JP7544064B2 - Submerged flat membrane element and its manufacturing method - Google Patents

Description

The present invention relates to a submerged flat membrane element of a membrane separation device used in a membrane bioreactor activated sludge process, and in particular to an element that prevents the entire separation membrane from breaking when peeling occurs from the permeable side of the water-stopping part by devising the peel strength and cross-sectional profile of the water-stopping part formed by bonding the separation membrane to the peripheral part of the filter plate, thereby maintaining the filtration performance of the flat membrane element for a long period of time and extending the service life by repairing only the permeable side of the water-stopping part.

In recent years, the quality and quantity of water required for domestic and industrial use has increased against the backdrop of global population growth, industrialization, urbanization, and improved living standards. The membrane separation activated sludge process uses a separation membrane to filter water treated with activated sludge, which allows for the production of clear filtrate. It also has advantages such as compact equipment and easy operation and management, and has become a technology that has attracted a lot of attention in recent years. Solid-liquid separation equipment used in the membrane separation activated sludge process includes, for example, a submerged membrane separation equipment as shown in Figure 1. In Figure 1, the membrane separation equipment 1 comprises a flat membrane element 2 and an aeration device 3.

As shown in Figure 2, the flat membrane element 2 has separation membranes 5 placed on the front and back of a filter plate 4 made of thermoplastic resin, which is bonded along the entire periphery of the filter plate 4. Water treated with activated sludge is filtered by the surface surrounded by the bonded parts of the separation membrane 5. The surface surrounded by the bonded parts is called the permeable part. The separation membrane 5 is composed of a nonwoven fabric base material impregnated with a membrane polymer. When the flat membrane element 2 is in operation, sludge and impurities adhere to and accumulate on the membrane surface of the permeable part due to suction filtration, but the membrane surface is washed by air bubbles sprayed from the air diffuser 3 and the resulting upward flow, removing the sludge and impurities that have adhered to and accumulated on the membrane surface, preventing a decline in the filtration performance of the flat membrane element.

However, collisions with these air bubbles and the upward flow can cause the separation membrane to vibrate, placing a load on the adhesive between the filter plate and the separation membrane, causing the separation membrane to peel off and sludge to leak into the permeate side. One method for adhering the filter plate and separation membrane is ultrasonic welding. With this method, a horn that emits ultrasonic waves is pressed against the area to be bonded, and the heat generated by the ultrasonic vibrations melts and welds the filter plate resin and the separation membrane.

As shown in Figure 3, if the welding between the filter plate and the separation membrane is weak, there is a problem that the separation membrane is likely to peel off from the filter plate due to the vibration of the membrane when the membrane surface is cleaned. Sludge leakage into the permeate side due to peeling of the separation membrane during activated sludge treatment is not practical unless more than 90% of the flat membrane elements in the membrane separation device are prevented. On the other hand, if the welding is too strong, the filter plate resin that melts during welding solidifies deep inside the separation membrane, resulting in a problem that the welded part between the filter plate and the separation membrane is likely to break.

To address these problems, Patent Document 1 discloses a technology that prevents the separation membrane from the filter plate and the rupture of the separation membrane by making the melting margin located on the outside of the filter plate higher than the melting margin located on the water permeable section side. However, with the invention described in Patent Document 1, once the melting margin is crushed, it is difficult to repair the partially peeled separation membrane and reuse it. In addition, multiple water stop parts must be provided independently on the same surface of the flat membrane element, making the work of manufacturing the flat membrane element complicated. As a result, during welding, poor contact of the ultrasonic oscillation horn is likely to occur, especially in the water stop part on the water permeable section side, and there is a problem that it is not possible to create a large difference in welding strength between multiple water stop parts.

Patent Document 2 also claims that rupture of the separation membrane during use is prevented by setting appropriate welding conditions that do not reach the nonwoven fabric layer. However, in reality, it is difficult to completely prevent rupture of the separation membrane; welding conditions that prevent rupture of the separation membrane make the separation membrane more likely to peel off, while welding conditions that prevent peeling of the separation membrane make the separation membrane more likely to rupture, making it difficult to prevent both simultaneously (Figure 3).

