Process for treating threonine waste liquors

Abstract

The present application relates to wastewater treatment technical field, specifically to a kind of threonine waste liquid processing technology, first add dicyandiamide-formaldehyde resin quaternary ammonium salt and nano hydroxyapatite composite modifier, adjust pH to complete chelation flocculation.Secondly, through light-enzyme synergistic degradation, electrochemical-ozone combined oxidation treatment, then use microbial electrofermentation system to convert threonine into short-chain fatty acid, finally through nanofiltration-RO membrane concentration, combined with vacuum cooling and melt crystallization recycle high-purity threonine and ammonium sulfate, realize pollutant removal and resource recovery.The present application is through multiple technology cooperation, efficiently removes COD, ammonia nitrogen and other pollutants in threonine waste liquid, simultaneously improves threonine recovery rate, product purity is high, compared with traditional process, with environmental protection and economic value, strong adaptability.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically a treatment process for threonine waste liquid. Background Technology

[0002] Threonine, an important essential amino acid, is widely used in medicine, food, and feed additives. Currently, industrially, threonine is mainly produced through microbial fermentation. However, the waste liquid generated after extraction and separation of the fermentation broth still contains a large amount of organic matter, inorganic salts, and residual threonine. These waste liquids are characterized by high COD concentrations (10,000-30,000 mg / L), high ammonia nitrogen content (1,000-5,000 mg / L), and complex compositions. Direct discharge of these waste liquids will not only cause serious environmental pollution but also lead to resource waste.

[0003] Traditional methods for treating threonine wastewater mainly include physical, chemical, and biological methods. While flocculation and sedimentation in physical methods can remove some suspended solids and colloidal substances, their effectiveness in removing dissolved organic matter is limited. Evaporation and crystallization methods are energy-intensive and prone to causing equipment scaling and corrosion. Chemical methods such as Fenton oxidation and ozone oxidation can effectively degrade organic matter, but they suffer from high oxidant consumption and high operating costs. Biological methods offer advantages such as low cost and environmental friendliness, but the high salt and ammonia nitrogen content of the wastewater inhibits microbial activity, leading to low treatment efficiency. Furthermore, traditional processes often focus on pollutant removal, neglecting the recovery and utilization of threonine and other resources in the wastewater, resulting in resource waste.

[0004] In recent years, new technologies such as membrane separation, advanced oxidation, and microbial fuel cells have been gradually applied to the field of industrial wastewater treatment. However, single technologies still have limitations in treating threonine wastewater. For example, membrane separation technology is susceptible to concentration polarization and membrane fouling, leading to a decrease in separation efficiency; advanced oxidation technology has insufficient mineralization capacity for certain recalcitrant organic compounds; and although microbial fuel cells can achieve energy recovery, their power generation efficiency is low, making it difficult to meet industrial needs. Therefore, developing an efficient, economical, and resource-recoverable threonine wastewater treatment process is of significant practical importance. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a process for treating threonine waste liquid.

[0006] A process for treating threonine waste liquid includes the following steps: S1: Add a composite modifier to the threonine waste liquid. The composite modifier is composed of dicyandiamide-formaldehyde resin quaternary ammonium salt and nano-hydroxyapatite in a mass ratio of 2:1, and adjust the pH to 8.0 to 9.0. The dicyandiamide-formaldehyde resin quaternary ammonium salt forms an electrostatic adsorption with threonine molecules through the quaternary ammonium groups, and the following chelation reaction occurs simultaneously: , S2: Add a composite photoenzyme system, wherein the composite photoenzyme system is an immobilized laccase-titanium dioxide nanotube array, the laccase loading is 50 to 80 mg / g, the titanium dioxide nanotube addition is 1.0 to 1.8 g / L, and the reaction is carried out under 365 nm ultraviolet light irradiation for 3 to 5 hours, with an ultraviolet light power density of 100 to 150 W / m². S3: The waste liquid is fed into a three-dimensional electrode reactor. The anode is an IrO2-Ta2O5 / Ti mesh electrode, the cathode is a nickel foam electrode, and activated carbon particles are used as the third electrode with a particle size of 0.5 to 1.0 mm. Ozone is introduced at a concentration of 15 to 20 mg / L and a flow rate of 3 to 5 L / min. The reaction is carried out for 45 to 75 minutes under the conditions of a current density of 20 to 30 mA / cm² and a pH of 10 to 11. S4: Connect the treatment liquid to the microbial electrofermentation system, inoculate the electrogenic bacteria Geobacter sulfurreducens at the anode, and add a mixed bacterial agent of nano-zero valent iron and acid-producing bacteria Clostridium acetobutylicum at the cathode. The nano-zero valent iron has a particle size of 50 to 100 nm. Connect an external 0.8 to 1.2V DC power supply and run for 96 to 120 hours. S5: A two-stage nanofiltration-reverse osmosis membrane system is used for concentration. The nanofiltration molecular weight cutoff is 150 to 200 Da, and the reverse osmosis operating pressure is 3.0 to 5.0 MPa. The concentrate is simultaneously separated from the threonine product and the ammonium sulfate byproduct through a vacuum cooling crystallization and melt crystallization coupling process. The vacuum cooling crystallization pressure is 5 to 10 kPa, the temperature is 10 to 15 °C, the melt crystallization heating rate is 2 to 5 °C / h, and the melting point temperature is 210 to 220 °C.

