Water treatment apparatus and water treatment method

Abstract

The present invention provides a water treatment device that can stably treat COD components while suppressing the generation of nitrite to a low level. [Solution] A water treatment apparatus comprising a biological treatment tank containing activated sludge containing bacteria capable of decomposing COD components, and into which water to be treated containing ammonia and thiocyanate ions is introduced. This water treatment apparatus comprises an aeration device that performs aeration in the biological treatment tank, and a measuring unit that measures at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by treating the water to be treated in the biological treatment tank. Furthermore, this water treatment apparatus comprises a control unit that adjusts the aeration airflow rate based on the difference between the ammonium concentration in the water to be treated and the ammonium concentration in the treated water measured by the measuring unit, and at least one index value selected from the group consisting of nitrite concentration, nitrate concentration, and the sum of nitrite concentration and nitrate concentration in the treated water measured by the measuring unit.

Description

[Technical Field]

[0001] The present invention relates to a water treatment apparatus and a water treatment method. [Background technology]

[0002] Coke, used in steelmaking to reduce iron oxide in iron ore, is produced by carbonizing coal in a coke oven. The gas generated during coal carbonization (coke oven gas) is purified in various equipment to remove impurities and recover components and heat for reuse. One example is the process of flushing the coke oven gas by spraying it with ammonia water to cool it and collect impurities. The condensate generated in this process (also called "ammonia water" or "coke oven wastewater") contains COD (chemical oxygen demand) components and high concentrations of ammonia.

[0003] In coke oven wastewater (ammonia water) treatment facilities, coke oven wastewater containing COD components is introduced into a biological treatment tank (also called an activated sludge tank or aeration tank) containing activated sludge, and the COD components in the coke oven wastewater are decomposed and reduced by the activated sludge method. For example, Patent Document 1 discloses a method for treating coke plant wastewater, characterized by first removing ammonia from the coke plant wastewater, then subjecting the treated liquid to coagulation and sedimentation treatment by adding ferrous salts, and finally treating it with activated sludge. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2000-84589 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] As mentioned above, coke oven wastewater treatment facilities remove COD components from coke oven wastewater through biological treatment using activated sludge. The main COD components include thiocyanate ions. After biological treatment of coke oven wastewater with activated sludge, treated water is generally obtained by separating the activated sludge using solid-liquid separation equipment such as sedimentation tanks or membrane separators. Since activated sludge mainly consists of aerobic microorganisms, the COD treatment performance of the biological treatment tank tends to improve when the oxygen level increases, and conversely, the COD treatment performance of the biological treatment tank tends to decrease when the oxygen level is insufficient.

[0006] On the other hand, when treating coke oven wastewater biologically with activated sludge, an increase in oxygen levels in the biological treatment tank can lead to a reaction in which ammonia is oxidized to nitrite ions (hereinafter sometimes simply referred to as "nitrite") by ammonia-oxidizing bacteria among the nitrifying bacteria in the activated sludge. Nitrite is measured as a COD component. Therefore, if the concentration of nitrite in the treated water increases, the COD in the treated water will be higher than when nitrite is not present. In addition, nitrite may have an effect that reduces the treatment performance of bacteria capable of decomposing COD components in wastewater.

[0007] Furthermore, when activated sludge is present and oxygen is deficient, nitrite can be reduced to produce nitrogen gas (denitrification), and this nitrogen gas can potentially cause sludge to float in solid-liquid separation equipment. One problem associated with this sludge floating is the outflow of floating sludge into treated water if the solid-liquid separation equipment is a sedimentation tank. For these reasons, it is desirable to minimize the generation of nitrite in biological treatment tanks as much as possible.

[0008] Therefore, the present invention aims to provide a water treatment apparatus and a water treatment method that can stably treat COD components, including thiocyanate ions and ammonia, in treated water by the activated sludge method while suppressing the generation of nitrite to a low level. [Means for solving the problem]

[0009] In other words, the present invention provides a water treatment apparatus comprising: a biological treatment tank containing activated sludge containing bacteria capable of decomposing COD components, into which water to be treated, containing ammonia and at least thiocyanate ions as the COD component, is introduced; an aeration device that performs aeration in the biological treatment tank to aerobically decompose the COD components in the water to be treated by the bacteria; a measuring unit that measures at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by treating the water to be treated in the biological treatment tank; and a control unit that adjusts the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the water to be treated and the ammonium concentration in the treated water measured by the measuring unit, and at least one index value selected from the group consisting of nitrite concentration, nitrate concentration, and the sum of nitrite concentration and nitrate concentration in the treated water measured by the measuring unit.

[0010] Furthermore, the present invention provides a water treatment method comprising: introducing water to be treated containing ammonia and at least thiocyanate ions as the COD component into a biological treatment tank containing activated sludge containing bacteria capable of decomposing COD components, and biologically treating the water to be treated with the activated sludge; aerating the biological treatment tank to aerobically decompose the COD components in the water to be treated by the bacteria; measuring at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by the biological treatment of the water to be treated; and adjusting the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the water to be treated and the ammonium concentration in the treated water measured as the concentration value, and at least one index value selected from the group consisting of nitrite concentration, nitrate concentration, and the sum of nitrite concentration and nitrate concentration in the treated water measured as the concentration value. [Effects of the Invention]

[0011] According to the present invention, when treating water to be treated containing a COD component containing thiocyanate ions and ammonia by the activated sludge method, it is possible to provide a water treatment apparatus and a water treatment method capable of stably treating the COD component while suppressing the generation of nitrous acid at a low level.

Brief Description of the Drawings

[0012] [Figure 1] It is a schematic block diagram showing the configuration of a water treatment apparatus according to an embodiment of the present invention. [Figure 2] It is a schematic block diagram showing the configuration of a water treatment apparatus according to another embodiment of the present invention. [Figure 3] It is a graph showing the results of Comparative Example 1 and Example 1. [Figure 4] It is a graph showing the results of Comparative Example 2 and Example 2. [Figure 5] It is a graph showing the results of Comparative Example 3 and Example 3.

Embodiments for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0014] The water to be treated containing a COD component is introduced into a biological treatment tank containing activated sludge containing bacteria capable of decomposing the COD component, and the COD component in the water to be treated is decomposed by the activated sludge method, thereby biologically treating the water to be treated. Further, after biologically treating the water to be treated containing a COD component with activated sludge, treated water is obtained by separating the activated sludge by solid-liquid separation equipment such as a sedimentation tank or a membrane separation device.

[0015] Since activated sludge mainly consists of aerobic microorganisms, aeration is carried out in the biological treatment tank to aerobically decompose the COD components in the treated water by bacteria. In the biological treatment tank containing activated sludge, when treating the treated water containing COD components, when the amount of oxygen increases due to aeration, the COD treatment performance of the biological treatment tank is likely to increase. Conversely, when oxygen is insufficient, the COD treatment performance of the biological treatment tank is likely to decrease.

[0016] When the treated water containing COD components contains thiocyanate ions (SCN - ), from the perspective of being able to effectively treat (decompose) thiocyanate ions, it can be said that it is preferable that a sufficient amount of oxygen exists due to aeration in the biological treatment tank. However, when the treated water containing thiocyanate ions further contains ammonia, in the biological treatment tank, when the amount of oxygen increases, ammonia may be oxidized to nitrite (NO2 - ) by ammonia-oxidizing bacteria among the nitrifying bacteria in the activated sludge. Since nitrite is measured as a COD component, when the concentration of nitrite in the treated water increases, the COD concentration in the treated water becomes higher than when nitrite does not occur. In addition, nitrite may have an impact on reducing the treatment performance of bacteria capable of decomposing the COD components in the treated water.

