JPH09276884A - Bacteria immobilized carrier separating method and treatment method of waste water utilizing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bacteria immobilized carrier separating method easy in maintenance, requiring no wide space and hard to damage a bacteria immobilized carrier and a waste water treatment method utilizing the same. SOLUTION: A centrifugal classifier 6 separating a bacteria immobilized carrier 5 from waste water by utilizing a head is installed in the vicinity of the outlet of a reaction tank in which a bacteria immobilized carrier 5 is charged or at the part between the reaction tank and a next reaction tank. By this constitution, the separated bacteria immobilized carrier 5 is returned to an upstream side and the concn. of the bacteria immobilized carrier is uniformized. This centrifugal classifier 6 requires no large power different from a pump and has no possibility of clogging and does not damage the bacteria immobilized carrier 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating a microorganism-immobilized carrier loaded in a sewage treatment tank or the like from wastewater, and a wastewater treatment method using the method.

[0002]

2. Description of the Related Art In wastewater treatment utilizing a biological reaction by microorganisms, a microorganism-immobilized carrier is put into wastewater to increase the biological concentration. And in order to prevent this microorganism-immobilized carrier from flowing out of the reaction tank,
It is usual to provide a screen at the outlet of the reaction tank to separate the microorganism-immobilized carrier. However, this screen separation method requires frequent cleaning work because contaminants in the wastewater tend to adhere to the screen or become entangled with the screen, which requires frequent cleaning work. There is a problem.

Further, as disclosed in Japanese Patent Application Laid-Open No. 63-278595, a sedimentation section having a low flow rate is provided near the outlet of the reaction tank into which the microorganism-immobilized carrier is charged, and the microorganism-immobilized carrier is provided therein. There is also known a method in which the microorganism-immobilized carrier is precipitated from the bottom of the precipitation part and the concentrated microorganism-immobilized carrier is taken out. However, this method has a space problem because it is necessary to provide a precipitation part inside the reaction tank.

Further, as disclosed in JP-A-5-261393, a method of sending waste water containing a microorganism-immobilized carrier to a fluid cyclone by a pump and centrifuging it is also considered. In this method, the pump is used. There is a problem that the microorganism-immobilized carrier is physically damaged when passing through the cell and the life of the microorganism-immobilized carrier is shortened.

[0005]

The present invention solves the above-mentioned problems of the prior art, is easy to maintain, does not require a wide space, and is a method for separating a microorganism-immobilized carrier which is less likely to damage the microorganism-immobilized carrier. And to provide a method for treating wastewater using this.

[0006]

The method for separating a microorganism-immobilized carrier according to the first aspect of the present invention, which has been made to solve the above-mentioned problems, comprises a reaction tank containing the microorganism-immobilized carrier and a reaction tank for the next step. Or a centrifuge classifier is installed near the outlet of the reaction tank in which the microorganism immobilization carrier is placed, and the microorganism immobilization carrier is used as waste water by utilizing the gentle flow in the centrifugal classifier caused by the drop between both reaction tanks. It is characterized by being separated and recovered from the.

The wastewater treatment method of the second invention made to solve the above problems is a treatment method of wastewater by a biological nitrification denitrification method using a microorganism immobilization carrier in a nitrification tank. Characterized by providing a centrifugal classifier near the exit of the nitrification tank or between the nitrification tank and the precipitation tank to separate the microorganism-immobilized carrier from the wastewater, and returning the separated microorganism-immobilized carrier to the inlet of the nitrification tank. It is what A third invention is a method for treating wastewater by a biological nitrification denitrification method using a microorganism immobilization carrier in a denitrification tank and a nitrification tank, which is near the outlet of the nitrification tank or between the nitrification tank and the precipitation tank. The method is characterized in that a centrifugal classifier is provided to separate the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is returned to the inlet of the denitrification tank. A fourth invention is a method for treating wastewater by a biological nitrification denitrification method using a microorganism immobilization carrier in a denitrification tank and a nitrification tank, which is provided near the outlet of the denitrification tank or between the denitrification tank and the nitrification tank. Also, a centrifugal classifier is installed near the exit of the nitrification tank or between the nitrification tank and the sedimentation tank to separate the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is connected to the inlet of the denitrification tank and the nitrification tank. It is characterized by returning each to the entrance.

