CN111747794A

CN111747794A - Method for preparing slow-release phosphate fertilizer by recycling phosphorus in sewage and application

Info

Publication number: CN111747794A
Application number: CN202010535602.2A
Authority: CN
Inventors: 薛利红; 褚清南; 潘纲; 杨林章
Original assignee: Jiangsu Academy of Agricultural Sciences
Current assignee: Jiangsu Academy of Agricultural Sciences
Priority date: 2020-06-12
Filing date: 2020-06-12
Publication date: 2020-10-09

Abstract

The invention belongs to the field of agriculture, and relates to a method for preparing a slow-release phosphate fertilizer by recycling phosphorus in sewage, which is characterized by comprising the following steps of: culturing microalgae in a bioreactor added with phosphorus-rich farm wastewater, and continuously aerating and maintaining illumination in the bioreactor; adding a flocculating agent into the cultured microalgae and stirring; filtering and collecting the flocculated microalgae, and mixing the flocculated microalgae with a citric acid solution; placing the mixture in a high-pressure reaction kettle for hydrothermal carbonization to obtain a hydrothermal carbon slow-release phosphate fertilizer; the microalgae is microcystis or chlorella. The invention provides a new technical approach for solving the problems of sewage dephosphorization and plant phosphate fertilizer supply, and can change the phosphorus in the sewage into valuable and prepare a novel sustained and controlled release phosphate fertilizer, thereby realizing the circulation of the phosphorus from the sewage to crops.

Description

Method for preparing slow-release phosphate fertilizer by recycling phosphorus in sewage and application

Technical Field

The invention belongs to the field of agriculture, and particularly relates to a method for recycling phosphorus in sewage and preparing a slow-release phosphate fertilizer and application.

Background

Phosphorus is one of the most important nutrients for maintaining crop growth, and phosphate fertilizer is of great importance to the contribution of crop yield and quality. The production of phosphate fertilizers depends mainly on the extraction and reprocessing of phosphate in phosphate ore, and the reserve of the phosphate ore on the earth is exhausted within 50-100 years. According to the analysis of the substance flow of phosphorus in global food production and consumption, only about 10-30% of phosphorus in phosphate fertilizer can be dissolved in soil solution for plants to utilize, so that a large amount of phosphorus remains in the soil and possibly enters rivers, lakes and the like through surface runoff or soil leaching to cause water body pollution phenomena such as water bloom and the like. Phosphorus itself is a macronutrient essential for crop growth and becomes a pollutant when lost to water. Compared with the shortage of storage of the phosphate ores on the earth, a large amount of phosphorus exists in breeding sewage, domestic sewage and polluted surface water.

Disclosure of Invention

The invention aims to overcome the defects of the prior art and provide a method and application for recycling phosphorus in sewage and preparing a slow-release phosphate fertilizer, wherein the phosphorus is recycled from the sewage to farmlands, the utilization rate of crops to phosphorus is improved, and the substitution of the fertilizer phosphorus and the yield increase are realized.

In order to solve the technical problem, the invention provides a method for preparing a slow-release phosphate fertilizer by recycling phosphorus in sewage, which comprises the following steps:

adding microalgae into a bioreactor of phosphorus-rich sewage for culture, and continuously aerating and maintaining illumination in the bioreactor;

adding a flocculating agent into the cultured microalgae and stirring;

filtering and collecting the flocculated microalgae, and mixing the flocculated microalgae with a citric acid solution;

placing the mixture in a high-pressure reaction kettle for hydrothermal carbonization to obtain a hydrothermal carbon slow-release phosphate fertilizer;

the microalgae is microcystis or chlorella.

Preferably, the culture method of the microalgae comprises the steps of adding the microalgae into a bioreactor of phosphorus-rich sewage, keeping the temperature constant at 25 ℃ and keeping the temperature constant at least 150 mu mol/m for 14-16 hours per day²Light intensity/s, two weeks of culture; the phosphorus-rich sewageIs waste water of a farm, domestic sewage, biogas slurry or a mixture thereof.

Preferably, the flocculant is a mixed solution of 1-5mg/L of chitosan, glacial acetic acid and polyaluminium chloride, and the concentrations of the chitosan, the glacial acetic acid and the polyaluminium chloride are the same.

Preferably, the flocculant has a pH of 3.

