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

Abstract

To suppress in-system concentration of a pH controlling agent even when target water having been added with a pH controlling agent is refluxed in a system of a membrane treatment apparatus.SOLUTION: An ultrapure water manufacturing method includes: a first step (step S11-S13) of adding a pH controlling agent for elevating basicity to target water to be supplied to a second reverse osmosis membrane apparatus, sending a part of treated water of the second reverse osmosis membrane apparatus to a downstream side as well as refluxing the other part of treated water to an upstream side with respect to an addition position of the pH controlling agent; and a second step (step S14-S15) of lowering an addition amount of the pH controlling agent to target water to be supplied to the second reverse osmosis membrane apparatus in accordance with changes of pre-set indexes of target water after refluxing treated water.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present disclosure relates to an ultrapure water production apparatus, an ultrapure water production method, and an ultrapure water production program. [Background technology]

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

[0003] Each membrane treatment device system may be configured as a multi-stage system with multiple membrane treatment devices included within the system. When the water to be treated is supplied to the membrane treatment device, permeated water, which is filtered after a predetermined membrane treatment, and concentrated water, which is discharged without filtration, are obtained as treated water. In this specification, both concentrated water and permeated water are referred to as the water to be treated.

[0004] As an example of a membrane treatment system, Patent Document 1 discloses a pure water production apparatus that includes a reverse osmosis membrane separation treatment apparatus that includes a raw water supply channel, a supply pump, and a reverse osmosis membrane that separates raw water into permeate and concentrated water. Patent Document 1 also discloses a return pipe that branches off from the permeate channel of the reverse osmosis membrane and serves as a circulating water channel that can return a portion of the permeate to the raw water supply channel. The circulating water channel is opened when the temperature of the raw water exceeds a reference temperature, and the amount of raw water supplied by the supply pump is controlled when the flow rate of the permeate falls outside a predetermined reference flow rate range. Patent Document 1 claims that even if the water temperature rises, it is possible to prevent the quality of the pure water from deteriorating and to prevent excessive production of pure water.

[0005] On the other hand, in pure water production plants, a process is carried out to adjust the pH of the water to a basic level, i.e., to increase the removal rate of weak electrolytes such as boron and silica by the reverse osmosis membrane. Specifically, a pH adjuster such as sodium hydroxide (NaOH) is added to the water.

[0006] For example, in Patent Document 2, a first permeate that has passed through a reverse osmosis membrane separation device (first RO device) as the first-stage membrane treatment device in a membrane treatment system is adjusted to a pH of 8.5 or higher by adding a basic pH adjuster. The first permeate adjusted to basicity is desalinated by being supplied under pressure to a reverse osmosis membrane separation device (second RO device) located downstream of the first RO device. The pH adjustment is said to remove silica, boron, and remaining carbon dioxide in an ionized state, as well as to remove impurities that can be removed under basic conditions.

[0007] In addition, in Patent Document 2, in order to increase the water recovery rate, the concentrated water from the second RO device is subjected to oxidation treatment in a separate treatment device and then returned to the upstream side of the first RO device. The returned concentrated water from the second RO device is then supplied again to the first RO device.

[0008] In Patent Document 3, raw water as water to be treated that has been subjected to pretreatment such as activated carbon treatment is adjusted to a pH of 6 or less and degassed, and then passed through a reverse osmosis membrane separation device (first RO device) as the first-stage membrane treatment device in a membrane treatment system. The permeate from the first RO device is adjusted to basicity by adding a pH adjuster such as sodium hydroxide (NaOH). The permeate from the first RO device, adjusted to basicity, is passed sequentially through a second-stage reverse osmosis membrane separation device (second RO device) and a third-stage reverse osmosis membrane separation device (third RO device).

[0009] In addition, in Patent Document 3, in order to efficiently remove carbonate components contained in raw water in the form of carbon dioxide (CO2), the concentrate from the second RO device and the concentrate from the third RO device are degassed together with their respective permeates in a membrane degassing device. The degassed concentrate from the second RO device and the third RO device are returned to the upstream side of the first RO device and are also supplied again to the first RO device. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2020-163254 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-015257 [Patent Document 3] Japanese Patent Application Laid-Open No. 2000-061464 Summary of the Invention [Problem to be solved by the invention]

[0011] However, after the treated water containing the added pH adjuster is treated in a membrane treatment device, if the treated water is returned upstream of the membrane treatment device, the amount of pH adjuster returned to the membrane treatment device system increases. This causes the pH adjuster to concentrate within the system, i.e., "in-system concentration." If the basicity of the treated water increases as the in-system concentration progresses, the membrane of the membrane treatment device is likely to clog and the pH of the permeate is likely to fluctuate beyond the design range. In particular, if in-system concentration occurs, unexpected problems such as membrane clogging may occur not only in the membrane treatment device directly supplied with the treated water containing the added pH adjuster, but also in other membrane treatment devices in the same system that are not directly supplied with the treated water containing the added pH adjuster.

[0012] In this regard, Patent Document 1 only improves the quality and production amount of pure water when the water temperature rises, and does not consider the concentration of the pH adjuster in the system. Similarly, Patent Documents 2 and 3 only disclose that a pH adjustment is performed to adjust the pH to basic, and do not consider the concentration of the pH adjuster in the system.

[0013] The present disclosure provides a technique that can suppress concentration of a pH adjuster in a membrane treatment system even when water to be treated, to which a pH adjuster has been added, is refluxed into the system. [Means for solving the problem]

[0014] The method for producing ultrapure water according to the first embodiment includes a first step of adding a pH adjuster to increase the basicity of the treated water 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 the position where the pH adjuster is added; and a second step of reducing the amount of pH adjuster added to the treated water supplied to the membrane treatment device in accordance with a change in a predetermined index of the treated water after the treated water has been returned.

[0015] In the first embodiment, in the first step, a pH adjuster that increases the basicity is added to the water to be treated that is supplied to the membrane treatment device. Furthermore, a portion of the treated water from the membrane treatment device is sent downstream, and another portion of the treated water is returned upstream of the position where the pH adjuster is added. Furthermore, in the second step, the amount of pH adjuster added to the water to be treated that is supplied to the membrane treatment device is reduced in accordance with a change in a preset index of the water to be treated after the treated water is returned.

[0016] In the first aspect, the amount of pH adjuster added to the water to be treated that is supplied to the membrane treatment device is reduced in accordance with a change in a preset index of the water to be treated after the treated water is returned, so that even if the water to be treated that has had the pH adjuster added is returned to the system of the membrane treatment device, concentration of the pH adjuster in the system can be suppressed.

[0017] In the second aspect, in the first aspect, the indicator is the pH of the treated water, and in the second step, if the basicity of the pH of the treated water after the treated water is returned is higher than a predetermined threshold value, the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced.

[0018] In the second aspect, the amount of pH adjuster added to the water to be treated is reduced in response to an increase in the basicity of the pH of the treated water being returned upstream of the addition position of the pH adjuster, thereby preventing the pH adjuster from concentrating in the system even when the water to be treated to which the pH adjuster has been added is returned to the membrane treatment system.

[0019] In a third aspect, the membrane treatment performed by the membrane treatment device in the first or second aspect is reverse osmosis membrane treatment.

[0020] According to the third aspect, in particular, concentration within the reverse osmosis membrane treatment device can be effectively suppressed.

