JPS6355998B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for treating organic wastewater, and particularly to a method for removing nitrogen and phosphorus from wastewater.

[Technical background of the invention and its problems] In recent years, the oxidation deutsch method (OD method) has been attracting attention as a medium-sized and small-scale sewage treatment facility. The reasons why this OD method is attracting attention include that it is an energy-saving system, can omit the settling tank in the previous stage, is easy to maintain, is resistant to load fluctuations, and generates little sludge.

On the other hand, in closed water areas such as lakes, marshes, and inner bays,
In recent years, there has been a movement to regulate nitrogen and phosphorus, which are the causes of pollution, with the aim of preventing eutrophication, and emission controls have already been implemented in some regions. In order to quickly respond to this situation, the present applicant has already proposed a biological denitrification and dephosphorization method that combines an anaerobic tank and OD (Japanese Patent Application No. 174328/1982). This method will be explained with reference to the wastewater treatment apparatus shown in FIG. 1 in the figure is an oxidation/dutch tank in which a partition plate 2 is arranged in the center. Such a partition plate 2
By arranging this in the deutch tank 1, a planar closed circuit is formed in the tank 1. Said Datetsu tank 1
An underwater propeller 3 for the purpose of water circulation and an aeration pipe 4 as an aerator for supplying oxygen are arranged. The underwater propeller 3 and the aeration pipe 4 are separated, and by arranging the aeration pipe 4 at an appropriate position, an anaerobic zone A and an aerobic zone B, which are indicated by diagonal lines in the Deitzch tank 1, are formed. I have made it possible. Further, the deutsch tank 1 is connected to a sedimentation tank 5. This sedimentation tank 5 is connected to an absolute anaerobic tank 6, and the anaerobic tank 6 is connected to the Deitzch tank 1. Note that it is necessary to install a submersible propeller or the like in the absolute anaerobic tank 6 to provide a flow velocity that prevents the sludge from settling.

In the above-mentioned device, the raw water 7 is treated with NOx (nitric acid,
The sludge is introduced into an absolute anaerobic tank 6 in which almost no nitrous acid is present, in which it is mixed with return sludge 8 from the settling tank 5, and then the mixed solution is introduced into the starting position of the anaerobic zone of the Deitzch tank 1. A mixed solution of raw water and returned sludge introduced into the deitsch tank 1 is circulated within the same tank 1. At this time, a mixed solution of raw water and returned sludge is introduced from anaerobic zone A to aerobic zone B, where a nitrification reaction occurs, and NH ₄ --N is oxidized to generate NOx. Furthermore, a denitrification reaction proceeds in the anaerobic zone using the BOD components in the raw water as an organic carbon source, and the generated NOx becomes nitrogen gas and is released into the atmosphere. The mixed liquid flowing out from the Deitz tank 1 is introduced into the settling tank 5, where the treated water 9 is discharged outside the system, and the settled sludge is returned to the absolute anaerobic tank 6 as return sludge 8. Through this circulation of Deitzch tank 1 → settling tank 5 → absolute anaerobic tank 6 → Deitzch tank 1, anaerobic → aerobic stress is applied to the sludge, and phosphorus is removed biologically. That is, the microorganisms used in such activated sludge absorb phosphorus under aerobic conditions and discharge phosphorus under anaerobic conditions. This anaerobic discharge of phosphorus serves as a source of organic carbon in the system.
It is promoted by the presence of BOD and inhibited by the presence of NOx. Therefore, in the absolute anaerobic tank 6, phosphorus is sufficiently discharged from the microorganisms in the returned sludge due to the presence of BOD in the raw water, and the mixed solution of raw water and returned sludge introduced into the Deitzch tank 1 enters the aerobic zone B. Once inside, the phosphorus is absorbed by microorganisms, and the phosphorus-absorbed state is introduced into the sedimentation tank 5. This sedimentation tank 5 is normally in an anaerobic state, but in a state where there is almost no BOD and no anaerobic biological reaction is progressing, microorganisms hardly discharge phosphorus. Therefore, phosphorus is not contained in the treated water 9 again and is not discharged. By applying anaerobic to aerobic stress to the sludge, such as the discharge of phosphorus in the absolute anaerobic tank 6 and the absorption of phosphorus in the Deitsch tank 1, the returned sludge is directly transferred to the Deitzch tank without installing the absolute anaerobic tank 6. The amount of phosphorus accumulated in sludge can be significantly improved compared to the method in which phosphorus is introduced into the sludge for treatment. Then, the sludge with increased phosphorus content is discharged outside the system as surplus sludge 10 and is treated separately.