Preferably, 1-5 mL of flocculant is dropped into 1-3L of microalgae culture solution during flocculation, and a glass rod is adopted for stirring for 1-3 minutes.

Preferably, the citric acid solution has a concentration of 1%. Preferably, the mixing ratio of the flocculated microalgae and the citric acid solution is 1: 1-4% (w/v).

Preferably, the hydrothermal carbonization is performed in a high-pressure reaction kettle at 260 ℃ for 1-2 hours.

Preferably, the pressure of the high-pressure reaction kettle is 5-10 MPa.

The invention also provides a slow-release phosphate fertilizer which is obtained by the preparation method.

The invention also provides application of the microcapsule algae hydrothermal carbon slow-release phosphate fertilizer in replacement of phosphatic fertilizers in farmlands, phosphorus reduction and synergism.

The invention also provides an application method of the microcystis hydrothermal carbon slow-release phosphate fertilizer in a farmland, and the slow-release phosphate fertilizer is adopted, and the application amount is 0.5 percent of the soil mass. . The nitrogen, phosphorus and potassium contents of the microcystis hydrothermal charcoal are 8.8% and 13.5% (P)₂O₅) And 0.8%. According to the recommended dosage of local conventional nitrogen, phosphorus and potassium, wherein the phosphate fertilizer adopts the microcystis hydrothermal carbon slow-release phosphate fertilizer, and the specific dosage is equal to phosphorus (P)₂O₅) The dosage is divided by 13.5 percent, and the nitrogen fertilizer and the potassium fertilizer adopt the conventional fertilizer varieties, but the nitrogen amount brought by the microcapsule algae hydrothermal carbon slow-release phosphate fertilizer is deducted. For example, the dosage of nitrogen and phosphorus in Jiangsu conventional wheat field is 270 kg ha and 90kg ha^-1If the amount of the microcapsule algae hydrothermal carbon slow-release phosphate fertilizer is 667kg ha^-1At the same time, 58.7kg/ha of nitrogen is brought in, and 211.3kg of N ha of nitrogen fertilizer is applied^-1. The potassium fertilizer is used in the same amount as the normal fertilizer.

The invention achieves the following beneficial effects:

because of the shortage of the storage of the phosphate ore, the shortage of the phosphate fertilizer is likely to appear in the near future, and the recycling of the phosphorus in the sewage into agriculture to provide phosphorus nutrition for the growth of crops is a win-win scheme for solving the environmental pollution and the limited storage of the phosphate ore.

The phosphorus content of microalgae enriched in sewage can reach 2-4%, which is far more than that of common organic fertilizers such as manure and sludge (0.8-2.3%). Microalgae can be enriched in phosphorus in sewage containing high concentrations of phosphorus above their physiological metabolic requirements. These over-ingested phosphorus is stored in the form of polyphosphate in the vacuole and some microvesicle structures. However, the polyphosphate releases available phosphorus at a low rate, and cannot effectively promote the utilization efficiency of phosphorus by crops. A method for efficiently preparing the phosphate fertilizer from the phosphate fertilizer is not found up to now. Through hydrothermal reaction in a closed space, liquid is used as a reaction medium, and raw materials are converted into a hydrothermal biochar material through a series of complex reactions at a relatively low temperature (200-260 ℃) and under self-generated pressure, so that complete hydrolysis of polyphosphate and organic phosphorus can be ensured. Furthermore, phosphorus chemical classification tests on hydrothermal carbon showed that orthophosphate exists mainly in a state of being adsorbed and bonded to iron-aluminum oxide. If hydrothermal charcoal is applied to the soil, the phosphorus adsorbed by the metal oxide can be gradually converted into dissolved or exchanged phosphorus for plant absorption, and is therefore considered to be a slow release type of phosphorus that can meet the long-term demand for phosphorus for plant growth. Moreover, the biochar prepared by the hydrothermal carbonization method has a plurality of microporous structures, so that the fixation of soil nutrients can be increased, and the loss of phosphorus in soil can be prevented. This suggests that microalgae may be converted into effective slow-release phosphate fertilizer by hydrothermal carbonization.

