JP2008253922A - Method for filtering suspension water - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for filtering suspension water, which achieves cost reduction and stable long-term operation. <P>SOLUTION: A module is used in which many porous multilayer hollow fiber membranes are integrated and housed. The porous multilayer hollow fiber membrane has at least two layers on the inside and outside thereof and consists of a thermoplastic resin. At least one layer (A) of the two layers has an isotropic and three-dimensional network structure and pores on the surface thereof the pore size of which is 0.6-1.4 times as large as that of central pores in the cross section and the other layer (B) of two layers has pores on the surface thereof the pore size of which is less than one-half of that of central pores in the cross section. The suspension water is made to flow from the surface of the layer (B) toward that of the layer (A) and filtered. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a method for filtering suspended water, such as river water, lake water, underground water, and the like, industrial water, sewage, secondary sewage water, industrial wastewater, domestic wastewater, human waste, seawater, and the like. Suspended water also includes activated sludge.

In recent years, a filtration method using a porous hollow fiber membrane having advantages of improving the safety of treated water and saving the space of equipment has been widely used as a filtration method for suspended water. Porous hollow fiber membranes have a wide range of operating conditions including high blocking performance that can reliably remove bacteria such as Cryptosporidium and turbid components, high water permeability for treating large amounts of water, chemical cleaning and high operating pressure. Three performances of high strength that can be used for a long time are required. Furthermore, wear resistance that can withstand surface wear due to fine sand or agglomerates that may be contained in the liquid to be treated may be required.
Much research has been done on these requirements. One representative example is a membrane having a relatively homogeneous three-dimensional network structure in which the change in pore diameter in the film thickness direction is small, and a high-strength hollow fiber membrane can be obtained (see, for example, Patent Document 1). However, since this membrane has almost no change in the pore diameter inside the membrane with respect to the surface pore diameter that determines the necessary blocking performance, the membrane has a certain blocking performance. The water permeability resistance as a whole film becomes large and the water permeability becomes low.

Another representative example is a structure in which the hole diameter continuously increases in the film thickness direction. With this structure, high water permeability can be achieved, but the mechanical strength of the entire membrane, that is, mechanical strength such as burst resistance and compression strength is significantly reduced.
An idea of obtaining a porous multilayer hollow fiber membrane having both high blocking performance and high water permeability by bonding a small pore blocking layer and a large pore strength support layer is disclosed in, for example, Patent Document 3. Yes. Specifically, a production method is disclosed in which a hollow thermoplastic extrudate is made into a porous multilayer hollow fiber membrane by a stretch opening method by melt extrusion without adding a solvent to a crystalline thermoplastic resin such as polyethylene. The stretch opening method is a method in which a lamellar crystal stack is cleaved and opened by performing high-strength stretching in the longitudinal direction of the hollow fiber-like extrudate (Non-patent Document 1). In Patent Document 1, crystalline thermoplastic resins having different mI (melt index) values are separately melt-extruded from two annular nozzles arranged concentrically. This is because resins having different mI values, that is, usually having different molecular weights, take advantage of the property of having different pore diameters when stretched and opened. As a result, a porous two-layer hollow fiber membrane in which the pore diameters of the outer layer and the inner layer of the hollow fiber membrane are different is obtained. However, this production method has the following disadvantages, and a high-strength porous multilayer hollow fiber membrane cannot be obtained.

(1) The strength in the direction of the stretching axis is increased by high-strength stretching. However, when performing filtration, the burst strength and compressive strength, which are the strength in the direction perpendicular to the stretch axis, are rather likely to decrease. (2) In principle, the molecular weight and polymer type must be changed between the outer layer and the inner layer. . However, the required physical properties such as chemical resistance and mechanical strength usually differ depending on the molecular weight and polymer type. Therefore, when a low-strength resin is used, there are difficulties such as a decrease in the strength of the entire film, and a high-strength film cannot be obtained. Moreover, the structure of the membrane obtained by this production method is a structure in which the pore diameter in the longitudinal direction of the hollow fiber is larger than the pore diameter in the film thickness direction, so that the membrane has a low burst strength and compressive strength.

Recently, the use of porous filtration membranes, especially hollow fiber filtration membranes that can increase the area of the packing membrane per unit volume, has been rapidly expanding, and the three levels of high strength, high blocking performance, and high water permeability are at a higher level. The appearance of a balanced hollow fiber membrane is desired. Furthermore, the appearance of a hollow fiber membrane excellent in wear resistance is desired.
JP-A-3-215535 Japanese Patent Laid-Open No. 2001-157827 JP 60-139815 A Editorial Committee for Encyclopedia of Plastics and Functional Polymer Materials, “Encyclopedia of Plastics and Functional Polymer Materials”, Industry Research Committee, February 2004, 672-679

  The present invention relates to a filtration method that enables low-cost and stable long-term operation in filtration of suspended water.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies, and as a result, the suspended water is a porous hollow fiber membrane composed of at least two layers inside and outside, and is composed of a thermoplastic resin. At least one of the layers (A) has an isotropic three-dimensional network structure, and the surface hole diameter is 0.6 to 1.4 times the cross-sectional hole diameter. (B) has a high treated water quality, a high treated water amount, and durability that can be used for a long period of time by performing filtration using a porous hollow fiber membrane whose surface pore diameter is less than ½ of the sectional pore diameter. Has been found to be effective for balancing at a high level, leading to the present invention.