[0007] Preferably, the process further includes ultrasonic pretreatment prior to S1: the waste liquid is placed in an ultrasonic device for 15 to 25 minutes, the ultrasonic device having a frequency of 40 kHz and a power density of 0.5 to 1.0 W / cm², thereby disrupting the colloidal structure in the waste liquid.

[0008] Preferably, the method further includes introducing magnetic nanoparticles Fe3O4 into the S2 photo-enzyme synergistic degradation process. The magnetic nanoparticles have a particle size of 10 to 20 nm and are added at an amount of 0.2 to 0.5 g / L. The photoenzyme system is rapidly separated and recycled by an external magnetic field with an external magnetic field strength of 0.3 to 0.6 T.

[0009] Preferably, in the S3 three-dimensional electrode reactor, the activated carbon particles are loaded with 5 to 10% MnO2 catalyst, which is prepared by impregnation-calcination method, with impregnation solution concentration of 0.1 to 0.3 mol / L and calcination temperature of 400 to 500℃.

[0010] Preferably, 0.05 to 0.15 g / L of the redox mediator methyl viologen is added to the cathode chamber of the S4 microbial electrofermentation system to promote electron transfer efficiency.

[0011] Preferably, the S5 nanofiltration membrane is a polypiperazine amide composite membrane, prepared by interfacial polymerization, and the water contact angle of the membrane surface is 60 to 70°.

[0012] Preferably, an electro-Fenton enhancement step is added between S3 and S4: ferrous oxalate is added to the waste liquid at an amount of 0.3 to 0.8 g / L, the pH is adjusted to 3 to 4, and the reaction is carried out for 30 to 45 minutes at a current density of 15 to 25 mA / cm².

[0013] Preferably, during the S5 vacuum cooling crystallization process, 0.01 to 0.03 g / L of seed crystals are added. The seed crystals are threonine crystals with a particle size of 100 to 200 μm, and the stirring speed is controlled at 80 to 120 rpm.

[0014] Preferably, after S5 melt crystallization, threonine is further purified by molecular distillation to remove residual trace impurities. The molecular distillation operation temperature is 180 to 200°C and the pressure is 0.1 to 1 Pa.

[0015] Preferably, the dicyandiamide-formaldehyde resin quaternary ammonium salt in the S1 composite modifier is prepared by the following method: dicyandiamide and formaldehyde are mixed in a molar ratio of 1:3 and reacted at 60 to 80°C for 2 hours. Trimethylamine is added for quaternization, with trimethylamine accounting for 10 to 15% of the total mass of the reactants. After reacting for 3 to 5 hours, the mixture is spray-dried to obtain the product.

[0016] Compared with existing technologies, the beneficial effects of this invention are: 1. The chelation-flocculation synergistic pretreatment process utilizes the synergistic effect of dicyandiamide-formaldehyde resin quaternary ammonium salt and nano-hydroxyapatite, which can not only effectively remove suspended solids and colloidal substances in waste liquid, but also selectively recover some threonine through chelation, reducing the burden of subsequent treatment.

[0017] 2. The photo-enzyme synergistic degradation process combines the specificity of biocatalysis with the high efficiency of photocatalysis, achieving deep oxidative decomposition of recalcitrant organic matter under mild conditions. The introduction of magnetic nanoparticles enables the photoenzyme system to be rapidly separated and recovered via a magnetic field, significantly reducing operating costs. The electrochemical-ozone combined oxidation process, through the synergistic effect of a three-dimensional electrode reactor and ozone, significantly increases the generation of hydroxyl radicals, enhances the mineralization capacity of organic matter, and effectively reduces ammonia nitrogen content.

