Method for modifying and recycling waste nanofiltration membranes

Abstract

The application relates to the technical field of nanofiltration membrane processing, in particular to a modification and recycling method of waste nanofiltration membranes, which comprises the following steps: 1) cyclic normal-temperature cleaning of the nanofiltration membranes by using alkaline agents or acid agents in combination with ultrasonic waves, and the cleaned nanofiltration membranes are washed clean by using deionized water or distilled water; 2) oxidation modification treatment in an oxidation reaction device by using an ozone oxidation method; 3) oxidation modification by using an oxidation agent in a modification device; 4) oxidation modification by using an acid medium oxidizing agent in the modification device; 5) hydroxylation modification by using a hydroxylation agent in the modification device; 6) modification and stabilization treatment by using a buffer stabilizing agent, and washing clean, so as to complete the modification and recycling treatment of the waste nanofiltration membranes. According to the modification and recycling method, the waste nanofiltration membranes can be utilized, the modification process is relatively simple, the membrane surface is not damaged after modification, the nanofiltration membrane flux is significantly improved after modification, and the interception rate meets the design and use requirements.

Description

TECHNICAL FIELD

[0001] The application relates to a method for modifying and recycling waste nanofiltration membranes and belongs to the technical field of nanofiltration membrane treatment. BACKGROUND

[0002] In recent years, nanofiltration membrane technology is widely applied in the field of wastewater treatment. Its typical feature is that, during operation, solutes (mainly macromolecular organic matters and ions with a valence of two or more, such as SO4 2- , Ca 2+ , Mg 2+ , etc.) are well retained by the nanofiltration membrane, especially ions with a valence of two or more, with a retention rate of more than 98%. Generally, the higher the retention rate, the higher the osmotic pressure on both sides of the nanofiltration membrane, the higher the operating pressure, and the more pollutants retained on the membrane surface. A large amount of pollutants will continuously accumulate on the membrane surface, and it is difficult to completely restore the initial flux state through chemical cleaning. As the use time increases, a series of problems such as rapid decline of membrane flux, shortening of chemical cleaning period, increase of cleaning agent dosage, weakening of membrane separation performance, and even failure of the effluent to meet the standard may occur.

[0003] At present, many high-concentration organic wastewater is treated by the process of "pretreatment + nanofiltration membrane". After long-term use, the nanofiltration membrane has a significant decline in membrane flux and retention rate, ultimately leading to problems such as failure to achieve the target treatment effect or unqualified overall process effluent. Therefore, in order to ensure normal production, enterprises generally replace the nanofiltration membrane after 2-3 years of use. A large amount of waste nanofiltration membranes are generally treated as hazardous waste, which increases the economic burden of enterprises and causes serious environmental pollution.

[0004] In view of the problems caused by a large amount of waste nanofiltration membranes, the existing technology process selects to simply oxidize and modify the waste nanofiltration membranes and repair them, so that the structural performance of the organic nanofiltration membrane material changes, such as an increase in membrane pore size and a decrease in divalent salt retention rate. In this way, a nanofiltration membrane that does not retain salt but only retains organic matter is obtained, which is used as a material membrane to realize the recycling economic value of waste.

[0005] However, the oxidation reagent used for membrane oxidation modification at present is mostly an organic oxidizing agent, which pollutes the environment. Meanwhile, the oxidation conditions and the quality of the modified membrane product cannot be effectively guaranteed. Moreover, the surface of the oxidized nanofiltration membrane is no longer smooth, which causes the cleaning period of the entire membrane element to be shortened and the service life to be greatly reduced due to unbalanced local pollution when the membrane is used again.

[0006] In addition, in order to realize the thorough removal of the organic matter on the surface of the discarded membrane, heating is often required in the oxidation modification process in the prior art, for example, CN113457456A, CN110127818A, CN106957089A and the like, but the heating process is not easy to control, and the membrane will be damaged at the same time when the organic matter is oxidized. Therefore, the oxidation modified membrane element in the prior art still has many problems, and it is of great significance to develop a stable and controllable modification and reuse method of discarded nanofiltration membranes. SUMMARY

[0007] The present application provides a modification and reuse method of discarded nanofiltration membranes to make the discarded nanofiltration membranes be reasonably utilized, reduce resource waste and environmental pollution, and the modification process is relatively simple, the surface structure of the modified membrane will not be damaged, and the flux of the modified nanofiltration membrane can be significantly improved, and the rejection rate meets the design and use requirements.

