Operating method for organic wastewater treatment apparatus and organic wastewater treatment apparatus

Abstract

An organic wastewater treatment apparatus is operated by switching between a first operation mode and a second operation mode. The first operation mode includes supplying organic wastewater to a dual-use tank and performing an anaerobic treatment process therein, performing a nitrification-denitrification process in a membrane bioreactor treatment tank, and extracting membrane-permeated water from a membrane separation device arranged in the membrane bioreactor treatment tank as treated water. The second operation mode includes supplying the organic wastewater to the dual-use tank and performing an aerobic treatment process therein, supplying mixed liquid containing activated sludge from the dual-use tank to both of the membrane bioreactor treatment tank and a sedimentation tank, performing a nitrification-denitrification process in the membrane bioreactor treatment tank, extracting membrane-permeated water from the membrane separation device arranged in the membrane bioreactor treatment tank as treated water, and also extracting solid-liquid separated liquid from the sedimentation tank as treated water.

Claims

1. A method for operating an organic wastewater treatment apparatus including a dual-use tank, a membrane bioreactor treatment tank, and a sedimentation tank, the method comprising: operating the organic wastewater treatment apparatus in a first operation mode, including: supplying organic wastewater to the dual-use tank and performing an anaerobic treatment process in the dual-use tank; then performing a nitrification-denitrification process in the membrane bioreactor treatment tank on the organic wastewater from the anaerobic treatment process; and extracting membrane-permeated water from a membrane separation device arranged in the membrane bioreactor treatment tank as treated water; and operating the organic wastewater treatment apparatus in a second operation mode, including: supplying the organic wastewater to the dual-use tank and performing an aerobic treatment process in the dual-use tank; then supplying mixed liquid from the dual-use tank to the membrane bioreactor treatment tank and the sedimentation tank, the mixed liquid containing activated sludge; then performing a nitrification-denitrification process in the membrane bioreactor treatment tank on the mixed liquid from the aerobic treatment process and extracting membrane-permeated water from the membrane separation device arranged in the membrane bioreactor treatment tank as treated water while extracting solid-liquid separated liquid from the sedimentation tank as treated water.

2. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the operating the organic wastewater treatment apparatus in the second operation mode further includes: returning the mixed liquid from the membrane bioreactor treatment tank to the dual-use tank.

3. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the operating the organic wastewater treatment apparatus in the second operation mode further includes: returning the mixed liquid from the sedimentation tank to the membrane bioreactor treatment tank.

4. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the operating the organic wastewater treatment apparatus in the second operation mode further includes: dividing and supplying the organic wastewater to the membrane bioreactor treatment tank and the dual-use tank.

5. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the operating the organic wastewater treatment apparatus in the second operation mode further includes: controlling an amount of the treated water withdrawn from the membrane bioreactor treatment tank and an amount of the treated water withdrawn from the sedimentation tank according to a position of a sludge interface in the sedimentation tank or quality of the treated water.

6. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the sedimentation tank is provided with a filtration device, such that filtered water that has passed through the filtration device is withdrawn as the treated water in the second operation mode.

7. The method for operating the organic wastewater treatment apparatus according to claim 1, wherein the operating the organic wastewater treatment apparatus in the first operation mode further comprises: adding a flocculant to the membrane bioreactor treatment tank, and wherein the operating the organic wastewater treatment apparatus in the second operation mode further comprises: adding a flocculant to the sedimentation tank or a flow inlet of the sedimentation tank.

8. The method for operating the organic wastewater treatment apparatus according to claim 7, further comprising: switching between the first operation mode and the second operation mode depending on any of the following indicators: a flow rate of the organic wastewater, a COD load, a NH.sub.4—N load, a transmembrane pressure difference, and a water temperature.

9. The method for operating the organic wastewater treatment apparatus according to claim 1, further comprising: performing a cleaning process to clean the sedimentation tank after switching from the second operation mode to the first operation mode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 A is a diagram illustrating a first operation mode in a method for operating an organic wastewater treatment apparatus according to the present invention.