The hydrothermal carbonization method can generate a hydrothermal carbon material through a hydrothermal reaction at a certain temperature and under a certain pressure by taking water as a reaction medium in a closed system. The pH of the water body in the hydrothermal reaction is controllable, so that the polyphosphate can be promoted to be hydrolyzed more thoroughly, the addition of the citric acid is favorable for promoting the acid hydrolysis of the polyphosphate in the hydrothermal reaction, the breakage of the long chain of the polyphosphate is accelerated, and the generation of more orthophosphate is facilitated. In addition, the citric acid is helpful for the carbonization degree of the hydrothermal carbon and increases the yield of the produced hydrothermal carbon. Moreover, the moisture content of the wet microalgae is about 50-90%, and the hydrothermal carbonization method can be carried out without dehydrating and drying the microalgae. In the produced hydrothermal carbon, orthophosphate exists mainly in a state of being adsorbed and bonded with iron-aluminum oxide to form a slow-release phosphate fertilizer.

The iron-aluminum combined phosphorus in the microalgae hydrothermal carbon slow-release phosphate fertilizer prepared by the invention can serve as a slow-release phosphorus reservoir, and effective phosphorus released prematurely in the fertilizer can not be completely absorbed and utilized by plants, but is assimilated into insoluble phosphorus by microorganisms, so that the phosphorus supply is insufficient in the later growth period of crops. The characteristic of the slow-release phosphate fertilizer of the invention can ensure that the rhizosphere soil of wheat has sufficient available phosphorus in the whole growth period, even in the mature period, so as to maintain the growth requirement of wheat. Compared with algae powder and chemical fertilizer, the microalgae hydrothermal carbon has more lasting and stronger phosphorus release capability. In addition, the microalgae hydrothermal carbon also improves the activity of alkaline phosphatase in soil and improves the carbon-nitrogen ratio of the soil. The invention uses the microalgae to circulate the phosphorus from the sewage to the farmland, promotes the utilization efficiency of the phosphorus by the wheat, and realizes the purposes of replacing phosphorus fertilizers, reducing phosphorus and increasing efficiency.

Drawings

FIG. 1 shows the removal rate of phosphorus from wastewater and the accumulation of biomass per se by using microcystis. DW dry weight

FIG. 2 shows the flocculation efficiency of microcystis and the change of Zeta potential in water body by using flocculant (pH 3) of chitosan, glacial acetic acid and polyaluminium chloride with the components of 1-5 mg/L.

FIG. 3 is a graph showing the effect of different temperatures and reaction solutions on the total phosphorus content of the microcystis hydrothermal charcoal. MS, original microcystis; MSHW 200: the microcystis hydrothermal carbon is fired at 200 ℃ by using deionized water as a reaction medium; MSHW 260: the microcystis hydrothermal carbon is fired at 260 ℃ by using deionized water as a reaction medium; MSHCA200 is microcapsule algae hydrothermal charcoal fired at 200 ℃ by using 1% citric acid solution as a reaction medium; MSHCA260 is microcapsule algae hydrothermal charcoal fired at 260 ℃ with 1% citric acid solution as reaction medium. According to the Duncan test of one-way anova, at P <0.05(n ═ 3), each column represents a significant difference in total phosphorus content with a different lower case letter.

FIG. 4 is a graph showing the effect of different temperatures and reaction solutions on the yield of the microcystis hydrothermal charcoal. According to the Duncan test of one-way anova, at P <0.05(n ═ 3), each column represents a significant difference in yield of hydrothermal char with a different lower case letter.

FIG. 5 is a graph showing the phosphorus classification of different microencapsulated algal pyrocarbons, including dissolved phosphorus, exchanged phosphorus, iron-aluminum bound phosphorus, calcium bound phosphorus and residual phosphorus. According to the Duncan test of one-way anova, a significant difference in the available phosphorus content is indicated in lower case letters at P <0.05(n ═ 3).

FIG. 6 is a graph showing the dynamic change in the effective phosphorus concentration of soil after application of Microcystis (MS) and microcystis hydrothermal charcoal (MSHCA260: microcystis hydrothermal charcoal fired at 260 ℃ with 1% citric acid solution as a reaction medium) to soil in a 120-day soil culture experiment.

FIG. 7 is the effect of primary Microcystis (MS) and Microcystis hydrothermal charcoal (MSHCA260: Microcystis hydrothermal charcoal fired at 260 ℃ in a 1% citric acid solution as a reaction medium) on Fe-Al bound phosphorus in rhizosphere soil at the tillering and maturation stages of wheat in a wheat potting experiment. According to the Duncan test of one-way ANOVA, at P.ltoreq.0.05 (n.3), each column is marked by a different capital letter for the significant difference compared at the tillering and mature stages at the same treatment and by a lower case letter for the significant difference between the different treatments at the same stage.

