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

Abstract

To provide an ultrapure water production method and ultrapure water production system capable of sufficiently reducing the number of fine particles such as iron and silica in ultrapure water without using complex equipment and without significantly reducing the water production volume.SOLUTION: A method for producing ultrapure water including filtration treatment using a filtration membrane, in which the filtration membrane comprises one or more types selected from ultrafiltration (UF) membrane, microfiltration (MF) membrane, and ion exchange membrane, and the filtration treatment is performed under conditions satisfying both (1) and (2). (1) The permeate flux during the filtration process is within the range of 0.5 times or more but not exceeding 2.0 times the optimal permeate flux determined based on the membrane pressure difference / permeate flux for the filtration membrane. (2) The water recovery rate in the filtration process is 50% or more but less than 80%.SELECTED DRAWING: Figure 1

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 comprising a primary water purification system and a secondary water purification system, in that order. In an ultrapure water production system, the primary water purification system produces primary pure water by removing total organic carbon (TOC) and ionic components from raw water or pretreated water using a reverse osmosis membrane device or an ion exchange device. The secondary water purification system produces ultrapure water by removing trace amounts of impurities from the primary pure water. A cross-flow ultrafiltration (UF) membrane device is typically installed at the end of the secondary water purification system and operated at a water recovery rate of 90% to 99% to remove nanometer-sized particles.

[0003] Various attempts have been made to increase the purity of ultrapure water production systems by extensively removing impurities such as particulates from water. For example, proposed methods include dead-end filtration using a particulate removal membrane with cationic functional groups at the end of a secondary water purification system (see, for example, Patent Document 1), arranging multiple ultrafiltration membrane devices in series at the end (see, for example, Patent Document 2), and providing parallel pumps that send pure water to the secondary water purification system and operating them at low power at all times (see, for example, Patent Document 3). Another proposed system includes a porous ion exchanger and a microfiltration membrane device, in that order, downstream of the ultrafiltration membrane device in the secondary water purification system (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-170406 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-064342 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-167661 [Patent Document 4] Japanese Patent Publication No. 2022-154537 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, with the remarkable progress in miniaturization and high integration of semiconductor circuits, the water quality requirements for ultrapure water used in semiconductor manufacturing processes have become increasingly stringent. For example, the International Technology Roadmap for Semiconductors (ITRS) requires that ultrapure water contain no more than 1 particle per milliliter of particles with a diameter of 10 nm or greater. However, the technologies described in Patent Documents 1 to 4 do not necessarily produce treated water quality that meets these requirements. For example, detailed studies by the present inventors have revealed that iron and silica remain in the treated water from a polisher (a non-regenerative mixed-bed ion exchange resin device) installed upstream of an ultrafiltration membrane device, resulting in an increase in the number of particles in the ultrapure water. In particular, it has been found that an increase in the number of particles (including colloids such as Fe, Cr, and Si) is likely to occur when a booster pump or heat exchanger is installed near the end of the secondary deionization system. In response to this, particle generation from gaskets and other components at the connection points of each device has also begun to be highlighted as a problem. Furthermore, in the past, it was thought that ultrafiltration membrane devices could be designed with a smaller molecular weight cutoff to remove even finer particles, but even if the molecular weight cutoff was reduced, the improvement in particle removal performance was slight, and the problem of reduced water production occurred.

[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 one or more selected from an ultrafiltration membrane (UF), a microultrafiltration 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 optimum 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%. [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], wherein the treated water is treated with water from a non-regenerative mixed-bed ion exchange resin device and subjected to the filtration treatment. [4] The method for producing ultrapure water is a method for 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 treatment is carried out at the end of the secondary water purification device. [5] The method according to any one of [1] to [4], wherein the iron concentration in the ultrapure water produced by the method is 1 ng / L or less and the silica concentration is 20 ng / L or less.

