A method for cleaning nanofiltration membranes using microbubbles

Abstract

The application discloses a method for cleaning nanofiltration membranes by using microbubbles, and belongs to the field of nanofiltration membrane cleaning in the membrane water treatment industry. The cleaning agent used in the method comprises acidic cleaning water, alkaline cleaning water and a cleaning repair agent. According to the pollution condition of the nanofiltration membrane, the cleaning agent can be flexibly selected to be used alone or in combination, microbubbles are added to assist cleaning, process parameters are strictly limited, and a cleaning process is precisely controlled, so that the purpose of directional cleaning of the nanofiltration membrane is achieved. The performance recovery is better than the traditional nanofiltration membrane cleaning and flushing effect, the cleaning period can be prolonged, the amount of reagent can be reduced, the service life of the nanofiltration membrane can be prolonged, and the operation cost can be reduced.

Description

Technical Field

[0001] This invention relates to a method for cleaning nanofiltration membranes using microbubbles, belonging to the field of nanofiltration membrane cleaning in the membrane water treatment industry. Background Technology

[0002] Nanofiltration membranes, a new type of separation membrane that emerged in the late 1980s, have a molecular weight cutoff between that of reverse osmosis and ultrafiltration membranes, approximately 100-2000 Da, and a pore size of approximately 1-10 nm, hence the name "nanofiltration." In the chemical industry, they are commonly used for brine separation and concentration. During use, nanofiltration membranes can become fouled due to various reasons, including inorganic salt scale, hydrated metal oxides, organic matter, and microorganisms. Fouling leads to a decline in membrane performance, such as reduced flux and desalination rate, necessitating membrane cleaning. Depending on the type of contaminant, appropriate cleaning water is typically prepared by adding specific chemicals to fresh water.

[0003] Cleaning is an essential part of membrane maintenance during the use of nanofiltration membranes. The appropriateness of the cleaning method directly affects the cleaning effect, frequency, and consumption of chemicals, which in turn affects the degree of membrane performance recovery, service life, and production and operating costs. Therefore, optimizing the cleaning method has always been an important research technology in the membrane maintenance process.

[0004] In the cleaning process of ultrafiltration and reverse osmosis membranes, the idea of ​​using dissolved air water for cleaning has been proposed. However, due to the different application ranges of ultrafiltration and reverse osmosis membranes compared to nanofiltration membranes, their membrane structures and performance differences also lead to differences in cleaning methods. Simple cleaning methods for ultrafiltration and reverse osmosis membranes are not entirely applicable to nanofiltration membrane cleaning, and may even exacerbate fouling and increase cleaning difficulty under certain specific cleaning process conditions. For cleaning membranes with different functions, the cleaning flow rate and cleaning pressure are key parameters. Excessive flow rate may cause significant damage to the membrane during chemical cleaning, while insufficient flow rate results in poor cleaning effectiveness. Excessive pressure can cause redeposition of contaminants or even their entry into the membrane interior. Furthermore, different bubble sizes also affect the cleaning effect on different membranes.

[0005] Chinese patent document CN109954406A discloses a reverse osmosis membrane cleaning process. The method mainly mentions using acid and alkali solutions combined with micro-nano bubbles and ultrasonic treatment to clean the reverse osmosis membrane. However, its application is limited to the field of reverse osmosis membrane cleaning, and the specific process conditions for micro-bubble cleaning are not mentioned in the entire document. However, the structure of nanofiltration membranes is different from that of reverse osmosis membranes, with larger filtration pore sizes. If the method provided by this patent is directly applied to nanofiltration membrane cleaning, it may cause the nanofiltration membrane to become more fouled.

[0006] Chinese patent document CN102210978A discloses a method and reagent for offline cleaning and repair of waste reverse osmosis membranes. This method is used for offline cleaning of reverse osmosis membranes and focuses on the use of reagents. The cleaning process only has one step of constant pressure high-speed cleaning. However, during the membrane cleaning process, large particles of contaminants are usually shed. The cleaning solution with large particles of contaminants can easily scratch the membrane during high-speed cleaning, causing permanent and irreversible damage to the membrane. Moreover, this method has a low degree of membrane performance recovery.

[0007] Chinese patent document CN112537823A discloses a method for enhancing chemical cleaning and controlling the aging of ultrafiltration membranes using micro-nano bubble technology. This method is mainly for ultrafiltration membranes, which are mostly hollow fiber or ceramic membranes. The hollow fiber membrane type determines that the membrane fibers will vibrate during bubble cleaning, thereby enhancing the decontamination effect. Ceramic membranes, due to their significantly superior mechanical properties compared to hollow fiber and spiral wound membranes, are often cleaned with high flow rates to improve the effect. However, nanofiltration membranes are mostly spiral wound membranes, and their cleaning methods are significantly different from those of ultrafiltration membranes. The size of the bubbles in the cleaning water, the flow rate of the cleaning water, and the cleaning pressure play important roles. Therefore, this method is not suitable for cleaning nanofiltration membranes. Summary of the Invention

[0008] The purpose of this invention is to provide a method for cleaning nanofiltration membranes using microbubbles, which can shorten the cleaning time, improve the cleaning effect, reduce reagent consumption, overcome the problem that dissolved gas cleaning methods for ultrafiltration and reverse osmosis membranes cannot be fully applied to nanofiltration membrane cleaning, and greatly improve the cleaning efficiency of nanofiltration membranes.

[0009] To achieve the above-mentioned objective, the method for cleaning nanofiltration membranes using microbubbles provided by the present invention includes the following steps:

[0010] (1) Prepare cleaning water, which includes: acidic cleaning water, alkaline cleaning water, a combination of acidic and alkaline cleaning water, a combination of acidic cleaning water and cleaning and repair agent, a combination of alkaline cleaning water and cleaning and repair agent, and a combination of acidic cleaning water, alkaline cleaning water and cleaning and repair agent;

[0011] (2) First rinse the nanofiltration membrane inside the membrane housing with fresh water, and control the flow rate of a single membrane housing to 3-4.5 m. 3 / h, flushing pressure 0.5-1 bar, until clear water flows steadily out of the membrane housing;

[0012] (3) Prepare dissolved air cleaning water with microbubble particle size of 20-90nm from acidic cleaning water, and then perform the following steps (4)-(6).

[0013] Alternatively, alkaline cleaning water can be made into dissolved air cleaning water with microbubble particle size of 20-90nm, and then the following steps (4)-(6) can be performed.

