A method for treating low-concentration PVA wastewater in a PVA production process

Abstract

This invention relates to the field of polyvinyl alcohol (PVA) wastewater degradation and treatment technology, and discloses a method for treating low-concentration PVA wastewater during PVA production. The method includes the following steps: S1, an adjustment stage, adjusting the wastewater temperature and pH; S2, a reaction stage, adding an oxidant to the wastewater and completing the reaction; and S3, a post-treatment stage, detecting the PVA concentration in the wastewater after the reaction and discharging the PVA wastewater. The low-concentration wastewater has a PVA concentration below 300 ppm, and a PVA removal rate greater than or equal to 80%. This solution, by limiting the order of PVA wastewater adjustment and oxidant addition, allows for temperature adjustment within the PVA production tank and pH adjustment via a pipeline connected to the reaction tank. Once the wastewater enters the reaction tank, the oxidant can be directly added to degrade the PVA, effectively increasing the reaction rate while further reducing energy loss in high-temperature PVA wastewater, saving energy and lowering treatment costs.

Description

Technical Field

[0001] This invention relates to the field of polyvinyl alcohol wastewater degradation and treatment technology, specifically to a method for treating low-concentration PVA wastewater during PVA production. Background Technology

[0002] Polyvinyl alcohol (PVA) is a unique polymer that is water-soluble and non-toxic, with properties falling between those of plastics and rubber. Due to its strong adhesion, gas barrier properties, abrasion resistance, emulsification, film-forming properties, and good resistance to oils and solvents, PVA is widely used in various industries such as textiles, food, pharmaceuticals, and construction.

[0003] The water solubility of PVA leads to the generation of large amounts of PVA-containing wastewater (approximately 50 ppm to 200 ppm) during its use, especially in the textile and embroidery industries. This wastewater is characterized by high outlet temperatures (above 80°C), large volumes, low concentrations, and difficulty in treatment. Because PVA is a recalcitrant organic compound, its accumulation in water bodies can cause environmental problems, such as increased foaming and viscosity, disrupting the ecological balance of natural water bodies.

[0004] Current technologies for treating PVA wastewater mainly include physicochemical methods, biochemical methods, and MBR membrane treatment. Physicochemical methods encompass chemical coagulation, electrocoagulation, and advanced oxidation technologies. Chemical coagulation often uses flocculants such as polyaluminum chloride and polyferric chloride, but this method has low removal rates of COD and polyvinyl alcohol in treating PVA wastewater, resulting in unsatisfactory effects. Electrocoagulation utilizes the dissociation of electricity to remove PVA from wastewater with the assistance of chemical flocculants, but this method is relatively complex and energy-intensive. Advanced oxidation technologies have significant advantages in treating COD in wastewater, but the cost of oxidants is high, and the operation and management requirements are strict, leading to unstable results. Biochemical methods remove organic pollutants from wastewater through the growth and metabolism of microorganisms. However, due to the poor biodegradability of PVA wastewater, microorganisms have high requirements for nutrients, temperature, pH, and other conditions, making it unsuitable for high-concentration PVA wastewater. Furthermore, it suffers from large footprint and complex management, thus limiting its practical application. If MBR membranes are used to treat this type of wastewater, trace amounts of PVA in the wastewater will accumulate in the downstream wastewater treatment plant, affecting the activity of sludge in the aeration tanks and easily clogging the MBR membrane, which has a significant impact on the stable and long-term operation of the wastewater treatment plant. Therefore, current methods for treating low-concentration PVA wastewater all have certain limitations, such as low removal efficiency, high energy consumption, high cost, and complex operation and management.

