Granulation method of struvite recycled from wastewater and application of struvite in water culture

Abstract

The invention relates to a granulation method of struvite recycled from waste water and application of the struvite recycled from the waste water in hydroponic.The granulation method comprises the following steps that S1, a binder solution is prepared, specifically, polyvinyl alcohol and sodium alginate are subjected to heating and ultrasonic treatment respectively, and struvite powder granulation glue is obtained; s2, struvite powder and glue are mixed, struvite powder and glue prepared in the step S1 are mixed according to the ratio of 5 g of struvite powder to 5 mL of glue, the mixture is kneaded to form large blocks, and struvite powder is obtained by being separated from pig manure wastewater through a chemical precipitation method; s3, granulation: preparing the block mass prepared in the step S1 into struvite particles with the size of 5 mm and the weight of 0.1-0.2 g / piece; the struvite is used as a slow-release fertilizer due to relatively low solubility, and the struvite recycled from the pig manure wastewater can completely provide sufficient phosphorus for plant growth, so that the problems of phosphorus pollution and phosphorus shortage in the future world are solved, and the environment is protected.

Description

Technical Field

[0001] The present invention belongs to the field of environmental protection and ecological hydraulic engineering technology, and relates to a granulation method of struvite recovered from wastewater and its application in hydroponics, and in particular to a granulation method of struvite recovered from pig manure wastewater and the efficient application of struvite in a hydroponic agricultural system. Background Art

[0002] As a slow-release fertilizer, struvite, due to its low solubility, can provide a long-term phosphorus source for crops and has good environmental benefits. Existing research shows that struvite may be more effective in promoting crop growth than commercial fertilizers. In addition, heavy metals and other elements introduced by feed additives during livestock and poultry farming have a lasting impact on the environment, especially bioaccumulation in the food chain, and toxic heavy metals may pose a safety threat to soil health and crop growth. Compared with commercial fertilizers, the use of struvite leads to less accumulation of Hg, Pb, Cr, and Ni in vegetable tissues.

[0003] A recent study showed that heavy metal levels in struvite recovered from piggery wastewater were below the standard limits set by the Korean Fertilizer and Raw Materials Guidelines. Analysis of samples from 24 European struvite production plants showed that heavy metal levels in struvite were below general fertilizer legal requirements, indicating that heavy metals are generally safe. Struvite produced using granulation technology has the advantage of slowing the release of heavy metals.

[0004] Therefore, there is an urgent need for a granulation method for struvite recovered from wastewater and its application in hydroponics to achieve phosphorus recovery from biological wastewater, solve the problem of insufficient phosphorus resources, and at the same time protect the environment, improve agricultural efficiency, save costs, and overcome the problem of small arable land area. Summary of the Invention

[0005] In view of this, in order to solve the above-mentioned problems of insufficient phosphorus resources, phosphorus pollution and phosphorus shortage, the present invention provides a granulation method of struvite recovered from wastewater and its application in hydroponics. The efficient application of struvite in the hydroponic agricultural system shows that the acidic matrix releases the most heavy metals, and too low pH is not conducive to controlling the release of heavy metals. Therefore, it is most effective to increase the water flow movement time of the hydroponic water pump.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] A method for granulating struvite recovered from wastewater comprises the following steps:

[0008] S1. preparing a binder solution: heating and ultrasonically treating polyvinyl alcohol and sodium alginate respectively to obtain struvite powder granulation glue;

[0009] S2. Mixing struvite powder with glue: The struvite powder prepared in step S1 is granulated into glue at a ratio of 5 g struvite powder to 5 mL glue, and the struvite powder and glue are mixed and kneaded to form larger agglomerates, wherein the struvite powder is separated from the pig manure wastewater by a chemical precipitation method;

[0010] S3, granulation: the agglomerates prepared in step S1 are made into struvite granules of 5 mm in size and weighing 0.1 to 0.2 g per granule.

[0011] Furthermore, S1 is specifically:

[0012] S11, adding 6 g of polyvinyl alcohol to 500 mL of water, and completely dissolving the polyvinyl alcohol by heating and ultrasonic treatment;

[0013] S12, adding 5g of sodium alginate to the polyethylene solution completely dissolved in step S11, and continuing heating and ultrasonic treatment to completely dissolve the sodium alginate;

[0014] S13, concentrating the mixed solution of polyethylene and sodium alginate completely dissolved in step S11 to 250 mL and cooling the solution to obtain struvite powder granulation glue.

[0015] Furthermore, the struvite granules prepared by the above-mentioned granulation method of struvite recovered from wastewater are used in hydroponic agricultural systems.

[0016] Furthermore, the above-mentioned hydroponic agricultural system includes PVC-U environmentally friendly water pipes for building plant growth racks. After the plant growth racks are built, a circular customized basket for cultivating plants is placed in the built plant growth racks. The cut sponges and prepared struvite particles are placed in the circular customized baskets. The PVC-U environmentally friendly water pipes and water pumps are connected with hoses so that the water pumps water into the water pipes.

