Water treatment method and water treatment system

Abstract

This water treatment system treats urea-containing raw water to produce treated water with a reduced urea concentration, the system comprising: an agent addition device (40) for adding an agent decomposing urea contained in the raw water, the agent addition device (40) adding the agent to the raw water at a point in a raw water tank (11) or in the upstream thereof, the raw water tank (11) temporarily storing the raw water; a reverse osmosis membrane device (20) having a reverse osmosis membrane (25) and receiving water discharged from the raw water tank (11); and an analysis device (51) for measuring the urea concentration in water to be analyzed, the analysis device (51) using concentrate water as the water to be analyzed, the concentrate water being discharged from the reverse osmosis membrane device (20) without passing through the reverse osmosis membrane (25). On the basis of measurement results from the analysis device (51), at least one of the following is controlled: the amount of the agent to be added to the raw water by the agent addition device (40); the temperature of the raw water; and the pH of the raw water.

Description

Water treatment method and water treatment system

[0001] The present invention relates to a water treatment method and a water treatment system for treating raw water containing urea to obtain treated water having a reduced urea concentration.

[0002] Pure water or ultrapure water is widely used for cleaning and other purposes in the manufacture of semiconductor devices. Pure water is generally produced by subjecting raw water to a combination of unit operations, such as filtration, activated carbon treatment, ion exchange, reverse osmosis membrane treatment, ultraviolet oxidation, and degassing. Ultrapure water can be obtained by further purifying the pure water thus produced, i.e., primary pure water.

[0003] With the advancement of miniaturization in semiconductor devices, there is a demand for further reduction in the concentration of TOC (total organic carbon) components contained in pure water or ultrapure water. Among the TOC components that may be present in raw water, urea is a component that is difficult to remove using the unit operations described above for producing pure water or ultrapure water. Therefore, in order to reduce the urea concentration in pure water or ultrapure water, it is necessary to decompose and remove urea from the raw water when the raw water is received, and then perform a process to produce pure water or ultrapure water. A method for decomposing and removing urea from water is known, using hypobromous acid. For example, Patent Document 1 discloses adding hypobromite or alkali bromide and hypochlorous acid to the water to be treated to decompose and remove urea from the water to be treated. Hypochlorite can also be used instead of hypochlorous acid. Examples of alkali bromides include sodium bromide and potassium bromide, and examples of hypochlorite include sodium hypochlorite and potassium hypochlorite. Patent Document 2 discloses that when sodium hypochlorite and sodium bromide are added to raw water used to produce pure water or ultrapure water to decompose and remove urea in the raw water, the amount of sodium hypochlorite and sodium bromide added is controlled by measuring the urea concentration in the raw water before adding the chemicals.

[0004] In order to control the urea concentration in pure water, it is necessary to be able to measure the urea concentration in raw water or in pure water produced from raw water. Patent Document 3 discloses a method for continuously quantifying trace amounts of urea in water by applying a flow injection analysis technique to the quantification of urea by a colorimetric method using diacetyl monoxime. Patent Document 4 discloses that, in order to remove interfering substances in the quantification of urea by a colorimetric method using diacetyl monoxime, a pretreatment for urea quantification is performed by treating the liquid to be measured with an ion exchange resin or by supplying the liquid to be measured to a reverse osmosis membrane and allowing it to pass through the membrane.

[0005] Japanese Patent Laid-Open No. 9-94585 Japanese Patent Laid-Open No. 2023-125145 Japanese Patent Laid-Open No. 2018-179545 Japanese Patent Laid-Open No. 2019-184436

[0006] When decomposing urea in raw water using hypobromous acid, chemicals such as sodium hypochlorite and sodium bromide are added to the raw water. Any excess chemicals not consumed in the decomposition of urea increase costs and impose a burden on downstream processes for producing pure water. Furthermore, because hypochlorous acid and its salts are oxidizing substances, excess chemicals can also cause damage to downstream equipment. Therefore, to decompose urea, it is necessary to add just the right amount of chemicals to the raw water. The method described in Patent Document 2 controls the amount of chemicals added based on the urea concentration before chemical addition. However, the urea decomposition rate is affected not only by the amount of chemicals added, but also by water temperature, raw water pH, and substances coexisting in the raw water, so adding the appropriate amount of chemicals is not always possible. It is also possible to control the amount of chemicals added by measuring the urea concentration in the raw water after chemicals have been added to decompose the urea. However, in this case, when the urea concentration in the raw water after urea decomposition approaches its lower limit of quantification, accurate quantification becomes impossible, and accurate control of the amount of chemical added becomes impossible. When accurate control of the amount of chemical added becomes impossible, particularly when the urea concentration required for the treated water is low, the above-mentioned problems occur, such as an increase in the urea concentration due to a lack of chemical or an excess of chemical. Furthermore, as the urea concentration decreases, the impact of interfering substances on the quantification value of urea also increases.