[0005] Therefore, developing a safe, effective, and simple method for treating low-concentration wastewater from the PVA production process not only effectively compensates for the shortcomings of existing technologies but also effectively reduces the environmental harm caused by low-concentration wastewater, thus achieving effective treatment of low-concentration wastewater generated during the PVA production process. Summary of the Invention

[0006] The present invention aims to provide a method for treating low-concentration PVA wastewater during PVA production, in order to solve the limitations of existing technologies for treating low-concentration PVA wastewater, such as low removal efficiency, high energy consumption, high cost, or complex operation and management.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for treating low-concentration PVA wastewater during PVA production, comprising the following steps: S1 adjustment stage, adjusting the wastewater temperature and pH; S2 reaction stage, adding an oxidant to the wastewater and completing the reaction; S3 post-treatment stage, detecting the PVA concentration in the wastewater after the reaction; the PVA concentration in the low-concentration wastewater before treatment is less than 300 ppm.

[0008] The principles and advantages of this scheme are:

[0009] 1. Compared to existing technologies with limited efficiency and effectiveness in treating PVA wastewater, this solution first adjusts the temperature and pH of the PVA wastewater to create a suitable reaction environment. This facilitates rapid degradation of PVA in the wastewater after the addition of the oxidant, reducing PVA and COD levels, thereby effectively improving the PVA degradation efficiency and overall wastewater treatment effect. Through long-term experiments, the applicant has found that this solution treats a batch of PVA wastewater (approximately 30 t / h) with a residence time of only 20–40 minutes in the pipes and reaction tank, effectively improving PVA wastewater treatment efficiency.

[0010] 2. Compared to existing technologies that treat high-concentration PVA wastewater, this solution targets low-concentration PVA wastewater. Due to the lower PVA content in low-concentration wastewater, its treatment efficiency is often lower, and the PVA in low-concentration wastewater is more difficult for the oxidant to capture and react with. Therefore, treating low-concentration PVA wastewater is more challenging. This solution limits the sequence of PVA wastewater conditioning and oxidant addition, allowing temperature regulation within the PVA production tank and pH adjustment via the delivery pipeline connected to the reaction tank. Once the wastewater enters the reaction tank, the oxidant can be directly added to degrade the PVA. This effectively increases the reaction rate while further reducing energy loss from high-temperature PVA wastewater, saving energy and lowering treatment costs.

[0011] 3. Due to the recalcitrant nature of low-concentration PVA, it cannot be removed during conventional anaerobic fermentation. Instead, it reduces the effectiveness of anaerobic fermentation by affecting wastewater viscosity. This solution effectively enhances the selectivity of the oxidant for PVA by adjusting the temperature and pH of the PVA wastewater before the reaction. This prioritizes the oxidation of PVA while having minimal impact on other organic waste, achieving targeted degradation of PVA in the wastewater. Specifically, by adjusting the temperature and pH of the PVA wastewater before the reaction and adding only a small amount of oxidant, this solution can reduce the PVA content in the wastewater to below 10 ppm, meeting the minimum standard for PVA in wastewater discharge. This allows for direct discharge to the wastewater treatment plant for further treatment, effectively protecting the environment. Although the wastewater treated with this solution still has relatively high COD levels, the absence of PVA's thickening and foaming effects allows it to be treated to discharge standards through anaerobic fermentation, oxidant treatment, and other methods. Furthermore, the absence of PVA significantly reduces the difficulty of wastewater treatment, resulting in a significant improvement in wastewater treatment efficiency.

[0012] Preferably, as an improvement, in the S2 reaction stage, the oxidant is sodium hypochlorite, and the amount of sodium hypochlorite added is 1‰ to 5‰ of the PVA concentration in the wastewater.

[0013] Preferably, as an improvement, in the S2 reaction stage, the dosage of sodium hypochlorite is determined according to the PVA concentration in the wastewater: when the PVA concentration in the wastewater is below 50 ppm, the dosage of sodium hypochlorite is 3-5‰ of the PVA concentration; when the PVA concentration in the wastewater is between 50 ppm and 100 ppm, the dosage of sodium hypochlorite is 2-4‰ of the PVA concentration; and when the PVA concentration in the wastewater is above 100 ppm, the dosage of sodium hypochlorite is 1-3‰ of the PVA concentration.

