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

Abstract

To provide an ultrapure water producing method and system capable of sufficiently reducing the number of fine particles of iron, silica, etc. in ultrapure water without using complicated equipment and without significantly decreasing the amount of water produced. [Solution] A method for producing ultrapure water including a filtration process using a filtration membrane, in which the filtration membrane is one or more types selected from an ultrafiltration membrane (UF), a microultrafiltration membrane (MF) and an ion exchange membrane, and the filtration process is carried out under conditions that satisfy both (1) and (2). (1) The permeate flux during filtration is in the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane, (2) The water recovery rate during the filtration is 50% or more and less than 80%.

Description

[Technical field]

[0001] The present invention relates to an ultrapure water producing method, an ultrapure water producing apparatus, and an ultrapure water producing system, and more particularly to an ultrapure water producing method, an ultrapure water producing apparatus, and an ultrapure water producing system that can reduce the iron concentration and silica concentration in ultrapure water. [Background technology]

[0002] Ultrapure water used in semiconductor manufacturing processes and the like is produced by an ultrapure water production system that includes a primary water purification device and a secondary water purification device in this order. In the ultrapure water production system, the primary water purification device produces primary pure water by removing total organic carbon (TOC) components and ionic components from raw water or pretreated water using a reverse osmosis membrane device or an ion exchange device. The secondary water purification device produces ultrapure water by removing extremely small amounts of impurities from the primary pure water. A cross-flow type ultrafiltration (UF) device is generally installed at the end of the secondary water purification device and operated at a water recovery rate of 90% to 99% to remove nanometer-sized particles.

[0003] In ultrapure water production systems, various attempts have been made to remove impurities such as fine particles from water to a high degree to increase its purity. For example, a method of performing total filtration using a fine particle removal membrane having a cationic functional group at the end of a secondary pure water system (see, for example, Patent Document 1), a method of arranging ultrafiltration membrane devices in series in multiple stages at the end (see, for example, Patent Document 2), and a method of providing a liquid delivery pump that sends pure water to the secondary pure water system in parallel and operating it at low power all the time (see, for example, Patent Document 3) have been proposed. Also proposed is a system in which a porous ion exchanger and a microfiltration membrane device are provided in this order downstream of the ultrafiltration membrane device of the secondary pure water system (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-170406 A [Patent Document 2] JP 2016-064342 A [Patent Document 3] JP 2006-167661 A [Patent Document 4] JP 2022-154537 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, with the remarkable progress of miniaturization and high integration of semiconductor circuits, the required water quality for ultrapure water used in semiconductor manufacturing processes has become increasingly strict. For example, according to the International Technology Roadmap for Semiconductors (ITRS), it is required that the number of particles with a diameter of 10 nm or more contained in ultrapure water be controlled to 1 particle / ml or less. However, the technology described in Patent Documents 1 to 4 does not necessarily provide treated water quality that satisfies such requirements. For example, detailed studies by the present inventors have revealed that iron and silica remain in the treated water of a polisher (non-regenerative mixed-bed ion exchange resin device) installed in the front stage of an ultrafiltration membrane device, which causes a problem of an increase in the number of particles in the ultrapure water. In particular, it has been found that when a booster pump or a heat exchanger is installed near the end of a secondary pure water device, the number of particles (including colloids such as Fe, Cr, and Si) is likely to increase. In response to this, the generation of particles from gaskets and the like at the connection parts of each device has also begun to be highlighted as a problem. In addition, in the past, it was thought that the molecular weight cutoff of ultrafiltration membrane devices could be reduced in order to remove even finer particles. However, even if the molecular weight cutoff was reduced, the improvement in the particle removal performance was only slight, and there was a problem that the amount of fresh water produced was reduced.

[0006] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide an ultrapure water producing method, an ultrapure water producing apparatus, and an ultrapure water producing system that can sufficiently reduce the number of fine particles such as iron and silica in ultrapure water without using complex equipment and without significantly reducing the amount of water produced. [Means for solving the problem]

[0007] An ultrapure water producing method, an ultrapure water producing apparatus, or an ultrapure water producing system according to an embodiment of the present invention is as follows. [1] In a method for producing ultrapure water, which includes a filtration process using a filtration membrane, The filtration membrane is an ultrafiltration membrane (UF), Secret The filtration process is carried out under conditions that satisfy both of the following (1) and (2): (1) The permeate flux during filtration is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more and less than 80%. [2] The method according to [1], wherein the water to be filtered has a resistivity of 17 MΩ·cm or more, an iron concentration of 3 ng / L to 100 ng / L, and a silica concentration of 10 to 1000 ng / L. [3] The method according to [1] or [2], in which the treated water from a non-regenerative mixed-bed ion exchange resin device is used as the water to be treated by the filtration treatment. [4] The method for producing ultrapure water comprises treating raw water in a primary water purification system and a secondary water purification system in this order, The manufacturing method according to any one of [1] to [3], wherein the filtration process is performed at the end of the secondary water purification device. [5] The method according to any one of [1] to [4], wherein the ultrapure water produced by the method has an iron concentration of 1 ng / L or less and a silica concentration of 20 ng / L or less.

[0008] [6] An ultrapure water production apparatus including a filtration membrane module having a filtration membrane, The filtration membrane is at least one selected from an ultrafiltration membrane, a microfiltration membrane, and an ion exchange membrane; The filtration process in the filtration membrane module is controlled to satisfy both of the following conditions (1) and (2): (1) The permeate flux of the filtration membrane module is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane module. (2) The water recovery rate in the filtration membrane module is 50% or more and less than 80%. [7] The ultrapure water production apparatus according to [6], comprising, in this order, a water pump, a non-regenerative mixed-bed ion exchange resin device, and the ultrafiltration membrane module. [8] The ultrapure water producing apparatus according to [6] or [7], wherein the treated water supplied to the filtration membrane module has a resistivity of 17 MΩ cm or more, an iron concentration of 3 ng / L or more, and a silica concentration of 100 ng / L or more. [9] A primary water purification system that treats raw water to produce primary pure water, and a secondary water purification system that treats the primary pure water, An ultrapure water producing system having the ultrafiltration membrane module described in any one of [6] to [8] at the end of the secondary pure water apparatus. In this specification, the symbol "~" indicates a numerical range including the numerical values ​​before and after it. Effect of the Invention

[0009] According to the ultrapure water producing method, ultrapure water producing apparatus, and ultrapure water producing system of the present invention, the number of fine particles such as iron and silica in ultrapure water can be sufficiently reduced without using complex equipment and without a significant decrease in the amount of water produced. [Brief description of the drawings]