[0017] Furthermore, when nitrite is generated in the biological treatment tank, in order to suppress the further generation of nitrite, aeration is suppressed to reduce the amount of oxygen, and when the environment becomes oxygen-deficient, nitrite may be reduced and a reaction (denitrification reaction) in which nitrogen gas is generated may occur. The generated nitrogen gas may cause the sludge to float in the solid-liquid separation equipment. In the case of a sedimentation tank, which is often used as the solid-liquid separation equipment, the floating sludge may flow out into the treated water. From such points, it is desirable to suppress the generation of nitrite in the biological treatment tank as much as possible.

[0018] The ammonia-oxidizing bacteria (also called nitrite-producing bacteria) that oxidize ammonia to nitrite are considered to be relatively vulnerable to competition from other bacteria in activated sludge. Therefore, it is thought that when there is surplus COD treatment capacity in the biological treatment tank, that is, when there is excess dissolved oxygen, the ammonia-oxidizing bacteria can use the oxygen and produce nitrite.

[0019] As mentioned above, nitrite is likely to be produced when the biological treatment tank is over-aerationd, but if the aeration airflow (amount of air supplied to the biological treatment tank) is insufficient, the COD treatment capacity of the biological treatment tank decreases. Therefore, the inventors considered it necessary to control the aeration airflow to the biological treatment tank within an appropriate range. To determine the appropriate range for the aeration airflow to the biological treatment tank, dissolved oxygen (DO) and oxidation-reduction potential (ORP) were considered as indicators. However, from the viewpoint of suppressing nitrite production, it is desirable for DO to be almost zero, but if DO is zero, the COD treatment capacity of the biological treatment tank is thought to decrease. On the other hand, ORP is suitable for determining whether the biological treatment tank is in an oxidizing or reducing state, but since the relationship between ORP and the nitrite production rate is not constant, it is thought that controlling the aeration airflow based solely on ORP cannot sufficiently suppress nitrite production.

[0020] The inventors conducted preliminary experiments and found that nitrite generation begins only after the COD components in the treated water flowing into the biological treatment tank have been sufficiently removed. Based on this, they discovered a water treatment apparatus and water treatment method that can stably treat thiocyanate ions while suppressing nitrite generation to a low level by controlling the aeration airflow rate to maintain a low nitrite concentration in the treated water.

[0021] Specifically, the aeration airflow rate in the biological treatment tank is adjusted based on a specific index value obtained by measuring the treated water, using a specific concentration value in the treated water. The concentration value used is at least one selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained when the treated water is treated in the biological treatment tank. The index value used is at least one selected from the group consisting of the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water, as well as the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water. By adjusting the aeration airflow rate in the biological treatment tank based on the index value, it is possible to stably treat thiocyanate ions while keeping nitrite generation at a low level. Specifically, regarding the suppression of nitrite generation, it is possible to suppress the nitrite concentration in the treated water to 30 mg-N / L or less.

[0022] <Water treatment equipment> Hereinafter, an embodiment of the water treatment apparatus will be described with reference to the drawings. Figure 1 is a schematic block diagram showing the configuration of a water treatment apparatus 10 according to one embodiment of the present invention. Figure 2 is a schematic block diagram showing the configuration of a water treatment apparatus 10 according to another embodiment of the present invention. Components common to both Figure 1 and Figure 2 are denoted by the same reference numerals, and their descriptions may be omitted. In addition, the thick arrows in Figures 1 and 2 represent the flow of liquid (water to be treated, treated water, or activated sludge) in the treatment process of the water to be treated.

[0023] As shown in Figures 1 and 2, the water treatment device 10 comprises a biological treatment tank 11, an aeration device 31, a measuring unit 41, and a control unit 51. The biological treatment tank 11 contains activated sludge containing bacteria capable of decomposing COD components. The water to be treated, which contains COD components including at least thiocyanate ions and ammonia, is introduced into the biological treatment tank 11. In this biological treatment tank 11, it is possible to decompose the COD components in the water to be treated using bacteria capable of decomposing COD components.

[0024] Examples of biological treatment tanks 11 include: a tank containing activated sludge; a tank containing activated sludge supported on a carrier that flows within the tank (e.g., sponge or plastic) (fluidized bed carrier type biological treatment tank); a tank in which activated sludge is fixed to a carrier fixed within the tank (fixed carrier) (fixed bed carrier type biological treatment tank); a membrane type activated sludge tank equipped with a membrane separation device for separating activated sludge and treated water within a tank containing activated sludge; and so on. Furthermore, biological treatment with activated sludge may be carried out in combination of two or more of the above-mentioned biological treatment tanks containing activated sludge. For example, an activated sludge tank containing activated sludge may be carried out in combination with the above-mentioned fluidized bed carrier type biological treatment tank. Moreover, biological treatment with activated sludge may also be a multiphase activated sludge method (e.g., a two-phase activated sludge method) in which the water to be treated is treated in a first biological treatment tank inhabited by dispersed bacteria, and the liquid treated in the first biological treatment tank is treated in a biological treatment tank (second biological treatment tank) containing activated sludge.

[0025] The water treatment device 10 may include a water treatment tank 12 connected to the biological treatment tank 11 for introducing water to be treated into the biological treatment tank 11. The water treatment tank 12 is a tank for storing water to be treated. The water treatment tank 12 can be connected to the biological treatment tank 11 using a pump 61 and piping 71 so that water to be treated can flow continuously into the biological treatment tank 11.

[0026] Furthermore, although not shown in the figures, the water treatment device 10 may also include a dilution tank that stores dilution water (for example, tap water and seawater, etc.) for diluting the water to be treated introduced into the biological treatment tank 11, if necessary. This dilution water dilutes the water to be treated in the biological treatment tank 11, and the COD components, etc., in the water to be treated may be adjusted to a predetermined concentration. The dilution tank may be connected to the biological treatment tank 11 via a pump and piping, etc., so that the dilution water can flow into the biological treatment tank 11 continuously, or it may be connected to the water to be treated tank 12 or the piping to the water to be treated tank via a pump and piping, etc., so that the dilution water can flow into the water to be treated tank 12 continuously.

[0027] The water treatment device 10 may also be equipped with solid-liquid separation equipment. The water treatment device 10 shown in Figure 1 is equipped with a sedimentation tank 21 as solid-liquid separation equipment. The biological treatment tank 11 can be connected to the sedimentation tank 21 using a pump 62 and piping 72 so that the treated liquid obtained in the biological treatment tank 11 can flow into the sedimentation tank 21 continuously. After the water to be treated is biologically treated with activated sludge in the biological treatment tank 11, the treated liquid obtained in the biological treatment tank 11 (a mixture containing treated water and activated sludge) can be introduced into the sedimentation tank 21. In the sedimentation tank 21, the activated sludge in the treated liquid can be settled and separated into activated sludge as a precipitate and treated water as a supernatant. The treated water obtained in the sedimentation tank 21 may be drawn out by a pump 63 and piping 73 and sent to the treated water tank 13. In this case, some of the treated water is sent to the measurement unit 41, which will be described later, via a pump 64 and piping 74. On the other hand, the sediment, mainly activated sludge, obtained in the sedimentation tank 21 may be withdrawn by the pump 65 and piping 75 and returned to the biological treatment tank 11.

