A method for reducing salt and increasing carbon and fertilization integration of water-saving rural domestic sewage in arid saline area and application

Abstract

The application belongs to the field of efficient utilization of agricultural water and soil resources and ecological improvement of saline-alkali land, and discloses a water-saving rural domestic sewage salt reduction and carbon increase and fertilization integrated method and application. First, the rural domestic sewage is collected in black and gray qualities, the black water is mixed with the pretreated gray water after anaerobic fermentation; the mixed sewage is subjected to biological contact oxidation and composite substrate subsurface flow artificial wetland ecological purification; biochar-based organic fertilizer, salt-tolerant functional microbial solid inoculum and water-saving salt reduction and efficiency increasing irrigation liquid are prepared; bottom application, inoculation, water-saving salt irrigation and crop planting are carried out on the saline-alkali land where the underground pipe drainage system and the underground drip irrigation system are laid; dynamic regulation is carried out according to the soil conductivity, organic carbon and available nutrients, and reuse or discharge is carried out according to the conductivity ratio of the leaching liquid to the irrigation liquid. The application realizes the synergistic closed loop of rural domestic sewage resource utilization, root zone salt control, directional salt discharge and soil carbon increase and fertilization, and is suitable for the improvement of arid and semiarid saline-alkali land and water-saving agriculture.

Description

Technical Field

[0001] This invention belongs to the field of efficient utilization of agricultural water and soil resources and ecological improvement of saline-alkali land. Specifically, it relates to an integrated method and application for water-saving rural domestic sewage desalination, carbon increase and fertilization in arid and saline-alkali areas, which combines the separate collection of rural domestic sewage, low-cost ecological purification, biochar-based carbon enrichment and fertilization, underground drip irrigation for salt washing, underground pipe salt discharge, and synergistic regulation of water, fertilizer, salt and carbon. Background Technology

[0002] my country has a large area of ​​saline-alkali land, with a significant portion located in arid and semi-arid regions of the north. These areas simultaneously face challenges such as freshwater scarcity, salt accumulation in the topsoil, low organic matter content, and the difficulty in managing dispersed rural domestic sewage. Traditional flood irrigation for salt leaching requires large amounts of freshwater, which can easily lead to groundwater level rise and salt return. While underground drainage can promote salt removal, its effectiveness is limited without a stable water source and root zone salt control measures. Biochar or organic fertilizer alone can improve soil structure but cannot address the issues of salt migration direction and insufficient irrigation water. Simply reusing treated rural domestic sewage for irrigation carries risks such as salt accumulation, unstable nutrient utilization, and unclear safety boundaries.

[0003] In existing technologies, rural domestic sewage treatment primarily aims to achieve standard discharge or general irrigation reuse, while saline-alkali land improvement technologies mainly focus on individual aspects such as engineering salt removal, chemical improvement, biological fertilization, or water-saving irrigation. These technological approaches lack synergistic design for arid and saline-alkali areas, particularly lacking a method to unify the water and nutrient resources in sewage, the carbon sequestration characteristics of biochar, the regulatory role of functional microorganisms, the salt control effect of underground drip irrigation in the root zone, and the directional salt removal effect of underground drainage into a single dynamic closed loop. Consequently, existing solutions struggle to simultaneously meet the requirements of water conservation, salt reduction, carbon increase, fertilization, low-cost treatment, and long-term stable operation.

[0004] Therefore, it is necessary to provide an integrated technical solution with clear process boundaries and control rules, so that rural domestic sewage is no longer just used as a general alternative water source, but as part of the carrier for reducing salinity, increasing carbon and enriching fertility, working in synergy with biochar-based materials, underground drip irrigation and underground drainage, so as to reduce soil salinity and improve fertility under low freshwater consumption conditions in arid and saline-alkali areas. Summary of the Invention