Patent No. 3576050 Patent No. 3778758

The present invention was devised in consideration of the problems of the prior art described above, and its purpose is to provide a submerged flat membrane element and its manufacturing method that can prevent rupture of the separation membrane even when only the permeable part side of the water-stopping part is partially peeled off, by devising the peel strength and cross-sectional profile of a single water-stopping part formed by bonding a separation membrane to the peripheral part of a filter plate, maintain filtration performance for a long period of time, and extend the service life by repairing peeling on the permeable part side of the water-stopping part.

In order to achieve the above-mentioned objective, the inventors conducted extensive research into the relationship between peel strength and the shape of the ultrasonic horn, and discovered that by using an ultrasonic horn that forms a specific cross-sectional shape to provide a continuous water-stopping section with a single concave cross-section and by allowing the strength of the welded parts of the water-stopping section to be appropriately set on the permeable section side and the outside of the filter plate, it is possible to easily manufacture a submerged flat membrane element that combines durability of the water-stopping section with ease of repair, thus completing the present invention.

That is, the present invention has the following configurations (1) to (5).
(1) A submerged flat membrane element in which a water-stopping portion is formed by bonding a resin separation membrane along the entire peripheral portion of a resin filter plate, the water-stopping portion being composed of a continuous line with a single concave cross-section, the peel strength at welded portion 1 forming the shoulder of the concave cross-section on the water permeable portion side of the water-stopping portion being 30 to 90% of the peel strength at welded portion 2 forming the shoulder of the concave cross-section on the outer side of the filter plate of the water-stopping portion, and the peel strength at welded portion 2 of the water-stopping portion being 8 N or more and 40 N or less.
(2) A submerged flat membrane element as described in (1), characterized in that the rising portion of the shoulder of the concave cross section on the water permeable section side of the water stop section forms a curve, the rising portion of the shoulder of the concave cross section on the outer side of the filter plate of the water stop section rises from the flat section at the outer end of the filter plate of the flat section between both shoulders of the concave cross section by a clear upward angle change, and the edge angle forming the angle change is 80 degrees or more and 135 degrees or less.
(3) The submerged type flat membrane element according to (1) or (2), wherein the peel strength of the welded portion 1 is 30 N or less.
(4) A method for producing a submerged flat membrane element according to any one of (1) to (3), comprising overlapping a resin separation membrane on a resin filter plate, and pressing an ultrasonic horn from the separation membrane side along the entire circumference of the overlapped peripheral portion to form a water-stopping portion composed of a continuous line with a single concave cross-section, characterized in that the portion pressed by the ultrasonic horn has a convex shape corresponding to the concave cross-section of the water-stopping portion.
(5) A manufacturing method described in (4), characterized in that the portion pressed by the ultrasonic horn further includes an iris portion continuing to a convex shaped portion corresponding to the concave cross section of the water-stopping portion.

(1) In the submerged flat membrane element of the present invention, the peel strength is appropriately controlled on the water permeable section side and the outside of the filter plate of the welded portion of the water stop section with a single concave cross section. This suppresses deterioration of the separation membrane due to heat during welding on the water permeable section side of the water stop section which is subjected to the greatest load during use, and prevents the separation membrane from rupturing during use. Furthermore, even if partial peeling occurs on the water permeable section side of the water stop section due to long-term use, the water stopping function is maintained on the outside of the filter plate, maintaining the filtration performance of the flat membrane element and enabling partial repairs to be performed to extend the service life.
(2) In the submerged flat membrane element of the present invention, the inner rising portion of the concave portion of the cross-sectional shape of the water-stopping portion forms a curve, and the rising portion of the shoulder portion of the concave cross-section on the outer side of the filter plate of the water-stopping portion rises from the flat portion at a clear upward angle change at the outer edge of the filter plate of the flat portion between the two shoulder portions of the concave cross-section, and the edge angle forming the angle change is controlled within a specific range. Therefore, inside the water-stopping portion, sufficient welding strength can be achieved to prevent peeling of the separation membrane while maintaining the structure of the separation membrane as much as possible, and the above-mentioned effects can be easily obtained.
(3) The manufacturing method of the submerged-type flat membrane element of the present invention uses an ultrasonic horn having a specific shape, so that a single concave water-stopping portion having the above-mentioned effects can be formed on the smooth surface of the filter plate by a single welding operation, thereby reducing the processing costs of the filter plate, the labor hours for welding, and the power supply capacity.

