JP2013202544A - Waste water treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waste water treatment method which can perform high load nitrification of waste water containing nitrogen while enabling reduction in a total running cost of equipment.SOLUTION: In a waste water treatment apparatus in which high concentration oxygen gas 10 is supplied into a gas phase of a nitrification tank 2 filled up with a carrier to which nitrifying bacteria is made to adhere and which can be sealed, an exhaust gas is discharged from the gas phase of the nitrification tank, and the gas in the gas phase in the nitrification tank is diffused into a liquid phase, which carries out oxidation treatment of NH-N and C-N in waste water to NObiologically, so that the dissolved oxygen concentration in the liquid phase in the nitrification tank is detected to maintain the dissolved oxygen concentration at a set value; the amount of air diffused into the liquid phase is controlled to detect the oxygen concentration of the exhaust gas in the nitrification tank, and the amount of an oxygen gas supplied to the gas phase part of the nitrification tank is controlled to maintain the oxygen concentration within a prescribed range and the prescribed range of the oxygen concentration in the gas phase part is set so that the oxygen use efficiency is 85% or less while maintaining the oxygen concentration in the gas phase part at 40% or more.

Description

  The present invention relates to a wastewater treatment method for biologically oxidizing wastewater containing ammoniacal nitrogen and organic nitrogen.

As a method for biologically treating wastewater containing ammonia nitrogen, a treatment for oxidizing ammonia nitrogen to nitrate nitrogen and nitrite nitrogen by nitrifying bacteria is generally performed. The nitrification reaction requires oxygen, and air is used as an oxygen source (for example, Patent Document 1).
In the nitrification reaction of ammonia nitrogen and organic nitrogen, about 4.6 kg of oxygen is required to nitrify 1 kg of nitrogen. Therefore, in the conventional wastewater treatment method using air as an oxygen source, A large amount of air must be supplied. Furthermore, in order to promote nitrification efficiently, it is necessary to maintain the DO of the mixed solution higher than that of the oxidative decomposition treatment of organic matter. When air with an oxygen concentration of 21% is used, the equipment can be downsized. There has been a limit to reducing the power cost of air supply facilities. As a method for solving such a problem, an oxygen activated sludge method using oxygen-enriched air having an oxygen concentration higher than that of air as an oxygen source necessary for the nitrification reaction is employed (for example, Patent Document 2).

In order to perform nitrification under high load conditions, it is necessary to increase the supply capacity of oxygen and hold a large amount of nitrifying bacteria in the nitrification tank. For that purpose, DO must be maintained high to increase the activity of nitrifying bacteria in the sludge, and the sludge concentration (MLSS concentration) of the mixed solution must be increased. In the case of the oxygen activated sludge method, there is a merit that the DO can be set high, but if the MLSS concentration is increased unnecessarily, there is a problem that it becomes difficult to separate the sludge and the treated water in the sedimentation basin as in the case of the standard activated sludge method. Arise. A membrane may be used for this solid-liquid separation, but an expensive membrane separation system is required, and there are many problems to be solved and improved such as clogging of the membrane. Furthermore, raising the MLSS concentration promotes the occurrence of scum in the aeration tank and settling basin, and there is a problem that operation management becomes complicated. In order to solve these problems, a method using a carrier on which microorganisms are fixed is widely used in nitrification treatment.
By combining the oxygen activated sludge method with excellent oxygen supply capability and the carrier nitrification method with excellent nitrification capability, nitrification treatment can proceed with compact equipment, and various device structures and methods have been studied (for example, Patent Document 2).

In the oxygen activated sludge method, a surface aeration method (mechanical aeration apparatus) is employed as an aeration method for the nitrification tank. However, the mechanical aeration apparatus cannot sufficiently perform the nitrification reaction for converting ammonia nitrogen into nitrite nitrogen or nitrate nitrogen, and as described in Patent Document 2, stirring for aeration In some cases, the nitrification carrier may be worn or disintegrated.
Moreover, in the case of the oxygen activated sludge method which employ | adopted the mechanical aeration apparatus, it is common to operate with oxygen utilization efficiency of 85-90% (nonpatent literature 1), and it is a well-known fact, and there is no doubt. It was common sense.
In order to solve the problem of carrier wear, there is a method (gas circulation aeration method) in which high-concentration oxygen gas is circulated in the reaction tank via a circulation blower instead of the surface aeration method. There is an example (for example, Non-Patent Document 2).
However, the method of circulating and aeration using a blower has fewer practical examples than the surface aeration method, and the implementation conditions for stabilizing the operation and operating efficiently have not been sufficiently studied. Gases having a higher oxygen concentration than air (hereinafter referred to as high-concentration oxygen gas) such as oxygen-enriched air and pure oxygen used in the oxygen activated sludge method are expensive to produce. In addition, the combined use of a blower that consumes a large amount of power further increases the running cost. To put into practical use a method of circulating air diffuser using a blower, an operating method that reduces the running cost as much as possible. It was necessary to consider.

JP 2000-31898 A JP-A-8-173983

Sewerage Facility Planning / Design Guidelines and Explanation / Part 2 [2009] 142 Sumiyama et al., 30th Sewerage Research Conference Lecture, p524 (1993)

  The present invention has been made in view of the above circumstances, and by using an oxygen activated sludge method combining a carrier nitrification method and a circulation aeration method, it is possible to reduce running costs and achieve a predetermined treatment performance. An object of the present invention is to provide a wastewater treatment method for biologically oxidizing nitrogen-containing wastewater that can be produced.

  In order to solve the above problems, in the present invention, high-concentration oxygen gas is supplied into a gas phase of a sealable nitrification tank filled with a carrier to which nitrifying bacteria are attached, and exhausted from the gas phase of the nitrification tank. In addition to discharging gas, the gas in the gas phase in the nitrification tank is diffused into the liquid phase, so that ammonia nitrogen and organic nitrogen in the waste water are biologically converted into nitrate nitrogen and nitrite nitrogen. In the wastewater treatment method for oxidation treatment, the dissolved oxygen concentration in the liquid phase in the nitrification tank is detected, and dispersed in the liquid phase so that the dissolved oxygen concentration is maintained at a set value based on the detection result. The amount of diffused air to be controlled is controlled, and the oxygen concentration of the gas phase portion in the nitrification tank or the exhaust gas is measured, and the oxygen concentration is maintained within a predetermined range based on the measurement result. Control the oxygen gas supply amount supplied to the gas phase part of the nitrification tank and The predetermined range of oxygen concentration to be maintained is set so that the gas concentration in the nitrification tank or the oxygen concentration of the exhaust gas is kept at 40% or more and the oxygen utilization efficiency is lower than 85%. The wastewater treatment method is characterized by the following.

