Pure water production device and pure water production method

Abstract

A pure water production device 10 produces pure water by sequentially treating water to be treated, and has: a first membrane separation device 11 having a reverse osmosis membrane for treating the water to be treated; one or more electric deionized water production devices 12 for desalinating permeated water from the first membrane separation device 11; and a second membrane separation device 13 having a reverse osmosis membrane or a nanofiltration membrane for treating deionized water from the one or more electric deionized water production devices 12.

Description

Apparatus and method for producing pure water

[0001] The present invention relates to an apparatus and a method for producing pure water.

[0002] Conventionally, in the manufacturing processes of semiconductor devices and liquid crystal devices, pure water (including ultrapure water) with highly removed impurities such as organic substances, ionic components, fine particles, bacteria, etc. has been used for applications such as cleaning electronic components. In recent years, with the increasing integration and miniaturization of semiconductor devices, the requirements for the quality of such pure water have been steadily increasing. In particular, it is required to reduce boron as trace impurities. As a method for highly removing boron, it is known to use an electrodialysis deionized water production apparatus, and multi-stage treatment is also performed in which a plurality of electrodialysis deionized water production apparatuses are connected in series to continuously treat the water to be treated (for example, see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2022-060806

[0004] On the other hand, in addition to boron, the reduction of total organic carbon (TOC) as trace impurities in pure water is also required. However, in the above-mentioned multi-stage treatment, although pure water with a boron concentration at the ng / L level can be obtained, non-ionic TOC cannot be removed by the electrodialysis deionized water production apparatus, so there is a limit to reducing the TOC concentration in pure water to the required level. Therefore, in order to remove such non-ionic TOC components, it is also conceivable to incorporate an ultraviolet oxidation apparatus into the above-mentioned multi-stage treatment system. However, in that case, hydrogen peroxide will be generated during the ultraviolet oxidation treatment for decomposing TOC. The generation of such by-products is not preferable in meeting the strict water quality requirements for pure water.

[0005] Therefore, an object of the present invention is to produce pure water with highly removed boron and total organic carbon while suppressing the generation of by-products.

[0006] To achieve the above-mentioned objectives, the present invention provides a pure water production apparatus that sequentially processes water to be treated to produce pure water, comprising: a first membrane separation apparatus having a reverse osmosis membrane for treating the water to be treated; at least one electrolytic deionized water production apparatus for desalinizing the permeate from the first membrane separation apparatus; and a second membrane separation apparatus having a reverse osmosis membrane or nanofiltration membrane for treating the deionized water from at least one electrolytic deionized water production apparatus.

[0007] Furthermore, the present invention relates to a method for producing pure water, which involves sequentially treating water to be treated to produce pure water, and includes the steps of: treating the water to be treated with a first membrane separation device having a reverse osmosis membrane; desalination of the permeate obtained in the treatment step using at least one electrolytic deionized water production device; and treating the deionized water obtained in the desalination step using a second membrane separation device having a reverse osmosis membrane or a nanofiltration membrane.

[0008] With this type of pure water production apparatus and method, components of total organic carbon (TOC) contained in the treated water that cannot be removed by the electrolytic deionized water production apparatus can be removed by the second membrane separation apparatus. Therefore, ultraviolet oxidation treatment to decompose TOC becomes unnecessary, and hydrogen peroxide is not generated as a by-product in that process. In addition, even if boron cannot be sufficiently removed by the first membrane separation apparatus and the electrolytic deionized water production apparatus alone, the remaining boron can be removed by the second membrane separation apparatus.

[0009] As described above, according to the present invention, it is possible to produce pure water from which boron and total organic carbon have been highly removed while suppressing the generation of by-products.

[0010] This is a schematic diagram of a pure water production apparatus according to one embodiment of the present invention. This is a schematic diagram of a conventional pure water production apparatus. This is a graph showing the change in TOC concentration in treated water over time in Example 2 and Comparative Example 3.

[0011] Embodiments of the present invention will be described below with reference to the drawings.

[0012] Figure 1 is a schematic diagram of a pure water production apparatus according to one embodiment of the present invention. It goes without saying that the configuration of the pure water production apparatus shown is merely an example and can be modified as appropriate depending on the purpose of use, application, and required performance of the apparatus.

