Method of operating reverse osmosis membrane device

Abstract

The present invention comprises a first reverse osmosis membrane device 1, a second reverse osmosis membrane device 2, and a feed water line 3. A feed water line 4 for treated water W2 and a discharge line 5 for concentrated water W3 are each connected to the second reverse osmosis membrane device 2. There is also an NaOH aqueous solution supply source 6 connected to the feed water line 3. In addition, a first pH measuring means 11 for measuring the pH of treatment water W1 is provided, and a second pH measuring means 12 for measuring the pH of the concentrated water W3 is provided. The pH of the treatment water W1 for the second reverse osmosis membrane device 2 is measured at the first pH measuring means 11, the pH of the concentrate W3 of the second reverse osmosis membrane device 2 is measured at the second pH measuring means 12, and the amount of NaOH aqueous solution added from the NaOH aqueous solution supply source 6 is controlled so that the pH of supplied water W1 ≤ the pH of the concentrate W3. According to such a method of operating reverse osmosis membrane devices, it is possible to improve the water quality, in particular, the boron removal rate of treated water of the reverse osmosis membrane devices when treatment water is being pH-adjusted and treated at the reverse osmosis membrane devices.

Description

Method for operating a reverse osmosis membrane device

[0001] The present invention relates to a method for operating a reverse osmosis membrane device that constitutes a pure water production system, and more particularly to a method for operating a reverse osmosis membrane device that improves the water quality of the treated water, particularly the boron removal rate, when the pH of the water to be treated is adjusted and treated with the reverse osmosis membrane device.

[0002] Conventionally, ultrapure water used in the semiconductor and other electronics industries is produced by treating raw water W4 in an ultrapure water production system 21, which is composed of a three-stage system, for example, a pretreatment system 22, a primary water purification system (pure water production system) 23, and a secondary water purification system (subsystem) 24, as shown in Fig. 7. Specifically, in the pretreatment system 22, the raw water W4 is subjected to pretreatment such as filtration, coagulation sedimentation, and turbidity reduction using a UF membrane, and suspended solids are mainly removed.

[0003] The primary water purification system 23 includes a water tank 31 for storing pretreated water (water to be treated) W5, a high-pressure pump 32 for pumping the pretreated water W5, a first reverse osmosis membrane device 33A, a pH adjusting means 34, a second reverse osmosis membrane device 33B, a deaeration membrane device 35 for removing dissolved gases with air, an ultraviolet oxidation device 36, and an electrodeionization device 37. The primary water purification system 33 removes most of the electrolytes, fine particles, live bacteria, etc. from the pretreated water W5 and also decomposes organic matter.

[0004] Here, the first reverse osmosis membrane device 33A and the second reverse osmosis membrane device 33B are often configured with multiple banks to conserve water. For example, in a three-bank configuration, the concentrated water from the first bank is used as the feed water (water to be treated) for the second bank, and the concentrated water from the second bank is used as the feed water (water to be treated) for the third bank, and the treated water from the first to third banks is combined to form the treated water for the reverse osmosis membrane device.

[0005] Subsystem 24 comprises a subtank 41 (pure water tank) downstream of electrodeionization device 37 for storing primary pure water W6 produced by primary pure water system 23, an ultraviolet oxidation device 42 (non-regenerative mixed-bed ion exchange device 43), and an ultrafiltration (UF) membrane 44 (membrane filtration device) for treating primary pure water W6 delivered from subtank 41 via a pump (not shown). Subsystem 24 may also include an RO membrane separator, if necessary. In this subsystem 4, the ultraviolet oxidation device 42 oxidizes and decomposes trace amounts of organic matter (TOC components) contained in primary pure water W6. The non-regenerative mixed-bed ion exchange device 43 then processes the water to remove residual carbonate ions, organic acids, anionic substances, and even metal ions and cationic substances by ion exchange. The ultrafiltration (UF) membrane 44 then removes particulates to produce ultrapure water W7, which is then supplied to point-of-use 25. Unused ultrapure water W7 is returned to subtank 41.

[0006] In the primary pure water system 23 of the ultrapure water production system 21 described above, pretreated water W5 is treated in a first reverse osmosis membrane device 33A or an ion exchange device, and this treated water is then treated in a subsequent second reverse osmosis membrane 33B. At this time, in order to improve the removal rate of boron and the like, alkali is added from a pH adjustment means 34 to the water to be treated (feed water) of the second reverse osmosis membrane 33B, and the pH is controlled to approximately 8 to 12 during treatment.