[0008] The technical scheme for solving the above technical problems is as follows: a modification and reuse method of discarded nanofiltration membranes, the modification and reuse method comprises the following steps:

[0009] S1, cleaning: using an aqueous solution of an alkaline agent or an aqueous solution of an acidic agent combined with ultrasonic to circulate and clean the nanofiltration membrane at room temperature, and then rinsing the cleaned nanofiltration membrane with deionized water or distilled water to obtain a membrane element A;

[0010] S2, oxidation modification treatment 1: using an ozone oxidation method to perform oxidation modification treatment on the membrane element A in an oxidation reaction device to obtain a membrane element B;

[0011] S3, oxidation modification treatment 2: using an oxidation agent to perform oxidation modification on the membrane element B by using a modification device, and treating the membrane element B by circulating immersion at room temperature, and then rinsing the treated membrane element B with deionized water or distilled water to obtain a membrane element C;

[0012] S4, oxidation modification 3: using an aqueous solution of an acidic medium oxidizing agent to perform oxidation modification on the membrane element C by using a modification device, and treating the membrane element C by circulating immersion at room temperature, and then rinsing the treated membrane element C with deionized water or distilled water to obtain a membrane element D;

[0013] S5, oxidation modification 4: using an aqueous solution of a hydroxylated agent to perform hydroxylated modification on the surface of the membrane element D by using a modification device, and then rinsing the treated membrane element D with deionized water to obtain a modified and reusable membrane element E;

[0014] S6, modification and stabilization treatment: the membrane element E is treated with a water solution of a buffer stabilizing agent using a stabilization device, and the membrane element E after soaking is cleaned with deionized water to obtain a modified and reusable membrane element F, i.e. the modification and reuse treatment of the discarded nanofiltration membrane is completed.

[0015] Further, in step S1, the mass concentration of the alkaline agent or the acidic agent in the water solution is 0.16%-1%, the cleaning time is 40-80min, the ultrasonic frequency is 45-55kHz, and the ultrasonic intensity is 0.3-1W·cm -2 .

[0016] According to the application environment of the discarded nanofiltration membrane, a corresponding alkaline agent or acidic agent is selected for cleaning. If there are more organic matters retained on the nanofiltration membrane, an alkaline agent is selected for cleaning. If there are more calcium and magnesium ions retained on the nanofiltration membrane, an acidic agent is selected for cleaning. The alkaline agent and the acidic agent can also be used in combination during the cleaning process.

[0017] Further, in step S1, the alkaline agent is one or a combination of sodium hydroxide, sodium dodecyl sulfate, EDTA, disodium EDTA, tetrasodium EDTA, sodium dodecyl benzene sulfonate, benzalkonium bromide, anionic polyacrylamide, sodium fatty alcohol acyl sulfate, and sodium ethoxylated fatty acid methyl ester sulfonate.

[0018] The acidic agent is one or a combination of citric acid, hydrochloric acid, phosphoric acid, nitric acid, and oxalic acid.

[0019] Further, in step S2, the ozone concentration is 0.2-0.8mg / L, and the oxidation time is 5-30min.

[0020] Further, in step S3, the mass concentration of the oxidizing agent is 0.005-1%, the oxidation time is 30-150min, and the circulating pump pressure is 8-15bar.

[0021] In step S3, the oxidizing agent is one or a combination of sodium hypochlorite and chlorine water.

[0022] Preferably, in step S3, the mass concentration of the oxidizing agent is 0.05-0.3%, the oxidation time is 30-60min.

[0023] Further, in step S4, the mass concentration of the acidic medium oxidizing agent in the water solution is 0.1‰-0.8‰, the oxidation time is 10-60min, and the circulating pump pressure is 8-15bar.

[0024] In step S4, the acidic medium oxidizing agent is one or a combination of H2O2, peroxyacetic acid, ammonium persulfate, and trimethylamine oxide.

