A multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation and its semi-permeable membrane.

Abstract

This invention discloses a multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation and the semi-permeable membrane thereof. The multi-stage treatment method includes the following steps: S1, primary treatment; S2, secondary treatment; S3, tertiary treatment. This method first performs primary treatment on the initially high-impurity fluoride-containing wastewater, which can rapidly degrade organic matter and heavy metals in the wastewater. Then, secondary treatment removes most of the heavy metal ions from the wastewater. Crystallization can effectively remove dissolved salts from the wastewater, while electrodialysis can concentrate and recover metal ions from low-concentration wastewater and desalinate the wastewater. Finally, tertiary treatment efficiently separates fluoride ions from the wastewater, and reverse osmosis filtration further removes the remaining fluoride ions and other dissolved salts, ensuring effluent quality. The semi-permeable membrane is a modified polyethersulfone membrane, and water quality is further purified by adding microorganisms.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically to a multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation and the semi-permeable membrane thereof. Background Technology

[0002] Fluoride-containing wastewater mainly originates from traditional fluorochemical production, including fluoride production and reprocessing applications. It is characterized by its wide distribution, high volume, and complex composition. With industrial development, the volume of industrial fluoride-containing wastewater has increased significantly. The main forms of fluoride in this wastewater are fluorosilicic acid and soluble fluoride salts. Furthermore, fluoride-containing wastewater often contains other pollutants, such as inorganic salts or organic matter, increasing the difficulty of treatment.

[0003] Membrane separation technology is commonly used in the purification of high-purity liquids containing impurities, with nanofiltration, reverse osmosis, and electrodialysis being the most prevalent methods. Nanofiltration uses nanomembranes to effectively block small organic molecules while allowing most inorganic salts to pass through. The nanomembranes also have surface charges, enabling the effective separation of ions with different valence states. Reverse osmosis utilizes the pressure difference across a small-pore reverse osmosis membrane to allow small molecules to pass through while large molecules cannot, thus purifying the water. Electrodialysis uses ion-exchange membranes to allow some ions to pass through, thereby separating different ions in an aqueous solution.

[0004] However, existing methods for treating fluoride-containing wastewater usually combine membrane separation technology with precipitation and other methods. However, a single combined treatment is not enough to render the fluoride-containing wastewater harmless. Multi-stage membrane separation is also required to filter out small molecule impurities and improve the treatment effect of fluoride-containing wastewater. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation and the semi-permeable membrane thereof.

[0006] The technical solution of this invention is: a multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation, comprising the following steps:

[0007] S1, Level 1 Processing:

[0008] An aluminum-magnesium alloy electrode was placed in fluoride-containing wastewater, and an injection current density of 5~10 mA / cm² was applied. 2 Electrocoagulation was performed using direct current, and 1~1.5g / L of biochar adsorbent was added to the fluoride-containing wastewater at the same time. The electrocoagulation treatment time was 45~55min.

[0009] After electrocoagulation treatment, the fluoride-containing wastewater is nanofiltration through a nanofiltration membrane to obtain the first filtrate.

[0010] S2, Secondary Processing:

[0011] Add 6-8 g / L of precipitant to the first filtrate obtained in step S1 and precipitate for 4-7 hours. Then, perform ultrafiltration once to obtain the second filtrate and precipitate it at 55-65V and 510-530A / m. 2 The current density is set at the first semi-permeable membrane for one electrodialysis treatment, and the treatment time is 50-60 minutes to obtain the first concentrated water.

[0012] The first concentrated water is then crystallized at 80~110℃ and -50~-60KPa for 2~3 hours, followed by a second ultrafiltration to obtain the third filtrate, which is then subjected to a voltage of 85~95V and an A / m voltage of 540~560A / m. 2 A second electrodialysis treatment was performed under the current density and the second semipermeable membrane for 70-80 minutes to obtain a second concentrated solution.

[0013] S3, Level 3 processing:

[0014] The second concentrated water obtained in step S2 is adsorbed through an anion exchange column, and the resulting fourth filtrate is filtered through a reverse osmosis membrane.

[0015] Furthermore, the precipitant is polyaluminum chloride, and the seed crystal is calcium fluoride.

[0016] Note: Polyaluminum chloride can significantly improve solid-liquid separation efficiency, enhance sedimentation filtration and sludge dewatering performance, shorten sedimentation tank retention time, and increase water production; calcium fluoride can provide core crystallization sites in wastewater, which is conducive to the formation of a large number of calcium fluoride crystals, promoting the crystallization and precipitation process of fluoride ions in water and accelerating the removal rate of fluoride ions.

