Wastewater Treatment Methods

Abstract

To provide a wastewater treatment method that fully exhibits a membrane cleaning effect by relaxation while improving the productivity of the treatment water.SOLUTION: While aerating a membrane cartridge (18), a filtration operation and a filtration stop operation are regarded as one cycle and the cycle is repeated. Using a degassing valve (62) connected to a gas retention chamber (52), the degassing valve (62) is temporarily opened and closed in accordance with the timing of the start of the filtration stop operation in each cycle.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a wastewater treatment method for producing treated water by subjecting wastewater to solid-liquid separation. [Background technology]

[0002] In wastewater treatment systems using the membrane bioreactor (MBR), it is inevitable that foulants will accumulate on the membrane sheet of the membrane cartridge during wastewater filtration, increasing the permeation resistance of the membrane cartridge. For this reason, the wastewater filtration operation is usually stopped periodically and only aeration is performed on the membrane cartridge to remove the foulants from the membrane sheet.

[0003] In other words, one cycle is aeration of the membrane cartridge while generating negative pressure on the secondary side (treated water side) of the membrane cartridge to filter the wastewater, followed by a filtration stop operation to stop the filtration operation, and this cycle is repeated. Here, the aeration during the filtration stop operation is called relaxation, and this cycle is called an intermittent filtration cycle.

[0004] In addition, the action of peeling off foulants from the membrane sheet is exerted by pressure pulsation in the membrane cartridge accompanying the oscillation of the membrane sheet against the filter plate of the membrane cartridge. Furthermore, during filtration operation, the higher the flux (permeation flux), the greater the transmembrane pressure difference in the membrane cartridge, and the greater the force that sticks the membrane sheet to the filter plate, resulting in a smaller pressure pulsation in the membrane cartridge. Therefore, in order to fully exert the membrane cleaning effect by relaxation, it is necessary to increase the proportion of the time of the filtration stop operation in the intermittent filtration cycle and ensure the time to bring the transmembrane pressure difference in the membrane cartridge to an equilibrium state (zero).

[0005] Patent Documents 1 and 2 disclose wastewater treatment methods using an air release valve installed in the middle of a treated water piping system connecting a membrane separation tank and a treated water tank. In the wastewater treatment methods according to the prior art, when filtration is stopped, the air release valve is opened to open a part of the treated water piping system to the atmosphere, and the transmembrane pressure difference of the membrane cartridge is balanced. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2000-288543 A [Patent Document 2] Japanese Patent Application Publication No. 9-75686 Summary of the Invention [Problem to be solved by the invention]

[0007] However, if the proportion of time during which the filtration is stopped in the intermittent filtration cycle is increased, the membrane cleaning effect through relaxation can be fully exerted, but the production (generation) amount of treated water per unit time decreases, resulting in a decrease in the productivity (production efficiency) of the treated water.

[0008] On the other hand, if the proportion of the filtration stop time in the intermittent filtration cycle is reduced, the time during which the transmembrane pressure difference of the membrane cartridge is in equilibrium after the start of the filtration stop operation becomes shorter, making it difficult to fully exert the membrane cleaning effect through relaxation.

[0009] Furthermore, as shown in FIG. 1, when the flux during the filtration operation becomes high, the transmembrane pressure difference of the membrane cartridge increases, and the transmembrane pressure difference of the membrane cartridge does not reach equilibrium after the start of the filtration stop operation, and the next filtration operation is started, which may result in the membrane cleaning effect by relaxation not being fully exerted.

[0010] On the other hand, in the wastewater treatment method according to the prior art, a part of the treated water piping system is open to the atmosphere when the filtering operation is stopped, so that the time until the filtering operation is restarted becomes long, and the productivity of the treated water decreases.

