Method of replacement of a filtration membrane of a filtration cartridge for activated sludge

Abstract: The invention concerns a method of replacement of a filtration membrane (2B) of a filtration cartridge for activated sludge in case the effective filtration region of the filtration membrane (2B) is damaged. Said filtration cartridge comprises an organic flat filtration membrane which is disposed so as to cover the surface of a filter plate (2A) made of resin, a welded part (2E) having an ultrasonically welded seal which is formed along the entire periphery of said filtration membrane (2B) and securing said filtration membrane (2B) to said filter plate (2A), and a region surrounded by said welded part which is defined as an effective filtration region. The method of replacement of a filtration membrane comprises the steps of cutting said filtration membrane (2B) along the inside of said welded part (2E) to form around the periphery of said filter plate (2A) a frame of said filtration membrane (2B) of a predetermined width (L), which includes said welded part (2E), as a bonding allowance (41); disposing a new felt (43) in the region of the inside of the welded part (2E); disposing a fresh filtration membrane (42) such that its edge overlaps with said bonding allowance (41); and applying an adhesive agent to said fresh filtration membrane along said bonding allowance (41) to seal the entire periphery of said used (2B) and fresh (42) filtration membranes by a layer of adhesive agent, which is formed by permeation of said adhesive agent through both membranes (2B,42) and its solidification in said bonding allowance (41).

Description: FIELD OF THE INVENTION The present invention relates to techniques for regenerating filtration membrane cartridges used in solid-liquid separators for high-concentration activated sludge and, in particular, to method of regenerating filtration membrane cartridges for activated sludge with which filtration membrane cartridges employing filtration membranes composed of an organic flat membrane are readily replaced at low cost. BACKGROUND OF THE INVENTION In activated sludge treatment facilities to perform solid-liquid separation of high-concentration activated sludge, an air diffuser disposed in a tank blows out aeration air to supply oxygen to activated sludge in the tank and the organic matter and nitrogen in raw water is treated with the activated sludge, and a mixed liquor in the tank, which is a mixture of water treated and the activated sludge, is subjected to solid-liquid separation by a membrane separator immersed in an aeration tank. The above membrane separator has a plurality of filtration membrane cartridges and filters a mixed liquid by using a head within the tank as a driving pressure. The air diffuser is disposed beneath the membrane separator and blows out aeration air upwardly. Updraft caused by air lift action of the bubbles of the aeration air flows along the membrane surfaces of filtration membrane cartridges to clean these membrane surfaces. This cleaning suppresses a reduction in the separation function of the filtration membrane cartridges, thereby preventing the membrane separator from resulting in malfunction. In the above filtration membrane cartridge, a filtration membrane composed of an organic flat membrane, such as ultrafiltration membrane or microfiltration membrane, is provided on the surface of a filter plate and the periphery of the filtration membrane is secured to the filter plate by welding. Its lifetime is generally between two and five years. When this filtration membrane cartridge is put into service, however, its periphery is flapped by updraft to deteriorate the strength of a welded part, which may result in a shorter lifetime than the normal lifetime as described. On the other hand, filtration membranes used in the filtration membrane cartridges are prepared by coating an organic macromolecular membrane material on both sides of a non-woven fabric serving as a substrate. Since such filtration membranes have durability, little or no breakage takes place, however, contamination due to activated sludge progresses with age, leading to a reduction in flux. Accordingly, it is a primary object of the invention to provide method of regenerating filtration membrane cartridges for activated sludge which effect recovery of the filtration function of contaminated filtration membrane cartridges, remedy of broken parts, and replacement of filtration membranes, with ease and at low costs. DISCLOSURE OF THE INVENTION A method of replacement of a filtration membrane of a filtration cartridge for activated sludge according to the invention, the filtration cartridge having an organic flat filtration membrane which is disposed so as to cover the surface of a filter plate made of a resin, a welded part having an ultrasonically welded seal which is formed along the entire periphery of said filtration membrane and securing said filtration membrane to said filter plate, and a region surrounded by said welded part which is defined as an effective filtration region, comprises the steps of: cutting said filtration membrane along the inside of said welded part to form around the periphery of said filter plate a frame of said filtration membrane of a predetermined width which includes said welded part, as a bonding allowance; disposing a new felt in the region of the inside of the welded part; disposing a fresh filtration membrane such that its edge overlaps with said bonding allowance; and applying an adhesive agent to said fresh filtration membrane along said bonding allowance; to seal the entire periphery of said used and fresh filtration membranes by a layer of adhesive agent which is formed by permeation of the adhesive agent through both membranes and its solidification in said bonding allowance. This method is effective for the case where the damage to the filtration membrane cartridge is due to the filtration membrane being partially broken in the effective filtration region or due