Ultrapure water production device, ultrapure water production method, and ultrapure water production program

Abstract

The method for producing ultrapure water comprises: a first step in which a pH adjusting agent for improving alkalinity is added to water to be treated supplied to a second reverse osmosis membrane device, a portion of the treated water from the second reverse osmosis membrane device is discharged downstream, and the other portion of the treated water is refluxed upstream of the position where the pH adjusting agent is added; and a second step for reducing the addition amount of the pH regulator to the water to be treated supplied to the second reverse osmosis membrane device in accordance with a change in a preset index of the water to be treated after the treated water has refluxed.

Description

Technical Field

[0001] The present disclosure relates to an ultrapure water production device, an ultrapure water production method, and an ultrapure water production process. Background Art

[0002] The pure water production equipment included in an ultrapure water production line is equipped with a membrane treatment system that performs membrane treatment on the water being treated (i.e., the treatment target water) for a specified purpose. Specific membrane treatments include reverse osmosis (RO), ultrafiltration (UF), microfiltration (MF), nanofiltration (NF), and electrodeionization (EDI).

[0003] Each membrane treatment system can be configured in multiple stages using multiple membrane treatment devices within the system. When water to be treated is supplied to the membrane treatment device, permeate water, which undergoes the specified membrane treatment and serves as filtered water, and concentrated water, which is discharged without filtration, serves as treated water. It should be noted that, in this specification, both the concentrated water and permeate water obtained by a previous membrane treatment device can be used as the water to be treated for supply to a subsequent membrane treatment device.

[0004] As a membrane treatment system, for example, in the pure water production device disclosed in Japanese Patent Publication No. 2020-163254, a reverse osmosis membrane separation treatment device is disclosed. This reverse osmosis membrane separation treatment device includes a raw water supply line, a supply pump, and a reverse osmosis membrane that separates the raw water into permeate water and concentrated water. In addition, Japanese Patent Publication No. 2020-163254 provides a return pipe as a circulating water line. This return pipe branches off from the permeate water line of the reverse osmosis membrane and allows a portion of the permeate water to flow back to the raw water supply line. Furthermore, when the temperature of the raw water, etc., exceeds a reference temperature, the circulating water line is opened, and when the flow rate of the permeate water deviates from a specified reference flow rate range, the raw water supply rate from the supply pump is controlled. Japanese Patent Publication No. 2020-163254 discloses that even when the water temperature rises, the water quality of the pure water can be maintained, and the production of pure water will not be excessive.

[0005] On the other hand, pure water production equipment involves adjusting the pH of the treated water to an alkaline level, i.e., increasing the pH, to promote the ionization of weak electrolytes such as boron and silica and improve the removal rate of reverse osmosis membranes. Specifically, a pH adjuster such as sodium hydroxide (NaOH) is added to the treated water.

[0006] For example, Japanese Patent Application Laid-Open No. 2000-015257 describes a system in which an alkaline pH adjuster is added to the first permeate water after the first RO unit, a reverse osmosis membrane separation unit serving as the first membrane treatment unit in a permeate membrane treatment system, to adjust the pH to 8.5 or above. The alkaline first permeate water is then pressurized and supplied to the second RO unit, a reverse osmosis membrane separation unit located downstream of the first RO unit, for desalination. The pH adjustment is designed to remove ionized silica, boron, and residual carbon dioxide, and to remove impurities removable under alkaline conditions.

[0007] Furthermore, in Japanese Patent Application Laid-Open No. 2000-015257, to improve water recovery, concentrated water from the second RO unit is oxidized in another treatment unit and then returned to the upstream side of the first RO unit. The returned concentrated water from the second RO unit is then supplied to the first RO unit again.

[0008] Furthermore, in Japanese Patent Application Laid-Open No. 2000-061464, raw water, which has undergone pre-treatment such as activated carbon treatment, is adjusted to a pH of 6 or less and degassed before being passed through a first RO unit, a reverse osmosis membrane separation unit serving as the first-stage membrane treatment unit in a membrane treatment system. The permeate from the first RO unit is adjusted to alkaline by adding a pH adjuster such as sodium hydroxide (NaOH). The alkaline permeate from the first RO unit is then passed through a second RO unit, a second-stage reverse osmosis membrane separation unit, and a third RO unit, a third-stage reverse osmosis membrane separation unit, in sequence.

[0009] Furthermore, Japanese Patent Application Laid-Open No. 2000-061464 discloses that, in order to efficiently remove carbonic acid components contained in raw water as carbon dioxide (CO2), the concentrated water from the second and third RO units, along with their respective permeate water, is degassed by passing through a membrane degassing device. The degassed concentrated water from the second and third RO units is then returned to the upstream side of the first RO unit and supplied again to the first RO unit.

[0010] Patent Document 1: Japanese Patent Application Publication No. 2020-163254

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2000-015257

[0012] Patent Document 3: Japanese Patent Application Laid-Open No. 2000-061464 Summary of the Invention

[0013] Problems to be solved by the invention

[0014] However, after the treated water to which the pH regulator is added is treated in the membrane treatment device, when the treated water obtained from the membrane treatment device flows back to the upstream side of the membrane treatment device, the amount of pH regulator returned to the system of the membrane treatment device increases. Therefore, the pH regulator will be concentrated in the system, that is, in-system concentration will occur. If the alkalinity of the treated water increases due to the progress of in-system concentration, it is easy to cause the membrane of the membrane treatment device to become clogged or the pH of the permeate water to fluctuate beyond the design range. In particular, when in-system concentration occurs, not only in the membrane treatment device that directly supplies the treated water to which the pH regulator is added, but also in other membrane treatment devices arranged in the same system that are not directly supplied with the treated water to which the pH regulator is added, unexpected adverse conditions (i.e., undesirable adverse conditions) such as membrane clogging may occur in conjunction.

[0015] Regarding this point, Japanese Patent Application Laid-Open No. 2020-163254 only discloses that the quality and production of pure water improve when the water temperature is increased, but does not investigate the in-system concentration of the pH adjuster. Furthermore, Japanese Patent Application Laid-Open Nos. 2000-015257 and 2000-061464 only disclose pH adjustment to alkaline pH, but do not investigate the in-system concentration of the pH adjuster.

[0016] The present invention provides a technology capable of suppressing the concentration of the pH adjuster in the system even when water to be treated to which the pH adjuster is added is refluxed into the system of a membrane treatment apparatus.

[0017] Means of solving problems

[0018] The method for producing ultrapure water involved in the first aspect includes: a first step, in which a pH regulator that increases alkalinity is added to the treated water supplied to the membrane treatment device, a portion of the treated water of the membrane treatment device is sent to the downstream side, and another portion of the treated water is refluxed to the upstream side relative to the addition position of the pH regulator; and a second step, in which the amount of pH regulator added to the treated water supplied to the membrane treatment device is reduced according to the change of a predetermined indicator of the treated water after the treated water is refluxed.

[0019] In the first aspect, in a first step, a pH adjuster that increases alkalinity is added to the treated water supplied to the membrane treatment device. Furthermore, a portion of the treated water from the membrane treatment device is discharged downstream, while another portion of the treated water is returned upstream of the pH adjuster addition point. Furthermore, in a second step, the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced based on changes in a predetermined indicator of the treated water after the returned treated water.

[0020] In the first aspect, the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced based on changes in a predetermined indicator of the treated water after the treated water is recirculated. Therefore, even when the treated water to which the pH adjuster has been added is recirculated into the membrane treatment device system, the concentration of the pH adjuster within the system can be suppressed.

[0021] In the second aspect, based on the first aspect, the indicator is the pH of the treated water. In the second step, when the alkalinity of the pH of the treated water after the treated water is refluxed is higher than a predetermined threshold value, the amount of the pH adjuster added to the treated water supplied to the membrane treatment device is reduced.

[0022] In the second aspect, the amount of pH adjuster added to the treated water is reduced as the pH alkalinity of the treated water flowing back upstream of the pH adjuster addition point increases. Therefore, even when the treated water to which the pH adjuster has been added is circulated back into the membrane treatment system, the concentration of the pH adjuster within the system can be suppressed.

[0023] In a third aspect, in addition to the first or second aspect, the membrane treatment performed by the membrane treatment device is a reverse osmosis membrane treatment.

[0024] According to the third aspect, the concentration within the reverse osmosis membrane treatment device can be suppressed particularly effectively.

