only started draining, when the water level exceeded the axis of the pipe, whereas, the subsurface pipe wrapped with geotextile and 10 cmmixed filter materials started draining only, after the water level reached the top of the pipe. In terms of the desalination performance, the soil desalination rate of both outer filter materials decreased gradually with the increase of depth. The average desalination rate of the soil with geotextile only was 11.45 percentage points lower than that with mixed filter materials. The average mineralization level of the water samples from the subsurface pipe with the mixed filter materials was reduced by 1.89 and 1.21 g/L, respectively, after subsurface pipe drainage in Test 2-1 (the first round of leaching test of subsurface pipe wrapped with geotextile+10 cm mixed filter material) and Test 2-2 (the second round of leaching test of subsurface pipe wrapped with geotextile+10 cm mixed filter material), compared with the geotextile only. The drainage flow rate also increased significantly, with the maximum drainage flow rate of 6.27 mL/s for Test 2-1 and 2.46 mL/s for Test 2-2. In the late stage of leaching, the drainage stopped under the delayed effect of gravity drainage, when the water level of the saturation zone approached the bottom of the subsurface pipe. The salt in the soil was distributed mainly in a concave curve that centered around the subsurface pipe. The findings can provide a strong reference for the subsurface pipe drainage reuse in the Yinbei irrigation district of Ningxia. Nevertheless, the experimental process cannot consider evaporation, which is one of the main factors of soil salt accumulation. At the same time, the test environment of the seepage tank was greatly varied in the actual farmland drainage from the subsurface pipe project. Therefore, better performance can be expected for the adsorption filter material on the farmland soil.