[0007] An object of the present invention is to provide a water treatment method and a water treatment system that can appropriately adjust the amount of chemicals added to decompose urea when treating raw water containing urea to obtain treated water with a reduced urea concentration, even if the urea concentration required for the treated water is low.

[0008] One aspect of the water treatment method of the present invention is a water treatment method for treating raw water containing urea to produce treated water, and includes: a decomposition step in which a chemical agent is added to the raw water to decompose the urea contained in the raw water; a reverse osmosis step in which water discharged from the decomposition step is treated using a reverse osmosis membrane; a measurement step in which the urea concentration in concentrated water that is discharged in the reverse osmosis step without passing through the reverse osmosis membrane; and a control step in which, based on the measurement results in the measurement step, the amount of chemical agent added to the raw water in the decomposition step, the water temperature in the decomposition step, and the pH in the decomposition step are controlled. The treated water is the permeated water that has passed through the reverse osmosis membrane in the reverse osmosis step, or water obtained by further treating the permeated water.

[0009] A water treatment system according to one embodiment of the present invention treats raw water containing urea to produce treated water, and includes a raw water tank for temporarily storing the raw water, a chemical addition device for adding a chemical to the raw water at the raw water tank or a position upstream of the raw water tank that decomposes the urea contained in the raw water, a reverse osmosis membrane device equipped with a reverse osmosis membrane and supplied with water discharged from the raw water tank, and an analyzer for measuring the urea concentration in concentrated water discharged from the reverse osmosis membrane device without passing through the reverse osmosis membrane. In this water treatment system, the amount of chemical added to the raw water by the chemical addition device is controlled in accordance with the measurement results of the analyzer, and the permeate that passes through the reverse osmosis membrane and is discharged from the reverse osmosis membrane device, or water obtained by further treating the permeate, is used as treated water.

[0010] According to the present invention, when a chemical agent is added to raw water containing urea to obtain treated water with a reduced urea concentration, the amount of chemical agent added to decompose urea can be made appropriate even if the urea concentration required for the treated water is low.

[0011] Fig. 1 is a flow sheet showing a water treatment system of one embodiment of the present invention. Fig. 2 is a diagram explaining concentration of a solute by a reverse osmosis membrane. Fig. 3 is a graph showing the relationship between the permeability coefficient of a reverse osmosis membrane and the concentration ratio of urea. Fig. 4 is a flow sheet showing a water treatment system of another embodiment. Fig. 5 is a flow sheet showing a water treatment system of another embodiment. Fig. 6 is a flow sheet showing a water treatment system of another embodiment.

[0012] Next, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows the configuration of a water treatment system according to one embodiment of the present invention. The water treatment system according to this embodiment treats raw water containing urea and produces treated water having a reduced urea concentration. The following description will be given by way of example of producing primary pure water as treated water, in which the urea concentration has been reduced to, for example, less than 0.5 ppb. A urea concentration of 0.5 ppb roughly corresponds to the lower limit of quantification when urea is quantified without concentrating the urea. Examples of raw water that can be used include city water, well water, river water, and industrial water, as well as reclaimed water obtained by treating domestic or industrial wastewater and fresh seawater obtained by desalination. These may be used alone or in combination.