[0005] The technical problem to be solved by this invention is: to address the issues of insufficient freshwater, easy salt re-accumulation, low soil organic carbon, high cost of decentralized treatment of rural domestic sewage, and lack of a closed-loop system for water, fertilizer, salt, and carbon synergy in arid and saline-alkali areas, and to provide a water-saving integrated method and application for reducing salt content, increasing carbon content, and improving fertility in rural domestic sewage.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a water-saving rural domestic sewage desalination, carbon increase, and fertilization method for arid and saline-alkali areas, comprising the following steps: S1. Black water and grey water are collected separately at the source for rural domestic sewage. The black water is subjected to solar-assisted heating and anaerobic fermentation and then separated by static settling to obtain an upper fermentation liquid and a bottom fermentation sediment. The grey water is pretreated by oil separation, sedimentation and homogenization. The upper fermentation liquid and the pretreated grey water are mixed at a volume ratio of 1:3 to 1:6 to obtain mixed sewage. S2. The mixed wastewater is sequentially introduced into a biological contact oxidation unit and a composite matrix subsurface flow constructed wetland unit for ecological purification to obtain ecologically purified effluent that meets the requirements of farmland irrigation water quality; wherein, the composite matrix layer of the composite matrix subsurface flow constructed wetland unit contains agricultural waste-derived biochar. S3. Prepare biochar-based salt-reducing, carbon-enhancing, and fertilizer-building carrier materials, wherein the carrier materials include biochar-based organic fertilizer, salt- and alkali-tolerant functional microbial solid inoculants, and water-saving, salt-reducing, and efficiency-enhancing irrigation solutions. S4. Lay a subsurface drainage system and an underground drip irrigation system in the target arid and saline land, apply the biochar-based organic fertilizer to the top 0-20 cm of soil, inoculate the salt-tolerant functional microbial solid inoculant into the soil of the planting area, and apply the water-saving, salt-reducing and efficiency-enhancing irrigation liquid in several applications through the underground drip irrigation system for pre-sowing salt leaching and irrigation during the growing season. S5. Establish a coordinated monitoring and dynamic regulation system for water, fertilizer, salt and carbon, and monitor the soil electrical conductivity, pH, organic carbon, available nitrogen, phosphorus, potassium and water content of the 0-20 cm and 20-40 cm soil layers in real time or periodically. S6. Collect the salt leachate through the underground drainage system and divert it according to the ratio of the conductivity of the leachate to that of the irrigation solution. S7. Plant salt-tolerant crops and implement crop rotation management. After the crops are harvested, the straw is used for returning to the field, composting, or agricultural waste biochar production, so as to form a closed loop of rural domestic sewage resource utilization, salt directional migration and discharge, and soil carbonization and fertilization.

[0007] Furthermore, in step S1, the anaerobic fermentation is carried out in a fermentation unit with a transparent covering layer or a solar thermal collection structure on top, the fermentation temperature is 15℃~40℃, the fermentation time is 24~72 h, and the static separation time is 6~24 h.

[0008] Further, in step S2, the packing material filling rate of the biological contact oxidation unit is 50%–70%, the hydraulic retention time is 6–10 h, and the dissolved oxygen is 2.0–4.0 mg / L; the hydraulic retention time of the composite matrix subsurface flow constructed wetland unit is 24–48 h, and the hydraulic load is 0.2–0.5 m. 3 / (m 2 ·d); The composite matrix layer is composed of the following raw materials in parts by weight: 20-35 parts zeolite, 15-25 parts volcanic rock, 30-45 parts biochar derived from agricultural waste, and 5-10 parts fly ash. During implementation, the proportions of each component can be adjusted according to the nitrogen and phosphorus concentrations in the wetland influent and the availability of local materials, without altering the main composition of the composite matrix layer.