[0028] In the water treatment apparatus 10 shown in Figure 1, instead of the sedimentation tank 21, a filtration device (membrane separator) using a microfiltration membrane and an ultrafiltration membrane may be used as solid-liquid separation equipment. The treated liquid obtained in the biological treatment tank 11 can be separated into activated sludge and treated water by the filtration device. The treated water obtained in the filtration device is sent to the measurement unit 41 and the treated water tank 13, which will be described later. On the other hand, the activated sludge obtained in the filtration device may be returned to the biological treatment tank 11.

[0029] Furthermore, as shown in Figure 2, the water treatment device 10 may also be equipped with a membrane separator 22 as a solid-liquid separation device. The water treatment device 10 shown in Figure 2 is equipped with a membrane-type activated sludge tank 11 as a biological treatment tank 11, and the membrane separator 22 is installed inside this biological treatment tank 11. A pump 66 and piping 76 can be connected to the membrane separator 22. The treated liquid (a mixture containing treated water and activated sludge) obtained in the biological treatment tank 11 can be drawn out by the pump 66 and piping 76 connected to the membrane separator 22, thereby separating the activated sludge captured by the separation membrane of the membrane separator 22 from the treated water that passes through the separation membrane. A portion of the treated water obtained in the membrane separator 22 is sent to the measurement unit 41, which will be described later, via the pump 64 and piping 74. Another portion of the treated water may be sent to the treated water tank 13. As for the separation membrane provided in the membrane separator 22, a microfiltration membrane or an ultrafiltration membrane can be used, similar to the filtration device described above.

[0030] The aeration device 31 is a device for performing aeration in the biological treatment tank 11 to aerate the COD components in the water to be treated by bacteria capable of decomposing the COD components in the water to be treated. The aeration device 31 is not particularly limited as long as it is capable of performing aeration in the biological treatment tank 11 and supplying oxygen-containing air. As shown in Figures 1 and 2, the aeration device 31 may include, for example, an aeration blower 32 for sending air into the biological treatment tank 11, and an aeration unit 33 connected to the aeration blower 32 and located in the biological treatment tank 11 from which air bubbles are blown out. Examples of the form of the aeration unit 33 include an aeration plate, an aeration cylinder, and an aeration ball. The aeration device 31 is provided to cooperate with the control unit 51, which will be described later. The aeration device 31 may also be provided with an airflow meter and an airflow adjustment valve (not shown), and the airflow meter and airflow adjustment valve may be provided to be controllable by the control unit 51.

[0031] The measuring unit 41 measures at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by treating the water to be treated in the biological treatment tank 11.

[0032] In this disclosure, ammonium concentration in water means ammoniacal nitrogen concentration in water. Ammoniacal nitrogen (NH3) + -N, NH4 + -N) is ammonia, ammonium salts, or ammonium (NH4) among various nitrogen compounds. + This refers to nitrogen that exists as ammonia. The ammonia nitrogen concentration can be measured using, for example, the method specified in JIS K0102:2019, section 42, or the ion electrode method.

[0033] Furthermore, in this disclosure, nitrite concentration in water means nitrite nitrogen concentration in water, and nitrate concentration in water means nitrate nitrogen concentration in water. Nitrite nitrogen (NO2-N) is nitrite contained in water (usually nitrite ions (NO2) in water). - It exists as nitrates (NO3-N). It refers to nitrogen in water. Nitrate nitrogen (NO3-N) is nitrates contained in water (usually, nitrate ions (NO3) exist in water). - It exists as ). It refers to nitrogen in ). The nitrite nitrogen concentration and nitrate nitrogen concentration can be measured using values ​​obtained by the method specified in JIS K0102:2019, section 43, for example. Alternatively, the nitrite nitrogen concentration and nitrate nitrogen concentration can be measured using values ​​obtained by measuring, for example, the absorption of light in the ultraviolet-visible region, or values ​​obtained by measuring the amount of nitrite (HNO2) produced when nitrite ions are protonated by an acid. Among these, a method for measuring nitrite nitrogen in measurements based on the amount of nitrite (HNO2) can be described as measuring based on the change in the pH of the electrolyte that occurs when nitrite that has passed through a diaphragm capable of allowing nitrite (HNO2) to pass through is absorbed into the electrolyte.

[0034] In this disclosure, the treated water to which the concentration value is measured can be at least one type of treated water selected from the group consisting of: treated water in a biological treatment tank; treated water discharged from a biological treatment tank; and treated water obtained by separating activated sludge from the treated liquid obtained in a biological treatment tank using a solid-liquid separation device. As the "treated water discharged from a biological treatment tank" mentioned above, for example, the treated water in the piping 76 discharged from the biological treatment tank 11 (see Figure 2) can be used. As the "treated water obtained by separating activated sludge from the treated liquid obtained in a biological treatment tank using a solid-liquid separation device" mentioned above, at least one of the treated water obtained by separating activated sludge from the liquid discharged from the biological treatment tank 11 using a sedimentation tank 21 (see Figure 1) and the treated water obtained by separating activated sludge from the biological treatment tank 11 using a membrane separator 22 (see Figure 2) can be used.

[0035] The measurement unit 41 can be configured, for example, with a sample water tank 42 into which a portion of the treated water flows for sampling and is stored as sample water, and a concentration meter capable of measuring the concentration value in the sample water (treated water) in the sample water tank 42. The concentration meter can be configured, for example, with a probe 43 that detects a signal to be measured by immersing it in the sample water (treated water) in the sample water tank 42, and a measuring instrument 44 to which the signal from the probe 43 is transmitted. Depending on the type of concentration value to be measured, two or three probes 43 may be provided. As the concentration meter, for example, a sensor, a photometer (absorbance photometry), and an ion meter can be used. It is preferable that the measurement of the concentration value by the measurement unit 41 is performed continuously. Also, depending on the type of concentration value to be measured and the measurement method, the pH of the sample water may be adjusted with a pH adjusting solution, and the pH adjusting solution may be supplied to the sample water tank 42. When the sample water (treated water) in the sample water tank 42 reaches a predetermined volume, it may be discharged or sent to the treated water tank 13.

[0036] It is preferable, from the viewpoint of simplicity and effectiveness, to use the nitrite concentration in the treated water as an indicator value used for adjusting the aeration airflow rate by the control unit 51 described later. For this reason, it is preferable to measure at least the nitrite concentration in the treated water using the measuring unit. Therefore, it is preferable that the concentration value includes at least the nitrite concentration in the treated water, and it is preferable that the water treatment device 10 is equipped with a measuring unit 41 for measuring the nitrite concentration in the treated water. It is even more preferable that the measuring unit 41 is equipped with a sample water tank 42 into which the treated water flows and is stored as sample water, and a nitrite concentration meter capable of measuring the nitrite concentration in the sample water (treated water). It is even more preferable that the nitrite concentration meter is equipped with a nitrite measuring probe 43 and a nitrite measuring device 44 to which the signal from the nitrite measuring probe 43 is transmitted.

[0037] The control unit 51 adjusts the aeration airflow rate of the aeration device 31 based on an index value using the concentration value measured by the measurement unit 41. Specifically, the control unit 51 adjusts the aeration airflow rate based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured by the measurement unit 41, and at least one index value selected from the group consisting of nitrite concentration, nitrate concentration, and the sum of nitrite and nitrate concentrations in the treated water measured by the measurement unit 41.

[0038] As mentioned above, nitrite is thought to be produced when ammonia in the treated water is oxidized by oxygen present in the treated water and ammonia-oxidizing bacteria (nitrite-producing bacteria) in the activated sludge. Therefore, the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water can serve as an indicator of nitrite production. Specifically, the smaller the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water, the less nitrite is likely to be produced. Conversely, the larger the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water, the more nitrite is likely to be produced. Thus, the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water can be used as an indicator value to adjust the aeration airflow rate in order to suppress nitrite production to a low level.