FIG. 1 is a schematic diagram of a prior art submerged membrane separation device. FIG. 1 is a schematic diagram of a flat membrane element according to the prior art. 1 is a schematic diagram showing a problem of peeling of a water blocking portion in the prior art; FIG. 1 is a diagram showing a configuration of a flat membrane element of the present invention. FIG. 2 is a schematic enlarged view of a part of the flat membrane element of the present invention. 1 is a cross-sectional photograph of a water blocking portion of a flat membrane element of the present invention. 1 shows a peel strength profile in a water blocking portion of a flat membrane element of the present invention. FIG. 2 is a cross-sectional schematic diagram of a water blocking part of the flat membrane element of the present invention. FIG. 2 is a schematic diagram of an ultrasonic horn used in the present invention. FIG. 4 is a schematic cross-sectional view of a water-stopping portion when an ultrasonic horn is applied thereto.

An embodiment of a submerged flat membrane element of the present invention will be described with reference to the drawings.

Figure 4 is a schematic diagram of an example of a submerged flat membrane element of the present invention. As shown in Figure 4, the submerged flat membrane element of the present invention is preferably configured such that a resin mesh is placed as a flow path material 11 in the center 10 of the filter plate 4, a buffer material 12 is placed on top of that, and a separation membrane 5 is further bonded on top of that along the peripheral portion 7 of the filter plate 4. In the present invention, since the separation membrane 5 is pressed against the flow path material 11 side during suction filtration, it is preferable to place a buffer material 12 that serves to protect the membrane between them. Although Figure 4 shows only the configuration of one side of the filter plate 4 of the submerged flat membrane element, in the present invention, it is preferable that the filter plate 4 has a similar configuration on both the front and back sides.

The flow of membrane permeate in the submerged flat membrane element of the present invention is explained using Figure 4. The liquid to be treated is brought into contact with the outer surface of the separation membrane 5, and suction filtration is performed from the outside to the inside of the separation membrane 5. The filtered membrane permeate passes through the buffer material 12 arranged between the separation membrane 5 and the filter plate 4, and the gaps in the flow path material 11, in that order, and flows in the direction of the permeate intake 13 attached to the end of the filter plate 4, and is discharged from the permeate intake 13 to the outside of the submerged flat membrane element.

In the submerged flat membrane element of the present invention, the peripheral portion 7 of the filter plate 4 is preferably 0.6 to 2 mm higher than the central portion 10, and the flow path material 11 and the buffer material 12 are placed in the recessed space of the central portion 10 formed thereby, and it is preferable that the step between the central portion 10 and the peripheral portion 7 when the flow path material 11 and the buffer material 12 are placed is 0.5 mm or less. The buffer material 12 is preferably placed so as to cover the flow path material 11 bonded to the filter plate 4, and is bonded to the flow path material 11. The separation membrane 5 is preferably bonded at the peripheral portion 7 so as to cover the filter plate 4 on which the flow path material 11 and the buffer material 12 are placed.

In the present invention, the material of the separation membrane 5 is not particularly limited as long as it is made of resin, but for example, a thermoplastic resin such as polyvinyl chloride, chlorinated polyvinyl chloride, polyethersulfone, polyvinylidene fluoride, or polytetrafluoroethylene, or a mixture thereof, can be appropriately selected. The separation membrane 5 may also be a composite membrane in which a separation layer made of the membrane material is impregnated or laminated on a nonwoven fabric or paper made of a thermoplastic resin such as PET or polypropylene. The thickness of the separation membrane 5 is preferably 50 μm to 300 μm, more preferably 80 μm to 150 μm. If this thickness is too large, uneven welding may occur easily or the membrane may peel off from the filter plate. If the thickness is too thin, the separation membrane is more likely to break.