A fifth invention made to solve the same problem is a method for treating wastewater by an anaerobic / anoxic / aerobic method using a microorganism-immobilized carrier in an aerobic tank. A centrifugal classifier is installed near the outlet of the tank or between the aerobic tank and the sedimentation tank to separate the microorganism-immobilized carrier from the wastewater and return the separated microorganism-immobilized carrier to the inlet of the aerobic tank. It is a feature. A sixth invention is a method of treating wastewater by an anaerobic / anoxic / aerobic method using a microorganism-immobilized carrier in an anoxic tank and an aerobic tank, which is used near the outlet of the aerobic tank or the aerobic tank. It is characterized in that a centrifugal classifier is provided between the sedimentation tank and the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is returned to the inlet of the anoxic tank.

A seventh aspect of the present invention is a method for treating wastewater by an anaerobic / anoxic / aerobic method using a microorganism-immobilized carrier in an anoxic tank and an aerobic tank. A centrifugal classifier is installed between the oxygen tank and the aerobic tank, or near the outlet of the aerobic tank or between the aerobic tank and the precipitation tank to separate the microorganism-immobilized carrier from the wastewater and immobilize the separated microorganisms. The chemical carrier is returned to the inlet of the anoxic tank and the inlet of the aerobic tank, respectively. Furthermore, the eighth invention is a method for treating wastewater by a standard activated sludge method using a microorganism-immobilized carrier in an aeration tank, wherein a centrifugal classifier is provided near the outlet of the aeration tank or between the aeration tank and the sedimentation tank. Is provided to separate the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is returned to the inlet of the nitrification tank.

In any of the inventions, it is preferable to use an air lift pump to return the separated and recovered microorganism-immobilized carrier, and the drop between the reaction tank provided with a centrifugal classifier and the reaction tank of the next step is 1 m or less. It is preferable that

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of each invention will be described below with reference to the drawings. FIG. 1 shows the first aspect of the invention, in which 21 is a reaction tank into which the microorganism-immobilized carrier 22 is charged, 23 is a reaction tank in the next step, and 24 is a centrifugal classifier installed between them. The water surface of the reaction tank 21 is lower than that of the reaction tank 23, but the level difference is 1 m or less. The centrifugal classifier 24 may be installed near the outlet of the reaction tank 21 inside the tank.

The microorganism immobilization carrier 22 has a specific gravity of 1.02.
Granules within the range of ~ 1.1 are commonly used,
The particle size is usually about 3 to 15 mm. As the material, polyvinyl alcohol (PVA), polyethylene glycol (PEG), or other hydrophilic gel, as well as sponge or plastic are used. On the surface of the microorganism-immobilized carrier 22, microorganisms such as nitrifying bacteria are loaded at high density.

Centrifugal classifier 24 has a height of 1/2 of diameter D
~ 2 D, inlet 26 diameter is 1/8 ~ 1/3 D, siphon tube 28
Has a diameter of 1/10 to 1 / 3D, the lower outlet 27 has a diameter of 1/3 to 1 / 6D, and the taper angle of the bottom is 45 ° to 90 °. Also siphon tube 28
It is desirable to provide a structure for controlling the flow such as a baffle plate that prevents the growth of the forced vortex generated when the wastewater flows out at the lower part of the. Wastewater containing the microorganism-immobilized carrier 22 in the reaction tank 21 flows in tangentially from the inlet 26 near the upper end thereof, and forms a gentle vortex flow inside the tank 25. Then, due to the centrifugal force (about 0.5 to 1.5 G) generated by this, the microorganisms-immobilized carrier 22 having a specific gravity slightly larger than that of the waste water gathers outside and is taken out from the outlet 27 at the bottom of the tank. Further, since a region containing almost no microorganism-immobilized carrier 22 is formed in the center of the tank 25, a siphon pipe 28 installed in the upper center of the tank 25 is used to remove wastewater containing no microorganism-immobilized carrier 22. Only is taken out to the reaction tank 23,
The microorganism-immobilized carrier 22 thus separated and recovered is returned to the inlet of the reaction tank 21 or the inlet of the reaction tank in the previous step, and the concentration of the microorganism-immobilized carrier 22 in the reaction tank 21 is maintained constant. It

The flow in the centrifugal classifier 24 is caused by the drop between the reaction tank 21 and the reaction tank 23.
As described above, this drop is 1 m or less. Therefore, the flow in the centrifugal classifier 24 becomes a gentle flow. Further, since it does not pass through the pump as in the conventional case, the microorganism-immobilized carrier 22 is not physically damaged. Further, since the centrifugal classifier 24 does not cause troubles due to blockage like a conventional screen, there is an advantage that maintenance work is not required.