[0011] Therefore, one aspect of the present invention aims to fully exert the membrane cleaning effect through relaxation while improving the productivity of treated water. [Means for solving the problem]

[0012] A wastewater treatment method according to one embodiment of the present invention is a wastewater treatment method in which aeration is performed on a membrane cartridge immersed in wastewater (water to be treated) in a membrane separation tank, while a filtering operation is performed by generating negative pressure on the secondary side (treated water side) of the membrane cartridge to filter the wastewater, and a filtering stop operation is performed to stop the filtering operation, and the cycle is repeated to separate the wastewater into solid-liquid separation and produce treated water.The method uses a valve connected to a gas retention chamber that retains gas contained in the treated water and for supplying fluid to the gas retention chamber, and opens and then closes the valve in accordance with the timing of the start of the filtering stop operation in each cycle.

[0013] According to the above configuration, the valve is opened once and then closed in accordance with the timing of the start of the filtration stop operation of each cycle. As a result, a fluid is supplied to the gas retention chamber in accordance with the timing of the start of the filtration stop operation of each cycle. Then, the negative pressure on the secondary side of the membrane cartridge is alleviated in a short time, and the transmembrane pressure difference of the membrane cartridge is balanced. In other words, the transmembrane pressure difference of the membrane cartridge can be balanced after the start of the filtration stop operation of each cycle while maintaining the state in which the treated water is continuously present in the treated water piping (a state in which a siphon is formed) without opening a part of the treated water piping system connecting the membrane separation tank and the treated water tank to the atmosphere. Therefore, the membrane cleaning effect by relaxation can be fully exhibited while improving the productivity (production efficiency) of the treated water.

[0014] In the wastewater treatment method according to one aspect of the present invention, the valve may be a gas vent valve disposed midway in a pipe connecting an ejector for generating negative pressure and the gas retention chamber.

[0015] According to the above configuration, the gas vent valve is opened once and then closed in accordance with the start timing of the filtration stop operation of each cycle, whereby treated water is supplied to the gas retention chamber in accordance with the start timing of the filtration stop operation of each cycle.

[0016] In the wastewater treatment method according to one aspect of the present invention, the valve may be an atmosphere release valve for opening the gas retention chamber to the atmosphere.

[0017] According to the above configuration, the atmosphere release valve is opened once and then closed in accordance with the start timing of the filtration stop operation of each cycle, whereby air is supplied to the gas retention chamber in accordance with the start timing of the filtration stop operation of each cycle.

[0018] In a wastewater treatment method according to one embodiment of the present invention, the valve may be opened once and then closed in accordance with the start of the filtration stop operation in each cycle, so as to prevent airlocks (air bubbles) from occurring in the treated water piping system connecting the membrane separation tank and the treated water tank that stores the treated water.

[0019] With this configuration, the valve is opened once and then closed in accordance with the start timing of the filtration stop operation of each cycle so as to prevent an airlock from occurring in the treated water piping system, thereby enabling the filtration operation of each cycle to be performed stably.

[0020] In a wastewater treatment method according to one aspect of the present invention, the valve may be opened once simultaneously with or immediately after the start of the filtration stop operation in each cycle, and the valve may be closed when the transmembrane pressure of the membrane cartridge becomes zero.

[0021] According to the above configuration, the valve is opened once simultaneously with or immediately after the start of the filtration stop operation of each cycle, and the valve is closed when the transmembrane pressure difference of the membrane cartridge becomes zero. This allows fluid to be supplied to the gas retention chamber in accordance with the start timing of the filtration stop operation of each cycle, and the transmembrane pressure difference of the membrane cartridge can be brought into an equilibrium state. Effect of the Invention

[0022] According to one aspect of the present invention, the productivity of treated water can be improved while the membrane cleaning effect by relaxation can be fully exerted. [Brief description of the drawings]