[0008] [6] An ultrapure water production apparatus including a filtration membrane module equipped with 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 system according to [6], comprising a water pump, a non-regenerative mixed-bed ion exchange resin device, and the ultrafiltration membrane module, in this order. [8] The ultrapure water production apparatus according to [6] or [7], 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. [9] A water purification system comprising a primary water purification device that treats raw water to produce primary pure water, and a secondary water purification device that treats the primary pure water; An ultrapure water production system having the ultrafiltration membrane module according to any one of [6] to [8] at the end of the secondary water purification device. In this specification, the symbol "to" indicates a range of values ​​including the values ​​before and after it. [Effects of the Invention]

[0009] The ultrapure water producing method, ultrapure water producing apparatus, and ultrapure water producing system of the present invention 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. [Brief explanation 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. [Figure 2] FIG. 2 is a diagram for explaining the state of capture of fine particles by the filtration membrane in this embodiment during filtration, and is a schematic diagram showing, from top to bottom, (a) the initial stage of filtration, (b) when filtration has progressed, and (c) when filtration has further progressed. [Figure 3] FIG. 10 is a block diagram schematically showing the configuration of a filtration membrane device in a secondary pure water system according to a first modified example. [Figure 4]FIG. 10 is a block diagram schematically showing a secondary deionizer according to a second modified example. [Figure 5] 1 is a block diagram schematically illustrating an ultrapure water producing system according to an embodiment. [Figure 6] FIG. 2 is a block diagram schematically showing a secondary deionizer 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 INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described. The method for producing ultrapure water according to the embodiment is a method for producing ultrapure water that includes a filtration process using a filtration membrane, and reduces iron and silica in the ultrapure water by performing the filtration process under predetermined conditions. The filtration membrane is one or more selected from an ultrafiltration membrane (UF), a micro-ultrafiltration membrane (MF), and an ion exchange membrane. Below, a case where an ultrafiltration membrane is used as the filtration membrane will be described, but the same applies when a micro-filtration membrane (MF) or an ion exchange membrane is used.

[0012] Figure 1 is a block diagram showing a schematic configuration of a secondary pure water system 10 used in an embodiment of the ultrapure water production method. The secondary pure water system 10 shown in Figure 1 is an apparatus for producing ultrapure water, and includes, in this order, 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 device (Polisher) 12, a booster pump (BSP) 13, and an ultrafiltration membrane device (UF) 14. 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 device 12 and the ultrafiltration membrane device 14, where they are sequentially treated to produce 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 via a water supply pipe 1a. The ultrapure water produced in the secondary pure water system 10 is sent to the point-of-use (POU) 15 via the water supply pipe 1a and used there. One end of a circulation pipe 1c branches off and is connected to the water supply pipe 1a. A valve V1a is provided in 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 in 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 installed inside a housing having a water inlet for feedwater, a permeate outlet, and a concentrate outlet. The openings of the module are connected to pipes to allow the water to be treated to pass through the ultrafiltration membrane. The ultrafiltration membrane module has the advantage that when the ultrafiltration membrane becomes clogged and its filtration performance deteriorates, the entire module can be replaced and the deteriorated ultrafiltration membrane can be cleaned separately. The ultrafiltration membrane installed in the ultrafiltration membrane module is sometimes 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 an ultrafiltration membrane device 14 including one ultrafiltration membrane module, the water flow conditions of the ultrafiltration membrane device 14 and 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 may be provided in the circulation pipe 1c, and ultrapure water may 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 system 10 will be described. First, the water pump P is activated to send pure water from the pure water tank 11 to the downstream stage. The pure water tank 11 is not particularly limited in material or shape, as long as it is free from rust, minimally leaching out of components from the container, and can stably store primary pure water. Materials such as fiber-reinforced plastics (FRP), polyethylene, SUS304, and those lined with fluororesin such as Teflon (registered trademark) are preferred for the pure water tank 11. Furthermore, the upper part of the pure water tank 11 is preferably purged with an inert gas such as pure nitrogen to prevent absorption of impurity gases such as carbon dioxide and dissolved oxygen. The pure water tank 11 can also store a mixture of the primary pure water and ultrapure water when circulating unused ultrapure water produced.