[0021] The method for producing ultrapure water according to the fourth aspect includes a first step of adding a pH adjuster to increase the basicity of the water to be treated that is 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 the addition position of the pH adjuster at a predetermined return rate, and a second step of reducing the amount of pH adjuster added to the water to be treated that is supplied to the membrane treatment device when the return rate to the water from the membrane treatment device is increased.

[0022] In a fourth aspect, in the first step, a pH adjuster that increases the basicity is added to the water to be treated that is supplied to the membrane treatment device. Furthermore, a portion of the treated water from the membrane treatment device is sent downstream, and another portion of the treated water is returned upstream of the position where the pH adjuster is added. Furthermore, in the second step, the amount of returned treated water is increased compared to the first step, and the amount of pH adjuster added to the water to be treated that is supplied to the membrane treatment device is reduced.

[0023] In other words, the amount of pH adjuster added to the water to be treated is reduced in accordance with an increase in the amount of treated water returned upstream of the addition position of the pH adjuster. Therefore, even if the first treated water to which the pH adjuster has been added is returned to the membrane treatment system, concentration of the pH adjuster in the system can be suppressed.

[0024] The ultrapure water manufacturing apparatus of the fifth embodiment comprises a membrane treatment device that applies membrane treatment to the water to be treated that is supplied to the membrane treatment device; a pH adjuster addition device that is arranged upstream of the membrane treatment device and adds a pH adjuster to the water to be treated that increases the basicity of the water to be treated; piping that sends a portion of the treated water from the membrane treatment device downstream; a return pipe that returns another portion of the treated water to the upstream side of the pH adjuster addition device; and a control device that controls the pH adjuster addition device to reduce the amount of pH adjuster added to the water to be treated that is supplied to the membrane treatment device in accordance with changes in a predetermined index of the water to be treated after return.

[0025] In the fifth embodiment, similarly to the first embodiment, even if the first treated water to which the pH adjuster has been added is refluxed into the membrane treatment device system, concentration of the pH adjuster in the system can be suppressed.

[0026] The ultrapure water manufacturing program of the sixth aspect causes a processor to execute a first process in which a pH adjuster that increases the basicity of the treated water supplied to the membrane treatment device is added, and a portion of the treated water from the membrane treatment device is sent downstream while another portion of the treated water is returned upstream of the position where the pH adjuster is added; and a second process in which the amount of pH adjuster added to the treated water supplied to the membrane treatment device is reduced in accordance with a change in a predetermined index of the treated water after the treated water has been returned.

[0027] In the sixth embodiment, similarly to the first embodiment, even if the first treated water to which the pH adjuster has been added is refluxed into the membrane treatment device system, concentration of the pH adjuster in the system can be suppressed. [Effects of the Invention]

[0028] According to the present disclosure, even when the water to be treated, to which a pH adjuster has been added, is refluxed into the membrane treatment system, the concentration of the pH adjuster in the system can be suppressed. [Brief explanation of the drawings]

[0029] [Figure 1] 1 is a block diagram illustrating an ultrapure water production apparatus according to an embodiment of the present invention. [Figure 2] 1 is a block diagram illustrating a pure water manufacturing apparatus according to an embodiment of the present invention. [Figure 3] 1 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. [Figure 4] 1 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. [Figure 5] 2 is a block diagram showing the hardware configuration of a processor of the ultrapure water production apparatus according to the present embodiment. FIG. [Figure 6] 1 is a flowchart illustrating a method for producing ultrapure water using the ultrapure water production system according to the present embodiment. [Figure 7] 10 is a flowchart illustrating a method for producing ultrapure water using an ultrapure water production system according to a modified example of the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0030] Embodiments of the present disclosure will be described below. In the following drawings, identical or similar parts are designated by the same or similar reference numerals. However, the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each device or component, etc., differ from the actual ones. Therefore, specific thicknesses and planar dimensions should be determined with reference to the following description. Furthermore, parts with different dimensional relationships and ratios are included among the drawings.

[0031] <Ultrapure water production equipment> First, an ultrapure water production system 2 equipped with a pure water production system 1 according to this embodiment will be described with reference to Figures 1 to 5. As shown in Figure 1, the ultrapure water production system 2 is equipped with a primary pure water system 21 and a secondary pure water system 22 through which water to be treated is passed.

[0032] (Untreated water) The water to be treated may be raw water such as groundwater, river water, city water, or other industrial water, or recovered ultrapure water recovered from semiconductor factories, liquid crystal factories, etc. The boron concentration in the water to be treated is preferably, for example, 5 μg / L to 200 μg / L, and particularly preferably 10 μg / L to 100 μg / L. The pH of the water to be treated is preferably near neutral, for example, in the range of pH 5 to 8.

[0033] In this specification, when a numerical range is expressed, such as "5 μg / L to 200 μg / L," it means that the lower limit and the upper limit are included in the range. 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.

[0034] When recycled water is used as the water to be treated, the pH of the recycled water is preferably adjusted to, for example, a range of pH 5 to 8 by ion exchange treatment or neutralization treatment. The water to be treated may be raw water or recycled water that has been treated in a pretreatment system or a similar device. The pretreatment system is composed of, for example, a coagulation sedimentation device, a pressure flotation device, a filtration device, an activated carbon device, etc., and removes turbidity components and the like from the raw water.

[0035] In addition, the total carbon dioxide (CO2 + HCO3 - +CO3 2- The total carbonate concentration is preferably 3 mg / L to 50 mg / L, and more preferably 30 mg / L or less. The total carbonate concentration is a CO2 equivalent concentration. When the total carbonate concentration is 3 mg / L to 50 mg / L, the carbonate ion concentration or bicarbonate ion concentration in the 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 water supplied to the reverse osmosis membrane device can be reduced.

[0036] 2, the pure water production apparatus 1 of the primary pure water system 21 preferably includes a decarbonation device for removing carbon dioxide gas from the water to be treated, located before the first reverse osmosis membrane device 11, i.e., upstream of the first reverse osmosis membrane device 11. The decarbonation device is not particularly limited as long as it can remove carbon dioxide gas, and a degassing tower, degassing membrane device, vacuum degassing device, etc. can be used.

[0037] In a pure water production system, when dissolved carbon dioxide is removed using a decarbonation device, an acid is generally injected into the feed water to the decarbonation device to set the pH of the feed water to 6 or less in order to increase the rate of removal of dissolved carbon dioxide. In the pure water production system 1 of this embodiment, it is preferable to add as little acid as possible to the feed water to the decarbonation device in order to prevent an increase in the amount of chemicals such as acids and bases used.

[0038] (Primary pure water system) 1, the primary pure water system 21 is configured, for example, by combining an electrodeionization unit (EDI) 23, an ultraviolet oxidation unit (TOC-UV) 24, and a regenerative mixed-bed ion exchange unit (MB) 25 downstream of the pure water production system 1. In the primary pure water system 21, the electrodeionization unit 23 removes trace amounts of ionic components from the treated water from which boron has been removed in the pure water production system 1.

[0039] Furthermore, after the remaining organic matter is decomposed and removed in the ultraviolet oxidation device 24, low molecular weight organic acids and the like produced 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 TOC concentration of the primary pure water is, for example, 10 μg C / L or less, and the resistivity of the primary pure water is, for example, 17 MΩ cm or more.

