U.S. patent application number 11/848082 was filed with the patent office on 2008-03-20 for method for membrane filtration purification of suspended water.
Invention is credited to Hiroshi Hatayama, Takashi Ikemoto, Noboru KUBOTA.
Application Number | 20080067126 11/848082 |
Document ID | / |
Family ID | 18536867 |
Filed Date | 2008-03-20 |
United States Patent
Application |
20080067126 |
Kind Code |
A1 |
KUBOTA; Noboru ; et
al. |
March 20, 2008 |
METHOD FOR MEMBRANE FILTRATION PURIFICATION OF SUSPENDED WATER
Abstract
A method for membrane filtration purification of suspended
water, comprising filtering the suspended water under an external
pressure through a porous hollow fiber membrane comprising a
polyolefin, a copolymer of olefin and halogenated olefin,
halogenated polyolefin or a mixture thereof and having an open area
ratio in an outer surface of not less than 20% and a pore diameter
in a minimum pore diameter layer of not smaller than 0.03 .mu.m and
not larger than 1 .mu.m.
Inventors: |
KUBOTA; Noboru;
(Moriyama-shi, JP) ; Ikemoto; Takashi;
(Moriyama-shi, JP) ; Hatayama; Hiroshi;
(Moriyama-shi, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Family ID: |
18536867 |
Appl. No.: |
11/848082 |
Filed: |
August 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10181355 |
Jul 17, 2002 |
|
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PCT/JP01/00263 |
Jan 17, 2001 |
|
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11848082 |
Aug 30, 2007 |
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Current U.S.
Class: |
210/650 |
Current CPC
Class: |
B01D 67/0027 20130101;
B01D 61/14 20130101; C02F 1/444 20130101; B01D 71/26 20130101; B01D
67/003 20130101; B01D 69/02 20130101; B01D 71/34 20130101; B01D
69/08 20130101; B01D 2323/12 20130101 |
Class at
Publication: |
210/650 |
International
Class: |
B01D 61/14 20060101
B01D061/14; C02F 1/00 20060101 C02F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 18, 2000 |
JP |
2000-008622 |
Claims
1. A method for membrane filtration purification of suspended
water, comprising filtering the suspended water under an external
pressure through a porous hollow fiber membrane comprising a
polyolefin and having an open area ratio in an outer surface of not
less than 20% and a pore diameter in a minimum pore diameter layer
of not smaller than 0.03 .mu.m and not larger than 1 .mu.m.
2. The method in accordance with claim 1, wherein the open area
ratio in the outer surface is not less than 23%.
3. The method in accordance with claim 1, wherein the open area
ratio in the outer surface is not less than 25%.
4. The method in accordance with any one of claims 1 to 3, wherein
the pore diameter in the minimum pore diameter layer is not larger
than 0.6 .mu.m and a pore diameter in an inner surface is not
smaller than 0.8 .mu.m.
5. The method in accordance with any one of claims 1 to 3, wherein
the porous hollow fiber membrane has the inner diameter of not
smaller than 0.5 mm and not larger than 3 mm and a thickness of not
thinner than 0.1 mm and not thicker than 1 mm.
Description
[0001] This application is a Divisional of co-pending application
Ser. No. 10/181,355, filed on Jul. 17, 2002, the entire contents of
which are hereby incorporated by reference and for which priority
is claimed under 35 U.S.C. .sctn. 120.
TECHNICAL FIELD
[0002] The present invention relates to a clarification method to
obtain clear water usable as drinking water, industrial water or
the like from natural water such as river water, lake and marsh
water and underground water as well as suspended water obtained by
treating natural water, and to provide regenerated water for
miscellaneous uses or the like, or clean water dischargeable into
the environment from domestic wastewater such as sewage water and
from suspended water obtained by treating domestic wastewater.
BACKGROUND ART
[0003] A procedure of solid-liquid separation (clarification
process) to remove suspended substances is indispensable for clear
water treatment to obtain drinking water or to obtain industrial
water from natural water sources such as river water, lake and
marsh water and underground water, and for sewage water treatment
to obtain regenerated water for miscellaneous uses or to obtain
dischargeable clean water by treating domestic wastewater such as
sewage water. There is a major need for clarification procedure to
remove suspended substances (such as clay, colloid and bacteria)
derived from water from natural water sources in the case of a
clear water treatment, and to remove suspended substances in sewage
water or suspended substances (such as sludge) in biologically
treated (secondary treated) water with activated sludge and the
like in the case of a sewage water treatment. Conventionally,
sedimentation method, sand filtration method, and coagulation
sedimentation plus sand filtration method have been predominantly
used for these clarification processes, but recently a membrane
filtration method has become most common. Merits of the membrane
filtration method include: (1) clarification level in resultant
water quality is higher and stable (enhanced safety of resultant
water), (2) installation space for filter unit is small, and (3) an
automated operation is easier, etc. For example, in a clear water
treatment, a membrane filtration method is employed as a substitute
for coagulation sedimentation plus sand filtration or as a means to
further improve the water quality of treated water which has
undergone coagulation sedimentation plus sand filtration by
arranging it after the coagulation sedimentation plus sand
filtration. In the case of a sewage water treatment, studies
regarding the use of the membrane filtration method for, for
example, sludge separation from secondary treated water of sewage
water etc., are also in progress.
[0004] In these clarifying procedures by membrane filtration, a
hollow fiber-like ultrafiltration membrane or microfiltration
membrane (with pore diameters in the range from several nm to
several hundred nm) is mainly used. Filtration systems using a
hollow fiber-like filtration membrane include two types, an
internal pressure filtration where water is filtered from inner
surface side toward outer surface side of the membrane and an
external pressure filtration where water is filtered from outer
surface side toward inner surface side of the membrane. The
external pressure filtration is, however, advantageous because it
enables membrane surface area on the side in contact with raw
suspended water to be larger, so that load of suspended substances
per unit area of membrane surface can be less.
[0005] Clarification by a membrane filtration method is prevailing
in the clear water treatment and the sewage water treatment as an
alternative or a complementary technique for conventional ones due
to the above described many advantages which a conventional
sedimentation or sand filtration method does not have. However,
wider spread of a membrane filtration method is hindered because a
technology enabling a long-term stable operation of membrane
filtration has not been established (see, Y. Watanabe, R. Bian,
Membrane, 24(6), 310-318 (1999)). The most common hinderance to a
stable operation of membrane filtration is the deterioration of the
permeability of a membrane. The first cause of the deterioration of
permeability is clogging of a membrane by suspended substances and
the like (fouling) (see, Y. Watanabe, R. Bian, Membrane, 24(6),
310-318 (1999)). In addition, a membrane surface may be abraded by
suspended substances to cause the deterioration of
permeability.
SUMMARY OF THE INVENTION
[0006] An object of the present invention is to provide a
clarification method comprising a membrane filtration process for
clarifying natural water, domestic wastewater and suspended water,
which is treated water thereof, said process is lowered in
deterioration of permeability due to fouling of membrane and in
deterioration of permeability due to abrasion on membrane surface
and is superior in filtration stability.
[0007] The present inventors, having made extensive efforts to
solve the above described problems, found out that the use of a
membrane with high open area ratio in an outer surface could reduce
the deterioration of permeability due to fouling and the
deterioration of permeability due to abrasion on a membrane surface
and enhance filtration stability of the membrane, and thus
accomplished the present invention.