  In the present invention, a high-concentration oxygen gas is supplied into the gas phase of a sealable nitrification tank filled with a carrier to which nitrifying bacteria are attached, and exhaust gas is discharged from the gas phase of the nitrification tank, A gas in the gas phase in the nitrification tank is diffused into the liquid phase, and the liquid phase and / or sludge in the nitrification tank is returned to the denitrification tank provided in the preceding stage of the nitrification tank, and ammonia in the waste water In the wastewater treatment method of biologically oxidizing organic nitrogen and organic nitrogen to nitrate nitrogen and nitrite nitrogen, and denitrifying treatment, the dissolved oxygen concentration in the liquid phase in the nitrification tank is detected, Based on the detection result, the amount of diffused air to be diffused into the liquid phase is controlled so that the dissolved oxygen concentration is maintained at a set value, and the gas in the gas phase section in the nitrification tank or the exhaust gas is controlled. The oxygen concentration is measured, and the oxygen concentration is maintained within a predetermined range based on the measurement result. In addition, the oxygen gas supply amount supplied to the gas phase portion of the nitrification tank is controlled, and the oxygen concentration to be maintained is set to a predetermined range of the oxygen concentration of the gas phase portion in the nitrification tank or the oxygen concentration of the exhaust gas. The wastewater treatment method is characterized in that the oxygen utilization efficiency is set to be lower than 85% while being kept at 40% or more.

  In the wastewater treatment method, the nitrification tank comprises a plurality of tanks partitioned by a partition wall, and for each tank of the plurality of tanks, gas in a gas phase is diffused into the liquid phase, and the plurality of nitrification tanks The dissolved oxygen concentration in the liquid phase is detected for each tank, and based on the detection result, the dissolved oxygen concentration is dispersed in the liquid phase of each tank so that the dissolved oxygen concentration is maintained at the set value for each tank. The amount of air diffused can be controlled, and the nitrification tank is composed of a plurality of tanks partitioned by a partition, and the gas phase part for maintaining the oxygen concentration within a predetermined range is the nitrification tank of the final tank. The gas phase part is preferably a gas phase part gas in the nitrification tank or the predetermined range of the exhaust gas oxygen concentration is a pollutant that requires oxidation treatment contained in the inflow wastewater flowing into the nitrification tank It can be varied according to the amount variation.

According to the wastewater treatment method of the present invention, the total running cost for wastewater treatment can be reduced in the oxygen activated sludge method combining the carrier nitrification method and the circulation aeration method. In recent years, the production cost of high-concentration oxygen gas by a high-concentration oxygen gas generator (for example, the PSA method) has become low, which may be lower than the power cost of the diffuser blower. On the contrary, by increasing the oxygen concentration of the gas or exhaust gas in the gas phase part of the nitrification tank and lowering the oxygen utilization efficiency, it is possible to suppress the amount of diffused air and to reduce the running cost. It is possible to reduce the total running cost for wastewater treatment.
In addition, the running cost can be further reduced by changing the predetermined range of the oxygen concentration in the gas phase portion in accordance with the change in the amount of pollutant in the inflow drainage.

The flow block diagram which shows an example of the waste water treatment equipment used for this invention. The flow block diagram which shows the other example of the waste water treatment equipment used for this invention. The graph which shows the change of the running cost by the diffuser and the diffused water depth. The graph which shows the change of the required oxygen amount and setting exhaust gas oxygen concentration by time. The graph which shows the relationship between DO and nitrification performance.

Hereinafter, the present invention will be described in detail.
In the present invention, first, in order to clarify the problems of the conventional surface aeration method, a comparative experiment between the surface aeration method and the gas circulation aeration method was performed using a carrier having a shape and material that is difficult to wear. . As a result, in the surface aeration method, even when the carrier itself was not worn, it was found that the nitrification performance was estimated to be caused by the inhibition of nitrifying bacteria adhering to the carrier surface by aeration stirring. On the other hand, in experiments conducted using the gas circulation aeration method only under the same conditions as above, it was found that good nitrification performance was obtained, and that the treatment performance was superior to the conventional surface aeration method. However, since the power cost of the air diffuser used to perform the gas circulation diffuser (for example, a blower) is high and increases the running cost of the entire system, the production cost of the high concentration oxygen gas and the power cost of the air diffuser are reduced. Taking this into consideration, we have come to the idea that it is essential to establish a processing method that can keep the running cost of the entire system low.

In the conventional oxygen activated sludge method, it has been understood that it is essential and important to operate with high oxygen utilization efficiency in order not to waste oxygen input for wastewater treatment. The present inventors also followed the idea at the beginning, and were also developing the oxygen activated sludge method combining the carrier nitrification method and the circulation aeration method with the aim of achieving high oxygen utilization efficiency.
However, while the present inventors are diligently pursuing research and development, achieving high oxygen utilization efficiency in the oxygen activated sludge method combining the carrier nitrification method and the circulation aeration method is not necessarily the running cost of the entire system. It has been found that it does not contribute to the decline of
In particular, when the oxygen utilization efficiency of 85% or more, which is known as the value of the existing technology, is adopted, the running cost of the entire system is significantly increased.

In recent years, the production cost of high-concentration oxygen gas by a high-concentration oxygen gas generator (for example, the PSA method) has become low, which may be lower than the power cost of the diffuser blower. There is no need to achieve this, by increasing the oxygen concentration of the gas or exhaust gas in the nitrification tank gas phase and lowering the oxygen utilization efficiency, it is possible to reduce the amount of diffused air and reduce the running cost, and the wastewater treatment It is possible to reduce the total running cost.
For these reasons, it is important for the present inventors to control the oxygen concentration of the gas or exhaust gas in the gas phase part of the nitrification tank so that the oxygen utilization efficiency is lower than 85% in order to reduce the running cost of the entire system. I found out.
In the present invention, an apparatus as shown in FIG. 1 can be used. This apparatus comprises a sealed nitrification tank into which a nitrification carrier having nitrifying bacteria fixed on the surface is charged, and an oxygen supply line and an exhaust gas line are connected to the nitrification tank. Aeration in the nitrification tank is performed by a gas circulation system using a circulation blower and an aeration device, and the amount of aeration is controlled and supplied by the nitrification tank DO. Further, the supply amount of the high concentration oxygen gas is controlled and supplied by the gas phase portion of the nitrification tank or the oxygen concentration of the exhaust gas.