[0040] The primary pure water system 21 may include other devices for increasing the purity of water in addition to or instead of the electrodeionization device 23, ultraviolet oxidation device 24, and regenerative mixed-bed ion exchange device 25. Examples of such devices include a membrane degassing device or vacuum degassing device for removing dissolved gases from water, an anion exchange resin device or cation exchange resin device for removing ionic components, and a boron resin device filled with boron-selective ion exchange resin, or an ion exchange device with a multi-layered bed of these ion exchange resins. The electrodeionization device 23 may be a multi-stage device in which multiple electrodeionization devices are connected in series.

[0041] (Secondary pure water system) The secondary pure water system 22 is an apparatus for removing trace amounts of organic matter and minute particles from the primary pure water produced by the primary pure water system 21. The secondary pure water system 22 can be configured 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, 5 μg C / L or less. The resistivity of the ultrapure water is also reduced to, for example, 17.5 MΩ cm or more. The boron concentration of the ultrapure water is also reduced to, for example, 1 ng / L or less.

[0042] <Pure water production equipment> Next, we will explain the pure water manufacturing apparatus 1 included in the primary pure water system 21 according to this embodiment. As shown in Figure 2, the pure water manufacturing 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.

[0043] The first reverse osmosis membrane device 11 performs reverse osmosis membrane treatment on the water to be treated to remove salts from the water. The second reverse osmosis membrane device 12 performs reverse osmosis membrane treatment on the permeate from the first reverse osmosis membrane device 11 (i.e., the first permeate) to remove impurities such as organic matter and fine particles, and boron remaining in the first permeate. The third reverse osmosis membrane device 13 performs reverse osmosis membrane treatment on the concentrate from the first reverse osmosis membrane device 11 (i.e., the first concentrate) to recover the water to be treated into the first reverse osmosis membrane device 11.

[0044] (1st reverse osmosis membrane device) The first reverse osmosis membrane device 11 includes a supply pipe 11a for introducing the water to be treated, a permeate pipe 11b, and a concentrate pipe 11c. The first permeate obtained in the first reverse osmosis membrane device 11 is supplied to the second reverse osmosis membrane device 12 via the permeate pipe 11b. The first concentrate is supplied to the third reverse osmosis membrane device 13 via the concentrate pipe 11c.

[0045] The upstream end of the supply pipe 11a is connected to the first pit 17. The water to be treated is supplied to the first reverse osmosis membrane device 11 from the first pit 17. A portion of the first permeate is returned to the first pit 17 via a permeate return pipe 11d that connects the first pit 17 and the permeate pipe 11b.

[0046] In the present disclosure, an addition device for adding a scale inhibitor or acid to the water to be treated may be provided at a position on the supply pipe 11a between the first pit 17 and the first reverse osmosis membrane device 11. Also, in the present disclosure, a pH measuring device may be provided at a position on 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 position of the supply pipe 11a. Note that a tank may be provided instead of the pit. The order of the position of the pit, the pH measuring device, and the pH adjuster adding device may be changed as desired within the scope of the present disclosure.

[0047] The first reverse osmosis membrane device 11 has a reverse osmosis membrane. The reverse osmosis membrane of the first reverse osmosis membrane device 11 performs reverse osmosis membrane separation treatment on the water to be treated under pressure, separating the water into a first concentrate in which salts have been concentrated and a first permeate in which salts have been removed. As the first reverse osmosis membrane device 11, any reverse osmosis membrane device commonly used for producing pure water can be used without any particular limitation. As the first reverse osmosis membrane device 11, for example, an ultra-low pressure type, low pressure type, medium pressure type, or high pressure type reverse osmosis membrane device can be used.

[0048] Examples of the reverse osmosis membrane of the first reverse osmosis membrane device 11 include a cellulose triacetate asymmetric membrane and a polyamide, polyvinyl alcohol, or polysulfone composite membrane. The membrane shape is not particularly limited, and may be a flat sheet membrane, a spiral membrane, a tubular membrane, a hollow fiber membrane, or the like. In particular, the reverse osmosis membrane is preferably a polyamide composite membrane, and particularly preferably a crosslinked wholly aromatic polyamide composite membrane, due to its high salt removal rate. The membrane shape is preferably a spiral membrane.

[0049] The salt removal ability of the first reverse osmosis membrane device 11 is preferably 95% or more, and more preferably 99.5% or more, in terms of, for example, the NaCl removal rate. Commercially available first reverse osmosis membrane devices 11 include TMG20, TM720, and TM800K manufactured by Toray Industries, Inc., and BW30 manufactured by Dow Chemical Company.

[0050] The supply pressure of the water to be treated to the first reverse osmosis membrane device 11 is preferably set in the range of 0.4 MPa to 6 MPa depending on the type of the first reverse osmosis membrane device, etc. For example, if the first reverse osmosis membrane device 11 is an ultra-low pressure type, the supply pressure of the water to be treated is preferably 0.4 MPa to 0.8 MPa, and more preferably 0.6 MPa to 0.7 MPa.

[0051] When the first reverse osmosis membrane device 11 is a low-pressure type, the supply pressure of the water to be treated is preferably more than 0.8 MPa and less than 2.0 MPa, 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 water to be treated is preferably 2 MPa to 4 MPa, more preferably 2 MPa to 3 MPa.

[0052] When the first reverse osmosis membrane device 11 is a high-pressure type, the supply pressure of the water to be treated is preferably more than 4 MPa and not more than 8 MPa, and more preferably more than 4 MPa and not more than 6 MPa. Therefore, it is preferable to provide a water supply pump before the first reverse osmosis membrane device 11, i.e., on the upstream side.

[0053] In reverse osmosis membranes, generally, the higher the water supply pressure, the better the salt removal rate, but the more likely it is that scaling will occur. For example, if the first reverse osmosis membrane device 11 is an ultra-low pressure type, the salt removal rate can be improved by setting the supply pressure of the water to be treated 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 system 1. Furthermore, by setting the supply pressure of the water to be treated to 6 MPa or lower, membrane clogging due to scaling can be suppressed, and as a result, pure water with reduced boron can be obtained stably for a long period of time.

[0054] In order to improve the boron removal rate in the pure water production system 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%.

[0055] The flow rate of the water to be treated supplied to the first reverse osmosis membrane device 11 is 100 m 3 / h~1000m 3 / h is preferable because continuous operation using the circulation flow path of the first reverse osmosis membrane device 11 can significantly reduce the amount of wastewater discharged due to flushing at the start-up of intermittent operation.

[0056] The water to be treated is treated in the first reverse osmosis membrane device 11 to obtain a first permeate and a first concentrate. The first concentrate contains a high concentration of salts due to concentration, and is therefore supplied to the third reverse osmosis membrane device 13 via the concentrate pipe 11c. The first permeate has a conductivity of, for example, 5 μS / cm or more.

[0057] In the present disclosure, an acid or alkali may be added to prevent scale formation in the first reverse osmosis membrane device 11. The scale inhibitor prevents hardness (i.e., calcium and magnesium) and silica in the raw water from adhering to the RO membrane as scale. The chemicals (i.e., scale inhibitor, acid, alkali) added to prevent scale formation in the first reverse osmosis membrane device are concentrated into concentrated water. The concentrated water is supplied to the third reverse osmosis membrane device 13 and also contributes to scale prevention in the third reverse osmosis membrane device 13.

