JP7643123B2 - Hollow fiber membrane and its manufacturing method, hollow fiber membrane module, wastewater treatment device, and wastewater treatment method - Google Patents

Description

The present invention relates to a hollow fiber membrane, a hollow fiber membrane module, a wastewater treatment device, and a wastewater treatment method.

Industrial wastewater and domestic wastewater are treated to remove organic matter contained in the wastewater, and then either reused as industrial water or discharged into rivers, etc. Typical methods for treating industrial wastewater include activated sludge treatment, in which the water to be treated is aerated and organic matter is decomposed by aerobic microorganisms. Such biological water treatment methods, such as activated sludge treatment, use aerobic microorganisms and denitrifying bacteria to remove nitrate nitrogen. In recent years, treatment has also been carried out using the membrane separation activated sludge (MBR) method, which combines activated sludge treatment with membrane filtration using a separation membrane module.

When using aerobic microorganisms to treat wastewater, it is necessary to aerate the water being treated with air or oxygen in order to maintain the activity of the microorganisms and improve the treatment capacity. Thus, a method using hollow fiber membranes is adopted as a method for efficiently aerating the water being treated. This hollow fiber membrane is used in various fields, such as wastewater treatment, drinking water production, water purification, etc.

Recently, a method has been proposed for treating wastewater using a hollow fiber membrane, in which a microbial layer (biofilm) derived from microorganisms in wastewater is formed on the outer surface of the hollow fiber membrane and wastewater treatment is performed in this state. In a bioreactor that forms a microbial layer on the outer surface of the hollow fiber membrane and supplies oxygen from the inner side of the hollow fiber membrane, a so-called membrane aeration biofilm reactor (MABR), an oxygen gradient is formed in the thickness direction of the microbial layer. As a result, aerobic treatment (BOD oxidation, nitrification of ammonia) progresses on the inner layer side of the microbial layer, while anaerobic treatment of nitrate (BOD oxidation, denitrification) progresses on the outer layer side of the microbial layer, making it possible to remove various pollutants in a single process. Therefore, compared to the conventional membrane separation activated sludge method, in which aerobic treatment and anaerobic treatment are performed in separate treatment tanks, equipment that requires less space can be realized.

In addition, the inclusion of a microbial layer in the MABR increases the oxygen dissolution efficiency compared to conventional systems, which reduces the operating load of the blower that supplies oxygen and reduces the generation of sludge, making it possible to reduce the running costs of the entire facility. Furthermore, the formation of a microbial layer ensures a large membrane surface area for the hollow fiber membrane, making it possible to perform stable treatment even with fluctuations in the inflow load of wastewater. Therefore, the introduction of the MABR is being considered in a wide range of wastewater treatment equipment.
For example, Patent Document 1 discloses a biological treatment device equipped with a hollow fiber membrane module in which hollow fiber membranes are arranged crossing each other horizontally and vertically. The hollow fiber membrane module of Patent Document 1 allows microorganisms to easily attach to the hollow fiber membranes, and a microbial layer is easily formed.

JP 2018-75538 A

However, in the hollow fiber membrane module described in Patent Document 1, condensed water (water contained in oxygen, air, etc., or water in wastewater condensed as water inside the membrane) tends to accumulate inside the horizontally arranged hollow fiber membranes. This makes it difficult for oxygen to pass through the hollow fiber membranes, reducing the effective surface area and causing a problem of reduced wastewater treatment capacity. Therefore, there is a demand for hollow fiber membranes to which microorganisms can easily attach and which can easily form a microbial layer, even when the hollow fiber membranes are not crossed.
An object of the present invention is to provide a hollow fiber membrane, a hollow fiber membrane module, a wastewater treatment device, and a wastewater treatment method in which a microbial layer is easily formed.

The present invention has the following aspects.
[1] A hollow fiber membrane for wastewater treatment, in which a microbial layer derived from microorganisms or bacteria in the wastewater is formed on the outer surface during the wastewater treatment,
The hollow fiber membrane has a surface zeta potential of -8 mV or more, as measured under the following measurement conditions.
(Measurement conditions)
A particle dispersion (pH=6 at 25° C.) prepared by diluting polystyrene latex having a solid content of 0.25% by mass and an average particle size of 500 nm 150 times with a 10 mM aqueous sodium chloride solution was used, and a plurality of the hollow fiber membranes were arranged and laid out on a sample mounting surface of a plate measurement cell without overlapping or gaps, and in close contact with each other. From the zeta potential measurement result of the polystyrene latex by electrophoresis, the surface zeta potential of the hollow fiber membranes was calculated.
[2] The hollow fiber membrane according to [1], wherein the hollow fiber membrane comprises a hollow membrane body and a coating layer formed on the outer surface of the membrane body, and the coating layer contains a cationic polymer and an anionic polymer.
[3] The hollow fiber membrane according to [2] above, wherein the cationic polymer comprises one or more selected from the group consisting of polyethyleneimine, polyallylamine hydrochloride, and polydiallyldimethylammonium chloride.
[4] The hollow fiber membrane according to [2] or [3], wherein the anionic polymer comprises one or more selected from polystyrene sulfonic acid and polyacrylic acid.
[5] The hollow fiber membrane according to any one of [2] to [4], wherein the cationic polymer is unevenly distributed on a surface of the coating layer.
[6] The hollow fiber membrane according to any of [2] to [5], wherein the membrane body has a multilayer structure including one or more porous layers and one or more non-porous layers, and the porous layers are disposed as outermost layers.
[7] The hollow fiber membrane according to [6], wherein the porous layer is made of a material containing one or more selected from a polyolefin-based resin, a polyurethane-based resin, and a fluorine-based resin.
[8] The hollow fiber membrane according to [6] or [7], wherein the non-porous layer is made of a material containing one or more selected from a styrene-based resin and a polyolefin-based resin.
[9] A hollow fiber membrane module comprising the hollow fiber membrane according to any one of [1] to [8].
[10] A wastewater treatment device comprising the hollow fiber membrane module according to [9].
[11] A wastewater treatment method for treating wastewater using the hollow fiber membrane module of [9] or the wastewater treatment device of [10], comprising:
The wastewater treatment method comprises forming the microbial layer derived from microorganisms or bacteria in the wastewater on the outer surface of the hollow fiber membrane, and then supplying an oxygen-containing gas into the hollow portion of the hollow fiber membrane.

The present invention provides a hollow fiber membrane, a hollow fiber membrane module, a wastewater treatment device, and a wastewater treatment method that are easy to form a microbial layer.

FIG. 1 is a partial perspective view showing a multi-layer structure of a hollow fiber membrane for typically explaining one embodiment of the hollow fiber membrane according to the present invention. FIG. 1 is a partial perspective view showing a state in which a microbial layer has been formed on the surface of a coating layer provided on a hollow fiber membrane, for illustrating one embodiment of the hollow fiber membrane according to the present invention. FIG. 1 is a schematic diagram showing an aerobic treatment region and an anaerobic treatment region in a microbial layer, for illustrating one embodiment of a hollow fiber membrane according to the present invention. FIG. 1 is a schematic diagram showing the overall configuration of an apparatus including a wastewater treatment tank, for illustrating an embodiment of a hollow fiber membrane module, a wastewater treatment apparatus, and a wastewater treatment method according to the present invention. FIG. 2 is a diagram illustrating a method for measuring the adhesion amount of a microbial layer.

Hereinafter, an embodiment of the hollow fiber membrane, hollow fiber membrane module, wastewater treatment device, and wastewater treatment method according to the present invention will be described in detail with reference to Figs. 1 to 4 as appropriate.
In addition, in each drawing used in the following description, characteristic parts may be enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones. Furthermore, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and can be appropriately changed and implemented within the scope of the present invention.

As shown in Figures 1 to 4, the hollow fiber membrane 1 of this embodiment, the hollow fiber membrane module 10 equipped with this hollow fiber membrane 1, the wastewater treatment device 100 equipped with a plurality of hollow fiber membrane modules 10, and the wastewater treatment method using the hollow fiber membrane module 10 or the wastewater treatment device 100 are used for treatment (wastewater treatment) to remove organic matter and the like contained in industrial wastewater, domestic wastewater, etc.

