Preparation method of biogas slurry-based organic aqueous solution fertilizer

Abstract

The invention discloses a preparation method of a biogas slurry-based organic aqueous solution fertilizer, which comprises the following steps: filtering fresh anaerobic digestion biogas slurry, adding a dispersing agent, adjusting the pH value, mixing with a composite high-temperature bacterial agent, carrying out primary fermentation, carrying out secondary electrochemical treatment, adjusting the content ratio of nitrogen to phosphorus to potassium, adding a seaweed extract, a water-retaining agent and a preservative, and carrying out secondary electrochemical treatment to obtain the biogas slurry-based organic aqueous solution fertilizer. Uniformly mixing and curing to obtain an organic water solution fertilizer; wherein the composite high-temperature fungicide is prepared by compounding bacillus licheniformis, sporotrichum thermophile and azotobacter chroococcum according to the viable count ratio of 3: 1: 1. The high-end functional liquid fertilizer product with market competitiveness is prepared by efficiently treating biogas slurry through staged and multi-technology cooperation, decomposing organic matters and plant toxic substances and passivating metals.

Description

Technical Field

[0001] This invention belongs to the field of biogas slurry resource utilization and organic water-soluble fertilizer preparation technology, specifically relating to a method for preparing biogas slurry-based organic aqueous solution fertilizer. Background Technology

[0002] Biogas slurry not only contains abundant macronutrients such as nitrogen, phosphorus, and potassium, as well as micronutrients such as zinc, but also 17 kinds of amino acids and active enzymes. These nutrients are primarily present in readily available forms, therefore, biogas slurry has a strong ability to provide rapid nutrition and a high nutrient utilization rate, making it a multi-element, fast-acting compound fertilizer that can be quickly absorbed and utilized by animals and crops. Theoretically, it is an ideal raw material for organic liquid fertilizer. However, significant bottlenecks exist in practical applications: insufficient stabilization and composting of biogas slurry pose risks when applied directly; and the presence of sodium (Na) in biogas slurry... + Cl - SO4 2- The content of heavy metals such as Cu and Zn may exceed the standard. If it is returned to the field or made into fertilizer without treatment, it will easily cause soil salinization and heavy metal pollution, posing a potential environmental safety hazard. The biogas slurry contains pathogens such as E. coli, Salmonella, and parasite eggs, as well as antibiotic residues. Long-term application may induce the spread of soil resistance genes (ARGs), threatening the ecological environment and public health.

[0003] In existing technologies, the closest approach is to use aerobic fermentation to further treat the biogas slurry. Aeration promotes the degradation of residual organic matter by microorganisms, reduces toxic substances, and improves fertilizer stability. For example, Chinese invention patent CN201811347891.2 discloses a process for producing high-concentration biogas slurry compound microbial liquid fertilizer. This process involves adding Bacillus subtilis and Bacillus licheniformis to the biogas slurry from cattle farms for fermentation to produce liquid fertilizer. Furthermore, the presence of compound fertilizer is necessary to obtain a nutrient-rich and balanced liquid fertilizer. Another common approach is physicochemical treatment. For example, Chinese utility model patent CN202020756812.X discloses a complete set of equipment for the resource utilization treatment of biogas slurry and biogas residue. The proposed complete set of equipment for the resource utilization treatment of biogas slurry processes biogas slurry and biogas residue through steps such as chemical hydrolysis, centrifugal separation, filtration, and batching (adding trace elements, nitrogen, phosphorus, and potassium fertilizers, and adding stabilizers). Finally, liquid or solid organic soil conditioners and organic fertilizer compound water-soluble fertilizers are produced, realizing the complete resource utilization of biogas slurry and biogas residue without secondary pollution.

[0004] However, these existing technologies have significant drawbacks. Simple aerobic fermentation has a long processing cycle (typically 15-30 days), requires a large area, and has limited effectiveness in removing residual pollutants such as heavy metals and antibiotics from the biogas slurry, potentially leading to environmental safety hazards in the final product. Furthermore, the fermentation process is easily affected by ambient temperature, exhibiting poor stability, especially with a sharp decline in efficiency at low temperatures. While purely physicochemical methods, such as chemical hydrolysis and separation, can quickly remove certain pollutants, they are costly to operate and may damage natural active substances in the biogas slurry (such as humic acid and amino acids), resulting in a product with limited fertilizer efficacy and losing its potential as a high-end liquid fertilizer. The root of these shortcomings lies in the failure to organically combine the thoroughness of biological treatment with the high efficiency of physicochemical treatment, and the lack of targeted stabilization strategies for the complex components of biogas slurry. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing biogas slurry-based organic aqueous solution fertilizer. Through a phased and multi-technology synergy, the method efficiently processes biogas slurry to decompose organic matter and phytotoxic substances, passivates metals, and prepares a high-end functional liquid fertilizer product with market competitiveness.