[0023] [Figure 1] FIG. 1 is a diagram for explaining a problem in the related art, showing the relationship between the transmembrane pressure difference of a membrane cartridge and time. [Diagram 2] FIG. 1 is a schematic diagram of a wastewater treatment system for carrying out a wastewater treatment method according to a first embodiment. [Diagram 3] FIG. 4 is a time chart showing the relationship between the aeration operation, the filtration operation, and the filtration stop operation and the opening and closing operation of the gas vent valve or the atmosphere release valve. [Figure 4] FIG. 1 is a schematic diagram of a wastewater treatment system for carrying out a wastewater treatment method according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] [Embodiment 1] Hereinafter, an embodiment of the present invention will be described with reference to Figures 2 and 3. Figure 2 is a schematic diagram of a wastewater treatment system for carrying out a wastewater treatment method according to embodiment 1. Figure 3 is a time chart showing the relationship between an aeration operation, a filtering operation, a filtering stop operation, and an opening and closing operation of a gas vent valve.

[0025] (Wastewater Treatment System 10) As shown in Fig. 2, a wastewater treatment system 10 for carrying out the wastewater treatment method according to the first embodiment is a system that performs solid-liquid separation of wastewater (water to be treated) E using a membrane bioreactor (MBR) to generate treated water (filtrate) T. The wastewater treatment system 10 includes a membrane separation tank 12 that stores the wastewater E. A water level gauge 14 that detects the water level of the wastewater E in the membrane separation tank 12 is provided at an appropriate position in the membrane separation tank 12.

[0026] A membrane module 16 for solid-liquid separation of wastewater E is disposed in the membrane separation tank 12, and the membrane module 16 is immersed in the wastewater E in the membrane separation tank 12. The membrane module 16 has a plurality of flat membrane type membrane cartridges 18 arranged side by side in an upright state. The membrane cartridge 18 has a filter plate 20 and a membrane sheet 22 provided on both sides of the filter plate 20. The peripheral portion of each membrane sheet 22 is fixed to the side of each filter plate 20. Since the portions of each membrane sheet 22 other than the peripheral portion are not fixed to each filter plate 20, each membrane sheet 22 can swing with respect to each filter plate 20. Between each filter plate 20 and each membrane sheet 22, and inside each filter plate 20, a passage is formed for circulating the treated water (filtrate) T that has permeated the membrane sheet 22.

[0027] An aeration device 24 that aerates the multiple membrane cartridges 18 is disposed below the membrane module 16 in the membrane separation tank 12, and is connected to a blower (not shown) that generates compressed air. The aeration device 24 not only supplies compressed air to the multiple membrane cartridges 18 to clean the membrane surfaces, but also supplies oxygen to the activated sludge contained in the wastewater. This creates an aerobic state inside the membrane separation tank 12, and the activated sludge performs nitrification treatment of the wastewater.

[0028] The wastewater treatment system 10 includes a treated water tank 26 for storing treated water T, and the treated water tank 26 is disposed at a position separated from the membrane separation tank 12. The water level of the treated water T in the treated water tank 26 is lower than the water level of the wastewater E in the membrane separation tank 12.

[0029] The wastewater treatment system 10 includes a treated water piping system 28 that connects the membrane separation tank 12 and the treated water tank 26. The specific configuration of the treated water piping system 28 is as follows.

[0030] One end of a first pipe 30 is connected to the membrane module 16, and the other end of the first pipe 30 is connected to a header pipe 32 that accommodates the treated water T produced by the membrane module 16. The header pipe 32 is located at a position higher than the water level of the treated water T in the membrane separation tank 12. In addition, one end of a second pipe 34 is connected to the header pipe 32, and the other end of the second pipe 34 is connected to the treated water tank 26. In the second pipe 34, a pressure gauge 36 that detects a pressure corresponding to the transmembrane pressure difference of the membrane cartridge 18, a flowmeter 38 that detects the flow rate of the treated water T, an adjustment valve 40, and a shutoff valve 42 are disposed in this order from the header pipe 32 side (upstream side).

[0031] One end of a third pipe 44 is connected to the second pipe 34 between the flow meter 38 and the adjustment valve 40, and the other end of the third pipe 44 is connected to the treatment water tank 26. A suction valve 46, a suction pump 48, and a shutoff valve 50 are disposed in this order in the third pipe 44 from the one end side thereof.