[Hollow fiber membrane]
Hereinafter, one embodiment of the hollow fiber membrane of the present invention will be described in detail.
As shown in Figs. 1 and 2, the hollow fiber membrane 1 of this embodiment has a microbial layer 6 derived from microorganisms or bacteria in the wastewater formed on its outer surface during wastewater treatment.
The hollow fiber membrane 1 in the illustrated example comprises a hollow membrane body 7 and a coating layer 8 formed on the outer surface of the membrane body 7 .

<Membrane body>
The membrane body 7 in the illustrated example has a multi-layer structure including two porous layers 2 and 4 and one non-porous layer 3, with the porous layer 4 disposed as the outermost layer. In the illustrated example, it has a three-layer structure in which the non-porous layer 3 is disposed between the two porous layers 2 and 4. In the following specification, the porous layer 2 disposed as the innermost layer is also referred to as the "first porous layer 2," and the porous layer 4 disposed as the outermost layer is also referred to as the "second porous layer 4."

The membrane body 7 in the illustrated example has a hollow portion 5 on the inner surface 2b side (the surface opposite to the side in contact with the non-porous layer 3) of the first porous layer 2, which is the innermost layer. By supplying oxygen to this hollow portion 5, oxygen can be transmitted from the inner surface 2b side toward the surface 4a of the second porous layer 4 on the outer layer side.

(Porous Layer)
In the membrane body 7 of this embodiment, the two porous layers, the first porous layer 2 and the second porous layer 4, are concentrically arranged via the non-porous layer 3. Although not shown in the figure, the first porous layer 2 and the second porous layer 4 are each composed of a membrane having a plurality of pores, and the pores are formed over the entire circumference of these porous layers 2, 4.

In this embodiment, the pores formed in the first porous layer 2 and the second porous layer 4 refer to holes that communicate at least from the inner surface side to the outer surface. For example, as shown in Fig. 3, in the first porous layer 2, the pores refer to holes that communicate from the inner surface 2b side to the outer surface (the surface in contact with the non-porous layer 3) 2a side, and in the second porous layer 4, the pores refer to holes that communicate from the inner surface (the surface in contact with the non-porous layer 3) 4b side to the surface 4a side.
Furthermore, the "pores" described in this embodiment refer to the gaps in the membrane substrate that form a three-dimensional structure, and refer to the portions that serve as paths through which oxygen passes when it permeates from the inner surface side to the surface (outer surface) side.

The porous layers 2 and 4 are formed, for example, by the melt-drawing method. In the melt-drawing method, the resin that is to be the material for the porous layers is first heated to a fluid state above its melting point and then extruded into a cylindrical shape. The extruded resin in a fluid state is then cooled to make it non-fluid and fix the shape. The resin with the fixed shape is then stretched under optimal conditions to form the porous structure.

The porous layers 2, 4 are preferably made of a material containing one or more selected from polyolefin resins, polyurethane resins, and fluorine resins, for example, from the viewpoint of further increasing oxygen permeability. By making the porous layers from a material containing one or more of the above resins, it becomes possible to form the porous structure as described above while controlling it, and oxygen permeability is further increased, and oxidation resistance, chemical resistance, heat resistance, and mechanical durability can be imparted. In addition, solubility in a solvent when preparing the film-forming solution is also improved.
The first porous layer 2 and the second porous layer 4 may be made of the same material or different materials. In particular, the first porous layer 2 and the second porous layer 4 are preferably made of a material containing a polyolefin resin.

The thickness of each of the first porous layer 2 and the second porous layer 4 is preferably such that the total thickness of the entire porous layer, i.e., the average thickness Dp of the total of the first porous layer 2 and the second porous layer 4 in the illustrated example, is 10 to 100 μm. If the average thickness Dp of the total of the porous layers is 10 μm or more, this contributes to ensuring sufficient mechanical strength in the entire hollow fiber membrane 1. On the other hand, if the average thickness Dp of the total of the porous layers is 100 μm or less, the outer diameter of the membrane does not become larger than necessary, and therefore it is possible to prevent the membrane packing amount from becoming small when the hollow fiber membrane 1 is modularized.
From the above viewpoint, the average total thickness Dp of all the porous layers is more preferably 15 to 90 μm, and further preferably 20 to 80 μm.

The average thickness of the porous layers 2 and 4 was determined by observing the cross section of the hollow fiber membrane with a scanning electron microscope (SEM), analyzing the image, measuring the thickness of the porous layers 2 and 4 at multiple locations (5 locations), and calculating the average value.

In the membrane body 7 of this embodiment, a porous layer is disposed in the outermost layer. In this way, by disposing a porous layer in the outermost layer, sufficient oxygen permeability can be ensured.

The average pore diameter of the pores formed in the porous layers 2 and 4 is preferably 0.01 to 5 μm. If the average pore diameter of the pores in the porous layers 2 and 4 is 0.01 μm or more, they are unlikely to act as a resistance that affects oxygen permeation. Furthermore, if the average pore diameter of the pores in the porous layers 2 and 4 is 5 μm or less, sufficient membrane strength can be obtained.
From the above viewpoint, the average pore size of the pores in the porous layers 2 and 4 is more preferably 0.03 to 4 μm, and further preferably 0.05 to 3 μm.

The average pore diameter of the multiple pores formed in the porous layers 2 and 4 was determined by observing the outer surface of the porous layers using a SEM, randomly selecting 30 pores, measuring the longest diameter of each pore, and averaging the longest diameters of these 30 pores.

(Non-porous layer)
In the membrane body 7 of the present embodiment, the non-porous layer 3 is provided as a single layer between the first porous layer 2 and the second porous layer 4 .

The non-porous layer 3 is preferably made of a material containing one or more selected from, for example, a styrene-based resin and a polyolefin-based resin. By making the non-porous layer 3 out of a material containing one or more of the above resins, it is possible to further increase the mechanical strength of the entire hollow fiber membrane 1 while ensuring sufficient oxygen permeability. Among the above resins, it is more preferable that the non-porous layer 3 be made of a material containing a styrene-based resin from the viewpoint of further increasing oxygen permeability.

The styrene-based resin is a polymer obtained by polymerizing a styrene-based monomer, or a polymer obtained by polymerizing a styrene-based monomer and another monomer.
Examples of the styrene monomer include styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, 1,3-dimethylstyrene, vinylnaphthalene, and vinylanthracene.
Examples of other monomers include maleic anhydride, acrylic acid, methacrylic acid, methyl acrylate, ethyl acrylate, propyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, and acrylonitrile.

The styrene resin preferably contains 50 to 100% by mass of styrene monomer units relative to the total mass of the monomer units constituting the styrene resin, more preferably 70 to 100% by mass, and even more preferably 90 to 100% by mass. In particular, the styrene resin preferably contains 50 to 100% by mass of styrene units relative to the total mass of the monomer units constituting the styrene resin, more preferably 70 to 100% by mass, and even more preferably 90 to 100% by mass.

The average thickness of the non-porous layer 3 is preferably 0.3 to 10 μm. If the average thickness of the non-porous layer 3 is 0.3 μm or more, no defects are generated during manufacturing, and stable production becomes possible. Furthermore, if the average thickness of the non-porous layer 3 is 10 μm or less, a decrease in oxygen permeability that would affect practical use can be suppressed.
From the above viewpoints, the average thickness of the non-porous layer 3 is more preferably from 0.5 to 8 μm, and further preferably from 1 to 6 μm.

The average thickness of the non-porous layer 3 was determined by observing the cross section of the hollow fiber membrane with a scanning electron microscope (SEM), analyzing the image, measuring the thickness of the non-porous layer 3 at multiple locations (five or more locations), and calculating the average value.