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

[0018] Pure water is supplied to a non-regenerative mixed-bed ion exchange resin device 12, where cations and anions are removed from the water. The non-regenerative mixed-bed ion exchange resin device 12 is constructed, for example, by filling a cylindrical sealed container with mixed-bed ion exchange resin. Mixed-bed ion exchange resins are typically a mixture of cation and anion exchange resins. In the non-regenerative mixed-bed ion exchange resin device 12, fine particles such as silica and iron, which are more likely to form colloids, are more likely to break through than strong ions such as sodium and chloride. In conventional methods, 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 system 10 of this embodiment, the passage of these fine particles through the ultrafiltration membrane device 14 can be suppressed by controlling the ultrafiltration membrane device 14 under the conditions described below. Iron, silica, and the like can exist in water in the form of ions, colloids, or fine particles. However, in this specification, they are referred to as fine particles without considering their 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 the ultrafiltration membrane device 14 with its supply pressure increased by a booster pump 13. In the method for producing ultrapure water of this embodiment, filtration 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) will be 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 through the filtration membrane. In this specification, the "K value" is defined as the "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 water flow conditions (1) in the ultrafiltration membrane device 14 are, in order to improve the removal rate of fine particles, 0.5 to 2.0 times the optimum 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 by the following formula (3) depending on the K value. Optimal permeate flux = -0.69 log 10 (K)+0.64 (3)

[0023] As a result of various preliminary experiments, the inventors found that the optimal permeate flux can be calculated using the above formula (3). The details of these preliminary experiments are as follows. First, commercially available 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 of each membrane module was measured when the transmembrane pressure was changed. From the measurement results, the "transmembrane pressure / transmembrane flux" ratio, i.e., the K value, was calculated for each transmembrane pressure. It was confirmed that the K value is approximately constant regardless of the transmembrane pressure or water recovery rate. In these preliminary experiments, the transmembrane pressure was changed under two to five conditions, and the K value was determined by arithmetic mean. Furthermore, from the "Experiment for Determining the K Value (Table 2)" described below, it was found that the K value is approximately constant regardless of the conditions. This is consistent with the fact that Darcy's law leads us to conclude 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" listed in the instruction manual for a commercially available membrane module were used to plot the "recommended filtrate volume (permeate volume)"—the maximum filtrate volume (maximum permeate volume) when operated under those operating conditions—on the vertical axis, i.e., the filtration flux (permeate flux), i.e., the filtrate volume (permeate volume) divided by the membrane area, on the horizontal axis. The graph in Figure 7 and the above equation (3) were then obtained using an automated program using the least squares method. These "recommended operating conditions" or "standard operating conditions" are generally determined by the manufacturer as conditions under which the membrane module at the end of an ultrapure water system can be operated without clogging or damage. Numerous preliminary experiments have confirmed that membranes other than those used in the "Experiment to Determine the K Value (Table 2)" below also exhibit a tendency similar to that of equation (3).

[0025] Next, the above condition (2) will be explained. 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), it is possible to remove fine particles with high precision in the ultrafiltration membrane device 14. This is thought to be because, as will be explained next, it is possible to prevent fine particles smaller than the pore size of the ultrafiltration membrane from passing through the ultrafiltration membrane, thereby maintaining a high removal rate.

[0026] Conventionally, it has been thought that filtration membrane devices, such as ultrafiltration membrane devices, capture and remove fine particles larger than the pore size of the ultrafiltration membrane from water, i.e., filtration proceeds mainly by the mechanism of surface filtration. Therefore, it was also thought that a sedimentary layer (cake) of fine particles forms on the surface of the ultrafiltration membrane over the course of a treatment period, and that this sedimentary layer of fine particles captures the fine particles, thereby improving the fine particle removal rate. Therefore, conventionally, in ultrafiltration membrane devices at the end of secondary pure water systems, in order to reduce the number of fine particles in the ultrapure water, methods have been used to increase the water recovery rate by reducing the amount of concentrated water, or to perform dead-end filtration (see, for example, Patent Document 1 and Patent Document 2, paragraph 0018).

