Method for preparing water-soluble fulvic acid fertilizer based on anaerobic fermentation residues, water-soluble fertilizer and application

Abstract

The present application relates to the field of agricultural waste resource utilization, and particularly relates to a method for preparing fulvic acid water-soluble fertilizer based on anaerobic fermentation residues. The method for preparing fulvic acid water-soluble fertilizer rich in fulvic acid based on anaerobic fermentation residues comprises the following steps: collecting anaerobic fermentation residues, performing solid-liquid separation to obtain biogas residue and biogas slurry, wherein the raw material of the anaerobic fermentation residues comprises livestock and poultry manure; performing membrane filtration concentration treatment on the biogas slurry to obtain concentrated retentate; mixing the biogas residue with the concentrated retentate, and performing hydrothermal reconstruction reaction to obtain the fulvic acid water-soluble fertilizer. The method of the present application precisely controls the membrane concentration and hydrothermal temperature, effectively overcomes the bottleneck that high-activity components represented by fulvic acid are prone to pyrolysis and loss at high temperature, realizes that the ratio of high-value fulvic acid to humic acid is close to 1, and ensures the abundance of high-activity and readily available components in the product, thereby laying a material foundation for producing high-value new full water-soluble fertilizer.

Description

Technical Field

[0001] This invention relates to the field of agricultural waste resource utilization technology, specifically to a method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residue, the water-soluble fertilizer, and its application. Background Technology

[0002] With the rapid development of modern urban agriculture, the discharge of livestock and poultry manure from large-scale farming has posed a severe challenge to ecological and environmental safety. Anaerobic fermentation, as one of the mainstream technologies for organic waste treatment, plays a crucial role in realizing the energy conversion and harmless utilization of manure in large-scale farming scenarios. However, the residues and liquids produced during anaerobic fermentation have significant limitations in fertilizer application. The high water content and low concentration of effective nutrients in the liquid lead to high transportation and storage costs. Furthermore, the residue itself contains recalcitrant macromolecules such as lignocellulose, making it difficult to provide readily available nutrients to the soil. The fermentation process also struggles to effectively remove heavy metals and some stubborn antibiotics, resulting in a high risk of secondary pollution with long-term, extensive application.

[0003] To overcome the aforementioned bottlenecks, biogas slurry concentration and hydrothermal treatment technologies have been introduced into the field of agricultural waste resource utilization. Membrane separation and other biogas slurry concentration technologies can effectively reduce volume, enrich nitrogen, phosphorus, and potassium nutrients and water-soluble small molecule precursors, significantly reducing storage and transportation costs. Hydrothermal treatment technology demonstrates unique advantages in treating high-moisture solid waste such as biogas residue. It eliminates the need for pre-drying and, in a subcritical aquatic environment, destroys lignocellulose macromolecules, kills pathogens, and passivates heavy metals through thermochemical reactions such as hydrolysis, decarboxylation, and Maillard reactions. Introducing hydrothermal treatment technology to directionally convert it into functional fertilizers rich in active molecules such as humic acid and fulvic acid has significant agricultural application implications. However, the above technologies, applied alone or simply in combination, still cannot meet the demand for creating high-quality fertilizers. Single concentration technology cannot solve the problem of rapid conversion of large molecules in biogas residue. Furthermore, direct hydrothermal mixing of untreated biogas slurry and biogas residue not only limits the humification efficiency due to the low concentration of organic substrate, but also easily leads to excessive condensation during the reaction, generating a large amount of insoluble solids, which severely restricts its engineering application as a water-soluble fertilizer.