[0018] 3. The microbial electrofermentation recovery process combines electrogenic and acid-producing bacteria, not only further degrading organic matter in the waste liquid but also converting threonine into high-value-added short-chain fatty acids. Through the synergistic effect of applied voltage and redox mediators, it significantly improves electron transfer efficiency and fermentation yield. The membrane-integrated crystallization coupled recovery process, through staged separation of nanofiltration and reverse osmosis combined with fine purification via vacuum cooling crystallization and melt crystallization, achieves efficient recovery and high-purity purification of threonine, while simultaneously producing ammonium sulfate as a byproduct, thus improving resource utilization.

[0019] 4. This process boasts advantages such as high treatment efficiency, high resource recycling rate, and environmental friendliness. Furthermore, it exhibits excellent adaptability, allowing for flexible adjustment of process parameters based on wastewater concentration and treatment requirements, thus providing threonine production enterprises with an efficient and sustainable wastewater treatment solution. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of a threonine waste liquid treatment method proposed in this invention; Figure 2 This is a line graph comparing the total COD removal rate and ammonia nitrogen removal rate of the examples and comparative examples; Figure 3 This is a bar chart comparing the threonine recovery rate and product purity of the examples and comparative examples; Figure 4 This is a radar comparison chart created by standardizing the performance data of the embodiments and comparative examples. Detailed Implementation

[0021] according to Figures 1 to 4 The specific embodiments of the present invention are as follows: Example 1: Conventional threonine wastewater treatment S1: Chelation-flocculation synergistic pretreatment Five liters of threonine waste liquid (COD = 18500 mg / L, ammonia nitrogen = 2300 mg / L, threonine content 3.2 g / L) from a fermentation plant were taken. A composite modifier (dicyandiamide-formaldehyde resin quaternary ammonium salt, molecular weight 6500 Da, nano-hydroxyapatite particle size 30-40 nm) was added at a dosage of 1.2 g / L. The pH was adjusted to 8.5 with 20% NaOH, and the reaction was carried out at 300 rpm for 20 minutes, then reduced to 100 rpm and continued for another 15 minutes. After standing and settling for 30 minutes, the COD of the supernatant decreased to 12800 mg / L, and the turbidity decreased from 450 NTU to 68 NTU.

[0022] S2: Photo-enzymatic synergistic degradation The pretreated supernatant was transferred to a photocatalytic reactor, and 1.5 g / L of immobilized laccase-titanium dioxide nanotube array (laccase loading 65 mg / g, TiO2 tube diameter 90-100 nm) and 0.3 g / L of Fe3O4 magnetic nanoparticles were added. A 365 nm UV lamp (power density 120 W / m²) was turned on, and the reaction was carried out at a constant temperature of 40 °C with stirring at 150 rpm for 4 hours. After the reaction, the photoenzyme system was separated using a 0.5 T magnetic field, and the COD was found to have decreased to 7200 mg / L, while the B / C ratio increased from 0.23 to 0.45.

[0023] S3: Electrochemical-ozone combined oxidation The waste liquid was pumped into a three-dimensional electrode reactor (effective volume 2L). The anode was an IrO2-Ta2O5 / Ti mesh electrode, and the cathode was a nickel foam electrode, filled with activated carbon particles (particle size 0.8-1.0mm, loaded with 8% MnO2). Ozone (concentration 18mg / L, flow rate 4L / min) was introduced, the current density was adjusted to 25mA / cm², the pH was 10.5, and the reaction was carried out at a constant temperature of 50℃ for 60 minutes. After the reaction, COD decreased to 2800mg / L, and ammonia nitrogen decreased to 420mg / L.

[0024] S4: Microbial electrofermentation recovery The treatment solution was connected to an MEC device (1L anode chamber, 0.5L cathode chamber). The anode was inoculated with *Geobacter sulfurreducens* culture (OD600=0.8), and the cathode was treated with a mixture of nano-zero valent iron (70-80nm particle size) and *Clostridium acetobutylicum*, along with 0.1g / L methyl viologen. An external 1.0V DC power supply was used, and the mixture was incubated at 35℃ for 108 hours. After fermentation, the acetic acid concentration in the liquid phase reached 12.5g / L, the propionic acid concentration was 4.3g / L, and the residual threonine content decreased to 0.2g / L.