[0013] The pure water production apparatus 10 produces pure water by sequentially processing raw water (water to be treated), and comprises a first reverse osmosis (RO) membrane apparatus 11, an electro-deionized water production apparatus (hereinafter also referred to as the "EDI apparatus") 12, and a second RO membrane apparatus 13.

[0014] The first RO membrane apparatus (first membrane separation apparatus) 11 is an apparatus that obtains permeate by reverse osmosis treatment of raw water, and has an RO membrane that separates the raw water into permeate from which impurities have been removed and concentrated water containing impurities. Specifically, the first RO membrane apparatus 11 consists of RO membrane modules and has at least one RO membrane element housed in a cylindrical vessel (pressure vessel). The RO membrane module constituting the first RO membrane apparatus 11 may have a single vessel, or it may be a unit formed by connecting multiple vessels in series or in parallel. Furthermore, the first RO membrane apparatus 11 may have multiple RO membrane modules connected in series or in parallel. "Connected in series" here means that the water to be treated is treated sequentially by multiple RO membrane modules. That is, in two adjacent RO membrane modules, the permeate separated in the upstream RO membrane module is supplied as the water to be treated to the downstream RO membrane module. In this case, the permeate separated by the upstream RO membrane module is further treated by the downstream RO membrane module, resulting in treated water of better quality.

[0015] The first RO membrane device 11 is connected to a water supply line L1 for supplying raw water to the first RO membrane device 11, a permeate line L2 for circulating permeate water from the first RO membrane device 11, and a first RO concentrated water line L3 for circulating concentrated water from the first RO membrane device 11 (hereinafter also referred to as "first RO concentrated water"). The water supply line L1 is equipped with a pressurizing pump 14 that pressurizes raw water stored in a raw water tank (not shown) and supplies it to the first RO membrane device 11. The pressurizing pump 14 may be equipped with an inverter (not shown) to control its rotation speed, thereby having a function to adjust the supply pressure of raw water to the first RO membrane device 11. The first RO concentrated water line L3 is branched into a drainage line L4 for discharging a portion of the first RO concentrated water to the outside and a recirculation water line L5 for recirculating the remainder to the water supply line L1 (specifically, upstream of the pressurizing pump 14). The drainage line L4 is provided with a valve V1 for adjusting the flow rate of the first RO concentrated water flowing through the drainage line L4. The recirculation line L5 is provided with a valve V2 for adjusting the pressure balance between the first RO concentrated water flowing through the drainage line L4 and the first RO concentrated water flowing through the recirculation line L5. The permeate line L2 may be provided with a tank for storing permeate from the first RO membrane device 11 and a pressurizing pump for pressurizing the permeate and supplying it to the EDI device 12. The downstream side of the recirculation line L5 may be connected to a raw water tank (not shown) provided on the water supply line L1 instead of being connected to the water supply line L1.

[0016] The EDI device 12 is connected in series to the first RO membrane device 11 via a permeate water line L2 and is a device that desalinates the permeate water from the first RO membrane device 11 to obtain deionized water. The EDI device 12 is connected to a deionized water line (first line) L6 that flows the deionized water from the EDI device 12 and supplies it to the second RO membrane device 13, a concentrated water discharge line L7 that discharges concentrated water (hereinafter also called "EDI concentrated water") from the EDI device 12 to the outside, and an electrode water discharge line L8 that discharges electrode water from the EDI device 12 to the outside. The deionized water line L6 may be provided with a tank for storing the deionized water from the EDI device 12 and a pressurizing pump that pressurizes the deionized water and supplies it to the second RO membrane device 13. Depending on the water quality, some or all of the EDI concentrated water may be returned to the water supply line L1 or a raw water tank (not shown). Therefore, a concentrated water return line may be connected to the concentrated water discharge line L7.

