Organic wastewater treatment device and method for operating the same

Abstract

To provide an organic wastewater treatment apparatus capable of suppressing an increase in nitrate-nitrogen concentration in an anoxic tank while avoiding membrane clogging in a membrane separator, even when the amount of sludge returned from the aerobic tank to the anoxic tank is increased.SOLUTION: An organic wastewater treatment apparatus comprises a biological treatment tank including at least an anoxic tank and an aerobic tank in which a membrane separator is immersed and provided in activated sludge, and a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, in which the membrane separator is configured to repeatedly operate and stop filtration, and organic waste water containing nitrogen is biologically treated in activated sludge. The organic wastewater treatment apparatus is equipped with an air discharge pipe in the aerobic tank and a switching mechanism for switching between air supply to an aeration device provided in the membrane separator and air supply to the air discharge pipe, and based on a DO value in the aerobic tank, selectively supplies the air to one of the air discharge pipe and the aeration device via the switching mechanism during filtration pause of the membrane separator.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an organic wastewater treatment device and an operating method for the organic wastewater treatment device. [Background technology]

[0002] Patent Document 1 proposes an organic wastewater treatment device capable of efficiently carrying out biological treatment according to the properties of nitrogen-containing organic wastewater.

[0003] The organic wastewater treatment device has a pair of biological treatment units, each consisting of an anoxic tank arranged upstream along the flow of the organic wastewater and an aerobic tank arranged downstream with a membrane separation device immersed in activated sludge, and is equipped with a biological treatment tank in which a plurality of biological treatment units are connected in series, a sludge return path for returning activated sludge from the aerobic tank arranged at the most downstream position to the anoxic tank arranged at the most upstream position, a raw water supply path for dividing and supplying the organic wastewater to the anoxic tanks of each biological treatment unit, and a treated water delivery path for delivering membrane permeate from the membrane separation device of each biological treatment unit as treated water.

[0004] The system is also equipped with measuring devices that measure at least one of the inflow rate of organic wastewater, the water tank liquid level, the transmembrane pressure difference of each membrane separation device, the TN of the treated water, the NO3-N concentration of the treated water, and the NH4-N concentration of the treated water, and a control device that uses the measured values ​​obtained by the measuring devices as indicators to determine whether to stop an operating membrane separation device or start a membrane separation device that is currently stopped, for each biological treatment unit.

[0005] Patent Document 2 proposes a cleaning air volume control system that can more appropriately determine the maintenance period while reducing the cleaning air volume.

[0006] The cleaning air volume control system is equipped with a representative pond pressure meter installed in a representative pond to which air is supplied at a rated air volume, and which measures the membrane differential pressure of a representative pond separation membrane unit that filters the treated water flowing into the representative pond, a target value setting unit installed in a control pond to which air is supplied at a predetermined air volume less than the rated air volume, and which sets a target value of the membrane differential pressure or membrane filtration resistance of the control pond based on an upper limit value of the membrane differential pressure or membrane filtration resistance of the control pond separation membrane unit that filters treated water similar to the treated water flowing into the representative pond, and information on the change over time in the membrane filtration resistance or the membrane differential pressure of the representative pond separation membrane unit calculated based on the membrane differential pressure of the representative pond separation membrane unit, and an air volume control unit that controls the air volume supplied to the control pond based on the target value. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2018-176016 A [Patent Document 2] JP 2020-199475 A Summary of the Invention [Problem to be solved by the invention]

[0008] In the organic wastewater treatment device disclosed in Patent Document 1, the actual residence time in the anoxic tank is shorter than that in a typical MBR (Membrane Bio Reactor), and the residence time in the anoxic tank where the water is substantially in an anoxic state is reduced by dissolved oxygen (hereinafter referred to as "DO (Dissolved Oxygen)") brought in from the aerobic tank, so that the denitrification process does not progress and the nitrate nitrogen concentration in the treated water is likely to increase.

[0009] For example, in an organic wastewater treatment device with two biological treatment units, the actual retention time in the anoxic tank is reduced to half that of a conventional MBR, and in an organic wastewater treatment device with four biological treatment units, it is reduced to one-quarter.

