JP2008155080A - Sewage treatment apparatus and its method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sewage treatment apparatus and method which effectively perform membrane surface cleaning and allow advanced treatment for removing nitrogen and phosphorus to satisfactorily function by effectively utilizing energy in MBR of sewage. <P>SOLUTION: A membrane separation tank 35 that communicates with an aerobic tank 34 at the bottom, has an aeration means 4d in the lower part, and has an immersed filtration membrane 10 for filtering and separating a mixture liquid of the aerobic tank 34 flowing into above the aeration means 4d, and a storage tank 36 that partially communicates with the membrane separation tank 35 and receives inflow of the membrane-separated mixture liquid of the aerobic tank are installed on the downstream side of the aerobic tank 34. The mixture liquid of the storage tank 36 is circulated to an anaerobic tank 32. The upper parts of the membrane separation tank 35 and the storage tank 36 are opened to atmosphere. The mixture liquid of the aerobic tank spontaneously flows into and fills the membrane separation tank 35 and the storage tank 36 in conjunction with the circulating liquid so that the water levels of the membrane separation tank and the storage tank maintains the water level of the aerobic tank. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a sewage treatment apparatus and method for biologically treating sewage containing nitrogen and phosphorus in addition to organic matter with activated sludge using a solid-liquid separation method by membrane filtration.

  In sewage treatment plants, advanced treatment methods have been introduced that use microorganisms called activated sludge to remove nitrogen and phosphorus in addition to organic matter. FIG. 10 is a diagram showing a configuration of an advanced treatment apparatus using both A2O and a sedimentation basin system. In this apparatus, the pretreated sewage 11 is composed of an anaerobic tank 32, an anaerobic tank 33, and an aerobic tank 34. Into the biological reaction tank 1 and mixed with the return sludge 18 containing the high-concentration activated sludge and subjected to a predetermined biological treatment to remove organic matter, nitrogen and phosphorus. The biological communication pipe 12 is solid-liquid separated in the sedimentation basin 5, the activated sludge is gravity settled, and the supernatant is discharged as treated water 16.

  In this advanced processing method, the circulating liquid 17 that is refluxed from the aerobic tank 34 to the anoxic tank 33 directly affects the nitrogen removal rate, and therefore may be set to several times the sewage flow rate. In order to adjust and maintain the sludge concentration in the biological reaction tank 1, the return sludge 18 is usually operated at a sewage flow rate or less. The sludge grown in the process is discharged out of the system as surplus sludge 19, and the amount of sludge in the entire process is managed.

  On the other hand, the membrane separation activated sludge method (hereinafter referred to as MBR) is being applied because the treated water is of good quality, a sedimentation basin is not required, and there is an expectation of downsizing by high sludge concentration treatment. This method includes a method of directly immersing the filtration membrane in a biological reaction tank that is several times the sludge concentration (˜3 g / L) of the conventional reaction tank shown in FIG. Various methods have been proposed, such as a solid-liquid separation by providing a membrane separation tank.

  In the submerged membrane method for advanced treatment, as described in [Patent Document 1], there is a proposal of extracting sludge from below the aeration tube of an aeration tank provided with a membrane separator and circulating it to an oxygen-free tank. This circulating sludge is a combination of the circulating fluid and return sludge shown in FIG.

  Moreover, the big subject of MBR is peeling the sludge adhering to a film surface, performing the film | membrane washing | cleaning which suppresses clogging efficiently, and maintaining the stable driving | operation. In the case of a submerged membrane, air is aerated from below the filtration membrane, an upward flow of the liquid to be separated is formed by an air lift action, and cleaning that suppresses the adhesion of the sludge to the membrane surface is performed using the shear force of the upward flow. ing. As an example, although not intended for advanced treatment, as described in [Patent Document 2], a membrane separation tank in which two or more filtration membranes are arranged with a partition wall interposed is separated from a biological reaction tank. Some are installed and the filtration equipment is alternately filtered and diffused (washed), and the mixed solution is returned to the original reaction tank. In addition, since a sufficient cleaning flow rate cannot be obtained only by the upward flow due to the bubble rising, a stirring means for enhancing the upward flow is newly installed between the air diffuser and the filtration membrane as described in [Patent Document 3]. A method has been proposed.