[0017] Furthermore, in step S2, the primary ultrafiltration uses an aromatic polyamide membrane with a pore size of 65-75 nm, and the secondary ultrafiltration uses an aromatic polyamide membrane with a pore size of 40-50 nm.

[0018] Note: Aromatic polyamide membranes have high transparency, good tear resistance, excellent low gas permeability, and good environmental performance; an electrodialysis treatment is performed between the primary and secondary ultrafiltration processes, thus the secondary ultrafiltration reduces the pore size and improves the interception of impurities.

[0019] Furthermore, the preparation method of the biochar adsorbent is as follows: pyrolyzed almond shells are impregnated in a potassium hydroxide solution with a mass concentration of 80-90% for 140-180 min, the solid-liquid ratio of the impregnation is 1:4-5, and then dried and granulated to obtain a core with a diameter of 8-10 mm.

[0020] The pyrolytic carbonized almond shells are then mixed with carbon aerogel and silica gel solution at a mass ratio of 1:0.1~0.2:2.5~3.5 to obtain a gel solution. The gel solution is then coated on the surface of the core to form a shell with a thickness of 0.3~0.4 mm, thus obtaining a biochar adsorbent.

[0021] Explanation: Immersing almond shells in a salt solution can affect the surface charge properties of the almond shells, thereby improving the selective adsorption of specific ions and enabling more effective selective removal in mixed ion systems. It can also enhance the stability of almond shells as biochar, improving their durability and recyclability. Furthermore, using almond shells to form an outer shell with carbon aerogel and silica gel can protect the inner core under the action of adsorption force, achieving a dual adsorption effect.

[0022] Furthermore, the pyrolysis and carbonization of the almond shells are carried out at a heating rate of 3~5℃ / min, a pyrolysis temperature of 450~500℃, and a pyrolysis time of 130~160min.

[0023] Explanation: Pyrolysis carbonization transforms almond shell biomass into biochar, thereby improving its adsorption effect on wastewater.

[0024] Further, in step S1, the nanofiltration membrane is a polyamide membrane with a pore size of 1~2 nm; in step S3, the reverse osmosis membrane is a cellulose acetate membrane with a pore size of 0.4~0.5 μm.

[0025] Note: Polyamide membranes have advantages such as high antifouling properties, pressure and flow stability, and a wide range of applications; cellulose acetate membranes have good film-forming properties, a smooth membrane surface, are not prone to scaling, and have good resistance to oxidation and free chlorine ions, and are highly selective.

[0026] The semipermeable membrane used in the above-mentioned multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semipermeable membrane separation includes a first semipermeable membrane and a second semipermeable membrane. Both the first and second semipermeable membranes are modified polyethersulfone membranes. The pore size of the first semipermeable membrane is 15~20nm, and the pore size of the second semipermeable membrane is 5~10nm.

[0027] Description: Polyethersulfone (PES) filter membranes are characterized by high throughput, enabling rapid and efficient separation and purification of biochemical molecules. Furthermore, the highly uniform pore size distribution of PES filter membranes allows for precise control of biochemical molecule separation and purification. The surface of PES filter membranes undergoes special treatment, resulting in high corrosion resistance, making them suitable for various wastewater treatment and industrial production applications.

[0028] Furthermore, the method for preparing the modified polyethersulfone film includes the following steps:

[0029] 1) After drying polyethersulfone with a mass concentration of 15-20% at 110-115℃ for 12-13 hours, it is dissolved in N-methylpyrrolidone at room temperature, and pore-forming agent and activator are added. The mixture is stirred at 50-90℃ for 6-10 hours and allowed to stand for degassing for 30-40 hours to obtain a uniformly mixed casting solution A; wherein the mass ratio of polyethersulfone:pore-forming agent:activator:N-methylpyrrolidone is 1:0.5-1.5:0.3-0.5:5.5-6.

[0030] 2) Dissolve Bacillus thuringiensis and Pseudomonas aeruginosa in calcium chloride solution at a mass ratio of 1:0.4~0.6:6~8, with the calcium chloride solution concentration being 15~20wt%, and magnetically disperse at 45~65℃ for 30~40min to obtain a suspension. Mix the suspension with casting solution A at a mass ratio of 1:2~3 to obtain casting solution B.