[0010] For this reason, it is necessary to suppress the dissolved oxygen concentration (hereinafter referred to as the "DO value") in the aerobic tank. However, in the case of organic wastewater treatment equipment with a low inflow load, it is expected that the DO value will not fall to the ideal concentration of around 1.0 mg / L unless not only the air volume of the auxiliary aeration device that supplies air to the aerobic tank but also the air volume used to clean the membrane by the aeration device installed in the membrane separation unit are suppressed.

[0011] Even in a standard MBR equipped with one anoxic tank and one aerobic tank in which a membrane separation device is immersed in the activated sludge, a similar situation may occur if the amount of returned sludge through the sludge return path is increased.

[0012] In general, the membrane cleaning air volume is set to a constant air volume or based on the membrane filtration resistance, regardless of the DO value of the aerobic tank, including a cleaning air volume control system that automatically adjusts the cleaning air volume based on information on changes over time in membrane filtration resistance, as disclosed in Patent Document 2.

[0013] Considering the increase in the DO value, it is possible to control the DO value by forcibly reducing the cleaning airflow when the dissolved oxygen concentration in the aerobic tank reaches the upper limit. However, if the membrane cleaning airflow is reduced based only on the DO value, there is a risk that the membrane fouling rate will increase rapidly.

[0014] An object of the present invention is to provide an organic wastewater treatment system and an operating method for the same, which can suppress an increase in the nitrate nitrogen concentration in the anoxic tank while avoiding clogging of the membrane of a membrane separation device, even when the amount of sludge returned from the aerobic tank to the anoxic tank is increased. [Means for solving the problem]

[0015] In order to achieve the above-mentioned object, a first characteristic configuration of an organic wastewater treatment device according to the present invention is an organic wastewater treatment device which comprises at least an anoxic tank and an aerobic tank in which a membrane separation device is immersed in activated sludge, and a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, and the membrane separation device is configured to repeatedly perform filtration operation and filtration pause, and which biologically treats nitrogen-containing organic wastewater in activated sludge, and which is provided with an air release pipe in the aerobic tank and a switching mechanism for switching between supplying air to an aeration device provided in the membrane separation device and supplying air to the air release pipe, and is configured to selectively supply air to either the air release pipe or the aeration device via the switching mechanism when filtration of the membrane separation device is paused based on the DO value of the aerobic tank.

[0016] The membrane separation equipment repeats at specified time intervals a membrane filtration operation in which treated water is obtained by membrane filtration while releasing fine bubbles by the aeration device, and a filtration pause in which the membrane filtration is stopped while continuing to release fine bubbles by the aeration device, and the membrane surface is purified by the upward flow of treated water generated by aeration. At this time, the fine bubbles released by the aeration device increase the DO value.

[0017] Therefore, when the DO value in the aerobic tank is high and denitrification in the anoxic tank is not progressing, leading to a situation where the nitrate nitrogen concentration in the treated water is likely to rise, the aeration by the aeration device is stopped and air is released from the air release pipe when filtration in the membrane separation unit is paused, thereby suppressing the rise in the DO value. The air released from the air release pipe forms large localized lumps in the aerobic tank and rises quickly through the liquid, so it does not raise the DO value like the fine air bubbles released from the aeration device. Because membrane filtration is not performed when filtration is paused, there is no risk of fouling substances adhering to the membrane surface.

[0018] When the DO value in the aerobic tank falls and the denitrification process in the anoxic tank begins, the membrane separation unit switches from releasing air from the air release pipe to diffusing air using the aeration device during relaxation operation, and the membrane surface is purified by the upward flow of treated water caused by the aeration.

[0019] The second characteristic configuration is, in addition to the first characteristic configuration described above, that the air discharge pipe is positioned upstream of the flow of organic wastewater or at a position separated from the sludge return path.

[0020] It is necessary to take care that the air discharged from the air discharge pipe is not returned to the anoxic tank via the sludge return path. Therefore, it is preferable to place the air discharge pipe upstream of the organic wastewater flow into which the activated sludge flows from the anoxic tank, or at a position separated from the sludge return path.

