Linen-based biological filler, preparation method, circulating biological tank and application

Abstract

The invention relates to a preparation method of a flax-based biological filler, which comprises the following steps: putting flax chips into an iron solution, fully stirring, transferring into a sealed low-vacuum container, fully soaking the flax chips in the iron solution, filtering the mixture, performing heat treatment at 450-850 DEG C in a nitrogen atmosphere, then performing heat treatment at 500-900 DEG C in a carbon dioxide atmosphere, and drying to obtain the flax-based biological filler. The flax magnetic biochar is obtained; mixing the flax magnetic biochar, sodium methyl cellulose and animal glue according to a mass ratio of (75-90): (8-20): (2-10), and preparing a magnetic particle filler; and modifying the surface of the magnetic particle filler by using a modifier to obtain the flax-based biological filler with the water content of 5-10%. The invention relates to application of a flax-based biological filler. The flax-based biological filler is applied to treatment of circulating aquaculture wastewater. The flax-based biological filler has the beneficial effects that the flax-based biological filler has an efficient pollutant removal effect and meets the current low-carbon and sustainable environmental governance requirements.

Description

Technical Field

[0001] This invention relates to the field of aquaculture wastewater technology, specifically to a flax-based biological packing material and its preparation method, a circulating biological tank, and its application. Background Technology

[0002] Flax fiber is a natural fiber known for its high strength, toughness, and abrasion resistance. Flax plants grow rapidly and require minimal fertilizers and pesticides, resulting in low environmental pollution and making it more sustainable than traditional coal, timber, or general crops. China is one of the world's major flax fiber producers, with a flax planting area of ​​8,200 hectares and a yield of 44,000 tons in 2020. Flax shavings are a byproduct of flax processing, primarily composed of holocellulose (up to 76.31%). Compared to other crop straws, flax shavings offer significant advantages, including higher cellulose content, more concentrated distribution, and lower cost. On the other hand, with the development of industrialized aquaculture, wastewater treatment has become a hot topic. Currently, filter media are commonly used to purify and recycle aquaculture water. However, due to the high number of water cycles and large volumes of water processed in aquaculture, fish have a much lower tolerance for concentrations of ammonia nitrogen and nitrite than typical wastewater treatment limits. This places higher demands on the formation of biofilms and the purification effect of the filter media. Furthermore, in existing biological filtration devices, the filter media is constantly submerged in water, preventing oxygen from penetrating to every corner, leading to reduced purification efficiency and easy accumulation of sludge in the tank. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a flax-based biological filler and its preparation method, a circulating biological tank and its application, so as to overcome the shortcomings of the prior art.

[0004] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: A method for preparing a flax-based biofiller includes the following steps: S10. Add flax shavings to the iron solution and stir thoroughly. Then transfer the mixture to a sealed low vacuum container and allow the flax shavings to soak fully in the iron solution. Filter the mixture and heat it at 450℃~850℃ for 45min~75min under a nitrogen atmosphere, and then heat it at 500℃~900℃ for 80min~100min under a carbon dioxide atmosphere to obtain flax magnetic biochar. S20. Mix flax magnetic biochar, sodium methylcellulose, and animal glue in a mass ratio of (75-90):(8-20):(2-10) and prepare magnetic particle filler. S30. Modify the surface of the magnetic particle filler with a modifier to obtain flax-based biological filler with a moisture content of 5% to 10%.

[0005] Based on the above technical solution, the present invention can be further improved as follows.

[0006] Furthermore, the specific details of S10 are as follows: S11. Add the dried flax shavings to a 0.2 mol / L to 2 mol / L iron solution, stir thoroughly, and then transfer to a sealed low vacuum container to allow the flax shavings to be fully soaked in the iron solution. S12. Filter the mixture and dry it; S13. Heat treatment at 450℃~850℃ for 45min~75min under nitrogen atmosphere to form a porous structure of carbon skeleton, while iron mineral particles are formed and embedded in carbon skeleton. S14. Continue heat treatment at 500℃~900℃ for 80min~100min in a carbon dioxide atmosphere to promote the formation of micropores and mesopores, thereby increasing the specific surface area and optimizing the pore structure. During the high-temperature activation process, the iron minerals are further converted into ferromagnetic particles. Cool to room temperature and pulverize into powder to obtain flax magnetic biochar.

[0007] Furthermore, the iron solution is prepared using ferric chloride, ferric sulfate, ferric nitrate, or ferric acetate, with a mass ratio of iron to flax shavings of (3-30):100. The flax shavings are fully soaked in the iron solution for 2-4 hours, and the pressure inside the low vacuum container is 50 Torr-200 Torr.

[0008] Furthermore, under 1 Tesla condition, the mass magnetization value of flax magnetic biochar is 3 Am. 2 / Kg~30Am 2 / Kg.

[0009] Furthermore, the specific details of S20 are as follows: S21. Dissolve sodium methylcellulose and animal glue in water to form a stable colloidal solution with a mass concentration of 40% to 60%. S22. Gradually add flax magnetic biochar powder and stir thoroughly for 30 min to 60 min. The water content of the mixture should be 10% to 15%. The mass ratio of flax magnetic biochar, sodium methylcellulose and animal glue should be (75 to 90): (8 to 20): (2 to 10). S23. Press the mixture into spherical particles with a particle size of 5 mm to 10 mm, and then vacuum dry it; S24. The spherical particles are heat-treated at 180℃~220℃ to form a stable carbonized skeleton of sodium methylcellulose and animal glue, thereby enhancing the structural stability and strength of the granulated particles and obtaining magnetic particle fillers.

