JP2015009173A - Nitrogen removal method and apparatus for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide, by targeting a method for removing a nitrogen component included within treatment target water based on the electrochemical reduction thereof, a method enabling a stable treatment for removing the nitrogen component while an efficient reaction control is being executed.SOLUTION: In a method for removing, via electrolysis, a nitrogen component within treatment target water without partitioning the gap between an anode and a cathode with a diaphragm, the concentration of the nitrogen component within the treatment target water is measured at a nitrogen component removal treatment step, and after a main reaction mechanism specific to the cathode has been estimated based on the concentration measurement results, the addition of a specified chemical required for the nitrogen removal treatment is controlled depending on the estimated reaction mechanism. Alternatively, the pH of the treatment target water is maintained within a specified range by using a pH adjuster, and after a main reaction mechanism specific to the cathode has been estimated based on the quantity of the pH adjuster used at a nitrogen component removal treatment step, the addition of a specified chemical is controlled depending on the estimated reaction mechanism.

Description

  The present invention relates to a nitrogen removal method and apparatus for removing nitrogen components in water to be treated by electrolysis.

  Treatment methods for nitrate nitrogen contained in wastewater include biological treatment methods that utilize the denitrification ability of microorganisms, physicochemical treatment methods such as ion exchange, reverse osmosis, and electrodialysis, and electrolysis There are electrochemical treatment methods using

  The biological treatment method is the most popular method because of its low running cost. However, since the reaction rate is low, a large treatment device is required to treat a large amount of waste water. In addition, this biological treatment method is difficult to apply to wastewater containing a high concentration of nitrate nitrogen of about 1 g / L or more, and fluctuations in the load on the treatment device such as changes in nitrate nitrogen concentration in the wastewater. Therefore, the processing performance tends to become unstable.

  The physicochemical processing method is a method in which a processing apparatus can be downsized and reliable processing can be expected. However, since this method is a method for separating and concentrating nitrogen in water, it is necessary to separately treat a liquid finally enriched with nitrogen, and the nitrogen is not fundamentally treated.

  The electrochemical treatment method using electrolysis is a method of fundamentally treating nitrogen components, which has a relatively large treatment capacity relative to the size of the device and is applicable to wastewater containing high concentrations of nitrogen. It is possible to expect stable processing against fluctuations in the load on the processing apparatus such as changes in nitrogen concentration.

  However, among conventional electrochemical nitrogen removal methods, particularly with respect to the treatment method of oxidized nitrogen such as nitrate and nitrite, there was a problem in reaction control according to the nitrogen concentration.

  For example, Patent Document 1 describes a technique for estimating the concentration of the nitrogen component of the water to be treated in the detected physical property value and controlling the current value of electrolysis based on the estimated concentration. Are ORP (redox potential) and absorbance. Regarding ORP, as described in Patent Document 1, it is detected that ammonia nitrogen is depleted and the residual chlorine concentration is increased. However, as will be described later as a comparative example in the present specification, it is not possible to confirm whether the oxidized nitrogen has been processed up to the processing target when the ammonia nitrogen is exhausted. Therefore, in ORP, even if it can be estimated that ammonia has been exhausted, the concentration of oxidized nitrogen cannot be accurately confirmed.

  Regarding the absorbance, it is described that a correlation between the nitrogen component concentration and the absorbance is obtained in advance, and the electrolysis current value is controlled based on the absorbance. However, since the absorbance may be affected by other components depending on the wavelength, the correlation may change depending on the wavelength and water quality. In this way, if the correlation changes depending on the water quality, stable treatment cannot be expected.

  Actually, this Patent Document 1 explains that the correlation between the absorbance and the nitrogen concentration differs depending on whether ammonia nitrogen is the main component or nitrate nitrogen. In many cases, ammonia nitrogen and nitrate nitrogen coexist in the water to be treated, and nitrate nitrogen is reduced at the cathode to ammonia nitrogen. It is inevitable in principle, and it is difficult to determine which is the main component.

  Patent Documents 2 and 3 disclose control methods that focus on changes in pH. Specifically, in Patent Document 2, when the pH shows a minimum value as seen from the reaction formula of the electrolytic reaction, the end point of the oxidized nitrogen reduction reaction is detected, and this is detected to terminate the electrolysis. Explained. However, it can be determined that the reduction of the oxidized nitrogen has been completed when the pH becomes minimum as described above, as described in Patent Document 2, it is possible to monitor the pH in a random manner. And only when the reaction described in the said literature is performed in the anode and the cathode.

  In that respect, when the concentration of oxidized nitrogen contained in the water to be treated is high, the amount of change in pH becomes large. . Therefore, in reality, the method of monitoring the pH in a practical manner is limited to a case where the amount of change in pH is small, that is, when an extremely low concentration of nitrogen treatment is performed.

  In addition, due to the influence of coexisting substances and the like, not only the reactions described in the document occur at the anode and the cathode. For example, when the reaction efficiency of the reduction reaction of oxidized nitrogen is remarkably reduced at the cathode, it is assumed that oxidized nitrogen remains even when the pH becomes a minimum value. Therefore, actually, even if the pH becomes a minimum value, the reduction reaction of oxidized nitrogen is not necessarily completed.

  Patent Document 2 and Patent Document 3 describe the principle of nitrogen removal reaction and the resulting pH change, but only the reduction reaction of oxidized nitrogen and the hydrogen generation reaction are described as reactions at the cathode. is there. Actually, in addition to that, when the anode and the cathode are not partitioned by the diaphragm, the reduction reaction of hypochlorous acid generated at the anode is particularly important as the cathode reaction. As will be described later, since the change in pH differs depending on which of the reactions is the main reaction, the change in pH is not necessarily uniform. In both of the above-mentioned documents, the current value is controlled according to the pH change, but the current value is controlled only by the pH change without confirming whether the target reaction is the main reaction at the cathode. Even if it does, stable control cannot be expected.

Japanese Patent No. 4408706 Japanese Patent No. 4349862 Japanese Patent No. 4349842

  Therefore, the present invention has been proposed in view of such circumstances, and in a method for electrochemically reducing and removing nitrogen components such as oxidized nitrogen contained in water to be treated such as waste water, It is an object of the present invention to provide a method capable of stably removing a nitrogen component while performing a typical reaction control.

  The nitrogen removal method according to the present invention is a nitrogen removal method in which nitrogen components in water to be treated are removed by electrolysis without partitioning between an anode and a cathode by a diaphragm. Measure the concentration of the nitrogen component in the treated water, estimate the main reaction mechanism at the cathode based on the measurement result of the concentration of the nitrogen component, and according to the estimated reaction mechanism, It is characterized by controlling the addition.

  The nitrogen removal method according to the present invention is a nitrogen removal method for removing nitrogen components in water to be treated by electrolysis without partitioning between the anode and the cathode by a diaphragm, wherein the pH of the water to be treated is adjusted. It is maintained within a predetermined range by addition of a pH adjusting agent, and the main reaction mechanism at the cathode is estimated based on the addition rate of the pH adjusting agent, and according to the estimated reaction mechanism, a predetermined agent required for nitrogen removal treatment is estimated. It is characterized by controlling the addition.

