JP2024056209A - Organic wastewater treatment method and organic wastewater treatment device - Google Patents

Abstract

The present invention provides a method for treating organic wastewater that can effectively prevent the growth of actinomycetes in nitrogen-containing organic wastewater without increasing equipment costs.
[Solution] An organic wastewater treatment method in which nitrogen-containing organic wastewater is biologically treated in activated sludge, the organic wastewater treatment apparatus is equipped with an anoxic tank in which a stirring mechanism is installed to stir supplied raw water and activated sludge, an aerobic tank in which a membrane separation device is immersed in the activated sludge, and a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, and by switching the operating state of the stirring mechanism, the anoxic tank is temporarily made to function as an anaerobic tank.
[Selected Figure] Figure 1

Description

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

Conventionally, the A2O method (UCT method) has been widely adopted as an organic wastewater treatment method that uses activated sludge to biologically treat organic wastewater containing nitrogen and phosphorus, such as sewage. In this method, an anaerobic tank, an anoxic tank, and an aerobic tank are arranged in that order, and sludge from the aerobic tank is circulated and supplied to the anaerobic and anoxic tanks. In recent years, MBR methods (such as UCT-MBR) have been attracting attention, in which a membrane separation device 4 is immersed in the aerobic tank 3 instead of a settling tank for solid-liquid separation, and an airlift type sludge return line 5 is provided to return part of the activated sludge from the aerobic tank 3 to the anoxic tank 2 and anaerobic tank 1, as shown in Figures 6(a) and (b).

Patent Document 1 discloses a method for suppressing foaming scum, which is characterized by suppressing the generation of foaming scum caused by actinomycetes, by setting the aerobic sludge retention time in the system to 15 days or less in a sewage treatment facility that employs a conventional activated sludge process in which wastewater is retained in an aeration tank for 24 hours or more, such as an oxidation ditch or extended aeration process.

By focusing on the fact that actinomycetes are aerobic bacteria, the amount of proliferation can be reduced by reducing the time during which actinomycetes can proliferate, even when the sludge retention time is long, through measures such as setting up an anaerobic period or an anaerobic zone in the aeration tank.

Japanese Patent Application Laid-Open No. 8-309383

Even in the above-mentioned MBR process, the activated sludge suspended solids (mg/liter) (hereafter referred to as "MLSS") is generally adjusted to 7,000 to 10,000 mg/liter, which tends to lengthen the sludge retention time (hereafter referred to as "SRT"). Furthermore, due to cleaning aeration, which is essential to avoid clogging of the membrane of the membrane separation device, the dissolved oxygen concentration (hereafter referred to as "DO concentration") in the aerobic tank tends to remain high. This creates an environment that is favorable for the growth of actinomycetes.

When actinomycetes occur and a large amount of scum accumulates near the water surface, the flow of the air lift pump installed in the sludge return line is obstructed, causing problems such as preventing the return of activated sludge from the aerobic tank to the anoxic tank. There is also a risk of problems occurring in which foamy scum leaks out of the aerobic tank, which can cause erroneous detection by sensors installed in the aerobic tank, etc.

Once actinomycetes have appeared, possible ways to deal with them include physically removing the scum, completely replacing the sludge, and shortening the SRT, but in large-scale facilities, the task of physically removing the scum is extremely cumbersome, and replacing the sludge or shortening the SRT is not easy. As a result, the reality is that operations have to continue without suppressing the actinomycetes.

One way to suppress the occurrence of such actinomycetes is to reduce the amount of easily decomposable organic matter flowing into the aerobic tank. In the above-mentioned UCT-MBR, in addition to promoting the denitrification reaction in the anoxic tank, a method has been put into practical use that encourages the consumption of easily decomposable organic matter by polyphosphate accumulating bacteria in the anaerobic tank. Polyphosphate accumulating bacteria have the characteristic of ingesting organic matter such as acetic acid and butyric acid in anaerobic conditions, releasing phosphate as polyphosphate in the process of accumulating internal reserve substances, and ingesting more phosphate as polyphosphate than they release in aerobic conditions.

In this case, it is necessary to install an anaerobic tank in addition to an anoxic tank, which requires a large facility area. It is also necessary to install agitators in both the anoxic and anaerobic tanks and a sludge return path from the anoxic tank to the anaerobic tank, which increases the facility costs.