As shown in FIG. 3, it is preferable that the total average thickness Dn of the non-porous layer 3, i.e., the average thickness Dn of the non-porous layer 3 formed as a single layer, and the total average thickness Dp of the porous layers 2 and 4, i.e., the total average thickness Dp of the first porous layer 2 and the second porous layer 4, satisfy the relationship expressed by the following equation: {0.005≦Dn/Dp≦1.0}.
When the relationship between the average thickness Dn of the non-porous layer 3 and the average thickness Dp of the porous layers 2 and 4 satisfies the relationship expressed by the above formula, the ratio of the thickness of the non-porous layer 3 to the total thickness of the porous layers 2 and 4 and the non-porous layer 3 becomes small. Therefore, the non-porous layer can be made thinner while ensuring sufficient mechanical strength suitable for practical use, thereby further improving oxygen permeability.
From the above viewpoint, the relationship between the total average film thickness Dn of the non-porous layer 3 and the total average film thickness Dp of the porous layers 2 and 4 is preferably expressed by the following formula: {0.007≦Dn/Dp≦0.9}, and more preferably expressed by the following formula: {0.01≦Dn/Dp≦0.8}.

<Coating Layer>
The coating layer 8 is a layer formed on the outer surface of the membrane body 7 (the surface 4a of the second porous layer 4 in this embodiment), and contains a cationic polymer and an anionic polymer.
By forming the coating layer 8 containing a cationic polymer and an anionic polymer on the outer surface of the membrane body 7, the surface zeta potential of the hollow fiber membrane 1 is likely to be high, the adhesion of the microbial layer 6 to the surface 8a of the coating layer 8 is increased, and the microbial layer 6 is likely to be formed in a short period of time. In particular, from the viewpoint of further increasing the surface zeta potential of the hollow fiber membrane 1 and facilitating the formation of the microbial layer 6 in a short period of time, it is preferable that the cationic polymer is unevenly distributed on the surface 8a of the coating layer 8.
As the wastewater treatment progresses, the microbial layer 6 swells, and it is therefore necessary to periodically peel off the microbial layer 6. Since the coating layer 8 is formed on the outer surface of the membrane body 7, the microbial layer 6 that has once been peeled off reattaches to the surface 8a of the coating layer 8 in a short period of time.
Here, the “surface 8 a of the coating layer 8 ” refers to the surface of the coating layer 8 opposite to the surface in contact with the second porous layer 4 .

The coating layer 8 preferably has a multi-layer structure in which layers containing a cationic polymer and layers containing an anionic polymer are alternately laminated. Since the surface 4a of the second porous layer 4 is usually negatively charged, it is preferable that the innermost layer of the coating layer 8, i.e., the layer in contact with the surface 4a of the second porous layer 4, is a layer containing a cationic polymer. It is also preferable that the outermost layer of the coating layer 8, i.e., the layer in contact with the microbial layer 6, is a layer containing a cationic polymer.
At the interface between the layer containing a cationic polymer and the layer containing an anionic polymer, the region consisting of the layer containing a cationic polymer and the region consisting of the layer containing an anionic polymer may slightly intersect with each other.

Examples of cationic polymers include compounds having functional groups capable of bearing a positive charge, such as quaternary ammonium groups and amino groups. Specific examples of such polymers include polyethyleneimine (PEI), polyallylamine hydrochloride (PAH), polydiallyldimethylammonium chloride (PDDA), polyvinylpyridine (PVP), polylysine, polyethylenemethylenediethylmethylamine (PSMDEMA), precursor of poly(phenylene vinylene) (Pre-PPV), polymethylpyridylvinyl (PMPyV), and protonated poly(p-pyridyl vinylene) (R-PHPyV). Among these, from the viewpoint of versatility, polyethyleneimine (PEI), polyallylamine hydrochloride (PAH), and polydiallyldimethylammonium chloride (PDDA) are preferred.
These cationic polymers may be used alone or in combination of two or more kinds.

Examples of anionic polymers include compounds having functional groups capable of carrying a negative charge, such as sulfonic acid, sulfuric acid, and carboxylic acid. Specific examples of the anionic polymer include polystyrene sulfonic acid (PSS), polyvinyl sulfate (PVS), dextran sulfate, chondroitin sulfate, hyaluronic acid, polyacrylic acid (PAA), polymethacrylic acid (PMA), polymaleic acid, polyfumaric acid, Poly(1-(4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido)-1,2-ethanediyl) (PAZO), Poly(anilinepropanesulfonic acid) (PAPSA), Sulfonated polyaniline (SPAN), Poly(thiophene-3-acetic acid) (PAPSA), and the like. Examples of suitable conductive polymers include poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PTAA), poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA), conductive polymers such as polythiophene and polyaniline, polymers having chromophores, liquid crystal polymers, and biopolymers such as DNA. Among these, polystyrene sulfonic acid (PSS) and polyacrylic acid (PAA) are preferred from the viewpoint of versatility.
These anionic polymers may be used alone or in combination of two or more kinds.

The average thickness of the coating layer 8 is preferably 1 to 30 μm. If the average thickness of the coating layer 8 is 1 μm or more, it becomes possible to stably produce the coating layer 8. Furthermore, if the average thickness of the coating layer 8 is 30 μm or less, it is possible to suppress a decrease in oxygen permeability that would affect practical use.
From the above viewpoints, the average thickness of the coating layer 8 is more preferably from 2 to 20 μm, and further preferably from 3 to 10 μm.

The average thickness of the coating layer 8 was determined by observing the cross section of the hollow fiber membrane with a scanning electron microscope (SEM), analyzing the image, measuring the thickness of the coating layer 8 at multiple locations (five or more locations), and calculating the average value.

<Microbial layer>
For example, when the hollow fiber membrane 1 is used as a hollow fiber membrane module described later for wastewater treatment, activated sludge already used in another wastewater treatment plant or the like is used as a seed to grow microorganisms or bacteria, and the hollow fiber membrane module is immersed in the solution to a predetermined concentration, whereby the microbial layer 6 is formed on the outer surface of the hollow fiber membrane 1, originating from the microorganisms or bacteria. In this embodiment, the microbial layer 6 is formed on the surface 8a of the coating layer 8. That is, in this embodiment, the surface 8a of the coating layer 8 is the outer surface of the hollow fiber membrane 1.

The above-mentioned activated sludge has various component compositions and ratios depending on the type of wastewater, but it is possible to use activated sludge that has multiplied by feeding on the BOD (organic matter) components and nutrients (nitrogen, phosphorus, etc.) contained in the wastewater. Then, a microbial layer 6 is formed on the surface 8a of the coating layer 8 as a layer derived from the microorganisms and bacteria contained therein.

Figure 3 is a schematic diagram showing the aerobic treatment area and the anaerobic treatment area in the microbial layer 6 (see also Figure 2). As shown in Figure 3, in the hollow fiber membrane 1 of this embodiment, by forming the above-mentioned microbial layer 6, oxygen that has permeated from the hollow part 5 of the hollow fiber membrane 1 through the first porous layer 2, the non-porous layer 3, the second porous layer 4, and the coating layer 8 in order is dissolved and diffused in the microbial layer 6, forming an oxygen gradient (concentration) in the thickness direction of the microbial layer 6, and the inner layer side is in an oxygen-rich aerobic state, while the outer layer side is in an oxygen-reduced anaerobic state. As a result, the microbial layer 6 is in a state in which an aerobic treatment area 6A is formed on the inner layer side and an anaerobic treatment area 6B is formed on the outer layer side.

In the aerobic treatment area 6A, the ammonia contained in the wastewater is oxidized to nitrate through aerobic treatment (BOD oxidation). Furthermore, in the anaerobic treatment area 6B, the nitrate produced in the aerobic treatment area 6A is converted to nitrogen by anaerobic bacteria through anaerobic treatment (BOD oxidation), and is denitrified. This allows both aerobic and anaerobic treatment to be carried out in a single process, which allows for space-saving equipment and improved treatment efficiency compared to the conventional system in which aerobic and anaerobic treatment were carried out in separate treatment tanks.

The thickness of the microbial layer 6 is not particularly limited, and when it reaches a certain thickness or a certain treatment time, operations such as air bubbling cleaning can be performed to adjust the thickness to a thickness that allows optimal aerobic and anaerobic treatment.