(2) FIG. 1B is a diagram illustrating a second operation mode in the method for operating the organic wastewater treatment apparatus according to the present invention.

(3) FIG. 2 is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with another embodiment.

(4) FIG. 3A is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(5) FIG. 3B is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(6) FIG. 4A is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(7) FIG. 4B is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(8) FIG. 5A is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(9) FIG. 5B is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(10) FIG. 6A is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(11) FIG. 6B is a diagram illustrating the second operation mode in the method for operating the organic wastewater treatment apparatus in accordance with yet another embodiment.

(12) FIG. 7 is a diagram illustrating a circulating-type MBR.

(13) FIG. 8 is a diagram schematically illustrating a method for operating a conventional organic wastewater treatment facility.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(14) A method for operating an organic wastewater treatment apparatus and the organic wastewater treatment apparatus according to the present invention will be described by referring to the drawings.

(15) As shown in FIGS. 1A and 1B, the organic wastewater treatment apparatus 100 is an apparatus for purifying by biologically treating organic wastewater such as sewage introduced as raw water, and discharging treated water into a river or the like. The apparatus 100 includes an initial sedimentation basin 10, a dual-use tank 20 which can be switchably used for an aerobic treatment process and an anaerobic treatment process, a membrane bioreactor treatment tank 30 which performs a nitrification-denitrification process, and from which membrane permeated water from a membrane separation device 60 immersed therein is extracted as the treated water, a final sedimentation basin 70, and a disinfection tank 90, among others.

(16) The dual-use tank 20 is provided with an agitator and an aeration device. When the aeration device is activated, the dual-use tank 20 is conditioned to an aerobic state and functions as an aerobic tank, while the aeration device is stopped, the dual-use tank 20 is conditioned to an anaerobic state and functions as an anaerobic tank in which the agitator is activated. In addition, the final sedimentation basin 70 functions as the sedimentation tank 70 of the present invention, and the sedimentation tank 70 may be provided with an inclined plate 71 to improve a sedimentation efficiency.

(17) The organic wastewater treatment apparatus 100 further includes a raw water supply path 1 for supplying the organic wastewater to the dual-use tank 20, a first mixed liquid path 2 for allowing the mixed liquid in the dual-use tank 20 to flow into the membrane bioreactor treatment tank 30, a second mixed liquid path 3 for allowing the mixed liquid in the dual-use tank 20 to flow into the sedimentation tank 70, a third mixed liquid path 4 for returning the mixed liquid in the membrane bioreactor treatment tank 30 to the dual-use tank 20, and a fourth mixed liquid path 5 for returning the mixed liquid in the sedimentation tank 70 to the membrane bioreactor treatment tank 30.

(18) The membrane bioreactor treatment tank 30 is a biological treatment tank in which organic bodies are decomposed by microorganisms, and may include two tanks: an anoxic tank 40 provided with an agitator, and an aerobic tank 50 provided with an aeration device (not shown in the figures) and a membrane separation device 60 immersed therein. A sludge return path 6 returns the sludge from the aerobic tank 50 to the anoxic tank 40, and nitrate-nitrogen, which has been nitrified from ammoniacal nitrogen in the aerobic tank 50, is also returned via the sludge return path 6 to the anoxic tank 40 in which a denitrification process is performed.

(19) The organic wastewater treatment apparatus 100 is configured to be operated in two different operation modes by switching between a first operation mode (see FIG. 1A) and a second operation mode (see FIG. 1B). In the first operation mode, as shown in FIG. 1A, the organic wastewater is supplied to the dual-use tank 20 in which an anaerobic treatment process is performed thereon, and then a nitrification-denitrification process is performed thereon in the membrane bioreactor treatment tank 30, and the membrane permeated liquid of the membrane separation device 60 disposed in the membrane bioreactor treatment tank 30 is extracted as treated water. In the second operation mode, as shown in FIG. 1B, the organic wastewater is supplied to the dual-use tank 20 in which an aerobic treatment process is performed thereon, and then mixed liquid (which is the aerobically-treated wastewater containing activated sludge) is supplied to the membrane bioreactor treatment tank 30 and to the sedimentation tank 70, and then a nitrification-denitrification process is performed in the membrane bioreactor treatment tank 30 and the membrane permeated liquid from the membrane separation device 60 disposed in the membrane bioreactor treatment tank 30 is extracted as the treated water, while solid-liquid separated liquid from the sedimentation tank 70 is also extracted as the treated water.