[0032] With the above configuration, when gravity filtration is performed, the control device consisting of a computer closes the suction valve 46 and opens the shutoff valve 42, and adjusts the opening of the adjustment valve 40 to adjust the amount of filtration. Then, negative pressure is generated on the secondary side (treated water side) of the multiple membrane cartridges 18 due to the hydraulic head difference between the membrane separation tank 12 and the treated water tank 26, and gravity filtration of the wastewater E is performed. Moreover, when a filtration stop operation is performed to stop the gravity filtration operation, the control device closes the shutoff valve 42.

[0033] When performing the suction filtration operation, the control device closes the shutoff valve 42 and opens the suction valve 46 and the shutoff valve 50. Then, the control device drives the suction pump 48. Then, a negative pressure is generated on the secondary side (treated water side) of the multiple membrane cartridges 18 by the suction force of the suction pump 48, and the suction filtration operation of the wastewater E is performed. When performing a filtration stop operation to stop the suction filtration operation, the control device stops driving the suction pump 48 and closes the shutoff valve 50.

[0034] The control device switches between gravity filtration and suction filtration based on the detection results from the water level gauge 14 and the flow meter 38.

[0035] A gas retention chamber 52 for retaining gas contained in the treated water T is provided at the upper part of the header pipe 32, and the gas retention chamber 52 is connected to the inside of the header pipe 32. A water level gauge 54 for detecting the water level of the treated water T in the gas retention chamber 52 is provided at an appropriate position in the gas retention chamber 52. The gas retention chamber 52 may be a gas retention tank that is separated from the header pipe 32 and disposed at a higher position than the header pipe 32. The gas contained in the treated water T is mainly air, but is not limited to air.

[0036] The wastewater treatment system 10 is provided with a degassing piping system 56 that connects the gas retention chamber 52 and the treated water tank 26. The specific configuration of the degassing piping system 56 is as follows.

[0037] One end of a fourth pipe 58 is connected to the gas retention chamber 52, and an ejector 60 that generates a negative pressure is connected to the other end of the fourth pipe 58. A gas vent valve 62 is provided in the middle of the fourth pipe 58 to vent the gas retained in the gas retention chamber 52. The gas vent valve 62 is connected to the gas retention chamber 52 via the fourth pipe 58. One end of a fifth pipe 64 is connected to the ejector 60, and the other end of the fifth pipe 64 is connected to the treatment water tank 26. A check valve 66 that prevents backflow to the treatment water tank 26 and an ejector pump 68 are provided in the fifth pipe 64 in this order from the ejector 60 side.

[0038] One end of a sixth pipe 70 is connected to the ejector 60, and the other end of the sixth pipe 70 is open to the atmosphere. A hopper 72 for recovering the treated water T is disposed below the other end of the sixth pipe 70. One end of a seventh pipe 74 is connected to the hopper 72, and the other end of the seventh pipe 74 is connected to the treated water tank 26.

[0039] With the above configuration, when the water level gauge 54 detects a predetermined first low water level, the control device drives the ejector pump 68 to open the gas vent valve 62. Then, the ejector 60 is supplied with high-speed treated water T, which generates negative pressure. As a result, the gas in the gas retention chamber 52 is sucked in and released to the atmosphere via the fourth pipe 58 and the sixth pipe 70. At this time, a part of the treated water T sucked in together with the gas in the gas retention chamber 52 is collected in the treated water tank 26 via the fourth pipe 58, the ejector 60, the sixth pipe 70, the hopper 72, and the seventh pipe 74. The predetermined first low water level is a water level lower than the reference water level and at which it is necessary to vent the gas in the gas retention chamber 52. The reference water level is a water level that is a reference when the gas vent valve 62 is closed.

[0040] When the water level gauge 54 detects the reference water level, the control device closes the gas vent valve 62 and stops driving the ejector pump 68. Then, the suction operation of the gas in the gas retention chamber 52 stops, and the treated water T accumulates between the gas vent valve 62 and the ejector 60 in the fourth piping 58.

