Membrane cleaning method and water treatment device

Abstract

To provide a membrane cleaning method and a water treatment apparatus using this method capable of controlling adverse effect on water treatment and ensure the effectiveness of membrane cleaning.SOLUTION: After the Membrane Bioreactor using the filtration membrane 21, a cleaning solution of nanobubble water containing bubbles of less than 1 μm in size is passed through the filtration membrane 21 in the opposite direction of the direction in which the treated water is passed in the MBR. At this time, the nanobubbles 61 enter between the filtration membrane 21 and the fouling material 100, and the degree of adhesion between the fouling material 100 and the filtration membrane 21 is reduced. On the other hand, the reaction tank 10 contains 40 carriers in suspension. Therefore, after the cleaning solution passes through the filtration membrane 21, the filtration membrane 21 is immersed in the suspension containing the carriers 40. When such carriers 40 strike the fouling material 100, the surface or pores of the filtration membrane 21, the fouling material 100, whose adhesion is reduced by the nanobubbles 61, is diverted from the filtration membrane 21.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a membrane cleaning method and a water treatment device using the same. [Background technology]

[0002] Conventionally, methods for performing water treatment using filtration membranes have been used. Patent Document 1 is one example. According to Patent Document 1, raw water (water to be treated) that has undergone biological treatment is filtered through a membrane (filtration membrane). The filtered water is stored in a filtrate tank. Meanwhile, the filtered water stored in the filtrate tank is also used for backwashing the membrane. Backwashing is a process in which filtered water is passed through a membrane in the opposite direction to the filtration of the raw water, thereby removing substances that have adhered to the membrane due to the filtration of the raw water. In Patent Document 1, a chemical agent is added to the filtered water during backwashing to improve the effectiveness of membrane cleaning. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-202260 Summary of the Invention [Problem to be solved by the invention]

[0004] As mentioned above, methods for enhancing the effectiveness of backwashing, such as using chemicals, have been implemented. However, for example, when backwashing is performed using chemicals, the chemical action of the chemicals may adversely affect the environmental conditions of the water treatment (such as microbial activity and post-treatment water quality). Thus, even if the membrane can be effectively cleaned by backwashing, depending on the backwashing method, subsequent water treatment may not be carried out properly.

[0005] An object of the present invention is to provide a membrane cleaning method that can suppress adverse effects on water treatment and ensure the effectiveness of membrane cleaning, and a water treatment device using the same. [Means for solving the problem]

[0006] The membrane cleaning method of the present invention is a method for cleaning a filtration membrane consisting of at least one of a microfiltration membrane and an ultrafiltration membrane after performing water treatment using the filtration membrane, and includes a backwashing step in which a liquid containing bubbles less than 1 μm in size is passed through the filtration membrane in a direction opposite to the direction in which the water to be treated is passed during the water treatment.

[0007] In addition, a water treatment device according to another aspect of the present invention is a device that performs water treatment by passing water to be treated through a filtration membrane consisting of at least one of a microfiltration membrane and an ultrafiltration membrane, and is equipped with a bubble generation unit that generates bubbles less than 1 μm in size, and a backwash unit that passes a liquid containing the bubbles generated by the bubble generation unit through the filtration membrane in a direction opposite to the direction in which the water to be treated is passed during the water treatment.

[0008] According to the present invention, bubbles less than 1 μm in size penetrate between the membrane and substances adhering to the filtration membrane, making it easier to peel off the substances. This makes it possible to enhance the effectiveness of backwashing. Furthermore, unlike other methods that use chemicals, methods that use bubbles tend to be less likely to affect the environmental conditions of water treatment, such as using air bubbles. Furthermore, even when using both chemicals and bubbles, the amount of chemical used can be reduced by the amount that the bubbles enhance the backwashing effect. Therefore, it is easier to suppress the impact on water treatment compared to using chemicals alone.

[0009] In addition, in the present invention, the water treatment may be a treatment based on a membrane bioreactor. The membrane bioreactor involves treating wastewater using microorganisms. Even in such a case, the present invention, which uses air bubbles, does not affect the activity of the microorganisms.

[0010] In the present invention, it is also preferable that the filtration membrane after the passage of the liquid containing the bubbles is immersed in a liquid containing a carrier that contacts and removes fouling substances adhering to the filtration membrane. In this way, the carrier comes into contact with the fouling substances that have been made easier to peel off by the bubbles, thereby enabling more effective removal of the fouling substances.