In the method described above, if the capacity of Deitzch tank 1 is sufficiently large (for example, the Deitzch pipe length is approximately 150 m),
As mentioned above, the underwater propeller 3 for the purpose of water circulation and the air diffuser 4 are separated,
By arranging the aeration pipe 4 at an appropriate position, the deutsch tank 1 is placed in an anaerobic zone A indicated by diagonal lines.
and aerobic zone B can be formed, and nitrification and denitrification reactions can proceed sufficiently. However, if the capacity of the Deitz tank is small (for example, the Deitz pipe length
(less than 150 m), it becomes difficult to form aerobic and anaerobic zones. In such a case, a method is adopted in which an aerobic zone 4 serving as an aerator is operated intermittently to alternately form an aerobic zone and an anaerobic zone in the Deitch tank 1 over time. Under aerobic conditions, organic matter removal and nitrification reactions (NH ₄ -N→
NOx-N) progresses, and denitrification reaction (NOx-N→N ₂ ↑) progresses under anaerobic conditions. During the denitrification reaction, an organic carbon source is required, and
It is rational to use BOD as an organic carbon source, but in the case of the method described above, an absolute anaerobic tank 6 is used.
Since most of the BOD is absorbed, the concentration of BOD in the mixed liquid flowing from the anaerobic tank 6 into the Deitzch tank 1 is low. As a result, the organic carbon source in the deitsch tank 1 becomes insufficient, and even if the deitsch tank 1 is intermittently set as an anaerobic zone, the denitrification rate decreases.
In other words, nitrogen removal within the deutsch tank 1 becomes insufficient. In addition, NOx that remains without being denitrified flows into the settling tank 5, which inhibits the phosphorus discharge reaction by microorganisms in the absolute anaerobic tank 6 to which sludge is returned from the settling tank 5, and removes phosphorus. Reduce efficiency. Furthermore, in the above method, since an aerobic zone is formed in the deutsch tank and an anaerobic zone is formed in the subsequent stage thereof, the oxygen supply (aeration) by the aeration pipe 4 cannot be increased arbitrarily. Therefore, when raw water with a high BOD concentration flows into the deitsch tank 1 through the absolute anaerobic tank 6 due to fluctuations in the BOD amount of the raw water, the
Since BOD removal is not performed sufficiently, sludge with a high BOD is discharged to the sedimentation tank 5, where a phosphorus discharge reaction by microorganisms occurs, and the amount of phosphorus in the treated water 9 increases.

Furthermore, during normal operation of the above treatment method, sufficient nitrogen and phosphorus can be removed. However, when the amount of sludge accumulated in the settling tank 5 increases and the residence time of the sludge becomes longer, the anaerobic biological reaction progresses in the settling tank 5, and the discharge of phosphorus from the organisms occurs, resulting in the removal of phosphorus from the treated water 9. This results in a problem of high phosphorus concentration. As a countermeasure to these problems, sedimentation ponds 5
A method of adjusting the amount of sludge drawn out by discharging the sludge out of the system, or a method of increasing the amount of sludge returned from the settling tank 5 to the absolute anaerobic tank 6 are adopted. However, when the amount of sludge extracted is increased, the amount of sludge (MLSS) in the Deitzch tank 1 decreases, and nitrification and denitrification reactions are not performed sufficiently. On the other hand, absolute anaerobic tank 6
If the amount of sludge returned to the tank is increased, the residence time in the anaerobic tank 6 will be reduced, and NOx
If the amount increases, the microorganisms will not be able to discharge enough phosphorus, resulting in a decrease in the rate of accumulation of phosphorus in the sludge, that is, in the removal efficiency of phosphorus.

[Purpose of the invention]

The present invention solves the shortage of organic carbon sources during denitrification reactions when aerobic zones and anaerobic zones are intermittently formed in sewage treatment waterways as the capacity of the waterways becomes smaller.
Moreover, it prevents a mixed liquid with a large amount of BOD from being discharged from the sewage treatment waterway to the settling tank, and further effectively prevents the progress of anaerobic biological reactions in the settling tank due to an increase in the amount of sludge accumulated in the settling tank. The purpose of the present invention is to provide a method for treating organic wastewater.

