Method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches

Abstract

The present invention discloses a method for controlling and purifying microplastic pollution at a confluence of a drainage ditch for farmland. Firstly, basic parameters of a main ditch and a branch ditch are obtained, and a three-dimensional digital twin model is constructed according to the basic parameters. Based on the three-dimensional digital twin model, a main ditch rectifying pile group and a branch ditch rectifying pile group are arranged upstream of the confluence of the main ditch and the branch ditch, a submerged shallow dam is arranged downstream of the confluence of the main ditch and the branch ditch, and a first microplastic purification unit is arranged on the side close to the back water surface of the submerged shallow dam to purify the microplastics in the main ditch. A non-submerged diversion dam is arranged downstream of the submerged shallow dam to divert the water flow of the main ditch, and a second microplastic purification unit is arranged on the side close to the front water surface of the non-submerged diversion dam to complete the control and purification of microplastic pollution at the confluence of the drainage ditch for farmland. The method for hydraulic control of microplastic pollution at the confluence of drainage ditch for farmland solves the problems of complex arrangement of interception devices and poor interception effect in the prior art.

Description

Technical Field

[0001] The present invention belongs to the technical field of agricultural water environment protection, and specifically relates to a method for controlling and purifying microplastic pollution at a confluence of a farmland drainage ditch. Background Art

[0002] Microplastics are defined as plastic particles smaller than 5mm, which have strong adsorption and are difficult to degrade. They are important carriers of various pollution. In agricultural production, due to the large-scale use of agricultural mulch and the wear of plastic agricultural tools, the microplastics produced are enriched in the soil, making farmland an important source of microplastics. The farmland runoff generated by precipitation, surface runoff and irrigation will carry microplastics in the soil through the main, agricultural, ditches and branch ditches, and finally be transported to natural rivers and lakes through the main ditch, causing serious harm to the aquatic ecosystem and posing the risk of enrichment in the ecosystem.

[0003] The current environmental management perspective for farmland drainage ditches mainly focuses on two soluble pollutants, total nitrogen and total phosphorus, while using technical means such as ecological plant planting to absorb pollutants. However, ecological measures such as planting plants cannot effectively remove microplastics in the water flow, and will increase the possibility of ingestion by organisms in the ditch. Therefore, effective treatment of microplastics in farmland drainage ditches is crucial to improving the efficiency and accuracy of environmental management of farmland drainage ditches. However, the current interception devices for microplastics below 5 mm in complex drainage ditch networks are complex to deploy and have poor interception effects. Therefore, it is urgent to propose a method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches, so as to more efficiently intercept microplastics at the confluence of farmland drainage ditches. Summary of the invention

[0004] The purpose of the present invention is to provide a method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches, which solves the problems in the prior art of complex layout of interception devices for microplastics with a diameter of less than 5 mm and poor interception effect in complex drainage ditch networks.

[0005] The technical solution adopted by the present invention is a method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches, and the specific steps are as follows:

[0006] S1. Obtain the basic parameters of the main ditch and branch ditch, and build a three-dimensional digital twin model based on the basic parameters;

[0007] S2. Based on the three-dimensional digital twin model in step S1, a main ditch rectifying pile group and a branch ditch rectifying pile group are set upstream of the confluence of the main ditch and the branch ditch to rectify the water flow of the main ditch and the branch ditch, a submerged dam is arranged downstream of the confluence of the main ditch and the branch ditch to raise the water level of the main ditch, and a first microplastic purification unit is arranged on the back side of the submerged dam to purify the microplastics in the main ditch;

[0008] S3. Based on the three-dimensional digital twin model in step S1, a non-submerged diversion dam is arranged downstream of the submerged dam to divert the water flow in the dry ditch. A second microplastic purification unit is arranged on the side close to the water-facing surface of the non-submerged diversion dam to perform secondary purification on the microplastics in the dry ditch, thus completing the control and purification of microplastic pollution at the confluence of the farmland drainage ditch.

[0009] The present invention is also characterized in that:

[0010] Step S2 is specifically as follows:

[0011] S201: In a three-dimensional modeling software, a three-dimensional digital twin model is established according to the dimensions and water flow parameters of the main ditch and the branch ditch. The specific dimensions are: main ditch width w1, branch ditch width w2, main ditch height d1, branch ditch height d2, main ditch and branch ditch confluence angle α, main ditch cross section S1, branch ditch cross section S2, upstream length L5 of the confluence of the main ditch and the branch ditch, and downstream length L6 of the confluence of the main ditch and the branch ditch;

[0012] The ratio of the upstream length L5 of the confluence of the main ditch and the branch ditch to the width w1 of the main ditch is 3-4:1, and the ratio of the downstream length L6 of the confluence of the main ditch and the branch ditch to the width w1 of the main ditch is 15-20:1;

[0013] S202: In FLUENT software, the three-dimensional digital twin model is initialized according to the water flow parameters of the main ditch and the branch ditch. The specific parameters are set as follows: the initial flow rate of the main ditch inlet water flow is set to the cross-sectional average flow velocity v1 of the main ditch, the initial flow velocity of the branch ditch inlet water flow is set to the cross-sectional average flow velocity v2 of the branch ditch, the water depth of the main ditch is set to h1, and the water depth of the branch ditch is set to h2;

[0014] S203: simulating the flow pattern of water at the confluence of the main ditch and the branch ditch to obtain the flow velocity distribution diagram of the main ditch and the branch ditch;

[0015] S204: Determine the confluence of the main ditch and the branch ditch, the location and range of the maximum velocity area, and the downstream boundary of the maximum velocity area from the velocity distribution diagram, and measure the horizontal distance L from the downstream boundary of the maximum velocity area at the 1 / 2 water depth section of the main ditch to the confluence origin.

[0016] In step S2, the main ditch rectification pile group and the branch ditch rectification pile group are specifically set as follows:

[0017] According to the flow pattern of the main ditch and the branch ditch and the position and range of the maximum flow velocity area at the confluence of the main ditch and the branch ditch, the arrangement positions of the main ditch rectifying pile group and the branch ditch rectifying pile group are determined upstream of the confluence of the main ditch and the branch ditch, wherein the main ditch rectifying pile group includes rectifying pile a, rectifying pile b, rectifying pile c, and the main ditch rectifying pile group is located on the left bank of the main ditch, and the branch ditch rectifying pile group includes rectifying pile d, rectifying pile e, rectifying pile f, and the branch ditch rectifying pile group is located on the left bank of the branch ditch;

[0018] The rectifying pile a in the main ditch rectifying pile group is connected to the left bank of the upstream main ditch where the main ditch and the branch ditch meet, and the rectifying piles b and c are arranged at intervals in the upstream main ditch where the main ditch and the branch ditch meet;

[0019] The rectifying pile d in the branch ditch rectifying pile group is connected to the left bank of the upstream branch ditch where the main ditch and the branch ditch meet, and the rectifying piles e and f are arranged at intervals in the upstream branch ditch where the main ditch and the branch ditch meet;

[0020] The angle between the connecting line of the cross-section centroids of the rectifying piles a, b and c and the left bank of the main ditch upstream of the confluence of the main ditch and the branch ditch is θg;

[0021] The angle between the centroid line of the cross section of the rectifying pile d, rectifying pile e, and rectifying pile f and the left bank of the branch ditch upstream of the confluence of the main ditch and the branch ditch is θz;

[0022] The cross sections of the rectifying pile a, rectifying pile b and rectifying pile c are all square, and the side lengths are all a1. The ratio of the side length a1 of the rectifying pile a, rectifying pile b and rectifying pile c to the dry ditch width w1 is 0.1~0.2:1;

[0023] The cross sections of the rectifying piles d, e and f are all square, and the side lengths are all a2. The ratio of the side length a2 of the rectifying piles d, e and f to the width w2 of the branch ditch is 0.1-0.2:1.

[0024] The centroid of the cross section of the rectifying pile a is taken as the zero point of the first local reference system, the ratio of the ordinate y2 of the centroid of the cross section of the rectifying pile b to the dry ditch width w1 is 0.2-0.3:1, and the calculation formula of the abscissa x2 of the centroid of the cross section of the rectifying pile b is as follows:

[0025] (1)

[0026] Where x2 is the abscissa of the centroid of the cross section of the rectifying pile b, θg is the angle between the centroid of the cross sections of the rectifying piles a, b, and c and the left bank slope of the dry ditch;

[0027] The ratio of the centroid y3 of the rectifying pile c to the dry ditch width w1 is 0.4~0.6:1. The calculation formula of the centroid x3 of the rectifying pile c section is as follows:

[0028] (2)

[0029] Where x3 is the abscissa of the centroid of the cross section of the rectifying pile c, θg is the angle between the centroid of the cross sections of rectifying pile a, rectifying pile b, and rectifying pile c and the left bank slope of the dry ditch;

[0030] The centroid of the cross section of the straightening pile d is taken as the origin of the second local reference system, the ratio of the ordinate y4 of the cross section centroid of the straightening pile e to the width w2 of the branch ditch is 0.2-0.3:1, and the calculation formula of the abscissa x4 of the cross section centroid of the straightening pile e is as follows:

[0031] (3)

[0032] Where x4 is the abscissa of the centroid of the cross section of the rectifying pile e, θz is the angle between the connecting line of the centroids of the cross sections of the rectifying piles d, e, and f and the left bank slope of the branch ditch;

