Urea treatment method and device

Abstract

The present invention provides a urea treatment method and apparatus, which can optimize the addition amount of chlorine-based oxidants such as NaClO required for urea decomposition. The urea treatment method generates hypobromite ions by adding a bromide salt and a chlorine-based oxidant to the water to be treated containing urea, thereby decomposing the urea in the water to be treated to obtain treated water, including: step a), respectively obtaining the residual free bromine concentration and the residual free chlorine concentration in the treated water; and step b), controlling the addition amounts of the bromide salt and the chlorine-based oxidant based on the residual free bromine concentration and the residual free chlorine concentration. As the apparatus (1), it has a bromide salt addition unit (11), a chlorine-based oxidant addition unit (12), a urea decomposition tank (2), a residual chlorine meter (3), and a control device (4) for controlling the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit based on the above method.

Description

Technical Field

[0001] The present invention relates to a urea treatment method and apparatus for decomposing urea in water to be treated to obtain treated water. Background Art

[0002] Generally, after treating raw water such as industrial water, tap water, and well water using a pretreatment system, pure water (primary pure water) is obtained by treating it with a primary pure water system, and ultrapure water is produced by treating the primary pure water with a secondary pure water system (subsystem). When producing ultrapure water, a method is known in which sodium bromide (NaBr) and sodium hypochlorite (NaClO) are added to water to be treated, and the urea in the water to be treated is decomposed using the generated hypobromite ions.

[0003] Regarding such a method, Patent Document 1 discloses the following: measuring the free residual chlorine concentration in the effluent water of the urea decomposition step, and controlling the addition amount of hypochlorite based on the measured value. The purpose of this method is to stably and reliably decompose and remove urea without adding an excessive or insufficient amount of the agent required for decomposing urea even when the water to be treated contains reaction-inhibiting substances such as ammonia, and without increasing the treatment cost.

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2019-63768 Summary of the Invention

[0007] (Problems to be Solved by the Invention)

[0008] Patent Document 1 describes the measurement of "free residual chlorine concentration" using a residual chlorine meter. However, in the measured value obtained from the residual chlorine meter (the indication value of the residual chlorine meter), it actually includes not only the free chlorine concentration but also the free bromine concentration. Therefore, in the method disclosed in Patent Document 1 for determining the addition amount of NaClO based on the indication value of the residual chlorine meter, since the accurate free chlorine concentration is not measured, the addition of NaClO may sometimes not be appropriate.

[0009] An object of the present invention is to provide a urea treatment method and apparatus capable of optimizing the addition amount of a chlorine-based oxidant such as NaClO required for urea decomposition.

[0010] (Technical Means for Solving the Problems)

[0011] According to one aspect of the present invention, there is provided a urea treatment method in which bromide salt and a chlorine-based oxidant are added to water to be treated containing urea to generate hypobromite ions, thereby decomposing the urea in the water to be treated to obtain treated water. The urea treatment method includes:

[0012] Step a), respectively obtaining the residual free bromine concentration and the residual free chlorine concentration in the treated water; and

[0013] Step b), controlling the addition amounts of the bromide salt and the chlorine-based oxidant based on the residual free bromine concentration and the residual free chlorine concentration.

[0014] In addition, according to another aspect of the present invention, there is provided a urea treatment device, comprising: a bromide salt addition unit for adding a bromide salt to the water to be treated containing urea; a chlorine-based oxidant addition unit for adding a chlorine-based oxidant to the water to be treated; a urea decomposition tank for reacting the water to be treated with the bromide salt and the chlorine-based oxidant to obtain treated water; a residual chlorine meter for measuring the residual chlorine in the treated water; and

[0015] a control device that calculates the residual free chlorine concentration in the sample based on a first measurement value obtained by measuring a sample obtained from the treated water by the residual chlorine meter and a second measurement value obtained by measuring the sample after adding glycine to the sample, and controls at least one of the addition amount of the bromide salt added by the bromide salt addition unit and the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit based on the first measurement value, the second measurement value, and the residual free chlorine concentration.

[0016] (Advantages of the Invention)

[0017] It is possible to optimize the addition amount of the chlorine-based oxidant required for urea decomposition. Description of the Drawings

[0018] Figure 1 is a process flow chart showing a schematic structural example of the urea treatment device of the present invention.

[0019] Figure 2 is a process flow chart showing a schematic structural example of an ultrapure water production system equipped with the urea treatment device of the present invention.

[0020] Figure 3 is a graph showing the relationship between the residual free chlorine concentration, the residual free bromine concentration, and the reaction rate constant. Detailed Embodiments

[0021] One aspect of the present invention relates to a urea treatment method, which decomposes urea in the water to be treated by adding a bromide salt and a chlorine-based oxidant to the water to be treated containing urea to generate hypobromite ions, thereby obtaining treated water.

[0022] Figure 1 is a process flow chart showing a schematic structural example of the urea treatment device in the urea decomposition method of the present invention.