[0006] To achieve the above objectives, the present invention provides a method for preparing biogas slurry-based organic aqueous solution fertilizer, comprising the following steps: (1) Pretreatment and preparation: After filtering the fresh anaerobic digestion slurry through a 100-mesh sieve, a dispersant is added and the pH is adjusted to 6.5-7.0 to obtain the prepared slurry; (2) Primary fermentation: The prepared biogas slurry is heated to 45°C and then a compound high-temperature bacterial agent is added to it. Fermentation is carried out by intermittent aeration (stirring speed 60-100 rpm) to obtain primary fermentation liquid; (3) Secondary electrochemical treatment: The primary fermentation broth is filtered under 0.5-1.0 MPa pressure (filter cloth pore size 200 mesh) and then electrochemically treated, and then finely filtered through a 5 μm filter cartridge to obtain the secondary treated broth; (4) Stabilization treatment: Adjust the nitrogen, phosphorus and potassium content ratio of the secondary treatment solution, add seaweed extract, water-retaining agent and preservative, stir at a constant speed for 2 hours to make it evenly mixed, and then seal and mature at 50℃ for 48 hours to obtain organic aqueous solution fertilizer.

[0007] Preferably, the volume ratio of the fresh anaerobic digester slurry to the dispersant in step (1) is 100:(3-7); the reagent for adjusting the pH value is any one of 0.1 mol / L citric acid solution, 0.1 mol / L malic acid solution and 0.1 mol / L lactic acid solution.

[0008] More preferably, the total solids content (TS) of the fresh anaerobic digestate is less than 3%; the dispersant is any one of sodium lignosulfonate, sodium naphthalenesulfonate formaldehyde condensate, and sodium polyacrylate.

[0009] Preferably, the volume ratio of the prepared biogas slurry to the compound high-temperature bacterial agent in step (2) is 100:(1-3); the intermittent aeration mode is aeration for 10-20 minutes and stopping for 40-50 minutes.

[0010] More preferably, the compound high-temperature bacterial agent is composed of Bacillus licheniformis, Laterophyton floccosum, and Azotobacter chrysogenum in a viable count ratio of 3:1:1, with a total viable count of 5 × 10⁻⁶. 9 -1×10 11 CFU / g.

[0011] More preferably, the aeration rate is 0.8-1.0 vvm (volume of gas / volume of liquid per minute).

[0012] Preferably, the stirring speed during the primary fermentation in step (2) is 60 rpm and the fermentation time is 72 h.

[0013] Preferably, the pressure of the pressure filtration in step (3) is 0.5-1.0 MPa, and the filter cloth pore size is 200 mesh.

[0014] Preferably, the electrochemical treatment conditions in step (3) are as follows: the electrode is a titanium-based lead dioxide electrode, and the current density is 10-20 mA / cm². 2 The electrode spacing is 1 cm, and the hydraulic residence time is 25-40 min.

[0015] More preferably, the electrochemical treatment uses a cyclone electrochemical reactor with a rotation speed of 300-500 rpm and a treatment flow rate of 5-10 L / h.

[0016] Preferably, the nitrogen-phosphorus-potassium ratio in step (4) is the mass ratio of N-P2O5-K2O (2-4):(1-2):(1.5-2.5), and the reagents for adjusting the nitrogen-phosphorus-potassium ratio are potassium dihydrogen phosphate and / or potassium sulfate.

[0017] Preferably, the volume ratio of the secondary treatment liquid in step (4) to the seaweed extract, water-retaining agent and preservative is 100:(0.3-0.7):(0.1-0.3):(0.03-0.07).

[0018] More preferably, the water-retaining agent is any one of polyglutamic acid, sodium carboxymethyl cellulose, and acrylamide-potassium acrylate copolymer; and the preservative is any one of potassium sorbate, sodium dehydroacetate, and sodium benzoate.

