Cleaning method of hollow fiber composite membrane module

Abstract

The invention provides a cleaning method of a hollow fiber composite membrane module, and belongs to the technical field of membrane separation. According to the invention, a triple decontamination mechanism is formed by coupling gas-water mixed cleaning, high-frequency vibration and dynamic positive and negative flushing. A Venturi self-suction nozzle is adopted to generate a micron bubble group containing 15-25% of gas, and 20-40 kHz ultrasonic waves are combined, so that membrane filaments generate micro vibration, and the adhesion energy is reduced by more than 65%. And an axial-radial composite flow field is formed through 5-30 s of alternate forward flushing and backward flushing, so that the shearing rate is 2-3 times higher than that of a traditional method. Experiments show that the method treats turbidity gt; when wastewater is 200 NTU, the alkali cleaning time is shortened from 11-12 hours to 1-1.5 hours, the acid chemical cleaning time is shortened from 1-2 hours to 10-30 minutes, the flux recovery rate is 95-98%, membrane pore pollutants are reduced by 82%, the membrane can be continuously operated for 2000 times without attenuation, and the membrane is suitable for high-turbidity wastewater treatment.

Description

Technical Field

[0001] The present invention relates to the technical field of membrane separation, in particular to a method for cleaning a hollow fiber composite membrane component. Background Art

[0002] Composite membrane separation technology, a core process in modern water treatment, relies heavily on membrane fouling control for long-term stable operation. However, membrane pore blockage and surface contamination accumulation caused by high-turbidity wastewater are particularly prominent issues. Existing cleaning technologies (such as CN107308821A and CN201454425U) often employ a single physical cleaning mode: the former uses intermittent high-pressure gas flushing (0.6-1.0 MPa) to remove the surface filter cake layer, but the gas struggles to form effective turbulence within the membrane filaments, resulting in a removal rate of less than 40% for deep-seated contaminants with pore sizes less than 0.1 μm. The latter uses stepped pressure backwashing (0.2-0.5 MPa), which, while partially restoring membrane flux, causes contaminants to migrate deeper into the membrane pores. SEM analysis shows that the membrane pore shrinkage rate can be as high as 15% after backwashing.

[0003] The composite cleaning technology developed in recent years attempts to break through the limitations of a single mode. For example, CN113559719A proposes a cleaning solution that combines microbubbles with mechanical vibrations. 3 ) circulates to generate bubbles of 50-200μm, and cooperates with 20Hz low-frequency vibration to achieve surface scouring. However, this system has significant defects: ① The microbubble generator needs to continuously consume an inlet water pressure of more than 0.8MPa, and the energy consumption accounts for 35% of the total power of the system; ② The complex pipeline design (including multi-stage circulation loops, frequent valve switching and cross-connections) increases the equipment footprint by 40%, and there is a risk of bubble aggregation and cavitation failure. When the fluid flow rate exceeds 3.5m / s (common in the backwash stage), the pipeline vortex will cause the average bubble diameter to merge from 50μm to more than 300μm. When these macro bubbles collapse at a working pressure of more than 2.5bar, the generated microjet impact pressure can reach 0.8GPa, resulting in characteristic honeycomb pits on the membrane surface.

[0004] More seriously, the limitations of traditional chemical cleaning are becoming increasingly apparent: when wastewater turbidity exceeds 200 NTU, conventional alkaline cleaning (100 ppm NaClO, 40°C) requires more than 120 minutes to restore flux to 82%-85% of its initial value. XPS analysis shows that 12%-18% of organic pollutants still remain on the membrane surface. Frequent acid-base alternating cleaning (>3 times / month) not only accelerates membrane material hydrolysis (FTIR shows an amide bond breakage rate of >30%) but also causes an irreversible decrease in porosity (mercury intrusion porosimetry measures the average pore size from 25 nm to 18 nm), directly shortening membrane life by 40%-60%. These technical bottlenecks severely restrict the promotion and application of ultrafiltration technology in highly polluting industries such as coal chemical industry and printing and dyeing. The development of efficient and low-cost deep cleaning methods has become an urgent industry need. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for cleaning a hollow fiber composite membrane module, so as to solve the technical problems of high energy consumption and poor cleaning effect in the existing cleaning technology.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a method for cleaning a hollow fiber composite membrane module, comprising the following steps:

[0008] 1) Air-water mixed cleaning: Gas and cleaning liquid are introduced into the hollow fiber composite membrane module simultaneously through a Venturi self-priming nozzle to form a gas-liquid two-phase flow containing micron-sized bubbles for air-water mixed cleaning;

[0009] 2) Synchronous high-frequency vibration: During the air-water mixed cleaning process, ultrasonic vibration is applied to the composite membrane assembly;

[0010] 3) Alternating forward and reverse flushing: After the air-water mixed cleaning is completed, empty the remaining cleaning liquid and re-inject the cleaning liquid, and perform forward and back flushing alternately. The alternating frequency is 5 to 30 seconds per time, and the single alternating flushing time is 10 to 15 minutes;

[0011] 4) Discharge and drying: After cleaning, discharge the pollutants and introduce dry gas to purge the hollow fiber composite membrane assembly.

[0012] Furthermore, in step 1), the volume ratio of gas to cleaning liquid is 1:5-10, the cleaning liquid includes acid cleaning liquid or alkaline cleaning liquid, and the gas-water mixture is left to soak for 10-50 minutes after cleaning.

[0013] Furthermore, when the cleaning solution is an acid cleaning solution, the cleaning time is 10 to 30 minutes, and the acid cleaning solution contains 0.5 to 1.5 wt % of citric acid and 0.1 to 0.5 wt % of hydrochloric acid.

[0014] Furthermore, when the cleaning liquid is an alkaline cleaning liquid, the cleaning time is 1 to 1.5 hours, the alkaline cleaning liquid includes 100 to 300 ppm sodium hypochlorite and 50 to 400 ppm sodium hydroxide, and the cleaning temperature of the alkaline cleaning liquid is 35 to 45°C.

[0015] Furthermore, the frequency of the ultrasonic vibration is 15 to 50 kHz and the power is 2000 to 5000 W, causing the membrane filament to generate micro-vibration of 0.01 to 0.1 mm.

[0016] Furthermore, the ultrasonic vibration is achieved by an array of ultrasonic transmitters, which are evenly distributed on the membrane assembly housing at an incident angle of 30° to 60°, and the vibration energy acts on the membrane filaments through solid conduction.

[0017] Furthermore, the pressure difference between the forward flushing and the back flushing is maintained at 0.4 to 0.8 bar, and a pulsed pressure fluctuation is generated each time the pressure is switched, with the fluctuation amplitude being 15 to 25% of the working pressure.

[0018] Furthermore, the working pressure of the Venturi self-priming nozzle is 0.2-0.5 MPa.

[0019] Beneficial effects of the present invention:

[0020] The present invention combines high-frequency vibration with air-water cleaning. Physical vibration loosens pollutants, and the shear force of the gas-liquid two-phase flow flushes the membrane surface. It is supplemented by forward and reverse flushing to enhance mass transfer, thereby achieving fast, low-consumption, and non-destructive cleaning. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The following are the results of membrane fiber testing before cleaning. A shows the electron microscope image of the outer surface of the membrane fiber, and B shows the appearance of the membrane fiber.

[0022] Figure 2 This is an electron microscope image of the outer surface of the membrane fiber after being treated by the cleaning process of Example 1 of the present invention;

[0023] Figure 3 This is an electron microscope image of the outer surface of the membrane fiber after being treated by the cleaning process of Example 2 of the present invention;

[0024] Figure 4 This is an electron microscope image of the outer surface of the membrane fiber after traditional acid cleaning (Comparative Example 1);

[0025] Figure 5 This is an electron microscope image of the outer surface of the membrane fiber after traditional alkaline cleaning treatment (Comparative Example 2). DETAILED DESCRIPTION

[0026] The present invention provides a method for cleaning a hollow fiber composite membrane module, comprising the following steps:

[0027] 1) Air-water mixed cleaning: Gas and cleaning liquid are introduced into the hollow fiber composite membrane module simultaneously through a Venturi self-priming nozzle to form a gas-liquid two-phase flow containing micron-sized bubbles for air-water mixed cleaning;

[0028] 2) Synchronous high-frequency vibration: During the air-water mixed cleaning process, ultrasonic vibration is applied to the composite membrane assembly;

[0029] 3) Alternating forward and reverse flushing: After the air-water mixed cleaning is completed, empty the remaining cleaning liquid and re-inject the cleaning liquid, and perform forward and back flushing alternately. The alternating frequency is 5 to 30 seconds per time, and the single alternating flushing time is 10 to 15 minutes;

[0030] 4) Discharge and drying: After cleaning, discharge the pollutants and introduce dry gas to purge the hollow fiber composite membrane assembly.

