1. A membrane element comprising: a membrane supporting member arranged along a flowing direction of a liquid to be treated; a filtration membrane formed of a flat sheet membrane that is arranged to cover surfaces on a front and a back of the membrane supporting member; a reversed portion of the filtration membrane folded to include an end on a downstream side of the membrane supporting member; and a seal portion formed in a peripheral edge portion of the filtration membrane.

2. The membrane element according to claim 1, wherein the seal portion comprises: a joined portion for joining an edge side portion on an upstream side of the filtration membrane to the membrane supporting member; and a sealing material for binding an edge side portion of the filtration membrane onto the membrane supporting member on both sides of the membrane supporting member along the flowing direction of the liquid to be treated.

3. The membrane element according to claim 1, wherein the seal portion comprises: a joined portion for joining edge side portions on an upstream side of the filtration membrane to each other, the edge side portions on the upstream side being opposed in a front-to-back direction of the membrane supporting member outside an end of the membrane supporting member; and a sealing material for binding an edge side portion of the filtration membrane onto the membrane supporting member on both sides of the membrane supporting member along the flowing direction of the liquid to be treated.

4. The membrane element according to claim .1, wherein the seal portion comprises a joined portion for joining the peripheral edge portion of the filtration membrane to the membrane supporting member over an entire periphery of the filtration membrane.

5. The membrane element according to claim 1, wherein the seal portion comprises a joined portion for joining the peripheral edge portions of the filtration membrane to each other, the peripheral edge portions being opposed in a front-to-back direction of the membrane supporting member outside an end of the membrane supporting member, over an entire periphery of the filtration membrane.

6. A membrane module comprising: at least one membrane element; and a pair of water collecting cases, wherein the membrane element comprises: a membrane supporting member arranged along a flowing direction of a liquid to be treated; a filtration membrane formed of a flat sheet membrane that is arranged to cover surfaces on a front and a back of the membrane supporting member; a reversed portion of the filtration membrane folded to include an end on a downstream side of the membrane supporting member; a joined portion for joining an edge side portion on an upstream side of the filtration membrane to the membrane supporting member; and a sealing material for binding an edge side portion of the filtration membrane onto the membrane supporting member on both sides of the membrane supporting member along the flowing direction of the liquid to be treated, and each of the water collecting cases comprises an opening portion communicating with a water collecting space in the water collecting case, and a side of the membrane element inserted in the opening portion is water-tightly kept by the sealing material.

7. A membrane module comprising: at least one membrane element; and a pair of water collecting cases, wherein the membrane element comprises: a membrane supporting member arranged along a flowing direction of a liquid to be treated; a filtration membrane formed of a flat sheet membrane that is arranged to cover surfaces on a front and a back of the membrane supporting member,- a reversed portion of the filtration membrane folded to include an end on a downstream side of the membrane supporting member; a joined portion for joining edge side portions on an upstream side of the filtration membrane to each other, the edge side portions on the upstream side being opposed in a front-to-back direction of the membrane supporting member via an end of the membrane supporting member; and a sealing material for binding an edge side portion of the filtration membrane onto the membrane supporting member on both sides of the membrane supporting member along the flowing direction of the liquid to be treated, and each of the water collecting cases comprises an opening portion communicating with a water collecting space in the water collecting case, and a side of the membrane element inserted in the opening portion is water-tightly kept by the sealing material. When permeate pushed up by aeration presses a filtration membrane, the reversed portion of the filtration membrane flexibly swells to allow the permeate to move and suppresses a load acting on the filtration membrane, thereby preventing the filtration membrane from being broken.

Description
MEMBRANE ELEMENT AND MEMBRANE MODULE
Technical Field
The present invention relates to a membrane element
and a membrane module that are used for filtration or
concentration in the water treatment of tap water,
wastewater, and the like.
Background Art
As a conventional membrane separator, for example, a
submerged membrane filtration apparatus having a
plurality of membrane elements arranged in parallel at
appropriate intervals is known. As the membrane element,
for example, there is a membrane element shown in FIGS.