[0010] [Figure 1] 1 is a block diagram showing a schematic configuration of an ultrapure water production apparatus used in an embodiment of the ultrapure water production method. [Diagram 2] FIG. 2 is a diagram for explaining the state of capture of fine particles by the filtration membrane in this embodiment, and is a schematic diagram showing, from the top, (a) the initial stage of filtration, (b) when filtration has progressed, and (c) when filtration has further progressed. [Diagram 3] FIG. 11 is a block diagram illustrating a schematic configuration of a filtration membrane device in a secondary pure water system according to a first modified example. [Figure 4]FIG. 11 is a block diagram illustrating a secondary pure water system according to a second modified example. [Diagram 5] 1 is a block diagram illustrating an ultrapure water producing system according to an embodiment. [Figure 6] FIG. 2 is a block diagram illustrating a secondary water purification device used in the examples. [Figure 7] 1 is a graph for determining the optimum permeate flux. [Figure 8] 1 is a graph showing the relationship between water recovery rate and iron removal rate when the permeate flux is changed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described. The ultrapure water production method of the embodiment is a method for reducing iron and silica in ultrapure water by performing a filtration process under predetermined conditions in an ultrapure water production method including a filtration process using a filtration membrane. The filtration membrane is an ultrafiltration membrane (UF), a purifying membrane (PWM), or a filtration membrane (PWM). Secret The filtration membrane is at least one selected from a microfiltration membrane (MF) and an ion exchange membrane. Hereinafter, the case where an ultrafiltration membrane is used as the filtration membrane will be described, but the same applies to the case where a microfiltration membrane (MF) or an ion exchange membrane is used.

[0012] Fig. 1 is a block diagram showing a schematic configuration of a secondary pure water system 10 used in an ultrapure water manufacturing method according to an embodiment. The secondary pure water system 10 shown in Fig. 1 is a system for manufacturing ultrapure water, and includes a pure water tank (TK) 11, a water pump P provided downstream of the pure water tank 11, a non-regenerative mixed bed ion exchange resin system (Polisher) 12, a booster pump (BSP) 13, and an ultrafiltration membrane system (UF) 14, in this order. In the secondary pure water system 10, pure water (primary pure water) stored in the tank 11 is supplied by the water pump P to the non-regenerative mixed bed ion exchange resin system 12 and the ultrafiltration system 14, which are sequentially processed to manufacture ultrapure water.

[0013] In the secondary pure water system 10, the permeate side of the ultrafiltration membrane device 14 is connected to a point-of-use (POU) 15 by a water supply pipe 1a. The ultrapure water produced by the secondary pure water system 10 is sent to the point-of-use (POU) 15 via the water supply pipe 1a and used here. One end of a circulation pipe 1c is branched and connected to the water supply pipe 1a. A valve V1a is provided on the path of the circulation pipe 1c. A drain pipe 1b is connected to the concentration side of the ultrafiltration membrane device 14, and a valve V1b is provided on the path of the drain pipe 1b. The other end of the circulation pipe 1c is connected to a tank 11, and these return ultrapure water not used at the point-of-use 15 to the tank 11.

[0014] The ultrafiltration membrane device 14 includes one or more ultrafiltration membrane modules, or one or more ultrafiltration membrane cartridges. The ultrafiltration membrane module includes an ultrafiltration membrane inside a housing having a water inlet for feed water, a permeate outlet, and a concentrated water outlet, and the openings of the module are connected to piping to pass the water to be treated through the ultrafiltration membrane. The ultrafiltration membrane module has an advantage that when the filtration performance is reduced due to the blockage of the ultrafiltration membrane, the module can be replaced as a whole, and the deteriorated ultrafiltration membrane can be washed for each module. The ultrafiltration membrane installed in the ultrafiltration membrane module may be called an ultrafiltration membrane cartridge. In the case of an ultrafiltration membrane cartridge, it is possible to replace only the cartridge in the ultrafiltration membrane module. In this embodiment, an ultrafiltration membrane device 14 including one ultrafiltration membrane module will be described as an example. In the ultrafiltration membrane device 14 including one ultrafiltration membrane module, the water passing conditions of the ultrafiltration membrane device 14 and the water passing conditions of the ultrafiltration membrane module are the same.

[0015] 1 shows only one use point 15, the pure water production system 10 may be connected to two or more use points 15. In this case, branch pipes for connecting to each use point are provided in the circulation pipe 1c, and ultrapure water can be supplied to each use point via the branch pipes.

[0016] Next, an embodiment of an ultrapure water manufacturing method using the secondary pure water device 10 will be described. First, the water pump P is operated to send the pure water in the pure water tank 11 to the rear stage. The material and shape of the pure water tank 11 are not particularly limited as long as it is free from rust, hardly dissolves components from the container, and can stably store the primary pure water. The material of the pure water tank 11 is preferably, for example, fiber reinforced plastics (FRP), polyethylene, SUS304, and those lined with fluororesin such as Teflon (registered trademark). In addition, the upper part of the pure water tank 11 is preferably purged with an inert gas such as pure nitrogen to prevent the absorption of impurity gases such as carbon dioxide gas and dissolved oxygen. The pure water tank 11 can also store a mixture of the primary pure water and the ultrapure water when circulating unused ultrapure water among the produced ultrapure water.

[0017] Pure water (primary pure water) can be obtained, for example, by treating raw water or pretreated water with a primary pure water apparatus described below. The quality of the pure water is, for example, a resistivity of 17 MΩ cm or more, an iron concentration of 3 ng / L to 5 ng / L, and a silica concentration of 10 ng / L to 1000 ng / L.

[0018] The pure water is supplied to the non-regenerated mixed bed ion exchange resin device 12, where cations and anions in the water are removed. The non-regenerated mixed bed ion exchange resin device 12 has a structure in which a mixed bed ion exchange resin is filled in, for example, a cylindrical sealed container. The mixed bed ion exchange resin is usually a mixture of a cation exchange resin and an anion exchange resin. In the non-regenerated mixed bed ion exchange resin device 12, fine particles such as silica and iron, which are easily colloidalized, are more likely to break through than strong ions such as sodium and chlorine. In the conventional method, it is considered that these fine particles pass through the ultrafiltration membrane device 14, resulting in an increase in the number of fine particles in the ultrapure water. According to the secondary pure water device 10 of this embodiment, by controlling the ultrafiltration membrane device 14 under the conditions described below, it is possible to suppress the passage of these fine particles through the ultrafiltration membrane device 14. Note that iron, silica, etc. can exist in water in the form of ions, colloids, or fine particles, but in this specification, they are described as fine particles without considering the form in water.