The method for separating the microorganism-immobilized carrier of the present invention comprises:
It is easy to maintain and manage, and the microorganism-immobilized carrier can be separated and recovered without requiring a large amount of power, and further, the damage to the carrier due to the operation can be almost eliminated.

Next, embodiments of the wastewater treatment methods of the second to eighth inventions will be described. FIG. 2 is a view showing an embodiment of the second invention, 1 is a reaction tank for biological nitrification and denitrification, 2 is a denitrification tank, 3 is a nitrification tank, and 4 is a precipitation tank. Is. In the first invention, the microorganism immobilization carrier 5 is added to the nitrification tank 3.
Has been turned on. 6 is a centrifugal classifier installed near the outlet of the nitrification tank 2. The liquid level of the settling tank 4 is lower than that of the nitrification tank 3, but the drop is 1 m or less. The centrifugal classifier 6 may be provided between the nitrification tank 3 and the precipitation tank 4.

When the wastewater in the nitrification tank 3 containing the microorganism-immobilized carrier 5 flows into the centrifugal classifier 6, the wastewater containing the microorganism-immobilized carrier 5 in a high density and the microorganism-immobilized carrier 5 as described above. Separated into wastewater that does not contain. Waste water containing no microorganism-immobilized carrier 5 is guided to the settling tank 4 from the siphon tube 7 above the centrifugal classifier 6, and waste water containing the microorganism-immobilized carrier 5 at high density is connected to the tube 8 below the centrifugal classifier 6. It is guided to the inlet of the nitrification tank 3 by an air lift pump (not shown). The microorganism-immobilized carrier 5 thus separated and recovered is used in the nitrification tank 3
The concentration of the microorganism-immobilized carrier 5 in the nitrification tank 3 is maintained uniform by returning it to the inlet of the. The ratio of the amount of water to the settling tank 4 and the amount of water to the inlet of the nitrification tank 3 is 1: 0.5 to 1: 3.
Is desirable. A part of the wastewater (nitrification solution) that does not contain the microorganism-immobilized carrier 5 is returned to the entrance of the denitrification tank 2 by the return pipe 9, and nitrification denitrification is performed by circulating the nitrification solution.

According to the present invention, the microorganism-immobilized carrier 5 can be separated and recovered, and the microorganism-immobilized carrier 5 in the nitrification tank 3 can be separated and recovered.
The operation can be performed while maintaining a uniform concentration. Further, since the microorganisms-immobilized carrier 5 is separated and collected by the centrifugal classifier 6 using the head, maintenance and management are easy unlike the case where the conventional screen is used, and moreover the physical damage of the microorganisms-immobilized carrier 5 is prevented. You can almost eliminate it.

FIG. 3 is a view showing an embodiment of the third invention, in which a common microbial immobilization carrier 5 is put in the denitrification tank 2 and the nitrification tank 3. The microorganism immobilization carrier 5 is separated from the wastewater by a centrifugal classifier 6 provided near the outlet of the nitrification tank 3 (or between the nitrification tank 3 and the precipitation tank 4), and the separated microorganism immobilization carrier 5 is denitrified. Return to 2 entrance. According to this method, in addition to the same effect as the second invention,
Since the nitrification solution is returned to the denitrification tank 2 together with the microorganism-immobilized carrier 5, there is an advantage that it can be used for circulation of the nitrification solution.

