JPH04267931A - Filtering method - Google Patents

Abstract

PURPOSE:To remove bacterial cells from a fermentation solution or to clarify said solution by performing full filtration while periodic backwashing is repeated using a micro-filter membrane module arranged so that the membrane surface thereof becomes parallel to a gravity direction. CONSTITUTION:When a raw fluid composed of a fluid containing suspended matter is supplied to a micro-filter membrane module A and filtered to be separated into the fluid and the suspended matter, periodic backwashing is employed in the full filtration method of conventional technique and, further, the suspended matter desorbed from the filter membrane 3 is discharged out of the filter system. In this full filtration periodic backwashing system, the membrane module A is arranged so that the surface of the membrane 3 becomes parallel to a gravity direction. As a result, a filtering method suitable for the removal of bacterial cells from a fermentation solution or the clarification of said solution is obtained. Further, a suspension whose concn. is several times as compared with a case performing filtration in such a state that the surface of the membrane is made parallel can be filtered.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a total filtration periodic backwashing system, and more particularly to a new total filtration periodic backwashing system in which backwashing is performed periodically to maintain a large membrane permeation flux. It is something. The total filtration periodic backwashing system of the present invention is applied to the separation, purification, recovery, and concentration of fluids containing or suspending various polymers, microorganisms, yeast, and fine particles, and is particularly applicable to fine particles that require filtration. It can be applied in any case where it is necessary to separate fine particles from a fluid containing fine particles, such as from various suspensions containing fine particles, fermentation liquids or culture liquids, as well as suspensions of pigments, etc. It is also applied to the separation of fine particles, and is also used to purify gas by separating and removing fine particles from suspended gas containing fine particles, for example, sterilizing nitrogen gas to be filled into pharmaceutical ampoules, and positive application to ultrapure water production equipment. It is also applied to the production of dust-free, sterile gas filled as pressurized gas, or dust-free, sterile air for air conditioning in IC manufacturing lines.

0002

[Prior Art] Conventionally, techniques for separating suspended solids from a raw fluid containing suspended solids using a membrane include, for example, reverse osmosis, ultrafiltration, precision filtration, etc. using pressure as a driving force. There are electrodialysis methods that use a potential difference as a driving force, and diffusion dialysis methods that use a concentration difference as a driving force. These methods can be operated continuously, can separate, purify, or concentrate without significantly changing temperature or pH conditions during the separation operation, and can separate a wide range of particles, molecules, ions, etc. It is economical because even small plants can maintain large processing capacity, and the energy required for separation operations is small.
Moreover, it is widely practiced because it is possible to treat low-concentration raw fluids that are difficult to use with other separation methods. The membranes used in these separation techniques include polymer membranes mainly made of organic polymers such as cellulose acetate, cellulose nitrate, regenerated cellulose, polysulfone, polyacrylonitrile, polyamide, and polyimide, as well as those with heat resistance, chemical resistance, etc. There are porous ceramic membranes with excellent durability.Ultrafiltration membranes are used mainly for filtering colloids, and precision filtration is used for filtering fine particles of 0.05 to 10μm. In this case, a microfiltration membrane with suitable micropores is used. By the way, in recent years, with the progress of biotechnology, higher purity, higher performance,
As higher precision is required, the fields of application of precision filtration or ultrafiltration technology are expanding. However, when separating fine particles using a membrane in microfiltration or ultrafiltration, a cake layer is formed due to the influence of concentration polarization, creating resistance to the flow of the permeate fluid, and the resistance increases due to membrane clogging. However, there is a problem in that the membrane permeation flux rapidly and significantly decreases, and this has been the biggest cause of hindering the practical application of microfiltration or ultrafiltration. Furthermore, the membrane used therein is easily contaminated, and measures to prevent this are required.