<Outer shape and average outer diameter D of hollow fiber membrane>
The outer shape of the hollow fiber membrane 1 is not particularly limited, but may be, for example, approximately cylindrical.
Here, the external shape of the hollow fiber membrane 1 described in this embodiment includes both the external shape in a state where the microbial layer 6 is removed, and the external shape in a state where the microbial layer 6 is formed on the surface 8a of the coating layer 8.
In addition, the term "approximately cylindrical" as used herein means a three-dimensional shape in which the shape of any cross section perpendicular to the longitudinal direction is, for example, a perfect circle, egg, oval, ellipse, or other oval shape.

When a non-porous layer 3 is disposed between the first porous layer 2 and the second porous layer 4 as shown in Figures 1 to 3, the area consisting of the porous layers 2 and 4 and the area consisting of the non-porous layer 3 may slightly intersect at the interface between them.

The outer diameter of the hollow fiber membrane 1 is not particularly limited, but it is preferable that the average outer diameter D excluding the microbial layer 6 is 0.01 to 3.0 mm. By making the average outer diameter D of the hollow fiber membrane 1 3.0 mm or less, it is possible to prevent the membrane packing amount from becoming small when forming a membrane module. Furthermore, by making the average outer diameter D of the hollow fiber membrane 1 0.01 mm or more, it is possible to ensure a sufficient inner diameter of the hollow portion 5, thereby reducing the effect of a decrease in the flow rate of oxygen flowing through the hollow portion 5 due to pressure loss, etc.
Furthermore, the average outer diameter D of the hollow fiber membrane 1 is more preferably a dimension that ensures a surface area sufficient for preventing the microbial layer 6 formed on the surface 8a of the coating layer 8 from peeling off.

The "average outer diameter D of the hollow fiber membrane" described in this embodiment means the diameter of the smallest circle that inscribes the outer edge of the cut surface when the hollow fiber membrane 1 before the formation of the microbial layer 6 is cut on any plane perpendicular to the longitudinal direction. The average outer diameter D of the hollow fiber membrane 1 can be calculated by measuring any 3 or more and 10 or less points on the cut surface and averaging the measurements.

<Surface zeta potential>
The surface zeta potential of the hollow fiber membrane 1 is −8 mV or more. When the surface zeta potential of the hollow fiber membrane 1 is −8 mV or more, the microbial layer 6 is easily formed on the outer surface of the hollow fiber membrane 1 (the surface 8a of the coating layer 8 in this embodiment).
The surface zeta potential of the hollow fiber membrane 1 is preferably −7 mV or more, more preferably −5 mV or more. The surface zeta potential of the hollow fiber membrane 1 is preferably 20 mV or less, more preferably 15 mV or less, and even more preferably 10 mV or less. That is, the surface zeta potential of the hollow fiber membrane 1 is preferably −8 to 20 mV, more preferably −7 to 15 mV, and even more preferably −5 to 10 mV.
The surface zeta potential of the hollow fiber membrane 1 can be easily controlled, for example, depending on the type of cationic polymer or anionic polymer constituting the coating layer 8 formed on the outer surface of the membrane body 7. In addition, as will be described in detail later, the surface zeta potential of the hollow fiber membrane 1 can also be easily controlled by the number of times that cationic polymers and anionic polymers are alternately laminated to form the coating layer 8 on the outer surface of the membrane body 7.

The surface zeta potential of the hollow fiber membrane 1 is measured under the following measurement conditions.
(Measurement conditions)
A particle dispersion (pH=6 at 25° C.) prepared by diluting polystyrene latex having a solid content of 0.25% by mass and an average particle size of 500 nm 150 times with a 10 mM aqueous sodium chloride solution was used, and a plurality of the hollow fiber membranes were arranged and laid out on a sample mounting surface of a plate measurement cell without overlapping or gaps, and in close contact with each other. From the zeta potential measurement result of the polystyrene latex by electrophoresis, the surface zeta potential of the hollow fiber membranes was calculated.
The surface of the polystyrene latex is preferably coated with, for example, hydroxypropyl cellulose (mass average molecular weight = 30,000) to suppress the zeta potential to approximately zero.

Specifically, the surface zeta potential of the hollow fiber membrane 1 is measured as follows.
A sample prepared by laying a plurality of hollow fiber membranes 1 on the sample mounting surface of a flat plate measurement cell without overlapping and without gaps, and attaching them with double-sided tape, and the particle dispersion liquid are set in a zeta potential/particle size measurement device, and the measurement position is moved up and down to measure the mobility at each cell position. There are seven measurement positions. Based on the data on the mobility of particles in the particle dispersion liquid at each cell position, approximation is performed by the least squares method, and the coefficients are compared to obtain the true mobility of the particles from the Mori-Okamoto formula represented by the following formula (1). The true mobility is then substituted for "U" in the Smoluchowski formula represented by the following formula (2) to obtain the surface zeta potential of the hollow fiber membrane 1.
U _obs (Z) = AU ₀ (Z/b) ² +ΔU ₀ (Z/b) + (1-A) U ₀ + U _p ... (1)
ζ=4πηU/ε...(2)
In formula (1), "U _obs (Z)" is the apparent velocity to be measured, "Z" is the distance from the center of the cell, "A" is a value calculated by 1/[(2/3)-(0.420166/k)] (where k=a/b: length of the sides (horizontal and vertical) of the cell cross section, a>b), "U _p " is the true mobility of the particle, "U ₀ " is the average flow velocity of the solvent on the top and bottom surfaces of the cell, and "ΔU ₀ " is the difference in flow velocity of the solvent on the top and bottom surfaces of the cell.
In equation (2), "ζ" is the surface zeta potential, "η" is the viscosity of the solvent, "U" is the electrical mobility, which is calculated by U = V/E (where V is the migration velocity and E is the electric field), and "ε" is the dielectric constant of the solvent.

<Method of manufacturing hollow fiber membrane>
Hereinafter, one embodiment of a method for producing the hollow fiber membrane 1 will be described.
The method for producing the hollow fiber membrane 1 of this embodiment includes a spinning step, a stretching step, and a coating layer forming step. It is preferable to include an annealing step between the spinning step and the stretching step.

(Spinning process)
The spinning step is a step in which a molten resin is spun to obtain a hollow fiber membrane precursor.
As shown in the figure, when manufacturing a three-layered membrane body 7 in which one non-porous layer 3 is sandwiched between two porous layers 2 and 4, a composite nozzle die in which an innermost layer nozzle part, a middle layer nozzle part, and an outermost layer nozzle part are arranged concentrically is used to perform the spinning process. Specifically, the material for forming the first porous layer 2 is supplied in a molten state to the innermost layer nozzle part. The material for forming the non-porous layer 3 is supplied in a molten state to the middle layer nozzle part. The material for forming the second porous layer 4 is supplied in a molten state to the outermost layer nozzle part. Next, each material is extruded from each nozzle part and wound up.
The discharge temperature when each material is extruded may be any temperature at which each material is sufficiently melted and can be spun.
The extrusion speed and winding speed are appropriately adjusted, and the unstretched non-porous layer precursor is cooled and solidified to obtain a hollow fiber membrane precursor. The hollow fiber membrane precursor has a three-layer structure in which one unstretched non-porous layer precursor is sandwiched between two porous layer precursors in a non-porous state.

(Annealing process)
The annealing step is a step of annealing (fixed length heat treatment) the hollow fiber membrane precursor obtained in the spinning step.
The hollow fiber membrane precursor obtained in the spinning step is preferably subjected to a fixed length heat treatment (annealing treatment) at a temperature equal to or lower than the melting point of each material before the stretching step. When polyethylene is used as the material for forming the porous layers 2, 4, the fixed length heat treatment is preferably performed at 105 to 130°C for 8 to 16 hours. If the temperature of the fixed length heat treatment is 105°C or higher, a hollow fiber membrane 1 with good quality is likely to be obtained. If the temperature of the fixed length heat treatment is 130°C or lower, sufficient elongation is likely to be obtained, stability during stretching is improved, and stretching at a high ratio is facilitated. In addition, if the treatment time is 8 hours or more, a hollow fiber membrane 1 with good quality is likely to be obtained.