[0017] Furthermore, the water pump is equipped with solar panels and lithium batteries for power supply.

[0018] Furthermore, the water pump is connected to a timer.

[0019] Furthermore, the method for using the hydroponic agricultural system comprises the following steps:

[0020] S51. Assemble and build equipment: Assemble environmentally friendly PVC-U water pipes, build a plant growth rack, place a custom round basket in the plant growth rack, place the cut sponges in the custom round basket, place the prepared struvite granules in the custom round basket, connect the environmentally friendly PVC-U water pipes and water pump with hoses, and assemble the solar panels and lithium batteries for power.

[0021] S52. Seed pretreatment: Soak the seeds for 1 hour. Place the soaked seeds in a wet towel to allow the seeds to blanch (germinate). Use sterilized tweezers and cotton swabs to bury the blanched seeds in the gaps of the cut sponge, ensuring that the seeds are buried to a depth of 1.5 cm and with the seed buds facing upwards.

[0022] S53. Initial Cultivation: Turn on the water pump and timer, and incubate the cabbage seedlings in an incubator for 7 days at a temperature of 30°C. The pH of the nutrient solution is 7.5-9.2, and the hydroponic cycle time is 9 hours. Lowering the pH can increase the release rate of struvite phosphorus, but lowering the pH of the nutrient solution is detrimental to the growth of the cabbage. Increasing the hydroponic cycle time promotes phosphorus release and cabbage growth. Struvite is a supplement to counteract substrate acidity.

[0023] S54. Transplanting and subsequent cultivation: Transplant the cabbage seedlings after 7 days of cultivation into a hydroponic equipment and continue to cultivate in the hydroponic equipment for 37 days. The nutrient solution in the hydroponic water tank mainly consists of 300 mg / L KNO3, 400 mg / L calcium carbonate, 10 mg / L Fe-EDTA, 3 mg / L boric acid, and 2 mg / L manganese sulfate.

[0024] The beneficial effects of the present invention are:

[0025] 1. This invention discloses a method for granulating struvite recovered from wastewater. Due to its low solubility, struvite is used as a slow-release fertilizer, providing a longer-term source of phosphorus for crop growth. This method offers superior slow-release effects and environmental benefits compared to traditional fertilizers. Furthermore, struvite recovered from actual pig manure wastewater can provide sufficient phosphorus for plant growth, effectively addressing the challenges of phosphorus pollution and shortages in the future world while also protecting the environment.

[0026] 2. The present invention discloses a method for granulating struvite recovered from wastewater. This soil cultivation system not only saves water by 85-90%, but also makes full use of indoor space. Only nutrient solution is needed for crop growth, thereby reducing the demand for arable land and labor costs.

[0027] 3. The disclosed method for granulating struvite recovered from wastewater recovers phosphorus from pig manure wastewater to produce struvite, achieving waste utilization while minimizing environmental pollution. With socioeconomic development, the scale of the pig farming industry will continue to expand, leading to an increase in pig manure wastewater discharge. This method ensures economic benefits while contributing to environmental protection.

[0028] 4. The present invention discloses a method for granulating struvite recovered from wastewater. The soilless cultivation technology can enable the development of my country's agriculture to move towards scale, mechanization, informatization and intelligence, improve agricultural efficiency, and greatly alleviate problems such as labor and soil health.

[0029] 5. The present invention discloses a granulation method for struvite recovered from wastewater. In actual production, the volume of nutrient solution is large, and the corresponding heavy metal concentration is too low. In addition, the struvite produced by real wastewater contains a large amount of organic matter, which has a passivating effect on heavy metals, thereby being more conducive to reducing the release of heavy metals and more conducive to the safe agricultural use of struvite fertilizer for crops.

[0030] Other advantages, objects, and features of the present invention will be described in part in the following description and, in part, will be apparent to those skilled in the art upon examination of the following description or may be learned from practice of the present invention. The objects and other advantages of the present invention may be realized and obtained through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention will be described in detail below with reference to the accompanying drawings, in which:

[0032] Figure 1 Schematic diagram of the structure of the hydroponic system of the present invention;

[0033] Figure 2 : is a graph showing changes in pH and PO4-P in the nutrient solution during the hydroponic culture process in this embodiment; Figure 2 a is the graph showing the changes in pH of 7 groups of hydroponic culture over time. Figure 2 b is the change of PO4-P in 7 groups of hydroponics with the increase of time;

[0034] Figure 3 Graph showing wet weight (a), dry weight (b), leaf area (c), chlorophyll and carotenoid content (d) of each treatment of Chinese cabbage in this embodiment; Figure 3 a is the wet weight of 7 groups of hydroponic cabbage, Figure 3 b is the dry weight of 7 groups of hydroponic cabbage, Figure 3 c is the leaf area of 7 groups of hydroponic cabbage, Figure 3 d is the chlorophyll and carotenoid contents of 7 groups of hydroponic cabbage.

