JP2021025970A - Method and device for measuring urea concentration, and device for producing pure water - Google Patents

Abstract

To provide a method and a device for measuring urea concentration in which in a pure water production line or the like, the urea concentration in sample water containing a minute amount of urea can be measured by a simple method stably for a long period of time and with high accuracy.SOLUTION: Disclosed is a method for measuring the concentration of urea in sample water containing urea and water. This method includes the steps of: adding an oxidizer composed of hypobromous acid and/or salt thereof to the sample water and decomposing the urea into ammonium under a condition of pH 3 to 6 to obtain the decomposition liquid containing ammonium ions; obtaining the concentrated liquid by circulating the decomposition liquid through a reverse osmosis membrane device and concentrating the ammonium ions; and measuring the concentration of the ammonium ions in the concentrated liquid and calculating the urea concentration in the sample water.SELECTED DRAWING: Figure 1A

Description

The present invention relates to a method for measuring urea concentration, a device for measuring urea concentration, and a device for producing pure water.

In recent years, in pure water used in a semiconductor manufacturing process, it has been required to reduce the concentration of total organic carbon (TOC) to the order of ppb. There are various organic components (hereinafter referred to as "TOC components") that serve as carbon sources for TOC, but it is known that urea is a compound that is difficult to remove among these TOC components.

For example, in the production of pure water, there are reverse osmosis membrane devices, ion exchange devices, ultraviolet oxidizing devices, etc. as means for reducing TOC in raw water by means such as separation, adsorption, and decomposition. The removal rate of urea was not always sufficient when used alone or in combination. Therefore, in the production of pure water, a method of adding hypobromous acid or a salt thereof to the water to be treated has been studied as a method of decomposing and removing urea in the water to be treated such as raw water (for example, Patent Document 1, Patent Document 1, See 2).

In such a method for producing pure water, it has been required to accurately measure the urea concentration in raw water or the obtained pure water in order to efficiently decompose and remove urea. That is, if the amount of hypobromous acid or a salt thereof to be added can be adjusted according to the concentration of urea contained in the raw water or the obtained pure water, it can be expected that the productivity will be greatly improved. Therefore, there is a demand for a method of continuously and accurately measuring and monitoring the urea concentration of raw water and pure water flowing through a pure water production line.

Here, the urea concentration in the raw water used for producing pure water is approximately 0.3 mass ppm or less. Although the urea concentration of such a low concentration sample water can be measured by a liquid chromatograph mass spectrometer in an analysis room, for example, it is not suitable for monitoring in a pure water production line. A simple measuring device suitable for monitoring on a pure water production line has low sensitivity and cannot directly use sample water for measurement. Therefore, in Patent Document 3, the sample water is distilled and concentrated to measure and use the urea concentration, but the distillation concentration requires a large-scale apparatus and the recovery rate does not reach 100%, so that it is sufficient in terms of accuracy and practical use. is not it.

Patent Document 4 describes a concentration measurement method using the decomposition of urea by an enzyme (urease), but there is a problem in the stability and handleability of the enzyme, and stable concentration measurement for a long period of time is not possible. Have difficulty. Patent Document 5 describes a method of hydrolyzing urea in a sample water by pressurization and heating and analyzing the obtained ammonium ion by an ion chromatograph, but it is simple because an apparatus for pressurization and heating is required. There are difficulties in sex and economy. In Patent Document 6, urea in sample water is concentrated and analyzed by an electrodeionization device or a reverse osmosis membrane device, but in these devices, the recovery rate does not reach 100%, so that an error becomes large. Further, in Patent Document 7, hydrogen is adsorbed on a platinum group metal catalyst, urea is adsorbed on the hydrogen, and then ionization is performed when urea is desorbed to obtain a sample, and the sample is obtained from the low efficiency meter detection value of the sample. A method for determining the urea concentration has been described, but this has been a problem in terms of practicality.

Japanese Unexamined Patent Publication No. 2015-100733 Japanese Unexamined Patent Publication No. 09-094585 Japanese Unexamined Patent Publication No. 08-192154 Japanese Unexamined Patent Publication No. 2000-189196 Japanese Unexamined Patent Publication No. 2003-344379 Japanese Unexamined Patent Publication No. 2004-07299 Japanese Unexamined Patent Publication No. 2012-08137

The present invention has been made to solve the above-mentioned problems, and in a pure water production line or the like, the urea concentration in a sample water containing a trace amount of urea can be stably increased by a simple method for a long period of time. It is an object of the present invention to provide a method for measuring a urea concentration capable of measuring with accuracy and a device for measuring a urea concentration. Another object of the present invention is to provide a pure water production apparatus equipped with the urea concentration measuring apparatus, which can produce pure water having a sufficiently controlled urea concentration.

The method for measuring the urea concentration of the present invention is a method for measuring the urea concentration in a sample water containing urea and water, in which an oxidizing agent composed of hypobromic acid and / or a salt thereof is added to the sample water to pH 3 Under the conditions of ~ 6, the step of decomposing the urea into ammonia to obtain a decomposition solution containing ammonium ions, the step of circulating the decomposition solution to a back-penetrating membrane device and concentrating ammonium ions to obtain a concentrated solution, and the above-mentioned. It includes a step of measuring the concentration of ammonium ions in the concentrate and calculating the urea concentration in the sample water.

In the method for measuring the urea concentration of the present invention, it is preferable that the pH condition is 4 to 6 in the step of obtaining the decomposition liquid. The method for measuring the urea concentration of the present invention is preferably used when the urea concentration in the sample water is 0.3 mass ppm or less.

In the method for measuring the urea concentration of the present invention, when the urea concentration in the sample water is 0.3 mass ppm or less, the amount of the oxidizing agent added is such that urea is present in the sample water at a concentration of 0.3 mass ppm. It is preferable that the amount is such that the total amount of urea can be decomposed.

In the method for measuring the urea concentration of the present invention, the reverse osmosis membrane in the reverse osmosis membrane apparatus is preferably made of a polyamide-based material or a cellulose triacetate-based material.

The method for measuring the urea concentration of the present invention preferably includes a step of adding a reducing agent to the decomposition solution before the step of obtaining the concentrated solution.

In the method for measuring the urea concentration of the present invention, it is preferable to measure the concentration of ammonium ions in the concentrate by an ion-selective electrode method, and the concentration of ammonium ions in the concentrate is 0.05 mass ppm or more. It is preferable to concentrate so as to become.

The urea concentration measuring device of the present invention is a device for measuring the urea concentration in a sample water containing urea and water, and is an oxidizing agent that supplies an oxidizing agent composed of hypobromic acid and / or a salt thereof to the sample water. A supply unit, a reaction tank in which the urea in the sample water is reacted with the oxidizing agent to decompose it into ammonia to make the sample water into a decomposition solution containing ammonium ions, and conditions under which the reaction has a pH of 3 to 6. A pH adjusting unit that adjusts the pH of the sample water as performed below, and a back-penetrating membrane device to which the decomposition liquid is supplied and the ammonium ions in the supplied decomposition liquid are concentrated and discharged as a concentrated liquid. It also has a measuring unit that measures the concentration of ammonium ions in the concentrated solution and calculates the urea concentration in the sample water.