[0011] In the present invention, it is also preferable that the bubbles contain ozone, which can further enhance the cleaning effect of the filtration membrane by the bubbles. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic configuration diagram of a water treatment device according to an embodiment of the present invention. [Figure 2] (a) A diagram showing fouling substances adhering to a ceramic flat membrane, (b) A diagram showing nanobubbles entering between the ceramic flat membrane and fouling substances, and (c) A diagram showing the carrier peeling off fouling substances. [Figure 3] 1 is a graph showing TMP during continuous operation of an MBR at 50 LMH while backwashing was performed using tap water, nanobubble water (NB water), or a sodium hypochlorite aqueous solution (NaClO). [Figure 4] 1 is a graph showing TMP during continuous operation of an MBR at 60 LMH while backwashing was performed using tap water, nanobubble water, or a sodium hypochlorite aqueous solution. [Figure 5] 1 is a graph showing DOC when tap water, nanobubble water, or an aqueous sodium hypochlorite solution is added to activated sludge. [Figure 6] FIG. 1 shows an EEM spectrum when tap water is added to activated sludge. [Figure 7] FIG. 1 shows an EEM spectrum when nanobubble water is added to activated sludge. [Figure 8] FIG. 1 shows an EEM spectrum when an aqueous sodium hypochlorite solution is added to activated sludge. [Figure 9] 1 is a graph showing the TMP during continuous operation of an MBR at 80 LMH while backwashing is performed using nanobubble water, a sodium hypochlorite aqueous solution, and a sodium hypochlorite aqueous solution containing nanobubbles (NB+NaClO). DETAILED DESCRIPTION OF THE INVENTION

[0013] A water treatment device 1 that uses a membrane cleaning method according to one embodiment of the present invention will be described with reference to Figures 1 and 2. The water treatment device 1 is a device that treats water based on an MBR (Membrane Bioreactor, a membrane separation activated sludge process). There are no particular limitations on the type of water to be treated by the water treatment device 1. Examples include sewage, industrial wastewater, domestic wastewater, and livestock wastewater. Note that water to be treated that has been subjected to pretreatment (such as microbial treatment) other than MBR treatment of raw water such as sewage may also be further treated by the water treatment device 1.

[0014] The water treatment device 1 includes a reaction tank 10, pipes 11 and 13, a membrane module 20, an aeration pipe 30, a carrier 40, and a backwash unit 50.

[0015] The reaction tank 10 contains a suspension containing activated sludge. The reaction tank 10 is connected to a pipe 11 equipped with a pump 12. The pipe 11 is connected to a supply source (raw water tank, etc.) of the water to be treated. In the reaction tank 10, organic matter in the water to be treated is decomposed by microorganisms contained in the activated sludge in the suspension.

[0016] The membrane module 20 includes a filtration membrane 21 and a water collection pipe 22. One or more membrane modules 20 are immersed in the suspension within the reaction tank 10. The filtration membrane 21 is a flat membrane. The filtration membrane 21 is at least one of a microfiltration membrane and an ultrafiltration membrane formed from an inorganic ceramic membrane or an organic membrane such as cellulose acetate, polyethylene, or polyamide. Depending on the characteristics of the water to be treated and the quality of the treated water, either a microfiltration membrane or an ultrafiltration membrane may be used, or both. The filtration membrane 21 has a large number of pores and an internal flow path through which liquid that passes through the pores from the outside of the filtration membrane 21 joins. By passing the suspension through these pores, the liquid undergoes solid-liquid separation. The filtration membrane 21 is connected to a water collection pipe 22. The space within the water collection pipe 22 is connected to the internal flow path of the filtration membrane 21. The liquid that has passed through the pores flows into the space within the water collection pipe 22 through the internal flow path of the filtration membrane 21. The water collection pipe 22 is connected to a pipe 13 equipped with a pump 14. The pipe 13 is connected to another treatment tank, a storage tank, or the like, which is provided downstream of the reaction tank 10. The size and number of membrane modules 20 are appropriately selected depending on the size of the reaction tank 10, the amount of water to be treated per unit time, etc.