  Further, the nitrogen removing apparatus according to the present invention is a nitrogen removing apparatus that removes nitrogen components in water to be treated by electrolysis without partitioning between the anode and the cathode by a diaphragm, and includes an anode and a cathode, An electrolytic cell in which water to be treated is accommodated, a direct current power source for supplying a direct current between the anode and the cathode, a regulating tank for adjusting the water quality of the treated water, and between the electrolytic cell and the regulating tank A circulation mechanism that circulates the water to be treated, a drug supply unit that supplies a drug required for nitrogen removal treatment, and a control unit that controls a supply amount of the drug supplied from the drug supply unit, A concentration measuring unit for measuring the concentration of nitrogen components in the water to be treated is provided, and in the course of nitrogen removal treatment, the concentration of nitrogen components in the water to be treated taken from the adjustment tank is measured. Nitrogen concentration measured at Based on the estimates main reaction mechanism at the cathode, and controls the supply amount of a given drug, needed nitrogen removal process from the drug supply unit according to the estimated reaction mechanism.

  Further, the nitrogen removing apparatus according to the present invention is a nitrogen removing apparatus that removes nitrogen components in water to be treated by electrolysis without partitioning between the anode and the cathode by a diaphragm, and includes an anode and a cathode, An electrolytic cell in which water to be treated is accommodated, a direct current power source for supplying a direct current between the anode and the cathode, a regulating tank for adjusting the water quality of the treated water, and between the electrolytic cell and the regulating tank A circulation mechanism that circulates the treated water, a pH adjuster supply unit that adds a pH adjuster for maintaining the pH of the treated water in a predetermined range, and nitrogen removal treatment other than the pH adjuster. A drug supply unit that supplies the required drug; and a control unit that controls a supply amount of the drug supplied from the drug supply unit. In the control unit, the pH adjustment agent added from the pH adjustment agent supply unit Calculate the addition rate and calculate the pH adjuster Based on the addition rate, to estimate the principal reaction mechanism at the cathode, and controls the supply amount of a given drug, needed nitrogen removal process from the drug supply unit according to the estimated reaction mechanism.

  According to the present invention, the reaction mechanism at the cathode is estimated based on the concentration of the nitrogen component and the amount of the pH adjuster used, and appropriate chemical addition control is performed accordingly. Therefore, the nitrogen component in the water to be treated can be removed stably.

It is a flowchart which shows the flow which estimates reaction mechanism from the measurement result of nitrogen concentration, and controls addition of a chemical | medical agent. It is a flowchart which shows the flow which estimates the reaction mechanism from the change of the usage-amount of a pH adjuster, and controls addition of a chemical | medical agent. It is a block diagram which shows an example of the nitrogen removal apparatus used for the nitrogen removal method. It is a block diagram of the electrolytic vessel in a nitrogen removal apparatus. It is a block diagram which shows an example of a nitrogen concentration measuring apparatus. 2 is a graph showing changes in nitrogen concentration during electrolytic treatment in Example 1. FIG. It is a graph which shows the change of the nitrogen concentration in to-be-processed water with progress of the electrolytic treatment time in the comparative example 1, and the change of ORP. It is a graph which shows the change of the nitrogen concentration during the electrolytic treatment in Example 2, and the change of the addition rate of caustic soda.

Hereinafter, specific embodiments to which the present invention is applied (hereinafter referred to as “the present embodiment”) will be described in detail in the following order. The present invention is not limited to the following embodiments, and various modifications can be made without departing from the gist of the present invention.
1. Nitrogen removal method 2. Electrolytic treatment reaction (nitrogen removal mechanism) for water to be treated 2-1. Basic nitrogen removal reaction mechanism 2-2. Change (change) of reaction mechanism
2-3. Drug supply control according to reaction mechanism estimation (reaction control)
3. 3. Nitrogen removing device 3-1. Configuration of nitrogen removing apparatus 3-2. 3. Drug supply control in the nitrogen removing device Example

<< 1. Nitrogen removal method >>
The nitrogen removal method according to the present embodiment performs an electrolysis process on water to be treated containing chloride ions (for example, wastewater discharged from a factory or the like), and is included in the water to be treated. The nitrogen component such as oxidized nitrogen is removed by electrochemical reduction treatment.

  In this removal method by electrochemical reduction treatment, water to be treated is accommodated in an electrolytic cell having an anode and a cathode, and a voltage is applied to the electrode to energize it. Chlorous acid is produced, and at the cathode, nitrogen components such as oxidized nitrogen are reduced to produce ammonia. Then, the oxidized nitrogen is removed as nitrogen gas by reacting the generated hypochlorous acid with ammonia. In particular, in the method according to the present embodiment, water to be treated is stored in a non-diaphragm electrolytic cell in which an anode and a cathode are not partitioned by a diaphragm such as an ion exchange membrane, and electrolytic decomposition is performed to remove nitrogen components. .

  Specifically, the nitrogen removal method according to the present embodiment measures the concentration of the nitrogen component in the water to be treated in the process of removing the nitrogen component, and estimates the main reaction mechanism at the cathode based on the concentration measurement result. According to the estimated reaction mechanism, the addition of a predetermined drug required for the nitrogen removal process is controlled.

  Alternatively, instead of estimating the reaction mechanism based on the measured concentration of the nitrogen component in the water to be treated, the pH of the water to be treated is maintained within a predetermined range with a pH adjuster, and the pH adjustment in the process of removing the nitrogen component Based on the amount of agent used, the main reaction mechanism at the cathode is estimated. And according to the estimated reaction mechanism, addition of the predetermined chemical | medical agent required for a nitrogen removal process is controlled, It is characterized by the above-mentioned.

  As described above, in the nitrogen removal method according to the present embodiment, the removal treatment process is based on the concentration of the nitrogen component in the treated water or the amount of the pH adjuster used to maintain the pH of the treated water in a predetermined range. It is important to estimate the main reaction mechanism at the cathode at a given point in time. And in this method, according to the estimated reaction mechanism, the addition of the chemical | medical agent required for the nitrogen removal process at the time is controlled. As a result, efficient reaction control can be performed, and nitrogen components such as oxidized nitrogen in the water to be treated can be stably removed.

≪2. About electrolytic treatment reaction (nitrogen removal mechanism) for water to be treated >>
First, the electrolytic treatment reaction of water to be treated generated in the electrolytic bath will be described more specifically.

<2-1. Basic nitrogen removal reaction mechanism>
In the electrolytic treatment of the water to be treated, when energization to the anode and the cathode provided in the electrolytic cell is started, the anode is oxidized from chloride ions to hypochlorite ions as shown in the following reaction formula 1. Is done. On the other hand, at the cathode, as shown in the following reaction formula 2, the nitrogen component (for example, nitrate nitrogen) in the water to be treated is reduced to ammonia (ammonia nitrogen). In addition, in the following reaction formula 1 and reaction formula 2, in order to compare the reaction equivalent per the same electric quantity, it represents with the reaction equivalent per 48 electron moles.

[Reaction Scheme 1: Reaction at the Anode]
_{^{24Cl - + 24H 2 O → 24ClO}} - + 48H + + 48e
[Reaction Formula 2: Reaction at the Cathode]
6NO ₃ ⁻ + 48e + 42H ₂ O → 6NH ₄ ⁺ + 60OH ⁻

In the nitrogen removal method according to the present embodiment, electrolysis is performed in a diaphragm membrane electrolytic cell that does not partition the anode and the cathode. From this, hypochlorous acid (ClO ⁻ ) generated at the anode and ammonia (NH ₄ ⁺ ) generated by the reduction reaction of oxidized nitrogen at the cathode are uniformly mixed in the electrolytic cell, and the following reaction formula The reaction is performed in situ according to No. 3 and removed as nitrogen gas (N ₂ ) to complete the denitrification reaction.

