Method for cleaning nanofiltration membrane

Abstract

The invention relates to a cleaning method of a nanofiltration membrane. The cleaning method comprises first alkali washing treatment, enzyme washing treatment and second alkali washing treatment which are sequentially carried out. According to the method, an alkali-enzyme-alkali cleaning sequence is adopted, organic matter, grease, protein and microorganisms in the nanofiltration membrane can be effectively removed, a channel is opened for enzyme washing treatment in the first alkali washing treatment, and enzyme washing is conducted after alkali washing, so that the cleaning effect of enzyme washing treatment can be remarkably improved; the cleaning method provided by the invention is scientific in cleaning sequence and deep in effect, and the flux of the nanofiltration membrane can be efficiently recovered.

Description

Technical Field

[0001] This invention relates to the field of filter membrane cleaning and filtration technology, and more particularly to a method for cleaning nanofiltration membranes. Background Technology

[0002] In the production of 1,3-propanediol, nanofiltration membranes, during long-term operation, trap organic matter, oils, proteins, and microorganisms in the solution, forming a dense and complex mixed fouling layer. This leads to a sharp and irreversible decline in membrane flux. Conventional cleaning methods typically employ a single alkaline or acid wash, or a simple "alkaline-then-acid" step, which is effective for mild fouling. However, for severe fouling with flux decline exceeding 50%, conventional methods often fail to effectively restore membrane performance.

[0003] CN111217425A discloses a cleaning method for submerged ultrafiltration membranes, comprising the following steps: 1) First alkaline wash: The ultrafiltration membrane tank is emptied, and an alkaline cleaning solution is introduced into the ultrafiltration membrane tank until the cleaning solution completely submerges the ultrafiltration membrane fibers, immersing the ultrafiltration membrane for 4-10 hours. During the immersion process, compressed air or a Roots blower is used to aerate and agitate the immersion solution and ultrafiltration membrane fibers in the ultrafiltration membrane tank for 1-5 minutes every 1-2 hours. The alkaline cleaning solution has a temperature of 20-38℃ and consists of sodium hydroxide and sodium hypochlorite. The mass fraction of sodium hydroxide is 1-3%, and the concentration of sodium hypochlorite (calculated as available chlorine) is 2000-5000 ppm. The remainder is reverse osmosis permeate or low-hardness tap water; 2) The ultrafiltration membrane tank... Washing: Discharge the alkaline cleaning solution from the ultrafiltration membrane tank, backwash the ultrafiltration membrane with ultrafiltration permeate until the pH and residual chlorine of the backwash water are close to those of the ultrafiltration permeate. Stop the backwashing and discharge the alkaline cleaning solution and backwash water separately. 3) First acid wash: Empty the ultrafiltration membrane tank, transfer the acidic cleaning solution into the ultrafiltration membrane tank to completely submerge the ultrafiltration membrane fibers, and soak the ultrafiltration membrane for 2-8 hours. During the soaking process, use compressed air or a Roots blower to aerate and agitate the soaking solution and ultrafiltration membrane fibers in the ultrafiltration membrane tank for 1-5 minutes every 0.5-1 hour. The acidic cleaning solution temperature is 20-38℃, the main component is citric acid or oxalic acid, mass fraction 1-2%, and the remainder is reverse osmosis permeate or low-hardness tap water. 4) Ultrafiltration membrane Backwashing: Discharge the acidic cleaning solution from the ultrafiltration membrane tank, and backwash the ultrafiltration membrane with ultrafiltration permeate until the pH value of the backwash water is close to that of the ultrafiltration permeate. Stop the backwashing process and discharge the acidic cleaning solution and backwash water separately. 5) Second alkaline wash: Empty the ultrafiltration membrane tank and transfer the alkaline cleaning solution into the ultrafiltration membrane tank until the ultrafiltration membrane fibers are completely submerged. Soak the ultrafiltration membrane for 4-8 hours. During the soaking process, use compressed air or a Roots blower to aerate and agitate the soaking solution and ultrafiltration membrane fibers in the ultrafiltration membrane tank for 1-5 minutes every 1-2 hours. The temperature of the alkaline cleaning solution is 20-38℃, and its main components are sodium hydroxide and tetrasodium ethylenediaminetetraacetate (EDTA). The mass fraction of sodium hydroxide is 0.6-1.5%, and the mass fraction of tetrasodium ethylenediaminetetraacetate is... 0.4-1%, with the remainder being reverse osmosis permeate or low-hardness tap water; 6) Backwashing of the ultrafiltration membrane tank: Discharge the alkaline cleaning solution from the ultrafiltration membrane tank, backwash the ultrafiltration membrane several times with ultrafiltration permeate, and empty the ultrafiltration membrane tank separately. When the pH value of the backwash water is close to that of the ultrafiltration permeate, stop the backwashing and discharge the alkaline cleaning solution and backwash water; 7) Second acid wash: Empty the ultrafiltration membrane tank, transport the acidic cleaning solution into the ultrafiltration membrane tank until the ultrafiltration membrane fibers are completely submerged, and soak the ultrafiltration membrane for 1.5-3 hours. During the soaking process, use compressed air or a Roots blower to aerate and stir the soaking solution and ultrafiltration membrane fibers in the ultrafiltration membrane tank for 1-2 minutes every 0.5-1 hour. The temperature of the acidic cleaning solution is 20-38℃, and the mass fraction of hydrochloric acid is 0.6-1%, with the remainder being reverse osmosis permeate or low-hardness tap water; 8) Ultrafiltration membrane tank backwashing: Discharge the acidic cleaning solution from the ultrafiltration membrane tank, backwash the ultrafiltration membrane with ultrafiltration permeate, and empty the ultrafiltration membrane tank separately. Stop backwashing when the pH of the backwash water is close to that of the ultrafiltration permeate, and discharge the acidic cleaning solution and backwash water. This cleaning method cannot remove proteins from the filter membrane contaminated during the 1,3-propanediol production process.

