JP7639856B2 - Pure water production system and method for producing pure water - Google Patents

Description

The present invention relates to a pure water production system and a method for producing pure water using the same, and in particular to a pure water production system that combines the efficient operating performance of the pure water production equipment with the performance of producing pure water with stable water quality, and a method for producing pure water using the same.

Conventionally, ultrapure water used in the semiconductor and other electronics industries is produced by treating raw water in an ultrapure water production system that is composed of a pretreatment system, a primary pure water production device (pure water production system), and a subsystem that treats the primary pure water (secondary pure water production device).

For example, as shown in FIG. 6, an ultrapure water production system 1 is composed of three stages of equipment: a pretreatment device 2, a primary pure water production device (pure water production system) 3, and a subsystem 4. In the pretreatment device 2 of this ultrapure water production system 1, the raw water W is pretreated by filtration, coagulation and sedimentation, and a microfiltration membrane, and mainly suspended solids are removed.

The primary pure water production system 3 has a two-stage reverse osmosis membrane device 5, 6 that treats the pretreated water W1, and an electrically regenerated ion exchange device 7, and a relay tank, water pump, or boost pump can be provided between these devices. It also has an ultraviolet oxidation device 8 in the downstream of the electrically regenerated ion exchange device 7. This ultraviolet oxidation device 8 may also be in the upstream of the electrically regenerated ion exchange device 7. This primary pure water production system 3 removes most of the electrolytes, fine particles, live bacteria, etc. in the pretreated water W1 and decomposes organic matter to obtain primary pure water (pure water) W2.

The subsystem 4 has a sub-tank 10, a supply pump 11, an ultraviolet oxidation device 12, a non-regenerative mixed-bed ion exchange device 13, and an ultrafiltration membrane (UF membrane) 14, and is configured to return water from the ultrafiltration membrane (UF membrane) 14 to the sub-tank 10 via a use point 15. In this subsystem 4, trace amounts of organic matter (TOC components) contained in the primary pure water W2 produced in the primary pure water production device 3 are oxidized and decomposed, and carbonate ions, organic acids, anionic substances, and even metal ions and cationic substances are removed using equipment filled with ion exchangers, and finally fine particles are removed using the ultrafiltration (UF) membrane 14 to produce ultrapure water W3, which is supplied to the use point 15, and unused ultrapure water W3 is returned to the sub-tank 10.

In the primary pure water production device 3 of the ultrapure water production system 1 as described above, in order to improve the treatment performance, such as reducing the concentration of boron in the reverse osmosis membrane device 6, an alkali such as NaOH is injected into the water supplied to the reverse osmosis membrane device 6 to adjust the pH to 8 or higher, promoting the ionization of various components and treating the water.

The electrical regeneration ion exchange device 7 used in the primary pure water production system 3 generally has cation exchange membranes and anion exchange membranes alternately arranged between a negative electrode (cathode) and an anode (anode), and these cation exchange membranes and anion exchange membranes are used to form compartments to form desalting compartments and concentration compartments, which are then filled with ion exchange resin. Examples of ion exchange membranes such as cation exchange membranes and anion exchange membranes include heterogeneous membranes formed by adding a binder such as polystyrene to powdered ion exchange resin, homogeneous membranes formed by polymerization of styrene-divinylbenzene, and membranes formed by graft polymerization of monomers with various anion exchange or cation exchange functions.

The deionization compartment is filled with a mixture or multiple layers of ion exchangers (anion exchangers and cation exchangers) made of ion exchange resins, ion exchange fibers, graft exchangers, etc. The concentration compartment, anode compartment, and cathode compartment are also filled with ion exchangers.

In the electrically regenerated ion exchange device that constitutes this pure water production system, the permeate water from the reverse osmosis membrane device 6 is used as the water supply for the electrically regenerated ion exchange device, or the permeate water from the reverse osmosis membrane device is treated in an ultraviolet oxidation device and then used as the water supply for the electrically regenerated ion exchange device.