[0033] The ratio of the centroid y5 of the rectifying pile f to the width w2 of the branch ditch is 0.4~0.6:1. The calculation formula of the centroid abscissa x5 of the cross section of the rectifying pile f is as follows:

[0034] (4)

[0035] Where x5 is the abscissa of the centroid of the cross section of the rectifying pile f, θz is the angle between the connecting line of the centroids of the cross sections of the rectifying piles d, e, and f and the left bank slope of the branch ditch;

[0036] The ratio of the height h3 of the dry ditch rectification pile group to the dry ditch water depth h1 is 0.5~1:1;

[0037] The ratio of the branch ditch rectification pile group height h4 to the branch ditch water depth h2 is 0.5~1:1;

[0038] The submerged dam in step S2 is specifically set as:

[0039] According to the flow pattern of the confluence of the main ditch and the branch ditch and the horizontal distance L between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch and the confluence origin, a submerged dam is arranged downstream of the confluence of the main ditch and the branch ditch;

[0040] The ratio of the x-direction distance L1 from the starting point of the submerged dam on the left bank of the main channel to the confluence origin (5) to the horizontal distance L from the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main channel to the confluence origin is 1.5~2:1;

[0041] The ratio of the width of the submerged dam w3 to the width of the dry ditch w1 is 0.1~0.2:1, and the ratio of the height of the submerged dam h5 to the depth of the dry ditch h1 is 0.5~0.75:1;

[0042] The angle θ2 between the submerged dam and the left bank slope of the main channel is equal to the angle θg between the connecting line of the cross-section centroids of the rectifying piles a, b and c and the left bank slope of the main channel.

[0043] In step S2, the first microplastic purification unit includes a first microplastic purification device, which is arranged on the side away from the right bank of the dry ditch behind the submerged dam. A first multi-stage filtering device is arranged in the first microplastic purification device. The side of the first microplastic purification device close to the submerged dam is connected in sequence to a pressurized pipeline inlet pipe containing a gate, a pressurized pipeline straight pipe, and a pressurized pipeline connecting pipe a. The other side of the first microplastic purification device is connected in sequence to a pressurized pipeline connecting pipe b and a pressurized pipeline outlet pipe containing a gate.

[0044] The first microplastic purification unit in step S2 is specifically configured as follows:

[0045] The height of the pressurized pipeline inlet pipe including the gate, the pressurized pipeline straight pipe, the pressurized pipeline connecting pipe a, the pressurized pipeline connecting pipe b, the pressurized pipeline outlet pipe including the gate, and the first microplastic purification device are all h6;

[0046] A pressurized pipeline inlet pipe including a gate, a pressurized pipeline straight pipe, a pressurized pipeline connecting pipe a, a pressurized pipeline connecting pipe b, a pressurized pipeline outlet pipe including a gate, and a ratio of the height h6 of the first microplastic purification device to the height h5 of the submerged dam is 0.4~1:1;

[0047] The inlet pipe of the pressurized pipeline with gate is a gradually expanding pipe;

[0048] The value range of the angle θ3 between the upstream side of the inlet pipe of the gated pressurized pipeline and the right bank slope of the dry ditch is 30°~50°;

[0049] The ratio of the angle θ4 between the downstream side of the inlet pipe of the gated pressurized pipeline and the right bank slope of the dry ditch and the angle θ2 between the submerged dam and the left bank slope of the dry ditch is 1:1;

[0050] The ratio of the width w4 of the straight pipe of the pressurized pipeline to the width w3 of the submerged dam is 1:1;

[0051] The outlet pipe of the pressurized pipeline with gate is straight;

[0052] The angle θ6 between the outlet pipe of the pressurized pipeline with gate and the right bank slope of the dry ditch is in the range of 25~35°;

[0053] The ratio of the width w5 of the outlet pipe of the pressurized pipeline with gate to the width w3 of the submerged dam is 1.2~1.5:1;

[0054] The width of the pressurized pipeline connecting pipe a and the pressurized pipeline connecting pipe b are both b, and the ratio of the width b of the pressurized pipeline connecting pipe a and the pressurized pipeline connecting pipe b to the width w3 of the submerged dam is 1:1;

[0055] The first microplastic purification device is a rectangular parallelepiped, and the ratio of the width w6 of the first microplastic purification device to the width w3 of the submerged dam is 3-5:1;

[0056] The ratio of the length L3 of the first microplastic purification device to the width w3 of the submerged dam is 1.5~2:1;

[0057] The non-submerged diversion dam in step S3 is specifically set as:

[0058] The ratio of the x-direction distance L2 from the starting point of the non-submerged diversion dam to the confluence origin 5 to the x-direction distance L from the downstream boundary of the maximum flow velocity area to the confluence origin is 3-4:1;

[0059] The angle θ6 between the non-submerged diversion dam and the left bank slope of the main channel ranges from 90° to 140°;

[0060] The ratio of the width of the non-submerged diversion dam w9 to the width of the submerged dam w3 is 1:1;

[0061] The ratio of the non-flooded diversion dam height h7 to the dry ditch height d1 is 1:1;

[0062] The ratio of the projection length w8 of the non-submerged diversion dam in the yz plane to the width w1 of the dry ditch is 0.25~0.75:1.

[0063] In step S3, the second microplastic purification unit includes a second microplastic purification device, in which a second multi-stage filtering device is arranged. The second microplastic purification device is arranged on the side away from the left bank of the dry ditch behind the back water surface of the non-submerged diversion dam. The side of the second microplastic purification device close to the non-submerged diversion dam is connected with an inlet slot containing a gate, a straight slot, and a connecting slot a. The other side of the second microplastic purification device is connected with a connecting slot b and a gate outlet slot.

[0064] In step S3, the second microplastic purification unit is specifically configured as follows: the projection dimensions of the gated inlet slot, the straight slot, the connecting slot a, the connecting slot b, and the gated outlet slot on the xy plane are respectively equal to the projection dimensions of the gated pressurized pipeline inlet pipe, the pressurized pipeline straight pipe, the pressurized pipeline connecting pipe a, the pressurized pipeline connecting pipe b, and the gated pressurized pipeline outlet pipe;

[0065] The height of the second microplastic purification device including the gate inlet slot, the straight slot, the connecting slot a, the connecting slot b, the gate outlet slot, and the second microplastic purification device is h8;

[0066] The ratio of the height h8 of the second microplastic purification device to the height d1 of the dry ditch is 1:1, including the gate inlet slot, the straight slot, the connecting slot a, the connecting slot b, and the gate outlet slot;

[0067] The second microplastic purification device is provided with a second multi-stage filtering device;

[0068] The second microplastic purification device is a rectangular parallelepiped, the ratio of the width w7 of the second microplastic purification device to the width w6 of the first microplastic purification device is 1:1, and the ratio of the length L4 of the second microplastic purification device to the width w7 of the second microplastic purification device is 2-3:1.

[0069] The beneficial effect of the present invention is that the method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches proposed by the present invention simplifies the layout of microplastic interception devices in complex drainage ditch networks, improves the interception effect of microplastics in complex drainage ditch networks, and ensures that the dry ditch water has a lower microplastic pollution level when entering the natural water body. BRIEF DESCRIPTION OF THE DRAWINGS

[0070] Figure 1 This is a schematic diagram of the arrangement of the device for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches of the present invention;

[0071] Figure 2 It is an xy plane view of the dry ditch rectification pile group in the present invention;

[0072] Figure 3 It is a yz plane view of the dry ditch rectification pile group in the present invention;

[0073] Figure 4 It is an xy plane view of the branch ditch rectification pile group in the present invention;

[0074] Figure 5 It is a yz plane view of the branch ditch rectification pile group in the present invention;

[0075] Figure 6 is a yz plane view of the submerged dam 7 of the present invention;

[0076] Figure 7 It is a yz plane view of the first microplastic purification unit in the present invention;

[0077] Figure 8 is a yz plane view of the non-submerged diversion dam in the present invention;

[0078] Fig. 9 is a yz plane view of the second microplastic purification unit of the present invention;

[0079] Fig.10 Schematic diagram of the maximum flow velocity area at the confluence of the main ditch and the branch ditch in Example 1 of the present invention;

[0080] Fig.11 This is a typical microplastic motion trajectory diagram when the observation time is 25s in working condition 1 of Example 1 of the present invention;

[0081] Fig.12 This is a typical microplastic motion trajectory diagram when the observation time is 25s in working condition 2 of Example 1 of the present invention;

[0082] Fig.13 This is a typical microplastic motion trajectory diagram when the observation time is 40s in working condition 1 of Example 1 of the present invention;

[0083] Fig.14 This is a typical microplastic motion trajectory diagram when the observation time is 40s in working condition 2 of Example 1 of the present invention;

[0084] Fig.15 This is a typical microplastic motion trajectory diagram when the observation time of working condition 1 is 80s in Example 1 of the present invention;

[0085] Fig.16 This is a typical microplastic motion trajectory diagram when the observation time is 80s in working condition 2 of Example 1 of the present invention;

[0086] Fig.17 It is a vertical line graph of the abundance of microplastics on the left bank of the dry ditch 4 in Example 1 of the present invention;

[0087] Fig.18 It is a vertical line graph of microplastic abundance in the middle of dry ditch 4 in Example 1 of the present invention;

[0088] Fig.19 It is a vertical line graph of microplastic abundance on the right bank of the dry ditch 4 in Example 1 of the present invention;

[0089] Fig. 20 This is a diagram showing the proportion of microplastic flux in each area of ​​the confluent channel when the observation time is 130 s in working condition 1 in Example 1 of the present invention;

[0090] Fig.21 This is a diagram showing the proportion of microplastic flux in each area of ​​the confluent channel when the observation time is 130s in working condition 2 in Example 1 of the present invention.