[0039] When using the difference between the ammonium concentration in the water to be treated and the ammonium concentration in the treated water as an indicator value for adjusting the aeration airflow rate, the water treatment device 10 can be equipped with an ammonium concentration meter for measuring the ammonium concentration in the water to be treated. The ammonium concentration meter for measuring the ammonium concentration in the water to be treated is preferably installed before the biological treatment tank 11, since it measures the water to be treated before it flows into the biological treatment tank 11, and more preferably installed in the water to be treated tank 12 or in the piping 71 connecting the water to be treated tank 12 and the biological treatment tank 11. Furthermore, it is preferable that the ammonium concentration meter for measuring the ammonium concentration in the water to be treated is equipped to transmit a signal regarding the measured ammonium concentration in the water to be treated to a control unit 51, which will be described later.

[0040] As mentioned above, ammonia is thought to be nitrified to nitrite by ammonia-oxidizing bacteria contained in activated sludge. However, when the ammonia concentration in the treated water is high, nitrite is not nitrified to nitrate by nitrite-oxidizing bacteria, and the reaction tends to almost stop at nitrite. The reason for this is not entirely clear, but one possible reason is that nitrite-oxidizing bacteria have weaker tolerance to ammonia than ammonia-oxidizing bacteria. However, when nitrification from nitrite to nitrate occurs, the nitrate concentration in the treated water can serve as an indicator of nitrite production. Specifically, it is thought that the higher the nitrate concentration in the treated water, the greater the amount of nitrite produced, and the lower the nitrate concentration in the treated water, the less nitrite produced. Therefore, the nitrate concentration in the treated water, or the sum of the nitrite and nitrate concentrations in the treated water, can be used as indicator values ​​to adjust the aeration airflow rate to suppress nitrite production to a low level.

[0041] As mentioned above, it is preferable to use the nitrite concentration in the treated water as an indicator value for adjusting the aeration airflow rate, as this is simple and effective. Therefore, it is preferable that the indicator value includes at least the nitrite concentration in the treated water, and the water treatment device 10 is preferably equipped with a control unit 51 that adjusts the aeration airflow rate based on the nitrite concentration in the treated water measured by the measuring unit 41 as the indicator value.

[0042] The control unit 51 can adjust the aeration airflow rate based on an index value as follows. First, a signal relating to the concentration value measured by the aforementioned measuring unit 41 is transmitted from the measuring unit 41 (preferably the measuring instrument 44) to the control unit 51 via the input line 81. The control unit 51 has a calculation function and is configured to calculate an index value by performing predetermined calculations as needed from the received signal relating to the concentration value. The control unit 51 is also configured to calculate a signal for adjusting the aeration airflow rate using the index value and transmit that signal to the aeration device 31 (preferably the aeration blower 32) via the control line 82. In this way, the control unit 51 can adjust the aeration airflow rate of the aeration device 31 based on the index value. Examples of signals for adjusting the aeration airflow rate include the output of the aeration airflow rate from the aeration device 31 and the rotation speed of the aeration blower 32 of the aeration device 31.

[0043] As a method for adjusting the aeration airflow rate based on the indicator value, it is preferable that the control unit 51 adjusts the aeration airflow rate so that the above-mentioned indicator value approaches a preset target value. The target value is preferably set through preliminary experiments depending on the water to be treated and the indicator value used. For example, if the concentration value and the indicator value include at least the nitrite concentration in the treated water, the target value for the nitrite concentration in the treated water is preferably 30 mg-N / L or less (0 to 30 mg-N / L), more preferably 20 mg-N / L or less (0 to 20 mg-N / L), and even more preferably 10 mg-N / L or less (0 to 10 mg-N / L).

[0044] For example, when the nitrite concentration in the treated water measured by the measurement unit 41 is within a predetermined target range, the control unit 51 can be configured to send a signal to the aeration device 31 via the control line 82 that causes the output of the aeration airflow to be set to a relatively high value or to remain at the current level. This allows the aeration airflow from the aeration device 31 to be increased or maintained at the current level. As a result, bacteria capable of decomposing COD components are activated in the biological treatment tank, and their ability to decompose COD components such as thiocyanate ions is enhanced. On the other hand, when the nitrite concentration in the treated water measured by the measurement unit 41 is higher than a predetermined target value, the control unit 51 can be configured to send a signal to the aeration device 31 via the control line 82 that causes the output of the aeration airflow to be reduced. This allows the aeration airflow from the aeration device 31 to be reduced. As a result, excessive aeration is suppressed in the biological treatment tank 11, which suppresses the reaction in which ammonia in the treated water is oxidized to nitrite by ammonia-oxidizing bacteria in the activated sludge. In this way, the control unit 51 adjusts the aeration airflow rate based on the indicator value, making it possible to stably process thiocyanate ions while suppressing the generation of nitrite to a low level.

[0045] When measuring the ammonium concentration in the treated water as the concentration value and using the difference between the ammonium concentration in the water being treated and the ammonium concentration in the treated water as the index value, the target value of this difference is preferably 30 mg-N / L or less (0 to 30 mg-N / L), more preferably 20 mg-N / L or less (0 to 20 mg-N / L), and even more preferably 10 mg-N / L or less (0 to 10 mg-N / L). Furthermore, when measuring the nitrate concentration in the treated water as the concentration value and using the nitrate concentration in the treated water as the index value, the target value of the nitrate concentration in the treated water is preferably 30 mg-N / L or less (0 to 30 mg-N / L), more preferably 20 mg-N / L or less (0 to 20 mg-N / L), and even more preferably 10 mg-N / L or less (0 to 10 mg-N / L). Furthermore, when measuring the nitrite and nitrate concentrations in the treated water as concentration values ​​and using the sum of the nitrite and nitrate concentrations in the treated water as an indicator value, the target value for the sum of the nitrite and nitrate concentrations in the treated water is preferably 30 mg-N / L or less (0 to 30 mg-N / L), more preferably 20 mg-N / L or less (0 to 20 mg-N / L), and even more preferably 10 mg-N / L or less (0 to 10 mg-N / L).

[0046] The control unit 51 can adjust the aeration airflow rate based on an index value by changing the output of the aeration airflow rate from the aeration device 31 using methods such as proportional control (P control), proportional-integral control (PI control), or proportional-integral-derivative control (PID control). This allows for automatic control of the aeration airflow rate based on the index value, enabling continuous aeration under control based on the index value.

[0047] As one aspect, it is more preferable that the control unit 51 adjusts the aeration air volume by proportional control represented by the following formula (1). b (output when the deviation e is 0) in the following formula (1) can be arbitrarily set according to the capacity of the aeration device and the like. For example, b can be set to a numerical value within the range of preferably 30 to 70%, more preferably 40 to 60%, and can usually be set to 50%. By performing proportional control on the aeration air volume, the index value can be made closer to the target value, and the nitrite concentration in the treated water can be maintained at a lower level. When using the nitrite concentration in the treated water as the index value, the following formula (1) becomes a formula using the nitrite concentration in the treated water measured by the aforementioned measurement unit 41, and e(t) in the formula can be expressed as the deviation = nitrite concentration in the treated water - target value of the nitrite concentration.

