Patent Publication Number: US-2016220958-A1

Title: Ultrapure water production apparatus

Description:
FIELD OF THE INVENTION 
     The present invention relates to ultrapure water production apparatuses and particularly relates to an ultrapure water production apparatus including a primary pure water system and a subsystem. 
     BACKGROUND OF THE INVENTION 
     Ultrapure water used for cleaning semiconductors is produced with an ultrapure water production apparatus including a primary pure water system, a subsystem (secondary pure water system), and the like. Optionally, a pretreatment system may be disposed upstream of the primary pure water system. 
     The pretreatment system, which includes a coagulation unit, a dissolved-air-flotation (sedimentation) unit, a filtration (membrane filtration) unit, and the like, removes suspended substances, colloidal substances, and the like contained in raw water. 
     The primary pure water system, which includes a reverse osmosis membrane separation device, a deaeration device, an ion-exchange device (mixed-bed type or 4-bed 5-column type), and the like, removes ions, organic components, and the like contained in water to produce primary pure water. The subsystem, which includes a low-pressure ultraviolet oxidation device, an ion-exchange pure water device, an ultrafiltration membrane (UF membrane) device, and the like, treats the primary pure water at a high level to produce ultrapure water. The UF membrane device, which is disposed at the end of the subsystem, removes microparticles generated from an ion-exchange resin and the like. 
     Recently, the control over the microparticles contained in water has been increasingly tightened due to the development of semiconductor production processes. International Technology Roadmap for Semiconductors requests that the certified value for the number of microparticles having a diameter of more than 11.9 nm being less than 1,000 particle/L (control value: less than 100 particle/L) be achieved by the year 2019. 
     A UF membrane device is commonly used as a membrane device disposed at the end of the subsystem. In order to remove microparticles through the UF membrane, it is desirable to use a membrane in which pores having a smaller diameter than the microparticles are formed. However, an infinite number of pores are present in the surface of the UF membrane, and the pores have different diameters. Therefore, it is not possible to completely remove microparticles having a diameter of about 10 nm. 
     The diameter of pores formed in a microfiltration membrane (MF membrane) is on the submicron order and is larger than that of the pores of a UF membrane. Therefore, it is difficult to control the number of microparticles contained in water that permeated through the MF membrane to be 100 particle/L or less (particle diameter &gt;10 nm). The diameter of pores formed in a reverse osmosis membrane (RO membrane) is smaller than that of the pores of a UF membrane. Therefore, in theory, it is considered that microparticles can be removed at a high level through an RO membrane. However, an RO membrane, which has a low degree of cleanliness as a module, may cause microparticles to be generated (e.g., dust particles may be generated from a potting member). Thus, it is not possible to use an RO membrane as a microparticle removal unit disposed at the end of the subsystem. 
     In order to reduce the number of microparticles contained in ultrapure water, two membrane separation devices may be disposed in series in the subsystem (Patent Literatures 1 to 4). FIGS. 2 and 3 in Patent Literature 1 illustrate a case where a UF membrane device and an ion-exchange-group-modified MF membrane device are disposed in series in this order at the end of an ultrapure water production apparatus. FIG. 4( a ) in Patent Literature 2 illustrates a case where a reverse osmosis membrane (RO membrane) device is disposed downstream of a UF membrane device disposed at the end of a secondary pure water system. 
     Patent Literature 3 describes a technique in which a secondary pure water system includes a UF membrane device and an anion-desorption membrane device having a pore diameter of 500 to 5000 Å. Patent Literature 4 describes a technique in which a prefilter that blocks particles having a diameter of 0.01 mm (10 μm) or more from permeating therethrough is disposed upstream of a UF or MF (microfiltration) membrane device used as a separation membrane module for producing ultrapure water. 
     When a UF membrane device and an ion-exchange-group-modified MF membrane are arranged in series as in Patent Literature 1, the ion-exchange group may detach from the ion-exchange-group-modified MF membrane to act as a source of the microparticles. 
     When a UF membrane device and an RO membrane device are arranged in series as in Patent Literature 2, and the quality of ultrapure water may be degraded since microparticles may be generated from the RO membrane. 
     The anion-desorption membrane described in Patent Literature 3 is specifically a hollow fiber membrane having a pore diameter of 0.2 μm (2000 Å), a porosity of 60%, and a thickness of 0.35 mm (Paragraph 0023). Although silica can be removed at a high level through this anion-desorption membrane, it is not possible to remove microparticles, which are required to be removed in the production of ultrapure water. 
     The prefilter described in Patent Literature 4 is provided in order to reduce the likelihood of dust particles having a size of 10 μm or more coming into collision with and breaking the UF or MF membrane disposed at the end. Thus, it is not possible to remove particles having a size of less than 10 μm through the prefilter described in Patent Literature 4. 
     As described above, Patent Literatures 1 to 4 describe techniques in which a plurality of membrane devices are disposed in series as a microparticle removal unit at the end of the subsystem. However, it is not possible to remove microparticles at a sufficient level by using any of these techniques. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Publication 2004-283710 A 
     Patent Literature 2: Japanese Patent Publication 2003-190951 A 
     Patent Literature 3: Japanese Patent Publication 10-216721 A 
     Patent Literature 4: Japanese Patent Publication 4-338221 A 
     SUMMARY OF INVENTION 
     An object of the present invention is to provide an ultrapure water production apparatus capable of producing high-quality ultrapure water from which microparticles have been removed at a high level. 
     An ultrapure water production apparatus of the present invention includes a subsystem that produces ultrapure water from primary pure water. The subsystem includes a membrane unit disposed at the end of the subsystem. The membrane unit is constituted by a plurality of membrane devices arranged in series. The first of the membrane devices is a UF membrane device, an MF membrane device, or an RO membrane device. The last of the membrane devices is a UF membrane device or an MF membrane that is not modified with an ion-exchange group. 
     It is preferable in the present invention that the membrane unit is constituted by two UF membrane devices arranged in series. The membrane unit may be constituted by an MF membrane device, an RO membrane device, and a UF membrane device arranged in this order. 
