Method for preventing and controlling farmland nitrogen and phosphorus source reduction process interception tail water reuse

Abstract

This invention discloses a method for controlling and reusing nitrogen and phosphorus pollution at the source in farmland by intercepting wastewater. The method includes: excavating furrows, collection ditches, and ecological purification ponds; constructing two dividing dams within the ecological purification pond to separate it into a water storage area, a facultative oxidation area, and a deep ecological treatment area; the water-facing side of the dividing dams is divided into a partially permeable structure and a permeable structure from bottom to top, with a water-stopping stepped transition structure between the two; the water-repellent side of the dividing dams is a fully permeable structure; and functional microbial communities and plant systems are deployed in the water storage area, facultative oxidation area, and deep ecological treatment area. This method enables synergistic collection and end-of-pipe purification of farmland wastewater, achieving gradient process interception of nitrogen and phosphorus in farmland, significantly improving the graded purification efficiency of nitrogen and phosphorus pollutants, and balancing process purification efficiency with system operational stability.

Description

Technical Field

[0001] This invention relates to the field of agricultural technology, specifically to a method for controlling and preventing nitrogen and phosphorus reduction at the source in farmland by intercepting and reusing wastewater. Background Technology

[0002] Agricultural non-point source pollution is a core control target in my country's current watershed water ecological environment protection and management. Among them, nitrogen and phosphorus runoff from farmland is the primary contributor to agricultural non-point source pollution. The whole-chain prevention and control of nitrogen and phosphorus loss from farmland, as well as the purification and safe utilization of farmland tailwater, are core research and application directions in the fields of agricultural environmental protection and farmland water conservancy engineering. Currently, the industry has formed a farmland nitrogen and phosphorus pollution prevention and control technology system with source reduction, process interception, and end-of-pipe purification as its core logic. Related technologies have been applied on a large scale in contiguous farmland non-point source pollution control projects across the country.

[0003] Existing conventional agricultural nitrogen and phosphorus pollution control and tailwater treatment technologies generally adopt a two-stage combination model of field water collection system and end-of-pipe ecological treatment pond. First, furrows are dug along the planting direction in the farmland, and water collection ditches are dug outside the field ridges to collect and transport farmland rainwater runoff and irrigation tailwater. At the end of the farmland water collection, ecological treatment ponds are dug. In each treatment unit of the pond, only general-purpose aquatic plants are scattered, or undifferentiated microbial agents are added. There is no targeted functional microbial community and plant system zoning based on the water purification law along the way, the hydraulic retention characteristics of each unit and the pollutant degradation law. The farmland tailwater can only achieve limited removal of nitrogen and phosphorus pollutants through natural sedimentation and simple adsorption in the pond.

[0004] It is clear from the above that existing technologies cannot achieve a coordinated matching of the entire process of nitrogen and phosphorus source water collection, process gradient interception, and end-of-pipe graded purification in farmland. Due to the mismatch between pond layout parameters and farmland water catchment characteristics, functional zoning and purification system layout and water body hydraulic path are disconnected, resulting in low efficiency of nitrogen and phosphorus process interception in farmland and poor nitrogen and phosphorus purification effect in tailwater, and the inability to form a stable and continuous whole-chain prevention and control system for nitrogen and phosphorus in farmland. Summary of the Invention

[0005] The present invention aims to at least partially solve the technical problems in the above-mentioned technologies.

[0006] Therefore, this invention discloses a method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland, characterized by comprising the following steps:

[0007] On the surface of the target farmland, lay out the center lines of the furrows; on the outer side of the field ridges, lay out the center lines of the drainage ditches; and at the ends of the fields, lay out the boundary lines of the ecological purification ponds.

[0008] Based on the total area of ​​the target farmland, the area of ​​the ecological purification pond accounts for 4.5% to 6.5%, and the length-to-width ratio is 1.5:1 to 2.5:1;

[0009] The furrows, the drainage ditches, and the ecological purification ponds were excavated sequentially, wherein...

[0010] The furrow and the water collection ditch are connected;

[0011] Two dividing dams are constructed inside the ecological purification pond, sequentially dividing it along the water flow direction into a water storage area, a facultative oxidation area, and a deep ecological treatment area.

[0012] The water collection ditch and the water storage area are connected, and the water flows into the downstream area step by step through the seepage effect of the two separating dams;

[0013] The water-facing side of the dividing dam is divided into a partially permeable structure and a permeable structure from bottom to top, with the preset normal water level of the pond as the boundary. A water-stopping stepped transition structure is set between the partially permeable structure and the permeable structure. The water-repellent side of the dividing dam is a fully permeable structure.

[0014] Functional microbial communities and plant systems are respectively deployed in the water storage area, the facultative oxidation area, and the deep ecological treatment area, wherein,

[0015] The functional microbial community is evenly distributed in the water and bottom substrate of the water storage area, the facultative oxidation area and the deep ecological treatment area;

[0016] The water storage area is planted with submerged and emergent plants, the facultative oxidation area is planted with emergent and floating plants, and the deep ecological treatment area is planted with submerged plants and emergent plants along the bank.

[0017] The method for preventing and controlling nitrogen and phosphorus pollution at the source in farmland by intercepting and reusing tailwater disclosed in this invention can achieve synergistic effects between farmland tailwater collection and end-of-pipe purification, realize gradient process interception of nitrogen and phosphorus in farmland, significantly improve the graded purification efficiency of nitrogen and phosphorus pollutants, and balance process purification efficiency and system operation stability.

[0018] In addition, the method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland disclosed in this invention may also have the following additional technical features:

[0019] Furthermore, the excavation of the furrows and the drainage ditches specifically includes:

[0020] The furrows are dug evenly along the crop planting direction of the target farmland, with a depth of 20cm to 35cm and a width of 25cm to 40cm.

[0021] The drainage ditch is dug along the outer side of the field, with a depth of 50cm to 80cm and a width of 1.0m to 1.5m.