FIG. 4 is a diagram showing an embodiment of the fourth invention, in which the microorganism immobilization carrier 5 is placed in each of the denitrification tank 2 and the nitrification tank 3. A centrifugal classifier 6a is provided near the exit of the denitrification tank 2 (or between the denitrification tank 2 and the nitrification tank 3), and near the exit of the nitrification tank 3 (or between the nitrification tank 3 and the precipitation tank 4). Is also provided with a centrifugal classifier 6b. Then, the microorganism-immobilized carrier 5 separated and collected from the wastewater by the centrifugal classifier 6a is returned to the inlet of the denitrification tank 2, and the centrifugal classifier
The microorganism-immobilized carrier 5 separated and collected from the wastewater by 6b is returned to the entrance of the nitrification tank 3. The present invention has an advantage that a dedicated microorganism immobilization carrier 5 can be used for the denitrification tank 2 and the nitrification tank 3.

FIG. 5 is a view showing an embodiment of the fifth invention, in which the microorganism-immobilized carrier 5 is put in an aerobic tank 10 for treating wastewater by an anaerobic / anoxic / aerobic method. . A centrifugal classifier 6 is provided near the outlet of the aerobic tank 10 (or between the aerobic tank 10 and the precipitation tank 4), and the microorganism immobilization carrier 5 recovered from the wastewater by the centrifugal classifier 6 is aerobic. Tank
Returned to 10 entrances. The function and effect are basically the same as those of the second invention.

FIG. 6 is a diagram showing an embodiment of the sixth invention, in which microorganisms are commonly immobilized in the anoxic tank 11 and the aerobic tank 10 for treating wastewater by the anaerobic / anoxic / aerobic method. The carrier 5 is loaded. Similar to the third invention, the microorganism immobilization carrier 5 is separated from the wastewater by the centrifugal classifier 6 provided near the outlet of the aerobic tank 10 (or between the aerobic tank 10 and the precipitation tank 4), and separated. The microorganism-immobilized carrier 5 thus prepared is returned to the inlet of the anoxic tank 11. The function and effect are basically the same as those of the third invention.

FIG. 7 is a diagram showing an embodiment of the seventh invention, in which microorganisms are immobilized in the anoxic tank 11 and the aerobic tank 10 for treating wastewater by the anaerobic / anoxic / aerobic method. The carrier 5 is loaded. As in the fourth aspect, a centrifugal classifier 6a is provided near the outlet of the anoxic tank 11 (or between the anoxic tank 11 and the aerobic tank 10), and near the outlet of the aerobic tank 10. A centrifugal classifier 6b is also provided (or between the aerobic tank 10 and the precipitation tank 4). Then, the microorganism-immobilized carrier 5 recovered by the centrifugal classifier 6a is returned to the inlet of the anoxic tank 11, and the microorganism-immobilized carrier 5 recovered by the centrifugal classifier 6b.
Are returned to the inlet of the aerobic tank 10. The function and effect are basically the same as those of the fourth invention.

FIG. 8 is a view showing an embodiment of the eighth invention, in which each microorganism immobilization carrier 5 is put in an aeration tank 12 for treating wastewater by the standard activated sludge method. The carrier 5 for immobilizing the microorganisms in the aeration tank 12 is the aeration tank 12
Have been returned to the entrance. The function and effect are basically the same as those of the second invention.

[0025]

Example (Example of the first invention) In the vicinity of the outlet of a nitrification tank in which a microorganism-immobilized carrier made of PVA having an average particle size of 4 mm and a specific gravity of 1.03 was added at a concentration of 10%,
A centrifugal classifier was installed to separate the microorganism-immobilized carrier. The centrifugal classifier has a diameter of 19 cm, a height of 17 cm, and a taper angle of 60 ° at the bottom, and the drop between the nitrification tank and the precipitation tank is 50 cm. The wastewater from the nitrification tank containing the microorganism-immobilized carrier flows into the centrifugal classifier at a flow rate of 50 L / min and a flow rate of 1 m / sec, and the wastewater from which the microorganism-immobilized carrier is separated is discharged to the sedimentation tank at a flow rate of 25 L / min. Was done. On the other hand, wastewater containing 20% of the microorganism-immobilized carrier was taken out from the outlet of the bottom of the centrifugal classifier and returned to the inlet of the tank. At this time, the ratio of the microorganism-immobilized carrier flowing out from the centrifugal classifier was 0.001%, and the microorganism-immobilized carrier was separated with extremely high efficiency. Moreover, the separated microorganism-immobilized carrier was observed, but no damage was observed at all.