[0010] Furthermore, sodium methylcellulose and animal glue mainly function as adhesives. Sodium methylcellulose is an important water-soluble cellulose ether derivative. Due to the introduction of highly polar sodium carboxymethyl ether into the cellulose molecule, it has adhesive and stabilizing colloidal properties. Animal glue is mainly derived from poultry and livestock processing. It is composed of peptide chains and contains amino and carboxyl groups. These functional groups endow it with good adhesive properties. At the same time, its good water solubility allows it to form a stable colloidal structure in aqueous solution.

[0011] Furthermore, the magnetic particle filler has a mechanical strength of 5 MPa to 10 MPa and a specific surface area of ​​150 m². 2 / g~250m 2 / g.

[0012] Furthermore, the specific details of S30 are as follows: S31. Under low vacuum conditions, tumble the magnetic particle filler, first uniformly spray a polysaccharide solution with a mass fraction of 3% to 6%, then spray a biological enzyme solution with a mass fraction of 0.5% to 1.5%. The mass ratio of magnetic particle filler: polysaccharide: biological enzyme is 300: (1.3 to 4.0): (0.15 to 0.1). S32. Continue to tumble the packing under low vacuum and then ventilate and dry it to obtain flax-based biological packing with a moisture content of 5% to 10%.

[0013] Furthermore, in order to further improve the biocompatibility of the magnetic particle packing, especially its ability to induce rapid biofilm growth in the early stages, the surface of the magnetic particle packing is modified. The modifiers include polysaccharides and bioenzymes.

[0014] Furthermore, the polysaccharides include one or more combinations of chitosan, sodium alginate, and cyclodextrin. Chitosan is extracted from the shells of crustaceans, sodium alginate is a hydrophilic polysaccharide from algae, and cyclodextrin is derived from starch enzymatic hydrolysis. These polysaccharides are widely available and contain abundant functional groups such as hydroxyl, carboxyl, and amino groups. They have high hydrophilicity and biocompatibility, which can improve the complexation and adsorption capacity of the magnetic particle packing surface for pollutants and promote the formation of biofilms on the magnetic particle packing surface. Biological enzymes include: peroxidase, protease and urease. Peroxidase can degrade drug residues, protease can decompose protein pollutants, and urease can degrade urea. Aquaculture wastewater contains a large amount of antibiotic and other drug residues, high-nitrogen feed residues and fish metabolites, i.e., the content of drug molecules, protein molecules and urea is high. By modifying these enzymes, the degradation of corresponding pollutants can be promoted and the biofilm can be accelerated. The mass ratio of peroxidase, protease and urease is (1-5): (65-75): (20-34).

[0015] Based on the above technical solution, the present invention also provides a flax-based biological filler, which is prepared by the above preparation method.

[0016] Based on the above technical solution, the present invention also provides an application of flax-based biological packing material, which is used to treat wastewater from recirculating aquaculture.

[0017] Based on the above technical solution, the present invention also provides a circulating biological tank, comprising: a packing support, a circulation device, a biological tank, and flax-based biological packing. The biological tank has a water circulation zone and a biological filtration zone. The circulation device is located in the water circulation zone of the biological tank and includes: an outer pipe, an inner pipe, and a circulation float. The upper end of the outer pipe is closed, and the lower end is provided with an orifice. A gap of a certain depth is formed between the lower end of the outer pipe and the bottom of the biological tank. The inner pipe is inserted into the outer pipe. The inner pipe is open at the top and bottom and penetrates the bottom of the biological tank. An external water passage is formed between the lower end of the outer pipe and the inner pipe. An internal water passage is formed between the upper end of the outer pipe and the inner pipe, which discharges to the outside of the tank. A circulation float surrounds the outer pipe. The circulating float is set with the same buoyancy as the gap between the lower opening of the outer pipe and the bottom of the biological tank; when the water level in the biological tank rises to the upper end of the inner pipe, the air trapped between the outer and inner pipes is expelled, thereby generating a siphon effect to discharge the water out of the tank. When the water level in the biological tank drops to near the opening at the lower end of the outer pipe, the siphon stops due to the blocking effect of the circulating float, and the water level rises and falls repeatedly. The circulating float includes a hollow cylinder, a specific gravity adjustment ring, and a stabilizing plate. The inner diameter of the hollow cylinder is 1.2 to 2.0 times the outer diameter of the outer tube 310. The specific gravity adjustment ring is used to adjust the weight of the circulating float so that it can touch the bottom or nearly touch the bottom when it is near the opening at the lower end of the outer tube, so as to prevent water from flowing to the outer tube. The weight of the specific gravity adjustment ring is equal to the specific gravity of the opening at the lower end of the outer pipe and the water depth at the bottom of the biological tank. The stabilizing plate is used to expand the lower surface of the circulation float. When the circulation float is lowered to near the opening of the outer pipe, it increases the water flow resistance through the lower surface of the circulation float, ensuring a rapid reduction in water volume or a rapid cessation of water flow. When the water level in the biological tank is high enough, there is enough space between the lower surface of the circulating float and the bottom of the biological tank. Water will not be subject to special resistance and will be drawn in from the opening of the outer pipe and discharged to the outside of the biological tank through the inner pipe, so that the water level of the circulating float and the water level of the biological tank are the same. When the water level in the biological tank drops, the circulating float also drops, reducing the space between the lower surface of the circulating float and the bottom of the biological tank. The lower surface of the circulating float creates resistance to the inflow of water, making it difficult for the water to flow. As it continues to drop, the side wall of the circulating float will block the water flow, reducing the amount of water flowing between the circulating float and the outer pipe, thus reducing the amount of water flowing into the outer pipe. The water level in the circulating float is lower than the water level in the biological tank. As the circulating float continues to descend to near the opening of the outer pipe, the amount of water flowing into the outer pipe decreases further, air is drawn into the outer pipe, the siphon mechanism cannot be maintained, the siphon stops, and the water level in the biological tank begins to rise. The packing support is located in the biological filtration zone of the biological tank. The flax-based biological packing is located on the packing support. The packing support at the bottom of the biological tank has a slope and tilts towards the water circulation zone. The minimum height of the packing support from the bottom of the biological tank is greater than the height from the lower end opening line of the outer pipe to the bottom of the biological tank. The lower end opening line of the outer pipe refers to the horizontal line of the highest point of the opening.