[0025] Further, in step S5, the mass concentration of the hydroxylated agent in the aqueous solution is 0.1 ‰-0.6 ‰, and the membrane element D is treated by a circulation method, and the circulating pump pressure is 8-15 bar, and the membrane element D is soaked at normal temperature by a circulation soaking method for 60-120 min.

[0026] Further, in step S5, the hydroxylated agent is one or a combination of hydroxypropyl methyl cellulose, polyethylene glycol-800, lauroyl diethanolamide, and diethylene glycol.

[0027] Further, in step S6, the mass concentration of the buffer stabilizing agent in the aqueous solution is 0.5-1.5 %, the water temperature is controlled by a heat exchanger to be 18-28 ℃, and the circulation soaking time is 120-240 min.

[0028] Further, in step S6, the buffer stabilizing agent is any one of sodium sulfate, Na2HPO4, KH2PO4, NaCl, KCl, and 2-morpholinoethanesulfonic acid.

[0029] The beneficial effects of the present application are:

[0030] 1) The modification and reuse method of the discarded nanofiltration membrane is stable and controllable, the discarded nanofiltration membrane is cleaned by a combination of circulation cleaning and ultrasonic waves, the cleaning speed is fast, the cleanliness is high, and the membrane surface is not damaged after cleaning, which is beneficial to the complete modification of the subsequent nanofiltration membrane, prevents the modification from being limited to the surface of the pollution layer, and prevents the non-uniform modification from causing the cleaning period to be shortened and the service life to be reduced when the nanofiltration membrane is used again;

[0031] 2) The discarded nanofiltration membrane is modified by the modification method, and the modification time, modification sequence, temperature, pressure and other conditions are strictly controlled, so that the ideal effect is achieved while the amount of oxidizing modification agent is saved;

[0032] 3) After the discarded nanofiltration membrane is modified by the modification method, the nanofiltration membrane has a high rejection rate for macromolecular organic matter and a low rejection rate for inorganic ions, and a nanofiltration membrane that does not reject salt is obtained, and the flux is also significantly improved, so that the discarded nanofiltration membrane can be used again, waste is turned into treasure, and the recycling economic value of waste reuse is realized;

[0033] 4) The method for modifying and recycling the waste nanofiltration membrane according to the present application, first, the ozone is used to pre-oxidize the organic matter in the membrane element, which can preliminarily remove part of the organic matter in the membrane element, and treat part of the macromolecular organic matter into small molecules, thereby reducing the processing pressure of subsequent reagent oxidation, without using high-concentration reagents for oxidation, while reducing the processing time and temperature of reagent oxidation, avoiding excessive oxidation of the reagent, causing damage to the nanofiltration membrane and causing delamination of the nanofiltration membrane, or causing damage to the surface of the nanofiltration membrane, and causing unevenness on the surface of the nanofiltration membrane, thereby realizing the recycling of the nanofiltration membrane.

[0034] 5) The conventional membrane oxidation treatment process needs to heat the oxidizing agent, which is conducive to more thorough oxidation treatment, but for the modification treatment of the nanofiltration membrane, heating can easily cause damage to the nanofiltration membrane, but if heating is not used, there can be a problem of incomplete modification treatment, but in the method according to the present application, ozone and reagent oxidation are combined to perform modification treatment at room temperature, avoiding damage to the nanofiltration membrane during the modification treatment, and the entire treatment process is controllable.

[0035] 6) After the oxidation treatment, the nanofiltration membrane is treated with a buffer stabilizer, which has a protective effect on the nanofiltration membrane. The reason is that the surface of the nanofiltration membrane after modification treatment is in a wet state, and if it is directly placed, it will not only be contaminated by microorganisms, causing the membrane surface to "mold" or block the membrane pores, but also carbon dioxide in the air will easily enter the moisture on the membrane surface to form a buffer system of H2CO3, causing changes in the pH of the membrane surface, thereby affecting the performance of the membrane material and causing damage to the membrane surface. The buffer stabilizer has the effects of sterilization and stable pH changes, and can have a good protective effect on the nanofiltration membrane, avoiding damage to the nanofiltration membrane in the air after modification treatment. In addition, 2-morpholinoethanesulfonic acid can easily remove the contaminants on the nanofiltration membrane, and 2-morpholinoethanesulfonic acid is a weak acid and is relatively mild, which will not damage the surface of the nanofiltration membrane. The buffer stabilizer has a good protective effect on the nanofiltration membrane. DETAILED DESCRIPTION

[0036] The specific embodiments of the present application will be described in detail below. The present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without departing from the spirit of the present application, therefore the present application is not limited by the disclosed specific embodiments.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terms used are only for describing specific embodiments, not for limiting the present application.