[0025] S5: Membrane Integration-Crystallization Coupling Recycling The fermentation broth was passed through a two-stage membrane system: an NF membrane (molecular weight cutoff 180 Da) operating at 1.8 MPa and an RO membrane operating at 4.0 MPa. The NF permeate was concentrated by RO to a threonine concentration of 250 g / L and then introduced into a vacuum cooling crystallizer (pressure 8 kPa, temperature 12 °C). 0.02 g / L seed crystals (particle size 150 μm) were added, and crystallization was carried out at 100 rpm for 3 hours. After centrifugation, the crystals were purified by melt crystallization (heating rate 3 °C / h, melting point 215 °C) and molecular distillation (190 °C, 0.5 Pa) to obtain 15.8 g of threonine product with a purity of 99.2%. 8.3 g of ammonium sulfate was recovered from the mother liquor through concentration and crystallization.

[0026] Example 2: Treatment of high-concentration threonine wastewater S1: Chelation-flocculation synergistic pretreatment Take 5L of high-concentration waste liquid with COD 32000mg / L, ammonia nitrogen 4100mg / L, and threonine content 5.8g / L. Add a composite modifier at a dosage of 1.5g / L, adjust the pH to 9.0 with 20% NaOH, and react at 300rpm for 20 minutes. Then reduce the stirring speed to 100rpm and continue the reaction for 15 minutes. After standing and settling for 30 minutes, the COD of the supernatant decreased to 21500mg / L, and the turbidity was 75NTU.

[0027] S2: Photo-enzymatic synergistic degradation The pretreated supernatant was transferred to a photocatalytic reactor, and 1.8 g / L of immobilized laccase-titanium dioxide nanotube array and 0.5 g / L of Fe3O4 magnetic nanoparticles were added. A 365 nm UV lamp was turned on at a power density of 150 W / m², the temperature was kept constant at 40℃, and the mixture was stirred at 150 rpm for 5 hours. After the reaction, the photoenzyme system was separated using a 0.5T magnetic field, and the COD was found to have decreased to 10800 mg / L, while the B / C ratio increased to 0.48.

[0028] S3: Electrochemical-ozone combined oxidation The waste liquid was pumped into a three-dimensional electrode reactor with an effective volume of 2L. The anode was an IrO2-Ta2O5 / Ti mesh electrode, and the cathode was a nickel foam electrode filled with activated carbon particles (0.8-1.0 mm in diameter) and loaded with 8% MnO2. Ozone was introduced at a concentration of 20 mg / L and a flow rate of 4 L / min. The current density was adjusted to 30 mA / cm², the pH was set to 10.5, and the reaction was carried out at a constant temperature of 50℃ for 75 minutes. After the reaction, COD decreased to 4200 mg / L, and ammonia nitrogen decreased to 580 mg / L.

[0029] S4: Microbial electrofermentation recovery The treatment solution was connected to the MEC device, with 1L in the anode chamber and 0.5L in the cathode chamber. The anode was inoculated with *Geobacter sulfurreducens* culture (OD600 = 0.8), and the cathode was inoculated with a mixture of nano-zero valent iron and *Clostridium acetobutylicum*, along with 0.15 g / L methyl viologen. An external 1.2V DC power supply was used, and the mixture was incubated at 35°C for 120 hours. After fermentation, the acetic acid concentration in the liquid phase reached 18.7 g / L, the propionic acid concentration was 6.2 g / L, and the residual threonine content decreased to 0.32 g / L.

[0030] S5: Membrane Integration-Crystallization Coupling Recycling The fermentation broth was passed through a two-stage membrane system: an NF membrane operating at 1.8 MPa and an RO membrane operating at 5.0 MPa. The NF permeate was concentrated by RO to a threonine concentration of 250 g / L and then introduced into a vacuum cooling crystallizer at 5 kPa and 12 °C. 0.02 g / L seed crystals (150 μm particle size) were added, and crystallization was carried out at 100 rpm for 3 hours. After centrifugation, the crystals were melt-crystallized at a heating rate of 5 °C / h to a melting point of 215 °C, and purified by molecular distillation at 190 °C and 0.5 Pa to obtain 27.5 g of threonine product with a purity of 99.4%. The mother liquor was concentrated and crystallized to recover 14.2 g of ammonium sulfate.

[0031] Example 3: Treatment of low-concentration threonine wastewater S1: Chelation-flocculation synergistic pretreatment Take 5L of low-concentration waste liquid with COD 9500mg / L, ammonia nitrogen 1200mg / L, and threonine content 1.5g / L. Add a composite modifier at a dosage of 0.8g / L, adjust the pH to 8.0 with 20% NaOH, and react at 300rpm for 20 minutes. Then reduce the stirring speed to 100rpm and continue the reaction for 15 minutes. After standing and settling for 30 minutes, the COD of the supernatant decreased to 6800mg / L, and the turbidity was 52NTU.