16 and 17. In FIGS. 16' and 17, in the membrane element,
filtration membranes 2 formed of an organic membrane are
arranged to cover the surfaces of a rectangular flat
filtration plate 1, which is a membrane supporting member,
and the filtration membranes 2 are jointed to the
filtration plate 1 in the peripheral edge of the
filtration membranes 2. The filtration plate 1 has a
water collecting port 3. For example, as the membrane
supporting member, the filtration plate 1 made of resin
is described as an example. However, in some case, a
membrane supporting member made of a flexible material
such as non-woven fabric or a net may be used.
The membrane element receives a driving pressure and
filters water to be treated with through the filtration
membrane 2. The membrane element is used for gravity
filtration using a head pressure in a tank as the driving
pressure or suction filtration with a negative pressure
given as the driving pressure to the inner side of the
filtration membrane 2.
As a method of joining the filtration plate 1 and the
filtration membranes 2 in such a membrane element, for
example, welding and bonding are performed. The welding
is a method of melting the resin of the filtration plate
1 using ultrasound to form a welded portion 4 and joining
the filtration plate 1 and the filtration membranes 2 in
the welded portion 4. The bonding is performed using an
adhesive to join the filtration plate 1 and the
filtration membrane 2.
As a method of joining such a filtration plate 1 and
filtration membranes 2, for example, there is a method
described in Japanese Patent No. 3010979. The method
involves arranging a filtration membrane on the surface
of a filtration plate, applying ultrasonic vibrations
from above the filtration membrane along the peripheral
edge of the filtration membrane, and welding the
filtration membrane to the filtration plate with the
frictional heat of caused by ultrasonic vibrations.
A method described in Japanese Patent Application
Laid-Open No. Hll-33370 involves stacking two flat
membranes via a spacer and welding or bonding the two
flat membranes end to end on both sides of the flat
membranes to form a filtration membrane body.
A method described in Japanese Patent No. 381.5645
involves forming a separator unit by attaching filtration
membranes via spacers on both surfaces of a member having
a water passing function and forming a hollow portion in
a joining member for joining membrane ends.
Disclosure of the Invention
Problems to be Solved by the Invention
When the above-described membrane separator is used,
the membrane separator is immersed in an activated sludge
mixture in an aeration tank, and air for aeration is
diffused from an air diffuser. In this state, a driving
pressure is applied to the membrane element to filter the
activated sludge mixture and permeate having permeated
through the filtration membrane is led out to the outside
of the tank as treated water.
At this point, upflow is caused by the air-lift
action of bubbles of the air for aeration diffused from
the air diffuser. The membrane surface of the filtration
membrane of the membrane element is aerated and cleaned
by this upflow. Thus a decrease in separating function
due to fouling is suppressed and the membrane separator
is prevented from being nonfunctional.
In this way, the process of aerating for cleaning is
necessary to prevent the membrane separator from fouling.
In this case, it has been known that a cleaning effect is
improved if aeration is performed in a state in which a
filtration operation is stopped.
However, as shown in FIG. 14, if the membrane
separator is left in the state in which the filtration
operation is stopped, permeate accumulates in the inside
of the membrane element, i.e., between the filtration
plate l and the filtration membranes 2 and the filtration
membranes 2 slightly swell.
As shown in FIG. 15, when only the aeration is
performed in the state in which the filtration operation
is stopped, the permeate having accumulated between the
filtration plate 1 and the filtration membranes 2 is
pushed upward by upflow. As a result, as shown in FIG. 13,
the filtration membrane 2 is folded in the welded portion
4 to form a swell 5 near the upper part of the membrane
element. This swell 5 acts as a resistance against the
upflow, and vibrations and stresses occur on the
filtration membrane 2. Therefore, the filtration membrane
2 is likely to peel off in the welded portion 4 or be
broken near the welded portion 4.