[0004] Humic acids are key to improving soil and enhancing its water and fertilizer retention capacity. Among them, fulvic acid, as the core water-soluble component with the smallest molecular weight and the richest oxygen-containing functional groups, has particularly outstanding agricultural application value. Fulvic acid is fully water-soluble, perfectly suited for integrated fertigation drip irrigation systems, and can directly penetrate plant cell walls for rapid absorption. Therefore, how to break down the large molecular barriers in high-intensity thermochemical reactions, block the formation of insoluble substances, and simultaneously avoid excessive pyrolysis and loss of highly active fulvic acid has become a key technological challenge in this field. In summary, exploring a deep treatment process that couples biogas slurry membrane concentration with hydrothermal humification is not only an effective way to overcome the bottlenecks in the harmless and reduced-volume disposal of fermentation residues, but also an important direction for creating high-value-added fulvic acid liquid fertilizers. Given the current lack of systematic research on this dual-effect synergistic mechanism, there is an urgent need in this field to develop a novel treatment method that can precisely control the humification process and achieve a large-scale enrichment of highly active components. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide a method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residue, the water-soluble fertilizer, and its application.

[0006] To achieve the above and other related objectives, the first aspect of this application provides a method for preparing humic acid-rich water-soluble fertilizer based on anaerobic fermentation residue, comprising the following steps:

[0007] 1) Collect the anaerobic fermentation residue, perform solid-liquid separation to obtain biogas residue and biogas slurry, wherein the raw materials of the anaerobic fermentation residue include livestock and poultry manure;

[0008] 2) The biogas slurry described in 1) is subjected to membrane filtration and concentration treatment to obtain concentrated retentate;

[0009] 3) Mix the biogas residue described in 1) with the concentrated retentate obtained in 2) and carry out a hydrothermal reconstruction reaction to obtain humic acid water-soluble fertilizer.

[0010] The second aspect of this application provides a humic acid water-soluble fertilizer prepared by the above method.

[0011] The third aspect of this invention provides the application of the above-mentioned humic acid water-soluble fertilizer in crop cultivation.

[0012] In summary, this invention provides a method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residues, the water-soluble fertilizer itself, and its application, achieving the following beneficial effects:

[0013] Firstly, in terms of improving the active components and fertilizer value, the high-pressure reverse osmosis concentration process can significantly improve the conversion efficiency of fulvic acid in organic matter. Its pretreatment is conducive to the directional generation of humic acid-like substances from organic waste through Maillard reaction in a hydrothermal environment, thereby maximizing the content of humic acid and fulvic acid in hydrothermal products.

[0014] Secondly, in terms of improving the physical properties and engineering applicability of fertilizers, the high-pressure reverse osmosis biogas slurry pretreatment process significantly improves the water solubility of hydrothermal products by blocking the formation path of insoluble substances in advance, reducing the insoluble solids yield to a low range of less than 0.36%, and eliminating the mechanical blockage risk in modern integrated water and fertilizer drip irrigation systems.

[0015] Ultimately, by precisely controlling the membrane concentration and hydrothermal temperature, the bottleneck of easy thermal decomposition and loss of highly active components, represented by fulvic acid, at high temperatures was effectively overcome. Furthermore, the ratio of high-value fulvic acid to humic acid was made close to 1, ensuring the abundance of highly active and fast-acting components in the product and laying the material foundation for the production of high-value-added new fully water-soluble fertilizers. Attached Figure Description

[0016] Figure 1 This is a flowchart of the method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residues according to the present invention.

[0017] Figure 2 Figure 1 shows the effect of biogas slurry concentration treatment and different reaction temperatures on CO2 free carbon release.

[0018] Figure 3 Figure 1 shows the effect of biogas slurry concentration treatment and different reaction temperatures on humic acid production.

[0019] Figure 4 Figure 1 shows the effect of biogas slurry concentration treatment and different reaction temperatures on fulvic acid yield.

[0020] Figure 5 This is a graph showing the effect of biogas slurry concentration treatment and different reaction temperatures on the solid content of hydrothermal products. Detailed Implementation

[0021] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Before further describing specific embodiments of the present invention, it should be understood that the scope of protection of the present invention is not limited to the specific embodiments described below; it should also be understood that the terminology used in the embodiments of the present invention is for describing specific embodiments and not for limiting the scope of protection of the present invention; in the specification and claims of the present invention, unless otherwise expressly stated in the text, the singular forms "a", "an" and "this" include the plural forms.