[0021] The third characteristic configuration is that, in addition to the first or second characteristic configuration described above, the discharge pressure of the air discharge pipe is adjusted to a discharge pressure equivalent to the discharge pressure of the aeration device.

[0022] If the discharge pressure of the air discharge pipe and the discharge pressure of the aeration device are different, there is a risk that the amount of air sent to other lines will fluctuate significantly depending on which one is switched to when filtration is suspended, making it impossible to operate properly. However, if the discharge pressure of the air discharge pipe is adjusted to the same discharge pressure as the aeration device, a stable state of air sending to other lines can be achieved. For this reason, for example, it is necessary to provide a throttle mechanism in the air discharge pipe or to interpose a constant flow valve that can supply a constant flow rate even if there are pressure fluctuations so that the discharge pressure is the same as the aeration device discharge pressure.

[0023] The third characteristic configuration is, in addition to any of the first to third characteristic configurations described above, in that the biological treatment tank has a plurality of biological treatment units connected in series, with the anoxic tank and the aerobic tank being treated as a pair of biological treatment units, the sludge return path is configured to return activated sludge from the aerobic tank located at the most downstream to the anoxic tank located at the most upstream, organic wastewater is divided and supplied to the anoxic tanks of each biological treatment unit, and the organic wastewater is biologically treated by repeating denitrification treatment in the anoxic tank and nitrification treatment in the aerobic tank, and based on the DO value of one of the aerobic tanks, air is selectively supplied to either the air release pipe or the aeration device via a switching mechanism provided in the membrane separation device when filtration of the membrane separation device provided in the aerobic tank is suspended.

[0024] By adopting such a biological treatment tank, the effective sludge circulation ratio through the sludge return path can be set to a large value, resulting in a high nitrogen removal rate. If the DO value increases, the DO value can be reduced while suppressing membrane fouling in the membrane separation device by selectively supplying air to either the air release pipe or the aeration device via the switching mechanism when filtration in the membrane separation device is paused.

[0025] A characteristic configuration of the operating method of an organic wastewater treatment device according to the present invention is a method of operating an organic wastewater treatment device having any of the first to fourth characteristic configurations described above, in which, when the DO value of the aerobic tank exceeds a predetermined threshold value, a switch is made to supply air from the aeration device to the air release pipe via the switching mechanism when filtration of the membrane separation device is paused. Effect of the Invention

[0026] As described above, according to the present invention, it is possible to provide an organic wastewater treatment device and an operating method for the organic wastewater treatment device that can suppress an increase in the nitrate nitrogen concentration in the anoxic tank while avoiding clogging of the membrane of the membrane separation device, even when the amount of sludge returned from the aerobic tank to the anoxic tank is increased. [Brief description of the drawings]

[0027] [Figure 1] FIG. 2A is a plan view of an organic wastewater treatment device according to the present invention, and FIG. 2B is a cross-sectional view of a main part of the organic wastewater treatment device according to the present invention. [Diagram 2] An explanatory diagram of a membrane element installed in a membrane separation device [Diagram 3] FIG. 11 is a cross-sectional view illustrating a main part of an organic wastewater treatment device according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] Hereinafter, an embodiment of an organic wastewater treatment apparatus and an operating method for the organic wastewater treatment apparatus according to the present invention will be described. [First aspect] 1(a) and (b) show a first embodiment of an organic wastewater treatment apparatus and an operating method for the organic wastewater treatment apparatus. The organic wastewater treatment device 1 is a water treatment device that biologically treats raw nitrogen-containing organic wastewater in activated sludge to obtain treated water, and is equipped with a biological treatment tank 2 in which a pair of anoxic tank 10 and aerobic tank 20 are used as a biological treatment unit, and multiple biological treatment units (four pairs of biological treatment units in this embodiment) are arranged in series along the flow of the organic wastewater in an endless manner.

[0029] In addition, multiple biological treatment units may be formed by dividing a single biological treatment tank 2 into multiple areas, or the biological treatment tank 2 may be formed by arranging multiple pairs of individual anoxic tanks 10 and aerobic tanks 20 along the flow of the organic wastewater.

