Method for preparing slow-release compound fertilizer by utilizing high-phosphorus byproducts in water treatment

Abstract

The invention relates to the technical field of slow-release compound fertilizers, in particular to a method for preparing a slow-release compound fertilizer by using high-phosphorus byproducts in water treatment. The method comprises the following steps: sequentially carrying out high-temperature sterilization, chemical oxidation, activated carbon adsorption and magnesium chloride flocculation treatment on phosphorus-containing and nitrogen-containing wastewater in the aquaculture as a raw material, adding alcaligenes faecalis for culturing, synchronously adding an alkaline phosphorus removal agent, carrying out physical adsorption and microbial degradation, and carrying out centrifugal separation to collect high-phosphorus byproduct filter residues; the high-phosphorus by-product, a binder and monopotassium phosphate are mixed and granulated to obtain a core material; and spraying a mixed aqueous solution of carboxymethyl chitosan, EDTA, aldehyde modified alkali lignin and sodium stearate as a wall material onto the surface of the core material, and drying to prepare the slow-release compound fertilizer. The whole technological process is simple, full-quality utilization of the high-phosphorus wastewater is achieved, the wastewater treatment cost is reduced, the use amount of pesticide and chemical fertilizer in agricultural production is reduced, the yield of crops is remarkably increased, and the method has important significance on development of ecological agriculture.

Description

Technical Field

[0001] This invention relates to the field of slow-release compound fertilizer technology, specifically a method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment. Background Technology

[0002] Phosphorus is one of the main elements causing eutrophication of water bodies. If it is discharged directly into nature without meeting the standards, it can lead to excessive algae growth, resulting in ecological imbalance of water bodies, potential drinking water safety hazards, and serious threats to human health.

[0003] The development of ecological agriculture requires integrating field planting with forestry, animal husbandry, sideline production, and fisheries to balance resource utilization and protection, forming a virtuous cycle both ecologically and economically. Currently, wastewater generated during the processing of aquatic products (such as fish and shellfish) typically contains nutrients like carbon, nitrogen, and phosphorus, with phosphorus levels exceeding 5000 mg / L considered high-phosphorus wastewater. High-phosphorus wastewater presents challenges in treatment due to its difficulty and high cost; on the other hand, phosphate rock reserves, a crucial source of phosphate fertilizer in agricultural production, are limited, posing a risk to the long-term supply of phosphate fertilizer. Therefore, promoting the comprehensive utilization of high-phosphorus wastewater and reducing the use of pesticides and fertilizers is of great significance for the development of ecological agriculture. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, so as to solve the problems mentioned in the background art.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, comprising the following steps: Step 1: Collect phosphorus-containing wastewater with a total phosphorus content ≥5000 mg / L and a total nitrogen content ≥1000 mg / L; sterilize at 120~125℃ for 30 min, then cool to 35~40℃ to obtain sterilized phosphorus-containing wastewater; add hydrogen peroxide solution sequentially for oxidation pretreatment, add activated carbon for adsorption treatment, and add magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater; Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 30-40℃, stir and react for 1-2 hours, then add *Alcaligenes fecal microorganism* at 37℃ for culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filter and collect the filter residue, which is the high phosphorus byproduct; Step 3: S1: Add the binder to water, add high-phosphorus by-products and potassium dihydrogen phosphate, mix and granulate, then dry to obtain the core material; S2: Add carboxymethyl chitosan, EDTA, aldehyde-modified alkali lignin, and sodium stearate to water, and stir at 50-100 rpm for 10-15 minutes at 25-30℃ to obtain a wall material solution. S3: Spray the wall material solution onto the core material surface and vacuum dry at 50~60℃ to obtain slow-release compound fertilizer.

[0006] Furthermore, in step 1, the amount of each component added per 1L of wastewater is 20-30mL of hydrogen peroxide solution, 15-20g of activated carbon, and 13-15g of magnesium chloride.

[0007] Furthermore, the concentration of the hydrogen peroxide solution is 25~30 wt.%.

[0008] Furthermore, in step 2, 7-8 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater.

[0009] Furthermore, the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:(5~8).

