Wastewater biological treatment device, and aeration air flow controlling device and method

Abstract

To attain reliable reduction of an emission amount of N2O produced by nitrification of ammonia nitrogen without adversely affecting decomposition of organic matter in an aerobic tank.SOLUTION: An aeration air quantity of an aeration device 30, a water amount of wastewater to be treated in an aerobic tank 4, and a concentration of dissolved N2O or gaseous N2O in the aerobic tank 4 are independently measured. An N2O emission amount per unit of the water amount to be treated is calculated on the basis of a measured value of the water amount, a measured value of the aeration air quantity, and a measured value of the concentration of dissolved N2O or gaseous N2O. A controller 10 operates the aeration device 30 so as to decrease the aeration air quantity when the N2O emission amount per unit of the water amount to be treated is equal to or higher than a predetermined threshold.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a biological wastewater treatment device that oxidizes and decomposes organic matter by aeration in an aerobic tank, and a device and method for controlling the aeration air volume. [Background technology]

[0002] Biological treatment equipment that utilizes aerobic microorganisms is used to treat wastewater, including sewage. Activated sludge, which is a flocculated form of aerobic microorganisms, floats in the aerobic tank (or aeration tank) of the biological treatment equipment. When organic wastewater is supplied to the aerobic tank together with air or other gases that contain oxygen, the organic matter in the wastewater is oxidized and decomposed, and the wastewater is purified.

[0003] However, supplying oxygen-containing gas, i.e. aeration, requires a lot of power. For example, about half of the electricity used in sewage treatment plants is due to aeration. The aeration volume is generally controlled to keep either the airflow, airflow ratio, or dissolved oxygen concentration (DO) constant.

[0004] There are several treatment methods for biological treatment equipment, such as the standard method, the AO method, and the A2O method, depending on the purpose. In the standard method and the AO method, which are the common treatment methods, when N-BOD becomes a problem, nitrification promotion operation is performed as a countermeasure. Nitrification promotion operation makes it possible to oxidize ammonia nitrogen in wastewater to nitrate nitrogen. However, in nitrification promotion operation, the aeration air volume increases to about 1.7 times that of nitrification suppression operation. Therefore, when N-BOD is not a problem, nitrification suppression operation is performed to save energy.

[0005] However, when nitrification suppression operation is performed, in some cases partial nitrification is allowed to proceed in order to use it as an indicator of whether organic matter has been sufficiently treated. In addition, partial nitrification may unintentionally proceed as a result of controlling the aeration air volume with constant air volume control, constant air volume ratio control, or constant DO control.

[0006] Figure 17 shows an example of water quality measurements at various points in an aerobic tank when wastewater is treated using the standard method. There are five measurement points: influent water, organic matter adsorption / decomposition area, organic matter decomposition area, ammonia nitrification area, and treated water. The measured water quality is organic matter concentration, DO, inorganic nitrogen, and dissolved nitrous oxide (N2O). As shown in this figure, oxygen supplied by aeration is first consumed for the oxidative decomposition of organic matter. Then, when the decomposition of organic matter is sufficiently achieved, DO increases, and nitrification of ammonia nitrogen progresses in that area. In the ammonia nitrification area where nitrification of ammonia nitrogen progresses, nitrite nitrogen and nitrate nitrogen are generated, and N2O is also generated.

[0007] As shown in Figure 18, there are two routes for the nitrification of ammonia nitrogen: one is to oxidize hydroxylamine to form nitrite nitrogen and then nitrate nitrogen, and the other is to oxidize hydroxylamine to form N2O as a by-product. Operation that promotes partial nitrification as described above causes the generation of N2O. N2O is a greenhouse gas with a warming potential approximately 300 times that of carbon dioxide, so it is necessary to reduce N2O emissions in order to prevent global warming.

[0008] Figure 19 shows an example of the results of monitoring N2O emissions over time. In this example, the aeration air volume is controlled with constant DO control. Although N2O emissions tend to decrease over time, they continue to increase and decrease and have not yet converged to a value close to zero. In other words, unless some kind of active control is implemented with the aim of reducing N2O emissions, there is a limit to how much N2O emissions can be reduced by natural processes.

[0009] Patent Document 1 discloses a technology for controlling the amount of N2O emissions. In the conventional technology disclosed in Patent Document 1, the concentration of gaseous N2O is monitored, and the aeration air volume supplied to the reaction tank is reduced based on the increase in the concentration. However, N2O is initially present in the liquid as a dissolved state, and is converted to a gaseous state by the difference in concentration with the aeration air or stripping due to gas-liquid contact, and is discharged into the atmosphere. Therefore, in addition to the difference in concentration between the liquid and the gas, the balance between the amount of wastewater and the amount of aeration air is also a factor that affects the concentration of gaseous N2O. In addition, the amount of water and the amount of aeration air change from moment to moment, and are not always constant. Therefore, the control of the aeration air volume based on the increase in the concentration of gaseous N2O does not necessarily ensure the absolute amount of N2O can be reduced, and there is a risk that the aeration air volume will be excessively reduced, which will have a negative effect on the decomposition of organic matter. In other words, there is room for improvement in the conventional technology disclosed in Patent Document 1.

[0010] In addition to Patent Document 1, Patent Documents 2 to 4 can be cited as prior art documents that can serve as references for understanding the technical level at the time of filing of the present application. However, like Patent Document 1, Patent Documents 2 to 4 do not disclose any technology that can reliably reduce the amount of NO emissions generated by the nitrification of ammonia nitrogen without adversely affecting the decomposition of organic matter in an aerobic tank. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 5140545 [Patent Document 2] Patent No. 5300827 [Patent Document 3] Japanese Patent Application Publication No. 57-027197 [Patent Document 4] Japanese Unexamined Patent Publication No. 55-151262 Summary of the Invention [Problem to be solved by the invention]

[0012] The present disclosure has been made in consideration of the above-mentioned problems, and aims to provide a technology that can reliably reduce the amount of NO emissions generated by the nitrification of ammonia nitrogen without adversely affecting the decomposition of organic matter in an aerobic tank. [Means for solving the problem]

[0013] The present disclosure provides a biological treatment device for wastewater. The biological treatment device of the present disclosure is a biological treatment device using an aerobic tank. The biological treatment device of the present disclosure includes an aeration device for aerating the inside of the aerobic tank, an air flow sensor, a water flow sensor, an N2O concentration sensor, and a control device. The air flow sensor measures the aeration air flow rate of the aeration device. The water flow sensor measures the water volume of the wastewater to be treated in the aerobic tank. The N2O concentration sensor measures the concentration of dissolved N2O or gaseous N2O in the aerobic tank. The control device is configured to control the aeration air flow rate by operating the aeration device. In detail, the control device is configured to calculate the amount of N2O emissions per unit amount of water treated based on the measured water volume, the measured aeration air flow rate, and the measured concentration of dissolved N2O or gaseous N2O, and to reduce the aeration air flow rate by operating the aeration device when the amount of N2O emissions per unit amount of water treated is equal to or greater than a predetermined threshold value.