[0022] At the junctions of the furrow and the collection ditch and the collection ditch and the water storage area, 25cm to 35cm of pebbles or gravel are filled to form a dirt-blocking and filtering structure.

[0023] Furthermore, the construction of the aforementioned dividing dam specifically includes:

[0024] First, excavate the dam foundation along the preset dividing axis of the ecological purification pond, and then compact the dam foundation in layers.

[0025] On the compacted dam foundation, the core dam material is filled in layers and compacted layer by layer to the preset dam height to form the main structure of the partition dam.

[0026] Using the preset normal water level as the boundary, an incompletely permeable structure and a permeable structure are constructed sequentially from bottom to top on the water-facing side of the dividing dam. First, a modified clay layer is laid below the boundary to form an incompletely permeable structure, and then a graded crushed stone layer is laid above the boundary to form a permeable structure.

[0027] At the boundary between the incompletely permeable structure and the permeable structure, the water-stopping stepped transition structure is poured.

[0028] On the backwater side of the dividing dam, a filter layer and a permeable gravel layer are laid sequentially from the core of the dam outwards to form a full-section permeable structure.

[0029] Furthermore, the construction of the water storage area, the facultative oxidation area, and the deep ecological treatment area specifically includes:

[0030] The bottom of the ecological purification pond is repaired and compacted in layers.

[0031] A seepage-proof layer is laid on the compacted bottom of the pond, and the seepage-proof layer is made of modified clay.

[0032] An initial purification matrix layer is laid above the impermeable layer corresponding to the water storage area, a microbial biofilm matrix layer is laid above the impermeable layer corresponding to the facultative oxidation zone, and a deep purification matrix layer is laid above the impermeable layer corresponding to the deep ecological treatment zone.

[0033] Furthermore, the thickness of the impermeable layer is 25cm to 35cm, the initial purification matrix layer is laid with adsorbent granular material with a thickness of 15cm to 25cm, the microbial biofilm matrix layer is laid with porous granular material with a thickness of 10cm to 20cm, and the deep purification matrix layer is laid with composite porous material with a thickness of 10cm to 20cm.

[0034] Furthermore, the deployment of the functional microbial community and the plant system specifically includes:

[0035] After the initial purification substrate layer, the microbial biofilm substrate layer and the deep purification substrate layer are laid, the functional microbial community is introduced into the water storage area, the facultative oxidation area and the deep ecological treatment area respectively, so that the functional microbial community is evenly attached to the substrate layer and water body in each area.

[0036] After the functional microbial community is introduced, submerged and emergent plants are planted on the initial purification substrate layer in the water storage area, emergent plants are planted on the microbial biofilm substrate layer in the facultative oxidation zone, floating plants are placed in the water body, submerged plants are planted on the deep purification substrate layer in the deep ecological treatment zone, and emergent plants are planted on the shore to form the plant system.

[0037] Furthermore, vegetation is planted on the dividing dam, specifically as follows:

[0038] Planting soil is laid on the top of the dividing dam and dam-stabilizing plants are planted.

[0039] Emergent plants are planted on the permeable structure on the water-facing side of the dividing dam, and planting soil is laid on the permeable gravel layer on the water-retaining side of the dividing dam, where soil-stabilizing herbaceous plants are planted.

[0040] Furthermore, the farmland straw from the target farmland, the plant residues from the ecological purification pond, and the silt from the bottom of the ecological purification pond are collected;

[0041] After the farmland straw, plant residues and pond bottom dredging are crushed, mixed and pretreated, composting agents are added and the moisture content is adjusted for composting and fermentation.

[0042] After fermentation, organic fertilizer is obtained and reused in the target farmland.

[0043] Furthermore, water is taken from the deep ecological treatment zone and recharged, specifically as follows:

[0044] A water intake device is installed on the side of the deep ecological treatment zone closest to the target farmland to regularly test the water quality of the deep ecological treatment zone;

[0045] When the water quality meets the standards for farmland irrigation, the purified water from the deep ecological treatment zone is extracted through the water intake device and transported to the farmland irrigation network for re-irrigation of the target farmland.

[0046] Furthermore, along the outer edge of the boundary line of the ecological purification pond, the construction axis is laid out and positioned. The foundation of the dam is excavated along the construction axis. The excavated foundation is compacted in layers. On the compacted dam foundation, dam material is filled in layers and compacted layer by layer to the preset height to form a protective dam surrounding the edge of the ecological purification pond.

[0047] Additional features and advantages of this invention will be set forth in the description which follows, or may be learned by practicing the invention. Attached Figure Description

[0048] The technical solution and beneficial effects of the present invention will become apparent and readily understood from the following description in conjunction with the accompanying drawings, wherein:

[0049] Figure 1 This is a plan view of the furrows, drainage ditches, and ecological purification ponds in the farmland nitrogen and phosphorus source reduction process interception and wastewater reuse control method of the present invention.

[0050] Figure 2 A plan view of the ecological purification pond for the farmland nitrogen and phosphorus source reduction process interception and wastewater reuse control method of the present invention;

[0051] Figure 3 This is a cross-sectional view of the dividing dam used in the method for controlling and preventing nitrogen and phosphorus source reduction in farmland by intercepting tailwater for reuse, as described in this invention.

[0052] As shown in the figure:

[0053] 101 - Field ridge, 102 - Furrow;

[0054] 200-water collection ditch;

[0055] 300 - Ecological purification pond, 301 - Water storage area, 302 - Facultative oxidation area, 303 - Deep ecological treatment area;

[0056] 400 - Dividing dam, 401 - Partially permeable structure, 402 - Fully permeable structure, 403 - Water-stopping stepped transition structure, 404 - Fully permeable structure. Detailed Implementation

[0057] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0058] The following description, with reference to the accompanying drawings, will illustrate the method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland disclosed in this invention.

[0059] Example

[0060] A method for controlling nitrogen and phosphorus source reduction in farmland by intercepting and reusing tailwater includes the following steps:

[0061] like Figure 1 As shown, the center line of the furrow 102 is marked on the surface of the target farmland, the center line of the drainage ditch 200 is marked on the outer side of the field ridge, and the boundary line of the ecological purification pond 300 is marked at the end of the field.