[0091] In the figure, 1. branch ditch rectification pile group; 2. branch ditch; 3. main ditch rectification pile group; 4. main ditch; 5. confluence origin; 6. submerged submerged dam starting point; 7. submerged submerged dam; 8. pressurized pipeline inlet pipe with gate; 9. pressurized pipeline straight pipe; 10. pressurized pipeline connecting pipe a; 11. first microplastic purification device; 12. first multi-stage filtration device; 13. pressurized pipeline connecting pipe b; 14. pressurized pipeline outlet pipe with gate; 15. non-submerged diversion dam; 16. non-submerged Starting point of diversion dam; 17. Inlet slot with gate; 18. Straight slot; 19. Connecting slot a; 20. Second microplastic purification device; 21. Second multi-stage filtration device; 22. Connecting slot b; 23. Exit slot with gate; 301. Rectifying pile a; 302. Rectifying pile b; 303. Rectifying pile c; 101. Rectifying pile d; 102. Rectifying pile e, 103. Rectifying pile f; 110. First microplastic purification unit; 120. Second microplastic purification unit. DETAILED DESCRIPTION

[0092] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0093] Microplastic pollution control and purification method at the confluence of farmland drainage ditches, combined with Figure 1 , follow the steps below:

[0094] 1. The specific steps of the method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches are as follows:

[0095] S1. Obtain basic parameters of the main ditch 4 and the branch ditch 2, and construct a three-dimensional digital twin model based on the basic parameters;

[0096] S2. Based on the three-dimensional digital twin model in step S1, a main ditch rectifying pile group 3 and a branch ditch rectifying pile group 1 are set upstream of the confluence of the main ditch 4 and the branch ditch 2 to rectify the water flow of the main ditch 4 and the branch ditch 2, and a submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2 to raise the water level of the main ditch 4, and a first microplastic purification unit 110 is arranged on the back side of the submerged dam 7 to purify the microplastics in the main ditch 4;

[0097] Specifically:

[0098] In the 3D modeling software, a 3D digital twin model is established according to the size parameters of the main ditch 4 and the branch ditch 2. The specific sizes are: the width w1 of the main ditch 4, the width w2 of the branch ditch 2, the height d1 of the main ditch 4, the height d2 of the branch ditch 2, the confluence angle α of the main ditch 4 and the branch ditch 2, the cross section S1 of the main ditch 4, the cross section S2 of the branch ditch 2, the upstream length L5 of the confluence of the main ditch 4 and the branch ditch 2, and the downstream length L6 of the confluence of the main ditch 4 and the branch ditch 2.

[0099] The ratio of the upstream length L5 of the confluence of the main ditch 4 and the branch ditch 2 to the width w1 of the main ditch 4 is 3-4:1, and the ratio of the downstream length L6 of the confluence of the main ditch 4 and the branch ditch 2 to the width w1 of the main ditch 4 is 15-20:1;

[0100] Use the built-in tools of FLUENT software to divide the 3D digital twin model;

[0101] In FLUENT software, the three-dimensional digital twin model is initialized according to the water flow parameters of the main ditch 4 and the branch ditch 2. The specific parameter setting method is to set the initial flow velocity of the water flow at the inlet of the main ditch 4 to the cross-sectional average flow velocity v1 of the main ditch 4, set the initial flow velocity of the water flow at the inlet of the branch ditch 2 to the cross-sectional average flow velocity v2 of the branch ditch 2, set the water depth of the main ditch 4 to h1, and set the water depth of the branch ditch 2 to h2;

[0102] Simulate the flow pattern of the water flow at the confluence of the main ditch 4 and the branch ditch 2 to obtain the flow velocity distribution diagram of the main ditch 4 and the branch ditch 2;

[0103] Determine the maximum velocity area at the 4 1 / 2 water depth section of the main channel from the velocity distribution diagram, referred to as the maximum velocity area, determine the location and range of the maximum velocity area and its downstream boundary, and calculate the x-direction distance L of the downstream boundary of the maximum velocity area from the confluence origin 5;

[0104] According to the flow pattern of the main ditch 4 and the branch ditch 2 at the confluence, the main ditch rectifying pile group 3 and the branch ditch rectifying pile group 1 are arranged;

[0105] Specifically:

[0106] According to the flow pattern of the main ditch 4 and the branch ditch 2 and the location and range of the maximum flow velocity area, the arrangement positions of the rectifying piles a301, b302, c303, d101, e102 and f103 are determined;

[0107] In the upstream of the confluence of the main ditch 4 and the branch ditch 2, the left bank of the main ditch 4 is provided with rectifying piles a301, rectifying piles b302, rectifying piles c303, forming a main ditch rectifying pile group 3; in the upstream of the confluence of the main ditch 4 and the branch ditch 2, the left bank of the branch ditch 2 is provided with rectifying piles d101, rectifying piles e102, rectifying piles f103, forming a branch ditch rectifying pile group 1;

[0108] Divert and rectify the flow of main and branch ditches to change the migration direction of microplastics in the water, increase the turbulent kinetic energy of the water flow, increase the flocculation rate of microplastics, and further promote the deposition of low-density microplastics.

[0109] The deployment scheme of the main ditch rectification pile group 3 and the branch ditch rectification pile group 1 is as follows:

[0110] The rectifying pile a301 in the dry ditch rectifying pile group 3 is connected to the left bank slope of the dry ditch 4, and the rectifying piles b302 and c303 are arranged in the dry ditch 4 at equal distances and intervals;

[0111] The rectifying pile d101 in the branch ditch rectifying pile group 1 is connected to the left bank slope of the branch ditch 2, and the rectifying piles e102 and f103 are arranged in the branch ditch 2 at equal distances and intervals;

[0112] The angle between the centroid line of the cross section of the rectifying pile a301, the rectifying pile b302, and the left bank slope of the main channel 4 is θg;

[0113] The angle between the centroid line of the cross section of the rectifying pile d101, the rectifying pile e102, and the left bank slope of the branch ditch 2 is θz;

[0114] The cross sections of the rectifying piles a301, b302 and c303 are all square, and the side lengths are all a1. The ratio of the side length a1 of the rectifying piles a301, b302 and c302 to the width w1 of the dry ditch 4 is 0.1-0.2:1.

[0115] The cross sections of the rectifying piles d101, e102 and f103 are all square, and the side lengths are all a2. The ratio of the side length a2 of the rectifying piles d101, e102 and f103 to the width w2 of the branch ditch 2 is 0.1-0.2:1.

[0116] The centroid of the cross section of the rectifying pile a301 is taken as the zero point of the first local reference system. The ratio of the centroid y2 of the cross section of the rectifying pile b302 to the width w1 of the dry ditch 4 is 0.2~0.3:1. The calculation function of the centroid x2 of the cross section of the rectifying pile b302 is shown in formula (1):

[0117] (1)

[0118] In formula 1, x2 is the abscissa of the centroid of the cross section of the rectifying pile b302, θg is the angle between the connecting line of the centroids of the cross sections of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0119] The ratio of the centroid y3 of the rectifying pile c303 to the width w1 of the dry ditch 4 is 0.4~0.6:1. The mathematical expression of the calculation method of the centroid abscissa x3 of the cross section of the rectifying pile c303 is shown in formula (2):

[0120] (2)

[0121] In formula (2), x3 is the abscissa of the centroid of the cross section of the rectifying pile c303, θg is the angle between the centroid of the cross section of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0122] The centroid of the cross section of the rectifying pile d101 is taken as the zero point of the second local reference system. The ratio of the centroid y4 of the cross section of the rectifying pile e102 to the width w2 of the branch ditch 2 is 0.2~0.3:1. The calculation function of the centroid x4 of the cross section of the rectifying pile e102 is shown in formula (3):

[0123] (3)

[0124] In formula (3), x4 is the abscissa of the centroid of the cross section of the straightening pile e102, θz is the angle between the connecting line of the centroids of the cross sections of the straightening piles d101, straightening piles e102, and straightening piles f103 and the left bank slope of the branch ditch 2;

[0125] The ratio of the centroid y5 of the rectifying pile f103 to the width w2 of the branch ditch 2 is 0.4~0.6:1. The mathematical expression of the calculation method of the centroid abscissa x5 of the cross section of the rectifying pile f103 is shown in formula (4):

[0126] (4)

[0127] In formula (4), x5 is the abscissa of the centroid of the cross section of the straightening pile f103, θz is the angle between the connecting line of the centroids of the cross sections of the straightening piles d101, straightening piles e102, and straightening pile f103 and the left bank slope of the branch ditch 2;

[0128] The ratio of the height h3 of the dry ditch rectification pile group 3 to the water depth h1 of the dry ditch 4 is 0.5~1:1;

[0129] The ratio of the height h4 of the branch ditch rectifying pile group 1 to the water depth h2 of the branch ditch 4 is 0.5~1:1;

[0130] A submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2, and a first microplastic purification unit 110 is arranged on the side close to the back water surface of the submerged dam 7, so that microplastics in the main ditch 4 are purified once by the first microplastic purification unit 110;

[0131] The deployment plan for submerged dam 7 is:

[0132] According to the flow pattern of the main ditch 4 and the branch ditch 2 at the confluence, and the x-direction distance L between the downstream boundary of the maximum flow velocity area and the confluence origin 5, a submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2;

[0133] The ratio of the x-direction distance L1 from the starting point 6 of the submerged dam on the left bank of the main channel 4 to the confluence origin 5 to the x-direction distance L from the downstream boundary of the maximum flow velocity area to the confluence origin 5 is 1.5~2:1;

[0134] The ratio of the width w3 of the submerged dam 7 to the width w1 of the dry ditch 4 is 0.1-0.2:1, and the ratio of the height h5 of the submerged dam 7 to the water depth h1 of the dry ditch 4 is 0.5-0.75:1;

[0135] The angle θ2 between the submerged dam 7 and the left bank slope of the main channel 4 is equal to the angle θg between the connecting line of the cross-section centroids of the rectifying pile a301, the rectifying pile b302, and the rectifying pile c303 and the left bank slope of the main channel 4.