[0025] The method for producing ultrapure water involved in the fourth aspect includes: a first step, in which a pH adjuster that increases alkalinity is added to the treated water supplied to the membrane treatment device, a portion of the treated water of the membrane treatment device is sent to the downstream side, and another portion of the treated water is refluxed to the upstream side relative to the addition position of the pH adjuster at a predetermined reflux rate; and a second step, in which the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced while increasing the reflux rate of the treated water to the membrane treatment device.

[0026] In the fourth aspect, in the first step, a pH adjuster that increases alkalinity is added to the treated water supplied to the membrane treatment device. Furthermore, a portion of the treated water from the membrane treatment device is discharged downstream, while another portion of the treated water is refluxed upstream of the pH adjuster addition point. Furthermore, in the second step, the amount of refluxed treated water is increased compared to the first step, and the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced.

[0027] Specifically, as the amount of treated water returned upstream of the pH adjuster addition point increases, the amount of pH adjuster added to the treated water decreases. Therefore, even when the first treated water to which the pH adjuster has been added is returned to the membrane treatment system, the concentration of the pH adjuster within the system can be suppressed.

[0028] The ultrapure water production device involved in the fifth aspect includes: a membrane treatment device, which performs membrane treatment on the supplied treated water; a pH adjuster adding device, which is arranged on the upstream side of the membrane treatment device and adds a pH adjuster to the treated water to increase the alkalinity of the treated water; a piping, which sends a part of the treated water of the membrane treatment device to the downstream side; a reflux pipe, which allows the other part of the treated water to reflux to the upstream side relative to the pH adjuster adding device; and a control device, which controls the pH adjuster adding device according to the change of a predetermined indicator of the treated water after the reflux, so as to reduce the amount of pH adjuster added to the treated water supplied to the membrane treatment device.

[0029] In the fifth aspect, similarly to the first aspect, even if the first treated water to which the pH adjuster is added is refluxed into the system of the membrane treatment apparatus, the concentration of the pH adjuster in the system can be suppressed.

[0030] The ultrapure water manufacturing procedure involved in the sixth aspect causes the processor to execute: a first process, which adds a pH adjuster to increase the alkalinity of the treated water supplied to the membrane treatment device, sends a part of the treated water of the membrane treatment device to the downstream side, and refluxes the other part of the treated water to the upstream side relative to the addition position of the pH adjuster; and a second process, which reduces the amount of pH adjuster added to the treated water supplied to the membrane treatment device according to the change of a predetermined indicator of the treated water after the treated water is refluxed.

[0031] In the sixth aspect, similarly to the first aspect, even when the first treated water to which the pH adjuster is added is refluxed into the system of the membrane treatment apparatus, the concentration of the pH adjuster in the system can be suppressed.

[0032] Effects of the Invention

[0033] According to the present disclosure, even when water to be treated to which a pH adjuster is added is refluxed into the system of a membrane treatment apparatus, it is possible to suppress the concentration of the pH adjuster in the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 This is a block diagram illustrating the ultrapure water production apparatus according to this embodiment.

[0035] Figure 2This is a block diagram illustrating the pure water production apparatus according to this embodiment.

[0036] Figure 3 This is a graph illustrating the relationship between the pH of the water to be treated and the boron removal rate of the treated water when the weak electrolyte to be measured is boron.

[0037] Figure 4 This is a graph illustrating the relationship between the boron concentration in the water to be treated and the pH required to achieve a boron concentration of 1 ppb in the treated water.

[0038] Figure 5 This is a block diagram showing the hardware configuration of a processor of the ultrapure water production apparatus according to this embodiment.

[0039] Figure 6 This is a flowchart illustrating a method for producing ultrapure water using the ultrapure water production apparatus according to this embodiment.

[0040] Figure 7 This is a flowchart illustrating a method for producing ultrapure water using an ultrapure water production apparatus according to a modification of the present embodiment. DETAILED DESCRIPTION

[0041] The following describes embodiments of the present disclosure. However, the present disclosure is not limited to the following embodiments. In the present disclosure, when an embodiment is described with reference to the accompanying drawings, the structure of the embodiment is not limited to the structure shown in the drawings. In addition, the sizes of the components in each figure are schematic, and the relative sizes of the components are not limited to this.

[0042] In the description of the following drawings, the same or similar parts are marked with the same or similar reference numerals. However, the drawings are schematic, and the relationship between thickness and plane size, the thickness ratio of each device and each component, etc. are different from the actual ones. Therefore, the specific thickness and plane size should be determined with reference to the following description. In addition, the drawings also contain parts with different dimensional relationships or ratios. In addition, unless otherwise specified in the specification, the number of each component disclosed in the present invention is not limited to one, and there may be multiple.

[0043] In the following embodiments, unless otherwise explicitly stated, the constituent elements (including element steps, etc.) are not essential. The same applies to numerical values and their ranges, and do not limit the present disclosure. In the present disclosure, in the numerical range expressed using "to", the numerical values recorded before and after "to" are included as the minimum and maximum values, respectively.

[0044] In the numerical ranges described in this disclosure, the upper limit or lower limit of one numerical range may be replaced by the upper limit or lower limit of another numerical range described in this disclosure. In addition, in the numerical ranges described in this disclosure, the upper limit or lower limit of the numerical range may be replaced by the value shown in the embodiments.

[0045] In the present disclosure, when a component is included, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in the composition, unless otherwise specified, the content rate or content of each component refers to the total content rate or content of the multiple substances present in the composition.

[0046] In the present disclosure, particles corresponding to each component may include multiple types of particles. When multiple types of particles corresponding to each component are present in the composition, the particle size of each component refers to the value for the mixture of the multiple types of particles present in the composition, unless otherwise specified.

[0047] In the present disclosure, the term "layer" or "film" includes not only the case where it is formed throughout the entire region when observing a region where the layer or film exists, but also the case where it is formed only in a portion of the region.

[0048] <Ultrapure Water Production Equipment>

[0049] First, refer to Figures 1 to 5 The ultrapure water production apparatus 2 including the pure water production apparatus 1 according to this embodiment will be described. Figure 1 As shown, the ultrapure water production apparatus 2 includes a primary pure water system 21 and a secondary pure water system 22 , each of which allows water to be treated to flow.

[0050] (treated water)

[0051] The water to be treated can be raw water such as groundwater, river water, tap water, or other industrial water, or recycled water obtained from ultrapure water used in semiconductor and liquid crystal manufacturing plants. The boron concentration in the treated water is preferably between 5 μg / L and 200 μg / L, and particularly preferably between 10 μg / L and 100 μg / L. The pH of the treated water is preferably near neutral, for example, within the range of pH 5 to pH 8.

[0052] It should be noted that in this specification, the expression "5 μg / L to 200 μg / L" in a numerical range means that the lower limit and the upper limit are included in the range. Therefore, for example, "5 μg / L to 200 μg / L" means "5 μg / L or more and 200 μg / L or less." The same applies to other numerical ranges.

[0053] When recycled water is used as the treated water, the pH of the recycled water is preferably adjusted to, for example, a pH range of 5 to 8 through ion exchange or neutralization. The treated water may be raw water or recycled water that has been treated through a pretreatment system or similar equipment. The pretreatment system, for example, comprises a coagulation and sedimentation system, a pressure flotation system, a filtration system, an activated carbon system, etc., and removes turbid components from the raw water.

[0054] In addition, the total carbonic acid (CO2+HCO3 - +CO3 2- The total carbonic acid concentration is preferably 3 mg / L to 50 mg / L, more preferably 30 mg / L or less. The total carbonic acid concentration is calculated as CO2. By adjusting the total carbonic acid concentration to 3 mg / L to 50 mg / L, the carbonate ion concentration or bicarbonate ion concentration in the feed water supplied to the reverse osmosis membrane device is reduced. As a result, the amount of pH adjuster or scale inhibitor required to adjust the pH of the feed water supplied to the reverse osmosis membrane device can be reduced.

[0055] Therefore, if Figure 2 As shown, the pure water production apparatus 1 of the primary pure water system 21 preferably includes a decarbonation device for removing carbon dioxide from the treated water, upstream of the first reverse osmosis membrane device 11. The decarbonation device is not particularly limited as long as it can remove carbon dioxide, and a degassing tower, a degassing membrane device, a vacuum degassing device, etc. can be used.