(Stretching process)
The stretching step is a step in which the hollow fiber membrane precursor obtained in the spinning step or the annealed hollow fiber membrane precursor is stretched to obtain the membrane body 7 .
In the stretching step, it is preferable to stretch the hollow fiber membrane precursor obtained in the spinning step or the annealed hollow fiber membrane precursor at a stretching temperature T that is equal to or lower than the Vicat softening point of the material forming the porous layers 2, 4. If the stretching temperature T is equal to or lower than the Vicat softening point of the material forming the porous layers 2, 4, the pore size of the membrane body 7 can be enlarged. If the stretching temperature T exceeds the Vicat softening point, the crystalline lamellar structure becomes easily broken, and conversely, the porous portion that was once opened may become blocked.

In the stretching step, it is preferable to carry out the above-mentioned annealing step as necessary and then cold stretching before stretching (hot stretching) at a stretching temperature T. Specifically, it is preferable to carry out two-stage stretching in which cold stretching is followed by hot stretching, or multi-stage stretching in which cold stretching is followed by hot stretching divided into two or more stages.
Cold stretching is a stretching method that causes destruction of the membrane structure and generates micro cracks at a relatively low temperature. The cold stretching temperature is preferably within a range from 0° C. to a temperature lower than the Vicat softening point −20° C.

The stretching ratio can be set appropriately depending on the material forming the non-porous layer 3 and the material forming the porous layers 2 and 4, but it is preferable to set the final ratio (total stretching ratio) for the unstretched hollow fiber membrane precursor to 2 to 5 times. If the total stretching ratio is 2 times or more, the porosity of the porous layers 2 and 4 is likely to be high, and excellent oxygen permeability is likely to be obtained. If the total stretching ratio is 5 times or less, the breaking elongation of the hollow fiber membrane 1 is likely to be high.

In order to improve the dimensional stability of the hollow fiber membrane, it is preferable to perform relaxation heat setting on the membrane body 7 after stretching while the membrane body 7 is in a fixed length state or in a state where it is slightly relaxed within a range of 70% or less of the fixed length. To perform the relaxation heat setting effectively, the relaxation heat setting temperature is preferably equal to or higher than the stretching temperature T. In addition, the relaxation heat setting temperature is preferably equal to or lower than the melting point of the lower of the material forming the non-porous layer 3 and the material forming the porous layers 2 and 4.

(Coating layer forming process)
The coating layer forming step is a step of forming a coating layer 8 on the outer surface of the membrane body 7 to obtain the hollow fiber membrane 1 .
In the coating layer formation step, it is preferable to alternately laminate the cationic polymer and the anionic polymer by a layer-by-layer method to form the coating layer 8. Specifically, a step (α) of immersing the membrane body 7 in an aqueous solution containing a cationic polymer and then washing the membrane body 7 and a step (β) of immersing the membrane body 7 in an aqueous solution containing an anionic polymer and then washing the membrane body 7 are alternately repeated to form the coating layer 8 on the outer surface of the membrane body 7. The coating layer 8 thus formed has a multilayer structure in which layers containing a cationic polymer and layers containing an anionic polymer are alternately laminated.

The coating layer forming process preferably starts with step (α) and ends with step (α). It is preferable to carry out a drying step after the last step (α). Also, a drying step may be carried out between step (α) and step (β). Examples of the drying method include vacuum drying, pressure drying, heat drying, and air drying. The temperature when air drying is not particularly limited, but it is preferable to carry out the drying at room temperature. Here, "normal pressure" means a temperature range of 20°C ± 15°C.
The method for washing the membrane body 7 is not particularly limited, but examples thereof include a method of immersing the membrane body 7 in water, a method of spraying water onto the membrane body 7, and the like.

The coating layer forming process may be performed immediately after the stretching process, or may be performed after multiple membrane bodies 7 are bundled together. When performing the coating layer forming process after multiple membrane bodies 7 are bundled together, it is preferable to, for example, make the multiple membrane bodies 7 into a sheet shape, fix both ends of the sheet with a housing to form a module precursor, and then perform the coating layer forming process to form a coating layer on the outer surface of the membrane bodies.

<Action and effect>
As described above, in the hollow fiber membrane 1 of this embodiment, the surface zeta potential of the hollow fiber membrane 1 is -8 mV or more, so that the adhesion of the microbial layer 6 to the outer surface of the hollow fiber membrane 1 (the surface 8a of the coating layer 8 in this embodiment) is increased, and the microbial layer 6 is formed in a short period of time. In addition, the retention of the microbial layer 6 is also increased. In particular, if the cationic polymer is unevenly distributed on the surface 8a of the coating layer 8, the activated sludge is more likely to be attracted to the hollow fiber membrane 1, and the microbial layer 6 is more likely to be formed on the surface 8a of the coating layer 8 in a short period of time. Oxygen is effectively diffused and dissolved in the thickness direction of the microbial layer 6, and further, an aerobic treatment region 6A rich in oxygen and an anaerobic treatment region 6B poor in oxygen are formed, so that aerobic treatment and anaerobic treatment can be performed in one process.
Furthermore, even if the enlarged microbial layer 6 is peeled off, the microbial layer 6 reattaches to the surface 8a of the coating layer 8 in a short period of time.
Furthermore, since the hollow fiber membrane 1 of this embodiment adopts a multilayer structure including the porous layers 2, 4 and the non-porous layer 3, it is possible to achieve both high oxygen permeability due to the porous layers 2, 4 and high membrane strength due to the non-porous layer 3, thereby obtaining excellent water treatment efficiency and mechanical properties.

<Other embodiments>
The hollow fiber membrane of the present invention is not limited to the above.
For example, the membrane body may include one porous layer, or may include three or more porous layers, although in either case, it is preferable that the porous layer is disposed as the outermost layer.
The membrane body may also include two or more non-porous layers.
Furthermore, the hollow fiber membrane may include layers other than the porous layer, non-porous layer, coating layer, and microbial layer (other layers). When adopting a configuration including other layers, it is preferable that the other layers have high oxygen permeability from the viewpoint of further increasing the oxygen permeability of the entire hollow fiber membrane.

[Hollow fiber membrane module and wastewater treatment device]
The hollow fiber membrane module 10 of this embodiment shown in FIG. 4 includes the hollow fiber membrane 1 of this embodiment.
The wastewater treatment device 100 of this embodiment is configured to include a hollow fiber membrane module 10 .
The hollow fiber membrane module 10 and the wastewater treatment device 100 of this embodiment will be described in detail below.

<Hollow fiber membrane module>
As shown in FIG. 4, the hollow fiber membrane module 10 of this embodiment is generally composed of a housing 12 (an upper housing 12A and a lower housing 12B) and a hollow fiber membrane sheet 11 in which a plurality of hollow fiber membranes 1 are formed into a sheet shape.

The hollow fiber membrane sheet 11 is formed by bundling together a plurality of hollow fiber membranes 1 according to the present embodiment as described above. Both ends of the hollow fiber membrane sheet 11 are inserted into the housing 12, the details of which will be described later, and are open, and the hollow fiber membrane sheet 11 is configured as a flat sheet overall.

The housing 12 is a substantially hollow member into which both ends of the hollow fiber membrane sheet 11 are inserted and fixed, and in the illustrated example, is composed of an upper housing 12A and a lower housing 12B. That is, the hollow fiber membrane sheet 11 is held in a sheet-like form between the upper housing 12A and the lower housing 12B.
In the illustrated example, a gas supply line 120 is connected to the upper housing 12A so that oxygen or air is supplied to the inside of the upper housing 12A.