  The nitrification performance of the carrier greatly depends on the DO of the nitrification tank, and 2 mg / L or more of DO was necessary to maintain high nitrification performance. On the other hand, the higher the DO, the higher the nitrification performance. Therefore, it is better to set the DO as high as possible. However, when the DO exceeds 12 mg / L, the nitrification performance reaches its peak (see FIG. 5), which is preferable. The DO condition of the liquid mixture in the nitrification tank liquid phase is 2 to 12 mg / L. Further, the high dependency of DO means that the nitrification performance can be arbitrarily adjusted by setting the optimal DO value according to the load condition and the required processing performance. In particular, when there is load fluctuation, the DO value is set low during the low load period, and conversely, the DO value is set high during the high load period. Performance can be demonstrated.

  Specifically, a method of changing the upper and lower limit set values of the dissolved oxygen concentration in the nitrification tank mixed solution in advance based on the fluctuation pattern of the ammonia nitrogen or organic nitrogen load with time can be employed. Since the load is calculated by the product of wastewater inflow and wastewater concentration, (drainage inflow) x (drainage concentration), both wastewater inflow and wastewater concentration are used as indicators. Is small, the drainage concentration may be used as an index, and conversely, when the fluctuation of the drainage concentration is small, the drainage inflow amount may be used as an index. Here, the liquid phase as an index is not limited to waste water. That is, following the load fluctuation, the concentration of ammonia nitrogen in the liquid phase such as the mixed liquid in the nitrification tank, the water in the sedimentation basin, and the treated water also changes. The upper and lower limit set values of the dissolved oxygen concentration of the nitrification tank mixture may be changed in advance.

  The liquid phase used as an indicator is not limited as long as the change in ammoniacal nitrogen concentration can be grasped. However, in order to better follow the load fluctuation pattern, the wastewater to be treated and the nitrification reaction progress. A nitrification tank is preferred. Furthermore, based on the detection result of measuring ammonia nitrogen or organic nitrogen load or ammonia nitrogen concentration in the liquid phase, it had a function to change the upper and lower limit set values of dissolved oxygen concentration in the nitrification tank mixture You may control automatically using a control apparatus. Examples of these detectors include an ammonia concentration meter and a nitrogen concentration meter. As for load fluctuations, changes in the amount of water affect the concentration, so it is sufficient to use a water meter together with these concentration meters, or use a load meter that measures the concentration and water volume and calculates the load. May be. Of course, the detector is not limited to these detectors as long as it is a detection means capable of grasping and predicting ammonia nitrogen or organic nitrogen load or liquid ammonia ammonia concentration.

There are no restrictions on the location of DO concentration measurement in the nitrification tank mixture to control the amount of air blower blower, as long as the operation of the blower and stabilization of the nitrification tank DO are possible. It is preferable that a DO meter and a blower are provided in each nitrification tank, and the blower is individually controlled by DO. This is because it is necessary to maintain an appropriate DO in all nitrification tanks in order to advance nitrification efficiently. The blower is controlled by increasing or decreasing the number of rotations of the blower with an inverter based on the detection result of the DO value, or by adjusting the opening of the air volume control valve.
The DO meter used in the present invention is not limited as long as it can be used for a long period of time without being affected by the measurement of the mixed liquid and wastewater components. Is preferred.

As a method for controlling the supply amount of oxygen gas, there is a method of supply control based on a change in pressure in the gas phase part of the nitrification tank or a change in the flow rate of exhaust gas, which is often employed in the surface aeration method.
However, in the circulation aeration method, when these methods are applied, the gas movement and pressure change are more remarkable than the surface aeration method, and stable operation cannot be performed. On the basis of this, it is preferable to employ a control method for supplying high-concentration oxygen gas.

In the method of the present invention, the high concentration oxygen gas is controlled and supplied based on the measurement result of the oxygen concentration of the nitrification tank gas phase part or the exhaust gas. Specifically, for example, when the oxygen concentration in the gas phase part falls below the lower limit value, the flow control valve (17 in FIG. 1) installed in the oxygen gas supply pipe (10 in FIG. 1) is opened, or the opening degree The amount of supplied oxygen can be increased by increasing. Conversely, when the oxygen concentration exceeds the upper limit, the amount of supplied oxygen can be reduced by closing the flow rate control valve installed in the oxygen gas supply pipe or reducing the opening. In this method, since the oxygen concentration in the gas phase can be set arbitrarily, it not only contributes to the operation of the blower and the stabilization of the nitrification tank DO, but also adjusts the oxygen utilization efficiency arbitrarily, so that stable and efficient operation is possible. Can do.
The oxygen concentration measurement location for controlling the high-concentration oxygen gas supply is not limited as long as the operation of the blower, stabilization of the nitrification tank DO, and adjustment of oxygen utilization efficiency are possible, but the oxygen utilization efficiency is adjusted. Therefore, it is preferable to control by the gas phase part of the most downstream nitrification tank or the oxygen concentration of the exhaust gas.

In the present invention, the target running cost can be easily reduced by setting the oxygen concentration of the nitrification tank gas phase gas or exhaust gas. In general, in the oxygen activated sludge method, oxygen utilization efficiency of about 85 to 90% is required. However, in the present invention, the oxygen concentration of the gas phase part gas or exhaust gas in the most downstream is 40% or more, preferably 45%. The oxygen utilization efficiency was set to be lower than 85%, preferably 80% or lower, by controlling to not less than 85%. Although this setting depends on the concentration of the high-concentration oxygen gas to be used, when the oxygen concentration is 40% or less, the advantage of using the high-concentration oxygen gas to reduce the amount of air diffused cannot be exhibited. In addition, when the oxygen utilization efficiency is 85% or more, it has been found through numerous tests that it is necessary to increase the amount of diffused air in order to supply the required amount of oxygen to the liquid phase. By finding out the superiority of setting by investigating and studying important factors such as constraints on treatment facilities.
Here, the oxygen utilization efficiency is represented by the following.
Oxygen utilization efficiency = Oxygen utilization (necessary oxygen) / Supplying oxygen x 100

The oxygen concentration meter used in the present invention is not limited as long as it can withstand long-term use without being affected by the measurement of the gas diffused and the components of the exhaust gas, but is included in the target gas at a high concentration. A zirconia oxygen concentration meter, a magnetic oxygen concentration meter, and an infrared oxygen concentration meter that are not easily affected by carbon dioxide are suitable.
As the high-concentration oxygen gas used in the method of the present invention, a gas having an arbitrary oxygen concentration in which the oxygen concentration is higher than that of air can be used. Examples of such a gas include oxygen-enriched air in which the oxygen gas concentration is increased using an oxygen enricher, and pure oxygen gas having an oxygen concentration close to 100%. It is preferable to use a high-concentration oxygen gas by the PSA method with about 90%. Such a high-concentration oxygen gas can be directly supplied to the gas phase part of the nitrification tank or the circulation aeration pipe. When supplied to the circulating air diffuser piping, the oxygen concentration in the nitrification tank gas phase and the DO responsiveness of the mixed solution were slightly accelerated.

