JP7181251B2 - Organic wastewater treatment method and organic wastewater treatment apparatus - Google Patents

Description

The present invention relates to an organic wastewater treatment method and an organic wastewater treatment apparatus.

In recent years, activated sludge treatment is generally used in many cases to treat organic wastewater with a high biochemical oxygen demand (BOD). Activated sludge treatment is easy to maintain and has low running costs. Activated sludge treatment enables stable removal of BOD in influent raw water, and has the advantage of always obtaining good treated water quality. Therefore, activated sludge treatment is widely used for treatment of various organic wastewaters such as domestic wastewater and industrial wastewater.

However, it is known that activated sludge treatment generates excess sludge due to BOD removal. In particular, wastewater with a high BOD concentration generates a large amount of excess sludge, so the ratio of the cost associated with the disposal of excess sludge to the total treatment has increased, and the reduction of excess sludge has become a major issue.

As a method for reducing excess sludge, there is a method of multi-stage treatment that combines high load and low load. For example, in Japanese Patent Application Laid-Open Nos. 2010-069482 and 2005-211879, dispersed bacteria are generated in the first biological treatment tank, which is the high-load tank in the previous stage, and the second biological treatment, which is the low-load tank in the latter stage. A method for reducing the volume of sludge through the food chain has been disclosed, in which predation by dispersed fungi of protozoa and metazoans in tanks is used to reduce the volume of sludge.

JP 2010-069482 A Japanese Patent Application Laid-Open No. 2005-211879

In the biological treatment methods described in Patent Documents 1 and 2, in order to improve treatment efficiency and reduce the amount of excess sludge generated, the BOD load and pH of the first biological treatment tank and the second biological treatment tank are set within a predetermined range. It is described that it is necessary to control within In this respect, when the properties of raw water are relatively stable, it is relatively easy to control the BOD load and pH of each biological treatment tank.

However, when a large concentration fluctuation occurs in the raw water for some reason, it becomes difficult to control the BOD load and pH of the biological treatment tank, and the generation of dispersed bacteria in the first biological treatment tank may become unstable. As a result, the effect of reducing the volume of sludge in the food chain by feeding on dispersed bacteria cannot be obtained sufficiently. In addition, if dispersed bacteria are generated excessively in the first biological treatment tank due to changes in the properties of the raw water, and dispersed bacteria remain in the second biological treatment tank, there is a risk of deteriorating the quality of the treated water.

In view of the above problems, the present invention provides an organic wastewater treatment method and an organic wastewater treatment apparatus capable of maintaining stable treated water quality at all times and reducing the amount of excess sludge generated.

As a result of intensive studies by the present inventors in order to solve the above problems, in order to always obtain stable treated water quality even if the properties of raw water change, aerobic treatment is performed in multiple stages, and the first aerobic treatment We have found that it is effective to pay attention to the total alkalinity of the tank.

In one aspect of the embodiment of the present invention completed based on the above findings, raw water containing organic matter is aerobicly treated in a first aerobic treatment tank adjusted to have a total alkalinity of 100 to 500 mg / L. a process of obtaining a first aerobic treatment liquid by aerobic treatment, and a second aerobic treatment liquid by aerobically treating the first aerobic treatment liquid in a second aerobic treatment tank holding a carrier. A method of treating organic wastewater comprising:

In one embodiment of the organic wastewater treatment method according to the embodiment of the present invention, the BOD volume load of the first aerobic treatment tank is adjusted to 5 to 20 kg/m ³ /d, and In the aerobic treatment consisting of the aerobic treatment tank and the second aerobic treatment tank, the BOD volume load is adjusted so that the BOD volume load of all tanks is 1-5 kg/m ³ /d.

In another embodiment of the organic wastewater treatment method according to the embodiment of the present invention, the floating sludge in the second aerobic treatment tank is subjected to the first aerobic treatment so that the flow rate is 10 to 300% of the raw water inflow. It also has a return sludge treatment for returning to the tank.

In still another embodiment of the method for treating organic wastewater according to the embodiment of the present invention, the amount of aeration is adjusted so that the dissolved oxygen concentration in the first aerobic treatment tank is 2-7 mg/L.

In still another embodiment of the organic wastewater treatment method according to the embodiment of the present invention, the carrier is added so that the carrier filling rate in the second aerobic treatment tank is 5 to 30% by volume.