[0019] The beneficial effects of this invention are as follows: 1. Utilizing a specific compound high-temperature microbial agent for efficient biodegradation of biogas slurry, it rapidly degrades residual volatile fatty acids, small-molecule alcohols and ketones, and other easily decomposable organic matter, eliminating phytotoxicity and initiating the conversion of organic nitrogen into a more stable form, achieving rapid start-up and initial stabilization. Simultaneously, the compound high-temperature microbial agent efficiently removes most easily decomposable organic matter and phytotoxic substances within a short time (72 hours), significantly shortening the time required for traditional aerobic fermentation, reducing energy consumption and land area. The nitrogen-fixing bacteria within also retain nitrogen to a certain extent, reducing ammonia volatilization loss.

[0020] 2. Low-intensity electrochemical treatment of the primary fermentation broth allows the active chlorine and hydroxyl free radicals generated during electrolysis to effectively degrade stubborn pollutants (such as antibiotics) that are difficult to break down in the preceding biological treatment, and to destroy the cell structure of pathogenic microorganisms, thereby efficiently killing pathogens and ensuring the biosafety of the product. Electrochemical action (electrocoagulation) breaks down free heavy metal ions (such as Cu) 2+ Zn 2+ The Fe(OH)3 and Al(OH)3 micro-flocs generated by the electrode reaction undergo co-precipitation or adsorption to achieve deep passivation. They are then removed by forming stable precipitates through electrocoagulation. This step solves the heavy metal risk problem that cannot be solved by biological methods alone, and is more environmentally friendly than adding chemical passivating agents, with no residue introduced.

[0021] 3. During the nutrient fortification stage, the addition of natural synergists such as seaweed extract and polyglutamic acid not only replenishes nutrients but also endows the product with additional functions such as promoting plant growth, improving stress resistance, and improving soil. This upgrades it from ordinary organic liquid fertilizer to a high-end functional liquid fertilizer product with market competitiveness. The product has stable physicochemical properties, no suspended solids or odor, and is easy to dilute and apply through drip irrigation and spraying systems.

[0022] 4. This invention avoids high pressure, high temperature or expensive advanced oxidation technologies (such as ozone, Fenton). The cyclone electrochemical device has relatively low energy consumption, compact equipment, simple operation, and reduced overall operating costs. Moreover, the entire process cycle is short, the cost is controllable, and there is no secondary pollution. Detailed Implementation

[0023] The technical solution of the present invention will be further explained and described below with reference to specific embodiments. It is worth noting that the following embodiments are only preferred embodiments of the present invention and should not be construed as limiting the present invention. The scope of protection of the present invention should be determined by the contents of the claims. Modifications and substitutions made by those skilled in the art to the technical solution of the present invention without creative effort all fall within the scope of protection of the present invention.

[0024] In the following examples, the fresh anaerobic digestion slurry is derived from the anaerobic digestion system of large-scale livestock and poultry farms (such as dairy farms and pig farms), and the total solids content (TS) is 2.5%.

[0025] Example 1 (1) Pretreatment and preparation: Take fresh anaerobic digestion liquid (total solids content (TS) of 2.5% (mass percentage)) from the anaerobic digestion system of a large-scale dairy farm and filter it through a 100-mesh sieve to remove large particulate suspended matter. Then add 5% sodium lignosulfonate by volume of the digestion liquid and add 0.1 mol / L citric acid solution to adjust the pH to 7.0 to obtain the prepared digestion liquid. (2) Preparation of compound high-temperature bacterial agent: Bacillus licheniformis, thermophilic laterospora, and azotobacter brownii were compounded in a live bacteria ratio of 3:1:1 to form a compound high-temperature bacterial agent, and the total live bacteria count of the agent was 5×10⁻⁶. 9 CFU / g; (3) Primary fermentation: The prepared biogas slurry is pumped into a closed fermentation reactor (with aeration and stirring device), heated to 45°C, and 2% of the volume of the prepared biogas slurry is added with compound high-temperature bacteria agent. Aeration is carried out intermittently at a rate of 0.9 vvm (aeration for 15 min, stop for 45 min), and fermentation is carried out at 60 rpm for 72 h to obtain primary fermentation liquid. (4) Secondary electrochemical treatment: The primary fermentation broth was pressure filtered (pressure 0.8 MPa, filter cloth pore size 200 mesh) to obtain a clear liquid, which was then pumped into a cyclone electrochemical reactor. The electrodes were titanium-based lead dioxide electrodes; the rotation speed was 400 rpm; the treatment flow rate was 8 L / h, and the current density was 15 mA / cm². 2 Electrochemical treatment was carried out with an electrode spacing of 1 cm and a hydraulic retention time of 30 min. After treatment, the solution was finely filtered through a 5 μm filter to obtain a secondary treated solution. (5) Stabilization treatment: Pump the secondary treatment liquid into the mixing tank, add potassium dihydrogen phosphate and potassium sulfate to make the mass ratio of N-P2O5-K2O 3:1:2, then add 0.5% of seaweed extract, 0.2% of polyglutamic acid and 0.05% of potassium sorbate by volume of the liquid, stir at a constant speed (80 rpm) for 2 hours to make it evenly mixed, and then seal and mature at 50℃ for 48 hours to obtain organic aqueous fertilizer.