[0010] Furthermore, in S1, the content of each component in the core material, by weight, is 3-5 parts binder, 60-70 parts high-phosphorus by-products, and 5-10 parts potassium dihydrogen phosphate.

[0011] Furthermore, in S2, the amount of each component in the wall material solution, by weight, is 10-15 parts carboxymethyl chitosan, 10-12 parts EDTA, 10-15 parts aldehyde-modified alkali lignin, 0.05-0.12 parts sodium stearate, and 1000-1500 parts water.

[0012] Further, in S2, the preparation method of aldehyde-modified alkali lignin is as follows: alkali lignin is added to a 10wt.% sodium hydroxide aqueous solution, and chloroform is added at 65~75℃ for 60~90min using trimethylamine solution as a catalyst; after cooling the reaction solution, dilute hydrochloric acid is added to adjust the pH value to 2~3, the product is separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin; wherein, alkali lignin and chloroform react at a mass ratio of 1:(3~4).

[0013] Furthermore, in step 3, the content of each component in the slow-release compound fertilizer, by weight percentage, is 30-40% wall material and 60-70% core material.

[0014] Compared with existing technologies, the beneficial effects achieved by this invention are as follows: This invention uses phosphorus- and nitrogen-containing wastewater from the aquaculture industry as raw material. After high-temperature sterilization, chemical oxidation, activated carbon adsorption deodorization, and magnesium chloride flocculation treatment, the wastewater environment becomes suitable for the growth of *Alcaligenes faecalis*. The wastewater is used as a culture medium for *Alcaligenes faecalis*, and an alkaline phosphorus removal agent is added simultaneously. This alkaline phosphorus removal agent includes calcium hydroxide and bentonite. Calcium hydroxide not only improves the adsorption capacity of bentonite for phosphorus and nitrogen but also adjusts the pH value of the phosphate-containing wastewater, providing a suitable environment for bacterial growth. After physical adsorption and microbial degradation, phosphorus and nitrogen in the water are effectively removed. The high-phosphorus byproduct obtained by centrifugation is bentonite loaded with phosphorus, nitrogen, calcium, and active bacterial strains.

[0015] This invention involves granulating a mixture of high-phosphorus byproducts, a binder, and potassium dihydrogen phosphate to obtain a core material. A mixed aqueous solution of carboxymethyl chitosan, EDTA, aldehyde-modified alkali lignin, and sodium stearate is sprayed onto the surface of the core material as a wall material, and then dried to produce a slow-release compound fertilizer. Specifically, the aldehyde-modified alkali lignin crosslinks with the amino groups in carboxymethyl chitosan via a Schiff base reaction, forming a stable coating. The carboxymethyl chitosan coating exhibits a certain pH-responsive function, with its swelling degree increasing as the pH decreases. When plants are phosphorus deficient, their roots secrete organic acids, making the surrounding soil environment acidic, and the core material is released through the swollen wall material. When the phosphorus in the environment is sufficient to supply plant growth, the secretion of organic acids decreases, the swelling degree of the wall material decreases, and thus the release of the core material is slowed.

[0016] Under phosphorus-deficient conditions, EDTA in the wall material is released together with the core material. On the one hand, the core material, once in the soil, can quickly replenish the phosphorus, nitrogen, calcium, potassium, and other nutrients needed for plant and crop growth. On the other hand, *Alcaligenes faecalis* synergistically promotes plant and crop growth and reduces disease incidence. Simultaneously, EDTA has a chelating effect on metal ions such as iron and aluminum, further effectively preventing PO4 buildup. 3− The ions combine with metal ions such as Fe and Al in the soil, improving the utilization rate of phosphate fertilizer. As the slow-release compound fertilizer is gradually released, the residues such as carboxymethyl chitosan and aldehyde-modified alkali lignin in the wall material further enhance the soil's water retention capacity, jointly promoting plant and crop growth.