(20) The switching between the first operation mode and the second operation mode may be performed using one of the following as an indicator: a flow rate of the organic wastewater, a COD load, a NH.sub.4—N load, a transmembrane pressure difference, and a water temperature. For example, the apparatus 100 may be configured to operate in the second operation mode when the flow rate of the organic wastewater increases excessively, when the transmembrane pressure difference of the membrane separation device 60 increases abnormally, or when the treatment efficiency decreases due to a water temperature drop in winter, and to operate in the first operation mode when the COD load and the NH.sub.4—N load increase abnormally.

(21) In the first operation mode, as shown in FIG. 1A, the apparatus 100 is configured as a UCT-type MBR performing an A2O treatment process, including the dual-use tank 20 which is functioning as an anaerobic tank, and the membrane bioreactor treatment tank 30 including the anoxic tank 40 and the aerobic tank 50.

(22) For example, the organic wastewater (post-sedimentation water) with a flow rate of 2Q, which has been solid-liquid separated in the initial sedimentation basin 10 provided in the raw water supply path 1, becomes mixed liquid (also referred to as mixed water) mixed with activated sludge in the dual-use tank 20 and is anaerobically treated therein, which is then denitrified in the anoxic tank 40 of the membrane bioreactor treatment tank 30 and aerobically treated in the aerobic tank 50. The thus treated mixed water is solid-liquid separated by the membrane separation device 60 and extracted as permeated water, and then is discharged to a river or the like as the treated water with a maximum flow rate of 2Q.

(23) The mixed liquid whose ammoniacal nitrogen is nitrified in the aerobic tank 50 is returned to the anoxic tank 40 with a flow rate of 3Q through the sludge return path 6, and the mixed liquid is denitrified in the anoxic tank 40. The mixed liquid is also returned, with a flow rate of 1Q, from the anoxic tank 40 to the dual-use tank 20 via the third mixed liquid path 4, and phosphorus in the mixed liquid is desorbed in the dual-use tank 20. As a result, the phosphorus contained in the organic wastewater is excessively absorbed in the aerobic tank 50 on the downstream side, and thus the phosphorus concentration of the membrane permeated water is greatly reduced.

(24) By gradually returning the sludge from the aerobic tank 50 to the dual-use tank 20 via the sludge return path 6 and the third mixed liquid path 4, the MLSS concentration in the dual-use tank 20 can be significantly lowered compared to that in the aerobic tank 50. In the example of FIG. 1A, the MLSS concentration in the aerobic tank 50 is 1000 mg/L, while the MLSS concentration in the anoxic tank 40 is 7500 mg/L, and the MLSS concentration in the dual-use tank 20 is 3750 mg/L.

(25) In the second operation mode of the apparatus 100, as shown in FIG. 1B, the aeration device 21 provided in the dual-use tank 20 is activated (i.e. operated) to make the dual-use tank 20 function as an aerobic tank, that is, an initial adsorption tank, and then the nitrification-denitrification process is conducted in the membrane bioreactor treatment tank 30 having the anoxic tank 40 and the aerobic tank 50.

