JP7498046B2 - Filtration device for oil-containing wastewater and method for cleaning filtration membrane for oil-containing wastewater - Google Patents

Description

The present invention relates to a filtering device for oil-containing wastewater and a method for cleaning a filtering membrane for oil-containing wastewater.

A technology is known for filtering oil-containing wastewater to reduce its volume. One example of oil-containing wastewater is scrubber wastewater discharged from a ship's exhaust gas scrubber. Oil-containing wastewater may be highly viscous, and when filtered through a filtration membrane, oil-containing solid matter (also called cake) consisting of oil and suspended matter may adhere to the membrane surface. The oil-containing solid matter gradually compacts and membrane blockage progresses, so it is desirable to periodically clean the filtration membrane. Patent Document 1 discloses a method for cleaning a filtration membrane, which includes a step of immersing the filtration membrane in a mixed solution of chloric acid and a surfactant. Patent Document 2 discloses a method for cleaning a permeable membrane, which includes a step of immersing the permeable membrane in alkaline water containing a surfactant and then a step of immersing the permeable membrane in oxalic acid.

JP 2013-031839 A JP 2016-185514 A

In the case of a filtration membrane that has filtered a high concentration of oil-containing wastewater, the water permeability of the filtration membrane may not be fully restored by cleaning. One possible reason for this is that the oil in the oil-containing wastewater penetrates into the pores inside the filtration membrane and reaches the secondary side. In this case, simply soaking the filtration membrane as disclosed in Patent Documents 1 and 2 may not provide a sufficient cleaning effect.

The present invention aims to provide a filtering device for oil-containing wastewater with excellent cleaning properties and a method for cleaning the filtering membrane for oil-containing wastewater.

The oil-containing wastewater filtration device of the present invention includes a membrane immersion tank into which oil-containing wastewater is introduced, a filtration membrane that is introduced into the membrane immersion tank and immersed in the oil-containing wastewater to filter the oil -containing wastewater, a suction line connected to the secondary side of the filtration membrane, a suction pump provided in the suction line to suck the filtrate to the secondary side, a return line branched from the suction line downstream of the suction pump and connected to the membrane immersion tank, and an air supply means connected to the return line, and the suction line and the return line constitute a circulation line connected to the filtration membrane and the membrane immersion tank . The suction pump cleans the filtration membrane by passing a cleaning solution from the primary side of the filtration membrane to the secondary side , and the circulation line returns the cleaning solution that has passed through the secondary side of the filtration membrane to the membrane immersion tank. The air supply means pushes the cleaning solution remaining in the circulation line after cleaning the filtration membrane into the membrane immersion tank by supplying air to the circulation line.

The cleaning method for a filtration membrane for oil-containing wastewater of the present invention is a cleaning method for a filtration membrane in a filtration device having a membrane immersion tank into which oil-containing wastewater is introduced, a filtration membrane immersed in the oil-containing wastewater introduced into the membrane immersion tank to filter the oil-containing wastewater, a suction line connected to the secondary side of the filtration membrane, a suction pump provided in the suction line to suck the filtrate to the secondary side, a return line branched from the suction line downstream of the suction pump and connected to the membrane immersion tank, and an air supply means connected to the return line, the suction line and the return line constituting a circulation line connected to the filtration membrane and the membrane immersion tank . This cleaning method includes: cleaning the filtration membrane by passing a cleaning solution from the primary side of the filtration membrane to the secondary side with the suction pump; returning the cleaning solution that has passed through the secondary side of the filtration membrane to the membrane immersion tank through the circulation line; and pushing out the cleaning solution remaining in the circulation line after cleaning the filtration membrane into the membrane immersion tank by supplying air from the air supply means to the circulation line .

The present invention provides a filtering device for oil-containing wastewater with excellent cleanability and a method for cleaning the filtering membrane for oil-containing wastewater.