[0035] Figure 4 This is a diagram showing the release pattern of heavy metals contained in struvite in the nutrient solution in this embodiment;

[0036] Figure 5This is a graph showing the change in the length of the upper part of the root system over time after germination of wheat in the control group CK and the experimental groups (a to k) with different treatments in this example;

[0037] Figure 6 This is a graph showing the release of NH4-N over time from fertilizers in the hydroponic box of the control group CK and the experimental groups (a-k) in this example;

[0038] Figure 7 This is a graph showing the release of PO4-P over time in the hydroponic box of the comparative group CK and the experimental groups (a-k) in this embodiment;

[0039] Figure 8 This is a graph showing the wheat root length of the control group CK and the experimental groups (a-k) on the 30th day in this example;

[0040] Figure 9 This is a graph showing changes in pH over time in the hydroponic culture of the control group CK and the experimental groups (a-k) in this embodiment;

[0041] Figure 10 This is a graph showing the release of Cu over time from the fertilizers in the hydroponic box of the control group CK and the experimental groups (a-k) in this embodiment;

[0042] Figure 11 This is a graph showing the release of Zn over time from the fertilizers in the hydroponic box of the control group CK and the experimental groups (a-k) in this embodiment;

[0043] Figure 12 This is a graph showing the heavy metal content absorbed by wheat roots and leaves in the control group CK and the experimental groups (a to k) in this example.

[0044] Figure numerals: PVC-U environmentally friendly water pipe 1, round customized basket 2, sponge 3, water pump 4, hose 5, solar panel 6, lithium battery 7, timer 8, plant 9. DETAILED DESCRIPTION

[0045] The following describes the embodiments of the present invention by means of specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and the following embodiments and features in the embodiments can be combined with each other without conflict.

[0046] A method for granulating struvite recovered from wastewater comprises the following steps:

[0047] S1. Prepare binder solution: Heat and ultrasonicate polyvinyl alcohol and sodium alginate respectively to obtain struvite powder granulation glue. Specifically:

[0048] S11. Add 6 g of polyvinyl alcohol to 500 mL of water, and completely dissolve the polyvinyl alcohol by heating and ultrasonic treatment.

[0049] S12. Add 5 g of sodium alginate and continue heating and ultrasonic treatment to completely dissolve the sodium alginate.

[0050] S13, concentrating the solution to 250 mL to obtain struvite powder granulation glue.

[0051] S2. Mixing struvite powder with glue: The struvite powder obtained in S1 is granulated into glue at a ratio of 5 g struvite powder to 5 mL glue, and the struvite powder and glue are mixed, and the mixture is kneaded to form larger lumps. The struvite powder is separated from pig manure wastewater by chemical precipitation.

[0052] S3. Granulation: The agglomerates obtained in S2 are made into 5 mm struvite granules, each weighing 0.1 to 0.2 g.

[0053] S4: Measure particle weight: The particles generated using synthetic wastewater were weighed, and the weight was 0.17±0.01g / piece; the particles generated using real wastewater were weighed, and the weight was 0.12±0.01g / piece.

[0054] The struvite particles prepared by the above method are used in a hydroponic system.

[0055] like Figure 1 The hydroponic system shown includes plants 9, a timer 8, a PVC-U environmentally friendly water pipe 1 and a circular custom basket 2. The sponge 3 and the prepared struvite particles placed in the circular custom basket 2 are connected to a water pump 4 through a hose 5 and the PVC-U environmentally friendly water pipe 1, wherein the water pump 4 is connected to a solar panel 6 and a lithium battery 7 for power supply.

[0056] The method for using the hydroponic system comprises the following steps:

[0057] S51. Assemble and build equipment: Assemble environmentally friendly PVC-U water pipes, build a plant growth rack, place a custom round basket in the plant growth rack, place the cut sponges in the custom round basket, place the prepared struvite granules in the custom round basket, connect the environmentally friendly PVC-U water pipes and water pump with hoses, and assemble the solar panels and lithium batteries for power.

[0058] S52. Seed pretreatment: Soak the seeds for 1 hour. Place the soaked seeds in a wet towel to allow the seeds to blanch (germinate). Use sterilized tweezers and cotton swabs to bury the blanched seeds in the gaps of the cut sponge, ensuring that the seeds are buried to a depth of 1.5 cm and with the seed buds facing upwards.

[0059] S53. Initial Cultivation: Turn on the water pump and timer, and incubate the cabbage seedlings in an incubator for 7 days at a temperature of 30°C. The pH of the nutrient solution is 7.5-9.2, and the hydroponic cycle time is 9 hours. Lowering the pH can increase the release rate of struvite phosphorus, but lowering the pH of the nutrient solution is detrimental to the growth of the cabbage. Increasing the hydroponic cycle time promotes phosphorus release and cabbage growth. Struvite is a supplement to counteract substrate acidity.

[0060] S54. Transplanting and subsequent cultivation: Transplant the cabbage seedlings after 7 days of cultivation into a hydroponic equipment and continue to cultivate in the hydroponic equipment for 37 days. The nutrient solution in the hydroponic water tank mainly consists of 300 mg / L KNO3, 400 mg / L calcium carbonate, 10 mg / L Fe-EDTA, 3 mg / L boric acid, and 2 mg / L manganese sulfate.