(Embodiment of the second invention) An average particle diameter of 4 m was added to a nitrification tank having a volume of 13 m ³ divided into 5 stages with a treatment amount of 70 L / min.
1.3 m ^{3 of} microorganism-immobilized carrier consisting of PVA with a specific gravity of 1.03
(10% of the volume of the nitrification tank) was charged, and a 140 L / min treatment centrifugal classifier was installed near the outlet (fifth stage) to separate the microorganism-immobilized carrier. When operating at 70 L / min to the settling tank and returning to the nitrification tank inlet at 70 L / min, the outflow rate of the microorganism-immobilized carrier into the wastewater toward the settling tank was 0.001%, and the microorganisms in the returned solution were The concentration of the immobilized carrier was about 20%. The concentration of the microorganism-immobilized carrier at each stage of the nitrification tank is the first stage.
10.5%, 2nd stage 10%, 3rd stage 9.5%, 4th stage 9.5%, 5
The concentration of the microorganism-immobilized carrier in each part of the nitrification tank became uniform at the 10.5% stage. Furthermore, no damage was observed on the microorganism-immobilized carrier even after continuous operation for 3 months.

(Embodiment of the third invention) An average particle size is applied to a denitrification tank having a volume of 8 m ³ divided into three stages of a treatment amount of 70 L / min and a nitrification tank having a volume of 13 m ³ divided into five stages. 4mm, specific gravity 1.03
2.1m ³ (10% of the total volume of both tanks) of microorganism-immobilized carrier consisting of PVA was added, and it was near the exit of the nitrification tank (5th stage).
A 140 L / min treatment centrifugal classifier was installed to separate the microorganism-immobilized carrier. 70 L / min to settling tank, 70 to return to denitrification tank inlet
When operated at L / min, the outflow rate of the microorganism-immobilized carrier into the wastewater toward the settling tank was 0.001%, and the concentration of the microorganism-immobilized carrier in the returned liquid was approximately 20%. The concentration of the microorganism-immobilized carrier at each stage of the denitrification tank is 10.5
%, 2nd stage 10%, 3rd stage 9.5%, the concentration of the microbial immobilization carrier at each stage of the nitrification tank is 1st stage 10%, 2nd stage 10%, 3rd stage 10%, 4th stage 9.5 %, It was almost unchanged at the 5th stage 10.5%, and the concentration of the microorganism-immobilized carrier in each part of the denitrification tank and the nitrification tank became uniform. Even after continuous operation for 3 months, no damage was found on the microorganism-immobilized carrier.

[0028]

As described above, according to the method for separating a microorganism-immobilized carrier of the first invention, the microorganism-immobilized carrier can be separated from wastewater without damaging the microorganism-immobilized carrier.
In addition, unlike the conventional screen separation method, there is no risk of clogging, so there is no need for maintenance, and there is an advantage that no running cost is required because a pump is not required. Further, since it is not necessary to provide a settling section in the tank, it has many advantages such as not requiring a large space. According to the wastewater treatment methods of the second to eighth inventions, the concentration of the microorganism-immobilized carrier in the reaction tank can be kept uniform by collecting and returning the microorganism-immobilized carrier in the wastewater. There are advantages.

[Brief description of drawings]

FIG. 1 is a flow sheet showing an embodiment of a first invention.

FIG. 2 is a flow sheet showing an embodiment of the second invention.

FIG. 3 is a flow sheet showing an embodiment of a third invention.

FIG. 4 is a flow sheet showing an embodiment of a fourth invention.

FIG. 5 is a flow sheet showing an embodiment of the fifth invention.

FIG. 6 is a flow sheet showing an embodiment of the sixth invention.

FIG. 7 is a flow sheet showing an embodiment of a seventh invention.

FIG. 8 is a flow sheet showing an embodiment of the eighth invention.