1 is a schematic diagram showing the configuration of a filtration device according to one embodiment of the present invention and the flow of oil-containing wastewater and treated water during normal operation. 2 is a schematic diagram showing the flow of backwash water during simple backwash and filtrate water during rinsing in the filtration device shown in FIG. 1 . FIG. 2 is a schematic diagram showing a flow of a cleaning liquid during cleaning in the filtration device shown in FIG. 1 . 1 is a graph showing the cleaning effects of Example 1 and Comparative Examples 1 and 2.

Below, an embodiment of the present invention will be described with reference to the drawings. The present invention is applied to a filtration device that filters oil-containing wastewater. Examples of oil-containing wastewater include scrubber wastewater discharged from ship exhaust gas scrubbers (not shown), as well as waste liquid containing oil cake discharged from food factories, and the present invention can be widely applied to liquids containing oil. Scrubber wastewater contains suspended solids (SS) in addition to oil. Hereinafter, oil and SS will be referred to as oil, etc.

Figure 1 shows a schematic diagram of a filtration device 1 according to one embodiment of the present invention. The thick lines indicate the flow of oil-containing wastewater and treated water during normal operation. The filtration device 1 for oil-containing wastewater has a membrane immersion tank 2 into which oil-containing wastewater is introduced and a filtration membrane 3 housed in the membrane immersion tank 2. The filtration membrane 3 is immersed in the oil-containing wastewater introduced into the membrane immersion tank 2 and filters the oil-containing wastewater. The filtration membrane 3 is a hollow fiber membrane filter, and multiple bundles of multiple hollow fiber membrane filters are provided between a common upper support part 4A and a lower support part 4B. The filtration membrane 3 is not covered by a casing or the like, and is exposed to the oil-containing wastewater in the membrane immersion tank 2. Figure 1 is a conceptual diagram and shows only two filtration membranes 3. The material of the filtration membrane 3 is not limited as long as it is highly hydrophilic, but examples include PTFE (polytetrafluoroethylene), PES (polyethersulfone), PAN (polyacrylonitrile), CA, etc. As described later, in this embodiment, an alkaline cleaning solution is used to clean the filtration membrane 3. Since the pH of the cleaning solution is 11 or more, an organic membrane that is resistant to alkali is preferable, and an organic membrane made of PTFE that is less likely to deteriorate even at a pH of 12 or more is particularly preferable. In addition, an immersion type filtration membrane 3 is more preferable because it has a lower possibility of pipe clogging than a module type filtration membrane.

A supply port 5 for the raw cleaning solution (chemical) is provided at the top of the membrane immersion tank 2. The raw cleaning solution in a container is supplied to the membrane immersion tank 2 from the supply port 5. A dilution solution supply line L8 for supplying a dilution solution for diluting the raw cleaning solution is connected to the side of the membrane immersion tank 2. The raw cleaning solution is diluted with the dilution solution to a predetermined concentration to become the cleaning solution. Although not shown, a supply line for the cleaning solution may be provided at the top or side of the membrane immersion tank 2. A mixture of a surfactant and caustic soda is used as the cleaning solution (chemical). There are ionic surfactants and nonionic surfactants, and ionic surfactants are classified into anionic surfactants, cationic surfactants, and amphoteric surfactants. In this embodiment, any type of surfactant can be applied, but nonionic surfactants or anionic surfactants are preferred from the viewpoint of affinity with oil and the like contained in oil-containing wastewater. Preferred nonionic surfactants are those whose main component is polyoxyalkylene alkyl ether (e.g., Cedran (manufactured by Sanyo Chemical Industries, Ltd.)), those whose main component is polyoxyethylene fatty acid diester or polyoxyethylene fatty acid monoester (e.g., IONET (manufactured by Sanyo Chemical Industries, Ltd.)), and preferred anionic surfactants are those whose main component is dodecyl dimethylamine oxide or sodium dodecyl sulfate (e.g., MCL-20 (manufactured by Sumitomo Electric Industries, Ltd.)). Also preferred are those that contain both nonionic and anionic surfactants (e.g., Pink Soap (manufactured by monotaRO)).