[0056] It should be noted that in pure water production apparatuses, when removing dissolved carbon dioxide by a decarbonation unit, acid is generally injected into the water supplied to the decarbonation unit to increase the removal rate of dissolved carbon dioxide, thereby setting the pH of the water supplied to the decarbonation unit to below 6. In the pure water production apparatus 1 of this embodiment, in order to suppress an increase in the amount of chemicals such as acids and alkalis used, the amount of acid added to the water supplied to the decarbonation unit is preferably as small as possible.

[0057] (Primary pure water system)

[0058] like Figure 1 As shown, the primary pure water system 21 is constructed by combining, for example, an electrodeionization device (EDI) 23, an ultraviolet oxidation device (in other words, TOC-UV) 24, and a regenerative mixed-bed ion exchange device (MB) 25 on the downstream side of the pure water production device 1. In the primary pure water system 21, the electrodeionization device 23 removes trace ions from the treated water after boron removal in the pure water production device 1.

[0059] After the remaining organic matter is decomposed and removed by the ultraviolet oxidation device 24, low-molecular-weight organic acids and the like generated in the ultraviolet oxidation device 24 are removed by the regenerative mixed-bed ion exchange device 25. As a result, primary pure water is produced in the primary pure water system 21. The total organic carbon (TOC) concentration of the primary pure water is, for example, 10 μgC / L or less, and the resistivity of the primary pure water is, for example, 17 MΩ·cm or greater.

[0060] Furthermore, the primary pure water system 21 may also include other devices for improving water purity in addition to or in place of the electrodeionization device 23, ultraviolet oxidation device 24, and regenerative mixed-bed ion exchange device 25. Examples of these other devices include membrane degassing devices or vacuum degassing devices for removing dissolved gases from water, anion exchange resin devices or cation exchange resin devices for removing ions, boron resin devices filled with boron-selective ion exchange resins, or ion exchange devices using these ion exchange resins as multi-layer beds. The electrodeionization device 23 may also be a multi-stage device comprising multiple electrodeionization devices connected in series.

[0061] (Secondary pure water system)

[0062] The secondary pure water system 22 is a device that removes trace organic matter and trace particulates from the primary pure water produced by the primary pure water system 21. The secondary pure water system 22 can be constructed by combining an ultraviolet oxidation device, a membrane degassing device, a non-regenerative mixed-bed ion exchange device, and an ultrafiltration device. The TOC concentration of the ultrapure water obtained by the secondary pure water system 22 is reduced to, for example, below 5 μgC / L. In addition, the resistivity of the ultrapure water is reduced to, for example, above 17.5 MΩ·cm. In addition, the boron concentration of the ultrapure water is reduced to, for example, below 1 ng / L.

[0063] <Pure Water Production Equipment>

[0064] Next, the pure water production device 1 included in the primary pure water system 21 according to this embodiment will be described. Figure 2 As shown, the pure water production apparatus 1 includes a first reverse osmosis membrane device (first RO) 11 , a second reverse osmosis membrane device (second RO) 12 , and a third reverse osmosis membrane device (third RO) 13 .

[0065] The first reverse osmosis membrane device 11 performs reverse osmosis membrane treatment on the water being treated, thereby removing salts from the water being treated. The second reverse osmosis membrane device 12 performs reverse osmosis membrane treatment on the permeate (i.e., first permeate) from the first reverse osmosis membrane device 11, thereby removing impurities such as organic matter, particulates, and boron remaining in the first permeate. The third reverse osmosis membrane device 13 performs reverse osmosis membrane treatment on the concentrated water (i.e., first concentrated water) from the first reverse osmosis membrane device 11, thereby returning the treated water to the first reverse osmosis membrane device 11.

[0066] (First reverse osmosis membrane device)

[0067] like Figure 2 As shown, the first reverse osmosis membrane device 11 includes a supply pipe 11a for introducing treated water, a permeate water pipe 11b, and a concentrated water pipe 11c. The first permeate water obtained by passing through the first reverse osmosis membrane device 11 is supplied to the second reverse osmosis membrane device 12 via the permeate water pipe 11b. The first concentrated water is supplied to the third reverse osmosis membrane device 13 via the concentrated water pipe 11c.

[0068] The upstream end of the supply pipe 11a is connected to the first pit 17. The treated water supplied to the first reverse osmosis membrane device 11 is supplied from the first pit 17. A portion of the first permeated water is returned to the first pit 17 via the permeated water return pipe 11d connecting the first pit 17 and the permeated water pipe 11b.

[0069] In the present disclosure, a device for adding a scale inhibitor or acid to the treated water may be installed at the supply pipe 11a between the first pit 17 and the first reverse osmosis membrane device 11. Furthermore, in the present disclosure, a pH measuring device may be installed at the supply pipe 11a between the first pit 17 and the first reverse osmosis membrane device 11, and the amount of pH adjuster added by the pH adjuster adding device 15 may be controlled based on the pH measured at the supply pipe 11a. It should be noted that a tank may be installed instead of the pit. The location of the pit, the pH measuring device, and the order of the pH adjuster adding device may be arbitrarily changed within the scope of the present disclosure.

[0070] The first reverse osmosis membrane device 11 includes a reverse osmosis membrane. The reverse osmosis membrane of the first reverse osmosis membrane device 11 allows for reverse osmosis membrane separation of the water being treated under pressure, thereby separating it into a first concentrated water from which salts are concentrated and a first permeate from which salts are removed. The first reverse osmosis membrane device 11 can be any reverse osmosis membrane device commonly used for pure water production, without particular limitation. For example, ultra-low pressure, low pressure, medium pressure, or high pressure reverse osmosis membrane devices can be used as the first reverse osmosis membrane device 11.

[0071] Examples of the reverse osmosis membrane included in the first reverse osmosis membrane device 11 include triacetylcellulose asymmetric membranes, polyamide-based, polyvinyl alcohol-based, or polysulfone-based composite membranes. The membrane shape is not particularly limited, and sheet-like flat membranes, spiral membranes, tubular membranes, and hollow fiber membranes can be used. In particular, polyamide-based composite membranes are preferred as the reverse osmosis membrane due to their high salt removal rate, and cross-linked wholly aromatic polyamide-based composite membranes are particularly preferred. The membrane shape is preferably a spiral membrane.

[0072] The first reverse osmosis membrane device 11 preferably has a salt removal capacity in the treated water, for example, a sodium chloride (NaCl) removal rate of 95% or greater, more preferably 99.5% or greater. Commercially available products for the first reverse osmosis membrane device 11 include TMG20, TM720, and TM800K manufactured by Toray Industries, Inc., and BW30 manufactured by Dow Chemical.

[0073] The supply pressure of the treated water to the first reverse osmosis membrane device 11 is preferably set within the range of 0.4 MPa to 6 MPa, depending on the model of the first reverse osmosis membrane device, etc. For example, when the first reverse osmosis membrane device 11 is an ultra-low pressure type, the supply pressure of the treated water is preferably 0.4 MPa to 0.8 MPa, and more preferably 0.6 MPa to 0.7 MPa.

[0074] When the first reverse osmosis membrane device 11 is a low-pressure type, the supply pressure of the treated water is preferably greater than 0.8 MPa and less than 2.0 MPa, and more preferably 1 MPa to 1.6 MPa. When the first reverse osmosis membrane device 11 is a medium-pressure type, the supply pressure of the treated water is preferably 2 MPa to 4 MPa, and more preferably 2 MPa to 3 MPa.

[0075] When the first reverse osmosis membrane device 11 is a high-pressure type, the supply pressure of the treated water is preferably greater than 4 MPa and less than 8 MPa, more preferably greater than 4 MPa and less than 6 MPa. Therefore, a water feed pump is preferably provided upstream of the first reverse osmosis membrane device 11.

[0076] In reverse osmosis membranes, higher feed water pressure generally increases the salt removal rate, but also tends to increase the formation of scale. For example, when the first reverse osmosis membrane device 11 is an ultra-low pressure type, the salt removal rate can be increased by setting the feed pressure of the treated water to the first reverse osmosis membrane device 11 to 0.4 MPa or higher, thereby further reducing the boron and silica concentrations in the pure water obtained by the pure water production apparatus 1. Furthermore, by setting the feed pressure of the treated water to 6 MPa or lower, membrane clogging due to scale can be suppressed, resulting in the stable and long-term production of pure water with reduced boron content.