JP 2004-835 JP JP 2001-276874 A JP 2003-251386 A

  The conventional technology described in [Patent Document 1] is a proposal in which an anaerobic tank or an anaerobic tank is used as a circulating liquid in an aerobic tank mixed solution with as little oxygen as possible because a microbial reaction proceeds in an oxygen-free atmosphere. Only air aeration is used to clean the membrane.

  The conventional technology described in [Patent Document 2] has a cleaning function for the filtration membrane by forming a cross flow on the filtration side with a swirling flow obtained by aeration for cleaning, but a return line is installed. When the filtration membrane on the side that is being used is being filtered, there is a problem that the swirling flow is weakened and the cleaning efficiency is reduced. In the conventional techniques described in [Patent Document 1] and [Patent Document 2], the rising flow or the swirling flow is formed only by aerated air. It is necessary to increase the amount of aerated air.

  Since the conventional technique described in [Patent Document 3] is provided with a stirrer that forms an upward flow, it is possible to improve the cleaning effect by surely increasing the upward flow velocity, but new equipment costs and running costs are generated. There's a problem.

  An object of the present invention is to provide a sewage treatment apparatus and a method for effectively performing an advanced treatment for effectively removing energy such as nitrogen and phosphorus by effectively using energy to clean a membrane surface.

  In order to achieve the above object, the sewage treatment apparatus and method of the present invention circulates an activated sludge mixed liquid in an aerobic tank at least in an anaerobic tank, and the bottom part is aerobic at the rear stage of the aerobic tank. A membrane separation tank in which a filtration membrane for filtering and separating the aerobic tank mixed liquid flowing into the upper part of the aeration tank is immersed, and a part of the membrane separation tank communicates with the tank. A residence tank into which the aerobic tank mixed liquid after membrane separation flows is provided, and the mixed liquid in the residence tank is circulated to the anaerobic tank.

  In addition, the membrane separation tank and the retention tank are open to the atmosphere at the top, so that the water surface position of both tanks keeps the water surface position of the aerobic tank in conjunction with the circulating liquid. The aerobic tank mixture naturally flows into the tank and is replenished.

  In addition, a plurality of membrane separation tanks are provided, and a membrane separation liquid mixture is sequentially introduced into the rear membrane separation tank, and a retention tank is provided for introducing the membrane separation liquid flowing out from the rearmost membrane separation tank. By circulating the membrane separation liquid mixture at least in the anaerobic tank, the flow rate of the liquid mixture flowing through the filtration membrane is improved.

  ADVANTAGE OF THE INVENTION According to this invention, the circulating fluid indispensable by an advanced process can be utilized effectively for washing | cleaning of an immersion film, without impairing the function of a circulating fluid, and there exists an effect which can reduce an installation cost and an operating cost.

  Examples 1 to 3 will be described below with reference to the drawings.

  A first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a configuration diagram of a sewage treatment apparatus according to the present embodiment, and FIGS. 2 and 3 are modifications thereof. In this embodiment, the activated sludge system of nitrification liquid circulation type is targeted.

  As shown in FIG. 1, the biological reaction tank 1 includes an oxygen-free tank 33 into which sewage 11 and circulating liquid 17 are injected, an aerobic tank 34, a membrane separation tank 35, and a retention tank 36.

  The anaerobic tank 33 and the aerobic tank 34 are separated by a partition wall 2b having a rectifying function. The anaerobic tank 33 is provided with a stirring means 3b, and the aerobic tank 34 is provided with a diffuser pipe installed at the bottom. Air 14 is aerated from 4c, and each liquid mixture is stirred. The dissolved oxygen concentration meter 21 is installed in the aerobic tank 34, and the measured value of the dissolved oxygen concentration meter 21 is fed back to the air adjusting means 14c connected to the air diffuser 4c.

  A partition wall 2c is provided between the aerobic tank 34 and the membrane separation tank 35, except that the bottom is partially opened. A filtration membrane 10 is immersed in the membrane separation tank 35, and an air diffuser 4 d is installed below the filtration membrane 10. A suction means 6 is connected to the filtration membrane 10 by piping, and a pressure gauge 22 is attached to the piping. The filtrate sucked by the suction means 6 is collected as treated water 16. The air diffusing pipe 4d is connected to the air adjusting means 14d, and a part of the air 14 is diffused to the filtration membrane 10.