As shown in FIG. 7, the reverse osmosis membrane device and the electric regenerative ion exchange device may be configured to treat the water to be treated W4 in a first reverse osmosis membrane device 21, add an alkaline chemical to the permeated water from an alkali addition means 22 to adjust the pH to 8-11 to produce conditioned water W5, which is then treated in a second reverse osmosis membrane device 23, and treat the permeated water W6 in an electric regenerative ion exchange device (CEDI) 24 to obtain treated water W7. In such a device, a low pressure reverse osmosis membrane (LPRO) is used in at least one of the first reverse osmosis membrane device 21 or the second reverse osmosis membrane device 23. In addition, in the electric regenerative ion exchange device 24, the feed water or treated water for the desalting compartment is used as the concentrated water, and the desalting compartment and the concentrating compartment are passed through in the same direction in all or part of the case.

However, while low-pressure reverse osmosis membranes (low-pressure RO) have a high impurity removal rate, they require an effective membrane surface pressure of 1.0 to 2.0 MPa to achieve an appropriate permeation flow rate, which necessitates large pump power and increases the amount of electricity required for operation.

The present invention has been made in consideration of the above problems, and aims to provide a pure water production system that combines the efficient operating performance of pure water production equipment with the performance of producing pure water with stable water quality, and a pure water production method using the same.

In view of the above object, the present invention first provides a pure water production system having a first ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa to which water to be treated is supplied, a pH adjustment device that adjusts the pH of the permeate from the first ultra-low pressure reverse osmosis membrane device to 8 to 11, a second ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa to which conditioned water pH-adjusted by the pH adjustment device is supplied, and an electrically regenerated ion exchange device to which the permeate from the second ultra-low pressure reverse osmosis membrane device is supplied (Invention 1).

According to this invention (Invention 1), in a pure water production system equipped with a two-stage reverse osmosis membrane device that alkalizes the water to be treated by the second-stage reverse osmosis membrane, an ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa is used as the first-stage reverse osmosis membrane device, so that the water to be treated (raw water to be treated) can be treated with an appropriate flux with lower power consumption than conventional low-pressure reverse osmosis membranes. After that, an alkaline chemical is added from a pH adjustment device to adjust the pH of the permeate from the first-stage reverse osmosis membrane device to 8 to 11 to promote ionization of difficult-to-remove components, and then an ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa is used as the second-stage reverse osmosis membrane device, so that the water to be treated (raw water to be treated) can be treated with an appropriate flux with lower power consumption than conventional low-pressure reverse osmosis membranes. Then, the permeate from this second-stage reverse osmosis membrane device is treated with an electrically regenerated ion exchange device (CEDI), so that pure water of stable water quality can be produced. This allows for a pure water production system that combines efficient operation of the pure water production equipment with the production of pure water with stable water quality.

In the above invention (Invention 1), it is preferable to pass water through the concentration compartment in a countercurrent manner to the direction of water flow in the desalting compartment of the electrically regenerated ion exchange device, and to pass the feed water or treated water of the electrically regenerated ion exchange device through the concentration compartment (Invention 2).

According to this invention (Invention 2), when the permeate from the second-stage reverse osmosis membrane device is treated with an electrical regenerative ion exchange device (CEDI), the feed water to the desalting compartment or treated water (desalted water) is used as the concentrated water of the electrical regenerative ion exchange device, and the water is passed countercurrently through the desalting compartment and the concentrating compartment, thereby mitigating the concentration difference between the desalting compartment and the concentrating compartment, and high-purity pure water can be stably produced.

In the above invention (Invention 1 or 2), it is preferable to have a resistivity meter or conductivity meter that measures the resistivity or conductivity of the treated water 10 seconds or more after it has been discharged from the desalting chamber of the electrically regenerated ion exchange device (Invention 3).

According to the above invention (Invention 3), the treated water (demineralized water) from the electrical regenerative ion exchange device is not in a state in which carbon dioxide dissociates in the water immediately after discharge, but rather the carbon dioxide dissociates and ionizes over time, lowering the resistivity value. Therefore, when measuring the quality of the treated water (demineralized water) from the electrical deionization device with a resistivity meter or conductivity meter, the quality of the demineralized water can be accurately measured by measuring after a certain period of time has elapsed, and the quality of the demineralized water from the electrical deionization device can be appropriately managed.

