JP7338682B2 - Hollow fiber membranes, filtration modules and wastewater treatment equipment - Google Patents

Description

TECHNICAL FIELD The present disclosure relates to hollow fiber membranes, filtration modules, and wastewater treatment devices.

The activated sludge method, which has high treatment efficiency, is often used in the purification of wastewater containing organic substances such as industrial wastewater, livestock wastewater, and sewage. In particular, the membrane separation activated sludge process (MBR process), in which treated water and sludge are separated by microfiltration membranes (MF membranes) or ultrafiltration membranes (UF membranes) in place of the conventional sedimentation process, has attracted attention. As one type of such filtration membranes, hollow fiber membranes in which a porous body is formed in a cylindrical shape with a relatively small diameter are sometimes used. As a material for the hollow fiber membrane, a material containing polytetrafluoroethylene as a main component, which is relatively excellent in mechanical strength and chemical stability, is known.

Since polytetrafluoroethylene has relatively high hydrophobicity, it tends to have low water permeability, and particularly when the pore diameter of the hollow fiber membrane is small, the filtration capacity tends to be insufficient. Therefore, a porous membrane formed of polytetrafluoroethylene is impregnated with an aqueous solution of a hydrophilic material such as polyvinyl alcohol, and the hydrophilic material is crosslinked to be insolubilized and fixed to the inner surface of the pores of the porous membrane. It has been proposed to hydrophilize porous membranes (see International Publication No. 2010/092938).

WO2010/092938

A hollow fiber membrane according to an aspect of the present disclosure is a hollow fiber membrane in which the outer peripheral surface and the inner peripheral surface are coated with a hydrophilic resin, the main component of the hydrophilic resin is polyvinyl alcohol, and the unit length in the longitudinal direction is The coating amount of the hydrophilic resin per shell is 0.31 mg/cm or more and 0.44 mg/cm or less.

A filtration module according to another aspect of the present disclosure includes a plurality of hollow fiber membranes whose outer and inner peripheral surfaces are coated with a hydrophilic resin, and a pair of holders that hold both ends of the plurality of hollow fiber membranes. wherein the main component of the hydrophilic resin is polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more and 0.44 mg/cm or less. is.

A wastewater treatment apparatus according to another aspect of the present disclosure includes a water tank that stores water to be treated, a filtration module that is housed in the water tank, and a water tank that is housed in the water tank and supplies air bubbles from below the filtration module. and a bubble supplier, wherein the filtration module comprises a plurality of hollow fiber membranes whose outer and inner peripheral surfaces are coated with a hydrophilic resin, and a pair of vertically holding both ends of the plurality of hollow fiber membranes. The main component of the hydrophilic resin is polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg / cm or more and 0.44 mg. / cm or less.

FIG. 1 is a schematic cross-sectional view showing a hollow fiber membrane according to one embodiment. FIG. 2 is a schematic partial cross-sectional view of the hollow fiber membrane of FIG. FIG. 3 is a schematic perspective view showing a filtration module according to one embodiment. FIG. 4 is a schematic diagram showing the configuration of a waste water treatment apparatus according to one embodiment.

[Problems to be Solved by the Present Disclosure]
Impurities adhere to the surface of the hollow fiber membrane as the filtration process is carried out. This impurity causes a reduction in the filtration efficiency of the liquid that should be filtered. Therefore, during the filtration process, for example, air bubbles are rubbed against the surface of the hollow fiber membrane at regular intervals, and the hollow fiber membrane is oscillated to remove impurities adhering to the surface of the hollow fiber membrane.

However, hollow fiber membranes that have been hydrophilized using polyvinyl alcohol or the like tend to swell during the filtration process, and the hollow fiber membranes tend to elongate in the longitudinal direction due to the application of tension during the filtration process. When the hollow fiber membrane is stretched excessively, it becomes saggy, and during the removal of impurities, an upward load is applied to the hollow fiber membrane due to rising air bubbles, and there is a risk of buckling, which causes a sudden large deflection. Even temporary buckling of the hollow fiber membranes may collapse the flow path and increase the transmembrane pressure difference, resulting in a decrease in filtration performance.

The present disclosure has been made based on such circumstances, and provides a hollow fiber membrane that can suppress excessive elongation during filtration treatment and improve filtration performance even if hydrophilization treatment with polyvinyl alcohol is performed. intended to provide

[Effect of the present disclosure]
The hollow fiber membrane according to one aspect of the present disclosure can suppress excessive elongation during filtration treatment and improve filtration performance even when hydrophilization treatment is performed with polyvinyl alcohol.

[Description of Embodiments of the Present Disclosure]
First, the embodiments of the present disclosure are listed and described.

A hollow fiber membrane according to an aspect of the present disclosure is a hollow fiber membrane in which the outer peripheral surface and the inner peripheral surface are coated with a hydrophilic resin, the main component of the hydrophilic resin is polyvinyl alcohol, and the unit length in the longitudinal direction is The coating amount of the hydrophilic resin per shell is 0.31 mg/cm or more and 0.44 mg/cm or less.