[Summary of the invention]

The present invention introduces raw water through an absolute anaerobic tank into a sewage treatment waterway that has a water circulator and an aerator and forms a closed circuit on a plane, and introduces sludge flowing out from the waterway into a settling tank. In treating organic wastewater, the treated water in the pond is discharged to the outside of the system, and at least a portion of the settled sludge is returned to the absolute anaerobic tank and reintroduced to the waterway together with the raw water for circulation.
A step of intermittently performing aeration into the waterway, and introducing a part of the raw water into the waterway when aeration of the waterway is stopped, and post-aeration during the introduction of sludge from the waterway into the settling pond. The process of
In addition to continuously returning the sludge to the absolute anaerobic tank, when the sludge level in the settling tank exceeds a predetermined value, another pump returns the sludge to the waterway, and when the sludge level rises further, the sludge is returned to the waterway. This method is characterized by comprising a step of stopping sludge return and discharging the sludge to a surplus tank.

Hereinafter, the present invention will be explained in detail with reference to FIG.

FIG. 1 is a schematic diagram showing one embodiment of an apparatus used for organic wastewater treatment of the present invention. This device is roughly divided into an oxidation deutsch tank 11 which has a small capacity as a sewage treatment waterway (for example, the detsch pipe length is less than 150 m), and an oxidation detsch tank 11 connected to this detsch tank 11 via a pipe 12a. a sedimentation tank 13 into which sludge flows;
The absolute anaerobic tank 14 is connected to the deutsch tank 11 via a pipe 12b, into which raw water flows in through a pipe 12c, and the raw water and returned sludge are supplied to the deitch tank 11 through a pipe 12d.

A partition plate 15 is arranged in the center of the deitch tank 11, and the partition plate 15 forms a planar closed circuit in the tank 11. Further, near the connection of the piping 12d of the deutsch tank 11, there is a submersible propeller 16 for the purpose of water circulation, and a submersible propeller 16 for supplying oxygen near the rear stage of the propeller 16 and at a location separated by a predetermined distance from the propeller 16. Two first and second aeration pipes 17 ₁ , 17 ₂ as aerators
are placed respectively. These diffuser pipes 17 ₁ , 1
A blower 18 is connected to 7 ₂ via a pipe 12e. Further, a post-aeration tank 19 having a third aeration pipe 17 ₃ is interposed in the pipe 12 a connecting the deitch tank 11 and the sedimentation tank 13 .
The blower 18 is connected to the diffuser pipe ₁₇₃ via a pipe 12f. The raw water is supplied to the absolute anaerobic tank 14 via the pipe 12c, and a part of it can be supplied to the deutch tank 11 via the pipe 12g.
This raw water pipe 12g has a first solenoid valve 20 ₁
is interposed therein, and a timer 21 is connected to the solenoid valve ₂₀₁ . Furthermore, the blower 18
A second solenoid valve 20 ₂ is interposed in the pipe 12 e that supplies air from the air to each of the diffuser pipes 17 ₁ and 17 ₂ , and the timer 21 is connected to the second solenoid valve 20 ₂ . has been done. These timers 21
When the first solenoid valve 20 ₁ of the pipe 12g is open, the second solenoid valve 20 ₂ of the pipe 12e is closed, and the first solenoid valve 20 of the pipe 12g is closed.
0 ₁ is in the closed state, the second electromagnetic valve 20 ₂ of the pipe 12e is set to be in the open state.
In other words, the first solenoid valve ₂₀₁ is opened by the timer 21, and raw water is supplied from the pipe 12g to the Deitzch tank 1.
1, the second solenoid valve ₂₀₂ of the pipe 12e is closed, and the first,
The supply of air to the second diffuser pipes 17 ₁ and 17 ₂ is stopped, while the timer 21 causes the first solenoid valve 20
₁ is closed and the supply of raw water to the deutsch tank 11 is stopped, the second electromagnetic valve ₂₀₂ of the pipe 12e is opened and the blower 18 is
Air is supplied to the air diffuser pipes 17 ₁ and 17 ₂ to intermittently bring the inside of the Deitzch tank 11 into an anaerobic state and an aerobic state. Note that air is constantly supplied from the blower 18 to the third aeration pipe 17 ₃ of the post-aeration tank 19 .
Further, a stirring propeller 2 is provided in the absolute anaerobic tank 14.
A second pipe is installed to provide a flow velocity that does not allow the sludge to settle.