[0004] Therefore, developing a method that can achieve deep, targeted cleaning and significantly restore the flux of nanofiltration membranes that are severely contaminated during the production of 1,3-propanediol has become an urgent problem to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a cleaning method for nanofiltration membranes. This invention employs an "alkali-enzyme-alkali" cleaning sequence, which effectively removes organic matter, oils, proteins, and microorganisms from the nanofiltration membrane. The first alkali wash opens the pathway for the enzyme wash; washing with alkali before enzyme wash significantly increases the cleaning effect of the enzyme wash. The cleaning method provided by this invention features a scientific cleaning sequence, deep penetration, and efficient restoration of the nanofiltration membrane's flux.

[0006] To achieve this objective, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for cleaning a nanofiltration membrane, the method comprising sequentially performing a first alkaline washing treatment, an enzyme washing treatment, and a second alkaline washing treatment.

[0007] In the existing technology, conventional cleaning methods usually employ a single alkaline wash or acid wash, or a simple "alkaline first, acid later" step, which is effective for mild contamination. However, for severely contaminated nanofiltration membranes, the existing cleaning methods have poor targeting, and a single agent cannot deal with complex mixed contaminants. Alkaline wash has limited ability to remove proteins, while acid wash is not effective in damaging organic matrix and biofilm.

[0008] This invention employs an "alkali-enzyme-alkali" cleaning sequence. The first alkaline wash is a "barrier-breaking" stage, designed to saponify oils and hydrolyze polysaccharide matrices, disrupting the macroscopic structure of the fouling layer and opening channels for subsequent cleaning. The subsequent enzyme wash serves as a "cleaning" stage, precisely degrading exposed protein contaminants that are difficult to remove with alkaline washing and deeply unclogging membrane pores. The second alkaline wash is a "sweeping" stage, designed to remove enzymatic hydrolysis products and residual agents, consolidating the cleaning effect. The cleaning method provided by this invention is not only applicable to nanofiltration membranes severely fouled during 1,3-propanediol production, but also to various nanofiltration membrane systems severely fouled by organic matter, proteins, and biofilms, with particularly broad application prospects in the food, pharmaceutical, and chemical wastewater resource recovery fields.