[0077] From the viewpoint of improving the boron removal rate in the pure water production apparatus 1 , the water recovery rate in the first reverse osmosis membrane device 11 is preferably 50% to 95%, more preferably 60% to 90%, and even more preferably 65% to 85%.

[0078] The flow rate of the water to be treated supplied to the first reverse osmosis membrane device 11 is preferably 100 m 3 / h~1000m 3 This is because continuous operation using the circulation flow path of the first reverse osmosis membrane device 11 significantly reduces the amount of water discharged by flushing at startup in the case of intermittent operation.

[0079] The first reverse osmosis membrane device 11 treats the water to be treated, producing first permeate water and first concentrated water. Since the first concentrated water contains a high concentration of salts due to concentration, it is supplied to the third reverse osmosis membrane device 13 via the concentrated water pipe 11c. The electrical conductivity of the first permeate water is, for example, 5 μS / cm or greater.

[0080] In the present disclosure, acids or bases are sometimes added to prevent scaling in the first reverse osmosis membrane device 11. Scale inhibitors prevent hardness (i.e., calcium and magnesium) and silica in the raw water from adhering to the RO membrane in the form of scale. The agents (i.e., scale inhibitors, acids, and bases) added to prevent scaling in the first reverse osmosis membrane device are concentrated in the concentrated water. The concentrated water, enriched with the scale inhibitors, is supplied to the third reverse osmosis membrane device 13, also helping to prevent scaling in the third reverse osmosis membrane device 13.

[0081] It should be noted that antiscalants, acids, and bases are sometimes added at the inlet of the third reverse osmosis membrane device 13 (i.e., to the concentrated water from the first reverse osmosis membrane device 11). Antiscalants can also be added to the second reverse osmosis membrane device. In this case, since the concentrated water is returned to the previous stage while preventing scaling in the second reverse osmosis membrane device, the antiscalant can also effectively prevent scaling in the first reverse osmosis membrane device, and thus in the third reverse osmosis membrane device supplied with the first concentrated water.

[0082] (Second reverse osmosis membrane device)

[0083] like Figure 2 As shown, the second reverse osmosis membrane device 12 is connected to the first reverse osmosis membrane device 11 via a permeate water pipe 11b. The second reverse osmosis membrane device 12 includes a permeate water pipe 12b, a concentrated water return pipe 12c, and a permeate water return pipe 12d. The permeate water pipe 12b corresponds to the "pipe for sending a portion of the treated water from the membrane treatment device downstream" in this disclosure. The permeate water return pipe 12d corresponds to the "return pipe" in this disclosure.

[0084] A second pit 18 is provided on the permeate water pipe 11b between the first reverse osmosis membrane device 11 and the pH adjuster addition device 15. The treated water supplied to the second reverse osmosis membrane device 12 is supplied from the second pit 18. In the present disclosure, the permeate water return pipe 12d may be returned to the first pit 17, eliminating the need for the second pit 18.

[0085] The second reverse osmosis membrane device 12 corresponds to the "membrane treatment device" in the present disclosure. That is, in this embodiment, reverse osmosis membrane treatment, which is the membrane treatment in the present disclosure, is implemented. It should be noted that in the present disclosure, membrane treatments may include, in addition to reverse osmosis membrane treatments, ultrafiltration membrane treatments, microfiltration treatments, nanofiltration treatments, and electrodeionization membrane treatments, for example.

[0086] In addition, while this embodiment illustrates a case where the treated water flowing back upstream is both permeate and concentrated water, this disclosure is not limited to this. In this disclosure, in reverse osmosis membrane treatment, the treated water flowing back upstream may be either permeate or concentrated water. Furthermore, in this disclosure, the treated water flowing back upstream in each of ultrafiltration membrane treatment, microfiltration treatment, nanofiltration treatment, and electrodeionization membrane treatment is concentrated water.

[0087] The concentrated water return pipe 12c is connected to the first pit 17. The permeate water return pipe 12d is connected to the second pit 18 located upstream between the first reverse osmosis membrane device 11 and the second reverse osmosis membrane device 12. If the second pit 18 is not provided, the permeate water return pipe 12d can be returned to the first pit 17.

[0088] A pH adjuster addition device 15 is connected to the permeated water pipe 11b, located downstream of the connection point between the permeated water return pipe 12d and the permeated water pipe 11b and upstream of the second reverse osmosis membrane device 12. A pH measuring device 14 is connected to the permeated water pipe 11b, located downstream of the connection point between the pH adjuster addition device 15 and the permeated water pipe 11b and upstream of the second reverse osmosis membrane device 12. In other words, the pH adjuster addition device 15 is located upstream of the second reverse osmosis membrane device 12. Furthermore, a flowmeter 16 is connected to the permeated water return pipe 12d to measure the return flow rate of the second permeated water.

[0089] The pH measuring device 14, the pH adjusting agent adding device 15, and the flow meter 16 are each connected to the control device 30. Data on the pH of the treated water supplied to the second reverse osmosis membrane device 12, data on the amount of pH adjusting agent added to the treated water, and data on the return flow rate of the second permeated water are continuously input to the control device 30.

[0090] A portion of the permeated water obtained in the second reverse osmosis membrane device 12 (i.e., second permeated water) is sent to the downstream side of the second reverse osmosis membrane device 12 via the permeated water pipe 12b. A portion of the second permeated water obtained in the second reverse osmosis membrane device 12 is returned to the second pit 18 on the upstream side of the second reverse osmosis membrane device 12 via the permeated water return pipe 12d. The concentrated water obtained in the second reverse osmosis membrane device 12 (i.e., second concentrated water) is introduced into the first pit 17 via the concentrated water return pipe 12c and is treated again in the first reverse osmosis membrane device 11.

[0091] The second reverse osmosis membrane device 12 includes a reverse osmosis membrane. The first permeated water is subjected to reverse osmosis membrane separation under pressure by the reverse osmosis membrane of the second reverse osmosis membrane device 12. Consequently, the first permeated water is separated into a second permeated water from which impurities such as organic matter and particulates, as well as boron, remaining in the first permeated water have been removed, and a second concentrated water from which these impurities and boron have been concentrated. The second reverse osmosis membrane device can be the same as the first reverse osmosis membrane device 11. The second reverse osmosis membrane device 12 can be any of the ultra-low pressure, low pressure, medium pressure, or high pressure types, but is preferably an ultra-low pressure or low pressure type.

[0092] From the viewpoint of improving the boron removal rate, the salt removal capability of the second reverse osmosis membrane device 12 is preferably 95% or higher, and more preferably 99.5% or higher, as, for example, the NaCl removal rate.

[0093] The water pressure supplied to the second reverse osmosis membrane device 12 is preferably set within the range of 0.4 MPa to 6 MPa, similar to the water pressure supplied to the first reverse osmosis membrane device 11, depending on the model of the second reverse osmosis membrane device 12. The preferred water pressure supplied when using an ultra-low pressure, low pressure, medium pressure, or high pressure reverse osmosis membrane device is the same as the water pressure supplied to the first reverse osmosis membrane device 11.

[0094] The pH of the first permeated water is continuously measured by a pH measuring device 14. It should be noted that in the present disclosure, other weak electrolyte measuring devices, such as a silica meter or a boron monitor, may be used in place of the pH measuring device 14. In the present disclosure, silica and boron are considered weak electrolytes. Other weak electrolyte measuring devices measure the concentration of weak electrolytes in the treated water. The pH of the treated water changes with changes in the weak electrolyte concentration.

[0095] For example, in boron-containing raw water, the pH of the treated water correlates with the boron concentration in the treated water, i.e., the permeate of the reverse osmosis membrane device. Specifically, the inventors of this disclosure generated raw water for experimental treatment by injecting boric acid into ultrapure water. The boron concentration of the resulting raw water was approximately 50 ppb.

[0096] In the experiment, boron was removed by allowing raw water to pass through a reverse osmosis membrane device corresponding to the second reverse osmosis membrane device 12 of this embodiment at a pressure of approximately 1.3 MPa and recovering the water. The reverse osmosis membrane of the reverse osmosis membrane device used in the experiment was BW30 manufactured by Dow. The recovery rate of raw water was approximately 75%. The boron removal rate in the treated water and the pH of the treated water were then measured. Figure 3 As shown in Figure 2, the lower the pH value of the treated water, the lower the boron removal rate in the treated water. In other words, the lower the pH value of the treated water, the more boron will remain in the treated water passing through the reverse osmosis membrane.