Actinomycetes are more likely to occur when the inflow load is low and the amount of excess sludge generated is reduced, resulting in a long SRT. Therefore, even in an MBR with a two-tank configuration of an aerobic tank and an anoxic tank, the required capacity of the anoxic tank is reduced, so diverting the surplus to the anaerobic tank can contribute to suppressing actinomycetes. However, if the tank is divided into an anaerobic tank and an anoxic tank, the equipment costs will be high as mentioned above, making it a relatively expensive measure to suppress actinomycetes.

The object of the present invention is to provide an organic wastewater treatment method and an organic wastewater treatment device that can effectively prevent the occurrence of actinomycetes in nitrogen-containing organic wastewater without increasing equipment costs.

In order to achieve the above-mentioned object, the first characteristic feature of the organic wastewater treatment method according to the present invention is an organic wastewater treatment method for biologically treating nitrogen-containing organic wastewater in activated sludge, which is provided with an anoxic tank in which a stirring mechanism for stirring supplied raw water and activated sludge is installed, an aerobic tank in which a membrane separation device is immersed in the activated sludge, and a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, and by switching the operating state of the stirring mechanism, the anoxic tank is temporarily made to function as an anaerobic tank.

By operating the stirring mechanism in the anoxic tank, the nitrate nitrogen returned from the sludge return line comes into contact with the denitrifying bacteria in the activated sludge and is denitrified, during which the easily decomposable organic matter contained in the raw water is consumed by the denitrifying bacteria. In addition, when the operating state of the stirring mechanism is switched, for example by stopping the stirring mechanism, the nitrate nitrogen contained in the sludge returned from the sludge return line comes into contact with the denitrifying bacteria in the process of natural settling in the tank and is denitrified, and the activated sludge eventually becomes in a highly anaerobic state in which no nitrate nitrogen is present. When the anaerobic state becomes high, the easily decomposable organic matter contained in the raw water is taken up by the polyphosphate accumulating bacteria and phosphoric acid is released as polyphosphate. In this way, the sludge in which the easily decomposable organic matter contained in the raw water has been consumed by the polyphosphate accumulating bacteria is supplied to the aerobic tank, so that the generation of actinomycetes is suppressed even in an aerobic tank with a high DO concentration. At the same time, the polyphosphate accumulating bacteria can also remove phosphorus by excessive intake.

The second characteristic configuration is that, in addition to the first characteristic configuration described above, the period during which the anoxic tank temporarily functions as an anaerobic tank is adjusted using either the redox potential, the T-N concentration, or the MLSS as an indicator.

When the anoxic tank is temporarily functioning as an anaerobic tank, if the potential detected by the oxidation-reduction potential sensor in the anoxic tank deviates from the range in which denitrification occurs, and an increase in the T-N concentration of the treated water detected by the T-N concentration sensor is detected, or the MLSS value detected by the MLSS sensor in the aerobic tank increases, the tank can be switched from temporarily functioning as an anaerobic tank to operating the agitation mechanism to promote denitrification, thereby preventing an increase in the T-N concentration in the treated water.

The first characteristic feature of the organic wastewater treatment device according to the present invention is that it is an organic wastewater treatment device that biologically treats nitrogen-containing organic wastewater in activated sludge, and is equipped with an anoxic tank in which a stirring mechanism for stirring raw water and activated sludge is installed, an aerobic tank in which a membrane separation device is immersed in the activated sludge, a sludge return path for returning activated sludge from the aerobic tank to the anoxic tank, and a control unit that temporarily causes the anoxic tank to function as an anaerobic tank by switching the operating state of the stirring mechanism.

By switching the operating state of the stirring mechanism with the control unit and making the anoxic tank where denitrification is performed function as an anaerobic tank, the easily decomposable organic matter contained in the raw water is consumed by polyphosphate accumulating bacteria. As a result, the amount of easily decomposable organic matter flowing into the aerobic tank can be reduced, and the occurrence of actinomycetes in the aerobic tank can be suppressed.

The second characteristic configuration is, in addition to the first characteristic configuration described above, that the raw water supply mechanism that supplies raw water to the anoxic tank is configured to supply raw water near the bottom of the anoxic tank, and the sludge return path is configured to return the sludge returned from the aerobic tank to near the water surface of the anoxic tank.

With the anoxic tank temporarily functioning as an anaerobic tank, raw water supplied near the bottom of the anoxic tank by the raw water supply mechanism rises inside the tank, and sludge returned from the sludge return line to near the water surface of the anoxic tank slowly mixes over time as it settles inside the tank. As the sludge settles to the bottom of the anoxic tank, nitrate nitrogen is no longer present in the treated water around the sludge, so the anaerobicity of the sludge increases, and the polyphosphate accumulating bacteria take up the organic acids contained in the raw water supplied from the bottom of the anoxic tank, consuming easily decomposable organic matter.