That is, the present invention
(1) A porous multi-layer hollow fiber membrane comprising at least two layers of suspended water, comprising a thermoplastic resin, and at least one of the two layers (A) is an isotropic three-dimensional network. It has a structure and the surface hole diameter is 0.6 to 1.4 times the cross-sectional central hole diameter, and the other layer (B) of the two layers has a surface hole diameter of less than ½ of the cross-sectional central hole diameter Filtration characterized by filtering from the surface of one layer (B) to the surface of one layer (A) using a module in which a large number of porous multilayer hollow fiber membranes are collected and stored Method and
(2) The filtration method according to (1), wherein the one layer (B) of the porous hollow fiber membrane has an isotropic three-dimensional network structure, and
(3) The filtration method according to (1) or (2), wherein the surface pore diameter of the one layer (B) of the porous hollow fiber membrane is 0.01 μm or more and less than 5 μm, and
(4) The filtration method according to any one of (1) to (3), wherein the cross-sectional central pore diameter of the porous hollow fiber membrane is 0.1 μm or more and 10 μm or less, and
(5) The filtration method according to any one of claims 1 to 4, wherein the surface porosity of the one layer (B) of the porous hollow fiber membrane is 20% or more and 80% or less. and,
(6) The thickness of the one layer (B) of the porous hollow fiber membrane is 1/100 or more and 40/100 or less of the film thickness, according to any one of (1) to (5) Filtration method and
(7) In any one of (1) to (6), the isotropic ratio of the one layer (A) and the one layer (B) of the porous hollow fiber membrane is 80% or more. The filtration method described, and
(8) The number of Qs where the parameter Q, which is a value representing the amount of change in the average pore diameter depending on the position of the inner surface from the outer surface of the porous hollow fiber membrane, is −0.2 ≦ Q ≦ 0.2. The filtration method according to any one of (1) to (7), characterized in that it is 80% or more based on the total average pore diameter measurement value, and
(9) The filtration method according to any one of (1) to (8), wherein the thermoplastic resin of the porous hollow fiber membrane is selected from polyolefins and polyvinylidene fluoride, and ,
(10) The filtration method according to any one of (1) to (9), wherein the porous hollow fiber membrane has an inner diameter of 0.4 mm to 5 mm and a film thickness of 0.2 mm to 1 mm. ,
It is.

  According to the present invention, low-cost and stable long-term operation is possible in filtration of suspended water.

Hereinafter, the present invention will be described specifically and in detail.
Suspended water in the present invention is water in which fine particles are suspended, such as river water, lake water, underground water, etc., industrial water, sewage, sewage secondary treated water, industrial wastewater, domestic wastewater, human waste, seawater Etc. Suspended water also includes activated sludge.
The porous hollow fiber membrane used in the filtration method of the present invention is a multilayer membrane made of a thermoplastic resin having at least two layers.
Hereinafter, the porous multilayer hollow fiber membrane of the present application will be described by taking a schematic diagram of the porous two-layer hollow fiber membrane (see FIG. 1) as an example.

Of the two layers, a layer having a large pore size is defined as one layer (A), and a layer having a small pore size is defined as one layer (B). In addition, one layer (A) will be described as an inner layer and one layer (B) as an outer layer. However, the present invention is not limited to this. For example, another layer may exist between one layer (A) and one layer (B), or another layer may be laminated on one layer (A) or one layer (B).
FIG. 1 (1) is a diagram showing a change in pore diameter in the film thickness direction when both the first layer (A) and the first layer (B) have an isotropic three-dimensional network structure, and FIG. FIG. 1B is a diagram showing changes in pore diameter when one layer (B) has an anisotropic three-dimensional network structure, and FIG. 1 (3) is a layer having a small pore diameter on the outer surface side of FIG. It is a figure which shows the hole diameter change in case the layer is formed. The graph showing the relationship between the film thickness of each hollow fiber membrane cross section and the cross-sectional pore diameter is shown in FIGS. In the graph, the vertical axis represents the ratio of the hole diameter in each cross section to the cross-sectional central hole diameter, and the horizontal axis represents the distance between the positions proceeding from the outer surface in the film thickness direction, assuming that the total film thickness is 1. Even if surface wear occurs, the blocking performance is unlikely to change, and it is preferable that both layers (A) and (B) have an isotropic three-dimensional network structure.

One of the two layers (A) is a so-called support layer. This support layer has a function to ensure high mechanical strength such as pressure resistance and to reduce water permeability as much as possible.
This one layer (A) has an isotropic three-dimensional network structure. The term “isotropic” as used herein means that the change in pore diameter is small and the structure is almost homogeneous in any of the film thickness direction, the film circumferential direction, and the film longitudinal direction. An isotropic structure is a structure in which weak portions such as macrovoids are not easily generated. Accordingly, it is possible to increase the mechanical strength such as pressure resistance while maintaining the water permeability of the porous multilayer hollow fiber membrane.

  Further, the three-dimensional network structure referred to in the present application schematically indicates a structure as shown in FIG. It can be seen that the thermoplastic resin a is joined to form a mesh, and the void b is formed. An example of a micrograph of an isotropic three-dimensional network structure in the actual porous two-layer hollow fiber membrane obtained in Example 1 is shown in FIG. The thickness of the thermoplastic resin forming the mesh is substantially constant. In this three-dimensional network structure, a lump of resin having a so-called spherulite structure as schematically shown in FIG. 3 is hardly seen. The void portion of the three-dimensional network structure is surrounded by a thermoplastic resin, and each portion of the void portion communicates with each other. Since most of the used thermoplastic resins form a three-dimensional network structure that can contribute to the strength of the hollow fiber membrane, a high-strength support layer can be formed. In addition, chemical resistance is improved. The reason for the improvement in chemical resistance is not clear, but the amount of thermoplastic resin that forms a network that can contribute to the strength is large. This may be because it does not have a major impact. On the other hand, in the so-called spherulite structure schematically shown in FIG. 3, since the resin is gathered in the lump, the amount of the thermoplastic resin that contributes to the strength is relatively small, so that a part thereof is affected by the chemical. It is thought that the strength of the entire layer is likely to be affected. For reference, a schematic diagram of the spherulite structure is shown in FIG. In FIG. 3, it can be seen that the spherulites c are partially dense, and the gaps between the dense parts of the spherulites c are voids d.

The surface pore diameter of one layer (A) is not less than 0.6 times and not more than 1.4 times the central hole diameter of the cross section. The fact that the surface pore diameter of one layer (A) is not less than 0.6 times and not more than 1.4 times the central hole diameter of the cross section is consistent with the fact that one layer (A) has an isotropic three-dimensional network structure. . If it is 0.6 times or more, the filtration resistance on the surface of the support layer does not become too large, and the membrane as a whole can exhibit practically sufficient high water permeability. Moreover, if it is 1.4 times or less, high mechanical strength can be expressed.
Unlike a flat membrane that is generally used on a support made of a mesh-like metal or plastic, the hollow fiber membrane needs to exhibit strength that can withstand filtration pressure. Therefore, it is particularly important to design a membrane structure that can exhibit strength in the filtration direction, that is, burst strength and compressive strength. By suppressing the increase in the hole diameter from the vicinity of the center of the cross section toward the inner surface of the hollow fiber, both low filtration resistance and high compressive strength can be achieved. By controlling the pore diameter in the membrane cross-sectional direction in this way, it becomes possible to balance the blocking performance, mechanical strength, and water permeability at a high level. The surface pore diameter of one layer (A) is preferably 0.7 to 1.3 times, more preferably 0.8 to 1.2 times the central hole diameter of the cross section.