[0023] The urea treatment apparatus 1 includes: a bromide salt addition unit 11 that adds a bromide salt to the water to be treated containing urea; a chlorine-based oxidant addition unit 12 that adds a chlorine-based oxidant to the water to be treated; a urea decomposition tank 2 that stores the water to be treated added with the bromide salt and the chlorine-based oxidant, and causes the urea in the water to be treated to react with the bromide salt and the chlorine-based oxidant to obtain treated water; a residual chlorine meter 3 that measures the residual chlorine concentration of the treated water obtained through the above reaction; and a control unit 4 that controls the addition amounts of the bromide salt and the chlorine-based oxidant based on the measurement value of the residual chlorine meter 3. In Figure 1 this case, the water to be treated containing urea in the pipeline L1 is added with a bromide salt by the bromide salt addition unit 11 and a chlorine-based oxidant by the chlorine-based oxidant addition unit 12, and is supplied to the urea decomposition tank 2. The water to be treated in the urea decomposition tank 2 reacts with the bromide salt and the chlorine-based oxidant to decompose urea, and is discharged as treated water from the pipeline L4. A residual chlorine meter 3 for measuring the chlorine concentration (residual chlorine concentration) in the treated water is connected to the pipeline L4. The control unit 4 controls the addition amount of the bromide salt added by the bromide salt addition unit 11 and the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit 12 based on the residual free bromine concentration and the residual free chlorine concentration calculated according to the measurement value of the residual chlorine meter 3.

[0024] As the water to be treated, water containing urea can be used as appropriate, for example, raw water for manufacturing ultrapure water such as industrial water, tap water, and well water can be used as appropriate. The water to be treated contains, for example, about 10 to 200 μg / L of urea. The urea concentration in the treated water obtained by urea treatment is, for example, about 1 μg / L or less.

[0025] As the added bromide salt, there are sodium bromide (NaBr), potassium bromide (KBr), etc. In addition, as the chlorine-based oxidant, there are sodium hypochlorite (NaClO), sodium perchlorate (NaClO4), calcium hypochlorite (Ca(ClO)2), etc. Hereinafter, the case of using NaBr and NaClO for the urea decomposition reaction will be described as a representative.

[0026] Generally, a residual chlorine meter measures the free chlorine concentration (displayed as an indication value). However, in the case where the inventors add a bromide salt and a chlorine-based oxidant to the water to be treated containing urea to generate hypobromite ions, thereby decomposing the urea in the water to be treated to obtain treated water, free bromine is contained in the treated water. The residual chlorine meter displays the total value of the free bromine concentration and the free chlorine concentration as the residual chlorine concentration, so the accurate concentration of free chlorine is not displayed. Therefore, it has been found that if the addition amount of NaClO is controlled based on the measured value (indication value) of the residual chlorine meter, sometimes an appropriate amount of NaClO cannot be added. As a result, an appropriate amount of NaBr cannot be added either, which may cause effects such as time-consuming urea decomposition reaction. For this reason, in the present invention, the free bromine concentration and the free chlorine concentration are calculated based on the measured value (indication value) of the residual chlorine meter, and the addition amounts of NaBr and NaClO are controlled based on the obtained free bromine concentration and free chlorine concentration.

[0027] It should be noted that in the present invention, the "residual chlorine concentration" refers to the value measured by the residual chlorine meter (the indication value of the residual chlorine meter), and the "residual free chlorine concentration" refers to the value obtained by subtracting the residual free bromine concentration from the value measured by the residual chlorine meter (the indication value of the residual chlorine meter, that is, the "free residual chlorine concentration" described in Patent Document 1).

[0028] Next, the control of the addition amounts of NaBr and NaClO in the present invention will be described.

[0029] Hypobromite ions are generated by the reaction of NaBr and NaClO, and urea is decomposed by the hypobromite ions.

[0030] The reaction for generating hypobromite ions (BrO － ) from NaBr and NaClO is considered to follow the following formula.

[0031] NaBr + NaClO → 2Na + + BrO － + Cl － (Formula 1)

[0032] The urea decomposition reaction based on hypobromite ions is considered to follow the following formula.

[0033] (NH2)2CO + 3BrO － → N2 + CO2 + 2H2O + 3Br － (Formula 2)

[0034] Based on Formula 1 and Formula 2, the following formula is derived.

[0035] (NH2)2CO + 3NaClO → N2 + CO2 + 2H2O + 3Na + + 3Cl － (Formula 3)

[0036] It can be considered that the concentration of BrO remains unchanged and is substantially constant before and after the urea decomposition reaction. － The concentration remains unchanged and is substantially constant.

[0037] When NaBr and NaClO are added to the water to be treated containing urea and reacted, and a part of the treated water obtained is collected as sample water and measured by a residual chlorine meter, the measured value (indicated value) is obtained as the total value of the free bromine concentration and the free chlorine concentration. On the other hand, a given compound (a compound having an amino group) is added to the sample water to convert the free chlorine in the sample water into combined chlorine. Since the residual chlorine meter cannot detect combined chlorine as a measured value, the value (indicated value) obtained by measuring the sample water added with the given compound using the residual chlorine meter is the free bromine concentration. Thus, the difference (A - B) between the measured value A (the total value of the free bromine concentration and the free chlorine concentration) of the residual chlorine meter of the sample water before adding the given compound and the measured value B (the free bromine concentration) of the residual chlorine meter of the sample water after adding the compound having an amino group becomes the free chlorine concentration in the treated water.