The hollow fiber membrane is subjected to a hydrophilic treatment in which the outer peripheral surface and the inner peripheral surface are coated with a hydrophilic resin containing polyvinyl alcohol as a main component, so that the outer peripheral surface and the inner peripheral surface of the hollow fiber membrane are made hydrophobic. As a result of being suppressed and imparted with hydrophilicity, water permeability is improved. On the other hand, when the outer and inner peripheral surfaces of the hollow fiber membrane are coated with a hydrophilic resin, the hollow fiber membrane expands due to swelling during water absorption and tension during filtration, and the length of the hollow fiber membrane in a dry state before filtration is increased. By comparison, the elongation rate exceeds 2%. If the length of the hollow fiber membrane exceeds 2%, sagging occurs. Therefore, when washing is performed by rubbing the air bubbles while rocking the hollow fiber membrane, the upward load caused by the air bubbles causes buckling of the hollow fiber membrane. The inventors have found that this is more likely to occur. The hollow fiber membrane is hydrophilized with polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction is 0.31 mg / cm or more and 0.44 mg / cm or less. , it is possible to suppress excessive elongation when shifting to a wet state during filtration treatment, and to improve filtration performance. The reason why such an effect occurs is presumed as follows, for example. When the outer and inner peripheral surfaces of the hollow fiber membrane are coated with a hydrophilic resin, the hydrophilic resin swells and shrinks when wet and dry, and the length of the hollow fiber membrane expands and contracts accordingly. By setting the hydrophilic resin coating amount of the hollow fiber membrane hydrophilized with polyvinyl alcohol to 0.31 mg / cm or more and 0.44 mg / cm or less, the elongation rate in the longitudinal direction during the filtration process can be appropriately adjusted. Can be controlled within range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance.

The main component of the hollow fiber membrane is preferably polytetrafluoroethylene. Since the main component of the hollow fiber membrane is polytetrafluoroethylene, the hollow fiber membrane is excellent in chemical resistance, heat resistance, weather resistance, nonflammability, and the like. In addition, by using hydrophobic polytetrafluoroethylene as the main component of the hollow fiber membrane, the effect of coating with the above-mentioned hydrophilic resin, that is, the hydrophobicity is suppressed and the water permeability is improved, can be exhibited more effectively.

A filtration module according to another aspect of the present disclosure includes a plurality of hollow fiber membranes whose surfaces are coated with a hydrophilic resin, and a pair of holding members that hold both ends of the plurality of hollow fiber membranes, The main component of the hydrophilic resin is polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more and 0.44 mg/cm or less.

The filtration module includes a plurality of hollow fiber membranes coated with a hydrophilic resin on the surface, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more. When it is 0.44 mg/cm or less, the elongation in the longitudinal direction during filtration can be controlled within an appropriate range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance.

A wastewater treatment apparatus according to another aspect of the present disclosure includes a water tank that stores water to be treated, a filtration module that is housed in the water tank, and a water tank that is housed in the water tank and supplies air bubbles from below the filtration module. The filtration module includes a plurality of hollow fiber membranes whose surfaces are coated with a hydrophilic resin, and a pair of holding members that vertically hold both ends of the plurality of hollow fiber membranes. The main component of the hydrophilic resin is polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more and 0.44 mg/cm or less. .

The wastewater treatment apparatus includes a filtration module having a plurality of hollow fiber membranes, the surface of which is coated with a hydrophilic resin. When it is 31 mg/cm or more and 0.44 mg/cm or less, the longitudinal elongation rate during filtration can be controlled within an appropriate range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance.

Here, the "main component" means a component with the highest mass content ratio, preferably a component containing 90% by mass or more.

[Details of the embodiment of the present disclosure]
Hereinafter, each embodiment of the present disclosure will be described in detail with reference to the drawings.

<Hollow fiber membrane>
A hollow fiber membrane is a porous membrane formed into a tubular shape that allows water to pass therethrough but blocks the permeation of particles contained in the water to be treated. The hollow fiber membrane prevents impurities from penetrating from the liquid to be treated existing on the outer peripheral side, and allows filtered water to permeate the inside of the membrane for filtration. FIG. 1 is a schematic cross-sectional view showing a hollow fiber membrane 2 according to one embodiment. FIG. 2 is a schematic partial cross-sectional view of the R region of the hollow fiber membrane 2 of FIG. As shown in FIGS. 1 and 2, the hollow fiber membrane 2 according to one embodiment of the present disclosure includes a cylindrical porous support layer 2a and a porous support layer 2a laminated on the outer peripheral surface of the support layer 2a. It includes a filter layer 2b and a hydrophilic resin that coats the inner peripheral surface of the support layer 2a and the outer peripheral surface of the filter layer 2b. That is, in the hollow fiber membrane 2, the filtration layer 2b directly covers the outer peripheral surface side of the support layer 2a. The hollow fiber membrane 2 includes a two-layer body of a support layer 2a and a filtration layer 2b, and a hydrophilic resin covering the surface of the two-layer body. The inner peripheral surface of the hollow fiber membrane 2 is made of a hydrophilic resin that coats the inner peripheral surface of the support layer 2a, and the outer peripheral surface of the hollow fiber membrane 2 is made of a hydrophilic resin that coats the outer peripheral surface of the filtration layer 2b. It is composed by By forming a multi-layered structure having the support layer 2a that ensures the strength of the hollow fiber membrane 2 and the filtration layer 2b that prevents the permeation of impurities in this way, both water permeability and mechanical strength can be achieved, and the surface can be cleaned by air bubbles. effect can be effective.

The main component of the support layer 2a and the filtration layer 2b that constitute the hollow fiber membrane is polytetrafluoroethylene (PTFE). Since the main component of the support layer 2a and the filter layer 2b of the hollow fiber membrane 2 is PTFE, the hollow fiber membrane 2 is excellent in chemical resistance, heat resistance, weather resistance, nonflammability, and the like. In addition, by using hydrophobic PTFE as the main component of the hollow fiber membrane, the effect of coating with the above-mentioned hydrophilic resin, that is, the hydrophobicity is suppressed and the water permeability is improved, can be exhibited more effectively. In addition to PTFE, the support layer 2a and the filtration layer 2b that constitute the hollow fiber membrane 2 may appropriately contain other polymers, additives such as lubricants, and the like.