     It is preferable in the present invention that the apparatus is provided with microparticle measuring means for measuring the number of microparticles contained in treated water for controlling microparticles in the treated water. The apparatus may be provided with microparticle measuring means for measuring the number of microparticles contained in treated water treated with a membrane device immediately before the last of the membrane devices, and/or microparticle measuring means for measuring the number of microparticles contained in water treated with the last of the membrane devices, whereby detecting leakage of microparticles from the membrane device or a decline in a rate of microparticles rejection, so that maintenance including changing a membrane is performed when necessary, resulting that a high grade management of controlling microparticles in ultrapure water produced by the device. 
     When the ultrapure water production apparatus includes microparticle measuring means capable of measuring the number of microparticles contained in water treated with each of two or more of the membrane devices, the microparticle measuring means may be provided for each of the membrane devices, or may be provided for a plurality of the membrane device, so that the number of microparticles contained in water treated with each of the membrane devices is measured using the microparticle measuring means by switching treated water samples one by one, the treated water samples being fed from the respective membrane devices to the microparticle measuring means in order to measure the number of the microparticles. 
     When the membrane device includes two or more membrane modules arranged in parallel, it is preferable that microparticles is controlled in each of the membrane modules. Accordingly, it is preferable that a water-sampling pipe including an automatic valve disposed thereon branches from a pipe through which water treated with each of the two or more membrane modules is taken, the water-sampling pipe being used for sampling water in order to measure the number of microparticles and feeding the water to the microparticle measuring means, and that the number of microparticles contained in water treated with each of the membrane modules is measured by switching the automatic valve. It is also preferable that water-sampling pipe including an automatic valve disposed thereon branches through which water treated with each of the two or more membrane modules is merged with one another in order to measure the number of microparticles in the water of the membrane apparatus. A manual valve may be disposed instead of the automatic valve. 
     Advantageous Effects of Invention 
     The ultrapure water production apparatus according to the present invention, which includes a subsystem including a plurality of membrane devices, such as a UF membrane device, disposed in series at the end of the subsystem, is capable of producing high-quality ultrapure water in which the number of microparticles has been markedly reduced. According to the present invention, it is possible to produce high-quality ultrapure water in which the number of microparticles having a diameter of 10 nm or more is less than 100 particle/L. 
     In the present invention, the last of the plurality of the membrane devices arranged in series is a UF membrane device or an MF membrane device that is not modified with an ion-exchange group. Thus, there is no risk of microparticles being generated from the membrane device as in an RO membrane device. Furthermore, the MF membrane device used in the present invention is not modified with an ion-exchange group. This eliminates the risk of the ion-exchange group detaching from the MF membrane device and acting as a source of the microparticles. 
     By providing microparticle measuring means for measuring the numbers of microparticles contained in water treated with a membrane device disposed immediately before the last membrane device and/or water treated with the last membrane device and optionally performing maintenance such as replacement of the membrane on the basis of the results of the measurement made by the microparticle measuring means, high-quality ultrapure water in which the number of microparticles having a diameter of 10 nm or more is less than 100 particle/L can be produced with certainty in a consistent manner. 
     Specifically, as the membrane device is operated, microparticles are accumulated on the surface of the membrane with time and may leak into the treated water. The leakage of microparticles may also occur when the membrane is broken under an external load. This may deteriorate the quality of the ultrapure water that is to be produced. Thus, by providing microparticle measuring means and monitoring the number of microparticles contained in the membrane-treated water by the microparticle measuring means, the leakage of microparticles into the treated water may be prevented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flow diagram illustrating an ultrapure water production apparatus according to an embodiment. 
         FIG. 2  is a flow diagram illustrating an ultrapure water production apparatus according to an embodiment. 
         FIG. 3  is a flow diagram illustrating an ultrapure water production apparatus according to an embodiment. 
         FIG. 4  is a flow diagram illustrating an ultrapure water production apparatus according to an embodiment which includes first and second membrane devices each including microparticle measuring means. 
         FIG. 5  is a flow diagram illustrating an ultrapure water production apparatus including microparticle measuring means according to another embodiment. 
         FIGS. 6 a  and 6 b    are graphs illustrating changes with time in the concentrations of microparticles in water treated in UF membrane modules  17 A and  17 B, respectively, in Example 8. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments are described below with reference to the attached drawings. 
     An ultrapure water production apparatus according to the present invention includes a subsystem including two or more membrane devices disposed in series at the end of the subsystem.  FIGS. 1 to 3  illustrate examples of the overall flow in the ultrapure water production apparatus including the subsystem. 
     The ultrapure water production apparatuses illustrated in  FIGS. 1 to 3  include a pretreatment system  1 , a primary pure water system  2 , and a subsystem  3  in common. 
     The pretreatment system  1 , which includes a coagulation unit, a dissolved-air-flotation (sedimentation) unit, a filtration (membrane filtration) unit, and the like, removes suspended substances and colloidal substances contained in raw water. The primary pure water system  2 , which includes a reverse osmosis (RO) membrane separation device, a deaeration device, and an ion-exchange device (mixed-bed type, 2-bed 3-column type, or 4-bed 5-column type), removes ions and organic components contained in raw water. The RO membrane separation device removes ionic and colloidal TOC components in addition to salts. The ion-exchange device removes salts. TOC components are also removed by the ion-exchange device by being desorbed on an ion-exchange resin or through ion exchange. The deaeration device (nitrogen deaeration or vacuum deaeration) removes dissolved oxygen. 