[0031] 3) The prepared casting solution B is coated onto a glass plate at room temperature to form a film. The glass plate is then immersed in a coagulation bath in a DC electrostatic field for synchronous polarization treatment. The DC electrostatic field strength for synchronous polarization is 650~750kV / m, the synchronous polarization time is 10~15min, the synchronous polarization temperature is 25~35℃, and the angle between the film surface and the direction of the electrostatic field is 45°~90°. After the casting solution is completely gelled, a modified polyethersulfone film with a thickness of 0.15~0.25mm is obtained.

[0032] Furthermore, the pore-forming agent is polyvinylpyrrolidone.

[0033] Note: Polyvinylpyrrolidone (PVP) can effectively control the size and distribution of membrane pores, thereby obtaining membrane materials with specific pore structures and properties; PVP can increase the density of membrane pores, improve membrane permeability, and give the membrane material better filtration and separation performance.

[0034] Furthermore, the active agent is potassium lauryl ether phosphate.

[0035] Note: Potassium lauryl ether phosphate has good compatibility with polyethersulfone (PES) materials and can be uniformly mixed with PES during the filter membrane preparation process, preventing phase separation and ensuring the stability of the filter membrane structure. Potassium lauryl ether phosphate can regulate pore size and porosity during filter membrane preparation, helping to control the permeability and filtration performance of the filter membrane. In addition, it can improve the hydrophilicity of PES filter membranes, making them easier to wet and reducing resistance during the filtration process.

[0036] The beneficial effects of this invention are:

[0037] (1) The multi-stage treatment method for fluoride-containing wastewater of the present invention firstly treats the initial high impurity content of fluoride-containing wastewater by electrocoagulation and adsorption nanofiltration, which can rapidly degrade pollutants such as organic matter and heavy metals in the wastewater; then, through precipitation-one ultrafiltration-one electrodialysis, and crystallization-second ultrafiltration-second electrodialysis, in the second stage of treatment, precipitation can remove most of the heavy metal ions in the wastewater, crystallization can effectively remove the dissolved salts in the wastewater, and electrodialysis can concentrate and recover metal ions in low-concentration wastewater and desalinate the wastewater. The combination of the two can reduce the energy consumption of electrodialysis; then, the third stage of treatment is carried out by anion exchange column adsorption and reverse osmosis. The anion exchange column can use its high affinity for fluoride ions and specific active groups to efficiently separate fluoride ions from the wastewater, and reverse osmosis filtration can further remove the remaining fluoride ions and other dissolved salts to ensure the quality of the effluent.

[0038] (2) The semipermeable membrane used in the multi-stage treatment method for fluoride-containing wastewater of the present invention adds a pore-forming agent and an active agent to polyethersulfone. Through their synergistic effect, the pore size of polyethersulfone is controlled and its permeability is improved. Bacillus thuringiensis and Pseudomonas have a large specific surface area and abundant surface functional groups, which enables them to efficiently adsorb pollutants in water. In addition to adsorption, these two microorganisms also have a certain biodegradation ability, which can degrade some organic pollutants into harmless substances and further purify the water quality.

[0039] Mixing bacterial culture with polyethersulfone (PES) enhances the filtration and interception capabilities of PES through adsorption synergy. Simultaneous polarization of the casting solution influences the phase separation process of the polymer solution, thereby optimizing the pore structure of the filter membrane. This allows for the control of pore size, distribution, and porosity, resulting in a more uniform and interconnected pore structure that improves filtration performance and separation efficiency. Furthermore, it enhances the interactions between polymer chains, improving the mechanical strength and stability of the filter membrane. Attached Figure Description

[0040] Figure 1 This is a graph showing the effect of various parameters on the fluoride ion removal rate in Experiment Example 1;

[0041] Figure 2 This is a graph showing the effect of various parameters on COD removal rate in Experiment Example 1;

[0042] Figure 3 This is a graph showing the effect of various parameters on the fluoride ion removal rate in Experiment Example 2;

[0043] Figure 4 This is a graph showing the effect of various parameters on COD removal rate in Experiment Example 2;

[0044] Figure 5This is a graph showing the effect of various parameters on the fluoride ion removal rate in Experiment Example 3;

[0045] Figure 6 This is a graph showing the effect of various parameters on the COD removal rate in Experiment Example 3. Detailed Implementation

[0046] The present invention will now be described in more detail with reference to specific embodiments, so as to better demonstrate the advantages of the present invention.