[0008] A core part of the present invention is to use a membrane
having an outer surface with high open area ratio for filtration.
It has not been known conventionally to use a membrane with high
open area ratio in an outer surface in order to suppress the
deterioration of permeability due to fouling or due to abrasion in
membrane surface.
[0009] The deterioration of permeability due to fouling has been
generally considered so far to be associated with levels of pure
water flux, porosity and further pore diameters, which are
fundamental properties of membrane. However, the present inventors
have found out that, as described practically in Examples later,
within a certain range of pore diameter, retention of permeability
(degree of deterioration of permeability; the lower the retention
of permeability is, the severer the deterioration is), in
filtration of suspended water, has no relation to levels of pure
water flux, porosities and pore diameter but is determined by the
degree of open area ratio in an outer surface. That is, the present
inventors have found that the larger the open area ratio in an
outer surface is, the larger the retention of permeability is. This
means that even the membranes having the same pure water flux,
porosities and pore diameters, may have different retentions of
permeability (degree of deterioration of permeability), if they
have different open area ratios in their outer surfaces, and thus
shows an importance of an open area ratio in an outer surface for
suppression of deterioration of permeability due to fouling.
[0010] Abrasion on membrane surface has been considered to occur
not during filtering operation but mainly during the process of
removing suspended substances accumulated on the outer membrane
surface in external pressure type filtration by air cleaning etc.
However, the phenomenon itself has not been well known and thus
there has been little development in technology addressing the
deterioration of permeability due to abrasion on membrane surface.
There has only been made mention that using a membrane having high
breaking strength is effective (see JP-A-1999-138164). The present
inventors have obtained knowledge that use of a membrane with high
open area ratio in an outer surface is also advantageous against
the deterioration of permeability due to abrasion on membrane
surface. The present invention has been accomplished based on this
knowledge as a core concept.
[0011] Namely, the present invention relates to:
[0012] (1) A method for membrane filtration purification of
suspended water comprising filtering the suspended water under an
external pressure through a porous hollow fiber membrane comprising
polyolefin, a copolymer of olefin and halogenated olefin, a
halogenated polyolefin or a mixture thereof and having an open area
ratio in an outer surface of not less than 20% and a pore diameter
in a minimum pore diameter layer of not smaller than 0.03 .mu.m and
not larger than 1 .mu.m.
(2) the method in accordance with the above described (1), wherein
the open area ratio in the outer surface of said porous hollow
fiber membrane is not less than 23%.
(3) the method in accordance with the above described (1), wherein
the open area ratio in the outer surface of said porous hollow
fiber membrane is not less than 25%.
[0013] (4) the method in accordance with any one of the above
described (1), (2) or (3), wherein said porous hollow fiber
membrane has the pore diameter in minimum pore diameter layer of
not larger than 0.6 .mu.m and at least a pore diameter of an inner
surface between pore diameters of the inner and outer surfaces is
not smaller than 0.8 .mu.m.
[0014] (5) the method in accordance with any one of the above
described (1), (2), (3) or (4), wherein the porous hollow fiber
membrane has the inner diameter of not smaller than 0.5 mm and not
larger than 3 mm and the membrane thickness of not thinner than 0.1
mm and not thicker than 1 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 shows a schematic diagram of equipment for filtration
through a hollow fiber membrane in an external pressure system,
wherein, 1 is raw water; 2 is peristaltic pump; 3 is pressure gauge
(inlet pressure); 4 is connector; 5 is hollow fiber membrane; 6 is
tube (inner diameter of 3 mm); 7 is epoxy resin; 8 is injection
needle; 9 is permeated water; 10 is silicon cap; 11 is pressure
gauge (exit pressure); 12 is valve; 13 is silicon tube; 14 is feed
water; and 15 is circulating water.
[0016] FIGS. 2A-E are electron micrographs of the porous hollow
fiber membrane prepared in Example 1, and their images after black
and white binarize process of a part of the photographs (black part
expresses a pore section and the white part expresses a non-pore
section), wherein, A is a photograph of outer surface; B is a
photograph of membrane cross-section (whole view); C is a
photograph of membrane cross-section (magnified view); D is a
photograph of inner surface; and E is a black and white binarized
image of the photograph of outer surface.
[0017] FIGS. 3A-C are electron micrographs of the porous hollow
fiber membrane prepared in Example 2, and their images after black
and white binarize process of a part of the photographs (black part
expresses a pore section and the white part expresses a non-pore
section), wherein, A is a photograph of outer surface; B is a
photograph of membrane cross-section (whole view); and C is a black
and white binarized image of the photograph of outer surface.
[0018] FIGS. 4A-C are electron micrographs of porous hollow fiber
membrane prepared in Example 3, and their images after black and
white binarize process of a part of the photographs (black part
expresses a pore section and the white part expresses a non-pore
section), wherein, A is a photograph of outer surface; B is a
photograph of membrane cross-section (whole view); and C is a black
and white binarized image of the photograph of outer surface.
[0019] FIGS. 5A-C are electron micrographs of porous hollow fiber
membrane prepared in Example 4, and their images after black and
white binarize process of a part of the photographs (black part
expresses a pore section and the white part expresses a non-pore
section), wherein, A is a photograph of outer surface; B is a
photograph of membrane cross-section (whole view); and C is a black
and white binarized image of the photograph of outer surface.
[0020] FIGS. 6A-E are electron micrographs of porous hollow fiber
membrane prepared in Example 5, and their images after black and
white binarize process of a part of the photographs (black part
expresses a pore section and the white part expresses a non-pore
section), wherein, A is a photograph of outer surface; B is a
photograph of membrane cross-section (whole view); C is a
photograph of membrane cross-section (magnified view); D is a
photograph of inner surface; and E is a black and white binarized
image of the photograph of outer surface.
[0021] FIGS. 7A-C are electron micrographs of porous hollow fiber
membrane prepared in Comparative Example 1, and their images after
black and white binarize process of a part of the photographs
(black part expresses a pore section and the white part expresses a
non-pore section), wherein, A is a photograph of outer surface; B
is a photograph of membrane cross-section (whole view); and C is a
black and white binarized image of the photograph of outer
surface.
[0022] FIGS. 8A-C are electron micrographs of porous hollow fiber
membrane prepared in Comparative Example 2, and their images after
black and white binarize process of a part of the photographs
(black part expresses a pore section and the white part expresses a
non-pore section), wherein, A is a photograph of outer surface; B
is a photograph of membrane cross-section (whole view); and C is a
black and white binarized image of the photograph of outer
surface.