[0023] When numerical ranges are given in the embodiments, it should be understood that, unless otherwise stated in the present invention, both endpoints of each numerical range and any value between the two endpoints may be selected. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art. In addition to the specific methods, apparatus, and materials used in the embodiments, based on the knowledge of the prior art possessed by one of ordinary skill in the art and the description of this invention, any prior art methods, apparatus, and materials similar to or equivalent to those described, apparatus, and materials in the embodiments of this invention may be used to implement the present invention.

[0024] This invention first provides a method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residue, comprising the following steps:

[0025] 1) Collect the anaerobic fermentation residue, perform solid-liquid separation, and obtain biogas residue and biogas slurry;

[0026] 2) The biogas slurry described in 1) is subjected to membrane filtration and concentration treatment to obtain concentrated retentate;

[0027] 3) Mix the biogas residue described in 1) with the concentrated retentate obtained in 2) and carry out a hydrothermal reconstruction reaction to obtain humic acid water-soluble fertilizer.

[0028] In this invention, the anaerobic fermentation residue is an organic product generated from the anaerobic fermentation of livestock and poultry manure. It not only contains abundant macroelements essential for plant growth—nitrogen (N), phosphorus (P), and potassium (K)—but also water-soluble medium and trace elements formed through natural fermentation—calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), and manganese (Mn). The biogas slurry also contains various amino acids, vitamins, proteins, gibberellins, auxins, sugars, nucleic acids, and antibiotics, many of which are physiologically active substances that regulate crop growth and development and prevent crop diseases. The ammonia, ammonium salts, and antibiotics in the biogas slurry can reduce egg laying on crops, inhibit insect egg growth, and reduce insect damage.

[0029] In some embodiments, the livestock manure may include any one or more of chicken manure, duck manure, goose manure, or pig manure. In some embodiments, preferably, the manure may be selected from pig manure.

[0030] In some embodiments of the present invention, the membrane filtration concentration includes one or more of ultrafiltration concentration, nanofiltration concentration, and high-pressure reverse osmosis concentration.

[0031] In this invention, ultrafiltration concentration refers to a method of concentrating a solution by using a membrane with a specific pore size as a selective barrier under pressure, allowing solvents (usually water) and small molecule solutes to permeate through the membrane while retaining large molecule solutes, thereby concentrating the latter in the solution. In practical operation, ultrafiltration membranes with pore sizes of 1-100 nm are typically used as the separation medium. Under a transmembrane pressure difference of 0.1-0.5 MPa, water and small molecules (salts, solvents, etc.) permeate through the membrane pores, while retaining large molecules (proteins, enzymes, colloids, polysaccharides, etc.) with molecular weights of 1-300 kDa. This membrane concentration method achieves the purpose of "dehydration-concentration" under ambient temperature and no phase change conditions. It can include dead-end ultrafiltration concentration, cross-flow ultrafiltration concentration, batch concentration, fed-batch / diafiltration concentration, diafiltration + concentration, and multi-stage concentration.

[0032] In this invention, nanofiltration concentration refers to the separation process of liquids using nanofiltration membranes with nanoscale micropores and charged membrane surfaces under pressure. Its separation principle combines molecular-size sieving and charge effects. The membrane surface is typically negatively charged, which is effective against divalent anions (such as SO42-). 2- It has high selective retention capacity, but is less effective against monovalent ions (such as Cl-). - Na + It has a low rejection rate or even partial permeation. This characteristic gives it an irreplaceable advantage in selective salt concentration.

[0033] In this invention, high-pressure reverse osmosis concentration refers to the use of a reverse osmosis membrane where solvent water molecules permeate under pressure to achieve concentration. Reverse osmosis, also known as reverse permeation, is the opposite of normal osmosis. Pressure is applied to the feed solution on one side of the membrane; when the pressure exceeds its osmotic pressure, the solvent will permeate in the opposite direction of natural osmosis. This process can be carried out at room temperature and does not involve phase changes, thus preserving the original properties of substances to a greater extent, improving concentration efficiency while reducing energy consumption. High-pressure reverse osmosis can effectively retain most inorganic salt ions and organic compounds with molecular weights greater than 100 Daltons, thereby achieving efficient retention and concentration of short-chain reducing sugars, amino acids, most volatile fatty acids, and metallic inorganic salts.