  In addition, the surface pore diameter of 1 layer (A) here means the average hole diameter of the hole observed on the surface which 1 layer (A) exposes when observing a hollow fiber membrane from the outside. This average pore diameter is measured as follows. First, using a scanning electron microscope, the surface on which one layer (A) of the porous multilayer hollow fiber membrane is exposed is photographed at a magnification such that the shape of a large number of holes can be clearly confirmed as much as possible. Next, on the photograph, five lines are drawn at almost equal intervals so as to be orthogonal to the vertical and horizontal directions, and the length of these lines across the hole in the photograph is measured. And the arithmetic average value of those measured values is calculated | required, and this is made into the average hole diameter. In order to increase the accuracy of the hole diameter measurement, it is preferable that the number of hole diameters traversed by the 10 lines in the vertical and horizontal directions is 20 or more. If the pore diameter is about 0.1 μm to 1 μm, it is appropriate to use an electron microscope image with a magnification of about 5000 times.

In addition, the cross-sectional central pore diameter referred to here is within a range of 10% of the total film thickness from the central position of the film thickness in the cross section when the porous multilayer hollow fiber membrane is cut in a cross section perpendicular to the length direction. A scanning electron micrograph was taken at an arbitrary magnification, and the arithmetic average value of the pore diameter was obtained using this photograph in the same manner as the above average pore diameter. Specifically, the cross-sectional central hole diameter is preferably 0.1 μm or more and 10 μm or less. A desirable balance between water permeability and mechanical strength can be achieved within this range. More preferably, they are 0.3 micrometer or more and 8 micrometers or less, More preferably, they are 0.6 micrometer or more and 6 micrometers or less, More preferably, they are 0.8 micrometer or more and 4 micrometers or less.
The porosity of the surface of the single layer (B) is not particularly limited as long as it is appropriately determined depending on the purpose, but is preferably 20% or more from the viewpoint of filtration stability of the liquid to be treated containing suspended substances. More preferably, it is 23% or more, and further preferably 25% or more. From the viewpoint of increasing the mechanical strength of the surface portion, the porosity is preferably 80% or less. More preferably, it is 60% or less, More preferably, it is 50% or less. For example, as described in International Publication No. PCT / WO01 / 53213 A1, the aperture ratio is obtained by overlaying a transparent sheet on a copy of an electron microscope image, painting the hole portion black using a black pen, etc. By copying the transparent sheet onto white paper, the hole portion is clearly distinguished from black and the non-hole portion is clearly distinguished from white, and thereafter, it can be obtained using commercially available image analysis software.

  The other one of the two layers (B) is a so-called blocking layer. A small surface pore diameter exerts a function of blocking the permeation of foreign substances contained in the liquid to be treated. The surface pore diameter of the first layer (B) mentioned here refers to the average pore diameter of the holes observed on the surface where the first layer (B) is exposed when the hollow fiber membrane is observed from the outside. The measurement of the surface pore diameter of one layer (B) may be performed using a scanning electron micrograph in the same manner as the measurement of the surface pore diameter of one layer (A). In addition, it is preferable that the specific surface pore diameter of 1 layer (B) is 0.01 micrometer or more and less than 5 micrometers. If it is 0.01 micrometer or more, the filtration resistance of a dense surface is small, and it will be easy to express practically sufficient water permeability. Moreover, if it is 5 micrometers or less, the expression of the turbidity which is an important required function of a filtration membrane will be attained. More preferably, they are 0.05 micrometer or more and 2 micrometers or less, More preferably, they are 0.05 micrometer or more and 0.5 micrometers or less, Most preferably, they are 0.1 micrometer or more and 0.5 micrometers or less.

This one layer (B) has a surface pore diameter of less than ½ of the cross-sectional central hole diameter. Thereby, one layer (B) functions as a desirable blocking layer. What is necessary is just to select suitably the minimum of the surface hole diameter according to the magnitude | size of the target object to block. From the viewpoint of ensuring water permeability, it is preferable that the cross-sectional central hole diameter is 1/1000 or more. More preferably, it is 1/3 or less and 1/500 or more, more preferably 1/4 or less and 1/100 or more of the central hole diameter of the cross section.
The thickness of one layer (B) is preferably 1/100 or more and less than 40/100 of the film thickness. By making the thickness of one layer (B) relatively thick in this way, it becomes possible to use even if the liquid to be treated contains insoluble substances such as sand and aggregates. This is because the surface pore diameter does not change even when worn slightly. If it is in the range of this thickness, desirable blocking performance and high water permeability can be balanced. More preferably, the film thickness is 3/100 or more and 20/100 or less, and further preferably 5/100 or more and 15/100 or less.

Further, unlike the first layer (A), the first layer (B) may have an anisotropic structure in which the diameter of each hole gradually increases from the surface toward the inside of the film. Or it is good also as an isotropic structure where the diameter of each hole becomes uniform irrespective of the distance from the surface similarly to 1 layer (A). A preferable structure of one layer (B) is an isotropic three-dimensional network structure similar to that of one layer (A). Thereby, the mechanical strength of the entire hollow fiber membrane can be increased while maintaining a desirable stopping power.
The thicknesses of the first layer (A) and the first layer (B) are obtained as follows. About each film thickness part, the cross-sectional hole diameter of each film thickness part is calculated | required by the method as described in (7) of an Example. From the center of the cross section toward the surface of one layer (B), the thickness up to the film thickness portion having a hole diameter closest to 0.7 times the central hole diameter of the cross section is taken as the boundary line of both layers, and one layer (A ) Is the thickness of one layer (A), and similarly, the distance from the boundary line to the surface of one layer (B) is the thickness of one layer (B). In addition, when there are a plurality of cross-sectional portions having a hole diameter closest to 0.7 times the cross-sectional central hole diameter, one layer (A) is set up to the point closest to the cross-sectional center.