[0009] Furthermore, in step S3, the biochar-based organic fertilizer is prepared by mixing the bottom fermentation sediment, agricultural waste and livestock manure aerobic compost with biochar from agricultural waste source at a mass ratio of 4:1 to 3:1. The salt-tolerant functional microbial solid inoculant is prepared from salt-tolerant functional microorganisms isolated and screened from soils in arid and saline-alkali areas. These microorganisms possess one or more of the following functions: salt tolerance, phosphorus solubilization, potassium solubilization, extracellular polymer production, or growth promotion. The effective viable count of the solid inoculant is not less than 2 × 10⁻⁶. 8 CFU / g; The water-saving, salt-reducing, and efficiency-enhancing irrigation solution is prepared by mixing the ecologically purified effluent with the biochar extract from agricultural waste at a volume ratio of 5:1 to 10:1 and adjusting the pH to 6.5 to 7.5. In the embodiments where it is necessary to enhance the aggregate structure and water retention capacity, potassium humate or fulvic acid at a mass-volume ratio of 0.01% to 0.05% can be added. Furthermore, the agricultural waste-derived biochar can be obtained by pyrolyzing and pulverizing crop straw, rice husks, fruit branches, or other lignocellulosic agricultural wastes available at the implementation site under limited oxygen conditions; in the embodiments, corn straw is preferably pyrolyzed at 500°C for 2 hours under limited oxygen and then pulverized to about 3 mm to produce biochar.

[0010] Furthermore, in step S4, the buried pipes of the underground drainage system are buried at a depth of 1.0–1.5 m, spaced at intervals of 8–12 m, and have a slope of 3‰–5‰; the drip irrigation tapes of the underground drip irrigation system are buried at a depth of 15–20 cm, and the single irrigation volume during the growing season is 10–20 m³. 3 / mu, the total irrigation during the growing season is 60-120 m³. 3 / acre / season.

[0011] Furthermore, in step S5, a preset soil electrical conductivity threshold based on the target crop type is used as the crop salt tolerance threshold; when the electrical conductivity of the 0-20 cm soil layer exceeds the threshold, an additional 10-20 m... 3 / mu of water-saving, salt-reducing, and efficiency-enhancing irrigation solution; when soil organic carbon is lower than the target value, apply 200-500 kg / mu of the biochar-based organic fertilizer; when readily available nutrients are low, prioritize supplementing the ecologically purified effluent through the underground drip irrigation system.

[0012] Furthermore, in step S6, the diversion specifically means that when the ratio is greater than 1.30, the leaching liquid is discharged to an evaporation pond or a centralized treatment unit; when the ratio is not greater than 1.30 and the leaching liquid meets the requirements for reuse water quality, the leaching liquid is mixed with the ecologically purified effluent and reused in the underground drip irrigation system.

[0013] Furthermore, in step S7, a drip irrigation planting mode under film or an integrated water, fertilizer and salt management mode is adopted. The target crop selection is based on the salt tolerance threshold, economic value and crop rotation system of the implementation area. In the examples, salt-tolerant corn and sunflower are used for verification.

[0014] The second aspect of the present invention provides the application of the above-mentioned method in the improvement of saline-alkali farmland soil in arid and semi-arid areas of Northwest China, ecological restoration of coastal saline-alkali land, water-saving agricultural production, or low-cost resource utilization of rural domestic sewage. This application is based on the premise that the synergistic regulation of water, fertilizer, salt and carbon and the closed-loop diversion of leachate can be implemented.

[0015] Unlike simple wastewater re-injection after reaching standards, biochar application alone to the soil, or underground drainage for salt removal, the core of this invention lies in: ensuring irrigation safety through the separate collection and ecological purification of black and gray materials; improving the soil carbon pool and rhizosphere environment with biochar-based organic fertilizer and salt-tolerant functional bacteria; enhancing the water retention and fertilization capacity of irrigation solution with biochar extract; driving salts away from the root zone through underground drip irrigation; receiving migrating salts through underground drainage; and determining supplementary irrigation, topdressing, reuse, or discharge based on the monitoring results of electrical conductivity, organic carbon, and readily available nutrients, thereby forming an executable closed-loop control chain of water, fertilizer, salt, and carbon.