[0038] The given compound is not particularly limited as long as it is a compound that converts free chlorine into combined chlorine. For example, compounds having an amino group can be cited. As the compound having an amino group, any organic compound having an amino group in its molecular structure can be used. For example, amino acids such as alanine, arginine, asparagine, aspartic acid, cysteine, cystine, glutamine, glutamic acid, glycine, histidine, isoflavone, chrysin, lysine, methionine, phenylalanine, proline, serine, threonine, theanine, ornithine, tryptophan, tyrosine, valine, and aminoalkylsulfonic acids such as taurine can be cited. Among them, from the aspect of high reactivity with hypochlorous acid or its salts, glycine, arginine, asparagine, glutamine, lysine, phenylalanine, proline, serine, and taurine are preferably used, and glycine, which is particularly easy to obtain and easy to operate, is more preferably used. The coexisting compound having an ammonium salt or an amino group can be in a solution state such as an aqueous solution, or in a solid state or a powder state.

[0039] As a specific method, the method described in Japanese Patent Laid-Open No. 2019-100781 can be used.

[0040] In addition, the residual free bromine concentration can also be obtained, for example, by measuring using a bromine measuring device such as the portable bromine meter HI96716 manufactured by Hanna Instruments.

[0041] In the present invention, the addition amounts of NaBr and NaClO are controlled based on the free bromine concentration and the free chlorine concentration thus obtained.

[0042] When controlling the addition amounts of NaBr and NaClO, first, a plurality of preliminary tests are conducted in which the addition amounts of NaBr and NaClO added to the water to be treated are changed as parameters to decompose urea. In each preliminary test, the reaction rate constant of the urea decomposition reaction is calculated based on the change in urea concentration over time. The reaction rate constant can be calculated by a general method, for example, it can be calculated according to the following formula.

[0043] C = C0e -kt (Equation 4)

[0044] Wherein,

[0045] t: The reaction time of the urea decomposition reaction

[0046] C: The urea concentration in the water to be treated (reaction solution) at the reaction time t

[0047] C0: The initial urea concentration of the water to be treated (the urea concentration at the reaction time t = 0)

[0048] k: The reaction rate constant of the urea decomposition reaction.

[0049] However, if the addition amounts of NaBr and NaClO added to the water to be treated change, the reaction rate constant k will also change. In other words, the reaction rate constant k is determined according to the addition amounts of NaBr and NaClO.

[0050] Previously, obtain the relationship (such as in the form of a graph or mathematical formula) between the reaction rate constant calculated according to Equation 4 and the free bromine concentration and free chlorine concentration obtained by the above method.

[0051] In addition, set the target urea concentration (target urea concentration) of the treated water and the target reaction time (target reaction time), and calculate the reaction rate constant (target reaction rate constant) required to reach the target urea concentration at the target reaction time based on the urea concentration of the water to be treated. Obtain the residual free bromine concentration (target residual free bromine concentration) and residual free chlorine concentration (target residual free chlorine concentration) of the treated water in the calculated target reaction rate constant according to the above relationship.

[0052] Therefore, as long as NaBr and NaClO are added in such a way that the residual free bromine concentration and residual free chlorine concentration in the treated water become the target residual free bromine concentration and target residual free chlorine concentration, respectively.

[0053] Specifically, the amount of NaBr added is increased or decreased by an amount corresponding to the difference between the residual free bromine concentration in the treated water obtained from the measured value (indicated value) based on the residual chlorine and the target residual free bromine concentration. Further, the amount of NaClO added is increased or decreased by an amount corresponding to the difference between the required free chlorine addition amount necessary to achieve the target residual free chlorine concentration after the increase or decrease in the amount of NaBr added and the initial free chlorine addition amount.

[0054] The target residual free bromine concentration can be set based on the relationship among the above reaction rate constant, free bromine concentration, and free chlorine concentration.

[0055] At this time, the required free chlorine addition amount can be obtained by adding the sum of the target residual free chlorine concentration and the value obtained by multiplying the target residual free bromine concentration by a given value within the range of 1.2 to 1.4 (1.54 to 1.80 when converted to Br － . The initial free chlorine addition amount can be obtained by adding the sum of the residual free chlorine concentration at this time and the value obtained by multiplying the residual free bromine concentration in the treated water before the change in the NaBr addition amount by a given value within the range of 1.2 to 1.4.

[0056] Accordingly, the accurate residual free bromine concentration and residual free chlorine concentration in the treated water can be grasped, and an appropriate amount of NaBr (bromide salt) and NaClO (chlorine-based oxidant) can be added during urea decomposition.

[0057] As described above, before and after the urea decomposition reaction, the BrO － concentration is theoretically unchanged, and thus is considered to be substantially constant, and the amount of NaBr added to the water to be treated can be made constant. In this case, from the viewpoint of promoting the urea decomposition reaction, the amount of NaBr added is preferably 0.5 mg / L or more, more preferably 1 mg / L or more, and further preferably 2 mg / L or more. Additionally, the amount of NaBr added is preferably 3 mg / L or less. Even if it exceeds 3 mg / L, the urea decomposition promotion effect does not increase significantly.

[0058] Further, the value obtained by multiplying the addition concentration of NaBr to the water to be treated by a given value within the range of 0.5 to 0.7 (0.64 to 0.90 when converted to Br － can be used as the residual free bromine concentration in the treated water.

[0059] The pH during the urea decomposition reaction is preferably 5 to 6.5. Compared with the case where the pH is neutral, there is a tendency for the urea decomposition rate to be faster. In addition, in the case of an alkaline pH of around 9, for example, compared with the neutral case, there is a tendency for the urea decomposition rate to be faster, but in this case, in order to perform the subsequent treatment (such as coagulation treatment) of the urea treatment, it is sometimes necessary to readjust the pH to neutral again. If urea decomposition is carried out at around pH 5 to 6.5, compared with the case carried out under alkaline conditions, it is possible to easily perform the pH readjustment to make the pH neutral, or the pH readjustment is not required. In order to decompose urea at a desired pH, an appropriate pH regulator can be added to the water to be treated as needed.