[0041] It should be noted that the control device may periodically open and close the gas vent valve 62 instead of opening and closing the gas vent valve 62 based on the detection result of the water level gauge 54 .

[0042] As described above, the treated water T accumulates between the gas vent valve 62 and the ejector 60 in the fourth piping 58. Therefore, when the gas vent valve 62 is opened, a portion of the treated water T accumulated between the gas vent valve 62 and the ejector 60 in the fourth piping 58 is supplied to the gas retention chamber 52. In other words, the gas vent valve 62 is a valve for releasing the gas retained in the gas retention chamber 52, and is also a valve for supplying the treated water T as a fluid to the gas retention chamber 52.

[0043] (Wastewater treatment method) 2 and 3, the wastewater treatment method according to the first embodiment is a method in which aeration is performed on a plurality of membrane cartridges 18 immersed in wastewater E in a membrane separation tank 12, and a cycle of a filtering operation and a filtering stop operation is repeated (intermittent filtering cycle) to separate the wastewater E into solid and liquid to produce treated water T. Here, the filtering operation refers to an operation of filtering the wastewater E by generating negative pressure on the secondary side of the plurality of membrane cartridges 18, and includes a gravity filtering operation and a suction filtering operation. The filtering stop operation refers to an operation of stopping the filtering operation.

[0044] In the wastewater treatment method according to embodiment 1, the control device opens and then closes the gas vent valve 62 in accordance with the start of the filtration stop operation of each intermittent filtration cycle so as to prevent the occurrence of an airlock (air bubbles) in the treated water piping system 28.

[0045] Specifically, the control device opens the gas vent valve 62 once at the same time as or immediately after the start of the filtration stop operation of each cycle, and then closes it after a predetermined time (e.g., 1 second) has elapsed. Here, the opening degree when the gas vent valve 62 is opened does not need to be full open, and a small opening degree is sufficient as long as an appropriate amount of treated water T can be supplied to the gas retention chamber 52.

[0046] When the water level gauge 54 detects a predetermined second low water level while the intermittent filtration cycle is being repeated, the control device drives the ejector pump 68 and opens the gas vent valve 62. This causes the gas in the gas retention chamber 52 to be vented, and the water level of the treated water T in the gas retention chamber 52 to rise. Thereafter, when the water level gauge 54 detects the reference water level, the control device closes the gas vent valve 62 and stops driving the ejector pump 68. Here, the predetermined second low water level is a water level lower than the predetermined first low water level and refers to a water level before an airlock occurs in the treated water piping system 28. In other words, the predetermined second low water level refers to a water level for avoiding an airlock in the treated water piping system 28.

[0047] Instead of opening the vent valve 62 once simultaneously with or immediately after the start of the filtration stop operation of each cycle and then closing it after a predetermined time has elapsed, the control device may do the following.

[0048] The control device opens the gas vent valve 62 once at the same time as or immediately after the start of the filtration stop operation of each intermittent filtration cycle, and closes the gas vent valve 62 when the transmembrane pressure difference between the multiple membrane cartridges 18 becomes zero. The control device determines that the transmembrane pressure difference between the multiple membrane cartridges 18 has become zero based on the detection result from the pressure gauge 36.

[0049] (Action and effect) As described above, the control device opens and then closes the gas vent valve 62 in accordance with the start timing of the filtration stop operation of each intermittent filtration cycle. As a result, a part of the treated water T accumulated between the gas vent valve 62 and the ejector 60 in the fourth piping 58 is supplied to the gas retention chamber 52 in accordance with the start timing of the filtration stop operation of each intermittent filtration cycle. Then, the negative pressure on the secondary side of the multiple membrane cartridges 18 is alleviated, and the transmembrane pressure difference of the multiple membrane cartridges 18 is balanced. That is, without opening a part of the treated water piping system 28 to the atmosphere, while maintaining a state in which the treated water is continuously present in the treated water piping system 28 (a state in which a siphon is formed), the transmembrane pressure difference of the multiple membrane cartridges 18 can be balanced in a short time after the start of the filtration stop operation of each intermittent filtration cycle. As a result, the flux (permeation flux) can be increased, and the productivity (production efficiency) of the treated water T by the wastewater treatment system 10 can be improved, while the membrane cleaning effect by relaxation can be fully exhibited.