In another aspect of the embodiment of the present invention, raw water containing organic matter is aerobically treated to obtain a first aerobic treatment liquid; Alkalinity adjusting means for adjusting the total alkalinity of the raw water to be 100 to 500 mg/L, and the first aerobic treatment liquid is aerobically treated using a carrier to obtain a second aerobic treatment liquid. and a second aerobic treatment tank.

ADVANTAGE OF THE INVENTION According to this invention, the organic wastewater treatment method and organic wastewater treatment apparatus which can always maintain stable treated water quality and can reduce the amount of excess sludge generated can be provided.

1 is a schematic diagram showing an example of an organic wastewater treatment apparatus according to a first embodiment of the present invention; FIG. FIG. 2 is a schematic diagram showing an example of an organic wastewater treatment apparatus according to a second embodiment of the present invention; It is a schematic diagram showing an example of a treatment flow of a conventional wastewater treatment apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. The embodiments shown below are examples of devices and methods for embodying the technical idea of the present invention. It is not specific.

The organic wastewater treatment apparatus according to the first embodiment of the present invention, as shown in FIG. a second aerobic treatment tank 1b for obtaining a second aerobic treatment liquid by aerobically treating the first aerobic treatment liquid using a carrier; and inside the first aerobic treatment tank 1a Alkalinity adjustment means 2 for adjusting the total alkalinity of the raw water to be 100 to 500 mg / L.

The raw water is not particularly limited as long as it contains organic matter, and various organic wastewaters containing organic matter such as domestic wastewater, sewage, food factories, chemical factories, and pulp factories are used. Although not limited to the following, foods with low inorganic floating substances (SS) (for example, SS is about 0 to 100 mg / L) and easily degradable organic matter concentration is high (for example, BOD value is about 1000 to 2000 mg / L) Raw water such as drinking waste water and domestic waste water is particularly preferably treated.

As the first aerobic treatment tank 1a and the second aerobic treatment tank 1b, activated sludge containing bacteria capable of decomposing organic matter (BOD) in the raw water flowing into the aerobic treatment tank 1, microorganisms preying on bacteria, etc. is housed therein and the raw water is suitably treated by aeration to obtain an aerobic treated liquid, but for example, an aeration tank or a fluidized carrier tank can be used.

The first aerobic treatment liquid treated in the first aerobic treatment tank 1a is introduced into the second aerobic treatment tank 1b and further treated to obtain treated water. In the first aerobic treatment tank 1a, it is extremely important to appropriately maintain various conditions such as BOD volume load, DO, and pH, which are important for growing microorganisms. In particular, in order to treat organic wastewater in consideration of fluctuations in concentration, flow rate, etc. of raw water, the amount of raw water inflow is And it is important to adjust the total alkalinity of the first aerobic treatment tank 1a.

The role of the first aerobic treatment tank 1a is to grow dispersible bacteria and the like in the first aerobic treatment tank 1a, remove BOD, and then prey on sludge with microorganisms living in the second aerobic treatment tank 1b. The purpose is to reduce the sludge volume by At that time, by maintaining the total alkalinity in the first aerobic treatment tank 1a under constant conditions, the pH is stabilized by the buffering action, so that the treated water quality can be constantly maintained and the excess sludge can be removed. It is possible to reduce the amount of generation.

In the first embodiment, the raw water flowing into the first aerobic treatment tank 1a is added with an alkali agent as appropriate so that the total alkalinity of the first aerobic treatment tank 1a is 100 to 500 mg / L. adjust the total alkalinity of The total alkalinity of the raw water in the first aerobic treatment tank 1a is preferably adjusted to 100-400 mg/L, more preferably 200-300 mg/L.

When the pH of the first aerobic treatment tank 1a fluctuates, the microflora of microorganisms in the aerobic treatment tank changes, which causes the quality of treated water to become unstable and the sludge volume reduction effect to decrease. For example, if the amount of alkali added is too small, there is concern that fungi will grow in the first aerobic treatment tank 1a and produce acid, which will lower the pH and prevent the preferential growth of dispersive bacteria from progressing. On the other hand, when the addition of alkali is excessive, the pH of the first aerobic treatment tank 1a becomes high and the sludge is easily dispersed. This makes it difficult for sludge to adhere to the carrier, resulting in deterioration of treated water quality and loss of sludge volume reduction effect. In addition, if the alkalinity is too low or too high, the pH of the treated water will be out of the range of the discharge standard value, so it will be necessary to adjust the pH before discharge. This incurs the cost of chemicals such as acids and alkalis. That is, from the viewpoint of the sludge reduction effect and maintenance of stable water quality, it is important to adjust the total alkalinity and maintain the pH buffering action so as not to cause pH fluctuation in the first aerobic treatment tank 1a. .