The urea concentration measuring device of the present invention preferably further includes a reducing agent supply unit that supplies a reducing agent to the decomposition liquid supplied to the reverse osmosis membrane device.

The pure water production apparatus of the present invention is a pure water production apparatus for producing pure water by treating raw water with a primary pure water system and a secondary pure water system, and includes the urea concentration measuring apparatus of the present invention.

In the pure water production apparatus of the present invention, the primary pure water system or the secondary pure water system may have a urea decomposition apparatus, and the urea concentration measuring apparatus may be provided before and / or after the urea decomposition apparatus. preferable.

According to the present invention, a method for measuring urea concentration and a urea concentration capable of measuring the urea concentration in a sample water containing a trace amount of urea with a simple method and stably for a long period of time in a pure water production line or the like. Measuring equipment can be provided. According to the present invention, it is possible to provide a pure water production apparatus capable of producing pure water in which the urea concentration is sufficiently controlled, further equipped with the urea concentration measuring apparatus.

It is a flow chart which shows an example of the measuring method of the urea concentration of an embodiment. It is a flow chart which shows another example of the measuring method of the urea concentration of an embodiment. It is a schematic block diagram which shows an example of the urea concentration measuring apparatus of embodiment. It is a schematic block diagram which shows another example of the urea concentration measuring apparatus of embodiment. It is a block diagram which shows the example of the pure water production apparatus of embodiment schematicly.

Hereinafter, an embodiment of the urea concentration measuring method, the urea concentration measuring device, and the pure water producing device of the present invention will be described with reference to the drawings. The present invention is not limited to these embodiments, and these embodiments can be modified or modified without departing from the spirit and scope of the present invention.

[Measurement method of urea concentration]
FIG. 1A is a flow chart showing an example of the method for measuring the urea concentration of the embodiment. The method for measuring the urea concentration of the embodiment is a method for measuring the urea concentration in a sample water containing urea and water, which comprises a urea decomposition step S1 (hereinafter, also referred to as “S1 step”) and ammonium ion (NH ₄ ^+). ) Concentration step S2 (hereinafter, also referred to as “S2 step”) and urea concentration measurement step S3 (hereinafter, also referred to as “S3 step”) are provided in that order. Hereinafter, the sample water containing urea and water is simply referred to as sample water.

In the urea decomposition step S1, an oxidizing agent composed of hypobromous acid and / or a salt thereof (hereinafter referred to as “oxidizing agent B”) is added to the sample water, and urea is decomposed into ammonia under the conditions of pH 3 to 6. This is a step of obtaining a decomposition solution containing ammonium ions. FIG. 1A shows that a pH adjuster is added in the S1 step. The pH of the sample water after the addition of the oxidant B depends on the pH of the sample water before the addition of the oxidant B and the amount of the oxidant B added. Usually, by adding a pH adjuster, the pH of the sample water after the addition of the oxidizing agent B is adjusted to 3 to 6. However, when the pH of the sample water after the addition of the oxidizing agent B is 3 to 6, the addition of the pH adjusting agent can be omitted.

The NH ₄ ⁺ concentration step S2 is a step of circulating the decomposition solution obtained in the S1 step to a reverse osmosis membrane apparatus and concentrating ammonium ions to obtain a concentrate. The urea concentration measuring step S3 is a step of measuring the concentration of ammonium ions in the concentrated solution obtained in the step S2 and calculating the urea concentration in the sample water.

As described above, conventionally, in the production of pure water, a method of decomposing and removing urea in water to be treated such as raw water by adding hypobromous acid or a salt thereof has been known. ^{In that case, the decomposition of urea by hypobromous acid ion (BrO −} ) produced from hypobromous acid or a salt thereof is generally represented by the following reaction formula (2).
(NH ₂ ) ₂ CO + 3BrO ⁻ → N ₂ + CO ₂ + 2H ₂ O + 3Br ⁻ … (2)

According to the literature (Journal of Synthetic Organic Chemistry vol 37 No. 7 1979, p25-38), the reaction of formula (2) is a kind of Hoffmann rearrangement reaction. According to this reaction mechanism, it is considered that hydrazine is produced as an intermediate when _{urea is decomposed into nitrogen (N 2).} This intermediate is alkaline, is dehydrogenated immediately hypochlorite or hypobromite, it is to be the final N ₂ as shown in Equation (2). For example, according to Patent Document 1, the preferable pH condition of the reaction is pH 9 or higher.

However, when the present inventors react urea with hypobromous acid ion (BrO ⁻ ), the mechanism is not clear, but the reaction of decomposing to _{N 2 does not proceed at pH 3 to 6.} We found that and completed the present invention. It is unclear what kind of reaction is occurring, but it is thought that ammonia is generated at the stage where the intermediate hydrazine is formed, or hydrazine is decomposed into ammonia. When the pH exceeds 6, the reaction of decomposing _{to N 2 is promoted.} On the other hand, if the pH is less than 3, the decomposition of urea itself will be delayed. By adjusting the pH to 3 to 6 in the S1 step in this way, urea is ^{decomposed into ammonia by hypobromous acid ion (BrO −} ), so that it is concentrated as ammonium ion in the reverse osmosis membrane device in the subsequent S2 step. It will be possible.

In the method for measuring the urea concentration of the present invention, when the hypobromous acid ion (BrO ⁻ ) is reacted with urea, the pH is adjusted to 3 to 6, so that the urea is _{not decomposed to N 2} and is intermediate. It has made it possible to control the decomposition reaction so that it stays at the stage of ammonia, which is the body. As a result, ammonia can be produced in high yield, and the urea concentration in the sample water can be measured with a simple method and stably for a long period of time with high accuracy.

In the method for measuring the urea concentration of the embodiment, any step may be provided in addition to the S1 step, the S2 step and the S3 step, if necessary. For example, it is preferable to have a step of adding a reducing agent to the decomposition solution obtained in the S1 step (hereinafter, also referred to as “reduction step S4” or “S4 step”) before the S2 step. FIG. 1B is a flow chart showing another example of the urea concentration measuring method of the embodiment having the S1 step, the S4 step, the S2 step, and the S3 step in that order in the urea concentration measuring method of the embodiment.

Hereinafter, each step included in the method for measuring the urea concentration of the embodiment will be described.

(Urea decomposition step S1)
The urea decomposition step S1 is a step of adding an oxidizing agent B to the sample water and decomposing urea into ammonia under the conditions of pH 3 to 6 to obtain a decomposition solution containing ammonium ions.

The sample water used in the method for measuring the urea concentration of the embodiment is water and sample water containing urea. The urea concentration measuring method of the embodiment is preferably used in applications where the measurement result of the urea concentration of the sample water is required to be quickly fed back. For example, in a pure water production apparatus, it is suitable for an application in which a urea concentration in a sample water containing a trace amount of urea is measured and the pure water production apparatus is controlled based on the result.