     In the ultrapure water production apparatus illustrated in  FIG. 1 , primary pure water (generally, pure water having a TOC concentration of 2 ppb or less) produced in the above-described manner is passed through a subtank  11 , a pump P, a heat exchanger  12 , a UV oxidation device  13 , a catalytic oxidizing substance decomposition device  14 , a deaeration device  15 , a mixed-bed deionization device (ion-exchange device)  16 , and first and second membrane devices  17  and  18  used for removing microparticles in order, and the resulting ultrapure water is fed to a point-of-use  19 . 
     The UV oxidation device  13  is generally a UV oxidation device capable of emitting UV radiation having a wavelength of about 185 nm, with which irradiation is performed in the ultrapure water production apparatus. For example, a UV oxidation device including a low-pressure mercury lamp may be used. By using the UV oxidation device  13 , TOC contained in the primary pure water is decomposed into organic acids, which are further decomposed into CO 2 . Furthermore, H 2 O 2  is generated from water due to excess UV radiation emitted from the UV oxidation device  13 . 
     Water treated with the UV oxidation device  13  is subsequently passed into the catalytic oxidizing substance decomposition device  14 . As a catalyst for decomposing oxidizing substances which is included in the catalytic oxidizing substance decomposition device  14 , noble metal catalysts known as redox catalysts may be used. Examples of the noble metal catalysts include palladium (Pd) compounds such as metal palladiums, palladium oxides, and palladium hydroxides and platinum (Pt). Among the above noble metal catalysts, a platinum (Pt) catalyst, which has high reducing ability, can be suitably used. 
     The catalytic oxidizing substance decomposition device  14  decomposes and removes H 2 O 2  generated in the UV oxidation device  13  and other oxidizing substances by using the catalyst with efficiency. In the decomposition of H 2 O 2 , water is produced, but oxygen is hardly produced as in the case where an anion-exchange resin or active carbon is used. Therefore, this does not increase the amount of DO. 
     Water treated with the catalytic oxidizing substance decomposition device  14  is subsequently passed into the deaeration device  15 . The deaeration device  15  may be a vacuum deaeration device, a nitrogen deaeration device, or a membrane deaeration device. The deaeration device  15  removes DO and CO 2  contained in the water with efficiency. 
     Water treated with the deaeration device  15  is subsequently passed into the mixed-bed ion-exchange device  16 . The mixed-bed ion-exchange device  16  may be a nonregenerative mixed-bed ion-exchange device filled with an anion-exchange resin and a cation-exchange resin, which are mixed together in accordance with the ionic load. The mixed-bed ion-exchange device  16  removes cations and anions contained in the water to increase the purity of the water. The mixed-bed ion-exchange device  16  may be replaced with a multiple-bed ion-exchange device, an electrically regenerative ion-exchange device, or the like. 
     The ultrapure water production apparatus illustrated in  FIG. 1  is merely an example of the ultrapure water production apparatus according to the present invention. In the ultrapure water production apparatus according to the present invention, various devices other than those described above may be used in combination. For example, as illustrated in  FIG. 2 , the UV-irradiation-treated water discharged from the UV oxidation device  13  may be directly introduced into the mixed-bed deionization device  16 . As illustrated in  FIG. 3 , the catalytic oxidizing substance decomposition device  14  may be replaced with an anion-exchange column  19 ′. 
     Although illustration is omitted, an RO membrane separation device may be disposed subsequent to the mixed-bed ion-exchange device. The ultrapure water production apparatus according to the present invention may further include a device that performs a thermal decomposition treatment of raw water under an acidic condition of pH 4.5 and in the presence of an oxidizer in order to decompose urea and other TOC components contained in the raw water and subsequently performs a deionization treatment. A plurality of UV oxidation devices, a plurality of mixed-bed ion-exchange devices, a plurality of deaeration devices, and the like may be arranged in series. The pretreatment system  1  and the primary pure water system  2  are not limited to those described above and may include various devices other than those described above. 
     A membrane included in the first membrane device  17  may be a UF membrane, an MF membrane, or an RO membrane. A membrane included in the second membrane device  18  is a UF membrane or an MF membrane that is not modified with an ion-exchange group. In other words, the following six combinations of the first membrane device  17  and the second membrane device  18  are possible. 
     (1) UF membrane—UF membrane 
     (2) UF membrane—MF membrane that is not modified with ion-exchange group 
     (3) MF membrane—UF membrane 
     (4) MF membrane—MF membrane that is not modified with ion-exchange group 
     (5) RO membrane—UF membrane 
     (6) RO membrane—MF membrane that is not modified with ion-exchange group 
     Alternatively, three or more membrane devices may be arranged in series. For example, an MF membrane device, an RO membrane device, and a UF membrane device, that is, three membrane devices, may be arranged in series. 
     In the case where the membrane devices  17  and  18  are an MF membrane device and a UF membrane device, the diameter of pores formed in the membranes is preferably 1 μm or less, is more preferably 0.001 to 1 μm, and is particularly preferably 0.001 to 0.5 μm. The thickness of the membranes is preferably 0.01 to 1 mm. Examples of a material of the membranes include polyolefins, polystyrenes, polysulfones, polyesters, polyamides, cellulosic materials, polyvinylidene fluoride, and polytetrafluoroethylene. 
     The above-described ultrapure water production apparatus includes a subsystem including a plurality of membrane devices, such as a UF membrane device, disposed in series at the end of the subsystem. This enables high-quality ultrapure water in which the number of microparticles has been markedly reduced to be produced. In addition, the last of the plurality of membrane devices is a UF membrane device or an MF membrane device that is not modified with ion-exchange group. This eliminates the risk of microparticles being generated from the membrane device as in the case where an RO membrane device is used. Furthermore, the MF membrane device used in the present invention is not modified with ion-exchange group. This eliminates the risk of the exchange group detaching from the MF membrane and acting as a source of the microparticles. 
     In the present invention, the membrane devices preferably employ a cross-flow system and are preferably operated at a recovery ratio of about 95% or less. If the flow rate of brine is further reduced, microparticles may be deposited on the membrane, which reduces the likelihood of the membrane blocking microparticles from permeating therethrough. Alternatively, the number of the membrane devices arranged in series may be changed depending on the quality of feedwater while the membrane devices are operated at a recovery ratio of about 95%. 
     In the case where two UF membrane devices are used, removal of microparticles is calculated using the following expressions. 
         C   1   =C   0 ×(1− Re/ 100)+ B  
 