[0097] In addition, according to Figure 3 The relationship between the boron concentration in the treated water and the pH required to obtain a boron concentration of 1 ppb in the treated water after passing through the reverse osmosis membrane was determined. Figure 4 As shown. Figure 4 As shown, it can be seen that the lower the boron concentration in the water to be treated, the lower the pH required to obtain a boron concentration of 1 ppb in the treated water.

[0098] It should be noted that in Figure 4 In the figure, the parameter describing the relationship between pH and boron concentration in the treated water is the pH required to achieve a boron concentration of 1 ppb in the treated water. However, the relationship between pH and boron concentration in the treated water also exhibits the same trend when other pH values are used to achieve boron concentrations. Furthermore, although not shown in the figure, the relationship between pH and concentration in the case of silica exhibits a similar trend as that in the case of boron. Furthermore, the relationship between pH and concentration of weak electrolytes other than boron and silica is similar to that in the case of boron and silica.

[0099] That is, as described above, pH alkalinity and weak electrolyte concentration correspond to the "predetermined indicators" of this disclosure. In this disclosure, the amount of pH adjuster added can also be adjusted based on the values measured by other measuring devices. During adjustment, for example, when the measured weak electrolyte concentration falls below a predetermined value, on-off control can be performed to stop the addition of the pH adjuster.

[0100] The second permeated water obtained in the second reverse osmosis membrane device 12 is sent downstream. The electrical conductivity of the second permeated water is, for example, 5 μS / cm or less. The sent second permeated water can be used directly. Alternatively, the second permeated water may be further processed to produce primary pure water, secondary pure water, or ultrapure water. The second permeated water may also be further processed in the reverse osmosis membrane device. In addition, the second concentrated water obtained in the second reverse osmosis membrane device 12 is introduced into the supply pipe 11a via the concentrated water return pipe 12c, and then returned to the first pit 17 via the concentrated water return pipe 12c. The second concentrated water returned to the first pit 17 is supplied to the first reverse osmosis membrane device 11.

[0101] The water recovery rate in the second reverse osmosis membrane device 12 is preferably 50% to 95%, more preferably 60% to 90%, further preferably 65% to 90%, and may be 65% to 85%.

[0102] In addition, as a combination of the first reverse osmosis membrane device 11 and the second reverse osmosis membrane device 12, from the perspective of improving the boron removal rate, it is preferred that one be of an ultra-low pressure type or a low pressure type, while the other be of a high pressure type or a medium pressure type. Furthermore, it is more preferred that the first reverse osmosis membrane device 11 be of a high pressure type or a medium pressure type, while the second reverse osmosis membrane device 12 be of a low pressure type or an ultra-low pressure type. Alternatively, in order to reduce operating pressure and operating costs, it is preferred that both the first reverse osmosis membrane device 11 and the second reverse osmosis membrane device 12 be of a low pressure type or an ultra-low pressure type.

[0103] (Third reverse osmosis membrane device)

[0104] like Figure 2 As shown, the third reverse osmosis membrane device 13 includes a concentrated water discharge pipe 13b and a permeate return pipe 13c. The first concentrated water supplied from the first reverse osmosis membrane device 11 undergoes reverse osmosis membrane treatment in the third reverse osmosis membrane device 13. The concentrated water obtained in the third reverse osmosis membrane device 13 (i.e., third concentrated water) is discharged to the outside of the system via the concentrated water discharge pipe 13b. The permeate return pipe 13c is connected to the supply pipe 11a located upstream of the first reverse osmosis membrane device 11. The permeate water obtained in the third reverse osmosis membrane device 13 (i.e., third permeate) is returned to the first pit 17 upstream of the first reverse osmosis membrane device 11 via the permeate return pipe 13c. The third permeate returned to the first pit 17 is supplied to the first reverse osmosis membrane device 11.

[0105] (pH adjuster adding device)

[0106] The pH adjuster adding device 15 adjusts the liquid property of the first permeated water to alkalinity. That is, the pH adjuster adding device 15 adds a pH adjuster to increase the alkalinity of the first permeated water.

[0107] The pH adjuster is water-soluble and can be used without particular limitation as long as it adjusts the pH of the first permeated water, which is the feed water supplied to the second reverse osmosis membrane device 12, to alkaline. Examples of pH adjusters for adjusting the pH to alkaline include alkali metal salts such as potassium hydroxide and sodium hydroxide. The pH adjuster may be used alone or in combination of two or more.

[0108] It should be noted that, in the present disclosure, the liquid properties of the first permeated water may be adjusted to alkaline by using an alkaline scale inhibitor as a pH adjuster. Furthermore, in the present disclosure, the pH adjuster addition device 15 may include a membrane clogging prevention agent addition device that adds a scale inhibitor or a slime control agent to the first permeated water as a membrane clogging prevention agent to prevent scale formation in the first reverse osmosis membrane device 11.

[0109] The adjusted pH value is not particularly limited, but is preferably between 8.5 and 10.5, and more preferably between 9 and 9.5. By setting the pH value within this range, weak electrolytes such as silica and boron can be ionized, thereby improving the removal rate in the first reverse osmosis membrane device 11. When the pH value is below 8.5, the removal rate of weak electrolytes decreases slightly. Furthermore, when the pH value exceeds 10.5, the amount of alkali metal in the added pH adjuster increases, thereby increasing the amount of alkali metal in the treated water. In this case, the removal rates of both silica and boron are above 90%.

[0110] In the present disclosure, there is no particular limitation on the scale of the apparatus, but the preferred treatment capacity (i.e., water supply capacity) is 100m 3 / h~1000m 3 / h device. In this case, it is particularly important to adopt a circulating operation in the second reverse osmosis membrane device and to operate the second reverse osmosis membrane device continuously to prevent deterioration of the treated water quality. This is because it can avoid the large amount of wastewater generated during startup operations when the second reverse osmosis membrane device is operated on and off.

[0111] (Control device)

[0112] Figure 2 The control device 30 is a computational control unit that controls the flow rate, temperature, pH, and other parameters associated with the treated water and the processed water in the pure water production apparatus 1. Specifically, when the pH alkalinity of the refluxed treated water exceeds a predetermined threshold, the control device 30 adds a pH adjuster in an amount less than the current pH adjuster addition amount to the treated water supplied to the second reverse osmosis membrane device 12.

[0113] In the present disclosure, the control device 30 controls the pH adjuster addition device 15 based on changes in a predetermined indicator of the treated water after reflow, so as to add a pH adjuster in an amount less than the current amount of pH adjuster added to the treated water supplied to the second reverse osmosis membrane device 12. In the present disclosure, the amount of pH adjuster added reduced by control includes zero.

[0114] like Figure 5 As shown, the control device 30 includes a CPU (Central Processing Unit) 31, a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a memory 34, a user interface 35, and a communication interface 36. The components of the control device 30 are connected to each other via a bus 37 so as to be communicable with each other.

[0115] CPU 31 is a central processing unit (CPU) that executes various programs and controls various components. Specifically, CPU 31 reads programs from ROM 32 or memory 34 and executes them using RAM 33 as a work area. CPU 31 controls the various components described above and performs various computations based on the programs stored in ROM 32 or memory 34. CPU 31 is the processor of the present disclosure.

[0116] In this embodiment, the ROM 32 or the memory 34 stores an ultrapure water production program. The ultrapure water production program is a calculation program for producing ultrapure water.

[0117] ROM 32 stores various programs and data. RAM 33 temporarily stores programs and data as a work area. Memory 34, which is composed of a hard disk drive (HDD) or a solid state drive (SSD), stores various programs and data, including the operating system.

[0118] The user interface 35 is an interface used by an operator operating the pure water production apparatus 1 to operate the control device 30. The user interface 35 may include, for example, at least one of a liquid crystal display having a touch panel that can be touched by the operator, a voice input receiving unit that receives voice input from the operator, and buttons that can be pressed by the operator.

[0119] The communication interface 36 is an interface for the control device 30 to communicate with other devices, and uses standards such as Ethernet (registered trademark), FDDI, and Wi-Fi (registered trademark).