[0013] The water treatment system includes a raw water tank 11 for temporarily storing raw water, a reverse osmosis membrane device (RO) 20 equipped with a reverse osmosis membrane 25 and supplied with water discharged from the raw water tank 11, a chemical addition device 40 for adding a chemical that decomposes urea contained in the raw water, and a urea analyzer 51 for measuring the urea concentration. Between the raw water tank 11 and the reverse osmosis membrane device 20 are provided a sand filtration device (SF) 12 to which water discharged from the raw water tank 11 is supplied, an intermediate storage tank 13 to which outlet water from the sand filtration device 12 is temporarily stored, an activated carbon device (AC) 14 to which water discharged from the intermediate storage tank 13 is supplied, an ion exchange device (IE) 15 to which outlet water from the activated carbon device 14 is supplied, and an intermediate storage tank 16 to which outlet water from the ion exchange device 15 is temporarily stored. Water discharged from the intermediate storage tank 16 is supplied to the reverse osmosis membrane device 20. The sand filtration device 12, the activated carbon device 14, and the ion exchange device 15 are all pretreatment devices that perform individual unit operations as pretreatment on the water to be supplied to the reverse osmosis membrane device 20. Some or all of these pretreatment devices may be removed as necessary, and the order of arrangement may be changed. Furthermore, pretreatment devices other than the sand filtration device 12, the activated carbon device 14, and the ion exchange device 15 may be arranged between the raw water tank 11 and the reverse osmosis membrane device 20.

[0014] The permeated water that passes through the reverse osmosis membrane 25 and is discharged from the reverse osmosis membrane device 20 is used as treated water for the water treatment system either directly or after further treatment. In the example shown here, the target is to obtain primary pure water as treated water, so downstream of the reverse osmosis membrane device 20 are provided an intermediate storage tank 31 for temporarily storing the permeated water, an ultraviolet oxidation device (UV) 32 to which water discharged from the intermediate storage tank 31 is supplied, an ion exchange device (IE) 33 to which outlet water from the ultraviolet oxidation device 32 is supplied, and a membrane degassing device (MD) 34 to which outlet water from the ion exchange device 33 is supplied. Primary pure water, which is treated water in this water treatment system, is discharged from the membrane degassing device 34.

[0015] In the concentrated water discharged from the reverse osmosis membrane device 20 without passing through the reverse osmosis membrane 25, the solutes contained in the water supplied to the reverse osmosis membrane device 20, i.e., the feed water, are concentrated, and urea is also concentrated. Regarding solutes, the concentration factor k is defined as the number of times the concentration of the solute in the concentrated water compared to the concentration in the feed water. The concentration factor k for urea in the reverse osmosis membrane device 20 is approximately 1.5 to 2.5. The concentrated water discharged from the reverse osmosis membrane device 20 and containing a large amount of solute components is used, for example, as facility water. A portion of the concentrated water is supplied to a urea analyzer 51 as the water to be analyzed, and the urea analyzer 51 measures the urea concentration in the supplied concentrated water, i.e., the water to be analyzed. The urea analyzer 51 may be, for example, that described in Patent Document 3. The urea concentration in the water to be analyzed can be determined by dividing the quantitative value of urea obtained by the urea analyzer 51 by the concentration factor k. Since urea is hardly removed by the sand filter 12, the activated carbon device 14, and the ion exchange device 15, the urea concentration determined in this manner can be said to represent the urea concentration in the water discharged from the raw water tank 11.

[0016] Here, the concentration of urea in the reverse osmosis membrane device 20 will be described with reference to FIG. 2. It is assumed that the feed water is pressurized by a pump 26 and supplied to the reverse osmosis membrane device 20 equipped with a reverse osmosis membrane 25. The flow rate of the feed water supplied to the reverse osmosis membrane device 20 is Q f , the flow rate of the concentrated water discharged from the reverse osmosis membrane device 20 is Q b , the flow rate of the permeated water discharged from the reverse osmosis membrane device 20 is Q p A portion of the concentrated water is used as the water to be analyzed. Equation (1) holds for the flow rates of the feed water, concentrated water, and permeated water, and the water recovery rate f in the reverse osmosis membrane device 20 is defined by equation (2).

[0017] Q f = Q b + Q p (1) f = Q p / (Q b + Q p ) = Q p / Q f (2)

[0018] Since solutes contained in the feed water generally do not permeate the reverse osmosis membrane 25, the solutes appear in the concentrated water. If the recovery rate f is increased, the flow rate Q of the concentrated water becomes relatively b becomes smaller, so the concentration of the solute in the concentrated water increases. When the rejection rate of the solute in the reverse osmosis membrane 25 is sufficiently high, the concentration factor k in the reverse osmosis membrane device 20 is expressed by equation (3). However, for non-ionic solutes with small molecular weights such as urea, the reverse osmosis membrane 25 does not have a sufficient rejection rate, and therefore the concentration factor k for urea is smaller than that expressed by equation (3).