[Reaction Formula 3]
6NH ₄ ⁺ + 9ClO ⁻ → 3N ₂ + 9Cl ⁻ + 9H ₂ O + 6H ⁺

  The overall reaction formula from the electrode reaction to the denitrification reaction described above is as shown in the following reaction formula 4.

[Reaction Formula 4]
6NO ₃ ⁻ + 15Cl ⁻ + 3H ₂ O → 3N ₂ + 15ClO ⁻ + 6OH ⁻

  In the nitrogen removal reaction described above, chlorine gas is generated from hypochlorous acid generated at the anode in the electrolytic cell when the pH of the water to be treated is lowered, and ammonia is volatilized when the pH is increased. Since there is a problem that nitrate ions are easily generated when ammonia and hypochlorous acid react with each other, the pH of the water to be treated is maintained at approximately 4.0 to 10.0 during the electrolytic treatment. Preferably it maintains at 5.0-9.0.

  In pH adjustment of to-be-processed water, pH is maintained in the above-mentioned range by adding alkalis, such as caustic soda, and acids, such as hydrochloric acid and a sulfuric acid, as a pH adjuster, for example. The pH adjuster is not particularly limited. However, when ammonia or ammonium salt is used as the surplus hypochlorous acid scavenger described later, the pH fluctuates due to electrolytic treatment, so alkali such as caustic soda is used. It is preferable to use it.

  By the way, as shown in the above reaction formula 4, the hypochlorous acid produced at the anode is in excess of the amount consumed in the denitrification reaction of the ammonia nitrogen produced at the cathode. Therefore, hypochlorous acid becomes surplus in the electrolytic solution, and the surplus hypochlorous acid is also present in the vicinity of the cathode in the diaphragm electrolyzer.

  In this way, when hypochlorous acid is present in the vicinity of the cathode that performs the reduction reaction in the diaphragmless electrolytic cell, hypochlorous acid has a strong oxidizing action, so that the oxidized nitrogen and the like at the cathode Inhibits the reduction reaction to the nitrogen component.

  For this reason, in the present embodiment, in order to prevent the excessive hypochlorous acid from inhibiting the reduction reaction at the cathode, surplus hypochlorous acid is consumed (captured and decomposed) in the electrolytic reaction. A scavenger (hereinafter referred to as “excess hypochlorous acid scavenger”) is allowed to be present in the electrolytic cell. This allows the hypochlorous acid to be decomposed by the surplus hypochlorous acid scavenger before surplus hypochlorous acid reaches the vicinity of the cathode, and the reduction reaction for oxidized nitrogen at the cathode proceeds efficiently. Can be made.

  The surplus hypochlorous acid scavenger is not limited as long as it is a compound that reacts rapidly with hypochlorous acid. For example, ammonia, ammonium salt, sulfite and the like can be used. Among them, it is preferable to use ammonia or an ammonium salt in consideration of reactivity with hypochlorous acid, economy, and the like. The surplus hypochlorous acid scavenger varies in pH during electrolysis depending on the type. For this reason, it is preferable to control the pH of the water to be treated by appropriately changing the type of pH adjuster that adjusts the pH of the water to be treated according to the type of surplus hypochlorous acid scavenger to be added.

  Here, more specifically, the reaction mechanism will be described by taking an example in which an ammonium salt is used as a hypochlorous acid scavenger.

  When an ammonium salt is added as a hypochlorous acid scavenger, hypochlorous acid and ammonia generated at the anode react according to the following reaction formula 5, and the overall reaction becomes the following reaction formula 6. As can be seen from the reaction formula 6 below, when an ammonium salt is used as the surplus hypochlorous acid scavenger, the pH varies due to the electrolytic treatment, so an alkali such as caustic soda is used as the pH adjuster.

[Reaction Formula 5]
10NH ₄ ⁺ + 15ClO ⁻ → 5N ₂ + 15Cl ⁻ + 15H ₂ O + 10H ⁺

[Reaction Formula 6]
6NO ₃ ⁻ + 10NH ₄ ⁺ → 8N ₂ + 18H ₂ O + 4H ⁺

<2-2. Changes in the reaction mechanism (transition)>
Each of the reaction formulas described above represents a reaction equivalent relationship when the electrolytic reaction at the anode and the cathode shown in the reaction formulas 1 and 2 proceeds at a current efficiency of 100%. In the electrolytic treatment, there is an influence of coexisting substances in the water to be treated, and the current efficiency is not necessarily 100% in the actual electrolytic treatment. The above-mentioned pH movement and the necessary amount of surplus hypochlorous acid scavenger are described above. The amount described in the reaction formula is different. Therefore, in the electrolytic treatment, it is necessary to proceed with the treatment while constantly or intermittently monitoring important water quality items such as nitrogen components and pH of the water to be treated.

  Specifically, for example, the higher the surplus hypochlorous acid scavenger concentration, the higher the reaction rate with hypochlorous acid, so the effect of promoting the reduction reaction of nitrogen components such as oxidized nitrogen is high. Therefore, it is preferable to continue the electrolysis while maintaining a surplus hypochlorous acid scavenger concentration sufficient to maintain the required reaction rate, but the required surplus hypochlorous acid scavenger concentration is less than the hypochlorous acid concentration at the anode. It depends on the generation speed. For this reason, it is necessary to increase the concentration of excess hypochlorous acid scavenger to be maintained as the production rate increases. In addition, the generation rate of hypochlorous acid at the anode is proportional to the amount of direct current applied to the anode and the cathode. Therefore, the necessary excess hypochlorous acid scavenger concentration varies depending on the amount of current.

  Thus, in the electrolytic treatment of the water to be treated, the concentration of surplus hypochlorous acid scavenger required due to various factors fluctuates, and the main concentration at the cathode depends on the concentration of surplus hypochlorous acid scavenger in the water to be treated. The reaction mechanism also changes (changes).

  Here, the transition of the reaction mechanism at the cathode will be described in more detail with a specific example. Table 1 shows the transition of the main reaction mechanism at the cathode according to the concentration (ammonia nitrogen concentration) (mg / L) in the treated water when ammonia or ammonium salt is added as a surplus hypochlorous acid scavenger. It is a summary.

  In addition, this Table 1 gives the example of the to-be-processed water which contains 5000 mg / L of nitrate nitrogen (oxidized nitrogen) as a nitrogen component, and the pH adjuster which maintains the pH of to-be-processed water in a predetermined range. Are summarized as examples using caustic soda. Therefore, the scavenger concentration of 3000 mg / L at which the reaction mechanism changes is an approximate concentration at which the reaction mechanism changes in this specific example, and the transition point of the reaction mechanism is not limited to this concentration.

  As shown in Table 1, when the ammonia nitrogen (scavenger) concentration is high at the initial stage of the reaction, for example, approximately 3000 mg / L or more (Phase I), the reduction efficiency of oxidized nitrogen at the cathode is almost 100%. Then, an oxidation reaction of ammonia generated along with a reduction reaction of oxidized nitrogen occurs, and a nitrogen component is removed as nitrogen gas.

  Next, when the reaction proceeds and the scavenger concentration in the treatment liquid decreases (phase II), the reduction efficiency of oxidized nitrogen at the cathode decreases. Specifically, when the scavenger concentration is less than 3000 mg / L, the reduction efficiency of oxidized nitrogen is lowered, and the reduction rate (decomposition rate) of the oxidized nitrogen concentration is reduced. Therefore, by measuring the concentration of oxidized nitrogen in the liquid to be treated, it is possible to detect that the reduction efficiency has decreased due to the amount of change over time.