[0009] Meanwhile, this invention only requires three cleaning steps—first alkaline washing, enzyme washing, and second alkaline washing—to effectively remove organic matter, grease, protein, and microorganisms from the nanofiltration membrane and efficiently restore its flux, without the need for acid washing and additional alkaline washing steps.

[0010] As a preferred technical solution of the present invention, the temperature of the first alkaline washing treatment is 30~45℃, for example, it can be 30℃, 33℃, 36℃, 39℃, 42℃ or 45℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0011] Preferably, the time for the first alkaline washing treatment is 24 to 48 hours, for example, it can be 24 hours, 30 hours, 36 hours, 42 hours or 48 hours, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 36 to 48 hours.

[0012] As a preferred embodiment of the present invention, an acidic protease solution is used in the enzyme washing treatment.

[0013] This invention selects an acidic protease solution for enzymatic washing, with the core advantage being its suitability for the microenvironment of the contaminated system and the characteristics of the target contaminants after the first alkaline wash. For example, in the complex contamination layer formed during the production of 1,3-propanediol, protein contaminants are prone to denaturation and aggregation under alkaline conditions. Although the trace alkaline substances remaining after the first alkaline wash are rinsed, local weakly alkaline micro-regions may still exist inside the membrane pores. If alkaline protease is used, its optimal pH value (usually 8-11) will cause the exposed proteins to further cross-link and solidify, which will exacerbate membrane pore blockage. On the other hand, the optimal pH value of acidic protease (3-5) can neutralize the weakly alkaline substances remaining in the membrane pores, while loosening the spatial structure of the denatured proteins, making it easier for enzyme molecules to attack peptide bonds and achieve efficient degradation. In addition, the acidic environment can inhibit the secondary reproduction of microorganisms and avoid the generation of new biological contamination during enzymatic hydrolysis, while alkaline protease is prone to bacterial growth during enzymatic washing, leading to enzyme activity decay and contamination layer regeneration. If alkaline protease solution is used for enzymatic washing, the protein degradation rate will be reduced to below 30%, the membrane flux recovery rate will be less than 50%, and the enzymatic hydrolysis products are prone to react with alkaline substances to form insoluble precipitates, causing secondary pollution.

[0014] As a preferred technical solution of the present invention, the temperature of the enzyme washing treatment is 35~40℃, for example, it can be 35℃, 36℃, 37℃, 38℃, 39℃ or 40℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0015] This invention precisely matches the optimal reaction temperature range for acidic proteases by limiting the enzyme washing treatment temperature to 35-40℃. Within this temperature range, the spatial conformation of acidic proteases is stable, the binding efficiency between the enzyme active site and the protein substrate is highest, and the peptide bond hydrolysis rate reaches its peak, enabling complete degradation of proteins deep within the membrane pores within 48-72 hours. If the enzyme washing treatment temperature is too low (e.g., below 35℃), the enzyme molecules will lack sufficient kinetic energy, resulting in a reduced collision frequency with the substrate and a decrease in the enzymatic hydrolysis rate of at least 50%. Even with an extended soaking time of 96 hours, protein contaminants within the membrane pores cannot be completely removed, and the membrane flux recovery rate is only about 60%. If the enzyme washing treatment temperature is too high (e.g., above 40℃), the peptide bonds of the acidic proteases will break, leading to irreversible loss of enzyme activity. When the temperature reaches 50℃, the enzyme activity decreases by at least 80%, rendering the enzyme washing treatment almost ineffective. Furthermore, high temperatures accelerate membrane material aging, causing a 5%-10% decrease in the rejection rate of the nanofiltration membrane.

[0016] Preferably, the enzyme washing treatment time is 48~72h, for example, it can be 48h, 54h, 60h, 66h or 72h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable, preferably 60~72h.

[0017] This invention ensures that the chemical agents have sufficient time to diffuse, penetrate, and react through a long-term sequential immersion cleaning process, achieving deep cleaning from the membrane surface to the interior of the membrane pores. This effectively removes contaminants deep within the membrane pores, fundamentally restores membrane performance, and significantly extends the service life of nanofiltration membranes.