  In the present invention, a gas having a high oxygen concentration is diffused into the mixed solution. However, the exhaust gas that has been diffused and has come out of the water surface has a higher oxygen concentration than the atmosphere, so this exhaust gas is not released into the atmosphere. The oxygen utilization efficiency could be increased by repeatedly aeration into the liquid phase through the blower. Therefore, the gas once diffused is not directly discharged to the outside of the tank, but can be sealed so that most of the gas other than the discharged gas can be supplied again to the blower and repeatedly diffused. There are no restrictions on the shape, number, arrangement, etc. of the tank. When the tank structure is simplified, a single tank may be used, or the tank may be divided into multiple tanks in consideration of the fluidity of the carrier. The division of the tank may be parallel or in series with the inflow of raw water. However, when oxygen is used particularly efficiently, a multistage structure as shown in FIG. Supply high-concentration oxygen (10) to the gas phase section or circulation aeration line (15), and arrange the gas to flow from the upstream tank to the downstream tank sequentially through the gas phase communication section, The most efficient method is to discharge the exhaust gas (11) out of the system from the most downstream water tank.

  The air diffuser used in the present invention does not cause nitrification carrier wear / breakage, nitrifying bacteria adherence inhibition, and the gas emitted from the water surface of the nitrification tank is repeatedly aerated to produce a sludge mist. Any porous tube, disk diffuser, sparger, etc. may be used as long as they are not easily clogged by dust or the like. In addition, by installing sludge mist in the circulating gas, mist separator for removing dust, fine dust, etc., gas filtration air, etc. on the suction side of the blower, a fine bubble diffuser with higher oxygen transfer efficiency An apparatus can also be used. Examples of such a diffuser include a diffuser plate and diffuser made of ceramic or synthetic resin, and a membrane diffuser.

Even when the same amount of oxygen is supplied to the sewage, the amount of air diffused varies depending on the depth of the diffused air and the diffuser used. A normal aeration tank has an aeration depth of 4 to 6 m. However, if the depth of the aeration is increased, the required amount of aeration is reduced even if the same aeration device is used. Even when an air diffuser with good oxygen dissolution efficiency is used, the amount of air required is reduced.
Therefore, the oxygen utilization efficiency at which the running cost is minimum differs depending on the depth of water to be diffused and the diffuser to be used. This is shown in the graph of the change in running cost according to the air diffuser and the water depth of FIG. In each condition, the running cost when the oxygen utilization efficiency is 85% is assumed to be 100, and the running cost changes depending on the change of the oxygen utilization efficiency.
However, in any case, there is a point where the running cost is minimized when the oxygen utilization efficiency is lower than 85%.

In an actual facility, it is not always possible to drive at a point where the running cost is minimized. To operate with low oxygen utilization efficiency requires so much high-concentration oxygen gas and many oxygen production apparatuses. Depending on the treatment plant, there may not be enough facility area to install the oxygen production device, and in that case, it will be operated with the oxygen utilization efficiency that minimizes the running cost within the range where the oxygen production device can be installed. Become. In addition, when the capacity of the oxygen production device is small and it is difficult to operate constantly under the conditions of the present invention, it is necessary, for example, referring to seasonal changes and fluctuations in the amount of pollutants that require oxidation treatment. The present invention may be applied to a part of the operation time such as a time zone in which the amount of oxygen is relatively low.
The present invention can be used with the various air diffusers described above, but when used in the circulating air diffuser, water drops and scum are mixed into the gas to be sucked, causing clogging of the air diffuser. In view of this, it is more preferable that the air diffuser is a perforated tube capable of providing large air holes.
The blower is not limited as long as it does not hinder hermeticity and continuous operation for a long time, but a Roots blower is preferable.

As the carrier used in the present invention, a binding-immobilized carrier that allows nitrifying bacteria to adhere to the surface of the carrier is suitable. Moreover, if the adherence of nitrifying bacteria is good and can hold a sufficient amount of nitrifying bacteria for treatment, and the flowability and durability are good, there is no limitation on the shape, material, and physical properties, but the shape is Since it has a large surface area and good wear resistance, it is preferably granular or spherical. The size is preferably a granule having a diameter of 1 to 10 mm, and the material preferably includes polyethylene glycol (PEG) or polyethylene glycol. The specific gravity is preferably in the range of 0.90 to 1.1 in terms of fluidity.
The binding immobilization carrier is suitable in that nitrifying bacteria adhere to the carrier spontaneously in the nitrification tank to form a biofilm. According to the study by the present inventors, the pH is 6 or less, and in some cases 5 This is because it has been found that even under extremely low conditions of about 5 or less, high nitrification performance can be exhibited by gradually adapting to the conditions. This makes it possible to proceed with nitrification only by adjusting the pH with a small amount of an alkaline agent without performing any neutralization treatment even in the oxygen activated sludge method in which the pH tends to decrease.

The target waste water of the present invention is not limited to sewage and industrial waste water, and may be water containing ammonia nitrogen and / or organic nitrogen, and the concentration of the waste water is not limited. For example, in the case of water lacking the alkalinity necessary for nitrification and other components such as phosphorus and iron necessary for the nitrification reaction, these may be added, and copper and hydrogen sulfide that inhibit the nitrification reaction About water that contains etc., it can be set as target drainage by performing abatement treatment.
The sewage into which domestic wastewater flows has periodic fluctuations in the flow rate and fluctuations in the pollutant concentration during the day depending on human life patterns. The flow increases and the pollutant concentration also increases in the morning and evening when activities are active. On the other hand, the flow rate and pollutant concentration decrease from midnight to early morning when human activity is reduced. However, since it takes time to flow from the sewage generation point to the sewage treatment plant, there is a flow rate variation and pollutant concentration variation specific to each treatment plant. The required amount of oxygen required for inflow raw water is mainly determined by the amount of pollutants contained in the inflow raw water (= load), and the load is expressed by the product of the flow rate and the concentration of pollutants. It is possible to further reduce the running cost of the entire system by changing the predetermined range of the oxygen concentration in accordance with the change of the load.