Secondly, the present invention provides a method for producing pure water, comprising a first reverse osmosis membrane treatment step of treating the water to be treated with a first ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa, a pH adjustment step of adjusting the pH of the permeate from the first ultra-low pressure reverse osmosis membrane device to 8 to 11, a second reverse osmosis membrane treatment step of treating the pH-adjusted water with a second ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa, and a deionization step of treating the permeate from the second ultra-low pressure reverse osmosis membrane device with an electrical regeneration ion exchange device to obtain treated water (Invention 4).

According to this invention (Invention 4), by using an ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa, the water to be treated (raw water to be treated) is treated with an appropriate flux with lower power consumption than conventional low-pressure reverse osmosis membranes, and then an alkaline chemical is added from a pH adjustment device to adjust the pH of the permeate from the first stage reverse osmosis membrane device to 8 to 11 to promote ionization of difficult-to-remove components, and then an ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa is used to treat the water to be treated (raw water to be treated) with an appropriate flux with lower power consumption than conventional low-pressure reverse osmosis membranes. Then, by treating the permeate from this second stage reverse osmosis membrane device with an electrical regenerative ion exchange device (CEDI), pure water can be produced stably. As a result, pure water can be produced by achieving both efficient operation of the pure water production facility and stable water quality.

In the above invention (Invention 4), in the deionization process, it is preferable to pass water through the concentration chamber in a countercurrent to the water flow direction of the desalting chamber of the electrically regenerated ion exchange device, and to pass the feed water or treated water of the electrically regenerated ion exchange device through the concentration chamber (Invention 5).

According to this invention (Invention 5), when the permeate water from the second-stage reverse osmosis membrane device is treated with an electrical regenerative ion exchange device (CEDI), the water supplied to the deionization compartment or the treated water (desalted water) is used as the concentrated water of the electrical regenerative ion exchange device, and the water is passed countercurrently through the deionization compartment and the concentrating compartment. This reduces the concentration difference between the deionization compartment and the concentrating compartment, making it possible to stably produce high-purity pure water.
High-purity pure water can be produced stably.

In the above inventions (Invention 4 or 5), it is preferable to measure the resistivity of the treated water obtained in the deionization process 10 seconds or more after the treated water is discharged from the desalting chamber of the electrically regenerated ion exchange device, and to control the operating conditions of the electrically regenerated ion exchange device so that the measured resistivity is 15 MΩ cm or more (Invention 6).

According to the above invention (Invention 6), the treated water (demineralized water) from the electrical regenerative ion exchange device is not in a state in which carbon dioxide has dissociated in the water immediately after discharge, but rather the carbon dioxide dissociates and ionizes over time, lowering the resistivity value. Therefore, when measuring the quality of the treated water (demineralized water) from the electrical deionization device with a resistivity meter or conductivity meter, the quality of the demineralized water can be accurately measured by measuring after a certain period of time has elapsed, and the quality of the demineralized water from the electrical deionization device can be appropriately managed.

According to the pure water production system of the present invention, in a pure water production system equipped with a two-stage reverse osmosis membrane device that alkalizes the water to be treated by the second-stage reverse osmosis membrane, ultra-low pressure reverse osmosis membrane devices with an effective membrane surface pressure of 0.4 to 0.9 MPa are used as the first-stage reverse osmosis membrane device and the second-stage reverse osmosis membrane device, and by treating the permeate water of this second-stage reverse osmosis membrane device with an electrically regenerative ion exchange device (CEDI), it is possible to achieve a good balance between efficient operation of the pure water production equipment and production of pure water with stable water quality. As a result, for example, pure water with high purity, with a resistivity value of 17 MΩ cm or more, can be produced with less electricity from the treated water of the electrically regenerative ion exchange device.