[0038] The method for modifying and recycling the waste nanofiltration membrane according to the present application comprises the following steps:

[0039] S1, cleaning: using an aqueous solution of an alkaline agent or an aqueous solution of an acidic agent combined with ultrasonic to circulate normal temperature cleaning of the nanofiltration membrane, the cleaned nanofiltration membrane is washed clean with deionized water or distilled water, to obtain the membrane element A;

[0040] S2, oxidation modification treatment 1: using ozone oxidation method to perform oxidation modification treatment of the membrane element A in the oxidation reaction device to obtain the membrane element B;

[0041] S3, oxidation modification treatment 2: using the modification device to perform oxidation modification of the membrane element B with an oxidation agent, circulating immersion at normal temperature to treat the membrane element B, the treated membrane element B is washed clean with deionized water or distilled water, to obtain the membrane element C;

[0042] S4, oxidation modification 3: using the modification device to perform oxidation modification of the membrane element C with an aqueous solution of an acidic medium oxidizing agent, circulating immersion at normal temperature to treat the membrane element C, the treated membrane element C is washed clean with deionized water or distilled water, to obtain the membrane element D;

[0043] S5, oxidation modification 4: using the modification device to perform hydroxylation modification of the membrane element D surface with an aqueous solution of a hydroxylation agent, the treated membrane element D is washed clean with deionized water, to obtain the modified reusable membrane element E;

[0044] S6, modification and stabilization treatment: using the stabilization device to perform modification and stabilization treatment of the membrane element E with an aqueous solution of a buffer stabilization agent, the treated membrane element E is washed clean with deionized water, to obtain the modified reusable membrane element F, that is, the modification and reuse treatment of the discarded nanofiltration membrane is completed.

[0045] In step S1, the mass concentration of the alkaline agent or the acidic agent in the aqueous solution is 0.16%-1%, the cleaning time is 40-80min, the ultrasonic frequency is 45-55kHz, and the ultrasonic intensity is 0.3-1W·cm -2 According to the application environment of the discarded nanofiltration membrane before, the corresponding aqueous solution of alkaline agent or acidic agent is selected for cleaning. If there are more organic matters retained on the nanofiltration membrane, the aqueous solution of alkaline agent is selected for cleaning. If there are more calcium and magnesium ions retained on the nanofiltration membrane, the aqueous solution of acidic agent is selected for cleaning. The alkaline agent and the acidic agent can also be used in combination during the cleaning process.

[0046] In step S1, the alkaline agent is one or a combination of sodium hydroxide, sodium dodecyl sulfate, EDTA, disodium EDTA, tetrasodium EDTA, sodium dodecylbenzenesulfonate, benzalkonium bromide, anionic polyacrylamide, sodium fatty alcohol acylsulfate, and sodium ethoxylated fatty acid methyl ester sulfonate;

[0047] The acid agent is one or a combination of citric acid, hydrochloric acid, phosphoric acid, nitric acid, and oxalic acid.

[0048] In step S2, the ozone concentration is 0.2-0.8mg / L, and the oxidation time is 5-30min.

[0049] In step S3, the mass concentration of the oxidizing agent is 0.005-1%, the oxidation time is 30-60min, and the circulating pump pressure is 8-15bar; the oxidizing agent is one or a combination of sodium hypochlorite and chlorine water.

[0050] In step S4, the mass concentration of the acid medium oxidizing agent in the aqueous solution is 0.1‰-0.8‰, the oxidation time is 10-60min, and the circulating pump pressure is 8-15bar; the acid medium oxidizing agent is one or a combination of H2O2, peroxyacetic acid, ammonium persulfate, and trimethylamine oxide.