         C   2   =C   1 ×(1− Re/ 100)+ B  
 
     where, 
     C 0 : Concentration of microparticles contained in water fed into the UF membrane [particle/mL] 
     C 1 : Concentration of microparticles contained in water treated with the first UF membrane [particle/mL] 
     C 2 : Concentration of microparticles contained in water treated with the second UF membrane [particle/mL] 
     Re: Ratio of rejection of microparticles by the UF membrane [%] 
     B: Number of microparticles generated from a material of the UF membrane [particle/mL] 
     The ratio of rejection of microparticles by a microparticle removal membrane is calculated by passing water containing model nanoparticles through the membrane and measuring the number of microparticles contained in water fed to the membrane and the number of microparticles contained in water treated with the membrane. 
     Although the diameter of pores of an MF membrane is larger than that of pores of a UF membrane, it is expected that an MF membrane has an adsorption effect due to the difference in the qualities of material between a UF membrane and an MF membrane. In the case where an MF membrane and a UF membrane are arranged in series, the UF membrane device is desirably, but not necessarily, disposed at the end, because a UF membrane is capable of blocking microparticles from permeating therethrough at a higher rejection ratio than an MF membrane. 
     Although an RO membrane has an advantage over a UF membrane in terms of microparticle rejection ratio, microparticles may be generated from the RO membrane or a potting member. Thus, in the case where an RO membrane device is used as a first membrane device, a UF membrane is preferably disposed at the end in order to remove microparticles at a high level. 
     A booster pump or a valve may be interposed between two membrane devices arranged in series or between each adjacent pair of three or more membrane devices arranged in series. For example, when a plurality of membrane devices are arranged in series, the amount of pressure drop is accordingly increased. Therefore, a pump may be interposed between the membrane devices in consideration of the pressure drop. In such a case, a UF membrane is preferably disposed at the end in order to remove microparticles generated from the pump or the valve. It is desirable that particle-filled equipment such as a mixed-bed ion-exchange device or a catalytic oxidizing substance decomposition device be not interposed between the membrane devices, because such particle-filled equipment may cause a fine powder to be generated due to pulverization of the particles. It is preferable that nothing other than a clean pipe be disposed downstream of the UF membrane disposed at the end. 
     In the device according to the present invention, an excessively high recovery ratio increases the risk of microparticles being deposited on the membrane. Accordingly, it is preferable to pay attention to the range of recovery ratio. It is preferable to determine the type of the membrane used for removing microparticles and the number of the membrane devices used on the basis of the diameter of microparticles that are to be removed, the flow rate of water-to-be-treated, and the targeted water quality. 
     As the membrane device is operated, microparticles are likely to accumulate on the surface of the membrane with time and may leak into the treated water. The leakage of the microparticles may also occur when the membrane is broken under an external load. This may deteriorate the quality of the ultrapure water that is to be produced. Accordingly, in the present invention, it is preferable to provide microparticle measuring means and to monitor the number of microparticles contained in the membrane-treated water by the microparticle measuring means in order to prevent the microparticles from leaking into the treated water. 
     A microparticle control system including the microparticle measuring means is described below with reference to  FIGS. 4 and 5 . In  FIGS. 4 and 5 , members having the same function are denoted by the same reference numeral. 
     The microparticle measuring means is not limited, and any commercially available microparticle measuring means may be used. 
       FIG. 4  is a flow diagram illustrating a system for controlling microparticles contained in the treated water, which includes a microparticle counter  31  that measures the number of microparticles contained in water treated with the first membrane device  17  and a microparticle counter  32  that measures the number of microparticles contained in water treated with the second membrane device  18 . 
     Hereinafter, treated water fed to the first membrane device  17  (e.g., in the ultrapure water production apparatuses illustrated in  FIGS. 1 to 3 , water treated with the mixed-bed deionization device  16 ) is referred to as “first-membrane feedwater”; water fed to the second membrane device  18  (normally, water treated with the first membrane device  17 ) is referred to as “second-membrane feedwater”; and water treated with the first membrane device  17  and water treated with the second membrane device  18  are referred to as “first-membrane-treated water” and “second-membrane-treated water”, respectively. 
     In  FIG. 4 , the first membrane device  17  has three membrane modules  17 A to  17 C arranged in parallel, and the second membrane device  18  has three membrane modules  18 A to  18 C arranged in parallel. 
     The first-membrane feedwater is introduced to the membrane modules  17 A to  17 C of the first membrane device  17  from a pipe  21  through the respective branch pipes  21   a ,  21   b , and  21   c . The first-membrane-treated water is fed to the second membrane device  18  through branch pipes  22   a ,  22   b , and  22   c  and a junction pipe  22 . Membrane-concentrated water is returned to the entry side of the subsystem (in the ultrapure water production apparatuses illustrated in  FIGS. 1 to 3 , the subtank  11 ) through branch pipes  23   a ,  23   b , and  23   c  and a junction pipe  23 . Similarly, the second-membrane feedwater (first-membrane-treated water) is introduced to the membrane modules  18 A to  18 C of the second membrane device  18  from the junction pipe  22  through the respective branch pipes  24   a ,  24   b , and  24   c . The second-membrane-treated water, that is, ultrapure water, is fed to a point-of-use through branch pipes  25   a ,  25   b , and  25   c  and a junction pipe  25 . Membrane-concentrated water is returned to the entry side of the subsystem (in the ultrapure water production apparatuses illustrated in  FIGS. 1 to 3 , the subtank  11 ) through branch pipes  26   a ,  26   b , and  26   c  and a junction pipe  26 . 