FIG. 2 is a flow diagram showing the configuration of a two-stage reverse osmosis membrane treatment device and an electrical regeneration type ion exchange device in a pure water production system according to one embodiment of the present invention. FIG. 2 is a schematic diagram showing an example of a water flow system of the electrically regenerated ion exchange device in the embodiment. FIG. 4 is a schematic diagram showing a method for measuring the resistivity of treated water from an electrically regenerated ion exchange device in the embodiment. FIG. 4 is a schematic diagram showing another example of a water flow system of the electrically regenerated ion exchange device in the embodiment. FIG. 2 is a flow diagram showing the configuration of a two-stage reverse osmosis membrane treatment device and an electrical regeneration type ion exchange device in the pure water production system used in Example 1. 1 is a flow diagram showing an ultrapure water production apparatus to which the pure water production system of the present invention can be applied. FIG. 1 is a flow diagram showing the configuration of a two-stage reverse osmosis membrane treatment device and an electrical regeneration type ion exchange device in a conventional pure water production system.

The pure water production system of the present invention will be described below with reference to the attached drawings.

[Pure water production system]
The pure water producing system of this embodiment may be any system having an electrical regeneration type ion exchange device downstream of a two-stage reverse osmosis membrane device that renders the treated water alkaline through the second-stage reverse osmosis membrane, and can be suitably applied, for example, to the primary pure water producing apparatus 3 in the ultrapure water producing system 1 shown in FIG. 5 described above.

Specifically, as shown in FIG. 1, the two-stage reverse osmosis membrane device has a first reverse osmosis membrane device (ULP-RO1) 31 for treating water W4, an alkali addition means 32 as a pH adjustment device that adds an alkaline chemical to the treated water (permeate) of this first reverse osmosis membrane device 31, a second reverse osmosis membrane device (ULP-RO2) 33 that treats conditioned water W5 whose pH has been adjusted to 8 to 11 by adding an alkaline chemical, and an electrically regenerated ion exchange device (CEDI) 34 that treats the treated water (permeate) W6 of this second reverse osmosis membrane device 33 to produce treated water W7.

(Reverse osmosis membrane)
An ultra-low pressure reverse osmosis membrane is used as each of the first reverse osmosis membrane device 31 and the second reverse osmosis membrane device 33. In this embodiment, the ultra-low pressure reverse osmosis membrane is a reverse osmosis membrane having a permeation flux of 0.4 to 0.9 ^m3 /( ^m2 ·day) at an effective membrane surface pressure (water temperature 25°C, pure water (RO permeate)) of 0.4 to 0.9 MPa, a NaCl removal rate of 90% or more, and a boron removal rate of 30% or more.

With low-pressure reverse osmosis membranes with an effective membrane surface pressure of more than 0.9 MPa, the purity of the pure water obtained is improved, but power consumption increases. On the other hand, with reverse osmosis membranes with an effective membrane surface pressure of less than 0.4 MPa, the quality of the pure water obtained is reduced.

The low-pressure reverse osmosis membrane is a reverse osmosis membrane having a permeation flux of 0.3 to 1.0 ^{m 3} /(m ² ·day) at an effective membrane surface pressure of 1.0 to 2.0 MPa, a NaCl rejection rate of 99% or more, and a boron rejection rate of 50% or more.

(Electrically regenerated ion exchange device)
In this embodiment, the electrical regeneration ion exchange device is configured such that cation exchange membranes and anion exchange membranes are alternately arranged between a cathode and an anode, and these cation exchange membranes form compartments to form a deionization compartment (compartment D) and a concentration compartment (compartment C) as well as an anode compartment and a cathode compartment at both ends, as shown in Fig. 2. Water for the deionization compartment (permeated water W6 from the second reverse osmosis membrane device 33) is supplied to the concentration compartment (compartment C), the anode compartment, and the cathode compartment. The permeated water W6 is supplied to the concentration compartment (compartment C) in the opposite direction (countercurrent) to the deionization compartment (compartment D).

[Method of producing pure water]
The method of operating the above-described pure water production system will now be described.

(First reverse osmosis membrane treatment step)
First, a water supply pump (not shown) is driven to supply the water to be treated W4 to the first reverse osmosis membrane device 31. When water is passed through the ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa, the first reverse osmosis membrane device 31 can treat the water with an appropriate flux and with lower power consumption than conventional low-pressure reverse osmosis membranes.