[0003] As a filtration method, there is a so-called total filtration method in which all the fluid to be filtered passes through a filter medium (filter cloth, membrane, etc.) and a cake layer to separate fine particles contained in the fluid. In this conventional total filtration method, a high permeation flux is obtained when the fluid passes through and the suspended solids are captured and separated inside the filtration membrane, but when the suspended solids are trapped on the surface of the filtration membrane, a high permeate flux is obtained. When a layer is formed and a large amount of raw fluid is processed, or when the specific resistance of the formed cake layer is extremely high, filtration resistance becomes large, and when such total filtration is performed, the membrane permeation flux becomes small. On the other hand, in fields such as wastewater treatment, water production, and pool water filtration, it is known that backwashing is performed to restore the permeation flux of a clogged filter. However, this method that combines total filtration and backwashing is a technology developed in the field of wastewater treatment, where the resistivity of the cake layer is relatively small, so it is difficult to remove fine particles with high resistivity, such as when separating bacterial cells from fermentation liquid. It was powerless to filter as it was. For this reason, a cross-flow filtration method was considered. In this cross-flow filtration system, the raw fluid to be filtered is passed parallel to the membrane surface of the filtration membrane, the fluid passes through the filtration membrane to the opposite side, and the flow of the raw fluid and the permeated fluid are perpendicular to each other. This is why it is called this way. In this cross-flow type filtration method, the cake layer formed on the membrane surface is stripped off by the flow of the raw fluid parallel to the membrane, so the membrane permeation flux is larger than in the conventional total filtration method, and a large amount of raw fluid is It is possible to directly and continuously separate, purify, and concentrate
When using a membrane with a high pure water permeation flux, that is, a membrane in the precision filtration range that removes particles of 0.05 μm or larger, the membrane permeation flux decreases rapidly and it is difficult to maintain the high membrane permeation flux at the beginning of filtration. As a result, when comparing the total filtration method and the total amount of permeate, the improvement effect was small and insufficient to obtain an economical permeate flux.

0004

[Problems to be solved by the invention] As mentioned above, the cross-flow filtration method is an advanced separation technology in principle, but the biggest problem, the membrane permeation flux, is lower than that of the conventional total filtration method. Although this method is large, there is a problem in that even if this cross-flow method is adopted as a precision filtration method, a sufficiently high membrane permeation flux cannot be obtained economically. In addition, looking at specific examples of conventional separation of suspended solids and fluids, for example, when separating bacterial cells from fermentation liquid, conventional centrifugation, diatomaceous earth filtration, Even if a cross-flow filtration method is used instead of a filtration method, the membrane permeation flux not only decreases as the filtration time passes due to a cake layer or clogging formed on the membrane surface, but also shear when circulating the raw fluid. There was a problem in that the activity of the bacterial cells was lost due to force.

[0005] As a method of increasing the permeation flux, the flow of raw fluid into the filtration membrane is intermittently stopped by using the cross-flow filtration method, or by closing the valve on the permeate side of the filtration membrane. By intermittently eliminating or reducing the pressure applied perpendicular to the membrane surface of the membrane, or by applying pressure from the permeate side of the filtration membrane and causing the fluid to flow from the permeate side to the raw fluid side, the raw fluid side of the filtration membrane can be Attempts have been made to intermittently remove the cake layer and adhesion layer deposited on the membrane surface, but when backwashing is performed, the suspended solids desorbed from the filtration membrane are also removed. If left in the filtration system, the concentration of suspended matter in the raw fluid will gradually increase, and in some cases, the viscosity of the raw fluid will also increase, so the membrane permeation flux will gradually decrease, even if backwashing is performed. There were problems such as insufficient recovery of permeation flux. Furthermore, when backwashing is performed using the permeated liquid, the amount of membrane permeation is reduced by the amount of backwashing, so there is a problem that it is not possible to secure enough backwash liquid to sufficiently recover the membrane permeation flux. On the other hand, as a method to not reduce the activity of bacterial cells, reducing the shearing force by lowering the cross-flow circulation flow rate is used, but reducing the shearing force reduces the effectiveness of the cross-flow filtration method, so If measures were taken not to reduce body activity, there was a problem that the membrane permeation flux would decrease. In addition, when a pump with a small shearing force such as a diaphragm pump is used to reduce the crushing of bacterial cells by the pump, there is a problem that the pump pulsates so much that the effect of the cross-flow filtration system is reduced.

[0006]

[Means for Solving the Problems] The present invention has been made to solve the problems of the prior art described above, and has a practical high membrane permeation flux, and is capable of transporting bacterial cells, etc. The objective is to provide a novel total filtration periodic backwashing system that reduces activity loss. That is, the present invention adds periodic backwashing to the entire filtration method of the prior art when a raw fluid consisting of a fluid containing suspended solids is supplied to a microfiltration membrane module and filtered to separate the fluid and suspended solids. This is achieved by installing the membrane module so that the membrane surface is parallel to the direction of gravity in a total filtration periodic backwashing system that discharges suspended solids desorbed from the filtration membrane by backwashing to the outside of the filtration system. It was done.