[0014] In the biological treatment device of the present disclosure, the N2O concentration sensor is a sensor that measures the concentration of dissolved N2O and may be installed in the water of the ammonia nitrification region of the aerobic tank, or the N2O concentration sensor is a sensor that measures the concentration of gaseous N2O and may be connected to a gas collection device installed on the water surface of the ammonia nitrification region of the aerobic tank.

[0015] In the biological treatment device of the present disclosure, the aerobic tank may be separated by a partition wall into at least the ammonia nitrification region and other regions. In this case, the aeration device may be configured to change the aeration air volume in the ammonia nitrification region independently of the other regions. The N2O concentration sensor may be disposed to measure the concentration of dissolved N2O or gaseous N2O in the ammonia nitrification region. The control device may be configured to reduce the aeration air volume in the ammonia nitrification region when the N2O emission per unit amount of water treated is equal to or greater than a threshold value.

[0016] In the biological treatment device of the present disclosure, the control device may be configured to control the aeration air volume so as to bring the aeration air volume close to a predetermined set value while suppressing the amount of NO emissions per unit amount of water treated below a threshold value. That is, the control device may perform constant air volume control under the constraint that the amount of NO emissions per unit amount of water treated is suppressed below a threshold value.

[0017] In the biological treatment device of the present disclosure, the control device may be configured to control the aeration air volume so as to bring the ratio of the measured aeration air volume to the measured treated water volume closer to a predetermined set value while suppressing the amount of NO emissions per unit treated water volume below a threshold value. That is, the control device may perform constant air blowing ratio control under the constraint that the amount of NO emissions per unit treated water volume is suppressed below a threshold value.

[0018] The biological treatment device of the present disclosure may further include a DO sensor that measures the DO of the aerobic tank. In this case, the control device may be configured to control the aeration air volume so as to bring the measured DO value closer to a predetermined set value while suppressing the amount of NO emission per unit amount of water treated below a threshold value. In other words, the control device may perform constant DO control under the constraint that the amount of NO emission per unit amount of water treated is suppressed below a threshold value.

[0019] The present disclosure also provides a control device for controlling the aeration air volume of wastewater. The control device for the aeration air volume of the present disclosure is a control device for controlling the aeration air volume of an aeration device that aerates an aerobic tank, and is configured to execute the following first to fifth processes. The first process is a process for acquiring a measurement value of the aeration air volume. The second process is a process for acquiring a measurement value of the water volume of wastewater to be treated in the aerobic tank. The third process is a process for acquiring a measurement value of the concentration of dissolved N2O or gaseous N2O in the aerobic tank. The order of execution of the first to third processes is arbitrary. The fourth process is a process for calculating the amount of N2O emission per unit of water volume treated based on the measurement value of the water volume, the measurement value of the aeration air volume, and the measurement value of the concentration of dissolved N2O or gaseous N2O. And the fifth process is a process for operating the aeration device to reduce the aeration air volume when the amount of N2O emission per unit of water volume treated is equal to or greater than a predetermined threshold value.

[0020] The present disclosure further provides a method for controlling the aeration air volume of wastewater. The method for controlling the aeration air volume of the present disclosure is a method for controlling the aeration air volume of an aerobic tank, and includes the following first to fifth steps. The first step is a step of measuring the aeration air volume. The second step is a step of measuring the amount of wastewater to be treated in the aerobic tank. The third step is a step of measuring the concentration of dissolved N2O or gaseous N2O in the aerobic tank. The first to third steps may be performed in any order. The fourth step is a step of calculating the amount of N2O emission per unit amount of water treated based on the measured value of the water volume, the measured value of the aeration air volume, and the measured value of the concentration of dissolved N2O or gaseous N2O. And the fifth step is a step of reducing the aeration air volume when the amount of N2O emission per unit amount of water treated is equal to or greater than a predetermined threshold value. Effect of the Invention

[0021] According to the technology disclosed herein, the amount of N2O emissions per unit amount of treated water can be calculated from the measured treated water volume, aeration air volume, and N2O concentration, thereby making it possible to accurately grasp the increase or decrease in the absolute amount of N2O emitted. Then, by reducing the aeration air volume when the accurately grasped amount of N2O emissions reaches or exceeds a threshold, it is possible to reliably reduce the amount of N2O emissions generated by the nitrification of ammonia nitrogen without adversely affecting the decomposition of organic matter in the aerobic tank. [Brief description of the drawings]

[0022] [Figure 1] FIG. 2 is a diagram showing the configuration of a biological treatment device according to the first embodiment. [Diagram 2] FIG. 1 is a graph showing the correlation between the concentration of dissolved N2O and the concentration of gaseous N2O. [Diagram 3] 1 is a flowchart of a general constant air volume control. [Figure 4] 11 is a flowchart of aeration air volume control in which constant air volume control is combined with N2O control, which is executed in the biological treatment device according to the first embodiment. [Diagram 5] 11 is a flowchart of a general constant air blowing ratio control. [Figure 6] 11 is a flowchart of aeration air volume control in which constant air blow ratio control executed in the biological treatment device according to the first embodiment is combined with N2O control. [Figure 7] 1 is a flowchart of a general DO constant control. [Figure 8] 1 is a flowchart of aeration air volume control in which constant DO control is combined with N2O control, which is executed in the biological treatment device according to the first embodiment. [Figure 9] FIG. 2 is a diagram showing the configuration of a first modified example of the biological treatment device according to the first embodiment. [Figure 10] FIG. 4 is a diagram showing the configuration of a second modified example of the biological treatment device according to the first embodiment. [Figure 11] FIG. 13 is a diagram showing the configuration of a third modified example of the biological treatment device according to the first embodiment. [Figure 12] FIG. 11 is a diagram showing the configuration of a biological treatment device according to a second embodiment. [Figure 13] 13 is a flowchart of aeration air volume control in which constant air volume control is combined with N2O control, which is executed in a biological treatment device according to a second embodiment. [Figure 14] 13 is a flowchart of aeration air volume control in which constant air blow ratio control executed in a biological treatment device according to a second embodiment is combined with N2O control. [Figure 15] 13 is a flowchart of aeration air volume control in which constant DO control is combined with N2O control, which is executed in a biological treatment device according to a second embodiment. [Figure 16] FIG. 11 is a diagram showing the configuration of a modified example of a biological treatment device according to the second embodiment. [Figure 17] FIG. 13 is a diagram showing an example of measurement of water quality at each point in an aerobic tank. [Figure 18] FIG. 1 shows the nitrification and denitrification reactions of ammonium nitrogen. [Figure 19] FIG. 1 shows the results of monitoring N2O emissions over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] 1. First embodiment 1-1. Biological treatment equipment configuration In this embodiment and the second embodiment described later, the standard method is adopted as the treatment method of the biological treatment device. However, the technology disclosed herein can be applied to treatment methods other than the standard method as long as the treatment method involves a series of processes of adsorption and decomposition of organic matter, decomposition, and nitrification in the aerobic tank. For example, the technology disclosed herein can be applied to a biological treatment device that uses the AO method as the treatment method.