(pH adjustment step)
Next, NaOH solution is added to the permeate from the first reverse osmosis membrane device 31 from the alkali adding means 32 to adjust the pH of the permeate from the first reverse osmosis membrane device 31 to 8 to 11, and components that are difficult to ionize, such as silica and carbon dioxide, are ionized.

(Second reverse osmosis membrane treatment step)
Subsequently, the conditioned water W5, whose pH has been adjusted to 8 to 11, is treated in the second reverse osmosis membrane device 33. When the second reverse osmosis membrane device 33 passes water through an ultra-low pressure reverse osmosis membrane device with an effective membrane surface pressure of 0.4 to 0.9 MPa, it can treat the water with an appropriate flux and with lower power consumption than conventional low-pressure reverse osmosis membranes.

(Deionization process)
Then, the permeate W6 from the second reverse osmosis membrane device 33 is treated with an electrically regenerative ion exchange device (CEDI) 34 to remove trace amounts of ionic impurities contained in the permeate W6, thereby obtaining treated water W7.

In this embodiment, the water flows through the desalting compartment of the electrically regenerated ion exchange device 34 in the opposite direction (countercurrent) to the concentrating compartment, which reduces the difference in impurity concentration between the desalting compartment and the concentrating compartment, allowing high-purity pure water to be produced stably. For example, high-purity pure water with a resistivity of 17 MΩ·cm or more for the treated water W7 of the electrically regenerated ion exchange device can be produced with less electricity.

The resistivity of the desalted water (treated water) W7 of the electrically regenerative ion exchange device 34 is preferably measured, for example, by a method as shown in Figure 3. In Figure 3, the multiple (four) series of electrically regenerative ion exchange devices 34A, 34B, 34C, and 34D are connected to branch pipes 41A-41D that supply the permeate water W6 of the second reverse osmosis membrane device 33 to the desalting chambers of the electrically regenerative ion exchange devices 34A-34D branched from a water supply pipe 41, and outlet pipes 42A-42D of the treated water (desalted water) W7 are connected to the outlets of the desalting chambers. These outlet pipes 42A-42D join at a junction pipe 42 to supply the treated water W7 to the downstream system. Resistivity meters 43A to 43D are provided in the outflow pipes 42A to 42D, respectively, and these resistivity meters 43A to 43D measure the resistivity of the treated water (desalinated water) W7 immediately after it is discharged from the desalting chambers of the electrically regenerated ion exchange devices 34A to 34D, thereby monitoring the quality of the treated water. A resistivity meter 43 is also provided in the junction pipe 42.

In such a system using an electric deionization device, the outlet pipes 42A-42D and the junction pipe 42 are preferably constructed entirely, at least up to the connection points of the resistivity meters 43A-43D, from pipe material made of a hard resin material with gas barrier properties such as polypropylene (PP) or polyvinyl chloride (PVD), or from a metal pipe material. These materials have excellent gas barrier properties, so by constructing the outlet pipes 42A-42D and the junction pipe 42 from such materials, it is possible to prevent the treated water discharged from the outlet of the desalination chamber from dissolving gas due to external factors and deteriorating in water quality before it reaches the resistivity meters 43A-43D, where the resistivity values are measured.

The water sampling tubes 44A-44D of the resistivity meters 43A-43D are made of a gas barrier material such as perfluoroalkoxyalkane (PFA) or nylon, and their length is preferably such that the desalted water (treated water) W7 discharged from the outlet of the desalting chamber of the electrically regenerated ion exchange device 34A-34D requires a residence time of 10 seconds or more, particularly 30 seconds or more, before reaching the measuring section of the resistivity meters 43A-43D. If the length is less than 10 seconds, the resistivity will be measured in a state where the carbon dioxide in the desalted water (treated water) is hardly dissociated, which is not preferable. The length of the water sampling tubes 44A-44D may be set to the desired residence time depending on the tube diameter and water flow rate of the water sampling tube.