[0017] The aeration pipe 30 is a pipe made of metal, resin, or the like, with holes or slits formed therein. The aeration pipe 30 is disposed on the inner bottom of the reaction tank 10, directly below the membrane module 20. The aeration pipe 30 is connected to an aeration blower (not shown) that supplies air. Air is supplied from the aeration blower to the aeration pipe 30, and air bubbles 31 are released into the suspension through the holes or slits. The oxygen in the air bubbles 31 allows microorganisms in the activated sludge to decompose organic matter in the water to be treated and suppresses clogging of the membrane (membrane fouling). The size and number of the aeration pipes 30 are selected appropriately depending on the aeration rate, the effective volume of the reaction tank 10, the size and number of the membrane modules 20, etc.

[0018] The carrier 40 is contained in the suspension contained in the reaction vessel 10. The carrier 40 is granular or sponge-like. The shape of the carrier 40 is a cylinder, a rectangular pillar, a sphere, or the like. The material of the carrier 40 is a synthetic resin such as polyethylene glycol, a wood-based material, or the like. The size of the carrier 40 is preferably 1 mm to 10 mm. The amount of the carrier 40 used is preferably about 1% to 30% of the effective volume of the reaction vessel 10. The material, size, and amount of the carrier 40 used are appropriately adjusted to enhance the backwashing effect described below.

[0019] The backwashing unit 50 includes a pipe 51, a pump 52, a nanobubble generator 55 (a bubble generating unit in the present invention), and a cleaning liquid storage tank 56. The cleaning liquid storage tank 56 stores a cleaning liquid for backwashing (hereinafter referred to as the cleaning liquid). For example, tap water may be used as the cleaning liquid. The cleaning liquid storage tank 56 is connected to a portion of the pipe 13 between the water collection pipe 22 and the pump 14 via a pipe 51 provided with a pump 52. The pipe 51 guides the cleaning liquid from the cleaning liquid storage tank 56 to the pipe 13. The nanobubble generator 55 is a device that generates bubbles (hereinafter referred to as nanobubbles 61) with a particle size of 1 nm or more and 1 μm or less in the cleaning liquid in the pipe 51. The bubbles are made of gas such as air, oxygen, or ozone. The particle size of the bubbles is such that they can pass through the pores of the microfiltration or ultrafiltration membrane used. Because the nanobubbles 61 have an extremely small particle size, they rise slowly and remain in the reaction vessel 10 for a long time. The method for generating nanobubbles is not particularly limited, and examples include a method in which a gas is dissolved in a liquid, the pressure is reduced, and the insoluble gas is precipitated in the form of bubbles; a method in which bubbles are generated by pulverizing a gas-containing liquid using a physical swirling force or the like; and a method in which bubbles are generated by ejecting gas from pores into a liquid. Depending on the membrane used as the filtration membrane 21, a method for generating bubbles with a particle size that can pass through pores formed in the membrane is appropriately selected. In addition to the nanobubbles 61, the cleaning liquid may contain bubbles with a particle size of less than 1 nm or 1 μm or more.

[0020] Water treatment device 1 is used as follows. First, pump 12 is driven to cause the water to be treated to flow into reaction tank 10 through pipe 11 and mix with the suspension in reaction tank 10. In reaction tank 10, organic matter and the like in the water to be treated are decomposed by microorganisms contained in the activated sludge of the suspension. Next, pump 14 is driven to apply negative pressure to filtration membrane 21 through water collection pipe 22, causing the suspension mixed with the water to be treated in reaction tank 10 to permeate filtration membrane 21 and undergo solid-liquid separation. The permeate after solid-liquid separation is sent through pipe 13 to a downstream treatment tank or the like.

[0021] Continuing to treat the target water using an MBR as described above will result in membrane fouling, in which fouling substances 100 adhere to the surface of the filtration membrane 21 or within the pores of the filtration membrane 21, as shown in Figure 2(a). The fouling substances 100 will reduce the filtration performance of the filtration membrane 21.

[0022] Therefore, before membrane fouling causes excessive degradation of water treatment performance (for example, periodically), a backwashing process for the membrane module 20 is carried out as follows. The operation of pumps 12 and 14 is temporarily stopped, and the operation of pump 52 is started. As a result, the cleaning liquid in the cleaning liquid storage tank 56 is sent from pipe 51 to pipe 13. Meanwhile, the nanobubble generator 55 generates nanobubbles 61 in the cleaning liquid in pipe 51. The cleaning liquid containing the nanobubbles 61 passes from pipe 51 to pipe 13 and the water collection pipe 22, passes through the pores of the filtration membrane 21, and flows out into the suspension in the reaction tank 10. In other words, the cleaning liquid is passed through the pores of the filtration membrane 21 in the opposite direction to the direction in which the water to be treated passes in the MBR.