[0016] The present invention has the following beneficial effects: 1. This invention incorporates water, nitrogen and phosphorus nutrients, and fermentation precipitates from rural domestic sewage into irrigation, fertilization, and biochar-based carrier preparation processes, respectively, to achieve full-component resource utilization of black water, grey water, and sewage treatment derivatives, thereby reducing the cost of decentralized rural sewage treatment.

[0017] 2. This invention controls salt concentration in the root zone through low-volume, high-frequency drip irrigation and deep, directional salt removal through underground pipe drainage, giving salt migration a clear direction. It also reduces the risk of salt accumulation in the system by using the ratio of the conductivity of the leaching solution to that of the irrigation solution for reuse or discharge.

[0018] 3. This invention utilizes biochar-based organic fertilizer, biochar extract, and salt-tolerant functional microbial agents to synergistically improve soil organic carbon, aggregate structure, cation exchange capacity, and microbial activity, so that desalination treatment no longer comes at the expense of soil fertility, and can simultaneously achieve desalination, carbon increase, and fertilization.

[0019] 4. This invention triggers supplemental irrigation, topdressing, and nutrient replenishment by using indicators such as soil electrical conductivity, organic carbon, available nutrients, and moisture content, thus avoiding the problem of insufficient adaptability of fixed irrigation systems under different salinity levels, different crops, and different climatic conditions.

[0020] 5. The raw materials required for this invention mainly come from rural domestic sewage, agricultural waste, livestock and poultry manure and native salt-tolerant microorganisms, which has the advantages of local raw material availability, low operating costs and regional replicability. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the overall process flow of the method of the present invention. Detailed Implementation

[0022] The present invention will be further described below with reference to embodiments. It should be understood that the embodiments are only used to illustrate the technical solutions of the present invention and do not limit the scope of protection of the present invention; without departing from the core concept of the present invention, those skilled in the art can make equivalent adjustments to the parameters according to the quality of rural domestic sewage, the type of saline-alkali land, the target crop and climatic conditions.

[0023] Example 1: Application in secondary salinized farmland in arid Northwest China A secondary salinized farmland of 50 mu (approximately 3.3 hectares) was selected in the Hetao Irrigation District of Inner Mongolia, with an annual precipitation of approximately 150 mm. Before the experiment, the total salt content of the 0–20 cm soil layer was 3.8 g / kg, pH 8.6, organic carbon content 5.8 g / kg, available nitrogen 42 mg / kg, and available phosphorus 8 mg / kg.

[0024] Step 1: Wastewater collection and pretreatment based on wastewater type. Approximately 3 cubic meters of blackwater are collected daily. 3 Approximately 9 m³ of grey water 3 Black water enters a solar-assisted anaerobic fermentation tank with a transparent top cover. After fermentation for 48 hours, it is allowed to stand and separate, and the upper fermentation liquid is collected. The ash water is treated by oil separation, sedimentation, and homogenization. The upper fermentation liquid and the treated ash water are mixed at a volume ratio of 1:4 to obtain mixed wastewater.

[0025] Step two, ecological purification. The mixed wastewater enters the biological contact oxidation tank, filled with polypropylene elastic packing material at a filling rate of 60%, with a hydraulic retention time of 8 hours and dissolved oxygen maintained at 2.5 mg / L; subsequently, it enters a composite matrix subsurface flow constructed wetland with a wetland area of ​​300 m². 2 The hydraulic retention time was 36 hours. The composite matrix layer was laid with a mixture of 30 parts zeolite, 20 parts volcanic rock, 35 parts corn straw biochar, 8 parts fly ash, and 7 parts acid-modified attapulgite, with a thickness of 50 cm. The purified effluent had a COD of 45 mg / L, NH3-N of 6.5 mg / L, and TP of 0.8 mg / L, meeting the water quality requirements for farmland irrigation.