The third characteristic configuration is, in addition to the second characteristic configuration described above, that the communication section through which the activated sludge flows from the anoxic tank to the aerobic tank is provided near the water surface of the anoxic tank.

Since the communication section is provided near the water surface of the anaerobic tank, wastewater supplied near the bottom of the anaerobic tank is prevented from short-passing and flowing into the aerobic tank. In addition, the sludge returned to the anaerobic tank settles and only the supernatant flows into the aerobic tank through the communication section, which contributes to increasing the MLSS concentration in the anaerobic tank and thus improving the anaerobicity.

The fourth characteristic configuration is that, in addition to the third characteristic configuration described above, the control unit is configured to adjust the period during which the anoxic tank temporarily functions as an anaerobic tank using either the redox potential, the T-N concentration, or the MLSS as an index.

The control unit temporarily operates the anoxic tank as an anaerobic tank, and then switches it to function as an anoxic tank using either the redox potential, T-N concentration, or MLSS as an indicator, thereby making it possible to suppress the occurrence of actinomycetes in the aerobic tank while avoiding the T-N concentration of the treated water from rising beyond the allowable range.

The fifth characteristic configuration is that, in addition to the fourth characteristic configuration described above, the potential sensor that measures the redox potential is installed at the boundary between the anaerobic region and the anaerobic region formed in the anoxic tank when the anoxic tank is made to function as the anaerobic tank.

By detecting the redox potential at the boundary between the anaerobic and anaerobic regions, if the redox potential begins to rise extremely, it is suspected that the rate at which the water mass rises due to the supply of wastewater has completely exceeded the settling rate of the activated sludge. By returning to normal operation and mixing the wastewater and activated sludge, nitrogen removal performance can be secured and action can be taken before the T-N concentration in the treated water deteriorates.

As described above, the present invention provides an organic wastewater treatment method and an organic wastewater treatment device that can effectively prevent the occurrence of actinomycetes in nitrogen-containing organic wastewater without increasing equipment costs.

1A is a plan view of an anoxic tank in a normal operating state; FIG. 1B is a front view of the anoxic tank in a normal operating state; FIG. 1C is a plan view of the anoxic tank in an anaerobic operating state; and FIG. 1D is a front view of the anoxic tank in a normal operating state. An explanatory diagram of a separation membrane housed in a membrane separation device. 1A is a plan view of the anoxic tank in a normal operating state; FIG. 1B is a front view of the anoxic tank in a normal operating state; FIG. 1C is a plan view of the anoxic tank in an anaerobic operating state; and FIG. 1D is a front view of the anoxic tank in a normal operating state. 1A is a plan view of the anoxic tank in a normal operating state; FIG. 1B is a front view of the anoxic tank in a normal operating state; FIG. 1C is a plan view of the anoxic tank in an anaerobic operating state; and FIG. 1D is a front view of the anoxic tank in a normal operating state. 1A is a front view of an anoxic tank in a normal operating state, and FIG. 1B is a front view of an anoxic tank in an anaerobic operating state. FIG. 1A is an explanatory diagram of a conventional MBR type organic wastewater treatment device equipped with an anaerobic tank and an aerobic tank; FIG. 1B is an explanatory diagram of a conventional MBR type organic wastewater treatment device equipped with an anaerobic tank, an anoxic tank, and an aerobic tank.

Embodiments of a wastewater treatment method and a wastewater treatment device according to the present invention will be described below with reference to the drawings. The wastewater treatment device according to the present invention is an organic wastewater treatment device that biologically treats organic wastewater, such as sewage, that contains nitrogen and phosphorus in activated sludge.

Figures 1(a) and (b) show an organic wastewater treatment device 100. The organic wastewater treatment device 100 includes an anoxic tank 2 equipped with an agitation mechanism 20, an aerobic tank 3 in which a membrane separation device 4 is immersed, a sludge return path 5 that returns sludge from the aerobic tank 3 to the anoxic tank 2, a raw water supply mechanism 6 that supplies raw water, which is organic wastewater containing nitrogen and phosphorus, to the anoxic tank 2, a control unit 8, and the like.