The isotropic rate of one layer (A) is preferably 80% or more. This means that one layer (A) has a very isotropic structure. If it is 80% or more, high strength can be expressed while maintaining high water permeability. The isotropic ratio of one layer (A) is more preferably 90% or more, and still more preferably 95% or more.
In addition, the isotropic ratio of one layer (A) is a cross-sectional hole diameter of 0.8 to 1.2 times the cross-sectional central hole diameter in each film thickness part included in one layer (A) measured above. It is a ratio obtained by dividing the number of film thickness parts by the total number of film thickness parts included in one layer (A).
Similarly, the isotropic rate of one layer (B) is preferably 80% or more. This also means that one layer (B) has a very isotropic structure. If it is 80% or more, high blocking performance can be expressed, and even if the surface of one layer (B) is worn out by insoluble material such as sand or agglomerates in the liquid to be treated, the decrease in blocking performance can be suppressed as much as possible. . The isotropic ratio of one layer (B) is the thickness of each layer included in one layer (B), with the cross-sectional hole diameter at the film thickness corresponding to 1/2 of the thickness of one layer (B) being the central hole diameter (B). Of these, the ratio is obtained by dividing the number of film thickness parts having a cross-sectional hole diameter of 0.8 times or more and 1.2 times or less of the center hole diameter (B) by the total number of film thickness parts included in one layer (B). . The isotropic ratio of one layer (B) is more preferably 90% or more, and still more preferably 95% or more. In addition, when one layer (B) is very thin with respect to the total film thickness, the number of points of the cross-sectional hole diameter of one layer (B) is increased and this measurement is performed. It is appropriate to measure cross-sectional pore diameters of 20 points or more in both layers.

Furthermore, it is most preferable that the isotropic rates of the first layer (A) and the first layer (B) are both 80% or more. If both are 80% or more, since the membrane structure is composed of the blocking layer and the strength support layer without waste, a membrane having a high balance of blocking performance, water permeability and strength can be most suitably obtained. The isotropic ratio of both layers is more preferably 90% or more, and still more preferably 95% or more.
The isotropy referred to in the present application can also be expressed by the parameter Q shown below.
The parameter Q is a value representing the change rate of the hole diameter at each position of the film thickness from the outer surface to the inner surface. Specifically, it is obtained as follows.
The cross-sectional hole diameters at each position of the film thickness are arranged in the order of the position from the outer surface to the inner surface.
Here, the outer surface pore size _{D 0, D 1,} D 2 in order to cross the pore diameter from the outer surface side, · · · _{D N,} the inner surface hole diameter and _{D i.}

At this time, Q is represented by the following general formula.
Q = (D _N −D _N−1 ) / D _N
Q = (D ₁ −D ₀ ) / D ₁ when calculating the outer surface pore diameter
Q = (D _i −D _N ) / D _i when calculating the inner surface pore diameter
It is.
In the porous multilayer hollow fiber membrane of the present invention, the number of Q satisfying −0.2 ≦ Q ≦ 0.2 is preferably 80% or more with respect to the total number of measured values of Q. More preferably, it is 85% or more, More preferably, it is 90% or more. If it is this range, since the part with which the hole diameter is equal occupies most of a film | membrane, a film | membrane with the balance of blocking performance, water permeability performance, and intensity | strength can be obtained.

It is also preferable that the number of Q satisfying −0.1 ≦ Q ≦ 0.1 is 50% or more with respect to the total number of measured values of Q. More preferably, it is 60% or more, More preferably, it is 70% or more.
It is shown that the change of the hole diameter depending on the film thickness position is particularly large in the part where the parameter Q is less than −0.2 or greater than 0.2.
In addition, the outer surface hole diameter and the inner surface hole diameter at this time are measured by the above-described method, and the cross-sectional hole diameter is measured by the measuring method (7) of the example.
One layer (A) and one layer (B) may be on the outside of the hollow fiber membrane and may be appropriately arranged according to the purpose, but the blocking layer is arranged on the outside of the hollow fiber membrane. Is preferable from the viewpoint of maintaining stable operation for a long period of time.

The inner diameter of the hollow fiber membrane is preferably 0.4 mm or more and 5 mm or less. If it is 0.4 mm or more, the pressure loss of the liquid flowing in the hollow fiber membrane does not become excessive, and if it is 5 mm or less, sufficient compressive strength and bursting strength are easily developed with a relatively thin film thickness. More preferably, they are 0.5 mm or more and 3 mm or less, More preferably, they are 0.6 mm or more and 1 mm or less.
The film thickness is preferably from 0.1 mm to 1 mm. If it is 0.1 mm or more, sufficient compressive strength and burst strength are easily developed, and if it is 1 mm or less, sufficient water permeability is easily developed. More preferably, they are 0.15 mm or more and 0.8 mm or less, More preferably, they are 0.2 mm or more and 0.6 mm or less.

The hollow fiber membrane having such a preferable structure balances water permeability performance, blocking performance and mechanical strength at a high level, and exhibits high performance while accommodating a wide range of operating conditions. Further, even if the liquid to be treated contains fine particles containing an inorganic component as a turbid component or an organic insoluble material, for example, an insoluble material such as sand or agglomerated material, the blocking performance hardly changes and has high wear resistance.
Particularly preferred film properties for the purposes of the present invention are: the blocking rate of uniform latex spheres of 0.2 μm is 95% or more, the water permeability of pure water is 5000 L / m ² /hr/0.1 mPa or more, and compression The film has a strength of 0.3 mPa or more.
As the thermoplastic resin constituting the hollow fiber membrane, polyolefin or polyvinylidene fluoride is preferably used from the physical properties of the membrane.

  In addition, in the present invention, when filtering suspended water, which is a fine particle containing an inorganic component as a turbid component or an organic insoluble matter, the filtration can be performed using a module in which a large number of membranes are integrated. The form of the module may be a casing type in which a hollow fiber membrane bundle is surrounded by an outer cylinder or a shellless type in which there is no outer cylinder and the hollow fiber membrane is exposed. Further, the cross-sectional shape of the module may be circular or square. Furthermore, the raw water may be filtered directly through a membrane, or may be filtered through a membrane after adding an oxidizing agent such as a flocculant or ozone as a pretreatment. The filtration method may be a whole amount filtration method or a cross flow filtration method. A pressure filtration system or a negative pressure filtration system may be used. Furthermore, as an operation method, air scrubbing and back pressure cleaning used for the purpose of removing the filtration object deposited on the membrane surface may be performed separately or simultaneously. Moreover, as a liquid used for back pressure washing | cleaning, oxidizing agents, such as sodium hypochlorite, chlorine dioxide, ozone, etc. can be used suitably.