Oil-containing wastewater that comes into contact with the side wall of the filtration membrane 3 from the outside is filtered by the side wall of the filtration membrane 3, and oil, etc. remain on the outside of the side wall, and the filtered water from which oil, etc. has been removed or reduced enters the inner space of the filtration membrane 3. In the following description, the outer space of the filtration membrane 3 (hollow fiber membrane filter) in the membrane immersion tank 2 is referred to as the primary side space 6, and may also be referred to as the primary side. The inner space of the filtration membrane 3 is referred to as the secondary side space 7, and the secondary side space 7 and the area connected to the secondary side space 7 (such as the suction line L2) may also be referred to as the secondary side. The membrane immersion tank 2 has a volume sufficiently larger than the installation space of the filtration membrane 3, and the space between the membrane immersion tank 2 and the filtration membrane 3 is prevented from being blocked by the high-viscosity oil components contained in the oil-containing wastewater. The membrane immersion tank 2 is connected to an oil-containing wastewater supply line L1 that supplies oil-containing wastewater. One example of the oil-containing wastewater is scrubber wastewater discharged from a ship's scrubber device. Oil-containing wastewater is supplied from the oil-containing wastewater supply line L1 in an amount sufficient to immerse the filtration membrane 3 in the oil-containing wastewater.

The suction line L2 is connected to the secondary space 7 of the filtration membrane 3. The suction line L2 is provided with a suction pump 8 for sucking the filtrate water to the secondary side. The suction pump 8 is, for example, a vacuum pump. A filtrate line L3 branches off from the suction line L2, and a filtrate tank 9 is connected to the filtrate line L3. A first valve V1 is provided to the filtrate line L3. A backwash line L4 is connected to the filtrate tank 9. A backwash pump 10 for pumping the backwash water and a second valve V2 are provided to the backwash line L4. The backwash line L4 merges with the suction line L2 between the filtration membrane 3 and the suction pump 8. A drainage line L5 for discharging oil-containing wastewater from inside is provided at the bottom of the membrane immersion tank 2.

During normal operation, the filtration device 1 operates as follows. The first valve V1 and the second valve V2 are closed. Oil-containing wastewater from the exhaust gas scrubber is supplied to the membrane immersion tank 2 through the oil-containing wastewater supply line L1. The suction force of the suction pump 8 moves the oil-containing wastewater from the primary space 6 to the secondary space 7 and turns it into filtered water, and oil and other substances are captured by the filtration membrane 3. The filtered water introduced into the suction line L2 can be discharged outside the system after it is confirmed that it meets the specified water quality standards. By opening the first valve V1, a portion of the filtered water is stored in the filtered water tank 9 through the filtered water line L3 and used as backwash water.

In order to remove oil and other substances adhering to the membrane surface of the filtration membrane 3, it is preferable to perform simple backwashing during operation. Specifically, as shown in FIG. 2, the first valve V1 is closed, the second valve V2 is opened, and the suction pump 8 is stopped. After that, the backwash pump 10 is operated for a certain period of time, and the filtrate stored in the filtrate water tank 9 is supplied to the secondary side of the filtration membrane 3 through the backwash line L4, as shown by the thick line. Oil and other substances adhering to the primary side membrane surface of the filtration membrane 3 are peeled off and released into the primary side space 6. In this way, by repeating normal operation (filtration) and simple backwashing as a set, the filtration process can be performed while maintaining the performance of the filtration membrane 3. If the salt concentration of the filtrate water is high, for example, if the total dissolved solids (TDS) is 3% or more, salt may precipitate in the filtrate water line L3 or the backwash line L4, which does not have a steady flow. In this case, desalted water can be used as the backwash water.