[0014] Technical Effects: This solution, employing the aforementioned setup, effectively reduces the PVA content in wastewater to below 10 ppm based on varying PVA concentrations, thus preventing PVA accumulation and environmental pollution, and minimizing wastewater treatment complexity. Through long-term experiments, the applicant discovered that, unlike existing technologies where higher PVA concentrations require higher oxidant dosages, this solution overcomes this bias. When PVA concentrations in wastewater exceed 100 ppm, the sodium hypochlorite dosage is actually reduced (specifically, by 1-3‰ of the PVA concentration). The inventors analyzed that this is because during the redox reaction between sodium hypochlorite and PVA, higher PVA concentrations in the wastewater result in a faster reaction and lower treatment difficulty, thus requiring less oxidant. This effectively reduces PVA concentration in the wastewater while significantly decreasing oxidant dosage and cost, resulting in unexpected technical benefits.

[0015] Preferably, as an improvement, the reaction time in the S2 reaction stage is 20 min to 40 min.

[0016] Technical effect: The above settings in this solution facilitate sufficient contact and reaction between the oxidant and PVA in the wastewater, thereby improving the PVA treatment effect.

[0017] Preferably, as an improvement, in the S2 reaction stage, the addition of an oxidant and the reaction also include stirring at a speed of 60–120 rpm.

[0018] Technical Effects: The above-mentioned setup facilitates uniform contact between the oxidant and PVA in the wastewater during the reaction stage, improving reaction efficiency and PVA treatment effectiveness. Through long-term experiments, the applicant discovered that excessively rapid stirring can actually cause wastewater agitation, affecting the reaction and potentially leading to wastewater overflow and unnecessary environmental pollution.

[0019] Preferably, as an improvement, the wastewater temperature is adjusted to 80-95℃ during the S1 adjustment stage.

[0020] Technical Effects: This solution employs the above-mentioned setup, facilitating rapid reaction and degradation of PVA after the oxidant comes into contact with it. Through long-term experiments, the applicant discovered that excessively high wastewater temperatures not only waste energy but also make PVA modification at high temperatures more difficult to oxidize and degrade; conversely, excessively low wastewater temperatures result in lower PVA wastewater temperatures during the reaction due to energy losses during equipment transport, significantly reducing the PVA oxidation and degradation effect.

[0021] Preferably, as an improvement, the pH of the wastewater is adjusted to 5-6 during the S1 adjustment stage.

[0022] Technical Effects: This solution, employing the aforementioned setup, facilitates rapid PVA degradation through contact with the oxidant. Through long-term experiments, the applicant discovered that if the wastewater pH is too low, the improvement in reaction rate and PVA degradation efficiency is not significant; instead, it leads to reagent waste, resulting in more harm than good. Conversely, if the wastewater pH is too high, the oxidant cannot rapidly generate free radicals through acidification, thereby reducing the oxidative degradation effect and impacting PVA removal efficiency.

[0023] Preferably, as an improvement, the pH of the wastewater is adjusted using one of concentrated sulfuric acid, concentrated hydrochloric acid, or concentrated nitric acid.

[0024] Technical Effects: The above-mentioned setup facilitates rapid pH adjustment of PVA wastewater, reduces heat loss during pH adjustment, thereby improving PVA wastewater treatment efficiency, reducing energy consumption, and enabling continuous discharge and treatment of PVA wastewater from PVA production tanks. This further enhances PVA wastewater treatment efficiency and, in turn, promotes PVA production efficiency within the plant.

[0025] Preferably, as an improvement, in the S3 post-treatment stage, if the PVA concentration in the post-reaction wastewater drops to 10 ppm or below, the post-reaction wastewater is discharged into the wastewater treatment plant; if the PVA concentration in the post-reaction wastewater is higher than 10 ppm, it needs to be recycled back to the front end for further treatment, which prolongs the time required for the same batch of wastewater and reduces treatment efficiency.

[0026] Technical effect: The above settings in this solution facilitate the thorough removal of PVA from low-concentration PVA wastewater, thereby ensuring the wastewater treatment effect.

[0027] Preferably, as an improvement, it also includes an S4 biochemical stage, where wastewater with a PVA concentration of less than 10 ppm undergoes further biochemical treatment before being discharged after meeting the standards.