The raw water supply mechanism 6 is arranged on one side of the anoxic tank 2 and is composed of a raw water supply pipe with an opening formed near the bottom. The sludge return line 5 is composed of an air lift pump AP provided downstream of the aerobic tank 3 and a sludge transport pipe that transports the sludge pumped by the air lift pump AP, and is arranged so that the opening of the sludge transport pipe is located near the water surface of the anoxic tank 2.

The stirring mechanism 20 includes a stirring blade disposed in the center of the anoxic tank 2 in plan view, and a motor M that drives the stirring blade. When the motor M is driven in the normal direction, the stirring blade creates a downward flow in the sludge and raw water in the tank, and when it reaches the bottom, a circulating flow that changes to an upward flow from the periphery.

A plurality of membrane separation devices 4 are immersed in the aerobic tank 3, and treated water that has permeated the separation membranes provided in each membrane separation device 4 is taken out by a suction pump P via a water collection pipe and a header pipe.
Each membrane separation device 4 incorporates a plurality of membrane elements 41. As shown in Fig. 2, each membrane element 41 has separation membranes 41b disposed on both sides of a resin membrane support 41a equipped with a water collection pipe 41c at the top. In this embodiment, the separation membrane 41b is a microfiltration membrane having a nominal pore size of about 0.4 µm and equipped with a porous organic polymer membrane on the surface of a nonwoven fabric.

The type of separation membrane 41b and the membrane element 41 are not limited to the above-mentioned embodiments, and any type of separation membrane and any form of membrane element (hollow fiber membrane element, tubular membrane element, monolith membrane element, etc.) can be used.

An aeration device is provided below the membrane separation device 4, and the surface of the separation membrane 41b is cleaned by the upward flow of air bubbles that are supplied from the aeration device and rise inside the tank. In addition, an auxiliary aeration device 40 is provided upstream of the area of the aerobic tank 3 where the membrane separation device 4 is immersed.

The raw water supplied from the raw water supply mechanism 6 to the anoxic tank 2 flows down to the aerobic tank together with the activated sludge through the communication part 21. An opening formed in the partition wall separating the anoxic tank 2 from the aerobic tank 3 near the water surface of the anoxic tank 2 functions as the communication part 21.

In the aerobic tank 3, ammonia nitrogen contained in the raw water is nitrified by nitritizing bacteria and nitrifying bacteria under aerobic conditions adjusted by the auxiliary air diffuser 40 to become nitrate nitrogen, and the nitrate nitrogen is sent together with activated sludge to the anoxic tank 2 via the sludge return line 5, where it is denitrified by denitrifying bacteria. In this embodiment, for 1Q of raw water flowing into the anoxic tank 2, 1Q of treated water is taken out from the membrane separation device 4, and the return amount is set so that a flow rate of 2Q is circulated via the sludge return line 5, and the MLSS concentration in the anoxic tank 2 is set to 6,670 [mg/liter] and the MLSS concentration in the aerobic tank 3 is set to 10,000 [mg/liter].

As shown in Figures 1(c) and (d), the control unit 8 switches the operating state of the stirring mechanism 20 provided in the anoxic tank 2 to temporarily function as an anaerobic tank. Thereafter, based on either the output of a potential sensor that measures the redox potential provided in the anoxic tank 2, the output of the MLSS concentration sensor provided in the aerobic tank 3, or the T-N concentration of the treated water detected by the T-N concentration sensor, the control unit 8 controls the stirring mechanism 20 to return to its original operating state so as to return from the state where it functions as an anaerobic tank to its normal state.

When the agitation mechanism 20 installed in the anoxic tank 2 is activated, the nitrate nitrogen returned from the sludge return line 5 comes into contact with the denitrifying bacteria in the activated sludge and is denitrified. In the normal treatment process, the easily decomposable organic matter contained in the raw water is consumed by the denitrifying bacteria.

Then, when the operating state of the agitation mechanism 20 is switched, for example by stopping the agitation mechanism, the nitrate nitrogen contained in the sludge returned from the sludge return line 5 naturally settles in the tank and comes into contact with the denitrifying bacteria, undergoing denitrification treatment, and the activated sludge eventually reaches a highly anaerobic state in which no nitrate nitrogen is present. When the degree of anaerobicity increases, easily decomposable organic matter such as acetic acid and butyric acid contained in the raw water is taken up by the polyphosphate accumulating bacteria, and phosphorus is released as polyphosphate.