(26) For example, the organic wastewater (post-sedimentation water) with a flow rate of 3Q (which is higher than the flow rate of 2Q of the organic wastewater in the first operation mode mentioned above), which has been solid-liquid separated in the initial sedimentation basin 10 provided in the raw water supply path 1, becomes the mixed liquid mixed with the activated sludge in the dual-use tank 20. After the initial adsorption process is performed in the dual-use tank 20, a half of the mixed liquid with the flow rate of 1.5Q is supplied to the membrane bioreactor treatment tank 30 via the first mixed liquid path 2, and the other half of the mixed water with a flow rate of 1.5Q is supplied to the sedimentation tank 70 via the second mixed liquid path 3.

(27) In the membrane bioreactor treatment tank 30, from among the mixed liquid which has been denitrified in the anoxic tank 40 on the upstream side and then aerobically treated in the aerobic tank 50, the mixed liquid with a flow rate of 2Q is returned to the anoxic tank 40 via the sludge return path 6, and then the mixed liquid in the anoxic tank 40 is returned therefrom, with a flow rate of 2Q, to the dual-use tank 20 via the third mixed liquid path 4.

(28) Then, the permeated water, which has been solid-liquid separated by the membrane separation device 60 in the aerobic tank 50, is discharged into a river or the like with a maximum flow rate of 2Q. In addition, the treated water, which has been solid-liquid separated in the sedimentation tank 70, is discharged through the disinfection tank 90 into a river or the like, while the mixed liquid is also returned, as return sludge, from the sedimentation tank 70 to the membrane bioreactor treatment tank 30 through the fourth mixed liquid path 5 with a flow rate of 0.5Q.

(29) By employing a lower circulation rate of the sludge from the membrane bioreactor treatment tank 30 to the dual-use tank 20 than that in the first operation mode, the MLSS concentration in the dual-use tank 20 can be further reduced, and by sending a portion of the sludge which has been treated by the initial adsorption process to the sedimentation tank 70, the solid-liquid separation performance in the sedimentation tank 70 can be maintained.

(30) In other words, by operating the organic wastewater treatment apparatus 100 in the first operation mode when the volume of the organic wastewater to be treated or the treatment load has a steady value, and in the second operation mode when the volume of the organic wastewater to be treated or the treatment load increases excessively, it becomes possible to avoid excessive investment in the facility, such as installing a number of membrane separation devices 60 corresponding to the peak processing volume of the organic wastewater, while reducing the BOD and the COD load of the treated water as a whole, without imposing an excessive load on the membrane separation device. Accordingly, even if the organic wastewater has the peak water volume, the amount of water permeating the membrane separation device 60 can be controlled, and thus it is also possible to reduce the frequency of chemical cleaning of the membranes of the membrane separation device 60.

(31) The Initial adsorption refers to such a process including a physical absorption in which fine particles and dissolved organic matter in the organic wastewater are physically absorbed onto the surface of the activated sludge by the sticky gelatin substance secreted by the aerobic microorganisms in the activated sludge, and a biosorption in which the physically absorbed organic matter is quickly taken up by the microorganisms, resulting in a significant decrease in the BOD within a few tens of minutes after the activated sludge and the organic wastewater come into contact each other.

(32) Since the mixed liquid is returned from the membrane bioreactor treatment tank 30 to the dual-use tank 20 via the third mixed liquid path 4 in both of the first and second operation modes, the mixed liquid returned from the membrane bioreactor treatment tank 30 promotes the anaerobic treatment, i.e., denitrification and/or dephosphorization process in the dual-use tank 20 during the first operation mode, while the mixed liquid returned from the membrane bioreactor treatment tank 30 promotes the aerobic treatment, i.e., the initial adsorption process in the dual-use tank 20 during the second operation mode.

(33) In addition, since the mixed liquid is returned from the sedimentation tank 70 to the membrane bioreactor treatment tank 30 via the fourth mixed liquid path 5, the MLSS concentration in the membrane bioreactor treatment tank 30 is prevented from decreasing by the mixed liquid returned from the sedimentation tank 70.