The present invention solves the problems and it is
an object of the present invention to provide a membrane
element and a membrane module in which peeling and
breakage are unlikely to occur on a filtration membrane.
Means for Solving the Problems
In order to attain the object, a membrane element
according to the present invention includes: a membrane
supporting member arranged along the flowing direction of
a liquid to be treated; a filtration membrane formed of a
flat sheet membrane that is arranged to cover the
surfaces on the front and back of the membrane supporting
member; the reversed portion of the filtration membrane
folded to include the end on the downstream side of the
membrane supporting member; and a seal portion formed in
the peripheral edge portion of the filtration membrane.
In the membrane element of the present invention, the
seal portion includes: a joined portion for joining the
edge side portion on the upstream side of the filtration
membrane to the membrane supporting member; and a sealing
material for binding the edge side portion of the
filtration membrane onto the membrane supporting member
on both sides of the membrane supporting member along the
flowing direction of the liquid to be treated.
Alternatively, in the membrane element of the present
invention, the seal portion includes: a joined portion
for joining the edge side portions on the upstream side
of the filtration membrane to each other, the edge side
portions on the upstream side being opposed in the front-
to-back direction of the membrane supporting member
outside the end of the membrane supporting member; and a
sealing material for binding the edge side portion of the
filtration membrane onto the membrane supporting member
on both sides of the membrane supporting member along the
flowing direction of the liquid to be treated.
Alternatively, in the membrane element of the present
invention, the seal portion includes a joined portion for
joining the peripheral edge portion of the filtration
membrane to the membrane supporting member over the
entire periphery of the filtration membrane.
Alternatively, in the membrane element of the present
invention, the seal portion includes a joined portion for
joining the peripheral edge portions of the filtration
membrane to each other, the peripheral edge portions
being opposed in the front-to-back direction of the
membrane supporting member outside the end of the
membrane supporting member, over the entire periphery of
the filtration membrane.
A membrane module of the present invention includes:
at least one membrane element; and a pair of water
collecting cases. The membrane element includes: a
membrane supporting member arranged along the flowing
direction of a liquid to be treated; a filtration
membrane formed of a flat sheet membrane that is arranged
to cover the surfaces on the front and back of the
membrane supporting member; the reversed portion of the
filtration membrane folded to include the end on the
downstream side of the membrane supporting member; a
joined portion for joining the edge side portion on the
upstream side of the filtration membrane to the membrane
supporting member; and a sealing material for binding the
edge side portion of the filtration membrane onto the
membrane supporting member on both sides of the membrane
supporting member along the flowing direction of the
liquid to be treated. Each of the water collecting cases
includes an opening portion communicating with a water
collecting space in the water collecting case, and the
side of the membrane element inserted in the opening
portion is water-tightly kept by the sealing material.
A membrane module of the present invention includes:
at least one membrane element; and a pair of water
collecting cases. The membrane element includes: a
membrane supporting member arranged along the flowing
direction of a liquid to be treated; a filtration
membrane formed of a flat sheet membrane that is arranged
to cover the surfaces on the front and back of the
membrane supporting member; the reversed portion of the
filtration membrane folded to include the end on the
downstream side of the membrane supporting member; a
joined portion for joining the edge side portions on the
upstream side of the filtration membrane to each other,
the edge side portions on the upstream side being opposed
in the front-to-back direction of the membrane supporting
member outside the end of the membrane supporting member;
and a sealing material for binding the edge side portion
of the filtration membrane onto the membrane supporting
member on both sides of the membrane supporting member
along the flowing direction of the liquid to be treated.
Each of the water collecting cases includes an opening
portion communicating with a water collecting space in
the water collecting case, and the side of the membrane
element inserted in the opening portion is water-tightly
kept by the sealing material.