[0051] In step S5, the mass concentration of the hydroxylated agent in the aqueous solution is 0.1‰-0.6‰, the membrane element D is treated by the circulating method, the circulating pump pressure is 8-15bar, and the membrane element D is soaked at room temperature by the circulating immersion method for 60-120min; the hydroxylated agent is one or a combination of hydroxypropyl methylcellulose, polyethylene glycol-800, lauroyl diethanolamide, and diethylene glycol.

[0052] In step S6, the mass concentration of the buffer stabilizing agent in the aqueous solution is 0.5-1.5%, the water temperature is controlled by the heat exchanger to be 18-28℃, and the circulating soaking time is 120-240min; the buffer stabilizing agent is any one of sodium sulfate, Na2HPO4, KH2PO4, NaCl, KCl, and 2-morpholinoethanesulfonic acid.

[0053] Example 1

[0054] 1) Cleaning: using a cleaning device, the nanofiltration membrane is cleaned by circulating the alkaline cleaning agent composed of sodium hydroxide, EDTA, and sodium dodecyl sulfate, and ultrasonic auxiliary cleaning is performed, wherein the mass concentration of sodium hydroxide is 0.1%, the mass concentration of EDTA is 0.05%, the mass concentration of sodium dodecyl sulfate is 0.05%, the cleaning time is 40min, the ultrasonic frequency is 50kHz, and the ultrasonic intensity is 0.5W·cm -2 The cleaned nanofiltration membrane is rinsed with deionized water or distilled water to obtain a membrane element A.

[0055] 2) Oxidation modification treatment 1: using an ozone reaction device to oxidize and modify the membrane element A with ozone, wherein the ozone concentration is 0.5 mg / L, the oxidation time is 15 min, and the membrane element A after oxidation is cleaned with deionized water or distilled water to obtain a membrane element B;

[0056] 3) Oxidation modification treatment 2: using a modification device to oxidize and modify the membrane element B with sodium hypochlorite, wherein the mass concentration of sodium hypochlorite is 0.3%, the oxidation time is 30 min, the circulating pump pressure is 10 bar, and the membrane element B is treated by circulating immersion at room temperature, and the membrane element B after immersion is cleaned with deionized water or distilled water to obtain a membrane element C;

[0057] 4) Oxidation modification 3: using a modification device to oxidize and modify the membrane element C with H2O2, wherein the mass concentration of H2O2 is 0.3‰, the oxidation time is 40 min, the circulating pump pressure is 10 bar, and the membrane element C is treated by circulating immersion at room temperature, and the membrane element C after immersion is cleaned with deionized water or distilled water to obtain a membrane element D;

[0058] 5) Oxidation modification 4: using a modification device to hydroxylate the surface of the membrane element D with polyethylene glycol-800, wherein the mass concentration of polyethylene glycol-800 is 0.2‰, the membrane element D is treated by circulating method, the circulating pump pressure is 10 bar, and the membrane element D is immersed by circulating immersion method at room temperature for 120 min, and the membrane element D after immersion is cleaned with deionized water to obtain a modified reusable membrane element E.

[0059] 6) Modification and stabilization treatment: using a stabilization device to modify and stabilize the membrane element E with 2-morpholinoethanesulfonic acid, wherein the mass concentration of 2-morpholinoethanesulfonic acid is 1%, the water temperature is controlled to be 18°C by a heat exchanger, the circulating time is 120 min, and the membrane element E after immersion is cleaned with deionized water to obtain a modified reusable membrane element F, i.e., the oxidation modification of the discarded nanofiltration membrane is completed.

[0060] In this embodiment 1, the membrane performance of the nanofiltration membrane before modification is as follows: NaCl rejection rate: 10%; flux: 16.2 L / m 2 .h; CODcr rejection rate: 76%; MgSO4 rejection rate: 95.8%.

[0061] In this embodiment 1, the membrane performance of the nanofiltration membrane after modification is as follows: NaCl rejection rate: 2.3%; flux: 66.3 L / m 2 .h; CODcr rejection rate: 68%; MgSO4 rejection rate: 41%.