The support layer 2a and the filter layer 2b have a large number of holes penetrating in the thickness direction. The hollow fiber membrane 2 is preferably made by stretching a PTFE sheet to make it porous. The stretching of the PTFE sheet may be uniaxial stretching or biaxial stretching.

The lower limit of the number average molecular weight of PTFE, which is the main component of the support layer 2a and the filter layer 2b, is preferably 500,000, more preferably 2,000,000. On the other hand, the upper limit of the number average molecular weight of PTFE is preferably 20,000,000. If the number average molecular weight of PTFE is less than the above lower limit, the mechanical strength of the hollow fiber membrane 2 may decrease. Conversely, if the number average molecular weight of PTFE exceeds the above upper limit, it may be difficult to form pores in the hollow fiber membrane 2 .

The lower limit of the isopropyl alcohol bubble point of the hollow fiber membrane 2 is preferably 80 kPa, more preferably 100 kPa. On the other hand, the upper limit of the isopropyl alcohol bubble point of the hollow fiber membrane 2 is preferably 140 kPa, more preferably 120 kPa. If the isopropyl alcohol bubble point of the hollow fiber membrane 2 is less than the above lower limit, the impurities may not be sufficiently separated. If the isopropyl alcohol bubble point of the hollow fiber membranes 2 exceeds the above upper limit, the water permeability of the hollow fiber membranes 2 may be insufficient, and the filtration efficiency of the hollow fiber membranes 2 may be reduced. Here, the "isopropyl alcohol bubble point" is a value measured in accordance with ASTM-F316-86 using isopropyl alcohol, and indicates the minimum pressure required to push out a liquid from a pore. It is an index corresponding to the average.

The lower limit of the average outer diameter of the hollow fiber membrane 2 is preferably 1 mm, more preferably 1.5 mm, and even more preferably 2 mm. On the other hand, the upper limit of the average outer diameter of the hollow fiber membrane 2 is preferably 6 mm, more preferably 5 mm, and even more preferably 4 mm. If the average outer diameter of the hollow fiber membranes 2 is less than the above lower limit, the mechanical strength of the hollow fiber membranes 2 may be insufficient. Conversely, if the average outer diameter of the hollow fiber membranes 2 exceeds the above upper limit, the ratio of the surface area to the cross-sectional area of the hollow fiber membranes 2 may decrease, resulting in a decrease in filtration efficiency. "Average outer diameter" refers to the average value of outer diameters at two arbitrary points.

The lower limit of the average inner diameter of the hollow fiber membrane 2 is preferably 0.3 mm, more preferably 0.5 mm, and even more preferably 0.9 mm. On the other hand, the upper limit of the average inner diameter of the hollow fiber membrane 2 is preferably 4 mm, more preferably 3 mm. If the average inner diameter of the hollow fiber membranes 2 is less than the above lower limit, the pressure loss may increase when the filtered water in the hollow fiber membranes 2 is discharged. Conversely, if the average inner diameter of the hollow fiber membrane 2 exceeds the above upper limit, the thickness of the hollow fiber membrane 2 may become small, resulting in insufficient mechanical strength and impermeability of impurities. "Average inner diameter" refers to the average value of inner diameters at two arbitrary points.

[Support layer]
In the hollow fiber membrane, a cylindrical support layer 2a containing PTFE as a main component is provided on the inner peripheral surface side of the filter layer 2b, and the average pore diameter of the support layer 2a is within the above range. is large enough and flexibility is high enough. Since the hollow fiber membrane 2 is provided with the support layer 2a on the inner peripheral surface side of the filter layer 2b, the support layer 2a functions as a reinforcing layer, and the filter layer 2b is less likely to break during the filtration operation. Furthermore, since the hollow fiber membrane 2 is provided with the support layer 2a on the inner peripheral surface side of the filtration layer 2b, the support layer 2a functions as a cushion layer, and the filtration layer 2b easily swings when rubbed by air bubbles. . As a result, the hollow fiber membrane 2 can easily remove impurities adhering to the outer peripheral surface of the filter layer 2b, and can easily maintain filtration performance.

As a minimum of the average thickness of the support layer 2a, 0.3 mm is preferable and 0.5 mm is more preferable. On the other hand, the upper limit of the average thickness of the support layer 2a is preferably 2.0 mm, more preferably 1.8 mm. If the average thickness of the support layer 2a is less than the above lower limit, the strength of the hollow fiber membranes 2 and thus the hollow fiber membranes 2 may be insufficient. Conversely, if the average thickness of the support layer 2a exceeds the above upper limit, the lumen diameter of the hollow fiber membrane 2 may become small and pressure loss may increase when the filtered water is discharged. "Average thickness" refers to the average value of arbitrary 10 thicknesses.

The lower limit of the average diameter of pores in the support layer 2a is preferably 1 μm, more preferably 1.5 μm. On the other hand, the upper limit of the average diameter of pores in the support layer 2a is preferably 3 μm, more preferably 2.5 μm. If the average diameter of the pores in the support layer 2a is less than the above lower limit, the hollow fiber membrane 2 may have insufficient water permeability. Conversely, if the average diameter of the pores in the support layer 2a exceeds the above upper limit, there is a possibility that the permeation of impurities cannot be sufficiently blocked. The average diameter of the pores means the average diameter of the pores on the outer peripheral surface (filter layer surface) of the hollow fiber membrane 2, and is measured by a pore size distribution measuring device (for example, "Porous Materials Automatic Pore Diameter distribution measurement system").