[0024] Fig. 1 is a diagram showing the configuration of a biological treatment device 101 according to this embodiment. The biological treatment device 101, which employs the standard method, has three tanks: a primary sedimentation tank 2, an aerobic tank 4, and a final sedimentation tank 6. Wastewater to be treated is sent to the aerobic tank 4 via the primary sedimentation tank 2. Wastewater treated in the aerobic tank 4 is sent to the final sedimentation tank 6, and discharged from the final sedimentation tank 6 as treated water. Sludge that settles in the final sedimentation tank 6 is returned to the inlet of the aerobic tank 4 by a return pump (not shown).

[0025] The aerobic tank 4 is divided into three regions from the upstream: the organic matter adsorption / decomposition region R1, the organic matter decomposition region R2, and the ammonia nitrification region R3. These three regions R1, R2, and R3 are divided based on the reactions that occur there. The organic matter adsorption / decomposition region R1 is the region where the adsorption and decomposition of organic matter occurs by activated sludge. The organic matter decomposition region R2 is the region where the decomposition of organic matter occurs by microorganisms. And the ammonia nitrification region R3 is the region where the nitrification of ammonia nitrogen occurs. However, there is no clear division between these three regions R1, R2, and R3, and they do not need to be strictly separated.

[0026] The biological treatment device 101 is equipped with an aeration device 30 for aerating the inside of the aerobic tank 4. The aeration device 30 is equipped with an aeration device 31 installed in the aerobic tank 4, a blower 34 for sending a gas such as air containing oxygen to the aeration device 31, and a valve 33 for adjusting the aeration air volume flowing through the aeration device 31.

[0027] The biological treatment device 101 is equipped with many sensors. An air volume sensor 21 is provided downstream of the valve 33 in the aeration device 31. The air volume sensor 21 is a sensor that measures the volume of air or other gas containing oxygen aerated into the aerobic tank 4 by the aeration device 30, i.e., the aeration air volume. A water volume sensor 22 is provided in a wastewater pipe that supplies wastewater to the primary sedimentation tank 2. The water volume sensor 22 is a sensor that measures the volume of wastewater treated in the aerobic tank 4. As the air volume sensor 21 and the water volume sensor 22, commonly used flow meters such as an orifice flow meter, an electromagnetic flow meter, and an ultrasonic flow meter can be used.

[0028] A DO sensor 23 and an N2O concentration sensor 24 are placed below the water surface WS of the aerobic tank 4. The DO sensor 23 is a sensor that measures the DO of wastewater in real time. The N2O concentration sensor 24 is a sensor that measures the concentration of dissolved N2O in the wastewater in real time. Both the DO sensor 23 and the N2O concentration sensor 24 are placed in the water of the ammonia nitrification region R3. However, the DO sensor 23 is not necessarily required depending on the aeration air volume control method. Specifically, the DO sensor 23 can be omitted in control other than aeration air volume control that combines constant DO control with N2O control, which will be described later.

[0029] The biological treatment device 101 is equipped with a control device 10. The control device 10 is a control device that controls the aeration air volume by operating the aeration device 30. Information obtained by an air volume sensor 21, a water volume sensor 22, a DO sensor 23, and an N2O concentration sensor 24 is input to the control device 10. The control device 10 controls the aeration air volume based on the input information.

[0030] In controlling the aeration air volume by the control device 10, first, the N2O emission amount per unit water volume to be treated (hereinafter, may be simply referred to as N2O emission amount) is calculated based on the air volume measured by the air volume sensor 21, the water volume measured by the water volume sensor 22, and the N2O concentration measured by the N2O concentration sensor 24. In this calculation, the dissolved N2O concentration measured by the N2O concentration sensor 24 is converted to a gaseous N2O concentration. There is a correlation between the dissolved N2O concentration and the gaseous N2O concentration as shown in Fig. 2. A conversion coefficient based on the correlation between the two is used to convert the dissolved N2O concentration to the gaseous N2O concentration.

[0031] The control device 10 monitors the amount of N2O emissions obtained by the above calculation. Since the air volume, water volume, and N2O concentration are all data that can be acquired continuously in real time, the control device 10 can continuously monitor the amount of N2O emissions in real time. Then, when the amount of N2O emissions reaches or exceeds a predetermined threshold, the control device 10 reduces the rotation speed of the blower 34 and reduces the opening of the valve 33. This operation reduces the aeration air volume.

[0032] By reducing the aeration air volume, excessive nitrification in the ammonia nitrification region is prevented, and the amount of N2O emissions, which is a greenhouse gas, can be reduced. In addition, by reducing the aeration air volume, it is possible to reduce the power consumption associated with driving the blower 34. Furthermore, this control reduces the aeration air volume to an extent that the amount of N2O emissions falls below the threshold, and does not reduce the aeration air volume to zero. Therefore, it is possible to avoid insufficient decomposition of organic matter due to insufficient aeration air volume.

[0033] Control of the aeration air volume aimed at reducing N2O emissions as described above is referred to as N2O control in this specification. The control device 10 executes N2O control in combination with conventional aeration air volume control. The control methods for aeration air volume control by the control device 10 include a control method that combines constant air volume control with N2O control, a control method that combines constant air ratio control with N2O control, and a control method that combines constant DO control with N2O control. The control device 10 is configured to be able to selectively execute any one of the control methods. However, when executing the control method that combines constant DO control with N2O control, the DO sensor 23 is essential.

[0034] Hereinafter, three control methods for aeration air volume control that can be implemented by the control device 10 of the biological treatment device 101 will be described in order with reference to the drawings.

[0035] 1-2. Aeration air volume control method 1-2-1. Control method combining constant airflow control with N2O control First, typical constant airflow control will be described with reference to the flowchart of Fig. 3. In typical constant airflow control, an airflow set value F is set (step S11). Then, aeration is started by operating the aeration device 30 (step S12).