However, since the effect of simple backwashing is limited, oil and the like will adhere to the membrane surface on the primary side of the filtration membrane 3 in the form of a cake over time. For this reason, in this embodiment, a cleaning means 11 for the filtration membrane 3 is provided to clean the filtration membrane 3. In FIG. 3, the thick line indicates the flow of the cleaning liquid during cleaning, that is, the circulation line CL through which the cleaning liquid circulates. The cleaning means 11 includes a suction line L2, a return line L6 branched from the suction line L2, and a suction pump 8 provided on the suction line L2. The suction pump 8 is shared during normal operation and cleaning, so the number of pumps does not increase in order to perform the cleaning of this embodiment. In this embodiment, the cleaning liquid is sucked by the suction pump 8 provided on the secondary side of the filtration membrane 3, so that the cleaning liquid can be reliably permeated through the filtration membrane 3 by negative pressure. The return line L6 is connected to the membrane immersion tank 2, that is, the primary side of the filtration membrane 3. The suction line L2 and the return line L6 form a circulation line CL that allows the cleaning liquid to permeate from the primary side to the secondary side of the filtration membrane 3 and returns the cleaning liquid that has permeated the secondary side of the filtration membrane 3 to the primary side. The return line L6 is provided with a third valve V3 and a fourth valve V4, which are closed during normal operation as shown in Figure 1 and during simple backwashing as shown in Figure 2, but are opened during cleaning. An air supply line L7 joins the return line L6 between the third valve V3 and the fourth valve V4.

The cleaning is performed in the following steps. First, the first valve V1 and the second valve V2 are closed, the backwash pump 10 is stopped, and the third valve V3 and the fourth valve V4 are opened. This forms the circulation line CL. After this step (or before or at the same time), the entire amount of oil-containing wastewater in the membrane immersion tank 2 is discharged from the drainage line L5. Next, the raw cleaning solution is supplied from the supply port 5 to the membrane immersion tank 2, and the dilution solution is supplied from the dilution solution supply line L8 to form a cleaning solution inside the membrane immersion tank 2. The filtration membrane 3 is immersed in the cleaning solution thus formed. By discharging the oil-containing wastewater in advance, a high concentration of the cleaning solution is brought into contact with the filtration membrane 3, and the cleaning effect of the filtration membrane 3 can be enhanced. It is also possible to discharge only a portion of the oil-containing wastewater and supply the membrane immersion tank 2 with a cleaning solution of the same amount as the discharged oil-containing wastewater.

Next, the suction pump 8 provided on the secondary side of the filtration membrane 3 is operated to circulate the cleaning liquid along the circulation line CL. The filtration membrane 3 is cleaned by passing the cleaning liquid from the primary side to the secondary side of the filtration membrane 3 with the suction pump 8. In the filtration membrane 3 that has treated oil-containing wastewater, oil and the like usually adhere to the membrane surface on the primary side. For this reason, the filtration membrane 3 has been conventionally cleaned by immersing the filtration membrane 3 in the cleaning liquid in the membrane immersion tank 2. However, the inventor has found that, in the filtration membrane 3 that has treated particularly high-concentration oil-containing wastewater, the oil and the like deforms and enters the pores of the filtration membrane 3, and then escapes to the secondary side, causing the inside of the pores and the secondary side to be contaminated with oil and the like. When the entire filtration membrane 3 is thus contaminated with oil and the like, it is difficult to efficiently remove the oil and the like that adheres to the inside of the pores and the secondary side with the conventional cleaning method that uses only immersion. In this embodiment, the cleaning liquid is forcibly passed from the primary side of the filtration membrane 3 to the secondary side by the suction force of the suction pump 8. As a result, the adhesion of the oil, etc., that has entered the pores of the filtration membrane 3 and the oil, etc., that has adhered to the membrane surface on the secondary side to the filtration membrane 3 is reduced, and the oil, etc., becomes easier to peel off. Since it is important that the cleaning liquid permeates the filtration membrane 3 from the primary side to the secondary side, that is, that the cleaning liquid contacts the membrane surface on the secondary side of the filtration membrane 3, the permeation flow rate of the cleaning liquid is not particularly limited and may be smaller than the filtration flow rate of the oil-containing wastewater through the filtration membrane 3 during normal operation. As a result, the power of the suction pump 8 can be reduced. In this embodiment, the cleaning liquid is circulated, so the amount of cleaning liquid can be reduced, but a one-through method in which used cleaning liquid is not reused may also be adopted.