[0023] FIGS. 9A-B are electron micrographs of porous hollow fiber
membrane prepared in Comparative Example 3, and their images after
black and white binarize process of a part of the photographs
(black part shows a pore section and the white part shows a
non-pore section), wherein, A is a photograph of outer surface; and
B is a black and white binarized image of the photograph of outer
surface.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Suspended water as a target of the present invention
includes natural water, domestic wastewater and treated water
thereof. Natural water includes river water, lake and marsh water,
underground water and sea water. Treated water of natural water,
having been subjected to sedimentation treatment, sand filtration
treatment, coagulation sedimentation plus sand filtration
treatment, ozone treatment and the like is also included in the
suspended water as a target water of the present invention. An
example of domestic wastewater is sewage water. A primary treated
water of sewage water having been subjected to screening filtration
or sedimentation treatment, a secondary treated water of sewage
water having been subjected to bioprocess treatment and further a
tertiary treated (highly treated) water of sewage water having been
subjected to coagulation sedimentation plus sand filtration
treatment, activated carbon treatment or ozone treatment are also
included in the suspended water as a target of the present
invention. Suspended substances consisting of fine organic
materials, inorganic materials and mixtures thereof, with a size of
not larger than .mu.m order (such as humus colloid, organic
colloid, clay and bacteria) are contained in these suspended water.
A method in accordance with the present invention is suitable to
clarify these suspended waters.
[0025] Water quality of the above described natural water, domestic
wastewater and treated water thereof as a target of clarification
of the present invention can generally be expressed by typical
indices, turbidity and concentration of organic materials, either
alone or in combination thereof. Water quality is roughly
classified by turbidity (not an instantaneous value but an average
value) into low turbidity water with a turbidity of less than 1,
medium turbidity water with a turbidity of not less than 1 but less
than 10, high turbidity water with a turbidity of not less than 10
but less than 50 and ultra-high turbidity water with a turbidity of
not less than 50. Water quality is also roughly classified by
concentration of organic materials (Total Organic Carbon (TOC):
mg/L) (also not an instantaneous value but an average value) into
low TOC water with a TOC of less than 1, medium TOC water with a
TOC of not less than 1 but less than 4, high TOC water with a TOC
of not less than 4 but less than 8 and ultra-high TOC water with a
TOC of not less than 8. Basically, water with higher turbidity or
TOC are more likely to plug filtration membrane and thus the
effects of the present invention become greater for the water with
higher turbidity or TOC. However, for water with extremely high
turbidity or TOC, the effects of the present invention become less
noticeable because a gel layer derived from suspended substances,
which are blocked and accumulated on filter surface (outer surface
in the case of the present invention) of filtration membrane,
becomes a controlling factor of filtration resistance.
[0026] The material used in the porous hollow fiber membrane of the
present invention includes a polyolefin, a copolymer of olefin and
halogenated olefin, a halogenated polyolefin or a mixture thereof.
Examples are polyethylene, polypropylene, polyvinyl alcohol,
ethylene-vinyl alcohol copolymer, ethylene-tetrafluoroethylene
copolymer, polyvinylidenefluoride and a mixture thereof. These
materials are superior as a membrane material because they are easy
to handle due to being thermoplastic and have sufficient toughness.
Among others, polyethylene, polypropylene, polyvinylidenefluoride
and a mixture thereof are preferable due to having superior water
resistance (mechanical strength in wet state), mechanical strength
and chemical strength (chemical resistance) due to their
hydrophobic property and high crystallinity, as well as good
moldability. In particular, polyethylene, polypropylene and a
mixture thereof are more suitable as a membrane material because
not only do they have particularly good moldability but they also
are easy in waste treatment due to their halogen-free composition
and low cost.
[0027] The open area ratio in the outer surface of the porous
hollow fiber membrane used in the present invention is not less
than 20%, preferably not less than 23%. By using a membrane with a
high open area ratio in the outer surface in filtration,
deterioration of permeability due to both fouling and abrasion on
membrane surface can be reduced and filtration stability can be
enhanced. In particular, since the effect of suppressing
deterioration of permeability due to abrasion on membrane surface
becomes significant when the open area ratio in outer surface is
not less than 25%, open area ratio in outer surface of not less
than 25% is particularly preferable. In a case where a halogenated
polyolefin such as polyvinylidenefluoride is used, the effect of
making open area ratio in the outer surface of not less than 25% is
particularly great. However, if open area ratio in outer surface is
too high, the mechanical strength of the membrane becomes too low,
which is not preferred, and thus the open area ratio in the outer
surface is preferably less than 50%, particularly preferably less
than 40% and more preferably less than 30%.
[0028] The open area ratio in the outer surface is determined by
subjecting an electron micrograph of outer surface to a black and
white binarize process to identify pore sections and non-pore
sections present in the outer surface and using the formula: Open
area ratio[%]=100.times.(area of pore section)/{(area of pore
section)+(area of non-pore section)}
[0029] The magnification of the electron micrograph should be large
enough so as to allow shapes of pores present in outer surface to
be clearly recognized. However, an area to be photographed should
be as large as possible to measure the open area ratio as
accurately as possible, and thus a too high magnification is not
suitable. A guideline for the magnification of photograph is
1,000-5,000 times when a real nucleus of pore diameter in outer
surface (pore diameter corresponding to a cumulative area of 50%)
is about 1-10 .mu.m, 5,000-20,000 times when it is about 0.1-1
.mu.m and 10,000-50,000 times when it is about 0.03-0.1 .mu.m. Upon
black and white binarizing, electron micrographs taken by these
magnifications may be used after enlarging by means of copier and
the like.
[0030] In this connection, by using a commercially vailable image
analysis system, black and white binarize process can be performed
in an apparatus of the system directly from an electron micrograph
or its duplicate. However, this method is not suitable because it
tends to induce an error in measuring open area ratio caused by an
incorrect recognition in the binarize process due to the following
reasons: in a general electron micrograph the edge of peripheral
part of pore may shine whitely or a non-pore part becomes black as
pore part does, depending on the way contrast is used in
photographing. Furthermore, the direct black and white binarize
process using an electron micrograph or its duplicate in the system
apparatus, may wrongly recognize an inner structure, which is in
fact not a surface region but is viewable from opening section of
surface, as a structure of a surface region to induce an error in
measuring open area ratio.
[0031] Therefore, when the open area ratio is determined by a black
and white binarize process, it is convenient to place a transparent
sheet on an electron micrograph or its copy, mark (transcript) the
pore section present on the surface with black ink of a black felt
tip pen or the like, then photocopy this transcribed sheet on a
white paper to clearly distinguish pore sections as black and
non-pore sections as white, and make a measurement of open area
ratio using a commercially available image analysis system and the
like.