[0026] Step 3: Preparation of carrier materials. 150 kg dry weight of blackwater fermentation sediment, 400 kg crushed corn stalks, and 300 kg well-rotted sheep manure were mixed, with a C / N ratio adjusted to 28:1 and a moisture content of 60%. After inoculation with a composting agent, the mixture was composted for 28 days, turning it 4 times during this period, and maintaining a high temperature of approximately 60°C for 10 days. After composting, it was mixed with corn stalk biochar that had undergone 500°C anaerobic pyrolysis for 2 hours and crushed to 3 mm at a mass ratio of 3.5:1 to obtain biochar-based organic fertilizer. Haloxytolerant Bacillus, Halomonas, and Pseudomonas were fermented by single-strain fermentation and then mixed in equal proportions and adsorbed onto a sterilized peat carrier to produce an effective viable count of 3.5 × 10⁻⁶ bacteria. 8 Solid microbial agent with CFU / g. The ecologically purified effluent and biochar extract were mixed at a volume ratio of 7:1, and 0.03% humic acid was added to adjust the pH to 7.0, resulting in a water-saving, salt-reducing, and efficiency-enhancing irrigation solution.

[0027] Step four: Field application and coordinated irrigation and drainage. After deep plowing to a depth of 35 cm, lay 100 mm diameter PVC / PE composite underground pipes at a spacing of 10 m and a depth of 1.2 m, with a slope of 4‰, and install soil salinity sensors at depths of 20 cm and 40 cm. Apply biochar-based organic fertilizer at 2800 kg / mu and rotary tillage it into the 0–20 cm soil layer; apply solid microbial agent at 8 kg / mu in sowing furrows; bury underground drip irrigation tape to a depth of 18 cm with drippers spaced 30 cm apart. Before sowing, apply water-saving, salt-reducing, and efficiency-enhancing irrigation solution at a depth of 42 m... 3 / mu is irrigated 3 times with drip irrigation; a total of 8 drip irrigations are applied during the growing season, with each irrigation covering 10-15 m². 3 / mu, total 95 m 3 / mu. The underground drainage system collects the leachate simultaneously. When the ratio of the conductivity of the leachate to that of the irrigation solution is greater than 1.30, it is discharged to the evaporation pond; when it is not greater than 1.30 and meets the reuse requirements, it is returned to the storage tank and mixed with the ecologically purified effluent for recycling.

[0028] Step 5, Dynamic Adjustment. The control platform presets the EC threshold for the 0–20 cm soil layer to be 4.0 dS / m; when this threshold is exceeded, an additional 10 m is automatically added. 3 / mu of water-saving, salt-reducing, and efficiency-enhancing irrigation solution. During the corn jointing and tasseling stages, based on the results of fast-acting nutrient testing, the ecologically purified effluent is used to perform two topdressing irrigations through a drip irrigation system, reducing the application of urea and diammonium phosphate by 25% compared to conventional planting.

[0029] Results: After the first corn harvest, the total salt content in the 0-20 cm soil layer decreased from 3.8 g / kg to 1.7 g / kg, a reduction of 55.3%; organic carbon increased from 5.8 g / kg to 8.0 g / kg, an increase of 37.9%; available nitrogen increased from 42 mg / kg to 56 mg / kg, an increase of 33.3%; and available phosphorus increased from 8 mg / kg to 11 mg / kg, an increase of 37.5%. The total water consumption during the entire growth period was 137 m³. 3 / mu, compared to approximately 250 m² of traditional local furrow irrigation for salt washing. 3 The water saving rate was 45.2% per mu; the corn yield was 562 kg per mu, which was 139.1% higher than the 235 kg per mu of the control field that did not use this method.

[0030] Example 2: Application in Severely Saline-Alkali Land in Oases of Southern Xinjiang A 30-mu (approximately 2 hectares) arid saline-alkali land area in the Aksu Oasis of southern Xinjiang was selected. Before the experiment, the total salt content of the 0–20 cm soil layer was 6.2 g / kg, pH 9.0, organic carbon content 3.5 g / kg, available nitrogen 28 mg / kg, and available phosphorus 5 mg / kg. Following the steps in Example 1, the following parameters were adjusted for extreme drought and severe salinity: the anaerobic fermentation time of the blackwater was extended to 72 h; the application rate of biochar-based organic fertilizer was increased to 3500 kg / mu; the volume ratio of ecologically purified effluent to biochar extract was adjusted to 6:1; the burial depth of the underground pipes was adjusted to 1.5 m; and the pre-sowing irrigation quota was 48 m³. 3 The planting of salt-tolerant sunflowers was carried out in three separate applications per acre using drip irrigation under mulch.