     Water-sampling branch pipes  27   a ,  27   b ,  27   c , and  27   d  are connected to the branch pipes  22   a  to  22   c  and a junction pipe  22 , respectively, through which water treated with the membrane modules  17 A to  17 C of the first membrane device  17  is taken from the membrane modules  17 A to  17 C. Through the water-sampling branch pipes  27   a ,  27   b ,  27   c , and  27   d , part of the treated water is sampled and fed to the microparticle counter  31 . The water samples taken through the branch pipes  27   a  to  27   d  are fed to the microparticle counter  31  through a junction water-sampling pipe  27 , and the number of microparticles contained in the water is measured. Similarly, water-sampling branch pipes  28   a ,  28   b ,  28   c , and  28   d  are connected to the branch pipes  25   a  to  25   c  and a junction pipe  25 , respectively, through which water treated with the membrane modules  18 A to  18 C of the second membrane device  18  is taken from the membrane modules  18 A to  18 C. Through the water-sampling branch pipes  28   a ,  28   b ,  28   c , and  28   d , part of the treated water is sampled and fed to the microparticle counter  32 . The water samples taken through the branch pipes  28   a  to  28   d  are fed to the microparticle counter  32  through a junction water-sampling pipe  28 , and the number of microparticles contained in the water is measured. 
     V 1  to V 18 , V 20 , and V 30  are automatic valves each disposed on a corresponding one of the above pipes. 
     The membrane module  17 C of the first membrane device  17  and the membrane module  18 C of the second membrane device  18  are auxiliary membrane modules; in normal times, the membrane modules  17 A and  17 B and the membrane modules  18 A and  18 B are used for removing microparticles. 
     Specifically, of the automatic valves V 1  to V 18 , V 20 , and V 30  each disposed on a corresponding one of the pipes, V 7  to V 9  and V 16  to V 18  are closed, and the automatic valves V 1 , V 2 , V 4 , V 5 , V 10 , V 11 , V 13 , and V 14  are opened. The automatic valves V 3 , V 6 , and V 20  are opened and closed one by one. Similarly, the automatic valves V 12 , V 15 , and V 30  are opened and closed one by one. 
     The first-membrane feedwater is introduced from the pipe  21  to the membrane modules  17 A and  17 B through the respective branch pipes  21   a  and  21   b  and subjected to a membrane treatment. The resulting treated water is fed to the second membrane device  18  through the branch pipes  22   a  and  22   b  and the junction pipe  22 . The concentrated water produced with the membrane modules  17 A and  17 B, which contains a high concentration of microparticles, is returned to the subtank disposed on the entry side of the subsystem through the branch pipes  23   a  and  23   b , respectively, and the junction pipe  23 . 
     The first-membrane-treated water is then introduced from the junction pipe  22  to the membrane modules  18 A and  18 B through the respective branch pipes  24   a  and  24   b  and subjected to a membrane treatment. The resulting treated water (ultrapure water) is fed to the point-of-use through the branch pipes  25   a  and  25   b  and the junction pipe  25 . The concentrated water produced with the membrane modules  18 A and  18 B, which contains a high concentration of microparticles, is returned to the subtank disposed on the entry side of the subsystem through the branch pipes  26   a  and  26   b , respectively, and the junction pipe  26 . 
     In the embodiment illustrated in  FIG. 4 , part of water treated with the membrane module  17 A, part of water treated with the membrane module  17 B, and a mixture thereof, that is, part of the first-membrane-treated water discharged from the first membrane device  17 , are fed to the microparticle counter  31  one by one by opening and closing the automatic valve V 3 , the automatic valve V 6 , and the automatic valve V 20  one by one. This enables the number of microparticles contained in water treated with the membrane module  17 A, the number of microparticles contained in water treated with the membrane module  17 B, which are used for removing microparticles, and the number of microparticles contained in a mixture thereof, that is, the first-membrane-treated water, one by one by using only one microparticle counter  31 . Similarly, part of water treated with the membrane module  18 A, part of water treated with the membrane module  18 B, and a mixture thereof, that is, part of the second-membrane-treated water discharged from the second membrane device  18 , are fed to the microparticle counter  32  one by one by opening and closing the automatic valve V 12 , the automatic valve V 15 , and the automatic valve V 30  one by one. This enables the number of microparticles contained in water treated with the membrane module  18 A, the number of microparticles contained in water treated with the membrane module  18 B, which are used for removing microparticles, and the number of microparticles contained in a mixture thereof, that is, the second-membrane-treated water, one by one by using only one microparticle counter  32 . 