Advantages of the Invention
According to the present invention, in a state in
which a filtration operation is stopped, permeate present
between the membrane supporting member and the filtration
membrane is pushed to the downstream side of the flow of
the liquid to be treated and is collected in the reversed
portion. The filtration membrane flexibly swells in the
reversed portion to allow the permeate to move. Further,
since the swell of the reversed portion occurs in the
downstream area of the end on the downstream side of the
membrane supporting member, the swell of the reversed
portion does not act as a resistance against the flow of
the liquid to be treated flowing along the membrane
supporting member. Vibrations and stresses do not occur
in the filtration membrane because of the swell of the
reversed portion. Therefore, it is possible to suppress a
load acting on the filtration membrane and prevent the
filtration membrane from being broken.
Brief Description of the Drawings
FIG. 1 is a perspective view showing a membrane
cassette in a first embodiment of the present invention;
FIG. 2 is a sectional view showing a main part in a
membrane module of the membrane cassette;
FIG. 3 is a perspective view showing a membrane
element having a first configuration in the first
embodiment of the present invention;
FIG. 4 is a perspective view showing the membrane
element having the first configuration in the first
embodiment of the present invention;
FIG. 5 is a front view showing the membrane element
having the first configuration in the first embodiment of
the present invention;
FIG. 6 is a perspective view showing a membrane
element having a second configuration in the first
embodiment of the present invention;
FIG. 7 is a perspective view showing the membrane
element having the second configuration in the first
embodiment of the present invention;
FIG. 8 is a perspective view showing a membrane
element in a second embodiment of the present invention;
FIG. 9 is a perspective view showing a membrane
element in a third embodiment of the present invention;
FIG. 10 is a perspective view showing the membrane
element in the third embodiment of the present invention;
FIG. 11 is a perspective view showing the membrane
element in the third embodiment of the present invention;
FIG. 12 is a schematic diagram showing an action of a
membrane element of the present invention,-
FIG. 13 is a schematic diagram showing an action of a
conventional membrane element;
FIG. 14 is a side view showing the conventional
membrane element;
FIG. 15 is a side view showing the conventional
membrane element;
FIG. 16 is an exploded perspective view showing the
conventional membrane element; and
FIG. 17 is a perspective view showing the
conventional membrane element.
Best Mode for Carrying Out the Invention
Embodiments of the present invention are explained
below on the basis of the drawings.
(First Embodiment)
In FIGS. 1 to 4, a membrane cassette 11 forming a
membrane separator includes a plurality of vertically-
stacked membrane modules 12. The membrane cassette 11 is
set and immersed in a liquid to be treated in a treatment
tank (not shown) . In the membrane cassette 11, an air
diffuser lla is arranged below the lower membrane module
12.
In the membrane module 12, a plurality of membrane
elements 13 are arranged in parallel at predetermined
intervals to form channels in a vertical direction
between the membrane elements 13. Both sides along the
flowing direction of the liquid to be treated of each of
the membrane elements 13 are water-tightly sealed
respectively to water collecting cases 14. Each of the
water collecting cases 14 is hollow-shaped and has a
water collecting space therein. Although the water
collecting case 14 is formed like a square box, the water
collecting case 14 may be in a shape other than a square.
In this embodiment, a configuration in which the
membrane elements 13 are arranged in the up-to-down
direction is explained. However, the arranging direction
of the membrane elements 13 is not limited to the up-to-
down direction. The membrane elements 13 only have to be
arranged along the flowing direction of the liquid to be
treated. Therefore, as explained later, it is also
possible to arrange the membrane elements 13 in the
horizontal direction or obliquely.
The membrane module 12 includes upper coupling
portions 23 provided on the upper end faces of the water
collecting cases 14 and lower coupling portions 24
provided on the lower end faces thereof. The upper
coupling portions 23 and the lower coupling portions 24
form channels and communicate with the water collecting
spaces of the water collecting cases 14.
The lower coupling portions 24 of the upper membrane
module 12 and the upper coupling portions 23 of the lower
membrane modules 12 are coupled. The upper coupling
portions 23 of the water collecting cases 14 of the upper
membrane module 12 communicate with water collecting
pipes 26 via tubes 25.