[0030] Alternatively, the biological treatment tank 2 may be configured in a straight line, and a waterway or pipeline may be provided separately as a sludge return path for returning activated sludge from the most downstream aerobic tank 20 to the most upstream anoxic tank 10.

[0031] Raw organic wastewater is divided into roughly equal amounts and supplied to each anoxic tank 10 via the raw water supply line 3, where it is subjected to denitrification treatment, an anaerobic process, before flowing into the downstream aerobic tank 20 for aerobically treated water. A membrane separation device 30 is immersed in each aerobic tank 20, and an auxiliary aeration device 40 for aerobic treatment is installed nearby.

[0032] An air lift pump AP is installed in the anoxic tank 10 (10a) on the most upstream side, and organic wastewater is sent to the aerobic tank 20 (20a) on the downstream side together with activated sludge by an upward flow in the air lift pipe generated by air bubbles supplied from a blower B through a valve V10, and then the organic wastewater flows down by gravity in the following order: anoxic tank 10 (10b), aerobic tank 20 (20b), anoxic tank 10 (10c), aerobic tank 20 (20c), anoxic tank 10 (10d), and aerobic tank 20 (20d). Because the air lift pump AP is installed in the anoxic tank 10, there is no increase in dissolved oxygen (DO) in the anoxic tank 10, compared to a case in which an air lift pump AP is installed in the aerobic tank 20 and liquid is sent to the anoxic tank 10.

[0033] In this embodiment, four pairs of biological treatment units are arranged endlessly along the flow of organic wastewater, and the aerobic tank 20 (20d) located at the most downstream position and the anoxic tank 10 (10a) located at the most upstream position are adjacent to each other separated by a partition, and a sludge return path 4 that returns activated sludge from the most downstream aerobic tank 20 (20d) to the most upstream anoxic tank 10 (10a) is formed in part of the partition.

[0034] A partition wall W1 is formed between the anoxic tank 10 and the aerobic tank 20, and a notch portion 11 (see Figure 1 (b)) is provided in a part of the upper end side of the partition wall W1 so that organic wastewater including activated sludge in the anoxic tank 10 can overflow into the aerobic tank 20.

[0035] A partition wall W2 is formed between the aerobic tank 20 and the anoxic tank 10, and an outlet 21 for organic wastewater containing activated sludge is provided at a position corresponding to the vicinity of the bottom of the membrane separation device 30 in the vertical direction. The upper end of the opening that serves as the outlet 21 is submerged, and is provided at a position 30 cm or less from the water surface in the aerobic tank 20. The outflow flow rate of the activated sludge from the outlet 21 is set to 0.5 m / sec or less. The outlet 21 formed in the most downstream aerobic tank 20 (20d) serves as the sludge return path 4 described above. The arrows indicated by two-dot chain lines in FIG. 1(a) indicate that activated sludge flows in biological treatment unit units to form a circulating flow.

[0036] The membrane separation device 30 includes a plurality of membrane elements 31 and an aeration device 32 installed below the membrane elements 31 (see FIG. 1(b)). The plurality of membrane elements 31 are arranged and housed in a casing at a certain interval in two upper and lower stages so that each membrane surface is in a vertical position.

[0037] 2, the membrane element 31 is configured by disposing separation membranes 31b on both sides of a resin membrane support 31a equipped with a water collection pipe 31c at the top. In this embodiment, the separation membrane 31b is configured as a microfiltration membrane having a nominal pore size of about 0.4 μm and equipped with a porous organic polymer membrane on the surface of a nonwoven fabric.

[0038] The type of separation membrane 31b and the membrane element 31 are not limited to the above-mentioned embodiments, and any type of separation membrane and any form of membrane element (hollow fiber membrane element, tubular membrane element, monolith membrane element, etc.) can be used.

[0039] The treated water that has permeated the separation membrane 31b flows along the grooves formed in the membrane support 31a to the water collection pipe 31c, and as shown in Figures 1(a) and (b), flows from the water collection pipe 31c through the header pipe 34 into the air separation tank 35, and is collected in the treated water tank 37 via the liquid delivery pipe 36 connected to the air separation tank 35.