[0060] The urea decomposition reaction can be carried out at normal temperature (for example, around 20 °C) and normal pressure (for example, around 1 atmosphere).

[0061] For example, as Figure 2 shown, the urea treatment device of the present invention can be used as a pretreatment for an ultrapure water production system. Figure 2 The ultrapure water production system shown consists of a pretreatment system, a primary pure water system, and a subsystem. In the pretreatment system, coagulation filtration is carried out by a coagulation filtration device. In the primary pure water system, the filtered water pretreated by the activated carbon device removes TOC and residues, and then the ion exchange device removes ions to generate demineralized water, and then the reverse osmosis membrane device removes ions and TOC to generate RO water. Then, after the ion exchange device further removes ions, the membrane degassing device removes dissolved oxygen to produce primary pure water. In the subsystem, through the decomposition of TOC by the ultraviolet (UV) oxidation device for the primary pure water, the removal of ions by the non-regenerative ion exchange device (CP: Cartridge Polisher), the removal of dissolved oxygen by the membrane degassing device, and the removal of fine particles by the ultrafiltration membrane (UF) device, ultrapure water is produced and sent to the point of use.

[0062] In Figure 2 the ultrapure water production system, the urea treatment device is provided upstream of the coagulation filtration device in the pretreatment system, but it can also be provided in the primary pure water system or the subsystem.

[0063] Examples

[0064] Hereinafter, the present invention will be described in detail using examples, but the present invention is not limited thereto.

[0065] (Example 1)

[0066] Urea was added to the tap water in Sagamihara City, Kanagawa Prefecture, Japan so that the urea concentration became 100 μg / L and it was used as the water to be treated. It should be noted that hereinafter, unless otherwise specified, during the urea decomposition reaction, the temperature was 20 °C, the pressure was approximately atmospheric pressure, and the pH was 6. Hydrochloric acid was appropriately added as a pH regulator.

[0067] Preliminary test

[0068] As a preliminary test, while changing the NaBr addition amount and the NaClO addition amount as parameters in the water to be treated, multiple reaction tests were carried out.

[0069] In each reaction test, the change over time of the urea concentration C in the water to be treated was investigated, and the reaction rate constant k was calculated. It should be noted that the initial time of the change over time was the time when NaBr and NaClO were added (reaction time t = 0). In the measurement of the urea concentration, a urea meter (LC: LC800 manufactured by GL Science, MSMS: 3200Q TRAP manufactured by AB SCIEX) was used. In addition, a residual chlorine meter (trade name: Portable Digital Residual Chlorine Meter HI96711C, manufactured by Hanna Instruments Japan Co., Ltd.) was used for the residual free chlorine concentration of the treated water after the reaction. In addition, when measuring the residual free bromine concentration of the treated water, glycine (an aqueous solution obtained by dissolving 10 g of glycine in 100 mL of water) was added to the water to be measured so that the glycine concentration was 1 g / L, and it was stirred for 1 minute.

[0070] The relationship between the residual chlorine concentration and the residual free bromine concentration obtained in the preliminary test and the reaction rate constant is shown in Figure 3 .

[0071] According to Figure 3 , when the residual free bromine concentration is 1.2 mg / L or more, the reaction rate constant is constant. That is, it does not affect the decomposition rate of urea. Therefore, its minimum value of 1.2 mg / L can be set as the target residual free bromine concentration. In addition, during the process of this preliminary test, it was confirmed that the value obtained by multiplying the addition concentration of NaBr added to the water to be treated by a given value in the range of 0.5 to 0.7 (0.64 to 0.90 when converted to Br － ) can be adopted as the residual bromine concentration in the treated water.

[0072] Urea decomposition treatment experiment

[0073] NaBr and NaClO were added to the water to be treated (urea concentration: 100 μg / L, 0.6 L) contained in a water tank so that their concentrations in the water to be treated became 1 mg / L, respectively, and the mixture was stirred for 3 minutes. The treated water after the reaction was measured using a residual chlorine meter (trade name: Portable Digital Residual Chlorine Meter HI96711C, manufactured by Hanna Instruments Japan Co., Ltd.). As a result, the indicated value of the residual chlorine meter was 0.8 mg / L.

[0074] Next, glycine (manufactured by Kanto Chemical Co., Inc.) at 1 g / L was added to the treated water. After stirring for 1 minute, the treated water was measured with a residual chlorine meter. As a result, the indicated value of the residual chlorine meter was 0.6 mg / L. Therefore, the concentration of residual free bromine in the treated water was 0.6 mg / L. The sum of the concentration of residual free bromine and the concentration of residual free chlorine in the treated water was 0.8 mg / L. Therefore, the concentration of residual free chlorine in the treated water was 0.8 - 0.6 = 0.2 mg / L.