[0120] When executing the ultrapure water production program, pure water production apparatus 1 uses the aforementioned hardware resources to implement various functions. The functional configuration implemented by pure water production apparatus 1 includes a pH adjuster addition unit, a treated water delivery unit, a treated water return unit, a pH comparison unit, a return flow rate comparison unit, and a pH adjuster addition amount reduction unit. Each of these functional configurations is implemented by CPU 31 reading and executing the ultrapure water production program stored in ROM 32 or memory 34.

[0121] <Method for producing ultrapure water>

[0122] Next, a method for producing ultrapure water using the ultrapure water production device 2 involved in this embodiment is described. First, a pure water production device 1 including a primary pure water system 21 included in the ultrapure water production device 2 is used to produce primary pure water. In this embodiment, an example is given of a case where the production of primary pure water is not performed by an operator but is automatically performed by computer control of a control device 30. It should be noted that in the present disclosure, the operator of the ultrapure water production device 2 can use the control device 30 to implement each process involved in the production of primary pure water.

[0123] Specifically, in Figure 2 In the pure water production apparatus 1, the treated water is supplied to the first reverse osmosis membrane device 11 via the supply pipe 11a, and the reverse osmosis membrane treatment is performed in the first reverse osmosis membrane device 11. In this embodiment, the temperature of the treated water supplied to the first reverse osmosis membrane device 11 is adjusted as needed. The temperature of the treated water can be, for example, approximately room temperature (i.e., 20°C to 30°C).

[0124] Next, the control device 30 of the ultrapure water production device 2 is Figure 6 In step S11, a pH adjuster is added to the first permeated water supplied to the second reverse osmosis membrane device 12 using the pH adjuster adding device 15. The amount of pH adjuster added to the first permeated water can be set so that the pH of the feed water supplied to the second reverse osmosis membrane device 12 is adjusted to a predetermined threshold value, for example, a value between 9.2 and 10. The threshold value can be set within a range having an upper limit and a lower limit, or can be set to a specific value.

[0125] It should be noted that in the present disclosure, the step of adding the scale inhibitor can be performed before or after the step of adding the pH adjuster, i.e., step S11, or simultaneously with step S11. The first permeate water, after the pH has been adjusted, is supplied to the second reverse osmosis membrane device 12 via the permeate water piping 11b, where it undergoes reverse osmosis membrane treatment.

[0126] Next, the control device 30 of the ultrapure water production device 2 is Figure 6In step S12, a portion of the second permeated water of the second reverse osmosis membrane device 12 is sent to the downstream side using the permeated water pipe 12b. Figure 6 In step S13, the other part of the second permeated water flowing in the permeated water pipe 12b is refluxed upstream of the pH adjuster addition position using the permeated water reflux pipe 12d branched from the permeated water pipe 12b.

[0127] Next, the control device 30 of the ultrapure water production device 2 measures the pH of the treated water after the second permeated water is refluxed using the pH measuring device 14 in step S14. The control device 30 then determines whether the alkalinity of the measured pH is higher than a preset threshold value. If the result of the determination is that the alkalinity of the pH of the treated water after the treated water is refluxed is higher than the preset threshold value, that is, if the alkalinity of the treated water is higher than that before the reflux, the process enters the process of Figure 6 Step S15 in .

[0128] In step S15, the control device 30 adds a smaller amount of pH adjuster than the amount added in step S11 to the treated water supplied to the second reverse osmosis membrane device 12. In other words, the pH of the previous treated water to which the pH adjuster was added is monitored, and the amount of pH adjuster added to the subsequent treated water is feedback-controlled based on the monitoring result.

[0129] It should be noted that feedback control is generally performed at locations where the quality of the treated water fluctuates significantly, such as when treating raw water in a first-stage reverse osmosis membrane device. On the other hand, in subsequent reverse osmosis membrane devices, such as the second-stage device that treats the first-stage permeate, the water quality fluctuates less, so the need for feedback control is generally less noticeable. However, in this embodiment, the amount of pH adjuster added in the second reverse osmosis membrane device 12 is intentionally feedback-controlled to suppress concentration within the system.

[0130] On the other hand, if the result of the determination in step S14 is that the pH alkalinity of the treated water after the treated water is refluxed is below the pre-set threshold value, the process does not proceed to step S15 and ends. In this embodiment, the series of steps S11 to S15 are repeatedly and continuously performed during the entire reverse osmosis membrane treatment. That is, when the process enters Figure 6 After the end of step S15, the process returns to Figure 6 The beginning of .

[0131] Therefore, the pH of the feed water supplied to the second reverse osmosis membrane device 12 is adjusted to a predetermined alkalinity threshold of 9.2 to 10. If the pH of the feed water supplied to the second reverse osmosis membrane device 12 is 9.2 or higher, the boron removal rate in the second reverse osmosis membrane device 12 can be significantly improved.

[0132] It should be noted that, in the present disclosure, the control device 30 may be configured to determine whether the alkalinity of the measured pH is lower than a preset threshold. Figure 6 If the result of step S14 is that the alkalinity of the measured pH is lower than the preset threshold value, that is, if the alkalinity of the treated water is lower than that before the reflux, the treatment may also enter step S2. Figure 6 In step S15, the control device 30 adds a ratio of Figure 6 The amount of pH adjuster added in step S11 is larger.

[0133] That is, in the present disclosure, feedback control to increase the amount of pH adjuster added can be performed not only when the alkalinity increases as shown in this embodiment, but also when the alkalinity decreases (in other words, when it decreases from a predetermined value or range serving as a threshold).

[0134] It should be noted that, in the present disclosure, the series of steps S11 to S15 described above does not necessarily need to be performed continuously throughout the entire reverse osmosis membrane treatment period. In the present disclosure, the series of steps S11 to S15 may also be performed at least once during the reverse osmosis membrane treatment period. Alternatively, the series of steps may be performed multiple times intermittently or continuously.

[0135] Then, the primary pure water produced by the pure water production device 1 is passed to the Figure 1 The secondary pure water system 22 shown supplies and performs predetermined treatments, thereby ultimately producing ultrapure water.

[0136] (Regarding other operating modes of the pure water production device)

[0137] Next, other operation modes of the pure water production apparatus 1 according to this embodiment will be described.

[0138] (On-off mode)

[0139] For example, Figure 2The case where the permeate flow rate of the second reverse osmosis membrane device 12 is set to 100, the permeate flow rate of 99 is flowing through the permeate water pipe 12b, and the permeate flow rate of 1 is flowing through the permeate water return pipe 12d is set as condition A. During the operation of the pure water production apparatus 1 under condition A, if the second permeate water cannot be supplied to the permeate water pipe 12b because the tank at the downstream stage of the second reverse osmosis membrane device 12 (not shown) is full, as condition B, for example, the permeate flow rate of the permeate water return pipe 12d is adjusted to 99, and the permeate flow rate of the permeate water pipe 12b is adjusted to 1.

[0140] After the operation of the pure water production apparatus 1 is started under condition B, if the water level in the downstream tank falls below a predetermined value, the operating conditions of the pure water production apparatus 1 are returned to condition A. In other words, control is performed such that conditions A and B are alternately changed according to the water level in the downstream tank.

[0141] Here, condition A may include a case where the permeate flow rate of the permeate water pipe 12b is set to 100, and the permeate flow rate of the permeate water return pipe 12d is set to 0 (zero). Furthermore, condition B may include a case where the permeate flow rate of the permeate water pipe 12b is set to 0 (zero), and the permeate flow rate of the permeate water return pipe 12d is set to 100. That is, one of the permeate flow rates of the permeate water pipe 12b and the permeate water return pipe 12d is set to 0 (zero), and the other is set to 100.

[0142] Switching between condition A and condition B when the permeate water flow rate of the permeate water pipe 12b or the permeate water return pipe 12d is 0 (zero) and the other is 100 is, in other words, on-off control. Even with on-off control, the conditions can be switched based on the predefined amounts of alkali added under conditions A and B. Switching between condition A and condition B as on-off control eliminates the need to control the permeate water flow rate, allowing for a simpler device configuration.

[0143] Furthermore, under condition B, the second permeated water hardly flows to the equipment downstream of the permeated water pipe 12b at the subsequent stage of the second reverse osmosis membrane device 12, and the second permeated water returning from the permeated water return pipe 12d returns to the supply side of the second reverse osmosis membrane device 12. Therefore, the quality of the water supplied to the second reverse osmosis membrane device 12 is improved. Therefore, the injection rate of the alkali as a pH adjuster can also be set to 0 (zero). Of course, under condition B, the addition rate of the pH adjuster can also be controlled.