[0026] Example 2 (1) Pretreatment and preparation: Take fresh anaerobic digestion liquid and filter it through a 100-mesh sieve to remove large suspended particles. Then add 3% sodium lignosulfonate by volume of the digestion liquid and adjust the pH to 6.5 with 0.1 mol / L citric acid solution to obtain prepared digestion liquid. (2) Preparation of compound high-temperature bacterial agent: Bacillus licheniformis, Thermophilic Lateral Floodspora, and Azotobacter chrysogenum were compounded in a viable count ratio of 3:1:1 to form a compound high-temperature bacterial agent, and the total viable count of the agent was approximately 5 × 10⁻⁶. 9 CFU / g; (3) Primary fermentation: The prepared biogas slurry is pumped into a closed fermentation reactor (with aeration and stirring device), heated to 45°C, 1% of the compound high-temperature bacterial agent of the prepared biogas slurry is added, and aeration is carried out intermittently at a rate of 0.8 vvm (aeration for 10 min, stop for 40 min), and fermentation is carried out at 60 rpm for 72 h to obtain the primary fermentation liquid. (4) Secondary electrochemical treatment: The primary fermentation broth was pressure filtered (pressure 0.8 MPa, filter cloth pore size 200 mesh) to obtain a clear liquid, which was then pumped into a cyclone electrochemical reactor (the electrode used was a titanium-based lead dioxide electrode, the rotation speed was 300 rpm, and the treatment flow rate was 5 L / h), and a current density of 10 mA / cm² was applied. 2 Electrochemical treatment was carried out with an electrode spacing of 1 cm and a hydraulic retention time of 25 min. After treatment, the solution was finely filtered through a 5 μm filter to obtain a secondary treated solution. (5) Stabilization treatment: Pump the secondary treatment liquid into the mixing tank, add potassium dihydrogen phosphate and potassium sulfate to make the mass ratio of N-P2O5-K2O 2:1:1.5, then add 0.3% seaweed extract, 0.1% polyglutamic acid and 0.03% potassium sorbate by volume of the liquid, stir at a constant speed (80 rpm) for 2 hours to make it evenly mixed, and then seal and mature at 50℃ for 48 hours to obtain organic aqueous fertilizer.

[0027] Example 3 (1) Pretreatment and preparation: Take fresh anaerobic digestion liquid and filter it through a 100-mesh sieve to remove large suspended particles. Then add 7% sodium lignosulfonate by volume of the digestion liquid and adjust the pH to 6.5 with 0.1 mol / L citric acid solution to obtain prepared digestion liquid. (2) Preparation of compound high-temperature bacterial agent: Bacillus licheniformis, thermophilic laterospora, and azotobacter brownii were compounded in a live bacteria ratio of 3:1:1 to form a compound high-temperature bacterial agent, and the total live bacteria count of the agent was approximately 1×10⁻⁶. 11 CFU / g; (3) Primary fermentation: The prepared biogas slurry is pumped into a closed fermentation reactor (with aeration and stirring device), heated to 45°C, 3% of the compound high-temperature bacterial agent of the prepared biogas slurry is added, and aeration is carried out intermittently at a rate of 0.8 vvm (aeration for 10 min, stop for 40 min), and fermented at 60 rpm for 72 h to obtain the primary fermentation liquid. (4) Secondary electrochemical treatment: The primary fermentation broth was pressure filtered (pressure 0.8 MPa, filter cloth pore size 200 mesh) to obtain a clear liquid, which was then pumped into a cyclone electrochemical reactor (the electrode used was a titanium-based lead dioxide electrode, the rotation speed was 500 rpm, and the treatment flow rate was 10 L / h), with a current density of 20 mA / cm². 2 Electrochemical treatment was carried out with an electrode spacing of 1 cm and a hydraulic retention time of 40 min. After treatment, the solution was finely filtered through a 5 μm filter to obtain a secondary treated solution. (5) Stabilization treatment: Pump the secondary treatment liquid into the mixing tank, add potassium dihydrogen phosphate and potassium sulfate to make the mass ratio of N-P2O5-K2O 4:2:2.5, then add 0.7% of seaweed extract, 0.3% of polyglutamic acid and 0.07% of potassium sorbate by volume of liquid, stir at a constant speed (100 rpm) for 2 hours to make it evenly mixed, and then seal and mature at 50℃ for 48 hours to obtain organic aqueous fertilizer.