[0032] A pore diameter in minimum pore diameter layer of the porous
hollow fiber membrane used in the present invention is not smaller
than 0.03 .mu.m and not larger than 1 .mu.m. Minimum pore diameter
layer means a layer having the most dense pores (small pore
diameter) in a membrane cross-section, which determines
clarification performance for suspended substances and affects an
initial permeability (or pure water permeability) greatly. Pore
diameter in a minimum pore diameter layer is an averaged pore
diameter of pores present in said layer. A pore diameter in minimum
pore diameter layer smaller than 0.03 .mu.m is not suitable because
an initial permeability is too low. A pore diameter over 1 .mu.m is
not suitable because clarification performance for suspended
substances to be removed lowers. A pore diameter in minimum pore
diameter layer is preferably not smaller than 0.05 .mu.m and not
larger than 0.6 .mu.m, more preferably not smaller than 0.1 .mu.m
and not larger than 0.4 .mu.m. Pore diameter in a minimum pore
diameter layer can be measured in accordance with ASTM: F316-86, a
method for measuring mean flow pore size (half-dry method in
another name). Mean flow pore size measured by half-dry method is a
flow averaged pore diameter in minimum pore diameter layer. In the
present invention, therefore, said flow averaged pore diameter
measured by half-dry method is used as pore diameter in a layer
with minimum pore diameter. In the present invention a measurement
by half-dry method is carried out using ethanol as a liquid for
hollow fiber membrane with length of about 10 cm and a measurement
at 25.degree. C. with pressure increasing rate of 0.01 atm/sec is
used as standard measurement conditions. A pore diameter in minimum
pore diameter layer (flow averaged pore diameter by half-dry
method) is determined by the following equation: Pore diameter in
minimum pore diameter layer[.mu.m]=2860.times.(surface tension of
liquid in use[dynes/cm])/(half-dry air pressure[Pa])
[0033] Since surface tension of ethanol at 25.degree. C. is 21.97
dynes/cm (see Handbook of Chemistry, edited by Chemical Society of
Japan, Fundamental Section, Revision 3, II-82 p, Maruzen, 1984),
the pore diameter in minimum pore diameter layer can be determined
by the following equation, under the standard measurement
conditions of the present invention: Pore diameter in minimum pore
diameter layer[.mu.m]=62834/(half-dry air pressure[Pa])
[0034] A preferable cross-sectional structure of porous hollow
fiber membrane used in the present invention is a sponge structure
with 3-dimensional network. A non-3-dimensional network structure,
that is, a structure with linear through holes in thickness
direction, or a non-sponge structure, that is, a structure where
such a macro-void, that may occupy not less than 1/4 of thickness
is substantially present (so-called void structure) in membrane
cross-section, generally gives a porous membrane with small
specific surface area and thus small specific surface area per
suspended substance load. On the other hand, a sponge structure
with 3-dimensional network generally provides a porous membrane
with large specific surface area, and thus large specific surface
area per suspended substance load, and results in enhanced ability
for suspended matter load in a membrane cross-section, contributing
to an improvement of filtration stability. Furthermore, a sponge
structure with 3-dimensional network is preferred to a void
structure since the 3-dimensional network has a higher compressive
strength than a void structure.
[0035] A suitable manufacturing method for a sponge structure with
3-dimensional network suitably used in the present invention
includes a thermally induced phase separation method. The thermally
induced phase separation method is a method wherein a thermoplastic
polymer and a latent solvent for the thermoplastic polymer, which
is a non-solvent at around room temperature but is a solvent at a
high temperature for the thermoplastic polymer, are heated and
mixed to a molten state at a high temperature (not lower than the
temperature at which both of them are mutually solved). The mixture
is then cooled down to a temperature not higher than the
solidification temperature of the thermoplastic polymer, inducing a
phase separation between a polymer rich phase and a polymer lean
(solvent rich) phase by utilizing lowering of solubility of the
thermoplastic polymer to the latent solvent during said cooling
process. Subsequently the latent solvent is removed by extraction
to give a porous body consisting of solidified body of the polymer
rich phase generated in the phase separation (see H. Matsuyama,
Chemical Engineering, 43 (1998) 453-464 or D. R. Lloyd, et. al.,
Journal of Membrane Science, 64 (1991) 1-11, etc). In this
connection, the thermally induced phase separation method also
includes a method comprising adding inorganic filler such as
pulverized silica particles to the mixture in addition to a
thermoplastic polymer and its latent solvent. The mixture is heated
and mixed and after cooling for solidification, the inorganic
fillers are extracted together with the latent solvent to obtain a
porous body. Examples of latent solvents, when a thermoplastic
polymer is, for example, polypropylene and polyvinylidenefluoride,
include phthalates such as dibutyl phthalate, dihexyl phthalate,
dioctyl phthalate, di(2-ethylhexyl)phthalate and diisodecyl
phthalate and a mixture thereof.
[0036] One of the suitable methods for obtaining porous hollow
fiber membrane using a thermally induced phase separation method is
as follows: A thermoplastic polymer as a membrane polymer material
and its latent solvent (optionally with an inorganic filler) are
heated and mixed to a melt using an extruder and the like, followed
by extruding said molten mixture through a spinneret (a nozzle
having a circular ring hole to extrude heated mixture in its
extrusion face and a round hole to inject a hollow part forming
fluid inside the circular ring hole) in a hollow shape with
injection of hollow part forming fluid into the hollow part,
cooling to solidify, and then removing by extraction the latent
solvent (and an inorganic filler). The hollow part forming fluid is
injected into said hollow part so that hollow part of a hollow
fiber-like extrudate does not collapse during cooling and
solidifying, and gas or liquid that is substantially inactive
(without inducing any chemical change) to the molten extrudate is
used as a hollow part forming fluid. Cooling and solidifying after
extrusion can be performed by air cooling, liquid cooling or the
combination thereof. Gas or liquid as a cooling medium is required
to be substantially inactive to the extrudate. Extraction of a
latent solvent (or an inorganic filler) is performed using a
volatile liquid or an aqueous solution that is substantially inert
to the materials solidified by cooling and superior in dissolving
power for the latent solvent (or inorganic filler).
[0037] An example of a suitable method for manufacturing porous
hollow fiber membrane having a sponge structure with 3-dimensional
network suitably used in the present invention includes the
following three types, (A)-(C), using a thermally induced phase
separation method and a combination thereof.
[0038] (A) A method comprising heating and mixing an inorganic
filler along with a membrane polymer material and its latent
solvent, and after cooling and solidifying, removing by extraction
the inorganic filler together with the latent solvent. Preferable
inorganic fillers are pulverized silica having an average primary
particle diameter of not smaller than 0.005 .mu.m and not larger
than 0.5 .mu.m and a specific surface area of not less than 30
m.sup.2/g and not larger than 500 m.sup.2/g. Since such pulverized
silica has good dispersibility during heated mixing, the resultant
membrane tends to have less structural defects and removal by
extraction can be easily performed with an alkaline aqueous
solution. Preferable amounts of a membrane material polymer in the
heated mixing is, in view of a balance between strength and opening
characteristics of the resultant membrane, from 15% by weight to
25% by weight for materials with a specific gravity of about 1
g/cm.sup.3, such as polyethylene and polypropylene, and from 25% by
weight to 45% by weight for materials with a specific gravity of
about 1.7 g/cm.sup.3, such as polyvinylidenefluoride, which is
about 1.7 times more than that of a case of material with a
specific gravity of 1. Furthermore, a weight ratio of latent
solvent/pulverized silica is, in view of a balance between strength
and opening characteristics of resultant membrane, not less than
1.0 and not more than 2.5, in particular, preferably not less than
1.2 and not more than 1.8.