[0018] The beneficial effects of this invention are: Biochar fillers prepared from flax flakes exhibit excellent material properties and environmental friendliness. This is because flax flakes are rich in fiber, which helps to form a stable carbon skeleton. They are also low in ash, which effectively reduces impurities during carbonization. Compared with traditional fuels (such as coal and wood), flax plants have a shorter growth cycle and require less fertilizer, making them a more sustainable biomass source. In addition, introducing magnetic materials into biochar can enhance the electron transfer capacity of microorganisms, reduce electron transfer resistance, and promote microbial metabolic activities, thereby improving the ability to degrade pollutants. However, because the planar layers of carbon atoms that make up biochar are disordered and irregular, their wear resistance and compressive strength are very poor. Furthermore, powdered biochar is inconvenient to use and transport. By granulating, it can be made to have a certain shape, improve strength, and be easy to apply in actual wastewater treatment. Fish delivery time has a significant impact on economic returns. Shortening the biofilm formation time of the packing material can reduce the preparation time for aquaculture. To solve the above problems, magnetic biochar was prepared from flax shavings and shaped into granular packing material. Further modification was carried out to improve biofilm formation efficiency. The prepared flax-based biological packing material with magnetic promotion effect was combined with a circulating biological tank. The circulation device in the biological tank can automatically raise and lower the water level repeatedly, improve oxygen penetration in the packing material, reduce installation and maintenance costs, and achieve low-carbon and green treatment of aquaculture wastewater. Benefits of Circulating Biotanks: The use of highly efficient and eco-friendly flax-based biological packing material, with its magnetic and modified components, attracts microbial aggregation, significantly accelerating biofilm formation. Magnetic materials enhance electron transfer capabilities, reduce electron transfer resistance during pollutant transformation, induce enzyme synthesis, and promote microbial metabolic activity, thereby improving the degradation of pollutants such as organic matter and ammonia nitrogen in aquaculture wastewater. Intermittent reoxygenation is achieved by altering the water level within the biotank through a water circulation system. Compared to continuous soaking, this improves oxygen diffusion and penetration within the packing material, significantly enhancing the activity of aerobic microorganisms. Furthermore, the simple structure of this circulation device requires no additional power, and even in large-scale water circulation systems, it can continuously and stably repeat water level rises and falls at regular intervals, effectively avoiding the accumulation of sludge at the bottom of continuous systems. This significantly reduces overall installation and maintenance costs and size. The innovative application of using more sustainable flax scraps to prepare the packing material and combining it with the circulating biotank for wastewater purification broadens the value chain of flax waste, contributing to a greener, lower-carbon water treatment system and providing new solutions for ecological sustainability. Attached Figure Description

[0019] Figure 1 This is a flowchart of the preparation method of flax-based biofiller in this invention; Figure 2 This is a structural diagram of the circulating biological tank in this invention; Figure 3 This is a top view of the biological tank in this invention; Figure 4 This is a structural diagram of the circulating float in this invention; Figure 5 This is a schematic diagram of the water level circulation in the circulating biological tank of the present invention.