The material of the filter plate 4 is not particularly limited as long as it is made of a resin that maintains the overall shape of the flat membrane element, but thermoplastic resins such as ABS resin, polyvinyl chloride resin, polycarbonate resin, etc. can be appropriately selected. The dimensions of the filter plate are preferably a rectangular shape with a height (length) of 300 mm to 1500 mm and a width of 200 mm to 550 mm, and a peripheral edge 7 with a width of 10 mm to 20 mm.

In the present invention, the filter plate and the separation membrane are preferably welded by ultrasonic welding at a position 0.5 mm or more outside the inner end of the peripheral portion 7 after the peripheral portion 7 of the filter plate and the separation membrane 5 are arranged so that they overlap with a width of about 5 to 15 mm. Figure 5 is an explanatory diagram showing the water stop portion 6 and the welding auxiliary portion 8 in the peripheral portion 7 in a state in which the filter plate 4 and the separation membrane 5 are shifted in order to make the welded portion of the filter plate 4 and the separation membrane 5 easier to understand. Examples of ultrasonic welding methods include the rotary welding method and the up-down method, but in the present invention, it is preferable to use an up-down horn by the up-down method to integrally form a single continuous water stop portion 6 (welded portion) with a width of 0.5 mm to 3 mm and a maximum depth of 50 μm to 500 μm all around the peripheral portion 7 of the filter plate 4. In addition, it is preferable to provide an iris-shaped welding auxiliary portion 8 on the outside of the water stop portion 6 to prevent the outer periphery of the separation membrane 5 from flapping during use.

Figure 6 is a cross-sectional photograph of the water-stopping portion 6 of the peripheral portion 7 cut in the width direction. In the present invention, the water-stopping portion 6 is composed of a continuous line having a concave cross section as indicated by the double-headed arrow A in Figure 6, and this line forms a closed rectangle along the peripheral portion 7. The water-stopping portion 6 is a welded portion formed by welding the filter plate 4 and the separation membrane 5, and the welded portion is a portion where peel strength is developed when the filter plate 4 and the separation membrane 5 are peeled off. Figure 7 is a peel strength profile when the separation membrane 5 is peeled off from the water-permeable portion side (inside the peripheral portion) toward the outside of the filter plate (outside the peripheral portion) in Figure 6. The separation membrane 5 stretches due to the tensile stress during the peel test, so the tensile distance at which peel strength is developed is greater than the actual length of A.

In the flat membrane element of the present invention, the peel strength profile of the water stop portion has two peaks (peak 1 and peak 2), as shown in Figure 7. Peak 1 is the peel strength at welded portion 1 that forms the shoulder of the concave cross section on the permeable portion side of the water stop portion shown in Figure 8, and peak 2 is the peel strength at welded portion 2 that forms the shoulder of the concave cross section on the outer side of the filter plate of the water stop portion. Such a peel strength profile is closely related to the shape of the up-down horn when welding the separation membrane to the peripheral portion, and is a property unique to the submerged type flat membrane element of the present invention.

In the flat membrane element of the present invention, as shown in FIG. 7, the peel strength (peak 1) of the welded portion 1 is set relatively weak, while the peel strength (peak 2) of the welded portion 2 is set stronger than the peel strength (peak 1) of the welded portion 1. That is, in the welded portion 1, the filter plate and the separation membrane are melted and bonded to each other widely and shallowly by using an ultrasonic horn having a specific shape on the welded portion 1 side as described later. Therefore, the welded portion 1 has a relatively weak peel strength and suppresses the deterioration of the separation membrane compared to the welded portion 2. This effectively prevents the separation membrane from breaking at the welded portion 1, which is the boundary between the water-stopping portion and the water-permeable portion and is subjected to the most stress when exposed to stress due to the oscillation of the separation membrane in the water-permeable portion during activated sludge treatment. On the other hand, in the welded portion 2, the welded portion 2 uses an ultrasonic horn having a specific shape on the welded portion 2 side, so the filter plate and the separation membrane are melted more than the welded portion 1 and are firmly integrated. For this reason, welded portion 2 has a relatively strong peel strength compared to welded portion 1. As a result, even if the separation membrane peels off at welded portion 1, the separation membrane and the filter plate can be prevented from easily peeling off at welded portion 2. With this configuration, even when the separation membrane peels off from the peripheral portion of the filter plate at welded portion 1, welded portion 2 makes it possible for the submerged flat membrane element to maintain its function.