FIG. 8 is the effect of primary Microcystis (MS) and Microcystis hydrothermal charcoal (MSHCA260: Microcystis hydrothermal charcoal fired at 260 ℃ in a 1% citric acid solution as a reaction medium) on dissolved phosphorus (A) and exchanged phosphorus (B) in rhizosphere soil at the tillering stage and the maturation stage of wheat in a wheat potting experiment. According to the Duncan test of one-way anova, at P.ltoreq.0.05 (n.3), each column is marked with different capital letters for the significance of the difference between the same treatment at the tillering stage and the mature stage and lower case letters for the significance of the difference between the different treatments at the same stage.

FIG. 9 is the effect of primary Microcystis (MS) and Microcystis hydrothermal charcoal (MSHCA260: Microcystis hydrothermal charcoal fired at 260 ℃ in a 1% citric acid solution as a reaction medium) on alkaline phosphatase activity in rhizosphere soil at the tillering stage and mature stage of wheat in wheat potting experiments. According to the Duncan test of one-way anova, at P.ltoreq.0.05 (n.3), each column is marked by different capital letters for the significance of the difference between the same treatment at the tillering stage and the mature stage and by lower case letters for the significance of the difference between the different treatments at the same stage.

FIG. 10 shows the effect of primary Microcystis (MS) and Microcystis hydrothermal charcoal (MSHCA260: Microcystis hydrothermal charcoal fired at 260 ℃ in 1% citric acid solution as reaction medium) on wheat phosphate utilization efficiency (A) and grain yield (B) in wheat potting experiments. According to the Duncan test of one-way ANOVA, at P ≦ 0.05(n ═ 3), each column represents a significant difference with a different lower case letter;

FIG. 11 is a flow chart of the method of the present invention.

Detailed Description

The present invention is further described with reference to the accompanying drawings, and the following examples are only for clearly illustrating the technical solutions of the present invention, and should not be taken as limiting the scope of the present invention.

Commercially available microencapsulated algal species (Microcystis sp., CCAP 1450/13) were cultured in a reactor filled with sewage from a chicken farm containing a high concentration of phosphorus (41.5mg L)^-1) Continuous aeration in the reactor (4 mLs)^-1) Keeping the temperature at 25 ℃ and the speed at 14 hours per day and 150 mu mol/m²Intensity of illumination in/s. Two weeks of culture. The biomass accumulation and phosphorus removal efficiency are shown in FIG. 1, and the biomass reached a maximum of 1.17g L after two weeks of cultivation^-188.4 percent of phosphorus in the sewage is removed, and the removal rate of the total phosphorus in the sewage reaches 2.65mg L^-1day^-1。

Flocculating the microcystis enriched with phosphorus in the wastewater, wherein the flocculating agent is a mixed solution (pH is 3) of 1-5mg/L of chitosan, glacial acetic acid and polyaluminium chloride. The concentrations of the chitosan, the glacial acetic acid and the polyaluminium chloride are the same. During flocculation, several drops of flocculant mixed solution are dropped into 1-3L of the microcystis solution, and the solution is stirred for 1-3 minutes by a glass rod, and the flocculation effect is shown in figure 2. Aiming at microcystis with different concentrations, the concentration of the flocculating agent fluctuates, but the selected flocculating agent can effectively flocculate and precipitate the microcystis or chlorella within the range of 1-5mg/L, the flocculation rate reaches more than 98 percent, and the Zeta potential in the solution is close to 0.

Filtering and collecting flocculated microcystis, and mixing the wet microcystis with deionized water or 1% citric acid solution at a ratio of 1:1-4 (w/v); placing the mixture into a high-pressure reaction kettle, and hydrothermally carbonizing at 200 ℃ and 260 ℃ and under the naturally generated pressure (about 5-10MPa) for 2 hours to obtain four kinds of microcystis hydrothermal carbon materials, including MSHW200 (microcystis hydrothermal carbon fired at 200 ℃ and with deionized water as a reaction medium), MSHW 260: the microcystis hydrothermal carbon is fired at 260 ℃ by using deionized water as a reaction medium; MSHCA200 is microcapsule algae hydrothermal charcoal fired at 200 ℃ by using 1% citric acid solution as a reaction medium; MSHCA260 is microcapsule algae hydrothermal charcoal fired at 260 ℃ with 1% citric acid solution as reaction medium.