[0027] However, the inventors conducted experiments based on the hypothesis that the fine particles observed in the terminal ultrapure water, such as silica and iron, are colloidal, that the fine particles may be smaller than the pore size of the ultrafiltration membrane, that the filtration of the fine particles proceeds by a depth filtration mechanism rather than the surface filtration described above, and that the formation of a sediment layer may not progress very much due to the small number of fine particles in the water.As a result, they found that the fine particle removal rate at the filtration membrane can be increased by filtering using the cross-flow method under the above-mentioned conditions.The reasons for this are not entirely clear, but are 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 top to bottom, (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, rectangles represent the membrane surface 21 and pore inner walls 22 of the filtration membrane, and small circles represent fine particles 23. Also, in FIG. 2, thick arrows indicate water flow, and thin arrows indicate the movement of the fine particles 23. In membrane filtration, fine particles 23 are captured and adsorbed at two possible locations: the membrane surface 21 and the pore inner walls 22. When a filtration membrane is new or nearly new, the number of fine particles 23 already adsorbed on the membrane surface 21 or pore inner walls 22 of the filtration membrane is small. Therefore, although both the membrane surface 21 and the pore inner walls 22 can adsorb the fine particles 23, the water to be treated first comes into contact with the membrane surface 21, and therefore, as shown in FIG. 2(a), the membrane surface 21 mainly adsorbs the fine particles 23. Subsequently, 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 walls 22 gradually progresses, as shown in FIG. 2(b).

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

[0031] Here, the fine particles 23 adsorbed to the membrane are repeatedly adsorbed and desorbed during the membrane filtration process, so if the membrane filtration is performed as 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 shown in Figure 2(a) can be maintained. This prevents fine particles smaller than the pore size of the ultrafiltration membrane from passing through the ultrafiltration membrane, which is thought to maintain a high removal rate. Here, the flow rate along the membrane surface can be increased by increasing the flow rate on the concentration side, i.e., by reducing 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 higher, the ultrafiltration membrane device 14 will not be able to sufficiently remove fine particles. This is thought to be because the small amount of concentrated water reduces the flow rate on the membrane surface, allowing fine particles smaller than the pore size of the ultrafiltration membrane to pass through the ultrafiltration membrane. If the water recovery rate is less than 50%, the water recovery rate will be too low, reducing practicality. In order to improve the fine particle removal rate and efficiently obtain ultrapure water, it is preferable that the water recovery rate in the ultrafiltration membrane device 14 be 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 of devices 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, microfiltration membrane device, and 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 from water, for example, fine particles with 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, but may also be a spiral membrane, tubular membrane, flat membrane, etc. The material of the ultrafiltration membrane is polysulfone, polyvinylidene fluoride, polyethylene, polypropylene, etc., and the nominal molecular weight cutoff is preferably 3000 to 8000, more preferably 4000 to 6000. The ultrafiltration membrane module may be either an internal pressure type or an external pressure type.

[0035] A 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 cartridges are equipped with ion exchange membranes, which may be cation exchange membranes, anion exchange membranes, or bipolar membranes that combine cation and anion exchange membranes.