In the flat membrane element of the present invention, the peel strength at welded portion 1 of the water stop portion is 30 to 90%, preferably 35 to 85%, of the peel strength at welded portion 2. If the peel strength ratio at welded portion 1 is smaller than the above range, the peel strength of welded portion 1 will be insufficient and welded portion 1 will easily peel off when the flat membrane element is handled. If the ratio is larger than the above range, the intended effect of the present invention cannot be fully obtained.

In the present invention, the peel strength of the welded portion 2 of the water stop portion is 8 N or more and 40 N or less, preferably 11 N or more and 35 N or less. If the peel strength of the welded portion 2 is too small, membrane peeling is likely to occur at the welded portion 2 when membrane peeling or the like occurs at the welded portion 1. On the other hand, if the interface between the separation membrane and the filter plate resin is melted too much in an attempt to increase the peel strength of the welded portion 2, the structure of the separation membrane at the welded portion will deteriorate and the rupture strength of the separation membrane will decrease significantly, making the membrane more likely to rupture.

In the present invention, the peel strength of the welded portion 1 of the water-stopping portion is preferably 30 N or less, more preferably 27 N or less. Also, the peel strength of the welded portion 1 of the water-stopping portion is preferably 4 N or more, more preferably 5 N or more. If the peel strength of the welded portion 1 is too high, the occurrence of membrane peeling is reduced, but membrane rupture is more likely to occur. On the other hand, if the peel strength of the welded portion 1 is too low, membrane peeling is more likely to occur.

As described above, by providing welded area 1, which has a relatively weak peel strength, on the permeable section side of the water stop section, and providing welded area 2, which has a relatively strong peel strength, on the outer side of the filter plate of the same water stop section, deterioration of the separation membrane during welding on the permeable section side of the water stop section can be prevented, and rupture of the separation membrane can be prevented on the permeable section side, which is subjected to the greatest load during use, while at the same time peeling of the separation membrane from the filter plate can be prevented, thereby maintaining the filtration performance of the flat membrane element for a long period of time. Furthermore, if partial peeling occurs between the separation membrane and the filter plate only at welded area 1, the life of the flat membrane element can be extended by repairing this during inspection.

The shape of the water stop part of the flat membrane element of the present invention has the following characteristics. That is, in the water stop part of the present invention, the rising part of the shoulder part (welded part 1) of the concave cross section on the water permeable part side of the water stop part forms a curve as shown in E of FIG. 10, and the rising part of the shoulder part (welded part 2) of the concave cross section on the outer side of the filter plate of the water stop part rises with a clear upward angle change at the outer end of the filter plate (C of FIG. 10) of the flat part (B of FIG. 10) between the two shoulder parts. The edge angle (X in FIG. 8) that forms the angle change of the rising part of this welded part 2 is preferably 80 degrees or more and 135 degrees or less. On the other hand, in order to form the cross section of the welded part 1, it is preferable to use an ultrasonic horn whose cross-sectional shape on the welded part 1 side has a curve with a radius of curvature of 0.3 to 3 mm and an arc length of 0.4 to 4 mm. Because the water-stopping portion has this type of shape, when the separation membrane is stretched due to the formation of a water-stopping portion with a concave cross-section during welding, the degree of stretching of the separation membrane is distributed over a wide range at the welded portion 1 of the water-stopping portion, so that deterioration of the separation membrane during welding can be effectively prevented. Furthermore, the effect of peeling of the welded portion 1 can be stopped at position C in Figure 10, and the progression of peeling between the separation membrane and the filter plate can be completely prevented at the welded portion 2, which has a stronger peel strength.