[0136] The first microplastic purification unit 110 includes a first microplastic purification device 11, which is arranged on the side away from the right bank of the dry ditch 4 behind the submerged dam 7, and introduces microplastics transported along the bottom of the ditch into the first microplastic purification device by using the upstream and downstream head difference; a first multi-stage filtering device 12 is arranged in the first microplastic purification device 11, and the side of the first microplastic purification device 11 close to the submerged dam 7 is connected in sequence to the pressurized pipeline inlet pipe 8 containing a gate, the pressurized pipeline straight pipe 9, and the pressurized pipeline connecting pipe a10, and the other side of the first microplastic purification device 11 is connected in sequence to the pressurized pipeline connecting pipe b13 and the pressurized pipeline outlet pipe 14 containing a gate;

[0137] The specific parameters in the first microplastic purification unit 110 are set as follows:

[0138] A pressurized pipeline inlet pipe 8 including a gate, a pressurized pipeline straight pipe 9, a pressurized pipeline connecting pipe a10, a pressurized pipeline connecting pipe b13, a pressurized pipeline outlet pipe 14 including a gate, and a first microplastic purification device 11 are all of height h6;

[0139] A pressurized pipeline inlet pipe 8 including a gate, a pressurized pipeline straight pipe 9, a pressurized pipeline connecting pipe a10, a pressurized pipeline connecting pipe b13, a pressurized pipeline outlet pipe 14 including a gate, and a ratio of a height h6 of the first microplastic purification device 11 to a height h5 of the submerged dam 7 is 0.4-1:1;

[0140] The inlet pipe 8 of the gated pressurized pipeline is a gradually expanding pipeline;

[0141] The value range of the angle θ3 between the upstream side of the inlet pipe 8 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 is 30°~50°;

[0142] The ratio of the angle θ4 between the downstream side of the inlet pipe 8 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 and the angle θ2 between the submerged dam 7 and the left bank slope of the dry ditch 4 is 1:1;

[0143] The ratio of the width w4 of the straight pipe 9 of the pressurized pipeline to the width w3 of the submerged dam 7 is 1:1;

[0144] The outlet pipe 14 of the pressurized pipeline including the gate is of straight type;

[0145] The angle θ6 between the outlet pipe 14 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 is in the range of 25 to 35°;

[0146] The ratio of the width w5 of the gated pressurized pipeline outlet pipe 14 to the width w3 of the submerged dam 7 is 1.2-1.5:1;

[0147] The widths of the pressurized pipeline connecting pipe a10 and the pressurized pipeline connecting pipe b13 are both b, and the ratio of the width b of the pressurized pipeline connecting pipe a10 and the pressurized pipeline connecting pipe b13 to the width w3 of the submerged dam 7 is 1:1;

[0148] The first microplastic purification device 11 is a rectangular parallelepiped, and the ratio of the width w6 of the first microplastic purification device 11 to the width w3 of the submerged dam 7 is 3-5:1;

[0149] The ratio of the length L3 of the first microplastic purification device 11 to the width w3 of the submerged dam 7 is 1.5-2:1;

[0150] A first multi-stage filtering device 12 is provided at the outlet of the first microplastic purification device 11, which can prevent microplastic particles from returning to the channel. In actual use, water is introduced through a pressurized pipe, and a water flow carrying a large amount of high-density microplastics enters the first microplastic purification device 11. First, some microplastics are intercepted by two large-scale coherent structures generated in the first microplastic purification device 11, and secondly, the remaining microplastics not retained by the coherent structures are efficiently intercepted by the filtering device at the outlet of the first microplastic purification device 11. The ingenious combination of these two interception mechanisms significantly improves the purification capacity of the purification device.

[0151] In addition, a first microplastic purification device cleaning port is provided on the top of the first microplastic purification device 11 , and the management personnel can regularly close the gates in the gated pressurized pipeline inlet pipe 8 and the gated pressurized pipeline outlet pipe 14 to perform targeted cleaning of the microplastics in the first microplastic purification device 11 .

[0152] S3. Based on the three-dimensional digital twin model in step S1, a non-submerged diversion dam 15 is arranged downstream of the submerged dam 7 to divert the water flow of the dry ditch 4, and a second microplastic purification unit 120 is arranged on the side close to the water-facing surface of the non-submerged diversion dam 15 to perform secondary purification on the microplastics in the dry ditch 4, thereby completing the control and purification of microplastic pollution at the confluence of the farmland drainage channel.

[0153] The specific configuration of the non-submerged diversion dam 15 is as follows:

[0154] The ratio of the x-direction distance L2 from the starting point 16 of the non-submerged diversion dam to the confluence origin 5 and the x-direction distance L from the downstream boundary of the maximum flow velocity area to the confluence origin 5 is 3-4:1;

[0155] The angle θ6 between the non-submerged diversion dam 15 and the left bank slope of the main ditch 4 ranges from 90° to 140°;

[0156] The ratio of the width w9 of the non-submerged diversion dam 15 to the width w3 of the submerged submerged dam 7 is 1:1;

[0157] The ratio of the height h7 of the non-flooded diversion dam 15 to the height d1 of the dry ditch 4 is 1:1;

[0158] The ratio of the projection length w8 of the non-submerged diversion dam 15 in the yz plane to the width w1 of the dry ditch 4 is 0.25~0.75:1;

[0159] The second microplastic purification unit 120 includes a second microplastic purification device 20, in which a second multi-stage filtering device 21 is arranged. The second microplastic purification device 20 is arranged on the back surface of the non-submerged diversion dam 15 and away from the left bank of the dry ditch 4. The side of the second microplastic purification device 20 close to the non-submerged diversion dam 15 is connected to an inlet groove 17 containing a gate, a straight groove 18, and a connecting groove a19. The other side of the second microplastic purification device 20 is connected to a connecting groove b22 and an outlet groove 23 containing a gate.

[0160] The specific configuration including the gate inlet slot 17, the straight slot 18, the connecting slot a19, the connecting slot b20 and the outlet slot 23 is as follows:

[0161] The projection dimensions of the gate-containing inlet groove 17, the straight groove 18, the connecting groove a19, the connecting groove b20, and the gate-containing outlet groove 23 on the xy plane are respectively equal to the projection dimensions of the gate-containing pressurized pipeline inlet pipe 8, the pressurized pipeline straight pipe 9, the pressurized pipeline connecting pipe a10, the pressurized pipeline connecting pipe b13, and the gate-containing pressurized pipeline outlet pipe 14;

[0162] The height of the second microplastic purification device 20 including the gate inlet slot 17, the straight slot 18, the connecting slot a19, the open slot b22, and the gate outlet slot 23 is h8;

[0163] The ratio of the height h8 of the second microplastic purification device 20 to the height d1 of the dry ditch is 1:1, including a gate inlet slot 17, a straight slot 18, a connecting slot a19, a connecting slot b22, and a gate outlet slot 23;

[0164] The second microplastic purification device 20 is provided with a second multi-stage filtering device 21;

[0165] The second microplastic purification device 20 is a cuboid, the ratio of the width w7 of the second microplastic purification device 20 to the width w6 of the first microplastic purification device 11 is 1:1, and the ratio of the length L4 of the second microplastic purification device 20 to the width w7 of the second microplastic purification device 20 is 2-3:1.

[0166] The second microplastic purification device 20 mainly focuses on intercepting low-density microplastics. Through the low-flow coherent structure formed and the second multi-stage filtration device 21, microplastic particles floating on the water surface can be intercepted. Management personnel need to regularly salvage the microplastics in the second microplastic purification device and properly dispose of them.