[0062] From the experimental data of Example 1, it can be seen that the rejection rate of the modified nanofiltration membrane to organic matter does not decrease obviously, but the rejection rate of the modified nanofiltration membrane to divalent salt decreases obviously, so as to achieve the purpose of not rejecting salt and only rejecting organic matter. This is also a kind of nanofiltration membrane that the market hopes to find, which does not reject divalent salt and only rejects organic matter.

[0063] Examples 2-6

[0064] The modification and reuse treatment of the waste nanofiltration membrane was carried out according to the same method as in Example 1. The specific process conditions and performance data of the nanofiltration membrane before and after modification are shown in Table 1 and Table 2.

[0065] Table 1 Process conditions and performance data of Examples 1-6

[0066]

[0067]

[0068] Table 2 Process conditions and performance data of Examples 7-9

[0069]

[0070] Comparative Examples 1-2

[0071] The modification and reuse treatment of the waste nanofiltration membrane was carried out according to the same method as in Example 1. The specific process conditions and performance data of the nanofiltration membrane before and after modification are shown in Table 1 and Table 2.

[0072] From the data comparison of Comparative Examples 1-2 and Example 1, it can be seen that if ozone oxidation is not used in the whole modification and reuse process, the performance of the modified membrane cannot meet the requirements of reuse as shown in the results of Comparative Example 2, and the flux is very low. In order to improve the performance of the modified membrane, the concentration of the reagent and the treatment time in steps 3) to 4) are increased in Comparative Example 1, but the performance of the modified membrane still cannot meet the requirements of reuse. In addition, it is found in the experiment that if the concentration of the reagent and the treatment time in steps 3) to 4) are further increased (exceeding the value range defined in the present application), the surface of the modified nanofiltration membrane appears obvious unevenness, and even the nanofiltration membrane appears delamination, which is damaged, so the nanofiltration membrane cannot be reused.

[0073] In the method, the nanofiltration membrane is first subjected to ozone oxidation treatment, so that the organic matter on the nanofiltration membrane is subjected to ozone oxidation treatment, and then combined with the oxidizing agent in steps 3) to 4) to realize the removal of pollutants under relatively mild conditions, improve the flux of the nanofiltration membrane, and avoid damage to the nanofiltration membrane under strong oxidation treatment conditions. If the ozone oxidation process is not increased, the low-concentration oxidizing agent cannot realize sufficient treatment of the pollutants on the nanofiltration membrane. If the concentration is increased, the high-concentration oxidizing agent directly contacts the surface of the nanofiltration membrane, which can damage the surface of the nanofiltration membrane.

[0074] Comparative Example 3-Comparative Example 4

[0075] The same method as in Example 1 was used for the modification and reuse treatment of the waste nanofiltration membrane, except that the oxidizing agent in steps 3) to 4) was not used. The specific process conditions and performance data are shown in Table 3.

[0076] From the data comparison of Comparative Examples 3-4 and Example 1, it can be seen that if the entire modification and reuse process does not use oxidizing agent oxidation, only ozone oxidation treatment is used, even if the ozone concentration and ozone oxidation treatment time are increased within the range described in the application, the flux of the nanofiltration membrane after oxidation treatment is low, and reuse cannot be realized. In addition, it was found in the experiment that if the ozone concentration is further increased (>0.8 mg / L) or the ozone treatment time is increased (>30 min), the surface of the nanofiltration membrane appears uneven, and cannot withstand the working pressure of the nanofiltration membrane. Therefore, the nanofiltration membrane cannot be reused. The reason is that the use of a certain ozone oxidation cannot completely remove all pollutants on the nanofiltration membrane, but if the ozone concentration or treatment time is increased, ozone is a strong oxidizing agent, which can cause the organic molecules of the nanofiltration membrane to break down, resulting in damage to the nanofiltration membrane.

[0077] Table 3 Process conditions and performance data of Comparative Examples 1-4

[0078]

[0079]

[0080] Comparative Example 5

[0081] The same method as in Example 1 was used for the modification and reuse treatment of the waste nanofiltration membrane, except that the order of steps 4) and 5) was changed. The specific process conditions and performance data are shown in Table 4.