The hollow fiber membrane module 10 is configured such that oxygen or air supplied from a blower (not shown) is sent through the housing 12 into the hollow portions of the multiple hollow fiber membranes 1, and as shown in Figures 2 and 3, for example, the oxygen or air permeates the first porous layer 2, the non-porous layer 3, the second porous layer 4, and the coating layer 8, and then dissolves and diffuses in the microbial layer 6 in the membrane thickness direction.

Although not shown in FIG. 4, it is more preferable to provide a pair of supports made of rod-shaped members or the like between the upper housing 12A and the lower housing 12B so as to connect both ends of the upper housing 12A and the lower housing 12B. By providing such a pair of supports and maintaining a constant distance between the upper housing 12A and the lower housing 12B, the surface shape of the hollow fiber membrane sheet 11 can be maintained to form a flat hollow fiber membrane module 10.

In addition, in the illustrated example, the hollow fiber membrane module 10 is shown in the form of a flat sheet, but this is not limited to this, and for example, the hollow fiber membrane module can also be configured in a cylindrical or rectangular tube shape, etc.

As described above, multiple membrane bodies may be formed into a sheet and both ends of the sheet may be fixed with a housing to form a module precursor, after which a coating layer formation step may be carried out to form a coating layer on the outer surface of the membrane body. In this case, hollow fiber membranes are obtained in the form of a hollow fiber membrane module.

<Wastewater treatment equipment>
The wastewater treatment device 100 of this embodiment is generally configured to include the hollow fiber membrane module 10 of this embodiment in the form of a single unit or a plurality of units. The wastewater treatment device 100 of the illustrated example is configured such that the hollow fiber membrane module 10 is accommodated inside a treatment tank 110.
An assembly of multiple hollow fiber membrane modules is also called a "hollow fiber membrane module unit."

The treatment tank 110 contains wastewater W, which is the water to be treated. As the treatment tank 110, any tank that has been conventionally used in this field, such as a large metal container-shaped treatment tank, can be used without any restrictions. Although detailed illustration is omitted in FIG. 4, the treatment tank 110 is connected to a wastewater inlet pipe for containing the wastewater, which is the water to be treated, and a discharge pipe for discharging the treated water out of the tank after treatment is completed.

A hollow fiber membrane module 10 is accommodated in the treatment tank 110, and the hollow fiber membrane module 10 is arranged so as to be immersed in the wastewater W. The hollow fiber membrane module 10 is preferably accommodated in the treatment tank 110 so that the longitudinal direction of the hollow fiber membrane 1 is vertical. If the hollow fiber membrane module 10 is accommodated in the treatment tank 110 so that the longitudinal direction of the hollow fiber membrane 1 is vertical, condensed water (water condensed as water in the membrane from moisture contained in oxygen, air, etc., and moisture in the wastewater) is less likely to accumulate inside the hollow fiber membrane 1, and the wastewater treatment capacity can be maintained better.
As described above, the gas supply line 120 is connected to the upper housing 12A of the hollow fiber membrane module 10, and oxygen, air, or a gas whose component ratio has been changed by a process of separating or concentrating air is supplied.

With the above configuration, the wastewater treatment device 100 of this embodiment can simultaneously carry out aerobic and anaerobic treatments on the wastewater W in the treatment tank 110 using the hollow fiber membranes 1 that make up the hollow fiber membrane module 10, and can treat the wastewater in a single process. This allows the device to be made smaller and more space-saving than conventional systems in which aerobic and anaerobic treatments are carried out in separate treatment tanks.

In addition, in the treatment tank 110, the hollow fiber membrane module 10 may be housed from the opening 111 side of the treatment tank 110 by, for example, a frame member (not shown) arranged so as not to interfere with the flow of the wastewater W. In this case, one end of the frame member is fixed near the opening 111 of the treatment tank 110, and the upper housing 12A provided on the hollow fiber membrane module 10 is fixed to the other end of this frame member.

<Action and effect>
The hollow fiber membrane module 10 of this embodiment includes the hollow fiber membrane 1 of the present invention described above, and thus, similarly to the above, the microbial layer 6 is easily formed. In addition, high oxygen permeability due to the porous layer and high membrane strength due to the non-porous layer can be achieved at the same time, and excellent water treatment efficiency and mechanical properties can be obtained, and aerobic treatment and anaerobic treatment can be performed in a single process. Moreover, it is possible to make the module even more compact and energy-efficient than conventional MABRs.

The wastewater treatment device 100 of this embodiment is equipped with the hollow fiber membrane module 10 of the present invention described above, and thus, as described above, the microbial layer 6 is easily formed. In addition, excellent water treatment efficiency and mechanical properties are obtained, and both aerobic treatment and anaerobic treatment, which were previously performed in separate devices, can be performed in the microbial layer alone, resulting in space saving. Furthermore, since the entire device can be made smaller according to the present invention, a wastewater treatment device 100 can be provided that is extremely suitable for applications requiring simple wastewater treatment or applications requiring wastewater treatment in a small space, for example.

In addition, since the hollow fiber membrane 1 of the present invention is easy to form a microbial layer, there is no need to cross the hollow fiber membrane 1 when using it, and the hollow fiber membrane 1 can be stored in the treatment tank 110 so that the longitudinal direction of the hollow fiber membrane 1 is vertical. Therefore, according to the hollow fiber membrane module 10 and wastewater treatment device 100 of this embodiment, condensed water (water contained in oxygen, air, etc., or water in wastewater condensed as water inside the membrane) is unlikely to accumulate inside the hollow fiber membrane 1, so the wastewater treatment capacity is unlikely to decrease.

[Wastewater treatment method]
The wastewater treatment method of the present embodiment is a method of treating wastewater using a hollow fiber membrane module 10 of the present embodiment or a wastewater treatment device 100 of the present embodiment as shown in FIG.

Specifically, in the wastewater treatment method of this embodiment, first, wastewater W, which is the water to be treated, is introduced into the treatment tank 110. At this time, the treatment tank 110 is filled with wastewater W so that the hollow fiber membrane module 10 arranged in the treatment tank 110 is immersed in the wastewater W.

Next, oxygen or air is supplied to the hollow fiber membrane module 10 from a blower (not shown) via a gas supply line 120, so that oxygen or air is transmitted from the hollow portion 5 of the hollow fiber membrane 1 to the first porous layer 2, the non-porous layer 3, the second porous layer 4, and the coating layer 8, as shown in Figs. 2 and 3, for example. In this embodiment, in such an initial stage of wastewater treatment, microorganisms, bacteria, etc. present in the wastewater W are attached to the surface 8a of the coating layer 8, and a microbial layer 6 derived from them is formed.

Then, by continuing to supply oxygen or air to the hollow fiber membrane module 10, the dissolution and diffusion of oxygen in the thickness direction of the microbial layer 6 proceeds as described above, and an aerobic treatment area 6A and an anaerobic treatment area 6B are formed as shown in Figure 3.

Then, in the aerobic treatment area 6A, ammonia contained in the wastewater is converted to nitrate through aerobic treatment. In addition, in the anaerobic treatment area 6B, the nitrate produced in the aerobic treatment area 6A is converted to nitrogen through anaerobic treatment, and is denitrified. In this way, by carrying out both aerobic treatment and anaerobic treatment in a single process, treatment efficiency is improved compared to when these are carried out in separate treatment tanks.

Next, after carrying out the above biological treatment for a predetermined period of time, the microbial layer 6 is detached from the hollow fiber membrane 1, for example, by a bubbling process using an air diffuser (not shown) arranged below the hollow fiber membrane module 10.
Thereafter, sludge containing the detached microbial layer is collected by using a solid-liquid separation method using a separation membrane (not shown) or the like, completing the wastewater treatment.

The pressure of the gas (oxygen or air) supplied from the inner surface 2b side of the first porous layer 2, i.e., the hollow portion 5, is not particularly limited, but is preferably 200 kPa or less. If the gas pressure is 200 kPa or less, it is possible to prevent oversupply of gas that does not contribute to the wastewater treatment effect, and to prevent damage to components due to this oversupply. The lower limit of the gas pressure may be, for example, 5 kPa or more, from the viewpoint of obtaining a wastewater treatment effect sufficient for practical use.