[0019] k = Q f / Q b (3)

[0020] A parameter that indicates the characteristics of a reverse osmosis membrane is the permeability coefficient A, which is expressed as the pure water permeation flux (flux) per unit effective pressure through the reverse osmosis membrane at a given temperature (usually 25°C). The permeability coefficient A is specific to each reverse osmosis membrane and does not depend on the operating conditions of the reverse osmosis membrane device. The pure water permeation flux is calculated by dividing the pure water permeation volume by the membrane area. As described in JIS (Japanese Industrial Standards) K3802:2015 "Membrane Terminology," "effective pressure" refers to the effective pressure acting on the membrane, calculated by subtracting the osmotic pressure difference and secondary side pressure from the average operating pressure. The average operating pressure is the average value of the pressure of the membrane feed water (operating pressure) and the pressure of the concentrated water (concentrated water outlet pressure) on the primary side of the membrane, and is expressed by the following equation (4):

[0021] Average operating pressure = (operating pressure + concentrated water outlet pressure) / 2 (4)

[0022] Generally, reverse osmosis membranes used in applications such as pure water production, e.g., ultra-low pressure membranes or low pressure membranes, have a large permeability coefficient A, while reverse osmosis membranes used in applications such as seawater desalination, e.g., high pressure membranes, have a small permeability coefficient A. Figure 3 shows the results of determining the urea concentration factor k for several types of reverse osmosis membranes with different permeability coefficients A when the water recovery rate is 95%. As shown in the figure, when the permeability coefficient A is greater than 0.7 m / d / MPa, the concentration factor k is approximately 2 regardless of the permeability coefficient A. However, when the permeability coefficient A is 0.7 m / d / MPa or less, the concentration factor k for urea is 2 or greater, and the smaller the permeability coefficient A, the larger the concentration factor k. While the concentration factor k also depends on the recovery rate f of the reverse osmosis membrane device, in the water treatment system of this embodiment, a reverse osmosis membrane with a large permeability coefficient A used in pure water production, e.g., a membrane with a permeability coefficient A of approximately 1 m / d / MPa, is used as the reverse osmosis membrane 25 of the reverse osmosis membrane device 20. Therefore, as described above, the concentration factor k for urea in the reverse osmosis membrane device 20 is about 1.5 to 2.5.

[0023] The chemical dosing device 40 adds a chemical to the raw water tank 11 or upstream of the raw water tank 11 to decompose urea in the raw water. The chemicals added are, for example, hypochlorous acid or hypochlorite and alkali bromide. In the illustrated example, as indicated by "HClO or NaClO" and "NaBr," an aqueous solution of hypochlorous acid or sodium hypochlorite and an aqueous solution of sodium bromide are supplied to the raw water tank 11 via pumps 41 and 42, respectively. In the raw water tank 11, hypochlorous acid or sodium hypochlorite reacts with sodium bromide to produce hypobromous acid, which then oxidizes and decomposes urea. The raw water tank 11 functions as a urea decomposition tank, where the oxidative decomposition of urea proceeds. The pumps 41 and 42 are controlled according to the urea measurement results from the urea analyzer 51. As described above, the urea concentration in the concentrated water from the reverse osmosis membrane device 20 is 1.5 to 2.5 times that of the urea concentration in the outlet water from the raw water tank 11. Therefore, even if the urea concentration in the outlet water from the raw water tank 11 is below the lower limit of quantification, the urea concentration in the concentrated water can be measured. Because substances interfering with urea quantification are largely removed by the activated carbon device 14 and the ion exchange device 15, the urea concentration measured by the urea analyzer 51 is highly accurate. Therefore, in this water treatment system, even if the required urea concentration in the treated water (in this case, primary pure water) is low and the urea concentration in the outlet water from the raw water tank 11 must be below the lower limit of quantification, it is possible to accurately control the amount of chemicals added to the raw water in accordance with the urea concentration in the concentrated water from the reverse osmosis membrane device 20. In other words, even if the required urea concentration in the treated water is low, the amount of chemicals added to decompose urea can be appropriate.