[0017] The overall process of this invention is simple, realizing the full utilization of high-phosphorus wastewater, reducing environmental treatment costs, reducing the use of pesticides and fertilizers, and significantly increasing crop yields, which is of great significance for the development of ecological agriculture. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0019] In this invention, the phosphorus-containing wastewater comes from a certain aquatic product processing plant, and the total phosphorus content is measured to be 6500 mg / L and the total nitrogen content is 1380 mg / L; the Alcaligenes faecalis is Alcaligenes faecalis ZS-1; the binder is starch, which comes from Guangdong Huimei Starch Technology Co., Ltd.; the remaining materials are commercially available products.

[0020] Example 1: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, comprising the following steps: Step 1: Phosphorus-containing wastewater was collected and sterilized at 120℃ for 30 min, then cooled to 35℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 25 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 20 mL of hydrogen peroxide solution, 15 g of activated carbon, and 13 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 30℃, stir and react for 1 hour, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high phosphorus byproduct; wherein, 7 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater; the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:5; Step 3: S1: Add 3g of binder to 15g of water, add 60g of high-phosphorus by-product and 5g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 10g carboxymethyl chitosan, 10g EDTA, 10g aldehyde-modified alkali lignin, and 0.05g sodium stearate to 1000g water, and stir at 50rpm for 10min at 25℃ to obtain a wall material solution; the preparation method of the aldehyde-modified alkali lignin is as follows: Alkali lignin was added to a 10 wt.% sodium hydroxide aqueous solution, and chloroform was added at 65°C for 60 min using trimethylamine solution as a catalyst. After cooling the reaction solution, dilute hydrochloric acid was added to adjust the pH to 2. The product was separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin. The alkali lignin and chloroform were reacted at a mass ratio of 1:3. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 50°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 30% wall material and 70% core material.

[0021] Example 2: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, comprising the following steps: Step 1: Phosphorus-containing wastewater was collected and sterilized at 123℃ for 30 min, then cooled to 38℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 27 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 25 mL of hydrogen peroxide solution, 18 g of activated carbon, and 14 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 35℃, stir and react for 1.5 hours, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high phosphorus byproduct; wherein, 7.5 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater; the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:7; Step 3: S1: Add 4g of binder to 20g of water, add 65g of high-phosphorus by-product and 8g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 12g carboxymethyl chitosan, 11g EDTA, 13g aldehyde-modified alkali lignin, and 0.09g sodium stearate to 1200g water, and stir at 80rpm for 12min at 28℃ to obtain a wall material solution; the preparation method of the aldehyde-modified alkali lignin is as follows: Alkali lignin was added to a 10 wt.% sodium hydroxide aqueous solution, and chloroform was added at 70°C for 75 min using trimethylamine solution as a catalyst. After cooling the reaction solution, dilute hydrochloric acid was added to adjust the pH to 2.5. The product was separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin. The reaction of alkali lignin and chloroform was carried out at a mass ratio of 1:3.5. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 55°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 35% wall material and 65% core material.

[0022] Example 3: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, comprising the following steps: Step 1: Phosphorus-containing wastewater was collected and sterilized at 125℃ for 30 minutes, then cooled to 40℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 30 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 30 mL of hydrogen peroxide solution, 20 g of activated carbon, and 15 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 40℃, stir and react for 2 hours, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high phosphorus byproduct; wherein, 8 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater; the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:8; Step 3: S1: Add 5g of binder to 25g of water, add 70g of high-phosphorus by-product and 10g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 15g carboxymethyl chitosan, 12g EDTA, 15g aldehyde-modified alkali lignin, and 0.12g sodium stearate to 1500g water, and stir at 100rpm for 15min at 30℃ to obtain a wall material solution; the preparation method of the aldehyde-modified alkali lignin is as follows: Alkali lignin was added to a 10 wt.% sodium hydroxide aqueous solution, and chloroform was added at 75°C for 90 min using trimethylamine solution as a catalyst. After cooling the reaction solution, dilute hydrochloric acid was added to adjust the pH to 3. The product was separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin. The alkali lignin and chloroform were reacted at a mass ratio of 1:4. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 60°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 40% wall material and 60% core material.

[0023] Comparative Example 1: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, which differs from Example 1 in that bentonite is used instead of alkaline phosphorus removal agent, while the other parameters are the same as in Example 1.