[0007] However, in the above-described ultrapure water producing system 21, the quality of the primary pure water W6 obtained in the primary pure water system 23, particularly the boron removal rate, sometimes declines. The present inventors conducted extensive research into the cause of this and found that the boron removal rate of the second reverse osmosis membrane 33B can vary depending on the relationship between the quality of the water (feedwater) to be treated by the second reverse osmosis membrane 33B, particularly the dissolved inorganic carbon (IC) concentration, and the pH of the water (feedwater). It is desirable to maintain the quality of the water treated by the second reverse osmosis membrane 33B, particularly the boron removal rate, regardless of the quality of the water (feedwater) to be treated, by controlling the pH of the water to be treated.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for operating a reverse osmosis membrane device that improves the water quality of the treated water, particularly the boron removal rate, when the pH of the water to be treated is adjusted and treated with the reverse osmosis membrane device.

[0009] In view of the above object, the present invention provides a method for operating a reverse osmosis membrane device that constitutes a pure water production system, the method comprising: a reverse osmosis membrane device that treats water to be treated; and a pH adjustment mechanism that adjusts the pH of the water to be treated, the method treating the water to be treated by adjusting the pH of the water to be treated so that the pH of the concentrated water from the reverse osmosis membrane device is equal to or higher than the pH of the water to be treated after pH adjustment (Invention 1).

[0010] According to this invention (Invention 1), when the pH of the water to be treated is adjusted to the alkaline side and treated with a reverse osmosis membrane device, the water quality of the treated water, particularly the boron removal rate, can be improved. This is due to the following reasons. Specifically, when the pH of the water to be treated (feedwater) is set to the alkaline side in a reverse osmosis membrane device consisting of multiple banks, the water to be treated in the subsequent banks is the concentrated water of the previous banks. Therefore, when the pH of the feedwater is adjusted to the alkaline side, particularly to pH 8 or higher, using an NaOH aqueous solution, the concentrated water may be affected by the balance and recovery rate of the dissolved inorganic carbon concentration and Na concentration in the treated water, and the pH of the treated water may decrease with each bank. This reduces the boron removal rate in that bank, and as a result, the boron removal rate of the treated water in the entire reverse osmosis membrane device decreases. After extensive research, the inventors discovered that adjusting the pH of the water to be treated (feedwater) so that the pH of the concentrated water from the reverse osmosis membrane device is equal to or higher than the pH of the treated water maintains the boron removal rate in all banks of the reverse osmosis membrane device, leading to the invention.

[0011] In the above invention (Invention 1), it is preferable to measure two or more of the pH of the water to be treated, the dissolved inorganic carbon concentration of the water to be treated, and the pH of the concentrated water, and adjust the pH of the water to be treated based on these measured values ​​so that the pH of the concentrated water from the reverse osmosis membrane is higher than the pH of the water to be treated (Invention 2).

[0012] According to this invention (Invention 2), by measuring two or more of the pH of the water to be treated, the dissolved inorganic carbon concentration of the water to be treated, and the pH of the concentrated water, it is possible to easily adjust the pH of the water to be treated so that the pH of the concentrated water from the reverse osmosis membrane is higher than the pH of the water to be treated.

[0013] In the above invention (Invention 1), the water to be treated in the reverse osmosis membrane device is permeate from a upstream reverse osmosis membrane device provided upstream of the reverse osmosis membrane device, and it is preferable that the pH of this permeate be adjusted to a pH of 8 to 12 by the pH adjustment mechanism (Invention 3).

[0014] According to the above invention (Invention 3), by performing the control of Invention 1 on the reverse osmosis membrane device in the latter stage of a two-stage reverse osmosis membrane device, boron and the like can be removed more stably to obtain treated water.

[0015] In the above invention (Invention 1), the water to be treated by the reverse osmosis membrane device is treated water from an ion exchange device provided upstream of the reverse osmosis membrane device, and it is preferable that the pH of this treated water be adjusted to a pH of 8 to 12 by a pH adjustment mechanism (Invention 4).

[0016] According to the above invention (Invention 4), by providing an ion exchange device upstream of the reverse osmosis membrane and using the treated water as the water to be treated by the reverse osmosis membrane downstream, boron and other substances can be removed more stably to obtain treated water.