[0062] Based on the total area of ​​the target farmland, the area of ​​the ecological purification pond 300 accounts for 4.5% to 6.5%, and the length-to-width ratio is 1.5:1 to 2.5:1;

[0063] 102 furrows, 200 drainage ditches, and 300 ecological purification ponds were excavated in sequence.

[0064] Furrow 102 and drainage ditch 200 are connected;

[0065] like Figure 2 As shown, two dividing dams 400 are constructed inside the ecological purification pond 300, dividing the pond into a water storage area 301, a facultative oxidation area 302, and a deep ecological treatment area 303 along the water flow direction.

[0066] The water collection ditch 200 and the water storage area 301 are connected, and the water flows into the downstream area step by step through the seepage effect of the two dividing dams 400.

[0067] like Figure 3 As shown, the water-facing side of the dividing dam 400 is divided into an incompletely permeable structure 401 and a permeable structure from bottom to top, with the preset normal water level of the pond as the boundary. A water-stopping stepped transition structure 403 is provided between the incompletely permeable structure 401 and the permeable structure. The back side of the dividing dam 400 is a fully permeable structure 404.

[0068] Functional microbial communities and plant systems were respectively deployed in the water storage area 301, the facultative oxidation area 302, and the deep ecological treatment area 303.

[0069] Functional microbial communities are evenly distributed in the water bodies and bottom substrates of the water storage zone 301, the facultative oxidation zone 302, and the deep ecological treatment zone 303;

[0070] Submerged and emergent plants are planted in water storage area 301, emergent and floating plants are planted in facultative oxidation area 302, and submerged and emergent plants are planted in deep ecological treatment area 303.

[0071] It should be noted that the excavation of furrow 102 and drainage ditch 200 is specifically as follows:

[0072] Dig furrows 102 evenly along the crop planting direction of the target farmland. The furrows 102 are 20cm to 35cm deep and 25cm to 40cm wide.

[0073] Dig a drainage ditch 200 along the outer side of the field. The drainage ditch 200 should be 50cm to 80cm deep and 1.0m to 1.5m wide.

[0074] At the junctions of the furrow 102 and the collection ditch 200, and at the junctions of the collection ditch 200 and the water storage area 301, fill the joints with 25cm to 35cm of pebbles or gravel to form a debris-blocking and filtration structure.

[0075] It should be noted that the construction of the 400-meter dividing dam is as follows:

[0076] First, excavate the dam foundation along the pre-set dividing axis of the ecological purification pond 300, and then compact the dam foundation in layers.

[0077] On the compacted dam foundation, the core dam material is filled in layers and compacted layer by layer to the preset dam height, forming the main structure of the 400-meter-high dam.

[0078] Using the preset normal water level as the boundary, the incompletely permeable structure 401 and the permeable structure are constructed sequentially from bottom to top on the water-facing side of the dividing dam 400. First, a modified clay layer is laid below the boundary to form the incompletely permeable structure 401, and then a graded crushed stone layer is laid above the boundary to form the permeable structure.

[0079] At the boundary between the incompletely permeable structure 401 and the permeable structure, a water-stopping stepped transition structure 403 is poured.

[0080] On the backwater side of the dividing dam 400, a filter layer and a permeable gravel layer are laid sequentially from the core of the dam to the outside, forming a full-section permeable structure 404.

[0081] It should be noted that the construction water storage area 301, the facultative oxidation area 302, and the deep ecological treatment area 303 are specifically as follows:

[0082] The bottom of the 300-meter ecological purification pond was repaired and compacted in layers.

[0083] After compacting the pond bottom, a seepage-proof layer is laid, which is made of modified clay.

[0084] An initial purification substrate layer is laid above the impermeable layer corresponding to the water storage zone 301, a microbial biofilm substrate layer is laid above the impermeable layer corresponding to the facultative oxidation zone 302, and a deep purification substrate layer is laid above the impermeable layer corresponding to the deep ecological treatment zone 303.

[0085] It should be noted that the thickness of the impermeable layer is 25cm to 35cm, the initial purification substrate layer is laid with adsorbent granular material with a thickness of 15cm to 25cm, the microbial biofilm substrate layer is laid with porous granular material with a thickness of 10cm to 20cm, and the deep purification substrate layer is laid with composite porous material with a thickness of 10cm to 20cm.

[0086] It should be noted that the establishment of functional microbial communities and plant systems specifically involves:

[0087] After the initial purification substrate layer, the microbial biofilm substrate layer and the deep purification substrate layer are laid, functional microbial communities are introduced into the water storage area 301, the facultative oxidation area 302 and the deep ecological treatment area 303 respectively, so that the functional microbial communities are evenly attached to the substrate layer and water body in each area.

[0088] After the functional microbial community is introduced, submerged and emergent plants are planted on the initial purification substrate layer in the water storage area 301, emergent plants are planted on the microbial biofilm substrate layer in the facultative oxidation zone 302, and floating plants are placed in the water body. Submerged plants are planted on the deep purification substrate layer in the deep ecological treatment zone 303, and emergent plants are planted on the shore to form a plant system.

[0089] It should be noted that vegetation was planted on the 400-meter dividing dam, specifically as follows:

[0090] Planting soil and dam-stabilizing plants were laid on the top of the 400-meter-wide dividing dam.

[0091] Emergent plants are planted on the permeable structure of the water-facing side of the dividing dam 400, and planting soil is laid on the permeable gravel layer on the water-retaining side of the dividing dam 400 to plant soil-stabilizing herbaceous plants.

[0092] It should be noted that the collection includes farmland straw from the target farmland, plant residues from the ecological purification pond 300, and dredging of the bottom of the ecological purification pond 300.

[0093] After crushing and mixing farmland straw, plant residues and pond bottom silt for pretreatment, composting agents are added and the moisture content is adjusted for composting fermentation.