[0014] Alternatively, acidic cleaning water and alkaline cleaning water can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), and then the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6) or first, the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), and then the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6).

[0015] Alternatively, acidic cleaning water and cleaning and repairing agent can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, use the dissolved air cleaning water of acidic cleaning water to perform the following steps (4)-(6), and then use the dissolved air cleaning water of cleaning and repairing agent to perform the following steps (4)-(6).

[0016] Alternatively, alkaline cleaning water and cleaning and repairing agent can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, use the dissolved air cleaning water of alkaline cleaning water to perform the following steps (4)-(6), and then use the dissolved air cleaning water of cleaning and repairing agent to perform the following steps (4)-(6).

[0017] Alternatively, acidic cleaning water, alkaline cleaning water, and cleaning and repairing agent can be respectively made into dissolved air cleaning water with microbubble particle size of 20-90nm. First, the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), then the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), and finally the dissolved air cleaning water of cleaning and repairing agent is used to perform the following steps (4)-(6). Or, first, the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), then the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), and finally the dissolved air cleaning water of cleaning and repairing agent is used to perform the following steps (4)-(6).

[0018] (4) Use the dissolved gas cleaning water obtained in step (3) to clean the nanofiltration membrane inside the membrane housing, and control the flow rate of the single membrane housing to 3-4.5 m. 3 Clean at a pressure of 0.5-1 bar per hour until all the fresh water in the membrane housing is replaced. Then start the dissolved air cleaning water circulation cleaning. After circulating for 20-30 minutes, stop the circulation and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 1-12 hours.

[0019] (5) After soaking, continue to clean with dissolved air cleaning water circulation, controlling the flow rate of the single membrane shell to 3-4.5m. 3 Cleaning pressure 0.5-1 bar, circulation 12-18 min; then increase the flow rate of the single membrane housing to 6-9 m / h. 3 / h, cleaning pressure to 1.5-2 bar, continue circulation for 30-60 minutes;

[0020] (6) After step (5) is completed, rinse the nanofiltration membrane inside the membrane housing with fresh water, and control the flow rate of the single membrane housing to 6-9 m. 3 The flushing pressure is 2-2.5 bar per hour until all the dissolved air cleaning water in the membrane housing is replaced.

[0021] The acidic cleaning water is an aqueous solution of any one of hydrogen chloride, citric acid, oxalic acid, aminosulfonic acid, and acetic acid; the alkaline cleaning water is an aqueous solution of any one of sodium hydroxide, sodium tripolyphosphate, and tetrasodium ethylenediaminetetraacetate; the cleaning and repair agent is an organic solvent solution composed of piperazine or trimesoyl chloride and any one of tannic acid, triethanolamine, polyetheramine, and 2,2'-oxodiethylamine, wherein the organic solvent is methanol, ethanol, or glycerol.

[0022] In steps (2) and (4), the flow velocity of the single membrane shell is 3-4 m / s. 3 / h, flushing pressure is 0.5-0.8 bar.

[0023] The microbubble particle size is 30-80 nm.

[0024] The soaking time in step (4) is 4-12 hours.

[0025] In step (5), the flow rate of the single membrane shell is increased to 7-9 m. 3 / h, cleaning pressure up to 1.8-2 bar.

[0026] In step (6), the flow rate is controlled to be 8-9 m / s. 3 The flushing pressure is 2.2-2.5 bar per hour.

[0027] Steps (1)-(6) are carried out at a temperature of 20-30℃.

[0028] The mass concentrations of the aqueous solutions of hydrogen chloride, citric acid, oxalic acid, aminosulfonic acid, and acetic acid are 0.2-0.3%, 2-3%, 2-3%, 0.5-0.8%, and 0.5-0.8%, respectively; the mass concentrations of the aqueous solutions of sodium hydroxide, sodium tripolyphosphate, and tetrasodium ethylenediaminetetraacetate are 0.1-0.2%, 0.5-0.7%, and 1-2%, respectively; the mass concentration of piperazine or trimesoyl chloride in the organic solvent solution is 0.1-0.3%, and the mass concentration of tannic acid, triethanolamine, polyetheramine, or 2,2'-oxodiethylamine in the organic solvent solution is 0.01-0.05%.

[0029] All pressures described in this invention are gauge pressures.

[0030] This invention can effectively improve the cleaning effect of nanofiltration membranes, with membrane flux recovery rate reaching over 98%, cleaning cycle extended by 19%-25%, and annual reagent consumption reduced by 16%-20%.

[0031] By combining appropriate cleaning agents with dissolved air devices, controlling the microbubble particle size, the inlet flow rate and circulation rate of the cleaning water, and simultaneously controlling the cleaning pressure, the optimal conditions for cleaning nanofiltration membranes can be met.

[0032] Depending on the type of fouling, different cleaning water formulations are used. For example, alkaline cleaning water is used for sulfate scale, acidic cleaning water is used for carbonate scale, and a combination of alkaline and acidic cleaning water is used for organic matter. If the membrane performance recovery is poor after cleaning, a cleaning repair agent is used for further cleaning.

[0033] Piperazine and pyromellitic trimethylolpropionate (PMPC) in the cleaning and repair agent are the main monomers for the aqueous (piperazine) and oil (PMPC) phases of nanofiltration membrane manufacturing and repair. They repair damaged membrane surfaces through surface coating reactions. The hydroxyl and amino functional groups in tannic acid, triethanolamine, polyetheramine, and 2,2'-oxodiethylamine impart hydrophilic modification to the nanofiltration membrane and can also react with piperazine and PMPC, exhibiting a synergistic effect. Methanol, ethanol, or glycerol, as solvents for the cleaning and repair agent, not only dissolve piperazine, PMPC, tannic acid, triethanolamine, polyetheramine, and 2,2'-oxodiethylamine, but also enhance the hydrophilic modification of the nanofiltration membrane, improve its antifouling ability, and promote the hydrophilic modification of the nanofiltration membrane by tannic acid, triethanolamine, polyetheramine, and 2,2'-oxodiethylamine.

[0034] The dissolved air cleaning water of this invention is formulated using a high-efficiency dissolved air device with a dissolved air efficiency of 70%-90% and a dissolved air pressure of 0.5-0.9MPa, which controls the microbubble particle size to 20-90nm. This is designed for the nanofiltration membrane pore size range, and too high or too low a pressure will affect the cleaning effect of the nanofiltration membrane.