The sludge from the settling tank 13 is transferred to the absolute anaerobic tank 14.
A return pump 23 is interposed in the pipe 12b that returns the water to the water. Further, a pipe 12h in which a control pump 24 is interposed is connected to the sedimentation tank 13,
The pipe 12h is connected to a first branch pipe 25 ₁ for returning surplus sludge to the deitch tank 11, and a second branch pipe 25 ₂ for discharging sludge to the surplus tank. Further, first to third level meters 26 ₁ to 26 ₃ are sequentially arranged in the sedimentation tank 13 from the bottom side of the sedimentation tank 13 .
Each of the level meters 26 ₁ to 26 ₃ is controlled by a controller 2.
7 is connected. This controller 27 is connected to the control pump 24. Further, the controller 27 includes a third solenoid valve 20 ₃ interposed in the first branch pipe 25 ₁ and a third electromagnetic valve 20 3 interposed in the second branch pipe 25 1 .
5 ₂ are respectively connected to fourth solenoid valves 20 ₄ interposed therein. In such a control system, the first
The level meter 26 ₁ is located at the height of the sludge level (interface) during normal operation, and when the sludge level in the settling tank 13 reaches the second level meter 26 ₂ , the level meter 26 ₂ In response to the signal, the controller 27 opens the third solenoid valve 20 ₃ of the first branch pipe 25 ₁ and outputs a signal to operate the control pump 24, thereby displacing the sludge into the pipe 12h.
The water is then returned to the deutch tank 11 through the first branch pipe ₂₅₁ . Further, when the sludge level in the sedimentation tank 13 further rises and reaches the third level meter ₂₆₃ , the signal from the level meter ₂₆₃ causes the controller 27 to send the third branch pipe ₂₅₁ to the third level meter 263. A signal is output that closes the solenoid valve 20 ₃ and opens the fourth solenoid valve 20 ₄ of the second branch pipe 25 ₂ , thereby causing the sludge to pass through the pipe 12h and the second branch pipe 25 _2. It is designed to be drained into a surplus tank. When the sludge level in the sedimentation tank 13 reaches the first level meter 26 ₁ due to such discharge to the surplus tank, the level meter 26 ₁
A return signal is output from the controller 27 to the control pump 24 and the fourth solenoid valve 20 ₄ in response to a signal from the controller 27 .
The operation of the control pump 24 is stopped, and the fourth
The solenoid valve ₂₀₄ is closed. Note that the return pump 23 is always operated regardless of the operation of the control system.
When activated, the sludge in the sedimentation tank 13 is returned to the absolute anaerobic tank 14 through the pipe 12b.

Next, a treatment method will be explained with reference to the above-mentioned wastewater treatment apparatus.

First, raw water is introduced from the sedimentation tank 13 through the pipe 12c to the absolute anaerobic tank 14 to which sludge is returned by the return pump 23 and the pipe 12b, and the mixed solution of raw water and returned sludge is introduced into the Deitzch tank 11 through the pipe 12d.
supply to. The mixed liquid introduced into the Deitzch tank 11 is circulated by an underwater propeller 16 through a closed circuit partitioned by a partition plate 15 in the Deitzch tank 1. At this time, a timer 21 activates, for example, the first solenoid valve 20 ₁ for a certain period of time. By opening the second solenoid valve ₂₀₂ , the air from the blower 18 flows into the pipe 1.
Air (oxygen) is supplied to the first and second aeration pipes 17 ₁ and 17 ₂ in the Deitzch tank 11 through the Deitzch tank 11 through 2e, and the inside of the Deitzch tank 11 becomes aerobic. A nitrification reaction takes place. Also, after a certain period of time has elapsed, the timer 21 causes the first solenoid valve 2 to
By opening the solenoid valve 20 ₁ and closing the second solenoid valve 20 ₂ , the first and second aeration pipes 17 ₁ and 17 ₂ are opened.
At the same time, a portion of the raw water is supplied into the deitch tank 11 from the pipe 12g. At this time, the interior of the deitsch tank 11 becomes anaerobic, and the raw water flows in as an organic carbon source.
The presence of BOD allows the denitrification reaction to proceed sufficiently. By intermittently bringing the inside of the Deitzch tank 11 into anaerobic and aerobic conditions, sufficient nitrification and denitrification reactions can proceed even if the Deitzch tank 11 has a small capacity, and nitrogen removal from the raw water can be accomplished. can.