The manufacturing method of the flat membrane element of the present invention preferably includes a step of stacking a separation membrane on a resin filter plate, pressing a horn (ultrasonic horn) that emits ultrasonic waves along the entire circumference of the overlapped peripheral portion with a constant pressure to generate frictional heat at the interface between the filter plate and the separation membrane, melting the resin that constitutes the filter plate and the separation membrane, and forming a water-stopping portion (welded portion) composed of a continuous line with a single concave cross section. The ultrasonic horn 9 is preferably composed of a portion that presses against the overlapped portion of the filter plate 4 and the separation membrane 5, which is a portion that forms the water-stopping portion 6 as shown on the right side of Figure 9, and a portion that forms an iris-shaped welding auxiliary portion 8 on the outside of the portion that presses against the overlapped portion of the filter plate 4 and the separation membrane 5. The ultrasonic horn 9 is pressed against the peripheral portion of the filter plate in the direction of the arrow in Figure 9 at a set horn speed. The molten resin on the peripheral portion of the filter plate flows to both sides of the pressed horn and rises up, as shown in Figure 10. In this case, the peel strengths of areas E, B, and D of the waterproofing portion in Figure 10 are more strongly affected by the ultrasonic horn in the order of area D > area E >> area B, resulting in a peel strength profile such as that shown in Figure 7.

In order to realize such a peel strength profile, an ultrasonic horn having the shape described above is used, and the ultrasonic frequency, ultrasonic oscillation time, pressing pressure, etc. are set to a specific suitable range. Specifically, the ultrasonic frequency is preferably 15 to 70 kHz, more preferably 15 to 50 kHz, and even more preferably 15 to 30 kHz. The pressing pressure is preferably 0.1 to 3.0 MPa, more preferably 0.5 to 2.5 MPa, and the oscillation time is preferably 0.3 to 1.5 seconds, more preferably 0.4 to 1.0 seconds. If the pressing pressure is small, the adhesion between the peripheral portion of the filter plate and the separation membrane is weak (the horn cannot sink into the filter plate), and uniform welding may not be obtained. If the pressing pressure is large, the ultrasonic oscillator may be loaded and its life may be shortened. If the oscillation time is small, heat generation at the friction interface between the separation membrane and the filter plate may be insufficient, resulting in uneven welding. If the oscillation time is long, the melting of the filter plate resin and the separation membrane becomes too great, which may result in a weakening of the peel strength.

Because the flat membrane element of the present invention is constructed as described above, the welded portion 1 on the permeable portion side of the water-stopping portion will not break, and even if the welded portion 1 peels off due to long-term use, the welded portion 2 on the filter plate side will not peel off and will remain so. Therefore, the life of the element can be extended by only partially repairing the welded portion 1.

The flat membrane element of the present invention will be illustrated by examples, but the present invention is not limited to these. The peel strength in the present invention was evaluated according to the following method.

(Peel Strength)
The peel strength of the welded parts 1 and 2 was measured using an unused flat membrane element after ultrasonic welding according to the following procedure. The welded part was cut so that the length of the water-stopping part was 15 mm wide, and the water-permeable part of the separation membrane 2 cm away from the water-stopping part was set in the upper grip of a tensile tester (Shimadzu Autograph AGS-J, 50N load cell), and the filter plate to which the separation membrane was welded was set in the lower grip. 180-degree peeling was performed at 20 mm/min, and the strength (N) at that time was measured. The peeling direction was from the water-permeable part side of the flat membrane element toward the peripheral part side. The peel strength of the welded part 1 was measured by reading the peak top on the water-permeable part side, and the peel strength of the welded part 2 was measured by reading the peak top on the outer side of the filter plate.

(Sludge leakage suppression rate to permeate side)
To determine the rate of sludge leakage inhibition to the permeate side, 10 flat membrane elements were used in continuous operation in an aeration tank for three months, and the transparent suction tube connected to the permeate outlet 13 was visually observed periodically during use to check for the presence or absence of flat membrane elements from which sludge had leaked to the permeate side. The percentage of flat membrane elements from which sludge had not leaked out of the 10 flat membrane elements used was taken as the rate of sludge leakage inhibition to the permeate side (%).