[0028] Technical Effects: This solution, employing the aforementioned setup, reduces the PVA concentration in wastewater to below 10 ppm before biochemical treatment, facilitating the thorough treatment of low-concentration wastewater from the PVA production process to a dischargeable level. Through long-term experiments, the applicant has found that this solution, by first adjusting the wastewater and pH, and then using an oxidant to degrade PVA, effectively reduces the PVA concentration in the wastewater to below 10 ppm, effectively avoiding the increased difficulty of biochemical treatment due to PVA, thereby improving wastewater treatment efficiency. The applicant also found that compared to adding large amounts of oxidant to treat wastewater to meet discharge standards for both PVA and COD, this solution, combining oxidant degradation of PVA with biochemical treatment of COD in the wastewater, not only effectively reduces treatment costs but also improves treatment efficiency and shortens treatment time. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the low-concentration wastewater treatment device used in the PVA production process in this embodiment of the invention. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto. Unless otherwise specified, the technical means used in the following embodiments and experimental examples are conventional means well known to those skilled in the art, and the materials and reagents used can all be obtained commercially.

[0031] The attached diagram is labeled as follows: PVA production tank 1, reboiler 11, hot water valve 111, wastewater reaction tank 2, reagent feed pipe 21, reagent regulating valve 211, drain pipe 22, reflux pipe 221, reflux valve 222, stirrer 23, thermometer 24, water inlet pipe 3, acid inlet pipe 31, acid regulating valve 32, pH meter 33, baffle 34, acid tank 4, reagent tank 5, controller 6.

[0032] Overview of the Plan

[0033] This solution provides a low-concentration wastewater treatment device for the PVA production process, such as... Figure 1 As shown, the system includes a controller 6, a PVA production tank 1, and a wastewater reaction tank 2. An inlet pipe 3 connects the PVA production tank 1 and the wastewater reaction tank 2. The inlet pipe 3 is equipped with an acid inlet, a baffle 34, and a pH meter 33. The acid inlet is connected to an acid inlet pipe 31, which is equipped with an acid regulating valve 32. The acid inlet pipe 31 is also connected to an acid tank 4. Both the pH meter 33 and the acid regulating valve 32 are electrically connected to the controller 6. The pH meter 33 is used to detect the pH of the wastewater in the inlet pipe 3, generating a pH signal and transmitting it to the controller 6. The controller 6 receives the pH signal and adjusts the opening of the acid regulating valve 32 according to the pH signal.

[0034] The baffle 34 includes several baffles spaced apart on both sides of the inner wall of the inlet pipe 3, which facilitates the thorough mixing of acid added from the acid inlet pipe 31 with the wastewater in the area of ​​the baffle 34, so that the pH meter can accurately measure the pH of the mixed wastewater, thereby improving the accuracy of acid adjustment and avoiding acid waste.

[0035] Wastewater reaction tank 2 is connected to a reagent inlet pipe 21 and a drain pipe 22. The reagent inlet pipe 21 is equipped with a reagent regulating valve 211, which is connected to a reagent tank 5. The reagent regulating valve 211 is electrically connected to a controller 6. The controller 6 is used to match the reagent addition amount according to the real-time input of the PVA concentration in the wastewater before treatment, and to control the valve opening of the reagent regulating valve 211 in stages according to the reagent addition amount, thereby regulating the reagent addition amount in real time. Specifically, the reagent used in this solution is sodium hypochlorite. An information storage module is installed within the controller 6, pre-entering reagent dosage tiered control information (specifically, three-level threshold control information) as follows: Level 1 threshold: When the PVA concentration in the wastewater before treatment is below 50 ppm, the sodium hypochlorite dosage is 3-5‰ of the PVA concentration; Level 2 threshold: When the PVA concentration in the wastewater before treatment is between 50 ppm and 100 ppm, the sodium hypochlorite dosage is 2-4‰ of the PVA concentration; Level 3 threshold: When the PVA concentration in the wastewater before treatment is above 100 ppm, the sodium hypochlorite dosage is 1-3‰ of the PVA concentration. Furthermore, the information storage module pre-sets the valve opening of the reagent regulating valve 211 corresponding to each level of sodium hypochlorite dosage, facilitating the matching of the valve opening of the reagent regulating valve 211 to the real-time input of the PVA concentration in the wastewater before treatment, thus achieving tiered control of the reagent regulating valve 211.