(34) The organic wastewater treatment apparatus 100 described above may further include a flocculant adding device 80, which is configured to switch the addition target such that the flocculant is added to the membrane bioreactor treatment tank 30 in the first operation mode, while the flocculant is added to the sedimentation tank 70 in the second operation mode. For example, the flocculant adding device 80 can be constructed with a chemical tank filled with the flocculant, supply paths provided from the chemical tank to the membrane bioreactor treatment tank 30 and the sedimentation basin 70, and valves provided in each supply path.

(35) In the first operation mode, the flocculant added to the membrane bioreactor treatment tank 30 causes the dissolved phosphorus to be absorbed by the activated sludge and then separated by the membrane filtration, while in the second operation mode, the dissolved phosphorus is absorbed by the activated sludge flowing into the sedimentation tank so as to be separated by sedimentation process in the sedimentation tank. Accordingly, in the both cases, the effect of phosphorus removal from the treated water can be increased. Inorganic flocculants containing aluminum salts such as polyaluminum chloride (PAC) and aluminum sulfate, and iron salts such as ferric chloride are suitable as the flocculants.

(36) The sedimentation tank 70 may be provided with a drain plug at a sludge accumulation section where the sludge is settled and accumulated, and a washing nozzle from which washing water is jetted out toward the sludge accumulation section. After the operation mode is switched from the second operation mode to the first operation mode, a cleaning process may be performed, in which the residual sludge is washed away from the sludge accumulation section by supplying water from the washing nozzle to the sedimentation tank 70 with the drain plug opened, where the drain plug is closed before switching back from the first operation mode to the second operation mode.

(37) When the operation mode is switched from the second operation mode to the first operation mode, the sludge may remain in the sedimentation tank 70 which has been operated in the second operation mode, where such residual sludge may become decomposed. However, even in such a case, the decomposition of the residual sludge can be avoided by performing the above-described cleaning process with opening and closing of the drain plug. Furthermore, it becomes unnecessary to install a separate sludge scraping mechanism in the sedimentation tank 70.

(38) Other embodiments of the present invention will be described below. In the following description, a flow rate balance of each treatment tank and the MLSS concentration in each tank are noted in the figures, and further explanation in the text description is omitted.

(39) As shown in FIG. 2, in the second operation mode, the post-sedimentation water may be divided and supplied to the dual-use tank 20 and the membrane bioreactor treatment tank 30. Depending on the degree to which the organic wastewater is treated by the initial absorption process in the dual-use tank 20, there may be such a risk that the anoxic tank 40 in the membrane bioreactor treatment tank 30 on the downstream side may not have sufficient BOD source to perform the biological treatment process appropriately.

(40) Even in such a case however, by dividing and distributing the organic wastewater (i.e., the post-sedimentation water) to both of the dual-use tank 20 and the anoxic tank 40 of the membrane bioreactor treatment tank 30, an appropriate biological treatment process can be performed in the membrane bioreactor treatment tank 30. Although the distribution ratio of the post-sedimentation water may be fixed, it is preferable to control the distribution ratio so as to correspond to the distribution ratio between the membrane separation device 60 and the sedimentation tank 70.

(41) By monitoring the sludge interface in the sedimentation tank 70, if a rising sludge interface poses a risk of failure in the solid-liquid separation, the amount of the permeated water through the membrane separation device 60 can be increased to suppress the amount of water distributed to the sedimentation tank 70 side, so as to avoid a sludge leakage from the sedimentation tank 70 and thus prevent the treated water quality from deteriorating. A human operator may visually observe the sludge interface, or a surveillance camera may be used to monitor the sludge interface and an automatic determination may be made by processing the images captured by the camera.

(42) Accordingly, in the second operation mode, the processing volume in the membrane bioreactor treatment tank 30 and that in the sedimentation tank 70 may be controlled according to the position of the sludge interface in the sedimentation tank 70 or the quality of the treated water.

(43) For example, when the sludge interface in the sedimentation tank 70 rises and performing a proper solid-liquid separation becomes difficult, increasing the treatment volume in the membrane bioreactor treatment tank 30 can stabilize the solid-liquid separation in the sedimentation tank 70. On the other hand, when the sludge interface in the sedimentation tank 70 falls and a proper solid-liquid separation is being performed, decreasing the treatment volume in the membrane bioreactor treatment tank 30 can reduce the load on the membrane separation device 60.