  Between the membrane separation tank 35 and the retention tank 36, a partition wall 2d having an upper portion located below the water surface is provided. A pipe is provided in the staying tank 36, and the activated sludge mixed liquid is sucked as the circulating liquid 17 from the lower part of the staying tank 36 by the circulation means 7 connected to the pipe and returned to the oxygen-free tank 33.

  In the oxygen-free tank 33, the sewage 11 and the circulating liquid 17 are stirred by the stirring means 3b, and nitric acid or nitrite nitrogen contained in the circulating liquid 17 is reduced using the organic matter in the sewage 11 to denitrify it. To do. In the aerobic tank 34, oxygen is supplied by the aerated air from the diffuser 4c, and organic matter decomposition and nitrification of activated sludge are promoted.

  In this embodiment, the activated sludge concentration in the biological reaction tank 1 varies in the flow direction, but is operated at about 5000 to 50000 mg / L. The circulating liquid 17 is an operation amount that influences the denitrification rate of the entire process, although it is for the circulating action of activated sludge (also referred to as sludge return).

  For example, when the total nitrogen in the sewage 11 is 40 mg / L, the nitrogen in the treated water 16 is 10 mg / L or less, and complete nitrification is performed in the aerobic tank 34, the flow rate of the circulating liquid 17 is the flow rate of the sewage 11. It is necessary to make more than 3 times.

  In addition, when the filtration membrane 10 is washed with diffused air, generally 10 to 40 times the amount of diffused air is supplied with respect to the flow rate of the sewage 11.

In this embodiment, since the membrane separation tank 35 and the retention tank 36 are partitioned by the partition wall 2d whose upper part is located below the water surface, the water surface is at the same position, and the water surface is the same level in the entire biological reaction tank 1. Therefore, when the circulating liquid 17 is sucked from the retention tank 36, the mixed liquid in the aerobic tank 34 flows into the membrane separation tank 35 by the same amount. The liquid mixture flowing into the membrane separation tank 35 flows in from the bottom of the partition wall 2 c provided between the aerobic tank 34 and the membrane separation tank 35, and ascends the primary flow path of the filtration membrane 10.

The ascending flow rate of this mixed solution is, for example, 0.02 m / s. That is, the amount of treated water is 3000
m ³ / day, the circulating fluid flow rate is 9000 m ³ / day, which is 3 times the amount of treated water, a flat membrane of 0.5 * 1 m is used for the filtration membrane, the filtration area is 1 m ² with ^two filtration membranes, If a flat membrane is installed with a primary flow path width of 5 mm at a distance of 5 mm and a filtration flux of 0.5 m / d, 6,000 flat membranes are required. Since the channel area of the entire membrane is 5 m ² , the flow rate between the membranes is 0.02 m / s.

  Since the flow rate between membranes for membrane cleaning is 0.1 to 1.0 m / s at the maximum, a flow rate of 20% to 2% can be secured by using the circulating fluid. The amount of air from 4d can be reduced.

  Thus, since the aerobic tank mixed liquid newly flows into the membrane separation tank in conjunction with the operation amount of the circulating liquid, the flow rate between the membranes in the membrane separation tank is improved in accordance with the amount of the circulating liquid. Since the circulating fluid is operated with several times the amount of sewage, including the return sludge, the flow rate between the membranes of the membrane is greatly increased, and the air aerated from below the membrane for membrane cleaning can be reduced. it can. In particular, in the existing advanced treatment process, a circulation means is laid, which can be dealt with by a partial modification such as provision of a membrane separation tank or a retention tank, and the existing equipment can be used effectively.

  The mixed liquid after membrane separation overflows from the membrane separation tank 35 into the retention tank 36. The air bubbles floating in the mixed solution at the time of overflowing are diffused to the atmosphere, and the dissolved oxygen also reaches the bottom which becomes the circulating liquid 17 and has an activity that is about 30% higher than the aerobic tank 34. Consumed rapidly with sludge. For this reason, the circulating liquid 17 with few air bubbles and dissolved oxygen is obtained, and the stable biological treatment can be maintained without impairing the function of the oxygen-free tank 33.