  On the other hand, the total chlorine (Cl) concentration in the water to be treated does not change even if the reaction proceeds. This is because only the change of Cl⇔ClO is repeated in the water to be treated, and chlorine does not come out of the system. Therefore, the oxidation efficiency at the anode does not change much during the process. As described above, when only the reduction efficiency at the cathode is lowered, the ratio of the oxidative decomposition reaction of ammonia derived from the reduction of oxidized nitrogen is lowered, and only the scavenger is decomposed. As a result, the overall main reaction changes from the above reaction formula 6 to the following reaction formula 7.

[Reaction Scheme 7]
16NH ₄ ⁺ + 24ClO ⁻ → 8N ₂ + 24Cl ⁻ + 24H ₂ O + 16H ⁺

Thus, overall main reaction is changed in Scheme 7 from Scheme 6, ClO ^- strong when the oxidation of ammonia by oxidation is the main reaction, pH modifiers with increasing amount of generated H ⁺ ( The amount of caustic soda used) will increase (addition rate will increase). Therefore, it can be detected that the reduction efficiency is lowered by measuring the addition rate of the pH adjuster.

  Even if the reduction efficiency decreases, the scavenger ammonia still remains in low concentration. Even if electrolysis is continued until the ammonia in the scavenger is exhausted, the reduction of the oxidized nitrogen cannot be sufficiently performed in a state where the reduction efficiency is lowered, so that the scavenger is wasted. Therefore, as described above, whether or not to add additional scavengers is determined at the stage where it has been detected that the reduction efficiency has been reduced based on the change in the concentration of oxidized nitrogen and the increase in the amount of pH adjuster used. It is desirable to do.

<2-3. Drug supply control according to estimation of reaction mechanism (reaction control)>
Therefore, in the nitrogen removal method according to the present embodiment, the concentration of the nitrogen component in the water to be treated is measured in the process of removing the nitrogen component, and the main reaction mechanism at the cathode is estimated based on the concentration measurement result. And according to the estimated reaction mechanism, addition of the predetermined chemical | medical agent required for a nitrogen removal process is controlled.

  Alternatively, in the process of removing nitrogen components while maintaining the pH of the water to be treated in a predetermined range with the pH adjuster, the main reaction mechanism at the cathode is determined based on the amount of use (supply amount) of the pH adjuster. presume. And according to the estimated reaction mechanism, addition of the predetermined chemical | medical agent required for a nitrogen removal process is controlled.

[When estimating reaction mechanism from measurement result of nitrogen concentration]
Specifically, when estimating the reaction mechanism from the measurement result of the nitrogen concentration in the water to be treated, as shown in the flowchart of FIG. 1, first, as step S11, the nitrogen concentration of the oxidized nitrogen in the water to be treated is measured. . Next, as step S12, a decrease rate (decomposition rate) of the nitrogen concentration with the lapse of the processing time is calculated based on the measured nitrogen concentration.

  Next, in step S13, it is determined whether or not the calculated decrease rate (decrease slope) of the nitrogen concentration is equal to or less than a predetermined value (set value). That is, it is determined whether or not the decomposition rate of oxidized nitrogen is decreasing. As a result of this determination, if the decrease rate is greater than the set value (No), the process returns to step S11 again. On the other hand, if it is determined that the decrease rate is equal to or less than the set value (Yes), the process proceeds to step S14.

  In step S14, it has been determined that the rate of decrease in nitrogen concentration is equal to or less than the set value, but whether or not the concentration of oxidized nitrogen in the water to be treated is equal to or lower than the target value, that is, the measured nitrogen concentration is treated. It is determined whether or not the nitrogen component that is equal to or less than the target value has been sufficiently removed. If the result of this determination is that the oxidized nitrogen concentration is below the target value (Yes), the nitrogen removal process is terminated. On the other hand, if the oxidized nitrogen concentration is higher than the target value (No), that is, if the nitrogen removal process has not been sufficiently performed, the process proceeds to step S15.

  In step S15, the rate of decrease in the concentration of oxidized nitrogen is reduced, and sufficient processing has not yet been performed to reach the target concentration. Therefore, it is determined that the reaction mechanism has changed at the cathode due to the decrease in the scavenger concentration. That is, it is determined that the reaction mechanism at the cathode has shifted to the phase II in Table 1 above, and an additional scavenger is added.

  Here, the method for measuring the nitrogen concentration in the water to be treated is not particularly limited. For example, it can be performed using an apparatus using an ultraviolet absorption photometry method using ultraviolet rays having a central wavelength of 254 nm. In addition, as will be described in detail later, chloramines are contained as intermediate products in the water to be treated such as factory effluent. Therefore, when the concentration is measured by absorptiometry, the absorbance is detected by the chloramines. It is preferable to decompose and remove chloramines in advance so as not to be inhibited.

[When estimating the reaction mechanism from changes in the amount of pH adjuster used]
Further, when estimating the reaction mechanism based on the change in the amount of the pH adjuster used, as shown in the flowchart of FIG. 2, first, as step S21, it is added to maintain the pH of the water to be treated within a predetermined range. The addition rate of a pH adjuster (such as caustic soda) is calculated. Next, in step S22, it is determined whether or not the calculated addition rate (the slope of the usage amount of the pH adjusting agent) is equal to or greater than a predetermined value (set value). That is, it is determined whether or not the usage amount of the pH adjuster is increasing. As a result of this determination, when the addition rate is smaller than the set value (No), the process returns to step S21 again. On the other hand, when it is determined that the addition rate is equal to or higher than the set value (Yes), the process proceeds to step S23.

  In step S23, the concentration of oxidized nitrogen in the water to be treated is measured. Then, in step S24, it is determined that the addition rate of the pH adjusting agent is equal to or higher than the set value, but whether or not the concentration of oxidized nitrogen in the water to be treated measured in step S23 is equal to or lower than the treatment target value. That is, it is determined whether or not the measured nitrogen concentration is equal to or lower than the processing target value and the nitrogen component to be processed can be sufficiently removed. As a result of this determination, when the oxidized nitrogen concentration is equal to or lower than the processing target value (Yes), the nitrogen removal processing is terminated. On the other hand, if the oxidized nitrogen concentration is higher than the treatment target value (No), that is, if the nitrogen removal treatment has not been sufficiently performed, the process proceeds to step S25.

  In step S25, the rate of addition of the pH adjuster is increased, and since sufficient treatment has not yet been achieved to the target concentration, it is determined that the reaction mechanism has changed at the cathode due to the decrease in the scavenger concentration. Then, it is determined that the reaction mechanism at the cathode has shifted to the phase II in Table 1 above, and an additional scavenger is added.

  Here, regarding the increase in the rate of addition of the pH adjusting agent, for example, when the slope of the amount of the pH adjusting agent used with respect to the processing elapsed time has changed more than twice, it is determined that the reaction mechanism has changed, It is possible to perform control for adding an additional scavenger.

  As described above, in the nitrogen removal method according to the present embodiment, the measurement result of the measured concentration of oxidized nitrogen in the water to be treated and the change in the usage amount of the pH adjuster according to the measurement result of the pH of the water to be treated. Based on the above, the main reaction mechanism in the cathode is estimated, and the supply of the chemical (hypochlorous acid scavenger) required for the removal of oxidized nitrogen is controlled according to the estimated reaction mechanism. According to such a nitrogen removal method, it is possible to accurately grasp the main reaction mechanism at the cathode in the electrolytic treatment of oxidized nitrogen and perform efficient reaction control, and stably perform nitrogen components such as oxidized nitrogen. Can be removed.