[0035] Before cleaning begins, rinse the nanofiltration membrane inside the membrane housing with fresh water to remove any residual material inside the housing or / and on the nanofiltration membrane, thus preventing it from mixing and reacting with the cleaning water and causing membrane damage or cleaning water failure.

[0036] When cleaning with low flow rate and low pressure, all the cleaning water flows on the concentrate side of the membrane, and there is no product water on the permeate side. This can effectively prevent the redeposition of pollutants on the membrane surface or even their entry into the nanofiltration membrane.

[0037] Soaking decomposes contaminants as much as possible. After soaking, low-flow-rate rinsing removes the contaminants with the rinsing water. Then, the flow rate and rinsing pressure are increased to continue the circulation. With most of the surface contaminants removed, deeper contaminants are further treated. Finally, a high-flow-rate fresh water rinse removes the remaining rinsing water and contaminants.

[0038] Compared with existing technologies, this invention optimizes the nanofiltration membrane cleaning process. By adding microbubbles to assist cleaning and strictly limiting process parameters, the cleaning process is precisely controlled to achieve the purpose of targeted cleaning of nanofiltration membranes. The performance recovery is better than that of traditional nanofiltration membrane cleaning and rinsing. At the same time, it can also extend the cleaning cycle, reduce the amount of reagents used, improve the life of nanofiltration membranes, and reduce operating costs. Attached Figure Description

[0039] Figure 1 This is a schematic diagram of a nanofiltration membrane cleaning device. Detailed Implementation

[0040] This invention provides a nanofiltration membrane cleaning device, as described above. Figure 1The nanofiltration membrane cleaning device includes a freshwater tank 1, an acidic cleaning water tank 2, an alkaline cleaning water tank 3, a cleaning and repair agent tank 4, a water pump 5, a security filter 6, a membrane housing 7, a dissolved air device 8, an air compressor 9, and a return water pipe 22. The inlet of the membrane housing 7 is connected to the outlet of the security filter 6 via a connecting pipe connected to a flushing valve 17. The inlet of the security filter 6 is connected to the outlet of the water pump 5. The outlet of the freshwater tank 1 is connected to the inlet of the water pump 5 via a pipe connected to a freshwater outlet valve 10. The outlet of the acidic cleaning water tank 2 is connected to... The pipeline connected to the acidic cleaning water outlet valve 11 is connected to the inlet of the water pump 5; the outlet of the alkaline cleaning water tank 3 is connected to the inlet of the water pump 5 via the pipeline connected to the alkaline cleaning water outlet valve 12; the outlet of the cleaning and repair agent tank 4 is connected to the inlet of the water pump 5 via the pipeline connected to the cleaning and repair agent outlet valve 13; the air inlet of the dissolved air device 8 is connected to the air compressor 9; the water inlet of the dissolved air device 8 is connected to the section of the connecting pipe located between the security filter 6 and the flushing valve 17 via the pipeline connected to the liquid inlet valve 18; the dissolved air device 8... The outlet of the membrane housing 7 is connected to the section of the connecting pipe between the flushing valve 17 and the membrane housing 7 via a pipe connected in series with a check valve 19. The check valve 19 ensures that the liquid inside the dissolved air device 8 flows out through the check valve 19, while the liquid outside the dissolved air device 8 cannot flow into the dissolved air device 8 through the check valve 19. A return main valve 21 and a drain valve 20 are connected in series on the return water pipe 22. A wastewater tank 23 is provided below the drain valve 20 to receive the liquid flowing out from the drain valve 20. The product water outlet and concentrate outlet of the membrane housing 7 are respectively connected to the return water pipe 22 via pipes. 2. The section between the main return valve 21 and the drain valve 20 is connected; the inlet of the cleaning and repair agent tank 4 is connected to the section of the return water pipe 22 located before the main return valve 21 via a pipeline connected in series with the cleaning and repair agent return valve 16; the inlet of the alkaline cleaning water tank 3 is connected to the section of the return water pipe 22 located before the main return valve 21 via a pipeline connected in series with the alkaline cleaning water return valve 15; the inlet of the acidic cleaning water tank 2 is connected to the section of the return water pipe 22 located before the main return valve 21 via a pipeline connected in series with the acidic cleaning water return valve 14. The dissolved air device 8 is an existing product, with a dissolved air efficiency of 70%-90% and a dissolved air pressure of 0.5-0.9MPa. The microbubble particle size of the dissolved air water produced by the dissolved air device can be controlled within 20-90nm. The direction in front of the main return valve 21 is determined according to the direction of liquid flow through the main return valve 21; the direction consistent with the liquid flow direction is the front.

[0041] The method for cleaning nanofiltration membranes using microbubbles described in this invention can be performed using the aforementioned nanofiltration membrane cleaning device, achieving offline cleaning of the nanofiltration membrane. Alternatively, online cleaning of the nanofiltration membrane can also be achieved by modifying the nanofiltration device and replacing the membrane housing. Figure 1 The membrane housing shown illustrates how online cleaning of nanofiltration membranes can be achieved by establishing a cleaning sideline in the nanofiltration unit.

[0042] The following is combined Figure 1The nanofiltration membrane cleaning apparatus shown herein provides a detailed explanation of the implementation of the method for cleaning nanofiltration membranes using microbubbles, through examples. Example 1

[0043] To clean nanofiltration membranes fouled by calcium carbonate scale, cleaning should begin when the membrane flux drops to 90% of the normal permeate flux. Before cleaning, all valves in the nanofiltration membrane cleaning unit should be closed. The cleaning steps are as follows:

[0044] (1) Prepare an aqueous solution containing 3% oxalic acid as acidic cleaning water, add the acidic cleaning water to acidic cleaning water tank 2, add fresh water to fresh water tank 1, and put the nanofiltration membrane to be cleaned into membrane shell 7.

[0045] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of a single membrane housing to 4.5 m. 3 The flushing pressure is 1 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0046] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.9 MPa and the dissolved air efficiency to 90%, so as to produce dissolved air cleaning water with microbubble particle size of 20 nm.

[0047] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4.5 m / s. 3 / h, cleaning pressure 1 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the acid cleaning water return valve 14, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 12 hours.