[0061] Safe agricultural use includes fertilizer efficiency testing and heavy metal testing technologies. To test fertilizer efficiency and heavy metals, this patent conducted two experiments. These included comparing the PO4-P release characteristics of granular struvite at different pH values and water circulation times, comparing the P release flux of struvite generated from real and synthetic wastewater, studying the effects of struvite addition on cabbage growth, exploring the release patterns of granular and powdered struvite, and detecting trace heavy metal levels in crops.

[0062] Experimental Example 1

[0063] This study compared the PO₄-P release characteristics of granular struvite at different pH values and water circulation times, compared the P release of struvite generated from real and synthetic wastewater, and examined the effects of struvite addition on the growth of Chinese cabbage. This experiment involved growing 24 cabbages in each of seven hydroponic systems, designated A through G, with Group E serving as a blank control. The detailed experimental design is shown in Table 1.1. This study employed 10L hydroponic tanks and selected fast-growing Chinese cabbage to investigate the phosphorus release characteristics of cabbage over a single growth cycle (from seedling to maturity, a 35-day period) and the fertilizer efficacy of struvite. To ensure this study more closely resembled actual vegetable hydroponic cultivation conditions, the nutrient solution in the circulating water tank was prepared using tap water and chemicals under the same conditions. The nutrient solution used in this study consisted primarily of 300mg / L KNO₃, 400mg / L calcium carbonate, 10mg / L Fe-EDTA, 3mg / L boric acid, and 2mg / L manganese sulfate. No chemicals containing phosphorus were added to the nutrient solution to avoid interference with struvite phosphorus release. The pH of experimental groups B and C was adjusted every seven days by adding 2% dilute hydrochloric acid to the circulating water tank to 4 and 5.5, respectively. The pH of the other five experimental groups remained unchanged. 10 mL of circulating water samples were taken from the upper pipe of the hydroponic system 3, 6, 9, 13, 20, 27, and 37 days after the cabbage seedlings were transplanted from the planting baskets in the incubator to the hydroponic system. These samples were used to test for PO₄-P and heavy metal Cu release from struvite.

[0064] Table 1.1: Greenhouse hydroponic experiment design

[0065]

[0066] All experimental groups were carried out under the same room temperature conditions. Figure 3 The cabbage was cultured in the hydroponic equipment until the 36th day. The cabbage on all the hydroponic equipment was picked and the dry weight and wet weight of the cabbage were weighed using a Melien electronic balance. The leaf area of the cabbage was measured using a YMJ-A plant leaf area meter. A 1cm diameter puncher was used to punch 5 holes in each cabbage leaf, and then the leaves were placed in a volumetric flask filled with ethanol (95%) and extracted in the dark for 14 hours. The chlorophyll content was measured using a 722S spectrophotometer at wavelengths of 665, 649, and 470nm, respectively. The relevant calculation formula is as follows. Among them, carotenoids (C x ) is in chlorophyll a (C a ) and chlorophyll b(C b ) is obtained by entering the calculation formula 1 to 3, and then the C a with C b Add them together to get the total amount of chlorophyll.

[0067] C a=13.95×A 655 -6.88×A 649 (1)

[0068] C b =24.96×A 649 -7.32×A 665 (2)

[0069] C x =(1000×A 470 -2.05×C a -114.8×C b ) (3)

[0070] like Figure 2 As shown in a, the pH value of the circulating water system of the two experimental groups (B and C) with lower pH values increases with the increase of release time. This is because the decrease in pH leads to an increase in the solubility of granular struvite fertilizer under acidic conditions, resulting in more NH4 + and PO4 3- Produced, forming a buffer system, that is, under acidic conditions, the PO4 produced by the dissolution of struvite 3- will bind to the protons (H + ) generates H2PO4 - , thereby increasing the pH value of the nutrient solution circulation system. Compared with soil cultivation, the nutrient solution of hydroponics is easier to adjust artificially to control the release of struvite nutrients, and Figure 2 The pH value change of the hydroponic circulating water system after adjustment shown in a is a dynamic process. Therefore, adjusting the pH every 7 days in the circulating water system will cause the struvite fertilizer to dissolve continuously, resulting in periodic pH fluctuations. Although the dissolution of struvite produces PO4 3- Will combine with H + With H2PO4 - The form of exists in the system, but the pH value of the experimental groups (A, D, E, F, G) without adjusting the pH changes very little.

[0071] like Figure 2 As shown in b, with the increase of time, the dissolution of struvite fertilizer leads to the increase of PO4 in the system. 3-The release rate of phosphorus (P) continued to increase. The order of phosphorus release rates for the experimental groups on the 25th and 35th days of hydroponics was C>F>B>A>D>G>E, from highest to lowest. It can be seen that, with the exception of Group E (which did not include struvite fertilizer), phosphorus was detected in the nutrient solutions of all other groups. Furthermore, the phosphorus release rate increased with increasing growth time and was influenced by factors such as the pH of the nutrient solution, water flow time, and struvite type. Specifically, Group C had the lowest pH and released the most phosphorus, which continued to increase over time. Group B had a pH slightly higher than Group C, and its corresponding phosphorus release rate was lower than Group C (C>B). Notably, Group B had a lower phosphorus release concentration than Group F (F>B), indicating that the phosphorus release rate of granular struvite fertilizer is not only closely related to pH but also to the circulation time of hydroponics. Increasing the circulation of the nutrient solution can promote the phosphorus release rate of struvite, and the effect is even better than that of the experimental group with a lower initial pH (Group B).