The lower limit of the porosity of the support layer 2a is preferably 55% by volume, more preferably 60% by volume. On the other hand, the upper limit of the porosity of the support layer 2a is preferably 90% by volume, more preferably 85% by volume. If the porosity is less than the lower limit, the flexibility of the support layer 2a may be insufficient, and the flexibility of the hollow fiber membrane 2 may be insufficient. Conversely, if the porosity exceeds the upper limit, the function of reinforcing the tensile strength of the filter layer 2b by the support layer 2a may become insufficient. Here, "porosity" means the ratio of the total volume of pores to the volume, and can be obtained by measuring the density according to ASTM-D-792.

[Filtration layer]
As a minimum of average thickness of filter layer 2b, 10 micrometers is preferred and 12 micrometers is more preferred. On the other hand, the upper limit of the average thickness of the filter layer 2b is preferably 100 µm, more preferably 80 µm. If the average thickness of the filter layer 2b is less than the above lower limit, it may not be possible to sufficiently block the permeation of impurities. Conversely, if the average thickness of the filtration layer 2b exceeds the above upper limit, the water permeability of the hollow fiber membrane 2 may be insufficient.

The lower limit of the average diameter of pores in the filter layer 2b is preferably 0.01 μm, more preferably 0.05 μm. On the other hand, the upper limit of the average diameter of pores in the filter layer 2b is preferably 0.45 μm, more preferably 0.3 μm. If the average diameter of the pores in the filtration layer 2b is less than the above lower limit, the hollow fiber membrane 2 may have insufficient water permeability. Conversely, if the average diameter of the pores in the filter layer 2b exceeds the above upper limit, there is a possibility that the permeation of impurities cannot be sufficiently prevented.

As a lower limit of the porosity of the filter layer 2b, 45 volume% is preferable and 55 volume% is more preferable. On the other hand, the upper limit of the porosity of the filter layer 2b is preferably 80% by volume, more preferably 70% by volume. If the porosity is less than the lower limit, the filtration efficiency of the hollow fiber membrane may be insufficient. Conversely, if the porosity exceeds the upper limit, the filter layer 2b may be easily broken.

[Hydrophilic resin]
The outer peripheral surface and the inner peripheral surface of the hollow fiber membrane 2 are coated with a hydrophilic resin 5 .

The main component of the hydrophilic resin layer 5 is polyvinyl alcohol. By coating the surface of the hollow fiber membrane 2 with a hydrophilic resin 5 containing polyvinyl alcohol as a main component, the hollow fiber membrane 2 containing PTFE as a main component is made hydrophilic by suppressing the water repellency thereof. By doing so, the water permeability can be improved. Moreover, the hydrophilic resin has a crosslinked structure of polyvinyl alcohol. Polyvinyl alcohol is one of the resins that are readily soluble in water, but it can be prevented from dissolving in water by converting linear polyvinyl alcohol into a three-dimensionally crosslinked net-like structure.

As a minimum of the average molecular weight of the said polyvinyl alcohol, 15000 is preferable and 20000 is more preferable. On the other hand, the upper limit of the average molecular weight of the polyvinyl alcohol is preferably 100,000, more preferably 50,000. If the average molecular weight of the polyvinyl alcohol is less than the lower limit, the hydrophilic resin may easily peel off. Conversely, if the average molecular weight of polyvinyl alcohol, which is the main component of the hydrophilic resin, exceeds the above upper limit, the production cost of the hollow fiber membrane 2 may increase due to difficulty in forming the hydrophilic resin. In addition, the "average molecular weight" refers to JIS-K7252-1 (2008) "Plastics-How to determine the average molecular weight and molecular weight distribution of a polymer by size exclusion chromatography-Part 1: General rule", gel permeation It refers to the weight average molecular weight measured using chromatography (GPC).

The lower limit of the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane 2 is 0.31 mg/cm, preferably 0.35 mg/cm. On the other hand, the upper limit of the coating amount of the hydrophilic resin in the hollow fiber membrane 2 is 0.44 mg/cm, preferably 0.40 mg/cm. When the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane 2 is less than the lower limit, the hollow fiber membrane 2 cannot be continuously coated, and the hollow fiber membrane 2 cannot be coated continuously. There is a possibility that the water permeability cannot be sufficiently improved. On the other hand, when the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane 2 exceeds the upper limit, the hollow fiber membrane 2 tends to swell, and tension is applied during the filtration process. The hollow fiber membrane 2 may be excessively stretched in the longitudinal direction, causing buckling and collapsing the flow path, thereby increasing the transmembrane pressure difference and degrading the filtration performance. In addition, blocking the pores of the support layer 2a and the filtration layer 2b of the hollow fiber membrane 2 may result in insufficient water permeability.

[Method for producing hollow fiber membrane]
(Formation of support layer)
As the support layer 2a, for example, a cylindrical tube obtained by extruding PTFE can be used. A polytetrafluoroethylene powder is compacted into a primary compact having a cylindrical shape by compression molding. Next, the primary molded body is molded into a cylindrical body by extrusion molding. This extrusion is performed at a temperature below the melting point of PTFE, generally at ambient temperature. The cylindrical body is stretched in the longitudinal direction while being heated. By using an extruded tube as the support layer 2a in this way, the support layer 2a can be provided with mechanical strength and holes can be easily formed.

(Formation of filtration layer)
The filter layer 2b is formed by spirally winding a strip made of PTFE, for example, around the support layer 2a so that both edges of the strip are overlapped. As the belt-like body to be wound around the support layer 2a, it is preferable to use a PTFE sheet made porous by stretching a PTFE sheet to form clefts. The step of forming the filtration layer includes, for example, a step of spirally winding a porous strip around the outer periphery of a cylindrical body that is a support layer so that both edges overlap, and a step of bonding the cylindrical body and the strip by heating. and In the bonding step, the strip forming the filter layer 2b is wound around the outer peripheral surface of the support layer 2a and heated to a temperature equal to or higher than the melting point of PTFE, thereby bonding the overlapping side edges of the strip. to form a continuous filter layer 2b and integrate this filter layer 2b with the support layer 2a.