[0023] At this time, nanobubbles 61 in the cleaning solution pass through the pores of the filtration membrane 21 and reach the vicinity of the openings of the pores where the fouling substances 100 are attached. As shown in FIG. 2(b), these nanobubbles 61 get between the filtration membrane 21 and the fouling substances 100, reducing the degree of adhesion between the fouling substances 100 and the filtration membrane 21. Meanwhile, the filtration membrane 21 is immersed in a suspension containing the carrier 40. As a result, after the cleaning solution passes through the filtration membrane 21, the carrier 40 collides with the fouling substances 100 or the surface or pores of the filtration membrane 21, as shown in FIG. 2(c), and the fouling substances 100, whose degree of adhesion has been reduced by the nanobubbles 61, are separated from the filtration membrane 21.

[0024] According to the water treatment device 1 described above, nanobubbles 61 with a size of less than 1 μm penetrate between the filtration membrane 21 and the fouling substances 100 adhering to the filtration membrane 21, making it easier to peel off the fouling substances 100. This makes it possible to enhance the effectiveness of backwashing. Furthermore, unlike other methods that use chemicals, the method using nanobubbles 61 uses nanobubbles 61 made of air or the like, which tends to have little effect on the activity of microorganisms in the suspension in the reaction tank 10. Furthermore, even when using a chemical and nanobubbles 61 in combination, for example, it is easier to suppress the impact on the MBR by reducing the amount of chemical used compared to using only the chemical.

[0025] Furthermore, the filtration membrane 21 is immersed in the suspension in the reaction tank 10 containing the carrier 40. Since backwashing is performed in this state, after the cleaning liquid passes through the filtration membrane 21 during backwashing, the filtration membrane 21 remains immersed in the suspension in the reaction tank 10 containing the carrier 40. This allows the carrier 40 to come into contact with the fouling substances 100 that have been made more likely to peel off using the nanobubbles 61, thereby effectively removing the fouling substances 100 from the filtration membrane 21. In this embodiment, the treatment of the water to be treated is performed with the carrier 40 present in the reaction tank 10. Therefore, even during treatment of the water to be treated, the membrane surface of the filtration membrane 21 is physically cleaned by the carrier 40, thereby suppressing the occurrence of membrane fouling.

[0026] [Example] Examples relating to the above-described embodiment will be described below. An MBR device according to this example (corresponding to the water treatment device 1 in the above-described embodiment) was installed at a treatment facility (Sosei River Water Reclamation Plaza) in Sapporo, and the facility's initial settling influent was used as the water to be treated. Excess sludge collected from another MBR device installed at the same facility was used as the activated sludge. The reaction tank 10 used had an effective volume of 7.5 L. The liquid temperature in the reaction tank 10 was 14°C. Air stones were used in the aeration pipe 30, and constant aeration was performed at an aeration rate of 12 L / min. Cylindrical granular carriers made of polyethylene glycol were used as the carriers 40. The amount of carriers 40 used was 5 v / v % of the effective volume of the reaction tank 10. The filtration membrane 21 had a membrane pore size of 0.1 μm and a membrane area of ​​0.02 mm. 2 Two ceramic flat membranes (Meidensha Corporation) were installed in the reactor 10. The filtration conditions were a flux of 50 LMH (L / m 2 The filter was operated intermittently with a filter load of 9 minutes per hour, an SRT (Solid Retention Time) of 20 days, and a one-minute pause in filtration. Backwashing was performed for one hour every six hours at a flux of 8 LMH. The cleaning solutions used in the backwashing were ordinary tap water, ordinary tap water in which air-based nanobubbles 61 were generated using a nanobubble generator 55 (ultrafineGaLF, IDEC Corporation) (hereafter referred to as nanobubble water), and a sodium hypochlorite solution (50 ppm) that did not contain nanobubbles 61. The nanobubble water contained approximately 90 million nanobubbles 61 per mL.

[0027] [Experimental Example 1] (MBR with flux 50LMH and backwashing with normal tap water, nanobubble water, or sodium hypochlorite solution) As in the above-mentioned examples, a continuous treatment experiment was conducted for three days using an MBR with backwashing using ordinary tap water (hereafter referred to as tap water), nanobubble water, and sodium hypochlorite aqueous solution. Figure 3 shows the time-dependent changes in TMP (trans-membrane pressure) for each cleaning solution. In the MBR using tap water for backwashing, TMP rose very quickly, and it became difficult to continue stable operation of the MBR after two days, leading to the shutdown of the MBR. In the MBR backwashing using sodium hypochlorite aqueous solution or nanobubble water, TMP decreased when backwashing was performed every six hours, suppressing the increase in TMP. Furthermore, a similar experiment using ozone bubbles as nanobubble water showed even more suppressed TMP increase compared to when air bubbles were used.