[0039] (B) A method wherein cooling and solidifying of the molten
mixture extruded from a spinneret is carried out in a liquid bath
composed of a latent solvent as an upper layer and water as a lower
layer. This method is applicable when the latent solvent is a
liquid with a specific gravity smaller than that of water and is
incompatible with water, such as di(2-ethylhexyl)phthalate, dioctyl
phthalate and diisodecyl phthalate. A thickness of the upper layer
is, in view of ensuring opening characteristics, not less than 1
mm, preferably not less than 5 mm. On the contrary, too thick upper
layer gives unfavorable effect in view of ensuring cooling ability
of a liquid bath, and the thickness is not larger than 30 cm,
preferably not larger than 10 cm, and more preferably not larger
than 2 cm. A lower water layer should have a thickness of not less
than 5 cm, preferably not less than 10 cm, in view of ensuring
cooling ability. Cooling in this two layer liquid bath system is a
process to ensure opening characteristics in an outer surface by
passing through an upper layer consisting of a latent solvent and
to ensure cooling and solidifying by passing through a lower layer
consisting of water, which is superior in cooling ability due to
its high heat content. In this connection, a time required for
extrudate to travel from a spinneret to liquid surface of a liquid
bath (aerial running time) should not be too long in order to
obtain sufficient effects brought about by passing through an upper
layer of a liquid bath, and is preferably not longer than 5 second,
more preferably not longer than 1 second. However, a condition
under which the aerial running time is zero, that is, a state in
which the spinneret contacts with liquid surface of the liquid
bath, is not preferable because temperatures of both the spinneret
and the liquid bath cannot be controlled. By a method using this
two layer liquid bath system, the opening characteristics in outer
surface is effected relatively easily. When this two layers liquid
bath system is used, preferable amounts of a membrane material
polymer used in heated mixing is, in view of a balance between
strength and opening characteristics of resultant membrane, from
15% by weight to 35% by weight for materials with a specific
gravity of about 1 g/cm.sup.3 such as polyethylene and
polypropylene, and from 25% by weight to 60% by weight for
materials with a specific gravity of about 1.7 g/cm.sup.3, such as
polyvinylidenefluoride which is about 1.7 times more than that of a
material with a specific gravity of 1. By the way, an aerial
running time is determined by the following equation based on
winding speed and an aerial running distance (a distance from
spinneret surface to liquid bath surface), when hollow fiber is
wound up at an exit of liquid bath without tension: Aerial running
time[second]=(aerial running distance[cm])/(winding
speed[cm/second]) (C) A method wherein a porous hollow fiber
membrane prepared by using a thermally induced phase separation
method is drawn in the longitudinal direction of the hollow fiber.
Drawing is conducted after cooling and solidifying and before or
after extraction of a latent solvent (and/or an inorganic filler).
With regard to the extent of the extension of hollow fiber by
drawing, too small of an extension results in insufficient security
of opening characteristics and too large of an extension results in
fracture of membrane structure. Therefore, drawing operation should
be controlled preferably within a range from 10% to 100% in a
residual elongation ratio. In this connection, the residual
elongation ratio is defined by the following equation based on a
fiber length before drawing and a fiber length after relaxation
when tension is removed after drawing. By subjecting a hollow fiber
membrane to drawing operation at such a low ratio (relatively low
residual elongation ratio), improvement in the opening
characteristics of the membranes with low opening characteristics
can be achieved. Residual elongation ratio[%]=100.times.[(fiber
length after relaxation)-(fiber length before drawing)]/(fiber
length before drawing)
[0040] With regard to a membrane structure, so-called anisotropic
cross-sectional structure is particularly preferable, in which a
pore diameters in other layers than a minimum pore diameter layer
are significantly larger than that in the minimum pore diameter
layer. However, it is not an indispensable constituent of the
present invention for a membrane to have an anisotropic
cross-sectional structure. In a membrane with an anisotropic
cross-sectional structure (hereinafter referred to as membrane with
anisotropic structure), pore diameter is not uniform (even) but
varies along a cross-sectional direction of membrane (thickness
direction). Initial permeability (or pure water permeability) of a
membrane depends on a thickness of minimum pore diameter layer. The
thicker the minimum pore diameter layer in a membrane is, the
larger the permeation resistance of the whole membrane and the
lower the initial permeability (or pure water permeability). In a
membrane with anisotropic structure, since a minimum pore diameter
layer is a part of the whole membrane thickness, it has a smaller
permeation resistance than a membrane with an isotropic structure
which have a minimum pore diameter layer throughout the thickness,
and thus an initial membrane permeability (or pure water
permeability) can be improved. Contrarily, a blocking ability for
suspended substances is the same as long as a pore diameter in a
minimum pore diameter layer is the same, irrespective of the
thickness of the minimum pore diameter layer. Therefore, membranes
with an anisotropic structure and with an isotropic structure
having the same pore diameter in their minimum pore diameter
layers, by comparison have the same blocking ability for suspended
substances, but the former has a higher initial permeability (or
pure water permeability). In a practical clarification operation,
it is generally conducted by a constant filtration operation, where
an amount of permeated water through membrane is kept constant in
filtration. A high initial permeability (or pure water
permeability) means an ability to operate filtration under a lower
filtration pressure at least during an initial stage of filtration
operation, and contributes to high filtration stability, that is an
objective of the present invention.
[0041] From the above viewpoint, a porous hollow fiber membrane
having an anisotropic structure in which at least the inner surface
side has coarser pores, wherein a pore diameter in the minimum pore
diameter layer is not larger than 0.6 .mu.m to provide a sufficient
ability to block suspended substances, and at least the pore
diameter in the inner surface of the pore diameters in the inner
and outer surfaces is not smaller than 0.8 .mu.m, can be suitably
used in the present invention. However, since too large pore
diameter in an inner surface lowers the membrane strength, it is
preferable to be not larger than 10 .mu.m. Pore diameter in outer
surface is not specially limited. Although an outer surface may
have a larger pore diameter and less dense structure than a minimum
pore diameter layer, desirably pore diameter in the outer surface
is not larger than 10 .mu.m, in view of strength.
[0042] Pore diameter in an inner surface is expressed by a pore
diameter corresponding to 50% cumulative area of pores observed on
the inner surface (present on an inner surface) in an image of
inner surface observed by electron microscope. The "pore diameter
corresponding to 50% cumulative area of pores" means, for the pores
observed on the surface (present on the surface), the diameter of
the pore up to which the subtotal of the pore areas reaches 50% of
the total of the areas of all the pores when adding up the areas of
the pores in an electromicroscopic image in order of diameter (for
the smallest diameter to larger diameters or from the largest
diameter to smaller diameters). As a diameter of a pore observed to
be a non-circular shape (such as ellipse), a diameter of a circular
shape, to which said pore is approximated, (a diameter of circle
having the same area as said pore) is adopted. In this connection,
the pore diameter, defined by a pore diameter corresponding to 50%
cumulative area, is different in definition from mean flow pore
size used in defining pore diameter of a minimum pore diameter
layer, and gives smaller value than mean flow pore size. However,
for measuring pore diameter in a surface, since pore diameter
corresponding to 50% cumulative area is easier to measure and more
accurate than mean flow pore size, inner surface pore diameter is
defined by pore diameter corresponding to 50% cumulative area in
the present specification.
[0043] Pore diameter in an inner surface is measured just as in
measurement of open area ratio in an outer surface described above,
by subjecting pore and non-pore sections present in the inner
surface to a black and white binarize process in an electron
micrograph of the inner surface, followed by determination of pore
diameter (pore diameter of approximated circle) and pore area of
each pore using a commercial image analyzing system or the like and
adding up area of each pore in order of diameter, from the smallest
pore to the larger pores or vice versa, in accordance with the
above described definition, and thus determining the diameter of
the pore up to which the subtotal of each pore area reaches 50% of
the total area of all pores.