(Coating with hydrophilic resin)
Coating with a hydrophilic resin (also referred to as hydrophilization treatment) includes, for example, a step of impregnating a laminate of the support layer 2a and the filter layer 2b with a solvent, It can be carried out by a step of introducing an aqueous solution of a certain polyvinyl alcohol, a step of cross-linking the polyvinyl alcohol, and a drying step.

In the impregnation step, the laminate of the support layer 2a and the filter layer 2b is impregnated with a water-compatible solvent. By the impregnation step, the surface of the highly hydrophobic fluororesin such as PTFE can be coated with the hydrophilic resin. As the solvent used in the impregnation step, any solvent can be used as long as it has high wettability with polytetrafluoroethylene and high affinity with water. For example, isopropyl alcohol can be used.

In the introduction step, the aqueous solution of polyvinyl alcohol is introduced into the pores of the laminate by, for example, immersing the laminate in the aqueous solution of polyvinyl alcohol. Here, since the pores of the laminate are filled with a solvent that is highly compatible with the aqueous solution of polyvinyl alcohol, the surface tension of the aqueous solution of polyvinyl alcohol is relaxed, and the aqueous solution of polyvinyl alcohol can be removed relatively easily. It can be introduced into the pores of the laminate.

The lower limit of the time for immersing the laminate in the polyvinyl alcohol aqueous solution is preferably 5 minutes, more preferably 30 minutes, although it depends on conditions such as the type of polyvinyl alcohol, the concentration of the aqueous solution, and the temperature. On the other hand, the upper limit of the time for immersing the laminate in the polyvinyl alcohol aqueous solution is preferably 6 hours, more preferably 9 hours. If the time for which the laminate is immersed in the polyvinyl alcohol aqueous solution is less than the above lower limit, there is a possibility that the polyvinyl alcohol cannot be sufficiently introduced into the pores of the laminate. Conversely, if the time for which the laminate is immersed in the aqueous solution of polyvinyl alcohol exceeds the upper limit, the isopropyl alcohol bubble point may increase due to excessive adhesion of the resin.

As a minimum of the content rate (solid content rate) of polyvinyl alcohol in the said aqueous solution, 0.5 mass % is preferable and 0.7 mass % is more preferable. On the other hand, the upper limit of the content of polyvinyl alcohol in the aqueous solution is preferably 1.0% by mass, more preferably 0.8% by mass. If the content of polyvinyl alcohol in the aqueous solution is less than the lower limit, it may not be possible to form a hydrophilic resin that continuously coats the surface of the laminate. Conversely, if the content of polyvinyl alcohol in the aqueous solution exceeds the upper limit, the aqueous solution may not be impregnated into the pores of the laminate, or the pores of the laminate may be clogged.

In the cross-linking step, a network structure is formed by cross-linking the polyvinyl alcohol by a chemical reaction such as acetalization after the introduction step. Thereby, the hydrophilic resin can be made insoluble in water. Since the crystalline region of polyvinyl alcohol has hydroxyl groups, chemical cross-linking can be performed using a cross-linking agent such as glutaraldehyde. The above-mentioned cross-linking agent is used as a cross-linking liquid by dissolving the cross-linking agent in a solvent. In the cross-linking step, for example, the laminate after the introduction step is immersed in a cross-linking liquid to cross-link the polyvinyl alcohol on the surface of the laminate. The cross-linking step is preferably carried out under acid catalysis.

The cross-linking agent is not particularly limited, and examples include aldehyde compounds such as formaldehyde, glutaraldehyde and terephthalaldehyde; ketone compounds such as diacetyl and chloropentanedione; bis(2-chloroethylurea)-2-hydroxy-4,6 -Compounds having reactive halogens such as dichloro-1,3,5-triazine; compounds having reactive olefins such as divinylsulfone; N-methylol compounds; isocyanates; aziridine compounds; Epoxy compounds; halogen carboxaldehydes such as mucochloric acid; dioxane derivatives such as dihydroxydioxane; inorganic cross-linking agents such as chromium alum, zirconium sulfate, boric acid, borates, and phosphates; diazo compounds such as 2-phenylethane; and bifunctional maleic acid imides. Among these cross-linking agents, aldehyde compounds such as glutaraldehyde and terephthalaldehyde are preferred because they are highly reactive at room temperature and the resulting crosslinked structure has good chemical resistance. These cross-linking agents can be used singly or in combination of two or more.

As a minimum of the content rate (solid content rate) of the crosslinking agent in the said crosslinking liquid, 2.0 mass % is preferable. On the other hand, the upper limit of the content of the cross-linking agent in the cross-linking liquid is preferably 4.0% by mass. If the content of the cross-linking agent in the cross-linking liquid is less than the above lower limit, it may not be possible to form a hydrophilic resin that continuously coats the surface of the laminate. Conversely, if the content of glutaraldehyde in the cross-linking solution exceeds the upper limit, the amount of coating may be excessive and may clog the pores of the support layer 2a and the filtration layer 2b of the hollow fiber membrane 2, resulting in insufficient water permeability. There is

In the drying step, after the cross-linking step, the laminate is washed with pure water, for example, and then dried at room temperature to 80° C. to produce a hollow fiber membrane.

According to the hollow fiber membrane, the hydrophilic resin coating amount of the hollow fiber membrane that has been hydrophilized with polyvinyl alcohol is 0.31 mg / cm or more and 0.44 mg / cm or less, so that during the filtration process The longitudinal elongation rate can be controlled within an appropriate range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance.