[0028] [Experimental Example 2] (Flux 60LMH MBR and backwashing with tap water, nanobubble water, or sodium hypochlorite solution) The experiment was conducted in the same manner as in Example 1, except that the operating conditions for the MBR in the above experimental example were changed to a flux of 60LMH. Figure 4 shows the change in TMP over time for each cleaning solution. Compared to Figure 3, the rate at which TMP rose was faster. When backwashing was performed using tap water, it became difficult to continue stable operation of the MBR equipment after one day, and operation was stopped. On the other hand, when backwashing was performed every six hours in the MBR using a sodium hypochlorite aqueous solution or nanobubble water, TMP decreased and the rise in TMP was suppressed, as in Figure 3.

[0029] [Experimental Example 3] (Effects of tap water, nanobubble water, or sodium hypochlorite solution on activated sludge) To 190 ml of the activated sludge used in the above examples, 8.1 ml of tap water, nanobubble water, or sodium hypochlorite aqueous solution as in Experimental Example 1 was added. The mixture was then stirred for 1 hour. The mixture was then centrifuged, and the supernatant was filtered through a 0.45 μm filter membrane. The filtrate was then collected, and the DOC (Dissolved Organic Carbon) concentration and EEM (Excitation-Emission Matrix) were measured.

[0030] As shown in Figure 5, the DOC concentration was 18.1 mg / ml when tap water was added to activated sludge. The DOC concentration was 15.8 mg / ml when nanobubble water was added to activated sludge. The DOC concentration was 20.9 mg / ml when nanobubble water was added to activated sludge. When sodium hypochlorite was added to activated sludge, the DOC concentration increased compared to tap water. The increased DOC caused by adding sodium hypochlorite to activated sludge may promote membrane fouling. When nanobubble water was added to activated sludge, the DOC concentration did not increase compared to tap water. Furthermore, the DOC concentration was lower in nanobubble water than in tap water. To investigate the effects of trace amounts of residual chlorine and measurement error, additional experiments were conducted using excess sludge collected from a different MBR unit. No clear difference in DOC concentration was observed when tap water or nanobubble water was added to activated sludge. Nanobubbles may not promote membrane fouling because they do not cause an increase in DOC when mixed with activated sludge.

[0031] The EEM spectra shown in Figures 6 to 8 indicate the presence of proteins (peaks within the excitation wavelength range of 200-250 nm and fluorescence wavelength range of 230-380 nm), SMPs (soluble microbial metabolites) (peaks within the excitation wavelength range of 250-280 nm and fluorescence wavelength range of 250-380 nm), and humic substances (peaks within the excitation wavelength range of 280-500 nm and fluorescence wavelength range of 380-500 nm). Comparing Figures 6 and 8, the addition of sodium hypochlorite solution to activated sludge resulted in an increase in SMP peak intensity compared to tap water. Comparing Figures 6 and 7, the addition of nanobubble water to activated sludge did not result in any change in peak intensity due to SMP compared to tap water. In other words, the EEM analysis results also indicate that nanobubbles have little effect on activated sludge.

[0032] [Experimental Example 4] (MBR with flux 80LMH and backwashing with nanobubble water, sodium hypochlorite solution, or sodium hypochlorite solution containing nanobubbles) A three-day continuous treatment experiment was conducted on an MBR with backwashing every six hours using nanobubble water, a sodium hypochlorite solution containing no nanobubbles 61, and a sodium hypochlorite solution containing nanobubbles 61. Figure 9 shows the time course of TMP for each cleaning solution. The MBR backwashed with a sodium hypochlorite solution containing nanobubbles 61 exhibited a more effective reduction in TMP and suppressed increases in TMP than the MBR backwashed with a sodium hypochlorite solution containing no nanobubbles 61. The results of this experiment demonstrated that the cleaning effect of backwashing can be enhanced by using a sodium hypochlorite solution containing nanobubbles 61 as the cleaning solution. This demonstrates that not only can nanobubbles 61 improve the effectiveness of backwashing, but further improvements can be expected by combining nanobubbles 61 with a chemical (sodium hypochlorite).