[0044] Such a porous hollow fiber membrane having an anisotropic
cross-sectional structure with a nondense structure at least in an
inner surface side, can be prepared by using a latent solvent as a
hollow part forming fluid in the above-described example method for
manufacturing porous hollow fiber membrane using a thermally
induced phase separation approach.
[0045] The inner diameter of porous hollow fiber membrane used in
the present invention is not smaller than 0.5 mm and not larger
than 3 mm, preferably not smaller than 0.5 mm and not larger than
1.5 mm. Too small an inner diameter is disadvantageous, because it
increases the resistance of liquid (pressure loss) flowing in a
hollow fiber tube, and on the contrary, too large a diameter is
also disadvantageous due to decrease in filled membrane area per
unit volume. The thickness of porous hollow fiber membranes used in
the present invention is not thinner than 0.1 mm and not thicker
than 1 mm. Too thin of a membrane is disadvantageous due to its
lowered membrane strength, and on the contrary, too thick of a
membrane is also disadvantageous due to its strong filtration
resistance. A thicker membrane also enables to provide a larger
specific area per membrane surface area and increase specific area
per suspended matter load and thus is preferable in view of
improvement of filtration stability. Therefore, a membrane
thickness of not less than 0.2 mm is particularly preferable.
EXAMPLES
[0046] Hereinbelow, examples of the present invention will be
described, but the present invention is not limited to these
examples.
[0047] In this connection, measurements for various property values
described in these examples were conducted in accordance with the
following procedures:
1) Open Area Ratio in Outer Surface and Pore Diameter in Inner
Surface:
[0048] Electron micrographs taken at the magnification from 1,000
to 10,000 times were photocopied with enlargement by 2 times in
length and breadth. A transparent sheet (a commercially available
OHP sheet) was then placed on said enlarged copy to mark the pore
sections present in the membrane surface with the black ink of a
felt tip pen. Then said marked sheet was black and white
photocopied on a white paper in such a way that pore sections
became black and non-pore sections became white, and the black and
white photocopied image was then input to a computer using a CCD
camera. Area and diameter (value corresponding to approximated
circle) of each pore were determined by using image analyzing
software "Quantimet 500", manufactured by Leica. Open area ratio
was determined by the following equation: Open area
ratio[%]=100.times.(sum of each pore area)/(area of analysis
object) wherein, (area of analysis object)=(sum of each pore
area)+(sum of each non-pore area). Pore diameter in the inner
surface was determined by adding up the area of each pore in the
inner surface in order of diameter, from the smallest pore
diameters to the larger pores, and determining a pore diameter of
the pore up to which the subtotal of each pore area reaches 50% of
the total area of all pores. 2) Pore Diameter in a Minimum Pore
Diameter Layer:
[0049] It was measured in accordance with ASTM F316-86 under the
standard measurement conditions described hereinabove.
3) Pure Water Flux:
[0050] A hollow fiber membrane with a length of about 10 cm was
immersed in ethanol and then several times in pure water
repeatedly. Thus wet-treated hollow fiber membrane was sealed in
one end, and an injection needle was inserted into a hollow section
in the other end. By injecting pure water at 25.degree. C. into a
hollow part under a pressure of 0.1 MPa at 25.degree. C. of ambient
temperature, an amount of pure water permeated from outer surface
was measured to determine a pure water flux by the following
equation: Pure water flux[L/m.sup.2/h]=60.times.(amount of
permeated water[L])/[.pi..times.(outer diameter of
membrane[m]).times.(effective length of
membrane[m]).times.(measurement time[min])] wherein, an effective
length of membrane is defined as a net membrane length excluding a
portion into which an injection needle is inserted. 4) Retention of
Water Permeability During Filtration of Suspended Water:
[0051] This value is an index to judge a degree of deterioration of
water permeability by fouling. A hollow fiber membrane was immersed
in ethanol and then several times in pure water repeatedly. Using
thus wet-treated hollow fiber membrane with an effective length of
11 cm, filtration was carried out by external pressure system (FIG.
1). Pure water was filtered under the pressure to provide 10
m.sup.3 of permeation per 1 m.sup.2 of membrane outer surface area
per day, thus permeated pure water was collected for two minutes
and determined as an initial permeation amount for pure water.
Then, river surface water (surface stream water of Fuji River:
turbidity of 2.2, TOC concentration of 0.8 ppm), as natural
suspended water, was filtered for 10 minutes under the same
filtration pressure as in the measurement of initial permeation
amount for pure water, and the permeated water was collected for 2
minutes from the eighth minute to the tenth minute after the
filtration started, to obtain a permeation amount in filtration of
the suspended water. Retention of water permeability in filtration
of suspended water was defined by the equation below. All of the
operations were conducted at 25.degree. C. and a linear velocity at
membrane surface of 0.5 m/sec. Retention of water permeability in
filtration of suspended water[%]=100.times.(Permeation amount in
filtration of suspended water[g])/(Initial permeation amount for
pure water[g]) wherein, Filtration pressure=[(Inlet pressure)+(Exit
pressure)]/2 Outer surface area of
membrane[m.sup.2]=.pi..times.(Outer fiber
diameter[m]).times.(Effective membrane length[m]) Linear velocity
at membrane surface[m/s]=4.times.(Amount of circulating
water[m.sup.3/s])/[.pi..times.(Tube
diameter[m].sup.2-.pi..times.(Outer membrane
diameter[m]).sup.2]
[0052] In this measurement, filtration pressure for suspended water
was not constant for each membrane but set at such a pressure that
an initial permeability for pure water (it is also a permeability
at the start of filtration of suspended water) provides permeation
of 10 m.sup.3 per 1 m.sup.2 of membrane outer surface area per day.
This is because, in practical treatment for tap water and sewage
water, the membrane is generally used in operation at a constant
filtration amount (a filtration operation system where filtration
pressure is adjusted so as to obtain a constant filtration amount
per a given time), and therefore, in the present measurement, a
comparison of the deterioration of membrane permeability can be
made under conditions which resemble as close as possible to those
of the operation at a constant filtration amount, within the limits
of using single hollow fiber membrane.
5) Ratio of Resistance to Membrane Surface Abrasion:
[0053] This value is an index for judging degree of deterioration
of permeability due to abrasion on the membrane surface. A
wet-treated hollow fiber membrane obtained by immersing hollow
fiber membrane in ethanol and then in pure water several times
repeatedly, was placed on a metal plate. Suspended water containing
20% by weight of fine sand (particle diameter of 130 .mu.m, Fuji
Brown FRR#120) was sprayed onto the outer surface of the membrane
by ejecting the suspended solution from a nozzle that is set at a
position 70 cm above said membrane with a pressure of 0.07 MPa.
After ten minutes of spraying, spraying for another ten minutes was
repeated after turning the membrane upside down. Pure water flux
was measured before and after the spraying and ratios of resistance
to membrane surface abration were determined by the following
equation: Ratio of resistance to membrane surface
abrasion[%]=100.times.(pure water flux after spraying)/(pure water
flux before spraying) 6) Porosity:
[0054] This value is an index showing pore characteristics not only
for surface of the membrane but also for the whole membrane.