When applied to a pure water production apparatus, the sample water may be, for example, raw water supplied to the pure water production apparatus, intermediate treated water in the pure water production apparatus, or pure water discharged from the pure water production apparatus. When the urea concentration is measured using raw water as sample water, highly productive pure water can be produced by changing the setting of the urea removing device in the pure water production device according to the result. When the urea concentration is measured using pure water as sample water, by monitoring the quality of pure water, it is possible to recognize when to replace the urea removing device in the pure water production device, and it becomes easy to control the quality of pure water.

Examples of raw water include city water, well water, groundwater, industrial water, and recovered and pretreated water (recovered water) used in semiconductor manufacturing factories and the like. When these waters contain suspended substances, water from which the suspended substances in the water has been removed by a sand filtration device, a microfiltration device, or the like may be supplied to the pure water production device as raw water. The sample water used in the method for measuring the urea concentration of the embodiment preferably does not contain a suspended substance.

If ammonia is contained in the sample water, it will be decomposed and removed in advance because it affects the measured value of the obtained urea concentration. Alternatively, by measuring the ammonium ion concentration in the sample water before the urea decomposition step S1 and subtracting it from the ammonium ion concentration measured in the urea concentration measuring step S3, only the concentration of ammonia generated in the urea decomposition step S1 can be obtained. calculate.

In addition, when the sample water contains other oxidizing agents such as hypochlorous acid, components other than ammonia may be generated in urea decomposition. In addition, since there is a possibility of decomposing the generated ammonia, it is desirable to use sample water from which these oxidizing agents have been removed in advance. Furthermore, when the reducing agent is contained in the sample water, it reacts with hypobromous acid, so it is desirable to remove the reducing agent in advance or increase the amount of hypobromous acid added.

In the raw water, the urea concentration is usually 0.3 mass ppm or less, and the urea concentration in the intermediate treated water in the pure water production apparatus and the pure water discharged from the pure water production apparatus is lower than the urea concentration in the raw water. Therefore, when the method for measuring the urea concentration of the embodiment is applied to the control of the pure water production apparatus, the urea concentration of the sample water is 0.3 mass ppm or less. The method for measuring the urea concentration of the embodiment is a method suitable for measuring the urea concentration of the sample water having a urea concentration of 0.3 mass ppm or less.

The detection limit of the urea concentration in the method for measuring the urea concentration in the embodiment is about 1 ppb. If the detection limit is 1 ppb, it can be sufficiently used in quality control of pure water. The detection limit of urea concentration is more preferably 0.1 ppb. Although the detection limit depends on the accuracy of the analyzer, a low detection limit can be achieved by increasing the concentration ratio of the reverse osmosis membrane device.

Oxidizing agent B consists of hypobromous acid and / or a salt thereof. Examples of the salt of hypobromic acid include alkali metal salts such as lithium, sodium and potassium, alkaline earth metal salts such as magnesium, calcium and barium, and other metal salts. Among these, potassium salt and sodium salt are preferable, and sodium salt is particularly preferable. Oxidizing agent B may consist of one of these or two or more.

In the S1 step, if necessary, an oxidizing agent other than the oxidizing agent B may be added to the sample solution as long as the effects of the present invention are not impaired. Furthermore, other components may be added as long as the effects of the present invention are not impaired. For example, as the sodium hypobromite, as shown in the following reaction formula (3), those obtained by reacting sodium hypochlorite with sodium bromide can be used. In this case, the oxidizing agent B, sodium hypobromite, may be added to the sample solution together with sodium chloride, unreacted sodium hypochlorite, and sodium bromide produced by the reaction formula (3). ..
NaClO + NaBr → NaBrO + NaCl… (3)

The amount of the oxidizing agent B added to the sample water is preferably an amount capable of decomposing the entire amount of urea when urea is present in the sample water at a concentration of 0.3 mass ppm. Specifically, the number of moles of hypobromous acid ion (BrO ⁻ ) per 1 mole of urea is preferably 3.5 to 1000 mol, more preferably 4 to 300 mol. ^{If the number of moles of BrO −} per 1 mol of urea is less than 3.5 mol, urea may not be sufficiently decomposed, and if it exceeds 1000 mol, the load on the S2 step becomes large, which is not preferable. If the oxidizing agent B is excessively added, it causes oxidative deterioration of the reverse osmosis membrane when the decomposition liquid is distributed to the reverse osmosis membrane apparatus in the S2 step, which is not preferable. Therefore, in the method for measuring the urea concentration of the embodiment, preferably, a reducing step S4 is provided in which a reducing agent is added before the step S2 to reduce the excess oxidizing agent B.

The pH of the sample water rises by adding the oxidizing agent B. The pH of the sample water is generally in the range of 5 to 8 for raw water when applied to a pure water production apparatus, for example. As a method for adjusting the pH of the sample water after the addition of the oxidizing agent B to 3 to 6, a method of adding a pH adjusting agent to the sample water is common. added to the sample water pH additives that pH increases, prior to the addition of the oxidizing agent B the reaction formula (2) decomposition in the decomposition reaction of urea in that proceeds until N ₂ from the viewpoint of suppressing It is preferable to do so.

Acids are preferred as the pH regulator. Specific examples thereof include hydrochloric acid, nitric acid, sulfuric acid, acetic acid, citric acid and the like, and hydrochloric acid is preferable. The amount of the pH adjuster added is such that the pH of the sample water after the addition of the oxidizing agent B is 3 to 6.

The pH range is more preferably 4 to 6 from the viewpoint of suppressing side reactions. If the pH of the sample water after the addition of the oxidant B is less than 3, the amount of the acid added is too large and is wasted. When the pH exceeds 6, it becomes difficult to sufficiently suppress the _{decomposition from proceeding to N 2} in the decomposition reaction represented by the reaction formula (2).

When decomposing urea in the sample water into ammonia, it is preferable to stir the sample water. As a method of stirring, a normal stirring machine can be applied. The reaction temperature for decomposing urea in the sample water into ammonia is preferably 10 to 40 ° C, more preferably 15 to 30 ° C. Specifically, the reaction temperature is the temperature of the sample water (hereinafter, also referred to as “reaction solution”) to which the oxidizing agent B is added and the pH is adjusted to a predetermined range. The method for raising the temperature of the reaction solution to the above range is not particularly limited. For example, the temperature of the reaction solution may be adjusted to the above temperature range by using a heater or the like, or the temperature of the reaction solution may be raised to the above temperature range by circulating the reaction solution by a pump. For example, stirring can be performed at the same time as heating, which is preferable from the viewpoint of efficiency.

If the reaction temperature is lower than 10 ° C., the reaction rate will decrease. If the reaction temperature exceeds 40 ° C, when the sample water after the reaction is treated with the reverse osmosis membrane device, ammonia gas may flow out to the permeated water side and sufficient concentration may not be possible. The operation of lowering is required.

The reaction time when decomposing urea in the sample water into ammonia is the time during which all of the urea in the sample water is decomposed, and is appropriately adjusted according to the urea concentration in the sample water and the reaction temperature. When the room temperature is 25 ° C., if the urea concentration of the sample water is 0.3 mass ppm or less, the reaction time at room temperature can be approximately 0.5 to 5 hours. If the reaction time is less than 0.5 hours, the decomposition reaction of urea in the sample water does not proceed sufficiently, and the undecomposed urea content is not reflected in the finally obtained urea concentration, so that highly accurate measurement may not be possible. is there. On the other hand, since the decomposition reaction proceeds sufficiently in 5 hours, it is not preferable in terms of efficiency if the time is exceeded.