The circulating air diffuser not only supplies DO but also has a role of causing the carrier to flow. If necessary, a stirrer may be used in combination to maintain the fluidity of the carrier. The stirrer to be used is not limited as long as it can flow the nitrification carrier without causing abrasion or damage of the nitrification carrier and inhibition of nitrifying bacteria adhesion.
Furthermore, the detailed conditions for the nitrification reaction under the low pH conditions revealed by the present inventors are that the alkalinity is important, specifically, the pH of the nitrification tank is 5 to 6, and the alkalinity is The minimum amount required for nitrification, preferably the alkalinity of the nitrification tank was 10 mg / L or more, more preferably the alkalinity of the nitrification tank was 30 mg / L or more. If it can be set in such a range, it is not necessary to use a decarboxylation treatment or a pH adjuster, and if the pH is further lowered, or if the alkalinity is insufficient, an alkali agent sufficient to meet the shortage is injected, What is necessary is just to perform the decarboxylation process of a required amount. In addition, as shown in FIG. 2, a denitrification step may be provided in the front stage of the nitrification tank, and the increase in alkalinity due to the denitrification reaction may be utilized by returning the liquid phase and / or sludge of the nitrification tank.

  In our research, with various effluents such as sewage and other industrial wastewater, a denitrification tank is installed in the front stage of the nitrification tank, and even if circulation nitrification denitrification equivalent to the returned sludge is performed, without using chemicals, It was possible to keep the alkalinity of the nitrification tank in a sufficiently suitable range. Of course, a circulation line may be provided in the nitrification tank and the denitrification tank for circulation. The circulation rate including the amount of returned sludge is preferably 0.3 times or more of the raw water amount, and more preferably 0.5 or more. Thus, by incorporating the denitrification step, not only the purpose of nitrogen removal can be achieved, but also the stabilization of nitrification performance by keeping the alkalinity suitable can be realized without using chemicals. The installation of a denitrification tank in front of the nitrification tank itself is a general method as a nitrification denitrification system, but alkalinity is supplied for the conditions of the oxygen activated sludge method using the nitrification carrier of the present invention. By doing so, it has a big role of stabilizing the performance at a low cost.

The suspended sludge in the nitrification tank mixed solution may have the removal of BOD coexisting in the waste water and the nitrification performance, so it is advantageous to coexist in the nitrification tank together with the carrier. However, if the treatment can be satisfied only by nitrification with a carrier, it is not necessary to use floating sludge. In such a case, the return sludge line is not necessary, and naturally, if the circulation type nitrification denitrification operation is not performed, the circulation line is also unnecessary.
According to the method of the present invention, since high nitrification performance can be exhibited, the nitrification tank capacity can be made compact, and the treatment performance can be satisfied with an extremely short residence time of about HRT 1.4-2 hr. .
The screen for separating the carrier installed in the nitrification tank is not limited as long as the carrier can be separated and damaged and worn, but the oxygen activated sludge method has a maintenance structure because the nitrification tank has a sealed structure. Good specification is suitable. As a result of the study by the present inventors, it was found that a cylindrical mechanical screen that can be installed and rotated, in particular, has a most preferable specification.

DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a flow configuration diagram showing an example of an apparatus used in the wastewater treatment method of the present invention. As shown in FIG. 1, the waste water treatment apparatus includes a sealable nitrification tank 2 storing a nitrification carrier 5 to which nitrifying bacteria are attached, a sedimentation basin 3, an oxygen gas supply line 10, an exhaust gas line 11, and a raw water supply. Line 1 and treated water outflow line 4 are provided. The oxygen gas supply line 10 is provided with a flow rate control valve 17, and the exhaust gas line 11 is provided with an oxygen concentration meter 14.
The nitrification tank 2 includes a gas circulation line 15 for circulating and diffusing the nitrification tank gas phase gas 12 between the liquid surface and the ceiling of the nitrification tank through the blower 9 and a dissolved oxygen concentration meter 13. ing.

Furthermore, the waste water treatment apparatus of the present invention is provided with a control device. The control device 16 functions as a control unit that controls the amount of air diffused by the blower 9 based on the detection result of the dissolved oxygen concentration meter 13. The blower 9 is controlled by increasing or decreasing the number of rotations of the blower based on the detection result of the DO value, or by adjusting the opening of the air volume control valve. On the other hand, the control device 18 functions as a control means for adjusting the opening / closing operation or the opening degree of the flow control valve 17 of the high concentration oxygen gas based on the detection result of the oximeter 14.
At this time, in the present invention, in order to reduce the running cost, the control device 18 sets the oxygen concentration of the exhaust gas in the nitrification tank to 40% or more and sets the oxygen utilization efficiency to be lower than 85%. Yes. By doing so, the dissolved oxygen concentration can be maintained at the set value even if the amount of air diffused is reduced, the power of the air diffuser can be reduced, and the cost of the entire system can be reduced.

Next, a wastewater treatment method using the wastewater treatment apparatus configured as described above will be described.
First, the waste water to be treated is introduced into the nitrification tank 2 via the line 1. The waste water introduced into the nitrification tank 2 is mixed with a carrier on which nitrifying bacteria in the nitrification tank 2 are fixed.
Next, high-concentration oxygen gas is supplied into the nitrification tank 2 through the line 10 to fill the gas phase part 12 with high-concentration oxygen. Then, by operating the blower 9, the gas in the gas phase portion 12 is once sucked, and this gas is sent to the diffusion tube 8 in the nitrification tank 2 and diffused into the mixed liquid in the nitrification tank 2.
In this way, oxygen supplied from the line 10 into the sealable nitrification tank 2 dissolves in the mixed liquid in the nitrification tank 2 more efficiently than air.
Next, the control device 18 is operated, the flow rate control valve 17 is controlled based on the indicated value of the oximeter 14, and a necessary amount of oxygen is always supplied, so that the oxygen concentration of the nitrification tank gas phase gas 12 is predetermined. Stable within range. Further, the control device 16 is operated, and the amount of air diffused in the blower 9 is controlled based on the indicated value of the dissolved oxygen concentration meter 13, so that the necessary amount of air is always diffused and the dissolved oxygen concentration of the mixed liquid is stabilized.