  In this way, the retention tank dissipates air bubbles aerated in the membrane separation tank to the atmosphere and can also reduce dissolved oxygen because of its high sludge concentration. Can be refluxed.

  In FIG. 1, the air adjusting means 14c and 14d are installed. However, the adjusting means 14c is a deviation between the measured value and the target value of the dissolved oxygen concentration meter 21 installed in the aerobic tank 1c, and the adjusting means 14d is filtered. An operation of correcting the deviation between the actual value of the pressure gauge 22 installed on the suction side of the membrane and the target value or the amount of circulating fluid can be applied.

  FIG. 2 is a modification of the embodiment shown in FIG. 1. In order to extract the circulating fluid and excess sludge from the retention tank 36, surplus sludge extraction means 9 is provided in the extraction line of the circulating fluid 17, and the excess sludge 19 is used as a system. I try to discharge outside. The surplus sludge 19 is drawn out at the same position as the circulating liquid 17 and can be easily changed from the embodiment shown in FIG. 1. Thus, the mixed liquid after membrane separation is used as the surplus sludge 19. Thus, since the flow rate obtained by adding excess sludge to the circulating liquid 17 flows through the membrane separation tank 35, the flow rate between the membranes is increased, and the cleaning effect is improved. Moreover, the mixed liquid after membrane separation has an increased sludge concentration. By using this mixed liquid as excess sludge, there is a synergistic effect that reduces the processing flow rate of the subsequent sludge treatment process (not shown).

  The surplus sludge 19 is operated by the amount of activated sludge in the entire process, and is set in consideration of, for example, the sludge concentration meter 23 of the biological reaction tank 1, the circulating liquid 17 or the sludge concentration in the surplus sludge 19.

  FIG. 3 is a modification of the embodiment shown in FIG. 1, and is an example applied to the A2O system. As shown in FIG. 3, an anaerobic tank 32 is provided in front of the anaerobic tank 33, and the circulating liquid 17 is provided from the lower part of the staying tank 36 by the circulation means 7 connected to a pipe provided in the staying tank 36. Is sucked and returned to the anaerobic tank 32 and the anaerobic tank 33. The distribution of the circulating fluid to the anaerobic tank 32 and the anaerobic tank 33 is operated in consideration of the activated sludge concentration and the nitrogen removal rate of each tank. By comprising in this way, activated sludge can be returned to the anaerobic tank 32. FIG.

  In the first to third embodiments in which the flow rate between the membranes is improved by the combination of the diffused air and the circulating fluid, it is necessary to form a swirl flow path between the membrane separation tank 35 and the filtration membrane 10. In this embodiment, the swirl flow path is not particularly defined, but if the swirl flow path width is too wide, the intermembrane flow velocity by the circulating fluid is not improved. In order to exert the improvement effect, it is desirable that the cross-sectional area of the swirl flow path is 1 to 1/50 of the cross-sectional area of the filtration membrane.

  A second embodiment of the present invention will be described with reference to FIGS. FIG. 4 is a configuration diagram of the sewage treatment apparatus according to the present embodiment, and FIGS. 5 to 7 are modifications thereof.

The present embodiment is configured in the same manner as in FIG. 1, but in this embodiment, a membrane filtration treatment tank 31 constituted by a membrane separation tank 35 and a retention tank 36 is separately provided at the subsequent stage of the biological reaction tank 1. . The biological reaction tank 1 and the membrane filtration treatment tank 31 communicate with each other at the bottom through the communication pipe 12, and treated water from the aerobic tank 34 flows into the bottom of the membrane filtration treatment tank 31. The operation of the membrane separation tank 35 and the retention tank 36 is the same as in the embodiment shown in FIG.

  In this embodiment, there are many sewage treatment plants having a plurality of biological reaction tanks 1, and thus installing the membrane filtration treatment tank 31 separately from the biological reaction tank 1 enables maintenance, cleaning, and handling various troubles. Maintenance management can be performed individually. Further, the MBR system can be improved without stopping the biological treatment operation of the existing plant. In addition, there is expandability such as expansion, and chemical cleaning that is implemented to prevent fluctuations in influent sewage and clogging of filtration membranes can be performed sequentially without stopping the plant.