[0047] Example 1: A multi-stage treatment method for photovoltaic fluoride-containing wastewater based on semi-permeable membrane separation, comprising the following steps:

[0048] S1, Level 1 Processing:

[0049] An aluminum-magnesium alloy electrode was placed in fluoride-containing wastewater, and an injection current density of 8 mA / cm² was applied. 2 The direct current was used for electrocoagulation treatment, and 1.3 g / L of biochar adsorbent was added to the fluoride-containing wastewater at the same time. The electrocoagulation treatment time was 50 min.

[0050] The preparation method of the biochar adsorbent is as follows: pyrolyzed almond shells are immersed in a potassium hydroxide solution with a mass concentration of 85% for 160 min, the solid-liquid ratio of the immersion is 1:4.5, and then dried and granulated to obtain a core with a diameter of 8.5~9.5 mm.

[0051] The pyrolytic carbonized almond shells are then mixed with carbon aerogel and silica gel solution at a mass ratio of 1:0.15:3.0 to obtain a gel solution. The gel solution is then coated on the surface of the core to form a shell with a thickness of 0.35 mm, thus obtaining a biochar adsorbent.

[0052] The almond shell pyrolysis and carbonization process is as follows: heating rate is 4℃ / min, pyrolysis temperature is 475℃, and pyrolysis time is 145min.

[0053] After electrocoagulation treatment, the fluoride-containing wastewater is nanofiltered using a polyamide membrane with a pore size of 1.5 nm to obtain the first filtrate.

[0054] S2, Secondary Processing:

[0055] Add 7 g / L polyaluminum chloride to the first filtrate obtained in step S1 for precipitation treatment for 5.5 h, and then perform ultrafiltration once to obtain the second filtrate. Then, heat the second filtrate at 60 V voltage and 520 A / m. 2 An electrodialysis treatment was performed under the current density and the first semipermeable membrane for 55 minutes to obtain the first concentrated water.

[0056] The first concentrated solution was then treated with 2.5 g / L calcium fluoride crystallization at 95°C and -55 kPa for 2.5 h. The resulting solution underwent secondary ultrafiltration to obtain the third filtrate, which was then subjected to a voltage of 90 V and an A / mA voltage of 550 A / mA. 2 A second electrodialysis treatment was performed under the current density and the second semipermeable membrane for 75 minutes to obtain a second concentrated solution.

[0057] The primary ultrafiltration uses an aromatic polyamide membrane with a pore size of 70 nm; the secondary ultrafiltration uses an aromatic polyamide membrane with a pore size of 45 nm.

[0058] Both the first and second semi-permeable membranes are modified polyethersulfone membranes. The pore size of the first semi-permeable membrane is 17 nm, and the pore size of the second semi-permeable membrane is 7 nm. The preparation method of the modified polyethersulfone membrane includes the following steps:

[0059] 1) After drying 18% polyethersulfone at 113℃ for 12.5h, it was dissolved in N-methylpyrrolidone at room temperature, and polyvinylpyrrolidone and potassium lauryl ether phosphate were added. The mixture was stirred at 70℃ for 8h, and then allowed to stand for 35h to remove bubbles, to obtain a uniformly mixed casting solution A; wherein the mass ratio of polyethersulfone:polyvinylpyrrolidone:potassium lauryl ether phosphate:N-methylpyrrolidone was 1:1:0.4:5.8.

[0060] 2) Bacillus thuringiensis and Pseudomonas aeruginosa were dissolved in calcium chloride solution at a mass ratio of 1:0.5:7 and magnetically dispersed at 50°C for 35 min. The concentration of calcium chloride solution was 17.5 wt% to obtain a suspension. The suspension was then mixed with casting solution A at a mass ratio of 1:2.5 to obtain casting solution B.

[0061] 3) The prepared casting solution B is coated onto a glass plate at room temperature to form a film. The glass plate is then immersed in a coagulation bath in a DC electrostatic field for synchronous polarization treatment. The DC electrostatic field strength for synchronous polarization is 700 kV / m, the synchronous polarization time is 13 min, the synchronous polarization temperature is 30℃, and the angle between the film surface and the direction of the electrostatic field is 60°. After the casting solution is completely gelled, a modified polyethersulfone film with a thickness of 0.20 mm is obtained.

[0062] S3, Level 3 processing:

[0063] The second concentrated water obtained in step S2 is adsorbed through an anion exchange column, and the resulting fourth filtrate is filtered by reverse osmosis using a cellulose acetate membrane with a pore size of 0.45 μm.

[0064] Example 2: This example differs from Example 1 in that, in step S1, the injection current density is 5 mA / cm². 2The wastewater was treated with direct current for electrocoagulation, and 1 g / L of biochar adsorbent was added to the fluoride-containing wastewater at the same time. The electrocoagulation treatment time was 45 min.