[0167] Example 1

[0168] 1. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches is as follows:

[0169] S1. Prototype observations were conducted on the main ditch 4 and the branch ditch 2, and relevant parameters of the main ditch 4 and the branch ditch 2 during spring irrigation were measured; the average flow velocity of the cross section S1 of the main ditch 4 was v1=0.3m / s, and the average flow velocity of the cross section S2 of the branch ditch 2 was v2=0.2m / s; the width of the main ditch 4 was w1=0.9m, and the width of the branch ditch 2 was w2=0.3m; the height of the main ditch 4 was d1=0.3m, and the height of the branch ditch 2 was d2=0.3m; the water depth of the main ditch 4 was h1=0.2m, and the water depth of the branch ditch 2 was h2=0.2m; the cross section S1 of the main ditch 4 was rectangular, and the cross section S2 of the branch ditch 2 was rectangular; the upstream length of the confluence of the main ditch 4 and the branch ditch 2 was L5=3.15m, and the downstream length of the confluence of the main ditch 4 and the branch ditch 2 was L6=15.3m; the confluence angle α of the main ditch 4 and the branch ditch 2 was 60°;

[0170] S2. Import the 3D digital twin model into FLUENT software to simulate the flow pattern of the confluence of the main ditch 4 and the branch ditch 2, and measure the x-direction distance L=2m between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch 4 and the confluence origin 5;

[0171] According to the flow pattern of the main ditch 4 and the branch ditch 2 at the confluence, a main ditch rectifying pile group 3 and a branch ditch rectifying pile group 1 are arranged upstream of the confluence of the main ditch 4 and the branch ditch 2;

[0172] The cross-sectional length of the main ditch rectification pile group 3 is a1=0.09m, and the cross-sectional length of the branch ditch rectification pile group 1 is a2=0.03m;

[0173] The rectifying pile a301 in the dry ditch rectifying pile group 3 is connected to the left bank slope of the dry ditch 4, and the rectifying piles b302 and c303 are arranged in the dry ditch 4 at equal distances and intervals;

[0174] The rectifying pile d101 in the branch ditch rectifying pile group 1 is connected to the left bank slope of the branch ditch 2, and the rectifying piles e102 and f103 are arranged in the branch ditch 2 at equal distances and intervals;

[0175] Taking the centroid of the cross section of the rectifying pile a301 as the zero point 304 of the first local reference system, the calculation function of the abscissa x2 of the centroid of the cross section of the rectifying pile b302 is shown in formula (1):

[0176] (1)

[0177] In formula (1), x2 is the abscissa of the centroid of the cross section of the rectifying pile b302, θg is the angle between the connecting line of the centroids of the cross sections of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0178] The mathematical expression of the calculation method of the centroid x3 of the cross section of the rectifying pile c303 is shown in formula (2):

[0179] (2)

[0180] In formula (2), x3 is the abscissa of the centroid of the cross section of the rectifying pile c303, θg is the angle between the centroid of the cross section of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0181] In formula (1) and (2), θg=63.4, then the local coordinates of the rectifying pile b302 are (0.09, 0.18), the local coordinates of the rectifying pile c303 are (0.18, 0.36), the local coordinates of the rectifying pile e102 are (0.03, 0.06), the local coordinates of the rectifying pile f103 are (0.06, 0.012), the height of the main ditch rectifying pile group 3 is h3=0.1m, and the height of the branch ditch rectifying pile group 1 is h4=0.1m;

[0182] A submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2, and a first microplastic purification unit 110 is arranged on the side close to the back water surface of the submerged dam 7, so that microplastics in the main ditch 4 are purified once by the first microplastic purification unit 110;

[0183] The x-direction distance from the starting point 6 of the submerged dam to the confluence origin 5 is L1=3m, the angle between the submerged dam 7 and the left bank slope of the dry ditch 4 is θ2=63.4°, the width of the submerged dam 7 is w3=0.1m; the height of the submerged dam 7 is h5=0.15m;

[0184] The height of the first microplastic purification unit 110 is h6=0.06m, and the width of the pressurized straight pipe 9 is w4=0.1m;

[0185] The angle θ3 between the upstream side of the inlet pipe 8 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 is 42.1°;

[0186] The angle θ4 between the downstream side of the gated pressurized pipeline inlet pipe 8 and the right bank slope of the dry ditch 4 is 63.4°.

[0187] The gated pressurized pipeline outlet pipe 14 is straight, and the width w5 of the gated pressurized pipeline outlet pipe 14 is 0.12m; the width b of the pressurized pipeline connecting pipe a10 is 0.1m, and the width b of the pressurized pipeline connecting pipe b13 is 0.1m;

[0188] The first microplastic purification unit 110 includes a first microplastic purification device 11;

[0189] The width w6 of the first microplastic purification device 11 is 0.3 m, the length L3 of the first microplastic purification device 11 is 0.15 m, and the height h6 of the first microplastic purification device 11 is 0.06 m;

[0190] S3. A non-submerged diversion dam 15 is arranged downstream of the submerged dam 7, and a second microplastic purification unit 120 is arranged on the side close to the water-facing surface of the non-submerged diversion dam 15. The microplastics in the dry ditch 4 are secondary purified by the second microplastic purification unit 120 to complete the hydraulic control of microplastic pollution at the confluence of the farmland drainage ditch.

[0191] The x-direction distance between the non-submerged diversion dam starting point 16 and the confluence origin 5 is L2=7m, the angle θ6 between the non-submerged diversion dam 15 and the left bank slope of the dry ditch 4 is 120°, the width w9=0.1m and the height h7=0.3m of the non-submerged diversion dam 15; the projection length w8=0.4m of the non-submerged diversion dam 15 on the yz plane;

[0192] The gate inlet slot 17, the straight slot 18, the connecting slot a19, the connecting slot b20, the gate outlet slot 23, the second microplastic purification device 20, and the gate outlet slot 23 all have a height of h8, h8=0.3m;

[0193] The second microplastic purification device 20 is a rectangular parallelepiped, with a width w7=w6=0.3 m and a length L4=2*0.3=0.6 m. A second multi-stage filtering device 21 is arranged in the second microplastic purification device 20 to effectively intercept microplastics.

[0194] In order to prove the effectiveness of this method, in Example 1, two working conditions were set. Working condition 1 was the movement of microplastics in the confluent channel of the main ditch 4 and the branch ditch 2 without microplastic treatment measures, and working condition 2 was the movement of microplastics in the confluent channel of the main ditch 4 and the branch ditch 2 after using this method.

[0195] For these two working conditions, microplastics made of PE and PS with a length of 1mm-4.5mm were dynamically released at the entrance of the main ditch 4 and the branch ditch 2 respectively. The release rate of microplastics at the entrance of the main ditch 4 was 300 pieces / s, and the release time was 60s. The release rate of microplastics at the entrance of the branch ditch 2 was 300 pieces / s, and the release time was 60s. The timing started from the time when the first microplastic entered the ditch. The motion characteristics of microplastics in the intersection ditch were captured by high-resolution instantaneous particle dynamic tracking technology. The observation time was 130s.

[0196] The observation results are as follows Figure 11-16 As shown in the figure, in both conditions 1 and 2, microplastics are affected by the hydrodynamic characteristics and deflect and move to varying degrees at the confluence of the main ditch 4 and the branch ditch 2. At the same time, dominated by the density of the microplastics themselves, polyethylene (PE) microplastics float in the surface water of the main ditch 4 and the branch ditch 2, and polystyrene (PS) microplastics gradually sink to the bottom of the ditch over time. However, the microplastic retention areas of conditions 1 and 2 are different. Figure 11-12It can be seen that in working condition 1, a large number of microplastics are retained in the flow separation zone, and in working condition 2, only a small amount of microplastics are retained near the main ditch rectifying pile group 3 and the branch ditch rectifying pile group 1. When the time is 25s, compared with working condition 1, in working condition 2, due to the layout of the rectifying pile group, the PS microplastics moving along the main ditch 4 and the branch ditch 2 are gradually diverted to the right bank of the main ditch 4, and the movement trajectory of the PS microplastics is significantly deflected. It can be seen that the layout of the main ditch rectifying pile group 3 and the branch ditch rectifying pile group 1 in working condition 2 significantly affects the movement of PS microplastics, creating conditions for the interception of microplastics;

[0197] As microplastics move further, the experimental results are as follows Figure 13-16 As shown in the figure, there is a huge difference in the movement of microplastics in working conditions 1 and 2. In working condition 1, microplastics are widely distributed in the dry ditch 4, and the two types of microplastics are mixed with each other, while in working condition 2, there are fewer microplastics in the dry ditch. Due to the layout of the submerged dam 7, the high-density PS microplastics are further transported to the first microplastic purification device on the right bank of the dry ditch 4, and are intercepted by the large-scale coherent structure and the first multi-stage filtration device in the first microplastic purification device, while the low-density PE microplastics cross the submerged dam 7 and are transported downstream. The pollution belt composed of low-density PE microplastics gradually approaches and compresses the left bank of the dry ditch 4; when the low-density PE microplastic pollution belt passes through the non-submerged diversion dam 17, the diversion and diversion effects of the non-submerged submerged dam 17 cause most of the PE microplastics to enter the second microplastic purification device with the water flow, and the PE microplastics are intercepted by the low-flow coherent structure and the second multi-stage microplastic filtration device. It can be seen that this method can effectively control the distribution of microplastics in the dry ditch 4 downstream of the confluence.

[0198] like Figure 17-19 The longitudinal distribution of microplastic abundance under the two working conditions at t=80s shows that compared with the movement of microplastics in the confluence of the main ditch 4 and the branch ditch 2 without any microplastic remediation measures, after the implementation of this plan, the abundance of microplastics in the downstream of the main ditch 4 was significantly reduced, and the abundance of microplastics in the middle of the main ditch 4 decreased most significantly;

[0199] like Fig. 20 In condition 1, 71% of microplastics are transported downstream through the dry ditch, which will endanger the water quality of the next level river system. Only 29% of microplastics are retained in the confluent ditch due to the influence of low flow velocity environments such as flow separation zones, gradually forming an unstable microplastic sink. When the channel flow conditions change, this part of microplastics is likely to re-enter the water body and be discharged into the natural water environment, threatening the stability of various ecosystems. Fig.21 In condition 2, only 17% of microplastics were transported downstream, 83% of microplastics were intercepted by the first and second microplastic purification devices, and the proportion of microplastics retained in the channel decreased by about 29% compared with condition 1. It can be seen that this method can not only significantly intercept microplastics transported along the confluent channel, but also reduce the risk of remobilization of microplastics at the bottom of the channel;

[0200] Finally, in order to evaluate the ecological risk at the downstream outlet of the dry ditch after using this method, the potential ecological risk index (PERI) was introduced to quantify the ecological risk. The background value of microplastic abundance in the environment was taken as 6.65 pieces / L, and the polymer hazard coefficients of PE and PS were taken as 11 and 30, respectively. Through analysis, it was found that the PERI at the downstream outlet of the dry ditch in Condition 1 was 12.2807, and the PERI at the downstream outlet of the dry ditch in Condition 2 was 0.56311, which was a decrease of 95.4% compared with Condition 1. It can be seen that the potential ecological risk of the confluence outlet area was also significantly reduced after using this method, and the exposure risk of aquatic organisms in this area was reduced accordingly.