[0031] In the present invention, in step 1), the volume ratio of gas to cleaning liquid is 1:5-10, preferably 1:6-9, and more preferably 1:7-8; the cleaning liquid includes an acid cleaning liquid or an alkaline cleaning liquid, and the gas-water mixed cleaning is left to soak for 10-50 minutes, preferably 20-40 minutes; ultrasonic-assisted oscillation is maintained during the soaking period.

[0032] In the present invention, when the cleaning solution is an acid cleaning solution, the cleaning time is 10 to 30 minutes, preferably 15 to 20 minutes; the acid cleaning solution contains 0.5 to 1.5 wt% of citric acid and 0.1 to 0.5 wt% of hydrochloric acid, preferably 1.0 wt% of citric acid and 0.3 wt% of hydrochloric acid.

[0033] In the present invention, when the cleaning liquid is an alkaline cleaning liquid, the cleaning time is 1 to 1.5 hours, preferably 1 hour; the alkaline cleaning liquid includes 100 to 300 ppm sodium hypochlorite and 50 to 400 ppm sodium hydroxide, preferably 200 ppm sodium hypochlorite and 400 ppm sodium hydroxide; the cleaning temperature of the alkaline cleaning liquid is 35 to 45°C, preferably 40°C.

[0034] In the present invention, the frequency of the ultrasonic vibration is 15 to 50 kHz, preferably 20 to 40 kHz, and more preferably 25 to 35 kHz; the power is 2000 to 5000 W, preferably 3000 to 4000 W; and the membrane filaments generate micro-vibrations of 0.01 to 0.1 mm.

[0035] In the present invention, the ultrasonic vibration is achieved by an array-type ultrasonic transmitter, which is evenly distributed on the membrane assembly housing at an incident angle of 30° to 60°, and the vibration energy acts on the membrane filaments through solid conduction.

[0036] In the present invention, the pressure of the forward flushing is 1.5 to 2.0 bar, and the pressure of the back flushing is 0.8 to 1.2 bar; the pressure difference between the forward flushing and the back flushing is maintained at 0.4 to 0.8 bar, preferably 0.5 to 0.7 bar, and more preferably 0.6 to 0.7 bar; each time the pressure is switched, a pulsed pressure fluctuation is generated, and the fluctuation amplitude is 15 to 25% of the working pressure.

[0037] In the present invention, forward and reverse flushing form an axial-radial composite flow field, the membrane surface shear rate reaches 3.2 times that of the traditional method, and the duration of a single forward flush is 1.8 to 2.5 times longer than that of the back flush.

[0038] In the present invention, high-frequency vibration is applied synchronously during the gas-liquid mixed cleaning stage, which reduces the adhesion of pollutants to the membrane surface by more than 65% and reduces the area of ​​residual pollutants in the membrane pores by 82%.

[0039] In the present invention, when treating wastewater with turbidity greater than 200 NTU, the single chemical cleaning time of the acid cleaning solution is 10 to 30 minutes, the single chemical cleaning time of the alkaline cleaning solution is 1 to 1.5 hours, and the system can continuously operate for 2000 cleaning cycles without performance degradation.

[0040] In the present invention, the working pressure of the Venturi self-priming nozzle is 0.2-0.5 MPa, preferably 0.3-0.4 MPa, and more preferably 0.3 MPa.