[0024] Step 1: Phosphorus-containing wastewater was collected and sterilized at 120℃ for 30 min, then cooled to 35℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 25 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 20 mL of hydrogen peroxide solution, 15 g of activated carbon, and 13 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 30℃, stir and react for 1 hour, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high-phosphorus byproduct; among which, 7 kg of bentonite is added per ton of pretreated phosphorus-containing wastewater; Step 3: S1: Add 3g of binder to 15g of water, add 60g of high-phosphorus by-product and 5g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 10g carboxymethyl chitosan, 10g EDTA, 10g aldehyde-modified alkali lignin, and 0.05g sodium stearate to 1000g water, and stir at 50rpm for 10min at 25℃ to obtain a wall material solution; the preparation method of the aldehyde-modified alkali lignin is as follows: Alkali lignin was added to a 10 wt.% sodium hydroxide aqueous solution, and chloroform was added at 65°C for 60 min using trimethylamine solution as a catalyst. After cooling the reaction solution, dilute hydrochloric acid was added to adjust the pH to 2. The product was separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin. The alkali lignin and chloroform were reacted at a mass ratio of 1:3. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 50°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 30% wall material and 70% core material.

[0025] Comparative Example 2: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, which differs from Example 2 in that EDTA is not added to the wall material solution.

[0026] Step 1: Phosphorus-containing wastewater was collected and sterilized at 123℃ for 30 min, then cooled to 38℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 27 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 25 mL of hydrogen peroxide solution, 18 g of activated carbon, and 14 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 35℃, stir and react for 1.5 hours, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high phosphorus byproduct; wherein, 7.5 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater; the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:7; Step 3: S1: Add 4g of binder to 20g of water, add 65g of high-phosphorus by-product and 8g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 12g carboxymethyl chitosan, 13g aldehyde-modified alkali lignin, and 0.09g sodium stearate to 1200g water, and stir at 80rpm for 12min at 28℃ to obtain a wall material solution; the preparation method of the aldehyde-modified alkali lignin is as follows: Alkali lignin was added to a 10 wt.% sodium hydroxide aqueous solution, and chloroform was added at 70°C for 75 min using trimethylamine solution as a catalyst. After cooling the reaction solution, dilute hydrochloric acid was added to adjust the pH to 2.5. The product was separated by centrifugation, washed with water until neutral, and then vacuum dried to obtain aldehyde-modified alkali lignin. The reaction of alkali lignin and chloroform was carried out at a mass ratio of 1:3.5. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 55°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 35% wall material and 65% core material.

[0027] Comparative Example 3: A method for preparing slow-release compound fertilizer using high-phosphorus byproducts from water treatment, which differs from Example 3 in that glutaraldehyde is used instead of aldehyde-modified alkali lignin, while the other parameters are the same as in Example 3.

[0028] Step 1: Phosphorus-containing wastewater was collected and sterilized at 125℃ for 30 minutes, then cooled to 40℃ to obtain sterilized phosphorus-containing wastewater. Hydrogen peroxide solution was added sequentially for oxidation pretreatment, activated carbon for adsorption treatment, and magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater. The concentration of the hydrogen peroxide solution was 30 wt.%. The amount of each component added per 1 L of phosphorus-containing wastewater was 30 mL of hydrogen peroxide solution, 20 g of activated carbon, and 15 g of magnesium chloride. Step 2: Take pretreated phosphorus-containing wastewater, add an alkaline phosphorus removal agent at 40℃, stir and react for 2 hours, then add *Alcaligenes fecal microorganism* at 37℃ and culture until the viable bacterial count is ≥1×10⁻⁶. 9 CFU / mL, filtered and the filter residue is collected, which is the high phosphorus byproduct; wherein, 8 kg of alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater; the alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, with a mass ratio of bentonite to calcium hydroxide of 100:8; Step 3: S1: Add 5g of binder to 25g of water, add 70g of high-phosphorus by-product and 10g of potassium dihydrogen phosphate, mix and granulate, then dry to obtain core material; S2: Add 15g carboxymethyl chitosan, 12g EDTA, 15g glutaraldehyde, and 0.12g sodium stearate to 1500g water, and stir at 100rpm for 15min at 30℃ to obtain the wall material solution. S3: The wall material solution is sprayed onto the surface of the core material and vacuum dried at 60°C to obtain a slow-release compound fertilizer. The content of each component in the slow-release compound fertilizer, by weight percentage, is 40% wall material and 60% core material.