[0034] In some embodiments of the present invention, the membrane filtration concentration is selected from high-pressure reverse osmosis concentration.

[0035] In some specific embodiments, in step 2), the high-pressure reverse osmosis concentration uses a reverse osmosis membrane with a pore size of 0.2~0.8 nm. Preferably, it is a polyamide composite material with an equivalent pore size in the sub-nanometer range (typically less than 1 nm).

[0036] Preferably, the pore size of the reverse osmosis membrane used for high-pressure reverse osmosis concentration can be approximately 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, or 0.8 nm. It also includes any range characterized by combinations of the above values, which will not be elaborated here.

[0037] In some specific embodiments, in step 2), the high-pressure reverse osmosis concentration uses a reverse osmosis membrane made of polyamide film composite material.

[0038] In some specific embodiments, in step 2), the temperature of the high-pressure reverse osmosis concentration is 20~40°C. Preferably, the temperature can be approximately 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40°C. Any range characterized by combinations of the above values ​​is also included, which will not be elaborated here.

[0039] In some specific embodiments, in step 2), the pressure during high-pressure reverse osmosis concentration is 70~120 bar.

[0040] Preferably, the pressure during high-pressure reverse osmosis concentration is 70~120 bar; for example, the pressure can be 70 bar, 80 bar, 90 bar, 100 bar, 110 bar or 120 bar.

[0041] More preferably, the pressure during high-pressure reverse osmosis concentration is 90 bar.

[0042] In a specific embodiment, in step 2), the concentration factor of high-pressure reverse osmosis concentration is 8.0-15.0;

[0043] In specific embodiments, the concentration factor is 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, or 15.0.

[0044] Furthermore, the concentration factor in step (2) is 8.0-15.0. After this process, the humic acid substances in the biogas slurry can be retained and highly concentrated, and the humic acid substances are enriched by high-pressure reverse osmosis treatment. Secondly, high-pressure reverse osmosis concentration can retain and concentrate inorganic metal salts such as potassium and calcium, which are tightly bound to the fulvic acid components to form salt bridge protection and improve the structural stability of fulvic acid.

[0045] In this invention, in step 3), the mixing ratio of the biogas residue and the concentrated retentate is 1:2 to 1:9.

[0046] Preferably, the mixing ratio of the biogas residue and the concentrated retentate can be 1:2, 1:3, 1:5, 1:7, 1:8, or 1:9. It also includes any range characterized by combinations of the above values, which will not be elaborated here.

[0047] More preferably, the mixing ratio of the biogas residue to the concentrated retentate is 1:3 to 9.

[0048] In some embodiments, the temperature of the hydrothermal reaction is 160-220°C; preferably, it can be 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, or 220°C. Any range characterized by combinations of the above values ​​is also included, which will not be elaborated here.

[0049] More preferably, the temperature of the hydrothermal reaction is 180~200℃.

[0050] In some embodiments, the hydrothermal reaction time is 1-3 hours; preferably, it can be 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours. Any range characterized by combinations of the above values ​​is also included, which will not be elaborated here.

[0051] More preferably, the hydrothermal reaction takes 2 hours.

[0052] The present invention also provides a fulvic acid water-soluble fertilizer prepared by the above method. In the fulvic acid water-soluble fertilizer, the mass concentration of fulvic acid is 120-160 g / L; and / or, the mass concentration of humic acid is 100-130 g / L; and / or, the mass ratio of fulvic acid to humic acid is 0.9-1.1.

[0053] In some embodiments, the mass concentration of fulvic acid in the fulvic acid water-soluble fertilizer can be 120~125 g / L, 125~130 g / L, 130~135 g / L, 135~140 g / L, 140~145 g / L, 145~150 g / L, 150~155 g / L, or 155~160 g / L.

[0054] In some embodiments, the mass concentration of humic acid in the fulvic acid water-soluble fertilizer can be 100~105g / L, 105~110g / L, 110~115g / L, 115~120g / L, 120~125g / L, or 125~130g / L.