<Filtration module>
The filtration module includes a plurality of hollow fiber membranes and a pair of holding members that hold both end portions of the plurality of hollow fiber membranes. A filtration module 10 according to an embodiment of the present disclosure, as shown in FIG. A pair of holding members (an upper holding member 3 and a lower holding member 4) for fixing both ends of a plurality of hollow fiber membranes 2 are provided.

[Upper holding member]
The upper holding member 3 is a member that holds the upper ends of the multiple hollow fiber membranes 2 . The upper holding member 3 has a discharge portion (water collection header) that communicates with the lumens of the plurality of hollow fiber membranes 2 and collects filtered water. A discharge pipe is connected to this discharge part, and the filtered water that permeates the interior of the plurality of hollow fiber membranes 2 is discharged. Although the shape of the upper holding member 3 is not particularly limited, for example, a square shape that facilitates molding and fixing of a plurality of hollow fiber membranes 2 is preferably adopted.

[Lower holding member]
The lower holding member 4 holds the lower ends of the multiple hollow fiber membranes 2 . The lower holding member 4 may form an internal space like the upper holding member 3, and may hold the lower ends of the hollow fiber membranes 2 in such a way as to close the openings of the hollow fiber membranes 2. . The lower holding member 4 may be a member for folding back the hollow fiber membranes 2 . That is, in the filtration module 10, the adjacent hollow fiber membranes 2 may be connected at their lower ends. The shape, material, etc. of the lower holding member 4 may be the same as those of the upper holding member 3 .

Further, the lower holding member 4 may have a configuration in which one hollow fiber membrane 2 is bent in a U shape and folded back. In this case, the upper holding member 3 holds both ends of the hollow fiber membrane 2 .

In order to facilitate handling (transportation, installation, replacement, etc.) of the filtration module 10, the upper holding member 3 and the lower holding member 4 may be connected by a connecting member. As this connection member, for example, a support rod made of metal, a casing (outer cylinder) made of resin, or the like can be used.

The hollow fiber membrane 2 includes a porous support layer 2a having a cylindrical shape, a porous filtration layer 2b laminated on the outer peripheral surface of the support layer 2a, and an inner peripheral surface of the support layer 2a and the filtration layer. It has a hydrophilic resin covering the outer peripheral surface of 2b. The support layer 2a and the filter layer 2b are mainly composed of polytetrafluoroethylene. The hydrophilic resin has a crosslinked structure of polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction is 0.31 mg/cm or more and 0.44 mg/cm or less. The details of each element contained in the hollow fiber membrane 2 are as described above.

In the filtration module 10, the lower limit of the average effective length along the central axis of the hollow fiber membrane 2 is preferably 1 m, more preferably 2 m. On the other hand, the upper limit of the average effective length of the hollow fiber membrane 2 is preferably 8 m, more preferably 7 m. If the average effective length of the hollow fiber membranes 2 is less than the above lower limit, the volumetric efficiency of the filtration module 10 may decrease. Conversely, if the average effective length of the hollow fiber membranes 2 exceeds the upper limit, there is a risk that the hollow fiber membranes 2 will bend too much due to the weight of the hollow fiber membranes 2. There is a possibility that handleability may deteriorate. Here, the "average effective length" of the hollow fiber membrane 2 means the average length of the portion exposed between the pair of holding members. More specifically, the above-mentioned "average effective length" means the average distance from the upper end fixed to the upper holding member 3 to the lower end fixed to the lower holding member 4, and one hollow fiber membrane 2 is curved in a U shape, and this curved portion is used as the lower end portion and fixed by the lower holding member 4, it means the average length of the portion exposed between the pair of holding members.

According to the filtration module, a plurality of hollow fiber membranes whose surfaces are coated with a hydrophilic resin are provided, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm. When it is 0.44 mg/cm or less, the elongation rate in the longitudinal direction during filtration can be controlled within an appropriate range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance. The filtration module can be suitably used in various fields as a solid-liquid separation apparatus.

<Wastewater treatment equipment>
The wastewater treatment apparatus decomposes organic matter in water to be treated, such as industrial wastewater, livestock wastewater, and sewage, using activated sludge, which is an aerobic microorganism, and separates and discharges impurities using a filtration module. That is, the wastewater treatment equipment is equipment for treating wastewater by the membrane separation activated sludge method. A wastewater treatment apparatus 20 according to an embodiment of the present disclosure shown in FIG. 12. The wastewater treatment apparatus 20 also includes a suction pump 14 for sucking filtered water filtered by the hollow fiber membranes 2 through a discharge pipe 13 connected to the discharge portion of each filtration module 10 . The upper holding member 3 of the filtration module 10 has a discharge portion for discharging filtered water filtered by the hollow fiber membrane 2, and a suction pump 14 is connected to a discharge pipe 13 connected to the discharge portion. be. Filtered water is discharged from the discharge pipe 13 by the suction pump 14 .

[Water tank]
The water tank 11 stores water to be treated and houses the plurality of filtration modules 10 . As the material of the water tank 11, for example, resin, metal, concrete, or the like can be used. In the wastewater treatment device 20, the plurality of filtration modules 10 are arranged side by side with a gap.

The filtration module 10 housed in the water tank 11 includes a plurality of hollow fiber membranes 2, an upper holding member 3 which is a pair of holding members for holding both ends of the plurality of hollow fiber 2 membranes in the vertical direction, and It has a lower holding member 4 .

In the wastewater treatment apparatus 20 of one embodiment of the present disclosure, the upper holding members 3 and the lower holding members 4 of the plurality of filtration modules 10 accommodated in the water tank 11 hold both ends of several hollow fiber membranes 2. It is arranged so as to be held vertically. By holding the plurality of hollow fiber membranes in the vertical direction, when combined with the gas supplier 12 that supplies the air bubbles B from below, the air bubbles B are formed on the surface of the hollow fiber membranes 2 held in the vertical direction. Since it rises along, the surface washing efficiency of the said waste water treatment apparatus 20 can be improved more effectively.
Details of the elements contained in filtration module 10 are provided above.