[0020] The attached diagram lists the components represented by each number as follows: 1. Flax-based biological packing material; 2. Packing material support; 3. Circulation device; 310. Outer pipe; 311. Perforation line; 320. Inner pipe; 330. Circulation float; 331. Hollow cylinder; 332. Specific gravity adjustment ring; 333. Stabilizing plate; 340. External water channel; 350. Internal water channel; 4. Biological tank; 410. Water circulation zone; 420. Biological filtration zone. Detailed Implementation

[0021] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0022] Example 1 like Figure 1 As shown, a method for preparing a flax-based biofiller includes the following steps: S10. Fine flax shavings (5mm-10mm in length) were added to a 0.4mol / L ferric nitrate solution at a mass ratio of 28:100 (iron to flax shavings). After thorough stirring, the mixture was transferred to a sealed low-vacuum container at a pressure of 50-200 Torr, allowing the flax shavings to soak in the iron solution for 2 hours. The mixture was then filtered and dried under vacuum. Heat treatment was then carried out at 780℃ for 65 minutes under a nitrogen atmosphere to form a porous carbon framework, while iron mineral particles formed and embedded within the carbon framework. Further heat treatment was carried out at 800℃ for 85 minutes under a carbon dioxide atmosphere to promote the formation of micropores and mesopores, thereby increasing the specific surface area, optimizing the pore structure, and further converting the iron minerals into ferromagnetic particles during the high-temperature activation process. The mixture was cooled to room temperature and pulverized into powder to obtain flax magnetic biochar. Testing showed that the mass magnetization of the flax magnetic biochar was 6.69 Am under 1 Tesla conditions. 2 / Kg; S20. Dissolve sodium methylcellulose and animal glue in water to form a stable colloidal solution with a mass concentration of 52%. Gradually add flax magnetic biochar powder and stir thoroughly for 30 minutes. The water content of the mixture is approximately 12%, and the mass ratio of flax magnetic biochar, sodium methylcellulose, and animal glue is 85:10:5. Use an extrusion granulator to press the mixture into spherical particles with a particle size of approximately 8 mm and vacuum dry them. Heat-treat the spherical particles at 200℃ to form a stable carbonized framework of organic molecules from sodium methylcellulose and animal glue, thereby enhancing the structural stability and strength of the granulated particles. The resulting magnetic particle filler has an average mechanical strength of 5.8 MPa and an average specific surface area of ​​156 m². 2 / g; S30. Under low vacuum conditions, the magnetic particle packing is tumbled, and a 3% (w / w) chitosan solution is sprayed evenly first, followed by a 0.5% (w / w) bio-enzyme solution. The mass ratio of magnetic particle packing to polysaccharide to bio-enzyme is 300:1.98:0.165. The polysaccharide includes one or more combinations of chitosan, sodium alginate, and cyclodextrin. The bio-enzyme includes peroxidase, protease, and urease, and the mass ratio of peroxidase, protease, and urease is 1.5:70.5:28. The packing is then tumbled again under low vacuum and ventilated to dry, thus obtaining flax-based bio-packing M-CG with a moisture content of 5% to 10%.

[0023] The effectiveness of the packing material of this invention was studied through a small-scale test using simulated wastewater. For comparison, a non-magnetic modified packing material MG was prepared using the same method as the above packing material but omitting the iron adhesion process, and a magnetic particle packing material MC was prepared using the same method but omitting the modification process. Additionally, it was compared with the commonly used elastic K5 packing material on the market. The small-scale test was conducted in a 5-liter plastic container with an inlet pipe at the bottom and an outlet pipe at the top. Packing materials with a bulk volume of 2 liters were weighed and introduced into simulated recirculating aquaculture system wastewater at a rate of 25 mL / min. The wastewater contained COD of 50 mg / L, ammonia nitrogen of 2 mg / L, and nitrite of 1.5 mg / L. The simulated wastewater was pre-oxygenated. The test was conducted continuously for 60 days, and the water indicators were monitored periodically. The results are shown in Table 1 below. Table 1. Pollutant treatment effect of different packing materials

[0024] Small-scale experimental results show that the prepared flax-based biological packing material M-CG can efficiently remove pollutants, especially ammonia nitrogen and nitrite, which have a significant impact on fish growth. Compared with the unmodified MC packing material, this packing material has a shorter biofilm formation time, and its removal effect is significantly improved compared with the unmagnetized MG packing material and the commonly used K5 packing material. This indicates that the biological removal effect of the flax-based packing material is enhanced by magnetization treatment, and rapid biofilm formation can be achieved through modification. In other words, the sustainable flax-based biological packing material M-CG used in this invention meets the requirements of rapid and efficient circulating water treatment in the current context.