The present invention will be described more specifically based on examples. Hereinafter, methods for measuring various physical properties will be described. In addition, all measurements were performed at 25 ° C. except for the measurement temperature.
(1) Thread diameter (mm), flatness The hollow fiber membrane was thinly cut with a razor or the like in a direction perpendicular to the longitudinal direction of the membrane, and the cross section was observed with a microscope. The major and minor diameters of the inner diameter of the hollow fiber and the major and minor diameters of the outer diameter were measured, and the inner diameter and the outer diameter were determined by the following equations, respectively.

The flatness was determined by dividing the major axis of the inner diameter by the minor axis of the inner diameter.
(2) Pure water permeability (L / m ² /hr/0.1 mPa)
The hollow fiber membrane was immersed in a 50% by mass ethanol aqueous solution for 30 minutes and then immersed in water for 30 minutes to wet the hollow fiber membrane. One end of a 10 cm long wet hollow fiber membrane was sealed, and an injection needle was inserted into the hollow part at the other end. Pure water was injected into the hollow part from the injection needle at a pressure of 0.1 mPa, and the amount of permeated water permeated to the outer surface was measured. Pure water permeability was determined by the following equation.

The effective membrane length here refers to the net membrane length excluding the portion where the injection needle is inserted.
(3) Breaking strength (mPa), elongation at break (%)
The load and displacement at the time of pulling and breaking were measured under the following conditions.
Sample: Wet hollow fiber membrane prepared by the method of (2) Measuring instrument: Instron type tensile tester (AgS-5D manufactured by Shimadzu Corp.) Distance between chucks: 5 cm
Tensile speed: 20 cm / min The breaking strength and breaking elongation were determined according to the following equations.

  Here, the film cross-sectional area is obtained by the following equation.

(4) Latex rejection (%)
A monodisperse latex with a particle diameter of 0.208 μm (manufactured by JSR Corporation, trade name: STADEX, solid content 1 mass%) is diluted with an aqueous 0.5 mass% SDS (sodium dodecyl sulfonate) solution. A suspension having a latex concentration of 0.01% by mass was prepared. 100 mL of latex suspension is placed in a beaker and supplied to a wet hollow fiber membrane having an effective length of about 12 cm with a tube pump at a linear velocity of 0.1 m / second from the outer surface at a pressure of 0.03 mPa. The latex suspension was filtered by discharging the permeate from both ends (open to the atmosphere) of the yarn membrane. The filtrate was returned to the beaker and filtered in a closed system. Ten minutes after the filtration, the permeate from both ends of the hollow fiber membrane and the feed solution from the beaker were sampled, the absorbance at 600 Nm was measured using an absorptiometer, and the latex rejection was determined by the following equation.

(5) Compressive strength (mPa)
One end of a wet hollow fiber membrane having a length of about 5 cm was sealed, the other end was opened to the atmosphere, pure water at 40 ° C. was pressurized from the outer surface, and permeate was discharged from the open end of the atmosphere. At this time, the whole amount was filtered without circulating the membrane feed water, that is, the whole amount filtration method was adopted. The pressurizing pressure was increased from 0.1 mPa in steps of 0.01 mPa, the pressure was maintained at each pressure for 15 seconds, and the permeated water that emerged from the open end of the atmosphere was sampled for 15 seconds. While the hollow part of the hollow fiber membrane is not crushed, the absolute value of the amount of permeated water (mass) increases as the pressurized pressure increases. However, when the pressurized pressure exceeds the compressive strength of the hollow fiber membrane, the hollow part is crushed and blocked. Therefore, the absolute value of the amount of permeated water decreases as the pressurizing pressure increases. The compression pressure at which the absolute value of the amount of permeated water was maximized was taken as the compressive strength.

(6) Inner and outer surface hole diameters and cross-sectional center hole diameters (μm)
Using a scanning electron microscope, the inner and outer surface pore diameters and the cross-sectional central pore diameter of the porous hollow fiber membrane were measured using photographs capable of confirming the shape of 20 or more pores. In order to divide the A4 version of the photograph into six parts each in vertical and horizontal directions, draw five lines perpendicular to the vertical and horizontal directions at equal intervals, measure the length of the lines across the holes in the picture, The average value was calculated by arithmetic average, and the inner and outer surface hole diameters and the cross-sectional center hole diameter were used. If the pore diameter is about 0.1 μm to 1 μm, a scanning microscope image with a magnification of about 5000 times is appropriate. In addition, about the cross-sectional center hole diameter, it measured considering the range of 10% of the total film thickness from the center position of a film thickness.

(7) Cross-sectional hole diameter and thickness of 1 layer (A) and 1 layer (B) in each film thickness part Photograph of a cross section of a hollow fiber membrane taken with a scanning electron microscope, and a photograph showing the shape of 20 or more holes Was used. On the A4 photo, 100 lines with the same distance from the outer surface (that is, a line connecting points having the same film thickness) are drawn at intervals equal to the total film thickness of 101. The length across the void was measured. The average value of the length was calculated by arithmetic average, and the cross-sectional pore diameter in each film thickness portion was obtained. When the magnification of the scanning electron micrograph is sufficiently high, lines having the same distance from the outer surface may be approximated by a straight line. From the center of the cross section toward the surface of one layer (B), the point closest to 0.7 times the cross-sectional center hole diameter is the boundary line of both layers, and the distance from the surface of the single layer (A) to the boundary line is 1 The thickness of the layer (A) and the distance from the surface of the first layer (B) to the boundary line were defined as the thickness of the first layer (B). If the pore diameter is about 0.1 μm to 1 μm, a scanning microscope image with a magnification of about 5000 times is appropriate. In the present application, the entire film thickness was divided into 14 images. That is, the above-mentioned measurement was performed using 14 electron micrographs of 5000 times the cross section of the hollow fiber membrane. In addition, since the magnification was sufficiently high, lines with the same distance from the outer surface were approximated by straight lines.