[0032] S2: Photo-enzymatic synergistic degradation The pretreated supernatant was transferred to a photocatalytic reactor, and 1.0 g / L of immobilized laccase-titanium dioxide nanotube array and 0.2 g / L of Fe3O4 magnetic nanoparticles were added. A 365 nm UV lamp was turned on at a power density of 120 W / m², the temperature was kept constant at 40℃, and the mixture was stirred at 150 rpm for 3 hours. After the reaction, the photoenzyme system was separated using a 0.5 T magnetic field, and the COD was found to have decreased to 3800 mg / L, while the B / C ratio increased to 0.42.

[0033] S3: Electrochemical-ozone combined oxidation The waste liquid was pumped into a three-dimensional electrode reactor with an effective volume of 2L. The anode was an IrO2-Ta2O5 / Ti mesh electrode, and the cathode was a nickel foam electrode filled with activated carbon particles (0.8-1.0 mm in diameter) and loaded with 8% MnO2. Ozone was introduced at a concentration of 15 mg / L and a flow rate of 4 L / min. The current density was adjusted to 20 mA / cm², the pH was set to 10.5, and the reaction was carried out at a constant temperature of 50℃ for 45 minutes. After the reaction, COD decreased to 1200 mg / L, and ammonia nitrogen decreased to 180 mg / L.

[0034] S4: Microbial electrofermentation recovery The treatment solution was connected to the MEC device, with 1L in the anode chamber and 0.5L in the cathode chamber. The anode was inoculated with *Geobacter sulfurreducens* culture (OD600 = 0.8), and the cathode was inoculated with a mixture of nano-zero ferric iron and *Clostridium acetobutylicum*, along with 0.05 g / L methyl viologen. An external 0.8V DC power supply was used, and the mixture was incubated at 35°C for 96 hours. After fermentation, the acetic acid concentration in the liquid phase reached 7.3 g / L, the propionic acid concentration was 2.1 g / L, and the residual threonine content decreased to 0.08 g / L.

[0035] S5: Membrane Integration-Crystallization Coupling Recycling The fermentation broth was passed through a two-stage membrane system: an NF membrane operating at 1.8 MPa and an RO membrane operating at 3.0 MPa. The NF permeate was concentrated by RO to a threonine concentration of 250 g / L and then introduced into a vacuum cooling crystallizer at 10 kPa and 12 °C. 0.02 g / L seed crystals (150 μm particle size) were added, and crystallization was carried out at 100 rpm for 3 hours. After centrifugation, the crystals were melt-crystallized at a heating rate of 2 °C / h to a melting point of 215 °C, and purified by molecular distillation at 190 °C and 0.5 Pa to obtain 7.1 g of threonine product with a purity of 99.1%. The mother liquor was concentrated and crystallized to recover 3.6 g of ammonium sulfate.

[0036] Comparative example: Traditional biochemical treatment process Flocculation and sedimentation: Take 5L of the same waste liquid, add 2g / L of polyaluminum chloride (PAC) and 0.05g / L of polyacrylamide (PAM), pH=7.5, and the COD drops to 14200mg / L after precipitation.

[0037] Anaerobic treatment: Inoculate with anaerobic sludge (MLSS=8000mg / L), react at 35℃ for 7 days, and COD will decrease to 8500mg / L.

[0038] Aerobic treatment: Inoculate with activated sludge (MLSS=4000mg / L), aeration rate of 1.5m³ / h, reaction time of 48 hours, COD drops to 3200mg / L.

[0039] Evaporation and crystallization: Concentrate to a threonine concentration of 200 g / L, cool and crystallize to obtain 12.3 g of threonine product with a purity of 92.5%.

[0040] The performance comparison between the examples and the comparative examples is shown in the table below: Table 1 .

[0041] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for treating threonine wastewater, characterized in that, Includes the following steps: S1: Add a composite modifier to the threonine waste liquid. The composite modifier is composed of dicyandiamide-formaldehyde resin quaternary ammonium salt and nano hydroxyapatite in a mass ratio of 2:1, and adjust the pH to 8.0 to 9.