Porosity was determined by the following equation:
Porosity[%]=100.times.[(Weight of wet-treated membrane[g])-(Weight
of dry membrane[g])]/(Membrane volume[cm.sup.3]) wherein,
wet-treated membrane means a state of membrane filled with water in
pores but not filled with water in a hollow part, and practically,
it was obtained as follows: Sample membrane with a length of 10-20
cm was immersed in ethanol to fill pores with ethanol, followed by
immersion into water 4-5 times repeatedly to sufficiently
substitute ethanol in pore with water. Then water in a hollow part
was removed by swinging the hollow fiber about five times with one
end of the hollow fiber gripped and further swinging about five
times with another end of the hollow fiber gripped. A dry membrane
was obtained by drying the above mentioned wet-treated membrane,
after weight measurement, in an oven at 80.degree. C. to have a
constant weight. Membrane volume was determined by the following
equation: Membrane volume[cm.sup.3]=.pi..times.[(Outer
diameter[cm]/2).sup.2-(Inner diameter[cm]/2).sup.2].times.(membrane
length[cm]). When an error is large in weight measurement due to
too light weight for single membrane, a plurality of membranes were
used in weight measurement.
Example 1
[0055] A mixture of 20 parts by weight of high density polyethylene
(SH800 from Asahi Kasei Corp.) and 80 parts by weight of diisodecyl
phthalate (DIDP) was heated and mixed to a molten state (at
230.degree. C.) in a twin screw extruder (TEM-35B-10/1V from
Toshiba Machine Co., Ltd.). Then the above molten mixture was
extruded through a circular ring hole for extrusion of molten
material, having an outer diameter of 1.58 mm and an inner diameter
of 0.83 mm, which is present in an extrusion face of a spinneret
for hollow fiber formation, mounted at the extrusion exit in a head
(230.degree. C.) of the extruder tip. DIDP was discharged as a
hollow part forming fluid through a round hole for injection of
hollow part forming fluid, having 0.6 mm of diameter, which is
present inside the circular ring hole for extrusion of molten
material, and injected into a hollow part of the hollow fiber-like
extrudate.
[0056] Hollow fiber-like extrudate extruded through the spinneret
into air and was introduced into a liquid bath consisting of a top
layer of di(2-ethylhexyl)phthalate (DOP) (1.5 cm thick; 50.degree.
C.) and a lower layer of water (50 cm thick; 30.degree. C.), via an
aerial running distance of 2.0 cm. The hollow fiber-like extrudate
was, after running through DOP layer with a thickness of 1.5 cm,
introduced into a water layer, and ran therethrough for a distance
of about 3 m, then through DOP layer again. The extrudate was taken
out from the liquid bath and wound up at a speed of 16 m/min
without tension. DIDP and DOP contained in the membrane and adhered
to the membrane were removed by extraction by immersing the hollow
fiber-like material thus obtained in methylene chloride, then the
membrane was dried at 50.degree. C. for half a day to obtain a
porous hollow fiber membrane made of polyethylene. Various property
values of the resultant membrane (open area ratio in outer surface,
pore diameter in minimum pore diameter layer, pore diameter in an
inner surface, fiber diameter, pure water flux, porosity, retention
of water permeability in filtration of suspended water) are shown
in Table 1, and electron micrographs and their images after black
and white binarize process are shown in FIGS. 2A-E.
Example 2
[0057] A mixture of 20 parts by weight of high density polyethylene
(Hizex Million 030S from Mitsui Chemical Co., Ltd.) and 80 parts by
weight of diisodecyl phthalate (DIDP) were heated and mixed to a
molten state (at 230.degree. C.), in a twin screw extruder
(TEM-35B-10/1V from Toshiba Machine Co., Ltd.). Then said molten
material was extruded through a circular ring hole for extrusion of
molten material, having an outer diameter of 1.58 mm and an inner
diameter of 0.83 mm, which is present in an extrusion face of a
spinneret for hollow fiber formation, mounted at extrusion exit in
a head (230.degree. C.) of extruder tip. DIDP was discharged as a
hollow part forming fluid through a round hole for injection of
hollow part forming fluid, having a diameter of 0.6 mm, which is
present inside the circular ring hole for extrusion of molten
material, and injected into a hollow part of hollow fiber-like
extrudate.
[0058] Hollow fiber-like extrudate extruded through the spinneret
into air was introduced into a water bath (30.degree. C.), via
aerial running distance of 5 cm. The hollow fiber-like extrudate
ran through water layer for a distance of about 3 m, then was taken
out from the water bath and wound up at a speed of 16 m/min without
tension. DIDP in the membrane was removed by extraction by
immersing the hollow fiber-like material thus obtained in methylene
chloride, and then the membrane was dried at 50.degree. C. for half
a day. A drawing procedure was applied to the resultant porous
hollow fiber membrane made of polyethylene, in which the membrane
with an original length of 20 cm was stretched up to 40 cm at
25.degree. C. under a tension, then the tension was removed. The
fiber length after releasing tension was 28 cm. Various property
values of the resultant membrane (open area ratio in outer surface,
pore diameter in minimum pore diameter layer, pore diameter in an
inner surface, fiber diameter, pure water flux, porosity, retention
of water permeability in filtration of suspended water) are shown
in Table 1, and electron micrographs and their images after black
and white binarize process are shown in FIG. 3A-C.
Example 3
[0059] A mixture of 25.5 parts by weight of pulverized silica
(R-972 from Nippon Aerosil Co., Ltd.) and 50.5 parts by weight of
dibutyl phthalate (DBP) were mixed in a Henschel mixer, then 24.0
parts by weight of high density polyethylene (SH800 from Asahi
Kasei Corp.) were further added thereto and mixed again in the
Henschel mixer. The mixture was pelletized using a twin screw
extruder. Pellets thus obtained were melted and mixed in a twin
screw extruder (at 220.degree. C.). The said molten material was
extruded through a circular ring hole for extrusion of molten
material, having an outer diameter of 1.58 mm and an inner diameter
of 0.83 mm, which is present in an extrusion face of a spinneret
for hollow fiber formation, mounted at extrusion exit in a head
(220.degree. C.) of extruder tip. Nitrogen gas was discharged as a
hollow part forming fluid through a round hole for injection of a
hollow part forming fluid, having a diameter of 0.6 mm, which is
present inside the circular ring hole for extrusion of molten
material, and injected into a hollow part of hollow fiber-like
extrudate. The extrudate was wound up at a speed of 10 m/min. The
hollow fiber-like extrudate thus obtained was immersed in methylene
chloride to remove DBP in hollow fiber-like material by extraction.
Next, it was immersed in ethyl alcohol and then in 20% by weight of
a NaOH aqueous solution at 70.degree. C. for 1 hr to remove by
extraction silica in hollow fiber-like material, followed by
washing with water and drying to obtain a porous hollow fiber
membrane made of polyethylene. Various property values of the
resultant membrane (open area ratio in an outer surface, pore
diameter in a minimum pore diameter layer, pore diameter in an
inner surface, fiber diameter, pure water flux, porosity, retention
of water permeability in filtration of suspended water and ratio of
surface abrasion resistance) are shown in Table 1, and electron
micrographs and images after black and white binarize process are
shown in FIGS. 4A-C.