[0048] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4.5 m. 3 After cleaning at a pressure of 1 bar per hour and circulating for 18 minutes, increase the flow rate of the single membrane housing to 9 m³ / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 60 minutes;

[0049] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m.3 At a pressure of 2.5 bar / h, the flushing pressure is maintained until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows steadily out from the drain valve 20. Then, the water pump 5 is turned off, the nanofiltration membrane is removed from the membrane housing, and the liquid in the acidic cleaning water tank 2 and the dissolved air device 8 is emptied to complete the cleaning operation.

[0050] The above cleaning process is carried out at a temperature of 30℃.

[0051] After calibration, the flux of the cleaned nanofiltration membrane was restored to 99% of the initial flux, and the retention performance was restored to more than 99% of the initial performance. The cleaning cycle from this cleaning to the next cleaning is 42 days, which is 25% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption is reduced by 20%. Example 2

[0052] To clean nanofiltration membranes contaminated with calcium carbonate scale, cleaning should begin when the membrane flux drops to 90% of the normal permeate flux. The cleaning process includes acid washing and remedial cleaning.

[0053] (a) Acid washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state. The steps are as follows:

[0054] (1) Prepare an aqueous solution containing 2% oxalic acid as acidic cleaning water, and prepare a methanol solution containing 0.1% piperazine and 0.01% tannic acid as cleaning and repair agent; add the acidic cleaning water to acidic cleaning water tank 2, add the cleaning and repair agent to cleaning and repair agent tank 4, add fresh water to fresh water tank 1, and put the nanofiltration membrane to be cleaned into membrane shell 7;

[0055] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with freshwater, controlling the flow rate of the single membrane housing to 3m. 3 The flushing pressure is 0.5 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0056] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.5MPa and the dissolved air efficiency to 70%, so as to produce dissolved air cleaning water with microbubble particle size of 90nm.

[0057] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3m. 3 / h, cleaning pressure 0.5 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and acid cleaning water return valve 14, and start dissolved air cleaning water circulation cleaning. After circulation cleaning for 20 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 1 hour;

[0058] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3m. 3 After cleaning at a rate of 0.5 bar / h and a cleaning pressure of 0.5 bar for 12 minutes, the flow rate of the single membrane housing is increased to 6 m / s. 3 / h, cleaning pressure up to 1.5 bar, continue circulating and cleaning for 30 minutes;

[0059] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 6m. 3 At a pressure of 2 bar / h, the flushing pressure is maintained until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, the nanofiltration membrane is removed from the membrane housing, and the liquid in the acid cleaning water tank 2 and the dissolved air device 8 is emptied to complete the acid washing operation.

[0060] After calibration, the nanofiltration membrane flux recovered to 97% of the initial flux, and the retention performance recovered to 98% of the initial performance, which did not meet expectations. Cleaning and repair agents were used to continue cleaning.

[0061] (II) Repair agent cleaning: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0062] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with fresh water after alkaline washing, controlling the flow rate of a single membrane housing to 3.5 m. 3 The flushing pressure is 0.7 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0063] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the cleaning and repair agent outlet valve 13, the liquid inlet valve 18 and the air compressor 9. After the cleaning and repair agent enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.7MPa and the dissolved air efficiency to 80%, and make dissolved air cleaning water with microbubble particle size of 50nm.

[0064] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3.5 m.3 / h, cleaning pressure 0.7 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water that stably flows out of the cleaning and repair agent from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the cleaning and repair agent return valve 16, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 25 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water of the cleaning and repair agent for 6 hours.

[0065] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3.5m. 3 After cleaning at a rate of 0.7 bar / h and a cleaning pressure of 0.7 bar for 15 minutes, the flow rate of the single membrane housing is increased to 8 m / s. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 40 minutes;

[0066] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3 The flushing pressure is 2.5 bar / h, until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the cleaning and repair agent tank 4 and the dissolved air device 8 is emptied to complete the cleaning and repair agent cleaning operation.

[0067] Pickling and cleaning with a repair agent are performed at a temperature of 25°C.

[0068] After final calibration, the flux of the cleaned nanofiltration membrane was restored to 98.5% of the initial flux, and the retention performance was restored to more than 99% of the initial performance. The cleaning cycle from this cleaning to the next cleaning is 41 days, which is 22% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption is reduced by 18%. Example 3

[0069] To clean nanofiltration membranes contaminated with sulfate scaling, cleaning should begin when the membrane flux drops to 90% of the normal permeate flux. Before cleaning, all valves in the nanofiltration membrane cleaning unit should be closed. The cleaning steps are as follows:

[0070] (1) Prepare an aqueous solution containing 2% sodium ethylenediaminetetraacetate as alkaline cleaning water, add the alkaline cleaning water to alkaline cleaning water tank 3, add fresh water to fresh water tank 1, and put the nanofiltration membrane to be cleaned into membrane shell 7.

[0071] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with freshwater, controlling the flow rate of the single membrane housing to 4m.3 The flushing pressure is 0.8 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0072] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.8 MPa and the dissolved air efficiency to 85% to produce dissolved air cleaning water with a microbubble particle size of 30 nm.

[0073] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.8 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulating for 30 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 10 hours.

[0074] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a rate of 0.8 bar per hour and a cleaning pressure of 0.8 bar for 16 minutes, the flow rate of the single membrane housing is increased to 8 m / s. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 60 minutes;

[0075] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3 The flushing pressure is 2 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the cleaning operation.

[0076] The above cleaning process is carried out at a temperature of 25℃.

[0077] After calibration, the flux of the cleaned nanofiltration membrane was restored to 98.2% of the initial flux, and the retention performance was restored to 99% of the initial performance. The cleaning cycle from this cleaning to the next cleaning was 41 days, which is 22% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption was reduced by 19%. Example 4

[0078] To clean nanofiltration membranes contaminated with sulfate scale, cleaning should begin when the membrane flux drops to 90% of the normal permeate flux. The cleaning process includes alkaline washing and remedial cleaning.

[0079] (a) Alkaline washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state. The steps are as follows:

[0080] (1) Prepare an aqueous solution containing 1% sodium ethylenediaminetetraacetate as alkaline cleaning water, and prepare a glycerol solution containing 0.1% trimesoyl chloride and 0.01% polyetheramine as cleaning and repairing agent; add the alkaline cleaning water to the alkaline cleaning water tank 3, add the cleaning and repairing agent to the cleaning and repairing agent tank 4, add fresh water to the fresh water tank 1, and put the nanofiltration membrane to be cleaned into the membrane shell 7;

[0081] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with freshwater, controlling the flow rate of the single membrane housing to 3m. 3 The flushing pressure is 0.5 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0082] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.6MPa and the dissolved air efficiency to 75% to produce dissolved air cleaning water with a microbubble particle size of 70nm.