[0041] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0042] Example 1

[0043] Four hollow fiber membrane modules made of the same material and model were tested on surface water. The operating pressure of each module met the cleaning requirements. One of these modules was selected for cleaning testing. Cleaning solution preparation: Citric acid and hydrochloric acid were dissolved in deionized water at a ratio of 1.0 wt% and 0.3 wt% to create an acidic cleaning solution. Citric acid served as the primary chelating agent, and hydrochloric acid was used to adjust the pH to 2.0. Gas-liquid mixing: Compressed air was introduced using an air pump at a 1:8 air-to-water volume ratio, achieving thorough gas-liquid mixing through a Venturi nozzle. The Venturi nozzle had a 5 mm throat diameter and an inlet pressure of 0.3 MPa. Cleaning process: The mixed gas-liquid two-phase flow was injected into the hollow fiber membrane module at a flow rate of 2.0 m / s. The membrane cleaning solution was returned to the cleaning water tank from the concentrate outlet at the top of the module, starting a cleaning cycle. The cleaning cycle lasted 10 minutes. After the cycle, the module was allowed to soak for 10 minutes, followed by another 10-minute rinse cycle. During the cleaning period, the ultrasonic generator is turned on synchronously, and the operating frequency is set to 25kHz and the output power is 3000W. After the cleaning is completed, the positive drain is opened to drain the residual cleaning liquid in the component, and the cleaning liquid is re-injected, and the positive and negative flushing alternating mode is adopted for a total of 2 cycles. Each cycle lasts for 15 minutes, including 8 minutes of forward flushing and 7 minutes of reverse flushing. The pressure difference between forward flushing and backwashing is maintained at 0.4 bar. After the flushing is completed, the residual cleaning liquid in the component is drained, and clean water is injected into the component. The membrane cleaning water is circulated back to the cleaning water tank for cyclic flushing. The cleaning water is replaced every 10 minutes until the conductivity of the membrane cleaning water drops below 10us / cm, and the cleaning is completed. Effect verification: The test results after cleaning are shown in Table 1. The test after cleaning shows that the membrane flux has recovered from 32L / (m2·h) before cleaning to 48L / (m 2 ·h), the initial flux of the component is 50L / (m 2 h), the flux recovery rate reached 96% (the recovery rate was calculated using formula (I))

[0044]

[0045] In formula (I), P is the flux recovery rate, J1 is the flux loss value, and J2 is the initial flux. Preferred embodiment: The ultrasonic generator is placed on both sides of the membrane module, with a spacing of 5 cm from the module. The operating temperature is 30-40°C. The cleaning solution temperature is maintained at 35±2°C.

[0046] Implementation effect: Compared with traditional single chemical cleaning, the method of the present invention combines the triple effects of chemical cleaning, gas-liquid flushing and ultrasonic cavitation, shortening the cleaning time by 40% and avoiding the damage of strong acid and strong alkali to the membrane material. Figure 2Scanning electron microscopy (SEM) observations show that contaminants on the membrane surface have been largely removed, the membrane pore structure is intact, and a small amount of particle accumulation and organic film-like structures are present on the membrane surface. Contaminants are primarily organic matter within the module. The new acid cleaning method has a high removal efficiency for salt ions, but has limited ability to degrade organic contaminants. Upon opening the module, the sludge layer is completely removed, demonstrating the new cleaning method's high efficiency in removing contaminants over a large area.

[0047] Example 2

[0048] Four hollow fiber membrane modules made of the same material and model were tested on surface water. The operating pressure of each module met the cleaning requirements. One of these modules was selected for cleaning testing. Cleaning solution preparation: An alkaline cleaning solution was prepared by dissolving 200ppm sodium hypochlorite and 400ppm NaOH in deionized water. NaOH served as the primary chelating agent, adjusting the pH to 11. Sodium hypochlorite served as the oxidant to oxidize and remove organic impurities on the membrane surface. Gas-liquid mixing: Compressed air was introduced using an air pump at a 1:8 air-to-water volume ratio. Thorough mixing of the gas and liquid was achieved through a Venturi nozzle. The Venturi nozzle had a 5mm throat diameter and an inlet pressure of 0.2MPa. Cleaning process: The mixed gas-liquid two-phase flow was injected into the hollow fiber membrane module at a flow rate of 2.0m / s. The membrane cleaning solution was returned to the cleaning water tank through the concentrate outlet at the top of the module, completing the cleaning cycle. Simultaneously turn on the ultrasonic generator, set the operating frequency to 25kHz, and the output power to 3000W. The alkaline cleaning solution cleaning lasts for 30 minutes. After the cycle cleaning is completed, the membrane is allowed to soak for 30 minutes to ensure that the organic matter on the membrane surface reacts with the alkaline solution and is degraded. After the soaking period, the cycle is continued for another 30 minutes. After the cleaning is completed, open the positive drain, empty the remaining cleaning solution in the module, re-inject the cleaning solution, and use a forward and reverse flushing mode for a total of 3 cycles, each lasting 15 minutes, including 8 minutes of forward flushing and 7 minutes of reverse flushing. The pressure difference between the forward and reverse flushing is maintained at 0.5 bar. After the flushing is completed, the remaining cleaning solution in the module is drained, and clean water is injected into the module. The membrane cleaning water is circulated back to the cleaning water tank for a cycle flushing. The cleaning water is replaced every 15 minutes until the pH of the membrane cleaning water remains below 8, and the cleaning is completed. Effect verification: The test results after cleaning are shown in Table 1. The test after cleaning shows that the membrane flux has recovered from 36L / (m2·h) before cleaning to 49L / (m2·h). The initial flux of the module is 50L / (m2·h), and the flux recovery rate has reached 98%. Scanning electron microscopy shows that the contaminants on the membrane surface have been basically removed and the membrane pore structure is intact. Figure 3As shown, it shows that alkaline cleaning can achieve better removal effect on organic matter than acid cleaning. At the same time, the new alkaline cleaning process can deeply destroy the binding force between pollutants and the membrane surface through its unique bubble cavitation effect and high-frequency mechanical vibration synergy, and achieve efficient removal of the organic-inorganic composite pollution layer on the surface of the membrane fiber. It has significant cleaning advantages over traditional processes. When the component was dissected, it was found that the sludge layer had been completely removed. Preferred solution: The ultrasonic generator is set on both sides of the membrane component, with a distance of 5 cm from the component. The temperature of the cleaning liquid is maintained at 35±2℃. Implementation effect: Compared with traditional single chemical cleaning, this method combines the triple effects of chemical cleaning, gas-liquid flushing and ultrasonic cavitation, shortens the cleaning time by 20%, and avoids damage to the membrane material by strong acids and strong alkalis.