[0058] The scale inhibitor, acid, and alkali may also be added to the inlet of the third reverse osmosis membrane device 13 (i.e., to the concentrated water of the first reverse osmosis membrane device 11). It is also possible to add the scale inhibitor at the position of the second reverse osmosis membrane device. In this case, since the concentrated water returns to the previous stage in addition to preventing scale in the second reverse osmosis membrane device, the scale inhibitor can also effectively prevent scale in the first reverse osmosis membrane device and, further, in the third reverse osmosis membrane device to which the first concentrated water is supplied.

[0059] (Second reverse osmosis membrane device) The second reverse osmosis membrane device 12 is connected to the first reverse osmosis membrane device 11 by a permeate pipe 11b. The second reverse osmosis membrane device 12 includes a permeate pipe 12b, a concentrate return pipe 12c, and a permeate return pipe 12d. The permeate pipe 12b corresponds to the "pipe that sends a portion of the treated water from the membrane treatment device downstream" in this disclosure. The permeate return pipe 12d corresponds to the "return pipe" in this disclosure.

[0060] A second pit 18 is provided in the permeate pipe 11b at a position between the first reverse osmosis membrane device 11 and the pH adjuster adding device 15. The water to be treated is supplied to the second reverse osmosis membrane device 12 from the second pit 18. In the present disclosure, the second pit 18 may not be provided and the permeate return pipe 12d may be returned to the first pit 17.

[0061] The second reverse osmosis membrane device 12 corresponds to the "membrane treatment device" of the present disclosure. That is, in this embodiment, reverse osmosis membrane treatment is performed as the membrane treatment of the present disclosure. Note that in the present disclosure, the membrane treatment may be, for example, ultrafiltration membrane treatment, microfiltration treatment, nanofiltration treatment, electrodeionization membrane treatment, etc., in addition to reverse osmosis membrane treatment.

[0062] In addition, although the present embodiment exemplifies a case in which the treated water returned to the upstream side is both permeate and concentrate, the present disclosure is not limited to this. In the present disclosure, in the reverse osmosis membrane treatment, the treated water returned to the upstream side may be either permeate or concentrate. In the present disclosure, the treated water returned to the upstream side in each of the ultrafiltration membrane treatment, microfiltration treatment, nanofiltration treatment, and electrodeionization membrane treatment is concentrate.

[0063] The concentrated water return pipe 12c is connected to the first pit 17. The permeated water return pipe 12d is connected to a 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 permeated water return pipe 12d may be returned to the first pit 17.

[0064] A pH adjuster adding device 15 is connected to the permeate pipe 11b at a position downstream of the connection position of the permeate return pipe 12d to the permeate pipe 11b and upstream of the second reverse osmosis membrane device 12. A pH measuring device 14 is connected to the permeate pipe 11b at a position downstream of the connection position of the pH adjuster adding device 15 to the permeate pipe 11b and upstream of the second reverse osmosis membrane device 12. In other words, the pH adjuster adding device 15 is disposed upstream of the second reverse osmosis membrane device 12. A flow meter 16 that measures the return flow rate of the second permeate is connected to the permeate return pipe 12d.

[0065] The pH measuring device 14, the pH adjuster adding device 15, and the flow meter 16 are each connected to a control device 30. Data on the pH of the water to be treated supplied to the second reverse osmosis membrane device 12, data on the amount of pH adjuster added to the water to be treated, and data on the return flow rate of the second permeate are input to the control device 30 continuously over time.

[0066] A portion of the permeate obtained in the second reverse osmosis membrane device 12 (i.e., second permeate) is sent downstream of the second reverse osmosis membrane device 12 via permeate piping 12b. A portion of the second permeate obtained in the second reverse osmosis membrane device 12 is returned to the second pit 18 upstream of the second reverse osmosis membrane device 12 via permeate return piping 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 concentrate return piping 12c and is treated again in the first reverse osmosis membrane device 11.

[0067] The second reverse osmosis membrane device 12 has a reverse osmosis membrane. The first permeate is subjected to reverse osmosis membrane separation treatment under pressure by the reverse osmosis membrane of the second reverse osmosis membrane device 12. As a result, the first permeate is separated into a second permeate from which impurities such as organic matter and fine particles remaining in the first permeate and boron have been removed, and a second concentrated water from which the impurities and boron have been concentrated. The second reverse osmosis membrane device may be the same as the first reverse osmosis membrane device 11. The second reverse osmosis membrane device 12 may be any of an ultra-low pressure type, a low pressure type, a medium pressure type, or a high pressure type, but is preferably an ultra-low pressure type or a low pressure type reverse osmosis membrane device.

[0068] The salt removal capacity of the second reverse osmosis membrane device 12 is preferably 95% or more, more preferably 99.5% or more, for example, in terms of the NaCl removal rate, in order to improve the boron removal rate.

[0069] The supply water pressure to the second reverse osmosis membrane device 12 is preferably set in the range of 0.4 MPa to 6 MPa depending on the type of the second reverse osmosis membrane device 12, similar to the supply water pressure to the first reverse osmosis membrane device 11. When using an ultra-low pressure, low pressure, medium pressure, or high pressure reverse osmosis membrane device, the preferred supply water pressure is the same as the supply water pressure to the first reverse osmosis membrane device 11.

[0070] The pH of the first permeate is continuously measured by a pH measuring device 14. In the present disclosure, other weak electrolyte measuring devices, such as a silica meter or a boron monitor, may be installed instead of the pH measuring device 14. In the present disclosure, silica and boron are weak electrolytes. The other weak electrolyte measuring devices measure the concentration of weak electrolytes in the water to be treated. The pH of the water to be treated fluctuates depending on the change in the concentration of weak electrolytes.

[0071] For example, in raw water containing boron, there is a correlation between the pH of the water and the boron concentration in the treated water, i.e., the permeate water of a reverse osmosis membrane device. Specifically, the present inventors produced raw water for experimental treatment by injecting boric acid into ultrapure water. The boron concentration of the produced raw water was approximately 50 ppb.

[0072] In the experiment, boron was removed by passing raw water 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 used in the experiment was a 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 water to be treated were then measured. As shown in Figure 3, the lower the pH of the water to be treated, the lower the boron removal rate in the treated water. In other words, the lower the pH of the water to be treated, the more boron remains in the treated water that has passed through the reverse osmosis membrane.

[0073] Furthermore, based on the results of Figure 3, we determined 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 that has passed through the reverse osmosis membrane. The relationship obtained is shown in Figure 4. As Figure 4 shows, it was found that the lower the boron concentration in the water to be treated, the lower the pH required to achieve a boron concentration of 1 ppb in the treated water.

[0074] In Figure 4, the parameter used to explain the relationship with the boron concentration in the treated water was the pH required to achieve a boron concentration of 1 ppb in the treated water, but the relationship with the boron concentration in the treated water also shows a similar trend for other pH values. Furthermore, although not shown, the relationship between pH and concentration in the case of silica shows a similar trend to that for boron. Furthermore, the relationship between pH and concentration for other weak electrolytes is similar to that for boron and silica.

[0075] That is, as in the above experiment, the basicity of pH and the concentration of weak electrolytes correspond to the "predetermined index" of the present disclosure. In the present disclosure, it is also possible to adjust the amount of pH adjuster added according to the measurement value of another measurement device. For example, when the measured value of the weak electrolyte concentration changes to or below a specified value, on-off control is also possible, such as stopping the addition of the pH adjuster.

[0076] The second permeate obtained in the second reverse osmosis membrane device 12 is sent downstream. The second permeate has a conductivity of, for example, 50 μS / cm or less. The sent second permeate is used as is. Alternatively, the second permeate is further processed to produce primary pure water, secondary pure water, or ultrapure water. The second permeate may be further processed in a reverse osmosis membrane device. 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.