In order to improve the cleaning efficiency, it is desirable to adjust the temperature of the cleaning liquid in the membrane immersion tank 2 to a range of preferably more than 30°C and less than 50°C, more preferably more than 35°C and less than 45°C. By adjusting the cleaning liquid to such a temperature range, the hydrolysis reaction of the cleaning liquid and the action of the surfactant are promoted. In addition, the effect of softening oil and the like and making it easier to peel off is also obtained. For this purpose, the filtration device 1 is equipped with a temperature adjustment means 12. The temperature adjustment of the cleaning liquid is usually performed by heating, although it depends on the temperature of the cleaning liquid. Therefore, the temperature adjustment means 12 that adjusts the temperature of the cleaning liquid is some kind of heat source or a heat exchanger connected to a heat source. In the case of a ship, in addition to heat sources related to the engine such as the heat of the engine itself, the heat of the engine cooling water, the heat of the exhaust, and the heat of the desulfurization wastewater, a boiler can be used as a heat source, so the heat exchanger method can be suitably applied. Desalted water or seawater can be used as the medium. In order to supply the membrane immersion tank 2 with a temperature-adjusted cleaning liquid in a short time, it is preferable to preheat the dilution liquid introduced into the membrane immersion tank 2. For this reason, in this embodiment, a heat exchanger 12 is provided in the dilution liquid supply line L8. Although it takes time to heat, a heat source such as a heater (not shown) may be installed in the membrane immersion tank 2. If the membrane immersion tank 2 is located far from the heat source, piping costs will be high, so this method may be advantageous.

Once the circulating cleaning with the cleaning liquid is complete, it is preferable to stop the suction pump 8 and immerse the filtration membrane 3 in the cleaning liquid. Immersion promotes the peeling and elution of oil and other substances adhering to the filtration membrane 3, enhancing the cleaning effect. Immersion is preferably performed for approximately 2 to 24 hours, more preferably approximately 6 to 24 hours, with no particular upper limit. After the immersion process is complete, the suction pump 8 can be started again and circulating operation can be performed. This allows the oil and other substances that have peeled off and been eluted during immersion to be removed from the filtration membrane 3.

After that, the cleaning liquid is discharged from the membrane immersion tank 2 through the drain line L5. Next, compressed air is supplied from the air supply line L7 to push out the cleaning liquid remaining in the circulation line CL and discharge it into the membrane immersion tank 2. The pressure of the compressed air is preferably, for example, about 50 to 100 kPa. The air supply line L7 connected to the circulation line CL and the compressed air source 14 (compressed air tank, compressor, etc.) connected to the air supply line L7 constitute the air supply means 13. Since the third valve V3 and the fourth valve V4 are open, air proceeds in both directions through the return line L6 from the junction of the air supply line L7 and the return line L6. The cleaning liquid remaining in the part on the primary side of the junction is pushed out to the membrane immersion tank 2, and the cleaning liquid remaining in the part on the secondary side of the junction is pushed out to the membrane immersion tank 2 through the filtration membrane 3. At this time, the cleaning liquid acts on the filtration membrane 3 in the same way as backwashing. In this way, the air supply means 13 supplies air to the circulation line CL to remove the cleaning liquid remaining in the circulation line CL after cleaning the filtration membrane 3. By performing this process, the subsequent rinsing process can be performed more efficiently. This process can also be performed before the cleaning liquid is discharged from the membrane immersion tank 2, i.e., while the cleaning liquid remains in the membrane immersion tank 2.