Next, another example of the apparatus used in the wastewater treatment method of the present invention will be described based on the flow configuration diagram of FIG. 2, the same reference numerals are given to the same or corresponding parts as in FIG. 1, and detailed description thereof will be omitted.
In FIG. 2, the nitrification tank is partitioned by a partition wall to form a two-stage structure in series with the inflow of raw water, and a denitrification tank 20 is provided in the front stage of the nitrification tank. The nitrification tank is divided into a liquid phase and a gas phase by a partition wall, but the liquid phase communicates with the carrier separation screen 24, while the gas phase also has a gas communication portion 25. Each nitrification tank is provided with a gas circulation line, and the oxygen supply line 10 is connected to the first nitrification tank.
The denitrification tank 20 biologically treats the introduced wastewater to be treated using activated sludge mainly composed of denitrifying bacteria. For example, floating activated sludge is accommodated in the denitrification tank 20, A stirring device 21 for stirring the waste water is provided.
In FIG. 2, the exhaust gas line 11 is provided with the oxygen concentration meter 14, but it can also be provided in the gas phase part of the nitrification tank 2 or the gas phase part of the nitrification tank 2 ′.

Next, a wastewater treatment method using the wastewater treatment apparatus of FIG. 2 will be described.
First, the wastewater to be treated is introduced into the denitrification tank 20 through the line 1. The wastewater introduced into the denitrification tank 20 is mixed with the activated sludge and stirred, so that the organic matter supplied from the raw water is used as a hydrogen donor, and nitrate nitrogen and nitrite nitrogen supplied from the return sludge 6 are used. Is decomposed into nitrogen gas.
The drained water after denitrification is sent in the order of the first nitrification tank 2 and the second nitrification tank 2 ′, and is mixed with the carrier to which nitrifying bacteria adhere, and nitrification proceeds.
The effluent after nitrification is sent to the sedimentation basin 3, and activated sludge is separated from the effluent after nitrification. The supernatant waste water from which the activated sludge has been separated is discharged as treated water 4. On the other hand, the separated and separated sludge is returned to the denitrification tank through the return sludge 6 line. Further, surplus separated sludge is discharged from the surplus sludge line 7 to the outside of the system.
The high-concentration oxygen gas is supplied from the line 10 to the gas phase portion 12 of the first nitrification tank 2 so that the gas phase portion 12 is filled with high-concentration oxygen. Then, by operating the blower 9, the gas in the gas phase portion 12 is once sucked, and this gas is sent to the aeration pipe 8 in the nitrification tank 2 and diffused into the waste water in the nitrification tank 2.

Next, the remaining oxygen gas discharged from the first nitrification tank 2 is supplied to the gas phase section 12 ′ of the second nitrification tank 2 ′, and the gas phase section 12 ′ is filled with high-concentration oxygen. Then, by operating the blower 9 ′, the gas in the gas phase section 12 ′ is once sucked, and this gas is sent to the diffuser pipe 8 ′ in the nitrification tank 2 ′ to be discharged into the waste water in the nitrification tank 2 ′. Scatter.
Next, the control device 18 is operated, the flow rate control valve 17 is controlled based on the indicated value of the oximeter 14, and a necessary amount of oxygen is always supplied, so that the oxygen concentration of the nitrification tank gas phase gas 12 ′ is increased. Stable within a predetermined range to reduce running costs. Further, the control devices 16 and 16 ′ are operated, and the blower 9 and 9 ′ aeration amount are controlled based on the indicated values of the dissolved oxygen concentration meters 13 and 13 ′, respectively, so that a necessary amount of aeration is always performed. The dissolved oxygen concentration in the nitrification tank mixture is stabilized.

In this way, the oxygen supplied from the line 10 into the sealable nitrification tank 2, 2 ′ dissolves in the waste water in the nitrification tank 2, 2 ′ more efficiently than air. Furthermore, the nitrification tank is made into a multistage in series, and oxygen is efficiently used from the upstream side by supplying oxygen to the upstream tank, and the oxygen concentration is lowered toward the downstream side, so that oxygen is efficiently used. be able to.
The purpose of supplying the high concentration oxygen gas from the line 10 to the gas phase portion 12 of the first nitrification tank 2 is to use oxygen efficiently. The injection point and gas flow are not limited to the first nitrification tank 2. For example, if the gas phase part of the second nitrification tank 2 ′ is also injected near the boundary with the gas phase part of the first nitrification tank 2, a part of the high-concentration oxygen gas is first from the second nitrification tank 2 ′. It was also supplied to the nitrification tank 2 and efficient operation was possible.

Hereinafter, the present invention will be described in detail with reference to examples, and the results obtained through experiments are shown in Tables 2 and 3 together with Comparative Examples.
Example 1
The nitrification carrier is put into the experimental apparatus (circulation amount 165 m ³ / day, nitrification tank capacity 10 m ³ , HRT 1.5 hr, return sludge amount 82.5 m ³ / day) based on the flow shown in FIG. For the sewage primary treated water (hereinafter referred to as raw water) of ammonia nitrogen (NH ₄ -N) concentration 16 to 25 mg / L and organic nitrogen (Org-N) concentration 3 to 11 mg / L shown in Table 1, A treatment experiment was conducted.

The specifications of the experimental apparatus are as follows.
Oxygen gas generator: PSA (pressure swing adsorption) system DO meter: Fluorescent dissolved oxygen meter Oxygen meter: Zirconia oxygen concentration Blower: Roots blower diffuser: Porous tube Carrier: Spherical PEG carrier Carrier filling rate: 20 % (Apparent volume per nitrification tank volume)
The experimental conditions are as follows.
O ₂ concentration of the supplied high concentration oxygen gas: 80 to 90%
High concentration oxygen gas supply amount: 34 to 66 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 66%
Aeration amount: 34 to 72 m ³ / h
Aeration water depth: 4m

As a result, the nitrifying bacteria adhering to the carrier gradually became acclimatized, and the NH ₄ -N concentration of the treated water gradually decreased, and on the 20th day from the start of the treatment, it became 0.2 to 0.9 mg / L. The processing performance was good. The DO and exhaust gas oxygen concentrations were successfully controlled and could be controlled as set. The oxygen concentration of the exhaust gas was constant at about 66%, and the oxygen utilization efficiency was 63%. Further, when the power cost of Comparative Example 1 in which the oxygen utilization efficiency was 85% was set to 1, the power cost was reduced to 0.85 because the blower power was reduced.