[0050] As described above, the control device opens and then closes the gas vent valve 62 in accordance with the start of the filtration stop operation of each intermittent filtration cycle so as to prevent an airlock from occurring in the treated water piping system. This allows the filtration operation of each intermittent filtration cycle to be performed stably.

[0051] Therefore, according to the wastewater treatment method of embodiment 1, it is possible to improve the productivity of the treated water T by the wastewater treatment system 10, while fully exerting the membrane cleaning effect by relaxation and stably performing the filtration operation of each intermittent filtration cycle.

[0052] [Embodiment 2] Hereinafter, a second embodiment of the present invention will be described with reference to Figs. 3 and 4. Fig. 3 is a time chart showing the relationship between an aeration operation, a filtration operation, a filtration stop operation, and an opening and closing operation of an atmospheric release valve. Fig. 4 is a schematic diagram of a wastewater treatment system for carrying out a wastewater treatment method according to the second embodiment. For ease of explanation, members having the same functions as those described in the embodiments of the present invention are denoted by the same reference numerals, and their explanation will not be repeated.

[0053] (Wastewater Treatment System) 4, a wastewater treatment system 10A for carrying out the wastewater treatment method according to the second embodiment is a system that performs solid-liquid separation of wastewater E using a membrane bioreactor (MBR) to generate treated water T. The wastewater treatment system 10A has a similar configuration to the wastewater treatment system 10, and only the configuration of the wastewater treatment system 10A that differs from the wastewater treatment system 10 will be described.

[0054] One end of an eighth pipe 76 is connected to the gas retention chamber 52, and the other end of the eighth pipe 76 is connected to an atmosphere release valve 78 for opening the gas retention chamber 52 to the atmosphere. The atmosphere release valve 78 is connected to the gas retention chamber 52 via the eighth pipe 76. When the atmosphere release valve 78 is opened, the gas retention chamber 52 is opened to the atmosphere and air (atmosphere) is supplied to the gas retention chamber 52. In other words, the atmosphere release valve 78 is a valve for opening the gas retention chamber 52 to the atmosphere and also a valve for supplying air as a fluid to the gas retention chamber 52.

[0055] (Wastewater treatment method) 3 and 4, the wastewater treatment method according to the second embodiment is a method in which aeration is performed on a plurality of membrane cartridges 18 immersed in wastewater E in a membrane separation tank 12, and a cycle (intermittent filtration cycle) of a filtration operation and a filtration stop operation is repeated to perform solid-liquid separation of the wastewater E to produce treated water T. In the wastewater treatment method according to the second embodiment, the control device opens and then closes the atmosphere release valve 78 in accordance with the start of the filtration stop operation of each intermittent filtration cycle so as to prevent airlock (air bubbles) from occurring in the treated water piping system 28.

[0056] Specifically, the control device opens the air release valve 78 once simultaneously with or immediately after the start of the filtration stop operation of each cycle, and then closes it after a predetermined time (e.g., one second) has elapsed. Here, the opening degree when the air release valve 78 is opened does not need to be full open, and a small opening degree is sufficient as long as an appropriate amount of air can be supplied to the gas retention chamber 52.

[0057] The control device may, instead of opening the atmospheric relief valve 78 once simultaneously with or immediately after the start of the filtration stop operation in each cycle and then closing it after a predetermined time has elapsed, perform the following operation.

[0058] The control device opens the atmosphere relief valve 78 once simultaneously with or immediately after the start of the filtration stop operation of each intermittent filtration cycle, and closes the atmosphere relief valve 78 when the transmembrane pressure difference between the multiple membrane cartridges 18 becomes zero.