≪3. Nitrogen removal equipment >>
Next, the nitrogen removal apparatus used for the nitrogen removal method according to the present embodiment will be described.

<3-1. Configuration of nitrogen removal device>
FIG. 3 is a schematic configuration diagram showing an example of a nitrogen removing apparatus (electrolytic treatment apparatus) used in the nitrogen removing method according to the present embodiment. As shown in FIG. 3, the nitrogen removing apparatus 10 includes an anode and a cathode, and an electrolytic cell 11 that contains water to be treated 20 containing a nitrogen component, and a DC power source that supplies a DC current between the anode and the cathode. 12, the adjustment tank 13 which adjusts the water quality of the to-be-processed water 20, and the circulation mechanism 14 (14a, 14b) which circulates the to-be-processed water 20 between the electrolytic vessel 11 and the adjustment tank 13. Further, the nitrogen removing apparatus 10 includes a measuring unit 15 that measures the pH and hypochlorous acid concentration of the water to be treated 20 and a nitrogen concentration measuring unit 16 that measures the nitrogen concentration in the water to be treated 20. Further, the nitrogen removing apparatus 10 includes a medicine supply unit 17 that supplies a medicine required for the nitrogen removal process, and a control unit 18 that controls a medicine supply amount supplied from the medicine supply unit 17.

(Electrolysis tank)
The electrolytic cell 11 is composed of one or more sets of anode 11a and cathode 11b as a pair. FIG. 4 is a front view (A) and a side view (B) of the electrolytic cell 11. As shown in FIG. 4, in the electrolytic cell 11, water to be treated 20 containing a nitrogen component such as oxidized nitrogen is accommodated, so that at least a part is immersed in the water to be treated 20. The anode 11a and the cathode 11b are arranged. Each electrode 11a, 11b is connected to a DC power source 12 to be described later. By supplying a DC current applied from the DC power source 12 to the anode 11a and the cathode 11b, the nitrogen component in the water to be treated 20 is reduced. Remove electrolytically.

  This electrolytic cell 11 is a non-diaphragm electrolytic cell in which the anode 11a and the cathode 11b are not partitioned by a diaphragm such as an ion exchange membrane. In this non-diaphragm electrolytic cell 11, as described above, hypochlorous acid generated at the anode 11 a and ammonia generated at the cathode 11 b are uniformly mixed and reacted in situ to form nitrogen gas. (Scheme 3 above). In the non-diaphragm electrolytic cell in which the anode 11a and the cathode 11b are not partitioned by a diaphragm, the problem that the hardness component contained in the water to be treated 20 is clogged in the diaphragm does not occur and there is no load such as raw water management. There are advantages.

  As the anode 11a, a commonly used insoluble electrode can be used. In the anode 11a, in order to generate hypochlorous acid from chloride ions contained in the water to be treated, it is desirable to employ an electrode having high chlorine generation efficiency among insoluble electrodes. High chlorine generation efficiency means that the oxidation reaction from chloride ions to hypochlorite ions takes place over other reactions at the anode. This leads to preventing the oxidized nitrogen reduced at the cathode 11b from being oxidized again.

  As described above, the cathode 11b is used to remove nitrogen components such as oxidized nitrogen in the water to be treated by electrolysis, and is a metal having a reducing performance with respect to nitrogen components such as oxidized nitrogen. An electrode made of can be used. Specifically, for example, an amorphous electrode formed by coating an alloy such as an iron-chromium-based alloy having a reduction performance with respect to oxidized nitrogen in an amorphous state can be used. By using such an amorphous electrode, even when a direct current is applied, the metal component constituting the cathode 11b does not elute into the solution, and the electrolytic treatment can be performed while maintaining the reduction performance.

(DC power supply)
The DC power source 12 supplies and applies a DC current between the anode 11 a and the cathode 11 b provided in the electrolytic cell 11. This DC power supply 12 is connected to a control unit 18 to be described later so that signals can be transmitted and received, and the energization amount of the DC current to be supplied can be controlled in accordance with the water quality of the treated water 20 and the amount of chemical supply. May be.

(Adjustment tank)
The adjustment tank 13 periodically takes in the water to be treated 20 subjected to the electrolytic treatment in the electrolytic tank 11 from the electrolytic tank 11 through a circulation mechanism 14 (14a) described later, and adjusts the pH of the water to be treated 20 and the next. Adjust water quality such as chlorous acid concentration.

  Specifically, the adjustment tank 13 is connected to a drug supply unit 17 including a pH adjuster supply unit 17a, an excess hypochlorous acid scavenger supply unit 17b, and the like, and the pH of the water to be treated 20 in the measurement unit 15 described later. Based on the measurement result of the hypochlorous acid concentration and the measurement result of the nitrogen concentration in the treated water 20 in the nitrogen concentration measuring unit 16, the pH adjuster, the surplus hypochlorous acid scavenger, etc. for the treated water 20 The chemical required for the nitrogen removal treatment is supplied and added to adjust the quality of the water 20 to be treated. That is, the pH of the water to be treated 20 is adjusted to a range suitable for electrolytic treatment, and surplus hypochlorous acid generated at the anode 11a is decomposed by a hypochlorous acid scavenger.

  In addition, the to-be-processed water 20 which sent the liquid to this adjustment tank 13 and added the chemical | medical agent as needed is circulated again and accommodated in the electrolytic cell 11 via the circulation mechanism 14 (14b).

(Circulation mechanism)
The circulation mechanism 14 mainly includes, for example, a circulation pipe 14 a that transfers the treated water 20 taken out from the electrolytic cell 11 to the adjustment tank 13, and a circulation pipe that returns the treated water 20 from the adjustment tank 13 back to the electrolytic cell 11 again. 14b. In the circulation mechanism 14, the treated water 20 is circulated by, for example, a pump 19a.

(Measurement part)
The measurement unit 15 is provided, for example, on the circulation pipe 14a that constitutes the circulation mechanism 14 described above, and is generated in a pH measurement device that measures the pH of the water to be treated 20 taken from the electrolytic cell 11 or the anode 11a. It consists of a hypochlorous acid concentration measuring device that measures the hypochlorous acid concentration. Thus, the measurement part 15 measures the water quality of the to-be-processed water 20 in a nitrogen removal process.

  The measurement unit 15 is connected to a control unit 18 to be described later, and signals of measurement results of the pH of the treated water 20 and the hypochlorous acid concentration in the measurement unit 15 can be transmitted to the control unit 18. ing. And in the control part 18, the chemical | medical agent supply with respect to the to-be-processed water 20 is performed in the adjustment tank 13 from the measurement results of water quality, such as those pH. That is, the control unit 18 adjusts the supply amount of the drug from the drug supply unit 17 based on the measurement result such as the pH of the treated water 20 received from the measurement unit 15, and is necessary via the drug supply unit 17. The chemical | medical agent used as is added to the to-be-processed water 20 in the adjustment tank 13. FIG. The medicine supply control in the control unit 18 will be described in detail later.

(Nitrogen concentration measurement part)
The nitrogen concentration measuring unit 16 takes in the water to be treated 20 accommodated in the adjustment tank 13 with the pump 19b and measures the concentration of nitrogen components such as oxidized nitrogen in the water to be treated 20.

  Further, the nitrogen concentration measuring unit 16 is connected to a control unit 18 which will be described later, and a signal of the measurement result of the oxidized nitrogen concentration in the treated water 20 in the measuring unit 15 can be transmitted to the control unit 18. Yes. The control unit 18 controls the supply of chemicals to the water to be treated 20 from the chemical supply unit 17 based on the measurement result of the nitrogen concentration. The medicine supply control in the control unit 18 will be described in detail later.