However, the lower coupling portions 24 of the lower
membrane module 12 can also communicate with the water
collecting pipes 26 via the tubes 25. Further, the lower
coupling portions 24 or the upper coupling portions 23
can also communicate with the water collecting pipes 26
not via the tubes 2 5 but directly.
In this embodiment, the lower coupling portions 24 of
the lower membrane module 12 are closed by plugs (not
shown). However, it is also possible to use a membrane
module having no lower coupling portions 24 as the lower
membrane module 12.
As shown in FIG. 2, the plurality of membrane
elements 13 are water-tightly joined by each of the water
collecting cases 14 via a sealing material (resin, etc.)
16 potted in an opening portion 15. However, the water
collecting case 14 is not limited to the configuration
shown in FIG. 2. There are various structures for water-
tightly joining the membrane elements 13 to the water
collecting case 14. For example, it is also possible to
form a plurality of slits instead of the single opening
portion 15 of the water collecting case 14, insert the
membrane elements 13 in the slits, and pot the sealing
material 16 of resin or the like in the slits.
Alternatively, it is also possible to arrange a seal
material such as rubber around the membrane elements 13.
(First configuration of the membrane element)
As shown in FIGS. 3 and 4, the membrane element 13
includes a filtration plate 17 made of resin, which forms
a membrane supporting member, and a filtration membrane
18 made of a flat sheet membrane (an organic membrane)
arranged to cover the principal surfaces on the front and
back of the filtration plate 17. In each of the membrane
elements 13, a permeate channel formed between the
principal surfaces on the front and back of the
filtration plate 17 and the filtration membrane 18
communicates with the water collecting space of the water
collecting case 14. In this embodiment, the filtration
plate 17 made of resin is explained as an example of the
membrane supporting member. However, in some case, a
membrane supporting member of a flexible material such as
non-woven fabric or a net is used.
The upper end side of the membrane element 13 is
located on the downstream side in the flowing direction
of the liquid to be treated. The lower end side of the
membrane element 13 is located on the upstream side in
the flowing direction of the liquid to be treated. The
filtration membrane 18 includes a reversed portion 20
folded to include an end 19 on the downstream side of the
filtration plate 17. A seal portion explained below is
formed in the peripheral edge portion of the filtration
membrane 18. The entire filtration membrane 18 may be one
sheet or the filtration membrane 18 may be formed by-
joining a plurality of membranes.
An edge side portion 21 on the upstream side of the
filtration membrane 18 is joined to the end on the
upstream side of the filtration plate 17 to form a joined
portion 22. The joined portion 22 forms a part of the
seal portion. The end edge of the filtration membrane 18
can also be bent and joined to an end face 2 7a of an end
27 on the upstream side of the filtration plate 17.
In this embodiment, the joined portion 22 is formed
by applying ultrasonic vibrations from above the
filtration membrane 18 and welding the filtration plate
17 and the filtration membrane 18 with frictional heat.
However, the filtration plate 17 and the filtration
membrane 18 may be joined with an adhesive.
The membrane elements 13 having this structure are
arranged in parallel at predetermined intervals, the
plurality of membrane elements 13 are water-tightly fixed
by the sealing material 16 that is arranged between the
membrane elements 13, the edge side portion of the
filtration membrane 18 is bound by the sealing material
16 on the filtration plate 17 on both sides of the
filtration plate 17 along the flowing direction of the
liquid to be treated, and a part of the seal portion is
formed by the sealing material 16.
However, as shown in FIG. 5, the sealing material 16
can be formed for each of the membrane element 13. The
edge side portion of the filtration membrane 18 is bound
by the sealing material 16 on the filtration plate 17 for
each of the membrane elements 13 on both sides of the
filtration plate 17 along the flowing direction of the
liquid to be treated. Thereafter, it is possible to
arrange the membrane elements 13 having this structure in
parallel at predetermined intervals and bind the
plurality of membrane elements 13 with the sealing
material (resin, etc.) 16 arranged between the membrane
elements 13. Further, as explained above, it is possible
to arrange a seal material such as a rubber material on
the filtration membrane 18 and join the filtration
membrane 18 to the filtration plate 17.