[0082] The nanofiltration membrane after the modification and reuse treatment in this comparative example 5 has an uneven surface, which cannot be reused. Because the surface of the nanofiltration membrane appears uneven, the nanofiltration membrane cannot withstand the normal working pressure requirements.

[0083] The other process conditions of Comparative Example 5 and Example 1 are the same, except that the order of step 4) and step 5) is changed, and the modified nanofiltration membrane after the reusing treatment has completely different performance, which shows that the treatment steps described in the application are used to obtain a reasonable reused nanofiltration membrane.

[0084] The oxidation modification treatment of steps 3) to 4) in the method described in the application is relatively strong, and can remove most of the pollutants on the surface of the nanofiltration membrane as much as possible. The hydroxylation modification of step 5) is relatively weak in oxidation performance, because most of the pollutants on the surface of the nanofiltration membrane have been removed by the time step 5) is reached, and the surface of the nanofiltration membrane is relatively clean. The use of a hydroxylation agent to contact the membrane surface for fine-tuning can achieve complete removal of pollutants on the nanofiltration membrane, and also avoid direct contact of the nanofiltration membrane with a strong oxidizing agent, which can cause damage to the membrane surface. In Comparative Example 5, the hydroxylation modification is performed first, and then the nanofiltration membrane is oxidized by an acidic medium oxidizing agent, which causes the strong oxidizing acidic medium oxidizing agent to directly contact the membrane surface more closely, and ultimately causes damage to the membrane, which cannot be reused. Therefore, using the method described in the application and reasonably arranging the sequence of strong oxidation and weak oxidation treatment is more conducive to obtaining a nanofiltration membrane that can be reused.

[0085] Comparative Example 6

[0086] The same method as in Example 1 is used to perform the modification and reuse treatment of the discarded nanofiltration membrane, except that the temperature in step 6) is controlled at 15°C. The specific process conditions and performance data are shown in Table 4.

[0087] Comparative Example 7

[0088] The same method as in Example 1 is used to perform the modification and reuse treatment of the discarded nanofiltration membrane, except that the temperature in step 6) is controlled at 30°C. The specific process conditions and performance data are shown in Table 4.

[0089] Comparative Example 8

[0090] The same method as in Example 1 is used to perform the modification and reuse treatment of the discarded nanofiltration membrane, except that step 6) is not performed. The specific process conditions and performance data are shown in Table 4.

[0091] Table 4 Process conditions and performance data of Comparative Examples 5-8

[0092]

[0093]

[0094] From the data comparison of Comparative Example 8 and Example 1, it can be seen that if the treatment of 2-morpholinoethanesulfonic acid buffer stabilizing agent is not performed, the performance of the modified nanofiltration membrane is relatively poor, because 2-morpholinoethanesulfonic acid can dissolve and remove the pollutants on the nanofiltration membrane, and 2-morpholinoethanesulfonic acid is a weak acid and is relatively mild, and will not damage the surface of the nanofiltration membrane, if the 2-morpholinoethanesulfonic acid buffer stabilizing agent is not used for treatment, the pollutants will still remain on the nanofiltration membrane, which will ultimately affect the performance of the nanofiltration membrane, and the 2-morpholinoethanesulfonic acid buffer stabilizing agent has a protective effect on the nanofiltration membrane, because the modified nanofiltration membrane can be damaged by the environment if it is directly placed, and the 2-morpholinoethanesulfonic acid buffer stabilizing agent can have a good protective effect on the nanofiltration membrane, avoiding damage to the modified nanofiltration membrane in the air. In addition, the temperature condition of step 6) in Comparative Example 6 and Comparative Example 7 is not within the temperature range described in the present application, so the performance of the modified nanofiltration membrane is relatively poor, because 2-morpholinoethanesulfonic acid only has better activity in the range of 18-28℃, which is more conducive to the removal of pollutants on the nanofiltration membrane.

[0095] The technical features of the above-described embodiments can be combined in any manner. For the sake of brevity, not all possible combinations are described, but it should be understood that any combination of the technical features is within the scope of the present disclosure.

[0096] Those skilled in the art will appreciate that, without departing from the concept of the present application, several modifications and improvements can be made, which are within the scope of the present application, and the scope of protection of the present application is subject to the appended claims.