Next, the rinsing process is performed. As shown in FIG. 2, the rinsing process is performed in the same manner as the simple backwashing. The first valve V1 is closed, the second valve V2 is opened, and filtrate water is supplied from the filtrate water tank 9 to the membrane immersion tank 2 in the same manner as the simple backwashing. When the filtration membrane 3 is immersed in the filtrate water, the backwash pump 10 is started, and the filtrate water is passed from the secondary side to the primary side of the filtration membrane 3. This allows the dirt loosened by the cleaning to be peeled off and removed from the filtration membrane 3. In addition, the cleaning solution remaining on the primary and secondary sides of the filtration membrane 3 and in the circulation line CL can be discharged and removed. The filtrate water tank 9, the backwash line L4, the suction line L2, and the backwash pump 10 constitute the rinsing means 15. As an alternative, the filtrate water can be supplied from the dilution liquid supply line L8 to the membrane immersion tank 2, and the suction pump 8 can pass the filtrate water from the primary side to the secondary side of the filtration membrane 3. In this case, the operation mode is the same as that during normal operation shown in FIG. 1. The filtrate water can be supplied, for example, from the filtrate water tank 9. However, performing the rinsing process by backwashing provides a greater rinsing effect on the filtration membrane 3. There are no particular limitations on the rinse water as long as it does not contain cleaning components, so oil-containing wastewater can also be used as the rinse water. In this case, the rinse operation is the same as the normal operation after cleaning, so normal operation can be resumed immediately after checking the water quality of the treated water. In this way, as long as a liquid that does not contain cleaning components can be supplied from the primary or secondary side during the rinsing process and can be permeated through the filtration membrane 3, the configuration of the rinsing means 15 is not limited.

(Example)
Raw water containing 30,000 ppm oil and 30,000 ppm suspended solids was passed through a PTFE filter membrane for about two weeks, causing the filter membrane to be almost blocked. In Comparative Example 1, the filter membrane was immersed in a cleaning solution at 20°C for cleaning. In Comparative Example 2, the filter membrane was immersed in a cleaning solution at 40°C for cleaning. In Example 1, the filter membrane was passed through a cleaning solution at 40°C and then immersed for cleaning. A cleaning solution containing 0.5% surfactant and 1% caustic soda was used for cleaning. The results are shown in Figure 4. In Comparative Example 2, the temperature of the cleaning solution was high, so better cleaning results were obtained than in Comparative Example 1. In Example 1, the initial water permeability was almost restored due to the effect of passing the cleaning solution through the filter membrane and the effect of heating the cleaning solution.

Although the present invention has been described above by way of embodiments and examples, the present invention is not limited to these. For example, an inorganic membrane can be used as the filtration membrane 3. The inorganic membrane is preferably a hydrophilic membrane. One or more inorganic membranes are housed exposed in the membrane immersion tank 2. The inorganic membrane is a flat membrane made of alumina, SiC (silicon carbide), or the like, and has multiple internal flow paths formed therein through which treated water from oil-containing wastewater flows. The flow paths form a secondary space 7 of the inorganic membrane. Both ends of the flow path are open on the side of the inorganic membrane, and both ends are connected to water collection pipes. The water collection pipe is connected to the suction line L2. The oil-containing wastewater permeates from the outer surface of the inorganic membrane through the countless pores of the inorganic membrane and is discharged into the internal flow path. Large particle size oil particles and the like remain on the outer surface of the inorganic membrane, small particle size oil particles and the like are captured in the pores, and treated water (filtered water) from which oil particles and the like have been sufficiently removed is collected in the internal flow path. By cleaning such an inorganic membrane with the cleaning method of the present invention, the same effect as that of the above embodiment can be obtained.

Although this embodiment is directed to a filtration membrane for filtering oil-containing wastewater, the present invention can also be applied to a filtration membrane for filtering liquids containing highly viscous substances other than oil. When highly viscous substances adhere to the membrane surface of the filtration membrane, the adhered substances cause membrane blockage or an increase in suction pressure, as in this embodiment. In addition, in this embodiment, the filtration membrane 3 is washed using the membrane immersion tank 2, but the filtration membrane 3 can also be washed in a dedicated washing tank. That is, when washing the filtration membrane 3, the filtration membrane 3 can be removed from the membrane immersion tank 2, placed in a washing tank, washed by the above-mentioned method, and returned to the membrane immersion tank 2. In addition, if continuous operation of the filtration device 1 is required, multiple membrane immersion tanks 2 may be installed in parallel. By isolating the immersion tank 2 in which the filtration membrane 3 that needs to be washed is installed, the operation of the filtration device 1 and the washing of the isolated filtration membrane 3 can be performed simultaneously. Furthermore, in order to promote the peeling of the cake, an air blowing section may be provided at the bottom of the membrane immersion tank 2 to perform an aeration operation. An agitator may be installed in the membrane immersion tank 2 to promote the peeling effect of the cake by the shear force of stirring.