[0017] In the above inventions (Inventions 1 to 4), it is preferable that the reverse osmosis membrane device is composed of a multi-stage bank of reverse osmosis membranes (Invention 5).

[0018] According to the above invention (Invention 5), in a reverse osmosis membrane device configured with multiple reverse osmosis membrane banks, by managing not only the pH of the feed water but also the pH of the concentrated water in the reverse osmosis membrane device, the pH of the treated water can be maintained high in all reverse osmosis membrane banks, thereby maintaining a high removal rate of boron and other substances.

[0019] The method of operating a reverse osmosis membrane device of the present invention adjusts the pH of the water to be treated by the reverse osmosis membrane to the alkaline side, and adjusts the pH of the water to be treated so that the pH of the concentrated water from the reverse osmosis membrane is higher than the pH of the water to be treated.Therefore, the water quality of the water treated by the reverse osmosis membrane device, particularly the boron removal rate, can be maintained at a high level.

[0020] FIG. 1 is a schematic diagram showing a method for operating a reverse osmosis membrane device according to a first embodiment of the present invention; FIG. 2 is a schematic diagram showing a method for operating a reverse osmosis membrane device according to a second embodiment of the present invention; FIG. 3 is a graph showing the relationship between the pH of feed water (water to be treated) to a reverse osmosis membrane device and the pH of concentrate when the dissolved inorganic carbon concentration of the water to be treated is 700 μg / L; FIG. 4 is a graph showing the relationship between the pH of feed water (water to be treated) to a reverse osmosis membrane device and the pH of concentrate when the dissolved inorganic carbon concentration of the water to be treated is 600 μg / L; FIG. 5 is a graph showing the relationship between the pH of feed water (water to be treated) to a reverse osmosis membrane device and the pH of concentrate when the dissolved inorganic carbon concentration of the water to be treated is 500 μg / L; and FIG. 6 is a graph showing the relationship between the pH of feed water (water to be treated) to a reverse osmosis membrane device and the pH of concentrate when the dissolved inorganic carbon concentration of the water to be treated is 400 μg / L. FIG. 1 is a schematic diagram showing an example of an ultrapure water producing apparatus to which the method for operating a reverse osmosis membrane device of the present invention can be applied.

[0021] Hereinafter, the method of operating the reverse osmosis membrane device of the present invention will be described with reference to the accompanying drawings.

[0022] [First Embodiment] (Pure Water Production System) The present invention is characterized by a method for operating a reverse osmosis membrane that constitutes a pure water production system (primary pure water system) used in an ultrapure water production system or the like. The pure water production system (primary pure water system) is not particularly limited, as long as it includes a water supply mechanism for supplying water to be treated and a reverse osmosis membrane device downstream of the water supply mechanism. The present invention can be applied to various pure water production systems. For example, the present invention can be suitably applied to a primary pure water system 23 in an ultrapure water production system 21 as shown in FIG. 7 .

[0023] (Reverse Osmosis Membrane Treatment System) The reverse osmosis membrane treatment system of this embodiment has a configuration as shown in FIG. 1. In FIG. 1, the reverse osmosis membrane treatment system includes a first reverse osmosis membrane device 1 as a front-stage reverse osmosis membrane device that treats raw water W, such as pretreated water; a second reverse osmosis membrane device 2, the operation of which is controlled in this embodiment; and a water supply line 3 through which treated water from the first reverse osmosis membrane device 1 is supplied as water to be treated (feed water) W1 to the second reverse osmosis membrane device 2. The second reverse osmosis membrane device 2 is connected to a water supply line 4 for treated water W2 from the second reverse osmosis membrane device 2 and a discharge line 5 for concentrated water W3. Furthermore, the water supply line 3 is connected via a supply pipe 7 to an NaOH aqueous solution supply source 6, which serves as a pH adjuster for adjusting the pH of the feed water W1 to the second reverse osmosis membrane device 2 to an alkaline pH, preferably a pH of 8 to 12. Furthermore, in this embodiment, a first pH measuring means 11 such as a pH meter for measuring the pH of the treated water W1 after pH adjustment is provided immediately before the second reverse osmosis membrane device 2 after the addition of the NaOH aqueous solution to the water supply line 3, and a second pH measuring means 12 for measuring the pH of the concentrated water W3 is provided in the discharge line 5 for the concentrated water W3. The first pH measuring means 11 and the second pH measuring means 12 are capable of transmitting information to a control means (not shown) such as a PLC, and this control means can control the amount of NaOH solution supplied from the NaOH aqueous solution supply source 6 based on the measured values ​​of the first pH measuring means 11 and the second pH measuring means 12.