[0059] (Action and effect) As described above, the control device opens and then closes the air release valve 78 in accordance with the timing of the start of the filtration stop operation of each intermittent filtration cycle. As a result, air is supplied to the gas retention chamber 52 in accordance with the timing of the start of the filtration stop operation of each intermittent filtration cycle. Then, the negative pressure on the secondary side of the plurality of membrane cartridges 18 is alleviated, and the transmembrane pressure difference of the plurality of membrane cartridges 18 is balanced. That is, without opening a part of the treated water piping system 28 to the atmosphere, while maintaining a state in which the treated water is continuously present in the treated water piping system 28 (a state in which a siphon is formed), the transmembrane pressure difference of the plurality of membrane cartridges 18 can be balanced in a short time after the start of the filtration stop operation of each intermittent filtration cycle. This increases the flux, improving the productivity (production efficiency) of the treated water T by the wastewater treatment system 10A, while fully exerting the membrane cleaning effect by relaxation.

[0060] As described above, the control device opens and then closes the air release valve 78 in accordance with the start of the filtration stop operation of each intermittent filtration cycle so as to prevent an airlock from occurring in the treated water piping system. This allows the filtration operation of each intermittent filtration cycle to be performed stably.

[0061] In other words, according to the wastewater treatment method of embodiment 2, it is possible to improve the productivity of the treated water T by the wastewater treatment system 10A, while fully exerting the membrane cleaning effect by relaxation and stably performing the filtration operation of each intermittent filtration cycle.

[0062] [Additional Notes] The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention. [Explanation of symbols]

[0063] 10 Wastewater Treatment System 12 Membrane separation tank 14 Water level gauge 16 Membrane module 18 Membrane Cartridge 20 Filter plate 22 Membrane Sheet 24 Aeration device 26 Treatment tank 28 Treated water piping system 30 First Pipe 32 Header pipe 34 Second Pipe 36 Pressure Gauge 38 Flow meter 40 Adjustment valve 42 Shut-off valve 44 Third Pipe 46 Suction valve 48 Suction Pump 50 Shut-off valve 52 Gas retention chamber 54 Water level gauge 56 Gas venting piping system 58 4th Pipe 60 Ejector 62 Gas release valve 64 5th Pipe 66 Check valve 68 Ejector pump 70 6th Pipe 72 Hopper 74 7th Pipe 10A Wastewater Treatment System 76 No. 8 Pipe 78 Atmospheric relief valve E. Wastewater (water to be treated) T Treated water (filtrate)

Claims

1. A wastewater treatment method for producing treated water by performing solid-liquid separation of the wastewater by repeating a cycle of a filtering operation in which a membrane cartridge immersed in wastewater in a membrane separation tank is aerated while generating negative pressure on a secondary side of the membrane cartridge to filter the wastewater, and a filtering stop operation in which the filtering operation is stopped, the cycle being repeated, A wastewater treatment method comprising the steps of: using a valve connected to a gas retention chamber that retains gas contained in the treated water and for supplying fluid to the gas retention chamber; and opening and then closing the valve in accordance with the start of the filtration stop operation in each cycle.

2. 2. The wastewater treatment method according to claim 1, wherein the valve is a gas vent valve disposed midway along a pipe connecting an ejector that generates a negative pressure and the gas retention chamber.

3. 2. The wastewater treatment method according to claim 1, wherein the valve is an atmosphere release valve for opening the gas retention chamber to the atmosphere.

4. A wastewater treatment method according to any one of claims 1 to 3, characterized in that the valve is opened once and then closed in accordance with the timing of the start of the filtration stop operation in each cycle, so as to prevent an airlock from occurring in the treated water piping system connecting the membrane separation tank and a treated water tank for storing the treated water.

5. The wastewater treatment method according to any one of claims 1 to 3, characterized in that the valve is opened once simultaneously with or immediately after the start of the filtration stop operation of each cycle, and the valve is closed when the transmembrane pressure difference of the membrane cartridge becomes zero.

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