[0036] Wastewater reaction tank 2 is equipped with a thermometer 24. A reboiler 11 is connected to the bottom of PVA production tank 1, and a hot water pipe is connected to the reboiler 11. A hot water valve 111 is installed on the hot water pipe. Both the reboiler 11 and the thermometer 24 are electrically connected to the controller 6. The thermometer 24 is used to detect the temperature of the wastewater in wastewater reaction tank 2, generate a temperature signal, and transmit the temperature signal to the controller 6. The controller 6 receives the temperature signal and adjusts the opening of the hot water valve 111 according to the temperature signal, thereby regulating the hot water flow rate in the reboiler 11 and thus regulating the wastewater temperature in PVA production tank 1.

[0037] As an improvement, the wastewater reaction tank 2 is equipped with a stirring device. For reference, the stirring device is a stirrer 23. The stirring speed of the stirrer 23 is controlled at 60-120 rpm. The stirrer 23 is electrically connected to the controller 6. The controller 6 is used to control the start and stop of the stirrer 23 and its rotation speed.

[0038] As an improvement, a return pipe 221 is connected between the drain pipe 22 and the wastewater reaction tank 2. A return valve 222 is provided on the return pipe 221. The return valve 222 is electrically connected to the controller 6. The controller 6 is used to adjust the valve opening of the return valve 222 according to the real-time input of the PVA concentration in the treated wastewater.

[0039] In this solution, controller 6 is a PLC controller 6. The PLC controller 6, pH meter 33, thermometer 24, reflux valve 222, stirrer 23, hot water valve 111, acid regulating valve 32 and reagent regulating valve 211 are all existing electrical equipment, and appropriate models are selected according to the usage requirements. They will not be described in detail here.

[0040] The specific implementation method is as follows:

[0041] The information storage module of controller 6 is pre-set with pH threshold, temperature threshold, reagent addition level control information, and reflux information.

[0042] The PVA wastewater generated by PVA production tank 1 is kept at a certain temperature by the reboiler 11. The wastewater flows through the inlet pipe 3 at the bottom of PVA production tank 1 and through the baffle 34 into wastewater reaction tank 2. During this process, the pH meter 33 detects the pH of the wastewater in the inlet pipe 3, generates a pH signal, and transmits the pH signal to the controller 6. After receiving the pH signal, the controller 6 compares the pH signal with the pH threshold preset in the controller 6 to adjust the valve opening of the acid regulating valve 32. Specifically, if the pH signal is lower than the pH threshold, the valve opening of the acid regulating valve 32 is reduced; if the pH signal is higher than the pH threshold, the valve opening of the acid regulating valve 32 is increased. The acid that enters the inlet pipe 3 through the acid inlet pipe 31 is fully mixed in the area of ​​the baffle 34 and is detected by the pH meter 33 to generate a pH signal, which is transmitted to the controller 6 for real-time adjustment of the valve opening of the acid regulating valve 32.

[0043] After the wastewater enters the wastewater reaction tank 2, the controller 6 adjusts the valve opening of the reagent regulating valve 211 according to the real-time input PVA concentration and the corresponding reagent addition amount. After the reagent is added, it is thoroughly mixed and reacted with the wastewater under the stirring action of the stirrer 23. During the reaction, the thermometer 24 detects the wastewater temperature in the wastewater reaction tank 2 in real time, generates a temperature signal, and transmits the temperature signal to the controller 6. After receiving the temperature signal, the controller 6 compares the temperature signal with the temperature threshold preset in the controller 6 and adjusts the valve opening of the hot water valve 111, thereby regulating the hot water flow rate in the reboiler 11 and thus regulating the wastewater temperature in the PVA production tank 1.