The reason for this configuration is as follows. That is, when the permeate W6 of the second reverse osmosis membrane device 33 is supplied to the desalination chamber of the electric regenerator ion exchange device 34A-34D, ionic impurities are removed and treated water (desalinated water) W7 is discharged. At this time, the carbon dioxide that is not dissociated in the treated water (supply water) W7 is not ionized immediately after it leaves the desalination chamber, so it does not affect the resistivity value, but the carbon dioxide dissociates and ions over time, decreasing the resistivity value of the treated water (desalinated water) W7. Therefore, by setting the length of the water sampling tubes 34A-34D to a length that allows the desalinated water (treated water) to remain for 10 seconds or more, preferably 30 seconds or more, from the outlet of the desalting chamber until it reaches the measuring section of the resistivity meter 43A-43D, the carbonate ion concentration of the desalinated water (treated water) can be measured with high accuracy. A length of less than 10 seconds until it reaches the measuring section is not preferable because it is not possible to accurately measure the carbonate ion concentration of the desalinated water (treated water). In addition, the resistivity of the treated water W7 that joins at the junction 42 can be measured with a resistivity meter 43 to confirm the resistivity value.

As described above, the quality of the desalted water from the electrically regenerated ion exchange device is measured, and the operating conditions of the electrically regenerated ion exchange device are controlled so that the measured resistivity is 15 MΩ·cm or more, thereby making it possible to maintain a favorable water quality for the treated water W7. Furthermore, the performance of the electrically regenerated ion exchange device can be accurately grasped, and it is possible to judge not only the suitability of the operating conditions of the electrically regenerated ion exchange device, but also its deterioration and lifespan. It is also possible to accurately grasp the load on downstream equipment and reflect this in the design.

The pure water production system and the pure water production method of the present invention have been described above with reference to the attached drawings, but the present invention is not limited to the above embodiment and various modifications are possible. For example, the electrically regenerated ion exchange device 34 may be configured to pass the treated water W7 in the opposite direction (counterflow) between the desalting compartment and the concentrating compartment as shown in FIG. 4. By adopting such a configuration, pure water of higher purity can be stably produced. In addition, in the above embodiment, the length of the resistivity meter's water sampling tube is adjusted, and the installation position of the resistivity meter is adjusted, but this is not limited to this. A retention section for desalted water may be provided in the outflow pipes 42A to 42D, and the resistivity of the desalted water may be measured by the resistivity meter after retention for 10 seconds or more, preferably 30 seconds or more. Also, a conductivity meter may be provided instead of the resistivity meter, and the quality of the desalted water from the electrically regenerated ion exchange device may be similarly managed by conductivity instead of resistivity.

The present invention will be described in more detail below with reference to specific examples, but the present invention is not limited to the following examples.

[Test pure water production equipment]
A pure water production system for testing was prepared as shown in Fig. 5. In Fig. 5, the pure water production system 1 has a structure in which a raw water tank 52, a first feed water pump (RO1 feed water pump) 53, a first reverse osmosis membrane (RO1) 54, a tank 55 for permeated water of the first reverse osmosis membrane 54, an alkali adding means 56 using a NaOH solution, a second feed water pump (RO2 feed water pump) 57, a second reverse osmosis membrane (RO2) 58, an electrical regenerative ion exchange device (CEDI) 59, and a treated water tank 60 for desalted water (treated water) W7 of the electrical regenerative ion exchange device 59 are sequentially provided.

[Reverse osmosis membrane device]
As the reverse osmosis membrane (RO membrane), an ultra-low pressure reverse osmosis membrane and a reverse osmosis membrane were used. The performance and operating conditions of these are shown below.

Ultra-low pressure reverse osmosis membrane: effective membrane surface pressure of 0.5 to 0.8 MPa, permeate volume of 0.5 to 0.8 m ³ /(m ² ·day), and water recovery rate of 83 to 87%
Low-pressure reverse osmosis membrane: effective membrane surface pressure 1.2-1.5 MPa, permeate volume 0.5-0.8 ^{m 3} /(m ² ·day), and water recovery rate 83-87%