The present invention will be explained in detail below. In conventional total filtration, when backwashing is performed, suspended solids desorbed from the filtration membrane gradually accumulate in the filter, and the permeation flux cannot be recovered sufficiently even with backwashing. In the present invention, the suspended solids desorbed from the filtration membrane can be easily discharged from the system together with the backwashing liquid, so that the periodic backwashing effect becomes remarkable. Also,
By using the total filtration periodic backwashing system of the present invention, the filtration system becomes simple, and unlike the cross-flow filtration system, there is no shearing force when circulating the raw fluid, making it possible to prevent a decrease in bacterial activity. Become. In order to desorb the cake deposited on the membrane and particles trapped inside the membrane during backwashing, and to effectively discharge the desorbed cake and particles out of the filtration system, the membrane surface should be aligned parallel to the direction of gravity. and a backwash liquid outlet at the bottom of the filter housing.

Backwashing is more effective when carried out with a liquid than with a gas.Although permeate may be used as the backwash liquid, not only does the amount of permeate decrease by the amount of permeate that is reversed, but the membrane permeation flow is If the amount of backwashing liquid equivalent to the amount of permeated liquid is required to fully recover the bundle, there is a risk that virtually no permeated liquid will be obtained, so cleaning liquid should be supplied from outside the filtration system to meet the needs. It is preferable to perform backwashing with a corresponding amount of backwashing liquid. Basically, any cleaning liquid supplied from outside the filtration system can be used as long as it does not degrade the properties of the filtration membrane or change the properties of the raw fluid, but if the raw fluid is an aqueous solution, sterile water is generally used. It is preferable. Furthermore, if it is not desired to leave the backwash liquid in the filtration system after the backwash is completed, it is preferable to perform dehydration using gas. When performing constant pressure filtration, if backwashing is performed after the membrane permeation flux becomes extremely low, as in the conventional "total filtration backwashing technology," the recovery of the membrane permeation flux after backwashing will be poor. Backwashing is performed before the permeation flux reaches 1/100 of the initial filtration flux. A higher permeation flux can be obtained by backwashing preferably before reaching 1/10 of the permeation flux at the initial stage of filtration. In addition, when performing constant-speed filtration, if backwashing is performed after the filtration transmembrane pressure difference becomes extremely high, the recovery of the filtration transmembrane pressure difference after backwashing, that is, the cleaning performance, will deteriorate, so It is preferable to perform backwashing before the pressure reaches 100 times the transmembrane pressure. More preferably, by backwashing before reaching 10 times the filtration transmembrane pressure difference at the initial stage of filtration, the permeation flux conditions can be further increased. The cleaning performance will be higher if the backwash liquid is passed through the filtration membrane in large quantities with a high membrane permeation flux.
The permeation flux of backwash liquid is 1×10-4m3/m2/sec
The backwashing time is preferably at least 1 second, particularly preferably from 2 seconds to 10 seconds.

FIG. 1 is a developed view showing the overall structure of a general microfiltration membrane module. A microfiltration membrane 3 and a smaller liquid-permeable sheet 4 are arranged between the primary support plate 1 and the secondary support plate 2 and overlapped with each other. The liquid-permeable sheet 4 is arranged in a recess 5 provided in the secondary support plate 2 and is designed to be flush with the sealing surface 10 of the support plate 2. The microfiltration membrane 3 is arranged at the peripheral edge 10. The microfiltration membrane is liquid-tightly sealed with support plates 1 and 2, and is either bonded to each other with adhesive to withstand the filtration pressure, or surrounded by bolts and nuts via gaskets or O-rings. It's tight. The secondary side support plate 2 is provided with a groove 6, a liquid passage hole 9, and a secondary side inlet/outlet 8 as means for collecting the filtrate and discharging it outside the module. As shown in FIG. 2, the primary support plate 1 is provided with a large number of small protrusions 11, which serve to support the membrane 3 during backwashing. The primary side support plate is further provided with a liquid passage hole (not shown) and a primary side inlet/outlet 7 as means for supplying the filtered stock solution.