Example 2
Experimental apparatus of circulation aeration system based on the flow shown in FIG. 2 (throughput 165 m ³ / day, nitrification tank capacity 10 m ³ , HRT 1.5 hr, denitrification tank capacity 5 m ³ , return sludge volume 82.5 m ³ / day) The nitrification carrier is added to the sewage primary treated water (hereinafter, referred to as “ammonia nitrogen (NH ₄ -N) concentration 16-25 mg / L, organic nitrogen (Org-N) concentration 3-11 mg / L) shown in Table 1”. A treatment experiment was conducted on the raw water.
The specifications of the experimental apparatus are the same as those in Example 1, and the experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High-concentration gas supply amount: 31 to 63 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 66%
Aeration amount: 45 to 62 m ³ / h
Aeration water depth: 4m

The nitrifying bacteria adhering to the carrier gradually became acclimatized, and the NH ₄ -N concentration of the treated water gradually decreased, and was 0.2 to 0.4 mg / L on the 20th day from the start of the treatment. The nitrification performance was even better than that. This is because the alkalinity of the nitrification tank could always be maintained at 30 mg / L or more due to the increase in alkalinity due to the denitrification reaction. The treatment performance was good, the DO and exhaust gas oxygen concentrations were successfully controlled, and the exhaust gas oxygen concentrations could be controlled as set values (66%). Since the oxygen utilization efficiency was set to 63%, the blower power was reduced, and the power cost was 0.80 when the power cost of Comparative Example 1 was 1, and Comparative Example 2 (power cost: 0) under the same experimental conditions. .95).

Example 3
Based on the flow shown in FIG. 2, a treatment experiment was performed on the treated water of Table 1. A treatment experiment was performed under the same conditions as in Example 2 except that the diffused water depth of the perforated tube was 5 m. The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High-concentration gas supply amount: 29 to 59 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 63%
Aeration amount: 34 to 47 m ³ / h
Aeration water depth: 5m
In this example, nitrifying bacteria adhering to the carrier gradually acclimatize, and the NH ₄ -N concentration of the treated water gradually decreases, and on the 20th day from the start of the treatment, it is 0.2 to 0.4 mg / L. There was good nitrification performance. The DO and exhaust gas oxygen concentrations were successfully controlled, and the exhaust gas oxygen concentration was 63% and could be controlled as set. In this example, since the diffused water depth was increased to 5 m, the optimum running cost point was changed, and the optimum oxygen utilization efficiency was 67%, which was higher than those in Examples 1 and 2. The power cost was 0.85 when the blower power was reduced and was lower than that of Comparative Example 3, and the cost of Comparative Example 3 was set to 1.

Example 4
Based on the flow shown in FIG. 2, a treatment experiment was performed under the same conditions as in Example 2 except that a membrane panel was used as a diffuser for the treated water of Table 1.
The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High concentration gas supply amount: 28 to 56 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 55%
Air diffuser: Membrane panel Air diffuser: 12-17m ³ / h
Aeration water depth: 4m
In this example, NH ₄ —N of the treated water was 0.2 to 0.4 mg / L, and the treatment performance was good. The DO and exhaust gas oxygen concentrations were successfully controlled, and the exhaust gas oxygen concentration was 55%, which was controlled as set. In this example, since the diffuser was a membrane panel, the optimum point of running cost was changed, and the optimum oxygen utilization efficiency was 78%. The power cost was 0.90 when the power of the blower power was reduced and the power cost of Comparative Example 4 was 1.

Example 5
A processing experiment based on the flow shown in FIG. 2 was performed. The exhaust gas oxygen concentration to be set was changed in accordance with the change in the amount of oxygen necessary for oxidizing the pollutant contained in the raw water.
FIG. 4 shows the required oxygen amount at each time as a ratio to the daily average value of the required oxygen amount. FIG. 4 shows a set value of the set exhaust gas oxygen concentration that is changed by the change in the required oxygen amount. Except for changing the set exhaust gas oxygen concentration according to the required oxygen amount of the raw water, a treatment experiment was conducted under the same conditions as in Example 2, and the experimental conditions were as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High-concentration gas supply amount: 36 to 72 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: As shown in FIG. 4, (varies between 60-72%)
Aeration amount: 39 to 53 m ³ / h
Aeration water depth: 4m
In this example, the set exhaust gas oxygen concentration was also changed by the change in the required amount of oxygen in the influent wastewater, but stable control was possible as in the other examples. From the start of operation, the NH ₄ —N concentration in the treated water gradually decreased, and on the 20th day from the start of the treatment, it was 0.2 to 0.4 mg / L, indicating good nitrification performance. The DO and exhaust gas oxygen concentrations were successfully controlled, and the exhaust gas oxygen concentrations could be controlled according to the set values shown in FIG. In this example, the oxygen utilization efficiency was 58%, but the power cost when the power cost of Comparative Example 1 was set to 1 was 0.70, and the set exhaust gas oxygen concentration was changed in the required oxygen amount of raw water. As a result of changing according to the above, it could be further reduced.

Comparative Example 1
The specifications of the experimental apparatus, the water to be treated, and the experimental apparatus were treated under the same conditions as in Example 1.
The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High-concentration gas supply amount: 28 to 55 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 46%
Aeration amount: 41 to 86 m ³ / h
Aeration water depth: 4m
This comparative example is a comparative example of the first embodiment, and controls the supply amount of the high concentration oxygen gas by the exhaust gas oxygen concentration as in the first embodiment. Also in this comparative example, the treatment performance was good, the DO and exhaust gas oxygen concentration were successfully controlled, and the exhaust gas oxygen concentration was controlled to 46% as set. However, since the oxygen utilization efficiency was set to 85%, the oxygen supply amount was reduced from the first embodiment in which the oxygen utilization efficiency was optimally set from the viewpoint of running cost. The cost was high.