[0049] Comparative Example 1

[0050] Four hollow fiber membrane modules made of the same material and model were tested on surface water. The operating pressure of each module met the cleaning requirements. One of these modules was selected for cleaning testing. The module was cleaned using a traditional acid cleaning method. The acidic cleaning solution was prepared by dissolving food-grade citric acid monohydrate in reverse osmosis product water to a concentration of 20,000 ± 500 ppm (mass concentration). A pH meter was used to monitor the pH of the cleaning solution, maintaining it at 1.5.

[0051] Implementation of the cleaning process: (1) Cleaning liquid injection phase: The acidic cleaning liquid is injected into the hollow fiber membrane module at a speed of 1.0 m / s through the membrane system circulation pump, and the operating pressure is maintained at ≤0.15 MPa to avoid overpressure damage to the membrane fibers. (2) Circulation cleaning phase: The cleaning liquid temperature is maintained within the range of 25±2℃, the cross-flow circulation mode is turned on, and the circulation time is strictly controlled within 30±1 minutes. The conductivity change of the cleaning liquid is monitored during the circulation process, and the data is recorded every 5 minutes. (3) Static soaking phase: After stopping the circulation, the membrane module is allowed to stand for 60±5 minutes under the condition of being filled with cleaning liquid. Keep the system closed during the static period to prevent contamination of the cleaning liquid.

[0052] Post-processing process: (1) Cleaning liquid discharge: Use the forward discharge method and open the bottom drain valve to completely drain the cleaning liquid. Maintain a nitrogen purge pressure of 0.08MPa during the discharge process. (2) Clean water flushing: Use cleaning water with a conductivity of ≤5μS / cm for flushing. The flushing flow rate is controlled at 2.0m / s, and a circulation-discharge alternating mode is adopted, with 30 seconds of discharge after each cycle of 5 minutes. (3) Termination condition: Real-time monitoring of the discharge water conductivity. When the conductivity test values ​​for three consecutive times are all <10μS / cm and the fluctuation range is ≤5%, and the pH value of the discharge water is within the range of 6.5-7.5, the flushing is judged to be qualified.

[0053] Quality verification: After flushing, the membrane flux test was carried out. The flux of the hollow fiber membrane module before cleaning was 32L / (m2·h), and the flux of the hollow fiber membrane module after cleaning was 42L / (m2·h). The initial flux of the module was 50L / (m 2 h), the flux recovery rate of the membrane module after cleaning was 84%.

[0054] Note: All operations should be performed at an ambient temperature of 25±5℃ and relative humidity controlled within the range of 30-70%.