Whether or not all of the urea has been decomposed can be confirmed, for example, by changing the concentration of the oxidizing agent B with time. At the initial stage of the reaction, the concentration of oxidant B decreases with time, but when urea is sufficiently decomposed, the concentration of oxidant B becomes almost constant. The concentration of the oxidizing agent B can be analyzed by, for example, a normal DPD (diethyl paraphenylenediamine) method or the like.

In the method for measuring the urea concentration of the embodiment, a decomposition liquid obtained by decomposing 1 mol of urea in the sample water into 2 mol of ammonia is obtained by the step S1 described above. In the decomposition liquid contains urea was obtained by decomposing ammonia as ammonium ion NH ₄ ^+. Further, depending on the amount of the oxidizing agent B added, an excess of the oxidizing agent B that was not used in the decomposition reaction of urea in the sample water may remain in the decomposition liquid.

(Reduction step S4)
In the method for measuring the urea concentration of the embodiment, it is preferable to have a reduction step S4 in which a reducing agent is added to the decomposition liquid in order to reduce the oxidizing agent B remaining in the decomposition liquid before the S2 step. As a result, the load on the reverse osmosis membrane device in the S2 step can be reduced.

The reducing agent may be any one capable of inactivating the oxidizing ability of the oxidizing agent B, and examples thereof include sodium hydrogen sulfite, sodium bisulfite (SBS), sodium sulfite and the like.

The amount of the reducing agent added to the decomposition liquid depends on the amount of the residual oxidizing agent B. The amount of the reducing agent added to the decomposition liquid is an amount capable of sufficiently deactivating the oxidizing ability of the oxidizing agent B remaining in the decomposition liquid, for example, 1.2 to a molar ratio with respect to the residual amount of the oxidizing agent B. It can be doubled.

(NH ₄ ⁺ concentration step S2)
The NH ₄ ⁺ concentration step S2 is a step in which the decomposition solution after the S1 step or the S4 step is circulated to the reverse osmosis membrane device to concentrate ammonium ions to obtain a concentrate. NH ₄ ⁺ is to obtain a concentrate which is concentrated in S3 process, it is possible to measure easily and quickly NH ₄ ⁺ concentration.

When the decomposition liquid is circulated through the reverse osmosis membrane device, a concentrated liquid in which permeated water that permeates the reverse osmosis membrane and salts and ions that do not permeate the reverse osmosis membrane are concentrated can be obtained. In the S2 process, by using the reverse osmosis membrane apparatus can be a NH ₄ ⁺ in the decomposition solution and easily concentrated to obtain a concentrate.

Examples of the constituent material of the reverse osmosis membrane included in the reverse osmosis membrane device include cellulose triacetate-based, polyamide-based, polyvinyl alcohol-based, and polysulfone-based materials. The membrane shape is, but is not limited to, a sheet flat membrane, a spiral membrane, a tubular membrane, a hollow fiber membrane, and the like. Examples of the material of the reverse osmosis membrane, a high NH ₄ ⁺ of rejection, i.e. NH ₄ ⁺ the viewpoint of high recovery for recovering the concentrated solution side does not transmit, cellulose triacetate based materials, and polyamide The material is preferred. As the polyamide-based material, a crosslinked total aromatic polyamide-based material is preferable. The membrane shape is preferably a spiral membrane or a hollow fiber membrane.

In the method of the embodiment, the pH of the treated water is 3 to 6, which is suitable as the pH condition for using the cellulose triacetate membrane. Further, since the cellulose triacetate membrane is excellent in the oxidizing agent, there is an advantage in that the reverse osmosis membrane is not deteriorated even if the oxidizing agent B that cannot be completely decomposed by the reducing agent remains.

The desalting rate of the reverse osmosis membrane device is preferably 96 to 99.8%. The desalination rate is 25 ° C., pH = 7, water supply with NaCl concentration of 0.2% by mass has a water recovery rate of 15%, and the water supply pressure is standard pressure (for example, 1.5 MPa for a low-pressure reverse osmosis membrane device, ultra-low pressure reverse osmosis). In the case of a membrane device, it can be measured as the removal rate of sodium ions when water is passed through the reverse osmosis membrane (0.75 MPa). Water recovery rate of the reverse osmosis membrane apparatus, salts and ions, in particular in terms of concentrating the _NH ^{4 +} efficiently, preferably 60-98%, more preferably 80 to 95%.

Since a decomposition solution having a pH of 6 or less is supplied to the reverse osmosis membrane device, since ammonia is almost 100% ionized under this condition, it remains in the concentrated water almost 100% in the reverse osmosis membrane device. , There is almost no effect on the measured value in the concentration process.

A plurality of reverse osmosis membrane devices may be used if necessary. In this case, concentrated water is supplied to yet another reverse osmosis membrane device to concentrate. By doing so, it is possible to lower the required lower limit of quantification.

The reverse osmosis membrane device may be any of an ultra-low pressure type, a low pressure type, and a high pressure type reverse osmosis membrane device, and may be an ultra low pressure type or a low pressure type reverse osmosis membrane device from the viewpoint of concentration efficiency. preferable.

The ultra-low pressure type reverse osmosis membrane has an operating pressure of 0.4 MPa to 0.8 MPa, preferably 0.6 MPa to 0.7 MPa. The low-pressure type reverse osmosis membrane has an operating pressure of more than 0.8 MPa and less than 2.5 MPa, preferably 1 MPa to 1.6 MPa. The high-pressure type reverse osmosis membrane has an operating pressure of more than 2 MPa and 8 MPa or less, preferably more than 5 MPa and 6 MPa or less. The operating pressure of the ultra-low pressure type, low pressure type, and high pressure type reverse osmosis membrane devices can be distinguished by the design pressure (standard pressure) at the time of manufacturing each reverse osmosis membrane, but in reality, it is in the above range. It may be operated at a pressure other than.

The reverse osmosis membrane device may be composed of a two-stage reverse osmosis membrane device in which two reverse osmosis membrane devices are connected in series. Thus, NH ₄ ⁺ concentrate concentration was further increased is obtained. Further, by circulating the circulating concentrate to the reverse osmosis membrane apparatus 1 groups may obtain NH ₄ ⁺ concentrate heightened in concentration of.

As commercially available reverse osmosis membrane devices, RE2521-BLF manufactured by Toray Industries, HA3110PM manufactured by Toyobo Co., Ltd., ES20-D2 manufactured by Nitto Denko Co., Ltd., etc. can be used as commercial products.

In S2 process, the degree of _NH ^{4 +} concentration is, by the performance of the instrument for measuring the concentration of _NH ^{4 +} is used in S3 process. That is, the degree of _NH ^{4 +} concentration is, _NH ^{4 +} concentration in the concentrate resulting solution is performed until the _NH ⁴ instruments for measuring the concentration of ⁺ detection limit than that used in step S3 step.