[0036] After aeration starts, an air volume measurement value F1 is acquired from the air volume sensor 21 (step S13). The air volume measurement value F1 is compared with the air volume setting value F (step S14). If the air volume measurement value F1 is equal to or less than the air volume setting value F, the aeration air volume is increased (step S15). On the other hand, if the air volume measurement value F1 is greater than the air volume setting value F, the aeration air volume is decreased (step S16). Then, the processing from step S13 onwards is repeated at a fixed cycle until the aeration ends (step S17).

[0037] Next, a control method combining constant airflow control with N2O control will be described with reference to the flow chart in Fig. 4. In this control method, an airflow setting value F and a threshold value K for the amount of N2O emissions (hereinafter referred to as the N2O threshold) are set (step S101). The N2O threshold K is preferably 20 mg-N2O / m 3 Less than 10mg-N2O / m 3 After these set values ​​are set, aeration is started by operating the aeration device 30 (step S102).

[0038] After aeration starts, an air volume measurement value F1 is obtained from the air volume sensor 21. In addition, an N2O concentration measurement value C1 is obtained from the N2O concentration sensor 24. However, the N2O concentration measurement value C1 is a value after conversion from the dissolved N2O concentration to the gaseous N2O concentration. In addition, a water volume measurement value F2 is obtained from the water volume sensor 22 (the above is step S103). Then, using the air volume measurement value F1, the N2O concentration measurement value C1, and the water volume measurement value F2, an N2O emission amount K1 is calculated by the following formula (step S104). K1 = (F1 × C1) / F2

[0039] However, while the measured air volume F1 represents the total aeration air volume of the aerobic tank 4, it is the aeration air volume of the ammonia nitrification region R3 that determines the nitrification reaction. Therefore, the calculation of the N2O emission amount K1 using the above formula uses the measured air volume F1 corrected by the ratio of the aeration air volume of the ammonia nitrification region R3 to the total aeration air volume of the aerobic tank 4. This is also common to other control methods described later.

[0040] The N2O discharge amount K1 is compared with the N2O threshold K (step S105). If the N2O discharge amount K1 is equal to or greater than the N2O threshold K, the aeration air volume is reduced (step S106). However, the aeration air volume is not suddenly reduced in one execution of step S106, but is gradually reduced each time the process of step S106 is executed. For example, the amount of reduction in the number of revolutions of the blower 34 and the amount of reduction in the opening of the valve 33 per one execution of step S106 are set in the control device 10. Each time the process of step S106 is executed, the control device 10 reduces the number of revolutions of the blower 34 by a predetermined number of revolutions and reduces the opening of the valve 33 by a predetermined opening. The method of reducing the aeration air volume described here is also common to other control methods described later.

[0041] The processes of steps S103 to S106 are repeated until the N2O emission amount K1 falls below the N2O threshold K. Then, when the N2O emission amount K1 falls below the N2O threshold K, the measured airflow amount F1 is compared with the airflow amount set value F (step S107). If the measured airflow amount F1 is equal to or less than the airflow amount set value F, the aeration airflow amount is increased (step S108). On the other hand, if the measured airflow amount F1 is greater than the airflow amount set value F, the aeration airflow amount is decreased (step S109). Then, the processes from step S103 onwards are repeated until the aeration is completed (step S110).

[0042] By controlling the aeration air volume using a control method that combines the above-mentioned NO control with constant air volume control, it is possible to remove organic matter and suppress NO emissions at the same time. In addition, since suppression of NO emissions is achieved by lowering the aeration air volume, the power required for aeration is also reduced, resulting in further energy savings and reduction of greenhouse gases.

[0043] 1-2-2. Control method combining constant air flow rate control with N2O control First, a typical constant air blowing ratio control will be described with reference to the flowchart of Fig. 5. In typical constant air blowing ratio control, an air blowing ratio set value M is set (step S21). The air blowing ratio means the ratio of the aeration air volume to the amount of wastewater supplied to the aerobic tank 4. After the air blowing ratio set value M is set, aeration is started by operating the aeration device 30 (step S22).

[0044] After the aeration starts, the air volume measurement value F1 is obtained from the air volume sensor 21. Also, the water volume measurement value F2 is obtained from the water volume sensor 22 (step S23). Then, the air volume measurement value F1 is divided by the water volume measurement value F2 to calculate the air blowing ratio M1 (step S24). The air blowing ratio M1 is compared with the air blowing ratio setting value M (step S25). If the air blowing ratio M1 is less than the air blowing ratio setting value M, the aeration air volume is increased (step S26). On the other hand, if the air blowing ratio M1 is equal to or greater than the air blowing ratio setting value M, the aeration air volume is decreased (step S27). Then, the processes from step S23 onwards are repeated at a constant cycle until the aeration ends (step S28).

[0045] Next, a control method combining constant air blowing ratio control with N2O control will be described with reference to the flowchart in Fig. 6. In this control method, an N2O threshold value K is set together with an air blowing ratio set value M (step S201). Then, aeration is started by operating the aeration device 30 (step S202).

[0046] After aeration starts, an air volume measurement value F1 is acquired from the air volume sensor 21. Also, an N2O concentration measurement value C1 is acquired from the N2O concentration sensor 24. Furthermore, a water volume measurement value F2 is acquired from the water volume sensor 22 (the above is step S203). Then, an N2O emission amount K1 is calculated using the air volume measurement value F1, the N2O concentration measurement value C1, and the water volume measurement value F2 (step S204).

[0047] The N2O discharge amount K1 is compared with the N2O threshold K (step S205). If the N2O discharge amount K1 is equal to or greater than the N2O threshold K, the aeration airflow rate is reduced (step S206).

[0048] The processes of steps S203 to S206 are repeated until the N2O discharge amount K1 falls below the N2O threshold K. Then, when the N2O discharge amount K1 falls below the N2O threshold K, an air blowing ratio M1 is calculated from the air volume measurement value F1 and the water volume measurement value F2 (step S207). The air blowing ratio M1 is compared with an air blowing ratio setting value M (step S208). When the air blowing ratio M1 is less than the air blowing ratio setting value M, the aeration air volume is increased (step S209). On the other hand, when the air blowing ratio M1 is equal to or greater than the air blowing ratio setting value M, the aeration air volume is decreased (step S210). Then, the processes from step S203 onwards are repeated until the aeration is completed (step S211).

[0049] By controlling the aeration air volume with a control method that combines the above-mentioned N2O control with constant airflow ratio control, it is possible to remove organic matter and suppress N2O emissions at the same time. In addition, since suppression of N2O emissions is achieved by lowering the aeration air volume, the power required for aeration is also reduced, resulting in further energy savings and reduction of greenhouse gases.