[0048] TIFF2026104130000001.tif21170u(t): Output (%) of the aeration air volume K P : Proportional gain e(t): Deviation (= index value - target value) b: Output when the deviation e is 0

[0049] Also, as one aspect, it is even more preferable that the control unit 51 adjusts the aeration air volume by proportional-integral control represented by the following formula (2). By performing proportional-integral control on the aeration air volume, the index value can be stably made closer to the target value, and the nitrite concentration in the treated water can be maintained at an even lower level. When using the nitrite concentration in the treated water as the index value, the following formula (2) becomes a formula using the nitrite concentration in the treated water measured by the aforementioned measurement unit 41, and e(t) in the formula can be expressed as the deviation = nitrite concentration in the treated water - target value of the nitrite concentration.

[0050] TIFF2026104130000002.tif2117o u(t): Output (%) of the aeration air volume K P [ : Proportional gain K I : Integral gain e(t): Deviation (= index value - target value)

[0051] The proportional gain K in equations (1) and (2) above P or the integral gain K in equation (2) above I This can be determined through preliminary experiments, depending on the degree of nitrite production, the degree of change in aeration airflow output, etc.

[0052] The control unit 51 can be a personal computer (PC) or a digital indicating controller, and preferably a digital indicating controller. The control unit 51 is equipped with a central processing unit (CPU), and preferably a microcomputer unit (MCU) or microprocessor unit (MPU) equipped with a CPU. The control unit may also be equipped with other components, such as a power supply unit, an input unit, an output unit, and a storage unit. For example, the concentration values ​​measured by the aforementioned measuring unit 41 and the ammonium concentration in the water to be treated can be stored in the storage unit of the control unit 51, and the MCU or MPU can use these to perform calculations and control the output of the aeration airflow.

[0053] As described above, in the water treatment apparatus 10 of this embodiment, the aeration airflow rate in the biological treatment tank 11 is adjusted by the control unit 51, so that aeration is performed to the extent that denitrification reactions hardly occur, and biological treatment is carried out under aerobic conditions of a certain level or higher. From this point of view, the water treatment apparatus 10 of this embodiment can be operated with a ratio of the total nitrogen concentration in the treated water to the total nitrogen concentration in the treated water (total nitrogen concentration in the treated water / total nitrogen concentration in the treated water) of 0.6 or higher. Generally, when the purpose is to treat nitrogen in the water to be treated, a treatment apparatus is used that uses both a nitrification reaction, which oxidizes ammonia in the water to nitrate via nitrite, and a denitrification reaction, which reduces nitrite and nitrate to nitrogen gas. In this treatment apparatus, the above ratio (total nitrogen concentration in the treated water / total nitrogen concentration in the treated water) is operated to be close to 0.

[0054] <Water Treatment Methods> Next, a water treatment method according to one embodiment of the present invention (hereinafter sometimes simply referred to as "this method") will be described. In this method, a biological treatment tank containing activated sludge with bacteria capable of decomposing COD components is used to treat water containing COD components including thiocyanate ions and ammonia. In this treatment method, it is preferable to use the water treatment apparatus 10 according to the present invention described above. Below, a water treatment method using the water treatment apparatus 10 will be described with examples.

[0055] First, an activated sludge containing bacteria capable of decomposing COD components, a biological treatment tank capable of containing it, and the water to be treated containing COD components including thiocyanate ions and ammonia are prepared.

[0056] The water to be treated is not particularly limited, as long as it contains COD components including at least thiocyanate ions and ammonia. Examples of such water to be treated include coke oven wastewater (ammonia water) generated during the carbonization of coal in a coke oven. Coke oven wastewater includes condensate generated when exhaust gas discharged during the production of coke from coal is cooled, and may also be scrubber wastewater after treatment with a scrubber or the like. Furthermore, these may be appropriately diluted with industrial water, river water, seawater, etc., until they reach a concentration suitable for biological treatment. In this disclosure, coke oven wastewater (ammonia water) includes the above-mentioned scrubber wastewater and diluted materials.

[0057] Since coke oven wastewater (ammonia) is preferred as the water to be treated, the water to be treated is preferably one that contains phenol and other substances in addition to thiocyanate ions as COD components. Furthermore, the water to be treated may be pretreated before flowing into the biological treatment tank. For example, the water to be treated may contain cyanide ions (CN) in addition to thiocyanate ions. - If it contains ), the water to be treated may be the water after it has been treated to reduce the amount of cyanide ions in the water to be treated.

[0058] Thiocyanate ions in the treated water usually exist in ionic form, but the treated water may also contain thiocyanic acid (HSCN) before ionization, or thiocyanate salts. Examples of thiocyanate salts include sodium thiocyanate, ammonium thiocyanate, calcium thiocyanate, and potassium thiocyanate.

[0059] In one embodiment of this method, the ammonium concentration (ammonia nitrogen concentration) of the water to be treated is preferably 30 to 2000 mg-N / L, more preferably 100 to 1500 mg-N / L, and even more preferably 200 to 800 mg-N / L. Also, thiocyanate ions (SCN) in the water to be treated are also preferred. - The concentration is preferably 20 to 1500 mg-SCN / L, and more preferably 50 to 400 mg-SCN / L. The thiocyanate ion concentration in water can be measured, for example, by a colorimetric method using iron nitrate or by ion chromatography. Furthermore, the COD concentration in the treated water (COD Mn The COD in water is preferably 100-6000 mg / L, and more preferably 300-1500 mg / L. Mn This can be measured, for example, by the method specified in JIS K0102:2019, item 17.

[0060] Furthermore, in one embodiment of this method, the pH of the water to be treated is preferably 4 to 10, and more preferably 4.5 to 9.5. Similarly, the temperature of the water to be treated is preferably 10 to 70°C, and more preferably 15 to 45°C.

[0061] The activated sludge contains bacteria capable of decomposing COD components. Examples of bacteria capable of decomposing COD components include thiocyanide-degrading bacteria and phenol-degrading bacteria. The activated sludge is cultured in the biological treatment tank 11. When the water to be treated is introduced into the biological treatment tank 11 containing the activated sludge and aerated, the activated sludge dispersed in the water to be treated can proliferate while decomposing COD components. It is preferable to use activated sludge from a water treatment facility that has biologically treated wastewater containing COD components, and it is even more preferable to use activated sludge from a coke oven wastewater treatment facility that treats coke oven wastewater containing COD components.

[0062] Next, activated sludge containing bacteria capable of decomposing COD components is placed in a biological treatment tank. Then, treated water containing COD components including thiocyanate ions and ammonia is introduced into the biological treatment tank 11 containing the activated sludge via a pump 61 and piping 71 and stored therein. This mixes the activated sludge and the treated water in the biological treatment tank 11. When introducing the treated water into the biological treatment tank 11, the treated water obtained by diluting the raw water (wastewater) with dilution water (for example, tap water and seawater) may be introduced into the biological treatment tank 11, or the dilution water may be introduced into the biological treatment tank 11 together with the raw water. The concentration of COD components, etc. in the treated water may be adjusted by using dilution water in these manners. It is preferable to continuously introduce the treated water into the biological treatment tank 11.

[0063] This method includes biological treatment of the water to be treated with activated sludge in a biological treatment tank 11. During this process, aeration is performed in the biological treatment tank 11 to aerobically decompose the COD components in the water to be treated by bacteria. This makes it possible to decompose thiocyanate ions in the water to be treated in the biological treatment tank 11 and reduce the COD in the water. The aforementioned aeration device 31 can be used for aeration.

[0064] The treatment time (reaction time), i.e., the hydrological residence time (HRT) in the biological treatment tank 11, is preferably 1 to 60 hours, more preferably 3 to 50 hours, and even more preferably 6 to 40 hours. The pH inside the biological treatment tank 11 is preferably 4.0 to 10.0, and more preferably 4.5 to 9.5. The water temperature inside the biological treatment tank 11 is preferably 10 to 45°C, and more preferably 15 to 40°C.