It is preferable that the BOD volume load of the first aerobic treatment tank 1a is determined to be 5 to 20 kg/m ³ /d, and the flow rate is adjusted. Further, the BOD volume load in the entire aerobic treatment tank 1 (all tanks) including the first aerobic treatment tank 1a and the second aerobic treatment tank 1b is adjusted to 1 to 5 kg/m ³ /d. It ^is preferable to adjust the BOD volume load of the second aerobic treatment tank 1b. Thus, it is effective to control the treatment conditions for BOD removal in the first aerobic treatment tank 1a.

The dissolved oxygen concentration (DO) in the first aerobic treatment tank 1a is preferably adjusted to 2 to 7 mg/L, more preferably 4 to 6 mg/L. more effective. The DO in the second aerobic treatment tank 1b is preferably adjusted to 2-7 mg/L, more preferably about 3-5 mg/L. DO affects the bacterial flora in the aerobic treatment tank, and DO control enables the maintenance of effective bacterial flora to reduce excess sludge and the maintenance of stable treated water quality. The pH of the first aerobic treatment tank 1a is preferably 6.0-8.5, more preferably 7.0-8.0. The MLSS of the first aerobic treatment tank 1a can be 1000-4000 mg/L, more typically 1000-2000 mg/L.

It is preferable to charge the carrier so that the carrier filling rate in the second aerobic treatment tank 1b is 5 to 30% by volume, more preferably 15 to 30% by volume. Examples of microorganism carriers include synthetic polymers such as polyethylene glycol (PEG), polyvinyl alcohol (PVA), polyacrylamide, and photocurable resins, gel carriers using polymers such as carrageenan and sodium alginate, polyethylene, polyurethane, polypropylene, and the like. and a fluid carrier. The shape of the carrier may be spherical, square or cylindrical, and its effective diameter is preferably a diameter that allows stable separation from the screen provided at the outlet of the second aerobic treatment tank 1b. Moreover, it is also possible to further perform solid-liquid separation on the treated liquid in the second aerobic treatment tank 1b, if necessary. Sedimentation tanks, membrane separation, and coagulation sedimentation are all effective as solid-liquid separation means.

When injecting the carrier, the surface of the carrier can be made hydrophilic by immersing the carrier in city water or aerobic treated water in advance. It is possible to attach microorganisms such as activated sludge to the surface.

The HRT in the second aerobic treatment tank 1b can be 8 to 30 hours, more preferably 12 to 24 hours. It is preferable to process the

The specific configuration of the alkalinity adjusting means 2 is not particularly limited. In order to adjust to L, an alkali addition means (not shown) equipped with a pipe, a pump, etc. for adding an alkali agent into the first aerobic treatment tank 1a and the supply of the alkali agent by the alkali addition means are controlled. A controller (not shown) or the like may be provided. For example, the first aerobic treatment tank 1a may be provided with measuring means or the like for measuring the total alkalinity of the raw water contained in the first aerobic treatment tank 1a. Based on this, the total alkalinity of the raw water in the first aerobic treatment tank 1a may be adjusted to an appropriate range.

As the alkaline agent, sodium hydrogen carbonate, sodium carbonate, sodium hydroxide, magnesium hydroxide, calcium hydroxide and the like are used. Especially, by using sodium bicarbonate, pH fluctuation is small, and the treatment in the first aerobic treatment tank 1a can be stabilized while downsizing the apparatus.

(Processing method)
In the organic wastewater treatment method according to the first embodiment, first, raw water is supplied into the first aerobic treatment tank 1a, and the raw water is aerobically treated in the first aerobic treatment tank 1a. In the first aerobic treatment tank 1a, although not limited to the following, for example, HRT is 5 hours, DO is 2 to 7 mg / L, total alkalinity is 200 mg / L, MLSS is 2000 mg / L, BOD volume load is 5 to 5 The raw water is aerobically treated at 20 kg/m ³ /d to obtain a first aerobic treated liquid.

Subsequently, the first aerobic treatment liquid obtained in the first aerobic treatment tank 1a is supplied into the second aerobic treatment tank 1b, and the first aerobic treatment liquid is further aerobically treated in the second aerobic treatment tank 1b. processed appropriately. In the second aerobic treatment tank 1b, although not limited to the following, for example, the HRT is 22 hours, the DO is 1 to 7 mg/L, the carrier filling rate is 15% by volume, and the MLSS is 1000 mg/L. is aerobically treated to obtain a second aerobic treatment liquid.