[0050] 1-2-3.Control method combining constant DO control with N2O control First, typical constant DO control will be described with reference to the flowchart of Fig. 7. In typical constant DO control, a target DO value V, which is a target value of DO, is set (step S31). After the target DO value V is set, aeration is started by operating the aeration device 30 (step S32).

[0051] After aeration starts, a DO measurement value V1 is acquired from the DO sensor 23 (step S33). The DO measurement value V1 is compared with a target DO value V (step S34). If the DO measurement value V1 is less than the target DO value V, the aeration airflow is increased (step S35). On the other hand, if the DO measurement value V1 is equal to or greater than the target DO value V, the aeration airflow is decreased (step S36). Then, the processing from step S33 onwards is repeated at a constant cycle until the aeration ends (step S37).

[0052] Next, a control method combining constant DO control with N2O control will be described with reference to the flowchart in Fig. 8. In this control method, a target DO value V and an N2O threshold value K are set (step S301). Then, aeration is started by operating the aeration device 30 (step S302).

[0053] After aeration starts, an air volume measurement value F1 is acquired from the air volume sensor 21. Also, an N2O concentration measurement value C1 is acquired from the N2O concentration sensor 24. Furthermore, a water volume measurement value F2 is acquired from the water volume sensor 22 (the above is step S303). Then, an N2O emission amount K1 is calculated using the air volume measurement value F1, the N2O concentration measurement value C1, and the water volume measurement value F2 (step S304).

[0054] The N2O discharge amount K1 is compared with the N2O threshold K (step S305). If the N2O discharge amount K1 is equal to or greater than the N2O threshold K, the aeration airflow rate is reduced (step S306).

[0055] The processes of steps S303 to S306 are repeated until the N2O discharge amount K1 falls below the N2O threshold K. Then, when the N2O discharge amount K1 falls below the N2O threshold K, a DO measurement value V1 is acquired from the DO sensor 23 (step S307). The DO measurement value V1 is compared with the target DO value V (step S308). When the DO measurement value V1 is less than the target DO value V, the aeration airflow is increased (step S309). On the other hand, when the DO measurement value V1 is equal to or greater than the target DO value V, the aeration airflow is decreased (step S310). Then, the processes from step S303 onwards are repeated at a fixed cycle until the aeration is completed (step S311).

[0056] By controlling the aeration air volume using a control method that combines the above-mentioned NO control with constant DO control, it is possible to both remove organic matter and suppress NO emissions. In particular, by combining with constant DO control, it is possible to suppress NO emissions while maintaining DO at an appropriate value. In addition, constant DO control is a stable control, and has the advantage that there is little waste of air volume because the air volume changes depending on the amount and quality of wastewater flowing into the aerobic tank 4. By combining constant DO control, which has such advantages, with NO control, the power required for aeration is further reduced, making it possible to achieve further energy savings and greenhouse gas reductions.

[0057] As explained above for the three control methods, the increase or decrease in the absolute amount of N2O emitted can be accurately grasped by calculating the amount of N2O emissions per unit amount of treated water from the treated water volume, aeration air volume, and N2O concentration. Then, by reducing the aeration air volume when the accurately grasped amount of N2O emissions exceeds a threshold value, it is possible to reliably reduce the amount of N2O emissions generated by the nitrification of ammonia nitrogen without adversely affecting the decomposition of organic matter in the aerobic tank 4.

[0058] In the biological treatment device 101, the dissolved N2O concentration is measured as the N2O concentration, but it is also possible to measure the gaseous N2O concentration instead of the dissolved N2O concentration and calculate the amount of N2O discharged per unit amount of water treated from the gaseous N2O concentration. In that case, conversion processing as in the case of using the dissolved N2O concentration is not necessary, and the value of the gaseous N2O concentration can be used directly to calculate the amount of N2O discharged. Below, as a modified example of the biological treatment device 101, a biological treatment device configured to measure the gaseous N2O concentration will be described.

[0059] 1-3. Modified biological treatment equipment 1-3-1. First modified example Fig. 9 is a diagram showing the configuration of a biological treatment device 102 as a first modified example of the biological treatment device 101. In Fig. 9, elements common to the above-mentioned biological treatment device 101 are denoted by the same reference numerals.

[0060] The biological treatment device 102 of the first modified example is equipped with an N2O concentration sensor 25. The N2O concentration sensor 25 is a sensor that measures in real time the concentration of gaseous N2O accumulated in the aerobic tank 4. In order to prevent the generated gaseous N2O from being released into the atmosphere, a cover 4a is provided on the aerobic tank 4. A gas intake port 25a of the N2O concentration sensor 25 is provided in the space between the water surface WS and the cover 4a.

[0061] In the biological treatment device 102 of the first modification, the average concentration of gaseous N2O in the space below the cover 4a is measured by the N2O concentration sensor 25. The control device 10 multiplies the air volume measurement value obtained from the air volume sensor 21 by the N2O concentration measurement value obtained from the N2O concentration sensor 25, and divides the multiplied result by the water volume measurement value obtained from the water volume sensor 22 to calculate the amount of N2O emission per unit water volume treated.

[0062] 1-3-2. Second modified example Fig. 10 is a diagram showing the configuration of a biological treatment device 103 which is a second modified example of the biological treatment device 101. In Fig. 9, elements common to the above-mentioned biological treatment device 101 or 102 are denoted by common reference numerals.

[0063] The biological treatment device 103 of the second modification is provided with a duct 25b that guides gas accumulated under the cover 4a of the aerobic tank 4 to the deodorization device. The N2O concentration sensor 25 is provided in the duct 25b. The gas inlet 25a of the N2O concentration sensor 25 also serves as the gas inlet of the duct 25b. In the biological treatment device, it is also possible to install the N2O concentration sensor 25 by utilizing the existing duct 25b in this way.

[0064] 1-3-3.Third modified example Fig. 11 is a diagram showing the configuration of a biological treatment device 104 as a third modified example of the biological treatment device 101. In Fig. 9, elements common to the above-mentioned biological treatment devices 101, 102, or 103 are denoted by common reference numerals.

[0065] The biological treatment device 104 of the third modified example is equipped with an N2O concentration sensor 25. The N2O concentration sensor 25 is a sensor that measures the concentration of gaseous N2O in real time. However, the aerobic tank 4 of the biological treatment device 104 is an open type that does not have a cover. Therefore, the N2O concentration sensor 25 is attached to a gas collection device 25c called an open chamber. The gas collection device 25c is installed on the water surface WS and is a device that directly collects gas that rises to the water surface WS. The gas collection device 25c is floated on the water surface of the ammonia nitrification region R3 and collects N2O generated in the ammonia nitrification region R3.