[0033] As shown by the results of Experimental Examples 1, 2, and 4, by combining nanobubbles 61 with either or both of a physical cleaning method using a solid object such as the carrier 40 and a cleaning method using a chemical agent, it is possible to realize a membrane cleaning method or a water treatment device with a higher backwashing effect.

[0034] <Other variations> The above is a description of a preferred embodiment of the present invention, but the present invention is not limited to the above-described embodiment, and various modifications are possible within the scope of the means for solving the problems.

[0035] In the above-described embodiment, backwashing is performed using nanobubble water and a carrier, but backwashing may also be performed by further mixing in a sodium hypochlorite aqueous solution. Other chemicals typically used for backwashing may also be used. Even when chemicals are used in combination in this way, the use of nanobubble water enhances the backwashing effect, allowing the amount of chemical used to be reduced.

[0036] In the above-described embodiment, a flat membrane is used as the filtration membrane 21. However, the filtration membrane may be of any suitable form, such as a hollow fiber type or a tubular type. Furthermore, the filtration membrane may be incorporated in any suitable form, such as a sheet type, spiral type, tubular type, or monolith type, depending on the type of the filtration membrane.

[0037] In the above-described embodiment, the backwashing effect is enhanced by the nanobubbles 61 contained in the cleaning liquid and the carriers 40 in the suspension in the reaction tank 10. However, as described above, since the fouling substances 100 are easily peeled off by the nanobubbles 61, it is possible to enhance the backwashing effect even without the carriers 40 in the suspension, compared to when a cleaning liquid not containing nanobubbles 61 is used.

[0038] The carrier 40 used in the above-described embodiment is intended to enhance the effect of backwashing when used in combination with the nanobubbles 61. In this regard, the carrier 40 may play only this role, or may play other roles as well. For example, in addition to the role in the above-described embodiment, the carrier 40 may or may not play the role of retaining microorganisms in the suspension.

[0039] In the above-described embodiment, tap water is used as the cleaning liquid for backwashing. However, the permeated water that has permeated the membrane module 20 may be stored and used as the cleaning liquid.

[0040] In the above-described embodiment, the nanobubbles 61 are formed of gases such as air, oxygen, and ozone. In this case, the bubbles may be formed of air, oxygen, or ozone alone, or may be formed of a combination of these gases. Furthermore, multiple types of bubbles formed of different gases may be used simultaneously as the nanobubbles 61.

[0041] The water treatment device 1 according to the above-described embodiment performs water treatment based on an MBR. However, the present invention may also be applied to other water treatment processes that use microfiltration membranes or ultrafiltration membranes. For example, the present invention may be applied to filtration processes that use microfiltration membranes or ultrafiltration membranes in various industrial wastewater treatment processes, pretreatment for seawater desalination, water treatment for water purification, and the like. [Explanation of symbols]

[0042] 1 Water treatment equipment 21 Filtration membrane 40 Carrier 50 Backwash section 55 Nanobubble Generator 100 Fouling substances

Claims

1. A method for cleaning a filtration membrane after performing water treatment using a filtration membrane consisting of at least one of a microfiltration membrane and an ultrafiltration membrane, A membrane cleaning method characterized by carrying out a backwashing process in which the filtration membrane is immersed in a liquid containing a carrier that comes into contact with and removes fouling substances adhering to the filtration membrane, and a liquid containing bubbles less than 1 μm in size is passed through the filtration membrane in a direction opposite to the direction in which the water to be treated is passed in the water treatment.

2. 2. The membrane cleaning method according to claim 1, wherein the water treatment is a treatment based on a membrane separation activated sludge method.

3. 3. The membrane cleaning method according to claim 1, wherein the bubbles contain ozone.

4. A membrane cleaning method described in any one of claims 1 to 3, characterized in that the filtration membrane is a ceramic flat membrane.

5. An apparatus for performing water treatment by passing water to be treated through a filtration membrane consisting of at least one of a microfiltration membrane and an ultrafiltration membrane, a bubble generating unit that generates bubbles having a size of less than 1 μm; a backwash unit that passes the liquid containing the bubbles generated by the bubble generating unit through the filtration membrane in a direction opposite to the direction in which the water to be treated is passed through in the water treatment, while the filtration membrane is immersed in a liquid containing a carrier that comes into contact with and removes fouling substances adhering to the filtration membrane.

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