In this way, the sludge in which the easily decomposable organic matter contained in the raw water has been consumed by the polyphosphate accumulating bacteria is supplied to the aerobic tank, so that the occurrence of actinomycetes is suppressed even in the aerobic tank 3, which has a high DO concentration. At the same time, the polyphosphate accumulating bacteria also have the effect of removing phosphorus through excessive intake.

In more detail, the raw water supply mechanism 6 is configured to supply raw water near the bottom of the anoxic tank 2, and the sludge return path 5 is configured to return the sludge returned from the aerobic tank 3 to near the water surface of the anoxic tank 2. Therefore, when the anoxic tank 2 is temporarily functioning as an anaerobic tank, that is, when the stirring mechanism 20 is stopped, the raw water supplied near the bottom of the anoxic tank 2 by the raw water supply mechanism 6 rises in the tank, and the sludge returned from the sludge return path 5 to near the water surface of the anoxic tank 2 settles in the tank, slowly mixing over time. In the process of the sludge settling to the bottom of the anoxic tank 2, nitrate nitrogen is no longer present in the treated water around the sludge, so the anaerobicity of the sludge increases, and the polyphosphate accumulating bacteria take up the organic acids contained in the raw water supplied from the bottom of the anoxic tank, consuming easily decomposable organic matter.

Since the communication part 21 is provided near the water surface of the anoxic tank 2, the raw water supplied near the bottom of the anoxic tank 2 is prevented from short-passing and flowing into the aerobic tank 3. In addition, the sludge returned to the anoxic tank 2 settles and only the supernatant flows into the aerobic tank 3 through the communication part 21, which contributes to increasing the MLSS concentration in the anoxic tank 2 and thus improving the degree of anaerobicity.

When the anoxic tank 2 is temporarily functioning as an anaerobic tank, if the potential detected by the oxidation-reduction potential sensor in the anoxic tank 2 deviates from the range in which denitrification occurs, and an increase in the T-N concentration of the treated water detected by the T-N concentration sensor is detected, or the MLSS value detected by the MLSS sensor in the aerobic tank 3 increases, the agitation mechanism 20 is operated to switch from the state in which the tank temporarily functions as an anaerobic tank to one that promotes denitrification, thereby preventing an increase in the T-N concentration in the treated water.

When using an oxidation-reduction potential sensor, it is preferable to install it at the boundary depth between the anaerobic and anaerobic regions formed in the anoxic tank 2 when the anoxic tank 2 is functioning as an anaerobic tank. For example, when the water surface of the anoxic tank 2 is set to about 5 m from the bottom, it is preferable to install it at a position about 2 to 3 m from the bottom.

By detecting the redox potential at the boundary between the anoxic and anaerobic regions, if the redox potential begins to rise extremely, it is suspected that the rate at which the water mass rises due to the supply of wastewater is completely exceeding the settling rate of the activated sludge. In such a case, by returning to normal operation and mixing the wastewater and activated sludge, nitrogen removal performance can be ensured and action can be taken before the T-N concentration of the treated water deteriorates.

The period during which the anoxic tank 2 is temporarily made to function as an anaerobic tank and then returned to its normal state does not need to be constant, and as described above, may be set taking into consideration the inflow load conditions, the state of treated water quality, the occurrence of actinomycetes, etc. Any of the redox potential, T-N concentration, and MLSS may be used as an indicator, or a combination of these may be used as an indicator.

Another embodiment of the organic wastewater treatment apparatus 100 will be described below.
3(a) to 3(d), the raw water supply mechanism 6 is composed of a plurality of raw water supply pipes that supply raw water evenly toward the bottom of the anoxic tank 2, and a branch pipe may be formed between the sludge return paths so that the returned sludge from the sludge return path 5 is returned evenly between the upstream and downstream of the anoxic tank 2. As the raw water supply mechanism 6, a raw water supply pipe having a plurality of openings formed in the pipe wall along the axial direction may be installed horizontally at the bottom of the anoxic tank 2.

When anoxic tank 2 is functioning as an anaerobic tank, the return sludge settles and the raw water rises, resulting in gentle mixing throughout the tank, efficiently creating an anaerobic state and consuming easily decomposable organic matter.

As shown in Figures 4(a) to (d), when the control unit 8 switches the operating state of the stirring mechanism 20 provided in the anoxic tank 2 to temporarily function as an anaerobic tank, the motor M may be configured to rotate in the reverse direction at a slower speed than when rotating forward. Normally, the shape of the stirring blade is set so that a large downward flow is generated when the stirring blade is driven in the forward direction. When such a stirring blade is driven in the reverse direction, no significant upward flow is generated, and only a gentle upward flow is generated around the rotation axis, and a gentle circulating flow is generated in the upper layer area. As a result, the returned sludge and raw water are gently mixed evenly in the upper layer area of the tank, an anaerobic state can be efficiently created, and easily decomposable organic matter is consumed.