Equipment for measuring the concentration of NH ₄ ⁺ is used in step S3 step is usually measurable simple instrument in situ, is used. As the concentration of _NH ^{4 +} in the detection limit corresponding to the instrument, for example, 0.05 mass ppm. Therefore, it is preferable that the _NH ^{4 +} concentration in the concentrated solution obtained in S2 step is not less than 0.05 mass ppm.

(Urea concentration measurement step S3)
The urea concentration measuring step S3 is a step of measuring the concentration of ammonium ions (NH ₄ ⁺ ) in the concentrated solution obtained in the S2 step and calculating the urea concentration in the sample water.

Methods for measuring the NH ₄ ⁺ concentration in the concentrate is not particularly limited. For example, a simple method that can measure on the spot or an NH ₄ ⁺ measuring instrument is used. As a method for measuring the NH ₄ ⁺ concentration in the concentrate, in particular, ion selective electrode method, indophenol blue absorptiometry, coulometric titration, and the like, ion-selective electrodes because it can be measured without reagent The method is preferred. In these methods, when measuring the _NH ^{4 +} concentration, the detection limit is generally 0.05 mass ppm.

_{Examples of the NH 4} ⁺ measuring instrument by the ion-selective electrode method include HC-200NH manufactured by Horiba Advanced Techno Co., Ltd. as a commercially available product.

From NH ₄ ⁺ concentration in the concentrate can be converted ammonia concentration in the concentrated solution. Then, in order to calculate the urea concentration in the sample water from the ammonia concentration in the concentrated solution, the following formula (4) can be used.
C _urea = C _NH3 / B × 60 / (2 × 17)… (4)
In the formula (4), C _urea indicates the urea concentration [mass ppm] in the sample water, C _NH3 indicates the ammonia concentration [mass ppm] in the concentrate, and B indicates the concentration ratio in the S2 step.

[Urea concentration measuring device]
The urea concentration measuring device of the embodiment is a device for measuring the urea concentration in the sample water containing urea and water, and FIG. 2 is a schematic configuration diagram showing an example of the urea concentration measuring device of the embodiment. FIG. 3 is a schematic configuration diagram showing another example of the urea concentration measuring device of the embodiment. The urea concentration measuring apparatus shown in FIGS. 2 and 3 is suitably used in the method for measuring the urea concentration of the embodiment.

The urea concentration measuring device 50A shown in FIG. 2 is a continuous measuring device that continuously measures the urea concentration in the continuously supplied sample water. The urea concentration measuring device 50B shown in FIG. 3 is a batch-type measuring device that measures the urea concentration of the sample water in a batch. FIG. 3A shows a state in which the urea decomposition step S1 is being executed, and FIG. 3B shows a state in which the NH ₄ ⁺ concentration step S2 and the urea concentration measurement step S3 are being executed. In FIGS. 2 and 3, the same configurations are designated by the same reference numerals, and the description of overlapping operations will be omitted.

The urea concentration measuring device 50A shown in FIG. 2 decomposes the sample water into ammonia by reacting the urea in the sample water with the oxidizing agent B and the oxidizing agent supply unit 1 that supplies the oxidizing agent B to the sample water. The reaction tank 3 for making a decomposition solution containing ammonium ions, the pH adjusting unit 2 for adjusting the pH of the sample water so that the above reaction is carried out under the conditions of pH 3 to 6, and the decomposition solution were supplied and supplied. It has a back-penetrating membrane device 5 that concentrates ammonium ions in the decomposition liquid and discharges it as a concentrated liquid, and a measuring unit 6 that measures the concentration of ammonium ions in the concentrated liquid and calculates the urea concentration in the sample water. ..

The urea concentration measuring device 50A further includes a reducing agent supply unit 4 that supplies a reducing agent to the decomposition liquid supplied to the reverse osmosis membrane device 5. However, in the urea concentration measuring device of the embodiment, the reducing agent supply unit is an arbitrary member.

Sample water is supplied to the measuring device 50A from the sample water supply unit 10 via the sample water supply pipe 20. As the sample water, the same sample water as the sample water described in the method for measuring the urea concentration of the embodiment can be applied. The sample water supply pipe 20 is arranged so as to connect the sample water supply unit 10 and the reaction tank 3. In the measuring device 50A, the pH adjusting unit 2 and the oxidizing agent supply unit 1 are arranged in the sample water supply pipe 20 in this order from the sample water supply unit 10 side. The order of the pH adjusting unit 2 and the oxidizing agent supply unit 1 does not matter, but it is preferable that the pH adjusting unit 2 and the oxidizing agent supply unit 1 are arranged in this order.

The pH adjuster 2 has a pH adjuster storage unit 12, a pH adjuster supply pipe 21 that connects the pH adjuster storage unit 12 and the sample water supply pipe 20, and a pH adjuster storage unit 12 by a measuring pump (not shown). It is a mechanism that can adjust the amount of the pH adjuster supplied to the sample water supply pipe 20 from.

The oxidant supply unit 1 has an oxidant storage unit 11, an oxidant supply pipe 22 that connects the oxidant storage unit 11 and the sample water supply pipe 20, and sample water from the oxidant storage unit 11 by a measuring pump (not shown). It is a mechanism that can adjust the amount of the oxidizing agent B supplied to the supply pipe 20. The amount of the pH adjuster and the oxidizing agent B added to the sample water can be the same as described in the method for measuring the urea concentration in the embodiment.

The reaction solution (sample water to which the oxidizing agent B is added and the pH is adjusted to a predetermined range) is supplied from the sample water supply pipe 20 to the reaction tank 3, and as described above, urea is N in the reaction formula (2). _As an intermediate reaction when the urea is decomposed into 2, the decomposition reaction of urea into ammonia by the oxidizing agent B is executed. The pH condition in the decomposition reaction is 3 to 6, preferably 4 to 6, as described in the method for measuring the urea concentration in the embodiment. Conditions such as reaction temperature and reaction time can be the same as described in the method for measuring urea concentration in the embodiment.

The material of the reaction tank 3 is not particularly limited as long as it is a material in the range of pH 3 to 6 and does not elute ammonia by the action of the oxidizing agent B. Examples of the material include general-purpose plastics such as polyvinyl chloride (PVC).

The measuring device 50A has a decomposition liquid supply pipe 23 for supplying the decomposition liquid obtained in the reaction tank 3 from the reaction tank 3 to the reverse osmosis membrane device 5. A reducing agent supply unit 4 is provided in the decomposition liquid supply pipe 23. The reducing agent supply unit 4 has a reducing agent storage unit 14, a reducing agent supply pipe 24 that connects the reducing agent storage unit 14 and the decomposition liquid supply pipe 23, and a decomposition liquid from the reducing agent storage unit 14 by a measuring pump (not shown). It is a mechanism that can adjust the amount of the reducing agent supplied to the supply pipe 23.