[0037] 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 rate of the valves V1a and V1b. The valves V1a and V1b can be configured as automatically controllable valves, and a control device can be provided in the secondary water purification system 10. The control device can automatically control the valves V1a and V1b so that the water recovery rate in 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 in the circulation pipe 1c in order to obtain sufficient water supply pressure to the use point 15, but it may also 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 portion in contact with the pure water is made of a material such as stainless steel that elutes small amounts of metal components such as iron and chromium, in this embodiment, the eluted metal components are removed by the ultrafiltration membrane device 14. Therefore, the material of the portion of the booster pump 13 that contacts the primary pure water has almost no effect on the quality of the ultrapure water produced. Therefore, a booster pump commonly used in pure water production can be used as the booster pump 13. Note that if the water 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. However, 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 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 can be provided between the non-regenerative mixed-bed ion exchange resin device 12 and the ultrafiltration membrane device 14 to increase the water supply pressure to the point of use 15. The booster pump 13 may be a 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. 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, a total organic carbon (TOC) concentration of 1 μgC / L or less, and a resistivity of 18 MΩ cm or more. 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. Figure 3 is a diagram schematically showing the 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 Figure 1 is configured in an array using three or more ultrafiltration membrane modules, and the other configuration is the same as that of the secondary pure water system 10 of the above-mentioned embodiment. In this modified example, 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 in 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 that supplies the permeated water of the first filtration membrane module 141 and a drain pipe 141b that discharges the concentrated water. The second filtration membrane module 142 is connected to a water supply pipe 142a that supplies the permeated water of the second filtration membrane module 142 and a drain pipe 142b that discharges the concentrated water. The third filtration membrane module 143 is connected to a water supply pipe 143a that supplies the permeated water of the third filtration membrane module 143 and a drain pipe 143b that discharges 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 from 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 discharge pipe 143b.

[0047] The operating conditions for the first filtration membrane module 141, the second filtration membrane module 142, and the third filtration membrane module 143 are all the same as the operating conditions for the ultrafiltration membrane device (ultrafiltration membrane module) in the secondary pure water system 10 of the above-described 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 the pump that supplies water to the filtration membrane module and by providing variable valves in the water supply pipe and the drain pipe, respectively, and adjusting the flow rate by the valve opening. (1) The permeate flux during filtration is in the range of 0.5 to 2.0 times the optimum 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%.

[0048] The permeated water from the first bank 140P and the second bank 140Q is supplied to a point of use 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 or a stage preceding the primary pure water system.

[0049] While the example described here has two banks, with two filtration membrane modules in the most upstream bank, the filtration membrane device may have three or more banks, with each bank having three or more filtration membrane modules. In this case, the three or more banks are arranged in series along the flow direction, and the filtration membrane modules in one bank are arranged in parallel along the flow direction. Furthermore, because concentrated water from the preceding bank is collected and treated by the filtration membrane module in the following bank, the amount of treated water in each bank decreases toward the downstream side. Taking this into consideration, the number of filtration membrane modules in the following stages can be determined depending on the water recovery rate of the filtration membrane module in the preceding stage.

[0050] In this way, when a filtration membrane device 140 is used in which three or more filtration membrane modules are configured as an array, the water recovery rate of the entire unit can be improved. For example, if 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 Figure 3 are all 50%, a water recovery rate of 75% can be achieved for the entire filtration membrane device 140. Therefore, both a high water recovery rate and a high particulate removal rate 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 schematically illustrating the 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 includes a circulation pipe 54c that returns concentrated water from the ultrafiltration membrane device 14 to the inlet side of the booster pump 53. However, the other configurations are similar. The secondary pure water system 150 of FIG. 4 includes a pure water tank 51, a booster pump 53, and an ultrafiltration membrane device (UF) 54, in that order. Although the non-regenerative mixed-bed ion exchange resin device, valves, use points, and circulation pipes downstream of the use points are not shown in FIG. 4, the secondary pure water system 150 includes these components, just like the secondary pure water system 10 of FIG. 1. Note that 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 point of use (not shown) via 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, thereby returning a portion of the concentrated water from the ultrafiltration membrane device 54 to the booster pump 53. Valves may be provided on the drain pipe 54b downstream of the circulation pipe 54c and on the circulation pipe 54c, and the flow rate from the drain pipe 54b to the circulation pipe 54c may be controlled by the valves to circulate a portion of the concentrated water to the booster pump 53.

[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-described 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 valves in the water supply pipe and the drain pipe, respectively, and adjusting the flow rate by the valve opening. (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%.