[0201] Example 2

[0202] The specific steps of the method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches are as follows:

[0203] S1. Prototype observations were conducted on the main ditch 4 and the branch ditch 2, and relevant parameters of the main ditch 4 and the branch ditch 2 during spring irrigation were measured; the average flow velocity of the cross section S1 of the main ditch 4 was v1=0.3m / s, and the average flow velocity of the cross section S2 of the branch ditch 2 was v2=0.2m / s; the width of the main ditch 4 was w1=0.9m, and the width of the branch ditch 2 was w2=0.3m; the height of the main ditch 4 was d1=0.3m, and the height of the branch ditch 2 was d2=0.3m; the water depth of the main ditch 4 was h1=0.2m, and the water depth of the branch ditch 2 was h2=0.2m; the cross section S1 of the main ditch 4 was rectangular, and the cross section S2 of the branch ditch 2 was rectangular; the upstream length of the confluence of the main ditch 4 and the branch ditch 2 was L5=3.15m, and the downstream length of the confluence of the main ditch 4 and the branch ditch 2 was L6=15.3m; the confluence angle α of the main ditch 4 and the branch ditch 2 was 60°;

[0204] S2. Import the 3D digital twin model into FLUENT software to simulate the flow pattern of the confluence of the main ditch 4 and the branch ditch 2, and measure the x-direction distance L=2m between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch 4 and the confluence origin 5;

[0205] According to the flow pattern of the main ditch 4 and the branch ditch 2 at the confluence, a main ditch rectifying pile group 3 and a branch ditch rectifying pile group 1 are arranged upstream of the confluence of the main ditch 4 and the branch ditch 2;

[0206] The cross-sectional length of the main ditch rectification pile group 3 is a1=0.18m, and the cross-sectional length of the branch ditch rectification pile group 1 is a2=0.06m;

[0207] The rectifying pile a301 in the dry ditch rectifying pile group 3 is connected to the left bank slope of the dry ditch 4, and the rectifying piles b302 and c303 are arranged in the dry ditch 4 at equal distances and intervals;

[0208] The rectifying pile d101 in the branch ditch rectifying pile group 1 is connected to the left bank slope of the branch ditch 2, and the rectifying piles e102 and f103 are arranged in the branch ditch 2 at equal distances and intervals;

[0209] Taking the centroid of the cross section of the rectifying pile a301 as the zero point 304 of the first local reference system, the calculation function of the abscissa x2 of the centroid of the cross section of the rectifying pile b302 is shown in formula (1):

[0210] (1)

[0211] In formula (1), x2 is the abscissa of the centroid of the cross section of the rectifying pile b302, θg is the angle between the connecting line of the centroids of the cross sections of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0212] The mathematical expression of the calculation method of the centroid x3 of the cross section of the rectifying pile c303 is shown in formula (2):

[0213] (2)

[0214] In formula (2), x3 is the abscissa of the centroid of the cross section of the rectifying pile c303, θg is the angle between the centroid of the cross section of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0215] The height of the main ditch rectification pile group 3 is h3 = 0.2m, and the height of the branch ditch rectification pile group 1 is h4 = 0.2m;

[0216] A submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2, and a first microplastic purification unit 110 is arranged on the side close to the back water surface of the submerged dam 7, so that microplastics in the main ditch 4 are purified once by the first microplastic purification unit 110;

[0217] The distance in the x direction between the starting point 6 of the submerged dam and the confluence origin 5 is L1=4m, the angle between the submerged dam 7 and the left bank slope of the dry ditch 4 is θ2=63.4°, the width of the submerged dam 7 is w3=0.1m; the height of the submerged dam 7 is h5=0.165m;

[0218] The height of the first microplastic purification unit 110 is h6=0.06m, and the width of the pressurized straight pipe 9 is w4=0.1m;

[0219] The angle θ3 between the upstream side of the inlet pipe 8 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 is 42.1°;

[0220] The angle θ4 between the downstream side of the gated pressurized pipeline inlet pipe 8 and the right bank slope of the dry ditch 4 is 63.4°.

[0221] The gated pressurized pipeline outlet pipe 14 is straight, and the width w5 of the gated pressurized pipeline outlet pipe 14 is 0.12m; the width b of the pressurized pipeline connecting pipe a10 is 0.1m, and the width b of the pressurized pipeline connecting pipe b13 is 0.1m;

[0222] The first microplastic purification unit 110 includes a first microplastic purification device 11;

[0223] The width w6 of the first microplastic purification device 11 is 0.3 m, the length L3 of the first microplastic purification device 11 is 0.15 m, and the height h6 of the first microplastic purification device 11 is 0.165 m;

[0224] S3. A non-submerged diversion dam 15 is arranged downstream of the submerged dam 7, and a second microplastic purification unit 120 is arranged on the side close to the water-facing surface of the non-submerged diversion dam 15. The microplastics in the dry ditch 4 are secondary purified by the second microplastic purification unit 120 to complete the hydraulic control of microplastic pollution at the confluence of the farmland drainage ditch;

[0225] The x-direction distance between the non-submerged diversion dam starting point 16 and the confluence origin 5 is L2=7m, the angle θ6 between the non-submerged diversion dam 15 and the left bank slope of the dry ditch 4 is 140°, the width w9=0.1m and the height h7=0.3m of the non-submerged diversion dam 15; the projection length w8=0.4m of the non-submerged diversion dam 15 on the yz plane;

[0226] The gate inlet slot 17, the straight slot 18, the connecting slot a19, the connecting slot b20, the gate outlet slot 23, the second microplastic purification device 20, and the gate outlet slot 23 all have a height of h8, h8=0.3m;

[0227] The second microplastic purification device 20 is a rectangular parallelepiped, with a width w7 of 0.3 m and a length L4 of 0.6 m. A second multi-stage filtering device 21 is arranged in the second microplastic purification device 20 to effectively intercept microplastics.

[0228] Example 3

[0229] The specific steps of the method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches are as follows:

[0230] S1. Prototype observations were conducted on the main ditch 4 and the branch ditch 2, and relevant parameters of the main ditch 4 and the branch ditch 2 during spring irrigation were measured; the average flow velocity of the cross section S1 of the main ditch 4 was v1=0.3m / s, and the average flow velocity of the cross section S2 of the branch ditch 2 was v2=0.2m / s; the width of the main ditch 4 was w1=0.9m, and the width of the branch ditch 2 was w2=0.3m; the height of the main ditch 4 was d1=0.3m, and the height of the branch ditch 2 was d2=0.3m; the water depth of the main ditch 4 was h1=0.2m, and the water depth of the branch ditch 2 was h2=0.2m; the cross section S1 of the main ditch 4 was rectangular, and the cross section S2 of the branch ditch 2 was rectangular; the upstream length of the confluence of the main ditch 4 and the branch ditch 2 was L5=3.15m, and the downstream length of the confluence of the main ditch 4 and the branch ditch 2 was L6=15.3m; the confluence angle α of the main ditch 4 and the branch ditch 2 was 60°;

[0231] S2. Import the 3D digital twin model into FLUENT software to simulate the flow pattern of the confluence of the main ditch 4 and the branch ditch 2, and measure the x-direction distance L=2m between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch 4 and the confluence origin 5;

[0232] According to the flow pattern of the main ditch 4 and the branch ditch 2 at the confluence, a main ditch rectifying pile group 3 and a branch ditch rectifying pile group 1 are arranged upstream of the confluence of the main ditch 4 and the branch ditch 2;

[0233] The cross-sectional length of the main ditch rectification pile group 3 is a1=0.0135m, and the cross-sectional length of the branch ditch rectification pile group 1 is a2=0.045m;

[0234] The rectifying pile a301 in the dry ditch rectifying pile group 3 is connected to the left bank slope of the dry ditch 4, and the rectifying piles b302 and c303 are arranged in the dry ditch 4 at equal distances and intervals;

[0235] The rectifying pile d101 in the branch ditch rectifying pile group 1 is connected to the left bank slope of the branch ditch 2, and the rectifying piles e102 and f103 are arranged in the branch ditch 2 at equal distances and intervals;

[0236] Taking the centroid of the cross section of the rectifying pile a301 as the zero point 304 of the first local reference system, the calculation function of the abscissa x2 of the centroid of the cross section of the rectifying pile b302 is shown in formula (1):

[0237] (1)

[0238] In formula (1), x2 is the abscissa of the centroid of the cross section of the rectifying pile b302, θg is the angle between the connecting line of the centroids of the cross sections of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0239] The mathematical expression of the calculation method of the centroid x3 of the cross section of the rectifying pile c303 is shown in formula (2):