[0077] The water recovery rate in the second reverse osmosis membrane device 12 is preferably 50% to 95%, more preferably 60% to 90%, even more preferably 65 to 90%, and may be 65 to 85%.

[0078] In terms of improving the boron removal rate, it is preferable that the combination of the first reverse osmosis membrane device 11 and the second reverse osmosis membrane device 12 be such that one is an ultra-low pressure or low pressure type and the other is a high pressure or medium pressure type. It is more preferable that the first reverse osmosis membrane device 11 is a high pressure or medium pressure type and the second reverse osmosis membrane device 12 is a low pressure or ultra-low pressure type. Alternatively, in order to lower the operating pressure and reduce operating costs, it is preferable that both the first reverse osmosis membrane device 11 and the second reverse osmosis membrane device 12 are low pressure or ultra-low pressure types.

[0079] (Third reverse osmosis membrane device) The third reverse osmosis membrane device 13 includes a concentrate drain pipe 13b and a permeate return pipe 13c. The first concentrate supplied from the first reverse osmosis membrane device 11 is treated by a reverse osmosis membrane in the third reverse osmosis membrane device 13. The concentrate obtained in the third reverse osmosis membrane device 13 (i.e., third concentrate) is discharged to the outside of the system via the concentrate drain 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 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.

[0080] (pH adjuster addition device) The pH adjuster adding device 15 adjusts the pH of the first permeated water to basic, that is, adds a pH adjuster to the first permeated water to increase its basicity.

[0081] There are no particular limitations on the pH adjuster that can be used as long as it is water-soluble and adjusts the pH of the first permeate, which is the supply water to the second reverse osmosis membrane device 12, to a basic value. Examples of pH adjusters that adjust H to a basic value include alkali metal salts such as potassium hydroxide and sodium hydroxide. One type of pH adjuster may be used alone, or two or more types may be used in combination.

[0082] In the present disclosure, the pH of the first permeate may be adjusted to be basic by using a basic scale inhibitor as a pH adjuster. Also, in the present disclosure, the pH adjuster adding device 15 may be equipped with a membrane blockage prevention agent adding device that adds a scale inhibitor or slime control agent that prevents scale formation in the first reverse osmosis membrane device 11 to the first permeate as a membrane blockage prevention agent.

[0083] The pH value after adjustment is not particularly limited, but is preferably 8.5 to 10.5, and more preferably 9 to 9.5. By setting the pH within this range, weak electrolytes such as silica and boron can be ionized, thereby increasing the removal rate in the first reverse osmosis membrane device 11. If the pH is less than 8.5, the removal rate will be slightly reduced. If the pH exceeds 10.5, the amount of alkali metal in the added pH adjuster will increase, resulting in an increase in the amount of alkali metal in the treated water. In this case, the removal rate will be 90% or more for both silica and boron.

[0084] In the present disclosure, the scale of the apparatus is not particularly limited, but a treatment volume (amount of supplied water) of 100 m 3 / h~1000m 3 / h is preferred. In this case, it is particularly important to employ a circulation operation for the second reverse osmosis membrane device so that the quality of the treated water does not deteriorate by continuously operating the second reverse osmosis membrane device. This is because it is possible to avoid the generation of a large amount of wastewater during start-up operations, which occurs when the second reverse osmosis membrane device is operated in an on-off cycle.

[0085] (Control device) The control device 30 is an arithmetic and control unit that controls the flow rate, temperature, pH, etc. of the water to be treated and the treated water in the pure water manufacturing system 1. Specifically, when the basicity of the pH of the water to be treated after reflux is higher than a preset threshold, the control device 30 adds a smaller amount of pH adjuster than the current amount of pH adjuster added to the water to be treated that is supplied to the second reverse osmosis membrane device 12.

[0086] In the present disclosure, the control device 30 controls the pH adjuster adding device 15 in accordance with a change in a preset index of the treated water after reflux so as to add a smaller amount of pH adjuster than the current amount of pH adjuster to the treated water to be supplied to the second reverse osmosis membrane device 12. In the present disclosure, the amount of pH adjuster added that is reduced by the control includes zero.

[0087] 5, the control device 30 includes a CPU (Central Processing Unit: processor) 31, a ROM (Read Only Memory) 32, a RAM (Random Access Memory) 33, a storage 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 able to communicate with each other.

[0088] The CPU 31 is a central processing unit that executes various programs and controls each part. That is, the CPU 31 reads a program from the ROM 32 or the storage 34 and executes the program using the RAM 33 as a work area. The CPU 31 controls each of the above components and performs various arithmetic processing according to the program recorded in the ROM 32 or the storage 34. The CPU 31 is a processor of the present disclosure.

[0089] In this embodiment, an ultrapure water production program is stored in the ROM 32 or the storage 34. The ultrapure water production program is a calculation program for producing ultrapure water.

[0090] The ROM 32 stores various programs and various data. The RAM 33 serves as a working area and temporarily stores programs or data. The storage 34 is configured with an HDD (Hard Disk Drive) or an SSD (Solid State Drive) and stores various programs including the operating system and various data.

[0091] The user interface 35 is an interface used by an operator operating the water purifying apparatus 1 to operate the control device 30. The user interface 35 may include, for example, at least one of a liquid crystal display equipped with a touch panel that allows the operator to perform touch operations, a voice input receiving unit that receives voice input from the operator, and a button that the operator can press.

[0092] 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).

[0093] When executing the ultrapure water production program, the pure water production system 1 uses the above hardware resources to realize various functions. The functional components realized by the pure water production system 1 include a pH adjuster addition unit, a treated water delivery unit, a treated water reflux unit, a pH comparison unit, a reflux amount comparison unit, and a pH adjuster addition amount reduction unit. Each functional component is realized by the CPU 31 reading and executing the ultrapure water production program stored in the ROM 32 or storage 34.

[0094] <Ultrapure water manufacturing method> Next, a method for producing ultrapure water using the ultrapure water production apparatus 2 according to this embodiment will be described. First, primary pure water is produced using the pure water production apparatus 1 of the primary pure water system 21 included in the ultrapure water production apparatus 2. In this embodiment, a case will be described as an example in which the production of primary pure water is performed automatically under computer control of the control device 30, rather than by an operator. Note that in the present disclosure, the operator of the ultrapure water production apparatus 2 may also use the control device 30 to perform each step related to the production of primary pure water.

[0095] Specifically, in the pure water manufacturing apparatus 1, the water to be treated is supplied to a first reverse osmosis membrane device 11 via a supply pipe 11a and undergoes reverse osmosis membrane treatment in the first reverse osmosis membrane device 11. In this embodiment, the temperature of the water to be treated supplied to the first reverse osmosis membrane device 11 is adjusted as necessary. The temperature of the water to be treated may be, for example, about room temperature (20°C to 30°C).

[0096] Next, in step S11 of Fig. 6, the control device 30 of the ultrapure water production system 2 adds a pH adjuster to the first permeate to be 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 permeate can be set so as to adjust the pH of the water supplied to the second reverse osmosis membrane device 12 to a preset threshold value, for example, a value of 9.2 to 10. The threshold value may be set as a range having an upper limit and a lower limit, or may be set as a specific value.