[0024] (Second reverse osmosis membrane device 2) In this embodiment, the second reverse osmosis membrane device 2 is configured with three banks of reverse osmosis membranes to improve recovery rate. In this three-bank reverse osmosis membrane device, concentrated water from the first bank is used as feed water (water to be treated) for the second bank, concentrated water from the second bank is used as feed water (water to be treated) for the third bank, and treated water from the first to third banks is combined to form treated water W2 for the second reverse osmosis membrane device 2, and concentrated water from the third bank is used as concentrated water W3 for the second reverse osmosis membrane device 2.

[0025] (Method of Operating a Reverse Osmosis Membrane Device) Next, a method of operating the reverse osmosis membrane device shown in FIG. 1 will be described. First, a high-pressure pump (not shown) is started to supply raw water W to be treated to the first reverse osmosis membrane device 1, where most of the electrolytes, fine particles, live bacteria, etc. are removed to obtain permeated water. This permeated water becomes the feed water (water to be treated) W1 for the second reverse osmosis membrane device 2. Since it is difficult for the first reverse osmosis membrane device 1 to sufficiently remove weakly ionic impurities such as boron, the pH of the water to be treated W1 is controlled to be alkaline, thereby improving the removal rate of these impurities in the second reverse osmosis membrane device 2. Specifically, by making the water alkaline, boron is converted into borate ions (B(OH)4 － ), improving the removal rate in the reverse osmosis membrane device. Therefore, an aqueous NaOH solution is added from the NaOH aqueous solution supply source 6 to the permeate of the first reverse osmosis membrane device 1 to adjust the pH of the water to be treated W1 to approximately 8 to 12, particularly 9 to 11. The pH of this water to be treated W1 may be set within a suitable range depending on the performance of the second reverse osmosis membrane device 2 and the desired boron removal rate. Treated water W2 from which boron has been removed can then be obtained by treating the water W2 with, for example, a degassing membrane device, an ultraviolet oxidation device, and an electrodeionization device, and then stored in a subtank.

[0026] In the method for operating a reverse osmosis membrane device as described above, in this embodiment, the pH of the water to be treated W1 supplied to the second reverse osmosis membrane device 2 is measured by the first pH measuring means 11, and the pH of the concentrated water W3 from the second reverse osmosis membrane device 2 is measured by the second pH measuring means 12, and the two values ​​are compared. The amount of NaOH aqueous solution added from the NaOH aqueous solution supply source 6 is then controlled so that the pH of the feed water W1 is equal to or less than the pH of the concentrated water W3. This makes it possible to maintain a high boron removal rate in the treated water W2.

[0027] The reason for controlling the pH of the water to be treated (feedwater) W1 and the concentrated water W3 in this manner is as follows. Even if the pH of the water to be treated W1 is maintained constant at a pH that allows the second reverse osmosis membrane device 2 to maintain a high boron removal rate, the balance between the dissolved inorganic carbon (IC) concentration in the feedwater W1 and the feedwater pH (Na concentration) may cause the pH of the concentrated water W3 to be lower than the pH of the water to be treated W1. This is due to the buffering effect of the dissolved inorganic carbon (IC) in the feedwater W1. In particular, since the second reverse osmosis membrane device 2 has a multi-bank (three-bank) configuration as in this embodiment, the water to be treated by the reverse osmosis membranes of the later banks (concentrated water of the earlier banks) is more susceptible to the drop in pH, resulting in a pH below the pH suitable for boron removal. The reverse osmosis membranes of such banks have a reduced boron removal rate in the treated water. When this treated water is combined with the treated water W2 of the second reverse osmosis membrane device 2, the boron removal rate of the treated water W2 is reduced. To prevent this, the Na concentration should be set to a level that exceeds the buffering effect of dissolved inorganic carbon (IC) in the feedwater W1. Therefore, as in this embodiment, it is effective to control the amount of NaOH aqueous solution added from the NaOH aqueous solution supply source 6 so that the pH of the feedwater W1 is equal to or less than the pH of the concentrated water W3.