[0072] The ratio A > D indicates that the phosphorus release rate of struvite recovered from real wastewater is lower than that of struvite recovered from synthetic wastewater. This is due to the different purities of the two types of struvite. During the continuous reactor reaction, the phosphorus recovery rate from real wastewater is lower than that from synthetic wastewater under the same conditions. Accordingly, the struvite precipitate recovered from real wastewater has a lower specific phosphorus content than that recovered from synthetic wastewater, resulting in a lower phosphorus release rate.

[0073] In this experiment, although lowering the pH of the nutrient solution increased the release rate of phosphorus from struvite, lowering the pH actually hindered cabbage growth, resulting in poorer quality. Compared to the pH-induced phosphorus release enhancement of struvite, water circulation time also significantly promoted phosphorus release and, in turn, cabbage growth. Therefore, increasing the pump run time, and thus the nutrient solution circulation time, could promote phosphorus release and cabbage growth.

[0074] No heavy metals were detected in the dried cabbage during one growth cycle. This may be because the volume of nutrient solution in actual production is large and the corresponding heavy metal concentration is too low. Figure 4 It can be seen that the release of heavy metals also increases with time. Consistent with the previous conclusion, more heavy metals are released under acidic conditions. The heavy metals released by Group F are less than those of Group C (28.12μg / L), and are equivalent to those of Group A, which means that increasing the water circulation time of hydroponics does not bring additional risks of heavy metal release. It also further proves that the release rates of heavy metals and phosphorus are not consistent. In addition, the content of heavy metals released by the struvite crystals re-formed in real wastewater is less than that of other synthetic struvite fertilizers. On the one hand, this is because the content of heavy metals carried by struvite products in real wastewater is relatively small.

[0075] On the other hand, it is possible that the struvite produced by real wastewater contains a large amount of organic matter, which has a passivating effect on heavy metals, thereby being more conducive to reducing the release of heavy metals and more conducive to the safe agricultural use of struvite fertilizers.

[0076] Experimental Example 2

[0077] To further investigate the release patterns of granular and powdered struvite, compare the release characteristics of struvite with those of other phosphate fertilizers, and examine the impact of struvite release on crop growth, we used authentic struvite recovered from the third batch of wastewater, Experimental Group a, and prepared struvite fertilizer from synthetic wastewater containing the same concentrations of heavy metals and struvite-forming ions. We also purchased two commercially available phosphate fertilizers: potassium dihydrogen phosphate (52% P₂O₅ and 34% K₂O) and superphosphate (P₂O₅≥16%), and compared their release characteristics with those of struvite. The specific experimental design is shown in Table 1.2.

[0078] Table 1.2: Incubator hydroponic experiment design

[0079]

[0080] In this experiment, wheat was cultivated in an incubator (20°C) for 30 days. A 1-liter incubation box was used, filled with 1 liter of ultrapure water. Germinated wheat seeds were repeatedly rinsed with ultrapure water and transplanted to a hydroponic box to ensure that all nutrients in the hydroponic nutrient solution came from the fertilizer added during this experiment. The pH of the water in the hydroponic box was measured on days 5, 8, 10, 14, 17, 20, 23, 25, and 30. 10 mL of water samples were removed from the incubation box and filtered through a 0.45 μm filter for nitrogen, phosphorus, and heavy metal analysis. On day 30, wheat roots and stems (all parts above the seeds) were cleaned, dried, and ground into a powder. The powder was weighed, digested with concentrated nitric acid, and then brought to volume with ultrapure water. The sample was filtered and used with an inductively coupled plasma mass spectrometer (ICP-MS, Jena, Germany) to measure trace heavy metals in the crop. Nitrogen and phosphorus measurements were performed using the same methods as previously tested.

[0081] The experiment used a thermostatic magnetic stirrer with a constant temperature control module, and all experiments were performed at a temperature of 25 ± 0.5 ° C. A Hach HQ40d dual-input multiparameter instrument (HANNA, Italy) was used for accurate measurement of pH. Heavy metals in the precipitate solution were measured by flame atomic absorption spectrometry (AAS, Agilent 240DuoAA, USA). NH4 + -N and PO4 3--P was measured based on the colorimetric method of the American Public Health Association (APHA, 2012). Morphological analysis and elemental identification of the precipitates were performed using a field-emission electron probe microanalyzer (EPMA) (JXA-8530F Plus, JEOL Co. Ltd., Japan) combined with energy-dispersive X-ray spectroscopy (EDS). Physical phase identification of the precipitates was performed by X-ray diffraction (XRD, XPert Pro, Panaco, Holland). Surface elemental chemical analysis was performed using X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, USA) and Fourier transform infrared spectroscopy (FTIR5700, Thermo Fisher Scientific, USA). Experimental data were processed using Matlab 2016, Python, and SPSS 19.0. Response surface methodology (RSM) experimental design and visualization were performed using Design Expert 9.0 software. The Minteq model method was described in our previous study (Wang et al., 2022b). We also tested the specific surface areas of three samples (commercial struvite, A2 (Mg / Ca=1 / 0) and A3 (Mg / Ca=1 / 3)) using a high-throughput surface area and porosity analyzer (TriStar II3020, USA).