[0075] Here, in order to decompose urea so that the urea concentration changes from 100 μg / L to 1 μg / L in 3 hours, according to Equation 4 (C = C0e -kt ), the reaction rate constant k needs to be 1.54 h -1 . Moreover, according to Figure 3 the graph of Curve C1, when the concentration of residual free bromine is 1.2 mg / L or more, the reaction rate constant is constant (i.e., it does not affect the decomposition rate of urea). Therefore, when setting its minimum value, i.e., 1.2 mg / L, as the target residual free bromine concentration, the target residual free chlorine concentration when the reaction rate constant k = 1.54 h -1 is 0.9 mg / L. From this, it can be known that the concentration of free bromine in the treated water is 0.6 mg / L and the concentration of free chlorine is 0.2 mg / L. Therefore, in order to decompose urea so that the urea concentration changes from 100 μg / L to 1 μg / L in 3 hours, it is necessary to add NaBr and NaClO so that the concentration of free bromine in the treated water is 1.2 - 0.6 = 0.6 mg / L and the concentration of free chlorine increases by 0.9 - 0.2 = 0.7 mg / L.

[0076] In addition, when wanting to suppress the amount of NaBr to be added, according to Figure 3 the graph of Curve C2, when the residual free bromine concentration is set to 0.6 mg / L, the reaction rate constant k = 1.54 h -1The residual free chlorine concentration at that time was 3.3 mg / L. It can be seen from this that the free bromine concentration in the treated water was 0.6 mg / L and the free chlorine concentration was 0.2 mg / L. Therefore, in order to decompose urea in such a way that the urea concentration changes from 100 μg / L to 1 μg / L in 3 hours, it is necessary to add NaBr and NaClO so that the free bromine concentration in the treated water is 0.6 - 0.6 = 0 mg / L, and the free chlorine concentration increases by 3.3 - 0.2 = 3.1 mg / L.

[0077] Next, the addition amounts of NaBr and NaClO were increased in the initially added 1 mg / L of NaBr and 1 mg / L of NaClO so that the residual free bromine concentration in the treated water obtained above increased by 0.6 mg / L and the residual free chlorine concentration increased by 0.7 mg / L. As a result, it took 2.9 hours for the urea concentration in the water to be treated to decompose from 100 μg / L to 1 μg / L. Similarly, the addition amounts of NaBr and NaClO were increased so that the residual free bromine concentration in the treated water obtained above increased by 0.4 mg / L and the residual free chlorine concentration increased by 4.8 mg / L. As a result, it took 3.0 hours for the urea concentration in the water to be treated to decompose from 100 μg / L to 1 μg / L.

[0078] (Example 2)

[0079] Similar to Example 1, urea was added to the tap water in Sagamihara City so that the urea concentration became 100 μg / L to prepare the water to be treated, and 6 mg / L of NaBr and 6 mg / L of NaClO were added to the water to be treated (urea concentration 100 μg / L, 0.6 L) contained in the water tank, and it was stirred for 3 minutes. The treated water after the reaction was measured using a residual chlorine meter (trade name: Portable Digital Residual Chlorine Meter HI96711C, manufactured by Hanna Instruments Japan Co., Ltd.). As a result, the indicated value of the residual chlorine meter was 4.8 mg / L.

[0080] Next, glycine (manufactured by Kanto Chemical Co., Inc.) was added to the treated water so that the glycine concentration became 1 g / L. After stirring for 1 minute, the treated water was measured with a residual chlorine meter. As a result, the indicated value of the residual chlorine meter was 3.6 mg / L. Therefore, the residual free bromine concentration in the treated water was 3.6 mg / L. The total value of the residual free bromine concentration and the residual free chlorine concentration in the treated water was 4.8 mg / L. Therefore, the residual free chlorine concentration in the treated water was 4.8 - 3.6 = 1.2 mg / L.

[0081] Here, in order to decompose urea in such a way that the urea concentration changes from 100 μg / L to 1 μg / L in 2.5 hours, according to Equation 4 (C = C0e -kt ), the reaction rate constant k needs to be 1.84 h -1 . And according to Figure 3Chart of curve C1. When the residual free bromine concentration is 1.2 mg / L or more, the reaction rate constant is constant (i.e., it does not affect the decomposition rate of urea). Therefore, when set to its minimum value of 1.2 mg / L, the reaction rate constant k = 1.84 h -1 The residual free chlorine concentration at this time is 2.1 mg / L. It can be seen from this that the residual free bromine concentration in the treated water is 3.6 mg / L and the residual free chlorine concentration is 1.2 mg / L. Therefore, in order to decompose urea in such a way that the urea concentration changes from 100 μg / L to 1 μg / L in 2.5 hours, it is necessary to reduce the addition amount of NaBr so that the free bromine concentration in the treated water is 1.2 - 3.6 = -2.4 mg / L, that is, reduce the addition amount of NaBr by an amount corresponding to a reduction of 2.4 mg / L, and add NaClO to increase the free chlorine concentration by 2.1 - 1.2 = 0.9 mg / L.

[0082] Next, in the initially added 6 mg / L of NaBr and 6 mg / L of NaClO, the addition amounts of NaBr and NaClO are increased or decreased so that the residual free bromine concentration in the treated water obtained above is reduced by 2.4 mg / L and the residual free chlorine concentration is increased by 0.9 mg / L. As a result, it took 2.4 hours for the urea concentration in the water to be treated to decompose from 100 μg / L to 1 μg / L.

[0083] (Example 3)

[0084] Similar to Example 1, urea was added to tap water in Sagamihara City to make the urea concentration 100 μg / L to prepare the water to be treated. 1 mg / L of NaBr and 1 mg / L of NaClO were added to the water to be treated (urea concentration 100 μg / L, 0.6 L) contained in a water tank, and it was stirred for 3 minutes. The treated water after the reaction was measured by a residual chlorine meter (trade name: Portable Digital Residual Chlorine Meter HI96711C, manufactured by Hanna Instruments Japan Co., Ltd.). As a result, the indicated value of the residual chlorine meter was 0.8 mg / L.