[0065] This method includes adjusting the aeration airflow rate of the above-mentioned aeration based on a specific indicator value. To use as an indicator value for adjusting the aeration airflow rate, this method measures the following concentration values. That is, this method includes measuring at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by biological treatment of the water to be treated. Various concentration values ​​can be measured by the method described in the description of the measuring unit 41 in the water treatment apparatus 10 above. In this method as well, it is preferable to measure at least the nitrite concentration in the treated water, and it is preferable that the concentration value includes at least the nitrite concentration in the treated water.

[0066] Furthermore, this method includes adjusting the aeration airflow rate based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured as a concentration value, and at least one indicator value selected from the group consisting of the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water measured as concentration values. In this method as well, it is preferable to use at least the nitrite concentration in the treated water as an indicator value, and it is preferable to adjust the aeration airflow rate based on the nitrite concentration in the treated water.

[0067] In this method as well, the aeration airflow rate can be adjusted by the method described in the explanation of the control unit 51 in the water treatment device 10 above. That is, as a method for adjusting the aeration airflow rate based on an indicator value, it is preferable to adjust the aeration airflow rate so that the indicator value approaches a preset target value. In this case, it is more preferable that the aeration airflow rate is adjusted by proportional control represented by the above-mentioned equation (1), and even more preferable that it is adjusted by proportional-integral control represented by the above-mentioned equation (2). As the target value used in equations (1) and (2), for example, when a preferred nitrite concentration is used as the indicator value, it is preferable to set the target value of the nitrite concentration in the treated water to 30 mg-N / L or less. The target value of the nitrite concentration in the treated water is more preferably 20 mg-N / L or less, and even more preferably 10 mg-N / L or less.

[0068] In one embodiment of this method, it is preferable to further include a step (solid-liquid separation step) in which, after biologically treating the water to be treated with activated sludge, the treated liquid (a mixture containing treated water and activated sludge) obtained in the biological treatment tank 11 is separated into solid and liquid components using a solid-liquid separation device. As the solid-liquid separation device, the aforementioned sedimentation tank 21 or membrane separation device 22 can be used. Furthermore, as shown in Figure 1, when using a solid-liquid separation device (sedimentation tank 21 or membrane separation device not shown) installed downstream of the biological treatment tank 11, it is preferable to return some or all of the activated sludge separated from the treated water in the solid-liquid separation step to the biological treatment tank 11 as return sludge. This allows for a continuous activated sludge biological treatment step and makes it easier to maintain the concentration of MLSS (activated sludge suspended solids) in the biological treatment tank 11 within a certain range. The concentration of MLSS in the biological treatment tank 11 is preferably 500 to 10000 mg / L. It is also preferable to use a membrane-type activated sludge tank as the biological treatment tank 11 and perform solid-liquid separation treatment within that tank.

[0069] In one embodiment of this method, the sludge retention time (SRT) is preferably 5 to 1000 days, and more preferably 20 to 600 days.

[0070] As described above, in this method, the aeration volume of the aeration performed in the biological treatment tank 11 is adjusted based on a specific indicator value, so that aeration is performed to the extent that denitrification reaction hardly occurs, and biological treatment is carried out under aerobic conditions of a certain level or higher. From this point of view, this method can perform treatment when the ratio of the total nitrogen concentration in the treated water to the total nitrogen concentration in the treated water (total nitrogen concentration in the treated water / total nitrogen concentration in the treated water) is 0.6 or higher. Generally, in treatment methods that use a nitrification process to oxidize ammonia in the treated water to nitric acid via nitrite and a denitrification process to reduce nitrite and nitric acid to nitrogen gas, which is intended to treat nitrogen in the treated water, the treatment is carried out so that the above ratio (total nitrogen concentration in the treated water / total nitrogen concentration in the treated water) is close to 0.

[0071] This method aims to decompose and remove COD components in the treated water, and therefore also aims to suppress the production of nitrite, a type of COD component. For this reason, unlike the above-mentioned methods which include nitrification and denitrification steps for nitrogen treatment of the treated water, this method does not require a denitrification step. In other words, this method does not require a denitrification step, which involves the biological treatment of the treated water with activated sludge followed by a denitrification reaction by anaerobic autotrophic bacteria, specifically Anammox bacteria (a reaction in which nitrite and nitrate are reduced, producing nitrogen gas). It should be noted that Anammox bacteria require anaerobic conditions both temporally and spatially in relation to other bacteria for growth. In this method, the aeration airflow is adjusted based on specific indicator values ​​in the biological treatment tank, making it difficult to achieve the above-mentioned anaerobic conditions. Therefore, it is considered unlikely that enough Anammox bacteria to significantly reduce nitrogen concentration will grow.

[0072] As described in detail above, in the water treatment apparatus 10 and water treatment method of this embodiment, by controlling the aeration airflow rate so that the nitrite concentration in the treated water obtained after biological treatment can be maintained at a low level, COD components can be stably treated while suppressing the generation of nitrite at a low level.

[0073] Furthermore, one embodiment of the present invention can have the following configuration. [1] A biological treatment tank containing activated sludge containing bacteria capable of decomposing COD components, into which water to be treated containing ammonia and at least thiocyanate ions as the COD component is introduced, The biological treatment tank includes an aeration device that performs aeration to decompose the COD components in the water to be treated aerobically using the bacteria, A measuring unit that measures at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by treating the water to be treated in the biological treatment tank, A control unit adjusts the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured by the measuring unit, and at least one index value selected from the group consisting of the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water measured by the measuring unit. A water treatment device equipped with the following features. [2] The water treatment apparatus according to [1] above, wherein the concentration value and the index value include at least the nitrite concentration in the treated water. [3] The water treatment apparatus according to [1] or [2] above, wherein the control unit adjusts the aeration airflow rate so that the index value approaches a preset target value. [4] The water treatment apparatus according to [3] above, wherein the control unit adjusts the aeration airflow rate by proportional control represented by the above formula (1). [5] The water treatment apparatus according to [3] above, wherein the control unit adjusts the aeration airflow rate by proportional-integral control represented by the above formula (2). [6] The concentration value and the index value include at least the nitrite concentration in the treated water, The water treatment apparatus according to any one of the above [3] to [5], wherein the target value as the nitrite concentration is 30 mg-N / L or less. [7] The water treatment apparatus according to any one of [1] to [6] above, wherein the water to be treated is coke oven wastewater. [8] Introduce water to be treated, which contains ammonia and at least thiocyanate ions as the COD component, into a biological treatment tank containing activated sludge with bacteria capable of decomposing COD components, and biologically treat the water to be treated with the activated sludge; Aeration is performed in the biological treatment tank to aerobically decompose the COD components in the water to be treated by the bacteria; To measure at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by biological treatment of the water to be treated; and Adjusting the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured as the concentration value, and at least one index value selected from the group consisting of the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water measured as the concentration value; A water treatment method that includes [a specific component]. [9] The water treatment method according to [8] above, wherein the concentration value and the index value include at least the nitrite concentration in the treated water.

[10] The water treatment method according to [8] or [9] above, wherein the aeration airflow rate is adjusted so that the index value approaches a preset target value.

[11] The water treatment method according to

[10] above, wherein the aeration airflow is adjusted by proportional control represented by the above formula (1).

[12] The water treatment method according to

[10] above, wherein the aeration airflow is adjusted by proportional-integral control represented by the above formula (2).