[0028] Comparative Example 1 The method and steps are the same as in Example 1, except that step (1) is omitted. Compound high-temperature bacterial agent is directly added to fresh anaerobic digestion slurry to prepare organic aqueous fertilizer.

[0029] Comparative Example 2 The method and steps are the same as in Example 1, except that the compound high-temperature bacterial agent in step (2) is changed to a compound high-temperature bacterial agent made by mixing Bacillus licheniformis and Thermophilus lateralis at a viable count ratio of 3:1, and the total viable count of the bacterial agent is 5×10⁻⁶. 9 Organic aqueous fertilizer was prepared by using CFU / g.

[0030] Comparative Example 3 The method and steps are the same as in Example 1, except that the compound high-temperature bacterial agent in step (2) is changed to a compound high-temperature bacterial agent made by mixing thermophilic laterospora and azotobacter brownii in a 1:1 ratio of viable bacteria, and the total viable bacteria count of the bacterial agent is 5 × 10⁻⁶. 9 Organic aqueous fertilizer was prepared by using CFU / g.

[0031] Comparative Example 4 The method and steps are the same as in Example 1, except that the compound high-temperature bacterial agent in step (2) is changed to a compound high-temperature bacterial agent made by mixing Bacillus licheniformis and Azotobacter brownii in a live bacteria ratio of 3:1, and the total live bacteria count of the bacterial agent is 5×10⁻⁶. 9 Organic aqueous fertilizer was prepared by using CFU / g.

[0032] Comparative Example 5 The method and steps are the same as in Example 1, except that step (2) primary fermentation is omitted, and the pretreated biogas slurry is directly subjected to step (3) electrochemical treatment.

[0033] Comparative Example 6 The method and steps are the same as in Example 1, except that step (4) is omitted and only fermentation is performed to prepare organic aqueous fertilizer.

[0034] Comparative Example 7 The method and steps are the same as in Example 1, except that the current density in step (4) is changed to 30 mA / cm. 2 (Exceeding the maximum limit), an organic aqueous solution fertilizer was prepared.

[0035] Comparative Example 8 The method and steps are the same as in Example 1, except that step (5) stabilization treatment is omitted to prepare organic aqueous fertilizer.

[0036] Example 4 The organic aqueous solution fertilizers prepared in the above examples and comparative examples were tested for their physicochemical properties according to the relevant requirements and regulations of "NY 1107-2021 Water-soluble Fertilizers with Macro-elements". The results are shown in Table 1: The organic aqueous solution fertilizer prepared in the above examples and comparative examples was diluted at a mass ratio of 1:100 (liquid fertilizer: water) and applied to the roots of potted plants. The plant type was potted tomato (variety: Zhongshu No. 4), and the application period was during the seedling stage (3-4 true leaves stage). After one application, the growth of the plants was observed for 30 days, including indicators such as plant height, root length, fresh weight, and dry weight. At the same time, the organic fertilizer granules were replaced with conventional compound fertilizer and water as a control. The results are shown in Table 2. Table 1 Physicochemical properties of organic aqueous solution fertilizer

[0037] Note: All percentages above are by mass.