[0085] Here, since a value obtained by multiplying the addition concentration of NaBr to the water to be treated by a given value in the range of 0.5 to 0.7 (0.64 to 0.90 when converted to Br － can be used as the residual free bromine concentration in the treated water, for example, when a value obtained by multiplying the addition concentration of NaBr to the water to be treated by 0.6 is used as the residual free bromine concentration in the treated water, the residual free bromine concentration in the treated water is 0.6 mg / L, and the residual free chlorine concentration is 0.8 - 0.6 = 0.2 mg / L.

[0086] Here, in order to decompose urea in such a way that the urea concentration changes from 100 μg / L to 1 μg / L in 3 hours, according to Equation 4 (C = C0e -kt) The reaction rate constant k needs to be 1.54 h -1 . Moreover, according to Figure 3 the graph of curve C1, when the residual free bromine concentration is 1.2 mg / L or more, the reaction rate constant is constant (i.e., it does not affect the decomposition rate of urea). Therefore, when setting its minimum value of 1.2 mg / L as the target residual bromine concentration, the reaction rate constant k = 1.54 h -1 The target residual free chlorine concentration when k = 1.54 h is 0.9 mg / L. It can be seen from this that the free bromine concentration in the treated water is 0.6 mg / L and the free chlorine concentration is 0.2 mg / L. Therefore, in order to decompose urea in such a way that the urea concentration changes from 100 μg / L to 1 μg / L in 3 hours, it is necessary to add NaBr and NaClO so that the free bromine concentration in the treated water is 1.2 - 0.6 = 0.6 mg / L and the free chlorine concentration increases by 0.9 - 0.2 = 0.7 mg / L.

[0087] (Comparative Example 1)

[0088] 9 mg / L of NaBr and 3 mg / L of NaClO were added to the water to be treated (urea concentration 100 μg / L, 0.6 L) contained in a water tank and stirred for 3 minutes. The treated water after the reaction was measured by a residual chlorine meter (trade name: Portable Digital Residual Chlorine Meter HI96711C, manufactured by Hanna Instruments Japan Co., Ltd.). As a result, the indicated value of the residual chlorine meter was 1.8 mg / L. At this time, it took 4.5 hours to decompose urea in the water to be treated from 100 μg / L to 1 μg / L. Thus, although it was judged based on the indicated value of the residual chlorine meter of the treated water that there was sufficient chlorine for the decomposition of urea, in fact, since the indicated value of the residual chlorine meter represents the total value of the free bromine concentration of 1.8 mg / L and the free chlorine concentration of 0 mg / L, the amount of NaClO was insufficient and the urea decomposition took a long time.

[0089] 1 Urea treatment device

[0090] 2 Urea decomposition tank

[0091] 3 Residual chlorine meter

[0092] 4 Control unit

[0093] 11 Bromide salt addition unit

[0094] 12 Chlorine-based oxidant addition unit.

Claims

1. A urea treatment method, in which bromide salt and a chlorine-based oxidant are added to the water to be treated containing urea to generate hypobromite ions, thereby decomposing the urea in the water to be treated to obtain treated water. The urea treatment method includes: Step a), respectively obtaining the residual free bromine concentration and the residual free chlorine concentration in the treated water; And Step b), controlling the addition amounts of the bromide salt and the chlorine-based oxidant based on the residual free bromine concentration and the residual free chlorine concentration, The step a) includes: Step a1), using a residual chlorine meter to measure a sample collected from the treated water; Step a2), adding a compound having an amino group to the sample collected from the treated water, and measuring the obtained sample with a residual chlorine meter to obtain the residual free bromine concentration in the obtained sample; and Step a3), calculating the residual free chlorine concentration based on the difference between the measured value of the residual chlorine meter obtained in the step a1) and the residual free bromine concentration obtained in the step a2), The step b) includes: Step b1), obtaining the required residual free bromine concentration and the required residual free chlorine concentration of the treated water to achieve a preset target urea concentration and a target reaction time; Step b2), increasing or decreasing the addition amount of the bromide salt by an amount corresponding to the difference between the residual free bromine concentration obtained in the step a2) and the required residual free bromine concentration obtained in the step b1); and Step b3), increasing or decreasing the addition amount of the chlorine-based oxidant by an amount corresponding to the difference between the residual free chlorine concentration obtained in the step a3) and the required residual free chlorine concentration obtained in the step b1), Before the step a), by performing a plurality of preliminary tests for decomposing urea while changing the addition amounts of the bromide salt to the water to be treated and the chlorine-based oxidant as parameters, calculating the reaction rate constant of the urea decomposition reaction, thereby obtaining the correlation values among the residual free chlorine concentration, the residual free bromine concentration in the water to be treated, and the reaction rate constant of the urea decomposition reaction, The step b1) includes: Step b1-1), obtaining the reaction rate constant required to achieve the target urea concentration and the target reaction time based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time; and Step b1-2), using the correlation values to obtain the required residual free chlorine concentration of the treated water based on the reaction rate constant obtained in the step b1-1) and the residual free bromine concentration obtained in the step a).