(Whether or not separation membrane is peeled off or broken)
The ten flat membrane elements used were visually inspected for peeling or breakage of the separation membrane, and if peeling or breakage occurred, the percentage of peeling or breakage was also indicated.

(Whether or not the item can be reused after repair)
Regarding the feasibility of reuse through repair, in the case where peeling or breakage of the separation membrane occurs, if the peeled or broken welded portion is re-welded and the peel strength of the welded portion recovers to 90% or more of its initial strength, it is deemed feasible to reuse, but if it is less than 90%, it is deemed unfeasible.

Example 1
(Separation membrane)
The PET paper was cut to a predetermined size and then fixed to a frame for film formation while being careful not to wrinkle. Next, the PET paper was gently immersed in a solution (film-forming stock solution) consisting of 7.5% by mass of chlorinated polyvinyl chloride, 63.3% by mass of tetrahydrofuran, 19% by mass of isopropanol, and 10.2% by mass of 1-butanol, and left for 1 minute. Thereafter, the PET paper impregnated with the film-forming stock solution was slowly pulled out and left for 10 minutes in a drying zone (in a constant temperature and humidity box) at a relative humidity of 75% and a temperature of 20°C to form a separation membrane.

(Preparation of filter plate)
An ABS resin filter plate (height 315 mm, width 225 mm) had its peripheral portion (width 12.5 mm) glued tightly to both sides to create a filter plate with a peripheral thickness of 6 mm and a peripheral portion 1 mm higher than the center.

(Preparation of flat membrane element)
A resin mesh (Nihon Filcon Co., Ltd. DOP-18K, height 290 mm, width 200 mm) was set as a flow path material in the center of the filter plate, and was bonded to the filter plate with a water-resistant adhesive to form a membrane permeate flow path. The filter plate and the resin mesh were bonded, and a PET nonwoven fabric (Hirose Paper Co., Ltd. 05TH-60, height 285 mm, width 195 mm) was set as a cushioning material on the upper surface of the resin mesh. Furthermore, a separation membrane (height 305 mm, width 215 mm, thickness 0.13 mm) was overlapped with the peripheral portion of the filter plate from above the nonwoven fabric, and the up-down horn shown in FIG. 9 was pressed against it to weld the peripheral portion and the separation membrane.
The up-down horn had a radius of curvature of 1 mm at the tip of the water permeable portion (inner rising portion), an arc length of 1.6 mm, and an edge angle of 90 degrees on the outside of the filter plate (outer rising portion). The pressing pressure of the up-down horn was 0.8 MPa, the ultrasonic oscillation frequency was 20 kHz, and the oscillation time was 0.475 seconds.
A separation membrane was similarly adhered to the back side to prepare a flat membrane element.

Example 2
A flat membrane element was produced in the same manner as in Example 1, except that the oscillation time was set to 0.50 seconds.

Example 3
A flat membrane element was produced in the same manner as in Example 1, except that the oscillation time was set to 0.65 seconds.

Example 4
A flat membrane element was prepared in the same manner as in Example 2, except that an up-down horn having a larger radius of curvature (2 mm) at the tip on the water permeable section side (inner rising section) than that used in Example 1 was used.

Example 5
A flat membrane element was prepared in the same manner as in Example 1, except that an up-down horn having a smaller radius of curvature (0.6 mm) at the tip on the water permeable section side (inner rising section) than that used in Example 1 was used.

Example 6
A flat membrane element was produced in the same manner as in Example 5, except that the oscillation time was set to 0.40 seconds.

(Example 7)
A flat membrane element was prepared in the same manner as in Example 1, except that an up-down horn having a smaller radius of curvature (0.8 mm) at the tip on the water permeable section side (inner rising section) than that used in Example 1 was used, and the oscillation time was set to 0.80 seconds.

(Comparative Example 1)
A flat membrane element was prepared in the same manner as in Example 1, except that an up-down horn with an edge angle of 90 degrees on the water permeable section side (inner rising section) and an edge angle of 90 degrees on the outside of the filter plate (outer rising section) was used, and the oscillation time was set to 0.40 seconds.