[0028] Second Embodiment Next, a second embodiment of the present invention will be described with reference to Figure 2. (Reverse Osmosis Membrane Treatment System) As shown in Figure 2, the reverse osmosis membrane treatment system of the second embodiment has the same configuration as the first embodiment described above, except that an ion exchange device 1A is provided instead of the first reverse osmosis membrane device 1 that treats the raw water W. This ion exchange device 1A can be one in which an anion exchange resin and a cation exchange resin are mixed or stacked and filled. Note that a softening device or an electrodeionization device mainly using a cation exchange resin can also be used as the ion exchange device 1A.

[0029] (Method of operating reverse osmosis membrane device) The present invention is characterized by the operation control of the second reverse osmosis membrane device 2. Therefore, the upstream stage of the second reverse osmosis membrane device 2 does not need to be a reverse osmosis membrane 1. Even if the ion exchange device 1A is used, the second reverse osmosis membrane device 2 may be controlled in the same manner as in the first embodiment.

[0030] Although the present invention has been described above based on the above embodiment, various modifications are possible. For example, the dissolved inorganic carbon (IC) concentration of the water W1 to be treated may be measured, the amount of NaOH solution to be added to the water W1 may be determined based on this value, and the amount of NaOH solution added may be controlled based on the pH of the concentrate W3 measured by the second pH meter 12. Alternatively, the amount of NaOH solution added may be controlled based on the dissolved inorganic carbon (IC) concentration of the water W1 to be treated, by monitoring this with the first pH meter 11 so that the pH of the water W1 exceeds the buffering effect of Na due to the dissolved inorganic carbon (IC). Furthermore, in this embodiment, the first reverse osmosis membrane device 1 and the ion exchange device 1A are located upstream of the second reverse osmosis membrane device 2 to be controlled, but these upstream treatments may not be necessary. Furthermore, the pure water production system equipped with a reverse osmosis membrane device is not limited to an ultrapure water production system.

[0031] The present invention will be described in more detail below based on specific examples, but the present invention is not limited to the following examples.

[0032] Example 1 Experimental Conditions Raw water W to be treated was obtained by subjecting industrial water to activated carbon treatment and UV sterilization treatment, and the water was treated in a two-stage reverse osmosis membrane device using the reverse osmosis membrane treatment system shown in Figure 1. The reverse osmosis membrane (RO membrane) used in the first reverse osmosis membrane device 1 and the second reverse osmosis membrane device 2 was K-RO-A-202U-FP00 (manufactured by Kurita Water Industries Ltd.).

[0033] The first reverse osmosis membrane device 1 had a three-bank configuration, with the first bank consisting of six 8-inch diameter RO membranes, the second bank treating the concentrated water from the first bank consisting of three 8-inch diameter RO membranes, and the third bank treating the concentrated water from the second bank consisting of six 4-inch diameter RO membranes. Water was passed through the first reverse osmosis membrane device 1 at a flux of 0.64 m / d and a recovery rate of 85%.

[0034] The second reverse osmosis membrane device 2 had a three-bank configuration, with the first bank consisting of four 8-inch diameter RO membranes, the second bank treating the concentrated water from the first bank consisting of three 8-inch diameter RO membranes, and the third bank treating the concentrated water from the second bank consisting of four 4-inch diameter RO membranes. Water was passed through the second reverse osmosis membrane device 2 to achieve a flux of 0.76 m / d and a recovery rate of 90%.

[0035] At this time, an aqueous NaOH solution was added to the treated water from the first reverse osmosis membrane device 1 to adjust the pH to 9.5, to prepare water to be treated (supply water for the second reverse osmosis membrane device 2) W1. The electrical conductivity of the raw water to be treated W (supply water for the first reverse osmosis membrane device 1) was approximately 10 to 15 mS / m, and the electrical conductivity of the water to be treated (supply water for the second reverse osmosis membrane device 2) W1 was approximately 0.1 to 0.3 mS / m, and the dissolved inorganic carbon (IC) concentration was approximately 300 to 600 μg / L as C.

[0036] <Results and Discussion> As a result of treating the water to be treated W1 using the second reverse osmosis membrane device 2 of the reverse osmosis membrane treatment system, the boron removal rate of the treated water W2 relative to the water to be treated W1 was 84%, which exceeded the estimated boron removal rate of 78% for a reverse osmosis membrane under these conditions.

[0037] In the treatment with the second reverse osmosis membrane device 2, attention was paid to the pH of each bank of the second reverse osmosis membrane device 2. While the pH of the water to be treated W1 was 9.5, the pH of the concentrated water of the first bank was 9.6, the pH of the concentrated water of the second bank was 9.7, and the pH of the concentrated water of the third bank (W3) was 9.7. As a result, the second reverse osmosis membrane device 2 was under conditions where the pH increased with each bank, resulting in a higher than expected boron removal rate.