[13] The concentration value and the index value include at least the nitrite concentration in the treated water, The water treatment method according to any one of the above

[10] to

[12] , wherein the target value as the nitrite concentration is 30 mg-N / L or less.

[14] The water treatment method according to any one of [8] to

[13] above, wherein the water to be treated is coke oven wastewater. [Examples]

[0074] The present invention will be specifically described below with reference to examples and comparative examples (which may be collectively referred to as "test examples"), but the present invention is not limited to the following examples.

[0075] (Test equipment) A test apparatus having a configuration corresponding to the water treatment apparatus 10 shown in Figure 2 was used. For reference, the corresponding components of the test apparatus (10) will be described below, with the corresponding components shown in Figure 2 indicated in parentheses. The test apparatus (10) comprises a water tank to be treated (12), an activated sludge tank (11), a treated water tank (13), an aeration device (31), a nitrite measuring unit (41), and a control device (51). The nitrite measuring unit (41) comprises a sample water tank (42), a nitrite measuring probe (43), and a measuring instrument (44). The aeration device (31) comprises an aeration blower (32) and a diffuser ball (33).

[0076] In the test apparatus (10), the activated sludge tank (11) is equipped with a separation membrane (22), and treated water is drawn out of the activated sludge tank (11) by a treated water pump (66) and piping (76) connected to the membrane, thereby separating the activated sludge from the treated water. The drawn-out treated water is stored in the treated water tank (13). A portion of the treated water sent to the treated water tank (13) is sent to the sample tank (42) by a sampling pump (64) and piping (74). The sample tank (42) is equipped with a nitrite measuring probe (43). The signal from the nitrite measuring probe (43) is sent to the nitrite meter (44), and the nitrite concentration in the treated water is measured.

[0077] Furthermore, the activated sludge tank (11) in the test apparatus (10) is aerated by an aeration device (31). Aeration is performed by an aeration blower (32) to which a diffuser ball (33) is connected on the air outlet side. When controlling the aeration airflow rate based on the measured value of nitrite concentration (in the example), a signal regarding the nitrite concentration in the treated water is transmitted from the nitrite meter (44) to the control device (51) via the input line (81). Based on the received signal regarding the nitrite concentration, the control device (51) performs a predetermined calculation regarding the output control of the aeration blower (32) for adjusting the aeration airflow rate, and transmits the calculation result to the aeration blower (32) via the control line (82), thereby controlling the aeration airflow rate.

[0078] The effective volume of the activated sludge tank (11) is 5L. A hollow fiber membrane (product name "Stellapore STNM424", manufactured by Mitsubishi Chemical Corporation) was used as the separation membrane (22) installed in the activated sludge tank (11). A nitrite ion measuring electrode (model name "9003K-SR", manufactured by Toko Chemical Research Institute Co., Ltd.) was used as the nitrite measuring probe (43), and a nitrite ion meter (product name "TiN-9003T", manufactured by Toko Chemical Research Institute Co., Ltd.) was used as the nitrite measuring instrument (44). A digital indicating controller (product name "SDC15", manufactured by Azbil) was used as the control device (51). A pump (product name "Masterflex L / S", manufactured by Thermo Fisher Scientific) was used as the aeration blower (32).

[0079] (Water flow test) A continuous water flow test was conducted using the above-described test apparatus (10). First, 5 L of activated sludge (MLSS concentration = 5000 mg / L) collected from an actual activated sludge facility that treats coke oven wastewater (wastewater obtained by diluting coke oven wastewater with seawater) was placed into the activated sludge tank (11). Subsequently, the water to be treated shown in Table 1 was continuously flowed into the activated sludge tank (11) from the water to be treated tank (12) at an inflow rate of 7.8 L / day, and the treated water was continuously withdrawn at the same rate as the inflow rate. At the same time, the activated sludge tank (11) was continuously aerated at the aeration airflow rate shown in Table 2. The activated sludge tank (11) was then operated in this manner. From the effective volume of the activated sludge tank (11) and the inflow rate, the hydraulic residence time (HRT) was calculated to be 15.4 hours. During this operation, a portion of the withdrawn treated water was transferred to the sample tank (42) of the nitrite measurement unit (41), and the nitrite concentration in the treated water was continuously measured using a nitrite measurement probe (43) and a nitrite meter (44). Also during this operation, the pH of the mixture of treated water and activated sludge in the activated sludge tank (11) was maintained at 8.0 using a pH meter (product name "pH Controller FP-01", manufactured by Tokyo Glass Instruments Co., Ltd.) with an aqueous sodium hydroxide solution as a pH adjuster.

[0080] (Water to be treated) The treated water was simulated wastewater (simulated ammonia water) prepared to the composition shown in Table 1. This was prepared by dissolving the components (solutes) shown in the left column of Table 1 to the concentrations shown in the right column of Table 1 in a mixture of artificial seawater and tap water in a volume ratio of 2:3.

[0081] TIFF2026104130000003.tif46170

[0082] TIFF2026104130000004.tif62170

[0083] JPEG2026104130000005.jpg96170

[0084] (Method for measuring thiocyanate ion concentration) In the following test examples, the thiocyanate ion concentration in water was measured periodically using a colorimetric method with iron nitrate. Specifically, a sodium thiocyanate (NaSCN) aqueous solution (10 g-SCN / L) was diluted to prepare a series of diluted samples for the calibration curve. To 2 mL of each diluted series sample, 160 μL of 2.5 mol / L nitric acid aqueous solution and 80 μL of iron(II) nitrate aqueous solution were added to induce color development. The absorbance at a wavelength of 460 nm was measured for each sample immediately after this color development procedure. This resulted in obtaining a linear calibration curve in the range of 0 to 30 mg-SCN / L. The treated water obtained in the test examples described below was measured as follows: First, the treated water was appropriately diluted with pure water so that the thiocyanate ion concentration was in the range of 0 to 30 mg-SCN / L to prepare diluted samples of the treated water. After performing the color development procedure described above on these diluted samples, the absorbance at a wavelength of 460 nm was measured. The thiocyanate ion concentration (mg-SCN / L) in the treated water was calculated from the absorbance of diluted samples of the treated water and the calibration curve described above. A spectrophotometer (product name "Ratio Beam Spectrophotometer U-1800", manufactured by Hitachi, Ltd.) was used to measure the absorbance.

[0085] (Method for measuring nitrite concentration) As described above, the nitrite (nitrite nitrogen, NO2-N) concentration in the treated water in the sample tank (42) was continuously measured using a nitrite measurement probe (43) and a nitrite meter (44). The nitrite measurement probe (43) is a probe capable of measuring nitrite ion concentration by measuring the amount of nitrite (HNO2) produced when nitrite ions are protonated by an acid, based on the change in the pH of the electrolyte that occurs when the nitrite that has passed through a diaphragm that allows nitrite (HNO2) to pass through is absorbed into the electrolyte. The flow rates of the transfer pumps for each liquid were adjusted so that the treated water and the pH adjustment solution were mixed in a volume ratio of 10:1 and continuously flowed into the sample tank (42). When the nitrite measurement probe (43) was immersed in the pH-adjusted treated water, a signal corresponding to the nitrite concentration in the treated water was sent to the nitrite meter (44), thereby measuring the nitrite nitrogen concentration. The pH adjusting solution was prepared by mixing 95 g of sodium sulfate with 442 mL of 1 mol / L (2N) sulfuric acid and diluting it to 500 mL with pure water.