[0018] As a preferred technical solution of the present invention, the pH value of the acidic protease solution is 3 to 5, for example, it can be 3, 3.5, 4, 4.5 or 5, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0019] As a preferred technical solution of the present invention, the concentration of the acidic protease solution used in the enzyme washing treatment is 5000~7000ppm, for example, it can be 5000ppm, 5500ppm, 6000ppm, 6500ppm or 7000ppm, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0020] This invention limits the concentration of the acidic protease solution to 5000-7000 ppm, based on the balance between enzyme adsorption and substrate saturation. At this concentration, enzyme molecules can form a monomolecular adsorption layer on the surface of the contaminant layer and within the membrane pores, precisely covering all exposed protein substrate active sites, thus maximizing enzymatic hydrolysis efficiency. If the concentration of the acidic protease solution is too low (e.g., below 5000 ppm), the number of enzyme molecules is insufficient to completely occupy the substrate active sites, resulting in a protein degradation rate of only 40%-50% and incomplete recovery of membrane flux. If the concentration of the acidic protease solution is too high (e.g., above 7000 ppm), excessive enzyme molecules will aggregate, forming multilayer adsorption. This not only fails to improve enzymatic hydrolysis efficiency but also causes a temporary decrease in membrane flux due to enzyme molecule aggregates clogging the membrane pores. It also increases cleaning costs, and the hydrolysis products of excessive enzyme molecules increase the processing load of the subsequent second alkaline wash.

[0021] Preferably, the enzyme activity of the acidic protease is 50,000 to 200,000 U / mL, for example, it can be 50,000 U / mL, 80,000 U / mL, 100,000 U / mL, 150,000 U / mL or 200,000 U / mL, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0022] This invention limits the enzyme activity of the acidic protease to 50,000~200,000 U / mL, which is crucial for ensuring enzyme washing efficiency and membrane safety. Within this enzyme activity range, a unit volume of solution can degrade 20~40g of protein within 1 hour, and can achieve deep removal of proteins from heavily contaminated nanofiltration membranes within 48~72 hours. If the enzyme activity of the acidic protease is too low (e.g., below 50,000 U / mL), the number of peptide bonds hydrolyzed per unit time is insufficient, and even with extended soaking time, it is difficult to break through the dense protein network within the membrane pores, resulting in a membrane flux recovery rate of only 55%~65%. If the enzyme activity of the acidic protease is too high (e.g., above 200,000 U / mL), the enzymatic reaction will be too vigorous, potentially attacking the functional layer (e.g., polyamide layer) on the nanofiltration membrane surface in addition to degrading the target protein, leading to a decrease in membrane retention performance and an increase in the loss rate of target products such as 1,3-propanediol by 3%~8%. Simultaneously, small molecule peptides generated by enzymatic hydrolysis can easily penetrate into the membrane material, making them difficult to remove through subsequent washing and affecting the long-term stability of the membrane.

[0023] As a preferred technical solution of the present invention, the temperature of the second alkaline washing treatment is 30~40℃, for example, it can be 30℃, 32℃, 34℃, 36℃, 38℃ or 40℃, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0024] Preferably, the second alkaline washing treatment time is 0.5 to 2 hours, for example, it can be 0.5 hours, 1 hour, 1.5 hours or 2 hours, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0025] As a preferred embodiment of the present invention, the alkaline solutions used in the first and second alkaline washing treatments each independently include a sodium hydroxide solution.