  Also in this example, the effect of reducing the amount of diffused air accompanying the improvement of the intermembrane flow velocity by the circulating fluid 17 is the same as that of the first embodiment.

  A modification of this embodiment will be described with reference to FIG. FIG. 5 is a configuration diagram of the sewage treatment apparatus of the present embodiment.

This embodiment is configured in the same manner as the embodiment shown in FIG. 4, but in this embodiment, a plurality of membrane separation tanks 35 are arranged, and the upper part is below the water surface between adjacent membrane separation tanks 35. The partition wall 2d located at the bottom and a partition wall 2c that completely partitions except for a part of the bottom opening are sequentially provided, and a bypass tank 37 is formed between the partition wall 2d and the partition wall 2c. A retention tank 36 is provided at the end of the membrane filtration processing tank 31.

  Each membrane separation tank 35 is provided with a filtration membrane 10 and an air diffuser 4d below the filter membrane 10, and each air diffuser 4d is supplied with air 14. The filtrate of each filtration membrane 10 sucked by the suction means 6 is collected as treated water 16.

  The treated water is allowed to flow below the uppermost stream portion of the membrane filtration treatment tank 31, and the circulating liquid 17 is drawn out from the bottom of the retention tank 36. The air diffuser 4d of each membrane separation tank 35 diffuses air 14A branched from the main air 14, and the filtration membrane 10 enters the same amount of treated water as the inflowing circulating liquid 17 into the most upstream membrane separation tank 35. Then, it overflows into the downstream bypass tank 37, enters the next membrane separation tank 35 from the lower part of the bypass tank 37, is sequentially repeated, and finally reaches the retention tank 36.

As described above, by providing a plurality of membrane separation tanks 35 and passing the mixed liquid in series, the flow rate between the membranes can be further increased, and the cleaning air can be reduced. In the calculation in Example 1, when the membrane separation tank 35 has five stages, the flow rate between the membranes is 5 times, the flow rate of the circulating fluid is the same, and the aeration air for cleaning the membrane surface can be greatly reduced. There are also situations where care is not required.

  A modification of this embodiment will be described with reference to FIG. This embodiment is configured in the same manner as the embodiment shown in FIG. 4, but in this embodiment, a plurality of membrane separation tanks 35 are arranged, and the upper part is below the water surface between adjacent membrane separation tanks 35. The partition wall 2d located at the bottom and the partition wall 2c that completely partitions except for a part of the bottom opening are alternately provided, and the membrane separation by the upward flow and the downward flow is alternately performed, and the rearmost membrane separation The membrane separation mixed liquid in the tank 35 is drawn out by the circulation means 7 and refluxed to the anaerobic tank 34 or the oxygen-free tank 33.

  Thus, by providing a plurality of membrane separation tanks 35 and passing the mixed liquid in series, the flow rate between the membranes can be further improved even in the case of the dynamic filtration method.

  A modification of this embodiment will be described with reference to FIG. This embodiment is configured in the same manner as the embodiment shown in FIG. 4, but in this embodiment, the membrane separation tanks 35 are stacked in the vertical direction, and each membrane separation tank 35 has a lower membrane separation mixing. A partition wall 2c provided with an opening through which the liquid flows is partitioned, and the final membrane separation tank 35 is formed by a partition wall 2e partially opened to the atmosphere. The treated water that has flowed into the membrane filtration tank 31 flows into the membrane filtration tank 31 through the communication pipe 12 and is sequentially separated into solid and liquid, and the circulating liquid is withdrawn from the atmosphere opening of the final membrane separation tank 35 to be refluxed.

  Thus, by providing a plurality of membrane separation tanks 35 and passing the mixed liquid in series, the flow rate between the membranes can be further improved even in the case of the dynamic filtration method.

  A third embodiment of the present invention will be described with reference to FIGS. FIG. 8 is a configuration diagram of the sewage treatment apparatus of the present embodiment, and FIG. 9 is a modification thereof.