[0066] In the biological treatment device 104 of the third modified example, the concentration of gaseous N2O generated in the ammonia nitrification region R3 is measured by the N2O concentration sensor 25. The control device 10 converts the air volume measurement value obtained from the air volume sensor 21 into the aeration air volume of the ammonia nitrification region R3. Then, the aeration air volume of the ammonia nitrification region R3 is multiplied by the N2O concentration measurement value obtained from the N2O concentration sensor 25, and divided by the water volume measurement value obtained from the water volume sensor 22 to calculate the amount of N2O emission per unit water volume treated.

[0067] 2. Second embodiment 2-1. Biological treatment equipment configuration Fig. 12 is a diagram showing the configuration of a biological treatment device 201 according to this embodiment. In Fig. 12, elements common to the above-mentioned biological treatment devices 101, 102, 103, or 104 are denoted by common reference numerals.

[0068] The aerobic tank 4 of the biological treatment device 201 is composed of an upstream tank 4A corresponding to the organic matter adsorption / decomposition region R1, a midstream tank 4B corresponding to the organic matter decomposition region R2, and a downstream tank 4C corresponding to the ammonia nitrification region R3. The upper spaces of the upstream tank 4A and the midstream tank 4B are separated by a partition wall 4b, and the upper spaces of the midstream tank 4B and the downstream tank 4C are separated by a partition wall 4c. However, the upstream tank 4A, the midstream tank 4B, and the downstream tank 4C are connected underwater.

[0069] The biological treatment device 201 is provided with aeration devices 30A, 30B, and 30C, each of which is independent for each tank. The aeration device 30A is provided with an aeration device 31A installed in the upstream tank 4A, a blower 34A that sends a gas such as air containing oxygen to the aeration device 31A, and a valve 33A for adjusting the aeration air volume flowing through the aeration device 31A. The aeration device 30B is provided with an aeration device 31B installed in the midstream tank 4B, a blower 34B that sends a gas such as air containing oxygen to the aeration device 31B, and a valve 33B for adjusting the aeration air volume flowing through the aeration device 31B. The aeration device 30C is provided with an aeration device 31C installed in the downstream tank 4C, a blower 34C that sends a gas such as air containing oxygen to the aeration device 31C, and a valve 33C for adjusting the aeration air volume flowing through the aeration device 31C.

[0070] The biological treatment device 201 is equipped with many sensors. Air volume sensors 21A, 21B, 21C are provided downstream of the valves 33A, 33B, 33C in the air diffusers 31A, 31B, 31C, respectively. A water volume sensor 22 is provided in the wastewater pipe supplying wastewater to the primary sedimentation tank 2. The DO sensor 23 and the N2O concentration sensor 25 are provided in the downstream tank 4C. The DO sensor 23 is placed below the water surface WS of the downstream tank 4C. The N2O concentration sensor 25 has its gas intake port 25a provided in the space between the water surface WS of the downstream tank 4C and the cover 4a.

[0071] The control device 10 of the biological treatment device 201 can independently control the aeration air volumes of the three tanks 4A, 4B, and 4C by separately operating the three aeration devices 30A, 30B, and 30C. In particular, the aeration air volume of the downstream tank 4C is controlled by constant air volume control, constant air magnification control, or constant DO control, as well as NO control.

[0072] In the N2O control of the aeration air volume of the downstream tank 4C, first, the measured air volume by the air volume sensor 21C, the measured water volume by the water volume sensor 22, and the measured N2O concentration by the N2O concentration sensor 25 are acquired. The measured air volume by the air volume sensor 21C is the aeration air volume of the ammonia nitrification region R3. The measured N2O concentration by the N2O concentration sensor 25 is the concentration of gaseous N2O in the ammonia nitrification region R3. The control device 10 calculates the amount of N2O emission per unit of treated water volume based on these acquired measured values. The control device 10 monitors the amount of N2O emission obtained by the above calculation. Then, when the amount of N2O emission becomes equal to or exceeds a predetermined threshold, the control device 10 reduces the rotation speed of the blower 34C and reduces the opening of the valve 33C. This operation reduces the aeration air volume of the downstream tank 4C, suppressing nitrification and thereby suppressing the generation of N2O.

[0073] The three aeration air volume control methods that can be implemented by the control device 10 of the biological treatment device 201 will be described below in order with reference to the drawings. In the biological treatment device 201, the aeration air volume is controlled individually for each of the tanks 4A, 4B, and 4C, so that it is possible to reduce the aeration air volume only in the ammonia nitrification region R3 while supplying sufficient oxygen to the organic matter adsorption / decomposition region R1 and the organic matter decomposition region R2. Therefore, the biological treatment device 201 can achieve a high level of both organic matter removal and suppression of N2O emissions.

[0074] 2-2. Aeration air volume control method 2-2-1. Control method combining constant airflow control with N2O control A control method combining constant airflow control with N2O control will be described with reference to the flow chart in Fig. 13. In this control method, an airflow set value Ff for the upstream tank 4A, an airflow set value Fc for the midstream tank 4B, and an airflow set value Fr for the downstream tank 4C are set. In addition, an N2O threshold value K is set (above, step S151). After these set values ​​are set, aeration is started by operating each aeration device 30A, 30B, and 30C (step S152).

[0075] After aeration starts, air volume measurements Ff1, Fc1, and Fr1 are acquired from the air volume sensors 21A, 21B, and 21C. In addition, an N2O concentration measurement C1 is acquired from the N2O concentration sensor 24. Furthermore, a water volume measurement F2 is acquired from the water volume sensor 22 (the above is step S153). Then, using the air volume measurement Fr1, N2O concentration measurement C1, and water volume measurement F2 of the downstream tank 4C, an N2O emission amount K1 is calculated by the following formula (step S154). K1 = (Fr1 × C1) / F2

[0076] The N2O emission amount K1 is compared with the N2O threshold K (step S155). If the N2O emission amount K1 is equal to or greater than the N2O threshold K, the aeration air volumes of the upstream tank 4A and the midstream tank 4B are kept constant, and only the aeration air volume of the downstream tank 4C is reduced (step S156). In detail, the control device 10 reduces the rotation speed of the blower 34C by a predetermined rotation speed while maintaining the rotation speed of the blowers 34A, 34B and the openings of the valves 33A, 33B, and also reduces the opening of the valve 33C by a predetermined opening. This process maintains the measured air volumes Ff1 and Fc1, while only the measured air volume Fr1 is reduced.