[0026] As a preferred technical solution of the present invention, the pH value of the alkaline solution is 10~12, for example, it can be 10, 10.5, 11, 11.5 or 12, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] As a preferred technical solution of the present invention, the cleaning method includes the following steps: (1) First alkaline washing treatment: At 30~45℃, the nanofiltration membrane is circulated and soaked for 24~48h using an alkaline solution with a pH of 10~12 to obtain the nanofiltration membrane after the first alkaline washing. (2) Enzyme washing treatment: Rinse the first alkaline washed nanofiltration membrane with water until the pH value is 6.5~7.5, and then use an acidic protease solution with a pH value of 3~5 at 35~40℃ to circulate and soak the first alkaline washed nanofiltration membrane for 48~72h to obtain the enzyme washed nanofiltration membrane. (3) Second alkaline washing treatment: Rinse the enzyme-washed nanofiltration membrane with water until the pH value is 6.5~7.5, and then use an alkaline solution with a pH value of 10~12 to circulate and clean the enzyme-washed nanofiltration membrane at 30~40℃ for 0.5~2h.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects: This invention cleans contaminated nanofiltration membranes using an "alkali-enzyme-alkali" cleaning sequence. The first alkaline wash acts as a "barrier-breaking" stage, aiming to saponify oils and hydrolyze the polysaccharide matrix, disrupting the macroscopic structure of the contaminant layer and opening channels for subsequent cleaning. The subsequent enzymatic wash acts as a "cleaning" stage, precisely degrading exposed protein contaminants that are difficult to remove with alkaline washing, deeply unclogging membrane pores. The second alkaline wash acts as a "sweeping" stage, aiming to remove enzymatic hydrolysis products and residual agents, consolidating the cleaning effect. The cleaning method provided by this invention has a scientific cleaning sequence, deep effects, and can efficiently restore the flux of nanofiltration membranes, increasing the flux of a single nanofiltration membrane from 10 L / min to over 26.1 L / min, with a recovery rate of over 87%. Detailed Implementation

[0029] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0030] The nanofiltration membrane used in the following examples is a nanofiltration membrane with a single-cell flux of 10 L / min used in the production process of 1,3-propanediol: When a 1,3-propanediol fermentation company used a nanofiltration membrane with a single-cell flux of 30 L / min to perform post-filtration of the prepared 1,3-propanediol clarified liquid, it significantly increased the system concentration factor in order to reduce the product loss rate. This operation resulted in an excessively high concentration of macromolecules such as proteins in the feed liquid in the later stage of operation, causing proteins to precipitate and deposit on the membrane surface and in the flow channel, and form dense complex fouling with other impurities in the feed liquid. This caused the nanofiltration membrane fouling to worsen rapidly. Long-term fouling accumulation caused the membrane flux of the nanofiltration membrane to continuously decrease, resulting in a nanofiltration membrane with a flux of 10 L / min.

[0031] Example 1 This embodiment provides a method for cleaning a nanofiltration membrane, the method comprising the following steps: (1) First alkaline washing treatment: At 40℃, the nanofiltration membrane was circulated and washed for 30 min with sodium hydroxide solution with pH value of 11 and then soaked for 40 h to obtain the nanofiltration membrane after the first alkaline washing. (2) Enzyme washing treatment: The first alkaline washed nanofiltration membrane was rinsed with water until the pH value was 7. Then, at 38°C, the first alkaline washed nanofiltration membrane was circulated and washed for 30 min with an acidic protease solution with a pH value of 4 and a concentration of 6000 ppm. It was then allowed to stand and soak for 65 h. During the soaking period, it was circulated and washed for 30 min every 6 h to obtain the enzyme-washed nanofiltration membrane. The enzyme activity of the acidic protease was 150000 U / mL. (3) Second alkaline washing treatment: The enzyme-washed nanofiltration membrane is rinsed with water until the pH value is 7, and then the enzyme-washed nanofiltration membrane is circulated and cleaned for 2 hours at 35°C using a sodium hydroxide solution with a pH value of 10.

[0032] Example 2 This embodiment provides a method for cleaning a nanofiltration membrane, the method comprising the following steps: (1) First alkaline washing treatment: At 30°C, the nanofiltration membrane was circulated and washed for 30 min with sodium hydroxide solution with pH value of 10 and then allowed to stand and soak for 46 h to obtain the nanofiltration membrane after the first alkaline washing. (2) Enzyme washing treatment: The first alkaline washed nanofiltration membrane was rinsed with water until the pH value was 7.5. Then, at 35°C, the first alkaline washed nanofiltration membrane was circulated and washed for 30 min with an acidic protease solution with a pH value of 3 and a concentration of 7000 ppm. It was then allowed to stand and soak for 48 h. During the soaking period, it was circulated and washed for 30 min every 6 h to obtain the enzyme-washed nanofiltration membrane. The enzyme activity of the acidic protease was 50000 U / mL. (3) Second alkaline washing treatment: The enzyme-washed nanofiltration membrane is rinsed with water until the pH value is 6.5, and then the enzyme-washed nanofiltration membrane is circulated and washed for 1 hour at 40°C using a sodium hydroxide solution with a pH value of 11.