REFERENCE SIGNS LIST 1 Filtration device 2 Membrane immersion tank 3 Filtration membrane 8 Suction pump 11 Cleaning means 12 Temperature adjustment means 13 Air supply means 15 Rinsing means CL Circulation line

Claims (7)

The apparatus comprises a membrane immersion tank into which oil-containing wastewater is introduced, a filtration membrane immersed in the oil-containing wastewater introduced into the membrane immersion tank and filtering the oil-containing wastewater, a suction line connected to a secondary side of the filtration membrane, a suction pump provided in the suction line and sucking filtered water to the secondary side, a return line branched off from the suction line downstream of the suction pump and connected to the membrane immersion tank, and an air supply means connected to the return line,
The suction line and the return line constitute a circulation line connected to the filtration membrane and the membrane immersion tank,
The suction pump cleans the filtration membrane by passing a cleaning solution from the primary side to the secondary side of the filtration membrane , and the circulation line returns the cleaning solution that has passed through the secondary side of the filtration membrane to the membrane immersion tank,
The air supply means supplies air to the circulation line to push out the cleaning solution remaining in the circulation line after cleaning the filtration membrane into the membrane soaking tank , in this oil-containing wastewater filtration apparatus. A rinsing means is connected to the primary side or the secondary side of the filtration membrane,
2. The filtration device according to claim 1, wherein the rinsing means supplies a liquid not containing a cleaning component from the primary side or the secondary side of the filtration membrane after the cleaning liquid is pushed out into the membrane immersion tank by the air supply means, and allows the liquid to permeate through the filtration membrane. 3. The filtration apparatus according to claim 1 , further comprising a temperature adjusting means for adjusting the temperature of the cleaning solution in the membrane immersion tank so that the temperature of the cleaning solution is higher than 30°C and not higher than 50°C. The filtration device according to any one of claims 1 to 3 , wherein the oil-containing wastewater is scrubber wastewater from a ship. A method for cleaning a filtration membrane in a filtration device comprising: a membrane immersion tank into which oil-containing wastewater is introduced; a filtration membrane immersed in the oil-containing wastewater introduced into the membrane immersion tank and filtering the oil-containing wastewater; a suction line connected to a secondary side of the filtration membrane; a suction pump provided in the suction line for sucking filtrate to the secondary side; a return line branched off from the suction line downstream of the suction pump and connected to the membrane immersion tank; and an air supply means connected to the return line, wherein the suction line and the return line constitute a circulation line connected to the filtration membrane and the membrane immersion tank, the method comprising:
Cleaning the filtration membrane by passing a cleaning solution from the primary side to the secondary side of the filtration membrane with the suction pump ;
Returning the cleaning solution that has permeated the secondary side of the filtration membrane to the membrane immersion tank through the circulation line;
A method for cleaning a filtration membrane of oil-containing wastewater, comprising : supplying air from the air supply means to the circulation line, thereby pushing out the cleaning solution remaining in the circulation line after cleaning the filtration membrane into the membrane immersion tank . The cleaning method according to claim 5 , wherein a flow rate at which the cleaning liquid permeates the filtration membrane is smaller than a filtration flow rate at which the oil-containing wastewater is filtered by the filtration membrane. 7. The cleaning method according to claim 5, further comprising the steps of: after the cleaning liquid is caused to permeate the filtration membrane by the suction pump, stopping the suction pump and immersing the filtration membrane in the cleaning liquid.

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