The hollow fiber membrane 2 includes a porous support layer 2a having a cylindrical shape, a porous filtration layer 2b laminated on the outer peripheral surface of the support layer 2a, and an inner peripheral surface of the support layer 2a and the filtration layer. It has a hydrophilic resin covering the outer peripheral surface of 2b. The support layer 2a and the filter layer 2b are mainly composed of polytetrafluoroethylene. The hydrophilic resin has a crosslinked structure of polyvinyl alcohol, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction is 0.31 mg/cm or more and 0.44 mg/cm or less. The details of each element contained in the hollow fiber membrane 2 are as described above.

<Air bubble supplier>
The air bubble supplier 12 supplies air bubbles B for washing the surface of the hollow fiber membranes 2 from below the filtration module 10 . The air bubbles B rise while scraping the surface of the hollow fiber membrane 2 to remove activated sludge or the like adhering to the hollow fiber membrane 2 or suppress adhesion of the activated sludge or the like to the surface. In addition, since the hollow fiber membranes 2 are oscillated by the air bubbles B, cleaning of the surface of the hollow fiber membranes 2 is promoted and clogging of the hollow fiber membranes 2 is suppressed.

The air bubble supplier 12 is immersed in a water tank 11 storing water to be treated together with the filtration module 10, and continuously or intermittently discharges gas supplied from a compressor or the like through an air supply pipe (not shown). Supply air bubbles B.

Such a bubble supplier 12 is not particularly limited, and a known air diffuser can be used. The air diffuser includes, for example, an air diffuser using a perforated plate or perforated tube in which a large number of holes are formed in a plate or tube made of resin or ceramics, and a jet type air diffuser that injects gas from a diffuser or a sparger. device, an intermittent air bubble injection diffuser that intermittently injects air bubbles, and a bubbling jet nozzle that injects air bubbles mixed with water flow.

The gas that forms bubbles supplied from the bubble supplier 12 is not particularly limited as long as it is a general inert gas, but it is preferable to use air from the viewpoint of running costs.

In addition, although not shown, the waste water treatment apparatus 20 includes an aeration apparatus for supplying air (oxygen) to the activated sludge, a sludge extraction apparatus for discharging excessive activated sludge, and a support for components in the water tank 11. It may have a frame, a controller, and the like.

According to the wastewater treatment apparatus, a filtration module having a plurality of hollow fiber membranes coated with a hydrophilic resin on the surface is provided, and the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is When it is 0.31 mg/cm or more and 0.44 mg/cm or less, the longitudinal elongation rate during filtration can be controlled within an appropriate range. As a result, it is possible to suppress the occurrence of buckling of the hollow fiber membrane and improve filtration performance.

[Other embodiments]
It should be considered that the embodiments disclosed this time are illustrative in all respects and not restrictive. The scope of the present disclosure is not limited to the configuration of the above-described embodiment, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims. be.

In the filtration module, the upper holding member may seal the hollow fiber membranes, and the lower holding member may have the discharge section.

The filtration module is not limited to the immersion type in which the water to be treated is permeated to the inner peripheral side by the negative pressure on the inner peripheral side. It can be of any type, such as an external pressure type that permeates the inner peripheral surface of the membrane, or an internal pressure type that presses the inner peripheral surface of the hollow fiber membrane and permeates the water to be treated to the outer peripheral surface of the hollow fiber membrane. can be done.

The filtration module may be used to filter water to be treated other than water containing activated sludge.

The wastewater treatment apparatus further includes a water tank for sedimenting suspended matter in the water to be treated, a water tank for decomposing organic matter with activated sludge, etc., in addition to the water tank for filtration treatment in which the filtration module is arranged. may be

EXAMPLES The present invention will be described in detail below based on examples, but the present invention is not limitedly interpreted based on the description of these examples.

<Test No. 1 to No. 5>
(Hollow fiber membrane)
A supporting layer made of polytetrafluoroethylene, having an average thickness of 600 μm, an average pore diameter of 2 μm, and a porosity of 80%; Hydrophilization treatment was performed on the surface of a cylindrical laminate having an average outer diameter of 2.3 mm and having a filter layer. Hydrophilization treatment first immersed the laminate of the support layer and the filtration layer in isopropyl alcohol for 1 hour or longer. Next, the operation of putting the film into an aqueous solution of isopropyl alcohol having a concentration of 0.75% by mass and immediately taking it out was repeated twice to wash the isopropyl alcohol. Next, the substrate was immersed in an aqueous solution of polyvinyl alcohol having a concentration shown in Table 1 for 2.5 hours or more for 6 hours, and the aqueous solution of polyvinyl alcohol was introduced. After that, excess liquid was removed from the surface of the membrane. Then, the polyvinyl alcohol was crosslinked by being immersed in a glutaraldehyde crosslinking solution having a concentration of 2.5% by mass for 6 hours or more. Next, the excess cross-linked product was washed with pure water, and the test No. 1 to No. No. 5 hollow fiber membrane was produced.

(filtration module)
No. 1 to No. A filtration module having 512 hollow fiber membranes with an average effective length of 200 cm was prepared using the hollow fiber membranes of No. 5. The average effective length is No. 1 to No. For each of 10 hollow fiber membranes of No. 5, the average length of the portion exposed between the pair of holding members of the filtration module was measured.