(8) Isotropic rate (%) of 1 layer (A)
Of the cross-sectional hole diameters in each film thickness part of one layer (A) measured in (7), the number of film thickness parts that are 0.8 times or more and 1.2 times or less the cross-sectional center hole diameter is 1 layer (A). The ratio divided by the total number of film thickness parts contained in the film was defined as the isotropic ratio of one layer (A).
(9) Isotropic rate (%) of 1 layer (B)
A line is drawn concentrically with the circle indicated by the cross section of the hollow fiber at intervals equal to the thickness of one layer (B) measured in (7), and the length of the line crossing the hole in the photograph is measured. And the average value of the length was computed by the arithmetic mean, and the cross-sectional hole diameter in each film thickness part of 1 layer (B) was calculated | required.
The cross-sectional hole diameter in the thickness part of 1/2 of the thickness of one layer (B) is defined as the cross-sectional central hole diameter (B), and among the measured film thickness parts of one layer (B), the cross-sectional central hole diameter (B) is 0. A ratio obtained by dividing the number of film thickness portions that is 8 times or more and 1.2 times or less by the total number 20 of film thickness portions included in one layer (B) was defined as the isotropic ratio of one layer (B).
(10) Maximum pore size (μm) (bubble point method)
The maximum pore size of the membrane was measured according to ASTm F316-03.
(11) Average pore diameter (μm) (half dry method)
Based on ASTm F316-03, the average pore size of the minimum pore size layer of the membrane was measured.

(12) Production of Pressurized Casing Type Hollow Fiber Membrane Module A pressurized hollow fiber membrane module (FIG. 4) having a membrane area of 50 m ² was produced as follows.
After bundling a plurality of porous hollow fiber membranes, the hollow portion of one end face of the hollow fiber bundle is sealed, and stored in a polysulfone cylindrical module case having an inner diameter of 150 mm and a length of 2000 mm. After placing a total of 24 sticks made of polypropylene having an outer diameter of 11 mm in parallel with the porous hollow fiber membrane on the other end not subjected to the sealing treatment, only the bonding jig is provided at the end which has been performed. A bonding jig was attached in a liquid-tight manner.
The module case with the bonding jig attached on both sides was centrifugally cast with a two-component epoxy resin.
After the centrifugal casting, the bonding jig and the polypropylene rod were removed, and the passage formed was used as an inlet for raw water and air bubbling air during external pressure filtration. After the epoxy adhesive portion was sufficiently cured, the adhesive end portion on the side subjected to the sealing treatment was cut to open the hollow fiber hollow portion, which was used as a permeate discharge hole during external pressure filtration.
As described above, a pressurized casing type hollow fiber membrane module comprising hollow fiber membrane bundles was obtained.

(13) Production of negative pressure type shellless type hollow fiber membrane module A negative pressure type hollow fiber membrane module (FIG. 5) having a membrane area of 25 m ² was produced in the same manner as described in Japanese Patent Application No. 2005-507221.
Both ends of a plurality of porous hollow fiber membranes are bonded and fixed with urethane resin, and a cartridge head fixed in a liquid-tight manner on the outer periphery of one end portion and a lower ring fixed in a liquid-tight manner on the outer periphery of the other end portion. A cylindrical hollow fiber membrane module was prepared. The effective length between the filtration part interfaces of the cartridge head side and the lower ring side adhesive fixing layer was 2000 mm. The diameter of the adhesive fixing layer at both ends of the hollow fiber was about 150 mm. The end of the hollow fiber on the cartridge head side opens to form a filtrate drain hole. A passage is provided in the lower ring side adhesive fixing layer in the same manner as in (12), and air is discharged for air bubbling below the lower ring. A hole is provided so that air can be once bubbled through the passage after the air is once received in the lower ring. The hollow fiber membrane on the lower ring side is bonded without sealing, and the end is sealed.
As described above, a negative pressure shellless type hollow fiber membrane module was produced.

(14) Experiment 1 for measuring water permeability of hollow fiber membrane module (pressurization)
Using the hollow fiber membrane module obtained in (12), river surface water having a turbidity of 5 to 10 degrees and a water temperature of 18 to 25 ° C. was used as raw water. The amount of water permeation was measured by pressurizing with a pump under the condition that the transmembrane differential pressure was stabilized at about 35 kPa in terms of 25 ° C. and stabilized for about 400 hours by the external pressure total filtration method.
The filtration operation was a cycle operation of filtration / (back washing and air bubbling). Each cycle is filtration / (backwash and air bubbling) time cycle: 29 minutes / 1 minute, the backwash flow rate during backwash is 2.3 L / min / module, and the air flow rate during air bubbling is 4.6 NL / min / module.

(15) Water permeability measurement experiment 2 of hollow fiber membrane module (negative pressure)
The hollow fiber membrane module obtained in (13) was used and immersed in an immersion bath having a volume of about 0.35 m ³ . Moreover, river surface water with a turbidity of 5 to 10 degrees and a water temperature of 18 to 25 ° C. was used as raw water. The amount of water permeation was measured under conditions where the membrane hollow portion was negative pressured by a suction pump and the transmembrane differential pressure was stable at about 30 kPa in terms of 25 ° C. for about 400 hours by a total filtration method.
The filtration operation was a cycle operation of filtration / (backwashing and air bubbling) / drain.
The drain is a step of discharging suspended substances accumulated in the tank in the physical washing (back washing and air bubbling) process and the filtration process to the outside of the tank.
Each cycle is filtration / (backwash and air bubbling) / drain time cycle: 28.5 min / 1 min / 0.5 min, and the backwash flow rate during backwash is 2.3 L / min / module The air flow rate during air bubbling was 4.6 NL / min / module.

(16) Water permeability measurement experiment 3 (negative pressure) of the hollow fiber membrane module
The hollow fiber membrane module obtained in (13) was used and immersed in an activated sludge tank having a volume of 8 m ³ . Moreover, the sugar factory waste water whose BOD is 750 mg / L was used as raw water. The mLSS concentration in the activated sludge was constant at about 10 g / L. The amount of water permeation was measured under the condition that the hollow portion of the membrane was set to a negative pressure by a suction pump, and the transmembrane differential pressure was about 20 kPa in terms of 25 ° C. and stable for about 200 hours by a total filtration method.
The filtration operation described above was a cycle operation of filtration / backwashing while constantly aeration of air with a membrane aeration amount of 6 Nm ³ / hour. The filtration / backwash time cycle was filtration / backwash: 9 minutes / 1 minute, and the backwash flow rate during backwashing was the same as the flow rate during filtration.