[0094] After fermentation, organic fertilizer is obtained and reused in the target farmland.

[0095] It should be noted that the water body in the deep ecological treatment zone 303 will be taken for recharge, specifically as follows:

[0096] A water intake device is installed on the side of the deep ecological treatment zone 303 closest to the target farmland, and the water quality of the deep ecological treatment zone 303 is tested regularly.

[0097] When the water quality meets the standards for farmland irrigation, purified water from the deep ecological treatment zone 303 is extracted through the water intake device and transported to the farmland irrigation network for re-irrigation of the target farmland.

[0098] It should be noted that the construction axis is laid out along the outer edge of the boundary line of the ecological purification pond 300, the foundation of the dam is excavated along the construction axis, the excavated foundation is compacted in layers, and the dam material is filled in layers on the compacted foundation, and each layer is compacted to the preset height to form a protective dam around the edge of the ecological purification pond 300.

[0099] This embodiment uses 100 mu (approximately 6.7 hectares) of contiguous dryland farmland in northern China as the implementation object. The farmland soil type is sandy loam, and the crops are planted on ridges 101. A wheat-corn rotation pattern is adopted, with an annual rainfall of 800-900 mm and a farmland runoff coefficient of 0.3-0.4 during the rainy season.

[0100] In the layout and preliminary survey stage, a comprehensive survey of the topography, hydrology, soil texture, farmland drainage direction, and groundwater level of the target farmland was first carried out to determine that the farmland water flow direction is from north to south.

[0101] The overall slope is 1.5‰, based on which precise surveying was completed:

[0102] On the surface of the field, along the direction of crop planting, mark the center line of furrow 102 at 1m intervals.

[0103] On the outer side of the field ridge, along the direction of farmland water flow, lay out the center line of the drainage ditch 200, with the axis of the drainage ditch 200 perpendicular to the direction of the furrow 102;

[0104] At the southern end of the field, the boundary line of the ecological purification pond 300 is laid out. Based on the total area of ​​100 mu of farmland, the total area of ​​the ecological purification pond 300 is 5 mu, accounting for 5% of the area. The length-to-width ratio of the pond is 2:1, with an east-west length of 80m and a north-south width of 41.7m.

[0105] The pre-set dividing axis of the two dividing dams 400 inside the ecological purification pond 300 and the construction axis of the protective embankment outside the ecological purification pond 300 were simultaneously laid out, and the deviation of all layout stakes was controlled within ±5cm.

[0106] For the excavation of furrow 102 and drainage ditch 200, the earthwork excavation was completed sequentially according to the marked positions:

[0107] Dig furrows 102 evenly along the crop planting direction to form field ridges 101 and primary water collection units in the ridge planting area. The furrows 102 are 30cm deep and 30cm wide, and the slope of the furrow bottom is consistent with the overall slope of the farmland to ensure that all farmland runoff flows into furrows 102.

[0108] A water collection ditch 200 is dug along the outer side of the field ridge to form a secondary water collection unit. The water collection ditch 200 is 70cm deep and 1.2m wide, with a bottom slope of 1.5‰, and slopes towards the water inlet of the water storage area 301 of the ecological purification pond 300. The water collection ditch 200 is connected to the ends of all the furrows 102 to form a two-level water collection network of furrows 102 and water collection ditch 200.

[0109] At the junction of furrow 102 and water collection ditch 200, and at the junction of water collection ditch 200 and water storage area 301, 30cm thick cobblestones with a particle size of 5-10cm are filled to form a two-stage pollution interception and filtration structure to intercept large particles of mud and sand and straw residue, and prevent siltation in ecological purification pond 300.

[0110] Simultaneously, on both sides of the slopes of furrow 102 and drainage ditch 200, Bermuda grass was planted by division at a density of 12 plants / m². 2 This enables the roots to stabilize the soil and prevent erosion.

[0111] For the excavation and zoning construction of the ecological purification pond 300, the overall earthwork excavation of the ecological purification pond 300 was completed according to the boundary line. The bottom of the pond was leveled and smoothed with a flatness error of ≤5cm and an overall slope of 1‰, sloping towards the outlet end of the pond.

[0112] By constructing two dividing dams 400 inside the ecological purification pond 300, the ecological purification pond 300 is divided into a water storage area 301, a facultative oxidation area 302, and a deep ecological treatment area 303 along the water flow direction (from north to south). The area ratios of the three areas are 45%, 35%, and 20%, respectively, corresponding to areas of 2.25 mu, 1.75 mu, and 1 mu.

[0113] After excavation, the effective water depth of the water storage area 301 is 1.8m, the effective water depth of the facultative oxidation area 302 is 1.5m, and the effective water depth of the deep ecological treatment area 303 is 1.0m.

[0114] The outlet of the water collection ditch 200 is connected to the inlet of the water storage area 301. After the farmland tailwater flows into the water storage area 301 through the water collection ditch 200, it flows by gravity into the facultative oxidation zone 302 and the deep ecological treatment zone 303 through the seepage effect of the two dividing dams 400, forming a gradient infiltration and purification hydraulic path.

[0115] Regarding the construction of the core dividing dam 400, the construction process for both dividing dams 400 is the same. Each dividing dam 400 has a crest width of 2m, a dam height of 2.3m, and a crest elevation 0.5m higher than the highest designed water level of the ecological purification pond 300. The upstream slope ratio is 1:2.5, and the downstream slope ratio is 1:3. The specific construction steps are as follows:

[0116] Remove the top 30cm of humus, weeds, and silt along the pre-defined dividing axis to expose the original soil. Then, use a vibratory roller to compact the soil in layers, achieving a compaction degree of ≥90%.

[0117] A continuous intercepting trench, 50cm deep and 80cm wide, was excavated along the dam foundation axis. Modified clay was backfilled in layers within the trench, with each layer having a loose thickness of ≤20cm. Each layer was then compacted, achieving a compaction degree ≥95% and a permeability coefficient ≤1×10⁻⁶. -7 cm / s, blocking short-circuit seepage in the dam foundation;

[0118] After the intercepting trench is backfilled, a 15cm thick medium-coarse sand leveling layer is laid across the entire dam foundation area to form a reverse filter transition and prevent the loss of soil particles from the dam body.