2. A urea treatment method, in which bromide salt and a chlorine-based oxidant are added to the water to be treated containing urea to generate hypobromite ions, thereby decomposing the urea in the water to be treated to obtain treated water. The urea treatment method includes: Step a), respectively obtaining the residual free bromine concentration and the residual free chlorine concentration in the treated water; And Step b), controlling the addition amounts of the bromide salt and the chlorine-based oxidant based on the residual free bromine concentration and the residual free chlorine concentration, The step a) includes: Step a1): Measuring a sample collected from the treated water using a residual chlorine meter; Step a2): Adding a compound having an amino group to the sample collected from the treated water, and measuring the resulting sample with a residual chlorine meter to obtain the residual free bromine concentration in the resulting sample; and Step a3): Calculating the residual free chlorine concentration based on the difference between the measured value of the residual chlorine meter obtained in Step a1) and the residual free bromine concentration obtained in Step a2); Step b) includes: Step b1): Obtaining the required residual free bromine concentration and required residual free chlorine concentration of the treated water to achieve a preset target urea concentration and target reaction time; Step b2): Increasing or decreasing the addition amount of the bromide salt by an amount corresponding to the difference between the residual free bromine concentration obtained in Step a2) and the required residual free bromine concentration obtained in Step b1); and Step b3): Increasing or decreasing the addition amount of the chlorine-based oxidant by an amount corresponding to the difference between the residual free chlorine concentration obtained in Step a3) and the required residual free chlorine concentration obtained in Step b1); Before Step a), keeping the addition amount of the bromide salt to the water to be treated constant, and calculating the reaction rate constant of the urea decomposition reaction by performing a plurality of preliminary tests for decomposing urea by varying the addition amount of the chlorine-based oxidant as a parameter, thereby obtaining the correlation value among the residual free chlorine concentration, residual free bromine concentration, and reaction rate constant of the urea decomposition reaction in the water to be treated; Step b1) includes: Step b1-1): Obtaining the reaction rate constant required to achieve the target urea concentration and target reaction time based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time; and Step b1-2): Using the correlation value to obtain the required residual free chlorine concentration of the treated water based on the reaction rate constant obtained in Step b1-1) and the residual free bromine concentration obtained in Step a).

3. A urea treatment method for decomposing urea in water to be treated containing urea by adding a bromide salt and a chlorine-based oxidant to generate hypobromite ions, thereby obtaining treated water, the urea treatment method including: Step a): Separately obtaining the residual free bromine concentration and residual free chlorine concentration in the treated water; And Step b): Controlling the addition amounts of the bromide salt and the chlorine-based oxidant based on the residual free bromine concentration and the residual free chlorine concentration; Step a) includes: Step a1): Measuring a sample collected from the treated water using a residual chlorine meter; Step a2'): Calculating a value obtained by multiplying the addition amount of the bromide salt to the water to be treated by a given value within the range of 0.5 to 0.7 as the residual free bromine concentration in the sample; and Step a3): Calculating the residual free chlorine concentration based on the difference between the measured value of the residual chlorine meter obtained in Step a1) and the residual free bromine concentration calculated in Step a2').

4. The urea treatment method according to claim 1 or 2, wherein, The compound having an amino group is an amino acid or an aminoalkylsulfonic acid.

5. The urea treatment method according to claim 1 or 2, wherein the compound having an amino group is glycine, arginine, asparagine, glutamine, lysine, phenylalanine, proline, serine or taurine.

6. The urea treatment method according to claim 1 or 2, wherein the compound having an amino group is glycine.

7. The urea treatment method according to claim 3, wherein the step b) includes: step b1), obtaining the required residual free bromine concentration and required residual free chlorine concentration of the treated water for achieving a preset target urea concentration and target reaction time; step b2), increasing or decreasing the addition amount of the bromide salt by an amount corresponding to the difference between the residual free bromine concentration obtained in the step a2') and the required residual free bromine concentration obtained in the step b1); and step b3), increasing or decreasing the addition amount of the chlorine-based oxidant by an amount corresponding to the difference between the residual free chlorine concentration obtained in the step a3) and the required residual free chlorine concentration obtained in the step b1).

8. The urea treatment method according to claim 7, wherein before the step a), by performing a plurality of preliminary tests for decomposing urea by varying the addition amounts of the bromide salt to the water to be treated and the chlorine-based oxidant as parameters, calculating the reaction rate constant of the urea decomposition reaction, and thereby obtaining the correlation values among the residual free chlorine concentration, residual free bromine concentration, and reaction rate constant of the urea decomposition reaction in the water to be treated, the step b1) includes: step b1-1), obtaining the reaction rate constant required for achieving the target urea concentration and target reaction time based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time; and step b1-2), obtaining the required residual free chlorine concentration of the treated water using the correlation values based on the reaction rate constant obtained in the step b1-1) and the residual free bromine concentration obtained in the step a).

9. The urea treatment method according to claim 7, wherein before the step a), keeping the addition amount of the bromide salt to the water to be treated constant, performing a plurality of preliminary tests for decomposing urea by varying the addition amount of the chlorine-based oxidant as a parameter, calculating the reaction rate constant of the urea decomposition reaction, and thereby obtaining the correlation values among the residual free chlorine concentration, residual free bromine concentration, and reaction rate constant of the urea decomposition reaction in the water to be treated, the step b1) includes: step b1-1), obtaining the reaction rate constant required for achieving the target urea concentration and target reaction time based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time; and step b1-2), obtaining the required residual free chlorine concentration of the treated water using the correlation values based on the reaction rate constant obtained in the step b1-1) and the residual free bromine concentration obtained in the step a).