The four microcystis hydrothermal carbon materials are taken out after the hydrothermal reaction kettle is naturally cooled, the four microcystis hydrothermal carbon materials are dried for standby, and the total phosphorus content in the hydrothermal carbon is determined by using a molybdenum blue colorimetric method, the result is shown in figure 3, the MSHCA260 hydrothermal carbon is enriched with phosphorus with the highest concentration, the phosphorus concentration is up to 5.8%, and the phosphorus concentration is increased by 65.7% compared with the original algae powder. Then weighing and calculating the yield of the hydrothermal carbon relative to the original microcystis after hydrothermal carbonization, as shown in fig. 4, the yields of the two hydrothermal carbons MSHCA200 and MSHCA260 prepared by using 1% citric acid solution as hydrothermal reaction medium are the highest, respectively 58.8% and 55.2%, while the yields of the two hydrothermal carbons MSHW200 and MSHW260 prepared by using deionized water as reaction medium are only 44.6% and 42.1%.

The chemical forms of phosphorus in the four hydrothermal carbon samples were analyzed, and the concentrations of dissolved phosphorus, exchanged phosphorus, iron-aluminum-bound phosphorus, calcium-bound phosphorus and residual phosphorus were analyzed by sequential extraction, as shown in fig. 5. The dissolved phosphorus and the exchanged phosphorus are phosphorus forms which can be directly absorbed by plants in simulated soil and can be regarded as an effective phosphorus library; the iron-aluminum combined phosphorus is dissolved in soil under an acidic condition or is converted into effective phosphorus under the action of microorganisms, so that the iron-aluminum combined phosphorus can be regarded as a slow-release phosphorus library; calcium-bound phosphorus mainly mimics apatite in soil, and residual phosphorus is a generic name of the class of phosphorus that is most difficult to decompose and utilize by plants, so calcium-bound phosphorus and residual phosphorus can be regarded as a steady-state phosphorus pool (not easily utilized by plants). The results in fig. 5 show that as the microcystis hydrothermally carbonized, the available phosphorus pool and the steady state pool decrease and the iron-aluminum bound phosphorus in the middle increases, indicating that the hydrothermal carbonization promotes the formation of the slow-release phosphorus pool, while the MSHCA260 has the highest concentration of iron-aluminum bound phosphorus in the four hydrothermally carbonized materials, up to 51.6% of the total phosphorus, and the recovery rate of phosphorus from the MSHCA260 is also higher, 91%.

MSHCA260, which had the highest total phosphorus and iron-aluminum bound phosphorus concentrations, was then applied to the soil and a 120-day soil culture experiment was performed to investigate the release kinetics of available phosphorus, as shown in FIG. 6, using fertilizer and original microencapsulated algae powder (MS) as a comparison. The result clearly shows that the content of available phosphorus in the soil 30 days after the fertilizer is applied is far higher than that of the MS and MSHCA260 treatment, but after 30 days, the slow release phosphorus performance of the MS and the MSHCA260 plays a role, the phosphorus concentration of the soil is continuously increased and exceeds that of the fertilizer, and compared with the slow release performance of the MSHCA260, the slow release performance of the MS and the MSHCA260 is stronger. Fertilizers release high concentrations of available phosphorus early on, but may not be utilized by plants for long periods of time as they slowly leach or are assimilated by microorganisms over time. The MSHCA260 with the characteristic of slow phosphorus release is more beneficial to providing long-term effective phosphorus for crops.