[Examples 1 to 3, Comparative Examples 1 to 5]
Raw water (water to be treated) W4 containing Na ions, carbonate ions, and boron (B) of 10 to 20 μg/L and having an electrical conductivity of about 20 mS/m was prepared. In the test device shown in FIG. 5, the raw water W4 was supplied from a raw water tank 52 to a first reverse osmosis membrane (RO1) 54 by a first feed water pump (RO1 feed water pump) 53, and the treated water (permeated water) of the first reverse osmosis membrane 54 was received in a tank 55. Next, the permeated water of the first reverse osmosis membrane 54 was supplied to a second reverse osmosis membrane (RO2) 58 by a second feed water pump (RO2 feed water pump) 57, and at this time, NaOH solution was added to the permeated water of the first reverse osmosis membrane 54 from an alkali addition means 56 as necessary to obtain adjusted water W5. This adjusted water W5 was treated with a second reverse osmosis membrane (RO2) 58 to obtain permeated water W6 of the second reverse osmosis membrane. This permeate W6 was treated in an electrically regenerated ion exchanger (CEDI) 59 to obtain treated water W7.

In the above-mentioned treatment, the water supply rate of the first feed water pump 53 was about 14 m ³ /h, the water recovery rate of the first reverse osmosis membrane 54 was 83-87%, the water supply rate of the second feed water pump 57 was about 12 m ³ /h, and the water recovery rate of the second reverse osmosis membrane 58 was 88-92%. In addition, the permeated water W6 of the second reverse osmosis membrane 58 was directly supplied to the electrically regenerative ion exchange device 59, which was operated at a constant current of 10 A with a water recovery rate of 93-97%, and the voltage fluctuated between 100 and 150 V. At this time, the amount of treated water W7 of the electrically regenerative ion exchange device 59 was about 10 m ³ /h in both cases.

When the pH of the water fed to the second reverse osmosis membrane 58 is made alkaline, 1% NaOH solution is added to the permeate of the first reverse osmosis membrane 54 from the alkali addition means 56 as necessary to adjust the water to a predetermined pH, and this is used as adjusted water W5.

In the electrically regenerated ion exchange device 59, the water flow method was switched between parallel flow, in which the water flow direction to the desalting compartment is the same as that to the concentration compartment, and countercurrent flow, in which the water flow direction is the opposite, depending on the conditions. In addition, the water supply to the concentration compartment was switched between passing the permeate W6 of the second reverse osmosis membrane 58, which is the same as the water supply to the desalting compartment of the electrically regenerated ion exchange device 59, and passing the treated water W7 of the electrically regenerated ion exchange device 59, depending on the conditions.

The power consumption of the RO1 feed water pump 53, the RO2 feed water pump, and the electrical regenerator ion exchanger 59 in the production process of treated water W7 were measured, and the total of these was calculated. In addition, the boron concentration of treated water W7, the resistivity immediately after discharge from the desalting compartment, and the resistivity 30 seconds after discharge were measured. These results are shown in Tables 1 to 3 together with the feed water pH and membrane type of the first reverse osmosis membrane 54, the feed water pH and membrane type of the second reverse osmosis membrane 58, the water flow system of the desalting compartment and the concentration compartment of the electrical regenerator ion exchanger 59, and the feed water type and recovery rate of the concentration compartment.

As is clear from Tables 1 to 3, in Examples 1 to 3 in which ultra-low pressure reverse osmosis membranes are used as the first reverse osmosis membrane 54 and the second reverse osmosis membrane 58, the pH of the feed water (adjusted water) W5 of the second reverse osmosis membrane 58 is set to 9 to 10, and the water flow method of the electrically regenerated ion exchange device 59 is countercurrent, it is possible to obtain treated water of high purity with less power consumption.

In contrast, in Comparative Examples 1, 2, and 4 in which an ultra-low pressure reverse osmosis membrane was used as the first reverse osmosis membrane 54 and the second reverse osmosis membrane 58, and the pH of the feed water of the second reverse osmosis membrane 58 was not adjusted, the water quality of the treated water W7 was worse than that of Examples 1 and 2, regardless of the water flow method of the electric regenerator ion exchange device 59. In Comparative Examples 3 and 5 in which a low pressure reverse osmosis membrane was used as the first reverse osmosis membrane 54 and an ultra-low pressure reverse osmosis membrane was used as the second reverse osmosis membrane 58, treated water with high purity was obtained, but the power consumption was increased by about 40% compared to Examples 1 to 3. Furthermore, in Comparative Examples 1 and 3 in which the water flow method of the electric regenerator ion exchange device 59 was a parallel flow type, the resistivity value after 30 seconds had passed from the outlet of the desalting compartment of the electric regenerator ion exchange device 59 further decreased. This is thought to be because molecular CO ₂ present in the treated water W7 of the electric regenerator ion exchange device 59 dissociated over time, lowering the resistivity value.