[Example 1]
Extruded using a vinylidene fluoride homopolymer as a thermoplastic resin, a mixture of di (2-ethylhexyl) phthalate and dibutyl phthalate as an organic liquid, finely divided silica as an inorganic fine powder, and a hollow fiber molding nozzle shown in FIG. Two-layer hollow fiber membranes were melt-extruded by two machines. As the melt-kneaded product (a) for the outer layer, the composition is vinylidene fluoride homopolymer: di (2-ethylhexyl) phthalate: dibutyl phthalate: fine powder silica = 34: 33.8: 6.8: 25.4 (mass ratio) )) As a melt-kneaded product (b) for the inner layer, and the composition is vinylidene fluoride homopolymer: di (2-ethylhexyl) phthalate: dibutyl phthalate: finely divided silica = 36: 35.3: 5.0 : 23.7 (mass ratio) of the melt-kneaded product and air as a hollow portion forming fluid, respectively, for forming hollow fibers having an outer diameter of 2.00 mm and an inner diameter of 0.92 mm at a resin temperature of 240 ° C. From the nozzle, extrusion was performed at a discharge linear velocity of 14.2 m / min, that is, a spin nozzle discharge parameter R of 440 / second, and an outer layer: inner layer thickness ratio = 10: 90. The outer diameter of the nozzle here refers to the outermost diameter of the discharge port in FIG. The inner diameter of the nozzle refers to the maximum diameter of the lower end of the partition wall between the inner layer melt-kneaded product discharge port and the hollow portion forming fluid discharge port.

The extruded hollow fiber-shaped extrudate was cooled and solidified by being introduced into a 40 ° C. water bath after running in the air of 60 cm, and wound up skein at a speed of 40 m / min. The obtained two-layer hollow fiber was immersed in methylene chloride to extract and remove di (2-ethylhexyl) phthalate and dibutyl phthalate, and then dried. Next, after being immersed in a 50% by weight ethanol aqueous solution for 30 minutes, then immersed in water for 30 minutes, then immersed in a 20% by weight sodium hydroxide aqueous solution at 70 ° C. for 1 hour, and further washed with water to repeat fine powder. Silica was extracted and removed.
The obtained porous two-layer hollow fiber membrane was wetted by the method (2), and added to a 4% by mass sodium hydroxide aqueous solution containing sodium hypochlorite having a free chlorine concentration of 0.5% by mass at room temperature. After immersing for 10 days and measuring the rupture elongation before and after immersing, the rupture elongation after immersing retains a value of 90% of the rupture elongation before immersing and has good chemical resistance. I found out.

  In addition, the electron microscope image of 5000 times magnification of the outer surface of the obtained porous two-layer hollow fiber membrane is shown in FIG. 7, the electron microscope image of 5000 times magnification near the outer surface of the section is shown in FIG. FIG. 9 shows an electron microscope image at a magnification of 1000 times in the vicinity, FIG. 10 shows an electron microscope image at a magnification of 5000 times at the center of the cross section, FIG. FIG. 12 shows an electron microscope image at a magnification of 5000 times, FIG. 13 shows an electron micrograph at a magnification of 70 times, and FIG. 14 shows an electron micrograph at a magnification of 300 times in FIG. 7 to 14 show that an outer layer having a small pore diameter and an inner layer having a large pore diameter are formed. In addition, the surface porosity of one layer (B) was 30%.

The obtained porous two-layer hollow fiber membrane had no interface disturbance and high roundness. According to cross-sectional observation with an electron microscope, both the blocking layer and the support layer had an isotropic three-dimensional network structure with no macrovoids. The physical property evaluation results of the obtained film are shown in Table 1. The pure water permeability, latex rejection, and various mechanical strengths all showed excellent values. Further, FIG. 18 shows the results of measuring the cross-sectional pore diameter by dividing the cross section of the obtained porous two-layer hollow fiber membrane into 100 equal parts. It can be seen that the film has a structure very close to that shown in FIG. Moreover, the value which measured the parameter Q is shown in FIG.
The obtained hollow fiber membrane was made into a module by the methods shown in (12) and (13), respectively, and the filtration operation was performed under the conditions shown in (14), (15) and (16). Table 1 shows the water permeability under the conditions (14) to (16).

[Example 2]
The both ends of the porous hollow fiber membrane obtained by extraction and removal of the organic liquid and inorganic fine powder obtained in Example 1 were held by hand, stretched twice, and then released from both ends. The yarn length was shortened by releasing the hand, and the final draw ratio was 1.3 times.
The obtained hollow fiber membrane was made into a module by the methods shown in (12) and (13), respectively, and the filtration operation was performed under the conditions shown in (14), (15) and (16). Table 1 shows the water permeability under the conditions (14) to (16).

[Comparative Example 1]
The outer layer melt-kneaded product (a) is not extruded, and the composition of the inner layer melt-kneaded product (b) is vinylidene fluoride homopolymer: di (2-ethylhexyl) phthalate: dibutyl phthalate: finely divided silica = 40.0 : The thickness of the entire membrane was the same as in Example 1 except that only the melt-kneaded product of 30.8: 6.2: 23.0 (mass ratio) was extruded from the slit on the inner layer side. Obtained. The obtained porous hollow fiber membrane had an isotropic three-dimensional network structure with no macrovoids, as observed by a cross section with an electron microscope. The physical property evaluation results of the obtained film are shown in Table 1. The latex rejection was high and high mechanical strength was exhibited, but the pure water permeability was extremely low.

FIG. 13 shows an electron microscope image of the outer surface of the obtained porous hollow fiber membrane at a magnification of 5000 times, FIG. 14 shows an electron microscope image at a magnification of 5000 times in the vicinity of the outer surface of the cross section, and FIG. FIG. 15 shows an electron microscope image, FIG. 16 shows an electron microscope image at a magnification of 5000 times near the inner surface of the cross section, and FIG. 17 shows an electron microscope image at a magnification of 5000 times on the inner surface.
The obtained porous hollow fiber membrane was wetted by the method (2) and was added to a 4% by mass sodium hydroxide aqueous solution containing sodium hypochlorite having a free chlorine concentration of 0.5% by mass at room temperature. When dipping for 10 days and measuring the breaking elongation before and after dipping, the breaking elongation after dipping retained 90% of the breaking elongation before dipping.
The obtained hollow fiber membrane was made into a module by the methods shown in (12) and (13), respectively, and the filtration operation was performed under the conditions shown in (14), (15) and (16). Table 1 shows the water permeability under the conditions (14) to (16).