[0119] At the top of the leveling layer, the core material of the dam body is filled in layers from bottom to top, with each layer having a loose thickness of ≤25cm. The same layer is laid and compacted simultaneously to avoid longitudinal joints and uneven settlement.

[0120] The dam core, extending from the water-facing side to the backwater side, is divided into three functional zones for simultaneous filling. The pre-construction infiltration and purification zone (water-facing side, 1.5m thick) uses a mixture of 40% in-situ native soil, 30% river sand, 20% zeolite, and 10% well-rotted straw, evenly mixed and spread. After compaction, the compaction degree is controlled at 85%-88%, with a porosity ≥30% and a permeability coefficient of 1×10⁻⁶. -4 cm / s;

[0121] The central seepage-stabilizing dam area (middle core, 3m thick) is constructed by uniformly mixing and spreading a mixture of 60% in-situ native soil, 25% river sand, and 15% graded crushed stone. After compaction, the compaction degree is controlled at 90%-92%, and the permeability coefficient is 5×10⁻⁶. -5 cm / s, which is the structural load-bearing core and seepage velocity control point of the 400-meter-long separation dam;

[0122] The post-biological purification zone (backwater side, 1.5m thick) is constructed by uniformly mixing and spreading a mixture of 20% in-situ native soil, 30% river sand, 25% volcanic rock, 15% biochar, and 10% well-rotted straw. After compaction, the compaction degree is controlled at 82%-85%, the porosity is ≥35%, and the permeability coefficient is 2×10⁻⁶. -4 cm / s;

[0123] After filling to the designed dam crest elevation, the dam crest is leveled and compacted to a compaction degree of ≥92%. After static settlement for 72 hours, the elevation and flatness are re-measured, and the main structure construction of the 400-meter-long separation dam is completed.

[0124] After the 400mm dam body of the dividing dam has settled and stabilized, the slope is trimmed by mechanical and manual means according to the designed slope ratio, and the slope flatness error is ≤5cm.

[0125] With the preset normal water level of the ecological purification pond 300 as the dividing elevation, the dividing dam 400 between the water storage area 301 and the facultative oxidation area 302 has a dividing elevation of 0.3m (1.2m) below the normal water level of 1.5m. The dividing dam 400 between the facultative oxidation area 302 and the deep ecological treatment area 303 has a dividing elevation of 0.3m (0.9m) below the normal water level of 1.2m.

[0126] After accurately marking the boundary position on the prepared water-facing surface, construction proceeds from bottom to top. Below the boundary, a 30cm thick layer of modified clay is laid, compacted in layers with a compaction degree ≥93% and a permeability coefficient controlled at 1×10⁻⁶. -6 -1×10 -7 cm / s, forming an incompletely permeable structure 401;

[0127] The area above the boundary is paved with 30cm thick graded crushed stone (particle size 20-50mm), with a porosity ≥40%, forming a fully permeable structure 402;

[0128] At the boundary between the partially permeable structure 401 and the fully permeable structure 402, a 20cm high plain concrete water-stop stepped transition structure 403 is poured to completely interlock the upper and lower sections of the structure and prevent seepage short circuits.

[0129] Simultaneously, a 10cm thick dry-laid stone slab protective layer is laid on the outside of the incompletely permeable structure 401, and a 20cm thick zeolite, 15cm thick planting soil and gravel mixed ecological protective layer is laid on the outside of the fully permeable structure 402.

[0130] On the repaired backwater surface, from the core of the dam to the outside, a 15cm thick layer of graded medium-coarse sand, a 10cm thick layer of fine gravel, and a 10cm thick layer of coarse gravel are laid in sequence to form a three-level reverse filter protection layer. The permeability coefficient of each layer is more than 10 times greater than that of the inner layer. Then, a 15cm thick layer of permeable gravel is laid on the outside of the reverse filter layer to form a full-section permeable structure 404, which ensures smooth seepage and avoids piping and soil erosion.

[0131] Simultaneously, a dry-laid stone drainage prism is constructed along the toe of the back slope. The prism is 0.5m high and 0.8m wide at the top, with an inner slope of 1:1 and an outer slope of 1:1.5. A 15cm thick medium-coarse sand filter layer is laid at the bottom to evenly discharge the 400 m seepage water from the dividing dam into the downstream pond, while supporting the toe of the dam to prevent landslides.

[0132] For the construction of the functional layer of the ecological purification pond 300, the bottom of the entire ecological purification pond 300 was first repaired and compacted in layers using a vibratory roller, with a compaction degree of ≥90%.

[0133] A 30cm thick modified clay impermeable layer is laid across the entire bottom of the compacted pond. The modified clay is a mixture of in-situ soil and 8% bentonite. It is compacted in layers, with each layer having a loose thickness of ≤20cm, a compaction degree of ≥93%, and a permeability coefficient of ≤1×10⁻⁶. - 7 cm / s;

[0134] Above the impermeable layer, differentiated matrix layers are laid in three functional zones. Above the 301 impermeable layer in the water storage zone, a 20cm thick zeolite and crushed stone mixed layer (volume ratio 3:1) is laid as the initial purification matrix layer.

[0135] A 15cm thick layer of zeolite and volcanic rock mixture (volume ratio 1:1) is laid above the 302 seepage barrier layer in the facultative oxidation zone as a substrate layer for microbial biofilm formation.

[0136] A 15cm thick layer of mixed ceramsite and volcanic rock (volume ratio 1:1) is laid above the 303 seepage-proof layer in the deep ecological treatment area as a deep purification matrix layer.