[evaluation]
(Coating amount of hydrophilic resin)
No. 1 to No. With respect to the hollow fiber membrane No. 5, the coating amount (mg/cm) of the hydrophilic resin per unit length in the longitudinal direction was measured by the following procedure.
The weight of the hollow fiber membrane before and after coating was measured with a precision electronic balance, and the amount of coating was calculated by subtracting the weight of the hollow fiber membrane before coating from the weight of the hollow fiber membrane after coating.

(longitudinal elongation)
No. 1 to No. Using a filtration module equipped with No. 5 hollow fiber membranes, no. 1 to No. The elongation rate (%) per unit length in the longitudinal direction of the hollow fiber membrane No. 5 was measured.
After the filtration operation, the hollow fiber membrane was cut out from the module, and the length was actually measured to calculate the elongation rate from the dry state before the operation. The elongation in the longitudinal direction is No. 1 to No. 5 shows the elongation rate (%) when the average effective length in the dry state is 100 after hydrophilization treatment for each of the hollow fiber membranes of No. 5.

(Buckling evaluation)
No. 1 to No. Using the hollow fiber membrane No. 5, buckling was evaluated by the following procedure.
The occurrence of buckling was visually confirmed. The occurrence of buckling was evaluated in three grades from A to C. The evaluation criteria for the occurrence of buckling were as follows. If the evaluation of occurrence of buckling is A, it is good. Table 1 shows the evaluation results.
A: No buckling at all.
B: It can be visually confirmed that the hollow fiber membrane is bent in the range of 25° or more and less than 30° from the vertical position.
C: It can be visually confirmed that the hollow fiber membrane is bent by 30° or more from the vertical position.

(Water permeability of hollow fiber membrane)
No. 1 to No. The water permeability of the hollow fiber membrane No. 5 was measured. The water permeation amount was measured by measuring the pure water flow rate (mL/min/cm ² ) of the hollow fiber membrane under an internal pressure of 100 kPa.

(Isopropyl alcohol bubble point)
No. 1 to No. The isopropyl alcohol bubble point (kPa) of the hollow fiber membrane No. 5 was measured using isopropyl alcohol according to ASTM-F316-86.

As shown in Table 1, no. 3 to No. The hollow fiber membrane No. 4 had a longitudinal elongation of 2.0% or less, did not undergo buckling, and had a good water permeability.
On the other hand, no. 1 to No. In the hollow fiber membrane of No. 2, the elongation in the longitudinal direction was kept small, but buckling occurred. Furthermore, No. 1, which has a hydrophilic resin coating amount of 0.15 mg/cm. The hollow fiber membrane of No. 1 had a small water permeability because the surface thereof was not sufficiently hydrophilized.
In addition, No. 3, which has a coating amount of the hydrophilic resin per unit length in the longitudinal direction of more than 0.44 mg/cm. The hollow fiber membrane of No. 5 had a large elongation in the longitudinal direction, buckling occurred, and the isopropyl alcohol bubble point was very large, so that the water permeability was small.

As described above, in the wastewater treatment device including the filtration module having the hollow fiber membrane, even if the hydrophilization treatment with polyvinyl alcohol is performed, the coating amount of the hydrophilic resin on the hollow fiber membrane is 0.31 mg / cm or more. It was confirmed that by setting the content to 0.44 mg/cm or less, the elongation rate in the longitudinal direction during the filtration process can be controlled within an appropriate range, and the filtration performance can be improved.

2 Hollow fiber membrane 2a Support layer 2b Filtration layer 3 Upper holding member 4 Lower holding member 5 Hydrophilic resin 10 Filtration module 11 Water tank 12 Air bubble supplier 13 Discharge pipe 14 Suction pump 20 Wastewater treatment device B Air bubbles

Claims (3)

A hollow fiber membrane in which the outer peripheral surface and the inner peripheral surface are coated with a hydrophilic resin,
The main component is polytetrafluoroethylene,
The average outer diameter is 1 mm or more and 6 mm or less,
The average inner diameter is 0.3 mm or more and 4 mm or less,
The isopropyl alcohol bubble point is 80 kPa or more and 140 kPa or less,
The main component of the hydrophilic resin is polyvinyl alcohol,
A hollow fiber membrane, wherein the coating amount of the hydrophilic resin per unit length in the longitudinal direction is 0.31 mg/cm or more and 0.44 mg/cm or less. a plurality of hollow fiber membranes coated with a hydrophilic resin on the outer peripheral surface and the inner peripheral surface;
A pair of holding members holding both ends of the plurality of hollow fiber membranes,
The hollow fiber membrane is mainly composed of polytetrafluoroethylene,
The hollow fiber membrane has an average outer diameter of 1 mm or more and 6 mm or less, an average inner diameter of 0.3 mm or more and 4 mm or less, and an isopropyl alcohol bubble point of 80 kPa or more and 140 kPa or less,
The main component of the hydrophilic resin is polyvinyl alcohol,
A filtration module in which the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more and 0.44 mg/cm or less. a water tank for storing water to be treated;
a filtration module housed in the water tank;
an air bubble supplier that is housed in the water tank and supplies air bubbles from below the filtration module,
The filtration module has a plurality of hollow fiber membranes whose outer and inner peripheral surfaces are coated with a hydrophilic resin, and a pair of holding members that vertically hold both ends of the plurality of hollow fiber membranes. ,
The hollow fiber membrane is mainly composed of polytetrafluoroethylene,
The hollow fiber membrane has an average outer diameter of 1 mm or more and 6 mm or less, an average inner diameter of 0.3 mm or more and 4 mm or less, and an isopropyl alcohol bubble point of 80 kPa or more and 140 kPa or less,
The main component of the hydrophilic resin is polyvinyl alcohol,
A waste water treatment device, wherein the coating amount of the hydrophilic resin per unit length in the longitudinal direction of the hollow fiber membrane is 0.31 mg/cm or more and 0.44 mg/cm or less.

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