[0017] The EDI apparatus 12 is a device that combines electrophoresis and electrodialysis, and simultaneously performs deionization (desalination) of the water to be treated using an ion exchanger and regeneration of the ion exchanger. Although simplified in the figure, the EDI apparatus 12, as an example, has an anode chamber E1 that houses the anode, a cathode chamber E2 that houses the cathode, a desalination chamber D located between the anode chamber E1 and the cathode chamber E2, and a pair of concentration chambers C1 and C2 located on both sides of the desalination chamber D via an ion exchange membrane. The desalination chamber D is filled with at least one of a cation exchanger and an anion exchanger, and permeate water from the first RO membrane apparatus 11 is supplied as the water to be treated through the permeate water line L2. In the concentration chambers C1 and C2, permeate water from the first RO membrane apparatus 11 is supplied as concentrated water, and in the electrode chambers (anode chamber and cathode chamber) E1 and E2, permeate water from the first RO membrane apparatus 11 is also supplied as electrode water.

[0018] When permeate from the first RO membrane apparatus 11 is supplied to the desalination chamber D, the ionic components in the permeate are adsorbed and removed by the ion exchanger in the desalination chamber D. The permeate from which the ionic components have been removed is supplied as deionized water to the second RO membrane apparatus 13 through the deionized water line L6. At this time, the ionic components removed in the desalination chamber D are detached from the ion exchanger by the potential difference generated by the DC voltage applied between the two electrodes and move to the concentration chambers C1 and C2 adjacent to the desalination chamber D. Specifically, anionic components are attracted to the anode side and move to the anode-side concentration chamber C1 adjacent to the anode side of the desalination chamber D via the anion exchange membrane. Cationic components are attracted to the cathode side and move to the cathode-side concentration chamber C2 adjacent to the cathode side of the desalination chamber D via the cation exchange membrane. The ionic components that have moved to the concentration chambers C1 and C2 are taken up by the concentrated water flowing through the concentration chambers C1 and C2 and discharged to the outside through the concentrated water discharge line L7. Furthermore, the electrode water supplied to electrode chambers E1 and E2 is also discharged to the outside through the electrode water discharge line L8. Meanwhile, in desalination chamber D, a water dissociation reaction (a reaction in which water dissociates into hydrogen ions and hydroxide ions) proceeds continuously, with hydrogen ions being exchanged with cationic components adsorbed on the cation exchanger, and hydroxide ions being exchanged with anionic components adsorbed on the anion exchanger. In this way, the ion exchanger packed in desalination chamber D is regenerated.

[0019] The second RO membrane apparatus (second membrane separation apparatus) 13 is connected in series to the EDI apparatus 12 via a deionized water line L6, and is a apparatus that performs reverse osmosis treatment on deionized water from the EDI apparatus 12 to obtain permeate as pure water. Similar to the first RO membrane apparatus 11, the second RO membrane apparatus 13 consists of an RO membrane module having at least one cylindrical vessel (pressure vessel) and at least one RO membrane element housed in each vessel. Similar to the first RO membrane apparatus 11, the number of RO membrane modules constituting the second RO membrane apparatus 13 may be multiple, and they may be connected in series or in parallel.

[0020] The second RO membrane device 13 is connected to a pure water line L9 that supplies permeate (pure water) from the second RO membrane device 13 to a point of use, and to a second RO concentrated water line (second line) L10 that supplies concentrated water (hereinafter also referred to as "second RO concentrated water") from the second RO membrane device 13. In the second RO membrane device 13, the deionized water from the EDI device 12 is further subjected to reverse osmosis treatment, so from the viewpoint of water quality, it is not necessarily required to discharge the second RO concentrated water to the outside. Therefore, from the viewpoint of efficient use of water (water conservation), it is preferable to return all of the second RO concentrated water to the water supply line L1. For this purpose, the downstream side of the second RO concentrated water line L10 is connected to the water supply line L1 (specifically, the upstream side of the pressure pump 14). Alternatively, the downstream side of the second RO concentrated water line L10 may be connected to a raw water tank (not shown) provided in the water supply line L1. Furthermore, the destination of the second RO concentrated water is not particularly limited as long as it is upstream of the EDI device 12, and does not necessarily have to be the water supply line L1. For example, if a tank is provided in the permeate water line L2, that tank may be used.