[0082] like Figure 5 As shown in the figure, the wheat height of the control group CK and the experimental groups (a-k) increased over time, but the growth trend of the experimental wheat in each group gradually decreased. Figure 6 The graphs show the time-dependent release of NH4-N from fertilizers in hydroponic boxes for the control group (CK) and experimental groups (a–k). In group b (no wheat was planted), the NH4-N concentration in the hydroponic box steadily increased over time, demonstrating that all groups treated with struvite fertilizer released NH4-N into the water. However, with the exception of group d (superphosphate fertilizer), the NH4-N concentration in the nutrient solution in each hydroponic box reached its highest concentration on the fifth day after fertilizer addition and then decreased continuously as the wheat grew. This demonstrates that wheat absorbs NH4-N during growth, resulting in a near-zero NH4-N concentration in the nutrient solution. This suggests that the NH4-N released by struvite fertilizer is insufficient to sustain crop growth.

[0083] according to Figure 7The graphs show the time-dependent release of PO4-P from fertilizers in the hydroponic culture box for the control group (CK) and experimental groups (a–k). No fertilizer was added to the hydroponic box in the control group (CK), resulting in undetectable phosphorus in the nutrient solution. Results from group b (no wheat was grown) show a gradual increase in phosphorus concentration in the nutrient solution. A comparison of groups a and f reveals that the phosphorus release rate of 1g of powdered struvite fertilizer is slightly higher than that of 1g of granular struvite. Notably, group h (which had the highest amount of struvite fertilizer added) showed a continuous increase in phosphorus release into the nutrient solution over time. This is due not only to the powdered fertilizer but also to the greater amount of struvite added compared to the other groups, resulting in the highest release rate of all struvite fertilizers. Therefore, the phosphorus release rate of powdered struvite is greater than that of granular struvite and is proportional to the amount added.

[0084] The release of phosphorus was detected in all other experiments. In the experiments of groups C and D, potassium dihydrogen phosphate and superphosphate were used as phosphorus fertilizers respectively. However, the nutrient solution of group C contained high phosphorus only on the 5th and 8th day. As the wheat grew longer, the phosphorus concentration gradually decreased, indicating that the wheat fully absorbed phosphorus. In the hydroponic box of group D, no phosphorus was detected. This may be related to the absorption of the wheat root system. The most direct evidence is that the wheat in this group had the longest root system on the 30th day ( Figure 8 ), that is, the phosphorus released by superphosphate greatly promotes the growth of wheat roots, and in turn, the rapid root growth accelerates the absorption of phosphorus. Ultimately, the remaining phosphorus concentration in the nutrient solution is very low.

[0085] Although the purity of struvite in real wastewater is relatively low, the experimental results of group e show that ( Figure 7 ), the phosphorus released by struvite recovered from real pig manure wastewater can also be detected, which shows that the struvite recovered from real pig manure wastewater can provide sufficient phosphorus for wheat growth. Similar to group d, the longer root system of wheat in group e may cause it to secrete more organic acids, which has a significant promoting effect on the release of struvite ( Figure 8 Comparing experimental groups a, i, j, and k that planted different amounts of wheat, it was found that granular struvite released excess phosphorus in the nutrient solution, which could meet the phosphorus needs of wheat.

[0086] In addition, the root lengths of the smaller crops in groups a and i were shorter than those in groups j and k, indicating that wheat growth was significantly affected by nitrogen shortages. It was observed that the smaller the number of cultivated wheat plants, the more adequate the root growth ( Figure 8). In short, for crop growth, struvite can provide both NH4-N and PO4-P nutrients. However, although the NH4-N content released by struvite (Mg(NH4)PO4·6H2O) is significantly higher than the other two phosphate fertilizers (combination c and group d), the NH4-N released by struvite in water is still insufficient to provide a reliable guarantee for crop growth compared to the PO4-P content required for crop growth. Therefore, struvite is a high-quality phosphate fertilizer, but it cannot be used entirely as a nitrogen fertilizer. It must be combined with other nitrogen fertilizers or supplemented with additional nitrogen fertilizers to meet the crop's demand for nitrogen nutrients.