Claims

1. A method for modifying and reusing a waste nanofiltration membrane, characterized by, The modified recycling method comprises the following steps: S1, cleaning: using an aqueous solution of an alkaline agent or an aqueous solution of an acidic agent combined with ultrasonic to clean the nanofiltration membrane at room temperature, and then washing the cleaned nanofiltration membrane with deionized water or distilled water to obtain a membrane element A; the mass concentration of the alkaline agent or the acidic agent in the aqueous solution is 0.16%-1%; S2, oxidation modification treatment 1: using an ozone oxidation method to perform oxidation modification treatment on the membrane element A in an oxidation reaction device to obtain a membrane element B; In step S2, the ozone concentration is 0.2-0.8 mg / L, and the oxidation time is 5-30 min; S3, oxidation modification treatment 2: using an oxidation agent to perform oxidation modification on the membrane element B by using a modification device, and treating the membrane element B by circulating immersion at room temperature, and then washing the treated membrane element B with deionized water or distilled water to obtain a membrane element C; In step S3, the oxidation agent is one or a combination of sodium hypochlorite, hypochlorous acid and chlorine water; In step S3, the mass concentration of the oxidation agent is 0.005-1%, the oxidation time is 30-60 min, and the circulating pump pressure is 8-15 bar; S4, oxidation modification 3: using a non-chlorine acidic medium oxidant aqueous solution to perform oxidation modification on the membrane element C by using a modification device, and treating the membrane element C by circulating immersion at room temperature, and then washing the treated membrane element C with deionized water or distilled water to obtain a membrane element D; In step S4, the acidic medium oxidant is one or a combination of H2O2, peroxyacetic acid and ammonium persulfate; In step S4, the mass concentration of the acidic medium oxidant in the aqueous solution is 0.1‰-0.8‰, the oxidation time is 10-60 min, and the circulating pump pressure is 8-15 bar; S5, oxidation modification 4: using a hydroxylated agent aqueous solution to perform hydroxylation modification on the surface of the membrane element D by using a modification device, and then washing the treated membrane element D with deionized water to obtain a modified reusable membrane element E; S6, modification and stabilization treatment: using a buffer stabilizing agent aqueous solution to perform modification and stabilization treatment on the membrane element E by using a stabilization device, and then washing the treated membrane element E with deionized water to obtain a modified reusable membrane element F, i.e. completing the modification and recycling treatment of the waste nanofiltration membrane; In step S4, the water temperature is controlled to be 18-28℃ by a heat exchanger.

2. The method for modifying and reusing the discarded nanofiltration membrane according to claim 1, characterized in that, In step S1, the cleaning time is 40-80 min, the ultrasonic frequency is 45-55 kHz, and the ultrasonic intensity is 0.3-1 W-cm -2 .

3. The method for modifying and reusing the discarded nanofiltration membrane according to claim 1, characterized in that, In step S1, the alkaline agent is one or a combination of sodium hydroxide, sodium dodecyl sulfate, sodium dodecyl benzene sulfonate, sodium fatty alcohol acyl sulfate and sodium ethoxylated fatty acid methyl ester sulfonate; The acidic agent is one or a combination of citric acid, phosphoric acid and oxalic acid.

4. The method for modifying and reusing the discarded nanofiltration membrane according to claim 1, characterized in that, In step S5, the mass concentration of the hydroxylated agent in the aqueous solution is 0.1‰-0.6‰, the membrane element D is treated by circulating method, the circulating pump pressure is 8-15 bar, and the membrane element D is immersed by circulating immersion method at room temperature for 60-120 min.

5. The method for modifying and reusing the discarded nanofiltration membrane according to claim 1, characterized in that, In step S5, the hydroxylated agent is diethylene glycol.

6. The method for modifying and reusing the discarded nanofiltration membrane according to claim 1, wherein, In step S6, the mass concentration of the buffering stabilizing agent in the aqueous solution is 0.5-1.5%, and the circulating soaking time is 120-240 min.

7. The method of claim 1, wherein the method is characterized by: In step S6, the buffering stabilizing agent is any one of Na2HPO4, KH2PO4 and 2-morpholinoethanesulfonic acid.

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