  As shown in FIG. 8, in the sewage treatment apparatus of the present embodiment, the biological reaction tank 1 is constituted by an anoxic tank 33 into which sewage 11 is injected, an aerobic tank 34 and a membrane separation tank 35. The anaerobic tank 33 and the aerobic tank 34 are separated by a partition wall 2b having a rectifying function. The anaerobic tank 33 is provided with a stirring means 3b, and the aerobic tank 34 is provided with a diffuser pipe installed at the bottom. Air 14 is aerated from 4c, and each liquid mixture is stirred.

  A partition wall 2c is provided between the aerobic tank 34 and the membrane separation tank 35, except that the bottom is partially opened. The filtration membrane 10 is immersed in the membrane separation tank 35. A suction means 6 is connected to the filtration membrane 10 by piping, and the filtrate sucked by the suction means 6 is collected as treated water 16. A pipe is provided above the filtration membrane 10 in the membrane separation tank 35, and the circulating liquid 17 is sucked back by the circulation means 7 connected to the pipe and returned to the oxygen-free tank 33.

  As described above, the present embodiment is different from the first embodiment in that the membrane separation tank 35 is not provided with an air diffuser in the membrane separation tank 35 and there is no residence tank. In this embodiment, the membrane separation tank 35 is formed by the partition wall 2c that is completely partitioned from the aerobic tank 34 except that the bottom part is partially opened, and is drawn from above the filtration membrane 10 in which the circulating liquid 17 is immersed. Therefore, the aerobic tank mixed liquid equivalent to the circulating liquid 17 drawn from above flows into the membrane separation tank 35 from the opening of the partition wall 2c, and the upward flow velocity corresponding to the amount of the circulating liquid and the flow passage area of the filtration membrane 10 Is obtained.

  A modification of this embodiment will be described with reference to FIG. This embodiment is configured in the same manner as the embodiment shown in FIG. 8, but in this embodiment, the upper portion is located below the water surface between the aerobic tank 34 and the membrane separation tank 35. A partition wall 2c is provided so as to be pulled out from below the filtration membrane 10 in which the circulating fluid 17 is immersed.

  The aerobic tank mixed liquid equivalent to the circulating liquid 17 drawn from below flows into the membrane separation tank 1d, and a downward flow velocity corresponding to the amount of the circulating liquid and the flow path area of the filtration membrane 10 is obtained.

  These upward flow velocity and downward flow velocity are the values calculated in Example 1 when the membrane separation tank 35 has one stage. This flow rate is low for cleaning the membrane surface, but is effective in the case of a so-called dynamic filtration method in which a sludge layer (cake layer) is formed on the membrane surface for filtration.

  In this method, it is necessary to peel off the cake layer that has become excessively thick. This is because, when the flow rate is increased, the necessary cake layer is also peeled off, so that operation at a relatively low flow rate is required. Depending on the required flow rate, a plurality of membrane separation tanks 35 may be installed as in the second embodiment. Since air is not diffused in the membrane separation tank 35, the circulating liquid 17 has a sufficiently low oxygen concentration, and the retention tank 36 is not required.

It is a block diagram of the sewage treatment apparatus which shows Example 1 of this invention. It is a block diagram which shows the modification of a present Example. It is a block diagram which shows the modification of a present Example. It is a block diagram of the sewage treatment apparatus which shows Example 2 of this invention. It is a block diagram which shows the modification of a present Example. It is a block diagram which shows the modification of a present Example. It is a block diagram which shows the modification of a present Example. It is a block diagram of the sewage treatment apparatus which shows Example 3 of this invention. It is a block diagram which shows the modification of a present Example. It is a block diagram of the conventional advanced processing system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Biological reaction tank 2 Partition wall 4 Aeration pipe 10 Filtration membrane 11 Sewage 12 Communication pipe 14 Air 16 Treated water 17 Circulating liquid 19 Excess sludge 21 Dissolved oxygen concentration meter 22 Pressure gauge 23 Sludge concentration meter 31 Membrane filtration treatment tank 32 Anaerobic tank 33 Anaerobic tank 34 Aerobic tank 35 Membrane separation tank 36 Residence tank 37 Diversion tank

Claims (12)