[0077] The processes of steps S153 to S156 are repeated until the N2O discharge amount K1 falls below the N2O threshold K. When the N2O discharge amount K1 falls below the N2O threshold K, the following constant airflow control is performed for each of the tanks 4A, 4B, and 4C.

[0078] For the upstream tank 4A, the measured airflow rate Ff1 is compared with the set airflow rate Ff (step S157). If the measured airflow rate Ff1 is equal to or less than the set airflow rate Ff, the aeration airflow rate of the upstream tank 4A is increased (step S158). On the other hand, if the measured airflow rate Ff1 is greater than the set airflow rate Ff, the aeration airflow rate of the upstream tank 4A is decreased (step S159).

[0079] For the midstream tank 4B, the measured airflow rate Fc1 is compared with the set airflow rate Fc (step S160). If the measured airflow rate Fc1 is equal to or lower than the set airflow rate Fc, the aeration airflow rate of the midstream tank 4B is increased (step S161). On the other hand, if the measured airflow rate Fc1 is greater than the set airflow rate Fc, the aeration airflow rate of the midstream tank 4B is decreased (step S162).

[0080] For the downstream tank 4C, the measured airflow rate Fr1 is compared with the set airflow rate Fr (step S163). If the measured airflow rate Fr1 is equal to or less than the set airflow rate Fr, the aeration airflow rate of the downstream tank 4C is increased (step S164). On the other hand, if the measured airflow rate Fr1 is greater than the set airflow rate Fr, the aeration airflow rate of the downstream tank 4C is decreased (step S165).

[0081] The N2O control process in steps S153 to S156 and the constant airflow control process in steps S157 to S165 are repeated until the aeration is completed (step S166).

[0082] 2-2-2. Control method combining constant air flow rate control with N2O control Next, a control method combining constant airflow ratio control with N2O control will be described with reference to the flow chart in Fig. 14. In this control method, the airflow ratio setting value Mf of the upstream tank 4A, the airflow ratio setting value Mc of the midstream tank 4B, and the airflow ratio setting value Mr of the downstream tank 4C are set. Also, the N2O threshold value K is set (above, step S251). After these setting values ​​are set, aeration is started by operating each aeration device 30A, 30B, and 30C (step S252).

[0083] After aeration starts, air volume measurements Ff1, Fc1, and Fr1 are acquired from the air volume sensors 21A, 21B, and 21C. In addition, an N2O concentration measurement C1 is acquired from the N2O concentration sensor 24. Furthermore, a water volume measurement F2 is acquired from the water volume sensor 22 (the above is step S253). Then, an N2O emission amount K1 is calculated using the air volume measurement Fr1, N2O concentration measurement C1, and water volume measurement F2 of the downstream tank 4C (step S254).

[0084] The N2O emission amount K1 is compared with the N2O threshold K (step S255). If the N2O emission amount K1 is equal to or greater than the N2O threshold K, the aeration air volume of the upstream tank 4A and the midstream tank 4B is kept constant, and only the aeration air volume of the downstream tank 4C is reduced (step S256). This process maintains the measured air volumes Ff1 and Fc1, while only the measured air volume Fr1 is reduced.

[0085] The processes of steps S253 to S256 are repeated until the N2O discharge amount K1 falls below the N2O threshold K. When the N2O discharge amount K1 falls below the N2O threshold K, the following constant air blowing ratio control is performed for each of the tanks 4A, 4B, and 4C.

[0086] For the upstream tank 4A, an air blowing ratio Mf1 is calculated from the measured air volume Ff1 and the measured water volume F2 (step S257). The air blowing ratio Mf1 is compared with a set air blowing ratio Mf (step S258). If the air blowing ratio Mf1 is less than the set air blowing ratio Mf, the aeration air volume of the upstream tank 4A is increased (step S259). On the other hand, if the air blowing ratio Mf1 is equal to or greater than the set air blowing ratio Mf, the aeration air volume of the upstream tank 4A is decreased (step S260).

[0087] For the midstream tank 4B, an air blowing ratio Mc1 is calculated from the air volume measurement value Fc1 and the water volume measurement value F2 (step S261). The air blowing ratio Mc1 is compared with the air blowing ratio setting value Mc (step S262). If the air blowing ratio Mc1 is less than the air blowing ratio setting value Mc, the aeration air volume of the midstream tank 4B is increased (step S263). On the other hand, if the air blowing ratio Mc1 is equal to or greater than the air blowing ratio setting value Mc, the aeration air volume of the midstream tank 4B is decreased (step S264).

[0088] For the downstream tank 4C, an air blowing ratio Mr1 is calculated from the air volume measurement Fr1 and the water volume measurement F2 (step S265). The air blowing ratio Mr1 is compared with the air blowing ratio setting value Mr (step S266). If the air blowing ratio Mr1 is less than the air blowing ratio setting value Mr, the aeration air volume of the downstream tank 4C is increased (step S267). On the other hand, if the air blowing ratio Mr1 is equal to or greater than the air blowing ratio setting value Mr, the aeration air volume of the downstream tank 4C is decreased (step S268).

[0089] The N2O control process in steps S253 to S256 and the constant air blowing ratio control process in steps S257 to S268 are repeated until the aeration is completed (step S269).

[0090] 3-2-3.Control method combining constant DO control with N2O control Next, a control method that combines constant DO control with N2O control will be described with reference to the flowchart in Fig. 15. In this control method, the N2O threshold value K is set together with the target DO value V (step S351). Then, aeration is started by operating the aeration device 30 (step S352).

[0091] After aeration starts, air volume measurements Ff1, Fc1, and Fr1 are acquired from the air volume sensors 21A, 21B, and 21C. In addition, an N2O concentration measurement C1 is acquired from the N2O concentration sensor 24. Furthermore, a water volume measurement F2 is acquired from the water volume sensor 22 (the above is step S353). Then, an N2O emission amount K1 is calculated using the air volume measurement Fr1, N2O concentration measurement C1, and water volume measurement F2 of the downstream tank 4C (step S354).

[0092] The N2O emission amount K1 is compared with the N2O threshold K (step S355). If the N2O emission amount K1 is equal to or greater than the N2O threshold K, the aeration air volume of the upstream tank 4A and the midstream tank 4B is kept constant, and only the aeration air volume of the downstream tank 4C is reduced (step S356). This process maintains the measured air volumes Ff1 and Fc1, while only the measured air volume Fr1 is reduced.

[0093] The processes of steps S353 to S356 are repeated until the N2O discharge amount K1 falls below the N2O threshold K. Then, when the N2O discharge amount K1 falls below the N2O threshold K, a DO measurement value V1 is acquired from the DO sensor 23 (step S357). The DO measurement value V1 is compared with the target DO value V (step S358). When the DO measurement value V1 is less than the target DO value V, the aeration air volume is increased throughout the aerobic tank 4 (step S359). On the other hand, when the DO measurement value V1 is equal to or greater than the target DO value V, the aeration air volume is decreased throughout the aerobic tank 4 (step S360).