[0024] Control of the amount of chemical added based on the measurement results of the urea analyzer 51 is feedback control. The residence time from the point in the raw water tank 11 where the chemical is added by the chemical addition device 40 to the outlet of the concentrated water from the reverse osmosis membrane device 20 acts as a delay time in this feedback control, so the residence time must be taken into consideration when controlling the amount of chemical added. When operating the water treatment system, it is preferable to check the residence time in advance. When the amount of chemical added is changed intermittently based on the measurement results of the urea analyzer 51, it is preferable to set the interval between the previous determination of the chemical injection amount and the next determination of the chemical injection amount to be equal to or greater than the residence time.

[0025] In the above description, both the amount of hypochlorous acid or sodium hypochlorite and the amount of sodium bromide added are controlled in accordance with the urea concentration measurement results from the urea analyzer 51, but it is also possible to control the amount of either one added. By controlling the amount of chemical added, it is possible to change the concentration of the chemical in the raw water. Furthermore, since the urea decomposition reaction is also affected by the water temperature and pH, it is also possible to control the amount of pH adjuster added to the raw water in accordance with the urea concentration measurement results from the urea analyzer 51, or to control the water temperature in the raw water tank 11, i.e., the temperature of the raw water. An acid or alkali is used as the pH adjuster.

[0026] FIG. 4 shows a water treatment system according to another embodiment. Similar to the water treatment system shown in FIG. 1 , this water treatment system produces treated water, which is primary pure water, from raw water containing urea. The water treatment system shown in FIG. 4 differs from the water treatment system shown in FIG. 1 in that it does not include an ion exchange device 15 as a pretreatment device, and instead of the reverse osmosis membrane device 20, it includes two-stage reverse osmosis membrane devices (RO) 21, 22, each equipped with a reverse osmosis membrane 25. Furthermore, the water treatment system shown in FIG. 4 includes, as an example, an electrodeionization (EDI) device 35 instead of the ion exchange device 33 in the water treatment system shown in FIG. 1 . Of course, the ion exchange device 33 may be provided downstream of the two-stage reverse osmosis membrane devices 21, 22 instead of the EDI device 35. The reverse osmosis membranes 25 used in the reverse osmosis membrane devices 21, 22 are the same as the reverse osmosis membrane 25 used in the reverse osmosis membrane device 20 in the water treatment system shown in FIG. 1 . 4, the outlet water of the activated carbon device 14 is temporarily stored in an intermediate storage tank 16, the water discharged from the intermediate storage tank 16 is supplied to a reverse osmosis membrane device 21 in the preceding stage, the permeated water of this reverse osmosis membrane device 21 is supplied to a reverse osmosis membrane device 22 in the following stage, and the permeated water of the reverse osmosis membrane device in the following stage is temporarily stored in an intermediate storage tank 31. The outlet water of the ultraviolet oxidation device 32 is supplied to an EDI device 35, and the outlet water of the EDI device 35 is supplied to a membrane degassing device 34.

[0027] The water treatment system shown in Figure 4 does not include an ion exchange device as a pretreatment device, so the concentrated water from the upstream reverse osmosis membrane device 21 contains large amounts of impurities and interfering substances, mainly ionic impurities, which can cause problems or measurement errors in the urea analyzer 51. Therefore, it is preferable to supply the concentrated water from the downstream reverse osmosis membrane device 22 to the urea analyzer 51 as the water to be analyzed and quantify urea. Note that while ionic impurities and interfering substances have been removed from the permeated water from the upstream reverse osmosis membrane device 21, approximately 10% to 20% of urea is also removed, depending on the type of reverse osmosis membrane 25 used in the upstream reverse osmosis membrane device 21. Therefore, when calculating the urea concentration in the outlet water of the raw water tank 11 as a numerical value, the urea removal rate in the upstream reverse osmosis membrane device 21 must also be taken into consideration.