According to the organic wastewater treatment method according to the first embodiment of the present invention, the BOD volume load of the first aerobic treatment tank 1a is increased, and the BOD volume load of the entire aerobic treatment tank 1 is appropriately adjusted. By maintaining the conditions, the activated sludge that removes BOD and the microorganisms that prey on the activated sludge can be maintained in the first aerobic treatment tank 1a in a constant amount and in a highly active state. In addition, since the activated sludge that removes BOD is always kept in the first aerobic treatment tank 1a at a constant amount and high activity, stable aerobic treatment is performed even if the BOD volume load fluctuates due to fluctuations in the amount and concentration of raw water. is possible.

Although the amount of excess sludge generated in the treated water varies depending on the amount of BOD removed, according to the organic wastewater treatment method and the organic wastewater treatment apparatus according to the first embodiment of the present invention, the activated sludge is preyed on. Since the amount and concentration of raw water and concentration of raw water fluctuates, stable sludge volume reduction is possible even if the amount of surplus sludge generated fluctuates. As a result, the treatment in the first aerobic treatment tank 1a can be stabilized even if the properties of the raw water change, and the amount of excess sludge generated can be reduced while always maintaining stable treated water quality.

The volume of the first aerobic treatment tank 1a is set according to the BOD-SS load, but according to the first embodiment, the volume of the first aerobic treatment tank 1a is the Since only about 1/3 to 1/10 of the volume is required, only by arranging the small first aerobic treatment tank 1a, wastewater treatment can be stably and efficiently performed while suppressing the enlargement of the entire apparatus. can be done.

(Second embodiment)
The organic wastewater treatment apparatus according to the second embodiment of the present invention, as shown in FIG. It differs from the organic wastewater treatment apparatus according to the first embodiment in that it further has a sludge return means 3 .

The suspended sludge in the second aerobic treatment tank 1b is characterized by high DO and high activity. Therefore, by returning the suspended sludge in the second aerobic treatment tank 1b to the first aerobic treatment tank 1a, the microbial activity in the first aerobic treatment tank 1a can be maintained at a higher level, This is expected to maintain stable treated water quality and effective bacterial flora for reducing excess sludge. The flow rate ratio of the floating sludge return flow rate to the raw water inflow rate to the first aerobic treatment tank 1a (floating sludge return flow rate/raw water inflow rate) is 10 to 300 flow rate%, more preferably 10 to 30 flow rate%, and further 10 It is preferable to adjust the flow rate to 20%.

The BOD volume load of the first aerobic treatment tank 1a is adjusted to 5 to 20 kg/m ³ /d, and the aerobic In the treatment tank 1, it is preferable to adjust the BOD volume load of each of the treatment tanks 1a and 1b so that the BOD volume load of all tanks is 1 to 5 kg/m ³ /d. As a result, after preferentially growing dispersible bacteria and the like in the first aerobic treatment tank 1a and removing BOD, the sludge is preyed on by microorganisms living mainly on the surface of the carrier in the second aerobic treatment tank 1b. It has the effect of reducing the amount of excess sludge generated. Others are substantially the same as the organic wastewater treatment apparatus according to the first embodiment, so description thereof is omitted.

According to the organic wastewater treatment method and treatment apparatus according to the second embodiment, the first aerobic treatment is performed by returning the suspended sludge in the second aerobic treatment tank 1b to the first aerobic treatment tank 1a. The microbial activity in tank 1a can be maintained in a more appropriate state. Accordingly, it is possible to provide an organic wastewater treatment method and an organic wastewater treatment apparatus capable of always maintaining stable treated water quality and reducing the amount of excess sludge generated, regardless of changes in the properties of raw water.

Although the present invention has been described by first and second embodiments, the statements and drawings forming part of this disclosure should not be understood to limit the present invention. Various alternative embodiments can be derived from this disclosure by those skilled in the art, and in the implementation stage, they can be modified and embodied without departing from the scope of the invention.

Examples of the present invention are presented below along with comparative examples, which are provided for a better understanding of the invention and its advantages and are not intended to be limiting of the invention.