     By measuring the number of microparticles contained in water treated with each of the membrane modules included in each membrane device, which are used for removing microparticles, and the number of microparticles contained in the entire membrane-treated water, the leakage of microparticles from each membrane module and a reduction in the microparticle rejection ratio of the membrane module can be detected. In addition, the overall performance of the membrane device can be monitored. In the case where the leakage of microparticles from any of the membrane modules or a reduction in the microparticle rejection ratio of any of the membrane modules is detected, feeding of water to the membrane module is stopped, and feeding of water to the auxiliary membrane module is started. Thus, the auxiliary membrane module is used for removing microparticles. Specifically, in the case where the leakage of microparticles into water treated with the membrane module  17 A or a reduction in the microparticle rejection ratio of the membrane module  17 A is detected, the automatic valves V 1 , V 2 , and V 3  are closed, the automatic valves V 7  and V 8  are opened, and the automatic valve V 9 , the automatic valve V 6 , and the automatic valve V 20  are opened and closed one by one such that the water treated with the membrane module  17 B and the membrane module  17 C are used for removing microparticles and such that part of the water treated with the membrane module  17 B, part of the water treated with the membrane module  17 C, and part of the first-membrane-treated water are sampled one by one and the number of microparticles contained in the water sample is measured using the microparticle counter  31 . Meanwhile, maintenance of the membrane module  17 A, such as replacement of the membrane, is performed. 
     The same treatment as in the first membrane device  17  is performed in the second membrane device  18 . 
     The frequency at which the automatic valves are switched for sampling water used for measuring the number of microparticles contained therein is not limited, but preferably such that the number of microparticles contained in water treated with each membrane module and the number of microparticles contained in water treated with the entire membrane device can be measured for 30 to 60 minutes. 
     As described above, by measuring the number of microparticles contained in water treated with each of the membrane modules arranged in parallel in each membrane device and the number of microparticles contained in water treated with the membrane device and switching flow channels as needed, the leakage of microparticles into the membrane-treated water can be prevented with certainty, which makes it possible to produce high-quality ultrapure water in a consistent manner. 
     The microparticle control system illustrated in  FIG. 5  has the same structure as that illustrated in  FIG. 4 , except that only one microparticle counter  30  is used instead of the two microparticle counters  31  and  32  illustrated in  FIG. 4 , and the water samples taken from water-sampling pipes  27   a  to  27   d  and water-sampling pipes  28   a  to  28   d  are fed to the microparticle counter  30  through a junction water-sampling pipe  29  one by one such that the number of microparticles contained in each treated water sample can be measured using only one microparticle counter  30 . 
     By providing only one microparticle counter for a plurality of membrane devices and measuring the number of microparticles contained in the treated water sample taken from each position one by one by switching the automatic valves, the number of the microparticle counters used can be reduced. Furthermore, an increase in the size of the ultrapure water production apparatus due to the attachment of microparticle counters to the ultrapure water production apparatus may be limited. This reduces the facility cost and the amount of maintenance work. 
     The number of the membrane modules included in the membrane device is generally, but not limited to,  2  to  20 . The number of the auxiliary membrane modules is not limited to one and may be two or more. 
     The number of microparticles contained in the membrane-treated water may be measured at the last membrane device or a membrane device immediately before the last membrane device. Alternatively, the number of microparticles contained in the treated water may be measured at each of the plurality of membrane devices arranged in series. 
     In general, the last membrane device is used for finishing the removal of microparticles, and the risk of microparticles leaking into water treated with the last membrane device can be eliminated when a certain degree of microparticle rejection ratio is achieved at a membrane device immediately before the last membrane device. Therefore, it is preferable to dispose the microparticle measuring means, which is used for measuring the number of microparticles contained in the membrane-treated water, at least in the membrane device immediately before the last membrane device. It is preferable to dispose microparticle measuring means in both membrane device immediately before the last membrane device and last membrane device such that the number of microparticles contained in water treated with each of these membrane devices can be measured. 
     In the above-described embodiment, the concentrated water (brine water) discharged from the first membrane device  17  and the concentrated water discharged from the second membrane device  18  are both returned to the subtank. However, the present invention is not limited to this. Alternatively, the concentrated water may be fed to an additional brine collection tank. 
     EXAMPLES 
     The present invention is described further in detail below with reference to Examples below. 
     In Examples below, the concentration of microparticles was determined by measuring the number of microparticles having a diameter of 10 nm or more in water by using a centrifugal filtration-SEM microparticle counter. 
     Example 1 
     Ultrapure water was produced using an ultrapure water production apparatus as illustrated in  FIG. 1  in which UF membrane devices (external-pressure-type hollow fiber membrane, material: polysulfone, nominal molecular weight cutoff: 6,000 (insulin), rejection ratio Re: 99.90%) were used as a first membrane device  17  and a second membrane device  18  disposed at the end of the subsystem. Table 1 describes the results and the like of the measurement of the concentration of microparticles contained in water fed to each of the membrane devices and the concentration of microparticles contained in water treated with each of the membrane devices. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Abbre- 
                   