[0025] Example 2 like Figure 1 As shown, a method for preparing a flax-based biofiller includes the following steps: S10. Fine flax shavings (5mm-10mm in length) were added to a 0.4mol / L ferric nitrate solution at a mass ratio of 25:100 (iron to flax shavings). After thorough stirring, the mixture was transferred to a sealed low-vacuum container at a pressure of 50-200 Torr, allowing the flax shavings to soak in the iron solution for 2 hours. The mixture was then filtered and vacuum-dried. Under a nitrogen atmosphere, it was heat-treated at 800℃ for 64 minutes to form a porous carbon skeleton, while iron mineral particles formed and embedded within the carbon skeleton. Further heat treatment at 800℃ for 95 minutes under a carbon dioxide atmosphere promoted the formation of micropores and mesopores, increasing the specific surface area and optimizing the pore structure. During the high-temperature activation process, the iron minerals were further converted into ferromagnetic particles. After cooling to room temperature, the mixture was pulverized into powder to obtain flax magnetic biochar. Testing showed that the mass magnetization of the flax magnetic biochar was 6.58 Am under 1 Tesla conditions. 2 / Kg; S20. Dissolve sodium methylcellulose and animal glue in water to form a stable colloidal solution with a mass concentration of 50%. Gradually add flax magnetic biochar powder and stir thoroughly for 30 minutes. The water content of the mixture is approximately 12%, and the mass ratio of flax magnetic biochar, sodium methylcellulose, and animal glue is 85:10:5. Use an extrusion granulator to press the mixture into spherical particles with a particle size of approximately 8 mm and vacuum dry them. Heat-treat the spherical particles at 205℃ to form a stable carbonized framework of organic molecules from sodium methylcellulose and animal glue, thereby enhancing the structural stability and strength of the granulated particles. The resulting magnetic particle filler has an average mechanical strength of 6.1 MPa and an average specific surface area of ​​162 m². 2 / g; S30. Under low vacuum conditions, the magnetic particle packing is tumbled, and a 4.3% (w / w) cyclodextrin solution is sprayed evenly first, followed by a 0.65% (w / w) bio-enzyme solution. The mass ratio of magnetic particle packing to polysaccharide to bio-enzyme is 300:2.15:0.33. The polysaccharide includes one or more combinations of chitosan, sodium alginate, and cyclodextrin. The bio-enzyme includes peroxidase, protease, and urease, and the mass ratio of peroxidase, protease, and urease is 3:72:25. The packing is then tumbled again under low vacuum and ventilated to dry, thus obtaining flax-based bio-packing M-CG2 with a moisture content of 5% to 10%.

[0026] like Figures 2-5 As shown, a circulating biological tank includes: flax-based biological packing material 1, packing support 2, circulation device 3, and biological tank 4; the biological tank 4 has a water circulation zone 410 and a biological filtration zone 420, and the circulation device 3 is located in the water circulation zone 410 of the biological tank 4. The circulation device 3 includes an outer pipe 310, an inner pipe 320, and a circulation float 330. The principle of water circulation is as follows: the upper end of the outer pipe 310 is closed, and the lower end is provided with an orifice. A gap of a certain depth is formed between the lower end of the outer pipe 310 and the bottom of the biological tank 4. The inner pipe 320 is inserted into the outer pipe 310. The inner pipe 320 is open at the top and bottom, and the lower end of the inner pipe 320 passes through the bottom of the biological tank 4. The lower end of the outer pipe 310 and the inner pipe 320 form an outer water passage 340. The upper end of the outer pipe 310 and the inner pipe 320 form an inner water passage 350 that discharges to the outside of the tank. The circulation float 330 is surrounded by the outer pipe 310. The circulation float 330 is equipped with buoyancy that is the same as the gap depth between the lower end opening of the outer tube 310 and the bottom of the biological tank 4. When the water level in the biological tank 4 rises to the upper end of the inner pipe 320, the air trapped between the outer pipe 310 and the inner pipe 320 is expelled, thereby generating a siphon effect to discharge the water out of the tank. When the water level in the biological tank 4 drops to near the opening at the lower end of the outer pipe 310, the siphon is stopped by the blocking action of the circulation float 330. This process of raising and lowering the water level is repeated. Figure 5 The middle stages 1 to 4 are continuously cyclical to achieve cyclic intermittent aeration. The circulating float 330 includes a hollow cylinder 331, a specific gravity adjustment ring 332, and a stabilizing plate 333. The inner diameter of the hollow cylinder 331 is 1.2 to 2.0 times the outer diameter of the outer tube 310. The specific gravity adjustment ring 332 is used to adjust the weight of the circulating float 330 so that it can touch the bottom or nearly touch the bottom when it is near the lower end opening of the outer tube 310, so as to prevent water from flowing into the outer tube 310. The weight of the specific gravity adjustment ring 332 is almost equal to the specific gravity of the opening line 311 at the lower end of the outer tube 310 and the water depth at the bottom of the biological tank 4. The function of the stabilizer 333 is to expand the lower surface of the float. When the circulation float 330 descends to the vicinity of the opening of the outer tube 310, the water flow resistance through the lower surface of the circulation float 330 increases, ensuring a rapid reduction in water volume or a rapid cessation of water flow. The stabilizing plate 333 adopts a flange structure. The circulation float 330 can be made by combining rigid plastic, polyvinyl chloride, expanded polystyrene, polystyrene and other materials. The process of stopping the siphon effect of the circulating float 330 is as follows: When the water level in the biological tank 4 is high enough, there is enough space between the lower surface of the circulating float 330 and the bottom of the biological tank 4. Water will not be subject to special resistance and will be drawn in from the opening of the outer pipe 310 and discharged to the outside of the biological tank 4 through the inner pipe 320. At this time, the water level of the circulating float 330 is the same as the water level of the biological tank 4. When the water level in the biological tank 4 drops, the circulating float 330 also drops, reducing the space between the lower surface of the circulating float 330 and the bottom of the biological tank 4. The lower surface of the circulating float 330 creates resistance to the inflow of water, making it difficult for the water to flow. As it continues to drop, the side wall of the circulating float 330 blocks the water flow, thus reducing the water flow between the circulating float 330 and the outer pipe 310, thereby reducing the amount of water flowing into the outer pipe 310. At this point, the water level in the circulating float 330 is lower than the water level in the biological tank 4. When the circulating float 330 continues to drop to near the opening of the outer pipe 310, the amount of water flowing into the outer pipe 310 decreases further, and air is drawn into the outer pipe 310, making it impossible to maintain the siphon mechanism. The siphon stops, and as a result, the water level in the biological tank 4 begins to rise. The packing support 2 is located in the biological filtration zone 420 of the biological tank 4, and the flax-based biological packing 1 is located on the packing support 2. The packing support 2 at the bottom of the biological tank 4 has a slope and is inclined towards the water circulation zone 410. The minimum height of the packing support 2 from the bottom of the biological tank 4 is slightly greater than the height from the lower end opening line 311 of the outer pipe 310 to the bottom of the biological tank 4. The lower end opening line 311 of the outer pipe 310 refers to the horizontal line of the highest point of the opening.