(44) When the quality of the treated water is determined to be significantly deteriorated, based on indicators such as the COD, SS, T-N, and T-P, the solid-liquid separation process in the sedimentation tank 70 can be stabilized by increasing the processing volume in the membrane bioreactor treatment tank 30, while the load on the membrane separation device 60 can be reduced by decreasing the processing volume in the membrane bioreactor treatment tank 30.

(45) FIG. 3A shows an example in which a pressure gauge is installed to detect the differential pressure of the MBR so as to maximize the processing capacity of the MBR. Based on the value of the pressure gage, the apparatus is operated by setting the amount of permeated water withdrawn by the membrane separation device 60 to the maximum value which does not cause the membrane clogged.

(46) FIG. 3B shows another example in which a sludge interface meter and/or a water quality meter are/is installed, such that the amount of treatment process in the membrane bioreactor treatment tank 30 and that in the sedimentation tank 70 are controlled according to the measurements.

(47) In FIG. 1B, such an example has been explained in which the dual-use tank 20 in the second operation mode functions as an aerobic tank, and the post-sedimentation water is supplied to the dual-use tank 20. However, as shown in FIG. 4A, it is possible that the dual-use tank 20 functions as an anoxic tank during the second operation mode in order to prioritize the denitrification reaction under clear weather conditions. In this case, pH adjustment in the anoxic tank 40 becomes unnecessary.

(48) As shown in FIG. 4B, both of the dual-use tank 20 and the anoxic tank 40 may be provided with respective aeration devices such that all of the tanks function as aerobic tanks so as to maximize the amount of water to be treated using the existing structure. In this case, an alkaline agent can be added to the anoxic tank 40, which functions as an aerobic tank, in order to keep the pH of the mixed liquid from dropping.

(49) As shown in FIG. 5A, it is also possible to supply the post-sedimentation water from the initial sedimentation basin 10 directly to the anoxic tank 40 without using the dual-tank 20 in the first operation mode, and to operate the membrane bioreactor treatment tank 30 using the AO method in which the anaerobic treatment process is only performed by the anoxic tank 40 and the aerobic treatment process is performed by the aerobic tank 50, while in the second operation mode, as shown in FIG. 5B, it is possible to supply the post-sedimentation water to the dual-use tank 20 and allow the dual-use tank 20 to function as an aerobic tank, such that overflow water from the anoxic tank 40 is led to the dual tank 20 for the initial adsorption treatment and then transferred to the sedimentation tank 70. That is, the function of the dual-use tank 20 can be switched such that the dual-use tank 20 functions as a bypassed tank (i.e., not in use) in the first operation mode, and functions as an aerobic tank in the first operation mode to perform the initial adsorption treatment.

(50) For example, in the first operation mode, as shown in FIG. 5A, the post-sedimentation water with a flow rate of 2Q is fed into the anoxic tank 40 of the membrane bioreactor treatment tank 30 in which the denitrification process is performed thereon, and after aerobically treated in the aerobic tank 50 and solid-liquid separated by the membrane separation device 60, the permeated water is discharged to a river or the like as the treated water with a maximum flow rate of 2Q.

(51) The mixed liquid in which ammoniacal nitrogen has been nitrified in the aerobic tank 50 is returned to the anoxic tank 40 with a flow rate of 3Q via the sludge return path 6, and then is denitrified in the anoxic tank 40. By returning the mixed liquid from the aerobic tank 50 to the anoxic tank 40 via the sludge return path 6 at a flow rate 3Q, the MLSS concentration in the anoxic tank 40 is 6000 mg/L, compared with the MLSS concentration of 8000 mg/L in the aerobic tank 50.