[0083] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3m. 3 / h, cleaning pressure 0.5 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 5 hours.

[0084] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3m. 3 After cleaning at a pressure of 0.5 bar and a cycle time of 15 minutes, increase the flow rate of the single membrane housing to 7 m³ / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 40 minutes;

[0085] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 7m. 3 The flushing pressure is 2 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the alkaline cleaning operation.

[0086] After calibration, the nanofiltration membrane flux recovered to 97.5% of the initial flux, and the retention performance recovered to 98% of the initial performance, which did not meet expectations. Cleaning and repair agents were used to continue cleaning.

[0087] (II) Repair agent cleaning: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0088] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with fresh water after alkaline washing, controlling the flow rate of a single membrane housing to 3.5 m. 3 The flushing pressure is 0.7 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0089] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the cleaning and repair agent outlet valve 13, the liquid inlet valve 18 and the air compressor 9. After the cleaning and repair agent enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.7MPa and the dissolved air efficiency to 85% to produce dissolved air cleaning water with a microbubble particle size of 50nm.

[0090] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3.5 m. 3 / h, cleaning pressure 0.7 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water that stably flows out of the cleaning and repair agent from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the cleaning and repair agent return valve 16, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 25 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water of the cleaning and repair agent for 6 hours.

[0091] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3.5m. 3 After cleaning at a rate of 0.7 bar / h and a cleaning pressure of 0.7 bar for 15 minutes, the flow rate of the single membrane housing is increased to 8 m / s. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 40 minutes;

[0092] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3 The flushing pressure is 2.5 bar / h, until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the cleaning and repair agent tank 4 and the dissolved air device 8 is emptied to complete the cleaning and repair agent cleaning operation.

[0093] Alkaline washing and cleaning and repair agent cleaning are carried out at a temperature of 20°C.

[0094] After final calibration, the flux of the cleaned nanofiltration membrane was restored to 98.8% of the initial flux, and the retention performance was restored to more than 99% of the initial performance. The cleaning cycle from this cleaning to the next cleaning is 40 days, which is 19% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption is reduced by 16%. Example 5

[0095] To clean nanofiltration membranes contaminated with organic matter, cleaning begins when the flux drops to 90% of the normal permeate flux. The cleaning process includes alkaline washing and acid washing.

[0096] (a) Alkaline washing: Before washing, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0097] (1) Prepare an aqueous solution containing 0.2% sodium hydroxide as alkaline cleaning water and an aqueous solution containing 0.3% hydrogen chloride as acidic cleaning water; add the alkaline cleaning water to the alkaline cleaning water tank 3, add the acidic cleaning water to the acidic cleaning water tank 2, add fresh water to the fresh water tank 1, and put the nanofiltration membrane to be cleaned into the membrane housing 7.

[0098] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with freshwater, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.9 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0099] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.9MPa and the dissolved air efficiency to 90%, and make dissolved air cleaning water with microbubble particle size of 20nm.

[0100] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.9 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulation cleaning for 26 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 8 hours;

[0101] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a pressure of 0.9 bar and a cycle time of 16 minutes, the flow rate of the single membrane housing was increased to 8 m / s. 3 / h, cleaning pressure up to 1.8 bar, continue circulating and cleaning for 50 minutes;

[0102] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3 The flushing pressure is 2.3 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the alkaline cleaning operation.

[0103] (ii) Acid washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0104] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane that has been washed with alkali inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.8 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0105] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.8MPa and the dissolved air efficiency to 90%, so as to produce dissolved air cleaning water with microbubble particle size of 30nm.

[0106] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.8 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the acid cleaning water return valve 14, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 6 hours.

[0107] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a pressure of 0.8 bar and a cycle time of 18 minutes, the flow rate of the single membrane housing is increased to 9 m / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 60 minutes;

[0108] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m. 3 The flushing pressure is 2.5 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the acid cleaning water tank 2 and the dissolved air device 8 is emptied to complete the acid cleaning operation.

[0109] Alkali washing and acid washing are carried out at a temperature of 30°C.

[0110] After calibration, the nanofiltration membrane recovered to 98% of its initial flux and 99% of its initial retention performance after alkaline and acid washing. The cleaning cycle from this cleaning to the next cleaning was 41 days, which is 22% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption was reduced by 18%. Example 6

[0111] To clean nanofiltration membranes contaminated with organic matter, cleaning begins when the flux drops to 90% of the normal permeate flux. The cleaning process includes alkaline washing, acid washing, and remedial cleaning.

[0112] (a) Alkaline washing: Before washing, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0113] (1) Prepare an aqueous solution containing 0.1% sodium hydroxide as alkaline cleaning water, prepare an aqueous solution containing 0.2% hydrogen chloride as acidic cleaning water, and prepare an ethanol solution containing 0.3% piperazine and 0.05% triethanolamine as cleaning and repair agent; add the alkaline cleaning water to the alkaline cleaning water tank 3, add the acidic cleaning water to the acidic cleaning water tank 2, add the cleaning and repair agent to the cleaning and repair agent tank 4, add fresh water to the fresh water tank 1, and put the nanofiltration membrane to be cleaned into the membrane shell 7;

[0114] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with freshwater, controlling the flow rate of the single membrane housing to 3m. 3 The flushing pressure is 0.5 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0115] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.6 MPa and the dissolved air efficiency to 70%, and make dissolved air cleaning water with microbubble particle size of 80 nm.

[0116] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3m. 3 / h, cleaning pressure 0.5 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulation cleaning for 20 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 4 hours.

[0117] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3m. 3 After cleaning at a pressure of 0.5 bar and a cycle time of 12 minutes, increase the flow rate of the single membrane housing to 7 m³ / h. 3 / h, cleaning pressure up to 1.8 bar, continue circulating and cleaning for 30 minutes;

[0118] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 7m. 3The flushing pressure is 2.1 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the alkaline cleaning operation.

[0119] (ii) Acid washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0120] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane that has been washed with alkali inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.8 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0121] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.7MPa and the dissolved air efficiency to 80% to produce dissolved air cleaning water with a microbubble particle size of 50nm.

[0122] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.8 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and acid cleaning water return valve 14, and start dissolved air cleaning water circulation cleaning. After circulating for 30 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 4 hours.