[0021] With this configuration, the second RO membrane device 13 is provided downstream of the EDI device 12, allowing the second RO membrane device 13 to remove nonionic components of TOC in the raw water that cannot be removed by the EDI device 12. Therefore, there is no need to provide an ultraviolet oxidation device to decompose TOC, and hydrogen peroxide is not generated as a byproduct during the oxidation treatment process by ultraviolet irradiation. Furthermore, even if boron cannot be sufficiently removed by the first RO membrane device 11 and the EDI device 12 alone, the remaining boron can be removed by the second RO membrane device 13. As a result, pure water with a high degree of boron and TOC removal can be produced while suppressing the generation of byproducts. In addition, it is known that TOC leaks into the treated water (deionized water) when the EDI device 12 restarts after being stopped for a certain period of time, but such TOC can also be removed by the second RO membrane device 13. This also suppresses deterioration of treated water quality when the pure water production device 10 restarts operation.

[0022] Furthermore, in the second RO membrane device 13, as described above, it is not always necessary to discharge the second RO concentrated water to the outside, and it can be recirculated to the water supply line L1 through the second RO concentrated water line L10. On the other hand, in a conventional configuration in which an EDI device is provided instead of the second RO membrane device 13, electrode water is constantly discharged to the outside from the EDI device. Therefore, in this embodiment, the amount of wastewater discharged to the outside can be reduced compared to such a conventional configuration, and the recovery rate of the entire system can be increased. The recovery rate of the entire system is expressed as a percentage of the ratio of the amount of treated water obtained by the system to the sum of the amount of treated water obtained by the system and the amount of wastewater discharged from the system (i.e., the amount of water supplied to the system).

[0023] The type of RO membrane used in the second RO membrane device 13 is not particularly limited, but if a pressure pump is not provided in the deionized water line L6, it is necessary to select the membrane considering the pressure resistance strength of the EDI device 12. That is, if the EDI device 12 and the second RO membrane device 13 are connected only by the deionized water line L6, the deionized water from the EDI device 12 will flow through the deionized water line L6 under its own pressure and be supplied to the second RO membrane device 13. Therefore, the supply pressure will not exceed the pressure of the deionized water at the outlet of the EDI device 12. Incidentally, the supply pressure of the water to be treated (persistent water from the first RO membrane device 11) to the EDI device 12 is adjusted so as not to exceed the pressure resistance limit of the EDI device 12, for example, to a maximum of 0.6 to 0.7 MPa. Also, the differential pressure of the water flowing through the EDI device 12 (desalination chamber D) is usually 0.1 to 0.2 MPa. Therefore, the supply pressure of the water to be treated (deionized water from the EDI device 12) to the second RO membrane device 13 is at most 0.4 to 0.5 MPa. Considering this, it is preferable to use an ultra-low pressure RO membrane with an operating pressure of 0.5 MPa or less as the RO membrane of the second RO membrane device 13. Conversely, by using such an ultra-low pressure RO membrane, it becomes unnecessary to install a pressure pump in the deionized water line L6, and the exhaust pressure of the EDI device 12 can be effectively utilized. Instead of using an ultra-low pressure RO membrane, a nanofiltration membrane (NF membrane), also called a loose RO membrane, may be used.

[0024] Furthermore, the second RO membrane device 13 is not capable of removing boron to a very high degree, and this tendency is particularly pronounced when the above-mentioned ultra-low-pressure RO membrane is used. For example, in order to obtain pure water with a boron concentration at the ng / L level, it is necessary to reduce the boron concentration in the water to be treated supplied to the second RO membrane device 13 to a certain extent. Therefore, depending on the boron concentration in the raw water and the performance of the EDI device 12, another EDI device may be installed between the EDI device 12 and the second RO membrane device 13. However, adding an EDI device will lead to an increase in equipment costs and running costs, including the addition of ancillary equipment such as DC power supplies and tanks. From this perspective, it is preferable that the EDI device 12 has the highest possible boron removal rate, for example, 99.8% or higher. This makes it possible to reduce the boron concentration in the treated water (deionized water from the EDI device 12) supplied to the second RO membrane device 13 to a desired level without adding a separate EDI device. In other words, in order to use an ultra-low pressure RO membrane for the second RO membrane device 13, it is preferable that the boron removal rate of the EDI device 12 be as high as possible.