[0028] In the water treatment system described above, the urea concentration in the concentrated water from the reverse osmosis membrane device 20, i.e., the water to be analyzed, may still be insufficient for accurate quantification of urea. In such cases, a preconcentrator 52 may be further provided to concentrate the urea contained in the water to be analyzed discharged from the reverse osmosis membrane device 20, and the water to be analyzed discharged from the preconcentrator 52 may be supplied to a urea analyzer 51. The water treatment system shown in FIG. 5 is the water treatment system shown in FIG. 1 , except that the preconcentrator 52 is disposed between the outlet of the concentrated water from the reverse osmosis membrane device 20 and the urea analyzer 51. Examples of the preconcentrator 52 that can be used include an ion exchange device, a membrane concentrator, an evaporative concentrator, and a membrane distillation device. It is particularly preferable to use a membrane concentrator consisting of a reverse osmosis membrane device for the preconcentrator 52. Using a reverse osmosis membrane device allows for a simple and inexpensive configuration of the preconcentrator 52.

[0029] As explained using FIG. 3 , when the permeability coefficient A of the reverse osmosis membrane is greater than 0.7 m / d / MPa, the concentration factor k for urea is approximately 2 regardless of the permeability coefficient A. However, when the permeability coefficient A is 0.7 m / d / MPa or less, the concentration factor k for urea is 2 or greater, and the smaller the permeability coefficient A, the greater the concentration factor k. If a urea concentration factor k of 2 or greater in the pre-concentrator 52 is considered practical, it can be seen that the permeability coefficient A of the reverse osmosis membrane used in the pre-concentrator 52 should be 0.7 m / d / MPa or less. The permeability coefficient A of the reverse osmosis membrane used in the pre-concentrator 52 is preferably 0.6 m / d / MPa or less, and more preferably 0.4 m / d / MPa or less. If the permeability coefficient A is 0.4 m / d / MPa, a concentration factor k of approximately 4.5 can be obtained.

[0030] In each of the water treatment systems described above, interfering substances interfering with the urea determination in the urea analyzer 51 are supposed to be removed in each pretreatment device located between the raw water tank 11 and the reverse osmosis membrane device 20. However, interfering substances may not be sufficiently removed, and may be present in the target water to be analyzed that is discharged as concentrated water from the reverse osmosis membrane device 20. To address such cases, an interfering substance removal device 53 may be provided to remove interfering substances from the target water to be supplied to the urea analyzer 51. The interfering substance removal device 53 may be located anywhere upstream of the urea analyzer 51. However, if a preconcentrator 52 is provided, the interfering substance removal device 53 may be provided upstream of the preconcentrator 52 so that only urea is concentrated in the preconcentrator 52. The water treatment system shown in FIG. 6 is the water treatment system shown in FIG. 5 , except that the interfering substance removal device 53 is located between the concentrated water outlet of the reverse osmosis membrane device 20 and the preconcentrator 52. The interfering substance removal device 53 may be, for example, an activated carbon device, an ion exchange device, or a reverse osmosis membrane device. When a reverse osmosis membrane device is used as the interfering substance removal device 53, the reverse osmosis membrane used should have a low urea rejection rate, for example, the same as the reverse osmosis membrane 25 of the reverse osmosis membrane device 20, and the permeated water from the reverse osmosis membrane device should be supplied to the urea analyzer 51.

[0031] Next, the present invention will be described in more detail with reference to examples. The water treatment system shown in FIG. 1 was assembled, and raw water containing 4.2 ppb of urea was supplied to the raw water tank 11. This water treatment system was then operated, and the amount of chemical added to the raw water by the chemical addition device 40 was controlled based on the measurement results from the urea analyzer 51. After the operation of the water treatment system stabilized, the urea concentration was measured in the outlet water of the raw water tank 11 and the concentrated water discharged from the reverse osmosis membrane device 20. The urea concentration was below the lower limit of quantification in the outlet water of the raw water tank 11, and 0.7 ppb in the concentrated water. Liquid chromatography-mass spectrometry (LC-MS) was used to measure the urea concentration. These results demonstrate that even if the urea concentration at the outlet of the raw water tank 11 (the urea decomposition tank) was below the lower limit of quantification, the urea concentration could be quantified in the concentrated water from the reverse osmosis membrane device 20, and the amount of chemical added could be controlled based on the urea concentration in the concentrated water. In other words, it has been found that the water treatment method and water treatment system according to the present invention can appropriately adjust the amount of chemicals used to decompose urea even when the required urea concentration in the treated water is low.