[0027] Benefits of circulating biological tanks: 1) High-efficiency and ecological flax-based biological packing material 1 is used. The magnetic substances and modified materials in the flax-based biological packing material 1 can attract the aggregation of microorganisms, greatly accelerate the formation of biofilm, enhance the electron transfer ability of microorganisms, reduce the electron transfer resistance during the transformation of pollutants, induce the synthesis of microbial enzymes, promote microbial metabolic activities, thereby improving the ability to degrade pollutants such as organic matter and ammonia nitrogen in aquaculture wastewater. 2) The water level in the biological tank 4 is changed by the water circulation device 3 to achieve intermittent reoxygenation. Compared with continuous soaking, it can improve the diffusion and penetration of oxygen in the packing material and significantly improve the activity of aerobic microorganisms. At the same time, the circulation device has a simple structure and does not require additional power. Even in a large water circulation system, it can continuously and stably repeat the rise and fall of the water level at a certain time interval, which can effectively avoid the phenomenon of easy accumulation of sludge at the bottom of the continuous device. The overall installation and maintenance cost and volume are greatly reduced. 3) By using more sustainable flax scraps to prepare fillers and combining them with circulating biological tanks to purify wastewater, this innovative application broadens the value chain of flax waste, helps to build a greener, low-carbon water treatment system, and provides new solutions for ecological sustainability.

[0028] In Example 2, a 40L circulating biological tank was prepared, with dimensions of 500mm (length), 300mm (width), and 265mm (height). A 15L flax-based biological packing material was installed on a packing support. The bottom of the support was slightly angled towards the water circulation zone, approximately 30mm from the bottom of the tank. Both the outer and inner pipes in the circulation zone were made of PVC. The outer pipe had a diameter of 63mm and a length of 257mm, with its lower part standing upright at the bottom of the tank. Three triangular openings, each 25mm high, were cut into the lower part of the outer pipe. The inner pipe had a diameter of 25mm and a length of 390mm, extending 320mm through the bottom of the tank. The circulation... The hollow tube of the float has an inner diameter of 90mm and a height of 135mm. A 40mm thick, 50mm high layer of foam plastic is bonded to the outside of the hollow tube. A 50mm wide rubber flange, weighing 600g, is used as a stabilizing plate. Wastewater is introduced into the circulating biological tank at an influent flow rate of 0.5L / min. After stabilization, the water level in the tank circulates approximately once every 4 seconds, ensuring sufficient oxygen supply to the packing material. A continuous flow device of the same volume was also prepared as a control group, with water entering and exiting from the top of the tank. To ensure oxygen supply to the packing material, an air pump was used at the bottom of the device. The experimental results are shown in Table 2 below. Table 2 Pollutant removal status of different devices

[0029] The results showed that the use of a circulating biological tank can achieve natural and effective oxygen supply in the packing material without the need for additional oxygen supply power. In contrast, in general devices, the packing material is immersed in water, but the air dissolution in water is limited, and oxygen cannot effectively penetrate to all corners of the packing material, resulting in insufficient pollutant removal. By controlling the siphon to achieve repeated changes in the water level in the tank, effective oxygen diffusion can be achieved. At the same time, combined with more sustainable hemp-based packing material, low-carbon and high-efficiency treatment of circulating aquaculture water can be achieved.