(52) In the second operation mode, as shown in FIG. 5B, the post-sedimentation water with a flow rate of 4.2Q (which is higher than the flow rate of organic wastewater of 2Q in the first operation mode) is fed into the anoxic tank 40, and the sludge, which has overflowed from the anoxic tank 40 into the dual-use tank 20 and has been treated with the initial absorption process, is sent to the sedimentation tank 70 at a flow rate of 2Q, and after the sedimentation process, the treated water with a flow rate of 1.8Q is sent to the disinfection tank 90, while the sludge with a flow rate of 0.2Q is returned to the aerobic tank 50 from the sedimentation tank 70.

(53) Then, the sludge with a flow rate of 1.0 Q is returned from the aerobic tank 50 to the anoxic tank 40, while the treated water with a flow rate of 2.4 Q is withdrawn from the membrane separation device 60 of the aerobic tank 50. Thus, as a total, the treated water with a flow rate of 4.2 Q is discharged into a river or the like. At this time, with respect to the MLSS concentration 1500 mg/L in the dual-use tank 20, the MLSS concentration in the anoxic tank 40 is 1500 mg/L, and the MLSS concentration in the aerobic tank 50 is about 1200 mg/L.

(54) FIGS. 6A and 6B show an example in which the membrane bioreactor treatment tank 30 in FIGS. 1A and 1B is replaced with a circulating-type MBR. FIG. 7 shows an example of the circulating-type MBR in which the dual-use tank 20 functions as an anaerobic tank in the first operation mode.

(55) A circulating-type MBR is a biological treatment tank in which a plurality of biological treatment units are connected in series, where each biological treatment unit includes a pair of an anoxic tank 40 located on an upstream side and an aerobic tank 50 located on a downstream side along the flow of organic wastewater, and a membrane separator 60 is immersed in activated sludge in the aerobic tank 50. The MBR is provided with a sludge return path 7 which returns the activated sludge from the aerobic tank 50 disposed at the most downstream side to the anoxic tank 40 disposed on the most upstream side, such that mixture (mixed liquid) of the organic wastewater and the activated sludge circulates through the MBR. Since the organic wastewater is supplied to the anoxic tank 40 formed next to the aerobic tank 50, a high performance of the denitrification process can be achieved under a high BOD concentration in the anoxic tank 40.

(56) As shown in FIG. 6A, by operating the apparatus 100 in the first operation mode during the summer or when the process load is low, the treated water having a very low T-N of about 3 mg/L can be obtained. As shown in FIG. 6B, on the other hand, by operating the apparatus 100 in the second operation mode during the winter or when the process load is high, in which the dual-use tank 20 functions as a nitrification tank, and the treated water obtained through the sedimentation tank 70 and the treated water obtained through the membrane separation device 60 are combined, the generally required treated water quality of T−N<10 mg/L and T−P<0.5 mg/L can be achieved.

(57) In this example, the sedimentation tank 70 is provided with a filtration device 72, such that the water filtered through the filtration device 72 is withdrawn as the treated water in the second operation mode. Accordingly, even in such a case where the MLSS concentration in the sedimentation tank 70 increases, the filtration device 72 can prevent the sludge from flowing into the treated water.

(58) The filtration device 72 may be composed of a filtering material having an apparent specific gravity less than 1, such as foamed polyethylene, foamed polystyrene, foamed polypropylene, and the like, which is formed into minute chips having unevenness of several millimeters, and a net member configured to hold and prevent the floating filter material from flowing out of the sedimentation tank 70. The sludge is caught by the group of minute chips held by the net member, whereby only the treated water is withdrawn from the sedimentation tank 70 and discharged to the disinfection tank 90.

(59) If the regulated T−P value is not so strict, the floating filter material in the sedimentation tank (the final sedimentation basin) 70 can be omitted or replaced with an inclined plate 71 shown in FIG. 6A.

(60) The above-mentioned examples are embodiments of the invention, and the present invention is not limited by the description above, and the specific configurations of each part can be designed and modified as appropriate within the scope of the effect of the invention.