[0055] In some embodiments, the mass ratio of fulvic acid to humic acid can be 0.9~1.0 or 1.0~1.1.

[0056] The present invention also provides the application of the above-mentioned humic acid water-soluble fertilizer in crop cultivation.

[0057] In some implementations, the crop may be selected from vegetables, fruits, and grains.

[0058] The water-soluble fertilizer of this invention contains a higher content of humic acid and fulvic acid compared to fertilizers using membrane separation or hydrothermal treatment technologies, and also has a higher fulvic acid / humic acid ratio. Fulvic acid has a small molecular weight, better water solubility, and high biological activity, allowing it to directly penetrate plant cell walls and be rapidly absorbed. The method of this invention avoids excessive pyrolysis and loss of highly active fulvic acid. High-pressure reverse osmosis concentration retains humic acid, fulvic acid, and inorganic metal salts. During the hydrothermal reaction, the inorganic salts tightly bind with fulvic acid, forming salt bridges for protection, enhancing the structural stability of fulvic acid, and thus increasing the fulvic acid content and FA / HA ratio in the water-soluble fertilizer.

[0059] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0060] Example 1: Comparison of methods for preparing humic acid water-soluble fertilizer

[0061] 1. Raw material pretreatment:

[0062] The biogas residue and biogas slurry were obtained from a large-scale livestock farm in Shanghai, derived from pig manure through anaerobic fermentation and then extracted from the fermentation tank. Samples were stored at 4℃ and brought to room temperature before the experiment.

[0063] The test procedure is as follows Figure 1 As shown, the fermentation residue is first separated into biogas residue and biogas slurry through solid-liquid separation. The separated biogas slurry is then treated by ultrafiltration (UF) and high-pressure reverse osmosis (RO) membranes, respectively, concentrating it by 5 times and 10 times compared to the original biogas slurry, to obtain high-pressure reverse osmosis biogas slurry as the liquid phase feedstock for hydrothermal treatment. The ultrafiltration membrane used is the CYJ-3019-1200 from Nanjing Aiyuqi Membrane Technology Co., Ltd., with an operating temperature of 20~40℃ and a pressure of 2 bar. The high-pressure reverse osmosis membrane is an imported membrane from DuPont Water Treatment, custom-assembled after membrane processing, with a high-pressure reverse osmosis concentration temperature of 20~40℃ and a pressure of 90 bar. This concentration process retains dissolved salt ions and small molecules, especially alkaline inorganic salt ions such as bicarbonate and carbonate. The biogas residue is not further treated and is used as the solid phase feedstock. The physicochemical properties of the biogas slurry obtained from different concentration treatments are shown in Table 1.

[0064] Table 1 Physicochemical properties of biogas slurry after different concentration treatments

[0065]

[0066] Note: The data in the table is in the format of "mean ± standard deviation". Different lowercase letters after the data in the same row indicate the size of a certain indicator between different treatments. For example, 'a' represents the maximum value of this indicator, followed by 'b' and 'c'.

[0067] 2. Hydrothermal reaction:

[0068] The test procedure is as follows Figure 1 As shown, untreated biogas residue and biogas slurry concentrated by high-pressure reverse osmosis were mixed. A mixture of biogas slurry concentrated by ultrafiltration and biogas residue, and a mixture of unconcentrated biogas slurry and biogas residue were used as comparative examples. The solid-liquid ratio of the hydrothermal reaction was fixed at 1:9, ensuring that the solid content in the reaction system reached 10% TS. The mixture was placed in the reaction system, and the hydrothermal reaction time was set to 2 hours. The hydrothermal reaction temperature was controlled within the range of 160℃, 180℃, 200℃, and 220℃.

[0069] 3. Implementation Results:

[0070] Hydrothermal products were prepared using the above-described method. The carbon dioxide gas yield of the hydrothermal products at each temperature stage was measured, and the contents of fulvic acid and humic acid, as well as the solid content of the hydrothermal products after each temperature treatment, were also determined. Simultaneously, the content of heavy metal elements in the hydrothermal products was determined.