[Explanation of symbols]

1 reaction tank, 2 denitrification tank, 3 nitrification tank, 4 precipitation tank, 5
Microorganism immobilization carrier, 6 centrifugal classifier, 6a centrifugal classifier, 6b centrifugal classifier, 7 siphon tube, 8 carrier return tube, 9 nitrification solution return tube, 10 aerobic tank, 11 anoxic tank,
12 aeration tank, 21 reaction tank, 22 microbial immobilization carrier, 23 next step reaction tank, 24 centrifugal classifier, 25 tank, 26 inlet,
27 outlet, 28 siphon tube

Claims (11)

[Claims] 1. A reaction tank containing a microorganism-immobilized carrier,
It is characterized in that a centrifugal classifier is provided between the reaction tank of the next step and the microorganism-immobilized carrier is separated and recovered from wastewater by utilizing the gentle flow in the centrifugal classifier caused by the drop between both reaction tanks. A method for separating a microorganism-immobilized carrier. 2. A method for treating wastewater by a biological nitrification denitrification method using a microorganism-immobilized carrier in a nitrification tank, wherein a centrifugal classifier is provided near the outlet of the nitrification tank or between the nitrification tank and the precipitation tank. A method for treating wastewater, comprising separating the microorganism-immobilized carrier from the wastewater and returning the separated microorganism-immobilized carrier to the inlet of the nitrification tank. 3. A method for treating wastewater by a biological nitrification denitrification method using a microorganism-immobilized carrier in a denitrification tank and a nitrification tank,
A centrifugal classifier is provided near the outlet of the nitrification tank or between the nitrification tank and the precipitation tank to separate the microorganism-immobilized carrier from the wastewater.
A method for treating wastewater, which comprises returning the separated microorganism-immobilized carrier to the inlet of a denitrification tank. 4. A method for treating wastewater by a biological nitrification denitrification method using a microorganism-immobilized carrier in a denitrification tank and a nitrification tank,
Centrifugal classifiers are installed near the outlet of the denitrification tank or between the denitrification tank and the nitrification tank, and between the outlet of the nitrification tank or between the nitrification tank and the precipitation tank to separate the microorganism-immobilized carrier from the wastewater and separate it. A method for treating wastewater, which comprises returning the immobilized microorganism-supporting carrier to the entrance of the denitrification tank and the entrance of the nitrification tank, respectively. 5. Anaerobic using a microorganism-immobilized carrier in an aerobic tank.
A method for treating wastewater by the anoxic / aerobic method, in which a microorganism-immobilized carrier is separated from the wastewater by installing a centrifugal classifier near the outlet of the aerobic tank or between the aerobic tank and the precipitation tank. A method for treating wastewater, which comprises returning the immobilized microorganism-supporting carrier to the inlet of an aerobic tank. 6. A method for treating wastewater by an anaerobic / anoxic / aerobic method using a microorganism-immobilized carrier in an anoxic tank and an aerobic tank, which is near the outlet of the aerobic tank or an aerobic tank and a precipitation tank. A method for treating wastewater, characterized in that a centrifugal classifier is provided between the separator and the separator to separate the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is returned to the inlet of the anoxic tank. 7. A method for treating wastewater by an anaerobic / anoxic / aerobic method using a microorganism-immobilized carrier in an anoxic tank and an aerobic tank, the method being in the vicinity of the outlet of the anoxic tank or in the anoxic tank and aerobic. A centrifugal classifier is installed between the tank and near the outlet of the aerobic tank or between the aerobic tank and the precipitation tank to separate the microorganism-immobilized carrier from the wastewater, and the separated microorganism-immobilized carrier is anoxic. A method for treating wastewater, characterized in that the wastewater is returned to the inlet of the tank and the inlet of the aerobic tank. 8. A method for treating wastewater by a standard activated sludge method using a microorganism-immobilized carrier in an aeration tank, wherein a centrifugal classifier is provided near the outlet of the aeration tank or between the aeration tank and the sedimentation tank. A method for treating wastewater, which comprises separating a microorganism-immobilized carrier from wastewater and returning the separated microorganism-immobilized carrier to the inlet of a nitrification tank. 9. The method for treating wastewater according to claim 2, wherein an air lift pump is used to return the separated and recovered microorganism-immobilized carrier. 10. The method for separating a microorganism-immobilized carrier according to claim 1, wherein the difference between the reaction tank provided with the centrifugal classifier and the reaction tank in the next step is 1 m or less. 11. The method for treating wastewater according to claim 2, wherein the drop between the reaction tank provided with the centrifugal separator and the reaction tank used in the next step is 1 m or less.