[0094] The N2O control process in steps S353 to S356 and the DO constant control process in steps S357 to S360 are repeated until aeration is completed (step S361).

[0095] In the biological treatment device 201, the gaseous N2O concentration is measured as the N2O concentration, but it is also possible to measure the dissolved N2O concentration instead of the gaseous N2O concentration as in the example shown in Fig. 1 and calculate the N2O emission amount per unit water volume to be treated from the dissolved N2O concentration. In that case, a process of converting the dissolved N2O concentration to the gaseous N2O concentration is required. Also, as in the example shown in Fig. 10, the N2O concentration sensor 25 may be provided in the duct 25b that leads the gas to the deodorizing device. Also, as in the example shown in Fig. 11, the N2O concentration sensor 25 may be provided in the gas collecting device 25c installed on the water surface WS.

[0096] 2-3. Modified biological treatment equipment Fig. 16 is a diagram showing the configuration of a biological treatment device 202 which is a modified example of the biological treatment device 201. In Fig. 16, elements common to the above-mentioned biological treatment device 201 are denoted by the same reference numerals.

[0097] In the biological treatment device 102 of the modified example, the blower 34 is shared between the aeration devices 30A, 30B, and 30C. The aeration air volume of each tank 4A, 4B, and 4C is basically controlled by manipulating the opening of each valve 33A, 33B, and 33C. For example, when reducing the aeration air volume of the downstream tank 4C in NO control, the opening of only the valve 33C is reduced while maintaining the openings of the valves 33A and 33B. According to this modified example, the number of blowers 34 is reduced, thereby making it possible to reduce initial costs and maintenance work. [Explanation of symbols]

[0098] 2: Primary sedimentation tank 4:Aerobic tank 4a: Cover 4b, 4c: Bulkhead 6: Final settling tank 10: Control device 21: Air flow sensor 22: Water level sensor 23: DO sensor 24: N2O concentration sensor (dissolved) 25: N2O concentration sensor (gas state) 25a: Gas inlet 25b: Duct 25c: Gas collection device 30, 30A, 30B, 30C: Aeration device 31, 31A, 31B, 31C: Diffuser 33, 33A, 33B, 33C: Valves 34, 34A, 34B, 34C: Blower 101, 102, 103, 104, 201, 202: Biological treatment equipment R1: Organic matter adsorption / decomposition area R2: Organic matter decomposition area R3: Ammonia nitrification region WS: Water surface

Claims

1. A biological treatment device using an aerobic tank, an aeration device for aerating the aerobic tank; an air volume sensor for measuring an aeration air volume of the aeration device; a water volume sensor for measuring the amount of wastewater to be treated in the aerobic tank; Dissolved N in the aerobic tank ２ O or gaseous N ２ Measure the concentration of O ２ An O concentration sensor; A control device for controlling the aeration air volume by operating the aeration device, The control device includes: The measured water volume, the measured aeration volume, and the dissolved N ２ O or gaseous N ２ Based on the measured value of the concentration of N per unit of treated water volume, ２ Calculating the amount of O emissions; N per unit of treated water volume ２ and reducing the aeration air volume when the O discharge amount is equal to or greater than a predetermined threshold. A biological wastewater treatment device comprising:

2. The biological treatment device according to claim 1, The N ２ The O concentration sensor measures dissolved N ２ A sensor for measuring the concentration of O is installed in the water in the ammonia nitrification region of the aerobic tank. A biological wastewater treatment device comprising:

3. The biological treatment device according to claim 1, The N ２ The O concentration sensor detects N gas ２ A sensor for measuring the concentration of O is connected to a gas collection device installed on the water surface of the ammonia nitrification region of the aerobic tank. A biological wastewater treatment device comprising:

4. The biological treatment device according to claim 1, The aerobic tank is at least separated from other areas by a partition wall, The aeration device is configured to be able to change the aeration air volume in the ammonia nitrification area independently of the other areas, The N ２ The O concentration sensor measures dissolved N in the ammonia nitrification region. ２ O or gaseous N ２ configured to measure a concentration of O; The control device is configured to control N ２ When the O discharge amount is equal to or greater than the threshold value, the aeration air volume in the ammonia nitrification area is reduced. A biological wastewater treatment device comprising:

5. The biological treatment device according to any one of claims 1 to 4, The control device is configured to control N ２ The aeration air volume is controlled so as to approach a predetermined set value while suppressing the O2 emission amount below the threshold value. A biological wastewater treatment device comprising:

6. The biological treatment device according to any one of claims 1 to 4, The control device is configured to control N ２ and controlling the aeration air volume so as to bring the ratio of the measured value of the aeration air volume to the measured value of the water volume closer to a predetermined set value while suppressing the O discharge amount below the threshold value. A biological wastewater treatment device comprising:

7. The biological treatment device according to any one of claims 1 to 4, Further comprising a dissolved oxygen concentration sensor for measuring the dissolved oxygen concentration in the aerobic tank; The control device is configured to control N ２ The aeration air volume is controlled so as to bring the measured value of the dissolved oxygen concentration close to a predetermined set value while suppressing the O discharge amount below the threshold value. A biological wastewater treatment device comprising:

8. A control device for controlling an aeration air volume of an aeration device that aerates an aerobic tank, obtaining a measurement of the aeration airflow; Obtaining a measurement of the volume of wastewater being treated in the aerobic tank; Dissolved N in the aerobic tank ２ O or gaseous N ２ obtaining a measurement of the concentration of O; The measured water volume, the measured aeration volume, and the dissolved N ２ O or gaseous N ２ Based on the measured value of the concentration of N per unit of treated water volume, ２ Calculating the amount of O emissions; N per unit of treated water volume ２ and operating the aeration device to reduce the aeration airflow when the O discharge amount is equal to or greater than a predetermined threshold. An aeration air volume control device characterized by the above.

9. A method for controlling an aeration air volume in an aerobic tank, comprising: Measuring the aeration air volume; Measuring the amount of wastewater being treated in the aerobic tank; Dissolved N in the aerobic tank ２ O or gaseous N ２ Measuring the concentration of O; The measured water volume, the measured aeration volume, and the dissolved N ２ O or gaseous N ２ Based on the measured value of the concentration of N per unit of treated water volume, ２ Calculating the amount of O emissions; N per unit of treated water volume ２ and reducing the aeration air volume when the O discharge amount is equal to or greater than a predetermined threshold. A method for controlling aeration air volume.

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