[0123] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a pressure of 0.8 bar and a cycle time of 18 minutes, the flow rate of the single membrane housing is increased to 9 m / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 50 minutes;

[0124] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m. 3 The flushing pressure is 2.5 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the acid cleaning water tank 2 and the dissolved air device 8 is emptied to complete the acid cleaning operation.

[0125] After calibration, the nanofiltration membrane flux recovered to 96.5% of the initial flux, and the retention performance recovered to 98% of the initial performance, which did not meet expectations. Cleaning and repair agents were used to continue cleaning.

[0126] (III) Repair agent cleaning: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0127] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the acid-washed nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.7 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0128] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the cleaning and repair agent outlet valve 13, the liquid inlet valve 18 and the air compressor 9. After the cleaning and repair agent enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.8MPa and the dissolved air efficiency to 85% to produce dissolved air cleaning water with a microbubble particle size of 30nm.

[0129] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.7 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water that stably flows out of the cleaning and repair agent from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the cleaning and repair agent return valve 16, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water of the cleaning and repair agent for 8 hours.

[0130] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a flow rate of 0.7 bar for 15 minutes, increase the flow rate to 8 m / h. 3 / h, cleaning pressure up to 1.9 bar, continue circulating and cleaning for 55 minutes;

[0131] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3The flushing pressure is 2.3 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the cleaning and repair agent tank 4 and the dissolved air device 8 is emptied to complete the cleaning and repair agent cleaning operation.

[0132] Alkaline washing, acid washing, and cleaning and repair agent cleaning are carried out at a temperature of 25°C.

[0133] After final calibration, the nanofiltration membrane, after being cleaned with alkaline washing, acid washing, and cleaning and repairing agent, had its flux restored to 99% of its initial flux and its retention performance restored to more than 99% of its initial performance. The cleaning cycle from this cleaning to the next cleaning was 41 days, which is 22% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption was reduced by 17%. Example 7

[0134] To clean nanofiltration membranes contaminated by both carbonate and sulfate scaling, cleaning begins when the flux drops to 90% of the normal permeate flux. The cleaning process includes acid washing and alkaline washing.

[0135] (a) Acid washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0136] (1) Prepare an aqueous solution containing 3% citric acid as acidic cleaning water and an aqueous solution containing 0.2% sodium hydroxide as alkaline cleaning water; add the acidic cleaning water to acidic cleaning water tank 2, add the alkaline cleaning water to alkaline cleaning water tank 3, add fresh water to fresh water tank 1, and put the nanofiltration membrane to be cleaned into membrane shell 7.

[0137] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of a single membrane housing to 4.5 m. 3 The flushing pressure is 1 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0138] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.9 MPa and the dissolved air efficiency to 90%, so as to produce dissolved air cleaning water with microbubble particle size of 20 nm.

[0139] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4.5 m / s. 3 / h, cleaning pressure 1 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the acid cleaning water return valve 14, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 9 hours.

[0140] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4.5 m. 3 After cleaning at a pressure of 1 bar per hour and circulating for 18 minutes, increase the flow rate of the single membrane housing to 9 m³ / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 60 minutes;

[0141] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m. 3 The flushing pressure is 2.5 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the acid cleaning water tank 2 and the dissolved air device 8 is emptied to complete the acid cleaning operation.

[0142] (II) Alkaline washing: Before washing, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0143] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the acid-washed nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.9 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0144] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.9 MPa and the dissolved air efficiency to 90%, so as to produce dissolved air cleaning water with microbubble particle size of 20 nm.

[0145] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.9 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulation cleaning for 26 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 8 hours;

[0146] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a pressure of 0.9 bar and a cycle time of 16 minutes, the flow rate of the single membrane housing was increased to 8 m / s. 3 / h, cleaning pressure up to 1.8 bar, continue circulating and cleaning for 50 minutes;

[0147] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3 The flushing pressure is 2.3 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the alkaline cleaning operation.

[0148] Pickling and alkaline washing are carried out at a temperature of 30°C.

[0149] After calibration, the nanofiltration membrane recovered to 98% of its initial flux and 99% of its initial retention performance after acid and alkali washing. The cleaning cycle from this cleaning to the next cleaning was 41 days, which is 22% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption was reduced by 18%. Example 8

[0150] To clean nanofiltration membranes contaminated by both carbonate and sulfate scaling, cleaning begins when the flux drops to 90% of the normal permeate flux. The cleaning process includes acid washing and alkaline washing.

[0151] (a) Acid washing: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0152] (1) Prepare an aqueous solution containing 3% citric acid as acidic cleaning water, prepare an aqueous solution containing 0.2% sodium hydroxide as alkaline cleaning water, and prepare a glycerol solution containing 0.3% trimesoyl chloride and 0.05% 2,2'-oxodiethylamine as cleaning and repair agent; add acidic cleaning water to acidic cleaning water tank 2, add alkaline cleaning water to alkaline cleaning water tank 3, add cleaning and repair agent to cleaning and repair agent tank 4, add fresh water to fresh water tank 1, and put the nanofiltration membrane to be cleaned into membrane shell 7;

[0153] (2) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of a single membrane housing to 3.5 m. 3 The flushing pressure is 0.8 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0154] (3) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the acidic cleaning water outlet valve 11, the liquid inlet valve 18 and the air compressor 9. After the acidic cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.8 MPa and the dissolved air efficiency to 85% to produce dissolved air cleaning water with a microbubble particle size of 30 nm.

[0155] (4) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (3) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3.5 m / s. 3 / h, cleaning pressure 0.8 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the acid cleaning water return valve 14, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 6 hours.

[0156] (5) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3.5m. 3 After cleaning at a rate of 0.8 bar / h and a cleaning pressure of 0.8 bar for 12 minutes, the flow rate of the single membrane housing is increased to 8 m / s. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 50 minutes;

[0157] (6) After the circulation cleaning in step (5) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 8m. 3The flushing pressure is 2.2 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the acid cleaning water tank 2 and the dissolved air device 8 is emptied to complete the acid cleaning operation.

[0158] (II) Alkaline washing: Before washing, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0159] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the acid-washed nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 3m. 3 The flushing pressure is 0.7 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0160] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the alkaline cleaning water outlet valve 12, the liquid inlet valve 18 and the air compressor 9. After the alkaline cleaning water enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.7MPa and the dissolved air efficiency to 80%, so as to produce dissolved air cleaning water with a microbubble particle size of 50nm.