[0019] The pure water that has passed through the non-regenerative mixed bed ion exchange resin device 12 is then supplied to an ultrafiltration membrane device 14 with its supply pressure increased by a booster pump 13. In the ultrapure water producing method of this embodiment, a filtration process is performed in the ultrafiltration membrane module of the ultrafiltration membrane device 14 under conditions that satisfy both of the following (1) and (2). (1) The permeate flux during filtration is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more but less than 80%.

[0020] The above conditions (1) and (2) are explained in detail below. First, the "optimum permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane" in the above condition (1) is explained. When the water supply pressure to the filtration membrane is constant, the transmembrane pressure difference during filtration is proportional to the thickness of the filtration membrane, and the permeate flux is inversely proportional to the thickness of the filtration membrane. Therefore, when the "transmembrane pressure difference / permeate flux" (hereinafter referred to as "K value") is calculated when water is passed through the filtration membrane under certain conditions, the "K value" is determined as a value specific to the filtration membrane, regardless of the water passing conditions in the filtration membrane. In this specification, the "K value" is defined as "transmembrane pressure difference (Kgf / m 2 ) / permeated water flux (m / h)”×10 -4 The value of "transmembrane pressure (Kgf / cm 2 ) / permeate flux (m / h).

[0021] The above water flow condition (1) in the ultrafiltration membrane device 14 is, in terms of improving the removal rate of fine particles, 0.5 to 2.0 times the optimal permeate flux determined according to the K value, preferably 0.7 to 1.6 times, and more preferably 0.8 to 1.2 times.

[0022] This optimum permeate flux is determined according to the K value by the following formula (3). Optimal permeate flux = -0.69log 10 (K)+0.64···(3)

[0023] As a result of various preliminary experiments, the inventors found that the optimal permeate flux can be obtained by the above formula (3). The contents of the preliminary experiments are as follows. First, commercial ultrafiltration membrane modules (nominal molecular weight cutoffs of 4000, 6000, and 10000), microfiltration membrane modules, and ion exchange membrane modules for secondary pure water production were obtained, and the permeate flux was measured for each membrane module when the transmembrane pressure was changed. From the measurement results, the "transmembrane pressure / transmembrane flux", i.e., the K value, was calculated at each transmembrane pressure. Here, it was confirmed that the K value is an approximately constant value regardless of the transmembrane pressure or water recovery rate. In this preliminary experiment, the transmembrane pressure was changed under two to five conditions to obtain the K value for each condition, and the K value was determined by the arithmetic mean of the K values. In addition, from the "Experiment to obtain the K value (Table 2)" described later, it was also found that the K value shows an approximately constant value regardless of the conditions. This is consistent with the fact that it can be derived from Darcy's law that the K value is a parameter of the same dimension as the thickness of the film.

[0024] Next, the "recommended operating conditions" or "standard operating conditions" shown in the instruction manual of the commercially available membrane module were referred to, and the "recommended amount of filtrate (permeate amount)" shown as the maximum amount of filtrate (maximum amount of permeate) when operated under the operating conditions was used to plot the filtration flux (permeate flux), i.e., the amount of filtrate (permeate amount) divided by the membrane area, on the vertical axis and the K value on the horizontal axis, and the graph in Figure 7 and the above formula (3) were obtained by an automatic program using the least squares method. These "recommended operating conditions" or "standard operating conditions" are generally determined by the distributor as conditions under which the membrane module at the end of the ultrapure water system can be operated without clogging or damage. It has been confirmed through many preliminary experiments that the same tendency as the above formula (3) is shown even for membranes other than those used in the "Experiment to determine the K value (Table 2)" described later.

[0025] Next, the above condition (2) will be described. By adopting the above condition (2) of a water recovery rate in the filtration process of 50% or more and less than 80% while satisfying the above condition (1), fine particles can be removed with high accuracy in the ultrafiltration membrane device 14. This is thought to be because, as will be described next, fine particles smaller than the pore size of the ultrafiltration membrane can be prevented from passing through the ultrafiltration membrane, thereby maintaining a high removal rate.

[0026] Conventionally, in a filtration membrane device, for example, an ultrafiltration membrane device, it was believed that fine particles larger than the pore size of the ultrafiltration membrane were captured and removed from the water, that is, filtration proceeded mainly by the mechanism of surface filtration. Therefore, it was also believed that a sedimentary layer (cake) of fine particles was formed on the surface of the ultrafiltration membrane as the treatment period passed, and this sedimentary layer of fine particles captured the fine particles, and therefore the formation of the sedimentary layer improved the removal rate of the fine particles. Therefore, conventionally, in an ultrafiltration membrane device at the end of a secondary pure water system, in order to reduce the number of fine particles in the ultrapure water, a method was adopted in which the amount of concentrated water was reduced to increase the water recovery rate or a total amount was filtered (for example, see paragraph 0018 of Patent Document 1 and Patent Document 2).

[0027] However, the inventors conducted an experiment based on the hypothesis that the fine particles observed in the terminal ultrapure water, such as silica and iron, are in a so-called colloidal state, that the fine particles may be smaller than the pore size of the ultrafiltration membrane, that the filtration of the fine particles proceeds not by surface filtration as described above, but by a mechanism of deep filtration, and that the formation of the sediment layer may not proceed very much because the number of fine particles in the water is small. As a result, they found that the removal rate of fine particles in the filtration membrane can be increased by filtering using the cross-flow method under the above-mentioned conditions. The reason for this is not necessarily clear, but is thought to be as follows.

[0028] FIG. 2 is a diagram for explaining the state of capture of microparticles by the membrane during membrane filtration in this embodiment, and is a schematic diagram showing, from the top, (a) the early stage of filtration, (b) when filtration has progressed (middle stage), and (c) when filtration has further progressed (final stage).

[0029] In FIG. 2, the rectangles represent the membrane surface 21 and the pore inner wall 22 of the filtration membrane, and the small circles represent the fine particles 23. In FIG. 2, the thick arrows represent the water flow, and the thin arrows represent the movement of the fine particles 23. In membrane filtration, the fine particles 23 are captured and adsorbed at two possible locations: the membrane surface 21 and the pore inner wall 22. When the filtration membrane is new or in a state close to new, the number of fine particles 23 already adsorbed on the membrane surface 21 or the pore inner wall 22 of the filtration membrane is small. Therefore, although both the membrane surface 21 and the pore inner wall 22 can adsorb the fine particles 23, the water to be treated first comes into contact with the membrane surface 21, so that the membrane surface 21 mainly adsorbs the fine particles 23, as shown in FIG. 2(a). After that, as the adsorption of the fine particles 23 on the membrane surface 21 progresses, the adsorption of the fine particles 23 on the pore inner wall 22 gradually progresses, as shown in FIG. 2(b).