Comparative Example 2
The experiment was performed under the same conditions as in Example 2 with respect to the specifications of the experimental apparatus, the water to be treated and the experimental apparatus.
The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High concentration gas supply: 25-50 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 46%
Aeration amount: 56-77 m ³ / h
Aeration water depth: 4m
This comparative example is a comparative example of Example 2. Like Example 2, the treatment performance was further improved by the increase in alkalinity due to the denitrification reaction, and the control was also good. However, since the exhaust gas oxygen concentration was controlled to 46% and the oxygen utilization efficiency was set to 85%, the oxygen supply amount decreased from Example 2 in which the oxygen utilization efficiency was optimally set from the viewpoint of running cost. The amount of air diffused increased and the power cost of the entire system increased.

Comparative Example 3
A treatment experiment was performed under the same conditions as in Comparative Example 2, except that the diffused water depth of the perforated tube was 5 m.
The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High concentration gas supply: 25-50 L / min
Setting DO: 7.5 mg / L
Aeration amount: 40 to 55 m ³ / h
Set exhaust gas O ₂ concentration: 46%
Aeration water depth: 5m
This comparative example is a comparative example of Example 3. The processing performance was good as in Example 3, and the DO and exhaust gas oxygen concentrations could be controlled as set values. However, since the exhaust gas oxygen concentration was controlled to 46% and the oxygen utilization efficiency was set to 85%, the oxygen supply amount decreased compared to Example 3 in which the oxygen utilization efficiency was optimally set from the viewpoint of running cost. As the amount of air diffused increased, the power cost of the entire system increased.

Comparative Example 4
A treatment experiment was performed under the same conditions as in Comparative Example 2, except that a membrane panel was used as the air diffuser.
The experimental conditions are as follows.
O ₂ concentration of supplied high concentration oxygen gas: 80 to 90%
High concentration gas supply: 25-50 L / min
Setting DO: 7.5 mg / L
Set exhaust gas O ₂ concentration: 46%
Air diffuser: Membrane panel Air diffuser depth: 4m
Aeration amount: 14 to 19 m ³ / h
This comparative example is a comparative example of the example 4. The processing performance was good as in the example 4, and the DO and exhaust gas oxygen concentrations could be controlled as set values. However, since the exhaust gas oxygen concentration was controlled to 46% and the oxygen utilization efficiency was set to 85%, the oxygen supply amount decreased compared to Example 4 in which the oxygen utilization efficiency was optimally set from the viewpoint of running cost. As the amount of air diffused increased, the power cost of the entire system increased.

  1: drainage, 2, 2 ′: nitrification tank, 3: sedimentation tank, 4: treated water, 5: nitrification carrier, 6: return sludge, 7: surplus sludge, 8, 8 ′: perforated pipe, 9, 9 ′: Blower, 10: High concentration oxygen gas, 11: Exhaust gas, 12, 12 ′: Nitrogen tank gas phase gas, 13, 13 ′: Dissolved oxygen concentration meter, 14: Oxygen concentration meter, 15, 15 ′: Gas circulation line, 16, 16 ': Control device, 17: Flow control valve, 18: Control device, 19: Stirrer, 20: Denitrification tank, 21: Stirrer, 24, 24': Screen, 25: Gas phase communication part

Claims (5)

  A high-concentration oxygen gas is supplied into the gas phase of a sealable nitrification tank filled with a carrier to which nitrifying bacteria are attached, exhaust gas is discharged from the gas phase of the nitrification tank, and the gas in the nitrification tank is discharged. In the wastewater treatment method, the gas in the phase is diffused into the liquid phase, and the ammonia nitrogen and organic nitrogen in the wastewater are biologically oxidized to nitrate nitrogen and nitrite nitrogen. Detecting the dissolved oxygen concentration in the liquid phase, and controlling the amount of air diffused in the liquid phase so that the dissolved oxygen concentration is maintained at a set value based on the detection result, and the nitrification tank An oxygen gas supply for supplying gas to the gas phase part of the nitrification tank so that the oxygen concentration of the gas in the gas phase part or the exhaust gas is measured and the oxygen concentration is maintained within a predetermined range based on the measurement result Controlling the amount and maintaining a predetermined range of the maintained oxygen concentration, Waste water treatment method which holds the gas or the oxygen concentration of the exhaust gas in the vapor phase in the reduction vessel above 40%, oxygen utilization efficiency is characterized in that set to be lower than 85%.   A high-concentration oxygen gas is supplied into the gas phase of a sealable nitrification tank filled with a carrier to which nitrifying bacteria are attached, and exhaust gas is exhausted from the gas phase of the nitrification tank. The gas inside is diffused into the liquid phase, and the liquid phase and / or sludge of the nitrification tank is returned to the denitrification tank provided in the preceding stage of the nitrification tank, so that ammonia nitrogen and organic nitrogen in the waste water In a wastewater treatment method of biologically oxidizing to nitrate nitrogen and nitrite nitrogen and denitrifying, the dissolved oxygen concentration in the liquid phase in the nitrification tank is detected, and based on the detection result Control the amount of diffused air to be diffused into the liquid phase so that the dissolved oxygen concentration is maintained at a set value, and measure the oxygen concentration of the gas phase portion or the exhaust gas in the nitrification tank, Based on the measurement result, the nitrification tank is maintained so that the oxygen concentration is maintained within a predetermined range. The oxygen gas supply amount supplied to the phase part is controlled, and the oxygen concentration to be maintained is maintained within a predetermined range of oxygen concentration of the gas phase part in the nitrification tank or the exhaust gas at 40% or more. The wastewater treatment method is characterized in that the oxygen utilization efficiency is set to be lower than 85%.   The nitrification tank is composed of a plurality of tanks partitioned by a partition wall, and for each tank of the plurality of tanks, gas in the gas phase is diffused into the liquid phase, and in the liquid phase in the plurality of nitrification tanks The dissolved oxygen concentration of each tank is detected for each tank, and based on the detection result, the amount of diffused air to be diffused in the liquid phase of each tank is controlled so that the dissolved oxygen concentration is maintained at a set value. The waste water treatment method according to claim 1 or 2.   2. The nitrification tank comprises a plurality of tanks partitioned by a partition wall, and the gas phase part in which the oxygen concentration is maintained within a predetermined range is a gas phase part of a nitrification tank of a final tank. The waste water treatment method according to 2 or 3.   The predetermined range of the gas in the gas phase in the nitrification tank or the oxygen concentration of the exhaust gas may be changed in accordance with the fluctuation of the amount of pollutants that need to be oxidized in the inflow wastewater flowing into the nitrification tank. The wastewater treatment method according to any one of claims 1 to 4, wherein