The decomposition liquid supply pipe 23 is provided with a pump 7 for adjusting the supply pressure of the decomposition liquid supplied to the reverse osmosis membrane device 5. The supply pressure of the decomposition liquid corresponds to the operating pressure of the reverse osmosis membrane device described above. As the reverse osmosis membrane device 5, the same reverse osmosis membrane device as described in the method for measuring the urea concentration of the embodiment can be used.

The reverse osmosis membrane device 5, a permeate discharge pipe 26 for discharging the permeate that has passed through the reverse osmosis membrane, salts and ions which does not transmit a reverse osmosis membrane, in particular for discharging concentrate NH ₄ ⁺ enriched The first concentrated liquid discharge pipe 25a is connected. The permeated water of the reverse osmosis membrane device 5 is discharged from the measuring device 50A via the permeated water discharge pipe 26. The first concentrated liquid discharge pipe 25a is branched into two in the middle, and one of them is connected to the measuring unit 6 as a second concentrated liquid discharge pipe 25b. The other is a third concentrated liquid discharge pipe 25c, which is connected to the decomposition liquid supply pipe 23. A switching valve (not shown) is provided at the branch portion of the first concentrated liquid discharge pipe 25a, and the flow of the concentrated liquid is set to the second concentrated liquid discharge pipe 25b side or the third concentrated liquid discharge pipe 25c side. It is a mechanism that can be adjusted.

When the concentrated solution discharged from the reverse osmosis membrane device 5 is concentrated to such an _{extent that the NH 4} ⁺ concentration can be measured in the measuring unit 6, for example, when the NH ₄ ⁺ concentration is 0.05 mass ppm or more. If there is, it is discharged to the second concentrated liquid discharge pipe 25b side and transferred to the measuring unit 6. Then, the measuring unit 6, in a manner similar to that described in the method of measuring urea concentration embodiments, NH ₄ ⁺ concentration in the concentrate is measured, urea concentration of the sample water is calculated. After the urea concentration is measured by the measuring unit 6, the concentrated liquid is discharged from the measuring unit 6 via the concentrated liquid discharge pipe 27.

On the other hand, when the concentrated solution discharged from the reverse osmosis membrane device 5 _{is not sufficiently concentrated so that the NH 4} ⁺ concentration can be measured in the measuring unit 6, for example, the NH ₄ ⁺ concentration is 0.05. If the mass is less than ppm, it is discharged to the third concentrated liquid discharge pipe 25c side and supplied to the reverse osmosis membrane device 5 again, and the obtained concentrated liquid is measured by the measuring unit 6 to the _{extent that the NH 4} ⁺ concentration can be measured. Concentration is repeated until it is concentrated.

The urea concentration measuring device 50B shown in FIG. 3 includes an oxidizing agent supply unit 1, a reaction tank 3, a pH adjusting unit 2, a reducing agent supply unit 4, a reverse osmosis membrane device 5, and a measuring unit 6. In the measuring device 50B, in the same manner as the measuring device 50A, the sample water whose pH is adjusted to a predetermined range by adding the oxidizing agent B to the reaction tank 3 from the sample water supply unit 10 via the sample water supply pipe 20. Is supplied as a reaction solution. In the measuring device 50B, the reducing agent supply unit 4 is connected to the reaction tank 3.

In the urea concentration measuring device 50B, the oxidizing agent supply unit 1 and the pH adjusting unit 2 may be connected to the reaction tank 3. In that case, the liquid supplied from the sample water supply unit 10 to the reaction tank 3 via the sample water supply pipe 20 is the sample water. That is, the urea concentration measuring device 50B may be a mechanism in which the sample water, the oxidizing agent B, and the pH adjusting agent are separately supplied to the reaction tank 3. The sample water, the oxidizing agent B, and the pH adjuster may be supplied to the reaction vessel 3 to prepare a reaction solution in the reaction vessel 3.

A first decomposition liquid supply pipe 23a for discharging the decomposition liquid from the reaction tank 3 is connected to the reaction tank 3. A pump 7 is arranged in the first decomposition liquid supply pipe 23a, and is further branched into two in the middle, and one of them is connected to another part of the reaction tank 3 as a second decomposition liquid supply pipe 23b. The other is a third decomposition liquid supply pipe 23c, which is connected to the reverse osmosis membrane device 5. A switching valve 31 is arranged at the branch portion of the first decomposition liquid supply pipe 23a, and whether the flow of the decomposition liquid is on the side of the second decomposition liquid supply pipe 23b or on the side of the third decomposition liquid supply pipe 23c. It is a mechanism that can be adjusted.

FIG. 3A is a diagram showing a state in which the measuring device 50B is executing the urea decomposition step S1 by setting the decomposition liquid to flow to the second decomposition liquid supply pipe 23b side by the switching valve 31. Is. _{FIG. 3B shows that the measuring device 50B performs the NH 4} ⁺ concentration step S2 and the urea concentration measurement step S3 by setting the decomposition liquid to flow to the third decomposition liquid supply pipe 23c side by the switching valve 31. It is a figure which shows the execution state.

Prior to the state shown in FIG. 3A, a step of supplying a predetermined amount of the reaction solution to the reaction vessel 3 is executed. After that, by operating the pump 7, the measuring device 50B is in the state shown in FIG. 3A, and the reaction liquid in the reaction tank 3 is the first decomposition liquid supply pipe 23a and the second decomposition liquid supply pipe 23b. It is circulated through. The pump 7 plays a role of heating the reaction solution by the heat generated by the operation of the pump at the same time as stirring the reaction solution. In this state, the urea decomposition step S1 is executed for a predetermined reaction time to obtain a decomposition liquid.

Then, for example, after confirming that all the urea has been decomposed, the reducing agent is supplied from the reducing agent supply unit 4 to the obtained decomposition liquid, and then the switching valve 31 is switched, and the measuring device 50B is shown in FIG. The state shown in b) is obtained. In the state shown in FIG. 3B, the decomposition liquid is supplied from the decomposition tank 3 through the first decomposition liquid supply pipe 23a and the third decomposition liquid supply pipe 23c, and the supply pressure is adjusted by the pump 7. It is supplied to the reverse osmosis membrane device 5. The permeated water of the reverse osmosis membrane device 5 is discharged from the measuring device 50B via the permeated water discharge pipe 26, and the concentrated liquid is supplied to the reaction tank 3 via the concentrated liquid discharge pipe 25.

The reaction tank 3 is connected to the measuring unit 6 and is designed so that _{the NH 4} ^{+ concentration of the liquid contained in the reaction tank 3 can be measured.} In the above, the concentrated liquid discharged from the reverse osmosis membrane device 5 is housed in the reaction tank 3.

When the concentrated solution discharged from the reverse osmosis membrane device 5 _{is not sufficiently concentrated so that the NH 4} ⁺ concentration can be measured in the measuring unit 6, for example, the NH ₄ ⁺ concentration is 0.05 mass ppm. If it is less than, it is supplied to the reverse osmosis membrane device 5 again, and the concentration is repeated until the obtained concentrate is concentrated to such an extent that the _{NH 4} ^{+ concentration can be measured in the measuring unit 6.}

On the other hand, if the concentrate is _NH ^{4 +} concentration is concentrated to an extent that can be measured in the measurement unit 6, for example, if _NH ^{4 +} concentration is more than 0.05 mass ppm, in the measuring section 6, the embodiment in the urea concentration the same method as described in the measurement method, NH ₄ ⁺ concentration in the concentrate is measured, urea concentration of the sample water is calculated. The reaction tank 3 is connected to the concentrate discharge pipe 27. After the urea concentration has been measured by the measuring unit 6, all the concentrated liquid is discharged from the reaction tank 3 via the concentrated liquid discharge pipe 27, and one urea concentration measurement is completed.