[0030] Furthermore, as the membrane filtration progresses and the adsorption of the fine particles 23 to the ultrafiltration membrane progresses, the amount of the fine particles 23 adsorbed to the membrane surface 21 or the pore inner walls 22 increases. As a result, the fine particles 23 are less likely to be newly adsorbed to the membrane, and the fine particles 23 begin to permeate the membrane, as shown in Fig. 2(c). In other words, the removal rate of the fine particles 23 decreases.

[0031] Here, since the fine particles 23 adsorbed to the membrane are repeatedly adsorbed and desorbed during the membrane filtration process, if the membrane filtration is a crossflow and the flow rate along the membrane surface 21 is increased to suppress an increase in the concentration of the fine particles 23 on the membrane surface 21, the initial state as shown in Fig. 2(a) can be maintained. This makes it possible to suppress fine particles smaller than the pore size of the ultrafiltration membrane from passing through the ultrafiltration membrane, and therefore it is believed that a high removal rate can be maintained. Here, the flow rate along the membrane surface can be increased by increasing the flow rate on the concentrated side, i.e., by decreasing the water recovery rate.

[0032] At the optimum permeate flux determined according to the K value, if the water recovery rate in the ultrafiltration membrane device 14 (ultrafiltration membrane module) is 80% or more, the ultrafiltration membrane device 14 cannot sufficiently remove fine particles. This is believed to be because the small amount of concentrated water reduces the flow rate on the membrane surface, and fine particles smaller than the pore size of the ultrafiltration membrane pass through the ultrafiltration membrane. If the water recovery rate is less than 50%, the water recovery rate becomes too small and the practicality decreases. In terms of improving the removal rate of fine particles and efficiently obtaining ultrapure water, it is preferable that the water recovery rate in the ultrafiltration membrane device 14 is 60% or more and 70% or less at the optimum permeate flux determined according to the K value.

[0033] As described above, one or more types selected from a microfiltration membrane device and an ion exchange membrane device may be used instead of the ultrafiltration membrane device 14. In this case, one type selected from the ultrafiltration membrane device, the microfiltration membrane device, and the ion exchange membrane device may be used alone, or two or more types may be used in combination. A plurality of ultrafiltration membrane devices, microfiltration membrane devices, and ion exchange membrane devices may be used. The ultrafiltration membrane device 14 may be used in combination with an ion exchange membrane device installed on the water supply pipe 1a.

[0034] The ultrafiltration membrane device 14 removes fine particles in water, for example, fine particles having a particle diameter of 50 nm or more (preferably 10 nm or more). The ultrafiltration membrane device is composed of one or more ultrafiltration membrane modules. The type of ultrafiltration membrane provided in the ultrafiltration membrane module is not particularly limited, and is usually a hollow fiber membrane, and may be a spiral membrane, a tubular membrane, a flat membrane, or the like. The material of the ultrafiltration membrane is polysulfone, polyvinylidene fluoride, polyethylene, polypropylene, or the like, and the nominal molecular weight cutoff is preferably 3000 to 8000, and more preferably 4000 to 6000. The ultrafiltration membrane module may be either an internal pressure type or an external pressure type.

[0035] The microfiltration membrane (MF) device is composed of one or more microfiltration membrane (MF) cartridges. The microfiltration membrane (MF) cartridge is equipped with a microfiltration membrane (MF). The material of the microfiltration membrane is polysulfone, polyethersulfone, polyvinylidene fluoride, polyethylene, polypropylene, polytetrafluoroethylene, etc., and the nominal pore size of the microfiltration membrane is, for example, 0.5 nm to 5 μm.

[0036] The ion exchange membrane device is composed of one or more ion exchange membrane cartridges. The ion exchange membrane cartridge is equipped with an ion exchange membrane, and the ion exchange membrane may be any of a cation exchange membrane, an anion exchange membrane, and a bipolar membrane that combines a cation exchange membrane and an anion exchange membrane.

[0037] In Fig. 1, the valves V1a and V1b are, for example, open / close valves or variable opening valves. The water recovery rate in the ultrafiltration membrane device 14 can be adjusted by opening / closing the valves V1a and V1b or adjusting the opening. The valves V1a and V1b can be configured as automatically controllable valves, and a control device can be provided in the secondary pure water system 10, so that the valves V1a and V1b can be automatically controlled by the control device so that the water recovery rate of the ultrafiltration membrane device 14 satisfies the above condition (2). Note that the valve V1a is provided downstream of the branch point to the use point 15 of the circulation pipe 1c in order to obtain sufficient water supply pressure to the use point 15, but it may be provided between the ultrafiltration membrane device 14 and the branch point to the use point 15.

[0038] The booster pump 13 is a general booster pump, and its configuration is not particularly limited. For example, even if the part in contact with the pure water is made of a material such as stainless steel, which dissolves a small amount of metal components such as iron and chromium, in this embodiment, the dissolved metal components are removed by the ultrafiltration membrane device 14. Therefore, the material of the part in contact with the primary pure water in the booster pump 13 has almost no effect on the water quality of the ultrapure water to be produced. Therefore, as the booster pump 13, a booster pump that is generally used in the production of pure water can be used. Note that, when the water supply pump P is installed between the pure water tank 11 and the non-regenerative mixed bed ion exchange resin device 12, trace metals may be generated from the pump, but most of the trace metals are adsorbed and removed by the ion exchange resin in the non-regenerative mixed bed ion exchange resin device 12. The control device may also control the water supply pump P and the booster pump 13 so that the permeate flux of the ultrafiltration membrane device 14 satisfies the above condition (1).

[0039] In the secondary pure water system 10 of this embodiment, a booster pump 13 is provided between the non-regenerative mixed bed ion exchange resin device 12 and the ultrafiltration membrane device 14, thereby increasing the water supply pressure to the point of use 15. As the booster pump 13, a booster pump whose contact portion with the water to be treated is made of a material that does not or does not easily elute metal components may be used. The booster pump 13 may also be installed upstream of the non-regenerative mixed bed ion exchange resin device 12.