[0010] When carrying out large-capacity filtration, a flat membrane laminated type module is used which can increase the membrane area. Regarding the structure of the flat membrane laminated module, see Japanese Patent Application Laid-Open No. 56-129016.
No., JP-A-63-80815, JP-A-63-2865
No. 4, Utility Model No. 64-4417, US 4,221,66
Many things have been proposed, including number 3. FIG. 3 shows an example of a flat membrane stacked module containing membrane elements with a large membrane area in a relatively small filter housing. When filtering, the filtration stock solution is the primary side entrance/exit 31
It enters from the membrane support 21 , passes through the microfiltration membrane disposed on the membrane support 21 , passes through a passage inside the support, and is discharged from the central through hole 25 to the secondary side entrance 32 . FIG. 4 is a partial view of the membrane support 21, and FIG. 5 is a view of the membrane support with the membrane 24 installed as seen from the II-II cross section in FIG. The membrane support 21 consists of a boss part 22 having a central through hole 25 in the axial direction and a disk-like part on the outside thereof. 2
Consists of 7. The liquid that has permeated through the membrane flows from the mesh portion to the inner passage 26 provided inside the inner flat portion 23 and the boss portion 22.
The central through hole 25 is reached through the central through hole 25. This flat membrane stacked module is arranged in the filtration line so that the filtration membrane surface is parallel to gravity, and the backwash liquid discharge port (here, the primary side entrance and exit port is used) 31 is located in the housing 30. Preferably, it is provided at the bottom.

[0011] When filtering a large volume of liquid, a pleated cartridge type module is also used in addition to the flat membrane laminated type module. 6 and 7 show an example of an exploded view of a pleated cartridge membrane element and the entire module. The filtration stock solution is supplied from the primary side entrance/exit 72 of the filter housing 70, passes through the outer protective sheet 62, microfiltration membrane 63, and inner protective sheet 64 of the pleated cartridge module in order, and enters the central through hole 68 through the hole provided in the core. , is discharged from the housing secondary side entrance/exit 73. In this module as well, the module is arranged so that the filtration membrane surface is parallel to the direction of gravity.
Backwash liquid outlet (here, the primary side inlet/outlet is used)
is preferably provided at the bottom of the housing.

FIG. 8 shows the flow of the total filtration periodic backwash system of the present invention. The filtered stock solution is sent to the filtration module 50 by the pump 51, and after being filtered, is sent to the filtrate storage tank. After the backwash liquid has been filtered for a certain period of time, the valve is switched and the pump 52 is used to transfer the backwash liquid to the filtration module 50.
and is discharged together with the cake from the primary side entrance/exit. Thereafter, the cleaning liquid remaining in the filtration system is discharged by gas pressure, and filtration is performed again. By repeating this cycle, it becomes possible to maintain a high permeation flux without increasing the concentration of suspended solids in the raw fluid. In this system,
When the filtration membrane surface is installed parallel to the direction of gravity, the captured particles that have been removed from the membrane surface and inside the membrane by backwashing must be washed out from the housing by thorough backwashing, otherwise they will be trapped inside the housing. When filtration is restarted, it is captured again in the membrane and prevents clogging. If the membrane is installed parallel to the direction of gravity and the backwash liquid outlet (primary side inlet/outlet) is located at the bottom of the filter housing,
Not only do particles that have detached from the membrane easily settle and be discharged, but also particles that remain in the housing without being completely discharged are completely removed together with the residual liquid in the housing during the next process of discharging the residual liquid using gas pressure. As a result, backwashing can be performed with a small amount of backwashing and a short backwashing time.