[0040] Each header pipe 34 is provided with a valve V5, V6, V7, V8 for adjusting the flow rate, and a suction pump P is disposed in the liquid feed pipe 36. The amount of membrane permeated water from each membrane separation device 30 is adjusted by adjusting the pressure by the suction pump P and the opening degree of the valves V5, V6, V7, V8.

[0041] In order to detect the transmembrane pressure difference of the membrane separation device 30, a pressure sensor Pm is provided upstream of the valves V5, V6, V7, and V8 in each header pipe 34. In the figure, the symbol M indicates a motor for adjusting the opening of the valve. The treated water discharge path is a path in which water flows from the water collection pipe 31c through the header pipe 34 into the air separation tank 35, and is collected in the treated water tank 37 via a liquid delivery pipe 36 connected to the air separation tank 35.

[0042] Four first auxiliary blower pipes Ts are branched and connected to a main blower pipe Tm connected to a blower B, and each of the first auxiliary blower pipes Ts is connected to an aeration device 32. The first auxiliary blower pipes Ts are provided with flow rate limiting valves V1, V2, etc. corresponding to the membrane separation tanks 30 installed in each aerobic tank 20, so that the aeration amount and the start and stop of aeration can be controlled.

[0043] In addition, a second auxiliary blower duct Ta is branched and connected to the main blower duct Tm, and air for the air lift is supplied through a valve V10 provided in the second auxiliary blower duct Ta. Furthermore, four third auxiliary air blowing pipes Tb are branched and connected to the main air blowing pipe Tm, and each of the third auxiliary air blowing pipes Tb is connected to an air discharge pipe 5 installed in each aerobic tank 20. Each of the third auxiliary air blowing pipes Tb is provided with a flow rate limiting valve Va, Vb, etc., so that the start and stop of air discharge can be controlled.

[0044] The organic wastewater is aerated together with the activated sludge in the aerobic tank 20 by the auxiliary aeration device 40, and the organic matter is decomposed and ammonia nitrogen is nitrified into nitrate nitrogen, and a portion of the wastewater is separated into solid and liquid form as treated water by the membrane separation device 30. The organic wastewater that has been nitrified in the aerobic tank 20 flows together with the activated sludge into the adjacent anoxic tank 10 on the downstream side, where denitrification treatment progresses, in which nitrate nitrogen is reduced and removed to nitrogen gas.

[0045] When the inflow rate of raw water per unit time is Q, the inflow rate of raw water to each anoxic tank 10 is Q / 4, a total amount of treated water with a permeate volume of Q is withdrawn from each membrane separation device 30, and 3Q of activated sludge from the most downstream aerobic tank 20 (20d) is returned to the most upstream anoxic tank 10 (10a) via the sludge return path, the effective sludge circulation ratio becomes 3 x 4, and a high circulation ratio of 12Q can be realized, resulting in a high nitrogen removal rate.

[0046] The organic wastewater treatment system 1 is provided with multiple measuring devices, such as a flow meter that measures the inflow rate of organic wastewater, a level gauge that measures the tank liquid level, pressure sensors that measure the transmembrane pressure difference of each membrane separation device, and a measuring device S that is provided in the treatment water tank 37 and measures the TN, NO3-N concentration, and NH4-N concentration of the treated water. Also provided is a control unit 60 that serves as a control device that controls the operation of the organic wastewater treatment system 1 based on the values ​​measured by these measuring devices. The control unit 60 is composed of a control panel equipped with a computer consisting of an arithmetic circuit, an input circuit, an output circuit, etc.

[0047] The control unit 60 repeatedly operates each membrane separation device 30 in two modes, a filtration operation mode and a relaxation operation mode, while monitoring the amount of raw water inflow measured by these measuring devices, the water level in the biological treatment tank 2, the values ​​of each pressure sensor Pm, and the value of the total nitrogen (TN) concentration measuring device S provided in the treated water tank 37.