[0137] For the functional microbial community and plant system setup, the microbial community should be introduced first. EM bacteria and photosynthetic bacteria compound inoculum should be cultured 72 hours in advance, with a viable count ≥1×10⁻⁶. 8 After the substrate layers for each zone are laid, the expanded compound microbial agent is sprayed onto the water storage zone 301, the facultative oxidation zone 302, and the deep ecological treatment zone 303 at a dosage of 500 mL / m³. 3 The bacteria are evenly attached to the substrate layer and water body in each area. The same dosage is added every 3 months.

[0138] After the bacterial colony is introduced, the plant system is arranged in different areas. In water storage area 301, submerged plants such as foxtail algae and hydrangea are planted evenly on the initial purification substrate layer at a planting density of 25 buds / clump with a clump spacing of 50cm. Emergent plants such as reeds and calamus are planted in the shallow water area of ​​the pond slope at a planting density of 12 buds / clump with a clump spacing of 80cm.

[0139] In facultative oxidation zone 302, cattails and water onions emergent plants are planted on the microbial biofilm substrate layer at a planting density of 12 buds / clump with a clump spacing of 80cm. Water hyacinth floating plants are enclosed in the middle of the pond with a fence, covering an area of ​​22% of the pond's water surface. They are harvested once a week to prevent spread.

[0140] In the deep ecological treatment zone 303, submerged plants such as Vallisneria natans and Ceratophyllum demersum are evenly planted on the deep purification substrate layer throughout the pond at a planting density of 25 buds / clump with a clump spacing of 50cm to create an underwater meadow. Emergent plants such as Canna indica and Lythrum salicaria are planted on the banks of the pond at a planting density of 12 buds / clump with a clump spacing of 80cm.

[0141] At the same time, silver carp and bighead carp were released into the deep ecological treatment zone 303 at a rate of 40 fish per mu (approximately 667 square meters), with a size of 100-150g per fish. Snails were also released at a rate of 12kg per mu to establish a complete food chain and control algae reproduction.

[0142] For the planting system layout of the 400 dividing dam, a 15cm thick layer of planting soil was laid on the top of the 400 dividing dam, and Amorpha fruticosa and Caragana korshinskii were planted in holes with a spacing of 1m×1m. The main root system penetrated into the central seepage stabilization and dam-stabilizing area of ​​the 400 dividing dam, forming a root-reinforced soil structure to improve the dam's anti-sliding and anti-cracking performance.

[0143] Reeds and cattails are planted on the ecological protective layer of the fully permeable structure 402 on the water-facing side, with a planting density of 12 buds / clump and a clump spacing of 50cm, to enhance the purification of seepage water and the stabilization of the slope.

[0144] A 15cm thick layer of planting soil was laid on the 404 permeable gravel layer of the full-section permeable structure on the back side of the water-retaining surface, and then fully sown with Bermuda grass for soil stabilization, at a sowing rate of 25g / m². 2 This prevents soil erosion on the slope and simultaneously purifies the effluent.

[0145] For the closed-loop construction of agricultural waste resource utilization, a 10cm thick gravel impermeable layer was laid on an idle plot of land near the ecological purification pond 300, after leveling and compacting the land, thus constructing a 60 m The aerobic composting platform is equipped with diversion ditches around its perimeter, and the leachate is returned to the facultative oxidation zone 302.

[0146] Collect wheat and corn stalks from the target farmland, aquatic plant residues harvested from the ecological purification pond (300 m³), ​​bottom silt from the pond, and expired purification substrate from the replacement of the pond body with the dividing dam (400 m³). Crush the stalks and plant residues to a length of ≤5 cm, mix the raw materials at a carbon-to-nitrogen ratio of 25:1, where the stalks and plant residues are the carbon source, and the bottom silt and pond plants are the nitrogen source. Add 2.5% composting agent, adjust the moisture content to 58%, and pile them into windrows 1.3 m high and 2.2 m wide. Turn the pile over every 4 days, keep the pile temperature above 55°C for 12 days, and allow it to compost for 30 days. After composting, obtain organic fertilizer, which can be directly reused in the target farmland to reduce chemical fertilizer use.

[0147] For the construction of the wastewater reuse system, a submersible pump station is set up as a water intake device on the side of the deep ecological treatment zone 303 closest to the target farmland. The outlet of the pump station is connected to the farmland irrigation network, and the pump head is matched to the farmland irrigation elevation. The water quality of the deep ecological treatment zone 303 is tested weekly. When the water quality meets the farmland irrigation water quality standards, the purified water is pumped out through the pump station and transported to the farmland irrigation network for re-irrigation of the target farmland, forming a closed loop of farmland water circulation of irrigation, drainage, purification and reuse.

[0148] For the construction of the outer protective dike, along the construction axis outside the 300-meter boundary line of the ecological purification pond, the dike foundation was excavated, the top 30cm of humus and silt was removed, exposing the original old soil, and then compacted in layers with a compaction degree of ≥90%.

[0149] On the compacted foundation, the original soil and graded crushed stone mixture is filled in layers, with each layer having a loose thickness of ≤25cm. Each layer is then rolled to the preset height, with a compaction degree of ≥92%, forming a protective embankment around the edge of the ecological purification pond 300. The embankment is 1.5m wide at the top, 1.5m high, and has a slope ratio of 1:2, preventing external rainwater and runoff from entering the pond and avoiding water overflow from the ecological purification pond 300.

[0150] Comparative Example 1

[0151] In this comparative study, a contiguous dryland farmland of 10 mu (approximately 1.65 acres) in northern China was selected. The soil type was sandy loam, the crop planting pattern was wheat-corn rotation, the annual rainfall was 800-900 mm, the farmland runoff coefficient during the rainstorm period was 0.3-0.4, and the total area of ​​the ecological purification pond was uniformly 1 mu (approximately 0.16 acres).

[0152] For the construction of farmland water collection systems, the farmland furrows and drainage collection ditches on the outer side of the field ridges are excavated according to conventional planting requirements. The furrows are 30cm deep and 30cm wide, and the collection ditches are 70cm deep and 1.2m wide. There are no ecological soil stabilization structures on the ditch slopes and no pollution interception and filtration structures at the ditch connections. The system only realizes the basic function of farmland drainage collection.