[0097] In the present disclosure, the step of adding the scale inhibitor may be performed either before or after step S11, which is the step of adding the pH adjuster, or simultaneously with step S11. The first permeate whose pH has been adjusted is supplied to the second reverse osmosis membrane device 12 via the permeate piping 11b, and then undergoes reverse osmosis membrane treatment in the second reverse osmosis membrane device 12.

[0098] Next, in step S12 of Fig. 6, the control device 30 of the ultrapure water production system 2 sends a portion of the second permeate from the second reverse osmosis membrane device 12 downstream using the permeate pipe 12b. In addition, in step S13 of Fig. 6, the control device 30 of the ultrapure water production system 2 returns another portion of the second permeate flowing in the permeate pipe 12b to the upstream side of the addition position of the pH adjuster using the permeate return pipe 12d branching off from the permeate pipe 12b.

[0099] Next, in step S14, the control device 30 of the ultrapure water production system 2 measures the pH of the water to be treated after the second permeate has been returned using the pH measuring device 14. The control device 30 then determines whether the basicity of the measured pH is higher than a preset threshold. If the determination result shows that the basicity of the pH of the water to be treated after the treated water has been returned is higher than the preset threshold, i.e., if the basicity of the water to be treated has increased compared to before the return, the process proceeds to step S15 in FIG. 6.

[0100] In step S15, the control device 30 adds a smaller amount of pH adjuster than the amount added in step S11 to the water to be treated that is to be supplied to the second reverse osmosis membrane device 12. In other words, the pH of the preceding water to be treated to which the pH adjuster has been added is monitored, and the amount of pH adjuster to be added to the subsequent water to be treated is feedback-controlled based on the monitoring results.

[0101] Feedback control is generally performed at a location where the quality of the water to be treated fluctuates significantly, such as when raw water is treated in a first-stage reverse osmosis membrane device. However, in a later-stage reverse osmosis membrane device, such as a second-stage device that treats the permeate from the first stage, the water quality fluctuates less, so the need for feedback control is generally not emphasized. However, in this embodiment, feedback control of the amount of pH adjuster added in the second reverse osmosis membrane device 12 is intentionally performed to suppress concentration in the system.

[0102] On the other hand, if the result of the determination in step S14 is that the basicity of the pH of the water to be treated after the treated water is returned is equal to or lower than the preset threshold, the process ends without proceeding to step S15. In this embodiment, the series of steps S11 to S15 described above are repeatedly and continuously performed throughout the entire reverse osmosis membrane treatment. That is, after the process moves to the end after step S15 in FIG. 6, the process returns to the start in FIG. 6 again.

[0103] For this reason, the pH of the water supplied to the second reverse osmosis membrane device 12 is adjusted to a predetermined basicity threshold of 9.2 to 10. If the pH of the 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.

[0104] In the present disclosure, the control device 30 may be configured to determine whether the basicity of the measured pH is lower than a preset threshold. For example, if the determination result in step S14 in Fig. 6 indicates that the basicity of the measured pH is lower than the preset threshold, i.e., if the basicity of the water to be treated has decreased compared to before reflux, the process may proceed to step S15 in Fig. 6. In step S15, the control device 30 adds a larger amount of pH adjuster to the water to be treated that is to be supplied to the second reverse osmosis membrane device 12 than the amount added in step S11 in Fig. 6.

[0105] In other words, in the present disclosure, feedback control can be performed to increase the amount of pH adjuster added not only when the basicity increases as in the present embodiment, but also when the basicity decreases (i.e., when it decreases from a predetermined value or range as a threshold value).

[0106] In the present disclosure, it is not essential that the series of steps S11 to S15 be performed continuously throughout the entire reverse osmosis membrane treatment. In the present disclosure, the series of steps S11 to S15 may be performed at least once during the reverse osmosis membrane treatment. Alternatively, the series of steps may be performed intermittently multiple times, or may be performed continuously.

[0107] The primary pure water produced using the pure water production apparatus 1 is then supplied to a secondary pure water system 22 as shown in FIG. 1 and subjected to a predetermined treatment, ultimately producing ultrapure water.

[0108] (Other operating patterns of the water purifier) Next, other operation patterns of the pure water manufacturing system 1 according to this embodiment will be described.

[0109] (On-off pattern) For example, a case where the permeate volume of the second reverse osmosis membrane device 12 is set to 100, and 99 permeate volumes are flowed through the permeate pipe 12b and 1 permeate volume is flowed through the permeate return pipe 12d is set as condition A. If, during operation of the pure water production system 1 under condition A, a tank (not shown) downstream of the second reverse osmosis membrane device 12 becomes full and the second permeate cannot be supplied to the permeate pipe 12b, condition B is set by adjusting the permeate volume of the permeate return pipe 12d to 99 and the permeate volume of the permeate pipe 12b to 1, for example.

[0110] After starting operation of the water purifying apparatus 1 under condition B, if the water level in the downstream tank falls below a predetermined value, the operating conditions of the water purifying apparatus 1 are returned to condition A. In other words, control is performed to alternate between condition A and condition B depending on the water storage state in the downstream tank.

[0111] Here, condition A may include a case where the permeate amount in the permeate pipe 12b is set to 100, and the permeate amount in the permeate return pipe 12d is set to 0. Condition B may include a case where the permeate amount in the permeate pipe 12b is set to 0, and the permeate amount in the permeate return pipe 12d is set to 100. That is, one of the permeate amount in the permeate pipe 12b and the permeate amount in the permeate return pipe 12d is 0, and the other is 100.

[0112] Switching between condition A and condition B when one of the permeate rate in the permeate pipe 12b and the permeate return pipe 12d is 0 and the other is 100 is, in other words, on-off control. In the case of on-off control, it is also possible to switch the conditions after determining in advance the amount of alkali added under each of condition A and condition B. In the case of switching between condition A and condition B using on-off control, there is no need to control the permeate rate, and therefore the device can be configured simply.

[0113] Furthermore, under condition B, almost no second permeate flows into the equipment downstream of the second reverse osmosis membrane device 12, including the permeate pipe 12b, and the second permeate returned from the permeate return pipe 12d returns to the supply side of the second reverse osmosis membrane device 12, improving the quality of the water supplied to the second reverse osmosis membrane device 12. For this reason, it is also possible to set the amount of alkali added as a pH adjuster to 0. Of course, under condition B, the amount of pH adjuster added may be controlled.

[0114] 2, when almost all of the second permeate is returned to the second pit 18 through the permeate return pipe 12d under condition B, the process of supplying the first permeate from the first reverse osmosis membrane device 11 to the second reverse osmosis membrane device 12 through the permeate return pipe 11b is almost unnecessary. Therefore, the first permeate is returned to the first pit 17 through the permeate return pipe 11d of the permeate pipe 11b.

[0115] Here, under condition B, if the amount of alkali added is fixed at a constant level as in the conventional case, the water supplied to the second reverse osmosis membrane device 12 will be almost the same as the second permeate from the permeate return pipe 12d. Therefore, the quality of the water supplied to the second reverse osmosis membrane device 12 will be better than the quality of the permeate from the first reverse osmosis membrane device 11, and as a result, the amount of alkali in the water supplied to the second reverse osmosis membrane device 12 will be excessive.