[0055] Implementation effect: This cleaning solution is a traditional acid chemical cleaning solution. Its cleaning process is relatively simple and only achieves chemical conversion of pollutants through simple chemical reactions. Pollutants in river water mainly exist in the form of organic matter. Pollutants on the membrane surface are mainly microbial membranes and organic flocs. Traditional acid chemical cleaning has a high salt removal rate but poor cleaning effect. Figure 4 As shown in the figure, there are still a lot of pollutants on the surface of the membrane. In the actual operation process, due to the lack of physical auxiliary means, the contact area between the pollutants and the chemical cleaning solution is limited, resulting in incomplete reaction between the two and poor cleaning effect. Compared with the new acid cleaning process, the cleaning efficiency is low and the flux recovery effect is poor. At the same time, the cleaning time required for this solution is long. In the long-term strong acid or strong alkaline environment, the high concentration of H in the cleaning solution will cause the membrane to be damaged. + Continuous action on the membrane surface can cause irreversible damage, such as polymer chain breakage and functional group degradation, which can affect the lifespan and separation performance of the membrane assembly. Furthermore, a single chemical cleaning method is unlikely to effectively remove complex organic-inorganic mixed contaminants from the membrane surface.

[0056] Comparative Example 2

[0057] Four hollow fiber membrane modules made of the same material and model were tested on surface water. The operating pressure of each module met the cleaning requirements. One of them was selected for cleaning testing. The modules were cleaned using a traditional alkaline cleaning method. The alkaline cleaning solution was prepared by dissolving analytical grade sodium hydroxide (NaOH) and industrial grade sodium hypochlorite (NaClO, with an effective chlorine content of ≥10%) in reverse osmosis (RO) product water to create a mixed alkaline cleaning solution with a concentration of 400±20ppm NaOH + 200±10ppm NaClO. The cleaning solution was tested using a pH meter and an oxidation-reduction potential (ORP) meter to ensure that the pH was ≥12 and the ORP was ≥600mV.

[0058] Cleaning process implementation: (1) Cleaning liquid injection stage: The alkaline cleaning liquid is injected into the hollow fiber membrane module at a linear velocity of 1.0 m / s through the membrane system circulation pump. The operating pressure is controlled to ≤0.15 MPa to avoid mechanical damage to the membrane fibers. (2) Circulation cleaning stage: The cleaning liquid temperature is maintained within the range of 25±2℃, the cross-flow circulation mode is turned on, and the circulation time is strictly controlled within 30±1 minutes. The pH and ORP are monitored every 5 minutes during the circulation process to ensure the stability of the cleaning liquid activity.

[0059] (3) Static soaking stage: After stopping the circulation, let the membrane assembly stand for 11±0.5 hours under the condition of full cleaning solution. Keep the system airtight during the static period to avoid light and air contact that may cause decomposition of sodium hypochlorite.

[0060] Post-treatment process: (1) Cleaning liquid discharge: Use forward discharge method, open the bottom drain valve to completely drain the cleaning liquid. During the discharge process, introduce nitrogen purge (0.08MPa) to ensure that there is no residue. (2) Clean water flushing: Use reverse osmosis (RO) produced water with a conductivity of ≤5μS / cm for flushing. The flushing flow rate is controlled at 2.0m / s, and a circulation-discharge alternating mode is adopted (circulation for 5 minutes, discharge for 30 seconds). (3) Termination condition: Real-time monitoring of the pH value of the discharged water. When the pH is ≤8.0 for three consecutive tests and the fluctuation range is ≤0.5, stop flushing and at the same time detect ORP ≤200mV to ensure that the oxidant is completely removed.

[0061] Quality verification: After flushing, the membrane flux test was carried out. The flux of the hollow fiber membrane module was 35L / (m2·h) before cleaning and 46L / (m2·h) after cleaning. The initial flux of the module was 50L / (m2·h). The flux recovery rate of the membrane module after cleaning was 92%.

[0062] Implementation effect: This cleaning scheme is a traditional alkaline chemical cleaning scheme. Pollutants in river water mainly exist in the form of dissolved organic matter (DOM) and colloidal organic matter. Traditional alkaline chemical cleaning (usually using alkaline cleaning agents such as NaOH) can effectively decompose organic pollutants on the membrane surface through saponification, hydrolysis and other actions, especially for the removal of biological macromolecules such as proteins and polysaccharides, as well as lipid substances. Compared with traditional acid cleaning (such as HCl, H2SO4, etc.), alkaline cleaning can more efficiently destroy the structure of the microbial membrane, and the component flux recovery effect is better, but the cleaning effect is inferior to the cleaning effect of the new alkaline cleaning process. The main reason is that there is a lack of physical auxiliary means in the traditional cleaning process, and the contact area between the pollutants and the chemical cleaning solution is limited, resulting in incomplete reaction between the two and poor cleaning effect. Long-term penetration soaking and water flushing are required to wash away the pollutants. However, long-term use of high-concentration alkali solution may lead to hydrolysis or oxidation of the membrane material.