[0161] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 3m. 3 / h, cleaning pressure 0.7 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water flows out stably from drain valve 20. Then close drain valve 20, open main return valve 21 and alkaline cleaning water return valve 15, and start dissolved air cleaning water circulation cleaning. After circulation cleaning for 25 minutes, stop water pump 5 and air compressor 9, close main return valve 21, stop circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in dissolved air cleaning water for 6 hours;

[0162] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 3m. 3 After cleaning at a pressure of 0.7 bar and a cycle time of 16 minutes, the flow rate of the single membrane housing is increased to 8 m / h. 3 / h, cleaning pressure up to 1.8 bar, continue circulating and cleaning for 50 minutes;

[0163] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m. 3 The flushing pressure is 2.3 bar / h until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the alkaline cleaning water tank 3 and the dissolved air device 8 is emptied to complete the alkaline cleaning operation.

[0164] After calibration, the nanofiltration membrane flux recovered to 97% of the initial flux, and the retention performance recovered to 97.5% of the initial performance, which did not meet expectations. Cleaning and repair agents were used to continue cleaning.

[0165] (III) Repair agent cleaning: Before cleaning, all valves of the nanofiltration membrane cleaning device are in the closed state; the steps are as follows:

[0166] (1) Open the freshwater outflow valve 10, flushing valve 17, and drain valve 20, start the water pump 5, and flush the acid-washed nanofiltration membrane inside the membrane housing 7 with fresh water, controlling the flow rate of the single membrane housing to 4m. 3 The flushing pressure is 0.8 bar / h, and the flushing ends when clean water flows steadily out of the drain valve 20.

[0167] (2) Close the fresh water outlet valve 10 and the flushing valve 17, and at the same time open the cleaning and repair agent outlet valve 13, the liquid inlet valve 18 and the air compressor 9. After the cleaning and repair agent enters the dissolved air device 8, control the dissolved air pressure of the dissolved air device 8 to 0.8MPa and the dissolved air efficiency to 85% to produce dissolved air cleaning water with a microbubble particle size of 30nm.

[0168] (3) Under the delivery of water pump 5, the dissolved gas cleaning water obtained in step (2) enters the membrane housing through check valve 19 to clean the nanofiltration membrane, and the flow rate of the single membrane housing is controlled at 4m. 3 / h, cleaning pressure 0.8 bar, until all the fresh water in the membrane housing is replaced with dissolved air cleaning water, i.e., dissolved air cleaning water that stably flows out of the cleaning and repair agent from the drain valve 20. Then close the drain valve 20, open the main return valve 21 and the cleaning and repair agent return valve 16, and start the dissolved air cleaning water circulation cleaning. After circulating cleaning for 30 minutes, stop the water pump 5 and the air compressor 9, close the main return valve 21, stop the circulation cleaning, and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water of the cleaning and repair agent for 10 hours.

[0169] (4) After soaking, start water pump 5 and air compressor 9, open the main return valve 21, and continue to circulate the dissolved air cleaning water, controlling the flow rate of the single membrane shell to 4m. 3 After cleaning at a flow rate of 0.8 bar for 18 minutes, increase the flow rate to 9 m / h. 3 / h, cleaning pressure up to 2 bar, continue circulating cleaning for 60 minutes;

[0170] (5) After the circulation cleaning in step (4) is completed, stop the air compressor 9, close the main return valve 21, open the fresh water outlet valve 10 and the drain valve 20, and then rinse the nanofiltration membrane with fresh water, controlling the flow rate of the single membrane housing to 9m. 3The flushing pressure is 2.5 bar / h, until all the dissolved air cleaning water in the membrane housing is replaced with fresh water, i.e., fresh water flows out steadily from the drain valve 20. Then, the water pump 5 is turned off, and the liquid in the cleaning and repair agent tank 4 and the dissolved air device 8 is emptied to complete the cleaning and repair agent cleaning operation.

[0171] Pickling, alkaline washing, and cleaning and repair agent cleaning are carried out at a temperature of 25°C.

[0172] After final calibration, the nanofiltration membrane, after being cleaned with acid, alkali and cleaning repair agent, recovered to 99.2% of its initial flux and more than 99% of its initial retention performance. The cleaning cycle from this cleaning to the next cleaning is 40 days, which is 19% longer than the cleaning cycle of traditional cleaning methods, and the annual reagent consumption is reduced by 17%. Example 9

[0173] The operation steps in this embodiment are the same as those in Embodiment 1. The difference is that the acidic cleaning water prepared is an aqueous solution containing 0.8% aminosulfonic acid. The cleaning effect is basically the same as that in Embodiment 1. Example 10

[0174] The operation steps in this embodiment are the same as those in Embodiment 2. The difference is that the acidic cleaning water prepared is an aqueous solution containing 0.5% aminosulfonic acid. The cleaning effect is basically the same as that in Embodiment 2. Example 11

[0175] The operation steps in this embodiment are the same as those in Embodiment 1. The difference is that the acidic cleaning water prepared is an aqueous solution containing 0.8% acetic acid. The cleaning effect is basically the same as that in Embodiment 1. Example 12

[0176] The operation steps in this embodiment are the same as those in Embodiment 2. The difference is that the acidic cleaning water prepared is an aqueous solution containing 0.5% acetic acid. The cleaning effect is basically the same as that in Embodiment 2. Example 13

[0177] The operation steps in this embodiment are the same as those in Embodiment 3, except that the alkaline cleaning water is an aqueous solution containing 0.7% sodium tripolyphosphate. The cleaning effect is basically the same as that in Embodiment 3. Example 14

[0178] The operation steps in this embodiment are the same as those in Embodiment 4, except that the alkaline cleaning water is an aqueous solution containing 0.5% sodium tripolyphosphate. The cleaning effect is basically the same as that in Embodiment 4. Example 15

[0179] The operation steps in this embodiment are the same as those in Embodiment 2. The difference is that the tannic acid content in the prepared cleaning and repair agent is 0.05%. The cleaning effect of the cleaning and repair agent is slightly improved compared with Embodiment 2, and the tannic acid consumption is increased accordingly. Example 16

[0180] The operation steps in this embodiment are the same as those in embodiment 4. The difference is that the content of polyetheramine in the prepared cleaning and repair agent is 0.05%. The cleaning effect of the cleaning and repair agent is slightly improved compared with that in embodiment 4, and the consumption of polyetheramine is increased accordingly. Example 17

[0181] The operation steps in this embodiment are the same as those in Embodiment 6. The difference is that the content of triethanolamine in the prepared cleaning and repair agent is 0.01%. The cleaning effect of the cleaning and repair agent is slightly reduced compared with Embodiment 6, and the consumption of triethanolamine is reduced accordingly. Example 18

[0182] The operation steps in this embodiment are the same as those in embodiment 8. The difference is that the content of 2,2'-oxodiethylamine in the prepared cleaning and repair agent is 0.01%. The cleaning effect of the cleaning and repair agent is slightly lower than that in embodiment 8, and the consumption of 2,2'-oxodiethylamine is reduced accordingly.