[0065] Example 3: This example differs from Example 1 in that, in step S1, the injection current density is 10 mA / cm². 2 The wastewater was treated with direct current for electrocoagulation, and 1.5 g / L of biochar adsorbent was added to the fluoride-containing wastewater at the same time. The electrocoagulation treatment time was 55 min.

[0066] Example 4: The difference between this example and Example 1 is that the pyrolyzed almond shells were soaked in an 80% potassium hydroxide solution for 140 minutes with a solid-liquid ratio of 1:4, and then dried and granulated to obtain kernels with a diameter of 8~8.5 mm.

[0067] Example 5: The difference between this example and Example 1 is that the pyrolyzed almond shells were soaked in a 90% potassium hydroxide solution for 180 minutes with a solid-liquid ratio of 1:5, and then dried and granulated to obtain kernels with a diameter of 9.5~10 mm.

[0068] Example 6: The difference between this example and Example 1 is that the pyrolytic carbonized almond shells are mixed with carbon aerogel and silica gel solution at a mass ratio of 1:0.1:2.5 to obtain a glue solution, and then the glue solution is coated on the surface of the core to form a shell with a thickness of 0.3 mm.

[0069] Example 7: The difference between this example and Example 1 is that the pyrolytic carbonized almond shells are mixed with carbon aerogel and silica gel solution at a mass ratio of 1:0.2:3.5 to obtain a glue solution, and then the glue solution is coated on the surface of the core to form a shell with a thickness of 0.4 mm.

[0070] Example 8: The difference between this example and Example 1 is that the pyrolysis and carbonization of almond shells is as follows: the heating rate is 3℃ / min, the pyrolysis temperature is 450℃, and the pyrolysis time is 130min.

[0071] Example 9: The difference between this example and Example 1 is that the pyrolysis and carbonization of almond shells is as follows: the heating rate is 5℃ / min, the pyrolysis temperature is 500℃, and the pyrolysis time is 130~160min.

[0072] Example 10: This example differs from Example 1 in that, in step S2, 6 g / L of precipitant is added to the first filtrate obtained in step S1 for precipitation treatment for 4 hours, followed by one ultrafiltration to obtain the second filtrate, which is then subjected to a voltage of 55V and an A / mA voltage of 510A / mA. 2 The current density was set at the first semipermeable membrane for one electrodialysis treatment, which lasted for 50 minutes, to obtain the first concentrated water.

[0073] Example 11: This example differs from Example 1 in that, in step S2, 8 g / L of precipitant is added to the first filtrate obtained in step S1 for precipitation treatment for 7 hours, followed by one ultrafiltration to obtain the second filtrate, which is then subjected to a 65V voltage and 530A / m 2 The current density was set at the first semipermeable membrane for one electrodialysis treatment, which lasted for 60 minutes, to obtain the first concentrated water.

[0074] Example 12: This example differs from Example 1 in that, in step S2, 2 g / L calcium fluoride is added to the first concentrated water at 80°C and -60 kPa for crystallization treatment for 2 hours. The resulting solution is then subjected to secondary ultrafiltration to obtain a third filtrate, which is then subjected to ultrafiltration at 85V and 540A / m. 2 A second electrodialysis treatment was performed under the current density and the second semipermeable membrane for 70 minutes to obtain a second concentrated solution.

[0075] Example 13: This example differs from Example 1 in that, in step S2, the first concentrated water is treated with 3 g / L calcium fluoride crystallization at 110°C and -50 kPa for 3 hours, followed by secondary ultrafiltration to obtain the third filtrate, which is then subjected to 95V voltage and 560A / m 2 A second electrodialysis treatment was performed under the current density and the second semipermeable membrane for 80 minutes to obtain a second concentrate.

[0076] Example 14: This example differs from Example 1 in that, in the preparation method of the modified polyethersulfone membrane, 1) a 15% polyethersulfone mass concentration is dried at 110°C for 12 hours, then dissolved in N-methylpyrrolidone at room temperature, and polyvinylpyrrolidone and potassium lauryl ether phosphate are added and stirred at 50°C for 6 hours. After standing for degassing for 30 hours, a uniformly mixed casting solution A is obtained; wherein, the mass ratio of polyethersulfone:polyvinylpyrrolidone:activator:N-methylpyrrolidone is 1:0.5:0.3:5.5.