(Comparative Example 2)
A flat membrane element was prepared in the same manner as in Example 3 (i.e., oscillation time 0.60 seconds), except that an up-down horn was used in which the edge angle on the water permeable section side (inner rising section) was 90 degrees and the edge angle on the outside of the filter plate (outer rising section) was 90 degrees.

(Comparative Example 3)
A flat membrane element was prepared in the same manner as in Example 3, except that the up-down horn had an edge angle of 90 degrees on the water permeable section side (inner rising section) and an edge angle of 160 degrees on the outside of the filter plate (outer rising section).

Details and evaluation results of the flat membrane elements of Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Table 1.

As is clear from the results in Table 1, in Examples 1 to 7, flat membrane elements with excellent durability and easy repair were obtained. In Examples 1, 2, and 4, the separation membrane peeled off from the filter plate at part of welded portion 1 during operation, but the treated liquid did not leak to the permeate side, and the flat membrane element could be reused by repairing it by re-welding welded portion 1. In Example 6, the sludge leakage suppression rate was high at 90%, but both welded portions 1 and 2 peeled off, but the flat membrane element could be reused by repairing it. On the other hand, in Comparative Example 1, in which the peel strength of welded portions 1 and 2 was low, membrane peeling occurred at both welded portions 1 and 2 in 60% of the flat membrane elements, and the sludge leakage suppression rate was low at 40%. In Comparative Example 2, rupture (membrane rupture) occurred at welded portion 1 in 30% of the flat membrane elements, possibly because the separation membrane deteriorated too much during welding at welded portion 1, which receives a load from rocking during use, and reuse by repair was impossible. In Comparative Example 3, the welded shapes of welded portion 1 and welded portion 2 were switched, but rupture (membrane rupture) was observed at welded portion 1 in 20% of the flat membrane elements, possibly because of rocking during use, as in Comparative Example 2, and reuse by repair was impossible.

The submerged flat membrane element of the present invention has an improved peel strength and cross-sectional profile for the water-stopping section with a single continuous concave cross-section formed by bonding the separation membrane to the peripheral portion of the filter plate, which allows for both durability and ease of repair, thereby extending the service life. Therefore, the submerged flat membrane element of the present invention is suitable for use in the water treatment field, particularly wastewater treatment.

REFERENCE SIGNS LIST 1 Membrane separation device 2 Flat membrane element 3 Air diffuser 4 Filter plate 5 Separation membrane 6 Water stopper 7 Periphery 8 Welding auxiliary 9 Ultrasonic horn 10 Center 11 Flow path material 12 Cushioning material 13 Permeate outlet

Claims (4)

a peel strength at welded portion 1 forming a shoulder of the concave cross section on the water permeable portion side of the water stop portion is 30 to 90% of the peel strength at welded portion 2 forming the shoulder of the concave cross section on the outer side of the filter plate of the water stop portion, the peel strength at welded portion 2 of the water stop portion is 8N or more and 40N or less, the rising portion of the shoulder of the concave cross section on the water permeable portion side of the water stop portion forms a curve, the rising portion of the shoulder of the concave cross section on the outer side of the filter plate of the water stop portion rises with a clear upward angle change from the flat portion at the outer edge of the filter plate of the flat portion between both shoulders of the concave cross section, and the edge angle forming the angle change is 80 degrees or more and 135 degrees or less. 2. The submerged type flat membrane element according to claim 1 , wherein the peel strength of the welded portion is 30 N or less. A method for manufacturing a submerged flat membrane element as described in claim 1 or 2, characterized in that it includes overlapping a resin separation membrane on a resin filter plate, and pressing an ultrasonic horn from the separation membrane side along the entire circumference of the overlapped peripheral portion to form a water-stopping portion composed of a continuous line with a single concave cross-section, wherein the portion pressed by the ultrasonic horn has a convex shape corresponding to the concave cross-section of the water-stopping portion. 4. The manufacturing method according to claim 3 , wherein the portion against which the ultrasonic horn is pressed further includes an iris portion continuing to a convex shaped portion corresponding to the concave cross section of the water-stopping portion.