1 Ultrapure water production system 2 Pretreatment device 3 Primary pure water production device (pure water production system)
4 Subsystem 5, 6 Reverse osmosis membrane device 7 Electrically regenerated ion exchange device 8 Ultraviolet oxidation device 10 Subtank 11 Supply pump 12 Ultraviolet oxidation device 13 Non-regenerative mixed bed ion exchange device 14 Ultrafiltration membrane (UF membrane)
15 Point of use 21 First reverse osmosis membrane device 22 Alkali adding means (pH adjusting device)
23 Second reverse osmosis membrane device 24 Electrically regenerative ion exchange device 31 First reverse osmosis membrane device 32 Alkali adding means 33 Second reverse osmosis membrane device 34, 34A to 34D Electrically regenerative ion exchange device 41 Water supply pipes 41A to 41D Branch pipe 42 Junction pipes 42A to 42D Outlet pipes 43, 43A to 43D Resistivity meters 44A to 44D Water sampling tube 51 Pure water production device (pure water production system)
52 raw water tank 53 first feed water pump 54 first reverse osmosis membrane 55 permeate tank 56 alkali addition means 57 second feed water pump 58 second reverse osmosis membrane 59 electrical regenerative ion exchange device (CEDI)
60 Treatment water tank W Raw water W1 Pre-treated water W2 Primary pure water (pure water)
W3 Ultrapure water W4 Water to be treated W5 Adjusted water W6 Permeate water from second reverse osmosis membrane device W7 Treated water from electrical regeneration ion exchange device

Claims (4)

a first ultra-low pressure reverse osmosis membrane device to which the water to be treated is supplied and which has an effective membrane surface pressure of 0.4 to 0.9 MPa;
A pH adjusting device that adjusts the pH of the permeate of the first ultra-low pressure reverse osmosis membrane device to 8 to 11;
a second ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa to which adjusted water having a pH adjusted by the pH adjuster is supplied;
an electrically regenerated ion exchange device to which permeated water from the second ultra -low pressure reverse osmosis membrane device is supplied ;
a resistivity meter or conductivity meter for measuring the resistivity or conductivity of the treated water 10 seconds or more after it is discharged from the desalting compartment of the electrical regeneration type ion exchange device;
A pure water production system having the above structure. The pure water production system according to claim 1, in which water is passed through the concentration chamber in a countercurrent direction to the water flow direction of the desalting chamber of the electrically regenerated ion exchange device, and the feed water or treated water of the electrically regenerated ion exchange device is passed through the concentration chamber. a first reverse osmosis membrane treatment step in which the water to be treated is treated with a first ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa;
a pH adjustment step of adjusting the pH of the permeate of the first ultra-low pressure reverse osmosis membrane device to 8 to 11;
a second reverse osmosis membrane treatment step in which the adjusted water having the pH adjusted by the pH adjustment step is treated by a second ultra-low pressure reverse osmosis membrane device having an effective membrane surface pressure of 0.4 to 0.9 MPa;
a deionization step of treating the permeate from the second ultra-low pressure reverse osmosis membrane device with an electric regenerator ion exchange device to obtain treated water,
A method for producing pure water, comprising: measuring the resistivity value of the treated water obtained in the deionization process 10 seconds or more after the treated water is discharged from the desalting compartment of the electrically regenerated ion exchange device; and controlling the operating conditions of the electrically regenerated ion exchange device so that the measured resistivity value is 15 MΩ cm or more . 4. The method for producing pure water according to claim 3, wherein in the deionization step, water is passed through a concentration compartment in a countercurrent manner to the direction of water flow through the desalting compartment of the electrically regenerated ion exchange device, and feed water or treated water from the electrically regenerated ion exchange device is passed through the concentration compartment .

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