[0040] The quality of the ultrapure water obtained through the ultrafiltration membrane device 14 is, for example, 50 pcs. / L or less in the number of particles with a particle diameter of 50 nm or more, 1 μgC / L or less in total organic carbon (TOC) concentration, and 18 MΩ cm or more in resistivity. Furthermore, the ultrapure water can have an iron concentration of 1 ng / L or less, preferably 0.1 ng / L or less, and a silica concentration of 20 ng / L or less, preferably 10 ng / L or less.

[0041] [Variation 1] Next, a first modified example of the secondary pure water system 10 of the embodiment will be described. Fig. 3 is a diagram showing a schematic configuration of a filtration membrane device 140 in the secondary pure water system of the first modified example. The filtration membrane device 140 of the first modified example is an example in which the ultrafiltration membrane device 14 of the secondary pure water system 10 of Fig. 1 is configured in an array using three or more ultrafiltration membrane modules, and other configurations are similar to those of the secondary pure water system 10 of the above-mentioned embodiment. In this modified example as well, one or more of a microfiltration membrane and an ion exchange membrane may be used instead of the ultrafiltration membrane.

[0042] 3, the filtration membrane device 140 has two banks, a first bank 140P and a second bank 140Q. The first bank 140P and the second bank 140Q are arranged in series along the flow direction.

[0043] The first bank 140P includes two ultrafiltration membrane modules, a first filtration membrane module 141 and a second filtration membrane module 142. The first filtration membrane module 141 and the second filtration membrane module 142 are arranged in parallel with respect to the flow direction. The second bank 140Q includes a third filtration membrane module 143.

[0044] The first filtration membrane module 141 is connected to a water supply pipe 141a for supplying the permeated water of the first filtration membrane module 141 and a drain pipe 141b for discharging the concentrated water. The second filtration membrane module 142 is connected to a water supply pipe 142a for supplying the permeated water of the second filtration membrane module 142 and a drain pipe 142b for discharging the concentrated water. The third filtration membrane module 143 is connected to a water supply pipe 143a for supplying the permeated water of the third filtration membrane module 143 and a drain pipe 143b for discharging the concentrated water. The drain pipe 141b and the drain pipe 142b are connected to the supply side of the third filtration membrane module 143.

[0045] As a result, the pure water supplied to the first bank 140P is filtered by the first filtration membrane module 141 and the second filtration membrane module 142, and the permeated water is sent to the water supply pipe 141a and the water supply pipe 142a, respectively. The concentrated water of the first filtration membrane module 141 and the second filtration membrane module 142 is discharged from the drain pipe 141b and the drain pipe 142b, respectively, and supplied to the second bank 140Q.

[0046] The concentrated water of the first bank 140P supplied to the second bank 140Q is filtered by the third filtration membrane module 143 and separated into permeate and concentrated water. The permeate of the third filtration membrane module 143 is sent to the water supply pipe 143a, and the concentrated water is discharged from the drain pipe 143b.

[0047] The operating conditions of the first filtration membrane module 141, the second filtration membrane module 142, and the third filtration membrane module 143 are all similar to the operating conditions of the ultrafiltration membrane device (ultrafiltration membrane module) in the secondary pure water system 10 of the above-mentioned embodiment. The first filtration membrane module 141, the second filtration membrane module 142, and the third filtration membrane module 143 are controlled so as to satisfy the following conditions (1) and (2) in each module. This control can be achieved by controlling the output of a pump that supplies water to the filtration membrane module, and by providing variable opening valves in the water supply pipe and the drain pipe, respectively, and adjusting the flow rate by the opening of the valves. (1) The permeate flux during filtration is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more and less than 80%.

[0048] The permeated water from the first bank 140P and the second bank 140Q is supplied to a use point via water pipes 141a, 142a, and 143a. The concentrated water from the second bank 140Q may be discharged to the outside of the system or returned to one or more locations, such as the inlet side of a booster pump, a pure water tank, or a primary pure water system provided upstream of the secondary pure water system and a stage preceding the primary pure water system.

[0049] Here, an example has been described in which two banks are provided and the most upstream bank is provided with two filtration membrane modules, but the filtration membrane device may have three or more banks, and each bank may have three or more filtration membrane modules. In this case, the three or more banks are arranged in series along the flow direction, and the multiple filtration membrane modules in one bank are arranged in parallel with respect to the flow direction. In addition, since concentrated water from the previous bank is collected and treated by the filtration membrane module of the subsequent bank, the amount of treated water in the bank decreases toward the downstream side. Taking this into consideration, the number of filtration membrane modules in the subsequent stage can be determined according to the water recovery rate in the filtration membrane module in the previous stage.

[0050] In this way, when using a filtration membrane device 140 in which three or more filtration membrane modules are used to form an array, the water recovery rate of the entire unit can be improved. For example, when the water recovery rates of the first filtration membrane module 141, the second filtration membrane module 142, and the third filtration membrane module 143 of the filtration membrane device 140 shown in Fig. 3 are all 50%, a water recovery rate of 75% is achieved in the entire filtration membrane device 140. Therefore, both a high water recovery rate and a high removal rate of fine particles can be achieved.

[0051] [Variation 2] Next, a second modified example of the secondary pure water system 10 of the embodiment will be described. FIG. 4 is a block diagram showing a schematic configuration of a secondary pure water system 150 of the second modified example. The secondary pure water system 150 of the second modified example differs from the secondary pure water system 10 of FIG. 1 in that it has a circulation pipe 54c that returns the concentrated water of the ultrafiltration membrane device 14 to the inlet side of the booster pump 53 in the secondary pure water system 10 of FIG. 1, but the other configurations are similar. The secondary pure water system 150 of FIG. 4 is equipped with a pure water tank 51, a booster pump 53, and an ultrafiltration membrane device (UF) 54 in this order. Although the non-regenerated mixed bed ion exchange resin device, valves, use points, and circulation pipes from the use points downstream are omitted in FIG. 4, the secondary pure water system 150 is equipped with these, as in the secondary pure water system 10 of FIG. 1. In this modified example, one or more of a microfiltration membrane and an ion exchange membrane may be used instead of the ultrafiltration membrane.

[0052] A water supply pipe 54a is connected to the permeation side of the ultrafiltration membrane device 54, and the permeated water is supplied to a use point (not shown) through the water supply pipe 54a. A drain pipe 54b is connected to the concentration side of the ultrafiltration membrane device 54, and one end of a circulation pipe 54c is connected to the path of the drain pipe 54b. The other end of the circulation pipe 54c is connected to the inlet side of the booster pump 53, so that a part of the concentrated water of the ultrafiltration membrane device 54 is returned to the booster pump 53. Valves may be provided on the downstream side of the drain pipe 54b from the circulation pipe 54c and on the circulation pipe 54c, and a part of the concentrated water may be circulated to the booster pump 53 by controlling the flow rate from the drain pipe 54b to the circulation pipe 54c using the valves.