[0048] The filtration operation state refers to a state in which the membrane permeate water is extracted as treated water from the water collection pipe 31c while aeration is being performed by the aeration device 32, and the relaxation operation state refers to a state in which the surface of the separation membrane 31b is cleaned by an upward flow generated by air bubbles by aeration performed by the aeration device 32 with the valve provided on the header pipe 34 closed or the suction pump P stopped. The relaxation operation state can also be expressed as a filtration pause state. The control unit 60 repeats a filtration operation for a first predetermined time (e.g., 9 minutes) and a relaxation operation for a second predetermined time (e.g., 1 minute).

[0049] In such an organic wastewater treatment device 1, the actual residence time in the anoxic tank 10 is reduced to one-fourth that of a typical MBR, so it is necessary to suppress the DO value in the aerobic tank 20 in order to maintain good denitrification treatment in the anoxic tank 10. The increase in the DO value becomes particularly noticeable when the load of the organic wastewater is low.

[0050] Therefore, the control unit 60 is configured to selectively supply air to either the air discharge pipe 5 or the aeration device 32 via the switching mechanism when the filtration of the membrane separation device 30 is suspended, based on the value of the DO sensor Sdo installed in the aerobic tank 20. In this embodiment, when the DO value exceeds 1.5 mg / L, the control unit 60 switches to blow air to the air discharge pipe 5 when the filtration of the membrane separation device 30 is suspended, and when the DO value falls below 0.8 mg / L, the control unit 60 switches to aeration from the aeration device 32 when the filtration of the membrane separation device 30 is suspended. That is, the appropriate range of the DO value is set to a range of 0.8 mg / L to 1.5 mg / L.

[0051] The above-mentioned valves V1, V2... and valves Va, Vb... are controlled to be opened and closed in a reciprocal manner by the control unit 60, so that when the valves V1, V2... are controlled to be open, the valves Va, Vb... are controlled to be closed, and when the valves Va, Vb... are controlled to be open, the valves V1, V2... are controlled to be closed.

[0052] In other words, the valves V1, V2,... and the valves Va, Vb,... constitute a switching mechanism that switches between supplying air to each aeration device 32 provided in the membrane separation apparatus 30 and supplying air to each air discharge pipe 5. Note that the switching mechanism may be constituted by a three-way valve that switches whether the air blown from the main blower pipe Tm is led to each aeration device 32 or to each air discharge pipe 5.

[0053] In either configuration, since the main blower pipe Tm blows air to multiple organic wastewater treatment devices, if the discharge pressure when leading to each air discharge pipe 5 is lower than the discharge pressure when leading to each aeration device 32, there is a risk that the amount of air sent to the other organic wastewater treatment devices will decrease. For this reason, the discharge pressure of the air discharge pipe 5 needs to be adjusted to a discharge pressure equivalent to the discharge pressure of the aeration device 32.

[0054] Therefore, in order to ensure that the discharge pressure is equivalent to the discharge pressure of the aeration device 32, a throttle mechanism is provided in the air discharge pipe 5, or a constant flow valve is interposed that can supply a constant flow rate even if there are pressure fluctuations.

[0055] The air release pipe 5 must be located in the aerobic tank 20 at a position upstream of the organic wastewater flow, away from the inlet of the sludge deflection path, or away from the opening 21 that actually functions as the sludge return path, in order to prevent the air released from the air release pipe 5 from being returned to the anoxic tank 10 via the sludge return path or the like.

[0056] The control unit 60 may also control the auxiliary air diffuser 40 to stop diffusing air at the same time as switching the supply of air from the aeration device 32 to the air discharge pipe 5 via the switching mechanism during relaxation operation of the membrane separation device 30.

[0057] When the DO value of the aerobic tank 20 exceeds the appropriate range, the control unit 60 switches to synchronously supplying air from the aeration device 32 to the air release pipe 5 for each cycle of relaxation operation for the membrane separation devices 30 of all of the aerobic tanks 20 constituting the organic wastewater treatment device 1, and when the DO value of the aerobic tank 20 falls within the appropriate range, the control unit 60 switches to synchronously supplying air from the air release pipe 5 to the aeration device 32 for each cycle of relaxation operation for the membrane separation devices 30 of all of the aerobic tanks 20 constituting the organic wastewater treatment device 1.