Claims

1. A method for cleaning nanofiltration membranes using microbubbles, characterized in that: The method includes the following steps: (1) Prepare cleaning water, which includes: acidic cleaning water, alkaline cleaning water, a combination of acidic and alkaline cleaning water, a combination of acidic cleaning water and cleaning and repair agent, a combination of alkaline cleaning water and cleaning and repair agent, and a combination of acidic cleaning water, alkaline cleaning water and cleaning and repair agent; (2) First rinse the nanofiltration membrane inside the membrane housing with fresh water, and control the flow rate of a single membrane housing to 3-4.5 m. 3 / h, flushing pressure 0.5-1 bar, until clear water flows steadily out of the membrane housing; (3) Prepare dissolved air cleaning water with microbubble particle size of 20-90nm from acidic cleaning water, and then perform the following steps (4)-(6). Alternatively, alkaline cleaning water can be made into dissolved air cleaning water with microbubble particle size of 20-90nm, and then the following steps (4)-(6) can be performed. Alternatively, acidic cleaning water and alkaline cleaning water can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), and then the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6) or first, the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), and then the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6). Alternatively, acidic cleaning water and cleaning and repairing agent can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, use the dissolved air cleaning water of acidic cleaning water to perform the following steps (4)-(6), and then use the dissolved air cleaning water of cleaning and repairing agent to perform the following steps (4)-(6). Alternatively, alkaline cleaning water and cleaning and repairing agent can be made into dissolved air cleaning water with microbubble particle size of 20-90nm respectively. First, use the dissolved air cleaning water of alkaline cleaning water to perform the following steps (4)-(6), and then use the dissolved air cleaning water of cleaning and repairing agent to perform the following steps (4)-(6). Alternatively, acidic cleaning water, alkaline cleaning water, and cleaning and repairing agent can be respectively made into dissolved air cleaning water with microbubble particle size of 20-90nm. First, the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), then the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), and finally the dissolved air cleaning water of cleaning and repairing agent is used to perform the following steps (4)-(6). Or, first, the dissolved air cleaning water of alkaline cleaning water is used to perform the following steps (4)-(6), then the dissolved air cleaning water of acidic cleaning water is used to perform the following steps (4)-(6), and finally the dissolved air cleaning water of cleaning and repairing agent is used to perform the following steps (4)-(6). (4) Use the dissolved gas cleaning water obtained in step (3) to clean the nanofiltration membrane inside the membrane housing, and control the flow rate of the single membrane housing to 3-4.5 m. 3 Clean at a pressure of 0.5-1 bar per hour until all the fresh water in the membrane housing is replaced. Then start the dissolved air cleaning water circulation cleaning. After circulating for 20-30 minutes, stop the circulation and let the nanofiltration membrane in the membrane housing soak in the dissolved air cleaning water for 1-12 hours. (5) After soaking, continue to clean with dissolved air cleaning water circulation, controlling the flow rate of the single membrane shell to 3-4.5m. 3 Cleaning pressure 0.5-1 bar, circulation for 12-18 minutes; then increase the single membrane housing flow rate to 6-9 m / h. 3 / h, cleaning pressure to 1.5-2 bar, continue circulation for 30-60 minutes; (6) After step (5) is completed, rinse the nanofiltration membrane inside the membrane housing with fresh water, and control the flow rate of the single membrane housing to 6-9 m. 3 The flushing pressure is 2-2.5 bar per hour until all the dissolved air cleaning water in the membrane housing is replaced.

2. The method for cleaning nanofiltration membranes using microbubbles as described in claim 1, characterized in that: The acidic cleaning water is an aqueous solution of any one of hydrogen chloride, citric acid, oxalic acid, aminosulfonic acid, and acetic acid; the alkaline cleaning water is an aqueous solution of any one of sodium hydroxide, sodium tripolyphosphate, and tetrasodium ethylenediaminetetraacetate; the cleaning and repair agent is an organic solvent solution composed of piperazine or trimesoyl chloride and any one of tannic acid, triethanolamine, polyetheramine, and 2,2'-oxodiethylamine, wherein the organic solvent is methanol, ethanol, or glycerol.

3. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, In steps (2) and (4), the flow velocity of the single membrane shell is 3-4 m / s. 3 / h, flushing pressure is 0.5-0.8 bar.

4. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, The microbubble particle size is 30-80 nm.

5. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, The soaking time in step (4) is 4-12 hours.

6. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, In step (5), the flow rate of the single membrane shell is increased to 7-9 m. 3 / h, cleaning pressure up to 1.8-2 bar.

7. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, In step (6), the flow rate of the single membrane shell is controlled to be 8-9 m / s. 3 The flushing pressure is 2.2-2.5 bar per hour.

8. The method for cleaning nanofiltration membranes using microbubbles according to claim 1, characterized in that, Steps (1)-(6) are carried out at a temperature of 20-30℃.

9. The method for cleaning nanofiltration membranes using microbubbles according to claim 2, characterized in that, The mass concentrations of the aqueous solutions of hydrogen chloride, citric acid, oxalic acid, aminosulfonic acid, and acetic acid are 0.2-0.3%, 2-3%, 2-3%, 0.5-0.8%, and 0.5-0.8%, respectively; the mass concentrations of the aqueous solutions of sodium hydroxide, sodium tripolyphosphate, and tetrasodium ethylenediaminetetraacetate are 0.1-0.2%, 0.5-0.7%, and 1-2%, respectively; the mass concentration of piperazine or trimesoyl chloride in the organic solvent solution is 0.1-0.3%, and the mass concentration of tannic acid, triethanolamine, polyetheramine, or 2,2'-oxodiethylamine in the organic solvent solution is 0.01-0.05%.

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