[0032] REFERENCE SIGNS LIST 11 raw water tank 12 sand filter device 13, 16, 31 intermediate water tank 14 activated carbon device 15, 33 ion exchange device 20, 21, 22 reverse osmosis membrane (RO membrane) device 25 reverse osmosis membrane 26, 41, 42 pump 32 ultraviolet oxidation device 34 membrane degassing device 35 electrodeionized water production device (EDI device) 40 chemical addition device 51 urea analysis device 52 preconcentration device 53 interfering substance removal device

Claims

1. A water treatment method for treating raw water containing urea to produce treated water with a reduced urea concentration, comprising: a decomposition step of adding a chemical to the raw water to decompose the urea contained in the raw water; a reverse osmosis step of treating water discharged from the decomposition step with a reverse osmosis membrane; a measurement step of measuring the urea concentration in the water to be analyzed, using concentrated water discharged in the reverse osmosis step without passing through the reverse osmosis membrane as the water to be analyzed; and a control step of controlling at least one of the amount of the chemical to be added to the raw water in the decomposition step, the water temperature of the raw water, and the pH of the raw water, based on the measurement results from the measurement step.

2. The water treatment method according to claim 1, further comprising a preconcentration step for concentrating urea contained in the water to be analyzed discharged from the reverse osmosis step, and supplying the water to be analyzed in which urea has been concentrated in the preconcentration step to the measurement step.

3. The water treatment method according to claim 2, wherein the pre-concentration step comprises a step of supplying the water to be analyzed discharged from the reverse osmosis step to a reverse osmosis membrane device for concentration equipped with a reverse osmosis membrane, and the concentrated water that does not pass through the reverse osmosis membrane of the reverse osmosis membrane device for concentration is supplied to the measurement step as the water to be analyzed in which urea has been concentrated.

4. A water treatment method according to any one of claims 1 to 3, further comprising a step of removing interfering substances from the water to be analyzed.

5. The water treatment method according to any one of claims 1 to 3, further comprising a pretreatment step in which the water discharged from the decomposition step is subjected to at least one of filtration, activated carbon treatment, and ion exchange treatment, and the water discharged from the pretreatment step is supplied to the reverse osmosis step.

6. A water treatment method according to any one of claims 1 to 3, wherein the chemicals are hypochlorous acid or a salt thereof and alkali bromide, and in the control step, at least one of the amount of hypochlorous acid or a salt thereof added and the amount of alkali bromide added is controlled.

7. A water treatment system that treats raw water containing urea to produce treated water with a reduced urea concentration, comprising: a chemical addition device that adds a chemical that decomposes the urea contained in the raw water to the raw water in a raw water tank that temporarily stores the raw water or at a position upstream of the raw water tank; a reverse osmosis membrane device that is equipped with a reverse osmosis membrane and to which water discharged from the raw water tank is supplied; and an analyzer that measures the urea concentration in the water to be analyzed, using concentrated water that is discharged from the reverse osmosis membrane device without passing through the reverse osmosis membrane as the water to be analyzed; and at least one of the amount of the chemical added to the raw water by the chemical addition device, the temperature of the raw water, and the pH of the raw water is controlled according to the measurement results of the analyzer.

8. The water treatment system according to claim 7, further comprising a preconcentrator that concentrates urea contained in the water to be analyzed, and the water to be analyzed, with urea concentrated and discharged from the preconcentrator, is supplied to the analyzer.

9. The water treatment system according to claim 8, wherein the pre-concentration device comprises a reverse osmosis membrane device for concentration, which is equipped with a reverse osmosis membrane and to which the water to be analyzed is supplied, and the concentrated water that does not pass through the reverse osmosis membrane of the reverse osmosis membrane device for concentration is supplied to the analysis device as the water to be analyzed in which urea has been concentrated.

10. A water treatment system according to any one of claims 7 to 9, further comprising an interfering substance removal device for removing interfering substances from the water to be analyzed.

11. A water treatment system as described in any one of claims 7 to 9, comprising a pretreatment device which is at least one of a filtration device, an activated carbon device, and an ion exchange device, and in which water discharged from the raw water tank passes through the pretreatment device before being supplied to the reverse osmosis membrane device.

12. A water treatment system as described in any one of claims 7 to 9, wherein the chemicals are hypochlorous acid or its salt and alkali bromide, and the chemical addition device controls at least one of the amount of hypochlorous acid or its salt and the amount of alkali bromide added.

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