[0153] For the construction of the treatment pond, three independent treatment units were excavated, namely a primary sedimentation pond, a secondary oxidation pond, and a tertiary stabilization pond, with a total area of ​​1 acre;

[0154] The pond body is hardened and seepage-proofed with C25 concrete, and there is no graded functional matrix layer at the bottom of the pond;

[0155] The three ponds are connected by conventional concrete overflow dams and manual gates. The dams are fully seepage-proof structures with no seepage purification path. The water flows into the downstream unit by gravity only through the overflow outlets.

[0156] As for the layout of the aquatic ecosystem, only reeds and foxtail algae were scattered in the three ponds, without a differentiated plant system for each zone, no functional microbial community was introduced, and no complete food chain was constructed.

[0157] For end-of-pipe treatment, the effluent from the three-stage stabilization pond is directly discharged into the surrounding water bodies, with no tailwater reused in the irrigation system.

[0158] The dredged bottom mud, harvested plant residues, and agricultural straw are all transported off-site for disposal, and there is no on-site resource-based composting system.

[0159] For the outer protection, conventional plain earth embankments are used to surround the pond, without standardized layered compaction and protective structures.

[0160] Comparative Example 2

[0161] Both of the dividing dams in this comparative example are conventional fully impermeable homogeneous earth dams. The entire cross-section of the water-facing side is covered with a 30cm thick modified clay impermeable layer. There is no upper and lower segmented structure with a pre-set normal water level boundary, that is, there is no incompletely permeable structure 401, no fully permeable structure 402, and no water-stopping stepped transition structure 403.

[0162] A fixed-size overflow outlet is set at the top of the dam, and the upstream and downstream water bodies are connected only through the overflow outlet, with no seepage purification path through the dam body;

[0163] The backwater side is simply reinforced with rammed clay, without a three-stage reverse filter layer or a full-section permeable structure 404. The other dam dimensions and filling materials are the same as in the previous example.

[0164] Comparative Example 3

[0165] The dividing dam in this comparative example adopts a construction process with the procedures reversed, specifically as follows:

[0166] After excavating the dam foundation and completing the backfilling of the intercepting trench, the core main structure of the dam was not prioritized for construction. Instead, the slope structures of the water-facing and water-repellent sides were constructed according to the designed slope ratio before the dam core was constructed in sections.

[0167] The core filling of the dam body did not adopt the process of simultaneous paving and compaction in the same layer. The three functional zones were filled separately, resulting in multiple longitudinal construction joints.

[0168] After the dam body was filled, no static settlement treatment was carried out, and the subsequent slope structure construction was carried out directly. All other materials, parameters, and structural designs were completely consistent with the example.

[0169] The table below compares the core performance of the examples and comparative examples. The data are the average test values ​​from 6 months of continuous operation:

[0170] Core detection indicators Example Comparative Example 1 Comparative Example 2 Comparative Example 3 CODcr removal rate 76.2 44.8 59.7 67.5 Total nitrogen removal rate 82.5 37.6 54.8 65.2 Total phosphorus removal rate 86.3 41.9 58.2 70.4 Heavy metal removal rate 91.4 34.7 61.8 72.1 Dam body core compaction 92.8 95.2 91.5 86.7 Safety factor for dam body anti-sliding stability (specification requirement ≥ 1.2) 1.58 1.45 1.22 1.08 The safety factor for seepage stability in the dam body (the specification requires ≥1.1). 2.12 2.05 1.35 1.02 Incidence of engineering defects within 6 months of operation 0 0 15% 100% Tailwater reuse rate 92.3 0 58.6 74.8 Annual evaporation loss rate of pond water 9.7 24.8 17.6 14.9 Construction cost per unit area of ​​pond 39.6 98.5 68.2 40.2 Annual maintenance costs 785 1480 1160 1950 Farmland fertilizer reduction rate 32.4 0 14.7 19.8

[0171] Table 1 Comparison of Core Performance

[0172] As can be seen from Table 1, compared with Comparative Example 1, this embodiment of the invention significantly improves the removal efficiency of core pollutants and significantly reduces construction costs through gradient infiltration purification process, segmented seepage control dam, and full-chain resource utilization closed loop. At the same time, it achieves near-full reuse of tailwater and a significant reduction in farmland fertilizer use, which to a certain extent solves the industry pain points of poor purification effect, high engineering cost, water waste, and lack of pollutant resource utilization closed loop in existing technologies.

[0173] Compared to Comparative Example 2, which lacks a core innovative structure, the segmented seepage control dam of this invention achieves adaptive seepage regulation at high and low water levels. This ensures both the hydraulic residence time and the dam's seepage purification effect at low water levels, while also preventing overflow at high water levels. The removal efficiency of core pollutants and the tailwater reuse rate are effectively improved, and the seepage and anti-sliding stability of the dam are significantly enhanced.

[0174] Compared to Comparative Example 3, which reverses the process, this invention strictly follows compliant procedures, with simultaneous filling and compaction at the same layer without construction joints. All stability indicators of the dam body meet the specifications, eliminating the risk of engineering defects. At the same time, it avoids the attenuation of purification effect caused by seepage short circuits, ensuring the long-term stable operation of the system and significantly reducing the later operation and maintenance costs.

[0175] In summary, the method for preventing and controlling nitrogen and phosphorus pollution at the source in farmland by intercepting and reusing tailwater disclosed in this invention can achieve synergistic effects between farmland tailwater collection and end-of-pipe purification, realize gradient process interception of nitrogen and phosphorus in farmland, significantly improve the graded purification efficiency of nitrogen and phosphorus pollutants, and balance process purification efficiency and system operation stability.