(Example 1)
Organic wastewater having an organic substance (BOD) concentration of 2000 mg/L was used as raw water, and the wastewater was treated under the basic treatment conditions shown in Table 1 using the organic wastewater treatment apparatus shown in FIG. In Example 1, the volume of the first aerobic treatment tank was 0.5 L, the volume of the second aerobic treatment tank was 2 L, and the raw water supply flow rate was 2.5 to 5 L / d. Sodium hydrogen carbonate was added as an alkaline agent so that the total alkalinity was 200 mg/L. In the first aerobic treatment tank, aerobic treatment was carried out under conditions of a BOD volume load of 5-20 kg/m ³ /d, an MLSS concentration of 1000 mg/L, and a DO of 2-7 mg/L. In the second aerobic treatment tank, PE carrier was added to 15% by volume, and after aerobically treated under the conditions of MLSS concentration of 4000 mg / L and DO of 2 to 7 mg / L, it was passed through a screen. to hold the carrier in the tank, and to obtain treated water. At this time, the BOD volume load in all the aerobic treatment tanks was 1-5 kg/m ³ /d.

In Example 1, as shown in Table 1, the total alkalinity of the first aerobic treatment tank was adjusted to 200 mg/L, the raw water inflow was adjusted to 2.5 to 5 L/d, and the BOD volume load was adjusted to 5. Up to 20 kg/m ³ /d.

Comparative Example 1 uses an aerobic treatment tank with a volume of 2.5 L based on the conventional organic wastewater treatment flow of FIG. The waste water was treated under the conditions shown in Tables 1 and 2. In Comparative Example 2, the total alkalinity of the first aerobic treatment tank was 50 mg/L, and in Comparative Example 3, the total alkalinity of the first aerobic treatment tank was 600 mg/L, and the wastewater was treated under the conditions shown in Tables 1 and 2. I have been there. The analysis of each item was performed according to the sewage test method (Japan Sewage Works Association 2012 edition).

In Example 1, the total alkalinity of the first aerobic treatment tank is 200 mg/L, the BOD volume load of the first aerobic treatment tank is 20 kg/m ³ /d, the BOD volume load of all tanks is 3 kg/m ³ /d, It shows the analysis results of the amount of sludge generated and the dissolved BOD in the treated water when the DO of the first aerobic treatment tank was adjusted to a suitable range of 2 to 7 mg/L.

Comparative Example 1 shows the analysis results of the amount of sludge generated and the dissolved BOD in the treated water when organic wastewater was treated by the conventional activated sludge treatment system with only one aerobic treatment tank.

In Comparative Example 2, the total alkalinity of the first aerobic treatment tank is 50 mg / L, in Comparative Example 3, the total alkalinity of the first aerobic treatment tank is 600 mg / L, and the BOD volume load of the first aerobic treatment tank is 20 kg/m ³ /d, BOD volume load of all tanks is 3 kg/m ³ /d, and DO of the first aerobic treatment tank is adjusted to a suitable range of 2 to 7 mg/L. 1 shows the results of analysis of soluble BOD of

In Example 1, the preferential growth of dispersive bacteria in the first aerobic treatment tank and the subsequent food chain by microorganisms in the second aerobic treatment tank progressed, and better treated water quality than in Comparative Example 1 was obtained. Furthermore, compared with the amount of sludge generated in Comparative Example 1, the amount of sludge generated was reduced by 24%, and a good sludge volume reduction effect was obtained.

In Comparative Example 2, when the total alkalinity is too low, fungi and the like are generated in the first aerobic treatment tank to generate acid, resulting in a decrease in pH, and the preferential growth of dispersive bacteria does not proceed, resulting in treated water quality. is worse than in Example 1, and the sludge volume reduction rate is reduced by 19%, which is worse than in Example 1. In addition, since it does not meet the discharge standards, pH neutralization is required at the time of discharge.

In Comparative Example 3, the excessive total alkalinity causes the pH to increase, and the sludge is easily dispersed. This makes it difficult for sludge to adhere to the carrier, resulting in deterioration of treated water quality and loss of sludge volume reduction effect. As a result, the treated water quality is worse than in Example 1, and the sludge volume reduction rate is reduced by 15%, which is worse than in Example 1. In addition, since it does not meet the discharge standards, pH neutralization is required at the time of discharge.

Thus, according to the present disclosure, in the first aerobic treatment tank for aerobically treating organic wastewater, when aeration treatment is performed under conditions of a total alkalinity of 100 to 500 mg / L, the first aerobic treatment Generation of excess sludge is reduced by predominantly growing dispersible bacteria and the like in the tank and allowing them to be preyed on by microorganisms in the second aerobic treatment tank.