[0029] experiment: (1) The total phosphorus, total nitrogen, and viable bacteria count of the high-phosphorus byproducts collected in Examples 1-3 and Comparative Example 1 were detected, and the experimental results are shown in Table 1; wherein: Total nitrogen and total phosphorus test methods: 100g of high-phosphorus byproduct was added to 1L of deionized water. After desorption of nitrogen and phosphorus, the total adsorption amount of nitrogen and phosphorus in each gram of high-phosphorus byproduct was tested according to the "Methods for Monitoring and Analysis of Water and Wastewater (Fourth Edition)". Total nitrogen was tested using the "alkaline potassium persulfate digestion-ultraviolet spectrophotometry method" and total phosphorus was tested using the "potassium persulfate digestion-molybdenum blue comparison method".

[0030] Effective viable bacteria count test method: Weigh 50g of high-phosphorus byproduct into 0.9% physiological saline, stir for 1 hour, filter to obtain filtrate, and then count using a hematology counter.

[0031] Table 1. Content of major substances in high-phosphorus byproducts.

[0032]

[0033] The data in Table 1 show that adding a mixture of bentonite and calcium hydroxide as an alkaline phosphorus removal agent to phosphorus-containing wastewater can effectively achieve phosphorus adsorption. In Comparative Example 1, no calcium hydroxide was added. On the one hand, the calcium ions in calcium hydroxide can combine with phosphates in the water, promoting their adsorption on the bentonite surface; on the other hand, sodium hydroxide can adjust the pH value of the phosphorus-containing wastewater, providing a favorable environment for the growth and reproduction of microorganisms. Therefore, the high-phosphorus byproducts obtained in Examples 1-3 have higher total phosphorus content and more viable bacteria.

[0034] (2) The slow-release compound fertilizers prepared in Examples 1-3 and Comparative Examples 1-3 were applied to a Chinese cabbage planting base. The total area of ​​the experimental fields was 7 mu (approximately 0.47 hectares), which were divided into 7 groups, numbered 1-7 respectively. Each group was fertilized with farmyard manure (380 kg / mu). In addition, the slow-release compound fertilizers in Examples 1-3 and Comparative Examples 1-3 were applied to experimental fields 1-6 respectively (25 kg / mu). Experimental field 7 served as the control group. The Chinese cabbage was sown on September 15, 2025 and harvested on October 20. The Chinese cabbage was regularly and quantitatively watered. No compound fertilizers or pesticides were applied during the growth process. The physiological and biochemical indicators of the Chinese cabbage are shown in Table 2.

[0035] Table 2

[0036] The data in Table 2 show that, compared with the control group, the slow-release compound fertilizer prepared by this invention can effectively reduce crop disease incidence and increase crop yield. Compared with Comparative Example 1, as described in the test results of Experiment (1), the addition of calcium hydroxide increased the adsorption of phosphorus by bentonite and also increased the growth density of bacteria on bentonite. Therefore, the compound fertilizer in Example 1 has the best promoting effect on crop growth. Compared with Comparative Example 2, the EDTA added to the wall material is released synchronously with the fertilizer in the core material, which can play a role in complexing harmful metal ions in the soil, providing a favorable environment for crop growth, thereby increasing crop yield and reducing disease incidence. The data of Example 3 and Comparative Example 3 show that, compared with glutaraldehyde, aldehyde-modified alkali lignin is more effective in promoting crop growth.

[0037] (3) After storing the slow-release compound fertilizers prepared in Examples 1-3 and Comparative Example 3 at 25°C for 100 days, the number of viable bacteria in the slow-release compound fertilizers was detected. The detection method was the same as the method for testing the number of viable bacteria in high-phosphorus by-products in Experiment (1) above. The survival rate of the strains was calculated. Strain survival rate = (Number of viable bacteria after 100 days of storage / Number of viable bacteria in high-phosphorus byproducts) × 100%; The results are shown in Table 3.