[0063] like Figure 5 As shown in the electron microscope image of the membrane filament after cleaning, a small amount of contaminants remain on the surface of the membrane filament. Figure 2 Electron micrographs of the membrane fibers after acid cleaning are similar to those of the new type, further demonstrating that combined air-water ultrasonic cleaning, through the synergistic effects of cavitation, high-frequency vibration, and hydraulic scouring, can deeply disrupt the binding force between contaminants and the membrane surface, achieving highly effective removal of membrane fouling. Its cleaning performance is comparable to, and even partially surpasses, traditional chemical cleaning methods, while also avoiding the potential damage to membrane materials caused by strong acids and bases, demonstrating significant technical advantages and engineering application value.

[0064] Table 1 Comparison of performance of different cleaning processes

[0065]

[0066] Table 1 compares key performance parameters of membrane modules with the same degree of contamination before and after treatment with different cleaning methods (new acid / alkaline vs. traditional acid / alkaline). From the perspective of cleaning efficiency, the new acid cleaning method reduces operating time by approximately 17% compared to traditional acid cleaning processes, while maintaining equivalent cleaning effectiveness. The new alkaline cleaning technology demonstrates a significant advantage, saving nearly 35% in time costs compared to traditional alkaline cleaning. Regarding flux recovery performance, despite the primary contaminants in the treated modules being organic matter, modules using the new acid cleaning process demonstrate excellent flux recovery, with a significantly higher recovery rate than traditional acid cleaning. Particularly noteworthy is that the new alkaline cleaning process achieves a 6 percentage point improvement in flux recovery compared to traditional alkaline cleaning, demonstrating the highly effective removal of various contaminants using the combined "bubble agitation + high-frequency overall vibration" cleaning mechanism. In terms of operational stability, components treated with the new cleaning process exhibited excellent pressure drop characteristics: test data showed that their transmembrane pressure (TMP) reduction rate generally exceeded 90%, and the stable operating TMP was maintained at an ideal level below 5kPa. In contrast, components treated with the traditional cleaning process not only had high operating TMP values ​​but also frequently experienced pressure fluctuations, reflecting the limitations of traditional cleaning methods in completely removing contaminants. These comparative data strongly demonstrate the comprehensive advantages of the new cleaning process in terms of cleaning efficiency, flux recovery, and operational stability.

[0067] Figure 1The following is a diagram showing the membrane filament inspection results before cleaning. Samples of contaminated modules were taken for testing and microscopic morphology analysis before cleaning. As shown in Figure B, the membrane filament surfaces within the module were yellow, indicating the presence of contaminant deposition. A large number of loose, flocculent contaminants were found adhering to the membrane filament surfaces. These flocculents, with their fibrous, interwoven structures, covered the membrane filament surfaces and penetrated deep into the membrane pores, causing varying degrees of blockage. Further observations using a scanning electron microscope (SEM), as shown in Figure A (an electron micrograph of the membrane filament's outer surface), revealed that these floccules were primarily composed of microbial metabolites, organic colloids, and inorganic particles. Their dense network structure not only significantly increased the membrane filtration resistance but also severely affected the recovery of membrane flux.

[0068] Figure 2 、 Figure 3 、 Figure 4 、 Figure 5 The SEM comparison of the microscopic morphology of the outer surface of the membrane after different cleaning methods. Figure 2 、 Figure 3 The two groups are the surface morphologies of the membrane fibers after being treated with a new cleaning process (bubble agitation + high-frequency vibration). Figure 2 This is an electron microscope image of the outer surface of the membrane after the new acid cleaning. Figure 3 This is an electron microscope image of the outer surface of the membrane after the new alkaline cleaning. Figure 4 、 Figure 5 The two groups are control samples after traditional acid and alkali cleaning respectively. Through comparative analysis, it can be clearly observed that the membrane fibers treated by traditional cleaning ( Figure 4 、 Figure 5 ) still has a large amount of dense pollutant adhesion layer on the surface, showing obvious particle accumulation and organic film-like covering structure, indicating that the traditional cleaning method has failed to effectively remove the pollutant layer; while the membrane filaments treated by the new cleaning process ( Figure 2 、 Figure 3 ) Surface contamination is significantly reduced, especially Figure 3 The samples showed a nearly clean membrane surface structure, with only scattered residual particles. This comparison result fully demonstrates that the new alkaline cleaning process, through its unique bubble cavitation effect and the synergistic effect of high-frequency mechanical vibration, can deeply destroy the binding force between contaminants and the membrane surface, effectively removing the organic-inorganic composite contamination layer on the membrane surface, and has significant cleaning advantages.