[0176] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for controlling nitrogen and phosphorus source reduction in farmland by intercepting and reusing tailwater, characterized in that, Includes the following steps: On the surface of the target farmland, lay out the center lines of the furrows; on the outer side of the field ridges, lay out the center lines of the drainage ditches; and at the ends of the fields, lay out the boundary lines of the ecological purification ponds. Based on the total area of ​​the target farmland, the area of ​​the ecological purification pond accounts for 4.5% to 6.5%, and the length-to-width ratio is 1.5:1 to 2.5:1; The furrows, the drainage ditches, and the ecological purification ponds are excavated sequentially, wherein... The furrow and the water collection ditch are connected; Two dividing dams are constructed inside the ecological purification pond, sequentially dividing it along the water flow direction into a water storage area, a facultative oxidation area, and a deep ecological treatment area. The water collection ditch and the water storage area are connected, and the water flows into the downstream area step by step through the seepage effect of the two separating dams; The water-facing side of the dividing dam is divided into a partially permeable structure and a permeable structure from bottom to top, with the preset normal water level of the pond as the boundary. A water-stopping stepped transition structure is set between the partially permeable structure and the permeable structure. The water-repellent side of the dividing dam is a fully permeable structure. Functional microbial communities and plant systems are respectively deployed in the water storage area, the facultative oxidation area, and the deep ecological treatment area, wherein, The functional microbial community is evenly distributed in the water and bottom substrate of the water storage area, the facultative oxidation area and the deep ecological treatment area; The water storage area is planted with submerged and emergent plants, the facultative oxidation area is planted with emergent and floating plants, and the deep ecological treatment area is planted with submerged plants and emergent plants along the bank.

2. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, The excavation of the furrows and the drainage ditches specifically includes: The furrows are dug evenly along the crop planting direction of the target farmland, with a depth of 20cm to 35cm and a width of 25cm to 40cm. The drainage ditch is dug along the outer side of the field, with a depth of 50cm to 80cm and a width of 1.0m to 1.5m. At the junctions of the furrow and the collection ditch and the collection ditch and the water storage area, 25cm to 35cm of pebbles or gravel are filled to form a dirt-blocking and filtering structure.

3. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, The construction of the aforementioned dividing dam specifically includes: First, excavate the dam foundation along the preset dividing axis of the ecological purification pond, and then compact the dam foundation in layers. On the compacted dam foundation, the core dam material is filled in layers and compacted layer by layer to the preset dam height to form the main structure of the partition dam. Using the preset normal water level as the boundary, an incompletely permeable structure and a permeable structure are constructed sequentially from bottom to top on the water-facing side of the dividing dam. First, a modified clay layer is laid below the boundary to form an incompletely permeable structure, and then a graded crushed stone layer is laid above the boundary to form a permeable structure. At the boundary between the incompletely permeable structure and the permeable structure, the water-stopping stepped transition structure is poured. On the backwater side of the dividing dam, a filter layer and a permeable gravel layer are laid sequentially from the core of the dam outwards to form a full-section permeable structure.

4. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, The construction of the water storage area, the facultative oxidation area, and the deep ecological treatment area specifically includes: The bottom of the ecological purification pond is repaired and compacted in layers. A seepage-proof layer is laid on the compacted bottom of the pond, and the seepage-proof layer is made of modified clay. An initial purification matrix layer is laid above the impermeable layer corresponding to the water storage area, a microbial biofilm matrix layer is laid above the impermeable layer corresponding to the facultative oxidation zone, and a deep purification matrix layer is laid above the impermeable layer corresponding to the deep ecological treatment zone.

5. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 4, characterized in that, The thickness of the impermeable layer is 25cm to 35cm, the initial purification matrix layer is laid with adsorbent granular material and has a thickness of 15cm to 25cm, the microbial biofilm matrix layer is laid with porous granular material and has a thickness of 10cm to 20cm, and the deep purification matrix layer is laid with composite porous material and has a thickness of 10cm to 20cm.

6. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 4, characterized in that, The deployment of the functional microbial community and the plant system is specifically as follows: After the initial purification substrate layer, the microbial biofilm substrate layer and the deep purification substrate layer are laid, the functional microbial community is introduced into the water storage area, the facultative oxidation area and the deep ecological treatment area respectively, so that the functional microbial community is evenly attached to the substrate layer and water body in each area. After the functional microbial community is introduced, submerged and emergent plants are planted on the initial purification substrate layer in the water storage area, emergent plants are planted on the microbial biofilm substrate layer in the facultative oxidation zone, floating plants are placed in the water body, submerged plants are planted on the deep purification substrate layer in the deep ecological treatment zone, and emergent plants are planted on the shore to form the plant system.

7. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 3, characterized in that, Planting vegetation on the dividing dam, specifically: Planting soil is laid on the top of the dividing dam and dam-stabilizing plants are planted. Emergent plants are planted on the permeable structure of the water-facing side of the dividing dam, and planting soil is laid on the permeable gravel layer on the water-retaining side of the dividing dam, where soil-stabilizing herbaceous plants are planted.

8. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, Collect farmland straw from the target farmland, plant residues from the ecological purification pond, and dredge the bottom of the ecological purification pond; After the farmland straw, plant residues and pond bottom dredging are crushed, mixed and pretreated, composting agents are added and the moisture content is adjusted for composting and fermentation. After fermentation, organic fertilizer is obtained and reused in the target farmland.

9. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, The water in the aforementioned deep ecological treatment zone is taken for recharge, specifically as follows: A water intake device is installed on the side of the deep ecological treatment zone closest to the target farmland to regularly test the water quality of the deep ecological treatment zone; When the water quality meets the standards for farmland irrigation, the purified water from the deep ecological treatment zone is extracted through the water intake device and transported to the farmland irrigation network for re-irrigation of the target farmland.

10. The method for intercepting and reusing tailwater during the nitrogen and phosphorus source reduction process in farmland as described in claim 1, characterized in that, Along the outer edge of the boundary line of the ecological purification pond, the construction axis is laid out and positioned. The foundation of the dam is excavated along the construction axis. The excavated foundation is compacted in layers. On the compacted dam foundation, dam material is filled in layers and compacted to the preset height to form a protective dam around the edge of the ecological purification pond.

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