[0044] During the reaction, it is also necessary to periodically test the PVA content in the wastewater before and after the reaction, and input both data into the controller 6. This allows the controller 6 to control the opening of the reagent regulating valve 211 in real time based on the PVA content in the wastewater before the reaction, and to adjust the opening and closing of the reflux valve 222 and its opening degree in real time based on the PVA content in the wastewater after the reaction. This facilitates the recirculation of wastewater with a PVA content higher than 10 ppm for further reagent oxidation treatment, avoiding excessively high PVA content from increasing the difficulty of biodegradability of the wastewater.

[0045] This solution also provides a method for treating low-concentration PVA wastewater during PVA production, relying on the aforementioned low-concentration wastewater treatment device for PVA production. The PVA content in the wastewater before treatment is less than 300 ppm, and the method includes the following steps:

[0046] In the S1 adjustment stage, the pH and temperature of the wastewater are adjusted.

[0047] As a reference, a reboiler is used to adjust the temperature of the PVA wastewater in the PVA production tank to 80–95°C. In the connecting pipeline that transports the PVA wastewater to the wastewater reaction tank, acid is added to adjust the pH of the wastewater to 5–6. The acid used to adjust the pH of the wastewater is one of concentrated sulfuric acid, concentrated hydrochloric acid, or concentrated nitric acid. As a reference, this scheme uses concentrated hydrochloric acid to adjust the pH of the wastewater.

[0048] In the S2 reaction stage, an oxidant is added to the wastewater to complete the reaction;

[0049] For reference, the oxidant in this scheme is sodium hypochlorite, and the dosage of sodium hypochlorite is 1‰ to 5‰ of the PVA concentration in the wastewater. For further reference, the dosage of sodium hypochlorite is determined based on the PVA concentration in the wastewater: when the PVA concentration in the wastewater is below 50 ppm, the dosage of sodium hypochlorite is 3‰ to 5‰ of the PVA concentration; when the PVA concentration in the wastewater is between 50 ppm and 100 ppm, the dosage of sodium hypochlorite is 2‰ to 4‰ of the PVA concentration; when the PVA concentration in the wastewater is above 100 ppm, the dosage of sodium hypochlorite is 1‰ to 3‰ of the PVA concentration.

[0050] When adding oxidant to the wastewater reaction tank, the stirring device is activated to agitate the wastewater in the tank at a speed of 60–120 rpm. Stirring continues throughout the entire reaction process; for reference, the reaction time in this scheme is 20–40 minutes. Specifically, this scheme treats a batch of PVA wastewater (approximately 30 t / h), with the PVA wastewater residing in the pipelines and reaction tank for only 20–40 minutes, effectively improving the PVA wastewater treatment efficiency.

[0051] In the S3 post-treatment stage, the PVA concentration in the wastewater after the reaction is detected, and the PVA wastewater after the reaction is discharged.

[0052] In the S4 biological treatment stage, wastewater with PVA concentration below 10 ppm undergoes further biological treatment before being discharged in compliance with standards.

[0053] As a reference, the PVA concentration and COD of the wastewater after the reaction are tested. If the PVA concentration in the wastewater after the reaction drops to 10 ppm or below, the wastewater is discharged into the wastewater treatment plant. If the PVA concentration in the wastewater after the reaction is higher than 10 ppm, it needs to be recycled back to the front end for further treatment.

[0054] Example 1

[0055] In this embodiment, the PVA concentration in the low-concentration PVA wastewater was 37 ppm. The reaction temperature was adjusted to 90℃, the pH of the wastewater was 5.3, the amount of sodium hypochlorite added was 4‰ of the PVA content, and the reaction time was 20 min.

[0056] Example 2

[0057] In this embodiment, the PVA concentration in the low-concentration PVA wastewater was 51 ppm, the pH of the wastewater was adjusted to 5.5, the reaction temperature was 95℃, the amount of sodium hypochlorite added was 3‰ of the PVA content, and the reaction time was 20 min.