Then, a wheat potting experiment was performed, the fertilizer control and the phosphorus application amount in the MS and MSHCA260 treatments were equal, samples were taken at the tillering stage (20 days after transplantation) and the maturation stage (120 days after transplantation) of wheat, and the change of phosphorus and alkaline phosphatase in different forms in soil with the growth of wheat at different stages was analyzed. As shown in FIG. 7, the Fe-Al bound phosphorus pool in the rhizosphere soil gradually decreased under the treatment of MS and MSHCA260 as the wheat went from the tillering stage to the mature stage, while the opposite result appeared in the fertilizer control. As shown in FIG. 8, as the wheat goes from the tillering stage to the mature stage, the concentration of phosphorus directly available to crops in the rhizosphere soil treated by MS and MSHCA260 gradually increased, while the concentration of dissolved phosphorus and exchanged phosphorus increased, and the opposite result was observed in the fertilizer control. These results demonstrate that as wheat grows and time passes, the available phosphorus in the originally applied phosphatic fertilizer in the control group gradually decreases, and that this decreased available phosphorus either has been lost before it is absorbed by wheat or is assimilated by soil microorganisms and is not continuously absorbed by plants. In contrast, although the effective phosphorus content of the hydrothermal carbon slow-release phosphate fertilizer is lower than that of a phosphatic fertilizer, phosphorus in the hydrothermal carbon can be slowly released into soil for long-term utilization of wheat. The slow release effect meets the long-term requirement of wheat growth. In addition, hydrothermal charcoal also promotes the activity of soil alkaline phosphatase, as shown in fig. 9, probably because the hydrothermal charcoal contains abundant small-molecule organic substances, which can promote the activity of microorganisms and release some active enzymes such as phosphatase, etc.

Therefore, the microcystis hydrothermal carbon realizes the effect of slowly releasing the phosphate fertilizer, and promotes the utilization efficiency of the phosphate fertilizer by wheat compared with the fertilizer. As shown in fig. 10, MS did not significantly improve phosphorus utilization compared to the fertilizer control group, whereas the utilization of the wheat phosphate treated with MSHCA260 increased significantly by 34.4%. Meanwhile, only MSHCA260 obviously increases the wheat yield, and is improved by 21.4% compared with the fertilizer. The two positive results prove that the invention can realize that the phosphorus in the sewage is transferred to the farmland by taking the microcystis as a medium to improve the utilization rate of the phosphate fertilizer of the crops, and the slow-release phosphate fertilizer prepared by a hydrothermal carbonization method realizes the substitution of the phosphate fertilizer and the phosphorus reduction and the efficiency improvement of the farmland, thereby relieving the dual problems of insufficient phosphate fertilizer raw materials and environmental pollution caused by phosphorus loss.

The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It should be understood by those skilled in the art that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by using equivalent alternatives or equivalent variations fall within the scope of the present invention.

Claims

1. A method for preparing a slow-release phosphate fertilizer by recycling phosphorus in sewage is characterized by comprising the following steps:

adding a flocculating agent into the cultured microalgae and stirring;

the microalgae is microcystis or chlorella.

2. The method as claimed in claim 1, wherein the cultivation method of the microalgae is to add the microalgae into the bioreactor of the phosphorus-rich sewage, keep the temperature at 25 ℃ and maintain at least 150 μmol/m for 14-16 hours per day²Light intensity/s, two weeks of culture; the phosphorus-rich sewage is farm wastewater, domestic sewage, biogas slurry or a mixture thereof.

3. The method for recycling phosphorus in sewage to manufacture a slow-release phosphate fertilizer according to claim 1, wherein the flocculant is a mixed solution of 1-5mg/L of chitosan, glacial acetic acid and polyaluminium chloride, the concentrations of the chitosan, the glacial acetic acid and the polyaluminium chloride are the same, the pH of the flocculant is 3, 1-5 mL of the flocculant is dropped into 1-3L of microalgae culture solution during flocculation, and the flocculant is stirred for 1-3 minutes.

4. The method for recycling phosphorus in sewage to produce slow-release phosphate fertilizer according to claim 1, wherein the concentration of the citric acid solution is 1%.

5. The method for recycling phosphorus in sewage to produce a slow-release phosphate fertilizer according to claim 1, wherein the mixing ratio of the flocculated microalgae to the citric acid solution is 1: 1-4% (w/v).

6. The method for recycling phosphorus in sewage to produce a slow-release phosphate fertilizer according to claim 1, wherein the hydrothermal carbonization is performed in a high-pressure reaction kettle at 260 ℃ for 1-2 hours.

7. The method for preparing the slow-release phosphate fertilizer by recycling phosphorus in sewage as claimed in claim 1, wherein the pressure of the high-pressure reaction kettle is 5-10 MPa.

8. A slow-release phosphate fertilizer, characterized in that it is obtained according to the preparation method described in claims 1-7.

9. The use of the slow-release phosphate fertilizer as defined in claim 8 for phosphorus reduction and efficiency enhancement in farmland.

10. A farmland fertilizing method, characterized in that the slow-release phosphate fertilizer of claim 8 is applied in an amount of 0.5% of the soil mass.