[Comparative Example 2]
The both ends of the porous hollow fiber membrane obtained by extraction and removal of the organic liquid and inorganic fine powder obtained in Comparative Example 1 were held by hand and stretched twice, and then released from both ends. The yarn length was shortened by releasing the hand, and the final draw ratio was 1.3 times.
The obtained hollow fiber membrane was made into a module by the methods shown in (12) and (13), respectively, and the filtration operation was performed under the conditions shown in (14), (15) and (16). Table 1 shows the water permeability under the conditions (14) to (16).

  According to the present invention, low-cost and stable long-term operation is possible in filtration of suspended water.

It is the schematic diagram which showed the example of the hole diameter change of the film thickness direction of a porous 2 layer hollow fiber membrane. It is a schematic diagram of an isotropic three-dimensional network structure. It is a schematic diagram of a spherulite structure. It is a schematic diagram of a pressurization type casing type module. It is a schematic diagram of a negative pressure type shellless type module. It is a figure which shows the example of a 2 layer hollow fiber shaping | molding nozzle, (a) Sectional drawing cut | disconnected in the surface parallel to a discharge direction, (b) The front view of a nozzle discharge port, (c) 2 layer hollow fiber shaped extrudate It is sectional drawing cut by the surface perpendicular | vertical to an extrusion direction. 2 is an electron micrograph of the outer surface of the porous two-layer hollow fiber membrane obtained in Example 1 at a magnification of 5000 times. 2 is an electron micrograph of a magnification of 5000 times of a cross section near the outer surface of the porous bilayer hollow fiber membrane obtained in Example 1. FIG. 2 is an electron micrograph at a magnification of 1000 times of a cross section near the outer surface of the porous two-layer hollow fiber membrane obtained in Example 1. FIG. 2 is an electron micrograph at a magnification of 5000 times of the central portion of the cross section of the porous two-layer hollow fiber membrane obtained in Example 1. FIG. 2 is an electron micrograph of a magnification of 5000 times of a cross section near the inner surface of the porous two-layer hollow fiber membrane obtained in Example 1. FIG. 2 is an electron micrograph at a magnification of 5000 times of the inner surface of the porous two-layer hollow fiber membrane obtained in Example 1. FIG. 2 is an electron micrograph at a magnification of 60 times of the entire annular cross section of the porous bilayer hollow fiber membrane obtained in Example 1. FIG. 3 is an electron micrograph of a magnification of 300 times of an annular cross section of a porous two-layer hollow fiber membrane obtained in Example 1. FIG. 4 is an electron micrograph of the outer surface of the porous hollow fiber membrane obtained in Comparative Example 1 at a magnification of 5000 times. 2 is an electron micrograph of a magnification of 5000 times of a cross section near the outer surface of the porous hollow fiber membrane obtained in Comparative Example 1. FIG. 2 is an electron micrograph at a magnification of 5000 times of the central portion of the cross section of the porous hollow fiber membrane obtained in Comparative Example 1. FIG. 3 is an electron micrograph of a magnification of 5000 times of a cross section near the inner surface of the porous hollow fiber membrane obtained in Comparative Example 1. FIG. 3 is an electron micrograph of the inner surface of the porous hollow fiber membrane obtained in Comparative Example 1 at a magnification of 5000 times. 2 is a graph of change in cross-sectional pore diameter of the porous bilayer hollow fiber membrane obtained in Example 1. It is a figure which shows the fluctuation | variation by the film thickness position of the parameter Q of the porous 2 layer hollow fiber membrane obtained in Example 1. FIG. The horizontal axis indicates the position of the film thickness when the total film thickness is 1, and the vertical axis indicates Q.

Claims (10)

Suspended water is a porous multilayer hollow fiber membrane consisting of at least two layers, inner and outer, made of a thermoplastic resin, and at least one of the two layers (A) has an isotropic three-dimensional network structure. And the surface hole diameter is 0.6 to 1.4 times the central hole diameter of the cross section, and the other layer (B) of the two layers has a surface hole diameter of less than ½ of the central hole diameter of the cross section. A filtration method comprising filtering from the surface of one layer (B) to the surface of one layer (A) using a module in which a large number of porous multilayer hollow fiber membranes are collected and stored. The filtration method according to claim 1, wherein the one layer (B) of the porous hollow fiber membrane has an isotropic three-dimensional network structure. The filtration method according to claim 1 or 2, wherein the surface pore diameter of the one layer (B) of the porous hollow fiber membrane is 0.01 µm or more and less than 5 µm. The filtration method according to any one of claims 1 to 3, wherein the pore diameter at the cross-section of the porous hollow fiber membrane is 0.1 µm or more and 10 µm or less. The filtration method according to any one of claims 1 to 4, wherein the surface porosity of the one layer (B) of the porous hollow fiber membrane is 20% or more and 80% or less. The filtration method according to any one of claims 1 to 5, wherein the thickness of the one layer (B) of the porous hollow fiber membrane is 1/100 or more and 40/100 or less of the film thickness. The filtration method according to any one of claims 1 to 6, wherein the isotropic ratio of the one layer (A) and the one layer (B) of the porous hollow fiber membrane is 80% or more. The number of Qs where the parameter Q, which is a value representing the change amount of the average pore diameter depending on the position of the inner surface from the outer surface of the porous hollow fiber membrane, is −0.2 ≦ Q ≦ 0.2 is the total average The filtration method according to any one of claims 1 to 7, wherein the filtration method is 80% or more based on the number of measured pore diameters. The filtration method according to any one of claims 1 to 8, wherein the thermoplastic resin of the porous hollow fiber membrane is selected from polyolefins and polyvinylidene fluoride. The filtration method according to any one of claims 1 to 9, wherein the porous hollow fiber membrane has an inner diameter of 0.4 mm to 5 mm and a film thickness of 0.2 mm to 1 mm.

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