[0058] Example 3

[0059] In this embodiment, the PVA concentration in the low-concentration PVA wastewater was 110 ppm, the pH of the wastewater was adjusted to 6, the reaction temperature was 90℃, the amount of sodium hypochlorite added was 2‰ of the PVA content, and the reaction time was 30 min.

[0060] Example 4

[0061] In this embodiment, the PVA concentration in the low-concentration PVA wastewater was 216 ppm, the pH of the wastewater was adjusted to 5, the reaction temperature was 80℃, the amount of sodium hypochlorite added was 2‰ of the PVA content, and the reaction time was 40 min.

[0062] The process conditions and implementation effects of treating low-concentration PVA wastewater in Examples 1-4 are shown in Table 1. Comparative Examples 1-12 are also added to Table 1 to illustrate the treatment effect of this scheme on low-concentration PVA wastewater when the parameters are taken outside the range of this scheme.

[0063] The PVA removal rate and COD removal rate in Table 1 are calculated using the following formulas:

[0064] PVA removal rate (%) = (PVA concentration before treatment - PVA concentration after treatment) / PVA concentration before treatment * 100%

[0065] Table 1 Implementation Conditions and Results

[0066]

[0067]

[0068] Experimental data show that this scheme, by limiting the sequence of steps such as temperature adjustment, pH adjustment, and oxidant addition for PVA wastewater, allows for pre-conditioning of the PVA wastewater temperature in the PVA production tank and pH adjustment in the delivery pipeline connected to the reaction tank. Once the wastewater enters the reaction tank, the oxidant can be directly added to degrade the PVA. This effectively increases the reaction rate while further reducing energy loss in high-temperature PVA wastewater, saving energy and lowering treatment costs.

[0069] Furthermore, the applicant's long-term experiments revealed that adjusting the temperature and pH of the PVA wastewater, as well as the order and parameters of adding oxidants, significantly reduces the treatment effect of PVA wastewater. Specifically, if the pH is too low (as in Comparative Example 1), it not only fails to significantly improve the reaction rate and PVA degradation efficiency but also wastes reagents, resulting in more harm than good. Conversely, failing to adjust the wastewater pH before the reaction (as in Comparative Example 11, where the oxidant was added first, followed by temperature and pH adjustment) or having an excessively high pH after adjustment (as in Comparative Example 2) both prevent the oxidant from rapidly acidifying and generating free radicals, thus reducing the oxidative degradation effect and affecting the removal of PVA, especially in the case of not adjusting the wastewater pH. Specifically, in Comparative Example 11, the wastewater contained 110 ppm of PVA with a pH of approximately 7-8. If the pH of the wastewater was not adjusted before the reaction, and instead 2‰ sodium hypochlorite was added before adjusting the temperature and pH, 76 ppm of PVA remained in the wastewater after 30 minutes of reaction. The PVA removal rate was only 30.9%, which not only failed to meet the PVA discharge standards and required further treatment, but also necessitated increasing the sodium hypochlorite dosage to 3‰ during the re-treatment process, resulting in wasted oxidant and reduced PVA treatment efficiency. This clearly demonstrates that the pH of PVA wastewater must be adjusted to the optimal reaction range before the oxidation reaction to effectively improve the PVA removal efficiency.

[0070] If the wastewater temperature is set too low (as in Comparative Example 3), the PVA wastewater temperature will be too low due to energy loss during equipment transport, which will significantly reduce the PVA oxidation and degradation effect. If the wastewater temperature is set too high (as in Comparative Example 4), not only will energy be wasted, but the PVA modification at high temperature will also make it more difficult to oxidize and degrade. Therefore, it can be seen that both excessively high and excessively low temperatures will reduce the PVA oxidation and degradation effect.