[0087] according to Figure 9 As shown in the graph of the change of pH of the hydroponic culture of the control group CK and the experimental groups (a~k) over time, it can be seen that the pH of the nutrient solution of the control group (CK) continues to decrease. The reason is that in addition to releasing large molecular weight sugar compounds, the plant roots also release small molecular weight organic acids, such as citric acid, malic acid, oxalic acid and other organic acids. In particular, 7 to 8 different organic acids can be separated from the root secretions of crops such as barley, wheat, and rice. These organic acids secreted by the roots will cause the pH of the hydroponic nutrient solution to continue to decrease. The pH of group b is the highest among all groups, and it is still increasing. This is because the struvite fertilizer continuously releases NH4-N and PO4-P in the hydroponic box where wheat is not planted, which increases the pH value of the solution. The result that struvite causes the pH of the nutrient solution to continue to rise is consistent with the results of the indoor experiment ( Figure 2 ).according to Figure 2 As shown in the figure, the pH of struvite in the indoor hydroponic nutrient solution (tap water and additional nutrients) is higher than that in the hydroponic box nutrient solution (pure water). It can be seen that the change in pH is closely related to the initial culture nutrient solution. Except for group C (applied with potassium dihydrogen phosphate fertilizer), the pH continued to decline and was the lowest (pH < 4) compared with the pH values of all groups. The reason is that potassium dihydrogen phosphate fertilizer is easily soluble in water, which causes it to release a large amount of H + .

[0088] In summary, there is a complex antagonistic relationship between crop root exudates and struvite release. Organic acids secreted by wheat roots promote the release of struvite fertilizer, while the release of struvite fertilizer increases the pH of the nutrient solution, hindering the release of struvite nutrients. Insufficient nutrient concentrations may also affect root growth, thereby affecting the release of organic acids.

[0089] Comparing the release of heavy metals in the two experimental groups a and b with and without crops, it was found that struvite released more heavy metals into the nutrient solution of the crops (group a). Figure 10 The release of Cu over time in the hydroponic box of the control group CK and the experimental group (a~k) is shown. Figure 11The Zn release of the control group CK and the experimental group (a-k) in the hydroponic box over time is shown, and the absorption of heavy metals in the wheat roots and stems and leaves is greater ( Figure 12 ). Therefore, the organic acids secreted by the roots of crops will make the heavy metals in struvite more easily released and able to be absorbed by plants. Except for the control group (CK) where no fertilizer was added and no heavy metals were released, the granular struvite fertilizer (Group e) recovered from real wastewater released the least heavy metals. The reason is that compared with the heavy metal content in fertilizers recovered from synthetic wastewater, the struvite recovered from real wastewater itself has a lower heavy metal content. In particular, the Zn content in struvite is very low, which means that even if it is applied to the nutrient solution in the form of powder fertilizer, it will not release a large amount of Zn.

[0090] It is worth noting that, referring to Figure 10 , comparing the release patterns of Cu in water bodies of two groups (e, g) of struvite recovered from real wastewater, it was found that powdered struvite fertilizer was more likely to release heavy metal Cu. Comparing the heavy metal content released by two groups (a, f) of struvite fertilizer recovered from synthetic wastewater, it was finally found that the heavy metal content released by powdered struvite was greater than the heavy metal concentration released by granular struvite. Therefore, when the heavy metal content in the fertilizer is high enough, its release rate is related to the form of the fertilizer. In short, the heavy metals present in struvite are closely related to the form of fertilizer (powdered and granular after granulation) and the type of phosphorus recovery wastewater (real wastewater and synthetic wastewater) during the release process. and Figure 4 The results of the indoor vegetable hydroponic experiment are consistent with those shown. The struvite recovered from real wastewater has lower heavy metal release and wheat absorption than synthetic struvite due to its low heavy metal content.

[0091] The heavy metal release rates of the two commercial fertilizers (c and d) were both low, and the corresponding heavy metal content in the wheat roots and stems and leaves was also the lowest. The heavy metal content absorbed by the wheat roots and leaves in experimental groups e and g was lower than that of the other struvite fertilizer groups, and was close to the heavy metal content transferred to the plants by commercial fertilizers. The struvite recovered from real wastewater had a heavy metal release rate and plant absorption rate that was basically similar to commercial fertilizers. The struvite recovered from synthetic wastewater had a higher content of heavy metals released and a higher amount of heavy metals absorbed by plants, indicating that there is a risk of release of heavy metals stored in struvite. This further illustrates that it is very necessary and meaningful to regulate the reduction of heavy metals in struvite.

[0092] In addition, the amount of heavy metals released into the hydroponic box by struvite produced from real wastewater was lower than that of struvite recovered from synthetic wastewater ( Figure 10 and 11This experiment used real struvite recovered from wastewater initially containing high heavy metal concentrations. In reality, most real pig manure wastewater has relatively low heavy metal concentrations (Table 1.1). This means that struvite recovered from real wastewater is comparable to commercial phosphate fertilizer for agricultural use and is relatively safe.