                   
               
               
                 Item 
                 viation 
                 Unit 
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Concentration of microparticles in 
                 C 0   
                 [Particle/L] 
                 1,000,000 
               
               
                 water fed to first membrane device 
               
               
                 17 
               
               
                 Rejection ratio 
                 Re 
                 [%] 
                 99.90 
               
               
                 Number of dust particles origi- 
                 B 
                 [Particle/L] 
                 50 
               
               
                 nating from UF membrane 
               
               
                 Concentration of microparticles 
                 C 1   
                 [Particle/L] 
                 1,050 
               
               
                 in water treated with first 
               
               
                 membrane device 17 
               
               
                 Concentration of microparticles 
                 C 2   
                 [Particle/L] 
                 51 
               
               
                 in water treated with second 
               
               
                 membrane device 18 
               
               
                   
               
            
           
         
       
     
     As described in Table 1, while the concentration of microparticles in water treated with the first membrane device  17 , was 1,000 particle/L or more, the concentration of microparticles in water treated with the second membrane device  18  was 51 particle/L. This confirms that using two UF membrane devices arranged in series reduces the concentration of microparticles to 100 particle/L or less. 
     Examples 2 to 6 
     Ultrapure water was produced as in Example 1, except that the combination of the first membrane device and the second membrane device was changed as described in Table 2. For each case, the concentration of microparticles was determined by measuring the number of microparticles contained in water. Table 2 describes the results. In addition to the UF membrane devices, the following membrane devices were used. 
     MF membrane device that is not modified with an ion-exchange group: external-pressure-type hollow fiber membrane, material: surface-modified PTFE, pore diameter: 50 nm 
     RO membrane device: spiral-wound type, material: polyamide 
     Example 7 
     Ultrapure water was produced as in Example 1, except that three membrane devices arranged in series, that is, an MF membrane device, an RO membrane device, and a UF membrane device, were used. The concentration of microparticles was determined by measuring the number of microparticles contained in water. Table 2 describes the results. The membrane devices used were the same as described above. 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Concentration of microparticles in water 
               