to replacement of the filtration membrane. That is, the fresh filtration membrane is not directly bonded to the filter plate by an adhesive agent but is secured to the filter plate via the used filtration membrane by utilizing the used filtration membrane as an adhesive application surface. With this arrangement, the welded part ensures the sealing property between the filter plate and the used filtration membrane, and the layer of adhesive agent ensures the sealing property between the used and new filtration membranes, thereby the effective filtration region in the new filtration membrane is sealed against the exterior by the welded part and the layer of adhesive agent. BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS Fig. 1 is a perspective view showing the arrangement of a membrane separator employing filtration membrane cartridges for activated sludge. Fig. 2 is a front view of the above filtration membrane cartridge. Fig. 3 is an exploded view illustrating the arrangement of the filtration membrane cartridge. Fig. 4 is a schematic view illustrating one procedure in welding the filtration membrane cartridge. Fig. 5 is a cross section of the filtration membrane cartridge. Fig. 6 is a schematic view illustrating procedure in regenerating a filtration membrane cartridge for activated sludge. Fig. 7 is a schematic view illustrating the procedure in regenerating a filtration membrane cartridge for activated sludge according to preferred embodiment. Fig. 8 is an enlarged cross section showing the junction structure of the above filtration membrane cartridge. EMBODIMENTS Preferred embodiments of the invention will be described by referring to the drawings. Fig. 1 is a perspective view of a membrane separator which functions as a solid-liquzd separator for high-concentration activated sludge. In Fig. 1, a membrane separator 1 comprises a plurality of plate-like filtration membrane cartridges for activated sludge (hereinafter referred to as a filtration membrane cartridge) 2, an air diffuser 3 by which air for cleaning membrane surface is blown out from a lower part of the membrane separator 1, and a casing 4 for encasing the filtration membrane cartridges 2 and air diffuser 3. To facilitate conveyance and maintenance, the casing 4 is formed in two parts, i.e., a membrane casing 5 in which a plurality of filtration membrane cartridges 2 in parallel arrangement are held in vertical direction, and a diffuser casing 6 for encasing the air diffuser 3. By the diffuser casing 6, the entire quantity of air for cleaning membrane surface which is blown out of the air diffuser 3 is introduced into the membrane casing 5. Referring to Figs. 2 and 3, in a filtration membrane cartridge 2, a filtration membrane 2B of an organic flat membrane is disposed on both sides of a filter plate 2A made of ABS resin, a felt 2C is interposed between the filter plate 2A and the filtration membrane 2B, and the periphery of the filtration membrane 2B is secured to the filer plate 2A by welding with ultrasonic wave. The filtration membrane 2B is prepared by coating an organic macromolecular membrane material onto both sides of a non-woven fabric serving as a substrate. Manufacturing steps of a filtration membrane cartridge 2 is described by referring to Figs. 3 to 5. A filter plate 2A has a welding auxiliary part 2D along its whole periphery, and a welded part 2E which is located at the inside of the welding auxiliary part 2D along the periphery of the part 2D. The welding auxiliary part 2D and welded part 2E are formed at a position higher than the surface of the filter plate 2A. A water collecting part 2F passes through both sides of the filter plate 2A, and a permeated liquid outlet 2G in communication with the water collecting part 2F is formed in an upper edge of the filter plate 2A. A felt 2C is disposed at position surrounded by the welded part 2E on the surface of the filter plate 2A, and a filtration membrane 2B is then overlapped so as to cover the welding auxiliary part 2D and the welded part 2E. In this state, ultrasonic wave is oscillated from an ultrasonic oscillator S to the welding auxiliary part 2D. With ultrasonic wave, the welding auxiliary part 2D is melted to weld between the part 2D and the periphery of the filtration membrane 2B, thereby the membrane 2B is tacked to the filter plate 2A. Thanks to the tack, when the filtration membrane 2B is pressed against the welded part 2E, the filtration membrane 2B lies flat without wrinkles in an effective filtration region A surrounded by the welded part 2E. Subsequently, ultrasonic wave is oscillated from the ultrasonic oscillator S to the welded part 2E. With ultrasonic wave, the welded part 2E is melted to weld between the welded part 2E and the filtration membrane 2B. Thereby, the welded part 2E effects sealing between the effective filtration region A and the exterior. Both sides of the filter plate 2A are subjected to the foregoing steps. The prepared filtration membrane cartridge 2 ensures a predetermined space between the filter plate 2A and the filtration membrane 2B by the felt. This space forms a permeated liquid passage. As shown in Fig. 1, each filtration membrane cartridge 2 is in communication with a collecting pipe 8 through a tube 7 connected to the permeated liquid outlet 2G. The collecting pipe 8 is in communication with a permeated liquid discharge pipe 9 that discharges a membrane-permeated liquid. A holding-down plate 10 prevents each filtration membrane cartridge 2 from floating. When the above membrane separator 1 is used in activated sludge treatment facilities, the membrane separator 1 is immersed in a mixed liquid of activated sludge and raw water which is stored in an aeration tank (not shown), and aeration air is blown out from a diffuser 3. In this state, the organic and nitrogen in the raw water are treated with the activated sludge, while the mixed liquid is filtered through the filtration membrane cartridge 2 by using the head within the tank as a driving pressure (Alternatively, forced suction filtration can be effected by interposing a suction pump in the permeated liquid discharge pipe 9), and the liquid that has permeated the membrane surface of the filtration membrane cartridge 2 is then discharged to the outside of the tank as a treated water, through the permeated liquid discharge pipe 9. At this time, the membrane surface of the filtration membrane cartridge 2 is cleaned by updraft which is caused by the bubbles of aeration air blown out from the diffuser 3 and their air lift action. This cleaning suppresses a reduction in separation function, preventing the membrane separator 1 from causing malfunction. During the use of the filtration membrane cartridge 2, a cake layer composed of an organic, e.g., activated sludge, is attached to the membrane surface of the filtration membrane cartridge 2, or the pores of the membrane become clogged by organic contaminants, thus deteriorating its membrane separation function. A procedure in remedying a filtration membrane cartridge 2 is described by referring to Figs. 6 to 8. In regenerating a filtration membrane cartridge 2 due to the damage to an effective filtration region A of a filtration membrane 2B, the membrane 2B is cut along the inside of the welded part 2E in a predetermined distance, to leave the filtration membrane 2B of a predetermined width L including the welded part 2E along the periphery of the filter plate 2A, as a bonding allowance 41. The reason for this is that upon removal of the filtration membrane 2B, the surface of the filter plate 2A becomes rough at a welding auxiliary part 2D, making it difficult to directly bond a new filtration membrane 42 to the filter plate 2A. Accordingly, instead of directly bonding the new filtration membrane 42 to the filter plate 2A, the used filtration membrane 2B forming a bond on the welded part 2E is utilized as a bonding allowance 41. Then, under the condition that the surface of the used filtration membrane 2B at the bonding allowance 41 is retained clean by cleaning, a new felt 43 is disposed in the region of the inside of the welded part 2E, and a new filtration membrane 42 is secured so as to cover the surface of the filter plate 2A. In this state, the periphery of the new filtration membrane 42 is brought into contact with the bonding allowance 41, and an adhesive agent is applied to the periphery of the new filtration membrane 42 which corresponds to the bonding allowance 41. This adhesive agent may be applied to the contact surface between the used filtration membrane 2B and the new filtration membrane 42. In this embodiment, however, a sufficient quantity of the adhesive agent is applied to the entire width of the bonding allowance 41, from the surface of the new filtration membrane 42. Here, one of soft polyurethane (esters) adhesive agents is employed. The adhesive agent permeates through the pores of both filtration membranes 2B, 42, and then solidified to become resin, thereby forming a layer of adhesive agent 44. The layer 44 seals the entire periphery of the filtration membranes 2B, 42. Thus, the effective filtration region A in the new filtration membrane 42 can be sealed against the exterior by fixing the new filtration membrane 42 to the filter plate 2A through the used filtration membrane 2B. As a result, the sealing property between the filter plate 2A and the used filtration membrane 2B is ensured by the welded part 2E and its welded layer 2H, and the sealing property between the membranes 42 and the used filtration membrane 2B is ensured by the layer of adhesive agent 44. When the layer of adhesive agent 44 composed of a polyurethane adhesive is placed in water, it contains water and then exhibits flexibility, thus serving as a buffer to absorb the vibration of the new filtration membrane 42. This prevents breakage between the used filtration membrane 2B and new filtration membrane 42 in the bonding allowance 41. As described, the steps of cutting the used filtration membrane 2B and bonding the new filtration membrane 42 permit to regenerate the filtration membrane cartridge 2 with ease and at low cost.

Claims: A method of replacement of a filtration membrane (2B) of a filtration cartridge for activated sludge, said filtration cartridge having an organic flat filtration membrane (2B) which is disposed so as to cover the surface of a filter plate (2A) made of resin, a welded part (2E) having an ultrasonically welded seal which is formed along the entire periphery of said filtration membrane (2B) and securing said filtration membrane (2B) to said filter plate (2A), and a region surrounded by said welded part which is defined as an effective filtration region, comprising the steps of: cutting said filtration membrane (2B) along the inside of said welded part (2E) to form around the periphery of said filter plate (2A) a frame of said filtration membrane (2B) of a predetermined width (L) which includes said welded part (2E), as a bonding allowance (41); disposing a new felt (43) in the region of the inside of the welded part (2E); disposing a fresh filtration membrane (42) such that its edge overlaps with said bonding allowance (41); and applying an adhesive agent to said fresh filtration membrane along said bonding allowance (41) to seal the entire periphery of said used (2B) and fresh (42) filtration membranes by a layer of adhesive agent which is formed by permeation of said adhesive agent through both membranes (2B, 42) and its solidification in said bonding allowance (41). The method according to claim 1, wherein the adhesive agent is a wet hardening soft adhesive.

Inventors: KENJI KUBOTA, KIYOSHI IZUMI, YOSHIO NIGARA, YUTAKA YAMADA, KENJI, KUBOTA, KIYOSHI, IZUMI, YOSHIO, NIGARA, YUTAKA, YAMADA, Kenji, Kubota, c/o KUBOTA Corporation, Kiyoshi, Izumi, c/o KUBOTA Corporation, Yoshio, Nigara, c/o KUBOTA Corporation, Yutaka, Yamada, c/o KUBOTA Corporation
Applicants: KUBOTA KK, KUBOTA CORPORATION
Application year: 1999
Main classification: B01D63
IPC codes: B01D63, B01D65