[0093] In summary, from the perspective of fertilizer efficiency testing technology, the two groups of experiments showed that struvite is a high-quality phosphate fertilizer, and the phosphorus it releases is sufficient to meet the phosphorus growth needs of crops. The pH in the hydroponic process continued to rise as the struvite dissolved, similar to the release experiment. Struvite has a very good slow-release performance and can still provide rich nutrients (N, P, Mg) after 35 days of cabbage growth. In addition, there is still a lot of struvite fertilizer that has not dissolved. It can still provide nutrients for future vegetable cultivation. The struvite fertilizer recovered from synthetic wastewater and real wastewater does have a significant promoting effect on the growth of cabbage (wet weight, dry weight, leaf area, chlorophyll and carotenoids, etc.). In addition, the struvite recovered from synthetic wastewater has a higher purity than the struvite recovered from real wastewater, so it has a better fertilizer effect.

[0094] Heavy metal testing technology shows that struvite recovered from real wastewater has good fertilizer efficacy. Compared to struvite recovered from synthetic wastewater, it releases fewer heavy metals into the nutrient solution, resulting in lower heavy metal absorption by crops, thus meeting agricultural safety requirements. Furthermore, because the heavy metal content was too low, it was not detected in the hydroponic system. This indicates that the heavy metals in struvite fertilizer do not pose a hazard.

[0095] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for granulating struvite recovered from wastewater, characterized in that: The following steps are involved: S1. preparing a binder solution: heating and ultrasonically treating polyvinyl alcohol and sodium alginate respectively to obtain struvite powder granulation glue; S2. Mixing struvite powder with glue: The struvite powder prepared in step S1 is granulated into glue at a ratio of 5 g struvite powder to 5 mL glue, and the struvite powder and glue are mixed and kneaded to form larger agglomerates, wherein the struvite powder is separated from the pig manure wastewater by a chemical precipitation method; S3, granulation: the agglomerates prepared in step S1 are made into struvite granules of 5 mm in size and weighing 0.1 to 0.2 g per granule.

2. A method for granulating struvite recovered from wastewater according to claim 1, characterized in that: The S1 is specifically: S11, adding 6 g of polyvinyl alcohol to 500 mL of water, and completely dissolving the polyvinyl alcohol by heating and ultrasonic treatment; S12, adding 5g of sodium alginate to the polyethylene solution completely dissolved in step S11, and continuing heating and ultrasonic treatment to completely dissolve the sodium alginate; S13, concentrating the mixed solution of polyethylene and sodium alginate completely dissolved in step S11 to 250 mL and cooling the solution to obtain struvite powder granulation glue.

3. Use of the struvite granules prepared by the granulation method of struvite recovered from wastewater according to any one of claims 1 to 2 in a hydroponic agricultural system.

4. The use of struvite granules in a hydroponic system according to claim 3, characterized in that: The hydroponic agricultural system comprises a PVC-U environmentally friendly water pipe (1) for building a plant growth frame, a circular customized basket (2) for cultivating plants (9) is placed in the built plant growth frame, cut sponges (3) and prepared struvite particles are placed in the circular customized basket (2), and a hose (5) is used to connect the PVC-U environmentally friendly water pipe (1) and a water pump (4) so that the water pump can pump water into the water pipe.

5. The use of struvite granules in a hydroponic system according to claim 4, characterized in that: A solar panel (6) and a lithium battery (7) are assembled on the water pump (4) for power supply.

6. The use of struvite granules in a hydroponic system according to claim 5, characterized in that: The water pump (4) is connected to a timer (8).

7. The use of the struvite granules in a hydroponic system according to claim 6, characterized in that: The method for using the hydroponic agricultural system comprises the following steps: S51. Assemble and build equipment: Assemble environmentally friendly PVC-U water pipes, build a plant growth rack, place a custom round basket in the plant growth rack, place the cut sponges in the custom round basket, place the prepared struvite granules in the custom round basket, connect the environmentally friendly PVC-U water pipes and water pump with hoses, and assemble the solar panels and lithium batteries for power. S52. Seed pretreatment: Soak the seeds for 1 hour. Place the soaked seeds in a wet towel to allow the seeds to blanch (germinate). Use sterilized tweezers and cotton swabs to bury the blanched seeds in the gaps of the cut sponge, ensuring that the seeds are buried to a depth of 1.5 cm and with the seed buds facing upwards. S53. Initial Cultivation: Turn on the water pump and timer, and incubate the cabbage seedlings in an incubator for 7 days at a temperature of 30°C. The pH of the nutrient solution is 7.5-9.2, and the hydroponic cycle time is 9 hours. Lowering the pH can increase the release rate of struvite phosphorus, but lowering the pH of the nutrient solution is detrimental to the growth of the cabbage. Increasing the hydroponic cycle time promotes phosphorus release and cabbage growth. Struvite is a supplement to counteract substrate acidity. S54. Transplanting and subsequent cultivation: Transplant the cabbage seedlings after 7 days of cultivation into a hydroponic equipment and continue to cultivate in the hydroponic equipment for 37 days. The nutrient solution in the hydroponic water tank mainly consists of 300 mg / L KNO3, 400 mg / L calcium carbonate, 10 mg / L Fe-EDTA, 3 mg / L boric acid, and 2 mg / L manganese sulfate.

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