In order to measure the urea concentration of the next batch, first, a step of supplying a predetermined amount of the reaction solution to the reaction vessel 3 is executed. Then, by performing the same operations described with reference to FIGS. 3 (a) and 3 (b) above, the urea concentration measurement of the next batch is completed. In this way, by using the batch type urea concentration measuring device 50B, the urea concentration in the sample water can be measured intermittently only when necessary.

According to the continuous type urea concentration measuring device 50A, for example, it is preferably used in a pure water production device when it is necessary to constantly monitor the urea concentration. When the urea concentration monitoring may be intermittent in the pure water production apparatus, the batch type urea concentration measuring apparatus 50B can measure the urea concentration which is economically advantageous.

[Pure water production equipment]
The pure water production apparatus of the embodiment is a pure water production apparatus for producing pure water by treating raw water with a primary pure water system and a secondary pure water system, and includes the urea concentration measuring apparatus of the above embodiment. FIG. 4 is a block diagram schematically showing an example of a pure water production apparatus including a urea concentration measuring apparatus 50A.

The pure water production apparatus 100 shown in FIG. 4 processes raw water in the order of the primary pure water system 101 and the secondary pure water system 102 to produce pure water, and supplies the obtained pure water to the use point (POU). The primary pure water system 101 includes a urea decomposition device 110 and a urea concentration measuring device 50A in front of the urea decomposition device 110.

The primary pure water system 101 usually includes various water treatment devices for removing impurities from raw water, for example, an ultraviolet oxidizing device, an electric deionizing device, a degassing film device, and an ion exchange device, in addition to the urea decomposition device 110. Various water treatment devices including the urea decomposition device 110 are connected in series in the primary pure water system 101 via, for example, a water pipe 120 to be treated. The raw water supplied to the primary pure water system 101 is circulated in the primary pure water system 101 via the water to be treated pipe 120, and is sequentially treated by various treatment devices including the urea decomposition device 110.

In FIG. 4, the description of the water treatment device other than the urea decomposition device 110 is omitted, and a state in which various water treatment devices are connected in series by a single point chain wire is shown. In the primary pure water system 101, the urea concentration measuring device 50A is connected to the water to be treated pipe 120 by the sample water supply pipe 20 in a form branched from the water to be treated pipe 120. In the primary pure water system 101, the sample water supply unit 10 to the urea concentration measuring device 50A is a branch portion in the water to be treated pipe 120 to which the sample water supply pipe 20 is connected.

The permeated water of the reverse osmosis membrane device 5 discharged from the urea concentration measuring device 50A via the permeated water discharge pipe 26 and the concentrated liquid discharged through the concentrated liquid discharge pipe 27 are the primary pure water system after the urea concentration is measured. It is discharged to the outside of 101. The dotted line from the measuring device 50A to the urea decomposition device 110 indicates the flow of data of the urea concentration measurement result. In the urea decomposition device 110, the setting is changed based on the urea concentration measurement result sent from the measuring device 50A.

As the urea decomposition device 110, for example, the urea decomposition device described in Patent Document 1 can be used. The urea decomposition device 110 supplies an alkaline agent to the water to be treated via the water pipe 120 to be treated to adjust the pH of the water to be treated to 9 or more, and a hypobromous acid to the water to be treated. It is equipped with a hypobromous acid supply device for adding bromic acid and / or a salt thereof and a urea decomposition tank. The urea decomposition apparatus 110 is set so that _{the urea in the water to be treated is completely decomposed to N 2} by the decomposition reaction of the reaction formula (2).

The urea concentration measurement result sent from the measuring device 50A is used to control the addition amount of hypobromous acid and / or a salt thereof in the hypobromous acid supply device. Thereby, the economic burden on hypobromous acid and / or a salt thereof can be reduced.

The pure water production apparatus shown in FIG. 4 is an example, and the pure water production apparatus of the present invention is not limited to the above. The urea concentration measuring device 50B may be used instead of the urea concentration measuring device 50A, or may not be used together with the urea decomposition device. Further, the setting location of the urea concentration measuring device in the pure water production device is not limited. That is, the urea concentration may be measured using water from any part of the pure water production apparatus as sample water. The pure water production apparatus of the present invention is a pure water production apparatus capable of producing pure water in which the urea concentration is sufficiently controlled by using the urea concentration measuring apparatus of the present invention.

Next, an embodiment will be described. The present invention is not limited to the following examples. The urea concentration in the sample water was measured using the same measuring device as the urea concentration measuring device 50A shown in FIG. Examples 1 and 2 are examples, and examples 3 and 4 are comparative examples.

(specification)
Reaction tank 3; made of PVC, diameter 250 mm, height 2.5 m, capacity 120 L
Reverse osmosis membrane device 5; Nitto Denko, ES20-D2, reverse osmosis membrane material is a crosslinked total aromatic polyamide material pH measuring instrument; Horiba Advanced Techno Co., Ltd., CP-200
NH ₄ ⁺ concentration measuring instrument; manufactured by Horiba Advanced Techno Co., Ltd., HC-200NH, detection limit of 0.1 mass ppm as ammonia nitrogen concentration
pH adjuster; 4% by mass aqueous solution of hydrochloric acid (HCl) (hereinafter, "HCl aqueous solution") or 4% by mass aqueous solution of sodium hydroxide (NaOH) (hereinafter, "NaOH aqueous solution")
Oxidizing agent B; 5% by mass aqueous solution of sodium hypobromite (NaBrO) prepared by reacting sodium hypochlorite with sodium bromide according to the reaction formula (3) (hereinafter, "NaBrO aqueous solution").
Reducing agent; 10% by mass aqueous solution of sodium bisulfite (NaHSO _{3 ) (hereinafter, "NaHSO 3} aqueous solution")

[Example 1]
As the sample water, sample water 1 in which urea was dissolved in pure water so that the urea concentration was 0.3 mass ppm was prepared. The sample water 1 was passed through the reaction vessel 3 via the sample water supply pipe 20 at 60 L / h. In the pH adjusting unit 2, an aqueous HCl solution was added to the sample water 1 in the sample water supply pipe 20 in an amount such that the pH in the reaction solution was 5.3. The pH of the reaction solution was measured immediately before the sample water supply pipe 20 was connected to the reaction vessel 3, and the pH value was confirmed.

Next, in the oxidizing agent supply unit 2, an amount of NaBrO aqueous solution having a NaBrO concentration of 12 mass ppm in the obtained reaction solution was added to the sample water 1 after addition of the HCl aqueous solution. The reaction solution supplied to the reaction vessel 3 in this manner was allowed to stay in the reaction vessel 3 for 2 hours, and urea was decomposed into ammonia to obtain _{a decomposition solution containing ammonium ions (NH 4} ⁺ ) (step S1).