[0116] The excess alkali in the water supplied to the second reverse osmosis membrane device 12 is supplied to the first pit 17 via the concentrated water return pipe 12c, resulting in the consumption of the acid added to the supply side of the first reverse osmosis membrane device 11. This increases the pH of the water supplied to the first reverse osmosis membrane device 11. As a result, in the first reverse osmosis membrane device 11 and the third reverse osmosis membrane device 13, the Langerer index of the water supplied increases by, for example, about 1 to 2, making it more likely for hard scale to form. However, in this embodiment, control is performed to reduce the amount of alkali injected, thereby suppressing the formation of hard scale in the first reverse osmosis membrane device 11 and the third reverse osmosis membrane device 13.

[0117] (Automatic control of circulation flow rate) Furthermore, in the present disclosure, for example, when the flow rate of the permeate pipe 12b is flexibly changed to maintain a constant water level in a tank downstream of the second reverse osmosis membrane device 12, the circulation flow rate of the permeate return pipe 12d is also flexibly changed. That is, the circulation flow rate is automatically controlled. In the present disclosure, the control device 30 controls the amount of pH adjuster added to the water to be treated supplied to the second reverse osmosis membrane device 12, so that flexible changes in the circulation flow rate of the permeate return pipe 12d can be effectively accommodated. In this case, the amount of pH adjuster added may be changed according to the circulation flow rate. Feedback control may also be performed according to the pH or amount of impurities in the feed water supplied to the second reverse osmosis membrane device 12.

[0118] In the present disclosure, when the circulation flow rate exceeds a threshold value, or when the supply flow rate to the downstream stage falls below a threshold value, the amount of alkali added may be set to a specified set amount or to 0. This is because, even if treated water is being supplied to the downstream stage, if the circulation flow rate is high, the quality of the treated water can be maintained without supplying a pH adjuster. For example, the allowable value of the supply water quality of the downstream device (e.g., EDI, etc.) will not be exceeded.

[0119] Furthermore, as with the on / off control of the permeate volume, there is no need to control the circulating water volume once the circulating flow rate exceeds a threshold, allowing for a simpler device configuration. It is also possible to minimize the amount of pH adjuster used. The amount of alkali added can be set to a specified amount or even to zero after a specified time has elapsed using a timer. This is to allow for the quality of the feedwater supplied to the second reverse osmosis membrane device 12 by circulation to improve and stabilize. [Example]

[0120] Next, examples of the present embodiment and comparative examples will be described. Specifically, the operating conditions of the examples and comparative examples were changed as follows, and experiments were carried out.

[0121] [Comparative Example] First, in a comparative example, pure water was produced under the following conditions using the pure water producing apparatus 1 according to this embodiment illustrated in FIG.

[0122] (Operating conditions) Water to be treated: Industrial water, conductivity: 90μS / cm, pH=6.8, flow rate: 500m 3 / h First reverse osmosis membrane device 11: 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%. Second reverse osmosis membrane device 12: 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 90%. pH adjuster adding device 15: Sodium hydroxide solution was added to adjust the pH to 9.2. Third reverse osmosis membrane device 13: 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 65%.

[0123] In the comparative example, the pure water production system 1 was operated for 15 days under the above operating conditions by the control device 30. During operation, the pure water production system 1 was switched, approximately every 3 to 4 hours, between supplying treated water to a downstream stage without circulation and circulating operation within the system of the pure water production system 1, using the on / off pattern described above.

[0124] [Example] Next, in the examples, as in the comparative examples, pure water was produced using the pure water production system 1 according to this embodiment illustrated in Fig. 2. Note that in the examples, during circulation operation, the addition of sodium hydroxide solution by the pH adjuster addition device 15 in the second reverse osmosis membrane device 12 was stopped. The operating conditions of the examples, other than the conditions related to the second reverse osmosis membrane device 12, were the same as those of the comparative examples.

[0125] [result] Next, as a result of the experiment, the change in the supply pressure to the third reverse osmosis membrane device 13 when the pure water production system 1 was operated for 15 days is shown in Table 1 below.

[0126] [Table 1]

[0127] As shown in Table 1, in the comparative example, 15 days after the start of operation, clogging of the membrane of the third reverse osmosis membrane device 13 occurred due to the influence of the operation of the second reverse osmosis membrane device 12, and the operating pressure required to achieve the specified water recovery rate increased. On the other hand, in the example, even 15 days after the start of operation, no membrane clogging that would affect the specified water recovery rate occurred. In the comparative example, while treated water was being supplied from the pure water production system 1 to the subsequent stage, the pH of the water supplied from the first reverse osmosis membrane device 11 to the subsequent stage (first treated water) was 6.8, but during circulation operation, the pH of the water supplied to the subsequent stage (first treated water) rose to 8.3.

[0128] (Action and effect) In this embodiment, in step S11, a pH adjuster that increases the basicity is added to the water to be treated that is supplied to the second reverse osmosis membrane device 12. In step S12, a portion of the treated water from the second reverse osmosis membrane device 12 is sent downstream, and in step S13, another portion of the treated water is returned upstream of the addition position of the pH adjuster.

[0129] Furthermore, in steps S14 and S15, if the basicity of the pH of the water to be treated after the treated water is returned is higher than a preset threshold, the amount of pH adjuster added to the water to be treated supplied to the second reverse osmosis membrane device 12 is reduced. That is, the amount of pH adjuster added to the water to be treated is reduced in accordance with the increase in the basicity of the pH of the treated water returned upstream of the addition position of the pH adjuster. Therefore, even if the water to be treated to which the pH adjuster has been added is returned to the system of the second reverse osmosis membrane device 12, concentration of the pH adjuster in the system can be suppressed.

[0130] One possible way to suppress the progression of concentration within the system is to stop the operation of the second reverse osmosis membrane device 12. However, when the second reverse osmosis membrane device 12 is restarted after being stopped, it takes time for the quality of the water to stabilize. Furthermore, when the second reverse osmosis membrane device 12 is stopped or restarted, the flow state of the water to be treated flowing through other treatment devices and piping in the production line of the ultrapure water production system 2 is likely to fluctuate beyond the design range, raising concerns about malfunctions in other water treatment processes. As a result, the quality of the ultrapure water is likely to deteriorate and the yield is likely to decrease.

[0131] In this regard, in this embodiment, there is no need to stop the operation of the second reverse osmosis membrane device 12, and therefore the burden of restarting the operation, deterioration of the quality of the ultrapure water, and reduction in yield are unlikely to occur.

[0132] Furthermore, in this 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 particularly effectively suppressed.

[0133] (Variation) In this embodiment, the amount of pH adjuster added to the water to be treated is reduced or increased depending on the increase in the basicity of the pH of the treated water returning upstream of the addition position of the pH adjuster. However, in this disclosure, the conditions for reducing the amount of pH adjuster added are not limited to this. For example, the amount of pH adjuster added to the water to be treated may be reduced depending on the increase in the amount of treated water returning upstream of the addition position of the pH adjuster.

[0134] The method for producing ultrapure water according to the modified example can also use the ultrapure water producing apparatus 2 of the present embodiment illustrated in Figures 1 to 5. In the method for producing ultrapure water according to the modified example, the step of adding a pH adjuster to increase the basicity of the water to be treated and supplied to the second reverse osmosis membrane device 12 (step S11 in Figure 7) is the same as step S11 in Figure 6 of the present embodiment.

[0135] In the modified example, the process of sending treated water from the second reverse osmosis membrane device 12 downstream (step S12 in FIG. 7) is the same as step S12 in FIG. 6 of the present embodiment. In addition, the process of reducing the amount of pH adjuster added to the water to be treated that is supplied to the second reverse osmosis membrane device 12 (step S15 in FIG. 7) is the same as step S15 in FIG. 6 of the present embodiment.