In the above embodiment, an example was shown in which an opening formed near the water surface of the anoxic tank 2 in the partition wall separating the anoxic tank 2 and the aerobic tank 3 functions as the communication section 21, but an overflow weir may also be provided in the partition wall separating the anoxic tank 2 and the aerobic tank 3. In this case, the amount of sludge retained in the anoxic tank 2 will increase, but the power required for circulating the sludge will also increase.

As shown in Figures 5(a) and (b), the installation area can be further reduced by providing a partition plate 9 that separates the treatment tank into upper and lower layers, with the lower layer being the anoxic tank 2 and the upper layer being the aerobic tank 3. An opening is formed at one end of the partition plate 9 to form a communication section 21, and an air lift pump that functions as the sludge return path 5 is provided at the other end of the partition plate. By using a submersible mixer as the agitation mechanism 20, the denitrification reaction can be promoted, and the generated nitrogen gas is released to the atmosphere via the sludge return path 5 and the aerobic tank 3.

In the case of a circulating MBR designed for a facility that uses first settling overflow of normal quality as the raw water supplied to the anoxic tank 2, the surface area load of the anoxic tank is about 50 m/d for the maximum daily wastewater volume. For this reason, it is quite likely that a situation will arise in which the settling speed of the sludge and the rising speed of the wastewater compete with each other during times when the inflow volume is low. In this case, the capacity of the anoxic tank 2 should be 2 to 3 h at HRT.

The above-described embodiments are merely examples of the present invention, and the present invention is not limited to these descriptions. It goes without saying that the specific configuration of each part can be appropriately modified and designed within the scope of the effects of the present invention. In addition, any one or more of the above-described embodiments may be appropriately combined.

2: Anoxic tank 3: Aerobic tank 4: Membrane separation device 5: Sludge return line 6: Raw water supply mechanism 8: Control unit 9: Partition plate 20: Stirring mechanism 21: Communication unit 100: Organic wastewater treatment device

Claims (7)

A method for treating organic wastewater by biologically treating nitrogen-containing organic wastewater in activated sludge, comprising:
An organic wastewater treatment apparatus comprising an anoxic tank in which a stirring mechanism for stirring supplied raw water and activated sludge is installed, an aerobic tank in which a membrane separation device is immersed in the activated sludge, and a sludge return path for returning the activated sludge from the aerobic tank to the anoxic tank,
The organic wastewater treatment method includes switching an operating state of the stirring mechanism so that the anoxic tank temporarily functions as an anaerobic tank. The organic wastewater treatment method according to claim 1, in which the period during which the anoxic tank temporarily functions as an anaerobic tank is adjusted using either the redox potential, the T-N concentration, or the MLSS as an indicator. An organic wastewater treatment apparatus for biologically treating nitrogen-containing organic wastewater in activated sludge,
An anoxic tank equipped with a mixing mechanism for mixing raw water and activated sludge;
an aerobic tank in which a membrane separation device is immersed in activated sludge;
a sludge return line for returning activated sludge from the aerobic tank to the anoxic tank;
a control unit that switches an operating state of the stirring mechanism to temporarily cause the anoxic tank to function as an anaerobic tank;
An organic wastewater treatment device comprising: The raw water supply mechanism for supplying raw water to the anoxic tank is configured to supply raw water to the vicinity of the bottom of the anoxic tank,
4. The organic wastewater treatment apparatus according to claim 3, wherein the sludge return line is configured to return the sludge returned from the aerobic tank to the vicinity of the water surface of the anoxic tank. The organic wastewater treatment device according to claim 4, wherein the communication section through which activated sludge flows from the anoxic tank to the aerobic tank is provided near the water surface of the anoxic tank. The organic wastewater treatment device according to claim 5, wherein the control unit is configured to adjust the period during which the anoxic tank temporarily functions as an anaerobic tank using one of the redox potential, T-N concentration, or MLSS as an index. The organic wastewater treatment device according to claim 6, wherein the potential sensor for measuring the oxidation-reduction potential is installed at the boundary between the anaerobic and anoxic regions formed in the anoxic tank when the anoxic tank is made to function as the anaerobic tank.