  An anaerobic tank into which sewage flows, an aerobic tank installed downstream of the anaerobic tank and divided by a partition wall, and a part of the bottom installed downstream of the aerobic tank communicated with the aerobic tank A membrane separation tank in which a diffuser is provided below, and a filtration membrane is immersed above the diffuser, and a partition wall that is installed at a subsequent stage of the membrane separator and communicates with the membrane separator. A sewage treatment apparatus comprising a divided retention tank and a circulation means for returning to the aerobic tank as a circulating liquid from a lower part of the retention tank.   An anaerobic tank into which sewage flows, an aerobic tank installed downstream of the anaerobic tank and divided by a partition wall, and a part of the bottom installed downstream of the aerobic tank communicated with the aerobic tank A sewage treatment apparatus comprising a membrane separation tank in which a filtration membrane is immersed, and circulation means for returning the membrane separation tank to the aerobic tank as a circulating liquid from an upper part or a lower part of the filtration membrane.   The sewage treatment apparatus according to claim 1 or 2, wherein the circulation means is connected to excess sludge extraction means for extracting excess sludge.   The sewage treatment apparatus according to claim 1, wherein an anaerobic tank is provided in front of the anaerobic tank, and the circulation means is configured to circulate to the anaerobic tank as a circulating liquid from a lower part of the staying tank.   The sewage treatment apparatus according to claim 1, wherein the aerobic tank, the membrane separation tank, and the retention tank are separately installed, and a part of the bottom is connected by a communication pipe.   The membrane separation tank is composed of a plurality of membrane separation tanks, and a bypass tank is provided between the plurality of membrane separation tanks to guide the activated sludge mixed liquid to the bottom and flow into the downstream membrane separation tank, and the last membrane separation. The sewage treatment apparatus according to claim 5, wherein the activated sludge mixed liquid in the tank flows into the staying tank.   The membrane separation tank is provided with a plurality of membrane separation tanks in a horizontal direction or a vertical direction, and the aerobic tank mixed liquid from the aerobic tank is introduced into the first stage, and the activated sludge mixed liquid is sequentially moved backward. Alternatively, the sewage treatment apparatus according to claim 5, wherein water is serially passed through an upper membrane separation tank, and a membrane separation mixed solution in the rearmost membrane separation tank is circulated to the anoxic tank.   The sewage treatment according to claim 1, wherein the upper part of the membrane separation tank and the retention tank is open to the atmosphere, and the water surface position of the membrane separation tank and the retention tank becomes the water surface position of the aerobic tank. apparatus.   The sewage treatment apparatus according to any one of claims 1 to 7, wherein a cross-sectional area of a swirling flow path formed between the filtration membranes of the membrane separation tank is set to 1 to 1/50 of the cross-sectional area of the filtration membrane.   Denitrified by an anaerobic tank into which sewage flows, and an aerobic tank separated by a partition wall that partially communicates with the latter part of the anaerobic tank to decompose the organic matter of activated sludge and convert it into an aerobic tank mixture by nitrification The aerobic tank mixing was introduced by a membrane separation tank in which a part of the bottom part communicated with the aerobic tank at the rear stage of the aerobic tank, and a diffuser was provided below, and a filter membrane was immersed above the diffuser Sewage treatment that separates the liquid into solid and liquid, part of the upper part communicates with the membrane separation tank, and the membrane separation mixed liquid is circulated to the anaerobic tank from the lower side of the retention tank into which the activated sludge mixed liquid of the membrane separation tank flows. Method.   The sewage treatment method according to claim 10, wherein the aerobic tank, the membrane separation tank, and the retention tank are separately installed, and a part of the bottom is connected by a communication pipe.   Denitrified by an anaerobic tank into which sewage flows, and an aerobic tank separated by a partition wall that partially communicates with the latter part of the anaerobic tank to decompose the organic matter of activated sludge and convert it into an aerobic tank mixture by nitrification In addition, a part of the bottom communicates with the aerobic tank at the rear stage of the aerobic tank, and the aerobic tank mixed liquid flowing in by the membrane separation tank in which the filtration membrane is immersed is solid-liquid separated, and the filtration membrane of the membrane separation tank A sewage treatment method for returning an aerobic tank mixed liquid as a circulating liquid to the aerobic tank from the upper part or the lower part of the aerobic tank as a circulating liquid.

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