[0053] The operating conditions of the ultrafiltration membrane device 54 are all the same as those of the secondary pure water system 10 of the above-mentioned embodiment. The ultrafiltration membrane device 54 is equipped with an ultrafiltration membrane module, and the water flow conditions are controlled so that filtration in the filtration membrane module satisfies the following conditions (1) and (2). This control is achieved by controlling the output of the booster pump and by providing variable opening valves in the water supply pipe and the drain pipe, respectively, and adjusting the flow rate by the opening of the valves. (1) The permeate flux during filtration is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more and less than 80%.

[0054] In this way, according to the secondary pure water system 150 in which the concentrated water of the ultrafiltration membrane device 54 is circulated to the inlet side of the booster pump, the use of the circulation piping 54c makes it possible to increase the flow rate of the drain pipe 54b while maintaining the flow rate of the water supply pipe 54a, thereby improving the water recovery rate of the entire secondary pure water system. Also, since the water recovery rate in the ultrafiltration membrane device 54 (filtration membrane module) can be reduced, it is possible to maintain the fine particle removal rate in the filtration membrane device 54 at a high level.

[0055] [Ultrapure water production system] Next, an ultrapure water producing system and an ultrapure water producing method according to the present embodiment will be described. The ultrapure water producing system according to this embodiment includes a primary pure water apparatus that performs a primary pure water treatment to convert raw water or pretreated water into primary pure water, and the above-mentioned secondary pure water apparatus.

[0056] 5 is a block diagram showing an outline of an ultrapure water production system 100 according to this embodiment. The ultrapure water production system 100 includes a pretreatment device 110, a primary pure water system 120, and a secondary pure water system 1130. The secondary pure water system 130 differs from the secondary pure water system 10 of the above-mentioned embodiment in that it further includes a heat exchanger 136, an ultraviolet irradiation device 137, and a membrane degassing device 138, but the other configurations are similar. Specifically, the secondary pure water system 130 includes a pure water tank 131, a water pump (not shown), a heat exchanger (HEX) 136, an ultraviolet irradiation device (UV) 137, a membrane degassing device 138, a non-regenerative mixed bed ion exchange resin device (polisher) 132, a booster pump 133, and an ultrafiltration membrane (UF) device 134, in this order.

[0057] (Pretreatment device) Raw water is supplied to the pretreatment device 110. The pretreatment device 110 is equipped with a coagulation sedimentation device, a sand filter, a membrane filter, etc., and these are used to clarify the raw water and produce pretreated water from which suspended solids and a part of organic matter have been removed. The raw water may be industrial water, tap water, groundwater, river water, etc. Furthermore, by adding a urea removal device consisting of a combination of a high-pressure reverse osmosis membrane (RO) device, a hypobromous acid addition device, and an ultraviolet ray irradiation device, or a high-pressure reverse osmosis membrane (RO) device and a boron removal device such as a boron-selective ion exchange device, etc., to the pretreatment device 110, brackish water or sewage can also be used as the raw water. As the raw water, one of these may be used, or two or more may be used in combination.

[0058] (Primary pure water equipment) The primary pure water system 120 performs purification treatment on the pretreated water to remove impurities from the pretreated water and produce primary pure water. Specifically, the primary pure water system 120 has various devices such as a demineralizer that removes impurity ions, a reverse osmosis membrane device that removes inorganic ions, organic matter, fine particles, etc., a vacuum degassing device or membrane degassing device that removes dissolved gases such as dissolved oxygen, a regenerative mixed-bed demineralizer that removes remaining ions, etc., and an electric deionization device.

[0059] (Pure water tank) The primary pure water obtained in the primary pure water system 120 is sent to a pure water tank 131. The pure water tank 131 temporarily stores the primary pure water obtained in the primary pure water system 120. A preferred configuration of the pure water tank 131 is similar to that of the pure water tank 11 in the secondary pure water system 10 of the above-described embodiment.

[0060] (heat exchanger) In the heat exchanger 136, the temperature of the primary pure water is adjusted by heat exchange (heating or cooling). As the heat exchanger 136, for example, a plate-type heat exchanger can be mentioned, but the specific structure is not particularly limited. The heat exchanger generally adjusts the water temperature to room temperature, for example, about 20°C. However, there are also cases where the water temperature is adjusted to, for example, 60°C to 80°C, and in this case, the produced ultrapure water is called hot ultrapure water. When producing hot ultrapure water, a further heat exchanger may be installed in addition to the heat exchanger 136, and in that case, the heat exchanger is installed, for example, between the non-regenerative mixed bed ion exchange resin device 132 and the ultrafiltration membrane device 134.

[0061] (Ultraviolet irradiation device) The primary pure water whose temperature has been adjusted by the heat exchanger 136 is supplied to the ultraviolet irradiation device 137. In the ultraviolet irradiation device 137, the primary pure water is irradiated with ultraviolet light to decompose organic matter in the primary pure water and kill (sterilize) live bacteria. The ultraviolet irradiation device 137 is equipped with an ultraviolet lamp capable of irradiating, for example, a wavelength of about 185 nm or a wavelength of about 254 nm, and can reliably decompose and sterilize organic matter in the primary pure water. There is no particular limitation on the ultraviolet lamp of the ultraviolet irradiation device 137, but a low-pressure mercury lamp is preferable in terms of ease of handling. In addition, the ultraviolet irradiation device 137 may be a flow-through type in which an ultraviolet lamp is arranged inside a housing along the flow path of the water to be treated, or an immersion type in which an ultraviolet lamp is immersed in a tank that stores the water to be treated, but the flow-through type is preferable in terms of treatment efficiency.

[0062] (Membrane degassing device) The membrane degassing device 138 removes gases, particularly dissolved oxygen, from the primary pure water using a gas separation membrane that is impermeable to water but permeable to gases.

[0063] (Non-regenerative mixed bed ion exchange resin device) The primary pure water from which the dissolved oxygen concentration has been removed by the membrane degassing device 138 is sent to the non-regenerative mixed bed ion exchange resin device 132. The non-regenerative mixed bed ion exchange resin device 132 has a configuration similar to that of the non-regenerative mixed bed ion exchange resin device 12 in the secondary pure water system 10 of the above-mentioned embodiment, and adsorbs and removes organic acids produced by decomposition of organic matter in the ultraviolet irradiation device 137 and impurity ions such as metal ions remaining in the water.