[0033] Example 3 This embodiment provides a method for cleaning a nanofiltration membrane, the method comprising the following steps: (1) First alkaline washing treatment: At 45℃, the nanofiltration membrane was circulated and washed for 30 min with sodium hydroxide solution with pH 12 and then soaked for 24 h to obtain the nanofiltration membrane after the first alkaline washing. (2) Enzyme washing treatment: The first alkaline washed nanofiltration membrane was rinsed with water until the pH value was 7. Then, at 40°C, the first alkaline washed nanofiltration membrane was circulated and washed for 30 min with an acidic protease solution with a pH value of 5 and a concentration of 5000 ppm. It was then allowed to stand and soak for 70 h. During the soaking period, it was circulated and washed for 30 min every 6 h to obtain the enzyme-washed nanofiltration membrane. The enzyme activity of the acidic protease was 200000 U / mL. (3) Second alkaline washing treatment: The enzyme-washed nanofiltration membrane is rinsed with water until the pH value is 6.5, and then the enzyme-washed nanofiltration membrane is circulated and cleaned for 0.5 h at 30°C using sodium hydroxide solution with a pH value of 12.

[0034] Example 4 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the soaking time of the first alkaline washing treatment in step (1) is adjusted from 40h to 10h. All other aspects are the same as in Embodiment 1.

[0035] Example 5 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the soaking time for the enzyme washing treatment in step (2) is adjusted from 65h to 10h, while the rest is the same as in Embodiment 1.

[0036] Example 6 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the acidic protease in step (2) is replaced with an alkaline protease of the same concentration and activity, and the pH value of the enzyme washing treatment is 10. Otherwise, the method is the same as that in Embodiment 1.

[0037] Example 7 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the temperature of the enzymatic washing process in step (2) is adjusted from 38°C to 30°C. All other aspects are the same as in Embodiment 1.

[0038] Example 8 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the temperature of the enzyme washing treatment in step (2) is adjusted from 38°C to 50°C. All other aspects are the same as in Embodiment 1.

[0039] Example 9 This embodiment provides a method for cleaning a nanofiltration membrane. The only difference from Embodiment 1 is that the concentration of the acidic protease solution in step (2) is adjusted from 6000ppm to 3000ppm. All other aspects are the same as in Embodiment 1.

[0040] Example 10 This embodiment provides a method for cleaning a nanofiltration membrane. The only difference from Embodiment 1 is that the concentration of the acidic protease solution in step (2) is adjusted from 6000ppm to 9000ppm. All other aspects are the same as in Embodiment 1.

[0041] Example 11 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the enzyme activity of the acidic protease solution in step (2) is adjusted from 150,000 U / mL to 30,000 U / mL. All other aspects are the same as in Embodiment 1.

[0042] Example 12 This embodiment provides a method for cleaning nanofiltration membranes. The only difference from Embodiment 1 is that the enzyme activity of the acidic protease solution in step (2) is adjusted from 150,000 U / mL to 300,000 U / mL. All other aspects are the same as in Embodiment 1.

[0043] Comparative Example 1 This comparative example provides a method for cleaning a nanofiltration membrane. The only difference from Example 1 is that step (2) is changed to acid washing: the first alkaline-washed nanofiltration membrane is rinsed with water until the pH value is 7, and then the first alkaline-washed nanofiltration membrane is circulated and cleaned for 30 minutes at 38°C using a hydrochloric acid solution with a pH value of 4 and a concentration of 6000ppm, and then left to stand and soak for 65 hours. During the soaking period, the membrane is circulated and cleaned for 30 minutes every 6 hours to obtain the acid-washed nanofiltration membrane. The rest is the same as in Example 1.