               
                   
                 treated with membrane device (particle/L) 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Combination 
                 First 
                 Second 
                 Third 
               
               
                   
                 of membrane 
                 membrane 
                 membrane 
                 membrane 
               
               
                 No. 
                 devices 
                 device 
                 device 
                 device 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 2 
                 UF-MF 
                 1,050 
                 100 
                   
               
               
                 Example 3 
                 MF-UF 
                 48,050 
                 98 
               
               
                 Example 4 
                 MF-MF 
                 48,050 
                 2,356 
               
               
                 Example 5 
                 RO-UF 
                 10,100 
                 60 
               
               
                 Example 6 
                 RO-MF 
                 10,100 
                 535 
               
               
                 Example 7 
                 MF-RO-UF 
                 48,050 
                 10,005 
                 60 
               
               
                   
               
            
           
         
       
     
     As described in Table 2, in Examples 2 to 7, high-quality ultrapure water in which the number of microparticles had been markedly reduced was produced by using two or three membrane devices. 
     Example 8 
     Ultrapure water was produced as in Example 1. In Example 8, microparticle counters (“NanoCount25+” produced by Lighthouse)  31  and  32  were disposed in the first membrane device  17  that was a UF membrane device and the second membrane device  18  that was a UF membrane device, respectively, as illustrated in  FIG. 4  in order to measure the number of microparticles contained in water treated with each of the first membrane device  17  and the second membrane device  18 . 
     The UF membrane devices used as the first membrane device  17  and the second membrane device  18  included UF membrane modules  17 A to  17 C and UF membrane modules  18 A to  18 C, respectively. The UF membrane modules  17 C and  18 C served as auxiliary membrane modules. In normal times, the UF membrane modules  17 A and  17 B and the UF membrane modules  18 A and  18 B were used for performing the treatment. 
     In the first membrane device  17 , water treated with the UF membrane module  17 A, water treated with the UF membrane module  17 B, and the first-membrane-treated water discharged from the first membrane device  17  were fed to the microparticle counter  31  one by one by switching the automatic valves V 3 , V 6 , and V 20  (at a frequency of once every 30 minutes) in order to measure the number of microparticles contained in the water. Similarly, in the second membrane device  18 , water treated with the UF membrane module  18 A, water treated with the UF membrane module  18 B, and the second-membrane-treated water discharged from the second membrane device  18  were fed to the microparticle counter  32  one by one by switching the automatic valves V 12 , V 15 , and V 30  (at a frequency of once every 30 minutes) in order to measure the number of microparticles contained in the water. 
       FIGS. 6 a  and 6 b    illustrate the changes with time in the concentrations of microparticles which were determined from the results of measurement of the number of microparticles contained in water treated with the UF membrane module  17 A and the number of microparticles contained in water treated with the UF membrane module  17 B, respectively. This confirms that the durability of the UF membrane module varied over lots even among UF membrane modules included in the same membrane device and that, in the UF membrane module  18 A, the leakage of microparticles started earlier than in the UF membrane module  18 B. 
     The treatment was continued by feeding the first-membrane feedwater to the UF membrane module  17 B and the auxiliary UF membrane module  17 C instead of the UF membrane module  17 A and the UF membrane module  17 B by switching the automatic valves immediately after the start of the leakage of microparticles from the UF membrane module  18 A. As a result, high-quality ultrapure water having a microparticle concentration of 100 particle/L or less was produced with the second membrane device  18  in a consistent manner for a long period of time, as in Example 1. 
     When the flow channels were not switched as described above, that is, the treatment was continued using the UF membrane module  17 A and the UF membrane module  17 B, even after the start of the leakage of microparticles from the UF membrane module  18 A, microparticles started leaking into water treated with the second membrane device  18  and it became impossible to satisfy the control value for the number of microparticles contained in ultrapure water 600 days after the start of the leakage of microparticles from the UF membrane module  17 A. 
     Although the present invention has been described in detail with reference to a particular embodiment, it is apparent to a person skilled in the art that various modifications can be made therein without departing from the spirit and scope of the present invention. 
     The present application is based on Japanese Patent Application No. 2013-209175 filed on Oct. 4, 2013, and Japanese Patent Application No. 2014-013478 filed on Jan. 28, 2014, which are incorporated herein by reference in their entirety. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  PRETREATMENT SYSTEM 
               2  PRIMARY PURE WATER SYSTEM 
               3  SUBSYSTEM 
               17  FIRST MEMBRANE DEVICE 
               17 A, 17 B, 17 C FIRST MEMBRANE MODULE 
               18  SECOND MEMBRANE DEVICE 
               18 A, 18 B, 18 C SECOND MEMBRANE MODULE 
               30 , 31 , 32  MICROPARTICLE COUNTER