[0086] (Test results) As a result of the above water flow tests, graphs are shown in Figures 3 to 5, with the time of the water flow test on the horizontal axis, the aeration airflow rate (L / min) on the left vertical axis, and the concentrations of nitrite (NO2-N) and thiocyanate ion (SCN) (mg / L) on the right vertical axis. Figure 3 is the graph for Comparative Example 1 and Example 1. Figure 4 is the graph for Comparative Example 2 and Example 2. Figure 5 is the graph for Comparative Example 3 and Example 3. In the above water flow tests, when conducting the test for Example 1, the test was first started under the conditions of aeration using the aeration airflow rate of Comparative Example 1 shown in Table 2, and then the aeration airflow rate for Example 1 shown in Tables 2 and 3 was adjusted based on the continuously measured nitrite concentration. Similarly, when conducting the test for Example 2, the test was first started under the conditions of aeration using the aeration airflow rate of Comparative Example 2 shown in Table 2, and then the aeration airflow rate for Example 2 shown in Tables 2 and 3 was adjusted based on the continuously measured nitrite concentration. Furthermore, when conducting the test in Example 3, the test was first started under the conditions of aeration using the aeration airflow rate of Comparative Example 3 shown in Table 2. Then, based on the continuously measured nitrite concentration values, the aeration airflow rate of Example 3 shown in Tables 2 and 3 was adjusted.

[0087] The results of Example 1 shown in Figure 3 confirm that by performing biological treatment of the treated water while adjusting the aeration airflow rate using proportional control to bring the measured nitrite concentration in the treated water closer to the target value, it is possible to suppress nitrite production to a low level while stably treating thiocyanate ions. By manually switching the aeration airflow rate at 100% output in the aeration device to conditions A, B, and C, it was possible to further suppress nitrite production to a low level. The nitrite concentration in the treated water stabilized at approximately 20 mg-N / L under condition A, approximately 15 mg-N / L under condition B, and approximately 5 mg-N / L under condition C. Thiocyanate ions remained low throughout the test period, indicating that sufficient aeration airflow rate (oxygen amount) was maintained for treating thiocyanate ions.

[0088] Furthermore, if the experiment in Comparative Example 1, in which aeration is performed under constant conditions with an aeration airflow of approximately 0.3 L / min, were to continue, the nitrite concentration in the treated water would continue to rise and stabilize at a high concentration. When the experiment, in which aeration is performed under constant conditions with an aeration airflow of approximately 0.3 L / min, was run for several days, the nitrite concentration in the treated water was approximately 80-100 mg-N / L.

[0089] The results of Example 2, shown in Figure 4, confirm that by performing biological treatment of the treated water while adjusting the aeration airflow rate using proportional-integral control to bring the measured nitrite concentration in the treated water close to the target value, it is possible to suppress nitrite production to a low level while stably treating thiocyanate ions. The nitrite concentration in the treated water of Example 2 fluctuated within the range of 0.3 to 15 mg-N / L. Furthermore, the thiocyanate ion level remained low throughout the test period, indicating that sufficient aeration airflow rate (oxygen amount) was maintained for treating thiocyanate ions.

[0090] The results of Example 3, shown in Figure 5, confirm that by performing biological treatment of the treated water while adjusting the aeration airflow rate using proportional-integral control to bring the measured nitrite concentration in the treated water closer to the target value, it is possible to suppress nitrite production to a low level while stably treating thiocyanate ions. The nitrite concentration in the treated water in Example 3 fluctuated within the range of 0.2 to 0.6 mg-N / L, confirming that it is possible to further suppress nitrite production to a low and stable level by lowering the target value of the nitrite concentration in the treated water. In addition, thiocyanate ions remained low throughout the test period, indicating that sufficient aeration airflow rate (oxygen amount) was maintained for treating thiocyanate ions. [Explanation of Symbols]

[0091] 10 Water treatment equipment 11. Biological treatment tank 31 Aeration equipment 41 Measuring part 51 Control Unit

Claims

1. A biological treatment tank containing activated sludge containing bacteria capable of decomposing COD components, and into which treated water containing ammonia and at least thiocyanate ions as the COD component is introduced, The biological treatment tank includes an aeration device that performs aeration to aerobically decompose the COD components in the water to be treated by the bacteria, A measuring unit that measures at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by treating the water to be treated in the biological treatment tank, A control unit adjusts the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured by the measuring unit, and at least one index value selected from the group consisting of the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water measured by the measuring unit. A water treatment device equipped with the following features.

2. The water treatment apparatus according to claim 1, wherein the concentration value and the index value include at least the nitrite concentration in the treated water.

3. The water treatment apparatus according to claim 1, wherein the control unit adjusts the aeration airflow rate so as to bring the index value closer to a preset target value.

4. The water treatment apparatus according to claim 3, wherein the control unit adjusts the aeration airflow rate by proportional control represented by the following formula (1). u(t): Output (%) of the aeration airflow. K Ｐ : Proportional gain e(t): Deviation (= the aforementioned index value - the aforementioned target value) b: Output when the deviation e is 0

5. The water treatment apparatus according to claim 3, wherein the control unit adjusts the aeration airflow rate by proportional-integral control represented by the following formula (2). u(t): Output (%) of the aeration airflow. K Ｐ : Proportional gain K Ｉ : Integral gain e(t): Deviation (= the aforementioned index value - the aforementioned target value)

6. The aforementioned concentration value and the aforementioned index value include at least the nitrite concentration in the treated water, The water treatment apparatus according to claim 3, wherein the target value as the nitrite concentration is 30 mg-N / L or less.

7. The water treatment apparatus according to any one of claims 1 to 6, wherein the water to be treated is coke oven wastewater.

8. A biological treatment tank containing activated sludge with bacteria capable of decomposing COD components is used to introduce water to be treated, which contains ammonia and at least thiocyanate ions as the COD component, and the water to be treated is biologically treated with the activated sludge; Aeration is performed in the biological treatment tank to aerobically decompose the COD components in the water to be treated by the bacteria; To measure at least one concentration value selected from the group consisting of ammonium concentration, nitrite concentration, and nitrate concentration in the treated water obtained by biological treatment of the water to be treated; and Adjusting the aeration airflow rate of the aeration based on the difference between the ammonium concentration in the treated water and the ammonium concentration in the treated water measured as the concentration value, and at least one index value selected from the group consisting of the nitrite concentration, nitrate concentration, and the sum of the nitrite and nitrate concentrations in the treated water measured as the concentration value; A water treatment method that includes [a specific component].

9. The water treatment method according to claim 8, wherein the concentration value and the index value include at least the nitrite concentration in the treated water.

10. The water treatment method according to claim 8, wherein the aeration airflow rate is adjusted so that the index value approaches a preset target value.

11. The water treatment method according to claim 10, wherein the aeration airflow rate is adjusted by proportional control represented by the following formula (1). u(t): Output (%) of the aeration airflow. K Ｐ : Proportional gain e(t): Deviation (= the aforementioned index value - the aforementioned target value) b: Output when the deviation e is 0

12. The water treatment method according to claim 10, wherein the aeration airflow rate is adjusted by proportional-integral control represented by the following formula (2). u(t): Output (%) of the aeration airflow. K Ｐ : Proportional gain K Ｉ : Integral gain e(t): Deviation (= the aforementioned index value - the aforementioned target value)

13. The aforementioned concentration value and the aforementioned index value include at least the nitrite concentration in the treated water, The water treatment method according to claim 10, wherein the target value as the nitrite concentration is 30 mg-N / L or less.

14. The water treatment method according to any one of claims 8 to 13, wherein the water to be treated is coke oven wastewater.

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