[0077] Example 15: This example differs from Example 1 in that, in the preparation method of the modified polyethersulfone membrane, 1) a 20% polyethersulfone mass concentration is dried at 115°C for 13 hours, then dissolved in N-methylpyrrolidone at room temperature, and polyvinylpyrrolidone and potassium lauryl ether phosphate are added and stirred at 90°C for 10 hours. After standing for degassing for 40 hours, a uniformly mixed casting solution A is obtained; wherein, the mass ratio of polyethersulfone:polyvinylpyrrolidone:potassium lauryl ether phosphate:N-methylpyrrolidone is 1:1.5:0.5:6.

[0078] Example 16: The difference between this example and Example 1 is that in the preparation method of the modified polyethersulfone membrane, 2) Bacillus thuringiensis and Pseudomonas aeruginosa are dissolved in calcium chloride solution at a mass ratio of 1:0.4:6 and magnetically dispersed at 45°C for 30 min. The concentration of calcium chloride solution is 15 wt%, and a suspension is obtained. The suspension is then mixed with casting solution A at a mass ratio of 1:2 to obtain casting solution B.

[0079] Example 17: The difference between this example and Example 1 is that in the preparation method of the modified polyethersulfone membrane, 2) Bacillus thuringiensis and Pseudomonas aeruginosa are dissolved in calcium chloride solution at a mass ratio of 1:0.6:8 and magnetically dispersed at 65°C for 40 min. The concentration of calcium chloride solution is 20 wt%, and a suspension is obtained. The suspension is then mixed with casting solution A at a mass ratio of 1:3 to obtain casting solution B.

[0080] Example 18: The difference between this example and Example 1 is that in the preparation method of the modified polyethersulfone membrane, the DC electrostatic field strength of synchronous polarization is 650kV / m, the synchronous polarization time is 10min, the synchronous polarization temperature is 25℃, and the angle between the membrane surface and the direction of the electrostatic field is 45°. After the casting solution is completely gelled, a modified polyethersulfone membrane with a thickness of 0.15mm is obtained.

[0081] Example 19: This example differs from Example 1 in that, in the preparation method of the modified polyethersulfone membrane, the DC electrostatic field strength of the synchronous polarization is 750kV / m, the synchronous polarization time is 15min, the synchronous polarization temperature is 35℃, and the angle between the membrane surface and the direction of the electrostatic field is 90°. After the casting solution is completely gelled, a modified polyethersulfone membrane with a thickness of 0.25mm is obtained.

[0082] Example 20: This example differs from Example 1 in that, in step S1, the nanofiltration membrane is a polyamide membrane with a pore size of 1 nm; in step S2, the first semi-permeable membrane has a pore size of 15 nm, the second semi-permeable membrane has a pore size of 5 nm, the primary ultrafiltration uses an aromatic polyamide membrane with a pore size of 65 nm, and the secondary ultrafiltration uses an aromatic polyamide membrane with a pore size of 40 nm; in step S3, the reverse osmosis membrane is a cellulose acetate membrane with a pore size of 0.4 μm.

[0083] Example 21: This example differs from Example 1 in that, in step S1, the nanofiltration membrane is a polyamide membrane with a pore size of 2 nm; in step S2, the first semi-permeable membrane has a pore size of 20 nm, the second semi-permeable membrane has a pore size of 10 nm, the primary ultrafiltration uses an aromatic polyamide membrane with a pore size of 75 nm, and the secondary ultrafiltration uses an aromatic polyamide membrane with a pore size of 50 nm; in step S3, the reverse osmosis membrane is a cellulose acetate membrane with a pore size of 0.5 μm.

[0084] Experimental Example

[0085] For the fluoride-containing wastewater treated in each embodiment, the fluoride ion removal rate and COD removal rate of each embodiment were tested five times to test the treatment effect of the method of the present invention. The average value of the five test results for each embodiment was taken as the test result of that embodiment. The specific investigation is as follows:

[0086] 1. For example Figure 1 and Figure 2 As shown, this study investigated the effects of parameters in each step of the primary treatment and the preparation of biochar adsorbent on the removal of fluoride and COD, specifically the effects of Examples 1-9 on the fluoride ion removal rate and COD removal rate. Insufficient or excessive parameters in the electrocoagulation treatment, the addition of biochar adsorbent, the preparation of almond shell nuclei, the preparation of almond shell outer layers, and the pyrolysis carbonization of almond shells all reduced the fluoride ion removal rate and COD removal rate. Therefore, considering all factors, the parameters in Example 1 were relatively more effective.