(Second configuration of the membrane element)
The membrane element 13 can have a structure shown in
FIGS. 6 and 7. In FIGS. 6 and 7, in the membrane element
13, the filtration membrane 18 includes the reversed
portion 20 folded to include the end 19 on the downstream
side of the filtration plate 17. The edge side portions
21 on the upstream side of the filtration membrane 18 are
opposed to each other in the front-to-back direction of
the filtration plate 17 outside the end 27 on the
upstream side of the filtration plate 17. The edge side
portions 21 on the upstream side are joined to each other
to form a joined portion 28. This joined portion 28 forms
a part of the seal portion and is formed by welding with
ultrasound or bonding with an adhesive.
The membrane elements 13 having this structure are
arranged in parallel at predetermined intervals, the
plurality of membrane elements 13 are water-tightly fixed
by the sealing material (resin, etc.) 16 arranged between
the membrane elements 13, the edge side portion of the
filtration membrane 18 is bound by the sealing material
16 on the filtration plate 17 on both sides of the
filtration plate 17 along the flowing direction of the
liquid to be treated, and a part of the seal portion is
formed by the sealing material 16.
Regardless of which of the membrane elements 13
having the first and second configurations is used, the
following operations and effects are realized in the
membrane module 12 of the present invention.
Normal operation
Air is diffused as a gas for aeration from the air
diffuser lla arranged below the lower membrane module 12.
Upflow of air-liquid mixtures is caused inside the
membrane cassette 11 by the air-lift action of bubbles of
the air. The liquid to be treated in a treatment tank
(not shown) is supplied between the membrane elements 13
by this upflow. A flow of the liquid to be treated along
the membrane surfaces of the membrane elements 13 is
formed. The liquid to be treated is supplied as a cross
flow with respect to the flow of permeate flowing to
permeate through the filtration membrane 18.
In this embodiment, the liquid to be treated is
supplied as the cross flow by the upflow caused by the
air-lift action. Therefore, the membrane elements 13 are
arranged in the up-to-down direction. However, when the
liquid to be treated is supplied as the cross flow
between the membrane elements 13 by a power unit such as
a pump, the membrane elements 13 can also be arranged in
the horizontal or oblique direction.
There are various systems for applying a driving
pressure to the membrane elements 13. In this embodiment,
an activated sludge mixture in the tank is gravity-
filtrated by the membrane elements 13 with a water head
in the tank as a driving pressure. Alternatively, a
suction pressure is applied to the membrane modules 12 of
the membrane cassette 11 as a driving pressure through
the water collecting pipes 2 6 and the tubes 25 by a
suction pump to suck and filter the activated sludge
mixture.
The permeate having permeated through the filtration
membranes 18 of the membrane elements 13 with the driving
pressure flows into the water collecting spaces of the
water collecting cases 14 through the permeate channels
between the filtration membranes 18 and the filtration
plates 17. The permeate having flowed into the water
collecting cases 14 of the lower membrane module 12 flows
into the water collecting cases 14 of the upper membrane
module 12 from the lower coupling portions 24 through the
upper coupling portions 23. The permeate having flowed
into the water collecting cases 14 of the upper membrane
module 12 is led out to the outside of the tank as
treated water from the upper coupling portions 23 through
the tubes 25 and the water collecting pipes 26.
During this operation, the activated sludge mixture
is supplied as a cross flow to the channels between the
membrane elements 13 by upflow. The membrane surfaces of
the membrane elements 13 are aerated and cleaned by the
upflow. Thus a decrease in separating function due to
fouling is suppressed and the membrane separator is
prevented from being nonfunctional.
Aeration and cleaning operation
In the case of the gravity filtration, valves (not
shown) provided in the water collecting pipes 26 are
closed. In the case of the suction filtration, the
suction pump is stopped to stop the filtration operation.