[0058] The criterion for "when the DO value of the aerobic tank 20 exceeds the appropriate range" can be the average value of the DO values ​​of multiple aerobic tanks 20 or the DO values ​​of all the aerobic tanks 20, but is preferably the DO value of any one of the aerobic tanks 20. Also, the criterion for "when the DO value of the aerobic tank 20 falls within the appropriate range" can be the average value of the DO values ​​of multiple aerobic tanks 20 or the DO values ​​of all the aerobic tanks 20, but is preferably the DO value of any one of the aerobic tanks 20.

[0059] As described above, the operating method of the organic wastewater treatment device according to the present invention operates such that, when the DO value in the aerobic tank 20 exceeds a predetermined threshold value, during relaxation operation of the membrane separation device 30, the switching mechanism is used to switch so that air is supplied from the aeration device 32 to the air discharge pipe 5. It goes without saying that during membrane filtration operation, air is supplied from the air discharge pipe 5 to the aeration device 32 via the switching mechanism.

[0060] Another embodiment will be described below. In the above-described embodiment, the organic wastewater treatment device 1 is configured with a biological treatment tank 2 in which a pair of anoxic tank 10 and aerobic tank 20 are used as biological treatment units, and four pairs of biological treatment units are arranged in series and endlessly along the flow of organic wastewater. However, the number of biological treatment units may be any number greater than or equal to two pairs.

[0061] [Second aspect] A second embodiment of the organic wastewater treatment device and the operating method thereof is shown in Fig. 3. The same functional blocks as those in the first embodiment are denoted by the same reference numerals. The organic wastewater treatment device 1 includes a biological treatment tank 2 consisting of an anoxic tank 10 and an aerobic tank 20 in which a membrane separation device 30 is immersed in activated sludge, and a sludge return path 4 for returning activated sludge from the aerobic tank 20 to the anoxic tank 10.

[0062] A first auxiliary air duct Ts is branched off and connected to a main air duct Tm connected to a blower B, and each aeration device 32 is connected to the auxiliary air duct Ts. A flow rate limiting valve V1 is provided in the auxiliary air duct Ts corresponding to the membrane separation tank 30 installed in the aerobic tank 20, so that the aeration amount and the start and stop of aeration can be controlled.

[0063] A third auxiliary air blower pipe Tb is branched and connected to the main air blower pipe Tm, and an air discharge pipe 5 installed in each aerobic tank 20 is connected to the third auxiliary air blower pipe Tb. A flow rate limiting valve Va is provided in the third auxiliary air blower pipe Tb, so that the start and stop of air discharge can be controlled.

[0064] The valves V1 and Va constitute a switching mechanism that switches between supplying air to the aeration device 32 provided in the membrane separation device 30 and supplying air to the air discharge pipe 5. As in the first embodiment, the switching mechanism can also be constituted by a three-way valve that switches between directing the air from the main blower pipe Tm to the aeration device 32 or to each air discharge pipe 5.

[0065] As in the first embodiment, the membrane separation device 30 is controlled by the control unit 60 so as to repeatedly perform filtration operation and filtration pause. If the amount of sludge returned via the sludge return path 4 is increased, the actual retention time in the anoxic tank is shortened, and it becomes necessary to suppress the DO value of the aerobic tank 20.

[0066] Therefore, the control unit 60 is configured to selectively supply air to either the air release pipe 5 or the aeration device 32 via a switching mechanism when filtration of the membrane separation device 30 is suspended, based on the value of the DO sensor Sdo installed in the aerobic tank 20. Based on the DO value of the aerobic tank 20, the control unit 60 is configured to selectively supply air to either the air release pipe 5 or the aeration device 32 via a switching mechanism when filtration of the membrane separation device 30 is suspended.

[0067] In this embodiment as well, when the DO value exceeds 1.5 mg / L, the system switches to blowing air to the air release pipe 5 when filtration in the membrane separation device 30 is suspended, and when the DO value falls below 0.8 mg / L, the system switches to aeration from the aeration device 32 when filtration in the membrane separation device 30 is suspended. In other words, the present invention can be applied even if the organic wastewater treatment device 1 is configured to include one anoxic tank 10 and one aerobic tank 20, and is provided with a sludge return path for returning activated sludge from the aerobic tank 20 to the anoxic tank 10.