[0031] Results: After the first harvest of sunflowers, the total salt content in the 0-20 cm soil layer decreased from 6.2 g / kg to 3.4 g / kg, a reduction of 45.2%; organic carbon increased from 3.5 g / kg to 4.9 g / kg, an increase of 40.0%; available nitrogen increased from 28 mg / kg to 38 mg / kg, an increase of 35.7%; and available phosphorus increased from 5 mg / kg to 7.5 mg / kg, an increase of 50.0%. The total irrigation water consumption for the entire growth period was 115 m³. 3 / mu, compared to the local traditional winter and spring irrigation of about 300 m 3 Water saving was 61.7% per mu; sunflower yield was 185 kg per mu, an increase of 164.3% compared to the control group of 70 kg per mu in unimproved saline-alkali land.

[0032] The above embodiments demonstrate that the present invention does not simply list sewage irrigation, biochar application, drip irrigation, and underground drainage in parallel. Instead, it forms a mutually restrictive technical closed loop through differentiated resource utilization, biochar-based carriers, root zone salt control in underground drip irrigation, underground drainage of salt, diversion of leachate conductivity, and dynamic regulation of water, fertilizer, salt, and carbon. This closed loop can achieve stable water-saving, salt-reducing, carbon-enhancing, fertilization, and yield-increasing effects through parameter adjustments under different salinity and drought conditions. Therefore, it has outstanding substantive features and significant progress compared to single sewage reuse, single biochar improvement, or single irrigation and drainage projects.

Claims

1. A water-saving method for integrated desalination, carbon increase, and fertilization of rural domestic sewage in arid and saline-alkali areas, characterized in that, Includes the following steps: S1. Black water and grey water are collected separately at the source for rural domestic sewage. The black water is subjected to solar-assisted heating and anaerobic fermentation and then separated by static settling to obtain an upper fermentation liquid and a bottom fermentation sediment. The grey water is pretreated by oil separation, sedimentation and homogenization. The upper fermentation liquid and the pretreated grey water are mixed at a volume ratio of 1:3 to 1:6 to obtain mixed sewage. S2. The mixed wastewater is sequentially introduced into a biological contact oxidation unit and a composite matrix subsurface flow constructed wetland unit for ecological purification to obtain ecologically purified effluent that meets the requirements of farmland irrigation water quality; wherein, the composite matrix layer of the composite matrix subsurface flow constructed wetland unit contains agricultural waste-derived biochar. S3. Prepare biochar-based salt-reducing, carbon-enhancing, and fertilizer-building carrier materials, wherein the carrier materials include biochar-based organic fertilizer, salt- and alkali-tolerant functional microbial solid inoculants, and water-saving, salt-reducing, and efficiency-enhancing irrigation solutions. S4. Lay a subsurface drainage system and an underground drip irrigation system in the target arid and saline land, apply the biochar-based organic fertilizer to the top 0-20 cm of soil, inoculate the salt-tolerant functional microbial solid inoculant into the soil of the planting area, and apply the water-saving, salt-reducing and efficiency-enhancing irrigation liquid in several applications through the underground drip irrigation system for pre-sowing salt leaching and irrigation during the growing season. S5. Establish a coordinated monitoring and dynamic regulation system for water, fertilizer, salt and carbon, and monitor the soil electrical conductivity, pH, organic carbon, available nitrogen, phosphorus, potassium and water content of the 0-20 cm and 20-40 cm soil layers in real time or periodically. S6. Collect the salt leachate through the underground drainage system and divert it according to the ratio of the conductivity of the leachate to that of the irrigation solution. S7. Plant salt-tolerant crops and implement crop rotation management. After the crops are harvested, the straw is used for returning to the field, composting, or agricultural waste biochar production, so as to form a closed loop of rural domestic sewage resource utilization, salt directional migration and discharge, and soil carbonization and fertilization.