When the air diffuser lla is operated to perform the
aeration and cleaning in this state, an excellent
cleaning effect can be obtained.
At this point, the upflow pushes the permeate in each
of the membrane elements 13 to the upper downstream side
between the filtration plate 17 and the filtration
membrane 18. However, as shown in FIG. 12, the filtration
membrane 18 includes the reversed portion 2 0 folded to
include the end 19 on the downstream side of the
filtration plate 17. Therefore, the permeate pushed to
the upper downstream side between the filtration plate 17
and the filtration membrane 18 is collected in the
revered portion 20. The filtration membrane 18 flexibly
swells in the reversed portion 2 0 to allow the permeate
to move. Further, since a swell 20a of the reversed
portion 2 0 is formed in the downstream area of the end 19
on the downstream side of the filtration plate 17, the
swell 20a of the reversed portion 20 does not act as a
resistance against the upflow flowing along the
filtration plate 17. Vibrations and stresses do not occur
in the filtration membrane 18 because of the swell 20a of
the reversed portion 20.
Therefore, a load acting on the filtration membrane
18 is suppressed to prevent the filtration membrane 18
from being broken because the reversed portion 2 0
flexibly swells to allow the permeate to move flexibly
and the swell 2 0a does not act as a resistance against
the upflow.
(Second Embodiment)
As shown in FIG. 8, a membrane module 12 may have a
configuration in which water collecting cases 14 are
arranged respectively on both sides of a single membrane
element 13. In this case, a plurality of the membrane
modules 12 are arranged in parallel to form channels in a
vertical direction between the membrane elements 13. The
membrane element 13 used in this case may be the membrane
element 13 in any one of the above-described forms.
In this configuration, it is possible to realize the '
same operations and effects as those in the first
embodiment.
(Third Embodiment)
As shown in FIGS. 9 to 11, in a membrane element 13,
a filtration membrane 18 includes a reversed portion 20
folded to include an end 19 on the downstream side of a
filtration plate 17. A peripheral edge portion 31 of the
filtration membrane 18 is joined to the principal
surfaces on the front and back of the filtration plate 17
and the end 19 on the downstream side to form a joined
portion 32 over the entire periphery of the filtration
membrane 18.
In this embodiment, the joined portion 32 is formed
by applying ultrasonic vibrations from above the
filtration membrane 18 and welding the filtration plate
17 and the filtration membrane 18 with frictional heat.
However, the filtration plate 17 and the filtration
membrane 18 may be joined with an adhesive. The
filtration plate 17 includes a water collecting port 33
on the side thereof and, in an area covered by the
filtration membrane 18, a permeate channel (not shown)
communicating with the water collecting port 33.
In this embodiment, the peripheral edge portion 31 of
the filtration membrane 18 is joined to the filtration
plate 17 over the entire periphery of the filtration
membrane 18. However, it is possible to form the
filtration membrane 18 larger than the filtration plate
17, extend the peripheral edge portion 31 of the
filtration membrane 18 from the peripheral edge of the
filtration plate 17 to the outer side, and join the
peripheral edge portions 31 of the filtration membranes
18 to each other, the peripheral edge portions 31 being
opposed in the front-to-back direction of the filtration
plate 17 outside the end of the filtration plate 17, over
the entire periphery of the filtration membrane 18.
The membrane element 13 having this configuration is
accommodated in a case 34 and is used. Specifically, a
plurality of the membrane elements 13 are arranged in
parallel in the case 34 to form channels between the
membrane elements 13.
During an operation, air is diffused as a gas for
aeration from an air diffuser (not shown) arranged under
the case 34 and upflow of an air-liquid mixture is caused
in the case 34 by the air-lift action of bubbles of the
air.
Even in this configuration, the same operations and
effects as those in the first embodiment can be realized.