Next, the sludge in the deitsch tank 11 is transferred to the pipe 12.
a and the post-aeration tank 19 into the sedimentation tank 13.
In this way, the sludge in the Deitsch tank 11 is transferred to the aeration pipe 17 _.
Even if sludge in an anaerobic state is introduced into the settling tank 13 from the Deitzch tank 11, which has been brought into an anaerobic state by introducing air (oxygen) into the settling tank 13 through the aeration tank 19, While passing through the aeration tank 19, the sludge becomes aerobic and can prevent anaerobic sludge from flowing into the settling tank 13. Furthermore, even if sludge with residual BOD flows out from the Deitz tank 11 by introducing raw water into the Deitz tank 11, the BOD is removed while passing through the post aeration tank 19 (that is, the post aeration tank 19 in an aerobic state). Therefore, it is possible to prevent sludge containing BOD from flowing into the settling tank 13. As a result, the sedimentation tank 13 becomes anaerobic and the discharge of phosphorus by microorganisms due to the inflow of BOD can be suppressed, and the phosphorus that has accumulated in the sludge is discharged into the treated water, increasing its phosphorus concentration. can be prevented. Furthermore, by providing a post-aeration tank 19, the deitsch tank 11 is brought into an anaerobic state.
The sludge (sludge that has undergone a phosphorus discharge reaction) can be brought into an aerobic state and phosphorus can be absorbed. It can be prevented.

In the settling tank 13 into which the sludge has been introduced, the treated water is discharged outside the system, and a portion of the settled sludge is transferred to the pipe 12.
b and return pump 23 to the absolute anaerobic tank 14.
will be returned to. Through this circulation of Deitzch tank 11 -> sedimentation tank 13 -> absolute anaerobic tank 14 -> Deitzch tank 11, anaerobic -> aerobic stress is applied to the sludge, and as described above, phosphorus is biologically accumulated in the sludge. In such a circulation, the sludge level in the sedimentation tank 13 exceeds the level during normal operation, and the sludge level in the sedimentation tank 13 exceeds the level during normal operation.
When the level of the second level meter 26 ₂ disposed at is reached, the controller 27 connected to the level meter 26 ₂ is activated to open the third solenoid valve 20 ₃ and operate the control pump 24. The sludge in the sedimentation tank 13 is returned to the deitch tank 11 through the pipe 12h and the first branch pipe ₂₅₁ . Further, when the sludge level in the sedimentation tank 13 rises further and reaches the third level meter ₂₆₃ , the controller 27 connected to the level meter ₂₆₃ closes the third solenoid valve _203. , and the fourth solenoid valve ₂₀₄ is opened to drain the sludge from the sedimentation tank 13 to the pipe 12h and the second branch pipe 2.
5 Discharge to surplus tank through ₂ . When the discharge to the surplus tank progresses and the sludge level in the sedimentation tank 13 reaches the first level meter 26 ₁ , the controller 27 is reset, the control pump 24 is stopped, and the fourth solenoid valve 20 ₄ is activated. becomes closed. As a result, a certain amount of sludge is always returned from the settling tank 13 to the absolute anaerobic tank 14, and as the amount of sludge accumulated in the settling tank 13 increases, unlike in the past, the sludge is returned to the anaerobic tank. It is possible to reduce the residence time of raw water in the anaerobic tank and prevent insufficient phosphorus discharge. Therefore, even if the sludge level in the sedimentation tank 13 increases, the anaerobic to aerobic stress caused by the circulation can be effectively carried out and phosphorus in the raw water can be sufficiently removed.
Furthermore, when the level of sludge from the sedimentation tank 13 rises by a considerable amount, the return to the Deitsch tank 11 is stopped and the sludge is discharged to the surplus tank. It is possible to suppress the accumulation of phosphorus in the sludge by being returned to the sludge.

Therefore, according to the present invention, the shortage of organic carbon source during the denitrification reaction when an aerobic zone and an anaerobic zone are intermittently formed in the Deitzch tank 11 due to the reduction in the capacity of the Deitzch tank 11 can be solved. , and the datesch tank 1
From 1, it is possible to prevent sludge with a large amount of BOD in an anaerobic state from being discharged into the settling tank, and also to effectively prevent the progress of anaerobic biological reactions in the settling tank due to an increase in the amount of sludge accumulated in the settling tank. It is possible to provide a method for treating organic wastewater that can effectively remove phosphorus and phosphorus.