[0038] Comparative Example 1 Experimental Conditions Water treatment was carried out in the same manner as in Example 1, except that an aqueous NaOH solution was added to the water to be treated W1 so that the pH was 9.0.

[0039] <Results and Discussion> As a result of treating the treated water W1 using the second reverse osmosis membrane device 2 of the reverse osmosis membrane treatment system, the boron removal rate of the treated water W2 relative to the treated water W1 was 57%, which was lower than the expected boron removal rate of 72% of the reverse osmosis membrane under these conditions.

[0040] In the treatment with the second reverse osmosis membrane device 2, attention was paid to the pH of each bank of the second reverse osmosis membrane device 2. While the pH of the water to be treated W1 was 9.0, the pH of the concentrated water of the first bank was 9.0, the pH of the concentrated water of the second bank was 8.9, and the pH of the concentrated water of the third bank (W3) was 8.5. As a result, the second reverse osmosis membrane device 2 was in a condition where the pH decreased with each bank, resulting in a lower than expected boron removal rate.

[0041] In Example 1, the dissolved inorganic carbon (IC) concentration of the treated water from the first reverse osmosis membrane device 1 was varied between 400 and 700 g / L as C, and the pH of the concentrated water W3 from the treated water W1 at the inlet of the second reverse osmosis membrane device 2 was measured at pH 8.9, pH 9.1, pH 9.4, and pH 9.6. The results are shown in Figures 3 to 6.

[0042] <Results and Discussion> As is clear from Figures 3 to 6, the higher the dissolved inorganic carbon (IC) concentration of the feedwater W1, the lower the pH of the feedwater, and the more pronounced the decrease in pH of the concentrate W3.

[0043] Depending on the balance between the dissolved inorganic carbon (IC) concentration and pH (Na concentration) of the feedwater W1, the feedwater pH may be less than or equal to the concentrated water pH, or the feedwater pH may be greater than or equal to the concentrated water pH. This depends on whether the Na concentration in the feedwater exceeds the buffering effect caused by the dissolved inorganic carbon (IC) concentration. If the Na concentration exceeds the buffering effect, it is estimated that the feedwater pH will be less than or equal to the concentrated water pH.

[0044] REFERENCE SIGNS LIST 1 First reverse osmosis membrane device 1A Ion exchange device 2 Second reverse osmosis membrane device 3 Water supply channel 4 Water supply channel 5 Discharge channel 6 NaOH aqueous solution supply source 7 Supply pipe 11 First pH measuring means 12 Second pH measuring means W Raw water to be treated W1 Water to be treated (feed water) W2 Treated water W3 Concentrated water

Claims

1. A method for operating a reverse osmosis membrane device that constitutes a pure water production system comprising a reverse osmosis membrane device that treats water to be treated and a pH adjustment mechanism that adjusts the pH of the water to be treated, wherein the pH of the water to be treated is adjusted so that the pH of the concentrated water from the reverse osmosis membrane device is equal to or higher than the pH of the water to be treated after pH adjustment, thereby treating the water to be treated.

2. A method for operating a reverse osmosis membrane device as described in claim 1, wherein two or more of the pH of the water to be treated, the dissolved inorganic carbon concentration of the water to be treated, and the pH of the concentrated water are measured, and the pH of the water to be treated is adjusted based on these measured values ​​so that the pH of the concentrated water from the reverse osmosis membrane is equal to or higher than the pH of the water to be treated.

3. The method for operating a reverse osmosis membrane device according to claim 1, wherein the water to be treated in the reverse osmosis membrane device is permeated water from a upstream reverse osmosis membrane device provided upstream of the reverse osmosis membrane device, and the pH of the permeated water is adjusted to a pH of 8 to 12 by the pH adjustment mechanism.

4. The method for operating a reverse osmosis membrane device according to claim 1, wherein the water to be treated by the reverse osmosis membrane device is treated water from an ion exchange device provided upstream of the reverse osmosis membrane device, and the pH of the treated water is adjusted to a pH of 8 to 12 by a pH adjustment mechanism.

5. The method for operating a reverse osmosis membrane device according to any one of claims 1 to 4, wherein the reverse osmosis membrane device is composed of multiple banks of reverse osmosis membranes.

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