[0071] The carbon dioxide gas production test results are provided by Figure 2It can be seen that at a relatively low hydrothermal temperature of 160℃, the decarboxylation and carbon release from the raw biogas slurry and biogas residue after hydrothermal treatment is dominant, while biogas slurry filtration methods such as ultrafiltration and high-pressure reverse osmosis significantly inhibit carbon dioxide release. Similar trends were observed at hydrothermal reaction temperatures of 200℃ and 220℃. However, the CO2 release mechanisms under low-temperature and high-temperature hydrothermal conditions are significantly different. Under low-temperature conditions, biogas slurry concentration treatment increases the viscosity of the liquid phase or changes the dielectric constant of the solvent, locking more carbon elements in the solid or liquid phase and blocking the initial gaseous volatilization. As the hydrothermal temperature increases, the RO biogas slurry group shows a significant reduction in CO2 production at 200℃, which may be mainly because the CO2 produced by organic matter decarboxylation undergoes an in-situ mineralization reaction with the calcium and magnesium ions rich in the liquid phase. When the reaction temperature is further increased to 220℃, the lignocellulose in the biogas residue begins to decompose in large quantities, producing more CO2, exceeding the carbon fixation capacity of the RO group, thus releasing more free carbon. The UF group, under high-temperature conditions, enhanced aromatization and cross-linking reactions, re-fixing free carbon and significantly reducing the loss of gaseous carbon. Notably, at 180℃, the CO2 release from the CK group was lower than that from the biogas slurry concentration group. For the UF group, the retentate was rich in large-molecule proteins and polysaccharides, and its CO2 release increased with the intensification of decarboxylation reactions at 180℃. In contrast, the alkaline inorganic salts enriched in the RO group acted as catalysts, significantly lowering the activation energy threshold of the decarboxylation reaction and accelerating the large-scale hydrolysis and bond-breaking reactions of organic matter. Overall, the mild conditions of 160℃-180℃ not only effectively limited the large-scale generation of pyrolysis toxic byproducts, ensuring biosafety after water-soluble fertilizer application, but also effectively limited the generation of free carbon and enhanced carbon nutrient retention.

[0072] Hydrothermal treatment is a rapid humification process. During this process, the biogas slurry and residue undergo complex hydrolysis, condensation, and Maillard reactions to generate humic acid-like substances rich in oxygen-containing functional groups. The content of fulvic acid and humic acid varies from... Figure 3 and Figure 4It was found that the hydrothermal co-treatment of the RO group's biogas slurry and residue resulted in the most significant increase in humic acid and fulvic acid content; at a hydrothermal reaction temperature of 160 degrees Celsius, the contents of fulvic acid and humic acid increased to 155.95 g / L and 127.77 g / L, respectively. In contrast, the UF group's biogas slurry showed similar results to the untreated slurry, maintaining only a concentration of 25-58 g / L. This is mainly because the dissolved organic matter directly involved in humic acid production is difficult to retain during the ultrafiltration stage. The RO membrane highly concentrated short-chain reducing sugars, amino acids, and volatile fatty acids in the biogas slurry, thus enriching humic acid products after high-pressure reverse osmosis treatment. Simultaneously, the abundant inorganic salts and the accompanying increased pH value in the RO group promoted the conversion of reaction intermediates towards oxygen-containing functional groups such as carboxyl and hydroxyl groups, contributing to humic acid production. Analysis of temperature changes revealed a significant decrease in the content of humic acid and fulvic acid as temperature increased, indicating that a milder hydrothermal reaction temperature is more suitable for the production of small-molecule liquid fertilizers. However, when the hydrothermal temperature reached 220℃, the previously generated humic acid and fulvic acid underwent secondary cracking and decarboxylation reactions, transforming into lower-grade small-molecule organic acids or gases, leading to a significant decrease in both humic and fulvic acid levels. Notably, the fulvic acid content in the RO biogas slurry treatment group remained relatively high and stable at 160℃, 180℃, and 200℃, only decreasing from 128 g / L to 78.8 g / L at 220℃. This demonstrates that RO biogas slurry treatment provides excellent protection and stabilization for highly active fulvic acid, broadening the process operating window.