Example 4
[0060] A mixture of 29 parts by weight of pulverized silica (R-972
from Nippon Aerosil Co., Ltd.) and 50 parts by weight of DBP were
mixed in a Henschel mixer, then 21 parts by weight of high density
polyethylene (SH800 from Asahi Kasei Corp.) were further added
thereto and mixed again in a Henschel mixer. The mixture was
pelletized using a twin screw extruder. Pellets thus obtained were
melted and mixed in a twin screw extruder (at 200.degree. C.). The
said molten material was extruded through a circular ring hole for
extrusion of molten material, having an outer diameter of 1.4 mm
and an inner diameter of 0.7 mm, which is present in an extrusion
face of a spinneret for hollow fiber formation, mounted at
extrusion exit in a head (200.degree. C.) of extruder tip. Nitrogen
gas was discharged as a hollow part forming fluid through a round
hole for injection of a hollow part forming fluid, which is present
inside the circular ring hole for extrusion of molten material, and
injected into a hollow part of hollow fiber-like extrudate. The
extrudate was wound up at a speed of 10 m/min. The hollow
fiber-like extrudate thus obtained was immersed in methylene
chloride to remove DBP in the hollow fiber-like material by
extraction. Next, it was immersed in ethyl alcohol and then in 20%
by weight of a NaOH aqueous solution at 70.degree. C. for 1 hr to
remove by extraction the silica in hollow fiber-like material,
followed by washing with water and drying to obtain a porous hollow
fiber membrane made of polyethylene. Various property values of the
resultant membrane (open area ratio in an outer surface, pore
diameter in a minimum pore diameter layer, pore diameter in an
inner surface, fiber diameter, pure water flux, porosity, retention
of water permeability in filtration of suspended water and ratio of
surface abrasion resistance) are shown in Table 1, and electron
micrographs and images after black and white binarize process are
shown in FIG. 5A-C.
Example 5
[0061] A mixture of 23.1 parts by weight of pulverized silica
(R-972 from Nippon Aerosil Co., Ltd.), 30.7 parts by weight of DOP
and 6.2 parts by weight of DBP were mixed in a Henschel mixer, then
40 parts by weight of polyvinylidenefluoride (KF#1000 from Kureha
Chem. Ind. Co., Ltd.) were further added thereto and mixed again in
the Henschel mixer. This mixture was pelletized using a twin screw
extruder. Pellets thus obtained were melted and mixed in a twin
screw extruder (at 250.degree.). The said molten material was
extruded through a circular ring hole for extrusion of molten
material, having an outer diameter of 1.7 mm and an inner diameter
of 0.9 mm, which is present in an extrusion face of a spinneret for
hollow fiber formation, mounted at extrusion exit in a head
(240.degree. C.) of extruder tip. Nitrogen gas was discharged as a
hollow part forming fluid through a round hole for injection of a
hollow part forming fluid, having 0.6 mm diameter, which is present
inside the circular ring hole for extrusion of molten material, and
injected into a hollow part of hollow fiber-like extrudate. The
extrudate was, via an aerial running a distance of 30 cm,
introduced into a water bath (40.degree. C.), ran therein for a
distance of about 3 m, and then was wound up at a speed of 10
m/min. The hollow fiber-like extrudate thus obtained was immersed
in methylene chloride to remove DOP and DBP in the hollow
fiber-like material by extraction, and dried. Next, it was immersed
in ethyl alcohol and then in 20% by weight of a NaOH aqueous
solution at 70.degree. C. for 1 hr to remove by extraction the
silica in the hollow fiber-like material, followed by washing with
water and drying to obtain a porous hollow fiber membrane made of
polyvinylidenefluoride. Various property values of the resultant
membrane (open area ratio in outer surface, pore diameter in a
minimum pore diameter layer, pore diameter in an inner surface,
fiber diameter, pure water flux, porosity, retention of water
permeability in filtration of suspended water) are shown in Table
1, and electron micrographs and their images after black and white
binarize process are shown in FIG. 6A-E.
Comparative Example 1
[0062] A porous hollow fiber membrane made of polyethylene was
obtained as in Example 2 except that the drawing procedure was not
carried out. Various property values of the resultant membrane
(open area ratio in an outer surface, pore diameter in a minimum
pore diameter layer, pore diameter in an inner surface, fiber
diameter, pure water flux, porosity, retention of water
permeability in filtration of suspended water) are shown in Table
1, and electron micrographs and their images after black and white
binarize process are shown in FIG. 7A-C.
Comparative Example 2
[0063] A porous hollow fiber membrane made of polyethylene was
obtained as in Comparative Example 1 except that the aerial running
distance was set to be 1.5 cm and temperature of water bath was set
at 40.degree. C. Various property values of the resultant membrane
(open area ratio in an outer surface, pore diameter in a minimum
pore diameter layer, pore diameter in an inner surface, fiber
diameter, pure water flux, porosity, retention of water
permeability in filtration of suspended water) are shown in Table
1, and electron micrographs and their images after black and white
binarize process are shown in FIG. 8A-C.
Comparative Example 3
[0064] A porous hollow fiber membrane made of polyethylene was
obtained as in Example 2 except that amount of polyethylene was 24
parts by weight, amount of DIDP was 76 parts by weight and
temperature of water bath was set at 40.degree. C. Various property
values of the resultant membrane (open area ratio in an outer
surface, pore diameter in a minimum pore diameter layer, pore
diameter in an inner surface, fiber diameter, pure water flux,
porosity, retention of water permeability in filtration of
suspended water) are shown in Table 1, and electron micrographs and
their images after black and white binarize process are shown in
FIG. 9A-B.
Comparative Example 4
[0065] A porous hollow fiber membrane made of polyethylene was
obtained as in Example 4 except that a composition of pellet was 28
parts by weight of SH800, 24 parts by weight of R-972 and 48 parts
by weight of DOP. Various property values of the resultant membrane
(open area ratio in outer surface, pore diameter in minimum pore
diameter layer, pore diameter in inner surface, fiber diameter,
pure water flux, porosity, retention of water permeability in
filtration of suspended water and ratio of surface abrasion
resistance) are shown in Table 1.
INDUSTRIAL APPLICABILITY
[0066] A method for clarification by membrane filtration in
accordance with the present invention is applicable to treatment of
clear water to obtain drinking water or industrial water or
treatment of sewage water to obtain regenerated water for
miscellaneous use. TABLE-US-00001 TABLE 1 Retention Open of water
Ratio of area Pore peameablility resistance ratio diameter Pore
Outer in to in in min. pore diameter diameter/ Pure filtration
membrane outer diameter in inner Inner water of suspended surface
surface layer surface diameter flux Porosity water abrasion [%]
[.mu.m] [.mu.m] [mm] [L/m.sup.2/h] [%] [%] [%] Example 1 25 0.33
1.51 1.34/0.71 2200 70 90 -- Example 2 22 0.30 1.65 1.21/0.67 4000
75 56 -- Example 3 20 0.17 0.62 1.21/0.69 800 67 60 15 Example 4 27
0.20 0.70 1.22/0.68 1100 65 83 44 Example 5 23 0.20 0.56 1.25/0.67
1050 65 65 -- Comparative 12 0.20 1.45 1.28/0.70 1100 70 25 --
Example 1 Comparative 11 0.38 1.60 1.41/0.79 1700 70 30 -- Example
2 Comparative 15 0.32 1.58 1.21/0.66 2700 73 32 -- Example 3
Comparative 15 0.15 0.45 1.23/0.68 440 60 -- 5 Example 4
* * * * *