[0087] 2. For example Figure 3 and Figure 4 As shown, the effects of parameters in each step of the secondary and tertiary treatments and the pore size of the semi-permeable membrane on the fluoride and COD removal efficiency were investigated, specifically the effects of Examples 10-13, 20-21 and Control Examples 1-4 on the fluoride ion removal rate and COD removal rate. Control Example 1 differed from Example 1 in that only one electrodialysis was performed; Control Example 2 differed from Example 1 in that the pore size of the semi-permeable membrane used for the first and second ultrafiltrations was the same; Control Example 3 differed from Example 1 in that the pore sizes of the first and second semi-permeable membranes were the same; and Control Example 4 differed from Example 1 in that only precipitation treatment was performed without crystallization treatment.

[0088] It can be seen that, compared with Examples 1, 10-13 and 20-21, the removal rates of fluoride ions and COD were significantly reduced in Comparative Example 1 (lacking secondary electrodialysis), Comparative Example 2 (lacking ultrafiltration pore size change), Comparative Example 3 (lacking pore size change of the semipermeable membrane used in electrodialysis), and Comparative Example 4 (lacking crystallization treatment). Therefore, it is indicated that the elements involved in Comparative Examples 1-4 all played a certain role in the removal of fluoride ions and COD.

[0089] Comparing the curves of Examples 1, 10-13, and 20-21 in the figures, it can be seen that excessively small or large parameters in the precipitation treatment-first ultrafiltration-first electrodialysis, excessively small or large parameters in the crystallization treatment-second ultrafiltration-second electrodialysis, and excessively small or large filter pores in the multi-stage filtration all reduce the fluoride ion removal rate and COD removal rate. Therefore, combining... Figure 2 In terms of the overall comparison of the curve trends, the parameter effect of Example 1 is relatively better.

[0090] 3. For example Figure 5 and Figure 6 As shown, the effects of the preparation of semipermeable membranes on the removal of fluoride and COD were investigated, namely the effects of Examples 14-19 and Control Examples 5-6 on the fluoride ion removal rate and COD removal rate. Among them, the difference between Control Example 5 and Example 1 is that the suspension does not contain Pseudomonas; the difference between Control Example 6 and Example 1 is that synchronous polarization treatment is not performed.

[0091] It can be seen that the lack of Pseudomonas in Control Example 5 and the lack of simultaneous polarization treatment in Control Example 6 significantly reduced the fluoride ion removal rate and COD removal rate compared with Examples 1 and 14-19. Therefore, it is indicated that the addition of Pseudomonas and simultaneous polarization treatment both have a certain effect on the removal of fluoride ions and COD.

[0092] Comparing the curves of Examples 1 and 14-19 in the figure, it can be seen that if the preparation parameters of casting solution A are too small or too large, the preparation parameters of casting solution B are too small or too large, and the parameters of simultaneous polarization treatment are too small or too large, the fluoride ion removal rate and COD removal rate will be reduced. Therefore, in summary, the parameters of Example 1 are relatively better.

[0093] Combination Figures 1-6 It can be seen that the fluoride ion removal rate is above 98.6% and the COD removal rate is above 97.3%.