[0073] In agricultural water-soluble fertilizer applications, fulvic acid has a higher application value than conventional humic acid due to its small molecular weight, better water solubility, and high physiological activity. Therefore, the ratio of fulvic acid to humic acid (FA / HA) directly determines the proportion of highly active and fast-acting components in the fertilizer and is a core indicator for evaluating the fertilizer application value of hydrothermal products. In the untreated CK biogas slurry and biogas residue co-hydrothermal reaction system, as the temperature increases, small-molecule fulvic acid, due to its poor thermal stability, is prone to secondary cracking and decarboxylation volatilization. When the hydrothermal reaction temperature rises from 160℃ to 200℃, the FA / HA ratio of the CK group decreases from 0.94 to 0.64. However, the RO-concentrated biogas slurry group exhibits a different FA / HA production pattern. RO concentration not only enriches organic precursors but also contains high concentrations of inorganic metal salts such as potassium and calcium in its concentrate. In the high-temperature and high-pressure hydrothermal environment, these inorganic salt ions can tightly bind with the newly generated fulvic acid, forming salt bridges for protection, thereby improving the structural thermal stability of fulvic acid. This allows the RO system to not only reach its peak fulvic acid yield and FA / HA ratio at 180℃, but also, even under high-temperature stress of 200℃, when the FA / HA ratio in the CK group dropped to 0.64, the RO system was still able to maintain a stable FA / HA ratio of 0.93. This demonstrates the unique advantages of RO concentration treatment of biogas slurry in terms of resistance to pyrolysis and preservation of activity.

[0074] In modern agricultural production, the water solubility of fertilizers is a core foundation for achieving integrated water and fertilizer management and precision, high-efficiency agriculture. Good fertilizer water solubility not only ensures that nutrients can be delivered evenly and precisely to the root zone of crops with the help of water, but also effectively improves the bioavailability of nutrients and the rate of absorption and conversion by crops. This places strict requirements on the solids content of water-soluble fertilizers. Testing the solids ratio of hydrothermal products reveals that, for example... Figure 5 The results show that the pretreatment process of biogas slurry and the hydrothermal reaction temperature have a significant impact on the water solubility of the products. The high-pressure reverse osmosis (RO) biogas slurry treatment group showed the most significant effect on improving fertilizer water solubility. At all test temperature gradients from 160℃ to 220℃, the solids content of the RO group remained in an extremely low range of 0.06% to 0.36%. This effect is far superior to the CK and UF groups. The underlying mechanism is that the RO membrane highly retains small-molecule organic matter and inorganic salts, which are extremely difficult to undergo large-scale condensation reactions to form insoluble solid carbon in a hydrothermal environment. In contrast, the reaction temperature played a decisive role in the evolution of water solubility in the CK and UF groups. Under the low-temperature hydrothermal conditions of 160℃, the solids content of the CK and UF groups was the highest, ranging from 4.5% to 4.9%. At this temperature, primary cross-linking of macromolecules occurred within the hydrothermal reaction system, generating a large amount of insoluble substances, posing a higher risk for engineering applications. However, as the reaction temperature increases to 200℃-220℃, the energy within the hydrothermal system is sufficient to break the chemical bonds of macromolecules, triggering intense secondary cracking, hydrolysis, and decarboxylation vaporization. This leads to a significant decrease in solid yield and increases the overall water solubility of the remaining liquid phase. In summary, RO concentration technology blocks the formation pathway of insoluble solid phases at the substrate level, making it the optimal solution for agricultural applications. For conventional biogas slurry or ultrafiltrate containing macromolecular precursors, appropriately increasing the hydrothermal temperature (≥180℃) is a key engineering control method to improve the water solubility of the product.