[0038] Table 2. Fertilizer efficacy of organic aqueous solution fertilizer (average value of 5 tomato seedlings)

[0039] The results are shown in Table 1: The pH values ​​of Examples 1-3 were stable at 6.5-7.0, total nitrogen 2.1-4.0%, total phosphorus 1.0-2.0%, potassium content 1.5-2.5%, active component (humic acid) 3.8-5.2%, and heavy metal Cu content ≤8.5mg / kg (meeting the requirements of national standard GB / T17419-2018); the heavy metal Cu content of Comparative Example 1 (untreated) was 25.6mg / kg (significantly higher than the examples, posing an environmental safety hazard), the active component of Comparative Example 5 (unfermented) was only 2.5%, and the active component of Comparative Example 7 (excessive current) was only 1.8% (activity destroyed).

[0040] The results, as shown in Table 2, indicate that the plant height (28.3-35.1 cm), root length (15.6-19.5 cm), fresh weight (38.9-48.2 g / plant), and dry weight (4.9-6.2 g / plant) of Examples 1-3 were significantly better than those of Comparative Examples 1-8 (plant height ≤ 23.1 cm, dry weight ≤ 3.8 g / plant) and commercially available compound fertilizer (plant height 25.6 cm, dry weight 4.3 g / plant). The water control (plant height 18.2 cm, dry weight 2.5 g / plant) was the worst, demonstrating the high efficiency of the liquid fertilizer of this invention.

Claims

1. A method for preparing a biogas slurry-based organic aqueous solution fertilizer, characterized in that: Includes the following steps: (1) Pretreatment and preparation: After filtering the fresh anaerobic digestion slurry, add a dispersant and adjust the pH value to 6.5-7.0 to obtain the prepared slurry; (2) Primary fermentation: After heating the prepared biogas slurry, add compound high-temperature bacteria agent to it, and stir while intermittently aerating to obtain primary fermentation liquid; (3) Secondary electrochemical treatment: After filtering the primary fermentation broth, electrochemical treatment is performed, and the secondary treated broth is obtained by fine filtration; (4) Stabilization treatment: Adjust the nitrogen, phosphorus and potassium content ratio of the secondary treatment solution, add seaweed extract, water-retaining agent and preservative, mix and mature to obtain organic aqueous solution fertilizer.

2. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 1, characterized in that: The volume ratio of fresh anaerobic digester slurry to dispersant in step (1) is 100:(3-7); the reagent for adjusting the pH value is any one of citric acid, malic acid and lactic acid solution.

3. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 2, characterized in that: The total solids content of the fresh anaerobic digestate is <3%; the dispersant is any one of sodium lignosulfonate, sodium naphthalenesulfonate formaldehyde condensate, and sodium polyacrylate.

4. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 1, characterized in that: The volume ratio of the prepared biogas slurry to the compound high-temperature bacterial agent in step (2) is 100:(1-3); the intermittent aeration mode is aeration for 10-20 minutes and stopping for 40-50 minutes.

5. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 4, characterized in that: The compound high-temperature bacterial agent is composed of Bacillus licheniformis, Thermophilic Lateral Glomerulosa, and Azotobacter chrysogenum in a viable count ratio of 3:1:1, with a total viable count of 5×10⁻⁶. 9 -1×10 11 CFU / g.

6. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 4, characterized in that: The aeration rate is 0.8-1.0 vvm.

7. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 1, characterized in that: The electrochemical treatment conditions described in step (3) are as follows: the electrode is a titanium-based lead dioxide electrode, and the current density is 10-20 mA / cm². 2 The electrode spacing is 1cm, and the hydraulic residence time is 25-40min.

8. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 1, characterized in that: The nitrogen-phosphorus-potassium ratio in step (4) is N-P2O5-K2O mass ratio of (2-4):(1-2):(1.5-2.5), and the reagents for adjusting the nitrogen-phosphorus-potassium ratio are potassium dihydrogen phosphate and / or potassium sulfate.

9. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 1, characterized in that: The volume ratio of the secondary treatment liquid, seaweed extract, water-retaining agent, and preservative in step (4) is 100:(0.3-0.7):(0.1-0.3):(0.03-0.07).

10. The method for preparing a biogas slurry-based organic aqueous solution fertilizer according to claim 9, characterized in that: The water-retaining agent is any one of polyglutamic acid, sodium carboxymethyl cellulose, and acrylamide-potassium acrylate copolymer; the preservative is any one of potassium sorbate, sodium dehydroacetate, and sodium benzoate.

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