[0025] Before giving specific examples to confirm the effects of the present invention, we will briefly describe the configuration of a conventional pure water production apparatus in which an EDI apparatus is provided instead of a second RO membrane apparatus, with reference to Figure 2. Figure 2 is a schematic diagram of a conventional pure water production apparatus. Hereafter, components similar to those in the pure water production apparatus of this embodiment shown in Figure 1 will be denoted by the same reference numerals and their descriptions will be omitted, with only the differences being described.

[0026] A conventional pure water production system 20 has a separate EDI device (second EDI device) 21 downstream of the EDI device (first EDI device) 12. The second EDI device 21 has the same configuration as the first EDI device 12 and is connected in series to the first EDI device 12 via a deionized water line L6 to further desalinate the deionized water from the first EDI device 12. The deionized water obtained in the second EDI device 21 flows through a pure water line L9 connected to the desalination chamber D and is supplied as pure water to the point of use. On the other hand, the concentrated water discharged from the concentration chambers C1 and C2 of the second EDI device 21 is returned through the EDI concentrated water line L21 to the feedwater line L1 or the raw water tank (not shown), or to a tank (not shown) optionally provided in the permeate water line L2. Furthermore, the electrode water discharged from the electrode chambers E1 and E2 of the second EDI device 21 is discharged directly to the outside through the electrode water discharge line L22.

[0027] Next, the effects of the present invention will be explained with reference to specific examples.

[0028] (Example 1) Using a test apparatus that omitted the first RO membrane apparatus from the pure water production apparatus shown in Figure 1, the treated water quality (resistivity, boron concentration, TOC concentration, and hydrogen peroxide concentration in the treated water) was measured after 100 hours of continuous operation. RO permeate (permeate obtained by reverse osmosis treatment) with a resistivity of 1.3 MΩ·cm, a boron concentration of 11 μg / L, a TOC concentration of 8 μg / L, and a hydrogen peroxide concentration of less than 3 μg / L was used as the water to be treated. An electro-deionized water production apparatus (model number: EDI-XP2-0500) manufactured by Organo Corporation was used as the EDI apparatus. In the EDI apparatus, the treatment flow rate (flow rate of RO permeate circulating in the desalination chamber) was set to 550 L / h, and the flow rates of concentrated water and electrode water were set to 55 L / h and 20 L / h, respectively. Furthermore, as the RO membrane module of the second RO membrane apparatus, two units were used, each containing one RO membrane element in a single vessel. The RO membrane elements used were RO membrane elements manufactured by Dow Chemical Co., Ltd. (product number: XLE-4040). Deionized water from the EDI apparatus was passed through one RO membrane module, and concentrated water from the other RO membrane module was passed through the other RO membrane module. The permeates from each RO membrane module were then combined to obtain the permeate from the second RO membrane apparatus. In the second RO membrane apparatus, the flow rates of the permeate and concentrated water were set to 500 L / h and 50 L / h, respectively. The pressure of the water to be treated supplied to the EDI apparatus was 0.55 MPa, and the pressure of the water to be treated supplied to the second RO membrane apparatus was 0.40 MPa.

[0029] (Comparative Example 1) The same measurements as in Example 1 were performed using a test apparatus that was the same as the pure water production apparatus shown in Figure 2 but without the RO membrane apparatus. RO permeate water as in Example 1 was used as the water to be treated, and an electro-deionized water production apparatus (model number: EDI-XP2-0500) manufactured by Organo Corporation was used as each EDI apparatus. In the first EDI apparatus, the processing flow rate was set to 570 L / h, and the flow rates of concentrated water and electrode water were set to 57 L / h and 20 L / h, respectively. In the second EDI apparatus, the processing flow rate was set to 500 L / h, and the flow rates of concentrated water and electrode water were set to 50 L / h and 20 L / h, respectively.