[0054] In this way, in the secondary pure water system 150 in which the concentrated water from 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 in the drain pipe 54b while maintaining the flow rate in 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 a high fine particle removal rate in the filtration membrane device 54.

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

[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 130. The secondary pure water system 130 differs from the secondary pure water system 10 of the above-described embodiment in that it further includes a heat exchanger 136, an ultraviolet irradiation device 137, and a membrane degassing device 138. However, the other components are similar. Specifically, the secondary pure water system 130 includes, in this order, 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 (UF) membrane device 134.

[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., which are used to clarify the raw water and produce pretreated water from which suspended solids and a portion of organic matter have been removed. The raw water can be industrial water, tap water, groundwater, river water, etc. Furthermore, by adding a urea removal device, such as a high-pressure reverse osmosis (RO) device combined with a hypobromous acid adder and an ultraviolet irradiation device, or a high-pressure reverse osmosis (RO) device and a boron removal device such as a boron-selective ion exchange device, to the pretreatment device 110, brackish water or sewage can also be used as the raw water. One of these devices may be used as the raw water, or two or more may be used in combination.

[0058] (Primary pure water equipment) The primary pure water system 120 purifies the pretreated water to remove impurities from the pretreated water and produce primary pure water. Specifically, the primary pure water system 120 includes 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 degasser or membrane degasser that removes dissolved gases such as dissolved oxygen, a regenerative mixed-bed demineralizer that removes remaining ions, etc., and an electrodeionization 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) The heat exchanger 136 adjusts the temperature of the primary pure water by heat exchange (heating or cooling). The heat exchanger 136 may be, for example, a plate-type heat exchanger, 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, the water temperature may also be adjusted to, for example, 60°C to 80°C, in which 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. In this 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. The ultraviolet irradiation device 137 irradiates the primary pure water 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, with a wavelength of around 185 nm or 254 nm, thereby ensuring the decomposition and sterilization of organic matter in the primary pure water. The ultraviolet lamp of the ultraviolet irradiation device 137 is not particularly limited, but a low-pressure mercury lamp is preferred in terms of ease of handling. 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 preferred 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-described embodiment. The booster pump 133 increases the water supply pressure of the primary pure water in the pure water tank 131 and sends 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 in place of the ultrafiltration membrane device. In addition, in the secondary pure water system 130, other treatment devices such as a catalytic resin packed tower may be installed as needed to remove hydrogen peroxide generated secondarily in the ultraviolet irradiation system 137, thereby obtaining ultrapure water of 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 conceivable that contact between the booster pump and the water to be treated will cause metal components to elute, increasing the metal concentration in the water to be treated, 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 after passing through the above-mentioned devices (each process) is delivered via a water delivery pipe 130a to a use point 135, such as a process point in a semiconductor manufacturing process. Any unused ultrapure water delivered is circulated to a pure water tank 131 via a circulation pipe 130c 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 particularly suitably used for producing ultrapure water to be supplied to process points in semiconductor manufacturing processes. [Example]

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

[0070] Figure 6 shows a schematic diagram of the secondary water purification system 500 used in Example 1. The secondary water purification system 500 shown in Figure 6 includes, in this order, 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. The permeate side of the ultrafiltration membrane module 504 is connected to a point-of-use (POU) 505 via a branched water pipe 5a, and a valve Va is provided in the path of the water pipe 5a downstream of the point-of-use. 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 the booster pump (BSP) 503 and the ultrafiltration membrane module (UF) 504, and the filter devices listed in Table 1, i.e., the ultrafiltration membrane module (UF), the microfiltration membrane module (MF), and the ion exchange membrane module, were respectively 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 of the filter devices listed in Table 1, and tests were conducted using this apparatus.