[0075] Example 2 Safety Testing of Water-Soluble Fertilizers

[0076] The heavy metal content in the water-soluble fertilizers obtained in Example 1 through ultrafiltration, high-pressure reverse osmosis concentration, and hydrothermal reaction at various temperature conditions was determined using the methods described in "Limits of Toxic and Hazardous Substances in Fertilizers" (GB 38400-2019). The results are shown in Table 2. Only 0.01-0.28 mg / kg of total arsenic was detected in the water-soluble fertilizers obtained under each concentration and hydrothermal reaction condition; all other arsenic was not detected. This indicates that the water-soluble fertilizer prepared by the method of this invention has good safety and can effectively prevent the negative impact of heavy metal toxic substances on crops.

[0077] Table 2. Heavy metal content in water-soluble fertilizers

[0078]

[0079] The embodiments of this invention are based on the synergistic effect of membrane pretreatment and hydrothermal reaction of anaerobic fermentation residues. Among various treatment methods involving biogas slurry concentration and hydrothermal reaction temperatures, the high-pressure reverse osmosis concentration treatment of biogas slurry and biogas residue exhibits the best hydrothermal performance. Furthermore, at a hydrothermal reaction temperature of 160℃-200℃, it not only maximizes the content of humic acid and fulvic acid in the hydrothermal products but also achieves the most significant improvement in product water solubility. This discovery provides a novel pathway for the high-value utilization of biogas slurry and biogas residue. Compared to conventional treatments, the high-pressure reverse osmosis biogas slurry treatment system not only reduces the insoluble solids yield to below 0.36%, eliminating the risk of drip irrigation clogging, but also effectively overcomes the bottleneck of easily pyrolyzed loss of highly active components at high temperatures. At the optimal reaction temperature of 180℃, it achieves a high-value fulvic acid to humic acid ratio close to 1, providing a feasible method and theoretical support for the high-value and closed-loop resource utilization of livestock and poultry breeding waste.

[0080] In summary, the above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing humic acid water-soluble fertilizer based on anaerobic fermentation residue, comprising the following steps: 1) Collect the anaerobic fermentation residue, perform solid-liquid separation, and obtain biogas residue and biogas slurry; 2) The biogas slurry described in 1) is subjected to membrane filtration and concentration treatment to obtain concentrated retentate; 3) Mix the biogas residue described in 1) with the concentrated retentate obtained in 2) and carry out a hydrothermal reconstruction reaction to obtain humic acid water-soluble fertilizer.

2. The method as described in claim 1, characterized in that, The anaerobic fermentation residue is an organic product generated from the anaerobic fermentation of livestock and poultry manure; preferably, the livestock and poultry manure includes any one or more of chicken manure, duck manure, goose manure, or pig manure.

3. The method as described in claim 1, characterized in that, The membrane filtration concentration includes one or more of ultrafiltration concentration, nanofiltration concentration, and high-pressure reverse osmosis concentration. Preferably, the membrane filtration concentration is selected from high-pressure reverse osmosis concentration.

4. The method as described in claim 3, characterized in that, Step 2) also includes one or more of the following features: 21) The high-pressure reverse osmosis concentration uses a reverse osmosis membrane with a pore size of 0.2~0.8 nm; 22) The high-pressure reverse osmosis concentration temperature is 20~40℃; 23) The pressure during high-pressure reverse osmosis concentration is 70~120 bar; 24) The concentration factor of the high-pressure reverse osmosis concentration is 8.0-15.

0.

5. The method as described in claim 1, characterized in that, In step 3), the mixing ratio of biogas residue and concentrated retentate is 1:2 to 1:

9.

6. The method as described in claim 1, characterized in that, In step 3), the temperature of the hydrothermal reaction is 160-220℃.

7. The method as described in claim 1, characterized in that, In step 3), the hydrothermal reaction takes 1-3 hours.

8. The humic acid water-soluble fertilizer prepared by the method according to any one of claims 1 to 7.

9. The humic acid water-soluble fertilizer as described in claim 8, characterized in that, In the fulvic acid water-soluble fertilizer, the mass concentration of fulvic acid is 120~160 g / L, and / or the mass concentration of humic acid is 100~130 g / L; and / or the mass ratio of fulvic acid to humic acid is 0.9~1.

1.

10. The application of the humic acid water-soluble fertilizer as described in claim 8 or 9 in crop cultivation.

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