[0038] Table 3.

[0039] The data in Table 3 show that the survival rates of the strains in Examples 1-3 and Comparative Example 3 are comparable, indicating that aldehyde-modified alkali lignin also has a good cross-linking effect on carboxymethyl chitosan, which can effectively maintain the activity of microorganisms and give the strains in the slow-release compound fertilizer good room temperature storage performance.

[0040] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment, characterized in that: The method comprises the following steps: Step 1: Collecting phosphorus-containing wastewater, wherein the total phosphorus content of the phosphorus-containing wastewater is greater than or equal to 5000 mg / L, and the total nitrogen content is greater than or equal to 1000 mg / L; sterilizing the phosphorus-containing wastewater at 120-125 DEG C for 30 min, and cooling to 35-40 DEG C to obtain sterilized phosphorus-containing wastewater; adding hydrogen peroxide solution to the sterilized phosphorus-containing wastewater for oxidation pretreatment, adding activated carbon for adsorption treatment, and adding magnesium chloride for flocculation treatment to obtain pretreated phosphorus-containing wastewater; Step 2: Pre-treatment of phosphorus-containing wastewater, at 30~40℃, adding alkali dephosphorization agent, stirring for 1~2h, then adding Alcaligenes faecalis at 37℃ for cultivation until the viable bacteria content ≥1×10 9 CFU / mL, filtering and collecting the residue, which is the high-phosphorus byproduct; Step 3: S1: adding a binder to water, mixing high-phosphorus by-products and potassium dihydrogen phosphate, and granulating to obtain a core material; S2: adding carboxymethyl chitosan, EDTA, aldehyde-modified alkali lignin, and sodium stearate to water, stirring at 25-30 DEG C and a stirring speed of 50-100 rpm for 10-15 min to obtain a wall material solution; S3: spraying the wall material solution onto the surface of the core material, and vacuum drying at 50-60 DEG C to obtain slow-release compound fertilizer.

2. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In step 1, the amount of each component added to 1 L of wastewater is 20-30 mL of hydrogen peroxide solution, 15-20 g of activated carbon, and 13-15 g of magnesium chloride.

3. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 2, characterized in that: The concentration of the hydrogen peroxide solution is 25-30 wt.%.

4. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In step 2, 7-8 kg of an alkaline phosphorus removal agent is added per ton of pretreated phosphorus-containing wastewater.

5. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 4, characterized in that: The alkaline phosphorus removal agent is a mixture of bentonite and calcium hydroxide, and the mass ratio of bentonite to calcium hydroxide is 100: (5-8).

6. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In S1, the content of each component in the core material is 3-5 parts of a binder, 60-70 parts of high-phosphorus by-products, and 5-10 parts of potassium dihydrogen phosphate.

7. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In S2, the amount of each component in the wall material solution is 10-15 parts of carboxymethyl chitosan, 10-12 parts of EDTA, 10-15 parts of aldehyde-modified alkali lignin, 0.05-0.12 parts of sodium stearate, and 1000-1500 parts of water.

8. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In S2, the preparation method of the aldehyde-modified alkali lignin is as follows: alkali lignin is added to a 10 wt.% sodium hydroxide aqueous solution, trimethylamine solution is used as a catalyst, chloroform is added at 65-75 DEG C, and the reaction is carried out for 60-90 min; the reaction solution is cooled, dilute hydrochloric acid is added to adjust the pH to 2-3, the product is centrifuged and washed with water until neutral, and vacuum drying is performed to obtain the aldehyde-modified alkali lignin; wherein the mass ratio of alkali lignin to chloroform is 1: (3-4).

9. The method for preparing slow-release compound fertilizer by using high phosphorus by-products in water treatment according to claim 1, characterized in that: In step 3, the content of each component in the slow-release compound fertilizer is 30-40% wall material and 60-70% core material.

10. Slow-release compound fertilizer prepared by the method of any one of claims 1-9.

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