  Specifically, the nitrogen concentration measuring unit 16 is not particularly limited as long as it can measure the concentration of oxidized nitrogen to be removed in the water 20 to be treated, but for example, ultraviolet absorption by ultraviolet rays having a central wavelength of 254 nm. It can be set as the apparatus using the photometric method. This central wavelength of 254 nm is a wavelength characterized by absorption of oxidized nitrogen but no absorption of ammonia.

Here, in this nitrogen removal treatment by electrolysis, in addition to nitrogen components such as oxidized nitrogen (for example, NO ₃ ⁻ ) as a concentration measurement target, monochloramine (NH ₂ Cl), dichloramine ( Chloramines such as NHCl ₂ ) and trichloramine (NCl ₃ ) are generated as intermediate products. When the concentration of nitrogen components is measured by UV absorption photometry for such treated water containing chloramines, the chloramines show an absorbance close to that of nitrogen components such as oxidized nitrogen. It becomes difficult to measure the concentration of the component.

  Therefore, in the case where the nitrogen concentration measuring unit 16 is an apparatus using an ultraviolet absorption photometry method, activated carbon treatment is applied to the water to be treated as a pretreatment to selectively decompose only chloramines in the water to be treated. Thus, it is preferable to perform concentration measurement using ultraviolet absorption photometry for the water to be treated from which chloramines have been decomposed and removed. This makes it possible to accurately measure the nitrogen concentration without causing measurement inhibition (interference) due to chloramines having an approximate absorbance.

  Specifically, FIG. 5 shows an example of the configuration of an apparatus using an ultraviolet absorptiometry as the nitrogen concentration measurement unit 16. In addition, in this FIG. 5, it shows as a structure including the adjustment tank 13 in the nitrogen removal apparatus 10, and the structure of the aspect which measures nitrogen concentration with respect to the to-be-processed water 20 taken out from the adjustment tank 13 is shown. As shown in FIG. 5, the nitrogen concentration measurement unit 16 using ultraviolet absorption spectrophotometry includes a pretreatment unit 31 that performs pretreatment on the water to be treated 20 prior to concentration measurement, and concentration measurement by ultraviolet absorption photometry. Part 32.

  The pretreatment unit 31 is composed of, for example, a column filled with activated carbon. The treated water 20 is taken into the column from the adjustment tank 13 by the pump 19b, and only chloramines contained in the treated water 20 are selectively decomposed and removed by activated carbon. To do. When the chloramines are decomposed and removed in the pretreatment unit 31, an acid such as sulfuric acid is added to the water to be treated 20 in advance to adjust the pH of the water to be treated 20 to about 3 or less. It is preferable to optimize the decomposition conditions.

  The concentration measuring unit 32 measures the concentration of nitrogen components such as oxidized nitrogen in the water to be treated 20 through which the chloramines are decomposed and removed through the pretreatment unit 31. The concentration measuring unit 32 is provided with an ultraviolet absorptiometer, and measures the concentration of oxidized nitrogen by irradiating the taken-in water 20 to be treated with, for example, ultraviolet light having a central wavelength of 254 nm. . A signal of the measurement result of the nitrogen concentration in the concentration measuring unit 32 is transmitted to the control unit 18 constituting the nitrogen removing apparatus 10.

  When the concentration of the nitrogen component is measured by the concentration measuring unit 32, the water 20 to be treated is returned to the adjustment tank 13 and circulated and transferred to the electrolytic tank 11 for electrolytic treatment. In addition, with respect to the to-be-processed water 20 put back in the adjustment tank 13, a pH adjuster is added suitably and the pH is adjusted to the range suitable for electrolytic treatment.

(Drug supply department)
The chemical supply unit 17 includes, for example, a pH adjuster supply unit 17a, an excess hypochlorous acid scavenger supply unit 17b, and the like, and contains chemicals required for a process for decomposing and removing nitrogen components such as oxidized nitrogen in the water 20 to be treated. Supply. This chemical supply unit 17 is connected to a control unit 18 to be described later, and the measurement result of water quality such as pH of the water to be treated 20 in the measurement unit 15 provided on the circulation path of the circulation pipe 14a, and the measurement of nitrogen concentration. Based on the measurement result of the oxidized nitrogen concentration in the unit 16, the chemical supply such as the chemical supply amount is controlled by the control unit 18.

  Here, in the nitrogen removing apparatus 10, nitrogen is removed while maintaining and controlling the pH of the water 20 to be treated within a predetermined range. Specifically, as described above, nitrogen is removed while maintaining the pH of the water to be treated 20 at approximately 4.0 to 10.0, preferably 5.0 to 9.0. Therefore, in the nitrogen removing device 10, the pH of the water to be treated 20 taken in from the electrolytic cell 11 and circulated is constantly measured by the measuring unit 15, and based on the measurement result, the pH adjusting agent from the pH adjusting agent supplying unit 17a is measured. The usage amount is changed, and the pH of the water to be treated 20 is maintained and controlled within a predetermined range.

More specifically, when the reaction mechanism in the cathode 11b of the electrolytic cell 11 changes and the amount of H ⁺ generated increases, the pH of the water to be treated 20 changes to the acidic side. In such a case, the usage amount of the pH adjusting agent from the pH adjusting agent supply unit 17a is increased based on the pH measurement result of the treated water 20 in the measuring unit 15 (measurement result that the pH has decreased). Thus, the pH of the treated water 20 is controlled to be maintained within a predetermined range.

(Control part)
The control unit 18 controls the amount of medicine supplied from the medicine supply unit 17. The control unit 18 measures the pH of the treated water 20 measured in the measuring unit 15 provided on the circulation path of the circulation pipe 14a, and the measurement result of the oxidized nitrogen concentration in the nitrogen concentration measuring unit 16. The amount of medicine supplied from the medicine supply unit 17 is controlled based on the measurement results.

<3-2. Drug supply control in nitrogen removal device>
Here, the chemical supply control (electrolytic reaction control) from the chemical supply unit 17 in the nitrogen removing apparatus 10 described above will be described.

  In the nitrogen removing device 10, as described above, when the electrolytic treatment is performed on the water to be treated 20 containing the component to be removed such as oxidized nitrogen in the electrolytic bath 11, the treatment is performed between the electrolytic bath 11 and the adjustment bath 13. Circulate water 20. At this time, with the circulation of the water to be treated 20, the measurement of the pH of the water to be treated 20 by the measurement unit 15 provided on the circulation path, and the oxidized nitrogen in the water to be treated 20 by the nitrogen concentration measurement unit 16. Nitrogen concentration is measured. In the nitrogen removing device 10, the removal process of oxidized nitrogen is performed based on the measurement result of the concentration of oxidized nitrogen measured at this time and the change in the amount of the pH adjuster used according to the measurement result of the pH of the water to be treated 20. The supply of chemicals (hypochlorous acid scavenger) required for the treatment is controlled.

[When supply control is performed from the measurement result of nitrogen concentration]
Specifically, when performing supply control of a chemical | medical agent based on the measurement result of a density | concentration of oxidized nitrogen, first, the oxidized nitrogen contained in the to-be-processed water 20 taken from the adjustment tank 13 by the nitrogen concentration measurement part 16 Measure the nitrogen concentration.

  Next, the measurement result is transmitted to the control unit 18 connected to the nitrogen concentration measurement unit 16, and the control unit 18 uses the measured nitrogen concentration to decrease the nitrogen concentration with the passage of processing time ( Decomposition rate) is calculated. Subsequently, the control unit 18 determines whether or not the calculated decrease rate (decrease slope) of the nitrogen concentration is equal to or less than a predetermined value (set value). That is, it is determined whether or not the decomposition rate of oxidized nitrogen is decreasing.