[0069] As can be seen from the above embodiments, the present invention provides a method for cleaning a hollow fiber composite membrane assembly. The present invention forms a triple decontamination mechanism by coupling gas-water mixed cleaning, high-frequency vibration and dynamic forward and backwashing. A venturi self-priming nozzle is used to generate a micron bubble group containing 15-25% gas, combined with 20-40kHz ultrasonic waves, so that the membrane filaments produce micro-vibrations that reduce the adhesion energy by more than 65%. An axial-radial composite flow field is formed by alternating forward and backwashing for 5-30s, making the shear rate 2-3 times higher than the traditional one. Experiments show that when this method treats wastewater with turbidity>200NTU, the alkaline cleaning time is shortened from 11-12 hours to 1-1.5 hours, and the acid chemical cleaning time is shortened from 1-2 hours to 10-30 minutes. The flux recovery rate is 95-98%, the membrane pore pollutants are reduced by 82%, and it can operate continuously for 2000 times without attenuation, which is suitable for the treatment of high turbidity wastewater.

[0070] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for cleaning a hollow fiber composite membrane module, characterized in that: The following steps are involved: 1) Air-water mixed cleaning: Gas and cleaning liquid are introduced into the hollow fiber composite membrane module simultaneously through a Venturi self-priming nozzle to form a gas-liquid two-phase flow containing micron-sized bubbles for air-water mixed cleaning; 2) Synchronous high-frequency vibration: Ultrasonic vibration is applied to the composite membrane assembly during the air-water mixing and cleaning process; 3) Alternating forward and reverse flushing: After the air-water mixed cleaning is completed, empty the remaining cleaning liquid and re-inject the cleaning liquid, and perform forward and back flushing alternately. The alternating frequency is 5 to 30 seconds per time, and the single alternating flushing time is 10 to 15 minutes; 4) Discharge and drying: After cleaning, discharge the pollutants and introduce dry gas to purge the hollow fiber composite membrane assembly.

2. The method for cleaning a hollow fiber composite membrane assembly according to claim 1, wherein: In the step 1), the volume ratio of gas to cleaning liquid is 1:5-10, the cleaning liquid includes acid cleaning liquid or alkaline cleaning liquid, and the gas-water mixture is left to soak for 10-50 minutes after cleaning.

3. The method for cleaning a hollow fiber composite membrane module according to claim 1 or 2, characterized in that: When the cleaning liquid is an acid cleaning liquid, the cleaning time is 10 to 30 minutes, and the acid cleaning liquid contains 0.5 to 1.5 wt % of citric acid and 0.1 to 0.5 wt % of hydrochloric acid.

4. The method for cleaning a hollow fiber composite membrane module according to claim 3, wherein: When the cleaning liquid is an alkaline cleaning liquid, the cleaning time is 1 to 1.5 hours, the alkaline cleaning liquid includes 100 to 300 ppm sodium hypochlorite and 50 to 400 ppm sodium hydroxide, and the cleaning temperature of the alkaline cleaning liquid is 35 to 45°C.

5. The method for cleaning a hollow fiber composite membrane module according to claim 1, 2 or 4, characterized in that: The frequency of the ultrasonic vibration is 15 to 50 kHz and the power is 2000 to 5000 W, causing the membrane filament to generate micro-vibration of 0.01 to 0.1 mm.

6. The method for cleaning a hollow fiber composite membrane module according to claim 5, characterized in that: The ultrasonic vibration is achieved by an array-type ultrasonic transmitter, which is evenly distributed on the membrane component housing at an incident angle of 30° to 60°, and the vibration energy acts on the membrane filaments through solid conduction.

7. The method for cleaning a hollow fiber composite membrane module according to claim 1 or 6, characterized in that: The pressure difference between the forward flushing and the back flushing is maintained at 0.4 to 0.8 bar, and a pulsed pressure fluctuation is generated each time the pressure is switched, with the fluctuation amplitude being 15 to 25% of the working pressure.

8. The method for cleaning a hollow fiber composite membrane module according to claim 1, wherein: The working pressure of the Venturi self-priming nozzle is 0.2-0.5 MPa.

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