[0144] It should be noted that Figure 2In the case where, under condition B, almost all of the second permeated water is returned to the second pit 18 via the permeated water return pipe 12d, the process of supplying the first permeated water from the first reverse osmosis membrane device 11 to the second reverse osmosis membrane device 12 via the permeated water pipe 11b becomes almost unnecessary. Therefore, the first permeated water is returned to the first pit 17 via the permeated water return pipe 11d of the permeated water pipe 11b.

[0145] Here, under condition B, when the amount of alkali added is kept constant as in the conventional method, for example, the feed water supplied to the second reverse osmosis membrane device 12 is substantially the second permeated water from the permeated water return pipe 12d. Therefore, the quality of the feed water supplied to the second reverse osmosis membrane device 12 is higher than that of the permeated water from the first reverse osmosis membrane device 11. As a result, the amount of alkali in the feed water supplied to the second reverse osmosis membrane device 12 becomes excessive.

[0146] The remaining alkali in the feed water supplied to the second reverse osmosis membrane device 12 is supplied to the first pit 17 via the concentrated water return pipe 12c. As a result, the acid added to the supply side of the first reverse osmosis membrane device 11 is consumed. Consequently, the pH of the feed water supplied to the first reverse osmosis membrane device 11 increases. As a result, the Langelier index in the feed water in the first and third reverse osmosis membrane devices 11 and 13 increases by, for example, 1 to 2, which increases the likelihood of hardness scale formation. However, in this embodiment, by controlling the amount of alkali injected to be reduced, the formation of hardness scale in the first and third reverse osmosis membrane devices 11 and 13 can be suppressed.

[0147] (Automatic control of circulation flow)

[0148] Furthermore, in the present disclosure, for example, when the flow rate in the permeate water piping 12b is flexibly varied to maintain a constant water level in the downstream tank of the second reverse osmosis membrane device 12, the circulation flow rate in the permeate water return piping 12d is also flexibly varied. In other words, automatic control of the circulation flow rate is performed. In the present disclosure, since the amount of pH adjuster added to the treated water supplied to the second reverse osmosis membrane device 12 is controlled by the control device 30, it is possible to effectively cope with flexible variations in the circulation flow rate in the permeate water return piping 12d. In this case, the amount of pH adjuster added can be varied according to the circulation flow rate. Furthermore, the amount of pH adjuster added can be feedback-controlled based on the pH or impurity level of the feed water supplied to the second reverse osmosis membrane device 12.

[0149] It should be noted that in the present disclosure, the amount of alkali added can also be set to a predetermined set amount or 0 (zero) when the circulation flow rate is above a threshold value or the supply flow rate to the subsequent stage is below a threshold value. This is because, even if treated water is supplied to the subsequent stage, when the circulation flow rate is high, the water quality of the treated water can be maintained without supplying a pH adjuster. For example, the water quality of the treated water does not exceed the allowable value of the water quality supplied to the subsequent stage device (such as EDI, etc.).

[0150] Furthermore, similar to the on-off control of the permeate flow rate, once the circulation flow rate reaches or exceeds the threshold, there is no need to control the circulating water flow rate, thus simplifying the device configuration. Furthermore, the amount of pH adjuster used can be kept to the minimum necessary. When the amount of alkali added is set to a predetermined set amount or 0 (zero), it can also be reset by a timer after a predetermined time has elapsed. This is done to allow the quality of the feed water supplied to the second reverse osmosis membrane device 12 to improve and stabilize through circulation.

[0151] Example

[0152] Next, examples and comparative examples according to the present embodiment will be described. Specifically, as described below, the operating conditions of the examples and the comparative examples were different, and experiments were performed.

[0153] [Comparative Example]

[0154] First, in the comparative example, Figure 2 The pure water production apparatus 1 according to the illustrated embodiment produces pure water under the following conditions.

[0155] (Operating conditions)

[0156] Treated water: industrial water (i.e. industrial water), conductivity: 90μS / cm, pH=6.8, flow rate: 500m 3 / h

[0157] The first reverse osmosis membrane device 11 was a low-pressure reverse osmosis membrane device (TM720, manufactured by Toray Industries, Inc.). The supply water pressure was adjusted so that the water recovery rate was 85%.

[0158] The second reverse osmosis membrane device 12: a low-pressure reverse osmosis membrane device (TM720, manufactured by Toray Industries, Ltd.), the supply water pressure was adjusted so that the water recovery rate was 90%.

[0159] pH adjuster adding device 15: sodium hydroxide water is added to adjust the pH to 9.2.

[0160] The third reverse osmosis membrane device 13: a low-pressure reverse osmosis membrane device (TM720, manufactured by Toray Industries, Ltd.), the supply water pressure was adjusted so that the water recovery rate was 65%.

[0161] In the comparative example, the pure water production apparatus 1 was operated for 15 days under the above-described operating conditions by the control device 30. During operation, the supply of treated water from the pure water production apparatus 1 to the subsequent stage without circulation and the circulation operation within the system of the pure water production apparatus 1 were switched by the above-described on / off mode in cycles of approximately 3 to 4 hours.

[0162] [Example]

[0163] Next, in the examples, similarly to the comparative examples, Figure 2 The pure water production apparatus 1 according to the illustrated embodiment produced pure water. It should be noted that in the example, during circulation operation, the addition of sodium hydroxide solution by the pH adjuster addition device 15 to the second reverse osmosis membrane device 12 was stopped. The operating conditions of the example, except for the conditions related to the second reverse osmosis membrane device 12, were the same as those of the comparative example.

[0164] [result]

[0165] Next, as experimental results, changes in the supply pressure to the third reverse osmosis membrane device 13 when the pure water production apparatus 1 was operated for 15 days are shown in Table 1 below.

[0166] [Table 1]

[0167]

[0168] As shown in Table 1, in the case of the comparative example, 15 days after the start of operation, the membrane of the third reverse osmosis membrane device 13 was clogged due to the influence of the operation of the second reverse osmosis membrane device 12, resulting in the operating pressure for obtaining a predetermined water recovery rate rising from 1.1 MPa at the start of operation to 1.5 MPa. On the other hand, in the embodiment, even after 15 days from the start of operation, no membrane clogging that affects the predetermined water recovery rate occurred. The operating pressure after 15 days was maintained at 1.1 MPa, the same as the operating pressure at the start of operation. It should be noted that, in the case of the comparative example, in the process of supplying treated water from the pure water manufacturing device 1 to the rear stage, the pH of the supply water (i.e., the first treated water) supplied from the first reverse osmosis membrane device 11 to the rear stage was 6.8, but during the circulation operation, the pH of the supply water (i.e., the first treated water) supplied to the rear stage rose to 8.3.

[0169] (Effect)

[0170] In this embodiment, Figure 6In step S11, a pH adjuster that increases alkalinity is added to the treated water supplied to the second reverse osmosis membrane device 12. Furthermore, in step S12, a portion of the treated water in the second reverse osmosis membrane device 12 is sent downstream, and in step S13, the remaining portion of the treated water is refluxed upstream of the pH adjuster addition point.

[0171] Furthermore, in steps S14 and S15, if the pH alkalinity of the treated water after the treated water is refluxed exceeds a predetermined threshold, the amount of pH adjuster added to the treated water supplied to the second reverse osmosis membrane device 12 is reduced. Specifically, the amount of pH adjuster added to the treated water is reduced as the pH alkalinity of the treated water refluxed upstream of the pH adjuster addition point increases. Therefore, even when the treated water to which the pH adjuster has been added is refluxed into the system of the second reverse osmosis membrane device 12, the concentration of the pH adjuster within the system can be suppressed.

[0172] As a means of suppressing the progression of concentration within the system, for example, stopping the operation of the second reverse osmosis membrane device 12 could be considered. However, restarting the second reverse osmosis membrane device 12 after stopping it takes time for the quality of the treated water to stabilize. Furthermore, with the shutdown or restart of the process, the flow conditions of the treated water flowing through other treatment devices or piping in the production line of the ultrapure water production device 2 are likely to exceed the designed range and fluctuate, potentially causing problems in other water treatment processes. This can result in a decrease in ultrapure water quality and yield.