[0030] (Comparative Example 2) Measurements were performed using the same test apparatus as in Comparative Example 1 and under the same conditions as in Comparative Example 1, except that an ultraviolet oxidation device was added between the two EDI devices. As the ultraviolet oxidation device, an ultraviolet oxidation device manufactured by Funatec Co., Ltd. (model number: FOV-6S) was used.

[0031] Table 1 shows the measurement results for Example 1, Comparative Example 1, and Comparative Example 2.

[0032]

[0033] Table 1 shows that in Example 1, good results were obtained for all indicators, whereas in Comparative Example 1, the TOC concentration did not decrease sufficiently, and in Comparative Example 2, the hydrogen peroxide concentration increased significantly compared to that of the treated water. On the other hand, the overall recovery rate of the system was 87% (= (500 / (500 + 55 + 20)) × 100)%) in Example 1, compared to 84% (= (500 / (500 + 57 + 20 + 20)) × 100)%) in Comparative Examples 1 and 2. Therefore, the effectiveness of installing an RO membrane device downstream of the EDI device was confirmed not only from the viewpoint of treated water quality but also from the viewpoint of water conservation.

[0034] (Example 2) Using the same test apparatus as in Example 1, two consecutive operations were performed for 75 hours and 35 hours, under the same conditions as in Example 1, and the change in TOC concentration in the treated water over time during the operation period was measured. An 89-hour shutdown period was included between the two consecutive operations.

[0035] (Comparative Example 3) The same measurement as in Example 2 was performed using the same test apparatus as in Comparative Example 1 and under the same conditions as in Comparative Example 1.

[0036] Figure 3 is a graph showing the measurement results for Example 2 and Comparative Example 3.

[0037] As is clear from Figure 3, in Example 2, the increase in TOC concentration at the start and restart of operation was significantly suppressed compared to Comparative Example 3. This is also considered to be an effect of installing the RO membrane device downstream of the EDI device.

[0038] 10 Pure water production apparatus 11 First RO membrane apparatus (first membrane separation apparatus) 12 EDI apparatus 13 Second RO membrane apparatus (second membrane separation apparatus) 14 Pressure pump D Desalination chamber C1, C2 Concentration chamber E1, E2 Electrode chamber L1 Water supply line L2 Permeate water line L3 First RO concentrated water line L4 Drainage line L5 Reflux water line L6 Deionized water line (first line) L7 Concentrated water discharge line L8 Electrode water discharge line L9 Pure water line L10 Second RO concentrated water line (second line) V1, V2 Valves

Claims

1. A pure water production apparatus for sequentially treating water to be treated to produce pure water, comprising: a first membrane separation apparatus having a reverse osmosis membrane for treating the water to be treated; at least one electrolytic deionized water production apparatus for desalinizing the permeate from the first membrane separation apparatus; and a second membrane separation apparatus having a reverse osmosis membrane or nanofiltration membrane for treating the deionized water from the at least one electrolytic deionized water production apparatus.

2. The pure water production apparatus according to claim 1, wherein the second membrane separation apparatus processes deionized water from at least one electrolytic deionized water production apparatus at an operating pressure of 0.5 MPa or less.

3. A pure water production apparatus according to claim 2, comprising a first line for supplying deionized water from at least one electrolytic deionized water production apparatus to the second membrane separation apparatus, wherein the deionized water is supplied to the second membrane separation apparatus by flowing through the first line under its own pressure.

4. A pure water production apparatus according to any one of claims 1 to 3, further comprising a second line for refluxing concentrated water from the second membrane separation apparatus to the upstream side of the at least one electro-deionized water production apparatus.

5. The pure water production apparatus according to any one of claims 1 to 3, wherein the at least one electrolytic deionized water production apparatus is one electrolytic deionized water production apparatus.

6. The electric deionized water production apparatus is a pure water production apparatus according to claim 5, wherein the boron removal rate is 99.8% or higher.

7. A method for producing pure water by sequentially treating water to be treated, comprising: a step of treating the water to be treated with a first membrane separation device having a reverse osmosis membrane; a step of desalination the permeate obtained in the first treatment step with at least one electrolytic deionization water production device; and a step of treating the deionized water obtained in the desalination step with a second membrane separation device having a reverse osmosis membrane or a nanofiltration membrane.

Images