10. A urea treatment device having: A bromide salt addition unit that adds a bromide salt to the water to be treated containing urea; A chlorine-based oxidant addition unit that adds a chlorine-based oxidant to the water to be treated; A urea decomposition tank that supplies the water to be treated with the bromide salt and the chlorine-based oxidant to react to obtain treated water; A residual chlorine meter that measures the residual chlorine in the treated water; And A control device that calculates the residual free chlorine concentration in the sample based on a first measurement value obtained by measuring a sample obtained from the treated water by the residual chlorine meter and a second measurement value obtained by measuring the sample after adding a compound having an amino group to the sample, and controls at least one of the addition amount of the bromide salt added by the bromide salt addition unit and the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit based on the first measurement value, the second measurement value, and the residual free chlorine concentration; The control device obtains the required residual free bromine concentration and required residual free chlorine concentration of the treated water required to achieve a preset target urea concentration and target reaction time, and increases or decreases the addition amount of the bromide salt by an amount corresponding to the difference between the second measurement value and the required residual free bromine concentration, and increases or decreases the addition amount of the chlorine-based oxidant by an amount corresponding to the difference between the residual free chlorine concentration and the required residual free chlorine concentration; Before separately obtaining the second measurement value and the residual free chlorine concentration in the treated water, the control device calculates the reaction rate constant of the urea decomposition reaction by performing a plurality of preliminary tests of urea decomposition by changing the addition amounts of the bromide salt to the water to be treated and the chlorine-based oxidant as parameters, thereby obtaining the correlation values among the residual free chlorine concentration, the second measurement value, and the reaction rate constant of the urea decomposition reaction in the water to be treated; When the control device obtains the required residual free bromine concentration and required residual free chlorine concentration of the treated water required to achieve a preset target urea concentration and target reaction time, Based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time, the reaction rate constant required to achieve the target urea concentration and target reaction time is obtained; And based on the obtained reaction rate constant and the obtained second measurement value, the required residual free chlorine concentration of the treated water is obtained using the correlation value.

11. A urea treatment device having: A bromide salt addition unit that adds a bromide salt to the water to be treated containing urea; A chlorine-based oxidant addition unit that adds a chlorine-based oxidant to the water to be treated; A urea decomposition tank that supplies the water to be treated with the bromide salt and the chlorine-based oxidant to react to obtain treated water; A residual chlorine meter that measures the residual chlorine in the treated water; And A control device calculates the residual free chlorine concentration in the sample based on a first measurement value obtained by measuring a sample obtained from the treated water with the residual chlorine meter and a second measurement value obtained by measuring the sample after adding a compound having an amino group to the sample, and controls at least one of the addition amount of the bromide salt added by the bromide salt addition unit and the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit based on the first measurement value, the second measurement value, and the residual free chlorine concentration. The control device obtains the required residual free bromine concentration and the required residual free chlorine concentration of the treated water required to achieve a preset target urea concentration and target reaction time, increases or decreases the addition amount of the bromide salt by an amount corresponding to the difference between the second measurement value and the required residual free bromine concentration, and increases or decreases the addition amount of the chlorine-based oxidant by an amount corresponding to the difference between the residual free chlorine concentration and the required residual free chlorine concentration. Before separately obtaining the second measurement value and the residual free chlorine concentration in the treated water, the control device keeps the addition amount of the bromide salt to the water to be treated constant, and calculates the reaction rate constant of the urea decomposition reaction by performing a plurality of preliminary tests for decomposing urea by changing the addition amount of the chlorine-based oxidant as a parameter, so as to obtain the correlation value among the residual free chlorine concentration, the second measurement value, and the reaction rate constant of the urea decomposition reaction in the water to be treated. When the control device obtains the required residual free bromine concentration and the required residual free chlorine concentration of the treated water required to achieve a preset target urea concentration and target reaction time, Based on the urea concentration of the water to be treated before adding the bromide salt and the chlorine-based oxidant, the target urea concentration, and the target reaction time, the reaction rate constant required to achieve the target urea concentration and target reaction time is obtained. And based on the obtained reaction rate constant and the obtained second measurement value, the required residual free chlorine concentration of the treated water is obtained by using the correlation value.

12. A urea treatment device, comprising: A bromide salt addition unit that adds a bromide salt to the water to be treated containing urea; A chlorine-based oxidant addition unit that adds a chlorine-based oxidant to the water to be treated; A urea decomposition tank for reacting the water to be treated with the added bromide salt and the chlorine-based oxidant to obtain treated water; A residual chlorine meter that measures the residual chlorine in the treated water; And A control device calculates the residual free chlorine concentration in the sample based on the difference between the measurement value obtained by measuring a sample obtained from the treated water with the residual chlorine meter and the product value obtained by multiplying the addition amount of the bromide salt to the water to be treated by a given value within the range of 0.5 to 0.7, and controls at least one of the addition amount of the bromide salt added by the bromide salt addition unit and the addition amount of the chlorine-based oxidant added by the chlorine-based oxidant addition unit based on the measurement value, the product value, and the residual free chlorine concentration.

13. The urea treatment device according to claim 10 or 11, wherein The compound having an amino group is an amino acid or an aminoalkylsulfonic acid.

14. The urea treatment device according to claim 10 or 11, wherein the compound having an amino group is glycine, arginine, asparagine, glutamine, lysine, phenylalanine, proline, serine or taurine.

15. The urea treatment device according to claim 10 or 11, wherein the compound having an amino group is glycine.

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