The oxygen-containing gas supplied to the hollow fiber membrane module 10 is preferably, for example, pure oxygen. By supplying highly pure oxygen to the hollow fiber membrane module 10 in this way, the concentration of oxygen dissolved and diffused in the microbial layer 6 becomes sufficient, allowing the above-mentioned aerobic and anaerobic treatments to proceed efficiently, enabling more effective wastewater treatment.

When supplying air containing oxygen to the hollow fiber membrane module 10, this air may be air from the atmosphere. In this way, using air from the atmosphere for wastewater treatment has the advantage of reducing running costs compared to using oxygen.

Furthermore, the oxygen-containing gas supplied to the hollow fiber membrane module 10 may be, for example, atmospheric air with a changed component ratio, which has been subjected to a process of separating or concentrating the air. In this way, when atmospheric air with a changed component ratio is used for wastewater treatment, as described above, the running costs are reduced compared to the case where oxygen is used, and treatment can be performed in accordance with the characteristics of the wastewater to be treated.

In addition, an aeration device (not shown) may be placed below the hollow fiber membrane module 10, and a gas containing a high concentration of nitrogen (high nitrogen concentration gas) may be supplied from the aeration device to aerate the wastewater W. The bubbles of the high nitrogen concentration gas come into contact with the microbial layer 6 formed on the outer surface of the hollow fiber membrane 1, further reducing the oxygen concentration in the anaerobic treatment area 6B. This makes the anaerobic treatment area 6B more anaerobic. In addition, the bubbles generated by the aeration device stir the wastewater W, creating an anaerobic environment overall in the wastewater W, so that anaerobic bacteria become dominant in the microbial flora of the flocs floating in the wastewater W.

<Action and effect>
According to the wastewater treatment method of this embodiment, a method is adopted in which the hollow fiber membrane module 10 or wastewater treatment device 100 of the present invention described above is used to form a microbial layer 6 on the outer surface of the hollow fiber membrane 1, and then an oxygen-containing gas is supplied from the hollow portion 5 on the inner surface 2b side of the hollow fiber membrane 1 to treat the wastewater W. As a result, as with the above, both aerobic treatment and anaerobic treatment, which have conventionally been performed in separate processes, can be performed in one process, resulting in excellent treatment efficiency and enabling the device to be made smaller.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.
The various measurements are as follows:

[Measurement method]
<Measurement of Average Thickness of Porous Layer and Non-Porous Layer>
The cross section of the hollow fiber membrane was observed with a scanning electron microscope (SEM), and the image was analyzed to measure the thickness of the porous layer and the non-porous layer at five points, and the average values were calculated.

<Measurement of the average pore size of the pores in the porous layer>
The outer surface portion of the porous layer was observed using an SEM, 30 pores were randomly selected, the longest diameter of each pore was measured, and the average value was calculated.

<Measurement of the average outer diameter of hollow fiber membrane>
Before the formation of a microbial layer, the hollow fiber membrane was cut at five random planes perpendicular to the longitudinal direction, and the diameter of the smallest circle inscribed on the outer edge of each cut plane was measured using a projector, and the average value was calculated.

<Measurement of surface zeta potential>
The surface zeta potential of the hollow fiber membrane 1 was measured as follows.
A polystyrene latex having a solid content concentration of 0.25% by mass, an average particle diameter of 500 nm, and a surface coated with hydroxypropyl cellulose (mass average molecular weight = 30,000) was diluted 150-fold with a 10 mM aqueous sodium chloride solution to prepare a particle dispersion (pH = 6 at 25°C).
A sample was prepared by placing a number of hollow fiber membranes on the sample mounting surface of a flat plate measurement cell in a line without overlapping or gaps, and attaching them with double-sided tape.
The sample and particle dispersion thus obtained were set in a zeta potential/particle size measurement system (manufactured by Otsuka Electronics Co., Ltd., product name "ELSZ-2000ZS"), and the measurement position was moved up and down to measure the mobility of the particles in the particle dispersion at each cell position. Seven measurement positions were used. Based on the mobility data at each cell position, approximation was performed using the least squares method, and the coefficients were compared to determine the true mobility of the particles from the Mori-Okamoto formula represented by the above formula (1). The true mobility was then substituted for "U" in the Smoluchowski formula represented by the above formula (2) to determine the surface zeta potential of the hollow fiber membrane.

<Measurement of adhesion amount of microbial layer>
5, the hollow fiber membrane module 10 was housed in a water tank 210, and wastewater W was stored in a wastewater tank 220. The water tank 210 and the wastewater tank 220, the wastewater tank 220 and the pump 230, and the pump 230 and the water tank 210 were each connected with a tube 240, and the pump 230 was operated to circulate the wastewater W between the water tank 210 and the wastewater tank 220 under the conditions shown below, exposing the hollow fiber membrane module 10 to the wastewater and forming a microbial layer on the outer surface of the hollow fiber membrane 1.
Wastewater W: Activated sludge having MLSS (Mixed Liquor Suspended Solids) of 1000 mg / L.
・Circulation flow: Upward flow (from bottom to top).
Circulation time: 2 hours.
- Membrane area: 0.002 m ² .
- Effective length of membrane: 200 mm.
- Activated sludge volume: 0.8L.

The wastewater W was circulated for 2 hours, after which the MLSS was measured.
Separately, without containing the hollow fiber membrane module 10 in the water tank 210, the wastewater W was circulated between the water tank 210 and the wastewater tank 220 for 2 hours, and the MLSS before and after the circulation was measured, and the difference therebetween (MLSS before circulation - MLSS after circulation) was calculated and used as the MLSS change in the blank test.
The amount of sludge attached per unit membrane area of the hollow fiber membrane (mg/m ² ) was calculated from the following formula (3), and this was taken as the amount of attached microbial layer (mg/m ² ).
a={(c ₀ -c _t +c _b )×b}/d...(3)
In formula (3), "a" is the amount of sludge attached (amount of microbial layer attached) per unit membrane area of the hollow fiber membrane, "c ₀ " is the MLSS before circulation (1000 mg/L), "c _t " is the MLSS after circulation for t hours (2 hours), "c _b " is the MLSS change in the blank test, "b" is the volume of activated sludge (0.8 L), and "d" is the membrane area (0.002 m ² ).

[Example 1]
A composite nozzle die having an innermost layer nozzle part, a middle layer nozzle part, and an outermost layer nozzle part arranged concentrically was used to carry out the spinning process as described below.
The innermost layer nozzle portion and the outermost layer nozzle portion were supplied with high-density polyethylene (manufactured by Asahi Kasei Corporation, product name "Suntech (registered trademark) B161") produced by homopolymerization using a Ziegler-Natta catalyst as a resin for forming the porous layer.
On the other hand, polyethylene (manufactured by Japan Polyethylene Corporation, product name "Harmolex NF324A") was supplied to the intermediate layer nozzle portion as a resin for forming the non-porous layer.
Then, melt spinning was performed at a discharge port temperature of 190° C. and a take-up speed of 100 m/min, and the obtained hollow fiber membrane precursor was taken up on a bobbin (spinning step).
The hollow fiber membrane precursor obtained in the spinning step was wound around a bobbin and subjected to a fixed length heat treatment (annealing treatment) at 108° C. for 12 hours (annealing step).
After the annealing step, the film was continuously cold-stretched at room temperature (20° C.) to a total stretch ratio of 150%, and then hot-stretched in a heating furnace heated to 106° C. until a total stretch ratio of 620% was reached. Further, relaxation heat setting was performed in a heating furnace heated to 115° C. so that a total stretch ratio of 400% was reached (stretching step), and a film body was obtained.
The average outer diameter D of the obtained membrane body was 0.28 mm (280 μm). The average total thickness Dp of the porous layers was 279 μm, the average thickness Dn of the non-porous layers was 1 μm, and Dn/Dp=0.003. The average pore diameter of the pores of the first porous layer was 0.1 μm, and the average pore diameter of the pores of the second porous layer was 0.1 μm.
Both ends of the twelve obtained membrane bodies were placed in a housing to manufacture a module precursor.