[0071] Regarding reaction time, if the reaction time is too short (e.g., Comparative Example 5), the PVA in the wastewater will not react completely, wasting oxidant and increasing the difficulty of subsequent wastewater treatment, potentially even causing environmental pollution due to excessive PVA levels during discharge. Conversely, if the reaction time is too long (e.g., Comparative Example 6), it will not significantly improve the final PVA removal rate but will actually reduce production efficiency. Regarding the amount of sodium hypochlorite added, if the amount is too small (e.g., Comparative Examples 7 and 8), although the PVA content in the wastewater before treatment is low, the reaction rate with the oxidant will be too slow due to the low PVA concentration, affecting the PVA removal efficiency and results. If the amount is too large (e.g., Comparative Examples 9 and 10), it will not only not significantly improve the final PVA removal rate but will also increase oxidant costs and reduce production efficiency.

[0072] Regarding the selection of oxidants, the applicant tried using persulfates (such as sodium persulfate used in Comparative Example 12). When the amount of oxidant used was the same as that in Comparative Example 3, not only was the residual PVA in the treated wastewater high, but also a large amount of persulfate remained in the wastewater, which could not meet the wastewater discharge standards.

[0073] In summary, this solution first adjusts the wastewater pH and uses an oxidant to degrade PVA, effectively reducing the PVA concentration in the wastewater to below 10 ppm. This effectively avoids the increased difficulty of wastewater biological treatment due to PVA, thereby improving wastewater treatment efficiency. The applicant also found that, compared to adding large amounts of oxidant to treat PVA in wastewater, which still fails to remove other organic matter to discharge standards, this solution combines oxidant degradation of PVA with biological treatment of COD in the wastewater. This not only effectively reduces treatment costs but also improves treatment efficiency and shortens treatment time.

[0074] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for treating low-concentration PVA wastewater during PVA production, characterized in that: The process includes the following steps: S1 adjustment stage, adjusting the wastewater temperature and pH; S2 reaction stage, adding oxidant to the wastewater and completing the reaction; S3 post-treatment stage, detecting the PVA concentration in the wastewater after the reaction and discharging the PVA wastewater after the reaction; the PVA concentration in the low-concentration wastewater before treatment is less than 300 ppm.

2. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: In the S2 reaction stage, the oxidant is sodium hypochlorite, and the amount of sodium hypochlorite added is 1‰ to 5‰ of the PVA concentration in the wastewater.

3. The method for treating low-concentration PVA wastewater during PVA production according to claim 2, characterized in that: In the S2 reaction stage, the dosage of sodium hypochlorite is determined according to the PVA concentration in the wastewater: when the PVA concentration in the wastewater is below 50 ppm, the dosage of sodium hypochlorite is 3-5‰ of the PVA concentration; when the PVA concentration in the wastewater is between 50 ppm and 100 ppm, the dosage of sodium hypochlorite is 2-4‰ of the PVA concentration; and when the PVA concentration in the wastewater is above 100 ppm, the dosage of sodium hypochlorite is 1-3‰ of the PVA concentration.

4. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: During the S2 reaction stage, the reaction time is 20 min to 40 min.

5. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: In the S2 reaction stage, the addition of oxidant and the reaction also include stirring at a speed of 60–120 rpm.

6. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: During the S1 adjustment stage, the wastewater temperature is adjusted to 80–95℃.

7. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: During the S1 adjustment stage, the pH of the wastewater is adjusted to 5-6.

8. The method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: The pH of the wastewater is adjusted using one of the following: concentrated sulfuric acid, concentrated hydrochloric acid, or concentrated nitric acid.

9. A method for treating low-concentration PVA wastewater during PVA production according to claim 1, characterized in that: In the S3 post-treatment stage, if the PVA concentration in the post-reaction wastewater drops to 10 ppm or below, the post-reaction wastewater is discharged into the wastewater treatment plant; if the PVA concentration in the post-reaction wastewater is higher than 10 ppm, it needs to be recycled back to the front end for further treatment.

10. A method for treating low-concentration PVA wastewater during PVA production according to any one of claims 1 to 9, characterized in that: It also includes the S4 biological treatment stage, where wastewater with PVA concentrations below 10 ppm undergoes further biological treatment before being discharged in compliance with standards.

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