  When the controller 18 determines that the nitrogen concentration decrease rate is equal to or lower than a predetermined set value and the nitrogen removal process has not yet been performed sufficiently, the decrease rate of the oxidized nitrogen concentration is increased. Since it is small and sufficient processing has not yet been achieved to the target concentration, it is determined that the reaction mechanism has changed in the cathode 11b due to the decrease in the scavenger concentration. That is, it is determined that the reaction mechanism in the cathode 11b has shifted to the phase II stage in Table 1 above. Based on the determination that the reaction mechanism has changed, the control unit 18 transmits a signal instructing the drug supply unit 17 to supply additional drug (excess hypochlorous acid scavenger), and the surplus The chemicals are additionally supplied from the hypochlorous acid scavenger supply unit 17b to the water to be treated 20 accommodated in the adjustment tank 13.

[When supply control of the drug is performed based on changes in the amount of the pH adjuster used based on the pH measurement results]
Next, when performing supply control of a chemical | medical agent based on the change of the usage-amount of the pH adjuster by the pH measurement result of the to-be-processed water 20, first of the to-be-processed water 20 taken out from the electrolytic vessel 11 in the measurement part 15 is shown. The pH is measured. And the measurement result of the pH is always transmitted to the control unit 18, and in the control unit 18, the pH adjusting agent from the pH adjusting agent supply unit 17a constituting the drug supply unit 17 based on the pH measurement result. To control the supply (addition).

  At this time, the control unit 18 calculates the addition rate of the pH adjusting agent from the pH adjusting agent supply unit 17a that varies depending on the measurement result of the pH of the water to be treated 20 as the processing time elapses.

  Next, the control unit 18 determines whether or not the calculated addition rate (the slope of the usage amount of the pH adjusting agent) is equal to or greater than a predetermined value (set value). That is, it is determined whether or not the usage amount of the pH adjuster is increasing.

  On the other hand, in the nitrogen removing device 10, the nitrogen concentration measuring unit 16 measures the nitrogen concentration of the oxidized nitrogen contained in the treated water 20 taken from the adjustment tank 13, and constantly transmits the measurement result to the control unit 18. . In the control unit 18, based on the measurement result of the nitrogen concentration, whether or not the concentration of oxidized nitrogen in the water to be treated 20 is equal to or lower than the target value, that is, the nitrogen concentration is equal to or lower than the target processing value. Therefore, it is determined whether or not the nitrogen component to be treated is sufficiently removed.

  When the controller 18 determines that the addition rate of the pH adjuster is equal to or higher than a predetermined set value and the nitrogen removal process has not been sufficiently performed, the addition rate of the pH adjuster is increased. Thus, since sufficient processing has not yet been achieved to the target concentration, it is determined that the reaction mechanism has changed at the cathode due to the decrease in the scavenger concentration. That is, it is determined that the reaction mechanism at the cathode has shifted to the phase II in Table 1 above. Based on the determination that the reaction mechanism has changed, the control unit 18 transmits a signal instructing the drug supply unit 17 to supply additional drug (excess hypochlorous acid scavenger), and the surplus The chemicals are additionally supplied from the hypochlorous acid scavenger supply unit 17b to the water to be treated 20 accommodated in the adjustment tank 13.

  As described above, the nitrogen removal apparatus 10 is based on the measured measurement result of the oxidized nitrogen concentration in the treated water 20 and the change in the usage amount of the pH adjuster according to the measured pH value of the treated water 20. Thus, the main reaction mechanism at the cathode is estimated, and the supply of a chemical (hypochlorous acid scavenger) required for the removal of oxidized nitrogen is controlled according to the estimated reaction mechanism. According to the nitrogen removing apparatus 10 as described above, it is possible to accurately grasp the main reaction mechanism at the cathode in electrolytic treatment of oxidized nitrogen and perform efficient reaction control, and stably perform nitrogen such as oxidized nitrogen. Components can be removed.

<< 4. Examples >>
Hereinafter, specific examples of the present invention will be described. Note that the scope of the present invention is not limited to any of the following examples.

[Example 1]
In Example 1, nitrogen removal treatment by electrolytic reaction of oxidized nitrogen was performed using a nitrogen removal device (electrolytic treatment device) 10 shown in FIG. In addition, the electrolytic cell 11 is the same as that of the structure of FIG.

In the electrolytic cell 11, a commercially available insoluble electrode for soda electrolysis was used as the anode 11a, and an electrode (amorphous electrode) in which an iron-chromium alloy was coated in an amorphous state on the surface of a titanium plate was used as the cathode 11b. And the to-be-processed water containing nitrate nitrogen with a density | concentration of 5000 mg / L is accommodated in this electrolytic tank 11, a direct current is sent between the anode 11a and the cathode 11b from the DC power supply device 12, and the oxidized nitrogen is contained. The water to be treated was subjected to electrolytic treatment for 5 hours. In addition, the area of the energization part of the anode 11a and the cathode 11b was set to 0.01 m ^2, and the current amount to be energized was set to 12A.

  Here, a solution in which ammonium chloride was used as the surplus hypochlorous acid scavenger and 10000 mg / L was added as the ammonia nitrogen concentration after addition to the water to be treated having a nitrate nitrogen concentration of 5000 mg / L accommodated in the electrolytic cell 11. 600 ml was put into the adjustment tank 13 in the nitrogen removing apparatus 10 and subjected to electrolytic treatment while decomposing excess hypochlorous acid. Further, during the electrolytic treatment, the pH of the water to be treated fluctuates due to the electrolytic reaction, so that caustic soda as a pH adjusting agent is sequentially added to the adjustment tank 13 in order to cope with the fluctuation, and the pH of the water to be treated is Was adjusted to be maintained within a predetermined range.

  Further, during the electrolytic treatment, the water to be treated was taken in from the adjustment tank 13 by a pump, and the nitrogen concentration in the water to be treated was measured by the nitrogen concentration measuring unit 16. As a measuring method of the nitrogen concentration in the nitrogen concentration measuring unit 16, the activated water was applied to the water to be treated as a pretreatment, and the measurement was performed using an ultraviolet absorption photometry method with a center wavelength of 254 nm. The central wavelength of 254 nm is the wavelength that has been most commonly used in the past, and is characterized by absorption of oxidized nitrogen but no absorption of ammonia, but chloramine, a decomposition product of ammonia. The class also has absorption near this wavelength, and unless it is pretreated, it cannot interfere and accurately measure the nitrogen concentration. Therefore, in Example 1, activated carbon treatment was performed as a pretreatment, and only chloramines were selectively decomposed to prevent interference with the absorptiometer so that an accurate nitrogen concentration could be measured.

  FIG. 6 is a graph showing changes in nitrogen concentration during electrolytic treatment. The larger the negative slope in the graph of FIG. 6, the greater the rate of decrease in nitrogen concentration. In Example 1, it can be seen that the rate of decrease is clearly smaller after the point A in the graph, and the reduction efficiency at the cathode is decreased after the point A.

  Therefore, in Example 1, ammonium chloride, which is a surplus hypochlorous acid scavenger, was additionally added at point B in the graph. Then, the nitrogen concentration further decreased by the addition of this scavenger.