[0240] (2)

[0241] In formula (2), x3 is the abscissa of the centroid of the cross section of the rectifying pile c303, θg is the angle between the centroid of the cross section of the rectifying pile a301, rectifying pile b302, and rectifying pile c303 and the left bank slope of the main channel 4;

[0242] The height of the main ditch rectification pile group 3 is h3 = 0.15m, and the height of the branch ditch rectification pile group 1 is h4 = 0.15m;

[0243] A submerged dam 7 is arranged downstream of the confluence of the main ditch 4 and the branch ditch 2, and a first microplastic purification unit 110 is arranged on the side close to the back water surface of the submerged dam 7, so that microplastics in the main ditch 4 are purified once by the first microplastic purification unit 110;

[0244] The distance in the x direction between the starting point 6 of the submerged dam and the confluence origin 5 is L1=4m, the angle between the submerged dam 7 and the left bank slope of the dry ditch 4 is θ2=63.4°, the width of the submerged dam 7 is w3=0.1m; the height of the submerged dam 7 is h5=0.165m;

[0245] The height of the first microplastic purification unit 110 is h6=0.06m, and the width of the pressurized straight pipe 9 is w4=0.1m;

[0246] The angle θ3 between the upstream side of the inlet pipe 8 of the gated pressurized pipeline and the right bank slope of the dry ditch 4 is 42.1°;

[0247] The angle θ4 between the downstream side of the gated pressurized pipeline inlet pipe 8 and the right bank slope of the dry ditch 4 is 63.4°.

[0248] The gated pressurized pipeline outlet pipe 14 is straight, and the width w5 of the gated pressurized pipeline outlet pipe 14 is 0.12m; the width b of the pressurized pipeline connecting pipe a10 is 0.1m, and the width b of the pressurized pipeline connecting pipe b13 is 0.1m;

[0249] The first microplastic purification unit 110 includes a first microplastic purification device 11;

[0250] The width w6 of the first microplastic purification device 11 is 0.3 m, the length L3 of the first microplastic purification device 11 is 0.15 m, and the height h6 of the first microplastic purification device 11 is 0.165 m;

[0251] S3. A non-submerged diversion dam 15 is arranged downstream of the submerged dam 7, and a second microplastic purification unit 120 is arranged on the side close to the water-facing surface of the non-submerged diversion dam 15. The microplastics in the dry ditch 4 are secondary purified by the second microplastic purification unit 120 to complete the hydraulic control of microplastic pollution at the confluence of the farmland drainage ditch;

[0252] The x-direction distance between the non-submerged diversion dam starting point 16 and the confluence origin 5 is L2=8m, the angle θ6 between the non-submerged diversion dam 15 and the left bank slope of the dry ditch 4 is 140, the width w9=0.1m and the height h7=0.3m of the non-submerged diversion dam 15; the projection length w8=0.4m of the non-submerged diversion dam 15 on the yz plane;

[0253] The gate inlet slot 17, the straight slot 18, the connecting slot a19, the connecting slot b20, the gate outlet slot 23, the second microplastic purification device 20, and the gate outlet slot 23 all have a height of h8, h8=0.3m;

[0254] The second microplastic purification device 20 is a rectangular parallelepiped, with a width w7 of 0.3 m and a length L4 of 0.6 m. A second multi-stage filtering device 21 is arranged in the second microplastic purification device 20 to effectively intercept microplastics.

Claims

1. A method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches, characterized in that: The specific steps are as follows: S1. Obtain basic parameters of the main ditch (4) and the branch ditch (2), and construct a three-dimensional digital twin model based on the basic parameters; S2. Based on the three-dimensional digital twin model in step S1, a main ditch rectifying pile group (3) and a branch ditch rectifying pile group (1) are arranged upstream of the confluence of the main ditch (4) and the branch ditch (2) to rectify the water flow of the main ditch (4) and the branch ditch (2); a submerged dam (7) is arranged downstream of the confluence of the main ditch (4) and the branch ditch (2) to raise the water level of the main ditch (4); a first microplastic purification unit (110) is arranged on the back side of the submerged dam (7) to purify the microplastics in the main ditch (4); S3. Based on the three-dimensional digital twin model in step S1, a non-submerged diversion dam (15) is arranged downstream of the submerged submerged dam (7) to divert the water flow of the dry ditch (4), and a second microplastic purification unit (120) is arranged on the side close to the water-facing surface of the non-submerged diversion dam (15) to perform secondary purification on the microplastics in the dry ditch (4), thereby completing the control and purification of microplastic pollution at the confluence of the farmland drainage channel; The step S2 is specifically as follows: S201: In a three-dimensional modeling software, a three-dimensional digital twin model is established according to the dimensions and water flow parameters of the main ditch (4) and the branch ditch (2), wherein the specific dimensions are: the width w1 of the main ditch (4), the width w2 of the branch ditch (2), the height d1 of the main ditch (4), the height d2 of the branch ditch (2), the confluence angle α between the main ditch (4) and the branch ditch (2), the upstream length L5 of the confluence of the main ditch (4) and the branch ditch (2), and the downstream length L6 of the confluence of the main ditch (4) and the branch ditch (2); The ratio of the upstream length L5 of the confluence of the main ditch (4) and the branch ditch (2) to the width w1 of the main ditch (4) is 3 to 4:1, and the ratio of the downstream length L6 of the confluence of the main ditch (4) and the branch ditch (2) to the width w1 of the main ditch (4) is 15 to 20:1; the water flow parameters of the main ditch (4) and the branch ditch (2) are: the average flow velocity v2 of the cross section S1 of the main ditch (4), and the average flow velocity v2 of the cross section S2 of the branch ditch (2); S202: In FLUENT software, the three-dimensional digital twin model is initialized according to the water flow parameters of the main ditch (4) and the branch ditch (2), and the specific parameters are set as follows: the initial flow rate of the water flow at the inlet of the main ditch (4) is set to the cross-sectional average flow velocity v1 of the main ditch (4), the initial flow velocity of the water flow at the inlet of the branch ditch (2) is set to the cross-sectional average flow velocity v2 of the branch ditch (2), the water depth of the main ditch (4) is set to h1, and the water depth of the branch ditch (2) is set to h2; S203: simulating the flow pattern of water at the confluence of the main ditch (4) and the branch ditch (2) to obtain a flow velocity distribution diagram of the main ditch (4) and the branch ditch (2); S204: Determine the confluence of the main ditch (4) and the branch ditch (2), the location and range of the maximum flow velocity area and the downstream boundary of the maximum flow velocity area from the flow velocity distribution map, and measure the horizontal distance L between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch (4) and the confluence origin (5); The main ditch rectifying pile group (3) and the branch ditch rectifying pile group (1) in step S2 are specifically configured as follows: According to the flow pattern of the water flow at the confluence of the main ditch (4) and the branch ditch (2) and the position and range of the maximum flow velocity area, the arrangement positions of the main ditch rectifying pile group (3) and the branch ditch rectifying pile group (1) are determined upstream of the confluence of the main ditch (4) and the branch ditch (2), the main ditch rectifying pile group (3) comprising rectifying piles a (301), rectifying piles b (302), and rectifying piles c (303), the main ditch rectifying pile group (3) being located on the left bank of the main ditch (4), the branch ditch rectifying pile group (1) comprising rectifying piles d (101), rectifying piles e (102), and rectifying piles f (103), and the branch ditch rectifying pile group (1) being located on the left bank of the branch ditch (2); The rectifying pile a (301) in the main ditch rectifying pile group (3) is connected to the left bank of the upstream main ditch (4) at the confluence of the main ditch (4) and the branch ditch (2), and the rectifying piles b (302) and the rectifying piles c (303) are arranged at intervals in the upstream main ditch (4) at the confluence of the main ditch (4) and the branch ditch (2); The rectifying pile d (101) in the branch ditch rectifying pile group (1) is connected to the left bank of the upstream branch ditch (2) at the confluence of the main ditch (4) and the branch ditch (2), and the rectifying piles e (102) and the rectifying piles f (103) are arranged at intervals in the upstream branch ditch (2) at the confluence of the main ditch (4) and the branch ditch (2); The angle between the connecting line of the centroids of the cross sections of the rectifying pile a (301), the rectifying pile b (302) and the rectifying pile c (303) and the left bank of the main ditch (4) upstream of the confluence of the main ditch (4) and the branch ditch (2) is θg; The angle between the connecting line of the centroids of the cross sections of the rectifying pile d (101), the rectifying pile e (102), and the rectifying pile f (103) and the left bank of the branch ditch (2) upstream of the confluence of the main ditch (4) and the branch ditch (2) is θz; The cross-sections of the rectifying pile a (301), the rectifying pile b (302), and the rectifying pile c (303) are all square, and the side lengths are all a1. The ratio of the side length a1 of the rectifying pile a (301), the rectifying pile b (302), and the rectifying pile c (303) to the width w1 of the dry ditch (4) is 0.1-0.2:1; The cross-sections of the rectifying pile d (101), the rectifying pile e (102), and the rectifying pile f (103) are all square, and the side lengths are all a2. The ratio of the side length a2 of the rectifying pile d (101), the rectifying pile e (102), and the rectifying pile f (103) to the width w2 of the branch ditch (2) is 0.1-0.2:

1. The centroid of the cross section of the rectifying pile a (301) is taken as the zero point of the first local reference system, the ratio of the ordinate y2 of the centroid of the cross section of the rectifying pile b (302) to the width w1 of the dry ditch (4) is 0.2-0.3:1, and the calculation formula of the abscissa x2 of the centroid of the cross section of the rectifying pile b (302) is as follows: （1） Wherein, x2 is the abscissa of the centroid of the cross section of the rectifying pile b (302), θg is the angle between the connecting line of the centroids of the cross sections of the rectifying pile a (301), the rectifying pile b (302), and the rectifying pile c (303) and the left bank slope of the dry ditch (4); The ratio of the centroid y3 of the rectifying pile c (303) to the width w1 of the dry ditch (4) is 0.4-0.6:

1. The calculation formula of the centroid x3 of the cross section of the rectifying pile c (303) is as follows: （2） Where x3 is the abscissa of the centroid of the cross section of the straightening pile c (303), θg is the angle between the connecting line of the centroids of the cross sections of the straightening pile a (301), the straightening pile b (302), and the straightening pile c (303) and the left bank slope of the main channel (4); The centroid of the cross section of the straightening pile d (101) is taken as the origin of the second local reference system, the ratio of the ordinate y4 of the centroid of the cross section of the straightening pile e (102) to the width w2 of the branch ditch (2) is 0.2-0.3:1, and the calculation formula of the abscissa x4 of the centroid of the cross section of the straightening pile e (102) is as follows: （3） Where x4 is the abscissa of the centroid of the cross section of the straightening pile e (102), θz is the angle between the connecting line of the centroids of the cross sections of the straightening pile d (101), straightening pile e (102), and straightening pile f (103) and the left bank slope of the branch ditch (2); The ratio of the centroid y5 of the rectifying pile f (103) to the width w2 of the branch ditch (2) is 0.4-0.6:

1. The calculation formula of the centroid x5 of the cross section of the rectifying pile f (103) is as follows: （4） Where x5 is the abscissa of the centroid of the cross section of the straightening pile f (103), θz is the angle between the centroid of the cross sections of the straightening pile d (101), the straightening pile e (102), and the straightening pile f (103) and the left bank slope of the branch ditch (2); The ratio of the height h3 of the dry ditch rectification pile group (3) to the water depth h1 of the dry ditch (4) is 0.5~1:1; The ratio of the height h4 of the branch ditch rectifying pile group (1) to the water depth h2 of the branch ditch (2) is 0.5~1:

1.

2. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 1, characterized in that: The submerged dam (7) in step S2 is specifically configured as follows: According to the flow pattern of the water flow at the confluence of the main ditch (4) and the branch ditch (2) and the horizontal distance L between the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main ditch (4) and the confluence origin (5), a submerged dam (7) is arranged downstream of the confluence of the main ditch (4) and the branch ditch (2); The ratio of the x-direction distance L1 from the starting point (6) of the submerged dam on the left bank of the main channel (4) to the confluence origin (5) to the horizontal distance L from the downstream boundary of the maximum flow velocity area at the 1 / 2 water depth section of the main channel (4) to the confluence origin (5) is 1.5-2:1; The ratio of the width w3 of the submerged dam (7) to the width w1 of the dry ditch (4) is 0.1-0.2:1, and the ratio of the height h5 of the submerged dam (7) to the water depth h1 of the dry ditch (4) is 0.5-0.75:1; The angle θ2 between the submerged dam (7) and the left bank slope of the dry ditch (4) is equal to the angle θg between the connecting line of the cross-section centroids of the rectifying pile a (301), the rectifying pile b (302), and the rectifying pile c (303) and the left bank slope of the dry ditch (4).

3. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 2, characterized in that: In step S2, the first microplastic purification unit (110) comprises a first microplastic purification device (11), which is arranged on the side of the submerged dam (7) away from the right bank of the dry ditch (4) behind the back surface of the submerged dam (7), and a first multi-stage filtering device (12) is arranged in the first microplastic purification device (11). The side of the first microplastic purification device 11 close to the submerged dam (7) is connected in sequence to a pressurized pipeline inlet pipe (8) containing a gate, a pressurized pipeline straight pipe (9), and a pressurized pipeline connecting pipe a (10), and the other side of the first microplastic purification device (11) is connected in sequence to a pressurized pipeline connecting pipe b (13) and a pressurized pipeline outlet pipe (14) containing a gate.

4. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 3 is characterized in that: In step S2, the first microplastic purification unit (110) is specifically configured as the pressurized pipeline inlet pipe (8) containing a gate, the pressurized pipeline straight pipe (9), the pressurized pipeline connecting pipe a (10), the pressurized pipeline connecting pipe b (13), and the pressurized pipeline outlet pipe (14) containing a gate, and the height of the first microplastic purification device (11) is h6; A pressurized pipeline inlet pipe (8) including a gate, a pressurized pipeline straight pipe (9), a pressurized pipeline connecting pipe a (10), a pressurized pipeline connecting pipe b (13), a pressurized pipeline outlet pipe (14) including a gate, and a ratio of a height h6 of the first microplastic purification device (11) to a height h5 of the submerged dam (7) of 0.4 to 1:1; The inlet pipe (8) of the gated pressurized pipeline is a gradually expanding pipeline; The value range of the angle θ3 between the upstream side of the inlet pipe (8) of the gated pressurized pipeline and the right bank slope of the dry ditch (4) is 30° to 50°; The ratio of the angle θ4 between the downstream side of the inlet pipe (8) of the gated pressurized pipeline and the right bank slope of the dry ditch (4) to the angle θ2 between the submerged dam (7) and the left bank slope of the dry ditch (4) is 1:1; The ratio of the width w4 of the straight pipe (9) of the pressurized pipeline to the width w3 of the submerged dam (7) is 1:1; The outlet pipe (14) of the pressurized pipeline including the gate is of a straight type; The angle θ6 between the outlet pipe (14) of the gated pressurized pipeline and the right bank slope of the dry ditch (4) is in the range of 25 to 35 degrees; The ratio of the width w5 of the outlet pipe (14) of the gated pressurized pipeline to the width w3 of the submerged dam (7) is 1.2-1.5:1; The widths of the pressurized pipeline connecting pipe a (10) and the pressurized pipeline connecting pipe b (13) are both b, and the ratio of the width b of the pressurized pipeline connecting pipe a (10) and the pressurized pipeline connecting pipe b (13) to the width w3 of the submerged dam (7) is 1:1; The first microplastic purification device (11) is a rectangular parallelepiped, and the ratio of the width w6 of the first microplastic purification device (11) to the width w3 of the submerged dam (7) is 3-5:1; The ratio of the length L3 of the first microplastic purification device (11) to the width w3 of the submerged dam (7) is 1.5-2:

1.

5. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 4, characterized in that: The non-submerged diversion dam (15) in step S3 is specifically configured as follows: The ratio of the x-direction distance L2 of the starting point (16) of the non-submerged diversion dam from the confluence origin 5 to the x-direction distance L of the downstream boundary of the maximum flow velocity area from the confluence origin (5) is 3 to 4:1; The angle θ6 between the non-submerged diversion dam (15) and the left bank slope of the dry ditch (4) ranges from 90° to 140°; The ratio of the width w9 of the non-submerged diversion dam (15) to the width w3 of the submerged submerged dam (7) is 1:1; The ratio of the height h7 of the non-submerged diversion dam (15) to the height d1 of the dry ditch (4) is 1:1; The ratio of the projection length w8 of the non-submerged diversion dam (15) on the yz plane to the width w1 of the dry ditch (4) is 0.25-0.75:

1.

6. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 5, characterized in that: In step S3, the second microplastic purification unit (120) comprises a second microplastic purification device (20), a second multi-stage filtering device (21) is arranged in the second microplastic purification device (20), the second microplastic purification device (20) is arranged on the back side of the non-submerged diversion dam (15) and away from the left bank of the dry ditch (4), the side of the second microplastic purification device (20) close to the non-submerged diversion dam (15) is connected to an inlet slot containing a gate (17), a straight slot (18), and a connecting slot a (19), and the other side of the second microplastic purification device (20) is connected to a connecting slot b (22) and an outlet slot containing a gate (23).

7. The method for controlling and purifying microplastic pollution at the confluence of farmland drainage ditches according to claim 6, characterized in that: In step S3, the second microplastic purification unit (120) is specifically configured as follows: the projection dimensions of the gated inlet slot (17), the straight slot (18), the connecting slot a (19), the connecting slot b (22), and the gated outlet slot (23) on the xy plane are respectively equal to the projection dimensions of the gated pressurized pipeline inlet pipe (8), the pressurized pipeline straight pipe (9), the pressurized pipeline connecting pipe a (10), the pressurized pipeline connecting pipe b (13), and the gated pressurized pipeline outlet pipe (14); The second microplastic purification device (20) includes a gate inlet slot (17), a straight slot (18), a connecting slot a (19), a connecting slot b (22), and a gate outlet slot (23), and the height of each of the second microplastic purification device (20) is h8; It includes a gate inlet groove (17), a straight groove (18), a connecting groove a (19), a connecting groove b (22), and a gate outlet groove (23), and the ratio of the height h8 of the second microplastic purification device (20) to the height d1 of the dry groove is 1:1; A second multi-stage filtering device (21) is provided in the second microplastic purification device (20); The second microplastic purification device (20) is a rectangular parallelepiped, the ratio of the width w7 of the second microplastic purification device (20) to the width w6 of the first microplastic purification device (11) is 1:1, and the ratio of the length L4 of the second microplastic purification device (20) to the width w7 of the second microplastic purification device (20) is 2-3:1.

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