[0064] (Booster pump) The booster pump 133 has the same configuration as the booster pump 13 in the secondary pure water system 10 of the above-mentioned embodiment. The booster pump 133 increases the water supply pressure of the primary pure water in the pure water tank 131 and supplies the water to the ultrafiltration membrane device 134.

[0065] (Ultrafiltration membrane device) The primary pure water from which impurity ions have been removed by the non-regenerative mixed-bed ion exchange resin device 132 is supplied to an ultrafiltration membrane device 134. The ultrafiltration membrane device 134 has a configuration similar to that of the ultrafiltration membrane device 14 in the secondary pure water system 10 of the above-mentioned embodiment, and removes fine particles to produce ultrapure water. In the ultrapure water producing system 100 of this embodiment, the ultrafiltration membrane device 134 is disposed at the end of the secondary pure water system 130, i.e., on the side closest to the POU 135 of the secondary pure water system 130. Also, in the ultrapure water producing system 100 of this embodiment, one or more of a microfiltration membrane device and an ion exchange membrane device may be used instead of the ultrafiltration membrane device. In addition, in the secondary pure water system 130, other treatment devices may be provided as necessary, such as a catalytic resin packed tower, for example, to remove hydrogen peroxide generated secondarily in the ultraviolet irradiation system 137, thereby making it possible to obtain ultrapure water having the desired purity.

[0066] Furthermore, in the secondary pure water system 130 of this embodiment, by providing a booster pump 133 between the non-regenerative mixed bed ion exchange resin system 132 and the ultrafiltration membrane system 134, it is possible to increase the water supply pressure to the ultrafiltration membrane system 134. However, although it is considered that metal components may be eluted due to contact between the booster pump and the water to be treated, and the metal concentration in the water to be treated increases, by using the method of this embodiment, the metal components that have increased due to elution can be removed by the filtration membrane system 134.

[0067] The secondary pure water (ultrapure water) obtained by the secondary pure water apparatus 130 through the above-mentioned devices (processes) is sent through a water supply pipe 130a to a use point 135, such as a process point in a semiconductor manufacturing process. Of the sent ultrapure water, any unused ultrapure water is circulated through a circulation pipe 130c to a pure water tank 131 and stored in the pure water tank 131 together with the primary pure water.

[0068] The ultrapure water producing system 100 can produce ultrapure water with an iron content of 1 ng / L or less and a silica concentration of 20 ng / L or less, and even ultrapure water with an iron concentration of 0.1 ng / L or less and a silica concentration of 10 ng / L or less. Therefore, it can be suitably used in producing ultrapure water to be supplied to process points in semiconductor manufacturing processes. EXAMPLES

[0069] Next, examples will be described, but the present invention is not limited to the following examples.

[0070] FIG. 6 shows a schematic diagram of the configuration of the secondary pure water system 500 used in Example 1. The secondary pure water system 500 shown in FIG. 6 includes a pure water tank (TK) 501, a non-regenerative mixed bed ion exchange resin system (Polisher) 502, a booster pump (BSP) 503, a water pump (not shown), and an ultrafiltration membrane module (UF) 504, in this order. The permeation side of the ultrafiltration membrane module 504 is connected to a point of use (POU) 505 by a branched water supply pipe 5a, and a valve Va is provided in the path downstream of the point of use of the water supply pipe 5a. A drain pipe 5b is connected to the concentration side of the ultrafiltration membrane module 504, and a valve Vb is provided in the path of the drain pipe 6b.

[0071] In Examples 2 to 5 and Comparative Examples 1 to 3, a branch pipe was provided between a booster pump (BSP) 503 and an ultrafiltration membrane module (UF) 504, and the filter devices listed in Table 1, i.e., an ultrafiltration membrane module (UF), a microfiltration membrane module (MF), and an ion exchange membrane module, were arranged in the path of the branch pipe, thereby constructing an apparatus in which the ultrafiltration membrane module 504 of the Examples was replaced with each filter device listed in Table 1, and tests were performed.

[0072] Pure water was passed from a pure water tank 501 to a non-regenerative mixed bed ion exchange resin device 502, a booster pump 503, and an ultrafiltration membrane module 504 in this order by a water pump (not shown) provided downstream of the pure water tank 501. At this time, the output of the booster pump 503 was adjusted so that the permeate flux (flux) of the ultrafiltration membrane module 504 became the value of "flux / optimum permeate flux" described in Table 1. In addition, the flow rates of the concentrated water and the permeate were adjusted by the valves Va and Vb to adjust the water recovery rate in the ultrafiltration membrane module 504 as described in Table 1. The same applies to the examples using a microfiltration membrane module and an ion exchange membrane module.

[0073] In Example 1, treated water (permeate) from the ultrafiltration membrane module 504 was collected to measure the iron (Fe) concentration and silica concentration. In other Examples and Comparative Examples, treated water (permeate) from each filter device was collected to measure the iron (Fe) concentration and silica concentration to determine each removal rate. In both Examples and Comparative Examples, measurements were taken at the beginning of water flow and 60 days after the start of water flow. The results are shown in Table 1.

[0074] The specifications of the filter devices used in the examples and comparative examples are as follows: Ultrafiltration membrane module (UF, nominal molecular weight cutoff 6000): OLT6036 (manufactured by Asahi Kasei Corporation), polysulfone hollow fiber membrane, membrane area 34 mm 2 , 1 Ultrafiltration membrane module (UF, nominal molecular weight cutoff 4000): OAT6036 (manufactured by Asahi Kasei Corporation), polysulfone hollow fiber membrane, membrane area 34 mm 2 , 1 Ultrafiltration membrane module (UF, nominal molecular weight cutoff 10,000): OLT3026 (manufactured by Asahi Kasei Corporation), polysulfone hollow fiber membrane, membrane area 10.7 mm 2 , 1 Microfiltration membrane module (MF): TRINZIK (Integris) 15 nm, hydrophilic PTFE membrane, 10-inch module, membrane area 1.1 mm 2 , 1 Ion exchange membrane module: PROTEGO PLUS (Integris), pore size 20 nm, cation exchange capacity, made of hydrophilic polysulfone, 10-inch module, membrane area 0.54 mm 2 , 1