  From this result, the change in nitrogen concentration in the treated water is measured, and when the nitrogen concentration decrease rate has decreased, if the nitrogen concentration in the treated water has not reached the treatment target, an additional scavenger is added. By doing so, it was found that the concentration of oxidized nitrogen can be further reduced. Thus, by estimating the reaction mechanism at the cathode based on the change in the nitrogen concentration, it is possible to control the appropriate chemical addition and to remove the nitrogen component in the water to be treated extremely stably. I understood.

[Comparative Example 1]
In Comparative Example 1, the same test as in Example 1 was performed, and the change in nitrogen concentration was measured. However, in this comparative example 1, the scavenger was not additionally charged based on the decrease rate of the nitrogen concentration, and the electrolytic treatment was terminated when the ORP was measured and the ORP increased.

  FIG. 7 is a graph showing changes in nitrogen concentration and ORP in the water to be treated as the electrolytic treatment time elapses. As shown in the graph of FIG. 7, it can be seen that even if the ORP increases, the oxidized nitrogen has not reached the treatment target. It has been found that the increase in ORP can only indirectly detect that ammonia has been depleted, and cannot detect whether the reduction efficiency of the oxidized nitrogen concentration has decreased. Thus, when the reaction is controlled by ORP, the determination of additional scavenger addition cannot be made until ammonia is depleted, and the determination of additional scavenger addition is delayed compared to Example 1, and the nitrogen concentration is treated. It has been found that it takes extra time to process to the target value.

[Example 2]
In Example 2, the same test as in Example 1 was performed, and the change in nitrogen concentration was measured. In Example 2, the addition rate of caustic soda as a pH adjusting agent was measured in the course of the treatment, and the addition of scavenger was judged at the change point of the addition rate.

  FIG. 8 is a graph showing a change in nitrogen concentration and a change in addition rate of caustic soda during electrolytic treatment. As shown in the graph of FIG. 8, the timing at which the reduction efficiency at the cathode decreases as determined from the change in nitrogen concentration in the water to be treated, that is, the timing at which the rate of decrease in nitrogen concentration decreases (A in the graph of FIG. 7). It can be seen that the point) coincides with the timing at which the caustic soda addition rate increases. From this, it can be understood that the reduction efficiency was reduced from the addition rate of caustic soda.

  Therefore, in Example 2, ammonium chloride, which is a surplus hypochlorite scavenger, was additionally added when it was confirmed that the addition rate of caustic soda was increased (point B in the graph of FIG. 8). Then, the nitrogen concentration further decreased by the addition of this scavenger.

  From this result, the addition rate of caustic soda to be added to the treated water is measured, and when the addition rate increases, if the nitrogen concentration in the treated water does not reach the treatment target, an additional scavenger is added. Thus, it was found that the concentration of oxidized nitrogen can be further reduced. Thus, by estimating the reaction mechanism at the cathode based on the addition rate of the pH adjusting agent, it is possible to control the appropriate chemical addition, and to remove nitrogen components in the water to be treated extremely stably. I understood that I could do it.

  DESCRIPTION OF SYMBOLS 10 Nitrogen removal apparatus (electrolytic processing apparatus), 11 Electrolysis tank, 11a Anode, 11b Cathode, 12 DC power supply, 13 Adjustment tank, 14 Circulation mechanism, 14a, 14b Circulation piping, 15 Measurement part, 16 Nitrogen concentration measurement part, 17 Chemicals Supply unit, 17a pH adjuster supply unit, 17b Surplus hypochlorous acid scavenger supply unit, 18 control unit, 19a, 19b pump, 20 treated water, 31 columns, 32 concentration measurement unit, 33 pump

Claims (9)

A nitrogen removing method for removing nitrogen components in water to be treated by electrolysis without partitioning between an anode and a cathode by a diaphragm,
Measure the concentration of the nitrogen component in the water to be treated in the process of removing the nitrogen component,
Based on the measurement results of the concentration of the nitrogen component, the main reaction mechanism at the cathode is estimated,
A nitrogen removal method characterized by controlling the addition of a predetermined agent required for the nitrogen removal process according to the estimated reaction mechanism.   The nitrogen removal method according to claim 1, wherein ammonia or ammonium salt is added as a surplus hypochlorous acid scavenger that captures and decomposes surplus hypochlorous acid generated at the anode.   3. The nitrogen removal according to claim 2, wherein when the rate of decrease in the concentration of the nitrogen component with respect to the processing elapsed time becomes equal to or less than a predetermined value, control is performed to increase the amount of the excess hypochlorous acid scavenger added. Method. A nitrogen removing method for removing nitrogen components in water to be treated by electrolysis without partitioning between an anode and a cathode by a diaphragm,
Maintain the pH of the water to be treated in a predetermined range by adding a pH adjuster,
Based on the addition rate of the pH adjusting agent, the main reaction mechanism at the cathode is estimated,
A nitrogen removal method characterized by controlling the addition of a predetermined agent required for the nitrogen removal process according to the estimated reaction mechanism.   The nitrogen removal method according to claim 4, wherein ammonia or ammonium salt is added as a surplus hypochlorous acid scavenger that captures and decomposes excess hypochlorous acid generated at the anode.   6. The nitrogen removal method according to claim 5, wherein when the rate of addition of the pH adjusting agent with respect to the processing elapsed time becomes equal to or higher than a predetermined value, control is performed to increase the amount of excess hypochlorous acid scavenger added. .   The nitrogen removal method according to claim 6, wherein when the rate of addition of the pH adjusting agent with respect to the elapsed processing time has changed by a factor of two or more, control is performed to increase the amount of surplus hypochlorous acid scavenger added. . A nitrogen removing device that removes nitrogen components in water to be treated by electrolysis without partitioning between an anode and a cathode by a diaphragm,
An electrolytic cell comprising an anode and a cathode, and containing treated water;
A direct current power source for supplying a direct current between the anode and the cathode;
An adjustment tank for adjusting the quality of the treated water;
A circulation mechanism for circulating the water to be treated between the electrolytic bath and the adjustment bath;
A drug supply unit for supplying a drug required for nitrogen removal treatment;
A control unit for controlling the supply amount of the drug supplied from the drug supply unit,
Furthermore, a concentration measuring unit for measuring the concentration of the nitrogen component in the treated water is provided, and in the process of removing nitrogen, the concentration of the nitrogen component in the treated water taken from the adjustment tank is measured,
The control unit estimates a main reaction mechanism at the cathode based on the nitrogen concentration measured by the concentration measurement unit, and according to the estimated reaction mechanism, a predetermined drug required for the nitrogen removal process from the drug supply unit is estimated. A nitrogen removing apparatus characterized by controlling a supply amount. A nitrogen removing device that removes nitrogen components in water to be treated by electrolysis without partitioning between an anode and a cathode by a diaphragm,
An electrolytic cell comprising an anode and a cathode, and containing treated water;
A direct current power source for supplying a direct current between the anode and the cathode;
An adjustment tank for adjusting the quality of the treated water;
A circulation mechanism for circulating the water to be treated between the electrolytic bath and the adjustment bath;
A pH adjuster supply unit for adding a pH adjuster for maintaining the pH of the water to be treated in a predetermined range;
A drug supply unit that supplies a drug required for nitrogen removal treatment other than the pH adjuster;
A control unit for controlling the supply amount of the drug supplied from the drug supply unit,
The controller calculates the addition rate of the pH adjuster added from the pH adjuster supply unit, estimates the main reaction mechanism at the cathode based on the calculated addition rate of the pH adjuster, and the estimated reaction mechanism The nitrogen removal apparatus characterized by controlling the supply amount of the predetermined medicine required for the nitrogen removal processing from the medicine supply section.

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