[0013] Microfiltration membranes are disclosed, for example, in US Pat.
, No. 341, No. 3,133,132, No. 2,944,
No. 017, Special Publication No. 15698, Special Publication No. 1972-1569, Special Publication No. 1977-3
No. 3313, No. 48-39586, No. 48-4005
As described in No. 0, etc., those manufactured using cellulose ester as a raw material, U.S. Patent No. 2,783,89
No. 4, No. 3,408,315, No. 4,340,479
No. 4,340,480, No. 4,450,126, German Patent DE 3,138,525, Japanese Unexamined Patent Publication No. 1983-
No. 37842, etc., manufactured using aliphatic polyamide as a raw material, US Pat. No. 4,196,
No. 070, No. 4,340,482, JP-A-55-99
934, those manufactured using polyfluorocarbon as raw materials as described in JP-A-58-91732, etc., JP-A-56-154051, JP-A-56-86, etc.
No. 941, JP-A-56-12640, JP-A-63-1
39930, JP-A No. 60-250049, etc., and those using polypropylene as a raw material, as described in German Patent OLS No. 3,003,400, etc. To produce a precision filtration membrane, the above polymer is dissolved in a good solvent, a mixed solvent of a good solvent and a non-solvent, or a mixture of multiple types of solvents with different degrees of solubility for the polymer to prepare a membrane-forming stock solution. death,
This is carried out by casting onto a support or directly into a coagulating solution, washing and drying. In this case, examples of solvents that dissolve the polymer include dichloromethane, acetone, dimethylformamide, dimethylacetamide, dimethylsulfoxide, 2-pyrrolidone, N-methyl-2-pyrrolidone, and sulfolane. Examples of nonsolvents added to the above solvent include cellosolves, alcohols such as methanol, ethanol, and isopropanol, ketones such as acetone and methyl ethyl ketone, ethers such as tetrahydrofuran and dioxane, polyethylene glycol, glycerin, and ethyl glycol. Examples include polyols. The ratio of the non-solvent to the good solvent may be in any range as long as the mixed liquid can maintain a uniform state, but is preferably 5 to 50% by weight.

[0014] Furthermore, an inorganic electrolyte, an organic electrolyte, a polymer electrolyte, etc. called a swelling agent may be added to control the porous structure. Electrolytes that can be used in the present invention include common salt, metal salts of inorganic acids such as sodium nitrate, potassium nitrate, sodium sulfate, zinc chloride, and magnesium bromide, organic acid salts such as sodium acetate, sodium formate, and potassium butyrate, and polystyrene sulfonic acid. Polymer electrolytes such as sodium, polyvinylpyrrolidone, and polyvinylbenzyltrimethylammonium chloride, and ionic surfactants such as sodium dioctyl sulfosuccinate and sodium alkylmethyltaurate are used. Although some of these electrolytes exhibit some effect even when added alone to a polymer solution, when these electrolytes are added as an aqueous solution, particularly remarkable effects may be exhibited. The amount of the aqueous electrolyte solution to be added is not particularly limited as long as the addition does not cause loss of uniformity of the solution, but is usually from 0.5% by volume to 10% by volume based on the solvent. Further, there is no particular restriction on the concentration of the electrolyte aqueous solution, and the higher the concentration, the greater the effect, but the concentration usually used is 1% by weight to 60% by weight. The polymer concentration as a membrane forming stock solution is 5 to 35% by weight, preferably 10 to 30% by weight.
It is. When it exceeds 35% by weight, the water permeability of the resulting microporous membrane becomes so small that it has no practical meaning;
When it is smaller than , a microfiltration membrane with sufficient separation ability cannot be obtained.

The membrane-forming stock solution prepared as described above is cast onto a support, and the polymer solution membrane together with the support is immersed in a coagulation solution immediately after casting or after a certain period of time. Water is most commonly used as the coagulating liquid, but organic solvents that do not dissolve the polymer may also be used, or two or more of these non-solvents may be mixed. The support can be arbitrarily selected from those that can be used as a support in the production of microfiltration membranes, but it is particularly preferable to use a nonwoven fabric since there is no need to peel off the support. The nonwoven fabric that can be used in the present invention is generally made of polypropylene, polyester, etc., and is not subject to any material limitations. The cast film on which the polymer has been precipitated in the coagulation bath is then washed with water, hot water, solvent, etc., and then dried.

[0016]

[Examples] The present invention will be explained in more detail by referring to specific examples of filtration, but the present invention is not limited to the examples unless it goes beyond the gist of the invention. Example 1 The filtration membrane was a cellulose acetate anisotropic microfiltration membrane with a nominal pore size of 1.2 μm (FM-120, manufactured by Fuji Photo Film Co., Ltd.)
The filtration flow device shown in Fig. 8 was assembled by using the single-layer flat membrane module shown in Fig. 1, which was equipped with a filtration system (manufactured by J.D.) and installed with the membrane surface parallel to the direction of gravity. Dissolve 20 ppm of tannic acid in commercially available beer and use it as a suspension to aggregate the protein, filtration flux 5 kl/m2/h, backwash flux 10 kl/m2/h, filtration time 54 seconds, backwash. When periodic backwash filtration was performed under the condition of 4 seconds of total filtration, the filtration pressure reached 3 kg/cm2 by 10 kl/m2.
A filtrate was obtained. On the other hand, when filtration was carried out using the same membrane with the membrane surface horizontal, the amount of filtrate obtained by the time the filtration pressure reached 3 kg/cm2 was only 2.5 kl/m2.