[0075] An experiment to determine the K value was carried out for each filter device as follows. The transmembrane pressure was changed as shown in Table 2, and the permeate flux was measured. From the measurement results, the "transmembrane pressure / transmembrane flux", i.e., the K value, was calculated for each transmembrane pressure. The transmembrane pressure was changed under two to five conditions for each filter device to determine the K value. The results are shown in Table 2. From Table 2, it was confirmed that the K value is a constant value regardless of the transmembrane pressure or water recovery rate. In addition, the arithmetic mean of the K values ​​obtained in Table 2 was calculated for each filter device, and the arithmetic mean was used to determine the optimal flow rate (optimum permeate flux) according to the above formula (3) and the graph in FIG. 7. The optimal flux determined according to the K value is as follows. Ultrafiltration membrane device (nominal molecular weight cutoff 6000): 0.33m / h Ultrafiltration membrane device (nominal molecular weight cutoff 4000): 0.28m / h Ultrafiltration membrane device (nominal molecular weight cutoff 10,000): 0.37m / h Precision filtration membrane device: 0.99m / h Ion exchange membrane device: 1.28m / h

[0076] Analytical methods used in the examples and comparative examples are as follows. Iron concentration: The sample water was concentrated by evaporation and analyzed by ICP-MS (inductively coupled plasma mass spectrometry). Silica concentration: The sample water was concentrated by evaporation and analyzed by ICP-MS.

[0077] [Table 1]

[0078] [Table 2]

[0079] In Table 1, the measurement was performed 60 days after the start of water flow, and in the examples, the removal rate was stable for at least 30 days thereafter. In contrast, in the comparative examples, the removal rate tended to gradually decrease. FIG. 8 shows the water recovery rate and the iron removal rate when the magnitude of the permeate flux (ratio to the optimal permeate flux) during treatment in a filter device (module) using an ultrafiltration membrane (UF) with a molecular weight cutoff of 6000 or 4000 was changed. In the graph shown in FIG. 8, the optimal permeate flux was the same as in Examples 1 to 3, 4, 6, and 7. From FIG. 8, it can be seen that when filtration was performed with a permeate flux exceeding 2.0 times the optimal permeate flux, there was no significant change in the iron removal rate regardless of the water recovery rate. When filtration is performed at a permeate flux less than 0.5 times the optimal permeate flux, the iron removal rate can be improved by setting the water recovery rate to between 50% and 80%, but if the permeate flux is too small, the amount of water produced will be small and it will be impractical.

[0080] Moreover, under all of the conditions shown in Table 1, the iron removal rate was 78% or more at the beginning of water flow. In other words, it can be seen that a high removal rate can be maintained in some cases and not in other cases depending on the operating conditions. If a high removal rate cannot be maintained, it is presumed that the membrane state transitions from (a) to (b) and then to (c) in FIG. 2 as explained above, due to the mechanism shown in FIG. 2, causing the removal rate to decrease. On the other hand, in the case of the example, the membrane state is maintained at (a) in FIG. 2, so it is considered that there is no or only a slight decrease in the removal rate.

[0081] From the above examples and comparative examples, it can be seen that by controlling the filtration process in an ultrafiltration membrane device installed near the end of the secondary pure water system so that the conditions (1) and (2) below are both satisfied, the concentration of fine particles in the treated water, particularly silica and iron, can be stably and significantly reduced. (1) The permeate flux during filtration is within the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more but less than 80%. [Explanation of symbols]

[0082] 10, 150, 500...Secondary pure water device, 11, 51, 501...Pure water tank, 12, 502...Non-regenerative mixed bed ion exchange resin device (polisher), 13, 53, 503...Booster pump, 14...Ultrafiltration membrane device, 54...Ultrafiltration membrane module, 504...Ultrafiltration membrane device, 15, 505...Use point (use point), V1a, Va, V1b, Vb...Valve, 1a, 5a, 54a...Water supply pipe, 1b, 5b, 54b...Drain pipe, 1c, 54c...Circulation pipe, 140...Filtration membrane device, 140P...First bank, 140Q...Second bank, 141...First filtration membrane module, 1 42...second filtration membrane module, 143...third filtration membrane module, 141a, 142a, 143a...water supply pipe, 141b, 142b, 143b...drainage pipe, 100...ultrapure water production system, 110...pretreatment device, 120...primary pure water device, 130...secondary pure water device, 131...pure water tank, 132...non-regenerative mixed bed ion exchange resin device (polisher), 136...heat exchanger, 137...ultraviolet irradiator, 138...membrane degasifier, 133...booster pump, 134...ultrafiltration membrane (UF) device, 135...point of use (use point), 130a...water supply pipe, 130c...circulation piping

Claims

1. A method for producing ultrapure water in which raw water is treated in a primary water purification system and a secondary water purification system in that order, A filtration process is performed using a filtration membrane at the end of the secondary pure water device; The filtration membrane is at least one selected from an ultrafiltration membrane (UF), a microfiltration membrane (MF) and an ion exchange membrane, and the filtration treatment is carried out under conditions that satisfy both of the following (1) and (2). (1) The permeate flux during filtration is in the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more and less than 80%.

2. 2. The method according to claim 1, wherein the filtered water has a resistivity of 17 MΩ·cm or more, an iron concentration of 3 ng / L to 100 ng / L, and a silica concentration of 10 to 1000 ng / L.

3. 3. The method according to claim 1, wherein the filtration treatment is carried out using treated water from a non-regenerative mixed-bed ion exchange resin device.

4. 3. The method according to claim 1, wherein the ultrapure water produced by the method has an iron concentration of 1 ng / L or less and a silica concentration of 20 ng / L or less.

5. An ultrapure water production system having a primary pure water device that treats raw water to produce primary pure water, and a secondary pure water device that treats the primary pure water, the ultrapure water production apparatus has a filtration membrane module equipped with a filtration membrane at an end of the secondary pure water apparatus; The filtration membrane is at least one selected from an ultrafiltration membrane, a microfiltration membrane, and an ion exchange membrane; The filtration process in the filtration membrane module is controlled to satisfy both of the following conditions (1) and (2): (1) The permeate flux of the filtration membrane module is in the range of 0.5 to 2.0 times the optimal permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane module. (2) The water recovery rate in the filtration membrane module is 50% or more and less than 80%.

6. 6. The ultrapure water producing system according to claim 5, comprising a water pump, a non-regenerative mixed bed ion exchange resin device, and the ultrafiltration membrane module, in this order.

7. 6. The ultrapure water producing apparatus according to claim 5, wherein the water to be treated supplied to the filtration membrane module has a resistivity of 17 MΩ·cm or more, an iron concentration of 3 ng / L or more, and a silica concentration of 100 ng / L or more.

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