Polydiallyldimethylammonium chloride (PDDA) as a cationic polymer was dissolved in water to a concentration of 0.1% by mass to prepare an aqueous PDDA solution.
Separately, polyacrylic acid (PAA) as an anionic polymer was dissolved in water to a concentration of 0.1% by mass to prepare an aqueous PAA solution.
The previously obtained module precursor was immersed in an aqueous PDDA solution for 30 minutes, and then washed three times by immersing it in pure water for one minute each (step (α)).
The module precursor was then immersed in the PAA aqueous solution for 30 minutes, and then subjected to a cleaning operation of immersing in pure water for 1 minute three times (step (β)).
The module precursor was then immersed in an aqueous PDDA solution for 30 minutes, and then washed three times by immersing it in pure water for one minute (step (α)).
Subsequently, the module precursor was air-dried at 40° C. for 2 hours, and a hollow fiber membrane in which a coating layer having a multilayer structure in which layers containing a cationic polymer and layers containing an anionic polymer were alternately laminated was formed on the outer surface of the membrane body was obtained in the form of a hollow fiber membrane module.
The hollow fiber membrane was removed from the obtained hollow fiber membrane module, and the surface zeta potential of the hollow fiber membrane was measured. The results are shown in Table 1.
The amount of attached microorganisms in the hollow fiber membrane module was also measured. The results are shown in Table 1.

[Example 2]
In forming a coating layer on the outer surface of the membrane body by alternately performing steps (α) and (β), the number of steps (α) was changed to 3 and the number of steps (β) was changed to 2, and the hollow fiber membrane was produced in the form of a hollow fiber membrane module in the same manner as in Example 1, and various measurements were carried out. The results are shown in Table 1.

[Example 3]
In forming a coating layer on the outer surface of the membrane body by alternately performing steps (α) and (β), the number of steps (α) was changed to four and the number of steps (β) to three, respectively, and the hollow fiber membrane was produced in the form of a hollow fiber membrane module in the same manner as in Example 1, and various measurements were carried out. The results are shown in Table 1.

[Example 4]
In forming a coating layer on the outer surface of the membrane body by alternately performing step (α) and step (β), the number of steps (α) was changed to 5 and the number of steps (β) to 4, but the hollow fiber membrane was produced in the form of a hollow fiber membrane module in the same manner as in Example 1, and various measurements were carried out. The results are shown in Table 1.

[Comparative Example 1]
A membrane body and a module precursor were produced in the same manner as in Example 1.
The obtained membrane body was used as a hollow fiber membrane, and the surface zeta potential of the membrane body was measured. The results are shown in Table 1.
The module precursor was also used as a hollow fiber membrane module to measure the amount of attached microorganism layer. The results are shown in Table 1.

[Comparative Example 2]
When the coating layer was formed on the outer surface of the membrane body by alternately performing the steps (α) and (β), the number of steps (α) was changed to 1 and the number of steps (β) to 0, i.e., the second step (α) was not performed, except that the hollow fiber membrane was produced in the form of a hollow fiber membrane module in the same manner as in Example 1, and various measurements were carried out. The results are shown in Table 1.

In Table 1, "PDDA" is polydiallyldimethylammonium chloride and "PAA" is polyacrylic acid.
As is clear from Table 1, the hollow fiber membranes obtained in Examples 1 to 4 were prone to the formation of a microbial layer on the surface.
On the other hand, the hollow fiber membranes obtained in Comparative Examples 1 and 2, which had a surface zeta potential of less than -8 mV, were less susceptible to the formation of a microbial layer on the surface, compared to the hollow fiber membranes obtained in each Example.

The hollow fiber membrane, hollow fiber membrane module, wastewater treatment device, and wastewater treatment method of the present invention are suitable for treating, for example, domestic wastewater and industrial wastewater because they facilitate the formation of a microbial layer and enable miniaturization and energy saving.

1 Hollow fiber membrane 2 First porous layer (porous layer)
2a outer surface 2b inner surface 3 non-porous layer 4 second porous layer (porous layer)
Reference Signs List 4a Surface 4b Inner surface 5 Hollow portion 6 Microbial layer 6A Aerobic treatment area 6B Anaerobic treatment area 7 Membrane body 8 Coating layer 8a Surface 10 Hollow fiber membrane module 11 Hollow fiber membrane sheet 12 Housing 12A Upper housing 12B Lower housing 100 Wastewater treatment device 110 Treatment tank 111 Opening 210 Water tank 220 Wastewater tank 230 Pump 240 Tube

Claims (13)

A hollow fiber membrane for wastewater treatment, in which a microbial layer derived from microorganisms or bacteria in the wastewater is formed on the outer surface during the wastewater treatment,
The surface zeta potential of the hollow fiber membrane measured under the following measurement conditions is −8 mV or more,
The hollow fiber membrane comprises a hollow membrane body and a coating layer formed on the outer surface of the membrane body,
The membrane body has a multilayer structure having one or more resin-based porous layers and one or more non-porous layers, and the porous layers are disposed as outermost layers of the membrane body;
The coating layer has a multilayer structure in which a layer containing a cationic polymer and a layer containing an anionic polymer are alternately laminated, and the outermost layer of the coating layer contains the cationic polymer .
(Measurement conditions)
A particle dispersion (pH=6 at 25° C.) prepared by diluting polystyrene latex having a solid content of 0.25% by mass and an average particle size of 500 nm 150 times with a 10 mM aqueous sodium chloride solution was used, and a plurality of the hollow fiber membranes were arranged and laid out on a sample mounting surface of a plate measurement cell without overlapping or gaps, and in close contact with each other. From the zeta potential measurement result of the polystyrene latex by electrophoresis, the surface zeta potential of the hollow fiber membranes was calculated. The hollow fiber membrane according to claim 1, wherein the coating layer has an average thickness of 1 to 30 μm. 3. The hollow fiber membrane according to claim 1 , wherein the cationic polymer comprises at least one selected from the group consisting of polyethyleneimine, polyallylamine hydrochloride, and polydiallyldimethylammonium chloride. The hollow fiber membrane according to any one of claims 1 to 3 , wherein the anionic polymer comprises one or more selected from polystyrene sulfonic acid and polyacrylic acid. The hollow fiber membrane according to any one of claims 1 to 4, wherein the cationic polymer is unevenly distributed on a surface of the coating layer. The hollow fiber membrane according to any one of claims 1 to 5, wherein the porous layer is made of a material containing at least one selected from the group consisting of polyolefin-based resins, polyurethane-based resins, and fluorine-based resins. The hollow fiber membrane according to any one of claims 1 to 6, wherein the non-porous layer is made of a material containing at least one selected from a styrene-based resin and a polyolefin-based resin. A hollow fiber membrane module comprising the hollow fiber membrane according to any one of claims 1 to 7 . A wastewater treatment device comprising the hollow fiber membrane module according to claim 8 . A wastewater treatment method for treating wastewater using the hollow fiber membrane module according to claim 8 or the wastewater treatment device according to claim 9 , comprising the steps of:
The wastewater treatment method comprises forming the microbial layer derived from microorganisms or bacteria in the wastewater on the outer surface of the hollow fiber membrane, and then supplying an oxygen-containing gas into the hollow portion of the hollow fiber membrane. A method for producing the hollow fiber membrane according to any one of claims 1 to 7,
A method for producing a hollow fiber membrane, comprising: a coating layer forming step of forming the coating layer by alternately laminating the cationic polymer and the anionic polymer on the outer surface of the membrane body by an alternating lamination method. 12. The method for producing a hollow fiber membrane according to claim 11, wherein in the coating layer formation step, a step (α) of immersing the membrane body in an aqueous solution containing the cationic polymer and then washing the membrane body and a step (β) of immersing the membrane body in an aqueous solution containing the anionic polymer and then washing the membrane body are alternately repeated to form the coating layer on the outer surface of the membrane body. The method for producing a hollow fiber membrane according to claim 12, wherein the coating layer forming step starts with the step (α) and ends with the step (α).

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