2. The method according to claim 1, characterized in that, In step S1, the anaerobic fermentation is carried out in a fermentation unit with a transparent covering layer or a solar thermal collection structure on top. The fermentation temperature is 15℃~40℃, the fermentation time is 24~72h, and the static separation time is 6~24h.

3. The method according to claim 1, characterized in that, In step S2, the packing material filling rate of the biological contact oxidation unit is 50%–70%, the hydraulic retention time is 6–10 h, and the dissolved oxygen is 2.0–4.0 mg / L; the hydraulic retention time of the composite matrix subsurface flow constructed wetland unit is 24–48 h, and the hydraulic loading is 0.2–0.5 m. 3 / (m 2 ·d); The composite matrix layer is composed of the following raw materials in parts by weight: 20-35 parts zeolite, 15-25 parts volcanic rock, 30-45 parts biochar from agricultural waste, and 5-10 parts fly ash.

4. The method according to claim 1, characterized in that, In step S3, the biochar-based organic fertilizer is prepared by mixing the bottom fermentation sediment, agricultural waste and livestock manure aerobic compost with biochar from agricultural waste source at a mass ratio of 4:1 to 3:

1. The salt-tolerant functional microbial solid inoculant is prepared from salt-tolerant functional microorganisms isolated and screened from soils in arid and saline-alkali areas. These microorganisms possess one or more of the following functions: salt tolerance, phosphorus solubilization, potassium solubilization, extracellular polymer production, or growth promotion. The effective viable count of the solid inoculant is not less than 2 × 10⁻⁶. 8 CFU / g; The water-saving, salt-reducing, and efficiency-enhancing irrigation solution is prepared by mixing the ecologically purified effluent with the biochar extract from agricultural waste at a volume ratio of 5:1 to 10:1 and adjusting the pH to 6.5 to 7.

5.

5. The method according to claim 1, characterized in that, In step S4, the buried pipes of the underground drainage system are buried at a depth of 1.0–1.5 m, spaced at intervals of 8–12 m, and have a slope of 3‰–5‰; the drip irrigation tapes of the underground drip irrigation system are buried at a depth of 15–20 cm, and the single irrigation volume during the growing season is 10–20 m³. 3 / mu, the total irrigation during the growing season is 60-120 m³. 3 / acre / season.

6. The method according to claim 1, characterized in that, In step S5, a preset soil electrical conductivity threshold based on the target crop type is used as the crop salt tolerance threshold; when the electrical conductivity of the 0-20 cm soil layer exceeds the threshold, an additional 10-20 m... 3 / mu of water-saving, salt-reducing, and efficiency-enhancing irrigation solution; when soil organic carbon is lower than the target value, apply 200-500 kg / mu of the biochar-based organic fertilizer; when readily available nutrients are low, prioritize supplementing the ecologically purified effluent through the underground drip irrigation system.

7. The method according to claim 1, characterized in that, In step S6, the diversion specifically involves: when the ratio is greater than 1.30, the leaching liquid is discharged to an evaporation pond or a centralized treatment unit; when the ratio is not greater than 1.30 and the leaching liquid meets the requirements for reuse water quality, the leaching liquid is mixed with the ecologically purified effluent and reused in the underground drip irrigation system.

8. The method according to claim 1, characterized in that, In step S7, drip irrigation under film or integrated water, fertilizer and salt management mode is adopted.

9. The application of the method according to any one of claims 1 to 8 in the improvement of saline-alkali farmland soil in arid and semi-arid areas of Northwest China, ecological restoration of coastal saline-alkali land, water-saving agricultural production, or low-cost resource utilization of rural domestic sewage.

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