Example 2 By the method disclosed in Japanese Patent Application Laid-Open No. 60-250049,
A polysulfone anisotropic membrane with an average pore diameter of 0.2 μm was formed. Using the single-layer flat membrane module shown in FIG. 1 equipped with this membrane, the filtration flow device shown in FIG. 8 was assembled by installing the membrane surface parallel to the direction of gravity. Escherichia coli (IFO
3301), glucose 10g/l, polypeptone 5g
A culture solution containing 5 g/l of yeast extract and 5 g/l of sodium chloride was used for permeation culture for 18 hours to obtain a filtered stock solution. The culture conditions were a temperature of 37°C and a pH of 7.0. Using this culture solution, the filtration flux was 0.5kl/m2/h.
When periodic backwash filtration was performed under the conditions of , backwash flux 2 kl/m2/h, filtration time 54 seconds, and backwash time 4 seconds,
1000l/ until the filtration pressure reaches 5kg/cm2
m2 of filtrate was obtained. On the other hand, when the same membrane was used for filtration with the membrane surface horizontal, only 200 l/m2 of filtrate was obtained by the time the filtration pressure reached 5 kg/cm2.

[0018]

[Effect of the invention] When performing total filtration while repeating periodic backwashing using a precision filtration membrane module whose membrane surface is installed parallel to the direction of gravity, compared to when filtration is performed with the membrane surface horizontal, It was possible to filter several times higher suspensions.

[Brief explanation of the drawing]

FIG. 1 is an exploded view showing the overall structure of a typical microfiltration flat membrane module.

FIG. 2: Flat membrane module primary support plate.

FIG. 3 is a cross-sectional view of a flat membrane stacked module.

FIG. 4: Membrane support of a flat membrane stacked module.

FIG. 5 is a cross-sectional view of the membrane support II-II of the flat membrane stacked module.

FIG. 6 is an exploded view of a pleated cartridge type membrane element.

FIG. 7: Pleated cartridge type module.

FIG. 8 is a flow diagram of a total filtration periodic backwashing system according to the present invention.

[Explanation of symbols] 1 Primary side support 2 Secondary side support 3 Microfiltration membrane 4 Liquid passage sheet 5 Liquid passage sheet receiver 6 Groove 7 Primary side entrance/exit 8 Secondary side entrance/exit 9 Liquid passage hole 10 Membrane sealing surface 11 Protrusion 12 Recess 13 Frame 20 Flat laminated membrane element 21 Membrane support 22 Boss 23 Inner flat part 24 Microfiltration membrane 25 Central hole 26 Internal passage 27 Rib 28 Outer flat part 29 Net part 30 Filter housing 31 Primary side entrance/exit 32 Secondary side entrance/exit 33 Air vent 50 Total filtration periodic backwashing module 51
Filtration pump 52 Backwash pump 60 Pleated cartridge membrane element 61
Guards 62, 64 Protective sheet 63 Microfiltration membrane 65 Core 66 End plate 67 Gasket 68 Center hole 70 Filter housing 71 Tightening nut 72 Primary side entrance/exit 73 Secondary side entrance/exit 74 Air bleed

Claims (3)

[Claims] Claim 1: In a total filtration periodic backwashing system in which filtration is performed while removing cake trapped on the surface of a filter medium by periodically repeating backwashing, the membrane surface of a flat membrane filter module used therein is oriented in the direction of gravity. A total filtration periodic backwash filtration method characterized by being installed in parallel to the filtration system. Claim 2: The flat membrane used in Claim 1 has an average pore diameter of 0.
A total filtration periodic backwash filtration method characterized by a microfiltration membrane of 0.05 to 5 μm. 3. The flat membrane used in claim 2 is an anisotropic membrane in which the pore diameter changes continuously in the thickness direction,
Total filtration periodic backwash filtration method.