0. The dicyandiamide-formaldehyde resin quaternary ammonium salt forms electrostatic adsorption with threonine molecules through quaternary ammonium groups. The dicyandiamide-formaldehyde resin quaternary ammonium salt in the S1 composite modifier is prepared by the following method: dicyandiamide and formaldehyde are mixed in a molar ratio of 1:3 and reacted at 60 to 80°C for 2 hours. Trimethylamine is added for quaternization, with trimethylamine accounting for 10 to 15% of the total mass of the reactants. After reacting for 3 to 5 hours, the mixture is spray-dried to obtain the product. S2: Add a composite photoenzyme system, wherein the composite photoenzyme system is an immobilized laccase-titanium dioxide nanotube array, the laccase loading is 50 to 80 mg / g, the titanium dioxide nanotube addition is 1.0 to 1.8 g / L, and react under 365 nm ultraviolet light irradiation for 3 to 5 hours, with an ultraviolet light power density of 100 to 150 W / m². 2 Magnetic nanoparticles Fe3O4 with a particle size of 10 to 20 nm and an addition amount of 0.2 to 0.5 g / L were introduced into the S2 photo-enzyme synergistic degradation process. The photoenzyme system was rapidly separated and recycled by an external magnetic field with an external magnetic field strength of 0.3 to 0.6 T. S3: The waste liquid is fed into a three-dimensional electrode reactor. The anode is an IrO2-Ta2O5 / Ti mesh electrode, the cathode is a nickel foam electrode, and activated carbon particles with a particle size of 0.5 to 1.0 mm are used as the third electrode. Simultaneously, ozone is introduced at a concentration of 15 to 20 mg / L and a flow rate of 3 to 5 L / min, with a current density of 20 to 30 mA / cm². 2 The reaction time is 45 to 75 minutes at a pH of 10 to 11. In the S3 three-dimensional electrode reactor, 5 to 10% of MnO2 catalyst is loaded on the surface of activated carbon particles and prepared by impregnation-calcination method, with impregnation solution concentration of 0.1 to 0.3 mol / L and calcination temperature of 400 to 500℃. S4: Connect the treatment liquid to the microbial electrofermentation system, inoculate the electrogenic bacteria Geobacter sulfurreducens at the anode, and add a mixed bacterial agent of nano-zero valent iron and acid-producing bacteria Clostridium acetobutylicum at the cathode. The nano-zero valent iron has a particle size of 50 to 100 nm. Connect an external 0.8 to 1.2V DC power supply and run for 96 to 120 hours. The S4 microbial electrofermentation system has 0.05 to 0.15 g / L of the redox mediator methyl viologen added to the cathode chamber to promote electron transfer efficiency. S5: A two-stage nanofiltration-reverse osmosis membrane system is used for concentration. The nanofiltration molecular weight cutoff is 150 to 200 Da, and the reverse osmosis operating pressure is 3.0 to 5.0 MPa. The concentrate is used to achieve simultaneous separation of threonine product and ammonium sulfate byproduct through a vacuum cooling crystallization and melt crystallization coupled process. The vacuum cooling crystallization pressure is 5 to 10 kPa, the temperature is 10 to 15℃, the melt crystallization heating rate is 2 to 5℃ / h, and the melting point temperature is 210 to 220℃. The S5 nanofiltration membrane is a polypiperazine amide composite membrane, prepared by interfacial polymerization, with a water contact angle of 60° to 70° on the membrane surface.

2. The treatment process for threonine waste liquid according to claim 1, characterized in that, This also includes ultrasonic pretreatment before S1: the waste liquid is placed in an ultrasonic device for treatment for 15 to 25 minutes at a frequency of 40 kHz and a power density of 0.5 to 1.0 W / cm³. 2 This disrupts the colloidal structure in the waste liquid.

3. The treatment process for threonine waste liquid according to claim 1, characterized in that, An electro-Fenton enhancement step is added between S3 and S4: ferrous oxalate is added to the waste liquid at a concentration of 0.3 to 0.8 g / L, the pH is adjusted to 3 to 4, and the current density is 15 to 25 mA / cm². 2 The reaction time is 30 to 45 minutes.

4. The treatment process for threonine waste liquid according to claim 1, characterized in that, During the S5 vacuum cooling crystallization process, 0.01 to 0.03 g / L of seed crystals are added. The seed crystals are threonine crystals with a particle size of 100 to 200 μm, and the stirring speed is controlled at 80 to 120 rpm.

5. The treatment process for threonine waste liquid according to claim 1, characterized in that, After S5 melt crystallization, threonine is further purified using molecular distillation technology to remove residual trace impurities. The molecular distillation operation temperature is 180 to 200℃ and the pressure is 0.1 to 1 Pa.

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