[0068] The present invention can also be applied to a cleaning air volume control system such as that described in Patent Document 2. That is, the present invention is incorporated into a cleaning air volume control system that includes a representative pond pressure gauge that is installed in a representative pond to which air is supplied at a rated air volume and that measures the membrane pressure difference of a representative pond separation membrane unit that filters the water to be treated flowing into the representative pond, a target value setting unit that is installed in a control pond to which air is supplied at a predetermined air volume less than the rated air volume and that sets a target value of the membrane pressure difference or membrane filtration resistance of the control pond based on an upper limit of the membrane pressure difference or membrane filtration resistance of the control pond separation membrane unit that filters the same water to be treated as the water to be treated flowing into the representative pond, and information on the change over time in the membrane filtration resistance or the membrane pressure difference of the representative pond separation membrane unit calculated based on the membrane pressure difference of the representative pond separation membrane unit, and an air volume control unit that controls the air volume supplied to the control pond based on the target value, and if the DO value is within an appropriate range, the air volume supplied to the control pond is controlled based on the cleaning air volume control system, and air is released from the air release pipe 5 only when the DO value exceeds the appropriate range.

[0069] The above-described embodiments are merely examples of the present invention, and the present invention is not limited to these descriptions. It goes without saying that the specific configurations of each part can be appropriately modified and designed within the scope in which the effects of the present invention are achieved. [Explanation of symbols]

[0070] 1: Organic wastewater treatment equipment 2: Biological treatment tank 3: Raw water supply channel 4: Sludge return route 5: Air release tube 10:Anoxic tank 11: Notch 20:Aerobic tank 21: Outflow section (sludge return path) 30: Membrane separation equipment 32: Aeration device 40: Auxiliary air diffuser 60: Control unit (control device)

Claims

1. An organic wastewater treatment apparatus comprising: a biological treatment tank including at least an anoxic tank and an aerobic tank in which a membrane separation device is immersed in activated sludge; and a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, the membrane separation device being configured to repeat filtration operation and filtration pause, and for biologically treating nitrogen-containing organic wastewater in the activated sludge, The aerobic tank is provided with an air discharge pipe, and a switching mechanism is provided for switching between supplying air to an aeration device provided in the membrane separation device and supplying air to the air discharge pipe, the organic wastewater treatment device being configured to selectively supply air to either the air release pipe or the aeration device via the switching mechanism when filtration of the membrane separation device is suspended based on the DO value of the aerobic tank.

2. 2. The organic wastewater treatment apparatus according to claim 1, wherein the air discharge pipe is disposed upstream of the flow of the organic wastewater or at a position separated from the sludge return path.

3. 3. The organic wastewater treatment apparatus according to claim 1, wherein the discharge pressure of said air discharge pipe is adjusted to be equal to the discharge pressure of said aeration device.

4. The biological treatment tank is a pair of biological treatment units, each consisting of an anoxic tank and an aerobic tank, and a plurality of biological treatment units are connected in series. The sludge return path is configured to return activated sludge from the aerobic tank disposed at the most downstream position to the anoxic tank disposed at the most upstream position. Organic wastewater is divided and supplied to the anoxic tanks of each biological treatment unit, and the organic wastewater is biologically treated by repeating denitrification treatment in the anoxic tank and nitrification treatment in the aerobic tank.

4. The organic wastewater treatment device according to claim 1, wherein, based on a DO value of any of the aerobic tanks, air is selectively supplied to either the air release pipe or the aeration device via a switching mechanism provided in the membrane separation device when filtration of the membrane separation device provided in the aerobic tank is suspended.

5. A method for operating an organic wastewater treatment apparatus according to any one of claims 1 to 4, A method for operating an organic wastewater treatment device, comprising: switching, via the switching mechanism, to supply air from the aeration device to the air release pipe during relaxation operation of the membrane separation device when the DO value of the aerobic tank exceeds a predetermined threshold value.

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