[0136] However, the modified example differs from the present embodiment in that it determines whether the return amount of treated water increases in step S14A in Figure 7, i.e., whether the return amount of treated water in step S14A is greater than the return amount of treated water in the preceding step S13A.

[0137] In this embodiment, the reflux amount of the treated water in step S13A corresponds to the "preset reflux amount" of the present disclosure. However, in the present disclosure, the preset reflux amount is not limited to this. For example, a specific reflux amount set based on an empirical rule, or a range having an upper limit and a lower limit, can be set as the preset reflux amount.

[0138] In a modified example, if the result of the determination in step S14A is that the return amount of the treated water is greater than the return amount in step S13A, the process proceeds to step S15 in Fig. 7. In step S15, the control device 30 of the ultrapure water production system 2 adds a smaller amount of pH adjuster than the amount added in step S11 to the water to be treated that is to be supplied to the second reverse osmosis membrane device 12.

[0139] On the other hand, if the result of the 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 equal to or less than the return flow rate of the treated water in step S13A, the process ends without proceeding to step S15. The other configurations in the modified example are the same as those of the present embodiment illustrated in Figures 1 to 6, and therefore redundant explanations will be omitted.

[0140] In this modification, if the return flow rate of the treated water is increased in steps S14A and S15 from the return flow rate in step S13A, the amount of pH adjuster added to the water to be treated supplied to the second reverse osmosis membrane device 12 is reduced. That is, the amount of pH adjuster added to the water to be treated is reduced in accordance with an increase in the return flow rate of the treated water upstream of the addition position of the pH adjuster. Therefore, even if the first treated water to which the pH adjuster has been added is returned to the system of the second reverse osmosis membrane device 12, concentration of the pH adjuster in the system can be suppressed. Other effects of the modification are similar to those of the present embodiment.

[0141] <Other embodiments> Although the present disclosure has been described by the following disclosed embodiments, the descriptions and drawings forming part of this disclosure should not be understood as limiting the present disclosure. It should be considered that various alternative embodiments, examples, and operating techniques will become apparent to those skilled in the art from this disclosure.

[0142] For example, although not shown in the drawings, a facility for subjecting concentrated water to ultraviolet oxidation or other treatment when returning the concentrated water to the upstream side may be added to the pure water production system. For example, when ultraviolet oxidation treatment is performed in the pure water production system, organic matter in the concentrated water can be oxidized and decomposed.

[0143] Furthermore, in the present disclosure, for example, the ultrapure water production process that is executed by loading software (programs) by the CPU 31 in the above embodiment may be executed by various processors other than a CPU. Examples of processors in this case include programmable logic devices (PLDs) such as field-programmable gate arrays (FPGAs) whose circuit configuration can be changed after manufacture, and dedicated electrical circuits such as application-specific integrated circuits (ASICs) that are processors with circuit configurations designed specifically for executing specific processes.

[0144] The ultrapure water production process may be performed by one of these various processors, or by a combination of two or more processors of the same or different types (for example, a plurality of FPGAs, or a combination of a CPU and an FPGA, etc.) The hardware structure of these various processors is, more specifically, an electric circuit that combines circuit elements such as semiconductor elements.

[0145] In addition, in each of the above embodiments, the ultrapure water production program is described as being pre-stored (installed) in the ROM 32 or the storage 34, but this is not limiting. The program may be provided in a form recorded on a recording medium such as a CD-ROM (Compact Disk Read Only Memory), a DVD-ROM (Digital Versatile Disk Read Only Memory), or a USB (Universal Serial Bus) memory. The program may also be downloaded from an external device via a network.

[0146] The present disclosure can also be configured by partially combining the configurations illustrated in Figures 1 to 7. As described above, the present disclosure includes various embodiments not described above, and the technical scope of the present disclosure is defined only by the invention-specifying matters in the claims that are appropriate from the above description. [Explanation of symbols]

[0147] 1 Pure water production equipment 2 Ultrapure water production equipment 11 1st reverse osmosis membrane device 11a Supply pipe 11b Permeated water piping 11c Concentrated water piping 11d Permeated water return piping 12 Second reverse osmosis membrane device 12b Permeated water piping 12c Concentrated water return piping 12d Permeated water return piping 13 Third reverse osmosis membrane device 13b Concentrate water drain pipe 13c Permeated water return piping 14 pH measuring device 15 pH adjuster adding device 16 Flow meter 17 First Pit 18 Second Pit 21 Primary pure water system 22 Secondary Pure Water System 23 Electrodeionization equipment 24 UV oxidation equipment 25 Regenerative mixed-bed ion exchanger 30 Control device 34 Storage 35 User Interface 36 Communication Interface 37 Bus

Claims

1. a first step of adding a pH adjuster to increase the basicity of water to be treated, which is to be supplied to a membrane treatment device, and sending a portion of the permeated water from the membrane treatment device downstream while returning another portion of the permeated water to the upstream side of the addition position of the pH adjuster; a second step of reducing the amount of the pH adjuster added to the water to be treated that is supplied to the membrane treatment device according to a change in a preset index of the water to be treated after the permeated water is refluxed; Including, a return pipe branching from a branch portion of a pipe connected downstream of the membrane treatment device and having a branch portion for sending a portion of the permeated water from the membrane treatment device downstream of the branch portion, said return pipe being connected downstream of the membrane treatment device and having a branch portion for sending a portion of the permeated water from the membrane treatment device downstream of the branch portion.

2. The indicator is the pH of the water to be treated, In the second step, when the basicity of the pH of the water to be treated after the permeated water is refluxed is higher than a predetermined threshold, the amount of the pH adjuster added to the water to be treated to be supplied to the membrane treatment device is reduced. The method for producing ultrapure water according to claim 1 .

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

4. a first step of adding a pH adjuster to increase the basicity of the water to be treated, which is to be supplied to a membrane treatment device, and sending a portion of the permeated water from the membrane treatment device downstream while returning another portion of the permeated water to the upstream side of the addition position of the pH adjuster at a preset reflux amount; a second step of increasing the amount of the permeate returned to the membrane treatment device compared to the first step and reducing the amount of the pH adjuster added to the water to be treated that is supplied to the membrane treatment device; Including, a return pipe branching from a branch portion of a pipe connected downstream of the membrane treatment device and having a branch portion for sending a portion of the permeated water from the membrane treatment device downstream of the branch portion, said return pipe being connected downstream of the membrane treatment device and having a branch portion for sending a portion of the permeated water from the membrane treatment device downstream of the branch portion.

5. a membrane treatment device that applies membrane treatment to the supplied water to be treated; a pH adjuster adding device that is disposed upstream of the membrane treatment device and that adds a pH adjuster to the water to be treated to increase the basicity of the water to be treated; a pipe connected to the downstream side of the membrane treatment device, the pipe having a branch portion and sending a portion of the permeated water of the membrane treatment device downstream of the branch portion; a reflux pipe that branches off from the branching portion of the piping and refluxes another portion of the permeated water upstream of the pH adjuster adding device; a control device that controls the pH adjuster adding device so as to reduce the amount of the pH adjuster added to the water to be treated that is supplied to the membrane treatment device in accordance with a change in a preset index of the water to be treated after reflux; An ultrapure water production apparatus comprising:

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