[0072] Pure water was passed from a pure water tank 501 through a non-regenerative mixed-bed ion exchange resin device 502, a booster pump 503, and an ultrafiltration membrane module 504 in that order by a water pump (not shown) installed 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 would be the value of "flux / optimum permeate flux" listed in Table 1. In addition, the flow rates of the concentrated water and permeate were adjusted by valves Va and Vb to adjust the water recovery rate in the ultrafiltration membrane module 504 as listed 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 sampled and the iron (Fe) concentration and silica concentration were measured. In other Examples and Comparative Examples, treated water (permeate) from each filter device was sampled and the iron (Fe) concentration and silica concentration were measured to determine the respective removal rates. 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 piece 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 piece 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 piece Microfiltration membrane module (MF): TRINZIK (Integris) 15 nm, hydrophilic PTFE membrane, 10-inch module, membrane area 1.1 mm 2 , 1 piece Ion exchange membrane module: PROTEGO PLUS (Integris), pore size 20 nm, cation exchange capacity, hydrophilic polysulfone, 10-inch module, membrane area 0.54 mm 2 , 1 piece

[0075] Experiments to determine the K value were conducted 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 / permeate flux" ratio, i.e., K value, was calculated for each transmembrane pressure. The transmembrane pressure was changed under two to five conditions for each filter device, and the K value was determined for each condition. The results are shown in Table 2. Table 2 confirms 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 for each filter device was calculated, and the arithmetic mean was used to determine the optimal flow rate (optimal permeate flux) using the above formula (3) and the graph in Figure 7. The optimal flux determined according to the K value is as follows: Ultrafiltration membrane device (nominal molecular weight cutoff 6000): 0.33 m / h Ultrafiltration membrane device (nominal molecular weight cutoff 4000): 0.28 m / h Ultrafiltration membrane device (nominal molecular weight cutoff 10,000): 0.37 m / h Microfiltration membrane device: 0.99m / h Ion exchange membrane device: 1.28m / h

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

[0077] [Table 1]

[0078] [Table 2]

[0079] In Table 1, measurements were performed 60 days after the start of water flow. In the Examples, the removal rate remained 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 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 (UF) membrane 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. It can be seen from FIG. 8 that when filtration was performed at 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 higher at the beginning of water flow. This shows that a high removal rate can be maintained in some cases and not in others, 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 Figure 2, as explained above, due to the mechanism shown in Figure 2, resulting in a decline in the removal rate. On the other hand, in the case of the example, the membrane state is maintained at (a) in Figure 2, and it is thought that there is no or only a slight decline in the removal rate.

[0081] From the above examples and comparative examples, it can be seen that by controlling the filtration process in the ultrafiltration membrane device installed near the end of the secondary pure water system so that it satisfies both of the following conditions (1) and (2), 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... Point of use (use point), V1a, Va, V1b, Vb... Valve, 1a, 5a, 54a... Water supply pipe, 1b, 5b, 54b... Drain pipe, 1c, 54c... Circulation piping, 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...Second pure water device, 131...Pure water tank, 132...Non-regenerative mixed bed ion exchange resin device (polisher), 136...Heat exchanger, 137...Ultraviolet irradiation device, 138...Membrane degassing device, 133...Booster pump, 134...Ultrafiltration membrane (UF) device, 135...Point of use (use point), 130a...Water supply pipe, 130c...Circulation piping

Claims

1. In a method for producing ultrapure water including a filtration process using a filtration membrane, The filtration membrane is one or more selected from an ultrafiltration membrane (UF), a microultrafiltration 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 optimum permeate flux determined according to the transmembrane pressure difference / permeate flux in the filtration membrane. (2) The water recovery rate in the filtration treatment is 50% or more but 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 apparatus.

4. The method for producing ultrapure water is a method for treating raw water in a primary water purification system and a secondary water purification system in this order, The manufacturing method according to claim 1 or 2, wherein the filtration treatment is performed at the end of the secondary water purification unit.

5. 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.

6. An ultrapure water production apparatus including a filtration membrane module equipped with 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 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%.

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

8. 7. The ultrapure water producing apparatus according to claim 6, 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.

9. The system comprises 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 production system having the ultrafiltration membrane module according to any one of claims 6 to 8 at the end of the secondary water purification device.

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