[0044] Comparative Example 2 This comparative example provides a cleaning method for a nanofiltration membrane. The only difference from Example 1 is that, except that step (2) is performed first and then step (1), i.e., enzyme washing, first alkaline washing, and second alkaline washing are performed sequentially, the rest are the same as in Example 1.

[0045] The membrane flux of the cleaned single nanofiltration membranes of Examples 1-12 and Comparative Examples 1-2 was tested according to GB / T 34242-2017, and the test results are shown in Table 1.

[0046] Table 1 The test results show that: (1) As can be seen from Examples 1 to 3, the present invention adopts the cleaning sequence of "alkali-enzyme-alkali". The cleaning method provided by the present invention has a scientific cleaning sequence, deep effect, and can efficiently restore the flux of nanofiltration membrane, so that the membrane flux of a single nanofiltration membrane can be restored from 10L / min to more than 26.1L / min, and the recovery rate can reach more than 87%.

[0047] (2) As can be seen from Examples 1 and 4-5, in Example 1 of the present invention, the soaking time for the first alkaline washing treatment was 40h and the soaking time for the enzyme washing treatment was 65h. After cleaning, the membrane flux of a single nanofiltration membrane was 27.6L / min and the recovery rate was 92%. In Example 4, the soaking time for the first alkaline washing treatment was 10h and the soaking time for the enzyme washing treatment was 65h. After cleaning, the membrane flux of a single nanofiltration membrane was 20.4L / min and the recovery rate was 68%. In Example 5, the soaking time for the first alkaline washing treatment was 40h and the soaking time for the enzyme washing treatment was 10h. After cleaning, the membrane flux of a single nanofiltration membrane was 19.8L / min and the recovery rate was 66%. It can be seen that short-time cyclic rinsing cannot allow the chemical agent to fully penetrate into the depth of the membrane pores and the interior of the fouling layer, and the cleaning is not thorough.

[0048] (3) As can be seen from Examples 1 and 6, after using alkaline protease in Example 6, the membrane flux recovery rate was only 55%, which was much lower than 92% in Example 1. This proves that alkaline protease cannot be adapted to the "alkali-enzyme-alkali" cleaning system and is prone to secondary solidification of proteins.

[0049] (4) As can be seen from Examples 1 and 7-8, the enzyme washing temperature in Example 1 was 38°C, and the membrane flux of a single nanofiltration membrane after washing was 27.6 L / min with a recovery rate of 92%. In Example 7, the enzyme washing temperature was 30°C, and the membrane flux of a single nanofiltration membrane after washing was 22.5 L / min with a recovery rate of 75%. In Example 8, the enzyme washing temperature was 50°C, and the membrane flux of a single nanofiltration membrane after washing was 17.1 L / min with a recovery rate of 57%. This shows that if the enzyme washing temperature is too low, the enzyme activity will be reduced, and if the temperature is too high, the enzyme will be inactivated and the membrane material will be damaged.

[0050] (5) As can be seen from Examples 1 and 9-10, in Example 1, the concentration of acidic protease solution was 6000 ppm, and the membrane flux of a single nanofiltration membrane after cleaning was 27.6 L / min, with a recovery rate of 92%. In Example 9, the concentration of acidic protease solution was 3000 ppm, and the membrane flux of a single nanofiltration membrane after cleaning was 21.3 L / min, with a recovery rate of 71%. In Example 10, the concentration of acidic protease solution was 9000 ppm, and the membrane flux of a single nanofiltration membrane after cleaning was 25.2 L / min, with a recovery rate of 84%. This shows that if the concentration of acidic protease solution is too low, the number of enzyme molecules is insufficient and cannot completely occupy the active site of the substrate. If the concentration is too high, enzyme aggregation and membrane pore blockage are likely to occur.