Claims

1. A multistage process for the treatment of photovoltaic fluorine-containing wastewater based on semi-permeable membrane separation, characterized in that, Comprising the following steps: S1, primary treatment: Aluminum-magnesium alloy electrode is placed in fluorine-containing waste water, and direct current with current density of 5-10 mA / cm 2 is injected to carry out electrocoagulation treatment, and 1-1.5 g / L biochar adsorbent is added into the fluorine-containing waste water at the same time, and the electrocoagulation treatment time is 45-55 min. The preparation method of the biochar adsorbent is: impregnating the pyrolysis carbonized almond shell in a potassium hydroxide salt solution with a mass concentration of 80-90% for 140-180 min, the solid-liquid ratio of impregnation is 1:4-5, drying and granulating to obtain a core body with a diameter of 8-10 mm; Then the pyrolysis carbonized almond shell is mixed with carbon aerogel and silica gel solution according to a mass ratio of 1:0.1-0.2:2.5-3.5 to obtain a glue solution, and then the glue solution is coated on the surface of the core body to form an outer shell with a thickness of 0.3-0.4 mm, thereby obtaining a biochar adsorbent; After the electrocoagulation treatment is completed, the fluorine-containing wastewater is subjected to nanofiltration through a nanofiltration membrane to obtain a first filtrate; S2, secondary treatment: The first filtrate obtained in step S1 is precipitated by adding 6-8 g / L precipitant for 4-7 h, and the second filtrate is obtained by once ultrafiltration, and the second filtrate is subjected to a second electrodialysis treatment under a voltage of 55-65 V and a current density of 510-530 A / m 2 The first filtrate obtained in step S1 is precipitated by adding 6-8 g / L precipitant for 4-7 h, and the second filtrate is obtained by once ultrafiltration, and the second filtrate is subjected to a second electrodialysis treatment under a voltage of 55-65 V and a current density of 510-530 A / m The first concentrated water is added with 2-3 g / L crystal seeds at 80-110°C and -50--60 KPa for 2-3 h for crystallization treatment, and a third filtrate is obtained through secondary ultrafiltration, and the third filtrate is subjected to electrolysis at 85-95 V and 540-560 A / m 2 The second concentrated water is subjected to secondary electrodialysis treatment at a current density and under a second semi-permeable membrane for 70-80 min. The first and second semi-permeable membranes are both modified polyether sulfone membranes, the pore size of the first semi-permeable membrane is 15-20 nm, and the pore size of the second semi-permeable membrane is 5-10 nm; The preparation method of the modified polyether sulfone membrane comprises the following steps: 1) After the polyether sulfone with a mass concentration of 15-20% is dried at 110-115℃ for 12-13h, it is dissolved in N-methyl pyrrolidone at room temperature, and a pore former and an active agent are added, and stirred at 50-90℃ for 6-10h, and then left to stand for 30-40h to remove bubbles, thereby obtaining a uniformly mixed casting solution A; wherein the mass ratio of polyether sulfone:pore former:active agent:N-methyl pyrrolidone is 1:0.5-1.5:0.3-0.5:5.5-6; 2) Bacillus thuringiensis and Pseudomonas are dissolved in a calcium chloride solution according to a mass ratio of 1:0.4-0.6:6-8, the concentration of the calcium chloride solution is 15-20wt%, and the suspension is obtained by magnetic dispersion at 45-65℃ for 30-40min, and then the suspension is mixed with the casting solution A according to a mass ratio of 1:2-3 to obtain a casting solution B; 3) The prepared casting solution B is coated on a glass plate at room temperature to form a film, and then the glass plate is immersed in a coagulation bath in a direct current electrostatic field for synchronous polarization treatment, the direct current electrostatic field strength of the synchronous polarization is 650-750kV / m, the synchronous polarization time is 10-15min, the synchronous polarization temperature is 25-35℃, and the film surface and the electrostatic field direction have an included angle of 45°-90°, and after the casting solution is completely gelled, a modified polyether sulfone membrane with a thickness of 0.15-0.25mm is obtained; S3, tertiary treatment: The second concentrated water obtained in step S2 is subjected to adsorption through an anion exchange column, and the fourth filtrate obtained is subjected to reverse osmosis filtration through a reverse osmosis membrane.

2. The photovoltaic fluorochemical wastewater multistage treatment method based on semi-permeable membrane separation according to claim 1, characterized in that, The precipitating agent is polyaluminum chloride, and the crystal seed is calcium fluoride.

3. The photovoltaic fluorochemical wastewater multistage treatment method based on semi-permeable membrane separation according to claim 1, characterized in that, In step S2, the primary ultrafiltration adopts an aromatic polyamide membrane with a pore size of 65-75nm, and the secondary ultrafiltration adopts an aromatic polyamide membrane with a pore size of 40-50nm.

4. The photovoltaic fluorochemical wastewater multistage treatment method based on semi-permeable membrane separation according to claim 1, characterized in that, The pyrolysis carbonization of the almond shell is: the heating rate is 3-5℃ / min, the pyrolysis temperature is 450-500℃, and the pyrolysis time is 130-160min.

5. The photovoltaic fluorochemical wastewater multistage treatment method based on semi-permeable membrane separation according to claim 1, characterized in that, In step S1, the nanofiltration membrane adopts a polyamide membrane with a pore size of 1-2nm; in step S3, the reverse osmosis membrane adopts a cellulose acetate membrane with a pore size of 0.4-0.5μm.

6. The photovoltaic fluorochemical wastewater multistage treatment method based on semi-permeable membrane separation according to claim 1, characterized in that, The pore forming agent is polyvinylpyrrolidone and the active agent is potassium lauryl etherphosphate. The pore forming agent is polyvinylpyrrolidone and the active agent is potassium lauryl etherphosphate. The pore forming agent is polyvinylpyrrolidone and the active agent is potassium lauryl etherphosphate. The pore

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