[0030] Example 3 like Figure 1 As shown, a method for preparing a flax-based biofiller includes the following steps: S10. Fine flax shavings (5mm-10mm in length) were added to a 0.3mol / L ferric nitrate solution at a mass ratio of 20:100 (iron to flax shavings). After thorough stirring, the mixture was transferred to a sealed low-vacuum container at a pressure of 50-200 Torr, allowing the flax shavings to soak in the iron solution for 2 hours. The mixture was then filtered and vacuum-dried. Under a nitrogen atmosphere, it was heat-treated at 810℃ for 60 minutes to form a porous carbon skeleton, while iron mineral particles formed and embedded within the carbon skeleton. Further heat treatment was carried out under a carbon dioxide atmosphere at 820℃ for 90 minutes to promote the formation of micropores and mesopores, thereby increasing the specific surface area and optimizing the pore structure. During the high-temperature activation process, the iron minerals were further converted into ferromagnetic particles. After cooling to room temperature, the mixture was pulverized into powder to obtain flax magnetic biochar. Testing showed that the mass magnetization value of the flax magnetic biochar was 5.48 Am under 1 Tesla conditions. 2 / Kg; S20. Dissolve sodium methylcellulose and animal glue in water to form a stable colloidal solution with a mass concentration of 55%. Gradually add flax magnetic biochar powder and stir thoroughly for 30 minutes. The water content of the mixture is approximately 12%, and the mass ratio of flax magnetic biochar, sodium methylcellulose, and animal glue is 83:12:5. Use an extrusion granulator to press the mixture into spherical particles with a particle size of approximately 8 mm and vacuum dry them. Heat-treat the spherical particles at 195℃ to form a stable carbonized framework of organic molecules from sodium methylcellulose and animal glue, thereby enhancing the structural stability and strength of the granulated particles. The resulting magnetic particle filler has a mechanical strength of 6.3 MPa and a specific surface area of ​​173 m². 2 / g; S30. Under low vacuum conditions, the magnetic particle packing is tumbled, and first a 4% sodium alginate solution is sprayed evenly, followed by a 0.5% bio-enzyme solution. The mass ratio of magnetic particle packing to polysaccharide to bio-enzyme is 300:2:0.25. The polysaccharide includes one or more combinations of chitosan, sodium alginate, and cyclodextrin. The bio-enzyme includes peroxidase, protease, and urease, and the mass ratio of peroxidase, protease, and urease is 2.5:70:27.5. The packing is then tumbled again under low vacuum and ventilated to dry, thus obtaining flax-based bio-packing M-CG3 with a moisture content of 5% to 10%.

[0031] A practical recirculating aquaculture system was constructed, including a culture tank, a rotary drum microfilter, a recirculating intermittent aeration biological tank, and a clear water tank. Water in the clear water tank, after disinfection, is returned to the culture tank, which is stocked with 500 bass fry. Water purification is primarily achieved through a recirculating biological tank containing packing material. The biological tank has a volume of 510 liters and contains approximately 200 liters of packing material. The device dimensions are 110mm × 850mm × 560mm. The bottom packing material support is slightly inclined towards the water circulation zone, with the bottom of the support approximately 40mm from the tank bottom. The inner and outer pipes of the water circulation zone are made of PVC. The outer pipe has a diameter of 125mm and a length of 520mm, with its lower end contacting the tank bottom and featuring four triangular openings approximately 35mm high. The inner pipe has a diameter of 50mm, a length of 920mm, and a length of 425mm penetrating the tank bottom. The hollow tube of the circulating float has an inner diameter of 160mm and a height of 215mm. A 60mm thick and 50mm high layer of foam plastic is adhered to the outside of the hollow tube. A 65mm wide rubber flange is used as a stabilizing plate, weighing 1940g. Wastewater is introduced into the circulating biological tank at a flow rate of 1.7L / min. After stabilization, the water level in the tank circulates approximately once every 4 seconds, ensuring sufficient oxygen for the packing material. The water in the aquaculture tank is circulated 10-12 times daily, with a feed coefficient of approximately 1.2. After nearly six months of continuous aquaculture, the circulating biological tank has operated stably, with no sludge accumulation at the bottom. The COD, ammonia nitrogen, and nitrite concentrations in the aquaculture tank are 15.23±2.56, 0.23±0.14, and 0.34±0.41, respectively, meeting the aquaculture requirements.

[0032] The flax-based biological packing material in this invention not only has a highly efficient pollutant removal effect, but also enables natural, non-powered oxygen supply when applied to a circulating biological tank. Furthermore, the repeated rise and fall of the water level can prevent the accumulation of sludge. In short, this invention meets the current requirements for low-carbon and sustainable environmental governance.

[0033] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a flax-based biofiller, characterized in that, Includes the following steps: S10. Add flax shavings to the iron solution and stir thoroughly. Then transfer the mixture to a sealed low vacuum container and allow the flax shavings to soak fully in the iron solution. Filter the mixture and heat it at 450℃~850℃ for 45min~75min under a nitrogen atmosphere, and then heat it at 500℃~900℃ for 80min~100min under a carbon dioxide atmosphere to obtain flax magnetic biochar. S20. Mix flax magnetic biochar, sodium methylcellulose, and animal glue in a mass ratio of (75-90):(8-20):(2-10) and prepare magnetic particle filler. S30. Modify the surface of the magnetic particle filler with a modifier to obtain flax-based biological filler with a moisture content of 5% to 10%.

2. The preparation method according to claim 1, characterized in that, The specific details of S10 are as follows: S11. Add the dried flax shavings to a 0.2 mol / L to 2 mol / L iron solution, stir thoroughly, and then transfer to a sealed low vacuum container to allow the flax shavings to be fully soaked in the iron solution. S12. Filter the mixture and dry it; S13. Heat treatment at 450℃~850℃ for 45min~75min under nitrogen atmosphere to form a porous structure of carbon skeleton, while iron mineral particles are formed and embedded in carbon skeleton. S14. Continue heat treatment at 500℃~900℃ for 80min~100min in a carbon dioxide atmosphere to promote the formation of micropores and mesopores, thereby increasing the specific surface area and optimizing the pore structure. During the high-temperature activation process, the iron minerals are further converted into ferromagnetic particles. Cool to room temperature and pulverize into powder to obtain flax magnetic biochar.