[0051] (6) As can be seen from Examples 1 and 11-12, the enzyme activity of the acidic protease solution in Example 1 was 150,000 U / mL, and the membrane flux of a single nanofiltration membrane after washing was 27.6 L / min, with a recovery rate of 92%. In Example 11, the enzyme activity of the acidic protease solution was 30,000 U / mL, and the membrane flux of a single nanofiltration membrane after washing was 18.9 L / min, with a recovery rate of 63%. In Example 12, the enzyme activity of the acidic protease solution was 300,000 U / mL, and the membrane flux of a single nanofiltration membrane after washing was 24.9 L / min, with a recovery rate of 83%. This shows that too low enzyme activity leads to insufficient degradation efficiency, while too high activity may damage the membrane functional layer.

[0052] (7) As can be seen from Example 1 and Comparative Example 1, the enzymatic washing treatment of the present invention has significant advantages over acid washing treatment. After replacing acidic protease with hydrochloric acid in Comparative Example 1, the recovery rate was only 51%, proving that acid washing cannot effectively degrade protein contaminants and may corrode the membrane surface.

[0053] (8) As can be seen from Example 1 and Comparative Example 2, the order of "alkali-enzyme-alkali" is the core of achieving efficient cleaning. The recovery rate of Comparative Example 2 after reversing the order is only 49%. This is because if enzyme washing is performed first, the organic pollutant matrix on the outer layer will block the agent from contacting the internal protein, greatly reducing the cleaning efficiency.

[0054] In summary, this invention cleans contaminated nanofiltration membranes using an "alkali-enzyme-alkali" cleaning sequence, which effectively removes organic matter, grease, protein, and microorganisms from the nanofiltration membrane. The cleaning method provided by this invention has a scientific cleaning sequence, works thoroughly, and can efficiently restore the flux of the nanofiltration membrane, increasing the membrane flux of a single nanofiltration membrane from 10 L / min to over 26.1 L / min, with a recovery rate of over 87%.

[0055] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for cleaning a nanofiltration membrane, characterized in that, The cleaning method includes a first alkaline washing treatment, an enzyme washing treatment, and a second alkaline washing treatment performed sequentially.

2. The cleaning method according to claim 1, characterized in that, The temperature of the first alkaline washing treatment is 30~45℃; Preferably, the first alkaline washing treatment time is 24~48h, more preferably 36~48h.

3. The cleaning method according to claim 1 or 2, characterized in that, An acidic protease solution is used in the enzymatic washing treatment.

4. The cleaning method according to any one of claims 1-3, characterized in that, The temperature for the enzyme washing treatment is 35~40℃; Preferably, the enzyme washing treatment time is 48-72 hours, and more preferably 60-72 hours.

5. The cleaning method according to claim 3, characterized in that, The pH value of the acidic protease solution is 3-5.

6. The cleaning method according to claim 3, characterized in that, The concentration of the acidic protease solution used in the enzyme washing treatment is 5000~7000ppm; Preferably, the enzyme activity of the acidic protease is 50,000 to 200,000 U / mL.

7. The cleaning method according to any one of claims 1-6, characterized in that, The temperature for the second alkaline washing treatment is 30~40℃; Preferably, the second alkaline washing treatment time is 0.5~2 hours.

8. The cleaning method according to any one of claims 1-7, characterized in that, The alkaline solutions used in the first and second alkaline washing treatments each independently include a sodium hydroxide solution.

9. The cleaning method according to claim 8, characterized in that, The pH value of the alkaline solution is 10-12.

10. The cleaning method according to any one of claims 1-9, characterized in that, The cleaning method includes the following steps: (1) First alkaline washing treatment: At 30~45℃, the nanofiltration membrane is circulated and soaked for 24~48h using an alkaline solution with a pH of 10~12 to obtain the nanofiltration membrane after the first alkaline washing. (2) Enzyme washing treatment: Rinse the first alkaline washed nanofiltration membrane with water until the pH value is 6.5~7.5, and then use an acidic protease solution with a pH value of 3~5 at 35~40℃ to circulate and soak the first alkaline washed nanofiltration membrane for 48~72h to obtain the enzyme washed nanofiltration membrane. (3) Second alkaline washing treatment: Rinse the enzyme-washed nanofiltration membrane with water until the pH value is 6.5~7.5, and then use an alkaline solution with a pH value of 10~12 to circulate and clean the enzyme-washed nanofiltration membrane at 30~40℃ for 0.5~2h.

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