In addition, although the aeration pipe was used as an aerator in the above method, it is not limited to this. For example, a similar effect can be achieved using mechanical methods (aerators, water wheels, etc.).

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described with reference to FIG. 1 mentioned above.

Raw water was treated under the conditions shown below using the wastewater treatment equipment shown in Figure 1 above, and Figure 3 shows the changes in phosphorus concentration in the raw water and treated water, and Figure 4 shows the changes in nitrogen concentration in the raw water and treated water. are shown respectively. In addition, the third
The figure shows the previously proposed method shown in Figure 2, which describes the 50 days after wastewater treatment (in which the capacity of the Deitzch tank is reduced to intermittently create anaerobic and aerobic conditions, and the sludge exceeds a predetermined level in the settling tank. is absolutely returned to the anaerobic tank)
The changes in phosphorus concentration due to the method of the present invention are shown below. In addition, Fig. 4 shows the change in nitrogen concentration according to the previously proposed method (same conditions as described above) shown in Fig. 2 for 50 days after wastewater treatment, and the change in nitrogen concentration according to the method of the present invention after that. show.

<Conditions for wastewater treatment> Raw water (sand treatment water) Sedimentation treatment amount: 2 m ³ /day Anaerobic tank residence time: 1 to 2 hours DO: 0 to 0.5 mg/Deitsch tank; Capacity: 6 m in Deitsch pipe length Timer Intermittent switching time: 30 minutes MLSS; approx. 4000mg/ BOD load: 0.05Kg/Kg・MLSS-day Post-aeration tank residence time: 2 hours Sedimentation tank residence time: 3 hours Return rate: 100% 1st and 2nd level gauges The height difference between the second and third level meters: 20 cm As is clear from Figures 3 and 4, the method of the present invention has excellent removal performance for both nitrogen and phosphorus. It turns out that it has.

[Effects of the Invention] As detailed above, according to the present invention, denitrification can be achieved when aerobic zones and anaerobic zones are intermittently formed in the deitsch tank as the capacity of the wastewater treatment channel (deitsch tank) is reduced. This eliminates the lack of organic carbon source during the reaction, prevents sludge with a large amount of BOD from being discharged from the Deitzch tank to the settling tank in an anaerobic state, and further improves the settling tank by increasing the amount of sludge accumulated in the settling tank. It is possible to provide a method for treating organic wastewater that can effectively prevent the progress of anaerobic biological reactions and effectively remove nitrogen and phosphorus.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing one form of a wastewater treatment device used in the organic wastewater treatment method of the present invention, and FIG.
The figure is a schematic diagram of a wastewater treatment device used in the organic wastewater treatment method already proposed by the applicant, and Figure 3 shows changes in the phosphorus concentration of raw water and treated water during wastewater treatment according to this embodiment and the method already proposed. A characteristic diagram showing
FIG. 4 is a characteristic diagram showing changes in the nitrogen concentration of raw water and treated water during wastewater treatment according to this embodiment and the previously proposed method. 11... Oxidation Deutsch tank, 13
... Sedimentation tank, 14 ... Absolute anaerobic tank, 16 ... Submersible propeller, 17 ₁ to 17 ₃ ... Diffusion pipe, 18 ... Blower, 19 ... Post aeration tank, 20 ₁ to 20 ₄ ... Solenoid valve, 21...Timer, 23...Return pump, 24
...Control pump, 26 ₁ to 26 ₃ ...Level meter, 2
7...Controller.

Claims (1)

[Claims] 1. Raw water is introduced through an absolute anaerobic tank into a sewage treatment waterway that has a water circulator and an aerator and forms a two-dimensional closed circuit, and the sludge flowing out from the waterway is introduced into a settling tank, where it is treated. While discharging the water out of the system, at least a portion of the settled sludge is returned to the absolute anaerobic tank and is reintroduced and circulated into the waterway together with the raw water.When treating organic wastewater, aeration into the waterway is intermittently carried out. a step of introducing a part of the raw water into the waterway when aeration to the waterway is stopped; a step of post-aerating the sludge from the waterway while introducing it into the settling tank; The settled sludge is constantly and continuously returned to the absolute anaerobic tank by a sludge return pump, and when the sludge level in the settling tank exceeds a predetermined value, the sludge is returned to the waterway by another pump, and the sludge level is further increased. 1. A method for treating organic wastewater, comprising the step of stopping sludge return to the waterway and discharging the sludge to a surplus tank when the sludge rises.