Even if the process of aerating and cleaning is performed
in a state in which a filtration operation remains
stopped, it is possible to realize a membrane element in
which breakage of a filtration membrane is unlikely to
occur, and obtain an excellent cleaning effect for
preventing fouling.
WE CLAIM:

1. A membrane element comprising: a membrane supporting
member arranged along a flowing direction of a liquid to
be treated; a filtration membrane formed of a flat sheet
membrane that is arranged to cover surfaces on a front
and a back of the membrane supporting member; a reversed
portion of the filtration membrane folded to include an
end on a downstream side of the membrane supporting
member; and a seal portion formed in a peripheral edge
portion of the filtration membrane.
2. The membrane element according to claim 1, wherein
the seal portion comprises: a joined portion for joining
an edge side portion on an upstream side of the
filtration membrane to the membrane supporting member;
and a sealing material for binding an edge side portion
of the filtration membrane onto the membrane supporting
member on both sides of the membrane supporting member
along the flowing direction of the liquid to be treated.
3. The membrane element according to claim 1, wherein
the seal portion comprises: a joined portion for joining
edge side portions on an upstream side of the filtration
membrane to each other, the edge side portions on the
upstream side being opposed in a front-to-back direction
of the membrane supporting member outside an end of the
membrane supporting member; and a sealing material for
binding an edge side portion of the filtration membrane
onto the membrane supporting member on both sides of the
membrane supporting member along the flowing direction of
the liquid to be treated.
4. The membrane element according to claim .1, wherein
the seal portion comprises a joined portion for joining
the peripheral edge portion of the filtration membrane to
the membrane supporting member over an entire periphery
of the filtration membrane.
5. The membrane element according to claim 1, wherein
the seal portion comprises a joined portion for joining
the peripheral edge portions of the filtration membrane
to each other, the peripheral edge portions being opposed
in a front-to-back direction of the membrane supporting
member outside an end of the membrane supporting member,
over an entire periphery of the filtration membrane.
6. A membrane module comprising: at least one membrane
element; and a pair of water collecting cases, wherein
the membrane element comprises: a membrane supporting
member arranged along a flowing direction of a liquid to
be treated; a filtration membrane formed of a flat sheet
membrane that is arranged to cover surfaces on a front
and a back of the membrane supporting member; a reversed
portion of the filtration membrane folded to include an
end on a downstream side of the membrane supporting
member; a joined portion for joining an edge side portion
on an upstream side of the filtration membrane to the
membrane supporting member; and a sealing material for
binding an edge side portion of the filtration membrane
onto the membrane supporting member on both sides of the
membrane supporting member along the flowing direction of
the liquid to be treated, and
each of the water collecting cases comprises an
opening portion communicating with a water collecting
space in the water collecting case, and a side of the
membrane element inserted in the opening portion is
water-tightly kept by the sealing material.
7. A membrane module comprising: at least one membrane
element; and a pair of water collecting cases, wherein
the membrane element comprises: a membrane supporting
member arranged along a flowing direction of a liquid to
be treated; a filtration membrane formed of a flat sheet
membrane that is arranged to cover surfaces on a front
and a back of the membrane supporting member,- a reversed
portion of the filtration membrane folded to include an
end on a downstream side of the membrane supporting
member; a joined portion for joining edge side portions
on an upstream side of the filtration membrane to each
other, the edge side portions on the upstream side being
opposed in a front-to-back direction of the membrane
supporting member via an end of the membrane supporting
member; and a sealing material for binding an edge side
portion of the filtration membrane onto the membrane
supporting member on both sides of the membrane
supporting member along the flowing direction of the
liquid to be treated, and
each of the water collecting cases comprises an
opening portion communicating with a water collecting
space in the water collecting case, and a side of the
membrane element inserted in the opening portion is
water-tightly kept by the sealing material.

When permeate pushed up by aeration presses a
filtration membrane, the reversed portion of the
filtration membrane flexibly swells to allow the permeate
to move and suppresses a load acting on the filtration
membrane, thereby preventing the filtration membrane from
being broken.