[0173] In this regard, in the present embodiment, since it is not necessary to stop the operation of the second reverse osmosis membrane device 12 , the burden of re-operation processing, the deterioration of ultrapure water quality, and the reduction in yield are unlikely to occur.

[0174] Furthermore, in the present embodiment, since the membrane treatment device to which the present disclosure is applied is the second reverse osmosis membrane device 12 , concentration within the system of the second reverse osmosis membrane device 12 can be effectively suppressed.

[0175] (Variation)

[0176] In this embodiment, the following example is given: the amount of pH adjuster added to the treated water is reduced or increased according to the increase in the alkalinity of the pH of the treated water flowing back to the upstream side relative to the addition position of the pH adjuster. However, in the present disclosure, the condition for reducing the amount of pH adjuster added is not limited to this. For example, Figure 7 As in the case of the ultrapure water production method according to the modified example illustrated in , the amount of pH adjuster added to the treated water may be reduced by increasing the amount of treated water refluxed upstream of the pH adjuster addition position.

[0177] In the method for producing ultrapure water according to the modification, it is also possible to use Figures 1 to 5 In the ultrapure water production method according to the modified example, a step of adding a pH adjusting agent to increase alkalinity to the water to be treated supplied to the second reverse osmosis membrane device 12 is performed. Figure 7 Step S11) in this embodiment Figure 6 The same as step S11 in .

[0178] In addition, in the modified example, the process of sending the treated water of the second reverse osmosis membrane device 12 to the downstream side ( Figure 7 Step S12) in this embodiment Figure 6 In addition, the step of reducing the amount of the pH regulator added to the treated water supplied to the second reverse osmosis membrane device 12 ( Figure 7 Step S15) in this embodiment Figure 6 The same as step S15 in .

[0179] However, the difference between the modified example and the present embodiment is that Figure 7 In step S14A, it is determined whether the return amount of the treated water has increased, that is, in step S14A, it is determined whether the return amount of the treated water is greater than the return amount of the treated water in the previous step S13A.

[0180] In a modified example, Figure 7 The reflux rate of treated water in step S13A corresponds to the "predetermined reflux rate" in this disclosure. It should be noted that, in this disclosure, the predetermined reflux rate is not limited thereto. For example, a specific reflux rate set based on empirical rules, or a range with an upper limit and a lower limit, may be set as the predetermined reflux rate.

[0181] In the modified example, if the result of determination in step S14A is that the reflux amount of the treated water is increased compared to the reflux amount in step S13A, the process enters Figure 7 In step S15 , the control device 30 of the ultrapure water production apparatus 2 adds a pH adjuster in an amount smaller than the amount added in step S11 to the water to be treated supplied to the second reverse osmosis membrane device 12 .

[0182] On the other hand, if the result of determination in step S14A is that the return flow rate of the treated water has not increased, that is, if the return flow rate of the treated water is less than the return flow rate of the treated water in step S13A, the process ends without proceeding to step S15. Figures 1 to 6 The present embodiment illustrated in FIG is the same as that in FIG, so repeated description is omitted.

[0183] In the modified example, in steps S14A and S15, if the amount of treated water refluxed is increased compared to the reflux amount in step S13A, the amount of pH adjuster added to the treated water supplied to the second reverse osmosis membrane device 12 is reduced. In other words, the amount of pH adjuster added to the treated water is reduced in response to the increase in the amount of treated water refluxed upstream of the pH adjuster addition point. Therefore, even if the first treated water to which the pH adjuster has been added is refluxed into the system of the second reverse osmosis membrane device 12, the concentration of the pH adjuster within the system can be suppressed. Other effects of the modified example are the same as those of the present embodiment.

[0184] <Other Implementation Methods>

[0185] The present disclosure is described by the following embodiments, but the discussion and drawings that constitute a part of this disclosure should not be construed as limiting the present disclosure. It should be considered that various alternative embodiments, examples, and application techniques will be apparent to those skilled in the art based on the present disclosure.

[0186] For example, although not shown in the present disclosure, when returning concentrated water upstream, a device for performing ultraviolet oxidation treatment or the like may be added to the pure water production apparatus. For example, when ultraviolet oxidation treatment is performed in the pure water production apparatus, organic matter in the concentrated water can be oxidized and decomposed.

[0187] In addition, for example, in the present disclosure, the ultrapure water manufacturing process performed by reading and executing the software (program) by the CPU 31 in the above-mentioned embodiment can also be performed by various processors other than the CPU. As examples of processors in this case, a programmable logic device (PLD) whose circuit structure can be changed after manufacturing, such as a field programmable gate array (FPGA), and an application specific integrated circuit (ASIC) having a circuit structure specially designed for performing specific processing, i.e., a dedicated circuit, etc. are exemplified.

[0188] Furthermore, the ultrapure water production process can be performed by one of these various processors, or by a combination of two or more processors of the same or different types (e.g., multiple FPGAs, or a combination of a CPU and an FPGA). Specifically, the hardware structure of these various processors is a circuit composed of a combination of circuit elements such as semiconductor elements.

[0189] In addition, in the above embodiments, the ultrapure water production program is described as being pre-stored (installed) in ROM 32 or memory 34, but the present invention is not limited thereto. The program may also be provided by recording it on a recording medium such as a compact disc read-only memory (CD-ROM), a digital versatile disc read-only memory (DVD-ROM), or a universal serial bus (USB) memory. Furthermore, the program may be downloaded from an external device via a network.

[0190] In addition, you can also Figures 1 to 7 As described above, the present disclosure includes various embodiments and the like that are not described above, and the technical scope of the present disclosure is determined only by the invention-specific matters of the appropriate claims based on the above description.

[0191] The disclosure of Japanese Patent Application No. 2023-004010 filed on January 13, 2023 is incorporated herein by reference in its entirety.

[0192] All documents, patent applications, and technical standards described herein are incorporated by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A method for producing ultrapure water, wherein: The method for producing ultrapure water comprises: a first step of adding a pH adjuster to increase alkalinity to water to be treated supplied to a membrane treatment device, sending a portion of the treated water from the membrane treatment device downstream, and returning another portion of the treated water upstream of a position where the pH adjuster is added; and The second step is to reduce the amount of the pH adjuster added to the water to be treated supplied to the membrane treatment device according to a change in a preset index of the water to be treated after the treated water has been refluxed.

2. The method for producing ultrapure water according to claim 1, wherein: The indicator is the pH of the treated water, In the second step, when the alkalinity of the pH of the water to be treated after the treated water has been refluxed exceeds a preset threshold value, the amount of the pH adjuster added to the water to be treated supplied to the membrane treatment device is reduced.

3. The method for producing ultrapure water according to claim 1 or 2, wherein: The membrane treatment performed by the membrane treatment device is reverse osmosis membrane treatment.

4. A method for producing ultrapure water, wherein: The method for producing ultrapure water comprises: a first step of adding a pH adjusting agent to increase alkalinity to water to be treated supplied to a membrane treatment device, sending a portion of the treated water from the membrane treatment device downstream, and returning another portion of the treated water to an upstream side relative to a position where the pH adjusting agent is added at a predetermined return flow rate; and The second step is to increase the amount of the treated water refluxed to the membrane treatment device and reduce the amount of the pH adjuster added to the treated water supplied to the membrane treatment device, compared to the first step.

5. An ultrapure water production device, wherein: The ultrapure water production device comprises: a membrane treatment device for performing membrane treatment on the supplied water to be treated; a pH adjusting agent adding device, the pH adjusting agent adding device being arranged on the upstream side of the membrane treatment device and adding a pH adjusting agent to the treated water to increase the alkalinity of the treated water; a pipe for sending a portion of the treated water of the membrane treatment device to a downstream side; a reflux pipe for returning another portion of the treated water to an upstream side relative to the pH adjuster adding device; and A control device controls the pH adjuster adding device to reduce the amount of the pH adjuster added to the treated water supplied to the membrane treatment device based on changes in a preset index of the treated water after reflow.

6. A process for producing ultrapure water, wherein: The ultrapure water production process causes the processor to perform: a first treatment comprising adding a pH adjusting agent to increase alkalinity to water to be treated supplied to a membrane treatment device, sending a portion of the treated water from the membrane treatment device downstream, and returning another portion of the treated water upstream of a position where the pH adjusting agent is added; as well as The second treatment is to reduce the amount of the pH adjuster added to the treated water supplied to the membrane treatment device according to a change in a preset index of the treated water after the treated water is returned.

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