Then, in the reducing agent supply unit 4 to the decomposition solution obtained, NaHSO ₃ concentration of the decomposition solution after addition was added aqueous NaHSO ₃ in an amount of 10 mass ppm (S4 step). The addition of the NaHSO ₃ aqueous solution was performed on the decomposition liquid in the decomposition liquid supply pipe 23.

The decomposition liquid after the S4 step is supplied to the reverse osmosis membrane device 5 through the decomposition liquid supply pipe 23 at a supply amount of 60 L / h at a supply pressure of 5.5 MPa using a pump 7, and at a discharge amount of 6 L / h. _NH ^{4 +} is in decomposition solution to obtain a concentrate enriched (S2 step). The concentration ratio can be calculated to be 10 times from the relationship between the supply amount of the decomposition liquid and the discharge amount of the concentrate in the reverse osmosis membrane device 5.

The NH ₄ ⁺ concentration of the obtained concentrate was measured with an _{NH 4} ^{+ concentration measuring device.} The resulting _NH ^{4 +} concentration in terms of the ammonia concentration was 1.6 mass ppm. The urea concentration in the sample water 1 obtained by inserting this value into the above formula (4) is 0.28 mass ppm, which is abbreviated as 0.3 mass ppm, which is the concentration of urea charged in the sample water 1. It was in agreement. _{The NH 4} ⁺ concentration of the decomposition solution after the S4 step was measured with an _{NH 4} ^{+ concentration measuring device.} The resulting _NH ^{4 +} concentration in terms of the ammonia concentration was 0.2 mass ppm.

[Example 2]
Sample water 2 in which urea was dissolved in pure water so that the urea concentration was 0.05 mass ppm was prepared. _{The NH 4} ⁺ concentration of the concentrated solution obtained by the same operation as in Example 1 with respect to the sample water 2 was measured with an _{NH 4} ^{+ concentration measuring device.} The resulting _NH ^{4 +} concentration in terms of the ammonia concentration was 0.3 mass ppm. The urea concentration in the sample water 1 obtained by inserting this value into the above formula (4) is 0.05 mass ppm, which is the concentration of urea charged in the sample water 2, which is 0.05 mass ppm. I was doing it. _{When the NH 4} ⁺ concentration of the decomposition solution after the S4 step _{was measured with an NH 4} ⁺ concentration measuring device, the NH ₄ ⁺ concentration (ammonia concentration) was below the detection limit.

[Examples 3 and 4]
Sample water 1 in which urea was dissolved in pure water so that the urea concentration was 0.3 mass ppm was prepared. Obtained by the same operation as in Example 1 except that the pH of the reaction solution was set to 8 (Example 3) or 10 (Example 4) by using an aqueous NaOH solution as a pH adjuster with respect to sample water 1. _{The NH 4} ⁺ concentration of the concentrated solution was measured with an _{NH 4} ^{+ concentration measuring device.} The resulting _NH ^{4 +} concentration in terms of ammonia concentration, in the example 3 was less than the detection limit in 0.1 mass ppm, Example 4. The urea concentration in the sample water 1 obtained by inserting this value into the above formula (4) was 0.02 mass ppm in Example 3, was below the detection limit in Example 4, and was the urea concentration in the sample water 1. It was significantly different from the charged concentration of 0.3 mass ppm. In Examples 3 and 4, when the NH ₄ ⁺ concentration _{of the decomposition solution after the S4 step was measured with an NH 4} ⁺ concentration measuring device, the NH ₄ ⁺ concentration (ammonia concentration) was below the detection limit.

The above results are shown in Table 1 together with the measurement conditions.

50A, 50B ... Urea concentration measuring device, 1 ... Oxidizing agent supply unit, 2 ... pH adjusting unit, 3 ... Reaction tank, 4 ... Reducing agent supply unit, 5 ... Reverse osmosis membrane device, 6 ... Measuring unit, 10 ... Sample Water supply unit, 20 ... Sample water supply pipe 100 ... Pure water production equipment, 101 ... Primary pure water system, 102 ... Secondary pure water system, 110 ... Urea decomposition equipment, 120 ... Water pipe to be treated

Claims (12)

It is a method of measuring the urea concentration in sample water containing urea and water.
A step of adding an oxidizing agent composed of hypobromous acid and / or a salt thereof to the sample water and decomposing the urea into ammonia under the conditions of pH 3 to 6 to obtain a decomposition solution containing ammonium ions.
A step of circulating the decomposition solution to a reverse osmosis membrane apparatus and concentrating ammonium ions to obtain a concentrated solution, and a step of measuring the concentration of ammonium ions in the concentrated solution and calculating the urea concentration in the sample water.
A method for measuring urea concentration. The measuring method according to claim 1, wherein in the step of obtaining the decomposition liquid, the pH condition is 4 to 6. The measuring method according to claim 1 or 2, wherein the urea concentration in the sample water is 0.3 mass ppm or less. The measuring method according to claim 3, wherein the amount of the oxidizing agent added is an amount capable of decomposing the entire amount of urea when urea is present in the sample water at a concentration of 0.3 mass ppm. The measuring method according to any one of claims 1 to 4, wherein the reverse osmosis membrane in the reverse osmosis membrane apparatus is made of a polyamide-based material or a cellulose triacetate-based material. The measuring method according to any one of claims 1 to 5, further comprising a step of adding a reducing agent to the decomposition liquid before the step of obtaining the concentrated liquid. The measuring method according to any one of claims 1 to 6, wherein the concentration of ammonium ions in the concentrated solution is measured by an ion-selective electrode method. The measuring method according to any one of claims 1 to 7, wherein the concentration of ammonium ions in the concentrated solution is concentrated to be 0.05 mass ppm or more. A device that measures the urea concentration in sample water containing urea and water.
An oxidant supply unit that supplies an oxidant composed of hypobromous acid and / or a salt thereof to the sample water,
A reaction tank that turns the sample water into a decomposition solution containing ammonium ions by reacting the urea in the sample water with the oxidizing agent and decomposing it into ammonia.
A pH adjusting unit that adjusts the pH of the sample water so that the reaction is carried out under the conditions of pH 3 to 6.
A reverse osmosis membrane device to which the decomposition liquid is supplied and which concentrates ammonium ions in the supplied decomposition liquid and discharges them as a concentrated liquid.
A urea concentration measuring device having a measuring unit for measuring the concentration of ammonium ions in the concentrated solution and calculating the urea concentration in the sample water. The measuring device according to claim 9, further comprising a reducing agent supply unit that supplies a reducing agent to the decomposition liquid supplied to the reverse osmosis membrane device. A pure water production apparatus for producing pure water by treating raw water with a primary pure water system and a secondary pure water system, the pure water production apparatus including the urea concentration measuring apparatus according to claim 9 or 10. The pure water production apparatus according to claim 11, wherein the primary pure water system or the secondary pure water system has a urea decomposition apparatus, and the urea concentration measuring apparatus is provided before and / or after the urea decomposition apparatus.

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