(Example 2)
In Example 2, the return sludge was introduced into the first aerobic treatment tank so that the ratio of the return sludge flow rate to the first aerobic treatment tank to the amount of raw water inflow was 300%, and the conditions shown in Table 3 were achieved. adjusted accordingly.

In Comparative Example 4, wastewater treatment was performed under the conditions shown in Tables 1 and 3 based on the conventional standard activated sludge treatment flow of FIG. 3 without installing the first aerobic treatment tank.

In Example 2, the return sludge flow rate ratio to the first aerobic treatment tank to the raw water inflow rate was 300%, the BOD volume load of the first aerobic treatment tank was 20 kg/m ³ /d, and the first aerobic treatment tank The total alkalinity of 200 mg / L, the DO of the first aerobic treatment tank is adjusted to a suitable range of 2 to 7 mg / L, the amount of sludge generated and the analysis results of soluble BOD in the treated water are shown. there is In Example 2, compared with Comparative Example 4, more stable treated water quality can be obtained.

Thus, according to the present disclosure, in the first aerobic treatment tank for aerobically treating organic wastewater, the BOD volume load is 5-20 kg/m ³ /d, the DO is 2-7 mg/L, and the total alkalinity is 100. When aeration treatment is carried out under the condition of ~500mg/L, the first aerobic treatment tank will predominantly grow disperse bacteria, etc., and the second aerobic treatment tank will prey on them with microorganisms to generate surplus sludge. is reduced. Furthermore, by introducing the suspended sludge in the second aerobic treatment tank into the first aerobic treatment tank at a flow rate of 10 to 300% of the raw water inflow, the amount of activated sludge retained in the first aerobic treatment tank is increased. Since it is maintained in a well-balanced manner, further BOD removal and activated sludge predation progress in the first aerobic treatment tank, making it possible to further suppress the generation of excess sludge. Since the returned sludge at this time is suspended sludge from the second aerobic treatment tank, it is characterized by a high DO. Therefore, when the returned sludge is returned to the first aerobic treatment tank, it is possible to maintain the DO in the first aerobic treatment tank in a more appropriate state, which is effective in maintaining stable treated water quality and reducing excess sludge. It is possible to maintain the effective bacterial flora.

Reference Signs List 1 Aerobic treatment tank 1a First aerobic treatment tank 1b Second aerobic treatment tank 2 Alkalinity adjusting means 3 Floating sludge returning means

Claims (6)

Raw water containing organic matter was adjusted to have a total alkalinity of 100 to 500 mg/L and a BOD volume load of 5 to 20 kg/m ³ /d . a process of obtaining a first aerobic treatment liquid by pneumatically treating;
a second aerobic treatment liquid is obtained by aerobically treating the first aerobic treatment liquid in a second aerobic treatment tank holding a carrier to obtain a second aerobic treatment liquid. Method. In the aerobic treatment comprising the first aerobic treatment tank and the second aerobic treatment tank, the BOD volume load is adjusted so that the BOD volume load of all tanks is 1 to 5 kg/m ³ /d. The method for treating organic wastewater according to claim 1, characterized by: A process of aerobically treating raw water containing organic substances in a first aerobic treatment tank adjusted to have a total alkalinity of 100 to 500 mg/L to obtain a first aerobic treatment liquid;
a process of aerobically treating the first aerobic treatment liquid in a second aerobic treatment tank holding a carrier to obtain a second aerobic treatment liquid;
Return sludge treatment for returning the floating sludge in the second aerobic treatment tank to the first aerobic treatment tank so that the flow rate is 10 to 300% of the raw water inflow
A method for treating organic wastewater, comprising : The method for treating organic wastewater according to any one of claims 1 to 3, wherein the amount of aeration is adjusted so that the dissolved oxygen concentration in the first aerobic treatment tank is 2 to 7 mg/L. . The method for treating organic wastewater according to any one of claims 1 to 4, characterized in that the carrier is added so that the carrier filling rate in the second aerobic treatment tank is 5 to 30% by volume. . a first aerobic treatment tank for aerobically treating raw water containing organic matter at a BOD volume load of 5 to 20 kg/m ³ /d to obtain a first aerobic treatment liquid;
alkalinity adjusting means for adjusting the total alkalinity of the raw water in the first aerobic treatment tank to be 100 to 500 mg/L;
and a second aerobic treatment tank for obtaining a second aerobic treatment liquid by aerobically treating the first aerobic treatment liquid using a carrier.

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