3. The preparation method according to claim 2, characterized in that, The iron solution is prepared using ferric chloride, ferric sulfate, ferric nitrate or ferric acetate. The mass ratio of iron to flax shavings is (3-30):

100. The flax shavings are fully soaked in the iron solution for 2-4 hours. The pressure inside the low vacuum container is 50 Torr-200 Torr.

4. The preparation method according to any one of claims 1 to 3, characterized in that, Under 1 Tesla conditions, the mass magnetization of magnetic flax biochar is 3 Am. 2 / Kg~30Am 2 / Kg.

5. The preparation method according to claim 1, characterized in that, The specific details of S20 are as follows: S21. Dissolve sodium methylcellulose and animal glue in water to form a stable colloidal solution with a mass concentration of 40% to 60%. S22. Gradually add flax magnetic biochar powder and stir thoroughly for 30 min to 60 min. The water content of the mixture should be 10% to 15%. The mass ratio of flax magnetic biochar, sodium methylcellulose and animal glue should be (75 to 90): (8 to 20): (2 to 10). S23. Press the mixture into spherical particles with a particle size of 5 mm to 10 mm, and then vacuum dry it; S24. The spherical particles are heat-treated at 180℃~220℃ to form a stable carbonized skeleton of organic molecules of sodium methylcellulose and animal glue, thereby enhancing the structural stability and strength of the granulated particles and obtaining magnetic particle fillers with a mechanical strength of 5MPa~10MPa and a specific surface area of ​​150m². 2 / g~250m 2 / g.

6. The preparation method according to claim 1, characterized in that, The specific details of S30 are as follows: S31. Under low vacuum conditions, tumble the magnetic particle filler, first uniformly spray a polysaccharide solution with a mass fraction of 3% to 6%, then spray a biological enzyme solution with a mass fraction of 0.5% to 1.5%. The mass ratio of magnetic particle filler: polysaccharide: biological enzyme is 300: (1.3 to 4.0): (0.15 to 0.1). S32. Continue to tumble the packing under low vacuum and then ventilate and dry it to obtain flax-based biological packing with a moisture content of 5% to 10%.

7. The preparation method according to claim 6, characterized in that, Polysaccharides include: One or more of chitosan, sodium alginate, and cyclodextrin; the biological enzymes include peroxidase, protease, and urease, and the mass ratio of peroxidase, protease, and urease is (1-5):(65-75):(20-34).

8. A flax-based biofiller, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. An application of a flax-based biological filler prepared by the preparation method according to any one of claims 1 to 7, or the flax-based biological filler according to claim 8, characterized in that, Flax-based biological packing materials are used to treat wastewater from recirculating aquaculture systems.

10. A circulating biological tank, characterized in that, include: The package includes a packing support (2), a circulation device (3), a bio-tank (4), and a flax-based biological packing material (1) prepared by any one of claims 1 to 7. The bio-tank (4) has a water circulation zone (410) and a biological filtration zone (420). The circulation device (3) is located in the water circulation zone (410) of the bio-tank (4). The circulation device (3) includes an outer pipe (310), an inner pipe (320), and a circulation float (330). The upper end of the outer pipe (310) is closed, and the lower end is provided with an orifice. The lower end of the outer pipe (310) is connected to the bio-tank (4). A gap of a certain depth is formed between the bottom of the tank; an inner pipe (320) is inserted into the outer pipe (310), the inner pipe (320) is open at the top and bottom and passes through the bottom of the biological tank (4), an outer water passage (340) is formed between the lower end of the outer pipe (310) and the inner pipe (320), and an inner water passage (350) is formed between the upper end of the outer pipe (310) and the inner pipe (320) to discharge to the outside of the tank, and a circulation float (330) is surrounded around the outer pipe (310); the circulation float (330) is set with buoyancy at the same depth as the gap between the lower end opening of the outer pipe (310) and the bottom of the biological tank (4); The circulating float (330) includes: a hollow cylinder (331), a specific gravity adjustment ring (332), and a stabilizing plate (333). The inner diameter of the hollow cylinder (331) is 1.2 to 2.0 times the outer diameter of the outer tube 310. The specific gravity adjustment ring (332) is used to adjust the weight of the circulating float (330) so that it can touch the bottom or nearly touch the bottom when it is near the lower end opening of the outer tube (310), so as to prevent water from flowing to the outer tube (310). The weight of the specific gravity adjustment ring (332) is equal to the specific gravity of the lower end opening of the outer tube (310) and the water depth at the bottom of the biological tank (4). The stabilizing plate (333) 3) Used to expand the lower surface of the circulating float (330); the packing support (2) is located in the biological filtration zone (420) of the biological tank (4), the flax-based biological packing (1) is located on the packing support (2), the packing support (2) at the bottom of the biological tank (4) has a slope and is inclined towards the water circulation zone (410), the minimum height of the packing support (2) from the bottom of the biological tank (4) is greater than the height from the lower end opening line (311) of the outer pipe (310) to the bottom of the biological tank (4), the lower end opening line (311) of the outer pipe (310) refers to the horizontal line of the highest point of the opening.

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